Dual nanofiber electrospinning was used to fabricate a series of tri-layer fuel cell membranes with high proton conductivity and low in-plane water swelling, using 825 EW perfluorosulfonic acid (PFSA) ionomer from 3M Company and poly(amide imide) (PAI) from Solvay S.A. The relative flow rates of the PFSA and PAI solutions, supplied from separate spinnerets (needles) during electrospinning, were adjusted to create three-layer mixed-fiber mats with different PFSA/PAI weight ratios. After electrospinning, the fiber mats were transformed into dense membranes by heating and compaction, where each layer consisted of a PFSA ionomer matrix with an embedded network of reinforcing PAI fibers. The surface layers of all membranes contained 95 wt.% PFSA and 5 wt.% PAI. The center layer, depleted in PFSA, contained either 75 wt.%, 60 wt.%, or 40 wt.% 825 EW PFSA, with a uniform or gradient PFSA/PAI composition. The thickness of the center 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. The best tri-layer membrane had a total thickness of 17 μm and a PFSA/PAI gradient morphology middle layer. This membrane had a sheet resistance 30% less than a commercial Nafion 211 membrane with a 3-fold decrease in areal water swelling, as compared to Nafion and excellent mechanical properties (a stress at break of 20 MPa versus 22 MPa for neat Nafion 211).

Acknowledgements. The authors thank the US DOE (EERE Cooperative Agreement No. DE-EE0006362) for funding this work and Dr. Michael Yandrasits at 3M Co. for providing the 825 EW PFSA used in this study.