Sustainable and Tunable Electrochemical CO2 Conversion to Chemicals and Fuels at Industrial Scale Using Metal-Oxide-Based Materials

Abstract

The most challenging deal we face these years is the need to lower greenhouse gas (GHG) emissions and tackle climate change; though calls to reduce it are growing louder yearly, emissions remain high. CO2 is the key contributor. For this reason, the synthesis of high-added-value products from CO2 conversion is a promising approach to mitigate the problem.1 Among the different alternatives, exploiting CO2 via electrochemical reduction under mild conditions (ambient pressure and temperature) represents an opportunity to support a low-carbon economy.2 The electrocatalytic (EC) CO2 reduction (CO2R) driven by renewable energy can be exploited for the future energy transition, for the carbon storage into valuable products like syngas (H2/CO mixtures), organic acids (formic acid) and chemicals/fuels (C1+ alcohols).3 A big challenge for the industrialisation of this technology is to find low-cost electrocatalysts, efficient reactors and process conditions. In the efforts to develop efficient, selective and stable materials, we have exploited the current knowledge of thermocatalytic CO2 hydrogenation to develop noble-metal-free CO2R electrocatalysts.4 For instance, Cu/Zn/Al synthesised catalyst producing methanol and CO from the CO2 thermocatalytic (TC) hydrogenation (at H2 pressure (P) of 30 bar and temperature (T) > 200 oC) promotes the formation of methanol (⁓32% of FE) during the EC CO2R in a gas-diffusion-electrode system; while operating in the liquid phase, the same catalyst produces syngas with a tunable composition (95% of FE at the most positive applied potential) and other liquid C2+ products (in both cases at ambient T, P).4 On the other hand, Cu/ZnO electrocatalyst has also been tested at industrially relevant current densities in liquid phase configuration.5 We demonstrated through ex-situ characterisations that the presence of ZnO nanoparticles in the mixed Cu/ZnO catalyst plays an important role in forming and stabilising mixed oxidation states of copper and Cu1+/Cu0 interfaces in the electrocatalyst (in bulk and surface). These interfaces seem to promote CO dimerisation to ethanol. Indeed, ethanol was produced with the Cu/ZnO catalyst, reaching ethanol productivity of about 5.3 mmol∙gcat -1∙h-1 in a liquid-phase configuration at ambient conditions. The Cu/Zn/Al and Cu/ZnO electrocatalysts also have been tested in a catholyte-free configuration with an increased selectivity to ethylene, reaching approx. 60% and 70% of FE, respectively. Here, Cu catalyst structure transformed, on average, completely to metallic with a very thin layer of Cu1+ during testing, which seems to promote the selectivity towards C2H4, demonstrating that the reaction pathways for EC CO2R are largely determined by transport limitations rather than only by the intrinsic properties of the electrocatalysts. Our results open a promising path for the prospective implementation of metal-oxide nanostructures for CO2 conversion to the chemicals and fuels of the future. Acknowledgements This work has received financial support by the EU H2020 SunCOChem Project, grant agreement: 862192. References Guzmán, H., Russo, N. & Hernández, S. CO2 valorisation towards alcohols by Cu-based electrocatalysts: challenges and perspectives. Green Chem. 23, 1896–1920 (2021).Romero Cuellar, N. S., Wiesner-Fleischer, K., Fleischer, M., Rucki, A. & Hinrichsen, O. Advantages of CO over CO2 as reactant for electrochemical reduction to ethylene, ethanol and n-propanol on gas diffusion electrodes at high current densities. Electrochim. Acta 307, 164–175 (2019).Guzmán, H., Farkhondehfal, M. A., Rodulfo Tolod, K., Russo, N. & Hernández, S. Photo/electrocatalytic hydrogen exploitation for CO2 reduction toward solar fuels production. in Solar Hydrogen Production Processes, Systems and Technologies 560 (Elsevier Inc., 2019). doi:10.1016/C2017-0-02289-9.Guzmán, H. et al. How to make sustainable CO2 conversion to Methanol: Thermocatalytic versus electrocatalytic technology. Chem. Eng. J. 417, 127973 (2021).Guzmán, H. et al. CO2 Conversion to Alcohols over Cu/ZnO Catalysts: Prospective Synergies between Electrocatalytic and Thermocatalytic Routes. ACS Appl. Mater. Interfaces 14, 517−530 (2022).