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

Anthropogenic activities have impacted the planet’s carbon cycle through the emissions of large amounts of greenhouse gases (GHGs), shifting the equilibrium of human history since the industrial revolution. CO 2 is the key contributor to global climate change in the atmosphere. For this reason, the synthesis of high-added-value products CO 2 conversion is a promising approach to mitigate climate change. 1 Among the different alternatives, exploiting CO 2 via electrochemical reduction under mild conditions (ambient pressure and temperature) represents an opportunity to support a low-carbon economy. 2 The electrocatalytic (EC) CO 2 reduction (CO 2 R) driven by renewable energy can be exploited for the future energy transition, for the carbon storage into valuable products like syngas (H 2 /CO mixtures), organic acids (formic acid) and chemicals/fuels (C 1+ 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 CO 2 hydrogenation to develop noble-metal-free CO 2 R electrocatalysts. 4 For instance, Cu/Zn/Al synthesised catalyst producing methanol and CO from the CO 2 thermocatalytic (TC) hydrogenation (at H 2 pressure (P) of 30 bar and temperature (T) > 200 o C) promotes the formation of methanol (⁓32% of FE) during the EC CO 2 R 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 C 2+ products (in both cases at ambient T, P). 4 On the other hand, Cu/ZnO electrocatalyst also has 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 Cu 1+ /Cu 0 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∙g cat -1 ∙h -1 in a liquid-phase configuration at ambient conditions. A Cu/ZnO electrocatalyst also has been tested in a catholyte-free configuration with an increased selectivity to ethylene, reaching approximately 70% of FE, demonstrating that the reaction pathways for EC CO 2 R 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 CO 2 conversion to the chemicals and fuels of the future. Acknowledgements This work has received financial support by the EU H2020 Project SunCOChem (grant agreement: 862192). References Guzmán, H., Russo, N. & Hernández, S. CO 2 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 CO 2 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 CO 2 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 CO 2 conversion to Methanol: Thermocatalytic versus electrocatalytic technology. Chem. Eng. J. 417 , 127973 (2021). Guzmán, H. et al. CO 2 Conversion to Alcohols over Cu/ZnO Catalysts: Prospective Synergies between Electrocatalytic and Thermocatalytic Routes. ACS Appl. Mater. Interfaces 14 , 517−530 (2022).