Mechanistic Study of Energy Efficient Electrochemical Capture and Release of CO2 in Ionic Liquid Complexes

There is an urgent need for the development of energy efficient methods for the capture and storage of carbon dioxide (CO₂). Several electrochemical agents which can capture CO₂ have been explored in our group over the last few years. The basic idea is to electrochemically reduce a precursor (P) to generate a potent nucleophile (N^-) which then reacts with CO₂ to form an adduct (N-CO₂^-). Later, this adduct is electrochemically oxidized to regenerate the precursor and the released CO₂ can then be captured. In most cases, the CO₂ is bound to either a nitrogen or a sulfur atom of the nucleophile. In the current work, we describe the mechanism for the electrochemistry of bipyridines and disulphides in presence of CO₂ in n-butyl-n-methylpyrrolidinium bis(trifluoromethylsulfonly)imide (BMP⁺ TFSI^-) ionic liquid using a combined approach employing experimental studies and digital simulation.

The work has been focused on using bipyridine and disulphide systems for electrochemical CO₂ capture. In these systems, the electrogenerated nucleophiles i.e.  bipyridinium radical anions (as shown in Figure 1) or thiolates reacts with CO₂ to form an adduct. Furthermore, the CO₂ adduct i.e. carbamate or thiocarbonate can be oxidized to release the precursor and CO₂. The oxidation potential of these adducts shifts far positive as compared to the oxidation of the nucleophiles. Based on the detailed comparison of the experimental data with simulations using possible mechanistic models, it is found that the electrogeneration of nucleophiles reacts with CO₂ to form an adduct and the direct oxidation of the adduct using EC mechanism releases the precursor and CO₂.

Acknowledgement

We acknowledge the support from Advanced Research Projects Agency (ARPA-E) for this work. 

References

1.  P.V. Danckwerts, Chemical Engineering Science, 34(4), 443-446 (1979).

2. G.P. Hammond and S.S.O. Akwe, International  Journal of Energy Research, 31(12), 1180-1201 (2007).

3.  H.W. Pennline et al., Fuel Processing Technology, 89(9), 897-907 (2008).

4.  S.S. Moganty and R.E. Baltus, Industrial & Engineering Chemistry Research, 49(12), 5846-5853 (2010).

5. M.B. Mizen and M.S. Wrighton, Journal of The Electrochemical Society, 136(4), 941-946, (1989).

6. H. Ishida, T. Ohba, T. Yamaguchi and K. Ohkubo, Chemistry Letters, 905-908 (1994).

7. E. Shouji and D. A. Buttry, Journal of Physical Chemistry B, 103(12), 2239-2247 (1999).

Figure-1 Experimental (black) and simulated (red) cyclic voltammograms for electrochemistry of 10 mM 4,4'-bipyridine in the presence of 10 mM CO₂ in BMP⁺ TFSI^- ionic liquid on glassy carbon electrode. Scan rate: 50 mV/s.