Abstract
Gasified rice husk carbon, which is a byproduct of power generation by gasification, can be converted to porous carbon (RHAC). This product is environmentally friendly, has excellent electrochemical performance, and represents a high value utilization of biomass resources. In this paper, the heterostructured nano composites were synthesized by a simple hydrothermal reaction. RHAC loaded sulfur-doped tin oxide was used to synthesize composites with a highly conductive porous structure, short ion/electron transport path, and enhanced pseudo capacitance kinetics. The specific capacitance of this composite was improved over that of biomass porous carbon RHAC. At a current density of 1.5 A/g, the specific capacitance of S-doped RHAC/SnO2 composite, RHAC/SnO2 composite, and RHAC were 215 F/g, 177 F/g, and 141 F/g, respectively. The current density was increased from 1 A/g to 5 A/g, and the specific capacity of the S-doped RHAC/SnO2 composite was maintained at 67% with good rate performance. At a current density of 0.4 A/g, the charge capacity was maintained at 78.5% after 5000 cycles of charge and discharge, indicating that the electrode has a long cycle life.
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Gasified Rice Husk Porous Carbon Loaded S-doped Tin Oxide Composite for Supercapacitor Electrode
Huilin Wang,a,b,c Dajun Wu,b Jianbin Zhou,a,* and Gang Yang b,*
Gasified rice husk carbon, which is a byproduct of power generation by gasification, can be converted to porous carbon (RHAC). This product is environmentally friendly, has excellent electrochemical performance, and represents a high value utilization of biomass resources. In this paper, the heterostructured nano composites were synthesized by a simple hydrothermal reaction. RHAC loaded sulfur-doped tin oxide was used to synthesize composites with a highly conductive porous structure, short ion/electron transport path, and enhanced pseudo capacitance kinetics. The specific capacitance of this composite was improved over that of biomass porous carbon RHAC. At a current density of 1.5 A/g, the specific capacitance of S-doped RHAC/SnO2 composite, RHAC/SnO2 composite, and RHAC were 215 F/g, 177 F/g, and 141 F/g, respectively. The current density was increased from 1 A/g to 5 A/g, and the specific capacity of the S-doped RHAC/SnO2 composite was maintained at 67% with good rate performance. At a current density of 0.4 A/g, the charge capacity was maintained at 78.5% after 5000 cycles of charge and discharge, indicating that the electrode has a long cycle life.
Keywords: Gasified rice husk active carbon; Composites; Supercapacitor electrode
Contact information: a: College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China; b: Changshu Institute of Technology, Changshu 215500, China; c: Suzhou Agricultural Modernization Research Center, Changshu 215500, China;
* Corresponding author: whl@cslg.edu.cn
INTRODUCTION
Mankind has entered a period of conflict between energy supply and demand, hence the use of green and renewable energy has become an inevitable trend of sustainable development. The exploitation of biomass energy is a very effective solution. Biomass sources have a large production volume, and the comprehensive utilization efficiency is relatively low. There has been much research on the development and utilization of biomass energy. The application of biomass-based carbon materials in the field of supercapacitors has become a research hotspot in the field of biomass resource utilization in recent years. The internal structure of biomass resources based on agricultural and forestry wastes makes it an ideal precursor for the production of porous carbon for the application to supercapacitor electrode materials (He et al. 2013; Chen et al. 2017; Tang et al. 2017). Supercapacitors are widely used as promising energy storage devices with the advantages of short charging time, high power density, and long cycle life (Lang et al. 2011). The performance of supercapacitors is closely related to electrode materials. Carbon materials have the advantages of good stability, low cost, long cycle life, and high specific power, but their energy density is relatively low (Lang et al. 2011; Kim et al. 2016).
Metal oxides undergo a reversible redox reaction that provides pseudo-capacitance, which increases the specific capacitance and specific energy of the electrode material (Zhou et al.2013a; Chuanxiang et al. 2016; Huang et al. 2016). SnO2 is a semiconductor material with abundant raw materials, low cost, high electrochemical activity, high power density, and environmental friendliness (Jin et al. 2011; Jiang et al. 2012). It forms rich nanostructures (Wang and Rogach 2014) and has been widely used in lithium ion batteries (Joshi et al. 2016; Ma et al. 2016; Madian et al. 2016; Peng et al. 2016; Xia et al. 2016). However, SnO2 has strong polarity, and therefore, SnO2 particles are prone to agglomeration, hindering the insertion and extraction of electrolyte ions, and low electrochemical utilization, which limits its application in supercapacitors (Lim et al. 2012; Deosarkar et al. 2013). When tin dioxide is compounded with other materials such as metal oxides (Zhou et al. 2018) and carbon-based materials (Li et al. 2012; Lim et al. 2013), the composite electrode combines the advantages of these materials, which is an effective method for improving the electrochemical performance of tin dioxide. Composites with high specific surface area and porosity are used to improve the defects of SnO2 structures (Hwang and Hyun 2007). SnO2 composites are prepared by different methods and applied to supercapacitor electrode materials. Hwang and Hyun (2007) impregnated the SnCl4 solution into the carbon aerogel electrode to obtain a SnO2/carbon aerogel composite, which is calculated by constant current charge and discharge method. The specific capacitance is 69.8 F/g at a current density of 10 mA/g. Rakhi et al. (2012) used a chemical method to load SnO2 on graphene nanosheets, where the specific capacitance of SnO2/graphene nanosheets was 195 F/g. Mu et al. (2011) made a heterostructure of SnO2/carbon nanofibers by electrospinning and template solvothermal method. The maximum specific capacitance of the composite sample was 187 F/g at a scan rate of 20 mV/s. Li et al. (2012) synthesized the SnO2/SWCNTs core-shell nanowires by electrodeposition technology with higher specific capacitance 320 F/g, at the scanning rate of 6 mV/s.
Although the above SnO2/carbon composites have high electrochemical performance, the preparation of expensive carbon-based materials and the complicated techniques for synthesizing composites hinder their industrial application. In this paper, based on the preparation of porous carbon (RHAC) from gasification power generation by-product gasified rice husk carbon, the composite of RHAC loaded SnO2 was further studied as a supercapacitor electrode. The S-doped micro-mesoporous SnO2/RHAC composites were synthesized by a simple hydrothermal method and used to prepare supercapacitor electrode materials. The in situ synthesis method is simple, with low costs, and the electrochemical performance of the electrodes is excellent.
EXPERIMENTAL
Materials
RHAC was prepared from the byproduct of rice husk gasification power generation boiled with KOH solution and activated by CO2, as previously described (Wang et al. 2018). SnCl2·2H2O, ethanediamine, CH4N2S, Ni foam, and absolute ethanol were obtained from Shanghai Sinopharm Chemical Co., Ltd. of Shanghai, China.
Methods
For the preparation of S-doped RHAC/SnO2 composites, 1 mmol SnCl2·2H2O and 2 mmol of NH2CSNH2 were dissolved in 30 mL of deionized (DI) water; 120 mg of RHAC was dissolved in 30 mL of deionized (DI) water by ultrasonication for 10 min and then added into the SnCl2 solution with continuous stirring. After stirring at room temperature for 2 h, the mixture was transferred into a Teflon-lined stainless steel autoclave. The sealed autoclave was kept at 160 °C for 12 h in an oven and then cooled to room temperature. The products were washed with DI water and absolute ethanol three times and finally dried in a vacuum oven at 70 °C. The product was annealed in N2 atmosphere at 400 °C for 2 h. For comparison, the RHAC/SnO2 composite was synthesized by the solvothermal method without NH2CSNH2, 1 mmol SnCl2·2H2O and 270 µL of ethanediamine were dissolved in 30 mL of glycol, and 120 mg of RHAC was dissolved in 30 mL of deionized (DI) water.
Characterization
The pore structure parameters of porous carbon were analyzed by Q10 automatic analyzer (Quantachrome Corporation, Boynton Beach, FL, USA). Using nitrogen gas as an adsorbent, the absorption/desorption experiment was performed at 77 K. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method, and the distribution was calculated by the density functional theory (DFT) method. The morphology of the product was analyzed by scanning electron microscopy (JSM-7600F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM, JEM-2100F, JEOL). The structure and phase of the sample were analyzed by X-ray powder diffraction (XRD; D/Max-2200/PC, Rigaku, Tokyo, Japan), and scanning was performed at a speed of 5 °/min and a range of 2 to 80°. X-ray photoelectron spectroscopy (XPS, escalab250xi, Thermo Scientific, Waltham, MA, USA) was used to study the surface chemical state of porous carbon composites, while the Gaussian function and Lorentz function were used to fit the curve for XPS analysis.
Electrochemical Measurements
The foamed nickel was cut into a size of 1 cm × 2 cm. It was ultrasonicated in acetone solution, 0.5 M HCl solution, deionized water, and ethanol for 15 min, respectively, to wash the surface oil impurities and oxide film. It was then dried in a vacuum drying oven. The prepared composite material was mixed with acetylene black and polytetrafluoroethylene according to a mass ratio of 8:1:1, and N- Methylpyrrolidone (NMP) was used as a solvent. The above mixture was stirred into a well-mixed slurry, then uniformly coated on a foamed nickel collector, dried in a vacuum oven at 80 °C for 12 h, and removed at room temperature. Tableting was carried out at a pressure of 10 MPa. The working electrode area was 1 cm x 1 cm, and the mass loading of the electrode material was approximately 4 mg. The traditional three-electrode test was carried out in the electrochemical workstation (Chenhua, Shanghai, China) containing 6.0 M KOH aqueous solution as the electrolyte. The S-doped RHAC/SnO2 composite, RHAC/SnO2 composite, and RHAC electrodes were used as the working electrode, respectively, and a standard calomel electrode (SCE) was used as the reference electrode and the platinum plate as counter electrode. The cyclic voltammetry (CV), constant current charge and discharge (GCD) and AC impedance (EIS) tests were performed. The mass specific capacitance at different charge and discharge current densities was determined according to Eq. 1,
C = I·t / (m·ΔV) (1)
where C is the specific capacitance (F/g), I is the discharge current (A), t is the discharge time (s), m is the mass of the active materials in the single electrode (g), and ΔV is the potential difference (V) in the discharge process.
RESULTS AND DISCUSSION
Morphology and Structure Analysis of RHAC/SnO2 and S-doped RHAC/SnO2
The XRD pattern of the composite material is shown in Fig. 1. It can be seen that the composite material of RHAC/SnO2 and the S-doped RHAC/SnO2 heterostructure composite obtained by hydrothermal reaction. Excluding the C(002) and C(100) diffraction peaks from RHAC, the XRD pattern of RHAC/SnO2 consisted of SnO2 and a small amount of SnO. Both RHAC/SnO2 and S-doped RHAC/SnO2 composites had significant SnO2 characteristic diffraction peaks at 2θ = 26.6°, 33.9°, 51.8°, 65.9°, 71.3°, and 78.7° (JCPDS No. 41-1445). In addition, there were distinct S-signal diffraction peaks at 2θ = 33.8°, 38.8°, 43.7°, and 70.8° (JCPDS No. 20-1225) in S-doped composites. The broad peak indicated the low crystal structure of SnO2 in the S-doped RHAC/SnO2 nanocomposite.
Fig. 1. XRD pattern of RHAC, RHAC/SnO2, and S doped RHAC/SnO2
Fig. 2. SEM and TEM of RHAC/SnO2 (a,b,c) and S-doped RHAC/SnO2 (d,e,f)
The morphology of the sample was characterized by scanning electron microscopy and transmission electron microscopy. The SEM and TEM images of RHAC/SnO2 and S-doped RHAC/SnO2 are shown in Fig. 2 (a-c) and (d-f), respectively. Figures 2a and 2d show two surface-like irregular nano particles composite materials that tin oxide uniformly coated on the surface of the porous carbon. As shown in Fig. 2b and Fig. 2e, the spherical particles are assembled from smaller nanoparticles. As shown in Fig. 2c and Fig. 2f, which are TEM (HRTEM) images of the composite, a lattice fringe spacing of 0.33 nm corresponding to the interplanar spacing of (110) of SnO2. The distortion of the lattice fringes further confirmed the low crystallization characteristics of SnO2 in the S-doped RHAC/SnO2 nanocomposites.
Fig. 3. (a) N2 adsorption/desorption isotherms and (b) Pore size distribution calculated by DFT methods of RHAC, S-doped RHAC/SnO2, RHAC/SnO2
Figure 3 shows the pore structure of RHAC, RHAC/SnO2, and S-doped RHAC/SnO2 composites. The specific surface area, and pore size distribution of porous carbon and its composites were analyzed by N2 adsorption/ desorption isotherm and pore size distribution curves. Figure 3a shows the I + IV isotherm (IUPAC classification) with high adsorption capacity of RHAC, indicating that the RHAC had abundant micropores and a certain amount of mesopores. When the relative pressure was less than 0.1, the sample adsorption amount rose rapidly, indicating that it had a developed microporous structure. In addition, capillary condensation occurred in the adsorbate, allowing the desorption isotherm to be above the adsorption isotherm, causing adsorption hysteresis. When the relative pressure exceeded 0.95, the adsorption amount rose again rapidly and failed to reach adsorption saturation at a higher partial pressure. Additionally, the adsorption-desorption hysteresis loop appears, showing a type IV isotherm, a representative H4 hysteresis loop, indicating the presence of a certain amount of mesopores (Yue et al. 2016).
The composite RHAC/SnO2 and S-doped RHAC/SnO2 have similar pore structure with RHAC. When the relative pressure was less than 0.1, the adsorption amount of the sample increased rapidly. When compared with RHAC, the adsorption amount of decreased, indicating that the micropores of the composite decreased. After SnO2 was loaded on RHAC, it filled most of the micropores in RHAC. As the relative pressure increases, the adsorption and desorption curves of the composites were lower than RHAC, but they still exhibited adsorption and desorption hysteresis loops. This was because after the SnO2 crystal particles were loaded on the RHAC, after filling some of the micropores and mesopores in the RHAC, the specific surface area and pore structure of the composite were greatly reduced, but the mesoporous structure was still preserved. One part of these mesopores were the original mesopores in RHAC that were not filled, and the other part were due to nano tin oxide filling most of the micropores in RHAC and generating mesopores.
The pore size distribution measured by the non-localized density function theory (NLDFT) is shown in Fig.3 b. According to the Brunauer-Emmett-Teller (BET) analysis, the specific surface areas of RHAC, S-doped RHAC/SnO2, and RHAC/SnO2 were 1383, 595, and 478, m2·g-1 respectively. The specific surface area of S-doped RHAC/SnO2 composites were better than those of RHAC/SnO2. It may be that excess thiourea is decomposed into H2S, NH3, CO2, and other gases at high temperature (Dong et al. 2011). These gases escape from the composite, resulting in a certain pore structure on the surface of the nano tin oxide. Therefore, this rich porosity S-doped composite is beneficial for the application of supercapacitor electrode materials.
Fig. 4. FTIR of RHAC/SnO2 and S-doped RHAC/SnO2
The composites were characterized by FTIR, as shown in Fig. 4. The peaks of RHAC/SnO2 and S-doped RHAC/SnO2 were mainly at 3493, 1620, 1385, 1120, and 645 cm-1, and functional groups were -OH, C=H, C-H /S=O, C-O /S=O, and C-H, respectively. The peaks of S-doped RHAC/SnO2 at 1385 and 1120 cm-1 were stronger than those of RHAC/SnO2, which was an expression of S=O, indicating that S was obviously doped.
X-ray photoelectron spectroscopy (XPS) was used to further evaluate the composition and chemical state of the elements on the surface of RHAC/SnO2 and S-doped RHAC/SnO2composites. The elemental composition of the composites obtained by XPS analysis is shown in Table 1.
Table 1. Element Composition of Composites Obtained by XPS Analysis
Figure 5a shows the XPS test spectrum of two composite materials. The RHAC/SnO2 consisted of three elements: Sn, O, and C, but four elements of Sn, S, O, and C can be observed in S-doped RHAC/SnO2. Both of them showed no significant impurities, which further confirms the formation of composite materials. Figure 5b shows the XPS lines of the Sn 3d elements of the two composites. Both samples showed two peaks at 487.6 and 496 eV, respectively, corresponding to Sn 3d5/2 and Sn 3d3/2. The peak distance between the two peaks is 8.4 eV, and the spin energy separation indicated that both sample surfaces were Sn4+ (Wang et al. 2015; Yue et al. 2016; Gu et al. 2017).
Fig. 5. (a) Survey, (b) Sn 3d region, (c) C 1s region, (d) O 1s region, and (e) S 2p region XPS spectra of S-doped RHAC/SnO2 and RHAC/SnO2
The XPS lines of the C 1s elements of the RHAC/SnO2 and S-doped RHAC/ SnO2 samples are shown in Fig. 5c. Each spectrum can be fitted to C-C/C=C (284.8 eV), C-O (286.1 eV), C=O (287.3 eV), and O-C=O (289.5 eV) (Figueiredo et al. 1999; Mooss and Athawale 2016), where the S-doped RHAC/SnO2 sample also had a C-S fit peak at the 286.1 eV position (Bhaskar et al. 2014). The XPS line of the C 1s element demonstrates the presence of RHAC in the composite. As shown in Table 2, the relative peak areas of C-C/C=C in the two composite samples were 66.6% and 64%, respectively, and the peak positions were basically the same, indicating that the carbons in the two samples had similar structures.
Figure 5d shows the XPS line of the O 1s element of the RHAC/SnO2 and S-doped RHAC/SnO2 samples. The fitting peak of the O 1s element in the RHAC/SnO2 sample consists of Sn-O (531.5 eV), C=O (532.8 eV) and O-C/O-H (534.2 eV). The fitted peak the O 1s element in the S-doped RHAC/SnO2 sample consists of Sn-O/S=O (531.1 eV), C=O (532 eV) and O-C/O-H (533.5 eV) (Zhou et al. 2013b; Bhaskar et al. 2014; Yue et al. 2016). Due to the doping of S, there is S=O fitting peak at the 531.1 eV position in the S-doped RHAC/SnO2 sample, as shown in Table 3. The C=O and O-C/O-H peaks are derived from RHAC, while the Sn-O peak belongs to SnO2.
Figure 5e shows the XPS line of the S 2p element of the S-doped RHAC/SnO2 sample. The high-resolution S 2p spectrum shows the S 2p double valence at 164 eV and 162.8 eV with a peak spacing of 1.2 eV. Corresponding to the spin orbitals S 2p1/2 and S 2p3/2 (Jönsson et al. 2003; Rufford et al. 2008; Bhaskar et al. 2014), respectively. The introduction of electron-rich S atoms in the carbon material improves the surface environment of the carbon material and increases the polarization and electron conductivity of the surface of the electrode material, which is beneficial to the electrochemical performance of the material. Conversely, S combines with C and O to form a sulfur-containing functional group, which undergoes a redox reaction during charge and discharge to generate a pseudo capacitance and stores more charge by generating a Faraday reaction at these sites, which has a positive effect on capacitance (Wang et al. 2014; An et al. 2016; Deng et al. 2018; Jiang et al. 2018; Ji et al. 2019).
Table 2. (a) XPS Peak Fitting Results of RHAC/SnO2 and (b) S-doped RHAC/SnO2 in the C1s Region
Table 3. (a) XPS Peak Fitting Results of RHAC/SnO2 and (b) S-doped RHAC/SnO2 in O1s Region
Electrochemical Properties of RHAC/SnO2 and S-doped RHAC/SnO2
The electrochemical properties of the product were investigated using a three-electrode device. The cyclic voltammetry (CV) curves for RHAC, RHAC/SnO2, and S-doped RHAC/SnO2 at the same scan rate of 20 mV/s are shown in Fig. 6a. The CV curve of RHAC shows no redox peaks, which is a typical electric double layer capacitor. While RHAC/SnO2 and S-doped RHAC/SnO2 exhibited a tiny hump, there was a redox reaction in the electrode material. Compared with pure biochar, the metal oxide SnO2 and S-doped have positive significance to specific capacitance. The integrated area of the CV curve of the RHAC/SnO2 material is smaller than that of the S-doped RHAC/SnO2, indicating that the S-doped composite has a higher specific capacitance. On one hand, due to the low crystallinity of the synthesized tin oxide, the low crystal or amorphous form can achieve faster charge transfer and better than the high crystal phase. More structural defects and disorder bring the better cycle stability (Chen et al. 2017; Owusu et al. 2017). Alternatively, the introduction of S heteroatoms in composites has a positive effect on the improvement of the capacitance (Tsubota et al. 2011; Zhao et al. 2012; Seredych et al. 2014).
Figure 6b shows the static voltage charge-discharge curves for three samples at a current density of 1.5 A/g. These curves are almost symmetrical during charging and discharging, exhibiting ideal capacitive behavior from the corresponding discharge. The specific capacitances that can be calculated from the discharge curve are 141 F/g, 177 F/g, and 215 F/g, respectively. Although RHAC has excellent rate performance (Wang et al. 2018), its capacitance is lower than 170 F/g. The composite material RHAC/SnO2 can provide a high capacitance of 177 F/g at a high current density of 1.5 A/g, and the specific capacitance of S-doped RHAC/SnO2 can reach 215 F/g. It is further demonstrated that the pseudo capacitance induced by the redox reaction can improve the overall capacitance strength of the composite material, so that the charging and discharging time is longer than that of the biomass carbon material, and the composite material doped with S heteroatom has better performance.
Fig. 6. (a) CV curves at 20 mV/s scan rates and (b) GCD curves at 1.5 A/g current densities of RHAC, RHAC/SnO2 and S-doped RHAC/SnO2
Figure 7a shows the CV plot of S-doped RHAC/SnO2 at the sweep speeds from 20 to 100 mV/s. These CV curves exhibit rectangular shapes at low scanning rates, indicating the charge storage mechanism of the surface of the electrode material. When the scanning rate was increased to 100 mV/s, the rectangular shape could still be maintained, showing a lower electrochemical polarization. Figure 7b shows the charge-discharge curves of S-doped RHAC/SnO2 composite at different current densities of 0.4, 1, 2, 4, and 8 A/g, which were almost symmetrical during charging and discharging. The ideal capacitive behavior, the small curvature of the charge and discharge curve reflected the pseudo capacitance behavior of the material, consistent with the results of the redox peak appearing in the CV curve.
The specific capacitance values S-doped RHAC/SnO2 composite at different current densities of 0.4, 1, 2, 4, and 8 A/g are shown in Fig. 7c, which were 283 F/g, 228 F/g, 186 F/g, 176 F/g, and 152 F/g, respectively. When the current density was increased from 0.4 A /g to 4 A /g, the specific capacity of the material still was able to maintain 62.2% of its maximum value with good rate performance. As shown in Fig. 7d, at a current density of 0.4 A/g, after 5000 cycles of charge and discharge, the specific capacitance is maintained at about 78.5%, indicating that this S-doped RHAC/SnO2 composite material electrode has a long cycle life, which is better than the 70% specific capacitance retention rate of RHAC/SnO2.
Fig. 7. (a) CV curves at scan rates from 20 to 100 mV/s, (b) GCD curves at current densities from 0.4 to 8 A/g, (c) Specific capacitance at different current densities and (d) Cycling performance at current density at 0.4 A/g of S-doped RHAC/SnO2
Figure 8 shows the Nyquist curves for the RHAC, S-doped RHAC/SnO2, and RHAC/SnO2 electrode materials. In the high frequency region, the intersection of the curve and the horizontal axis of the coordinate is the equivalent series resistance (Rs) of the material, and Rs includes the resistance of the electrode, the ionic resistance of the electrolyte, and the contact resistance of the active material and the collector interface. The lower the Rs value, the higher the electronic conductivity. The Rs of the three materials are all low, about 0.5, indicating that the porous carbon RHAC and its composites have good electronic conductivity. In the intermediate frequency region, the diameter of the arc corresponds to the charge transfer resistance (Rct), and the arcs diameters of the RHAC, S-doped RHAC/SnO2, and RHAC/SnO2 increase sequentially, indicating that the ion transport and electron conduction properties of the material surface are sequentially weakened. RHAC has faster adsorption/desorption at the electrode surface.
The impedance curve in the low frequency region was linear, indicating the resistance of the electrolyte ions to the electrochemically active sites inside the electrode material, and the Warburg impedance (W), the small slope of the oblique line indicates that the electrolyte ions diffuse to the active site surface at a lower rate. By comparison, it was found that RHAC porous carbon material had abundant mesopores and macropores. This situation favored the diffusion of ions inside the electrode material, so the linear slope was the largest. After RHAC was loaded with tin oxide, some of the pores were filled with tin oxide crystal particles, and the pores were filled. This reduces the diffusion speed of ions inside the material, so that the linear slope of the low frequency region decreases. The S-doped RHAC/SnO2 composite material had lower crystallinity and higher specific surface area than the undoped composite material, so the linear slope of the low frequency region was high and close to RHAC.
Fig. 8. Nyquist plots of RHAC, S-doped RHAC/SnO2 and RHAC/SnO2 electrodes
CONCLUSIONS
- Synthetic heterostructured nanocomposites with highly conductive porous matrix, large surface area, and short ion/electron transport paths showed enhanced pseudo capacitance kinetics, and composite specific capacitance properties were improved over biomass porous materials. In terms of specific surface area, surface polarity, and active sites for participation in the Faraday reaction, the S-doped RHAC/SnO2 electrode materials were superior to the RHAC/SnO2 electrode materials. The doped S atom combined with the C and O atoms in the material to form a new functional group, which participated in the reaction of the pseudocapacitor, so that the specific capacitance of the material increased.
- The specific capacitances of RHAC, RHAC/SnO2, and S-doped RHAC/SnO2 electrode materials were 141 F/g, 177 F/g, 215 F/g at a high current density of 1.5 A/g. The specific capacitance of S-doped RHAC/SnO2 composite at different current densities of 0.4, 1, 2, 4, and 8 A/g were 283 F/g, 228 F/g, 186 F/g, 176 F/g, and 152 F/g respectively.
- The current density is increased from 0.4 A/g to 4 A/g, and the specific capacity of the S-doped RHAC/SnO2 composite could be maintained at 62.2% of its maximum value with good rate performance. At a current density of 0.4 A/g, the charge capacity was maintained at about 78.5% after 5000 cycles of charge and discharge, indicating that the electrode has a long cycle life.
- The nanostructured metal oxides loaded on gasified rice husk carbon based porous carbon, providing an example for efficient and cost effective supercapacitor electrode materials.
ACKNOWLEDGEMENTS
This work was supported by the National Natural Science Foundation of China (51776100), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the National Key Research and Development Plan of China (2016YFE0201800).
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Article submitted: April 4, 2019; Peer review completed: May 26, 2019; Revised version received: June 3, 2019; Accepted: June 5, 2019; Published: June 10, 2019.
DOI: 10.15376/biores.14.3.5964-5979