Abstract
Activated carbons (ACs) were obtained from roots of Salvia miltiorrhiza Bunge (Danshen) followed by chemical activation with potassium hydroxide (KOH). The ACs were characterized by nitrogen adsorption, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and electrochemical testing. Results showed that ACs from cultivation sites in Pingyi (ASPY), possessed many micropores, a dominant amount of mesopores, and few macropores. They achieved a value of 1615 m2/g of specific surface area calculated by the Brunauer-Emmett-Teller (BET) method with numerous oxygen-containing functional groups. Electrodes fabricated with ASPY particles exhibited excellent double-layer specific capacitance (245 F/g), while the other two marked as ASFC (ACs from Fangcheng) and ASZJ (ACs from Zhongjiang) reached values of 163 F/g and 81 F/g, respectively, at a current density of 1 A/g in 6 mol/L KOH electrolytes. Electrodes made by ASPY had a good cyclic stability for 1000 cycles at a potential range from -1 V to 0 V and lower equivalent series resistance (ESR). Meanwhile, the authors found that roots of Danshen that grew in different cultivation sites were remarkably different while they were activated, which was the same with electrochemical properties of the electrodes fabricated by ACs.
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Performance of Activated Carbon from Salvia miltiorrhiza Roots as the Electrode Material for Supercapacitors
Kui Zhao,a Zhengjun Shi,b Qiuyuan Li,c Qing Li,a Fan Gao,d and Zhuyun Yan a,*
Activated carbons (ACs) were obtained from roots of Salvia miltiorrhiza Bunge (Danshen) followed by chemical activation with potassium hydroxide (KOH). The ACs were characterized by nitrogen adsorption, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and electrochemical testing. Results showed that ACs from cultivation sites in Pingyi (ASPY), possessed many micropores, a dominant amount of mesopores, and few macropores. They achieved a value of 1615 m2/g of specific surface area calculated by the Brunauer-Emmett-Teller (BET) method with numerous oxygen-containing functional groups. Electrodes fabricated with ASPY particles exhibited excellent double-layer specific capacitance (245 F/g), while the other two marked as ASFC (ACs from Fangcheng) and ASZJ (ACs from Zhongjiang) reached values of 163 F/g and 81 F/g, respectively, at a current density of 1 A/g in 6 mol/L KOH electrolytes. Electrodes made by ASPY had a good cyclic stability for 1000 cycles at a potential range from -1 V to 0 V and lower equivalent series resistance (ESR). Meanwhile, the authors found that roots of Danshen that grew in different cultivation sites were remarkably different while they were activated, which was the same with electrochemical properties of the electrodes fabricated by ACs.
Keywords: Salvia miltiorrhiza; Activated carbon; Electrode properties; Different cultivation sites; Supercapacitor
Contact information: a: Pharmacy College, Chengdu University of TCM, 1166 Liutai Road, Chengdu 611137, P. R. China; b: Institute of Chemical Engineering, Southwest Forestry University, Kunming 650224, P. R. China; c: Department of Psychiatry, the First People’s Hospital of Honghe State, Mengzi 661199, P. R. China; d: School of Materials Science and Engineering, Yunnan University, Kunming 650091, P. R. China; *Corresponding author: cdtcmyan@126.com
INTRODUCTION
Energy storage plays a great role in daily life. Important storage devices include batteries and electrochemical capacitors. Supercapacitors, or ECs, are novel devices in the electrical energy storage field, possessing many particular advantages, such as higher specific power density and more durable charge-discharge life than commercial batteries. With a higher specific energy density and working current, the capacitors are extensively used in portable electronics, hybrid power public vehicles of electric energy, pulse power in defense industries or aviation, and uninterruptible power supply (Simon and Gogotsi 2008; Boukhalfa et al. 2012).
On the basis of fundamental mechanisms, various carbon materials, such as activated carbons, graphene (Zhang et al. 2012), carbon nanotubes (Al-Zubaidi et al. 2012), and carbon nanofibers (Kim et al.2011), depend on electrostatic ion adsorption on electrode/electrolyte interface via the formation of electric double layers. Hierarchical porous ACs with high Brunauer-Emmett-Teller (BET) areas, especially within a range of 1000 m2/g to 3000 m2/g, are the most promising materials to make electrodes for electric double-layer capacitors (EDLCs) in the supercapacitor industry. This is due to their superior electrochemical stability and electrical conductivity. To develop high-performance supercapacitors, activated carbons (ACs) are extensively activated by physical or chemical methods to improve the specific surface area of the electroactive materials. The AC materials with large specific surface areas are more accessible to electrolyte ions. Therefore, the method concentrates on forming a perfect electric double-layer by optimizing the pore structure, pore size, and chemical bonds in functional groups affiliated on the surface to enrich charge in the electrode/electrolyte interface. The selection of the electrode material is the most important of all links to enhance capacitance of the supercapacitors in mass and volume.
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Fig. 1. Digital image of dried SMRs (inset) and the SEM images of extracted raw materials: (a) Pingyi; (b) Zhongjiang; and (c) Fangcheng
To avoid fossil fuel use, researchers increasingly utilize biomass from forestry and crop residues as carbonaceous precursors to obtain ACs, particularly in some agricultural countries with vastly available biomass residues such as fungi (Zhu et al. 2011) and banana fibers (Subramanian et al. 2007). Those raw botanical materials, including cotton stalks (Deng et al. 2010), corncobs (El-Hendawy 2003), fruit stones, walnut (Aygün et al. 2003), and coconut shells (Daud and Ali 2004), are good candidates for low-cost and readily available feedstocks. Each of the previous listed sources, when processed into AC, possesses different surface areas and electrochemical performance.
Compared with the above-mentioned biomass, roots of the medicine plant Salvia miltiorrhiza Bunge (Danshen) (SMRs) are abundant in China. Several diploid species belonging to the family Lamiaceae that are native to most regions of China and highly prized are enriched in diterpene compounds due to the presence of the lipid-solubles, known as tanshinones, along with the water-soluble compounds of rosmarinic acid, salvianolic acid, and lithospermic acid (Luo et al.2014). The tanshinones have been shown to promote blood circulation and antiphlogosis, while salvianolic acid has demonstrated efficacy as a treatment for some cardiovascular ailments and acts as a protector against ischemia-reperfusion injury to the brain (Zhou et al.2005). Tanshinones accumulate in the periderm of the reddish roots of S. miltiorrhiza specifically (Cui et al. 2015). The SMRs contain a great deal of pits and sieve tubes for nutrient transmission, as well as a large number of vessels and cellulose in the tissue, as shown in Fig. 1. Simultaneously, in Fig. 1(a) were longer diameters in width and thinner walls of vessels with more closely reticulated structure than others. There were lots of amorphous cellulose revolving around vessels in Fig. 1(b). As shown in Fig. 1(c) rows of small round holes are distinctly distributed on the wall of vessels.
The authors believe that the specific structure leads to the generation of the pores after carbonization, and these pores are conducive to prepare the porous AC through KOH activation at high temperatures (Ahmadpour and Do 1996; Deng et al. 2010). Herein, ACs were synthesized from the SMRs by carbonization and chemical activation with potassium hydroxide. The morphology, pore structure, and specific surface area of the ACs were detected by scanning electron microscopy (SEM) and BET methods. Furthermore, crystalline structure and oxygen functionality on the surface of carbon were measured using X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. Finally, the performance on electrochemistry was detected via cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) in KOH electrolyte.
EXPERIMENTAL
Materials
Synthesis of activated carbon
The SMR was collected from three different cultivation sites (Pingyi, Fangcheng, and Zhongjiang, China). Each sample was sieved to small pieces between 0.5 cm to 1 cm, and extracted by 95% ethanol for 2 h, then hot water for 2 h, and washed by distilled water three times to obtain herb residues. The three residues of SMRs were dried at 105 C for 6 h, and then soaked in KOH with a blending ratio of 3:1 wt% for 2 h. The mixture was dried at 105 C, and then placed in a furnace heated to 850 C at the rate of 5 C/min with a constant N2 (99.99%) flow of 300 cm3/min. The activating process was maintained for 2 h. The product was immersed with 1 mol/L HCl along with ultra-pure water repeatedly until a neutral pH was achieved. Finally, the active substance marked as either ASPY (precursors from Pingyi), ASFC (precursors from Fangcheng), or ASZJ (precursors from Zhongjiang), was ground, sieved to a particle size of 220-mesh, and then placed at 105 C for approximately 2 h for further analysis. The yield of the active product was defined as the weight in the last round. The percent value was calculated from Eq. 1,
(1)
where m and M are the dry weight (g) of the activated carbon and the precursor, respectively.
Electrode fabricated
The negative electrodes were prepared by following the method reported by Lang et al. (2012). Typically, 80% mass fraction of activated carbon material was blended with 7.5% of acetylene black (> 99.9%) along with 7.5% of conducting graphite in a mortar until homogeneous black powder was obtained. Then, 10% of 60% polytetrafluoroethylene was quickly added into this mixture with a few drips of ethanol as the disperser. After the evaporation came to an end at room temperature, the precast sheet was pressed at 10 MPa and wrapped in sulfate paper into a nickel foam (pretreated with 6 M HCl) of 1 cm × 2 cm. Thus, each electrode had an active area of approximately 1 cm2 that made the calculation shortcut. The pressed electrodes were weighed after being oven-dried for 24 h at 80 C.
Methods
Characterization of structure
The features of the activated products, particularly morphology and microstructure, were detected under a field emission scanning electron microscope (FESEM; Novanano 450; FEI, Hillsboro, OR, USA). Crystallite structures were analyzed by a powder X-ray diffractometer (XRD; Empyrean, PANalytical, Almelo, Netherlands) with Cu Kα radiation from 5 ° to 90 °. The chemical characteristics of the SMR-based carbon materials were analyzed with an FT-IR spectrometer (Nicolet IS10; Thermo Fisher, Waltham, USA). The samples were performed on a nitrogen adsorption-desorption isotherm measurement (ASAP 2020 Plus analyzer, Micrometitics, Atlanta, GA, USA) at 77 K. All samples underwent a degassing process at 300 C for 6 h to take the surface gaseous contaminants off before the adsorption/desorption experiments. The BET method was used to compute the specific surface area individually, and the pore volume as a whole was estimated from the value adsorbed at the relative pressure of P/P0 = 0.99. The pore width distributions of the three ACs were calculated via density functional theory (DFT) measurement (Li et al. 2011).
Boehm’s acidity
To evaluate the acidic and basic properties of each of the surface modified activated carbons, a scheme suggested by Boehm that measures the amount of acid/base uptake by carbon by neutralizing titration was followed (Noh and Schwarz 1990). The reagents used were HCI, NaOH, NaHCO3, and Na2CO3 solutions. Four parts, about 100 mg each in weight, of a sample were placed in four separate glass bottles containing 20 mL of 0.05 mol/L solution of each reagent mentioned. The containers were gently shaken for 24 h on a Shoker (TS-100B, Bozhen Corporation, Shanghai, China). The solutions were then filtered with filter paper. The amount of each reagent adsorbed was determined by back titration of the solution. Hydrochloric acid was the titer for the three basic solutions, and sodium hydroxide was used to titrate the hydrochloric acid solution.
Point of zero charge (PZC)
To measure the PZC of the samples, three different initial pH solutions were prepared using 0.1 mol/L HNO3 and 0.1 mol/L NaOH; for example, pH = 3, 6, or 11. Six bottles were filled with 20 mL of the solution, and different amounts of carbon sample were added (0.05 g, 0.25 g, 0.5 g, 1.0 g, 2.0 g, 2.5 g) each initial pH (Noh and Schwarz 1990). The equilibrium pH was measured after being shaken for 24 h on a pH-meter (CT-6021A, Liyuan Corporation, Shenzhen, China).
Electrochemical tests in a three-electrode system
Electrodes were immersed in 6 mol/L KOH electrolytes for 12 h to make electrolyte ions sufficiently diffuse into the porous carbon.
All of the tests of each electrode were performed on an electrochemical working station (CHI660D; Chenhua Instrument Co., Ltd., Shanghai, China) at room temperature. A platinum slice electrode (9 cm2) served as the positive, a saturated calomel electrode served as the reference electrode, and the working electrode served as the negative, correspondingly.
The GCD was underway at current densities from 0.5 A/g to 10 A/g. The CV measurements were conducted at a sweeping rate that ranged from 5 mV/s to 100 mV/s. Electrochemical impedance spectroscopy (EIS) tests collected data from 100 kHz to 10 mHz with an alternate current amplitude of 5 mV. The specific capacitance was calculated from Eq. 2,
(2)
where C is the specific capacitance (F/g), I is the value of the constant current, V and t stand for the window of the potential and times of discharging in one cycle (s), and m is the mass (g) of the corresponding electrode material. The coulombic efficiency was calculated from Eq. 3,
(3)
where td and tc are discharging and charging time (s) of one cycle, respectively.
RESULTS AND DISCUSSION
Microstructure Characterizations of AC Materials
Porous carbon materials were synthetized by KOH activating the carbonaceous precursor.
When the temperature was increased to over 700 C, the plausible products of the activation reactions could include the carbon dioxide, hydrogen, carbon monoxide, potassium oxide, potassium carbonate, and metallic potassium carbonization and oxidation conditions (El-Hendawy 2006).
The following reactions take place between KOH and carbon (Marsh and Rodríguez-Reinoso 2006):
First: 6KOH + 2C = 2K + 3H2 + 2K2CO3 (4)
Second: K2CO3 + 2C = 2K + 3CO (5)
Third: K2CO3 = K2O + CO2 (6)
The results of the percent yield of SMRs carbon are listed in Table 1. The yield of ASPY was 15.1% less than that of ASZJ. Images of SMRs carbon are displayed in Fig. 2. They showed some bumps and rough microstructure for the three ACs material in this paper.
After chemical activation, the AC samples exhibited a texture composed of particles on the surface of carbon materials with differences between individuals. Although the activated primary structure with pores of all three ACs in the mixed electrode materials was broken, the mixture of ASPY and ASFC with porous and open cell active particles diffused while the samples of ASZJ particles (shown in Fig. 2(b)) were visibly dense and smooth.
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Fig. 2. SEM of electrode materials: (a) ASPY; (b) ASZJ; and (c) ASFC
Nitrogen adsorption-desorption isotherms of both SMR-based carbon materials at 77 K are shown in Fig. 3(a). All samples were represented with the characteristic of type IV. The sharp increase at low relative pressure was attributed to the existence of micropores, and the desorption hysteresis at medium relative pressure suggests the presence of developed mesoporosity (Bansal and Meenakshi 2005). Furthermore, the results were observed from the pore size distribution in Fig. 3(b). The porous material of ASPY consisted mostly of micropores and mesopores. The mesoporous rate was calculated as 71%, 68%, and 44% in ASPY, ASFC, and ASZJ, respectively. However, in contrast, Fig. 3(b) illustrates that AC material was composed of mesopores and macropores. As shown in Fig. 4 and Table 1, the shapes for ASZJ, ASPY, and ASFC were very distinctive, and the specific surface area of ASPY (1615 m2/g) was higher than that of ASFC (1487 m2/g) and ASZJ (525 m2/g). Meanwhile, an apparent increase in pore volume was observed from 0.341 cm3/g for ASZJ to 0.997 cm3/g for ASPY in Table 1, which indicated that the activation reactions proceeded faster at ideal precursors and more pore channels were developed. Additionally, those pore widths between approximately 0.8 nm and 1 nm occupied almost all of the micropores in ASZJ from data of pore width distribution.
Table 1. Textural Characteristics and Yield of ACs
Values are the average of three replications; SBET– total area; SMes-area of mesopores;
VMic and VMes– micro- and mesopore volumes; and DAp– average pore radius
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Fig. 3. (a) Nitrogen adsorption-desorption isotherms of the ASFC, ASPY, and ASZJ samples; (b) Pore size distributions of the ASFC, ASPY, and ASZJ samples
Characteristics of ACs and Function Groups
The FT-IR spectra are shown in Fig. 4(a) for the carbonized ASPY, ASFC, and ASZJ samples activated with KOH of the three carbon materials. The carbon materials of ASPY and ASFC possessed similar chemical structural characteristics, which indicated more intense infrared peaks compared to the ASZJ sample with KOH activation. Based on Boehm’s acidity and PZC analysis in Table 2, the order of oxygen groups content was phenolic > carboxyl > lactonic group. Meanwhile, the order of acid oxygen groups contents in total was ASFC > ASPY > ASZJ. Combined with pHPZC value, ACs could defined as acido-basic property because of certain number of acid oxygen groups. In FT-IR spectra, the O-H stretching vibration (3436 cm-1) and the bending vibration (1630 cm-1) were due to surface hydroxyl groups and bound water, indicating that the O-H groups rounded into hydrogen bonds as reported (El-Hendawy 2006). Thus the ASPY and ASFC samples might have a higher probability to attract alkaline electrolyte than the ASZJ samples. Noticeable changes contrasted by ASZJ were also found in the spectra of the ASPY and ASFC samples. Although, all the strong peaks at 3500 cm-1 might be partly due to samples that were not dry enough.
Table 2. Contents of Acid Oxygen Groups on the Carbon Surface and pHPZC
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