A Highly Conductive and Mechanically Robust OH– Conducting Membrane for Alkaline Water Electrolysis

Abstract

In an alkaline water electrolysis cell, a membrane is needed between the cathode and the anode to avoid mixing of hydrogen and oxygen products while enabling OH- transport. Hydroxide ion conductivity and membrane mechanical properties are both important parameters that determine material constraints on low electrical resistance of a membrane versus sufficient structural integrity. Herein, we demonstrate a strategy to make membranes with both high OH- conductivity and mechanical strength. A chemically tailored OH- conducting polymer (qPPO) was synthesized via amination and subsequent quaternization of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and was blended with poly(vinyl alcohol) (PVA) to provide an environment analogous to basic water solutions. The -OH groups in PVA provide high-density Grotthuss mechanism conduction sites similar to water, which may be the key reason for the observed high OH- conductivity of the membranes. The PVA backbone was cross-linked to form a semi-interpenetrating network (semi-IPN) of PVA and qPPO; the resulting material contains PVA chemical cross-links and hydrogen bonds between PVA and qPPO and between PVA with itself, all of which are believed to contribute to a high tensile strength. By tuning the PVA/qPPO ratio, the transport and mechanical properties were optimized. The membrane with 30% qPPO possesses both extraordinary conductivity (151 mS/cm at room temperature) - about 2.7 times as high as Nafion 117 in acidic conditions - and high ultimate tensile strength (126 MPa (dry), 41 MPa (wet)). This highly conductive polymer membrane also exhibits stability in alkaline water electrolysis at room temperature, a property that makes qPPO an interesting and potentially translational material for the design of hydroxide-based electrochemical cells.