**3. Effect of carbon dioxide additive**

## **3.1 Cyclic voltammetry**

The cyclic voltammograms of 0.1 M FeCl3 in reline DES at different temperatures with and without 0.1 MPa CO2 are shown in **Figure 1**. The result indicates that in the range of −0.7–0.9 V with the scan rate of 25 mV s<sup>−</sup><sup>1</sup> , there are two peaks: one is the oxidation peak and the other one is the reduction peak, and the position does not change basically where the peaks appear. For example, in the case of 25°C, the pristine electrolyte's oxidation and reduction peaks appeared at 0.32 and 0.032 V, respectively. After the CO2 is being introduced, the electrolyte's oxidation peak appeared at 0.318 V and the reduction peak appeared at 0.033 V. This means that the introduction of CO2 does not affect the electrolyte composition because there is no generation of new material and it does not substantially affect the redox reversibility of the electrolyte. (The physical absorption of CO2 is also confirmed in reline DES.) On the basis of the redox peak currents, after the CO2 has been introduced, the peak current does not change significantly. In the case of 25°C, the pristine electrolyte showed the oxidation peak current density of 0.176 mA cm<sup>−</sup><sup>2</sup> , and in this case, the reduction peak

#### **Figure 1.**

*Cyclic voltammograms of 0.1 M FeCl3 in reline DES with and without 0.1 MPa CO2 at different temperatures: (a) 25°C, (b) 35°C, and (c) 45°C.*

**79**

**Figure 2.**

*equivalent circuit: (a) 25°C, (b) 35°C, and (c) 45°C.*

*Effects of Electrolyte Additives on Nonaqueous Redox Flow Batteries*

Fe(II)/Fe(III) redox couple in DES changes little when CO2 is being added in.

Electrochemical impedance spectroscopy (EIS) was carried out to further investigate how the addition of CO2 influences the electrochemical performance of Fe(III) ion in DES electrolyte. The Nyquist plots of Fe(III) in DES electrolyte at different temperatures with and without CO2 are shown in **Figure 2**. Each plot shows a similar illustration: in the high-frequency region, there is a semicircle and in the low-frequency region, there is a straight line upward. They correspond to the transfer reaction of charge at the interface of electrode/electrolyte and the diffusion of iron species in the electrolyte, respectively, and this result suggests that electrochemical reaction and diffusion steps mix-control the Fe(III)/Fe(II) redox reaction [36]. There is a distance from the crossing of the semicircle's left end and the abscissa to the origin, which is the ohmic resistance of the electrolyte, and the semicircle's diameter is the electrochemical reaction resistance of the electrolyte. In order to determine the ohmic resistance and the electrochemical reaction resistance precisely before and after adding 0.1 MPa CO2 to the reline DES which contains 0.1 M FeCl3 in case of different temperatures, the data were fitted and the straight line in **Figure 2** showed the results. **Figure 2** shows a simplified equivalent circuit; the resistance of the ion migration process in the solution is represented as *Rs*, that is to say, the ohmic resistance of the solution. *Rt* is the resistance of electrochemical reaction, the resistance of the electron transfer step. *CPE* represents

*Nyquist plots of the electrolyte with and without CO2 at different temperatures and the corresponding* 

. After introducing CO2, they became 0.178

, respectively. The result shows that the rate of redox reaction of

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

**3.2 Electrochemical impedance spectroscopy**

current density was −0.158 mA cm<sup>−</sup><sup>2</sup>

and −0.16 mA cm<sup>−</sup><sup>2</sup>

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

current density was −0.158 mA cm<sup>−</sup><sup>2</sup> . After introducing CO2, they became 0.178 and −0.16 mA cm<sup>−</sup><sup>2</sup> , respectively. The result shows that the rate of redox reaction of Fe(II)/Fe(III) redox couple in DES changes little when CO2 is being added in.

## **3.2 Electrochemical impedance spectroscopy**

*Redox*

in the electrolyte. The CVs were measured at 25, 35, and 45°C, respectively, and at each temperature the CV was tested three times. Simultaneously, in the electrochemical impedance spectroscopy measurements, the sinusoidal excitation voltage suitable for the cells was 5 mV. The frequency was in the range of 0.01 Hz–100 kHz.

The cyclic voltammograms of 0.1 M FeCl3 in reline DES at different temperatures with and without 0.1 MPa CO2 are shown in **Figure 1**. The result indicates that in the

oxidation peak and the other one is the reduction peak, and the position does not change basically where the peaks appear. For example, in the case of 25°C, the pristine electrolyte's oxidation and reduction peaks appeared at 0.32 and 0.032 V, respectively. After the CO2 is being introduced, the electrolyte's oxidation peak appeared at 0.318 V and the reduction peak appeared at 0.033 V. This means that the introduction of CO2 does not affect the electrolyte composition because there is no generation of new material and it does not substantially affect the redox reversibility of the electrolyte. (The physical absorption of CO2 is also confirmed in reline DES.) On the basis of the redox peak currents, after the CO2 has been introduced, the peak current does not change significantly. In the case of 25°C, the pristine electrolyte showed the oxida-

*Cyclic voltammograms of 0.1 M FeCl3 in reline DES with and without 0.1 MPa CO2 at different temperatures:* 

, there are two peaks: one is the

, and in this case, the reduction peak

The potential was settled at 0.15 V in order to ensure similar polarization.

**3. Effect of carbon dioxide additive**

range of −0.7–0.9 V with the scan rate of 25 mV s<sup>−</sup><sup>1</sup>

tion peak current density of 0.176 mA cm<sup>−</sup><sup>2</sup>

**3.1 Cyclic voltammetry**

**78**

**Figure 1.**

*(a) 25°C, (b) 35°C, and (c) 45°C.*

Electrochemical impedance spectroscopy (EIS) was carried out to further investigate how the addition of CO2 influences the electrochemical performance of Fe(III) ion in DES electrolyte. The Nyquist plots of Fe(III) in DES electrolyte at different temperatures with and without CO2 are shown in **Figure 2**. Each plot shows a similar illustration: in the high-frequency region, there is a semicircle and in the low-frequency region, there is a straight line upward. They correspond to the transfer reaction of charge at the interface of electrode/electrolyte and the diffusion of iron species in the electrolyte, respectively, and this result suggests that electrochemical reaction and diffusion steps mix-control the Fe(III)/Fe(II) redox reaction [36]. There is a distance from the crossing of the semicircle's left end and the abscissa to the origin, which is the ohmic resistance of the electrolyte, and the semicircle's diameter is the electrochemical reaction resistance of the electrolyte. In order to determine the ohmic resistance and the electrochemical reaction resistance precisely before and after adding 0.1 MPa CO2 to the reline DES which contains 0.1 M FeCl3 in case of different temperatures, the data were fitted and the straight line in **Figure 2** showed the results. **Figure 2** shows a simplified equivalent circuit; the resistance of the ion migration process in the solution is represented as *Rs*, that is to say, the ohmic resistance of the solution. *Rt* is the resistance of electrochemical reaction, the resistance of the electron transfer step. *CPE* represents

#### **Figure 2.**

*Nyquist plots of the electrolyte with and without CO2 at different temperatures and the corresponding equivalent circuit: (a) 25°C, (b) 35°C, and (c) 45°C.*


#### **Table 1.**

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

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

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.
