Nanocrystalline or amorphous cobalt oxyhydroxides (CoCat) are promising electrocatalysts for the oxygen evolution reaction (OER). While having the same short range order, CoCat phases possess different electrocatalytic properties. This phenomenon is not conclusively understood, as multiple interdependent parameters affect the OER activity simultaneously. Herein, a layered cobalt borophosphate precatalyst, Co(H2O)2[B2P2O8(OH)2]∙H2O, is fully reconstructed into two different CoCat phases. In marked contrast to previous reports, this reconstruction is not initiated at the surface but at the electrode substrate to catalyst interface. Ex- and in situ investigations of the two borophosphate derived CoCats, as well as the prominent CoPi and CoBi identify differences in the Tafel slope/range, buffer binding and content, long-range order, number of accessible edge sites, redox activity, and morphology. Considering and interconnecting these aspects together with proton mass transport limitations, we provide a comprehensive picture explaining the different OER activities. The most decisive factors are the buffers used for reconstruction, the number of edge sites that are not inhibited by irreversibly bond buffers, and the morphology of the catalysts. With this acquired knowledge, an optimized OER system is realized operating in near neutral potassium borate media at 1.62±0.03 VRHE yielding 250 mA/cm² at 65 °C for one month without degrading performance.
Oxygen Evolution Activity of Amorphous Cobalt Oxyhydroxides