Highly Efficient Electric Power Storage Using Fe-CI Redox System

Abstract

From a viewpoint of electric power storage, the following iron (III)-iron(II) redox reaction was electrochemically studied.
By use of a flow cell with gas diffusion electrodes, graphite cloth/FeCl₂ (aq), H₂ (aq)/ion exchange membrane/FeCl₃ (aq), H₂(g)/Pt loaded graphite cloth, the voltage-current charatteristics on charging and discharging of the cell were examined at 70°C as a function of (i) total Fe²⁺/Fe³⁺ concentration (1.2-2.4 mol/kgH₂O), (ii) HCl concentration (1-8 mol/kgH₂O), and (iii) the nature of the ion exchange membrane (Neosepta C-66-5 T and Nafion-125)(Figs.2 and 3).
The voltage -current characteristics were greatly influenced by the HCl concentration. A high HCl concentration favors the charging process because of the cell voltage decrease, while a low HCl concentration favors the discharging process because of the cell voltage increase. Factors (i) and (iii) did not exhibit significant effects on the voltage-current characteristics.
The power storage efficiency was estimated to be 93-68% at current density of 25-100 mA/cm² for a solution composition FeCl₂/FeCl₃/HCl=0.6 /0.6 /5.0 (mol/kgH₂O), and 90-57% at 25-100 mA/cm² for FeCl₂/FeCl₃/HCl=1.2/1.2/2.5 (mol/kgH₂O) (Table 1). If the cell is charged at higher HCl concentration provided by waste heat and discharged at a lower one, higher efficiencies (>100%) could be achieved: this has been experimentally confirmed (Fig.4).