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

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.