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Co-based catalysts are promising candidates to replace Ir/Ru-based oxides for oxygen evolution reaction (OER) catalysis in an acidic environment. However, both the reaction mechanism and the active species under acidic conditions remain unclear. In this study, by combining surface-sensitive soft X-ray absorption spectroscopy characterization with electrochemical analysis, we discover that the acidic OER activity of Co-based catalysts are determined by their surface oxidation/spin state. Surfaces composed of only high-spin CoII are found to be not active due to their unfavorable water dissociation to form CoIII-OH species. By contrast, the presence of low-spin CoIII is essential, as it promotes surface reconstruction of Co oxides and, hence, OER catalysis. The correlation between OER activity and Co oxidation/spin state signifies a breakthrough in defining the structure-activity relationship of Co-based catalysts for acidic OER, though, interestingly, such a relationship does not hold in alkaline and neutral environments. These findings not only help to design efficient acidic OER catalysts, but also deepen the understanding of the reaction mechanism. Co-based catalysts are promising candidates for oxygen evolution reaction catalysis in an acidic environment, but the structure-activity relationship is unclear. Here, the authors discover that their acidic water oxidation activity is determined by the surface oxidation/spin state. [ABSTRACT FROM AUTHOR]

In this work, results of structural studies, performed as a function of temperature (13-1073 K) of NaCoO cathode material, are presented, together with microstructural measurements, including detailed transmission microscopy examination, as well as electrochemical studies, showing current rate-dependent charge/discharge characteristics and also voltammetry tests. It was found that thermal expansion of P2 type ( P6/ mmc) NaCoO differs along a- and c-axes. Microstructural studies showed platelet-like grains of the synthesized sodium cobaltate, while detailed transmission microscopy measurements indicated coexistence of well-defined layered regions with regions having somewhat disturbed atomic arrangement. For higher current densities, characteristic plateaus observed on Na/Na/NaCoO charge/discharge curves diminish, and the voltage dependence on the sodium concentration is smoothed out. Results of the electrochemical tests, performed on cathodes without any carbon additives, indicated that in fact, such additives influence electrochemical behavior during first charge considerably. Additional voltammetry investigations confirmed presence of nine characteristic anodic and cathodic peaks, corresponding well with plateaus observed during charge/discharge of the cells. [ABSTRACT FROM AUTHOR]