Chemical and electrochemical synthesis of cobalt hydroxides: selective phase transformation and application to distinct electrocatalytic reactions

Individual cobalt hydroxides with clearly identified crystal structures can aid in discerning the relationship between the crystal structure and catalytic activity. Herein, the crystal structure of alpha-Co(OH)(2) and its transformations to other cobalt hydroxides are fully investigated. The alpha-Co(OH)(2) structure comprised a layered structure with anion intercalation, [(Co0.77Co0.23Td)-Co-Oh(OH)(1.77)](0.23+)[Cl-0.23 center dot 0.64H(2)O], which was identified by Rietveld refinement analysis using X-ray diffraction. Phase transformations of metastable alpha-Co(OH)(2) into various other cobalt hydroxides, such as beta-Co(OH)(2), gamma-CoOOH, and beta-CoOOH, were selectively performed through a variation in the chemical environment, and their structures were fully characterized. In addition, electrochemical oxidative transformations of alpha-Co(OH)(2) and beta-Co(OH)(2) into gamma-CoOOH and beta-CoOOH, respectively, were successfully accomplished. The prepared cobalt hydroxides were applied to the oxygen evolution reaction (OER) and chloride oxidation reaction (COR) to investigate the correlation between the crystal structure and activity. The gamma-CoOOH showed better OER catalytic activity than beta-CoOOH. In contrast, beta-CoOOH showed higher COR selectivity than gamma-CoOOH. Interestingly, the faradaic efficiency of the COR was over 97% on beta-CoOOH at a concentration of 0.1 M NaCl, and this value is the highest COR faradaic efficiency compared to all other electrodes. The importance of the crystal structure for each electrocatalytic reaction is elucidated.