Proton conducting zeolite composite membrane boosts the performance of vanadium redox flow battery

Abstract

In a vanadium redox flow battery (VRFB), the selectivity of protons over vanadium ions for an ion-conducting separator is crucial to achieving capacity retention in charge/discharge cycles. Conventionally used perfluorinated or hydrocarbon-based membranes are not able to resolve cross-contamination of vanadium during VRFB operation. Engineered inorganic materials like zeolites have inherent selectivity towards specific ions based on the size and charge of the ions. In this work, ZSM-5 zeolites with different Si/Al ratios were modified by a facile ion-exchange process to obtain proton rich zeolites, followed by successful infiltration into the SPEEK matrix. The composite membranes demonstrated excellent oxidative stability in 2 M H2SO4 containing 1.5 M VO2+ ions as well as favourable electrochemical properties. In a single-cell VRFB, the membrane with the highest proton conductivity i.e., SPEEK-ZSM 5 (25) 2H+ was able to achieve an average Coulombic efficiency of 97.0% and average energy efficiency of 78.0% over 300 charge/discharge cycles at 80 mA cm−2 current density. The polarization curve and self-discharge experiment results illustrated 21.7% higher peak power density and 2.5 times slower self-discharge compared to state-of-the-art Nafion-117 in identical operating conditions indicating zeolite-composite membranes as a potential candidate to enhance the durability and performance of VRFBs.