Proton Conducting Silicon Oxide Membranes as a Fluorine Free Alternative to Nafion for Low Temperature Water Electrolysis

Driven by environmental and health concerns related to per- and polyfluoroalkyl substances (PFAS), there has been growing interest in developing fluorine-free proton (H⁺) exchange membrane (PEM) materials for fuel cells and water electrolyzers. In this study, we present a side-by-side comparison of the key transport properties of submicron thick, PFAS-free amorphous silicon dioxide (SiO₂) membranes to Nafion, a fluorinated polymer electrolyte membrane that represents the industry standard for PEM fuel cells and electrolyzers. Measurements of proton (H⁺) conductivity (σ_H⁺), hydrogen (H₂) permeability (<i>P</i>_H₂), and electrical resistivity (ρ_e^-) were conducted using model thin films comprised of SiO₂ membranes deposited by atomic layer deposition (ALD). Although the H⁺ conductivity of the SiO₂ membranes is 2-3 orders of magnitude lower than Nafion, the addition of phosphorus dopants (PO_<i>x</i>) improves H⁺ conductivity such that the area specific membrane resistance of thin (<50 nm) PO_<i>x</i>-doped SiO₂ membranes is more than an order of magnitude lower than Nafion-117. Importantly, the safe operation of such nanoscale membranes within a PEM electrolyzer is feasible thanks to the low H₂ permeability of dense SiO₂-based membranes, which are predicted to limit H₂ crossover rates to acceptable levels for pressures up to ≈ 100 bar. As a proof-of-principle demonstration, a chip-scale water electrolyzer based on 100 nm thick PO_<i>x</i>-SiO₂ membrane is shown to achieve a current density of 2 A cm^-2 at a potential of 2.5 V. If this technology can be successfully scaled up, H⁺ conducting oxide membranes offer an attractive PFAS-free alternative to Nafion for efficient and durable water electrolysis and fuel cell technologies.