Advanced Spatiotemporal Voltammetric Techniques for Kinetic Analysis and Active Site Determination in the Electrochemical Reduction of CO₂

Abstract

Conspectus Electrochemical reduction of the greenhouse gas CO2 offers prospects for the sustainable generation of fuels and industrially useful chemicals when powered by renewable electricity. However, this electrochemical process requires the use of highly stable, selective, and active catalysts. The development of such catalysts should be based on a detailed kinetic and mechanistic understanding of the electrochemical CO2 reduction reaction (eCO2RR), ideally through the resolution of active catalytic sites in both time (i.e., temporally) and space (i.e., spatially). In this Account, we highlight two advanced spatiotemporal voltammetric techniques for electrocatalytic studies and describe the considerable insights they provide on the eCO2RR. First, Fourier transformed large-amplitude alternating current voltammetry (FT ac voltammetry), as applied by the Monash Electrochemistry Group, enables the resolution of rapid underlying electron-transfer processes in complex reactions, free from competing processes, such as the background double-layer charging current, slow catalytic reactions, and solvent/electrolyte electrolysis, which often mask conventional voltammetric measurements of the eCO2RR. Crucially, FT ac voltammetry allows details of the catalytically active sites or the rate-determining step to be revealed under catalytic turnover conditions. This is well illustrated in investigations of the eCO2RR catalyzed by Bi where formate is the main product. Second, developments in scanning electrochemical cell microscopy (SECCM) by the Warwick Electrochemistry and Interfaces Group provide powerful methods for obtaining high-resolution activity maps and potentiodynamic movies of the heterogeneous surface of a catalyst. For example, by coupling SECCM data with colocated microscopy from electron backscatter diffraction (EBSD) or atomic force microscopy, it is possible to develop compelling correlations of (precatalyst) structure–activity at the nanoscale level. This correlative electrochemical multimicroscopy strategy allows the catalytically more active region of a catalyst, such as the edge plane of two-dimensional materials and the grain boundaries between facets in a polycrystalline metal, to be highlighted. The attributes of SECCM-EBSD are well-illustrated by detailed studies of the eCO2RR on polycrystalline gold, where carbon monoxide is the main product. Comparing SECCM maps and movies with EBSD images of the same region reveals unambiguously that the eCO2RR is enhanced at surface-terminating dislocations, which accumulate at grain boundaries and slip bands. Both FT ac voltammetry and SECCM techniques greatly enhance our understanding of the eCO2RR, significantly boosting the electrochemical toolbox and the information available for the development and testing of theoretical models and rational catalyst design. In the future, it may be possible to further enhance insights provided by both techniques through their integration with in situ and in operando spectroscopy and microscopy methods.

Thousand-fold increase in O2 electroreduction rates with conductive MOFs

“…Given a droplet radius of 25 nm, this current translates to a geometric j ORR value of −1273 mA cm –2 , 38-fold greater than the highest current densities observed under air using the GDE (Figure S25). The larger current densities observed in SECCM versus the GDE or RRDE configurations are consistent with the short diffusion pathways enforced by the nanoscopic dimensions of SECCM. ,,− …”

“…High-resolution scanning electrochemical cell microscopy (SECCM; Figure A) − lent critical insight into the origin of the plateau in activity observed in our GDE studies (Figure S23 and the Supporting Information for experimental details). By confinement of the entirety of the electrode contact area to the footprint of a droplet at the end of a nanopipet, SECCM offers the fastest gas mass transport rates experimentally accessible for electrocatalysis: N 2 or O 2 can rapidly traverse the nanoscale droplet electrolyte and the porous Ni 3 (HITP) 2 particles, with the maximum diffusion length of gaseous species to the catalyst surface being set by the droplet radius. , For example, a hemispherical droplet with a radius r d = 25 nm has a sub-microsecond diffusion time, about 6 orders of magnitude higher than that in the RRDE studies.…”

Rational Manipulation of Intermediates on Copper for CO2 Electroreduction Toward Multicarbon Products

“…In particular, experimental observations can provide modeling guidance for theoretical calculations, in turn enabling the high-throughput screening of highly active catalysts with intermediates with suitable binding energies [176]. To enable the precise customization of active sites on Cu-based catalysts, there is an urgent need to develop high temporal and spatial resolu-tion characterization techniques to accurately detect the intermediates [177,178]. 2) Rational design of electrolyzers.…”

Screening Surface Structure–Electrochemical Activity Relationships of Copper Electrodes under CO2 Electroreduction Conditions

“…In this way, electrochemical analysis of multiple crystallographic orientations and grain boundaries, found on a polycrystalline metal surface, is achieved 30 , 31 as demonstrated by studies of an increasing diversity of systems. 24 , 28 , 32 − 38 Recent studies have used the combination of SECCM and EBSD to resolve CO 2 electroreduction activity at grain boundaries of Au, 39 − 41 and SECCM alone has been used to study CO 2 electroreduction at tin/reduced graphene oxide interfaces. 42 …”

“…To rapidly access the electrochemical characteristics of a wide variety of crystallographic sites, we introduced “pseudo-single crystal” electrochemistry, using high-resolution scanning electrochemical cell microscopy (SECCM) − to map out the voltammetry (with several thousand spatially resolved voltammograms easily achievable) , on a polycrystalline surface, whose crystallographic structure is determined by co-located electron backscatter diffraction (EBSD). In this way, electrochemical analysis of multiple crystallographic orientations and grain boundaries, found on a polycrystalline metal surface, is achieved , as demonstrated by studies of an increasing diversity of systems. ,,− Recent studies have used the combination of SECCM and EBSD to resolve CO 2 electroreduction activity at grain boundaries of Au, − and SECCM alone has been used to study CO 2 electroreduction at tin/reduced graphene oxide interfaces …”