(334d) Active Structures and Species of Modified Oxide Catalysts for the Oxygen Evolution Reaction (OER)

A major cause of energy losses in photocatalytic systems for solar fuels, and also in a number of other emergent technologies such as rechargeable metal-air batteries, water splitting devices, and unitized regenerative fuel cells, is the oxygen evolution reaction (OER). 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. IrO_x and RuO_x).

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 (CuO_x) [1] and Ni-modified cobalt (oxy)hydroxides [2] to reveal the OER active species and structures of these catalysts. In the case of Ce-modified CuO_x, Ce incorporation (6.9 at%) into CuO_x led to 3.3 times greater OER activity compared to pure CuO_x 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 CoO_xH_y, 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 CoO_xH_y, reveal the importance of chemical state and local structure considerations for the rational design of improved oxide-based OER catalysts.

[1] Z. Chen, C.X. Kronawitter, X. Yang, Y.-W. Yeh, N. Yao, and B.E. Koel, Phys. Chem. Chem. Phys., 2017, 19, 31545-31552.

[2] Z. Chen, L. Cai, X. Yang, C. Kronawitter, L. Guo, S. Shen, B. E. Koel, ACS Catal., 2018, 8, 1238−1247.