# A Comprehensive Study on Hydroxyl Multiwalled Carbon Nanotubes Used as Catalysts for VO2+/VO2+ Reaction in Vanadium Redox Flow Battery

Authors: Qiang Li; Anyu Bai; Zeqiang Qu; Tianyu Zhang; Jie Li; Xiaochen Zhang; Mingfu Yu; Zhichao Xue; Hong Sun
Journal: Journal of Chemistry
 (2019)

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## Abstract

A comprehensive study on the hydroxyl multiwalled carbon nanotubes (hydroxyl MWCNTs) as catalysts in a positive reaction was performed to improve the efficiency of the vanadium redox flow battery (VRFB). The physicochemical properties of the hydroxyl MWCNT-modified electrode were characterized by using a scanning electron microscope (SEM), conductivity measurement, Brunner–Emmet–Teller (BET) measurement, X-ray photoelectron spectroscopy (XPS) analysis, cyclic voltammetry (CV) tests, electrochemical impedance spectroscopy (EIS) analysis, and charge-discharge tests. The prepared composite electrode possesses a huge amount of oxygen-containing groups, high-specific surface area, high electrical conductivity, and high catalytic activity towards the VO2+/VO2+ reaction based on physicochemical characterization. The hydroxyl MWCNT-modified graphite felt (hydroxyl MWCNTs/GF) shows the best cell performance with the energy efficiency of 79.74% and remains in high stability after 50 cycles. The improved cell performance is probably ascribed to the increase in active sites, fast charge transfer, and mass transfer rate of the introduced hydroxyl MWCNTs.


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## Body

## 1. Introduction

With the gradual depletion of fossil energy, people around the world pay more and more attention to the development and utilization of renewable energy resources. However, renewable energy power generation, such as wind power and solar energy, has obvious discontinuities, instability, and uncontrollability. Therefore, the development and the use of large-scale and high-efficiency energy storage technologies are the basis for future changes in the energy system [1–5]. As one of the promising approaches for energy storage, VRFB has drawn great attention since it possesses advanced characteristics such as flexible battery assembly design, long cycle life, large battery capacity, low self-discharge, and no cross contamination [6–12].In VRFB, carbonaceous materials have been widely used as electrode materials for the catalysis of the electrode reaction. GF is the most popular electrode in VRFB [13–15]. However, GF shows poor kinetic reversibility and electrochemical activity toward electrode reactions. Hence, most investigations are focused on the modification of GF to improve its electrochemical performance. Various approaches including thermal treatment, acid oxidation, ammoniated treatment, and nanoparticles modification of GF have been reported to form more active sites for the improvement of cell performance [16–19].Carbon nanotubes (CNTs) or functionalized CNTs have been proved to be an excellent catalyst in batteries due to its superior electrical conductivity, mechanical strength, and chemical stability. Furthermore, the surface of CNTs is inherently more active than other graphitic variants and has faster electron transfer rate. Some literatures have focused on the investigation of CNTs as catalysts for VRFB electrodes. The application of single-walled CNTs used as catalysts in VRFB was firstly reported by Li et al. [20]. The kinetic properties of V2+/V3+ and VO2+/VO2+ redox couples are improved by employing the graphite/CNT composite as electrodes. Moreover, functionalized CNTs exhibit higher electronic conductivity and thermal/chemical stability. Hence, more interests have been focused on the functionalized CNTs due to their unique physicochemical properties. Various efforts have been paid on the modification of CNTs for VRFB applications, including KOH activation [21], acid treatment [22], nitrogen and sulfur doping [23, 24], and combination with nanosheets or nanoparticles [25–27]. Among various kinds of catalysts, hydroxyl MWCNTs exhibit remarkable electrochemical performance. Li et al. investigated the MWCNTs functionalized with hydroxyl groups for the VO2+/VO2+ redox reaction [28]. Compared with the pristine MWCNTs, the hydroxyl MWCNTs used as positive-electrode reaction catalysts exhibit better storage efficiency. However, the catalytic mechanism of the hydroxyl modified MWCNTs for the VO2+/VO2+ redox reaction still remains unclear. A comprehensive study on physiochemical characteristic variation of electrodes after the modification of hydroxyl MWCNTs should be further investigated to reveal the catalytic mechanism of hydroxyl MWCNTs.In this paper, hydroxyl MWCNTs were used as electrode catalysts, while pristine MWCNTs were used as a comparison. The characteristics of the prepared hydroxyl MWCNTs/GF were tested by using SEM, four-point probes, BET method, and XPS. The electrochemical performance of the hydroxyl MWCNT-modified electrodes for VO2+/VO2+ redox couple was analyzed by carrying out the tests of CV, EIS, and charge-discharge.


## 2. Experimental

### 2.1. Reagents and Materials

All chemical reagents which were of analytical reagent grade were directly used without purification. Deionized water was used to wash the modified GF and prepare the electrolyte. Sulfuric acid with the concentration of 98 wt% was used as the supporting electrolyte. Nafion 117 membrane, GF with a thickness of 5 mm, and graphite plates were used for the assembling of the cell unit. The MWCNTs and hydroxyl MWCNTs were purchased from Beijing DK nanotechnology Co. Ltd.


### 2.2. Preparation

#### 2.2.1. Immobilization of MWCNTs and Hydroxyl MWCNTs on GCE

A glassy carbon electrode (GCE, diameter 3 mm) was firstly polished with aqueous slurries of alumina powders on a polishing microcloth. Then, the electrode was rinsed by deionized water and ultrasonic processed in ethanol for 3 times. The suspension containing catalysts was prepared by ultrasonically mixing 10 mL dimethyl formamide with 10 mg MWCNTs and 10 mg hydroxyl MWCNTs, respectively. The decorated GCE was achieved by dropping 20μL prepared suspension onto the surface of GCE, followed by evaporation in an oven at 45°C for 8 h. The GCE without modification was used as a comparison.


#### 2.2.2. Preparation of MWCNTs/GF and Hydroxyl MWCNTs/GF

Prior to the modification of the GF electrode, a GF (3 cm × 3 cm) was washed by ultrasonication for 30 min in deionized water and then dried in an oven at 100°C for 4 h. The purified GF was immersed in the prepared suspension as mentioned in Section2.2.1. Then, the modified GF was purified by deionized water and dried in an oven at 100°C for 24 h. The composite electrode with the modification of hydroxyl MWCNTs was also prepared as stated above.


### 2.3. Characterization of Modified Electrode

#### 2.3.1. Material Characterization

The surface morphology and the specific surface area of the prepared electrode were measured by SEM (Hitachi, S4800) and BET method (Micromeritics TriStar II 3020), respectively. The conductivity of the prepared electrode was measured by four-point probes (RTS-8, Four Probes Tech). The nitrogen doping level and doping type in the modified composite electrode were evaluated by XPS (Thermo Fisher Scientific, EscaLab 250 XI system).


#### 2.3.2. Electrochemical Tests

The CV results were obtained by PARSTAT 4000+ workstation over the voltage range from −0.2 V to 1.6 V. The EIS tests were operated by applying an alternating voltage of 5 mV over the frequency range of 0.01 Hz to 100 kHz. A three-electrode cell was employed to characterize the electrochemical performance of hydroxyl MWCNT-modified electrode. The three-electrode cell was equipped with the hydroxyl MWCNT-modified glassy carbon electrode (hydroxyl MWCNTs/GCE) as a working electrode, a Pt electrode as a counter electrode, and a saturated calomel electrode as a reference electrode. The electrolyte used in the cell was 0.1 M VOSO4 + 2 M H2SO4 aqueous solution.Charge-discharge tests were conducted by using an assembly home-made static battery test system. The prepared hydroxyl MWCNT-modified electrode was used as a positive electrode in charge-discharge tests. The anolyte for VRFB was 1 M V3+ + 4.2 M H2SO4, and the catholyte was 1 M VO2+ + 4.2 M H2SO4. The upper and lower voltage limits for the charge-discharge test were 1.65 V and 0.8 V, respectively. The durability of the prepared hydroxyl MWCNTs/GF electrode was evaluated by 50 cycles of charge-discharge tests.


## 2.1. Reagents and Materials

All chemical reagents which were of analytical reagent grade were directly used without purification. Deionized water was used to wash the modified GF and prepare the electrolyte. Sulfuric acid with the concentration of 98 wt% was used as the supporting electrolyte. Nafion 117 membrane, GF with a thickness of 5 mm, and graphite plates were used for the assembling of the cell unit. The MWCNTs and hydroxyl MWCNTs were purchased from Beijing DK nanotechnology Co. Ltd.


## 2.2. Preparation

### 2.2.1. Immobilization of MWCNTs and Hydroxyl MWCNTs on GCE

A glassy carbon electrode (GCE, diameter 3 mm) was firstly polished with aqueous slurries of alumina powders on a polishing microcloth. Then, the electrode was rinsed by deionized water and ultrasonic processed in ethanol for 3 times. The suspension containing catalysts was prepared by ultrasonically mixing 10 mL dimethyl formamide with 10 mg MWCNTs and 10 mg hydroxyl MWCNTs, respectively. The decorated GCE was achieved by dropping 20μL prepared suspension onto the surface of GCE, followed by evaporation in an oven at 45°C for 8 h. The GCE without modification was used as a comparison.


### 2.2.2. Preparation of MWCNTs/GF and Hydroxyl MWCNTs/GF

Prior to the modification of the GF electrode, a GF (3 cm × 3 cm) was washed by ultrasonication for 30 min in deionized water and then dried in an oven at 100°C for 4 h. The purified GF was immersed in the prepared suspension as mentioned in Section2.2.1. Then, the modified GF was purified by deionized water and dried in an oven at 100°C for 24 h. The composite electrode with the modification of hydroxyl MWCNTs was also prepared as stated above.


## 2.2.1. Immobilization of MWCNTs and Hydroxyl MWCNTs on GCE

A glassy carbon electrode (GCE, diameter 3 mm) was firstly polished with aqueous slurries of alumina powders on a polishing microcloth. Then, the electrode was rinsed by deionized water and ultrasonic processed in ethanol for 3 times. The suspension containing catalysts was prepared by ultrasonically mixing 10 mL dimethyl formamide with 10 mg MWCNTs and 10 mg hydroxyl MWCNTs, respectively. The decorated GCE was achieved by dropping 20μL prepared suspension onto the surface of GCE, followed by evaporation in an oven at 45°C for 8 h. The GCE without modification was used as a comparison.


## 2.2.2. Preparation of MWCNTs/GF and Hydroxyl MWCNTs/GF

Prior to the modification of the GF electrode, a GF (3 cm × 3 cm) was washed by ultrasonication for 30 min in deionized water and then dried in an oven at 100°C for 4 h. The purified GF was immersed in the prepared suspension as mentioned in Section2.2.1. Then, the modified GF was purified by deionized water and dried in an oven at 100°C for 24 h. The composite electrode with the modification of hydroxyl MWCNTs was also prepared as stated above.


## 2.3. Characterization of Modified Electrode

### 2.3.1. Material Characterization

The surface morphology and the specific surface area of the prepared electrode were measured by SEM (Hitachi, S4800) and BET method (Micromeritics TriStar II 3020), respectively. The conductivity of the prepared electrode was measured by four-point probes (RTS-8, Four Probes Tech). The nitrogen doping level and doping type in the modified composite electrode were evaluated by XPS (Thermo Fisher Scientific, EscaLab 250 XI system).


### 2.3.2. Electrochemical Tests

The CV results were obtained by PARSTAT 4000+ workstation over the voltage range from −0.2 V to 1.6 V. The EIS tests were operated by applying an alternating voltage of 5 mV over the frequency range of 0.01 Hz to 100 kHz. A three-electrode cell was employed to characterize the electrochemical performance of hydroxyl MWCNT-modified electrode. The three-electrode cell was equipped with the hydroxyl MWCNT-modified glassy carbon electrode (hydroxyl MWCNTs/GCE) as a working electrode, a Pt electrode as a counter electrode, and a saturated calomel electrode as a reference electrode. The electrolyte used in the cell was 0.1 M VOSO4 + 2 M H2SO4 aqueous solution.Charge-discharge tests were conducted by using an assembly home-made static battery test system. The prepared hydroxyl MWCNT-modified electrode was used as a positive electrode in charge-discharge tests. The anolyte for VRFB was 1 M V3+ + 4.2 M H2SO4, and the catholyte was 1 M VO2+ + 4.2 M H2SO4. The upper and lower voltage limits for the charge-discharge test were 1.65 V and 0.8 V, respectively. The durability of the prepared hydroxyl MWCNTs/GF electrode was evaluated by 50 cycles of charge-discharge tests.


## 2.3.1. Material Characterization

The surface morphology and the specific surface area of the prepared electrode were measured by SEM (Hitachi, S4800) and BET method (Micromeritics TriStar II 3020), respectively. The conductivity of the prepared electrode was measured by four-point probes (RTS-8, Four Probes Tech). The nitrogen doping level and doping type in the modified composite electrode were evaluated by XPS (Thermo Fisher Scientific, EscaLab 250 XI system).


## 2.3.2. Electrochemical Tests

The CV results were obtained by PARSTAT 4000+ workstation over the voltage range from −0.2 V to 1.6 V. The EIS tests were operated by applying an alternating voltage of 5 mV over the frequency range of 0.01 Hz to 100 kHz. A three-electrode cell was employed to characterize the electrochemical performance of hydroxyl MWCNT-modified electrode. The three-electrode cell was equipped with the hydroxyl MWCNT-modified glassy carbon electrode (hydroxyl MWCNTs/GCE) as a working electrode, a Pt electrode as a counter electrode, and a saturated calomel electrode as a reference electrode. The electrolyte used in the cell was 0.1 M VOSO4 + 2 M H2SO4 aqueous solution.Charge-discharge tests were conducted by using an assembly home-made static battery test system. The prepared hydroxyl MWCNT-modified electrode was used as a positive electrode in charge-discharge tests. The anolyte for VRFB was 1 M V3+ + 4.2 M H2SO4, and the catholyte was 1 M VO2+ + 4.2 M H2SO4. The upper and lower voltage limits for the charge-discharge test were 1.65 V and 0.8 V, respectively. The durability of the prepared hydroxyl MWCNTs/GF electrode was evaluated by 50 cycles of charge-discharge tests.


## 3. Results and Discussion

### 3.1. Material Property Characterization

#### 3.1.1. SEM Tests

The morphology of pristine GF and hydroxyl MWCNTs/GF was characterized by using SEM. As shown in Figure1, the surface of pristine GF is smooth. After treatment, the surface of GF is covered by attachments, indicating that the hydroxyl MWCNTs have been successfully attached to GF.Figure 1
SEM images of pristine GF and (a) hydroxyl MWCNTs/GF (b).
(a)
(b)


#### 3.1.2. Conductivity Measurement

The conductivities of pristine GF, MWCNTs/GF, and hydroxyl MWCNTs/GF were measured by four-point probes (RTS-8, Four Probes Tech). The results are listed in Table1. It is obvious that the conductivity of GF is significantly increased due to the modification of MWCNTs. Compared to MWCNTs/GF, the conductivity of hydroxyl MWCNTs/GF has a slightly lower value of 54.58 S/cm, suggesting that the modification of hydroxyl MWCNTs does not lead to an obvious decrease in conductivity. The increase in the conductivity of electrode is owing to the modification of hydroxyl MWCNTs which is a benefit for electron transfer during the electrochemical reaction, resulting in a decrease in ohmic polarization.Table 1
Resistivity and conductivity of different samples.
Measured samples
Conductivity (S/cm)
Pristine GF
47.62
MWCNTs/GF
62.50
Hydroxyl MWCNTs/GF
66.67


#### 3.1.3. Specific Surface Area Test

The specific surface area of electrode has a significant influence on cell performance since electrode with high specific area attracts more vanadium ions and provides more active sites for the electrochemical reaction. The influence of hydroxyl MWCNTs modification on the specific surface area of GF was evaluated by the BET method. The nitrogen adsorption-desorption isotherms of GF, MWCNTs/GF, and hydroxyl MWCNTs/GF are presented in Figure2. It is clear that there is a typical IV isotherm for all samples, indicating the presence of the porous structure. The measured specific surface areas of GF, MWCNTs/GF, and hydroxyl MWCNTs/GF were 0.86 m2/g, 6.19 m2/g, and 11.10 m2/g, respectively. The results indicate that the specific surface area of the prepared electrode is significantly increased due to the decoration of hydroxyl MWCNTs. The specific area of hydroxyl MWCNTs/GF is higher than that of MWCNTs/GF. The reason may due to the fact that the hydroxyl groups in hydroxyl MWCNTs are a benefit for the adsorption of hydroxyl MWCNTs on the GF surface, resulting in a large number of hydroxyl MWCNTs adsorbed on the GF surface during the immersion process.Figure 2
Nitrogen adsorption-desorption isotherms.(1) XPS Test. In order to compare the main element contents on the surface of pristine GF, MWCNTs/GF, and hydroxyl MWCNTs/GF, XPS tests were performed in the binding energy range of 0–1350 eV. Figure 3 is the scanning spectra of pristine GF, MWCNTs/GF, and hydroxyl MWCNTs/GF. There are two peaks located at 284 eV and 532 eV, corresponding to C and O elements, respectively. The content of C, O, and N elements calculated from XPS scan data is listed in Table 2. As shown in Table 2, the content of O element increases from 2.17% to 13.02% after the decoration of hydroxyl MWCNTs, suggesting that the oxygen-containing groups have been successfully decorated onto the GF surface. Figure 3(f) is the C1s plot of the hydroxyl MWCNTs/GF that can be fitted by convolving into four peaks, namely, C=C (284.5 eV), C-C (286.2 eV), C-O (287.5 eV), and C=O (287.9 eV). Comparing Figure 3(f) with Figure 3(d), it can be seen that the content of the C-O bond in the hydroxyl MWCNTs/GF is significantly improved. The content of the C-O bond in the hydroxyl MWCNTs/GF is 16.2%, which is much higher than that in MWCNTs/GF (4.60%) and pristine GF (3.64%). The results indicate that the hydroxyl MWCNTs/GF has more oxygen-containing groups, especially the C-O bond containing groups, which show high catalytic activity toward the VO2+/VO2+ reaction.Figure 3
XPS spectra and curve fit of C1s spectra from pristine GF (a, b), MWCNTs/GF (c, d), and hydroxyl MWCNTs/GF (e, f).
(a)
(b)
(c)
(d)
(e)
(f)Table 2
Content of C, O, and N elements in hydroxyl MWCNTs/GF, MWCNTs/GF, and pristine GF.
Tested samples
Carbon content (%)
Oxygen content (%)
Nitrogen content (%)
Pristine GF
97.31
2.17
0.52
MWCNTs/GF
95.83
3.56
0.61
Hydroxyl MWCNTs/GF
86.25
13.02
0.74


### 3.2. Electrochemical Performance

#### 3.2.1. Cyclic Voltammetry Test

Cyclic voltammetry tests were carried out in 0.1 M VOSO4 + 2 M H2SO4 electrolyte from −0.2 V to 1.6 V. The CV curves of GCE, MWCNTs/GCE, and hydroxyl MWCNTs/GCE at different scan rates are shown in Figures 4(a)–4(c). It is obvious that only the oxidation peak of the VO2+/VO2+ redox reaction is observed when the pristine GCE is used as a working electrode, indicating that an irreversible VO2+/VO2+ redox reaction is taken place on the pristine GCE. After the modification of MWCNTs, the electrode has symmetric oxidation and reduction peaks. Furthermore, the reduction peak becomes obvious and the oxidation peak increases, as shown in Figure 4(b). For hydroxyl MWCNTs/GCE, the peak potential separation is almost unchanged with increasing scan rate, meaning that the VO2+/VO2+ redox reaction on the hydroxyl MWCNTs/GCE surface is a quasireversible reaction. Moreover, it can be seen that the redox peak currents of hydroxyl MWCNTs/GCE are the highest among these samples, while the potential separation is the lowest, indicating that the electrode modified with hydroxyl MWCNTs has the best performance.Figure 4
Cyclic voltammetry curves of GCE (a), MWCNTs/GCE (b), and hydroxyl MWCNTs/GCE (c) under different scan rates in 0.1 M VOSO4 + 2 M H2SO4 and the relationship between oxidation peak currents and square root of scan rate (d).
(a)
(b)
(c)
(d)Since the mass transfer plays an important role on cell performance, the mass transfer properties of active species in the modified electrodes were evaluated by plotting the oxidation peak currents versus the square root of scan rate, as shown in Figure4(d). The peak currents are approximately proportional to the square root of scan rate for all tested samples, indicating that the reactions in all electrodes are mass transfer controlled. In Figure 4(d), the slope of the fitted line positively represents the mass transfer velocity of active species. It should be noted that the slope for the hydroxyl MWCNTs/GCE is the largest, indicating that the hydroxyl MWCNTs potentially facilitate the diffusion of active species, which is a benefit for the decrease in concentration polarization. The reason may be due to the introduction of hydroxyl MWCNTs which show large numbers of oxygen-containing groups, resulting in a high hydrophilicity and high catalytic activity of the modified electrode.The CV tests of different electrodes at a scan rate of 20 mV/s are shown in Figure5. By comparing the voltammetric behavior of different electrodes, the results show that the electrochemical activity of GCE is significantly enhanced after the modification of hydroxyl MWCNTs.Figure 5
CV curves of different electrodes at a constant scan rate of 20 mV/s.The peak currents and peak potential separation (ΔEp) values are listed in Table 3. The electrode modified with hydroxyl MWCNTs show the best catalytic activity with the oxidation peak current Ipa of 64.34 μA and the reduction peak current Ipc of −53.61 μA, respectively. In comparison to the MWCNTs/GCE, the hydroxyl MWCNTs/GCE has abundant hydroxyl groups, indicating that the hydroxyl MWCNTs are electrochemically more accessible for electrode reactions, resulting in an improvement in VRFB performance. Furthermore, ΔEp decreases after the modification with hydroxyl MWCNTs, meaning that the modification of hydroxyl MWCNTs is also a benefit for the improvement of electrochemical reversibility. Compared with MWCNTs/GCE, hydroxyl MWCNTs/GCE delivers better electrochemical reversibility toward the positive redox couple. The results indicate that plenty of hydroxyl groups attached on hydroxyl MWCNTs can efficiently facilitate the positive-reaction rate and decrease the electrochemical polarization for the VO2+/VO2+ reaction.Table 3
Parameters obtained from CV tests with different catalysts at a scan rate of 20 mV/s.
Samples
Peak potential separation (ΔEp (mV))
Reduction peak current (Ipc (μA))
Oxidation peak current (Ipa (μA))
GCE
1100.13
−15.31
30.45
MWCNTs/GCE
153.43
−38.22
45.53
Hydroxyl MWCNTs/GCE
81.52
−53.61
64.34


#### 3.2.2. EIS Test

The electrochemical impedance spectroscopy tests were carried out to further investigate the effect of hydroxyl MWCNTs on the positive VO2+/VO2+ reaction. The corresponding Nyquist plot is shown in Figure 6. For cells with hydroxyl MWCNT-decorated electrode, the Nyquist plot is composed of a semicircle and a linear part in the frequency range from 0.01 Hz to 100 kHz. The semicircle part at high frequency represents the charge transfer process, and the linear part at low frequency reflects the mass diffusion process during the electrochemical reaction. The results demonstrate that the VO2+/VO2+ couple reaction is both controlled by charge transfer and mass transfer. The spectra can be fitted by an equivalent circuit. In the equivalent circuit, R1 is the ohmic resistance and R2 represents the charge transfer resistance across the electrode/electrolyte interface. Q is the constant phase element, reflecting the electric double-layer capacitance of the electrode/electrolyte interface. W is the Warburg impedance, which is relevant to vanadium ion adsorption and diffusion in electrodes.Figure 6
EIS spectra of cells with different electrodes and fitted equivalent circuit.The parameters obtained from the equivalent circuit are listed in Table4. A significant decrease in R2 is observed, indicating that the charge transfer resistance at the electrode/electrolyte interface decreases after the decoration of MWCNTs and hydroxyl MWCNTs. The hydroxyl MWCNTs/GF shows the lowest charge transfer resistance because the oxygen-containing groups on hydroxyl MWCNTs decrease the electrochemical polarization and facilitate the charge transfer across the solution/electrode interface, thus confirming the beneficial role of hydroxyl MWCNTs. According to Table 4, the value of Q increases after the treatment of GF. The increase in the Q value indicates that the modified electrodes are more adsorptive for vanadium ions, resulting in a decrease in mass transfer resistance. The hydroxyl MWCNTs/GF shows the largest Q value because the relatively high negative charge density of oxygen-containing groups in hydroxyl MWCNTs potentially accelerate the adsorption for vanadium ions by electrostatic force and facilitate the mass transfer of active species [29, 30]. The Warburg coefficient for different samples is depicted in Table 4. The hydroxyl MWCNTs/GF shows the largest value of the Warburg coefficient, implying that the active species diffusion is fast in hydroxyl MWCNTs/GF. Both EIS and CV results show that reaction kinetics of VO2+/VO2+ on the surface of the hydroxyl MWCNTs/GF electrode is faster than that of other electrodes.Table 4
Fitted parameters resulting from the equivalent circuit model.
Different electrodes
R
1 (Ω·cm2)
Q (CPE)
R
2 (Ω·cm2)
W (S·s5·cm2)
Q/Y⋅(S·s−n)−1
N (0 < n < 1)
Pristine GF
0.40
1.19 × 10−3
0.74
1.26
0.11
MWCNTs/GF
0.27
2.34 × 10−3
0.86
0.16
0.32
Hydroxyl MWCNTs/GF
0.23
4.53 × 10−3
0.94
0.14
0.44


#### 3.2.3. Charge-Discharge Test

For a further understanding on the influence of MWCNTs on the performance of VRFB, the charge-discharge tests were conducted by cells equipped with different electrodes, as shown in Figure7. All cells were charged to the upper voltage of 1.65 V and discharged to the lower voltage of 0.8 V at a constant current density of 80 mA/cm2. The thickness of GF was 5 mm, and the flow velocity of electrolyte was 30 ml/min. As observed, the charge-discharge curves reveal that the cell employing hydroxyl MWCNTs as catalysts shows the best cell performance, resulting in a lowest charge voltage and a largest discharge voltage. Compared to the cell equipped with the pristine electrode, the battery capacities of cells equipped with MWCNTs and hydroxyl MWCNT-modified electrodes increase by 18.71% and 27.60%, respectively. The significant increase in capacity is attributed to the high specific surface, the high conductivity, and high catalytic activity of hydroxyl MWCNTs/GF electrode, leading to a better cell performance.Figure 7
Charge-discharge curves of different electrodes.The current efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of cells with different electrodes are shown in Table5. The cell with hydroxyl MWCNT-modified electrode exhibits substantially improved VE of 83.05% and EE of 79.74%, respectively. The difference in EE reflects the difference in the catalytic activity and mass transfer property of electrode materials. The improved performance for hydroxyl MWCNTs/GF may be ascribed to the high conductivity and high specific surface area of the modified hydroxyl MWCNTs. Furthermore, the introduction of oxygen-containing groups in hydroxyl MWCNTs significantly decrease the charge transfer resistance between electrolyte/electrode interface while increase the mass diffusion rate of vanadium ions during the electrochemical reaction.Table 5
Efficiency of cells equipped with different electrodes.
Different electrodes
Current efficiency (CE) (%)
Voltage efficiency (VE) (%)
Energy efficiency (EE) (%)
Pristine GF
96.21
75.87
72.99
MWCNTs/GF
95.92
81.32
78.01
Hydroxyl MWCNTs/GF
96.01
83.05
79.74


#### 3.2.4. Cyclic Stability Test

Cyclic stability tests were carried out at a constant current density of 80 mA/cm2 to evaluate the durability of hydroxyl MWCNT catalysts. The cycling performance including CE, EE, and discharge retention is shown in Figure 8. It is obvious that the CE of two cells remains nearly stable during the whole cyclic stability test. The CEs for two cells are higher than 95%, demonstrating good airtightness of the battery setup. As for EE, the VRFB with hydroxyl MWCNTs/GF shows much higher EE than the GF-based VRFB. The EE of VRFB with GF decreases from 74.61% to 71.45%, while the EE of VRFB with hydroxyl MWCNTs/GF remains nearly stable with the value of 79.88%. Consequently, the initial discharge capacity of VRFB with hydroxyl MWCNTs/GF is higher than that of GF-based VRFB, as shown in Figure 8(b). The discharge capacity of VRFB with hydroxyl MWCNTs/GF is 736.20 mAh at the 1th cycle compared to 543.70 mAh of GF-based VRFB, while the values decrease to 556.8 mAh and 344.80 mAh at the 50th cycle, respectively. The stable EE and the slow capacity decay demonstrate the excellent cycling stability of hydroxyl MWCNT catalysts under VRFB operation conditions, which is ascribed to the enhanced electrochemical catalytic activity and reversibility of the hydroxyl MWCNTs/GF electrode.Figure 8
Variation of cell efficiency (a) and discharge capacity (b) after 50 cycles of charge-discharge tests for cells equipped with GF and hydroxyl MWCNTs/GF electrode.
(a)
(b)Figure9 shows the energy efficiency variation of pristine GF and hydroxyl MWCNTs/GF-based single cells operated at different current densities. The current density was ranged from 80 mA/cm2 to 150 mA/cm2. It can be noted that the energy efficiencies of pristine GF and hydroxyl MWCNTs/GF-based cells are decreased with the current density increasing due to the increase in overvoltage. The energy efficiency of hydroxyl MWCNTs/GF-based cell decreases from 81.67% to 73.42% with the current density ranging from 80 mA/cm2 to 150 mA/cm2. Compared with the cell with pristine GF, the cell equipped with hydroxyl MWCNTs/GF shows higher efficiencies at different current densities. The higher energy efficiency of the hydroxyl MWCNTs/GF-based cell is attributed to the enhanced conductivity and increased reactive sites of hydroxyl MWCNTs/GF electrode, resulting in improved overall cell performance [31, 32].Figure 9
Variation of the EE of pristine GF and hydroxyl MWCNTs/GF-based single cells operating at the current density range of 80 mA/cm2 to 150 mA/cm2.


## 3.1. Material Property Characterization

### 3.1.1. SEM Tests

The morphology of pristine GF and hydroxyl MWCNTs/GF was characterized by using SEM. As shown in Figure1, the surface of pristine GF is smooth. After treatment, the surface of GF is covered by attachments, indicating that the hydroxyl MWCNTs have been successfully attached to GF.Figure 1
SEM images of pristine GF and (a) hydroxyl MWCNTs/GF (b).
(a)
(b)


### 3.1.2. Conductivity Measurement

The conductivities of pristine GF, MWCNTs/GF, and hydroxyl MWCNTs/GF were measured by four-point probes (RTS-8, Four Probes Tech). The results are listed in Table1. It is obvious that the conductivity of GF is significantly increased due to the modification of MWCNTs. Compared to MWCNTs/GF, the conductivity of hydroxyl MWCNTs/GF has a slightly lower value of 54.58 S/cm, suggesting that the modification of hydroxyl MWCNTs does not lead to an obvious decrease in conductivity. The increase in the conductivity of electrode is owing to the modification of hydroxyl MWCNTs which is a benefit for electron transfer during the electrochemical reaction, resulting in a decrease in ohmic polarization.Table 1
Resistivity and conductivity of different samples.
Measured samples
Conductivity (S/cm)
Pristine GF
47.62
MWCNTs/GF
62.50
Hydroxyl MWCNTs/GF
66.67


### 3.1.3. Specific Surface Area Test

The specific surface area of electrode has a significant influence on cell performance since electrode with high specific area attracts more vanadium ions and provides more active sites for the electrochemical reaction. The influence of hydroxyl MWCNTs modification on the specific surface area of GF was evaluated by the BET method. The nitrogen adsorption-desorption isotherms of GF, MWCNTs/GF, and hydroxyl MWCNTs/GF are presented in Figure2. It is clear that there is a typical IV isotherm for all samples, indicating the presence of the porous structure. The measured specific surface areas of GF, MWCNTs/GF, and hydroxyl MWCNTs/GF were 0.86 m2/g, 6.19 m2/g, and 11.10 m2/g, respectively. The results indicate that the specific surface area of the prepared electrode is significantly increased due to the decoration of hydroxyl MWCNTs. The specific area of hydroxyl MWCNTs/GF is higher than that of MWCNTs/GF. The reason may due to the fact that the hydroxyl groups in hydroxyl MWCNTs are a benefit for the adsorption of hydroxyl MWCNTs on the GF surface, resulting in a large number of hydroxyl MWCNTs adsorbed on the GF surface during the immersion process.Figure 2
Nitrogen adsorption-desorption isotherms.(1) XPS Test. In order to compare the main element contents on the surface of pristine GF, MWCNTs/GF, and hydroxyl MWCNTs/GF, XPS tests were performed in the binding energy range of 0–1350 eV. Figure 3 is the scanning spectra of pristine GF, MWCNTs/GF, and hydroxyl MWCNTs/GF. There are two peaks located at 284 eV and 532 eV, corresponding to C and O elements, respectively. The content of C, O, and N elements calculated from XPS scan data is listed in Table 2. As shown in Table 2, the content of O element increases from 2.17% to 13.02% after the decoration of hydroxyl MWCNTs, suggesting that the oxygen-containing groups have been successfully decorated onto the GF surface. Figure 3(f) is the C1s plot of the hydroxyl MWCNTs/GF that can be fitted by convolving into four peaks, namely, C=C (284.5 eV), C-C (286.2 eV), C-O (287.5 eV), and C=O (287.9 eV). Comparing Figure 3(f) with Figure 3(d), it can be seen that the content of the C-O bond in the hydroxyl MWCNTs/GF is significantly improved. The content of the C-O bond in the hydroxyl MWCNTs/GF is 16.2%, which is much higher than that in MWCNTs/GF (4.60%) and pristine GF (3.64%). The results indicate that the hydroxyl MWCNTs/GF has more oxygen-containing groups, especially the C-O bond containing groups, which show high catalytic activity toward the VO2+/VO2+ reaction.Figure 3
XPS spectra and curve fit of C1s spectra from pristine GF (a, b), MWCNTs/GF (c, d), and hydroxyl MWCNTs/GF (e, f).
(a)
(b)
(c)
(d)
(e)
(f)Table 2
Content of C, O, and N elements in hydroxyl MWCNTs/GF, MWCNTs/GF, and pristine GF.
Tested samples
Carbon content (%)
Oxygen content (%)
Nitrogen content (%)
Pristine GF
97.31
2.17
0.52
MWCNTs/GF
95.83
3.56
0.61
Hydroxyl MWCNTs/GF
86.25
13.02
0.74


## 3.1.1. SEM Tests

The morphology of pristine GF and hydroxyl MWCNTs/GF was characterized by using SEM. As shown in Figure1, the surface of pristine GF is smooth. After treatment, the surface of GF is covered by attachments, indicating that the hydroxyl MWCNTs have been successfully attached to GF.Figure 1
SEM images of pristine GF and (a) hydroxyl MWCNTs/GF (b).
(a)
(b)


## 3.1.2. Conductivity Measurement

The conductivities of pristine GF, MWCNTs/GF, and hydroxyl MWCNTs/GF were measured by four-point probes (RTS-8, Four Probes Tech). The results are listed in Table1. It is obvious that the conductivity of GF is significantly increased due to the modification of MWCNTs. Compared to MWCNTs/GF, the conductivity of hydroxyl MWCNTs/GF has a slightly lower value of 54.58 S/cm, suggesting that the modification of hydroxyl MWCNTs does not lead to an obvious decrease in conductivity. The increase in the conductivity of electrode is owing to the modification of hydroxyl MWCNTs which is a benefit for electron transfer during the electrochemical reaction, resulting in a decrease in ohmic polarization.Table 1
Resistivity and conductivity of different samples.
Measured samples
Conductivity (S/cm)
Pristine GF
47.62
MWCNTs/GF
62.50
Hydroxyl MWCNTs/GF
66.67


## 3.1.3. Specific Surface Area Test

The specific surface area of electrode has a significant influence on cell performance since electrode with high specific area attracts more vanadium ions and provides more active sites for the electrochemical reaction. The influence of hydroxyl MWCNTs modification on the specific surface area of GF was evaluated by the BET method. The nitrogen adsorption-desorption isotherms of GF, MWCNTs/GF, and hydroxyl MWCNTs/GF are presented in Figure2. It is clear that there is a typical IV isotherm for all samples, indicating the presence of the porous structure. The measured specific surface areas of GF, MWCNTs/GF, and hydroxyl MWCNTs/GF were 0.86 m2/g, 6.19 m2/g, and 11.10 m2/g, respectively. The results indicate that the specific surface area of the prepared electrode is significantly increased due to the decoration of hydroxyl MWCNTs. The specific area of hydroxyl MWCNTs/GF is higher than that of MWCNTs/GF. The reason may due to the fact that the hydroxyl groups in hydroxyl MWCNTs are a benefit for the adsorption of hydroxyl MWCNTs on the GF surface, resulting in a large number of hydroxyl MWCNTs adsorbed on the GF surface during the immersion process.Figure 2
Nitrogen adsorption-desorption isotherms.(1) XPS Test. In order to compare the main element contents on the surface of pristine GF, MWCNTs/GF, and hydroxyl MWCNTs/GF, XPS tests were performed in the binding energy range of 0–1350 eV. Figure 3 is the scanning spectra of pristine GF, MWCNTs/GF, and hydroxyl MWCNTs/GF. There are two peaks located at 284 eV and 532 eV, corresponding to C and O elements, respectively. The content of C, O, and N elements calculated from XPS scan data is listed in Table 2. As shown in Table 2, the content of O element increases from 2.17% to 13.02% after the decoration of hydroxyl MWCNTs, suggesting that the oxygen-containing groups have been successfully decorated onto the GF surface. Figure 3(f) is the C1s plot of the hydroxyl MWCNTs/GF that can be fitted by convolving into four peaks, namely, C=C (284.5 eV), C-C (286.2 eV), C-O (287.5 eV), and C=O (287.9 eV). Comparing Figure 3(f) with Figure 3(d), it can be seen that the content of the C-O bond in the hydroxyl MWCNTs/GF is significantly improved. The content of the C-O bond in the hydroxyl MWCNTs/GF is 16.2%, which is much higher than that in MWCNTs/GF (4.60%) and pristine GF (3.64%). The results indicate that the hydroxyl MWCNTs/GF has more oxygen-containing groups, especially the C-O bond containing groups, which show high catalytic activity toward the VO2+/VO2+ reaction.Figure 3
XPS spectra and curve fit of C1s spectra from pristine GF (a, b), MWCNTs/GF (c, d), and hydroxyl MWCNTs/GF (e, f).
(a)
(b)
(c)
(d)
(e)
(f)Table 2
Content of C, O, and N elements in hydroxyl MWCNTs/GF, MWCNTs/GF, and pristine GF.
Tested samples
Carbon content (%)
Oxygen content (%)
Nitrogen content (%)
Pristine GF
97.31
2.17
0.52
MWCNTs/GF
95.83
3.56
0.61
Hydroxyl MWCNTs/GF
86.25
13.02
0.74


## 3.2. Electrochemical Performance

### 3.2.1. Cyclic Voltammetry Test

Cyclic voltammetry tests were carried out in 0.1 M VOSO4 + 2 M H2SO4 electrolyte from −0.2 V to 1.6 V. The CV curves of GCE, MWCNTs/GCE, and hydroxyl MWCNTs/GCE at different scan rates are shown in Figures 4(a)–4(c). It is obvious that only the oxidation peak of the VO2+/VO2+ redox reaction is observed when the pristine GCE is used as a working electrode, indicating that an irreversible VO2+/VO2+ redox reaction is taken place on the pristine GCE. After the modification of MWCNTs, the electrode has symmetric oxidation and reduction peaks. Furthermore, the reduction peak becomes obvious and the oxidation peak increases, as shown in Figure 4(b). For hydroxyl MWCNTs/GCE, the peak potential separation is almost unchanged with increasing scan rate, meaning that the VO2+/VO2+ redox reaction on the hydroxyl MWCNTs/GCE surface is a quasireversible reaction. Moreover, it can be seen that the redox peak currents of hydroxyl MWCNTs/GCE are the highest among these samples, while the potential separation is the lowest, indicating that the electrode modified with hydroxyl MWCNTs has the best performance.Figure 4
Cyclic voltammetry curves of GCE (a), MWCNTs/GCE (b), and hydroxyl MWCNTs/GCE (c) under different scan rates in 0.1 M VOSO4 + 2 M H2SO4 and the relationship between oxidation peak currents and square root of scan rate (d).
(a)
(b)
(c)
(d)Since the mass transfer plays an important role on cell performance, the mass transfer properties of active species in the modified electrodes were evaluated by plotting the oxidation peak currents versus the square root of scan rate, as shown in Figure4(d). The peak currents are approximately proportional to the square root of scan rate for all tested samples, indicating that the reactions in all electrodes are mass transfer controlled. In Figure 4(d), the slope of the fitted line positively represents the mass transfer velocity of active species. It should be noted that the slope for the hydroxyl MWCNTs/GCE is the largest, indicating that the hydroxyl MWCNTs potentially facilitate the diffusion of active species, which is a benefit for the decrease in concentration polarization. The reason may be due to the introduction of hydroxyl MWCNTs which show large numbers of oxygen-containing groups, resulting in a high hydrophilicity and high catalytic activity of the modified electrode.The CV tests of different electrodes at a scan rate of 20 mV/s are shown in Figure5. By comparing the voltammetric behavior of different electrodes, the results show that the electrochemical activity of GCE is significantly enhanced after the modification of hydroxyl MWCNTs.Figure 5
CV curves of different electrodes at a constant scan rate of 20 mV/s.The peak currents and peak potential separation (ΔEp) values are listed in Table 3. The electrode modified with hydroxyl MWCNTs show the best catalytic activity with the oxidation peak current Ipa of 64.34 μA and the reduction peak current Ipc of −53.61 μA, respectively. In comparison to the MWCNTs/GCE, the hydroxyl MWCNTs/GCE has abundant hydroxyl groups, indicating that the hydroxyl MWCNTs are electrochemically more accessible for electrode reactions, resulting in an improvement in VRFB performance. Furthermore, ΔEp decreases after the modification with hydroxyl MWCNTs, meaning that the modification of hydroxyl MWCNTs is also a benefit for the improvement of electrochemical reversibility. Compared with MWCNTs/GCE, hydroxyl MWCNTs/GCE delivers better electrochemical reversibility toward the positive redox couple. The results indicate that plenty of hydroxyl groups attached on hydroxyl MWCNTs can efficiently facilitate the positive-reaction rate and decrease the electrochemical polarization for the VO2+/VO2+ reaction.Table 3
Parameters obtained from CV tests with different catalysts at a scan rate of 20 mV/s.
Samples
Peak potential separation (ΔEp (mV))
Reduction peak current (Ipc (μA))
Oxidation peak current (Ipa (μA))
GCE
1100.13
−15.31
30.45
MWCNTs/GCE
153.43
−38.22
45.53
Hydroxyl MWCNTs/GCE
81.52
−53.61
64.34


### 3.2.2. EIS Test

The electrochemical impedance spectroscopy tests were carried out to further investigate the effect of hydroxyl MWCNTs on the positive VO2+/VO2+ reaction. The corresponding Nyquist plot is shown in Figure 6. For cells with hydroxyl MWCNT-decorated electrode, the Nyquist plot is composed of a semicircle and a linear part in the frequency range from 0.01 Hz to 100 kHz. The semicircle part at high frequency represents the charge transfer process, and the linear part at low frequency reflects the mass diffusion process during the electrochemical reaction. The results demonstrate that the VO2+/VO2+ couple reaction is both controlled by charge transfer and mass transfer. The spectra can be fitted by an equivalent circuit. In the equivalent circuit, R1 is the ohmic resistance and R2 represents the charge transfer resistance across the electrode/electrolyte interface. Q is the constant phase element, reflecting the electric double-layer capacitance of the electrode/electrolyte interface. W is the Warburg impedance, which is relevant to vanadium ion adsorption and diffusion in electrodes.Figure 6
EIS spectra of cells with different electrodes and fitted equivalent circuit.The parameters obtained from the equivalent circuit are listed in Table4. A significant decrease in R2 is observed, indicating that the charge transfer resistance at the electrode/electrolyte interface decreases after the decoration of MWCNTs and hydroxyl MWCNTs. The hydroxyl MWCNTs/GF shows the lowest charge transfer resistance because the oxygen-containing groups on hydroxyl MWCNTs decrease the electrochemical polarization and facilitate the charge transfer across the solution/electrode interface, thus confirming the beneficial role of hydroxyl MWCNTs. According to Table 4, the value of Q increases after the treatment of GF. The increase in the Q value indicates that the modified electrodes are more adsorptive for vanadium ions, resulting in a decrease in mass transfer resistance. The hydroxyl MWCNTs/GF shows the largest Q value because the relatively high negative charge density of oxygen-containing groups in hydroxyl MWCNTs potentially accelerate the adsorption for vanadium ions by electrostatic force and facilitate the mass transfer of active species [29, 30]. The Warburg coefficient for different samples is depicted in Table 4. The hydroxyl MWCNTs/GF shows the largest value of the Warburg coefficient, implying that the active species diffusion is fast in hydroxyl MWCNTs/GF. Both EIS and CV results show that reaction kinetics of VO2+/VO2+ on the surface of the hydroxyl MWCNTs/GF electrode is faster than that of other electrodes.Table 4
Fitted parameters resulting from the equivalent circuit model.
Different electrodes
R
1 (Ω·cm2)
Q (CPE)
R
2 (Ω·cm2)
W (S·s5·cm2)
Q/Y⋅(S·s−n)−1
N (0 < n < 1)
Pristine GF
0.40
1.19 × 10−3
0.74
1.26
0.11
MWCNTs/GF
0.27
2.34 × 10−3
0.86
0.16
0.32
Hydroxyl MWCNTs/GF
0.23
4.53 × 10−3
0.94
0.14
0.44


### 3.2.3. Charge-Discharge Test

For a further understanding on the influence of MWCNTs on the performance of VRFB, the charge-discharge tests were conducted by cells equipped with different electrodes, as shown in Figure7. All cells were charged to the upper voltage of 1.65 V and discharged to the lower voltage of 0.8 V at a constant current density of 80 mA/cm2. The thickness of GF was 5 mm, and the flow velocity of electrolyte was 30 ml/min. As observed, the charge-discharge curves reveal that the cell employing hydroxyl MWCNTs as catalysts shows the best cell performance, resulting in a lowest charge voltage and a largest discharge voltage. Compared to the cell equipped with the pristine electrode, the battery capacities of cells equipped with MWCNTs and hydroxyl MWCNT-modified electrodes increase by 18.71% and 27.60%, respectively. The significant increase in capacity is attributed to the high specific surface, the high conductivity, and high catalytic activity of hydroxyl MWCNTs/GF electrode, leading to a better cell performance.Figure 7
Charge-discharge curves of different electrodes.The current efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of cells with different electrodes are shown in Table5. The cell with hydroxyl MWCNT-modified electrode exhibits substantially improved VE of 83.05% and EE of 79.74%, respectively. The difference in EE reflects the difference in the catalytic activity and mass transfer property of electrode materials. The improved performance for hydroxyl MWCNTs/GF may be ascribed to the high conductivity and high specific surface area of the modified hydroxyl MWCNTs. Furthermore, the introduction of oxygen-containing groups in hydroxyl MWCNTs significantly decrease the charge transfer resistance between electrolyte/electrode interface while increase the mass diffusion rate of vanadium ions during the electrochemical reaction.Table 5
Efficiency of cells equipped with different electrodes.
Different electrodes
Current efficiency (CE) (%)
Voltage efficiency (VE) (%)
Energy efficiency (EE) (%)
Pristine GF
96.21
75.87
72.99
MWCNTs/GF
95.92
81.32
78.01
Hydroxyl MWCNTs/GF
96.01
83.05
79.74


### 3.2.4. Cyclic Stability Test

Cyclic stability tests were carried out at a constant current density of 80 mA/cm2 to evaluate the durability of hydroxyl MWCNT catalysts. The cycling performance including CE, EE, and discharge retention is shown in Figure 8. It is obvious that the CE of two cells remains nearly stable during the whole cyclic stability test. The CEs for two cells are higher than 95%, demonstrating good airtightness of the battery setup. As for EE, the VRFB with hydroxyl MWCNTs/GF shows much higher EE than the GF-based VRFB. The EE of VRFB with GF decreases from 74.61% to 71.45%, while the EE of VRFB with hydroxyl MWCNTs/GF remains nearly stable with the value of 79.88%. Consequently, the initial discharge capacity of VRFB with hydroxyl MWCNTs/GF is higher than that of GF-based VRFB, as shown in Figure 8(b). The discharge capacity of VRFB with hydroxyl MWCNTs/GF is 736.20 mAh at the 1th cycle compared to 543.70 mAh of GF-based VRFB, while the values decrease to 556.8 mAh and 344.80 mAh at the 50th cycle, respectively. The stable EE and the slow capacity decay demonstrate the excellent cycling stability of hydroxyl MWCNT catalysts under VRFB operation conditions, which is ascribed to the enhanced electrochemical catalytic activity and reversibility of the hydroxyl MWCNTs/GF electrode.Figure 8
Variation of cell efficiency (a) and discharge capacity (b) after 50 cycles of charge-discharge tests for cells equipped with GF and hydroxyl MWCNTs/GF electrode.
(a)
(b)Figure9 shows the energy efficiency variation of pristine GF and hydroxyl MWCNTs/GF-based single cells operated at different current densities. The current density was ranged from 80 mA/cm2 to 150 mA/cm2. It can be noted that the energy efficiencies of pristine GF and hydroxyl MWCNTs/GF-based cells are decreased with the current density increasing due to the increase in overvoltage. The energy efficiency of hydroxyl MWCNTs/GF-based cell decreases from 81.67% to 73.42% with the current density ranging from 80 mA/cm2 to 150 mA/cm2. Compared with the cell with pristine GF, the cell equipped with hydroxyl MWCNTs/GF shows higher efficiencies at different current densities. The higher energy efficiency of the hydroxyl MWCNTs/GF-based cell is attributed to the enhanced conductivity and increased reactive sites of hydroxyl MWCNTs/GF electrode, resulting in improved overall cell performance [31, 32].Figure 9
Variation of the EE of pristine GF and hydroxyl MWCNTs/GF-based single cells operating at the current density range of 80 mA/cm2 to 150 mA/cm2.


## 3.2.1. Cyclic Voltammetry Test

Cyclic voltammetry tests were carried out in 0.1 M VOSO4 + 2 M H2SO4 electrolyte from −0.2 V to 1.6 V. The CV curves of GCE, MWCNTs/GCE, and hydroxyl MWCNTs/GCE at different scan rates are shown in Figures 4(a)–4(c). It is obvious that only the oxidation peak of the VO2+/VO2+ redox reaction is observed when the pristine GCE is used as a working electrode, indicating that an irreversible VO2+/VO2+ redox reaction is taken place on the pristine GCE. After the modification of MWCNTs, the electrode has symmetric oxidation and reduction peaks. Furthermore, the reduction peak becomes obvious and the oxidation peak increases, as shown in Figure 4(b). For hydroxyl MWCNTs/GCE, the peak potential separation is almost unchanged with increasing scan rate, meaning that the VO2+/VO2+ redox reaction on the hydroxyl MWCNTs/GCE surface is a quasireversible reaction. Moreover, it can be seen that the redox peak currents of hydroxyl MWCNTs/GCE are the highest among these samples, while the potential separation is the lowest, indicating that the electrode modified with hydroxyl MWCNTs has the best performance.Figure 4
Cyclic voltammetry curves of GCE (a), MWCNTs/GCE (b), and hydroxyl MWCNTs/GCE (c) under different scan rates in 0.1 M VOSO4 + 2 M H2SO4 and the relationship between oxidation peak currents and square root of scan rate (d).
(a)
(b)
(c)
(d)Since the mass transfer plays an important role on cell performance, the mass transfer properties of active species in the modified electrodes were evaluated by plotting the oxidation peak currents versus the square root of scan rate, as shown in Figure4(d). The peak currents are approximately proportional to the square root of scan rate for all tested samples, indicating that the reactions in all electrodes are mass transfer controlled. In Figure 4(d), the slope of the fitted line positively represents the mass transfer velocity of active species. It should be noted that the slope for the hydroxyl MWCNTs/GCE is the largest, indicating that the hydroxyl MWCNTs potentially facilitate the diffusion of active species, which is a benefit for the decrease in concentration polarization. The reason may be due to the introduction of hydroxyl MWCNTs which show large numbers of oxygen-containing groups, resulting in a high hydrophilicity and high catalytic activity of the modified electrode.The CV tests of different electrodes at a scan rate of 20 mV/s are shown in Figure5. By comparing the voltammetric behavior of different electrodes, the results show that the electrochemical activity of GCE is significantly enhanced after the modification of hydroxyl MWCNTs.Figure 5
CV curves of different electrodes at a constant scan rate of 20 mV/s.The peak currents and peak potential separation (ΔEp) values are listed in Table 3. The electrode modified with hydroxyl MWCNTs show the best catalytic activity with the oxidation peak current Ipa of 64.34 μA and the reduction peak current Ipc of −53.61 μA, respectively. In comparison to the MWCNTs/GCE, the hydroxyl MWCNTs/GCE has abundant hydroxyl groups, indicating that the hydroxyl MWCNTs are electrochemically more accessible for electrode reactions, resulting in an improvement in VRFB performance. Furthermore, ΔEp decreases after the modification with hydroxyl MWCNTs, meaning that the modification of hydroxyl MWCNTs is also a benefit for the improvement of electrochemical reversibility. Compared with MWCNTs/GCE, hydroxyl MWCNTs/GCE delivers better electrochemical reversibility toward the positive redox couple. The results indicate that plenty of hydroxyl groups attached on hydroxyl MWCNTs can efficiently facilitate the positive-reaction rate and decrease the electrochemical polarization for the VO2+/VO2+ reaction.Table 3
Parameters obtained from CV tests with different catalysts at a scan rate of 20 mV/s.
Samples
Peak potential separation (ΔEp (mV))
Reduction peak current (Ipc (μA))
Oxidation peak current (Ipa (μA))
GCE
1100.13
−15.31
30.45
MWCNTs/GCE
153.43
−38.22
45.53
Hydroxyl MWCNTs/GCE
81.52
−53.61
64.34


## 3.2.2. EIS Test

The electrochemical impedance spectroscopy tests were carried out to further investigate the effect of hydroxyl MWCNTs on the positive VO2+/VO2+ reaction. The corresponding Nyquist plot is shown in Figure 6. For cells with hydroxyl MWCNT-decorated electrode, the Nyquist plot is composed of a semicircle and a linear part in the frequency range from 0.01 Hz to 100 kHz. The semicircle part at high frequency represents the charge transfer process, and the linear part at low frequency reflects the mass diffusion process during the electrochemical reaction. The results demonstrate that the VO2+/VO2+ couple reaction is both controlled by charge transfer and mass transfer. The spectra can be fitted by an equivalent circuit. In the equivalent circuit, R1 is the ohmic resistance and R2 represents the charge transfer resistance across the electrode/electrolyte interface. Q is the constant phase element, reflecting the electric double-layer capacitance of the electrode/electrolyte interface. W is the Warburg impedance, which is relevant to vanadium ion adsorption and diffusion in electrodes.Figure 6
EIS spectra of cells with different electrodes and fitted equivalent circuit.The parameters obtained from the equivalent circuit are listed in Table4. A significant decrease in R2 is observed, indicating that the charge transfer resistance at the electrode/electrolyte interface decreases after the decoration of MWCNTs and hydroxyl MWCNTs. The hydroxyl MWCNTs/GF shows the lowest charge transfer resistance because the oxygen-containing groups on hydroxyl MWCNTs decrease the electrochemical polarization and facilitate the charge transfer across the solution/electrode interface, thus confirming the beneficial role of hydroxyl MWCNTs. According to Table 4, the value of Q increases after the treatment of GF. The increase in the Q value indicates that the modified electrodes are more adsorptive for vanadium ions, resulting in a decrease in mass transfer resistance. The hydroxyl MWCNTs/GF shows the largest Q value because the relatively high negative charge density of oxygen-containing groups in hydroxyl MWCNTs potentially accelerate the adsorption for vanadium ions by electrostatic force and facilitate the mass transfer of active species [29, 30]. The Warburg coefficient for different samples is depicted in Table 4. The hydroxyl MWCNTs/GF shows the largest value of the Warburg coefficient, implying that the active species diffusion is fast in hydroxyl MWCNTs/GF. Both EIS and CV results show that reaction kinetics of VO2+/VO2+ on the surface of the hydroxyl MWCNTs/GF electrode is faster than that of other electrodes.Table 4
Fitted parameters resulting from the equivalent circuit model.
Different electrodes
R
1 (Ω·cm2)
Q (CPE)
R
2 (Ω·cm2)
W (S·s5·cm2)
Q/Y⋅(S·s−n)−1
N (0 < n < 1)
Pristine GF
0.40
1.19 × 10−3
0.74
1.26
0.11
MWCNTs/GF
0.27
2.34 × 10−3
0.86
0.16
0.32
Hydroxyl MWCNTs/GF
0.23
4.53 × 10−3
0.94
0.14
0.44


## 3.2.3. Charge-Discharge Test

For a further understanding on the influence of MWCNTs on the performance of VRFB, the charge-discharge tests were conducted by cells equipped with different electrodes, as shown in Figure7. All cells were charged to the upper voltage of 1.65 V and discharged to the lower voltage of 0.8 V at a constant current density of 80 mA/cm2. The thickness of GF was 5 mm, and the flow velocity of electrolyte was 30 ml/min. As observed, the charge-discharge curves reveal that the cell employing hydroxyl MWCNTs as catalysts shows the best cell performance, resulting in a lowest charge voltage and a largest discharge voltage. Compared to the cell equipped with the pristine electrode, the battery capacities of cells equipped with MWCNTs and hydroxyl MWCNT-modified electrodes increase by 18.71% and 27.60%, respectively. The significant increase in capacity is attributed to the high specific surface, the high conductivity, and high catalytic activity of hydroxyl MWCNTs/GF electrode, leading to a better cell performance.Figure 7
Charge-discharge curves of different electrodes.The current efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of cells with different electrodes are shown in Table5. The cell with hydroxyl MWCNT-modified electrode exhibits substantially improved VE of 83.05% and EE of 79.74%, respectively. The difference in EE reflects the difference in the catalytic activity and mass transfer property of electrode materials. The improved performance for hydroxyl MWCNTs/GF may be ascribed to the high conductivity and high specific surface area of the modified hydroxyl MWCNTs. Furthermore, the introduction of oxygen-containing groups in hydroxyl MWCNTs significantly decrease the charge transfer resistance between electrolyte/electrode interface while increase the mass diffusion rate of vanadium ions during the electrochemical reaction.Table 5
Efficiency of cells equipped with different electrodes.
Different electrodes
Current efficiency (CE) (%)
Voltage efficiency (VE) (%)
Energy efficiency (EE) (%)
Pristine GF
96.21
75.87
72.99
MWCNTs/GF
95.92
81.32
78.01
Hydroxyl MWCNTs/GF
96.01
83.05
79.74


## 3.2.4. Cyclic Stability Test

Cyclic stability tests were carried out at a constant current density of 80 mA/cm2 to evaluate the durability of hydroxyl MWCNT catalysts. The cycling performance including CE, EE, and discharge retention is shown in Figure 8. It is obvious that the CE of two cells remains nearly stable during the whole cyclic stability test. The CEs for two cells are higher than 95%, demonstrating good airtightness of the battery setup. As for EE, the VRFB with hydroxyl MWCNTs/GF shows much higher EE than the GF-based VRFB. The EE of VRFB with GF decreases from 74.61% to 71.45%, while the EE of VRFB with hydroxyl MWCNTs/GF remains nearly stable with the value of 79.88%. Consequently, the initial discharge capacity of VRFB with hydroxyl MWCNTs/GF is higher than that of GF-based VRFB, as shown in Figure 8(b). The discharge capacity of VRFB with hydroxyl MWCNTs/GF is 736.20 mAh at the 1th cycle compared to 543.70 mAh of GF-based VRFB, while the values decrease to 556.8 mAh and 344.80 mAh at the 50th cycle, respectively. The stable EE and the slow capacity decay demonstrate the excellent cycling stability of hydroxyl MWCNT catalysts under VRFB operation conditions, which is ascribed to the enhanced electrochemical catalytic activity and reversibility of the hydroxyl MWCNTs/GF electrode.Figure 8
Variation of cell efficiency (a) and discharge capacity (b) after 50 cycles of charge-discharge tests for cells equipped with GF and hydroxyl MWCNTs/GF electrode.
(a)
(b)Figure9 shows the energy efficiency variation of pristine GF and hydroxyl MWCNTs/GF-based single cells operated at different current densities. The current density was ranged from 80 mA/cm2 to 150 mA/cm2. It can be noted that the energy efficiencies of pristine GF and hydroxyl MWCNTs/GF-based cells are decreased with the current density increasing due to the increase in overvoltage. The energy efficiency of hydroxyl MWCNTs/GF-based cell decreases from 81.67% to 73.42% with the current density ranging from 80 mA/cm2 to 150 mA/cm2. Compared with the cell with pristine GF, the cell equipped with hydroxyl MWCNTs/GF shows higher efficiencies at different current densities. The higher energy efficiency of the hydroxyl MWCNTs/GF-based cell is attributed to the enhanced conductivity and increased reactive sites of hydroxyl MWCNTs/GF electrode, resulting in improved overall cell performance [31, 32].Figure 9
Variation of the EE of pristine GF and hydroxyl MWCNTs/GF-based single cells operating at the current density range of 80 mA/cm2 to 150 mA/cm2.


## 4. Conclusions

Based on the experimental results, VRFB assembled with hydroxyl MWCNTs/GF show significantly improved electrochemical performance. The results support that the hydroxyl MWCNTs modified on GF reduce the electrochemical polarization of the cell, decrease the ohmic and mass transfer resistance, and improve the electrochemical reversibility. The enhanced performance for hydroxyl MWCNTs/GF-based battery is not only related to the introduction of oxygen-containing groups on the hydroxyl MWCNTs surface but also due to the high conductivity and high specific surface area of the hydroxyl MWCNTs. The investigation demonstrates that the newly developed hydroxyl MWCNT-modified GF holds great promise in the application of VRFB.



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*Source: 3258342-2019-11-30.xml*