(Invited) Mechanism of Formic Acid synthesis in low pH during CO2 Reduction via electrified plasma/liquid interface
Authors: Mota-Lima Andressa 
Published: May 2020

Electrochemical discharger over liquid water enriched with dissolved carbon dioxide (CO2) is capable of synthesizes formate (HCO2
-) and oxalate (C2O4
2
-), although in very low faradaic selectivity toward organic molecules ca. 9% [1]. The efficient CO2 reduction (CO2-R) toward these organic molecules via electrochemical discharge remains a challenge. This contribution devotes to find possible ways to produce these products efficiently. In this context, the mechanism of electrosynthesis for both formate and oxalate is postulated and used from a theoretical perspective [2] to figure out which parameters are key to reach as maximum selectivity as possible.
Figure 1a shows the chemical reaction network (CRN) postulated for CO2-R via electrochemical discharge over water. The discharge promotes a periodic perturbation on the local concentration of the solvated electron within the nanoreactor (volume of less than one nanoliter beneath the plasma liquid interface), firing the set of reaction displayed in this CRN. Three final products are formed, one volatile and two dissolved in the liquid phase. Gaseous hydrogen is a volatile product synthetized majorly via recombination of two solvated electrons, reaction (1); other two routes for hydrogen synthesis described in reference 2 are negligible over uninterrupted long lasting discharge. On the other hand, organic molecules requires the carboxyl radical anion {CO2
-∙
(aq)} as precursor. In view of this CRN, the generalist model in reference (2) were used for implement the algorithm related to the chemical dynamical system. The computational experiments employed Wolfram language, 10-
6
s step, a stationary current at level 9.5 mA (the same used by the researchers in ref 1) and the parameters displayed in table 1. The selectivity toward a given product is the ratio of its quantity of mole with respect to that of injected electrons. Figure 1b shows time evolution for the concentrations of the main reactants, the intermediate (carboxyl radical anion) and the final products. Figure 1c shows the selectivity toward each final product considering two span of continuous stationary discharge.
The computational experiments demonstrated that the chemical dynamics driven by a stationary injection of electron (term qe in the model) in water is mostly governed by the formation reaction of CO2
-∙
(aq) within the early 43 ms span of time. Within this period, full content of CO2 in the nanoreactor depletes as result of the fastest reaction rate for the formation of CO2
-∙
(aq) . The large local concentration (“local” qualifies the concentration inside the nanoreactor) of CO2
-∙
(aq) favors the oxalate formation, being the oxalate the major final product. Formate is only synthetized at significant formation rate between 0.040 and 0.043 s, when the concentration of both solvated electrons and CO2
-∙
(aq) start to be less dissimilar as consequence of the depletions of the aqueous CO2 in the nanoreactor. Anytime thereafter, the solvated electrons has increased concentration, and the hydrogen passes to form majorly. Considering the 300,000 s (5 minutes used in reference 1), about 7.72% faradaic efficiency toward organic molecules is obtained, corroborating the low efficiency for experiments in real life.
In conclusion, this work postulate the mechanism of CO2 reduction via electrochemical discharge, and it suggests the employment of short span of stationary discharge to obtain faradaic efficiency toward organic molecules of 98.46%, being 96.89% selective toward oxalate.

Figure 1: (a) postulated chemical reaction network for CO2-R via electrochemical discharge over water, (b) time evolutions for the concentrations of either reactants or products , and (c) selectivity toward formate (yellow), oxalate (orange) and hydrogen (pink) at STP CO2 saturation.

Table 1: parameters for solving the dynamical model [3]

Parameter

Value

Unit

Ref

k1

6.0×109

L mol−1 s−1

3

k2

9.0×109

L mol−1 s−1

3

k3

1.0×1010

L mol−1 s−1

3

k4

1.26×109

L mol−1 s−1

3

qe (9.5 mA)

98.46×10-9

mol s-
1

1

V

2.36×10-9

L

2

[H+]0

1.00×10-7

mol L-1

[CO2]0

40.00×10-3

mol L-1

References

[1]Rumbach, P., R. Xu, and D.B. Go, Electrochemical Production of Oxalate and Formate from CO2 by Solvated Electrons Produced Using an Atmospheric-Pressure Plasma. Journal of The Electrochemical Society, 2016. 163(10): p. F1157-F1161.
[2] Mota-Lima, A., et al., Electrosynthesis via Plasma Electrochemistry: Generalist Dynamical Model To Explain Hydrogen Production Induced by a Discharge over Water. The Journal of Physical Chemistry C, 2019. 123(36): p. 21896-21912.
[3] Neta, P., J. Grodkowski, and A.B. Ross, Rate Constants for Reactions of Aliphatic Carbon‐Centered Radicals in Aqueous Solution. Journal of Physical and Chemical Reference Data, 1996. 25(3): p. 709-1050.

Figure 1