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Facile nano-templated CO2 conversion into highly interconnected hierarchical porous carbon for high-performance supercapacitor electrodes
Accepted Manuscript Facile nano-templated CO2 conversion into highly interconnected hierarchical porous carbon for high-performance supercapacitor electrodes Yun Kon Kim, Jae Hyun Park, Jae W. Lee PII:
S0008-6223(17)31013-8
DOI:
10.1016/j.carbon.2017.10.020
Reference:
CARBON 12457
To appear in:
Carbon
Received Date: 18 July 2017 Revised Date:
20 September 2017
Accepted Date: 7 October 2017
Please cite this article as: Y.K. Kim, J.H. Park, J.W. Lee, Facile nano-templated CO2 conversion into highly interconnected hierarchical porous carbon for high-performance supercapacitor electrodes, Carbon (2017), doi: 10.1016/j.carbon.2017.10.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Facile Nano-templated CO2 Conversion into Highly Interconnected Hierarchical Porous Carbon for High-performance
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Supercapacitor Electrodes Yun Kon Kim§, Jae Hyun Park§ and Jae W. Lee*
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Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea Both authors contributed equally to this work
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§:
*Corresponding author. Tel: +82-42-350-3940. E-mail: [email protected]
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Supercapacitor.
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KEYWORDS: CO2 conversion, CaCO3 template, Hierarchical porous carbon, Boron doping,
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ACCEPTED MANUSCRIPT Abstract Hierarchical porous carbon materials have been derived through CO2 conversion by using NaBH4 as a reducing agent and CaCO3 as a nano-template. The CaCO3-templated
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porous carbons (CPCs) feature an interconnected three-dimensional structure with hierarchical pores favorable for electrochemical energy storage. Notably, CPC1_700 prepared with an identical mass of CaCO3 and NaBH4 at 700 °C shows a very high capacitance of 270
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F/g at 1 A/g and retains its capacitance up to 170 F/g at 20 A/g in 6 M KOH aqueous electrolyte. Moreover, it presents an outstanding normalized capacitance of 21.4 µF/cm2 even
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in the absence of pseudocapacitive behavior, and a fast frequency response with a low relaxation time constant of 0.27 s. Concerning the cycle stability, more than 90% of the initial capacitance is maintained after 10000 consecutive cycles at high current densities (20 A/g and 30 A/g). The major fundamental insights underlying this performance are closely related
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to the interconnected hierarchical pore architecture generated by the concurrent template and CO2 activation effect, which leads to increased surface area, fast ionic transport, and efficient ionic storage. The proposed route of CO2-to-carbon with the template affords a facile,
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efficient, and sustainable strategy to synthesize hierarchical porous carbon for high-
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performance supercapacitors.
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ACCEPTED MANUSCRIPT 1. Introduction As global warming is an issue of great concern, renewable energy is attracting attention as a potential alternative to fossil fuels [1, 2]. In addition, the emerging market of
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numerous electronic devices such as cell phones, cameras and hybrid electric vehicles has generated strong demand for reliable and sustainable energy sources [3]. To meet the requirements, various types of electrochemical systems including fuel cells, lithium-ion
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batteries, and supercapacitors are considered promising candidates. Among them, the supercapacitors offer many benefits such as high power density, fast charge-discharge rate,
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and stable life cycle [4-6].
Electrode materials for supercapacitors are typically classified into transition metal oxides, conductive polymers, and porous carbon based materials. The former two materials
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provide the advantage of high capacitance but also have drawbacks of low rate capability, short life cycle, and high cost, limiting practical applications for commercialization. On the
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other hand, porous carbon based materials have attracted a great deal of attention due to their long-term and stable life cycle, as well as large surface area, high conductivity, low
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production cost, and superior thermal, mechanical, and chemical stabilities [5, 7-10]. The specific capacitance of carbon based materials is mainly determined by the specific surface area and the porosity of the structure [11]. Micropores contribute to the improved surface area of the porous material, mesopores provide a transport channel through which ions can readily pass [5, 12, 13], and macropores function as a reservoir for the ion storage and shorten the transport distance [14, 15]. Thus, the optimal porous carbon structure is an interconnected network of the variously sized pores to increase the effective ion storage area and lower the ion 3
ACCEPTED MANUSCRIPT migration resistance [16].
Recently, CO2 conversion into porous carbon materials has emerged as a key
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technology since CO2 is an abundant carbon source, and this CO2 utilization can contribute to the mitigation of global warming [17]. Numerous CO2 transformation strategies have been proposed to produce CNTs (at 70 MPa) [18], porous carbon (at 32 MPa) [19], and graphene
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[20] using supercritical CO2 or dry ice. However, these extreme conversion conditions require large energy consumption, and cause the reproduction of CO2. For a more energy
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efficient CO2 conversion process, sodium borohydride (NaBH4) has been used as a CO2 reducing agent to produce boron doped porous carbon at atmospheric pressure and 500 °C [17, 21-24]. Moreover, porous carbon materials were derived from a reaction of synthetic flue gas (85 % N2 and 15 % CO2) with NaBH4, followed by KOH activation [25]. Although these
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studies demonstrated promising technologies for CO2 conversion into porous carbon materials, the suggested approaches still have some disadvantages of limited electro-
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capacitive performance and multi-step preparation.
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In this study, we propose a single step CO2 conversion into hierarchical porous
carbon by using NaBH4 as a reducing agent and CaCO3 as a nano-template. The CaCO3 nanoparticles have been previously investigated for application as a hard template in carbon precursors such as melamine-formaldehyde resin and sucrose [26, 27]. However, the combination of a nano-template with CO2 conversion has never been explored for the production of hierarchically porous carbon materials. It will be shown that without any chemical treatment such as KOH activation [8, 25], the as-prepared hierarchical porous 4
ACCEPTED MANUSCRIPT carbons possess a large surface area (1262 m2/g), a high pore volume (3.35 cm3/g), and an interconnecting pore structure, leading to enhanced electrochemical properties. From various electrochemical analyses, it will be demonstrated that the porous carbons exhibit excellent
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supercapacitive performance in terms of specific capacitance, rate capability, and cycle stability. This study opens up a new way of connecting CO2 utilization and the production of hierarchical porous carbon via a facile one-step templating method for effective
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supercapacitive energy storage.
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2. Experimental 2.1 Materials
Argon (Ar, > 99.9%) and carbon dioxide (CO2, > 99.9%) were purchased from Deokyang Co., Ltd. Sodium borohydride (NaBH4, > 99%), polytetrafluoroethylene (PTFE,
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60 wt% dispersion in water), and potassium hydroxide (KOH, > 90%) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl, 37 wt% in water) was available in Junsei Chemical Co., Ltd. Calcium carbonate nanoparticles (CaCO3, 15 - 40 nm) were acquired from
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SkySpring Nanomaterials. All of the chemicals were used without further purification.
2.2 Synthesis of CaCO3-templated porous carbon (CPC) and non-templated porous carbon (NPC)
In a typical synthesis of CPCs_700 (Figure 1), 4g of NaBH4 was mechanically mixed with CaCO3 at different mass ratios (CaCO3/NaBH4 = 0.5, 1 and 2). The mixture of NaBH4 and CaCO3 was heated up to 500 °C (5 °C/min) under a CO2 flow of 75 ml/min and 5
ACCEPTED MANUSCRIPT maintained at this temperature for 2 h. The temperature was then increased up to 700 °C (5 °C/min), in which CO2 gas was switched to a 50 ml/min Ar flow at 600 °C, and the temperature was held at 700 °C for 2 h. The resultant was transferred into a beaker, and the
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residue including the template was washed several times with hot aqueous solutions of 5 M HCl, deionized (DI) water, and ethanol in sequence. The sample was dried in an oven at 95 °C for 12 h. The as-prepared carbons were denoted as CPCx_700, where x indicates the
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CaCO3/NaBH4 weight ratio (x = 0.5, 1 and 2). In the case of CPC1_600, the sample was heated until 600 °C under only a CO2 flow, and then cooled to room temperature without the
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the addition of a CaCO3 template.
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Ar treatment. NPC_700 was prepared following the same procedure as CPCx_700, except for
Figure 1. Schematic diagram for one-step nano-templated CO2 conversion into porous carbons, CPCs_700.
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ACCEPTED MANUSCRIPT 2.3 Characterization The morphology of the porous carbon was investigated by a scanning electron microscope (SEM, Hitachi SU8230). Transmission electron microscopy (TEM) images were
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obtained with a JEOL JEM-2100F at 200 kV. The sample for TEM was dispersed in ethanol, followed by dropwise addition of the suspension onto a copper grid. The nitrogen adsorption– desorption isotherms were measured at 77 K using a Micromeritics Triflex after degassing of
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the sample under a high vacuum at 200 °C for 8 h. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) theory by using a relative pressure range of 0.02 – 0.2.
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The correlation coefficients for BET plots were above 0.9999. The micropore and mesopore size distributions were determined by the Horvath-Kawazoe (H-K) and Barrett-JoynerHalenda (BJH) analyses, respectively. The total pore size distributions were obtained from non-local density functional theory (NLDFT). X-ray diffraction (XRD) patterns were
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measured on a RIGAKU Smart lab with Cu Kα (λ = 1.5406 Å) set at 45 kV and 200 mA. Fourier transform infrared (FTIR) spectra were recorded from 400-4000 cm-1 with a Thermo Nicolet iS50 using KBr pellets. RAMAN spectra of the samples were collected on a
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LabRAM HR Evolution Visible_NIR (HORIBA) at 514 nm of an Ar ion laser. X-ray photoelectron spectroscopy (XPS) was performed through a K-alpha with a microfocused
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monochromator X-Ray source to investigate the surface chemical properties. Elemental analyses (EA) were conducted on a Thermo EA 1112 to determine the oxygen content.
2.4 Electrochemical measurement Electrochemical analyses, including cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) measurements, and electrochemical impedance spectroscopy (EIS) were 7
ACCEPTED MANUSCRIPT conducted on the 3-electrode system in 6 M aqueous KOH electrolyte at 25 °C with a BioLogic 6002D potentiometer and EC-Lab software. The platinum and calomel electrodes were used as counter and reference electrodes, respectively. The working electrode was prepared
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by mixing 2.0 mg of active materials and PTFE with a mass ratio of 19:1 in water, which was dispersed on a 1 cm × 1 cm nickel foam and pressed, followed by drying in an oven at 95 °C for 12 h. The CV test was performed at a scan rate of 10 mV/s within a voltage range of -1.0
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to 0.0 V. The GCD analysis was carried out on the same voltage range as CV and varying the current density from 1 to 20 A/g. The Nyquist plots were obtained from EIS measurements,
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with a sinusoidal signal of 10 mV and a frequency range from 100 kHz to 0.05 Hz.
3. Results & Discussion
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3.1 Structural characteristics of the porous carbon
The morphology of the prepared CO2 derived porous carbon was investigated by scanning electron microscopy (SEM). As illustrated in Figure 2a, the non-templated sample
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NPC_700 has a blade-like shape consisting of different sizes of pores, and the pores are surrounded by a thick carbon wall structure. Using CaCO3 nanoparticles as a carbon template,
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a highly porous structure was clearly observed in CPCs_700 (Figure 2b and also refer to Figure S1 in the Supplementary material). In particular, CPC1_700 and CPC2_700 reveal a three-dimensional architecture interconnecting the overall carbon framework. The inset image of CPC1_700 indicates the hierarchical pore structure. Before washing with HCl (Figure 2c), non-washed CPC1_700 shows pores filled with the CaCO3 template and byproducts. Comparing Figures 2b with 2c, it is confirmed that the formation of pores is due to the removal of the fillers by the washing step. 8
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The internal pore networking was identified with the aid of TEM observation. For the non-templated sample of NPC_700 in Figure 2d, irregularly shaped pores are placed in a
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plate-like carbon structure. On the other hand, when the template was used, circular shaped pores (Figures 2e and 2f) were dominantly created. Comparing the structures of CPC1_600 and CPC1_700, the Ar heat treatment impacts the morphology and the pore interconnection.
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In the case of CPC1_600 prepared without the treatment, the structure appears to be porous, but the degree of interconnection is low. However, CPC1_700 not only shows a highly
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interconnected pore network, but also more distinct templated porosity over the whole carbon framework, because 2 h of thermal treatment sufficiently leads to re-organization of the carbon structure with a more rigid templating effect. These characteristics are also revealed in
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CPC2_700 (Figure S1), demonstrating the combined effect of circular and interconnected
Figure 2. SEM images of (a) NPC_700, (b) CPC1_700 (Inset: high magnification SEM image of CPC1_700) and (c) non-washed CPC1_700. TEM images of (d) NPC_700, (e) CPC1_600 and (f) CPC1_700. 9
ACCEPTED MANUSCRIPT pores coming from both the CaCO3 template and the Ar heat treatment.
The pore characterizations of NPC_700 and CPCs were analyzed with N2 adsorption
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isotherms in Figure 3. All of the isotherms (Figure 3a) have a shape of type IV with a sharp uptake at low relative pressure (P/P° < 0.001) and an obvious hysteresis loop at high pressure (0.4-1.0 P/P°) [5]. Although they show different adsorption amounts, the occurrence of micro
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and mesopore condensation was the same. The textural properties of the porous carbon are listed in Table 1. As the ratio of the nano-template increases, the total pore volume gradually
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increases. The largest total pore volume is 3.35 cm3/g in CPC2_700. As seen in Figure 3b, compared to NPC_700, the CPCs show a very large mesopore volume in a broad range of pore width from 10 to 50 nm, demonstrating the mesopore derivation effect of the nanotemplate. In the case of the surface area, CPC1_700 has the highest BET surface area, 1262
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m2/g, which is due to the abundant micropores. The micropore size distributions (Figure 3c) were calculated through the Horvath-Kawazoe method [28, 29]. NPC_700 exhibits a sharp and narrow peak centered at 0.5 nm, while CPCs_700 represent different types of peaks
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covering a wide pore width. In the case of CPC1_600, these two characteristic peaks were observed at the same time, which implies that some of the micropores in CPCs do not
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originate from those of NPC_700, but are generated by another mechanism. This mechanism is related to the decomposition of CaCO3 into gaseous CO2 at the high temperature [27]. The possibility of this decomposition reaction in the experimental temperature of 700 °C was supported by the TGA results (Figure S3). The generated CO2 reacts with the carbon around the decomposed CaCO3, which leads to forming micropores and the thin-wall structure (refer to the TEM images of Figures 2e and 2f) according to the following reactions [30].
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ACCEPTED MANUSCRIPT CaCO3 → CO2 + CaO
(1)
CO2 + C → 2CO (CO2 activation)
(2)
In more detail, the measured micropore volume of CPC1_700 is higher than that of
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CPC1_600 by 27 %, verifying that the above micropore formation mechanism mainly occurs through the CO2 activation originating from CaCO3 during the Ar heat treatment at 700 °C. Additionally, from the distribution result of CPC0.5_700, which has a lower micro-pore
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volume than NPC_700 (Figure 3c), it can be deduced that the template also hinders the propagation of the inherent micropores derived by the NaBH4-aided CO2 conversion.
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Consequently, the mass balance ratio between NaBH4 and CaCO3 template is an important factor for the construction of an effective micropore structure, and CPC1_700 provides optimal conditions for micropore generation. The NLDFT pore size distributions of the samples in Figure 3d also show that CPC1_700 has more micro- and mesopores than the
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other samples.
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Table 1. Textural characteristics of NPC_700, CPCs_700 and CPC1_600. BET surface area (m2/g)
Total pore volumea (cm3/g)
Mesopore volumeb (cm3/g)
Micropore volumec (cm3/g)
NPC_700
955
1.62
1.223
0.397
CPC0.5_700
910
2.17
1.789
0.381
CPC1_700
1262
3.21
2.682
0.528
CPC2_700
1218
3.35
2.843
0.507
CPC1_600
944
2.68
2.292
0.388
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Sample
a
Total pore volume up to P/P0 ~ 0.995. b Mesopore volume determined by the BJH method.
c
Micropore volume determined by the H-K method. 11
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Figure 3. (a) N2 sorption isotherms at 77 K (The isotherms for CPC0.5_700, CPC1_700, CPC1_600, and CPC2_700 samples were offset by 300, 600, 900, and 1200 cm3/g, respectively.), and (b) BJH and (c) H-K pore size distributions for NPC_700, CPCs_700, and CPC1_600 (Inset is a cumulative volume for micropores). (d) NLDFT pore size distributions for NPC_700, CPC1_700 and CPC1_600.
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3.2 Chemical properties of the porous carbon The crystalline structure of the porous carbon was analyzed by XRD patterns. As
shown in Figure 4a, all porous carbons have similar XRD patterns with two broad peaks at around 24° and 43°, corresponding to (002) and (10) diffraction profiles of a partially turbostratic structure [31]. There is no crystalline structure, indicating the removal of the crystalline template and by-products during the washing step. In order to investigate the 12
ACCEPTED MANUSCRIPT chemical state of the nano-template during the synthetic step, the non-washed CPC1_700 was immediately sampled out from the furnace, and subjected to an XRD analysis. The sample exhibits complex XRD patterns (Figure 4b), which do not match those of pristine CaCO3
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nanoparticles. To confirm the thermal behavior of CaCO3, the nanoparticles alone were
Figure 4. XRD spectra of (a) NPC_700, CPCs_700, and CPC1_600, (b) non-washed CPC1_700 and pristine CaCO3 nanoparticle, and (c) consecutive CO2 & Ar heat-treated CaCO3 nanoparticle.
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heated under the same conditions, as in the synthesis of CPC1_700, and the XRD result is presented in Figure 4c. The patterns consist mostly of CaCO3 and a small amount of CaO [27], verifying that there is a small amount of decomposed nanoparticles during the heat
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treatment. It thus can be inferred that the complicated peaks for the non-washed CPC1_700 may mainly originate from the reaction between CaCO3 and NaBH4. However, the peaks
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marked with arrows correspond to the intrinsic peaks of CaO, meaning that some of the CaCO3 was still decomposed into CaO and CO2. Considering these findings together with the results for the pore distributions, CaCO3 and generated CO2 contribute to the formation of various sized mesopores and micropores in the carbon framework.
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ACCEPTED MANUSCRIPT Raman spectroscopy is a non-destructive and effective method to characterize sp2 disordered carbon structures. The Raman spectra of NPC_700 and CPCs are shown in Figure 5. For all the prepared porous carbons, the two distinct peaks indicate the G band at around
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1590 cm-1 derived from the bond stretching of sp2 pairs, and the D band at around 1350 cm-1 from the breathing modes of sp2 atoms in rings, respectively. The measured I(D)/I(G) values,
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the ratio of peak heights, are indicated in Figure 5. The CPC1_600 has the smallest I(D)/I(G)
Figure 5. Raman spectra of NPC_700, CPCs_700, and CPC1_600.
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ratio of 0.72, while the others show almost identical values between 0.87 and 0.91. Previously,
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Ferrari et al. reported ordering trajectory in which the I(D)/I(G) ratio increases from the amorphous carbon (20 % sp3) to nanocrystalline graphite (0 % sp3) [32, 33]. Considering the higher heat treatment temperature of 700 °C under the Ar flow, the higher I(D)/I(G) ratio for NPC_700 and CPCs_700 than CPC1_600 is related to re-organization of the amorphous structures as they are exposed to more severe thermal treatments [34-36]. This confirms that the addition of the CaCO3 template is not related to the ordering or disordering, and that the heat treatment under the Ar atmosphere contributes to the formation of ordered sp2 carbon structure. 14
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The surface chemical information and the bonding state of elements were investigated using XPS. All the porous carbon materials mostly consist of carbon and oxygen,
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among which NPC_700 and CPC1_600 also contain a small amounts of boron. The O1s spectra for the samples are shown in Figure 6a (also refer to Figure S5). All the porous carbons have similar O1s spectra, and the binding energies around 531 eV, 532 eV, and 536
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eV are assigned to C=O, C-O, and chemisorbed water, respectively [37]. The C1s spectra also support the presence of oxygen functional groups (refer to Figure S6). The measured oxygen
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content and the specific composition of oxygen functionalities are listed in Table 2. Compared to the elemental analysis (EA) results, the oxygen content trends were similar but the quantities were different, confirming that the oxygen functional groups are more abundant near the surface. Furthermore, the higher C=O composition in CPCs than NPC_700 implies
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the participation of CaCO3 in the formation of the oxygen functional groups.
Figure 6. XPS (a) O1s spectra of NPC_700, CPC1_700 and CPC1_600, and (b) B1s spectra of NPC_700 and CPC1_600. 15
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In the case of boron, NPC_700 and CPC1_600 (Figure 6b) show weak, but obvious B1s spectra, while the other CPCs_700 present B1s spectra that are not well-recognized, as
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revealed in Figure S5 and Table 2. It is evident that boron and calcium complexes are mainly formed at 700 °C, and they are washed out by HCl from CPCs_700. For NPC_700 and CPC1_600, B1s spectra can be deconvoluted into three peaks at ca. 187 eV, 189 eV, and 192
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eV, which are assigned to BC3, BC2O and BCO2, respectively [38, 39]. The presence of B-C bonding is additionally confirmed by the deconvolution of C1s spectra (Figure S6). The
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incorporation of boron into the carbon network could contribute to the generation of pseudocapacitance, as reported in previous studies [21, 25, 40].
Table 2. Oxygen and boron content and composition of oxygen species determined by fitting the O1s core level XPS spectra for NPC_700, CPCs_700 and CPC1_600.
Sample EAa
CPC0.5_700
C-O (At %)
H2 O (At %)
Boron contentb (At %)
8.1
21.9
68.5
9.6
2.52
3.1
6.6
30.2
62.9
6.9
0.46
4.8
7.5
32.4
58.8
8.8
0.45
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CPC1_700
6.1
C=O (At %)
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NPC_700
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Oxygen content (At %)
CPC2_700
4.4
7.7
33.0
56.8
10.2
0.50
CPC1_600
6.5
9.1
37.0
54.3
8.7
2.22
a
Oxygen content obtained by elemental analysis.
b
Boron content obtained by XPS spectra.
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3.3 Supercapacitor electrochemical analyses The as-prepared NPC and CPCs were evaluated for the performance as supercapacitor electrode materials. Figure 7a shows the CV graph of the porous carbon at a
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scan rate of 10 mV/s. CPCs_700 exhibit a quasi-rectangular shape of CV graph indicating the electrical double-layer behavior [41]. Furthermore, CPCs_700 do not show pseudocapacitive behavior, because few B atoms are found at the surface of the materials (Figure S5). On the
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other hand, NPC_700 indicates a broad hump (Figure 7a) associated with the
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pseudocapacitive behavior presented in the mid-voltage region due to the incorporation of B atoms into the carbon structure [40, 42-44]. Although XPS B1s spectra show similar amounts of B in NPC_700 and CPC1_600, pseudocapacitive properties are not observed in CPC1_600.
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This will be discussed in more detail on the basis of EIS analyses.
The accurate capacitance was obtained using galvanostatic charge-discharge (GCD) experiments at a current density of 1 A/g, and the results are presented in Figure 7b. The
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calculated capacitance (F/g) and normalized capacitance (µF/cm2) of the synthesized porous carbons are listed in Table 3. Generally, as the specific surface area increases, the capacitance
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tends to rise [11], which can be the reason for CPC1_700 having the highest capacitance. Regarding the surface normalized performance, NPC_700 shows relatively high normalized capacitance of 21.7 µF/cm2 due to the effect of the pseudocapacitance from the boron doped sites. However, CPCs_700 also exhibits comparable normalized capacitance to that of NPC_700, which could be ascribed to the superior ion transport ability by the hierarchical porous structure even at very low boron content, as revealed in the XPS results. In the case of 17
ACCEPTED MANUSCRIPT CPC1_600, the lowest specific and normalized capacitances are revealed among the samples despite the comparable surface area with that of NPC_700 and CPC0.5_700. In terms of ion transport and storage, the lowest capacitance of CPC1_600 may be closely related to the low
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utilization efficiency of the porous structure due to the poor pore interconnection coming from the absence of the Ar heat treatment. Hence, for CPC1_600, a large portion of mesopore volume formed by the template remains isolated, and the mesopores do not function as ion
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reservoirs, effectively. This also can be related to the superior capacitance of CPC1_700 and CPC2_700. Both materials have not only high meso- and macropore volume with the aid of
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the template, but also sufficient micropore interconnection for high utilization of the porous carbon surface. Moreover, the higher capacitance of CPC1_700 than CPC2_700 is ascribed to the optimum mass ratio between CaCO3 and NaBH4, because although the CaCO3 template basically constructs new pores, excessive addition of it can inhibit the pore connection and
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the formation of intrinsic pores derived by CO2 conversion. From a practical point of view, the volumetric capacitance of the synthesized porous carbons was also calculated by using the material density (Table 3). Their densities are measured according to the previously
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reported palletization method [45-47].
Table 3. Three types of capacitance and material density of NPC_700 and CPCs. Normalized capacitance (µF/cm2) 21.7
Volumetric capacitance (F/cm3) 60.4
CPC0.5_700
195
21.4
38.3
0.1966
CPC1_700
270
21.4
50.7
0.1877
CPC2_700
238
19.5
46.3
0.1946
CPC1_600
154
16.3
75.7
0.4918
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NPC_700
Specific capacitance (F/g) 207
Sample
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Density (g/cm3) 0.2919
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The pore utilization was additionally confirmed by the EIS analysis (Figure 7c), which provides the frequency response of a supercapacitor system including electrode
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materials [48]. The Nyquist plots for the porous carbon can be divided into three parts: the intersection with the real x-axis in the high frequency region, the semicircular portion in the mid-frequency region, and the linear region in the low frequency region, which correspond to
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the internal resistance, the charge-transfer resistance, and the mass transport resistance of the
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ions, respectively [49].
For the internal resistance, all the materials have an x-axis intersection point at around 0.25 Ω, indicating that they have similar conductivity. As can be seen in the inset of Figure 7c, NPC_700 and CPCs_700 showed a similar shape of semi-circle, but CPC1_600
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indicated a large radius of curvature. This means that the ion migration resistance of CPC1_600 is much larger than that of the other materials due to the closed and highly
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tortuous structure of CPC1_600, corresponding to the TEM image (Figure 2e). This resistance behavior ascribed to the closed and tortuous pore structure was also found in
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previous studies [50, 51]. In addition, the slope of CPCs_700 was nearly vertical, confirming that it has rather low mass transport resistance [52] and almost an ideal double layer capacitance, as shown in the quasi-rectangular CV graph (Figure 7a) [51, 53].
In the cases of NPC_700 and CPC1_600, relatively low slopes were observed. This suggests that both carbon electrode materials have slow mass transport rates due to a less 19
ACCEPTED MANUSCRIPT effective porous structure. However, there are main differences between them. NPC_700 has a semi-circle with the same shape as that of CPCs_700 in the mid-frequency region, but the slope gradually decreased in the low frequency regions, suggesting a more distinct
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pseudocapacitive behavior than CPC1_600. On the other hand, CPC1_600 showed a large radius of curvature, followed by a large slope in the low frequency region. This phenomenon explains that the main resistance of CPC1_600 originated not from the pseudocapacitive
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behavior but also from the closed structure. Thus, for CPC1_600, the absence of the pseudocapacitive characteristics from the CV analysis is because the closed structure could
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interfere with the contact between B and the electrolyte.
For a more detailed investigation, Bode plots of the phase angle verses frequency were introduced from the EIS results (Figure 7d). The relaxation time constant is defined as
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the inverse of the frequency at a phase angle of 45 degrees, and it can be used as a quantitative reference of how quickly an electrode can discharge [54, 55]. CPC1_700 exhibits the lowest relaxation time constant of 0.27 s, while the other materials show a relaxation time
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of 0.28 s (CPC2_700), 0.31 s (CPC0.5_700), 0.44 s (NPC_700), and 1.15 s (CPC1_600),
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respectively. Compared to previously reported carbon based materials (0.47 s to 2.45 s) [5659], CPC1_700 has a lower relaxation time constant, signifying a very fast frequency response due to its highly interconnected carbon structure with hierarchical pores and its favorable electrical conductivity.
The specific capacitances obtained from the GCD experiments at different current densities are shown in Figure 7e. The figure clearly displays that CPC1_700 has the highest 20
ACCEPTED MANUSCRIPT specific capacitance over the whole range of current densities. The capacitance of CPC1_700 is 170 F/g at 20 A/g, indicating 63 % retention of the specific capacitance at 1 A/g. The capacitance retentions from the same current range were 33 %, 36 %, 65 %, and 62 % for
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NPC_700, CPC1_600, CPC0.5_700, and CPC2_700, respectively.
Figure 7. (a) Cyclic voltammetry curves, (b) galvanostatic charge-discharge curves, (c) Nyquist plots (the inset is an enlarged view of the Nyquist plots), (d) Bode plots of phase angle verses frequency and (e) specific capacitances in various current densities for NPC_700, CPCs_700, and CPC1_600. 21
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In particular, NPC_700 and CPC1_600 showed a rapid decrease of capacitance with an increase of current density, indicating that as the charge-discharge rate increases, the
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ability to store the charge significantly becomes lower than that of the other materials. NPC_700 has a slow pseudocapacitive behavior due to a slow interaction between B doped sites and ions in the electrolyte. Thus, the pseudocapacitive behavior does not occur
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completely under the high current density conditions. For CPC1_600, as the current density increases, the ionic transport is prominently disturbed by the closed and tortuous structural
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resistance, as confirmed by the EIS analysis.
The high utilization efficiency of the surface area was confirmed by a comparison of the area-normalized capacitance of CPC1_700 with results reported in the literature using
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other porous carbons, as listed in Table 4 [60, 61]. Among the various carbons, CPC1_700 shows superior normalized capacitance of 21.4 µF/cm2, even in the absence of
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pseudocapacitive performance. This suggests that the actual charge storage area can be different from the calculated BET specific surface area depending on the porous structure and
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the interconnection. This clearly supports the efficient surface area utilization of CPC1_700 due to its hierarchical pore distribution and pore networking architecture. Furthermore, CPC1_700 provides very stable properties during consecutive charge-discharge cycles at 20 A/g and 30 A/g (Figure 8). For both current densities, the capacitance is maintained above 90 % after 10000 charge-discharge cycles. The charge capacitance is gradually reduced, while the discharge curve is almost unaltered as shown in the inset of Figure 8, indicating that an irreversible redox reaction occurred during the initial charging process. Hence, as the 22
ACCEPTED MANUSCRIPT cycle repeats, the coulombic efficiency increases until the end of the irreversible redox reaction (Table S1). The high discharge performance retention is closely related to the electrical double layer behavior of CPC1_700. This indicates that the ions have indirect
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contact with the electrode material due to the ion accumulation outside of the inner Helmholtz plane [62]. Thus, the electrode material can maintain its structure without significant damage during repeated charge-discharge cycles [63]. Therefore, combined with
efficient and stable supercapacitor electrode material.
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simple and sustainable CO2 utilized production, CPC1_700 is a good candidate as a very
CPC1_700 FC-NH4Cl700-1.0 ACS-4.0800
Surface area (m2/g)
C (µF/cm2)
C (F/g)
Scan rate / Current density
Electrolyte
Remarks
Ref.
1262
21.4
270
1.0 A/g
6M KOH
0.45% B
This study
1180
20.5
242
1.0 A/g
6M KOH
10.12% N, 1.54% Fe
[1]
1622
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Sample
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Table 4. Electrochemical performance comparison for different porous carbon electrode materials.
17
162
1.0 A/g
6M KOH
-
[2]
19.1
197
1.0 A/g
1M H2SO4
7.7% N
[3]
20.6
283
1.0 A/g
6M KOH
-
[5]
10.3
300
1.0 A/g
6M KOH
0.52% Co, 0.37% N
[6]
1085
HPC-1.5
1371
PANKCo5
2921
NGPC750℃ / 1h
3124
8.6
270
1.0 A/g
6M KOH
1.95% N
[8]
CMC-2
1994
17.7
353
1.0 A/g
6M KOH
4.2% N, 12.5% S
[9]
BC11-850
1134
13.7
155
1.0 A/g
6M KOH
-
[25]
BC13-850
2771
8.5
236
1.0 A/g
6M KOH
-
[25]
MC-Q
729
39.6
289
20 mA/g
1M H2SO4
9% N
[64]
Y-AN
1680
20.2
340
2 mV/s
1M H2SO4
6% N
[42]
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NCC-2h
23
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7.6
187
0.1 A/g
1M TEABF4
-
[48]
ASC-4 h
2102
7.8
163
1.0 A/g
1M H2SO4
-
[65]
B-AC
2841
11.6
330
1.0 mV/s
2M KOH
-
[66]
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AC-C700
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Figure 8. Cyclic stability of CPC1_700 at current density of 20 A/g and 30 A/g after 10000 cycles (the inset is a comparison of the galvanostatic charge-discharge curves for 1st, 1000th, and 10000th cycles at 20 A/g).
4. Conclusions
Hierarchical porous carbons (CPCs_700) have been synthesized via a one-step
CaCO3 nano-templated CO2 conversion, followed by Ar heat treatment. The consecutive conversion and heat treatment process induced a highly interconnected porous carbon structure with hierarchical micro-, meso- and macropores. The novel porous architectures mainly originated from the templating and CO2 activation associated with CaCO3 24
ACCEPTED MANUSCRIPT nanoparticles. CPC1_700 showed the highest capacitance, 270 F/g (1 A/g), among the synthesized porous carbons due to its optimized pore construction, which leads to effective ionic mass transport and efficient utilization of the surface area. This superior performance
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was additionally confirmed by its excellent normalized capacitance (21.4 µF/cm2). Moreover, CPC1_700 still exhibited a high capacitance of 170 F/g even at a high current density of 20 A/g, and more than 90 % of the capacitance was maintained during 10000 consecutive
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charge-discharge cycles. In combination with its excellent capacitive performance, as well as the simple and CO2 utilizing green synthesis method, the as-prepared CPCs_700 provide
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potential for the practical application of supercapacitor electrode materials.
Acknowledgement
This work was supported by the National Research Foundation of Korea via the
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NNFC-KAIST-Drexel Nano Co-op Center (NRF-2016K1A4A3945039) and the Korea CCS
Planning.
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References
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R&D center (no. NRF-2014M1A8A1049297) funded by Ministry of Science, ICT, Future
[1] J.Y. Zhu, D. Xu, C.C. Wang, W.J. Qian, J. Guo, F. Yan, Ferric citrate-derived N-doped hierarchical porous carbons for oxygen reduction reaction and electrochemical supercapacitors, Carbon 115 (2017) 1-10.
[2] D.D. Zhang, J.H. Zhao, C. Feng, R.J. Zhao, Y.H. Sun, T.T. Guan, B.X. Han, N. Tang, J.L. Wang, K.X. Li, J.L. Qiao, J.J. Zhang, Scalable synthesis of hierarchical macropore-rich activated carbon microspheres assembled by carbon nanoparticles for high rate performance supercapacitors, J Power Sources 342 (2017) 363-370. [3] L. Li, Q.F. Zhong, N.D. Kim, G.D. Ruan, Y. Yang, C.T. Gao, H.L. Fei, Y.L. Li, Y.S. Ji, J.M. Tour, 25
ACCEPTED MANUSCRIPT Nitrogen-doped carbonized cotton for highly flexible supercapacitors, Carbon 105 (2016) 260-267. [4] L.N. Han, X. Wei, Q.C. Zhu, S.M. Xu, K.X. Wang, J.S. Chen, Nitrogen-doped carbon nets with micro/mesoporous structures as electrodes for high-performance supercapacitors, J Mater Chem A 4(42) (2016) 16698-16705. [5] D.Z. Zhu, Y.W. Wang, W.J. Lu, H. Zhang, Z.Y. Song, D. Luo, L.H. Gan, M.X. Liu, D.M. Sun, A novel performance supercapacitor electrodes, Carbon 111 (2017) 667-674.
RI PT
synthesis of hierarchical porous carbons from interpenetrating polymer networks for high [6] G.M. Kim, J.H. Park, J.W. Lee, Formation of Hollow Co3O4 Nanoparticles on Nitrogen-doped Porous Carbons for Highly Capacitive Performance, Chemistryselect 1(3) (2016) 560-566.
[7] M. Rana, K. Subramani, M. Sathish, U.K. Gautam, Soya derived heteroatom doped carbon as a promising platform for oxygen reduction, supercapacitor and CO2 capture, Carbon 114 (2017)
SC
679-689.
[8] K.T. Cho, S.B. Lee, J.W. Lee, Facile Synthesis of Highly Electrocapacitive Nitrogen-Doped
M AN U
Graphitic Porous Carbons, J Phys Chem C 118(18) (2014) 9357-9367.
[9] Y. Boyjoo, Y. Cheng, H. Zhong, H. Tian, J. Pan, V.K. Pareek, S.P. Jiang, J.F. Lamonier, M. Jaroniec, J. Liu, From waste Coca Cola (R) to activated carbons with impressive capabilities for CO2 adsorption and supercapacitors, Carbon 116 (2017) 490-499.
[10] S. Baik, J.W. Lee, Effect of boron-nitrogen bonding on oxygen reduction reaction activity of BN Co-doped activated porous carbons, Rsc Adv 5(31) (2015) 24661-24669. [11] G.P. Wang, L. Zhang, J.J. Zhang, A review of electrode materials for electrochemical
TE D
supercapacitors, Chem Soc Rev 41(2) (2012) 797-828.
[12] L.L. Zhang, Y. Gu, X.S. Zhao, Advanced porous carbon electrodes for electrochemical capacitors, J Mater Chem A 1(33) (2013) 9395-9408. [13] W. Xing, C.C. Huang, S.P. Zhuo, X. Yuan, G.Q. Wang, D. Hulicova-Jurcakova, Z.F. Yan, G.Q. Lu, (2009) 1715-1722.
EP
Hierarchical porous carbons with high performance for supercapacitor electrodes, Carbon 47(7) [14] D.W. Wang, F. Li, M. Liu, G.Q. Lu, H.M. Cheng, 3D aperiodic hierarchical porous graphitic
AC C
carbon material for high-rate electrochemical capacitive energy storage, Angew Chem Int Edit 47(2) (2008) 373-376.
[15] Y. Mun, C. Jo, T. Hyeon, J. Lee, K.-S. Ha, K.-W. Jun, S.-H. Lee, S.-W. Hong, H.I. Lee, S. Yoon, J. Lee, Simple synthesis of hierarchically structured partially graphitized carbon by emulsion/blockcopolymer co-template method for high power supercapacitors, Carbon 64 (2013) 391-402. [16] L. Xie, G. Sun, F. Su, X. Guo, Q. Kong, X. Li, X. Huang, L. Wan, W. song, K. Li, C. Lv, C.-M. Chen, Hierarchical porous carbon microtubes derived from willow catkins for supercapacitor applications, J Mater Chem A 4(5) (2016) 1637-1646. [17] J.S. Zhang, J.W. Lee, Production of boron-doped porous carbon by the reaction of carbon dioxide with sodium borohydride at atmospheric pressure, Carbon 53 (2013) 216-221. 26
ACCEPTED MANUSCRIPT [18] M. Motiei, Y.R. Hacohen, J. Calderon-Moreno, A. Gedanken, Preparing carbon nanotubes and nested fullerenes from supercritical CO2 by a chemical reaction, J Am Chem Soc 123(35) (2001) 8624-8625. [19] L.Z. Wei, Q.W. Chen, X.K. Kong, Lithium Storage Properties of Porous Carbon Formed Through the Reaction of Supercritical Carbon Dioxide with Alkali Metals, J Am Ceram Soc 94(9) (2011) 3078-3083.
RI PT
[20] A. Chakrabarti, J. Lu, J.C. Skrabutenas, T. Xu, Z.L. Xiao, J.A. Maguire, N.S. Hosmane, Conversion of carbon dioxide to few-layer graphene, J Mater Chem 21(26) (2011) 9491-9493.
[21] J.S. Zhang, J.W. Lee, Supercapacitor Electrodes Derived from Carbon Dioxide, Acs Sustain Chem Eng 2(4) (2014) 735-740.
[22] Y. Kim, W. Lee, G.M. Kim, J.W. Lee, Boron-manganese-carbon nanocomposites synthesized
SC
from CO2 for electrode applications in both supercapacitors and fuel cells, Rsc Adv 6(60) (2016) 54889-54897.
M AN U
[23] S. Baik, B.L. Suh, A. Byeon, J. Kim, J.W. Lee, In-situ boron and nitrogen doping in flue gas derived carbon materials for enhanced oxygen reduction reaction, J Co2 Util 20 (2017) 73-80. [24] J. Zhang, J.W. Lee, Progress and prospects in thermolytic dehydrogenation of ammonia borane for mobile applications, Korean Journal of Chemical Engineering 29(4) (2012) 421-431. [25] A. Byeon, W. Lee, G.M. Kim, J.W. Lee, Hierarchically porous heteroatom-doped carbon derived from flue gases for electrochemical energy storage, J Co2 Util 16 (2016) 420-427. [26] B. Xu, L. Peng, G.Q. Wang, G.P. Cao, F. Wu, Easy synthesis of mesoporous carbon using nano-
TE D
CaCO3 as template, Carbon 48(8) (2010) 2377-2380.
[27] G.W. Yang, H.Y. Han, T.T. Li, C.Y. Du, Synthesis of nitrogen-doped porous graphitic carbons using nano-CaCO3 as template graphitization catalyst, and activating agent, Carbon 50(10) (2012) 3753-3765.
[28] S.U. Rege, R.T. Yang, Corrected Horvath-Kawazoe equations for pore-size distribution, Aiche J
EP
46(4) (2000) 734-750.
[29] G. Horvath, K. Kawazoe, Method for the Calculation of Effective Pore-Size Distribution in
AC C
Molecular-Sieve Carbon, J Chem Eng Jpn 16(6) (1983) 470-475. [30] F. Rodriguezreinoso, M. Molinasabio, M.T. Gonzalez, The Use of Steam and Co2 as Activating Agents in the Preparation of Activated Carbons, Carbon 33(1) (1995) 15-23. [31] N. Iwashita, Chapter 2 - X-ray Powder Diffraction A2 - Inagaki, Michio, in: F. Kang (Ed.), Materials Science and Engineering of Carbon, Butterworth-Heinemann2016, pp. 7-25. [32] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Phys Rev B 61(20) (2000) 14095-14107. [33] A.C. Ferrari, J. Robertson, Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond, Philos T R Soc A 362(1824) (2004) 2477-2512. [34] J. McDonald-Wharry, M. Manley-Harris, K. Pickering, Carbonisation of biomass-derived chars 27
ACCEPTED MANUSCRIPT and the thermal reduction of a graphene oxide sample studied using Raman spectroscopy, Carbon 59 (2013) 383-405. [35] A. Byeon, J. Park, S. Baik, Y. Jung, J.W. Lee, Effects of boron oxidation state on electrocatalytic activity of carbons synthesized from CO2, J Mater Chem A 3(11) (2015) 5843-5849. [36] J. Zhang, A. Byeon, J.W. Lee, Boron-doped electrocatalysts derived from carbon dioxide, J Mater Chem A 1(30) (2013) 8665-8671.
RI PT
[37] D. Hulicova-Jurcakova, M. Seredych, G.Q. Lu, T.J. Bandosz, Combined Effect of Nitrogen- and Oxygen-Containing Functional Groups of Microporous Activated Carbon on its Electrochemical Performance in Supercapacitors, Adv Funct Mater 19(3) (2009) 438-447.
[38] T. Kwon, H. Nishihara, H. Itoi, Q.H. Yang, T. Kyotani, Enhancement Mechanism of Nanochannels, Langmuir 25(19) (2009) 11961-11968.
SC
Electrochemical Capacitance in Nitrogen-/Boron-Doped Carbons with Uniform Straight
[39] Z.H. Sheng, H.L. Gao, W.J. Bao, F.B. Wang, X.H. Xia, Synthesis of boron doped graphene for
M AN U
oxygen reduction reaction in fuel cells, J Mater Chem 22(2) (2012) 390-395.
[40] D.W. Wang, F. Li, Z.G. Chen, G.Q. Lu, H.M. Cheng, Synthesis and Electrochemical Property of Boron-Doped Mesoporous Carbon in Supercapacitor, Chem Mater 20(22) (2008) 7195-7200. [41] X.L. Yu, J.M. Lu, C.Z. Zhan, R.T. Lv, Q.H. Liang, Z.H. Huang, W.C. Shen, F.Y. Kang, Synthesis of activated carbon nanospheres with hierarchical porous structure for high volumetric performance supercapacitors, Electrochim Acta 182 (2015) 908-916.
[42] C.O. Ania, V. Khomenko, E. Raymundo-Pinero, J.B. Parra, F. Beguin, The large electrochemical 17(11) (2007) 1828-1836.
TE D
capacitance of microporous doped carbon obtained by using a zeolite template, Adv Funct Mater [43] H.L. Guo, Q.M. Gao, Boron and nitrogen co-doped porous carbon and its enhanced properties as supercapacitor, J Power Sources 186(2) (2009) 551-556. [44] J.P. Paraknowitsch, A. Thomas, Doping carbons beyond nitrogen: an overview of advanced
EP
heteroatom doped carbons with boron, sulphur and phosphorus for energy applications, Energ Environ Sci 6(10) (2013) 2839-2855.
AC C
[45] Y. Gogotsi, P. Simon, True Performance Metrics in Electrochemical Energy Storage, Science 334(6058) (2011) 917-918.
[46] Q. Wang, J. Yan, Z.J. Fan, Carbon materials for high volumetric performance supercapacitors: design, progress, challenges and opportunities, Energ Environ Sci 9(3) (2016) 729-762. [47] S. Murali, N. Quarles, L.L. Zhang, J.R. Potts, Z. Tan, Y. Lu, Y. Zhu, R.S. Ruoff, Volumetric capacitance of compressed activated microwave-expanded graphite oxide (a-MEGO) electrodes, Nano Energy 2(5) (2013) 764-768. [48] L. Wei, M. Sevilla, A.B. Fuertes, R. Mokaya, G. Yushin, Hydrothermal Carbonization of Abundant Renewable Natural Organic Chemicals for High-Performance Supercapacitor Electrodes, Adv Energy Mater 1(3) (2011) 356-361. 28
ACCEPTED MANUSCRIPT [49] L. Qie, W.M. Chen, H.H. Xu, X.Q. Xiong, Y. Jiang, F. Zou, X.L. Hu, Y. Xin, Z.L. Zhang, Y.H. Huang, Synthesis of functionalized 3D hierarchical porous carbon for high-performance supercapacitors, Energ Environ Sci 6(8) (2013) 2497-2504. [50] P. Cheng, S.Y. Gao, P.Y. Zang, X.F. Yang, Y.L. Bai, H. Xu, Z.H. Liu, Z.B. Lei, Hierarchically porous carbon by activation of shiitake mushroom for capacitive energy storage, Carbon 93 (2015) 315324.
RI PT
[51] D. He, J. Niu, M. Dou, J. Ji, Y. Huang, F. Wang, Nitrogen and oxygen co-doped carbon
networks with a mesopore-dominant hierarchical porosity for high energy and power density supercapacitors, Electrochim Acta 238 (2017) 310-318.
[52] C.X. Zhu, D. Hu, Z. Liu, Interconnected three-dimensionally hierarchical heterostructures with electrodes, Electrochim Acta 229 (2017) 155-165.
SC
homogeneously-dispersed V2O5 nanocrystals and carbon for high performance supercapacitor [53] Y.H. Zhao, M.X. Liu, X.X. Deng, L. Miao, P.K. Tripathi, X.M. Ma, D.Z. Zhu, Z.J. Xu, Z.X. Hao, L.H.
M AN U
Gan, Nitrogen-functionalized microporous carbon nanoparticles for high performance supercapacitor electrode, Electrochim Acta 153 (2015) 448-455.
[54] P.L. Taberna, P. Simon, J.F. Fauvarque, Electrochemical characteristics and impedance spectroscopy studies of carbon-carbon supercapacitors, J Electrochem Soc 150(3) (2003) A292A300.
[55] Q. Wang, J. Yan, Y.B. Wang, T. Wei, M.L. Zhang, X.Y. Jing, Z.J. Fan, Three-dimensional flower-like and hierarchical porous carbon materials as high-rate performance electrodes for supercapacitors,
TE D
Carbon 67 (2014) 119-127.
[56] C. Portet, G. Yushin, Y. Gogotsi, Effect of carbon particle size on electrochemical performance of EDLC, J Electrochem Soc 155(7) (2008) A531-A536. [57] X.X. Sun, P. Cheng, H.J. Wang, H. Xu, L.Q. Dang, Z.H. Liu, Z.B. Lei, Activation of graphene aerogel with phosphoric acid for enhanced electrocapacitive performance, Carbon 92 (2015) 1-10.
EP
[58] H.J. Wang, X.X. Sun, Z.H. Liu, Z.B. Lei, Creation of nanopores on graphene planes with MgO template for preparing high-performance supercapacitor electrodes, Nanoscale 6(12) (2014) 6577-
AC C
6584.
[59] Y.X. Xu, Z.Y. Lin, X. Zhong, X.Q. Huang, N.O. Weiss, Y. Huang, X.F. Duan, Holey graphene frameworks for highly efficient capacitive energy storage, Nat Commun 5 (2014). [60] H. Nishihara, T. Kyotani, Templated Nanocarbons for Energy Storage, Advanced Materials 24(33) (2012) 4473-4498.
[61] J. Yan, Q. Wang, T. Wei, Z. Fan, Recent Advances in Design and Fabrication of Electrochemical Supercapacitors with High Energy Densities, Adv Energy Mater 4(4) (2014) n/a-n/a. [62] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem Soc Rev 38(9) (2009) 2520-2531. [63] H. Choi, H. Yoon, Nanostructured Electrode Materials for Electrochemical Capacitor 29
ACCEPTED MANUSCRIPT Applications, Nanomaterials-Basel 5(2) (2015) 906-936. [64] M. Kodama, J. Yamashita, Y. Soneda, H. Hatori, K. Kamegawa, I. Moriguchi, Structure and electrochemical capacitance of nitrogen-enriched mesoporous carbon, Chem Lett 35(6) (2006) 680-681. [65] L. Wei, G. Yushin, Electrical double layer capacitors with activated sucrose-derived carbon electrodes, Carbon 49(14) (2011) 4830-4838.
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[66] C. Peng, X.B. Yan, R.T. Wang, J.W. Lang, Y.J. Ou, Q.J. Xue, Promising activated carbons derived from waste tea-leaves and their application in high performance supercapacitors electrodes,
AC C
EP
TE D
M AN U
SC
Electrochim Acta 87 (2013) 401-408.
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DOI:
10.1016/j.carbon.2017.10.020
Reference:
CARBON 12457
To appear in:
Carbon
Received Date: 18 July 2017 Revised Date:
20 September 2017
Accepted Date: 7 October 2017
Please cite this article as: Y.K. Kim, J.H. Park, J.W. Lee, Facile nano-templated CO2 conversion into highly interconnected hierarchical porous carbon for high-performance supercapacitor electrodes, Carbon (2017), doi: 10.1016/j.carbon.2017.10.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Facile Nano-templated CO2 Conversion into Highly Interconnected Hierarchical Porous Carbon for High-performance
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Supercapacitor Electrodes Yun Kon Kim§, Jae Hyun Park§ and Jae W. Lee*
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Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea Both authors contributed equally to this work
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§:
*Corresponding author. Tel: +82-42-350-3940. E-mail: [email protected]
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Supercapacitor.
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KEYWORDS: CO2 conversion, CaCO3 template, Hierarchical porous carbon, Boron doping,
1
ACCEPTED MANUSCRIPT Abstract Hierarchical porous carbon materials have been derived through CO2 conversion by using NaBH4 as a reducing agent and CaCO3 as a nano-template. The CaCO3-templated
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porous carbons (CPCs) feature an interconnected three-dimensional structure with hierarchical pores favorable for electrochemical energy storage. Notably, CPC1_700 prepared with an identical mass of CaCO3 and NaBH4 at 700 °C shows a very high capacitance of 270
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F/g at 1 A/g and retains its capacitance up to 170 F/g at 20 A/g in 6 M KOH aqueous electrolyte. Moreover, it presents an outstanding normalized capacitance of 21.4 µF/cm2 even
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in the absence of pseudocapacitive behavior, and a fast frequency response with a low relaxation time constant of 0.27 s. Concerning the cycle stability, more than 90% of the initial capacitance is maintained after 10000 consecutive cycles at high current densities (20 A/g and 30 A/g). The major fundamental insights underlying this performance are closely related
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to the interconnected hierarchical pore architecture generated by the concurrent template and CO2 activation effect, which leads to increased surface area, fast ionic transport, and efficient ionic storage. The proposed route of CO2-to-carbon with the template affords a facile,
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efficient, and sustainable strategy to synthesize hierarchical porous carbon for high-
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performance supercapacitors.
2
ACCEPTED MANUSCRIPT 1. Introduction As global warming is an issue of great concern, renewable energy is attracting attention as a potential alternative to fossil fuels [1, 2]. In addition, the emerging market of
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numerous electronic devices such as cell phones, cameras and hybrid electric vehicles has generated strong demand for reliable and sustainable energy sources [3]. To meet the requirements, various types of electrochemical systems including fuel cells, lithium-ion
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batteries, and supercapacitors are considered promising candidates. Among them, the supercapacitors offer many benefits such as high power density, fast charge-discharge rate,
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and stable life cycle [4-6].
Electrode materials for supercapacitors are typically classified into transition metal oxides, conductive polymers, and porous carbon based materials. The former two materials
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provide the advantage of high capacitance but also have drawbacks of low rate capability, short life cycle, and high cost, limiting practical applications for commercialization. On the
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other hand, porous carbon based materials have attracted a great deal of attention due to their long-term and stable life cycle, as well as large surface area, high conductivity, low
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production cost, and superior thermal, mechanical, and chemical stabilities [5, 7-10]. The specific capacitance of carbon based materials is mainly determined by the specific surface area and the porosity of the structure [11]. Micropores contribute to the improved surface area of the porous material, mesopores provide a transport channel through which ions can readily pass [5, 12, 13], and macropores function as a reservoir for the ion storage and shorten the transport distance [14, 15]. Thus, the optimal porous carbon structure is an interconnected network of the variously sized pores to increase the effective ion storage area and lower the ion 3
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Recently, CO2 conversion into porous carbon materials has emerged as a key
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technology since CO2 is an abundant carbon source, and this CO2 utilization can contribute to the mitigation of global warming [17]. Numerous CO2 transformation strategies have been proposed to produce CNTs (at 70 MPa) [18], porous carbon (at 32 MPa) [19], and graphene
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[20] using supercritical CO2 or dry ice. However, these extreme conversion conditions require large energy consumption, and cause the reproduction of CO2. For a more energy
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efficient CO2 conversion process, sodium borohydride (NaBH4) has been used as a CO2 reducing agent to produce boron doped porous carbon at atmospheric pressure and 500 °C [17, 21-24]. Moreover, porous carbon materials were derived from a reaction of synthetic flue gas (85 % N2 and 15 % CO2) with NaBH4, followed by KOH activation [25]. Although these
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studies demonstrated promising technologies for CO2 conversion into porous carbon materials, the suggested approaches still have some disadvantages of limited electro-
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capacitive performance and multi-step preparation.
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In this study, we propose a single step CO2 conversion into hierarchical porous
carbon by using NaBH4 as a reducing agent and CaCO3 as a nano-template. The CaCO3 nanoparticles have been previously investigated for application as a hard template in carbon precursors such as melamine-formaldehyde resin and sucrose [26, 27]. However, the combination of a nano-template with CO2 conversion has never been explored for the production of hierarchically porous carbon materials. It will be shown that without any chemical treatment such as KOH activation [8, 25], the as-prepared hierarchical porous 4
ACCEPTED MANUSCRIPT carbons possess a large surface area (1262 m2/g), a high pore volume (3.35 cm3/g), and an interconnecting pore structure, leading to enhanced electrochemical properties. From various electrochemical analyses, it will be demonstrated that the porous carbons exhibit excellent
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supercapacitive performance in terms of specific capacitance, rate capability, and cycle stability. This study opens up a new way of connecting CO2 utilization and the production of hierarchical porous carbon via a facile one-step templating method for effective
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supercapacitive energy storage.
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2. Experimental 2.1 Materials
Argon (Ar, > 99.9%) and carbon dioxide (CO2, > 99.9%) were purchased from Deokyang Co., Ltd. Sodium borohydride (NaBH4, > 99%), polytetrafluoroethylene (PTFE,
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60 wt% dispersion in water), and potassium hydroxide (KOH, > 90%) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl, 37 wt% in water) was available in Junsei Chemical Co., Ltd. Calcium carbonate nanoparticles (CaCO3, 15 - 40 nm) were acquired from
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SkySpring Nanomaterials. All of the chemicals were used without further purification.
2.2 Synthesis of CaCO3-templated porous carbon (CPC) and non-templated porous carbon (NPC)
In a typical synthesis of CPCs_700 (Figure 1), 4g of NaBH4 was mechanically mixed with CaCO3 at different mass ratios (CaCO3/NaBH4 = 0.5, 1 and 2). The mixture of NaBH4 and CaCO3 was heated up to 500 °C (5 °C/min) under a CO2 flow of 75 ml/min and 5
ACCEPTED MANUSCRIPT maintained at this temperature for 2 h. The temperature was then increased up to 700 °C (5 °C/min), in which CO2 gas was switched to a 50 ml/min Ar flow at 600 °C, and the temperature was held at 700 °C for 2 h. The resultant was transferred into a beaker, and the
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residue including the template was washed several times with hot aqueous solutions of 5 M HCl, deionized (DI) water, and ethanol in sequence. The sample was dried in an oven at 95 °C for 12 h. The as-prepared carbons were denoted as CPCx_700, where x indicates the
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CaCO3/NaBH4 weight ratio (x = 0.5, 1 and 2). In the case of CPC1_600, the sample was heated until 600 °C under only a CO2 flow, and then cooled to room temperature without the
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the addition of a CaCO3 template.
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Ar treatment. NPC_700 was prepared following the same procedure as CPCx_700, except for
Figure 1. Schematic diagram for one-step nano-templated CO2 conversion into porous carbons, CPCs_700.
6
ACCEPTED MANUSCRIPT 2.3 Characterization The morphology of the porous carbon was investigated by a scanning electron microscope (SEM, Hitachi SU8230). Transmission electron microscopy (TEM) images were
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obtained with a JEOL JEM-2100F at 200 kV. The sample for TEM was dispersed in ethanol, followed by dropwise addition of the suspension onto a copper grid. The nitrogen adsorption– desorption isotherms were measured at 77 K using a Micromeritics Triflex after degassing of
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the sample under a high vacuum at 200 °C for 8 h. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) theory by using a relative pressure range of 0.02 – 0.2.
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The correlation coefficients for BET plots were above 0.9999. The micropore and mesopore size distributions were determined by the Horvath-Kawazoe (H-K) and Barrett-JoynerHalenda (BJH) analyses, respectively. The total pore size distributions were obtained from non-local density functional theory (NLDFT). X-ray diffraction (XRD) patterns were
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measured on a RIGAKU Smart lab with Cu Kα (λ = 1.5406 Å) set at 45 kV and 200 mA. Fourier transform infrared (FTIR) spectra were recorded from 400-4000 cm-1 with a Thermo Nicolet iS50 using KBr pellets. RAMAN spectra of the samples were collected on a
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LabRAM HR Evolution Visible_NIR (HORIBA) at 514 nm of an Ar ion laser. X-ray photoelectron spectroscopy (XPS) was performed through a K-alpha with a microfocused
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monochromator X-Ray source to investigate the surface chemical properties. Elemental analyses (EA) were conducted on a Thermo EA 1112 to determine the oxygen content.
2.4 Electrochemical measurement Electrochemical analyses, including cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) measurements, and electrochemical impedance spectroscopy (EIS) were 7
ACCEPTED MANUSCRIPT conducted on the 3-electrode system in 6 M aqueous KOH electrolyte at 25 °C with a BioLogic 6002D potentiometer and EC-Lab software. The platinum and calomel electrodes were used as counter and reference electrodes, respectively. The working electrode was prepared
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by mixing 2.0 mg of active materials and PTFE with a mass ratio of 19:1 in water, which was dispersed on a 1 cm × 1 cm nickel foam and pressed, followed by drying in an oven at 95 °C for 12 h. The CV test was performed at a scan rate of 10 mV/s within a voltage range of -1.0
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to 0.0 V. The GCD analysis was carried out on the same voltage range as CV and varying the current density from 1 to 20 A/g. The Nyquist plots were obtained from EIS measurements,
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with a sinusoidal signal of 10 mV and a frequency range from 100 kHz to 0.05 Hz.
3. Results & Discussion
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3.1 Structural characteristics of the porous carbon
The morphology of the prepared CO2 derived porous carbon was investigated by scanning electron microscopy (SEM). As illustrated in Figure 2a, the non-templated sample
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NPC_700 has a blade-like shape consisting of different sizes of pores, and the pores are surrounded by a thick carbon wall structure. Using CaCO3 nanoparticles as a carbon template,
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a highly porous structure was clearly observed in CPCs_700 (Figure 2b and also refer to Figure S1 in the Supplementary material). In particular, CPC1_700 and CPC2_700 reveal a three-dimensional architecture interconnecting the overall carbon framework. The inset image of CPC1_700 indicates the hierarchical pore structure. Before washing with HCl (Figure 2c), non-washed CPC1_700 shows pores filled with the CaCO3 template and byproducts. Comparing Figures 2b with 2c, it is confirmed that the formation of pores is due to the removal of the fillers by the washing step. 8
ACCEPTED MANUSCRIPT
The internal pore networking was identified with the aid of TEM observation. For the non-templated sample of NPC_700 in Figure 2d, irregularly shaped pores are placed in a
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plate-like carbon structure. On the other hand, when the template was used, circular shaped pores (Figures 2e and 2f) were dominantly created. Comparing the structures of CPC1_600 and CPC1_700, the Ar heat treatment impacts the morphology and the pore interconnection.
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In the case of CPC1_600 prepared without the treatment, the structure appears to be porous, but the degree of interconnection is low. However, CPC1_700 not only shows a highly
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interconnected pore network, but also more distinct templated porosity over the whole carbon framework, because 2 h of thermal treatment sufficiently leads to re-organization of the carbon structure with a more rigid templating effect. These characteristics are also revealed in
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CPC2_700 (Figure S1), demonstrating the combined effect of circular and interconnected
Figure 2. SEM images of (a) NPC_700, (b) CPC1_700 (Inset: high magnification SEM image of CPC1_700) and (c) non-washed CPC1_700. TEM images of (d) NPC_700, (e) CPC1_600 and (f) CPC1_700. 9
ACCEPTED MANUSCRIPT pores coming from both the CaCO3 template and the Ar heat treatment.
The pore characterizations of NPC_700 and CPCs were analyzed with N2 adsorption
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isotherms in Figure 3. All of the isotherms (Figure 3a) have a shape of type IV with a sharp uptake at low relative pressure (P/P° < 0.001) and an obvious hysteresis loop at high pressure (0.4-1.0 P/P°) [5]. Although they show different adsorption amounts, the occurrence of micro
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and mesopore condensation was the same. The textural properties of the porous carbon are listed in Table 1. As the ratio of the nano-template increases, the total pore volume gradually
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increases. The largest total pore volume is 3.35 cm3/g in CPC2_700. As seen in Figure 3b, compared to NPC_700, the CPCs show a very large mesopore volume in a broad range of pore width from 10 to 50 nm, demonstrating the mesopore derivation effect of the nanotemplate. In the case of the surface area, CPC1_700 has the highest BET surface area, 1262
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m2/g, which is due to the abundant micropores. The micropore size distributions (Figure 3c) were calculated through the Horvath-Kawazoe method [28, 29]. NPC_700 exhibits a sharp and narrow peak centered at 0.5 nm, while CPCs_700 represent different types of peaks
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covering a wide pore width. In the case of CPC1_600, these two characteristic peaks were observed at the same time, which implies that some of the micropores in CPCs do not
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originate from those of NPC_700, but are generated by another mechanism. This mechanism is related to the decomposition of CaCO3 into gaseous CO2 at the high temperature [27]. The possibility of this decomposition reaction in the experimental temperature of 700 °C was supported by the TGA results (Figure S3). The generated CO2 reacts with the carbon around the decomposed CaCO3, which leads to forming micropores and the thin-wall structure (refer to the TEM images of Figures 2e and 2f) according to the following reactions [30].
10
ACCEPTED MANUSCRIPT CaCO3 → CO2 + CaO
(1)
CO2 + C → 2CO (CO2 activation)
(2)
In more detail, the measured micropore volume of CPC1_700 is higher than that of
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CPC1_600 by 27 %, verifying that the above micropore formation mechanism mainly occurs through the CO2 activation originating from CaCO3 during the Ar heat treatment at 700 °C. Additionally, from the distribution result of CPC0.5_700, which has a lower micro-pore
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volume than NPC_700 (Figure 3c), it can be deduced that the template also hinders the propagation of the inherent micropores derived by the NaBH4-aided CO2 conversion.
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Consequently, the mass balance ratio between NaBH4 and CaCO3 template is an important factor for the construction of an effective micropore structure, and CPC1_700 provides optimal conditions for micropore generation. The NLDFT pore size distributions of the samples in Figure 3d also show that CPC1_700 has more micro- and mesopores than the
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other samples.
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Table 1. Textural characteristics of NPC_700, CPCs_700 and CPC1_600. BET surface area (m2/g)
Total pore volumea (cm3/g)
Mesopore volumeb (cm3/g)
Micropore volumec (cm3/g)
NPC_700
955
1.62
1.223
0.397
CPC0.5_700
910
2.17
1.789
0.381
CPC1_700
1262
3.21
2.682
0.528
CPC2_700
1218
3.35
2.843
0.507
CPC1_600
944
2.68
2.292
0.388
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Sample
a
Total pore volume up to P/P0 ~ 0.995. b Mesopore volume determined by the BJH method.
c
Micropore volume determined by the H-K method. 11
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ACCEPTED MANUSCRIPT
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Figure 3. (a) N2 sorption isotherms at 77 K (The isotherms for CPC0.5_700, CPC1_700, CPC1_600, and CPC2_700 samples were offset by 300, 600, 900, and 1200 cm3/g, respectively.), and (b) BJH and (c) H-K pore size distributions for NPC_700, CPCs_700, and CPC1_600 (Inset is a cumulative volume for micropores). (d) NLDFT pore size distributions for NPC_700, CPC1_700 and CPC1_600.
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3.2 Chemical properties of the porous carbon The crystalline structure of the porous carbon was analyzed by XRD patterns. As
shown in Figure 4a, all porous carbons have similar XRD patterns with two broad peaks at around 24° and 43°, corresponding to (002) and (10) diffraction profiles of a partially turbostratic structure [31]. There is no crystalline structure, indicating the removal of the crystalline template and by-products during the washing step. In order to investigate the 12
ACCEPTED MANUSCRIPT chemical state of the nano-template during the synthetic step, the non-washed CPC1_700 was immediately sampled out from the furnace, and subjected to an XRD analysis. The sample exhibits complex XRD patterns (Figure 4b), which do not match those of pristine CaCO3
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nanoparticles. To confirm the thermal behavior of CaCO3, the nanoparticles alone were
Figure 4. XRD spectra of (a) NPC_700, CPCs_700, and CPC1_600, (b) non-washed CPC1_700 and pristine CaCO3 nanoparticle, and (c) consecutive CO2 & Ar heat-treated CaCO3 nanoparticle.
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heated under the same conditions, as in the synthesis of CPC1_700, and the XRD result is presented in Figure 4c. The patterns consist mostly of CaCO3 and a small amount of CaO [27], verifying that there is a small amount of decomposed nanoparticles during the heat
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treatment. It thus can be inferred that the complicated peaks for the non-washed CPC1_700 may mainly originate from the reaction between CaCO3 and NaBH4. However, the peaks
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marked with arrows correspond to the intrinsic peaks of CaO, meaning that some of the CaCO3 was still decomposed into CaO and CO2. Considering these findings together with the results for the pore distributions, CaCO3 and generated CO2 contribute to the formation of various sized mesopores and micropores in the carbon framework.
13
ACCEPTED MANUSCRIPT Raman spectroscopy is a non-destructive and effective method to characterize sp2 disordered carbon structures. The Raman spectra of NPC_700 and CPCs are shown in Figure 5. For all the prepared porous carbons, the two distinct peaks indicate the G band at around
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1590 cm-1 derived from the bond stretching of sp2 pairs, and the D band at around 1350 cm-1 from the breathing modes of sp2 atoms in rings, respectively. The measured I(D)/I(G) values,
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the ratio of peak heights, are indicated in Figure 5. The CPC1_600 has the smallest I(D)/I(G)
Figure 5. Raman spectra of NPC_700, CPCs_700, and CPC1_600.
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ratio of 0.72, while the others show almost identical values between 0.87 and 0.91. Previously,
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Ferrari et al. reported ordering trajectory in which the I(D)/I(G) ratio increases from the amorphous carbon (20 % sp3) to nanocrystalline graphite (0 % sp3) [32, 33]. Considering the higher heat treatment temperature of 700 °C under the Ar flow, the higher I(D)/I(G) ratio for NPC_700 and CPCs_700 than CPC1_600 is related to re-organization of the amorphous structures as they are exposed to more severe thermal treatments [34-36]. This confirms that the addition of the CaCO3 template is not related to the ordering or disordering, and that the heat treatment under the Ar atmosphere contributes to the formation of ordered sp2 carbon structure. 14
ACCEPTED MANUSCRIPT
The surface chemical information and the bonding state of elements were investigated using XPS. All the porous carbon materials mostly consist of carbon and oxygen,
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among which NPC_700 and CPC1_600 also contain a small amounts of boron. The O1s spectra for the samples are shown in Figure 6a (also refer to Figure S5). All the porous carbons have similar O1s spectra, and the binding energies around 531 eV, 532 eV, and 536
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eV are assigned to C=O, C-O, and chemisorbed water, respectively [37]. The C1s spectra also support the presence of oxygen functional groups (refer to Figure S6). The measured oxygen
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content and the specific composition of oxygen functionalities are listed in Table 2. Compared to the elemental analysis (EA) results, the oxygen content trends were similar but the quantities were different, confirming that the oxygen functional groups are more abundant near the surface. Furthermore, the higher C=O composition in CPCs than NPC_700 implies
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the participation of CaCO3 in the formation of the oxygen functional groups.
Figure 6. XPS (a) O1s spectra of NPC_700, CPC1_700 and CPC1_600, and (b) B1s spectra of NPC_700 and CPC1_600. 15
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In the case of boron, NPC_700 and CPC1_600 (Figure 6b) show weak, but obvious B1s spectra, while the other CPCs_700 present B1s spectra that are not well-recognized, as
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revealed in Figure S5 and Table 2. It is evident that boron and calcium complexes are mainly formed at 700 °C, and they are washed out by HCl from CPCs_700. For NPC_700 and CPC1_600, B1s spectra can be deconvoluted into three peaks at ca. 187 eV, 189 eV, and 192
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eV, which are assigned to BC3, BC2O and BCO2, respectively [38, 39]. The presence of B-C bonding is additionally confirmed by the deconvolution of C1s spectra (Figure S6). The
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incorporation of boron into the carbon network could contribute to the generation of pseudocapacitance, as reported in previous studies [21, 25, 40].
Table 2. Oxygen and boron content and composition of oxygen species determined by fitting the O1s core level XPS spectra for NPC_700, CPCs_700 and CPC1_600.
Sample EAa
CPC0.5_700
C-O (At %)
H2 O (At %)
Boron contentb (At %)
8.1
21.9
68.5
9.6
2.52
3.1
6.6
30.2
62.9
6.9
0.46
4.8
7.5
32.4
58.8
8.8
0.45
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CPC1_700
6.1
C=O (At %)
XPS
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NPC_700
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Oxygen content (At %)
CPC2_700
4.4
7.7
33.0
56.8
10.2
0.50
CPC1_600
6.5
9.1
37.0
54.3
8.7
2.22
a
Oxygen content obtained by elemental analysis.
b
Boron content obtained by XPS spectra.
16
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3.3 Supercapacitor electrochemical analyses The as-prepared NPC and CPCs were evaluated for the performance as supercapacitor electrode materials. Figure 7a shows the CV graph of the porous carbon at a
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scan rate of 10 mV/s. CPCs_700 exhibit a quasi-rectangular shape of CV graph indicating the electrical double-layer behavior [41]. Furthermore, CPCs_700 do not show pseudocapacitive behavior, because few B atoms are found at the surface of the materials (Figure S5). On the
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other hand, NPC_700 indicates a broad hump (Figure 7a) associated with the
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pseudocapacitive behavior presented in the mid-voltage region due to the incorporation of B atoms into the carbon structure [40, 42-44]. Although XPS B1s spectra show similar amounts of B in NPC_700 and CPC1_600, pseudocapacitive properties are not observed in CPC1_600.
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This will be discussed in more detail on the basis of EIS analyses.
The accurate capacitance was obtained using galvanostatic charge-discharge (GCD) experiments at a current density of 1 A/g, and the results are presented in Figure 7b. The
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calculated capacitance (F/g) and normalized capacitance (µF/cm2) of the synthesized porous carbons are listed in Table 3. Generally, as the specific surface area increases, the capacitance
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tends to rise [11], which can be the reason for CPC1_700 having the highest capacitance. Regarding the surface normalized performance, NPC_700 shows relatively high normalized capacitance of 21.7 µF/cm2 due to the effect of the pseudocapacitance from the boron doped sites. However, CPCs_700 also exhibits comparable normalized capacitance to that of NPC_700, which could be ascribed to the superior ion transport ability by the hierarchical porous structure even at very low boron content, as revealed in the XPS results. In the case of 17
ACCEPTED MANUSCRIPT CPC1_600, the lowest specific and normalized capacitances are revealed among the samples despite the comparable surface area with that of NPC_700 and CPC0.5_700. In terms of ion transport and storage, the lowest capacitance of CPC1_600 may be closely related to the low
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utilization efficiency of the porous structure due to the poor pore interconnection coming from the absence of the Ar heat treatment. Hence, for CPC1_600, a large portion of mesopore volume formed by the template remains isolated, and the mesopores do not function as ion
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reservoirs, effectively. This also can be related to the superior capacitance of CPC1_700 and CPC2_700. Both materials have not only high meso- and macropore volume with the aid of
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the template, but also sufficient micropore interconnection for high utilization of the porous carbon surface. Moreover, the higher capacitance of CPC1_700 than CPC2_700 is ascribed to the optimum mass ratio between CaCO3 and NaBH4, because although the CaCO3 template basically constructs new pores, excessive addition of it can inhibit the pore connection and
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the formation of intrinsic pores derived by CO2 conversion. From a practical point of view, the volumetric capacitance of the synthesized porous carbons was also calculated by using the material density (Table 3). Their densities are measured according to the previously
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reported palletization method [45-47].
Table 3. Three types of capacitance and material density of NPC_700 and CPCs. Normalized capacitance (µF/cm2) 21.7
Volumetric capacitance (F/cm3) 60.4
CPC0.5_700
195
21.4
38.3
0.1966
CPC1_700
270
21.4
50.7
0.1877
CPC2_700
238
19.5
46.3
0.1946
CPC1_600
154
16.3
75.7
0.4918
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NPC_700
Specific capacitance (F/g) 207
Sample
18
Density (g/cm3) 0.2919
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The pore utilization was additionally confirmed by the EIS analysis (Figure 7c), which provides the frequency response of a supercapacitor system including electrode
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materials [48]. The Nyquist plots for the porous carbon can be divided into three parts: the intersection with the real x-axis in the high frequency region, the semicircular portion in the mid-frequency region, and the linear region in the low frequency region, which correspond to
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the internal resistance, the charge-transfer resistance, and the mass transport resistance of the
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ions, respectively [49].
For the internal resistance, all the materials have an x-axis intersection point at around 0.25 Ω, indicating that they have similar conductivity. As can be seen in the inset of Figure 7c, NPC_700 and CPCs_700 showed a similar shape of semi-circle, but CPC1_600
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indicated a large radius of curvature. This means that the ion migration resistance of CPC1_600 is much larger than that of the other materials due to the closed and highly
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tortuous structure of CPC1_600, corresponding to the TEM image (Figure 2e). This resistance behavior ascribed to the closed and tortuous pore structure was also found in
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previous studies [50, 51]. In addition, the slope of CPCs_700 was nearly vertical, confirming that it has rather low mass transport resistance [52] and almost an ideal double layer capacitance, as shown in the quasi-rectangular CV graph (Figure 7a) [51, 53].
In the cases of NPC_700 and CPC1_600, relatively low slopes were observed. This suggests that both carbon electrode materials have slow mass transport rates due to a less 19
ACCEPTED MANUSCRIPT effective porous structure. However, there are main differences between them. NPC_700 has a semi-circle with the same shape as that of CPCs_700 in the mid-frequency region, but the slope gradually decreased in the low frequency regions, suggesting a more distinct
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pseudocapacitive behavior than CPC1_600. On the other hand, CPC1_600 showed a large radius of curvature, followed by a large slope in the low frequency region. This phenomenon explains that the main resistance of CPC1_600 originated not from the pseudocapacitive
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behavior but also from the closed structure. Thus, for CPC1_600, the absence of the pseudocapacitive characteristics from the CV analysis is because the closed structure could
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interfere with the contact between B and the electrolyte.
For a more detailed investigation, Bode plots of the phase angle verses frequency were introduced from the EIS results (Figure 7d). The relaxation time constant is defined as
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the inverse of the frequency at a phase angle of 45 degrees, and it can be used as a quantitative reference of how quickly an electrode can discharge [54, 55]. CPC1_700 exhibits the lowest relaxation time constant of 0.27 s, while the other materials show a relaxation time
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of 0.28 s (CPC2_700), 0.31 s (CPC0.5_700), 0.44 s (NPC_700), and 1.15 s (CPC1_600),
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respectively. Compared to previously reported carbon based materials (0.47 s to 2.45 s) [5659], CPC1_700 has a lower relaxation time constant, signifying a very fast frequency response due to its highly interconnected carbon structure with hierarchical pores and its favorable electrical conductivity.
The specific capacitances obtained from the GCD experiments at different current densities are shown in Figure 7e. The figure clearly displays that CPC1_700 has the highest 20
ACCEPTED MANUSCRIPT specific capacitance over the whole range of current densities. The capacitance of CPC1_700 is 170 F/g at 20 A/g, indicating 63 % retention of the specific capacitance at 1 A/g. The capacitance retentions from the same current range were 33 %, 36 %, 65 %, and 62 % for
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NPC_700, CPC1_600, CPC0.5_700, and CPC2_700, respectively.
Figure 7. (a) Cyclic voltammetry curves, (b) galvanostatic charge-discharge curves, (c) Nyquist plots (the inset is an enlarged view of the Nyquist plots), (d) Bode plots of phase angle verses frequency and (e) specific capacitances in various current densities for NPC_700, CPCs_700, and CPC1_600. 21
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In particular, NPC_700 and CPC1_600 showed a rapid decrease of capacitance with an increase of current density, indicating that as the charge-discharge rate increases, the
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ability to store the charge significantly becomes lower than that of the other materials. NPC_700 has a slow pseudocapacitive behavior due to a slow interaction between B doped sites and ions in the electrolyte. Thus, the pseudocapacitive behavior does not occur
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completely under the high current density conditions. For CPC1_600, as the current density increases, the ionic transport is prominently disturbed by the closed and tortuous structural
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resistance, as confirmed by the EIS analysis.
The high utilization efficiency of the surface area was confirmed by a comparison of the area-normalized capacitance of CPC1_700 with results reported in the literature using
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other porous carbons, as listed in Table 4 [60, 61]. Among the various carbons, CPC1_700 shows superior normalized capacitance of 21.4 µF/cm2, even in the absence of
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pseudocapacitive performance. This suggests that the actual charge storage area can be different from the calculated BET specific surface area depending on the porous structure and
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the interconnection. This clearly supports the efficient surface area utilization of CPC1_700 due to its hierarchical pore distribution and pore networking architecture. Furthermore, CPC1_700 provides very stable properties during consecutive charge-discharge cycles at 20 A/g and 30 A/g (Figure 8). For both current densities, the capacitance is maintained above 90 % after 10000 charge-discharge cycles. The charge capacitance is gradually reduced, while the discharge curve is almost unaltered as shown in the inset of Figure 8, indicating that an irreversible redox reaction occurred during the initial charging process. Hence, as the 22
ACCEPTED MANUSCRIPT cycle repeats, the coulombic efficiency increases until the end of the irreversible redox reaction (Table S1). The high discharge performance retention is closely related to the electrical double layer behavior of CPC1_700. This indicates that the ions have indirect
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contact with the electrode material due to the ion accumulation outside of the inner Helmholtz plane [62]. Thus, the electrode material can maintain its structure without significant damage during repeated charge-discharge cycles [63]. Therefore, combined with
efficient and stable supercapacitor electrode material.
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simple and sustainable CO2 utilized production, CPC1_700 is a good candidate as a very
CPC1_700 FC-NH4Cl700-1.0 ACS-4.0800
Surface area (m2/g)
C (µF/cm2)
C (F/g)
Scan rate / Current density
Electrolyte
Remarks
Ref.
1262
21.4
270
1.0 A/g
6M KOH
0.45% B
This study
1180
20.5
242
1.0 A/g
6M KOH
10.12% N, 1.54% Fe
[1]
1622
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Sample
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Table 4. Electrochemical performance comparison for different porous carbon electrode materials.
17
162
1.0 A/g
6M KOH
-
[2]
19.1
197
1.0 A/g
1M H2SO4
7.7% N
[3]
20.6
283
1.0 A/g
6M KOH
-
[5]
10.3
300
1.0 A/g
6M KOH
0.52% Co, 0.37% N
[6]
1085
HPC-1.5
1371
PANKCo5
2921
NGPC750℃ / 1h
3124
8.6
270
1.0 A/g
6M KOH
1.95% N
[8]
CMC-2
1994
17.7
353
1.0 A/g
6M KOH
4.2% N, 12.5% S
[9]
BC11-850
1134
13.7
155
1.0 A/g
6M KOH
-
[25]
BC13-850
2771
8.5
236
1.0 A/g
6M KOH
-
[25]
MC-Q
729
39.6
289
20 mA/g
1M H2SO4
9% N
[64]
Y-AN
1680
20.2
340
2 mV/s
1M H2SO4
6% N
[42]
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NCC-2h
23
ACCEPTED MANUSCRIPT 2457
7.6
187
0.1 A/g
1M TEABF4
-
[48]
ASC-4 h
2102
7.8
163
1.0 A/g
1M H2SO4
-
[65]
B-AC
2841
11.6
330
1.0 mV/s
2M KOH
-
[66]
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AC-C700
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Figure 8. Cyclic stability of CPC1_700 at current density of 20 A/g and 30 A/g after 10000 cycles (the inset is a comparison of the galvanostatic charge-discharge curves for 1st, 1000th, and 10000th cycles at 20 A/g).
4. Conclusions
Hierarchical porous carbons (CPCs_700) have been synthesized via a one-step
CaCO3 nano-templated CO2 conversion, followed by Ar heat treatment. The consecutive conversion and heat treatment process induced a highly interconnected porous carbon structure with hierarchical micro-, meso- and macropores. The novel porous architectures mainly originated from the templating and CO2 activation associated with CaCO3 24
ACCEPTED MANUSCRIPT nanoparticles. CPC1_700 showed the highest capacitance, 270 F/g (1 A/g), among the synthesized porous carbons due to its optimized pore construction, which leads to effective ionic mass transport and efficient utilization of the surface area. This superior performance
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was additionally confirmed by its excellent normalized capacitance (21.4 µF/cm2). Moreover, CPC1_700 still exhibited a high capacitance of 170 F/g even at a high current density of 20 A/g, and more than 90 % of the capacitance was maintained during 10000 consecutive
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charge-discharge cycles. In combination with its excellent capacitive performance, as well as the simple and CO2 utilizing green synthesis method, the as-prepared CPCs_700 provide
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potential for the practical application of supercapacitor electrode materials.
Acknowledgement
This work was supported by the National Research Foundation of Korea via the
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NNFC-KAIST-Drexel Nano Co-op Center (NRF-2016K1A4A3945039) and the Korea CCS
Planning.
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References
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R&D center (no. NRF-2014M1A8A1049297) funded by Ministry of Science, ICT, Future
[1] J.Y. Zhu, D. Xu, C.C. Wang, W.J. Qian, J. Guo, F. Yan, Ferric citrate-derived N-doped hierarchical porous carbons for oxygen reduction reaction and electrochemical supercapacitors, Carbon 115 (2017) 1-10.
[2] D.D. Zhang, J.H. Zhao, C. Feng, R.J. Zhao, Y.H. Sun, T.T. Guan, B.X. Han, N. Tang, J.L. Wang, K.X. Li, J.L. Qiao, J.J. Zhang, Scalable synthesis of hierarchical macropore-rich activated carbon microspheres assembled by carbon nanoparticles for high rate performance supercapacitors, J Power Sources 342 (2017) 363-370. [3] L. Li, Q.F. Zhong, N.D. Kim, G.D. Ruan, Y. Yang, C.T. Gao, H.L. Fei, Y.L. Li, Y.S. Ji, J.M. Tour, 25
ACCEPTED MANUSCRIPT Nitrogen-doped carbonized cotton for highly flexible supercapacitors, Carbon 105 (2016) 260-267. [4] L.N. Han, X. Wei, Q.C. Zhu, S.M. Xu, K.X. Wang, J.S. Chen, Nitrogen-doped carbon nets with micro/mesoporous structures as electrodes for high-performance supercapacitors, J Mater Chem A 4(42) (2016) 16698-16705. [5] D.Z. Zhu, Y.W. Wang, W.J. Lu, H. Zhang, Z.Y. Song, D. Luo, L.H. Gan, M.X. Liu, D.M. Sun, A novel performance supercapacitor electrodes, Carbon 111 (2017) 667-674.
RI PT
synthesis of hierarchical porous carbons from interpenetrating polymer networks for high [6] G.M. Kim, J.H. Park, J.W. Lee, Formation of Hollow Co3O4 Nanoparticles on Nitrogen-doped Porous Carbons for Highly Capacitive Performance, Chemistryselect 1(3) (2016) 560-566.
[7] M. Rana, K. Subramani, M. Sathish, U.K. Gautam, Soya derived heteroatom doped carbon as a promising platform for oxygen reduction, supercapacitor and CO2 capture, Carbon 114 (2017)
SC
679-689.
[8] K.T. Cho, S.B. Lee, J.W. Lee, Facile Synthesis of Highly Electrocapacitive Nitrogen-Doped
M AN U
Graphitic Porous Carbons, J Phys Chem C 118(18) (2014) 9357-9367.
[9] Y. Boyjoo, Y. Cheng, H. Zhong, H. Tian, J. Pan, V.K. Pareek, S.P. Jiang, J.F. Lamonier, M. Jaroniec, J. Liu, From waste Coca Cola (R) to activated carbons with impressive capabilities for CO2 adsorption and supercapacitors, Carbon 116 (2017) 490-499.
[10] S. Baik, J.W. Lee, Effect of boron-nitrogen bonding on oxygen reduction reaction activity of BN Co-doped activated porous carbons, Rsc Adv 5(31) (2015) 24661-24669. [11] G.P. Wang, L. Zhang, J.J. Zhang, A review of electrode materials for electrochemical
TE D
supercapacitors, Chem Soc Rev 41(2) (2012) 797-828.
[12] L.L. Zhang, Y. Gu, X.S. Zhao, Advanced porous carbon electrodes for electrochemical capacitors, J Mater Chem A 1(33) (2013) 9395-9408. [13] W. Xing, C.C. Huang, S.P. Zhuo, X. Yuan, G.Q. Wang, D. Hulicova-Jurcakova, Z.F. Yan, G.Q. Lu, (2009) 1715-1722.
EP
Hierarchical porous carbons with high performance for supercapacitor electrodes, Carbon 47(7) [14] D.W. Wang, F. Li, M. Liu, G.Q. Lu, H.M. Cheng, 3D aperiodic hierarchical porous graphitic
AC C
carbon material for high-rate electrochemical capacitive energy storage, Angew Chem Int Edit 47(2) (2008) 373-376.
[15] Y. Mun, C. Jo, T. Hyeon, J. Lee, K.-S. Ha, K.-W. Jun, S.-H. Lee, S.-W. Hong, H.I. Lee, S. Yoon, J. Lee, Simple synthesis of hierarchically structured partially graphitized carbon by emulsion/blockcopolymer co-template method for high power supercapacitors, Carbon 64 (2013) 391-402. [16] L. Xie, G. Sun, F. Su, X. Guo, Q. Kong, X. Li, X. Huang, L. Wan, W. song, K. Li, C. Lv, C.-M. Chen, Hierarchical porous carbon microtubes derived from willow catkins for supercapacitor applications, J Mater Chem A 4(5) (2016) 1637-1646. [17] J.S. Zhang, J.W. Lee, Production of boron-doped porous carbon by the reaction of carbon dioxide with sodium borohydride at atmospheric pressure, Carbon 53 (2013) 216-221. 26
ACCEPTED MANUSCRIPT [18] M. Motiei, Y.R. Hacohen, J. Calderon-Moreno, A. Gedanken, Preparing carbon nanotubes and nested fullerenes from supercritical CO2 by a chemical reaction, J Am Chem Soc 123(35) (2001) 8624-8625. [19] L.Z. Wei, Q.W. Chen, X.K. Kong, Lithium Storage Properties of Porous Carbon Formed Through the Reaction of Supercritical Carbon Dioxide with Alkali Metals, J Am Ceram Soc 94(9) (2011) 3078-3083.
RI PT
[20] A. Chakrabarti, J. Lu, J.C. Skrabutenas, T. Xu, Z.L. Xiao, J.A. Maguire, N.S. Hosmane, Conversion of carbon dioxide to few-layer graphene, J Mater Chem 21(26) (2011) 9491-9493.
[21] J.S. Zhang, J.W. Lee, Supercapacitor Electrodes Derived from Carbon Dioxide, Acs Sustain Chem Eng 2(4) (2014) 735-740.
[22] Y. Kim, W. Lee, G.M. Kim, J.W. Lee, Boron-manganese-carbon nanocomposites synthesized
SC
from CO2 for electrode applications in both supercapacitors and fuel cells, Rsc Adv 6(60) (2016) 54889-54897.
M AN U
[23] S. Baik, B.L. Suh, A. Byeon, J. Kim, J.W. Lee, In-situ boron and nitrogen doping in flue gas derived carbon materials for enhanced oxygen reduction reaction, J Co2 Util 20 (2017) 73-80. [24] J. Zhang, J.W. Lee, Progress and prospects in thermolytic dehydrogenation of ammonia borane for mobile applications, Korean Journal of Chemical Engineering 29(4) (2012) 421-431. [25] A. Byeon, W. Lee, G.M. Kim, J.W. Lee, Hierarchically porous heteroatom-doped carbon derived from flue gases for electrochemical energy storage, J Co2 Util 16 (2016) 420-427. [26] B. Xu, L. Peng, G.Q. Wang, G.P. Cao, F. Wu, Easy synthesis of mesoporous carbon using nano-
TE D
CaCO3 as template, Carbon 48(8) (2010) 2377-2380.
[27] G.W. Yang, H.Y. Han, T.T. Li, C.Y. Du, Synthesis of nitrogen-doped porous graphitic carbons using nano-CaCO3 as template graphitization catalyst, and activating agent, Carbon 50(10) (2012) 3753-3765.
[28] S.U. Rege, R.T. Yang, Corrected Horvath-Kawazoe equations for pore-size distribution, Aiche J
EP
46(4) (2000) 734-750.
[29] G. Horvath, K. Kawazoe, Method for the Calculation of Effective Pore-Size Distribution in
AC C
Molecular-Sieve Carbon, J Chem Eng Jpn 16(6) (1983) 470-475. [30] F. Rodriguezreinoso, M. Molinasabio, M.T. Gonzalez, The Use of Steam and Co2 as Activating Agents in the Preparation of Activated Carbons, Carbon 33(1) (1995) 15-23. [31] N. Iwashita, Chapter 2 - X-ray Powder Diffraction A2 - Inagaki, Michio, in: F. Kang (Ed.), Materials Science and Engineering of Carbon, Butterworth-Heinemann2016, pp. 7-25. [32] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Phys Rev B 61(20) (2000) 14095-14107. [33] A.C. Ferrari, J. Robertson, Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond, Philos T R Soc A 362(1824) (2004) 2477-2512. [34] J. McDonald-Wharry, M. Manley-Harris, K. Pickering, Carbonisation of biomass-derived chars 27
ACCEPTED MANUSCRIPT and the thermal reduction of a graphene oxide sample studied using Raman spectroscopy, Carbon 59 (2013) 383-405. [35] A. Byeon, J. Park, S. Baik, Y. Jung, J.W. Lee, Effects of boron oxidation state on electrocatalytic activity of carbons synthesized from CO2, J Mater Chem A 3(11) (2015) 5843-5849. [36] J. Zhang, A. Byeon, J.W. Lee, Boron-doped electrocatalysts derived from carbon dioxide, J Mater Chem A 1(30) (2013) 8665-8671.
RI PT
[37] D. Hulicova-Jurcakova, M. Seredych, G.Q. Lu, T.J. Bandosz, Combined Effect of Nitrogen- and Oxygen-Containing Functional Groups of Microporous Activated Carbon on its Electrochemical Performance in Supercapacitors, Adv Funct Mater 19(3) (2009) 438-447.
[38] T. Kwon, H. Nishihara, H. Itoi, Q.H. Yang, T. Kyotani, Enhancement Mechanism of Nanochannels, Langmuir 25(19) (2009) 11961-11968.
SC
Electrochemical Capacitance in Nitrogen-/Boron-Doped Carbons with Uniform Straight
[39] Z.H. Sheng, H.L. Gao, W.J. Bao, F.B. Wang, X.H. Xia, Synthesis of boron doped graphene for
M AN U
oxygen reduction reaction in fuel cells, J Mater Chem 22(2) (2012) 390-395.
[40] D.W. Wang, F. Li, Z.G. Chen, G.Q. Lu, H.M. Cheng, Synthesis and Electrochemical Property of Boron-Doped Mesoporous Carbon in Supercapacitor, Chem Mater 20(22) (2008) 7195-7200. [41] X.L. Yu, J.M. Lu, C.Z. Zhan, R.T. Lv, Q.H. Liang, Z.H. Huang, W.C. Shen, F.Y. Kang, Synthesis of activated carbon nanospheres with hierarchical porous structure for high volumetric performance supercapacitors, Electrochim Acta 182 (2015) 908-916.
[42] C.O. Ania, V. Khomenko, E. Raymundo-Pinero, J.B. Parra, F. Beguin, The large electrochemical 17(11) (2007) 1828-1836.
TE D
capacitance of microporous doped carbon obtained by using a zeolite template, Adv Funct Mater [43] H.L. Guo, Q.M. Gao, Boron and nitrogen co-doped porous carbon and its enhanced properties as supercapacitor, J Power Sources 186(2) (2009) 551-556. [44] J.P. Paraknowitsch, A. Thomas, Doping carbons beyond nitrogen: an overview of advanced
EP
heteroatom doped carbons with boron, sulphur and phosphorus for energy applications, Energ Environ Sci 6(10) (2013) 2839-2855.
AC C
[45] Y. Gogotsi, P. Simon, True Performance Metrics in Electrochemical Energy Storage, Science 334(6058) (2011) 917-918.
[46] Q. Wang, J. Yan, Z.J. Fan, Carbon materials for high volumetric performance supercapacitors: design, progress, challenges and opportunities, Energ Environ Sci 9(3) (2016) 729-762. [47] S. Murali, N. Quarles, L.L. Zhang, J.R. Potts, Z. Tan, Y. Lu, Y. Zhu, R.S. Ruoff, Volumetric capacitance of compressed activated microwave-expanded graphite oxide (a-MEGO) electrodes, Nano Energy 2(5) (2013) 764-768. [48] L. Wei, M. Sevilla, A.B. Fuertes, R. Mokaya, G. Yushin, Hydrothermal Carbonization of Abundant Renewable Natural Organic Chemicals for High-Performance Supercapacitor Electrodes, Adv Energy Mater 1(3) (2011) 356-361. 28
ACCEPTED MANUSCRIPT [49] L. Qie, W.M. Chen, H.H. Xu, X.Q. Xiong, Y. Jiang, F. Zou, X.L. Hu, Y. Xin, Z.L. Zhang, Y.H. Huang, Synthesis of functionalized 3D hierarchical porous carbon for high-performance supercapacitors, Energ Environ Sci 6(8) (2013) 2497-2504. [50] P. Cheng, S.Y. Gao, P.Y. Zang, X.F. Yang, Y.L. Bai, H. Xu, Z.H. Liu, Z.B. Lei, Hierarchically porous carbon by activation of shiitake mushroom for capacitive energy storage, Carbon 93 (2015) 315324.
RI PT
[51] D. He, J. Niu, M. Dou, J. Ji, Y. Huang, F. Wang, Nitrogen and oxygen co-doped carbon
networks with a mesopore-dominant hierarchical porosity for high energy and power density supercapacitors, Electrochim Acta 238 (2017) 310-318.
[52] C.X. Zhu, D. Hu, Z. Liu, Interconnected three-dimensionally hierarchical heterostructures with electrodes, Electrochim Acta 229 (2017) 155-165.
SC
homogeneously-dispersed V2O5 nanocrystals and carbon for high performance supercapacitor [53] Y.H. Zhao, M.X. Liu, X.X. Deng, L. Miao, P.K. Tripathi, X.M. Ma, D.Z. Zhu, Z.J. Xu, Z.X. Hao, L.H.
M AN U
Gan, Nitrogen-functionalized microporous carbon nanoparticles for high performance supercapacitor electrode, Electrochim Acta 153 (2015) 448-455.
[54] P.L. Taberna, P. Simon, J.F. Fauvarque, Electrochemical characteristics and impedance spectroscopy studies of carbon-carbon supercapacitors, J Electrochem Soc 150(3) (2003) A292A300.
[55] Q. Wang, J. Yan, Y.B. Wang, T. Wei, M.L. Zhang, X.Y. Jing, Z.J. Fan, Three-dimensional flower-like and hierarchical porous carbon materials as high-rate performance electrodes for supercapacitors,
TE D
Carbon 67 (2014) 119-127.
[56] C. Portet, G. Yushin, Y. Gogotsi, Effect of carbon particle size on electrochemical performance of EDLC, J Electrochem Soc 155(7) (2008) A531-A536. [57] X.X. Sun, P. Cheng, H.J. Wang, H. Xu, L.Q. Dang, Z.H. Liu, Z.B. Lei, Activation of graphene aerogel with phosphoric acid for enhanced electrocapacitive performance, Carbon 92 (2015) 1-10.
EP
[58] H.J. Wang, X.X. Sun, Z.H. Liu, Z.B. Lei, Creation of nanopores on graphene planes with MgO template for preparing high-performance supercapacitor electrodes, Nanoscale 6(12) (2014) 6577-
AC C
6584.
[59] Y.X. Xu, Z.Y. Lin, X. Zhong, X.Q. Huang, N.O. Weiss, Y. Huang, X.F. Duan, Holey graphene frameworks for highly efficient capacitive energy storage, Nat Commun 5 (2014). [60] H. Nishihara, T. Kyotani, Templated Nanocarbons for Energy Storage, Advanced Materials 24(33) (2012) 4473-4498.
[61] J. Yan, Q. Wang, T. Wei, Z. Fan, Recent Advances in Design and Fabrication of Electrochemical Supercapacitors with High Energy Densities, Adv Energy Mater 4(4) (2014) n/a-n/a. [62] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem Soc Rev 38(9) (2009) 2520-2531. [63] H. Choi, H. Yoon, Nanostructured Electrode Materials for Electrochemical Capacitor 29
ACCEPTED MANUSCRIPT Applications, Nanomaterials-Basel 5(2) (2015) 906-936. [64] M. Kodama, J. Yamashita, Y. Soneda, H. Hatori, K. Kamegawa, I. Moriguchi, Structure and electrochemical capacitance of nitrogen-enriched mesoporous carbon, Chem Lett 35(6) (2006) 680-681. [65] L. Wei, G. Yushin, Electrical double layer capacitors with activated sucrose-derived carbon electrodes, Carbon 49(14) (2011) 4830-4838.
RI PT
[66] C. Peng, X.B. Yan, R.T. Wang, J.W. Lang, Y.J. Ou, Q.J. Xue, Promising activated carbons derived from waste tea-leaves and their application in high performance supercapacitors electrodes,
AC C
EP
TE D
M AN U
SC
Electrochim Acta 87 (2013) 401-408.
30