Effect of Activating a Nickel–Molybdenum Catalyst in an Anion Exchange Membrane Water Electrolyzer

Water electrolysis using anion exchange membranes is promising for hydrogen production, and Ni–Mo catalysts have shown high activity for alkaline hydrogen evolution reaction (HER). However, their performance has been mostly tested in a half-cell setup and rarely studied in a single-cell setup with a membrane electrode assembly (MEA) structure, which is used for practical applications. With Ni3Mo as the cathode, a single cell was fabricated using non-noble metal catalysts exclusively. Interestingly, the activation procedure significantly affected the cell performance. The single cell performed better than that with the Pt/C catalyst when the Ni3Mo catalyst was mildly activated. The distribution of Mo in electrodes, membrane, and electrolytes was estimated, confirming Mo dissolution from the cathode. Once the cell was activated, the cell performance was stable without degradation in long-term chronopotentiometry operation, but the performance was degraded by sudden voltage change such as imposing open circuit voltage (OCV). The surface structure and reaction mechanism were studied with density functional theory: the Mo-dissolved Ni3Mo(101) surface could promote H2O dissociation, while MoO3 stably adsorbed on the surface weakened H* adsorption, promoting HER. This study provides important insights into the development of efficient catalysts for large-scale hydrogen production.