Improved proton conductivity and structure stability of an SPEEK/SPPS blend membrane for vanadium redox flow batteries

Abstract

Sulfonated polyphenylene sulfide (SPPS) containing a rigid main chain is introduced into a SPEEK-based proton exchange membrane to obtain high proton conductivity and dimensional stability. With varied contents of SPPS, the SPEEK/SPPS blend membranes present increased proton conductivity, and at 15 wt% SPPS, the SPEEK/SPPS-15% membrane shows higher proton conductivity (30.9 mS cm⁻¹) and ion selectivity (10.5 × 10³ S min cm⁻³) than Nafion 117 (26.8 mS cm⁻¹, 4 × 10³ S min cm⁻³). Moreover, a cell assembled with the SPEEK/SPPS-15% membrane demonstrates better voltage efficiency (VE: 92.5–75.0%) and energy efficiency (EE: 90.0–74.2%) in the current density range of 60–150 mA cm⁻². During a 100-time charge–discharge cycle at 100 mA cm⁻², excellent cycling stability and high charge capacity retention are proved, and the SPEEK/SPPS-15% blend membrane proves its outstanding structural stability and durability after 300 cycles at 150 mA cm⁻². This shows that the structural similarity of the membrane matrix is beneficial to develop a high-performance SPEEK/SPPS blend membrane with desirable structural stability.