Journal of Membrane Science, Vol.573, 107-116, 2019
Electrospun tri-layer membranes for H-2/Air fuel cells
Dual nanofiber electrospinning was used to fabricate a series of multi-layer fuel cell membranes with high proton conductivity and low in-plane water swelling, using 825 EW perfluorosulfonic acid (PFSA) ionomer and polyamide-imide (PAI). The relative flow rates of the two polymer solutions were adjusted during electrospinning to create multiple layers with different PFSA/PAI weight ratios. After electrospinning, fiber mats were transformed into dense membranes with a total thickness of 20 mu m by heating and compaction, where each layer had the same composite morphology: a PFSA ionomer matrix with an embedded network of reinforcing PAI fibers. Tri-layer films were fabricated with surface layers composed of 95 wt% PFSA and 5 wt% PAI and an inner (reinforcing) layer that was enriched in PAI (25-60 wt% PAI), with either a uniform PFSA/PAI composition or with a symmetric PFSA/PAI compositional gradient. The thickness of the inner layer (uniform or gradient composition) was adjusted so that the effective/average composition of the entire membrane was 80 wt% PFSA and 20 wt% PAI. As compared to a single layer membrane with a uniform distribution of PAI fibers in a PFSA matrix, the tri-layer membrane structures exhibited dramatically less in-plane swelling (5-6% vs. 11%) with no loss in proton conductivity. For a slightly different system, with alternating layers of neat PFSA and 70/30 PFSA/PAI, increasing the number of layers from 3 to 5, 7, or 9 had no effect on proton conductivity (because the overall membrane composition was held constant) and did not further reduce the membrane in-plane swelling.