(643a) Multi-Objective Optimization of an Electrochemical CO2 Utilization Process

CO₂ utilization has several advantages including reduction in the level of atmospheric CO₂ through reuse and a cleaner production process for products that are conventionally produced from hydrocarbon-based feedstocks. While many technologies for CO₂ utilization are being investigated, the electrochemical reduction technology has high potential. Due to the direct usage of the electric power in this technology, this process can take advantage of cheap electricity, when available, from renewable energy sources.

For improved commercial feasibility of the electrochemical conversion technology, it is important to increase the energy efficiency, conversion, and selectivity. However there is a strong tradeoff between these objectives. Single pass conversion through an electrochemical cell can be improved if the cell terminal voltage can be increased; however that results in a lower energy efficiency (Feaster et. al). A higher single pass conversion also typically lowers the selectivity. These tradeoffs are difficult to analyze and mitigate in electrochemical cells due to the complex ionic reactions and molecular and ionic transport that take place in the cells. Concentration of the products, anolyte and catholyte chemistry and their concentration, other operating conditions, and the cell design details affect the loss mechanisms in these cells affecting the conversion and yield through complex interaction among the activation overpotential, concentration overpotential, and Ohmic resistance (Delacourt et. al., 2010). While considerable work in this area has been done on the development of the cell material and electrolyte chemistry including several experimental on various types of cell designs, it is difficult to optimize these cells experimentally due to the trade-offs noted above, large number of decision variables, and their complex interactions and lack of measurements that can capture the spatial variation of transport variables. Therefore, a model-based multi-objective optimization study is conducted for improving the economics of the electrochemical CO₂-utilization processes.

The electrochemical CO₂-utilization process that is investigated here produces formate. Studies show that reduction to formate/ formic acid has the highest current efficiencies (around %) (Whipple & Kenis, 2010). This along with the many uses of formate salts (as de-icing agents) and formic acid (as a storage medium for hydrogen gas, replacement for mineral acids in steel picking, and so on) make formate/ formic acid a very good choice as a product. One notable study on an electrochemical cell producing formic acid was conducted by Li and Oloman (Li & Oloman, 2008). They developed a model of trickle-bed reactors with a tin electro-catalyst and a cation membrane separator.

This work extends the model developed by Li and Oloman (Li & Oloman, 2008) by incorporating more detailed and generic chemistry and electrolyte flow models so that the corresponding degrees of freedom can be optimized. The electrolyte chemistry plays a critical role in the performance of these cells. Since CO₂ must be dissolved in the aqueous catholyte for participating in the electrochemical reactions, reaction rates can be enhanced by improved solubility of CO₂ in the catholyte thus reducing the concentration overpotential (Li & Oloman, 2006). However, reduction of water concentration in the catholyte can result in an increase in the Ohmic resistance. The model is developed so that it can also capture the effect of the electrolyte chemistry on the rates of undesired side reactions that produce H₂ and CO. Other than modeling the cathode and anode chambers, ionic transport through the membrane was modeled by considering osmotic drag and migration. The system of partial differential algebraic equations is discretized in space and optimized by a nonlinear programming solver. Both energy efficiency and yield of formate are maximized for desired extent of CO₂ conversion. The multi-objective optimization problem is solved by a lexicographic programming approach (Zykina, 2004). In addition to the cell dimensions, operating conditions such as the catholyte and anolyte flows and their inlet concentrations are optimized for different electrolyte chemistries.

References:

1. Feaster, J.T., Shi, C., Cave, E., Hatsukade, T., Abram, D., Kuhl, K., Hahn, C., Nørskov, J., & Jaramillo, T. (2017). Understanding Selectivity for the Electrochemical Reduction of Carbon Dioxide to Formic Acid and Carbon Monoxide on Metal Electrodes. ACS Catalysis 2017 7 (7), 4822-4827

2. Delacourt, C., Ridgway, P. L., & Newman, J. (2010). Mathematical Modeling of CO₂ Reduction to CO in Aqueous Electrolytes. Journal of The Electrochemical Society, 157(12)

3. Whipple, D. T., & Kenis, P. J. A. (2010). Prospects of CO2 Utilization via Direct Heterogeneous Electrochemical Reduction. The Journal of Physical Chemistry Letters, 1(24), 3451–3458.

4. Oloman, C., & Li, H. (2008). Electrochemical Processing of Carbon Dioxide. ChemSusChem, 1(5), 385–391. doi: 10.1002/cssc.200800015

5. Li, H., Oloman, C. Development of a continuous reactor for the electro-reduction of carbon dioxide to formate – Part 1: Process variables. J Appl Electrochem 36, 1105 (2006).

6. Zykina, A.V. A Lexicographic Optimization Algorithm. Automation and Remote Control 65, 363–368 (2004). https://doi.org/10.1023/B:AURC.0000019366.84601.8e

7. Georgopoulou, C., Jain, S., Agarwal, A., Rode, E., Dimopoulos, G., Sridhar, N., & Kakalis, N. (2016). On the modelling of multidisciplinary electrochemical systems with application on the electrochemical conversion of CO2 to formate/formic acid. Computers & Chemical Engineering, 93, 160–170. doi: 10.1016/j.compchemeng.2016.06.012