Engineering Co2+ coordination in α-Co(OH)2 and its conversion to Co3O4 nanoparticles for application in asymmetric supercapacitors.

Owing to unique redox behaviour and structural versatility, cobalt hydroxide/cobalt oxide-based nanomaterials have emerged as promising materials for energy storage. Relation between coordination environment of Co2+ and its effect on electrochemical behaviour remains unexplored. α-Co(OH)₂ contains Co2+ in octahedral coordination (Co2+Oh). Engineering Co2+ coordination to tetrahedral (Co2+Td) can significantly affect 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 Co2+Oh,  whereas at higher concentration (50 mmol), blue colored α-Co(OH)₂ nanorods (Co(OH)₂-BP) are formed with both Co2+Oh and Co2+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. α-Co(OH)₂ was thermally decomposed to obtain Co3O4 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, which showed 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. Insights from this study will significantly impact development of advanced energy storage materials.