Realizing Breakthrough Round-Trip Efficiency of the Anion-Exchange Membrane-Unitized Regenerative Fuel Cell Double-Layer Oxygen Electrode with a Carbon Nanotube Skeleton

The oxygen electrode, where both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) occur, is an important contributor to the round-trip efficiency (RTE) of anion-exchange membrane-unitized regenerative fuel cells (AEM-URFCs). Herein, we propose an oxygen electrode comprising double catalyst layers with a macroporous OER outer layer to increase the RTE and operating current density. Two structural characteristics, a double-layer (DL) design and a macroporous outer layer, were applied to this oxygen electrode. Separate ORR and OER layers in the oxygen electrode improve the catalytic utilization. The macroporous outer layer of the OER, in which electrocatalyst nanoparticles (NPs) are dispersed onto carbon nanotube (CNT) scaffolds with square macropores, can enhance the performance of the OER. Consequently, the DL oxygen electrode exhibited a higher RTE at 20 mA cm^–2 (59.2%) than a conventional electrode (55.4%) due to the oxygen electrode’s featured design. The RTE achieved in this study is the highest reported in the literature for AEM-URFCs despite operating at the ultrahigh current density of 750 mA cm^–2.