Probing the Effects of Defects in Ultrathin Oxide Overlayers on the Selectivity of Encapsulated Electrocatalysts

Abstract

Application of ultrathin oxide encapsulation layers on co-catalysts employed in z-scheme photocatalysis has the potential to increase overall efficiency through the prevention of undesirable back reactions and increased charge separation. [1,2] However, defects within the semipermeable oxide coatings – such as pinholes, cracks or particle protrusions – have been postulated to facilitate locally high rates of undesirable reactions by creating pathways for facile transport of undesired reactants to exposed active sites.[3] Local probe measurements, such as scanning electrochemical microscopy (SECM), can estimate the relative rates of production of product species generated from competing electrochemical reactions with high spatial resolution over the electrode. This can be leveraged to determine the influence of local defects on global performance metrics, such as the apparent permeability of the overlayer and selectivity of the electrode. In this presentation, we report the use of SECM to determine the influence of overlayer defects on the performance of a model silicon oxide (SiOx) -encapsulated Pt thin film electrocatalyst when operated under conditions where two competing reactions can occur. Motivated by Z-scheme photocatalysis, the hydrogen evolution reaction (HER) and Fe(III)/Fe(II) redox reaction were studied. After introducing new methodology to determine local selectivity towards HER using SECM tip current, the resulting selectivity maps are compared against defects seen in optical images, scanning electron micrographs, and atomic force microscopy images. This analysis reveals that that certain types of defects in the oxide overlayer can be responsible for a large percentage of the partial current density towards the undesired Fe(III) reduction reaction. Correcting for the defect contributions to the undesired reaction, it is determined that the true Fe(III) permeability values for the SiOx overlayers are almost two orders of magnitude lower than permeabilities determined from conventional analysis that ignored the presence of defects. Finally, different types of defects were studied revealing that the defect morphology can have varying influence on both the selectivity and calculated permeability. This work highlights the need for local measurements in addition to macroscopic measurements of oxide encapsulated catalysts as they are applied to more complex geometries such as particle photocatalysts.