Suppression of Hydrogen Evolution in Acidic Electrolytes by Electrochemical CO₂ Reduction

Abstract

In this article we investigate the electrochemical reduction of CO2 at gold electrodes under mildly acidic conditions. Differential electrochemical mass spectroscopy (DEMS) is used to quantify the amounts of formed hydrogen and carbon monoxide as well as the consumed amount of CO2. We investigate how the Faradaic efficiency of CO formation is affected by the CO2 partial pressure (0.1–0.5 bar) and the proton concentration (1–0.25 mM). Increasing the former enhances the rate of CO2 reduction and suppresses hydrogen evolution from proton reduction, leading to Faradaic efficiencies close to 100%. Hydrogen evolution is suppressed by CO2 reduction as all protons at the electrode surfaces are used to support the formation of water (CO2 + 2H+ + 2e– → CO + H2O). Under conditions of slow mass transport, this leaves no protons to support hydrogen evolution. On the basis of our results, we derive a general design principle for acid CO2 electrolyzers to suppress hydrogen evolution from proton reduction: the rate of CO/OH– formation must be high enough to match/compensate the mass transfer of protons to the electrode surface.

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

“…A FE of 90% is achieved at pH 4, at 100 mA cm −2 . In terms of selectivity, these results not only show that large scale CO 2 R to CO can be accomplished in acidic media, but are also in agreement with the model recently proposed by Bondue et al based on small-scale DEMS measurements 10 . 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.…”

“…The development of CO 2 to CO electrolyzers at relatively large scale (GDE area > 5 cm 2 ) is recent, and in most work reported so far the reaction is performed in neutral to alkaline media. In general, the local concentration of various species (such as CO 2 , HCO 3 − , CO 3 2− OH − , and H + ) has shown to play a crucial role on the competition between CO 2 reduction and hydrogen evolution, through proton 10,11 , bicarbonate 12 , or water 13 reduction. This is relevant as, for instance, the activity of the water reduction reaction at high overpotentials may limit the window in which high current densities can be achieved while sustaining high CO current efficiencies.…”

“…Because of the high activity of proton reduction at low overpotentials, the current efficiencies found for CO in acidic media are usually low, especially on electrodes with low roughness. In fact, only in a few works in the literature, CO 2 R has been investigated (at small scale) in acidic media 10,11,20,21 . Recent work from Bondue et al 10 has indicated that proton reduction can be fully suppressed in acidic media, as long as the rate of CO/OH − formation from CO 2 R is high enough to compensate the mass transfer of protons to the electrode surface.…”

“…In fact, only in a few works in the literature, CO 2 R has been investigated (at small scale) in acidic media 10,11,20,21 . Recent work from Bondue et al 10 has indicated that proton reduction can be fully suppressed in acidic media, as long as the rate of CO/OH − formation from CO 2 R is high enough to compensate the mass transfer of protons to the electrode surface. Differential electrochemical mass spectrometry (DEMS) was used to quantify the formation of CO/H 2 and consumption of CO 2 .…”

Spatial reactant distribution in CO2 electrolysis: Balancing CO2 utilization and Faradaic Efficiency

“…In cases where CO2 utilizations are increased, spatial variations in performance and selectivity will occur when areas of the catalyst no longer have access to sufficient CO2, and produce unwanted H2 instead (see Fig. 1b for representation) [26][27][28] . To begin assessing this trade-off we first collected a data set under varying flow rate conditions for CO2 conversion to carbon-monoxide (CO) on a silver (Ag) catalyst in a membrane-electrode assembly with a serpentine flow field of 5 cm 2 geometric area (Fig.…”