Probing Interfacial Reactivity in Metal/Metal-Oxide Electrocatalysts

Our research group is broadly interested in the synthesis and characterization of catalysts for electrochemical hydrogen production and utilization. A major focus is on nonprecious catalysts for hydrogen evolution/oxidation in the long-term pursuit of efficient renewable energy storage via alkaline unitized regenerative fuel cells. This field has gained increased interest recently due to the attractiveness of alkaline operating conditions for capital cost reductions and the emerging availability of excellent nonprecious oxygen evolution/reduction catalysts and alkaline anion exchange membranes.

Ni–Mo composites are widely studied for electrochemical and thermochemical transformations involving hydrogen intermediates—including the hydrogen evolution reaction and hydrotreating processes, respectively. Our lab has developed a Ni–Mo alkaline hydrogen evolution catalyst that consists of a Ni-rich alloy core surrounded by Mo-rich oxide shell. We have executed environmental transmission electron microscopy (ETEM) measurements, which show that this alloy@oxide structure results from spontaneous phase segregation from a single-phase parent oxide. By correlating catalyst composition with bulk activity measurements, we conclude that the oxide shell inhibits electron-transfer from particle to particle and may enhance the intrinsic activity of the underlying Ni–Mo alloy via a bifunctional mechanism.

To further investigate the role of metal/metal-oxide interfaces in electrochemical catalysis, we are developing new routes to generate well-defined thin film and nanoparticulate materials. For thin films, we are adapting techniques from semiconductor nanofabrication to synthesize polycrystalline composites whose surface roughness is on the order of a single bond length, thereby enabling the use of atomically resolved surface probe measurements to study structure-activity relationships. For nanoparticles, we are using a technique called the salt-encapsulation method to generate uniform mixtures of transition metals with disparate oxophilicities followed by thermal treatments to induce oxidative phase segregation, which can be directly imaged via ETEM. This degree of dimensional control is expected to enable new insights into electrocatalyst structural evolution under operating conditions.