Electrochemical Capture and Release of CO2 in Aqueous Electrolytes Using an Organic Semiconductor Electrode

Developing efficient methods for capture and controlled release of carbon dioxide is crucial to any carbon capture and utilization technology. Herein we present an approach using an organic semiconductor electrode to electrochemically capture dissolved CO2 in aqueous electrolytes. The process relies on electrochemical reduction of a thin film of a naphthalene bisimide derivative, 2,7-bis(4-(2-(2-ethylhexyl)thiazol-4-yl)phenyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (NBIT). This molecule is specifically tailored to afford one-electron reversible and one-electron quasi-reversible reduction in aqueous conditions while not dissolving or degrading. The reduced NBIT reacts with CO2 to form a stable semicarbonate salt, which can be subsequently oxidized electrochemically to release CO2. The semicarbonate structure is confirmed by in situ IR spectroelectrochemistry. This process of capturing and releasing carbon dioxide can be realized in an oxygen-free environment under ambient pressure and temperature, with uptake efficiency for CO2 capture of ∼2.3 mmol g–1. This is on par with the best solution-phase amine chemical capture technologies available today.


3-ethylheptane nitrile (1)
2.73 g (0.042 mol) of KCN, 7.73 g (0.056 mol) of K 2 CO 3 and 150 ml of DMF were added to a 250 ml three-neck-round-bottom flask. The mixture was hetaed to 50 °C and 5.43 g (0.028 mol) of 2-ethylhexyl bromide was added dropwise. The reaction mixture was stirred at 90 °C for 24 hours. Then, after cooling down to room temperature it was poured into 150 ml of water and extracted with toluene. The collected organic phases were dried over MgSO 4 . Then, the organic solvent was removed yielding 3.00 g (75 %) of colorless oil, that was used in the next step without further purification. 1

3-ethylheptane amide (2)
An aqueous solution of NaOH (1 M, 25 ml) was added to a stirred solution of 0.50 g (0.0036 mol) of 3-ethylheptane nitrile (1) in ethyl alcohol (50 ml). An aqueous solution of H 2 O 2 (30 %, 25 ml) was then added dropwise. After being stirred for 1 h at room temperature the reaction mixture was poured into 50 ml of water and extracted with CHCl 3 (3×30 ml). The combined organic phases were dried over MgSO 4 and concentrated in vacuo. The crude product was purified by column chromatography on silica gel, using CHCl 3 as an eluent yielding 0.50 g (89 %) of white crystalline solid. 1

3-ethylheptane amide (2) to 3-ethylheptane thioamide (3).
S-4 2.00 g (0.0130 mol) of (2) and 2.63 g (0.0065 mol) of Lawesson's reagent were dissolved in 150 ml of toluene. The reaction mixture was heated in 70-80 °C for 10 h. The progress of the reaction was controlled by TLC. After complete disappearance of the substrate, the reaction mixture was cooled down to room temperature and the solvent was evaporated. The crude product was purified by column chromatography on silica gel, using toluene and the mixture of toluene and CHCl 3 (9:1, v/v) as eluents yielding 1.50 g (67 %) of orange oil. 1

Electrode preparation and electrochemical characterization:
10mg of NBIT was dissolved in 1mL chlorobenzene to obtain a 12.4 mM solution. Glassy carbon plates were first mechanically polished with alumina to remove any surface impurites and then washed thoroughly with distilled water and 2-propanol respectively. Later the glassy carbon plates were sonicated in deionized water for 30 min. The cleaning procedure was completed by electrochemical cleaning in 0.1M HClO 4 via potentiodynamic cycling. A thin film of NBIT was spin coated from its solution at 1000 rpm and dried under N 2 flow.
All electrochemical characterizations were done in 0.1M Na 2 SO 4 under inert atmosphere (under N 2 or CO 2 ). NBIT coated glassy carbon electrode was used as working electrode with Pt plate acting as counter electrode and a Ag/AgCl (3M KCl) as the reference electrode in an H-cell geometry.    Figure S6. Corresponding equivalent electrical circuits for EIS data fitting of NBIT under (a) Ar atmosphere and (b) CO 2 atmosphere at -1.0V and below, consisting of the ohmic resistance of the electrolyte solution (R s ), the double layer capacitance (C dl ) and the charge transfers resistance (R ct ). While under Ar saturation the EIS data can be well explained by a single R/C element in series with the R s , [1,2] two R/C elements have to be used, in series with the R s , to fit the spectra under CO 2 saturation at -1.0V and below.