Improvement of proton conductivity and efficiency of SPEEK-based composite membrane influenced by dual-sulfonated flexible comb-like polymers for vanadium flow battery

A sulfonated poly(ether ether ketone)-based composite membrane (S/SMA-SN) is developed by introducing dual-sulfonated flexible comb-like polymer (SMA-SN), varying from 0.1 to 1.5 wt%, which was prepared from the poly(styrene-co-maleic anhydride)-ethylenediamine through a main-/side-chain two-step sulfonation procedure. The S/SMA-SN composite membranes demonstrate ultrahigh proton conductivity and excellent cell performance at ultralow contents of SMA-SN polymers. Synergistic main-/side-chain proton channels formed by SPEEK and SMA-SN offer the highest proton conductivity (49.2 mS cm-1) and the lowest vanadium ion permeability (19.6 × 10-7 cm2 min-1) for S/SMA-SN-0.5, which is superior to SPEEK and Nafion membranes. Moreover, S/SMA-SN-0.5 displays its remarkable energy efficiency (72–81.3%) at 100–200 mA cm-2 current density, further illustrating the tradeoff effect of conductivity and resistance. Even at 150 mA cm-2 current density, stable coulombic efficiency (97%) and energy efficiency (80%) maintained after 500 charge-discharge cycles confirms the stable membrane structure and increased chemical stability, which originated from the main-/side-chain synergistic structure. This opens a promising method to explore the high-performance proton exchange membrane for vanadium flow battery application by the side-chain structuralized comb-like polymer at ultralow concentrations.