Transition metal doping effects in Co-phosphate catalysts for water splitting studied with XAS

Metal oxides as oxygen evolution reaction (OER) catalysts for water splitting are ubiquitous in research and application. Pure and doped (or hybrid) Co oxide systems are of particular interest due to their good efficiency. However, the electronic effects of different dopants are still unclear in many of these systems. We present a study of doped Co-phosphate (Pi) films deposited electrochemically from aqueous solutions of neutral pH using an X-ray absorption spectroscopy (XAS), a technique that can reveal important information about catalytically active states. These hybrid films, obtained from solutions containing both Co ions and another transition metal (TM) ion (TM = Mn, Fe, Ni), were analyzed with XAS at the TM L-edges and the O K-edge. We find that a large concentration of Co3+-ions in the films and a low-lying edge of the O 2p conduction band (CB) are good indicators for the OER efficiency of the films. Our results show that native Co-Pi is close to optimal for the OER activity at low deposition potential. However, Mn- and Ni-doped systems have promising properties when deposited at higher potentials because these ions tend to stabilize the Co3+-state in the films as well as the position of the O 2p-edge CB (a few tenths of 1 eV), in contrast to native Co-Pi films.