Fast in situ synthesis of CoFe layered double hydroxide onto multi-layer graphene for electrochemical capacitors

Homogeneous CoFe layered double hydroxide (LDH) was in situ synthesized onto the surfaces of multi-layer graphene (MLG) based on π-π interaction method just for 1-h reaction. The morphology, crystal structure, and electrochemical properties of the composite were systematically studied. The results showed that crystalline CoFe-LDH grown onto MLG was composed of two layers. The bottom layer was parallel to graphene sheet to increase the contact with graphene. The external layer was vertically assembled to allow the easy immersion of electrolyte into the pores between CoFe-LDH sheets. When being tested as electrochemical capacitor electrode, the composite could deliver a specific capacitance of 882.5 F g−1 at a current density of 1 A g−1. When prolonging the reaction time to 3 h, homogeneous CoFe oxide nanoparticles with a size of about 5 nm would be formed on the surface of CoFe-LDH. Its specific capacitance decreased to 462.5 F g−1 at 1 A g−1, but its cycle stability and rate capability were both improved. All CoFe-LDH sheets were transformed into CoFe oxide nanoparticles when the reaction time reaches 10 h, resulting in a further decrease of specific capacitance to 82.5 F g−1. Thus, to obtain a high electrochemical performance, the reaction time should be controlled because the phases and morphologies of the products are dependent on it.