Assessing Proton and Hydrogen Permeabilities for Nanoscopic Oxide Coatings Using a Rotating Disk Electrode

The transport of protons and molecular hydrogen (H₂) are of relevance to a wide range of electrochemical applications ranging from fuel cells and electrolyzers to sensors and photoelectrochemical cells. One way to modulate the flux of these species to improve device performance such as selectivity and efficiency is to encapsulate electrodes with semi-permeable oxide coatings. [1-3] Thus, knowing the permeabilities of these species within thin oxide layers is of great importance for guiding the design of electrodes and devices. Towards this end, we have employed a modified rotating disk electrode (RDE) set-up to quantify proton and hydrogen permeabilities through sub- 20 nm thick silicon oxide coatings and understand how these transport properties change as a function of the structural and compositional characteristics of the coatings. Oxide coatings were fabricated by both atomic layer deposition (ALD) and photochemical deposition, and their physical and chemical properties characterized by X-Ray Photoelectron spectroscopy and ellipsometry. Based on measurements of the mass transfer limited current density for the hydrogen evolution and hydrogen oxidation reactions, permeabilities were computed. Species permeabilities are furthermore correlated with membrane density and composition, revealing structure-property-performance relationships that can be used to guide the selection of oxide thickness and processing conditions that will optimize performance for an application of interest.