Acid-Doped Hydrogels for Robust Ion Transport in Electrochemical Catalyst Layers

Abstract

Polymer-electrolyte membranes (PEMs) are ubiquitous to state-of-the-art electrochemical devices. Polymer electrolyte materials have been historically limited to a small class of perfluorinated sulfonic-acid ionomers (PFSAs). Structural changes in PFSA ionomers observed under low humidity and confined length scales (<100’s of nanometers) limit proton conduction and mass transport in the catalyst layer, restricting current density in state-of-the-art devices. Heterogeneous catalyst layer morphology and complex formulation-performance relationships make these limitations difficult to assess. We propose an alternative class of polymer electrolyte based on a thermally crosslinked poly(acrylic acid)-poly(vinyl alcohol) scaffold doped with sulfuric acid, with ion-exchange capacity (IEC) tuned via the dopant concentration. We achieved IEC over 10 meq g-1, well above the upper-limit for PFSAs, and proton conductivity as high as 350 mS cm-1. Notably, proton conductivity was robust across the length (<100 nm film thickness) and humidity scales relevant to electrochemical device operation. The doped-hydrogel electrolyte was employed in a microelectrode vapor-fed water electrolyzer and achieved current densities up to 8 orders of magnitude higher than Nafion under low humidity operation. In this poster, I will present a facile method for fabricating electrolyte-doped hydrogel thin-films with tunable IEC. Furthermore, I will discuss transport properties of these materials across the range of humidity and thicknesses relevant for electrochemical catalyst layers. Finally, I will discuss the use of an electrochemical model system to isolate and systematically study ionomer performance under relevant operating conditions, providing a crucial link between material properties and performance in-operando. The results presented here underscore the potential value of tailoring materials chemistry to specific requirements of individual device components in membrane-electrode assemblies.