(Invited) Recent Developments in Intermediate-Temperature Reversible Fuel Cells

Abstract

Intermediate-temperature fuel cells have potential to be the cleanest and most efficient option for direct conversion to electricity of a wide variety of fuels, from hydrogen to hydrocarbons, coal gas, and bio-derived fuels. When operated in the reverse mode, on the other hand, they are very efficient for low-cost production of hydrogen from splitting water. Thus, a reversible fuel cell is ideally suited for large-scale energy storage and conversion, which is vital to the deployment of renewable energies. However, the commercialization of these systems hinges on rational design of novel materials of exceptional functionalities at lower temperatures to dramatically reduce the cost while enhancing performance and durability. A key challenge is the development of bifunctional catalysts, especially those for the oxygen electrode, for efficient dual-mode operation. Further, the reversible fuel cells must have high durability when cycled between the fuel cell and the electrolysis modes while maintaining high Faraday efficiency and roundtrip energy efficiency. The electrolyte materials must have high ionic transference number and excellent stability under the operating conditions. To accomplish these goals, it is imperative to gain a fundamental understanding of the mechanisms of charge and mass transport along surfaces, across interfaces, and through porous electrodes. Further, new protocols must be developed to control materials structure, composition, and morphology over multiple length scales. This presentation will highlight the critical scientific challenges facing the development of a new generation of reversible fuel cells based on oxygen ion- or proton- conducting electrolyte, the latest development of bifunctional electrodes and electrolyte materials, the strategies for enhancing electrode activity and durability of hydrogen evolution/oxidation, and oxygen reduction/evolution reactions in reversible fuel cells, the latest developments in modeling, simulation, and in situ characterization techniques for unraveling charge and mass transport mechanisms, and the outlook for future-generation energy storage systems that exploit nano-scale materials of significantly improved performance.