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| Fig. 1: CO2 being reduced to formate. (Source: J. Li, after Zeng et al. [2]) |
Due to limited development in renewable energy such as solar and wind, and continuous worries of safety issues in nuclear energy, fossil fuels still play the most important role in energy supplying for industry and daily life. However, the excessive utilization of traditional fossil fuels has broken the balance of carbon cycling in nature, and lead to an elevating CO2 level in the atmosphere. CO2 is a green house gas and it is linked to a series of environmental threats including rising sea level, global warming, and extreme climate events. Thus, a lot of efforts have been put into developing practical methods for valorization of CO2 in air. [1]
One consideration of solving the problem caused by too much CO2 emission is to convert CO2 into useful fuels, which is a reverse process of fuel burning with the assistance of proper energy input including heat, electricity or light, as well as corresponding catalysts for each condition. Electricity assisted CO2 conversion, which is usually named as Electro-catalytic CO2 Reduction Reaction (CO2 RR), is a promising method because of its high efficiency and mild reaction condition, making it possible for this technique to have practical application in large scale in the future. In addition, when combined with renewable energies such as solar and wind, electricity needed for CO2 RR could be satisfied by a purely sustainable energy source. This way, the issue of climate change and energy shortage could be mitigated at the same time.
Formic acid is a common and useful product via CO2 RR. Compared with common gas products such as carbon monoxide which is highly toxic, and methane which is another green house gas, formic acid is an easily transportable and storable compound. There are fuel cells designed specially for directly burning of formic acid to generate electricity. Formic acid is also an excellent hydrogen storage medium, as well as important feedstock for chemical synthesis. Thus, producing formic acid in large scale and high efficiency is an attracting study. Usually formic acid could be produced via CO2 reduction with high efficiency on the surface of Tin, Indium and Bismuth. For example, a recent study has reported a modulated tin catalyst (Fig. 1) achieving 93% Faradaic efficiency at 55 mA/cm2, with a duration as long as 40 hours. [2] Further study will continue focusing on the improvement of current density and the elongation of catalyst life time.
Ethanol is a more popular fuel that could be produced via CO2 RR. But producing ethanol could be much difficult because it contains 2 carbon atoms in the molecule and a C-C coupling process have to be involved. Best ethanol generating catalyst for mow are copper and its complexes, due to a medium binding energy between Cu and C according to DFT calculation. Maximum efficiency of producing ethanol is about 30% according to present reports, with current density usually lower than 10 mA/cm2. [3] Selectivity and stability of producing ethanol from CO2 still need to be optimized in the future.
© Jiachen Li. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. A. Olajire, "Valorization of Greenhouse Carbon Dioxide Emissions into Value-Added Products by Catalytic Processes," J. CO2 Util. 3-4, 74 (2013).
[2] X. Zheng et al., "Sulfur-Modulated Tin Sites Enable Highly Selective Electrochemical Reduction of CO2 to Formate," Joule 1, 794 (2017).
[3] D. Ren, B. S.-H. Ang, and B. S. Yeo, "Tuning the Selectivity of Carbon Dioxide Electroreduction toward Ethanol on Oxide-Derived CuxZn Catalysts," ACS Catal. 6, 8239 (2016).