Voltammetric Mapping of Hydrogen Evolution Reaction on Pt Locally via Scanning Electrochemical Cell Microscopy

Abstract

The advancement in nanoscale electrochemical tools has offered the opportunity to better understand heterogeneity at electrochemical interfaces. Scanning electrochemical cell microscopy (SECCM) has been increasingly used for revealing local kinetics and the distribution of active sites in electrocatalysis. Constant-contact scanning and hopping scanning are the two commonly used modes. The former is intrinsically faster, whereas the latter enables full voltammetry at individual locations. Herein, we revisit a less used mode that combines the advantages of hopping and constant-contact scan, resulting in a faster voltammetric mapping. In this mode, the nanodroplet cell in SECCM maintains contact with the surface during the scanning and makes intermittent pauses for local voltammetry. The elimination of frequent retraction and approach greatly increases the speed of mapping. In addition, iR correction can be readily applied to the voltammetry, resulting in more accurate measurements of the electrode kinetics. This scanning mode is demonstrated in the oxidation of a ferrocene derivative on HOPG and hydrogen evolution reaction (HER) on polycrystalline Pt, serving as model systems for outer-sphere and inner-sphere electron transfer reactions, respectively. While the kinetics of the inner-sphere reaction is consistent spatially, heterogeneity is observed for the kinetics of HER, which is correlated with the crystal orientation of Pt.

The New Era of High-Throughput Nanoelectrochemistry

“…Further work by Wang et al reintroduced the rarely used continuous scanning voltammetric mode of SECCM, originally developed by Guëll et al, 253 and obtained similar results to Guëll on (ferrocenylmethyl) trimethylammonium oxidation on the basal surface of HOPG. 207 They further used the technique to reveal the grain orientation dependence of hydrogen evolution reaction (HER) on platinum, employing the powerful combination of SECCM and electron backscatter diffraction (EBSD), which is becoming an increasingly popular methodology. 254 The local electrochemical current flowing through a thin organic coating is strongly dependent on the coating's electrical conductivity.…”

“…If the substrate is a conductor, it is usually connected as the working electrode (WE) and an additional voltage, applied between the QRCEs and the WE, facilitates the implementation of a variety of electrochemical techniques, within the confines of the miniature cell. Voltammetry experiments − and chronoamperometry experiments , are used to investigate the local electrochemical activity and to induce and monitor phase changes (Figure C). At the same time, the ionic current informs of any change occurring in the meniscus, e.g., the nucleation and growth of gas bubbles .…”

“…With vertical tip position modulation SECCM, the surface topography can be tracked in a constant-distance mode (Figure B), where the probe does not break contact with the substrate . In this case, the potential versus the surface is kept stable, or swept while lateral movement is briefly interrupted. − …”

“… 204 In this case, the potential versus the surface is kept stable, or swept while lateral movement is briefly interrupted. 205 − 207 …”

“…Further work by Wang et al reintroduced the rarely used continuous scanning voltammetric mode of SECCM, originally developed by Güell et al, 253 and obtained similar results to Güell on (ferrocenylmethyl) trimethylammonium oxidation on the basal surface of HOPG. 207 They further used the technique to reveal the grain orientation dependence of hydrogen evolution reaction (HER) on platinum, employing the powerful combination of SECCM and electron backscatter diffraction (EBSD), which is becoming an increasingly popular methodology. 254 …”