Preparation of porous carbon from candlenut (Aleurites moluccana) and its utilization as a cathode for lithium ion capacitor (LIC)

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1 Preparation of porous carbon from candlenut (Aleurites moluccana) and its utilization as a cathode for lithium ion capacitor (LIC) Dina Junipuspita 1,*, Nursyafarinah Nursyafarinah 1, Wahyu Widayatno 2, Agus Wismogroho 2, Muhammad Ikhlasul Amal 2, Slamet Priyono 2, Dahlang Tahir 1 and Bidayatul Armynah 1 1 Physics Departement, Hasanuddin University, Makassar, Sulawesi Selatan 2 Indonesian Institute of Science (LIPI), Serpong, Banten. * junipuspitadina@gmail.com Abstract. The necessity of highly porous carbon from inexpensive sources as a cathode for some energy storage devices has led the exploration of various renewable biomass waste as a carbon precursor. This research is focused on the making of activated carbon from candlenut shell for lithium ion capacitors (LIC) cathode. Activated carbon was made using ultrasonic impregnation method using KOH 3,5 M and ZnCl 2 3,5 M at 80 C for 2 hours, and followed by carbonization at 500 C and 650 C for 1,5 hours. The pore characteristics of the obtained activated carbon was analyzed by Brunauer Emmett Teller (BET) method. The cathode was assembled using the mixture of activated carbon, Super P and Polyvinylidene Fluoride (PVDF) with the ratio of 85:5:10 (wt%) respectively. The electrochemical properties of LIC were analyzed using Cyclic Voltammetry (CV) and calculation of capacitance. The experimental results indicated the potential use of carbonized candlenut shell as a precursor material of cathode for LIC. Keywords: Lithium ion capacitor, Cathode, Activated carbon, Ultrasonic impregnant, Candlenut shell 1. Introduction The depletion of fossil energy and greenhouse effect make society choose the renewable energy sources. Utilization of electrical energy from these renewable sources make an efficient energy storage system such as battery and supercapacitor [1]. Therefore, lithium ion battery and supercapacitor considered as a promising system [2], but both have their respective limitations [3]. Electrochemical energy storage system such as lithium ion battery (LIB) has high energy densities ( Wh kg -1 ) because their storage mechanisms are based on oxidation-reduction (redox) reactions, but their power density are low (below 100 W kg -1 ). While the supercapacitor has a high power density (10 kw kg -1 ), because it stores electrical charge in an electrical double layer (EDL), but the energy density is low (5-10 Wh kg -1 ). Accordingly, the storage mechanism of LIB and supercapacitor combined into lithium ion capacitor (LIC) as shown in fig. 1 [2,3,4,5,6].

2 (a) (b) (c) Figure 1. cell configuration for (a) Lithium ion battery (b) EDLC, and (c) LIC [9] The structure of LIC is composed of two electrodes, the positive electrode by activated carbon and the negative electrode uses lithium ion. Electrode positive as a cathode to ensure high power density and electrode negative to provide high energy density [6,7]. In this study, we would like to investigate the relationship between the proportion pore of activated carbon candlenut shell with the capacitance of cathode LIC from candlenut shell. 2. Experimental section 2.1. Carbonization of the candlenut shell Candlenut shell was cut into small pieces (in length 5 mm) and then clean by aquades to remove dust and dirt. For activated, 5 gr of the sample by using impregnation in the ultrasonic cleaner with KOH 3,5 M and ZnCl2 3,5 M 20 ml at 80ºC for 2 hours. After then, carbonized by the furnace for 90 minutes at 500ºC and 650ºC, with a rate increase of 10 C/min. Carbon obtained is crushed with mortar. Then, washed with HCl 0.5 M to ph 2 and aquades until the ph is neutral (ph 7). To separate the liquid and activated carbon used the centrifuge for 30 minutes with a speed of rpm. Furthermore, activated carbon is dried in the oven for 24 hours at 100ºC. After dried, activated carbon is sieved to 200 mesh. 2.2 Characterization of activated carbon Activated carbon is characterized by BET (Brunauer Emmett Teller) to determine the surface area, total pore volume, pore distribution, and isotherm adsorption. 2.3 Coating process Activated carbon (85%), Polyvinylidene Fluoride (PVDF) (10%), and Super p (Carbon black) (5%) and Dimethyl Acetamide (DMAC) solution (2.9 ml) were weighed in accordance with predetermined compositions. The material is mixed with a DMAC solution to obtain a homogeneous slurry mixture. Then the slurry is printed with a thickness of 0.2 mm on the aluminum foil by doctor blade at a speed of 3.5 mm/ s. The sheet is dried by a dry box with a temperature of 70ºC for 30 minutes. In fig. 2a can be seen cathode sheet designed. After dried, the cathode sheet is cut into a circle with a diameter of 1.6 cm as shown in fig. 2b. Figure 2a. Cathode sheet designed Figure 2b. Cell cathode designed

3 2.4 Assembling component of supercapacitor LIC The LIC component is arranged in sequence from the base, cathode, separator, LTO anode, and cover as shown in Figure 4. The lithium hexafluorophosphate (LiPF 6 ) electrolyte is dripped for 65 μl on the separator. Furthermore, the process of punch to the cased supercapacitor tightly closed. Figure 3. Components of LIC 2.5 Electrochemical Characterization Electrochemical characterization of LIC was analyzed by Cyclic Voltammetry (CV) and calculation of capacitance. The CV test was performed to confirm the presence of a redox reaction. The capacitance of LIC can be determined by the equation as follow [1]: (1) where C single is specific capacitance (F/g); I is discharge current (ma); t is total discharge time (s); m is the mass of the electrode cell (g); ΔV is the potential difference during discharge (V). 3. Result and discussion The effect of activator variation and the temperature of carbonization resulting in varying pore size, pore volume, and surface area can be observed in Table 1. The effect of temperature on the BET surface area with the activator of ZnCl 2 is reported to be optimum at 500ºC, while temperatures high at 500ºC made the structure of pore is amorphous [9]. The same happened in this study, where increasing carbonization temperature affected a decrease in surface area. This occurs because increased temperatures result in the high percentage of ash content on the pore structure of activated carbon. Sample KOH having a larger surface area at 650ºC this is due to impurities being easily released at higher temperatures. Table 1. Porosity data Sample Activator Carbonization Temperature (ºC) Pore size (nm) Pore volume (cm 3/ g) S BET (m 2 /g) KO KOH 3,5M KO Zn ZnCl 2 3,5M Zn Based on Figure 4, the distribution of pores produced in samples belongs to micropore and mesopore. According to IUPAC (International Union of Pure and Applied Chemical), the pore size can be categorized into micropore (<2 nm), mesopore (2-50 nm), and macropore (> 50 nm) [1]. Pore characterization using BET in which the activated carbon sample is given gas of N 2 to analyzed the size of the pore, volume and surface area owned by activated carbon. In the process, N 2 gas serves to push the impurities and prevent the formation of tar on the surface of the pore. The greater gas of N 2 that can be absorbed by the activated carbon the larger the surface area and the resulting pore size. When the number of pores owned activated carbon more, then the pore volume will be smaller. It is unique to the KO 2 sample because it does not show the surface area and the pore size, but has a pore volume. This is rare in activated carbon studies. This may occur if the increase in the volume of N 2 gas against the given pressure is unable to open the pore surface of activated carbon. However, the distribution graph shows that the KO2 sample has a distribution are mesoporous and microporous sizes. Another possibility that happens if something happens to the sample during the measurement is done.

4 Figure 4. The pore distribution In fig. 5 we analyzed that the maximum adsorption volume of nitrogen gas can be absorbed by activated carbon. It occurs at the relative pressure at scale 1 and obtained a maximum volume of cm 3 /g for the KO3 sample. The sample that activated by KOH has a larger pore volume peak at 650ºC, where the higher the pore volume, the absorption capacity of the gas also increases. Can be observed in Figure 5 shows a curve of type I based on IUPAC classification, where this type belongs to the category of micropore. Figure 5. N 2 adsorption isotherm Figure 6. LIC cell Cyclic Voltammetry The CV curve is a type of hysteresis type curve that expresses a relationship between the voltage (V) and the resultant electric current (I). In figure 6, the curve shows the presence of an upper peak, which is analyzed as the oxidation peak or ion intercalation process into the cathode cell. The bottom peak sediment is analyzed as the reduction peak where the absorbed ion subsequently undergoes deintercalation. Deintercalation is the process of release of ion or charge during cycle formation. The

5 resulting capacitance depends on the magnitude of the distance between the oxidation peaks and the reduction. Based on the results obtained KO2 sample has the distance between the peak reduction and oxidation the largest. The magnitude of the distance between the reduction peak and oxidation affects the value of the capacitance. The value of capacitance will increase with the magnitude of the current distance that is formed in the cycle. Table 2. Specific capacitance Sample m k (g) I (A) t (s) V (v) C sp (mf/g) KO KO Zn Zn This is supported by the calculation data of LIC cathode-specific capacitance in Table 2 which shows that the KO2 sample has the largest capacitance value of 717,587 mf / g. Figure 7 shows the comparison between the active carbon surface area and the resulting LIC capacitance. Sample KO2 has the highest capacitance, where the high capacitance value is also affected by the mesoporous proportions generated higher than the micropore (figure 4), thus facilitating the occurrence of intercalation. Intercalation is a reversible process of insertion of ions into the interlayer of an easily expanding structure (between silicone montmorillonite layers) without destroying its structure. Figure 7. Relationship of Surface area S BET (m 2 /g) and capacitance C sp (mf/g) The performance of the supercapacitor is influenced by several factors of activated carbon pore structure as previously described, including surface area, pore size, and pore volume. The quality of good pore structure can result from the selection of appropriate methods to produce good active carbon as well so that a high capacitance LIC capacitor cathode will be obtained. 4. Conclusion LIC supercapacitor cathode has been produced from activated carbon of candlenut shell. Sample KO2 that carbonized at 500ºC has the highest capacitance, the high capacitance value is also affected by mesoporous proportions generated higher than micropore, thus facilitating the occurrence of intercalation. The performance of sample KO2 shows the capacitance specific is mf/ g. The experimental results indicated the potential use of carbonized candlenut shell as a precursor material of cathode for LIC.

6 Acknowledgments Authors acknowledge to the Research Center of Physic Indonesian Institute of Sciences (LIPI) and all of the elements who have supported this research. References [1] Abioye M A and Farid N A 2015 Renewable and Sustainable Energy Reviews [2] Zhang S, Chen L, Xiong Z, Xianzhong S, Kai W and Yanwei M 2017 Appl. Mater. & Interface [3] Liu C, Bhaskar B K, Ling L, Satya E, Cuanlong W, and Leon L S 2016 Carbon [4] Xiao P W, Zhao L, Li J, Wei Z and Han B. H. Materials & Design [5] Gamby J, Taberna P L, Simon P, Fauvarque J F and Chesneau M 2001 Journal of Power Source [6] Lu M 2012 Ebook Supercapacitor: Material, System, and Application (Australia: Wiley-VCH) p 99 [7] Gualous H, Alcicek G, Diab Y, Hammar A, Venet P, Adams K, Akiyama M and Marumo C 2008 Supercapacitors and Applications 2 [8] Gonzales A, Eider G, Jon A B and Roman M 2016 Renewable and Sustainable Energy Reviews [9] Demiral I, Samdan C A and Demiral H 2015 Desalination and Water Treatment 4