Methanol tolerable ultrathin proton exchange membrane fabricated via in-situ ionic self-crosslinking strategy for high-performance DMFCs

Proton exchange membranes (PEMs) with high proton conductivity, mechanical stability, and methanol barrier capability is urgently needed for direct methanol fuel cell (DMFCs). In response, a series of highly sulfonated polybenzimidazoles (SPBI) were synthesized, followed by the creation of a unique in-situ ionic self-crosslinking mechanism via acid-base pair interactions between –SO₃^- and protonated N within the imidazolium rings of SPBI in an acidic milieu. The in-situ ionic self-crosslinking not only significantly enhances the mechanical stability of the prepared membrane, but also constructs a microstructure with a free volume radius smaller than the molecular dimensions of methanol, subsequently imparting unparalleled resistance to methanol permeation. After being reduced to an ultrathin thickness of 15 μm, the optimal SPBI-SO₃H-200 % membrane obtains remarkable high specific proton conductivity of 33.48 S cm⁻². Furthermore, the assembled DMFC demonstrates an exceptionally low methanol crossover current density of 188.25 mA/cm² alongside high power density of 109.92 mW cm⁻² within 2 M methanol fuel, significantly outperforming the methanol crossover current density of 386.06 mA/cm² and power density of 87.13 mW cm⁻² achieved by a single cell assembled with Nafion 115 membrane.