|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.
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.  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.
There are two main parts of electrochemical conversion: absorption from the atmosphere and electrolysis or conversion into fuel.
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.  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.  Yet another method of absorption is a low-temperature method which requires a bit more energy, about 430KJ/mol of CO2.  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.
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.  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.  The equation to calculate the input energy of the cell is:
|G = nFE||(1)|
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.
|Table 1: Calculations of the number of mols of electrons for an electrochemical cell. The mol fractions for all of the species were estimated from current lab-based data (see ref ). Note that hydrogen, which comes from the splitting of water, is produced in large quantities.|
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.
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.
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