Precious Metal-Free CoP Nanorod Electrocatalyst as an Effective Bifunctional Oxygen Electrode for Anion Exchange Membrane-Unitized Regenerative Fuel Cells

In this study, noble metal-free Co(OH)F and CoP nanorod electrocatalysts were prepared and explored as bifunctional oxygen electrodes (BOE) in anion exchange membrane-unitized regenerative fuel cells (AEM-URFCs). A CoP nanorod was synthesized from Co(OH)F via the hydrothermal treatment of cobalt nitrate, ammonium fluoride, and urea, followed by phosphorization. The crystal structures, surface morphologies, pore distributions, and elemental statuses of the obtained catalysts were analyzed to identify the changes caused by the incorporation of fluorine and phosphorus. The presence of F and P was confirmed through EDS and XPS analyses, respectively. Using these catalysts, the AEM-based URFCs were operated with hydrogen and oxygen in the fuel cell mode and pure water in the electrolysis mode. In addition, the electrocatalytic activities of the catalysts were evaluated using cyclic voltammetry and electrochemical impedance spectroscopy. In the AEM-URFC test, the CoP catalyst in the BOE delivered the best performance in the fuel cell mode (105 mA cm(-2) at 0.3 V), and Co(OH)F was suitable for the water electrolyzer mode (30 mA cm(-2) at 2.0 V). CoP and Co(OH)F exhibited higher round trip efficiency (RTE) and power densities than the conventional Co3O4 catalyst.