Popcorn-Derived Porous Carbon for Energy Storage and CO2

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Popcorn-Derived Porous Carbon for Energy Storage and CO2 Capture Ting Liang,†,‡ Chunlin Chen,*,† Xing Li,‡ and Jian Zhang*,† †

Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, 1219 Zhongguan West Road, Ningbo 315201, China ‡ Faculty of Materials Science and Chemical Engineering, Ningbo University, 818 Fenghua Road, Ningbo 315211, China

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S Supporting Information *

ABSTRACT: Porous carbon materials have drawn tremendous attention due to its applications in energy storage, gas/ water purification, catalyst support, and other important fields. However, producing high-performance carbons via a facile and efficient route is still a big challenge. Here we report the synthesis of microporous carbon materials by employing a steam-explosion method with subsequent potassium activation and carbonization of the obtained popcorn. The obtained carbon features a large specific surface area, high porosity, and doped nitrogen atoms. Using as an electrode material in supercapacitor, it displays a high specific capacitance of 245 F g−1 at 0.5 A g−1 and a remarkable stability of 97.8% retention after 5000 cycles at 5 A g−1. The product also exhibits a high CO2 adsorption capacity of 4.60 mmol g−1 under 1066 mbar and 25 °C. Both areal specific capacitance and specific CO2 uptake are directly proportional to the surface nitrogen content. This approach could thus enlighten the batch production of porous nitrogen-doped carbons for a wide range of energy and environmental applications. mere,3 willow catkins,14 yogurt,15 etc. A new synthesis strategy is required to swell cellulose, lignocellulose, polysaccharide, or other precursors into a porous or open structure before the carbonization process, allowing a high fraction of exposed carbon atoms to the pore-making or functionalization reagents. One successful example is the microwave popping of corn followed by thermal carbonization,16 albeit with the difficulties to be industrialized due to the limited processability of commercial microwave apparatus. Herein, we reported an efficient and sustainable method to swell corn by employing the steam-explosion method and successfully produce high-surface-area and nitrogen-incorporated carbons by subsequent KOH activation. The excellent textural properties with a large SSA up to 1489 m2 g−1 and high microporosity up to 90% as well as controllable nitrogen doping level (0.88−1.62 at. %) endowed the activated porous carbons as a very promising materials in both energy storage and CO2 capture applications. According to the quantitative description of the structure−effect relationship of the areal specific capacitance or volumetric specific CO2 uptake capacity versus surface nitrogen content, our finding demonstrated that the synergistic effect of textural parameters and surface chemistry was crucial to both charge storage and CO2 uptake. The fitted empirical equations provide guidelines for materials design and structural optimization for high-performance carbon materials. These results suggest the widespread and efficient

1. INTRODUCTION Energy storage and environmental protection are two determining factors to the sustainable development of living and industries. The exceptional properties of porous carbon materials, such as high surface area, controllable pore structure, excellent thermal and chemical stability, wide availability of raw materials, superior environmental friendliness, high amenability to surface functionalization, and heterogeneous doping, stimulate fundamental researches toward the applications in lithium batteries, supercapacitors, CO2 capture/sequestration, catalysis, and so forth. Both high porosity and surface functionality are necessary to achieve a high performance of carbons as supercapacitor electrode materials and gas adsorbent. Large porosity can provide more active sites for charge accommodation and accessibility of electrolyte ions,1−3 and CO2 capture is closely dependent on the volume of pores that are smaller than a certain diameter.4,5 Surface functionality may generate the extra pseudocapacitance derived from redox reactions of surface groups6 and improve the adsorption capacity due to the enhanced interaction force with CO2 molecules. Numerous researches are being conducted to use abundant and clean resources to directly synthesize functional carbon materials. Raw biomass naturally contains nitrogen and other heteroatoms that will be anchored inside of or around the graphitic structure to endow carbon materials with a variety of functionalities. With the aid of template chemicals, nitrogendoped carbons with a large specific surface area can be obtained from various biomass like rice,7 wood,8 gelatin,9 human hair,10 silk proteins,11 soybeans,12 pomelo peel,13 disposable cash© 2016 American Chemical Society

Received: May 23, 2016 Revised: July 15, 2016 Published: July 25, 2016 8042

DOI: 10.1021/acs.langmuir.6b01953 Langmuir 2016, 32, 8042−8049

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Figure 1. Schematic diagram for the preparation of PC-xK. alloy as counter electrode. Electrochemical performances were evaluated by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements. All of CV and GCD measurements were evaluated using a CHI 760E instrument, while the EIS was measured by a Zahner Zennium electrochemical system. In three-electrode system, the specific gravimetric capacitance (Cs, F g−1) can be calculated from GCD curves according to the formula 1:

utilization of biomass as a treasure resource for sustainable development.

2. EXPERIMENTAL SECTION 2.1. Preparation of Popcorn-Derived Porous Carbon. Briefly, popcorn was obtained by a traditional method using an orient artifact as depicted in Figure 1. It is essentially a pressure vessel manufactured in Shengdi Machinery Plant, Wenling City, Zhejiang Province, China. Corn with 15%−25% moisture and ∼70% starch named Bainong 5 was purchased from Hubei Kangnong Seed Industy Limited Company. Typically, 800 g of corn was sealed in the vessel and then heated on a flame for 6−8 min to reach the pressure of 1.0 MPa. At this point the water inside the hot corn should be vaporized and the starch liquefied. The corn popped to form popcorn when the pressure relieved suddenly by opening the lip of vessel. After that, popcorn was crushed into powders. The white powders were precarbonized under a N2 atmosphere at 400 °C for 1 h with the heating rate of 5 °C/min. Hereafter, the obtained black powders were grounded and mixed with a different volume of 1 M KOH aqueous solution, and then the mixture was treated with ultrasonic for 10 min. Afterward, the black mixture was dried in oven at 80 °C to remove the excess water. The solid mixture was further grounded into powders and subsequently transferred to tube furnace for activation at 800 °C for 1 h with the protection of nitrogen. After that, the as-prepared activated material was washed with suitable amounts of HCl (1 M) to remove any inorganic impurities, then washed with deionized water until the filtrate was neutral, and dried at 110 °C for the whole night. To investigate the effect of KOH dosage on the microstructure of the asobtained samples, a sequence of experiments were carried out under the same conditions. The obtained products were marked as PC-xK (x = 0.5, 1, 2), when x is the mass ratio of KOH and precarbonization product. For comparison, the sample directly carbonized at 800 °C without activation was noted as PC. 2.2. Characterization and CO2 Adsorption. Scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, Tecnai F20) were employed to investigate the morphology and microstructure of the as-prepared materials. The Xray photoelectron spectroscopy (XPS) tests were carried out on an AXIS ULTRADLD multifunctional X-ray photoelectron spectroscope with an Al Kα radiation source at room temperature and under a vacuum of 10−7 Pa. Raman spectra were recorded using a Renishaw inVia Reflex (532 nm laser) system. The textural analysis was carried out at 77 K using a Micromeritics ASAP 2020 instrument after degassing at 300 °C for 7 h prior to analysis. The CO2 adsorption isotherms of the obtained materials were measured on Micromeritics ASAP 2020 instrument at 0 and 25 °C. 2.3. Electrochemical Measurements. The working electrode was prepared by mixing the active materials, carbon black, and binder poly(tetrafluoroethylene) with the mass ratio of 8:1:1 in ethanol, followed by ultrasonic treatment. Then the mixture was coated onto the nickel foam current collector with coating area of 1 cm2, which was further dried at 100 °C overnight. Electrochemical measurement was performed in a standard three-electrode system in 6 M KOH aqueous solution with a saturated calomel electrode (SCE) as reference electrode and a 2 × 2 cm titanium foil plated with iridium−tantalum

Cs =

I Δt mΔV

(1)

where I (A), Δt (s), m (g), and ΔV (V) are GCD current, discharge time, loading mass of active material on the electrode, and voltage change excluding the IR drop during the discharge process, respectively. The electrochemical measurement of the symmetric supercapacitor (SC) was carried out in a 2032 stainless steel coin cell. Two pieces (diameter of 1.5 cm) of nickel foam were used as the current collectors, using a glass fiber membrane as separator and 6 M KOH aqueous solutions as the electrolyte. The working electrodes were prepared by the same procedure in the above three-electrode system with nearly identical loading mass of active materials. The electrochemical performance of SC was evaluated by the CV and GCD. The specific capacitance of the SC cell (Ccell, F g−1) was calculated from GCD curves based on the formula Ccell =

I Δt mΔV

(2)

where I (A) is the GCD current, m (g) is the total mass of active materials on the anode and cathode, Δt (s) is the discharge time, and ΔV (V) is the voltage change excluding the IR drop during the discharge process. The power and energy density were calculated based on the total mass of active materials loading on both electrodes. The energy density (E, Wh kg−1) was calculated from the formula E=

1 Ccell(ΔV )2 2

(3) −1

The average power density (P, W kg ) during the discharge time of the symmetrical supercapacitor systems was calculated according to

P=

E Δt

(4)

3. RESULTS AND DISCUSSION The history of popcorn can be traced back to 5000 years ago, and Chenda Fan, a Chinese poet, also described the homemade process that was popular in the 1150s.17 As illustrated in Figure 1, corn, millet or rice is heated in a sealed container, and then the water is vaporized to keep a balance of pressure across the outer shell. As the overall pressure inside the container reached 5−10 atm, the lid is suddenly opened to cool down the whole system. Rapid temperature drop will cause a large pressure difference to lead the fast explosion of the grain. The obtained 8043

DOI: 10.1021/acs.langmuir.6b01953 Langmuir 2016, 32, 8042−8049

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Figure 2. (a) N2 adsorption−desorption isotherms and (b) pore size distribution curves of obtained materials. (c) Raman ID/IG mapping of PC-2K. (d) XPS survey spectra of all samples. (e) TEM, (f) HRTEM, and (g) STEM HAADF image and elemental maps of PC-1K. Inset to (d): the highresolution N 1s spectra of PC.

at 800 °C without activation was noted as PC. The textural properties of all samples were investigated in detailed by nitrogen physisorption. Tiny N2 absorbance and no distribution in H−K region revealed that the PC sample only contained macropores with the size of a few micrometers, as shown in Figure 2a,b. After the KOH activation, the PC-xK samples exhibited characteristic I type isotherms and H4 type hysteresis curves, being typical for micropores dominated texture with little amount of mesopores. Such a hierarchical pore feature was reflected in the narrow distribution of pore size (Figure 2b) by the Barrett−Joy−Halenda (BJH) and Horvath−Kawazoe (H− K) methods. The slight rise in isotherms at the relatively high pressure (0.95−1 p/p0) implied the retention of some slitshaped macropores, which can be assigned to the remained

popcorn displayed an open feature with a honeycomb-like structure, and the productivity of popcorn is estimated to around 50 kg h−1 by using an automatic commercial machine. This demonstrates a great potential for large-scale nonpollution production of porous carbon precursors. Then we carbonized the popcorn powder with the assistance of KOH as activation agent to improve the porosity of the carbon materials. The products exhibited excellent textural properties with a specific surface area (SSA) up to 1489 m2 g−1, high microporosity of ∼90%, and a controllable nitrogen doping level (0.88−1.62 at. %). The final carbon materials were marked as PC-xK (x = 0.5, 1, 2), where x is the mass ratio of KOH to popcorn before carbonization. For comparison, the sample directly carbonized 8044

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Figure 3. Electrochemical performances in a three-electrode system in 6 M KOH electrolyte: (a) CV profiles at 5 mV s−1; (b) specific capacitances at different current densities and the inset shows GCD curves at the current density of 0.5 A g−1; (c) Nyquist plots and their corresponding expanded views of the high frequency range (inset); (d) the linear relationship between areal specific capacitance and the surface nitrogen content.

debris.19 KOH activation did not crush the interconnected macropores but produced sufficient porosity to construct a hierarchical pore structure (Figure 2e and Figure S2), in accordance with the physisorption results. High-resolution TEM images of the boundary area depicted a long-range disordered structure with short-range curved graphitic lattices (Figure 2f). Uniform distribution of both nitrogen and oxygen atoms throughout the carbon framework can be seen in the elemental maps by the STEM in Figure 2g. Owning to the highly porous structure with abundant surface functionalities, the activated PC-xK samples were investigated for their potential uses as the electrode in supercapacitor and absorbent for CO2 capture. The electrochemical capacitive properties of the four samples were evaluated by conducting the cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) tests in a three-electrode system with 6 M KOH aqueous solution. The CV profiles at scan rate of 5 mV s−1 (Figure 3a) showed that the activated samples were of the better reversibility because their CV curves were much closer to a rectangular shape than that of untreated PC. The activated samples also exhibited a greatly improved capacitance than PC, as revealed by the longer charge/discharge times in the GCD profiles at low current density of 0.5 A g−1 (inset to Figure 3b). The calculation by using the discharge branches demonstrated that the activation of 1 equiv of KOH can efficiently enhanced the specific capacitance from 60 to 245 F g−1. Despite the capacitance inferior to some elaborately manufactured carbons,20−22 the activated samples were superior to many other biomass-derived carbon materials in the literature (detailed comparison in Table S2). Of all activated samples,

honeycomb-like walls and stacking carbon particles. The BET surface area increased from 39 to 1489 m2 g−1, and the total pore volume reached 0.706 cm3 g−1, benefiting from an open and thin-walled structure of the popcorn precursor. The pore diameter of the most probable aperture increased from 0.57 to 0.59 nm with the dosage of KOH. This phenomenon is related to the chemical etching mechanism by potassium ions at elevated temperatures,18 in accordance with the raised weight loss from 37.3% to 49.4% with the increasing amount of KOH. Raman mapping is a useful technique to elucidate the lattice ordering of carbon materials in a large scope. Two distinct peaks positioned at around 1337 and 1594 cm−1 (Figure S1) were assigned to the characteristic D-bands (defects and disorder) and G-bands (graphitic) of carbon, respectively. The Raman ID/IG mapping of PC-2K in Figure 2c showed a narrow distribution of ID/IG ratio with an average value at 0.93. The slight fluctuation of the entire area revealed that a similar degree of graphitization varied to a limited extent in space, while the spatial inhomogeneity may originate from the coexistence of surface functionalities and well-graphitized domains. Surface chemical composition was determined by Xray photoelectron spectroscopy (XPS), and the survey curves in Figure 2d contain three major peaks centering at 284.6, 401, and 533 eV, corresponding to C 1s, N 1s, and O 1s, respectively. Deconvolution of the N 1s spectrum of PC (inset to Figure 2d) identified four nitrogen-containing components: pyridinic N (398.4 eV), pyrrolic N (400.4 eV), graphitic N (401.5 eV), and oxidized N (402.8 eV). The surface content of nitrogen dropped from 2.47% to 0.88% with the increasing dosage of KOH (Table S1), suggesting that the chemical etching process by potassium ions may start from nitrogen-rich 8045

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Figure 4. Electrochemical performance of PC-1K based symmetric supercapacitor using 6 M KOH solution as electrolyte: (a) CV profiles at 5−200 mV s−1 scan rates; (b) GCD curves at 0.1−20 A g−1; (c) GCD cycling stability at 5 A g−1 and rate performance at 0.1−20 A g−1 (inset); (d) Ragone plot comparison with reported biomass-derived carbons.3,27−31

outstanding EDLC behavior, lowest Rs, and appropriate Rct. Aiming to make clear the effect of heteroatoms on capacitance, we normalized the mass specific capacitance (F g−1) of the activated samples by their specific surface areas to obtain the areal specific capacitance (F m−2). As depicted in Figure 3d, the areal specific capacitance at a fixed current density linearly dependent on the surface nitrogen content with the least squares of 0.961−0.996. When the charge/discharge current density was reduced from 100 to 0.5 A g−1 to decrease the transfer resistance, the slope of the fitted line increased from 0.087 ± 0.004 to 0.159 ± 0.008. This phenomenon clearly indicated that the electron-rich nitrogen atoms are apt to modulate the intrinsic charge storage ability of carbon materials due to the improved electrical conductivity, wettability, and additional pseudocapacitance.26 The outstanding performance of PC-1K in three-electrode system inspired us to fabricate a symmetric supercapacitor by using a stainless steel 2032 coin cell. The CV profiles in Figure 4a are much closer to reversible rectangular shape with characteristic of mirror-image symmetry even at a high scan rate of 200 mV s−1, indicating a more ideal EDLC behavior due to the reduced distance of two electrodes (the distance between the working electrode and contrast electrode in three-electrode system). The robust reversibility was also demonstrated by GCD profiles featuring with nearly isosceles triangle appearance (Figure 4b) as well capacitance retention of 65% at a current density of 20 A g−1 (inset to Figure 4c). Both superior rate performance in a wide range of current density and GCD cycling stability under the relatively fast charge/discharge condition are required to construct a high-performance supercapacitor. As seen in Figure 4c, PC-1K displayed a high

PC-1K displayed the highest value, and there is no proportional relationship between the capacitance and KOH dosage. The rate performance at a range of current density from 0.5 to 100 A g−1 reported that the capacitance retention of all activated samples is 57−58% while that of untreated PC sample is only 5% (Figure 3b). The significantly improved rate capability can be related with the hierarchical pore structure of PC-xK samples. The mesopores and macropores are able to furnish a convenient transport and diffusion channels for electrolyte ions, while the micropores provide the intrinsic area for charge accommodation.23 Note that the size of different ions in KOH aqueous electrolyte is presented as follows: OH− < K+ ≈ H3O+ (3.62−4.2 Å).24 The physisorption results of PC-xK samples in Figure 2b show that the most probable pore size is slightly larger than those of all ions in KOH electrolyte to reach a high matching degree. The EIS tests were conducted to investigate the capacitive behavior of all samples (Figure 3c). The low-frequency region of Nyquist spectrum of PC-xK samples is much closer to the imaginary axis in order of PC-1K ≈ PC-2K > PC-0.5K ≫ PC, indicating a superior electric double layer capacitor (EDLC) behavior. It is known that the power output capability of supercapacitors strongly depends on both intrinsic capacitance and resistance.25 The expanded view of the high frequency range in the inset presents the solution resistances (Rs) obtained from the left intercept at real axis were measured to be 0.58, 0.93, 0.38, and 0.77 Ω, while the charge transfer resistances (Rct) determined from the diameter of the semicircle were 0.17, 0.45, 0.40, and 0.24 Ω for PC, PC-0.5K, PC-1K, and PC-2K, respectively (details shown in Figure S3). The best capacitance of PC-1K can be interpreted by its 8046

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Figure 5. (a) CO2 uptake isotherms at 25 °C and 0−1066 mbar; (b) the relationship between volumetric CO2 uptake capacity and surface nitrogen content; (c) isosteric heat of CO2 adsorption at different CO2 loadings; (d) the comparison of CO2 capture capacity with other reported carbon materials (details shown in Table S3).

pore volume of 0.706 cm3 g−1 provided the lowest CO2 adsorption capacity among the activated samples, suggesting that the surface chemical interaction may play a dominant role in CO2 capture. Previous research pointed that the microporous structure and surface chemistry are the two most important factors in CO2 adsorption.5 We herein plotted the CO2 uptake per cubic centimeter of micropore volume versus the surface nitrogen content of PC-xK samples to elucidate the effects of textural parameters and heteroatomic modification. As depicted in Figure 5b, the volumetric specific CO2 uptake capacity is directly proportional to the nitrogen content with a correlation of y = 0.90 + 6.84x (r2 = 0.85), indicating that the sorption behavior of PC-xK samples was governed by the physisorption moderated by pore structure and the chemisorption strengthened by acid−base or hydrogen-bonding interactions between the functionalized carbon surface and CO2 molecules.5,34 By fitting the adsorption isotherms conducted at 25 and 0 °C (details in Figure S5), the isosteric heats of adsorption (Qst) were calculated according to Clausius− Clapeyron equation to provide the insight into the surface adsorption affinity,35 being in the range 31.9−47.1, 15.8−24.3, 19.0−25.0, and 19.9−25.6 kJ mol−1 for PC, PC-0.5K, PC-1K, and PC-2K, respectively (Figure 5c). Therefore, the inferior performance of nitrogen-enriched macroporous PC sample can

capacitance retention of 97.8% after 5000 consecutive cycles with 16.7 h duration at 5 A g−1, demonstrating an excellent cycling stability and high reversibility. Furthermore, as depicted in the Ragone plot (Figure 4d), the symmetric device exhibited a maximum energy density of 6.0 Wh kg−1 at a power density 52.2 W kg−1, which is much better than the most of the reported symmetric supercapacitors based on biomass-derived carbons. Highly porous carbon materials with basic nitrogen functionalities are also promising for CO2 capture and storage, being of great importance for alleviating the greenhouse effect. Adsorption tests of these samples were carried out at 25 °C, and the isotherms are shown in Figure 5a. Different from the supercapacitor results, PC-0.5K exhibited the highest adsorption performance with a quantitative CO2 uptake capacity of 4.60 mmol g−1 at 1066 mbar, while PC-1K, PC-2K, and PC acquired 4.55, 4.32, and 2.21 mmol g−1, respectively. Although some nitrogen-rich carbons were reported with capacity up to 5.8 mmol g−1 (1 bar, 25 °C)32 and 8.99 mmol g−1 (1 bar, 0 °C),33 the three activated samples surpassed most of biomassderived carbons, polymer-derived carbons and other typical carbon materials with higher SSA and larger pore volume, as presented in Figure 5d as well as Figure S4 and Table S3. Interestingly, PC-2K with the largest SSA of 1489 m2 g−1 and 8047

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Zhejiang Provincial Natural Science Foundation of China (R16B030002), International Science and Technology Cooperation Program of Ningbo City (2014D10004), and Natural Science Foundation of Ningbo City (2014A610108).

be understood. Furthermore, a linear relationship between volumetric CO2 uptake capacity (y, mmol cm−3) and the surface nitrogen content (x, at. %) can be fitted by the empirical equation y = 0.90 + 6.84x to quantitatively describe the role of nitrogen. This comprehension of the structure−performance correlation would favor a more careful fine-tuning of textural and surface properties of carbonaceous adsorbent to meet the industrial demands for carbon capture and sequestration.

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4. CONCLUSIONS In summary, we have successfully put forward a novel hierarchical honeycomb-like porous carbon derived from sustainable popcorn as natural and nitrogen-containing carbon precursors, which exhibited large specific surface area (867− 1489 m2 g−1), improved pore volume (0.406−0.706 cm3 g−1), and extraordinary microporous porosity (83.6−91.1% in volume). The unique structural features endowed the activated porous carbon with outstanding performance in supercapacitor application and CO2 capture. The activated sample PC-1K displayed a high specific capacitance of 245 F g−1 at 0.5 A g−1 and good rate capability with 57% retention at 100 A g−1 in a three-electrode system. An excellent performance was also demonstrated in symmetric coin cell supercapacitor measurement with a remarkable stability of 97.8% retention after 5000 GCD cycles at 5 A g−1 as well as a maximum energy density of 6 Wh kg−1 at a power density 52.2 W kg−1. The activated sample PC-0.5K exhibited the highest CO2 adsorption capacity of 4.60 mmol g−1 under an atmospheric pressure and 25 °C condition, overriding most of doped or biomass-derived porous carbon materials. The quantitative description of the structure− effect relationship of the areal specific capacitance or volumetric specific CO2 uptake capacity versus surface nitrogen content revealed the synergistic effect of textural parameters and surface chemistry on performance of charge and CO2 storage. Importantly, the two fitted empirical equations provide guidelines for materials design and structural optimization for high-performance capacitor and gas adsorbent applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01953. Raman spectra, SEM images, EIS analysis, CO2 uptake isotherms at 0 °C, structural properties, comparison of the gravimetric capacitances, and CO2 capture capacity with references (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.C.). *E-mail: [email protected] (J.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the technical assistance from Binbin Lv, Guangyuan Yang, and Yan Liu (Ningbo Institute of Materials Technology & Engineering, CAS) as well as the financial support from National Natural Science Foundation of China (51422212, 21403261), Science Technology Department of Zhejiang Province (2015C31118), 8048

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DOI: 10.1021/acs.langmuir.6b01953 Langmuir 2016, 32, 8042−8049