**5. Effect of antimony ions**

Inspired by the use of antimony ions as the additive in VRFBs, the same ions are tried to be the additive in DES electrolyte nonaqueous RFBs. The active materials used in negative side electrolyte are V(III)/V(II) redox ions.

#### **5.1 Cyclic voltammetry**

The cyclic voltammetry curves of negative electrolyte containing 0.1 mol L<sup>−</sup><sup>1</sup> VCl3 with different concentrations of SbCl3 are shown in **Figure 6**, and the scanning rate is 25 mV s<sup>−</sup><sup>1</sup> . It shows two obvious peaks corresponding to V(III)/V(II) redox couple. A small peak appears at the position of −0.2 V, and since there is no peak in DES, thus the small peak at −0.2 V should be caused by impurities in the raw materials. In addition, there is no new peak, which further proves that after

**83**

*Effects of Electrolyte Additives on Nonaqueous Redox Flow Batteries*

the addition of Sb3+ ions, there is no chemical reaction with V(III) ions to generate a new substance. For the pristine electrolyte, the peak current densities were 3.583

the concentration of Sb3+ ions is 15 mM. This indicates that the introduction of Sb3+ ions can accelerate the redox reaction rate of the battery and increase the collision between ions which makes it easier to overcome the activation energy and realize

The influence of SbCl3 on the electrochemical properties of negative electrolyte was further investigated by EIS. The Nyquist plots of electrolyte without additive and with different concentration of SbCl3 are shown in **Figure 7**. The combination of semicircle and the straight line upward suggests that the redox reaction of vanadium is mix-controlled by electrochemical polarization and concentration polarization [38]. **Figure 5** shows the corresponding simplified equivalent circuit, where *Ws* and *CPE* represent the concentration polarization impedance and double-layer

It can be seen that the *Rs* and *Rt* of electrolyte decrease with the addition of different concentrations of SbCl3 and reach the minimum when the concentration is 15 mM. The corresponding parameters of the equivalent circuit obtained by the Z-view simulation are shown in **Table 2**. The *Rs* and *Rt* of the solution with

electrochemical reaction resistance indicates that the charge transfer process of the electrolyte is accelerated, which reflects the higher electrochemical reaction rate. This is probably owing to the adhesion of Sb to the electrode and its catalytic effect. The increased *CPE* and *Ws* suggest that the Sb3+ in the electrolyte is able to promote the absorption and diffusion of V ions. The results further confirm that the addition

In order to investigate the influence of the addition of SbCl3 on the power density of the battery, the polarization curve of the flow battery was measured (the active material is FeCl3 in positive side electrolyte). As shown in **Figure 8**, when

15 mM SbCl3 were the smallest; they were 19.41 and 8.95 ohm cm<sup>−</sup><sup>2</sup>

lower than that of the pristine electrolyte (22.03 and 11.57 ohm m<sup>−</sup><sup>2</sup>

of Sb3+ can improve the electrochemical reaction of V(III)/V(II).

increase and reach the maximum (4.589 and −4.764 mA cm<sup>−</sup><sup>2</sup>

*Nyquist plot of 0.1 M VCl3 in DES with different concentrations of Sb3+ ions.*

, respectively. After the introduction of Sb3+ ions, both of them

, respectively) when

, respectively,

). The reduced

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

and −3.691 mA cm<sup>−</sup><sup>2</sup>

**Figure 7.**

the electrochemical reaction [37].

**5.2 Electrochemical impedance spectroscopy**

capacitance of the solution, respectively.

**Figure 6.** *Effect of Sb3+ on CV curves of 0.1 M VCl3 in DES at the scanning rate of 25 mV s<sup>−</sup><sup>1</sup> .*

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

*Redox*

**Figure 5.**

**5. Effect of antimony ions**

**5.1 Cyclic voltammetry**

ning rate is 25 mV s<sup>−</sup><sup>1</sup>

**82**

**Figure 6.**

*Effect of Sb3+ on CV curves of 0.1 M VCl3 in DES at the scanning rate of 25 mV s<sup>−</sup><sup>1</sup>*

Inspired by the use of antimony ions as the additive in VRFBs, the same ions are tried to be the additive in DES electrolyte nonaqueous RFBs. The active materials

The cyclic voltammetry curves of negative electrolyte containing 0.1 mol L<sup>−</sup><sup>1</sup> VCl3 with different concentrations of SbCl3 are shown in **Figure 6**, and the scan-

redox couple. A small peak appears at the position of −0.2 V, and since there is no peak in DES, thus the small peak at −0.2 V should be caused by impurities in the raw materials. In addition, there is no new peak, which further proves that after

. It shows two obvious peaks corresponding to V(III)/V(II)

used in negative side electrolyte are V(III)/V(II) redox ions.

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

*.*

**Figure 7.** *Nyquist plot of 0.1 M VCl3 in DES with different concentrations of Sb3+ ions.*

the addition of Sb3+ ions, there is no chemical reaction with V(III) ions to generate a new substance. For the pristine electrolyte, the peak current densities were 3.583 and −3.691 mA cm<sup>−</sup><sup>2</sup> , respectively. After the introduction of Sb3+ ions, both of them increase and reach the maximum (4.589 and −4.764 mA cm<sup>−</sup><sup>2</sup> , respectively) when the concentration of Sb3+ ions is 15 mM. This indicates that the introduction of Sb3+ ions can accelerate the redox reaction rate of the battery and increase the collision between ions which makes it easier to overcome the activation energy and realize the electrochemical reaction [37].

#### **5.2 Electrochemical impedance spectroscopy**

The influence of SbCl3 on the electrochemical properties of negative electrolyte was further investigated by EIS. The Nyquist plots of electrolyte without additive and with different concentration of SbCl3 are shown in **Figure 7**. The combination of semicircle and the straight line upward suggests that the redox reaction of vanadium is mix-controlled by electrochemical polarization and concentration polarization [38]. **Figure 5** shows the corresponding simplified equivalent circuit, where *Ws* and *CPE* represent the concentration polarization impedance and double-layer capacitance of the solution, respectively.

It can be seen that the *Rs* and *Rt* of electrolyte decrease with the addition of different concentrations of SbCl3 and reach the minimum when the concentration is 15 mM. The corresponding parameters of the equivalent circuit obtained by the Z-view simulation are shown in **Table 2**. The *Rs* and *Rt* of the solution with 15 mM SbCl3 were the smallest; they were 19.41 and 8.95 ohm cm<sup>−</sup><sup>2</sup> , respectively, lower than that of the pristine electrolyte (22.03 and 11.57 ohm m<sup>−</sup><sup>2</sup> ). The reduced electrochemical reaction resistance indicates that the charge transfer process of the electrolyte is accelerated, which reflects the higher electrochemical reaction rate. This is probably owing to the adhesion of Sb to the electrode and its catalytic effect. The increased *CPE* and *Ws* suggest that the Sb3+ in the electrolyte is able to promote the absorption and diffusion of V ions. The results further confirm that the addition of Sb3+ can improve the electrochemical reaction of V(III)/V(II).

In order to investigate the influence of the addition of SbCl3 on the power density of the battery, the polarization curve of the flow battery was measured (the active material is FeCl3 in positive side electrolyte). As shown in **Figure 8**, when

#### *Redox*


**Table 2.**

*The parameters obtained from fitting the EIS plots with the equivalent circuit for antimony ion additive.*

**Figure 8.** *Polarization curves of batteries with different concentrations of Sb3+ ions.*

the concentration of Sb3+ ions increases, the maximum current density and maximum power density increase first and then decrease, and when the concentration reached 15 mM, they reached the maximum (16.25 mA cm<sup>−</sup><sup>2</sup> and 4.04 mW cm<sup>−</sup><sup>2</sup> , respectively), higher than those of pristine electrolyte (12.75 mA cm<sup>−</sup><sup>2</sup> and 3.08 mW cm<sup>−</sup><sup>2</sup> ). From these experimental results, it can be seen that the electrochemical performance of V(III)/V(II) redox couple has been improved due to the adhesion of Sb ion to the electrode and its catalytic effect.

#### **5.3 Physicochemical measurements**

To explore the mechanism of Sb ions to improve the electrochemical performance of electrolytes, the surface morphology of graphite felts obtained after charging-discharging cycle was characterized by SEM. **Figure 9** shows that some particles adhere to the surface of graphite felt after the addition of SbCl3.

With the increased concentration of SbCl3, the particles on the surface of graphite felt increased. In order to analyze the specific composition of the observed ions, the energy-dispersive X-ray spectroscopy (EDX) was used to identify them. As shown in **Figure 10**, the transverse coordinate is the energy, and the vertical coordinate is the relative content of elements. The results of EDX show that the elements on the pristine surface of graphite felt are only C, O, V, and Cl and the particle detected after the addition of additives is Sb. Corresponding to the energy of 4.0 keV, the content of Sb on graphite felt increases gradually with the increased concentration of

**85**

**Figure 10.**

*(d) 15 mM Sb3+, and (e) 20 mM Sb3+.*

**Figure 9.**

*(d) 15 mM Sb3+, and (e) 20 mM Sb3+.*

*Effects of Electrolyte Additives on Nonaqueous Redox Flow Batteries*

*The FESEM images of graphite felt electrode after cycling: (a) pristine, (b) 5 mM Sb3+, (c) 10 mM Sb3+,* 

*EDX spectrogram of ions on the surface of graphite felts: (a) pristine, (b) 5 mM Sb3+, (c) 10 mM Sb3+,* 

*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 9.**

*Redox*

**Table 2.**

**Concentration of Sb3+** *Rs* **(ohm cm<sup>−</sup><sup>2</sup>**

**84**

**Figure 8.**

3.08 mW cm<sup>−</sup><sup>2</sup>

*Polarization curves of batteries with different concentrations of Sb3+ ions.*

reached 15 mM, they reached the maximum (16.25 mA cm<sup>−</sup><sup>2</sup>

adhesion of Sb ion to the electrode and its catalytic effect.

**5.3 Physicochemical measurements**

respectively), higher than those of pristine electrolyte (12.75 mA cm<sup>−</sup><sup>2</sup>

the concentration of Sb3+ ions increases, the maximum current density and maximum power density increase first and then decrease, and when the concentration

chemical performance of V(III)/V(II) redox couple has been improved due to the

To explore the mechanism of Sb ions to improve the electrochemical performance of electrolytes, the surface morphology of graphite felts obtained after charging-discharging cycle was characterized by SEM. **Figure 9** shows that some

With the increased concentration of SbCl3, the particles on the surface of graphite felt increased. In order to analyze the specific composition of the observed ions, the energy-dispersive X-ray spectroscopy (EDX) was used to identify them. As shown in **Figure 10**, the transverse coordinate is the energy, and the vertical coordinate is the relative content of elements. The results of EDX show that the elements on the pristine surface of graphite felt are only C, O, V, and Cl and the particle detected after the addition of additives is Sb. Corresponding to the energy of 4.0 keV, the content of Sb on graphite felt increases gradually with the increased concentration of

particles adhere to the surface of graphite felt after the addition of SbCl3.

). From these experimental results, it can be seen that the electro-

**)** *Rt* **(ohm cm<sup>−</sup><sup>2</sup>**

Pristine 22.03 11.57 7.218 × 10<sup>−</sup><sup>4</sup> 0.5338 5 mM 20.72 9.89 9.86 × 10<sup>−</sup><sup>4</sup> 0.5381 10 mM 20.67 9.50 1.01 × 10<sup>−</sup><sup>3</sup> 0.5478 15 mM 19.41 8.95 1.15 × 10<sup>−</sup><sup>3</sup> 0.5581 20 mM 21.15 9.11 1.06 × 10<sup>−</sup><sup>3</sup> 0.5492

*The parameters obtained from fitting the EIS plots with the equivalent circuit for antimony ion additive.*

**)** *CPE* **(F cm<sup>−</sup><sup>2</sup>**

**)** *Ws*

and 4.04 mW cm<sup>−</sup><sup>2</sup>

and

,

*The FESEM images of graphite felt electrode after cycling: (a) pristine, (b) 5 mM Sb3+, (c) 10 mM Sb3+, (d) 15 mM Sb3+, and (e) 20 mM Sb3+.*

#### **Figure 10.**

*EDX spectrogram of ions on the surface of graphite felts: (a) pristine, (b) 5 mM Sb3+, (c) 10 mM Sb3+, (d) 15 mM Sb3+, and (e) 20 mM Sb3+.*

SbCl3. The results suggest that the enhancement of the battery performance is owing to the catalytic effect of Sb ions. However, when the concentration was 20 mM, the accumulation of ions is more serious, which would result in partial pore blockage of graphite felt. Therefore, when the concentration of Sb3+ ions further increases, the electrochemical performance of the battery decreases slightly.
