A study on the preferable preparation method of SPEEK/BPO4 composite membranes via an in situ sol–gel process

Abstract Sulfonated poly(ether ether ketone) (SPEEK)/boron phosphate (BPO 4 ) composite membranes for polymer electrolyte fuel cells (PEFCs) were prepared via an in situ sol–gel process in our previous study [P. Krishnan, J.-S. Park, C.-S. Kim, Preparation of proton-conducting sulfonated poly(ether ether ketone)/boron phosphate composite membranes by an in situ sol–gel process, J. Membr. Sci. 279 (2006) 220]. Proton conductivity of the composite membranes increased with increasing the amount of BPO 4 up to 40 wt.% due to the increasing BPO 4 particle size. In this study, the effect of several variables such as reaction time, reaction temperature and ionic form of solution-casting SPEEK polymers on the relationship between the size of inorganic particles and proton conductivity of the membranes was investigated for the preparation of SPEEK/BPO 4 composite membranes. The composite membranes were characterized using FT-IR, SEM, DSC, TGA and proton conductivity. Among the variables, reaction time and reaction temperature were not dependent on the proton conductivity and size of BPO 4 in the in situ sol–gel process. However, the ionic form of solution-casting polymers for the composite membranes significantly influenced the size of BPO 4 particles. At the same amount of BPO 4 in the composite membranes, the composite membrane using H + -form polymer made the BPO 4 particle size five to six times higher than that using Na + -form polymer. Water uptake of the former composite membrane (in particular, an amount of freezable water) was higher than the latter due to the bigger size of BPO 4 , and the proton conductivity of the former composite membrane increased. As a result, it can be recommended that the preparation of the composite membranes using H + -form polymer is preferable via the in situ sol–gel process.