Niobium Oxide Aerogel-Supported Bifunctional Oxygen Electrocatalysts for Unitized Regenerative Fuel Cells

Proton exchange membrane unitized regenerative fuel cells (URFCs) combine the ability to both produce power (in fuel cell mode) and generate fuel and oxidant (in electrolysis mode) from the same cell. The further development and utilization of URFCs is hindered by the high costs and low stability of bifunctional oxygen electrocatalysts. Distributing the oxygen electrocatalyst on a high surface area support can reduce the loading of the active noble metal (e.g., platinum and iridium) and influence the efficiency and stability of the catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Carbon, which is typically used as a support material, is highly unstable under the conditions required for OER which makes carbon unfeasible for long-term use within URFCs. Therefore, alternative carbon-free catalyst supports are needed. Niobium oxide, Nb 2 O 5 , is stable in the oxidative potentials and highly corrosive acidic conditions of URFCs; however, the low electronic conductivity of Nb 2 O 5 significantly limits its use as a catalyst support material. We investigated approaches to obtain high surface area and conductive niobium oxides, NbO x , as stable supports for bifunctional oxygen electrocatalysts. The effects of synthesis conditions, drying methods, and temperature/atmosphere treatments on the structure, morphology, surface area, and electronic conductivity were evaluated. The structure, morphology, composition, and surface area were determined by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and nitrogen physisorption analysis. Sol-gel synthesis and supercritical drying resulted in a NbO x aerogel with high surface area and porous morphology. Thermal treatment of the NbO x aerogel under H 2 increased the electronic conductivity of NbO x when compared to the as-prepared NbO x . Different methods to deposit noble metals on the NbO x support were evaluated. The NbO x -supported catalysts were tested as bifunctional ORR/OER electrocatalysts to determine their activity and stability. The development supported bifunctional oxygen electrocatalysts that can provide lower noble metal loadings, higher activity and improved stability under harsh acidic conditions and highly oxidizing potentials furthers the development of URFCs with lower cost, improved performance, and enhanced durability.