Tuning the Composition of Silicon Oxide Overlayers to Control the Performance of Platinum Electrocatalysts

Abstract

Membrane coated electrocatalysts (MCECs) have shown to be an attractive approach for improving the activity, stability, and selectivity of various electrochemical reactions. However, little is known about how overlayer structure impacts different catalytic properties and ultimate performance. By analyzing the affect that overlayer composition has on the electrochemical behavior for various probe reactions, guiding relationships that correlate MCEC structure and performance can be derived. For a silicon oxide (SiOx)|Pt MCEC, limiting the conversion of polydimethyl-siloxane (PDMS) to SiOx during synthesis can result in varied concentrations of carbon in the overlayer film, allowing composition to be manipulated and studied directly. These changes in composition consequently affect the hydrophobicity, density, and silicon-oxygen coordination of the overlayer, which in turn influences the final transport behavior of the SiOx membrane. Specifically, platinum electrodes with partially converted SiOxCy overlayers demonstrate a decreased permeability for protons when studying the hydrogen evolution reaction (HER), compared to the less carbon rich, fully converted Pt|SiOx electrodes. Surprisingly, the opposite trend was observed for oxygen permeability during the acidic oxygen reduction reaction (ORR), where the fully cured overlayers free of carbon suppress the transport of oxygen to the buried interface. These contrasting trends in transport suppression present a unique opportunity for future designs to dial in specific, intermediate concentrations of common co-reactants like oxygen and protons, potentially allowing for increased selectivity, poisoning resistance, and stability of the underlying catalyst simply by altering the overlayer’s final composition. Collectively, these results demonstrate the broadened design space that MCEC-style electrocatalysts introduce for future electrochemical systems, and further extend their applicability and relevance to a broader range of reactions.