(502a) Developing Models to Understand Performance Trends of Electrochemically-Mediated Carbon Capture Systems

Society-wide decarbonization is a grand challenge of the 21^st century which will require wide-scale deployment of various carbon-neutral and carbon-negative technologies. Carbon dioxide (CO₂) capture is one promising method as it provides a means to offset emissions from existing fossil-fuel-driven processes and hard-to-decarbonize sectors.^1,2 State-of-the-art approaches for carbon capture typically utilize temperature-driven processes, where CO₂ is captured and released at lower and higher temperatures, respectively.^3,4 While such methods have been demonstrated, even at commercial scales, broad adoption is hampered by drawbacks such as large energetic penalties, high capital costs, complex process designs, and limited scalability. Electrochemically-mediated carbon capture has recently gained interest as this approach can enable higher energetic efficiencies, modular deployment, and operation at ambient conditions. These technologies can also be readily coupled with renewable energy sources, avoiding emissions associated with electricity generation from fossil fuels. However, this is a relatively nascent field of research, and system performance and design principles necessary for feasible operation are not well understood.

At this early stage of development, modeling frameworks hold value in defining achievable performance bounds and describing important relationships between molecular property sets, system design, and operating conditions. Previous analyses have derived thermodynamic relations to determine theoretical system efficiencies, identifying effective material property values which balance tradeoffs and optimize efficiencies.^5–7 These studies have also highlighted how system configuration, a process design characteristic, can significantly increase energy requirements. While this work has led to an improved understanding of performance trends and design criteria, these modeling frameworks do not account for the large energetic penalties which can arise from mass transport, kinetics, ohmics, and other factors. To this end, low-dimensional models can be used as a concise tool to probe the impact of such phenomena, and therefore better characterize electrochemically-mediated carbon capture technologies

In this talk, we describe idealized reactor models which are used to determine cell overpotentials under constant current operation. More specifically, we represent electrodes with continuously stirred-tank reactor and plug-flow reactor approximations, which may serve as lower and upper performance bounds, respectively. We use this modeling framework to explore the impact of molecular/electrolyte properties (e.g., CO₂ solubility), cell design factors (e.g., electrodes, membranes, flowfields), and operating conditions (e.g., current density). One key finding of this work is that overpotentials due to mass transport and ohmics can significantly exceed the minimum energy requirements predicted by thermodynamics. In these scenarios, design criterion derived from thermodynamics need to be expanded to capture operationally relevant properties. We also find that electrode potentials may be asymmetric due to boundary layer phenomena. To further probe this, we apply one-dimensional models to understand the interplay between species transport, heterogeneous and homogeneous reaction rates, and applied current densities within this region. Overall, this work aims to establish connections between material properties, operating conditions, system design factors, and observed performance. Both within the project and more broadly, this modeling approach provides a framework to establish design guidelines for electrochemically-mediated CO₂ separation systems, which can aid in the development and deployment of these technologies.

Acknowledgements

This research was supported by the Alfred P. Sloan Foundation.

References

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2. Allen, M. et al. IPCC, 2018: Summary for Policymakers. (2018).

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5. DuBois, D. L., Miedaner, A., Bell, W. & Smart, J. C. Electrochemical Concentration of Carbon Dioxide. in Electrochemical and electrocatalytic reactions of carbon dioxide 94–117 (Elsevier Science Publishers B.V., 1993).

6. Shaw, R. A. & Hatton, T. A. Electrochemical CO₂ capture thermodynamics. Int. J. Greenh. Gas Control 95, (2020).

7. Clarke, L. E., Leonard, M. E., Hatton, T. A. & Brushett, F. R. Thermodynamic Modeling of CO2 Separation Systems with Soluble, Redox-Active Capture Species. Ind. Eng. Chem. Res. (2022) doi:https://doi.org/10.1021/acs.iecr.1c04185.