Functionality of Fe(NO3)3 salts as both positive and negative pseudocapacitor electrodes in alkaline aqueous electrolyte

For the first time, we designed a Fe(NO3)3 salt supercapacitor in alkaline aqueous electrolyte, which can be served as active components of both positive and negative electrodes and has high potential window of 1.6 V, higher than the theoretical operating voltage of water (1.23 V). After undergoing the chemical coprecipitation and electrochemical redox reactions in KOH electrolyte, Fe3+ cations were in-situ crystallized into high electrochemical active goethite FeOOH colloids. The charge storage mechanism observed in both positive and negative electrodes is pseudocapacitive reaction between FeOOH and Fe(OH)2 phases. In the three-electrode system, Fe(NO3)3 electrodes showed wider negative potential window (−1.2−0 V) compared to positive potential window (0−0.45 V). The Fe(NO3)3 negative electrodes exhibited 120 F/g capacitance, 23.6 Wh/kg energy density and 0.6 kW/kg power density, while the Fe(NO3)3 positive electrodes displayed 393 F/g, 10.4 Wh/kg and 0.22 kW/kg at the current density of 1 A/g. As positive and negative electrodes, Fe cation utilization ratios can reach 72% and 60%, respectively. The assembled symmetric Fe(NO3)3 supercapacitor device can exhibit remarkable energy density of 11.9 Wh/kg at the power density of 0.4 kW/kg. For pure iron oxides/hydroxides electrodes, these values are larger than that reported in previous literatures. The present results demonstrated the high performances of Fe(NO3)3 symmetric supercapacitors and the potentialities of using Fe(NO3)3 electrodes as negative and/or positive electrodes for supercapacitors with high energy densities.