Thermodynamic Modeling of CO2 Separation Systems with Soluble, Redox-Active Capture Species
Authors: Clarke Lauren E. , Leonard McLain E. , Brushett Fikile R. 
Published: July 2021

Deep society-wide decarbonization is a grand challenge of the 21st century requiring the development, manufacture, and deployment of transformative carbon-neutral and carbon-negative technologies on a global scale. Carbon dioxide (CO2) capture coupled with storage or conversion is projected to play a key role in mitigating and even reversing carbon emissions [1]. Present-day carbon capture processes rely on thermochemical cycles where solvents/sorbents absorb and release CO2 at lower and higher temperatures respectively. While functional, these embodiments are energetically-intensive and typically rely on fossil fuel derived heat for CO2 desorption, ultimately limiting effectiveness [2]. Electrochemical approaches enable lower energy CO2 separations, as electrode potential can be modulated to selectively activate sorbents rather than temperature and pressure swings which impact the entire capture media. Moreover, electrochemical systems enable direct integration of renewables, modular deployment, and safe operation at ambient conditions [3]. Current research efforts for electrochemical CO2 separation are largely exploratory, focusing on molecular discovery and proof-of-concept demonstrations [4]. As such, modeling work to identify favorable combinations of molecular properties and device characteristics could benefit the field by providing design criteria.

Here, expanding upon a prior thermodynamic modeling framework, we assess the energetics of CO2 separation processes that use soluble, redox-active capture species, and how this is impacted by certain tunable molecular properties [5]. To guide the analysis, we contemplate four distinct system configurations representative of different practical embodiments of the envisioned technology. One example is a 4-stage configuration, where the capture species is activated and deactivated in the electrochemical cell, and absorption and desorption steps are carried out in separate process units, as demonstrated in Figure 1(a). We evaluate thermodynamic and electron efficiencies, as measures of energetic and materials utilization efficiencies, respectively, to investigate how molecular properties and system configuration affect performance. This modeling work reveals a tradeoff between these two efficiencies, as shown in Figure 1(b) for a 4-stage configuration. To further explore this tradeoff, we introduce the combined efficiency, defined as the product of thermodynamic and electron efficiencies. More specifically, an electrochemical CO2 separation system that maximizes combined efficiency is anticipated to be successful due to a suitable balance of energy and materials costs. We find the combined efficiency metric is maximized when certain thermodynamic properties are optimized, such as CO2 binding affinity and CO2 solubility. Accordingly, optimal properties that lead to the maximization of the combined efficiency, will also be dependent upon system configuration and intended application (e.g., post-combustion or direct air capture). Overall, the introduction of combined efficiency as a thermodynamic metric enables assessment of candidate capture molecule and electrolyte pairs as well as the identification of the application and process configuration most suitable for the pairing. These results offer a framework of thinking and design guidelines for capture molecule synthesis as well as the selection of supporting salt and solvent.

Acknowledgement

We gratefully acknowledge funding support from the Alfred P. Sloan Foundation.

References

[1] M. Allen et al., IPCC, 2018: Summary for Policymakers, p. 1–24, (2018).

[2] M. Fasihi, O. Efimova, and C. Breyer, J. Clean. Prod., 224, 957–980 (2019).

[3] Y. Yuan and L. Aiwen, Nat. Commun., 11 (2020).

[4] J. H. Rheinhardt, P. Singh, P. Tarakeshwar, and D. A. Buttry, ACS Energy Lett., 2, 454–461 (2017).

[5] R. A. Shaw and T. A. Hatton, Int. J. Greenh. Gas Control, 95 (2020).

Figure 1