**2.1 Preparation of electrolyte**

*Redox*

synthesis steps, cost, and availability.

studied by Xu group [23, 24].

Ionic liquids are salts which can melt at room temperature. These salts have a potential window as wide as organic solvents. Compared with other solvents, ionic liquids have high thermal and electrochemical stability, and the conductivity is higher than aqueous electrolytes [7]. Because of these advantages, ILs have been applied to a lot of fields, such as lithium-ion batteries [8], dye-sensitized solar cells [9], electrolytes in sensors [10], electrochemical capacitors [11], lead acid batteries [12], and fuel cells [13], and even applied to flow batteries recently as electrolyte solutions [14]. In 2015, the applicability of ionic liquids has been explored by Ejigu et al. as solvents of metal complex-containing redox flow battery [15]. Zhang et al. applied TEAPF6 and EMIPF6, two ionic liquids, to nonaqueous redox flow batteries. The results of charge and discharge tests showed that the coulombic efficiencies range from 43.46 to 57.44% [16]. However, there still exist some problems which have become the limitation for their large-scale applications, such as complex

The deep eutectic solvent (DES) can be recognized as a peculiar ionic liquid; it is consisting of a conjunction with a stoichiometric ratio of acceptors of hydrogen bond (like quaternary ammonium salts) and donors of hydrogen bond (like compounds of amides, carboxylic acids, and polyols) into the eutectic mixture. We can prepare and use DES under ambient conditions; low cost is the main advantage of DES, which is cheaper than conventional ionic liquids to an order of magnitude; DES is easy to prepare and its overall biodegradability is an advantage too [17–20]. People have carried out preliminary works on the DES. Lloyd et al. had studied the kinetics of electron transfer of the Cu(I)/Cu(II) redox couple with chronoamperometry at a platinum electrode; they named cyclic voltammetry and impedance spectroscopy in a deep eutectic solvent, which consist of choline chloride and ethylene glycol as ethaline [21]. Thereafter, an all-copper hybrid redox flow battery was demonstrated in the ethaline DES [22]. Nevertheless, because of the mass transport limitations and the poor electrolyte conductivity, the energy efficiencies could only

reach to 52 and 62% when the current densities were 10 and 7.5 mA cm<sup>−</sup><sup>2</sup>

cell performance [33], but few are reported in redox flow batteries.

the electrochemical activity and redox kinetics of V(III)/V(II).

tively. In recent researches, the electrochemical and transport characteristics of Fe(II)/Fe(III) as well as V(II)/V(III) redox couples in the DES electrolytes had been

When people use pristine DES as electrolyte, the main issues are large viscosity and its small diffusion coefficient, which will lead to large pumping loss and cause low efficiency for RFBs [25]. In order to overcome these problems, researchers proposed ways of adding appropriate additives into the electrolyte, like gas and ionic additives, and tuning the active materials by adopting molecular engineering [26, 27]. In 2015, the viscosity changes of ionic liquids after adding SO2 had been studied by Zeng et al. [27]. The results indicate that when the concentration of SO2 increases, the viscosity of conventional ionic liquids will decrease sharply. Also, some studies have shown that the adoption of high-pressure CO2 in DES can certainly improve the physical properties of the RFB system, and this can also increase its electrical conductivity [28–32]. However, there are not too much literature about how the gas additives influence on the electrochemical performance of nonaqueous flow batteries. Moreover, some supporting electrolytes are widely used in lithiumion batteries for the reduction of electrolyte viscosity as well as the enhancement of

Metal ions are also widely used as additives of electrolytes for electrochemical energy system. Antimony (Sb) has the advantages of low cost, good chemical stability, and high catalytic activity, such that it is widely used in the field of electrocatalysis and battery [23, 34]. In 2015, Shen et al. introduced SbCl3 into a vanadium redox flow battery (VRFB) [35]. The work shows that the added SbCl3 can improve

, respec-

**76**

The DES electrolyte could be prepared by combining choline chloride (Aladdin, 98%) and urea (Sinopharm Reagent 57-13-6, 99%), and the molar ratio is 1:2. The solution at 120°C with a magnetic stirrer is heated and stirred until it formed a colorless and transparent liquid, this mixture is also known as "reline," and the temperature of liquid was also reduced to the room temperature. With long-term placement, the DES would appear some white crystalline precipitate, so it needed to be heated over 50°C in advance of the experiment; this process should proceed on the magnetic stirrer for near 30 minutes, and the rotor's rotation promotes the crystalline precipitate's dissolution. When finishing the experiment, DES should be kept in the sealed glass jar timely to avoid pollution caused by water vapor and oxygen in the air. The active material FeCl3 (Sinopharm Reagent 7705-08-0, 99%) with a concentration of 0.1 mol L<sup>−</sup><sup>1</sup> was added into reline, and the mixture at 120°C was heated and stirred to obtain the electrolyte. Vacuum dried the prepared electrolyte for 24 hours before the start of experiment. Divide the solution into two portions, one portion serving as the pristine and the other one being fed with 0.1 MPa of CO2 (with the purity of 99.99%). The EC (Macklin 98%) and DMC (Macklin 99%) mixture were prepared by 1:1 vol.% and stirred evenly when adding into the DES electrolyte. For the ion additive, the SbCl3 (99%, Sinopharm Chemical Reagent Co., Ltd.) was added into the negative DES electrolyte at a concentration of 5, 10, 15, and 20 mM, respectively.
