(472g) Tuning the Composition of Silicon Oxide Overlayers to Control the Performance of Platinum Electrocatalysts

Membrane coated electrocatalyst (MCEC) architectures have been demonstrated to be an attractive approach for improving the activity, stability, and selectivity of electrocatalysts. However, little is known how overlayer structure impacts different electrocatalytic properties in these systems. To better understand the relationship between MCEC structure and performance, it is important to decipher what impact overlayer composition has on electrochemical behavior. For a silicon oxide (SiO_x)|PtMCEC, limiting the conversion of polydimethyl-siloxane (PDMS) to SiO_x during synthesis results in varied concentrations of carbon and silicon-oxygen coordination in the overlayer film, allowing these factors to be manipulated and studied directly. These changes in composition are shown to affect the hydrophobic character, density, and permeability of the overlayers, which in turn affects the overall transport of active species for different electrochemical reactions. By changing the extent conversion of PDMS, the limiting current observed for both the hydrogen evolution reaction and the oxygen reduction reaction can be either enhanced or suppressed. These differences in limiting current are ascribed to differences in species permeability through overlayers of differing compositions, where proton permeability is enhanced as carbon content decreases and density increases, with inverse trends being observed for O₂. The observed transport differences seen for these species further support the hypothesis that O₂ diffuses through the available free volume of the overlayer, while H⁺ transport is facilitated by interactions with SiO_x moieties directly through a facilitated transport mechanism. Collectively, these observations offer insights into how the composition and structure of MCEC overlayers can be tailored to improve the selectivity of Pt for different electrochemical reactions, and serve as a basis for design rules to guide the development of future electrocatalyst architectures.