Studying Electrocatalytic Activity Using Scanning Electrochemical Microscopy

The use of single crystal electrodes in electrocatalysis have allowed researchers to understand some of the elementary steps in different reactions and to establish some relationships between the activity of an electrocatalyst and its surface structure. But the precise identification of the particular type of crystallographic site on a nanoparticle sized electrocatalyst where the reaction takes place at a higher rate (steps, kinks, or basal planes) still represents a challenge. Scanning electrochemical microscopy is an excellent tool for studying electrocatalytic reactions at the nanometer scale, although it has not been much utilized for this purpose. So far, most of the electrocatalysis studies reported using SECM are mainly focused on the relationship between bulk chemical composition and electrochemical activity. This article presents some examples of electrocatalytic reactions studied by SECM using the tip generation-substrate collection (TG-SC), substrate generation-tip collection (SG-TC), redox competition (RC), and micropipette delivery-substrate collection (MD-SC) modes of SECM. Electrocatalytic reactions1 are generally considered heterogeneous inner-sphere reactions, where the reactants, intermediates, and/or products are specifically adsorbed on the electrode surface, allowing a decrease in the reaction energetic barrier. A variety of crystallographic sites may be present at the surface of each single crystal electrode as shown in Fig. 1. In the early 1980s, the use of single crystal electrodes in electrocatalysis was well established, thanks to the flame annealing cleaning method developed by Clavilier, et al.,2 which facilitated obtaining a reproducible electrochemical response at single crystal electrodes while ensuring proper control of the type of site available on the surface. The use of single crystal electrodes, which, for instance in the case of platinum electrodes, need only three numbers to define the corresponding crystallographic Miller indexes (namely (111), (100), and (110)) for the three platinum basal planes, have allowed researchers to understand some of the elementary steps in different electrocatalytic reactions and to establish relationships between the electrocatalyst activity and electrocatalyst surface structure.3 While a majority of these studies have been primarily devoted to the comparison of the three low-index Pt basal surfaces, efforts have also been made to study mixed surfaces — i.e., surfaces with (111) terraces separated by monatomic (110) steps or surfaces with (110) terraces and (111) steps — to identify the most active site at the electrode surface. All these studies have exhibited some limitations due to the difficulty associated with scaling up the single crystal electrode fabrication process. Given the rapid development in the syntheses of nanomaterials, understanding particle size and surface structure effects on the electrocatalytic activity of unsupported and supported nanoparticles has become the next frontier in electrocatalysis. Nanoparticle electrocatalysts are very versatile and can be readily scaled for industrial applications. The electrocatalytic properties of nanoparticles are primarily determined by a set of physical parameters that include particle size, chemical composition (at the surface and in the bulk), and particle shape/ surface structure. In particular, the effect of the crystallographic domains (facets) at the surface of the nanoparticles is currently a hot topic. The effect of the particle shape and, consequently, surface atomic arrangement and coordination, assumes particular importance given the significant progress in the synthesis of shape-controlled metal nanoparticles that has been achieved over the past decade. Is it possible to investigate the catalytic activity provided by one single atomic step or kink on the surface of a catalytic nanoparticle? Identifying the particular zone on a single nanoparticle where the specific adsorption occurs during the redox reaction and where the activity is maximum for each particular electrocatalytic reaction would definitely help develop the surface structure versus reactivity relationship in electrocatalysis. This knowledge, if acquired at the nanometric scale, would be of considerable interest for further elaboration and design of future electrocatalytic nanomaterials. The unique