Preparation of Bamboo-Bad Activated Carbon Jubing Zhang, Zhaoping Zhong, Houkun Guo, Xiaoxiang Jiang
Thermoenergy Engineering Rearch Institute Southeast University
Nanjing 210096,Jiangsu Province, P.R.China;
Abstract—Experiment study on activated carbon preparation from bamboo with chemical activation and by K2CO3 as activating agent was carried out in this paper. The effects of activation temperature, impregnation ratio and activation time on specific surface area, volume resistivity and ash content were systematically investigated. Specific surface area was estimated by BET surface area analysis. SEM analysis was ud to study the structural changes among different activated carbons. XRD analysis was applied to investigate the structural parameters of the activated carbon generated from different temperatures. Optimum working condition was as follows: activation temperature of 900℃, activation time of 2h, and impregnation ratio of 1 and the experiment results showed that specific surface area, volume resistivity, and ash content of activated carbon could reach 1264 m2.g-1, 1569μΩ.m and 7.14% under optimum condition, respectively.
acquirementkickbackKeywords-Activated carbon; Specific surface area; Volume resistivity; Ash content
I.I NTRODUCTION
Direct carbon fuel cell (DCFC) is a device which converts chemical energy of carbon into electric energy through chemical oxidization directly [1-5]. A solid carbon fuel for DCFC must be characteristic of high surface area, high content of oxygen functional groups, low volume resistivity and low ash content. High surface area can increa contract area of three phas. High content of oxygen functional groups may accelerate anodic reaction becau more active site is produced during desorption [6]. Low volume resistivity is beneficial to reduce ohmic polarization [1, 5]. While low ash content should prolong the life of fuel cell [7].
Activated carbon is a form of carbon with developed inner pore structure, high surface area porous and strong adsorption capacity and its characteristic depends mainly on the physical and chemical properties of raw materials and activation methods ud. Activated carbon prepared with high surface area, low volume resistivity or low ash content has been reported largely [8-10], and even effects of characteristics of activated carbon on performance of DCFC had been studied by Xiang Li [11]. However, how to produce activated carbon with high surface area, high content of oxygen functional groups, low volume resistivity and low ash content for DCFC has hardly been investigated.
With totally irregular microcrystalline structure, activated carbon is very suitable for anodic reaction of DCFC. Besides the adventures pointed out above, biomass-bad activated carbon also has low ash content, which further fits the demands of DCFC. In addition, volume resistivity of bamboo-derived activated carbon is rather low contrast to activated carbon derived from other biomass, such as oak wood scobs and rice husk. In this study, biomass-bad activated carbon with high surface area, low volume resistivity and low ash content is prepared from bamboo sawdust [12-15] with chemical activation and by K2CO3 [16-21] as activating agent.
II.E XPERIMENT
A.Material
Bamboo ud in this study was collect from the city of Nanjing in Jiangsu Province, China. Prior to experiment, bamboo scrap with a particle size range of 0.5-1.0 mm was prepared with a high-speed rotary cutting mill. The characteristics of the bamboo were summarized in Table.1. Potassium carbonate was provided by Sinopharm Chemical Reagent Co., LtdS.
B.Instrument and Equipment
The specific surface areas of activated carbon samples were measured by a Micromeritics ASAP2010 apparatus.
Volume resistivity of activated carbon sample was detected by a multifunctional resistivity determinator (GM-Ⅱ).
The microstructure of activated carbon and was obrved using a field-emission scanning electron microscope (Sirion 2000) at an accelerating voltage of 20 kV.
The X-ray diffraction analysis (XRD) was conducted to investigate the structural parameters of the activated carbon by using a XD-3A X-Ray diffractometer, and the measurements were performed by using Cr Kα radiation (25 kV, 40 mA, k =
1.54A˚).
C.Preparation of activated carbon
As can be en in TGA of bamboo (Fig.1), volatile of bamboo volatilizes a lot between 250℃and 400℃, and reaches its maximum at 355℃. When temperature exceeds 400℃, volatilization reduces greatly, so 400℃ is t as carbonization temperature. The schematic diagram of activated carbon pr
eparation system is shown in Fig 2. Carbonization of bamboo was carried out in a high temperature tube furnace at 400℃ for 1h. Then, the whole system was cooled down to the room temperature without being forced. During the process of carbonization and cooling nitrogen (99.99%) was supplied in order to prevent oxidation.
Table.1 Main characteristics of bamboo (air-dry basis) Proximate analysis (wt. %) Ultimate analysis (wt. %)
M V A FC C H O N
7.7076.10 1.2214.98 46.69 5.46 47.630.22
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978-1-4244-4813-5/10/$25.00 ©2010 IEEE
youdao
-10-8-6-4-202T G / %
Temperature /℃英语学习聊天
D T G / (%·m i n
-1
)
Fig.1 TGA of bamboo
Fig.2 Schematic diagram of experimental apparatus
for activated carbon preparation Carbonized material was mixed well with K 2CO solution according to different impregnation ratios, and the mixture was then dried in an oven at 120℃ for 12h to prepare the impregnated sample. The impregnation ratio is given by (1). In this study, 0.5, 1, 1.5, 2, 2.5 impregnation ratios were lected. In activation process, the mixture prepared above was heated up to the tting temperature with a high temperature tube furnace under 200 mL/min nitrogen flow controlled by a rotameter at the heat rate of 10℃/min, and temperature ud here w as varied as 8
00℃℃ and When the activation temperature reached, it was controlled invariably for a given period of time, such as 1h, 1.5h, 2h, 2.5h and 3h. After that, the sample w down to the room temperature under nitrogen flow and was washed and filtered veral times until filtrate was Impregnation ratio=The effect of activation temperature on specific surface area is shown in Tab.2. Activation time is 2h and impregnation ratio is 1 for all the samples, which are lected according to other experiments done before. As can be en in Tab.2, specific surface area first increas gradually from 975m 2.g -1 to 1264m 2.g -1 with increa in activation temperature from 800℃ to 900℃, then decreas from 1264m 2.g -1 to 1062 m 2.g -1 with increa in activation temperature from 900℃ to 1000℃. It is believed that in the process of activated carbon preparation reactions, activation agent etches carbon atoms, and pores are left when residual activation agent in the places is washed off. In addition, CO 2 decompod by K 2CO 3 further boosts up activation through a physical activation. And higher activation temperature is beneficial to the reactions. In addition, when activation temperature reached the boiling point of potassium 1035 K, potassium would diffu into the layer of carbon, also causing increa in specific surface area. However, exorbitant temperature may cau excessive erosion of activated carbon, even collap of some pore structure, and that is why specific surface area reduces when temperature exceeds 900℃.
Increa in activation temperature can accelerate activation reaction, more carbon atoms react with activating agent, while ash in activated carbon remain invariant, thus ash content of activated carbon ris.
Physical change happens in internal structure of activated carbon with increa in activation temperature. Development of minicrystal is promoted and orientation of minicrystal becomes uniformly, and the changes improve graphitization degree, thus volume resistivity reduces.
B. Effect of activation time on characteristics of activated carbon
The effect of activation time on specific surface area is shown in Tab.3. Activation temperature of 900℃ and impregnation ratio of 1 is lected in this ction. The specific surface area of activated carbon increas with an increa in activation time, reaching a maximum at activation time of 2h, and then decreas with a further increa in activation time. With the increa of activation time from 1h to 2h, the specific surface area quickly increas due to pore creation related to activation reaction. And the decrea in specific surface area with the increa in activation time (after 2h) is possibly becau of some of the pores being burnt off for a prolonged activation time.
圣诞节 英文A prolonged activation time leads to ash accumulation due
to consume of carbon, which caus increa in ash content.
Graphite crystallite increas gradually with the increa of
activation time, which reduces volume resistivity of activated
carbon. However ash accumulation ascribed to a prolonged activation time increas volume resistivity. It is believed that before activation time of 2.5h, the effect of graphite crystallite predominates over the effect of ash accumulation, while the latter predominates over the former when activation time exceeds 2.5h.
Tab.3 Effect of activation time on characteristics of activated carbon sample 6# 7# 8# 9# 10# Time / h 1 1.5 2 2.5 3
Specific surface area / m 2.g -1 873 1022 1264 1176 1092 Ash content / % 3.73 5.35 7.14 10.25 12.53 Volume resistivity / μΩ·m 2123 1738 1569 1546 1884
Tab.4 Effect of impregnation ratio on characteristics of activated carbon
sample 11# 12# 13# 14# 15# Impregnation ratio 0.5 1 1.5 2 2.5 Specific surface area / m 2.g -1 964 1264 1200 1145 1033
Ash content / % 6.35 7.14 10.28 12.48 15.19 Volume resistivity / μΩ·m 1466 1569 1760 2079 2120
C. Effect of impregnation ratio oncharacteristics of activated carbon
The activation temperature and activation time are fixed at 900℃ and 2h according to above discussions. As is shown in Tab 4, it is easy to find that the effect of impregnation ratio on specific surface area is similar to that of activation temperature and activation time. It is sure that more chemical agents react with more carbon atoms, and more reactions produce more pores, which increas specific surface area. Nevertheless, further increasing in chemical agent may cau incorporation of small pores and thus formation of large pores, which leads to decrea in specific surface area.
Higher impregnation ratio brings higher ash content attributed to consumption of more carbon atoms in activation reaction.
Rising in impregnation ratio increas ash content, which reduce conductivity of activate carbon.
Through the discussions above, the optimum condition for activated carbon preparation is obtained as follows: activation temperature of 900℃, activation time of 2h, and impregnation ratio of 1, specific surface area, volume resistivity, and ash content of activated carbon under this condition could reach 1264m 2.g -1, 1569μΩ.m and 7.14%, respectively.
D. Microscopic examination of carbonaized material and activated carbon
Scanning electron micrographs of carbonized material and activated carbon prepared at different impregnation ratios are shown in Fig.3. During the carbonization process, pores are left after volatilization of moisture and volatile, which can be found in Fig.3a. In the activation process, pore size and pore volume increa and new pores are created due to reaction of carbon and activating agent, therefore specific surface area increas (Fig.3b). With increa in impregnation ratio, pore size enlarges continuously and new micropore generates in hole wall, which caus further increa in specific surface area. When impregnation ratio equals 1, specific surface area reaches maximum and pore structure is the most developed (Fig.3c). Nevertheless, excessive activating agent caus collap of some pore structure, which leads to the decrea in specific surface area (Fig.3d).
(a) (b)
riot
(c) (d)
Fig.3 Scanning electron micrographs of carbonized material and activated carbon prepared at different impregnation ratios: (a)
carbonized material, (b) 0.5, (c) 1 and (d) 2.5
2 theta (Cr Ka )
950O
C
1000O
C
900O
C
Fig.4 XRD pattern of activated carbon prepared at different activation temperature
E. XRD characterization of activated carbon
The XRD profiles for the activated carbon generated from different temperatures are prented in Fig.4. It can be clearly en that there are two diffraction peaks around 2θ = 39° and 2θ = 66° in each spectrum corresponding to the diffraction of (002) and (100), respectively. With increa in acti
vation temperature, the intensities of both peaks increa and diffraction peak became narrow. And the changes in the peaks indicate that a more disordered amorphous structure for activated carbon at 900℃. As a result, both specific surface area and volume resistivity decrea with the increa of temperature from 900℃ to 1000℃, which is consistent with the result derived from Tab.3.
IV. C ONCLUTION
In this study, experimental studies on activated carbon preparation from bamboo for DCFC with chemical activation by K 2CO 3 as activating agent were carried out. The effects of activation temperature, activation time and impregnation ratio
javeon the specific surface area of activated carbon were systematically investigated. And the optimum condition was: activation temperature of 900℃, activation time of 2h, and impregnation ratio of 1. Meanwhile, specific surface area, volume resistivity, and ash content of activated carbon reach 1264m2/g, 1569μΩ.m and 7.14%, respectively.
A CKNOWLEDGMENT
The support of the National Natural Science Fund Program of China (50776019), the Doctorial Subject Science &Technology Fund Program of State Education Ministry of China (200802860038) and the Science &Technology Foundation of Southeast University of China (XJ0703267) are gratefully acknowledged.
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