Experimental and computational study of proton and methanol permeabilities through composite membranes

To design direct methanol fuel cells, proton permeability and methanol crossover have to be evaluated. A study of the transport of methanol and protons through composite membranes of poly(ethylene glycol) (PEG) and polysulfone (PSf) was performed and permeabilities of these components were determined. PSF was treated with dilute sulfuric acid to enhance hydrophilicity. PEG was found to be a good material for the active layer, because it contains OH hydrophilic groups which combine with hydrated protons. A composite membrane made of 15 wt.% PSf and 40-50 wt.% PEG showed a lower methanol crossover (1.0E−06 cm2 s−1) than the commercial reference NAFION® 117. Maximal proton conductivity is also lower than NAFION® 117. A mathematical deterministic model, considering transport by diffusion through the composite membrane and equilibrium at the membrane-reservoir interfaces, was derived. However, the PEG layer did not present any pores and diffusion in the dense membrane was estimated using a transport probability. On the other hand, the porous PSf layer required an effective diffusivity that is a function of physical properties such as porosity and tortuosity. The contribution made by each mass transfer phenomenon to the total permeation was calculated by an association of mass transfer resistances.