Preparation and Optimization of Novel Anisotropic Proton-Exchange Membranes with Enhanced through-Plane Conductivity

Abstract

Maximizing through-plane (TP) conductivity, as opposed to in-plane (IP), is desired for proton exchange membranes (PEMs) in fuel cells, electrolysis and electrodialysis. Nafion, the benchmark PEM material, owes conductivity to randomly oriented ion-channels. Channel alignment in electrospun nanofibers was shown to enhance conductivity along the fibres by an order of magnitude, but it proved challenging to convert this to desired stable TP conductance. Another challenge is that orientation proves unstable and returns to random after nanofiber consolidation.Previously, we reported fabrication of electrospun nanofiber-based membrane with a stable anisotropic TP alignment of proton conducting channels prepared by co-spinning, buckling and fusion of Nafion and PVDF nanofibers (Odess, Li et al, ACS Appl Mat Interfaces, 2021), as shown in Figure 1. PVDF nanofibers serve as a mechanical reinforcement as well as a stabilizing component to safeguard Nafion TP alignment during annealing and swelling, akin to a solid template. However, the prepared 50% Nafion membrane still showed a TP conductivity slightly inferior to Nafion.In this report, we systematically optimized the Nafion content in the 50-80% range, as well as buckling and consolidation procedure and its temperature regime, to achieve a maximal and sustained conductance, exceeding that of pure Nafion. It was found that the TP proton conductivity of the membrane with 60% Nafion already exceeded that of Nafion 117 and further increased with Nafion content (Figure 1), with decrease in anisotropy (TP/IP conductance ratio). As another attractive feature, compared with Nafion 117, the TP conductivity of aligned membranes decreased less at lower humidity, i.e., was less sensitivity to dehydration, apparently due to better channel connectivity. Anisotropic structure and nanoscopic alignment were further confirmed by TEM and SAXS. The results demonstrate the potential of the new buckling approach for making novel nanofiber-based anisotropic membranes with significantly improved performance in electromembrane applications. Figure 1