A colloidal pseudocapacitor: direct use of Fe(NO₃)₃ in electrode can lead to a high performance alkaline supercapacitor system.

An electrochemical system including functioned Fe(NO3)3 and alkaline electrolyte is constructed to study the charge storage mechanism upon both thermodynamic calculations and electrochemical measurements. The thermodynamic calculation results demonstrate that increasing KOH concentration of alkaline electrolyte can enhance electrolyte activity but decrease the peak potential, which agrees well with that of electrochemical measurement. The present results indicate that the proposed pseudocapacitive redox reactions are between Fe(3+) and Fe(2+) in our salt electrode of Fe(NO3)3 system, in which the solid Fe(3+) in FeOOH colloids serves as the active anode component and free Fe(3+) on the colloidal electrodes serves as the cathode component. The active central ions of Fe(3+) in Fe(NO3)3⋅9H2O can be fixed on the electrodes by the surrounding ligands (OH(-) and NO3(-)) and can be in-situ transformed into colloidal Fe4NO3(OH)11 and goethite (α-FeOOH). Electrochemical results indicate that the current proposed colloidal pseudocapacitor system warrants the high-efficiency utilization of electroactive central Fe(3+) ions, showing high energy density of 58.4 Wh/kg at the power density of 8.4 kW/kg as an anode material. Meanwhile, our designed pseudocapacitor system can function well as a supercapacitor cathode. This colloidal pseudocapacitor system can offer a facile and efficient route for the design of advanced supercapacitors.