Facile Synthesis of Highly Porous Carbon from Rice Husk

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Facile Synthesis of Highly Porous Carbon from Rice Husk Yeru Liang, Chen Yang, Hanwu Dong, Wenqi Li, Hang Hu, Yong Xiao, Mingtao Zheng, and Yingliang Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01315 • Publication Date (Web): 24 Jun 2017 Downloaded from http://pubs.acs.org on June 25, 2017

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ACS Sustainable Chemistry & Engineering

Facile Synthesis of Highly Porous Carbon from Rice Husk Yeru Liang∗, Chen Yang, Hanwu Dong, Wenqi Li, Hang Hu, Yong Xiao, Mingtao Zheng, Yingliang Liu* College of Materials and Energy, South China Agricultural University, Guangzhou 510642, P. R. China KEYWORDS: highly porous carbon, rice husk, activation-free, no more post-treatment, energy storage

ABSTRACT: Highly porous carbon materials have attracted great interest for a wide range of important applications. Many examples for their synthesis exist, but these synthetic processes can be quite complex and also very time consuming. There is still a major challenge to develop a facile yet versatile conceptual approach to produce them. Here, we present an efficient, activation-free, posttreatment-free strategy for the synthesis of highly porous carbon by a simple carbonization of a mixture of rice husk and polytetrafluoroethylene (PTFE) powder. PTFE employed here can in-situ generate HF to etch out natural silica during carbonization treatment of rice husk. This strategy not only reduces synthesis procedure by combining carbonization and post-silica-removal into a single step, but also completely eliminates the usage of hazardous HF or corrosive NaOH or KOH. The as-synthesized carbon materials exhibit a BET surface area as high as 2051 m2/g without any activation treatment, which is about 20 times enhancement in porosity compared to the traditional carbon materials from rice husk. With combination of the highly porosity and the valuable hierarchical porous structure, the as*

Corresponding author, E-mail: [email protected] (Y. Liang), [email protected] (Y. Liu).

ACS Paragon Plus Environment

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prepared porous carbon materials serve well as electrodes for supercapacitive energy storage, including large capacitance of 317 F/g, good rate performance and high capacitances per surface area. These findings could provide a new avenue for the facile production of high-performance porous carbon materials with promising applications in various areas.

Porous carbons have attracted significant scientific and technological interest because of their unique features including large specific surface area, extraordinary electrical and thermal properties.1-5 These intriguing characteristics lead to a range of important applications such as energy storage,6-9 adsorption,10,

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separation,12,

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drug delivery14,

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and catalysis.16-19 Due to the decisive role of the

surface area in determining porous materials’ performance in many applications, development of highly porous carbons has been one of the most active areas in the carbonaceous material science.20, 21 Many methods for the synthesis of high-surface-area carbon exist, including activation,22-26 direct carbonization of porous organic materials,21, 27-32 halogenation of porous carbides,33, 34 and templating strategies (Figure S1).35-39 Among various methods, chemical activation is a powerful and well-established technique to construct highly porous structure with large surface area (e.g., above 2000 m2/g).40-43 However, it usually involves usage of high-amount corrosive activators such as KOH, which is inconsistent with the urgent needs of energy conservation and environmental protection. Furthermore, in ordered to remove impurities derived from activators, repeated post-purification treatments are often required after activation. For example, plenty of water is often required to wash off the impurities from the product, inevitably leading to generation of tremendous waste liquid pollution. Despite remarkable advances of templating methodologies, the most common hard templates, i.e., silica-based materials, are facing a significant bottleneck.44, 45 The removal of the silica template after carbonization always requires harsh repeated chemical post-treatments, such as hydrofluoric acid (HF) or NaOH etching. HF is hazardous and NaOH post-treatment is usually time-consuming and not-cost efficient because of its being handled


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at elevated temperatures.44,

ACS Sustainable Chemistry & Engineering 46-48

Though direct carbonization and high-temperature chlorination to

prepare high-surface-area carbon have been reported as alternate methods, they involve usage of costly carbon sources or expensive catalysts. Besides, some of these starting porous materials are prepared with complicated multiple synthetic steps or under rigorous conditions (e.g., anhydrous and anaerobic), leading to scale-up challenging.49-51 As cheap and environmental friendly sustainable resource, many biomasses have already been explored as attractive raw materials for fabricating carbon materials.43, 52-56 Particularly, porous carbons fabricated from waste rice husks (RHs) have shown promising applications as electrodes, adsorbents, catalyst supports, and others. Nevertheless, the problem is that activation method is still a general even essential way to obtain those highly porous carbon materials (HPCs) from biomass including RHs. There is very limited simple and satisfactory process for the production of biomass-based HPCs to date. Hence, establishing a facile and efficient synthesis protocol using low-cost precursors for preparation of high-surface-area porous carbons is still a great challenge but very desirable for their facile preparation and broad applications. Herein, we report development of a cost-effective, activation-free and post-treatment-free way to prepare a class of HPCs by in-situ etching natural silica during carbonization treatment of RHs (Figure 1). The key to this preparation strategy is utilization of polytetrafluoroethylene (PTFE), which can insitu generate HF to etch out the natural nano-silica during carbonization treatment of RHs. As illustrated in Figure 1, the overall strategy is very straightforward. The HPCs can be obtained by simply mixing the PTFE powder and the RHs, followed by direct carbonization. Compared with the reported methods of producing porous carbon, this strategy demonstrates several dominant characteristics. First of all, the carbonization and silica removal are strategically accomplished simultaneously, reducing the synthetic steps when compared to normal templating procedures to porous carbon. Moreover, the post-treatment for removal silica, i.e., the usage of hazardous HF or repeated NaOH washes is eliminated. In addition, the surface area of the as-obtained carbon materials can be up to 2051 m2/g without any activation


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treatment, which is 20 times enhancement in porosity compared to the traditional RHs-based carbon materials. The last but not the least, RHs employed here are abundant and renewable materials source with a huge supply in the world; and thus this is energy-efficient and easy to scale-up. With combination of the highly porosity and the valuable hierarchical porous structure, the as-prepared HPCs serve well when used as electrode materials for supercapacitor. These findings will provide a new avenue for facile preparation of high-surface-area carbon materials from biomass for challenging environmental and energy issues.

Figure 1. Schematic illustration of comparison of the conventional and our synthetic route developed here to HPCs. Grey arrow shows fabrication of conventional carbon materials by carbonization of RHs (). This method is complicated and usually leads to carbon products with low surface areas. Thus, chemical activation treatments are required to increase their surface areas. Red arrow shows fabrication


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of HPCs via in-situ removal of natural silica during carbonization treatment of RHs. This facile route can combines the carbonization and post-silica-removal in a single step and leads to generation of a well-developed hierarchical porous structure upon carbonization. Results and discussion As a class of naturally occurring biomass, RHs are composed of nano-silica, cellulose, lignin and very little other materials including fats, protein (Figure 2a).57, 58 As the major constituent, small and large silica nanoparticles are aggregated and formed on the intercellular layers and surface of cell walls.59 Thermogravimetric analysis (TGA) of RHs shows the content of the main inorganic material, i.e., nanosilica, is about 14 wt.% (Figure S2). In order to remove these silica compound, post-etching with corrosive HF or chemical activation are normally needed after carbonization of RH (Figure 1). In contrast to the conventional synthesis methods with two-step process, our route combines the carbonization and post-silica-removal in a single step with complete avoidance of post-treatment. The RHs are mixed with PTFE powder and then directly carbonized through heating under N2 atmosphere, readily leading to formation of target sample HPC-RH-1. Scanning electron microscope (SEM) images at different magnifications are displayed in Figures 2b and 2c. It is found that after a simple carbonization treatment at 900 oC, the obtained HPC-RH-1 reveals a typical hierarchical porous morphology. Magnified SEM image in Figure 2c shows that after the high temperature carbonization treatment, the resulting carbon frameworks are fractured on the nanometer scale to form abundant carbon nanoparticles with diameters in the range of 10~40 nm. The close and loose aggregation of these carbon nanoparticles lead to formation of mesopores and macropores, respectively. Transmission electron microscope (TEM) observation confirms the highly porous structure in HPC-RH-1 (Figure 2d). The absence of a Si peak in energy-dispersive X-ray spectroscopy (EDS) of HPC-RH-1 carbon demonstrates the complete removal of nano-silica component (Figure S3). For comparison, a control carbon sample is prepared through a traditional two-step strategy. EDS analyses


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demonstrate the chemical compositions of HPC-RH-1 and control carbon sample reveal few difference, and the silica of the control sample is also removed by HF post-etching (Figures S3~S8). However, as shown in Figures 2e and 2f, the control carbon sample displays a significantly poorer porous structure when compared with HPC-RH-1.

Figure 2. SEM images of (a) RH, (b, c) HPC-RH-1 and (e) control carbon sample; the inset digital photo in (a) shows the macroscopical form of RH. TEM images of (d) HPC-RH-1 and (f) control carbon sample; the inset in (d) shows the graphite-like microcrystalline structure in HPC-RH-1. N2 adsorption measurement is employed to quantitatively illuminate the pore characteristics of the asprepared samples. As shown in N2 adsorption-desorption isotherms of Figure 3a, the HPC-RH-1 displays a high adsorption uptake at low relative pressure (P/P0), indicating the presence of plentiful micropores. After that, the adsorption amount continuously increases, and but still does not reach a


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plateau near the P/P0 of 1.0, implying the existence of mesopores and macropores. The DFT pore size distribution in Figure 3b shows the micropores center at 0.8 and 1.3 nm, and the meso-/macropores range from 2 to 120 nm with a maximum at 17 nm. Brunauer-Emmett-Teller (BET) specific surface area (SBET) and pore volume of HPC-RH-1 are calculated to be as high as 2051 m2/g and 1.36 cm3/g, respectively, and its micropore surface area and meso-/macropore surface area are measured to be 1274 and 777 m2/g, respectively. It is noteworthy that this high-surface-area HPC-RH-1 with good reproducibility outperforms the majority of porous carbon without any extra activation technique, and is even comparable to a lot of activated carbons which are produced through complicated chemical activation procedures (Figure S9 and Table S1). In sharp contrast, the control carbon sample has much smaller SBET of 101 m2/g and pore volume of 0.16 cm3/g (Figure 3a), which are in good agreement with the observation from SEM and TEM images. The maximum pore size distribution peaks for the control carbon sample basically fix at 4 and 37 nm (Figure S10). Such a difference in pore structure is mainly resulted from the formation of additional pores by the removal of volatile oxygen-containing substances upon further pyrolysis. It is demonstrated that when a silica-containing hybrid is mixed with PTFE and heated in a N2 atmosphere with a slow rate (e.g., 5 oC/min), simultaneous carbonization and silica removal occur within 650 oC.48 That is to say, silica component of RH can be completely removed below 650 oC by releasing SiF4, CO2 and other oxygenate gases (Figure S11), leading to creation of porous carbonaceous materials. When further raising temperature until 900 oC and maintaining 900 oC for several hours, more mass loss occurs because the porous structure provides quite more rooms for large quantities of volatile noncarbon elements and carbon-containing compounds to effectively release. Therefore, more nanopores are formed, leading to a high-surface-area carbonaceous structure in a single step. On the contrary, for the control carbon sample, the presence of a tight silica-carbon interface during the whole carbonization process inhibits the formation of such additional rooms for release of volatile products (Figures S7 and S8), resulting in a relatively low surface area.


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Figure 3. Pore structures of typical samples. (a) N2 adsorption-desorption isotherms of HPC-RH-1 (red) and control carbon sample (gray). (b) DFT pore size distribution curve of HPC-RH-1. (c) SBET and (d) pore volume of HPCs obtained at various carbonization time. The carbon framework structure of HPC-RH-1 is studied by wide-angle X-ray diffraction (XRD) patterns, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The XRD pattern exhibits two broad peaks centered at 2θ ≈ 22o and 44o, demonstrating the formation of low crystallinity degree of carbon framework (Figure S12). The Raman spectrum show two characteristic peaks centered at about 1350 cm-1 and 1600 cm-1, which are attributed to the D and G bands, respectively (Figure S13). A broader D-band further indicates that HPC-RH-1 possesses low graphite crystallinity of carbon framework, whose carbon nanosheets and clusters possess a microcrystalline plane crystal size (i.e., La) of 2.4 nm and then form abundant micropores (the inset of Figure 2d). An integrated illustration of the


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chemical composition of HPC-RH-1 from XPS is shown in Figure S14. It is shown that the HPC-RH-1 is mainly composed of carbon, oxygen and hydrogen elements. No other elements like silicon is detected, further indicative of the complete removal of silica component for the final HPC-RH-1 sample. The nanostructure of HPCs can be readily adjusted through tuning the fabrication conditions (Table S2, Figures S15 and S16). For example, it is found that carbonization time play an important role in altering BET surface area. When extending the carbonization time from 4 to 20 h, both BET surface area and pore volume decrease (Figures 3c and 3d). In addition, the BET surface area increases from 1095 to 2051 m2/g with increasing the silica/PTFE weight ratio from 0.03 to 0.055. While further increasing the weight ratio to 0.09, the BET surface area decreases to 1383 m2/g (Figures S17 and S18). Furthermore, calculated from the Raman spectra of the samples obtained at various carbonization time, the values of La of HPCs gradually increase when increasing carbonization time (Table S3, Figures S13 and S19). Owing to their well-developed porosity and robust hierarchical porous morphology, HPCs developed in this study hold great promise for their application in many fields. Here, we focus on the electrochemical properties of HPCs used as the electrodes of supercapacitors. Supercapacitor, an emerging class of energy storage device with high power density, long cycle life and short charging time, is highly attractive as power source for portable electronic devices.60-63 Currently, the core technology of high-performance supercapacitors is development of carbon-based electrodes with both high capacitance and fast ion/electron transport properties. To analyse the usability of HPCs for promising supercapacitor electrode, the HPC-RH-1 is evaluated as a proof-of-concept demonstration in a two-electrode symmetrical cell by using 6 mol/L KOH aqueous solution as electrolyte. Typical quasi-rectangular shapes in cyclic voltammetry (CV) curves and triangular profiles in galvanostatic charge-discharge curves both demonstrate the ideal electrochemical capacitive characteristics (Figures S20 and S21). The capacitances calculated from galvanostatic charge-discharge measurements at various densities are summarized in Figure 4a. It is clearly seen that HPC-RH-1


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exhibits excellent charge storage capacity. For example, HPC-RH-1 possesses specific capacitances as high as 317 F/g at a current density of 50 mA/g, significantly higher than 202 F/g for the commercially benchmark activated carbon YP-50 with a BET surface area of 1713 m2/g. Even under the high current density of 10000 mA/g, high capacitance of 245 F/g could still be retained for HPC-RH-1, giving good rate capability (Figure 4b). These values are also clearly higher than those of many other reported highperformance carbon-based electrodes (Table S4), demonstrating that the HPC-RH-1 developed here is very attractive in electrochemical energy storage. The significant superiority of HPC-RH-1 in terms of the electrochemical performances is repeatable (Figure S22), which is mainly attributed to its more advanced porous structure. For activated carbon YP-50, most of its pores are micropores and/or smallsized mesopores (Figure S23), a large proportion of which are located in the micron/millimeter-scaled carbon particles. Due to absence of efficient ion diffusion pathways, many of its inner micropores are hard to be immersed by electrolyte, leading to a poor utilization of the surface area and a decrease in capacitance. On the contrary, HPC-RH-1 simultaneously possesses a high surface area and a valuable hierarchical porous morphology. The more robust porosity in HPC-RH-1-based electrode especially its microporosity enhances the formation of electric double layer for higher charge storage when compared with YP-50-based electrode (Figure S24). Meanwhile, the hierarchally meso- and macroporous structure offers minimized diffusive resistance to electrolyte ion transport to access the micropores, thus having lower impedance (Figure S25) and better specific capacitance per surface area when compared with YP50 (Figures 4b and S26).


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Specific Capacitance (F/g)

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Current Density (mA/g) Figure 4. Comparison of supercapacitive performance for typical samples. (a) Specific capacitances of HPC-RH-1 and commercially benchmark activated carbon YP-50. (b) Capacitance retention ratios (solid) and specific capacitances per surface area (hollow) of HPC-RH-1.


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Conclusions In conclusion, we have developed a versatile and effective approach for synthesis of highly porous carbons by a simple carbonization of a mixture of rice husk and PTFE powder. The PTFE is capable of in-situ etching natural silica component of rice husk during carbonization process, thus eliminating the tedious post-treatment and then decreasing the synthetic steps. The as-obtained carbon material possesses high surface area of up to 2051 m2/g, which exhibits excellent supercapacitive energy storage performance. It is envisioned that further tuning of pore structures could be achieved by variation of precursor type and size, carbonization temperature and heating rate. We hope that this work could provide new opportunities for the efficient production of high-performance porous materials with promising applications in various fields such as supercapacitors, lithium-sulphur batteries, adsorption and catalysis. Methods Materials. Rice husk (RH) was obtained as a by-product of rice harvested in the suburbs of Xiantao City in P. R. China, and directly used as the starting material without any pre-treatment. Polytetrafluoroethylene (DuPont Company, 7A), hydrofluoric acid (HF, Guangzhou Chemical Reagent Factory) and other chemicals were used as received. Synthesis of HPCs-RH. In a typical process, RH powder was obtained by smashing about 50 g of RH in a family-use electric grinder (Laifu, LFP-800T) for 20 minutes with a revolving speed of 29000 r/min. For preparation of HPC-RH-1, 0.5 g of RH powder was intimately mixed with 1.28 g of PTFE powder by grinding in a mortar. The mixture was subsequently transferred to an alumina boat. The boat was then placed inside a tubular furnace and then heated to 900 oC at 5 oC/min for 4 h under N2 flow, leading to formation of HPC-RH-1 product. For fabrication of other HPC-RHs, the overall procedures were exactly the same as those of HPC-RH-1 except for the carbonization time and RH/PTFE weight ratio. Detail fabrication parameters are listed in Table S2.


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Synthesis of control carbon sample. RH powder was placed inside a tubular furnace and then heated to 900 oC at 5 oC/min for 4 h under N2 flow. After carbonization, the as-obtained control carbon/silica composites were washed using 40 wt% HF solution to remove the silica, followed by filtration off and repeated flushing with distilled water until the filtrate is neutral. After dried at 100 oC, the control carbon sample was obtained. Structural characterization. The microstructure of the samples was investigated by a Quanta 400F scanning electron microscope (SEM) and a FEI Tecnai G2 Spirit transmission electron microscope (TEM). Raman measurement was carried out with inVia-Reflex Renishaw Raman system. Thermogravimetric (TG) analysis was performed under flowing air condition at a heating rate of 20 o

C/min. TG coupled with Fourier transform infrared spectrometry (TG-FTIR) measurement was

performed under N2 flow at a heating rate of 10 oC/min. The TG and FTIR spectra were recorded on a Netzsch TG-209 and a Bruker Vector-22 infrared spectroscopy, respectively. N2 adsorption measurements were carried out using a Micromeritics ASAP 2020 analyzer at 77K. The BET surface area (SBET) was estimated by Brunauer-Emmett-Teller (BET) theory. The micropore surface area (Smic) was analyzed by t-plot method, and then the external surface area (Sext) was obtained by subtracting the Smic from the SBET. The pore size distribution was determined by original density functional theory (DFT) combined with non-negative regularization and medium smoothing. The total pore volume (Vt) was calculated according to the amount adsorbed at a relative pressure P/P0 of about 0.99. XRD patterns were conducted on a D-MAX 2200 VPC diffractometer using Cu K radiation (40 kV, 30 mA). Raman measurements were carried out with inVia-Reflex Renishaw Raman system. An ESCALAB250 instrument was used to record the XPS data. Fabrication and measurements of supercapacitors. The electrodes in the form of round sheet with a diameter of 1.2 cm were obtained by pressing a mixture film of 87 wt.% carbon sample, 5 wt.% carbon black with a BET surface area of 62 m2/g and 8 wt.% PTFE. The average mass of carbon sample for an electrode was controlled at about 3 mg. The specific surface area of the electrode (Selectrode) was


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calculated according to the equation Selectrode = Scarbon − sample × 87% + Scarbon −black × 5% + S PTFE × 8% , where Scarbon-sample, Scarbon-black and SPTFE, represent the BET surface area of carbon sample, carbon black and PTFE, respectively. Nickel foam was used as the current collector. 6 mol/L KOH aqueous solution was used as the electrolyte. The electrochemical measurements were characterized with the assembled cointype supercapacitor. Cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) measurements were carried out using a Chenhua electrochemical workstation. The signal amplitude in EIS measurement was 5 mV.

AUTHOR INFORMATION Corresponding Authors [email protected] (Y.R. Liang); [email protected] (Y.L. Liu). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT: The authors gratefully acknowledge financial support from the projects of the National Natural Science Foundation of China (51602107 and U1501242), Program for Pearl River New Star of Science and Technology in Guangzhou (201710010104) and the Cultivation Fund of Scientific and Technical Youth Talents of South China Agricultural University. Supporting Information Available: Additional information about material characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

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61. Zhang, L.; You, T.; Zhou, T.; Zhou, X.; Xu, F., Interconnected Hierarchical Porous Carbon from Lignin-Derived Byproducts of Bioethanol Production for Ultra-High Performance Supercapacitors. ACS Appl. Mat. Interfaces 2016, 8, 13918-13925. 62. Yu, Z. N.; Tetard, L.; Zhai, L.; Thomas, J., Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions. Energy Environ. Sci. 2015, 8, 702-730. 63. Simon, P.; Gogotsi, Y., Materials for electrochemical capacitors. Nat. Mater. 2008, 7, 845-854.


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For Table of Contents Use Only

Facile Synthesis of Highly Porous Carbon from Rice Husk Yeru Liang*, Chen Yang, Hanwu Dong, Wenqi Li, Hang Hu, Yong Xiao, Mingtao Zheng, Yingliang Liu* Highly porous carbons were synthesized through a facile carbonization of mixture of rice husk and polytetrafluoroethylene powder.


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Facile Synthesis of Highly Porous Carbon from Rice Husk

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