Controlling the Relative Fluxes of Protons and Oxygen to Electrocatalytic Buried Interfaces with Tunable Silicon Oxide Overlayers

Modifying an electrocatalyst surface by encapsulating it with a nanoscale oxide membrane is an attractive approach for improving its activity, stability, and selectivity. However, little is known about how the composition and structure of such overlayers impact the properties and performance of the underlying electrocatalyst. The results presented herein demonstrate that the density and carbon content of carbon-modified silicon oxide (SiOxCy) overlayers can be systematically varied using a room-temperature photochemical conversion process to precisely control the flux of protons and molecular oxygen (O2) to the buried interface of an encapsulated platinum electrode. Correlations between species permeabilities, overlayer composition, and overlayer structure show that proton transport is greatly enhanced as the carbon content decreases and overlayer density increases, while the inverse trend is observed for O2. These observations are found to be consistent with O2 diffusing through the free volume in the SiOxCy matrix, while proton diffusion proceeds through a facilitated transport mechanism associated with the hydrogen bond network of silicon oxide moieties. Importantly, this study indicates that overlayer composition can be a potentially powerful control knob for tuning the fluxes of redox species to active sites as well as modifying local properties at electrocatalytic buried interfaces.