Phase and morphology evolution of ultrathin Co(OH)2 nanosheets towards supercapacitor application

Ultrathin α-Co(OH)2 nanosheets (NSs) were hydrothermally synthesized without using surfactants and templates. Interestingly, the ultrathin α-Co(OH)2 NSs evolved from graphene-like NSs to hexagonal nanoplates (NPs) and then transformed into a needle-like Co(CO3)0.5(OH)·0.11H2O phase with increasing reaction temperature and time. The phase and morphology evolution depended strongly on the CO32− ions in the solution. We ruled out the possibility that CO32− originated from the adsorption of CO2 in air and proposed the possible phase transition mechanism from α-Co(OH)2 to Co(CO3)0.5(OH)·0.11H2O, which was due to the decomposition of hexamethylenetetramine driven by dynamics and thermodynamics. The morphology evolution could be regarded as the dissolution–recrystallization–redissolution–recrystallization of α-Co(OH)2. The electrochemical behavior tests demonstrated that the ultrathin α-Co(OH)2 electrode exhibited an excellent specific capacitance of 833.4 F g−1 at 5 A g−1; however, the specific capacitance of the needle-like Co(CO3)0.5(OH)·0.11H2O electrode was 247.2 F g−1 at the same current density. This excellent electrochemical property is attributed to the crystalline phase and ultrathin nature of the materials.