Metal–Organic Framework-Derived Electrocatalysts for Rechargeable Zinc–Air Batteries

Abstract

Rechargeable zinc–air batteries (ZABs) are still impeded by the intrinsically sluggish kinetics of oxygen reduction and evolution reactions (ORR/OER) and by the instability or prohibitive price of state-of-the-art noble metal catalysts. Metal–organic frameworks (MOFs) have recently emerged as versatile sacrificial templates for next-generation air–cathode electrocatalysts. By programming pyrolytic or chemical conversion pathways, MOFs can be quantitatively transformed into hierarchically porous, heteroatom-doped carbon scaffolds that embed uniform metal, alloy, or metal-oxide nanodomains. The resulting architectures couple metallic conductivity with molecular-scale active site tunability, delivering exceptional ORR/OER activity, stability, and mass transport properties. This review critically examines the most recent advances in MOF-derived electrocatalysts for ZABs, establishing quantitative structure–composition–performance relationships across mono-, bi-, and multi-metallic systems. Emphasis is placed on deciphering how framework topology, metal–ligand coordination, and post-synthetic parameters dictate the density, electronic structure, and accessibility of surface-active moieties during catalyst evolution. We further dissect engineering strategies that enhance intrinsic activity via electronic modulation, bolster durability through encapsulation effects, and optimize hierarchical porosity for rapid O₂/water transport. This article concludes by outlining unresolved challenges and future research directions, including atomically precise active site construction, multi-scale compositional control, long-term reversibility under realistic ZABs cycles, scalable and green synthesis, providing a roadmap for translating MOF-derived catalysts from laboratory curiosities to commercially viable air–cathode materials.

1. Introduction

ZABs are increasingly recognized as a promising next-generation energy storage technology due to their high theoretical energy density (1218 Wh/kg), environmental sustainability, and cost-effectiveness [1]. Unlike traditional batteries, ZABs utilize oxygen from the ambient air as a freely available reactant at the cathode, significantly reducing material costs and weight while maintaining intrinsic safety. Their simple configuration, reliance on abundant zinc metal, and low environmental impact further underline their suitability for achieving sustainable energy transition goals [2,3,4]. However, the practical application of ZABs faces critical hurdles, primarily linked to the sluggish kinetics of the ORR during discharge and the OER during charge. These reactions involve complex multi-electron transfer pathways, leading to high overpotentials, energy losses, and limited cycle stability. The absence of efficient, durable, and cost-effective bifunctional catalysts remains the primary obstacle to unlocking the full potential of ZABs technology [5].

Typically, a cell consists of a zinc anode, a porous air cathode, and an alkaline or neutral electrolyte enabling ionic transport. During discharge, zinc at the anode is oxidized to zincate or zinc oxide, releasing electrons that flow through the external circuit. Concurrently, oxygen from ambient air diffuses into the air electrode and is reduced at catalytic sites to form hydroxide ions, which then migrate to the anode to complete the electrochemical cycle [6]. The key electrode reactions in alkaline electrolytes are as follows [6,7,8,9]:

Anode (discharge):

$$\mathrm{Z}\mathrm{n}+4{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{Z}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_4^{2-}+2{\mathrm{e}}^{-}$$

(1)

Followed by

$$\mathrm{Z}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_4^{2-}\to \mathrm{Z}\mathrm{n}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{O}\mathrm{H}}^{-}$$

(2)

Combined approximate overall:

$$\mathrm{Z}\mathrm{n}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}=\mathrm{Z}\mathrm{n}\mathrm{O}+2{\mathrm{O}\mathrm{H}}^{-}$$

(3)

Cathode (discharge, ORR):

$${\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to 4{\mathrm{O}\mathrm{H}}^{-}$$

(4)

Cathode (charge, OER):

$$4{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}$$

(5)

MOFs have emerged as a versatile and innovative class of materials ideally suited for the development of advanced electrocatalysts, particularly in ZAB applications (Figure 1) [10,11,12,13,14,15,16,17]. MOFs consist of metal ions or clusters coordinated with organic ligands, forming highly porous and crystalline frameworks with extraordinary structural tunability [18]. The ability to tailor their chemical, electronic, and textural properties through judicious choice of metal centers and organic linkers offers unprecedented control over catalyst design. Compared to traditional catalysts, MOFs and their derived materials provide distinct advantages, including exceptionally high surface areas, tunable pore sizes, and uniformly distributed active sites [19,20,21]. These features enable precise optimization of reaction pathways, improved interaction with oxygen species, and enhanced mass transport, significantly boosting ORR and OER performance [22]. Furthermore, MOF-derived materials obtained through thermal treatment or chemical conversion retain the structural advantages of the parent material (high specific surface area, high porosity, etc.). They can serve as conductive carriers to achieve favourable dispersion of catalytically active materials while providing the high electrical conductivity and porosity required for charge transfer and mass transport. This enables the conductive properties, stability, and catalytic activity demanded for demanding ZAB operations [22,23,24].

The primary objective of this review is to comprehensively explore the synthesis strategies employed to develop MOF-derived electrocatalysts for ZABs and their influence on catalytic performance. Various synthetic techniques, such as pyrolysis, doping, hybridization with conductive supports, and defect engineering, are critical in determining the structural, electronic, and morphological characteristics of the resultant materials. For instance, controlled pyrolysis can yield carbon-based derivatives with a hierarchical pore structure, enhancing oxygen diffusion and electron transfer. Doping with heteroatoms such as nitrogen, sulfur, or phosphorus introduces additional active sites and modulates electronic properties, further improving bifunctional catalytic activity. Hybridization strategies, including the integration of MOF-derived catalysts with carbon nanotubes or graphene, enhance conductivity and mechanical stability. This review emphasizes the correlation between synthesis methodologies, the resulting structural traits, and the electrocatalytic performance of MOF-derived materials. The insights provided aim to guide future research in designing cost-effective, high-performance catalysts, accelerating the commercialization of ZABs for grid-scale and portable energy applications.

2. Characteristics of MOF-Based Electrocatalysts

MOFs and their derivatives have demonstrated significant potential in the field of electrocatalysis, particularly for critical reactions such as the ORR and OER. As a class of highly designable porous materials, MOFs leverage their unique physical and chemical properties to offer innovative pathways for the development of efficient and cost-effective electrocatalysts [18,25]. In this section, the discussion expands on the key features of MOFs, the transformation of MOFs into electrocatalysts, and the correlations between their structure and catalytic performance in detail.

2.1. Key Characteristics of MOFs in Electrocatalysis

High Porosity and Tunable Pore Size. The high porosity of MOFs, along with their uniformly distributed porous structures, represents a fundamental advantage for their use as electrocatalytic materials [26]. MOFs can significantly increase the density of active sites within the material, thereby enhancing catalytic efficiency. Their ordered channels promote the effective diffusion of reactants and products, reducing diffusion limitations during reactions. Furthermore, the pore size of MOFs can be tailored by adjusting the metal nodes and organic linkers, optimizing reactant accessibility and mass transport [27,28].

Metal Sites as Catalytic Active Centers. The metal centers in MOFs form robust coordination frameworks with organic linkers, contributing to their structural diversity and providing abundant catalytically active sites [29,30,31,32]. Transition metals such as Fe, Co, and Ni in MOFs, known for their favorable electronic configurations and redox properties, demonstrate superior catalytic performance [33,34]. For instance, Fe-, Co-, or Ni-based electrocatalysts obtained from MOFs exhibit excellent activity in electrocatalytic applications, such as ORR and OER, under alkaline conditions, making them ideal candidates for energy storage and conversion technologies [35,36,37,38,39,40].

Customizable Chemical Functionalities. The modular nature of MOF offers remarkable versatility for chemical modifications. Functionalization can be achieved through the selection of organic linkers or by introducing heteroatoms (e.g., N, P, S) [41,42,43,44,45]. These modifications significantly enhance conductivity and catalytic activity. For instance, nitrogen-doped MOFs exhibit remarkable electrocatalytic activity due to their ability to tune charge distribution and optimize the interaction between catalysts and intermediates in complex electrochemical reactions [46,47].

2.2. Structure–Property Relationships of MOF-Derived Catalysts

The catalytic performance of MOFs and their derivatives depends on several structural parameters that influence their activity, selectivity, and stability:

(1)

Pore Structure for Facilitated Mass Transport: Hierarchical porosity in MOF-derived catalysts ensures efficient mass transport of reactants and intermediates. In OER applications, well-designed pores can reduce the overpotential and optimize oxygen gas diffusion.

(2)

Metal Dispersion and Active Sites: The uniform distribution of metal nodes within MOFs enables superior utilization of active sites. This uniformity can be preserved in derived catalysts, ensuring enhanced reaction kinetics. For instance, nanoscale metal particles generated during pyrolysis exhibit significant improvements in intrinsic catalytic activity due to their high surface-to-volume ratio.

(3)

Heteroatom Doping and Electronic Structure Optimization: Introducing heteroatoms (e.g., N, P, or S) during synthesis or transformation can alter the local electronic structure around the active sites, optimizing the adsorption energy of intermediates. For example, nitrogen doping enables favorable redistribution of charge density, enhancing ORR and OER performance with improved selectivity and durability.

(4)

Dynamic Evolution and Stability during High-Temperature Processing: Controlling the pyrolysis conditions (e.g., temperature, atmosphere) ensures the formation of thermally stable derivatives with optimal structures. Retaining the hierarchical order and preventing metal particle agglomeration are critical strategies to enhance long-term performance under typical electrocatalytic conditions.

2.3. Applications in OER and ORR

MOF-derived catalysts have garnered significant attention for ORR in fuel cells due to their high activity, stability, and cost-effectiveness [48,49,50,51,52,53,54]. MOF-derived materials demonstrate superior ORR efficiency in alkaline media, as they reduce overpotential and promote specific reaction pathways. In water electrolysis applications, MOF-derived materials offer low overpotentials and high charge transfer efficiencies. Multi-metallic MOF derivatives often exhibit remarkable activity due to the synergistic effects of diverse active centers. MOF-derived catalysts provide significant advantages in OER performance, combining high activity with long-term stability [31,55,56].

3. Synthesis Strategies for MOF-Derived Electrocatalysts

The development of MOF-derived electrocatalysts has revolutionized the field of renewable energy technologies, especially by advancing ORR, OER, and their implementation in ZABs. Theoretically, the activity of electrocatalysts can be enhanced through two approaches [49]: increasing the intrinsic activity of each catalytic active site by exploiting electronic effects, and boosting the overall number of active sites (i.e., extrinsic activity) via geometric effects. This section delves into advanced synthetic strategies for developing improved MOF-derived electrocatalysts, including pyrolysis techniques, doping and functionalization, hybridization, nanostructure control, and atomic layer deposition (ALD) enhancements, providing a systematic overview of their mechanisms and significance.

3.1. Annealing Route for Transforming MOFs into Electrocatalysts

Although MOFs themselves can serve as catalysts, their derivatives often exhibit enhanced stability, conductivity, and activity, especially when subjected to post-synthesis transformations [57,58,59].

(1)

Traditional thermal decomposition

Through controlled high-temperature pyrolysis, MOFs can be transformed into a variety of catalytically active materials, including metallic nanoparticles, metal oxides, carbides, nitrides, and metal–nitrogen–carbon (M–N–C) frameworks [60,61,62,63,64,65]. Among these, Fe-, Co-, and Ni-based M–N–C materials are especially attractive for oxygen electrocatalysis due to their high surface area, uniform metal dispersion, and favorable electronic structures. During pyrolysis, the inherent porous architecture of MOFs is often retained, generating carbon frameworks with hierarchical porosities. These structures improve the adsorption and transport of reactants, facilitating rapid charge transfer and intermediate stabilization at the active sites. This process also introduces synergistic effects; for example, doping can modify the binding energy of oxygen intermediates and improve the efficiency of OER catalysts [66,67,68,69,70,71,72].

Rational control over the morphology of MOF-derived carbons is pivotal for tailoring their electrocatalytic function, as it directly governs mass transport, active site accessibility, and structural stability. A key design strategy leverages the gaseous products in situ generated during pyrolysis to engineer porosity (Figure 2a) [73]. By employing volatile ligands or encapsulating sacrificial agents (e.g., thiourea), the rapid release of gases upon decomposition can create expanded, foam-like, or honeycomb-like porous networks with ultrahigh surface areas, significantly enhancing reactant diffusion. Beyond gas-mediated pore-forming, the thermal transformation of MOFs can be directed toward constructing interconnected conductive scaffolds. A prominent example is the in situ growth of carbon nanotubes (CNTs), where metal nanoparticles (e.g., Co, Fe), reduced from the MOFs metal nodes, catalyze the graphitization of surrounding carbon species (Figure 2b) [74]. The density and morphology of the CNTs can be precisely tuned by adjusting pyrolysis parameters, offering a versatile route to optimize the catalyst’s conductive network for bifunctional oxygen electrocatalysis.

(2)

Joule heating decomposition

Compared to traditional thermal methods, the Joule heating technique offers the following key advantages: (1) Joule heating achieves heating and cooling rates in milliseconds to seconds, which minimizes prolonged exposure to high temperatures. This avoids sintering or degradation of active sites, preserving the structural integrity and activity of the final electrocatalyst [75]. (2) By utilizing direct and uniform heating, Joule heating ensures uniform energy distribution, reducing thermal gradients and protecting sensitive structures [76]. (3) This method enables precise tuning of material properties such as defect density, particle size distribution, crystallinity, and doping homogeneity [77,78]. In Figure 3a–c, a fast and effective Joule heating activation method is proposed using nickel–cobalt layered double hydroxide-derived nanometal particles as templates [77].

Therefore, the application of Joule heating technology in the preparation of MOF-based electrocatalysts has garnered considerable attention in recent years [80]. For example, a facile synthesis strategy was established for fabricating self-supported Fe₃C nanoparticles on carbon sheets (Fe₃C@C) through modulation of the Fe-MOFs and chitosan (CS) composite film (Fe-MOFs@CS) and subsequent carbonization. Structural characterizations reveal that the anchored Fe₃C nanoparticles create abundant heterogeneous interfaces, resulting in a porous architecture with interconnected conductive networks, as shown in Figure 3d [79,81]. This innovative method offers significant advantages, including rapid heating, high energy efficiency, and precise temperature control. These features make it a transformative approach in enhancing the synthesis and performance of MOF-based materials for various electrocatalytic applications, such as hydrogen evolution reaction (HER), OER, and ORR [79,81,82,83].

(3)

Laser sintering technology

Through Joule heating decomposition, laser sintering has emerged as a transformative technology for synthesizing MOF-based electrocatalysts, offering unique advantages such as precise energy control, rapid thermal processing, and minimized energy consumption compared to conventional techniques [84,85]. Laser sintering enables controlled structural transformation of MOFs by applying localized, high-energy irradiation [86]. Unlike traditional thermal treatments, laser-induced methods can adjust material properties on micro- to nanoscales, enhancing their catalytic activity. Laser-induced instantaneous temperatures prevent aggregation and maintain porosity, critical for retaining MOF-based materials’ surface dynamics and active sites. Unlike bulk heating, laser sintering maintains the unique porous framework characteristic of MOFs, facilitating ion transport and reactant accessibility [87,88]. Laser parameters such as wavelength, pulse width, and energy flux can be tailored to optimize transformation efficiency and catalyst performance.

Recent studies have demonstrated the feasibility of using laser-induced annealing to fabricate nanoscale MOF derivatives with remarkable electrocatalytic properties. In Figure 4, Zhou and colleagues prepared porous carbon-coated metal nanoparticle electrocatalysts by CO₂ laser carbonization (10.6 µm wavelength) of 12 MOFs under ambient atmosphere [89]. The enhanced activity was attributed to laser-induced Ni doping on the Fe₃O₄ surface, which established stable active sites. Density functional theory calculations also demonstrated that this laser-induced synthesis enhances active site exposure and accelerates catalytic kinetics compared to conventional pyrolysis. Xu and coworkers reported the integration of ultrafast laser direct writing with an external magnetic field (MF) for scalable synthesis of graphene-coated ultrafine cobalt nanoparticles supported on 3D porous carbon from MOFs precursors, achieving 5 × 5 cm² samples in 10 s. The magnetic field-assisted picosecond laser scribing rapidly reduces metal ions while simultaneously aligning nanoparticles into an ultrafine, uniform distribution. The work established a versatile bottom-up strategy for synthesizing metamaterials with high throughput using a simple experimental setup [90].

3.2. Doping and Functionalization

Doping techniques introduce non-metallic atoms like N, S, or P into the carbon framework. These elements act as electron donors or acceptors, improving charge redistribution and tuning the electronic structure of material surfaces. Iron-based single-atom catalysts (SACs) represent efficient electrocatalysts for oxygen electrocatalysis, attributed to their high intrinsic activity, maximized metal atom utilization, well-defined active sites, tunable coordination environments, and adjustable electronic structures [91]. Despite considerable performance improvements achieved for Fe-based SACs, the fundamental principles governing electronic structure modulation to enhance catalytic activity remain elusive. In Figure 5, a facile strategy was developed for the in situ assembly of NiFe-MOFs nanosheets on heteroatom-doped porous activated carbon spheres. The resulting electrocatalyst (NP-ACSs@NiFe-MOFs) exhibited outstanding bifunctional oxygen electrocatalysis performance, achieving a remarkably low potential gap (ΔE = 0.61 V) in 0.1 M KOH, defined as the difference between the OER potential at 10 mA cm⁻² and the ORR half-wave potential. The work presented a facile approach for rationally integrating different catalytically active components, which can be extended to fabricate other highly competitive multifunctional electrocatalysts [92].

Sulfur doping combined with nitrogen enhances conductivity and promotes multi-functional active site development [93,94]. Xia and coworkers presented a Co@NG-S2 composite catalyst comprising cobalt-based sulfides on nitrogen-doped graphene, synthesized via a two-step sulfurization strategy. The first sulfurization enabled S and N co-doping while enhancing the conductivity and activity of the graphene support. The subsequent step promoted uniform distribution of cobalt-based sulfides and strengthened their interfacial coupling with graphene, thereby significantly improving bifunctional catalytic performance [95]. Co-doping maximizes synergistic effects by introducing multiple heteroatoms (e.g., N and P, or N and S), enhancing activity and selectivity for ORR and OER [93,96,97]. This tuning optimizes active site functionality, charge transport, and binding energy modulation for oxygen and reaction intermediates.

Optimization of metal–nitrogen–carbon (MNC) active sites is the focus for achieving single-atom-level efficiency [98,99]. A deprotonation strategy via alkaline treatment of polymerization intermediates was proposed to enrich Fe-N₄ active sites [100]. Theoretical calculations demonstrate Gibbs free energy changes of −3.70 and −26.99 kcal mol⁻¹ for Fe–N coordination at pH 0 and 7, respectively, confirming that deprotonation thermodynamically facilitates Fe–N bond formation. This approach achieves a two-fold increase in Fe–N₄ site density, endowing the resultant Fe–N–C catalyst with substantially enhanced ORR activity and superior Zn–air battery performance. This universal deprotonation strategy is extendable to various amine precursors and transition metal ions, providing a general avenue for developing advanced non-precious metal carbon catalysts.

Paired Fe single-atom sites coordinated with Se and N atoms, supported on N,S-codoped carbon (FeN_xSe_y@SNC), are designed and synthesized as an efficient bifunctional catalyst for ZABs (Figure 6) [101]. The catalyst demonstrates exceptional oxygen electrocatalytic activity, achieving a high ORR half-wave potential (E_1/2) of 0.92 V vs. RHE, an onset potential (E_onset) of 1.057 V vs. RHE, and a low OER overpotential of 346 mV at 10 mA cm⁻². Consequently, the FeN_xSe_y@SNC-based ZABs deliver a peak power density of 307 mW cm⁻² at 475 mA cm⁻². Density functional theory calculations reveal that the asymmetric Fe-Se-Fe bridging bonds and S-atom coordination in the Fe₂N₅SeS moiety enable precise modulation of the electronic structure of Fe active sites, optimizing the adsorption energies of oxygen intermediates and thereby enhancing bifunctional catalytic performance.

3.3. Hybridization with Other Materials

Hybridization strategies combine MOF-derived materials with CNTs, graphene, or carbon nanofibers to enhance both electrical conductivity and mechanical stability [102,103,104,105,106,107,108]. Carbonaceous materials act as conductive networks, improving electron transfer and mitigating issues like poor catalytic active site accessibility. MOF-on-MOF Composite Strategies were also reported [109,110,111]. Fabricating hierarchical structures by integrating multiple MOFs allows for enhanced structural complexity and density of active sites.

Yang and coworkers developed a novel strategy for synthesizing two-dimensional (2D) N-doped carbon nanobelts with exceptional electrocatalytic activity via pyrolysis of a dual-MOFs superstructure precursor [112]. Van der Waals-driven self-assembly enabled the formation of a 2D ZIF-8/ZIF-67 superstructure through a facile ice-templating co-assembly approach. During pyrolysis, the N-doped carbon matrix derived from ZIF-8 immobilizes cobalt species leached from adjacent ZIF-67, leading to the coexistence of atomically dispersed Co single atoms and nanoparticles embedded within the carbon nanobelt, which synergistically enhanced the intrinsic ORR activity. Consequently, the resulting nanobelt delivered outstanding alkaline ORR performance with a half-wave potential (E₁/₂) of 0.888 V vs. RHE, and Zn–air batteries assembled with this catalyst exhibit a peak power density of 179 mW cm⁻². The work underscored the efficacy and versatility of the ice-templating co-assembly approach for designing high-performance 2D carbon-based superstructure electrocatalysts.

Hollow and core–shell architectures introduce hierarchical porosity and internal voids that significantly enhance gas diffusion, electrolyte infiltration, and mechanical buffering. A notable example is the use of ZIF-67 as a self-sacrificing template to construct hollow NiCo₂O₄/Co–N–CNT nanocages. The hollow interior shortens diffusion paths and increases three-phase contact, while the carbonaceous shell confines active species and suppresses agglomeration—collectively yielding bifunctional ORR/OER performance superior to benchmark catalysts (E_1/2 ≈ 0.86 V, E_j10 ≈ 1.57 V in alkaline media) [15,113,114]. Similarly, core–shell MOFs@MOFs constructs (e.g., ZIF-8@ZIF-67 or ZIF-67@Co-MOFs-74) serve as protective shells that tailor the local chemical environment and mitigate surface degradation during prolonged cycling [115].

Heterostructure-based catalysts, incorporating metal–metal oxide, or carbon–metal oxide interfaces, enable superior bifunctionality. Such architectures exploit dissimilar electronic properties to promote robust catalysis pathways for ORR/OER. For example, CoCo₃O₄ heterojunctions enclosed in porous carbon achieved enhanced charge transfer, improving long-term cycles [116].

3.4. Templating and Nanostructure Control

By employing silica nanoparticles or micelles during precursor preparation, precise morphological control is achieved. Soft-templating techniques utilize self-assembling molecules, such as surfactants, to create hollow or rod-like structures. These methods ensure uniform metal dispersion and active site exposure. Hard templating, on the other hand, involves removing sacrificial structures post pyrolysis, yielding tailored nanostructures like hollow spheres [117,118,119]. The molten salt strategy offers compelling advantages for fabricating carbon-based electrocatalysts, encompassing environmental sustainability, recyclability, enhanced graphitization, efficient heteroatom doping, and exceptional structural tunability. Despite these merits, recent investigations have overwhelmingly prioritized ORR performance optimization, while OER activity has remained largely unaddressed. This unbalanced bifunctionality consequently constrains the practical performance of such materials in ZABs [120].

Moreover, hierarchical porous carbon structures with engineered interconnected networks allow faster ion diffusion and greater catalyst utilization [121,122,123,124]. ZAB performance is significantly enhanced by frameworks with hierarchical porous structures and multi-scale porosity. Macro-, meso-, and micro-level pores synergistically balance reactant diffusion (macro), surface area enhancement (meso-), and active site density. Nitrogen-doped carbon nanospheres (N–C) were synthesized via a facile soft-templating strategy as electrocatalysts for ZABs. The distinctive architecture affords a robust spherical morphology with a high specific surface area. Nitrogen doping modulates the electronic structure and generates abundant defect sites, endowing the N–C catalyst with exceptional ORR activity and durability. Consequently, ZABs incorporating the N–C catalyst achieve a high peak power density of 167.43 mW cm⁻², with a large specific capacity of 719.61 mA h g_Zn [125]. This work offers a novel approach for fabricating nitrogen-doped porous carbon electrocatalysts rich in active defect sites for high-performance electrochemical energy conversion applications.

3.5. Atomic Layer Deposition and Post-Synthetic Modifications

ALD allows for the precise coating of nanoscale films, such as metal oxides, directly onto MOF-derived materials. This technique enhances stability and avoids performance decay under harsh alkaline conditions typical of ZABs. For instance, ALD-modified CoFe oxide layers significantly improved durability and bifunctional performance [126,127]. ALD offers atomic-level control over shell thickness, preventing active site blockage while maintaining emitter accessibility. Post-synthetic modifications like acid etching, chemical oxidation, or ammonia annealing are employed to fine-tune the structure of electrocatalysts. Specific treatments increase surface roughness, activate previously inert sites, or modify coordinatively unsaturated active catalytic centers.

MOF-derived electrocatalysts represent a versatile platform for state-of-the-art energy applications like ZABs. By leveraging advanced strategies such as doping, pyrolysis, hybridization, templating, and ALD, researchers can design materials with enhanced catalytic functionalities, bringing us closer to efficient, durable, and cost-effective renewable energy solutions. As the field matures through the integration of theoretical and experimental insights, MOF-derived catalysts are poised to play a transformative role in advancing future clean energy systems.

4. Performance Metrics in ZABs

The focus on ZABs stems from their distinctive advantage over other metal–air systems: aqueous ZABs can operate as truly reversible, rechargeable cells, whereas magnesium–air and aluminum–air batteries suffer from severe self-corrosion, which fundamentally limits their viability for charging applications. ZABs stand out among energy storage technologies due to their high energy density, low costs, and environmental benignity. However, challenges related to ORR and OER kinetics significantly impede efficiency. Performance metrics in ZABs applications are critical for evaluating and engineering the electrocatalysts needed to overcome these hurdles. This section comprehensively examines three performance dimensions: electrocatalytic activity, cycling stability, and durability, emphasizing the correlation with synthesis strategies of MOF-derived catalysts.

The electrocatalytic activity of MOF-derived catalysts is a primary determinant of their suitability for ZABs applications since it directly impacts critical reactions at the cathode, the ORR, and OER. Several metrics are commonly used to evaluate this activity, including overpotential, specific activity, and Tafel slope [128]. Each of these factors correlates with specific structural and chemical features governed by synthesis strategies. Overpotential refers to the extra voltage required to drive a reaction and serves as a quantitative measure of reaction efficiency [60]. Ideal MOF-derived catalysts for ZABs aim to minimize both overpotential for ORR and OER. The density and dispersion of active metal sites, often highlighted in atomically dispersed catalysts (e.g., single-atom M–N–C structures), are critical for minimizing overpotentials [129,130,131]. Specific activity, defined as the catalytic activity normalized by the catalyst’s electrochemical surface area (ECSA), highlights the intrinsic efficiency of active sites [132]. The Tafel slope serves as an indicator of reaction kinetics, where lower values denote faster kinetics [133]. MOF-derived catalysts frequently achieve competitive Tafel slopes for ORR and OER due to tailored geometries, electronic structures, and defect engineering.

Cycling stability represents a critical aspect of ZABs performance, as real-world applications demand materials capable of repeated charge–discharge cycles without performance degradation [134,135,136,137,138]. Durability, particularly in alkaline environments, remains a key challenge for MOF-derived catalysts. Repeated cycling in ZABs can result in catalyst deactivation due to agglomeration of metallic nanoparticles, structural collapse, or blockage of active sites. MOF-derived catalysts have demonstrated excellent stability through several innovative synthesis approaches. Hierarchically macro–meso–microporous ZIF-67/nori-derived electrocatalysts were synthesized via a multi-templating approach, utilizing single-cell nori and ZIF-67 as macroporous and microporous templates, respectively, with KOH activation to create meso/micropores. The optimized ZIF-67/nori-800 catalyst delivered Zn–air battery performance, achieving a maximum power density of 294 mW cm⁻² [139]. Ultrafine bean-pod-like ZnCo/N-doped electrocatalysts synthesized via a multifunctional single-cell-chain biomass precursor are reported for Zn–H₂O₂ fuel cells. The catalyst features a wrinkled interface that exposes abundant active sites, while the integrated root-like carbon nanotube arrays facilitate rapid electron/ion transport. Consequently, the optimized catalyst exhibits exceptional ORR activity, achieving a high half-wave potential (E_1/2) of 0.90 V and an onset potential of 1.01 V. Significantly, the Zn–air battery delivers a record-breaking peak power density of 363 mW cm⁻² [140].

The performance metrics of MOF-derived electrocatalysts for ZABs applications are listed for their transformative potential in energy storage technology in

Table 1. Comparative electrochemical performance of recently reported MOF-derived bifunctional oxygen electrocatalysts.

Catalyst	Precursor MOF Type	ORR@E1/2 (V)	OER@ 10 mA cm−2 (V)	Pmax (mW·cm−2)	Tafel (mV dec−1)	Ref.
ZIF-67/nori-800	ZIF-67/nori	0.85	1.46	284	82	[139]
Bean-pod-like ZnCo/N-C	ZIF-67/Spirulina	0.90	1.57	363	67.1	[140]
Fe-SAs/NPS-HC	ZIF-8 + poly	0.912	-	195	36	[141]
Fe/P/N co-doped C	ZIF-8	0.88	-	157.5	-	[142]
Fe1-N4SO2/N-C	ZIF-8 with Fe precursor	0.91	-	282.4	-	[143]
Fe,W-N-C	Phthalocyanine-based MOFs	0.90	1.56	252-	87	[144]
NiCo2O4/Co,N-CNTs nanocages	ZIF-67	0.862	1.569	173.7	60	[113]
Fe, Co@N-C	ZIF-8/67	0.896	1.60	150	72	[145]
Fe2Co1-N-C	Fe-Co	0.882	-	234	60	[146]
CoMoN@NCNTs	ZIF-8	0.85	1.63	146.0	76.5	[15]
FeS/Fe3C@NS-C	Fe-MOF	0.78	1.52	90.9	94	[147]
Fe-N-CNBs	Non-MOF	0.875	-	257	71.4	[148]
NiFe-LDH@Co-NC/CC	Co-MOF	0.77	-	52.91	47.91	[149]
Cu/Fe/N–CNS	Cu-MOF	0.91	-	76.4	66	[150]
AlNiCoFeCrMoV@CoNC	ZIF	0.65	1.484	162	61.5	[151]
Co3O4-C-NA/NF	ZIF-67	0.83	1.54	118	90	[152]
FeNxSey@SNC	ZIF-8	0.92	1.58	307	72.1	[101]
FeZn–N–C	MOF-5	0.87	-	294	57	[153]
Fe-CoNi@C	CoNi-MOF	0.71	1.54	307.7	61.8	[154]
ZTB-NSCR-FePc	Pillar-layer MOF	0.89	-	198.9	40.7	[155]
FeSA@NC/CNT	ZIF-8	0.87	1.60	188	69	[156]
IrFe@NC	ZIF-8@IrFe	0.92	1.58	113.9	63	[157]

. Future development should focus on refining synthesis techniques to further optimize active sites, enhance long-term cycling durability, and bridge performance gaps between research benchmarks and commercial requirements.

Advanced multimodal characterization techniques were employed to unequivocally verify the atomic-level dispersion of Ir and Fe species on the nitrogen-doped carbon catalyst, corroborating the formation of heteronuclear Ir–Fe atomic pairs [157]. The IrFe–N–C catalyst exhibits substantially enhanced electrocatalytic activity for both the ORR and OER relative to its monometallic analogues (Ir–N–C and Fe–N–C), thereby demonstrating exceptional bifunctional performance. The dual-atom centers adopt an IrFeN₆ configuration, wherein both Ir and Fe are fourfold-coordinated by N atoms with two bridging N atoms shared between the IrN₄ and FeN₄ subunits. Mechanistic investigations reveal that the Fe site predominantly facilitates the ORR pathway, whereas the Ir site primarily governs the OER kinetics.

To address these critical concerns regarding testing inconsistencies in ZAB research, the community must adopt unified testing frameworks specifying (a) cell geometry (air electrode area, zinc electrode thickness, inter-electrode distance); (b) electrolyte specifications (concentration, pH, volume, refreshment protocol); (c) environmental controls (temperature, relative humidity, O₂ partial pressure); (d) current normalization criteria (geometric area vs. catalyst mass vs. zinc capacity); (e) cut-off voltages for discharge/charge cycling.

5. Conclusions and Future Challenges

MOF-derived electrocatalysts play an indispensable role in addressing ZAB performance challenges, laying the technical foundation for high-efficiency, environmentally friendly energy storage and conversion systems. Progress has been evident in overcoming sluggish oxygen reaction kinetics, improving cycle stability, and reducing costs. Three key design strategies have driven recent progress: (I) atomic-level active site engineering to mitigate sluggish ORR/OER kinetics; (II) hierarchical pore engineering to enhance oxygen and electrolyte mass transport under practical Zn–air battery conditions; (III) strengthened metal–carbon interactions and controlled graphitization to improve durability against metal dissolution and carbon corrosion during prolonged cycling. As new synthesis techniques and interdisciplinary analysis tools advance, the industry is poised for breakthroughs in scaling up MOF-derived catalysts. Furthermore, the inherent versatility of MOF materials offers promising cross-domain applications, setting the stage for next-generation energy technologies emphasizing multifunction and sustainability. MOF-derived materials are expected to play a decisive role in the field of energy chemistry, offering transformational solutions for global energy transitions.

Despite the excellent performance of MOF-derived materials under laboratory conditions, significant challenges remain in translating these advancements to large-scale production [158,159]. Advanced synthesis techniques often depend on complex procedures such as pyrolysis or controlled synthesis environments. These methods may suffer from low yields and high costs when scaled up. Additionally, maintaining batch-to-batch consistency is a substantial hurdle. Potential solutions include the development of automated, modular reactors to standardize the production process of MOF-derived electrocatalysts.

Innovative synthesis techniques are also welcome. Recent advancements, such as microwave synthesis, Joule heating, and laser sintering, have gained increasing attention in the field of MOF-derived materials. Compared to traditional pyrolysis, the innovative methods provide uniform heat distribution and kinetic advantages, significantly reducing reaction times while enhancing atomically dispersed interfaces. In addition, in situ MOF synthesis methods improve structural consistency and controllability by enabling the molecular-level assembly of precursors during crystal growth, avoiding damage from post-processing steps. Machine learning-guided catalyst design stands as another cutting-edge area, employing data-driven tools to predict active site distribution and optimize reaction mechanisms. By employing computational predictions, the electronic properties and surface stability of novel catalysts can be rapidly explored, compressing the timeline from experimental screening to industrial production.