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RuO2.nH2O Nanoparticles anchored on Carbon Nano-onions: an Efficient Electrode for Solid State Flexible Electrochemical Supercapacitor vedi kuyil azhagan muniraj, Chaitanya Krishna Kamaja, and Manjusha V. Shelke ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01627 • Publication Date (Web): 18 Mar 2016 Downloaded from http://pubs.acs.org on March 23, 2016

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ACS Sustainable Chemistry & Engineering

RuO2.nH2O Nanoparticles anchored on Carbon Nano-onions: an Efficient Electrode for Solid State Flexible Electrochemical Supercapacitor M. Vedi Kuyil Azhagana, Chaitanya Krishna kamaja abc, Manjusha V. Shelke*abc a

Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune-11008,

MH, India. E-mail: [email protected]; [email protected] b

CSIR-Network Institute for Solar Energy, CSIR-National Chemical Laboratory, Pune-411008,

MH, India c

Academy of Scientific and Innovative Research (AcSIR), Chennai, 600113, TN, India

KEYWORDS: Electrochemical Capacitor, Carbon Nano-onions, Hydrous Ruthenium Oxide Nanoparticles, flexible conducting substrate, flexible energy storage device

ABSTRACT: A flexible solid state electrochemical capacitor based on hydrous RuO2 nanoparticles, supported onto the nonporous and highly accessible ion adsorptive Carbon nanoonions (CNOs) is fabricated in a novel process of modifying a conducting carbon paper to flexible conducting substrate, separated with polyvinyl alcohol/H2SO4 gel electrolyte. Sol-gel technique tend to form homogeneously dispersed RuO2 nanoparticles with the average size of ~2.3 nm on the positive surface curvatures of multilayer fullerene (CNOs) which helped high diffusivity of ions in both the aqueous and solid state gel electrolytes. The flexible substrate 1 ACS Paragon Plus Environment


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worked excellently as an electrical conductor as well as a stable mechanical support. This solid state flexible energy storage device showed maximum energy density of 10.62 Whkg-1 and the maximum power density of 4.456 kWkg-1 for hydrous RuO2/CNOs nanocomposite with 94.47% cycling stability even after 4000 cycles.

INTRODUCTION Ever evolving microelectronics consumer industry is fascinated by cheap, wearable, flexible components/devices and therefore there is huge interest for pliable films of functional materials. Almost all portable, wearable devices require equally flexible and lightweight energy support systems. Electrochemical supercapacitors (ESC) are promising energy storage devices with their capability to deliver high specific power, fast charging/discharging, long cycle lifetime and are being hailed in many new technological applications1. ESC systems work on two types of energy storage mechanisms; an electrochemical double layer capacitor (EDLC) where charges are stored physically at the electrode/electrolyte interface and a pseudocapacitor where charge transfer occurs via faradaic redox reactions across the electrode/electrolyte interface2. However these systems conventionally use liquid electrolytes hence robust sealing techniques are required to avoid electrolyte leakage subsequently raising concerns regarding portability and safety. In addition, existing ESC systems lack comparable energy storage density as conventional batteries; therefore, improving energy density without compromising power density and cyclability is another research concern for ESCs. Pseudocapacitive materials like transition metal oxides, conducting polymers etc., are being evaluated in composition with carbon based materials as electrodes for ESCs so as to achieve above goals via usage of both EDLC and redox mechanisms of charge storage3. The electrochemical performance of ESCs is greatly dependent on the electroactive materials such as double layer capacitive carbon allotropes and pseudocapacitive 2 ACS Paragon Plus Environment

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electroactive oxides like V2O5, Co2O3, Fe2O3, MnO2, RuO2 or conductive polymers etc4-8. Ruthenium Oxide (RuO2) is the most promising pseudo-capacitive electrode material among the transition metals with remarkably high specific capacitance, high chemical, thermal stability and good electrochemical redox properties9. Amorphous hydrous RuO2 exhibits large specific capacitance than its crystalline counterpart as the intergranular water surface structure involves proton transport whereas the oxide allows electronic conduction10,11. Though hydrous ruthenium oxide displays excellent pseudocapacitive behavior with high reversibility and very high specific capacitance, less availability as well as high cost of this material poses limitations for its application. Loading of RuO2 in suitable carbon morphology can minimize its use, simultaneously enhancing the required electrochemical properties of ESCs. Carbon nanostructures like CNTs or carbon nano onions (CNOs) are unique electrode materials as they are much less limited by mass transfer kinetics, show lower resistance and improved performance due to exohedral curvature effect12. They can provide high specific power due to fast ion sorption/desorption on their outer surface and are ideal to enhance energy density by combining with RuO2. In the present study we report a nanocomposite of CNOs with hydrous ruthenium oxide as an electrode for ESC. We also demonstrate new flexible supercapacitor device based on this composite electrode. At first the composite material is tested in 0.5M H2SO4 aqueous electrolyte and demonstrate specific capacitance as high as 570 Fg-1 at current density of 1 A/g. Keeping in view of requirement of flexible, lightweight and efficient energy storage systems, we fabricated a novel, flexible, solid state ESC system. Wherein a conducting carbon paper (CCP) was modified into flexible CCP without disturbing its interconnected textural framework characteristics and facilitated excellent mass transfer. This solid state flexible device demonstrates specific capacitance of 305 Fg-1 at current density of 1 A/g. 3 ACS Paragon Plus Environment


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EXPERIMENTAL SECTION Materials. Ruthenium chloride trihydrate (RuCl3.3H2O) from Loba Chemie Pvt Ltd, Polyvinylidene fluoride (PVDF), Poly(vinyl alcohol) (PVA) from Sigma Aldrich, Sodium Hydroxide (NaOH), N-Methyl-2-pyrrolidone (NMP), sulfuric acid (H2SO4) from S. D. FineChem Ltd, India, Polydimethylsiloxane (PDMS) were used as received. Synthesis of RuO2/CNOs composite. CNOs were synthesized by low temperature CNO synthesis process using clarified butter13. The composite of RuO2/CNOs was synthesized by environment friendly sol-gel method using Ruthenium salt. Briefly, CNOs were dispersed in DI water, stirred at room temperature till the homogeneous dispersion occurred and RuCl3.3H2O was added to it. The pH level of the mixture was adjusted to neutral by adding NaOH; in aqueous media this NaOH reacts with the ruthenium precursor and forms RuO2 nanoparticles supported on the CNOs surface. Then the solution was continuously stirred for 6 hrs and was vacuum filtered. After washing several times with ethanol and DI water, the filter cake was dried at room temperature and calcined at 150 °C for 6hrs. Preparation of flexible conducting substrate and device assembly. Commercially available conducting carbon paper with the thickness of 370 µm was cut into strips of 1cm width and mechanically pressed using hot rolling machine at room temperature (gap between rolls was fixed to 14µm). In this process, the conducting carbon paper composed of carbon fibers, loses its brittleness and becomes flexible. A fine mixture of 10:1:2 ratios of Polydimethyl siloxane (PDMS) precursor, Curing agent and Acetylene Black (AB) was then coated on one of the sides of conducting carbon paper to hold the flexibility and subsequently it was dried on hot plate for 10 min at 95 °C. (Process depicted in Schematic 1.).

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To prepare polymer gel electrolyte (PGE), 1g of polyvinyl alcohol (PVA) was dissolved in 10 ml DI water and stirred at 85°C continuously for 1hr. Then 257µl of H2SO4 was added to it and kept for additional 1hr stirring. RuO2/CNOs composite with 5% PVDF binder in NMP was coated on the modified conducting flexible carbon paper. After drying, the electrodes were dipped into PGE and dried at room temperature overnight. Before making contact the electrodes were again wetted with PGE and brought into contact face to face with gel electrolyte sandwiched between two flexible electrode assemblies

Scheme 1. Schematic illustration of stepwise preparation of flexible conducting substrate and solid state supercapacitor device assembly using RuO2/CNOs electrodes separated by PVA/H2SO4 gel electrolyte. Structural and elemental characterization. X-ray diffraction (XRD) was obtained from diffractometer system, XPERT-PRO with Goniometer, PW3050/60 (θ/θ) equipped with CuKα radio generator and the scan range (2θ) was between 10 and 80°. Morphology of CNOs and RuO2/CNOs was studied by high resolution Transmission electron microscopy (HR-TEM) using 5 ACS Paragon Plus Environment


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Tecnai F30 FEG machine and Thermal gravimetric analysis (TGA) was performed on Perkin Elmer STA 6000 simultaneous thermal analyzer. Raman spectrum was recorded on HR 800 Raman spectrometer (Jobin–Yvon, Horiba, France) using 632.8 nm red laser (NRS1500W). Electrochemical measurements were all carried out by SP-300 multichannel electrochemical workstation (Biologic Science Instruments) at room temperature. Electrochemical studies based on Cyclic Voltametry, Electrochemical Impedance Spectra (EIS) and Galvanostatic charge discharge were all conducted through two electrodes system in aqueous 0.5 M of H2SO4 and solid state gel PVA/ H2SO4 electrolyte. All the calculated values were normalized to the weight of the active electrode material. RESULTS AND DISCUSSIONS Physicochemical characterizations. Fig 1a shows the Powder XRD spectra of CNOs and RuO2/CNOs nanocomposite. Prominent graphitic peaks of (002) and (101/100) observed in CNOs are almost faded after anchoring RuO2 nanoparticles on it. No diffraction peaks are observed for RuO2 due to amorphous nature of hydrous RuO2 which was calcined at low temperature of 150° C 14. Vibrational Raman spectrum was recorded for synthesized materials. Figure1b shows the Raman spectra of CNOs and hydrous RuO2/CNOs. D-band and G-band corresponding to CNOs are observed at 1325 cm-1 and 1600 cm-1 respectively in both the materials. In case of composite material three additional peaks for Eg, A1g, and B2g, modes of RuO2 are also observed at 513, 625 and 690 cm-1 respectively-1 15,16.


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Figure 1. (a) Powder XRD spectra (b) Raman spectra of CNOs and RuO2/CNOs (c) XPS spectra and (d) TGA curve of RuO2/CNOs Fig 1c shows the XPS spectra of Hydrous RuO2/CNOs composite which exhibits two major signals of C1s and O1s at 351 and 284 eV. As the metallic Ruthenium oxidizes, the binding energy of Ru 3d5/2 peak corresponding to the Ru4+ oxidation state shifts to higher binding energy than that of metallic Ru which was observed at 281.1 eV in the composite and it slightly overlapped with the C1s peak of supported CNOs17. This shift towards higher binding energy gives evidence of the formation of hydrous RuO2 in the composite18. Moreover, this hydrous nature of RuO2 in the composite was confirmed with the high resolution Ru 3d peaks (supporting information Fig S1) at two distinct binding energies 281.1 and 284.5 eV corresponding to Ru 7 ACS Paragon Plus Environment


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3d5/2 and Ru 3d3/2 respectively18. The O1s peak further contains discrete peaks of Ru-O-Ru at 529.1 eV and Ru-O-H at 530.56 eV19. In addition to these, 3p doublets at 463.7 and 485.8 eV, 3s signal at 587.5 eV, 4s at 76.3 eV and 4p at 46.1 eV are also observed for hydrous RuO2 in the XPS spectrum20-22. A low intensity Na peak was observed at 496 eV that can be attributed to the use of NaOH as one of the reaction precursors19.

Figure 2. HRTEM images of (a) CNOs and (b, c) RuO2 nanoparticles decorated on CNOs As of TGA curve, the total weight loss was examined from 25° to 1000° C, the initial weight loss was observed up to 142 °C due to physisorbed water. A slower weight loss as a result of chemisorbed and crystalline water was noted from 143 °C to 236 °C, which strongly envisage that the RuO2.nH2O was not crystallized when it was calcined at 150 °C for 6 h23,24. Further, weight loss was attributed to carbon and hence the overall weight percentage of RuO2 present in the composite was found to be 45.85% as shown in the fig 1d. HRTEM images (fig 2b, c) show the homogeneously distinguishable hydrous RuO2 nanoparticles on the surface of the CNOs with the average size of ~2.3nm. For comparison pristine CNO is also shown in the figure 2a. The strong interactions between RuO2 nanoparticles and CNOs surface is also observed as the oxygen-containing functional groups present on the surface of the CNOs are involved in the formation of highly dispersed RuO2 nanoparticles on the positive surface curvature of CNOs14. 8 ACS Paragon Plus Environment

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Electrochemical characterizations. For comparison we have carried out electrochemical measurements of as synthesized composite electrode material in an aqueous electrolyte at first (0.5M H2SO4).

Figure 3. (a) Cyclic voltametry curves for RuO2/CNOs at various scan rates ranging from 100 to 10 mV/s in the voltage range of 0 to 1 V respectively, (b) Galvanostatic charge/discharge plots of RuO2/CNOs at different constant currents ranging from 1 to 15 Ag-1 in the voltage range of 0-1 V in 0.5M H2SO4 electrolyte. (c) Nyquist plots of RuO2/CNOs composites with a frequency range from 0.1MHz to 0.1Hz. And the inset shows the high frequency region of impedance spectra and an equivalent Randle circuit (d) capacitance retention percentage of RuO2/CNOs in 0.5M H2SO4 electrolyte for 10000 cycles at the constant current density of 15 Ag-1 9 ACS Paragon Plus Environment


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The CV plots (Fig 3a) showing nearly rectangular mirror-image curves were taken at the scan rates of 10, 20, 50 and 100 mV/s indicating the facile ionic diffusion and low resistivity of the amorphous hydrous RuO2/CNOs nanocomposite. Fig 3b shows the corresponding Galvanostatic charge-discharge curves for the hybrid RuO2/CNOs electrode at various current densities of 1, 2, 5, 10 and 15 A/g with operating voltage from 0 to 1 V. The specific capacitance was obtained from Galvanostatic charge/discharge experiments by using the equation Ccd=2I/(-[ܸ݀/݀‫]ݐ‬m) ; Where Ccd is the specific capacitance (Fg-1) from Galvanostatic charge/discharge, I is the current applied, m is the active mass loaded on working electrode and [ܸ݀/݀‫ ]ݐ‬is the slope obtained from the discharge curve. The specific capacitances of 570, 557, 531, 516 and 509 Fg-1 were obtained at the current densities 1, 2, 5, 10 and 15 Ag-1 respectively. To evaluate the contribution of RuO2 to the electrochemical performance of hybrid RuO2/CNOs nanocomposite electrode, the specific capacitance of only RuO2 (45.85%) is calculated. It is found to be 1110 Fg-1 at the current density of 1 Ag-1 which was calculated by subtracting the contribution of pristine CNOs using the equation CRuO2 = [Ccd - CCNOs x (1- 45.85%)] / 45.85% 25. This higher specific capacitance of RuO2 is due to the homogeneously dispersed RuO2 nanoparticles (~2.3nm) on the CNOs which provide excellent contact of electrolyte and electrode interface. In fig 3c Nyquist plot of RuO2/ CNOs is shown which was used to evaluate internal resistance, charge transfer resistance and ion diffusion route. This electrochemical impedance spectroscopy (EIS) was carried out within the frequency range from 100 kHz to 100 mHz. At higher frequency, the internal resistance due to electrolyte solution and the contact of active material to current collector was 2.2 Ω and the semi-circle implies the charge transfer resistance of 0.25 Ω. Accordingly, the vertical straight line closest to the imaginary impedance axis called Warburg 10 ACS Paragon Plus Environment

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impedance occurred in the low frequency region which proves the dynamic diffusion of ions through the active electrode material under ac modulation. An electrical equivalent circuit or Randles circuit was used to fit the impedance spectra shown in the Fig 3c inset, comprising of 6 elements, including a constant phase element (CPE) which takes place of a capacitor due to the inhomogeneity of the film and it also accounts for double-layer capacitance26. Hence the total impedance can be written as ZTotal= Rs+Cdl/(Rct+W)+CL/RL Where Rs is the solution resistance, Cdl is the double layer capacitance, Rct is the faradic charge transfer resistances, W represents the Warburg impedance, CL is the leakage capacitance, and RL is the leakage resistance respectively. The cyclability test was conducted at a current density of 15 Ag-1, hydrous RuO2/CNOs exhibited excellent cycle life, retaining 88.7 % of initial capacitance even after 10000 cycles (Fig 3d) and the Coulombic efficiency was maintained at almost 100%. In addition to the double layer charge storage on the multilayer fullerene, the uniformly distributed metal oxide nanoparticles are most likely to be responsible for low resistive charge transfer and thus deliver a very high reversibility with good electrochemical stability. A symmetric solid state flexible supercapacitor device was fabricated using two RuO2/CNOs electrodes, where the polymer mixture of PDMS/AB holds the flexibility of the device which can therefore, be bent as well as twisted. As a result, the device will not be structurally destroyed or deformed as shown in the illustrated scheme 1 and the conducting carbon substrate supported by the polymer can act as a current collector.



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Figure 4. (a) cyclic voltametry curves for the flexible solid state supercapacitor device with RuO2/CNOs nanocomposite electrodes and PVA/H2SO4 gel electrolyte at various scan rates ranging from 100 to 1 mV/s in the voltage range of 0 to 1 V respectively, (b) Galvanostatic charge/discharge plots at different constant currents ranging from 1 to 15 Ag-1 in the voltage range of 0-1 V in PVA/H2SO4 gel electrolyte. (c) Nyquist plots with a frequency range from 0.1MHz to 0.1Hz. And the inset shows the high frequency region of impedance spectra and an equivalent Randles circuit. (d) Capacitance retention percentage of flexible solid state RuO2/CNOs supercapacitor electrolyte for 4000 cycles at the constant current density of 15 Ag-1 The CV curves of the flexible solid state supercapacitor were obtained at various scan rates of 10, 20, 50 and 100 mV/s. As seen in Fig 4a CV curves are almost rectangular which demonstrate the excellent electrochemical performance and fast redox reaction of RuO2/CNO nanocomposite 12 ACS Paragon Plus Environment

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electrode with PVA/H2SO4 solid state gel electrolyte. Galvanostatic charge-discharge (Fig 4b) was conducted for the device at the current densities varying from 1 to 2, 5, 10 and 15 Ag-1 and the specific capacitances calculated as 305, 294, 280, 251 and 232 Fg-1 respectively. Before the first Galvanostatic charge-discharge cycle the EIS was carried out in the frequency range from 100 KHz to 100 mHz which showed the internal resistance of 4.69 Ω and increased after 4000 cycles to 5.6 Ω as shown in Nyquist plot of Fig 4c. Inset shows the equivalent series circuit used to fit the impedance spectra of the device. Further the cyclability test was carried out at 15 A/g scan rate up to 4000 cycles, which showed retention of 94.47% of initial capacity. First 500 cycles were measured by keeping the device in flat position and then it was bent for next 500 cycles as shown in fig 4(d) and again kept flat for remaining cycles. Due to the mechanical strain the capacitance decreases slightly and 8% loss in capacitance was observed after bending but still it retained 88.9% of initial capacitance with the Coulombic efficiency above 96% over all the cycles. Energy density and power density are the other important assessable parameters of electrochemical performance. The competence with which a capacitor performs work can be defined as the energy density where as the power density is said to be the rate of energy delivered per unit time.



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Figure 5. Ragone plot of RuO2/CNOs nanocomposite for the measurements carried out in aqueous electrolyte and in solid state device assembly. The energy and power densities were calculated from the Galvanostatic charge/discharge curves by using the formulae E= 1/8 CcdV2 and P= E/t respectively, where, Ccd is the specific capacitance and V is the potential window and t is discharge time in hrs after the IR drop. Fig 5 illustrates the Ragone plot of RuO2/CNOs nanocomposite in aqueous 0.5 M H2SO4 electrolyte and solid state PVA/H2SO4 gel electrolyte for the flexible device. In the case of solid state gel electrolyte the highest energy density of 10.59 Whkg-1 and the maximum power density of 4.475 kWkg-1 is obtained for hydrous RuO2/CNOs nanocomposite where as in 0.5 M H2SO4 aqueous electrolyte the attained highest energy density and power density is 19.78 Whkg-1 and 4.782 kWkg-1 respectively. CONCLUSION The nanocomposite of hydrous RuO2/CNOs was successfully synthesized by sol-gel method which led to the formation of homogeneously dispersed RuO2 nanoparticles on the CNOs surface. The homogeneity of the RuO2 nanoparticles onto the CNOs surface led to reaching a maximum capacitance of 570 Fg-1 for composite electrode at the current density of 1 Ag-1 in 14 ACS Paragon Plus Environment

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aqueous 0.5M H2SO4. A flexible solid-state energy storage device is fabricated based on this hydrous RuO2/CNOs nanocomposite electrode which displayed excellent electrochemical performance and 94.47% cycling stability even after 4000 cycles while maintaining the Coulombic efficiency at almost 96%. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XPS Spectra, FESEM images, HRTEM images, Charge-discharge curves and Calculation of Specific capacitance of RuO2 AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Author Contributions VKA carried out synthesis and electrochemical characterization. CKK carried out structural characterization of material. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Funds from CSIR-TAPSUN program on energy conversion and storage (Grant no. NWP0056) used to support the research of the manuscript. ACKNOWLEDGMENT



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Authors acknowledge the Council of Scientific and Industrial Research, New Delhi, India for providing financial support for this work through the CSIR-TAPSUN program on energy conversion and storage (Grant no. NWP0056). REFERENCES (1)

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For Table of Contents Use Only

RuO2.nH2O Nanoparticles anchored on Carbon Nano-onions: an Efficient Electrode for Solid State Flexible Electrochemical Supercapacitor M. Vedi Kuyil Azhagana, Chaitanya Krishna kamaja abc, Manjusha V. Shelke*abc

Synopsis

Sustainable, inexpensive flexible supercapacitor was fabricated with excellent electrochemical performance using hydrous RuO2/CNOs (Carbon nano-onions), separated with polymer gel electrolyte.


RuO2Â·nH2O Nanoparticles Anchored on Carbon ... - ACS Publications

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