FABRICATION OF NANOPOROUS CARBON AS ELECTRODES FOR SUPERCAPACITORS Pitchuda Suwannasarn,a,b, Sujitra Wongkasemjit a,b, Thanyalak Chaisuwan a,b a The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand b Center of Excellence on Petrochemical and Materials Technology, Bangkok, Thailand Keywords : Polybenzoxazine, Nanoporous Carbon, Electrodes, Supercapacitors ABSTRACT Electrode material is a key element in determining the ability of energy storage for supercapacitors which should have high specific surface area (SSA) with an appropriate porous structure and also good conductivity. In this work, nanoporous carbons were prepared via pyrolysis and physical activation of polybenzoxazine which synthesized by using difference types of amine including ethylene diamine (EDA) and tetraethylenepentamine (TEPA) to study the pore structure and pore size distribution. In addition, silica nanoparticles were used as hard templates to create uniform mesoporous structure. The relationships between the specific capacitance and pore structure of the nanoporous carbon electrodes were investigated in 1.0 M of H2SO4. The results from both of non-template and templated-nanoporous carbons showed that CO2 activation led to better capacitive performances. At a scan rate of 1 mv/s, non-template nanoporous carbon showed the highest specific capacitance of 337.54 F/g due to its non-uniform mesopore size which was appropriate for electrolyte ions to transfer into the pores, However, it was found that at higher scan rate, the pore size had to be larger in order to facilitate ions in the pores. In this case, templated-nanoporous carbon from AS-40 silica nanoparticles showed the highest specific capacitance of 83.72 F/g at a scan rate of 25 mv/s due to its largest mesopore size from template facilitating the ion mobility. thanyalak.c@chula.ac.th INTRODUCTION Supercapacitors are the most attractive power sources due to their high power density, long life cycle and moderate energy compared to conventional dielectric capacitors and batteries (Lota, et al., 2016). The performance of SCs is mainly evaluated from a power energy densities (>10 Wh kg-1), an excellent cycleability, fast charge/discharge processes within second, safe operation and low cost (Zang and Zhao, 2009). Due to the construction of SCs consists of electrode material, electrolyte and separator, the things that determine SCs performance is electrode material which should have four major requirements; including high surface area, low electrical resistance, good polarizability and no participation in faradic reaction at the applied voltage (Pekala, et al., 1998). Carbon materials are remarkable as the first candidate electrode materials for EDLCs. Mostly, porous carbon materials, especially microporous activated carbon, have been widely selected as the electrode materials due to their high surface area, excellent electrical conductivity, high thermal stability and chemical stability and low cost. However, in order Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 1
to be quick electrolyte ions transportation during charge/discharge process, the intrinsically small micropores (<2nm) of the activated carbons restrict this process (Wan, et al., 2014). So, the surface of the micropores cannot be effectively utilized, which leads to low specific capacitance and poor rate capability. On the other hand, mesopores and macropores are more effectively utilized which will facilitate fast access of the electrolyte to the accumulated surface. Therefore, the essential keys in order to have the promising electrode materials for supercapacitors, the combination of advantages of microporous carbons and meso/macroporous carbons which are high specific surface area, large pore volume, tunable pore structure, controllable surface chemistry, and good electrical conductivity are required. Polybenzoxazine (abbreviated as PBZ) has been introduced as a new polymeric precursors for carbon gels which is recently developed as a class of phenolic resin (Lorjai, et al., 2011). Due to its unique properties, including low water adsorption, low shrinkage upon polymerization, no catalyst requirement, and high dimensional stability, PBZ has become an attractive materials. Polybenzoxazine has a wide range of molecular design flexibility (Jubsilp, et al., 2011). Moreover, by varying different types of amines, it can be changed the pore structure and pore volume of PBZ-based carbon. In this work, nanoporous carbons were prepared via pyrolysis and physical activation of polybenzoxazine which synthesized by using difference types of amine which are ethylene diamine (EDA) and tetraethylenepentamine (TEPA) to study the pore structure and pore size distribution. Besides using difference types of amine, I also studied the effect of template by using silica nanoparticles as hard templates with TEPA based nanoporous carbon to create uniform mesopore size and compare the result with the non-template materials from EDA based nanoporous carbon which created non-uniform mesopore size by itself. The relationship between the specific capacitance and pore structure of the nanoporous carbon electrodes was investigated in1.0 M of H 2 SO 4. EXPERIMENTAL A. Materials EDA-based nanoporous carbons were synthesized via sol-gel process by using phenol, paraformaldehyde, ethylenediamine (EDA) and Dimethylformamide (DMF) as a solvent. Phenol detached crystals (99.99%) was purchased from Fisher Chemical Company. Paraformaldehyde (powder, 95%) and EDA (99%) were purchased from Sigma-Aldrich Co., Ltd. DMF was purchased from RCI Labscan Co., Ltd. TEPA-based nanoporous carbons were synthesized by using bisphenol-a (BA), formaldehyde, triethylenepentamine (TEPA), silica nanoparticles which was used as hard template and hexadecyltrimethylammonium bromide (CTAB) used as stabilizer. BA (97%) was purchased from Sigma-Aldrich Co., Ltd. Formaldehyde was purchased from Merck Limited. TEPA were purchased from Sigma-Aldrich Co., Ltd. LUDOX AS-40 colloidal silica solution (40%) with an average particle size of 24 nm and LUDOX AS-30 colloidal silica solution (30%) with an average particle size of 12 nm were also purchased from Sigma-Aldrich Co., Ltd. CTAB was purchased from Fluka. Ethanol (EtOH), acetone and sulfuric acid (H 2 SO 4, 98%) were purchased from RCI Labscan Co., Ltd. All chemicals were used without further purification. B. Synthesis of polybenzoxazines Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 2
EDA-based benzoxazine was synthesized via solventless method reported by Ishida (Ishida, 1996). Phenol, paraformaldehyde and EDA at a 2:4:1 molar ratio, were mixed together and heated at 110 o C. The mixture of phenol, paraformaldehyde and EDA were heated into melting state and stirred continuously for 1 h. at 110 o C until the mixture became a transparent yellow colour. The obtained viscous liquid was left in an ambient condition to cool the temperature down and turn into a brittle solid. EDA-based benzoxazine solutions were prepared by dissolving the precursor in selected solvent and kept concentration at 30wt%. EDA-BZ solution was transferred into the vial and sealed, then place in an undisturbed condition at 150 o C in oil bath until finish the gelation process. The following step, PBZ-EDA gels were then cut into disc and immerse in acetone to exchange the original solvent, acetone was renewed for every 12 h, 3 times. PBZ-EDA organogels were obtained by conditionally dried at 100 o C overnight. The resulting EDA-based PBZ organogel was completely polymerized by step-raising temperature at 160, 180, 190, 200 and 220 o C for 1, 2, 6, 3 and 0.5 h, respectively, to obtain the high porosity fullypolymerized PBZ. EDA-based PBZ organogels were pyrolyzed under N 2 atmosphere flow rate of 600 cm 3 /min at 800 o C for 2 h, using following heating step: 30-250 o C for 1 h, 250-600 o C for 6h, 600-800 o C for 2h and hold at 800 o C for 2 h, followed by cooling to room temperature under nitrogen atmosphere. Then, you will obtain EDA-based nanoporous carbon. Here, the sample is labeled as C_EDA. TEPA-based benzoxazine precursors were synthesized by dissolving Bisphenol-A in N,Ndimethylformamide (DMF) in the glass bottle and continuously stirred until the clear solution was obtained. Formaldehyde was then added into the solution and mixed with 30% colloidal silica and cetyltrimethylammonium bromide (CTAB), respectively. In addition, colloidal silica was used in two size of the particle which are 12nm and 25nm, noted as AS30 and AS40,respectively. The following step, Tetraethylenepentamine (TEPA) in DMF was added dropwise into the previous mixture solution and continuously stirred continuously for approximately 30 min before putting in a closed system and heating at 80 o C for 2 days in an oil bath for the gelation process. After that, CTAB and solvent were removed out by Soxhlet extraction using ethyl alcohol as a carrier phase under condensation temperature of 10 o C and heating temperature of 100 o C. Then, the PBZ-silica composites were heated at 80 o C for 24 h to remove all the residual ethyl alcohol. The obtained PBZ-silica composite gel was then fully polymerized by step curing in an oven start at room temperature to 100 o C in 30 min. C. Preparation of nanoporous carbon electrodes The fully-polymerized PBZ-silica composites were carbonized under N 2 flow of 600 cm 3 /min, using the following cycles step: 30-250 o C for 1 h, 250-600 o C for 6 h, 600-800 o C for 1 h and held at 800 o C for 2 h, and the oven was then cooled to room temperature under N 2 atmosphere. The next step is the removal which is done by using 15% of HF solution which was prepared in the mixture of 50water:50ethyl alcohol. Carbon-silica composites were soaked in the as-prepared 15%HF solution for 24 h. Here, the samples are labeled as C_TEPA_AS30 and C_TEPA_AS40. The activated nanoporous carbon electrodes were further pyrolyzed at 900 C under a CO 2 flow rate of 350 cm 3 /min for 3 hours. These were labeled as AC_TEPA_AS30 and AC_TEPA_AS40. D. Characterizations of nanoporous carbon electrodes Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 3
The porous structure were characterized by Quantachrome Autosorb-1 MP using N2 adsorption-desorption. The cyclic voltammetry (CV) were used to study the electrochemical properties. The testing cell for the CV measurements consisted of the graphite rods as current collectors, two carbon electrodes separated by a porous polyester sheet, and 1.0 M H 2 SO 4 as the electrolyte solution. RESULTS AND DISCUSSION A. Chemical structure of polybenzoxazines Chemical Structure of Polybenzoxazine derived from EDA without template The FTIR spectra of EDA-based benzoxazine precursors were shown in Figure 4.1 (a). The three characteristic absorption bands assigned to EDA-based benzoxazine structure appeared at 1230 1236 and 1028 1036 cm -1 which represent to the asymmetric stretching and symmetric stretching of Ph O C, respectively. CH 2 wagging of benzoxazine appeared at 1340 1380 cm -1 (Lorjai, et al., 2011). These three peaks disappeared after ring-opening polymerization of oxazine ring. (b) PBZ_EDA Absorbance (a) BZ_EDA 4000 3000 2000 1000 Wavenumber (cm -1 ) Figure 1. FTIR spectra of EDA-based benzoxazine monomer and EDA-based polybenzoxazine organogel. Chemical Structure of Polybenzoxazine derived from TEPA silica nanoparticles as hard template The FTIR spectra of partially-polymerized and fully-polymerized TEPA-based polybenzoxazine incorporating with AS-40 and AS-30 colloidal silica as silica template were shown in Figure 1 and 2, respectively. The characteristic absorption bands assigned to partially-polymerized TEPA-based polybenzoxazine incorporating with silica template was observed at 1479 cm -1 due to trisubstituted benzene ring, at 1360 cm -1 due to CH 2 wagging of oxazine ring, the broad peak at 1104 cm -1 corresponding to -Si-O-C- linkage confirm the presence of silica in the polybenzoxazine matrix and around 880-995 cm -1 represents the out of plane bending vibration of =C-H. In case of fully-polymerized TEPA-based polybenzoxazine incorporating with silica template, the new absorption peak appears at 1654 cm -1 due to the tetra-substituted benzene ring which confirms the ring opening polymerization of TEPA-based polybenzoxazine. Further the intensity of the adsorption band at 1104 cm -1 increased due to the formation of silica nanoparticles into polybenzoxazine matrix increased (Agag and Takeichi, 2011). Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 4
Absorbance (b) Fully-polymerized (a) Partially-polymerized 4000 3000 2000 1000 Wavenumber (cm -1 ) Figure 2. FTIR spectra of (a) partially-polymerized and (b) fully-polymerized TEPAbased polybenzoxazine incorporating with silica nanoparticles. B. Porous structure of polybenzoxazine-based carbon electrodes Porous Structure of EDA-based non-templated nanoporous carbon In order to investigate the porosity and the effect of CO 2 activation process of EDA-based nanoporous carbon, the N 2 physisorption using Quantachrome Autosorb-1MP surface area analyzer was used. The microstructure characteristics of non-templated polybenzoxazinebased nanoporous carbon materials derived from EDA-based PBZ precursors are summarized in Table 1. The result showed that the surface area (S BET ) of the nanoporous carbon materials increased after activated with CO 2 at 900 C due to the development of the micropores in the materials during the activation process (Taer, et al., 2010). As a result, the average pore diameter decreased from 9.30 nm to 7.45 nm after the activation due to the activation opening the closed pores, drilling new narrow micropores and widening the preexistent (Xia, et al., 2008). Table 1 Textural characteristics of carbon materials derived from polybenzoxazine precursor. S BET S micro V micro V meso APD Microporosity Mesoporosity Sample (m 2 /g) (m 2 /g) (cm 3 /g) (cm 3 /g) (nm) (%) (%) C_EDA 355 137 0.07 0.76 9.30 8.22 91.78 AC_EDA 460 244 0.12 0.72 7.45 14.33 85.62 Porous Structure of TEPA-based templated nanoporous carbon The microstructure characteristics of TEPA-based nanoporous carbon are summarized in Table 2. The surface area (S BET ) of the nanoporous carbon materials increased after activated with CO 2 at 900 C. As mentioned before, it came from the development of the micropores in the materials during the activation process. Moreover, it was found that the average pore size of all sample was between 2-50 nm which is classified as a mesoporous material. By using the 30 wt% of AS-30 colloidal silica nanoparticles with diameter 12 nm, the resulting nanoporous carbon was found that it has the pore size a little bit larger than the size of silica template; whereas, using the 15 wt% of AS-40 colloidal silica nanoparticles with diameter 25 nm, the resulting nanoporous carbon was found that it had pore size close to the size of silica template. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 5
Table 2 Pore characteristics of templated nanoporous carbon derived from TEPA-based polybenzoxazine obtained after removal of silica nanoparticle templates Sample S BET S micro V micro V meso APD a meso Microporosity Mesoporosity (m 2 /g) (m 2 /g) (cm 3 /g) (cm 3 /g) (nm) (%) (%) C_TEPA_blank 135 114 0.07 0.04 3.15 63.64 36.36 AC_TEPA_blank 839 819 0.42 0.04 2.19 91.30 8.70 C_TEPA_AS30 739 - - 1.49 15-100 AC_TEPA_AS30 961 448 0.08 1.63 15 4.68 95.32 C_TEPA_AS40 408 - - 0.83 25.9-100 AC_TEPA_AS40 656 145 0.10 0.94 25.9 9.38 90.71 C. Electrochemical characterizations of polybenzoxazine-based carbon electrodes Cyclic voltammetry (CV) of all samples at a scan rate of 1 mv/s show in Figure 3(a) and (b). CV curves of all nanoporous carbon materials, excepted C_TEPA_blank, showed a rectangular, symmetric, and reversible shape with less deviation in the voltage range, suggesting ideal EDLC behavior. Moreover, There is also an increase in the induced current caused by activation process with CO 2 atmosphere at 900 o C as seen in both nontemplate nanoporous carbons and templated-nanoporous carbons due to the improvement in wettability between surface of nanoporous carbons and electrolyte ions which leading to increment of specific capacitance value. As seen in Table 3 and 4, the highest specific capacitance was found for AC_EDA (337.54 F/g), having high surface area with proper pore size for charge accumulation. Additionally, as scan rate was increased a larger pore size will be a crucial factor for ion mobility and facilitate electrolyte ions into pores of electrode materials. Therefore, If the electrode can retain a rectangular shape even at high scan rate, implying good capacitive performance for quick charge/discharge operation (Wan, et al., 2014). Resulting in templated-nanoporous carbons, the highest specific capacitance at scan rate of 25 mv/s is AC_TEPA_AS40 (83.72 F/g) due to its largest mesopore size from template. Table 3 Specific capacitance at different voltage scan rates for TEPA-based templated nanoporous carbon electrode and its activated nanoporous carbon Specific capacitance (C sp, F/g) Scan rate(mv/s) 1 2 5 10 C_EDA 150.42 124.9 83.48 37.94 AC_EDA 337.54 134.26 87.86 46.42 Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 6
.03.02 (a).03.02 (b).01.01 Current (A/g) 0.00 -.01 Current (A/g) 0.00 -.01 -.02 -.03-1.5-1.0 -.5 0.0.5 1.0 1.5 Potential (V) C_EDA AC_EDA -.03-1.5-1.0 -.5 0.0.5 1.0 1.5 Potential (V) Figure 3 Cylic voltammograms of non-template nanoporous carbons (a) and templatednanoporous carbons (b) at a scan rate of 1 mv/s -.02 Table 4 Specific capacitance at different voltage scan rates for TEPA-based templated nanoporous carbon electrode and its activated nanoporous carbon Specific capacitance (C sp, F/g) Scan rate (mv/s) 1 2 5 10 20 25 C_TEPA_blank 14.96 11.66 8.32 4.5 3.5 2.64 C_TEPA_AS30 161.16 142.14 122.78 105.82 71.02 58.76 C_TEPA_AS40 145.8 133.4 116.14 102 81.5 72.22 AC_TEPA_blank 183.02 155.7 104.02 57.84 24.7 17.74 AC_TEPA_AS30 178.44 170.92 144.84 118.86 84.32 73.48 AC_TEPA_AS40 164.6 152.68 137.14 116.4 94.18 83.72 CONCLUSIONS Polybenzoxazines were successfully synthesized using both EDA and TEPA as amines. The S BET of the nanoporous carbon materials increased after activated with CO 2 at 900 C due to the development of the micropores in the materials during the activation process. The higher SSA of AC, the higher ability for charge accumulation; resulting in the higher C sp. The activated nanoporous carbon material derived from EDA- based PBZ showed the highest C sp of 337.54 F/g at a scan rate of a mv/s in 1.0 M H 2 SO 4 due to its high SSA and proper mesopore size for electrolyte ions mobility and ions accumulations. However, as the scan rates were increased up to 25 mv/s, the larger pore size would be the best candidate to be the electrode material to provide fast electrolyte ions transfer rate (Kwon, et al., 2014). ACKNOWLEDGEMENTS This thesis is funded by the Petroleum and Petrochemical College, the Ratchadapisek Sompote Endowment Fund of Chulalongkorn University and the Center of Excellence on Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 7
Petrochemical and Materials Technology. In addition, the authors would like to thank Professor Suwabun Chirachanchai for the electrochemical measurement. REFERENCES Lota, K., Acznik, I., Sierczynska, A., Lota, G. (2016). The capacitance properties of activated carbon obtained from chitosan as the electrode material for electrochemical capacitors. Material Letters, 173, 72-75. Zang, L. and Zhao, S. (2009). Carbon-based materials as supercapacitor electrodes. Chemical Society Reviews, 38, 2520-2531. Pekala, R., Farmer, J., Alviso, C., Tran, T., Mayer, S., Miller, J., Dunn, B. (1998). Carbon aerogels for electrochemical applications. Journal of Non-Crystalline Solids, 225, 74-80. Wan, L., Wang, J., Xie, L., Sun, Y., and Li, K. (2014). Nitrogen-Enriched Hierarchically Porous Carbons Prepared from Polybenzoxazine for High-Performance Supercapacitors. ACS Applied Materials & Interfaces, 6, 15583-15596. Lorjai, P., Wongkasemjit, S., Chisuwan, T., Jemieson, A. (2011). Significant enchancement of thermal stability in the non-oxidative thermal degradation of bisphenol-a/aniline based polybenzoxazine aerogel. Polymer Degradation and Stability, 96, 708-718. Jubsilp, C., Takeichi, T., Rimdusit, S. (2011). Property enhancement of polybenzoxazine modified with dianhydride. Polymer Degradation and Stability, 96, 1047-1053. Ishida,H. (1996). Process for preparation benzoxazine compounds in solventless systems, US Patent 5, 543, 516. Agag, T., and Takeichi, T. (2011). Stnthesis ans characterization of benzoxazine resin- SiO 2 hybrids by sol-gel process: The role of benzoxazine-functional silane coupling agent. Polymer, 52, 2757-2763. Taer, E., Deraman, M., Talib, I.A., Umar, A.A., O, M., and Yunus, R.M. (2010) Physical, electrochemical and supercapacitive properties of activated carbon pellets from pre-carbomized rubber wood sawdust by CO 2 activation. Current Applied Physics, 10, 1071-1075. Xia, K., Gao, Q., Jiang, J., and Hu, J. (2008). Hierarchical porous carbons with controlled micropores and mesopores for supercapacitor electrode materials. Carbon, 46, 1718-1726. Kwon, H.S., Lee, E., Kim, B.S., Kim, S.G., Lee, B.J., Kim, M.S., and Jung, J.C. (2014). Activated carbon aerogel as electrode material for coin-type EDLC cell in organic electrolyte. Current Applied Physics, 14, 603-607. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 8