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Unitized regenerative fuel cells (URFC) convert electrical energy to and from chemical bonds in hydrogen. URFCs have the potential to provide economical means for efficient long-term, seasonal, energy storage and on-demand conversion back to electrical energy. We first optimize the catalyst layer for discrete electrolyzer and fuel cell and then configure the URFC. Two possible configurations of URFCs are compared, which emphasize the advantages of the unconventional constant-electrode (CE) URFC over the traditional constant-gas (CG) configuration. We also study the stability via accelerated stress tests (ASTs) and demonstrate steady state operation in a daily cycle for day to night energy shifting. The goal is to identify a competitive configuration for URFCs, and demonstrate it in terms of upper limit of round trip efficiencies (RTEs). From the investigations, the optimum composition of the URFC anode catalyst layer is 90 at% Ir-black balanced by Pt-black for both CE and CG configurations. At 80 °C and 1 A/cm2, the optimized CE URFC achieves 58 and 61% RTE with air and O2 as the reductant gases, respectively. We then evaluated the differences in durability using an AST over 10k charge-discharge cycles; the results reveal that the wider potential window at the anode in CE (0.05-1.55 V) has minimal effect on catalyst layer stability compared to CG (0.55-1.55 V). Furthermore, there was no degradation up to the range of 2k-5k cycles; beyond that the fuel cell (discharge) performance degraded while the electrolyzer (charge) performance was still stable. The observations here indicate substantial potential to employ URFCs as efficient and cost-effective bidirectional energy-conversion devices within energy storage and utilization systems after appropriate technological and operational optimizations.

A low temperature unitized regenerative fuel cell realizing 60% round trip efficiency and 10 000 cycles of durability for energy storage applications