Tailoring Electrocatalytic Pathways: A Comparative Review of the Electrolyte’s Effects on Five Key Energy Conversion Reactions

Abstract

The advancement of efficient energy conversion and storage technologies is fundamentally linked to the development of electrochemical systems, including fuel cells, batteries, and electrolyzers, whose performance depends on key electrocatalytic reactions: hydrogen evolution (HER), oxygen evolution (OER), oxygen reduction (ORR), carbon dioxide reduction (CO₂RR), and nitrogen reduction (NRR). Beyond catalyst design, the electrolyte microenvironment significantly influences these reactions by modulating charge transfer, intermediate stabilization, and mass transport, making electrolyte engineering a powerful tool for enhancing performance. This review provides a comprehensive analysis of how fundamental electrolyte properties, including pH, ionic strength, ion identity, and solvent structure, affect the mechanisms and kinetics of these five reactions. We examine in detail how the electrolyte composition and individual ion contributions impact reaction pathways, catalytic activity, and product selectivity. For HER and OER, we discuss the interplay between acidic and alkaline environments, the effects of specific ions, interfacial electric fields, and catalyst stability. In ORR, we highlight pH-dependent activity, selectivity, and the roles of cations and anions in steering 2e⁻ versus 4e⁻ pathways. The CO₂RR and NRR sections explore how the electrolyte composition, local pH, buffering capacity, and proton sources influence activity and the product distribution. We also address challenges in electrolyte optimization, such as managing competing reactions and maximizing Faradaic efficiency. By comparing the electrolyte’s effects across these reactions, this review identifies general trends and design guidelines for enhancing electrocatalytic performance and outlines key open questions and future research directions relevant to practical energy technologies.

1. Introduction

1.1. Importance of Electrocatalysis in Energy Conversion and Storage

The 21st century faces challenges such as the depletion of non-renewable energy sources, environmental harm caused by fossil fuels, and increasing global energy demand. Renewable energy sources, such as solar, wind, and biomass, are being explored. However, they are intermittent and require efficient energy transformation and storage. Fuel cells and electrolyzers are key for electrochemical energy transformation, while lithium-ion and redox flow batteries are leading efficient storage technologies. These technologies rely on key reactions, including the hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and oxygen evolution reaction (OER), which face challenges due to slow reaction kinetics and high energy losses. Electrocatalysts are, therefore, crucial for enhancing the aforementioned (HER, OER, and ORR) and other vital electrochemical processes in energy systems by reducing energy barriers and boosting performance across renewable energy conversion and storage applications [1]. Electrocatalysts are critical components in electrolyzers, as they accelerate the HER and OER—the two key processes that enable efficient water splitting into hydrogen and oxygen. The sluggish reaction kinetics of HER/OER are a barrier to producing green hydrogen on a large scale. By lowering the activation energy barriers, these catalysts enhance reaction kinetics, reduce energy losses, and improve the overall efficiency of hydrogen production.

A fuel cell (FC) is an electrochemical device that converts the chemical energy of a fuel (typically hydrogen) and an oxidant (usually oxygen) directly into electricity through controlled redox reactions, producing water and heat as byproducts. Unlike batteries, fuel cells operate continuously as long as fuel and oxidant are supplied, making them ideal for applications like electric vehicles, stationary power generation, and portable electronics. Categorized by their electrolyte type and operating temperature, FCs include hydrogen FCs, phosphoric acid FCs, molten carbonate FCs, and formic acid FCs. ORR and hydrogen oxidation (HOR) reactions are the key cathodic and anodic half-reactions in FCs. Therefore, electrocatalysts are indispensable in fuel cells for accelerating the sluggish ORR and HOR, which enable efficient electricity generation from chemical energy. Platinum-based catalysts remain the benchmark for both reactions, but their high cost and scarcity drive research into alternatives like alloy nanoparticles, single-atom catalysts, and metal-free carbon materials. Researchers are developing nanostructured and composite materials to enhance catalytic performance and durability in next-generation energy storage systems. For a deeper understanding of recent progress in electrocatalyst and electrode development, readers are advised to consult a concise review by the authors of [2].

As already mentioned, hydrogen, a high-energy-density and emission-free fuel, can be produced by the electrochemical electrolysis of water. This process involves HER and OER at the electrodes, requiring a theoretical minimum energy of 1.23 V. However, practical water splitting requires higher energy due to cell resistances and inefficiencies, requiring an effective electrocatalyst to minimize overpotential and enhance efficiency. Up until now, the most successful electrocatalysts for water electrolysis have been those made from precious metals, for example, Pt for HER and RuO₂ and IrO₂ oxides for OER. However, their scarcity and high cost make it challenging to scale up their use in industrial applications. In hot, arid coastal regions, seawater electrolysis represents a practical alternative to freshwater for hydrogen production, although its high salinity and impurities pose challenges to catalyst stability. Thus, the implementation of direct seawater electrolysis again requires the development of suitable electrocatalysts for stable performance. As summarized in [3], the design of OER/HER bifunctional electrocatalysts is preferred for direct seawater electrolysis due to their efficient utilization and simplified electrolytic equipment system without cross-contamination.

Electrocatalytic OER is a pivotal electrochemical process, serving as the essential second half-reaction in both water electrolysis and CO₂ reduction (CO₂RR) systems. In water splitting, OER complements hydrogen evolution at the cathode, while in CO₂RR, it balances the anodic reaction needed for complete electrolytic processes. This dual role makes OER critical for enabling renewable energy storage through green hydrogen production and for facilitating the synthesis of carbon-neutral fuels. However, OER faces significant challenges, including its high overpotential, sluggish four-electron (4e⁻) transfer kinetics, and dependence on scarce noble metal catalysts (e.g., IrO₂ and RuO₂). Recent advances in transition metal-based catalysts (particularly NiFe and CoFe oxides) and innovative material designs show promise for improving efficiency and reducing costs. Optimizing these electrocatalysts along with system parameters remains crucial for advancing both water splitting and CO₂RR technologies toward commercial viability [4].

The ORR is the cathodic half-reaction in fuel cells and metal–air batteries (MABs), enabling renewable energy storage and conversion with applications in electric vehicles and grid storage. However, its sluggish kinetics demand efficient catalysts to compete with combustion-based energy systems. Modern electrocatalyst design focuses on maximizing the 4e⁻ pathway (O₂ to H₂O) while minimizing costly platinum-group metals (PGMs) through innovative materials like atomically dispersed Fe-N-C catalysts and strained alloy nanostructures. However, challenges persist in balancing activity, stability, and cost—particularly under acidic conditions for proton exchange membrane fuel cells. Recent advances in operando characterization and machine learning-guided catalyst discovery are accelerating the development of next-generation ORR electrocatalysts to meet industrial demands.

Electrocatalytic CO₂RR is another pivotal electrochemical process, offering a sustainable pathway to convert greenhouse gases into valuable fuels and chemicals while addressing global carbon emissions. It is a critical process for addressing the over 30 gigatons of CO₂ emitted annually from fossil fuels, which disrupts the carbon cycle and accelerates climate change. By converting CO₂ into fuels and value-added chemicals using renewable energy, this method enables a sustainable, carbon-neutral cycle while supporting a circular carbon economy. The electrochemical approach is particularly advantageous due to its scalability, mild operating conditions, and ability to tune product selectivity by applying an external potential. However, challenges such as complex multi-step reactions, diverse product distribution (e.g., CO, formic acid, methane, and ethylene), and high energy barriers due to the stability of CO₂ hinder its efficiency. To overcome these limitations, the development of highly selective and efficient electrocatalysts, along with optimized reaction conditions, is essential for making CO₂RR a viable large-scale solution [5].

The electrocatalytic nitrogen reduction reaction (NRR) is another key electrochemical process, offering a sustainable pathway to convert atmospheric N₂ into ammonia (NH₃) under mild conditions, which could revolutionize fertilizer production and reduce reliance on the energy-intensive Haber–Bosch process. It is a critical strategy for addressing the over 150 million tons of NH₃ produced annually, which accounts for ~1–2% of global CO₂ emissions due to the fossil fuel-dependent synthesis process. By utilizing renewable electricity to drive N₂ fixation, this method provides a carbon-neutral alternative for ammonia synthesis, supporting a decentralized energy economy. The electrochemical approach is particularly attractive due to its scalability, ambient operation, and potential for tunable activity through the design of catalysts. However, challenges such as the extreme stability of N₂, competing HER, and low Faradaic efficiencies (FEs) hinder practical implementation. To overcome these limitations, the development of highly active, selective, and stable electrocatalysts, along with optimized electrolytes and reactor designs, is essential for advancing NRR toward industrial viability [6].

Over the past few decades, extensive research has been focused on developing effective and stable electrocatalysts for electrochemical processes. Despite advancements in various effective modification techniques, energy conversion technologies like water splitting and fuel cells remain a long way from practical applications [1].

1.2. How to Use This Review

This review is structured to allow both comprehensive reading and targeted consultation. Although broad in scope, this review emphasizes comparative insights across systems and is structured to allow both deep and cross-sectional reading. Readers with a broad interest in the electrolyte’s effects on electrocatalysis are encouraged to read Section 1 and Section 2 for background and theoretical context. Section 3, Section 4, Section 5, Section 6 and Section 7 provide focused discussions on five key reactions (HER, OER, ORR, CO₂RR, and NRR), each written to be largely self-contained. Thus, if reading in full, some repetitions might appear. If your research concerns only one of these reactions, feel free to go directly to the relevant section. We note that the present review article typically addresses aqueous electrolytes. Other types of electrolytes, such as those based on deep eutectic solvents and molten salts, will not be considered here.

Section 8 offers a cross-cutting analysis, extracting general trends and outlining design principles that emerge from comparing all five reactions. Section 9 highlights current knowledge gaps, future research directions, and practical implications for the development of energy technology. Readers seeking a broader perspective or interested in guiding their own electrolyte or catalyst design efforts may wish to begin with Section 8 and Section 9.

We also note that the topic of this review has received increasing attention in recent years, and other reviews can be found in the literature, with varying focuses or approaches to the same topic [7,8,9,10].

1.3. Role of the Electrolyte in Electrocatalytic Reactions

Electrolytes play a crucial role in electrochemical processes, such as batteries, fuel cells, and electrolyzers. They provide the necessary medium for charge transfer, enabling electrochemical reactions to occur and ensuring the circuit’s continuity. The nature of the electrolyte ions is also crucial, as they modulate the activity and selectivity of target products through what is often termed the “ion effect”. Thus, the electrolyte’s properties, such as its pH, ion identity, ionic strength, and solvent characteristics, can modulate the adsorption of reactants and products on the catalyst surface. Moreover, specific ions within the electrolyte can directly participate in the reaction, altering the reaction pathway.

The electrolyte influences the formation and the structure of the electric double layer (EDL) at the electrode–electrolyte interface, which is critical for charge transfer. Furthermore, it can affect the local pH and the stability of reaction intermediates. Therefore, selecting the proper electrolyte is crucial for optimizing electrocatalytic performance, as it not only acts as a passive conductor (as usually presented in electrochemistry textbooks) but also plays an active role in shaping the outcomes of electrocatalytic processes. Recent research has demonstrated that the interactions between the electrolyte and the electrocatalyst significantly influence the electrocatalytic properties. The electrolyte’s impact on electrochemical reactions occurs through two mechanisms: (i) via chemisorption of adsorbents in the inner Helmholtz layer involving electron transfer and (ii) through weak van der Waals interactions between the electrode and spectator (supporting) ions in the electrolyte at the outer Helmholtz layer [11,12].

Across diverse electrochemical reactions, such as HER, OER, ORR, CO₂RR, and NRR, the electrolyte’s role is both essential and complex. Subsequent sections will address the specific influence of the electrolyte on each of these five electrochemical processes individually.

1.4. Scope of This Article: Five Key Electrocatalytic Reactions

A thorough understanding of the five key electrochemical reaction pathways is crucial for developing effective catalytic materials. This section of this review will highlight the significance of the five key electrocatalytic reactions introduced previously. It will also emphasize the importance of understanding their fundamental mechanisms and the impact of electrolytes on each reaction, aiming to establish a thorough foundational understanding.

Understanding the intricate mechanisms of HER is essential for advancing electrocatalysis. This reaction serves as a model system for studying fundamental electrochemical processes. Research into HER has led to the development of better electrocatalysts, which can enhance the efficiency of various electrochemical reactions. As the cathodic half-reaction in water electrolyzers (2H⁺ + 2e⁻ → H₂ in acid; 2H₂O + 2e⁻ → H₂ + 2OH⁻ in base), HER enables scalable H₂ generation when paired with renewable electricity, offering a sustainable alternative to fossil-fuel-derived hydrogen. The HER operates through distinct mechanisms influenced by electrolyte pH. Under acidic conditions, the Volmer–Tafel or Volmer–Heyrovsky pathways dominate, involving proton adsorption and subsequent hydrogen molecule formation, whereas alkaline media necessitate a water dissociation step. While platinum remains the gold standard for HER due to its optimal hydrogen adsorption energy, the reaction kinetics are significantly impacted by pH, with acidic electrolytes generally yielding faster rates than alkaline or neutral electrolytes. Furthermore, the presence of specific cations, such as Li⁺, can modulate the interfacial water structure, thereby affecting proton availability and influencing HER performance. Recent advancements have explored the use of buffered electrolytes, such as phosphate solutions, to stabilize the pH near the electrode surface during high-current operation, thereby mitigating pH fluctuations and enhancing overall efficiency.

The ORR is essential in electrocatalysis because it powers energy conversion devices, such as FCs and MABs, which are crucial for sustainable energy solutions. ORR (O₂ + 4H⁺ + 4e⁻ → 2H₂O in acid; O₂ + 2H₂O + 4e⁻ → 4OH⁻ in base) drives electricity production through oxygen reduction, yet its slow kinetics due to intricate 4e⁻ transfers limit system efficiency. ORR is both foundational for renewable energy progress and a model for studying multi-step electrocatalysis, cementing its status as vital for clean energy advancements. This positions ORR as a primary target for catalyst development, where slight enhancements in performance can significantly reduce costs and increase energy output [13,14]. Its dependence on the electrolyte’s properties (e.g., pH and competing anions) and catalyst design (e.g., Pt-based or Fe-N-C systems) highlights its role in optimizing devices like proton exchange membrane (PEM) FCs or alkaline FCs. Thus, studying the role of electrolytes in the ORR is crucial for optimizing its performance and product selectivity by understanding how the electrolyte’s properties affect reaction kinetics and pathways, while taking into consideration complexity of its mechanism in different electrolytes [15]. This knowledge also enhances catalyst stability and provides insights into the fundamental reaction mechanisms, ultimately driving the development of more efficient electrochemical devices.

The electrochemical CO₂ reduction process aims to transform atmospheric CO₂ into valuable fuels and chemicals through a complex, multi-step process involving electron and proton transfers, with product selectivity heavily influenced by catalyst design and the electrolyte’s properties. Efficient catalyst design and reaction optimization are crucial for overcoming these hurdles [5]. The pursuit of efficient CO₂ reduction centers on creating better catalysts. While Cu can yield hydrocarbons, it often produces a mix of products, limiting its usefulness. Researchers are exploring nanostructured forms of copper, as well as gold and silver, to enhance the formation of specific products. However, concerns remain regarding cost and diverse outputs. Single-atom catalysts offer promise in precisely controlling reaction pathways for targeted outputs. Catalyst morphology (e.g., porosity, nanostructuring) and electrolyte composition (pH-dependent proton availability) critically influence active sites, mass transport, and product distribution, with alkaline conditions favoring CO production and acidic media promoting formic acid production. The electrolyte plays a critical role in determining the local pH and the availability of protons and electrons at the catalyst surface. Cation effects, especially those of alkali metal ions, have been shown to influence CO₂ activation and product selectivity. Additionally, reaction parameters like temperature and pressure further tune kinetics and selectivity, underscoring the multifaceted optimization required for practical CO₂RR systems.

The NRR holds significant promise for sustainable ammonia production (N₂ + 6H⁺ + 6e⁻ → 2NH₃ in acid; N₂ + 6H₂O + 6e⁻ → 2NH₃ + 6OH⁻ in base), a vital chemical for fertilizers and energy storage. Studying the NRR is essential for developing environmentally friendly ammonia production methods to replace the energy-intensive Haber–Bosch process. However, its practical application is hindered by the inherent stability of the triple N≡N bond and the complex multi-proton, multi-electron transfer process, resulting in low efficiency and selectivity. Moreover, understanding how the electrolyte’s properties and catalyst design influence the reaction pathway and efficiency is key to unlocking the potential of electrochemical nitrogen fixation for a more sustainable future. Research is focused on developing catalysts with high selectivity for ammonia production and minimizing the competing HER. The effects of electrolytes and ions are crucial in determining the reaction pathway and efficiency. Aqueous electrolytes are preferred due to their high proton availability. However, they also pose challenges due to the competitive HER. Thus, investigating the role of electrolytes in the NRR is critical for enhancing its activity and selectivity towards ammonia by elucidating how the electrolyte’s properties influence reaction mechanisms and kinetics, ultimately paving the way for sustainable ammonia synthesis and related electrochemical technologies.

Therefore, the fundamental electrocatalytic reactions of HER, OER, ORR, CO₂RR, and NRR are central to advancing clean energy technologies and sustainable chemical production in aqueous media. Understanding the complex reaction mechanisms of each, along with the critical influence of the electrolyte’s properties and catalyst design, is paramount for achieving efficient and selective transformations. Ongoing research focuses on developing advanced catalytic materials and optimizing reaction conditions, particularly the electrolyte environment, to overcome current limitations and realize the full potential of these electrochemical processes for a more sustainable future.

2. Fundamental Aspects of the Electrolyte’s Effects

2.1. Influences of pH, Ionic Strength, and Ion Identity

The effectiveness of an electrocatalytic reaction largely depends on the electrode surface composition and structure. A fundamental understanding of the processes at the electrode–electrolyte interface is also essential for optimizing catalytic performance. These factors are crucial for advancing cost-effective, efficient, and sustainable hydrogen technologies, such as electrolyzers, fuel cells, and batteries [16].

HER research employs diverse electrolytes, varying in pH and solvent, with most studies focusing on extreme acidic or alkaline conditions. However, near-neutral pH is gaining attention. While strong acidic or alkaline environments enhance reaction rates and ionic conductivity, they introduce challenges such as corrosion and the need for expensive materials [1,7]. Using neutral or near-neutral electrolytes in electrocatalytic reactions offers multiple advantages to overcome current limitations, as outlined in a recent review by Fayuan et al. (2024) [1]. HER studies utilize a range of electrolytes, including common acids such as H₂SO₄ and HCl, as well as bases like NaOH and KOH, and alternative media, including LiOH, solid electrolytes, and ionic liquids [17].

The electrolyte pH has a significant influence on the HER. In acidic media, HER is driven by H₃O⁺ reduction, while alkaline conditions shift the mechanism to water reduction, often requiring higher overpotentials due to sluggish kinetics [18]. In neutral/near-neutral electrolytes, HER/ORR/OER mirror acidic/alkaline mechanisms but face added complexity due to thermodynamically unfavorable water dissociation, requiring extra overpotential to generate scarce H⁺/OH⁻ ions from water molecules [1,19]. The process involves the dissociation of water, followed by the transport of protons. However, the evolution of the H-bond network and local pH changes hinder hydroxide diffusion, leading to proton–hydroxide recombination that impedes the HER and increases the overpotential [7]. Near-neutral pH introduces additional complexity. For instance, the HER relies on water dissociation, progressing through dual pathways: initial H₃O⁺ reduction at low overpotentials, followed by water reduction at higher potentials [18]. Additional challenges include buffer group adsorption (e.g., phosphoric acid in phosphate-buffered saline—PBS), which poisons catalysts like Pt/C [20], coupled with inferior ionic conductivity and O₂ solubility compared to extreme pH systems, potentially shifting the rate-determining steps [21]. However, buffered electrolytes (e.g., phosphate and borate) may mitigate pH gradients, stabilizing activity by providing proton donors, such as H₂PO₄⁻ [22,23]. Yet, hydrogen binding energy (HBE) remains pH-sensitive, weakening in acidic media but strengthening under alkaline conditions, thereby modulating kinetics [24,25].

Cations in the EDL profoundly impact reaction kinetics. Smaller cations (e.g., Li⁺) enhance HER activity on Pt due to lower proton transfer barriers, while larger cations (e.g., Cs⁺) disrupt interfacial water networks, increasing kinetic overpotentials [26,27]. Alkali metal cations (AM⁺s) also alter the ORR activity, with the trend Cs⁺ > K⁺ > Na⁺ > Li⁺ linked to cation hydration energies [28]. In mixed-cation systems (e.g., Li⁺/K⁺), synergistic effects emerge, where K⁺ can amplify ORR activity beyond predictions from noncovalent interaction models [27]. Such effects are electrode-specific: Li⁺ suppresses HER on Pt but enhances it on Au, highlighting metal–cation interactions as a key design parameter [7,27].

Anion identity affects catalysis through its impact on adsorption strength and the stabilization of intermediates. Weakly adsorbing anions (e.g., ClO₄⁻) minimize site blocking, making them ideal for benchmarking Pt-based HER/ORR, whereas strongly adsorbing species (e.g., Cl⁻ and SO₄²⁻) suppress activity by competing with reactive intermediates [7,29]. In ORR, HClO₄ outperforms H₂SO₄ due to weaker anion–Pt interactions, while Cl⁻ poisoning shifts HER overpotentials to higher values [29]. Anions also modulate local electrostatic fields, altering HBE and intermediate adsorption energies, as demonstrated by DFT studies [30].

The structure of EDL, dictated by the ion composition and pH, governs catalytic efficiency. At the potential of maximum entropy (PME), disordered interfacial water reduces kinetic barriers, facilitating reactions [31]. Cation tuning (e.g., Na⁺/K⁺ ratios) can linearly adjust the PME, enabling the optimization of activity in near-neutral electrolytes [16]. Practical systems must strike a balance between ion-specific effects and cost, as the choice of electrolyte impacts both performance and durability. Advances in water-in-salt electrolytes (e.g., lithium bis(trifluoromethane sulfonyl)imide (LiTFSI)) were found to expand stability windows but introduce secondary effects like LiF passivation, underscoring the need for holistic design [32].

While the effects of cations are increasingly understood, the roles of anions remain debated, with studies reporting a minimal influence in some systems (e.g., CNTs) but significant impacts in others (e.g., Pt) [29]. The interplay between ion-specific interfacial structuring and catalyst geometry (e.g., low-coordinated Pt sites) warrants further study [33]. Integrating in operando techniques and computational models will be key to unraveling these complexities for next-generation electrocatalysts.

2.2. Solvent Effects, Double-Layer Structure, and Hydrogen Bonding

Aqueous electrocatalysis plays a pivotal role in achieving carbon neutrality by transforming renewable electricity into value-added chemical fuels. Water serves as the versatile backbone of electrocatalytic systems, acting simultaneously as a solvent, reactant, and catalytic modifier to control mass transport, active site chemistry, and reaction pathways. Its multifunctional nature directly shapes critical performance outcomes—from catalytic activity and product selectivity to long-term stability and energy efficiency. Beyond these roles, water’s H-bonding network and solvent effects create dynamic interfacial environments that can accelerate or hinder reactions, making it both a participant and architect of the electrocatalytic process [34].

The EDL serves as the primary reaction zone in electrocatalysis, where its microstructure governs the interfacial reaction kinetics at the solid/liquid interface in all electrochemical processes. The EDL forms the critical reaction zone in electrocatalysis, emerging at the dynamic interface between water and the electrode surface. The Gouy–Chapman–Stern (GCS) model, developed in the 20th century, classically describes EDL through three distinct regions: the inner Helmholtz plane (IHP), outer Helmholtz plane (OHP), and diffuse layer. Although this framework has proven effective for macroscopic experimental interpretation, its oversimplified treatment of the molecular-scale structure and interactions within the EDL limits the mechanistic understanding of electrocatalysis and leads to observed deviations from model predictions [35,36,37].

For instance, understanding the Helmholtz layer’s capacitive response demands the precise characterization of chemisorbed water molecule orientations at electrode surfaces [38], highlighting how water’s molecular arrangement mediates the interfacial charge distribution. In aqueous electrocatalysis, interfacial water molecules (IWMs) within the EDL play critical roles in both surface reactions and mass transport, with their spatial arrangement, concentration, orientation, H-bonding networks, and structural rigidity directly governing catalytic kinetics. These fundamental relationships make understanding the IWM structure and dynamics a central challenge in electrochemical surface science, particularly given their profound influence on reaction mechanisms. However, characterizing IWMs remains very challenging due to the overwhelming signals from bulk water and the inherent complexity of solid–liquid interfacial environments. To date, researchers have employed various advanced spectroscopic techniques, such as infrared/Raman spectroscopy, sum-frequency generation spectroscopy, Terahertz (THz) spectroscopy, and X-ray spectroscopy, to investigate these complex interfacial phenomena [37].

The strong electric field within the EDL significantly influences the structure of the IWMs due to water’s dipole moment, as revealed by soft X-ray absorption spectroscopy (XAS). XAS studies indicate a lower concentration of H-bonds near the electrode surface, within the first two to three water layers, compared to bulk water [39], with IWMs exhibiting both saturated and broken H-bonds depending on their orientation within the electric field across the EDL. However, these soft X-ray measurements, which require a high vacuum and pose the potential for X-ray-induced water radiolysis, present challenges for the accurate in situ analysis of IWMs [40]. Solvated ions within the EDL significantly disrupt the structure of the IWMs, thereby affecting electrocatalytic reaction rates. A study combining in situ shell-isolated Raman spectroscopy and molecular dynamics simulations identified H-bonded water and sodium-ion-hydrated water (Na⁺·H₂O) within close proximity to the electrode surface. Under negative potentials, a structural ordering of IWMs was observed, resulting in enhanced electron transfer and improved HER rates. Research has revealed a positive correlation between HER activity and the population of Na⁺·H₂O, suggesting that this ordered structure enhances the reaction. Koper et al. proposed that cations such as Na⁺ stabilize the transition state of the rate-determining Volmer step in HER by forming water–ion clusters (*H–OH^δ−-cat⁺) [41], a mechanism debated against alternative theories where ions and IWMs act independently. Through ab initio molecular dynamics simulations (AIMDS) and in situ surface-enhanced infrared spectroscopy (ISSE-IR), Li et al. (2025) [42] demonstrated that the cation size reshapes the EDL, altering proton transfer energy barriers and HER kinetics in alkaline media. They found that large cations disrupt the interfacial water structure, hindering proton transport—overturning previous views that the effects of cations stemmed mainly from direct interactions with reaction intermediates [42]. This work redefines our understanding of cation-specific influences in electrocatalysis through an interfacial perspective. The type of solvated ions crucially influences hydration shell structures at the electrode surface, highlighting the significant roles of both ions and interfacial water in electrocatalysis. Using THz spectroscopy, Havenith et al. revealed that Cl⁻ ions retain their hydration shell at low positive potentials, while Na+ ions lose part of their hydration shell at low negative potentials in an NaCl electrolyte at the Au surface [43]. This observation challenges the GCS model, which overlooks specific ion–water and water–water interactions, highlighting the importance of considering these interactions for accurate interface modelling.

In general, increasing the electrolyte concentration compresses the EDL by reducing the Debye length, which enhances charge screening and shifts ion distributions closer to the electrode surface. This affects the interfacial electric field, alters the orientation and dynamics of IWMs, and modulates the adsorption energies of intermediates. High concentrations can also lead to ion overcrowding near the surface, impacting mass transport and potentially stabilizing or destabilizing reactive species. Moreover, in highly concentrated or “water-in-salt” electrolytes, the solvent structure and activity coefficients change significantly, leading to EDL behavior that departs from classical models. These universal effects of concentration are therefore central to electrolyte optimization across all major electrocatalytic processes discussed in this review.

In addition to electric fields and cations, surface-adsorbed species like OH_ads groups and reaction intermediates significantly modulate the H-bond networks of IWMs within the EDL, affecting the reactivity of water dissociation. A study employing surface-enhanced infrared spectroscopy revealed an H-bond gap between OHP and the diffuse layer on Pt in alkaline media, hindering proton transfer and reducing HER activity [44] while also demonstrating OH_ad’s unexpected enhancement of HER kinetics by strengthening H-bond connectivity. Another computational study relevant to the alkaline HER on Pt(100) further suggested that at high pH, negatively charged Pt repels water adsorption, increasing HBE and decreasing HER performance [45]. Similar pH-dependent activity has also been observed in the ORR [46], and was attributed to varying IWM configurations within the EDL, which affect H-bond formation between oxygenated intermediates and IWMs, thus regulating proton-coupled electron transfer.

Moreover, solvent polarity, oxygen solubility, dielectric constant, and viscosity impact EDL formation and the kinetics of electron transfer. Polar solvents, for instance, can stabilize charged intermediates and transition states, facilitating reactions involving ion transfer [29]. Kamat et al. found that HClO₄ has the highest mass transfer limited current density, followed by HNO₃ and H₂SO₄, which could be attributed to minor differences in oxygen solubility and electrolyte viscosity [29].

H-bonding, a specific type of intermolecular interaction, profoundly affects the adsorption of reactants and the stability of intermediates on the catalyst surface. Solvents capable of forming strong H-bonds, like water or alcohols, can alter the local environment at the electrode–electrolyte interface, influencing the adsorption energy of reactants and products. This can lead to changes in reaction pathways and selectivity [47]. Furthermore, H-bonding can stabilize or destabilize reaction intermediates, affecting their lifetime and reactivity. In proton-coupled electron transfer (PCET) reactions, H-bonding networks can facilitate proton transfer, a critical step in many electrocatalytic processes [48]. The solvent’s ability to form H-bonds can also influence the structure and dynamics of the catalyst surface, potentially leading to surface reconstruction or the formation of specific solvation shells that affect catalytic activity [49]. Therefore, understanding and controlling solvent effects, as well as H-bonding, are essential for optimizing electrocatalytic systems.

3. Hydrogen Evolution Reaction (HER)

3.1. Mechanism and Rate-Determining Steps

Water electrolysis involves two simultaneous electrochemical reactions, the HER at the cathode and the OER at the anode [50,51]. Depending on the type of electrolyte employed, water electrolysis can take place under acidic, pH-neutral, or alkaline conditions. Equations (1) and (2) outline the corresponding overall HER reactions in acidic and alkaline electrolytes, respectively.

2H⁺ + 2e⁻ → H₂

(1)

2H₂O + 2e⁻ → H₂ + 2OH⁻

(2)

The mechanism of the HER, which is influenced by the pH of the solution [52], can be summarized in acidic environments as follows:

H⁺ + e⁻ + * → H_ads

(3)

H_ads + H_ads → H₂ + 2*

(4)

H⁺ + H_ads + e⁻ → H₂ + *

(5)

In an alkaline and neutral media, the HER mechanism can be presented similarly:

H₂O + e⁻ + * → H_ads + OH⁻

(6)

H_ads + H_ads → H₂ + 2*

(7)

H₂O + H_ads + e⁻ → H₂ + OH⁻ + *

(8)

Within the above-presented mechanisms, * signifies an unoccupied active site on the catalyst’s surface. The reaction initiates with a Volmer step (Equations (3) and (6)), and the intermediate species (H_ads) is subsequently eliminated from the surface via either the Tafel reaction (Equations (4) and (7)) or the Heyrovsky reaction (Equations (5) and (8)).

In acidic media, the HER proceeds via either the Volmer–Tafel mechanism (dominant at high H_ads coverage due to frequent surface recombination) or the Volmer–Heyrovsky pathway (favored at low H_ads coverage where electrochemical desorption prevails) [53]. The operative mechanism depends critically on both H_ads surface coverage and the applied overpotential, with the Volmer–Tafel route becoming statistically more probable as H_ads coverage increases [46]. The Volmer–Tafel pathway predominates at low overpotentials, whereas the Volmer–Heyrovsky pathway dominates at higher overpotentials [54]. These mechanisms involve three potential rate-determining steps (RDSs): Volmer, Heyrovsky, or Tafel. Both pathways exhibit exponential current growth with overpotential, but the rate varies with the rate-determining step (RDS), enabling mechanistic discrimination [53]. While some studies identify Volmer as the RDS for HER/HOR on polycrystalline Pt [55,56,57,58,59], others propose that the Heyrovsky step controls the kinetics of the HER [60,61]. Parts et al. [62] developed a method to determine the dominant HOR/HER mechanism by analyzing experimental data while accounting for H_ads coverage and mass transport effects. Correcting mass transport with the Koutecky–Levich equation helps extract kinetic currents, but determining the mechanism may also require H_ads coverage or activation energy data. For reversible systems like Pt, transfer coefficients can be fitted to a Butler–Volmer equation, whereas less active catalysts, such as Au, require a separate HOR/HER analysis. The study proposes flowcharts to determine the mechanism and suggests the Heyrovsky step as rate-limiting for Pt under both acidic and alkaline conditions.

The Tafel slope reveals the RDS and probable HER mechanism by characterizing electron transfer kinetics. Lower Tafel slopes correspond to faster kinetics, yielding higher current density (j) at reduced overpotentials. Exchange current density (j₀) refers to the charge transfer rate under equilibrium conditions, where elevated j₀ indicates faster charge transfer and lower activation barriers [25]. Therefore, superior electrocatalysts generally exhibit smaller Tafel slopes and larger j₀ values [33]. Within the HER potential window at 25 °C, theoretical Tafel slopes vary by the RDS: −120 mV/dec (Volmer), −30 mV/dec (Tafel), and −40 mV/dec (Heyrovsky). Most materials experimentally show −120 mV/dec, reflecting slow proton discharge or H_ads desorption kinetics. However, PGMs in acidic media often display −30 mV/dec due to accelerated Volmer steps. Although −120 mV/dec traditionally indicates Volmer-limited HER, Shinagawa et al. demonstrated it can also arise from Heyrovsky-controlled kinetics at high H_ads coverage (>0.6) [61]. Thus, one should be very careful in assigning the HER mechanism solely based on the Tafel slope.

Additionally, cases exist where reaction steps occur at comparable rates, which complicates the interpretation of Tafel slopes [63]. Watzele et al. examined the mechanistic uncertainties of HER, employing electrochemical impedance spectroscopy to quantify the concurrent Volmer–Heyrovsky and Volmer–Tafel pathway contributions across varying potentials and pH values. Their data revealed neither pathway’s dominance, with both contributing equally to the overall reaction [64]. Electrocatalytic kinetics differ fundamentally from outer-sphere reactions because of adsorbed intermediates. The Butler–Volmer equation must incorporate potential-dependent surface coverage of these species, which complicates the interpretation of Tafel slopes, particularly for HER and related reactions. Consequently, the Tafel slope-based HER mechanistic determination is not only challenging but potentially highly deceptive.

Figure 1 illustrates the HER mechanism (in addition to the ORR and OER discussed later), highlighting the importance of the M–H_ads bond strength for electrocatalyst kinetics, as per Sabatier’s principle.

However, Sun challenges the conventional understanding of the HER mechanism, which has been based mainly on Trasatti’s work correlating j_o with metal work functions. Instead, Sun argues that a metal’s electronegativity (EN) and valence electronic configuration (VEC)—particularly the presence of two or more partially filled orbitals (PFOs)—are key determinants of HER catalytic activity, with high EN and multiple PFOs enabling higher j_o values [65]. This new perspective suggests a revised HER mechanism, emphasizing the critical roles of atomic EN and PFOs in catalytic behavior, particularly for transition metals. In this study, Sun proposes a revised mechanism for the HER on bare electrodes, where the initial step involves a single electron transfer (H⁺ + e⁻ → H﮲) forming atomic hydrogen radicals (H﮲), followed by a rate-determining radical dimerization (H﮲ + H﮲ → H₂). However, this uncatalyzed process suffers from inefficiency due to slow H﮲ diffusion and parasitic side reactions with water or other species. To explain the catalytic role of transition metals, Sun highlights the critical role of transition metal catalysts, where EN and VEC, particularly the presence of two or more PFOs, enable covalent-like orbital interactions that stabilize H· radicals on the metal surface. The proposed mechanism diverges from traditional V-T and V-H pathways, offering a new perspective on HER catalysis. The new suggested mechanism of the HER on active catalytic sites is illustrated in Figure 2a–e.

Although the present review focuses primarily on the hydrogen evolution reaction, we include limited references to the hydrogen oxidation reaction (HOR) where they are mechanistically informative. Given that the HER and HOR are electrochemical reverse reactions on Pt and share surface-bound intermediates, mechanistic insights derived from HOR studies, especially concerning interfacial proton transfer, H_ads dynamics, and hydrogen binding energy, can help elucidate the electrolyte’s effects relevant to the HER. Care has been taken to exclude phenomena specific to the HOR (such as OH adsorption at higher potentials) that are not operative in the HER potential window.

3.2. Impact of the Electrolyte Composition (Acidic vs. Alkaline Media)

Electrolytes used for HER studies vary, and they can differ in terms of pH (acidic, neutral, or alkaline) or solvent (such as organic solvents or ionic liquids) [66]. The HER’s efficiency is strongly influenced by the nature of the electrolyte, with acidic solutions accelerating the reaction on PGMs but limiting catalyst choices and facing challenges related to diffusion. The electrolyte pH drastically alters reaction mechanisms and kinetics in electrocatalysis. In acidic media (e.g., 0.1 M HClO₄), proton (H⁺) availability dominates, enabling direct PCET steps (H⁺ + e⁻ → ½ H₂). Even within the same (acidic, alkaline, or neutral) environments, the composition and the concentration of electrolytes are found to strongly affect the HER [7] and OER in electrolytic water splitting [67]. According to Arminio-Ravelo, higher catalytic performances were observed in HClO₄ electrolytes in comparison to H₂SO₄, with little effect of the concentration of HClO₄ [67]. The HER on Pt and PGMs is rapid and reversible at low pH, but cost and scalability issues demand alternative catalysts. Cheaper metals like Ni hold potential as alternatives to Pt for the HER, but they exhibit lower performance and stability issues under acidic conditions [7,17,68].

In alkaline media (e.g., 0.1 M KOH), the reaction requires water dissociation (H₂O + e⁻ → H_ads + OH⁻) before proton reduction, introducing a kinetic bottleneck [69]. In alkaline and pH-neutral media, the HER is notably slower, even on Pt, mainly due to the sluggish H₂O dissociation step (Equation (6)). PGMs still have excellent HER performance in alkaline environments. However, their activity declines by approximately two orders of magnitude compared to acidic media [63], emphasizing the impact of the electrolyte on the HER. Alkaline electrolytes, while slower, enable the use of cost-effective non-PGM catalysts.

On the other hand, neutral electrolytes offer reduced corrosion and broaden catalyst options, eliminating the need for specialized materials. The electrochemical performance of catalysts again varies across different neutral electrolyte solutions, leading to distinct catalytic pathways, benefits, and limitations [1]. Two primary types of neutral electrolytes, including buffered and unbuffered electrolytes, can be used, depending on the type of electrolyte. The same catalyst may exhibit varying catalytic activity in buffered and unbuffered electrolytes, resulting in a range of benefits and drawbacks for different electrolytes. Merrill et al. [23] found that protonated weak acids in a microbial electrolysis cell (MEC) promote the HER through weak acid catalysis and reduced solution resistance. Their research highlighted the importance of buffer selection, showing that phosphate and acetate work better under acidic conditions, while carbonate works better at higher pH due to increased conductivity. However, the impact of buffer groups on catalytic mechanisms remains largely unexplored, limiting our understanding of their roles. Furthermore, the lower ionic conductivity, solubility, and oxygen diffusion in neutral electrolytes compared to extreme pH electrolytes can significantly affect the rate-determining step of the catalytic process.

Recent research has shown that the interactions between the electrolyte and the electrocatalyst have a significant influence on the electrocatalytic properties. The electrolyte’s impact on electrochemical reactions occurs through two mechanisms: (i) via chemisorption of adsorbents in the IHL involving electron transfer, and (ii) through weak van der Waals interactions between the electrode and spectator (supporting) ions in the electrolyte at the outer Helmholtz plane (OHL) [11,12].

3.3. Effects of Specific Ions and the Interfacial Electric Field

The effectiveness of an electrocatalytic reaction largely depends on the electrode surface composition and structure. A fundamental understanding of the processes at the electrode–electrolyte interface is also essential for optimizing catalytic performance. These factors are crucial for advancing cost-effective, efficient, and sustainable hydrogen technologies, such as electrolyzers, fuel cells, and batteries [16].

The HER is strongly influenced by electrolyte pH, especially for highly active metals like Pt, Ir, and Pd, while non-precious metals like Au, Ni, or Cu are less affected, with activity decreasing as pH increases. Under acidic conditions, H₃O⁺ drives the HER, but at higher pH, water reduction dominates, making the HER pH-independent beyond a certain pH and potential. Near-neutral pH requires a significant overpotential due to its reliance on water for hydrogen production, which progresses through two phases: initial H₃O⁺ reduction and later H₂O-driven reduction at higher overpotentials [18].

Experimental data confirms declining HER performance with rising pH across various electrocatalysts. Pt’s HER performance at neutral pH deviates from thermodynamic predictions due to kinetic effects caused by pH variations at the electrode surface [19]. Studies suggest that HER activity in near-neutral solutions depends on the nature of reactants, electrolyte state, and concentration, with low H₃O⁺ levels limiting reactivity [18,70,71]. However, buffered electrolytes (phosphate, borate, and carbonate) can enhance HER performance by stabilizing pH near the electrode, achieving onset potentials similar to acidic or alkaline conditions [22,23]. Weak acid species like H₂PO₄⁻ and HPO₄²⁻ may serve as proton sources [23,70,72], influencing reaction kinetics through mass transport, though their exact roles are still debated. Some researchers argue that HBE determines HER activity across pH ranges, with higher pH increasing the HBE and slowing kinetics, while OH⁻ ions further modulate HBE’s effects [73,74]. Shao et al.’s spectroscopic analysis demonstrated that Pt’s HBE is influenced by electric fields, adsorbate coverage, and water interactions, weakening at higher pH and slowing HER kinetics, highlighting HBE’s key role in pH-dependent activity [75]. However, HBE alone cannot fully explain HER behavior on Pt(111) surfaces, where adsorbed hydroxyl (OH_ads) molecules also critically impact the reaction dynamics [76,77,78]. Marković et al. proposed that in alkaline environments, HER/HOR requires separate sites for H_ads and OH_ads, with OH_ads altering kinetics via site competition or adsorption energy shifts [78,79], suggesting that optimizing H adsorption and OH desorption boosts activity. The bi-functional tuning of Pt with Ni(OH)₂, optimizing OH_ads and H_ads interactions, was found to significantly improve the HER in both alkaline and pH-neutral solutions [24,25].

Trassati’s pioneering research in acidic media identified HBE as a critical factor controlling HER kinetics, a concept later reinforced by Sheng et al., who showed that an underpotential deposited hydrogen (H_UPD) on Pt(100) and Pt(110) exhibited pH-dependent desorption peaks linked to HBE, with higher pH strengthening HBE and reducing Pt’s HER activity [74]. Surface-dependent kinetics further reveal the influence of pH, as Pt single-crystal surfaces display varying HER rates under alkaline conditions; the highly defective Pt(110) surface shows superior activity, attributed to the crystal surface structure influencing the adsorption of the hydroxyl and H_UPD species [80]. Marković et al. demonstrated that Pt(111) surfaces modified with Pt islands achieve 5–6 times higher HER activity in alkaline media compared to just 1.5 times in acidic conditions, highlighting how pH alters reaction mechanisms [79]. These findings collectively emphasize that low-coordinated Pt atoms in alkaline environments accelerate the HER by facilitating water dissociation, underscoring the interplay between the surface structure, HBE, and pH in electrocatalytic performance.

The identity and strength of ions, more specifically AM⁺, is also known to influence the HER. Recent studies reveal that alkaline HER/HOR kinetics on Pt exhibit a strong cation dependence (Li⁺ > Na⁺ > K⁺ > Cs⁺), with this effect extending to other key reactions like the OER/ORR and CO₂RR, sparking significant research interest in cation-specific electrocatalytic phenomena. This widespread influence of cations across diverse electrochemical processes highlights their fundamental importance in understanding and optimizing electrocatalyst performance [42].

Early studies attributed the kinetics of alkaline hydrogen electrocatalysis to cation interactions with adsorbed hydrogen (H_ads). However, contradictory evidence from acidic systems revealed pH-dependent but cation-independent H_ads behavior [60,75], redirecting research toward cation interactions with adsorbed hydroxyl (OH_ads) and interfacial OH⁻ species instead. The varying hydration energies of AM⁺ lead to distinct interactions with OH_ad/OH⁻ intermediates, enabling the cation identity to critically influence alkaline HER/HOR kinetics by modulating water dissociation/formation [27], the adsorption/desorption OH_ads/OH⁻ [81], the transfer of OH⁻ species in the interfacial region [82], and the number of surface OH_ads species acting as proton mediators [83]. This multifaceted cation-dependent regulation directly impacts the alkaline Volmer–Heyrovsky steps through both interfacial species dynamics and proton transfer processes. Cations in electrolytes, particularly in alkaline media, have been shown to weaken the metal–OH_ad bond, causing an increase in interfacial pH and a positive shift in the H_UPD desorption peak [84]. Strmcnik et al. showed that non-covalent interactions between hydrated AM⁺ (Cs⁺, K⁺, Na⁺, and Li⁺) and OH_ads species influence Pt(111) ORR activity, with performance following the trend Cs⁺ > K⁺ > Na⁺ > Li⁺—inversely correlating with cation hydration energies. This highlights how the cation hydration strength modulates oxygen reduction kinetics [85]. Xue et al. found that HER activity on Pt electrodes, Pt(111), Pt(221), and Pt_pc, in alkaline electrolytes consistently followed the cation trend Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺, with LiOH delivering four times higher current densities than CsOH across all surfaces. This demonstrates that the cation size strongly influences HER performance, regardless of the surface structure [86]. Koper et al. proposed that cation-specific kinetics in alkaline hydrogen reactions arise from active site-blocking effects, where larger and weakly hydrated cations (e.g., K⁺) preferentially adsorb onto electrode surfaces, hindering catalytic activity [41,87].

Recent research has established that AM⁺s critically influence alkaline hydrogen reaction kinetics by modifying interfacial environments, moving beyond early theories like Frumkin’s OHP effects [88]. Shao-Horn and colleagues advanced this understanding by applying the Marcus–Hush–Chidsey theory, revealing that cation-dependent kinetics arise primarily from differences in interfacial water reorganization energy [26,89] rather than simple thermodynamic considerations. Research has shown that cations present in the electrolyte—and consequently within the EDL—can significantly impact reaction kinetics, particularly in neutral or alkaline environments [90]. Some studies reveal that the electrolyte composition often dominates over electrode design in catalytic activity, as aqueous electrolytes directly steer reaction pathways and kinetics through EDL structuring [91,92]. Despite being electrochemically inert, cations in the EDL have been observed to significantly influence key interfacial properties, including electric field strength, IWM arrangement, and localized pH conditions [93].

The EDL’s organization—particularly cation arrangement and interfacial water dipole alignment—dictates catalytic efficiency by modulating energy barriers and interfacial interactions. In a recent study, Chen et al. found that the connectivity of the H-bond network in the EDL governs the pH-dependent kinetic behavior of HER/HOR processes on Pt–water interfaces. Compared to alkaline media, acidic conditions promote greater continuity in the interfacial H-bond network, leading to superior HER/HOR activity [94]. Li et al. recently showed that Na⁺ accumulation in the EDL during alkaline hydrogen electrocatalysis disrupts the interfacial water structure and H-bonding, significantly slowing HER/HOR kinetics [94]. Sun et al.’s follow-up research further confirmed these findings, strengthening the understanding of how cation arrangement affects reaction rates [95]. Through AIMDS and ISSI-IR spectroscopy, Li et al. (2025) demonstrated that cation size variations modify EDL configurations, influencing proton transfer energy barriers and HER kinetics in alkaline environments [94]. Their findings show that bulky cations fragment the interfacial water network and weaken H-bonding, impeding proton diffusion—a mechanism that challenges prior assumptions about cation-mediated surface reaction energetics. These insights present a fresh framework for understanding how cation identity governs electrocatalytic behavior at the electrode–electrolyte interface.

Recent studies highlight the need to understand better how AM⁺s influence electrocatalytic processes. Ding et al. [96] investigated this using laser-induced current transient (LICT) measurements on polycrystalline Au (Au_pc) electrodes at varying pH levels (2, 4, 6, 8, and 10) in Na₂SO₄ and K₂SO₄ electrolytes, revealing that the PME showed minimal cation dependence under acidic conditions but shifted dramatically near neutral pH (pH 4–6) (Figure 2). Notably, K⁺-containing electrolytes exhibited a sharper PME shift (≈1 V increase) compared to Na⁺, underscoring cation-specific interfacial behavior in neutral-to-alkaline environments. Moreover, the PME for Au_pc electrodes in Ar-saturated 0.5 M K₂SO₄ at pH 8 (≈1.30 V vs. RHE) was significantly higher than in Na₂SO₄ (≈0.60 V vs. RHE) (Figure 3), reaffirming the cation-dependent trend observed under near-neutral pH conditions.

Moreover, interfacial H-bond network dynamics are now recognized as key mediators of interfacial PCET kinetics, directly modulating the rate and product distribution of electrocatalytic reactions through water structure reorganization. The H-bond network connectivity in the EDL critically influences HER/HOR electrocatalysis, though the cation-specific impact on this connectivity remains poorly understood. Tang et al. [93] demonstrated through AIMDS that cations at Pt(111)/water interfaces modulate the H-bond network structure, with Na⁺ and Ca²⁺ sharply reducing connectivity as surface charge density turns more negative, while Mg²⁺ slightly enhances it. Their results revealed a distinct water gap zone near the electrode surface that substantially disrupts H-bond network connectivity. These divergent effects stem from the interplay between cation hydration and interfacial water reorganization, revealing a key mechanistic link between cation identity and H-bond network dynamics in EDLs [93].

Suo et al. demonstrated that lithium bis(trifluoromethane sulfonyl)imide (LiTFSI)-based aqueous electrolytes under water-in-salt conditions can expand the water stability window to ~2.9 V. However, high TFSI⁻ concentrations promote LiF layer formation on stainless steel electrodes that selectively blocks H⁺ diffusion [97]. Guha et al. later revealed that Li⁺’s influence on HER activity extends beyond LiF’s effects, showing anion-independent suppression on Pt but enhancement on Au electrodes at high Li⁺ concentrations that are linked to metal-specific cation interactions [98]. Gas chromatography confirmed that this Au/Li⁺ synergy boosts hydrogen production, highlighting how the electrode–cation interplay divergently modulates HER performance. Additionally, recent studies have shown that alkali metal cation-containing electrolytes, particularly Li+, can enhance electrochemical reactions, including those in batteries, carbon dioxide reduction, and nitrogen reduction [12].

As already summarized in our previous review article [7], there are conflicting views on the influence of electrolyte anions on the electrocatalytic process. Some studies suggest that their effects are insignificant, while others reveal that electrolytes can significantly influence the HER of metal catalysts. Ref. [99] shows that in contrast to the HOR, the HER current densities, which have been examined in low overpotential and underpotential sites, were found to be independent of the nature of the supporting electrolyte (HClO₄, H₂SO₄, and HCl). Similarly, Guha et al. [100] revealed that the effect of the nature of anions (TFSI⁻, OTf⁻, ClO₄⁻, Cl⁻, and OH⁻) on the HER of the carbon nanotubes (CNTs) was minor. Kamat et al. [29] studied the influence of acid electrolyte anions on Pt catalyst performance at pH 1, observing variations in ORR and OER activity but consistent HER/HOR performance. They found that the ORR activity was significantly higher in HClO₄, followed by HNO₃ and H₂SO₄. Moreover, the study suggested that HNO₃ can be a viable alternative to HClO₄ for studying ORR, as trace Cl⁻ in HClO₄ may obscure experimental results. Using DFT calculations, they revealed that weaker anion adsorption on Pt(111) correlates with enhanced ORR/OER activity. Our recent study demonstrated that both cations and anions, particularly Cl⁻ ions, significantly affect the HER of metals. Cl⁻ ions in HCl resulted in a broader overpotential range for the HER compared to ClO₄⁻ ions, altering the volcano plot shape (Figure 4) [17].

This difference is attributed to Cl⁻ ion poisoning versus the non-adsorbing nature of perchlorate ClO₄⁻ ions. The effect of electrolytes was also evident in neutral solutions, with NaCl exhibiting higher HER overpotentials than KH₂PO₄. Electrolyte anions influence catalytic activity by modulating the binding energy of reaction intermediates, particularly HBE in the HER. Different anions can alter HBE, affecting hydrogen adsorption and desorption [7]. Studies revealed that HClO₄ has been chosen as the standard electrolyte for benchmarking performance on Pt due to its high performance and stability, attributed to weak interactions between ClO₄⁻ ions and the Pt surface. Conversely, ion species in a double-layer microenvironment negatively impact Pt activity for the ORR, with hydrogen halide acids (e.g., HCl, HBr, and HI) and inorganic acids (e.g., H₂SO₄ and H₃PO₄) suppressing activity mainly due to competitive adsorption effects [29]. DFT calculations can help elucidate these interactions, highlighting the importance of anion selection for optimizing catalyst performance.

3.4. Interim Summary on HER

The HER is widely regarded as a model reaction for electrocatalysis, yet significant ambiguities remain concerning its mechanistic interpretation, particularly across different pH regimes. While it is generally accepted that acidic media offer more favorable kinetics, the situation in alkaline and near-neutral environments is far less understood. Discrepancies persist in identifying the rate-determining step, with various studies attributing sluggish performance to water dissociation, hydroxide recombination, or interfacial restructuring. Moreover, reliance on Tafel slopes for mechanistic inference can be misleading, as surface coverage effects and electrochemical impedance are rarely accounted for. The role of cations, previously considered secondary, has now emerged as a primary factor in modulating interfacial fields, water orientation, and hydrogen bonding. However, current approaches often fail to decouple these effects from catalyst-specific behaviors. We argue that the field would benefit from an integrative framework that combines surface science, solvation dynamics, and electric double-layer modeling to describe HER kinetics more universally, particularly in systems designed for practical water electrolysis applications.

The decline in HER activity on platinum with increasing pH remains one of the most debated phenomena in electrocatalysis. While the need for water dissociation in alkaline media is frequently identified as a kinetic bottleneck, growing evidence suggests that it is not the sole factor. Several studies have demonstrated that interfacial water structure, hydrogen bonding networks, and reorganization energy play crucial roles in modulating proton transfer and reaction kinetics at high pH levels. Moreover, the weakening of hydrogen binding energy on Pt in alkaline environments and changes in electric field distributions at the interface further complicate the picture. Importantly, OH adsorption on platinum does not occur within the potential window relevant to HER, and thus should not be considered mechanistically relevant in this context. Based on the current evidence, we contend that while water dissociation is a necessary prerequisite for alkaline HER, it is not the primary cause of the observed decrease in activity. Instead, the interplay of interfacial solvation dynamics, electric field effects, and altered proton availability should be considered as co-dominant factors.

4. Oxygen Evolution Reaction (OER)

4.1. Mechanistic Pathways in Acidic and Alkaline Media

The water electrolysis process splits water into its constituent gases, offering a clean energy solution. The overall reaction is represented by the following equations (see

Table 1. The overall reaction of water splitting in acidic and alkaline media. E° indicates the standard electrode potential, while E°_cell is the standard cell potential.

	Under Acidic Conditions (pH = 0)	Under Alkaline Conditions (pH = 14)
Cathode reaction	2H+ + 2e− → H2        E° = 0 V	2H2O + 2e− → H2 + 2OH−   E° = 0.83 V
Anode reaction	2H2O → O2 + 4H+ + 4e−   E° = 1.23 V	4OH− → O2 + 2H2O + 4e−     E° = 0.40 V
Overall reaction	2H2O → 2H2 + O2      E°cell = 1.23 V

).

The thermodynamic minimum voltage required for water splitting is 1.23 V, as shown in Table 1, but additional overpotential is needed to overcome kinetic barriers during electrolysis. While the HER involves a simpler 2e⁻ transfer process, OER’s complex 4e⁻ multi-step mechanism creates higher energy barriers, demanding significantly larger overpotentials [101]. Moreover, it is evident from the thermodynamic viewpoint that it would be best to conduct the HER in acidic media and OER in alkaline media with a theoretical voltage of 0.4 V [102]. The OER, which extracts oxygen from water molecules, is an energy-intensive, rate-limiting process crucial for electrochemical energy conversion systems, as it supplies electrons for transforming renewable electricity into chemical fuels [103,104]. This fundamental anodic process, which oxidizes water or hydroxide ions, serves as the operational cornerstone for key technologies, including electrolyzers [105,106], CO₂ reduction systems [107,108], and MABs [109], where its performance dictates overall device efficiency through coupled cathode processes.

Among water-splitting technologies, proton exchange membrane (PEM) water electrolyzers are highly promising for renewable energy-driven hydrogen production due to their rapid response, high efficiency, and scalability. However, their widespread adoption is hindered by the harsh acidic conditions (pH < 1), high corrosion potentials (>1.5 V vs. RHE), and slow OER kinetics at the anode [110,111]. PEM-based acidic water electrolysis offers high efficiency due to excellent proton conductivity and low resistance but necessitates expensive precious metal OER catalysts (e.g., IrO₂/RuO₂). While a high proton concentration facilitates proton transport, it also accelerates catalyst corrosion [112] and unwanted surface reactions [113]. Furthermore, limited water dissociation hinders intermediate formation [114], illustrating the efficiency–stability trade-offs in acidic electrolyzers. Currently, the majority of OER electrocatalyst research focuses on alkaline media due to its more favorable reaction kinetics and lower energy barriers compared to acidic conditions. The OER pathway in alkaline media is relatively direct, involving OH⁻ adsorption and O-O bond formation, leading to faster kinetics. However, OER studies in acidic media are equally important for specific applications. Electrocatalyst design for the OER differs significantly between alkaline and acidic media; alkaline research focuses on active site optimization and reconstruction, while acidic design prioritizes corrosion resistance and stability through material selection and structural control to inhibit unwanted reactions [115,116].

Two main OER mechanisms are proposed (Figure 1): the adsorption evolution mechanism (AEM) and lattice oxygen mechanism (LOM) [1]. The AEM for the OER proceeds via proton-coupled electron transfer, forming sequential oxygen intermediates (*OH → *O) on metal active sites [116,117]. From the *O state, two pathways emerge: (1) mononuclear AEM (which assumes a single metal active site), where *O reacts with either H₂O (in acidic media) or OH⁻ (in alkaline media) to form *OOH before the release O₂, and (2) binuclear AEM (which assumes two metal centers), where two *O species couple directly to produce O₂. These pathways illustrate how the electrolyte pH (acidic vs. alkaline) and active site geometry (single vs. dual metal centers) influence OER kinetics.

In acidic media,

H₂O(l) + * → OH* + H⁺ + e⁻

(9)

OH* → O* + H⁺ + e⁻

(10)

O* + H₂O(l) → OOH* + H⁺ + e⁻

(11)

OOH* → * + O₂(g) + H⁺ + e⁻

(12)

or   2O* → O₂(g) + 2*

(13)

In alkaline media,

OH⁻ + * → OH* + e⁻

(14)

OH* + OH⁻ → O* + H₂O + e⁻

(15)

O* + OH⁻ → OOH* + e⁻

(16)

OOH* + OH⁻ → * + O₂(g) + H₂O(l) + e⁻

(17)

or   2O* → O₂(g) + 2*

(18)

The OER proceeds through the cyclic oxidation of active sites and oxygen intermediate (*OH, *O, and *OOH) adsorption/desorption, where the binding strengths of these species critically determine catalytic efficiency in mononuclear mechanisms. Since the formation of each intermediate represents a thermodynamically unfavorable step, the overall reaction overpotential depends on the Gibbs free energy changes (ΔG) across the four-electron process, with the highest-energy step acting as the rate-limiting barrier. This energy landscape directly governs the OER kinetics and electrocatalyst performance [118].

The oxygen evolution reaction (OER) has a thermodynamic potential of 1.23 V, requiring 4.92 eV of free energy (ΔG) for the four-electron process, with ideal catalysts exhibiting equal energy steps at zero potential [119]. However, DFT reveals a fundamental constraint: oxygen intermediates (*OH, *O, and *OOH) follow linear scaling relationships (ΔG_*OOH = ΔG_*OH + 3.2 ± 0.2 eV), enforcing a minimum overpotential of ~370 mV and limiting optimization within the conventional AEM [120]. Recent studies challenge this framework, observing structural reconstruction in catalysts and overpotentials that exceed theoretical predictions, suggesting that the AEM cannot fully explain the experimental behavior [1]. The interdependence of oxygen intermediates creates a trade-off, where weakening *OH → *O binding strengthens *O → *OOH, and vice versa, making the oxygen binding energy difference (ΔG_*O − ΔG_*OH) a key OER descriptor that follows a volcano trend [115]. To improve OER efficiency, catalyst design must optimize binding energies while overcoming the inherent limitations of linear scaling relationships in the AEM [118].

While advancements in OER catalysts have been made, some observed catalytic behaviors deviate from the traditional AEM mechanism, including activity exceeding theoretical limits and pH-dependent behavior on the RHE scale [121,122,123]. To explain these phenomena, the LOM has been proposed, suggesting that oxygen from the catalyst lattice participates in O-O bond formation, bypassing the *OOH intermediate and potentially leading to lower overpotentials [124]. The adsorbed H₂O undergoes two consecutive deprotonations to generate an O* intermediate, much like the first two stages of the AEM. Oxygen molecules and oxygen vacancies are subsequently formed by the electrocatalysts’ lattice oxygen and O* intermediates. Meanwhile, the formed oxygen vacancies will be recovered by a refilling process. Several LOM mechanisms have been proposed, involving direct coupling with adsorbed oxygen or nucleophilic attack by hydroxide ions on activated lattice oxygen, both of which result in oxygen vacancy formation and refilling.

The variety of these proposed LOM pathways makes the rational design of highly efficient OER electrocatalysts more complex. The LOM involves dynamic changes on the catalyst surface through the oxidation, exchange, and release of lattice oxygen, a process influenced by the covalency of the metal–oxygen bond. A highly covalent metal–oxygen bond, resulting from weaker metal binding, favors a shift from the AEM to the LOM. The relative energy levels of the O 2p and metal d bands serve as indicators of oxygen activity; an elevated O 2p band or downshifted metal d band promotes oxygen redox and lattice oxygen release [125]. Experimental studies complement theoretical work in differentiating OER mechanisms, with the LOM exhibiting pH-dependent kinetics due to non-concerted proton-electron transfers—a key marker differentiating it from the AEM’s pH-independent behavior [126]. Further evidence for the LOM includes isotopic labeling (confirming lattice oxygen involvement) and spectroscopic detection of oxygen vacancies or peroxo-like intermediates. Isotope labeling with ¹⁸O, combined with in situ differential electrochemical mass spectrometry (DEMS), directly tracks lattice oxygen participation in the OER by detecting mixed ¹⁶O¹⁸O and pure ¹⁸O₂ evolution from labeled catalysts in ¹⁶O electrolytes, confirming the LOM through direct O–O coupling [127,128]. Therefore, the LOM differs from the AEM by having decoupled proton and electron transfers, resulting in a strong pH dependence and potentially lower theoretical overpotential due to direct lattice oxygen coupling. However, the LOM’s reliance on lattice oxygen oxidation risks catalyst degradation through structural destabilization, as oxygen loss can trigger surface reconstruction or phase transitions during the OER [125].

Understanding the AEM and LOM is crucial for designing efficient OER electrocatalysts in both acidic and alkaline environments [129]. In acidic media, the OER pathway is more complex, requiring significant energy to break the O-H bond of water and potentially involving the LOM. While most metal catalysts exhibit better stability in less corrosive alkaline solutions, acidic media pose a significant challenge to catalyst durability, and the LOM pathway might further accelerate degradation by altering the catalyst structure and active sites [130]. Researchers continue to explore novel catalysts and mechanisms to enhance OER performance. A deep understanding of the OER mechanism in acidic environments provides a theoretical foundation for designing efficient and stable electrocatalysts by identifying RDSs and developing more durable materials in corrosive acidic conditions [131,132].

4.2. Roles of Cations and Anions in Stabilizing Intermediates

In the OER, both cations and anions significantly impact catalytic performance by stabilizing intermediates and influencing activity. Cations in the electrolyte can modify the catalyst’s electronic structure and the EDL, thereby affecting the adsorption energies of OER intermediates and overall reaction kinetics. Anions, on the other hand, can directly interact with these intermediates, further influencing their stability and reactivity during the OER process. Early investigations into cations’ effects on the OER, conducted by Erdey-Gruz and Shafarik in acidic media [133] and by Kozawa in alkaline media [134], revealed distinct influences on catalytic activity. Erdey-Gruz and Shafarik observed a specific activity order, K⁺ < Al³⁺ < NH₄⁺ < Zn²⁺ < Na⁺ < Mg²⁺ < Li⁺, on Pt, while Kozawa noted an increase in overpotential for the OER on several metals in the presence of 0.1–10 mM Ba²⁺, Cr²⁺, and Ca²⁺ [134]. These early studies laid the groundwork for understanding the complex role of cations in modulating OER performance across different pH environments. Cations can influence the OER mechanism by affecting the stability of key intermediates like OH*, O*, or OOH* [135]. In some instances, an accumulation of cations near the electrode surface, known as “cation overcrowding”, can hinder the OER process. Furthermore, certain cations can integrate into or detach from the catalyst’s structure, which can alter its stability and overall catalytic activity. Research has shown that cation effects significantly influence OER activity at NiOOH thin films, with varying trends observed in purified and Fe-containing alkaline electrolytes—Cs⁺ > K⁺ > Na⁺ ≈ Li⁺ in purified solutions but K⁺ ≈ Na⁺ > Cs⁺ > Li⁺ with Fe impurities (Figure 5) [136].

Interestingly, Heijden et al. [137] discovered that adding small amounts of Li⁺ to Fe-free NaOH or KOH electrolytes enhances the OER activity of NiFeOOH compared to single-cation electrolytes, and this activation persists even after returning to pure NaOH. This activation by Li⁺ was primarily non-kinetic, resulting from the enhanced intercalation of sodium, water, and hydroxide within the catalyst structure, which facilitates the OER, particularly at higher current densities and catalyst loadings, suggesting a promising strategy for improving OER rates using mixed electrolytes with distinct cation roles [137]. Zaffran et al. observed that alkaline earth cations (Mg²⁺ and Ca²⁺) dramatically decrease the OER activity of NiOOH thin films in both purified and Fe-containing alkaline electrolyte solutions, a modern echo of the results reported by Kozawa more than 50 years ago [138]. The study by Rao et al. (2021) [139] revealed that alkali cations critically influence OER activity on RuO₂(110) in alkaline media, with K⁺ yielding the highest performance due to its weaker stabilization of -O intermediates and promotion of isolated OH⁻ species compared to Li⁺ and Na⁺. Surface-sensitive techniques showed that K⁺ creates an interfacial environment with more unbonded water molecules and less ordered hydration structures at OER potentials, which facilitates the rate-determining -O + OH⁻ → -OOH step. These findings demonstrate how cation-specific water restructuring at electrified interfaces directly governs catalytic activity, providing molecular-level insights for designing optimized electrochemical systems.

Electrochemical reaction rates can be finely tuned through the electrolyte composition, as reactions occur within the EDL where dielectric properties differ dramatically from the bulk solution—for instance, water’s dielectric constant (ε) drops from 78.4 to just 6 near the electrode surface [140,141]. Recent theoretical work confirms this ε reduction at solid interfaces [142]. At the same time, experimental studies demonstrate how hydrated cations (Li⁺, Na⁺, K⁺, and Cs⁺) modulate local solvation environments and OER overpotentials by altering the EDL structure and interacting with intermediates, as observed for NiOOH and RuO₂ catalysts [89,142,143]. The cation effect involves multiple interdependent factors, including intermediate stabilization, double-layer restructuring, and catalyst intercalation [137,139,144], which require sophisticated AIMD for accurate atomistic modeling of solvent effects [145]. While powerful, these simulations remain computationally prohibitive for standard DFT approaches, highlighting the need for more efficient computational strategies to unravel these complex interfacial phenomena. The electrolyte environment critically influences OER catalysis by modulating interfacial solvation effects, where Wang et al. [146] demonstrated that dielectric properties linearly correlate with overpotential in NiFe oxyhydroxides (γ-Ni_1−xFe_xOOH), revealing how tuning the local solvation environment can enhance activity. Their hybrid computational approach, combining quantum and kinetic modeling for single/dual-site pathways, shows that realistic electrolyte–catalyst interactions (including doping and solvation) are key to optimizing OER performance on transition metal oxides. This strategy provides a practical framework for designing efficient electrocatalysts by controlling the electrode–electrolyte interface. The electrolyte composition critically modulates OER activity by governing interfacial interactions, as demonstrated by Li et al. [147], who observed a 6.5-fold IrOx activity enhancement in concentrated KOH (4.0 M vs. 0.1 M), where outer-sphere Na⁺-OH⁻ H-bonding reduces OH^─ mobility while simultaneously improving oxygen transport through increased conductivity. However, inert additives like NaNO₃ disrupt this balance, degrading both performance and stability by altering the delicate equilibrium between inner- and outer-sphere processes. These findings necessitate a reevaluation of traditional EDL models to incorporate dynamic pseudo-capacitive behavior and outer-sphere effects when designing electrolytes for optimal OER catalysis [147].

Anions significantly influence OER kinetics by modifying the catalyst’s electronic properties and stabilizing intermediates through surface adsorption or intercalation, which can disrupt *OH/*OOH scaling relations [29]. Anions alter active site electronic structures, affecting the adsorption and transformation of key intermediates like *OH and *OOH, while intercalation may enable alternative reaction pathways [130,148]. Their presence thus directly impacts intermediate stability and overall OER efficiency. Sulfate anions (SO₄²⁻) have been shown to stabilize the *OOH intermediate, enhancing OER activity. Functional anions like squaric acid anions can stabilize OH⁻ through H-bond interactions, maintaining high interface alkalinity. Organic anions can influence the redox potential of the catalyst, impacting OER activity [148].

4.3. Electrolyte-Dependent Activity and Stability of Catalysts

The OER proceeds through a 4e⁻ transfer mechanism involving multiple intermediate species, making it inherently kinetically slow and requiring substantial overpotentials. Acidic OER catalysis exhibits particularly slow kinetics because water dissociation introduces an extra step alongside its multi-electron transfer mechanism. Additionally, the highly corrosive acidic environment frequently causes catalyst degradation through dissolution and corrosion, compromising long-term stability [149].

The electrolyte environment critically governs the activity–stability trade-off in OER catalysts, as demonstrated by RuO₂’s high activity but poor durability in acidic media versus IrO₂’s superior stability but lower performance [150,151,152]. This inverse relationship persists across pH conditions, with Ru-based materials exhibiting exceptional activity but rapid degradation in both acidic and alkaline electrolytes, while more stable alternatives, such as Ir oxides, sacrifice catalytic efficiency [150,153]. These universal trends underscore the need for innovative approaches, such as protective coatings or dynamic surface stabilization, to overcome the inherent trade-off between catalyst activity and electrolyte-driven degradation.

The presumed inverse activity–stability correlation in OER catalysts faces notable exceptions, as RuO₂ facet studies reveal site-specific decoupling, where high activity coexists with stability [154]. Meanwhile, noble metals like Pd/Au [155] and bimetallic Ir-Ni systems [156] exhibit composition-dependent behavior, violating conventional trends. These findings demonstrate that catalyst performance depends on complex interfacial factors, including the crystallographic orientation, elemental mixing, and operational surface dynamics, rather than following universal scaling relationships. Such complexity necessitates case-specific optimization strategies that account for each material’s unique degradation pathways and active site geometries.

4.4. Interim Summary of the OER

The OER is intrinsically more complex than the HER due to its multistep, four-electron mechanism, which imposes large thermodynamic and kinetic barriers. While the traditional adsorbate evolution mechanism (AEM) has served as a useful descriptor framework, it fails to fully account for observed behavior in several oxide-based systems, particularly those exhibiting anomalously low overpotentials or dynamic surface restructuring. Recent evidence for the lattice oxygen mechanism (LOM) has expanded the mechanistic landscape, yet distinguishing between these pathways remains experimentally challenging. The influence of the electrolyte composition, particularly pH and anion identity, is often neglected in mechanistic interpretations, despite clear evidence of their roles in intermediate stabilization, surface reconstruction, and catalyst degradation. Moreover, cation effects, although less pronounced than in the HER or ORR, have been shown to modulate redox transitions and alter the catalyst’s hydration environment. A more rigorous understanding of electrolyte–catalyst interactions is needed, particularly under acidic conditions where catalyst stability is a key limitation. In our view, future progress in OER design hinges on bridging atomistic simulations with an in operando structural analysis to clarify how electrolytes shape both activity and degradation.

5. Oxygen Reduction Reaction (ORR)

5.1. Two-Electron vs. Four-Electron Pathways

ORR is a crucial electrochemical reaction occurring at the cathode of energy conversion technologies like FCs and MABs, which are emerging as promising zero-emission technologies and alternatives to combustion engines. PEM fuel cells electrochemically combine hydrogen and oxygen to produce only water as the end product, while MABs substitute oxidizable metals (Li, Zn, Mg, and Al) as anodes while retaining atmospheric oxygen reduction at the cathode. Both technologies achieve zero-emission operation, with their performance fundamentally governed by ORR kinetics at the cathode interface [109,157,158]. The ORR is the RDS in these technologies, affecting overall system performance.

Studies identify three ORR mechanisms: the first is the direct 4e⁻ pathway without H₂O₂ formation, the second is a series pathway involving H₂O₂ as either a final product (2e⁻) or intermediate (4e⁻), and the third is parallel/interactive combinations of these routes [159]. However, the ORR is reported to proceed typically via the two distinct pathways: (1) a direct four-electron transfer process that converts O₂ to OH⁻ in alkaline conditions (Equations (19)–(23)) or to H₂O in acidic environments (Equations (20′)–(23′)), and (2) a two-electron transfer route that produces HO₂⁻ in basic media (Equation (29)) or H₂O₂ in acidic solutions (Equation (29′)) [157,160].

In alkaline and neutral media, the ORR converts O₂ molecules to OH⁻ ions, either through a rapid, single-step four-electron (direct 4e⁻) process or a sluggish, two-step two-electron (2e⁻ + 2e⁻) process. For the 2e⁻ + 2e⁻ pathway, a two-electron oxygen reduction befalls first to generate HO₂^─, followed by another two-electron reduction of HO₂⁻, or the chemical disproportionation of HO₂⁻ without electron transfer with the electrodes.

The kinetic pathways of ORR are complex, involving multiple intermediate and elementary steps that vary with the nature of the catalyst and electrolyte. In alkaline media, ORR proceeds via either associative or dissociative mechanisms [1,157], in which the associative pathway initiates with O₂’s associative adsorption on the catalyst surface through the following reaction sequence:

O₂(g) + * → O₂*

(19)

O₂* + H₂O(l) + e⁻ → OOH* + OH⁻

(20)

OOH* + e⁻ → O* + OH⁻

(21)

O* + H₂O(l) + e⁻ → OH* + OH⁻

(22)

OH* + e⁻ → * + OH⁻

(23)

Here, O*, OH*, and OOH* are the intermediates adsorbed on the electrocatalyst active surface *. The complete 4e⁻ ORR pathway occurs when O₂ accepts four electrons, generating four OH⁻ ions.

O₂ + 2H₂O + 4e⁻ → 4OH⁻   (E⁰ = 0.401 V vs. SHE, pH = 14)

(24)

In alkaline media, the dissociative mechanism follows a more direct pathway: rather than proceeding through Reactions (20)–(22), the adsorbed O₂ spontaneously cleaves into two O* species. This process similarly consumes four electrons, achieving the complete 4e⁻ ORR, as represented in the following equations:

½ O₂ + * → O*

(25)

O* + H₂O + e⁻ → OH* + OH⁻

(26)

OH* + e⁻ → OH⁻ + *

(27)

However, if an electron transfers to the adsorbed OOH*, it may desorb as peroxide ions (HO₂⁻), terminating the reaction early via the two-electron pathway. While the first step remains the same (Equation (19)), the 2e⁻ pathway in alkaline media proceeds as follows:

*O₂ + H₂O + e⁻ → OOH* + OH⁻

(28)

OOH* + e⁻ → HO₂⁻ + *

(29)

The overall reaction of the 2e⁻ pathway in alkaline media is as follows:

O₂ + H₂O + 2e⁻ → H₂O⁻ + OH⁻ (E⁰ = −0.076 V vs. SHE, pH = 14)

(30)

In acidic media, the first step (Equation (19)) remains the same, while an analogous associative ORR mechanism takes place where proton availability modifies the reaction pathway to

O₂* + H⁺ + e⁻ → OOH*

(20′)

OOH* + H⁺ + e⁻ → O* + H₂O (RDS)

(21′)

O* + H⁺ + e⁻ → OH*

(22′)

OH* + H⁺ + e⁻ → H₂O + *

(23′)

The process consumes four protons and four electrons, reducing O₂ to two H₂O molecules.

O₂ + 4H⁺ + 4e⁻ → 2H₂O (E⁰ = 1.229 V vs. SHE, pH = 0)

(24′)

In acidic media, the dissociative mechanism proceeds in the following way:

½ O₂ + * → O*

(25′)

O* + H⁺ + e⁻ → OH*

(26′)

OH* + H⁺ + e⁻ → H₂O + *

(27′)

The initial step (Equation (19)) is the same for the 2e⁻ pathway in acidic media. The next steps proceed as follows:

O₂* + H⁺ + e⁻ → OOH* (RDS)

(28′)

OOH* + H⁺ + e⁻ → H₂O₂ + *

(29′)

The overall reaction of the 2e⁻ pathway in acidic media is as follows:

O₂ + 2H⁺ + 2e⁻ → H₂O₂   E⁰ = 0.695 V vs. SHE, pH = 0

(30′)

The 2e⁻ pathway’s key step, in both alkaline and acidic media, is *OOH acquiring an electron to produce HO₂⁻/H₂O₂, a vital consideration in the rational design of catalysts favoring this specific pathway. However, these products are susceptible to further reduction reactions [161]. The relevant reactions are

HO⁻ + H₂O + 2e⁻ → 3OH⁻

(31)

H₂O₂ + 2H⁺ + 2e⁻ → 2H₂O

(32)

Consequently, when initially designing catalysts for the 2e⁻ ORR, it is also important to take into account strategies for preventing the subsequent reaction of the desired products [162].

The 4e⁻ ORR associative mechanism remains fundamentally similar under both alkaline (Equations (19)–(27)) and acidic conditions ((20′)–(27′)). However, the key distinction between 2e⁻ and 4e⁻ ORR pathways stems from how oxygen adsorbs to active sites, triggering different reaction mechanisms [163]. When O₂ binds strongly to the active site, it adsorbs in parallel across two centers, forming a bridging oxygen configuration (*O-O*). The adsorbed oxygen then combines with a proton, first forming *OH before being finally reduced to H₂O. Conversely, when O₂ adsorbs in an end-on configuration, it participates in a PCET process, leading to the formation of a *OOH intermediate. Alternatively, through an electron transfer pathway, the sequential formation of *O and *OH species occurs, ultimately leading to the formation of H₂O. The end-on configuration demonstrates higher efficiency and faster reaction kinetics compared to the bridging pathway, making it crucial for optimizing catalyst design and performance [13,164].

The ORR, utilizing either a 4e⁻ or 2e⁻ pathway, is fundamentally important for both renewable energy conversion systems and some essential chemical processes. The 4e⁻ ORR pathway is crucial for FCs and MABs due to its high energy efficiency, while the 2e⁻ route offers a sustainable method for electrochemical H₂O₂ production using renewable electricity. Both mechanisms play vital roles in clean energy conversion and green chemical synthesis [162]. Moreover, H₂O₂, being a green oxidant, is increasingly used in cascade reactions. After its formation, it reacts with substrates to degrade pollutants or produce valuable chemicals [165].

The ORR process involves the 2e⁻ and 4e⁻ pathways, where the adsorption and desorption of *OH and *OOH are critical [166,167,168]. The catalyst’s ability to adsorb *OOH determines whether the reaction follows a 2e⁻ or 4e⁻ pathway [101,164]. Unlike typical reactions, both pathways are equally important, with their relevance depending on the application. The 2e⁻ ORR relies heavily on *OOH adsorption, making the binding energy (ΔG_*OOH) a critical screening parameter for catalysts. Ideal catalysts balance ΔG_*OOH—neither too strong nor too weak—mirroring the requirements in the HER [169]. The 4e⁻ ORR involves three key intermediates (*O, *OH, and *OOH), each with distinct binding energy (ΔG_*O, ΔG_*OH, and ΔG_*OOH) requirements that are closely interrelated. Researchers must optimize the catalyst’s electronic structure and active sites to balance these ΔG values for efficient 4e⁻ ORR performance. Different types of metals exhibit varying adsorption behaviors toward these intermediates, allowing selectivity control through proper metal selection [170,171]. However, the unexpected preference for H₂O₂ formation over water during the ORR on weak binding electrodes like Au [13,172,173] challenges conventional thermodynamic models based solely on PCETs. Through AIMDs and free energy calculations on Au(111), Diesen et al. demonstrated that competing reaction pathways, including both PCET and non-Faradaic chemical steps like desorption and surface dissociation, reveal significant kinetic barriers that selectively inhibit water formation while permitting H₂O₂ production. These barriers persist even under reducing conditions, with Au’s inherent “nobleness” crucially preventing O−O bond cleavage, illustrating that selectivity is governed by intrinsic chemistry rather than the applied potential. Their findings underscore the critical kinetic competition between PCET and non-PCET steps, which cannot be predicted by simple thermodynamic scaling relationships [167]. Moreover, Lou and Koper [174] introduced a novel ORR descriptor, the electrochemical reversibility of the *O ↔ *OH transition, which captures electrolyte-dependent activity trends that conventional *OH binding energy fails to explain. This kinetic descriptor, measurable through voltammetry, complements thermodynamic models by linking ORR rates to both the *O → *OH transition dynamics and local chemical environment (anions/cations/ionomers), while also rationalizing pH-dependent behavior on different Pt surface structures.

In acidic media, for catalysts that exhibit weak oxygen binding, the RDS is either O–O bond cleavage (in the dissociative mechanism, Equation (25′)) or the proton–electron transfer to O₂* (Equation (20′)). In contrast, for catalysts with strong oxygen binding, the limiting step becomes the proton–electron transfer to O* or OH* (Equations (26′) and (27′)) [163]. Following Sabatier’s principle, catalytic activity follows a volcano-shaped trend with oxygen binding strength as the descriptor, placing Pt near the peak due to its near-optimal binding energy and making it the top-performing 4e⁻-ORR catalyst. Pt demonstrates the highest intrinsic ORR activity among transition metals, achieving the lowest overpotential for O₂-to-H₂O conversion, followed by Pd, Ir, and Rh [163]. Despite Pt’s superior activity, substantial overpotentials remain unavoidable due to linearly correlated binding energies of multiple reaction intermediates [163,174], creating inherent constraints for catalyst optimization. At the equilibrium potential (1.23 V vs. RHE), the first ORR intermediate forms favorably, but subsequent steps become thermodynamically uphill. Due to linear scaling relations, tuning one intermediate’s binding energy shifts others predictably, forcing a trade-off—applying higher overpotentials boosts driving force but sacrifices efficiency. For Pt(111), DFT calculations reveal a limiting potential of 0.78 V, exposing a fundamental efficiency gap even for top catalysts. Optimizing catalyst performance for both ORR pathways requires the precise modulation of intermediate binding energies—a task complicated by inherent scaling relations that intrinsically link these energetics.

As previously discussed, optimal electrocatalyst design plays a pivotal role in steering the reaction mechanism toward this preferential route [157]. The ORR proceeds via a 2e⁻ pathway when the electrode cannot further reduce H₂O₂, as observed on Au single crystals in acidic media [49]. Conversely, electrodes capable of reducing H₂O₂ to H₂O facilitate the 4e⁻ pathway, exemplified by polycrystalline Pt [159] and Pd [175]. This dichotomy in reaction pathways directly depends on the electrode’s catalytic activity toward H₂O₂ reduction. A higher Tafel slope observed for the ORR on Rh(poly) in H₂SO₄ versus HClO₄ reflects slower reaction kinetics that are directly attributable to sulfate anion adsorption inhibiting the catalytic activity [160]. The ORR on Rh(poly) primarily follows a 4e⁻ pathway, with a minor contribution from a 2e⁻ route, likely due to Rh(poly)’s reduced ability to reduce H₂O₂ intermediates in the presence of specifically adsorbed sulfate anions. Thus, poor catalytic selectivity, sluggish kinetics, and high cost of ORR catalysts remain key obstacles, severely limiting the energy efficiency and commercial viability of ORR-based technologies.

Because of the unusually strong bond energy of the O=O bond (498 kJ mol⁻¹) and the multielectron transfer processes (Equations (26)–(28)), the kinetics of the ORR in alkaline and neutral media are very slow, requiring a significant overpotential to drive the ORR at a feasible rate. Based on DFT simulations, Nørskov et al., established a description of the free energy (ΔG) landscape of various chemical intermediates during the ORR in order to pinpoint the source of the overpotential [163].

5.2. pH-Dependent Activity and Selectivity

The ORR displays pronounced pH-sensitive characteristics, showing substantial variations in both catalytic activity and product selectivity across acidic and alkaline conditions. Under alkaline conditions, enhanced reaction rates are typically observed owing to improved OH⁻ adsorption kinetics and reduced energy barriers for O₂ bond cleavage, favoring the 4e⁻ reduction pathway that generates H₂O. For instance, the enhanced ORR performance of Rh(pc) in alkaline solutions was attributed to the excessive amount of specifically adsorbed OH⁻ species and extended RhOH formation across a wider potential range, which simultaneously lower the reaction overpotential and promote the 4e⁻ pathway [160]. Furthermore, the superior ORR activity of Rh in alkaline versus acidic media, compared to Pt, is ascribed to its weaker adsorption of OH_ads and O_ads species compared to Pt [13]. A similar enhancement occurs on Au(100) surfaces, where OH_ads species boost ORR performance under alkaline conditions [176]. One study demonstrated that single-crystal Au electrodes exhibit facet-dependent ORR activity, with Au(100) uniquely producing both H₂O (>−0.2 V) and H₂O₂ (<−0.2 V) in basic media, while other facets exclusively formed H₂O₂. In acidic conditions, all surfaces generated only H₂O₂, though Au(100)’s potential-dependent selectivity in alkaline solutions was linked to its distinctive surface charge inhibiting HO₂⁻ reduction. These performance variations directly correlate with each crystal face’s specific adsorption properties for HO₂⁻ intermediates, highlighting the critical role of atomic surface structure in determining reaction pathways [177].

Furthermore, the pH environment critically affects the stability of key reaction intermediates (*OOH and *OH), thereby governing the overall mechanistic pathway. Strbac examined the ORR and hydrogen peroxide reduction (HPRR) on Pt(poly) in sulfate solutions across the broad pH spectrum using rotating disc electrodes. The reaction onset potentials aligned with PtOH formation and shifted negatively with increasing pH, with 4e⁻ pathways dominating below pH 3.0 (acidic) and above pH 10.0 (alkaline). Between pH 3.0 and 6.0, the electron transfer numbers dropped below the theoretical values (ORR < 4e⁻, HPRR < 2e⁻) due to local pH changes causing double-wave polarization curves and Tafel slope variations, reflecting a transition between acidic and alkaline reaction mechanisms [159]. Golubovic et al. revealed that Rh(poly) exhibits superior ORR activity in alkaline media (0.1 M NaOH) compared to acidic solutions (0.1 M HClO₄ > 0.05 M H₂SO₄), as evidenced by half-wave potentials, with OH⁻ adsorption enhancing kinetics while sulfate adsorption inhibits it. The reaction follows a 4e⁻ pathway in all media, showing Tafel slopes of 60 mV dec⁻¹ at low overpotentials and 120 mV dec⁻¹ at high overpotentials in both HClO₄ and NaOH electrolytes [160]. Steady-state ORR measurements on Rh(poly) demonstrated identical reaction mechanisms in acidic and alkaline media, exhibiting Tafel slopes of 60 mV dec⁻¹ at low overpotentials and 120 mV dec⁻¹ at high overpotentials [159]. Recall that a shift in the Tafel slope typically indicates an alteration in the reaction’s rate-determining step.

5.3. Roles of Specific Anions and Cations in Catalyst Performance

The catalytic performance of the ORR is profoundly influenced by the electrolyte composition, with anions playing a decisive role in acidic media. Strongly adsorbing anions like sulfate (SO₄²⁻) and chloride (Cl⁻) poison active sites and modify intermediate binding energies while weakly coordinating anions (e.g., perchlorate, ClO₄⁻) enable higher activity. Zhu et al. [178] systematically investigated how different anions affect ORR kinetics on Pt/C and Pd/C catalysts in alkaline media. While F⁻, Cl⁻, and ClO₄⁻ showed no adsorption or poisoning effects, CO₃²⁻ and SO₄²⁻ significantly reduced Pt/C activity but had minimal impacts on Pd/C. Citrate anions underwent self-dissociation on both catalysts, with the oxidation of adsorbed species occurring at higher potentials on Pd/C, leading to greater ORR activity suppression. Kamat et al. [29] studied the influence of acidic electrolyte anions on Pt catalyst performance at pH 1, observing variations in ORR and OER activity but consistent HER/HOR performance. The study revealed that the ORR activity was significantly higher in HClO₄, followed by HNO₃ and H₂SO₄. Moreover, the study suggested that HNO₃ can be a viable alternative to HClO₄ for studying the ORR, as trace Cl⁻ in HClO₄ may obscure experimental results. Using DFT calculations, they revealed that weaker anion adsorption on Pt(111) correlates with enhanced ORR/OER activity.

Additionally, it was suggested that local electrostatic interactions between anions and ORR intermediates can influence the adsorption free energy of ORR intermediates [29]. Unlike weakly adsorbing perchlorate anions that minimally perturb surface processes, strongly adsorbed sulfate anions occupy the double layer and significantly disrupt both hydrogen adsorption/desorption and oxide formation/reduction reactions [160]. According to Tripkovic et al., hydrohalic acids (HCl, HBr, and HI) and inorganic acids (H₂SO₄ and H₃PO₄) significantly suppress Pt’s ORR activity through the competitive adsorption of their anions at active sites within the double layer microenvironment [179].

The reaction microenvironment—particularly the electrolyte composition—is pivotal for optimizing catalytic performance, where cation engineering (e.g., alkali and alkaline earth metals) emerges as a powerful strategy. These cations tune ORR activity by restructuring interfacial water, modulating electric fields, and stabilizing intermediates, though balancing activity–selectivity trade-offs remains challenging. Such effects directly influence oxygen adsorption and intermediate activation, demonstrating electrolyte design as complementary to catalyst engineering. Strmcnik et al. showed that non-covalent interactions between hydrated AM⁺s (Cs⁺, K⁺, Na⁺, and Li⁺) and adsorbed OH species influence Pt(111) ORR activity, with performance following the trend Cs⁺ > K⁺ > Na⁺ > Li⁺—inversely correlating with cation hydration energies [85]. AM⁺s also alter ORR activity in alkaline media, with trends Cs⁺ > K⁺ > Na⁺ > Li⁺ linked to cation hydration energies [85]. This highlights how the cation hydration strength modulates oxygen reduction kinetics. Rademaker et al. systematically examined the electrolyte’s effects on ORR activity in PCN-224(Co) MOF, finding modest cation-dependent variations (activity changed 1.3–1.6 times between Li⁺, Na⁺, and Cs⁺ acetates) at high ionic strength but no anion size, buffer, or O₂ concentration dependence. However, the study emphasized that the reaction is governed by the Co site’s intrinsic ORR kinetics rather than mass/charge transport limitations. This suggests that catalytic improvements should focus on electronic modulation of the Co centers rather than electrolyte engineering [180].

Understanding the ORR activity as a function of cation molar ratios is also crucial for optimizing both interfacial structure and cost-effective electrolyte selection in practical applications. Suntivich et al. demonstrated that varying Li⁺/K⁺ ratios in KOH-LiOH mixtures on Pt/C revealed enhanced ORR activity with higher K⁺ concentrations, suggesting K⁺ modulates Li⁺’s influence—a phenomenon not fully explained by noncovalent interaction models [181]. These findings underscore the need to explore cation-specific effects beyond existing theoretical frameworks to tailor electrocatalytic systems effectively. Sarpey et al. demonstrated that the ORR activity of Au electrodes can be precisely tuned by adjusting the Na⁺/K⁺ molar ratio in sulfate electrolytes, with PME showing a linear correlation with activity, confirming the hypothesized PME–activity link. At pH 8, a tenfold activity enhancement was observed in mixed Na₂SO₄/K₂SO₄ electrolytes compared to pure Na₂SO₄, highlighting the significant impact of the cation composition on catalytic performance [16].

Recent research highlights the critical role of interfacial H-bond networks in regulating PCET kinetics by restructuring interfacial water molecules, thereby controlling reaction rates and selectivity. For example, Wang et al. showed that protic ionic liquids in an interfacial layer of Pt/Au electrodes strengthen H-bonds with ORR intermediates, increasing activity by up to five times [182], while similar H-bond networks in diaminotriazine–Cu–porphyrin systems enhance CO₂RR efficiency by stabilizing intermediates [183]. Additionally, cations modulate these H-bond networks, steering selectivity toward either the 4e⁻ (H₂O) or 2e⁻ (H₂O₂) ORR pathways by facilitating or disrupting proton transfer [85]. Cations influence electrocatalytic ORR through distinct interfacial field and solvation effects, where larger alkali ions (K⁺ and Cs⁺) enhance H₂O₂ selectivity by creating a positive interfacial field that repels protons from the reaction zone, while divalent cations (Mg²⁺ and Ca²⁺) alter kinetics via double-layer compression—despite the carbonate precipitation risks in alkaline media [184,185]. Zhang et al. demonstrated that AM⁺s in acidic electrolytes significantly enhance H₂O₂ production in flow electrolyzers [186], with AIMD simulations revealing that adsorbed Na⁺ creates a local coordination environment that repels H⁺ from the electrode surface, thereby suppressing H₂O₂ reduction. However, they noted that this cation effect becomes undetectable in rotating ring disc electrode (RRDE) measurements due to solution agitation and insufficient cation accumulation at the electrode interface. Hübner et al. [187] demonstrated that AM⁺s dramatically enhance the ORR activity of carbon electrodes in acidic media, shifting the half-wave potential positively by 260 mV (from −0.48 to −0.22 V_RHE). This switchable catalytic enhancement, governed by the electrode’s PZC, arises from AM⁺-induced local field effects that stabilize the *OOH intermediate, as confirmed by voltammetry, in situ X-ray photoemission spectroscopic (XPS), and DFT calculations. Their work establishes a revised mechanistic framework for the 2e⁻ ORR to H₂O₂, highlighting the critical role of interfacial AM⁺ effects in acidic ORR catalysis. The study reveals that while the *OOH intermediate maintains a fixed distance from the catalyst surface, large cations like K⁺ induce a positive local field that reorients the hydrogen atom away from the surface. This geometric rearrangement lowers ΔG_*OOH by ∼0.8 eV under applied positive fields, enhancing catalytic efficiency [187].

This dichotomy in the ORR pathways directly depends on the electrode’s catalytic activity toward H₂O₂ reduction. The catalytic activity for ORR in acidic media depends strongly on anion adsorption, where sulfate species inhibit H₂O₂ reduction more severely than perchlorate, as evidenced by higher Tafel slopes in sulfuric acid [160]. On Rh(poly) surfaces, the reaction predominantly proceeds via a 4e⁻ pathway, with only minor contributions from the 2e⁻ route, since sulfate adsorption particularly impairs the catalyst’s ability to reduce H₂O₂ intermediates. Hydrated non-specifically adsorbed anions positioned in the OHP are typically viewed as inactive toward intrinsic ORR activity, whereas hydrated cations have been shown to influence ORR kinetics in alkaline solutions [188]. However, Lou and Koper demonstrated that non-specifically adsorbed anions significantly influence ORR activity. They discovered that non-specifically adsorbed anions unexpectedly influence ORR kinetics on Pt(111), deviating from conventional *OH binding energy descriptors. They introduced a new voltammetric descriptor—the reversibility of the *O ↔ *OH transition—which correlates with ORR activity across various electrolytes (anions in acidic solutions, cations in alkaline solutions, and ionomer effects). Their model connects ORR rates to both *O → *OH transition kinetics and *OH binding energy, explaining distinct activity trends on stepped Pt surfaces (Figure 6) in acidic versus alkaline media [174].

The ORR in alkaline media shows greater sensitivity to reaction conditions like the electrode potential and electrolyte composition [85,181,189] compared to acidic environments. On Pt surfaces, the dominant 4e⁻ inner-sphere (IS) pathway proceeds through sequential PCETs, converting O₂ to H₂O via adsorbed intermediates (O₂⁻_ads, HO_2,ads, OH_ads, and O_ads) [122,190], though this route faces inhibition by oxide species (OH_ad/O_ad) [189]. Recent studies propose competing parallel mechanisms where both IS and outer-sphere (OS) pathways operate simultaneously [190,191,192]; while IS begins with O₂ adsorption, OS involves long-range electron transfer to solvated O₂, forming O₂⁻ₛₒₗ [190]. Notably, the OS route enables 2e⁻ peroxide formation [190,192], making the IS/OS balance critical for controlling 4e⁻ versus 2e⁻ selectivity in catalyst design. In alkaline media, electrolyte cations at the OHP critically modulate OH_ads stability: Li⁺ stabilizes it by 0.1 eV while K⁺ destabilizes it by ≈0.1 eV [85,193], with OH_ads coverage inversely correlating with ORR activity by blocking O₂ adsorption [194]. Recent work further reveals that cations regulate IS-PCET efficiency by controlling the kinetics and reversibility of the OH_ads ↔ O_ads transition [174]. Although OH_ads alters the double-layer structure and promotes OS-ORR [190], the cation’s role in OS mechanisms remains unclear, with proposed (but unconfirmed) dual influences—stabilizing OH_ads for IS pathways and promoting the formation of solvated species (e.g., O_2,sol) for OS pathways [195,196].” Kumeda et al. [197] combine experiments, simulations, and theory to reveal how cations govern the alkaline ORR on Pt(111), showing that cations simultaneously govern (i) the electrode surface state via Pt–O/Pt–OH interconversion and (ii) inner- vs. outer-sphere PCET pathway competition. These results provide definitive evidence of electrolyte-driven ORR modulation, with significant implications for the design of electrochemical systems and the screening of computational catalysts [197].

5.4. Interim Summary of the ORR

Despite decades of research, the ORR remains one of the most kinetically challenging and poorly understood electrochemical reactions. The sluggish nature of oxygen activation and the competition between two-electron and four-electron pathways continue to hinder the development of efficient catalysts. While pH has long been known to influence both the reaction pathway and selectivity, the specific roles of cations and anions in steering intermediate formation and adsorption remain insufficiently resolved. Strong anion adsorption can poison active sites, yet its impact is often substrate-dependent and difficult to generalize. Cations, on the other hand, modulate electric fields and interfacial solvation, particularly in alkaline media, where their influence on OH⁻ adsorption and transition state stabilization becomes critical. Notably, the ORR exhibits strong sensitivity to the structure of the electric double layer, which can shift the potential of maximum entropy and alter water structuring near the electrode. However, many studies treat the electrolyte as a static medium, overlooking its dynamic interactions with evolving surface states. We believe that a unified theory of the ORR must account for electrolyte-specific interfacial restructuring as a primary factor controlling activity and selectivity.

6. Carbon Dioxide Reduction Reaction (CO₂RR)

6.1. Influence of the Electrolyte on Product Selectivity

The electrolyte plays a critical role in determining product selectivity during the electrochemical CO₂ reduction reaction (CO₂RR). The electrolyte composition, including pH, cation identity, and nature of anions, significantly impacts the reaction pathways and resulting product distribution. By altering the local environment at the electrode surface, electrolytes can influence the adsorption of intermediates, proton availability, and electron transfer kinetics, thereby steering the reaction toward specific products. This section of this review article emphasizes the importance of understanding these effects of the electrolyte for designing efficient and selective CO₂RR systems, ultimately paving the way for practical applications in sustainable fuel and chemical production. The electrolyte’s pH critically governs CO₂ reduction selectivity: alkaline conditions (high OH⁻ concentration) stabilize CO intermediates, favoring CO production, while acidic media (abundant H⁺) promote formic acid (HCOOH) generation via PCET. This pH-dependent proton availability directly modulates the reaction’s free energy landscape, steering product distributions.

Aqueous CO₂ electrochemical reduction faces Faradaic efficiency losses from competing HERs—proton reduction (2H⁺ + 2e⁻ → H₂) or water reduction (2H₂O + 2e⁻ → H₂ + 2OH⁻) [198]—with water reduction dominating at high pH due to proton scarcity. However, this effect can extend to mildly acidic conditions where cathodic proton depletion occurs during the HER [199]. The pathways exhibit pH-dependent kinetics and onset potentials, as seen in Pt-catalyzed HER [44,76], while CO₂ further complicates this competition by acting as a pH buffer [199], adsorbate [200], or proton mediator [201], each uniquely altering HER behavior. Thus, optimizing CO₂ reduction requires probing HER pathways under controlled mass transport and local pH conditions at the electrode interface. Ooka et al. [202] established that water reduction, rather than proton reduction, serves as the primary competing HER route in CO₂ reduction on Cu electrodes, even in relatively acidic environments (pH 2.5), since proton reduction only becomes significant at potentials where CO₂ conversion is negligible. The study identifies adsorbed CO, generated during CO₂ reduction, as selectively inhibiting water reduction—a critical factor sustaining Cu’s elevated Faradaic efficiency under neutral pH conditions, verified through in situ FTIR spectroscopic analyses [202].

Thus, CO₂ reduction often reaches near-100% Faradaic efficiency under neutral pH conditions [203,204], primarily because the HER proceeds via water reduction rather than proton reduction. In acidic electrolytes, however, proton reduction begins at more positive potentials than water reduction, leading to significantly lower Faradaic efficiencies for CO₂ reduction [202,205]. The technical advantages of acidic electrolytes, such as superior conductivity, enhanced OER kinetics [123], optimized electrolyzer configurations, and the prevention of HCO₃⁻ crossover [206], make high Faradaic efficiency CO₂ reduction at low pH highly desirable. Bondue et al. [207] investigated CO₂ reduction on Au electrodes under mildly acidic conditions, demonstrating that increased CO₂ pressure (0.1–0.5 bar) enhances CO production (CO₂ + 2H⁺ + 2e⁻ → CO + H₂O) while suppressing competing proton reduction (2H⁺ + 2e⁻ → H₂), achieving ~100% Faradaic efficiency by depleting interfacial protons. They proposed a design principle for acidic CO₂ electrolyzers: the rate of CO/OH⁻ production must equal or exceed proton mass transfer to the electrode to inhibit parasitic H₂ evolution fully [207].

Carbon dioxide has limited solubility in water (~35 mM under standard conditions), which can be further reduced when salts are added to create electrolytes due to salting-out effects. This constrained solubility, when relying only on dissolved CO₂, can cause mass transport limitations at high current densities. Gas diffusion electrodes and membrane electrode assemblies (MEAs) address CO₂ solubility limitations by creating a triple-phase boundary, where gaseous CO₂, the electrolyte, and electrocatalyst directly interact [208]. Non-aqueous electrolytes, with their superior CO₂ solubility, present a viable alternative to aqueous systems for electrochemical CO₂ reduction. Their ability to suppress the competing HER further enhances CO₂ conversion efficiency and selectivity toward desired products [208,209]. While generally stable, these electrolytes require rigorous stability testing to confirm that the reaction products are derived from CO₂ rather than electrolyte decomposition. Moreover, the introduction of water or blended electrolytes has significant impacts on the product distribution and system stability, underscoring the need for careful formulation optimization [208]. Another general concern is electrolyte purity, as even trace metal impurities can significantly impair catalytic performance by reducing both reaction efficiency and product selectivity. To address this challenge, purification methods, particularly pre-electrolysis, are routinely implemented to eliminate contaminants before experimental use [208,210].

Recent advances have shown that highly concentrated electrolytes can simultaneously suppress competitive hydrogen evolution and steer CO₂RR pathways toward multicarbon products. For instance, Zhao et al. demonstrated that water-in-salt electrolytes, such as concentrated LiTFSI, significantly reduce the availability of free water, thereby suppressing HER activity by impeding proton transfer through reduced water activity and an altered interfacial structure [211]. This effect widens the HER overpotential window, creating a more favorable environment for the selective reduction of CO₂. Complementarily, Lin et al. showed that increasing the concentration of potassium formate (HCOOK) on polycrystalline Cu electrodes from 1 M to 7.1 M markedly enhances the Faradaic efficiency toward C₂H₄, with the C₂H₄/CO ratio increasing from ~2.2 to ~18.3 [212]. This enhancement was attributed to a cooperative effect of interfacial K⁺ enrichment and a modified solvation structure, which collectively stabilize key *CO intermediates and lower the activation barrier for C–C coupling via the OCCO transition state. These findings highlight how high-concentration electrolytes alter the electric double layer and the local reaction microenvironment, simultaneously suppressing the HER and promoting C₂₊ product formation in CO₂RR. Generally, the strategies for suppressing the HER in CO₂ reduction have been reviewed recently by Khalil et al. [213].

6.2. Buffering Capacity and Local pH Effects

The electrolyte’s buffering capacity is highly important in the CO₂RR, as it maintains a stable pH by resisting acid or base addition, which is vital due to the PCET nature of the CO₂RR. Inadequate buffering can lead to significant pH fluctuations at the electrode surface, altering the reaction pathway and product distribution. A high buffering capacity ensures a consistent supply of protons, facilitating proton-dependent reduction steps and stabilizing intermediates. This stability is particularly crucial for reactions requiring multiple proton and electron transfers, such as the formation of hydrocarbons. The choice of buffer, its concentration, and its pK_a value directly influence the local pH and, consequently, the selectivity and activity of the catalyst [214].

The selectivity of the CO₂RR is strongly dependent on both the local pH environment and CO₂ concentration at the reaction interface. As demonstrated by Hori et al., electrolyte-dependent pH changes alter product distributions on Cu electrodes [215]. While proton availability (governed by local pH) determines CH₄ formation, C₂H₄ formation remains pH-independent. These findings established pH as a key mechanistic variable linking specific products to proton concentrations [216]. Despite the bulk electrolyte’s pH being well-controlled, significant local pH variations can occur at the electrode surface during the CO₂RR. This is due to the consumption or generation of protons during the electrochemical reactions. For instance, the reduction of CO₂ to formate consumes protons, leading to an increase in the local pH, while the formation of CO or hydrocarbons can also alter the local pH [217,218]. These pH gradients can significantly affect the adsorption of reactants and intermediates, the kinetics of electron transfer, and the stability of the catalyst. Moreover, the local pH is influenced by the current density, the rate of proton consumption or generation, and the diffusion of protons from the bulk electrolyte [219].

The local pH near the electrode surface directly impacts the selectivity of CO₂RR products. Different products are favored under varying pH conditions. For example, a higher local pH favors the formation of products requiring fewer protons, like CO, while a lower pH promotes the formation of products requiring more protons, such as methane or ethylene. The protonation state of intermediates is highly pH-dependent, and this directly influences the subsequent reaction pathways. Furthermore, the kinetics of competing reactions, such as the HER, are also affected by the local pH [220]. Understanding and controlling the local pH is, therefore, crucial for achieving high selectivity toward the desired products. This principle has been utilized in alkaline electrolyzers for CO₂ reduction, producing C₂ products such as ethylene and ethyl alcohol with high FEs, high current densities, and low overpotentials [221].

Several strategies can be employed to mitigate the adverse effects of local pH variations. Optimizing the electrolyte’s buffering capacity is essential for maintaining a stable pH environment. Incorporating proton donors or reservoirs near the electrode surface can also help replenish protons and stabilize the local pH [222]. Microfluidic systems and flow cells can enhance mass transport and facilitate the delivery of protons to the reaction site, thereby improving the efficiency of the process. Additionally, modifying the catalyst surface with proton-conducting materials or functional groups can improve proton availability and buffer the local pH changes [223].

6.3. Effects of Cations on CO₂ Activation and Reaction Pathways

Beyond pH, the identities of cations and anions within the electrolyte significantly contribute to the local environment at the electrode surface. Cations, such as alkali or alkaline earth metals, can influence the adsorption of CO₂ and reaction intermediates through electrostatic interactions. These interactions can alter the activation energy barriers for specific reaction pathways. For example, some cations can stabilize specific intermediates, leading to enhanced selectivity toward certain products. Anions, on the other hand, can affect the EDL structure and the potential distribution at the electrode–electrolyte interface. Additionally, they can participate in specific adsorption processes or form ion pairs with intermediates, further influencing the reaction’s selectivity. Patniboon et al. computationally demonstrated that electrolyte anions compete with adsorbates and poison active sites on single metal atom coordinated N-doped carbon (M/N/C, MN₄) catalysts during ORR but not CO₂RR, where water and gas molecules dominate the interactions instead. Their study reveals that catalytic performance depends critically on both the identity of the metal center and the electrolyte composition, with the 2D nature of the MN₄ structure enabling simultaneous anion adsorption on one surface while maintaining catalytic activity on the opposite side [224].

Beyond their role in charge balance, cations actively participate in CO₂RR mechanisms, directly interacting with CO₂ molecules and intermediates to alter adsorption energies, proton transfer rates, and the local pH, ultimately governing product distribution and electrode stability [139,209]. While the mechanistic role of alkali cations in the CO₂RR remains debated, emerging in situ/operando techniques (e.g., surface-enhanced infrared absorption spectroscopy (SEIRAS)) and multiscale techniques (DFT and AIMDS) now permit atomistic tracking of cation–electrode interactions. Recent atomic-scale modeling of the CO₂RR has elucidated key cation mechanisms, particularly their positioning within the EDL and associated catalytic effects. Recent experimental characterizations and theoretical simulations have allowed researchers to revisit the underlying mechanisms of cations’ effects on the CO₂RR. A recent review by Qin et al. has summarized the effect of cations on the CO₂RR, focusing on interfacial electric field-related interactions, coordinating reaction intermediates, altering the interfacial water structure, and modulating the local CO₂ concentration and pH [225]. Yet, comprehending cations’ impacts on aqueous–metal interfaces remains complex, as these vary with the catalyst choice, desired reaction route, and precise experimental parameters. A consensus remains elusive, with conflicting results sometimes observed in identical reaction setups.

Recent studies highlight that cations in the CO₂RR significantly influence the reaction efficiency through interfacial electric field effects, where AM⁺s (e.g., K⁺ and Cs⁺) enhance the local field strength, promoting CO₂ activation and stabilizing key intermediates [225]. The electric field generated by cations modifies the double-layer structure, reducing activation barriers for CO₂ → CO conversion while suppressing the competing HER, though the effects vary with the cation size and concentration [226]. DFT and experimental studies of Ag(111) reveal that cation-induced electric fields enhance *CO₂ (* = adsorption state) and *COOH adsorption in the CO₂-to-CO pathway, driven by adsorbate dipole moments and polarizability. While the field negligibly affects the HER (H_ads has a zero dipole), it significantly stabilizes CO₂RR intermediates like *CO₂, *OCCO, *OCCHO, and *CO due to their substantial dipole moments and polarizability [227,228]. Moreover, the identity of electrolyte cations plays a crucial role in determining CO₂RR activity and product distribution, where larger AM⁺s like K⁺ and Cs⁺ preferentially promote the formation of C₂ products (ethylene/ethanol) on Cu electrodes [227,229]. The origin of these effects remains debated: some studies attribute them to cation adsorption at the OHP, which alters local electric fields and intermediate stabilization, while others emphasize cation-induced surface pH shifts due to varying buffer capacities, which modify reaction pathways or surface CO₂ concentrations [227,230]. This fundamental disagreement highlights the complex, multifaceted nature of cation influences in electrochemical CO₂ reduction systems.

Beyond electric field effects, cations impact the CO₂RR through direct coordination with intermediates. Monteiro et al. combined experimental and theoretical work, revealing no CO production, demonstrating that the CO₂RR requires trace Cs⁺. Partially desolvated Cs⁺ stabilizes the initial intermediate (CO₂) via short-range electrostatic interactions. The research demonstrates that CO generation on Au, Ag, or Cu exclusively occurs if a metal cation is added to the electrolyte during the CO₂RR [231]. Subsequent studies by the same group revealed that multivalent cations (Ba²⁺ and Nd³⁺) strongly stabilize *CO₂ intermediates through coordination, facilitating spontaneous protonation to COOH. AM⁺s such as K⁺ can stabilize specific intermediates, favoring certain reaction pathways. The size and charge density of the cation play a crucial role in determining the strength of these interactions. Smaller and higher-charged cations tend to exhibit stronger interactions, leading to more pronounced effects on CO₂ activation [47]. Studies reveal a consistent trend in CO₂RR activity, Li⁺ < Na⁺ < K⁺ < Cs⁺, linked to hydrated cation radii (Li⁺ > Na⁺ > K⁺ > Cs⁺) and their hydration free energies. This highlights the role of cation size in modulating reaction kinetics [225,226,229] (Figure 7). A successful CO₂RR depends on three cation-mediated factors: intermediate stabilization (*CO₂⁻), HER suppression, and cation enrichment at the OHP. These effects are governed by cation acidity, highlighting its multifunctional role in reaction kinetics [232].

Beyond electric fields and coordination effects, cations influence the CO₂RR by modifying interfacial water networks, affecting the H-bonding network, proton diffusion pathways, and proton transfer steps. Li et al. demonstrated via SEIRAS that cation-induced field effects are negligible for CO selectivity. Rather, they found that larger cations influence product selectivity by disrupting the H-bonding between surface-adsorbed CO and IWMs [47]. Furthermore, cations also regulate proton transport in acidic electrolytes, hindering the HER while boosting CO₂ conversion to form formic acid, CO, and multicarbon products [225,226,233]. Chen et al. [234] combined theoretical and experimental approaches to examine how the applied potential and cationic species influence CO₂RR efficiency in a model Ag nanocluster system (Ag₁₅(C≡C–CH₃)⁺). It was revealed that the applied potential exposes active Ag sites while Na⁺ ions critically enhance the CO₂RR by boosting *CO₂ activation and promoting its conversion to COOH and CO through Na⁺–CO₂(COOH) complexes, simultaneously suppressing the HER. Their electrochemical tests confirmed that optimal Na⁺ concentrations (0.1 M NaCl) increase CO selectivity to ~96%, demonstrating the pivotal role of cations in promoting the CO₂RR. Recent research utilizing SEIRAS and AIMDS on Cu has highlighted the critical role of interfacial water structures in the cation-facilitated CO₂RR. Contrary to prevailing assumptions, it was demonstrated that Li⁺ outperforms larger cations in CO₂ activation, despite its rigid hydration shell impeding proton transfer to *CO₂ [235]. Through combined electrokinetic, isotopic, and computational studies, Yu et al. [236] demonstrated that smaller, more acidic cations (Li⁺ > Na⁺ > K⁺ > Cs⁺) markedly improve CO₂-to-CH₃OH conversion on Co, contrasting the trends observed for CO on Ag or C₂₊ on Cu. This activity trend correlates with H-bonding interactions around *CHO, where the cation’s hydration shell serves as a proton donor during the rate-limiting *CHO formation step. The study reveals that cation solvation structures, beyond mere stabilization, play a decisive role in PCET during the CO₂RR.

Untangling the effects of cations on CO₂RR kinetics is complicated by intertwined electrolyte factors (mass transport, conductivity, and pH) [227,237], with studies reporting conflicting dominant mechanisms: surface pH control under mass transfer limits [237] versus interfacial field dominance in mass transport-free systems [227,237]. It was reported that a higher local pH promotes intermediate dimerization and C₂ product formation [238], yet contrary to the expectations, electrolytes with large cations (higher buffering capacity) enhance C₂ yields more than small cations [227], indicating that factors beyond pH modulation influence CO₂RR performance. Shah et al. conducted a systematic investigation of the CO₂RR using carbonate electrolytes containing different AM⁺s (Li⁺–Cs⁺) on Cu electrodes (−0.1 to −1.1 V vs. RHE), identifying three key cation influences: hydrated cation adsorption, preferential hydrolysis, and an interaction between the cation and adsorbed species, where the predominant effect depends on the applied potential and cation type. According to this study, at low cathodic potentials, changes in the OHP potential and cation–adsorbate interactions within the OHP significantly affect performance, particularly for small cations (Li⁺ and Na⁺). At high cathodic potentials, variations in cation hydrolysis and surface adsorption become the dominant influencing factors, especially for larger cations (K⁺, Rb⁺, and Cs⁺). Their study establishes an integrated model that connects cation-mediated electrolyte behavior to fundamental CO₂RR kinetics, demonstrating how these interactions govern electron transfer, surface adsorption, and reactant transport phenomena [239].

6.4. Interim Summary of the CO₂RR

Among the reactions covered in this review, the CO₂RR is perhaps the most sensitive to the electrolyte composition, owing to its diverse product landscape and the central role of proton-coupled electron transfer. Selectivity toward C₁ or C₂⁺ products depends not only on the nature of the catalyst but also on the local pH, cation identity, buffering capacity, and electrolyte concentration. While high pH and large alkali cations tend to favor C–C coupling, the mechanisms behind this trend are still debated, with various studies attributing it to field effects, intermediate stabilization, or interfacial solvation. Water-in-salt electrolytes and buffered near-neutral systems offer additional levers for modulating activity, yet often introduce mass transport or conductivity penalties. Moreover, competition with the HER remains a central challenge, particularly under conditions where proton availability and CO₂ solubility vary simultaneously. From a design standpoint, the key issue is that many observed trends are highly system-specific, making cross-study comparisons difficult. We argue that advancing CO₂RR selectivity will require a more predictive understanding of how the electrolyte composition shapes the interfacial microenvironment at high current densities.

7. Nitrogen Reduction Reaction (NRR)

7.1. Challenges in Achieving High Faradaic Efficiency

Environmentally benign ammonia production is crucial to mitigate the carbon emissions and high energy consumption linked to traditional chemical methods like the Haber–Bosch process, which is energy-intensive and releases significant amounts of CO₂ [240,241]. The electrocatalytic nitrogen reduction reaction, NRR, with the overall reaction N₂ + 3H₂ → 2NH₃ (Δ_fH° = −45.9 kJ mol⁻¹, at ambient conditions) has emerged as a promising alternative due to its mild operation, use of water as a hydrogen source, and absence of CO₂ emissions [242]. Electrochemical NH₃ synthesis via the NRR utilizes renewable electricity and N₂, with protons sourced from water splitting, to produce NH₃ sustainably. This process relies on efficient electrocatalysts that adsorb and reduce N₂ on the electrode surface, enabling scalable ammonia generation. The resulting green ammonia can support decarbonized industrial and transportation applications [243].

Ammonia production via electrochemical N₂ reduction can occur through indirect or direct pathways. The indirect mechanism employs aprotic nonaqueous electrolytes, generating metal nitride intermediates that undergo hydrolysis to produce ammonia. Because of the limited proton content, the indirect approach effectively suppresses the HER and side reactions, leading to higher ammonia yields and FEs [244,245]. Due to the instability, poor recyclability, and limited scalability of nonaqueous electrolytes, indirect ammonia synthesis fails to meet current industrial requirements. The direct pathway, on the other hand, is categorized by the type of electrolyte used: nonaqueous or aqueous. Nonaqueous systems utilize sacrificial proton donors, such as alcohols or cationic complexes, to protonate adsorbed N₂ into ammonia [242,246], effectively suppressing the competing HER due to limited proton availability.

In contrast, aqueous electrolytes, using water as the proton source under acidic, alkaline, or neutral conditions, offer a more feasible route for sustainable ammonia synthesis. Direct ammonia synthesis in aqueous media primarily follows dissociative and associative pathways, differentiated by the initial step of N₂ bond breaking or protonation, respectively. The dissociative N₂ reduction pathway, requiring direct cleavage of the strong N≡N bond, faces a substantial kinetic barrier [247], contrasting with the associative mechanism’s stepwise protonation of adsorbed N₂. The hydrogenation of the dissociatively adsorbed N usually occurs with the H₂ produced from steam reforming of natural gas (methane, CH₄). This process generates CO₂ as a by-product, contributing to environmental concerns. The associative pathway converts N₂ to NH₃ through six PCETs while maintaining the N–N bond until partial/complete hydrogenation occurs, facilitated by end-on N₂ adsorption on the catalyst surface. Furthermore, the associative mechanism is classified into two pathways, the associative alternating pathway (AAP) and the associative distal pathway (ADP), based on the sequential addition of hydrogen to the activated dinitrogen, as shown in

Table 2. Reaction mechanism for the electrocatalytic NRR [243].

Mechanism	Primary Reaction Stages
Dissociative pathway	N2 + 2* → 2*N
	2*N + 2e− + 2H+ → 2*NH
	2*NH + 2e− + 2H+ → 2*NH2
	2*NH2 + 2e− + 2H+ → 2NH3 + 2*
Associative distal pathway	N2 + * → *N2
	*N2 + e− + H+ → *NNH
	*NNH + e− + H+ → *NNH2
	*NNH2 + e− + H+ → *N + NH3
	*N + e− + H+ → *NH
	*NH + e− + H+ → *NH2
	*NH2 + e− + H+ → NH3 + *
Associative alternating pathway	N2 + * → *N2
	*N2 + e− + H+ → *NNH
	*NNH + e− + H+ → *NHNH
	*NHNH + e− + H+ → *NHNH2
	*NHNH2 + e− + H+ → *NH2NH2
	*NH2NH2 + e− + H+ → *NH2 + NH3
	*NH2 + e− + H+ → NH3 + *

and Figure 8.

The AAP simultaneously hydrogenates both nitrogen atoms before the release of ammonia (Figure 8b), while the ADP sequentially hydrogenates one nitrogen, releases NH₃, then protonates the second nitrogen (Figure 8c) [248]. The Mars–van Krevelen (MvK) mechanism represents another distinct pathway that preferentially occurs on transition metal nitride surfaces (Figure 8d). This mechanism involves protonation of surface nitrogen to form NH₃, creating vacancies that are subsequently refilled by either external N₂ (via distal pathway) or bulk nitrogen migration. While this process enables NH₃ generation, the bulk-to-surface nitrogen transfer induces structural changes in the electrocatalyst, ultimately compromising its stability [243,249].

The practical implementation of the electrocatalytic NRR faces challenges, including sluggish nitrogen adsorption kinetics, limited catalyst current efficiency, and low ammonia yield. These limitations stem from the stability of the strong N≡N triple bond, which decreases the FE while promoting the HER [243,250,251]. Research demonstrates that the HER’s lower limiting potential compared to the NRR enables it to dominate at lower applied potentials, severely competing with nitrogen reduction [252]. Kinetically, protons and electrons preferentially form H₂ instead of NH₃, explaining the poor FE and suppressed NH₃ yields in electrocatalytic systems [253]. An effective NRR requires strong HER suppression, as catalyst active sites preferentially bind H⁺ ions due to electronic interactions, severely limiting NH₃ yield and FE. To enhance NRR performance, electrocatalysts must achieve superior N₂ selectivity over competitive proton reduction. Moreover, electrolyte engineering [254,255] has been one of the promising strategies for suppressing the HER.

At the electrode–electrolyte interface, while electrocatalysts enable electron transfer to N₂, the electrolyte serves multiple critical functions: facilitating reactant (N₂) and product (NH₃) transport while controlling proton (H₂O) transfer rates. Electrolyte engineering proves particularly valuable for addressing the fundamental challenge of similar NRR and HER equilibrium potentials, enabling selective nitrogen reduction through strategic regulation. Current approaches, including cation selection, organic solvents, and ionic liquids, each uniquely modify NRR efficiency and selectivity, as systematically reviewed by Jiang et al. (2025) [256]. These electrolyte modifications create tailored interfacial environments that preferentially stabilize NRR intermediates while suppressing the parasitic HER. Cations accumulate at charged electrode surfaces, forming an EDL that critically influences electrocatalytic kinetics by modulating interfacial potential, electric fields, pH, and geometric microenvironments [257,258]. Notably, cations suppress the HER by reducing proton availability through solvation/steric effects while enhancing NH₃ selectivity in the NRR, with their concentration directly impacting electrode–electrolyte interface dynamics. Salt-based electrolytes are classified as either dilute (salt-in-water, SIW) or concentrated (water-in-salt, WIS, C > 5 M), each exhibiting distinct interfacial behaviors that govern NRR efficiency. Unlike standard SIW electrolytes, WIS systems feature exceptionally high salt concentrations, drastically reducing free water molecules through intense ion solvation, thereby suppressing water activity and subsequent hydrolysis reactions [97]. Shen et al. [255] demonstrated record-breaking NRR performance by combining Se-vacancy-rich WSe_2−x with a WIS electrolyte, where the WIS system suppressed the HER, enhanced N₂ adsorption, and strengthened π-back-donation, achieving 62.5% FE and 181.3 μg h⁻¹ mg⁻¹ NH₃ yield in 12 M LiClO₄. The same group [258] further advanced this approach using a single-atom Rh/MnO₂ catalyst in a 9 M K₂SO₄ WIS electrolyte, reaching exceptional performance metrics (73.3% FE, 271.8 μg h⁻¹ mg⁻¹) through optimized N₂ enrichment and hydrogenation kinetics, as confirmed by operando spectroscopy and DFT calculations. These studies highlight the dual role of WIS electrolytes in simultaneously modifying interfacial environments and stabilizing reactive intermediates. By integrating defect-engineered catalysts with tailored electrolytes, their work establishes a transformative strategy for overcoming the intrinsic limitations in electrochemical ammonia synthesis.

7.2. Proton Source and Competition with the HER

In the electrochemical NRR, protons play a vital role, but the competing HER can disrupt the reaction. Improving NRR selectivity requires regulating the proton supply and optimizing catalysts to prioritize N₂ reduction over the HER. Theoretical studies indicate that the NRR limiting potential (UL) is approximately −1 V for most catalysts, which is significantly more negative than that of the HER [252]. Consequently, the HER tends to dominate over the NRR when the potential is reduced.

Experimental studies reveal that NRR activity, including both FE and NH₃ yield rate, typically peaks at low overpotentials (~200 mV) before declining sharply at higher potentials, a trend observed across diverse catalysts such as transition metals, single-atom catalysts (SACs), metal oxides, and non-metal systems [259,260,261]. This behavior leads to NRR currents falling far below the theoretical mass transfer limit (~1 mM N₂ in aqueous solutions [252]), suggesting kinetic limitations rather than N₂ availability as the primary constraint. The early decline in activity at moderate overpotentials highlights the challenge of balancing catalytic enhancement with competing side reactions such as the HER.

Enhancing NRR selectivity can be achieved by strategically modifying catalyst electronic structures and introducing defects, which suppress the HER by weakening hydrogen adsorption while promoting N₂ activation [262,263]. Additionally, lowering proton availability, either by reducing proton concentration in the electrolyte or replacing water with aprotic solvents, effectively limits HER competition by hindering proton transport and water reduction kinetics [264]. These approaches collectively shift the catalytic preference toward the NRR, improving both activity and selectivity under electrochemical conditions.

The choice of proton donor is critical in the Li-based NRR (Li-NRR), as it governs both the protonation of Li₃N and the solid–electrolyte interphase (SEI) formation, directly impacting long-term stability and NH₃ selectivity [265,266]. Ethanol, the most widely studied proton source, faces limitations due to irreversible consumption via tetrahydrofuran (THF) oxidation and the formation of lithium ethoxide, leading to system instability [265]. Alternative alcohols like 1-butanol and isopropanol show promise, with 1-butanol demonstrating superior NH₃ selectivity, while isopropanol avoids THF degradation [265,267]. Additionally, studies using Kamlet–Taft parameters reveal that linear aliphatic alcohols, with optimal H-bonding properties, enhance proton delivery, with peak FE observed at 0.25 M concentration, linked to SEI permeability and mass transfer kinetics [268,269]. The synergistic optimization of proton donors and lithium salts is essential for achieving high-efficiency Li-NRR systems.

7.3. Role of Non-Aqueous Electrolytes in Enhancing Selectivity

The HER presents a major challenge for electrochemical NRR by severely compromising NH₃ selectivity and yield. While acidic electrolytes would theoretically optimize NRR by facilitating hydrogenation, their practical implementation requires precise proton management to suppress competing HER kinetics. Currently, effective electrolyte optimization strategies to simultaneously inhibit the HER and enhance the NRR remain critically underdeveloped [254]. Non-aqueous solvents, including organic solvents and ionic liquids (ILs), boost NRR performance by enhancing N₂ solubility while suppressing the HER, though organic solvents may generate by-products. Optimal water–organic mixtures balance improved NH₃ production with stability, while fluorinated ILs offer superior conductivity and N₂ solubility. Combining ILs with aprotic solvents maximizes N₂ dissolution and HER inhibition, achieving a peak NH₃ yield and Faradaic efficiency.

To overcome limited N₂ solubility in aqueous electrolytes, non-aqueous organic solvents have been employed, significantly increasing N₂ availability for the NRR while their structural polarity disrupts H-bonding, raising HER kinetic barriers and enhancing NRR selectivity [270]. Recent studies demonstrate that mixing these organic solvents with water (as a sustainable proton source) effectively balances N₂ solubility and system stability [256]. Guo et al. [254] developed HER-suppressing electrolytes using hydrophilic poly(ethylene glycol) (PEG) as the electrolyte additive, whose molecular crowding effect enhanced NRR performance by inhibiting HER kinetics. Using a TiO₂ nanoarray electrode in PEG-containing acidic electrolytes, they achieved a 32.13% NH₃ FE and a 1.07 µmol cm⁻² h⁻¹ yield, which are improvements of 9.4 and 3.5 times over pure acidic electrolytes, respectively. These enhancements, which were also consistently observed across Pd/C and Ru/C catalysts and alkaline conditions, were attributed to PEG’s ability to suppress the HER’s Heyrovsky step by slowing proton diffusion through H-bonding between PEG’s oxygen and H₃O⁺/H₂O’s hydrogen. This restriction on proton movement enhances the NRR over the HER. Moreover, ILs show great potential for the NRR due to their excellent conductivity, stability, and wide electrochemical stability windows [271].

7.4. Interim Summary of the NRR

The NRR is arguably the least mature and most experimentally ambiguous of the electrocatalytic reactions considered here. The extreme inertness of N₂, coupled with the thermodynamic favorability of the HER under most conditions, presents a formidable barrier to practical ammonia synthesis. The electrolyte composition plays a central role in determining whether the NRR can proceed at all, as the presence of protons, buffers, and cations influences both reactant activation and the suppression of competing side reactions. However, due to the low reaction rates and high susceptibility to contamination, the field suffers from reproducibility issues and false positives. While some studies have explored the roles of proton donors and solvent structuring in stabilizing intermediates, a comprehensive mechanistic picture remains lacking. Electrolyte engineering has potential to enhance selectivity, particularly through the use of aprotic solvents, non-aqueous electrolytes, or engineered ionic environments, but few systematic comparisons exist. We believe that the electrolyte’s effects on the NRR should be studied in conjunction with rigorous control experiments and standardized protocols, as progress in this field depends as much on eliminating artifacts as it does on enhancing activity.

8. Comparative Discussion and General Trends

8.1. Common Effects of Electrolytes on Different Electrocatalytic Reactions

Across all five electrocatalytic reactions, the electrolyte’s effects are mediated through combinations of electric field modulation, interfacial structure, and intermediate stabilization. The HER and ORR are particularly sensitive to cation–water interactions, while the CO₂RR and NRR exhibit a stronger dependence on solvation and proton management. Importantly, HER competition is a limiting factor in all reductive reactions (ORR, CO₂RR, and NRR), but the mechanisms of suppression differ, from buffering (NRR) to kinetic competition (CO₂RR). This comparison reveals shared constraints and points to electrolyte microengineering as a cross-cutting optimization strategy. In our view, the dominant effects of the electrolyte on the discussed reaction are summarized in

Table 3. A wide overview of the dominant electrolyte’s effects on the considered electrocatalytic reactions.

Reaction	Key Electrolyte Factors	Most Sensitive to	HER Competition	Cation Role
HER	pH, cation hydration	Interfacial water structure	—	Strong (Li+ slows, Cs+ speeds)
OER	Anion adsorption, pH	Surface oxidation	No	Moderate (Fe/Ni shifts)
ORR	pH, anion competition	Adsorption site blocking	Yes (esp. acidic)	Strong (Pt activity cation-dependent)
CO2RR	pH, cation size, concentration	C–C coupling barriers	Yes (dominant)	Critical (Cu/C2+ tuning)
NRR	Proton donors, solvation	Competing HER	Major limitation	Emerging topic

.

The pH of an electrolyte critically influences electrochemical reactions by affecting proton availability, reaction kinetics, and catalyst stability. Acidic environments favor proton-dependent reactions, such as the HER and NRR, where high H⁺ concentrations facilitate efficient reduction processes on catalysts like Pt. Similarly, the ORR in fuel cells proceeds more readily in acidic media, favoring a direct 4e⁻ pathway on Pt-based catalysts. However, reactions like the OER and CO₂RR typically suffer in strong acids due to catalyst instability or competing HER dominance. In acidic media, the competing HER is strongly favored, often limiting the FE of the NRR. A recent review by Jiang et al. [272] highlights that the acidic CO₂RR can overcome key challenges such as carbonate formation and CO₂ crossover seen in alkaline or neutral media, although it faces strong HER competition. Their work emphasizes that catalyst–electrolyte interface engineering, through electrolyte tuning, catalyst modification, and surface optimization, can suppress the HER and improve CO₂RR efficiency. Additionally, exploring interfacial properties such as the electronic structure, ion adsorption, and surface hydrophobicity under electric fields offers further potential for enhancing acidic CO₂RR performance.

Alkaline environments enhance electrochemical reactions involving hydroxide (OH⁻) mediation, particularly the OER, where OH⁻ stabilizes *OOH intermediates on Ni/Fe/Co catalysts, and the ORR, enabling efficient 4e⁻ pathways on non-precious metal catalysts. While alkaline conditions improve OER and ORR performance, they hinder the HER due to slow water dissociation kinetics, unlike acidic media, which favor the proton-driven HER on Pt. Alkaline electrolytes also benefit the CO₂RR by suppressing competing the HER, though carbonate formation remains a limitation. Conversely, the NRR performs poorly in strong alkaline solutions due to inadequate N₂ protonation, preferring neutral or mildly acidic conditions. While most electrocatalysis research focuses on highly acidic or alkaline media due to their superior activity, these systems face challenges such as corrosion, costly membranes, and side reactions. A promising alternative is neutral or near-neutral electrolytes, although limited attention has slowed progress in this area. A recent review by Lai et al. systematically examines the reaction mechanisms, electrolytes, catalysts, and modification strategies for the HER, ORR, and OER in neutral media while also summarizing advanced characterization techniques. Their work highlights key challenges and future directions, aiming to spur further research and development in neutral electrocatalysis [1]. In neutral/near-neutral electrolytes, the HER/ORR/OER follow similar fundamental steps as in acidic/alkaline systems but face greater complexity due to the thermodynamically unfavorable water dissociation step, requiring an additional overpotential to generate protons/hydroxides from the scarce H⁺/OH⁻ ions [1,19]. Unlike extreme pH conditions where these ions are abundant, neutral media rely entirely on water molecules as the proton/hydroxide source for the HER and OER, fundamentally altering the reaction dynamics. The catalytic process in neutral electrolytes begins with water dissociation on the catalyst surface, followed by reaction and desorption steps, as exemplified by the HER, where proton transport at the electrode–electrolyte interface is critical; however, the evolving H-bond network and rising local pH hinder hydroxide diffusion, causing proton–hydroxide recombination that impedes HER progression and introduces an additional overpotential, partly explaining the lower activity compared to acidic/alkaline media [19]. Another key distinction is the specific adsorption of buffer groups (e.g., phosphoric acid in PBS) onto catalyst surfaces, which can poison active sites, such as Pt/C, and degrade performance. However, such effects remain understudied [70]. Further complicating neutral electrocatalysis are the poorer ionic conductivity, O₂ solubility, and mass transport compared to extreme pH electrolytes, which potentially alter the rate-determining steps [71]. These factors collectively differentiate neutral mechanisms from conventional acid/alkaline pathways, revealing significant knowledge gaps that demand a more profound investigation to advance the field.

Cations play a critical role in electrochemical reactions by modulating interfacial electric fields, stabilizing reaction intermediates, and influencing local pH and reactant concentrations. Their size, charge density, and hydration strength determine their impact. Cations also govern the double-layer structure, proton/water dynamics, and catalyst stability, making their selection pivotal for optimizing activity and selectivity in both acidic and alkaline media. Small and/or highly charged cations like Li⁺ and Mg²⁺ perform exceptionally well in reactions requiring a precise water layer organization, particularly in the alkaline OER, where they help stabilize critical *OOH intermediates. Their larger counterparts (K⁺ and Cs⁺) prove more effective for the HER in alkaline solutions by disrupting water’s H-bonds to speed up dissociation while also improving CO₂ conversion (in the CO₂RR) through interfacial CO₂ accumulation. Although compact cations can aid in supplying protons for the CO₂RR in neutral/alkaline environments, they unfortunately intensify the competing HER under acidic conditions. The strategic choice of cations thus becomes paramount. Smaller cations control the OER and pH-dependent processes, while larger cations excel in the HER and CO₂RR by manipulating interface chemistry. This size-dependent behavior creates distinct optimization pathways for different electrochemical systems, with alkaline media generally favoring larger ions for water-splitting and gas-phase reactions. A comprehensive review by Ringe highlights that cations disrupt H-bonds of both surface-adsorbed water and the OHP by reorganizing surrounding water molecules. Importantly, the effects of cations are intrinsically linked to pH, as both phenomena depend on cation accumulation at the interface. However, the specific impact and mechanism of cations vary significantly with the experimental conditions, including pH, electrode oxophilicity, applied potential, as well as the rate-determining step and proton source involved in the reaction [273]. Moreover, as demonstrated by Goyal et al., cations can selectively enhance HER activity at Pt step sites while minimally affecting terraces, underscoring the need for precise control over the catalyst structure and electrolyte composition to optimize performance [274].

Electrolyte anions play a role in electrocatalysis by directly interacting with the electrode surface and influencing the EDL. Specific anions can either hinder reactions like the HER and ORR by blocking active sites (e.g., SO₄²⁻ and Cl⁻) or enhance performance by reducing competitive adsorption (e.g., ClO₄⁻). In alkaline media, anions affect catalyst stability and hydroxide availability for the OER and HER, while in the CO₂RR, anions such as HCO₃⁻ and halides (Br⁻ and I⁻) can modulate the local pH and catalyst morphology to favor specific products. Moreover, electrolyte ions significantly affect reaction outcomes not only by influencing the stability of transition states but also by the availability of key reactants like protons. The Biddinger group [275] showed that hydrophobic anions (TFSI⁻) can suppress the HER in CO₂ reduction by limiting water transport, while hydrophilic anions (C₂N₃⁻) promote it. Likewise, it was demonstrated that concentrated salt solutions reduce water activity, suppressing the HER and enhancing C₂₊ product selectivity in CO₂ reduction, highlighting the importance of ion–solvent interactions [276]. A recent review by the Hall group emphasizes a growing focus on the intricate electrochemical interface formed by the dynamic interactions between the electrode and electrolyte components, including solvents, ionomers, and ions. This interface is further modulated by both inherent and externally applied electric fields, highlighting the complexity of explaining the effect of anions on electrochemical systems [277]. Yoo et al. [278] demonstrated that anions play a crucial role in determining the selectivity and activity of electrochemical CO₂RR on Au catalysts, with weakly adsorbing anions like propionate enabling optimal CO₂ reduction kinetics and near 100% FE for CO production. Their combined experimental and computational approach revealed that the free energy of anion physisorption serves as a key descriptor, where minimal anion adsorption enhances reaction performance despite the electrode’s negative charge. These findings highlight the importance of carefully selecting anions in aqueous electrolytes to maximize electrocatalytic CO₂RR efficiency.

8.2. Trade-Offs Between Reaction Rates and Selectivity

Electrocatalytic systems inherently struggle with the trade-off between achieving high reaction rates and maximizing selectivity toward a desired product. Conditions that accelerate kinetics, such as strong reactant adsorption or abundant active sites, often inadvertently promote the formation of undesired by-products. Conversely, achieving high selectivity typically necessitates precise intermediate stabilization and tailored reaction pathways, which can intrinsically slow down the overall reaction rate due to restrictive binding or diminished activation of reactants. For example, in the CO₂RR, weakly adsorbing electrolytes like propionate enhance CO selectivity, while strongly adsorbing species (e.g., sulfate) may boost rates but reduce FE by promoting competing pathways like the HER. Similarly, in the ORR, alkaline electrolytes accelerate kinetics but can favor H₂O₂ formation over H₂O.

These trade-offs arise from shared active sites, competing adsorption equilibria, and scaling relations in intermediate energetics. Electrolyte engineering represents a crucial avenue for navigating these limitations. Electrolyte engineering, through ion selection, pH tuning, and double-layer modulation, offers a tool to balance these factors. By carefully selecting electrolyte components, including anions and cations, it is possible to influence interfacial charge transfer, reactant concentrations, and intermediate stabilization, thereby mediating the balance between rate and selectivity. For instance, high-concentration KOH improves HER rates but may increase parasitic reactions, while tailored ionomer coatings in acidic media can enhance selectivity for H₂O₂ in the ORR, though with some activity loss.

Advancing beyond these limitations requires integrating electrolyte design with catalyst engineering. Operando studies reveal how the local pH and cation effects (e.g., Li⁺ vs. Cs⁺ in the CO₂RR) alter intermediate stabilization, while microkinetic models predict optimal ion–catalyst pairings. For example, buffered electrolytes mitigate pH swings to maintain selectivity in nitrate reduction, and hydrophobic ions suppress the HER in the CO₂RR. Future efforts should combine computational descriptors (e.g., adsorption energetics) with in situ tools to decouple kinetic and thermodynamic constraints. Innovations such as dynamic field control or spatially modulated active sites could further break scaling relations, as observed in pulsed electrolysis strategies that enhance ethylene selectivity in the CO₂RR. Ultimately, synergistic electrolyte–catalyst design promises to unlock systems that exhibit both high rates and selectivity, which are critical for sustainable energy applications. Future advancements in in situ characterization and computational modeling will be vital for establishing predictive relationships between electrolyte properties and catalytic performance, paving the way for smarter electrocatalyst–electrolyte pairings that optimize both reaction rates and product selectivity for sustainable energy conversion.

8.3. Design Principles for Optimizing Electrocatalytic Performance

Optimizing electrocatalytic performance requires an integrated approach that considers the electrolyte not as a passive medium but as a co-designer of the catalytic interface. Across all five reactions discussed here, HER, OER, ORR, CO₂RR, and NRR, it is evident that the electrolyte composition, particularly ion identity and concentration, profoundly influences interfacial electric fields, hydrogen-bonding networks, and the stabilization of reaction intermediates. The pH of the electrolyte governs fundamental mechanistic pathways, with acidic media generally favoring faster HER and ORR kinetics due to high proton availability, while alkaline conditions are more suitable for the OER and enable the use of non-precious catalysts. Cations such as Li⁺, Na⁺, and K⁺ modulate the electric double layer (EDL) structure, affect water dissociation kinetics, and influence the binding energies of intermediates, often with reaction- and surface-specific trends. Solvent effects, particularly the structure and orientation of interfacial water molecules, play a decisive role in proton-coupled electron transfer processes, impacting both activity and selectivity. In near-neutral media, buffering agents such as phosphate or borate help stabilize the local pH under operating conditions, thereby enhancing reproducibility and mitigating performance losses due to pH gradients.

Additionally, engineering the EDL by controlling ion distributions and local dielectric environments offers a powerful means of tuning catalytic behavior. These effects must be balanced against the need for long-term catalyst stability and product selectivity, as aggressive conditions or lattice oxygen participation (as in the LOM pathway) may improve activity at the cost of degradation. Ultimately, combining advanced operando characterization with theoretical modeling (e.g., DFT and AIMD) offers critical insights into complex interfacial phenomena, facilitating the rational design of catalyst–electrolyte pairs. Together, these principles highlight the importance of electrolyte engineering as a key strategy in developing high-performance, durable, and selective electrocatalytic systems for energy conversion.

9. Conclusions and Future Perspectives

9.1. Summary of the Key Findings

In proton-dependent reactions like the HER, acidic electrolytes enhance kinetics by providing abundant protons, whereas alkaline conditions slow down reaction rates due to limited proton availability, though additives like buffers can mitigate this limitation. Alkaline media promote the 4e⁻ ORR pathway to H₂O (but excessive alkalinity may promote H₂O₂ formation) and enhance C₂₊ production in the CO₂RR while suppressing the HER, but these benefits are inherently linked to intertwined pH and cation effects. Beyond reactivity and selectivity, electrolytes also impact catalyst durability. Corrosion in the acidic HER or dissolution in the alkaline OER underscores the need for careful electrolyte–catalyst pairing. The electrode–electrolyte interface fundamentally shapes electrocatalytic processes through ion-specific effects. Moreover, cation selection (e.g., Li⁺ vs. K⁺) significantly influences cathodic reactions such as the HER and CO₂RR by altering adsorption energetics, while anionic species in the OER can either promote hydroxide oxidation or inhibit activity through site-blocking. For more complex electrocatalytic reactions, such as the CO₂RR and NRR, electrolytes have a critical influence on the product distribution. In the NRR, non-aqueous electrolytes improve the ammonia yield by minimizing the parasitic HER, whereas aqueous acidic conditions overwhelmingly favor the HER.

In the HER, larger cations like Cs⁺ accelerate kinetics by disrupting water networks, while in the OER, alkali metals stabilize *OOH intermediates to boost activity. For the ORR, cation size dictates 2e⁻ vs. 4e⁻ selectivity, and in the CO₂RR, hydrated cations (K⁺ and Cs⁺) enhance C₂+ production by affecting the local CO₂ concentration and electric field. In the NRR, cations like Li⁺ suppress the HER and stabilize N₂, showcasing their universal role in tuning reactivity. However, their proton-modulating behavior presents trade-offs: while beneficial in alkaline media, small cations inadvertently accelerate the competing HER in acidic conditions. These universal yet reaction-dependent roles make cation selection a powerful lever for tuning activity, selectivity, and stability across electrocatalytic systems.

Weakly adsorbing salts (e.g., KCl and KHCO₃) enhance C₂⁺ formation in the CO₂RR by suppressing the HER, while buffering agents help stabilize key intermediates such as *CO. Koper’s group [174] discovered that non-specifically adsorbed anions (like ClO₄⁻) significantly influence ORR kinetics on Pt(111), revealing the *O ↔ *OH transition as a key kinetic descriptor. This breakthrough descriptor also effectively explained how PFSA ionomers and cations in an alkaline environment affect Pt(111) ORR activity, providing new insights into interfacial electrocatalysis. Additionally, a recent study revealed that weakly adsorbing anions (e.g., propionate) maximize CO₂RR efficiency on Au catalysts, achieving near-100% CO selectivity by minimizing interfacial interference. Their study identified anion physisorption energy as a critical performance descriptor, showing how subtle electrolyte interactions govern catalytic behavior. This work provides a design principle for optimizing the CO₂RR through strategic anion selection in aqueous electrolytes [278].

A recent review by Xu et al. [30] demonstrates that tailored modulation of the catalytic interface microenvironment effectively increases both reaction kinetics and product selectivity. Strategic control of the electrolyte composition, including pH adjustment, ion selection, and interfacial hydrophilicity/hydrophobicity tuning, effectively enhances reaction rates, product selectivity, and system stability in electrocatalytic processes. By optimizing the catalyst–interface microenvironment through these approaches, significant improvements in performance and durability have been achieved, particularly in fuel cell applications. Furthermore, precise modulation of local conditions, such as the cation concentration and reactant ratios (e.g., local CO₂/H₂O in the CO₂RR), enables efficient multi-carbon production in CO₂ reduction, demonstrating the transformative potential of microenvironment engineering for practical electrochemical systems. The interfacial microenvironment significantly impacts HER/HOR kinetics through non-Nernstian pH effects, primarily by modifying the EDL structure and interfacial water networks. These interface-induced changes alter local electric fields and proton transfer dynamics, creating deviations from traditional electrochemical expectations. Understanding these mechanisms is crucial for both developing efficient hydrogen energy devices and establishing fundamental structure–activity relationships in electrocatalysis. Chen et al. investigated pH-dependent ORR activity on FeCo-N₆-C dual-atom catalysts through simulations and in situ spectroscopy, revealing that acidic and alkaline environments create distinct EDL structures. These structural differences alter the interfacial water orientation and H-bonding with oxygen intermediates, ultimately governing the observed variations in ORR kinetics [279]. Shen et al. achieved breakthrough NRR performance using Se-vacancy-rich WSe_2−x and WIS electrolytes by suppressing the HER and enhancing N₂ activation [255], later surpassing these results with Rh/MnO₂ in 9 M K₂SO₄ through optimized N₂ hydrogenation [258]. Their work demonstrates how WIS electrolytes synergize with defect-engineered catalysts to modify interfacial environments and stabilize intermediates, offering a transformative approach to overcome fundamental limitations in electrochemical ammonia synthesis.

Future advances in electrochemical systems require integrated electrolyte design strategies that simultaneously address pH, ion effects, and concentration to optimize performance metrics. Cutting-edge in situ/operando characterization techniques now enable real-time observations of reaction intermediates and microenvironment dynamics, while computational methods provide valuable mechanistic insights. Together, these experimental and theoretical approaches will deepen our understanding of interfacial phenomena and accelerate the development of efficient energy conversion systems.

Although intrinsically impossible, we have attempted to summarize the main effects reported here for the five electrocatalytic reactions discussed in the present work (

Table 4. An attempt at a summary of the electrolyte’s effects on the considered electrocatalytic reactions.

Reaction	pH Effect	Cation’s Effect	Anion’s Effect	Solvent/Buffering	Notable Outcomes
HER	Strong pH dependence; faster in acid but influenced by the EDL in base 1	Hydrated cations (e.g., Li+) may inhibit activity in base; larger cations (e.g., K+) often enhance activity 2	Adsorbing anions may block active sites (e.g., SO42−) 3	Interfacial water structure is critical; buffering stabilizes the local pH 4	Activity modulated by both outer-sphere (double layer) and inner-sphere interactions
OER	Faster in base due to easier OH− activation; pH affects the pathway (AEM vs. LOM) 5	Larger cations (K+ and Cs+) can promote activity via EDL effects 6	Some anions (e.g., PO43−) adsorb and poison the surface 7	Buffering is important at a high current density; it affects stability	Electrolytes impact lattice oxygen participation and catalyst durability
ORR	pH governs selectivity (2e− vs. 4e−); faster in acid but more stable in base	Cation size and hydration impact O2 binding and the interfacial field	Cl− and other adsorbing anions suppress activity on Pt	Weakly coordinated solvents enhance O2 solubility and transport	The electrolyte composition controls activity and product selectivity
CO2RR	Local pH affects selectivity (CO vs. hydrocarbons); lower pH promotes the HER	Alkali cations stabilize *CO2− and intermediates; Cs+ > K+ > Na+ > Li+ 8	Weakly coordinating anions (e.g., ClO4−) are beneficial; strong adsorption blocks sites	Buffering is essential to maintain the pH near the interface; water-in-salt systems explored	Cation effects dominate; the interfacial environment controls the product distribution
NRR	Mildly acidic to neutral favored; high pH may suppress N2 adsorption	Weakly hydrated cations (e.g., Cs+) may facilitate N2 activation 9	Strongly adsorbing anions hinder the reaction	The proton source and water activity must be finely tuned	Electrolyte selection is key to suppress the HER and enable NRR selectivity

¹ The HER rate is significantly higher in acidic media due to abundant proton availability. ² Larger cations tend to weaken water binding at the surface, facilitating the Volmer step in base. ³ Anions like SO₄²⁻ and Cl⁻ can competitively adsorb and block active metal sites. ⁴ Structured water layers impact both PCET and the effective proton availability at the interface. ⁵ In alkaline media, the OER mechanism often involves lattice oxygen (LOM) and adsorbed OH⁻ intermediates. ⁶ Larger cations may enhance the local electric field and lower activation barriers. ⁷ Phosphate and sulfate anions are known to form surface complexes that inhibit OER activity. ⁸ Larger alkali cations better stabilize CO₂RR intermediates through non-covalent interactions. ⁹ Weak hydration allows closer approach to the surface, aiding in N₂ activation and suppressing the HER.

).

9.2. Open Questions and Potential Research Directions

Despite significant advances in understanding the effects of electrolytes on electrocatalytic reactions, numerous questions remain open, presenting compelling directions for future research. A major challenge lies in decoupling the intertwined influences of the cation size, hydration energy, and specific adsorption on the reaction kinetics and selectivity, particularly in systems where multiple ions interact simultaneously with the electrode interface. The role of anions remains less clearly understood than that of cations, with conflicting reports on their impacts across different reactions and catalyst surfaces, necessitating more systematic and comparative studies. Additionally, the structure and dynamics of interfacial water, especially under operating conditions, are not yet fully understood, and their roles in mediating proton-coupled electron transfer and stabilizing intermediates require further investigation using operando spectroscopic and computational tools. Another critical gap is the lack of generalized descriptors that can link electrolyte properties to catalytic performance across diverse systems, which hinders the rational design of electrolytes. The interplay between the electrolyte composition and catalyst degradation, particularly in pathways involving lattice oxygen participation or strongly adsorbing ions, also demands further attention to ensure long-term stability in practical applications.

Furthermore, the development of advanced electrolyte formulations, such as water-in-salt systems, ionomer/electrolyte composites, or hybrid aqueous–organic solvents, opens new possibilities but also introduces complexities that require a careful mechanistic understanding. Lastly, bridging the gap between fundamental studies in model systems and the performance of realistic devices remains a pressing task, calling for multi-scale investigations that connect atomistic insights with macroscopic performance metrics. Addressing these challenges will be essential for guiding the next generation of integrated catalyst–electrolyte systems toward scalable, efficient, and selective energy conversion technologies.

9.3. Implications for Practical Applications in Energy Conversion and Sustainability

Understanding and controlling the effects of electrolytes have profound implications for advancing practical energy conversion technologies. Tailored electrolytes can significantly improve reaction efficiency, reduce overpotentials, and enhance selectivity, enabling the use of earth-abundant catalysts in place of scarce precious metals. This is particularly relevant for water electrolysis, fuel cells, and CO₂/NRR systems, where optimized electrolytes can suppress competing reactions, such as the HER in the case of the CO₂RR, stabilize active sites, and extend catalyst lifetimes. Moreover, electrolyte engineering enables operation under milder conditions (e.g., near-neutral pH), thereby reducing system corrosion and facilitating the integration of more sustainable, lower-cost materials. From a sustainability perspective, leveraging benign, recyclable, and non-toxic electrolyte components is essential for the scalability and environmental viability of electrochemical technologies. Thus, electrolyte optimization is not merely a tool for improving lab-scale performance but a central strategy for translating electrocatalytic innovations into durable, cost-effective, and sustainable energy solutions.

From a practical perspective, translating electrolyte insights into functional energy devices requires addressing several critical challenges. Key issues include electrolyte stability under operating conditions, compatibility with catalysts and membranes, and maintaining ionic conductivity while minimizing parasitic side reactions. For instance, the integration of optimized electrolytes into fuel cell systems must consider evaporation, degradation, and scaling limitations in long-term operation. Moreover, the often narrow electrochemical windows of aqueous electrolytes pose constraints for high-voltage applications. Future efforts must also bridge the gap between fundamental electrolyte design and system-level implementation, especially in high-current-density electrolysis, CO₂ electrolyzers, and NRR setups, where managing mass transport and buffering is crucial. Developing robust, scalable electrolytes tailored to specific devices and operational environments will be crucial to accelerating the commercialization and real-world adoption of these electrocatalytic technologies.