Electrolyte Effects on the Stability of Ni‐Mo Cathodes for the Hydrogen Evolution Reaction

Water electrolysis to form hydrogen as a solar fuel requires its catalysts to be highly effective. Ni‐Mo is known as such a material. In this work, we perform theoretical and experimental studies on the activity and stability of Ni‐Mo cathodes. Density functional theory studies show various Ni‐Mo facets to be active for the hydrogen evolution reaction, Ni segregation to be thermodynamically favorable, and Mo vacancy formation to be favorable even without an applied potential. Electrolyte effects on the long‐term stability of Ni‐Mo cathodes have been determined. We compared Ni‐Mo before and after up to 100 h of continuous operation. We show that Ni‐Mo is unstable in alkaline media due to Mo leaching in the form of MoO42‐, ultimately leading to a decrease in absolute overpotential. We found that the electrolyte, the alkali cations and pH all influence Mo leaching. Changing from Li to Na to K influences the surface segregation of Mo, and pushes the reaction towards Mo dissolution. Decreasing the pH decreases the OH‐ concentration and in this manner inhibits Mo leaching. Of the electrolytes studied, in terms of stability, the best to use is LiOH at a pH of 13. Thus, we present a mechanism for Mo leaching as well as ways to influence the stability and make the Ni‐Mo material more viable for renewable energy storage in chemical bonds.