Acid-Doped Hydrogel Electrolytes for Electrocatalyst Interfaces

Polymer-electrolyte membranes (PEMs) are ubiquitous to state-of-the-art electrochemical devices (i.e., fuel cells and water electrolyzers). Stable PEMs have been historically limited to a small class of perfluorinated sulfonic acid ionomers (PFSAs), which are expensive, difficult to synthesize, and have several key mass-transport limitations that restrict the achievable current densities in PEM devices. Specifically, structural changes in PFSA ionomers observed under low humidity and confined length scales (<100 nm thin films) limit proton conduction and mass-transport rates in the catalyst layer (CL) of PEM devices. We propose an alternative class of polymer electrolyte based on a low-cost polymer scaffold cross-linked and doped with an inorganic acidic charge carrier. As a model material system, we demonstrate a thermally cross-linked poly(acrylic acid)-poly(vinyl alcohol) network doped with sulfuric acid, with charge carrier concentration tuned via the dopant concentration. By implementing a chemically cross-linked scaffold and high dopant concentrations, we achieved a proton concentration of over 15 mmol g-1, more than 10 times higher than available PFSAs. The dopant concentration strongly impacted the polymer's water swelling ratio, higher than 70% in a highly doped material, and proton conductivity, as high as 350 mS cm-1. Notably, the high proton conductivity was maintained even in confined thin films and across humidity levels relevant to PEM fuel cells (PEMFC) and electrolyzer operation. The doped hydrogel electrolyte was employed in a microelectrode vapor-fed water electrolyzer and achieved current densities up to several orders of magnitude higher than Nafion, the most widely implemented PFSA. The results demonstrate the potential for improved polymer electrolyte devices by utilizing low-cost materials tailored to specific device requirements.