Energy Efficient CO2 Reduction to CO on Gold Gas Diffusion Electrodes in Acidic Media

Abstract

The electrochemical reduction of CO2 to CO, is a promising technology for replacing production processes that The employ fossil fuels. However, low energy efficiencies hinder the sustainable production of this important building block at commercial scale. Although CO2 electrolysis, to date, has only been carried out in neutral or alkaline media, recent work at small scale has pointed out that high selectivity's for CO can also be achieved in acidic media, as long as the rate of CO/OH− formation from CO2 reduction is high enough to compensate the mass transfer of protons to the electrode surface.1 In view of that, we have performed CO2 electrolysis at different acidic pH (2-4) using 10 cm² gold gas diffusion electrodes (GDEs), as shown in Fig. 1a.2 We observe that the high rates of CO2 reduction, and consequent high production of OH− at high current densities, hinder the competing hydrogen evolution reaction by neutralizing the protons before they can reach the catalytic interface. The later allows for faradaic efficiencies between 80-90% for CO to be achieved in acidic media at current densities up to 200 mA cm-2. Additionally, we find that the cation identity is crucial for enabling CO2 electrolysis in acidic media, due to the high CO2 rates accomplished in electrolytes containing weakly hydrated cations as Cs+ and K+.3 A direct comparison of the CO2 electrolysis performance in acidic and neutral media (Fig. 1b-d), shows that running CO2 electrolysis in Cs2SO4 at pH 4 instead of KHCO3 (both 1 M) leads to an energy saving of 24, 25 and 30% at 100, 150, and 200 mA cm-2, respectively. Considering, for example, an electricity price of 0.03 dollars per kWh, running the reaction in Cs2SO4 at pH 4 instead of KHCO3 (at 200 mA cm-2) could lead to saving 1343 dollars per ton of CO produced. As this is the first time CO2 electrolysis is carried out at > 5 cm² scale in acidic media, further research is of course necessary in order to lower energy costs even more, achieve higher current densities and fully optimize the process. Our work, however, lays down a new path towards the development of acidic CO2 electrolysers. (1) Bondue, C. J.; Graf, M.; Goyal, A.; Koper, M. T. M. Suppression of Hydrogen Evolution in Acidic Electrolytes by Electrochemical CO2 Reduction. J. Am. Chem. Soc. 2021, 143 (1), 279–285.(2) Monteiro, M. C. O.; Philips, M. F.; Schouten, K. J. P.; Koper, M. T. M. Efficiency and Selectivity of CO2 Reduction to CO on Gold Gas Diffusion Electrodes in Acidic Media. Nat. Commun. 2021, 12 (1), 4943.(3) Monteiro, M. C. O.; Dattila, F.; Hagedoorn, B.; García-Muelas, R.; López, N.; Koper, M. T. M. Absence of CO2 Electroreduction on Copper, Gold and Silver Electrodes without Metal Cations in Solution. Nat. Catal. 2021. Figure 1