Large-scale deployment of conventional proton exchange membrane (PEM) water electrolyzers is hindered by higher levelized cost of H₂ production versus carbon-intensive steam methane reforming. To decrease this cost, cell efficiencies can be improved by decreasing membrane thickness, which reduces the ohmic overpotential required to drive H+ transport across a membrane. Work has been done to reduce thickness of Nafion membranes to 50-200 um while keeping H₂ crossover below safety limits, but a fundamental limit is apparent. One potential replacement is submicron proton conducting oxide membranes (POM). Here, orders of magnitude lower H⁺ conductivities, σ_H+, than Nafion are acceptable if a POM also has orders of magnitude lower H₂ permeabilities, P_H2. In this context, membrane H+ resistance is minimized while electrolyzer efficiency maximized by maximizing the σ_H/P_H2 ratio. To that end, we compared wet chemical (WC) and atomic layer deposition (ALD) films of SiO₂ with Nafion. Baseline SiO₂ POMs had poor σ_H, but much lower P_H2 relative to Nafion117 (212 nm), with ALD having a promising σ_H/P_H2 ratio, as shown in Figure 1. To improve σ_H, phosphate (PO₄^3-) incorporation was explored with novel P ALD precursors, motivated by Kohl and co-workers’ research with P-doped silica glass membranes. In this work, PO₄^-3 was doped into ALD films across a wide temperature range. The best-performing membrane is 0.65% PO₄^-3 doped SiO₂ ALD film deposited at 100 °C with σ_H of 2.2x10^-3 S cm^-1 measured at 25 °C, almost an order of magnitude higher than other ceramic membranes, approaching lower σ_H values of Nafion, 8.0x10^-2 S cm^-1. P_H2after PO₄^-3 doping remained < 1x10^-10 cm²s^-1, ~3 orders of magnitude less than Nafion, and electronic conductivities remained sufficiently low. The film has a σ_H/P_H2 ratio of 2.7x10⁷ S*sec cm^-3, ~2 orders of magnitude higher than Nafion, Figure 1. These results point towards the viability of PO₄^-3 doped SiO₂ ALD films as a promising material to realize submicron high σ_H ceramic membranes for efficient and safe water electrolysis to enable a competitive green H₂ market.

Figure 1: H<sup>+</sup> conductivity to H<sub>2</sub> permeability ratio vs. membrane type.

Figure 1: H⁺ conductivity to H₂ permeability ratio vs. membrane type.

Arrelaine Dameron

Appearances