Gelatin-Based Microporous Carbon Nanosheets as High Performance

Research Article pubs.acs.org/journal/ascecg

Gelatin-Based Microporous Carbon Nanosheets as High Performance Supercapacitor Electrodes Huailin Fan†,‡ and Wenzhong Shen*,† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P.R. China University of Chinese Academy of Sciences, Beijing 100049, P.R. China

‡

S Supporting Information *

ABSTRACT: Microporous carbons nanosheets with controllable thicknesses were synthesized using gelatin biomass as both carbon and nitrogen precursors and graphene oxide (GO) without any auxiliary reagent. The pore structures, surface chemical compositions, and morphology of the microporous carbon material were analyzed and characterized by nitrogen adsorption isotherms, thermogravimetric analysis, Fourier transform infrared spectrum, X-ray photoelectron energy spectrum, and transmission electron microscope and scanning electron microscope images. The carbon nanosheets showed average thicknesses from 10 ± 4 to 30 ± 8 nm with tuning mass ratios of gelatin to GO from 20/1 to 100/1. The microporous carbon nanosheets exhibited high specific capacitance and excellent rate capability with a capacitance retention of 76% at 20 A/g in a 6 mol/L KOH aqueous electrolyte because of the shorter diffusion distance, large surface area, and excellent electrical conductivity. KEYWORDS: Porous carbon nanosheets, Gelatin, Supercapacitor, Nanostructure

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INTRODUCTION Recently, considerable efforts have been made to address the preparation of porous carbon materials due to their low cost, physicochemical and thermodynamic stability, high surface area, abundant pore framework, etc.1,2 Porous carbon materials have been widely utilized as absorbents, catalyst supports, and energy storage materials.3−6 Their application properties highly depend on their morphology, pore structure, chemical composition, and surface properties.7 Pore structure could be designed and developed by many methods, which are physical8 or chemical activation,9 polymer blend carbonization,10 and soft template11 and hard template12 associated with carbonization or activation. Chemical activation is an effective and low-cost way to develop pore structure, which is operated at lower temperatures and in shorter activation times. KOH,13 NaOH,14 H3PO4,15 and ZnCl216 are generally selected as activation agents for chemical activation, and porous carbon with narrower micropore distribution can be prepared by KOH activation. The chemical composition and surface chemical groups of porous carbon take on important roles on its application. However, there is less functional groups on the porous carbon internal and external hydrophobic surfaces due to carbonization or activation at high temperatures. The hydrophilic nature of porous carbon could be reinforced by oxygen or nitrogen heteroatoms incorporated into a carbon matrix.17,18 Nitrogencontaining groups generally provide basic properties, which can © XXXX American Chemical Society

enhance the interactions between the carbon surface and acid molecules by dipole−dipole, hydrogen bonding, and covalent bonding, and nitrogen-containing porous carbon materials have been widely prepared and investigated.19 Nitrogen-rich precursors, such as acrylonitrile,20 melamine,21 cyanamid,22 and quinoline pitch23 are generally selected to prepare nitrogen-containing porous carbon. However, the above carbon precursors are expensive, and the exhausts are noxious to the environment. Nitrogen-rich biomass precursors are gradually being used for carbon and nitrogen sources because it is renewable and produces less pollution in the preparation process.25,26 Carbon materials having various morphologies, such as carbon cubes,27 carbon microtrees,28 carbon cones,29 and flower-like carbon30 were reported to optimize their application in energy storage capacities, adsorption, and catalytic reaction fields. Because of their ultrahigh surface-to-volume ratio, carbon nanosheets (CNSs) with nanoscale thicknesses have been obtained and utilized as biosensors,31 lithium-ion batteries,32 fuel cells,33 hydrogen-storage materials,34 catalyst supports,35 and ultracapacitor electrodes.24 For example, leaf-like carbon nanosheets ranging from hundreds of nanometers to several micrometers in length have been prepared by mixing Received: October 22, 2015 Revised: December 2, 2015

A

DOI: 10.1021/acssuschemeng.5b01354 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

12 h. After cool naturally, the products were washed three times with deionized water, freeze-dried overnight in vacuum, and activated at 800 °C for 1 h with KOH at 1:1 weight ratios. The final samples were obtained after washing with diluted hydrochloric acid and deionized water until neutral pH, and they were dried at 80 °C overnight. These resultants were denoted as H-G/G-x and A-G/G-x; H and A represent the hydrothermal treatment and activation processes; x is the mass ratio of gelatin to GO. For comparison, gelatin was activated with KOH (weight ratio of 1:1) at 800 °C, and the final sample was marked as A-G. Structure Characterization. Nitrogen adsorption−desorption isotherms of the samples were measured at −196 °C by a Micromeritics ASAP 2020 adsorption apparatus. The adsorption branch isotherms were adopted to calculate the surface areas and pore size distributions of the samples using the Brunauer−Emmett−Teller (BET) method (P/P0 of 0.01 to 0.2) and nonlocal density functional theory (NLDFT) method (slit pore model). The total pore volumes (Vtotal) were counted based on the adsorption amount at P/P0 of 0.99. Micropore volumes (Vmicro) were calculated using the t-plot method. The SEM images were observed using an S-4800 field emissionscanning electron microscope (Hitachi, Japan) manipulated at 1 kV. Transmission electron microscopy (TEM) was carried out with a JEM2000 FX instrument operating at a 200 kV accelerating voltage. Fourier transform infrared (FT-IR) spectra of samples were obtained on a Nicolet FT-IR 380 spectrometer by the conventional KBr pellet technique. The thermogravimetric behavior of the sample was carried out under Ar flow from 30 to 800 °C with a 10 °C/min heating rate by a thermogravimetric analyzer (Rigaku, TG-DTA 8120, Japan). The Xray photoelectron spectra (XPS) were recorded with an ESCALAB 250 (Thermo Electron); the X-ray excitation was provided by a monochromatic Al Kα (1486.6 eV) source. Survey scans were received using a 100 eV pass energy, while high resolution scans of specific elements were obtained using a 20 eV pass energy. The emitted photoelectrons were detected by performing perpendicular to the surface sample. Data quantification was achieved on the advantage program. X-ray diffraction (XRD) patterns were recorded on a Rigaku MiniFlexII X-ray diffractometer with Cu Kα radiation at a scanning speed of 4°/min. Rounded carbon flakes (≈ diameter of 12 mm, thickness of 0.2 mm) were obtained by pressing the carbon powders applying a pressure of 5 MPa. Electrical conductivity was measured on a carbon flake by a 4-point probe resistivity measurement instrument (Suzhou Tongchuang, SZT-2A). Electrochemical Measurements. A 6 mol/L KOH aqueous solution was used as the electrolyte. The working electrode was prepared by mixing as-prepared carbon materials, PTFE, and carbon black (mass ratio of 85:5:10) in 5 mL of ethanol, followed by ultrasonication for 20 min. A slurry of the mixture was rolled into a film, cutting into a plate followed by placing it on a nickel foam current collector drying at 120 °C for 6 h (mass loading of electrode: ≈5 mg cm−2). The capacitive performance was tested on an electrochemical workstation (CH Instruments, Inc., Shanghai, China, CHI 660C). Cyclic voltammetry (CV) and galvanostatic charge−discharge cycling (GCD) measurements were carried out under ambient conditions with a conventional three-electrode electrochemical setup, in which the asprepared carbon materials served as the working electrode, and a platinum wire and Hg/HgO were used as the counter electrode and reference electrode, respectively. The potential window was chosen in the range from −0.9 to 0 V. Specific capacitance (C, F/g) of the single electrode was calculated from the discharge curve according to following equation.

abandoned plastics containing polypropylene, polyethylene, and polystyrene; they exhibited fast adsorption and unprecedented adsorption capacity of 769.2 mg/g for the removal of methylene blue from wastewater.36 The carbon nanosheet structures often overlap due to the high surface energy and interlayer van der Waals attractions; thus, aggregation or restacking are unavoidable, which significantly reduces and degrades the particular properties of the individual nanosheet. Gelatin with 14 wt % nitrogen content is a typical biomass, and it is usually selected as the carbon precursor to synthesize porous carbon using MgO,37 CaCO3,38 colloidal silica,39 SBA15,40 and Mg−Zn41 as the template or NaOH as activation agents;42 the resultant porous carbons exhibited outstanding cycling and rate performances as energy storage materials. Recently, as a class of excellent energy storage devices, supercapacitors have attracted intense interest due to their super power capacity, long cycle life, and cleanness. It is well known that abundant micropores could enhance electric double layer capacitance, but the electrolyte ions are difficult to penetrate into micropores. Low rate capacity is usually produced, so it is important to improve ion transferring in porous carbon especially at high current density.43−45 Creating a hierarchical pore structure in a carbon matrix has been worked on to improve the ion transferring rate in micropore carbon materials.40 Compared with hierarchical porous carbon, two-dimensional carbon nanosheets offer shorter diffusion paths for contacting with the electrolyte and facile strain relaxation because they enable large surface-to-volume ratios, continuous conducting pathways, and show excellent application properties in biochemistry, energy storage/conversion, and chemical reactions.7 To the best of our knowledge, there have been no reports on the use of graphene oxide (GO) without an auxiliary reagent to obtain gelatin-derived carbon nanosheets. In this work, nitrogen-containing microporous carbon nanosheets with controllable thicknesses were synthesized by the self-assembly of GO and gelatin without any auxiliary reagent under hydrothermal carbonization and activation at high temperature, and the detailed roles of GO to biomass gelatin carbon were investigated. The structure and chemical composition of the microporous carbon nanosheet were characterized, and its outstanding electronic chemistry property was investigated as a supercapacitor electrode. The effect of GO on structuredirecting and enhancing the electrical conductivity of the micropore carbon nanosheet were analyzed. Outstanding electrical conductivity and shorter diffusion paths for the electrolyte ion endowed the gelatin-based three-dimensional microporous carbon nanosheets with excellent supercapacitor performance at high current density and will potentially lay the foundation for practical supercapacitor applications and other applications that demand rapid electrons and/or mass transport.

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EXPERIMENTAL SECTION

Synthesis of Porous Carbon Nanosheets. GO was prepared by a modified Hummers’ method using natural flake graphite as the starting material.46 The solid GO (2.5 g) was dispersed in water (500 mL) to prepare a GO aqueous dispersion (5 mg/mL) solution. The detailed procedure of carbon nanosheet synthesis is as follows. First, 2, 6, and 10 g of gelatin and 20 mL of GO aqueous (0.1 g of GO solid contents) were dispersed in 50 mL of water under magnetic stirring at 70 °C for 1 h to obtain a homogeneous mixture. The weight ratios of gelatin to GO were 20:1, 60:1, and 100:1, respectively. Then, the mixture was transferred to a Teflon autoclave and treated at 180 °C for

C=

i × Δt m × ΔV

(1)

where i is the constant discharging current (A), Δt is the discharge time (s), ΔV is the voltage window (here, ΔV = 0.9 V), and m is the mass of the as-prepared carbon material (g). In a two-electrode system, two electrodes with the same weight and size were used as the test electrode and counter electrode and were separated by a polypropylene membrane. CV and GCD behaviors were also measured in a 6 mol/L KOH electrolyte. The electroB


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ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of 3D Porous Carbon Nanosheets

Figure 1. SEM images of A-G/G-20 (a, b), A-G/G-60 (d, e), and A-G/G-100 (g, h). TEM images of A-G/G-20 (c), A-G/G-60 (f), and A-G/G-100 (i). chemical impedance spectrum (EIS) was recorded in the range from 0.01 Hz to 0.1 MHz with a signal amplitude of 5 mV at the open circuit potential. Gravimetric specific capacitances of the cells Ccell were calculated at different current densities according to GCD profiles based on the following equations: Ccell =

i × Δt mtotal × ΔV

discharge time. GCD is performed by 5000 cycles at the current density of 2 A/g.

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RESULTS AND DISCUSSION The typical synthetic route for preparing gelatin/GO-derived porous carbons nanosheets is illustrated in Scheme 1. A homogeneous and stable colloid was obtained by mixing a certain amount of gelatin and GO−water dispersion at 70 °C for 1 h, which was supported by the noncovalent interaction, e.g., hydrogen bond from the nitrogen and oxygen species of gelatin with GO and van der Waals force. Meanwhile, an in situ reaction was carried out between an amino group of gelatin and a carboxylic acid group on GO, and the interactions between the carbon source and GO were reinforced by hydrothermal precarbonization. Cylindrical carbon materials were obtained by a hydrothermal method (Figure S1a), which were similar to the hydrogel by using GO.47 So gelatin bonded with the surface of GO almost were not impacting the hydrothermal reduction self-assembly of GO. For further confirmation of the function of GO, hydrothermal gelatin without GO was attempted to prepare, but only dark brown liquid was obtained (Figure S1b) without solid prepolymer carbon. The unexpected result showed GO not only as the framework supporting porous

(2)

where i is the constant discharging current (A), Δt is the discharge time (s), ΔV is the voltage window, and mtotal is the total mass of active materials in the two-electrode cell. The specific capacitance of single electrode could be obtained by multiplying the Ccell by four. Energy density and power density of the cells were calculated based on the following equations: Ecell =

Ccell × ΔV 2 × 1000 2 × 3600

(3)

Pcell =

Ecell × 3600 Δt

(4)

where Ecell and Pcell are the energy density and power density of two electrode cells, respectively, ΔV is the voltage window from the end of internal resistance (iR) drop to the end of discharge, and Δt is the C


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Figure 2. (a) N2 adsorption isotherms. (b) NDFT pore size distributions of A-G, A-G/G-x.

For all A-G/G-x samples, the adsorption−desorption isotherms exhibited typical characteristics of type I adsorption isotherms, indicating microporous carbons characteristics. The knee of the isotherm of A-G/GO-x was wider than that of A-G, which suggested a larger pore and wider pore size distribution existed in A-G/GO-x and was in accord with pore size distribution of A-G/G-x and A-G. The samples pore size distribution concentrated in the range of 0.7−2.5 nm. The specific surface areas and pore volumes of A-G/GO-20, A-G/GO-60, A-G/ GO-100, and A-G were 1620, 2252, 1853, and 1686 m2/g and 0.841, 1.116, 0.942, and 0.808 cm3/g, respectively. The contents of their micropore surface areas and volumes were 86%, 92%, 92%, and 95% and 72%, 80%, 79%, and 86%, respectively, and this suggested that there was the least micropore in A-G/G-20. This indicated that gelatin/GO tend to be activated easily, forming larger pore at the same condition due to its shorter path for KOH diffusion. According to the SEM image, A-G/G-20 should have the least diffusion resistance in all of the prepared samples. The thermal gravimetric curves and differential curves of GO and H-G/G-x were investigated to understand their thermal behaviors and are presented in Figure S3. For GO, the first weight loss at 70−150 °C was related to the loss of adsorbed water. The major mass loss at 150−250 °C can be attributed to the labile oxygen-containing groups, and a slower mass loss was observed higher than 250 °C.49,50 There was marked weight loss taking place from 220 to 400 °C, and this could be ascribed to the pyrolysis of gelatin (decomposing of aliphatic chains and forming condensed cross-linking structure). FTIR spectra of gelatin, GO, H-G/G-20, and A-G/G-20 are shown in Figure 3. The bands appearing around 3250−3580, 2925, 1380, and 1120 cm−1 can be ascribed to the O−H, C−H, C−OH, and C−O−C stretching in all samples, respectively.

carbon but also that it assisted gelatin into carbon materials. The 3D nanosheet networks were maintained at further activation, which prevented porous carbon from agglomeration. Porous carbon from gelatin was grafted layer by layer onto the surface of GO via chemical bond and π−π interactions. Abundant nitrogen and oxygen species in the gelatin carbon source were sufficient for forming self-assembly organization with GO, which avoided the addition of a “bridging group” such as asparagine43 or ethylenediamine.48 The nanostructures of A-G/G-x and A-G were investigated by SEM and TEM (Figure 1). On the whole, A-G/G-x possessed a typical nanosheet structure, and carbon nanosheets of A-G/G-x supported each other forming microcosmic 3D networks and avoiding agglomeration due to high surface energy (red arrow). The microcosmic 3D network structures were similar to hydrothermally reduced graphene oxide.47 The supporting structure ensured that the low mass transfer resistance advantage of the nanosheet structure got maximum play and charge transferred smoothly into porous carbon. The obtained samples were composed of thin carbon nanosheets with average thicknesses from 10 ± 4 to 30 ± 8 nm, as shown in the SEM. The thickness increased with increasing tuning mass ratios of gelatin to GO from 20/1 to 100/1. The thinnest parts of sample A-G/GO-20 (Figure 1a, b) were transparent nanosheets with ripples and wrinkles. With increasing the thickness to 20 ± 5 nm (Figure 1d, e) carbon the nanosheets became nontransparent and still corrugated. A relatively smooth surface was obtained with the ratio of gelatin to GO at 100 (Figure 1g, h). TEM images showed that A-G/GO-20 had a thin nanosheet, A-G/GO-60 displayed accumulational nanosheets, and A-G/GO-100 exhibited compact bulk (Figure 1c, f, i). The microcosmic morphology of the carbon nanosheets could be adjusted by tuning mass ratios of gelatin to GO. Sample A-G showed a particular size about of 0.5−3 μm (Figure S2). The N2 adsorption−desorption isotherms and pore size distributions calculated by the density functional theory method are shown in Figure 2 and summarized in Table 1. Table 1. Structure Parameters of A-G and A-G/G-x

A-G A-G/G-20 A-G/G-60 A-G/G-100

SBET (m2/g)

Smic (m2/g)a

Vtotal (cm3/g)b

Vmic (cm3/g)

Vmic/Vtotal %

1686 1620 2252 1853

1603 1399 2084 1718

0.808 0.841 1.116 0.942

0.695 0.603 0.892 0.742

86 72 80 79

a

Micropore surface area was derived from the t-plot method. bTotal pore volume was obtained at P/P0 = 0.99.

Figure 3. FTIR spectra of gelatin, GO, and obtained H-G/G-20. D


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ACS Sustainable Chemistry & Engineering The bands of 1730 and 1580 cm−1 can be assigned to CO and aromatic ring stretching in GO. After hydrothermal carbonization, the band of CO was moved to 1660 cm−1 due to the formation of an amide group between GO and the gelatin. A new band appeared at 1330 cm−1, and the strengthening of 1410 cm−1 was the C−N stretching, which provided additional evidence of reaction between GO and gelatin during hydrothermal carbonization. The bands of 1660 and 1410 cm−1 were weaker in A-G/G-20. These chemical groups existed in A-G/-G-60 and A-G/G-100 (Figure S4). This indicated that there were oxygen- and nitrogen-containing groups in prepared microporous carbon nanosheets. To confirm the chemical composition of A-G/G-x and A-G, X-ray photoelectron spectroscopy (XPS) and elemental analysis were conducted. As shown in Figure S5, three typical peaks corresponding to the binding energies of C 1s, N 1s, and O 1s were observed in the wide X-ray photoelectron spectroscopy survey scan for A-G/G-x and A-G. XPS microscanning measurements were performed to further gain insight into the pathway of nitrogen doping on the surface of the porous carbon. The high-resolution N 1s spectrum can be deconvoluted into four peaks with the binding energies centered at 397.9 ± 0.1, 400 ± 0.1, 401.5 ± 0.2, and 403 ± 0.2 eV, which were assigned to pyridine-like nitrogen (N-6), pyrrole-like nitrogen (N-5), graphite-like nitrogen (N-Q), and pyridine-N-oxide (N-X), respectively.50 As shown in Figure 4

and Table 2, the pyridine-like nitrogen (N-6) content decreased from 53.1% for A-G to 17.5−25.4 for A-G/G-x, and graphite-like nitrogen (N-Q) increased from 5.5% for A-G to 20.4%−32.5% for A-G/G-x. It is found that the introduction of GO to A-G/G-x resulted in the increase in N-Q content and decrease in N-6 content. GO may be in favor of fixing the nitrogen into the carbon matrix and changed the nitrogen species. The high-resolution O 1s spectrum (Figure S6) can be deconvoluted into four peaks with the binding energies centered at 530.7, 531.8, 532.8, and 533.7 eV, which were assigned to quinone, CO, C−O, and O−H, respectively. The presence of O- and N-containing groups on the carbon surface was also confirmed by the C 1s spectra. The C 1s spectra of samples in Figure S7 can be divided into four components corresponding to graphite carbon (284.7 eV), carbon in C−O or C−N (285.6 eV), carbon in CN (286.9 eV), and carbon in CO (289.3 eV). The difference of C−O or C−N and C N or CO part contents was small (Table S1). The primary peak at 284.7 eV corresponding to the graphitic carbon dominated 60.2−64.9%, suggesting that most of the C atoms were arrayed in conjugated hexagon lattices. Compared to A-G, A-G/G-x possessed more than around 4% of sp2 carbon− carbon bonds corresponding to graphite carbon at 284.6 eV, which can be ascribed to the presence of graphene layers from reduced GO. To further investigate the dispersion of oxygen and nitrogen in the surface of A-G/G-x and A-G, SEM elemental mapping analyses were performed to obtain vivid elemental distribution information. As shown in Figure S8, the oxygen and nitrogen species have homogeneous distributions. It is well known that the electrical conductivity of carbon materials could be insignificantly improved when GO was incorporated into the carbon matrix.51 In order to examine the effect of GO on electrical conductivity resultant microporous carbon nanosheets, the electrical conductivities of A-G/G-x and A-G were tested, and they are listed in Table 2. A-G/G-x had high conductivity, nearly 3 orders of magnitude in comparison to that of A-G, resulting from the introduction of reduced GO and more graphite-like nitrogen. The good conductivity and unique compositional and structural features endowed A-G/Gx with a rapid charge migration and mass transfer as potential electrode material. Taking into account the large surface area offered by abundant microporous structures, the short ion diffusion pathway from nanosheet structures and high electrically conductive networks from reduced GO and A-G/GO-x are expected to achieve advanced performances for supercapacitors. The electrochemical performances of A-G and A-G/G-x were evaluated using the technologies of CV, GCD, and EIS in a 6 mol/L KOH aqueous solution. Figure 5a shows the cyclic voltammograms (CVs) of A-G and A-G/G-20 at a low scan rate; they exhibit ideal rectangular shapes that resulted from electrical double-layer capacitance.

Figure 4. XPS high-resolution spectra of N 1s for A-G and A-G/G-x.

Table 2. Elemental Composition, Content of N Groups, and Electrical Conductivity of Samples elemental composition (wt %)a

a

content of N groups (%)c b

sample

N

C

H

O

A-G A-G/G-20 A-G/G-60 A-G/G-100

1.91 1.69 1.51 1.81

83.81 87.17 89.22 86.59

1.10 0.59 0.35 0.78

13.18 10.55 8.92 11.72

N-6

N-5

N-Q

N-X

electrical conductivity (s/m)

53.1 17.5 18.5 25.4

38.9 42.4 38.8 50.3

5.5 29.6 32.5 20.4

2.5 10.5 10.4 3.9

0.071 388 44.3 14.8

Data were from elemental analysis. bDetermined by difference. cData were from XPS. E


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Figure 5. CV of 5,10 mV/s (a), 50,100 mV/s (b), and 200,500 mV/s (c) in the three-electrode cell.

Figure 6. Galvanostatic charge−discharge curves of A-G (a), A-G/G-20 (b), A-G/G-60 (c), and A-G/G-100 (d) in the three-electrode cell.

quick electrolyte ion diffusion at a high rate. Figure S9 shows the CV curves for A-G/G-60 and A-G/G-100 electrodes that still remained roughly rectangular in shape, showing good capacitive performance for quick charge/discharge operations at different nanosheet thicknesses. Figure 6 shows the GCD curves of all electrodes prepared using A-G and A-G/G-x at a current density of 0.5−50 A/g. The specific capacities of A-G, A-G/G-20, A-G/G-60, A-G/G100 were calculated as 233, 272, 306, 283 F/g at 0.5 A/g and 213, 252, 280, 272 F/g at 1 A/g, respectively. At lower current density, the specific capacities are closely related to specific surface areas and pore volumes. The GCD curves showed a trend deviation from the linear characteristics due to the Faradaic reactions of N and O atoms during charge/discharge at low current densities. This was consistent with the results of cyclic voltammetry at 5 and 10 mV/s. It is important that the pore structure and electroconductivity determined the capacitance of porous carbon. A-G/G-x possessed better EDLC behavior than that of A-G due to its developed pore structure and high electroconductivity higher inner micropore surface, and the largest pore volume of A-G/G-60 was more electrochemically accessible for electrolyte ions and more charges accumulating in micropores. At a high current density of 50 A/g (Figure 7), the capacitance retention of A-G was only

Moreover, they also display broad oxidation and reduction peaks, which were derived from redox reactions of the N and O heteroatoms in the carbon matrix, and pseudocapacitance appeared. The incorporated nitrogen-containing groups into the carbon matrix could effectively enhanced the space-charge capacitance, and high pseudocapacitance was considered as fast Faradaic redox reactions of “edge” nitrogen (N-6 and N-5).52 Meanwhile, oxygen-containing groups also played a role in increasing pseudocapacitance.39 When increasing the scan rate to 100 mV/s, A-G and A-G/G-20 retained rectangular voltammogram profiles, and the degree of deviation from ideal rectangular shape for A-G was more significant than that of A-G/G-20. Even when increasing the scan rate to 500 mV/s, A-G/G-20 retained roughly rectangular voltammograms profiles, in contrast with a poorly performing A-G, which can be ascribed to the increasing electric conductivity resulting from the introduction of reduced GO. The A-G/GO-20 electrode showed a larger rectangular curve corresponding to higher capacitance, which resulted from better pore size and larger pore volume in the case of similar specific surface area. This indicated that 3D nanosheet networks of A-G/G-20 facilitated faster ion mobility and showed smaller resistance within the electrode than that of A-G, and microporous carbon possessing narrow micropore size distribution merely was unfavorable for F


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Figure 7. iR drops of A-G (a) and A-G/G-20 (b). Specific capacitance of A-G and A-G/G-x measured at various current densities (c).

Figure 8. CV of A-G (a) and A-G/G-20 (b). GCD curve of A-G (c) and A-G/G-20 (d) in the two-electrode cell.

carbon nanosheet usually possesses higher capacitance for more micropore structures at lower current density, and the thinner carbon nanosheets show excellent rate performance for shorter diffusion pathways. To better understand the electrochemical behavior of A-G and A-G/G-20 electrodes as real capacitors in a 6 mol/L KOH solution electrolyte, symmetrical two-electrode cells reproducing the physical configuration, internal voltages, and charge transfer resistance were constructed. Figure 8 shows the CV curves of the A-G/G-20 electrode at different scan rates (5− 200 mV/s) under a two-electrode system. This revealed that the current densities evidently enlarged with an increase in scan rates and showed quasi-rectangular shapes without distortion even at a high scan rate of 200 mV/s. As for A-G electrodes (Figure 8a), they exhibited distorted CV shapes due to their high resistance. GCD curves of A-G/G-20 (Figure 8d) show symmetric shapes without an obviously iR drop, and A-G/G-20 shows excellent reversibility and high Coulombic efficiency and specific capacitance (262 F/g at 0.5 A/g). This suggests that AG/G-20 has a good rate capability. The 3D porous nanosheet

36% (84 F/g), while higher capacitance retentions can be achieved for A-G/G-x. This indicated that the good electroconductivity and fast ion diffusion rate were more necessary for electronic electrode material at high charge/discharge current density. The nanosheets of A-G/G-20 that had a 10 ± 4 nm thickness were the minimum thickness nanosheets, resulting in the shortest diffusion pathway and having maximum electroconductivity, so they exhibited the most excellent rate capability that retained a higher capacitance retention of 81% at 10 A/g (219 F/g) and 68% at 50 A/g (183 F/g) compared with that at 0.5 A/g (271 F/g). This illustrated that A-G/G-20 was suitable for high-rate operation. These results showed that the A-G/G20 electrode has advantages at high rate performance compared to other reported carbonaceous nanosheet electrode materials in an aqueous electrolyte (Table S2). In addition, the iR drop as a parameter of equivalent series resistance, is another important index of internal resistance especially at large current density. The iR drops of A-G and A-G/G-20 at 10, 20, and 50 A/g are shown in Figure 7. The iR drops of A-G/G-20 were near 50% for A-G at 10, 20, and 50 A/g because of the rapid diffusion pathway and high electric conductivity. Overall, the thicker G


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Figure 9. Nyquist plots in the frequency range from 0.01 Hz to 100 kHz of A-G and A-G/G-20 (a). Cycling stability of the A-G/G-20 electrode at a current density of 2 A/g (b) (inset: CV curves of the first and 5000th cycles of the A-G/G-20 electrode at a scan rate of 50 mV/s) in a two-electrode system.

structure of A-G/G-20 was favorable for electrolyte ion transporting and charge transferring. The EIS test could reveal the ion and electron transport behavior in the two-electrode cell. Figure 9a presents the Nyquist plots of a two-electrode supercapacitor using A-G and A-G/G-20 as electrode materials in a range of 0.01 Hz−100 kHz. The EIS can be divided into three parts, which are in a semicircle in the high frequency region, a line with a slope about 45° in middle frequency region, and a nearly vertical line in the low frequency region. The intercept in the high frequency region at the real axis represents the equivalent series resistance (ESR) corresponding to the ohmic resistance derived from the electrolyte and the contact between the current collector and the active material. ESRs of A-G/G-20 and A-G were 0.3 Ω. The semicircle shape depends on the adsorption kinetics of the electrolyte ions at the microporous carbon electrode, series resistance of porous carbon, and charge transfer resistance inside the carbon structure at higher frequency.53 Compared with that of A-G, the semicircle of AG/G-20 was not obviously observed; it may be that the fast ion diffusion toward the surface of the A-G/G-20 electrode results from its high electric conductivity. Moreover, it can be found that the 45° slope line of the A-G/G-20 supercapacitor showed a shorter diffusion pathway compared with A-G due to its nanosheet structure. This phenomenon was consistent with iR drops of the electrode GCD and electrical conductivity. The nearly vertical line in the low frequency region indicated that the electrode had a near-ideal electric double layer-capacitor behavior (Figure 9a). Cycle life is another critical factor for the practical application of supercapacitors. In order to examine the long-term stability of the A-G/G-20//A-G/G-20 symmetric supercapacitor, a galvanostatic charge−discharge cycling experiment was carried out at a current density of 2 A/g over 5000 cycles, as depicted in Figure 9b. The specific capacitance of A-G/G-20 still remained 92% of initial capacitance after 5000 cycles, suggesting superb electrochemical stability and outstanding reversibility. The CV curves inserted in Figure 9b show the first and 5000th cycles at a scan rate of 50 mV/s. It is noticeable that there was a peak at 0.9 V at the initial scan rate, which was related to the redox reactions, and the sharp peak became smoother, which probably implied the doped nitrogen and oxygen species was lost partially during recycling GCD, resulting in Faradaic pseudocapacitance decreasing. Figure 10 demonstrates the relationship between the power density and energy density measured in the symmetrical A-G or A-G/G-20 two-electrode cell obtained in a 6 mol/L KOH

Figure 10. Ragone plots of the A-G and A-G/G-20 measured in the two-electrode cell.

electrolyte. The energy density of A-G/G-20 reached 7.43 Wh/ kg at a power density of 263.5W/kg, which was higher than that of A-G (5.49 Wh/kg, 221.7 Wk/g), and A-G/G-20 maintained a relative higher energy density than that of A-G at a high power density.

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CONCLUSIONS

In summary, a nitrogen- and oxygen-doping 3D microporous carbon nanosheet network with developed pore structure was fabricated using gelatin as the precursor without any auxiliary reagent. The surface oxygen-containing groups of GO could reacted with the amino of the gelatin during the hydrothermal carbonization process. The pyrolyzed carbon enwrapped GO and constructed a 3D network, and the thickness of microporous carbon nanosheets can be controlled below 30 nm by adjusting the ratio of gelatin to GO. The role of GO can be summarized as follows: (a) improving the gelatin into a precarbonization product in the hydrothermal process, (b) forming nanosheet structures as shape-directing agents, (c) fixing nitrogen into the carbon matrix, and (d) enhancing the conductivity of the carbon layers. The A-G/G-20 carbon nanosheets exhibited high specific capacitance and excellent rate capability with a capacitance retention of 68% at 50 A/g (183 F/g) in a 6 mol/L KOH aqueous electrolyte because of the shorter diffusion distance, large surface area, and excellent electrical conductivity, which ensures it as an electrode material for supercapacitors at high current density especially. Besides supercapacitor electrode materials, a broad range of applications demanding rapid electrons and/or mass transport can benefit from such structure integration. H


Research Article


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01354. Optical images, SEM image, TG and DTG curves, FTIR spectra, X-ray photoelectron spectroscopy, elemental mapping, electrochemical results, and comparison of the supercapacitor electrode. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-351-4041153. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of this work by the National Basic Research Program of China (No. 2012CB626806), International Science & Technology Cooperation Program of China (No. 2011DFA51980), National Science Foundation of China (No. 21276266), Shanxi Coal Based Key Scientific and Technological Project (MD2014-09), and Zhengzhou Tobacco Research Institute of CNTC project (GY-KF-[2014]-001).

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Gelatin-Based Microporous Carbon Nanosheets as High Performance

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