Electrochemical Study of the Effect of Prussian Blue As a Mediator in a Solid-State Supercapacitor

Abstract

In the present study, electrochemical, X-ray photoelectron spectroscopy (XPS), and In-situ X-ray absorption spectroscopy (XAS) measurements were performed to investigate the charge/discharge process in a solid-state mediator supercapacitor (SC) or redox electrolyte SC. Prussian blue (PB) was synthesized using a wet chemistry method. A solid-state asymmetrical supercapacitor was fabricated with the configuration of (active carbon) /polymer electrolyte membrane/ (active carbon + PB). A KCl saturated Ag/AgCl reference electrode was inserted in the cell to monitor the potentials of the positive and negative electrodes during the charge/discharge process. Both electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements indicated that the supercapacitor had a capacitor-like behavior. Upon negative polarization of the PB-containing electrode the specific capacitance of the SC was increased 80% while upon positive polarization, there was much less enhancement of the specific capacitance. The potentials of the two electrodes versus the reference electrode were displaced proportionally with increasing the voltage of the cell. The XPS results indicated that the potassium concentration in PB is 0.5 atomic % suggesting that iron ions had valence close to +3, which is consistent with the measured XPS binding energy of the Fe 2p3/2 line and the X-ray absorption Fe K-edge energy when compared with the appropriate reference compounds. This indicated that instead of a nominal chemical formula KFeFe(CN)6, the formula was actually closer to K0.07FeFe(CN)6. In-situ XAS results (Figure 1) confirmed that upon positive polarization of the PB-containing electrode there was only small degree of redox activity occurring. On the other hand, upon negative polarization of the PB-containing electrode there was significant redox activity occurring due to reduction of Fe3+ to Fe2+. The XAS results also confirmed that the redox activity is completely reversible between cycles 4 and 5. Figure 1