Nafion/microporous titanosilicate ETS-4 composite membranes for effective methanol crossover reduction in direct methanol fuel cells

Microporous Engelhard titanosilicate-4 (ETS-4) particles are synthesized by means of the hydrothermal method using a gel with the following molar composition: 5H2O2:0.5TiO2:10SiO2:18NaOH:675H2O. The as-synthesized ETS-4 product is comprised of large dumbbell-shaped particles consisting of fine needlelike crystallites. These ETS-4 particles are finely ground with a ball mill and added to a 5 wt% Nafion solution to produce the Nafion/ETS-4 composite membranes for direct methanol fuel cells (DMFCs). The methanol permeability of the composite membranes decreases with increasing ETS-4 contents up to 15 wt% and then increases again upon further addition of ETS-4. The proton conductivity of the composite membranes decreases upon increasing ETS-4 contents. These results show that the best membrane selectivity is achieved with the NE10 membrane (where ‘NEx denotes a Nafion/ETS-4 composite membrane containing x wt% of ETS-4). The proton diffusion coefficients obtained with 1H pulsed field gradient nuclear magnetic resonance (PFG-NMR) are found to be much smaller than that of pure bulk water and to decrease with increasing ETS-4 contents in the membrane, which is in good agreement with the results obtained from the proton conductivity.