Air-expansion induced hierarchically porous carbonaceous aerogels from biomass materials with superior lithium storage properties

Abstract

Traditional methods for the preparation of carbon aerogels, such as a sol–gel method, hydrothermal method, freeze-drying method and the direct carbonization of biomass materials, have been more and more limited in their applications due to their high cost, complex processes and the accompanying volume shrinkage in the preparation process. In this paper, we developed a novel air-expansion method for the preparation of porous carbonaceous aerogels with hierarchically macroporous, mesoporous and microporous structures from rice. The main advantages of an air-expansion method are large-scale preparation, low cost, a simple technique and most importantly it keeps the initial shape/structure and avoids shrinkage of the carbon aerogels owing to the air-expansion process of rice generating many macroporous structures for supporting the aerogel framework. When used as an anode for lithium ion batteries, rice-based carbonaceous aerogels exhibit a superior specific capacity and possess a good rate capability. This study gives a better insight into the preparation of carbonaceous aerogels from other grains as well as their potential applications in lithium ion batteries.

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1. Introduction

In order to solve the increasing environmental issues and finite fossil-fuel supply problem, it is very important and urgent to develop alternative energy conversion and storage resources. Lithium ion batteries have been the main power source of portable electronic devices in recent years due to their high energy density.^1,2 It’s well known that graphite is currently the most commonly used anode carbon material due to its low and flat potential plateaus, high coulombic efficiency and low cost. However, its limited theoretical capacity (up to 372 mA h g⁻¹) cannot completely satisfy the requirements of new applications such as electric and hybrid electric vehicles. Other anode materials such as metal oxides and silicon have high theoretical capacities, but the high price, environmental pollution and bad long term cycling characteristics limit their further application.^3,4

In the last ten years, graphene has been considered as the ideal candidate for next generation anode materials owing to its excellent mechanical and electrical properties.^5–7 As a two-dimensional all-sp²-hybridized carbon material, graphene carbon atoms are arranged in a honeycomb lattice with a high accessible surface area (2630 m² g⁻¹).⁸ However, the most commonly used method for the preparation of graphene through chemical oxidation has some drawbacks, such as a complex and tedious synthesis process, and the production of environmentally hazardous wastes, as well as a very low yield, which results in a high price of graphene and limits its practical application.^9–11 Furthermore, most reports show that the actual electrochemical performances of graphene and graphene-based anode materials are much lower than the theoretical values because of the facile aggregation and restacking of graphene sheets through van der Waals interactions among individual graphene sheets.^12,13 Hence, the exploitation of low cost and effective carbon anode materials with high-rate and good cycling stability is an urgent and key requirement for lithium-ion battery technology. Recently, porous carbon and carbon aerogels derived from biomass with natural micro-nano holes have attracted much attention for use as electrode materials for supercapacitors and lithium ion batteries because of their low cost, reproducibility, environmental friendliness and large specific surface area.^14–17 Therefore much effort has been made to synthesize the porous carbon materials obtained from all kinds of biomass materials. For example, Wu et al. prepared low-cost and sponge-like carbonaceous flexible hydrogels and aerogels using crude watermelon biomass as the carbon source.¹⁸ The final magnetite carbon aerogels exhibit excellent capacitance properties and an outstanding cycling stability after 1000 cycles of a charge/discharge process. Tang et al. developed a new method to synthesize graphene from eggshell as a high-performance electrode material for energy storage using a simple magnesiothermic reduction reaction.¹⁹ Besides, other biomass materials such as rice husks,²⁰ peanut shells,²¹ banana peels,²² pomelo peels,²³ coconut shells²⁴ and starch³ have been successfully used as carbon sources for preparing all kinds of porous carbon materials for use as new type electrode materials for capacitors, and lithium and sodium ion batteries.

In this paper, we demonstrate a novel air-expansion method for the preparation of hierarchically macro-, meso- and micro-porous carbonaceous aerogels using rice as the carbon source. Puffed rice is to rice as popcorn is to corn. During the puffing process, a porous spongy texture can be formed by the reaction of both starch and moisture when heated within the shell of the grain. The typical method of puffing rice is “gun puffing”, where the rice is exposed to a suitable moisture level and pressurised to around 200 pressure per square inch (PSI). When the pressure is suddenly released, the pressure stored inside the kernel causes it to puff out. This method produces puffed rice which is spongy in texture. After this high temperature carbonization process, a novel carbonaceous aerogel with hierarchically macroporous, mesoporous and microporous structures was obtained. The air-expansion method used here thoroughly solved the high cost, complex gelatinization and drying processes, and shrinkage problems of the traditional methods for forming carbon aerogels, such as the sol–gel method,²⁵ forming graphene based aerogels using a hydrothermal method or a freeze-drying method,²⁶ and the single-step carbonization of biomass materials method.²⁷ Most importantly, the air-expansion method generates a porous structure from natural non-porous biomass materials and exploits the scope of selecting biomass materials for the preparation of carbonaceous aerogels. The obtained porous carbonaceous aerogel exhibits an excellent electrochemical performance when used as anode materials for lithium ion batteries due to its extraordinary porous architecture and N-doped structure. This method not only opens a new way to prepare carbonaceous aerogels from grain, but also finds a potential application as anode materials for lithium ion batteries.

2. Experimental

2.1 Preparation of hierarchically porous carbonaceous aerogels

The puffed rice used in this research was prepared by “gun puffing”, where the rice was sealed into a pressure-tight metallic container and exposed to a suitable moisture level and pressurised to around 200 pressure per square inch (PSI). When the pressure was suddenly released, the pressure stored inside the kernel caused it to puff out. This method produced puffed rice which is spongy in texture. Then the sample was put into a tubular furnace under flowing argon for a pyrolysis process at different temperatures of 500, 600 and 700 °C for 1 h with a heating rate of 5 °C min⁻¹. The obtained carbon was then ground into powders and refluxed with 2 M HCl solution at 120 °C for 1 h to remove the remaining impurities. The prepared product was thoroughly rinsed with DI water to ensure it was neutral and then dried at 80 °C overnight in a vacuum oven. The obtained carbonaceous aerogels at 500, 600 and 700 °C were named as PFC-500, PFC-600 and PFC-700, respectively.

2.2 Characterization

The morphology of all samples was characterized using a scanning electron microscope (SEM, JEOL 7500F) and transmission electron microscope (TEM, JEOL JEM-2100F). X-ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCALab220i-XL electron spectrometer from VG Scientific. Al-Kα radiation was used as the X-ray source and operated at 300 W. Thermogravimetric analysis (TGA) was performed using a STA409PC (NETZSCH) from 50 to 800 °C in air. N₂ (at 77.3 K) and CO₂ (at 273 K) sorption measurements were performed using a QUADRASORB SI/MP and an Autosorb-1MP from Quantachrome Instruments, respectively. Samples were outgassed at 200 °C under vacuum for 24 h before the measurements. Brunauer–Emmett–Teller (BET) and nonlinear density functional theory (NLDFT) methods were used for measuring the surface area and pore size distributions (PSDs) using the N₂ adsorption data, and the Grand canonical Monte Carlo (GCMC) method was used for CO₂ adsorption data analysis. The crystalline structure was characterized using X-ray diffraction (XRD) on a Micscience M-18XHF (with Cu-Kα radiation) instrument. Raman spectra were taken using a confocal HR800 spectrometer of HORIBA Jobin Yvon.

2.3 Electrochemical measurements

The carbon electrodes for electrochemical lithium insertion were prepared by blade-coating a slurry of 80% PFC, 10% sodium alginate (SA) and 10% super P dispersed in a defined amount of DI water onto copper foil, followed by drying at 70 °C for 6 h and punching into circular discs for coin-cell fabrication. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (in a volume ration of 1 : 1 : 1). To test the electrochemical properties, 2032 type coin cells were assembled in an argon-filled glove box using lithium metal for the counter electrodes. The cells were electrochemically cycled between 0.01 V and 3 V (at 0.1 C under a constant current mode for both charge and discharge in the first cycle and cycled at different current rates thereafter using a cycle tester). The galvanostatic charge–discharge test was conducted using a Wu-Han land CT2001A testing system. The electrochemical impedance spectra (EIS) were measured using a Solatron 1260 Impedance Analyzer.

3. Results and discussion

As exhibited in Fig. 1a, a puffing and carbonization strategy was designed to prepare a hierarchically porous carbonaceous aerogel from rice. Firstly, the rice swelled to form popcorn through puffing treatment, which developed many macroporous structures as the skeleton of the carbonaceous aerogel. After carbonizing under an argon atmosphere, lightweight carbonaceous aerogels were obtained with only about ten percent of the weight of the popcorn but they maintained their original shape and had a larger volume containing plenty of microporous and mesoporous structures. Forming a carbonaceous aerogel using this air-expansion method greatly reduced the shrinkage of the aerogel in comparison to the method of direct carbonization of biomass materials. Since rice is the main grain in the world with a great yield, this method opens a new way to prepare a carbonaceous aerogel with a hierarchically porous structure for potential application in energy storage.

Fig. 1 Schematic illustration of the air-expansion method for a hierarchically porous carbon aerogel from rice.

The SEM images of the puffed rice and carbonized aerogels are presented in Fig. 2. It can be seen that after the puffing treatment the rice shows a laminated structure with a smooth surface. However, Fig. 2b–d show the typical images of the puffed rice powders carbonized at different temperatures. There are many pores of different diameters, which can remarkably enhance the active surface area for lithium ion storage. For a relatively low temperature (500 °C, Fig. 2b), the carbonaceous products roughly retain a multilayer structure and as a result of the pyrolysis a number of macropores of different sizes formed on the carbon layer surface which is favorable for the intercalation of lithium ions. In contrast, the random layers from before are destroyed and melt together more or less but more visible pores are found in the samples carbonized at 600 °C and 700 °C. The transmission electron microscopy (TEM) and HRTEM images in Fig. 2e and f reveal that there are many macropores, mesopores and a large amount of micropores within PFC-600. Among them, the walls of the macropores and large mesopores consist of an amorphous texture and micropores (Fig. 2f), forming an interconnected structure. In the hierarchically porous structure, the mesopores of the carbonaceous aerogel can provide a short ion-transport pathway, with a minimized inner-pore resistance. The interconnected micro-, meso-, and macropores can provide low-resistant ion channels to facilitate ion transportation, which can effectively improve the capacity of a carbonaceous aerogel anode.

Fig. 2 SEM images of (a) original puffed rice, (b) PFC-500, (c) PFC-600 and (d) PFC-700, and (e and f) TEM image and high resolution image of PFC-600.

Fig. 3a shows the XRD patterns of the carbonaceous aerogels. The typical peaks of the three samples centered at 25° and 43° can be assigned to the (002) and (101) reflections of the planes of hexagonal graphite, respectively.²⁸ Moreover, no sharp peaks in the XRD patterns of the three carbon aerogels indicates they are in their amorphous state, which is beneficial for Li⁺ intercalation and deintercalation.²⁹ The Raman spectra in Fig. 3b demonstrate that the three samples show an obvious G band (∼1595 cm⁻¹) and D band (∼1380 cm⁻¹) which are ascribed to the vibration of sp²-bonded carbon atoms in a 2D hexagonal lattice and disordered carbon or defective graphitic structures, respectively. The I_D/I_G intensity ratio is a measure of the disorder degree of the obtained carbon aerogels. The I_D/I_G ratio of PFC-500 (0.79) is lower than that of HPC-600 (0.78) and HPC-700 (0.87), suggesting a higher degree of defects and a lower degree of graphitization at higher carbonization temperatures.

Fig. 3 XRD and Raman spectra of PFC-500, PFC-600 and PFC-700.

Fig. 4 shows the X-ray photoelectron spectra of the three carbonized samples, which were used to investigate the nature of the doped nitrogen species on the surface of a carbonaceous aerogel. The different states of N existing in the carbon matrix have been well-documented. The high-resolution N1s core level XPS spectra of all samples can be fitted to three different peaks, at 398.6, 400.1 and 400.9 eV, representing pyridinic N (N-6), pyrrolic or pyridonic N (N-5) and quaternary N (N-Q), respectively. It has been reported that N-5 and N-6 functional groups in N-doped porous carbons are the most important functional groups for the improvement of the energy storage performance. PFC-600 contains more pyridinic N than the samples PFC-500 and PFC-700 (Table S1†), which is more favorable than pyrrolic N for lithium ion storage to increase the reversible capacity.

Fig. 4 N1s XPS spectra for PFC-500, PFC-600 and PFC-700.

The effect of carbonization on the specific surface area and microstructure of carbonaceous aerogels was further studied using both N₂ (77.3 K) and CO₂ (273 K) sorption. As is shown in Fig. 5c, all the N₂ adsorption isotherms of the PFC samples did not close upon desorption in the measured pressure range, thus presenting extensive pressure hysteresis. It is probably owing to N₂ sorption at 77.3 K that a broad size range of micro- and mesopores can be analysed. However, in the case of our materials, there may be more ultramicropores (<0.8 nm), hence N₂ sorption was affected by kinetic restrictions and could not provide exact surface area data.³⁰ But even so, we can still obtain some mesopore information from the PSDs (Fig. 5d) calculated from the N₂ isotherms which presented mesopore size distributions in a range of about 10 nm. In contrast, the CO₂ sorption isotherms at 273 K in Fig. 5a show good adsorption kinetics without hysteresis compared to N₂ adsorption, due to the smaller kinetic diameter of CO₂ (D(CO₂) = 0.33 nm and D(N₂) = 0.36 nm) and higher thermal energy at a higher measurement temperature of 273 K.³¹ It can be seen that all CO₂ adsorption isotherms acquired at 273 K show a leap in gas uptake, correlated with the elimination of volatile species. PFC-700 shows a continuous increase in adsorption capacity in the whole pressure range compared to PFC-500 and PFC-600, and the surface areas increase with the rise in carbonization temperature since more volatile species form at higher temperatures for PFC-500 (384.31 m² g⁻¹), PFC-600 (389.19 m² g⁻¹) and PFC-700 (461.61 m² g⁻¹). Pore size distributions (PSDs, Fig. 5b) showed an ultramicropore range of 0.3 to 0.7 nm. From the SEM images and PSDs, it can be concluded that the porous carbonaceous aerogels possess hierarchically macro-, meso- and microporous structures.

Fig. 5 (a and c) CO₂ and N₂ sorption isotherms and (b and d) PSDs calculated from CO₂ and N₂ sorption isotherms, respectively.

The electrochemical performance of carbonaceous aerogel PFC-600 as an anode material for lithium ion batteries was characterized using cyclic voltammetry (CV) and charge–discharge cycling. The CV curves recorded in the first three cycles for PFC-600 (Fig. 6a) are typical profiles for carbonaceous anode materials with the shape matching well with the charge–discharge profiles.²⁸ Fig. 5b–d show the representative charge–discharge curves of PFC-500, PFC-600 and PFC-700 at a current density of 37 mA h g⁻¹, respectively. In the first cycle, the charge–discharge curves of PFC-600 exhibit high first-cycle discharge and charge capacities of 1180 and 750 mA h g⁻¹ and a coulombic efficiency 63.6% higher than that of PFC-500 and PFC-700 (1340/680 mA h g⁻¹ and 745/482 mA h g⁻¹, respectively) (Fig. 6b and c). The initial coulombic efficiency of the three electrodes is not high because of the large irreversible capacity during the first cycle for reasons such as the decomposition of electrolyte and some irreversible processes such as the formation of a solid-electrolyte interface layer on the surface of the electrodes.³² In the second cycle, the PFC-600 electrode shows a discharge capacity of 696 mA h g⁻¹ followed by a charge capacity of 648 mA h g⁻¹, accompanying an increased coulombic efficiency of about 93.1%. After 30 cycles, the reversible capacity of PFC-600 is maintained at 458 mA h g⁻¹ with a stabilized coulombic efficiency around 98.4%, while the other two samples show apparently reduced capacities of 373 mA h g⁻¹ and 342 mA h g⁻¹.

Fig. 6 (a) Cyclic voltammograms of PFC-700 at a scan rate of 0.1 mV s⁻¹; charge and discharge curves of (b) PFC-500, (c) PFC-600 and (d) PFC-700 at 0.1 C.

Fig. 7a shows the cycling performance of PFC-500, PFC-600 and PFC-700. Obviously, PFC-600 shows a better cycling stability than PFC-500 and PFC-700 at a constant current density of 0.1 C. It can be seen that PFC-600 still retains 505 mA h g⁻¹ after 110 cycles while the reversible capacity of PFC-700 and PFC-500 fades rapidly to lower than 400 mA h g⁻¹. In other reported research that uses biomass materials as a carbon source, their materials can maybe show a higher specific capacity than ours,^20–24 but almost all of them use KOH or other chemical agents as pore-agents, and if they carbonized their materials directly without KOH, a relatively lower specific capacity of about only 200 mA h g⁻¹ would be obtained. Consequently, our environmentally friendly air-expansion method can indeed induce pores into materials and improve their electrochemical properties without the use of any chemical reagents.

Fig. 7 (a) Cycling performance of PFC-500, PFC-600 and PFC-700; (b) the capacity performance of PFC-500, PFC-600 and PFC-700 at different cycling rates.

Of the reasons for the improved specific capacity through this air-expansion method, the first and most crucial one is the induced porous structure as presented in the SEM images and N₂/CO₂ adsorption isotherms, which is beneficial for Li⁺ storage. The other reason is heteroatom dopants such as H atoms and N atoms, since Li atoms can bind in the vicinity of the H atoms on the hydrogen-containing carbons and have a stronger interaction with N atoms because of the hybridization of nitrogen lone pair electrons with the π electrons. There is an optimum carbonization temperature since high temperatures can develop more pores in the structure and a high surface area, but meanwhile, more advantageous heteroatoms like H atoms and N atoms will be lost as the temperature rises (Table S1†). This is believed to be related to a trade off between increasing the surface area and the loss of useful heteroatoms. As a result, PFC-600 balanced the two factors, and showed the best electrochemical properties.

Fig. 7b shows the rate capabilities of three samples at various current densities from 0.1 C to 10 C, each for 10 cycles. It is clear that all examples show an excellent cyclic capacity retention at each current density. Especially for PFC-600, when the charge–discharge current density increases from 0.1 C to 10 C and then returns back to 0.1 C, the reversible capacities are still as high as 534 mA h g⁻¹. The carbonaceous aerogels present a good cycling and rate performance which is mainly attributed to their hierarchically macroporous, mesoporous and microporous structures, which can provide a good channel for Li⁺ intercalation and deintercalation.

The Nyquist diagrams in Fig. 8 show the impedance variation of three carbonaceous aerogel examples. All three electrodes show similar plots consisting of a depressed semicircle in the high-medium frequency regions and a straight line in the low-frequency region, corresponding to the SEI film resistance (or contact resistance) and the charge transfer resistance on the interface of the electrode and the electrolyte (R_SEI), and the semi-infinite diffusion of the lithium ions in the carbonaceous aerogel electrodes (R_e), respectively.^33–35 The value of R_SEI is the diameter of the semicircle in the real part, which is the sum of the resistance of the electrochemical cell system and one of the limiting factors for the power density of electrodes. It is obvious that the diameter of the semicircle of PFC-700 is smaller than that of the PFC-500 and PFC-600 electrodes, implying that it possesses the highest electrical conductivity and the easiest charge transfer reaction for lithium ion insertion and extraction of all the three samples. The equivalent circuit has been set up as shown in the inset of Fig. 8.

Fig. 8 Typical Nyquist plots of PFC-500, PFC-600 and PFC-700.

4. Conclusions

In summary, a facile and economic air-expansion method was employed to prepare a hierarchically porous nitrogen-rich carbonaceous aerogel using rice as a carbon source. The obtained porous carbonaceous aerogel retains the pristine structure of popcorn and displays a large specific surface area due to its hierarchically macroporous, mesoporous and microporous structure, which exhibits a high specific capacity and ultra-high rate capability. This approach could become an effective way to prepare carbonaceous aerogels from other grains, such us corn, soybean and wheat using the air-expansion method, also making such porous carbonaceous aerogels promising anode materials for application in lithium ion batteries or supercapacitors.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC) (Grant No. 21304059), National High-tech R&D Program (863) (2015AA03A204), Guangdong Doctor Startup fund (S2013040015706) and Shenzhen Science and Technology Research Grant (JCYJ20140418091413553, JCYJ20150625102750478, JCYJ20150529164656097).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22435g

‡ These authors contributed equally.

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