Sterically-encumbered ionenes as hydroxide ion-conducting polymer membranes

Hydrogen/oxygen-based electrochemical energy conversion cells that operate under highly alkaline conditions deploy inexpensive electrocatalysts compared to their acidic counterparts. Solid polymer electrolyte (SPE) cells offer a reduced system footprint, and an additional reduction in capital cost. Alkaline membrane, SPE systems are attractive because they offer a synergistic combination of the two cost savings. Durable, hydroxide-conducting SPEs operating in highly caustic media are lacking because organic molecules and polymers, particularly their cationic derivatives, are inherently unstable to caustic conditions. This review focuses on an emerging class of alkaline SPE's, lonenes: polymers that incorporate cations directly into the polymer backbone. The purpose of this opinion piece is to highlight the fact that fixed cationic charges may be incorporated into the polymeric backbone to provide membranes with high conductivity, mechanical strength, and stability, and to dispel the widely-held view that distancing a cationic group from the polymer backbone is a necessary requirement.