Unitized regenerative fuel cells: Fundamental challenges and advancements

A unitized regenerative fuel cell (URFC) refers to a single cell that can operate as both an electrolyzer and a fuel cell. URFCs, therefore, could play a significant role in the shift to a sustainable energy future, decreasing the cost of implementing hydrogen technologies across applications from off-grid to distributed energy systems using renewables. However, to date, the high cost associated with the practical use of URFCs remains a significant barrier. Existing URFC stacks suffer from limited efficiency. Noble metals such as Pt, Ir, and Ru limit possible cost reduction and to date, only these noble metals can effectively catalyze the URFC reactions in a reversible way. Non-precious metal catalysts experience severe degradation and low intrinsic catalytic activity for both the hydrogen/air reaction in fuel cell mode and water splitting in electrolysis mode. This review provides a summary of the latest advancements in URFCs, highlighting areas for improvements and explores how separate state-ofthe-art developments in electrolyzers and fuel cells could potentially enhance the efficiency of URFCs. The history and recent performance of URFC stacks, flooding and corrosion mechanisms, thermal and water management systems are discussed. Various optimization strategies for improving the performance of URFC components, such as the membrane, catalyst layer, bipolar plate, gas diffusion layer, and flow field designs, are also reviewed.