A chemically robust, compatible, and low-swelling ionomer blend for medium-temperature proton conduction: old dogs with a new trick

Medium-temperature proton exchange membranes (PEMs) with high proton conductivity and low fuel crossover have received ever-growing research interest. Herein, a novel ionomer blend was designed and developed to address the issues that traditional PEMs are facing with: low membrane selectivity, trade-off between proton conductivity and mechanical strength, chemical degradation, phase-separation, and over-swelling. A poly(1,2-benzimidazole) (PBESK) is blended with a sulfonated polymer (SPEEK) to achieve good compatibility ascribed to acid-base interactions, π-π interactions, and hydrogen bonds. This membrane exhibits better mechanical strength, lower swelling ratio, and better durability of proton conductivity than SPEEK while displaying higher chemical stability than both PBESK and SPEEK. At 180°C, the proton conductivity of the PBESK/SPEEK(60%) membrane reaches 0.147 S cm -1 , 0.053 S cm -1 , and 0.00583 S cm -1 at 100% RH, 50% RH, and 0% RH, respectively. This membrane also shows low methanol permeability, low H 2 /O 2 permeance, and thus high membrane selectivity. This effective design paves the way for the development of next-generation medium-temperature PEMs. • Intermolecular interactions improved compatibility and avoided phase separation. • N-H bond-free copolymer improved conductivity and alleviated oxidative degradation. • The blend membranes showed good mechanical property and dimensional stability. • High proton conductivity achieved at low relative humidity with good durability. • High proton conductivity and low fuel crossover led to high membrane selectivity.