Electrolytic Conversion of Bicarbonate Solutions to CO at >500 mA cm-2 and 2.2 V

Abstract

Electrolyzers that reduce carbon dioxide (CO2) into chemicals and fuels often use high-purity gaseous CO2 feedstocks that need to be isolated from upstream carbon capture units. If CO2 were to be captured directly from air, the eluent is likely to be an aqueous solution rich in bicarbonate ions (HCO3-). This scenario provides the impetus to electrolytically reduce these bicarbonate-rich carbon capture solutions into the same products as a CO2 electrolyzer. We report here an electrolyzer configuration that couples the conversion of bicarbonate to CO at the cathode with hydrogen oxidation at an anode. This unique system is capable of reaching a commercially-relevant current density of 500 mA cm-2 at merely 2.2 V, which is >0.5 V more efficient than any other reported electrolyzer that reduces HCO3- or CO2 at these current densities.

Advancing integrated CO2 electrochemical conversion with amine-based CO2 capture: a review

“…8c) is superior to the Ag-based gas-diffusion electrodes in evolving CO from bicarbonate solutions, especially at current densities > 100 mA cm -2 . 63 These findings confirm that a hydrophilic porous electrode is likely applicable to CO 2 conversion in the tertiary amines than electrode structures used in gas-fed electrolysis.…”

“…57 Similar to direct bicarbonate reduction, CO 2 conversion in a bicarbonate-dominated CO 2 -rich capture medium requires a supply of protons to produce CO 2 from bicarbonate ions. The proton flux is usually current-dependent and supplied from either anode reaction via a proton-exchange membrane 63,64 or the bipolar membrane 29,65,66 under a reversed bias. Protons could also cause an acidic local reaction environment close to the catalyst surface and contribute to the unwanted hydrogen evolution reaction.…”

Continuum Modeling of Porous Electrodes for Electrochemical Synthesis

“…For electrochemical synthesis reactions that target reduced products, water oxidation is commonly used as the anodic reaction. The use of green hydrogen (from water electrolysis) or blue hydrogen (from steam methane reforming with carbon capture) to mediate the hydrogen oxidation reaction at the anode has the potential to lower the open circuit potential (defined as the difference between the anodic and cathodic potentials) of the overall cell and reduce the energy costs for operating electrochemical synthesis devices . Glycerol oxidation is another promising cosynthesis reaction because it occurs at a lower potential than water oxidation and can generate valuable hydrocarbon products …”

Materials and system design for direct electrochemical CO2 conversion in capture media

“…15 To reduce device voltage and thereby energy input, hydrogen oxidation reaction was used to replace oxygen evaluation reaction (OER) at the anode. 27 As a result, the electrolysis of bicarbonate to CO reached a commercial-relevant current density of 500 mA cm -2 at a full-cell voltage of 2.2 V. In comparison, the full-cell voltage of the similar cell configuration that operates OER and bicarbonate reduction was 4.4 V to maintain the current density of 100 mA cm -2 .…”