Harvesting Capacitive Carbon by Carbonization of Waste Biomass in

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Harvesting Capacitive Carbon by Carbonization of Waste Biomass in Molten Salts Huayi Yin,† Beihu Lu,† Yin Xu,† Diyong Tang,† Xuhui Mao,*,† Wei Xiao,† Dihua Wang,*,† and Akram N. Alshawabkeh‡ †

School of Resource and Environmental Science, Wuhan University, Wuhan 430072, P.R. China Civil and Environmental Engineering Department, Northeastern University, Boston, Massachusetts 02115, United States

‡

S Supporting Information *

ABSTRACT: Conversion of waste biomass to value-added carbon is an environmentally benign utilization of waste biomass to reduce greenhouse gas emissions and air pollution caused by open burning. In this study, various waste biomasses are converted to capacitive carbon by a single-step molten salt carbonization (MSC) process. The as-prepared carbon materials are amorphous with oxygen-containing functional groups on the surface. For the same type of waste biomass, the carbon materials obtained in Na2CO3-K2CO3 melt have the highest Brunauer−Emmett− Teller (BET) surface area and specific capacitance. The carbon yield decreases with increasing reaction temperature, while the surface area increases with increasing carbonization temperature. A working temperature above 700 °C is required for producing capacitive carbon. The good dissolving ability of alkaline carbonate molten decreases the yield of carbon from waste biomasses, but helps to produce high surface area carbon. The specific capacitance data confirm that Na2CO3K2CO3 melt is the best for producing capacitive carbon. The specific capacitance of carbon derived from peanut shell is as high as 160 F g−1 and 40 μF cm−2, and retains 95% after 10 000 cycles at a rate of 1 A g−1. MSC offers a simple and environmentally sound way for transforming waste biomass to highly capacitive carbon as well as an effective carbon sequestration method.

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and hemicellulose,16 containing around 50% carbon by weight. Therefore, so far there is no general or accepted process for converting waste biomass to liquid fuels and syngas aside from direct combustion. Extraction of value-added carbon is an alternative way for the utilization of waste biomass.18−21 The most common method is a thermochemical process, also called pyrolysis, which involves direct thermal decomposition of organic matter in absence of oxygen at elevated temperatures. Two categories of processes have been employed to prepare porous carbon materials by carbonization of waste biomass: physical (thermal) and chemical activation processes.22 The physical activation includes two steps: precursors are first activated at high temperature (600−1000 °C) in N2 or Ar atmosphere followed by activation with CO2, steam, air, or their mixtures. For the chemical activation process, precursors are carbonized and activated together at temperatures between 400 to 900 °C in the presence of some chemical catalysts (e.g., KOH, NaOH, K2CO3, ZnCl2, H3PO4, and carboxylic acids). Currently, value-added carbon has been produced from waste biomass such as sunflower seed shell,23 waste coffee bean,24 walnut shell,25 seaweed,26 rice husk,27 sugar cane bagasse,28 firewood,29 and

INTRODUCTION Globally, more than a hundred billion metric tons of biomass is generated every year from agriculture.1 This volume of biomass is converted to a substantial amount of food and raw materials, and at the same time, produces different forms of residual waste biomass, such as stalks, straw, leaves, roots, husk, nut shells, and waste wood. Direct incineration of waste biomass is a common disposal method in developing countries and rural regions, including areas in China.2 However, the open burning of waste biomass releases significant quantities of greenhouse gases (GHGs) and dust, leading to formidable pollution including haze, smog, and acid rain.3−8 Recent studies also showed that particulate matter concentrations may increase dramatically in downwind suburban and countryside locations as a result of postharvest burning of waste biomass like rice straw and maize straw.9 Innovative strategies and pathways are urgently needed to reduce the adverse impact of burning of waste biomass on the environment. Transforming waste biomass into biofuels and bioderived materials rather than incineration is attracting increasing attention.10−14 Traditionally, the research focuses on methods for converting biomass (e.g., wheat and corn) to gas and liquid fuel, such as gasification and liquefaction of biomass through thermochemical, biochemical, and agrochemical processes.15−17 However, unlike the harvested biomass consisting of starch and polysaccharide, waste biomass mainly consists of cellulose, lignin © XXXX American Chemical Society

Received: April 8, 2014 Revised: June 12, 2014 Accepted: June 23, 2014

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dx.doi.org/10.1021/es501739v | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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Figure 1. (a) XRD pattern, (b) EDX spectrum, (c, d) SEM, (e, f) TEM, and (e) HR-TEM images of the peanut shell-derived carbon obtained in Na2CO3-K2CO3 melt at 850 °C. Insets in (a) are the peanut shells before and after carbonization).

bamboo waste30 through these two processes. More recently, the microwave-assisted pyrolysis (MAP),12 autogenic reactions,31 single-step pyrolysis,32 hydrothermal carbonization process33 and fast pyrolysis of MgCl2 loaded waste biomass34 have been employed to produce fuels and functional carbon used for lithium ion batteries and electrochemical capacitors. Among these methods, the working temperature of MAP is significantly reduced to between 150 and 300 °C to produce syngas and liquid fuels.12 However, the temperature is hard to control. The autogenic reactions are designed to pyrolyze organic compounds, for example, mesitylene and plastics, at an elevated temperature and a high pressure, producing spherical graphitic carbon after being post-treated at much higher temperatures.31 The single-step pyrolysis process takes advantage of the natural elements existed in the leaves as the in situ catalysts to produce porous carbon exhibiting good capacitive performance.32 Carbon based materials with intrinsic physical and chemical properties have been widely used as functional resources such as energy storage materials, adsorbents, catalysis support, cell biology, etc. For example, carbon is one of the most important candidates for electrochemical double layer capacitor (EDLC) which stores charge in the electric double layer between the electrolyte and a high-surface area electrode. Therefore, a lowcost carbon resource and a green process for production of highly capacitive carbon are crucial to expanding practical applications for energy storage systems. In this respect, waste biomass possesses the merits of low cost, abundance and environmental advantage. Based on the experience of our research in molten salt chemistry in the past decade, molten salt is proposed as a bridge to link the gap between waste biomass and carbon production.35 High-temperature molten salt has low

vapor pressure and low solubility of oxygen and water: it can provide an inert reaction environment for pyrolysis of waste biomass. Conversion of biomass to syngas in molten salt,36,37 and activating carbon with molten salt have been reported.38−42 In comparison with the conventional thermal or chemical carbonization processes, molten salt process is more attractive because of its diversity and catalytic properties, which provides new possibilities to produce high-performance materials. In this study, we demonstrate a single-step molten salt carbonization (MSC) process to extract value-added capacitive carbon, instead of syngas, from agricultural waste biomass. The process is investigated to produce high-performance capacitive carbon at an optimal condition. A eutectic Na2CO3-K2CO3 melt is found to be a superior medium for the carbonization of a variety of agriculture waste biomasses. Due to the unique catalytic properties and enhanced dissolution capability of this molten salt system, carbon materials with high specific capacitance were produced.

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EXPERIMENTAL SECTION Carbonization of Waste Biomass in Molten Na2CO3K2CO3. A sealed stainless steel (SS) tube (inner diameter: 10 cm, height: 60 cm) equipped with circular cooling on top was used as the reactor for carbonization. An alumina crucible (inner diameter: 8 cm, height: 13 cm) containing 500 g salts was placed into the reactor and heated by a tube furnace. Anhydrous Na2CO3-K2CO3 (Na: K = 59:41 mole fraction, > 99.9%, Sinopharm Chemical Regent Co. Ltd., China) salts were dried at 250 °C in air for 24 h. The reactor was then purged with N2 and heated to working temperature. Afterward, carbonization of waste biomass was conducted. The waste biomass was first B



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predried at 140 °C for 12 h. Then the predried waste biomass was wrapped with a nickel foam sheet and attached onto a SS rod (6 mm diameter). After the salts were liquefied and the temperature remained constant, the wrapped waste biomass was placed into the steel reactor and subsequently immersed into the molten salt and kept in the molten salt for 1 h. Then the product was taken out from the melt and cooled down in the headspace of the reactor under N2 or Ar atmosphere. Finally, the product was taken out of the reactor, grounded with an agate mortar and pestle, washed with water and 0.1 M HCl to remove the absorbed salts, and vacuum-dried at 100 °C for 10 h. Moreover, CaCl2, CaCl2−NaCl (Ca:Na = 1:1 mole fraction), and Li2CO3− Na2CO3-K2CO3 (Li:Na:K = 43.5:31.5:25 mole fraction) melts were respectively employed as a reaction medium. All salts were of more than 99.9% purity and purchased from Sinopharm Chemical Regent Co. Ltd., China. Electrochemical Measurements. Electrochemical measurements were performed in 1 mol L−1 H2SO4 solution using a three-electrode system. A carbon electrode film working electrode (1 cm2, 10 mg) comprised of 80 wt % as-prepared carbon, 10 wt % acetylene black and 10 wt % PTFE binder and was pressed on a titanium mesh. A 3 cm2 platinum plate and a saturated calomel electrode (SCE) were used as counter and reference electrode, respectively. Cyclic voltammetry and galvanostatic charge−discharge tests were performed with an electrochemical workstation (CHI 1140a, Shanghai Chenhua Instrument Co. Ltd., China). Electrochemical impedance measurement was performed on another electrochemical workstation (Autolab302N, Eco Chemie, Utrecht, Netherlands) by sweeping frequencies from 100 kHz to 10 mHz at open circuit potential with an amplitude of 5 mV. A symmetric carbon/ carbon 2-electrodes supercapacitor was also assembled for specific capacitance measurement using Teflon Swagelok type cells which were built with nickel foam collectors, glassy fibrous separator and 6 mol L−1 KOH. The loading of PSDC on each electrode was 3.08 mg, and specific capacitance was calculated according to the reported equation.43 Characterization. Compositions of the waste biomass were characterized by elemental analysis (Elementar Analysen systeme Gmbh VarioEL III, Germany), and the pyrolysis process was investigated by thermogravimetric analysis (TG) and differential scanning calorimeters (DSC) analysis (NETZSCHSTA 409 C/CD, Germany). The obtained carbon was characterized by X-ray diffraction spectroscopy (XRD, Shimadzu X-ray 6000 with Cu Kα1 radiation at λ = 1.5405 Å), scanning electron microscopy (SEM, FEI Sirion field emission gun), energy-dispersive X-ray analysis (EDX, EDAX GENESIS 7000), high-resolution transmission electron microscope (HRTEM, JEOL2010), Fourier transform infrared spectroscopy (FT-IR, NICOLETAVATAR360) and Brunauer−Emmett− Teller (BET) surface area analysis (Gemini V 5331).

elements, which are detected from the peanut shell precursor (see Figure S1 and Table S1 in the Supporting Information (SI)), are no longer detectable. Meanwhile, according to the FTIR spectrum in Figure S2 in the SI, the oxygen element should be attributed to the oxygen-containing functional groups of carbon.44 All peaks of the absorptions in the IR spectrum are weak, revealing that only minor functional groups exist on the surface of the obtained carbon. Morphologies of the PSDC were characterized by SEM and TEM. Carbon sheets shown in Figure 1c were synthesized by carbonization of peanut shells in Na2CO3-K2CO3 melt at 850 °C. The carbon sheets have a smooth and clean surface and their thickness is between 50 and 200 nm (Figure 1d). A typical carbon sheet can be clearly observed from the TEM image. It is almost transparent, indicating the sheet is thin and uniform (Figure 1e). The edge of the carbon sheet is uneven, indicating a porous surface of PSDC (Figure 1f). No lattice fringe is found in the HR-TEM image of the edge (Figure 1g), agreeing well with the XRD data that the carbon is amorphous. The pore size distribution plot (see Figure S3 in the SI) shows a narrow peak in the region of 3−10 nm. To the best of our knowledge, the thin carbon sheet with meso pores may have better electronic conductance and facilitate ion transfer, which is good for its application in EDLC.45 Effect of Salt on the Carbonization of Peanut Shell. The effect of salt on the carbonization of peanut shells was investigated to select an optimal medium for MSC. The BET surface area of PSDC obtained in four kinds of melts, Na2CO3K2CO3, Li2CO3−Na2CO3-K2CO3, CaCl2 and CaCl2−NaCl, at 850 °C is 408, 380, 390, 316 m2 g−1, respectively. In general, the surface area is closely associated with some intrinsic properties, like specific capacity, absorptive ability, catalytic activity, etc. With respect to this point, Na2CO3-K2CO3 melt appears the best among the four melts for producing high-performance carbon. Another criterion that should be considered is the carbon yield of waste biomass by MSC. No significant difference of carbon yield is observed in the four kinds of melts with varied operation times (from 1 to 5 h) at 850 °C. The carbon yields are all around 20%. This observation indicates that neither the composition of melt nor the operation time affects the carbon yield. In case of the morphologies of derived carbons, SEM results show that the carbon obtained in K2CO3−Na2CO3 melt has a smoother surface than that from other three melts (SI Figure S4), suggesting that the salt may take part in or catalyze the carbonization of peanut shell. Effect of Temperature on the Carbonization of Peanut Shell. The effect of temperature on the carbonization of peanut shell was investigated, and a ternary salt (Li2CO3−Na2CO3− K2CO3) with lower melting point was applied as the target melt. The carbon yield decreases with increasing reaction temperatures, while the BET surface area increases with increasing reaction temperature in the range of 450−850 °C (Figure 2). When the temperature is higher than 650 °C, the carbon yield significantly decreases while the specific surface area increases. Therefore, a temperature above 650 °C is necessary for producing porous carbon by MSC process. According to the calculated Gibbs free energies, reactions 1 and 2 are both thermodynamically spontaneous above 700 °C.

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RESULTS AND DISCUSSION Harvest Carbon from Peanut Shell in Molten Na2CO3K2CO3 at 850 °C. Peanut shells were converted to black carbon after carbonizing in molten Na2CO3-K2CO3 for 1 h at 850 °C (inset of Figure 1a). Carbon was confirmed by XRD and EDX spectrum (Figure 1a, b). XRD pattern shows the peanut shell derived carbon (PSDC) is amorphous and has the similar character to activated carbon (Figure 1a). The peak of copper arises from the substrate of sample holder, and only a trace amount of oxygen and sodium are observed in addition to the carbon peak (Figure 1b). After carbonization, the Mg and Si

C + H 2O(g) = CO(g) + H 2(g)ΔG = − 25.2kJ mol−1(850°C) (1) C + CO2 (g) = 2CO(g) ΔG = − 26.2kJ mol−1(850°C) C

(2)



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°C, and the difference of weight loss becomes more evident, from 7% to 9%, at temperatures above 650 °C. Therefore, the molten salts remarkably affect the pyrolysis at higher temperatures, decreasing the apparent carbon yield and producing the porous carbon. This difference is due to the good dissolving ability of alkaline carbonate molten salt. The inorganic components, for example, the silicate and metals ions,46 of the biomass can be dissolved by the molten salts, resulting in larger weight loss, lower carbon yield and probably more meso pores. In contrast, the pyrolysis in Ar or other inert atmosphere may not extract the nonvolatile inorganic composition in the waste biomasses, and an extra process (or ingredient) may be needed for activating the carbon. Considering both salt and temperature on the carbonization process of peanut shells, as well as the cost of Li2CO3, the Na2CO3-K2CO3 binary melt is more suitable for the MSC process since the working temperature should be higher than 700 °C for the production of porous carbon. According to the DSC curve, the pyrolysis process is exothermic indicating the released heat during pyrolysis can be utilized to heat the molten salt. Two exothermic peaks respectively occurred at 345 and 600 °C suggest that two exothermic reactions take place in the carbonization process. In combination with the TG, both decomposition and activation processes are exothermic. The total heat released in the pyrolysis process is 435 J g−1, which is calculated by integrating the DSC curve. As the heat capacities of Na2CO3 and K2CO3 are 1.79 and 1.51 J K−1 g−1,47 respectively, the heat capacity of the eutectic melt, Na2CO3-K2CO3, is estimated to be 1.65 J K−1 g−1. Assuming that the heat released from carbonization of waste biomass in molten salt and in Ar is the same, and the weight of molten salt is 5 times of the waste biomass, 1 g molten salt can receive 87 J from carbonation of 1 g waste biomass. Due to the high heat conductivity and capacity of molten salt, the energy released during the pyrolysis can be effectively absorbed, stored, and distributed in the molten salt bath. As a result, the MSC can be a self-heated process taking advantage of the heat of the pyrolysis process, without the need of external energy supply to keep the MSC reactor working at high temperature. Technically, energy is required to melt the salts, but not requisite for a continuous operation of the carbonation of waste biomass. Renewable energy source like concentrated solar heating is an option to offset this part of energy consumption, since molten salt is a favorable heat-transfer medium. Electrochemical Capacitive Performance of the Peanut Shell Derived Carbon (PSDC). The carbon produced by pyrolysis of peanut shell in molten Na2CO3-K2CO3 at 850 °C shows good capacitive performance in 1 mol L−1 H2SO4. Galvanostatic charge−discharge measurements show the PSDC has a good specific capacitance of 160 F g−1 at a current density of 500 mA g−1 (Figure 4). The rectangular shape of CVs, even at a scan rate of 50 mV s−1, is typical of a stable double layer capacitor (Figure 4a). A small hump around 0.4 V may be associated with Faradaic reactions corresponding to the pseudocapacitance caused by some functional groups on the surface of the carbon,48 which has been verified by the FT-IR spectrum (Figure S2 in the SI). The galvanostatic charge− discharge curves at different current densities show no significant difference of the specific capacitance, indicating the PSDC has good rate capability (Figure 4b). Symmetric characteristic of the curves reveals low charge transfer resistance and good electrical conductance of the electrode. The specific capacitance is as high as 123.6, 133.5, 149, 160 F g−1 under a charge−discharge current density of 2, 1, 0.5, 0.25 A g−1, respectively. Even with a

Figure 2. Plots of carbon yield and BET surface area with melt temperature. Data are recorded for the peanut shells treated in Li2CO3− Na2CO3-K2CO3 melt.

As temperature is higher than 700 °C, the generated H2O and CO2 in the carbonization process react with the carbon, resulting in the formation of porous carbon. This is called the activation process, where the carbon is in situ activated by the products produced during the carbonization process. In addition, the molten salt also plays an important role in the carbonization process. After the gas products leave and enter into the molten salt, voids are created and subsequently occupied by the liquid molten salts, which can dissolve or react with the inorganic components of the biomass. As a result, the molten salts not only function as the media for heat and mass transfer, but also help to shape the porous structure of carbon. The pyrolysis of peanut shell was investigated by TG and DSC measurements in Ar atmosphere (Figure 3a). It shows that the

Figure 3. (a) TG and DSC curves of the peanut shell recorded in Ar atmosphere (heating rate: 10 K min−1), (b) carbon yields by MSC process and TG measurement.

onset temperature of weight loss is in the range of 230−240 °C. The main weight loss ends at 375 °C (53% weigh loss), followed by a slow and continuous weight loss. As the temperature is higher than 600 °C, the weight loss (∼68%) levels off, indicating that the pyrolysis almost completed. For the MSC process, the carbon yield ranges from 34.9% to 29% between 450 and 650 °C, while it is from 39% to 32.2% in pure Ar (Figure 3b). The weight loss in molten salt is 4.1% to 3.2% higher than in Ar below 650 D



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Figure 4. (a) CVs, (b) charge−discharge curves, (c) Nyquist plot, and (d) Ragone plot recorded from the PSDC in 1 mol L−1 H2SO4.

requisite for producing capacitive carbon, agreeing well with the result of BET surface area measurement. Additionally, the carbon obtained in different melts at the same temperature also has different capacitance, indicating the melt affects the carbonization process and influences the electrochemical properties of the carbon products (Figure S6b in the SI). Among the four different melts, the carbon obtained in Na2CO3K2CO3 melt has the highest specific capacitance. As reported, some alkaline and alkaline-earth metals ions can catalyze the pyrolysis creating pores in the produced carbon.32,53 In this study, the molten carbonate is demonstrated to be better than molten chloride for producing capacitive carbon. Due to the high cost and low solubility in water, Li2CO3 is not favorable for producing high capacitive carbon compared with Na2CO3K2CO3. In chloride melt, some chlorine and hydrochloric gases may generate during MSC process. Therefore, Na2CO3-K2CO3 melt is a good candidate for carbonization of waste biomass to produce highly capacitive carbon in an environmentally sound and cost-affordable manner. Versatility of MSC Process. Besides peanut shells, 13 other kinds of waste biomasses have been converted to porous carbon in K2CO3−Na2CO3 melt at 850 °C. The precursory waste biomass and their corresponding carbon are shown in Figure S7 in the SI. The derived carbon materials partly keep the morphologies of their precursory materials, indicating the process of carbonization involves solid to solid process. The elemental analysis and carbon yield of the waste biomass, as well as the specific capacitance measured at a current density of 500 mA g−1 are listed in Table 1. The different carbon yields of biomasses, ranging from 10 wt % to 30 wt %, may arise from the different water contents, chemical compositions or grain sizes of the starting materials. The carbon yields of some biomasses, such as bulrush and rice husk, are relatively low. Converting these biomasses with low carbon yields to other value-added products like syngas and liquid fuels will be another route for this technology. Note that all of the biomass-derived carbons have a specific capacitance higher than 78.5 F g−1. The rice husk, which is separated from the rice grains in milling process, has the highest specific capacitance of 186.5 F g−1. As a result, the MSC process may be engineered a general method for producing capacitive carbon from waste biomasses, as illustrated by schema S1 in the SI. The released gas or other chemicals from MSC

symmetric carbon/carbon 2-electrodes configuration, the PSDC still shows 154 F g−1 specific capacitance at 0.5 A g−1 current density with 6 mol L−1 KOH electrolyte. Electrochemical impedance spectroscopy (EIS), a powerful technique to test the electrochemical behaviors of the electrode, is presented in Figure 4c. There is a semicircle in the high frequency range, followed by a straight line at the low frequency region. A small resistance, 0.5 ohm/cm2, at the high frequency is the resistance of solution and a small radius of the semicircle indicates low interfacial charge transfer resistance, ∼2 ohm/cm2, indicating good electrical conductance of the electrode. At lower frequencies, a more vertical line means that the electrode is closer to an ideal capacitor.49,50 In order to evaluate the energy density (E) and power density (P) of PSDC electrode, the Ragone plot is presented in Figure 4d, E and P are calculated by the equation of E = CV2/2 and P = E/t (C, V, and t are specific capacitance, potential range and a full discharge time). The energy density of the carbon electrode ranges from 17 Wh kg−1 to 22 Wh kg−1 and the power density is between 1000 W kg−1 to 125 W kg−1, which are comparable to the reported data in the literature.49 Very recently Biswal et. al reported the carbon produced by dead leaves (needle leaf) had high specific capacity of 400 F g−1 and 32 μF cm−2.32 Note that the component of the leaf significantly affects the result. Moreover, the amount of the active materials apparently influences their capacitive performance.51,52 In order to minimize measurement errors, the test electrode should have a capacity over 0.25 F with the mass of active material on the order of 10 mg or more. Nonetheless, the carbon derived from waste biomass in molten salt can basically satisfy the current demands for supercapacitors. More importantly, the PSDC shows good cyclability: the retention of specific capacitance is 95% of the first cycle after 10 000 cycles (Figure S5 in the SI). The effects of temperature and molten salt on the specific capacitance of the carbon products were investigated. It is found that the specific capacitance of PSDC increases with increasing reaction temperature in both molten Li2CO3−Na2CO3-K2CO3 ternary melt and Na2CO3-K2CO3 binary melt (see Figure S6a in the SI). The carbon produced in Li2CO3−Na2CO3-K2CO3 melt at temperatures lower than 700 °C has nearly no capacitance, and it increases significantly by elevating the temperature from 650 to 750 °C. Thus, a working temperature above 700 °C is E



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S2); the pore size distribution plot of the obtained carbon (Figure S3); the SEM images of the PSDC in different molten salts (Figure S4); the plot of the capacitance retention as a function of the cycle number (Figure S5); the specific capacitance of the PSDC prepared at different molten salt temperatures, and in different salt systems (Figure S6); the photos of typical waste biomasses and their derived carbons (Figure S7); the schematic illustration for a general MSC process and the one coupled with a solar heating system (Schema S1, Schema S2). This material is available free of charge via the Internet at http://pubs.acs.org.

Table 1. Elemental Analysis and Carbon Yield of the Waste Biomasses (% by Weight), and the Specific Capacitance (F g−1) of the Obtained Carbon Measured at 500 mA cm−2 waste biomass peanut shell chestnut shell leaf of phoenix tree corn core bulrush wormwood pomelo peel corn straw wheat straw rice husk rice straw saw dust orange peal sunflower shell

C (%)

H (%)

N (%)

carbon yield (%)

specific capacitance (F g−1)

50.53 48.63 48.05

5.95 5.96 5.79

0.69 0.79 0.67

19.7 30 20

148 106 106

45.98 45.75 44.93 43.31 42.82 41.79 39.27 38.76 46.99 43.95 47.21

5.98 5.93 5.93 5.88 5.61 5.51 5.14 5.33 6.63 3.75 6.09

0.37 0.0 0.0 0.72 1.30 0.5 0.0 0.53 1.47 0.38 0.35

18 10 15 22 18 10.5 11 16.5 14.5 17.8 16

83.3 163 117.5 143 127 165.5 186.5 115 78.5 129.5 105.5

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Corresponding Authors

*Phone: +86-27-6877-5799; e-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the NSFC (Grant Nos. 51071112, 51325102, 51278386), MOE of China (NCET-08-0416) and the Fundamental Research Funds for Central Universities of China for financial support.

process can be collected for further analysis, which will be considered in our future research. Implications for Waste Biomass Utilization and Abatement of Air Pollution. In the present study, a number of waste biomasses, such as rice husk, peanut shell, straw, pomelo peel, chestnut hull, and corn core are converted to highly capacitive carbons through a single-step MSC process. The conversion from wastes to value-added products is a more environmentalfriendly and cost-effective way to replace the direct incineration of waste biomass, which helps to reduce air pollution caused by the emissions from biomass incineration. The profit from the value-added carbon products is also beneficial to the collection of waste biomass in rural area. To put the MSC process in a broader perspective, CO2 is actually the starting material which is first fixed by photosynthesis and then utilized by carbonization in molten salt. Unlike the conventional carbonization processes, molten salt is an ionic liquid that can dissolve inorganic components of waste biomass, providing a simple route to prepare capacitive carbon materials. The exothermic pyrolysis process of biomass also promises a self-heated molten system, reducing the external energy needed for heating the molten salt. In addition, owing to the high heat capacity and wide operating temperature window, molten salt is a good heat absorbent and working fluid for concentrated solar power (CSP) systems.36,54,55 Therefore, it is favorable to couple solar heating system with MSC process, further decreasing the energy demand of the molten salt system. The concept of the solar heated molten salt carbonization (SH-MSC) is schematically illustrated in Scheme S2 in the SI. In this process, CO2 is the starting material, carbon and oxygen are the products (assuming no gas produced during the pyrolysis), solar heat provides energy both for photosynthesis and heating molten salt, and molten salt is the medium for heat storage and pyrolysis. In this way, the MSC process can be developed as a green technology that bridges the gap between photosynthesis and pure carbon production using solar heating.

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AUTHOR INFORMATION

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The EDS spectrum of the peanut shell (Figure S1); the elemental composition of peanut shell derived from the EDS spectrum (Table S1); the FT-IR spectrum of the PSDC (Figure F



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