Activated Carbon Nanochains with Tailored Micro-Meso Pore

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Activated Carbon Nano-Chains with Tailored Micro-Meso Pore Structures and Their Application for Supercapacitors Miao Zhang, Chunnian He, Enzuo Liu, Shan Zhu, Chunsheng Shi, Jiajun Li, Qingfeng Li, and Naiqin Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05480 • Publication Date (Web): 26 Aug 2015 Downloaded from http://pubs.acs.org on September 3, 2015

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The Journal of Physical Chemistry

Activated Carbon Nano-Chains with Tailored Micro-Meso Pore Structures and Their Application for Supercapacitors

Miao Zhang ┼, Chunnian He ┼, Enzuo Liu ┼, Shan Zhu ┼, Chunsheng Shi ┼, Jiajun Li ┼, Qingfeng Li *, ╫ and Naiqin Zhao *┼,╪ ┼

School of Materials Science and Engineering and Tianjin Key Laboratory of

Composite and Functional Materials, Tianjin University, Tianjin 300072, China. ╪

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China. ╫

Department of Energy Conversion and Storage, Technical University of Denmark, Kemitorvet 207, DK 2800 Lyngby, Denmark.

*

Corresponding authors. Fax: +86 27891371. E-mail: [email protected] (NQ Zhao) and [email protected] (FQ Li)

ACS Paragon Plus Environment


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Abstract: Carbon nano-chains (CNCs) were synthesized by a facile chemical vapor deposition process consisting of a one-dimensional chain of interconnected carbon nano-onions for potential application in supercapacitors. In this study, the CNCs were further activated by a chemical method using potassium hydroxide (KOH) as the activation agent to obtain micro-meso pore structures. To improve the specific surface area (SSA) and optimize the pore size distribution (PSD) in order to enhance the capacitance performance, the activation parameters, including the KOH content, temperature and duration, were investigated. The results indicated that CNCs with a hierarchical pore structure and high SSA could be achieved using an activation process with a KOH-to-CNC ratio of 2 at 900 °C for 20 hours. The mechanism is also discussed. The activation temperature and duration affect the promotion of the carbon graphitization and exaggeration of the carbon etching. The CNCs activated using the optimal parameters exhibited a high capacitance performance of 112.7 F g-1 at 50 mV s-1 with excellent stability in 6 M KOH electrolyte, which was due to the improved surface and micro-mesoporosity without sacrificing their electronic transmission properties.

1．．Introduction Carbon is among the most attractive materials for technological applications. It is capable of forming many allotropes with structures of zero to three dimensions, such as diamond, graphite, fullerenes, nanotubes (CNTs), nanofibers (CNFs) and graphene. On a macroscale, the materials can be prepared in the form of powders, membranes and foams. 2


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A supercapacitor is a high-capacity electrochemical device for energy storage that exhibits a high power density, long cycling life, and fast charge–discharge rate. Supercapacitors are used in applications requiring many rapid charge/discharge cycles, such as electrical vehicles, power grids, and aerospace applications. carbon nanotubes (CNTs)

5-7

, activated carbon

8

1-4

and graphene

Carbon materials, such as

9, 10

, are the most popular

candidates for electrode materials in supercapacitors. These materials possess good electric conductivity, stable physicochemical properties, and more importantly, high specific surface area (SSA) and suitable porosity. When the carbon materials act as electrodes in supercapacitors, the charge storage mechanism is based on the electrochemical double layer capacitance and therefore depends on the SSA. However, in practice, conventional carbon materials with high specific surface areas (e.g., more than 1000 m2 g-1) exhibit poor performance in the typical range of a few tens of F g-1. Most of the surface area of the activated carbon is on the microscale

11

, and pores of this size are poorly accessible to

electrolyte ions. In terms of porosity, a hierarchical structure consisting of multiple levels of pore sizes is beneficial for electrochemical capacitive properties because the micropores with a size of approximately 0.7~2 nm enhance the capacitance by increasing the electrode/electrolyte interfaces.

12-15

However, the mesopores in a range from a few to 50 nm

provide ionic pathways that improve the electrolyte accessibility.

16, 17

Based on

orientationally tailored and tip structured carbon nanotubes and edge-functionalized graphene sheets, recent developments have demonstrated significantly improved capacitance performance from well above 100 F g-1

18

towards the highest intrinsic double-layer

capacitance of 550 F g-1 for carbon-based electrodes 19. One of the development challenges is 3



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the aggregation of these CNTs and graphene materials, which tends to result in a loss of surface area and inferior device performance. 18, 20 As an important carbon materials, carbon nano-onions (CNOs) have been envisioned to be promising supercapacitor electrode materials with high power density due to their nonporous outer shells that are easily accessible to electrolyte ions. However, the traditional zero-dimensional CNOs always trend to agglomerate during the preparation and purification processes, leading to limited contact area with the electrolyte and reduced electrochemical performance for supercapacitors. Recently, a novel carbon structure consisting of carbon nano-chains (CNCs) has been successfully synthesized using a simple chemical vapor deposition method

21

. The nanostructure is a one-dimensional chain of joined carbon

nano-onions of multi-shelled nanoparticles (i.e., hollow cores with concentric graphitic layers). Although traditional zero-dimensional CNOs suffer from agglomeration, the chain structure is expected to significantly improve the stability of the structure. Currently, the prepared CNCs exhibit a relatively low SSA (99 m2 g-1), limiting the electrode performance for supercapacitor applications. A method to achieve CNCs with high SSAs and appropriate porosity is highly desirable to enable a high performance supercapacitor. Chemical activation via a chemical process between carbon and an activated agent is an effective approach for industrially generating micropores and improving the SSA of carbon materials

22, 23

. Merino et al.

24

used potassium hydroxide (KOH) as the activating agent to

increase the SSA and porosity (micropores and mesopores) of CNFs, improving the capacitance of CNFs from 1 to 60 F g-1. Yang et al. 25 demonstrated that KOH-activated CNT electrodes exhibited a doubled capacitance of approximately 50 F g-1. Xing et al. 16 attempted 4


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to optimize the KOH activation process to compromise the collapse of the ordered mesostructures of carbon materials and reported a capacitance of 180 F g-1. In the current study, further activation of the synthesized CNCs was explored. The investigated process factors include the effects of the amount of potassium hydroxide (KOH), activation temperature and time on the SSA and pore size distribution (PSD) of CNCs were studied. The mechanism was also discussed in detail. Finally, the activated CNCs with different SSAs and PSDs were evaluated to determine their capacitance performance.

2. Experimental 2.1. Chemicals and preparation of CNCs Iron nitrate (Fe (NO3) 3 · 9H2O), nickel nitrate (Ni (NO3) 2 · 6H2O), magnesium oxide (MgO), ethanol, potassium hydroxide (KOH) and dilute hydrochloric acid (1 M HCl) were purchased from Tianjin Chemical Reagent Company (Tianjin, China). All of the chemicals were of analytical grade and were used without further purification. The synthesis of the CNCs was performed according to our previously published protocol. 21 2.1.1. Preparation of the Fe-Ni catalyst supported on MgO (Fe-Ni/MgO) Fe-Ni catalyst is the Fe and Ni oxide nanoparticles which catalyses the carbon source to form the carbon products. The Fe-Ni catalyst with the Fe-to-Ni mole ratio of 1:1 was prepared via an impregnation method. First, 8.08 g of the Fe (NO3)3 · 9H2O powder and 5.816 g of the Ni (NO3)2 · 6H2O powder were dissolved in 800 mL of ethanol with constant stirring to achieve complete dissolution. Then, 25 g of MgO were added to the mixed solution consisting of Fe (NO3)3 and Ni (NO3)2. Next, the ethanol in the solution was evaporated by 5



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heating in a water bath to yield MgO supported Fe and Ni oxide nanoparticles (Fe-Ni/MgO). 2.1.2. Growth of carbon products In a typical procedure, 2 g of Fe-Ni/MgO were placed in a quartz boat positioned inside horizontal tubular quartz tube of that was 100 mm in inner diameter, which was then placed into a tubular furnace. First, Fe-Ni/MgO was reduced at 550 °C for 1 h in the flow consisting of a mixture of hydrogen (99.99% purity) and argon (99.99 % purity) at a flow rate ratio of 100/100 mL min-1. Then, the furnace was ramped to 750 °C, and CH4 (99.99 % purity) with a flow rate of 60 mL min-1 was introduced into the quartz tube for 30 min. Then, the sample was cooled to ambient temperature in argon. The as-prepared sample was further treated with excess HCl (1 M) and repeatedly flushed with deionized water to remove the MgO. 2.2. Activation of CNCs The obtained CNCs were mixed with KOH powder with various KOH/CNCs weight ratios of 1.0 to 4.0. The mixture was placed in a Ni boat in a horizontal tubular quartz tube. The activation conditions (i.e., activation temperature (800~1000 °C) and time (5~30 h)) were investigated. The entire activation process was performed under an argon atmosphere with an Ar flow rate (99.99 % purity) of 200 mL min-1. Then, the activated CNCs were washed using excess HCl (1 M) to remove the residual KOH and Fe-Ni catalysts followed by repeated flushing with deionized water until a pH of 7 was reached. Finally, the activated CNCs were dried for 12 h in air at 80 °C. The samples were named using the values of the KOH/CNCs weight ratio, activation temperature (in °C) and activation time (in minute). For example, the CNC activated at 900 °C for 20 min with a KOH/CNCs weight ratio of 2 is named “2-900-20”. 6


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2.3. Characterization The morphology and crystallographic structures of the CNCs were studied via transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) using an FEI Tecnai G2 F20 microscope. Scanning electron microscope (SEM) investigations were performed using a TDCLS-4800 SEM (Hitachi). Raman spectra of the carbon products were recorded on a LabRAM HR Raman spectrometer using laser excitation at 514.5 nm from an argon ion laser source to validate the presence of carbon in the nanospheres. The crystallographic structures of the products were determined on a powder X-ray diffraction system (XRD, Rigaku D/max-2500, Cu−Kα radiation) at room temperature. The JADE5 software was utilized for data analysis. The X-ray photoelectron spectroscopy (XPS) was done using a PHI Quantera SXM Scanning X-ray Microprobe with a base pressure of 5 × 10-9 Torr. The nitrogen adsorption isotherms of the carbon products were measured at 77 K using an autosorb instrument (Quantachrome U. S.). The total surface area was calculated using the Brunauer-Emmett-Teller (BET) method, and the pore size distribution data were calculated using a density functional theory (DFT) method based on the adsorption and desorption data. 2.4. Electrochemical measurements A paste was prepared from 75 wt. % activated CNCs, 15 wt. % acetylene black as a conductive additive and 10 wt. % polytetraﬂuoroethylene (PTFE) as a binder dispersed in deionized water. The paste was coated on nickel foam (95 % porosity, 130~25 pores per linear inch (PPI), Advanced Technology Materials Co., Ltd.) and dried under vacuum at 80 °C overnight. Furthermore, a pressure of 10 MPa must be conducted to the CNCs/Ni foam to form the electrodes for the supercapacitors. The CNCs/Ni foam, platinum plate and 7



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mercury/mercury oxide (Hg/HgO) electrodes were used as the working, counter and reference electrodes, respectively. Each working electrode contained about 4 mg of active materials with a geometric surface area of about 1 cm2. The cyclic voltammetry (CV) and the galvanostatic charge-discharge (GCD) measurements were carried out in a 6 M KOH electrolyte using a three-electrode cell and a CHI 660D (Chenhua China) electrochemical workstation at ambient temperature. The electrochemical impedance spectra (EIS) of the CNCs electrodes were recorded in a frequency range from 10 mHz to 100 kHz.

3. Results and discussion 3.1. Activation and mechanism The TEM images of the original CNCs and activated CNCs with different KOH/CNCs weight ratios (marked as RKOH/CNC) prepared at 900 °C for 20 h are shown in Figure 1. One carbon nano-chain of the pristine CNCs synthesized in this experiment was formed by closely linked individual hollow quasi-CNO. The onion joints are marked in Figure 1a to illustrate the chain structure of the CNCs. The pristine CNCs have a high purity and regular framework, indicating that the CNCs have a high crystallinity in agreement with the XRD results. For the sample prepared with RKOH/CNC = 1, a small number of defects were observed on the visibly rough outer layers comparing to the pristine sample (Figure 1 and Figure S1, Supporting Information). For the sample with a RKOH/CNC of 2, the surface layers became rough, indicating the formation of surface defects (Figure 1c). As the ratio was further increased, larger pores were formed and aggregated around the joints of the chained onions. Therefore, the chain structure of the CNCs was nearly disassembled with pores of large sizes, as shown 8


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in Figure 1d. Meanwhile, the SEM images are in accord with the trend (Figure S2).

Figure 1. TEM images of original CNCs (a) and activated CNCs with different RKOH/CNC at 900 °C for 20 h (RKOH/CNC=1 (b), 2 (c), and 4 (d)).

The TEM and SEM images of activated CNCs with RKOH/CNC = 2 at different temperatures for an activation period of 20 h are shown in Figure 2 and Figure S3, respectively. A higher activation temperature aggravates the etching. It is important to note that the enhanced etching at elevated temperatures appeared to result in an increase in the number of pores. In addition, the sample at 1000 °C exhibited a significant thinning in the carbon wall.

9



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Figure 2. TEM images of activated CNCs (RKOH/CNC = 2) at 800 °C (a), 900 °C (b) and 1000 °C (c) for 20 h.

By keeping the other factors unchanged (i.e., RKOH/CNC of 4 at 900 °C), the effects of the activation time on the morphologies were also studied. As shown in Figure 3, the outer shells became increasingly rough as the activation time increased. For an activation period of more than 25 h, significant surface defects were observed, and the concentric structure of the CNCs was destroyed due to the formation of a large quantity of surface defects. The SEM images are also shown to prove the homogeneity of products (Figure S4).

Figure 3. TEM images of activated CNCs (RKOH/CNC = 4) at 900 °C for 5 h (a), 10 h (b), 15 h (c), 20 h (d), 25 h (e) and 30 h (f). 10


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The crystallographic structures of the products have been examined by XRD, and the patterns of pristine CNCs and activated CNCs under different conditions are shown in Figure 4. The black pattern in Figure 4a represents the XRD of pristine CNC samples, where the peak at 2θ = 26.11° corresponding to graphite (002) is sharp and intense. Other patterns in Figure 4a are the results of activated CNCs with different RKOH/CNC values (1, 2 and 4) at 900 °C for 20 h. The intensity of the graphite peaks decreased as the amount of KOH is increased due to the defects introduced by etching. For high RKOH/CNC ratios the peak corresponding to the Fe-Ni alloy at 2θ = 43.6° disappeared. After KOH activation, the sample was treated with the HCl solution. The residual Fe-Ni catalyst in the CNCs was removed by the following HCl treatment because the CNC structures became more open after the activation. However, further graphitization of the activated CNCs occurred at the activation temperature. This graphitization was observed for the samples with low RKOH/CNC ratios, and Figure 4a shows the improvement in the peak ratio of the graphite (002) (graphite, P63/mmc JCPDS No.41-1487, a =b =0.2470 nm, c =0.6724 nm) to the carbon (graphite, R3 JCPDS No.26-1079, a =b =0.2456 nm, c =1.0044 nm). However, at higher RKOH/CNC ratios, the trend appears to be the opposite. During activation, the graphitization and chemical etching occur simultaneously. Graphitization of carbon occurs at high temperatures

26

. With a low KOH amount, the graphitic layers are

favored, and the etching is suppressed during the activation. Therefore, the increase in the graphitization degree is more obvious than the etching. In contrast, when the content of KOH increased, the etching was enhanced and destroyed the graphite structure, simultaneously leading to a less ordered graphitic structure and creating disorders and defects in the form of a 11



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large number of pores. Figure 4b shows the XRD patterns of the pristine CNCs and activated CNCs treated at different temperatures for 20 h (with RKOH/CNC of 2). The graphitization of the CNCs during the activation continued as the temperature increased, which is consistent with the TEM observation (Figure 2). For this intermediate RKOH/CNC value (RKOH/CNC = 2 in this experiment), the temperature effect promotes the graphitization more than boosting the KOH etching. Therefore, the graphitization degree is magnified as the temperature increased. However, a high temperature leads to an increase in both the number and size of the pores, as observed in the TEM images (Figure 2).

Figure 4. XRD patterns of original CNCs and activated CNCs under different conditions: (a) with RKOH/CNC of 1, 2 and 4 at 900 °C for 20 h, (b) with RKOH/CNC of 2 at 800 °C, 900 °C and 1000 °C for 20 h, and (c) with RKOH/CNC of 4 at 900 °C for 5~30 h. 12


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The XRD tests have also been performed in pristine CNCs and activated CNCs for different treatment times (RKOH/CNC is 4 and the temperature is 900 °C) (Figure 4c). The peak strength of graphite (002) gradually decreased as the time increased, implying that KOH etching steadily proceeded during the entire activation process leading to increasing superposition of the micropores, as shown in Figure 3. In addition, when the activation time increased to 5 h, the metallic elements of the catalyst alloy could be completely removed by the following HCl treatment (as proved by XPS in Supporting Information). These alloy particles were previously wrapped by the graphitic layers and not accessible to the acid. After activation, the sheathing structure became more open and porous, allowing for removal of the alloy particles by the acid. In summary, an intermediate RKOH/CNC ratio of approximately 2 appears to be for achieving the proper pore structure and ultimate graphitization degree. To confirm the previous analysis, the nitrogen adsorption/desorption isotherms were measured. The isotherm of the pristine sample is of type IV with a mesoporous feature even though only a few mesopores were observed in the PSD result (Figure S6). In addition, the pore volume and SSA of the pristine samples were determined to be 0.473 cm3 g-1 and 99 m2 g-1, respectively. The results of the activated CNCs are shown in Figure 5. The isotherms of the activated CNCs are also of type IV, which most likely reflects the mesostructure being maintained even after severe activation. In Figure 5, as the RKOH/CNC increased, the SSA of the activated CNCs increased from 235.1 to 469.9 m2 g-1 and then decreased to 426.5 m2 g-1. The same trend was observed for the micropore area and external surface area. The chemistry of the KOH activation is not well understood. For the activation of petroleum coke with KOH below 700 °C, Otowa et al.

27

observed various products, such as

13



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H2, H2O, CO, CO2, potassium oxide (K2O), and potassium carbonate (K2CO3). Among the many proposed reactions, the key processes are briefly outlined here. KOH dehydrates to form K2O at 400 °C: 2KOH = K2O + H2O Reaction of carbon with water vapor generates hydrogen, CO and/or CO2. Then, K2O may further react with CO2 forming K2CO3 and be reduced by CO to produce metallic potassium at temperatures above 700 °C. An overall expression has been suggested as follows 28, 29: 6KOH + 2C →2K +2K2CO3 +3H2. The standard Gibbs free energy change of this overall reaction was estimated to become negative above 570 °C 30. A few remarks regarding the KOH activation are as follows: a) the K2CO3 formation occurs at approximately 400 °C, b) KOH is completely consumed at 600 °C, and c) significant decomposition of K2CO3 into CO2 and K2O starts above 700 °C and completes at ca. 800 °C. Based on this discussion, the proposed mechanisms for the KOH activation of carbon most likely involve the formation of metallic potassium via carbothermal reduction of K2CO3 and/or K2O. Potassium is a liquid at temperatures above 64 °C and has a boiling point of 759 °C. Below its boiling point, potassium most likely efficiently intercalates into the carbon lattices, creating high microporosity and a large specific surface area after the final removal of the metal. Simultaneously, the water that formed from the KOH dehydration also contributes to further developing the porosity via the gasification of carbon. In the reaction, if the amount of carbon is sufficient, the yield of metallic potassium increases with the RKOH/CNC ratio. 31 At low RKOH/CNC ratios (1 in the current study), the active 14


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metallic potassium congregates in the defects of the CNCs via capillary interaction, and then, more micropores are most likely created when the metal is removed. As the reaction continues, the unconnected micropores grow, and new mesopores are formed after prolonged activation. At high RKOH/CNC values, both potassium intercalation and carbon gasification are exaggerated, resulting in extended etching of the graphitic layers and increased microporosity mesostructure, as shown in Figure 6. It is important to note that the peaks corresponding to the micropores (approximately 0.8 nm) progressively shift right as the amount of KOH increased, indicating that the micropores have become larger. These larger micropores favor electronic transmission and ion diffusion in supercapacitor applications. However, the elevated activation temperature results in a higher SSA, micropore area and external surface area of the activated CNCs, (Figures 5c and d). At low temperatures, the relatively mild and slow process of KOH activation decreased the etching process and gradually generated microporous structures. At higher temperatures, the activation was intensified and severe etching occurred, resulting in the formation of mesostructures (Figure 6). In the terms of the activated time, the variation in the SSA, the micropore area and the external surface area is similar that observed for the temperature effect. In other words, steadily increased etching can be obtained as the reaction proceeds. This etching leads opening of the formed obstructions and to more porous structures. Therefore, the micropore area and external surface area increased, resulting in an improved SSA. As the time increased, the micropore structure might collapse as the micropores gradually merge into the mesopores or eventually macropores. Therefore the micropore area and external surface area simultaneously decrease, as shown in Figure 6. 15



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Based on the above discussion and the morphological characteristics listed in Table 1, the activation with a suitable amount of KOH for an appropriate time can improve the specific surface area and result in hierarchical pores with connected micropores and mesopores in the CNCs. This result may also be extended to the activation of other carbon materials with a high graphitization degree, such as CNTs, CNFs and graphene.

Figure 5. N2 sorption isotherms and pore size distribution of CNCs after KOH activation (a) (b) with different RKOH/CNC, (c) (d) at different temperature, and (e) (f) for different times.

16


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Figure 6. Illustration of KOH activation of CNCs.

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Table 1. Characteristics of the original CNCs and the activated CNCs. Sample

SAA

Micropore area

Pore volume

Micropore volume

Microporosity

(m2 g-1)

(m2 g-1)

(cm3 g-1)

(cm3 g-1)

(surface area) (%)

Original

99.0

0

0.47

0

0

1-900-20

235.1

7.9

0.57

0.004

3.4

2-900-20

469.9

99.6

0.83

0.05

21.2

4-900-20

426.5

27.9

1.03

0.012

6.5

2-800-20

252.6

19.0

0.67

0.011

7.5

2-900-20

469.9

99.6

0.83

0.050

21.2

2-1000-20

342.8

67.3

0.76

0.036

19.6

4-900-5

283.2

6.5

0.84

0.002

2.3

4-900-10

422.8

28.1

1.06

0.013

6.7

4-900-15

472.2

27.7

1.18

0.011

5.9

4-900-20

426.5

27.9

1.03

0.012

6.5

4-900-25

388.0

11.1

1.06

0.003

2.9

4-900-30

248.2

9.3

0.81

0.004

3.7

3.2. Improvement in the electrochemical properties The supercapacitor performance of the CNCs was evaluated using the three-electrode system from 5 to 200 mV s-1, as shown in Figure S7. Figure 7 shows the CV curves of the activated CNCs at 100 mV s-1, which exhibited typical characteristics of a quasi-rectangular shape, indicating that the activated CNCs acted as electrode materials. As the current density increased from 0.5 to 20 A g-1, the specific capacitance of all of the activated CNCs exhibited little attenuation, which indicated that the activated CNCs perform with excellent rate 18


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capability (i.e., the stability of the capacitance values under the different discharge rate). However, the original CNCs electrode exhibited a remarkably lower rate capability, their specific capacitances calculated from the galvanostatic discharge curves were 24.4 F g-1 at a low current density of 0.5 A g-1, while only 21.1 F g-1 of the capacitance remained when the current density increased to 20 A g-1. These results further confirm the predominance of the beneficial porous structure of activated CNCs for energy storage (Figure S8).

Figure 7. Electrochemical performance of activated CNCs using a three electrode cell in a 6 M KOH electrolyte: (a, b and c) CV curves at a potential scan rate of 100 mV s-1 and (d, e and f) capacitances of activated CNCs as a function of the current density.

Figure 8a shows the relationship between the supercapacitor performance and the specific surface area for the electrode materials. In Figure 8a, the trend for the variation in the specific capacitance is consistent with the change in the specific surface area, and this trend is not 19



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dependent on the activation conditions. In addition, the hierarchical structure with micropores and mesopores can improve the performance of supercapacitors. Abundant micropores lead to a large specific surface area and provide fast ionic transportation pathways, and the mesopores act as electrolyte ion channels. In activated CNCs, the double layers form on the outer surfaces as well as in the pores within the carbon shells. In other words, the hierarchical pore structure is beneficial for supercapacitor performance. Moreover, the relationship between the supercapacitor performance and the pore structure for the materials is shown in Figure 8b. When both pore volume and micropore volume are high, the large pore volume is advantaged for the improvement of capacitance. However, if the pore cannot include large percentage of micropores, the capacitance is low. It is declared that the mesopores are accessible to electrolyte ions and the micropores enhance the capacitance by increasing the electrode/electrolyte interfaces, which is accord with previous studies. 12, 13 In order to explore the internal resistance and the performance of electrode materials, EIS experiments were conducted. To properly describe the action of an alternating potential input on supercapacitors, one can in principle consider at least two coupled interface processes influencing the impedance of the system: the electron transfer process across the electrolyte/electrode interface and the double-layer effect. The equivalent circuit typically has been schematized in Figure 8c, constituted by a solution resistance (Ru), a charge transfer resistance (Rct) and a double-layer capacitance (Cdl). Ru represents the uncompensated resistance of the electrolyte and other possible ohmic resistances, whereas Rct represents the ohmic drop that can be associated to the electron transfer process. The double-layer defect, which roughly consists of charge separation in the electrode/electrolyte interphase as a result 20


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of charge migration, can be assimilated to a capacitor of capacitance Cdl. Moreover, the time constant (τ), which defines the limit between the resistive and the capacitive behaviors, can be obtained by the EIS tests. 31 Table 2 shows the resistances, the capacitances and the time constants of different samples from the EIS tests. The low Ru of electrodes can be kept during the activation process, which is advantaged for high-power supercapacitor applications. Time constant obtained from the EIS plots of supercapacitors reveals the ion transport within electrodes, thus is highly associated with the porosity of electrode materials. The hierarchical structure with micropores and mesopores can achieve short time constants, as shown in Table 2. The trend for the variation in the time constants is consistent with the change in the porosity of CNOs. It is attributed to that the abundant micropores provide fast ionic transportation pathways, and the mesopores act as electrolyte ion channels. The 2-900-20 sample shows an excellent time constant of 2.5 ms. The outstanding porosity and conductivity of the carbon electrodes increase the specific capacitance and decrease the resistance, respectively.

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Table 2. EIS data of the original CNCs and the activated CNCs. Sample

Ru (Ω)

Cdl (F)

Rct (Ω)

τ (ms)

Original

0.627

0.571

0.225

128.2

1-900-20

0.521

0.056

0.145

8.1

2-900-20

0.347

0.050

0.051

2.5

4-900-20

0.602

0.022

0.132

2.9

2-800-20

0.683

0.165

0.096

15.8

2-900-20

0.357

0.050

0.051

2.5

2-1000-20

0.613

0.031

0.082

2.5

4-900-5

0.631

0.0203

0.185

3.7

4-900-10

0.582

0.032

0.092

2.9

4-900-15

0.567

0.038

0.072

2.7

4-900-20

0.602

0.022

0.132

2.9

4-900-25

0.621

0.021

0.162

3.4

4-900-30

0.576

0.074

0.191

14.1

The cycle stability of the activated CNCs has also been evaluated. Due to its high specific capacitance, the 2-900-20 sample was selected for use in the galvanostatic charge-discharge studies at a high current density of 5 A g-1. As shown in Figure 8d, after 5000 cycles, approximately 100 % of the initial capacitance was preserved. The micropore volume of the 2-900-20 sample was the highest compared to all of the studied samples. With abundant micropores, the electrode still exhibited good capacitive performance and excellent cycle stability, which confirms the importance of a hierarchical pore structure for supercapacitors. Meanwhile, compared with other carbons reported in the literatures, the activated CNCs have a great potential for supercapacitors in the future (Table S1 in Supporting Information). 22


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Figure 8. Specific capacitance and (a) surface area (b) pore structure of the original and activated CNCs as a function of KOH/CNCs, activation temperature and time and (c) the equivalent circuit of the EIS system, as well as the cycle performance of the 2-900-20 sample at a current density of 5 A g-1.

4. Conclusions In conclusion, carbon nano-chains (CNCs) were synthesized by the CVD method and further activated by the KOH process. The activated CNCs contained abundant hierarchical pores and exhibited a high specific surface area (SSA). The improvement in the SSA of the activated CNCs was obtained relative to the pristine samples, reaching 470 m2 g-1. The formation of the micro-meso pores can be tailored by the activation parameters (i.e., the amount of KOH, activation time and temperature). This micro-meso structure is advantageous due to ion transport through the mesopores and increased capacitance due to the electrical double layers formed in the micropores. During the KOH activation, graphitization and etching simultaneously occurred, allowing for a compromise in the 23



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materials properties by optimizing the process. The CNCs activated with a KOH/CNCs weight ratio of 2 at 900 °C for 20 h exhibited an improved electro-capacity performance of 112.7 F g-1 at a scan rate of 50 mV s-1. The materials exhibited excellent performance stability due to the improved surface and porosity with little sacrifice in the electronic transmission feature. The excellent electrochemical performance of the activated CNCs indicates their potential for use in supercapacitors.

Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 51272173 and No. 51472177), Natural Science Foundation of Tianjin City (No. 12ZCZDGX00800).

Supporting Information Avaiable The high-resolution TEM images of original CNCs and SEM images of activated CNCs are shown. The N2 adsorption/desorption and DFT pore size distribution of CNCs with and without activation are also shown. The XPS results, CV curves at different scan rates of activated CNCs and the electrochemical data of original CNCs are shown in the fraction. Moreover, the performances of CNCs with that of carbons in the literature is compared in a table. This information is available free of charge via the Internet at http:// pubs.acs.org.

References 24


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Table of Contents Graphics

29



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Figure 1. TEM images of original CNCs (a) and activated CNCs with different RKOH/CNC at 900 °C for 20 h (RKOH/CNC=1 (b), 2 (c), and 4 (d)). 59x38mm (300 x 300 DPI)


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Figure 2. TEM images of activated CNCs (RKOH/CNC = 2) at 800 °C (a), 900 °C (b) and 1000 °C (c) for 20 h. 49x31mm (300 x 300 DPI)



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Figure 3. TEM images of activated CNCs (RKOH/CNC = 4) at 900 °C for 5 h (a), 10 h (b), 15 h (c), 20 h (d), 25 h (e) and 30 h (f). 66x50mm (300 x 300 DPI)


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Figure 4. XRD patterns of original CNCs and activated CNCs under different conditions: (a) with RKOH/CNC of 1, 2 and 4 at 900 °C for 20 h, (b) with RKOH/CNC of 2 at 800 °C, 900 °C and 1000 °C for 20 h, and (c) with RKOH/CNC of 4 at 900 °C for 5~30 h. 73x184mm (300 x 300 DPI)



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Figure 5. N2 sorption isotherms and pore size distribution of CNCs after KOH activation (a) (b) with different RKOH/CNC, (c) (d) at different temperature, and (e) (f) for different times. 78x97mm (300 x 300 DPI)


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Figure 6. Illustration of KOH activation of CNCs. 82x82mm (300 x 300 DPI)



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Figure 7. Electrochemical performance of activated CNCs using a three electrode cell in a 6 M KOH electrolyte: (a, b and c) CV curves at a potential scan rate of 100 mV s-1 and (d, e and f) capacitances of activated CNCs as a function of the current density. 84x44mm (300 x 300 DPI)


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Figure 8. Specific capacitance and (a) surface area (b) pore structure of the original and activated CNCs as a function of KOH/CNCs, activation temperature and time and (c) the equivalent circuit of the EIS system, as well as the cycle performance of the 2-900-20 sample at a current density of 5 A g-1. 57x33mm (300 x 300 DPI)



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The activated carbon nano-chains synthesized through chemical vapor deposition with KOH activation has great potential application in supercapacitors due to the improved surface and porosity with little sacrifice in the electronic transmission feature. 50x50mm (300 x 300 DPI)


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Activated Carbon Nanochains with Tailored Micro-Meso Pore

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