A Novel Layered Sedimentary Rocks Structure of the Oxygen

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A novel layered sedimentary rocks structure of the oxygenenriched carbon for ultrahigh rate performance supercapacitors Lin-Lin Zhang, Huan-huan Li, Yan-Hong Shi, Chao-Ying Fan, XingLong Wu, Hai-Feng Wang, Hai-Zhu Sun, and Jingping Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12484 • Publication Date (Web): 27 Jan 2016 Downloaded from http://pubs.acs.org on January 29, 2016

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ACS Applied Materials & Interfaces

A novel layered sedimentary rocks structure of the oxygen-enriched carbon for ultrahigh rate performance supercapacitors Lin-Lin Zhang, Huan-Huan Li, Yan-Hong Shi, Chao-Ying Fan, Xing-Long Wu, Hai-Feng Wang, Hai-Zhu Sun,* Jing-Ping Zhang* Faculty of Chemistry, National & Local United Engineering Laboratory for Power Batteries, Northeast Normal University, Changchun 130024, China. To whom correspondence should be addressed. Email:[email protected];[email protected]; Fax: 86-431-85099668. KEYWORDS：layered sedimentary rocks structure, biomass, oxygen-enriched, ultrahigh rate performance, symmetric supercapacitor

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ABSTRACT：In this paper, gelatin as a natural biomass was selected to successfully prepare an oxygen-enriched carbon with layered sedimentary rocks structure, which exhibited ultrahigh rate performance and excellent cycling stability as supercapacitors. The specific capacitance reached 272.6 F g-1 at 1 A g-1 and still retained 197.0 F g-1 even at 100 A g-1 (with high capacitance retention of 72.3%). The outstanding electrochemical performance resulted from the special layered structure with large surface area (827.8 m2 g-1) and high content of oxygen (16.215 wt%), which effectively realized the synergistic effects of the electrical double-layer capacitance and pseudocapacitance. Moreover, it delivered an energy density of 25.3 Wh kg-1 even with a high power density of 34.7 kW kg-1 and ultra-long cycling stability (with no capacitance decay even over 10,000 cycles at 2 A g-1) in a symmetric supercapacitor, which are highly desirable for their practical application in energy storage devices and conversion.


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1. Introduction Supercapacitors, as an advanced energy-storage devices, show highly promising applications due to its high power density and super long-cycle life.1-4 Among various supercapacitor electrode materials,5-10 activated carbon-based materials play an important role in commercial supercapacitors because of its low cost, high conductivity, and good stability.11-15 However, traditional activated carbon (e.g., carbon nanotube,16-18 graphene-based carbon aerogels19-25) as the commercially manufactured supercapacitor electrode materials shows low energy density, limited rapid charge-discharge performance and slowly kinetic processes especially at high current densities, which cannot meet the ever-growing energy requirement. To solve these problems, energy storage based on heteroatom-doped carbon materials (N, O, B) have been quickly developed.26-33 Such kinds of supercapacitors can combine the electrical double-layer capacitor (EDLC) with pseudocapacitance to successfully realize the ultra-rapid charge-discharge process, excellent long-cycle stability, and good specific capacitance. To obtain heteroatom-doped carbon materials, renewable sources or biomass (such as, pomelo peel,34 sugarcane bagasse,35 sucrose36 and cellulose37) have attracted extensive concerns due to its ecofriendly process and high yield.38 Doping of oxygen or nitrogen in these sources can possibly develop a positive charge density along the neighboring carbon atoms due to their large electronegativity, which contributes to good conductivity, larger active sites and additional pseudocapacitance. Moreover, various amino acids and oxygen-containing functional groups in the biomass are beneficial for the multi-heteroatom-doped carbon materials. Therefore, many researchers have developed various methods to obtain these carbon materials through the use of biomass sources.39-43 For example, Xia et al. fabricated an oxygen-rich hierarchical porous carbon derived from artemia cyst shells to achieve good specific capacitance, which exhibited


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349 F g-1 at a current density of 0.5 A g-1 through KOH chemical activation.44 Wang et al. fabricated a flower-like structure carbon through carbonization of the assembly of D-fructose with soft templates (the triblock copolymer Pluronic F127), which showed about 200 F g-1 at 1 A g-1.45 Although much process has been achieved, the complex and/or time consuming process with the help of various templates limits its wide application and large-scale production. On the other hand, it remains a great challenge to achieve ultra-high rate performance in intelligent devices. As a consequence, developing an eco-friendly and template-free process to obtain novel carbon materials from renewable sources is of great significance in achieving the excellent rate performance. Herein, we fabricated an oxygen-enriched activated carbon with layered sedimentary rocks structure and abundant micropores by simple activating calcinations of the compound of gelatin and critic acid. Gelatin as an ideal biomass source is a polypeptide molecule full of amino groups and hydroxy groups, which can give dual-doping of oxygen and nitrogen. Furthermore, critic acid as an important organic acid provides chemical bonding through the reaction between carboxyl and amino groups to enhance the oxygen content. The obtained oxygen-enriched carbon as the electrode material exhibited excellent rate performance, with specific capacitance of 197 F g-1 even at 100 A g-1 (with high capacitance retention of 72.3% at 1 A g-1) in threeelectrode system. As a symmetric supercapacitor, the specific capacitance showed 181.7 F g-1 at current density of 0.5 A g-1 and still retained 140.4 F g-1 even at 30 A g-1. There was almost no capacitance decay over 10,000 cycles, indicating its excellent cycling stability in practical application. 2. Experimental 2.1 Materials


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Gelatin and citric acid were purchased from Aladdin. All other chemicals were used as received without any further purification. 2.2 Preparation of layered sedimentary rocks structure activated carbon 3.0 g gelatin and 1.0 g citric acid were dissolved in 30.0 mL of water. The mixture was stirred for 5 h at 90oC to obtain a uniform transparent solution. The solution was dried in an oven at 60oC for 12 hours, resulting in a light yellow gel. The obtained gel was heated to 300oC maintained for 1 h and then heated to 800oC maintained for 2 h under N2 atmosphere in a tube furnace. The resulted black powder was carbon precursor (GA).The as-prepared GA was mixed with KOH solution under magnetic stirring overnight with a KOH/GA mass ratio of 3 and dried at 60oC in an oven. The obtained grey powder was heated to 650/750/850oC for 2 h at a ramp rate of 5oC min-1 under nitrogen atmosphere. The obtained products were washed with 1 M HCl to remove any inorganic salts and dried in an oven at 60oC, which was named GA650, GA750, or GA850. The contrastive activated carbon derived from pure gelatin or citric acid was also fabricated similar to the GA650. 2.3 Characterization Typical X-ray diffraction (XRD) patterns (Rigaku P/max 2200VPC) were recorded with Cu Kα radiation. Resonant Raman scattering spectra were recorded at room temperature with a JY HR-800 Lab Ram confocal Raman microscope in a backscattering configuration with an excitation wavelength of 633 nm. The morphologies were characterized by field-emission scanning electron microscopy (FESEM, XL 30 ESEM-FEG, FEI Company) equipped with energy-dispersive X-ray analysis. To confirm the existence of hierarchical pores in the hybrid composites, nitrogen adsorption isotherms were carried out at -196ºC using a micromeritics ASAP 2020 analyzer. Prior to adsorption, the samples were degassed at 150ºC for 10 h. X-ray


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photoelectron (XPS) test using Mg-Kα excitation (1253.6 eV) of as-prepared composites was collected with a VG ESCALAB MKII spectrometer. Binding energy calibration was based on C 1s at 284.6 eV. 2.4 Electrode prepared and electrochemical measurement The electrodes were prepared by mixing acetylene black, polyvinylidene fluoride with activated carbon (8:1:1) in N-methyl-2-pyrrolidone (NMP) to form a slurry. The slurry was coated onto the stainless steel and dried at 80ºC overnight. The diameter and thickness of the electrode was about 1 cm and 0.1 mm, respectively. The mass loading of the electrode is 1.25 mg (1cm×1cm). In three electrode system, platinum foil as counter electrode and saturated calomel electrode as reference electrode in 1 M H2SO4 with potential window of 0-0.9 V. In twoelectrode system, two electrodes with the same size and mass were sandwiched together and tested in 1 M H2SO4 with potential window of 0-1.2 V. The specific capacitance was calculated from galvanostatic charge−discharge curves using

C = 4 I∆t / m∆V (in two-electrode system)

(1)

C = I∆t / m∆V (in three-electrode system)

(2)

Where I (A) is the current density, m (g) is the total mass of both electrodes in two-electrode system or the single electrode mass in three electrode system, t (s) is the discharge time, and V is the voltage window. Electrochemical impedance spectroscopy (EIS) was conducted at open circuit voltage in the frequency range from 0.01 Hz to 100 kHz with a 5 mV AC amplitude. 3. Results and discussion Layered sedimentary rock structure with abundant micropores carbon was fabricated by a template free simple physical mixing gelatin and critic acid, which is shown in Scheme 1. The dual-doping of oxygen and nitrogen derives from the abundant amino group and hydroxyl or


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carboxyl groups in gelatin and critic acid. The obtained special structure provides continuous and short ion transport pathway as well as extra pseudocapacitance to achieve excellent rate performance.

Scheme 1. Schematic diagram of the preparation of the oxygen-enriched activated carbon with layered sedimentary rock structure. The SEM images of the overall layered activated carbon with irregular monoliths obtained at different calcination temperature are shown in Figure1a-1c. Their size ranges from 5 to 10 µm. The inset high magnification SEM image of GA650 in Figure 1a and TEM image in Figure S1 clearly reveal the stacked plate-like structure is similar to the natural sedimentary rock, which plays an important role in the large surface active area and fast ion transport as well as charge storage, especially at high current density. TEM image of GA650 in Figure 1d further shows the irregular lattice fringes at the edge layer with a 0.386 nm interplanar spacing (the inset), which confirms the good conductivity of the graphitized carbon materials to some extent. XRD pattern of as-prepared samples in Figure 1e shows two broad diffraction peaks centered at around 23.4º and 43.3º. Compared with the standard 2θ=26.6º of graphite (the 002 diffraction peak), the strong and broadening peak around 23.4º shifts to the left. The inter-lamellar spacing of 0.386 nm is larger than that of graphite (0.334 nm) according to Bragg′s equation: 2d ⋅ sin θ = λ


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.Another weak diffraction peak centered at 43.3º of the three samples attributes to the 100 diffraction of graphite carbon, demonstrating the low graphitization degree.

Figure 1. SEM images of (a) GA650, the inset is the high magnification SEM image; (b) GA750; (c) GA850. (d) TEM image of GA650, the inset in Figure 1d is the HR-TEM image. (e) XRD patterns and (f) Raman spectra of the GA650, GA750, and GA850. Figure 1f presents the Raman spectra of GA650, GA750, and GA850. Two characteristic peaks located around 1338 cm-1 (D-band) and 1584 cm-1 (G-band) of carbon materials can be observed. The intensity ratio (ID/IG) is widely used to determine the graphite with respect to defects in heteroatom-doped carbon. The ID/IG ratios are 1.10, 1.24, and 1.33 for GA650, GA750, and GA850, respectively. As the calcinations temperature increases, the enhanced intensity of ID/IG means more defects which probably result from the decomposition of O- and N- functional groups at high temperature. Therefore, the highest graphitization degree of GA650 among all carbon materials plays an important role in improving the conductivity and high retention of oxygen functional groups, which is in favor of extra pseudocapacitance and excellent rate performance at rapid charging process.


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To further confirm this decomposition phenomenon of the functional groups and the elemental composition in as-prepared materials, XPS survey spectra are performed in Figure 2.

Figure 2. (a) XPS spectra of the GA650, GA750, and GA850. The deconvoluted peaks of the C 1s (b), O 1s (c), and N 1s spectra (d) of GA650. As shown in Figure 2a, a characteristic carbon peak locates at ~284 eV, oxygen and nitrogen peaks locate at ~533 eV and ~400 eV, respectively. The deconvoluted C 1s spectrum of GA650 in Figure 2b is composed of sp2 C-C, sp3 C-C, and oxygen functional groups (-C-O, 286.6 eV), (O-C=O, 288.5 eV), which is consistent with the deconvoluted O 1s spectrum: -C=O (~531.5 eV), -O-C=O (~532.2 eV), and -OH (~533.4 eV) in Figure 2c. The uniform distribution of abundant oxygen functional groups is beneficial for the pseudocapacitance in electrochemical process. Figure 2d shows the XPS spectrum of N element, which is dominated by pyrrolic


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nitrogen (N-5, ~399.6 eV) and pyridinic nitrogen (N-6, ~400.8 eV). Dual-doping of the negatively charged oxygen and nitrogen functional groups with additional free electrons is beneficial for enhancing the wettability and the faradic pseudocapacitive in aqueous electrolyte. In order to quantify the oxygen and nitrogen element contents and investigate the formation mechanism of the high oxygen content activated carbon, the elemental analysis of the asprepared samples and carbon obtained from the pure critic acid and gelatin under the same experimental conditions are showed in Table S1. As shown in Table S1, N (4.322 wt%) and O (12.378 wt%) co-exist in the carbon obtained from pure gelatin, which attributes to its abundant amino functional groups and carboxylic groups. Residual O content of 18.288 wt% is obtained in the activated carbon from citric acid due to its hydroxyl and carboxyl groups, which plays an important role in high O content of activated carbon. It is concluded that O-enriched (16.215 wt%) activated carbon with N-doping (2.378 wt%) simultaneously can be obtained under the condition of coexistence of gelatin and citric acid, which is key for the formation of the oxygenenriched carbon. We have reported that the natural viscosity of citric acid in the mixture can extend and lead void in the monolithic materials.46 Therefore, the novel activated carbon with sedimentary rock structure may form along with the water evaporation during the calcinations process. The SEM images of activated carbon from pure gelatin or citric acid only shows monolithic morphologies with several microns in Figure S3a and S3b. In order to further discuss the nitrogen and oxygen elemental distribution in GA650, SEM image and elemental mapping of C, O, and N are shown in Figure S4a-d. The dual-doping N and O derived from biomass gelatin and citric acid and its homogeneous distribution as well as the layered structure will contribute to the excellent


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electrochemical performance. The porosities of the samples are further investigated by nitrogen adsorption isotherms. All as-prepared samples exhibit I-type adsorption-desorption isotherms with strong steep increase of N2 adsorption at relative low pressure, revealing the high microporosity in Figure 3a. The pore size distribution curves are calculated from the Barrett-Joyner-Halenda (BJH) method. Table S2 summarizes the specific surface area and pore volumes of the as-prepared samples.

Figure 3. (a) The N2 adsorption-desorption isotherms and (b) pore size distribution curves calculated from BJH of GA650, GA750, and GA850. Figure 3b compares the pore size distribution of the obtained activated carbon. Besides mesopores (2~50 nm), abundant micropores (

A Novel Layered Sedimentary Rocks Structure of the Oxygen

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