Synergistic Catalysis of SnO2-CNTs Composite for VO2+/VO2+ and V2+/V3+ Redox Reactions

In this study, a SnO2-carbon nanotube (SnO2-CNT) composite as a catalyst for vanadium redox flow battery (VRFB) was prepared using a sol-gel method. The effects of this composite on the electrochemical performance of VO2+/VO2+, and on the V2+/V3+ redox reactions and VRFB performance were investigated. The SnO2-CNT composite has better catalytic activity than pure SnO2 and CNT due to the synergistic catalysis of SnO2 and the CNT. SnO2 mainly provides the catalytic active sites and the CNTs mainly provide the three-dimensional structure and high electrical conductivity. Therefore, the SnO2-CNT composite has a larger specific surface area and an excellent synergistic catalytic performance. For cell performance, it was found that the SnO2-CNT cell shows a greater discharge capacity and energy efficiency. In particular, at 150 mA cm−2, the discharge capacity of the SnO2-CNT cell is 28.6 mAh higher than that of the pristine cell. The energy efficiency of the modified cell (7%) is 7.2% higher than that of the pristine cell (62.8%). This study shows that the SnO2-CNT is an efficient and promising catalyst for VRFB.


INTRODUCTION
With the rapid development of society, humanity is consuming increasingly more natural energy from finite resources, such as coal and natural gas Wang T. et al., 2020;. Furthermore, environmental pollution and energy shortages are becoming exacerbated (Cheng C. et al., 2020;Chuanchang et al., 2020;Huang et al., 2020a;Yang et al., 2020). Therefore, new energy development technologies have received widespread attention (Huang et al., 2020b;Liu et al., 2021;Nie et al., 2021). However, new resources, such as solar and tidal energies, cannot provide a continuous source of energy. It is therefore necessary to develop a large-scale electric energy storage system to integrate renewable energy (Kou et al., 2020;. Vanadium redox flow batteries (VRFBs) have the merits of flexible design, short response time, and long cycle life, resulting in them becoming popular large-scale energy storage devices (Wu et al., 2016;Ye et al., 2020;Jiang et al., 2021a).
VRFBs are mainly composed of electrolytes, ion exchange membranes, and electrodes (Jiang et al., 2021b). The positive and negative electrolytes are composed of VO + 2 /VO 2+ and V 2+ /V 3+ solutions, respectively (Cheng D. et al., 2020). The proton exchange membrane prevents electrolyte cross-contamination and proton transfer. The electrode is where the electrochemical reaction takes place and is therefore an important part of a VRFB (Lv et al., 2021). Carbon-based materials [carbon paper, carbon felt, graphite felt (GF), and so on] are popularly used as electrode materials for VRFB due to their good conductivity, large specific surface area, and low cost. However, their poor electrochemical activity limits the potential improvement in electrode performance (Yu et al., 2019).
In recent years, researchers have made excellent progress in improving the electrochemical performance of electrodes, including the introduction of catalysts. Currently, catalysts primarily include metals, metal oxides, and carbon-based materials. Regarding metal catalysts [e.g., Cu (Wei et al., 2016), Ir (Wang et al., 2007) and Sb (Kou et al., 2020)], Zhou et al. (2020) used semi-embedded carbon felt with bismuth nanospheres, which had good catalytic activity and could effectively promote the redox reaction of the negative electrode. For metal oxides [e.g., Nd 2 O 3 (Fetyan et al., 2018), MnO 2 (Ma et al., 2018), and PbO 2 (Wu et al., 2014)], Bayeh et al. (2020) modified GF with cubic CeO 2 nanowires and used it as a catalyst for VRFB. This resulted in abundant defects on the electrode surface and increased active sites, meaning that the CeO 2 nanowires could significantly improve the VO + 2 /VO 2+ redox process. Regarding carbon-based materials [e.g., carbon nanosheets , carbon nanofibers , and graphene (Etesami et al., 2018)], Park et al. (2013) demonstrated that carbon nanofiber/nanotube composite catalysts had good electrocatalytic performance in VRFB. Compared with the untreated electrode, the discharge capacity of the modified electrode increased by 64% at 40 mA cm −2 . This excellent performance is attributed to the increase in electron transfer rate by the modification treatment. The methods mentioned earlier accelerated the kinetics of redox reactions.
SnO 2 is an amphoteric oxide with wide bandgap semiconductor properties and has been widely used in the field of electrocatalysis (Liu et al., 2016;Zhang et al., 2020). For example, Qiu et al. (2020) introduced SnO 2 nanoparticles into carbon foam through an electrodeposition method as an anode for K-ion battery. This promoted electrolyte permeation and the transfer of K ions, resulting in an excellent cycle stability of the modified cell. Also, Mehboob et al. (2018) reported the electrocatalytic effect of SnO 2 in VRFB. Carbon felt was modified with nano-SnO 2 through a hydrothermal method. The stable and efficient performance of SnO 2 was proven and it had excellent catalytic performance, although its conductivity was poor.
To solve this problem, a SnO 2 -carbon nanotube (SnO 2 -CNT) composite was prepared in this study using a solgel method. This composite had better catalytic activity than SnO 2 and CNT separately due to its synergistic advantages. SnO 2 primarily provided the catalytic active sites while the CNT provided the three-dimensional (3D) structure and high electrical conductivity. As a result, this SnO 2 -CNT composite had a large specific surface area and excellent synergistic catalytic properties, thereby having an excellent electrocatalytic performance as a bifunctional catalyst in VRFBs.

Preparation of Materials
H 2 SO 4 (98% purity) was purchased from China Beijing Chemical Reagent Co., Ltd. (Beijing, China). N, N-dimethylformamide (DMF, 99.5% purity) was purchased from Tianjin Yongda Chemical Reagent Co., Ltd. (Tianjin, China). CNT (300 mg) was dipped in concentrated sulfuric acid at 80 • C for 8 h. After washing and filtering until neutral, the CNTs were put in an oven to dry for 24 h and were ground for later use. For the preparation of the active CNT, SnCl 2 ·2H 2 O was first added to 20 ml of anhydrous ethanol, stirred at 80 • C for 2 h, and aged for 24 h. Finally, the precursor of the tin sol was obtained. Activated CNT (SnCl 2 ·2H 2 O and CNT at a molar mass of 1:13, respectively) and sol were mixed ultrasonically for 40 min and dried at 60 • C. The mixture was heated at 500 • C for 2 h in a nitrogen atmosphere in a tubular furnace. To obtain the SnO 2 -CNT composite, 149.7 mg of SnCl 2 ·2H 2 O were added to 50 ml of anhydrous ethanol, stirred, and aged. Then, the precursor of tin sol was prepared. After drying, it was put into a tubular furnace and heated with nitrogen. This process was used to obtain SnO 2 .

Characterization
The crystallographic phase of the sample was studied using X-ray diffraction (XRD) using a D8 Advance A25 Instrument (Bruker, Berlin, Germany). The morphology of the material was investigated using a scanning electron microscopy (SEM, JSM-IT100, JEOL, Japan). An X-ray photoelectron spectroscopy (XPS) was carried out using a Thermo Scientific (Waltham, MA, USA) ESCALAB 250Xi (Xi+) instrument.

Electrochemical Measurements
An electrochemical test was performed with an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Co., Ltd., Beijing, China) using a three-electrode system. The glassy carbon electrode was the working electrode. The counter and reference electrodes were Pt and saturated calomel electrodes, respectively. The 10 mg sample was added to 5 ml of DMF and then sonicated for 3 h to make it uniformly mixed. A volume of 20 µl of the dispersion was transferred to the syringe, with 1-2 µl of the dispersion was added each time to the working electrode. The electrode was then dried for 4 h at indoor temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were carried out in a 1.6 M VO 2+ + 3 M H 2 SO 4 solution for the positive reaction and a 1.6 M V 3+ + 3 M H 2 SO 4 solution for the negative reaction. The positive voltage range was.2 to 1.5 V and the negative voltage range was −0.8 to −0.1 V. EIS was conducted with a frequency range of 10 −1 to 10 6 Hz. Polarization voltages of 0.85 and −0.45 V were used for the positive and negative redox reactions, respectively.

Charge-Discharge Tests
To compare the cells' performance, modified and pristine cells were assembled. Charge-discharge tests were carried out by the battery test system (CT2001A, Wuhan, China). The voltage window was 0.7-1.65 V. The SnO 2 -CNT was used to modify the negative and positive electrodes of the modified cell. Original GF was used for the negative and positive electrodes of the pristine cell. The two poles of the cell were separated by a membrane. To prepare the modified GF, 10 mg of the SnO 2 -CNT was added to 10 mL of DMF. The GF (3 × 3 cm 2 ) was soaked in this solution, ultrasonically dispersed for 3 h, and then dried in an oven. The original and modified GFs were soaked in an electrolyte of 0.8 M  V 3+ + 0.8 M VO 2+ + 3 M H 2 SO 4 in advance to completely assimilate the electrolyte. To balance the electrolytes, the cell was first charge-discharge for three cycles at a current density of 10 mA cm −2 . Five charge-discharge tests were then performed at each current density of 50, 75, 100, 125, and 150 mA cm −2 .

RESULTS AND DISCUSSION
SEM characterization was used to research the morphology of SnO 2 and SnO 2 -CNT. It can be seen from Figures 1a,b that the samples are all nanoscale. SnO 2 exhibits a certain agglomeration but this is not obvious for the SnO 2 -CNT. XRD was used to study the crystal structure of the samples. As seen in Figure 1c, the CNTs have characteristic peaks at 26 • and 42 • , which prove that their purity is high. The peaks observed for SnO 2 are consistent with the standard values (Nie et al., 2021) and there are no characteristic impurity peaks. The observed peaks of the SnO 2 -CNTs correspond with No. 00-041-1445. It can be seen that the peak of the composite material includes both pure CNTs and SnO 2 peaks, indicating that the sample is a SnO 2 -CNTs composite material. The composite presents a long tubular morphology and SnO 2 is attached to the CNTs, as shown in Figure 1d. Such a structure can provide a larger specific surface area, thus enhancing the catalytic performance. The composite combines the advantages of the two materials and greatly improves the electrochemical catalytic performance.
XPS was used to investigate the elemental composition of the SnO 2 -CNTs, as shown in Figure 2A. The sample has obvious peaks corresponding to C, Sn, and O, indicating that it mainly contains three elements. Figures 2B-D show the fitting results for each element. As seen in Figure 2B, the spectrum of C 1s is divided into four peaks at 284.8, 286.5, 289, and 291.2 eV, corresponding to the functional groups of C-C, C-O, C=O, and C=O-O, respectively (Tang et al., 2015). As seen in Figure 2C, Sn 3d 5/2 and Sn 3d 3/2 constitute the Sn 3d spectrum of the SnO 2 -CNTs. The peaks at 487.6 and 496 eV are assigned to Sn 3d 5/2 and Sn 3d 3/2 , respectively (Tang et al., 2015). The splitting energy of the two peaks is 8.4 eV, indicating that the Sn element exists in the form of Sn 4+ . Figure 2D displays the O 1s spectrum of the composite. The three peaks of the O 1s spectrum are assigned to the Sn-O (531.5 eV), O-H (532.9 eV), and C=O (534 eV) functional groups (Cheng et al., 2017). Figure 3A demonstrates the CV curves of the different electrodes. The order of electrochemical kinetics for the VO + 2 /VO 2+ redox reaction is SnO 2 -CNTs > CNTs > SnO 2 . The redox peak current of the CNTs is higher than that of SnO 2 due to the poor electrical conductivity of SnO 2 . The oxidation (3.3 mA) and reduction peak currents (1.7 mA) of the SnO 2 -CNTs both are the highest. This is due to the synergistic catalysis of SnO 2 and the CNTs. SnO 2 mainly provides the catalytic active sites while the CNTs provides the 3D structure and high electrical conductivity. The composite electrode shows the best electrocatalytic activity for the VO + 2 /VO 2+ redox reaction. Figures 3B-D show the CV curves of the CNTs, SnO 2, and SnO 2 -CNTs, respectively. With increasing scan rate, the peak shape of the CV curves remains good, which proves that the electrodes have good electrochemical stability. Also, the redox peak current and peak potential difference increase with increasing scan rate. Figure 3E shows the redox peak current vs. the square root of the scan rate. The peak current is proportional to the square root of the scan rate for all samples, proving that the redox reaction is dominated by ion diffusion. The SnO 2 -CNTs have the highest linear slope. This may be due to the synergistic catalytic effect of composite material, which provides active sites for vanadium ions and promotes the mass transfer process of vanadium ions in solution. Figure 3F shows the Nyquist diagrams for the three electrodes, each consisting of a semicircle of high frequency and a linear part of low frequency, corresponding to charge transfer and diffusion processes, respectively. A smaller semicircle diameter means a smaller charge transfer resistance. The charge transfer resistance of the SnO 2 -CNTs is much smaller than that of the other two samples. This may be due to the fact that the composite has a larger active surface area and more active sites than the CNT, which improves the catalytic performance of the vanadium redox reaction. Figure 4A shows the negative CV curves of the CNTs and SnO 2 -CNTs. Both electrodes have obvious redox peaks and the peak redox current of the composite is larger than that of the CNTs, indicating that the SnO 2 -CNT electrode also shows better electrocatalytic activity for the V 2+ /V 3+ redox reaction. Figures 4B,C show a series of CV curves for the CNTs and SnO 2 -CNTs composite, respectively, at different scan rates. The shape of the redox peak of the two electrodes remains unchanged. Under the influence of electrochemical polarization, the redox peak potential difference of the two electrodes increases with increasing scan rate.
The curves of the redox peak current vs. the square root of the scan rate can be seen in Figures 4D,E. The redox peak current is proportional to the square root of the scan rate. This shows that the V 2+ /V 3+ redox reaction is controlled by a mass transfer process. The higher the linear slope, the higher the mass transfer rate. The linear slope of the SnO 2 -CNTs is higher than the CNTs. This may be due to the fact that the addition of SnO 2 endows the CNTs with higher catalytic activity and more active sites. Figure 4F shows that the Nyquist diagrams of both the CNTs and SnO 2 -CNTs are composed of a semicircle of a high-frequency part and an oblique line of a low-frequency part. They correspond to the charge transfer and diffusion processes, respectively. The charge transfer resistance of the composite is lower, which indicates that it has a larger active surface area and more active sites than the CNTs, thus improving the kinetics of the vanadium redox reaction.
The comparison of the charge and discharge rate performance of the pristine cell and the cell using the SnO 2 -CNTs was studied. As shown in Figure 5A, the discharge capacity gradually decreases because of the more severe electrochemical polarization at the high current density. The discharge capacity of the SnO 2 -CNT cell is 28.6 mAh higher than the pristine cell at 150 mA cm −2 , indicating that the composite improves the utilization rate of the electrolyte. Figure 5B shows the efficiency of the pristine and modified cells. The coulombic efficiency (CE) represents the ratio of the   discharge capacity to charge capacity. The CE of the pristine cell is slightly larger than that of the SnO 2 -CNT cell. This is because of the longer charge-discharge process and the larger charge loss in the SnO 2 -CNT cell. The voltage efficiency (VE) reveals the electrochemical activity of the electrode. The VE of the modified cell is 8.1% larger than the pristine cell (63.5%) at 150 mA cm −2 . The energy efficiency (EE) depends on both the CE and VE. Both the EE and VE can be used to evaluate the electrochemical polarization of electrodes. The EE attenuation phenomenon occurs in both cells as the current density increases, but the cell with the SnO 2 -CNTs has a smaller attenuation than the pristine cell. The EE of the pristine and modified cells are 62.8% and 7% at 150 mA cm −2 , respectively. These results demonstrate that the composite can reduce electrochemical polarization and improve electrochemical activity. Figures 5C,D show the charge-discharge curves of the pristine and SnO 2 -CNTs modified cells, respectively. The modified cell has a higher discharge voltage platform and a lower charge voltage platform, illustrating that the SnO 2 -CNTs composite can decrease electrochemical polarization and increase the energy density of the cell.

CONCLUSION
In this study, the SnO 2 -CNT catalyst was prepared using a sol-gel method. The effects of the catalyst on the redox reaction and the cell performance were investigated. Compared with pure SnO 2 and pure CNTs, the SnO 2 -CNTs show better electrocatalytic activity and reversibility for the VO + 2 /VO 2+ and V 2+ /V 3+ reactions. This is due to the synergistic catalysis of SnO 2 and the CNTs. SnO 2 mainly provides the catalytic active sites while the CNTs mainly provides the 3D structure and high electrical conductivity. Therefore, this composite has larger specific surface area and excellent synergistic catalytic performance. At current densities of 50-150 mA cm −2 , the discharge capacity of the modified cell is higher than the pristine cell, indicating that a modified cell has a higher electrolyte utilization rate and better electrochemical stability. The EE of the modified cell (7%) is 7.2% higher than the pristine cell (62.8%) at 150 mA cm −2 . The results show that this composite can provide significant benefits for the improvement of the VRFB performance.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

AUTHOR CONTRIBUTIONS
XF was mainly responsible for experimental operations and drafting paper. JX was mainly responsible for the collecting and processing experimental data. TZ was mainly responsible for collecting information. ZZ was mainly responsible for drafting paper. CH was mainly responsible for making important modifications to the manuscript. LD was mainly responsible for reviewing the final manuscript for publication. LW was mainly responsible for designing the experiment. ZH are mainly responsible for the paper guidance. All authors contributed to the article and approved the submitted version.