Combined CO₂ capture and electrochemical conversion in non-aqueous environments

Abstract

Carbon capture, utilization, and storage (CCUS) technologies have a central role to play in mitigating rising CO₂ emissions and enabling sustainable power generation. Most industrially mature CCS technologies based on amine chemisorption are highly energy-intensive, consuming up to 30% of the power generating capacity of the plant in order to thermally regenerate the sorbents for continued capture. Moreover, the released CO₂ must additionally be compressed and stored permanently, which adds additional energy penalties and potential risks of release. To address these challenges, this thesis develops a new strategy for integrating CO₂ capture and conversion into a single process stream.

Such an approach, which employs CO₂ in the captured state as the reactant for subsequent electrochemical reactions, eliminates the need for energetically-intensive sorbent regeneration and CO₂ release between capture and utilization steps while potentially providing new solutions for the storage challenge. In the first part of this thesis, a proof-of-concept demonstration of combined CO₂ capture and conversion within a Li-based electrochemical cell is presented. To develop this system, new electrolyte systems were first designed to integrate amines (used in industrial CO₂ capture) into nonaqueous electrolytes. The resulting systems were found to be highly effective in both capturing and activating CO₂ for subsequent electrochemical transformations upon discharge of the cell.

This activity was particularly well-demonstrated in solvents such as DMSO where CO₂ normally is completely inactive, in which the amine-modified electrolytes containing chemisorbed CO₂ were found to enable discharge at high cell voltages (~2.9 V vs. Li/Li⁺) and to high capacities (> 1000 mAh/gc), converting CO₂ to solid lithium carbonate. Formation of a densely-packed, solid phase product from CO₂ is not only logistically attractive because it requires less storage space, but also eliminates the costs and safety risks associated with long-term geological storage of compressed CO₂. In addition, the conversion process generates electricity at point-of-capture, which may help to incentivize integration of the technology with existing point-source emitters. While promising, this initial system exhibited several challenges including slow formation of the active species in solution.

To address this, a suite of experimental and computational methods were employed to elucidate the influence of the electrolyte on electrochemical reaction rates. Reduction kinetics were found to be influenced by alkali cation desolvation energetics, which favors larger alkali cations such as potassium. Through further development, amine-facilitated CO₂ conversion was also demonstrated to be transferrable to other amine- and solvent- systems, opening a potentially large design space for developing improved electrolytes. Furthermore, the effect of operating temperature was investigated to evaluate the potential of this technology to integrate with practical CO₂ capture needs. While higher temperatures (40°C<T<70°C) improve the conversion kinetics of CO₂-loaded amines, device-level performance - especially in the low-rate regime - remains largely governed by the Li-electrolyte stability at elevated temperatures, which needs to be addressed in future work.

Lastly, CO₂ discharge activity as a function of electrolyte composition was also investigated in non-amine electrolytes for rechargeable Li-CO₂ batteries. In these systems, increased availability of the Li⁺ cation was found to be critical for supporting CO₂ activation and sustaining discharge to high capacities.Overall, the central advance of this thesis is the successful demonstration of using amine sorbents in an electrochemical context to activate new modes of CO₂ reactivity, establishing the feasibility of integrated CO₂ capture-conversion. This work not only provides a new reaction platform, but also proposes post-combustion storage concepts of CO₂ in solid phases that simultaneously achieve permanent CO₂ fixation and power delivery.