Efficient and durable composite membranes based on polydopamine-mediated sulfonated graphene oxide for direct methanol fuel cells

Abstract

Sulfonated graphene oxide (SGO) is already utilized as an effective filler in proton exchange membranes (PEMs) for performance enhancement in direct methanol fuel cells (DMFCs). Previously, SGO has been mostly prepared via functionalization based on the intrinsic groups of graphene oxide (GO), which may cause inhomogeneity/insufficient sulfonation and possibly limit the improvement of membrane performance. To overcome this widely overlooked problem, herein, a new type of GO sulfonation product, SDGO, is fabricated by carrying out the sulfonation on polydopamine (PDA) deposited GO, which results in improved uniformity/sufficient sulfonation on SDGO compared to PDA-free SGO. Therefore, the sulfonated poly(ether ether ketone) (SPEEK) composite PEMs filled with SDGO show better matrix–filler compatibility, larger hydrophilic domains and less voids than those using PDA-free SGO, which lead to much improved proton conductivity, methanol barrier and mechanical properties. SPEEK–SDGO based DMFCs largely outperform Nafion 212 based DMFCs, achieving an output of 177.93 mW cm⁻² (80 °C, 1 M methanol/O₂), one of the most competitive DMFC power densities reported for DMFCs based on SGO composite membranes. Moreover, the radical scavenging ability of PDA boosts the stability of SPEEK–SDGO composite membranes in chemical accelerated stress tests. Mechanical accelerated stress tests and constant voltage experiments further confirm the superior durability of SPEEK–SDGO composite membranes during DMFC operations.