The identification of structure-activity correlations and increased knowledge about the catalytically-active state of electrocatalysts for oxygen evolution reaction (OER) precedes the development of improved synthesis routes to prepare active, stable and inexpensive electrocatalysts for the electrochemical water splitting. This is ultimately necessary to commercialize water electrolysis for an efficient transformation of electrical energy from renewable power sources into the chemical energy of hydrogen bonds. Oxides and (oxy)hydroxides of 3d transition metals represent a promising alternative to noble metal based catalyst for OER for application in neutral and alkaline electrolyte.[1-2]

In this work, the structure of the catalytically-active state of four differently-coordinated Co oxides was determined and structure-activity correlations were identified. The morphology, crystal and local atomic structure as well as the electronic structure of the four Co oxides were investigated in the as-prepared state as well as in an OER conditioned state using electron microscopy, diffraction, and X-ray absorption spectroscopy recorded at the Co K- and L₃- as well as O K-edge. Additionally, the near-surface composition was determined using synchrotron-based X-ray photoelectron spectroscopy. We confirmed the synthesis of Co oxides with only Co²⁺ O_h, Co²⁺ T_d, and Co³⁺ O_h ions in the oxide as well as the mixed-valent Co₃O₄. The electrocatalytic activity and the Co oxide redox behavior were investigated in neutral, phosphate-containing electrolyte using quasi-stationary potential step experiments and cyclic voltammetry. The structure of the catalytically-active state was determined using in situ X‑ray diffraction and absorption spectroscopy.

With these investigations, a common structural motif in the catalytically-active state of the Co oxides independently from their initial Co coordination was unraveled. For the crystalline Co₃O₄ a near-surface amorphization of the crystallites at elevated oxygen evolution was identified.[3] Furthermore, a correlation between the near-surface electronic and local atomic structure, the Co oxide redox chemistry and the electrokinetic properties of the Co oxides was found. Our findings allow conclusion regarding the electrochemical and spectroscopic identification of activity-determining properties of the Co oxides and furthermore, might pave the road towards new synthesis concepts to design improved electrocatalysts for oxygen evolution reaction and electrochemical water splitting.

[1]        W. T. Hong, M. Risch, K. A. Stoerzinger, A. Grimaud, J. Suntivich, Y. Shao-Horn, Energy Environ. Sci. 2015, 8, 1404-1427.

[2]        M. W. Kanan, D. G. Nocera, Science 2008, 321, 1072-1075.

[3]        A. Bergmann, E. Martinez-Moreno, D. Teschner, P. Chernev, M. Gliech, J. F. d. Araújo, T. Reier, H. Dau, P. Strasser, Nat. Commun. 2015, 6:8625.