Valence-engineered MoNi4/MoOx@NF as a Bi-functional electrocatalyst compelling for urea-assisted water splitting reaction

Abstract

Valence engineering was initially studied to achieve advanced bi-functional electrocatalysts for optimum performance. For this purpose, TMoO4 nanorods (T = Ni, Mn, Cu, and Zn) were grown on nickel foam, which upon reduction treatments, transformed into a composite system MoNi4/MoOx@NF with alloy nanoparticles MoNi4 anchored on MoOx at 3D nickel foam. This composite system was demonstrated to show distinct mixed valence features of Mo0/Mo2+/Mo4+/Mo5+/Mo6+. When taken as a bifunctional electrocatalyst, this composite gives a lower surface charge transfer resistance, a lower Tafel slope, and an excellent electro-catalytic performance with 0.01 V for hydrogen evolution reaction and 1.29 V for urea oxidation reaction vs. reversible hydrogen electrode (RHE) at 10 mA cm−2. Valence engineering of Mo species achieved by tuning the reduction conditions makes the electron density of Mo improved and Tafel-Heyrovsky step significantly increased. Under a reduction atmosphere for 2 h, the composite shows an optimum performance as represented by a much better electron conduction efficiency, a faster rate of Heyrovsky step, and a long-term stability at 10 mA cm−2.