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Abstract: Cobalt hydroxide and other first-row transition metal hydroxides have gained significant attention as pseudocapacitor materials due to their rapid and reversible redox processes. Their layered structures facilitate interactions between electrolyte anions and cobalt cation sites within the bulk of the material, enabling higher charge density and extending redox activity beyond the particle surface. By controlled precipitation under hydrothermal conditions, the structure and morphology of cobalt hydroxides can be optimized to enhance electrochemical performance. Challenging conventional assumptions, surface area alone is not the primary factor driving increased pseudocapacitive performance. The hexagonal hydrotalcite-like structure, characterized by lower skeletal density and larger basal plane spacing, outperforms the monoclinic cobalt carbonate hydroxide structure, achieving an order of magnitude higher capacitance. In situ X-ray absorption spectroscopy provides critical insights into the pseudocapacitive behavior, revealing enhanced accessibility of Co2+ sites for electrochemical oxidation. While monoclinic cobalt carbonate hydroxide exhibits minimal changes in the Co2+ oxidation state, indicative of surface-limited redox activity, the hydrotalcite-like cobalt hydroxides show substantial shifts in the Co K-edge position, highlighting oxidation of Co2+ sites throughout the bulk.
Keywords: pseudocapacitors, layered-double hydroxides, cobalt hydroxide, redox processes, in situ x-ray absorption spectroscopy
Published in RUNG: 14.03.2025; Views: 327; Downloads: 6
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Abstract: Functional layered double hydroxide (LDH) usually contains different cationic substitutes to increase the activity of the oxygen evolution reaction (OER). The intrinsic OER activity of LDH materials is connected with the chemical composition and dispersion of metal cations substitutions in the matrix phase. The potential induced phase transitions, in particular hydroxide-to-oxyhydroxide transitions, are a predisposition for the high OER activity of LDH materials and can be followed by coupling the electrochemical experiments with spectroscopic techniques. The understanding of LDH catalysts under electrochemical conditions also allows an understanding of the behavior of OER catalysts based on transition metals, metal-chalcogenides, -pnictides, -carbides, and metal–organic frameworks. The surfaces of those materials are intrinsically poor OER catalysts. However, they act as precursors to catalysts, which are oxidized into a metal (oxy)hydroxide. This review summarizes the use of in situ techniques for the characterization of LDH-based OER electrocatalysts and presents the influence of these techniques on the understanding of potential induced phase transitions, identification of active sites, and reaction mechanisms.
Keywords: oxygen evolution reaction, layered double hydroxide, in-situ characterization
Published in RUNG: 14.07.2023; Views: 3106; Downloads: 16
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