Engineering Co 2+ Coordination in α‐Co(OH) 2 and its Conversion to Co 3 O 4 Nanoparticles for Application in Asymmetric Supercapacitors

Abstract Owing to their unique redox behaviour and structural versatility, cobalt hydroxide/cobalt oxide‐based nanomaterials have emerged as promising materials for energy storage. However, the interrelation between coordination environment of Co 2+ and its effect on their electrochemical behaviour remains unexplored. α‐Co(OH)₂ contains Co 2+ in octahedral coordination (Co 2+ Oh ). However, careful engineering of Co 2+ coordination to tetrahedral (Co 2+ Td ) can significantly affect the supercapacitive performance. Herein, a simple homogeneous precipitation method is used to achieve this transformation. At low concentration of Co salt (5 mmol), pink‐coloured α‐Co(OH)₂ nanoflakes (Co(OH)₂‐PP) are formed with only Co 2+ Oh , whereas at higher concentration of cobalt salt (50 mmol), blue colored α‐Co(OH)₂ nanorods (Co(OH)₂‐BP) are formed with both Co 2+ Oh and Co 2+ Td . The maximum specific capacity reached 167.5 C g −1 for Co(OH)₂‐BP which showed ~200 % increment as compared to α‐Co(OH)₂‐PP at 10 mV s −1 . The enhancement results from favourable transformation of Co 2+ Td to electroactive Co 3+ in CoOOH, high surface area (99 m 2  g −1 ) and small crystallite size (23.5 nm) of Co(OH)₂‐BP. α‐Co(OH)₂ was thermally decomposed to obtain Co 3 O 4 nanoparticles. The specific capacity of Co₃O₄ nanoparticles derived from Co(OH)₂‐BP and Co(OH)₂‐PP are 136.3 C g −1 and 110.7 C g −1 , respectively, the fomer showing only a marginal increase in specific capacity. An asymmetric supercapacitor device based on Co(OH)₂‐BP//rGO exhibits peak energy density of 14.6 W h kg −1 and peak power density of ~12 kW kg −1 . The insights from this study will significantly impact the development of advanced energy storage materials.