Oxygen Evolution Activity of Amorphous Cobalt Oxyhydroxides: Interconnecting Precatalyst Reconstruction, Long-Range Order, Buffer-Binding, Morphology, Mass Transport, and Operation Temperature

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)]center dot H2O, is fully reconstructed into two different CoCat phases. In contrast to previous reports, this reconstruction is not initiated at the surface but at the electrode substrate to catalyst interface. Ex situ 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, a comprehensive picture is provided 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 bonded buffers, and the morphology. With this acquired knowledge, an optimized OER system is realized operating in near-neutral potassium borate medium at 1.62 +/- 0.03 V-RHE yielding 250 mA cm(-2) at 65 degrees C for 1 month without degrading performance.

Automated summary

In this study, a layered cobalt borophosphate precursor is reconstructed into two different cobalt oxyhydroxide phases, and the electrocatalytic properties of various cobalt oxyhydroxides and phosphate derivatives are compared. The choice of buffer, the number of edge sites not inhibited by buffers, and the morphology are identified as important factors affecting the oxygen evolution reaction activity.