Theory and experiments for voltammetric and SECM investigations and application to ORR electrocatalysis at nanoelectrode ensembles of ultramicroelectrode dimensions.

Theoretical and experimental approaches to characterizing nanoelectrode (NE) ensembles of ultramicroelectrode dimensions (UME-NEEs) as a function of fraction of active area and random NE distribution are described. UME-NEEs were fabricated by addressing microregions of a gold-filled polycarbonate membrane through the UMEs of an underlying microfabricated addressable array. Results of Comsol Multiphysics 3D simulations based on randomly spaced NEs of 15 nm radius on a UME disk geometry of radii up to 5 μm are shown for steady-state voltammetry (SSV) and scanning electrochemical microscopy (SECM) experiments. Analytical equations were developed to describe the diffusion-limited steady-state current and steady-state voltammogram at an UME-NEE. These equations are shown to be in good agreement with the simulations and enabled evaluation of experimental SSVs. Comparison of experimental and simulated SECM approach curves, images, and tip voltammograms enabled the fraction of active area and distribution of NEs to be visualized and determined for individual UME-NEEs. Gold UME-NEEs are shown to be unique platforms for electrodeposition in forming nanoparticle electrodes (UME-NPEs). Electrocatalysis results for the oxygen reduction reaction (ORR) on Pt UME-NPEs in 0.1 M H2SO4 are also shown.