Direct Observation of the Local Reaction Environment during the Electrochemical Reduction of CO₂

Abstract

ABSTRACT:The electrochemical reduction of carbon dioxide is sensitive to electrolyte polarization, which causes gradients in pH and the concentration of carbon dioxide to form near the cathode surface. It is desirable to measure the concentration of reaction-relevant species in the immediate vicinity of the cathode because the intrinsic kinetics of carbon dioxide reduction depend on the composition of the local reaction environment. Meeting this objective has proven difficult because conventional analytical methods only sample products from the bulk electrolyte. In this study, we describe the use of differential electrochemical mass spectrometry to measure the concentration of carbon dioxide and reaction products in the immediate vicinity of the cathode surface. This capability is achieved by coating the electrocatalyst directly onto the pervaporation membrane used to transfer volatile species into the mass spectrometer, thereby enabling species to be sampled directly from the electrode−electrolyte interface. This approach has been used to investigate hydrogen evolution and carbon dioxide reduction over Ag and Cu. We find that the measured CO 2 reduction activity of Ag agrees well with what is measured by gas chromatography of the effluent from an H-cell operated with the same catalyst and electrolyte. A distinct advantage of our approach is that it enables observation of the depletion of carbon dioxide near the cathode surface due to reaction with hydroxyl anions evolved at the cathode surface, something that cannot be done using conventional analytical techniques. We also demonstrate that the influence of this relatively slow chemical reaction can be minimized by evaluating electrocatalytic activity during a rapid potential sweep, thereby enabling measurement of the intrinsic kinetics. For CO 2 reduction over Cu, nine products can be observed simultaneously in real time. A notable finding is that the abundance of aldehydes relative to alcohols near the cathode surface is much higher than that observed in the bulk electrolyte. It is also observed that for increasingly cathodic potentials the relative abundance of ethanol increases at the expense of propionaldehyde. These findings suggest that acetaldehyde is a precursor to ethanol and propionaldehyde and that propionaldehyde is a precursor to n-propanol.

Insights into the carbon balance for CO2 electroreduction on Cu using gas diffusion electrode reactor designs

“…41 In addition, a disproportionate amount of acetaldehyde crossed to the anolyte (it appears as if the crossover ratio of acetaldehyde was relatively high) and near 20% ethanol FE (dominant liquid product in Figure 3) entail that while CO2 reduction produced a significantly higher amount of acetaldehyde at the cathode in comparison with the detected results ( Figure 3) after finishing electrolysis, most of acetaldehyde was further reduced to ethanol as the catholyte is recycled during electrolysis. 42…”

Reactor design and integration with product detection to accelerate screening of electrocatalysts for carbon dioxide reduction

“…11 Differential electrochemical mass spectrometry (DEMS) is an analytical technique that combines an electrochemical reactor with a mass spectrometer. [18][19][20] It has been widely utilized to provide "real-time" detection of electrochemical reactants/products/intermediates. However, common issues associated with DEMS include the need for extensive electrode preparation, 21 the influence of mass transport at the working electrode surface, and a complicated cell configuration.…”

Electrochemical Real‐Time Mass Spectrometry (EC‐RTMS): Monitoring Electrochemical Reaction Products in Real Time

“…[14] At ca. Thes ignal for methanol was changing only slightly during pulsing (Supporting Information, enlarged view at Figure S1 a), so its formation was marginally above the detection limit.…”