Abstract
Electrochemical CO(2) separations, which use electricity rather than thermal energy to reverse sorption of CO(2) from concentrated point sources or air, are emerging as compelling alternatives to conventional approaches given their isothermal, ambient operating conditions, and ability to integrate with renewable energy inputs. Despite several electrochemical approaches proposed in previous studies, further explorations of new electrochemical CO(2) separation methods are crucial to widen choices for different emissions sources. Herein, we report an electrochemical cation-swing process that is able to reversibly modulate the CO(2) loading on liquid amine sorbents in dimethyl sulfoxide (DMSO) solvent. The process exploits a reversible carbamic acid-to-carbamate conversion reaction that is induced by changing the identity of Lewis acid cations (e.g. K(+), Li(+), Ca(2+), Mg(2+), and Zn(2+)) coordinated to the amine-CO(2) adduct in the electrolyte. Using ethoxyethylamine (EEA) as a model amine, we present NMR-based speciation studies of carbamic acid-to-carbamate conversion as a function of amine/salt concentrations and cation identity. The reaction is further probed using gas-flow reaction microcalorimetry, revealing the energetic driving forces between cations and the amine-CO(2) adduct that play a key role in the described re-speciation. A prototype electrochemical cell was further constructed comprising a Prussian white (PW) potassium (K(+)) intercalation cathode, zinc (Zn) foil anode, and EEA/DMSO electrolyte containing a dual KTFSI/Zn(TFSI)(2) salt. A low CO(2) separation energy of ∼22-39 kJ/mol CO(2) (0.1-0.5 mA cm(-2)) was achieved with a practical CO(2) loading delta of ∼0.15 mol CO(2)/mol amine. Further optimizations in electrolyte design and cell architectures toward continuous CO(2) capture-release are expected to enhance rate performance while retaining favorable separation energies.