Efficiency, Cost and Sustainability: Electrocatalysts for State-of-the-Art and Emerging Electrolysis Technologies

Abstract

Water electrolysis is a key technology for sustainable hydrogen production and a cornerstone of future low-carbon energy systems. However, large-scale deployment is constrained not only by efficiency and cost, but increasingly by the sustainability and availability of materials used in electrocatalysts and membranes. This review provides a materials-centric assessment of state-of-the-art and emerging electrocatalysts for alkaline (AEL), proton exchange membrane (PEM), and solid oxide electrolysis (SOEC) technologies, emphasizing the interdependence of performance, durability, cost, and sustainability. Electrocatalyst activity and stability are linked to cell- and stack-level efficiency, energy demand, and the levelized cost of hydrogen. Life cycle assessment (LCA) and resource criticality analyses are integrated to quantify environmental impacts, supply risks, and recycling potential of key materials, including platinum group metals, nickel, rare earth elements, and ceramic oxides. Particular attention is given to recycling and circularity strategies, which are essential for mitigating material scarcity and reducing upstream emissions, especially in PEM electrolyzers. Emerging catalyst concepts such as single-atom catalysts, high-entropy alloys, and noble-metal-free systems are discussed as promising pathways to reduce critical material dependence. The review concludes by highlighting the need for integrated material–technology–system approaches to enable efficient, scalable, and truly sustainable hydrogen production.

1. Introduction

Hydrogen (H₂) has emerged as a strategic pillar in the global transition towards a low-carbon future, offering a versatile energy vector capable of decarbonizing the energy, industrial, and mobility sectors [1]. Its ability to store and transport renewable energy, replace fossil feedstocks, and serve as a clean fuel positions hydrogen at the forefront of efforts to achieve climate neutrality.

Steam methane reforming (SMR) and coal gasification are the established hydrogen production methods today and contribute approximately 920 Mt CO₂ annually from the conversion of fossil feedstocks [2]. More sustainable and carbon-neutral production methods such as water electrolysis using low-carbon energy sources only produce 2% of worldwide hydrogen [3]. Water electrolysis stands out as a cornerstone technology for a clean hydrogen economy and is poised to become a key part of the future energy system. Electrolysis of water is an electrochemical process that splits water into H₂ and O₂ gases using an external electrical energy source. The overall reaction is as follows:

H₂O (l) → H₂ (g) + ½ O₂ (g)

(1)

When the electricity required for water electrolysis is generated from renewable sources, the resulting H₂ production exhibits low carbon intensity. This pathway is particularly attractive for integrating variable renewable energy sources into the energy system while ensuring a sustainable H₂ supply chain. Additionally, since the majority of hydrogen utilization pathways ultimately yield water as the principal reaction product, the use of water as a feedstock represents a more sustainable and circular resource compared to fossil-derived alternatives. Notably, the International Panel on Climate Change (IPCC) emphasizes that employing H₂, when produced via electrolysis powered by low-carbon or renewable electricity, offers effective mitigation pathways for decarbonizing hard-to-electrify industrial sectors [4].

The assessment of the sustainability of H₂ energy systems is critical to ensure that their deployment contributes meaningfully to climate mitigation goals, while balancing environmental performance, economic viability, and social acceptance across the entire value chain. In 1987, the United Nations Brundtland Commission defined sustainability as “meeting the needs of the present without compromising the ability of future generations to meet their own needs” [5]. This includes environmental, economic and social factors. With regard to H₂ value chains, the main aspects that define a sustainable value chain are as follows:

• Raw material demand: Mining and processing impacts for catalysts, membranes, steel, and other materials (e.g., platinum group metals, rare earths, and carbon fiber).
• Water consumption: Quantity and quality of water required for electrolysis or other H₂ production methods, and potential competition with local water needs.
• Global warming potential (GWP): Full life cycle greenhouse gas emissions from production, transport, storage, and utilization.
• Energy source for production: Share of renewable vs. fossil electricity; renewable integration reduces GWP.
• Land use: Footprint of renewable energy installations, production plants, and transport infrastructure.
• Air, soil, and water pollution: Potential leaks, by-products, or chemical use in synthesis.
• Economic performance.
• Levelized cost of hydrogen (LCOH): Production, conversion, transport, and storage costs.
• Capital and operating expenditures: Affordability and investment recovery time.
• Domestic value chains: Local manufacturing, service industries, and technology export potential.
• Long-term market stability: Demand certainty in mobility, industry, and power sectors.
• Job creation: Number and quality of jobs across the value chain (manufacturing, installation, operation, and maintenance).
• Public acceptance: Safety perceptions, transparency, and engagement with local communities.
• Equitable access: Ensuring benefits (energy access and jobs) are shared and do not exacerbate inequalities.
• Safety and health: Risk management for hydrogen handling, transport, and use.
• Supply chain ethics: Responsible sourcing of critical minerals, avoiding human rights violations.

While all of these dimensions are relevant for assessing the overall sustainability of a H₂ value chain, the majority of existing studies on the production [6,7,8,9,10,11,12], transport [13,14,15,16,17], storage [18,19,20,21] and utilization [22,23,24,25,26,27,28,29] of H2 concentrate on the environmental and economic impacts of H2 systems. These impact categories are strongly shaped by the choice and performance of electrocatalysts and membranes employed in the electrolysis process. These components are the critical functional elements of the electrolyzer, as they constitute the sites where the electrochemical water-splitting reactions take place. In essence, they represent the core of the device, around which all other system components are generally designed and integrated.

The performance of electrocatalysts critically determines the kinetics of the electrochemical reactions occurring at the electrode surfaces. When these reactions proceed sluggishly, they induce significant overpotentials [30]. This raises the specific energy demand of water electrolysis. Since the economic sustainability of electrolytic H2 production is directly tied to its energy requirements, electrocatalyst efficiency exerts a profound influence on overall process economics. Operating costs (OPEX) represent between 50 and 95% of the cost structure of electrolysis, with electricity expenses representing the largest share [31,32,33,34]. Consequently, any reduction in energy demand not only lowers the amount of renewable electricity required per unit of H₂ produced, but also substantially decreases the levelized cost of H₂ (LCOH), reinforcing both environmental and economic sustainability.

The stability of electrocatalysts [35,36,37] and membranes [38,39] is a key determinant of electrolyzer durability. Improved stability extends component lifetimes and reduces the frequency of replacements, thereby lowering material consumption and minimizing waste generation. From an LCA perspective, this directly translates into reduced impacts in categories such as resource depletion, waste generation, and energy demand for recycling of spent materials. By mitigating the need for frequent replacement cycles, enhanced durability improves resource efficiency, reduces environmental burdens associated with material extraction and end-of-life treatment, and strengthens the overall sustainability profile of hydrogen electrolysis systems.

This review provides a material-centric, comparative analysis of electrocatalysts for sustainable hydrogen production. It examines how different materials influence activity, stability, scalability, and life cycle impacts, offering insights into trade-offs between performance and sustainability. The article is structured to first introduce the fundamental principles of water electrolysis methods, followed by a detailed evaluation of state-of-the-art and emerging materials for electrodes, life cycle analyses of electrode materials, and concludes with perspectives on future research directions.

2. Materials in Electrolysis Technologies

At present, three main types of electrolyzers are commonly reported: alkaline (AEL), proton exchange membrane (PEM), and solid oxide electrolysis (SOEC). Each of these technologies still faces significant challenges for large-scale implementation; nevertheless, the efficiencies of AEL, PEM, and SOEC electrolyzers are reported to be around 60%, 73%, and 90%, respectively [40,41,42]. In Section 2.1, an overview of these technologies will be presented.

2.1. Types of Electrolyzers

2.1.1. Alkaline Electrolyzer (AEL)

The AEL stands out as the most mature electrolyzer technology and is commercially available. A typical AEL configuration integrates an electrolyte, such as an aqueous potassium hydroxide (KOH) solution, nickel-based electrodes, and a diaphragm to prevent the mixing of the generated gases [43] and to transport the OH- ions from the cathode to the anode. Figure 1 presents a schematic representation of this technology, including the chemical reactions occurring at the anode and the electrodes. An AEL operates at relatively low temperatures (<100 °C), pressures close to atmospheric and offers low manufacturing costs; however, despite these advantages, challenges such as its larger size compared with PEM electrolyzers, operational difficulties under pressure, and slow startup remain under investigation [12].

2.1.2. Proton Exchange Membrane (PEM)

PEM consists of electrodes, typically made of noble metals such as platinum, iridium, and ruthenium, and an electrolyte (a solid polymer electrolyte membrane, such as Nafion). In PEM systems, H⁺ species are generated at the anode and migrate to the cathode through the electrolyte (see Figure 1). Regarding operational conditions, PEM operates at high pressures (15–30 bar) and moderate temperatures (50–90 °C) [45]. PEM has a compact design. It also offers high proton conductivity, rapid response time, and the production of ultra-high-purity hydrogen (99.999%) [46]. As a drawback, the initial cost of PEM technologies is high, and advanced material development is still required to improve durability under acidic and oxidative conditions [47,48]. Current research is focused on the development of novel catalysts with better electrocatalytic activity, higher stability, and lower cost to reduce investment costs. In addition, efforts are being directed toward exploring higher operating temperatures to further enhance the efficiency, durability, and versatility of PEM-based systems [49,50].

2.1.3. Solid Oxide Electrolysis (SOEC)

SOEC technology differs considerably from the previously discussed cases, as its operating temperatures typically range between 800 and 1000 °C. SOEC is recognized as the most efficient electrolysis technology; it can also be operated at high pressure, and its solid oxide electrolyte is usually a ceramic material, such as yttria-stabilized zirconia (YSZ), which conducts oxygen ions (O²⁻), as shown in Figure 1. Regarding the electrodes, common materials include strontium-doped lanthanum manganite for the anode and Ni–YSZ ceramic–metal composites (cermets) for the cathode [51]. While the AEL is already a commercially mature technology, SOEC is still under development in the electrolysis market due to its higher capital costs and material-related challenges [52]. Some of the main barriers to large-scale implementation include the limited lifetimes of components in SOEC, which are prone to oxidation and corrosion during long-term high-temperature operation, as well as the demanding requirements for adequate stack sealing [53]. Nevertheless, the possibility of integrating hydrogen production with other industrial processes makes SOE a promising technology for enhancing overall energy efficiency.

The electrolyzer technologies described in this work have already reached a certain level of maturity, and nowadays it is possible to find cases where AELs and PEM and SOEC electrolyzers operate on-site, providing hydrogen for different purposes.

Table 1. Summary of hydrogen production facilities currently in operation employing AELs and PEM and SOEC electrolyzers.

Tech	Company/Project	Location	Capacity	H2 Prod. (t/yr)	Pressure	COD	Electricity Source	End-Use	Reference
AEL	Ovako—Hitachi Energy	Hofors, Sweden	2.5 MW	500–600 t/yr	30 bar (w/compressor)	2023	Solar + wind	Steel plant heating + trials	[54]
AEL	Cepsa + Thyssenkrupp Nucera (Andalusian Green H2 Valley)	Huelva, Spain	300 MW	48,000 t/yr	30 bar	2026 (planned)	Solar + wind	Refining, chemicals, shipping fuels	[55]
AEL	H2 Green Steel (Stegra)	Boden, Sweden	740 MW	>100,000 t/yr	30 bar	2026–2027 (under construction)	Hydro + wind	Direct reduction of iron (DRI)	[56]
PEM	Air Liquide—Bécancour (Cummins)	Quebec, Canada	20 MW	3000 t/yr	30 bar	2021	Hydropower	Industrial gases + mobility	[57]
PEM	Shell—REFHYNE (ITM Power)	Rheinland, Germany	10 MW	1300 t/yr	30 bar	2021	Renewable electricity	Refinery desulfurization	[58]
PEM	Iberdrola + Fertiberia	Puertollano, Spain	20 MW	3000 t/yr	30 bar	2022	Solar PV	Green ammonia (fertilizers)	[59]
SOEC	Sunfire + Salzgitter—GrInHy2.0	Salzgitter, Germany	720 kW	200 t/yr	Ambient (SOEC stack, downstream compression)	2022	Waste heat + grid electricity	Steel production (DRI, H2 blending)	[60]
SOEC	Sunfire + Neste—MULTIPLHY	Rotterdam, Netherlands	2.6 MW	400–450 t/yr	Ambient	2023–2024	Renewable electricity	Refinery hydrotreating	[61]

presents representative examples of each technology, including the associated project or company, facility capacity, hydrogen production rate, electricity source, and utilization. In this study, the commercial operation date (COD) considered ranges from 2021 to the present (September 2025). Notably, the Stegra plant is expected to achieve a capacity of over 100,000 t/yr by 2027 using AEL technology. Applications of SOEC technologies have also been identified, for instance, in Sunfire projects. Although the capacities in these cases are relatively modest (about 2.6 MW for Sunfire), their operation represents a promising step forward in the development of such technologies. It is also worth noting that, in all cases, renewable electricity sources have been integrated in pursuit of more sustainable processes.

2.2. Functional Materials for Each Architecture

2.2.1. Electrocatalysts

The electrolyzer industry has experienced significant growth in recent years, particularly in European countries, where commercial applications at several megawatt scales are already in operation. Electrocatalysts are a key component of AEL, PEM and SOEC technologies. In the case of PEM electrolyzers, commercial Pt/C and IrO₂/RuO₂ are regarded as the most effective electrocatalysts for acidic HER and OER, respectively, showing superior catalytic activity and stability compared to noble-metal-free catalysts in acidic media [62]. Nevertheless, the scarcity of reserves and the high cost of noble metals seriously limit their widespread industrial application. In this context, AEL and SOEC technologies can operate with electrocatalysts composed of more accessible materials.

Table 2. Overview of electrode catalytic materials in AEL, PEM and SOEC water electrolysis systems.

Catalyst Classification	Role in Electrolyzer(s)	Relevant Trends
Noble metals (Pt, Ir, Ru)	PEM electrolyzers, Pt and IrO2 or Ir–RuO2 are used as cathode and anode materials, respectively, due to their chemical stability and resistance to corrosion.	-Reduction in Ir loading [63,64].
Non-noble transition metals (Ni, Fe, Co) and their alloys/oxides	AELs -HER (cathode): Ni and Ni–Mo, Ni–Fe, Ni–Co. -OER (anode): NiFe (oxy)hydroxide, Co oxides and perovskites.	-Improvement of long-term stability of Ni-based alloys [65]. -Enhancement of abundance and stability of active Fe sites [66].
Perovskites & spinels (mixed-metal oxides)	AEL electrolyzers -OER (anode): Co3O4 (spinel), NiFe2O4, and perovskites (LaNiO3/LaCoO3). SOEC electrolyzer -Oxygen electrode: Perovskites with rare earths (La1−xSrxMnO3 and La1−xSrxCo1−γFeγO3) are extensively used due to high-temperature stability and ionic/electronic transport.	-Alternatives to degradation issues of rare-earth perovskites during long-term operation [51,67].
Rare-earth & ceramic oxides (YSZ/ScSZ, LSM/LSCF, GDC)	SOEC electrolyzer -YSZ (yttria-stabilized zirconia) and ScSZ (scandia-stabilized zirconia) provide oxygen ion conduction and thermal robustness. -Oxygen electrode (anode): Lanthanum strontium manganite (LSM) and lanthanum strontium cobaltite ferrite (LSCF), often combined with gadolinium-doped ceria (GDC) infiltration to improve surface exchange.	-Improvement of long-term durability of SOECs required for large-scale industry adoption [53]. -Further studies on microstructural instability due to strontium (Sr) segregation, chemical degradation, and limited performance at lower temperatures [68].

provides an overview of electrode catalytic materials in AEL, PEM and SOEC water electrolysis systems, highlighting key aspects of performance and relevant trends aimed at enabling broader deployment.

2.2.2. Membranes: Polymer and Ceramic Types

Membranes are a critical component of water electrolyzers for hydrogen production, as they simultaneously separate the anode and cathode compartments and enable selective ion transport (H⁺, OH⁻, or O²⁻ depending on the technology). From a materials perspective, membranes are generally classified into polymeric and ceramic types. Polymeric membranes used in electrolyzers are typically divided into PFSA-based proton exchange membranes (PEMs), hydrocarbon-based PEMs, and anion exchange membranes (AEMs).

In PEM electrolyzers, membranes are designed for operation at relatively low temperatures (50–80 °C), where they exhibit high proton conductivity and allow for compact, efficient system design. The most widely used membrane is Nafion™, a perfluorosulfonic acid (PFSA) ionomer that serves as the commercial benchmark. Under industrially relevant conditions, Nafion membranes typically sustain 2–3 A cm⁻² at cell voltages below 2.0 V, combining high ionic conductivity with mechanical durability and low gas crossover. However, Nafion’s main drawbacks include its high production cost, gradual chemical and mechanical degradation under harsh operating conditions, and limited thermal stability above 90 °C [69,70,71]. Hydrocarbon-based PEMs are developed as fluorine-free, lower-cost alternatives to traditional Nafion^® membranes. Representative materials include sulfonated poly(ether ether ketone) (SPEEK) and polybenzimidazole (PBI) blends, both of which offer competitive proton conductivity and can achieve 1–2 A cm⁻² in optimized membrane–electrode assemblies. However, these membranes generally exhibit higher swelling, lower chemical stability, and shorter operational lifetimes compared to PFSA-based membranes [72,73].

Anion exchange membranes (AEMs) are polymeric materials functionalized with cationic groups such as piperidinium, imidazolium, or quaternary ammonium. Commercial examples include FAA3-50, Sustainion^®, Aemion™, XION Composite, and PiperION™, among others. Their primary function is to conduct hydroxide ions (OH⁻) across the electrolyzer while maintaining separation between anode and cathode gases. Current state-of-the-art AEM systems demonstrate 0.5–1.0 A cm⁻² performance with hydroxide conductivities measured at 30–60 °C. Nevertheless, durability remains a significant challenge [74,75,76].

A general classification of ceramic membranes used in electrolyzers can be divided into three main groups: oxide-ion conducting electrolytes, protonic ceramic electrolytes, and mixed ionic–electronic conducting (MIEC) membranes. Oxide-ion conducting electrolytes are ceramic materials that transport oxide ions (O²⁻) through their crystal lattice and are primarily employed in solid oxide electrolysis cells (SOECs). Yttria-stabilized zirconia (YSZ) and scandia-stabilized zirconia (ScSZ) have been the most extensively studied materials in this category due to their relatively high ionic conductivity and chemical stability at elevated operating temperatures (700–900 °C), where they can achieve stack current densities approaching 2 A cm⁻² under pressurized conditions [77,78]. Regarding proton-conducting ceramic membranes, these materials belong to a class of perovskite-based mixed oxides that exhibit substantial proton conductivity in hydrogen or water vapor-rich atmospheres and operate at intermediate temperatures (400–700 °C). Their main advantages include reduced ohmic losses compared to oxide-ion conductors and the possibility of direct pressurized hydrogen production. However, they still face significant challenges, particularly in terms of long-term durability and chemical stability. BCZYYb (BaCe_0.4Zr_0.4Y_0.1Yb_0.1O_3–δ) is one of the most widely studied proton-conducting ceramic electrolytes for this application [79,80,81]. Finally, mixed ionic–electronic conducting (MIEC) membranes are oxygen separation membranes fabricated from perovskite-type materials such as lanthanum strontium cobalt ferrite (LSCF) and Ba_xSr_1−xCo_γFe_1−γO_3–δ (BSCF). These materials are considered a promising component for next-generation electrolyzers because they enable simultaneous ionic and electronic transport, potentially simplifying device architecture by combining electrolyte and electrode functionality. However, MIEC membranes are still at an early research and development stage, facing key challenges including electronic leakage, mechanical stability, and sealing complexity, which must be addressed before large-scale commercialization is viable [82,83].

3. Electrocatalyst Performance and System Efficiency

This chapter compares low-temperature PEM, AEL, and high-temperature SOEC from the perspective of cell/stack-level electrocatalyst performance and the resulting system efficiency. In this review, only technical cells/stacks are considered, rather than results from half-cell catalyst ink tests or model electrodes. System-level consequences are summarized with specific electrical energy (kWh·kg⁻¹ H₂) and Faradaic efficiency (FE).

At standard conditions (25 °C, 1 bar), the thermodynamic minimum voltage required is given by the Gibbs free energy change (ΔG₀), corresponding to 1.23 V. The total reaction enthalpy (ΔH₀) is 285.8 kJ mol⁻¹, equivalent to an energy demand of about 39.4 kWh per kilogram of H₂. In practical systems, kinetic overpotentials at the electrodes and ohmic resistances, along with mass transport limitations, raise the cell voltage to 1.6–2.0 V, increasing the real energy requirement to approximately 50–55 kWh kg⁻¹ H₂. This efficiency gap highlights the importance of advanced electrocatalyst and membrane materials to lower polarization overpotentials, reduce ohmic losses, and improve overall system performance.

At comparable industrial loads, PEM stacks commonly operate around 2–3 A·cm⁻² with cell voltages around 1.8 to 1.9 V, mapping to around 51 kWh·kg⁻¹ H₂ electrical energy demand and an FE higher than 99%, provided that differential pressure and temperature are controlled [84,85]. Pressurized operation improves downstream compression needs but increases the risk of gas crossover and membrane stress. High-pressure measurements (up to 11.3 MPa cathode) confirm rising H₂ crossover and irreversible mechanical deformation [84,86].

AEL stacks typically run at 0.2–0.6 A·cm⁻² and 1.8–2.2 V per cell, with FE around 95–98%. For AWE stacks, energy consumption around 56 kWh·kg⁻¹ H₂ is reported when the plant boundary includes 30 bar H₂.

State-of-the-art SOEC stacks, operated at 700–850 °C with steam feed and heat integration, demonstrate electrical consumptions of around 36 to 40 kWh·kg⁻¹ H₂ (plus thermal), and low apparent overpotential.

For PEM stacks/MEAs, EIS separates ohmic and charge-transfer losses and yields polarization resistance contributions consistent with high OER overpotentials dominating kinetic losses at industrial current densities; these losses increase under transient duty cycles. For AWE, internal-reference EIS and dynamic tests show larger transport loss contributions (separator/diaphragm) and lower apparent kinetic current densities at a given cell voltage than PEMs; under variable load, the apparent kinetics degrade more quickly due to bubble and surface oxidation effects. For SOEC stacks, periodic I–V and EIS measurements during long-term operation identify low activation losses at high temperatures, emphasizing the role of ohmic/transport evolution (e.g., interconnect/electrode interfaces) in limiting the stack performance.

Single-cell exchange current densities (HER/OER) provide upper bounds on intrinsic kinetics under relevant temperatures. Cell/stack operation introduces additional ohmic and mass transport penalties. The observed overpotential and FE trends are summarized in

Table 3. Cell/stack metrics of different electrolyzer technologies.

Metric	PEM	Alkaline (AWE)	SOEC
Aggregate overpotential at representative load	~0.6–0.8 V at 2–4 A·cm−2 [87]	~0.35–0.45 V at 0.2–0.6 A·cm−2 [88]	~0.1–0.2 V near 1.3–1.5 V at 700–850 °C
Stack FE (typical)	≈96–99% (pressure/temperature-dependent, reduced at high Δp due to H2 crossover) [89]	≈75–98% (diaphragm crossover- and shunt current-dependent)	≈90–98% (steam utilization/sealing-dependent)

.

4. Stability, Durability, and Degradation Pathways

The long-term performance of PEM, AEL, and SOEC electrolyzers is challenged by degradation phenomena that reduce efficiency and compromise sustainability. Each of the aforementioned electrolyzer technologies offer distinct advantages and face unique degradation challenges. Understanding these degradation pathways is essential to improving device longevity, reducing life cycle costs, and ensuring the environmental sustainability of hydrogen production. Table 4 provides an overview of the dominant degradation mechanisms.

4.1. Degradation in PEM Electrolyzers

PEM electrolyzers operate under acidic conditions and high current densities, which accelerate several degradation processes. One of the most critical issues is the dissolution and migration of noble metal catalysts such as Ir and Pt. Mass balance studies show that Ir sinks in the membrane and even the cathode, linking dissolution to rising overpotential [90]. Iridium (Ir) dissolves from the anode catalyst layer in PEM water electrolyzers primarily due to high anodic potentials and aggressive oxidative conditions during operation, leading to catalyst degradation and performance loss. During oxygen evolution reactions (OERs), the anode operates at potentials typically above 1.6 V vs. RHE. These high potentials oxidize metallic Ir (Ir⁰) to higher oxidation states like Ir³⁺ and Ir⁴⁺, forming soluble species such as IrO₂⁺ or IrO₄²⁻. These higher oxides are soluble in acidic media and can diffuse away from the anode through the membrane to the cathode [90,91].

In proton exchange membrane water electrolyzers (PEMWEs), the membrane plays a crucial role in separating the anode and cathode while facilitating proton conduction. However, under harsh operating conditions, it can also become a site of chemical degradation. One of the key degradation pathways involves membrane-mediated chemical attack, particularly through the formation of peroxide radicals. This process is closely linked to gas crossover, which refers to the unintended diffusion of hydrogen and oxygen gases through the membrane [84,92].

When hydrogen from the cathode and oxygen from the anode meet within the membrane or at its interfaces, they can react to form hydrogen peroxide (H₂O₂). This compound, although relatively stable, can further decompose into highly reactive species such as hydroxyl radicals (•OH) and peroxyl radicals (•OOH). These radicals are capable of attacking the polymer chains of the membrane, leading to thinning, the formation of pinholes, and eventual mechanical failure. Moreover, they can oxidize the catalyst support materials—often carbon-based—and cause the detachment or dissolution of catalyst particles such as iridium, as already discussed above.

The presence of these radicals not only degrades the membrane but also accelerates the loss of catalyst material and the oxidation of its support. This creates a feedback loop: gas crossover leads to radical formation, which in turn damages the membrane and catalyst, thereby increasing the rate of gas crossover. This self-reinforcing cycle significantly compromises the durability and efficiency of the electrolyzer [84,90,93]. In addition, fluoride emission and rising H₂ crossover correlates with accelerated failure at higher temperatures [84,92].

Another degradation side is the porous transport layers (PTLs) at the anode side, which are often composed of titanium and therefore can undergo surface oxidation and passivation, finally increasing the ohmic losses of the cell [94,95].

To mitigate these effects, strategies such as incorporating radical scavengers into the membrane [96,97], using more stable catalyst supports like antimony-doped tin oxide [98], reinforced membranes, protective coatings, and optimizing operating protocols to reduce start-stop cycling and pressure differentials are explored. These approaches aim to interrupt the cycle of degradation and extend the operational life of PEMWEs.

Diagnostic techniques such as EIS and post-mortem microscopy are used to monitor degradation and guide material improvements.

4.2. Degradation in Alkaline Electrolyzers

Alkaline water electrolyzers (AELs) are a mature technology known for their use of earth-abundant materials and long operational lifetimes. However, they are not immune to degradation. One major issue is the formation of carbonate species in the electrolyte, which can reduce ionic conductivity [99]. Electrode corrosion, particularly under fluctuating load conditions, also contributes to performance loss [100]. Nickel-based catalysts, commonly used for both hydrogen and oxygen evolution reactions, undergo structural and compositional changes under variable operation. Reverse currents during shutdowns can lead to dissolution, phase segregation, and loss of conductivity [101].

The Zirfon-type diaphragms, while robust, can suffer from pore enlargement and wettability changes under fluctuating loads and high temperatures. In situ testing has revealed increased gas crossover and electrolyte permeation over time [102]. This can lead to the formation of flammable gas mixtures and necessitates frequent shutdowns, which in turn accelerates electrode degradation through reverse currents [100].

Mechanical and chemical degradation of cell components, including current collectors and seals, also contribute to performance loss. High-pressure operation and intermittent cycling exacerbate these effects, leading to reduced efficiency and increased maintenance requirements [103].

4.3. Degradation in SOEC

SOECs operate at elevated temperatures (typically >700 °C), enabling efficient conversion of electrical energy into chemical fuels. Despite their advantages, SOECs suffer from rapid degradation during prolonged operation, which limits their industrial viability.

The delamination of the anode is widely recognized as the most catastrophic degradation mode. Recent studies using transmission electron microscopy (TEM) and density functional theory (DFT) have revealed that oxygen ions accumulate at the electrode–electrolyte interface, leading to anisotropic lattice strain, dislocation formation, and nanopore development. These structural changes precede crack formation and eventual delamination [104].

Cation migration (e.g., Sr and Co) from the anode into the electrolyte can form insulating phases at the interface, disrupting ionic transport. Mixed ionic–electronic conducting (MIEC) electrodes such as LSCF and BSCF are particularly vulnerable to oxygen vacancy depletion and irreversible phase transitions under anodic conditions [105].

The Nickel–yttria-stabilized zirconia (Ni-YSZ) electrodes (cathode) are prone to degradation under high steam partial pressures, where Ni oxidation can occur. Additionally, Ni agglomeration and microcracking reduce electronic conductivity and catalytic activity, since agglomeration leads to a loss of active triple-phase boundaries (TPBs), especially in electrodes with coarse microstructures [106].

The YSZ electrolytes may undergo phase transformations under thermal cycling, leading to volume changes and electrode delamination. Doping strategies (e.g., ScSZ and LSGM) aim to stabilize the cubic phase and enhance ionic conductivity, but interfacial reactions with electrodes remain a concern [105].

An additional well-known degradation source in SOECs is the chromium poisoning of the air electrode by volatile chromium species ((CrO₃ or CrO₂(OH)₂)) originating from the metallic interconnect and/or gas supply tubing [107,108].

Table 4. Dominant degradation mechanisms by component and technology.

Component	PEM	Alkaline (AWE)	SOEC
OER/HER catalysts	Ir dissolution/redeposition; oxide restructuring; poisoning (impurities) [90,91]	Ni/Fe/Co oxidation–reduction cycling, dissolution, film delamination; poisoning (carbonates) [101,109,110]	Ni agglomeration/oxidation; cathode delamination [105]
Electrolyte/separator	PFSA chemical thinning/cracking via radical attack; H2/O2 crossover [84]	PPS-based diaphragm aging (permeability/ASR); thermal softening (≈100–130 °C) [111]	YSZ cracking/strain; oxygen-ion accumulation effects; seal degradation [105]
Supports/interconnects	Ti PTL/BPP oxidation on anode side; stainless steel corrosion if used [94]	Nickel foam corrosion; current collector passivation [112]	Chromium evaporation/poisoning from interconnects; thermal mismatch [105,107]
Operational stressors	Intermittent load, high Δp, an increase in temperature accelerate Ir loss and crossover	Load cycling and air exposure accelerate Ni degradation; pressure/temperature excursions stress diaphragm	Thermal/redox cycling and pressure/steam transients drive delamination [105]

Table 4. Dominant degradation mechanisms by component and technology.

Component	PEM	Alkaline (AWE)	SOEC
OER/HER catalysts	Ir dissolution/redeposition; oxide restructuring; poisoning (impurities) [90,91]	Ni/Fe/Co oxidation–reduction cycling, dissolution, film delamination; poisoning (carbonates) [101,109,110]	Ni agglomeration/oxidation; cathode delamination [105]
Electrolyte/separator	PFSA chemical thinning/cracking via radical attack; H2/O2 crossover [84]	PPS-based diaphragm aging (permeability/ASR); thermal softening (≈100–130 °C) [111]	YSZ cracking/strain; oxygen-ion accumulation effects; seal degradation [105]
Supports/interconnects	Ti PTL/BPP oxidation on anode side; stainless steel corrosion if used [94]	Nickel foam corrosion; current collector passivation [112]	Chromium evaporation/poisoning from interconnects; thermal mismatch [105,107]
Operational stressors	Intermittent load, high Δp, an increase in temperature accelerate Ir loss and crossover	Load cycling and air exposure accelerate Ni degradation; pressure/temperature excursions stress diaphragm	Thermal/redox cycling and pressure/steam transients drive delamination [105]

5. Material Sustainability and Resource Risk

H₂ production by water electrolysis is frequently portrayed as a “green” energy technology. Yet this claim is only valid if the materials that enable electrolysis—catalysts, membranes, electrolytes—are themselves sourced and processed in a sustainable manner. Life cycle assessment (LCA) provides a systematic framework for quantifying these upstream impacts. By including the energy intensity of mining and refining, greenhouse gas (GHG) emissions, water demand, toxicity risks, and end-of-life recovery potential, LCA ensures that electrolyzer deployment does not merely shift environmental burdens from fossil fuel combustion to extractive industries.

GHG emissions are reported in terms of carbon dioxide equivalents (CO₂e). This convention enables the aggregation of different greenhouse gases—such as methane (CH₄), nitrous oxide (N₂O), and fluorinated gases—into a single, comparable metric. Each gas is weighted by its global warming potential (GWP) over a defined time horizon (typically 100 years, as established by the IPCC), reflecting its relative contribution to radiative forcing compared with CO₂. Expressing emissions in CO₂e thus provides a standardized unit for comparing and summing climate impacts across diverse sources and processes, which is essential for consistency in LCAs and international reporting frameworks.

LCA is the internationally standardized method for evaluating the environmental impacts of products and processes across their entire life cycle. The framework is codified in ISO 14040 standards [113]. It consists of four key steps:

• Goal and Scope Definition—Clarify the research question, system boundaries (e.g., cradle-to-gate for material extraction vs. cradle-to-grave including use and disposal), and the functional unit (e.g., 1 kg H₂ produced).
• Life Cycle Inventory (LCI)—Collect quantitative data on energy and material inputs, emissions, and waste flows for each process stage.
• Life Cycle Impact Assessment (LCIA)—Translate inventory data into impact categories such as global warming potential (GWP, kg CO₂e), eutrophication, acidification, human/ecotoxicity, and abiotic resource depletion.
• Interpretation—Analyze results, identify hotspots, and discuss uncertainties and limitations.

For electrolysis, LCAs can be carried out at two levels [114]:

• Material-level LCA: Evaluates the upstream mining, refining, and processing of catalysts, membranes, and balance-of-plant components.
• System-level LCA: Considers the electricity source, electrolyzer efficiency, hydrogen compression/storage, and end-of-life treatment.

For example, the carbon footprint of H₂ depends heavily on whether the electricity input is renewable or fossil-based [114], while the criticality of materials like iridium or nickel emerges in the resource depletion categories. In the following chapter, the focus will be placed on the sustainability aspects of the electrocatalysts.

5.1. Electrocatalyst Materials: Reserves and Supply Security

Electrolyzers are highly material-dependent technologies: PEM devices rely on scarce platinum group metals (PGMs), AEL cells use nickel and cobalt alloys, while SOEC stacks depend on yttria-stabilized zirconia (YSZ) and perovskite oxides containing rare earth elements (REEs). Each of these materials carries unique sustainability and geopolitical risks that could constrain large-scale deployment.

Table 5. Overview of the currently used electrocatalysts in PEM, AEL and SOE electrolyzers.

Electrolyzer	Typical Catalyst	Key Metal(s)	Main Producing/Reserve Locations	Known Reserves (World)	Geopolitical/Supply Notes	Sources
PEM—HER (cathode)	Pt/C	Platinum (Pt)	Reserves concentrated in South Africa (SA) with ~75% of global reserves; also, Russia, Zimbabwe	0.081 Mt PGMs globally; SA ~0.063 Mt of PGM reserves (group)	High risk: Supply concentrated in South Africa (labor unrest, electricity shortages) and Russia (sanctions risk). Recycling critical (>95% possible). Classified as EU critical and strategic material.	[115,116]
PEM—OER (anode)	IrO2	Iridium (Ir)	Same PGM districts (SA Bushveld; Russia)	As above, within PGM group	Extreme scarcity: Ir < 0.001% of PGM ores; scaling PEM deployment severely constrained. Strategic vulnerability due to co-production. Classified as EU critical and strategic material.	[115,116]
AEL—OER	NiFe oxyhydroxide	Nickel (Ni)	Indonesia, Australia, Russia, Canada	>130 Mt Ni globally (Indonesia ~55 Mt; Australia ~24 Mt)	Moderate risk: Indonesia dominates extraction. Russia exposed to sanctions. Australia stable supplier. Classified as EU strategic material.	[116,117]
AEL—OER	NiFe oxyhydroxide	Iron (Fe)	Australia, Russia, China	87,000 Mt globally (Australia 27,000 Mt, Russia 14,000 Mt, China 7000 Mt)	Low risk: Iron is abundant (resources ~280,000 Mt). Risks mainly environmental (large-scale mining).	[118]
AEL—HER	NiMo	Molybdenum (Mo) (Ni see above)	China, USA, Chile, Peru	~15 Mt Mo globally	Moderate risk: Mo is a by-product of copper mining (supply tied to copper demand). China and Chile dominate.	[119]
AEM—OER	NiFe/Co spinels	Cobalt (Co) (Ni see above)	Democratic Republic of Congo (DRC), Indonesia, Russia, Australia	11 Mt Co globally (DRC ~6 Mt)	Very high risk: >50% of supply from DRC (political instability, child labor concerns). Indonesia emerging, but also high ESG risk. Classified as EU critical and strategic material.	[116,120]
SOEC—fuel electrode	Ni–YSZ cermet	Rare earths: Yttrium (Y), Lanthanum (La)	Australia, China, Brazil, Canada and India	Estimated 110 Mt (ca. 44 Mt in China)	High risk: >60% REE refining capacity in China. Western countries expanding REE projects (Australia, Canada). Recycling limited. Classified as EU critical and strategic material.	[116,121]
SOEC—fuel electrode	Ni–YSZ cermet	Zirconium (Zr)	Mostly Australia (ca. ⅔ of resources), SA, Senegal	55 Mt	Low–moderate risk: Abundant, but processing dominated by few countries. Supply generally stable.	[122]
SOEC—oxygen electrode	LSM (La1−xSrxMnO3)	Manganese (Mn)	South Africa, Australia, Gabon	~1700 Mt Mn (70% of resources in SA)	Moderate risk: Concentrated in South Africa (political/economic volatility). Abundant overall, so low scarcity. Classified as EU critical and strategic material.	[116,123]
SOEC—oxygen electrode	LSM (La1−xSrxMnO3)	Strontium (Sr)	China, Iran, other countries	>1000 Mt (estimated)	Medium risk: Data uncertain; China dominates production, potential export restrictions possible. Classified as EU critical and strategic material.	[116,124]

synthesizes key data on the main electrocatalysts used in AEL, PEM and SOEC electrolyzers. It includes their functional role, geographic distribution of the main active elements, and reserve estimates. This comparison highlights the stark differences between metals such as platinum (with extreme scarcity) and more abundant, but less electrocatalytically active materials such as nickel and manganese.

In summary, the comparison of electrocatalyst materials across PEM, AEL, and SOEC technologies illustrates both the opportunities and vulnerabilities inherent in scaling electrolytic hydrogen production. PGMs remain indispensable for PEM devices but are marked by extreme scarcity and geopolitical concentration. Nickel and iron are comparatively abundant, yet their extraction poses environmental challenges and, in the case of cobalt, severe ethical and political risks. Rare earths and zirconium, though available in significant quantities, are tied to processing monopolies that concentrate on supplying security concerns in a handful of countries. Notably, seven of the ten elements considered in Table 5 are officially designated as critical or strategic raw materials by the European Union under the Critical Raw Materials Act [116], reflecting their high economic importance and significant supply risk due to geographic concentration or limited substitution potential. This designation underscores their strategic role in the energy transition but also highlights the urgency of diversifying supply chains, enhancing recycling, and fostering sustainable mining practices. These patterns demonstrate that the deployment of large-scale electrolyzers is not limited by technical or economic performance alone but equally by the sustainability and security of material supply chains.

5.2. Electrocatalyst Recycling

Catalyst recycling is increasingly recognized as a cornerstone for enabling a sustainable hydrogen economy. Recycling and recovery strategies are essential to reduce demand for virgin material, which has been extracted in primary mining processes, lower GHG emissions, and secure long-term supply. Several recycling approaches are under active development, ranging from pyro- and hydrometallurgical processes to direct catalyst regeneration methods and novel electrocatalyst recovery pathways. However, the urgency of recycling differs markedly between electrolyzer technologies. For PEM electrolyzers, the reliance on Pt and Ir—two of the rarest and most precious metals in the Earth’s crust—makes recycling indispensable for any large-scale deployment. By contrast, AEL and SOEC systems predominantly use more abundant transition metals, such as nickel, iron, cobalt, and manganese, or oxides like yttria-stabilized zirconia. While these materials are not without environmental and geopolitical risks, their greater abundance means that recycling does not represent as critical a bottleneck as it does for PGMs. Thus, recycling is strategically vital for PEM technologies and will be discussed in more detail in this chapter, whereas in other electrolyzer types it plays a more supportive role within broader sustainability strategies.

For AEL electrolysis, studies by Zhao et al. [125] and by Lotric et al. [126] include recycling strategies in LCA studies, highlighting the positive effects. A more detailed study by Hoppe et al. [127] evaluates the environmental benefits of recycling and reuse strategies in a 5 MW AEL system. The LCA demonstrates that the incorporation of recycled materials in the construction of a 5 MW alkaline water electrolyzer can reduce the global warming potential (GWP) by approximately 50% relative to reliance on primary raw materials. Notably, around 77% of the system’s material inventory, including stainless steel and nickel, is amenable to recycling or reuse, thereby offering considerable opportunities to mitigate environmental impacts. Nevertheless, the analysis also identifies specific components with disproportionately high contributions, particularly inverters and nickel, which emerge as critical drivers of greenhouse gas emissions as well as other environmental burden categories [127].

For SOEC systems, recycling has been reviewed recently, e.g., by Biswas et al. [128], and by Sarner et al. [129]. SOEC recycling will not be reviewed in more detail here.

The recycling of PGMs relies on several distinct strategies that differ in maturity, selectivity, and environmental performance. The most established approach is pyrometallurgical recovery, in which PGM-containing materials are smelted at very high temperatures together with fluxes and collectors [130,131,132,133]. In this process, the PGMs concentrate in a metallic phase, while gangue materials form a slag. Pyrometallurgy is robust and well suited to heterogeneous feedstocks, and recovery yields for Pt, Pd, and Rh typically exceed 95%. Its drawbacks are a high energy demand, limited selectivity, and the requirement for further refining, which together contribute to a significant carbon footprint.

A complementary route is hydrometallurgical leaching, which dissolves PGMs in strong oxidizing acid systems such as aqua regia, hydrochloric acid combined with chlorine, or peroxide-based mixtures, Figure 2. The dissolved metals are subsequently separated and purified by solvent extraction, ion exchange, or precipitation. Hydrometallurgy provides high selectivity and purity, often exceeding 99%, and is widely employed as a refining step after pyrometallurgical pre-concentration. However, it depends on hazardous reagents, and careful treatment of chemical waste streams is essential to avoid secondary environmental burdens [130,131,132,134,135,136,137].

In addition to chemical methods, mechanical and physical separation techniques are increasingly being investigated. These include milling, flotation, and more novel approaches such as wettability-based separation of catalyst components. Their role is primarily as pre-treatment steps that reduce chemical demand and processing costs by concentrating the precious metals prior to hydrometallurgical recovery. While such methods are relatively benign from an environmental standpoint, they cannot recover PGMs directly and thus require integration with downstream chemical refining [134,138].

Finally, direct catalyst regeneration offers a fundamentally different pathway. Instead of extracting metals, this approach seeks to restore the activity of deactivated PGMs by removing poisons, redistributing nanoparticles, or re-supporting catalyst layers. By preserving the nanostructure of the catalyst, regeneration avoids the energy- and emission-intensive recovery cycle and can dramatically reduce resource demand. The limitation is that regeneration is not always feasible when structural degradation is severe, and the field remains at an early research stage [139].

Taken together, these strategies span a spectrum from highly mature industrial practices to emerging laboratory concepts. In the context of proton exchange membrane (PEM) electrolyzers, where platinum and iridium are indispensable yet extremely scarce, the development of efficient recycling and regeneration technologies is essential. Today, industrial practice relies on pyro- and hydrometallurgical combinations, but future sustainability will increasingly depend on the maturation of electrochemical recovery, mechanical pre-separation, and direct regeneration approaches that enable a more circular use of PGMs.

Table 6. Electrocatalyst recycling process overview of the platinum group metals.

Approach/Method	Process Principle	Recovery Yield	Purity of Recovered Metal	Environmental/Energy Footprint	Advantages	Limitations	References
Pyrometallurgical recovery	High-T smelting of catalyst layers to concentrate PGMs	~95% Pt, >90% Ir	High (>99%)	High energy demand; large CO2 footprint	Industrially proven; robust	Energy-intensive; less selective; loss of volatile elements	[130,131,132,133]
Hydrometallurgical leaching	Acid/chloride leaching + solvent extraction/precipitation	>90% for Pt and Ir achievable	High (>99%)	Lower energy than pyro; chemical waste issues	High selectivity; scalable	Requires hazardous reagents; effluent management	[130,131,132,135,136,137]
Mechanical/physical separation (e.g., wettability-based ultrafine particle recovery)	Separation based on hydrophilic/hydrophobic contrast (IrOx vs. Pt/C)	~97% separation efficiency (TiO2 proxy for IrOx), ~99% carbon black (Pt carrier)	Dependent on feedstock purity	Very-low energy demand; avoids chemicals	Environmentally friendly; simple	Limited to separation; metals still need refining	[138]
Direct catalyst regeneration	Rejuvenation of nanoparticle activity without full recovery	Activity restoration > 80% reported	N/A (material reused directly)	Very-low energy compared to primary extraction	Preserves nanostructure; low energy	Technology readiness low; incomplete recovery	[139]

presents an overview of the main recycling strategies of PGMs used in PEM systems.

There are several projects aimed at the development of PGM and/or MEA recycling technologies.

Table 7. Overview of current projects aimed at the recycling of platinum group metals from spent PEM electrolyzers and fuel cells.

Name	Type	Scope	Focus (Metals/Components)	Key Methods	Reported Results	Reference
BReCycle	Research project	PEMFC (fuel cells)	Pt, Ru; stacks/MEAs; design for recycling	Closed-loop concept; separation of coatings; handling PFSA (Nafion) issues; pre-treatment & hybrid (pyro + hydro) routes	Project developing tailored process; industrial-scale process not yet available (as of kickoff)	[140]
BEST4Hy (H2020)	Research project	PEMFC (fuel cells)	Pt; ionomer/membrane; MEA/GDL	Hydrometallurgy; alcohol dissolution; electro-leaching + electrodeposition	≈80% Pt (hydromet); ≈90% Pt + ≈80% ionomer (alcohol); up to ≈95% Pt (electro-leach)	[141]
LYDIA—Recycling Critical Metals from Fuel Cells	Research/innovation (EIT RawMaterials)	Fuel cells & electrolyzers (MEAs)	PGMs (Pt, Ru, Ir); PFSA membranes	Recovery from EoL MEAs; catalyst re-manufacture; membrane recovery	Targets: ~200 kg PGM catalysts & ~36,000 m2 Nafion from ~3M MEAs (project plan)	[142]
ST2P (Stack-to-Piece)	Research project	PEMFC (fuel cells)	Stack dismantling; component reuse	Automated, scalable disassembly; flexibly configurable recycling line; LCA evaluation	Processes & chains under development for component-specific processing	[143]
RECYCALYSE (H2020)	Research project	PEM electrolyzers (PEMWE)	Ir (OER), Pt (HER); catalysts, electrodes, CCM; recycling scheme	New sustainable OER catalysts (Ir thrifting) + recycling scheme for PEMWE system parts	Aims to reduce use of critical PGMs, esp. Ir; establish recycling routes (project outputs)	[144]
Conceptual Recycling Chain for PEM Water Electrolyzers	Peer-reviewed review	PEM electrolyzers	Ir, Pt; CCM, membranes; stack model	Hydrometallurgy, pyrometallurgy, selective electrochemical dissolution; model stack approach	Framework & pathways summarized; recent literature review	[136]
Johnson Matthey—PGM refining for FC/PEMWE	Company (refining & recycling)	PEMFC & PEMWE	PGMs from MEAs, stacks; Ir, Pt, Ru	Complex sampling; refining to 99.95% purity; full-stack homogenization	Commercial refining & recycling services; global operations	[145]
Heraeus Precious Metals—Hydrogen Systems Recycling	Company (recycling & catalysts)	PEMFC & PEMWE	Ir, Pt, Ru from MEA, CCM, bipolar plates	EoL recovery; integrated catalyst & CCM know-how; logistics & metal management	Commercial services; partnerships with OEMs (e.g., Freudenberg)	[146]
Hensel Recycling + Mastermelt (collaboration)	Companies (collection/refining)	PEMFC & PEMWE	PGMs from fuel cells & electrolyzers	Collection/dismantling & mechanical processing (Hensel) + complex refining (Mastermelt)	European collaboration to advance FC/PEMWE recycling	[147]
Umicore—PGM recycling capabilities	Company (refining & materials)	PGM recycling (incl. fuel cell components)	Platinum group metals; FC components (historic technical notes)	Special fuel-cell pre-treatment + hydromet; integration with existing refining	Process concept for fluorine-containing FC components (historic presentation)	[148]

provides a comprehensive overview of research projects and industrial actors active in the recycling of Pt, Ir, and Ru electrocatalysts in PEM fuel cells and electrolyzers. PEM fuel cell recycling is included here on purpose, since the recycling strategies used are very similar to PEM electrolyzer recycling.

5.3. Energy Demand and Mining Impacts of Key Materials

5.3.1. PEM Electrolyzers—Platinum Group Metals

The energy demand of mining and refining is highly dependent on ore grade, deposit depth, mineralogy, and local energy systems. Among the materials required for water electrolysis technologies, the PGMs represent the most extreme case of energy- and emission-intensive production. Pt and Ir, which are indispensable for PEM electrolyzers, occur at exceptionally low ore grades—often below 10 g per ton of ore in the South African Bushveld Complex, which hosts the majority of global reserves. Such scarcity necessitates the processing of vast quantities of rock, resulting in disproportionally high energy inputs. Life cycle assessments indicate cradle-to-gate energy requirements on the order of 400–500 MJ per gram Pt, making it one of the most energy-intensive metals in industrial use [149]. Furthermore, platinum production in South Africa, where the energy mix is dominated by coal, results in GHG intensities as high as 63 t CO₂-eq per kg Pt, compared with only 2–3 t CO₂-eq per kg Pt in Canada, where hydroelectricity prevails [150,151]. Iridium, obtained exclusively as a by-product of Pt and Ni ores, inherits this high energy intensity; although produced in only minute volumes, its effective energy and emission burden per kilogram is extreme.

PGM mining and refining are extremely water intensive. In South Africa, where most PGMs are mined, water consumption is estimated at 70–150 m³ per kg of refined Pt, depending on the ore grade and mine depth [151,152]. Water is used extensively in underground dust suppression, milling, flotation, and high-temperature refining. Ir, being a by-product, shares this water footprint proportionally. Recycling of Pt and Ir, on the other hand, requires significantly less water—primarily for hydrometallurgical leaching and precipitation steps—resulting in a footprint reduction by more than 80% compared to primary production [149]. This underlines the centrality of catalyst recycling not only for energy and carbon savings, but also for mitigating water stress in PGM-producing regions, such as South Africa, where water scarcity is acute.

5.3.2. AELs—Nickel, Iron and Cobalt

In contrast, the metals underpinning AELs, particularly Ni and Fe, are far more abundant and accessible. Ni ores are mined at grades of 1–3% Ni, significantly higher than PGM concentrations, though still requiring energy-intensive beneficiation and refining. Global energy intensity values for nickel production are estimated at 150–200 MJ per kg Ni, with associated emissions of 10–15 kg CO₂-eq per kg Ni depending on ore type and refining technology [149]. Sulfide ores generally demand less energy than laterites, but the latter dominate new reserves, particularly in Indonesia, where coal-based electricity further elevates emissions. Iron ore, by comparison, is among the least energy-intensive metals. With ore grades often exceeding 30–60%, Fe is in large hematite deposits, and energy demand is typically 20–25 MJ per kg Fe, with GHG intensities of only 1.5–2 kg CO₂-eq per kg Fe. Thus, while Fe extraction is associated with very large absolute emissions due to high production volumes, the specific intensity per kilogram is low [153,154]. Molybdenum is primarily obtained as a by-product of Cu mining, with porphyry deposits in China, the USA, and Chile being dominant. Its energy demand is closely tied to copper production, with estimates of 100–150 MJ/kg Mo and ~7–9 kg CO₂-eq/kg Mo depending on the deposit type and refining [149]. Because it is a by-product, its supply and environmental footprint are sensitive to copper demand.

Nickel laterite processing is water-intensive due to pressure acid leaching and other hydrometallurgical methods, with reported water use of 5–10 m³ per ton of ore processed, translating to 1–2 m³ per kg of Ni [152]. Iron ore, conversely, is associated with relatively low water intensity per unit mass of product, at 0.1–0.3 m³ per kg Fe, though the absolute water footprint is high given global production volumes [153]. Molybdenum, mined largely as a by-product of copper, exhibits a moderate water demand of 2–5 m³ per kg Mo, again concentrated in the flotation and leaching steps. Recycling nickel and iron from scrap steel and alloys drastically lowers the water footprint, requiring only fractions of the volumes consumed in primary mining, with typical savings of >70%.

5.3.3. SOEC Electrolyzers—Rare Earths, Zirconium, and Manganese

For SOECs, the situation is intermediate. These devices rely on ceramic electrolytes such as YSZ and perovskite-type oxides that include rare earth elements (REEs). The energy demand for rare earths is substantial because they occur in low concentrations and require complex chemical separation. Reported cradle-to-gate energy intensities for rare earth oxides are typically in the range of 200–400 MJ per kg, with GHG intensities of 15–70 kg CO₂-eq per kg, depending on the element and the separation route [149,155]. In addition, the environmental footprint is amplified by the prevalence of production in China, where coal-fired electricity dominates. Zirconium, by contrast, is relatively abundant and mined from heavy mineral sands with grades sufficient to keep energy intensities comparatively moderate. Estimates suggest specific demands of 50–70 MJ per kg Zr, with GHG intensities of 3–5 kg CO₂-eq per kg [156]. Manganese, employed in perovskite-type SOEC electrodes, is considerably more abundant. Large deposits in South Africa and Australia are exploited at relatively high ore grades. The energy demand is moderate, with values of ~20–25 MJ/kg Mn [149] and GHG intensities of 1.5–2.5 kg CO₂-eq/kg Mn, similar to iron. Strontium, used in lanthanum strontium manganite (LSM) perovskites, is mined primarily as celestite (SrSO₄). Although less frequently assessed in LCA databases, available studies suggest relatively modest energy demand compared to PGMs or REEs. Estimates point to <50 MJ/kg Sr with GHG intensities of 2–3 kg CO₂-eq/kg, though data remain sparse and region-dependent [157].

REE mining and separation is water-intensive, due to multi-stage chemical leaching and solvent extraction processes. The production of REEs from bastnäsite or monazite ores typically consumes ~50–200 m³ per ton REO [155,158], with additional water demand for the production of the REE from the oxides. Zirconium production from heavy mineral sands is comparatively less water-intensive, requiring around 0.5–1 m³ per kg Zr, largely for mineral separation and washing processes [156]. Manganese mining exhibits a similar profile to iron ore, with relatively low unit water demand (~0.2–0.4 m³ per kg Mn) but significant absolute impacts due to large-scale production [153]. Recycling REEs and zirconium remains technically challenging, but, when feasible, it reduces water demand by over 80% relative to virgin mining, as closed-loop chemical systems minimize water throughput [159].

Taken together, these comparisons underscore that the energy and emission intensity of metal production varies by orders of magnitude across electrolyzer technologies. PEM systems are uniquely exposed to the unsustainable energy burden of PGM mining, while alkaline and SOEC technologies, though not without challenges, are based on more abundant metals with lower specific footprints. This disparity underlines the importance of recycling strategies for PGMs and rare earths and highlights the advantages of material efficiency and substitution in minimizing the environmental costs of hydrogen production technologies.

Metal recycling offers a powerful means to mitigate environmental burdens. In some cases, recycling can reduce impacts so dramatically that net negative emissions are reported. This outcome arises from the EEMRIO (Environmentally Extended Multi-Regional Input–Output) methodology, which accounts not only for the direct activities undertaken but also for those avoided. A negative balance indicates that the emissions prevented by substituting secondary materials for primary mining exceed the emissions generated in the recycling process. For instance, the greenhouse gas footprint of recycled Pt has been reported at −0.7 t CO₂-eq [150], i.e., a net negative value. Similar findings of net negative emissions from recycling pathways have been reported for batteries by Dunn et al. [160], Mohr et al. [161], and Richa et al. [162], reinforcing the conclusion that recycling can deliver substantial climate benefits when compared to virgin extraction.

Together, these insights highlight the complex interplay of geology, energy systems, and circularity strategies in shaping the sustainability profile of electrolyzer materials.

Table 8. Summary of greenhouse gas emissions, primary energy demand and water use of the extraction and recycling of selected metals.

Metal	Extraction Scenario	GHG (t CO2e/kg Metal)	Primary Energy Demand (MJ/kg Metal)	Water Use (m3/kg Metal)	Notes	Sources
Pt	Primary extraction in South Africa (coal-intensive grid)	63,000	494,563	0.3	International Platinum Group Metals Association (IPA) global average (mass/economic allocation mix).	[163]
	Primary extraction in Canada	2300	-	-	Illustrates grid/location sensitivity.	[150]
	Secondary production using recycled Pt	477	10,564	0.002	IPA secondary production average values.	[163]
Ir	PGM mines (global, primary)	42,100	566,069	40.8	Ir co-produced with PGMs; very high energy intensity.	[163]
	Recycled	-	-	-	Data scarce. Likely similar benefits as Pt.	-
Ni	From Ni sulfide ore	8–16	150–200	1–2	GHG varies widely by route (sulfide vs. laterite).	[164]
	From Ni laterite	30–45	100–150	0.1–0.2	-	[164,165]
	Recycled Ni	One order of magnitude lower than primary extraction GHG emissions	20–50	Ca. 0.01	-	[152,166,167]
Fe	Primary (hematite ore → steel)	1.5–2	20–30	0.1–0.3	Converted from ore basis to refined Fe metal.	[153,154]
	Recycling (scrap steel)	0.4–0.6	5–10	0.01–0.05	Emissions depend on electric arc furnace power mix.	[153,154]
Mo	Primary	7–9	100–150	0.002–0.005	As by-product of Cu extraction, values vary with allocation.	[149,168]
Co	Primary	28	560	1.0	By-product of Cu/Ni; water impacts high.	[169]
	Recycled (from used battery)	3–6	30–60	0.02–0.05	70–90% lower impacts; closed-loop potential.	[170]
Rare Earths	Primary (monazite/bastnäsite route)	65	900	0.05–0.2	Separation is very water- and chemical-intensive.	[159]
	Recycling (magnets/EoL products)	10–20	150–250	0.01–0.03	Reduces water + energy by 70–80%.	[159]
Mn	Primary (FeMn alloy)	1.8–2.0	20–25	0.2–0.4	Data from ferroalloy LCAs.	[149,171]
Zr	Primary (zircon sand → ZrO2)	0.32	4–5	0.005–0.01	-	[156,172]

presents an overview of the greenhouse gas emissions associated with different metal extraction pathways. GHG emissions are cradle-to-gate emissions of extraction and processing.

Why This Matters for Electrolyzer Sustainability.

Taken together, the life cycle impacts of electrolyzer materials highlight a paradox: while water electrolysis is a zero-emission pathway at the point of use, its upstream material footprint can be substantial. Without recycling, circular economy strategies, or substitution, future gigawatt-scale deployment risks shifting environmental burdens from fossil emissions to metal mining, refining, and geopolitical hotspots.

For instance, scaling PEM electrolyzers to tens of gigawatts could strain the iridium supply chain, which currently produces only ~7–8 t per year globally. Similarly, the reliance of SOECs on REEs ties their growth to China-dominated supply chains with high environmental externalities. Nickel, though abundant, raises questions about tropical laterite mining’s deforestation impacts in Indonesia.

Thus, life cycle assessment is not just a tool for academic benchmarking but a strategic necessity to ensure hydrogen technologies deliver genuine climate benefits across their entire supply chain. By quantifying energy demand, mining impacts, and recycling potential, LCA provides the evidence base for developing next-generation, sustainable catalysts and membranes.

6. Emerging Catalyst and Materials Innovations

6.1. Next-Generation Catalyst Concepts

6.1.1. Single-Atom Catalysts (Pt, Ir, Ru)

The sustainability of electrolytic H₂ production is closely tied to the availability and efficiency of catalyst materials. Single-atom catalysts (SACs) have emerged as a promising approach to mitigate these constraints by reducing the required metal loading while maintaining or even enhancing catalytic activity. Critical catalyst materials such as Pt, Ir and Ru are utilized primarily in PEM electrolyzers [173,174,175]. Therefore, SACs are particularly attractive for PEM electrolyzers. The main sustainability advantage of SACs lies in their maximized metal atom utilization. In traditional nanoparticle catalysts, only surface atoms are catalytically active, while interior atoms contribute little to performance. By contrast, SACs expose every atom as an active site, enabling loadings that are often an order of magnitude lower than conventional systems. Moreover, SACs benefit from strong metal–support interactions, which modulate the electronic structure of active sites, tune adsorption energies of intermediates, and improve both activity and stability. This directly translates into reduced material intensity, lower greenhouse gas emissions from mining and recycling, and improved scalability of electrolysis technology.

Early landmark works such as Zhu et al. (2018) [174] have shaped the field by articulating the fundamental principles of SACs for water splitting, emphasizing strong metal–support interactions and electronic structure modulation. For PEM electrolysis, electrocatalysts for PEM water electrolysis rely on PGMs such as Pt, Ir, and Ru. These elements exhibit exceptional catalytic activity but are costly, scarce, and associated with high environmental footprints (see Section 5). Recent studies show that SACs can achieve ultra-low loadings compared to conventional electrocatalysts. While traditional PEM anodes require 1.5–2.0 mg Ir cm⁻², single-atom Ir catalysts can reduce this by more than an order of magnitude. For instance, Ir or Pt single atoms stabilized on conductive supports have demonstrated high OER and HER activities with loadings as low as 0.1–0.3 mg cm⁻², in some cases achieving mass activities 20–30 times higher than commercial catalysts [176,177].

The sustainability of SACs in PEM electrolysis has become a key research focus as the field seeks to reconcile the promise of efficient hydrogen production with the pressing constraints of resource availability and environmental impact. SACs are distinguished from conventional nanoparticle catalysts by their maximized atomic efficiency: nearly every atom is accessible as a catalytic site, in contrast to nanoparticles, where the majority of noble metal atoms remain buried and inactive. This translates into drastically lower noble metal loadings while maintaining or even enhancing catalytic performance. In practice, SACs can reduce PGM requirements by more than an order of magnitude, a critical advantage considering the extreme scarcity, geopolitical concentration, and carbon intensity associated with mining Pt, Ir, and Ru [178]. By lowering the dependence on these materials, SACs directly address one of the main barriers to the large-scale deployment of PEM electrolysis.

From an environmental perspective, the reduced demand for PGMs contributes to lower life cycle impacts of hydrogen production technologies. Mining and processing of iridium and platinum in particular are associated with high energy demand and greenhouse gas emissions, reaching values in the range of 30–60 t CO₂e per kilogram of extracted metal. By improving the utilization of these atoms, SACs diminish the embodied emissions per unit of catalytic activity. In addition, the improved durability of SACs—arising from strong metal–support interactions and the stabilization of active sites in unsaturated coordination environments—further enhances sustainability. Prolonged catalyst lifetimes reduce the frequency of replacement in PEM stacks, thereby minimizing both material throughput and waste generation [178].

However, the sustainability profile of SACs is not without challenges. A major concern lies in the synthesis routes. Techniques such as high-temperature pyrolysis, chemical vapor deposition, or atomic layer deposition are often required to achieve stable single-atom dispersion. These processes can be energy-intensive and may offset some of the environmental benefits obtained through atomic efficiency. Furthermore, the use of nitrogen-doped carbons, graphene derivatives, or covalent organic frameworks as supports can involve costly or resource-demanding precursors. Scaling these methods from laboratory to industrial scale without compromising the green credentials of the technology remains a formidable challenge. A holistic life cycle assessment (LCA) of SAC production is still largely missing, underscoring the need for future work to include energy input, precursor toxicity, and solvent usage in sustainability evaluations.

Circularity also emerges as a central aspect in the sustainable deployment of SACs. While the ultra-low loadings of noble metals reduce the urgency of recycling on a per-device basis, the cumulative demand at terawatt-scale hydrogen production remains non-negligible. Strategies that integrate catalyst recycling and regeneration with SAC design are therefore essential. Potential pathways include low-temperature leaching and redeposition techniques, direct electrochemical stripping, or controlled thermal treatments that allow the recovery of noble atoms from supports without significant losses. Coupling SACs with supports derived from renewable or waste biomass sources could further reinforce the sustainability case by reducing reliance on fossil-based carbons [178].

Finally, sustainability considerations extend beyond materials into system-level impacts. SACs not only reduce the material footprint but also improve performance at lower overpotentials, thus lowering the energy consumption per kilogram of hydrogen produced. This indirect effect—higher efficiency of electrolysis—amplifies the ecological benefits by reducing the renewable electricity required for green hydrogen generation. Taken together, SACs embody a convergence of material efficiency, operational durability, and energy savings that can significantly improve the sustainability profile of PEM electrolysis. Yet, realizing this potential will require integrating material innovation with scalable synthesis, robust recycling pathways, and comprehensive LCA frameworks.

6.1.2. High-Entropy Materials

High-entropy materials (HEMs), first conceptualized by Cantor and Yeh in 2004 [179,180], represent a new paradigm in materials science in which five or more elements are combined in near-equiatomic ratios to maximize configurational entropy [181,182]. Their unique “four core effects”—high entropy stabilization, lattice distortion, sluggish diffusion, and the cocktail effect—endow them with remarkable stability, tunable electronic structures, and synergistic catalytic activity. For electrocatalysis, especially water splitting, these properties open a path to reduce or even eliminate reliance on scarce and geopolitically sensitive PGMs, such as Pt, Ir, and Ru. This makes HEMs a highly promising strategy for developing sustainable, high-performance catalysts for hydrogen production.

One of the central sustainability advantages of HEMs lies in their compositional flexibility. By incorporating abundant transition metals (Fe, Co, Ni, Mn, Cu, etc.) into multi-component systems, the reliance on critical PGMs can be minimized while still achieving high catalytic activity. For example, PdMoGaInNi HEAs have demonstrated excellent HER activity, with overpotentials as low as 13 mV at 10 mA cm⁻² and long-term stability over 200 h [183]. Similarly, the non-precious high-entropy chalcogenides reported by Jo et al. [184] (consisting of Co, Fe, Ni, Mo, W, and Te) achieved efficient OER and HER activity with superior durability, highlighting the potential of entirely PGM-free HEMs. Several studies have highlighted that HEMs can approach or surpass the activity of conventional PGM-based catalysts.

From a life cycle perspective, the atom-efficient nature of HEMs contributes to lower material intensity per unit of catalytic performance, thereby mitigating the CO₂ footprint of catalyst manufacturing. Their corrosion resistance and high stability extend catalyst lifetimes, reducing replacement rates and resource demand. Recent sustainability-focused analyses argue that HEMs align with circular economy principles by offering opportunities for recycling and realloying, although large-scale recycling strategies for HEM nanomaterials remain underdeveloped [182].

Several studies have highlighted that HEMs can approach or surpass the activity of conventional PGM-based catalysts. Chen et al. [185] reported an Ir-free HEM catalyst delivering efficient OER at industrially relevant current densities (>500 mA cm⁻²), with activity retention over extended testing for more than 1000 h. Yang et al. developed HEM nanowire–CNT hybrid electrodes with enhanced conductivity and mechanical robustness, showing excellent long-term operation stability [186]. These works demonstrate that the compositional tuning of HEMs provides a pathway to acidic-stable, low-PGM or PGM-free catalysts, addressing a major bottleneck of PEM electrolysis.

Downscaling HEMs to the nanoscale—HEM nanoparticles, nanowires, or 2D sheets—further enhances atom utilization and active site density. Yan et al. [182] reviewed nano-HEMs and emphasized that dimensionality control significantly influences catalytic performance, with ultrafine PtNiFeCoCu HEM nanoparticles achieving mass activities exceeding commercial Pt/C by an order of magnitude [187].

6.1.3. Other Noble-Metal-Free Electrocatalysts

The development of efficient, stable, and affordable electrocatalysts based on earth-abundant elements represents a cornerstone of the global hydrogen economy. Moving away from noble metals such as Ir, Ru, or Pt not only addresses cost barriers but also supports the sustainability and scalability of renewable energy conversion technologies. Recent research has demonstrated that multi-component, non-noble-metal systems—particularly oxides, antimonates, chalcogenides, phosphides, and carbides—can achieve catalytic performance approaching that of precious metals, while offering major advantages in abundance, circularity, and life cycle footprint. Detailed reviews summarize the latest developments [174,188,189,190,191].

The sustainability dimension provides one of the most compelling motivations for replacing noble metals. Precious metals such as iridium, platinum, and ruthenium are among the scarcest elements in earth’s crust, with natural abundances below 0.001 ppm. Their extraction is energy-intensive, geographically concentrated (primarily in South Africa and Russia), and environmentally damaging due to high greenhouse gas emissions and toxic tailings. In contrast, transition metals like Fe, Mn, Ni, and Co are several orders of magnitude more abundant and already embedded in established mining and recycling infrastructures.

7. Integrated Material–Technology–System Development

7.1. Multi-Criteria Evaluation of Sustainable Electrolysis

Sustainable water splitting requires optimization simultaneously at the material, stack, and plant levels. For PEM electrolysis, Ir at the anode is the critical pinch point: lowering Ir loading improves materials criticality and cost exposure but can compromise activity and durability if not paired with better utilization and electrode architecture. System analyses show that meeting large-scale PEM deployment without exhausting Ir supply requires both drastic loading reduction and high end-of-life recycling rates (≥90%) [63,192]. In other words, the objective function spans efficiency (kWh kg⁻¹ H₂), stack lifetime (h), specific metal demand (g PGM kW⁻¹), and levelized hydrogen cost (LCOH), with binding constraints from the global Ir supply curve and policy targets. Recent TEA/benchmarking from DOE/NREL further indicates that reaching low LCOH requires concurrent progress in efficiency, durability, and capex [193,194].

7.2. Strategies for Techno-Economic Optimization via Materials Innovation

Material levers with the strongest propagated impact on LCOH include: (i) Ir utilization (activity per gram via better dispersion, conductive scaffolds, and thin, robust ACLs), (ii) durability (stability under high current density and dynamic operation), and (iii) membrane/CCM integration (lower ohmic loss at reduced noble metal loadings). At the stack level, roll-to-roll manufacturing and optimized CL–PTL interfaces enable both performance and yield improvements at the production scale [64,195]. System-level TEA suggests three synergistic pathways: (1) Ir thrifting (ultra-low loadings and non-Ir OER co-catalysts), (2) closed-loop recycling of PGM from manufacturing scrap and end-of-life CCMs, and (3) lifetime extension through degradation-resilient microstructures—together flattening the effective metal intensity curve while preserving efficiency. Reviews of Ir-based OER catalysts emphasize that thinning the anode catalyst layer must be co-designed to support corrosion resistance and ionomer distribution to avoid accelerated decay [64,195].

7.3. Role of Data-Driven Discovery and AI-Guided Optimization

Data-centric methods now link atomistic discovery to process-aware design: ML accelerates screening of OER/HER chemistries and support materials; learned interatomic potentials provide near-DFT fidelity for exploring dissolution pathways and defect chemistries; and Bayesian optimization integrates performance–stability trade-offs with synthetic constraints [196]. Critically, AI is moving beyond “materials only” to closed-loop labs that couple ML-suggested formulations with automated synthesis/testing, enabling rapid mapping of composition–processing–structure–property relationships under relevant operating windows (acidic, high j, and high T) [197]. Emerging work also targets the inverse design of porous electrodes (pore networks, tortuosity, and ionomer fraction) and surrogate TEA models that translate measured catalyst/CCM metrics into stack- and plant-level KPIs to guide decision-making [198].

7.4. Policy Instruments for Sustainable Material Development

Policy can de-risk innovation by (i) setting performance + durability + cost targets together (avoiding single-metric optimization), (ii) supporting critical materials circularity (collection, advanced refining, and product passport tracking for CCMs), and (iii) incentivizing material substitution and Ir thrifting via public R&D and demand-pull policies (standards and procurement). The EU’s Critical Raw Materials Act (CRMA [199]) and related JRC frameworks [200] explicitly call for diversified supply, domestic processing/recycling, and Safe-and-Sustainable-by-Design (SSbD) assessment—well-aligned with electrolyzer catalyst value chains. Aligning CRMA with NZIA-style manufacturing targets and DOE technical targets would tie TRL progress to TEA outcomes, prioritizing platforms that meet efficiency, lifetime, and metal intensity thresholds simultaneously [201].

8. Summary and Outlook

This review highlights the interdependence of efficiency, cost, and sustainability in the development of electrocatalysts and membranes for water electrolysis technologies. Hydrogen production via electrolysis represents a cornerstone of the emerging low-carbon energy system, yet its scalability is intrinsically linked to the material foundations of the electrolyzer itself. The comparative analysis of alkaline electrolyzer (AEL), proton exchange membrane (PEM), and solid oxide electrolysis (SOEC) systems reveals distinct performance–cost–sustainability trade-offs that must be optimized concurrently rather than in isolation.

From a technological perspective, AELs remain the most mature and cost-effective platform, operating with abundant Ni-based catalysts but limited by slower dynamics and lower power density. PEM electrolyzers offer superior compactness, dynamic response, and hydrogen purity but rely heavily on scarce and geopolitically concentrated platinum group metals (PGMs), especially Pt and Ir. SOEC devices, operating at high temperatures, deliver the highest electrical efficiency but face durability constraints due to thermal and chemical degradation of ceramic electrodes and electrolytes.

The functional materials of each architecture—electrocatalysts, membranes, and supports—ultimately govern the efficiency and lifetime of the electrolyzer. Reducing kinetic overpotentials through advanced catalyst design and minimizing ohmic losses via improved membranes are central strategies for lowering the specific energy consumption of hydrogen production. In PEM systems, catalyst dissolution and membrane thinning remain dominant degradation pathways, while in AEL and SOEC technologies, corrosion, carbonate formation, and interfacial delamination dictate long-term stability.

The sustainability assessment underscores that the environmental footprint of electrolysis is largely defined by its materials. Life cycle and criticality analyses show that PGMs exhibit the highest energy, emission, and water intensities of production, whereas Ni, Fe, and Mn are comparatively benign but pose other geopolitical and environmental challenges. Recycling and recovery of Pt and Ir are therefore indispensable to establishing circular material flows, as evidenced by ongoing European initiatives such as BReCycle, BEST4Hy, LYDIA, and RECYCALYSE.

Emerging materials—single-atom catalysts, high-entropy alloys, and noble-metal-free oxides—offer promising routes to alleviate resource constraints while enhancing activity and durability. Integrating such material innovations with techno-economic modeling, data-driven discovery, and policy frameworks (e.g., the EU Critical Raw Materials Act) will be essential for achieving both performance and sustainability at the gigawatt scale. Ultimately, progress toward truly green hydrogen production demands an integrated material–technology–system approach, in which advances in catalyst efficiency, lifetime, and recyclability are co-optimized with manufacturing scalability and supply chain resilience.