(475f) Probing the Synergy in Nickel-Molybdenum Hydrogen Evolution Catalysts

Authors: 
Schwalbe, J., Stanford University
McKay, I., Stanford University
Willis, J., Stanford University
Goodman, E., Stanford University
Cargnello, M., Stanford University
Majumdar, A., Stanford University
Electrochemical production of hydrogen offers an alternative to fossil fuel intensive methane reforming. On a commercial scale, water electrolysis is done with either proton exchange membrane based systems (acidic) or in alkaline electrolyzers. In alkaline environments, nickel and nickel alloys are widely used. While nickel-molybdenum is known to be highly active, the reason for this high activity is not understood, nor are the potential degradation mechanisms. To investigate the synergy that exists in this system, we fabricated well defined nickel electrodes with preformed nanocrystals and explored the catalytic activity with and without the influence of molybdenum. First, we looked at the promoting effect of molybdenum substrates. We observe a synergistic increase in catalytic activity as well as a strong wetting effect of the nickel on molybdenum. When we compare the activity of different size nanocrystals on molybdenum, we see that large particles are more active on a mass basis than small particles. This does not scale with either the surface area or interface length, suppors the existence of a bulk interaction between nickel and molybdenum. To expand on these observations, we directly investigated the role of molybdate deposition on nickel by performing chronoamperometry experiments with molybdate added to the electrolyte. The molybdate experiments show that a mixed molybdenum-nickel phase can achieve even higher activtity. Over the course of the molybdate deposition experiments, we observe a slow increase in the activity followed by an eventual decline. By performing ex-situ SEM and auger depth profiling on electrodes at various timepoints, we are able to identify the most active morphology of nickel molybdenum. Our observations support the existence of a bifunctional mechanism, in which one species acts to stabilize water as a proton is abstracted and another site acts to combine these protons into hydrogen gas. While nickel can perform this alone, it is more effective in the presence of molybdenum.