Solar water splitting offers a sustainable and clean alternative energy source compared to fossil fuels, which are diminishing in supply and dangerously affecting our environment.  In order to improve the viability of photoelectrochemical water splitting as a frontier technology of the future, it is critical to understand the fundamental mechanisms involved in the oxygen and hydrogen evolution reactions, and focus on systems that use cheap earth abundant materials.  There have been significant advances in recent years on the oxygen evolution half-reaction. In particular, new catalysts with remarkable activity have been discovered to be coupled to semiconductor photoanodes [1,2].  However, the mechanism for how these catalysts perform in photoelectrochemical systems is not uniformly agreed upon.  In this study, we have investigated the electrochemical, optical, and catalytic properties of two borate-based catalysts: cobalt-borate (Co-Bi) and nickel-borate (Ni-Bi). We also compared their solar water splitting performance as co-catalysts when combined with spray deposited BiVO₄.

            Although the two catalysts we studied are structurally similar, it has been suggested that they oxidize water via different mechanisms. In particular, the cobalt-borate oxygen evolution catalyst (OEC) performs two oxidation steps, first rising from the 2+ to 3+ state, and then to the 4+ state where it is reported to be catalytically active.  This is, however, not the case for the Ni-Bi catalyst. After oxidizing from the 2+ to 3+ state, there are conflicting interpretations of the catalytically active state that efficiently oxidizes water; 3+ or 4+.  In order to determine this, we prepared the catalyst at different electrochemical potentials with respect to the observed oxidation peaks.  The electronic, optical, and morphological properties of both catalysts prepared in different oxidation states were examined by XPS, in-situ UV-vis, and AFM.  The results indicated that appreciable changes in the physical properties of both catalysts resulted from the potential at which they were prepared.  We were able to observe two changes in oxidation state for the Co-Bi catalyst, but only observed a single change in the Ni-Bi catalyst,.  This implies that the two borate catalysts work via two different mechanisms, and thus may find different preferential applications for use as electrocatalysts and co-catalysts with semiconductor photoanodes.