This poster presents our recent findings on “shelf-life” stability of 50 wt% carbon-supported Ni-Mo composites when stored in ambient aerobic environments. Prolonged exposure to atmospheric oxygen, alters surface morphology and chemical composition; leading to a monotonic, exponential decay in HER activity with a half-life of ~1 week, as characterized by thin-film rotating disk electrode voltammetry. This loss attributed to the formation and growth of oxide layers over the metallic active sites of the nanoparticles.
Catalyst samples were also examined using powder X-ray diffraction and X-ray photoelectron spectroscopy. We observed a distinct increase in the intensity of features associated with oxidation over time but signals attributable to metallic Ni were still evident even after months of shelf storage. Transmission electron microscopy revealed major changes in the morphology of the catalyst, which can be summarized as disintegration of the nanoparticulate structure. Temperature programmed oxidation was performed over a range of temperatures on fresh Ni-Mo/C composites; the results indicate that catalyst oxidation proceeds in at least two distinct stages that may be consistent with an initial, fast surface reaction followed by continued oxidation under diffusion control. These experiments yield important insights about the basic physics of non-PGM catalyst degradation that is relevant to the development of ink formulation and coating strategies that diverge from methods widely used for oxidatively robust PGMs.