Nickel-molybdenum alloy catalysts for the hydrogen evolution reaction: Activity and stability revised

Ni-Mo alloys and compounds are considered as highly active catalysts for the hydrogen evolution reaction (HER) in alkaline electrolytes. The origin of the reported high activity is, however, not yet fully understood. In this study, the fundamental activity for the HER and stability of metallurgically prepared Ni, Mo and different Ni-Mo alloy electrodes with comparable surface areas in alkaline electrolyte are studied in detail. Interestingly, the intrinsic activities of polished Ni-Mo electrodes were slightly lower than those of pure Ni electrodes. Moreover, using an electrochemical flow cell that is coupled to a mass spectrometer we show that Mo leaches out of the prepared samples and dissolves at HER relevant conditions. The Ni to Mo ratio of surface atoms was found to increase after electrochemical treatment, additionally confirming leaching of the less noble Mo. On the basis of our results we conclude that some of the previously reported high activities might be attributed to high surface areas that result from the manufacturing process and/or electrochemical de-alloying, rather than to intrinsic properties of the alloy.