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.