Ultrathin composite membrane of alkaline polymer electrolyte for fuel cell applications

Abstract

To minimize the ohmic loss in the cell voltage of fuel cells, the electrolyte should be made as thin as possible, in particular when alkaline polymer electrolytes (APEs) are employed, where both the mobility and the concentration of OH⁻ are relatively low. A practical strategy for fabricating thin APE membranes is to impregnate APE ionomers into an ultrathin, rigid framework (such as a porous PTFE film), so that high ion conduction is achieved by the APE with a high ion-exchange capacity (IEC), while good mechanical stability is provided by the robust host. Our previous study has realized a prototype of an APE fuel cell (APEFC) using this kind of composite membrane but we found later that the APE component, quaternary ammonium polysulfone (QAPS), will leach out gradually under fuel cell operating conditions because of the poor interaction between the QAPS guest and the PTFE host. To address this problem, we demonstrate in the present work a new approach for making ultrathin composite membranes of APEs. The APE ionomer (TQAPS) is impregnated into a porous PTFE film, followed by a self-crosslinking process, so as to form a semi-interpenetrating network. The resulting ultrathin composite membrane (xQAPS@PTFE, 25 μm thick) is highly tolerant to leaching in 80 °C water and possesses low area resistance (0.09 Ω cm²), a low swelling degree (3.1% at 60 °C) and high mechanical strength (31 MPa). Making use of such an xQAPS@PTFE membrane, the H₂–O₂ APEFC exhibits a peak power density of 550 mW cm² at 60 °C under 0.1 MPa of back pressure.