Nanostructured electrocatalysts for zero-emission green hydrogen production: from atomic-level design to scalable water electrolysis

1 Introduction

Hydrogen (H₂) is increasingly recognized as a key energy carrier for deep decarbonization because it can store renewable electricity in chemical form and support sectors where direct electrification remains technically or economically difficult (Gupta et al., 2026). However, its value relies heavily on the manner in which it is produced (Niu et al., 2025). Green H₂ is produced from water electrolysis using renewable or very-low-carbon electricity, where the boundaries include the electricity supply, water treatment, electrolyzer components, catalyst manufacturing, membrane manufacturing, replacement schedule, and end-of-life considerations (Odenweller and Ueckerdt, 2025). Therefore, zero-emission cannot be interpreted as describing the electrolyzer stack only but rather as a system-level goal (Suwaileh et al., 2025). The production of H₂ via water electrolysis does not directly result in any CO₂ emissions at the point of use. Still, the actual climate benefits of H₂ are dependent on the carbon intensity, durability, efficiency, and material footprint of the complete production chain (Boettcher, 2024).

Several platforms of water electrolysis are being developed, each with different materials and operating conditions (Ruan et al., 2025; Vedrtnam et al., 2025; Wu Y. et al., 2025). The most developed and commercialized route is alkaline water electrolysis, but further development is needed to increase the activity, durability, and scale to industrial current densities (Tüysüz, 2024). Anion-exchange membrane (AEM) electrolysis is a potential way to associate low-cost alkaline catalyst chemistry with a small-size cell design based on membranes. Nevertheless, there are still important issues relating to membrane stability, ion transport, and electrode integration (Li Q. et al., 2023). High-temperature electrolysis offers potential for higher thermodynamic efficiencies for the conversion of electricity and heat to H₂. Still, it comes with other issues such as ceramic stability, redox cycling, and system complexity (Liu et al., 2024). More design options can be generated by combining H₂ evolution with the conversion of renewable energy or by other anodic reactions (hybrid or self-powered electrolysis concepts) (Ren et al., 2024). On the other hand, solar-driven electrolysis emphasizes the importance of considering the three factors of intermittency, device matching, and catalyst durability as a whole rather than as individual performance parameters (Zhao and Yuan, 2023).

In all these systems, electrocatalysts control the H₂ evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. OER is defined here before its first abbreviation because its four-electron kinetics often dominates voltage loss. Meaningful comparison requires geometric and electrochemically active area normalization, iR correction stated explicitly, Faradaic efficiency, mass loading, electrolyte purity, and stability at device-relevant current density; a small overpotential in a three-electrode cell serves as an insufficient evidence of stack suitability (McCrory et al., 2013; Seh et al., 2017; Suen et al., 2017). The nanostructure undergoes changes more than the surface area. Feature size, facet exposure, strain, defect population, local coordination, and support coupling alter adsorption and charge transfer; pore size, wettability, and roughness also govern electrolyte access and bubble detachment. These benefits can be conflicting: aggressive nanostructuring may lower the apparent overpotential while increasing corrosion, phase reconstruction, ionomer blockage, or contact resistance. Atomic-scale interpretation must, therefore, be combined with operando verification of the working state and electrode-scale transport analysis (Man et al., 2011; Louie and Bell, 2013; Dionigi et al., 2020).

Nanostructuring has had a similar impact on the HER.MoS₂ nanocatalysts have demonstrated the crucial role of edge-site-dominated HER activity and paved the way for the design of 2D transition metal dichalcogenides with maximized active edges (Jaramillo et al., 2007). Furthermore, strain and exfoliation of WS₂ have shown the modulation of HER activity on nanosheet structure and lattice distortion, respectively, and the conductivity of MoS₂ nanosheets has been improved by engineering them into the metallic phase (Lukowski et al., 2013; Voiry et al., 2013). Transition-metal phosphides are potential non-precious catalysts for the HER: nickel phosphide nanoparticles are active in acids (Popczun et al., 2013), and cobalt phosphide nanoparticles have demonstrated the general potential of phosphide-based catalyst families (Popczun et al., 2014). Hence, in this review, we examine nanostructured electrocatalysts for zero-pollutant green H₂ production from a device-relevant point of view. It links all the key aspects of catalyst composition, nanoscale architecture, reaction mechanism, benchmarking practice, and electrolyzer operating environment to determine which progress is most likely to transfer from the laboratory electrode to durable, efficient, and scalable H₂ technologies.

Recent reviews commonly organize the field by electrolyzer type, catalyst family, or H₂-system analysis. These treatments are valuable, yet they rarely examine whether an electronic descriptor for a model surface survives potential-induced reconstruction, catalyst-layer processing, and multi-A cm^-2 operation. The distinctive framework of this review follows the assertion of performance across four linked levels: quantitative atomic descriptors, the operando active state, nanoscale-to-electrode transport, and device/manufacturing constraints. This review uses representative catalyst systems rather than an exhaustive catalog, distinguishes primary device evidence from review-level summaries, and identifies where literature comparisons remain non-equivalent. In 2024, the global H₂ demand approached 100 Mt, yet low-emission routes supplied less than 1% and new applications represented less than 1% of the demand. The gap defines the scale at which catalyst loading, stack efficiency, lifetime, and manufacturing yield must be judged. Figure 1 separates the current demand from the low-emission production pipeline. In this review, green H₂ denotes water electrolysis powered by renewable electricity within an explicitly stated lifecycle boundary; zero emission is treated as an aspirational system outcome rather than a description of production.

2 Fundamentals and evidence boundaries for renewable-electrolysis H₂

Green H₂ refers to H₂ produced by water electrolysis using renewable electricity, while its life-cycle greenhouse-gas intensity depends on system boundaries and electricity sourcing. When powered by fully decarbonized energy, proton-exchange membrane (PEM), anion-exchange, or alkaline electrolysis can achieve near-zero operational emissions and substantially lower climate impacts compared to gray or blue H₂ (Gerhardt-Mörsdorf et al., 2024; Koj et al., 2025). The need for thermodynamic voltage, kinetic overpotentials, electrolyte-dependent reaction pathways, product purity, and safety constraints govern performance and scalability. Wider deployment and technological learning may reduce electricity consumption, material demand, and capital costs, particularly at the megawatt scale (Kolahchian Tabrizi et al., 2023; Patel et al., 2024). However, the overall benefits remain susceptible to electricity mix, renewable additionality, water and land use, critical-material supply chains, and end-of-life emissions. Green H₂ can, therefore, support net-zero energy systems only when coupled with efficient electrolyzers, genuinely low-carbon electricity, and responsible supply-chain management (Weidner et al., 2023).

2.1 Definition, system boundary, and carbon-neutrality criteria

Green H₂ certification is a system-level assessment determined by the electricity source, life-cycle boundaries, and upstream and embodied emissions rather than by water electrolysis alone (Boettcher, 2024; Ren et al., 2024). Carbon accounting should include renewable electricity generation, water treatment, stack manufacture and operation, component replacement, balance-of-plant consumption, and end-of-life processes. Temporal and spatial matching between renewable generation and electrolyzer operation is also critical because grid balancing, storage losses, curtailment, and regional electricity mixes can substantially alter the emission benefit of nominally renewable electricity (Zhao and Yuan, 2023; Tüysüz, 2024). Renewable-power variability also affects electrochemical durability. Starts, stops, idle potentials, and rapid current ramps alter surface coverage, catalyst redox state, gas saturation, differential pressure, and local temperature. Consequently, renewable compatibility cannot be inferred from steady-state galvanostatic activity alone; studies should report the power profiles, conditioning and shutdown protocols, cycling history, and post-cycling performance.

Catalyst evaluation must likewise extend beyond half-cell overpotential to device-relevant behavior. Low intrinsic overpotential does not ensure high stack efficiency in conditions of inadequate water transport, gas removal, ionic conduction, or catalyst-layer stability (Wang C. R. et al., 2025). Conversely, moderately active catalysts may become technologically viable when incorporated into porous, mechanically robust electrodes that minimize contact resistance and promote bubble release (Liu et al., 2025). Sustainability assessments should further consider membrane and catalyst replacement, material supply chains, synthesis complexity, thermal demand, precursor availability, and solvent use (Seh et al., 2017; Li Q. et al., 2023). Scalable designs should, therefore, prioritize use of earth-abundant materials, reproducible low-impact fabrication, spray- or roll-coating compatibility, and tolerance to feedwater impurities (Indah Sari et al., 2025). Long-term performance also depends on the stability of metal–oxide, sulfide–oxyhydroxide, catalyst–support, and catalyst–ionomer interfaces, which govern the local electric fields, adsorption energies, and reconstruction pathways (Jiang et al., 2025). Mechanistic claims should, therefore, be supported by operando or post-operation characterization. Although low-current-density measurements can reveal intrinsic kinetics, high-current-density operation is increasingly controlled by ohmic losses, gas blockage, local pH shifts, water supply, membrane hydration, cracking, and heat management (He et al., 2025). Powder-catalyst activity alone, therefore, serves as an insufficient evidence of stack readiness.

2.2 Renewable-power quality and catalyst stress

Renewable-powered electrolysis requires technologies that tolerate start–stop operation, wide turndown, pressure fluctuations, non-uniform current distribution, and partial-load operation without accelerating catalyst, electrode, or membrane degradation. Platform selection, therefore, determines the relevant material requirements. Alkaline water electrolysis uses mature, non-noble electrodes; proton-exchange membrane electrolysis provides a compact and rapid response but relies on Ir and Pt; anion-exchange membrane electrolysis combines alkaline catalysts with a zero-gap architecture; ceramic electrolysis reduces the electrical demand through high-temperature operation while introducing distinct material constraints. Coupling electrolyzers with renewable electricity is central to production of low-emission H₂ (Niederhofer et al., 2025). Solar-driven systems require effective photovoltaic-electrolyzer voltage–current matching and electrodes that remain stable during intermittent irradiation (Tan et al., 2025). Wind power can provide high energy yields, but its variability demands fast response, broad load-following capability, and control strategies that limit stress during cycling and partial-load operation (dos Santos Ramos et al., 2025). Consequently, catalyst evaluation must extend beyond steady-state overpotential to durability under transient currents, repeated cycling, and changing gas-evolution rates (Ross et al., 2026).

Dispatchable hydropower and geothermal energy can increase electrolyzer utilization and reduce the shutdown frequency, although their economic and environmental benefits depend on regional resources, water use, infrastructure, and life-cycle impacts (Pastore et al., 2025). Hybrid renewable system portfolios and storage can further stabilize electricity supply and reduce stack stress (Brucksch et al., 2025), improving H₂ quality, compression requirements, and downstream storage or transport at a large scale (Li et al., 2025b). Performance should, therefore, be assessed using kinetic, transport, and durability metrics, including potential at defined current densities, Tafel slope, active surface area, Faradaic efficiency, impedance, and post-operation structural changes (Perez Bakovic et al., 2021). Such system-relevant testing prevents roughness, impurities, conductivity, or transient reconstruction from being mistaken for intrinsic activity and favors robust, directly grown catalyst architectures with efficient transport, gas release, adhesion, and electrical contact.

2.3 Thermodynamic voltage floor and efficiency accounting

At 25 °C under standard conditions, water electrolysis has a reversible cell potential of approximately 1.23 V and a thermoneutral voltage of approximately 1.48 V. The reversible cell potential represents the minimum electrical work required under ideal conditions and provides a fundamental benchmark for electrolyzer efficiency (Mazloomi et al., 2012). Thermoneutral voltage additionally accounts for reaction enthalpy and indicates whether the cell must absorb or release heat. Consequently, the reported efficiencies should specify the higher- or lower-heating-value basis, operating pressure, auxiliary energy consumption, and system boundaries to enable meaningful comparison. Practical electrolyzers operate above the reversible cell potential because of activation overpotentials, ohmic resistance, mass-transfer limitations, and gas-management losses. Heat generation and overall performance, therefore, depend on the current density, catalyst activity, membrane conductivity, electrolyte composition, internal resistance, and operating pressure (Ahmad et al., 2025). Higher temperatures generally improve reaction kinetics and reduce electrical energy demand but may accelerate membrane degradation, catalyst dissolution, corrosion, and water-management problems. The optimal temperature is, therefore, technology-specific and reflects a balance between efficiency and durability (Hauch et al., 2008). Pressurized electrolysis can reduce downstream H₂-compression requirements, but it may increase gas crossover, membrane stress, safety risks, and system complexity (Hayat et al., 2020; Al-Mahgari et al., 2023). Moreover, HER and OER activation barriers influence cell voltage, particularly at low current densities, whereas resistance, gas accumulation, local pH shifts, water depletion, membrane dehydration, catalyst-layer damage, and thermal gradients become increasingly important at high current densities (Song et al., 2025). Nanostructured electrodes may reduce kinetic losses, but their practical value depends on long-term structural stability and effective integration into complete cells (Pak et al., 2025). Thermodynamic analysis must, therefore, be combined with electrochemical diagnostics, durability testing, and post-operation structural characterization.

2.4 Kinetics, pathways, and the origin of voltage loss

Thermodynamic voltage defines the minimum energy required for water splitting, but it does not predict the reaction rate. Practical operating voltages also include activation, ohmic, and mass-transport losses, each requiring distinct materials and electrode-design solutions. For the HER, kinetics proceed through Volmer, Heyrovsky, and Tafel steps and depend on the pH, H₂ adsorption energy, interfacial water structure, and catalyst electronic properties (Ghazi et al., 2018; Luo et al., 2026). Although the HER proceeds rapidly on Pt in acidic media, alkaline HER is slower because water must first dissociate before H₂ intermediates can adsorb and recombine. Interfaces between metals, oxides, hydroxides, and supports can provide complementary sites for water activation and H₂ adsorption (Zai et al., 2026). The OER generally imposes larger kinetic penalty because it involves four coupled proton–electron transfers and O–O bond formation (Yang et al., 2026). Catalysts must stabilize OH, O, and OOH* intermediates without inhibiting oxygen release. However, scaling relationships among these intermediates restrict independent optimization of their adsorption energies (Do and Lee, 2025). Heteroatom doping, defects, strain, amorphization, mixed-metal sites, and interfacial coupling are, therefore, used to modify adsorption energetics and reaction pathways.

Catalytic activity must be evaluated cautiously because nanostructuring can enhance the surface area, conductivity, and transport while obscuring intrinsic activity. Reliable comparisons should report defined-current potentials, Tafel slopes, exchange currents, uncompensated resistance, electrochemically active surface area (ECSA), loading, Faradaic efficiency, electrolyte conditions, impedance, bubble coverage, and the relevant current range. Catalyst reconstruction is especially important during the OER and alkaline HER, where sulfides, phosphides, nitrides, carbides, and metals may transform into active oxide or oxyhydroxide layers (Nayem et al., 2021; Akter et al., 2022; Huang et al., 2026; Tariq et al., 2026). Operando characterization and post-reaction analysis are, therefore, essential. At device-relevant currents, bubble accumulation, electrolyte depletion, local pH gradients, ionomer distribution, and membrane resistance further alter apparent kinetics, requiring assessment within realistic electrode architectures and full-cell operation.

2.5 Electrolyte-dependent pathways and active-state uncertainty

Electrolyte identity governs more than conductivity: acidic, alkaline, anion-exchange, and ceramic environments alter the charge carrier, interfacial-water organization, adsorbate coverage, corrosion thermodynamics, and catalyst composition. Consequently, no universal surface mechanism applies across electrolyzer types. In alkaline HER, near-optimal H₂ adsorption does not guarantee rapid kinetics because Volmer water-dissociation may dominate; hydroxide–metal interfaces can specifically accelerate this process (Zhao et al., 2023; Que et al., 2026). In alkaline OER, nominal Ni- and Co-based precatalysts frequently undergo reconstruction into potential-dependent oxyhydroxide surfaces, while trace electrolyte impurities, particularly Fe, can substantially modify the active state. Conversely, the strongly acidic PEM-water-electrolysis conditions restrict viable OER catalysts because the activity is coupled to severe oxidative dissolution (Kwon et al., 2026). Therefore, ex situ structure–activity correlations remain hypotheses until the working phase is identified under bias. Mechanistic analysis must establish when adsorbate evolution, lattice-oxygen participation, reconstruction, or dissolution occurs at a specified potential, pH, and electrolyte purity. Isotope-labeled products, operando spectroscopy or diffraction, and quantitative dissolution measurements should be combined; a Tafel slope alone cannot uniquely identify a pathway.

2.6 Product purity, gas crossover, and safety boundaries

Catalyst performance is meaningful only when H₂ and oxygen remain effectively separated. Gas crossover increases at low current density, differential pressure, membrane defects, and local dehydration, creating safety risks and reducing product quality. Electrolyzer monitoring should, therefore, include H₂-in-O₂ and O₂-in-H₂ concentrations, pressure, water removal, and Faradaic efficiency. Membrane thickness, permeability, recombination catalysts, gas recirculation, and automated shutdown systems can limit gas crossover, although these measures may affect the efficiency and durability. H₂ purity depends on membrane selectivity, pressure gradients, current density, gas-production rates, water management, and component degradation. Minimizing H₂/O₂ crossover is essential both for safe operation and for protecting sensitive downstream processes, particularly fuel cells and catalytic conversion systems (Deng et al., 2025). Product contamination may also arise from membrane degradation, catalyst dissolution, electrolyte crossover, and impurity accumulation, making electrode stability and membrane compatibility system-level requirements rather than solely electrochemical considerations (Arun et al., 2025).

Storage and transport further influence the efficiency, cost, safety, and lifecycle emissions of green H₂. Compressed gas, liquefied H₂, metal hydrides, and ammonia differ in energy demand, volumetric density, reversibility, infrastructure needs, and system complexity; the preferred option, therefore, depends on the scale, transport distance, end use, and acceptable efficiency losses (Beyazit, 2025). Similarly, pipelines, tube trailers, liquefied-H₂ distribution, ammonia transport, and chemical carriers impose different infrastructure and energy requirements. In contrast, compression, liquefaction, reconversion, and transport can contribute to emissions beyond those associated with H₂ production (Niazi et al., 2026). End-use sectors also impose different purity, pressure, storage, and supply requirements. Fuel cells require especially strict contaminant limits, whereas ammonia synthesis, petroleum refining, steel reduction, and synthetic-fuel production may tolerate different impurity levels and operating conditions (Srinivasan et al., 2026). Scalable systems must, therefore, integrate durable catalysts, stable membranes, efficient gas separation, reliable compression, and suitable storage and distribution infrastructure (Segovia-Hernández et al., 2025). Advanced nanostructured electrodes are valuable only when they maintain accessible active sites, strong adhesion, effective electron and ion transport, and rapid gas release under realistic current densities, water quality, pressure, and cycling conditions. As illustrated in Figure 2, green H₂ functions most effectively within a broader net-zero emission system that includes renewable energy, carbon capture, energy storage, smart grids, efficient buildings, electric vehicles, and fuel cells.

3 Main electrolyzer technologies

Each electrolyzer is defined by its mobile ion, separator or electrolyte, electrode environment, operating temperature, and water-/gas-management strategy. These variables determine both the credible catalyst candidates and the dominant failure modes. The comparison below, therefore, couples device fundamentals, component choices, advantages, and limitations rather than treating a catalyst as transferable between platforms (El Kattel et al., 2025).

3.1 Alkaline water electrolysis

Alkaline water electrolysis (AWE) enables economic large-scale H₂ production with concentrated KOH, porous diaphragms, Ni-based electrodes, electrolyte circulation, and stacks using earth-abundant catalysts (Henkensmeier et al., 2020). OH⁻ migrates from the cathode to the anode; the diaphragm conducts ions and limits gas mixing. Common cathodic materials include Raney nickel, porous nickel, nickel–molybdenum alloys, and nickel phosphides. Typical anodic materials include nickel–iron oxyhydroxides, nickel–cobalt oxides, and cobalt–iron oxides. Their performance is influenced by several factors, including catalyst loading and surface roughness, electrolyte concentration and temperature, separator properties and full-cell design, electrode adhesion and contact resistance, tolerance to impurities, surface wetting and water access, bubble removal and gas management, and stability under repeated load cycling (Li M. et al., 2023; Tüysüz, 2024). Assessment requires voltage at current density, Tafel slope, ECSA, Faradaic efficiency, impedance, loading, electrolyte composition, temperature, and post-stability structure (Xu L. et al., 2024; Zuo et al., 2025). The dealloyed Ni mesh reached 2 A cm^-2 at 1.85 V and 80 °C but lacked deployment-level durability (Schalenbach et al., 2018). ACAC-assisted gelation–deflocculation yielded sub-10 nm NiFe-LDH, surpassing IrO_x and achieving 74.7% MEA energy efficiency at 1.0 A cm^-2 in 1 M KOH at 80 °C (Koshikawa et al., 2020). Figure 3a illustrates the conversion of sol–gel-derived NiFe₂O₄ into NiFe alloys through H₂ reduction at 500 and 700 °C. In Figure 3b, both NiFe catalysts substantially outperform bare Ni foam for OER, requiring only 319 and 307 mV for NiFe-500 and NiFe-700, respectively, compared with 517 mV for Ni foam at 10 mA cm⁻² (Raimundo et al., 2023). NiFe-700 exhibits the highest activity, which was attributed to its single FCC structure, whereas NiFe-500 contains an additional, less catalytically active BCC phase. The drawbacks are gas crossover, caustic handling, ohmic loss, slow dynamics, and renewable-coupled high-current degradation (Ahmed et al., 2025). Catalyst layers should be compatible with scalable manufacturing methods, such as spray coating and roll-to-roll processing, while remaining resistant to impurities and mechanical stress. Interfaces between metals and oxides, sulfides and oxyhydroxides, catalysts and supports, catalysts and ionomers, and catalysts and separators can influence adsorption, local electric fields, surface reconstruction, and bubble behavior (Zhang et al., 2026b). These interfaces operate under combined stresses from high current, heat, pH gradients, and gas bubbles. Therefore, operando characterization and post-mortem analysis are essential for understanding material changes and failure mechanisms. Low-current three-electrode tests mainly evaluate reaction kinetics. In contrast, industrial operation involves high current densities, where the performance is also affected by electrical resistance, gas blockage, alkalinity gradients, electrolyte circulation, flooding, cracking, separator degradation, and temperature gradients. Therefore, durability, manufacturability, and mass transport become critical under practical operating conditions.

3.2 Proton-exchange membrane water electrolysis

PEM water electrolysis (PEMWE), a zero-gap acidic electrolyzer, comprises an Ir-/Ru–Ir-oxide anode, PEM, Pt cathode, porous transport layers (PTLs), bipolar plates, and water/gas management; the OER produces O₂, the protons cross hydrated PEM, and HER yields separated H₂ (Figure 4a) (Wang C. R. et al., 2025). Compact high-current-density operation, rapid response, differential pressure, and high-purity H₂ favor use of renewables (Henkensmeier et al., 2020). Performance couples electron/proton/mass transport to ionomer coverage, water/O₂ management, Ti–-PTL contact, hydration, and catalyst–ionomer/support/PTL/membrane interfaces (Figure 4b). Low Ir loading, high current density, and suboptimal ionomer content can accelerate catalyst dissolution, pore blockage, conductivity loss, ionomer degradation, PTL corrosion, membrane thinning, interfacial failure, and mass-transport limitations (Feng et al., 2025; Sajid et al., 2026). Although nanostructured IrO_x, RuO₂-derived, mixed, supported, heterointerface-engineered, and oxyanion-modified catalysts show promise, their performance must be validated in MEAs under practical operating conditions, including realistic feed, temperature, pressure, and current density (Jiang et al., 2026). For example, RuIrO_x–CeO² achieved a performance of 3 A cm^-2 at a voltage of 1.75 V and operated for 2,000 h at 5 A cm^-2. Large-scale deployment will require concurrent advances in cost, durability, efficiency, manufacturability, Ir recycling, corrosion-resistant PTLs, and durable, reproducible membranes (Venkatesan et al., 2025; Wang C. R. et al., 2025). Standardized reporting of catalyst loading, ECSA, impedance, Faradaic efficiency, membrane thickness, PTL properties, degradation rates, and post-test structural evolution is also essential. Operando spectroscopy, microscopy, and accelerated stress testing can further clarify the catalyst reconstruction and failure mechanisms (Chee et al., 2023).

3.3 Anion-exchange membrane water electrolysis

AEM water electrolysis (AEMWE) combines OH⁻-conducting membrane-electrode assemblies and zero-gap gas separation with alkaline Ni/NiMo/NiFe/ferrite/transition-metal-oxide/nitride and low/non-PGM catalysis, positioning it between AWE and PEMWE (Ghorui et al., 2024; Volk et al., 2024). NiMo cathodes and NiFe-based (oxy)hydroxide anodes lead. Still, the performance depends on catalyst–ionomer contact, hydration, water distribution, carbonate uptake, compression, hydrophilicity/pore structure, conductivity, adhesion, and gas removal and not on powder activity alone (Jo et al., 2024). Membranes must maintain chemical and mechanical stability under hydroxide and radical attack, hydration and pH gradients, exposure to CO₂-derived carbonate and bicarbonate species, and operation at low supporting-electrolyte concentrations. In addition, a non-uniform ionomer distribution can lead to electrical and ionic isolation of catalyst sites while restricting hydroxide-ion and gas transport. Reconstructed metals, oxides, nitrides, phosphides, and sulfides may undergo transformation into catalytically active oxyhydroxide phases during operation; however, such transformations should be verified through operando characterization and rigorous post-test analysis. Catalyst performance should, therefore, be evaluated under representative device-level operating conditions rather than relying primarily on short-duration three-electrode measurements (Usman et al., 2021; Lv et al., 2022). Devices reached a performance of 10 A cm^-2 at a voltage of 2.3 V for >800 h (Zheng et al., 2025). Figure 5 contrasts ion transport in alkaline anion-exchange membrane fuel cells (AAEMFCs) and proton-exchange membrane fuel cells (PEMFCs). In AAEMFCs, OH⁻ ions generated at the cathode migrate through the positively functionalized anion-exchange membrane toward the anode, where they participate in H₂ oxidation and water formation (Ouma et al., 2022). In PEMFCs, H⁺ ions instead move from the anode to the cathode through the proton-exchange membrane. The enlarged AAEM structure shows polymer chains bearing fixed positively charged groups that facilitate OH⁻ transport while maintaining membrane electroneutrality and limiting fuel crossover. Successful commercialization requires durable and impurity-tolerant operation under realistic conditions, including variations in water quality, CO₂ concentration, temperature, pressure, dynamic loading, and membrane hydration. It also depends on the use of earth-abundant catalysts, low-temperature and solvent-efficient synthesis methods, and scalable continuous-coating processes. To enable meaningful comparison and reproducibility, studies should systematically report the membrane chemistry, ionomer fraction, catalyst loading, electrode compression, operating temperature, feed composition, current density, voltage efficiency, performance decay or degradation rate, Faradaic efficiency, and observed failure modes.

3.4 Oxygen-ion solid oxide electrolysis cells and proton-conducting ceramic electrolysis cells

Solid oxide electrolysis cells (SOECs) use industrial-waste/nuclear/concentrated-solar/renewable heat, improving electrical efficiency/co-electrolysis (Lahrichi et al., 2024). Oxygen-ion cells (650 °C–850 °C) reduce steam on porous-Ni-YSZ/Ni-ceramic electrodes, transport O₂ ⁻ through dense YSZ/ScSZ electrolytes, and produce oxygen on perovskites. Cell performance is governed by reaction kinetics, gas diffusion and fuel utilization, ionic and electronic conductivity, triple-phase-boundary density, polarization losses, and electrode–electrolyte microstructure and interface quality. Long-term stability further depends on thermomechanical compatibility and resistance to Ni coarsening, redox damage, delamination, Cr poisoning, seal degradation, and thermal cycling (Sun et al., 2019; Yousaf et al., 2025). A cell sustained for 4,400 h at a current density of 800 °C/1 A cm⁻². Protonic ceramic electrolysis cells (PCECs) (350 °C–650 °C) use BaZr/Ce electrolytes, oxygen-electrode steam, and dry-H₂ production; Choi et al. achieved a PCEC performance of 1.80 A cm⁻² at 1.3 V/600 °C, 76% Faradaic efficiency, and 500-h operation at 550 °C (Choi et al., 2019), but protonic-kinetics/electronic-leakage/interfacial-reactions/chemical-stability were found to lag. Symmetrical solid oxide cells (SSOCs) simplify reversible operation/fabrication (Liu K. et al., 2026): Figure 6a illustrates the reversible operation of solid oxide cells (SOCs) for electricity generation and water electrolysis (Fan et al., 2024). In Figure 6b, conventional SSOC electrode fabrication requires separate calcination, printing, and prolonged sintering steps. In contrast, Figure 6c shows the extreme heat treatment (EHT) strategy, where sprayed precursors are rapidly calcined and sintered in a single step using carbon paper as the heating source. As summarized in Figure 6d, EHT provides much faster heating and cooling, reduces fabrication from over 40 h to seconds, and lowers processing cost, enabling rapid electrode manufacturing.

Symmetric electrodes, although fabrication-/redox-tolerant, remain specialized; practical-use comparison favors employment of lower-temperature PCECs (potentially dry H₂) compared to mature oxygen-ion cells (stronger long-duration validation and higher thermal-management demand). SOEC durability is limited by Ni-coarsening/delamination/Cr-poisoning/redox cycling/silica or impurity migration/oxygen electrode instability/seal degradation/start-up–shutdown stress, governed by temperature/steam use/current–density/gas-composition/thermal cycling (Liu K. et al., 2026); nanostructuring increases the initial area/activity but causes coarsening. Steam–CO₂ co-electrolysis produces syngas for synthetic fuels/chemical and carbon utilization (Yang et al., 2023). However, the operation remains constrained by carbon deposition, the reverse water–gas shift reaction, fuel–electrode instability, and the need for precise H₂/CO ratio control (Bimpiri et al., 2023). Further progress requires balancing the catalytic activity, phase stability, interfacial compatibility, mechanical integrity, and manufacturability. These material advances must be integrated with improved stack design, thermal management, impurity control, operating strategies, diagnostics, and long-duration validation.

3.5 Hybrid and emerging systems

Hybrid/paired electrolysis combines cathodic H₂ evolution with photoelectrodes; thermochemical/waste heat steps; or oxidation of biomass derivatives, alcohols, urea, glycerol, and sulfite, replacing OER and resulting in production of chemicals (Xie et al., 2022). Evaluating its merit requires energy/carbon balances, Faradaic efficiency, conversion/selectivity, product distribution, separation energy, H₂ purity, feed variability, poisoning, durability, safety, scalability, and lifetime; anode potential is inadequate, and metrics cannot be equated with water–electrolyzer efficiency (Benghanem et al., 2024). Biomass salts, lignin fragments, sulfur species, proteins, and impurities undermine the model-feed results (Bilge et al., 2025). Seawater/wastewater systems face operational challenges such as chloride oxidation, precipitation, biofouling, membrane contamination, and poisoning; protective layers, selective membranes, local pH control, impurity-tolerant electrodes, and real-feed testing are essential (Chen et al., 2024; Huang L. et al., 2024). Renewable devices require photovoltaic–electrolyzer electrical/thermal/safety/degradation compatibility. Interfaces govern adsorption, selectivity, reconstruction, and transport (Li et al., 2025a). Table 1 summarizes the technologies/configurations.

TABLE 1. Representative research data for major electrolyzer technologies and device configurations.

4 Representative nanostructured electrocatalysts and mechanistic design principles

Nanostructured electrocatalysts for water electrolysis should be understood as integrated active site–electrode systems whose performance is governed by multiple, strongly coupled factors. At the atomic and electronic levels, the coordination environment and electronic structure determine the adsorption and transformation of reaction intermediates. Under operating conditions, catalyst reconstruction and phase evolution may further alter the identity and activity of the active sites. At the mesoscale, pore architecture, surface wettability, and active-site density influence reactant and ion transport, while also controlling bubble nucleation, contact angle, coalescence, detachment, surface blockage, and local concentration gradients. At the electrode level, fabrication method, catalyst adhesion, ionomer distribution, interfacial contact, electrical conductivity, and structural stability critically affect the overall performance and durability. Accordingly, rigorous evaluation of electrocatalysts for the HER and OER should report overpotential, catalyst loading, geometric and electrochemically active surface area, electrolyte composition, pH, temperature, mass-transport conditions, cell configuration, Faradaic efficiency, impedance characteristics, and post-operation structural and chemical changes. Such a comprehensive assessment is essential for translating promising catalyst-level activity into practical water-electrolysis devices (Parvin et al., 2026).

4.1 Noble-metal catalysts and noble-metal minimization

Pt benchmarks the HER via near-thermoneutral H* adsorption/rapid charge transfer; IrO₂/RuO₂ benchmark acidic OER, although Ru oxides undergo oxidative dissolution and Ir oxides are stable but scarce/costly (Samanta et al., 2022). Minimization involves use of Pt nanoparticles, ultrathin films, alloys, core–shells, clusters, atomically dispersed sites (Correia and de Sá, 2025), low-Ir mixed oxides, Ir atoms/clusters, Ir–Ru solid solutions, antimony/tin-oxide supports, and interfaces suppressing Ir dissolution/stabilizing high-valent states. The HER requires alkaline water dissociation, acidic proton transport, H₂ bubble removal, dispersion/adhesion, pore/ionomer accessibility, loading, turnover, roughness-normalized activity, and start–stop/high-current durability (Huang X. et al., 2024; Shah et al., 2026). Single atoms/subnanometer clusters can approach 100% utilization, but they require coordination/charge-density/metal–support control, electrical/ionic access, and resistance to migration, agglomeration, dissolution, pH, gas, temperature, and potential changes (Chen et al., 2022). Doped carbon, conductive oxides, nitrides, and carbides tune d-state occupancy/adsorption energies, but the OER supports may undergo corrosion (Liu et al., 2022). Contaminants induce reconstruction, passivation, electrodeposition, and dissolution (Figure 7a); STEM-EELS, ICP-OES, XPS, in situ Raman spectroscopy, and AFM resolve the effects (Figure 7b); Cl⁻ promotes chlorine evolution/anode corrosion, whereas Ca²⁺/Mg²⁺ cause cathodic precipitation, requiring pH, surface, and electrical double layer control (Figure 7c) (Parvin et al., 2026). Evaluation must combine (i) site activity/utilization, (ii) cycling-induced dissolution/redistribution, and (iii) low-loading MEA retention at specified current/lifetime, using chronopotentiometry, accelerated degradation, dissolution analysis, and operando spectroscopy; rotating-disk activity may fail via ionomer poisoning, migration, corrosion, dissolution, or poor porous transport layer contact.

4.2 Representative alkaline HER systems: NiMo and MoS₂

Scalable alkaline water electrolysis requires inexpensive, earth-abundant Ni-, Co-, Fe-, Mo-, and W-based catalysts, including oxides, hydroxides, oxyhydroxides, sulfides, phosphides, nitrides, carbides, and their combinations (Kawashima et al., 2023). Their metal centers, anions, defects, and interfaces can regulate water dissociation and H, OH⁻, and OER-intermediate adsorption, although conductivity, reconstruction, leaching, and durability must be evaluated under operating conditions. Ni-based materials can catalyze the HER and OER depending on the phase, electrolyte, and potential. For the HER, Ni promotes water activation, while Mo, W, P, S, or N modulates H adsorption. NiMo exemplifies this bifunctional design: Ni-rich sites contribute to water-splitting, Mo regulates H₂ adsorption and interfacial charge, and high-area active regions must remain connected to a conductive, adherent framework (Manickavasakam et al., 2026). Because Mo loss, oxidation, substrate contact, and apparent electrochemical area can distort the interpretation, the composition alone is insufficient. Under anodic polarization, Ni commonly undergoes reconstruction into active NiOOH-type phases, complicating the assignment of the synthesized material as the true OER catalyst (Han et al., 2025). Co oxides, hydroxides, phosphides, sulfides, and spinels similarly undergo alterations in the valence and phase; heteroatom incorporation, defects, conductive supports, and hierarchical structures may improve activity (Xu F. et al., 2024), but Co dissolution, surface oxidation, and reconstruction require operando and post-test analysis (Zhang et al., 2026a). Fe incorporation strongly enhances Ni hydroxide/oxyhydroxide OER activity by altering the electronic structure, redox behavior, and intermediate adsorption (Dinda et al., 2024); therefore, electrolyte purity, pretreatment, Fe content, and post-operation electrolyte composition must be reported because trace Fe impurities can affect the performance (Nusrat et al., 2026).

Mo- and W-based chalcogenides, carbides, nitrides, and phosphides are promising HER catalysts. In MoS₂, edges are generally more active than ideal basal planes, motivating phase control, vacancies, strain, fewer layers, and conductive supports; however, defect-induced oxidation and increased edge density can mimic intrinsic kinetic improvement, requiring operando phase identification and site-normalized turnover frequencies. Li et al. synthesized MoS₂ nanoparticles directly on reduced graphene oxide (rGO)by solvothermal treatment in DMF at 200 °C (Li et al., 2011). Figure 8a illustrates the hydrothermal synthesis of CTAB-assisted MoS₂ followed by ultrasonic coupling with rGO to form the CTAB-MoS₂/rGO sensing layer (Li et al., 2022). In Figure 8b, the different work functions of rGO and MoS₂ drive charge redistribution upon contact, producing band bending and a heterojunction at equilibrium. This interfacial electronic interaction promotes electron transfer from MoS₂ toward rGO, while CTAB increases accessible surface area and rGO improves conductivity, collectively enhancing NO₂ sensing performance. Related NiMo, CoMo, NiMoN, and MoS₂ heterostructures balance water activation and H₂ binding (Jia et al., 2025), but practical-use catalysts must also permit scalable synthesis, porous-electrode integration, electrolyte penetration, electron transport, mechanical integrity, and sustained operation at industrial current densities.

4.3 Representative alkaline OER system: NiFe (oxy)hydroxides

Oxides, hydroxides, and oxyhydroxides dominate the alkaline OER via redox flexibility, metal–oxygen bonding, defects, and anodically generated high-valency sites; reconstructed oxyhydroxides become operative, requiring operando/post-reaction analysis. NiFe LDHs are benchmarks (Mirghni et al., 2026a): porous nanosheets improve edge/electrolyte access (Su et al., 2026), while Fe-modified NiOOH-like motif nuclearity/coordination and electrolyte-derived Fe vary with the composition, preparation, potential, and electrolyte purification (Dionigi et al., 2020). LDH low-conductivity/restacking/mechanical instability necessitates OER-stable nickel foam/carbon cloth/metal oxide/doped carbon supports. Figure 9a shows that 1.0-LDH/3MXNF delivers the best OER activity, requiring only 247 mV at 100 mA cm⁻² (He et al., 2024). In Figure 9b, it also exhibits the lowest Tafel slope of 67.7 mV dec⁻¹, indicating faster kinetics. Figure 9c shows a higher double-layer capacitance (Cdl), while Figure 9d reveals the smallest charge-transfer resistance (0.51 Ω), confirming improved active-site accessibility and electron transport. The multi-step and long-term tests in Figures 9e,f demonstrate rapid response and stable operation, and Figure 9g summarizes its superior overpotential–Tafel performance. Spinels (Co₃O₄, NiCo₂O₄, FeCo₂O₄, and mixed-metal phases) exploit tetrahedral/octahedral cation distribution, substitution, vacancies, morphology, and reconstruction, although excessive defects promote dissolution/collapse (Tatarchuk, 2024). Perovskites follow B-site occupancy, orbital filling, oxygen vacancies, and metal–oxygen covalency; lattice-oxygen participation must be balanced against leaching/amorphization at high current density (Xu et al., 2022). Amorphous oxyhydroxides offer adaptive, unsaturated motifs but require X-ray absorption, Raman spectroscopy, isotope labeling, and electron microscopy for structural analysis (Guo B. et al., 2023; Kang et al., 2023). Comparisons should quantify Fe uptake/loss, Ni redox charge, electrochemically active area, dissolved metals, operating-phase persistence, porous-electrode performance, and high-current durability; pristine morphology, ex situ oxidation state, and geometric-current activity cannot identify active sites.

4.4 Reconstructing X-ide precatalysts

Transition-metal sulfides, selenides, phosphides, nitrides, carbides, and borides are potential-dependent X-ide catalysts/precatalysts governed by conductivity/covalency, anion chemistry, purity, termination, defects, morphology, and reconstruction. The HER may preserve the frameworks, although oxidation, anion loss, and electrolyte-dependent termination require verification. In MoS₂/WS₂, less-active 2H basal planes benefit from exposed edges, metallic/distorted phases, vertical nanosheets, defects, heteroatom doping, strain, and graphene/carbon coupling; intrinsic gains and metastable durability require verification (Van Nguyen et al., 2023). Ni₂P, CoP, FeP, MoP, and mixed phosphides provide metal-site H₂ adsorption and P-assisted proton/water activation, yet they form phosphate-modified oxyhydroxides during the OER (Maity et al., 2025). Mo₂C, WC, VN, MoN, and Ni₃N offer metal-like conductivity and H₂ adsorption/charge transfer in thick/porous electrodes. Still, anodic oxidation, N/C loss, and conversion can yield active shells on conductive cores (Liu L. et al., 2026). Selenides may show better conduction than sulfides; borides show metallicity and strong metal–B interactions. Figure 10a highlights the pH-dependent OER pathways. In alkaline media, OH⁻ adsorption generates M–OH, M–O, and M–OOH intermediates, whereas acidic OER may proceed through AEM or lattice-oxygen participation via LOM (Trębala and Łamacz, 2025). Figure 10b summarizes MOF-based catalyst design, where pristine MOFs can be integrated with conductive supports or converted into carbides, hydroxides, phosphides, and sulfides. These strategies address the intrinsically low conductivity and stability of MOFs while exploiting their high porosity, large surface area, and tunable structures for improved OER activity. Reconstruction should be controlled and reproducible (Somaily, 2024); scalability requires safe, nontoxic, low-cost synthesis, strong adhesion, and high current density stability.

4.5 Atomically dispersed and compositionally complex catalysts

Single- and dual-atom catalysts maximize utilization and tune HER/OER sites through identity/coordination, heteroatoms, vacancies, strain, conductivity, and charge distribution (Bai and Guan, 2023; Kment et al., 2024). Isolated active sites can facilitate H₂ adsorption, water dissociation, and stabilization of oxygenated intermediates, while homo- and heteronuclear pairs can modulate H₂, OH, O, and OOH* binding and potentially overcome conventional scaling relationships. However, bias-induced reconstruction, migration, dissolution, aggregation, and support-derived activity must be carefully distinguished. Robust validation, therefore, requires loading-dependent microscopy, operando coordination-sensitive and X-ray absorption spectroscopy, durability testing, post-test mass balance, and rigorous exclusion of cluster or nanoparticle contributions.

High-entropy alloys/oxides/sulfides/phosphides diversify adsorption through lattice distortion, electronic interactions, and entropy-stabilized phases (Lyu et al., 2026), but segregation/selective dissolution obscures the active sites. A study of FeCoNiCrZn linked reconstruction and selective Cr/Zn loss to the OER using pulsed-laser synthesis, operando analysis, differential electrochemical mass spectrometry, and computation, reporting an overpotential of 233 mV at a current density of 10 mA cm⁻² and stability for 94 h at a current density of 250 mA cm⁻² (Liang et al., 2026), which is below the commercial lifetime. MXenes provide conductive, termination-tunable two-dimensional supports/heterostructures but face oxidation, restacking, and synthesis-dependent chemistry (Thalji et al., 2025); this review defines the design space, while further studies are required for defining mechanisms. Descriptor screening, high-throughput synthesis, and machine learning aid discovery, but reconstruction requires validation (Wang et al., 2023). Deployment requires reproducible synthesis, loading/site density, coordination analysis, and stable conductivity/ion access/mechanical contact/composition/phase at device-relevant current density in hierarchical electrodes.

4.6 Defect, strain, and interface engineering

Defect, strain, interface, and morphology engineering tune adsorption, charge transfer, and transport but may accelerate corrosion, dissolution, reconstruction, or degradation; activity must balance durability (Guo Z.-Y. et al., 2023). Vacancies, grain boundaries, dislocations, heteroatom substitutions, and undercoordinated centers, including O, S, and cation vacancies, modify coordination and HER/OER adsorption, although defects can destabilize lattices (Xu Y. et al., 2024). Compressive/tensile strain caused by mismatch, core–shell structures, epitaxy, alloying, or deformation alters bonds, orbitals, intermediates, and transition states but may relax operando (Mao et al., 2023). Metal/oxide, sulfide/oxyhydroxide, phosphide/reconstructed-shell, catalyst/support, and catalyst/ionomer interfaces promote charge redistribution, electric fields, water activation, and phase stabilization, yet they risk delamination, corrosion, blockage, and gas-evolution damage (Meng et al., 2023). Nanosheets, nanowires, hollow/porous structures, aerogels, foams, and three-dimensional electrodes improve access and bubble release but may suffer from limitations of fragility, flooding, gas trapping, and non-uniform current (Pang et al., 2024). Combined architectures improve performance but complicate attribution (Muhammad et al., 2024); claims require quantified defects/strain, intrinsic activity, stability/dissolution evidence, interface controls, operando analysis, finite-element transport modeling, and high-current-density durability testing.

4.7 HER and OER mechanisms and operando active-state identification

Mechanistic design must align kinetics, products, operando structure, computation, transport, and durability at one operating state because nanostructured-electrode current also reflects the area/environment/architecture (Zhang et al., 2023). Tafel slopes primarily reflect the sensitivity of reaction rates to the applied potential and should not be interpreted as definitive evidence for a unique rate-determining step, particularly when surface coverage, uncompensated resistance, bubble accumulation, mass-transport limitations, or catalyst reconstruction vary during measurement. Likewise, isotope-labeling experiments and differential or online mass spectrometry can help identify oxygen sources, while pH- and cation-dependent trends can provide insights into interfacial electric field effects; however, neither approach alone constitutes conclusive evidence for the involvement of lattice oxygen. For the HER, ΔG_H* should approach 0: negative values hinder H₂ release, while positive values hinder adsorption. Pt binds H near thermoneutrality; alkaline/neutral non-noble catalysts often need bifunctional oxophilic/hydroxide-tolerant interfaces for Volmer water dissociation and adjacent H-binding/proton–electron-transfer/desorption/bubble release sites. For the OER, OH/O/OOH diagrams at specified potential/pH define overpotential by the largest uphill proton–electron transfer step; linear OH/OOH scaling limits single-site adsorbate evolution, requiring explicit intermediates and validated alternatives (Man et al., 2011). Metal–oxygen covalency/occupancy, redox flexibility, and lattice oxygen are relevant, but activity may trade against dissolution/vacancies/amorphization/lattice oxygen loss. Because phosphides, sulfides, nitrides, carbides, and oxides often undergo substantial structural and chemical reconstruction under operating conditions, no single operando characterization technique can independently provide definitive identification of the true catalytically active sites (Huang Y.-C. et al., 2024). XAS, including XANES and EXAFS, can distinguish bulk-phase retention from the formation of surface oxyhydroxide shells (Dionigi et al., 2020); Raman spectroscopy resolves NiFe/CoFe reconstruction and oxide/oxyhydroxide/anion/adsorbate vibrations (Bo et al., 2020); RuO₂ ATR-FTIR/XAFS links interfacial water/adsorbates and Ru–Ru/Ru–O bonding to an oxide pathway (Yang B. et al., 2025); AP-XPS resolves surface states/species, including NiFe OER chemistry (Ali-Löytty et al., 2016). Rigorous operando analysis requires careful control of signal averaging, self-absorption, beam- or laser-induced effects, reference selection, fitting covariance, fluorescence contributions, local pH variations, resonance enhancement, spectral dominance, band overlap, water absorption, selection-rule constraints, thin-layer mass transport, pressure, electrolyte thickness, radiation dose, and probe escape depth. These measurements should be complemented by isotope labeling and product analyses, conducted under realistic cell conditions, and cross-validated using X-ray absorption spectroscopy, Raman and infrared spectroscopy, microscopy, ICP-MS or ICP-OES, and quantitative gas analysis.

Trends derived from d-band descriptors and projected density of states (PDOS) are not universally transferable as they depend on electronic filling, bandwidth, orbital symmetry, spin state, solvation, adsorbate coverage, electrode potential, surface termination, and the coupling among metal d, ligand p, and adsorbate orbitals. Similarly, the Bader charge represents a method-dependent partitioning of the electron density and should not be interpreted directly as a formal oxidation state. Computational studies should, therefore, explicitly report the assumptions underlying the computational H₂ electrode model, together with adsorbate coverage, solvent and electric-field treatments, dispersion corrections, spin configuration, convergence criteria, and associated uncertainties. Calculated adsorption free energies, Bader charges, density-of-states features, and PDOS trends should be correlated with operando measurements of bonding and oxidation states, Faradaic efficiency, and catalyst dissolution or material loss. Moreover, under industrially relevant current densities, ohmic resistance, gas-bubble coverage, local pH gradients, water transport limitations, membrane hydration, catalyst layer cracking, and thermal effects may dominate the cell voltage, thereby necessitating validation at the device level (Kwon et al., 2024). Table 2 summarizes HER-/OER-/device-relevant catalysts. Effective designs integrate intermediate binding/site accessibility/electron–ion transport/bubble removal/stable reconstruction/dissolution or agglomeration resistance, standardized activity, quantified degradation, defined operating states, and realistic loadings/current densities (Srinivasan, 2025), linking molecular descriptors to durable zero-emission production of H₂.

TABLE 2. Representative nanostructured electrocatalysts for HER, OER, and device-relevant water splitting.

5 Device integration, industrial performance, and sustainability of nanostructured electrocatalysts

Device integration, industrial performance, and sustainability of nanostructured electrocatalysts for green H₂ production are tightly interwoven when moving from lab-scale concepts to grid-scale deployment. In terms of device integration, recent advances emphasize self-supported, 3D-foam and 3D framework electrodes that improve electrical connectivity, gas-bubble management, and mechanical stability, enabling higher current densities and better durability under industrial operating conditions (Chauhan et al., 2025), with design strategies including catalyst anchoring, heterostructuring, and scalable fabrication approaches suited for large-scale electrolyzers (Yap et al., 2023). Industrial performance and sustainability analyses increasingly couple material innovations with life-cycle assessment (LCA) and techno-economic analysis (TEA), showing that earth-abundant, non-precious catalysts (e.g., Ni-, Mo-, and carbon-based systems) can reduce energy input and lifecycle environmental impacts at scale. However, challenges remain in achieving industrial current densities, long-term stability, and fully representative LCA across the value chain for green H₂ production, with ongoing work on integrating advanced electrocatalysts with renewable-energy-powered electrolyzers and seawater electrolysis to enhance the sustainability metrics (Perera et al., 2024; Xu et al., 2025). Where nuance exists, some sources stress on remaining barriers in OER kinetics, seawater compatibility, and scale-up economics, underscoring the need for holistic device-level optimization, robust material engineering, and comprehensive LCA/TEA to realize commercially viable, low-emission H₂ production from nanostructured electrocatalysts. Figure 11 shows the production of green H₂ through an electrolyzer powered by renewable energy sources such as solar, wind, hydropower, geothermal, biomass, and ocean energy. It illustrates how water is split into H₂ and oxygen using clean electricity, highlighting green H₂ as an important pathway for low-carbon energy storage and decarbonization.

5.1 Catalyst-coated electrodes and membrane electrode assemblies

Catalyst-layer structure, interface engineering, and MEA processing are critical for the transfer of nanostructured electrocatalysts from the laboratory half-cell to the real-world electrolyzer device (Mo et al., 2023). Although they provide mechanical stability and direct electrical contact with the porous transport layers, the routes may also result in non-uniform contact with the membrane if the catalyst layer is rough, thick, or non-conforming to the membrane in some places. Catalyst-coated membrane approaches, on the other hand, can decrease interfacial resistance and enhance ionic contact but require more stringent conditions in the composition of the ink, choice of the solvent, drying characteristics, ionomer distribution, and lamination conditions (Ma and Mu, 2023; Ferdous et al., 2024; Zeng et al., 2024). These factors make it clear that intrinsic catalytic activity is not the only criterion by which the performance of electrodes can be assessed. Rather, it should be considered a coupled function of catalyst loading, morphology, roughness, electrolyte environment, membrane contact, temperature, and cell configuration. The key issue in the design of MEA is ensuring that the catalyst layer has properties of electronic conductivity, ionic accessibility, water transport, and removal of gases. The amount of ionomer, in particular, is of great importance since it determines the transport of protons/hydroxides and affects porosity and access to reactants (Jo et al., 2024). A lack of ionomers can result in loss of contact between active sites and ionic sites. In contrast, an abundance of ionomers can cause blocking of inactive surfaces, hindered gas transport, and increased mass-transfer resistance. In this way, an active nanostructured catalyst can be less effective in a full cell when its active sites are in poorly wetted areas or when the ionomer is unable to access the catalytically relevant domain (Avasthi and Balakrishnan, 2019; Abubaker et al., 2024). On the other hand, a material with moderate intrinsic activity could produce abundant activity in the device if it is grown directly on a mechanically stable, conductive, and porous substrate where contact resistance is low and where bubbles can be released easily. Figure 12 illustrates the hierarchical design of catalyst-coated electrodes, showing how nanostructured catalyst particles, ionomer pathways, and porous transport layers interact within an MEA. It also highlights key performance and sustainability challenges, including interfacial resistance, degradation pathways, catalyst loading, lifetime, recyclability, and critical-material intensity.

Hot pressing, hydration history, and lamination chemistry must also be carefully controlled in the fabrication of MEA. The main factors affecting catalyst-layer compaction, membrane deformation, interlayer adhesion, and overall ohmic resistance are temperature and pressure; in addition, poor processing can lead to delamination during operation (Choi et al., 2018; Wang et al., 2018; Shah et al., 2025). These effects are especially important for nanostructured catalysts, which have a large surface area, defect density, and hierarchical porosity, which can all be altered during ink drying, pressing, and operation. Interfaces between different materials such as metals, oxides, sulfides, oxyhydroxides, supports, ionomers, and membranes can lead to generation of local electric fields, modification of adsorption energetics, and steering of surface reconstruction (Karatum et al., 2022; Wu et al., 2025b). These interfacial effects are advantageous only when they are stable under the conditions of current load, hydration gradients, pH changes, heat production, and mechanical stress caused by the formation of bubbles. Thus, a single overpotential value should not be the sole indicator for a device-relevant evaluation. The following parameters need to be reported for reliable comparison: potentials at defined current densities, Tafel slope, electrochemically active surface area, catalyst loading, Faradaic efficiency, impedance, gas crossover, durability, and post-test structural characterization (Karki et al., 2025; Mirghni et al., 2026b). Kinetic barriers can be the controlling factor at low current density, and three-electrode measurements can be useful for providing mechanistic insights. Under high current density, however, the cell voltage can be dictated by ohmic losses, local pH gradients, membrane dehydration, blockage of the fuel and/or air, cracking of catalyst layers, water starvation, and thermal gradients (Kim et al., 2016). Therefore, the activity level of the powder does not necessarily correspond to the stack being “ready,” unless backed up by MEA level testing, operando evidence, and long-term stability data.

5.2 Scale-up, catalyst loading, and manufacturing

Scaling nanostructured electrocatalysts requires shifting from activity optimization to manufacturable catalyst-layer design in proton-exchange membrane electrolyzers; precious metal use reduction lowers cost only when utilization, electronic connectivity, ionic accessibility, wetting, adhesion, and durability are maintained (Feng et al., 2025; Wang C. R. et al., 2025). Loading must balance kinetics, thickness, porosity, ionomer fraction, substrate, transport, and integrity; high loading creates inaccessible material, whereas low loading increases mass activity. Spray, slot-die, screen-printing, decal-transfer, and roll-to-roll methods must ensure reproducible, uniform, large-area MEAs. Spray coating depends on solvent volatility, ink stability, droplet size, drying rate, substrate temperature, and catalyst–ionomer interactions; poor control produces thickness variation, pinholes, cracks, coffee-ring effects, and ionomer segregation (Mardle et al., 2023; Jo et al., 2024). Electrodeposition yields binder-free, electrically contacted films, but scale-up requires controlling the current distribution, bath chemistry, nucleation density, thickness, adhesion, and composition (Sharma et al., 2020; Ehsan et al., 2022). Self-supported foams, meshes, felts, and porous metals reduce contact resistance and improve gas release but complicate normalization, mass transfer, compression, and mechanical stability; comparisons should distinguish the apparent current density, geometric area, ECSA, and mass activity (Sing, 1985; Scherdel et al., 2010). Figure 13 presents a sequential flow: Step 1 ensures reproducible synthesis and critical material loading, targeting >95% utilization. Step 2 involves preparation of the catalyst–ionomer–solvent ink with specified solids, dispersant, rheology, and ionomer distribution. Step 3 involves coating of the membrane or porous transport substrate while preserving gas pathways. Step 4 involves drying the layer in a heated conveyor oven, limiting cracking, segregation, pore collapse, and contact resistance. Step 5 involves assembling and compressing gas-diffusion layers, cathode, proton-exchange membrane, and anode while detecting defects. Step 6 involves integration of cells into stacks with fuel and air/oxidant pathways and minimal gas blocking. Step 7 involves application of quality control, mechanical testing, and durability tests under current, pressure, temperature, and cycling stress. Step 8 closes the loop through failure analysis, critical-material recovery, reuse, and process adjustment. Scale-up changes the residence time, mixing, heat transfer, coating, drying, calendaring, compression, and quality control; composition, particle-size distribution, surface chemistry, and electrochemical performance must remain within the batch windows. Spatial diagnostics show how coating, compression, manifold flow, contact resistance, and temperature variations create hot-spots and non-uniform degradation. Practical catalysts should minimize critical material use; tolerate feedwater impurities; reduce solvent and thermal burdens; and retain accessible active sites, continuous electronic pathways, stable ionic interfaces, open gas transport, and strong adhesion (Shah et al., 2024b; Zhang Q. et al., 2026). Readiness metrics include batch variability, coating yield, catalyst utilization, degradation rate, recyclability, and stack compatibility.

5.3 Mass transport and gas bubble management

At practical current densities, nanostructure–chemistry–transport coupling governs electrolysis. High-area catalysts (Feng et al., 2025; Liu et al., 2025; Zhang Q. et al., 2026) can trap bubbles, reducing wetted area/electrolyte-ion access and causing reactant/OH− depletion and pH/concentration gradients; in alkaline/AEM cells, these alter NiFe speciation, accumulate carbonate, and increase resistance. The coupled effects of surface energy, feature size, pore connectivity, wettability, and surface roughness govern gas-bubble nucleation, detachment, and blockage. These processes can be further controlled through graded pore architectures, patterned hydrophilic–hydrophobic domains, optimized flow-field designs, and appropriate electrode compression. Durable aerophobicity must withstand reconstruction/impurities/gas evolution stress. PTL thickness/pore distribution/tortuosity/chemistry/compression balance water delivery, H₂/O₂ removal, membrane contact, and current uniformity; overcompression restricts gas flow, undercompression increases interfacial resistance (Fang and Liu, 2014; Hu et al., 2020). Acidic and proton exchange membrane operation imposes additional requirements for controlling water distribution, membrane hydration, and species crossover. Under operating gradients, oxides, sulfides, phosphides, nitrides, and oxyhydroxides may undergo substantial surface and bulk reconstruction, making operando and post-operando characterization essential. Ohmic and reaction-induced heating, together with non-uniform current distribution, can generate localized hot-spots across large-area electrodes and stacks. These thermal and electrochemical heterogeneities affect hydration, catalyst dissolution, ionomer stability, gas solubility, and mechanical stress (Senkov et al., 2011; Juan et al., 2015; Khan et al., 2026). Electrode performance should be validated using a comprehensive set of operando and long-term metrics, including gas saturation, pressure decrease, high-frequency resistance, local current density or potential mapping, gas flux and flow rate dependence, pressure, impedance, thermal profiles, gas purity, Faradaic efficiency, and voltage stability. Contact-angle measurements alone are insufficient. Effective electrodes must simultaneously manage water delivery, gas evacuation, ionic and electronic conduction, and heat dissipation while maintaining high reaction kinetics.

5.4 Durability and degradation mechanisms

Durability should be reported using test duration, current density, voltage decay rate, cycling protocol, and a defined failure criterion. Representative device studies report operation of more than 800 h for a high-current AEMWE, 2,000 h at 5 A cm⁻² for a PEMWE catalyst system, 4,400 h at 1 A cm⁻² and 800 °C for an oxygen-ion SOEC, and 500 h at 550 °C for a PCEC (Choi et al., 2019; Sun et al., 2019; Sun et al., 2025; Zheng et al., 2025). Because these conditions are not equivalent and do not independently establish commercial lifetime, terms such as “long-term” or “device-level” are inadequate without complete operating details. Nanostructured electrocatalysts may achieve high activity through increased surface area and defect density, but these features can also accelerate dissolution, oxidation, restructuring, coarsening, agglomeration, poisoning, detachment, and potential-driven reconstruction. Under acidic or oxidative conditions, start–stop cycling, and transient high-voltage events, dissolved metals may migrate, redeposit, contaminate membranes, or become electronically isolated (Jin et al., 2019; Wang K. et al., 2025). Apparent phase stability should, therefore, be confirmed by long-term operation, post-test compositional analysis, and mass balance.

Durability is governed by the entire catalyst–support–ionomer–membrane assembly rather than by any isolated component (Shah M. Z. U. et al., 2024; Volk et al., 2024). Carbon supports can undergo corrosion during oxygen evolution or high-potential excursions, causing catalyst loss, reduced conductivity, and increased interfacial resistance, whereas metallic supports may undergo passivation, oxidation, dissolution, or embrittlement. Oxide and ceramic supports improve oxidative stability but may introduce conductivity limitations or interfacial strain (Choi et al., 2019). Anion-exchange membranes are vulnerable to nucleophilic attack, carbonation, radicals, and mechanical stress, while proton-exchange membranes may undergo radical degradation, dehydration, thinning, and gas crossover (Mazloomian et al., 2023; Nissar et al., 2026). Membrane and ionomer degradation alters water uptake and ion transport, increases ohmic resistance, and releases species that poison or restructure catalyst surfaces; consequently, stability must be evaluated in the MEA environment rather than only in liquid-electrolyte half-cells (Feng et al., 2025; Yang L. et al., 2025).

Electrochemical and mechanical stresses are strongly coupled. Gas evolution, bubble-induced current hot-spots, swelling–drying cycles, thermal gradients, pressure fluctuations, and start–stop operation can crack catalyst layers, disrupt ionomer networks, collapse pores, detach films, corrode catalysts during shutdown, and increase contact resistance (Alharbi et al., 2023; Shah et al., 2024a; Ross et al., 2026). These changes may be absent from short-term polarization data yet dominate long-term voltage rise and stack failure. Meaningful accelerated stress tests must, therefore, reproduce service-relevant failure modes and include steady operation at industrial current density, renewable-load cycling, start–stop and idle periods, low-load gas-crossover conditions, differential pressure, feedwater impurities, impedance monitoring, microscopy, spectroscopy, dissolution analysis, gas-purity assessment, and post-test mass balance. Percentage activity retention alone is insufficient because it may conceal absolute voltage increase, catalyst loss, or mechanical degradation.

5.5 Techno-economics and life-cycle assessment

Techno-economic and LCA will be used to decide if nanostructured electrocatalysts could have a meaningful impact on zero-emission H₂ production. Energy efficiency is one of the most critical connections between catalyst performance and H₂ cost and is directly related to cell voltage (Jiang et al., 2023; Pedersen et al., 2023; Bukar et al., 2026). A more favorable overpotential measured in a laboratory half-cell does not necessarily translate into a lower system-level cost. For practical evaluation, the more relevant metric is the sustained operating voltage of a complete electrolyzer cell or stack at an industrially meaningful current density and under realistic conditions. These conditions include operating temperature and pressure, water quality, and fluctuations in electrical load. Economic performance must also be interpreted in relation to the capacity factor. When electrolyzers are coupled with renewable energy sources, their utilization may vary because of intermittent power availability, electricity prices, grid constraints, and energy-storage requirements. Under high-capacity-factor operation, durability and energy efficiency become particularly important because even modest reductions in cell voltage can produce substantial savings in cumulative electricity over the lifetime of the system. Start–stop tolerance, rapid response, degradation under cycling, and balance-of-plant flexibility are other important considerations under intermittent operation (Zhang et al., 2015; Wu et al., 2025a; Wang C. et al., 2026). Therefore, the “best” structure of the catalyst and electrode can vary depending on the operation of the electrolyzer, with baseload, grid following, and renewable curtailed options.

For any realistic sustainability evaluation, critical raw materials need to be taken into account too. Iridium, platinum, nickel, cobalt, fluoropolymers, rare-earth elements, and specialty membranes have varying supply risks, environmental impacts, geopolitical considerations, and recycling issues (Mirghni et al., 2026a). While reducing precious metal loading is important, substitution should not compromise service life, efficiency, or recyclability. The use of a catalyst containing less critical metals might also lead to a poorer life-cycle profile if the catalyst needs energy-intensive synthesis, toxic solvents, frequent replacement, and/or difficult separation at the end of life (Nel et al., 2006; Sani et al., 2021; Shah, 2023). It is, therefore, essential that the catalysts, membranes, and metallic components are recovered in a closed-loop cycle for sustainable deployment. The life cycle should encompass all stages from material extraction to synthesis of the catalyst, manufacturing of electrodes, assembly of stacks, use, water treatment, maintenance, replacement, recycling, and disposal. In addition, it should take into consideration the source of electricity, the lifetime of the plants, the frequency of replacing the stacks, the degradation rate, and the need for H₂ compression/purification. From this point of view, the most sustainable nanostructured catalysts would not only be highly active with low material intensity, extended lifetime, and easy scaling up of the production process. In addition, they would also not use any harmful inputs and would have an easy recovery route. The validity of techno-economic analysis claims should, therefore, be backed by performance at the device level, realistic durability data and transparency over the assumptions for the electricity price, capacity factor, stack lifetime, and recycling efficiency (Nair et al., 2008; Shah M. Z. U. et al., 2024; Volk et al., 2024; Wang J. et al., 2026).

5.6 Industrial deployment barriers

Commercialization of nanostructured electrocatalysts requires more than high laboratory activity. Apparent three-electrode measurement performance may arise from surface roughness, low catalyst loading, clean electrolytes, iR correction, or transiently reconstructed phases. Additional losses occur during ink formulation, ionomer deposition, coating, and drying because these steps can block active sites, alter wettability, create cracks, generate composition gradients, and increase contact resistance. At multi-A cm⁻² current densities, water, ion, heat, and bubble transport become critical, while dynamic operation, pressure, impurities, and thousands of hours of use reveal dissolution and component interactions. Reliable comparison also requires standardized electrolyte composition, temperature, reference electrode calibration, iR correction, catalyst loading, electrode geometry, current density normalization, cell configuration, and durability protocols (Anantharaj and Noda, 2022; Tan et al., 2025; Mirghni et al., 2026b). Commercial evidence should include intrinsic activity, operando catalyst identity, dissolution behavior, realistic electrode loading, full-cell voltage, Faradaic efficiency, degradation rate, manufacturing reproducibility, material intensity, cost, and life-cycle performance.

Practical value also depends on balance-of-plant components, operating flexibility, water purity, impurity tolerance, and compatibility with existing stacks (Momina et al., 2018; Eixenberger et al., 2024). Carbonate, metal ions, chloride, and other contaminants can foul active sites, block pores, reduce ionomer conductivity, accelerate membrane aging, and promote corrosion. Bankability further requires predictable degradation, warranty data, supply-chain security, serviceability, recyclability, safety compliance, scalable synthesis, reproducible large-area coating, stack validation, accelerated stress testing, and end-of-life recovery (Temsamani et al., 2018; Liu et al., 2021; Bai et al., 2023). Figure 14 summarizes this commercialization pathway, and Table 3 compares device-level integration, industrial performance, and sustainability indicators.

TABLE 3. Device-level integration, industrial performance, and sustainability indicators for representative studies.

6 Future prospective

Future progress in nanostructured electrocatalysts for green H₂ production should be evaluated through coupled, device-relevant evidence rather than isolated activity records. Atomic-scale modeling must account for realistic surface coverage, solvation, electric fields, and catalyst reconstruction, while synchronized operando spectroscopy, gas analysis, dissolution measurements, and post-mortem characterization should identify true active phases and degradation pathways. Because many OER and alkaline HER catalysts undergo transformation during operation, the most valuable materials will be those that reconstruct into stable, conductive, selective, and mechanically robust surfaces. Catalysts must also be assessed as components of integrated electrodes and electrolyzers. Standardized testing should report catalyst loading, electrochemically active area, Faradaic efficiency, gas purity, impedance, electrolyte and membrane conditions, structural evolution, and degradation at industrial current densities. Half-cell measurements should be complemented by membrane-electrode assembly tests, accelerated stress protocols, and long-duration operation under realistic water quality, pressure, temperature, impurities, and intermittent renewable power.

Material priorities differ by technology. Alkaline and anion-exchange membrane electrolyzers require earth-abundant, impurity-tolerant catalysts integrated with durable membranes, ionomers, and porous transport layers. PEM electrolysis should reduce iridium demand through use of ultrathin films, supported nanostructures, mixed oxides, clusters, single atoms, corrosion-resistant supports, and recycling without sacrificing acidic OER durability. Solid oxide electrolysis requires thermochemically stable, redox-tolerant electrodes resistant to coarsening, delamination, poisoning, seal failure, and thermal stress. The electrode architecture must balance accessible active sites, conductivity, wettability, ionomer distribution, adhesion, hierarchical porosity, and bubble removal while limiting gas trapping, concentration gradients, cracking, crossover, and ohmic losses. Coupled electrochemical, fluidic, thermal, and mechanical modeling should, therefore, guide the electrode design. Manufacturability and sustainability must be considered from the outset. Scalable processing, reproducibility, LCA, techno-economic analysis, critical-material use, water treatment, recycling, and manufacturing yield are as important as catalytic activity. Data-driven discovery will be useful only when datasets include standardized conditions, uncertainty, and negative results. Ultimately, progress should be measured by the verified material and energy intensity, durability, stack voltage, product purity, and scalable production rather than the lowest initial overpotential.

7 Conclusion

Green H₂ production through renewable electricity-driven water electrolysis offers a credible pathway for decarbonizing energy, transport, industry, and chemical manufacturing. However, this review demonstrates that progress cannot be judged solely by the lowest overpotential or highest activity reported for a catalyst in a laboratory half-cell. The practical value of nanostructured electrocatalysts depends on how effectively atomic-scale properties, active phase evolution, electrode architecture, transport processes, and device engineering are integrated under realistic operating conditions. Nanostructuring provides powerful opportunities to increase accessible active sites, modulate adsorption energies, regulate electronic structure, promote charge transfer, improve water activation, and facilitate gas bubble release. Noble metal catalysts remain important benchmarks, particularly under acidic conditions, while nickel, iron, cobalt, molybdenum, sulfide, phosphide, nitride, oxyhydroxide, atomically dispersed, and compositionally complex materials offer promising routes toward lower critical material dependence. Nevertheless, many catalysts undergo substantial reconstruction during operation, meaning that the initially synthesized phase may not represent the true active state. Reliable mechanistic understanding, therefore, requires characterization during operation, quantitative dissolution analysis, product measurements, and careful examination after testing. The comparison of alkaline water electrolysis, proton-exchange membrane water electrolysis, anion-exchange membrane water electrolysis, oxygen ion solid oxide electrolysis, and proton conducting ceramic electrolysis shows that no single catalyst design is universally applicable. Each technology presents distinct requirements related to acidity or alkalinity, temperature, membrane chemistry, ionic transport, corrosion, gas separation, water quality, pressure, and dynamic power input. Catalyst development must, therefore, be aligned with the specific chemical and mechanical environment of the intended electrolyzer. A central conclusion is that electrode and device integration often determine whether promising catalyst chemistry can deliver meaningful performance. Catalyst layer thickness, loading, porosity, wettability, adhesion, ionomer distribution, electrical contact, membrane compatibility, water transport, thermal management, and bubble removal strongly influence cell voltage and durability. Standardized reporting should include catalyst loading, current density, cell voltage, energy efficiency, Faradaic efficiency, gas purity, impedance, degradation rate, operating conditions, and structural changes. Long-duration testing under industrial current densities, realistic feedwater, pressure variations, impurities, repeated start and stop cycles, and intermittent renewable power is essential. These requirements also clarify the meaning of zero emission. Electrolysis produces no carbon dioxide at the point of H₂ generation, yet its climate benefit depends on electricity sourcing, component manufacture, replacement frequency, water preparation, compression, storage, transport, recycling, and end-of-life management. Accordingly, LCA and economic analysis should accompany performance testing from early stages of material development, ensuring that efficiency gains are not offset by energy-intensive synthesis, scarce precursors, or short service life. Future advances should combine predictive modeling, automated synthesis, data-driven screening, characterization during operation, and multiscale device simulation. Equal emphasis must be placed on scalable manufacturing, reproducibility, recycling, critical material use, lifecycle emissions, water treatment, and economic feasibility. Ultimately, successful green H₂ technologies will emerge not from isolated catalyst records but from coordinated optimization of materials, electrodes, membranes, stacks, and renewable energy systems. Progress should be measured by durable efficiency, verified emission reduction, resource responsibility, product safety, and manufacturability at scale.

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Footnotes

Edited by: Sehrish Mehdi, The Women University, Multan, Pakistan

Reviewed by: Sovann Khan, Kyushu University, Japan

Mohammad R. Thalji, Korea Institute of Energy Technology (KENTECH), Republic of Korea