The carbon dioxide electroreduction reaction (CO₂RR) powered by renewable electricity offers a promising route to generate valuable chemicals and fuels.¹ However, current CO₂RR systems – typically relying on cathodic CO₂RR coupled with the anodic oxygen evolution reaction (OER) – suffer from high full-cell voltages, limited CO₂ utilization, and, in cases, carbonate formation due to the use of alkaline/neutral media.^2-8 Each of these leads to energy penalties when considering the overall system.

 CO₂RR in acidic media addresses the CO₂ loss issue resulting from carbonate formation and crossover in alkaline/neutral media during reaction.^9-12 However, hydrogen evolution reaction (HER) under acidic media is kinetically favored relative to CO₂RR, leading to low selectivity towards carbon-based products in CO₂RR.^6,9,10 Thus, to achieve high CO₂ utilization efficiency practically, one must suppress HER and promote Faradaic efficiency (FE) to desired products in acidic CO₂RR.

 As noted above, CO₂RR systems today typically rely on OER on the anode, which has a standard potential of 1.23 V vs. the reversible hydrogen electrode.¹⁴ Replacing OER with another anodic reaction that requires a lower thermodynamic potential offers to decrease the full-cell voltage and thus the electricity consumption of CO₂RR system. It also provides a means to electroproduce economically valuable products at the anode.

 Here we report an efficient electrocatalysis system with low energy consumption: cathodic CO₂-to-CO in acidic media coupled to anodic allyl alcohol oxidation reaction (AOR) to acrolein (a precursor in the manufacture of acrylic acid), where the cathode chamber and anode chamber are separated by the cation exchange membrane (CEM). In this design, using acidic catholyte and CEM circumvents the CO₂ loss arising from the carbonate formation and crossover of carbonate ions from the cathode to anode (Figure 1a below).

 In cathodic CO₂RR, we lever the K⁺ strategy reported previously,¹¹ and achieve a CO FE of (96 ± 1)% on an Ag cathode prepared by sputtering a layer of Ag on a carbon-based gas diffusion layer. In anodic AOR, through anode catalyst screening and reaction condition optimization, we achieve an acrolein FE of (85 ± 1)% on a Pt anode at a current density of 100 mA cm⁻². We reach a CO₂ single pass utilization (SPU) of 84% in this CO₂RR-AOR system, fully a 6× improvement compared to the best values reported in prior acidic CO₂RR-to-CO studies above 100 mA cm⁻².^7,8,11,15,16 At 100 mA cm⁻², the average full-cell voltage of this paired electrolysis system is 3.2 V during 10 h operation, and the energy consumption for producing 1 kg of CO is 44 MJ; these represent a decrease in voltage of 0.7 V and a reduction of 1.6× in energy consumption relative to the most energy-efficient prior reported acidic ambient-temperature CO₂-to-CO studies (Figure 1b).

 This thus points to strategies for CO₂RR systems with lowered energy consumption.

 If you are interested in our work, you may find the full paper here: https://www.nature.com/articles/s41893-024-01363-1

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