An Advanced Composite Membrane for the All-Vanadium Redox Flow Battery

Abstract

Redox flow batteries (RFBs) are a promising technology for grid scale stationary energy storage to complement renewable energy systems. These batteries have a relatively low energy density; however, they offer important advantages, including: long life-time; decoupled energy (arbitrarily large electrolyte volume) and power (electrode area); high round-trip efficiency; scalability and design flexibility; fast response; and low environmental impacts. These advantages make them superior to many energy storage technologies for stationary applications [1-4]. Among the various types of RFBs, vanadium RFBs (VRFBs) are an emerging technology for grid scale energy storage and the integration of renewable energy generation [5]. The membrane is a key component of a VRFB that separates the two half-cell electrolytes and prevents cross-mixing, while allowing the transport of ions during charge-discharge cycles [6]. The VRFB membrane should exhibit low vanadium ion permeability to minimize self-discharge, low cost, and long‐term chemical stability under normal operating conditions. A high proton conductivity and low vanadium ion crossover are known to improve the efficiency of VRFBs [6-7]. In this study, we present a novel composite Nafion based membrane that results in a significant increase in the VRFB performance. The composite membrane has been characterized for its chemical, structural, and thermal properties using appropriate analytical techniques. The battery performance was evaluated in a flow cell using a ‘zero-gap’ design with an electrode area of 5 cm2. The electrolytic solution, 1.6 M VOSO4 in 3 M H2SO4, was circulated through the cell. Thermally treated carbon papers were used as the cathode and anode electrodes. For charge-discharge experiments, a constant current density (10 to 80 mA cm−2) was applied with upper and lower voltage cut-offs of 1.65 and 0.8 V, respectively. The stability of the battery using the composite membrane was evaluated over 100 cycles. Figures 1 and 2 show the energy efficiency and capacity retention during 100 charge-discharge cycles. The results reveal that the energy efficiency was improved from 51% to 63% by using the composite membrane. In addition, the charge-discharge capacity and capacity retention improved by around 200% and 25%, respectively. This improvement can be attributed to a higher proton conductivity and lower vanadium permeability of the composite membrane.