1-Butyl-3-Methylimidazolium Tetrafluoroborate Ionic Liquid As an Electrolyte for Electrochemical Reduction of Nitrous Oxide.
Authors: Kim Hyeonsu , Lim Taeho , Kim Jae Jeong 
Published: December 2020

Nitrous oxide (N2O), of which global warming potential is 310 times of CO2, is a powerful greenhouse gas [1]. The concentration of N2O has increased steadily from various agricultural and industrial activities such as cultivating crops in low-pH soil and manufacturing semiconductor devices [2,3]. Therefore, the importance of studying N2O reduction is emphasized. Thermal decomposition methods focusing on thermal catalyst have been reported, but there are limits such as process temperature and low efficiency [4,5].

Electrochemical reduction of N2O can be an attractive method since it is conducted at low temperatures and has low energy consumption. However, in aqueous solutions, the electrochemical reduction of N2O inevitably accompanies hydrogen evolution reaction (HER), which is the main side reaction. There were many trials to reduce HER through metal and metal oxide catalysts, but they also had limits in terms of efficiency and high cost of noble metal materials [6-9].

Ionic liquids are promising electrolytes in gas reduction field replacing water because they can avoid hydrogen evolution and enhance the solubility of gaseous reactants. Some imidazolium-based ionic liquids were adopted to electrochemical reduction of CO2 because of their favorable properties such as wide electrochemical potential window and high CO2 dissolution capacity [10,11]. Cations of ionic liquids are known to accept oxygen from CO2, and then return to initial state at counter electrode emitting oxygen [12]. According to the same principle, ionic liquids may also be suitable for the electrochemical reduction of N2O.

In this study, an ionic liquid/organic solvent mixture was adopted as an electrolyte for the electrochemical reduction of N2O. The ionic liquid and organic solvent used for electrolyte were 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and propylene carbonate (PC), respectively. The use of [BMIM][BF4] in place of aqueous solutions could improve the solubility of N2O and also avoid the hydrogen evolution reaction, which is the main side reaction in an aqueous solution.

Herein we observed the difference of LSV curves in aqueous solution and [BMIM][BF4]. Hydrogen evolution reaction was excluded, but the low current density was obtained in [BMIM][BF4]. The low electrical conductivity and high viscosity of [BMIM][BF4], the main disadvantages in the context of electrochemical reactions, were compensated on introducing PC. We measured the viscosity and electrical conductivity in various [BMIM][BF4]/PC composition, and the composition of the [BMIM][BF4]/PC was optimized considering both the electrical conductivity and N2O stability in the solution. The current and faradaic efficiency could be evaluated during chronoamperometry in aqueous solution and [BMIM][BF4]/PC. For the optimized [BMIM][BF4]/PC, high current efficiency and faradaic efficiency (>90%) were achieved for N2O reduction to N2.

Figure captions

Figure 1. LSV curves for Cu electrode in (a) 0.3 M K2SO4 aqueous solution and (b) [BMIM][BF4]. The scan rate was 50 mV s-1.

Figure 2. (a) Viscosity (black line) and conductivity (blue line) changes for [BMIM][BF4]/PC according to [BMIM][BF4] content (b) Current efficiency–time curves recorded at –1.3 V vs. SCE in 0.3 M K2SO4 aqueous solution (blue line) and at –2.27 V vs. Fc/Fc+ in 75 vol% [BMIM][BF4] (red line).

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