Engineering Co²⁺ Coordination in α-Co(OH)₂ and its Conversion to Co₃O₄ Nanoparticles for Application in Asymmetric...

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 Co²⁺ and its effect on their electrochemical behaviour remains unexplored. α-Co(OH)₂ contains Co²⁺ in octahedral coordination (Co²⁺ _Oh). However, careful engineering of Co²⁺ coordination to tetrahedral (Co²⁺ _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 Co²⁺ _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 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. The enhancement results from favourable transformation of Co²⁺ _Td to electroactive Co³⁺ in CoOOH, high surface area (99 m² g^-1) and small crystallite size (23.5 nm) of Co(OH)₂-BP. 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, 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.