Radiation grafted membranes for fuel cells containing styrene sulfonic acid and nitrile comonomers

Radiation grafted membranes with controlled monomer content were prepared to investigate the role of nitrile containing comonomers on the properties of proton conducting membranes. The membranes consist of a partially fluorinated backbone of ETFE with grafted chains containing styrene sulfonic acid and its comonomer, methacrylonitrile (MAN) or acrylonitrile (AN). Upon grafting, the comonomers show a tendency to form an alternating copolymer with styrene. The ex situ properties of both types of co-grafted membrane, i.e., proton conductivity, water uptake, and dimensional stability, are largely insensitive to the type of comonomer but mainly governed by the ion exchange capacity (IEC). Styrene/AN co-grafted membranes undergo substantial nitrile hydrolysis during the sulfonation procedure compared to styrene/MAN co-grafted membranes. The substituent at the alpha position of the comonomer determines the susceptibility of the nitrile to hydrolysis during membrane preparation as well as during fuel cell operation. Despite the change in the chemical properties as a result of hydrolysis, the mechanical integrity of the co-grafted membranes is retained. However, the styrene/AN co-grafted membrane showed considerably inferior performance in the fuel cell compared to the styrene/MAN co-grafted membrane. These findings bring about a design rationale for membrane materials with enhanced stability against hydrolysis.