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High Performance Sodium Ion Capacitor Based On MoO2@rGO Nano Composite and Goat Hair Derived Carbon Electrodes Kiruthiga Ramakrishnan, Chandrasekaran Nithya, and Ramasamy Karvembu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00284 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 31, 2018
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High Performance Sodium Ion Capacitor Based On MoO2@rGO Nano Composite and Goat Hair Derived Carbon Electrodes Kiruthiga Ramakrishnan, Chandrasekaran Nithya*, Ramasamy Karvembu a
National Institute of Technology, Tiruchirappalli, India – 620 015
KEYWORDS Reduced graphene oxide, Molybdenum dioxide, Goat hair, Activated carbon, Nano Composite, Hybrid capacitor.
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ABTRACT 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 battery type anode and offer power by capacitor type cathode. Accomplishing high energy and power density in a single device is significant interest which can probable merely by making hybrid devices. Herein we have synthesized biomass (goat hair) derived activated carbon cathode with high surface area of 2042 m2g−1 and MoO2@rGO composite anode materials. Goat hair, keratin rich bio mass has great impact economically and produced over 40 million tons per year. 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. INTRODUCTION Owing to fast growing technologies and increasing environmental issues, there is a need of sustainable, efficient and renewable energy storage system. This scenario has forced researchers to embark on electrochemical energy storage technologies like rechargeable batteries and supercapacitors which can fulfil the needs in long run. Especially, supercapacitors (rich in power density) are categorised as pseudo capacitors and electrical double layer capacitors (EDLC). The energy density of supercapacitors is an order of enormity lower than that of rechargeable batteries1-5. To overcome this issue, new efforts have been triggered of late called sodium ion
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capacitors (NICs) which includes, battery type anode and supercapacitor type cathode. Sodium based system has potential for mid-to large-scale energy storage application due to their high energy and power densities, long cycle life, cost effectiveness and rich abundance of sodium in nature6. The incorporation of battery type anode into a supercapacitor will enhance the energy density of the system, since faradaic process accomplished by the anode. So far very few attempts have been taken in this research area that comprises metal oxides7-9, titanates10, phosphate11, carbide12 and peanut shell derived carbon13. To accomplish a higher energy density at anode part, researchers put more efforts onto transition metal oxides as mentioned earlier. MoO2 has been attracted significant interest as active electrode for supercapacitors due to its low resistivity and high specific capacitance, metallic conductivity, cost effective and environmental benignity14-16. However, the practical use of MoO2 material is hampered by the poor cycling stability results inferior capacity retention during prolonged charge-discharge cycling. Possible reasons for the inferior performance may lie in the lacking of efficient electronic/ionic transport pathways which can overcome by making composite with reduced graphene oxide. MoO2@rGO composite can afford ideal designs towards high performance electrodes and has been extensively pursued17,18. The utilization of activated, disordered porous carbon specifically from natural precursors is brought about by their high electrical conductivity, surface area, superior performance, 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 play a key role in developing environmental friendly energy storage devices19-24. In this regard, here we demonstrated the synthesis of activated carbon from goat hair 3 ACS Paragon Plus Environment
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(keratin rich bio mass). Higher vertebrates (mammals, birds and reptiles) and human’s epithelial cells contain Keratin which is durable and fibrous protein. It provides strength to body and structurally exists in feathers, horn and wool. Keratin is produced from wool industry which is around 40 million tons/year and few millions of tons from food industry particularly from slaughterhouse and meat market. The chemical constituents of Ketatin are 2-5% sulphur, 15-18% nitrogen, 3.20% mineral elements, 1.27% fat and the remaining 90% are proteins 25,26. In this work, for the very first time, we demonstrate high performance 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 as device fabricated from synthesized cathode and anode materials rather than commercial one. EXPERIMENTAL SECTION Synthesis of Activated carbon Goat skin was collected from local meat shop. Then hair fibres were removed from the skin. The collected hair fibres 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 fibres were cut into small pieces with the size of 2-4 mm. Then in the pre-carbonization process, the sample was heated at 450°C for 6 hours (heat flow rate is 5° min-1) in an Argon atmosphere followed by activation process, in which pre-carbonized sample was mixed with KOH (the weight ratio of KOH and carbon are maintained as 2), then kept in the same atmospheric condition with the same flow rate at 800°C for 2 hours. After cooling, the activated sample was dispersed in distilled water, stirred for 18 hrs and sonicated for 8 hrs. Then washed with dil. HCl
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to remove the impurities, followed by water and ethanol, until it attains the pH=7. Finally sample was dried at 60°C in the hot air oven for 48 hrs and ground well. Synthesis of MoO2, rGO, MoO2@rGO nanocomposite All the chemicals used in the experiment were analytical grade without further purification. The MoO2 was synthesized via hydrothermal method, in which 2g of ammonium hepta molybdate was dissolved in 20ml of distilled water. Then 0.5g of tartaric acid was added drop wise into this solution under stirring. The colour of the mixture was changed into bluish green after 2hr of stirring. After that the mixture was transferred into Teflon coated stainless-steel made autoclave which is heated at 180°C for 28 hrs. The resultant black colour precipitate was washed several times with distilled water and ethanol. Then obtained precipitate was dried at 80°C for overnight. Finally it was calcined at 450°C for 6 hrs in an Ar atmosphere to obtain a desired product. Graphene oxide (GO) was prepared by modified Hummers method
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. 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 hrs. Then sodium borohydride (NaBH4) was added to it under sonication. After the addition of NaBH4, it was stirred vigorously for 8 hrs. The precipitate thus formed was filtered, dried at 60oC for 48 hrs 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 0.15g of rGO was dispersed under sonication for 1 hr. 0.85g of MoO2 was added and allowed for sonication about 2 hours. Further the mixture was stirred for 24 hours and the resulting solid was separated by filtration and washed with large amounts of DI water and ethanol to remove most residual ions. The 5 ACS Paragon Plus Environment
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resultant sample is dried in hot air oven at 60°C for 24 hrs, the product was obtained as a fine powder after grinding. Material characterization The surface morphological features of synthesized materials were analyzed by FESEM (CARL ZEISS -Neon 40 microscope) and HRTEM (FEI Technai- 20 G2 microscope). To confirm the crystal structures, XRD analysis was performed using 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° 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 material was investigated by X-ray photoelectron spectroscopy (XPS). The spectra were obtained using Thermoelectron spectrometer with AlKα radiation with the scan range of 1200 eV. The Raman spectra was 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 homogenous slurry. The prepared slurry was then coated onto a copper foil (for anode) and aluminium 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 glove box using Na 6 ACS Paragon Plus Environment
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foil as reference electrode. The electrolyte was 0.75M 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 electrochemical workstation (Biologic, SP-250 and SP-240) and Galvanostatic charge/discharge cycling were 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. C = I∆t/m∆V
(1)
where C (F/g) is the specific capacitance. I (A) is the discharge current, ∆t (s) is the discharge time, m (g) is the total mass of active material in the electrode, and ∆V (V) is the voltage window during the discharge process
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. The energy density and power density of NIC device was
calculated by using the following equations. E = (1/2) C(∆V)2
(2)
P = E/t
(3)
Where E (W hkg-1) is the specific energy density, P (W kg-1) is the specific power density, C (F/g) is the specific capacitance. ∆V (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 XRD pattern of as synthesized graphene oxide (GO), reduced graphene oxide (rGO), MoO2 and MoO2@rGO composite. The peak at 2θ = 10.56o, typical for GO phase corresponds to 002 plane which shifted to 25.15o, from which it can be concluded that GO is completely reduced into rGO and broadening of peak reveals that the existence of disordered graphene sheets
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. In the case of MoO2, the diffraction patterns are indexed to the monoclinic
structure (ICSD ref.code:01-073-1249) with the phase group of P21/c and lattice parameters a=5.6118 Å, b=4.8554 Å, c=5.6291 Å. The peaks at 2θ=25.97o, 36.91o, 41.54o, 49.47 o, 53.49 o, 60.39 o, 66.45o, correspond to the (011), (-211), (-212), (-302), (-311), (013), (-402), (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. Fig. 1b represents XRD pattern of goat hair derived carbon shows two broad peaks with low intensity at 24.5o and 43.5o which corresponds to 002 and 100 planes are an evidence of amorphous, disordered, non-graphitic nature of activated carbon material. The d-spacing value of (002) plane is 0.39 nm. The calculated R value (peak to background ratio) of 002 plane (see Supporting Information, Figure S1) is 1.6 which indicates that arbitrary arrangement of layers. The low value of R (
