(84ba) Highly Efficient Redox Flow Batteries Using Low Cost Materials

Redox flow batteries (RFBs) have gained prominence as energy storage devices in recent years. This is mainly because of their unique design that permits independent scaling of energy and power outputs for grid-scale renewable energy storage. Despite the pronounced advantages RFBs have over other electrochemical energy devices, they are still hindered by material cost relative to performance. The widely commercialized vanadium redox flow battery (VRFB) is restricted by the limited availability and expensive cost of the vanadium transition metal as well as the Nafion^TM membrane which accounts for about 40% of the cell stack cost. Thus, to increase the viability of flow batteries for mainstream grid scale renewable energy storage, there is a need to utilize cheaper and more efficient materials. Previously, we investigated the effect of redox-active carbon electrodes on the reaction kinetics at the electrode-electrolyte interface of low cost redox chemistries. Carbon electrodes containing redox-active iron nanoparticles yielded approximately 2.5x higher power densities and 57% higher energy densities compared to inert carbon electrodes in coin cells using an all iron redox chemistry electrolyte. Moreover, we showed that iron functionalized carbon electrodes improve the power density of the low cost and highly electroactive zinc iodine redox flow battery (ZIRFB) by 100% compared to inert carbon electrodes and had 90% lower charge transfer resistances.

In this work, we seek to further develop a high performing and low cost redox flow battery technology by examining the effect of the cation exchange membrane (CEM) on the electrochemical performance of a zinc iodine RFB. Sulfonated Poly (Ether Ether Ketone) SPEEK membranes were prepared by sulfonation of PEEK powder with various degrees of sulfonation (DS). The DS of SPEEK was characterized by proton nuclear magnetic resonance (¹H NMR). Additionally, the ion exchange capacity (IEC) and equilibrium water uptake (EWU) of the membranes were measured. Lab scale ZI RFBs were assembled with 1.0 M electrolyte concentration, iron functionalized carbon electrodes, SPEEK membranes and then with Nafion^TM 212 as a comparison of the membrane’s performance. Battery testing included conditioning at 5.6 mA/cm², polarization analysis and cycling at 17 mA/cm². SPEEK membranes were observed to have higher equilibrium water uptakes and ion exchange capacities than nafion 212 indicating that SPEEK is ideal for cation transport. In the ZI RFB, SPEEK had a 370% increase (8.0 Wh/L to 1.7 Wh/L) in energy density compared to nafion 212. Additionally, the cycling columbic efficiency was 92.9% with SPEEK compared to 75.9% with nafion 212. From polarization analysis, a 24% increase in peak power density was obtained. This confirms SPEEK’s high cationic selectivity and makes it an ideal CEM for the ZI RFB. The results coupled with the economic merits of SPEEK, and the zinc iodine redox chemistry provide a pathway for low cost, highly efficient grid scale renewable energy storage.