The effects of growth time and interface between the iron oxyhydroxide (FeOOH) and carbon materials (carbon nanotubes (CNT) and reduced graphene oxide (RGO)) to form an asymmetric supercapacitor was studied by X-ray absorption spectroscopy (XAS) and electrochemical measurements. FeOOH/CNT (FCNT) and FeOOH/RGO (FRGO) were successfully synthesized by a simple spontaneous redox reaction with FeCl3. The RGO functions as an ideal substrate, providing rich growth sites for FeOOH, and it is believed to facilitate the transport of electrons/ions across the electrode/electrolyte interface. FRGO has been identified as a supercapacitor and found to exhibit significantly greater capacitance than FCNT. To gain further insight into the effects of growth times and the interface of FeOOH for FCNT and FRGO, the electronic structures of FCNT and FRGO with various FeOOH growth times were elucidated by XAS. The difference between the surface electronic structures of CNT and RGO yields different nucleation and growth rates of FeOOH of FeOOH. RGO with excellent interface properties arises from a high degree of covalent functionalization, and/or defects make it favorable for FeOOH growth. FRGO is therefore a promising electrode material for use in the fabrication of asymmetric supercapacitors. In this work, coupled XAS and electrochemical measurements reveal the electronic structure of the interface between FeOOH and the carbon materials and the capacitance performance of asymmetric supercapacitors, which are very useful in the fields of nanomaterials and nanotechnology, especially for their applications in storing energy
