In Situ Study on Ni-Mo Stability in a Water-Splitting Device: Effect of Catalyst Substrate and Electric Potential

Nickel-molybdenum (Ni-Mo) alloys are well studied as highly effective electrocatalyst cathodes for water splitting. Understanding deactivation pathways is a key to improving the performance of these catalysts. In this study, in situ characterization by UV/Vis spectroscopy and AFM of the morphology and Mo leaching of an Ni-Mo electrocatalyst was performed with the goal of understanding the stability and related Mo leaching mechanism. Switching the potential towards higher overpotentials results in a nonlinear change in Mo leaching. Multiple processes are proposed to take place, such as a decrease in the extent of Mo oxidation at the cathode induced by more strongly reducing potentials, while simultaneously the increase in the local pH at the cathode due to the hydrogen evolution reaction causes more Mo leaching. The change in capacitance of these materials depends strongly on the change in surface composition and not only on the surface area. In situ UV/Vis spectroscopy showed that Mo leaching is a continuous process over the course of 4 h of operation. Finally, the material was deposited on different substrates and the effect on Ni-Mo stability was studied. The substrate has a significant, albeit complex, influence on the stability and activity of Ni-Mo cathodes. In terms of stability in 1 m KOH, Ni-Mo was found to be best deposited on stainless steel substrates operated at low overpotentials, on which it showed nearly no change in capacitance and exhibited low Mo leaching.