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Iron-group oxides are among the best earth abundant catalysts for the oxygen evolution reaction (OER) in alkaline and neutral electrolytes [1],[2]. Such oxides have been prepared with an amorphous or a crystalline structure, using thermal salt decomposition [3], autoclave synthesis [4], photochemical reactions [5], sol-gel synthesis [6], metal electrodeposition and subsequent oxidation, [7] and direct oxide electrodeposition [1], [8]. This work focuses on well-defined epitaxial CoOx [9] electrocatalysts grown by direct electrodeposition on Au(111) according to an approach similar to that introduced by Switzer [8]. The growth modes of catalysts and the mechanisms of deposition will be discussed based on electrochemical characterizations, XRD characterizations and microscopic observations. In particular to explain that CoOOH may be obtained at a potential of +1V/RHE by oxidation of a Co(II) complex diluted in an alkaline solution while thermodynamics predicts that Co3O4 should be formed. The electrochemical properties in pre-OER and within OER will be discussed as a function of the surface structure and morphology of layers. In particular to show that the large pseudocapacitance measured at the electrode is not related to its ECSA but to electrical charging within the bulk of the electrode. [1] M. W. Kanan and D. G. Nocera, Science 321, 1072 (2008). [2] C. C. L. McCrory, S. Jung, I. M. Ferrer et al., Journal of the American Chemical Society 137, 4347 (2015). [3] C. Pirovano and S. Trasatti, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 180, 171 (1984). [4] D. Yuming, H. Kun, Y. Lin et al., Nanotechnology 18, 435602 (2007). [5] S. R. Alvarado, Y. Guo, T. P. A. Ruberu et al., The Journal of Physical Chemistry C 116, 10382 (2012). [6] A. Bergmann, E. Martinez-Moreno, D. Teschner et al., Nature Communications 6, 8625 (2015). [7] M. W. Louie and A. T. Bell, Journal of the American Chemical Society 135, 12329 (2013). [8] J. A. Koza, Z. He, A. S. Miller et al., Chem. Mat. 24, 3567 (2012). [9] F. Reikowski, F. Maroun, I. Pacheco et al., ACS Catalysis 9, 3811 (2019).

Iron-group oxides are among the best precious-metal-free catalyst for the oxygen evolution reaction (OER) in alkaline media [1] and thus of great interest for large-scale electrochemical water splitting into O2 and H2. A wide variety of different catalysts, based on oxides of Co, Fe, and Ni, have been synthesized and studied, exhibiting rather diverse nanoscale morphologies and usually an unknown surface structure. This makes it difficult to compare their reactivity and correlate it with ab initio theoretical studies, which hinders the development of clear structure-reactivity relationships and unambiguous determination of the OER reaction mechanism. In this work we report in situ and operando surface x-ray diffraction (SXRD) studies of well-defined Co3O4 and b-CoOOH electrocatalysts [2]. Ultrathin epitaxial films (10-20 nm thickness) of these materials were electrodeposited on Au single crystals from a dilute Co(II) tartrate solution of pH 14, using a modified version of the procedure described in Ref. [3]. Characterization by SXRD and AFM revealed that epitaxial Co3O4(111) and β-CoOOH(001) deposits were obtained in 2 M and 5 M NaOH, respectively. The film orientation is independent of the substrate and in the surface plane aligned with the Au(111) and Au(001) substrate lattice. The β-CoOOH(001) film deposits were found to be smooth with clearly visible atomic steps. The Co3O4 films exhibits a more island-like morphology, which can be controlled by the deposition parameters. The potential-dependent structure of these model oxide electrocatalysts during cyclic voltammograms was studied at pH values between 7 and 13 over a wide potential range up to OER current densities of 150 mA/cm2. For this, a custom designed electrochemical cell was used that allows high-resolution SXRD characterization together with electrochemical and optical reflectivity measurements while gas evolution proceeds. The two oxide materials show very different behavior. The CoOOH(001) samples were perfectly stable over a wide potential range. In the case of Co3O4(111), fast and fully reversible structural changes are observed, in good agreement with a previous operando study of polycrystalline Co3O4 catalysts [4]. Specifically, the near surface region of Co3O4(111) is converted to a new phase, which is assigned to highly disordered CoOOH. This restructuring starts at potentials 300 mV negative of the onset of the OER, indicating that the process is related to the thermodynamically predicted Co3O4 / β-CoOOH(001) transition rather than to the catalytic reaction. The formed skin layer is of defined thickness, which changes linearly with applied potential. In addition, continuous changes of the Co3O4 unit cell volume are found over a wide potential range, which are assigned to changes in the oxide bulk, related to the pseudo-capacitive charging current in the pre-OER potential range. The kinetics of these structural changes is fast and highly reversible. Furthermore, systematic studies of Co3O4(111) films with different morphology show a smaller skin layer thickness and lower magnitude of the bulk oxide changes for smoother deposits, suggesting an important role of more open oxide facets for the structural conversion. Surprisingly, the catalytic activity of the skin layer covered Co3O4(111) and that of the smooth β-CoOOH(001) are almost identical, if the true microscopic surface area is taken into account. This indicates that the number of OER active sites on the two oxides is similar, despite the very different defect density, and is at variance with previous suggestions that di-µ-oxo bridged Co cations are exclusively responsible for the OER activity of Co oxides. For the smooth β-CoOOH(001) a turnover frequency of 4.2 s-1 per surface atom is determined at an overpotential of 400 mV, which exceeds that estimated in most studies of polycrystalline Co oxide electrocatalysts. [1] C. C. L. McCrory et al., Journal of the American Chemical Society 2015, 137, 4347. [2] F. Reikowski, et al., ACS Catalysis, 2019, 9, 3811 [3] J. A. Koza et al., Chem. Mat. 2012, 24, 3567. [4] A. Bergmann, et al., Nature Communications, 2015, 6, 8625. Figure 1