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 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 research team, led by Setia Budi and including Raudhatul Hadawiyah, Mokhamad Ali Rizqi Maulana, Suci Winarsih, Mohammad Hamzah Fauzi, and Muhammad Fathar Aulia, set out to tame a well-known nuisance in alloy plating called anomalous co-deposition. In NiCo electrodeposition, cobalt, the less noble metal, preferentially deposits over nickel even though nickel is thermodynamically easier to reduce. The culprit is a family of metal hydroxide intermediates that form at the cathode surface: as water is reduced, hydroxide ions accumulate locally and react with dissolved metal ions. Cobalt hydroxide intermediates adsorb more strongly than their nickel counterparts, suppressing nickel deposition and skewing the alloy composition away from the optimum for catalysis.

Saccharin turns out to be an elegant fix. The researchers prepared four NiCo films by potentiostatic electrodeposition at minus 1.5 volts onto flexible indium tin oxide-coated PET substrates, using an electrolyte of nickel sulphate, cobalt sulphate, and boric acid, with sodium saccharin added at concentrations of 0, 0.5, 1, and 2 grams per liter. Energy-dispersive X-ray analysis showed that as saccharin concentration rose, the nickel fraction of the deposits steadily increased, from Ni70Co30 in the additive-free film to Ni75Co25 at the highest loading. A composition ratio analysis confirmed that cobalt’s preferential deposition weakened progressively with saccharin, evidence that the additive suppresses the anomalous co-deposition that has long complicated NiCo plating.

The mechanism, the authors propose, hinges on saccharinate ions adsorbing onto the cathode surface. This adsorption suppresses the competing hydrogen evolution that occurs during plating, stabilizes the local interfacial pH, and reduces the formation of the oxide and hydroxide species from which cobalt hydroxide intermediates arise. With fewer of these intermediates, cobalt loses its unfair advantage and nickel deposition becomes relatively more favorable. Saccharin also forms metal-saccharinate complexes in the electrolyte and blocks active growth sites, further modulating deposition kinetics and nucleation behavior. The team notes that this mechanism is proposed on the basis of their results and prior literature, and that future in-situ local pH monitoring would be needed for direct validation.

X-ray diffraction revealed a second, equally important effect: grain refinement. All films showed the face-centered cubic NiCo alloy phase with no impurity peaks, but the crystallite size shrank steadily from 18.69 nanometers without saccharin to 14.18 nanometers at 2 grams per liter. At the same time, microstrain and dislocation density rose with additive concentration, indicating an increasingly defect-rich crystal structure packed with dislocations, lattice distortions, and vacancies. In electrocatalysis, such defects are not flaws but assets: low-coordination surface atoms at dislocations and strain fields act as additional active sites, and lattice distortion can tune local electronic structure to optimize the binding of hydrogen intermediates and lower charge-transfer barriers.

Field-emission scanning electron microscopy made the refinement visible to the eye. The additive-free film displayed large, compact, agglomerated spherical grains, while increasing saccharin concentrations produced progressively smaller, more dispersed, and more homogeneous grains. Raman spectroscopy added a further nuance: all films showed a dominant band near 538 wavenumbers from NiO and CoO stretching vibrations, confirming that the alloy surfaces are partially oxidized under ambient conditions, and the highest-saccharin film exhibited the strongest, best-defined bands, indicating a more ordered surface oxide layer. The key point, the authors stress, is that saccharin changes microstructure and morphology without altering the fundamental crystal phase of the deposit.

The payoffs appeared clearly in electrochemical testing in 0.5 molar potassium hydroxide. The Ni75Co25 film, grown with 2 grams per liter of saccharin, required an overpotential of just 253 millivolts to drive a current density of 10 milliamperes per square centimeter, the lowest of all samples, and delivered a Tafel slope of 70 millivolts per decade. That value falls within the range associated with a Volmer-Heyrovsky mechanism, in which water is first electrochemically dissociated into adsorbed hydrogen and hydroxide, and the adsorbed hydrogen is then desorbed electrochemically as hydrogen gas. The Tafel slopes fell monotonically from 163 to 70 millivolts per decade as nickel content rose, tracing steadily improving reaction kinetics across the series.

Kinetic analysis reinforced the picture. The Ni75Co25 catalyst showed the highest charge-transfer coefficient, 0.84, and the highest exchange current density, 0.898 milliamperes per square centimeter, signaling the most efficient electron transfer at the electrode-electrolyte interface and the fastest intrinsic reaction rate at equilibrium. Electrochemical impedance spectroscopy told a consistent story: charge-transfer resistance dropped from 29 ohms for the additive-free film to 8.5 ohms for Ni75Co25, while solution resistance fell from 13.0 to 7.8 ohms, reflecting better electrode-electrolyte contact and faster charge transport throughout.

Surface-area measurements explained why the refined film performs so well geometrically. The double-layer capacitance of Ni75Co25 was roughly double that of the additive-free film, yielding the largest electrochemically active surface area, 1.79 by 10 to the minus 2 square centimeters, and the highest roughness factor. Interesting subtleties emerged when current was normalized to active area: the Ni73Co27 film showed the highest intrinsic activity per site, while Ni75Co25 owed its superior overall performance mainly to a larger population of accessible active sites. In alkaline media, nickel-rich sites accelerate the water dissociation of the Volmer step, while cobalt fine-tunes the electronic structure to optimize hydrogen intermediate adsorption and desorption, a synergy that saccharin’s compositional control helps to maximize.

The broader lesson is that modest, inexpensive process chemistry can substitute for exotic materials engineering. Sodium saccharin, a commodity additive, simultaneously polished the alloy composition, shrank the grains, seeded beneficial defects, and enlarged the active surface, all within a one-step electrodeposition compatible with flexible substrates and scalable plating baths. The resulting Ni75Co25 thin film achieves respectable hydrogen evolution performance from entirely earth-abundant metals, offering a template for cost-effective electrocatalyst design at a time when green hydrogen production must expand rapidly to displace fossil-derived supply. As the authors conclude, saccharin-assisted electrodeposition stands as an effective, simple strategy for developing high-performance, low-cost catalysts for the hydrogen economy.

Subject of Research: Saccharin-assisted electrodeposition of nickel-cobalt thin film electrocatalysts for the hydrogen evolution reaction

Article Title: Saccharin-assisted microstructural refinement enables enhanced hydrogen evolution reaction in electrodeposited NiCo thin films

Article References: Budi, S., Hadawiyah, R., Maulana, M. A. R., Winarsih, S., Fauzi, M. H., & Aulia, M. F. (2026). Saccharin-assisted microstructural refinement enables enhanced hydrogen evolution reaction in electrodeposited NiCo thin films. Results in Chemistry, 30, Article 103824. https://doi.org/10.1016/j.rechem.2026.103824

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103824

Keywords: hydrogen evolution reaction, nickel-cobalt alloy, electrodeposition, saccharin additive, anomalous co-deposition, grain refinement, water splitting, electrocatalysis, thin films, defect engineering, alkaline media, green hydrogen