S or N-monodoping and S,N-codoping effect on electronic structure and electrochemical performance of tin dioxide: Simulation calculation and experiment validation

S or N-monodoping and S,N-codoping effect on electronic structure and electrochemical performance of tin dioxide: Simulation calculation and experiment validation

Electrochimica Acta 340 (2020) 135950 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electa...

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Electrochimica Acta 340 (2020) 135950

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

S or N-monodoping and S,N-codoping effect on electronic structure and electrochemical performance of tin dioxide: Simulation calculation and experiment validation Jing Xu, Yakui Mu, Chaohui Ruan, Pengxi Li, Yibing Xie* School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 December 2019 Received in revised form 17 February 2020 Accepted 23 February 2020 Available online 24 February 2020

Sulfur and nitrogen codoped tin dioxide (S,N-codoped SnO2) has been grown on porous carbon fiber (PCF) via assisted hydrothermal and then one-pot codoping strategies. First-principles density functional theory and electrochemical investigations have revealed S or N-monodoping and S,N-codoping effect on SnO2. N doping contributes to capacitance enhancement, while S doping favours rate capability. Thus, S,N-codoped SnO2 shows more narrow bandgap (1.2 eV) and homogeneous electron density distribution than S or N-monodoped SnO2 (2.0 and 1.7 eV) and SnO2 (3.2 eV). Consistently, S,N-codoped SnO2, Sdoped SnO2, N-doped SnO2 and SnO2 electrodes show specific capacitance (332.5, 123.8, 185.2 and 83.76 F g1 at 1 A g1) and rate capability (64.8%, 61.1%, 47.9% and 36.5% from 1 to 10 A g1). The wireshaped microsupercapacitor using S,N-codoped SnO2/PCF positive electrode and TiN negative electrode delivers considerable energy density of 0.55 mWh cm3. So, the consistent corroboration of simulation calculation and experiment validation indicates that high-performance S,N-codoped SnO2/PCF can act well as an effective energy storage electrode material. © 2020 Elsevier Ltd. All rights reserved.

Keywords: S,N-codoped SnO2 S or N-Monodoping Density functional theory calculations Electronic structure Electrochemical performance

1. Introduction With the development and wide application of many emerging technologies such as portable electronic equipment and hybrid vehicles, the demands for energy storage and conversion system of high energy and power density are increasing rapidly. Supercapacitors, also known as 1e3 orders of magnitude longer, longer life cycle, and capabilities in fast charging-discharging when compared to conventional energy storage devices [1e3]. Supercapacitors store the charges via the fast-reversible electrolyte ions adsorption in electrical double layer and/or fast faradaic redox reactions, making them promising candidate for supplementing or replacing backup sources for various uninterrupted power supplies [4e6]. The electrode materials become one of critical factors to determine the energy-storage efficiency, which include various carbon, metal oxides/sulfates, conductive polymers [7e10]. Thus, it is especially important to enhance the capacitance, stability and cycling performance of supercapacitor electrode materials [11e14]. Noticeably, tin dioxide (SnO2), a sort of metal oxides, has been

* Corresponding author. E-mail address: [email protected] (Y. Xie). https://doi.org/10.1016/j.electacta.2020.135950 0013-4686/© 2020 Elsevier Ltd. All rights reserved.

widely investigated for its considerable theoretical specific capacitance (780 mA h g1), low-50cost and environmentally protection [15,16]. Despite the above superiorities, there are certain existing drawbacks, such as poor electronic conductivity and large volume variation, resulting in limited rate capability and poor durability [17]. It has been hindered applying SnO2 for high-performance energy storage and conversion system. Many modification strategies are given to address the aforementioned issues to develop advanced SnO2-based electrode [18e22]. In order to enhance the electronic conductivity, an electronically conductive substrate has been introduced to support SnO2 [23e25]. As a highly conductive two-dimensional material with high stiffness and surface area, porous carbon fiber (PCF) is an ideal substrate to enhance the electronic conductivity of SnO2 [26e28]. To further improve the electrochemical performance, other strategies have also been applied such as nanostructuring and atomic doping. Much progress has been made in the synthesis of SnO2 with different morphologies so far. For example, zero-dimensional SnO2 quantum dots [29,30], one-dimensional nanoribbons [31,32], twodimensional nanosheets [33,34], and three-dimensional nanoflower [35,36] have been obtained by different methods. As a result, the performance of SnO2-based electrode in supercapacitors has been enhanced. Other than the morphology control, another

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effective way in improving the intrinsic performance of SnO2 is the incorporation of heteroatoms [37e39]. Recent progress indicated that substitutional doping of SnO2 with heteroatoms could significantly modify the electrical conductivity, chemical reactivity and surface activity for SnO2[40e42]. However, the most reported dopings are confined to single element incorporating as yet. The codoped with different heteroatoms and their disparate effects in the system have been rarely reported. Among numerous heteroatoms, nitrogen (N) and sulfur (S) were proposed to enhance the electronic conductivity and tune the electronic properties of the electrodes [43e45]. Here, we design SnO2 nanosheet array, S or N-monodoped SnO2 and S,N-codoped SnO2 nanoflower array grown on porous carbon fiber (PCF) substrate via the novel methods. The resulting binderfree S,N-codped SnO2/PCF electrode delivered high electrochemical performance coupled with considerable capacitance, rate capability and cycling stability. Under the guidance with density functional theory (DFT) calculations and experimental investigations, the obtained S,N codoped-SnO2 shows macroconstructing and micro-modifying of SnO2 based on the different roles of S and N. Nitrogen doped in SnO2 prioritize alters the internal charge distribution and decreases the band gap, thus benefitting the capacitance enhances, while sulfur doped predominantly contributes to more uniform electron density distribution of S,N-codoped SnO2, hence facilitating the electrolyte ion and electrons transfer. Such a recombination of S and N codoping could enable the S,N-codoped SnO2/PCF electrode to deliver superior electrochemical performance. As a result, a wire-shaped microsupercapacitor in all-solid-state composed of S,N-codped SnO2/PCF and TiN as positive and negative electrodes and sulfuric acid-phosphate-polyvinyl alcohol (H2SO4-PVA) as gel electrolyte demonstrated superior energy/power density and elongated cycling life. Hence, S,N-codoped SnO2/PCF can act as a positive electrode for effective energy storage application with great potentialities.

a typical method, the mixed aqueous solution containing 0.095 M SnCl4$5H2O and 0.450 M NH4F was conducted the magnetic stirring for 30 min and followed transferred into the Teflon-lined autoclave. Then, putting the obtained PCF to the resulting aqueous solution and hydrothermally heated to 180  C for 24 h, cooled to room temperature and rinsed it using ethanol and distilled water. Finally pure SnO2 was obtained on PCF substrate after drying at 90  C for 12 h.

2. Experimental section

Asymmetric microsupercapacitor in all-soild-state was constructed employing SeN codoped SnO2/PCF and TiN as positive and negative electrodes, respectively, PVA-H2SO4 as gel electrolyte, polyethylene heat-shrinkable material and non-woven fabric as package tube and separation layer, respectively. TiN electrode was synthesized via anodization and then procedural calcination in an ammonia atmosphere of Ti wire [49]. The PVA-H2SO4 gel electrolyte was obtained by mixing PVA, H2SO4 in distilled water at the optimum mass ratios of 1: 1.6: 10. The mixed solution was then placed in a vacuum oven and dehydrated at 80  C for 3 h, finally a homogeneous gel was obtained [50].

2.1. Material Stannic chloride hydrated (SnCl4$5H2O, 99%), ammonium fluoride (NH4F, 98%), nickel nitrate hexahydrate (Ni(NO3)2, 98%) and sublimed sulfur (S, 99%) were obtained from Sigma-Aldrich. Concentrated sulfuric acid (H2SO4, 98%) was purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Synthesis

2.2.3. Synthesis of S or N- monodoped and S,N-codoped SnO2 nanoflower array on PCF S-doped SnO2 was conducted in the chemical vapor deposition furnace. Briefly, putting the ceramic boat contained as-prepared SnO2/PCF in the intermediate position of tube furnace. And another ceramic boat loaded sulfur powder was placed on the inlet of tube furnace, approximately 10 cm from the ceramic boat contained sample. The S-doping reaction was allowed to proceed in a nitrogen atmosphere (100 sccm) at 200  C for 30 min with heating rate of 5  C min1. N-doped SnO2 was achieved via nitriding of SnO2. Briefly, obtained SnO2/PCF was progressive annealed using tubular furnace from the room temperature to 200  C for 30 min and then up to 400  C for 4 h with heating rate of 5  C min1 at an ammonia atmosphere of 100 sccm. S,N-codoped SnO2 was prepared through one-step codoping strategy in an ammonia atmosphere. Typically, putting the ceramic boat contained as-prepared SnO2/PCF in the intermediate position of tube furnace. And another ceramic boat loaded sulfur powder was placed on the inlet of tube furnace, approximately 10 cm from the ceramic boat contained sample. And the codoping reaction was proceed in a progressive annealed ammonia atmosphere (100 sccm) from the room temperature to 200  C for 30 min and then up to 400  C for 4 h with a heating rate of 5  C min1. 2.3. Assembly of wire-shaped microsupercapacitor

2.2.1. Synthesis of the porous carbon fiber (PCF) The porous carbon fiber (PCF) was performed through a simple chemical etching method with Ni. Firstly, the origin carbon fiber performed ultrasonic process in ethanol solution for 1 h and immersed in 1 M Ni(NO3)2 for 12 h at room temperature. Secondly, the Ni(NO3)2 coated carbon fiber was dried at 60  C for12 h and then heated at 1000  C for 2 h under N2 atmosphere to get PCF/Ni composite. Finally, the PCF was achieved by sufficiently immersed the above composite into 50 mL of 1 M H2SO4 at 90  C for 2 h for the residual Ni particles removal.

2.4. Theoretical calculation

2.2.2. Synthesis of SnO2 nanosheet array on PCF Metal oxides could be prepared through typical hydrothermal reaction on conductive substrate [46,47]. Here, bare SnO2 was grown on PCF substrate via a facile ammonium fluoride-assisted hydrothermal process, tin (IV) chloride and NH4F were used as tin source and morphology controlling agents, respectively [48]. In

2.5. Characterization and electrochemical measurement

The theoretical simulations were conducted by CASTEP procedure according to density functional theory (DFT) calculations. The plane wave fundamental cut-off energy was set to 300 eV. And the convergence criteria acting on the optimized geometry were configd as: each atom force and self-consistent field energy tolerance were 0.05 eV nm1 and 2.0*106 eV per atom, respectively. The valence electrons involved in the calculation were: Sn 5s25p2, S 3s23p4, O 2s22p4and N 2s22p3.

The crystal structures of samples were attested by X-ray diffractometer with equipment (XRD, D8 ADVANCE, Cu Ka radiation, l ¼ 1.54056 Å). The elemental distributions inside the samples were analyzed employing energy dispersive X-ray photoelectron

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spectroscopy on the equipment (XPS, ESCALAB 250 X-ray photoelectron spectrometer) and energy-dispersive X-ray spectra on the equipment (EDX, Phenom ProX, The Netherlands). The microstructures and morphologies of samples were evaluated via scanning electron microscopy with equipment (SEM, JEOL JSM-6700). Raman spectrum was tested employing Raman spectrometer with equiment (Renishaw inVia Reflex System, HeeNe lase) at an excitation wavelength of 532 nm. The electrochemical behaviors of SnO2/PCF, S or N- monodoped SnO2/PCF and S,N-codoped SnO2/PCF were surveyed using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectrum (EIS) measurements in the three-electrode system. Electrochemical impedance spectrum (EIS) was conducted in frequency ranging from 10 mHz to 100 kHz. The electrochemical measurements of SnO2/PCF, S or N-monodoped SnO2/PCF and S,N-codoped SnO2/PCF working electrodes were conducted on CHI 760C and IMe6x equipment. The saturated calomel electrode (SCE) and platinum foil were acted as reference and counter electrodes, respectively. 1 M H2SO4 solution was employed as aqueous electrolyte for measurement. 3. Results and discussion The formation process of S,N-codoped SnO2 nanoflower array grown on PCF substrate can be concluded using three steps, as schematically illustrated in Fig. 1. Firstly, PCF substrate with highly porous architecture is synthesized by a facile Ni-assisted chemical etching. Then, the formation of S,N-codoped SnO2 nanoflower array on PCF substrate involves the following reactions: SnCl4 þ 4H2O / Sn(OH)4 þ 4HCl

(1)

Sn(OH)4 þ NH4F / SnO2 þ 2H2O

(2)

NH4F þ HCl / NH4Cl þ HF

(3)

SnO2 þ S(g) þ NH3 / SnOxSyNz (x þ y þ z ¼ 2)

(4)

Tin (IV) chloride undergoes fast process of hydrolysing to form Sn(OH)4 under hydrothermal conditions (reaction 1), which followed dehydrates into SnO2 crystallites with NH4F as the adjuvant and morphology control agent (reaction 2). Subsequently, excessive addition of NH4F will be reacted (reaction 3). Finally, S,N-codoped SnO2 is formed via one-step chemical vapor deposition in an ammonia atmosphere on PCF substrate at programmed

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temperature (reaction 4). Interestingly, driven by surface energy minimization, SnO2 nanosheet array is assembled into a threedimensional nanoflower array on PCF substrate after S,N-codoping. The crystal phase and compositions of S or N- monodoped, S,Ncodoped SnO2 and SnO2 on PCF substrate were characterized by Xray diffractometer (XRD) in Fig. 2a. All the samples exhibit a similar phase structure, indicating the structure was maintained when SnO2 is doped by S and/or N. The XRD pattern of samples all exhibit the standard diffraction peaks at around 2q ¼ 22 , indexing to (002) crystal plane of graphitic carbon. Otherwise, three main characteristic peaks at 26.6 , 33.8 and 38.0 are attributed to the (110), (101) and (200) crystal faces of SnO2 according to JCPDS card numbered 41e1445. The similarity in XRD patterns of S or Nmonodoped and S,N-codoped SnO2 confirm that the pristine phase was not altered. Compared with the pristine SnO2, S or N monodoped SnO2, S,N-codoped SnO2 exhibits smaller crystallite size because of larger FWHM. This may be ascribed to that the incorporating of sulfur and nitrogen into SnO2 hinder the crystalline phase growing. The parameters of lattice were calculated as a ¼ 4.74 Å, b ¼ 4.74 Å, c ¼ 3.18 Å for pristine SnO2 and a ¼ 4.76 Å, b ¼ 4.76 Å, c ¼ 3.15 Å for the SeN codoped SnO2. The enhanced unit-cell volume of S,N-codoped SnO2 might attributed to replacing of O2 (r ¼ 1.36 Å) by N3 (r ¼ 1.46 Å) and S2 (r ¼ 1.84 Å), facilitating the insertion/extraction of electrolyte ion. The interactions between SnO2-based nanoparticle array and PCF substrate were investigated by Raman spectrum. Fig. 2b exhibits the Raman curves of SnO2, S or N-monodoped SnO2, and S,N-codoped SnO2 on PCF substrate. Two characteristic peaks of 1350 and 1591 cm1 both demonstrated in four samples, indexing to the vibrations of D and G bands in carbon substrate. Area ratio R (R ¼ PD/PG) represents the degree of graphitization in carbon. It is seen that S or N monodoped SnO2 and S,N-codoped SnO2 exhibit greater R value than SnO2, suggesting smaller conjugate fields of sp2 in S or N- monodoped and S,N-codoped SnO2. It is mainly due to more free electrons flowing from doped SnO2 to PCF substrate [51e53]. The chemical sates of S,N-codoped SnO2/PCF and SnO2/PCF were conducted by X-ray photoelectron spectra (XPS) in Fig. 3. Full XPS curve in Fig. 3a represents N and S have successfully incorporated into SnO2 lattice at a content of 4.2 and 2.1 atom%, respectively. Fig. 3b shows the high-resolution Sn 3d peaks of S,N-codoped SnO2. Specifically, the characterized peaks of 486.5 eV and 495.1 eV are consistent with standard peaks Sn4þ 3d5/2 and Sn4þ 3d3/2 of SnO2, respectively. The other two weak peaks of 485.9 eV and 494.7 eV are ascribed to Sn2þ 3d5/2 and Sn2þ 3d3/2, respectively. The appearance of Sn2þ signifies the doping with nitrogen and sulfur is

Fig. 1. The fabrication process of S,N-codoped SnO2/PCF electrode.

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Fig. 2. (a) XRD and (b) Raman-scattering spectrums of S,N-codoped SnO2/PCF, S or N- monodoped SnO2/PCF and pristine SnO2/PCF.

Fig. 3. (a) Full XPS curves of S,N-codoped SnO2/PCF and pristine SnO2/PCF; (b) high-resolution Sn 3d XPS peaks of S,N-codoped SnO2; (c) high-resolution O 1s XPS peaks of S,Ncodoped SnO2 and pristine SnO2; (d) and (e) high-resolution N 1s and S 2p XPS peaks of S,N-codoped SnO2, respectively; (f) EDX spectrum of S,N-codoped SnO2/PCF.

accompanied by forming a lower oxidation state of Sn atoms. Fig. 3c shows the O 1s peak of S,N-codoped SnO2 and pristine SnO2. The O 1s peaks can be fitted by two peaks at 531.2 eV and 532.9 eV assigned to SneO bonds and oxygen vacancies, respectively. It is worth mentioning that increasing the oxygen vacancies by S and N codoping can further enhance the weak conductivity of SnO2. The corresponding N 1s and S 2p peaks were shown in Fig. 3d and e, the dominant peaks at 399.4 and 163.1 eV are indexed to SneN and SneS bonds, respectively, suggesting that nitrogen and sulfur were successfully incorporated into SnO2 lattice. The weaker peaks of 403.2 and 168.9 eV are indexed to OeN and OeS bonds, respectively. Fig. 3f exhibits the energy-dispersive X-ray spectroscopy (EDX) spectrum and element mapping pattern of S,N-codoped SnO2 on PCF substrate. The relevant results of Sn, O, S and N elements by elemental mapping in S,N-codoped SnO2 are showed in the inset of Fig. 3f, suggesting S,N-codoped SnO2 involves the uniform distribution of these four elements. The atomic percentages of sulfur and

nitrogen by EDX tests were about 2.5% and 4.7%, respectively. Considering the results of XPS and EDX measurement, S and N elements are successfully incorporated into SnO2 lattice by one-pot codoping strategy. The surface morphology of SnO2, S or N- monodoped SnO2 and S,N-codoped SnO2 on PCF substrate was characterized by SEM in Fig. 4. The SEM images in Fig. 4a and b confirm that PCF demonstrates hierarchical porous architecture ranging from dozens of nanometers to several hundred nanometers after nickel-assisted chemical etching. The enhanced porous of PCF could act as a rapid transferring channel for ions during the electrochemical reactions as well as extra space for storage of active materials [54]. Fig. 4c and d exhibit the scanning images of SnO2 nanosheet array grown on PCF substrate. Two-dimensional SnO2 nanosheet array with a regular shape and size of ~200 nm was obtained through NH4F-assisted hydrothermal treatment. The SEM images in Fig. 4eej clearly elucidate their differences in morphology,

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also consistent with XRD measurements. And no structural collapse and agglomeration occurred. Such unique and well-ordered 3D confined architectures can shorten ions diffusion distance and provide abundant accommodation sites, enabling the excellent capacitance performance, rate capability and cycling stability. It is confirmed by electrochemical performance. In recent years, many electrochemical methods have been utilized in various electrodes for determining their behaviors in eletronic, biological and environmental analysis [55e61]. Here, electrochemical measurements are performed to validate S or Nmonodoping and S,N-codoping effect on the capacitance performance of SnO2. Fig. 5 shows the electrochemical behaviours of PCF, SnO2/PCF, S or N-monodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes evaluated by various electrochemical measurements concluding cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) and cycling tests with 1.0 M H2SO4 as electrolyte based on a typical three-electrode system. Fig. 5a compares typical CV curves of PCF, SnO2/PCF, S or N-monodoped SnO2/PCF and S,Ncodoped SnO2/PCF electrodes conducted at 10 mv s1. The distinctly Faradaic redox peaks around þ0.5 V ~ þ0.7 V were observed in CV curves of S or N-monodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes. Because, electronic species will exchange with the oxygen vacancies when S and N are incorporated into SnO2 lattice. It is clearly expressed by the CV curves that a larger redox peak and integral area were observed for the S,N-codoped SnO2 electrode, indicating more oxygen vacancies and higher capacitance with S,Nsubstitution in SnO2. Fig. 5b displays the GCD curves of PCF, SnO2/ PCF, S or N-monodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes at 1.0 A g1. The lowest pressure drop of S,N-codoped SnO2/PCF electrode among as-prepared electrodes indicates the best conductivity and lowest band gap. It is attributed to the codoping of sulfur and nitrogen atoms into SnO2 lattice. Fig. 5c and d shows the specific capacitances of SnO2/PCF, S or N-monodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes calculated from the curves of CV and GCD, respectively. The mass specific capacitances Cm (F g1) were calculated according to GCD results via equation (1).

Cm ¼

Fig. 4. SEM images of (a) and (b) PCF; (c) and (d) SnO2 nanosheet array grown on PCF substrate; (e) and (f) mono S-doped SnO2; (g)and (h) mono N-doped SnO2 and (i) and (j) S,N-codoped SnO2 nanoflower array grown on PCF substrate at different magnifications.

attributing to different doped states. Fig. 4e, f and g, h reveal the morphology of S or N- monodoped SnO2 in PCF substrate, respectively. SnO2 nanosheet array exhibit the tendency to agglomerate and extend into 3D architectures to form nanoflower architecture after S or N doping. Furthermore, by comparing SEM images of S and N- monodoped SnO2, we found that S-doped SnO2 is preferentially extended to 3D nanoflower with particle size of ~150 nm, enabling well electrolyte wettability for faster ions diffusion. Whereas N doping SnO2 is preferred to form tinier 3D nanoflower with smaller particle size of ~100 nm, which can shorten the electrolyte ions transportation and enhance the structure stability. However, it is insufficient that S or N monodoping can lead a small amount of particle structural collapse or agglomeration, respectively. Thus, based on the above characteristics of S or N monodoping, the morphological details of S,N-codoped SnO2 are shown in Fig. 4i and j. As expected, S,N-codoped SnO2 contained 3D nanoflower array with smaller particle size of ~100 nm, which is

It △V  m

(1a)

In which I (mA) regards as current during charging-discharging, t (s) represents the time of discharging, DV (v) represents the potential windows, and m (mg) represents the active mass of electrode materials. As summarized in Fig. 5, doping S element can effectively improve the weak rate performance of SnO2 attributing to the formed nanoflower structure, which could regard as “ionbuffering reservoirs” to facilitate electrolyte ions diffusion and promote electrons transportation. And doping N element mainly enhances the capacity performance of SnO2-based electrode attributed to the reduced band gap, which will be proved by DFT calculations. Obviously, S and N elements can achieve appropriate synergy in a codoped-SnO2 system, enabling a good rate capability and high capacitance performance of S,N-codoped SnO2 on PCF substrate. The schematics illustrating the electrons transporting and electrolyte ions diffusing paths of SnO2 and S,N-codoped SnO2 are compared in Fig. 6a and b. For SnO2 nanosheet array, electrolyte ions (Hþ and SO2 4 ) exhibit difficult infiltration into the internal structure due to the concentrated two-dimensional structure, resulting in the poor rate capability. On the contrary, S,N-codoped SnO2 nanoflower array make electrolyte ions and electrons migrated readily into the interval spaces, facilitating faster ions diffusion and electrons transportation. S,N-codoped SnO2 nanoflower array can achieve the electrode with superior rate

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Fig. 5. (a) CV curves of PCF, SnO2/PCF, S or N-monodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes at 10 mv s1; (b) GCD curves of PCF, SnO2/PCF, S or N-monodoped SnO2/ PCF and S,N-codoped SnO2/PCF electrodes at 1.0 A g1; (c) specific capacitance of PCF, SnO2/PCF, S or N-monodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes at 10 mv s1, 50 mv s1 and 100 mv s1; (d) specific capacitance of PCF, SnO2/PCF, S or N-monodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes in trems of current density.

performance as follows: (ⅰ) The formed nanoflower structure of S,Ncodoped SnO2 could act as unique “ion-buffering reservoirs”, shorting electrolyte ions diffusion and electrons transportation paths; (ⅱ) The formed nanoflower structure exhibit more accessible electroactive sites, enabling well electrolyte wettability for faster ions diffusion; (ⅲ) The nanoflower structure of S,N-codoped SnO2 also plays a confined role, which is favourable for relieving the volume expansion and structure collapse during long-time cycling test. In summary, the three-dimensional nanoflower structure formed by codoping with sulfur and nitrogen atoms plays a vital role in enhancing the poor rate and cycle performance of SnO2. The kinetic behaviours of electrodes have been investigated through electrochemical impedance spectrum (EIS) measurements. Fig. 6c shows Nyquist plots and their relevant fitting results of SnO2/PCF, S or N-monodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes with frequency varying from 1 mHz to 100 kHz. Nyquist plots enlarged at high-frequency region are shown in inset pattern. Nyquist plots of electrodes all present vibrio-shaped and oblique line at high and low frequency region, respectively. In highfrequency region, S,N-codoped SnO2/PCF electrode shows lowest value of intrinsic ohmic resistance (Ro) of 1.85 U among as-prepared electrodes, reflecting its best conductivity. It is corresponding to the lowest voltage drop of GCD curves. Otherwise, N-doped SnO2/PCF

electrode exhibits lower Ro value (2.27 U) than that of S-doped SnO2/PCF (2.35 U), indicating that N dopant with higher concentration is more preferred to enhance the weak electrical conductivity of SnO2-based electrode. In the low-frequency region, Sdoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes exhibit significant increase of Nyquist plots, presenting superior capability for ion diffusion because of the 3D nanoflower array structure. The correlation between electrochemical characteristics and frequency of SnO2/PCF, S or N-monodoped SnO2/PCF and S,N-codoped SnO2/ PCF electrodes can be clearly demonstrate through the capacitancefrequency curves [62]. The capacitance of real (C0 ) and imaginary (C’0 ) parts are calculated based on equations (2) and (3). 00

C ¼

00

C ¼

Z

00

ujZj2 Z0

ujZj2

(2a)

(3a)

In which |Z| represents the module of impedance, u defined as 2pG, G indicates the frequency in EIS test. In Fig. 6d, S,N-codoped SnO2/PCF electrode shows the highest initial C0 value among the

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Fig. 6. Schematics illustrating electrolyte ions diffusing and electrons transporting paths of (a) SnO2 and (b) S,N-codoped SnO2 in H2SO4 electrolyte; (c) Nyquist plots of SnO2/PCF, S or N-monodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes (the inset displays the enlarged plots in high-frequency range); (d) the real and (e) the imaginary parts of capacitance of SnO2, S or N-monodoped SnO2 and S,N-codoped SnO2 electrodes.

as-prepared electrodes, indicating the best capacitance performance due to proper synergy between S and N dopants. As illustrated in Fig. 6e, the C’0 value obtains its highest value at a frequency G0. Defining time constant t0 ¼ 1/G0 to evaluate the reversibility of the electrodes during charging and discharging process. S,Ncodoped SnO2/PCF electrode exhibits the lowest time constant of 1.04 s, representing the best rate capability due to the 3D nanoflower structure among the as-prepared electrodes. This matches well with the CV and GCD results. Specifically, Fig. 7a exhibits the CV curves of S,N-codoped SnO2/

PCF electrode conducted on various scanning rates from 5 mV s1 to 200 mV s1. All CV curves demonstrate rectangular-shaped with apparent redox peaks around þ0.5 V ~ þ0.7 V, ascribing to the redox reaction between electronic species and oxygen vacancies of S,N-codoped SnO2. Furthermore, The CV results also display no curve polarization even at high scanning rates above 100 mV s1. Because well-ordered nanoflower architectures enabled the reactive protons sufficient diffusing into the interior of structure even at fast scanning rates. Fig. 7b exhibits the GCD curves of S,N-codoped SnO2/PCF electrode varying from 1.0 A g1 to 10 A g1. A slight

Fig. 7. (a) CV curves of S,N-codoped SnO2/PCF electrode conducted on various scannng rates; (b) GCD curves of S,N-codoped SnO2/PCF electrode conducted on various current desities; (c) multi-step results during charging-discharging of SnO2/PCF, S or N-monodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes on various current densities; (d) cycling stability of SnO2/PCF, S or N-monodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes at 5 A g1 for 5000 cycles.

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curvature of GCD curves also suggests the pseudocapacitive behaviour of S,N-codoped SnO2. The mass specific capacitances of S,N-codoped SnO2/PCF electrode were calculated on the basis of equation (1). The obtained specific capacitances of this electrode are 332.5, 310.4, 285.6, 260.8, 235.4 and 215.5 F g1 at 1.0, 2.0, 3.0, 5.0, 8.0 and 10.0 A g1, respectively. Fig. 7c shows the multiple-step charge-discharge results of SnO2/PCF, S or N- monodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes conducted on various current densities. The capacitance retentions of SnO2/PCF, S or Nmonodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes are 79.7%,100.5%, 99.8% and 103.2% as recovering to 1.0 A g1, respectively. The excellent rate capability of S,N-codoped SnO2/PCF electrode is attributed to that 3D nanoflower architecture can act as unique “ion-buffering reservoirs”, enabling the short diffusion distance for electrolyte ions migration. Cycling stability was another important factor of electrode. Fig. 7d exhibits the longterm cycling results of SnO2/PCF, S or N-monodoped SnO2/PCF and S,N-codoped SnO2/PCF electrodes at 5 A g1. SnO2/PCF electrode exhibits the weakest stability of 74.3% after 5000 cycles. This is due to the aggregation and shedding of SnO2 nanosheet array on PCF substrate. More interestingly, the long-term cyclic tests indicate the stability of N-doped SnO2/PCF and S,N-codoped SnO2/PCF electrodes exceeds 100%. The fluctuation in capacitance retain can be attributed to the activation with the nanoflower architectures of the electrode during circulations, enabling more active points and thus achieving enhanced specific capacitance of electrode. Anyway, the synergistic effect of S and N dopants was regarded as the

fundamental factor in superior electrochemical performance of SnO2-based electrodes. Thus, S,N-codoped SnO2/PCF electrode displays excellent capacitance performance, rate capability and cycling stability, presenting potential application value for highperformance supercapacitor. In principle, substitutional and interstitial nitrogen and sulfur atoms at the content of 4.17 and 2.08 atom% are existing in SnO2 lattice. Based on this, several possible models of N- and S-monodoped SnO2 were introduced in Fig. 8. Fig. 8aec shows three possible N-doped SnO2 supercells concluded interstitial two N atoms (N(in)), substituting two N atoms to O atoms(N(O)) or to Sn atoms(N(Sn)), respectively. And Fig. 8def shows three possible Sdoped SnO2 model of S(in), S(O)and S(Sn) with a S atom, respectively. Density functional theory (DFT) calculations conducted on these models to deeper elucidate the reaction mechanism. The relative stability of N and S- monodoped SnO2 supercells is compared based on the value of the formation energies (Eform), as calculated and presented in Table 1. The formation energy of N and S- monodoped SnO2 supercells can be calculated based on Equations (4) to (9) [63],respectively. E(N(in))form ¼ E(Sn16O32N2) e E(Sn16O32) e 2E(N)

(4a)

E(N(O))form ¼ E(Sn16O30N2) e E(Sn16O32) þ 2E(O) e 2E(N)

(5)

E(N(Sn))form ¼ E(Sn14O32N2) e E(Sn16O32) þ 2E(Sn) e 2E(N)

(6)

Fig. 8. The possible models of N-doped SnO2 with (a) two interstitial N atoms (N(in)); (b) two substitutional N atoms to Sn atoms (N(Sn)) and (c) to O atoms (N(O)); The possible models of S-doped SnO2 with (d) one substitutional S atom; (e) one substitutional S atom to a Sn atom (S(Sn)) and (f) one substitutional S atom to a O atom (S(O)).

J. Xu et al. / Electrochimica Acta 340 (2020) 135950 Table 1 The calculated energy and the relative formation energy of the possible N or S-doped SnO2 models. Model

Total energy (eV)

Formation energy (eV)

N(O) supercell N(Sn) supercell N(in) supercell S(O) supercell S(Sn) supercell S(in) supercell

5169.7 4751.4 4553.1 4965.2 4632.4 4459.8

1.5 2.9 4.6 2.1 3.5 5.2

E(S(in))form ¼ E(Sn16O32S) e E(Sn16O32) eE(S)

(7)

E(S(O))form ¼ E(Sn16O31S) e E(Sn16O32) þ E(O) e E(S)

(8)

E(S(Sn))form ¼ E(Sn15O32S) e E(Sn16O32) þ E(Sn) e E(S)

(9)

Where E(Sn16O30N2), E(Sn14O32N2), E(Sn16O32N2), E(Sn16O31S), E(Sn15O32S) and E(Sn16O32S) present the total energy of system. The energy of single N, O, S and Sn atoms are represented by E(N), E(O), E(S) and E(Sn), respectively. Among the N- or S- monodoped SnO2 supercells, N(O) and S(O) supercells are energetically-favourable because of their lowest energy (5169.7 eV, 4965.2 eV) and formation energy (1.5 eV, 2.1 eV), respectively. On the contrary, N(in) and S(in) supercells are very difficult with the highest energy (4553.1 eV and 4459.8 eV) and formation energy (4.6 eV and 5.2 eV) because of the mismatching of system and dopants. Therefore, N and S atoms are preferentially substituting the O sites in SnO2 [64,65]. The lowest energy and formation energy of N(O) among these mono doped SnO2 supercells demonstrates nitrogen element is more easily for substituting oxygen atoms and more valuable for system stability when compared with S element. It is corresponding to the results of XPS and cycling measurements. According to the above results, the optimized supercells of 2  2  2 SnO2, N-or S- monodoped SnO2 and S,N-codoped SnO2 supercells were constructed in Fig. 9aed. The codoped system was created by simultaneous substitution of oxygen atoms with a sulfur atom and two nitrogen atoms at the content of 2.08 and 4.17 atom%, respectively. It is approximately matching with the doping concentration of N and S in this experiment. To further understand the correlation between the electronic structure and electrochemical properties of monodoped and codoped SnO2, DFT calculations were

9

introduced on these four models. The distributions of electron density are directly demonstrated in Fig. 9eeh for SnO2, S or Nmonodoped SnO2 and S,N-codoped SnO2 supercells. This reflects the valence bond and electron transfer of atoms in interface layer. The red colour demonstrates electrons depletion, while blue colour represents electrons accumulation and collection, respectively. The substitutional N and S are bound to adjacent Sn atoms S,N-codoped SnO2 supercells. Fig. 9b and c shows the distributions of differential electron density at longitudinal slice of S-doped SnO2 and N-doped SnO2 supercells, respectively. So, compared with the electron density distribution of pristine SnO2, it could be seen from that of doped SnO2 as follows: (ⅰ) S-doped SnO2 supercell displays more uniform distribution of electron density than that of N-doped SnO2, meaning more efficient charge transfer and better rate performance. It is matched with the electrochemical performance of Sdoped SnO2; (ⅱ) the electron clouds overlap between Sn, O, and dopants in S,N-codoped SnO2 systems, offering the reactive sites in interface to form SneS, SneN and weaker OeS, OeN bonds, which is corresponding to XPS results; (ⅲ) the charge density becomes more uniform and the oxygen vacancies are more abundant after codoping S and N elements in SnO2, enabling better rate performance and electronic conductivity. It matches with the electrochemical performance S,N-codoped SnO2. The band structures of SnO2, S or N-monodoped SnO2 and S,Ncodoped SnO2 could reflect the correlation between electrochemical properties and electronic conductivity. As shown in Fig. 10a, the low edge in conduction band and high edge in valence band of SnO2 are both located in point G belong to Brillouin zone, suggesting SnO2 based semiconductor demonstrate a direct bandgap of 3.2 eV. However, the obtained band gap of SnO2 exhibit lower value than 3.6 eV in theoretical because of the overestimated energy of Sn 5s in calculations, resulting the stronger correlation between Sn 5s and O 2p and eventually leading the lower bandgap. For mono doped SnO2 cell in Fig. 10b and c, S or N substituted in SnO2 diminished the bandgap by occupying conduction and valence bands near Fermi level, faciling electronic conduction and excitation. The mono N-substituted supercell (1.7 eV) exhibit lower bandgap than mono S-substituted (2.0 eV). It is consistent with lower Ro value and IR drop of N-doped SnO2. Fig. 10d exhibits the energy band structure of S,N-codoped SnO2. The bandgap of codoped SnO2 system (1.2 eV) shows much lower than that of mono doped SnO2, indcating best electronic conductivity.

Fig. 9. Supercell and the distribution of differential electron density at longitudinal slice of (a),(e) SnO2; (b),(f) S-doped SnO2; (c),(g) N-doped SnO2 and (d),(h) S,N-codoped SnO2, respectively.

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J. Xu et al. / Electrochimica Acta 340 (2020) 135950

Fig. 10. Band structures of (a) SnO2, (b) S-doped SnO2, (c) N-doped SnO2 and (d) S,N-codoped SnO2 supercells, respectively.

To get insight into the origin of performance improvement through S and N doping effect, the electronic distributions of SnO2, S or N-monodoped SnO2 and S,N-codoped SnO2 are shown in Fig. 11. As illustrated in Fig. 11a, O 2p electronic states in SnO2 occupied mostly near Fermi level, while the unoccupied electronic states result from hybridizing of Sn 5s and 5p. In S,N-codoped SnO2 cell, as shown in Fig. 11d, the strong hybridization and orbital coupling of S 3p, N 2p and O 2p electronic states occupied around Fermi level. Otherwise, O 2p and Sn 5p electronic states form the peaks of 2 ~ 5 eV in coupling. Most electronic states around

Fermi level of S,N-codoped SnO2 could be observed by comparing the DOS of four cells, indicating a highest electronic donation of S,N-codoped SnO2 crystal for electrochemical reactions [52]. It matches with the best electrochemical performance of S,Ncodoped SnO2 among the as-prepared electrodes. To demonstrate the potential application of S,N-codoped SnO2 electrode, the asymmetric microsupercapacitor in all-solid-state was assembled in wire shape using S,N-codoped SnO2/PCF and TiN as positive and negative electrodes, respectively, as schematically shown in Fig. 12. The schematic illustration for the assembled

Fig. 11. Total and partial density of states of (a) SnO2, (b) S-doped SnO2, (c) N-doped SnO2 and (d) S,N-codoped SnO2 supercells, respectively.

J. Xu et al. / Electrochimica Acta 340 (2020) 135950

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Fig. 12. Schematic illustrations for the (a) assembly and (b) construction of S,N-codoped SnO2/PCF/TiN microsupercapacitor (the inset showed the optical photograph of as-prepared microsupercapacitor).

S,N-codoped SnO2/PCF//TiN supercapacitor is shown in Fig. 12a. TiN electrode was synthesized through anodization and then procedural calcination in an ammonia atmosphere of Ti wire. The microsupercapacitor in wire shape was fabricated by twisting S,Ncodoped SnO2/PCF electrode around TiN electrode with non-woven fabric and polyethylene heat-shrinkable tube as separation layer and packaging material, respectively. Fig. 12b displays the construction of S,N-codoped SnO2/PCF//TiN microsupercapacitor, consisting of TiN and S,N codoped SnO2/PCF electrodes, PVA-H2SO4 gel electrolyte, separation and packaging layer. The total volume of this microsupercapacitor is around 0.374 cm3. The electrochemical behaviors of TiN wire are investigated in a three-electrodes system in 1.0 M H2SO4 aqueous electrolyte. Fig. 13a and b shows CV curves at different scan rates and GCD curves at different current densities, respectively. TiN wire presents the wide potential window of 1.2 V and small intrinsic ohmic resistance of 1.1 U. The specific capacitance of TiN wire was 108.3 F g1 at 1.0 A g1. Fig. 13c shows the CV curves of S,N codoped SnO2 and TiN at a scan rate of 10 mV s1 with a voltage range from 0.6~ þ 0.6 V and 0 ~ þ1.0 V in the three-electrode configuration, respectively. The potential window of this microsupercapacitor can be expanded to 1.6 V or more. Fig. 13d shows a series of CV measurements of the microsupercapacitor at a scan rate of 5 mV s1 with different voltages varying from 1.0 V to 1.8 V. The presence of obvious redox peaks indicates that the pseudocapacitive properties of assembled device originate from the two electrodes. Furthermore, with an increase of the operating potential to 1.8 V, more polarization occurred. So, a voltage window of 0 ~ þ1.6 V is selected for the subsequent research work. Moreover, Fig. 13c displays the CV curves of TiN and S,N codoped SnO2/PCF electrodes at 10 mV s1 with the voltages of 0.6 ~ þ 0.6 V and 0 ~ þ1.0 V, respectively. Considering the respective potential of TiN and S,N codoped SnO2/PCF electrodes, the working voltage of the microsupercapacitor could be achieved to 1.6 V or more. Fig. 13f exhibits the CV curves of S,N-codoped SnO2/PCF/TiN microsupercapacitor at 5 mV s1 with a voltage varying from 1.0 V to 1.8 V. However, more polarization occurred as the voltage up to

1.8 V. Thus, 1.6 V was obtained as the optimum operating voltage for this microsupercapacitor. Fig. 13e exhibits that the GCD curves of S,N-codoped SnO2/PCF/TiN microsupercapacitor are also still well maintained at the voltage window of 1.6 V at 10 mA cm3, suggesting good electrochemical reversibility. Fig. 13f exhibits the CV curves with consistent redox peaks at various scan rates from 5 to 200 mV s1, revealing a combination of pseudocapacitive properties of TiN and S,N codoped SnO2/PCF electrodes. Fig. 13g shows the GCD measurements conducted varying from 10 mA cm3 to 100 mA cm3. The volumetric capacitance of this microsupercapacitor, based on the device volume, dropped from 1.5 F cm3 to 0.9 F cm3, demonstrating the superior capacitance retention of 60.0%. Fig. 13h exhibits the long-term cycling test of S,N codoped SnO2/PCF//TiN microsupercapacitor at 50 mA cm3. The capacitance of this device could maintain 98.7% after 2000 cycles. This demonstrates the excellent cyclability of SnO2/PCF//TiN microsupercapacitor. The inset in Fig. 13h exhibits pictures of a red light-emitting-diode (LED) lamp bulb with the nominal voltage of 1.8 V and electric fan with the nominal power of 1.0 W handled by S,N codoped SnO2/PCF//TiN microsupercapacitor, holding potential for the supercapacitors with high-performance. Overall, the excellent performance of this microsupercapacitor is mainly ascribed to the superior electrochemical behaviors of electrodes and the matching effect between the electrode and gel electrolyte. The Ragone plots of S,N-codoped SnO2/PCF//TiN microsupercapacitor and other reported supercapacitors are obtained and shown in Fig. 13i. Table 2 lists Ragone data of S,N-codoped SnO2/PCF//TiN in comparison with other reported supercapacitors. The maximum volumetric power density of 80 mW cm3 and volumetric energy density of 0.55 mW h cm3 for this microsupercapacitor were achieved under a wide voltage window of 1.6 V, which are higher than other reported supercapacitors, including SnOxNy/ACF//TiN (0.43 mW h cm3, PVA/ H2SO4) [39], MnO2/HeZnO//CFs (0.04 mW h cm3, PVA/LiCl) [66], MnO2/CFs-based SC (0.22 mW h cm3, PVA/H3PO4) [67], TiN-based SC (0.05 mWh cm3, PVA/KOH) [68]. The superior performance of S,N-codoped SnO2/PCF//TiN device indicates its potential

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J. Xu et al. / Electrochimica Acta 340 (2020) 135950

Fig. 13. (a) and (b) CV and GCD curves of TiN electrode at various scan rates and current densities, respectively; (c) CV curves of TiN and S,N-codoped SnO2/PCF electrodes at 10 mVs1; (d) and (e) CV and GCD curves of S,N-codoped SnO2/PCF//TiN microsupercapacitor at 5 mV s1 and 10 mA cm3 in various voltage windows, respectively; (f) and (g) CV and GCD curves of S,N-codoped SnO2/PCF//TiN microsupercapacitor at various scan rates and current densities, respectively; (h) Cycling performance of S,N-codoped SnO2/PCF//TiN microsupercapacitor at 50.0 mA 3 for 2000 cycles. (the inset showed S,N-codoped SnO2/PCF//TiN microsupercapacitor powering a red LED and driving small electric fan). (i) Ragone plots of S,N-codoped SnO2/PCF//TiN microsupercapacitor and in comparison with other reported supercapacitors.

Table 2 Ragone data of S,N-codoped SnO2/PCF//TiN in comparison with other reported supercapacitors. Supercapacitor

E (mW h cm3)

P (mW cm3)

Ref.

SnOxNy/ACF//TiN MnO2/HeZnO/CFs MnO2/CFs-based SC TiN-based SC S,N-codoped SnO2/PCF//TiN

0.43 0.04 0.22 0.05 0.55

8.00 2.44 8.00 1.25 8.00

[39] [66] [67] [68] This work

application value.

4. Conclusions DFT-based calculations and electrochemical investigations of S or N-monodoped and S,N-codoped SnO2 have helped us to get insight into S or N-monodoping and S,N-codoping effect on SnO2. As corroborated by simulation calculation and experiment validation, N doping benefits the capacitance enhancement due to the

lower bandgap, while S doping predominantly facilitates the electron transfer due to favoring the uniform electron density distribution. The S,N-codoped SnO2 shows most narrow bandgap (1.2 eV) and homogeneous electron density distribution among S or N-monodoped SnO2 (2.0 and 1.7 eV) and bare SnO2 (3.2 eV). As a result, the remarkable specific capacitance, rate capability and cycle life were obtained in S,N-codoped SnO2/PCF electrode. A microsupercapacitor based on S,N-codoped SnO2/PCF and TiN electrodes delivers superior energy densities of 0.55 mW h cm3 at a power densities of 8 mW cm3, while maintaining a high cycle stability of 98.7% after 2000 cycles at 50 mA cm3. S,N-codoped SnO2/PCF electrode is believed to hold potential for the next generation of supercapacitors with high-performance. The results of DFT calculations and experimental measurements confirm favourable dianion doping for SnO2-based electrodes, providing an effective way to fabricate SnO2-based materials for energy storage. Credit author statement CRediT

authorship

contribution

statement,

Jing

Xu:

J. Xu et al. / Electrochimica Acta 340 (2020) 135950

Conceptualization, Methodology, Software, Investigation, Writing Original Draft. Yakui Mu: Validation, Formal analysis, Data Curation. Chaohui Ruan: Validation, Formal analysis, Data Curation. Pengxi Li: Validation, Formal analysis, Data Curation. Yibing Xie: Supervision, Conceptualization, Formal analysis, Data Curation, Writing Review & Editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21373047), the Jiangsu Graduate Innovation Program, and the Priority Academic Program Development for Jiangsu Higher Education Institutions. References [1] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797e828. [2] P.X. Li, C.H. Ruan, J. Xu, Y.B. Xie, A high-performance asymmetric supercapacitor electrode based on a three-dimensional ZnMoO4/CoO nanohybrid on nickel foam, Nanoscale 11 (2019) 13639e13649. [3] Y. Xie, Electrochemical performance of transition metal-coordinated polypyrrole: a mini review, Chem. Rec. 19 (2019) 1370e1384. [4] L. Guardia, L. Suarez, N. Querejeta, V. Vretenar, P. Kotrusz, V. Skakalova, T.A. Centeno, Biomass waste-carbon/reduced graphene oxide composite electrodes for enhanced supercapacitors, Electrochim. Acta 298 (2019) 910e917. [5] G.A. Snook, P. Kao, A.S. Best, Conducting-polymer-based supercapacitor devices and electrodes, J. Power Sources 196 (2011) 1e12. [6] G.P. Wang, L. Zhang, J.J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797e828. [7] P. Li, C. Ruan, J. Xu, Y. Xie, Enhanced capacitive performance of CoO-modified NiMoO4 nanohybrid as advanced electrodes for asymmetric supercapacitor, J. Alloys Compd. 791 (2019) 152e165. [8] Y. Xie, C. Yao, Electrochemical performance of RuO2-TiO2 nanotube hybrid electrode material, Mater. Res. Express 6 (2019) 125550. [9] Y. Xie, Y. Zhou, Enhanced capacitive performance of activated carbon paper electrode material, J. Mater. Res. 34 (2019) 2472e2481. [10] Y. Mu, C. Ruan, P. Li, J. Xu, Y. Xie, Enhancement of electrochemical performance of cobalt (II) coordinated polyaniline: a combined experimental and theoretical study, Electrochim. Acta 338 (2020) 135881. [11] Y. Xie, Y. Zhang, Electrochemical performance of carbon paper supercapacitor using sodium molybdate gel polymer electrolyte and nickel molybdate electrode, J. Solid State Electrochem. 23 (2019) 1911e1927. [12] Y. Wang, Y. Xie, Electroactive FeS2-modified MoS2 nanosheet for highperformance supercapacitor, J. Alloys Compd. 284 (2020) 153936. [13] C. Ruan, P. Li, J. Xu, Y. Xie, Electrochemical performance of hybrid membrane of polyaniline layer/full carbon layer coating on nickel foam, Prog. Org. Coating 139 (2020) 105455. [14] P. Li, C. Ruan, J. Xu, Y. Xie, Supercapacitive performance of CoMoO4 with oxygen vacancy porous nanosheet, Electrochim. Acta 330 (2020) 135334. [15] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Tin-based amorphous oxide: a high-capacity lithium-ion-storage material, Science 276 (1997) 1395e1397. [16] X. Zhou, Z. Dai, S. Liu, J. Bao, Y.-G. Guo, Ultra-uniform SnOx/carbon nanohybrids toward advanced lithium-ion battery anodes, Adv. Mater. 26 (2014) 3943e3949. [17] S.H. Lee, M. Mathews, H. Toghiani, D.O. Wipf, C.U. Pittman Jr., Fabrication of carbon-encapsulated mono- and bimetallic (Sn and Sn/Sb alloy) nanorods. Potential lithium-ion battery anode materials, Chem. Mater. 21 (2009) 2306e2314. [18] W. Wang, Q. Hao, W. Lei, X. Xia, X. Wang, Graphene/SnO2/polypyrrole ternary nanocomposites as supercapacitor electrode materials, RSC Adv. 2 (2012) 10268e10274. [19] S. Ren, Y. Yang, M. Xu, H. Cai, C. Hao, X. Wang, Hollow SnO2 microspheres and their carbon-coated composites for supercapacitors, Colloids Surf., A 444 (2014) 26e32. [20] S.P. Lim, N.M. Huang, H.N. Lim, Solvothermal synthesis of SnO2/graphene nanocomposites for supercapacitor application, Ceram. Int. 39 (2013) 6647e6655. [21] S.N. Pusawale, P.R. Deshmukh, J.L. Gunjakar, C.D. Lokhande, SnO2-RuO2 composite films by chemical deposition for supercapacitor application, Mater.

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