The crystalline structure of the powder samples was identified by a XRD diffractometer (Stoe STADI P) with Mo Kα radiation (λ=0.7093 Å). Rietveld refinements were then conducted using General Structure Analysis System (GSAS) II. The morphology of the powder samples was observed by a scanning electron microscope (SEM, JEOL JSM-7600F) and a transmission electron microscope (TEM, JEOL JEM-2100). Infrared data was collected using a Fourier transform infrared spectroscopy (Bruker ALPHA compact FT-IR spectrometer). The chemical composition of the samples was collectively determined by thermogravimetric analysis (TGA, Netzsch TG 209 F1 Libra) at a rate of 5 K min -1 in nitrogen, microwave plasma atomic emission spectroscopy (MP-AES, Agilent 4210 MP-AES), and elemental analysis (Thermo Flash 2000). The Raman spectra of the electrodes were recorded with 633 nm laser (Renishaw Raman spectrometer). The XRD patterns of the electrodes were obtained by an XRD diffractometer (Stoe STADI P) with Cu Kα radiation (λ=1.5406 Å). The K/Fe ratio of the electrodes was measured by energy dispersive X-ray spectroscopy (EDS) attached to the SEM. X-ray absorption spectroscopy (XAS) at Fe K-edge were recorded at the B18 beamline of Diamond Light Source (UK) using Si (111) double crystal monochromator. The electrode samples were measured as the original films. The pristine samples and reference samples were pelletized before the measurements. The spectra were collected in fluorescence mode. Data of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) was processed using Athena program.