Mechanism of CO₂ Capture and Release on Redox-Active Organic Electrodes

Abstract

The mechanism of faradaic electro-swing for CO 2 capture/release on a redox-active organic electrode is studied from the point of view of realizing a reversible process for CO 2 separation. First, the cyclic voltammograms (CVs) of two redox-active organic monomers, anthraquinone (AQ) and 2,1,3-benzothiadiazole (BTZ), were measured under a CO 2 atmosphere. The waveforms of CVs of the two redox-active organic monomers are altered under a CO 2 atmosphere relative to a N 2 atmosphere. There is a change in the number of redox waves from two to one for AQ and a change from reversible to irreversible waves for BTZ. To further understand the mechanisms of CO 2 capture/release on redox-active organic compounds, redox-active polymer electrodes coated with polyanthraquinone (PAQ) and polybenzothiadiazole (PBTZ) were investigated using a spectroscopic analysis known as in situ attenuated total reflectance–surface-enhanced infrared absorption spectroscopy (ATR–SEIRAS) as well as density functional theory (DFT) calculations. The CVs measured with two redox-active polymer electrodes have more positive shifts of reduction potentials for PAQ and PBTZ under a CO 2 atmosphere than under an N 2 atmosphere, as measured versus Ag/AgNO 3: −1.8 and −1.4 V → −1.4 and −0.9 V for PAQ and −2.5 V → −2.1 and −1.9 V for PBTZ. In addition, the oxidation current on the PBTZ electrode disappears only under a CO 2 atmosphere. DFT calculations indicate that the positive shifts of reduction potentials in the two electrodes under CO 2 conditions are due to an exergonic adsorption reaction of CO 2 onto the redox-active organic compounds. To clarify the reaction behavior between CO 2 and the redox-active organic electrode, an ATR–SEIRAS spectroscopic analysis was performed. Infrared peaks are observed at 2200 and 2100 cm –1 for PAQ and PBTZ electrodes, respectively, under a CO 2 atmosphere, which have been confirmed by measurements under a 13 CO 2 atmosphere to have adsorbed CO 2 . The wavenumbers corresponding to the CO 2 molecules adsorbed on the two electrodes are different. These findings indicate that one electron reduction and CO 2 adsorption for each redox-active organic compound are occurring simultaneously. The calculated adsorption energy of CO 2 for two redox-active organic electrodes indicates that the adsorption energies of two CO 2 molecules for AQ and BTZ are −7.3 and −36.3 kJ/mol. The larger adsorption energy in BTZ than that in AQ is clearly related to the disappearance of the oxidation current. However, both adsorption energies indicating adsorption of CO 2 onto organic units are less than the covalent bond energies of common organic compounds, indicating that such weak adsorptions are suitable for the faradaic electro-swing of CO 2 capture/release on the redox-active organic units.

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