Bioresource Technology xxx (2014) xxx–xxx

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Short Communication

Humic acids-based hierarchical porous carbons as high-rate performance electrodes for symmetric supercapacitors Zhi-jun Qiao, Ming-ming Chen ⇑, Cheng-yang Wang, Yun-cai Yuan Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China Synergetic Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, PR China

h i g h l i g h t s  HAs are used to prepare HPCs via a simple KOH activation process.  Though the mass ratio of KOH/HAs is only 1.5, the obtained HPCs possess high SSA.  The yield of LHA-HPC reaches to 51 wt% which is expected to be commercialized.  HPCs are successfully applied as electrode materials in supercapacitors.

a r t i c l e

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Article history: Received 17 February 2014 Received in revised form 19 April 2014 Accepted 25 April 2014 Available online xxxx Keywords: Humic acids Hierarchical porous carbons Supercapacitor Activation

a b s t r a c t Two kinds of hierarchical porous carbons (HPCs) with specific surface areas of 2000 m2 g1 were synthesized using leonardite humic acids (LHA) or biotechnology humic acids (BHA) precursors via a KOH activation process. Humic acids have a high content of oxygen-containing groups which enabled them to dissolve in aqueous KOH and facilitated the homogeneous KOH activation. The LHA-based HPC is made up of abundant micro-, meso-, and macropores and in 6 M KOH it has a specific capacitance of 178 F g1 at 100 A g1 and its capacitance retention on going from 0.05 to 100 A g1 is 64%. In contrast, the BHA-based HPC exhibits a lower capacitance retention of 54% and a specific capacitance of 157 F g1 at 100 A g1 which is due to the excessive micropores in the BHA-HPC. Moreover, LHA-HPC is produced in a higher yield than BHA-HPC (51 vs. 17 wt%). Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Biomass materials (BM) such as porous starch (Du et al., 2013), banana peels (Lv et al., 2012), animal bone (Huang et al., 2011) and fish scales (Chen et al., 2010) have been used to prepare hierarchical porous carbons (HPCs) and these materials have been successfully applied as electrode materials in supercapacitors. These BM-HPC materials have a number of advantages such as environmentally friendly synthesis processes, and porous structures with interconnected micro-, meso- and macropores. The micropores (pore sizes less than 2 nm) provide large surface areas which ensure high capacitance, the mesopores (2–50 nm) facilitate the diffusion of the electrolyte ions into the available surface area and the macropores (>50 nm) act as ion-buffers to ensure adequate penetration of the electrolyte into the electrode materials. ⇑ Corresponding author at: Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China. Tel.: +86 22 27890481. E-mail address: [email protected] (M.-m. Chen).

The most common methods to synthesize HPCs are template methods which require the synthesis and the removal of the templates. Chen et al. (2010) reported that fish scale-based HPCs were achieved by a self-templating method and exhibited a specific capacitance of 130 F g1 at a current density of 40 A g1 in 7 M KOH solution. However, the fish scales only contained 41% organic matter (Ikoma et al., 2003) which resulted in a low HPC yield. Lv et al. (2012) synthesized banana peel-based HPCs using a self-templating and chemical activation method but banana peels have a low residual carbon content (31.6 wt%) and the HPC preparation method was complex. Thus, the commercial viability of HPCs is hindered by the current preparation methods and by starting materials with low organic or residual carbon contents. Humic acids (HAs) as a green, renewable, inexpensive and amphiphilic biomass material has more than 40 wt% carbon content (Stevenson, 1994). It consists of a skeleton of alkyl/aromatic units inter-molecularly or intra-molecularly cross-linked mainly by carboxylic acid, phenolic and alcoholic hydroxyls, ketone, and quinone groups. In previous work, an amphiphilic precursor could

http://dx.doi.org/10.1016/j.biortech.2014.04.095 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Qiao, Z.-j., et al. Humic acids-based hierarchical porous carbons as high-rate performance electrodes for symmetric supercapacitors. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.04.095

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Z.-j. Qiao et al. / Bioresource Technology xxx (2014) xxx–xxx

well-dispersed in aqueous KOH, which results in a nano-scale contact between the KOH and precursor, facilitating a homogeneous chemical (Guo et al., 2014). The aromatic structure of HAs could construct the carbon skeleton of HPC after activation, while the heteroatoms (including N, S, and O) provide more ‘‘active sites’’ for activation to facilitate the formation of hierarchical porous structure. Therefore, leonardite humic acids (LHA) and biotechnology humic acids (BHA) were respectively employed as the precursor to prepare HPCs via a mild KOH activation (KOH/HAs = 1.5 by weight). The porous structures and electrochemical performances of the two HPCs were characterized and each material was used as the electrode in a symmetric supercapacitor. 2. Experimental 2.1. Preparation of the raw materials BHA and natural LHA were provided by Beijing Zimingyuanyi Co., Ltd. Pure BHA and LHA were obtained by three steps: (1) The HA was dissolved in alkaline solution (pH = 11) and then filtrated; (2) the filtrate was acidified to pH = 3 with 0.1 M hydrochloric acid and then a HA gel was formed after 8 h; (3) the HA gel was repeatedly washed with distilled water until the filtrate reached a neutral pH value and then the gel was dried at 60 °C. 2.2. Preparation of the hierarchical porous carbons Potassium hydroxide and pure LHA (KOH/LHA = 1.5:1, mass ratio) were mixed and dissolved in water, and then dried in air at 80 °C for 24 h. The dried sample was activated in a tube furnace under a N2 atmosphere at 800 °C for 1 h with a heating/cooling rate of 3 °C min1. The obtained black solid was then thoroughly washed with 1 M HCl at 80 °C for 6 h. After that, the material was dried at 80 °C for 24 h. The HPC sample was labeled LHAHPC. The BHA-HPC sample was prepared using the same method except BHA was used in place of LHA. 2.3. Characterization Thermogravimetric analysis (TGA) was carried out on an NETZSCH TG 209 apparatus with a heating rate of 10 °C min1 under N2. XPS analysis was carried out on a Perkin Elmer PHI-1600 electron system (America PE Company) using a Mg K X-ray source at 250 W. The surface areas and the porous textures of the HPCs were examined using N2 sorption at 77 K (ASAP2020). The macropores were measured using the AutoPore IV 9510 mercury porosimetry analyzer from the Micrometrics Company. The morphology of products was observed using a Philips XL30 scanning electron microscopy (SEM). 2.4. Supercapacitors fabrication preparation and electrochemical measurements The symmetric capacitors were a combination of HPC/acetylene black/polytetrafluoroethylene (mass ratio of 8:1:1) electrodes (13 mm diameter and about 80 lm thick) with a polypropylene membrane as the separator and 6 M KOH aqueous solution as the electrolyte. They were constructed in a vacuum box. Galvanostatic charge/discharge analysis was carried out on an Arbin MSTAT instrument. The specific capacitance of the supercapacitor was calculated based on charger/discharger according to the formula: C g ¼ 2IDt=ðDV m Þ, where Cg is the gravimetric specific capacitance (F g1), I is the current (A), Dt is the discharge time (s), m is the mass (g) of the HPC in the signal electrode and DV is the potential change during the discharging process (excluding the IR drop).

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were carried out using a PARSTAT 2273 system.

3. Results and discussion First the LHA and BHA were analyzed to determine their suitability to prepare a BM-HPC material. LHA is a natural humic acids derived from leonardite, while BHA is a regenerated humic acids derived from biodegradation of straws. LHA has more aromatic and cyclic hydrocarbon than BHA, which is identified by the broad band at 1605 cm1 (aromatic C@C stretching) in FT-IR spectrum (Fig. S1). The bulk and surface compositions of the samples were determined by XPS and the results are listed in Table 1. Both samples have a high oxygen content, which implies the existence of many oxygen-containing groups. TG analysis of the two HA samples after carbonization at 1000 °C was also performed (Fig. S2) and there is a significant mass loss between 180 and 600 °C which is a typical temperature range for the thermal decomposition of oxygen-containing groups. The mass loss between 600 and 1000 °C can be attributed to decomposition of the polyaromatic structures. The residual carbon percentage in LHA was 60.1 wt%, indicating that this precursor is a suitable biomass material for the preparation of porous carbon. The residual carbon content of BHA was only 38.2 wt%. To further determine the HA surface carbon and oxygen functional components, the XPS carbon and oxygen peaks were deconvoluted and the results are shown in Fig. S3. Most of the oxygencontaining functional groups in the HAs are C(@O)O (carbonyls and carboxyls) and CAO bonds (phenols and ethers). These functional groups are effective at dispersing humic acids in aqueous alkaline solutions. When the two HAs were dispersed in aqueous KOH (with a mass ratio of KOH/HAs = 1.5), colloids were formed. The zeta potentials of the colloids were 59.1 mV for LHA and 62.6 mV for BHA, indicating that both colloids are stable. The stable colloidal solutions visibly elucidate that HAs presented nanometer in aqueous KOH and the nano-scale contact of KOH and HAs facilitate homogeneously KOH activation. After KOH activation, the structural parameters of the HAHPC samples were determined and the results are summarized in Table 2. The surface functional groups of the HAs improved the efficiency of the KOH activation which increased the specific surface area (SSA). Both samples have similar SSA, around 2000 m2 g1, but the porous textures of the two HPCs are obviously difference. The Vtot, Vmic, Vmes, Vmac and porosity data for BHA-HPCs indicate that the sample possesses many micropores, whereas the data for LHA-HPCs indicates that this sample has higher porosity and abundant meso-, meso- and micropores. In addition, the yield of LHA-HPC was 51 wt% which is much higher than the yield for BHA-HPC (17 wt%). For comparison, the yields of several raw materials for porous carbon production are as follows: ginkgo shells (24 wt%) (Jiang et al., 2013), sugar cane bagasse (34.8 wt%) (Rufford et al., 2010), firwoods (20 wt%) (Wu et al., 2004), bamboo scaffolding waste (22.5 wt%) (Choy et al., 2005). The pore size and the macropore distributions are shown in Fig. S4(b) and (c) respectively. For LHA-HPC, the pore size distribution is broad and can be divided into three regions: (1) micropores (0.4–2 nm); (2) mesopores (2–50 nm) with a maximum peak at 10 nm; and (3) macropores (>50 nm) with three peaks at 63 nm, 2.1 lm and 33 lm. For BHA-HPC, the pore sizes are primarily centralized in the micropore region between 0.4–2 nm although there are a few exterior pores with mesopores (2–4 nm) and macropores (>1 lm). The SEM images in Fig. S5 show that LHA-HPC has numerous macro-, meso- and micropores, while BHA-HPC possesses excessive micropores and a few macropores. The different porous

Please cite this article in press as: Qiao, Z.-j., et al. Humic acids-based hierarchical porous carbons as high-rate performance electrodes for symmetric supercapacitors. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.04.095

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Z.-j. Qiao et al. / Bioresource Technology xxx (2014) xxx–xxx Table 1 Elemental bulk and surface compositions of BHA and LHA. Sample

BHA LHA a b

Ash (wt%)b

Elemental analysis (wt%) a

C

H

N

S

O

51.6 66.2

5.6 4.1

1.2 0.5

3.2 0.3

38.6 28.9

3.17 2.13

Surface atom analysis (%) C

O

N

S

71.0 78.4

25.7 19.8

1.5 1.4

1.8 0.4

Oxygen determined by difference. The ash were obtained after heating at 1000 °C for 2 h in air.

Table 2 Yields and structural parameters of the HPCs. Sample

Yield (wt%)

SBET (m2/g)

Smic (m2/g)

Vtot (cm3/g)

Vmes (cm3/g)

Vmic (cm3/g)

Smac (m2/g)

Vmac (mL/g)

Porosity (%)

BHA-HPC LHA-HPC

17 51

2060 2040

1503 1164

1.05 1.35

0.27 0.62

0.74 0.62

13.4 41.7

6.4 12.7

37.6 78.1

The specific surface areas were calculated by the Brunauer–Emmett–Teller method. The microporous suface areas (Smic) and micropore volumes (Vmic) were calculated using the t-plot method. The mesopore volumes (Vext) were calculated using the Barrett-Joyner-Halenda method. The total pore volumes (Vtot) were calculated from the amount of N2 adsorbed at a relative pressure (P/P0) of 0.99. The macropore area (Smac), macropore volumes (Vmac) and porosity were measured by mercury porosimetry analyzer.

structure of HPCs are determined by chemical structure and heteroatoms of precursors, because the carbon structure of HAs construct the carbon skeleton of HPC, while the heteroatoms provide ‘‘active sites’’ for activation. On the basis of these results, it can be concluded that not only was the LHA-HPC produced in a higher yield, but it also has a good hierarchical porous structure with abundant meso-, macro- and micropores. This indicates that LHA is a more appropriate precursor to produce HPCs and the porous texture of the resultant LHA-HPCs may be helpful for ion diffusion and storage.

The electrochemical performances of the HPCs as supercapacitor electrodes were evaluated by CV, EIS and galvanostatic charge/discharge measurements in 6 M KOH aqueous solution. The CV curves (Fig. 1a) of the symmetric capacitors exhibit relatively rectangular shapes without any redox peaks at scanning rates of 50 and 200 mV s1, proving that the sample electrodes are suitable for quick charge/discharge operations. Fig. 1b shows the galvanostatic charge/discharge curves of the supercapacitors (between 0 and 1 V) measured at current densities of 10, 20, 40 and 100 A g1. The charge/discharge curves are almost

Fig. 1. Electrochemical performance of the HPC electrodes: (a) CV curves at scanning rates of 50 and 200 mV s1; (b) charge/discharge curves at current densities of 10, 20, 40 and 100 A g1; (c) specific capacitance at different discharge rates; (d) cyclic life test based on galvanostatic charge–discharge at 100 A g1; (e) electrochemical impedance spectra measured at 100 A g1 and the equivalent circuit model of the porous carbons electrodes.

Please cite this article in press as: Qiao, Z.-j., et al. Humic acids-based hierarchical porous carbons as high-rate performance electrodes for symmetric supercapacitors. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.04.095

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symmetrical isosceles lines, demonstrating that both HPC materials have typical porous carbon supercapacitive behavior and stable electrochemical properties. The analyses of the electrochemical impedance spectra are shown in Fig. 1e, and the inset shows the equivalent circuit for the sample electrodes. According to the equivalent circuit, the whole capacitor circuit is made up of Rs, W, Rct, C and Q where, Rs is the equivalent series resistance which includes the contact resistance of the interface active material, the current collector, the electrolyte solution resistance and the intrinsic resistance of the electrode materials (Liu et al., 2007); Rct is the resistance for blocking the movement of ions from the solution entering the separator to the pores of the electrode material, which usually shows as a semicircle loop in the EIS spectrum (Toupin et al., 2005); W is the Warburg resistance which is a vertical line for an ideal capacitor (Farma et al., 2013); and C and Q are the capacitor layers formed in the charge/discharge processes respectively. At a current density of 100 A g1, BHA-HPC possesses larger Rs and Rct values than LHA-HPC, whether after one cycle or after 10,000 cycles. BHA-HPC exhibits typical features with a 45° region, where the length of the 45° segment is related to the resistance caused by the ion diffusion into the bulk of the electrode (Okajima et al., 2005). However for the LHA-HPC has no signal 45° line that indicate low ion diffusion resistance. The specific capacitances of the supercapacitors obtained at various charge/discharge rates are shown in Fig. 1c. The specific capacitances of the BHA-HPC and LHA-HPC supercapacitor electrodes reached 290 and 277 F g1 at a current density of 0.05 A g1, respectively. The higher specific capacitance for the BHA-HPC electrode is due to the higher micropore surface area which enhances charge accumulation. This is because at low current densities the ions have enough time to diffuse into the surface micropores. At relatively high current densities, it is harder for the ions to penetrate into the internal micropores which lead to a drop in the specific capacitance. At 100 A g1, LHA-HPC electrode exhibited a higher specific capacitance of 178 F g1 compared to 157 F g1 for BHA-HPC. This is due to the abundance of mesoand macropores that favor the transport of ions through these pores to the surface micropores. The LHA-HPC electrode also exhibited a fast charge–discharge rate and an excellent capacitance retention of 64% on going from 0.05 to 100 A g1. After 10,000 cycles, both electrodes still showed a stable performance at 100 A g1 (Fig. 1d). Although the capacitance of LHA-HPC is not the highest value at 0.05 A g1 compared to those of other biomass based porous carbons summarized by Elmouwahidi et al. (2012), its specific capacitance could be kept at 178 F g1 even at a current density of 100 A g1 which is higher than the conventional amphiphilic carbonaceous material (ACM) based HPC (Guo et al., 2014). Compared to LHA-HPC electrode, the specific capacitances of commercial activated carbon YP17 (Kuraray Chemical Corporation) are 158 F g1 at 0.05 A g1 and only 109 F g1 at 10 A g1 (Huang et al., 2011). In summary, the above results demonstrate that the supercapacitor with the LHA-HPC electrode has excellent specific capacitance and good reversible electrochemical behavior in 6 M KOH.

with relatively low KOH ratios (mass ratio of alkali/HAs = 1.5). The symmetric supercapacitor cell with the LHA-HPC electrode had good specific capacitance and rate performance, indicating that LHA is an appropriate precursor for making high-performance hierarchical porous carbons-based supercapacitors. Acknowledgements This study was supported by the National Natural Science Foundation of China and the Natural Science Foundation of Tianjin, 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.2014.04. 095. 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. Choy, K.K.H., Barford, J.P., McKay, G., 2005. Production of activated carbon from bamboo scaffolding waste-process design, evaluation and sensitivity analysis. Chem. Eng. J. 109, 147–165. Du, S.H., Wang, L.Q., Fu, X.T., Chen, M.M., Wang, C.Y., 2013. Hierarchical porous carbon microspheres derived from porous starch for use in high-rate electrochemical double-layer capacitors. Bioresour. Technol. 139, 406–409. 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. Farma, R., Deraman, M., Awitdrus, A., Talib, I.A., Taer, E., Basri, N.H., Manjunatha, J.G., Ishak, M.M., Dollah, B.N.M., Hashmi, S.A., 2013. Preparation of highly porous binderless activated carbon electrodes from fibres of oil palm empty fruit bunches for application in supercapacitors. Bioresour. Technol. 132, 254–261. Guo, Y., Shi, Z.Q., Chen, M.M., Wang, C.Y., 2014. Hierarchical porous carbon derived from sulfonated pitch for electrical double layer capacitors. J. Power Sources 252, 235–243. Huang, W.T., Zhang, H., Huang, Y.Q., Wang, W.K., Wei, S.C., 2011. Hierarchical porous carbon obtained from animal bone and evaluation in electric doublelayer capacitors. Carbon 49, 838–843. Ikoma, T., Kobayashi, H., Tanaka, J., Walsh, D., Mann, S., 2003. Microstructure, mechanical, and biomimetic properties of fish scales from Pagrus major. J. Struct. Biol. 142, 327–333. Jiang, L., Yan, J.W., Hao, L.X., Xue, R., Sun, G.Q., Yi, B.L., 2013. High rate performance activated carbons prepared from ginkgo shells for electrochemical supercapacitors. Carbon 56, 146–154. Liu, C.L., Dong, W.S., Cao, G.P., Song, J.R., Liu, L., Yang, Y.S., 2007. Influence of KOH followed by oxidation pretreatment on the electrochemical performance of phenolic based activated carbon fibers. J. Electroanal. Chem. 611, 225–231. Lv, Y.K., Gan, L.H., Liu, M.X., Xiong, W., Xu, Z.J., Zhu, D.Z., Wright, D.S., 2012. A selftemplate synthesis of hierarchical porous carbon foams based on banana peel for supercapacitor electrodes. J. Power Sources 209, 152–157. Okajima, K., Ohta, K., Sudoh, M., 2005. Capacitance behavior of activated carbon fibers with oxygen-plasma treatment. Electrochim. Acta 50 (11), 2227–2231. Rufford, T.E., Jurcakova, D.H., Khosla, K., Zhu, Z.H., Lu, G.Q., 2010. Microstructure and electrochemical double-layer capacitance of carbon electrodes prepared by zinc chloride activation of sugar cane bagasse. J. Power Sources 195, 912–918. Stevenson, F.J., 1994. Humus Chemistry: Genesis, Composition, Reactions, second ed. John Wiley & Sons Ltd., New York. Toupin, M., Daniel, B., Hill, I.R., Quinn, D., 2005. Performance of experimental carbon blacks in aqueous supercapacitors. J. Power Sources 140, 203–210. Wu, F.C., Tseng, R.L., Hu, C.C., Wang, C.C., 2004. Physical and electrochemical characterization of activated carbons prepared from firwoods for supercapacitors. J. Power Sources 138, 351–359.

4. Conclusions Two kinds of humic acids-based hierarchical porous carbons with different porous networks were obtained via KOH activation

Please cite this article in press as: Qiao, Z.-j., et al. Humic acids-based hierarchical porous carbons as high-rate performance electrodes for symmetric supercapacitors. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.04.095