High Energy Flexible Supercapacitor - Synergistic Effects of

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High Energy Flexible Supercapacitor - Synergistic Effects of Polyhydroquinone and RuO·xHO with Micro-sized, Few Layered, Self-Supportive Exfoliated-Graphite Sheets 2

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Vedi Kuyil Azhagan Muniraj, Pravin Kumari Dwivedi, Parikshit Tamhane, Sabine Szunerits, Rabah Boukherroub, and Manjusha V. Shelke ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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ACS Applied Materials & Interfaces

High Energy Flexible Supercapacitor - Synergistic Effects of Polyhydroquinone and RuO2·xH2O with Micro-sized,

Few

Layered,

Self-Supportive

Exfoliated-Graphite Sheets Vedi Kuyil Azhagan Muniraj†,‡,§, Pravin Kumari Dwivedi†,‡, Parikshit Shivaji Tamhane†, Sabine Szunerits§, Rabah Boukherroub*,§ and Manjusha Vilas Shelke*,†,‡ †Physical

and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune-411008,

MH, India ‡Academy of Scientific and Innovative Research (AcSIR), Gaziabad-201002, UP, India §Univ. Lille, CNRS, Centrale Lille, ISEN, Univ. Valenciennes, UMR 8520 – IEMN, F-59000 Lille, France KEYWORDS: Graphite exfoliation, ruthenium oxide, polyhydroquinone, ternary composite, solid-state flexible supercapacitor

ABSTRACT: An effective and straightforward route for tailoring the self-supporting, exfoliated flexible graphite substrate (E-FGS) is proposed using electrochemical anodization. E-FGS has essential features of highly exfoliated, few-layered, two dimensional graphite sheets with the size

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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of several tens of micrometres, interconnected along the axis of the substrate surface. The novel hierarchical porous structural morphology of E-FGS enables large active sites for efficient electrolyte ion and charge transport when used for a supercapacitor. In order to effectively utilize this promising E-FGS electrode for energy storage purpose, a ternary composite is further synthesized with pseudocapacitive polyhydroquinone (PHQ) and hydrous RuO2 (hRO). hRO is synthesized via a sol-gel route, while electro-polymerization is utilized for the electrodeposition of PHQ over E-FGS. Ultimately, the fabricated self-supporting E-FGS based flexible supercapacitor is capable of delivering areal specific capacitance values as high as 378 mF cm-2 at a current density of 1 mA cm-2. The addition of the pseudocapacitive component to the E-FGS texture leads to ~10 times increase of the electrochemical charge storage capability. The imposition of mechanical forces to this flexible supercapacitor device results in trivial changes in electrochemical properties and is still capable of retaining 91 % of the initial specific capacitance after 10,000 cycles. Alongside, the fabricated symmetrical solid-state flexible device exhibited a high energy density of 8.4 Wh cm-2. The excellent performance along with the ease of synthesis and fabrication process of the flexible solid-state supercapacitor device using PHQ/hRO/E-FGS holds promise for the large scale production.

 INTRODUCTION In recent years, tremendous efforts have been devoted to the development of high energy and power density electrochemical energy storage and/or conversion devices such as rechargeable batteries, fuel cells, solar cells and electrochemical supercapacitors (ES). In most cases, nanostructured materials were utilized owing to their high surface to volume ratio, leading to improved electrode material/electrolyte interface. Among these energy storage devices, ESs store


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charges either by fast kinetic, non-faradic capacitive, ionic adsorption or by surface redox pseudocapacitive faradic reactions. Both these charge storage mechanisms are associated with the interfaces of electrolyte solution and the active electrode material.1 However, the prospective handy, light weight, compact devices such as bendable displays, implantable, wireless medical contrivances require improvements in replacing conventional capacitors by mechanically flexible ESs. Electrochemical performance of ESs greatly depends on the electroactive materials such as electrical double layer capacitive (EDLC) carbon allotropes, pseudocapacitive metal oxides or their hydrous oxides like MnO2, RuO2, Fe3O4 and conductive polymers (CPs), etc.2, 3 Graphite, an allotrope of carbon, is a two dimensional (2D) planar structure consisting of π stacked graphene sheets that exhibit about 3.34 Å layer interspacing.4 Graphene is already well-known to be a promising electrode material for supercapacitor applications, mainly due to its incredible electronic properties as compared to other carbon-based materials. The combination of good electrical conductivity, low internal resistance, and high surface area makes graphene a competitive material for EDLCs. However, utilization of such graphene-based EDLCs is hampered by their low specific energy due to restacking of graphene layers on charge recycling.5 In the case of pseudocapacitive conductive polymers (CPs), when oxidation occurs, ions in electrolyte are transferred through the conjugated double bonds in the polymer backbone and these ions are released back into the electrolyte when reduction takes place.6 Therefore, in electrochemical capacitors based on CPs, pseudocapacitance by Faradic redox is dominant, although double-layer capacitance is included in the total specific capacitance.7 CPs are comparatively cheaper than metal oxides, but exhibit a low cyclability due to swelling and shrinking encountered during charge/discharge cycles.8 Commonly used and extensively studied


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conducting polymers as pseudocapacitor materials are polyaniline (PANI), polyacetylene, polythiophene, polypyrrole, and poly (3, 4 ethylenedioxythiophene).3, 9, 10

Figure 1. Schematic illustration of redox reaction of polyhydroquinone. Among the known electro-active CPs, polyhydroquinone (PHQ) is rarely studied for ESs applications, although it has certainly the highest theoretical capacitance of 3034 F g-1, because PHQ suffers from its poor conductivity.11,

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As shown in Figure 1, PHQ consists of two

hydroxyl groups at the 1, 4 positions; hence, the overall redox reaction of PHQ consists of a twoelectrons transfer and a four-protons addition/elimination reaction, and thus the specific capacitance value for PHQ is larger compared to other conducting polymers.11 Conversely, hydrous RuO2 (hRO) is a widely studied proton induced pseudocapacitive metal oxide; it affords higher specific capacitance than that of the crystalline RuO2 counterpart.13 This highly reversible pseudocapacitance is due to the hydrous moiety involved in proton permeability and RuO2 allows electronic conduction, expressed as follows:14 RuOa(OH)b+δH+ + δe-RuOa-δ(OH)b+δ

Considering pros and cons of the above discussed materials i.e. graphene, PHQ and hydrous RuO2, we attempted to design a novel electrode material to effectively utilize their charge storage capabilities by overcoming their limitations. Before the fabrication of a solid-state flexible supercapacitor device using the ternary composite of PHQ/hRO/E-FGS, it is essential to know independent electro-active capacitive behaviour of exfoliated graphene in binary


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compositions with PHQ and hRO. More clearly, a highly conductive flexible graphite substrate (FGS) is first prepared through electrochemical exfoliation and results in direct transformation of graphite to a few layered graphene. Each ordered 2D graphene layer is interconnected to the bulk of the substrate, facilitating maximum electronic conductivity. Hierarchical porous structure of exfoliated graphite layers enables efficient accessibility of electrolyte ions. Further, introduction of hRO nanoparticles protects the exfoliated graphene layers from restacking on longer galvanostatic recharging. Concurrently, the electro-polymerized PHQ improves surface wettability of pristine graphene. The synergistic effects of PHQ and hydrous RuO2 with the exfoliated graphite increased drastically the energy density of ES upon making their ternary composition. Unlike aqueous electrolytes used in the conventional supercapacitors, solid state polymer gel electrolytes such as poly (ethylene oxide) (PEO)/LiClO4, PEO/LiAlO2, PVA-H2SO4/H3PO4, Nafion, etc are being used in flexible supercapacitors.15-18 Among these, PVA is widely used polymer for solid state electrolytes as it is a hydrogel comprising of synthetic polymer and it can adhere to porous and water-absorbent surfaces, which possess good ionic conductivity, and are non-toxic and chemically stable.  EXPERIMENTAL SECTION Materials. Carbon black and ruthenium chloride trihydrate (RuCl3·3H2O) are purchased from Alfa Aesar. Graphite flakes, hydroquinone, ethyl cellulose, poly(vinyl alcohol) (PVA), Nmethyl-2-pyrrolidone (NMP) and Phosphate Buffered Saline (PBS) are obtained from SigmaAldrich. Sodium hydroxide (NaOH) and sulphuric acid (H2SO4) are purchased from S. D. Fine


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Chem Ltd., India. Sylgard 184 polydimethylsiloxane (PDMS) was obtained from Dow corning India Pvt Ltd. All chemicals and reagents were used as received. Fabrication of Flexible Graphite Sheets. A particulate composition of graphite flakes (coarse), graphite powder (ball milled) and conductive carbon black was thoroughly mixed with 2 % binder and ethyl cellulose in NMP to form uniform thick slurry by using mortar pestles. The composition of the composite was experimentally optimized to 51:4:2 of graphite flakes, graphite powder and C black. This slurry was then coated on copper foil by ‘screed coating’ technique. The copper foil with coating was dried on a hot plate at 130 C for 1 h till full solvent evaporation. It was then mechanically pressed using rolling machine at room temperature to produce well packed interconnected graphite flakes. A fine mixture of 10:1 ratio of PDMS precursor and curing agent was prepared and coated on to the surface of the graphite composite and left for 5 min till PDMS settles over the material. Subsequently, it was cured on a hot plate at 100 C for 20 min. After complete curing of PDMS, copper foil was peeled off to form a graphite composite coating on PDMS layer as conductive flexible graphite substrate (FGS). Exfoliation of Flexible Graphite Substrate. Graphite was exfoliated electrochemically by using a two-electrodes system. Highly conductive FGS was immersed into 1 M H2SO4 electrolyte solution and a platinum foil was used as an auxiliary electrode where graphite was connected to anode. Chronoamperometry method was used in this case and the potentials were applied in five steps 1, 3, 6, 8 and 10 V. The exfoliation of graphite on the FGS surface could be observed by naked eye as it readily turns black. Then the exfoliated FGS (E-FGS) was thoroughly rinsed with DI water until the pH of the electrode became neutral. Synthesis of hRO/E-FGS. E-FGS was immersed into 10, 20 and 40 mM of RuCl3.3H2O precursor solutions and kept under sonication for 1 h; the pH of the solution was adjusted to


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neutral by adding NaOH. The solution was continuously kept under agitation for an additional hour and then washed several times with ethanol and DI water. The hRO/E-FGS composite was then annealed at 150 C for 2 h. Preparation of Polyhydroquinone Modified Exfoliated Flexible Graphite Substrate (PHQ/E-FGS). Initially, PHQ polymer was electrodeposited over E-FGS and its electrochemical performance was assessed thoroughly. E-FGS was connected to anode and immersed into the electrolyte solution containing various concentrations (50, 80, 100, 120 and 140 mM) of HQ monomer in 0.1 M of PBS buffer solution. The pH of the electrolyte was 7.4. HQ was electrochemically polymerized by using three electrodes system at an over oxidation potential of 2 V. Ag/AgCl was used as reference electrode and Pt foil as an auxiliary electrode. Chronoamperometry method was performed at the said potential 2 V and the electrodeposition of polymerized HQ (PHQ) on E-FGS surface was controlled over various coulombic charges of 20, 25 and 30 C. Preparation of PHQ/hRO/E-FGS. The ternary composite was synthesized through electrodeposition of PHQ over hRO/E-FGS electrode surface which was used as anode; the oxidation potential was 2 V and the total applied charge was 25 C. During this electrochemical oxidation reaction, a colour change occurred in the electrolyte solution from transparent to brown, indicating the formation of PHQ (Figure S1). Preparation of PVA/H2SO4 Polymer Electrolyte Gel (PEG). 1 g of PVA was dissolved in 10 mL of DI water and kept at 85 C under continuous stirring for 1 h. Then 257 µL of H2SO4 was added and kept for an additional hour under stirring. Fabrication of a Symmetric Flexible Supercapacitor. The synthesized PHQ/hRO/E-FGS electrode was then cut into strips of 1 cm in width for further device fabrication. The electrodes


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were dipped into PVA/H2SO4 polymer electrolyte gel and dried at room temperature. The electrodes were wetted with PEG again and made into contact face to face. Electrochemical measurements were all carried out using an Autolab Potentiostat at room temperature. Cyclic voltammetry, electrochemical impedance spectra (EIS), and galvanostatic charge−discharge (GCD) curves were all conducted in a three-electrodes system in aqueous 1 and 4 M of H2SO4 where Ag/AgCl was used as reference electrode. A two-electrodes system was used for solid state device using PVA/H2SO4 gel electrolyte. All of the calculated values were normalized to the area of the active electrode material. Structural and Elemental Characterization. Scanning electron microscopy (SEM) was performed using FEI, Nova Nano SEM 450. Transmission electron microscopy (TEM) imaging was recorded using FEI, Tecnai F30, FEG system. X-ray diffraction (XRD) was conducted with a diffractometer system, XPERT-PRO with Goniometer, PW3050/60 (θ/θ) equipped with a CuKα radio generator, and the scan range (2θ) was between 10 and 80°. X-ray photoelectron spectroscopy (Thermo Scientific Kα+) equipped with a monochromatic AlKα X-ray (50eV) source was used to assess the chemical composition of the samples. UV absorption spectra were recorded on Analytik Jena (SPECORD 210 PLUS) UV-vis spectrometer. Sheet resistance of FGS was measured with a four‐probe conductivity meter (constant current source CCS-01), SES Instruments Pvt Ltd., India. Electrochemical Methods. Exfoliation of FGS and electrochemical measurements including CV and GCD for E-FGS, PHQ/E-FGS and hRO/E-FGS samples were carried out using PGSTAT-12 electrochemical workstation (Autolab, The Netherlands BV). Electrochemical measurements and GCD durability for PHQ/hRO/E-FGS were all carried out by an SP-300 multichannel electrochemical workstation (Biologic Science Instruments) at room temperature.


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 RESULTS AND DISCUSSION Commercially available carbon paper is commonly used as a current collector in supercapacitors owing to its high conductivity. However, it is extremely brittle and hence supercapacitors using carbon paper as a current collector find limited application in wearable, miniaturized and flexible electronic devices.19 Despite their relatively low conductivity, conducting polymers (CPs) represent an interesting alternative to carbon paper. To achieve a conductivity comparable to the existing current collectors, it is necessary to use carbon based material like graphite. It offers an alternative, which is concurrently cheaper to make it costeffective. As described in the experimental section, a highly conducting and flexible graphite substrate was fabricated and the resistivity measurement, carried out by four probe conductivity method, revealed a resistivity of 0.103 Ω cm. Figure S2 (a, b, c) depicts the SEM images of the flexible carbon current collector made up of highly conducting graphitic flakes, where the graphitic flakes are highly oriented and have ordered inter-connections without any breaks all over the substrate. The lateral size of the graphite flakes is several tens of micrometers. The cross-sectional view, shown in Figure S2d, evidences that the conducting carbon film has well adhered to the PDMS and thus, the polymer provides flexibility and mechanical support to the film. As reported earlier, the electrochemical exfoliation was carried out by applying a stepwise voltage (1, 3, 6, 8 and 10 V). In order to wet the graphite and allow the hydrated (OH-) ions to intercalate between graphite sheets, low bias potentials of 1 and 3 V were first applied to FGS (working electrode) for 30 s.20 Gradual exfoliation of FGS begun at the potential range from 6 V where SO42- ions intercalate into weakly bonded grain boundaries of graphite and bias potential was further increased to 8 and 10 V.21 The high DC voltage (10 V) was maintained for 30 s until


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the exfoliation process was completed. During this anodization process, excess of oxidized graphite exfoliated from FGS surface started dispersing into the electrolyte solution; E-FGS surface became black with a spongy-like texture, as shown in Figure 2a. Since H2SO4 was used as electrolyte, laterally large micro size layered graphene sheets were obtained (Figure 2).22 From the cross-sectional SEM images in Figure 2b-d, we can observe outreached 2D graphene sheets, which are highly exfoliated into numerous layers due to the large c-axis expansion.

Figure 2. SEM images of E-FGS, (a) surface topography of E-FGS with outreached exfoliated graphite sheets and (b, c and d) showing micro sized exfoliated 2D graphite sheets at different magnifications. TEM analysis showed evidence for the formation of characteristic hexagonal crystalline structure of few layered graphene sheets, most frequently observed for two to few layers (Figure 3). The interplanar distance measured from TEM is ~0.36 nm, which corresponds to the (0 0 2) plane of graphite. Furthermore, the crystallographic orientation of exfoliated graphite sheets was investigated with Selected Area Electron Diffraction (SAED) patterns obtained at different grains of the same sample. Figure 3c reveals the most commonly found Moiré patterns of


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bilayered graphene as two sets of six fold symmetrical hexagonal lattice patterns twisted with an angle of ~7. This means that bilayer graphene exists in polycrystalline structure with near/minimal amount of AB or Bernal stacking order and absence of AA-stacked graphene sheets23, 24. On the other hand, the diffraction pattern, recorded at different domains (Figure 3f), depicts multi-stacked hexagonal patterns of graphene layers with less preferred angular orientation (twisting angles

High Energy Flexible Supercapacitor - Synergistic Effects of

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