Article Cite This: ACS Appl. Energy Mater. 2018, 1, 841−850
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High-Performance Sodium Ion Capacitor Based on MoO2@rGO Nanocomposite and Goat Hair Derived Carbon Electrodes Kiruthiga Ramakrishnan, Chandrasekaran Nithya,* and Ramasamy Karvembu National Institute of Technology, Tiruchirappalli 620 015, India S Supporting Information *
ABSTRACT: Sodium ion based energy storage system is a rising alternative for imminent energy need. Especially sodium ion hybrid supercapacitors have attracted much attention because they store energy through batterytype anode and offer power by capacitor-type cathode. Accomplishing high energy and power densities in a single device is of significant interest, which can probably be done merely by making hybrid devices. Herein we have synthesized biomass (goat hair) derived activated carbon cathode with a high surface area of 2042 m2g−1 and MoO2@rGO composite anode materials. Goat hair, keratin rich biomass, has a great impact economically, and over 40 million tons per year is produced. Besides, reduced graphene oxide (rGO) has been used to facilitate the chemical stability, mechanical strength, and feasible pathway for electrochemical reactions of MoO2. Each electrode individually (half-cell) and combinedly (full-cell) showed good electrochemical performance which is almost equal to previously reported sodium ion based hybrid supercapacitors. This combination of supercapacitor can travel over the existing energy storage system to the next level. KEYWORDS: reduced graphene oxide, molybdenum dioxide, goat hair, activated carbon, nanocomposite, hybrid capacitor
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INTRODUCTION Owing to fast growing technologies and increasing environmental issues, there is a need for a sustainable, efficient, and renewable energy storage system. This scenario has forced researchers to embark on electrochemical energy storage technologies such as rechargeable batteries and supercapacitors which can fulfill the needs in the long run. Especially, supercapacitors (rich in power density) are categorized as pseudocapacitors and electrical double layer capacitors (EDLCs). The energy density of supercapacitors is an order of enormity lower than that of rechargeable batteries.1−5 To overcome this issue, new efforts have been triggered of late called sodium ion capacitors (NICs) which include a battery-type anode and supercapacitor-type cathode. A sodium based system has the potential for mid- to large-scale energy storage application due to its high energy and power densities, long cycle life, cost effectiveness, and rich abundance of sodium in nature.6 The incorporation of a battery-type anode into a supercapacitor will enhance the energy density of the system, since a faradaic process is accomplished by the anode. So far very few attempts have been taken in this research area that comprise metal oxides,7−9 titanates,10 phosphate,11 carbide,12 and peanut shell derived carbon.13 To accomplish a higher energy density at the anode part, researchers put more efforts onto transition metal oxides as mentioned earlier. MoO2 has attracted significant interest as an active electrode for supercapacitors due to its low resistivity and high specific capacitance, metallic conductivity, cost-effectiveness, and environmental benignity.14−16 However, the practical © 2018 American Chemical Society
use of MoO2 material is hampered by the poor cycling stability which results in inferior capacity retention during prolonged charge−discharge cycling. Possible reasons for the inferior performance may lie in the lack of efficient electronic/ionic transport pathways which can be overcome by making a composite with reduced graphene oxide (rGO). MoO2@rGO composite can afford ideal designs toward high-performance electrodes and has been extensively pursued.17,18 The utilization of activated, disordered porous carbon specifically from natural precursors is brought about by their high electrical conductivity, surface area, superior performance, and chemical and cycle stability. In the past few years, notable improvements in performance have been achieved through recent advances. The up to date progress in the synthesis of such activated carbon materials with desirable structure facilitates the enhanced specific capacitance in a wide variety of electrolytes and plays a key role in developing environmentally friendly energy storage devices.19−24 In this regard, here we demonstrated the synthesis of activated carbon from goat hair (keratin rich biomass). Higher vertebrates (mammals, birds, and reptiles) and human’s epithelial cells contain keratin, which is a durable and fibrous protein. It provides strength to body and structurally exists in feathers, horn, and wool. Keratin is produced from the wool industry, being around 40 million tons/year, and a few millions of tons from food industry, Received: December 14, 2017 Accepted: January 30, 2018 Published: January 30, 2018 841
DOI: 10.1021/acsaem.7b00284 ACS Appl. Energy Mater. 2018, 1, 841−850
Article
ACS Applied Energy Materials
material was investigated by X-ray photoelectron spectroscopy (XPS). The spectra were obtained using a Thermoelectron spectrometer with Al Kα radiation with the scan range of 1200 eV. The Raman spectra were recorded on a Renishaw InVia laser Raman microscope with a He−Ne laser (λ = 633 nm). Electrochemical Characterization. The working electrodes were prepared by a standard slurry coating technique. In detail, the 80% active material, 10% poly(vinylidene fluoride) (PVDF) binder , and 10% super P carbon were mixed in N-methyl-2-pyrrolidone (NMP) to form an homogeneous slurry. The prepared slurry was then coated onto a copper foil (for anode) and aluminum foil (for cathode). The mass loading of each electrode active material is 1.5−2.5 mg. Electrochemical measurements were performed using CR-2032 coin-type cells. The cells were assembled in an argon filled glovebox using Na foil as reference electrode. The electrolyte was 0.75 M NaPF6 [in ethylene carbonate (EC) and diethyl carbonate (DEC) {1:1 (v/v)}] for all electrochemical studies. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out by using an electrochemical workstation (Biologic, SP-250 and SP-240), and galvanostatic charge/discharge cycling was studied using a battery tester (NEWARE). The anode and cathode are tested separately as half-cells where Na foil was used as the reference electrode. The potential windows for anode, cathode, and full-cell are 0.01−3, 1.5−4.3, and 0.01−3 V, respectively. The specific capacitance of the both half-cell and full-cell was calculated by using the following equation.
particularly from the slaughterhouse and meat market. The chemical constituents of ketatin are 2−5% sulfur, 15−18% nitrogen, 3.20% mineral elements, 1.27% fat, and the remaining 90% proteins.25,26 In this work, for the very first time, we demonstrate a highperformance sodium ion hybrid supercapacitor by combining battery-type anode (MoO2@rGO) and capacitor-type cathode (goat hair derived activated carbon (CGH)). This is a new effort to develop a device fabricated from synthesized cathode and anode materials rather than a commercial one.
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EXPERIMENTAL SECTION
Synthesis of Activated Carbon. Goat skin was collected from local meat shop. Then hair fibers were removed from the skin. The collected hair fibers were put into boiling water to kill the parasites. Further it was washed with isoproponal followed by acetone to get clean hair without any impurities. The obtained hair fibers were cut into small pieces with the size of 2−4 mm. Then in the precarbonization process, the sample was heated at 450 °C for 6 h (heat flow rate is 5 deg/min−1) in an argon atmosphere followed by an activation process, in which precarbonized sample was mixed with KOH (the weight ratio of KOH and carbon are maintained as 2), and then kept in the same atmospheric condition with the same flow rate at 800 °C for 2 h. After cooling, the activated sample was dispersed in distilled water, stirred for 18 h, and sonicated for 8 h and then washed with diluted HCl to remove the impurities, followed by water and ethanol, until it attains pH = 7. Finally the sample was dried at 60 °C in a hot air oven for 48 h and ground well. Syntheses of MoO2, rGO, and MoO2@rGO Nanocomposite. All the chemicals used in the experiment were analytical grade without further purification. MoO2 was synthesized via hydrothermal method, in which 2 g of ammonium heptamolybdate was dissolved in 20 mL of distilled water. Then 0.5 g of tartaric acid was added dropwise into this solution under stirring. The color of the mixture changed to bluish green after 2 h of stirring. After that the mixture was transferred into a Teflon coated stainless steel made autoclave which was heated at 180 °C for 28 h. The resultant black color precipitate was washed several times with distilled water and ethanol. Then the obtained precipitate was dried at 80 °C for overnight. Finally it was calcined at 450 °C for 6 h in an Ar atmosphere to obtain a desired product. Graphene oxide (GO) was prepared by modified Hummers method.27,28 Reduced graphene oxide (rGO) was prepared by reduction of graphene oxide. In this step, the prepared GO was dispersed in double distilled water and sonicated for 2 h. Then sodium borohydride (NaBH4) was added to it under sonication. After the addition of NaBH4, it was stirred vigorously for 8 h. The precipitate thus formed was filtered, dried at 60 °C for 48 h, and ground well. Molybdenum dioxide reduced graphene oxide composite (MoO2@ rGO) was synthesized in the ratio of 85:15 from as prepared MoO2 and rGO. In DI water 0.15 g of rGO was dispersed under sonication for 1 h. A 0.85 g amount of MoO2 was added and allowed for sonication about 2 h. Further the mixture was stirred for 24 h, and the resulting solid was separated by filtration and washed with large amounts of DI water and ethanol to remove most residual ions. The resultant sample was dried in the hot air oven at 60 °C for 24 h; the product was obtained as a fine powder after grinding. Material Characterization. The surface morphological features of synthesized materials were analyzed by FESEM (CARL ZEISSNeon 40 microscope) and HRTEM (FEI Technai20 G2 microscope). To confirm the crystal structures, XRD analysis was performed using an X-ray diffractometer, Bruker D8 with Cu Kα radiation (λ = 1.5406 Å), keeping the operating voltage at 40 kV and current of 20 mA of between 10° and 90° at a scan rate of 2 deg/min−1. The porous structure of carbon was investigated by N2 adsorption−desorption measurements with BEL Sorp-II mini, (BEL Japan Co.) analyzer. The specific surface area was calculated by the conventional Brunauer− Emmett−Teller (BET) method using BEL Master-data evaluation software. The oxidation states and chemical composition of the carbon
(1)
C = I Δt /mΔV
where C (F/g) is the specific capacitance. I (A) is the discharge current, Δt (s) is the discharge time, m (g) is the total mass of the active material in the electrode, and ΔV (V) is the voltage window during the discharge process.29 The energy density and power density of the NIC device were calculated by using the following equations.
E = (1/2)C(ΔV )2
(2) (3)
P = E /t −1
−1
where E (W h kg ) is the specific energy density, P (W kg ) is the specific power density, and C (F/g) is the specific capacitance. ΔV (V) is the cell voltage for charging and discharging, and t (h) is the discharge time correspondingly.
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RESULTS AND DISCUSSION X-ray diffraction studies were carried out to find the crystal structure of as synthesized materials. Figure 1a displays the XRD pattern of as synthesized GO, rGO, MoO2, and MoO2@ rGO composite. The peak at 2θ = 10.56°, typical for GO phase, corresponds to the 002 plane which shifted to 25.15°, from which it can be concluded that GO is completely reduced into rGO and broadening of the peak reveals the existence of disordered graphene sheets.30 In the case of MoO2, the diffraction patterns are indexed to the monoclinic structure (ICSD reference code: 01-073-1249) with the phase group of P21/c and lattice parameters a = 5.6118 Å, b = 4.8554 Å, and c = 5.6291 Å. The peaks at 2θ = 25.97°, 36.91°, 41.54°, 49.47°, 53.49°, 60.39°, and 66.45°, correspond to the (011), (21̅ 1), (21̅ 2), (30̅ 2), (31̅ 1), (013), (4̅02), and (400) planes.31,32 The crystallite size is about 41.27 nm determined using Scherrer’s formula. Since the rGO content in the composite is low, the diffraction peaks of MoO2 have been dominated. Figure 1b represents XRD pattern of goat hair derived carbon showing two broad peaks with low intensity at 24.5° and 43.5°, which correspond to 002 and 100 planes and are evidence of the amorphous, disordered, nongraphitic nature of activated carbon material. The d-spacing value of the (002) plane is 0.39 nm. The calculated R value (peak to background ratio) of the 002 plane (see the Supporting Information, Figure S1) is 1.6 which indicates an arbitrary 842
DOI: 10.1021/acsaem.7b00284 ACS Appl. Energy Mater. 2018, 1, 841−850
Article
ACS Applied Energy Materials
Figure 1. XRD patterns of (a) GO, rGO, MoO2, and MoO2@rGO composite and (b) CGH.
Figure 2. Raman spectra of (a) CGH and (b) MoO2@rGO composite.
Figure 3. (a) N2 adsorption−desorption isotherm and (b) pore size distribution curve of CGH.
arrangement of layers. The low value of R (
