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Hierarchical porous carbon microspheres derived from porous starch for use in high-rate electrochemical double-layer capacitors
Bioresource Technology 139 (2013) 406–409
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Hierarchical porous carbon microspheres derived from porous starch for use in high-rate electrochemical double-layer capacitors Si-hong Du, Li-qun Wang, Xiao-ting Fu, Ming-ming Chen ⇑, Cheng-yang Wang Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Biomass waste porous starch was
utilized to fabricate hierarchical porous carbon. The nanocarbon contains inherent macropores and KOH-creating mesoand micropores. The capacitance could be maintained at 197 F g1 even at 180 A g1.
a r t i c l e
i n f o
Article history: Received 10 February 2013 Received in revised form 21 April 2013 Accepted 23 April 2013 Available online 30 April 2013 Keywords: Porous starch Hierarchical porous carbon microspheres Electric double layer capacitors High rate
a b s t r a c t Porous starch was used as a precursor for hierarchical porous carbon microspheres. The preparation consisted of stabilisation, carbonisation and KOH activation, and the resultant hierarchical porous carbon microspheres had a large BET surface area of 3251 m2 g1. Due to the large surface area and the hierarchical pore structure, electrodes made of the hierarchical porous carbon microsphere materials had high specific capacitances of 304 F g1 at a current density of 0.05 A g1 and 197 F g1 at a current density of 180 A g1 when used in a symmetric capacitor with 6 M KOH as the electrolyte. After 10,000 cycles, the capacitor still exhibited a stable performance with a capacitance retention of 98%. These results indicate that porous starch is an excellent precursor to prepare high performance electrode materials for EDLCs. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction An electric double-layer capacitor, which is sometimes termed a supercapacitor and which relies on the accumulation of charges at the electrodes via purely electrostatic forces (Frackowiak and Beguin, 2001), is a fast energy storage device compared with lithium ion batteries (Choi et al., 2012), which involve Li+/Li electrochemical reactions. Hence, unlike batteries, an electric double-layer capacitor has a long charge-cycling life of over several million cycles, a high specific capacitance and a high rate of energy ⇑ Corresponding author. Tel./fax: +86 22 27890481. E-mail address: [email protected] (M.-m. Chen). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.04.085
delivery, which are characteristics that scientists place many emphasis on (Kötz and Carlen, 2000; Sherrill et al., 2011). Currently, the final electrochemical performance of the available electric double-layer capacitors depends on the nanostructure of the carbonaceous electrode material. Ordered mesoporous carbon (Korenblit et al., 2010) and hierarchical porous carbon (Wang et al., 2008) have become hot topics in this research field because these structures enable fast ion diffusion, assuming a high-rate performance, and provide abundant micropores, leading to a high specific capacitance. In recent years, biomaterials have been shown to be advantageous for fabricating the above-mentioned structures (White et al., 2009). In 2007, Kijima and Kobayashi developed a new way
S.-h. Du et al. / Bioresource Technology 139 (2013) 406–409
to control pore distribution but not pore size, which was highlighted in chemical science of RCS publishing. Recently, his group used the biomaterial alkaline lignin to prepare micropore-based highly porous carbons, which were considered to be commendably applied to EDLC applications (Kijima et al., 2011). In addition, cassava peel waste (Ismanto et al., 2010), Argania spinosa seed shells (Elmouwahidi et al., 2012), bamboo (Kim et al., 2006), waste coffee beans (Rufford et al., 2008) and sunflower seed shells (Li et al., 2011) have all been used as precursors for carbon materials. Although these materials were prepared using completely different precursors and were activated by different chemical agents (KOH for cassava peel waste, Argania spinosa seed shells and sunflower seed shells, H2O for bamboo, ZnCl2 for waste coffee beans), all of the activated carbons are microporous; this property holds true for all plant-derived carbon. And these types of carbons can be termed group 1. Active carbons in group 1 generally have large micropore surface areas, over 1000 m2 g1, consequently giving excellent specific capacitances of approximately 150–300 F g1 at current densities less than 100 mA g1 in an aqueous electrolyte. Due to the dominance of their microporous characteristic, the capacitance retention of these carbons decreases quickly as the current density increases. Additionally, the use of non-aqueous electrolytes greatly reduces the specific capacitance of group 1 materials. For example, Kim et al. (2006) reported that in 1 M Et4NBF4/PC solution, a specific capacitance of only 60 F g1 was achieved at a scan rate of 1 mV s1. For this group of carbon materials, Li et al. (2011) reported that if the precursors are activated directly without pre-carbonisation specific capacitances of 244 F g1 at a current density of 0.25 A g1 and of 171 F g1 at a current density of 10 A g1, which is 70% of the former capacitance, can be achieved. Another group of biomaterial carbons consists of hierarchical structures that contain macro-, meso- and micropores (Chen et al., 2010; Huang et al., 2011). In this group, the nanostructure is hierarchical, i.e. is not micropore-dominated, and the capacitance retention of the carbon electrode is quite good (a specific capacitance of 130 F g1 was maintained even at a high current density of 40 A g1 in the work of Chen et al. (2010), and a capacitance retention of 70% was obtained by Huang et al. (2011) when the current density was increased from 0.05 to 100 A g1). These results indicate that a carbonaceous hierarchical electrode in an electric double-layer capacitor could produce a high rate of energy delivery. Porous starch is a type of bio-product that has macropores and a fixed carbon percentage of approximately 41%, which is higher than those of the other biomasses summarised by Ismanto et al. (2010). In this paper, the potential of utilizing porous starch biomass in the field of energy storage is discussed. First, a hierarchical structure based on a porous starch was developed by combining (1) the macropores inherited from the precursor with (2) the micro- and mesopores created by KOH activation. The importance of the macropores and the primary shape of the microspheres are discussed as well as the relationship between the hierarchical structure and the energy storage performance of the EDLC fabricated from the porous carbon microspheres.
2. Experiment 2.1. Sample preparation Hierarchical porous carbon microspheres were prepared from porous starch. A commercial porous starch powder (corn starch, 200 mesh, Lida Biotechnology, Liaoning, China) was impregnated with a 10 wt.% (NH4)2HPO4 aqueous solution for 1 h, dried at 40 °C and then stabilised at 210 °C for 3 h in a nitrogen atmosphere. After carbonisation at 600 °C for 2 h and KOH activation
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(KOH/starch mass ratio = 4:1) at 800 °C for 2 h, nanoporous carbon microspheres were obtained. 2.2. Characterisation The morphology of the porous carbon material was observed using a scanning electron microscope (SEM, Nano SEM 430, FEI, America). N2 adsorption–desorption isotherms were measured using a Micromeritics ASAP 2020 instrument at 77 K. The specific surface area (SBET) was calculated from the nitrogen adsorption isotherms using the Brunauer–Emmett–Teller (BET) method, and the micropore surface area (Smicro) was determined using the t-plot method. The pore size distribution was studied using original density functional theory (DFT).The macropores were measured using the AutoPore IV 9510 mercury porosimetry analyser from the Micrometrics Company. 2.3. Electrode preparation and electrochemical measurements The electrochemical performances were measured using symmetric two-electrode cells in a 6 M KOH aqueous electrolyte. The electrode was fabricated by pressing a mixture of the obtained porous carbon sample (80 wt.%), black carbon (10 wt.%) and polytetrafluoroethylene (10 wt.%) onto a nickel foam current collector. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed using a Princeton PARSTAT2273 electrochemical workstation. Different scan rates, between 50 and 300 mV s1, were used in the CV tests with the voltage varying from 0 to 1.0 V. Nyquist plots were evaluated at frequencies from 100 kHz to 1 mHz. Galvanostatic charge–discharge cycles were conducted over the range from 0.05 to 180 A g1 using an Arbin battery test instrument. The gravimetric capacitance C of the symmetric capacitor was calculated from the charge–discharge curves using the formula C ¼ IDt=mDV, where I is the set discharge current, Dt is the discharge time, m is the total mass of the active materials in the symmetric capacitor and DV is the voltage drop upon discharge (excluding the IR drop). The single electrode capacitance Cg is one quarter of the gravimetric capacitance of the symmetric capacitor. 3. Results and discussion The SEM micrographs of the raw material and of the hierarchical porous carbon microspheres are shown in Fig. S1. These show that the granular shape of the precursor was well preserved during the preparation of the microspheres, although there was some shrinkage in the particle sizes (Fig. S1(a) and (c)). In addition, the holes with diameters of 1–2 lm in the precursor were maintained in the hierarchical porous carbon microspheres (Fig. S1(b) and (d)). These macropores are not only on the surface but also go deep inside the granule in a ladder-like manner (Fig. S1(e)). It is suggested that the granule shape can be maintained with the aid of (NH4)2HPO4, although the starch-based material has to undergo a harsh heat treatment of up to 600 °C afterwards. The KOH activation of the carbon microspheres caused a new porous structure to appear on the surface of the ladder-like channel (Fig. S1(f)). The pores created by KOH along with the macropores inherited from the precursor give the porous carbon a 3D hierarchical structure. To further investigate the structure of the pores in the prepared hierarchical porous carbon microspheres, N2 adsorption–desorption and mercury porosimetry measurements were conducted, and the results are shown in Fig. S2. According to IUPAC, the isotherm of the hierarchical porous carbon microspheres (Fig. S2(a)) can be classified as a mixture of type I and IV, indicating the existence of both micropores and macropores. In addition, when the
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relative pressure (p/p0) increased from 0 to 0.4, the amount of N2 adsorption increased smoothly, indicating the presence of mesopores (Liu et al., 2009). The SBET and Smicro are 3251 and 826 m2 g1, respectively. The pore size distributions are shown in Fig. S2(b) and (c). There are four main regions, including (1) the ultrafine 0.4– 0.7 nm micropore region and (2) the 0.9–1.3 nm micropore region. According to Largeot et al. (2008), when the pore size of the electrode materials is close to the size of ion in the electrolyte, a maximum capacitance will be obtained. Taking into account the fact that the solvated ion size of K+ is between 0.36 nm and 0.42 nm (Eliad et al., 2001), it can be predicted that the pores of these first two regions will contribute the most to the formation of the electric double-layer, leading to a high capacitance. The other two regions are (3) the 1.3–3.4 nm micro/mesopore region, which plays a role in electrolyte ion diffusion paths, and (4) the >50 nm macropore region, which serves as ion-buffering reservoirs and thus decrease the ion transport distance during electrochemical processes. Most of the macropores were inherited from the precursor. The 4– 14 lm pores in the precursor porous starch contribute more to the material’s total volume than they do in the hierarchical porous carbon microspheres. The predominate pores in the hierarchical porous carbon microspheres are the 0.7–4 lm ones, most likely due to shrinkage during carbonisation. Additionally, the 50–95 nm macropores, shown in Fig. S2(b), contribute to the slight rise in the mercury injection, as shown in the left part of the curves in Fig. S2(c).
From the SEM, N2 adsorption–desorption isotherms and mercury porosimetry measurements, it can be concluded that these hierarchical porous carbon microspheres should have improved ion movement and better charge storage during electrochemical processes. This implies they also should have excellent performance and a high capacitance when is used as an electrode material. The electrochemical performance of the prepared hierarchical porous carbon microspheres as a supercapacitor electrode material was evaluated using EIS, CV and galvanostatic charge–discharge measurements and the results are shown in Fig. 1. EIS spectra (Fig. 1c) can be used to estimate the ionic resistance (Rionic) of the porous carbon microspheres (Iwama et al., 2012). For the 1st and 10,000th charge–discharge cycles at a current density of 40 A g1, the Rionic values are almost the same, approximately 0.05 X. This low Rionic demonstrates the high diffusion rate of the ions, which is beneficial to the rate capacity of the supercapacitor. Fast ion diffusion was also demonstrated by the CV tests. All of the curves are rectangular shaped at scan rates of 50, 100, 200 and 300 mV s1 (Fig. 1a). Even at a high scan rate of 300 mV s1, the hierarchical porous structure still facilitates an easy and smooth movement of the electrolyte ions, presenting no obvious polarisation (distortion from the standard rectangle) in the CV curve. The equivalent series resistance (ESR) in the EIS spectra changes slightly from 0.17 X for the 1st cycle to 0.2 X for the 10,000th charge–discharge cycle at a current density of 40 A g1. Such small
Fig. 1. The electrochemical performances of a symmetric supercapacitor with a hierarchical porous carbon microsphere electrode in 6 M KOH electrolyte: (a) the CV curves at scan rates of 50, 100, 200 and 300 mV s1, (b) the charge–discharge curves at current densities of 20, 40, 60, 100, 140 and 180 A g1, (c) the Nyquist plots for the hierarchical porous carbon microsphere electrode, (d) the specific capacitance at different discharge rates, (e) the cyclic life test based on galvanostatic charge–discharge at 40 A g1.
S.-h. Du et al. / Bioresource Technology 139 (2013) 406–409
ESR values ensure that all of the charge–discharge curves (Fig. 1b) in the current range of 20–180 A g1 exhibit an isosceles triangle shape with a small IR drop at the beginning of the discharge cycle. The slight variation in the ESR values corresponds to a discharge capacitance retention of 98% (Fig. 1d inset) after 10,000 cycles. The Nyquist plot shows a vertical line in the low frequency region, demonstrating a short Warburg diffusion region and the ideal capacitive behaviour of the hierarchical porous carbon microspheres. Fig. 1d shows the specific capacitance obtained at different discharge rates. A high specific capacitance of 304 F g1 is obtained at a current density of 0.05 A g1. In the case of a current density less than 20 A g1, the capacitance rapidly decreases with increasing current density, whereas the capacitance only decrease a little when the current density rises from 20 to 180 A g1. Even at a current density of 180 A g1, the supercapacitor still retains a specific capacitance of 197 F g1, displaying an excellent rate capacity for the hierarchical porous carbon microspheres. Based on this result, it can be concluded that most of the micropores for ion storage lie on the shallow surfaces of the microspheres and only a few micropores exist deep in the material. This configuration would lead to the satisfactory capacitance retention of 65% when the current density is increased 3600 times (from 0.05 to 180 A g1). Additionally, after 10,000 cycles of the charge–discharge, the electrode composed of the hierarchical porous carbon microspheres is still stable (Fig. 1e). This excellent electrochemical performance of the carbon microsphere can be attributed to their hierarchical porous structure, which includes the inherited, hollow, ladder-like inner structure that is combined with the micropores lying on the shallow surface and with the inherited macropores and mesopores acting as connections between the micro- and macropores. Such a 3D framework structure significantly enhances the availability of the electrode surface to the electrolyte ions, even at a high charge– discharge rate of 180 A g1. 4. Conclusions High performance electrode materials for use in EDLCs were prepared from porous starch. Based on the inherent macropores in the precursor, mesopores and micropores were created using KOH activation to form a 3D pore network. It is found that the 3D pore network is beneficial for enhancing the capability and rate performance of the EDLCs. Therefore, porous starch is a very promising precursor for synthesising hierarchical porous carbon electrode materials for use in EDLCs. Additionally, KOH activation is an effective way to produce pores in biomaterials. Acknowledgments This work has been supported by the National Basic Research Program of China (No. 2012CB720302), the National Natural Science Foundation of China (general program, No. 51172160), the
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National High Technology Research and Development Program of China (No. 2011AA11A232), and the Natural Science Foundation of Tianjin City of China (Key program, No. 12JCZDJC27000). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2013. 04.085. References Chen, W., Zhang, H., Huang, Y., Wang, W., 2010. A fish scale based hierarchical lamellar porous carbon material obtained using a natural template for high performance electrochemical capacitors. J. Mater. Chem. 20, 4773–4775. Choi, N.S., Chen, Z., Freunberger, S.A., Ji, X., Sun, Y.K., Amine, K., Yushin, G., Nazar, L.F., Cho, J., Bruce, P.G., 2012. Challenges facing lithium batteries and electrical double-layer capacitors. Angew. Chem., Int. Ed. 51, 9994–10024. Eliad, L., Salitra, G., Soffer, A., Aurbach, D., 2001. Ion sieving effects in the electrical double layer of porous carbon electrodes: estimating effective ion size in electrolytic solutions. J. Phys. Chem. B 105, 6880–6887. Elmouwahidi, A., Zapata-Benabithe, Z., Carrasco-Marín, F., Moreno-Castilla, C., 2012. Activated carbons from KOH-activation of argan (Argania spinosa) seed shells as supercapacitor electrodes. Bioresour. Technol. 111, 185–190. Frackowiak, E., Beguin, F., 2001. Carbon materials for the electrochemical storage of energy in capacitors. Carbon 39, 937–950. Huang, W., Zhang, H., Huang, Y., Wang, W., Wei, S., 2011. Hierarchical porous carbon obtained from animal bone and evaluation in electric double-layer capacitors. Carbon 49, 838–843. Ismanto, A.E., Wang, S., Soetaredjo, F.E., Ismadji, S., 2010. Preparation of capacitor’s electrode from cassava peel waste. Bioresour. Technol. 101, 3534–3540. Iwama, E., Taberna, P.L., Azais, P., Brégeon, L., Simon, P., 2012. Characterization of commercial supercapacitors for low temperature applications. J. Power Sources 219, 235–239. Kötz, R., Carlen, M., 2000. Principles and applications of electrochemical capacitors. Electrochim. Acta 45, 2483–2498. Kijima, M., Hirukawa, T., Hanawa, F., Hata, T., 2011. Thermal conversion of alkaline lignin and its structured derivatives to porous carbonized materials. Bioresour. Technol. 102, 6279–6285. Kim, C., Lee, J.W., Kim, J.H., Yang, K.S., 2006. Feasibility of bamboo-based activated carbons for an electrochemical supercapacitor electrode. Korean J. Chem. Eng. 23, 592–594. Korenblit, Y., Rose, M., Kockrick, E., Borchardt, L., Kvit, A., Kaskel, S., Yushin, G., 2010. High-rate electrochemical capacitors based on ordered mesoporous silicon carbide-derived carbon. ACS Nano 4, 1337–1344. Largeot, C., Portet, C., Chmiola, J., Taberna, P.L., Gogotsi, Y., Simon, P., 2008. Relation between the ion size and pore size for an electric double-layer capacitor. J. Am. Chem. Soc. 130, 2730–2731. Li, X., Xing, W., Zhuo, S., Zhou, J., Li, F., Qiao, S.Z., Lu, G.Q., 2011. Preparation of capacitor’s electrode from sunflower seed shell. Bioresour. Technol. 102, 1118– 1123. Liu, Y., Li, K., Wang, J., Sun, G., Sun, C., 2009. Preparation of spherical activated carbon with hierarchical porous texture. J. Mater. Sci. 44, 4750–4753. Rufford, T.E., Hulicova-Jurcakova, D., Zhu, Z., Lu, G.Q., 2008. Nanoporous carbon electrode from waste coffee beans for high performance supercapacitors. Electrochem. Commun. 10, 1594–1597. Sherrill, S.A., Rubloff, G.W., Lee, S.B., 2011. High to ultra-high power electrical energy storage. Phys. Chem. Chem. Phys. 13, 20714–20723. Wang, D.-W., Li, F., Liu, M., Lu, G.Q., Cheng, H.-M., 2008. 3D aperiodic hierarchical porous graphitic carbon material for high-rate electrochemical capacitive energy storage. Angew. Chem. 120, 379–382. White, R.J., Budarin, V., Luque, R., Clark, J.H., Macquarrie, D.J., 2009. Tuneable porous carbonaceous materials from renewable resources. Chem. Soc. Rev. 38, 3401– 3418.
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Hierarchical porous carbon microspheres derived from porous starch for use in high-rate electrochemical double-layer capacitors Si-hong Du, Li-qun Wang, Xiao-ting Fu, Ming-ming Chen ⇑, Cheng-yang Wang Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Biomass waste porous starch was
utilized to fabricate hierarchical porous carbon. The nanocarbon contains inherent macropores and KOH-creating mesoand micropores. The capacitance could be maintained at 197 F g1 even at 180 A g1.
a r t i c l e
i n f o
Article history: Received 10 February 2013 Received in revised form 21 April 2013 Accepted 23 April 2013 Available online 30 April 2013 Keywords: Porous starch Hierarchical porous carbon microspheres Electric double layer capacitors High rate
a b s t r a c t Porous starch was used as a precursor for hierarchical porous carbon microspheres. The preparation consisted of stabilisation, carbonisation and KOH activation, and the resultant hierarchical porous carbon microspheres had a large BET surface area of 3251 m2 g1. Due to the large surface area and the hierarchical pore structure, electrodes made of the hierarchical porous carbon microsphere materials had high specific capacitances of 304 F g1 at a current density of 0.05 A g1 and 197 F g1 at a current density of 180 A g1 when used in a symmetric capacitor with 6 M KOH as the electrolyte. After 10,000 cycles, the capacitor still exhibited a stable performance with a capacitance retention of 98%. These results indicate that porous starch is an excellent precursor to prepare high performance electrode materials for EDLCs. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction An electric double-layer capacitor, which is sometimes termed a supercapacitor and which relies on the accumulation of charges at the electrodes via purely electrostatic forces (Frackowiak and Beguin, 2001), is a fast energy storage device compared with lithium ion batteries (Choi et al., 2012), which involve Li+/Li electrochemical reactions. Hence, unlike batteries, an electric double-layer capacitor has a long charge-cycling life of over several million cycles, a high specific capacitance and a high rate of energy ⇑ Corresponding author. Tel./fax: +86 22 27890481. E-mail address: [email protected] (M.-m. Chen). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.04.085
delivery, which are characteristics that scientists place many emphasis on (Kötz and Carlen, 2000; Sherrill et al., 2011). Currently, the final electrochemical performance of the available electric double-layer capacitors depends on the nanostructure of the carbonaceous electrode material. Ordered mesoporous carbon (Korenblit et al., 2010) and hierarchical porous carbon (Wang et al., 2008) have become hot topics in this research field because these structures enable fast ion diffusion, assuming a high-rate performance, and provide abundant micropores, leading to a high specific capacitance. In recent years, biomaterials have been shown to be advantageous for fabricating the above-mentioned structures (White et al., 2009). In 2007, Kijima and Kobayashi developed a new way
S.-h. Du et al. / Bioresource Technology 139 (2013) 406–409
to control pore distribution but not pore size, which was highlighted in chemical science of RCS publishing. Recently, his group used the biomaterial alkaline lignin to prepare micropore-based highly porous carbons, which were considered to be commendably applied to EDLC applications (Kijima et al., 2011). In addition, cassava peel waste (Ismanto et al., 2010), Argania spinosa seed shells (Elmouwahidi et al., 2012), bamboo (Kim et al., 2006), waste coffee beans (Rufford et al., 2008) and sunflower seed shells (Li et al., 2011) have all been used as precursors for carbon materials. Although these materials were prepared using completely different precursors and were activated by different chemical agents (KOH for cassava peel waste, Argania spinosa seed shells and sunflower seed shells, H2O for bamboo, ZnCl2 for waste coffee beans), all of the activated carbons are microporous; this property holds true for all plant-derived carbon. And these types of carbons can be termed group 1. Active carbons in group 1 generally have large micropore surface areas, over 1000 m2 g1, consequently giving excellent specific capacitances of approximately 150–300 F g1 at current densities less than 100 mA g1 in an aqueous electrolyte. Due to the dominance of their microporous characteristic, the capacitance retention of these carbons decreases quickly as the current density increases. Additionally, the use of non-aqueous electrolytes greatly reduces the specific capacitance of group 1 materials. For example, Kim et al. (2006) reported that in 1 M Et4NBF4/PC solution, a specific capacitance of only 60 F g1 was achieved at a scan rate of 1 mV s1. For this group of carbon materials, Li et al. (2011) reported that if the precursors are activated directly without pre-carbonisation specific capacitances of 244 F g1 at a current density of 0.25 A g1 and of 171 F g1 at a current density of 10 A g1, which is 70% of the former capacitance, can be achieved. Another group of biomaterial carbons consists of hierarchical structures that contain macro-, meso- and micropores (Chen et al., 2010; Huang et al., 2011). In this group, the nanostructure is hierarchical, i.e. is not micropore-dominated, and the capacitance retention of the carbon electrode is quite good (a specific capacitance of 130 F g1 was maintained even at a high current density of 40 A g1 in the work of Chen et al. (2010), and a capacitance retention of 70% was obtained by Huang et al. (2011) when the current density was increased from 0.05 to 100 A g1). These results indicate that a carbonaceous hierarchical electrode in an electric double-layer capacitor could produce a high rate of energy delivery. Porous starch is a type of bio-product that has macropores and a fixed carbon percentage of approximately 41%, which is higher than those of the other biomasses summarised by Ismanto et al. (2010). In this paper, the potential of utilizing porous starch biomass in the field of energy storage is discussed. First, a hierarchical structure based on a porous starch was developed by combining (1) the macropores inherited from the precursor with (2) the micro- and mesopores created by KOH activation. The importance of the macropores and the primary shape of the microspheres are discussed as well as the relationship between the hierarchical structure and the energy storage performance of the EDLC fabricated from the porous carbon microspheres.
2. Experiment 2.1. Sample preparation Hierarchical porous carbon microspheres were prepared from porous starch. A commercial porous starch powder (corn starch, 200 mesh, Lida Biotechnology, Liaoning, China) was impregnated with a 10 wt.% (NH4)2HPO4 aqueous solution for 1 h, dried at 40 °C and then stabilised at 210 °C for 3 h in a nitrogen atmosphere. After carbonisation at 600 °C for 2 h and KOH activation
407
(KOH/starch mass ratio = 4:1) at 800 °C for 2 h, nanoporous carbon microspheres were obtained. 2.2. Characterisation The morphology of the porous carbon material was observed using a scanning electron microscope (SEM, Nano SEM 430, FEI, America). N2 adsorption–desorption isotherms were measured using a Micromeritics ASAP 2020 instrument at 77 K. The specific surface area (SBET) was calculated from the nitrogen adsorption isotherms using the Brunauer–Emmett–Teller (BET) method, and the micropore surface area (Smicro) was determined using the t-plot method. The pore size distribution was studied using original density functional theory (DFT).The macropores were measured using the AutoPore IV 9510 mercury porosimetry analyser from the Micrometrics Company. 2.3. Electrode preparation and electrochemical measurements The electrochemical performances were measured using symmetric two-electrode cells in a 6 M KOH aqueous electrolyte. The electrode was fabricated by pressing a mixture of the obtained porous carbon sample (80 wt.%), black carbon (10 wt.%) and polytetrafluoroethylene (10 wt.%) onto a nickel foam current collector. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed using a Princeton PARSTAT2273 electrochemical workstation. Different scan rates, between 50 and 300 mV s1, were used in the CV tests with the voltage varying from 0 to 1.0 V. Nyquist plots were evaluated at frequencies from 100 kHz to 1 mHz. Galvanostatic charge–discharge cycles were conducted over the range from 0.05 to 180 A g1 using an Arbin battery test instrument. The gravimetric capacitance C of the symmetric capacitor was calculated from the charge–discharge curves using the formula C ¼ IDt=mDV, where I is the set discharge current, Dt is the discharge time, m is the total mass of the active materials in the symmetric capacitor and DV is the voltage drop upon discharge (excluding the IR drop). The single electrode capacitance Cg is one quarter of the gravimetric capacitance of the symmetric capacitor. 3. Results and discussion The SEM micrographs of the raw material and of the hierarchical porous carbon microspheres are shown in Fig. S1. These show that the granular shape of the precursor was well preserved during the preparation of the microspheres, although there was some shrinkage in the particle sizes (Fig. S1(a) and (c)). In addition, the holes with diameters of 1–2 lm in the precursor were maintained in the hierarchical porous carbon microspheres (Fig. S1(b) and (d)). These macropores are not only on the surface but also go deep inside the granule in a ladder-like manner (Fig. S1(e)). It is suggested that the granule shape can be maintained with the aid of (NH4)2HPO4, although the starch-based material has to undergo a harsh heat treatment of up to 600 °C afterwards. The KOH activation of the carbon microspheres caused a new porous structure to appear on the surface of the ladder-like channel (Fig. S1(f)). The pores created by KOH along with the macropores inherited from the precursor give the porous carbon a 3D hierarchical structure. To further investigate the structure of the pores in the prepared hierarchical porous carbon microspheres, N2 adsorption–desorption and mercury porosimetry measurements were conducted, and the results are shown in Fig. S2. According to IUPAC, the isotherm of the hierarchical porous carbon microspheres (Fig. S2(a)) can be classified as a mixture of type I and IV, indicating the existence of both micropores and macropores. In addition, when the
408
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relative pressure (p/p0) increased from 0 to 0.4, the amount of N2 adsorption increased smoothly, indicating the presence of mesopores (Liu et al., 2009). The SBET and Smicro are 3251 and 826 m2 g1, respectively. The pore size distributions are shown in Fig. S2(b) and (c). There are four main regions, including (1) the ultrafine 0.4– 0.7 nm micropore region and (2) the 0.9–1.3 nm micropore region. According to Largeot et al. (2008), when the pore size of the electrode materials is close to the size of ion in the electrolyte, a maximum capacitance will be obtained. Taking into account the fact that the solvated ion size of K+ is between 0.36 nm and 0.42 nm (Eliad et al., 2001), it can be predicted that the pores of these first two regions will contribute the most to the formation of the electric double-layer, leading to a high capacitance. The other two regions are (3) the 1.3–3.4 nm micro/mesopore region, which plays a role in electrolyte ion diffusion paths, and (4) the >50 nm macropore region, which serves as ion-buffering reservoirs and thus decrease the ion transport distance during electrochemical processes. Most of the macropores were inherited from the precursor. The 4– 14 lm pores in the precursor porous starch contribute more to the material’s total volume than they do in the hierarchical porous carbon microspheres. The predominate pores in the hierarchical porous carbon microspheres are the 0.7–4 lm ones, most likely due to shrinkage during carbonisation. Additionally, the 50–95 nm macropores, shown in Fig. S2(b), contribute to the slight rise in the mercury injection, as shown in the left part of the curves in Fig. S2(c).
From the SEM, N2 adsorption–desorption isotherms and mercury porosimetry measurements, it can be concluded that these hierarchical porous carbon microspheres should have improved ion movement and better charge storage during electrochemical processes. This implies they also should have excellent performance and a high capacitance when is used as an electrode material. The electrochemical performance of the prepared hierarchical porous carbon microspheres as a supercapacitor electrode material was evaluated using EIS, CV and galvanostatic charge–discharge measurements and the results are shown in Fig. 1. EIS spectra (Fig. 1c) can be used to estimate the ionic resistance (Rionic) of the porous carbon microspheres (Iwama et al., 2012). For the 1st and 10,000th charge–discharge cycles at a current density of 40 A g1, the Rionic values are almost the same, approximately 0.05 X. This low Rionic demonstrates the high diffusion rate of the ions, which is beneficial to the rate capacity of the supercapacitor. Fast ion diffusion was also demonstrated by the CV tests. All of the curves are rectangular shaped at scan rates of 50, 100, 200 and 300 mV s1 (Fig. 1a). Even at a high scan rate of 300 mV s1, the hierarchical porous structure still facilitates an easy and smooth movement of the electrolyte ions, presenting no obvious polarisation (distortion from the standard rectangle) in the CV curve. The equivalent series resistance (ESR) in the EIS spectra changes slightly from 0.17 X for the 1st cycle to 0.2 X for the 10,000th charge–discharge cycle at a current density of 40 A g1. Such small
Fig. 1. The electrochemical performances of a symmetric supercapacitor with a hierarchical porous carbon microsphere electrode in 6 M KOH electrolyte: (a) the CV curves at scan rates of 50, 100, 200 and 300 mV s1, (b) the charge–discharge curves at current densities of 20, 40, 60, 100, 140 and 180 A g1, (c) the Nyquist plots for the hierarchical porous carbon microsphere electrode, (d) the specific capacitance at different discharge rates, (e) the cyclic life test based on galvanostatic charge–discharge at 40 A g1.
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ESR values ensure that all of the charge–discharge curves (Fig. 1b) in the current range of 20–180 A g1 exhibit an isosceles triangle shape with a small IR drop at the beginning of the discharge cycle. The slight variation in the ESR values corresponds to a discharge capacitance retention of 98% (Fig. 1d inset) after 10,000 cycles. The Nyquist plot shows a vertical line in the low frequency region, demonstrating a short Warburg diffusion region and the ideal capacitive behaviour of the hierarchical porous carbon microspheres. Fig. 1d shows the specific capacitance obtained at different discharge rates. A high specific capacitance of 304 F g1 is obtained at a current density of 0.05 A g1. In the case of a current density less than 20 A g1, the capacitance rapidly decreases with increasing current density, whereas the capacitance only decrease a little when the current density rises from 20 to 180 A g1. Even at a current density of 180 A g1, the supercapacitor still retains a specific capacitance of 197 F g1, displaying an excellent rate capacity for the hierarchical porous carbon microspheres. Based on this result, it can be concluded that most of the micropores for ion storage lie on the shallow surfaces of the microspheres and only a few micropores exist deep in the material. This configuration would lead to the satisfactory capacitance retention of 65% when the current density is increased 3600 times (from 0.05 to 180 A g1). Additionally, after 10,000 cycles of the charge–discharge, the electrode composed of the hierarchical porous carbon microspheres is still stable (Fig. 1e). This excellent electrochemical performance of the carbon microsphere can be attributed to their hierarchical porous structure, which includes the inherited, hollow, ladder-like inner structure that is combined with the micropores lying on the shallow surface and with the inherited macropores and mesopores acting as connections between the micro- and macropores. Such a 3D framework structure significantly enhances the availability of the electrode surface to the electrolyte ions, even at a high charge– discharge rate of 180 A g1. 4. Conclusions High performance electrode materials for use in EDLCs were prepared from porous starch. Based on the inherent macropores in the precursor, mesopores and micropores were created using KOH activation to form a 3D pore network. It is found that the 3D pore network is beneficial for enhancing the capability and rate performance of the EDLCs. Therefore, porous starch is a very promising precursor for synthesising hierarchical porous carbon electrode materials for use in EDLCs. Additionally, KOH activation is an effective way to produce pores in biomaterials. Acknowledgments This work has been supported by the National Basic Research Program of China (No. 2012CB720302), the National Natural Science Foundation of China (general program, No. 51172160), the
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