New sulfonated polybenzimidazole (SPBI) copolymer-based proton-exchange membranes for fuel cells

Abstract

The synthesis and characterization of sulfonated polybenzimidazole (SPBI) copolymers as proton-exchange membranes (PEMs) for fuel cells are described in this paper. A one-step high temperature polymerization method was used to synthesize the SPBI copolymers from 3,3′-diaminobenzidine (DABD), 4,4′-oxybis(benzoic acid) (OBBA), and two sulfonated dicarboxylic acids, i.e., 5-sulfoisophthalic acid (SIPA) and 4,8-disulfonyl-2,6-naphthalenedicarboxylic acid (DSNDA), using polyphosphoric acid (PPA) as the solvent. The pure SPBI copolymer membrane was prepared by the traditional solution-casting technique (technique 1), and the SPBI/H 3PO 4 blend membrane was prepared through a new direct hot-casting and in situ phase inversion technique (technique 2). The membranes prepared from both techniques possessed desirable mechanical, chemical, thermal, and hydrolytic stability. However, the pure SPBI copolymer membranes from technique 1 exhibited very poor proton conductivities since most of the sulfonated groups in SPBIs were neutralized by the basic imidazole groups and became deprotonated sulfonated groups. On the other hand, the SPBI/H 3PO 4 blend membrane from technique 2 showed extremely high conductivities (>0.1 S/cm) at high temperatures (>120 °C) and low relative humidities (RHs) (even at anhydrous conditions), which has great potential to be used as high temperature and low humidity PEMs for fuel cells.