Sulfonated derivatives of polyparaphenylene as proton conducting membranes for direct methanol fuel cell application

Proton conducting polymers derived from polybenzoyl-l,4-phenylene (PBP) and poly-p-phenoxybenzoyl-l,4-phenylene (PPBP) were synthesized by the 'Colon synthesis technique'. The sulfonation of these proton conducting polymers was carried out using either sulphuric acid or tetramethylsiliylchlorosulfonate (TMSC1) as sulfonating agent, and their thermal properties were evaluated. Both sulfonated PBP and PPBP are thermally stable up to at least 215°C. The sulfonated sPPBP exhibited good conductivity as proton conducting membranes at room temperature and were tested as electrolyte membranes for a single direct methanol fuel cell (DMFC) in terms of water absorption, methanol permeability and electrical performance. The water uptake of the sPPBP was found to be larger than that of the sPBP, i.e., 65 and 43 mol%, respectively. The permeability to methanol was found to be 10 times lower than sPPBP and sPBP compared to a Nafion&reg membrane. In spite of this, performance in a single DMFC was found to be twice inferior to that with Nafion&reg 117. Optimisation of the sulfonation level and of the electrode-membrane interfaces was lead to better results.