Evaluation of the Performance of an Iron-Chloride Redox Flow Battery for Large Scale Energy Storage
During discharge of the iron chloride flow battery, iron (III) chloride is converted to iron (II) chloride in the positive electrode and iron is oxidized to iron (II) chloride at the negative electrode (Eq. 1, 2). The reverse reactions happen during battery charging.
Positive electrode: 2FeCl3 + 2e- 2FeCl2 + 2Cl- Eo= 0.77V (1)
Negative electrode: Fe + 2Cl- FeCl2 + 2e- Eo= -0.44V (2)
Overall cell reaction: Fe + 2 FeCl3 3FeCl2 Ecell= 1.21V (3)
Negative electrode side reaction: 2H+ + 2e- H2 Eo= 0V (4)
The reversible potential of the Fe/Fe2+ couple is more negative compared to the reversible potential for hydrogen evolution (Eq. 4). As a result, in addition to the reduction of Fe2+ ions to metallic iron, hydrogen evolution also takes place on the negative electrode during battery charging. This hydrogen evolution results in very low charging efficiency of the negative electrode. The amount of hydrogen evolved on the negative electrode during charging is dependent upon the electrolyte pH, charging current and the presence of any electrolyte additives. The pH of the electrolyte increases due to the evolution of hydrogen and results in the precipitation of Fe2+ and Fe3+ions in the electrolyte. Operating at low charging efficiencies is therefore highly undesirable for stable long-term operation.
The faradaic efficiency of iron deposition in the presence of various additives was studied at a glassy carbon rotating disc electrode with a platinum wire counter electrode and a silver/silver chloride reference electrode. The baseline electrolyte consisted of 3M ferrous chloride (FeCl2.4H2O) and 2M ammonium chloride (NH4Cl). The ammonium chloride served as a supporting electrolyte. Full-cell studies were performed in an in-house designed flow cell with graphite electrodes and an anion exchange membrane that separated the positive and negative electrode compartments. In addition, reference electrodes were placed in the positive and negative electrode compartments to facilitate the characterization of the polarization characteristics of the individual electrodes in the flow cell .
The faradaic efficiency for iron deposition in the presence of different electrolyte additives at various current densities is shown in Figure 1. It is observed that the “round-trip” plating/stripping efficiency with the baseline electrolyte was less than 40%. In the presence of ascorbic acid or citric acid as buffering agents, the electrolyte could be stabilized at pH = 2 without the precipitation of Fe2+ and Fe3+ions. The round-trip plating efficiency with an electrolyte pH of 2 and ascorbic acid additive was about 92% (Figure 1).
The charging efficiency of the negative electrode of an iron-chloride redox flow battery under different operating conditions is shown in Figure 2. The effect of charging current density, electrolyte pH, electrolyte flow rate and electrolyte additives on the charging efficiency and cycling performance of the iron-chloride flow battery will be discussed.
The authors acknowledge the financial support for this research from US Army RDECOM CERDEC CP&I. We also thank Donald Wiggins and Mike Cowan of the USC Dornsife Machine Shop for support with the fabrication of the flow cell.
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