Design and Synthesis of Novel Solid Sorbents for Electrochemically Mediated Carbon Capture

Abstract

Carbon capture is a critical tool across industrial, energy, and environmental sectors. Electrochemically mediated carbon capture (EMCC) stands out as an energy-efficient approach. It uses redox-active materials to bind and release CO2 through electrochemical switching, avoiding the thermal energy costs of conventional amine scrubbing. Solid porous adsorbents are especially attractive for EMCC in fixed-bed configurations. Their high surface area concentrates active sites, their rigid porous structure enables gas flow, and their tunability allows precise chemical modification. However, translating these materials from lab-scale demonstrations to realistic deployment remains difficult. Two fundamental challenges limit practical deployment. First, buried active sites within bulk material particles are poorly accessible to electrochemical processes. Second, O2 present in real industrial flue gas competes directly with CO2 capture by consuming electrons to form superoxide (O2·–) at a more positive potential than most carriers, lowering capture efficiency. This superoxide further attacks the reduced carrier, electrolyte, and solvent, causing capacity fade and degrading active sites over time. This thesis addresses these challenges through two independent molecular design strategies, advancing the practical deployment of solid-phase EMCC systems. Chapter 1 provides an overview of electrochemically mediated carbon capture using redox-active carriers, with particular emphasis on solid adsorbents for fixed-bed systems. The thermodynamic criteria governing carrier selection are outlined, including CO2 binding affinity, energy consumption, O2 stability, and reaction kinetics. Solution-phase and solid sorbent platforms are surveyed, and key performance gaps are identified across material classes, including O2 sensitivity, limited cycling stability, and the disconnect between intrinsic material capacity and device-level utilization. The chapter closes with an outlook on structural design strategies for nanoporous frameworks as a path toward resolving these challenges. Chapter 2 addresses the active site accessibility challenge through a bottom-up synthetic approach. Bulk COF powders suffer from poor electrical contact and buried active sites that resist electrochemical access. To overcome this, the chapter introduces colloidal conjugated macrocycles as a new material class. These nitrogen-rich macrocycles adopt two types of linkages: square-planar Ni coordinated by four nitrogen atoms, or phenazine-based connections. Both preserve the ordered porosity and strong interlayer stacking of crystalline frameworks, while offering the processability of nanomaterials. Among the two macrocycle variants, the phenazine-based macrocycles serve as the platform for EMCC studies. Their colloidal dispersion yields smaller particle sizes that improve electrical contact with the electrode and increase electrochemical accessibility of active sites, resulting in better capture performance relative to bulk COF powders. Chapter 3 addresses the O2 stability challenge by translating molecular design principles into a solid framework. Isoindigo was first studied as a small molecule, where its lactam N-H sites demonstrated that hydrogen bonding improves EMCC performance. Building on this insight, the chapter reports the incorporation of isoindigo into the first COF with accessible free lactam N-H sites. A Boc-protection strategy prevents premature aggregation during synthesis. In situ deprotection yields the target COF with open N-H binding sites. The material achieves competitive CO2 capture efficiency and retains performance under O2 conditions simulating real flue gas, with minimal change across multiple cycles. The energy cost of capture falls below the current O2-tolerant COF benchmark. These results establish isoindigo-based COFs as a viable platform for O2-stable, energy-efficient electrochemical carbon capture. Appendix A reports the synthesis and preliminary characterization of glycol-functionalized Ni macrocycles, laying the groundwork for evaluating mixed ionic-electronic conduction in the conjugated macrocycle platform introduced in Chapter 2. Glycol substitution is confirmed to be structurally compatible with the macrocycle architecture, with full functional characterization planned as future work. Appendix B extends the macrocycle design strategy introduced in Chapter 2 to the ketoenamine linkage system. β-Ketoenamine COFs have been demonstrated in electrochemically mediated CO2 capture, making them a natural reference point for evaluating whether the macrocycle form factor offers improved active site accessibility and electrochemical performance. This appendix reports the synthesis and preliminary structural characterization of a family of β-ketoenamine-based macrocycles, laying the groundwork for a direct performance comparison with the bulk COF counterpart.