Composite proton conductive membranes composed of sulfonated poly(ether ether ketone) and phosphotungstic acid-loaded imidazole microcapsules as acid reservoirs

Imidazole microcapsules loaded with phosphotungstic acid (IMCs-HPW) were prepared and incorporated into the sulfonated polyether ether ketone (SPEEK) matrix to fabricate composite proton conductive membranes for potential use in direct methanol fuel cells. The hollow imidazole microcapsules (IMCs) were prepared via distillation-precipitation polymerization, immersed into HPW solution to obtain the acid loaded microcapsules (IMCs-HPW) which were subsequently embedded into membrane as acid-reservoirs. The Fourier transform infrared spectroscopy and the concentration-absorption UV-vis spectrum of the IMCs-HPW revealed the tight immobilization of HPW in the IMCs. The introduction of HPW greatly enhanced the thermal stability of the IMCs. The as-prepared SPEEK/IMCs-HPW membranes exhibited reduced swelling degree, reduced methanol crossover and in particular dramatically enhanced proton conductivity. The maximum conductivity at room temperature was 0.0316 S cm−1 for the SPEEK/IMCs-HPW composite membrane with a doping content of 15 wt%, which was nearly three times higher than that of the pristine SPEEK membrane. Incorporation of the IMCs-HPW acid reservoirs not only rendered the composite membranes with improved water-retention property but also provided additional proton-transfer pathways, leading to a superior proton conduction even under 20% relative humidity which was two orders of magnitude over the pure SPEEK membranes.