Nanoparticles were then mixed with Vulcan carbon CX72 with a mass ratio of C:XFe2O4 = 40:60. The as-prepared C/XFe2O4 (XFe2O4 = 60 w%) material and polyvinylidene difluoride (PVDF) were mixed in a mass ratio of 90:10 and dispersed in N-methyl pyrrolidone (NMP). The resulting slurry was pasted onto nickel foam substrate (1 cm × 1 cm) with a spatula. After drying at 80 oC for 12 hours, the electrode was pressed at a 10 MPa pressure. Electrochemical Characterization The electrochemical measurements were performed using the three-electrode method in 1 M Na2SO4 electrolyte within the potential window of 0 ~ 0.9 V vs Ag/AgCl. Platinum foil and Ag/AgCl electrode (filled with saturated KCl) were used as counter and reference electrodes, respectively. The cyclic voltammetry curves were collected on a Solartron 1287A potentiostat at the scan rate of 5 m
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