Bifunctional electrocatalysts for unitized regenerative fuel cells

Abstract

During the transition from conventional energy sources to sustainable alternatives, the role of energy storage and conversion is pivotal. Unitized regenerative fuel cells (URFCs) have attracted significant attention as promising energy storage and conversion devices owing to their ability to operate in both fuel cell (FC) and electrolyser (EL) modes within a compact single cell. They possess several advantages, including high power density, high specific energy density, light-weight design, low-cost production, high efficiency, long lifespan, and near-zero environmental impact. The development of efficient and durable bifunctional electrocatalysts is essential for the practical implementation and outstanding performance of URFCs. In URFCs, oxygen holds greater significance than hydrogen because its electrochemical reactions at the interface between the electrolyte and the electrode tend to be slow and intricate, primarily due to the pronounced irreversibility of oxygen-related processes. This chapter begins with a brief introduction to various types of FCs, setting the context for the subsequent discussion on URFCs. It provides an overview of bifunctional electrocatalysts in URFCs, highlighting their role in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). It also discusses bifunctional oxygen catalysts (BOCs) reported in the literature, along with a brief discussion of bifunctional hydrogen catalysts (BHCs). The chapter systematically summarizes the recent advances, challenges, and prospects related to bifunctional catalysts, which are critical for the performance of URFCs.