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Activated carbons derived from tamarind seeds for hydrogen storage
Journal of Energy Storage 4 (2015) 89–95
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Activated carbons derived from tamarind seeds for hydrogen storage T. Ramesh, N. Rajalakshmi* , K.S. Dhathathreyan Centre for Fuel Cell Technology, International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), IIT-M Research Park, Taramani, Chennai 600113, Tamil Nadu, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 July 2015 Received in revised form 16 September 2015 Accepted 16 September 2015
To make hydrogen economy a reality, the key technical challenges that need to be addressed are Hydrogen generation, transmission and storage for application in fuel cells. Among them, H2 storage presents a major challenge to material scientists to meet the USDOE target of 5.5 wt%. The present paper investigates the hydrogen storage capacity of activated carbons derived from tamarind seeds by thermal, microwave and by chemical KOH activation treatment. The various parameters optimised to obtain high hydrogen storage capacity are KOH concentration, duty cycle of microwave pulsing, carbonization, hydrogen adsorption conditions etc., The surface area, micropore volume, pore size and nitrogen adsorption measurements were determined for all the samples and found that the carbonization temperature and concentration of KOH play a major role in increasing the surface area, micropore volume and pore size. We have obtained a high surface area activated carbon of 1785 m2 g1, micropore volume of 0.94 cm3 g1 and pore size of around 0.8–1.1 nm. The maximum hydrogen storage capacity of these activated carbons from tamarind seeds at RT and 4.0 MPa was found to be 4.73 wt%, which is about 80% of the USDOE target. The samples also show good cyclic stability for hydrogen adsorption and desorption studies. These results suggest that activated carbons fabricated from tamarind seeds with high surface area and micropore volume will be an ideal candidate for hydrogen storage. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Activated carbons Tamarind seeds Hydrogen storage Surface area Micropore
1. Introduction Hydrogen is light and clean fuel, acts as an efficient energy carrier in the hydrogen economy. In recent years, hydrogen infrastructure in terms of production and storage play an important role in the commercialisation of fuel cells. A fuel cell combines hydrogen and oxygen to produce electricity, heat, and water. In order to have on board hydrogen storage to operate fuel cells using hydrogen, storage containers/materials with high volumetric and gravimetric density are preferable. Recently, lightweight carbon adsorbent materials, attracted lot of interest for possible use in a hydrogen storage system [1,2]. None of the storage methods viz., compressed gas, liquefaction, metal hydrides., etc. satisfy the criteria of capacity, size, efficiency, cost and safety. Recently many research groups are focussing on the H2 adsorption/desorption properties of ‘activated’ carbon materials, prepared from various natural precursors. These materials were typically obtained by thermochemical processing and show different types of carbon structures in terms of porosity, density, defect sites etc. These materials are found to be more economically
* Corresponding author. E-mail address: [email protected] (N. Rajalakshmi). http://dx.doi.org/10.1016/j.est.2015.09.005 2352-152X/ ã 2015 Elsevier Ltd. All rights reserved.
attractive compared to compressed hydrogen and metal hydride alloys. These activated carbon materials were found to be an ideal candidate for hydrogen storage medium due to their advantages like light weight, high specific surface area, complete reversibility and low cost. They can be produced in large quantities from a carbon rich precursor by a physical or chemical activation process. Activated carbons (AC) are generally fabricated from biomass resources like oil palm shell [3], almond shell, hazelnut shell [4], coconut shell [5], pistachio shells [6], olive stones [7], dates seeds [8], coffee beans [9], apricot seeds [10], grape seeds [11] etc., and have been studied for various applications. These activated carbons with micro pores, are considered for gas adsorption as well as for hydrogen storage applications due to their high specific surface areas and high adsorption properties [12,13]. These materials are also generally used for the gas separation and purification from polluted water streams. However, the pore structure of these activated carbon is complex and not well defined. Activated carbons (AC) are obtained by chemical activation, where the raw material is impregnated with activating agents like KOH, NaOH, Na2CO3, K2CO3, H3PO4, ZnCl2 etc., and are then heated in an inert atmosphere, where the carbonization and activation occurs simultaneously. The various activating agents increase the porosity and thereby the surface area. During this process, certain dehydrating agents are also employed that influences the
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temperature of pyrolysis decomposition that inhibit the formation of tar, thereby enhancing the yield of AC [14–16]. It has been reported that these properties are essential for gas storage, especially microporosity and high surface area [17]. Two different class of materials viz., zeolites and activated carbons have been studied extensively for gas storage and has been reported that ACs are very good adsorbents with the highest storage capacities for natural gas [18]. Carpetis and Peschka [19] have reported that hydrogen could be stored on porous carbon materials at low temperatures. They have studied many high surface area carbon materials and reported that porous carbon materials tend to adsorb more hydrogen than the denser carbons. Physically activated carbons commonly use bituminous coal or coconut shells as a starting material. High surface areas in excess of 3000 m2 g1 can be obtained by treating them with chemical agents like KOH. In the two-stage activation process of carbonization, the oxygen and hydrogen are burned and the char is heated in steam or carbon dioxide atmosphere to create a highly porous structure. The hydrogen storage capacity also found to increase if the carbon materials possess high specific surface area and micropore volume [20–23]. Although various carbons show a weight capacity of 0.1–7.5 wt%, at 77 K and at 1 MPa, the highest hydrogen storage capacity at RT was not found to be more than 1 wt% at 298 K, 1 MPa, while the targets by DOE for hydrogen storage is 5.5% in 2015. ACs based on rice hull, activated through chemical activation which developed pores and large specific surface area has been studied as a promising material for hydrogen storage. The mechanism for hydrogen storage was found to be the van der Waals forces between the huge surface area of ACs and hydrogen molecules. Hence large specific surface area is found to be beneficial for the storage of hydrogen in the said pressure range and the storage capacity increased with decreasing temperature and increasing pressure. Recently rice hull was studied for its hydrogen storage capacity and found to be 7.7 wt%, at 1.2 MPa and 77 K after carbonization and sodium hydroxide activation. The BET surface area was found to be 3969 m2 g1, using toxic reagents and complicated synthesis procedures [24]. It was observed that among the various agriculture residues, tamarind seeds are found to be interesting, as they are available in various forms viz. seeds, seed husk, Kernel powder etc. The elemental composition along with surface area is given in Table 1. The Tamarind Kernel Powder is normally used in industries like textile, dying and Printing, jute, card board, mosquito coil, food additive, oil well drilling, paper industry etc. [25,26]. The botanical name is Tamarindus indica L. and India is the major producer of tamarind on a large scale. The other countries are Thailand, Indonesia, Myanmar, and the Philippines. Tamarind is a brown pod-like fruit, consists of pulp and hard-coated seeds. The brown coloured seeds are hard, with square shape and rounded corners and edges. The seeds are 30–40%, while the shell, fibre and pulp constitute the rest. The present work aims to prepare activated carbon from tamarind seeds (TSC) by thermal, microwave heating method and to study the influence of chemical activation treatment for the hydrogen storage capacity. The carbon prepared Table 1 Elemental composition of agricultural material seeds. Material
C%
N%
O%
H%
SBET m2 g1
Mango Apricot Mahogang Longan Cherry Grapes Tamarind
60.01 51.5 48.14 46.5 53.9 62.2 51.7
– 0.2 0.28 1.4 0.3 1.6 35.5
– 41.9 45.15 45.4 38.4 28.2 12.4
– 6.3 6.4 6.3 7.1 7.8 -
2435.2 630-1175 – 835 – 485 2673
from microwave using pulsed method has many advantages compared to thermal treatment as it decreases the carbonization temperature as well the time required for the whole process. The samples were characterized by scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), IR etc. We have observed an increased surface area and micropore volume with KOH chemical activation. The hydrogen storage capacity of TSC was measured at various pressures ranging from 0.1 to 4 MPa and at RT and found that the hydrogen storage capacity increases from 1 to 4.6 wt%, while the surface area increases from 750 to 1775 m2 g1 for the sample of microwave treated and microwave treated with chemical activation respectively. These samples also showed cyclic stability for hydrogen adsorption and desorption. 2. Methods The tamarind (T. indica) seeds were used as a precursor for the preparation of activated carbon. The tamarind fruits were purchased from local market in Chennai. Seeds were removed from the fruit, washed and dried. Dried seeds were processed to obtain activated carbon as described below. The analytical grade potassium hydroxide (KOH) pellets were purchased; High pure nitrogen gas (99.999%) was used for inert atmosphere during activation. 2.1. Synthesis of activated carbon Carbonization of tamarind seeds were done by both microwave and thermal treatment. The microwave oven of power 900 W at 2.4 GHz in a partially closed system in air atmosphere was used for microwave carbonization. A handful of tamarind seeds were taken in a cleanpetri dish. The seeds were cleaned with water and were rinsed with 1-propanol, and were covered with another petri dish. The seeds were carbonised in ‘microwave’ mode for 30 s and switched off for 15 s interval. This process was repeated for 50 times. The process was repeated for 50, 75 and 100 cycles to make carbon from tamarind seed (1 cycle = 30 s ON + 15 s OFF). The prepared carbons were labelled as MWTSC-50, MWTSC-75 and MWTSC-100. The carbonized seeds were then crushed to powder using a hand held rock crusher. The crushed powder was put in a thimble made of thick filter paper. The impurities were extracted by Soxhlet extraction using iso Propanol at 80 C. All the samples were dried in an oven to remove the moisture. The carbons were activated in order to increase the surface area as well to increase the pore structure. The sample MWTSC-50 was activated to increase the surface area by treating them with KOH in the ratio of 1:3 and sufficient amount of deionised water in a plastic container. The slurry was stirred for about 16 h at ambient temperature, and then heated to desired activation temperature of 700 C in an alumina crucible for 1 h under flowing nitrogen gas. After completion of activation, it was cooled down to room temperature [27]. The resultant product was collected by washing with deionised in soxhlet extraction setup at 100 C until the filtrate was free from alkali and it was labelled as MWTSC-50-3-700. Thermal treatment of tamarind seeds was done by using tubular furnace at 300 C for 1 h in the Nitrogen flow of 100 ml min1. The activation was performed at 500 C for 1 h and the KOH/carbon ratio was varied from 1:1, 1:2 and 1:3. All the samples were powdered well, washed with deionised water and dried. The samples are labelled as TSC-1, TSC-2 and TSC-3, and the schematic is shown in Fig. 1. 2.2. Characterization A typical carbon sample obtained from tamarind seeds are characterized by Rigaku XRD unit and SEM by Hitachi SU1510, for
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Fig. 2. XRD pattern of MWTSC-50-3-700. Fig. 1. Synthesis of activated carbon from tamarind seeds using microwave and thermal methods.
morphology studies. The textural properties of the samples were studied by N2-sorption at 77 K with automatic instrument ASAP2020, Micromeritics, over a wide relative pressure range from about 106 to 0.995. Prior to the measurements, the samples were degassed at 250 C for 24 h. The specific surface area was measured by standard BET method using adsorption data in the relative pressure ranging from 0.02 to 0.25. The total pore volume was calculated by converting the amount of N2 adsorbed at a relative pressure (P/P0) of 0.99 to the volume of liquid. The micropore volume was calculated by the t-plot equation. Pore size distributions (PSD) were calculated by using the Density Functional Theory (DFT) Plus Software, which is based on calculated adsorption isotherms for pores of different sizes. In the DFT calculations, the equilibrium model of carbon slit-shaped pores was applied [28]. Thermogravimetric (TGA) and differential thermal analysis (DTA) measurements were carried out using a NETZSCH STA 449 F1 Jupiter (Netzsch, Germany) in the range from 25 to 1000 C at a heating rate of 100 C min1. The measurements were carried out in an open alumina crucible under air atmosphere at a flow rate of 40 ml min1. An empty, open crucible was used as a reference [29].
3. Results and discussion The XRD of activated carbon from tamarind seeds is shown in Fig. 2 and shows that MWTSC-50-3-700 sample is semi crystalline in nature as reported [31]. Two main diffraction peaks for the (0 0 2) and (1 0 1) planes of graphite structure at 2u = 23.36 and 43.03 are found in MWTSC-50-3-700. The SEM image of MWTSC50-3-700 sample is shown in Fig. 3 and one can see that the pores were uniformly distributed. Thermo gravimetric analysis was performed in air atmosphere at a heating rate of 10 C min1. The TGA of all carbon samples prepared from tamarind seeds are shown in Fig. 4. From Fig. 4, one can observe a weight loss of 10% within 100 C, indicating the evaporation of water and organic solvents in the sample. All the samples were stable up to 300 C, and further increase in temperature decreases mass of the sample due to decomposition. All the samples exhibit same kind of decomposition with a minor change in the complete decomposition temperature, and the residual mass for microwave treated samples were found to be less than 5% at higher temperatures. The surface area was measured for all these samples and the N2 adsorption isotherm is shown in Fig. 5 for MWTSC-50 and MWTSC50-3-700. From Fig. 5 one can see that
2.3. Hydrogen adsorption studies Hydrogen adsorption/desorption studies were performed for the samples microwave treated and activated carbon sample in the pressure ranges 0–4 MPa and at 303 K using a high-pressure Automated Sievert’s apparatus from Advanced Material Corporation, USA. The leakage test of the instrument has been carried out with high pure hydrogen (99.999%) at high pressure before starting the experiment. Initially all samples were evacuated at 103 mbar for 6 h at 423 K. After cooled down to room temperature hydrogen adsorption measurements were performed at 303 K over the pressure range of 0–4 MPa. The hydrogen adsorption measurements were carried out by passing the hydrogen from a calibrated vessel at a particular pressure and then the hydrogen was allowed to the sample chamber of known volume. The pressure composition isotherms were obtained by change in volume of the sample at a particular pressure. The experiments were repeated many times to check the reproducibility of the samples. Tap density of material was used for hydrogen adsorption/desorption calculation [30].
Fig. 3. SEM image of MWTSC-50-3-700.
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Fig. 4. TGA curve of MWTSC-50, MWTSC-75, MWTSC-100 and MWTSC-50-3-700.
Fig. 6. Pore size distribution of MWTSC-50-3-700.
the N2 adsorption isotherm of MWTSC-50 sample may be attributed to the type-III [32] isotherm and MWTSC-50-3-700 may be attributed to type-I isotherm [33]. These isotherms indicate that the MW treated sample is non-porous while, sample treated with KOH is of microporous nature. The carbon obtained using KOH treatment at 700 C shows a high BET specific surface area of 1784 m2 g1. Fig. 6 shows the pore size distribution of activated carbons MWTSC-50-3-700, revealing that the pores are in the range 0–2 nm which are microporous. The total pore volume and micropore volume was found to be 0.93 cm3 g1 and 0.62 cm3 g1. The micropores are not seen in all the microwave carbonized samples viz., MWTSC-50, MWTSC-75 and MWTSC-100. Similarly the N2 adsorption isotherms and pore size distribution of TSC-1, TSC-2 and TSC-3 samples are given in Figs. 7 and 8, respectively. As one can see from the figures that the surface area was found to increase from 353 to 785 m2 g1 and micropore volume from 0.14 to 0.31 cm3 g1. These samples also showed mesopores in the range 2–10 nm compared to microwave treated samples which showed micropores in the range 0–2 nm. These results indicate that activation of carbon performed using KOH at 700 C developed the pore structure in microwave treated tamarind seeds and increased the surface area with micropores suitable for hydrogen storage.
The hydrogen adsorption study was carried out using Sievert’s apparatus and Pressure was measured when hydrogen was introduced to the system containing known weight of carbon material (P1). Before carrying out the experiment, the system was checked for its leakage under both vacuum and at high pressure at 473 K for 24 h. A hydrogen uptake study, without the sample, was also carried out at 8 MPa and at RT. The study showed no hydrogen adsorption within the limits of the lowest count of the instrument. The sample tube and the connecting tubes were evacuated and left under the vacuum for 8 h. There was no change in the pressure. Finally the tube was filled with gas at 6 MPa and left for 4 h. After ascertaining zero leakage, the instrument was filled with known weight of carbon. The sample tube was evacuated and heated to 473 K for 24 h. When no change in the pressure was observed, the main experiment started as mentioned above. The system was left for 24 h to attain the equilibrium. After 24 h the decrease in pressure was measured (P2). The amount of hydrogen adsorbed by carbon material was calculated from the pressure difference, i.e. from (P1–P2). As the volume of the system and weight of carbon were known, the amount of hydrogen in the unit of wt% was calculated. The quantity of hydrogen adsorbed was calculated using the general gas law as modified by Van der Waal's (Van der
Fig. 5. N2-sorption isotherms at 77 K of ( ) MWTSC-50 and (&) MWTSC-50-3-700.
Fig. 7. N2-sorption isotherms at 77 K of ( ) TSC-1, ( ) TSC-2 and (!) TSC-3.
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Table 2 Hydrogen uptake of microwave treated tamarind seed at different temperature & 6 MPa. Sample
MWTSC-50 MWTSC-75 MWTSC-100
ON time (s)
30 30 30
OFF time (s)
15 15 15
Hydrogen storage at 6 MPa 303 K
323 K
348 K
373 K
0.67 1.19 1.36
0.73 0.7 0.88
0.27 1.02 1.14
0.29 0.96 0.81
Fig. 8. Pore size distribution of MWTSC-50-3-700.
Waal's gas law), using the following equation. " 2 # n ðV r nbÞ ¼ nRT Pr þ a Vr The constants ‘a’ and ‘b’ have positive values and are characteristic of the individual gas. For hydrogen gas ‘a’ is 2.45 102 Pa m3 and ‘b’ is 26.61 1026 m3/mol. Hydrogen adsorption was measured by using hydrogen upto 6 MPa pressure and all the experiments have been carried out a minimum of 3–5 times to ascertain the reproducibility. The hydrogen storage capacities of these samples were measured at RT and at various pressures. The hydrogen storage capacity of TSC-1 is shown in Fig. 9 and found that the maximum hydrogen storage capacity is 0.08 wt% at RT and at 0.8 MPa. Similarly the hydrogen storage capacities of MW treated samples are measured at various temperatures ranging from RT to 373 K and they are given in Table 2. From Table 2, one can see that the hydrogen adsorption capacity was found to be 1.36 wt% at RT and at 6 MPa. Hydrogen adsorption capacity was also measured for the KOH activated sample viz., MWTSC-50-3-700 and the results are given in Fig. 10, along with hydrogen storage capacity for all the MW treated samples viz., MWTSC-50, MWTSC-75 and MWTSC100. From Fig. 10, one can see that the hydrogen storage capacity increases with increase in microwave pyrolysis duty cycle and found to be 0.54 wt%, 0.73 wt% and 1.00 wt% for MWTSC-50,
Fig. 9. Hydrogen uptake of TSC-1 at different temperature and 0.8 MPa.
Fig. 10. Hydrogen adsorption/desorption isotherms of samples studied at 303 K and 4 MPa.
MWTSC -75 and MWTSC -100, respectively at 303 K and 4 MPa. However, the hydrogen storage capacity of the KOH activated sample showed a very high capacity of 4.73 wt% at 303K and 4 MPa. From the literature, we have observed that LaNi5 alloy showed 1.3 wt% of hydrogen uptake at room temperature and at 1 MPa [34] while the FeTi alloy showed a hydrogen adsorption capacity of 1.02 wt% under the same conditions [35]. Hence when these hydrogen storage capacity values are compared with activated carbons, the activated samples from carbon show a very high
Fig. 11. Hydrogen adsorption/desorption isotherms of MWTSC-50-3-700, (*) Batch 1 and ( ) Batch 2 sample studied at 303 K and 4 MPa.
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activation. From surface area and pore size measurements for all the samples reveal that micropores are very important parameter, in addition to high surface area for hydrogen storage compared to mesopores as obtained for thermally treated samples. Acknowledgements The authors would like to acknowledge Dr. G. Sundararajan, Director ARCI for his constant support and encouragement. The financial support from MNRE, Govt of India, New Delhi is acknowledged herewith. References
Fig. 12. Hydrogen storage cyclic stability test at 303 K and 4 MPa.
hydrogen uptake. In addition, these activated carbon materials also show a high hydrogen uptake of 7.7 wt% at 77 K and at 1 MPa [26]. This may be attributed to the high specific surface area and micropore volume of the samples which adsorbs hydrogen by physisorption, while the metal hydrides adsorb hydrogen by chemisorption process [36].In order to confirm the hydrogen storage capacity of these samples, two batch samples of MWTSC50-3-700 were prepared and hydrogen adsorption capacity was measured and found to be in good agreement with previous batch sample as shown in Fig. 11, showing the reproducibility. These activated carbons are amorphous materials with large surface areas and pore volumes. One of the criteria for hydrogen storage materials is their cyclic stability for hydrogen uptake at the said temperature and pressure. Hence the cyclic stability of these samples were also tested for MWTSC-50-3-700 sample upto 30 cycles. The sample showed the same 4.7 wt% of hydrogen uptake at room temperature and 4 MPa for all the 30 cycles and is shown in Fig. 12. This clearly reveals that the activated carbon samples from tamarind seeds after activation is suitable for high hydrogen uptake, reproducibility and cyclic stability for adsorption and desorption. 4. Conclusion In the present work, the hydrogen adsorption and desorption capacity of activated carbons, derived from tamarind seeds, carbonized by both thermal and by microwave treatment, and activated by KOH as an activating agent, are reported. The increase in microwave pyrolysis time of carbonization also increases the hydrogen adsorption. The time taken to obtain highly porous activated carbon sample by microwave treatment was much less compared to thermal treatment. It was observed that tamarind seed based activated carbons treated with KOH gave a hydrogen storage capacity of 4.73 wt% at RT and at 4 MPa, which is 80% of the target hydrogen storage capacity set by USDOE. It was also observed that the process of carbonization namely microwave treatment gives a highly porous carbon compared to thermally treated tamarind seeds. This is reflected in the increased BET surface area and higher total pore volume. The BET surface area and total pore volume of the activated MWSTC-50-3-700 is 1784 m2 g1 and 0.93 cm3 g1. The hydrogen storage capacity was found to increase from 0.54 wt% to 4.73 wt%, due to KOH
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Activated carbons derived from tamarind seeds for hydrogen storage T. Ramesh, N. Rajalakshmi* , K.S. Dhathathreyan Centre for Fuel Cell Technology, International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), IIT-M Research Park, Taramani, Chennai 600113, Tamil Nadu, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 July 2015 Received in revised form 16 September 2015 Accepted 16 September 2015
To make hydrogen economy a reality, the key technical challenges that need to be addressed are Hydrogen generation, transmission and storage for application in fuel cells. Among them, H2 storage presents a major challenge to material scientists to meet the USDOE target of 5.5 wt%. The present paper investigates the hydrogen storage capacity of activated carbons derived from tamarind seeds by thermal, microwave and by chemical KOH activation treatment. The various parameters optimised to obtain high hydrogen storage capacity are KOH concentration, duty cycle of microwave pulsing, carbonization, hydrogen adsorption conditions etc., The surface area, micropore volume, pore size and nitrogen adsorption measurements were determined for all the samples and found that the carbonization temperature and concentration of KOH play a major role in increasing the surface area, micropore volume and pore size. We have obtained a high surface area activated carbon of 1785 m2 g1, micropore volume of 0.94 cm3 g1 and pore size of around 0.8–1.1 nm. The maximum hydrogen storage capacity of these activated carbons from tamarind seeds at RT and 4.0 MPa was found to be 4.73 wt%, which is about 80% of the USDOE target. The samples also show good cyclic stability for hydrogen adsorption and desorption studies. These results suggest that activated carbons fabricated from tamarind seeds with high surface area and micropore volume will be an ideal candidate for hydrogen storage. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Activated carbons Tamarind seeds Hydrogen storage Surface area Micropore
1. Introduction Hydrogen is light and clean fuel, acts as an efficient energy carrier in the hydrogen economy. In recent years, hydrogen infrastructure in terms of production and storage play an important role in the commercialisation of fuel cells. A fuel cell combines hydrogen and oxygen to produce electricity, heat, and water. In order to have on board hydrogen storage to operate fuel cells using hydrogen, storage containers/materials with high volumetric and gravimetric density are preferable. Recently, lightweight carbon adsorbent materials, attracted lot of interest for possible use in a hydrogen storage system [1,2]. None of the storage methods viz., compressed gas, liquefaction, metal hydrides., etc. satisfy the criteria of capacity, size, efficiency, cost and safety. Recently many research groups are focussing on the H2 adsorption/desorption properties of ‘activated’ carbon materials, prepared from various natural precursors. These materials were typically obtained by thermochemical processing and show different types of carbon structures in terms of porosity, density, defect sites etc. These materials are found to be more economically
* Corresponding author. E-mail address: [email protected] (N. Rajalakshmi). http://dx.doi.org/10.1016/j.est.2015.09.005 2352-152X/ ã 2015 Elsevier Ltd. All rights reserved.
attractive compared to compressed hydrogen and metal hydride alloys. These activated carbon materials were found to be an ideal candidate for hydrogen storage medium due to their advantages like light weight, high specific surface area, complete reversibility and low cost. They can be produced in large quantities from a carbon rich precursor by a physical or chemical activation process. Activated carbons (AC) are generally fabricated from biomass resources like oil palm shell [3], almond shell, hazelnut shell [4], coconut shell [5], pistachio shells [6], olive stones [7], dates seeds [8], coffee beans [9], apricot seeds [10], grape seeds [11] etc., and have been studied for various applications. These activated carbons with micro pores, are considered for gas adsorption as well as for hydrogen storage applications due to their high specific surface areas and high adsorption properties [12,13]. These materials are also generally used for the gas separation and purification from polluted water streams. However, the pore structure of these activated carbon is complex and not well defined. Activated carbons (AC) are obtained by chemical activation, where the raw material is impregnated with activating agents like KOH, NaOH, Na2CO3, K2CO3, H3PO4, ZnCl2 etc., and are then heated in an inert atmosphere, where the carbonization and activation occurs simultaneously. The various activating agents increase the porosity and thereby the surface area. During this process, certain dehydrating agents are also employed that influences the
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temperature of pyrolysis decomposition that inhibit the formation of tar, thereby enhancing the yield of AC [14–16]. It has been reported that these properties are essential for gas storage, especially microporosity and high surface area [17]. Two different class of materials viz., zeolites and activated carbons have been studied extensively for gas storage and has been reported that ACs are very good adsorbents with the highest storage capacities for natural gas [18]. Carpetis and Peschka [19] have reported that hydrogen could be stored on porous carbon materials at low temperatures. They have studied many high surface area carbon materials and reported that porous carbon materials tend to adsorb more hydrogen than the denser carbons. Physically activated carbons commonly use bituminous coal or coconut shells as a starting material. High surface areas in excess of 3000 m2 g1 can be obtained by treating them with chemical agents like KOH. In the two-stage activation process of carbonization, the oxygen and hydrogen are burned and the char is heated in steam or carbon dioxide atmosphere to create a highly porous structure. The hydrogen storage capacity also found to increase if the carbon materials possess high specific surface area and micropore volume [20–23]. Although various carbons show a weight capacity of 0.1–7.5 wt%, at 77 K and at 1 MPa, the highest hydrogen storage capacity at RT was not found to be more than 1 wt% at 298 K, 1 MPa, while the targets by DOE for hydrogen storage is 5.5% in 2015. ACs based on rice hull, activated through chemical activation which developed pores and large specific surface area has been studied as a promising material for hydrogen storage. The mechanism for hydrogen storage was found to be the van der Waals forces between the huge surface area of ACs and hydrogen molecules. Hence large specific surface area is found to be beneficial for the storage of hydrogen in the said pressure range and the storage capacity increased with decreasing temperature and increasing pressure. Recently rice hull was studied for its hydrogen storage capacity and found to be 7.7 wt%, at 1.2 MPa and 77 K after carbonization and sodium hydroxide activation. The BET surface area was found to be 3969 m2 g1, using toxic reagents and complicated synthesis procedures [24]. It was observed that among the various agriculture residues, tamarind seeds are found to be interesting, as they are available in various forms viz. seeds, seed husk, Kernel powder etc. The elemental composition along with surface area is given in Table 1. The Tamarind Kernel Powder is normally used in industries like textile, dying and Printing, jute, card board, mosquito coil, food additive, oil well drilling, paper industry etc. [25,26]. The botanical name is Tamarindus indica L. and India is the major producer of tamarind on a large scale. The other countries are Thailand, Indonesia, Myanmar, and the Philippines. Tamarind is a brown pod-like fruit, consists of pulp and hard-coated seeds. The brown coloured seeds are hard, with square shape and rounded corners and edges. The seeds are 30–40%, while the shell, fibre and pulp constitute the rest. The present work aims to prepare activated carbon from tamarind seeds (TSC) by thermal, microwave heating method and to study the influence of chemical activation treatment for the hydrogen storage capacity. The carbon prepared Table 1 Elemental composition of agricultural material seeds. Material
C%
N%
O%
H%
SBET m2 g1
Mango Apricot Mahogang Longan Cherry Grapes Tamarind
60.01 51.5 48.14 46.5 53.9 62.2 51.7
– 0.2 0.28 1.4 0.3 1.6 35.5
– 41.9 45.15 45.4 38.4 28.2 12.4
– 6.3 6.4 6.3 7.1 7.8 -
2435.2 630-1175 – 835 – 485 2673
from microwave using pulsed method has many advantages compared to thermal treatment as it decreases the carbonization temperature as well the time required for the whole process. The samples were characterized by scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), IR etc. We have observed an increased surface area and micropore volume with KOH chemical activation. The hydrogen storage capacity of TSC was measured at various pressures ranging from 0.1 to 4 MPa and at RT and found that the hydrogen storage capacity increases from 1 to 4.6 wt%, while the surface area increases from 750 to 1775 m2 g1 for the sample of microwave treated and microwave treated with chemical activation respectively. These samples also showed cyclic stability for hydrogen adsorption and desorption. 2. Methods The tamarind (T. indica) seeds were used as a precursor for the preparation of activated carbon. The tamarind fruits were purchased from local market in Chennai. Seeds were removed from the fruit, washed and dried. Dried seeds were processed to obtain activated carbon as described below. The analytical grade potassium hydroxide (KOH) pellets were purchased; High pure nitrogen gas (99.999%) was used for inert atmosphere during activation. 2.1. Synthesis of activated carbon Carbonization of tamarind seeds were done by both microwave and thermal treatment. The microwave oven of power 900 W at 2.4 GHz in a partially closed system in air atmosphere was used for microwave carbonization. A handful of tamarind seeds were taken in a cleanpetri dish. The seeds were cleaned with water and were rinsed with 1-propanol, and were covered with another petri dish. The seeds were carbonised in ‘microwave’ mode for 30 s and switched off for 15 s interval. This process was repeated for 50 times. The process was repeated for 50, 75 and 100 cycles to make carbon from tamarind seed (1 cycle = 30 s ON + 15 s OFF). The prepared carbons were labelled as MWTSC-50, MWTSC-75 and MWTSC-100. The carbonized seeds were then crushed to powder using a hand held rock crusher. The crushed powder was put in a thimble made of thick filter paper. The impurities were extracted by Soxhlet extraction using iso Propanol at 80 C. All the samples were dried in an oven to remove the moisture. The carbons were activated in order to increase the surface area as well to increase the pore structure. The sample MWTSC-50 was activated to increase the surface area by treating them with KOH in the ratio of 1:3 and sufficient amount of deionised water in a plastic container. The slurry was stirred for about 16 h at ambient temperature, and then heated to desired activation temperature of 700 C in an alumina crucible for 1 h under flowing nitrogen gas. After completion of activation, it was cooled down to room temperature [27]. The resultant product was collected by washing with deionised in soxhlet extraction setup at 100 C until the filtrate was free from alkali and it was labelled as MWTSC-50-3-700. Thermal treatment of tamarind seeds was done by using tubular furnace at 300 C for 1 h in the Nitrogen flow of 100 ml min1. The activation was performed at 500 C for 1 h and the KOH/carbon ratio was varied from 1:1, 1:2 and 1:3. All the samples were powdered well, washed with deionised water and dried. The samples are labelled as TSC-1, TSC-2 and TSC-3, and the schematic is shown in Fig. 1. 2.2. Characterization A typical carbon sample obtained from tamarind seeds are characterized by Rigaku XRD unit and SEM by Hitachi SU1510, for
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Fig. 2. XRD pattern of MWTSC-50-3-700. Fig. 1. Synthesis of activated carbon from tamarind seeds using microwave and thermal methods.
morphology studies. The textural properties of the samples were studied by N2-sorption at 77 K with automatic instrument ASAP2020, Micromeritics, over a wide relative pressure range from about 106 to 0.995. Prior to the measurements, the samples were degassed at 250 C for 24 h. The specific surface area was measured by standard BET method using adsorption data in the relative pressure ranging from 0.02 to 0.25. The total pore volume was calculated by converting the amount of N2 adsorbed at a relative pressure (P/P0) of 0.99 to the volume of liquid. The micropore volume was calculated by the t-plot equation. Pore size distributions (PSD) were calculated by using the Density Functional Theory (DFT) Plus Software, which is based on calculated adsorption isotherms for pores of different sizes. In the DFT calculations, the equilibrium model of carbon slit-shaped pores was applied [28]. Thermogravimetric (TGA) and differential thermal analysis (DTA) measurements were carried out using a NETZSCH STA 449 F1 Jupiter (Netzsch, Germany) in the range from 25 to 1000 C at a heating rate of 100 C min1. The measurements were carried out in an open alumina crucible under air atmosphere at a flow rate of 40 ml min1. An empty, open crucible was used as a reference [29].
3. Results and discussion The XRD of activated carbon from tamarind seeds is shown in Fig. 2 and shows that MWTSC-50-3-700 sample is semi crystalline in nature as reported [31]. Two main diffraction peaks for the (0 0 2) and (1 0 1) planes of graphite structure at 2u = 23.36 and 43.03 are found in MWTSC-50-3-700. The SEM image of MWTSC50-3-700 sample is shown in Fig. 3 and one can see that the pores were uniformly distributed. Thermo gravimetric analysis was performed in air atmosphere at a heating rate of 10 C min1. The TGA of all carbon samples prepared from tamarind seeds are shown in Fig. 4. From Fig. 4, one can observe a weight loss of 10% within 100 C, indicating the evaporation of water and organic solvents in the sample. All the samples were stable up to 300 C, and further increase in temperature decreases mass of the sample due to decomposition. All the samples exhibit same kind of decomposition with a minor change in the complete decomposition temperature, and the residual mass for microwave treated samples were found to be less than 5% at higher temperatures. The surface area was measured for all these samples and the N2 adsorption isotherm is shown in Fig. 5 for MWTSC-50 and MWTSC50-3-700. From Fig. 5 one can see that
2.3. Hydrogen adsorption studies Hydrogen adsorption/desorption studies were performed for the samples microwave treated and activated carbon sample in the pressure ranges 0–4 MPa and at 303 K using a high-pressure Automated Sievert’s apparatus from Advanced Material Corporation, USA. The leakage test of the instrument has been carried out with high pure hydrogen (99.999%) at high pressure before starting the experiment. Initially all samples were evacuated at 103 mbar for 6 h at 423 K. After cooled down to room temperature hydrogen adsorption measurements were performed at 303 K over the pressure range of 0–4 MPa. The hydrogen adsorption measurements were carried out by passing the hydrogen from a calibrated vessel at a particular pressure and then the hydrogen was allowed to the sample chamber of known volume. The pressure composition isotherms were obtained by change in volume of the sample at a particular pressure. The experiments were repeated many times to check the reproducibility of the samples. Tap density of material was used for hydrogen adsorption/desorption calculation [30].
Fig. 3. SEM image of MWTSC-50-3-700.
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Fig. 4. TGA curve of MWTSC-50, MWTSC-75, MWTSC-100 and MWTSC-50-3-700.
Fig. 6. Pore size distribution of MWTSC-50-3-700.
the N2 adsorption isotherm of MWTSC-50 sample may be attributed to the type-III [32] isotherm and MWTSC-50-3-700 may be attributed to type-I isotherm [33]. These isotherms indicate that the MW treated sample is non-porous while, sample treated with KOH is of microporous nature. The carbon obtained using KOH treatment at 700 C shows a high BET specific surface area of 1784 m2 g1. Fig. 6 shows the pore size distribution of activated carbons MWTSC-50-3-700, revealing that the pores are in the range 0–2 nm which are microporous. The total pore volume and micropore volume was found to be 0.93 cm3 g1 and 0.62 cm3 g1. The micropores are not seen in all the microwave carbonized samples viz., MWTSC-50, MWTSC-75 and MWTSC-100. Similarly the N2 adsorption isotherms and pore size distribution of TSC-1, TSC-2 and TSC-3 samples are given in Figs. 7 and 8, respectively. As one can see from the figures that the surface area was found to increase from 353 to 785 m2 g1 and micropore volume from 0.14 to 0.31 cm3 g1. These samples also showed mesopores in the range 2–10 nm compared to microwave treated samples which showed micropores in the range 0–2 nm. These results indicate that activation of carbon performed using KOH at 700 C developed the pore structure in microwave treated tamarind seeds and increased the surface area with micropores suitable for hydrogen storage.
The hydrogen adsorption study was carried out using Sievert’s apparatus and Pressure was measured when hydrogen was introduced to the system containing known weight of carbon material (P1). Before carrying out the experiment, the system was checked for its leakage under both vacuum and at high pressure at 473 K for 24 h. A hydrogen uptake study, without the sample, was also carried out at 8 MPa and at RT. The study showed no hydrogen adsorption within the limits of the lowest count of the instrument. The sample tube and the connecting tubes were evacuated and left under the vacuum for 8 h. There was no change in the pressure. Finally the tube was filled with gas at 6 MPa and left for 4 h. After ascertaining zero leakage, the instrument was filled with known weight of carbon. The sample tube was evacuated and heated to 473 K for 24 h. When no change in the pressure was observed, the main experiment started as mentioned above. The system was left for 24 h to attain the equilibrium. After 24 h the decrease in pressure was measured (P2). The amount of hydrogen adsorbed by carbon material was calculated from the pressure difference, i.e. from (P1–P2). As the volume of the system and weight of carbon were known, the amount of hydrogen in the unit of wt% was calculated. The quantity of hydrogen adsorbed was calculated using the general gas law as modified by Van der Waal's (Van der
Fig. 5. N2-sorption isotherms at 77 K of ( ) MWTSC-50 and (&) MWTSC-50-3-700.
Fig. 7. N2-sorption isotherms at 77 K of ( ) TSC-1, ( ) TSC-2 and (!) TSC-3.
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Table 2 Hydrogen uptake of microwave treated tamarind seed at different temperature & 6 MPa. Sample
MWTSC-50 MWTSC-75 MWTSC-100
ON time (s)
30 30 30
OFF time (s)
15 15 15
Hydrogen storage at 6 MPa 303 K
323 K
348 K
373 K
0.67 1.19 1.36
0.73 0.7 0.88
0.27 1.02 1.14
0.29 0.96 0.81
Fig. 8. Pore size distribution of MWTSC-50-3-700.
Waal's gas law), using the following equation. " 2 # n ðV r nbÞ ¼ nRT Pr þ a Vr The constants ‘a’ and ‘b’ have positive values and are characteristic of the individual gas. For hydrogen gas ‘a’ is 2.45 102 Pa m3 and ‘b’ is 26.61 1026 m3/mol. Hydrogen adsorption was measured by using hydrogen upto 6 MPa pressure and all the experiments have been carried out a minimum of 3–5 times to ascertain the reproducibility. The hydrogen storage capacities of these samples were measured at RT and at various pressures. The hydrogen storage capacity of TSC-1 is shown in Fig. 9 and found that the maximum hydrogen storage capacity is 0.08 wt% at RT and at 0.8 MPa. Similarly the hydrogen storage capacities of MW treated samples are measured at various temperatures ranging from RT to 373 K and they are given in Table 2. From Table 2, one can see that the hydrogen adsorption capacity was found to be 1.36 wt% at RT and at 6 MPa. Hydrogen adsorption capacity was also measured for the KOH activated sample viz., MWTSC-50-3-700 and the results are given in Fig. 10, along with hydrogen storage capacity for all the MW treated samples viz., MWTSC-50, MWTSC-75 and MWTSC100. From Fig. 10, one can see that the hydrogen storage capacity increases with increase in microwave pyrolysis duty cycle and found to be 0.54 wt%, 0.73 wt% and 1.00 wt% for MWTSC-50,
Fig. 9. Hydrogen uptake of TSC-1 at different temperature and 0.8 MPa.
Fig. 10. Hydrogen adsorption/desorption isotherms of samples studied at 303 K and 4 MPa.
MWTSC -75 and MWTSC -100, respectively at 303 K and 4 MPa. However, the hydrogen storage capacity of the KOH activated sample showed a very high capacity of 4.73 wt% at 303K and 4 MPa. From the literature, we have observed that LaNi5 alloy showed 1.3 wt% of hydrogen uptake at room temperature and at 1 MPa [34] while the FeTi alloy showed a hydrogen adsorption capacity of 1.02 wt% under the same conditions [35]. Hence when these hydrogen storage capacity values are compared with activated carbons, the activated samples from carbon show a very high
Fig. 11. Hydrogen adsorption/desorption isotherms of MWTSC-50-3-700, (*) Batch 1 and ( ) Batch 2 sample studied at 303 K and 4 MPa.
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activation. From surface area and pore size measurements for all the samples reveal that micropores are very important parameter, in addition to high surface area for hydrogen storage compared to mesopores as obtained for thermally treated samples. Acknowledgements The authors would like to acknowledge Dr. G. Sundararajan, Director ARCI for his constant support and encouragement. The financial support from MNRE, Govt of India, New Delhi is acknowledged herewith. References
Fig. 12. Hydrogen storage cyclic stability test at 303 K and 4 MPa.
hydrogen uptake. In addition, these activated carbon materials also show a high hydrogen uptake of 7.7 wt% at 77 K and at 1 MPa [26]. This may be attributed to the high specific surface area and micropore volume of the samples which adsorbs hydrogen by physisorption, while the metal hydrides adsorb hydrogen by chemisorption process [36].In order to confirm the hydrogen storage capacity of these samples, two batch samples of MWTSC50-3-700 were prepared and hydrogen adsorption capacity was measured and found to be in good agreement with previous batch sample as shown in Fig. 11, showing the reproducibility. These activated carbons are amorphous materials with large surface areas and pore volumes. One of the criteria for hydrogen storage materials is their cyclic stability for hydrogen uptake at the said temperature and pressure. Hence the cyclic stability of these samples were also tested for MWTSC-50-3-700 sample upto 30 cycles. The sample showed the same 4.7 wt% of hydrogen uptake at room temperature and 4 MPa for all the 30 cycles and is shown in Fig. 12. This clearly reveals that the activated carbon samples from tamarind seeds after activation is suitable for high hydrogen uptake, reproducibility and cyclic stability for adsorption and desorption. 4. Conclusion In the present work, the hydrogen adsorption and desorption capacity of activated carbons, derived from tamarind seeds, carbonized by both thermal and by microwave treatment, and activated by KOH as an activating agent, are reported. The increase in microwave pyrolysis time of carbonization also increases the hydrogen adsorption. The time taken to obtain highly porous activated carbon sample by microwave treatment was much less compared to thermal treatment. It was observed that tamarind seed based activated carbons treated with KOH gave a hydrogen storage capacity of 4.73 wt% at RT and at 4 MPa, which is 80% of the target hydrogen storage capacity set by USDOE. It was also observed that the process of carbonization namely microwave treatment gives a highly porous carbon compared to thermally treated tamarind seeds. This is reflected in the increased BET surface area and higher total pore volume. The BET surface area and total pore volume of the activated MWSTC-50-3-700 is 1784 m2 g1 and 0.93 cm3 g1. The hydrogen storage capacity was found to increase from 0.54 wt% to 4.73 wt%, due to KOH
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