The core problem the review tackles lies in the chemistry of water electrolysis itself. Splitting water into hydrogen and oxygen involves two half-reactions: the hydrogen evolution reaction, which produces hydrogen gas at one electrode, and the oxygen evolution reaction, which releases oxygen at the other. Of the two, oxygen evolution is by far the more sluggish. Its kinetics are slow, demanding a large extra voltage, called an overpotential, beyond the thermodynamic minimum required to split water. That wasted energy translates directly into higher electricity bills and lower overall system efficiency, and it has been a persistent bottleneck for every electrolyzer design on the market.

Compounding the problem, the catalysts that currently tame the oxygen evolution reaction are precious. Conventional proton exchange membrane electrolyzers depend on noble metal catalysts, typically iridium and platinum group materials, which are scarce, expensive, and geographically concentrated. The review also points to a second vulnerability: these systems degrade under the intermittent, fluctuating power inputs that come hand in hand with wind and solar generation. An electrolyzer that only performs well when fed perfectly steady current is a poor match for a renewable grid, so cost and durability problems reinforce one another.

Transition metal chalcogenides offer a way around both constraints. These are compounds of transition metals, such as nickel, cobalt, iron, or molybdenum, combined with chalcogen elements like sulfur, selenium, or tellurium. Because their constituent elements are abundant in the Earth’s crust, they sidestep the supply chain risks and price volatility that plague noble metal catalysts. More importantly, their electronic structure gives them genuine catalytic talent. The review emphasizes that partially filled d-orbitals in these materials enable optimized adsorption of the reaction intermediates that form and vanish in the course of both the hydrogen and oxygen evolution reactions. Getting that adsorption energy right is the central art of electrocatalysis, because intermediates that bind too weakly never react, while those that bind too strongly poison the surface. Chalcogenides hit a favorable middle ground, lowering both the kinetic and thermodynamic barriers for the two half-reactions.

The most consequential property highlighted in the review is bifunctionality. Most electrolyzers to date have required different catalysts at the anode and the cathode, each optimized for its own half-reaction. A bifunctional transition metal chalcogenide can catalyze both the hydrogen evolution reaction and the oxygen evolution reaction within a single material system. That simplifies electrolyzer design considerably, reduces the number of distinct materials that must be synthesized, tested, and manufactured at scale, and opens the door to symmetric or reversible cell architectures in which the same electrode can switch roles. For a technology trying to drive down capital costs, collapsing two catalyst systems into one is a meaningful engineering advantage.

Conductivity is the third pillar of the case. Many abundant catalyst candidates fail not because they lack catalytic activity but because they are poor conductors, so electrons cannot reach the active sites efficiently. The review notes that transition metal chalcogenides possess metallic or semi-metallic conductivity, which improves charge transfer kinetics at the electrode surface. Faster charge transfer means less resistive loss inside the cell, which matters enormously when the goal is operation at high current density, the regime commercial electrolyzers must reach to produce hydrogen at scale. The materials also show enhanced resistance to corrosion, a critical trait in the harsh oxidative environment of the oxygen evolution electrode, where many otherwise promising compounds dissolve or crumble within hours.

Structurally, the review frames these advantages as arising from tunable electronic structures. Because the metal and chalcogen identities, stoichiometry, and crystal phase can all be varied, researchers have a broad design space in which to optimize adsorption energies and electronic properties. This tunability is what separates chalcogenides from a simple substitution story: rather than merely replacing iridium with something cheaper, designers can engineer the catalyst’s electronic landscape to suit specific operating conditions, electrolyzer configurations, and current density targets. The review consolidates synthesis strategies reported in recent literature and evaluates how the resulting catalysts perform across various electrolyzer configurations, providing a map of which approaches have delivered the strongest results so far.

The authors are candid that the field is not yet ready for prime time. Three challenges dominate their assessment: catalyst stability, scalability, and integration into commercial systems. Stability remains the hardest test, since laboratory measurements are often taken over hours while commercial electrolyzers must run for tens of thousands of hours under fluctuating load. Scalability raises the question of whether synthesis routes that work on milligram quantities in a beaker can be translated to the square meters of electrode area a commercial stack requires, without losing the nanostructured features that give the catalysts their activity. Integration, meanwhile, encompasses everything from electrode fabrication and membrane compatibility to how bifunctional catalysts behave in full devices rather than half-cell tests.

The significance of the review lies in its attempt to bridge these knowledge gaps and channel them into rational design principles. By systematically connecting synthesis strategies, electronic structure, and measured performance, it aims to give researchers a framework for designing cost-effective, high-performance bifunctional electrocatalysts rather than discovering them by trial and error. The timing is pointed. As nations commit to net-zero emission targets, demand for low-carbon hydrogen is projected to grow sharply, and the economics of production will determine whether that demand is met with genuinely green hydrogen or with fossil-derived alternatives. Catalyst cost is one of the levers that most directly shapes those economics.

The work also carries a regional dimension. The research team, based at De La Salle University in Manila and supported by a grant from the Philippine Department of Science and Technology through its Council for Industry, Energy, and Emerging Technology Research and Development, reflects growing scientific engagement with the hydrogen economy in Southeast Asia, a region whose island geography and renewable potential make energy storage and clean fuels particularly salient. If bifunctional transition metal chalcogenides can clear the stability and scalability hurdles the review identifies, the payoff would be electrolyzers that run at high current density, tolerate the stop-and-go rhythm of renewable power, and cost a fraction of today’s noble metal systems. That combination, the authors argue, is what it will take to move green hydrogen from a promising cornerstone to a load-bearing one.

Subject of Research: Bifunctional transition metal chalcogenide electrocatalysts for high current density water electrolysis and green hydrogen production

Article Title: Harnessing bifunctional transition metal chalcogenides for high current density sustainable electrolyzer technologies

Article References: Owogowog, M. B. B., Flaviano, A. L., Morente, P. R. O., & Ortenero, J. R. (2026). Harnessing bifunctional transition metal chalcogenides for high current density sustainable electrolyzer technologies. Discover Sustainability. https://doi.org/10.1007/s43621-026-04758-6

Image Credits: AI Generated

DOI: 10.1007/s43621-026-04758-6

Keywords: green hydrogen, water electrolysis, electrocatalysis, transition metal chalcogenides, oxygen evolution reaction, hydrogen evolution reaction, bifunctional catalysts, proton exchange membrane, overpotential, catalyst stability, renewable energy, electrolyzer technology

The elegance of the approach lies in what the researchers did not do. There is no multi-step synthesis, no high-temperature calcination, no hydrothermal autoclave, and no template removal. Instead, the team applied a direct current to a copper electrode immersed in a plating bath containing nickel salts, and they tuned the chemistry of that bath with two familiar ions: nitrate and chloride. Each ion plays a distinct role. Nitrate is known in electroplating circles for its ability to modify deposit morphology and even darken nickel films by altering how the metal nucleates and grows on the surface. Chloride, meanwhile, influences particle size and film compactness. By adjusting the balance between the two, the researchers could jointly control two things that usually fight each other in catalyst design: the ratio of metallic nickel to nickel hydroxide in the final film, and the mesoporous architecture that determines how much of the catalyst surface is actually reachable by electrolyte.

Why does that dual control matter so much? The answer lies in the complementary weaknesses of the two phases. Metallic nickel is an excellent electrical conductor, but on its own it is a mediocre oxygen evolution catalyst; the reaction intermediates bind to its surface with the wrong energetics, and the kinetics stall. Nickel hydroxide, by contrast, offers far more favorable adsorption sites for the oxygen-containing intermediates that form during water oxidation, but it is a poor conductor and, in many preparations, suffers from structural reconstruction during operation, gradually converting into other phases whose activity can drift over time. The hybrid electrode sidesteps both problems. Metallic nickel domains act as an internal electron highway, shuttling charge to and from the catalytically active hydroxide regions, while the low-crystallinity nickel hydroxide provides the reactive interface where water molecules are oxidized. The result is a composite in which each phase covers for the other’s deficiencies.

The performance numbers back up this design logic. At the benchmark current density of 10 milliamperes per square centimeter, the Ni/Ni(OH)2 electrode required an overpotential of just 254 millivolts, meaning the researchers had to apply only that much extra voltage beyond the thermodynamic minimum to get water oxidation underway at a useful rate. Equally telling is the Tafel slope, a measure of how quickly current rises as additional voltage is applied. The hybrid electrode delivered a Tafel slope of 50.3 millivolts per decade, indicating markedly accelerated reaction kinetics compared with conventional nickel-based catalysts. In practical terms, a lower Tafel slope means the catalyst does not need ever-larger voltage penalties as the electrolyzer is pushed toward industrial current densities, which is precisely where energy efficiency is won or lost.

Durability, the quiet killer of promising electrocatalysts, was also addressed head-on. Many laboratory catalysts post impressive initial numbers only to degrade within hours as their active layers dissolve, delaminate, or restructure. The Jingdezhen team subjected their electrode to a 100-hour chronopotentiometry test at a demanding 50 milliamperes per square centimeter, a current density far closer to real electrolyzer operation than the standard 10-milliamp benchmark. The electrode held its performance throughout, a result the researchers attribute to the in-situ nature of the fabrication: because the hybrid film is grown directly on the copper substrate rather than deposited as a powder and glued on with a polymer binder, the catalyst-substrate interface is intimate and mechanically robust, resisting the bubble evolution and mechanical stress that peel weaker electrodes apart.

Mechanistic analysis of the composite revealed why the interface between metallic nickel and low-crystallinity nickel hydroxide is so productive. The two phases form a coupled system in which electrons generated during the oxidation of hydroxide species can transfer efficiently across the phase boundary into the metallic network, avoiding the charge accumulation that throttles pure hydroxide electrodes. The low crystallinity of the hydroxide component is itself an asset rather than a defect: disordered structures typically expose a higher density of under-coordinated metal sites, edges, and defects that serve as active centers for the four-electron water oxidation sequence. Meanwhile, the anion-regulated deposition produced a mesoporous morphology with finely dispersed particles, maximizing the electrochemically accessible surface area and ensuring that bubbles generated during operation can escape easily rather than blanketing the surface.

The choice of copper as the substrate deserves its own note. Copper is cheap, conductive, and mechanically robust, and it is increasingly favored in the electrocatalysis literature as a support that can even participate chemically in the catalysis, promoting the transformation of nickel hydroxide into higher-valent nickel species that are considered the true active phase for oxygen evolution. By growing the catalyst directly on copper foil or mesh in a single electrochemical step, the researchers eliminated the need for expensive conductive additives, binders, and the energy-intensive processing steps that inflate the cost of conventional electrode manufacturing. For an industry trying to drive down the capital cost of alkaline water electrolyzers, a fabrication route that is essentially room-temperature electroplating is a genuinely attractive proposition.

The broader context makes the result more than a laboratory curiosity. Alkaline water electrolysis is the most mature hydrogen production technology, but its efficiency is capped by the large overpotential of the oxygen evolution reaction at the anode, which can waste hundreds of millivolts of electrical energy per cell. Every millivolt saved translates directly into kilowatt-hours saved per kilogram of hydrogen produced. Nickel-based catalysts have long been the workhorse candidates for replacing precious metals in this role, and recent years have seen an explosion of strategies, from nickel-iron layered double hydroxides to doped nitrides and oxyhydroxides, aimed at squeezing more activity out of earth-abundant elements. What distinguishes the new work is its emphasis on process simplicity: rather than adding complexity, it uses the intrinsic chemistry of two common anions to achieve phase control, morphology control, and interface engineering in a single deposition bath.

The anion-regulation concept also opens a tunable design space that other groups can explore. Because nitrate and chloride each exert distinct effects on nucleation density, particle size, and phase composition, varying their relative concentrations offers a continuous dial for optimizing the catalyst, rather than a discrete choice between recipes. The same principle could plausibly extend to other transition metal systems, including nickel-iron and nickel-cobalt chemistries, where anion-directed electrodeposition might similarly couple conductive metallic phases with active hydroxide phases. The researchers suggest that their work offers a facile route to low-cost, high-performance non-noble-metal OER electrodes with direct implications for industrial alkaline water electrolysis, and the combination of a 254-millivolt overpotential, a 50.3-millivolt-per-decade Tafel slope, and 100 hours of stable operation at 50 milliamperes per square centimeter gives that claim real weight.

Challenges remain before such electrodes see service inside a commercial electrolyzer stack. Long-term testing at even higher current densities, in flowing industrial electrolyte, and under the intermittent loads characteristic of renewable-powered hydrogen plants will be needed to confirm that the hybrid structure survives years rather than days. Scaling from small copper substrates to meter-scale electrode plates will test whether the anion-regulated deposition remains uniform across large areas. Still, the study is a reminder that some of the most consequential advances in clean energy chemistry come not from exotic new materials but from smarter control of the ones we already have. Two cheap ions, one electric current, and a copper plate may turn out to be an unlikely recipe for making green hydrogen a little more affordable.

Subject of Research: Anion-regulated one-step electrodeposition of Ni/Ni(OH)2 hybrid electrocatalysts for the alkaline oxygen evolution reaction

Article Title: NO₃⁻/Cl⁻-regulated one-step electrodeposition for in-situ fabrication of Ni/Ni(OH)₂ hybrid structures toward high-efficiency alkaline oxygen evolution reaction

Article References: Shi, J., Fan, J., Xv, J., Sun, L., Gong, Q., Huang, Y., Zhang, K., & Zhang, H. (2026). NO₃⁻/Cl⁻-regulated one-step electrodeposition for in-situ fabrication of Ni/Ni(OH)₂ hybrid structures toward high-efficiency alkaline oxygen evolution reaction. Ionics. https://doi.org/10.1007/s11581-026-07490-3

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07490-3

Keywords: oxygen evolution reaction, water electrolysis, green hydrogen, electrodeposition, nickel catalyst, nickel hydroxide, nitrate regulation, chloride regulation, copper substrate, Tafel slope, electrocatalyst durability, alkaline media

The material at the heart of the study, dubbed NiCeS/NF, is not a single crystalline compound but a deliberately messy multiphase framework. Using a molten-salt synthesis route, the researchers grew an array of ultrathin nanosheets in situ on a nickel foam substrate, producing a composite that contains nickel sulfide, nickel oxysulfide, and sulfate species all interwoven with one another. This phase diversity is not an accident of sloppy chemistry; it is the point. The boundaries where these different phases meet, known as heterointerfaces, create regions of structural strain and electronic mismatch that are rich in defects. Structural analyses confirmed that the resulting surface environment is defect-dense, providing exactly the kind of chemically active terrain where water oxidation reactions prefer to occur.

The real innovation, however, lies in the cooperative roles played by cerium and sulfur. Cerium, a rare-earth element famous in catalysis circles for its ability to shuttle between the +3 and +4 oxidation states, is a well-known promoter of oxygen vacancy formation in oxide materials. Sulfur, meanwhile, brings its own talents: it can modulate the electronic structure of nickel sites, and sulfur-treated metal catalysts have repeatedly shown enhanced OER activity in prior studies. When the two are combined, spectroscopic measurements revealed that they do more than add their individual effects. The synergistic coupling of Ce and S effectively redistributes the local electronic structure around the nickel centers, enriches the surface with oxygen-deficient species, and, crucially, accelerates the process of electrochemical surface reconstruction.

Surface reconstruction is a concept that has reshaped how electrochemists think about pre-catalysts. Many as-synthesized materials are not the true active phase at all; under the oxidizing potentials of the OER, they transform in situ into oxyhydroxide species, typically nickel oxyhydroxide, which is where the real catalysis happens. The rate and completeness of this transformation can make or break performance. In the NiCeS/NF system, the defect-rich, electronically modulated surface appears to fast-track this reconstruction, allowing the active oxyhydroxide phase to form more readily and more uniformly. In other words, the cerium and sulfur do not merely decorate the catalyst; they choreograph its evolution into its most active form under operating conditions.

The performance figures reported by the team are striking by any standard. In alkaline media, the NiCeS/NF electrode required an overpotential of just 187 millivolts to drive a current density of 10 milliamperes per square centimeter, a benchmark figure of merit for OER catalysts that reflects how much extra voltage beyond thermodynamic minimum must be supplied. The Tafel slope, which describes how rapidly current increases with applied voltage and offers a window into reaction kinetics, came in at a lean 44.1 millivolts per decade, indicating favorable reaction dynamics. These numbers place the material firmly in the company of the best non-precious-metal OER catalysts described in the recent literature.

Perhaps even more consequential for real-world electrolysis is the durability result. Industrial alkaline electrolyzers do not operate at the gentle 10 milliampere benchmark; they push currents of hundreds of milliamps to more than an ampere per square centimeter, and few laboratory catalysts survive such abuse for long. The NiCeS/NF electrode, however, sustained a punishing 1000 milliamperes per square centimeter for approximately 400 hours without catastrophic degradation. That combination of industrial-level current density and long-term stability is rare, and it addresses one of the most persistent criticisms of defect-engineered catalysts: that the very vacancies and interfaces that boost activity can also be points of structural weakness that dissolve or collapse under prolonged operation.

The findings also contribute to a growing body of evidence on the special role of rare-earth elements in reconstructive electrocatalysis. Recent studies have shown that cerium doping can induce lattice distortion in nickel-iron layered double hydroxides, and that cerium oxide can accelerate surface reconstruction of cobalt selenides into active oxyhydroxide interfaces. The Chengdu-led work extends this picture by pairing cerium with sulfur in a multiphase sulfide-oxysulfide-sulfate matrix, suggesting that the cooperative interplay of a redox-flexible rare-earth cation and a chalcogen that reshapes nickel’s electronic landscape may be a general design principle. If so, it opens a rational pathway for tuning not just what a catalyst is, but what it becomes under working conditions.

The broader context makes the advance timely. Alkaline water electrolysis is one of the leading candidate technologies for producing green hydrogen at scale using renewable electricity, and its cost competitiveness hinges on replacing or minimizing precious-metal catalysts such as iridium and ruthenium oxides, which are scarce and expensive. Nickel-based electrocatalysts, grown directly on conductive nickel foam substrates that double as current collectors, offer a self-supported electrode architecture that avoids the binders and additives that can add resistance and fail mechanically. By demonstrating that defect and interfacial engineering can push such earth-abundant systems to both high activity and industrial-grade durability, the study strengthens the case that hydrogen production need not depend on critical raw materials.

There are, of course, the usual caveats that separate laboratory triumphs from commercial deployment. The 400-hour stability test, while impressive, is still short compared with the multi-year lifetimes expected of commercial electrolyzer stacks, and the molten-salt synthesis route will need to prove itself scalable and economical at the electrode areas relevant to industry. The multiphase nature of the catalyst, so central to its performance, also complicates the task of understanding exactly which sites do the catalytic work, a question the authors address through spectroscopic evidence of electronic redistribution and oxygen-deficient species rather than a single atomic-scale structure. Continued operando studies, of the kind that have illuminated surface reconstruction in nickel sulfide and hydroxide systems, will be needed to pin down the mechanism in full detail.

Even so, the work offers a compelling demonstration that the chemistry of imperfection can be a source of perfection in electrocatalysis. By deliberately engineering oxygen vacancies through the joint action of cerium and sulfur, the researchers have shown that a catalyst’s defects can be designed, controlled, and exploited rather than merely tolerated. As the hydrogen economy matures, strategies like this one, which marry defect chemistry with interfacial engineering and self-supported electrode design, may prove essential to closing the gap between laboratory benchmarks and the gigawatt-scale electrolyzers that a decarbonized energy system will demand. The oxygen half of the water-splitting equation has long been the slow partner; with defect-rich, rare-earth-assisted nickel catalysts now performing at industrial current densities for hundreds of hours, that partnership is finally speeding up.

Subject of Research: Defect-engineered nickel-based electrocatalysts with cerium- and sulfur-induced oxygen vacancies for the oxygen evolution reaction in alkaline water electrolysis

Article Title: Ce and S co-induced oxygen vacancies for enhanced alkaline water electrolysis

Article References: Lei, R., Zhang, Y., Chen, L., Wu, Y., Chen, H., Chen, S., Zhu, Y., & Deng, J. (2026). Ce and S co-induced oxygen vacancies for enhanced alkaline water electrolysis. Ionics. https://doi.org/10.1007/s11581-026-07448-5

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07448-5

Keywords: oxygen evolution reaction, alkaline water electrolysis, oxygen vacancies, nickel-based electrocatalyst, cerium and sulfur co-doping, surface reconstruction, green hydrogen, electrocatalysis, nickel foam, molten-salt synthesis, rare-earth catalysis, Tafel slope

The thermodynamics of water splitting look deceptively simple. Whether the electrolyte is acidic or alkaline, the minimum cell voltage needed to drive the reaction is 1.23 volts. In practice, electrolyzers demand considerably more, because the OER is a four-electron, multi-step process riddled with energy barriers, whereas the hydrogen evolution reaction at the cathode is a comparatively easy two-electron affair. That extra voltage, the overpotential, is pure waste heat and lost efficiency. In acidic systems, proton exchange membranes deliver low resistance and high gas purity, but the corrosive environment forces the use of scarce, expensive iridium and ruthenium oxides at the anode. Alkaline electrolysis, by contrast, opens the door to first-row transition metals that cost a fraction as much and can match, or even exceed, the performance of precious metal systems.

Understanding why the OER is so sluggish begins with its mechanism. The dominant framework, the adsorbate evolution mechanism, describes a sequence in which hydroxide species adsorb onto metal sites, convert into oxygen intermediates, and ultimately release molecular oxygen. Each step carries a Gibbs free energy cost, and the largest one sets the rate-determining step and thus the overpotential. The problem is that the adsorption energies of the intermediates are not independent. Because the *OOH and *OH species bind to the surface through a single oxygen atom, their energies scale linearly across virtually all oxide surfaces. The difference between the oxygen and hydroxide binding energies, roughly 3.2 electron volts on average, cannot be compressed below about 1.6 electron volts, while an ideal catalyst would need only 1.23. That mismatch imposes a theoretical overpotential floor of roughly 0.37 electron volts, and it turns catalyst design into a game of climbing a volcano-shaped activity plot, where the sweet spot lies at neither too strong nor too weak oxygen binding.

Some catalysts, however, refuse to obey the volcano. Their activity exceeds what the scaling relationship permits, and their kinetics shift with electrolyte pH in ways the adsorbate mechanism cannot explain. The resolution is the lattice oxygen mechanism, in which oxygen atoms belonging to the catalyst’s own crystal lattice participate directly in forming the O–O bond, bypassing the hydroperoxide intermediate entirely. The evidence is compelling: when researchers label lattice oxygen with the isotope oxygen-18 and run the reaction in an oxygen-16 electrolyte, differential electrochemical mass spectrometry detects mixed isotopic oxygen molecules, proof that lattice oxygen is being released as product gas. The mechanism is favored when the metal–oxygen bond is highly covalent, which is governed by the position of the oxygen 2p band relative to the metal 3d band. Raising the oxygen p-band center toward the Fermi level activates oxygen redox chemistry, promotes vacancy formation, and unlocks the lattice oxygen pathway, offering a route around the scaling-relation ceiling.

Armed with these mechanistic insights, catalyst designers have developed a toolkit of activity descriptors that predict performance without simulating every reaction step. Beyond the oxygen binding energy difference, the d-band center theory links adsorbate binding strength to the energy of the metal’s d electrons, while the occupancy of e_g orbitals in octahedrally coordinated transition metals provides a remarkably simple rule of thumb. Shao-Horn and colleagues showed that perovskite oxides perform best when e_g filling is near unity, with the perovskite Ba0.5Sr0.8Co0.2Fe0.2O3−δ sitting at the volcano apex. The oxygen p-band center itself has emerged as a descriptor that captures both activity and the instability that sometimes accompanies it. Perhaps the most striking demonstration of descriptor-guided design came from Sargent’s group, which gelled iron, cobalt, and tungsten into an atomically homogeneous oxyhydroxide. Iron binds oxygen too strongly, cobalt and tungsten too weakly, but interpolation among the three achieved near-optimal binding, delivering a record-low overpotential of 191 millivolts at 10 milliamperes per square centimeter and 500 hours of stable operation.

One of the field’s most consequential realizations is that many catalysts are not what they appear to be. Sulfides, selenides, nitrides, and phosphides that show impressive alkaline OER activity are frequently oxidized in situ under the high anodic potentials of operation, transforming into metal (oxy)hydroxides that are the true active phases. The as-synthesized materials are, in effect, precatalysts. Operando and in situ characterization techniques have confirmed that most transition metal cations undergo surface reconstruction during the OER, typically involving rising oxidation states and the leaching of specific cations or anions. Rather than fighting this transformation, researchers have learned to choreograph it. Nickel substitution in zinc cobaltate spinels pushes the oxygen p-band above the metal d-band, triggering reconstruction into an active oxyhydroxide surface and a sevenfold activity boost that surpasses the iridium oxide benchmark. Iron substitution in otherwise inert cobalt aluminate initiates reconstruction that then self-terminates, preserving the underlying structure.

Crystallinity matters too. Amorphous materials, with their abundant defects and open structures, reconstruct far more readily than their crystalline counterparts. An amorphous NiFeMo oxide prepared by fast co-precipitation rebuilt itself into an oxygen-vacancy-rich oxyhydroxide layer within moments of the reaction starting, requiring only 280 millivolts of overpotential, while the crystalline version needed 358 millivolts because molybdenum ions diffuse sluggishly through the ordered lattice. Deliberate leaching has become a design strategy in its own right. Activating an inactive cobalt chromite spinel at high potential oxidizes chromium from the 4+ to the 6+ state, driving it out of the lattice and leaving behind defects and vacancies that let cobalt reconstruct into active oxyhydroxide, an activity gain of more than an order of magnitude. In another approach, molybdenum sulfide nanosheets coated with nickel-iron layered double hydroxide served as sacrificial templates; five cycles of voltammetry stripped the molybdenum into the growing oxyhydroxide, yielding a catalyst that needed just 242 millivolts at 10 milliamperes per square centimeter.

The review’s survey of recent multi-metal catalysts reveals how these principles converge in practice. Tensile lattice strain engineered into NiFe2O4 spinel ferrites flattened the electronic bands near the Fermi level and tuned e_g occupancy at iron sites, cutting the overpotential to 180 millivolts. Co-doping ruthenium and zinc into Co3O4 selectively activated lattice oxygen while preventing over-oxidation of ruthenium, achieving 172 millivolts and 100 hours of stability at a punishing 500 milliamperes per square centimeter. Vanadium leaching from Co2Fe0.25V0.75O4 left behind a stable vanadium oxide underlayer that compensated charge and prevented cobalt dissolution, sustaining 600 hours at 500 milliamperes per square centimeter. High-valence dopants such as molybdenum, chromium, niobium, and tungsten stabilize low-valence iron and nickel species, and the resulting Fe2+ fraction correlates directly with activity; NiFeMo oxyhydroxide reached 180 millivolts. Anchoring single molybdenum atoms onto NiFe oxyhydroxide nanoarrays strengthened metal–oxygen covalency and activated lattice oxygen, delivering 193 millivolts. Non-oxide systems joined the race as well: cobalt-nickel phosphide nanosheets hit 209 millivolts, vacancy-rich NiFe sulfide reached 213 millivolts and performed at 235 millivolts under quasi-industrial conditions of 6 molar potassium hydroxide at 60 degrees Celsius, and a hierarchical CoMnP/Ni2P array worked as a bifunctional catalyst for both half-reactions, splitting water at just 1.54 volts.

The newest frontier is high-entropy materials, in which five or more elements mix in near-equal proportions to form single-phase solid solutions. The compositional disorder stabilizes the phase, and the so-called cocktail effect of synergistic elemental interactions tunes adsorption energies toward the Sabatier optimum. A high-entropy sulfide of chromium, manganese, iron, cobalt, and nickel outperformed every unary through quaternary sulfide comparison, reaching 100 milliamperes per square centimeter at 295 millivolts, with charge migration from manganese and iron optimizing the cobalt active sites. High-entropy perovskites and spinels followed suit, and in the spinel (CrFeCoNiMo)3O4 the high-valence cobalt and molybdenum shifted the reaction pathway toward the lattice oxygen mechanism, verified by isotope labeling. An amorphous NiFeCoMnAl oxide, dealloyed into a nanoporous architecture, posted 190 millivolts. The authors of the review are candid about what remains: the synergistic interactions among multiple elements are still incompletely understood, stability at industrial current densities is not guaranteed, and bridging from laboratory electrodes to scalable electrolyzer architectures is the decisive next step. But the trajectory is unmistakable. By combining electronic structure engineering, dynamic surface reconstruction, and compositional complexity, researchers are turning the OER from the weakest link in electrochemistry into a solvable design problem, and with it, the prospect of hydrogen made cleanly, cheaply, and at scale moves measurably closer.

Subject of Research: Design strategies and recent advances in 3d transition metal-based multi-metal electrocatalysts for the alkaline oxygen evolution reaction

Article Title: An overview of design strategies and recent advancements in complex 3d transition metal-based electrocatalysts for alkaline oxygen evolution reaction

Article References: Lee, S., Shin, Y., Yeom, K., Shim, J., & Sung, Y.-E. (2025). An overview of design strategies and recent advancements in complex 3d transition metal-based electrocatalysts for alkaline oxygen evolution reaction. Advances in Industrial and Engineering Chemistry, 1(1), Article 2. https://doi.org/10.1007/s44405-025-00001-4

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00001-4

Keywords: oxygen evolution reaction, electrocatalysis, water electrolysis, green hydrogen, transition metal catalysts, surface reconstruction, lattice oxygen mechanism, activity descriptors, high-entropy materials, layered double hydroxides, alkaline media, multi-metal catalysts

The numbers tell the story. Laboratory studies of water-splitting catalysts are typically conducted below 10 milliamperes per square centimeter, a gentle regime where materials behave politely. Commercial alkaline electrolyzers are benchmarked above 0.2 amperes per square centimeter, while proton exchange membrane (PEM) and anion exchange membrane (AEM) systems are expected to deliver roughly 1.6 and 1 ampere per square centimeter respectively. At those loads, steep potential gradients, vigorous gas bubbling, local pH swings and mechanical stress conspire to tear catalysts apart. Redox conversions, surface reconstructions, phase transformations, dopant leaching and outright decomposition can occur at or even before the onset of catalysis, sometimes creating entirely new phases that turn out to be the true active sites.

Conventional ex-situ tools such as X-ray diffraction, X-ray photoelectron spectroscopy, electron microscopy and infrared spectroscopy can catalog what a catalyst looked like before the storm and what remains afterward, but they cannot witness the transformation itself. Worse, samples removed from an operating cell and exposed to air can develop artifacts that mislead interpretation entirely. The review’s central thesis is that operando Raman spectroscopy, which collects vibrational spectra from a fully functioning device while simultaneously recording its electrochemical performance, offers the molecular-level window the field has been missing. The distinction between in-situ and operando matters here: in-situ means real-time measurement under catalytically relevant conditions, while operando means the measurement happens inside a genuinely working cell, directly tying a catalyst’s chemical state to its output.

Raman spectroscopy has a particular advantage in water electrolysis that infrared techniques lack: water, the electrolyte itself, has a very low Raman cross-section, so aqueous environments do not swamp the signal. The technique works by shining monochromatic laser light onto the sample; most photons scatter elastically at the same wavelength (Rayleigh scattering), but a small fraction exchange energy with molecular vibrations and emerge shifted in wavelength. These Stokes and anti-Stokes shifts act as fingerprints of bond stretching, bending and lattice vibrations, revealing oxidation states, surface-bound intermediates and structural reorganizations without destroying the sample. Advanced variants amplify this capability dramatically: surface-enhanced Raman spectroscopy (SERS), shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and tip-enhanced Raman spectroscopy (TERS) can boost sensitivity enough to catch trace intermediates and nanoscale heterogeneity that conventional Raman would miss.

The review highlights striking case studies of catalyst self-reconstruction captured live. In one, researchers tracking a nickel-iron oxide catalyst watched the Raman signature of iron oxide fade with increasing potential while defect-related nickel oxide vibrations grew, until at 600 to 700 millivolts only two peaks remained, corresponding to the bending and stretching modes of nickel oxyhydroxide (NiOOH). The mixed-phase starting material had rebuilt itself into NiOOH, the phase actually doing the oxygen-evolving work. In another study on a molybdenum-doped nickel sulphide coupled with vanadium dioxide, the NiOOH signature appeared at 1.30 volts, earlier than in the undoped material, and a sulphate band emerged at lower potential, showing that the vanadium additive accelerated reconstruction while improving stability.

Composition, it turns out, plays an unexpectedly subtle role in these transformations. In FeOOH-decorated nickel molybdate nanowires, the NiOOH phase emerged slightly later than in the pristine material, reflecting iron’s inhibition of trivalent nickel formation, yet the reconstructed phase was ultimately more stable and active. Meanwhile, a rigorously controlled study of nickel-iron oxyhydroxides under iron-free conditions found that the intrinsic activity of pure beta-NiOOH is significantly lower than earlier reports suggested, and that the celebrated activity boost attributed to iron incorporation stems from electronic interactions between nickel and iron sites rather than conductivity gains alone. These contradictions, the review notes, would remain invisible without operando measurements, and they underscore how promoter elements govern catalytic behavior in ways that static characterization cannot untangle.

On the hydrogen evolution side, Raman has resolved equally fine details. Work on ruthenium surfaces in alkaline media distinguished hydrogen species adsorbed on metallic ruthenium from those bound to ruthenium oxide: after electrochemical reduction, the oxide band vanished and only the Ru(0)-H feature at 1825 wavenumbers remained, proving that a neighboring peak belonged to hydrogen on oxidized sites. On atomically flat palladium single crystals, operando Raman combined with computation showed that hydrogen-bonded and sodium-hydrated interfacial water reorganizes from a random to an ordered structure under bias, enhancing electron transfer and boosting activity. High-speed compressive Raman imaging of layered iridate and cobaltate oxygen-evolution electrodes revealed that at high current density, oxygen evolution outpaces cation exchange, confining charge compensation to the surface, whereas at lower overpotentials cations intercalate into the bulk lattice.

The technique also reads the electrolyte itself, the third leg of the catalyst-interface-electrolyte triad. In methanol oxidation on niobia, only one polymorph developed a distinctive Raman peak associated with short niobium-oxygen bonds in edge-sharing octahedra, and that in-situ-generated feature correlated precisely with the highest activity and formate selectivity, identifying the true active site. Similar studies have tracked formate formation during glycerol oxidation and mapped intermediates in 5-hydroxymethylfurfural oxidation to the plastic-recycling feedstock 2,5-furandicarboxylic acid. Because the three-dimensional porous substrates used in industrial electrodes, metal foams, meshes and carbon fabrics, do not obscure Raman signals and even allow deeper laser penetration, the method can capture gradients across electrode thickness under realistic loads.

Honest limitations remain. Raman scattering is inherently weak, fluorescence from dyes and pigments can bury signals, laser heating can damage sensitive materials, and gas bubbles disrupt optical paths at high current densities. The review points to engineering answers: carefully chosen laser wavelengths and power densities, longer relaxation times between scans, specialized temperature-monitoring probes, and cell architectures ranging from windowless thin-layer designs to flow cells with gas-diffusion electrodes that sweep bubbles away, and fiber-optic probes inserted directly into membrane electrode assemblies. Looking forward, the authors envision embedded Raman probes in commercial electrolyzer stacks feeding real-time data into digital durability dashboards, machine-learning spectral deconvolution for predictive maintenance, and multimodal platforms pairing Raman with X-ray absorption or mass spectrometry. Standardized protocols and reference spectral libraries for benchmark catalysts such as iridium dioxide, ruthenium dioxide and nickel-iron layered double hydroxides would be essential for industrial adoption.

The stakes are considerable. For hydrogen to compete with fossil-derived alternatives, electrolyzers must run reliably at industrial current densities for lifetimes exceeding 60,000 to 80,000 hours, whether the failure mode is ruthenium dissolution in PEM systems, nickel and iron leaching in alkaline devices or ionomer decomposition in AEM units. By catching degradation markers as they form rather than reconstructing them afterward, operando Raman spectroscopy promises to shift catalyst development from trial-and-error toward rational, data-driven engineering. If the vision of Raman-equipped smart electrolyzers materializes, the technique that lets chemists watch molecules vibrate could become the quality-control backbone of a global hydrogen economy.

Subject of Research: Operando Raman spectroscopy for monitoring catalyst degradation and structural dynamics in industrial green hydrogen water electrolyzers

Article Title: Operando Raman spectroscopy for exploring catalyst structural and interfacial dynamics in industrial green hydrogen electrolyzers

Article References: Rajani, N., Bhakhar, S. A., Pataniya, P. M., & Sumesh, C. K. (2026). Operando Raman spectroscopy for exploring catalyst structural and interfacial dynamics in industrial green hydrogen electrolyzers. Discover Electrochemistry, 3(1), Article 55. https://doi.org/10.1007/s44373-026-00142-1

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00142-1

Keywords: green hydrogen, water electrolysis, Raman spectroscopy, operando characterization, electrocatalysis, catalyst degradation, PEM electrolyzer, alkaline electrolyzer, AEM electrolyzer, oxygen evolution reaction, hydrogen evolution reaction, surface reconstruction

Nickel occupies a privileged position among non-precious metals for the hydrogen evolution reaction, the cathodic half-reaction that produces molecular hydrogen from water. Its electronic structure places it near the top of the volcano plot relating exchange current density to hydrogen binding energy, meaning it adsorbs hydrogen atoms with nearly ideal strength, close to the thermoneutral condition demanded by Sabatier’s principle. Nickel is also inexpensive and reasonably corrosion-resistant in both acidic and alkaline media. Previous work has shown that alloying nickel with cobalt, molybdenum, or iron can slash the overpotential required to drive meaningful current densities, but such multi-component catalysts typically demand complex synthesis routes that are difficult to integrate into continuous electrode manufacturing.

The Indian team chose a different path. They used a Watt’s bath, the workhorse formulation of industrial nickel electroplating, containing nickel sulfate, nickel chloride, and boric acid at a pH of 3.64. The substrate was titanium foil, chemically etched in a sulfuric-nitric acid mixture to strip away the native oxide layer and expose a rough, oxide-free metallic surface studded with step-edge nucleation sites. Titanium is an underutilized but compelling support: its stable native oxide resists corrosion across the full pH range, its rigidity allows precise geometric area definition for accurate benchmarking, and it is already the material of choice for bipolar plates and current collectors in proton exchange membrane electrolysers. By contrast, the nickel foam, stainless steel, and carbon cloth substrates common in the literature each suffer from oxidation, passivation, or poor adhesion under operating conditions.

The optimization proceeded in three stages. First, the team varied the deposition current density from minus one to minus fifty milliamperes per square centimeter at a fixed sixty-second deposition time. Scanning electron microscopy with energy-dispersive X-ray spectroscopy revealed that nickel content climbed monotonically with current density, reaching roughly ninety-six weight percent at the highest current. But loading alone proved a poor predictor of performance. At the lowest current, deposits were discontinuous islands with large gaps and poor electrical connectivity. At minus ten milliamperes per square centimeter, a kinetically balanced regime emerged in which nucleation, growth, and competing hydrogen evolution at the cathode proceeded at comparable rates, producing compact, fine-grained particles threaded with nanoscale voids. These voids, far from being defects, are templates left behind by hydrogen bubbles that nucleated on the growing surface and later detached.

The high-current electrodes told a cautionary tale. Despite their heavy nickel loading, coatings deposited at minus twenty-five and minus fifty milliamperes per square centimeter chipped, flaked, and delaminated during hydrogen evolution testing, a failure the authors attribute to compressive stress from lattice mismatch, trapped hydrogen, and supersaturated growth. The lesson is counterintuitive but practically important: maximizing catalyst loading is not a viable strategy, and the optimal compromise sits at moderate current density, where the electrode achieved an overpotential of 148 millivolts at ten milliamperes per square centimeter in sulfuric acid.

The second stage held the total deposited charge constant at 0.6 coulombs per square centimeter while varying deposition time from thirty to one hundred eighty seconds, thereby decoupling morphology from loading. The deposits followed a Volmer-Weber three-dimensional island growth mode, driven by the large lattice mismatch and weak adhesion between nickel and titanium. Short depositions produced a burst of tiny nuclei around 160 nanometers in a near-continuous film with few open voids. At one hundred twenty seconds, the growth-dominated regime took over, particles coarsened to about 307 nanometers, and the film achieved the sweet spot: full electrical connectivity with a preserved network of interparticle voids for electrolyte access. The overpotential dropped to 143 millivolts, a 56-millivolt improvement over the thirty-second electrode, while extending to one hundred eighty seconds brought no further benefit as voids began to close.

The third and most striking stage replaced continuous current with pulsed galvanostatic deposition. The total on-time of one hundred twenty seconds was split into one, four, eight, or twelve equal pulses separated by thirty-second open-circuit rests. During each rest interval, two things happen: nickel ions depleted at the cathode diffuse back from the bulk, resetting the concentration before the next pulse, and adherent hydrogen bubbles detach from the surface, eliminating the large voids and discontinuities they would otherwise imprint. Single-pulse deposition yielded a thick, featureless film. Eight pulses produced the morphological optimum, with uniformly distributed particles, the narrowest size distribution, and a regular, well-connected void network. Twelve pulses tipped into coalescence, as overly short pulses allowed adjacent nuclei to merge before the next interruption. X-ray diffraction confirmed the deposits were purely metallic nickel, with a contracted lattice parameter of 3.494 angstroms versus the bulk value of 3.524, evidence of in-plane compressive stress inherited from the nickel-titanium interface.

Electrochemical impedance spectroscopy quantified the kinetic gains. Bare titanium is essentially inert toward hydrogen evolution, with a charge-transfer resistance near 9.08 times ten to the fourth ohms per square centimeter; even a suboptimal nickel film dropped that figure by more than an order of magnitude. Interestingly, the electrode with the largest electrochemically active surface area, the four-pulse sample, was not the best catalyst. Its fine, well-separated particles created interparticle electrical resistance. The eight-pulse electrode, with better-connected particles and lower charge-transfer resistance, struck the superior balance between site accessibility and conductivity, a reminder that surface area alone does not determine electrocatalytic performance.

Across all three electrolytes, the optimized electrode outperformed bare titanium. In 0.5 molar sulfuric acid it delivered a Tafel slope of 149 plus or minus 3 millivolts per decade, and in 1 molar potassium hydroxide a nearly identical 151 plus or minus 4, values consistent with a Volmer-Heyrovsky mechanism in which the desorption step is partially rate-limiting. The near-identical acid and alkaline slopes suggest a common dominant rate-determining step across both media. In neutral phosphate-buffered saline, kinetics deteriorated sharply, with a Tafel slope of 329 plus or minus 18 millivolts per decade reflecting mass-transport limitations, low proton activity, high solution resistance, and competitive phosphate adsorption, yet the nickel coating still beat bare titanium. Stability tests showed negligible drift in acid over three-hour holds at current densities up to minus one hundred milliamperes per square centimeter, with the potential actually improving by about sixty millivolts at the highest current, indicating in-situ activation rather than degradation. Post-operation X-ray diffraction revealed titanium hydride formation in the substrate, evidence that atomic hydrogen diffuses into the titanium under sustained cathodic polarization, but no catalytically deleterious nickel hydroxide or hydride phases.

The most consequential benchmark came against a commercial nickel mesh. Run side by side at minus fifty milliamperes per square centimeter in sulfuric acid for eighteen hours, the pulse-deposited planar electrode initially lagged slightly behind the three-dimensional mesh, whose hierarchical porosity offers greater active area and faster bubble release. But after an activation period of wetting and surface restructuring, it stabilized at a potential comparable to the commercial product, with neither electrode showing measurable degradation. For the authors, this validates the central thesis: catalytic performance is limited not by the quantity of nickel but by the quality and accessibility of the active surface. A simple Watt’s bath, an eight-pulse protocol, and a planar titanium foil, all processed at room temperature with earth-abundant materials, can rival a commercial electrode. As emerging electrolyser architectures, from seawater splitters to decoupled water-splitting devices, demand catalysts that survive near-neutral conditions, careful process engineering of the deposition step may prove as important as catalyst chemistry itself.

Subject of Research: Pulse electrodeposition of nickel coatings on titanium for pH-universal hydrogen evolution electrocatalysis

Article Title: Pulse electrodeposition of Ni on Ti substrates for hydrogen evolution across acidic, alkaline, and neutral electrolytes

Article References: Kandwal, A., Choudhary, V., Tyagi, A., Konathala, S. K., Prakashaiah, B. G., Sinha, A. K., & Dhiman, M. (2026). Pulse electrodeposition of Ni on Ti substrates for hydrogen evolution across acidic, alkaline, and neutral electrolytes. Discover Electrochemistry, 3(1), Article 61. https://doi.org/10.1007/s44373-026-00149-8

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00149-8

Keywords: hydrogen evolution reaction, nickel electrodeposition, pulsed deposition, titanium substrate, electrocatalysis, green hydrogen, water electrolysis, Tafel analysis, nanostructured electrodes, electrochemical impedance spectroscopy, earth-abundant catalysts, Watt's bath

The team, led by Eric M. Garcia at the Federal University of São João del-Rei, focused on the oxygen evolution reaction, or OER, the half of water electrolysis that has long been the technology’s Achilles heel. While hydrogen gas bubbles off the cathode with relative ease, the anode must tear four electrons away from two water molecules to release one molecule of oxygen, and that multi-step process is kinetically sluggish. Electrolyzers must therefore push the anode several hundred millivolts beyond the thermodynamic requirement, an excess called overpotential that is paid for directly in electricity. Because roughly a million tons of lithium-ion batteries are expected to reach end-of-life by 2030, the researchers saw an opportunity to attack two problems at once: recovering cobalt from a growing mountain of electronic waste and using it to lower the cost of green hydrogen production.

The recycling chemistry begins with a battery dismantled by hand, its cathode material gently scraped from the aluminum foil current collector. After a thermal treatment at 400 degrees Celsius and a thorough washing to remove residual electrolyte, 5.3 grams of recovered active material were leached in 3.0 molar sulfuric acid at 100 degrees Celsius in the presence of 1.0 molar hydrogen peroxide. The peroxide plays a crucial chemical role: cobalt in lithium cobalt oxide sits in the +3 oxidation state, which dissolves poorly, but the reducing agent converts it to the far more soluble Co2+ species. Atomic absorption spectroscopy of the resulting leachate showed a cobalt concentration of 0.054 moles per liter, corresponding to about 3.18 grams of cobalt recovered from a theoretical content of roughly 3.19 grams, an apparent leaching efficiency of approximately 99.7 percent.

With the cobalt in solution, the researchers then raised the pH to 10 by slowly adding potassium hydroxide, precipitating cobalt hydroxide with a gravimetrically determined yield of about 98 percent. X-ray diffraction identified the product as predominantly the hexagonal brucite-like beta phase of Co(OH)2, in which charge-neutral layers of octahedrally coordinated Co2+ ions are stacked through hydrogen bonding. Applying the Scherrer equation to five diffraction peaks gave apparent crystallite sizes between roughly 5 and 10 nanometers, averaging 7.5 nanometers, a small domain size that typically favors catalytic activity by maximizing exposed surface. Infrared spectroscopy added nuance: broad hydroxyl stretching bands and the absence of the sharp 3645 per centimeter feature characteristic of well-ordered beta-phase material pointed to local structural disorder and hydration, likely influenced by residual sulfate species carried over from the acid leaching step, which energy-dispersive X-ray spectroscopy confirmed as trace sulfur alongside potassium from the precipitant.

Rather than coating this powder onto a conventional substrate, the team pressed it directly into the body of an electrode. Graphite powder was mixed with the recycled cobalt hydroxide at loadings ranging from 5 to 80 percent by weight, dispersed in ethanol, ultrasonicated for two hours and dried. The powder was then blended into molten paraffin wax at 80 degrees Celsius and compacted under ten tons of uniaxial pressure into a self-supported cylindrical pellet of two square centimeters. The paraffin serves as a hydrophobic binder that gives the pellet mechanical cohesion, while the graphite provides electrical percolation throughout the bulk. The approach deliberately avoids polymer binders such as Nafion and skips film-deposition steps entirely, meaning the catalyst is distributed through the electrode volume rather than confined to a thin surface layer.

Cyclic voltammetry screening revealed a clear optimum at 40 percent cobalt hydroxide, where the current density at 0.6 volts versus Ag/AgCl peaked before declining at higher loadings. The researchers attribute this maximum to a trade-off between catalytic content and conductivity: too much insulating hydroxide disrupts the graphite network through which electrons must travel, and the hydrophobic paraffin matrix already partially restricts electrolyte penetration to particles buried deep in the composite. The voltammograms also displayed a distinctive anodic process beginning near 0.2 volts, matched by a cathodic peak on the reverse scan, corresponding to the reversible oxidation of Co(OH)2 to cobalt oxyhydroxide, CoOOH. This transformation is widely regarded as the activation step in cobalt-based oxygen evolution catalysis, because the oxyhydroxide phase is more conductive and hosts the active sites where hydroxide ions are adsorbed and oxidized.

Quantitatively, the optimized electrode delivered a current density of 10 milliamperes per square centimeter at an overpotential of approximately 310 millivolts in 1.0 molar potassium hydroxide, with a Tafel slope of 75 millivolts per decade. Both figures compare favorably with literature benchmarks: the overpotential is lower than values reported for nickel phosphide on glassy carbon at 390 millivolts, carbon-supported NiCo nanospheres at 330 millivolts, and cobalt hydroxide on glassy carbon electrodes at 414 millivolts, though it does not match the 236 millivolts achieved by a sophisticated NiFe alloy nanotube catalyst. For an electrode whose active ingredient was pulled from a dead mobile phone and bound in candle wax, the performance is striking. The researchers also measured a reaction order of 0.87 with respect to hydroxide ions by varying the KOH concentration from 0.1 to 6.0 molar, a near-unity value indicating that hydroxide adsorption participates directly in the rate-determining step, consistent with the classical Bockris-Otagawa mechanism in which an adsorbed hydroxyl intermediate reacts with a second hydroxide ion to form a peroxide-like species before oxygen release.

Interface diagnostics reinforced the picture. Double-layer capacitance, a proxy for the electrochemically accessible surface area, rose from 1.69 to 14.2 microfarads per square centimeter upon incorporating the recycled hydroxide, an 8.4-fold increase. Polarization resistance, measured near the equilibrium potential, fell from about 11.4 to 3.15 kiloohm square centimeters, a 72 percent reduction. Most importantly for any practical device, the electrode held a current density close to 10 milliamperes per square centimeter for 180 hours of continuous operation at the 310 millivolt overpotential, with a slight upward drift in current that the authors interpret as progressive electrochemical activation of the cobalt surface under anodic polarization.

The authors are careful about the limits of their evidence. They did not measure Faradaic efficiency for oxygen, perform post-mortem microscopy or spectroscopy, or run electrochemical impedance spectroscopy, so the long-term test demonstrates operational stability of the current rather than proving that the electrode’s structure survived intact. They likewise note that definitive identification of the CoOOH-like active phase would require operando spectroscopy, since cobalt surfaces are known to reconstruct substantially under oxygen-evolving conditions, changing oxidation state and coordination as they work. Trace impurities below the detection limits of their analytical methods also cannot be excluded.

Even with those caveats, the study offers a compelling template for a circular hydrogen economy. It demonstrates that a battery destined for the shredder can pass through a single acid leach and a single precipitation step and emerge as a competitive water-oxidation anode, without phase engineering, nanostructure synthesis or noble metals. As electrolyzer deployments scale alongside renewable power, and as the first great wave of lithium-ion batteries reaches retirement, routes like this one suggest that yesterday’s phones could help supply the electrodes that split tomorrow’s water.

Subject of Research: Recycling cobalt from spent lithium-ion battery cathodes into low-cost electrodes for the oxygen evolution reaction in alkaline water electrolysis

Article Title: Recycled Co(OH)₂ from spent LiCoO₂ cathodes as a low-cost graphite-based composite electrode for the oxygen evolution reaction

Article References: Garcia, E. M., Taroco, H. A., Melo, J. O. F., & Taroco, C. G. (2026). Recycled Co(OH)₂ from spent LiCoO₂ cathodes as a low-cost graphite-based composite electrode for the oxygen evolution reaction. Ionics. https://doi.org/10.1007/s11581-026-07540-w

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07540-w

Keywords: lithium-ion battery recycling, cobalt hydroxide, oxygen evolution reaction, electrocatalysis, water electrolysis, green hydrogen, LiCoO2 cathode, graphite composite electrode, cobalt recovery, Tafel analysis, alkaline medium, circular economy

The core of the discovery is a structural detail measured at the scale of a single bond. By engineering molybdenum and nickel atoms into asymmetric pairs separated by just 2.85 angstroms, the researchers created an active site geometry that changes the fundamental pathway of the oxygen evolution reaction. In the traditional adsorbate evolution mechanism, oxygen–oxygen bonds form through a sequence of adsorbed intermediates whose energetics are locked together by so-called scaling relations, meaning that improving one step inevitably worsens another. The shortened Mo–Ni distance instead enables an oxide path mechanism with direct O–O coupling, in which the oxygen–oxygen bond forms without passing through the full ladder of adsorbed intermediates. Because this route bypasses the scaling relations entirely, the catalyst escapes the theoretical ceiling that has constrained activity and stability trade-offs for decades.

Density functional theory simulations, performed by a team at Wuhan University led by Xiang-Kui Gu, provided the mechanistic blueprint before the material was fully characterized. The calculations showed that the asymmetric pairing of a high-valence molybdenum atom with a nickel neighbour redistributes electronic density across the active site, lowering the energy barrier for direct O–O coupling while keeping the nickel centre from over-oxidizing and dissolving. This dual role matters because catalyst degradation at high current densities usually stems from exactly that failure mode: nickel oxyhydroxide phases can restructure, dissolve, or passivate when pushed to the potentials required for industrial operation. The molybdenum partner acts as an electronic stabilizer, absorbing and buffering charge so the nickel site can cycle through oxidation states without collapsing structurally.

Experimental characterization confirmed the predicted geometry. X-ray absorption spectroscopy, electron microscopy, and complementary structural probes verified that the molybdenum atoms sit in the intended coordination environment adjacent to nickel, with the 2.85 angstrom interatomic distance intact after synthesis. The catalyst was fabricated as an electrode material rather than a powder that would need binding to a substrate, an engineering choice that matters for real electrolysers where catalyst layers must withstand gas evolution, bubble formation, and mechanical stress at current densities far beyond what laboratory half-cell tests typically explore. The fabrication and characterization work was carried out at Shanghai Jiao Tong University under the supervision of Yancai Yao and Lizhi Zhang, with Xingyue Zou and Jinzhe Zhang contributing equally as lead authors.

The performance numbers are what set this catalyst apart from the crowded field of oxygen evolution materials. In laboratory-scale testing, the anode delivered exceptional durability for over 2,000 hours at 3 amperes per square centimetre, a current density that corresponds to the operating regime of commercial alkaline and anion-exchange membrane electrolysers rather than the gentle conditions of academic benchmarking. Most reported catalysts are evaluated at 10 milliamperes per square centimetre, three hundred times lower, where degradation mechanisms barely have time to manifest. Sustaining stable oxygen evolution at industrial current density for nearly three months of continuous operation is a stress test that very few non-precious-metal catalysts have passed, and it directly addresses the gap between laboratory records and electrolyser requirements.

The researchers then integrated the catalyst into a laboratory-scale anion-exchange membrane water electrolyser, a device architecture that combines some of the advantages of proton-exchange membrane systems with the cheaper, non-precious-metal chemistry possible in alkaline conditions. The anode produced low cell voltages of 1.78 to 1.95 volts at current densities of 2 to 3 amperes per square centimetre, alongside more than 1,200 hours of stable operation at the upper end of that range. Cell voltage is the single most important determinant of electrolyser energy efficiency, because every additional tenth of a volt translates directly into extra kilowatt-hours per kilogram of hydrogen. Voltages in this range at such high current densities represent a competitive operating point against commercial benchmarks that typically rely on more expensive electrode materials.

To prove commercial viability, the team scaled the technology into a 71 square centimetre per cell anion-exchange membrane electrolyser stack, developed with industrial partners including Headway Technology Group in Qingdao. The stack produced hydrogen with purity exceeding 99 percent while cutting energy consumption and hydrogen production costs by 10 to 17 percent compared with commercial benchmarks. Those percentages may sound incremental, but in the economics of hydrogen production, where electricity is the dominant cost and margins are thin, a double-digit reduction in energy demand per kilogram of output is the difference between green hydrogen being subsidized and being competitive. The researchers also conducted a technoeconomic analysis alongside the electrochemical testing to quantify the cost implications, and the results suggested the catalyst’s nickel-and-molybdenum composition avoids the price volatility of iridium and ruthenium that burdens acidic electrolyser anodes.

Perhaps the most consequential step is the one most often skipped in catalyst papers: manufacturing. The team established a mass-production line for the oxygen evolution reaction electrodes, demonstrating that the synthesis route can be executed at industrial scale rather than only on small laboratory substrates. Scalability has quietly become the graveyard of electrocatalysis research, with countless high-performing materials depending on synthesis conditions, precursor costs, or deposition techniques that cannot be translated to square-metre electrode areas. By proving scalable manufacturing viability in parallel with performance, the researchers addressed the full chain from atomistic design to deployable hardware, which is what distinguishes an engineering advance from a laboratory curiosity.

The scientific significance extends beyond this single material. The oxide path mechanism with direct O–O coupling, enabled by a precisely tuned interatomic distance, offers a design principle that other researchers can pursue: instead of searching for adsorbate intermediates with perfect energetics, engineer the active site geometry so the rate-limiting bond forms directly between two oxygen species. Related strategies have shown promise in acidic water oxidation on ruthenium-based systems, but demonstrating the principle on an abundant, inexpensive nickel–molybdenum platform in alkaline conditions, at industrial current densities, and in a working electrolyser stack substantially broadens its practical reach. It suggests that the scaling relations, long treated as an iron law of oxygen evolution catalysis, can be sidestepped through structural design rather than elemental substitution.

Challenges remain before this catalyst reshapes the hydrogen economy. The reported demonstrations, while impressive, involve a 71 square centimetre cell in a stack, whereas commercial electrolysers operate with much larger electrode areas and thousands of hours of field exposure across variable renewable power inputs. Long-term behaviour under fluctuating loads, impurity tolerance in real water feeds, and the economics of integrating the electrodes into complete electrolyser systems at gigawatt scale all require further validation. Still, the combination of 2,000-hour stability at 3 amperes per square centimetre, low cell voltages in a membrane electrolyser, double-digit cost reductions in a commercial-scale stack, and an operating production line represents one of the most complete demonstrations of an oxygen evolution catalyst to date. As governments and industry commit hundreds of billions of dollars to hydrogen infrastructure, advances that attack the anodic bottleneck with cheap, durable, scalable materials are exactly the kind of unglamorous chemistry on which the zero-emission fuel future will ultimately depend.

Subject of Research: Asymmetric Mo–Ni atom pair electrocatalysts for durable oxygen evolution in industrial alkaline water electrolysis

Article Title: Asymmetric Mo–Ni atom pairs enable scalable water oxidation at industrial current densities

Article References: Zou, X., Zhang, J., Xie, J., Dong, H., Zhou, B., Dai, J., Zhao, L., Wang, J., Zhan, G., Lian, W., Cao, X., Liu, B., Gu, X.-K., Yao, Y., & Zhang, L. (2026). Asymmetric Mo–Ni atom pairs enable scalable water oxidation at industrial current densities. Nature Sustainability. https://doi.org/10.1038/s41893-026-01942-4

Image Credits: AI Generated

DOI: 10.1038/s41893-026-01942-4

Keywords: green hydrogen, water electrolysis, oxygen evolution reaction, electrocatalysis, nickel catalyst, molybdenum, anion-exchange membrane, oxide path mechanism, industrial current density, catalyst durability, Nature Sustainability, energy transition

The core insight concerns a mechanism that electrochemists have courted for a decade: lattice-oxygen redox. In conventional metal oxide and oxyhydroxide catalysts, oxygen atoms in the crystal framework sit passively as spectators; every bond made and broken during water oxidation happens at adsorbed intermediates such as *OH and *O. That is safe but slow, because the adsorption energies of these intermediates are locked together by rigid scaling relationships. Make *OH binding stronger to speed up one step, and *O binding strengthens too, penalizing another. The scaling relations impose a theoretical floor on the overpotential, a tax every conventional catalyst must pay. Lattice-oxygen redox breaks that tax. When framework oxygen atoms themselves participate in the redox chemistry, oxygen-oxygen bonds can form through direct coupling of lattice oxygen, sidestepping the adsorbate scaling conundrum entirely.

The catch is stability, and it has been brutal. Oxidizing lattice oxygen means ripping electrons from the very orbitals that hold the crystal together. Once activated indiscriminately, that oxidation delocalizes across the lattice, holes accumulate on framework oxygen, metal-oxygen bonds soften and break, and the material dissolves or amorphizes. Many catalysts that unlock lattice oxygen in the laboratory die within hours under the punishing polarization of a real electrolyzer. The literature is littered with tantalizing activity reports that collapse under accelerated ageing. The Singapore-led team, whose senior authors include Junmin Xue, Haoyin Zhong, Bin Tian and Xiaopeng Wang, asked a different question: what if lattice oxygen could be activated not all at once, but in a spatially ordered, electronically programmed sequence?

Their answer begins with an unlikely precursor: a composite of chromium(III) oxide and nickel diselenide, Cr2O3/NiSe2. When the electrode is held under anodic polarization in alkaline electrolyte, the reconstruction unfolds in two chemically distinct stages, and the order matters enormously. In the first stage, selenium leaches out of the NiSe2 phase. Losing the chalcogen opens up the nickel 3d band manifold, specifically broadening the antibonding eg band, the orbital set that overlaps with oxygen 2p states. A broader eg band strengthens nickel-oxygen covalency, and stronger covalency does two things at once: it accelerates the deprotonation of *OH intermediates, a known rate-limiting step on nickel oxyhydroxide surfaces, and it begins to tilt the electronic structure toward oxygen participation.

Then comes the second, decisive stage. Chromium begins to leach from the oxide component, and crucially, the chromium departure selectively depletes oxygen atoms that were coordinated to nickel across the composite interface. What remains is a modified nickel oxyhydroxide, SRA-NiOOH, in which oxygen non-bonding states, oxygen 2p orbitals stripped of their bonding partners, are generated not randomly across the lattice but in spatially confined pockets. This is the heart of the strategy. Confined non-bonding states act as local redox reservoirs for lattice oxygen, close enough to the surface to join the catalytic cycle, yet electronically quarantined so that oxidation holes cannot wander freely through the framework. Destructive redox delocalization, the killer mechanism of lattice-oxygen catalysts, is suppressed by construction. The oxygen-redox activity and the lattice stability, usually two ends of a seesaw, are decoupled.

The experimental evidence for this picture is layered across multiple structural and spectroscopic probes. X-ray absorption spectroscopy traces the changes in nickel coordination and oxidation state as reconstruction proceeds, while synchrotron-based measurements carried out at the Singapore Synchrotron Light Source document the emergence of the modified oxyhydroxide phase. Density functional theory calculations, performed with the ASTAR Computational Resource Centre and the National Supercomputing Centre in Singapore, complement the experiments, showing how the broadened eg* band and the confined oxygen non-bonding states reshape the energy landscape of the oxygen evolution mechanism. The team further demonstrates that the approach generalizes: the sequential-reconstruction activation paradigm is not a one-off trick of one composition but a design framework that can be exported to related systems, a claim their fourth figure addresses directly.

The device-level performance is where the work will command the most attention. Anion-exchange-membrane water electrolyzers are the commercial sweet spot for many of these nickel-based catalysts, because AEM technology can use earth-abundant transition metals instead of the iridium that proton-exchange-membrane electrolyzers demand. But AEM electrolyzers still need to run at high current density, above 2 amperes per square centimeter, to reach the cost targets that would make green hydrogen competitive with fossil-derived hydrogen. At 4.1 A cm-2 at 1.8 V in 1 M KOH at 80 degrees Celsius, the SRA-NiOOH anode sits squarely in industrially relevant territory, and the nearly 3,500-hour stability test under accelerated ageing at 60 degrees Celsius suggests the catalyst does not merely sprint; it runs a marathon. Sustained oxygen-redox operation at high current density, the authors argue, is precisely what the sequential electronic reconstruction buys: a durable oxygen-redox window that stays open under demanding polarization.

The conceptual contribution may outlast the specific numbers. For years, the field has treated catalyst reconstruction as something to minimize, a necessary evil on the path to stable operation, or at best a one-shot activation. This work reframes reconstruction as something to choreograph. By choosing a precursor whose constituents leach in a controlled order, selenium first, chromium second, the researchers wrote the electronic structure of the final catalyst as a timed sequence rather than a static composition. Each leaching event performed a specific electronic function, band broadening here, oxygen non-bonding state generation there, and the final NiOOH inherited the benefits of both without the instability that an uncontrolled surface transformation would bring. It is a programming philosophy applied to surfaces, and it echoes a broader shift in electrocatalysis toward operando-informed precatalyst design, in which the working electrode is understood as a dynamic, self-assembling entity rather than a fixed material.

There are, of course, caveats and open questions. The reported metrics come from laboratory-scale AEM cells and accelerated ageing protocols, and the road from such benchmarks to kilowatt-scale stacks involves membrane durability, flow uniformity, and cost questions that no single anode catalyst can answer alone. The precise atomic-scale geography of the confined non-bonding states, and how far the confinement holds as reconstruction continues over thousands of hours, will reward further study. But the bar the paper clears is a meaningful one: it shows that the activity-stability trade-off, long treated as a law of nature for lattice-oxygen catalysis, can be suspended by electronic design. If sequential reconstruction proves as general as the authors suggest, the oxygen evolution bottleneck that has constrained green hydrogen may finally have an engineering-grade exit, built not from a miracle material but from a catalyst taught to rebuild itself in the right order.

Subject of Research: Sequential electronic reconstruction of a Cr2O3/NiSe2 precursor to enable durable lattice-oxygen redox in anion-exchange-membrane water electrolysis

Article Title: Sequential electronic reconstruction enables durable oxygen redox in water electrolysis

Article References: Yu, J., Zhong, H., Zhang, Q., Zhang, X., Ye, Y., Diao, C., Yu, Z. G., Xi, S., Tian, B., Wang, X., & Xue, J. (2026). Sequential electronic reconstruction enables durable oxygen redox in water electrolysis. Nature Sustainability. https://doi.org/10.1038/s41893-026-01940-6

Image Credits: AI Generated

DOI: 10.1038/s41893-026-01940-6

Keywords: water electrolysis, oxygen evolution reaction, lattice oxygen redox, nickel oxyhydroxide, green hydrogen, anion-exchange membrane, electrocatalysis, catalyst reconstruction, eg band, catalyst stability, nickel selenide, Nature Sustainability

The central insight of the study concerns how adjacent RuO6 octahedra, the basic building blocks of ruthenium dioxide, are connected to one another. Conventional rutile-type RuO2, the workhorse acidic OER catalyst, arranges its octahedra in corner-sharing and edge-sharing motifs that do not provide the most favorable electronic geometry. In the newly synthesized 1T phase, the octahedra share edges in a configuration that aligns the ruthenium 4d orbitals in parallel across neighboring clusters. This parallel alignment opens a highway for intersite electron transport: electrons generated during the four-proton, four-electron OER sequence can hop efficiently from one ruthenium center to the next rather than becoming localized at a single active site. The result is a catalyst in which electronic communication, rather than isolated site chemistry, becomes the rate-enhancing factor.

The performance numbers reported by the team are striking by any benchmark in the field. In acidic electrolyte, the 1T-RuO2 single layer requires an overpotential of just 77 millivolts to drive a current density of 10 milliamperes per square centimeter, a figure that places it among the most active acidic OER catalysts described to date. At a potential of 1.50 volts versus the reversible hydrogen electrode, the material achieves a mass activity of 3,743.43 amperes per gram of ruthenium and a turnover frequency of 23.99 per second, both substantially exceeding the corresponding values for rutile-RuO2 measured under the same experimental framework. These metrics matter because ruthenium, while cheaper than the iridium that currently dominates commercial PEM anodes, is still a precious metal; extracting more catalytic turnover per gram directly reduces the loading, and therefore the cost, of the precious metal in each device.

Activity alone has never been the hard part of acidic water oxidation. The acidic OER environment is brutally corrosive: highly oxidizing potentials, a flood of protons and reactive oxygen intermediates combine to dissolve most transition-metal oxides within hours. Ruthenium dioxide is comparatively robust, yet even it tends to degrade through ruthenium dissolution and lattice-oxygen-mediated pathways that corrode the catalyst from within. The durability demonstrated by the 1T single layer is therefore arguably the most consequential result of the study. When integrated into a full proton-exchange membrane water electrolyser, the catalyst sustained a current density of approximately 2.9 amperes per square centimeter at a cell voltage of 1.70 volts for more than 1,100 hours of continuous operation. Industrial electrolysers typically operate at current densities in the range of one to a few amperes per square centimeter, so the demonstration sits squarely in the regime that matters for real-world deployment.

Reaching that level of performance required first solving a synthesis problem that has long frustrated materials chemists. The 1T phase of RuO2 is metastable, meaning it is not the thermodynamically favored arrangement of ruthenium and oxygen atoms; left to its own devices, the material prefers to settle into the rutile structure. Metastable phases in two-dimensional materials, from the 1T polymorphs of molybdenum disulfide to exotic iridium oxide phases, often carry enhanced catalytic properties precisely because their unusual geometries reshape the electronic structure, but stabilizing them at scale is notoriously difficult. The Soochow-led team designed and fabricated a mechanothermal reactor, developed by co-author Mingwang Shao, to drive the formation of the single-layer 1T phase, and confirmed the resulting structure through a battery of characterization techniques including synchrotron-based X-ray absorption spectroscopy performed at facilities in Taiwan and Hefei.

The spectroscopic evidence was central to establishing why the edge-sharing geometry works. Using in situ ruthenium L3-edge X-ray absorption spectroscopy, together with total-electron-yield measurements at other absorption edges, the researchers probed the electronic and geometric state of the catalyst while it was actively evolving oxygen. Complementary density functional theory calculations, carried out by Zhiwei Hu, Jing Zhou, Chang-Yang Kuo and colleagues, rationalized the observations: the edge-sharing motif aligns the ruthenium 4d orbitals across adjacent octahedra, lowering the barriers for electron delocalization and stabilizing the reaction intermediates along the OER pathway. Attenuated-total-reflectance infrared measurements tracked the surface intermediates during operation, allowing the team to connect the macroscopic electrochemical performance to specific molecular events at the active sites.

The broader significance of the work lies in its reframing of catalyst design for acidic water oxidation. Much of the recent effort in the field has focused on compositional strategies: doping ruthenium oxide with rhenium, rhodium or tantalum, creating high-entropy oxides, or stabilizing iridium in perovskite-derived frameworks. Those approaches tune the electronic structure of essentially rutile-like lattices. The new study demonstrates that the connectivity of the octahedral network itself, an architectural variable rather than a compositional one, can be the decisive lever. By choosing a phase in which edge-sharing octahedra create parallel orbital alignment, the researchers achieved both the activity and the stability that compositional tuning has struggled to deliver simultaneously. This suggests a rich design space of metastable oxide phases, particularly in two-dimensional form, that the community has only begun to explore.

There are, of course, caveats and next steps. The 1T phase is metastable, and while the electrolyser test exceeded 1,100 hours, commercial PEM systems are expected to operate for tens of thousands of hours; long-term phase stability under thermal and potential cycling remains to be proven. Scaling the mechanothermal synthesis from laboratory batches to the kilogram quantities needed for stack manufacturing will require engineering development. And the precise balance between adsorbate-evolution and lattice-oxygen mechanisms on the 1T surface, a question the operando spectroscopy begins to answer, will continue to be refined as more theoretical and experimental data accumulate. Nevertheless, the combination of a 77-millivolt overpotential, a turnover frequency approaching 24 per second and sustained industrial-scale current density in a working electrolyser represents a level of integrated performance that few acidic OER catalysts have approached.

If the durability and manufacturability hurdles can be cleared, the implications for the hydrogen economy are substantial. PEM electrolysers are prized for their fast response and high output pressure, making them natural partners for intermittent renewable electricity, but their reliance on iridium, one of the scarcest elements on Earth, has been viewed as a hard ceiling on global deployment. A ruthenium-based anode catalyst that is both more active and more stable than rutile RuO2, and that has already demonstrated more than a thousand hours of operation at 2.9 amperes per square centimeter, offers a credible path toward easing that constraint. The edge-sharing single layer of ruthenium dioxide may thus be remembered less as a single catalyst and more as a proof of principle: that in the search for efficient acidic water splitting, the way atoms are wired together can matter as much as which atoms are chosen.

Subject of Research: Edge-sharing single-layer 1T-RuO2 electrocatalysts for acidic oxygen evolution in proton-exchange membrane water electrolysis

Article Title: Edge-sharing RuO2 single layer for stable and low overpotential acidic water electrolysis

Article References: Zhu, W., Zhou, J., Ma, M., Liu, H., Liao, F., Huang, H., Kuo, C.-Y., Lin, Y., Pao, C.-W., Chang, Y.-C., Haw, S.-C., Hsu, S.-Y., Chen, J.-M., Ni, M., Liu, Y., Shao, M., Hu, Z., Kang, Z., Huang, X., & Shao, Q. (2026). Edge-sharing RuO2 single layer for stable and low overpotential acidic water electrolysis. Nature Nanotechnology. https://doi.org/10.1038/s41565-026-02255-5

Image Credits: AI Generated

DOI: 10.1038/s41565-026-02255-5

Keywords: ruthenium dioxide, water electrolysis, oxygen evolution reaction, green hydrogen, proton-exchange membrane, electrocatalysis, two-dimensional materials, metastable phase, overpotential, PEM electrolyser, single-layer catalyst, acidic water oxidation