Solvothermal synthesis of Ni(HCO3)2/graphene composites toward supercapacitors and the faradiac redox mechanism in KOH solution

In this work, a series of composites consisting of Ni(HCO3)2 and graphene nanosheets (GS) have been prepared by a facile solvothermal method, and then their application as electrode materials for supercapacitors has been investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge tests. Morphological and structural analyses by field-emission scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy indicated that Ni(HCO3)2 particles deposited on the GS and formed a loosely packed microstructure, actualizing the successful combination of Ni(HCO3)2 particles with GS. Among the prepared composites, the sample of Ni(HCO3)2/GS2 exhibited the highest capacitance of 1200 F g−1 at a current density of 4 A g−1, illustrating that such composite is a promising candidate as electrode material for supercapacitors. Moreover, the Faradic redox mechanism of the Ni(HCO3)2/GS composite was further studied in virtue of XRD analysis, which revealed that the Ni(HCO3)2 phase could be quickly transformed into Ni(OH)2 phase by an electrochemically induced phase transformation process during the galvanostatic charge-discharge tests.