Highly active Pd-doped ZnCo₂Se₄ spinel nanoelectrocatalysts for synergistic hydrogen evolution and methanol/urea oxidation–assisted water splitting validated by DFT

Highly active palladium-doped spinel zinc cobalt selenide (PZCS)-coated nickel foam (ZnPd_xCo_2−xSe₄ (x ≤ 0.08)@NF) nanoelectrocatalysts were prepared via a two-step hydrothermal method. The spinel phases of the engineered samples were confirmed by X-ray diffraction, revealing a microsphere surface morphology with a dense nanofeather configuration. Chronopotentiometry measurements of the prepared nanoelectrocatalysts revealed their excellent hydrogen evolution reactivity, low overpotential (∼64 mV, similar to that of the benchmark Pt catalyst), low Tafel slope (∼55.61 mV dec⁻¹), and long-term stability exceeding 24 h. An electrochemical study with 6.0% Pd doping indicated higher hydrogen evolution efficacy, attributed to the increased surface area and accelerated charge-transfer kinetics at the semiconductor electrolyte interface. Methanol oxidation (MO), urea oxidation (UO), and MO-/UO-assisted water splitting analyses confirmed the optimal performance with 6.0% Pd. Density functional theory calculations were performed to evaluate the effects of Pd dopants on the catalytic activity of ZnCo₂Se₄ (ZCS). The hydrogen, water, and methanol adsorption energies on ZCS and PZCS demonstrated consistently exothermic interactions at all surface sites. Therefore, the incorporation of Pd enhanced hydrogen and water adsorption while preserving the weak binding toward methanol that remained non-competitive with hydrogen adsorption. The computed hydrogen adsorption free energies suggested near-optimal values (approximately −0.44 eV) at the Pd site, designating it as the active center for hydrogen evolution reactivity. The simulated electronic density of states indicated supplementary energy-level contributions from Pd near the Fermi level, improving electronic conductivity and charge transfer during electrocatalysis. Thus, Pd-doped ZCS enhanced the electronic properties of the nanoelectrocatalyst active surface, providing suitable candidates for coupled hydrogen evolution and MO-/UO-assisted water splitting.