Ultralow Ru-doped NiMoO4@Ni3(PO4)2 core-shell nanostructures for improved overall water splitting

Abstract

The potential of sustainable hydrogen production technology through water splitting necessitates the rational design of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) bi-functional electrocatalysts. In this context, we initially synthesized and empirically evaluated ultralow Ru-doped NiMoO₄@Ni₃(PO₄)₂ core-shell nanostructures on nickel foam (Ru-NiMoO₄@Ni₃(PO₄)₂/NF). The hydrous NiMoO₄ nanopillars were hydrothermally grown on NF, followed by successive RuCl₃ etching and subsequent phosphorylation processes, leading to the final Ru-NiMoO₄@Ni₃(PO₄)₂/NF. The catalyst demonstrated impressive HER overpotential values of −14.8 and −57.1 mV at 10 and 100 mA cm⁻², respectively, with a Tafel slope of 35.8 mV dec⁻¹. For OER at 100 mA cm⁻², an overpotential of 259.7 mV was observed, with a Tafel slope of 21.6 mV dec⁻¹. The cell voltage required for overall water splitting was 1.43 V at 10 mA cm⁻² and 1.68 V at 100 mA cm⁻². Moreover, the catalyst exhibited superior stability for 150 h, emphasizing its practical utility for long-term applications. Subsequent density functional theory calculations aligned with these empirical findings, indicating a low water dissociation energy barrier (ΔG_b = 0.46 eV), near-zero free adsorption energy for HER (ΔG_*H = 0.02 eV), and suitable free adsorption energy for OER (ΔG_*OOH − ΔG_*OH = 2.74 eV), alongside a high density of states near the Fermi level. These results, informed by both experimental evaluation and theoretical validation, highlight the potential of Ru-NiMoO₄@Ni₃(PO₄)₂/NF as a high-performance catalyst for water splitting, setting a solid foundation for advancements in sustainable energy technologies.