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Master's Thesis. Synthesis of N-doped Carbon Materials with Controlled Pore Sizes from Double-Network Hydrogels.. Suhyeon Hwang Department of Chemistry. Graduate School of UNIST 2017.

Synthesis of N-doped Carbon Materials with Controlled Pore Sizes from Double-Network Hydrogels.. Suhyeon Hwang. Department of Chemistry. Graduate School of UNIST.





Abstract. A series of nitrogen-doped carbon materials with different specific surface areas (SSA) and controlled pore size distributions (PSD) were synthesized through a facile and low-cost method. Double network hydrogels (DN gel) were synthesized from Agarose as the 1st network and polyacrylamide as the 2nd network and used as the precursor. To afford the micro porous structure, potassium oxalate (K2C2O4) was used as activating agent. The nitrogen doped carbon foam obtained from the DN gel has a higher surface area than a single network. The highest surface area (~2447.4 m2/g) was obtained at 700 °C when the precursor to activating agent ratio was 1:1 (g:g). The pore size distribution can be controlled by changing the time or temperature. The transformation of micro pores to mesopores was investigated by increasing the heat treatment temperature from 600 oC to 800 oC. As a metal-free electrocatalyst, it was examined for oxygen reduction reaction (ORR) in alkaline media. As a result, N-doped carbon materials from the double network showed higher electrocatalytic performance than single networks..



Contents 1. Introduction ......................................................................................................................................... 1. 1-1. Porous carbon materials ............................................................................................................... 1. 1-2. Template and activating gent ....................................................................................................... 2. 1-3. Double network hydrogels (DN gels) .......................................................................................... 3. 1-4. Application for electrochemical catalyst ..................................................................................... 4 1-4-1. Fundamentals of fuel cell ................................................................................................... 4 1-4-2. Oxygen reduction reaction (ORR)...................................................................................... 5 1-4-3. Development of an electrocatalyst ..................................................................................... 6. 1-5. Objective...................................................................................................................................... 8. 2. Experimental section ......................................................................................................................... 10. 2-1. Chemicals and Materials............................................................................................................ 10. 2-2. Synthesis of double network gels .............................................................................................. 10. 2-3. Synthesis of porous carbon materials ........................................................................................ 11. 2-4. Physical and chemical characterization ..................................................................................... 11. 2-5. Synthesis of catalyst ink ............................................................................................................ 12. 2-6. Electrochemical characterization ............................................................................................... 12.

3. Result and Discussion ....................................................................................................................... 13 3-1. Effect of double network gel as a precursor ............................................................................ 13 3-1-1. Comparison between single network and double network gel with NaCl ....................... 13 3-1-2. Increasing the template amount. ....................................................................................... 17. 3-2. Different type of template or activating agent. (Ca(OAc)2 and K2C2O4)................................... 19. 3-3. Activating agent: Potassium oxalate (K2C2O4) .......................................................................... 24 3-3-1. Increasing the activating agent ......................................................................................... 24 3-3-2. Decreasing the activating agent ........................................................................................ 28 3-3-3. Compare to single network and double network .............................................................. 31. 3-4. Expanding the heating period .................................................................................................... 34. 3-5. Increasing the temperature ......................................................................................................... 37 3-6. Application – Oxygen Reduction Reaction (ORR) ................................................................... 41 3-6-1. Comparison the single network and double network ....................................................... 41 3-6-2. Comparison the agarose and acrylamide weight ratio ...................................................... 43 3-6-3. Comparison amount of activating agent ........................................................................... 44 3-6-4. Comparison different temperature.................................................................................... 47. 4. Conclusion ........................................................................................................................................ 50. 5. Reference .......................................................................................................................................... 51.

List of Figure Figure 1. Illustration of proton exchange membrane (PEM) fuel cell .................................................... 4 Figure 2. Preparing the metal-free nitrogen-doped and transition metal-derived carbon catalysts ........ 6 Figure 3. Different nitrogen configuration and XPS pattern................................................................... 8 Figure 4. Schematic illustration of the one-pot method for preparing hybrid-linked DN gels ............... 9 Figure 5. XRD pattern. Comparison between single network and double network gel with NaCl ...... 14 Figure 6. SEM image of before heat-treatment..................................................................................... 14 Figure 7. XRD pattern. Difference of template concentration and double network ratio ..................... 17 Figure 8. SEM image of carbon material with Ca(OAc)2 and K2C2O4 before and after heat-treatment .............................................................................................................................................................. 19 Figure 9. PXRD pattern. Difference of precursor ratio with Ca(OAc)2 and K2C2O4 ............................ 20 Figure 10. N2 adsorption-desorption isotherms and pore size distributions about carbon material with Ca(OAc)2 and K2C2O4........................................................................................................................... 21 Figure 11. PXRD pattern. Different precursor ratio with 1:3 K2C2O4 .................................................. 24 Figure 12. N2 adsorption-desorption isotherms and pore size distributions about different precursor ratio with 1:3 K2C2O4. ........................................................................................................................... 25 Figure 13. PXRD Pattern. Difference of the activating agent concentration ........................................ 28 Figure 14. N2 adsorption-desorption isotherms and pore size distributions about difference of the activating agent concentration .............................................................................................................. 29 Figure 15. PXRD Pattern. Comparison between single network and double network with 1:3 K 2C2O4 .............................................................................................................................................................. 31 Figure 16. Comparison of surface area and pore size distributions between single network and double network gel with K2C2O4 ...................................................................................................................... 32 Figure 17. PXRD Pattern. Comparison of heating period .................................................................... 34 Figure 18. N2 adsorption-desorption isotherms and pore size distributions for comparison of heating period .................................................................................................................................................... 35 Figure 19. Illustration of increasing the micro pore depending on the heating period ......................... 36 Figure 20. PXRD Pattern. Comparison of the different temperature .................................................... 37.

Figure 21. N2 adsorption-desorption isotherms and pore size distributions for comparison of the different temperature ........................................................................................................................... 38 Figure 22. Illustration about pore size transition depending on the temperature .................................. 39 Figure 23. TEM images of the porous carbon material depending on the temperature ........................ 39 Figure 24. Cycle voltammetry curves and Linear sweep voltammograms for comparison the single network and double network ................................................................................................................. 42 Figure 25. Cycle voltammetry curves and Linear sweep voltammograms for comparison the agarose to acrylamide weight ratio .................................................................................................................... 43 Figure 26. Cycle voltammetry curves for comparison amount of activating agent .............................. 45 Figure 27. Linear sweep voltammograms for comparison amount of activating agent ...................... 46 Figure 28. Cycle voltammetry curves and Linear sweep voltammograms for comparison different temperature ........................................................................................................................................... 48.

List of Table Table 1. IUPAC classification of pore size and adsorption mechanism ................................................. 1 Table 2. Textural properties of the carbon material for comparison between single network and double network gel with 4 wt% aqueous NaCl solution ................................................................................... 15 Table 3. Elemental composition of the materials for comparison between single network and double network gel with 4 wt% aqueous NaCl solution ................................................................................... 16 Table 4. Textural properties of the carbon materials with 20 wt% aqueous NaCl solution.................. 18 Table 5. Elemental composition of the carbon materials with 20 wt% aqueous NaCl solution ........... 18 Table 6. Textural properties of the carbon materials with 20 wt% aqueous Ca(OAc)2 and K2C2O4 solution.................................................................................................................................................. 22 Table 7. Elemental composition of the carbon materials with 20 wt% aqueous Ca(OAc)2 and K2C2O4 solution.................................................................................................................................................. 23 Table 8. Textural properties of the carbon materials with 1:3 K2C2O4 ................................................. 26 Table 9. Elemental composition of the carbon materials with 1:3 K2C2O4 .......................................... 27 Table 10. Textural properties of the carbon materials with different amount of K2C2O4 ..................... 30 Table 11. Elemental composition of the carbon materials with different amount of K2C2O4............... 30 Table 12. Textural properties of the carbon materials for comparison between single network and double network gel with 20 wt% aqueous K2C2O4 solution ................................................................. 33 Table 13. Elemental composition of the carbon materials for comparison between single network and double network gel with 20 wt% aqueous K2C2O4 solution ................................................................. 33 Table 14. Textural properties of the carbon materials for comparison of heating period ..................... 35 Table 15. Elemental composition of the carbon materials for comparison of heating period .............. 36 Table 16. Textural properties of the carbon materials for comparison of the different temperature .... 38 Table 17. Elemental composition of the carbon materials for comparison of the different temperature .............................................................................................................................................................. 40 Table 18. Summary of half-wave potential(E1/2), onset potential (Eonset) and reduction current density (i0) for comparison the single network and double network ................................................................. 42 Table 19. Summary of half-wave potential(E1/2), onset potential (Eonset) and reduction current density (i0) for comparison the agarose to acrylamide weight ratio .................................................................. 44.

Table 20. Summary of half-wave potential(E1/2), onset potential (Eonset) and reduction current density (i0) for comparison amount of activating agent..................................................................................... 47 Table 21. Summary of half-wave potential(E1/2), onset potential (Eonset) and reduction current density (i0) for comparison different temperature ............................................................................................. 48 Table 22. Summary of ORR performance of N-doped carbon catalysts obtained from RDE ............ 49.

1. Introduction 1.1. Porous carbon materials Porous carbon materials have received increasing attention for potential use in a variety of applications. Porous carbon materials, which typically display useful properties that included high hydrophobicity, high surface areas, large pore volumes, high chemical inertness, good mechanical stability, and good thermal stability1, can be incorporated into gas separation and, water purification membranes, catalyst supports, chromatography columns, storage vessels for natural gas, used as electrodes various capacitors2 and other energy-demanding devices3. The International Union of Pure and Applied Chemistry (IUPAC) classifies porous materials, according to their pore sizes: microporous, mesoporous and macroporous.4 (Table 1) Table 1 IUPAC classification of pore size and adsorption mechanism.5 Pore type. Size regime. Condensation mechanism. Micropore. <2 nm. Three-dimensional. Mesopore. ≥2≤50 nm. Capillary. Macropore. >50 nm. No condensation. While the total specific surface area (SSA) is generally discussed in terms of their specific applications, the effective specific surface area (E-SSA) often determines the fundamental performance.6 The E-SSA depends on the total SSA and the pore size distribution (PSD) of the respective materials. Therefore, the discovery of new methods to control both SSA and PSD of carbon materials should facilitate material design and improve performance. For example, Chen and coworkers prepared various carbon materials with different SSA and controlled PSD via a facile and low-cost method by using different carbon sources such as cork or phenolic resin and changing various parameters, including carbon source, activation agent, amount of activation agent and activation temperature.7 While many materials can be prepared in industry scale, finding a cheaper precursor and more facile method is still an interesting and challenging task.. 1.

1.2. Template and activating gent It has been challengeable to synthesize the uniform porous carbon materials.5 Many kinds of inorganic templates have been employed in order to synthesize carbon materials with uniform pore sizes.8 Knox and co-workers pioneered the template synthesis of porous carbons.9 Hard templates can possess porous structure and are usually infiltrated with carbon precursor, followed by carbonization and etching of the template. Thus the general template synthetic procedure for porous carbons is as follows: 1) preparation of the carbon pre-cursor/inorganic template composite, 2) carbonization, and 3) removal of the inorganic template. Various inorganic materials, including silica materials, zeolites, Ni(OH)2, MgO, anodized aluminum oxide, natural sepiolite have been used as templates respectively. However, most of the hard templates are expensive and difficult to remove all of them after carbonization of the precursors. Recently, NaCl and calcium acetate were investigated as hard templates respectively10. Chemical activation using activation agent such as KOH is also being widely investigated because it can be used to afford porous carbon materials with ultra-large surface areas (1500–3500 m2 g−1) from various precursors (coal, polymers, biomass or hydrothermally carbonized biomass, graphene, etc.)11,12. However due to the high corrosiveness of KOH and its toxicity, the final product is expensive. And also because KOH is strong base, which could break the hydrogen bonding. It is hard to be used as agent for Agarose-polyacrylamide double-network gel heat treatment. Fuertes and coworkers found a novel activating agent, potassium oxalate, which produced highly microporous carbons with BET surface areas of 2600–3000 m2 g−1, pore volumes of 1.3–1.6 cm3 g−1 and pore size distributions in the super micropore-small meso pore (<3 nm) region9. For these reasons, we explored the use of potassium oxalate as activation agent in double-network system.. 2.

1.3. Double network hydrogels (DN gels) Hydrogels, which feature, three dimensional cross-linked, water-swollen polymer networks, often offer superior biocompatibility, good biodegradability and tunable porous structures. They have attracted considerable interest for use in a wide range of applications, such as contact lenses, tissue engineering, diagnostics and drug delivery.13 Because even though double network hydrogels have 80 to 90 weight percent (wt %) of water, it has strong mechanical properties which occurred by sacrificing the rupture of the covalent bonds of the brittle 1st network and allowing the 2nd network to turn back to its initial configuration.14 Many hydrogels such as alginate, chitosan, dextran, agarose, proteins (collagen and gelatin) and synthetic polymers (for example, poly(ethylene glycol), poly(vinyl alcohol) etc.) have been explored. Agarose is particularly interesting because it forms the supporting structure in the cell walls of algae with a typical sol–gel phase transition: the agar forms gels upon cooling to 30–40 °C, while melting to sols upon heating to 90–95 °C. Some other attractive properties of agarose gels include high mechanical strength, even at very low concentrations (as low as 0.15%), and excellent tolerance to extreme pH. Li and co-workers reported a homogeneous hydrogel-based bottom-up strategy to build three dimensional porous carbon-supported electrocatalysts.10 Polyacrylamide (PAM) is also a highly waterabsorbent polymer polymerized from acrylamide, with high contents of carbon and nitrogen elements, PAM can be expected to be an excellent candidate for producing porous carbons doped with nitrogen.15 Zheng and co-workers synthesized the agarose-PAM double network hydrogels via a facile one-pot method.16. 3.

1.4. Application for electrochemical catalyst. 1.4.1 Fundamentals of fuel cell As rapidly increasing consumption of fossil fuels, the energy source from fossil fuels is gradually running low. The alternative energy sources have been studied to prevent and solve this problem. As a promising application, fuel cells have received attention because they convert chemical energy into electrical energy directly, producing water, electricity and heat during the reaction. Therefore, it is an environment-friendly and high efficient device.17 An illustration of proton exchange membrane (PEM) fuel cell is shown in Figure 1. When the hydrogen comes into the fuel cell at the anode, hydrogen is decomposed into protons and electrons. Electrons flow from anode to cathode through the wires to generate an electrical current. On the other hand, at cathode, oxygen enters and combines with the protons which pass through the polymer electrolyte membrane (PEM) and electron came from the anode, foaming the water as the product.18. Figure 1. Illustration of proton exchange membrane (PEM) fuel cell. 4.

Electrochemical reaction occurred at the surface of the electrode. At the anode, hydrogen oxidation reaction (HOR) take a place, whereas at the cathode, oxygen reduction reaction (ORR) take place. The reactions at the electrode are as follows.18 H2 → 2H+ + 2e-. Anode Reaction : Cathode Reaction :. 1/2O2 + 2H+ + 2e- → H2O. Overall reaction of fuel cell :. 𝐸𝑎0 = 0.00 𝑉. (1.1). 𝐸𝑐0 = 1.229 𝑉. (1.2). H2 + 1/2O2 → H2O. (1.3). Where 𝐸𝑎0 is an anode potential and 𝐸𝑐0 is a cathode potential. Both are under standard condition versus SHE. And equation 1.3 is with the equilibrium standard electromotive force calculated to be 1.229 V.18. 1.4.2. Oxygen reduction reaction (ORR) In a fuel cell, at the anode, the hydrogen molecule can split easily, whereas at the cathode, it is hard to split the oxygen molecule and it lead to loss of activation.18 The ORR activity and mechanism is different depending on the type of carbon as electrocatalyst. For a glassy carbon electrode, there is the mechanism as follows :19. O2 → O2(ads). (1.4). O2(ads) + e- → [O2(ads)]-. (1.5). [O2(ads)]- → O2(ads) -. (1.6). O2(ads) - + H2O → HO2-(ads). (1.7). HO2-(ads) → HO2-. (1.8). The subscripts “ads’’ represent that the corresponding species are adsorbed on the surface of glassy carbon electrode. In reaction 1.6, the [O2(ads)]- species as a reactant are adsorbed on an inactivity graphite site, forming stable state relatively. The O2(ads) – species as a product are the same as the reactant, but absorbed on an active site which is an edge planes of the carbon materials, by migrating. It is the rate determining step when the pH is over 10.20 However, if pH is less than 10, the reaction 1.5 is the rate determining step which the first electron transfer to oxygen.21. 5.

1.4.3. Development of an electrocatalyst. To improve the ORR activities at the cathode which are much slower than the anode reaction, the platinum based material has been used as a best catalyst among the pure metal due to high performance.22 However, because the platinum is expensive and rare metal, it affect to the cost of the fuel cell. In addition, at the high voltage, platinum can be able to react with water and/or oxygen and then form the oxide layer on the catalytic active site. Consequently, it lead to use the more loading of platinum to make up for the flaw.23 Besides there are several drawbacks such as fuel crossover and CO poisoning effect.24 Therefore, it has been challenged to replace from the platinum based catalyst. Researchers have done a lot of work to reduce or replace the platinum based catalyst. As shown in figure 225, there are general methods to synthesize the transition metal derived carbon and metal-free nanocomposites. In the 1960s, Jasinski studied about cathode catalyst in the fuel cell by using the phthalocyanines with cobalt, comparing with nickel, platinum and copper.26,27 Afterward, macrocylic transition-metal compounds like a phthalocyanines has been explored.27 One of the strategies is that polyaniline mixed with cobalt and iron, as polyaniline-FeCo-C, synthesized as a catalyst by Wu et al.28 This macrocyclic complex showed high ORR performance because polyaniline contain the nitrogen and after heat-treatment, it has graphitic property come from the aromatic ring structure. In addition, cobalt helps to increase the activity and stability of performance.27, 15. Figure 2. Effect of synthetic parameters on performance of metal-free nitrogen-doped (N-C) and transition metal-derived carbon (M– N–C) catalysts.29,30,25. 6.

Although using transition-metal catalyst is beneficial, the transition metals are toxic and it needs the process for removing the transition metal. Consequently, it brings about high cost and harmful environmental effect and there are other disadvantages such as low selectivity and poor durability.31 Therefore, developing the catalyst for cathode in fuel cell has focused on metal-free catalyst which has high performance. The carbon nanomaterials such as fullerenes, carbon nanotubes (CNTs), graphene are appropriate for metal-free catalyst due to thermal stability, corrosion resistance, high surface area and large pore volme.31 To improve the catalytic ability, heteroatoms can be doped in the carbon network. But among the other heteroatoms, nitrogen has the most potential of becoming dopant. Because the nitrogen atom size is similar to carbon, it can dope into the carbon network easily. The carbon atoms neighboring the nitrogen which is more electronegative than carbon is electron deficient and then it lead the oxygen adsorption on the carbon structure. In addition, as nitrogen is an n-dopant due to the lone pair electron, the Fermi level shift to conduction band closely and the n-doped carbon material make more electronically conductive.25 As seen in figure 3a, when the nitrogen doped in the carbon network, there are commonly three positions depending on the configuration and in figure 3b, to distinguish the different types of nitrogen, X-ray photoelectron spectroscopy (XPS) is used and we can be aware of the nitrogen configuration in accordance with binding energy. If nitrogen is connected with two sp2 carbon at the edge sites, it is a pyridinic nitrogen and it give the graphitic π system with one P π electron. (binding energy of ~398.6 eV) In graphitic nitrogen case, it contributes to graphitic π system with two Pπ electorns. (binding energy of ~401.4 eV) When the nitrogen was in the five-membered ring, it is a pyrrolic nitrogen which provide the graphitic π system with two Pπ electrons and it is thermally unstable due to five-membered ring so when the heating temperature is above 800 °C, pyrrolic nitrogen can convert into a pyridinic or graphitic nitrogen configuration. (binding energy of ~400.5 eV)25 The binding energy of nitrogen oxide functionalities is between 402 and 405 eV. 32 For the oxygen reduction reaction, the pyridinic and/or graphitic nitrogen are tend to improve the efficiency but so far, it is unclear and engaged in debates which configuration is more influence.. 7.

Figure 3. (a) Nitrogen doping into carbon plane at different location.25,33 (b) Typical binding energies of nitrogen atoms in different doping environment determined by X-ray photoelectron spectroscopy.25, 34. 1.5. Objective Double network gel is used for the bio-field as tissue engineering or drug and biomolecule carriers. Agarose and polyacrylamide are common precursors to synthesize the double network gel. Agarose help to synthesize the double network gel easily by one-pot method and polyacrylamide has the graphitic like structure which related to conductivity and nitrogen which improve the electrocatalytic performance. But no one use the double network gel as a precursor to porous carbon material. So we supposed if double network gel synthesized by using agarose and polyacrylamide use as a precursor, it can contain both advantages and because it is more denser than only single network due to interpenetrating network, it have more carbon source which can react with template or activating agent at the same volume (Figure 4). We wanted to investigate that it can be a precursor for nitrogen doped porous carbon material and this carbon material can get a high surface area and pore volume with high nitrogen content. To confirm the ability of the nitrogen doped porous carbon material, the oxygen reduction reaction was studied in the alkaline media, comparing to platinum catalyst.. 8.

Figure 4. Schematic illustration of the one-pot method for preparing hybrid-linked DN gels with the first network being physically linked and the second network being chemically linked.. 9.

2. Experimental section 2.1. Chemicals and Materials Agarose (BioReagent, for molecular biology, low EEO) and. calcium acetate hydrate. ((CH3COO)2Ca·xH2O, ≥99%) were purchased from Sigma Aldrich. Acrylamide (98%), N,N’methylenebisacrylamide (99%), potassium oxalate monohydrate (C2K2O4·H2O, ACS, 98.8-101.0 %) ammonium peroxydisulfate (98%), platinum nominally 40% on carbon black and nafion D-520 dispersion ( 5% w/w in water and 1-propanol, |>1.00 meq/g exchange capacity) were purchased from Alfa Aesar. Sodium chloride (NaCl, 99%) was purchased from Daejung. All chemicals were used without further purification.. 2.2. Synthesis of double network gels Double network gels were prepared using a one-spot method. Agarose (Ag) was used as the first network precursor, acrylamide (Am) as second network precursor and N,N’-methyenebisacrylamide (MBAA) as the second network’s cross linker was mixed in the vial and template or activating agent aqueous solution (NaCl, Ca(OAc)2, K2(C2O4) was added into that vial. It was kept in the oven for 20 minute at 115 ºC to make agarose sol-form like liquid state and then cooled down until it is at 40~45 ºC. When the mixture is liquid state and its temperature is around 40~45 ºC, Ammonium persulfate (APS) solution as initiator was added into mixture. After injected the APS solution, let it cool down to room temperature to form the first network and nitrogen gas blow into vial to remove the air because the oxygen in the air can react with initiator. To synthesize the second network, mixture was kept in the oven at 65 ºC for 3.5 hours. If the time is less than 3.5 hours, the second network polymerization is not finished and if time is over than 3.5 hours, second network can broke the first network. To remove the water, it was lyophilized for 2 days. The amount of the reagent, APS : 1 mol % of Acrylamide, MBAA: 0.1 mol % of Acrylamide, solution : 50 times of Agarose mass. 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑎𝑓𝑡𝑒𝑟 𝑓𝑖𝑙𝑡𝑒𝑟𝑒𝑑 (𝑔) 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑝𝑟𝑒𝑐𝑢𝑟𝑠𝑜𝑟 (𝑔). Yield =. 10.

2.3. Synthesis of porous carbon materials Freeze-dried double network gel was put into alumina boat and it was entered into the quartz tube. The sample was placed in the middle of furnace, followed by purging with vacuum (3.5×10-3 torr) for 30 minute. After vacuum stopped, argon gas flowed until it approach atmosphere pressure and then it made vacuum again for 20 min. Finally, after argon gas flowed as same condition, vacuum was keep for 10 min. With a heating rate of 2 ºC min-1, freeze-dried double network gel were heated at 600, 700 and 800 ºC for 3 hours under argon flow, respectively. After heated, let the porous carbon material cool down to room temperature. To remove the template or activating agent clearly, first, ball milling was introduced (Pulverisette 7 planetary ball mill). 6 tungsten carbide balls and 20 ml of tungsten carbide bowl were used. It ran for 15 min as 500 rpm. After ball milling, fine powders were immersed in milli-Q water and stirred for 6 hours at 85 ºC. In Ca(OAc)2 case, instead of milli-Q water, hydrogen chloride solution was used to remove the calcium carbonate (CaCO3). That powder was filtered by using 0.2 µm polytetrafluoroethylene (PTFE) membrane filter and then dried at 115 ºC. The carbon materials were labeled x template (y), where x = each template (NaCl, Ca(OAc)2 and K2C2O4) weight percent solution or sum of agarose and acrylamide : template weight ratio and y=(Ag:Am) weight ratio. The product yield was calculated as the mass of heat-treated carbon material after filtering/ the mass of double network gel precursor.. 2.4. Physical and chemical characterization Scanning electron microscopy (SEM) images were taken with on a FEL Verios 460L scanning electron microscope using 0.8 kV accelerating voltage. X-ray diffraction (XRD) patterns were obtained on Rigaku SmartLab powder X-ray diffractometer with Cu Kβ radiation. The surface area (SBET) and pore structure of the carbon sample were determined by N2 adsorption-desorption isotherms at 77K (Quantachrome Autosorb-iQ automated gas sorption analyzer) after degassing at 200 ºC for 10 hours and calculated by the Brunauer-Emmett-Teller (BET) method. The total pore volume (Vp) was determined from the amount of nitrogen adsorbed at a relative pressure (p/p 0) of ~0.99. The pore size distribution was calculated by using the slit/cylindrical pores Quenched-Solid Density Functional Theory (QSDFT) method. The micropore areas and external surface areas were calculated by t-plot method. Elemental analysis (C,N,O,H and S) of the sample was carried out on a Thermo Flash 2000 CHNSO elemental analyzer. Transmission electron microscopy (TEM) images were recorded on a JEOL JEM 2100 and acceleration voltage is at 200 kV. 11.

2.5. Synthesis of catalyst ink The Pt/C and carbon catalyst ink were prepared by dispersing 10 mg of the Pt/C or carbon catalyst powder in mixed solution which consist of 60 μl nafion, 20 μl D.I water and 700 μl of 2-propanol. Pt/C solution was sonicated for 2 min and carbon catalyst solution was sonicated for 10 min. After then, catalyst ink was stirred with 350 rpm.. 2.6. Electrochemical characterization One drop of the catalyst was loaded by 7 μL micropipette onto a glassy carbon (GC) electrode of 5 mm in diameter and dried at room temperature for 20 min. Cyclic Voltammetry (CV) and Rotating Disk Electrode (RDE) were carried out by Autolab PGSTAT128N. Three-electrode was used; the GC electrode, Pt wire and Ag/AgCl electrodes were used as working, counter and reference electrode, respectively. Before measuring the argon CV, the electrolyte which is 0.1 M KOH aqueous solution was purged by argon for 20 min. Ar gas kept bubbling in the alkaline medium during the measurement. After changing from Ar gas to O2 gas, O2 gas kept bubbling into electrolyte to saturate for 20 min and then Oxygen CV was performed. In CV case, a GC electrode as the working electrode was scanned at a rate of 50 mV S-1 in potential range from 0.0 to 1.0 V (vs. RHE). In RDE case, a GC electrode as the working electrode rotated with 1600 rpm at a scanning rate of 5 mV s -1 in potential range from 0.0 to 1.0 V (vs. RHE).. 12.

3. Result and Discussion 3.1. Effect of double network gel as a precursor 3.1.1. Comparison between single network and double network gel with NaCl Agarose is used as a 1st network. Because of gelation hysteresis, when the temperature is higher than 85 °C, it turns to sol phase and when the temperature is less than 40 °C, the state is changed to gel phase. Using this property, when the agarose is sol phase at 45 °C, initiator to synthesize the 2nd network can add into a single pot. Therefore, double network gel synthesized by one-pot method and heating and cooling method. To investigate double network gel (DN gel) can be a precursor for porous carbon material, we also synthesized the single networks: agarose gel and polyacrylamide gel. In the double network gel cases, the gel were synthesized as the agarose (Ag) to acrylamide (Am) weight ratio is 1 to 5. We also prepared the gels with or without 4 wt% NaCl aqueous solution as template.. Figure 5 shows powder X-ray diffraction (XRD) data that were collected with a scan range of 5-90°. Even though the heat-treated carbon material with 4 wt% NaCl aqueous solution was purified by stirring and washing with milli-Q water, NaCl could not be remove completely. We thought the NaCl was coated by carbon materials therefore to make fine powder, the samples were ground by ball milling. Because the carbon materials were collected by 0.2 μm polytetrafluoroethylene (PTFE) membrane filter, the particle size is over 0.2 μm. As shown in After ball milling, DN gel (1:5) sample’s result, there is no intensity of NaCl peaks. Therefore we can get the sample without template. The XRD pattern of the Am+4 wt% NaCl sample shows broad peak at 2θ values of about 25.7 ° and weak peak at 43.3 °, which corresponded to the (002) and (100) plane of carbon, respectively.35,36 It reveals that the the carbon material has low graphitic-like amorphous carbon structure. The DN gel (1:5) sample which before and after ball milling also shows the same intensity of peaks in the XRD pattern. It suggest that graphitic peak came from the acrylamide. Therefore by adding the 2nd network, there is a graphitic like carbon property in the 1st network which consist of agarose.. 13.

Figure 5. XRD pattern. Comparison between single network and double network gel with 4 wt% NaCl aqueous solution and between before and after ball milling. Top to bottom: Agarose (Ag) with 4 wt% NaCl aqueous solution. Acrylamide (Am) with 4 wt% NaCl aqueous solution. Double network gel which the agarose to acrylamide weight ratio is 1 to 5 (DN gel (1:5)) with 4 wt% NaCl aqueous solution. After ball milling, it is the same sample as DN gel (1:5).. The figure 6 shows the scanning electron microscope (SEM) image before heat-treatment. The agarose has large size of pore (figure 6a) and that pore is filled with the polyacrylamide as shown in figure 6b.. Figure 6. SEM image of before heat-treatment. a) Agarose. b) Double network gel (1:9). 14.

All of the surface area is too small to analyze the surface area and porosity by Brunauer-EmmettTeller equation. For that reason, it was done at a relative pressure (p/p0) of 0.35 and the result is only surface area. When we see the table 2, agarose has highest surface area among them. But It is hard to afford high surface arear porous structure using NaCl as a template.. Table 2. Textural properties of the carbon material Sample Agarose Agarose with 4 wt% NaCl Acrylamide Acrylamide with 4 wt% NaCl Double network gel Double network gel with 4 wt% NaCl a. T. Yield. SBETa. ( ºC). (%). (m2 g-1). 600. 10. 153.8. 600. 11. 486.8. 600. 17. 45.0. 600. 30. 5.2. 600. 27. 42.4. 600. 31. 20.8. SBET is obtained from the isotherm by applying the Brunauer-Emmett-Teller equation at a relative pressure of 0.35.. 15.

After heat-treatment, elemental composition, especially nitrogen, was obtained by elemental analysis. It is important to possess high nitrogen content and low oxygen content to use application such as electrode or supercapacitor. Table 3 shows that Acrylamide sample’s nitrogen content is 15 wt% and Double network gel sample’s nitrogen content is 12.9 wt%. Since acrylamide served the nitrogen source as 2nd network, double network gel can be a nitrogen doped carbon materials. Compare to double network gel without NaCl, double network gel with NaCl also has similar nitrogen content. (13.28 wt%). Table 3. Elemental composition of the materials. (wt %) Sample. C. N. O. H. S. Remains. N/C (%). Agarose. 87.25. 0. 5.5. 1.62. 0. 5.63. 0. 88.81. 0. 2.47. 1.46. 0. 7.26. n.d. 72.20. 15.00. 5.18. 1.60. 0. 6.02. 20.8. 68.52. 15.58. 5.33. 1.5. 1.71. 7.36. 22.7. 74.56. 12.90. 5.47. 1.64. 0. 5.43. 17.3. 67.19. 13.28. 9.81. 2.23. 0.42. 7.07. 19.8. Agarose with 4 wt% NaCl Acrylamide Acrylamide with 4 wt% NaCl Double network gel Double network gel with 4 wt% NaCl. 16.

3.1.2. Increasing the template amount. To get high surface area and porosity, we increased the concentration form 4 wt% aqueous NaCl solution to 20 wt% aqueous NaCl solution. To know which agarose to acrylamide weight ratio formed the double network gel is proper to make a porous carbon materials, we used 3 types of double network gel. Agarose to acrylamide weight ratio is 1 to 2.5, 5 and 9. A bracket represents agarose to acrylamide weight ratio. From the Figure 5, there were peaks related the graphitic-like amorphous carbon materials at 2θ values of about 25.5 ° and weak peak at 43.6 ° which come from acrylamide as 2nd network.. Figure 7. XRD pattern. Difference of template concentration and double network ratio.. In 4 wt% NaCl (1:5) sample case, the surface area is too small to analyze its surface area and porosity by Brunauer-Emmett-Teller equatio. For that reason, it was done at a relative pressure (p/p0) of 0.35 and the result is only surface area. Table 4 shows that all of samples has the surface area less than 100 m2/g. But increasing the NaCl amount leads to high surface area. (70.05 m2/g). 17.

Table 4. Textural properties of the carbon material. NaCl/ Sample. Precusor (g/g). T. Yield. SBETa. Pore sizeb. Vtotalc. ( ºC). (%). (m2 g-1). (nm). (cc g-1). 4 wt % NaCl (1:5). 0.2/1. 600. 23. 20.8d. -. -. (1:2.5). 0.9/1. 600. 32. 69.6. 14.52. 0.2525. (1:5). 1.0/1. 600. 30. 70.5. 12.47. 0.2196. (1:9). 1.1/1. 600. 32. 53.8. 22.30. 0.3001. 20 wt % NaCl a. SBET is the surface area obtained from the isotherm by applying the the Brunauer–Emmett–Teller equation. b Pore size. distribution is calculated by quenched solid density functional theory (QSDFT). cVtotal is total pore volume calculated at a relative pressure of 0.99. dSBET is the surface area at a relative pressure of 0.35.. Nitrogen content is similar (13 wt%) between the 4 wt% and 20 wt% in table 5. When NaCl was used as a template, the nitrogen content doesn’t change much compare to nitrogen content of acrylamide.. Table 5. Elemental composition of the materials. (wt %) Sample. C. N. O. H. S. Remains. N/C (%). 4 wt % NaCl (1:5). 67.19. 13.28. 9.81. 2.23. 0.42. 7.07. 19.8. (1:2.5). 71.46. 13.00. 8.24. 2.11. 0.32. 4.88. 18.2. (1:5). 70.77. 13.57. 7.33. 2.09. 0.27. 5.96. 19.2. (1:9). 70.09. 13.93. 6.89. 1.76. 0.78. 6.55. 19.9. 20 wt % NaCl. In conclusion, because of agarose’s gelation, double network gel synthesized by one-spot method. Double network gels are graphitic-like structure and have nitrogen content due to acrylamide. Although NaCl was used to get high surface area, overall surface area was still less than 100 m2/g.. 18.

3.2. Different type of template or activating agent. (Ca(OAc)2 and K2C2O4) We tried to use different template or activating agent to increase the surface area. One is calcium acetate (Ca(OAc)2). At 300 ~ 500 °C, calcium acetate can change to calcium carbonate (CaCO3) which can act as a hard template. Due to decomposition of calcium acetate, acetone can produce and it can help to form porous carbon material.15 Another is potassium oxalate (K2C2O4). Until now, the prominent activating agent has been potassium hydroxide (KOH). Zhang, L. et al. showed that after preparing the mixture of carbon precursor obtained by hydrothermal carbonization process and activating agent, the carbon precursors were heated to 900 °C for 1 hour. When several activating agent (KOH, NaOH, K2CO3, ZnCl2, H3PO4 and CaO) was used, the highest BET specific surface area got by using KOH and it is ~3400 m2/g.7 In our case, template or activating agent added into agarose gel by one-pot method. However, because KOH is a strong base, the hydrogen bond in the agarose gel can be easily broken and it leads to weak 1st network. So compare to KOH, potassium oxalate (K2C2O4) is adequate activating agent. At 500~600 °C, the K2C2O4 start to decompose. (K2C2O4→K2CO3+CO) Over 800°C, carbon can react with carbonate. (K2C2O4+2C→2K+3CO or 2K2CO3+C→4K+3CO2)37,38 Potassium produce the both mesopores and micropores by reacting with the carbon network. Using K 2C2O4 might form more porous carbon material than KOH because not only does potassium work but also oxalate helps. The 20 wt% of Ca(OAc)2 aqueous solution and 20 wt% of K2C2O4 aqueous solution was used. There is morphology of carbon material with Ca(OAc)2 (figure 8a) and K2C2O4.(figure 8b) When using the Ca(OAc)2 as a template, the carbon material looks like film and solid but in K 2C2O4 case, it shows that some part seems like honeycomb.. Figure 8. SEM image of carbon material before and after heat-treatment. a) 20 wt% Ca(OAc)2 (1:5) b) 20 wt% K2C2O4 (1:5). 19.

Figure 9 shows the powder XRD pattern. In 20 wt% of Ca(OAc)2 aqueous solution case, the intensity of peak corresponded to graphitic carbon (25. 5 °) is distinct. But when 20 wt% of K2C2O4 aqueous solution used, the intensity of peak (25.1 °) is very slight. The carbon material got from Ca(OAc)2 is more graphitic-like structure than K2C2O4.. Figure 9. PXRD pattern. a) Difference of precursor ratio at 20 wt% Ca(OAc) 2. b) Difference of precursor ratio at 20 wt% K2C2O4. The Brunauer-Emmett-Teller (BET) surface area of samples was investigated by a N2 adsorptiondesorption isotherm measured at 77K and pore size distribution was calculated by using a slit/cylindrical. pores, adsorption branch, Quenched Solid State Functional Theory (QSDFT) model. It means geometrically and chemically disordered micro-mesoporous carbon. Slit pore model is related to micropore and cylindrical pore is related to mesopore. Figure 10 shows the N2 adsorption-desorption isotherms and pore size distribution about 20 wt% Ca(OAc)2 and K2C2O4. The adsorption-desorption isotherms of the samples which used Ca(OAc)2 (Figure 10a) belongs to the type IV isotherms according to the IUPAC classification. Those sample exhibit an H2 type hysteresis loop, which many porous adsorbent usually reveal. The H2 loop is hard to explain the pore size distribution and shape as not well-defined, but it tend to have the pore with narrow necks and wide bodies, called as ‘ink bottle’ pores, because of the condensation and evaporation process. So, N2 as an adsorbate can enter to mesopores only through micropores.39 About the samples which used K2C2O4, the adsorption-desorption isotherms (Figure 10c) also belongs to the type IV, but hysteresis loops is H4 type differently than Ca(OAc)2. This type of pore is associated with narrow slit-like pores and indicative of microporosity.4 Figure 10b shows the variable size of pore exist but pore volume is less than 0.1 cc g-1·nm-1 however figure 10d shows the pore less than 2 nm is tend to contribute to total pore volume. 20.

Figure 10. N2 adsorption-desorption isotherms measured at 77K. (a) 20 wt% Ca(OAc)2, c) 20 wt% K2C2O4 ), Pore size distributions calculated using DFT method. (b) 20 wt% Ca(OAc)2, d) 20 wt% K2C2O4).. 21.

Table 6 shows the textural properties of porous carbon material which used Ca(OAc) 2 and K2C2O4. From the result, the BET surface area of samples which formed by using K 2C2O4 is higher than by using Ca(OAc)2.(~1278 m2/g) And when K2C2O4 is employed, the carbon material has more micropore (~55 %) than Ca(OAc)2. Because microporosity may break the graphitic carbon structure, the intensity of peak in the K2C2O4 sample’s PXRD pattern is weak.. Table 6. Textural properties of the carbon material. Activating Sample. agent/. T. Yield. SBETa. Smicrob. Precursor. ( ºC). (%). (m2 g-1). (m2g-1). (g/g). 20 wt% Ca(OAc)2. 20 wt% K2C2O4. Pore sizec (nm). Vtotald. Vmicroe. (cc g-1). (cc g-1). (1:2.5). 0.9/1. 600. 23. 151.7. 80.9. 7.500. 0.285. (1:5). 1.0/1. 600. 20. 126.1. 40.4. 7.672. 0.242. (1:9). 1.1/1. 600. 21. 54.5. 2.93. 17.17. 0.234. (1:2.5). 0.9/1. 600. 20. 1126. 1010. 2.664. 0.750. (1:5). 1.0/1. 600. 23. 1278. 1162. 2.652. 0.847. (1:9). 1.1/1. 600. 19. 495. 231.5. 3.604. 0.446. 0.0.5 (12)f 0.021 (9) 0.001 (4) 0.410 (55) 0.462 (50) 0.107 (24). a. SBET is the surface area obtained from the isotherm by applying the Brunauer–Emmett–Teller equation. bSmicro is the. microporous surface area calculated from the t-plot method. c Pore size distribution is calculated by quenched solid density functional theory (QSDFT). dVtotal is total pore volume calculated at a relative pressure of 0.99.eVtotal is micropore volume calculated by the t-plot method. f This value is the microporosity. (The micropore volume obtained from t-plot method / The total pore volume obtained from QSDFT method).. 22.

To know how much of nitrogen exist in the nitrogen doped carbon material, elemental composition of the materials was examined by the EA (Table 7). In Ca(OAc)2 case, nitrogen content is around 11 wt% but in K2C2O4 case, it is around 5 wt%. Because when carbon material activated with K2C2O4, activating agent emit not only the CO or CO2 but also NOx which oxygen react with nitrogen, nitrogen content can decrease. However, Oxygen content increased compare to NaCl. As both Ca(OAc)2 and K2C2O4 contain the oxygen, it may increase the oxygen contents of the carbon networks.. Table 7. Elemental composition of the materials. (wt %) Sample 20 wt% Ca(OAc)2. 20 wt% K2C2O4. C. N. O. H. S. Remains. C/N (%). (1:2.5). 64.30. 11.14. 17.22. 1.99. 0.42. 6.03. 17.4. (1:5). 63.61. 11.84. 18.42. 2.16. 0.38. 3.59. 18.6. (1:9). 63.34. 11.24. 16.38. 2.55. 0. 6.49. 17.7. (1:2.5). 62.00. 5.92. 20.45. 2.07. 0. 9.56. 9.5. (1:5). 65.42. 5.89. 18.79. 1.40. 0.26. 8.24. 9.0. (1:9). 66.74. 6.87. 18.78. 2.22. 0.38. 5.01. 10. In conclusion, the nitrogen doped carbon material which created by activating agent (K2C2O4) has high surface area (1278 m2/g) and roughly 50 % of microporosity.. 23.

3.3 Activating agent: Potassium oxalate (K2C2O4) 3.3.1. Increasing the activating agent. Comparing with the template and activating agent, using potassium oxalate gives the high surface area (1278 m2/g) than others. From now on, we focused on the potassium oxalate. According to the result that when increasing the template aqueous solution concentration, from 4 wt% NaCl aqueous solution to 20 wt%, the surface area increased, we assume that if we use more activating agent, we can get a more porous carbon material. So we added the potassium oxalate as 3 times of precursor weight. The samples were labeled ‘1:3 K2C2O4’ that the precursor to K2C2O4 weight ratio is 1 to 3. When is it in 20 wt% K2C2O4 aqueous solution, the precursor to K2C2O4 weight ratio is 1 to 1.0. Figure 11 shows the powder XRD pattern. All of the sample with different precursor weight ratio shows the graphitic-like amorphous carbon structure because of the intensity of graphitic carbon peak. In 1:3 K2C2O4 (1:2.5) and 1:3 K2C2O4 (1:9) sample case, this peak exhibit at 2θ = 25.2 °, but 1:3 K2C2O4 (1:5) sample case, this peak is slightly shift to 2θ = 26.2.. Figure 11. PXRD pattern. Different precursor ratio with 1:3 K2C2O4.. 24.

N2 adsorption-desorption isotherms of sample which used 3 times amount of K2C2O4 belongs to the type IV isotherms and H4-type hysteresis loop, which indicates the existence of micropores in the carbon material. (Figure 12a) When we see the pore size distribution, 1:3 K2C2O4 samples have similar distribution as 20 wt% K2C2O4 sample. (Figure 12b) But pore volume of the K2C2O4 samples is smaller than 20 wt% K2C2O4 (1:5) sample.. Figure 12. a) N2 adsorption-desorption isotherms measured at 77K b) Pore size distributions calculated using DFT method.. 25.

Table 8 shows that even though K2C2O4 was used as 3 time amounts, the carbon material which used 20 wt% K2C2O4 aqueous solution (1.0 times amounts) has higher surface area. In 1:3 K2C2O4 samples case, the Ag to Am weight ratio which is 1 to 9 has higher surface area (881.3 m2/g) than 1 to 2.5 and 1 to 5. However, comparing to 20 wt% K2C2O4 aqueous solution, the surface area of 1:3 K2C2O4 samples is lower. We assume there were too much of activating agent to form the 2nd network well.. Table 8. Textural properties of the carbon material K2C2O4/ Sample. Precusor (g/g). 1:1 K2C2O4. 1:3 K2C2O4. T. Yield. SBETa. Smicrob. ( ºC). (%). (m2 g-1). (m2g-1). Pore size. c. (nm). Vtotal d. Vmicroe. (cc g-1). (cc g-1). (1:5). 1/1. 600. 23. 1278. 1162. 2.652. 0.847. 0.462 (55)f. (1:2.5). 3/1. 600. 8. 478.9. 397.7. 2.354. 0.282. 0.171 (55). (1:5). 3/1. 600. 4. 712.9. 630.5. 2.294. 0.409. 0.263 (24). (1:9). 3/1. 600. 6. 881.3. 734.8. 3.448. 0.760. 0.309 (40). a. SBET is the surface area obtained from the isotherm by applying the Brunauer–Emmett–Teller equation. bSmicro is the. microporous surface area calculated from the t-plot method. c Pore size distribution is calculated by quenched solid density functional theory (QSDFT). dVtotal is total pore volume calculated at a relative pressure of 0.99.eVmicro is micropore volume calculated by the t-plot method. f This value is the microporosity. (The micropore volume obtained from t-plot method / The total pore volume obtained from QSDFT method).. 26.

Table 9 shows the elemental composition analyzed by elemental analysis. The 1:3 K2C2O4 samples shows less than the samples using 20 wt% K2C2O4 (1:5) sample. During K2C2O4 decomposing, nitrogen detached from the carbon network and oxygen insert into the carbon network. Because 3 times of amount used, nitrogen contents are around 4 wt% and oxygen contents are around 22 wt%. .. Table 9. Elemental composition of the materials. (wt %) Sample 1: 1 K2C2O4 1:3 K2C2O4. C. N. O. H. S. Remains. C/N (%). (1:5). 65.42. 5.89. 18.79. 1.40. 0.26. 8.24. 9.0. (1:2.5). 65.69. 5.44. 19.08. 2.24. 0.36. 7.19. 8.3. (1:5). 65.87. 5.22. 18.01. 2.17. 0.47. 8.26. 7.9. (1:9). 63.86. 6.84. 15.72. 2.5. 0.41. 10.67. 10.7. 27.

3.3.2. Decreasing the activating agent. To synthesize the n-doped porous carbon materials with high surface area, we increased the amount of activating agent. However, it does not work well to get a higher surface area and nitrogen contents. Instead of increasing the activating agent, we tried to decrease the activating agent amount. The solution concentration is 4, 8, 12 or 16 wt% K2C2O4 aqueous solutions. The K2C2O4 to precursor weight ratio is 0.2, 0.4, 0.6 and 0.8, respectively. Because when agarose to acrylamide weight ratio is 1 to 5, the surface area and pore size is higher than others. From now on, we focused on the 1 to 5. We omitted the ‘aqueous solution’ in the sample name.. Figure 13 shows that all of sample has some graphitic like carbon structure, because of the peak at 24.6°. 0.2 K2C2O4 and 0.4 K2C2O4 samples’ peak can be distinguishable than others, because it is a small amount of K2C2O4 to broke the graphitic like structure.. Figure 13. PXRD Pattern. Dfference of the activating agent concentration.. 28.

In 0.2 K2C2O4 sample case, it is hard to analyze the surface area and pore size distribution. So, this sample shows only surface area in a relative pressure of 0.35. Figure 14a shows that all of samples belongs to the type IV isotherms and H4-type hysteresis loop as a 1.0 K2C2O4 sample. The pore size distribution of all of samples reveals it is concentrated in the micropore range due to K2C2O4. (Figure 14b) The surface area increased by using the 0.4 K2C2O4 sample, from to 1089 m2/g. (Table 10) In the 0.6 K2C2O4 sample case, the surface area is highest (1290 m2/g) and there is not quite difference over 0.6 K2C2O4. From that result, the porous carbon material can be synthesized by less amount of activating agent. (K2C2O4 /precursor weight ratio=0.4~0.6) M. Sevilla et al. got 1670 m2/g of surface area when they use that precursor to activating agent weight ratio is 1 to 1.2.37 (Since M. Sevilla et al. used hydrochar/potassium oxlate/melamine weight ratio is 1/3.6/2, I calculated that weight ratio by assuming both hydrochar and melamine as carbon precursors.) And the microporosity is around 50 %, so it is mesoporous and microporous carbon material.. Figure 14. a) N2 adsorption-desorption isotherms measured at 77K b) Pore size distributions calculated using DFT method.. 29.

Table 10. Textural properties of the carbon material. K2C2O4/ Sample. Precursor (g/g). T. Yield. SBETa. Smicrob. ( ºC). (%). (m2 g-1). (m2g-1). 121.8f. 1. 0.2/1. 600. 16. 2. 0.4/1. 600. 20. 3. 0.6/1. 600. 21. 4. 0.8/1. 600. 5. 1.0/1. 600. 1086. Pore size. c. (nm). Vtotal d. Vmicroe. (cc g-1). (cc g-1). 1004. 2.700. 0.733. 1290. 1194. 2.829. 0.912. 21. 1268. 1108. 3.294. 1.046. 23. 1278. 1162. 2.652. 0.847. (821.5)f. 0.395 (54)g 0.477 (52) 0.445 (43) 0.462 (55). a. SBET is the surface area obtained from the isotherm by applying the Brunauer–Emmett–Teller equation. bSmicro is the. microporous surface area calculated from the t-plot method. c Pore size distribution is calculated by quenched solid density functional theory (QSDFT). dVtotal is total pore volume calculated at a relative pressure of 0.99.eVmicro is micropore volume calculated by the t-plot method. f is the surface area at a relative pressure of 0.35. g This value is the microporosity. (The micropore volume obtained from t-plot method / The total pore volume obtained from QSDFT method).. As shown in Table 11, the nitrogen content decreased from 8.39 wt% to 4.64 wt%. However, the oxygen content increased from 14.47 wt% to 21.63 wt%. It might reveal when we increase the K2C2O4 amount, the nitrogen can be NOx gas and oxygen can enter into carbon network. Table 11. Elemental composition of the materials. (wt %) K2C2O4/ Sample. Precursor. C. N. O. H. S. Remains C/N (%). (g/g) 1. 0.2/1. 68.33. 8.39. 14.47. 2.03. 1.98. 6.78. 12.3. 2. 0.4/1. 69.11. 7.34. 17.24. 1.84. 1.42. 3.05. 10.6. 3. 0.6/1. 66.24. 4.64. 20.98. 2.01. 0.48. 5.65. 7.0. 4. 0.8/1. 68.81. 5.48. 21.63. 1.68. 0.12. 2.28. 8.0. 5. 1.0/1. 65.42. 5.89. 18.79. 1.40. 0.26. 8.24. 9.0. 30.

3.3.3 Compare to single network and double network Even though we studied properties compare between single network and double network with NaCl as template, we tried again to know at the K2C2O4 condition. Figure 15 shows Am + 20 wt% K2C2O4 sample has stronger graphitic carbon peak intensity (2θ = 26 °) than others. DN gel (1:5) + 20 wt% K2C2O4 sample has weak peak, but Ag + 20 wt% K2C2O4 sample doesn’t. In K2C2O4 condition, agarose gel provides a one-pot method as a 1st network and polyacrylamide serve the graphitic like structure as 2nd network, same as NaCl experiment.. Figure 15. PXRD Pattern. Comparison between single network and double network with K2C2O4. Top to bottom : Double network gel (agarose to acrylamide weight ratio is 1 to 5) with 20wt% K2C2O4 aqueous solution. Polyacrylamide with with 20wt% K2C2O4 aqueous solution. Agarose with 20wt% K2C2O4 aqueous solution.. 31.

20wt% K2C2O4 aqueous solution was used for DN gel (1:5), Agarose and Polyacrylamide. However, the K2C2O4/ Precursor ratio is 1.0, 6.2 and 1.2, respectively. Figure 16a shows that how surface area values are changed according to K2C2O4/precursor weight ratio.(g/g) Even though Ag + 20wt% K2C2O4 sample has 927.7 m2/g, the activating agent used 6.2 times more than precursor. Similarly, Am+ 20wt% K2C2O4 sample has 1021 m2/g because the activating agent used 1.2 times more. From that result, DN gel (1:5) has advantage as a precursor because it can produce porous carbon material using small amount of activating agent. In 0.4 K2C2O4 sample case, the K2C2O4/precursor ratio is 0.4 and surface area has 1086 m2/g. (Table 12) The pore size distributions (Figure 16b) of Am + 20wt% K2C2O4 sample and DN gel + 20wt% K2C2O4 sample is similar to each other (both samples have around 50% of microporosity), however, the Ag + 20wt% K2C2O4 sample shows different pore size distribution. Thus we assume polyacrylamide as a 2nd network dominates the pore size and pore volume in the carbon material obtained from DN gels. Another advantage of DN gel precursor is that the yield of porous carbon material from DN gel is higher than that from single network gels.. Figure 16. a) Comparison of surface area between single network and double network gel. b) Pore size distributions calculated using QSDFT method.. 32.

Table 12. Textural properties of the carbon material. K2C2O4/ Sample. Pore. T. Yield. SBETa. Smicrob. ( ºC). (%). (m2 g-1). (m2g-1). 6.2/1. 600. 10. 927.7. 731.2. 5.370. 1.246. 1.2/1. 600. 12. 1021. 912.1. 2.579. 0.658. 1.0/1. 600. 23. 1278. 1162. 2.652. 0.847. Precursor (g/g). size. c. (nm). Vtotal d. Vmicroe. (cc g-1). (cc g-1). Ag + 20 wt% K2C2O4 Am + 20 wt% K2C2O4 DN gel + 20 wt% K2C2O4. 0.298 (24)f. 0.378 (57). 0.462 (55). a. SBET is the surface area obtained from the isotherm by applying the Brunauer–Emmett–Teller equation. bSmicro is the. microporous surface area calculated from the t-plot method. cPore size distribution is calculated by quenched solid density functional theory (QSDFT). dVtotal is total pore volume calculated at a relative pressure of 0.99.eVmicro is micropore volume calculated by the t-plot method. f This value is the microporosity. (The micropore volume obtained from t-plot method / The total pore volume obtained from QSDFT method).. Table 13 shows that although the nitrogen content is lower than polyacrylamide, it is the nitrogen doped carbon material due to the nitrogen contents. (5.89 wt%) So, the nitrogen in the carbon material can help to increase the electrocatalytic performance.. Table 13. Elemental composition of the materials. (wt%) Sample Ag + 20 wt% K2C2O4 Am + 20 wt% K2C2O4 DN gel + 20 wt% K2C2O4. C. N. O. H. S. Remains. N/C (%). 69.42. 0. 22.11. 2.27. 0. 6.20. -. 58.51. 8.01. 17.90. 2.50. 0.49. 12.59. 13.7. 65.42. 5.89. 18.79. 1.40. 0.26. 8.24. 9.0. 33.

The carbon material obtained from double network gel using K2C2O4 as activating agent can be easily synthesized due to agarose property. As comparison between single network and double network, the carbon material obtained from double network gel is nitrogen doped graphitic-like structure because of the good precursor candidate, polyacrylamide. The high surface area could be obtained by using small amount of activating agent.. 3.4. Expanding the heating period. We studied to know how the nitrogen doped porous carbon materials differ depending on heating period. So far, all of the samples were heated at 600 °C for 3 hours. In this section, we investigated the carbon materials by changing the heating period like 1, 2 and 3 hours. We labeled the sample as a heating time. For example, 1 hour sample mean DN gel (1:5) that K2C2O4 /precursor ratio is 1.0 was heat treated at 600 °C for 1 hour. 2hours sample is heat-treated for 2 hours. Figure 17 shows that the intensity of graphitic like carbon peak (2θ=25.4°) is broad as time went by.. Figure 17. PXRD Pattern. Comparison of heating period.. 34.

When it was heated for 3 hours, the amount adsorbed increased (Figure 18a) and the surface area is higher than others. (1278 m2/g) (Table 14) All of the samples have similar pore size distribution each other but when time is longer, the micro pore volume increased, resulting increase of microporosity (55%). (Figure 18b and Table 14). Figure 18. a) N2 adsorption-desorption isotherms measured at 77K b) Pore size distributions calculated using DFT method.. Table 14. Textural properties of the carbon material.. Sample. T. Yield. SBETa. Smicrob. Pore sizec. Vtotal d. Vmicroe. ( ºC). (%). (m2 g-1). (m2g-1). (nm). (cc g-1). (cc g-1). 1 hour. 600. 25. 949.4. 846.6. 3.775. 0.8961. 2 hours. 600. 21. 901.5. 798.8. 3.761. 0.8473. 3 hours. 600. 23. 1278. 1162. 2.652. 0.8472. 0.334 (38)f 0.320 (38) 0.462 (55). a. SBET is the surface area obtained from the isotherm by applying the Brunauer–Emmett–Teller equation. bSmicro is the. microporous surface area calculated from the t-plot method. cPore size distribution is calculated by quenched solid density functional theory (QSDFT). dVtotal is total pore volume calculated at a relative pressure of 0.99.eVmicro is micropore volume calculated by the t-plot method. f This value is the microporosity. (The micropore volume obtained from t-plot method / The total pore volume obtained from QSDFT method).. 35.

As figure 19 show the illustration, when heating time increased at 600 °C, the carbon material tend to have more micro pore inside.. Figure 19. Illustration of increasing the micro pore depending on the heating period.. Table 15 shows nitrogen content is around 5~6 %. The nitrogen content is not much different each other but oxygen content is decreased.. Table 15. Elemental composition of the materials. (wt %) Sample. C. N. O. H. S. Remains. N/C (%). 1 hour. 63.71. 5.94. 25.77. 3.49. 0.19. 0.9. 9.3. 2 hours. 60.27. 5.45. 22.94. 2.57. 0.22. 8.55. 9.0. 3 hours. 65.42. 5.89. 18.79. 1.40. 0.26. 8.24. 9.0. 36.

3.5. Increasing the temperature At high temperature, the K2C2O4 can react with carbon network as itself or K2CO3 more. We expect to get a high surface area and porosity. We labelled the sample name as a heating temperature. The PXRD patterns imply the carbon material is almost amorphous structure because it is hard to distinguish the intensity of graphitic carbon peak (2θ=25.6°), (Figure 20).. Figure 20. PXRD Pattern. Comparison of the different temperature.. There was a significant difference of the N2 adsorption-desorption isotherms. (Figure 21a) When the temperature increased from 600 to 700 ºC, absorbed amount increased and Table 16 shows that surface area of 700 ºC sample (2447 m2/g) is twice as much as 600 ºC samples. (1278 m2/g) However, from 700 to 800 ºC, both isotherms intersect at roughly relative pressure of 0.2, indicating that the adsorbed amount of 800 ºC sample about the micropore is smaller than 700 ºC sample. It confirms that microporosity of 800 ºC sample is smaller (29%) but also the surface area of 800 ºC sample (2340 m2/g) is similar to 700 ºC sample. Figure 21b reveals that when the temperature increased from 600 ºC to 700 ºC, the pore volume is increased at the same pore size. But, from 700 ºC to 800 ºC, the pore size distribution seems like transfer to right side. It means that the pore size is larger than before, but the pore volume is similar as before.. 37.

Figure 21. a) N2 adsorption-desorption isotherms measured at 77K b) Pore size distributions calculated using DFT method.. Table 16. Textural properties of the carbon material. T. Yield. SBETa. Smicrob. Pore sizec. Vtotal d. Vmicroe. ( ºC). (%). (m2 g-1). (m2g-1). (nm). (cc g-1). (cc g-1). 1.0 K2C2O4 (1:5). 600. 23. 1278. 1162. 2.652. 0.8472. 1.0 K2C2O4 (1:5). 700. 16. 2447. 2187. 2.850. 1.716. 1.0 K2C2O4 (1:5). 800. 10. 2340. 1317. 3.613. 2.114. Sample. 0.462 (55)f 0.899 (52) 0.612 (29). a. SBET is the surface area obtained from the isotherm by applying the Brunauer–Emmett–Teller equation. bSmicro is the. microporous surface area calculated from the t-plot method. cPore size distribution is calculated by quenched solid density functional theory (QSDFT). dVtotal is total pore volume calculated at a relative pressure of 0.99.eVmicro is micropore volume calculated by the t-plot method. f This value is the microporosity. (The micropore volume obtained from t-plot method / The total pore volume obtained from QSDFT method).. 38.

Shiratori et al. suggested by using spherical particle ‘microdomain’ which is fundamental unit for activated carbon fiber. At the beginning during activating process, the microdomain on the uppermost surface produced the micropores. As the micropores were widened, size of the microdomain was reduced at the same time and generating the micropores progressed from uppermost surface to core part. And then it is the mesopores occurred between microdomain which became reduced the size.40 From 600 to 700 ºC, volume of micropores increased in the carbon material and it result the high surface area. But from 700 to 800 ºC, the mesopore more produced due to widening the micropores, including fusing and wall collapsing.7 So, even though total pore volume increased, the volume of micropores decreased and the surface area of 800 ºC sample was similar as 700 ºC.. Figure 22. Illustration about pore size transition from micropore to mesopore depending on the temperature.. Figure 23 is the image of transmission electron microscope (TEM). It indicates how to change the pore size in the carbon material depends on the temperature. The pore size is similar each other at 600 ºC and 700 ºC (figure 21a, b) but at 800 ºC, we can notice the pore size is larger than others.. Figure 23. TEM images of the porous carbon material. a) at 600 ºC. b) at 700 ºC. c) at 800 ºC.. 39.

As Table 17 shown, the nitrogen content decreased to 0.99 wt%. Even though the surface area is high around 2000 m2/g, it could be disadvantage because nitrogen content is around 1 wt% and oxygen content is high as 13 wt%.. Table 17. Elemental composition of the materials. (wt %) Sample 1.0 K2C2O4 (1:5) at 600 ºC 1.0 K2C2O4 (1:5) at 700 ºC 1.0 K2C2O4 (1:5) at 800 ºC. C. N. O. H. S. Remains. N/C (%). 65.42. 5.89. 18.79. 1.40. 0.26. 8.24. 9.0. 76.57. 1.47. 12.82. 0.78. 0.00. 7.28. 1.9. 77.04. 0.99. 13.53. 1.10. 0.06. 8.36. 1.3. 40.

3.6. Application – Oxygen Reduction Reaction (ORR). With the high surface area and nitrogen-doped carbon materials in hand, the oxygen reduction reaction was investigated. If the surface area is high, there are many active sites which have chance to absorb the oxygen. If the nitrogen is doped in the carbon network, it can help to promote oxygen adsorption because it is more electronegative than carbon and to make the carbon material more electronically conducive due to lone pair electron by lying close to the conduction band.25 To know the electrochemical performances of the nitrogen doped porous carbon material for oxygen reduction reaction (ORR), the experiment carried out. The cyclic voltammetry (CV) was measured in an Ar-saturated and O2-saturated aqueous solution of 0.1 M KOH at scanning rate of 50 mV S-1 and in a potential range from 0 to 1.1 V (vs. RHE). Linear sweep voltammograms (LSV) was measured in an O2-saturated aqueous solution of 0.1 M KOH at scanning rate of 5 mV S-1 and rotating speed of 1600 rpm.. 3.6.1. Comparison the single network and double network. Figure 24a shows the Cycle voltammetry (CV) curves and the value of the cathodic peak potential of Agarose, polyacrylamide and 1.0 K2C2O4 (1:5) sample is 0.63, 0.60 and 0.71V (vs. RHE), respectively. The peak shifts suggest that the carbon material which synthesize from double network gel has a more efficient reduction reaction of oxygen than single network. The linear sweep voltammograms (LSV) shows the onset potential and reduction current density of single network, single network with activating agent and double network with activating agent. (Figure 24b and Table 18) The onset potential of carbon material with the activating agent is more positive value (0.81-0.82 V (vs. RHE)) than without the activating agent (0.72-0.74 V (vs. RHE)). It reveals that high surface area obtained from the activating agent is able to affect shifting the onset potential. With activating agent, the onset potential of single network and double network is not so different. Even though the nitrogen content of Am+20wt% K2C2O4 sample is higher (8 wt%) than Ag+20wt% K2C2O4 sample (0 wt%) and DN gel (1:5)+20wt% K2C2O4 sample (5.89 wt%), reduction current density of DN gel (1:5)+20wt% K2C2O4 samples as double network is slightly higher than others.(-9.93 A/g) The average of pore size of Ag+20wt% K2C2O4 sample is lager (5.37 nm) and total pore volume also higher (1.246 cc/g). Because mesopores which the pore size is usually 2 to 8 nm can increase the kinetic process of the ion diffusion in the electrodes and enhance the electrocatalytic performance as current densities41 and mesopores of Ag+20wt% K2C2O4 sample were 0.948 cc/g (76 %), Ag+20wt% K2C2O4 sample has higher reduction current density at 0.5 to 0.8 V (vs. RHE). However, because DN gel (1:5)+20wt% K2C2O4 sample has similar pore volume (0.8472 cc/g) and high surface area (1278 m2/g), it exhibits the higher reduction current density at 0 to 0.5 V (vs. RHE). 41.

Figure 24. a) Cycle voltammetry curves of Agarose, Polyacrylamide and DN gel (1:5) in Ar(black curves) and O2 –saturated (red curves) 0.1M KOH aqueous solution at a scan rate of 50 mV S-1. b) Linear sweep voltammograms of the Ag, Am, Ag+20wt% K2C2O4, Am+20wt% K2C2O4, DN gel (1:5)+20wt% K2C2O4 and Pt/C in O2 –saturated 0.1M KOH aqueous solution at a scan rate of 5 mV S-1 and rotating speed of 1600 rpm.. Table 18. Oxygen reduction reaction (ORR) measured by rotating disk electrode (RDE) voltammetry. E1/2 is half-wave potential obtained from LSV. Onset potential (Eonset) is defined as the voltage where the reduction current density of -0.25 A/g is reached. i0 is the reduction current density at 0 V vs. RHE. sample. E1/2 (vs. RHE). Eonset (vs. RHE). i0 (A/g). Pt/C. 0.92. 1.05. -13.9. Ag. 0.60. 0.72. -8.14. Am. 0.57. 0.74. -7.92. Ag+20wt% K2C2O4. 0.69. 0.82. -9.22. Am+20wt% K2C2O4. 0.61. 0.81. -8.05. DN gel (1:5)+20wt% K2C2O4. 0.61. 0.83. -9.93. 42.

3.6.2. Comparison the agarose and acrylamide weight ratio. Figure 25a shows the CV of the precursors with different agarose to acrylamide weight ratio with 20 wt% K2C2O4 aqueous solution at 600 °C for 3 hours. The agarose to acrylamide weight ratios are 1:2.5, 1:5 and 1:9 in parentheses. The cathodic peak potential of DN gel (1:2.5) and DN gel K2C2O4 (1:5) sample is same as at 0.71 V (vs. RHE) and it is hard to find the cathodic peak potential of DN gel (1:9) sample. The onset potential and reduction current density can be confirmed from the LSV in figure 25b and Table 19. The onset potential is similar each other at 0.83-0.85 V (vs. RHE). But the reduction current density of the DN gel (1:2.5) sample is higher than others.(9.93 A/g) Because all of the samples has similar amount of nitrogen content (around 5 wt%), the result affected by the high surface area (1278 m2/g) and high pore volume (0.8472 cc/g).. Figure 25. a) Cycle voltammetry (CV) curves of DN gel (1:2.5), DN gel (1:5) and DN gel (1:9) in Ar-(black curves) and O2 –saturated (red curves) 0.1M KOH aqueous solution at a scan rate of 50 mV S-1. b) Linear sweep voltammograms (LSV) of the DN gel (1:2.5), DN gel (1:5) and DN gel (1:9) and Pt/C in O2 –saturated 0.1M KOH aqueous solution at a scan rate of 5 mV S-1 and rotating speed of 1600 rpm.. 43.