A low-cost green lignosulfonic acid/SPEEK composite membrane for iron–chromium redox flow batteries with high performance

Abstract

Iron–chromium redox flow batteries (ICRFBs) have attracted considerable attention due to their high safety, stability, and cost effectiveness. However, the commonly used Nafion-212 ion exchange membrane (IEM) suffers from high cost and low ion selectivity, which hinders its practical application. In this study, lignosulfonic acid (LSA) was synthesized from sodium lignosulfonate (SLS), a byproduct of the paper industry with abundant and renewable sources. Then we combined LSA with sulfonated poly (ether ether ketone) (SPEEK) to fabricate a green and low-cost composite membrane in a single cell. The abundant sulfonic and polar functional groups in LSA provided active sites for proton conduction while effectively suppressing the crossover of Fe³⁺ and Cr³⁺ ions. As a result, the membrane demonstrated excellent coulombic efficiency (CE: 99.67%) and energy efficiency (EE: 79.45%), surpassing those of the SPEEK IEM (CE: 97.24%, EE: 75.69%) after 100 cycles at a current density of 100 mA cm⁻², although its EE is slightly lower than that of the Nafion-212 IEM (CE: 95.96% and EE: 80.8%). In addition, the self-discharge time of the cell was extended to 45 h, significantly outperforming the cells using other two membranes. Our research provided a strategy of fabricating a green and cost-effective composite membrane for high-performance ICRFBs.