Unitized regenerative fuel cell technologies: Development, challenges, applications, and perspectives

Unlike discrete regenerative fuel cell systems that employ separate stacks, unitized regenerative fuel cells (URFCs) share common electrodes, membranes, and flow fields, offering reduced weight, volume, and capital cost. This review summarizes the historical development of URFCs across five generations of technology. Significant progress has been achieved in bifunctional catalyst design, component optimization, and system integration. However, material degradation, limited cycling durability, and high costs still hinder widespread applications in aerospace, grid-scale energy storage, unmanned vehicles, and portable power. Future efforts for bifunctional catalysts design should reduce platinum-group metal(PGM) loading below 0.5 mg/cm 2 while maintaining balanced catalytic activity and durability. Component optimization requires improving ionic conductivity and chemical stability of membranes, designing amphiphilic porous transport layers for bifunctional wettability, and enhancing bipolar plate corrosion resistance under dual-mode operation. System-level challenges include water management and thermal regulation during mode transitions. Adaptive control strategies are needed to minimize transition losses and couple URFCs with intermittent renewable energy sources. To achieve commercial viability, voltage round-trip efficiency should exceed 50%, and cycling durability requires minimal performance degradation beyond 10,000 cycles. Furthermore, the paper examines diverse applications of URFCs, including aerospace, grid-scale energy storage, and portable power solutions. It concludes with a summary of key findings, identifies critical research gaps, and discusses future perspectives for the development and commercialization of URFC technologies.