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 study, published in the Journal of Nanoparticle Research, was designed around a deceptively simple question: how does the copper fraction in a nickel oxide film change its ability to catalyze the methanol oxidation reaction in alkaline solution? The team deposited four compositions on fluorine-doped tin oxide glass, a transparent conducting substrate commonly used in photoelectrochemistry. A DC power source sputtered the nickel oxide target at a fixed 200 watts, while a separate radio-frequency power supply drove a copper target at 0, 40, 50, and 60 watts, corresponding to nominal copper contents of 0, 2.5, 4, and 7 percent. Everything else was held constant: a working pressure of 5 by 10 to the minus 3 torr, 400 seconds of deposition, 20 standard cubic centimeters per minute of argon, and a target-to-substrate distance of 14 centimeters. This kind of physical vapor deposition matters because it produces adherent, pinhole-free films whose composition and thickness can be dialled in precisely, unlike many solution-based routes where morphology is hard to reproduce.

Structural characterization showed that the copper was not forming a separate metallic phase. X-ray diffraction revealed only the cubic bunsenite phase of nickel oxide, with reflections from the (111), (200), and (220) crystal planes, and no peaks attributable to metallic copper. Instead, copper appeared to occupy interstitial and substitutional positions within the nickel oxide lattice, filling defects in the nonstoichiometric framework. The evidence was written into the peak shapes: the diffraction peaks broadened and weakened as copper content rose, and the average crystallite size, calculated with the Debye-Scherrer equation, shrank from 18.12 nanometers for pristine nickel oxide to 7.42 nanometers at 7 percent copper. Lattice strain climbed from 2.1 by 10 to the minus 4 to 5.31 by 10 to the minus 4, and the dislocation density rose nearly sevenfold, both signatures of a lattice that has been subtly disrupted by foreign atoms. Electron microscopy confirmed the trend, showing progressively rougher, more compact, and more agglomerated surfaces as the dopant level increased.

One consequence of the co-sputtering recipe deserves particular attention because it complicates the interpretation of catalytic data. Film thickness grew systematically with copper loading, from 53 nanometers for pristine nickel oxide to 90, 121, and 180 nanometers for the 2.5, 4, and 7 percent films. More material on the electrode naturally means more geometric current, so the researchers took the unusual and commendable step of normalizing their methanol oxidation currents in two additional ways: per unit of film thickness, and per unit of electrochemically active surface area estimated from double-layer capacitance measurements. The capacitance values rose from 0.226 to 0.44 millifarads per square centimeter across the composition series, yielding active surface areas between 5.65 and 11.12 square centimeters. Even after this careful normalization, the 7 percent copper film remained roughly 2.5 times more active than pristine nickel oxide, while the 2.5 and 4 percent films showed the highest currents per unit thickness and per active area respectively. The conclusion was nuanced: copper incorporation, enlarged active area, film growth, and improved interfacial kinetics all contributed, and no single factor explains the whole story.

Optical measurements pointed to a fundamental electronic reason for the improvement. Ultraviolet-visible spectroscopy combined with Tauc analysis showed that the optical band gap narrowed steadily as copper content increased, from 3.64 electron volts for pure nickel oxide to 3.24 electron volts at 7 percent doping. Band gap narrowing in doped nickel oxide is generally attributed to localized dopant states overlapping the band edges and to improved crystallization, and narrower gaps translate directly into better electronic conductivity through the film. For a p-type semiconductor like nickel oxide, which must shuttle holes and electrons between the current collector and the electrolyte interface during every catalytic cycle, that conductivity gain is not an academic nicety. It means less internal resistance, faster delivery of charge to surface reaction sites, and lower overpotentials, which is exactly what the electrochemical measurements went on to confirm.

Cyclic voltammetry in 1 molar sodium hydroxide exposed the redox machinery of the films. All four compositions showed anodic and cathodic peaks associated with the nickel(II) to nickel(III) and copper(I) to copper(II) couples, meaning that both metals contribute electroactive centers. Peak currents scaled linearly with the square root of the scan rate, indicating that hydroxide diffusion into the film pores, rather than some surface limitation, governed the redox process at the electrode-electrolyte boundary. When methanol was added, every film responded, but the doped films responded dramatically more, with anodic and cathodic current densities climbing in step with copper content. Onset potentials shifted to more negative values as methanol concentration rose from 0.25 to 3 molar, and the 7 percent copper film recorded the highest current density in 1 molar methanol, around 32 milliamperes per square centimeter.

Kinetic analysis tightened the picture. The team recorded linear sweep voltammograms at a deliberately slow scan rate of 5 millivolts per second, a quasi-steady condition that minimizes capacitive contributions from the nickel hydroxide to nickel oxyhydroxide transformation and yields Tafel slopes that reflect true faradaic kinetics. Pristine nickel oxide gave a slope of 178 millivolts per decade. Doping with just 2.5 percent copper dropped it to 127, and 7 percent copper brought it down to 120 millivolts per decade, a total reduction of about 32.5 percent. That final value corresponds to a charge-transfer coefficient of 0.5, consistent with the first electron transfer step dominating the reaction kinetics. Interestingly, the largest kinetic gain arrived at the lowest dopant loading, with the incremental benefit tapering off at 4 and 7 percent, suggesting the population of accessible surface redox sites begins to saturate. The authors attribute the enhancement to a bifunctional synergy: nickel centers cycle into the active nickel oxyhydroxide species that dehydrogenate methanol, while copper sites, present as mixed copper(I) and copper(II) states according to X-ray photoelectron spectroscopy, adsorb hydroxyl species at lower overpotentials and thereby strip carbonaceous intermediates from neighboring nickel sites before they can block them.

Electrochemical impedance spectroscopy, recorded at 0.6 volts in 1 molar sodium hydroxide with 1 molar methanol, backed this mechanism quantitatively. The fitted charge-transfer resistance decreased steadily with doping level, reflecting the greater density of redox-active nickel and copper sites on the film surfaces. A carbon monoxide stripping experiment delivered perhaps the most convincing evidence of poisoning resistance: the 7 percent copper film showed a clean stripping peak at 0.43 volts, and, strikingly, no reverse oxidation peak appeared during the backward scan, indicating negligible accumulation of adsorbed carbon monoxide intermediates. That absence of a backward peak is exactly the behavior fuel cell engineers want to see, because it means the catalyst surface stays clear and available for fresh fuel molecules rather than becoming choked with its own reaction byproducts.

Durability, the graveyard of many promising electrocatalysts, was tested on three fronts. Chronoamperometry at 0.6 volts for a full 12 hours showed only a 5.7 percent decline in current for the 7 percent copper film, with the small initial drop attributable to intermediate adsorption and local methanol depletion near the electrode surface. After 1000 cyclic voltammetry cycles the electrode retained 94.3 percent of its current, reaching 30.2 milliamperes per square centimeter, although the anodic oxidation feature shifted from roughly 0.40 to 0.49 volts. The authors interpret this shift not as degradation but as the gradual accumulation of adsorbed carbonaceous intermediates and reorganization of the surface oxyhydroxide layer, a reading supported by their post-mortem analyses. Inductively coupled plasma optical emission spectroscopy of the electrolyte after testing found nickel concentrations below the detection limit and copper at just 0.051 parts per million, and X-ray photoelectron spectra taken before and after operation showed the nickel and copper oxidation states essentially intact, confirming that the redox-active surface survives chemically.

What makes this work resonate beyond a single materials system is its recipe-like transferability. Rather than hunting for exotic compounds, the researchers demonstrated that a mainstream physical deposition technique, a cheap transition metal dopant, and careful normalization of electrochemical data can jointly push a simple nickel oxide film to performance levels that compete with far more elaborate catalysts. The band gap engineering, the bifunctional nickel-copper mechanism, and the documented poison tolerance together outline a practical roadmap for alkaline direct methanol fuel cells that sidestep platinum entirely. The authors point toward the logical next steps: integrating these optimized films into membrane electrode assemblies and testing them in full fuel cell stacks, where real operating conditions will judge whether laboratory elegance translates into engineered power. If it does, the humble nickel oxide lattice, nudged by a few percent of copper, may find itself at the heart of a new generation of liquid-fuel clean energy devices.

Subject of Research: Copper-doped nickel oxide thin film electrocatalysts for the methanol oxidation reaction in direct methanol fuel cells

Article Title: Tuning methanol oxidation activity: impact of Cu content on sputtered NiO thin film electrocatalysts

Article References: Tuning methanol oxidation activity: impact of Cu content on sputtered NiO thin film electrocatalysts. (n.d.). https://doi.org/10.1007/s11051-026-06765-0

Image Credits: AI Generated

DOI: 10.1007/s11051-026-06765-0

Keywords: Cu-doped NiO, methanol oxidation reaction, direct methanol fuel cells, electrocatalysis, co-sputtering deposition, thin films, band gap engineering, Tafel analysis, alkaline electrolyte, nickel oxide, carbon monoxide tolerance, electrochemical impedance spectroscopy