Function-graded proton exchange membranes (G-PEMs) based on poly(tetrafluoroethylene-co-hexafluoropropylene) were fabricated for direct methanol fuel cells (DMFCs) via electron beam-grafting using the heterogeneous energy deposition technique. The G-PEMs had a water uptake gradient in the proton transfer direction, originating from the sulfonic acid group gradient. The distribution of sulfonic acid groups in the various G-PEMs was evaluated using X-ray photoelectron spectroscopy. Four types of PEMs (flat-type, strong-gradient, meso-gradient, and weak-gradient types) were fabricated. By varying the direction of the G-PEMs, the methanol permeation test and DMFC operation were performed with two orientations of the sulfonic acid group gradient, decreasing from the methanol injection (anode) side (decrease-type) or the other (cathode) side (increase-type). The methanol permeability of the strong-gradient, meso-gradient, and weak-gradient G-PEMs was lower than that of Nafion®117 and the flat-type PEM. The "increase-type" orientation of the strong-gradient G-PEM resulted in the lowest methanol permeability. The DMFC performance of the G-PEMs was influenced by the thickness direction, such as "decrease-type" and "increase-type." The performance of the "decrease-type" assembly was higher than that of the "increase-type." The "decrease-type" assembly with P-200 k (weak-gradient G-PEM) exhibited the highest performance of the fabricated PEMs, comparable to that of Nafion®117.
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