National Institute of Standards and Technology,
Keywords: heterogeneous electrocatalysis, fluorescence microscopy, structure/activity relationships
Summary:Heterogeneous catalytic materials and electrodes are used for (electro)chemical transformations, including those important for energy storage and utilization. Due to the heterogeneous nature of these materials, activity measurements with sufficient spatial resolution are needed to obtain structure/activity correlations across the different surface features (exposed facets, step edges, lattice defects, grain boundaries, etc.). Advanced measurements that provide such structure/activity correlations will reveal underlying reaction mechanisms and enable engineering of more active materials. Because (electro)catalytic surfaces restructure with changing environments, it is important to perform measurements in operando. So far, spatially resolved activity measurements have been made on electrocatalysts only using scanning electrochemical microscopy methods. Sub-diffraction fluorescence microscopy is well suited to this problem because it can operate in solution with resolution down to a few nm and single-molecule sensitivity. We have applied sub-diffraction fluorescence microscopy to a thin cell containing an electrocatalyst and a solution containing a redox sensitive dye to characterize the reaction at the solid-liquid interface. Our chosen dye switches between a nonfluorescent reduced state and a one-electron oxidized bright state, a process that occurs at the electrode surface. This scheme is used to investigate the activity differences on the surface of polycrystalline Pt since it is used as an electrocatalyst for many chemical transformations. The focus of this measurement, in particular, is to differentiate reactivity at grain faces and grain boundaries. A diagram of the experimental scheme is shown below. Ultimately, this method will be extended to study other dye systems and electrode materials.