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Conversion of model biomass to carbon-based material with high conductivity by using carbonization
Energy 188 (2019) 116089
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Conversion of model biomass to carbon-based material with high conductivity by using carbonization Melih Soner Celiktas*, Fikret Muge Alptekin Solar Energy Institute, Biomass Energy Systems & Technology Center, Ege University, 35100, Bornova, Izmir, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2019 Received in revised form 2 September 2019 Accepted 7 September 2019 Available online 9 September 2019
Biomass materials are renewable sources that abundant worldwide due to natural plants and living organisms. Lignocellulosic biomass can be categorized as hardwood, softwood, agricultural wastes, and grasses. Agricultural residues those which of them have importance due to being produced in huge amounts in the worldwide annually. Food wastes and agricultural wastes are utilized either alternative use such as generating energy, fuels or disposal. However, disposal of these residues is follow out either scraping or burning way. This study aims to convert industrial agricultural origin biomass by using hydrothermal carbonization method to carbon-based material having high conductivity for use in supercapacitor production by using different activating chemicals. Hydrothermal carbonization was applied to different biomass samples such as nutshell, hazelnut shell, and corn cob. The elemental analysis of the obtained biochar was carried out and it was determined that the highest source of biomass was corn cob. The selected biochar has been chemically activated with different chemicals such as KOH, NaOH, H3PO4 and, ZnCl2. Advanced carbonization of activated biochar was carried out at 500, 600, 700 and 800 C with 1, 1.5 and 2-h retention times. The resulting carbon-based products were mixed with KBr and identical pellets were prepared and their electrical conductivity values were measured. Electrical conductivity results, NaOH-800 C-2h and ZnCl2-700 C-1.5 h obtained from the process prepared from the biocidal pellets were determined to have the highest conductivity value. BrunauereEmmetteTeller (BET) and Scanning Electron Microscope (SEM) analyses of the samples with the highest conductivity values were performed and surface morphologies were examined. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Supercapacitor Advanced material Electrical conductivity Carbon-based material
1. Introduction For human development energy is a crucial issue. Fossil-based energy consumption and production affect both the World economy and ecology [1]. By increasing the cost of fossil fuels that are limited sources, pollution, global warming, and geopolitical concerns became a serious problem that modern societies have met last years [2]. To overcome these problems, sustainable carbon-free or low-carbon energy sources and energy storage systems should be increased to solve energy concern [3]. Development of sustainable and renewable energy sources such as wind and solar energy have become crucial sources because of
* Corresponding author. Tel.: þ90 232 3111110; Fax: þ90 232 3886027 E-mail address: [email protected] (M.S. Celiktas). https://doi.org/10.1016/j.energy.2019.116089 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
these problem concerns. Though energy storage devices are required since these energy sources show intermittent properties in nature [4]. As a consequence, energy storage and accumulation is an essential part of renewable energy sources [5]. Among the various alternative energy storage technologies, supercapacitors are one of the most promising electrochemical energy storing devices [6]. A supercapacitor (SC) which is an energy storage device has several advantages such as having a high power density, fast charging and discharging opportunity and long life cycle when it is compared to batteries and capacitors [7e9]. Nowadays, SC as an energy storage devices is used in a wide variety of applications like wind power, tidal energy, solar energy, start-up and emergency power supply of heavy machinery [10], and in fuel cells [11e13]. An electrode is the most crucial part of a supercapacitor. Metal oxides, polymers, and carbon materials are the most widely used
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materials for supercapacitor electrode. Having a high surface area, good thermal and electric conductivity, high flexibility [14] the low production cost of activated carbon make its more appropriate materials to prepare electrode for supercapacitor. Due to all these properties, carbonaceous materials and activated carbon have been widely investigated by researchers [15,16] to be an electrode of supercapacitor [17]. Activated carbon which has a large specific area, porosity, high physicochemical stability, and high surface reactivity is a carbonaceous material that enables users for different applications. Several kinds of biomass such as waste coffee beans [18e22], apricot shell, sugarcane bagasse, sewage sludge and sawdust [20e24], rice husk [15,25] feedstock are commonly used for producing activated carbon that is a low-cost and sustainable alternative precursor to fossil-based. There are different kinds of research which agricultural wastes are used as feedstock to produce carbon-based material for supercapacitor production in the literature. Rufford et al. [18] produced activated carbon by using coffee grounds by applying ZnCl2. The findings showed that the supercapacitor electrode which was prepared from waste coffee ground carbon presented good energy densities and stability at high charge-discharge rates. The other study was carried out by Ismanto et al. [17]. They used Cassava peel as feedstock for producing activated carbon to prepare carbon-based electrodes to use in supercapacitor application. Electrodes were prepared by using both chemical and physical activation. Four different types of oxidative chemical agents were used in the study showed that HNO3 modified activated carbon electrode presented 72.6% increase of specific capacitance when compared to the initial one. Xu et al. [19] studied generating activated carbon from an apricot shell by using NaOH activation. They investigated the effects of carbonization temperature on the carbon porosity and electrode density. According to their finding, 500 C temperature gives both high gravimetric capacitance (339 F g-1) and 0.504 g cm-3 of high electrode density. Due to their capability of converting biomass to biochar, various thermochemical conversion applications have been used by researchers [26,27]. Pyrolysis, hydrothermal carbonization, flash carbonization, and gasification can be given as examples for thermochemical conversion methods that have many benefits of producing biochar to be used for carbon sequestration, soil amendment, soil productivity improvement and pollution control [28]. The objectives of the study is that determines the highest carbon content of biomass of agricultural origin, show the influence of process variables such as chemical agents, carbonization temperature, retention time, the effect of using hydrothermal carbonization method and obtain carbon-based material with high conductivity for use in the production of a supercapacitor. In this study, by using hydrothermal carbonization method, it is aimed to convert the biomass of agricultural origin to a carbonbased material with high conductivity for use in the production of a supercapacitor as a result of treatment with different activating chemicals.
2.1. Pretreatment Nutshells, hazelnut shells, and corn cobs were washed with deionized water to eliminate impurities and dust resulting from the accumulation of materials. After washing, the materials were dried at 105 C for 24 h. The purified and dried materials were ground to small particles and sieved. 1 mm particulate size was selected for this study. 2.2. Proximate analysis The analyses of moisture, ash content, volatile material, and fixed carbon were conducted for selected materials. The elemental analysis of the materials was carried by Elemental Analyzer Tuespec CHN Leco to determine carbon (C), hydrogen (H), and nitrogen (N) contents of samples at pure oxygen atmosphere. The oxygen content of the samples was calculated by a simple calculation from obtained C, H and N [29]. Also, the elemental analysis was done for samples that were subjected to hydrothermal carbonization. The samples were named related to their material, residence time as NS (NS-2-4-6-8 h), HS (HS-2-4-6-8 h) and CC (CC-2-4-6-8 h). The analyses were carried out with two replicates for each sample. The moisture content of materials was analyzed with AND MX50 analyzer. In ash content analysis, approximately 2 g of each sample was weighed and dried at 105 C for 1 h. Ash content analysis was carried out by MAGMA THERM brand muffle furnace. The samples were placed to a suitable number of crucibles. The crucibles which are identified with porcelain marker were places to the muffle furnace at 550 C (575 ± 25 C) for 3 h. After burning was completed, the crucibles from the furnace were removed directly to desiccator for cooling. The ash contents of samples were calculated by using Eq. (1).
Ash ð%Þ ¼
g1 *100 g2
(1)
where; g1: the weight of ash, (g) g2: the weight of dried samples that in the furnace, (g) Volatile analysis, 1 g-weighted sample that was dried at openair were burned at 950 C for 7 min. The processed samples were cooled at desiccator and then measured. The volatile matter amount of the samples were calculated by Eq (2).
Volatile Matter Amount ð%Þ ¼
g1 g2 M *100 g1
(2)
where; g1: Weight of used sample, (g) g2: Weight of sample after drying, (g) M: Moisture percent of the used sample Fixed carbon amount, was calculated as Eq (3).
2. Material and method
Fixed carbon ð%Þ ¼ 10 ðmoisture þ ash þ volatile matterÞ (3)
Three different kinds of agricultural-based biomass sources were used to produce carbon-based material to produce a supercapacitor. These were; nutshell, hazelnut shell, and corn cob. The hazelnut shells were supplied from the Gençler Fındık San. Ve Tic. A.S. The nutshells and corn cob was supplied from a local bazaar in _ Izmir. In Fig. 1, the method of the study is demonstrated.
2.3. Hydrothermal carbonization (HTC) Hydrothermal carbonization was carried out in a 100 ml
M.S. Celiktas, F.M. Alptekin / Energy 188 (2019) 116089
3
Fig. 1. The flow process chart of the study.
stainless steel reactor. Prepared materials were subjected to carbonization process at 250 C for 2, 4, 6 and 8 h in PLC brand drying oven which maximum of operating temperature is 250 C and it has constant temperature function. Weighed 10 g biomass and 90 ml deionized water were put in the reactor and then sealed. After then, the carbonization process was conducted for determining temperature values and different reaction times for each material. The experiments were carried out with two replicates. After processes were ended up C, H, N and O contents of the materials were determined from produced carbonbased materials. According to the findings, corn cob was determined to have the highest carbon content. Corn cob which has the highest carbon content was conducted to chemical activation and consecutively carbonization processes. 2.4. Chemical activation process The corn cob was activated with potassium hydroxide (KOH), sodium hydroxide (NaOH), phosphoric acid (H3PO4) and zinc chloride (ZnCl2). The ratio of chemical to the obtained carbonized material corn cob was determined as 4:1 [30e32]. Following the determined ratio, solutions were prepared for each chemical. To prepare a solution with KOH, NaOH, and ZnCl2 which were solid, the mixer was used to provide soluble chemicals in the water. Sufficient amount of three chemicals were measured and put into a volumetric flask and then added deionized water to prepare a solution. For aqueous H3PO4, the ratio of chemical/carbon-based material was calculated to prepare a solution. For 2 g each carbonized sample, 50 ml prepared solution was used. The samples were activated by separately NaOH, H3PO4, and ZnCl2 for 24 h with continuous stirring.
Chemical activation, differing from the other three chemicals, was performed by continuous stirring with KOH for 40 min [33]. After the activation process completed, activated samples were filtered with filter paper and kept in the furnace at 105 C for 24 h to evaporate their moisture. 2.5. Post carbonization The post carbonization of the samples which were chemically activated was carried out in Protherm Furnaces according to experiment list that was obtained from Design Expert software (version 7.0). The samples that were activated with KOH, NaOH, H3PO4, and ZnCl2 were carbonized at 500, 600, 700, and 800 C for 1, 1.5 and 2 h under continuous N2 gas flow in a horizontal furnace. After the carbonization process, the samples were washed with deionized water and then dried at for 24 h to evaporate their moisture content. The solid conversion yield of the samples was calculated from Eq (4).
m Conversion yield ð%Þ ¼ 100 m1 2 *100 m1
(4)
where; m1: activated sample, (g) m2: the samples which obtained after carbonization process, (g)
2.6. Electrical conductivity analysis In order to measure electrical conductivity value, pellets were
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prepared with the carbonized material and potassium bromide (KBr). 30 mg carbon-based material and 770 mg KBr were mixed homogeneously and pressed at Specac Hydraulic Press at a pressure of 5 N. They were converted to a pellet that has 10 mm Radius and 0.38 mm height. The analysis of electrical conductivity was carried out by a 4probe analyzer. The pellets were delivered to the probe from a source of direct current (DC) by Keithley 2400 model. From Eq (5), electrical specific resistivity and Eq (6), electrical conductivity value was calculated from obtained electrical resistivity values
p ¼ R*
A I
(5)
where; R: electrical resistivity, (R, U) A: area of pellet, (mm2) I: current density, (mA)
1 s¼ p
3. Results and discussion 3.1. Proximate analysis As a proximate analysis, moisture, ash content, volatile matter content, fixed carbon and elemental analysis of biomass sources were carried out. The proximate analysis of biomass sources is shown in Fig. 2. As a result of the analysis, the moisture content of nutshell, hazelnut shell, and corn cob was determined as 9.04%, 6.05%, and 6.98%, respectively. As a result of the analysis, the highest volatile matter content was found in a nutshell with 15.98%. When the results were investigated, the lowest volatile matter content was found in a corn cob. 3.2. Elemental analysis
(6)
where;
s: electrical conductivity, (Scm1)
2.7. Characterization The morphology of the obtained carbon-based samples was characterized by scanning electron microscope (SEM, Thermo Scientific Apreo S). Nitrogen adsorption and desorption isotherms of 8 samples that have the highest electrical conductivity value were carried out by Micromeritics brand Gemini 2369 model supported with computer device at liquid nitrogen gas (N2) environment. Before beginning the analysis, the samples were exposed to gas at 100 C for 24 h. The specific surface area was calculated using the Brunauer-Emmet-Teller (BET) method and porosity distribution was determined using the Barrett-Joyner-Halenda (BJH) method.
Pre elemental content of biomass sources is given in Table 1. The elemental analysis results of biomass that were subjected to carbonization at 250 C and different residence times are given in Table 2. According to the results, the percent carbon content of biomass sources increased through carbonization process with carrying out different residence time. After the carbonization process, the samples had a carbon-rich content while the hydrogen and oxygen content of the samples decreased [34]. The obtained results showed that with increasing carbonization time, the carbon yield increased
Table 1 Pre elemental analysis of biomass sources. Biomass
C (%)
H (%)
N (%)
O (%)
HS_1 HS_2 NS_1 NS_2 CC_1 CC_2
50,24 50,19 50,21 50,08 45,19 45,21
6,14 6,10 6,07 6,02 6,17 6,17
0,07 0,08 0,56 0,52 0,26 0,28
43,62 43,71 43,72 43,9 48,64 48,62
Fig. 2. The proximate analysis of biomass sources.
M.S. Celiktas, F.M. Alptekin / Energy 188 (2019) 116089 Table 2 The elemental analysis of biomass sources after carbonization process with different residence time. Biomass
C (%)
H (%)
N (%)
O (%)
HS_2H1_1 HS_2H1_2 HS_2H2_1 HS_2H2_2 HS_4H1_1 HS_4H1_2 HS_4H2_1 HS_4H2_2 HS_6H1_1 HS_6H1_2 HS_6H2_1 HS_6H2_2 HS_8H1_1 HS_8H1_2 HS_8H2_1 HS_8H2_2 NS_2H1_1 NS_2H1_2 NS_2H2_1 NS_2H2_2 NS_4H1_1 NS_4H1_2 NS_4H2_1 NS_4H2_2 NS_6H1_1 NS_6H1_2 NS_6H2_1 NS_6H2_2 NS_8H1_1 NS_8H1_2 NS_8H2_1 NS_8H2_2 CC_2H1_1 CC_2H1_2 CC_2H2_1 CC_2H2_2 CC_4H1_1 CC_4H1_2 CC_4H2_1 CC_4H2_2 CC_6H1_1 CC_6H1_2 CC_6H2_1 CC_6H2_2 CC_8H1_1 CC_8H1_2 CC_8H2_1 CC_8H2_2
53,63 53,57 55,41 55,13 69,94 70,15 68,88 69,50 69,15 68,94 70,27 70,33 69,55 69,21 70,86 70,76 65,46 65,67 63,15 63,24 70,23 69,83 70,21 69,89 70,27 67,90 69,48 70,77 70,10 70,37 70,98 69,95 58,96 60,73 56,13 56,56 70,25 69,66 70,29 70,73 70,93 72,05 72,04 72,06 73,61 71,00 71,51 72,32
5,71 5,74 5,65 5,57 4,87 4,95 4,93 4,93 4,95 4,92 4,92 5,01 4,77 4,84 4,90 5,01 5,24 5,30 5,64 5,65 5,08 5,03 5,10 5,08 5,06 0,84 9,75 5,01 5,09 5,10 5,09 5,03 5,55 5,52 5,77 5,74 5,19 5,18 5,01 5,07 5,12 5,17 5,05 5,07 5,45 5,22 4,93 5,01
0,01 0,01 0,05 0,01 0,18 0,16 0,18 0,18 0,21 0,22 0,14 0,15 0,16 0,16 0,19 0,20 0,04 0,08 0,66 0,67 0,13 0,12 0,18 0,20 0,16 0,18 0,20 0,15 0,11 0,08 0,13 0,08 0,34 0,35 0,18 0,31 1,09 1,07 0,73 0,73 1,21 1,21 1,06 1,13 1,61 1,52 0,59 0,70
40,66 40,69 38,94 39,3 25,19 24,9 26,19 25,57 25,9 26,14 24,81 24,66 25,68 25,95 24,24 24,23 29,3 29,03 31,21 31,11 24,69 25,14 24,69 25,03 24,67 31,26 20,77 24,22 24,81 24,53 23,93 25,02 35,49 33,75 38,1 37,7 24,56 25,16 24,7 24,2 23,95 22,73 22,91 22,87 20,94 23,78 23,56 22,67
5
pore-size distribution, and electrical conductivity [21]. In order to obtain high capacitance, carbon based material must be subjeced to appropriate activation. Porous network structure is responsible fort he performance of the supercapacitor and in order to improve performance, micro and mesoporous structure is needed. The porous structure and surface area improved by the application of chemical activation significantly contribute to the improvement of the capacitance performance [28]. The combustion data and combustion yields of the samples which were subjected to chemical activation and then advanced carbonization process are given in Table 3. The samples subjected to carbonization at 500 C from the samples which had been carbonized at different temperatures at the same holding times yielded more solid product yield. By increasing carbonization temperature, decreasing of solid product yield occurred. By evaluation of samples after the carbonization process, the highest carbon yields were observed in samples which were activated with ZnCl2 and H3PO4. H3PO4 is a chemical that is widely used in order to produce activated carbon from lignocellulosic materials. It is integrated into the internal wall of the cell of lignocellulosic materials in the carbonization process. Thanks to that, biomass material turns into a solid product with developed porosity structure. Due to the effect of H3PO4 hydrolysis and dehydration occurring above 300 C, the solid product amount increases [35]. On the other hand, zinc chloride increase porosity development of carbon-based materials by increasing thermal energy during the thermal process. Besides, by fixing volatile materials it helps to increase carbon yield [36]. In chemical activation by ZnCl2, it behaves as a dehydrating agent that effects pyrolytic decomposition and prevents tar formation. As a consequence, a higher yield of carbon is produced when compared to biochars [37]. Briefly, the core parameters for carbonization can be summarized as follows; Carbonization temperature has an important effect on carbon yield. Increasing temperature concluded with decreasing carbon yield. Chemical agent influence carbon yield. Chemical activation with ZnCl2 and H3PO4 have resulted in the highest carbon yield for every each temperature and retention time. Retention time also affects carbon yield that combined with a high level of temperature gave the lowest carbon yield.
3.4. The results of electrical conductivity for all the samples. It was determined that the highest carbon yields were achieved with 8 h carbonization time for all the samples. According to the results of elemental analysis, corn cob showed the highest % C content in selected biomass sources. It was determined through proximate analysis, corn cob had 45.20% carbon yield at the beginning. After 8 h of carbonization process, this was increased by approximately 60%, and it reached to 71.92%. While hazelnut and walnut had the same carbon content higher than that of corn cob, corn cob showed increasing carbon content with increasing detention time. When their carbon content was compared, corn con was determined as the main biomass source which was subjected to the chemical activation and advanced carbonization processes. 3.3. Combined combustion yields as a result of advanced carbonization Carbon sources and activation conditions specify the electrochemical performance which related to surface area of carbon,
The electrical conductivity results of samples that were conducted to different chemical activation in different residence time and at different temperature are shown in Table 4. According to the data of electrical conductivity measurements, the highest conductivity value was achieved with NaOH/800-2 h as 4.24 103 S/ohm. When examined the other electrical conductivity values, 2,31 103 S/ohm of ZnCl2/700-1,5 h, 1,85 103 S/ ohm of KOH/800-1,5 h, 1,35 103 S/ohm of ZnCl2/700-1 h, 1,17 103 S/ohm of KOH/800-2 h, 16 103 S/ohm of ZnCl2/8002 h, 7,51 10-4 S/U of H3PO4/800-1 h and 5.70 104 S/ohm of ZnCl2/800-1.5 h were gained. When results were examined, it is observed that electrical conductivity increased by increasing carbonization temperature. Additionally, the samples which were activated with different chemicals and different residence times gave varied electrical conductivity values. The highest electrical conductivity values were achieved at high carbonization temperature (700 and 800 C). Similar results were observed at Kennedy et al.'s study. Kennedy et al., [38], were examined electrical conductivity of activated
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Table 3 Combustion data and combustion yields obtained as a result of advanced carbonization. Chemically Activated Samples
Temperature ( C)
Time (s)
Sample (g)
After advanced carbonization (g)
Carbon Yield (%)
KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4
500 500 500 500 600 600 600 600 700 700 700 700 800 800 800 800 500 500 500 500 600 600 600 600 700 700 700 700 800 800 800 800 500 500 500 500 600 600 600 600 700 700 700 700 800 800 800 800
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
2,09 2,03 2,10 2,06 2,03 2,05 2,10 2,02 2,01 2,05 2,03 2,04 2,02 2,01 2,02 2,05 2,02 2,05 2,03 2,04 2,00 2,14 2,08 2,04 2,07 2,07 2,02 2,07 2,11 2,09 2,02 2,00 2,02 2,00 2,09 2,03 2,04 2,05 2,06 2,11 2,00 2,03 2,05 2,02 2,04 2,09 2,04 2,03
0,44 0,26 1,42 1,35 0,47 0,33 1,01 1,16 0,47 0,67 0,77 1,06 0,359 0,692 0,707 1096 0,65 0,42 1,49 1,31 0,874 0,502 1102 1444 0,46 0,45 0,73 1,01 0,504 0,666 0,648 0,980 0,68 0,65 1,46 1,40 0,67 0,74 1,27 1,22 0,539 1031 0,780 1110 0,41 0,27 0,74 0,95
21,05 12,81 67,62 65,53 23,15 16,09 48,09 54,42 23,38 32,68 37,93 51,96 17,78 34,43 35,19 53,51 32,18 20,48 73,40 64,22 43,73 23,44 53 70,79 22,22 21,74 36,13 47,26 23,91 31,86 32,10 49,02 33,66 32,5 69,85 68,97 32,84 36,10 61,65 57,82 26,94 50,78 38,06 54,96 20,10 12,92 36,27 46,80
Table 4 The results of electrical conductivity of samples. Sample
R (ohm/sq)
Res (ohm-cm)
Electrical conductivity (ohm-1)
NaOH/800-2 h ZnCl2/700-1,5 h KOH/800-1,5 h ZnCl2/700-1 h KOH/800-2 h ZnCl2/800-2 h H3PO4/800-1 h ZnCl2/800-1,5 h
114 209 262 358 413 417 644,5 849
16,01 29 36,93 50 58,23 59 90,74 120
4,24 103 2,31 103 1,85 103 1,35 103 1,17 103 1,16 103 7,51 104 5,70 104
carbon that was supplied from rice husk at the different pressures. They decided to electrical conductivity value of activated carbon which was obtained at 700, 800 and 900 C changed with increasing temperature. 3.5. The result of SEM and BET analyses ZnCl2, H3PO4, KOH, K2CO3, etc. are chemicals that can be used as
activators [33]. According to electrical conductivity results, 8 samples with the best electrical conductivity were investigated. Fig. 3 shows the scanning electron microscopy images which could display the morphology and microstructure of the samples. Fig. 3 a and b, the sample which was activated by NaOH and carbonized at 800 C for 2 h. Two factors affect the performance of supercapacitor; the first one is structure, the other one is functional groups. Variations in the structure lead to differences in ion exchange and electron transport capabilities, which in turn affect the capacitance performance of the material. These factors occur as a result of the constitution of the precursor and different activation mechanism of the activator [39]. In Fig. 3 a and b, it can be easily seen that huge amount of porous structure occurred. NaOH/800-2 h exhibited a foam-like structure. The average mean size of the porous structure was calculated to be 1.98 nm. According to the pore distribution of the sample, its pore width range is between 1.9 nm and 135.9 nm. Fig. 3 c and d show the sample which was activated by ZnCl2 and carbonized at 700 C for 1.5 h. It exhibited a spherical structure. According to the electrical conductivity analysis, it showed higher
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7
Fig. 3. 2 nm and 50 nm SEM images of (a)e(b) NaoOH/800-2 h, (c)e(d) ZnCl2/700e1.5 h, (e)e(f) KOH/800-1,5s, (g)e(h) ZnCl2/700-1s, (i)e(j) KOH/800-2s, (k)e(l) ZnCl2/800-2s, (m)e(n) H3PO4/800-1s, (o)e(p) ZnCl2/800-1,5s.
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Fig. 3. (continued).
M.S. Celiktas, F.M. Alptekin / Energy 188 (2019) 116089
electrical conductivity value than activated with the same chemical and carbonized at 700 C for 1 h. And also, it exhibited more uniform porous structure than ZnCl2/700-1 h. Additionally, the average pore size width was calculated as 1.71 nm and its width range change between 1.70 nm and 136.6 nm. In Fig. 3 e and f SEM images of the sample that was activated by KOH and then carbonized at 800 C for 1.5 h is shown. KOH/8001.5 h exhibited a similar uniform porous structure. Its average pore size was calculated as 1.77 nm and its width range is determined between 1.70 nm and 148.6 nm. Fig. 3 g and h show the SEM images of the sample that was activated with ZnCl2 and carbonized at 700 C for 1 h. When SEM images of the sample were examined, the spherical porous structure could be seen. Its average pore size width was calculated as 1.68 nm and it showed changing 1.60 nme157.8 nm pore width size distribution. Fig. 3 i and j display SEM images of the sample that was activated by KOH and substituted to 800 C carbonization process for 2 h. The sample displays a rough porous structure. Its average pore size was calculated as 1.74 nm and its pore size width showed variations from 1.7 nm to 168.6 nm. Comparing its porous structure properties with the sample which is shown in Fig. 3 e and f, alteration of a structure was observed. Contrary to this, the sample which exists in Fig. 3 e and f had a more uniform porous structure. Based on literature investigation, it is found out that the uniform porous structure can achieve to increase electrical conductivity [30]. The sample which was activated by ZnCl2 and substituted to 800 C carbonizations for 2 h is shown in Fig. 3 g and h. ZnCl2/8002 h had 1.66 nm average pore width. Its pore width range changes from 1.9 nm to 137.5 nm. Fig. 3 m and n exhibited the SEM images of H3PO4/800-1 h. The highest average pore width was observed in this sample as 4.72 nm. Fig. 3 o and p show the sample of ZnCl2/800-1,5 h. The high hole structure was observed in the sample. Additionally, it showed a smaller electrical conductivity value than the other sample. The average pore size width was calculated as 1.76 nm for the sample. In Table 5, activation materials and specific surface area, pore size and total pore volume of the carbon materials are shown. When the results of SSA are compared, NaOH/800-2 h that has the highest electrical conductivity value achieved relatively low SSA. The highest SSA value was reached by ZnCl2/800-1,5 h as 1177.31 m2g1. Even if it had high SSA value, it showed the less electrical conductivity property than the other. SSA is a crucial parameter to achieve high specific capacitance. According to some researchers [40], carbon-based material that has low SSA can show high specific capacitance property. Lipka [40], had prepared carbon-based materials which had SSA values of 0.40, 0.35 and 0.70 m2g1 but measurement results show that they exhibited specific capacitance value of 275 F/g, 310 F/g and 190 F/g high, respectively. On the other hand, Zhao et al. [41], had prepared activated carbon that has 3 times larger SSA (3089.2 m2g1) than conventional activated
Table 5 Activation materials and specific surface area, pore size and total pore volume of the carbon materials. Sample
SBET (m2/g)
Vtot (cm3/g)
Pore size (nm)
NaOH/800-2 h ZnCl2/700-1.5 h ZnCl2/700-1 h KOH/800-1.5 h KOH/800-2 h ZnCl2/800-2 h H3PO4/800-1 h ZnCl2/800-1.5 h
381.81 902.57 1039.02 709.43 1002.25 846.09 12.47 1177.31
0.189 0.399 0.441 0.314 0.438 0.352 0.014 0.520
1.982 1.773 1.772 1.772 1.749 1.666 4.723 1.769
9
carbon. Their activated carbon exhibited the maximum capacitance value of 41.4 F/g. These results show that specific capacitance is not only related to SSA of carbon-based material but related to porosity, structure, and surface functional groups as well. High specific SSA that occurred in microporous structure can create an interface to the storage of ion although mesoporous and macroporous structure can not emerge SSA, they can create a favorable pathway to transform ions that exist in the material [31]. In literature, there are various studies about activated carbon for using in supercapacitor application that is derived from a different kind of biomass precursor. Song et al. [42] have produced three dimensional nanostructured hierarchical porous carbons (3DHPCs) from corn husk. 3D-HPCs were obtained KOH pre-treatment which KOH solution at different concentrations (w/v), 5%, 7% and 9% and direct pyrolysis at 800 C for 1 h. According to their findings, the highest specific surface area was achieved with KOH pretreated with 5% concentration to be 928 m2/g while the pore volume was 0.53 cm3/g. Uçar et al. [37] have studied on preparation activated carbon from pomegranate seeds by chemical activation with ZnCl2. They investigated the effects of carbonization temperature and impregnation ratio that are process variables on textural and chemical-surface properties of the prepared activated carbon. They founded that 2.0 impregnation ratio at 600 C gave the high specific surface area of the carbon as 978.8 m2/g. Chen et al. [43] have studied in the preparation of activated carbon from cotton stalk for application in supercapacitor. They used a chemical activation method with using phosphoric acid (H3PO4) and cotton stalk with a mass ratio of 4:1 at 800 C activation temperature for 2 h. According to the result of the experiment, activated carbon with a specific surface area of 1.481 cm2/g and micropore volüme of 0.0377 cm3/g was achieved. When a comparison of the other study results, it can be seen clearly that process conditions such as impregnation ratio, carbonization temperature, activation agent, activation time and biomass materials affect the activated carbon properties. 4. Conclusion . Corn cob as one of the agricultural biomass materials is a renewable and suitable carbon-based material. In this study, chemical activation and post carbonization have been conducted by different chemicals at different carbonization time and temperature levels. Point of the study can be summarized as follows; 1. Foam like micropore activated carbon-based material has been successfully synthesized by carbonizing and chemical activation by NaOH from the corn cob. 2. Corn cob has shown the highest carbon yield compared to other biomass sources even if its pre-carbonized content was the lowest. Its carbon content has increased 60% from beginning to after hydrothermal carbonization. 3. Pre-treatment and post-treatment have an effect on specific surface area and pore size distribution which determine the electrochemical performance. . Thanks to hydrothermal carbonization, lignocellulosic material decomposition their main component and oxidative groups was realized. Via applying post-treatment which were chemical activation and advanced carbonization desired properties such as pore size distribution, specific surface area, and carbon yield were increased. 4. The best electrical conductivity value has been achieved by NaOH activation at 800 C for 2 h. 5. The obtained materials present a specific surface area up to 381.81 (m2g1) and with a micropore volume of 0.189 (cm3g1) and pore diameter of 1.98 nm.
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6. When the results were investigated activated carbon-based materials which were activated by different chemicals showed different specific surface area and pore volumes from each other. 7. Even if the highest specific surface area value has been obtained by ZnCl2 at 700 C for 1.5 h, it has represented the lowest electrical conductivity value. In post-study, the production of electrode material for the production of supercapacitors from the carbon-based materials which are obtained in this previous phase of the study can be carried out. Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Acknowledgment This study is supported by Ege University Scientific Research Projects Coordination Unit. Project number:FYL-2018-20039. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2019.116089. References [1] Poonam Sharma K, Arora A, Tripathi SK. Review of supercapacitors: materials and devices. J Energy Storage 2019. https://doi.org/10.1016/j.est.2019.01.010. lez A, Goikolea E, Barrena JA, Mysyk R. Review on supercapacitors: [2] Gonza technologies and materials. Renew Sustain Energy Rev 2016. https://doi.org/ 10.1016/j.rser.2015.12.249. [3] Dehghani-Sanij AR, Tharumalingam E, Dusseault MB, Fraser R. Study of energy storage systems and environmental challenges of batteries. Renew Sustain Energy Rev 2019. https://doi.org/10.1016/j.rser.2019.01.023. [4] Thangavel R, Kannan AG, Ponraj R, Thangavel V, Kim DW, Lee YS. High-energy green supercapacitor driven by ionic liquid electrolytes as an ultra-high stable next-generation energy storage device. J Power Sources 2018. https://doi.org/ 10.1016/j.jpowsour.2018.02.037. ca J, Vondr M. Supercapacitors: properties [5] Libich J, Ma ak J, Cech O, Sedlaríkova and applications. J Energy Storage 2018. https://doi.org/10.1016/ j.est.2018.03.012. [6] Kumar R, Savu R, Joanni E, Vaz AR, Canesqui MA, Singh RK, et al. Fabrication of interdigitated micro-supercapacitor devices by direct laser writing onto ultrathin, flexible and free-standing graphite oxide films. RSC Adv 2016. https:// doi.org/10.1039/c6ra17516c. [7] Borenstein A, Hanna O, Attias R, Luski S, Brousse T, Aurbach D. Carbon-based composite materials for supercapacitor electrodes: a review. J Mater Chem A 2017. https://doi.org/10.1039/c7ta00863e. [8] Liu X, Li S, Mi R, Mei J, Liu LM, Cao L, et al. Porous structure design of carbon xerogels for advanced supercapacitor. Appl Energy 2015. https://doi.org/ 10.1016/j.apenergy.2015.01.141. [9] Stoller MD, Ruoff RS. Best practice methods for determining an electrode material's performance for ultracapacitors. Energy Environ Sci 2010. https:// doi.org/10.1039/c0ee00074d. [10] Yang Z, Tian J, Yin Z, Cui C, Qian W, Wei F. Carbon nanotube- and graphenebased nanomaterials and applications in high-voltage supercapacitor: a review. Carbon N Y 2019. https://doi.org/10.1016/j.carbon.2018.10.010. [11] Li T, Huang L, Liu H. Energy management and economic analysis for a fuel cell supercapacitor excavator. Energy 2019. https://doi.org/10.1016/ j.energy.2019.02.016. ~ o P, Fern [12] Corral-Vega PJ, García-Trivin andez-Ramírez LM. Design, modelling, control and techno-economic evaluation of a fuel cell/supercapacitors powered container crane. Energy 2019. https://doi.org/10.1016/ j.energy.2019.115863. [13] Luta DN, Raji AK. Optimal sizing of hybrid fuel cell-supercapacitor storage system for off-grid renewable applications. Energy 2019. https://doi.org/ 10.1016/j.energy.2018.10.070. [14] Tehrani Z, Thomas DJ, Korochkina T, Phillips CO, Lupo D, Lehtim€ aki S, et al. Large-area printed supercapacitor technology for low-cost domestic green energy storage. Energy 2017. https://doi.org/10.1016/j.energy.2016.11.019.
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Conversion of model biomass to carbon-based material with high conductivity by using carbonization Melih Soner Celiktas*, Fikret Muge Alptekin Solar Energy Institute, Biomass Energy Systems & Technology Center, Ege University, 35100, Bornova, Izmir, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2019 Received in revised form 2 September 2019 Accepted 7 September 2019 Available online 9 September 2019
Biomass materials are renewable sources that abundant worldwide due to natural plants and living organisms. Lignocellulosic biomass can be categorized as hardwood, softwood, agricultural wastes, and grasses. Agricultural residues those which of them have importance due to being produced in huge amounts in the worldwide annually. Food wastes and agricultural wastes are utilized either alternative use such as generating energy, fuels or disposal. However, disposal of these residues is follow out either scraping or burning way. This study aims to convert industrial agricultural origin biomass by using hydrothermal carbonization method to carbon-based material having high conductivity for use in supercapacitor production by using different activating chemicals. Hydrothermal carbonization was applied to different biomass samples such as nutshell, hazelnut shell, and corn cob. The elemental analysis of the obtained biochar was carried out and it was determined that the highest source of biomass was corn cob. The selected biochar has been chemically activated with different chemicals such as KOH, NaOH, H3PO4 and, ZnCl2. Advanced carbonization of activated biochar was carried out at 500, 600, 700 and 800 C with 1, 1.5 and 2-h retention times. The resulting carbon-based products were mixed with KBr and identical pellets were prepared and their electrical conductivity values were measured. Electrical conductivity results, NaOH-800 C-2h and ZnCl2-700 C-1.5 h obtained from the process prepared from the biocidal pellets were determined to have the highest conductivity value. BrunauereEmmetteTeller (BET) and Scanning Electron Microscope (SEM) analyses of the samples with the highest conductivity values were performed and surface morphologies were examined. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Supercapacitor Advanced material Electrical conductivity Carbon-based material
1. Introduction For human development energy is a crucial issue. Fossil-based energy consumption and production affect both the World economy and ecology [1]. By increasing the cost of fossil fuels that are limited sources, pollution, global warming, and geopolitical concerns became a serious problem that modern societies have met last years [2]. To overcome these problems, sustainable carbon-free or low-carbon energy sources and energy storage systems should be increased to solve energy concern [3]. Development of sustainable and renewable energy sources such as wind and solar energy have become crucial sources because of
* Corresponding author. Tel.: þ90 232 3111110; Fax: þ90 232 3886027 E-mail address: [email protected] (M.S. Celiktas). https://doi.org/10.1016/j.energy.2019.116089 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
these problem concerns. Though energy storage devices are required since these energy sources show intermittent properties in nature [4]. As a consequence, energy storage and accumulation is an essential part of renewable energy sources [5]. Among the various alternative energy storage technologies, supercapacitors are one of the most promising electrochemical energy storing devices [6]. A supercapacitor (SC) which is an energy storage device has several advantages such as having a high power density, fast charging and discharging opportunity and long life cycle when it is compared to batteries and capacitors [7e9]. Nowadays, SC as an energy storage devices is used in a wide variety of applications like wind power, tidal energy, solar energy, start-up and emergency power supply of heavy machinery [10], and in fuel cells [11e13]. An electrode is the most crucial part of a supercapacitor. Metal oxides, polymers, and carbon materials are the most widely used
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materials for supercapacitor electrode. Having a high surface area, good thermal and electric conductivity, high flexibility [14] the low production cost of activated carbon make its more appropriate materials to prepare electrode for supercapacitor. Due to all these properties, carbonaceous materials and activated carbon have been widely investigated by researchers [15,16] to be an electrode of supercapacitor [17]. Activated carbon which has a large specific area, porosity, high physicochemical stability, and high surface reactivity is a carbonaceous material that enables users for different applications. Several kinds of biomass such as waste coffee beans [18e22], apricot shell, sugarcane bagasse, sewage sludge and sawdust [20e24], rice husk [15,25] feedstock are commonly used for producing activated carbon that is a low-cost and sustainable alternative precursor to fossil-based. There are different kinds of research which agricultural wastes are used as feedstock to produce carbon-based material for supercapacitor production in the literature. Rufford et al. [18] produced activated carbon by using coffee grounds by applying ZnCl2. The findings showed that the supercapacitor electrode which was prepared from waste coffee ground carbon presented good energy densities and stability at high charge-discharge rates. The other study was carried out by Ismanto et al. [17]. They used Cassava peel as feedstock for producing activated carbon to prepare carbon-based electrodes to use in supercapacitor application. Electrodes were prepared by using both chemical and physical activation. Four different types of oxidative chemical agents were used in the study showed that HNO3 modified activated carbon electrode presented 72.6% increase of specific capacitance when compared to the initial one. Xu et al. [19] studied generating activated carbon from an apricot shell by using NaOH activation. They investigated the effects of carbonization temperature on the carbon porosity and electrode density. According to their finding, 500 C temperature gives both high gravimetric capacitance (339 F g-1) and 0.504 g cm-3 of high electrode density. Due to their capability of converting biomass to biochar, various thermochemical conversion applications have been used by researchers [26,27]. Pyrolysis, hydrothermal carbonization, flash carbonization, and gasification can be given as examples for thermochemical conversion methods that have many benefits of producing biochar to be used for carbon sequestration, soil amendment, soil productivity improvement and pollution control [28]. The objectives of the study is that determines the highest carbon content of biomass of agricultural origin, show the influence of process variables such as chemical agents, carbonization temperature, retention time, the effect of using hydrothermal carbonization method and obtain carbon-based material with high conductivity for use in the production of a supercapacitor. In this study, by using hydrothermal carbonization method, it is aimed to convert the biomass of agricultural origin to a carbonbased material with high conductivity for use in the production of a supercapacitor as a result of treatment with different activating chemicals.
2.1. Pretreatment Nutshells, hazelnut shells, and corn cobs were washed with deionized water to eliminate impurities and dust resulting from the accumulation of materials. After washing, the materials were dried at 105 C for 24 h. The purified and dried materials were ground to small particles and sieved. 1 mm particulate size was selected for this study. 2.2. Proximate analysis The analyses of moisture, ash content, volatile material, and fixed carbon were conducted for selected materials. The elemental analysis of the materials was carried by Elemental Analyzer Tuespec CHN Leco to determine carbon (C), hydrogen (H), and nitrogen (N) contents of samples at pure oxygen atmosphere. The oxygen content of the samples was calculated by a simple calculation from obtained C, H and N [29]. Also, the elemental analysis was done for samples that were subjected to hydrothermal carbonization. The samples were named related to their material, residence time as NS (NS-2-4-6-8 h), HS (HS-2-4-6-8 h) and CC (CC-2-4-6-8 h). The analyses were carried out with two replicates for each sample. The moisture content of materials was analyzed with AND MX50 analyzer. In ash content analysis, approximately 2 g of each sample was weighed and dried at 105 C for 1 h. Ash content analysis was carried out by MAGMA THERM brand muffle furnace. The samples were placed to a suitable number of crucibles. The crucibles which are identified with porcelain marker were places to the muffle furnace at 550 C (575 ± 25 C) for 3 h. After burning was completed, the crucibles from the furnace were removed directly to desiccator for cooling. The ash contents of samples were calculated by using Eq. (1).
Ash ð%Þ ¼
g1 *100 g2
(1)
where; g1: the weight of ash, (g) g2: the weight of dried samples that in the furnace, (g) Volatile analysis, 1 g-weighted sample that was dried at openair were burned at 950 C for 7 min. The processed samples were cooled at desiccator and then measured. The volatile matter amount of the samples were calculated by Eq (2).
Volatile Matter Amount ð%Þ ¼
g1 g2 M *100 g1
(2)
where; g1: Weight of used sample, (g) g2: Weight of sample after drying, (g) M: Moisture percent of the used sample Fixed carbon amount, was calculated as Eq (3).
2. Material and method
Fixed carbon ð%Þ ¼ 10 ðmoisture þ ash þ volatile matterÞ (3)
Three different kinds of agricultural-based biomass sources were used to produce carbon-based material to produce a supercapacitor. These were; nutshell, hazelnut shell, and corn cob. The hazelnut shells were supplied from the Gençler Fındık San. Ve Tic. A.S. The nutshells and corn cob was supplied from a local bazaar in _ Izmir. In Fig. 1, the method of the study is demonstrated.
2.3. Hydrothermal carbonization (HTC) Hydrothermal carbonization was carried out in a 100 ml
M.S. Celiktas, F.M. Alptekin / Energy 188 (2019) 116089
3
Fig. 1. The flow process chart of the study.
stainless steel reactor. Prepared materials were subjected to carbonization process at 250 C for 2, 4, 6 and 8 h in PLC brand drying oven which maximum of operating temperature is 250 C and it has constant temperature function. Weighed 10 g biomass and 90 ml deionized water were put in the reactor and then sealed. After then, the carbonization process was conducted for determining temperature values and different reaction times for each material. The experiments were carried out with two replicates. After processes were ended up C, H, N and O contents of the materials were determined from produced carbonbased materials. According to the findings, corn cob was determined to have the highest carbon content. Corn cob which has the highest carbon content was conducted to chemical activation and consecutively carbonization processes. 2.4. Chemical activation process The corn cob was activated with potassium hydroxide (KOH), sodium hydroxide (NaOH), phosphoric acid (H3PO4) and zinc chloride (ZnCl2). The ratio of chemical to the obtained carbonized material corn cob was determined as 4:1 [30e32]. Following the determined ratio, solutions were prepared for each chemical. To prepare a solution with KOH, NaOH, and ZnCl2 which were solid, the mixer was used to provide soluble chemicals in the water. Sufficient amount of three chemicals were measured and put into a volumetric flask and then added deionized water to prepare a solution. For aqueous H3PO4, the ratio of chemical/carbon-based material was calculated to prepare a solution. For 2 g each carbonized sample, 50 ml prepared solution was used. The samples were activated by separately NaOH, H3PO4, and ZnCl2 for 24 h with continuous stirring.
Chemical activation, differing from the other three chemicals, was performed by continuous stirring with KOH for 40 min [33]. After the activation process completed, activated samples were filtered with filter paper and kept in the furnace at 105 C for 24 h to evaporate their moisture. 2.5. Post carbonization The post carbonization of the samples which were chemically activated was carried out in Protherm Furnaces according to experiment list that was obtained from Design Expert software (version 7.0). The samples that were activated with KOH, NaOH, H3PO4, and ZnCl2 were carbonized at 500, 600, 700, and 800 C for 1, 1.5 and 2 h under continuous N2 gas flow in a horizontal furnace. After the carbonization process, the samples were washed with deionized water and then dried at for 24 h to evaporate their moisture content. The solid conversion yield of the samples was calculated from Eq (4).
m Conversion yield ð%Þ ¼ 100 m1 2 *100 m1
(4)
where; m1: activated sample, (g) m2: the samples which obtained after carbonization process, (g)
2.6. Electrical conductivity analysis In order to measure electrical conductivity value, pellets were
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prepared with the carbonized material and potassium bromide (KBr). 30 mg carbon-based material and 770 mg KBr were mixed homogeneously and pressed at Specac Hydraulic Press at a pressure of 5 N. They were converted to a pellet that has 10 mm Radius and 0.38 mm height. The analysis of electrical conductivity was carried out by a 4probe analyzer. The pellets were delivered to the probe from a source of direct current (DC) by Keithley 2400 model. From Eq (5), electrical specific resistivity and Eq (6), electrical conductivity value was calculated from obtained electrical resistivity values
p ¼ R*
A I
(5)
where; R: electrical resistivity, (R, U) A: area of pellet, (mm2) I: current density, (mA)
1 s¼ p
3. Results and discussion 3.1. Proximate analysis As a proximate analysis, moisture, ash content, volatile matter content, fixed carbon and elemental analysis of biomass sources were carried out. The proximate analysis of biomass sources is shown in Fig. 2. As a result of the analysis, the moisture content of nutshell, hazelnut shell, and corn cob was determined as 9.04%, 6.05%, and 6.98%, respectively. As a result of the analysis, the highest volatile matter content was found in a nutshell with 15.98%. When the results were investigated, the lowest volatile matter content was found in a corn cob. 3.2. Elemental analysis
(6)
where;
s: electrical conductivity, (Scm1)
2.7. Characterization The morphology of the obtained carbon-based samples was characterized by scanning electron microscope (SEM, Thermo Scientific Apreo S). Nitrogen adsorption and desorption isotherms of 8 samples that have the highest electrical conductivity value were carried out by Micromeritics brand Gemini 2369 model supported with computer device at liquid nitrogen gas (N2) environment. Before beginning the analysis, the samples were exposed to gas at 100 C for 24 h. The specific surface area was calculated using the Brunauer-Emmet-Teller (BET) method and porosity distribution was determined using the Barrett-Joyner-Halenda (BJH) method.
Pre elemental content of biomass sources is given in Table 1. The elemental analysis results of biomass that were subjected to carbonization at 250 C and different residence times are given in Table 2. According to the results, the percent carbon content of biomass sources increased through carbonization process with carrying out different residence time. After the carbonization process, the samples had a carbon-rich content while the hydrogen and oxygen content of the samples decreased [34]. The obtained results showed that with increasing carbonization time, the carbon yield increased
Table 1 Pre elemental analysis of biomass sources. Biomass
C (%)
H (%)
N (%)
O (%)
HS_1 HS_2 NS_1 NS_2 CC_1 CC_2
50,24 50,19 50,21 50,08 45,19 45,21
6,14 6,10 6,07 6,02 6,17 6,17
0,07 0,08 0,56 0,52 0,26 0,28
43,62 43,71 43,72 43,9 48,64 48,62
Fig. 2. The proximate analysis of biomass sources.
M.S. Celiktas, F.M. Alptekin / Energy 188 (2019) 116089 Table 2 The elemental analysis of biomass sources after carbonization process with different residence time. Biomass
C (%)
H (%)
N (%)
O (%)
HS_2H1_1 HS_2H1_2 HS_2H2_1 HS_2H2_2 HS_4H1_1 HS_4H1_2 HS_4H2_1 HS_4H2_2 HS_6H1_1 HS_6H1_2 HS_6H2_1 HS_6H2_2 HS_8H1_1 HS_8H1_2 HS_8H2_1 HS_8H2_2 NS_2H1_1 NS_2H1_2 NS_2H2_1 NS_2H2_2 NS_4H1_1 NS_4H1_2 NS_4H2_1 NS_4H2_2 NS_6H1_1 NS_6H1_2 NS_6H2_1 NS_6H2_2 NS_8H1_1 NS_8H1_2 NS_8H2_1 NS_8H2_2 CC_2H1_1 CC_2H1_2 CC_2H2_1 CC_2H2_2 CC_4H1_1 CC_4H1_2 CC_4H2_1 CC_4H2_2 CC_6H1_1 CC_6H1_2 CC_6H2_1 CC_6H2_2 CC_8H1_1 CC_8H1_2 CC_8H2_1 CC_8H2_2
53,63 53,57 55,41 55,13 69,94 70,15 68,88 69,50 69,15 68,94 70,27 70,33 69,55 69,21 70,86 70,76 65,46 65,67 63,15 63,24 70,23 69,83 70,21 69,89 70,27 67,90 69,48 70,77 70,10 70,37 70,98 69,95 58,96 60,73 56,13 56,56 70,25 69,66 70,29 70,73 70,93 72,05 72,04 72,06 73,61 71,00 71,51 72,32
5,71 5,74 5,65 5,57 4,87 4,95 4,93 4,93 4,95 4,92 4,92 5,01 4,77 4,84 4,90 5,01 5,24 5,30 5,64 5,65 5,08 5,03 5,10 5,08 5,06 0,84 9,75 5,01 5,09 5,10 5,09 5,03 5,55 5,52 5,77 5,74 5,19 5,18 5,01 5,07 5,12 5,17 5,05 5,07 5,45 5,22 4,93 5,01
0,01 0,01 0,05 0,01 0,18 0,16 0,18 0,18 0,21 0,22 0,14 0,15 0,16 0,16 0,19 0,20 0,04 0,08 0,66 0,67 0,13 0,12 0,18 0,20 0,16 0,18 0,20 0,15 0,11 0,08 0,13 0,08 0,34 0,35 0,18 0,31 1,09 1,07 0,73 0,73 1,21 1,21 1,06 1,13 1,61 1,52 0,59 0,70
40,66 40,69 38,94 39,3 25,19 24,9 26,19 25,57 25,9 26,14 24,81 24,66 25,68 25,95 24,24 24,23 29,3 29,03 31,21 31,11 24,69 25,14 24,69 25,03 24,67 31,26 20,77 24,22 24,81 24,53 23,93 25,02 35,49 33,75 38,1 37,7 24,56 25,16 24,7 24,2 23,95 22,73 22,91 22,87 20,94 23,78 23,56 22,67
5
pore-size distribution, and electrical conductivity [21]. In order to obtain high capacitance, carbon based material must be subjeced to appropriate activation. Porous network structure is responsible fort he performance of the supercapacitor and in order to improve performance, micro and mesoporous structure is needed. The porous structure and surface area improved by the application of chemical activation significantly contribute to the improvement of the capacitance performance [28]. The combustion data and combustion yields of the samples which were subjected to chemical activation and then advanced carbonization process are given in Table 3. The samples subjected to carbonization at 500 C from the samples which had been carbonized at different temperatures at the same holding times yielded more solid product yield. By increasing carbonization temperature, decreasing of solid product yield occurred. By evaluation of samples after the carbonization process, the highest carbon yields were observed in samples which were activated with ZnCl2 and H3PO4. H3PO4 is a chemical that is widely used in order to produce activated carbon from lignocellulosic materials. It is integrated into the internal wall of the cell of lignocellulosic materials in the carbonization process. Thanks to that, biomass material turns into a solid product with developed porosity structure. Due to the effect of H3PO4 hydrolysis and dehydration occurring above 300 C, the solid product amount increases [35]. On the other hand, zinc chloride increase porosity development of carbon-based materials by increasing thermal energy during the thermal process. Besides, by fixing volatile materials it helps to increase carbon yield [36]. In chemical activation by ZnCl2, it behaves as a dehydrating agent that effects pyrolytic decomposition and prevents tar formation. As a consequence, a higher yield of carbon is produced when compared to biochars [37]. Briefly, the core parameters for carbonization can be summarized as follows; Carbonization temperature has an important effect on carbon yield. Increasing temperature concluded with decreasing carbon yield. Chemical agent influence carbon yield. Chemical activation with ZnCl2 and H3PO4 have resulted in the highest carbon yield for every each temperature and retention time. Retention time also affects carbon yield that combined with a high level of temperature gave the lowest carbon yield.
3.4. The results of electrical conductivity for all the samples. It was determined that the highest carbon yields were achieved with 8 h carbonization time for all the samples. According to the results of elemental analysis, corn cob showed the highest % C content in selected biomass sources. It was determined through proximate analysis, corn cob had 45.20% carbon yield at the beginning. After 8 h of carbonization process, this was increased by approximately 60%, and it reached to 71.92%. While hazelnut and walnut had the same carbon content higher than that of corn cob, corn cob showed increasing carbon content with increasing detention time. When their carbon content was compared, corn con was determined as the main biomass source which was subjected to the chemical activation and advanced carbonization processes. 3.3. Combined combustion yields as a result of advanced carbonization Carbon sources and activation conditions specify the electrochemical performance which related to surface area of carbon,
The electrical conductivity results of samples that were conducted to different chemical activation in different residence time and at different temperature are shown in Table 4. According to the data of electrical conductivity measurements, the highest conductivity value was achieved with NaOH/800-2 h as 4.24 103 S/ohm. When examined the other electrical conductivity values, 2,31 103 S/ohm of ZnCl2/700-1,5 h, 1,85 103 S/ ohm of KOH/800-1,5 h, 1,35 103 S/ohm of ZnCl2/700-1 h, 1,17 103 S/ohm of KOH/800-2 h, 16 103 S/ohm of ZnCl2/8002 h, 7,51 10-4 S/U of H3PO4/800-1 h and 5.70 104 S/ohm of ZnCl2/800-1.5 h were gained. When results were examined, it is observed that electrical conductivity increased by increasing carbonization temperature. Additionally, the samples which were activated with different chemicals and different residence times gave varied electrical conductivity values. The highest electrical conductivity values were achieved at high carbonization temperature (700 and 800 C). Similar results were observed at Kennedy et al.'s study. Kennedy et al., [38], were examined electrical conductivity of activated
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Table 3 Combustion data and combustion yields obtained as a result of advanced carbonization. Chemically Activated Samples
Temperature ( C)
Time (s)
Sample (g)
After advanced carbonization (g)
Carbon Yield (%)
KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4 KOH NaOH ZnCl2 H3PO4
500 500 500 500 600 600 600 600 700 700 700 700 800 800 800 800 500 500 500 500 600 600 600 600 700 700 700 700 800 800 800 800 500 500 500 500 600 600 600 600 700 700 700 700 800 800 800 800
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 1,5 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
2,09 2,03 2,10 2,06 2,03 2,05 2,10 2,02 2,01 2,05 2,03 2,04 2,02 2,01 2,02 2,05 2,02 2,05 2,03 2,04 2,00 2,14 2,08 2,04 2,07 2,07 2,02 2,07 2,11 2,09 2,02 2,00 2,02 2,00 2,09 2,03 2,04 2,05 2,06 2,11 2,00 2,03 2,05 2,02 2,04 2,09 2,04 2,03
0,44 0,26 1,42 1,35 0,47 0,33 1,01 1,16 0,47 0,67 0,77 1,06 0,359 0,692 0,707 1096 0,65 0,42 1,49 1,31 0,874 0,502 1102 1444 0,46 0,45 0,73 1,01 0,504 0,666 0,648 0,980 0,68 0,65 1,46 1,40 0,67 0,74 1,27 1,22 0,539 1031 0,780 1110 0,41 0,27 0,74 0,95
21,05 12,81 67,62 65,53 23,15 16,09 48,09 54,42 23,38 32,68 37,93 51,96 17,78 34,43 35,19 53,51 32,18 20,48 73,40 64,22 43,73 23,44 53 70,79 22,22 21,74 36,13 47,26 23,91 31,86 32,10 49,02 33,66 32,5 69,85 68,97 32,84 36,10 61,65 57,82 26,94 50,78 38,06 54,96 20,10 12,92 36,27 46,80
Table 4 The results of electrical conductivity of samples. Sample
R (ohm/sq)
Res (ohm-cm)
Electrical conductivity (ohm-1)
NaOH/800-2 h ZnCl2/700-1,5 h KOH/800-1,5 h ZnCl2/700-1 h KOH/800-2 h ZnCl2/800-2 h H3PO4/800-1 h ZnCl2/800-1,5 h
114 209 262 358 413 417 644,5 849
16,01 29 36,93 50 58,23 59 90,74 120
4,24 103 2,31 103 1,85 103 1,35 103 1,17 103 1,16 103 7,51 104 5,70 104
carbon that was supplied from rice husk at the different pressures. They decided to electrical conductivity value of activated carbon which was obtained at 700, 800 and 900 C changed with increasing temperature. 3.5. The result of SEM and BET analyses ZnCl2, H3PO4, KOH, K2CO3, etc. are chemicals that can be used as
activators [33]. According to electrical conductivity results, 8 samples with the best electrical conductivity were investigated. Fig. 3 shows the scanning electron microscopy images which could display the morphology and microstructure of the samples. Fig. 3 a and b, the sample which was activated by NaOH and carbonized at 800 C for 2 h. Two factors affect the performance of supercapacitor; the first one is structure, the other one is functional groups. Variations in the structure lead to differences in ion exchange and electron transport capabilities, which in turn affect the capacitance performance of the material. These factors occur as a result of the constitution of the precursor and different activation mechanism of the activator [39]. In Fig. 3 a and b, it can be easily seen that huge amount of porous structure occurred. NaOH/800-2 h exhibited a foam-like structure. The average mean size of the porous structure was calculated to be 1.98 nm. According to the pore distribution of the sample, its pore width range is between 1.9 nm and 135.9 nm. Fig. 3 c and d show the sample which was activated by ZnCl2 and carbonized at 700 C for 1.5 h. It exhibited a spherical structure. According to the electrical conductivity analysis, it showed higher
M.S. Celiktas, F.M. Alptekin / Energy 188 (2019) 116089
7
Fig. 3. 2 nm and 50 nm SEM images of (a)e(b) NaoOH/800-2 h, (c)e(d) ZnCl2/700e1.5 h, (e)e(f) KOH/800-1,5s, (g)e(h) ZnCl2/700-1s, (i)e(j) KOH/800-2s, (k)e(l) ZnCl2/800-2s, (m)e(n) H3PO4/800-1s, (o)e(p) ZnCl2/800-1,5s.
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Fig. 3. (continued).
M.S. Celiktas, F.M. Alptekin / Energy 188 (2019) 116089
electrical conductivity value than activated with the same chemical and carbonized at 700 C for 1 h. And also, it exhibited more uniform porous structure than ZnCl2/700-1 h. Additionally, the average pore size width was calculated as 1.71 nm and its width range change between 1.70 nm and 136.6 nm. In Fig. 3 e and f SEM images of the sample that was activated by KOH and then carbonized at 800 C for 1.5 h is shown. KOH/8001.5 h exhibited a similar uniform porous structure. Its average pore size was calculated as 1.77 nm and its width range is determined between 1.70 nm and 148.6 nm. Fig. 3 g and h show the SEM images of the sample that was activated with ZnCl2 and carbonized at 700 C for 1 h. When SEM images of the sample were examined, the spherical porous structure could be seen. Its average pore size width was calculated as 1.68 nm and it showed changing 1.60 nme157.8 nm pore width size distribution. Fig. 3 i and j display SEM images of the sample that was activated by KOH and substituted to 800 C carbonization process for 2 h. The sample displays a rough porous structure. Its average pore size was calculated as 1.74 nm and its pore size width showed variations from 1.7 nm to 168.6 nm. Comparing its porous structure properties with the sample which is shown in Fig. 3 e and f, alteration of a structure was observed. Contrary to this, the sample which exists in Fig. 3 e and f had a more uniform porous structure. Based on literature investigation, it is found out that the uniform porous structure can achieve to increase electrical conductivity [30]. The sample which was activated by ZnCl2 and substituted to 800 C carbonizations for 2 h is shown in Fig. 3 g and h. ZnCl2/8002 h had 1.66 nm average pore width. Its pore width range changes from 1.9 nm to 137.5 nm. Fig. 3 m and n exhibited the SEM images of H3PO4/800-1 h. The highest average pore width was observed in this sample as 4.72 nm. Fig. 3 o and p show the sample of ZnCl2/800-1,5 h. The high hole structure was observed in the sample. Additionally, it showed a smaller electrical conductivity value than the other sample. The average pore size width was calculated as 1.76 nm for the sample. In Table 5, activation materials and specific surface area, pore size and total pore volume of the carbon materials are shown. When the results of SSA are compared, NaOH/800-2 h that has the highest electrical conductivity value achieved relatively low SSA. The highest SSA value was reached by ZnCl2/800-1,5 h as 1177.31 m2g1. Even if it had high SSA value, it showed the less electrical conductivity property than the other. SSA is a crucial parameter to achieve high specific capacitance. According to some researchers [40], carbon-based material that has low SSA can show high specific capacitance property. Lipka [40], had prepared carbon-based materials which had SSA values of 0.40, 0.35 and 0.70 m2g1 but measurement results show that they exhibited specific capacitance value of 275 F/g, 310 F/g and 190 F/g high, respectively. On the other hand, Zhao et al. [41], had prepared activated carbon that has 3 times larger SSA (3089.2 m2g1) than conventional activated
Table 5 Activation materials and specific surface area, pore size and total pore volume of the carbon materials. Sample
SBET (m2/g)
Vtot (cm3/g)
Pore size (nm)
NaOH/800-2 h ZnCl2/700-1.5 h ZnCl2/700-1 h KOH/800-1.5 h KOH/800-2 h ZnCl2/800-2 h H3PO4/800-1 h ZnCl2/800-1.5 h
381.81 902.57 1039.02 709.43 1002.25 846.09 12.47 1177.31
0.189 0.399 0.441 0.314 0.438 0.352 0.014 0.520
1.982 1.773 1.772 1.772 1.749 1.666 4.723 1.769
9
carbon. Their activated carbon exhibited the maximum capacitance value of 41.4 F/g. These results show that specific capacitance is not only related to SSA of carbon-based material but related to porosity, structure, and surface functional groups as well. High specific SSA that occurred in microporous structure can create an interface to the storage of ion although mesoporous and macroporous structure can not emerge SSA, they can create a favorable pathway to transform ions that exist in the material [31]. In literature, there are various studies about activated carbon for using in supercapacitor application that is derived from a different kind of biomass precursor. Song et al. [42] have produced three dimensional nanostructured hierarchical porous carbons (3DHPCs) from corn husk. 3D-HPCs were obtained KOH pre-treatment which KOH solution at different concentrations (w/v), 5%, 7% and 9% and direct pyrolysis at 800 C for 1 h. According to their findings, the highest specific surface area was achieved with KOH pretreated with 5% concentration to be 928 m2/g while the pore volume was 0.53 cm3/g. Uçar et al. [37] have studied on preparation activated carbon from pomegranate seeds by chemical activation with ZnCl2. They investigated the effects of carbonization temperature and impregnation ratio that are process variables on textural and chemical-surface properties of the prepared activated carbon. They founded that 2.0 impregnation ratio at 600 C gave the high specific surface area of the carbon as 978.8 m2/g. Chen et al. [43] have studied in the preparation of activated carbon from cotton stalk for application in supercapacitor. They used a chemical activation method with using phosphoric acid (H3PO4) and cotton stalk with a mass ratio of 4:1 at 800 C activation temperature for 2 h. According to the result of the experiment, activated carbon with a specific surface area of 1.481 cm2/g and micropore volüme of 0.0377 cm3/g was achieved. When a comparison of the other study results, it can be seen clearly that process conditions such as impregnation ratio, carbonization temperature, activation agent, activation time and biomass materials affect the activated carbon properties. 4. Conclusion . Corn cob as one of the agricultural biomass materials is a renewable and suitable carbon-based material. In this study, chemical activation and post carbonization have been conducted by different chemicals at different carbonization time and temperature levels. Point of the study can be summarized as follows; 1. Foam like micropore activated carbon-based material has been successfully synthesized by carbonizing and chemical activation by NaOH from the corn cob. 2. Corn cob has shown the highest carbon yield compared to other biomass sources even if its pre-carbonized content was the lowest. Its carbon content has increased 60% from beginning to after hydrothermal carbonization. 3. Pre-treatment and post-treatment have an effect on specific surface area and pore size distribution which determine the electrochemical performance. . Thanks to hydrothermal carbonization, lignocellulosic material decomposition their main component and oxidative groups was realized. Via applying post-treatment which were chemical activation and advanced carbonization desired properties such as pore size distribution, specific surface area, and carbon yield were increased. 4. The best electrical conductivity value has been achieved by NaOH activation at 800 C for 2 h. 5. The obtained materials present a specific surface area up to 381.81 (m2g1) and with a micropore volume of 0.189 (cm3g1) and pore diameter of 1.98 nm.
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6. When the results were investigated activated carbon-based materials which were activated by different chemicals showed different specific surface area and pore volumes from each other. 7. Even if the highest specific surface area value has been obtained by ZnCl2 at 700 C for 1.5 h, it has represented the lowest electrical conductivity value. In post-study, the production of electrode material for the production of supercapacitors from the carbon-based materials which are obtained in this previous phase of the study can be carried out. Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Acknowledgment This study is supported by Ege University Scientific Research Projects Coordination Unit. Project number:FYL-2018-20039. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2019.116089. References [1] Poonam Sharma K, Arora A, Tripathi SK. Review of supercapacitors: materials and devices. J Energy Storage 2019. https://doi.org/10.1016/j.est.2019.01.010. lez A, Goikolea E, Barrena JA, Mysyk R. Review on supercapacitors: [2] Gonza technologies and materials. Renew Sustain Energy Rev 2016. https://doi.org/ 10.1016/j.rser.2015.12.249. [3] Dehghani-Sanij AR, Tharumalingam E, Dusseault MB, Fraser R. Study of energy storage systems and environmental challenges of batteries. Renew Sustain Energy Rev 2019. https://doi.org/10.1016/j.rser.2019.01.023. [4] Thangavel R, Kannan AG, Ponraj R, Thangavel V, Kim DW, Lee YS. High-energy green supercapacitor driven by ionic liquid electrolytes as an ultra-high stable next-generation energy storage device. J Power Sources 2018. https://doi.org/ 10.1016/j.jpowsour.2018.02.037. ca J, Vondr M. Supercapacitors: properties [5] Libich J, Ma ak J, Cech O, Sedlaríkova and applications. J Energy Storage 2018. https://doi.org/10.1016/ j.est.2018.03.012. [6] Kumar R, Savu R, Joanni E, Vaz AR, Canesqui MA, Singh RK, et al. Fabrication of interdigitated micro-supercapacitor devices by direct laser writing onto ultrathin, flexible and free-standing graphite oxide films. RSC Adv 2016. https:// doi.org/10.1039/c6ra17516c. [7] Borenstein A, Hanna O, Attias R, Luski S, Brousse T, Aurbach D. Carbon-based composite materials for supercapacitor electrodes: a review. J Mater Chem A 2017. https://doi.org/10.1039/c7ta00863e. [8] Liu X, Li S, Mi R, Mei J, Liu LM, Cao L, et al. Porous structure design of carbon xerogels for advanced supercapacitor. Appl Energy 2015. https://doi.org/ 10.1016/j.apenergy.2015.01.141. [9] Stoller MD, Ruoff RS. Best practice methods for determining an electrode material's performance for ultracapacitors. Energy Environ Sci 2010. https:// doi.org/10.1039/c0ee00074d. [10] Yang Z, Tian J, Yin Z, Cui C, Qian W, Wei F. Carbon nanotube- and graphenebased nanomaterials and applications in high-voltage supercapacitor: a review. Carbon N Y 2019. https://doi.org/10.1016/j.carbon.2018.10.010. [11] Li T, Huang L, Liu H. Energy management and economic analysis for a fuel cell supercapacitor excavator. Energy 2019. https://doi.org/10.1016/ j.energy.2019.02.016. ~ o P, Fern [12] Corral-Vega PJ, García-Trivin andez-Ramírez LM. Design, modelling, control and techno-economic evaluation of a fuel cell/supercapacitors powered container crane. Energy 2019. https://doi.org/10.1016/ j.energy.2019.115863. [13] Luta DN, Raji AK. Optimal sizing of hybrid fuel cell-supercapacitor storage system for off-grid renewable applications. Energy 2019. https://doi.org/ 10.1016/j.energy.2018.10.070. [14] Tehrani Z, Thomas DJ, Korochkina T, Phillips CO, Lupo D, Lehtim€ aki S, et al. Large-area printed supercapacitor technology for low-cost domestic green energy storage. Energy 2017. https://doi.org/10.1016/j.energy.2016.11.019.
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