Proton exchange membranes with monolayer graphene nanosheet nanofillers for direct methanol fuel cells

High-performance proton exchange membranes (PEMs) with excellent proton conductivity, mechanical robustness, and methanol-barrier properties are critically needed for advancing direct methanol fuel cell (DMFC) technology. This study addresses the challenges of high methanol crossover in traditional perfluorosulfonic acid (PFSA)-based membranes. Monolayer graphene nanosheets offer excellent proton conductivity while effectively blocking fuel molecules, making them ideal nanofillers for enhancing PEM performance in DMFCs. The strong ionic interactions between the -NH₂ groups of the polyethyleneimine (PEI)-functionalized monolayer graphene nanosheets and the -SO₃H groups of the PFSA matrix, combined with the highly impermeable nature of the nanosheets, effectively reduce methanol crossover and membrane swelling. Despite the lower IEC values resulting from the formation of acid-base pairs, the composite membranes presented high proton conductivity due to the creation of fast and continuous proton transport pathways. The graphene/PFSA composite membrane with 15 wt% nanosheet loading achieved the highest proton conductivity (~208 mS/cm) and the lowest methanol permeability (~1.98 × 10 −6 cm 2 /s), with a selectivity (proton conductivity/methanol permeability) of ~9.85 × 10 4 S·s/cm 3 . This composite membrane outperformed the commercially available Nafion-117 membrane in terms of DMFC performance under the same testing conditions, achieving a maximum power density of ~90.8 mW/cm 2 . Therefore, this work demonstrates the effective use of atomically thin crystals as highly proton-conductive and fuel-impermeable materials, thus facilitating the development of high-performance PEMs for fuel cell applications. • A simple vacuum filtration assembly was used to develop graphene/PFSA composite membranes. • Acid–base pairs formed between the -NH 2 groups and the -SO 3 H groups serve as proton hopping sites. • Monolayer graphene nanosheets enhance the dimensional stability and selectivity of composite membranes. • The graphene/PFSA-15 wt% membrane outperformed commercial Nafion-117 in DMFC applications.