Highly efficient SnO2-dispersed sulfonated poly(ether ether ketone) composite membrane for vanadium redox flow batteries

Abstract

The vanadium redox flow battery (VRFB) is a promising technology for large-scale energy storage, supporting the integration of renewable energy into power grids. In this study, sulfonated poly(ether ether ketone) (SPEEK) membranes incorporating different loadings of tin oxide (SnO2) nanoparticles are fabricated to enhance ion selectivity and reduce cost. The membranes are characterized by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and physicochemical analyses, including water uptake, swelling ratio, proton conductivity, and vanadium ion permeability. Electrochemical evaluation in a VRFB single cell shows that the composite membrane with an optimal SnO2 content (S-2) exhibits superior performance compared to the perfluorosulfonic acid (PFSA) membrane, achieving an energy efficiency of 86.4 % at 100 mA/cm2, longer self-discharge duration, and mitigated capacity fading. The improved performance is attributed to reduced vanadium ion crossover, stemming from strong interfacial interactions between SPEEK's sulfonic acid groups and the uniformly dispersed SnO2 nanoparticles, which also reinforce mechanical and oxidative stability. These findings demonstrate that the SPEEK/SnO2 composite membrane provides a cost-effective and durable alternative to PFSA membranes for VRFB systems. Moreover, the design strategy highlights the potential of hybrid hydrocarbon-based membranes for broader electrochemical energy applications.