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