Electrochemical CO₂ Conversion

Fig. 1: Representation of a electrolysis cell used to convert CO2 into fuel in a laboratory setting. The battery provides the electricity to convert the CO2 dissolved in the solvent into a fuel at the electrode.

The electrochemical conversion of CO₂ into fuel (i.e., methane, ethylene, ethanol, gasoline) could be used to decrease the concentration of CO₂ in our atmosphere. The mechanism for electrochemical reduction of CO₂ would essentially entail reversing a fuel cell. Instead of using fuel to make electricity, one would use electricity to make fuel. This paper aims to give a broad overview of the CO₂ reduction process and give rough estimates of the energy required for the conversion.

Electrochemical Conversion vs. Gas Phase Conversion

An ideal industrial electrochemical system would have few processes; it would be scalable and would operate at ambient temperature in order to easily adjust to real-time production demands. [1] An electrochemical CO₂ reduction system has an advantage over its gas phase counterpart in that it can operate at standard temperature and pressure. Thus it is flexible and easier to scale.

The Main Parts of Electrochemical Conversion

There are two main parts of electrochemical conversion: absorption from the atmosphere and electrolysis or conversion into fuel.

Absorption

One of the many techniques for CO₂ absorption uses amine compounds such as monoethanolamine (MEA). In this technique, a CO₂-rich gas stream would be circulated in a bath of MEA. [2] The MEA would bind to the CO₂ and form an intermediate compound, while the remaining gas, most likely N₂, would flow through unabsorbed. The amines would have to be regenerated by heating the solution, in which case the CO₂ would be liberated. Another temperature-based absorption method is to use limestone and solar heating. This method requires 320KJ/mol of CO₂. [1] Yet another method of absorption is a low-temperature method which requires a bit more energy, about 430KJ/mol of CO₂. [1] This electrochemical method uses KOH, which is turned into K₂CO₃, upon absorption of CO₂. Electrodialysis is then used to remove the CO₂ from the K₂CO₃ compound and regenerate the KOH. Thus, for estimation purposes, the 430KJ/mol value will be used as the energy it takes to absorb CO₂ from the atmosphere.

CO₂ to Fuel

The electrochemical conversion of CO₂ to fuel has been studied in a lab setting. The most promising and reliable data has come from the lab of Hori et al. [3] Using a copper catalyst, he was able to generate 5mA/cm² of product. The product distribution was a mix of methane, ethylene, carbon monoxide and formate, all of which are fuels. Formate can be used directly in a fuel cell while, methane, ethylene and carbon monoxide can be used in a combustion engine. [4] The equation to calculate the input energy of the cell is:

Where n is the number mols of electrons, F is Faraday's constant, 96,485 (C/mol) and E is the potential of the cell. The input potential of a full cell is estimated to be at 3V, which includes a 1V overpotential or energetic loss for the CO₂ reduction as well as energetic losses due to other factors in the cell. Calculating the number of mols of electrons has to be done individually and is shown below.

Eight moles of electrons are needed per mol of CO₂, and with the 3V potential of the cell as well as with Faraday's constant, the total input to the cell is 2316 KJ/mol.

The energetic output of the products that are combustible (CO, COOH, C₂H₄ and CH₄) can be estimated calculating the LHV, which is the industry standard calculation for energy output, even though it does not account for entropy. Fifty percent of the LHV was used as a generous estimate of efficiency for combustion devices. Formate can be used in a fuel cell; in such case, the output was calculated using the Gibbs free energy as in Equation1. Given those calculations, the energetic output is estimated to be 434 KJ/mol of CO₂.

Efficiencies

The energy efficiency of the process, given the rough calculations, is estimated to be 18.7%. This efficiency would be terrible if the electrons or electricity were taken from coal. However, if the electrons were used from wind or solar, the 18.7% could be tolerable. In either case, an economic incentive would almost certainly have to be put in place to offset the lack of energetic efficiency of the process.

Conclusion

The dream of electrochemical conversion to fuel is to convert CO₂ into a liquid fuel such as ethanol or diesel fuel, since those liquid fuels do not require compression, which is an energetically substantial drain for gas based fuel. Also, liquid fuels can be easily transported. As the analysis above shows, there is lots of room for improvements. There is currently little research in this area, but perhaps with more focus on the problem, advancements could be made to make it a useful means to convert CO₂ into fuel.

© Etosha Cave. 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.

References

[1] M. Gattrell, N. Gupta, and A. C. Co, "Electrochemical Reduction of CO₂ to Hydrocarbons as a Method to Store Renewable Electrical energy and Upgrade Biogas," Energy Conversion and Management 48, 1255 (2007).

[2] D. Mignard et al., "Methanol Synthesis from Flue-Gas CO₂ and Renewable Electricity: a Feasibility Study," International Journal of Hydrogen Energy 28, 455 (2003).

[3] Y. Hori, A. Murata, and R. Takahashi, "Formation of Hydrocarbons in the electrochemical Reduction of Carbon Dioxide at a Copper Electrode in Aqueous Solution," J. Chem. Soc., Faraday Transactions 1: Physical Chemistry in Condensed Phases 85, 2309 (1989).

[4] C. Rice et al., "Direct Formic Acid Fuel Cells," J. Power Sources 111, 83 (2002).