Electrospun Nanofiber Composite Membranes for Hydrogen/Air Fuel Cells

Abstract

The fabrication of highly charged and highly durable proton-exchange membranes (PEMs) is a critical step toward improving the performance and lifetime of hydrogen fuel cells. State of the art commercial PEMs from perfluorosulfonic acid ionomers suffer from excessive swelling when exposed to water. The present work’s objective was to investigate the use of electrospinning for the fabrication of highly charged (highly proton conductive) composite PEMs with low in-plane swelling. A series of membranes consisting of a low equivalent weight (i.e. highly charged) ionomer and an uncharged reinforcing polymer were prepared using dual fiber electrospinning (where ionomer and uncharged polymer are separately electrospun) and single fiber electrospinning (where ionomer and uncharged polymer are electrospun from a common solution). For the dual fiber membranes, the distribution of the uncharged reinforcing polymer in the thickness direction was varied (leading to a multi-layered membrane) or the average diameter of the reinforcing polymer was varied. For both the single fiber and dual fiber membranes, ionomers of varying equivalent weights were Incorporated into films with the same effective concentration of fixed charge groups. The dependence of membrane structure, ionomer equivalent weight and side chain chemistry on membrane conductivity and swelling was investigated, and the relevance of the results to fuel cell performance was discussed. The addition of a small amount of uncharged reinforcing polymer in an electrospun membrane led to a significant reduction in swelling, while conductivity generally followed a simple ionomer weight fraction mixing rule. Dual fiber membranes exhibited the best combination of properties, achieving low in-plane water swelling (as low as 4%) with high conductivities (up to 0.1 S/cm in 25°C water).