Effect of zeolites on chitosan/zeolite hybrid membranes for direct methanol fuel cell

Zeolites including 3A, 4A, 5A, 13X, mordenite, and HZSM-5 were incorporated into chitosan (CS) matrix to fabricate the hybrid membranes for direct methanol fuel cell (DMFC). Due to the presence of hydrogen bonds between CS and zeolite, the hybrid membranes displayed desirable thermal and mechanical stabilities. Through free volume characteristics analysis by positron annihilation lifetime spectroscopy (PALS) technique, it was found that incorporation of hydrophilic zeolites would increase the free volume cavity size whereas incorporation of hydrophobic zeolites would decrease the free volume cavity size. Through the investigations on water/methanol uptake, swelling, and methanol permeability, it was found that the membrane performance was highly dependent on the zeolite particle and pore size, content, and hydrophilic/hydrophobic nature. Based on the solution–diffusion mechanism, it was found that incorporation of hydrophobic zeolites increased the diffusion resistance of methanol and consequently decreased the methanol permeability, whereas incorporation of hydrophilic zeolites decreased the diffusion resistance of methanol and consequently increased the methanol permeability. Moreover, under the identical conditions, all the as-prepared membranes exhibited much lower methanol permeability than Nafion® 117 while the proton conductivity of the membranes remained high enough for DMFC applications.