# Tin Oxide/Nitrogen-Doped Graphene Quantum Dots Composite Nanotubes: An Efficient Electrode for Supercapacitors

Authors: Seshendra Reddy Ch; Ravindra N. Bulakhe; Jeevan Kumar Reddy Modigunta; G. Murali; Reddy Sivasankar A.; Jiyeong Kim; Eunji Park; Byungin Kang; Insik In; Arup Kumer Roy; Kiruthiga Ramakrishnan; Udayabhaskar Rednam
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

Tin oxide (SnO2) and nitrogen-doped graphene quantum dots (N-GQDs) composite nanotubes (SnO2/N-GQD NTs) were fabricated by the electrospinning technique and followed by the thermal annealing method for the application in supercapacitor as an electrode. SnO2/N-GQD NTs with different ratio of N-GQDs were prepared by adding different ratios of N-GQDs along with tin chloride during the electrospinning process. The prepared composite's structure and morphological properties were characterized by using different techniques like XRD, FE-SEM, TEM, and XPS. The supercapacitor performance of the SnO2/N-GQD NTs composite was analyzed by the electrochemical studies such as cyclic voltammetry and galvanostatic charge-discharge (GCD) measurement in 2 M KOH solution as electrolyte. The electrochemical analyses of SnO2/N-GQD NTs was tested at different scan rates and current densities. SnO2/N-GQD NTs prepared using 3 wt.% of N-GQDs showed an excellent capacity retention even after 5000 GCD cycles and exhibited a maximum specific capacitance of 420 mF g-1 at a current density of 8 mA cm-2 in comparison to pure SnO2 NTs (230 mF cm-2).


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## Body

## 1. Introduction

In the past few decades, the continuous growth in the demand for energy thrived researchers to find an alternative energy storage and conversion systems for the depleting fossil fuels which are affecting the global environment. Among various available energy storage and conversion devices, such as batteries, fuel cells, and supercapacitors (SCs), SCs provide an instant high-power density for the required applications [1–5]. SCs have gained a wide attention in the interdisciplinary fields due to their amazing characteristics like, high efficiency at prolonged cycles [6], high-power density, and high-energy density [7, 8]. In general most of the metal oxide semiconductors (MOS) such as RuO2 [9, 10], NiO [11], SnO2 [12], MnO2 [13, 14], Fe3O4 [15], CuO [16], ZnO [17], and CoO [18] are having a significant contribution for the SC applications [19]. Tin oxide (SnO2) is well-known among MOS due to its vast applications such as in gas sensors [20, 21], biosensors [22, 23], solar cells [24, 25], Li-ion battery materials [26, 27], photocatalytic applications [28], and SCs [29, 30]. Among the MOS, SnO2 is a non-toxic and easily synthesizable nanomaterial [31]. Although SnO2 performs well for SCs, the limiting factors such as poor electrical conductivity, poor transportation of electrolyte ions within the SnO2 matrix, and huge volume expansion hinder its practical applicability [32]. In order to improve SnO2 as an efficient electrode material for the SCs, the combination of SnO2 with other conductive materials containing a high surface area and good morphologies is one of the most suitable choices. The composites made of a conductive material such as carbon-based materials with SnO2 and related composites are one of the good choices among many available methods in order to enhance the performance as electrode for SCs.In recent years, carbon-based materials are among the eco-friendly, low-cost, and easily available sources for improving the conductivity of MOS along with different morphologies with high surface area [33]. Carbon-based nanomaterials and MOS nanocomposites have been reported to be good electrode materials for energy conversion and storage applications [26, 34]. The preparation of 2D fiber or tube-like morphologies supports the improvement of internal conductivity in the materials which contains interconnecting branching networks in the structures. These type of morphologies can be easily fabricated using the electrospinning method [35].The composites of carbon-based materials contribute to the improvement of the pseudocapacitance of the materials in electrochemical analysis. In recent times, heteroatom-doped carbon-based materials such as N-doped graphene and N-doped graphene quantum dots (N-GQDs) are among the carbon nanostructures that have been studied majorly [36–38]. N-GQDs are used as the electrodes for SCs and found that they exhibited superior capacitance and rate performance [39]. These properties make N-GQDs as one of the suitable materials for making composites with MOS for improving the electrochemical performance. The fabrication of these nanotube (NT) composite materials with nanometer scale morphology is one of the challenging processes to obtain a highly active sites and good conductive networks. However, the N-GQDs incorporated SnO2 NTs can be a good suitable electrode material for the improvement of electrochemical performance of SnO2 in SC applications.In this work, the SnO2 NTs and SnO2/N-GQD NTs have been prepared by two-step process in which first the solution of tin chloride and N-GQDs (with 3 and 5 wt.%) along with polyvinylpyrrolidone in DMF solutions was subjected to electrospinning followed by thermal annealing. This process is cheaper, easy to handle, and has the advantage of easy morphological control. The electrochemical performance of synthesized SnO2/N-GQD NTs was studied by fabricating a SC device with a three-electrode system configuration and aqueous potassium hydroxide as electrolyte.


## 2. Experimental

### 2.1. Materials

Tin (II) chloride dihydrate (SnCl2·2H2O), N, N-dimethylformamide (DMF), ethanol, polyvinylpyrrolidone (PVP, Mw=1,300,000gmol−1), citric acid monohydrate, urea, polyvinylidene fluoride (PVdF), and N-methyl pyrrolidone (NMP) were procured from Sigma Aldrich, USA. Conductive additive super p carbon and Ni-foam were procured from MTI Corporation, South Korea. All the chemicals purchased were used as received without any further purification.


### 2.2. Synthesis of N-GQDs

Citric acid monohydrate (0.42 g) and urea (0.36 g) were mixed in 12 mL deionized water and sonicated for 60 min. The mixture was then transferred to a Teflon-lined autoclave (50 mL) and heated at 180 °C for 3 h. After cooling the autoclave to room temperature, the resultant product was filtered and washed with ethanol and centrifuged at 8000 rpm for 5 min. The solid precipitate obtained was dried at 50 °C for 10 h to obtain the nitrogen-doped graphene quantum dots (N-GQDs) [38].


### 2.3. Synthesis of SnO2 Nanotubes and N-GQD Incorporated SnO2 Nanotubes

SnCl2·2H2O (0.7 g) was dissolved in 4.2 mL ethanol and sonicated adequately for 1 h; simultaneously, 0.44 g of PVP was dissolved in DMF and stirred for 3 h. Finally, both clear solutions were mixed and stirred for 4 h to obtain the precursor liquid for electrospinning. N-GQDs (3 and 5 wt.%) incorporated tin chloride NTs were prepared by adding the solution of tin chloride with pre-prepared N-GQD solution with stirring for several hours until the solution becomes uniform. The obtained solution was loaded into the syringe and electrospinned. For the preparation of the pristine tin chloride and N-GQD incorporated tin chloride NTs, a constant voltage of 12 kV and 15 kV was applied, respectively. The distance between the syringe needle and the electrospun mat collector was fixed at 12 cm. The specimens were collected and dried in vacuum oven for overnight. Pure SnO2 and N-GQDs incorporated SnO2 nanotube composites were obtained by annealing the dried electrospun mats at 600 °C for 2 h and 500 °C for 4 h, respectively. The samples were denoted as S1 for pure SnO2 NTs, S2 for SnO2 NTs with 3 wt.% of N-GQDs, and S3 for SnO2 NTs with 5 wt.% N-GQDs-loaded samples. The detailed synthesis procedure is shown in Scheme 1.Scheme 1
Schematic diagram for the synthesis of SnO2 nanotubes and N-GQDs incorporated SnO2 nanotubes by electrospinning followed by thermal annealing.


### 2.4. Materials Characterization

The structural properties of the prepared samples (S1, S2, and S3) were characterized by the patterns obtained from X-ray diffraction (Bruker, D8 ADVANCE, USA). The surface morphology and microstructure of the samples were characterized by field emission scanning electron spectroscopy (FE-SEM, JEOL, JSM-6700F, Japan) and transmission electron microscopy (TEM, Tecnai G2 F20, FEI Corporation, Japan). The chemical bonds in the samples were characterized using X-ray photoelectron spectra (XPS, Scientific Sigma Probe Multi-2000 spectrometer, Thermo VG Scientific Co., USA).


### 2.5. Electrochemical Characterization

The electrochemical analyses were carried by using a Ni foam electrode coated with the prepared active materials. For electrode preparation, 80 wt.% active material, 10 wt.% polyvinylidene fluoride (PVDF) and 10 wt.% conductive carbon were mixed in n-methyl-2-pyrrolidone (NMP). The mixture was made as fine slurry, and then, it was coated on Ni foam (1 × 4 cm) in 1 × 1 cm size and then dried in vacuum oven for 12 h at 100 °C. The loading of the active material in the electrodes is approximately 2.5 mg cm-2. Electrochemical measurements such as cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) were performed at room temperature with the help of Wonatech, Korea, instrument using three-electrode configuration in 2 M KOH electrolyte solution. Electrochemical impedance spectra (EIS) analyses were performed using frequency range between 0 Hz and 50 kHz. CV was carried out at various scan rates (5 to 100 mV/s) in the potential window of 0-0.5 V. GCD was performed at different current densities (1 to 8 mA g-1) in the voltage window of 0-0.5 V. The specific capacitance of the SCs is calculated by using the formula:
(1)SpecificcapacitanceCsp=I×∆tm×∆V,where IA is discharge current, Δts is the discharge time consumed for in the potential range of V, mg is the mass of the active material, and ΔVV is the potential window used for the charge-discharge. Energy density and power density of the fabricated materials are determined by using the following equations:
(2)EnergydensityE=Csp×∆V22,where CspFg−1 is the specific capacitance calculated from charge-discharge process and ΔV is the potential window used in the charge-discharge process.
(3)PowerDensityP=E∆t,where E is the energy density calculated from the specific capacitance and Δt discharge time of the supercapacitor.


## 2.1. Materials

Tin (II) chloride dihydrate (SnCl2·2H2O), N, N-dimethylformamide (DMF), ethanol, polyvinylpyrrolidone (PVP, Mw=1,300,000gmol−1), citric acid monohydrate, urea, polyvinylidene fluoride (PVdF), and N-methyl pyrrolidone (NMP) were procured from Sigma Aldrich, USA. Conductive additive super p carbon and Ni-foam were procured from MTI Corporation, South Korea. All the chemicals purchased were used as received without any further purification.


## 2.2. Synthesis of N-GQDs

Citric acid monohydrate (0.42 g) and urea (0.36 g) were mixed in 12 mL deionized water and sonicated for 60 min. The mixture was then transferred to a Teflon-lined autoclave (50 mL) and heated at 180 °C for 3 h. After cooling the autoclave to room temperature, the resultant product was filtered and washed with ethanol and centrifuged at 8000 rpm for 5 min. The solid precipitate obtained was dried at 50 °C for 10 h to obtain the nitrogen-doped graphene quantum dots (N-GQDs) [38].


## 2.3. Synthesis of SnO2 Nanotubes and N-GQD Incorporated SnO2 Nanotubes

SnCl2·2H2O (0.7 g) was dissolved in 4.2 mL ethanol and sonicated adequately for 1 h; simultaneously, 0.44 g of PVP was dissolved in DMF and stirred for 3 h. Finally, both clear solutions were mixed and stirred for 4 h to obtain the precursor liquid for electrospinning. N-GQDs (3 and 5 wt.%) incorporated tin chloride NTs were prepared by adding the solution of tin chloride with pre-prepared N-GQD solution with stirring for several hours until the solution becomes uniform. The obtained solution was loaded into the syringe and electrospinned. For the preparation of the pristine tin chloride and N-GQD incorporated tin chloride NTs, a constant voltage of 12 kV and 15 kV was applied, respectively. The distance between the syringe needle and the electrospun mat collector was fixed at 12 cm. The specimens were collected and dried in vacuum oven for overnight. Pure SnO2 and N-GQDs incorporated SnO2 nanotube composites were obtained by annealing the dried electrospun mats at 600 °C for 2 h and 500 °C for 4 h, respectively. The samples were denoted as S1 for pure SnO2 NTs, S2 for SnO2 NTs with 3 wt.% of N-GQDs, and S3 for SnO2 NTs with 5 wt.% N-GQDs-loaded samples. The detailed synthesis procedure is shown in Scheme 1.Scheme 1
Schematic diagram for the synthesis of SnO2 nanotubes and N-GQDs incorporated SnO2 nanotubes by electrospinning followed by thermal annealing.


## 2.4. Materials Characterization

The structural properties of the prepared samples (S1, S2, and S3) were characterized by the patterns obtained from X-ray diffraction (Bruker, D8 ADVANCE, USA). The surface morphology and microstructure of the samples were characterized by field emission scanning electron spectroscopy (FE-SEM, JEOL, JSM-6700F, Japan) and transmission electron microscopy (TEM, Tecnai G2 F20, FEI Corporation, Japan). The chemical bonds in the samples were characterized using X-ray photoelectron spectra (XPS, Scientific Sigma Probe Multi-2000 spectrometer, Thermo VG Scientific Co., USA).


## 2.5. Electrochemical Characterization

The electrochemical analyses were carried by using a Ni foam electrode coated with the prepared active materials. For electrode preparation, 80 wt.% active material, 10 wt.% polyvinylidene fluoride (PVDF) and 10 wt.% conductive carbon were mixed in n-methyl-2-pyrrolidone (NMP). The mixture was made as fine slurry, and then, it was coated on Ni foam (1 × 4 cm) in 1 × 1 cm size and then dried in vacuum oven for 12 h at 100 °C. The loading of the active material in the electrodes is approximately 2.5 mg cm-2. Electrochemical measurements such as cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) were performed at room temperature with the help of Wonatech, Korea, instrument using three-electrode configuration in 2 M KOH electrolyte solution. Electrochemical impedance spectra (EIS) analyses were performed using frequency range between 0 Hz and 50 kHz. CV was carried out at various scan rates (5 to 100 mV/s) in the potential window of 0-0.5 V. GCD was performed at different current densities (1 to 8 mA g-1) in the voltage window of 0-0.5 V. The specific capacitance of the SCs is calculated by using the formula:
(1)SpecificcapacitanceCsp=I×∆tm×∆V,where IA is discharge current, Δts is the discharge time consumed for in the potential range of V, mg is the mass of the active material, and ΔVV is the potential window used for the charge-discharge. Energy density and power density of the fabricated materials are determined by using the following equations:
(2)EnergydensityE=Csp×∆V22,where CspFg−1 is the specific capacitance calculated from charge-discharge process and ΔV is the potential window used in the charge-discharge process.
(3)PowerDensityP=E∆t,where E is the energy density calculated from the specific capacitance and Δt discharge time of the supercapacitor.


## 3. Results and Discussions

Figure1 Shows the XRD patterns of S1, S2, and S3 NTs with the major peaks corresponding to the (110), (101), (200), (211), (220), (002), (310), (112), (301), (202), and (321) lattice planes of the rutile structure of SnO2. XRD patterns are in good agreement with the reference patterns of SnO2 with JCPDS number 41-1445, which are compatible with previous reports [40]. The lack of N-GQD peaks in XRD might be due to the dominant concentration of the SnO2 matrix in the samples. The effect of N-GQDs incorporation into the samples is studied by considering the (110) diffraction peak as reference candidate to calculate the crystallite size. The crystallite size of the samples is calculated using the Scherrer’s formula. The calculated average crystallite size of the S1, S2, and S3 are 10.1 nm, 12.3 nm, and 14.5 nm, respectively, showing change in the crystallite size caused by the N-GQDs incorporation during the synthesis process.Figure 1
XRD spectral data of S1 (SnO2) nanotubes, S2 (SnO2/N-GQDs-3 wt.%) nanotubes, and S3 (SnO2/N-GQDs-5 wt.%) nanotubes.The surface elemental composition of the S2 is analyzed by XPS as shown in Figure2. The survey scan spectrum (Figure 2(a)) detected elements Sn, C, N, and O. The peaks of Sn 3d, Sn 3p, and MNN were detected from SnO2. The Sn 3d (3d3/2 and 3d5/2) peaks were observed at 495.2 and 486.7 eV (Figure 2(b)) corresponding to Sn4+ in the rutile SnO2 structure. In Figure 2(c), the O 1s spectra of the S2 shows the peaks at 532.1, 531.2, and 530.4 eV; corresponding to oxygen absorption (O2-), oxygen vacancy (O-), and oxygen binding (O2-), this designates the formation of oxygen vacancies in S2. XPS spectra of the S2 sample have an obvious peak at 399.8 eV corresponding to N 1s (Figure 2(d)) due to the presence of nitrogen functional groups (from N-GQDs). The peak of C1s can be attributed mainly to GQDs as shown in Figure 2(e). The C1s spectrum shows a strong peak center at 284.9 eV that is attributed for the C=C/C–C bonds, and peaks at 285.5 and 286.7 eV revealing the presence of C=N (sp2) and C–N (sp3) bonds, respectively. The XPS analysis provides evidence for the formation of N-doped GQDs and their composites in combination with SnO2 [41].Figure 2
XPS (a) survey scan spectrum and high resolution deconvoluted spectra for (b) Sn 3d, (c) O 1s, (d) N 1s, and (e) C 1s, of sample S2 (SnO2/N-GQDs-3 wt.%).
(a)(b)(c)(d)(e)FE-SEM and EDS spectral images of S1, S2, and S3 NTs are shown in Figures3(a)–3(f). In all samples, NTs are randomly oriented in all directions with the rough surface. Figures 3(a)–3(b), Figures 3(c)–3(d), and Figures 3(e)–3(f) represent the FE-SEM images and EDS spectra of S1, S2, and S3 samples after calcination, respectively. The elemental composition of pristine and composite SnO2 NTs is determined by EDS analysis that indicates the presence of Sn, O, and C elements as shown in Figure 3. Pristine and N-GQDs incorporated SnO2 long NTs are having the diameter in the range of 15–40 nm. From FE-SEM and EDS results, it is observed that almost there were no trances of PVP-based amorphous carbon after thermal annealing. Although PVP created some porous vacancies during the annealing process, the nanotube structures were retained during the calcination process due to the crystallization of SnO2. From FE-SEM images, it can also be observed that the S1 and S2 are thin and almost uniform. In the case of S3 sample, as the N-GQDs concentration is about 5 wt.%, the NTs were broken, and this may be due to the increase in the viscosity of the electrospinning precursor solution.Figure 3
FE-SEM images and EDS spectrum of samples (a, b) S1 (pure SnO2 nanotubes), (c, d) S2 (SnO2/N-GQDs-3 wt.% nanotubes), and (e, f) S3 (SnO2/N-GQDs-5 wt.% nanotubes).
(a)(b)(c)(d)(e)(f)Figure4 shows the TEM, HR-TEM, and SAED images of S1 and S2 samples. Figures 4(a)–4(c) display the TEM, HR-TEM, and SAED images of S1, i.e., pristine SnO2 NTs. The size of pristine SnO2 nanoparticles is about 20 nm. The nanoparticle network has been continued in the form of nanotube by the mutual sharing of the crystal edges of each particle. The crystal structure of S2, i.e., SnO2/N-GQD NTs (Figures 4(d)–4(f)), could be verified in the high-resolution TEM. The SnO2 NTs consist of crystalline nanoparticles-shaped SnO2, and the N-GQDs were tightly incorporated in the SnO2 NTs. Moreover, the HR-TEM images demonstrate clear lattice fringes with d-space of 0.34 nm and 0.27 nm, which are in good agreement with that of the (101) and (110) planes of SnO2. The SAED images of the S1 and S2 represent the monocrystalline structure and that is in good agreement with the XRD analysis as discussed earlier.Figure 4
TEM, HR-TEM, and SAED pattern images of (a–c) SnO2 (S1) and (d–f) SnO2/N-GQDs composite material (S2).
(a)(b)(c)(d)(e)(f)The electrochemical analysis of the prepared samples was investigated using CV, EIS, and GCD cycling through a three-electrode configuration. In the three-electrode system devices, active material-loaded Ni foam is used as working electrode, Ag/AgCl as the reference electrode, and platinum strip as counter electrode. Aqueous 2 M KOH was used as the electrolyte for the analyses. The CV, EIS, and GCD analysis results for S1, S2, and S3 are shown in Figure5.Figure 5
Electrochemical performance evolution of the S1, S2, and S3 electrodes: (a–c) cyclic voltammetry (CV) curves at different scan rates from 5 to 100 mV s-1, (d–f) Nyquist plots (EIS) analyzed in the frequency range 0.1 to 10 kHz (inset, selective magnified scale in the high-frequency region), and (g–i) galvanostatic charge-discharge (GCD) curves at different current densities from 1 to 8 mA cm-2.
(a)(b)(c)(d)(e)(f)(g)(h)(i)Figures5(a)–5(c) show the CV curves of S1, S2, and S3 electrodes for comparison in the potential window of 0 to 0.6 V in 2 M KOH aqueous electrolyte at different scan rates from 5 to 100 mV s-1. CV curves of S1, S2, and S3 have shown noticeable redox peaks at ~0.38 and 0.27 V. The CV curve for S1 electrode has clearly shown the characteristic redox peaks for SnO2 material (Figure 5(a)). The CV curves of S2 has shown well-defined redox peaks similar to S1, indicating the presence of quasi-reversible faradaic reactions at different scan rates as shown in Figure 5(b). The redox peaks of S2 sample are broadened with increase of the scan rate from 5 to 100 mV s−1 might be due to the fact that the quasi-reversible redox reactions at the surface of SnO2 material along with the conductive N-GQDs in the electrodes that might help to enhance the internal conductivity of the electrodes. Whereas in the case S3 electrode, the CV redox peak positions were similar to the previous electrodes, but the peak broadening was not observed as that of S2 (Figure 5(c)). The possible redox reactions of the SnO2/N-GQD NT electrodes during the CV process (between Sn(III)/Sn(IV) in reaction with OH−) are described in Equations (4) and (5) [42].
(4)SnO2+OH−↔SnOOH+e−(5)SnOOH+OH−↔SnO2+H2O+e−The electron/ion transport behavior of the nanocomposites is understood by using EIS measurements. EIS measurements also carried to compare the resistance at the interface of the electrode materials interface and the electrolyte in the devices fabricated using electrodes made-up of S1, S2, and S3 materials as shown in Figures5(d)–5(f). The Nyquist plots of the nanocomposites were showing the semi-half circle followed by the immediate raise in the diffusion resistance in the high-frequency region (as shown in the inset of Figures 5(d)–5(f)) and a linear slope in the low-frequency region. The charge transfer resistance (Rct) was observed for all the samples, and the inclined line is attributed to the anomalous diffusion of the ions in the electrolyte to the modified electrode. The Rct of the electrodes indicates the presence of more resistance due to the semiconducting SnO2 in S1, whereas the Rct value decreases for S2 electrodes and further increases for S3 as shown in Figures 5(d)–5(f). The device with S2 electrode showed a least resistance in comparison to S1 and S3. The above electrochemical results imply that S2 electrode is a high potential candidate for the application of SCs in comparison to the S1 and S3 electrodes.The N-GQDs will not take part in the redox reaction directly, and it will enhance the electrical/ionic conductivity of the composites with faster kinetics of positive ions during the redox process by means of electric double layer (EDL) formation. The GCD profiles of S1, S2, and S3 electrodes at different current densities ranging from 1 to 8 mA cm-2 are shown in Figures 5(g)–5(i). The corresponding areal capacitance values at different current densities were calculated, and the results were compared for S1, S2, and S3. The areal capacitances of the S2 electrode are calculated at different current densities from 1 to 8 mA cm2 and are varying from 420 to 200 mF cm-2, respectively, as shown in Figure 5(h). S2 electrode delivers higher areal capacitance of 420 mF cm-2 at current density of 1 mA cm-2, as compared to S1 (230 mF cm-2) and S3 (320 mF cm-2) as shown in Figures 5(g)–5(i). The S2 electrode revealed good electrochemical performance in comparison to S1 and S3 electrodes; this might be due to the improved conductivity of the S2 electrode. The CV curves with redox peaks and charge-discharge plots with lower internal resistance indicate that the materials are suitable for SC applications.Figure6(a) shows the specific capacitance of the electrodes S1, S2, and S3 with respect to scan rates used in the CV analysis. Similarly, Figure 6(b) shows the specific capacitance with change in current densities varied from 1 to 8 mA cm-2. From these electrochemical analyses, it can be corroborated that electrodes prepared using S2 sample are having the best possible combination of N-GQDs and SnO2 NTs for the application in SCs.Figure 6
Electrochemical performance of S1 (SnO2 NTs), S2 (SnO2/N-GQD-3 wt.% NTs), and S3 (SnO2/N-GQD-5 wt.% NTs) electrodes: (a) specific capacitance vs. scan rate, (b) plots of specific capacitance vs. current density, and (c) capacity retention efficiency with respect to cycle number. (d) Ragone plot of energy density vs. power density for supercapacitor with S2 electrodes.
(a)(b)(c)(d)The specific capacitance is decreased at higher scan rates, which could be due to the approach of positive ions (K+) only at the outer surface of the electrode during the EDL formation and/or quasi-reversible reactions. Whereas at lower scan rates, positive ions could easily diffuse into all the available spaces of electrode, thus leading to sufficient insertion reaction to form quasi-reversible reactions and/or EDL formation or to show a high specific capacitance [43, 44]. The capacitance retention of S1, S2, and S3 electrodes studied by the GCD measurements at 10 mA cm-2 current density for 5000 cycles is shown in Figure 6(c). The capacitance retention for S2 electrode (65%) is higher in comparison to S1 (51%) and S3 (44%) as shown in Figure 6(c). These results indicate the higher electrochemical cycling stability of S2 composite electrode. Figure 6(d) shows the Ragone plot of energy density vs. power density of the SC with S2 electrode. The energy density of 820 mWh cm-2 and the power density of 4.10 W cm-2 were obtained for the S2 electrode. Similar work was reported by Barik et al., which is found to have the energy density of 80 mWh cm-2 and power density of 4.87 W cm-2 for SnO2@carbon nanofiber synthesized by the electrospinning method [45]. The schematic for the possible mechanism involved in the SnO2/N-GQD NT composite electrodes during charge-discharge process is shown in Figure 7.Figure 7
The schematic of the possible mechanism involved in the SnO2/N-GQD NT composite electrodes during the charge-discharge processes.The schematic mechanism (Figure7) shows the movement of ions in electrolyte during the charge-discharge process of the SnO2/N-GQD NT electrodes. The K+ and OH- ions in the electrolyte move towards the opposite charged electrodes. The electrons in the OH- ions adsorbed onto the surface of SnO2/N-GQD electrodes undergo chemical reactions as discussed above in Equations (4) and (5). Whereas the K+ ions adsorbed on the surface of the electrodes take electron released during SnO2 reversible reactions and form an EDL during the charge process, in the charging process, both EDL and reversible chemical reactions both occur at the surface of the electrodes. The reversible reactions during the discharge process will retain the ionic combination of electrolyte resulting in neutral bulk electrolyte. The performance enhancement of S2 electrodes can be attributed to spacer effect of SnO2 particles in the NTs incorporated with N-GQDs. The highly reversible faradaic redox process of SnO2 also enhances the performance of SCs. The specific capacitance performance of the S2 electrode was higher in comparison to S1 and S3 and might be due to the factors discussed above such as spacer effect, good conductivity of N-GQDs, and good internal contacts between SnO2 particles and N-GQDs in the NTs. In the case of S1 electrodes, there are no additional conducting agents such as N-GQDs, whereas for S3 electrode, the N-GQDs concentration has hindered the adhesive property between SnO2 crystals leading to damage of NT structures subsequently losing the conductivity advantage. In contrast, S2 has shown a good structural stability and enhanced conductivity for electrodes along with more suitable sites for the surface chemical reactions on SnO2, therefore resulted as a good performing electrode than others.From the above experimental results, the N-GQDs incorporated SnO2 NTs have shown a considerable better improvement in the areal capacitance of the SnO2 NTs due to the enhanced electrons and ionic diffusion with increase of more active sites. The moderate (3 wt.%) addition of N-GQDs into SnO2 NTs has demonstrated an excellent electrochemical behavior due to the tubular porous morphology along with decrease in electrical resistance compared to pure SnO2 NTs and higher N-GQDs containing samples. The improved capacitive behavior could be attributed to the decrease in the resistance of the electrodes and increase in the ionic diffusion from the bulk electrolyte solution to the surface of the electrodes due to the incorporation of N-GQDs. Based on these above studies, it is evident that SnO2 NTs and N-GQDs composites are one of the good futuristic materials for the energy storage and conversion application and in many other interdisciplinary fields.


## 4. Conclusions

In summary, a simple and cost-effective one-step electrospinning technique followed by thermal annealing was adopted for the fabrication of pristine and N-GQDs incorporated SnO2 NTs. The structural, morphology, and elemental studies of as-prepared samples were analyzed by different techniques. The combination of tubular microstructures, chemical stability, and electrochemical properties of the S2 (SnO2/N-GQDs-3 wt.%) composite showed a good performance as an electrode in the supercapacitor device. This work demonstrates a simple and facile synthesis method for the preparation of SnO2 NTs and their composites that can be efficiently used in the energy storage and conversion applications.



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