Proceedings of nanoGe Fall Meeting 2018 (NFM18)
Publication date: 6th July 2018
In photoelectrolyzers for storage of solar energy in form of molecular hydrogen, the choice of alkaline environment favours the oxygen evolution reaction (OER) and stabilizes the catalyst material. On the other hand, it introduces severe disadvantages: it is corrosive and hinders the use of efficient ion transfer membranes. This increases the importance of development of water-oxidation catalysts active in the neutral pH range. One of the most prominent examples of OER catalysts operated in the near-neutral pH regime 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 CoII/III and CoIII/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 mechanistic changes that favour oxygen production at alkaline pH, we performed in-situ and quasi-in-situ (freeze-quench) X-ray absorption spectroscopy (XAS) during electrochemistry. XAS measurements revealed a reduction of the potential of Co oxidation with pH increase. For the two Co oxidation steps (CoII/III and CoIII/IV), we identified different types of pH dependencies (Figure). We also found that at alkaline pH the majority of CoII atoms are oxidized at very low potential, through an irreversible slow process that is coupled to an increase in the size of the CoOx fragments. Another factor improving the performances in alkaline electrolyte is the presence of hydroxide ions that can act as proton acceptor preventing the build up of local pH gradients. We used in-situ Raman spectroscopy to assess the local pH at different distances (μm scale) from the catalyst surface, in the case of neutral electrolyte we observed the formation of a pH gradient.
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