Electrochemically Induced CO2 Capture Enabled By Aqueous Organic Redox Chemistry

Abstract

Investigations are ongoing to determine whether electrochemically-driven CO2 capture with inexpensive redox-active molecules in aqueous electrolyte in a scalable flow cell system has the potential to cost-effectively reach impactful scale.Water-soluble redox organic molecules “Q” can potentially capture CO2 from diffuse and point sources in an electrochemical flow cell. For example, proton-coupled electron transfer of these molecules (Q+2H++2e- → QH2) can raise the electrolyte pH, leading to the capture of CO2 from air; subsequent electrochemical oxidation of the reduced molecules (QH2 → Q+2H++2e-) acidifies the electrolyte, resulting in the release of pure CO2 [1-2]. The thermodynamic analysis indicates that an idealized cycle requires the minimum work input varying from 16 to 75 kJ/molCO 2 as throughput per cycle increases, with the potential to go substantially lower if CO2 capture or release is performed simultaneously with electrochemical reduction or oxidation. The method appears safe and scalable, as it utilizes non-toxic, non-corrosive, non-volatile and potentially low-cost organic molecules.An alternative CO2 capture mechanism, which appears to operate in parallel to this pH swing mechanism, is a nucleophilicity swing in which the electrochemically reduced species spontaneously forms a CO2 adduct, Q(CO2)2 2 -, which releases its CO2 upon electrochemical oxidation.In spite of the advantages in thermodynamic minimum and sustainable electrochemical processes, oxygen intolerance of the reduced molecules (QH2) has limited its application to direct air capture (DAC) due to reversible chemical oxidation of QH2 back to Q by atmospheric oxygen. Here, we report reduced Q species with improved resistance to oxidation by air. The air tolerance of the reduced Q with captured CO2 was evaluated under air exposure and tracked by 1H NMR. From day-1 to day-9, an NMR sample was loosely capped and stored in lab air. There was no sign of chemical changes in the NMR spectra for the first 5 days, and the peaks of Q in the oxidized state did not appear before the ninth day. The air stability of Q(CO2)2 2 - within the first five days suggests the feasibility of utilizing Q for a CO2capture/release cycle in the presence of oxygen that takes << 5 days.We will report the results of molecular engineering efforts to further improve molecular stability of the reduced form under oxygen.[1] Jin, S.; Wu, M.; Gordon, R. G.; Aziz, M. J.; Kwabi, D. G. pH swing cycle for CO2 capture electrochemically driven through proton-coupled electron transfer, Energy Environ. Sci., 2020, 13, 3706-3722.[2] Jin, S.; Wu, M.; Jing, Y.; Gordon, R. G.; Aziz, M. J. Low energy carbon capture via electrochemically induced pH swing with electrochemical rebalancing, Nature Communications, 2022, 13, 2140.