Facet-dependent electrocatalysis and surface electrochemical processes on polycrystalline platinum

The (electro)chemical properties of polycrystalline materials used in energy applications is intimately affected surface structure, particularly the orientation of the constituent crystallographic facets (grains). The design more efficient energy materials calls for a better understanding of the structure-function relationship of such materials beyond the low-index facets that are usually the subject of conventional macroscopic "bulk" single crystal studies. Scanning electrochemical cell microscopy (SECCM) is a droplet-cell based scanning probe technique which is able to measure the electrochemical activity of any electrochemically active material with down-to nanoscale resolution. Using SECCM, the orientation-dependent electrochemical properties of poly-crystalline platinum (poly-Pt) have been investigated, focusing on the electrocatalytic (hydrogen evolution action, HER, and oxygen evolution reaction, OER) and surface (platinum oxide reduction, POR, underpotential deposition of hydrogen, HUPD) processes that take place in aqueous sulfuric acid between-0.29 and 1.48 V vs. the saturated calomel electrode (SCE). Probing a total of 28 crystallographically distinct grains, the following general trends are established; HER: (110) > (100), HUPD1: (110) > (100), HUPD2: (100) > (110), OER: (100) > (110), POR: (110) > (100), where HUPD1 and HUPD2 take place at less and more positive potentials, respectively. Interestingly, high-index grains [i.e., those with significant contributions from all three of the (100), (110) and (111) low-index facets] at times possess activities that deviate from the generally established trends based simply on the low-index contributions. Overall, this work demonstrates the ability of correlative SECCM probe electrochemical structure-function information for multiple processes in a single experiment, enabling further optimization and improvement of crucial electrode materials and processes.