In photoelectrolyzers for storage of solar energy in form of molecular hydrogen, the choice of alkaline environment favors the oxygen evolution reaction (OER) strongly (500 times higher current densities) and stabilizes the catalyst material. On the other hand, it introduces severe disadvantages: it is corrosive and impedes the coupling of OER with CO₂ reduction. This increases the importance of development of water-oxidation catalysts active in the neutral pH range. One prominent example is the amorphous Co–based and phosphate containing catalyst (CoCat) proposed in 2008 by Kanan and Nocera [1]. Previous investigations of CoCat films on conducting electrodes revealed metal atoms assembled in fragments of edge-sharing CoO₆ octahedra, which are accumulating oxidation equivalents through Co^II/III and Co^III/IV oxidation before O₂ formation can take place [2].

We investigated the electrochemical performances of this amorphous oxide in both neutral and alkaline pH regimes. At parity of overpotential, we observed a strong increase in catalytic activity in alkaline electrolyte. In order to understand the factors that favors oxygen production at alkaline pH, we performed in-situ time-resolved X-ray absorption spectroscopy (XAS) during electrochemistry, which revealed an improvement in both the energetics and kinetics of Co redox reactions. The lower onset of OER in alkaline electrolyte is caused by a shift in the equilibrium potential of the Co^III ↔ Co^IV reaction, which was assigned to easier Co^III-OH deprotonation. The faster oxidation state changes observed where attributed to better electron and proton transport inside the bulk of the material, which are due to a crystallization process.

We found that the amorphous CoCat undergoes a spontaneous increase in order resembling transition from an amorphous to a crystalline material when exposed to alkaline electrolyte; the crystallization is coupled with a major change in Co average oxidation state. Raman spectroscopy and analysis of EXAFS data were employed to follow the structural changes, which resulted in a structure with bigger CoOx clusters, less defects and strong similarities to CoO(OH). The loss of amorphicity improved electron transfer but reduced number of Co active sites. We propose a technique for stopping the crystallization process, by applying a strong oxidative potential immediately after exposing the catalyst to alkaline solution.