Abstract
Proton exchange membrane (PEM) fuel cells provide high efficiencies and clean power generation, however to enable wide scale adoption approaches are needed to reduce costs, improve durability, and provide on-demand hydrogen generation. Unitized regenerative fuel cells (URFCs) that combine power generation with fuel/oxidant generation within a single system can reduce costs and lower weight and volume compared with separate fuel cell and electrolyzer systems. Improved bifunctional oxygen electrodes (BOEs) that have lower overpotentials for oxygen reduction reaction (ORR) and oxygen evolution reactions (OER), higher stabilities for extended operating times, and lower costs are needed for URFCs. For extended durability, non-carbon-supported catalysts are required for BOEs since carbon oxidation occurs at high positive potentials leading to significant performance degradation. Our group has demonstrated metallic two-dimensional (2D) nanoframes can be derived from controlled temperature/atmosphere treatments of metal hydroxide nanosheets. Metallic Ni-Pt 2D nanoframes exhibit a unique porous nanoarchitecture that allows the use of unsupported (carbon-free) electrodes that have shown significantly higher ORR activities and stabilities compared with Pt/C. Incorporation of Ir and/or Ru in an integrated 2D NiPtxMy (M=Ru, Ir) nanoframe was investigated to obtain bifunctional (ORR and OER) electrocatalyst nanoarchitectures. Electrochemical analysis of unsupported metallic 2D nanoframes using rotating disc electrode experiments shows that incorporating Ni within the structure significantly improves both ORR and OER activities. Further, we have investigated using transition metal oxide-supported catalysts to reduce the precious metal loading while maintaining high activity and stability. The ability to control the structure and properties of 2D nanoframes provides a route to develop bifunctional electrocatalysts with high activity, extended durability, and lower cost.
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