Studies of Earth Abundant Metal Complexes for Near Neutral Aqueous Redox Flow Battery (RFB) for Grid Storage

Abstract

Among the many options available for Large-Scale Grid Energy Storage, Redox Flow Batteries (RFB) are leading candidates because of the decoupled design of power and energy1 which offers tremendous design flexibility and modularity. As part of the Department of Energy’s Office of Electricity (OE) energy storage program at Sandia National Laboratories, we are investigating earth abundant transition metal coordination complexes for aqueous redox flow batteries at close to pH 7. The aqueous-based systems offer complete freedom from the thermal instabilities that plague non-aqueous systems, such as lithium-ion. Furthermore, the use of neutral pH electrolytes engenders intrinsic safety and low/no corrosion, unlike the highly corrosive and toxic electrolytes used for the vanadium redox flow battery or the zinc-bromine systems. In this abstract we discuss the electrochemical properties of some of the complexes under development, and the performance of full cells containing the optimized redox couples will be presented at the meeting. Performance of Fe(2+)(tris-BPY)Cl2-0.4M Na2SO4 We have performed dc and ac voltammetry to characterize a number of iron coordination complexes, and Figure 1 shows the dc voltammetry curves at a 10 mV/s scan rate of one of these. As seen, the peak separation is ~60 mV, close to the theoretical value. We have also performed DC voltammetry at different scan rates up to 200 mV/s, and the peak separation varies between 60 and 64 mV which clearly demonstrate highly reversible behavior as well. Additionally, this couple exhibits higher redox voltage which will assure a RFB with a higher cell voltage and energy when appropriately configured with another redox couple. Figure 2 shows a plot of redox peak potentials vs. square root of scan rate. The linear increase of peak value vs. square root of san rate indicates that this couple is highly reversible. We also performed AC voltammetry, which is more sensitive than the DC measurement, just to make sure that there are no other redox peaks buried around the solvent decomposition voltage. We observed only one reversible peak (Figure 3) which corroborates the DC measurement. Similar observations were observed for other couples and will be described in detail at the meeting. Reference: B. Dunn, B., H. Kamath, H. & Tarascon, J.-M. “Electrical energy storage for the grid: a battery of choices” Science 334, 928–935(2011). Acknowledgment: Sandia Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation for the U. S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL 85000. The authors would like to thank Dr. Imre Gyuk of the DOE Office of Electricity for the support. Figure 1