S Codoped Flexible Graphene Paper for High

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Self-assembled N/S co-doped Flexible Graphene Paper for High Performance Energy Storage and Oxygen Reduction Reaction Taslima Akhter, Md. Monirul Islam, Shaikh Nayeem Faisal, Enamul Haque, Andrew I. Minett, Hua Kun Liu, Konstantin Konstantinov, and Shixue Dou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10545 • Publication Date (Web): 04 Jan 2016 Downloaded from http://pubs.acs.org on January 4, 2016

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ACS Applied Materials & Interfaces

Self-assembled N/S co-doped Flexible Graphene Paper for High Performance Energy Storage and Oxygen Reduction Reaction

Taslima Akhter, a Md. Monirul Islam, a Shaikh Nayeem Faisal, b Enamul Haque, b Andrew I. Minett, b Hua Kun Liu, a Konstantin Konstantinov, a* and Shi Xue Dou a a

Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials (AIIM)

Facility, University of Wollongong, Innovation Campus, North Wollongong, NSW 2522, Australia. b

Laboratory for Sustainable Technology, School of Chemical & Bimolecular

Engineering, The University of Sydney, NSW, Australia. *

Email: [email protected]

Abstract A novel flexible three-dimensional (3D) architecture of nitrogen and sulfur co-doped graphene has been successfully synthesized via thermal treatment of a liquid crystalline graphene oxide – doping agent composition, followed by a soft self-assembly approach. The high temperature process turns the layer-by-layer assembly into a high surface area macroand nanoporous free-standing material with different atomic configurations of graphene. The interconnected 3D network exhibits excellent charge capacitive performance of 305 F g-1 (at 100 mV s-1), an unprecedented volumetric capacitance of 188 F cm-3 (at 1 A g-1) and outstanding energy density of 28.44 Wh kg-1 as well as cycle life of 10000 cycles as a freestanding electrode for an aqueous electrolyte, symmetric supercapacitor device. Moreover, the resulting nitrogen/sulfur doped graphene architecture shows good electrocatalytic performance, long durability, and high selectivity when they are used as metal-free catalyst for the oxygen reduction reaction. This study demonstrates an efficient approach for the 1 ACS Paragon Plus Environment


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development of multifunctional as well as flexible 3D architectures for a series of heteroatom-doped graphene frameworks for modern energy storage as well as energy source applications. Keywords Liquid crystalline, Self-assembly, Co-doped graphene, Flexible, Supercapacitor, Oxygen reduction reaction. Introduction The increasing demand for modern, portable, and flexible electronic devices has highlighted the research to develop miniature energy sources, such as supercapacitors, fuel cells, Li-ion batteries, etc.1-3 The practical application of these energy sources has necessitated the quest for lightweight, high-performance, and cost-effective multifunctional electrode materials. Modern research has led to the development of various important new electrode materials and enhanced the performance of classical materials to obtain both higher energy and higher power densities for fabricating high performance energy sources.4-6 Considering the various materials, carbonaceous materials have gained the most attention as promising electroactive components for power generation sources.7-9 Graphene is a novel carbonaceous nanomaterial consisting of sp2-hybridized single layer carbon atoms packed in a hexagonal lattice. It exhibits fascinating physicochemical and electronic properties.10 These outstanding properties of graphene have made it a heavily explored and most promising electroactive material among all the carbonaceous materials applicable to energy conversion, energy storage, electrocatalysis, sensors, and modern electronics.9,

11-12

Alternatively, these properties can be manipulated to obtain enhanced

electrochemical properties by substitutional chemical doping via insertion of heteroatoms (N,

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S, B, and P) into the sp2-hybridized carbon framework.13-17 The dopant atom generates an activation region on the graphene surface by influencing the spin density and the charge distribution of neighboring carbon atoms,16, 18 which tailors the electroactivity of the surface by providing charged sites favorable for O2 adsorption and makes it suitable for electrocatalytic applications such as promoting the oxidation reduction reaction (ORR).14, 18-19 Moreover, this activation intensifies the native electric double layer capacitance (EDLC) of graphene through the generation of additional pseudocapacitance at the interface and promotes its application as an energy storage material.15, 20-22 Among the heteroatoms doped into the graphene lattice, nitrogen doped graphene (NGr) has shown superior electrochemical properties by forming three major bonding configurations of pyridinic-N, pyrrolic-N, and quarternary-N, which play important roles in increasing the conductivity and the pseudocapacitance as well as the electrocatalytic activity.13-17, 20, 23-27 Besides nitrogen, the effects of boron-doped graphene (B-Gr) and sulfurdoped graphene (S-Gr) have rarely been studied for charge storage application, although Sgraphene can act as an efficient metal-free cathode catalyst for the ORR in alkaline media.2831

Alternatively, co-doping with two different heteroatoms (especially, but exclusively,

nitrogen/sulfur) can induce more powerful active regions on the graphene surface for better electrocatalytic and electrocapacitive activity.32-35 Nevertheless, most of the reported techniques to produce heteroatom doped or co-doped graphene result in a final material in powder form and require binding agents such as Nafion® or polyvinylidene difluoride (PVDF) for electrode fabrication, which may have effects on the actual performance of the active materials and are hard to scale up for portable device fabrication.13,

16

Compare to

powdery material, free-standing heteroatom-doped graphene paper prepared as electrode material by an easily scaled-up process avoids the conventional tedious processing involved in electrode fabrication and thus, shows promising potential for practical applications.36 3 ACS Paragon Plus Environment


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Different synthesis methods have been applied to prepare heteroatom-doped graphene paper, although most of them are substrate-dependent, require several steps or external force to assemble, require multiple precursors, and are hard to scale up and unsuitable for portable or flexible device fabrication.23,

33, 36-39

The synthesis of substrate free N/S co-doped self-

assembled flexible graphene paper for capacitive energy storage and electrocatalytic applications has not been previously reported, however, to the best of our knowledge. In this present work, a flexible nitrogen/sulfur co-doped graphene paper (N/Sgraphene paper) has been prepared in a one-pot thermal treatment from liquid crystalline graphene oxide (LC GO) and an N/S dual precursor. 3-aminobenzenesulfonic acid (H2NC6H4SO3H), which was used as the novel dual doping agent, as it is easily soluble and truly miscible with a crystalline aqueous dispersion of graphene oxide (GO). The outstanding self-assembly approach at lower temperature for these ultra-large LC GO sheets results in a self-assembled layer-by-layer architecture, in which the doping precursor is entrapped. The interlayer precursor decomposes to reactive N/S species at higher temperature, which directly react with oxygenate groups of GO. In addition, the layer-bylayer self-assembly results in a porous, flexible, and free-standing three-dimensional (3D) architecture due to the evolution of interlayer N/S species. The obtained binder-free 3D architecture of N/S-graphene electrode resulted in excellent conductivity, promising energy storage capability, and efficient electrocatalytic activity due to the synergistic effects of N and S co-doping, making it a simply processed, multifunctional flexible paper, composed of an N/S co-doped graphene network for application in flexible integrated devices.


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Experimental Materials Liquid crystalline graphene oxide (LC GO) was prepared by following our previously reported method.40-42 3-aminobenzenesulfonic acid was purchased from Sigma-Aldrich and used as supplied. Preparation of N/S co-doped flexible graphene paper Nitrogen/sulfur co-doped flexible graphene paper (N/S-GP) was prepared from the homogeneous dispersion of ultra-large LC GO and 3-aminobenzenesulfonic acid (ABS) via thermal treatment, followed by a simple amphiphilic self-assembly approach. A given amount of LC GO aqueous dispersion (50 mg, 5 mg/mL) was mixed with ABS in a 1:1 ratio by stirring in open air for 12 h to form a homogeneous dispersion. The composite dispersion was then placed on a ceramic quartz boat in a tubular furnace that was first heated to 80 °C for 2 h and then 850 °C for 1 h under flowing argon with a heating rate of 5°/min to obtain the N/S co-doped flexible graphene paper, herein denoted as N/S-GP. Graphene paper (GP) was prepared from the pure LC GO dispersion under similar conditions for comparison. Graphene oxide paper (GO paper) was cast on a Teflon mold from LC GO at room temperature to compare the structural morphology. The resultant N/S-GP was cut into 2 cm × 1 cm sizes (weight ≈ ± 2 mg) to use directly as a free-standing electrode for the threeelectrode electrochemical study and 1 cm × 1 cm sizes (weight ≈ ± 1 mg) to use directly as free-standing working electrodes in the supercapacitor device. Materials and electrochemical characterization The cross-sectional and surface morphology of the thermally treated N/S co-doped layer-bylayer graphene structure was revealed by field emission scanning electron microscopy (FESEM, JEOL JSM-7500FA) and atomic force microscopy (AFM, JEOL ARM200F) using an 5 ACS Paragon Plus Environment


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Ultrascan charge coupled device (CCD) camera and an energy-dispersive X-ray spectroscopy (EDS) detector (Bruker, Massachusetts, USA). X-ray diffraction (XRD) patterns were collected on an X-ray diffractometer (XRD; GBC MMA, Cu Kα radiation, λ = 1.5406 Å). Xray photoelectron spectroscopy (XPS) was conducted on a PHOIBOS 100 hemispherical analyzer (pass energy of 26.00 eV, 45° take-off angle, and a beam size of 100 mm). Raman spectroscopy was conducted on a JOBIN YVON HR800 Confocal Raman system with 632.8 nm diode laser excitation on a 300 lines/mm grating at room temperature. Specific surface area was estimated using Brunauer–Emmett–Teller (BET) analysis on a NOVA 1000 (Quantachrome, Boynton Beach, Florida, USA). Thermogravimetric analysis (TGA) was carried out on a Mettler Toledo TGA/DSC1 under nitrogen atmosphere. The as-prepared N/S-GP cut to a size of 20×10 mm to examine the surface conductivity with JANDEL fourpoint-probe resistivity system (model RM3) at room temperature and 20 nA current. Electrochemical analysis was performed at standard temperature and pressure (STP) on a VMP3 Bio-Logic electrochemical workstation. The three-electrode measurements were carried out using a clamp-type stainless steel working electrode, an Ag|AgCl reference electrode, a platinum mesh counter electrode in a electrolyte solution of 6 M KOH, and a voltage range of 0.0 to –1.0 V. Symmetric supercapacitors (SCs) were assembled by sandwiching two identical pieces of N/S-GP (1 cm × 1 cm) electrode on indium tin oxide – polyethylene terephthalate (ITO-PET) film as current collector with Celgard membrane as the separator, with a potential window of 0.0 to 0.8 V. The separator was pre-dipped in a 6 M KOH electrolyte solution. For the oxygen reduction reaction analysis, small piece of film (GP or N/S-GP) was gently crushed in isopropanol with mortar and pestle. Required amount of isopropanol was added and sonicated for 1 h in bath sonication, followed by 15 min sonication in an ultrasonicator (Sonics, VC505), to make a proper dispersion of 1 mg mL-1 concentration. Certain amount of dispersion (12 µL = 12 µg) of required sample was


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deposited on a glassy carbon electrode (GCE) by simple drop casting method to develop film. The surface area of the GCE was 0.196 cm2. Further electrocatalytic analysis was performed in a N2-saturated or O2-saturated 0.1 M KOH solution at room temperature. Commercial Pt/C (20 wt%) electrode was used to compare the performance with the dual doped material.

Results and discussion Fabrication of the N/S co-doped flexible graphene paper The intrinsic self-assembly approach for nematic phase amphiphilic liquid crystalline (LC) materials allows the fabrication of a naturally aligned architecture in layer-by-layer form.43-44 Liquid crystalline graphene oxide (LC GO) sheets are also suitable for alignment in a specific direction due to the higher degree of anisotropy and high aspect ratio of graphene oxide (GO) sheets within the LC dispersion.45-47 Their amphiphilic nature and tendency to form an aligned layer-by-layer structure are more pronounced in the case of ultra-large LC GO sheets; they exhibit a high affinity towards minimizing the free energy of the system through the interactions of graphenic domains, so that a free-standing flexible paper can easily be fabricated by a simple casting technique.45, 48 Such beneficial properties can also be exploited to fabricate self-aligned heteroatom-doped composites with a higher degree of orientation, when the component contents do not interrupt the nematic phase morphology of the liquid crystals. An aqueous homogeneous dispersion of ultra-large LC GO was mixed with the nitrogen/sulfur containing reagent 3-aminobenzene sulfonic acid (ABS) to obtain a pure dispersion of LC GO-ABS [Figure 1(a)], while maintaining the LC property of the GO sheets.45 During this process, ABS interacts with functional groups on the GO surface. The ultra-large sheet structure of GO was maintained by avoiding ultrasonication. Upon thermal treatment of this LC GO-ABS dispersion in inert atmosphere, the inherent self-assembly characteristic of large LC GO sheets directed the formation of an aligned layered architecture 7 ACS Paragon Plus Environment


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at lower temperature (40-80 °C), and the doping agent ABS was entrapped between GO layers [Figure 1(b)] due to the hydrogen bonding with sulfonic groups and the electrostatic interaction with positively charged amino groups.22 At higher temperature (300-800 °C), the decomposition of the N/S-precursor generated active N/S species which react with oxygenate groups on the GO sheet surface to form doped graphene. Moreover, the unidirectional selfassembled layered structure turns into a microporous flexible 3D architecture due to the effect of interlayer pressure originating from the N/S species evolution [Figure 1(c)]. The beneficial mechanical properties of the ultra-large N/S-doped graphene sheets ensure the possibility of processing them directly into free-standing as well as flexible N/S co-doped graphene paper (N/S-GP) with a large number of microporous channels, without any need for subsequent forces. 49

Figure 1. Schematic illustration of the preparation of nitrogen/sulfur dual-doped flexible graphene paper: (a) aqueous dispersion of LC GO and ABS, (b) self-assembled layer-bylayer structure of ultra-large GO and ABS at lower temperature, (c) highly porous 3D architecture of N/S-doped graphene after thermal treatment, (d) photograph of the flexible and free-standing N/S-GP (6 cm × 2 cm), and (e) atomic structure of the dual-doped graphene. 8 ACS Paragon Plus Environment

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Structural analysis of the N/S co-doped flexible graphene paper To better understand the evolution of N/S-doped graphene, the thermal profile of GO-ABS composite was studied by thermogravimetric analysis [Supporting information (SI), Figure S1]. Based on this analysis, the calcination temperature was set at 850 °C, so that GO could be effectively reduced to graphene and the doping precursor could decompose completely into nitrogen/sulfur-containing species 22, 29 that could provide the nitrogen/sulfur sources for finally evolving into N/S-GP. The cross-sectional morphology obtained by the FESEM images in Figure 2 reveals the free-standing interconnected framework of N/S-doped graphene sheets with a discernible porous structure. The pore size can be visualized as ranging from a few hundred nanometers to several micrometers [Figure 2(a, b)] and should result in an excellent specific surface area. The 3D architecture with meso- and micropores shows a type IV nitrogen adsorption-desorption isotherm with a distinct hysteresis loop [Figure S2(a)] 32, and the surface area was determined to be 385 m2 g-1 (calculated from the adsorption branch over relative pressure, P/P0 = 0.4 – 1), with an average pore diameter of 4 to 70 nm [measured from the Barrett-Joyner-Halenda (BJH) pore size distribution curve in Figure S2(b)]. In the case of graphene paper (GP), the sheet restacking and strong π–π interaction of the pristine graphene sheets during the high temperature deoxygenation process result in a layer-by-layer structure that is almost the same as that of self-assembled GO paper (Figure S3), which turns into a highly packed structure without porosity that has a small specific surface area of 89 m2 g-1 (Figure S3 and Table S1). The surface morphology of the doped graphene layers remains smooth [Figure 2(c)], like those of as-prepared thermally treated GP and graphene oxide paper (GO paper) (Figure S3), revealing that the evolution of



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interlayer active species only changes the atomic structure of graphene without any further effect on the layer surface.

Figure 2. (a-b) Cross-sectional FESEM images of the N/S-doped flexible graphene paper (inset: the high magnification FESEM image reveals the nonporous feature), (c) surface morphology of the doped graphene, (d) TEM image of the N/S-GP, and (e) high resolution (HR)TEM image of a dual-doped graphene sheet (inset: the corresponding SAED pattern of N/S-GP). N/S dual-doped graphene features a transparent sheet structures with wrinkled and folded features in the transmission electron microscope (TEM) images (Figure 2d). The sheet morphology is similar to those of the as-prepared GO and GP (Figure S3), and its partially crumbled nature may originate from deformation of the layered structure during heteroatom doping. The typical selected area electron diffraction (SAED) pattern with the six-fold symmetry of the corresponding doped graphene [Figure 2(e)] reveals the existence of fully ordered single layer, dual-doped graphene sheets.27,

35

The elemental mapping and atomic


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composition of S, N, and C in the doped graphene sheets is revealed by the EDS mapping in Figure 3. A homogeneous distribution of the elements S, N, and C is clearly observed on the graphene surface.

Figure 3. (a) Dark field TEM image of N/S-GP with the corresponding [C] carbon, [O] oxygen, [N] nitrogen, and [S] sulfur elemental mapping images. (b) EDS spectrum of the N/S-GP. XRD patterns of the flexible graphene architecture along with the graphene oxide paper are presented in Figure 4 to clarify the interlayer spacing and packing effects from the self-assembly approach. The characteristic diffraction peak of GO at 10.9° points to typical interlayer spacing (0.81 nm) arising from the interlayer oxygen functional groups on the graphene surface.

45, 49

In the case of dual-doped N/S-GP, the diffraction peaks appearing at

24.7° represent a decrease in the interlayer spacing back to 0.36 nm, revealing the removal of oxygen functional groups and the de-intercalation of doped graphene sheets in a similar way to pure graphene film (peaks at 26°, dspacing = 0.34 nm) in Figure 4.35 In addition, the thermal



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energy evolves during the high temperature treatment, and structural change is expected to occur in the basal plane of the carbon skeleton.

Figure 4. XRD patterns of (1) GO, (2) GP, and (3) N/S-GP. The relevant structural modification, elemental composition, and doped-bonding configurations in the N/S-GP were further studied by XPS (Figure 5, Figure S4, Table S2 and Table S3) and Raman spectroscopy (Figure S5). The XPS survey spectrum of N/S-GP (Figure 5a) compared to GP (Figure S4) shows four characteristic peaks at 164, 285, 400, and 533 eV that correspond to S 2p, C 1s, N 1s, and O 1s, respectively.

29, 32-33, 35

The high

resolution C 1s spectrum of N/S-GP [Figure 5(b)] in comparison with the C 1s spectrum of GP (Figure S4) can be deconvoluted into several peaks corresponding to C=C (284.4 eV, 52.89 at. %), C-C (285.2 eV, 30.84 at. %), C-S-C (283.8 eV, 2.75 at. %), C-N-C (287.3 eV, 5.62 at. %), C-O (286.6 eV, 6.91 at. %), and C=O/O-C=O (289.2 eV, 0.99 at. %).29, 32-33, 35, 48, 50

To further demonstrate the composition of the N-doped sites, the high resolution N 1s

spectrum of N/S-GP [Figure 5(c)] was fitted with three different component peaks at 398.1 eV for pyridinic N (44.17 at. %), 400.1 eV for pyrrolic N (35.04 at. %), and 401.5 eV for graphitic N (20.79 at. %) species.24, 26, 33, 35 The high resolution S 2p spectrum highlights the 12 ACS Paragon Plus Environment

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formation of thiophene-S structure between carbon and sulfur atoms, owing to their spin orbit coupling. The fitted S 2p spectrum of N/S-GP [Figure 5(d)] establishes three peaks at the binding energies of 163.9, 165.2, and 168.8 eV, in good agreement with S 2p3/2 (54.52 at. %), S 2p1/2 (29.18 at. %), and oxidized sulfur at the edges (-SOn-), respectively.

29, 32, 38

Based on

the above structural analysis, we designed the atomic structural scheme of the as-prepared N/S-GP in Figure 1e.

Figure 5. XPS spectra of N/S-GP: (a) total XPS survey spectra, (b) high resolution C 1s region (c), N 1s region, (d) S 2p region. The Raman spectra of as-prepared GO, GP, and N/S-GP in Figure S5 feature prominent peaks around 1602, 1591, and 1584 cm-1, respectively, which represent the characteristic G band due to first order scattering of sp2 carbon atoms, while the D band at 1344, 1333, and 1326 cm-1 for GO, GP, and N/S-GP, respectively, reflects the defective or disordered sites of sp3carbon atoms.34-35 The shifting of the G and D peaks is attributed to the 13 ACS Paragon Plus Environment


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restoration of the conjugated structure during pyrolysis and the electron-donating nature of the heteroatoms.51 Moreover, the intensity ratios of the D to the G bands (ID/IG) for GO, GP, and N/S-GP are 0.99, 1.09, and 1.28, respectively. The increase in the ID/IG ratio is an effect that arises from the defects due to the heteroatom doping on the graphene sheets and is further confirmation of successful dual doping.33, 52 Capacitive performance of the N/S co-doped graphene paper A three-electrode system as well as cyclic voltammetry (CV) was first used to explore the electrochemical performance of the N/S-GP free-standing electrode and the effects of N/Sdoping on the flexible graphene paper (Figure S6). A comparison of the cyclic voltammograms (CVs) of N/S-GP and GP electrodes in 6 M KOH electrolyte at a scan rate of 100 mV s-1 (Figure S6) shows a capacitive response that reflects ideal double layer capacitive behavior during the charge/discharge process, with no remarkable peak in the curves. The current separation observed for the doped graphene paper (N/S-GP) has increased massively compared to the pristine graphene paper (GP), revealing that the dual doping makes the paper electrode a more electroactive material. The specific capacitance (Cs) measured from the CVs of the three-electrode set-up was 362 Fg-1 for N/S-GP, whereas the capacitance of GP was 119 F g-1, as determined by using Equation S1 (Supporting Information) at a scan rate of 100 mV s-1 (Table S4). Such electrochemical behavior and the 3-fold increase of the specific capacitance of N/S-GP compared to GP clearly identify the dual doping by heteroatoms and its successful electron spin effect in the graphene structure.13 The three-dimensionally interconnected N/S-doped graphene network (evidenced in Figure 2), the highly porous conductive structure, including micropores or nanopores created by the high temperature doping after the liquid crystalline soft self-assembly approach (Figure S2), and the synergistic electrochemical phenomena 14 ACS Paragon Plus Environment

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resulting from the native electrochemical double layer capacitance, along with pseudocapacitance that is generated from the different bonding configuration in the doped graphene lattice, is a possible explanation for the electrochemical performance of the asprepared N/S-GP electrodes. Moreover, the insertion of nitrogen atom as Pyridinic N, Pyrrolic N and Graphitic N in the graphene atomic structure establish a positive charge density on the adjacent carbon atoms due to the higher electronegativity of nitrogen compare to carbon.16-17 On the other hand, sulfur atom including thiophene-S and oxidized S in the graphene atomic structure resonate charge density with highly active electron configuration to neighbor carbon atoms as the electronegativity of sulfur and carbon are similar.29 As a result of this N/S-doping, the planner atomic structure especially the carbon atoms of N/S-GP experience higher electron spin density as well as enhanced electroactive sites compare to pure graphene or activated carbon and reveal excellent electrochemical activity in addition electrical conductivity of 167 S cm-1 (listed and compared with previously reported related materials in Table S4). The inhibition of doped graphene sheet restacking (evidenced by XRD in Figure 4) in N/S-GP help to increase the surface area (385 m2 g-1) to ensure better electrode-electrolyte contact at the interface, a huge amount of charge storage space, and super-fast pathways for electrolyte ion penetration throughout the whole electroactive surface of the free-standing electrode by shortening the ion diffusion path even at higher scan rates.41 However, having lower specific surface area, as-prepared N/S-GP results excellent energy storage ability compare to activated carbon (specific capacitance ≈ 100 F g-1).53-54 Commercial activated carbons encompass very high surface area (≈ 1500 m2 g-1) but the greater the surface area the weaker the electrical conductivity and another issue is to access of whole internal surface area by the electrolyte ions which results inferior capacitive performance.55 Having dual heteroatom doped graphene of electron dense active carbon atomic structure, three-dimensionally interconnected continuous conductive channels for



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electrolyte ion shuttle as well as impact of faradic pseudocapacitance, N/S-GP reveals this superior charge capacitive performance. Thus, the optimum electrochemical activity and reversibility of the charge storage capability of N/S-GP electrodes were recorded at different scan rates (Figure S6), and not only did the current separation increase at higher scan rates, but the ideal CV curves also reflect the stability of ionic channels throughout the electrodes. The good stability of this free-standing N/S-GP electrode at very high scan rates (200, 500, and 1000 mV s-1) is due to the strong interactions among the ultra-large doped graphene sheets in the network and the 3D architecture with a huge amount of pores and conductive channels for electrolyte ion insertion at higher rates.

Figure 6. Charge capacitive performance of the assembled supercapacitor device using the free-standing dual-doped graphene electrodes in 6 M KOH electrolyte: (a) cyclic


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voltammograms of the device at various scan rates, (b) specific capacitance obtained from the CV study, (c) charge/discharge profiles of the device at different current densities, (d) specific capacitance of results from the charge/discharge study, (e) Ragone plots for the N/SGP symmetric supercapacitor device compared with other recently reported values for supercapacitor devices fabricated with doped graphene as well as aqueous and ionic electrolytes (inset: photograph of the assembled flexible symmetric supercapacitor); and (f) cycling performance of the assembled device at a current density of 5 A g-1 (inset: last 10 cycles of the study). To explore the performance of as-prepared N/S-doped flexible graphene architecture, a symmetrical two-electrode flexible supercapacitor device was fabricated with 6 M KOH electrolyte. The CV study of the assembled device at various scan rates in Figure 6(a) reveals near-ideal voltammetric responses and the electric double layer capacitive (EDLC) properties of the flexible doped-graphene framework in aqueous electrolyte. The slight deviation from ideal EDLC behavior could be the effect of pseudocapacitance generated from the dual heteroatom doping and the electron spin density effect in the graphitic structure.15,

20

The

specific capacitance (Cs) was calculated from the specific area of the CV curves at different scan rates and plotted in Figure 6(b). The flexible 3D architecture of the N/S-doped binderfree graphene electrodes in a supercapacitor device results in a specific capacitance of 305 F g-1 at 100 mV s-1, one of the highest capacitive values for a heteroatom-doped graphene material for supercapacitor application (Table S4). Notably, the capacitance shows only a slight decrease to 228 F g-1, even at the very high scan rate of 1000 mV s-1, suggesting excellent rate capability of the N/S-GP electrode in real applications. The galvanostatic charge/discharge (CD) study of the supercapacitor device at different current densities in Figure 6(c) shows that the anodic charging points are asymmetric to their cathodic discharging counterparts within a potential window of 0 to 0.8 17 ACS Paragon Plus Environment


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V. The slow discharging step with an extended tail as compared with charging process of the device exhibits higher Columbic efficiency (>100%) and high capacitive reversibility of the N/S-doped graphene electrodes.56 The electrolyte ions penetrate trough the nanopores of electrodes during charging process and form an interaction with the electroactive sites of the graphitic structure produced from the N/S-doping. Due to this effect, Faraday pseudocapacitive behavior may results on the surface of the electrodes to hold the charge and longer the time to diffuse back to the electrolyte solution leading to an extension for the discharge process as well as extended tail for the discharge curves.56-57 From the discharge step of the charge-discharge analysis, the specific capacitance (Cs) was calculated as 281, 235, and 199 F g-1 at current densities of 1, 5, and 10 A g-1 respectively [Figure 6(d)]. The minute decrease in the capacitance with increasing current density indicates the applicability and potential of the binder-free N/S-GP electrode for high-power operation.22 The energy density (E) and power density (P) of the symmetric supercapacitor device were calculated based on the total cell capacitance (CT, estimated from the galvanostatic discharge rates with equation S4) by using equation S6 and S7 (Supporting Information), and plotted on a Ragone plot [Figure 6(e)]. The highest energy density of 28.44 Wh kg-1 was achieved at the power density of 930 W kg-1, and an energy density of 17.76 W h kg-1 was retained even at the high power density of 16000 W kg-1. This is remarkably higher than the values reported for a series of heteroatom-doped graphene materials used as electrode for aqueous electrolyte based supercapacitors [Figure 6(e)]. Besides the gravimetric specific capacitance (Cs), the volumetric specific capacitance (Cvs) is an important parameter for these electrodes that is relevant for the design and fabrication of practical energy storage devices. The average density of the binder-free flexible electrode was calculated, based on the thickness measured by FESEM (Figure S7), to be ~ 0.68 g cm-3, resulting in the volumetric capacitance of 188 F cm-3 (at 1 A g-1), which is very 18 ACS Paragon Plus Environment

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promising for future practical supercapacitor application and comparable to previously reported nanostructured graphene-based supercapacitor materials, such as activated graphene, 58

graphene film,

sandwich,

62

59

graphene hydrogel

60

, laser scribed graphene,

and graphene-carbon nanotube (CNT) composites

61

63-64

porous carbon/graphene

. The superior capacitive

performance of the N/S-GP electrodes in aqueous solution could be attributed to the lightweight flexible skeleton of the ultra-large doped-graphene sheets, as well as high contents of the various nitrogen configurations in the hexagonal carbon structure.22,

24

Moreover, the configurations of S with its high surface area and large pore volume provided excellent electrical conductivity and pseudocapacitance, with favorable electrolyte penetration towards exceptional volumetric capacitance.16 To reveal the electrochemical consistency of the assembled device, we further conducted a CD study over 10000 cycles at constant current density of 5 A g-1 [Figure 6(f)]. The electrode retained around 95.4% of its initial specific capacitance performance after 10000 cycles. The charge/discharge curves for the last ten cycles [inset of Figure 6(f)] again shed light on the stability and high degree of reversibility of the N/S-GP electrodes in a supercapacitor device.



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Figure 7: Electrocatalytic performance of the N/S-doped graphene architecture: (a) cyclic voltammograms for N/S-GP electrodes in O2 and N2 saturated 0.1 M KOH solution at a scan rate of 50 mV/s, (b) LSV curves of N/S-GP electrodes in an O2 saturated 0.1 M KOH solution at different rpm and scan rate of 10 mV/s (inset: K–L plot of the N/S-GP electrodes based on the LSV curves), (c) comparison of LSV curves between GP, N/S-GP, and commercial Pt/C electrode at 1600 rpm rotation in oxygen-saturated 0.1 M KOH at a scan rate of 10 mV/s, and (d) chronoamperometric response of N/S-GP upon addition of various concentrations of methanol. Electrocatalytic performance of the N/S co-doped graphene paper The resonance effect obtained from the co-doping of N-doped carbon nanomaterials with secondary heteroatom, such as S, tailor the electronic structure of graphene to create active charged sites for oxygen interaction which is an ultimate way to trigger the electrocatalytic activity.19, 65 To evaluate the influence of dual heteroatom doping for oxygen interaction with doped active sites, the electrocatalytic properties of N/S-GP were explored with a view to its application as an electrocatalyst for the oxygen-reduction reaction (ORR) under alkaline conditions. Cyclic voltammetry of N/S-GP deposited GCE was conducted in both argon- and oxygen-saturated 0.1 M KOH solution at a scan rate of 50 mV s-1. In the argon-saturated electrolyte chamber, no oxygen reduction peak was observed from the cyclic voltammogram in Figure 7(a). In oxygen-saturated electrolyte, electroactive sites of N/S-GP interact with oxygen and results a well-defined peak at ~ –0.35 V which can be noted as oxygen reduction peak.13 This observation evidenced the creation of electron dense active sites on N/S-doped graphene surface and the suitability of the as-prepared flexible architecture as metal free electrocatalyst for oxygen reduction reaction. The electrocatalytic activities of N/S-GP and the related material GP were further investigated using rotating disk electrodes (RDE). Figure 7(b) shows the linear sweep voltammograms (LSV) of N/S-GP at different revolutions per 20 ACS Paragon Plus Environment

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minute (rpms) from 400 to 2025 in oxygen-saturated electrolyte at a scan rate of 10 mV s-1. An increase in the limiting diffusion current with the rotation rate was observed due to the shortened diffusion distance in the oxygen-saturated electrolyte at high rotation rates.14 The KouteckyLevich (K-L) plot (calculated from the K-L formula in the Supporting Information) of J-1 vs. ω–1/2 at a potential of –0.4 on the N/S-GP electrode [Figure 7(b) inset] exhibited good linearity with the R2 value of 0.998. The electron transfer number was 3.94 at –0.4 V, which indicated that a four-electron process, with water as the product, was the preferred pathway. A comparison of N/S-GP with GP and commercial Pt/C was conducted via linear sweep voltammograms at 1600 rpm [Figure 7(c)]. The ORR performance of N/SGP with respect to onset potential and limiting diffusion current appeared remarkably higher than that of GP and close to that of commercial Pt/C electrode. The generation of electroactive sites from the dual (nitrogen and sulfur) doping on the graphene structure and the free pathways in the three-dimensionally porous electrode surface (evidenced by the FESEM images in Figure 2) facilitate the penetration of the electrolyte ions, as well as promoting the transportation of oxygen and –OH to yield rapid kinetics and a large cathodic current response.23, 29, 32 The onset potential of N/S-GP at 1600 rpm was found to be -0.13 V and the limiting current -3.72 mA cm-2, which was close to those of commercial Pt/C (with the onset potential of Pt/C -0.08 V and limiting current of -4.56 mA cm-2) at the same rpm [Figure 7(c)]. The stability against methanol poisoning was tested by chronoamperometry, by inserting a high concentration of methanol over a period of time into an oxygen-saturated electrolyte [Figure 7(d)]. The generation of current after flowing oxygen was observed for N/S-GP and the successive additions of 1, 2, and 3 M methanol showed some alteration in the current immediately after the addition, but there was no definite impact on catalytic activity towards the ORR, whereas the catalytic activity of commercial Pt/C was destroyed after the addition of methanol.35 Even though the onset potential and current density of N/S-GP were



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slightly lower than for commercial Pt/C, the stability of N/S-GP against methanol crossover can make it suitable for practical application and make it a good candidate electrode for the cathode side in alkaline as well as direct methanol fuel cells.

Conclusions In summary, we have explored a simple, yet cost-effective self-assembly approach for the development of novel nitrogen/sulfur heteroatom-doped, three-dimensionally interconnected flexible graphene papers via thermal treatment of a liquid-crystal-mediated composite of graphene oxide with the dopant precursor. This efficient process reveals the simultaneous atomic incorporation of three types of nitrogen groups, pyridinic-N, pyrrolic-N, and graphitic-N, along with thiophene-like S in single graphene sheets. The resulting 3D network of N/S-doped graphene frameworks exhibits exceptional charge capacitive performance (volumetric capacitance of 188 F cm-3, energy density of 28.44 Wh kg-1, and 95.4% capacity retention after 10000 charge/discharge cycles) as a binder-free electrode for flexible aqueouselectrolyte-based symmetric supercapacitors. As an active electrocatalytic material for the oxidation reduction reaction, N/S-GP revealed high selectivity, long durability, and excellent electrocatalytic activity. The lightweight N/S-doped graphene framework and substrate-free electrode preparation make this approach suitable for large-scale electrode fabrication for advanced portable electronics, as well as wearable or portable modern energy devices. Acknowledgements The authors are grateful for their financial support from the Commonwealth of Australia, the Automotive CRC 2020, and the Australian Research Council (ARC) through Linkage Infrastructure, Equipment and Facilities (LIEF) grants, (LE130100051, LE120100104), and to the Institute for Superconducting and Electronic Materials (ISEM) for the use of its


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infrastructure as part of its in-kind support. Supporting information available: Detail discussion on thermogravemetric analysis, FESEM imags of graphene oxide as well as graphene paper, specific surface area anaysis by BET method, quantitative analysis by XPS and raman spectrum have provided in the supporting information. Comparison study of heteroatom-doped graphene materials used for asymmetric or symmetric supercapacitor electrode along with the equations to calculate the electrochemical performance of the single electrode and assembled supercapacior device have listed in the supporting information. This information is available free of charge via the Internet at http://pubs.acs.org/.

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