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 (σH+), hydrogen (H2) permeability (PH2), and electrical resistivity (ρe-) 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 (POx) improves H+ conductivity such that the area specific membrane resistance of thin (x-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 POx-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.
Jin et al. (Fri,) studied this question.