Sulfonated graphene oxide doped sulfonated polybenzothiazoles for proton exchange membrane fuel cells

A series of sulfonated polybenzothiazoles (sPBT-E) were synthesized by direct polycondensation of 3,3′-disulfonate-4,4′-dicarboxylbiphenyl (DSBP), 4,4′-dicarboxylic diphenyl ether (DCPE) and 2,5-diamino-1,4-benzenedithiol dihydrochloride (DABDT) in polyphosphoric acid (PPA). The sPBT-E series showed considerable solubility in common organic solvents due to the presence of flexible ether groups in the polymer backbone. Sulfonated graphene oxide (SGO) with different loadings (1, 2, 3, and 4 wt%) was doped into the sPBT-E matrix with the sulfonated monomer ratio of 62.5% to prepare composite PEMs (denoted as sPBT-E62.5/SGO) for the first time. sPBT-E membranes with different sulfonated monomer ratios (55, 57.5, 60, and 62.5) were also prepared for comparison. sPBT-E and sPBT-E62.5/SGO series membranes exhibit excellent dimensional stability, appropriate water uptake, high thermal stabilities, and mechanical properties. Additionally, SGO can provide additional ion-transporting sites (resulting in continuous proton transfer channels) to enhance proton conduction of the sPBT-E62.5/SGO membranes. Therefore, the proton conductivity and power density of sPBT-E62.5/SGO3 were up to 0.139 S cm−1 and 519.9 mW cm−2 at 80 °C and 100% RH, which are superior to sPBT-E series and Nafion 212 (0.133 S cm−1, 413.2 mW cm−2). After 20 h continuous operation, the output voltage of sPBT-E62.5/SGO3 only decreased by 13.2%, which is better than sPBT-E62.5 (22.7%). Hence, the sPBT-E62.5/SGO3 membrane exhibits excellent potential as an alternative PEM for PEMFCs.