New developments in proton conducting membranes for fuel cells

•The key technical challenge for hydrogen/air PEM fuel cells is high proton conductivity at low humidity.•Fluorinated ionomers continue to dominate the PEM fuel cell landscape.•Low EW semi-crystalline PFIA ionomers from 3M Co. show great promise for hydrogen/air fuel cells.•Porous reinforcing mats limit in-plane swelling/shrinking of ionomers and improve membrane durability in a hydrogen/air fuel cell.•A promising approach for direct methanol fuel cells: pre-stretched recast Nafion films.

Research efforts on proton conducting polymeric membranes for fuel cells are discussed and future R&D directions are identified. The key membrane performance issues for hydrogen/air fuel cells are high proton conductivity under dry conditions, low gas crossover, and good mechanical/chemical stability. For direct liquid methanol fuel cells, there is a need for highly durable membranes with a high proton conductivity and low methanol crossover. Fluorinated ionomers continue to dominate the fuel cell membrane landscape and it is unlikely that a hydrocarbon polymer will supplant these materials anytime soon, especially for automotive applications. Promising strategies for improving membrane performance and durability in a hydrogen/air fuel cell include the use of ultra-low equivalent weight semicrystalline ionomers, covalently attaching conductive and water retaining particles to an ionomer, and adding a porous/nanofiber reinforcements to minimize in-plane swelling and shrinking. For direct methanol fuel cells, the best commercial material is Nafion 117, but pre-stretched films of recast Nafion outperform all commercial membranes and show great potential.