Synthesizing individual cobalt hydroxides from an identical starting material may aid in discerning the relationship between the crystal structure and catalytic activity. Herein, the crystal structure of α-Co(OH)2 and its transformation into other cobalt hydroxides were investigated. In particular, α-Co(OH)2 comprised a layered structure with anion intercalation, which was identified by Rietveld refinement analysis. This corresponds to the chemical formula, [CoOh0.77CoTd0.23(OH)1.77]0.23+[Cl0.230.64H2O]. Chemical phase transformations of metastable α-Co(OH)2 into various other cobalt hydroxides, such as β-Co(OH)2, γ-CoOOH, and β-CoOOH, were selectively performed, and their structures were characterized. In addition, electrochemical oxidative phase transformations of α-Co(OH)2 and β-Co(OH)2 into γ-CoOOH and β-CoOOH, respectively, were successfully accomplished. Interestingly, electrochemical cation intercalation occurs only when the structure changes from α-Co(OH)2 to γ-CoOOH. The cations in the electrolyte were selectively inserted into the layered structure to form the γ-CoOOH phase. The prepared cobalt hydroxides were subjected to the oxygen evolution reaction (OER) and chloride oxidation reaction (COR) to discern the correlation between the crystal structure and activity. The γ-CoOOH showed better OER catalytic activity than β-CoOOH. In contrast, β-CoOOH showed higher COR selectivity than γ-CoOOH at low chloride concentrations. The importance of the crystal structure for each electrocatalytic reaction is elucidated.

Chemical and electrochemical phase transformations of cobalt hydroxide for electrochemical water splitting

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