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Bruce Koel1

1, Princeton University, Princeton, New Jersey, United States

The oxygen evolution reaction (OER) is a major cause of energy losses in photocatalytic systems for solar fuels, but also in a number of other emergent technologies such as rechargeable metal-air batteries, water splitting devices, and unitized regenerative fuel cells. The low energy efficiencies of OER are the result of its sluggish reaction kinetics and large overpotentials. Significant improvements to the OER activity of transition metal oxides (TMOs) have been made by tailoring the morphology and crystal structure of the catalysts, incorporating dopants, as well as using conductive supports. However, clear structure-activity correlations remain elusive because of the complex composition and structure of TMO catalysts. Further understanding of these relationships for these promising catalysts for oxygen evolution may lead to additional developments that will enable TMOs to replace precious metal-based OER catalysts (e.g. IrOx and RuOx). Insights from two recent examples of our studies will be discussed. We have utilized a range of spectroscopic techniques for characterization of Ce-modified copper oxide (CuOx) and Ni-modified cobalt (oxy)hydroxides to reveal the OER active species and structures of these catalysts. In the case of Ce-modified CuOx, Ce incorporation (6.9 at%) into CuOx led to 3.3 times greater OER activity compared to pure CuOx and this was coincident with significant structural changes due to an increasing amount of disorder. By combining X-ray photoelectron and Raman spectroscopy techniques, a strong correlation between OER performance with tetravalent Ce (Ce4+) ions was observed up to a concentration corresponding to CeO2 phase segregation. We propose a strong promoting effect of Ce4+ for OER in this system. In the case of Ni-modified CoOxHy, operando Raman spectroscopy was used to reveal a drastic transformation of a spinel Co3O4 like structure into a more active (oxy)hydroxide structure under applied potential. Such a transformation was only observed in the presence of uniformly distributed Ni ions. These two examples, i.e. the promoting effect of Ce4+ and the formation of active OER structures in Ni-modified CoOxHy, reveal the importance of chemical state and local structure considerations for the rational design of improved oxide-based OER catalysts.

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