Electrochemical reduction of CO2 in the captured state using aqueous or nonaqueous amines

Abstract

CO₂ capture and its electrochemical conversion have historically developed as two distinct technologies and scientific fields. Each process possesses unique energy penalties, inefficiencies, and costs, which accrue along the mitigation pathway from emissions to product. Recently, the concept of integrating CO₂ capture and electrochemical conversion, or "electrochemically reactive capture," has aroused attention following early laboratory-scale proofs-of-concept. However, the integration of the two processes introduces new complexities at a basic science and engineering level, many of which have yet to be clearly defined. The key parameters to guide reaction, electrolyte, electrode, and system design would, therefore, benefit from delineation. To begin this effort, this perspective outlines several crucial physicochemical and electrochemical considerations, where we argue that the absence of basic knowledge leaves the field of designing metaphorically in the dark. The considerations make clear that there is ample need for fundamental science that can better inform design, following which the potential impacts of integration can be rigorously assessed beyond what is possible at present.

Keywords: chemistry; electrochemical energy conversion; electrochemistry; engineering.

Graphical abstract

Figure 1

Schematic and free energy landscape of integrated CO₂ capture and electrochemical conversion (A) Conceptual schematic of an integrated CO₂ capture and electrochemical conversion process. (B) Free energy landscape of such a process using amines (typical sorption enthalpies of −60 to −80 kJ/mol_CO2). “Pure CO₂” represents separated CO₂ at 1 atm, with further compression to high pressure as shown. The minimum work to separate dilute CO₂ (12-14% in flue gas) to pure CO₂ at 1 atm is on the order of 5-10 kJ/mol CO₂ depending on starting and ending purities (Wilcox, 2012). The minimum work of compression of pure CO₂ to ∼150 atm is 12 kJ/mol (Renfrew et al., 2020). Production of formally reduced products (chemicals, fuels) from either post-separation CO₂ or carbamates is energetically uphill (electrolytic cell). Electrochemically induced mineralization requires electron transfer to the carbamate to balance alkali cation transfer, producing solid (Li, Na, Mg, or Ca-based) carbonates, and is a downhill (galvanic) reaction electrochemically. As a rough (lower) estimate of scale, typical CO₂ mineralization enthalpies of magnesium carbonate and calcium carbonate with respect to CO₂ and their oxides (MgO, CaO) are −180 to −240 kJ/mol (Gadikota, 2020). For ease of representation, energy levels are not to scale, co-reactants (such as water, Mg²⁺ or Ca²⁺) and products (e.g., O₂ for electrolysis) are omitted, and “+e^–” does not imply stoichiometry given that multiple processes and products are allowed. “Reduced products” refers to chemical or fuel outputs such as CH₄, CH₃OH, or CO and is represented generically as a range. Red arrows denote thermal processes, while green arrows denote processes that can be electrified (B) is inspired in part by (Heldebrant et al., 2022).

Figure 2

Parameters governing integrated CO₂ capture-electrochemical conversion Multiple key factors are shown in call-out boxes, any of which can be rate-limiting, and have direct impacts on reactant-state and product formation. A⁺ = monovalent alkali or organic cation; alkaline earth (A²⁺) cations may also be utilized but are omitted for simplicity.

Figure 3

Electrochemical CO₂ reduction with amines in aqueous solution (A) Schematic (top) and linear sweep voltammogram (bottom) of electrochemical CO₂ reduction in aqueous media with MEA (30%), using an In electrode with 0.1 wt% surfactant at 22°C (Chen et al., 2017). The asterisk denotes a surface-adsorbed state. Cetyltrimethylammonium bromide (CTAB), SDS and 4-octylphenol polyethoxylate (Triton X-100)) refer to surfactants. Reprinted with permission from Ref (Chen et al., 2017). Copyright (2017), John Wiley and Sons. (B) Schematic (top) and averaged current density (bottom) resulting from potentiostatic polarizations in aqueous CO₂ capture solutions of 30 wt% MEA and 2 M KCl at an Ag electrode. Results were obtained at 60°C in a flow cell (Lee et al., 2021). Reprinted with permission from Ref (Lee et al., 2021). Copyright (2021), Springer Nature. Both schematics omit explicit labeling of product RNH₂, as indicated in the noted reactions, for clarity of viewing. Gray species are non-participating.

Figure 4

Electrochemical CO₂ reduction with amines in nonaqueous solution (A) Schematic (top) and cyclic voltammograms (CV, bottom) on carbon with 0.1 M EEA-CO₂ and 0.3 M LiClO₄/DMSO (Khurram et al., 2018). (B) Schematic (top) and CVs (bottom) using glassy carbon with 0.1 M amine-CO₂, 0.1 M 1,1,3,3-tetramethylgunaidine (TMG) in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) in acetonitrile at 0.1 V s⁻¹ (Bhattacharya et al., 2020b). (C) Schematic (top) and CV (bottom) using a Pb electrode with 1 M AMP-CO₂ in 0.7 M TEACl/PC (Pérez-Gallent et al., 2021). Reprinted with permission from (Pérez-Gallent et al., 2021). Copyright (2021), American Chemical Society. Schematics omit labeling of product RNH₂ for clarity. Gray species are non-participating.

Figure 5

CO₂ absorption capacity and reactant speciation in aqueous electrolyte (A) CO₂ absorption capacity in water (Hansen, 2007; Yoo et al., 2013), organic solvents (PC, DMSO, ACN (acetonitrile)) (Gennaro et al., 1990; Hansen, 2007), and aqueous amines (HMD (hexamethylenediamine), DETA (diethylenetriamine), TAEA (tris (2-aminoethyl) amine), TETA (triethylenetetramine)) (Singh et al., 2011). (B) pH-dependent CO₂ equilibria in aqueous media. Reactant state speciation of (C) primary/secondary amines and (D) tertiary amines in aqueous media as a function of time, as determined from operando NMR measurements. Adapted with permission from (Kortunov et al., 2015a). Copyright (2015), American Chemical Society.

Figure 6

Reactant speciation in nonaqueous electrolytes (A) Dynamic speciation of primary/secondary amines in nonaqueous media, as determined from ¹H NMR after introducing alkali salt to a solution of carbamic acid. (B) Proportion of carbamate in the presence of alkali or tetrabutyl ammonium (TBA⁺) cations under the same experimental conditions. The anion of Na⁺, K⁺, and TBA⁺ is ClO₄⁻. (C) Variable temperature (VT) ¹H NMR spectra of 0.05 M EEA-CO₂ in DMSO-d₆, 24 h after 0.3 M LiClO₄ was introduced to the solution. (D) Equilibrium proportion of carbamates from (C). A is adapted and B, C, and D are reproduced with permission from Ref (Khurram et al., 2019, 2020). Copyright (2019, 2020), American Chemical Society.

Figure 7

Transport and electrode-electrolyte interface considerations (A) Diffusion coefficients of CO₂ (Cussler, 1997; Haas et al., 2021), ions (Joung and Cheatham, 2009; Semino et al., 2014), and amines in water (blue background) and DMSO (yellow background). The diffusion coefficients of amines with 0 and 0.3 M LiClO₄ D₂O or DMSO-d₆ were measured using diffusion-ordered spectroscopy (DOSY) NMR. Note that MEA/MEAH⁺ signifies a mixture of lean amine and ammonium species. (B) Electrical double layer of aqueous amine electrolyte without (top) and with (bottom) supporting salt cations. Blue shaded adsorbates (top) and pink spheres (bottom) at the electrode surface are ammonium species and alkali cations, respectively, (B) is adapted with permission from (Lee et al., 2021). Copyright (2021), Springer Nature.