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We perform a thermodynamic analysis of the energetic cost of CO 2 separation from flue gas (0.1 bar CO 2 (g)) and air (400 ppm CO 2 ) using a pH swing created by electrochemical redox reactions involving proton-coupled electron transfer from molecular species in aqueous electrolyte. In this scheme, electrochemical reduction of these molecules results in the formation of alkaline solution, into which CO 2 is absorbed; subsequent electrochemical oxidation of the reduced molecules results in the acidification of the solution, triggering the release of pure CO 2 gas. We examined the effect of buffering from the CO 2 –carbonate system on the solution pH during the cycle, and thereby on the open-circuit potential of an electrochemical cell in an idealized four-process CO 2 capture-release cycle. The minimum work input varies from 16 to 75 kJ mol CO2 −1 as throughput increases, for both flue gas and direct air capture, with the potential to go substantially lower if CO 2 capture or release is performed simultaneously with electrochemical reduction or oxidation. We discuss the properties required of molecules that would be suitable for such a cycle. We also demonstrate multiple experimental cycles of an electrochemical CO 2 capture and release system using 0.078 M sodium 3,3′-(phenazine-2,3-diylbis(oxy))bis(propane-1-sulfonate) as the proton carrier in an aqueous flow cell. CO 2 capture and release are both performed at 0.465 bar at a variety of current densities. When extrapolated to infinitesimal current density we obtain an experimental cycle work of 47.0 kJ mol CO2 −1 . This result suggests that, in the presence of a 0.465 bar/1.0 bar inlet/outlet pressure ratio, a 1.9 kJ mol CO2 −1 thermodynamic penalty should add to the measured value, yielding an energy cost of 48.9 kJ mol CO2 −1 in the low-current-density limit. This result is within a factor of two of the ideal cycle work of 34 kJ mol CO2 −1 for capturing at 0.465 bar and releasing at 1.0 bar. The ideal cycle work and experimental cycle work values are compared with those for other electrochemical and thermal CO 2 separation methods. 

We demonstrate the electrochemical oxidation of an anthracene derivative to a redox-active anthraquinone at room temperature in a flow cell without the use of hazardous oxidants or noble metal catalysts. The anthraquinone, generated in situ , was used as the active species in a flow battery electrolyte without further modification or purification. This potentially scalable, safe, green, and economical electrosynthetic method is also applied to another anthracene-based derivative and may be extended to other redox-active aromatics. 

Abstract A highly stable phosphonate‐functionalized viologen is introduced as the redox‐active material in a negative potential electrolyte for aqueous redox flow batteries (ARFBs) operating at nearly neutral pH. The solubility is 1.23mand the reduction potential is the lowest of any substituted viologen utilized in a flow battery, reaching −0.462 V versus SHE at pH = 9. The negative charges in both the oxidized and the reduced states of 1,1′‐bis(3‐phosphonopropyl)‐[4,4′‐bipyridine]‐1,1′‐diium dibromide (BPP−Vi) effect low permeability in cation exchange membranes and suppress a bimolecular mechanism of viologen decomposition. A flow battery pairing BPP−Vi with a ferrocyanide‐based positive potential electrolyte across an inexpensive, non‐fluorinated cation exchange membrane at pH = 9 exhibits an open‐circuit voltage of 0.9 V and a capacity fade rate of 0.016% per day or 0.00069% per cycle. Overcharging leads to viologen decomposition, causing irreversible capacity fade. This work introduces extremely stable, extremely low‐permeating and low reduction potential redox active materials into near neutral ARFBs.