Evaluation of the Performance of an Iron-Chloride Redox Flow Battery for Large Scale Energy Storage

Highly efficient and inexpensive batteries are essential for the successful integration of renewable energy sources like solar and wind into the electricity grid [1, 2]. The present study focuses on the evaluation of the performance of an iron-chloride redox flow battery for such large scale energy storage applications. An iron-chloride redox flow battery is based on inexpensive and globally-abundant materials – iron and chloride. The all-iron redox flow battery was first reported by Hruska and Savinell in 1981[3]. While the economics and eco-friendly nature of this battery system is very promising, successful commercialization of the iron-chloride redox flow battery has been hindered by several technical challenges such as low charging efficiency of the negative electrode and poor cycle life.

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^-              E^o= 0.77V             (1)

Negative electrode:                             Fe + 2Cl^-  FeCl₂ + 2e^-                                   E^o= -0.44V           (2)

Overall cell reaction:                           Fe + 2 FeCl₃  3FeCl₂                         E_cell= 1.21V          (3)

Negative electrode side reaction:                      2H⁺ + 2e^-  H₂                                                       E^o= 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 [4].

 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.

Acknowledgement:

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.

References: 

1. Accommodating High Levels of Variable Generation, North American Electric Reliability Corporation, April 2009.
2. B. Dunn, H. Kamat, J-M. Tarascon, Science, 334, 928 (2011).
3. L. W. Hruska, R. F. Savinell, J. Electrochem. Soc., 128, 18, (1981).
4. A. K. Manohar, K. M. Kim, E. Plichta, M. Hendrickson, S. Rawlings, G. K. S. Prakash, S. Narayanan, Studies on the Iron-Chloride Redox Flow Battery for Large-Scale Energy Storage, The Electrochemical Society Meeting Abstracts, MA2013-01, 141-141, (2013).