**4. Effect of EC/DMC supporting electrolyte**

### **4.1 Cyclic voltammetry**

The cyclic voltammograms of 0.1 M FeCl3 in reline DES without and with EC/ DMC are shown in **Figure 3**. The volume of the tested DES electrolyte is 40 ml. The result indicates that with the scan rate of 50 mV s<sup>−</sup><sup>1</sup> in the range of −0.7–1.2 V, there appear only one oxidation peak and only one reduction peak. The position of the peak does not distinctly change with the addition of EC/DMC. After the introduction of EC/DMC (4 ml, 10% vol.), the oxidation peak and the reduction peak shift left and right for <50 mV, respectively. This suggests that the introduction of EC/DMC neither affects the electrolyte composition nor substantially affects the redox reversibility of the electrolyte. In terms of the redox peak currents, after the introduction of EC/DMC, the peak current increases for <10%. The result implies that the rate of redox reaction of Fe(II)/Fe(III) redox couple in DES does not change remarkably with the addition of EC/DMC.

#### **4.2 Electrochemical impedance spectroscopy**

The Nyquist plots of Fe(III) in DES electrolyte with and without EC/DMC are shown in **Figure 4**. For the pristine DES, the plot shows a semicircle in the high-frequency region, and in the low-frequency region, there is a straight line upward; the semicircle corresponds to the charge transfer reaction at the electrode/ electrolyte interface, and the straight line upward corresponds to the diffusion of iron species in the electrolyte, suggesting electrochemical reaction and diffusion steps mix-control the Fe(III)/Fe(II) redox reaction. With the addition of EC/DMC,

**81**

895 cm<sup>−</sup><sup>1</sup>

**Figure 4.**

**Figure 3.**

*50 mV s<sup>−</sup><sup>1</sup> .*

*Effects of Electrolyte Additives on Nonaqueous Redox Flow Batteries*

*CV curves of different concentrations of EC/DMC additive in DES with 0.1 mol L<sup>−</sup><sup>1</sup>*

the radius of semicircle reduces, suggesting the charge transfer reaction becomes diffusion-controlled. With addition of more EC/DMC, the ohmic resistance (*Rs*) of the DES electrolyte becomes smaller. For example, for the pristine DES, the *Rs* is 137.9 Ω; for the electrolyte with 3 ml EC/DMC, it is 122.2 Ω; and for the DES with 5 ml EC/DMC, it reduces to 103.4 Ω. The electrochemical reaction resistance (*Rt*)

*AC impedance spectra for different concentrations of EC/DMC in DES with 0.1 mol L<sup>−</sup><sup>1</sup>*

The Raman spectroscopy shows that the introduction of EC/DMC does not change the shape of spectrum significantly (**Figure 5**). The peak near 3000 cm<sup>−</sup><sup>1</sup>

result of the overlap of the characteristic peaks of choline chloride and ethylene glycol (the main components of DES), while a new characteristic peak appears around

EC/DMC should be the result of the stretching vibration of C-O-C bond.

 when EC/DMC is added into the DES electrolyte, which can be attributed to the symmetrical stretching vibration of C-O-C bond when aliphatic ethers exist in the nonaqueous solution [33]. The reduction of ohmic resistance after the addition of

is a

 *FeCl3 with a scan rate of* 

 *FeCl3.*

almost keeps unchanged with the addition of EC/DMC.

*DOI: http://dx.doi.org/10.5772/intechopen.88476*

*Effects of Electrolyte Additives on Nonaqueous Redox Flow Batteries DOI: http://dx.doi.org/10.5772/intechopen.88476*

#### **Figure 3.**

*Redox*

**Table 1.**

the double-layer capacitance of the interface, simulating the process of the double layer in case of charge and discharge. *Ws* stands for the concentration polarization

*The parameters obtained from fitting the EIS plots with the equivalent circuit for carbon dioxide additive.*

**Temperature (°C)** *Rs* **(ohm)** *Rt* **(ohm)**

 1867 1409 1337 1446 834.7 757.5 560.6 587.3 390.2 373.2 251.3 277.8 233.6 227.6 129.3 149

**Pristine 0.1 MPa CO2 Pristine 0.1 MPa CO2**

The Z-view simulation helps to obtain the parameters of the equivalent circuit, which are listed in **Table 1**. It can be found that the resistance *Rs* of the electrolyte decreases after the addition of 0.1 MPa CO2. In detail, at the temperature of 25°C, it decreases from 1867 to 1409 ohm. The solubility of CO2 in the electrolyte will decrease when the temperature increases, so the decline percentage of *Rs* becomes small, and it is consistent with the trend of viscosity. What is more, the electrochemical reaction resistance of the solution slightly increases, implying that the addition of CO2 would slow down the charge transfer process of the electrolyte solution. The redox flow battery with DES electrolyte adding CO2 has a littlechanged overall performance compared with the pristine DES electrolyte.

The cyclic voltammograms of 0.1 M FeCl3 in reline DES without and with EC/ DMC are shown in **Figure 3**. The volume of the tested DES electrolyte is 40 ml.

there appear only one oxidation peak and only one reduction peak. The position of the peak does not distinctly change with the addition of EC/DMC. After the introduction of EC/DMC (4 ml, 10% vol.), the oxidation peak and the reduction peak shift left and right for <50 mV, respectively. This suggests that the introduction of EC/DMC neither affects the electrolyte composition nor substantially affects the redox reversibility of the electrolyte. In terms of the redox peak currents, after the introduction of EC/DMC, the peak current increases for <10%. The result implies that the rate of redox reaction of Fe(II)/Fe(III) redox couple in DES does not change

The Nyquist plots of Fe(III) in DES electrolyte with and without EC/DMC are shown in **Figure 4**. For the pristine DES, the plot shows a semicircle in the high-frequency region, and in the low-frequency region, there is a straight line upward; the semicircle corresponds to the charge transfer reaction at the electrode/ electrolyte interface, and the straight line upward corresponds to the diffusion of iron species in the electrolyte, suggesting electrochemical reaction and diffusion steps mix-control the Fe(III)/Fe(II) redox reaction. With the addition of EC/DMC,

in the range of −0.7–1.2 V,

impedance, which simulates the liquid phase's mass transfer.

**4. Effect of EC/DMC supporting electrolyte**

The result indicates that with the scan rate of 50 mV s<sup>−</sup><sup>1</sup>

remarkably with the addition of EC/DMC.

**4.2 Electrochemical impedance spectroscopy**

**4.1 Cyclic voltammetry**

**80**

*CV curves of different concentrations of EC/DMC additive in DES with 0.1 mol L<sup>−</sup><sup>1</sup> FeCl3 with a scan rate of 50 mV s<sup>−</sup><sup>1</sup> .*

#### **Figure 4.**

*AC impedance spectra for different concentrations of EC/DMC in DES with 0.1 mol L<sup>−</sup><sup>1</sup> FeCl3.*

the radius of semicircle reduces, suggesting the charge transfer reaction becomes diffusion-controlled. With addition of more EC/DMC, the ohmic resistance (*Rs*) of the DES electrolyte becomes smaller. For example, for the pristine DES, the *Rs* is 137.9 Ω; for the electrolyte with 3 ml EC/DMC, it is 122.2 Ω; and for the DES with 5 ml EC/DMC, it reduces to 103.4 Ω. The electrochemical reaction resistance (*Rt*) almost keeps unchanged with the addition of EC/DMC.

The Raman spectroscopy shows that the introduction of EC/DMC does not change the shape of spectrum significantly (**Figure 5**). The peak near 3000 cm<sup>−</sup><sup>1</sup> is a result of the overlap of the characteristic peaks of choline chloride and ethylene glycol (the main components of DES), while a new characteristic peak appears around 895 cm<sup>−</sup><sup>1</sup> when EC/DMC is added into the DES electrolyte, which can be attributed to the symmetrical stretching vibration of C-O-C bond when aliphatic ethers exist in the nonaqueous solution [33]. The reduction of ohmic resistance after the addition of EC/DMC should be the result of the stretching vibration of C-O-C bond.

**Figure 5.** *Raman spectra of solvents without and with EC/DMC additive.*
