Synthesis of activated carbon derived from rice husks for improving hydrogen storage capacity

Journal of Industrial and Engineering Chemistry 31 (2015) 330–334

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Synthesis of activated carbon derived from rice husks for improving hydrogen storage capacity Young-Jung Heo, Soo-Jin Park * Department of Chemistry, Inha University, 100 Inharo, Incheon 402-751, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history: Received 4 December 2014 Received in revised form 2 July 2015 Accepted 6 July 2015 Available online 14 July 2015

In this work, we prepared activated carbon derived from rice husks (RHC) using chemical activation with KOH ratio. The results showed that significant increase in specific surface area and optimum pore size for hydrogen storage of RHC by KOH activation. Then, it was interesting note to that the best hydrogen storage capacity of 2.85 wt.% was observed in the RHC1 sample (KOH ratio of 1). The hydrogen storage capacity was strongly influenced by the nanometered size distribution and micropore volume than the specific surface area or total pore volume. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

Keywords: Hydrogen storage capacity Microporous carbons KOH activation Rice husks

Introduction Recently, hydrogen has emerged as one of the most promising alternative energy source to replace fossil hydrocarbon resources, due to its high abundance, high combustion heat, non-pollutant emission while combustion. Many studies have been performed on the effective and safe use for hydrogen energy. Research has been carried out targeting 5.5 wt.% (revised in 2009) of a hydrogen storage system to use hydrogen as transportation energy as suggested by the U.S. Department of Energy, in order to utilize hydrogen storage on certain materials. Many methods for hydrogen storing have been suggested such as liquid, solid-state storage, metal hydride and microporous materials. Among others, microporous materials are attractive hydrogen energy carrier, due to its high specific surface area, owning small pore for storing gas molecule. Recently, researchers have been actively researching materials for hydrogen storage using microporous materials, such as metal–organic frameworks (MOFs), zeolite and carbon materials [1–4]. The hydrogen molecule interacts weakly with the storing materials surface. So, interaction mechanism between hydrogen molecule and storage materials surface highly depends on physisorption mechanism, then chemisorption mechanism. The hydrogen molecule can easily be desorbed with adsorbent, due to its weak van der Waals interactions between hydrogen molecule

* Corresponding author. Tel.: +82 32 860 8438; fax: +82 32 860 8438. E-mail address: [email protected] (S.-J. Park).

and surface of adsorbent materials. Eventually, the physisorption mechanism on porous materials is an attractive method for hydrogen storage, due to its rapid adsorption and desorption speed [2,3]. Researchers are paying attention to the possibility of MOFs, the microporous crystal compounds formed after organic bridging ligand connects metal ions or metal clusters through coordination bond, for heterogeneous catalysts, separation, sensors, electrons and gas storage. Researchers are actively investigating the potential use of MOFs as materials for storing hydrogen. As MOFs shows properties such as framework interpenetration, framework flexibility and chirality. It also have a high specific surface area and are microporous. Further, MOFs have nanoscaled cavities and open channels to small molecules can give access with inherent conditions. However, MOFs are vulnerable to water, go through a complex synthesis process and use an expensive precursor. In addition, it is difficult for them to control the adsorption and desorption speed [1,5,6]. Zeolite is a general term for aluminosilicates with high crystallinity. Zeolite has an ample cage and channel structure, high heat stability and ion exchange capacity, and huge potential as a reservoir of non-polar gas. Gas Reversible closure of zeolite, which is a widely known symptom, is very effective about gas storage [7]. As zeolite is vulnerable to water and hard to reproducible, there has been a great deal of research into ways to complement those aspects [8,9]. There are various carbon materials, and they can be classified based on their synthesis types into carbon black, carbon nanotubes, carbon fibers and activated carbons, and their

http://dx.doi.org/10.1016/j.jiec.2015.07.006 1226-086X/ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

Y.-J. Heo, S.-J. Park / Journal of Industrial and Engineering Chemistry 31 (2015) 330–334

application range is very wide. Carbon materials are chemically very stable and have high thermal conductivity, electro conductivity, high strength and high modulus, so they are good in terms of structural stability as well. As it is easy to activate activated carbons and to treat its surface, in particular, revelation of the specific surface area and porosity is possible, and activated carbons have various structural and surface properties depending on the synthesis methods [10–15]. As a result, activated carbons have been the most actively researched materials for gas adsorption and storage. Researchers can freely choose the materials of activated carbons, and the working processes are simple, so activated carbons are highly practical and have additional physical and chemical properties from chemical treatment. As such, scientists are actively researching the application of activated carbons in the fields of gas storage and harmful gas removal [16–20]. Each year, Korea produces about 700,000 tons of rice husks, as an agricultural by-product. Most of these are used simply as fertilizer, or for warming and water treatment. As amorphous matters containing 30–40% carbons and 20–30% SiO2, rice husks are composed of silicates and carbons. Rice husks have a complex structure of carbons and silicates after organic substances are removed after carbonization, so they are appropriate materials for activated carbons. KOH, widely known as an activation reagent, reacts to silicon that is an atom of the same group [21], as well as carbons of rice husks, and when it can be activated, higher activation efficiency can be expected from KOH than existing carbon materials [22,23]. In this research, activated carbon materials were synthesized using rice husks and chemically activated by KOH. Chemical activation using KOH was effective in developing optimal micropores for hydrogen storage. The aim of this experiment was to examine the effect of KOH amount and pore size on the hydrogen storage behaviors of the synthesized activated carbon derived from rice husks. The textural properties of activated carbons were correlated with the hydrogen storage to figure out the factor that could enhance the property of rice husks based hydrogen storage materials. Experimental Materials and sample preparation For this research, rice husks grown in Goheung, South Jeolla Province, were used as a carbon precursor. Prior to the experiment, rice husks in natural conditions were washed with distilled water to remove impurities. The moisture was removed from the washed rice husks in an 80 8C oven for 24 h. The rice husks were placed in a solution with a fixed weight ratios of KOH/rice husks = 0.5, 1, 2, and 3, respectively. The mixtures were dried at 80 8C oven for 24 h, and then transferred to an alumina boat in a furnace under an N2 gas flow of 200 mL min1. The samples were heated to 900 8C for 1 h, with a heating rate of 2 8C/min. Rice husks were washed with distilled water until their pH reached around 7. The samples were finally dried at 80 8C oven for 24 h. The chemically further activated carbon samples are labeled RHC0.5, RHC1, RHC2 and RHC3 according to their KOH ratios. Characterization Thermo gravimetric analyzers (TGA, SDT-Q600, NETSCH) was used to figure out carbonization temperature of rice husks. Chemical properties of the surface of rice husk-based activated carbons was analyzed using X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific). AlKa was used as a light source for XPS measurement, and pressure within the chamber was controlled at 108–109 torr. X-ray Diffraction (XRD, D2 PHASER,

331

Bruker) investigated the transition of the activated carbons derived from rice husks that occurred in the crystalline phase and in their lattice distortions. The surface morphology was observed by Scanning Electron Microscope (SEM, S-4300SE, Hitachi Co.). The textural property was measured using N2/77 K isothermal adsorption equipment (BELSORP, BEL Co.). The specific surface area was calculated using the BET equation (Brunauer–Emmett– Teller equation). The amounts of N2 adsorbed at relative pressures (P/P0 = 0.99) were used to investigate the total pore volumes. The mesopore volume was calculated through the BJH equation (Barret–Joyner–Halenda equation) and the micropore volume was calculated after deducting the mesopore volume from the total pore volume. Hydrogen storage capacity The hydrogen storage capacity experiment was conducted under H2/77 K at 1 bar. For pretreatment measurement, the system was completely exhausted at 473 K for 6 h. After the system was cooled to room temperature, hydrogen was injected until pressure reached at 1 bar. The influence of moisture or other impurities on measurement was minimized using ultra-high pure hydrogen (99.9999%). Finally, the hydrogen storage capacity was measured using a method of volume measurement method at 77 K/1 bar. Results and discussion Activation mechanism and carbonization process The activation mechanism between KOH and rice husks is as follows [21]. Considering the decomposition of KOH and the removal of carbons and silicon, the following reactions take place at the same time. 4KOH þ C ! K2 CO3 þ K2 O þ 2H2

(1)

2KOH ! K2 O þ H2 O

(2)

C þ H2 O ! H2 þ CO

(3)

CO þ H2 O ! H2 þ CO2

(4)

K2 O þ CO2 ! K2 CO3

(5)

H2 O þ H2 ! 2K þ H2 O

(6)

K2 O þ C ! 2K þ CO

(7)

K2 CO3 þ 2C ! 2K þ 3CO

(8)

K2 O þ SiO2 ! K2 SiO3

(9)

It has been reported that KOH activation actively occurs at temperatures of 600 8C or higher in general [3], and it has been confirmed that the carbonization of rice husks was almost complete at around 500 8C through the TG curve of Fig. 1. Through this, carbonization of rice husks and KOH activation took place at the same time to synthesize activated carbon derived from rice husks. Characterization XPS was analyzed to confirm the effects of KOH activation on the chemical composition of the RHC surface, and the results are shown in Fig. 2. As can be seen, three characteristic peaks of 102– 105 eV (Si2p), 283–286 eV (C1s) and 529–534 eV (O1s) appear on the surface of RHC. A weak Si2p peak is observed in RHC0.5, and it

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100

TG Weight loss (%)

80

60

40

20

0 0

200

400

600

800

o

Temperature ( C) Fig. 1. TG curve of rice husks (N2 flow).

Fig. 3. XRD patterns of RHC samples as a function of KOH amount.

was observed that the Si2p peak completely disappeared in RHC1, RHC2 and RHC3. It is thought that Si is removed and the Si2p peak appearing in 283–286 eV disappears as KOH activation on C and Si and causes removal, as shown in rice husk precursors and KOH activation mechanism [21,24]. In addition, it was confirmed that the O1s peak reduces as K and C react and remove CO and CO2, and the C1s peak increases the C–C bonds increases. An X-ray diffraction analysis was performed to obtain information about the fine structure and crystal structure of RHC samples as a function of KOH amount, and the result is as shown in Fig. 3. In RHC without KOH treatment, a wide peak appeared in 2u = 228 (0 0 2) and 2u = 448 (1 0 0, 1 0 1) and it was confirmed that it had a typical amorphous graphite crystal structure [20]. The peak of (0 0 2) disappeared in 2u RHC0.5 and the strength of the (1 0 1) peak weakened. This was apparently because of layer structure defects caused by K intercalation through the graphite crystalloids and the evaporation of C [24,25]. As the amount of treated KOH increases, the peaks of (0 0 2) and (1 0 1) completely disappear. It is thought that the increase in the amount of K intercalation into graphite crystalloids aggravated the destruction of the molecule crystal structure [3,4]. Fig. 4 shows SEM images that investigated the surface properties of RHC samples as a function of KOH amount. Fig. 2(a) shows samples without KOH treatment. Fig. 2(b)–(d) show the surface properties of samples activated with different

Relative intensity

RHC RHC0.5 RHC1 RHC2 RHC3

600

400

200

Binding energy (eV) Fig. 2. XP spectra of RHC samples as a function of KOH amount.

0

ratios of KOH. While the surface of Fig. 2(a) without KOH treatment is smooth, Fig. 2(b)–(d) with KOH treatment have many pores on their surfaces. As a function of KOH amount increases, the size of the pores formed on the surface increases as well [26–28]. Fig. 3 shows the obtained N2/77 K adsorption/desorption isotherms of the sample studied. According to the graph, the adsorption quantity of samples without KOH treatment (RHC) is low, which means that thermal treatment to the rice husk precursors cannot form pores without activators. On the other hand, the adsorption amount of samples with KOH treatment significantly increased. As observed through Figs. 3 and 4, the specific surface area increased due to the destruction of graphite layer crystallinity and surface pore development. RHC0.5, RHC1 and RHC2 show Langmuir isotherms with a combination of microporous formed Type I and Type IV with developed mesopores according to classification recommended by IUPAC classification. In addition, hysteresis loop was observed at the relative pressures (P/P0) range of 0.3–1.0, 0.4–1.0 and 0.5–1.0, respectively. While an H-4 hysteresis loop caused by capillary condensation to mesopores can be observed in RHC1, also H-3 hysteresis loop caused by capillary opening condensation to large pores can be observed in RHC0.5 andRHC2 [5,6]. RHC3 shows Type I isotherms with high microporosity, but it can be observed that RHC3 has lower adsorption than RHC1 or RHC2 because of the destruction of mesopores and micropores that takes place due to excessive KOH content. Also there is no capillary condensation. Table 1 shows the specific surface area, the total pore volume, the mesopore volume and the micropore volume of RHC samples calculated from adsorption–desorption isotherms, and it can be confirmed that the specific surface area and pore properties remarkably increased because of KOH activation. As the treated KOH ratio increased, the specific surface area and the total pore volume also increase. The micropore volume shows the highest figure when the KOH ratio was 1. The specific surface area and the micropore volume decreased when the treated KOH ratio was 3, and it is apparently because the destruction of the crystal structure caused by excessive KOH treatment destroys the pore structure, as observed in Fig. 3. Micropore fraction is the highest in RHC3, and this is apparently due to the rapid decrease in the mesopore volume caused by the destruction of the pore structure, as observed previously. Except this, micropore fraction follows the tendency of micropore volume according to as a function of KOH amount, and it is thought that the micropore volume is 0.792 cm3 g1 and micropore fraction is 40.4% in RHC1, the highest in gas adsorption.

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Fig. 4. SEM images of RHC(a), RHC0.5(b), RHC1(c), RHC2(d).

Pore size distribution (PSD) was calculated using the Horvath– Kawazoe (HK) method and the result is shown in Fig. 6. There are almost no micropores in RHC without KOH treatment. Micropores sized 0.5–0.9 nm intensively developed in RHC1, and micropores sized 0.6–0.9 nm intensively developed in RHC0.5, RHC2 and RHC3. Those micropores satisfy the size condition of 0.6–0.8 nm, which is reportedly the optimum pore size for hydrogen storage [2,23], and it is thought that all samples excluding RHC have the optimum microporosity for hydrogen storage [3,10].

77 K/1 bar and the result is shown in Fig. 7. As to hydrogen storage at 77 K, hydrogen adsorption behaviors highly depend on physisorption mechanism caused by the textural properties of adsorbent materials [29–31]. Especially, for the microporous carbon materials, hydrogen uptakes are dominant on the micropore volume and pore size [32–34]. From the results, The hydrogen uptake was as low as 0.72 wt.% in RHC, a sample without KOH treatment, but samples with KOH treatment generally show a hydrogen uptake of 2.5 wt.% or higher. As observed in Figs. 5 and 6, plenty of optimum pore size for hydrogen storage developed due to

Hydrogen storage capacity The hydrogen uptakes of RHC samples as a function of KOH amount was measured through volume measurement method at Table 1 Textural properties of RHC samples as a function of KOH amount. Specimens

SBETa (m2 g1)

Vtotalb (cm3 g1)

Vmesoc (cm3 g1)

Vmicrod (cm3 g1)

Fmicroe (%)

RHC0 RHC0.5 RHC1 RHC2 RHC3

16 1855 2682 3044 2232

0.029 1.511 1.963 2.250 1.240

0.026 0.934 1.171 1.475 0.584

0.003 0.577 0.792 0.775 0.656

10.0 36.7 40.4 34.4 52.9

a

SBET: specific surface area calculated using BET equation. Vtotal: total pore volume is estimated at a relative pressure P/P0 = 0.99. c Vmeso: mesopore volume determined from the BJH equation. d Vmicro: micropore volume determined from the subtraction of mesopore volume from total pore volume. e FMicro: fraction of micropore volume = (micropore volume/total pore volume)  100. b

Fig. 5. N2 adsorption/desorption isotherms of RHC samples as a function of KOH amount.

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0.792 cm3 g1, when the ratio of rice husks to KOH was 1:1, and the hydrogen storage capacity also was the highest, 2.85 wt.%, at 77 K/ 1 bar. Structural properties in the synthesis of activated carbons using existing KOH were the best when the KOH rate was 3 or 4. On the other hand, in this study it was shown that it is possible to synthesize activated carbons which has high microporosity using a small KOH amount, indicating that this study is relatively ecofriendly and economically profitable. The existing method of synthesizing activated carbons derived from rice husks had to go through processes of KOH treatment and activated carbon synthesis after carbonizing rice husks.

5 RHC RHC0.5 RHC1 RHC2 RHC3

dV/dd0

4

3

2

1

Acknowledgements 0 0.5

1.0

1.5

2.0

2.5

Pore size (nm)

This work was supported by the Industrial Strategic technology development program (10050953) funded by the Ministry of Trade, Industry & Energy (MI, Korea).

Fig. 6. Pore size distribution of RHC samples as a function of KOH amount.

References

Hydrogen uptakes (wt.% 77 K)

3.0 2.5 2.0

RHC RHC0.5 RHC1 RHC2 RHC3

1.5 1.0 0.5 0.0 0.0

0.2

0.4

0.6

0.8

1.0

Pressure (bar) Fig. 7. Hydrogen storage capacity of RHC samples as a function of KOH amount.

the KOH activation of rice husks. It was confirmed that the hydrogen storage capacity was high in RHC1 and RHC2, where the micropore volume was high and pore size distribution of the optimum size for hydrogen storage was the highest. The hydrogen storage capacity in RHC2 was 2.78 wt.% and the figure was 2.85 wt.%, the highest, in RHC1. Conclusions This study investigated the hydrogen storage capacity of activated carbon derived from rice husks as a function of KOH amount. As the treated KOH amount increased, the specific surface area and microporosity also increased. The micropore volume and distribution of pores sized 0.6–0.8 nm, the most important factors for physisorption on hydrogen storage, developed appropriately in all KOH activated samples. The micropore volume was the highest,

[1] N.L. Rosi, J. Eckert, M. Eddaoudi, D.T. Vodak, J. Kim, M. O’Keeffe, O.M. Yaghi, Science 16 (2003) 1127. [2] S.J. Park, S.Y. Lee, Int. J. Hydrogen Energ. 35 (2010) 13048. [3] S.Y. Lee, S.J. Park, Int. J. Hydrogen Energ. 384 (2012) 116. [4] L.Y. Meng, S.J. Park, J. Colloid Interface Sci. 352 (2010) 498. [5] R. Lotfi, Y. Saboohi, Comput. Theor. Chem. 1044 (2014) 36. [6] Y. Feng, H. Jiang, M. Chen, Y. Wang, Powder Technol. 249 (2013) 38. [7] C.O. Ania, V. Khomenko, E. Raymundo-Pinero, J.B. Parra, F. Beguin, Adv. Funct. Mater. 17 (2007) 1828. [8] J. Zhou, W. Li, Z. Zhang, X. Wu, W. Xing, S. Zhou, Electrochim. Acta 89 (2013) 763. [9] J.L.C. Rowsell, O.M. Yaghi, J. Am. Chem. Soc. 128 (2006) 1304. [10] S.J. Park, Y.S. Jang, J.W. Shim, S.K. Ryu, J. Colloid Interface Sci. 119 (2003) 211. [11] M.Z. Figueroa-Torres, C. Dominguez-Rı´os, J.G. Cabanas-Moreno, O. Vega-Becerra, A. Aguilar-Elguezabal, Int. J. Hydrogen Energ. 37 (2012) 10743. [12] T. Ren, Y. Si, J. Yang, B. Ding, X. Yang, F. Hong, J. Yu, J. Mater. Chem. 22 (2012) 15919. [13] M.C. Tellez-Jua´rez, V. Fierro, W. Zhao, N. Ferna´ndez-Huerta, M.T. Izquierdo, E. Reguera, A. Celzard, Int. J. Hydrogen Energ. 39 (2014) 4996. [14] S.J. Park, M.K. Seo, Y.S. Lee, Carbon 41 (2003) 732. [15] L. Wang, Z. Zhang, Y. Qu, Y. Guo, Z. Wang, X. Wang, Colloids Surf. A Physicochem. Eng. Asp. 447 (2014) 183. [16] Y. Si, T. Ren, Y. Li, B. Ding, J. Yu, G. Sun, J. Mater. Chem. 22 (2012) 4619. [17] M. Zhou, F. Pu, Z. Wang, S. Guan, Carbon 68 (2014) 185. [18] A. Hruzewicz-Kolodziejczyk, V.P. Ting, N. Bimbo, T.J. Mays, Int. J. Hydrogen Energ. 37 (2012) 2728. [19] J. Zhang, L. Jin, X. He, S. Liu, H. Hu, Int. J. Hydrogen Energ. 36 (2011) 8978. [20] S.J. Park, K.D. Kim, J. Colloid Interface Sci. 212 (1999) 186. [21] Y. Guo, S. Yang, K. Yu, J. Zhao, Z. Wang, H. Xu, Mater. Chem. Phys. 74 (2002) 320. [22] V. Jimenez, J.A. Diaz, S. Paula, P. Sanchez, J.L. Valverde, A. Romero, Chem. Eng. J. 155 (2009) 931. [23] S.Y. Lee, S.J. Park, Int. J. Hydrogen Energ. 35 (2010) 6757. [24] B.J. Kim, Y.S. Lee, S.J. Park, Int. J. Hydrogen Energ. 33 (2008) 2254. [25] P. Osvaldo Jr., A.L. Cazetta, I.P.A.F. Souza, K.C. Bedin, C.M. Alessandro, T.L. Silva, V.C. Almeida, J. Ind. Eng. Chem. 20 (2014) 4401. [26] F. Galindo-Herna´ndez, P. Benjamin, M.D. Jose´, D. Aneles-Beltra´n, J. Power Sources 269 (2014) 69. [27] A. Minoda, S. Oshima, H. Iki, E. Akiba, J. Alloys Compd. 606 (2014) 112. [28] C.Y. Chen, J.K. Chang, W.T. Tsai, Int. J. Hydrogen Energ. 37 (2012) 3305. [29] Y. Xiao, H. Dong, C. Long, H. Zhang, B. Lei, Y. Liu, Int. J. Hydrogen Energ. 39 (2014) 11661. [30] N. Qiu, Z. Tian, Y. Guo, C. Zhang, Y. Luo, Y. Xue, Int. J. Hydrogen Energ. 39 (2014) 9307. [31] S. Dutta, J. Ind. Eng. Chem. 20 (2014) 1148. [32] B.J. Kim, Y.S. Lee, S.J. Park, Int. J. Hydrogen Energ. 33 (2008) 4112. [33] N.P. Stadie, J.J. Purewall, C.C. Ahn, B. Fultz, Langmuir 19 (2010) 15481. [34] M. Armandi, B. Bonelli, F. Geobaldo, E. Garrone, Microporous Mesoporous Mater. 132 (2010) 414.