Preparation of proton-conducting sulfonated poly(ether ether ketone)/boron phosphate composite membranes by an in situ sol–gel process

Composite membranes of sulfonated poly(ether ether ketone) (SPEEK)/boron phosphate (BPO4), with a BPO4 content up to 40 wt%, were prepared by introducing BPO4 via an in situ sol–gel process. Tripropylborate (C3H7O)3B and phosphoric acid (H3PO4) were used as precursors for the formation of BPO4. The composite membranes were characterized using X-ray diffraction, FT-IR, SEM and DSC/TGA. XRD studies indicated the existence of a certain level of crystallinity in the SPEEK, as well as the composite membranes. The FT-IR study revealed considerable interaction between the sulfonic acid functions of SPEEK and BPO4. SEM cross-sectional features showed that the BPO4 particles were uniformly embedded in the membrane matrix and the domain size of the BPO4 phase increased with increasing concentration of BPO4 precursors. Composite membranes had higher water uptake when compared with SPEEK and the SPEEK/BP10 composite membrane exhibited controlled swelling up to 75 °C. DSC/TGA studies established the good thermal stability of composite membranes. Proton conductivity of the membranes was measured under 100% relative humidity conditions up to 80 °C. A six-fold increase in conductivity was observed for the composite membrane with 30 wt% BPO4. The noticeable improvement in conductivity was mainly due to the uniform disposition of BPO4 throughout the membrane matrix, thus establishing continuous conduction pathways in composite membranes.