Electrochemical Upgrade of Captured Carbon from Amine Capture Solution

Like Comment
Read the Paper

A reduction in emissions of CO2 into the atmosphere is required to slow global warming. CO2 capture and electrolysis technologies are a promising approach to reduce CO2 emission and generate renewable chemicals. The COcapture process, which relies on alkylamine solvents, requires significant energy input, typically involving thermally cycling the system to 120-150°C to generate a pure stream of CO2.1-3 The subsequent valorization of the gas-phase COto value-added products introduces further energy losses and system complications.

We pursued therefore on a new scheme involving the electrochemical upgrade of the chemisorbed CO2 directly from an amine capture electrolyte.4

 

Amine solutions capture CO2 via reaction:

2RNH2 + CO2  → RNHCOO- + RNH3+

(1)

where the nucleophilic N and electrophilic carbon form a bond.

 

The direct electrochemical upgrade of amine-CO2 (RNHCOO-) to value-added products is of intense interest; yet, in early studies, we found that this goal was curtailed when the adduct of the amine molecules with CO2 fails to place the COsufficiently close to the site of the heterogeneous reaction.

 

We turned our attention to the composition of electrochemical double layer to investigate poor heterogeneous electron transfer dynamics. Since heterogeneous electron transfer is an inner sphere reaction5, the distance between RNHCOO- and the electrode in the electrochemical double layer is expected to play a crucial role in the electrolysis reaction. Systematic tuning of the electrochemical double layer can be achieved through the introduction of properly sized electrolyte ions. Here we focused on alkali cations with the goal of disrupting the undesired charge-blocking layer and achieving electron transfer to carbamate, and thus direct electrolysis.

Fig. 2
a, MEA–CO2 electrolyte. b, MEA–CO2 with alkali salt electrolyte.

We examined a suite of alkali cations (Li+, Na+, K+, Rb+ and Cs+) to gain insight into how the size of the cationic species affects the electrochemical double layer and the reduction of the amine-CO2 adduct to CO. The CO Faradaic efficiency for the Li+ and Na+ electrolyte was similar to those of the amine electrolyte, which is below 5%. Cscontaining electrolyte shows the best performance, with 30% CO FE at –0.66 V vs. RHE. To explore how tailoring the electrochemical layer by cationic species affects the electron transfer, we investigated the interfacial electric field strength via the Stark tuning. The Stark tuning slope of the frequency shift depends on the local electric field strength, which allows a direct reflection of the interfacial electric field with respect to the cationic species. 6-8 We can correlate this electric field strength with the thickness of the electrochemical double layer.9 The Stark tuning slopes for K+, Rb+ and Cs+ are significantly larger than amine-CO2 adduct (RNH3+), Li+ and Na+, which agrees with the picture that the thickness of the electrochemical layer for K+, Rb+ and Cs+ is lower than that of RNH3+, Li+ and Na+. We found that different constituents of the electrochemical double layer can hinder or promote the heterogenous electron transfer, a finding we attribute to the distance between the reactants and the electrode.

Fig. 4
a, The FE for different alkali cation salt solutions in the MEA electrolyte at the applied potentials of –0.58 V and –0.66 V versus RHE. The error bars represent the standard deviation of measurements over three independent samples. b, Cation effects on the COads stretching frequency shift at different applied potentials. The error bars indicate the standard deviation of the COads frequency based on three independent measurements.

We then sought to improve the current density and Faradaic efficiency to reach meaningful levels of utilization. With the aid of an alkali cation and accelerated mass transport achieved through improved system design – temperature (~80°C) and concentration (~2.5 M of amine-CO2) – we achieved amine-CO2 conversion to CO with 72% Faradaic efficiency at 50 mA/cm2. The performance is competitive with that of existing CO2RR systems such as alkaline flow cells52,  gas-fed membrane electrode assemblies56, and direct carbonate reduction systems10 (Table 1). 

System

Flow cell

MEA

Direct

CO32-

Direct

amine-CO2

CO2 utilization (%)

17

35

100

90

Carbonate formation (%)

45

0

0

10

Crossover (%)

2

30

0

0

Exit CO2 (%)

36

35

0

0

CO2 regeneration

(kJ/mol of prod.)

206

100

0

0

Electrolysis

(kJ/mol of prod.)

485

643

2572

643

Product separation (kJ/mol of prod.)

147

71

25

25

Total energy

(kJ/mol of prod.)

838

814

2597

668

Total energy

(kJ/tonne of prod.)

29

29

91

23

Table 1. Energy cost for an alkaline flow cell, gas-fed MEA cell, direct carbonate electrolysis and direct amine-COelectrolysis

This work indicates interest in amine-CO2 systems: the approach levers the ability to perform electrolysis directly from a capture liquid and can reduce the energy cost (and carbon footprint) of the CO­2 capture process and simplify the process flow.  

  1. Boot-Handford ME, et al. Carbon Capture and Storage Update. Energy Environ. Sci. 7, 130-189 (2014). 
  2. Wang Y, Zhao L, Otto A, Robinius M, Stolten D. A Review of Post-combustion CO2 Capture Technologies from Coal-fired Power Plants. Energy Procedia 114, 650-665 (2017).
  3. Haszeldine RS. Carbon Capture and Storage: How Green Can Black Be? Science 325, 1647 (2009).
  4. Lee G, et al. Electrochemical upgrade of CO2 from amine capture solution. Nature Energy, (2020).
  5. Schmickler ESW. Electrochemical Electron Transfer: From Marcus Theory to Electrocatalysis. John Wiley & Sons, Inc. (2008).
  6. Li J, Li X, Gunathunge CM, Waegele MM. Hydrogen bonding steers the product selectivity of electrocatalytic CO reduction. Proc. Natl. Acad. Sci. 116, 9220-9229 (2019).
  7. Ringe S, et al. Understanding cation effects in electrochemical CO2 reduction. Energy Environ. Sci. 12, 3001-3014 (2019).
  8. Chattopadhyay A, Boxer SG. Vibrational Stark Effect Spectroscopy. J. Am. Chem. Soc 117, 1449-1450 (1995)
  9. Bard AJ, Faulkner LR. Electrochemical Methods: Fundamentals and Applications, 2nd Edition. Wiley Textbooks (2000).
  10. Li YC, et al. CO2 Electroreduction from Carbonate Electrolyte. ACS Energy Lett. 4, 1427-1431 (2019).

Geonhui Lee

phD candidate, University of Toronto