Understanding the Voltammetry of Bulk CO Electrooxidation in Neutral Media through Combined SECM Measurements

Abstract

Recently, the bulk electrooxidation of CO on gold or platinum has been used to detect CO produced during CO 2 reduction in neutral media. The CO bulk oxidation voltammetry may show two distinct peaks depending on the reaction conditions, which up to now have not been understood. We have used scanning electrochemical microscopy (SECM) to probe CO oxidation and pH in the diffusion layer during CO 2 reduction. Our results show that the two different peaks are due to diffusion limitation by two different species, namely, CO and OH – . We find that between pH 7 and 11, CO oxidation by water and OH – gives rise to the first and second peak observed in the voltammetry, respectively. Additional rotating disc experiments showed that specifically in this pH range the current of the second peak is diffusion limited by the OH – concentration, since it is lower than the CO concentration.

Efficiency and selectivity of CO2 reduction to CO on gold gas diffusion electrodes in acidic media

“…However, in our case, at low current densities, and low pH, due to the lower mass transport limitation in the GDE configuration, the activity for CO (and corresponding production of OH − ) is not high enough to neutralize the protons diffusing towards the catalyst surface, favouring hydrogen production. On the other hand, at current densities above 50 mA cm −2 , especially for bulk pH 3 or 4, the local acidic environment can be rapidly neutralized 23 , 24 , enabling FEs for CO above 80%. We see that when starting from bulk pH 2, higher current densities are required to achieve this suppression of proton reduction.…”

Probing the local activity of CO2 reduction on gold gas diffusion electrodes: effect of the catalyst loading and CO2 pressure

“…Similarly, in our recent studies, we have used a Pt-UME in the surface-generation tip-collection (SG-TC) mode to detect CO produced during CO 2 RR on Au, Cu and Ag electrodes. 25,26 Although these previous measurements were performed on non-permeable substrates, we could also demonstrate that probing the local hydroxide and water activities during the oxygen reduction reaction (ORR) and CO 2 RR on GDEs can be achieved with SECM by shear-force positioning of Pt nanoelectrodes at ~100 nm above the GDE surface. [23,24] This experimental approach allows for simultaneously deriving modulations of the local pH value in correlation with the current density, and the topography in situ and with high resolution.…”

“…In contrast, scanning probe techniques are powerful tools for investigating activity locally with high resolution. [19][20][21][22][23][24][25][26] Mayer et al 22 used a Pt ultramicroelectrode (Pt-UME) to detect formate, CO and H 2 during CO 2 RR on Sn/SnO x arrays. However, the tip-to-surface distance was determined using O 2 reduction diffusion limitation, which is only practical for probing flat electrodes.…”

Probing the Local Reaction Environment During High Turnover Carbon Dioxide Reduction with Ag‐Based Gas Diffusion Electrodes

“…In addition to the local ion activity changes, one also needs to consider that the predominant product of the CO 2 RR at a Ag‐GDE is CO, which may poison the Pt tip surface. However, by sweeping the potential anodically, adsorbed CO is removed, which re‐establishes a clean Pt tip in each potentiodynamic cycle [29] . We found that ultraclean Pt tips are necessary for the anticipated measurements above CO 2 ‐converting electrodes, since otherwise often ambiguous Pt voltammograms are observed (Figure S3).…”

“…However, by sweeping the potential anodically,a dsorbed CO is removed, which re-establishes ac lean Pt tip in each potentiodynamic cycle. [29] We found that ultraclean Pt tips are necessary for the anticipated measurements above CO 2 -convertinge lectrodes, since otherwise often ambiguous Pt voltammograms are observed (Figure S3). The cathodic shift of the PtO red.…”