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

The development of efficient and cost-effective electrocatalysts for the hydrogen evolution reaction (HER) is critically important for sustainable hydrogen production. In this study, NiCo thin film electrocatalysts were fabricated using electrodeposition technique, and the role of sodium saccharin as an additive in tuning their structural and electrocatalytic properties was systematically investigated. X-ray diffraction analysis revealed that increasing saccharin concentration refined the NiCo microstructure, producing a defect-rich nanocrystalline alloy with reduced crystallite size and increased dislocation density and microstrain. These structural modifications significantly enhanced the electrochemically active surface area, as indicated by the increased double-layer capacitance. Among the prepared catalysts, Ni 75 Co 25 exhibited the best HER activity, delivering a current density of 30.43 mA cm −2 at 0.5 V, a low overpotential of 253 mV, and a Tafel slope of 70 mV dec −1 , together with the highest ECSA and lowest R ct . The enhanced catalytic performance follows a Volmer–Heyrovsky mechanism and is attributed to the synergistic effects of defect-engineered nanostructure, optimized Ni Co composition, and increased exposure of electrochemically active sites. These results highlight additive-assisted electrodeposition as an effective strategy for tailoring NiCo thin films for efficient hydrogen evolution reaction.