Gold is one of the most selective catalysts for the electrochemical reduction of CO2 (CO2RR) to CO. However, the concomitant hydrogen evolution reaction (HER) remains unavoidable under aqueous conditions. In this work, a rotating ring disk electrode (RRDE) setup has been developed to study quantitatively the role of mass transport in the competition between these two reactions on the Au surface in 0.1 M bicarbonate electrolyte. Interestingly, while the faradaic selectivity for CO formation was found to increase with enhanced mass transport (from 67% to 83%), this effect is not due to an enhancement of the CO2RR rate. Remarkably, the inhibition of the competing HER from water reduction with increasing disk rotation rate is responsible for the enhanced CO2RR selectivity. This can be explained by the observation that, on the Au electrode, water reduction improves with more alkaline pH. As a result, the decrease in the local alkalinity near the electrode surface with enhanced mass transport suppresses HER due to the water reduction. Our study shows that controlling the local pH by mass transport conditions can tune the HER rate, in turn regulating the CO2RR and HER competition in the general operating potential window for CO2RR (-0.4 to -1 V vs RHE).

https://scholarlypublications.universiteitleiden.nl/access/item%3A2965675/view

Competition between CO2 Reduction and Hydrogen Evolution on a Gold Electrode under Well-Defined Mass Transport Conditions

Electrochemical CO2 reduction has received an increased amount of interest in the last decade as a promising avenue for storing renewable electricity in chemical bonds. Despite considerable progress on catalyst performance using nanostructured electrodes, the sensitivity of the reaction to process conditions has led to debate on the origin of the activity and high selectivity. Additionally, this raises questions on the transferability of the performance and knowledge to other electrochemical systems. At its core, the discrepancy is primarily a result of the highly porous nature of nanostructured electrodes, which are vulnerable to both mass transport effects and structural changes during the electrolysis. Both effects are not straightforward to identify and difficult to decouple. Despite the susceptibility of nanostructured electrodes to mass transfer limitations, we highlight that nanostructured silver electrodes exhibit considerably higher activity when normalized to the electrochemically active surface in contrast to gold and copper electrodes. Alongside, we provide a discussion on how active surface area and thickness of the catalytic layer itself can influence the onset potential, selectivity, stability, activity and mass transfer inside and outside of the three dimensional catalyst layer. Key parameters and potential solutions are highlighted to decouple mass transfer effects from the measured activity in electrochemical cells utilizing CO2 saturated aqueous solutions.

/pdf/electrochemical-co2-reduction-on-nanostructured-metal-jw2z21zaaz.pdf

Electrochemical CO2 reduction on nanostructured metal electrodes: fact or defect?

Here, we demonstrate that small molecules such as H2, CO, HCOO− and their mixtures, generated from the electroreduction of CO2 (CO2RR), can be detected and quantified on a rotating ring-disc electrode (RRDE). We describe a series of systematic calibration protocol to quantify CO2RR products on four model electrocatalysts (Pt, Au, Sn, [NiII(cyclam)]2+). In an RRDE assembly, products generated at the disc are convectively transported and detected at the ring detector before diffusing into the bulk solution, circumventing the need for product pre-concentration. As the electrochemical fingerprints of these small molecules and their mixtures on the Pt detector are unique, RRDE also excludes the need for separation. Thus, using the rotating ring detector significantly minimizes the time delay between product generation and detection compared to conventional techniques such as GC, LC and NMR. An RRDE allows faster screening of catalysts that can help accelerate catalyst discovery for CO2RR. It also enables time-dependent mechanistic studies of CO2RR catalysis.

https://iopscience.iop.org/article/10.1149/1945-7111/ab7a80/pdf

Rapid Product Analysis for the Electroreduction of CO2 on Heterogeneous and Homogeneous Catalysts Using a Rotating Ring Detector

Reduction of carbon dioxide at copper(I) oxide photocathode activated and stabilized by over-coating with oligoaniline

Elucidation of activity of copper and copper oxide nanomaterials for electrocatalytic and photoelectrochemical reduction of carbon dioxide

Using the rotating ring (platinum)—disk (glassy carbon) electrode methodology, electrocatalytic activity of the microstructured copper centers (imbedded within the polyvinylpyrrolidone polymer matrix and deposited onto the glassy carbon disk electrode) has been monitored during electroreduction of carbon dioxide both in acid (HClO4) and neutral (KHCO3) media as well as diagnosed (at Pt ring) with respect to formation of the electroactive products. Combination of the stripping-type and rotating ring-disk voltammetric approaches has led to the observation that, regardless the overlapping reduction phenomena, the reduction of carbon dioxide at copper catalyst is, indeed, operative and coexists with hydrogen evolution reaction. Using the fundamental concepts of surface electrochemistry and analytical voltammetry, the reaction products (thrown onto the platinum ring electrode) could be considered and identified as adsorbates (on Pt) under conditions of the stripping-type oxidation experiment. Judging from the potentials at which the stripping voltammetric peaks appear in neutral CO2-saturated KHCO3 (pH 6.8), formic acid or carbon monoxide seem to be the most likely reaction products or intermediates. The proposed methodology also permits correlation between the CO2 electroreduction products and the potentials applied to the disk electrode. By performing the comparative stripping-type voltammetric experiments in acid medium (HClO4 at pH 1) with the adsorbates of formic acid, ethanol and acetaldehyde (on Pt ring), it can be rationalized that, although C2H5OH or CH3CHO are very likely CO2-reduction electroactive products, formation of some HCOOH, CH3OH or even CO cannot be excluded.

Rotating ring-disk voltammetry: Diagnosis of catalytic activity of metallic copper catalysts toward CO 2 electroreduction

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Rotating ring-disk voltammetry: Diagnosis of catalytic activity of metallic copper catalysts toward CO 2 electroreduction