(Invited) Pathways Towards High-Performance Reversible Alkaline Membrane Fuel Cells

Abstract

Reversible fuel cells have advantages over rechargeable batteries (Ni/MH or Li-ions) due to their high energy density, minimal self-discharge, and low cost. Reversible alkaline membrane fuel cells (AMFC) are attractive due to their ability to work with platinum-group-metal (PGM)-free catalysts. Major factors that limit the application of reversible AMFCs are low performance of bifunctional oxygen catalysts, alkaline membrane stability, and water management, which lead to high cost and low round-trip efficiency (RTE). We have developed a variety of high-performance metal oxides/nanocarbon bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts, which have demonstrated remarkable reversibility and stability under harsh voltage cycling from 0.0 V to 1.9 V, providing a solid foundation for enabling reversible AMFCs. However, during membrane and electrode (MEA) development, it was found that the alkaline membrane and ionomer degraded rapidly at high electrolysis voltages. Thus, hydroxide ion transfer paths across the membrane and in the electrodes were largely blocked. Although the introduction of diluted carbonate salt (e.g., NaHCO3) or base (e.g., KOH) solutions can help to retain the electrolysis operation to some degree, it leads to flooding and complicates the reversible fuel cell system. In this work, we will present some solutions to the challenges of reversible AMFCs, thus realizing high-efficiency reversible fuel cell operations. First, oxidation-resistant alkaline membranes and ionomers that are stable under oxidation conditions will be discussed, to eliminate the need of carbonate salt (e.g., NaHCO3) or base (e.g., KOH) solutions. Second, high-performance bifunctional catalysts for hydrogen evolution reaction (HER)/hydrogen oxidation reaction (HOR) and ORR/OER catalysts will be elaborated. Furthermore, advanced MEAs will be designed to minimize water transport-associated reversible fuel cell performance loss. These combined efforts can improve the RTE to 50% at an appreciable current density (>500 mA/cm2). Acknowledgement: The project is financially supported by the Department of Energy’s Fuel Cell Technology Office under the Grant DE-EE0006960.