Phase transformation induced capacitance activation for 3D graphene-CoO nanorod pseudocapacitor

Development of a pseudocapacitor over the integration of metal oxide on carbonaceous materials is a promising step towards energy storage devices with high energy and power densities. Here, a self-assembled cobalt oxide (CoO) nanorod cluster on three-dimensional graphene (CoO-3DG) is synthesized through a facile hydrothermal method followed by heat treatment. As an additive-free electrode, CoO-3DG exhibits good electrochemical performance. Compared with CoO nanorod clusters grown on Ni foam (i.e., CoO-Ni, ≈680 F g^-1 at 1 A g^-1 and ≈400 F g^-1 at 20 A g^-1), CoO-3DG achieves much higher capacitance (i.e., ≈980 F g^-1 at 1 A g ^-1 and ≈600 F g^-1 at 20 A g^-1) with excellent cycling stability of 103% retention of specific capacitance after 10 000 cycles. Furthermore, it shows an interesting activation process and instability with a redox reaction for CoO. In addition, the phase transformation from CoO nanorods to Co₃O₄ nanostructures was observed and investigated after charge and discharge process, which suggests the activation kinetics and the phase transformable nature of CoO based nanostructure. These observations demonstrate phase transformation with morphological change induced capacitance increasement in the emergent class of metal oxide materials for electrochemical energy storage device. An ex situ method demonstrates that the capacity increase of cobalt oxide nanorod cluster on 3D graphene (CoO-3DG) pseudocapacitors is due to phase transformation from nanorods to nanoflowers with a valence change of Co. This evidence could help to understand the activation process phenomenon in metal oxide pseudocapacitors.