Biomass derived hierarchical 3D graphene framework for high performance energy storage devices

Journal of Electroanalytical Chemistry 849 (2019) 113388

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Biomass derived hierarchical 3D graphene framework for high performance energy storage devices Masoud Amiri ⁎, Farhad Golmohammadi Department of Chemistry, Faculty of Science, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran

a r t i c l e

i n f o

Article history: Received 7 May 2019 Received in revised form 3 July 2019 Accepted 16 August 2019 Available online 17 August 2019 Keywords: Biomass Graphene Porous carbon Hierarchical carbon composite

a b s t r a c t Graphene and biomass-derived porous carbon materials are the basic materials for designing of many electrochemical energy storage devices. In this regard, we presented an environment-friendly, simple and scalable method for combining the porous carbon, derived from Typha domingensis, as an almost freely available biomass source, and graphene to produce a hierarchical carbon composite with high porosity and conductivity. The physical characteristics of composite were studied by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), nitrogen adsorption/desorption isotherms, thermogravimetric analysis (TGA), Raman spectroscopy, field emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM). The electrochemical behaviors of the composite were investigated with two and three electrode systems. The composite used as a positive electrode, in two-electrode assembly, against a Vulcan XC-72R as the negative electrode in 6 M KOH as electrolyte. The specific capacitance of electrode was 277.5 F g−1 and it retained more than 94% of the initial specific capacitance after 5000 cycles of successive charge/discharge. The two-electrode capacitor had a specific capacitance of 45.8 F g−1 while it retained 93% of the initial specific capacitance after 5000 cycles of successive charge/discharge. Two cells in series lighted up LED lamps brightly. © 2019 Published by Elsevier B.V.

1. Introduction Our unlimited demand for energy leads us to find (advanced) materials that aside from their high ability for storage or energy converting, have environmentally friendly, simple and inexpensive preparation method, especially for large-scale production. Among all energy storage devices, the electrochemical supercapacitors, are considered as the most promising candidate presenting the superior capacitance, fast charge and discharge, with high power density as well as excellent cycling stability [1–5]. However, in large scale and practical application, supercapacitors still suffer from the low energy density. Since the energy density (E) changes with the increase of capacity of a capacitor (C) and its window's potential (V) (based on E ≡ CV2), presenting an electrode materials that meet these requirements, is the first and most important step for achieving an ideal energy storage device [6–8]. Graphene, which is well-known for its single layer of carbon atoms structure with honeycomb crystal lattice [9,10], plays a vital role in different fields of technology such energy storage, electrochemical biosensors, fuel cell catalysts, biological application etc. The reasons that have made the carbon materials, like graphene, a basic source for such field of researches are their high conductivity, excellent mechanical flexibility, significant high surface area and unique ability to form different ⁎ Corresponding author. E-mail address: [email protected] (M. Amiri).

https://doi.org/10.1016/j.jelechem.2019.113388 1572-6657/© 2019 Published by Elsevier B.V.

composites with wide list of materials. Due to these characteristics and well-dispersing properties in water, especially in its oxidized form (GO), graphene is considered as a most demanding material for electrical double-layer capacitors (EDLCs) [2,11–14]. The porous graphenebased materials with the flat open atomic structure can also facilitate the access of ions and electrolytes to the surface resulting in faster charging and discharging rate [15,16]. Nevertheless, the high tendency of graphene sheets for aggregation and restacking among the sheets hinders the further applications [17–19]. It means, graphene with theoretical specific surface area of 2630 m2 g−1 just shows, experimentally, a surface area far below this value [20,21]. So far, many methods been developed for synthesis of porous graphene based materials to reduce restacking resulting to higher surface area [22]. For example, by applying bacteria, chelated metal or nanotubes as a buffer (separator) between the graphene sheets, some porous graphene composites have been reported [10,23,24]. Aside from these facts that many of these reports are indeed interesting and scientifically valuable, these reports practically have a complex and very expensive procedure making them impractical for large-scale production. However, the procedure of preparation of many conventional methods still have some toxic materials and solutions and cannot be categorized in environmentally friendly or simple and inexpensive methods. Fortunately, nature, with long history of evolution, can inspire us as a generous source of (bio)materials to achieve many novel structures with extraordinary properties [25]. Here, biomass-derived materials

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such as chicken feathers, wheat straw, waste tea leaves, sunflower seed shell, corn grains, etc., can be used as precursors for producing hierarchical carbon-based materials [1,26]. For example, the mixture of melamine/potato residue, cotton/urea-melamine, graphene/oil palm empty fruit bunch, graphene/coffee grounds-derived, activated biomassderived from glucose-based-polymer and many other methods have been used for application in high performance supercapacitor [16,27–30]. These materials are almost freely available, and with renewable and sustainable sources, the harmful environmental impacts of many other conventional methods for producing a given product can be reduced [15,31]. A hierarchical porous carbon structures have especial porosity, from micro- to meso- and macropores, creating the best situations for high-performance supercapacitors and other electrochemical devices. In fact, the bigger pores (macro-pores) act as an accessible reservoir for an electrolyte, facilitating the diffusion via reducing the distance between the ions and pores, while the smaller pores (in size of meso- and micro) increase the surface area resulting in higher charge accommodation [32–35]. However, it has been reported carbon-based materials composited with (at least) two different materials, even have significantly better electrochemical performances especially as supercapacitors [12,36]. For an instance, applying some conductive carbons materials, with graphene such as carbon nanotubes create stable network, and in comparison with bare graphene, shows higher surface area and better ions transporting [18,37–39]. The porous material with such unique structures and special network is a base for engineering other composites. They can be composited with the catalysts components or the electroactive materials (such as transition metals) to create an especial catalyst or a powerful pseudo-capacitor [40,41]. Hence, developing these carbon base materials (composites) is a base for many technologies which relying on these kinds of materials. Here, we present a scalable template-free approach to hybrid biomass (derived from southern cattail or Typha domingensis or as it is well-known in Kermanshah, Iran, Karrapo (KP) that is a highly available plant in almost all around the world) and graphene to produce a novel hierarchical porous carbon graphene framework material. In this regard, we first prepared a gel-like material of KP and then mixed it with GO and after freeze-drying and pyrolyzing the final product was completed. As an application, the electrochemical properties of the composite, as supercapacitor, in form of two and three electrode assembly were investigated. The new carbon composite, as supercapacitor has a specific capacitance of 277.5 F g−1 (at current density of 0.5 A g−1) in 6 M KOH. After 5000 cycle of successive charge/discharge at current density of 8 A g−1, this electrode shows more than 94% of the specific capacitance of the first cycle. The two-electrode capacitor ability (similar to an asymmetric capacitor with two different electrode materials) of this composite, in window potential between 0 and 1.6 V, has an ultra-high specific capacitance of 45.8 F g−1 at the current density of 0.33 A g−1 (or 1 mA cm−2) and 40.6 F g−1 at the current density of 1.67 A g−1 (or 5 mA cm−2). The asymmetric electrode can retain the 93% of its initial specific capacitance after 5000 cycles. The results prove that the KP-GO-F-P composite has a unique electrochemical behavior, with practical environmentally friendly features suitable for supercapacitors. It also can be a promising composite for other electrochemical applications like biosensors or catalysts.

water was used to prepare all the aqueous solutions. KP was picked up from the bank of a local river in Kermanshah, Iran. 2.2. Synthesis of GO The graphene oxide (GO) prepared by modified hummer method [46]. 2.3. Synthesis of KP-GO-F-P First, the KP fibers were separated from its flower and (4.0 g) was mixed in KOH for 2 h at 80 °C. Then the mixture (had a dark red color) separated by a centrifuge (4000 rpm, 4 min) and dispersed in deionized water and centrifuged (five times repeated) to remove the remains of KOH and moderate the pH of KP (about = 10–11) and at the end, it was dispersed in 100 ml deionized water. The 0.2 g GO was dispersed in 25 ml deionized water and sonicated for 2 h. The 20 ml of alkaline treated KP was added to GO mixture and stirred for 1 h. This mixture was freeze-dried and finally was pyrolyzed in a tube furnace at 700 °C for 3 h under argon atmosphere. 2.4. Material characterization The compositional and structural information of samples were investigated by X-ray diffraction (XRD) and Fourier Transform infrared spectroscopy (FTIR). The surface morphology was studied by a field emission scanning electron microscopy (FESEM) equipped with an energy-dispersive X-ray spectrometer (EDS) and transmission electron microscopy (TEM). Thermogravimetric analysis (TGA) was carried out with a thermogravimetric analyzer at a heating rate of 10 °C min−1 from 30 to 980 °C in oxygen to estimate the carbon content of the KPGO-F-P composite. Nitrogen adsorption and desorption isotherms studies were carried out at 77 K and the pore volume, pore size distribution and specific surface area were determined via the Brunauer-EmmettTeller (BET) method and Barrett-Joyner-Halenda (BJH) method. Raman spectra were recorded at room temperature with a microRaman system using an excitation wave-length of 514 nm. 2.5. Electrochemical measurement An electrochemical cell with three electrode system with 6 M KOH aqueous solution as the electrolyte at room temperature was used for electrochemical studies. A set of VSP 300 electrochemical workstation was used for all the electrochemical measurements. A platinum wire and a SCE electrode were employed as a counter electrode and a reference, respectively. The samples of KP-GO-F-P were mixed with polytetrafluoroethylene (PTFE), and ethanol as solvent to form a homogeneous slurry. Using the solution casting method, the as prepared slurry (1.0 mg) was coated on the 1 × 1 cm2 area Ni foam electrode and dried overnight at 120 °C to obtain the working electrodes. The electrochemical tests were cyclic voltammetry (CV), galvanostatic charge-discharge (CD) and electrochemical impedance spectroscopy (EIS) measurements. The EIS studies were measured in a frequency range of 100 kHz–0.01 Hz at open circuit voltage of 5 mV amplitude. The specific capacitance (Cs) of all the electrodes were calculated from CD curves according to the following Eq. (1) [47]:

2. Experimental section I  Δt ms  ΔV

ð1Þ

2.1. Chemicals and materials

Cs ¼

The chemicals, such as KOH, HCl, absolute ethanol (Aldrich), Vulcan XC-72R (Cabot) and Polytetrafluoroethylene (PTFE) were analytical grades. The Ni foam (~ × 1 cm) was cleaned with water/ethanol and then sonicated in solution of HCl (~1 M) for 5 min and finally washed with excessive deionized water successively [42–45]. The deionized

where Cs represents the specific capacitance of the electrodes (F g−1), I is the discharging current (in ampere), Δt, ms and ΔV are the discharge time (s), mass of the active materials (g) and the potential window (V), respectively.

M. Amiri, F. Golmohammadi / Journal of Electroanalytical Chemistry 849 (2019) 113388

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Scheme 1. Schematic figure of the formation process of KP-GO-F-P composite.

2.6. Fabrication of the asymmetric supercapacitor (ASC) The ASC device was prepared by using KP-GO-F-P and commercial Vulcan XC-72R (VC) as the positive and negative electrode material, respectively, which is specified as KP-GO-F-P//VC. For asymmetric system, a solution of 6 M KOH was used as the electrolyte with a cellulose paper as a separator. In order to balance the charge quantity (Q) the two positive and negative electrodes the optimal mass ratio of electrodes was calculated according to Eq. (2) [48]: mþ C −  ΔV − ¼ m− C þ  ΔV þ

ð2Þ

where C (F g−1) is the specific capacitance of electrodes while the ΔV (V) and m (g) are, discharge potential range and active mass of each

single electrode. The electrochemical performance of VC electrode presented in Fig. S1. The specific capacitances in the ASC device (CA, F g−1) (based on CD results) can be calculated according to Eq. (3) [48]:

CA ¼

I  Δt mt  ΔV

ð3Þ

where I (A) is the current value and Δt (s) is the time during the discharge process. mt is the total active weight on both the negative electrode and positive electrode (m− + m+); ΔV (V) represents the potential change excluding the potential drop. The energy densities (E, Wh kg−1) and power densities (P, W kg−1) of the ASC device were estimated based on following Eqs. (4) and (5),

Fig. 1. A) The SEM image of GO-F-P. B) The SEM image of KP. C) The SEM image of KOH-treated KP. D) The SEM image of KP-GO-F. E) The SEM images of KP-GO-F-P in two magnifications. F) The TEM image of KOH-treated and pyrolyzed KP. G) The TEM image of KP-GO-F-P. H) The TEM image of GO-F-P.

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3. Results and discussion

respectively [48]:

E ¼ CA 

P¼

ðΔV Þ2 2  3:6

3600  E Δt

3.1. Fabrication of hierarchical carbon materials ð4Þ

ð5Þ

where CA (F g−1) is the calculated specific capacitance value of the ASC, ΔV (V) and Δt (s) are the voltage variation and lasting time during the discharge process.

The fabrication of hierarchical structure of carbon material is started from KOH treated KP fibers (Scheme 1) to form a gel-like material. First, the KP was mixed in KOH for 2 h at 80 °C and then it was separated by a centrifuge (4000 rpm, 4 min) and dispersed in deionized water and centrifuged (five times repeated) to remove the remain of KOH and moderate the pH of KP (about = 10–11). During this process the chemical reaction between KP and KOH resulted in formation of more carbonyl and carboxylic groups [49]. Unlike the alkaline treated KP, the GO (prepared by hummer method) has an acidic nature pH (about 3–4). Hence,

Fig. 2. A) The BET and pore distribution (inset) of KP-GO-F-P. B) The FT-IR spectrums GO. KOH-treated KP, KP-GO-F and KP-GO-F-P. C) The Raman spectrums of GO and KP-GO-F-P. D) The XRD pattern of GO-F-P and KP-GO-F-P. E) The TGA diagrams of KP-GO-F-P and GO-F-P. F) The EDX diagram of KP-GO-F-P.

M. Amiri, F. Golmohammadi / Journal of Electroanalytical Chemistry 849 (2019) 113388

the addition of acidic GO to the alkaline treated KP can simply form a complex via hydrogen bonds or electrostatic forces. Subsequently, the freeze-drying was applied to produce a macro-porous complex. Finally, the pyrolysis step leads to increase further porosity and conductivity of KP-GO-F-P. 3.2. Characterization of KP-GO-F-P As it presented in Scheme 1, the preparation of KP-GO-F-P has three steps. As it was discussed before, one major issue with graphene is the restacking of its sheets. Fig. 1A is the SEM image of graphene prepared with same method as the KP-GO-F-P, except in this case, the KP was omitted (GO-F-P). As the image illustrates, the graphene does not

5

show its characteristic separated sheets and it seems that most of the graphene sheets accumulated together which, in turn, can reduce the porosity of the resultant graphene. Fig. 1B is the SEM image of KP before the treatment of KP in KOH, the KP fibers showing semi-cylinder shape structures, but after KOH processing, the KP has turned into a shrunk structure (Fig. 1C). Interestingly, after socking the KP in KOH solution, it formed a gelatin form. This gel can act as a buffer (separator) to prevent the restacking of graphene sheets. After mixing the KP and GO, the SEM image (Fig. 1D) shows the GO sheets warped around the KP fibers (KP-GO-F). Finally, the morphology of freeze-dried and pyrolyzed KP-GO-F-P is presented in Fig. 1E illustrating a porous 3D structure with separated sheets. The TEM images reveal more details about the KP-GO-F-P composite. The alkaline treated KP, after process of pyrolysis

Fig. 3. A) The CVs of GO-F-P and KP-GO-F-P electrodes in solution of 6.0 M KOH with scan rate of 100 mV s−1. B) The CVs of KP-GO-F-P in different scan rates. C) The CDs of KP-GO-F-P electrode at various current densities from 0.5 to 25 A g−1 (up and right: illustration of iR drop). D) The rate capability of KP-GO-F-P electrode, at current densities of 0.5 to 25 A g−1. E) The long-term cycling stability of KP-GO-F-P and GO-F-P electrodes after 5000 cycles. F) The Nyquist plot of experimental impedance data (scattering dots) and fitting result (solid line) for the KP-GO-F-P electrode. The inset show the electrical equivalent circuit used for fitting the impedance spectra.

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(prepared in process analogous to the process of KP-GO-F-P but without graphene (Fig. 1F)) has a highly porous structure with pore size of 10– 40 nm. Fig. 1G shows the KP-GO-F-P composite after compositing the alkaline treated KP and GO and pyrolyzing at 700 °C. As the TEM study shows, the highly porous KP, was wrapped by the graphene sheets. In fact, a comparison between this image and the TEM image of freezedried and pyrolyzed, GO-F-P (Fig. 1 H), proves that the graphene sheets are almost transparent and well-formed while the KP in the KP-GO-F-P composite is wrapped by graphene sheets. The graphene with its high conductivity and high surface area can significantly improve the function of a supercapacitor device. The composite of highly porous KP and graphene can even create the unique condition for an electrochemical supercapacitor, which its functionality is directly improved by increasing of conductivity and surface area. The porous structure of KPGO-F-P was studied BET experiment indicating very high surface area (815 m2 g−1) with the relevant pore diameter about 11–15 nm (Fig. 2 A). The FT-IR spectrums (Fig. 2B) of GO and KP (treated with KOH), KPGO-F are contained some bands at about 1685 cm−1 and 3440 cm−1 which can be attributed to the stretching vibration of CO and COOH groups. After the pyrolysis, the intensity of these bands due to the reduction in the oxygen content of KP-GO-F-P is dramatically decreased. The exiting of oxygen, during the pyrolysis process, can be resulted in the reduction of GO and decrease of the band intensities [50]. The Raman spectroscopy (Fig. 2C) shows the Raman spectrums of GO and KP-GO-F-P. The D (1379 cm−1) and G (1624 cm−1) bands are corresponded to the sp3 and sp2 respectively. The ratio of ID/IG in GO is about 0.77 but after the pyrolysis in KP-GO-F-P this ratio is up to 1.3 indicating to increase of sp2 hybridization in structure of the carbon atoms which has a better conductivity [51]. The XRD pattern of GO-F-P (Fig. 2D) has wide diffraction peak (002) at the 2ϴ about 22.0° indicating an interlayer of 0.39 nm that is higher than pristine graphene [20]. The XRD pattern of KP-GO-F-P shows the characteristics of porous carbon materials with the complicated pattern, however, the wide peak of graphene can be observed at a same 2ϴ regain [52,53]. In order to investigate the mass composition of KP-GO-F-P and GO-F-P the TGA measurement were conducted. Fig. 2E is the TGA diagrams of KP-GO-F-P and GO-F-P. The two related curves, have three zones: the first zone can be attributed to the exiting of water, and the second part is related to the combustion of KP-GO-F-P or GO-F-P (more than 60 present of KP-GOF-P weight is lost in these two parts, before 680 °C). The last zone (after 680 °C) in which the losing in the weight of samples are almost stopped, is related to the formation of the metal oxides (ashes) of KPGO-F-P or GO-F-P. The metal remaining from the synthesis step of GO (hummer method) or some nature metal in structure of KP (like Si or Ca). The EDX results are in agreement with TGA's results indicating the existence of C, O, Na, K, etc. (Fig. 2 F).

3.3. Application as electrochemical supercapacitor One of the most interesting applications of carbon-based material, is related to the devices like electrochemical capacitors demanding materials with high surface area and with high conductivity. The composite of KP-GO-F-P due to its high surface area and porosity, with simple and green method of preparation and hierarchical structure, could be an excellent candidate for use in designing of a supercapacitor. The electrochemical performances of KP-GO-F-P, as a supercapacitor, were studied with cyclic voltammetry (CV), charge-discharge (CD) and electrochemical impedance spectroscopy (EIS). Fig. 3.A shows the CVs of KP-GO-F-P and GO-F-P in solution of 6.0 M KOH with scan rate of 100 mV s−1. The CV of GO-F-P electrode shows high current due to the high surface area and conductivity of graphene. The CV of KP-GOF-P electrode in comparison to the GO-F-P electrode has the significantly higher current that can be attributed to the higher surface area of KP-GO-F-P composite. This behavior illustrates that the accessibility of KP-GO-F-P composite for electrolyte is dramatically increased, and as shown by the SEM and TEM images, the composite of porous KP and graphene, in form of a 3D structure, has led to higher active surface area. This improvement in surface area and electrochemical behavior can be assigned to the ability of KP-GO-F-P composite to prevent the graphene sheets from restacking as well as the role of highly porous KP as a part of KP-GO-F-P composite. The CVs of KP-GO-F-P in different scan rates (Fig. 3 B) show that the voltammograms retain their overall shape indicating of excellent rate capability for the capacitor [54]. Moreover, the related CV of KP-GO-F-P electrode has an almost rectangular shape suggesting the capacitor behavior the composite [16,29,33]. The CD studies of KP-GO-F-P and GO-F-P (Fig. S2) also prove the higher discharge time of KP-GO-F-P composite illustrating the improved electrochemical abilities for energy storage applications. Based on CD studies the rate capability and the specific capacitances (0.5 to 25 A g−1 and window potential = −1.0 to 0 mV) of KP-GO-F-P were estimated (Fig. 3C). Since, the KP-GO-F-P and GO-F-P electrodes are made of carbon, hence, the main discharge current of the electrode come from capacitance behavior of carbon materials (graphene and KP). As Fig. 3C illustrates, the CD curves show low iR drops indicating of low internal resistance which proves the excellent conductivity of and high rate capability of the KP-GO-F-P and electrode [55]. The rate capability of KP-GO-F-P electrode, at current densities of 0.5 to 25 A g−1, via plotting against specific capacitance, was investigated (Fig. 3D). Due to restacking (or self-aggregation) graphene oxide, the highest reported theoretical capacitances are about 550 F g−1 [56,57]. The interesting specific capacitance of 277.5 F g−1 (at current density of 0.5 A g−1) and 265 (at current density of 1.0 A g−1) for KP-GO-F-P illustrates that this composite has unique properties for an electrochemical supercapacitor. Moreover, the retaining of more than 76% of

Table 1 Comparison of the electrochemical performance of KP-GO-F-P electrode with some biomass-derived carbon electrodes. Carbon electrode Hierarchical porous nitrogen-doped carbon nanosheets derived from silk porous graphene-like nanosheets from coconut shell Microporous activated carbons from coconut shells Mesoporous carbons derived from protein Hierarchically porous carbon by activation of shiitake mushroom porous carbon derived from cotton fabric Nitrogen-doped carbonized cotton Graphene-embellished biological fiber KP-GO-F-P electrode a b c d e f

Specific capacitance. Current density. Maximum power density. Electrolyte type. d1-Ethyl-3-methylimidazolium tetrafluoroborate. Tetraethylammonium tetrafluoroborate.

Ca (F g−1)

Cdb (A g−1)

Pc (W k g−1)

242 196 258 80 149 360 207 291 277.5 265

0.1 1.0 1.0 0.25 0.5 0.5 1.0 1.0 0.5 1.0

8000 10,000 – – 13,000 – 3823 1000 21,000

Eld

Ref.

EMIMBF4e 6 M KOH 1 M H2SO4 1 M H2SO4 TEABF4f 6 M KOH 1 M H2SO4 1 M H2SO4 6 M KOH

[26] [15] [58] [25] [31] [59] [60] [61] This work

M. Amiri, F. Golmohammadi / Journal of Electroanalytical Chemistry 849 (2019) 113388 Fig. 4. A) The different CVs with a window potential from 0.70 V to 1.60 V for KP-GO-F-P//VC device (scan rate = 100 mV s−1, in 6 M KOH). B) The CVs of KP-GO-F-P//VC device at scan rates from 40 to 200 mV s−1. C) The CD studies at different current densities, of KP-GO-F-P//VC device. D) The rate capability of KP-GO-F-P//VC and GO-F-P//VC device, at current densities of 1 to 17 A g−1. E) The long-term cycling stability of KP-GO-F-P//VC device, after 5000 cycles in current density of 10 A g−1. F) The comparison between the few starting (in green) and last (in red) CD cycles). G) The Ragone plot KP-GO-F-P//VC device. H) The Nyquist plot of the KP-GO-F-P//VC device. I) The LEDs lighted up with charged KP-GO-F-P//VC device. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 7

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M. Amiri, F. Golmohammadi / Journal of Electroanalytical Chemistry 849 (2019) 113388

capacitance by increasing current density from 0.5 to 25 A g−1, suggests the excellent rate capability of KP-GO-F-P supercapacitor. Table 1 shows some electrochemical features of other previously reported carbon electrodes with for KP-GO-F-P electrode. As Table 1 shows, the specific capacitance (and power density) of the KP-GO-F-P electrode is more than (or at least comparable with) other reports proving high electrochemical performance of this composite. The long-term cycling stability of KP-GO-F-P and GO-F-P electrodes was monitored after 5000 cycles (Fig. 3E). After 5000 cycles the KPGO-F-P and GO-F-P electrodes have 94.5% and 85% of their first at current density of 8 A g−1 which proves the excellent cycling stability of KP-GO-F-P pointing out the important rule of porous composite of KP and GO that has even better electrochemical behavior than GO-F-P electrode. The few initial cycles and final cycles are presented in Fig. S3. The electrochemical behavior of KP-GO-F-P supercapacitor was also studied by EIS technique. Fig. 3F shows the Nyquist plot of with the assigned equivalent circuit presented in inset. In the equivalent circuit, the RESR represents the electrolyte bulk resistance, ionic resistance in micro-pores and nano-pores are shown with R1 and R2, and the double layer capacitances correspond to Cdl. The low value of RESR (0.5 Ω) proves the low internal resistance of KP-GO-F-P supercapacitor and very low ionic resistance for micro-pores (~1.5 Ω) and nano-pores (~7 Ω) demonstrate the excellent accessibility of pores [54,62]. 3.4. Electrochemical performance of the KP-GO-F-P//VC Using two different electrode made of different material (one serves as an anode and another is a cathodes) can create a situation in which the related voltage of water decomposing reach more than its thermodynamic potential (due to high overpotential of carbon base electrodes) [55,63]. Therefore, the evaluating the asymmetric performance of KPGO-F-P in a supercapacitor device is of great importance. Hence, in this part the electrochemical performance of KP-GO-F-P, as a positive electrode, against a VC as negative electrode was studied with CV, CD and EIS techniques. (For more information about electrochemical behavior of KP-GO-F-P and VC electrodes see supporting information: Fig. S1D and Fig. S4.) Fig. 4A presents the different CVs with a window potential from 0.70 V to 1.60 V for KP-GO-F-P//VC device in solution of 6 M KOH with scan rate of 100 mV s−1. As the CV's studies suggest, the wide stable range of 1.6 V for KP-GO-F-P//VC device is significantly higher than many other conventional asymmetric capacitors which mostly have a range of about 0.8–1.0 V [64]. The effective large surface area of KPGO-F-P composite containing accessible pores to electrolyte, dramatically have increased the electric double layer ability of the asymmetric capacitor making it excellent material for such applications. The effect of increasing scan rate from 40 to 200 mV s−1 on shape of CVs is presented in Fig. 4B. As the voltammograms demonstrate, the overall shape of CVs has excellent shape retention after increasing of scan rate. The CD studies (Fig. 4C), at different current densities, show that the shape of the CD curves is remain almost intact, while the KP-GOF-P//VC device shows an insignificant iR drop, contributing very low internal resistance with unique reversibility. The results illustrate the high rate capability of the KP-GO-F-P composite for electrochemical applications in field energy storage. The variation of specific capacitance (calculated based on CD measurements) of KP-GO-F-P//VC and GO-F-P//VC (for comparison) with increasing of current density is presented in Fig. 4D. The high specific capacitance of 45.8 F g−1 which was recorded at the current density of 0.33 A g−1 (or 1 mA cm−2) or 40.6 F g−1 at the current density of 1.67 A g−1 (or 5 mA cm−2). The results also show that the electrode has a 72% capacitance retention at current density of 17 A g−1 (or 50 mA cm−2). A comparison between KP-GO-F-P//VC and GO-F-P//VC shows the GO-F-P electrode has a significantly lower specific capacitance at the same condition (=26.9 F g−1 at 0.33 A g−1 or 1 mA cm−2) with capacitance retention about 60%. The long-term

stability and durability KP-GO-F-P//VC was monitored after 5000 CD cycles at current density of 10 mA cm−2 or 10 A g−1 (Fig. 4E). After 5000 cycle at this high current density, the as-prepared device retains more than 93% of its first specific capacitance (few first and last CDs are presented in Fig. 4F), suggesting the excellent cycling stability of KP-GO-F-P//VC device. The Ragone Plot of KP-GO-F-P system, plotting power density and energy density, is presented in Fig. 4G. The maximum energy density of 48.9 Wh kg−1 is obtained at the power density of about 426 W kg−1 (and maximum power density. 21.3 kW kg−1 at density of 19.0 Wh kg−1). This energy density is higher (or at least comparable) than most of reported values for carbon-based porous or graphene based capacitors [26,31,56,65,66]. Finally, the EIS study was perform for further investigation of KP-GO-F-P//VC electrochemical properties (Fig. 4H). As the Nyquist plot of the KP-GO-F-P//VC shows, the plot is vertical curve at low frequency indicating the capacitance behavior of device. Moreover, the KP-GO-F-P//VC has a low internal resistance (less than 0.1 Ω) which suggests excellent properties for an electrochemical capacitor. The high porosity of KP creates these unique electrochemical characteristics of KP-GO-F-P which proved by two and three electrode systems, can be ascribed to some basic traits of this composite. The high porosity of KP gives rise to a very large surface area. Aside from the high surface area of graphene the nature of its high conductivity can increase the conductivity of KP-GO-F-P composite as well. The special form of KP and GO during process of preparation, as the TEM and SEM studies reveled, can significantly reduce the aggregation and restacking of graphene sheets leading to superb performance of KP-GO-F-P electrode. Finally, two KP-GO-F-P//VC devices which connected in could light up some LEDs (light-emitting diodes), more than 7 min (Fig. 4I). 4. Conclusions Very simple and environmentally friendly method of preparation of carbon-based material is presented. High porous biomass derived material was successfully composited with graphene to produce a high conductive composite with ultra-porosity. The KP-GO-F-P composite has inexpensive procedure without using any toxic materials or solvents. The prepared composite shows excellent electrochemical properties that make it an excellent choice for electrochemical energy storage applications. Based on these findings, the composite can be effectively use for related application such as catalysts, Li/Li-ion batteries electrochemical (bio)sensors, fuel cells, and etc. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jelechem.2019.113388. References [1] J. Chen, K. Fang, Q. Chen, J. Xu, C.P. Wong, Integrated paper electrodes derived from cotton stalks for high-performance flexible supercapacitors, Nano Energy 53 (2018) 337–344, https://doi.org/10.1016/j.nanoen.2018.08.056. [2] W. Zhang, C. Ma, J. Fang, J. Cheng, X. Zhang, S. Dong, L. Zhang, Asymmetric electrochemical capacitors with high energy and power density based on graphene/CoAlLDH and activated carbon electrodes, RSC Adv. 3 (7) (2013) 2483–2490, https:// doi.org/10.1039/C2RA23283A. [3] L. Shen, J. Wang, G. Xu, H. Li, H. Dou, X. Zhang, NiCo2S4 nanosheets grown on nitrogen-doped carbon foams as an advanced electrode for supercapacitors, Adv. Energy Mater. 5 (3) (2015)https://doi.org/10.1002/aenm.201400977. [4] Y. Zhang, D. Du, X. Li, H. Sun, L. Li, P. Bai, W. Xing, Q. Xue, Z. Yan, Electrostatic selfassembly of sandwich-like CoAl-LDH/polypyrrole/graphene nanocomposites with enhanced capacitive performance, ACS Appl. Mater. Interfaces 9 (37) (2017) 31699–31709, https://doi.org/10.1021/acsami.7b04792. [5] M. Faraji, A. Abedini, Fabrication of electrochemically interconnected MoO3/GO/ MWCNTs/graphite sheets for high performance all-solid-state symmetric supercapacitor, Int. J. Hydrog. Energy 44 (5) (2019) 2741–2751, https://doi.org/10. 1016/j.ijhydene.2018.12.015. [6] J. Wang, Y. Song, Z. Li, Q. Liu, J. Zhou, X. Jing, M. Zhang, Z. Jiang, In situ Ni/Al layered double hydroxide and its electrochemical capacitance performance, Energy Fuel 24 (12) (2010) 6463–6467, https://doi.org/10.1021/ef101150b.

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