Reduced Graphene Oxide Embedded V2O5 Nanorods and Porous Honey Carbon as High Performance Electrodes for Hybrid Sodium-ion Supercapacitors

Accepted Manuscript Title: Reduced Graphene Oxide Embedded V2 O5 Nanorods and Porous Honey Carbon as High Performance Electrodes for Hybrid Sodium-ion Supercapacitors Authors: R. Kiruthiga, C. Nithya, R. Karvembu, B. Venkata Rami Reddy PII: DOI: Reference:

S0013-4686(17)32147-3 https://doi.org/10.1016/j.electacta.2017.10.049 EA 30435

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

21-8-2017 6-10-2017 7-10-2017

Please cite this article as: R.Kiruthiga, C.Nithya, R.Karvembu, B.Venkata Rami Reddy, Reduced Graphene Oxide Embedded V2O5 Nanorods and Porous Honey Carbon as High Performance Electrodes for Hybrid Sodium-ion Supercapacitors, Electrochimica Acta https://doi.org/10.1016/j.electacta.2017.10.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Reduced Graphene Oxide Embedded V2O5 Nanorods and Porous Honey Carbon as High Performance Electrodes for Hybrid Sodium-ion Supercapacitors R. Kiruthigaa, C. Nithyaa*, R. Karvembua, B. Venkata Rami Reddyb a

National Institute of Technology, Tiruchirappalli, India – 620 015

b

CSIR-Central Electrochemical Research Institute, Karaikudi, India – 630 006.

*Corresponding author: [email protected] GRAPHICAL ABSTRACT

RESEARCH HIGHLIGHTS



High surface area porous carbon cathode from honey uses as a cathode.



Nanorods of V2O5 anchored reduced graphene oxide uses as an anode.



Cathode and anode delivers high specific capacitance of 224 Fg-1 and 289 Fg-1 respectively in half-cell.



Non-aqueous sodium-ion hybrid supercapacitor is assembled by prepared electrodes.



Assembled NIC device delivers maximum energy density and power density.

ABSTRACT Attaining high energy density and power density in a single energy storage device is still a major challenge for electrochemical energy storage research community. Sodium-ion hybrid supercapacitor is a sustainable energy storage system which accomplishes the gap between battery and supercapacitor comprises of high energy density-battery type faradaic anode and high power density-supercapacitor type non-faradaic cathode. Here we have reported high surface area (1554 m2g-1) activated porous carbon obtained from naturally occurring viscous liquid honey as a cathode and sol-gel derived, V2O5 nanorods anchored reduced graphene oxide (rGO) nanocomposite as an anode for non- aqueous sodium-ion capacitor. When explored honey derived carbon as a non-faradaic cathode, it exhibits a higher specific capacitance of 224 F g-1 and V2O5@rGO anode delivers the maximum capacitance of 289 F g-1 at 0.01 A g-1 vs Na/Na+. The prepared V2O5@rGO anode has long stable cycle life (V2O5 nanorods@rGO retains 85% of the initial capacitance (112.2 F g-1) at the current density of 0.06 A g-1 after 1000 cycles). The assembled sodium-ion capacitor (NIC) using honey derived activated carbon (AC) and V2O5@rGO anode delivers the energy density of ≈65 Wh kg-1 and power density of ≈72 W kg-1 at 0.03 A g-1. The capacity retention is 74% after 1000 cycles at the current density of 0.06 Ag-1. The assembled sodium-ion hybrid capacitor delivers maximum energy and power density and exhibits very long stable cycle life. Key words: Honey derived carbon, V2O5@rGO, insertion electrode, sodium-ion capacitor, nanocomposite.

1. Introduction

Today our life style becomes smarter by touching the devices and swiping the cards. Human life moves parallel with the fast growing technology (portable smart electronic devices). It is incredible to visualize our day today events without those technologies. Concurrently this technology development strives for energy storage devices such as batteries and supercapacitors. Supercapacitor, a kind of energy storage device, also called electrochemical capacitor or ultra capacitor has been extensively studied and deliberated to be one of the most auspicious alternative energy storage systems for future sustainable energy supply, due to its fast charging/discharging rate, high power density, long cycle life, and long term operation stability. This storage process in supercapacitor is accomplished by either non-faradaic process (EDLC) or faradaic process (pseudo capacitor) [1-5]. Habitually secondary rechargeable batteries (LIB, NIB) and supercapacitors are lacking in terms of power density and energy density respectively. In order to accomplish batteries with high power density and supercapacitors with high energy density, copious efforts are devoted to the development of advanced electrode materials. The hybrid capacitor is a new breed of device which comprises of battery type electrode as an anode, supercapacitor type electrode as a cathode in a single device. The combination of two electrodes, one that stores charge non-faradaically, and the other stores charge faradaically. It is a promising device for future energy demand and fulfils the gap between batteries and supercapacitors [6-8]. Very recently, a hot topic in energy research is sodium ion hybrid supercapacitor (NIC). In recent years, sodium ion charge haulier has drawn the attention of electrochemical energy storage research community, even though it was scrutinized along with lithium system. In contrast to lithium, sodium resources are interminable everywhere, and sodium is one of the most profuse

elements in the Earth’s crust. Furthermore, sodium is the inexpensive, second-lightest and smallest alkali metal consequently to lithium. Therefore, sodium system has again provoked a great covenant of interest recently. On the basis of material abundance and standard electrode potential, sodium system is the ideal alternative to Lithium system. Compared with Li, Na has similar physical and chemical properties [9-14]. Research on sodium ion capacitor energy storage system is in budding stage. Very few reports are available so far, which focused on metal oxides NiCo2O4 [15], V2O5 [16], Nb2O5 [17], sodium titanate [18], sodium phosphate [19], vanadium carbide [20] and peanut shell derived carbon [21]. Seeking suitable electrode materials and achieving high energy density, high power density in a single device is still a major challenge. In this work we have attempted and reported honey derived activated carbon as a cathode and V2O5 nanorods anchored rGO nano composite as an anode for sodium ion capacitor. While considering the cathode material which should store charge non-faradaically, activated carbon comes to a mind, since it plays a vibrant role as an electrode material in an energy storage device. In recent years, researchers are more interested in the synthesis of activated carbon from natural sources like agricultural waste and bio mass [22-26] etc., To improve the energy density and specific capacitance of supercapacitor, activated carbon with high surface area and porosity can be a better option. The porous network will provide feasible way for ion transport. Since honey is a viscous liquid carbon rich source we picked up it for the precursor for synthesizing activated carbon. Honey is nature’s novel sweetener. It is a supersaturated, glutinous sugar solution, which consists of 80–85% carbohydrate (mainly glucose and fructose), 15–17% water, 0.1– 0.4% protein, 0.2% ash and minor quantities of amino acids, enzymes and vitamins. Each of these minor constituents is known to have unique nutritional or medicinal properties and the unique blend accounts for the varied and different applications of natural honey [27, 28].

Along with astonishing medicinal properties, it has been used for energy storage application as a cathode for sodium ion capacitor for the very first time. In consideration of anode material, the metal oxide V2O5 was preferred one, because of its pseudocapacitive behaviour, miscellaneous oxidation state, layered structure, low cost and foremost capability of being as a host for sodium ion. Although it holds favourable key points to deem, poor electronic conductivity is still a major issue which leads to capacitance fading and poor cyclability [29-31]. It can surmount by the addition of conductive carbon matrix. Here we introduced reduced graphene oxide as a matrix to enhance the performance of V2O5. The rGO network will boost mass transport, electronic conductivity, and cycle stability of V2O5. 2. Experimental 2.1. Synthesis of Activated carbon Fresh, pure, original and natural honey was used as a precursor to synthesize activated carbon. It was collected in the attakatti forest (through honey hunter), Coimbatore district, Tamilnadu, India. The concentrated sulphuric acid was added [32] drop wise in to the beaker contained pure honey, till it fully turned into charr. The charr were kept in hot air oven at 150°C, for 24 hrs. After that it was washed several times with double distilled water followed by drying and grinding. Then in the pre-carbonization process, the obtained brown coloured powder was kept at 350°C in an Argon atmosphere for 4 hours in the tubular furnace with a heating rate of 5°/min followed by activation process, in which pre-carbonized sample was mixed with KOH (weight KOH:weight carbon= 2), then kept in the same atmospheric condition with the same flow rate at 800°C for 2 hours. After cooling, the activated sample was dispersed in distilled water, stirred for 24 hrs and sonicated for 3 hrs. Then washed with dil. HCl to remove the impurities, followed by water and ethanol, until it attains the pH= 7. Finally sample was dried at 60°C in the hot air oven for 48 hrs and ground well.

2.2. Synthesis of V2O5, rGO, V2O5@rGO nano composite V2O5 was prepared by using sol-gel technique. 0.1M of ammonium meta vanadate and citric acid were dissolved in double distilled water separately. Then citric acid (chelating agent) was added drop wise to ammonium meta vanadate solution. The sol was heated at 70-80o C till the formation of gel and pH of the solution was maintained at 3. The gel was dried in an oven at 100oC for 12hrs, then it was calcined at 450oC for 5 hrs in an air atmosphere. Finally brownish yellow colour product was obtained. The obtained product was ground well and subjected to further characterization. Graphene oxide (GO) was prepared by modified Hummers method [33, 34]. Reduced graphene oxide (rGO) was prepared by reduction of graphene oxide. In this step, the prepared GO was dispersed in double distilled water and sonicated for 2 hrs. Then sodium borohydride (NaBH4) was added to it under sonication. After the addition of NaBH4, it was stirred vigorously for 8 hrs. The precipitate thus formed was filtered, dried at 60oC for 48 hrs and ground well. Vanadium pentoxide-reduced graphene oxide composite (V2O5@rGO) was synthesized in the ratio of 85:15 from as prepared V2O5 and rGO. 0.15g of rGO was dispersed in DI water under sonication for 1 hr. 0.85g of V2O5 was added and allowed for sonication about 2 hours to form V2O5 on the surface of rGO. The mixture was stirred for 24 hours and the resulting solid was separated by filtration and washed with large amounts of DI water and ethanol to remove most residual ions. Once it is fully dried at 60oC for 24 hrs, the product was obtained as a fine powder after grinding. 2.3. Structural Characterization X-ray diffraction pattern obtained on a X-ray diffractrometer (Bruker D8) using Cu-Kα (λ = 1.5406 Å) radiation at a scan rate of 2° min-1 while the voltage and current were held at 40kV and 20 mA (2θ = 10-90°). The X-ray photoelectron spectrum was recorded by using

Thermoelectron spectrometer. The powder sample was affixed into a holder as a pellet. All spectra were recorded by using AlKα radiation with as scan range of 1200 eV. The collected XPS spectra were investigated with XPS peak fitting software program (XPS peak version 4.1) The Raman spectra was recorded on a Renishaw InVia laser Raman microscope with a He–Ne laser (λ=633 nm). Nitrogen adsorption desorption isotherm was recorded on BEL Sorp-II mini, (BEL Japan Co). The pore size distribution plots were recorded based on the Barret-Joyner-Halenda (BJH) model. HRTEM images were captured on FEI Technai- 20 G2 microscope. FESEM images were collected on CARL ZEISS (Neon 40) microscope. 2.4. Electrochemical characterization Electrochemical performances of as-prepared V2O5@rGO nano composite and activated carbon electrodes were investigated with two-electrode coin-type cells (CR 2032). The working electrodes were prepared by making a slurry with active material, super-P carbon (as conducting additive) and polyvinylidene fluoride (PVDF, as binder) with a weight ratio of 80:10:10 in N-methyl pyrrolidinone (NMP, as solvent) were coated on copper foil (for anode) and aluminium foil (for cathode) and dried at 80°C for 12h. The mass loading of electrode and active material is 3 and 2.5 mg respectively. 0.75M NaPF6 in ethylene carbonate (EC)– diethyl carbonate (DEC) (1:1 in volume) was used as an electrolyte and a polypropylene film as the separator. Cells were fabricated in an argon-filled glove box with an oxygen and water less than 1ppm. In half-cell studies, both anode and cathode was tested separately with metallic sodium was used as the reference electrode. Sodium ion capacitor device (full cell) was assembled using V2O5@rGO composite as an anode and activated carbon as cathode. In the full cell configuration the mass ratio between anode and cathode is 2:1. Electrochemical

charge/discharge measurements were performed by using NEWARE battery cycler with a potential range of 1.5-4.3V for cathode 0.01-3V for anode and NIC device (full cell). Cyclic voltammetry and impedance measurements were performed on Bio-Logic SP-240 & SP-200

respectively. The specific capacitance of the both half cell and full cell was calculated by using the following equation. 𝐶=

𝐼∆𝑡 𝑚∆𝑉

(1)

where C (F/g) is the specific capacitance. I (A) is the discharge current, Δt (s) is the discharge time, m (g) is the total mass of active material in the electrode, and ΔV (V) is the voltage window during the discharge process [35, 36]. The energy density and power density of NIC device was calculated by using the following equations.

1

𝐸 = 2 𝐶(∆𝑉)2

(2)

𝑃=

𝐸 𝑡

(3)

Where E (Whkg-1) is the specific energy density, P (Wkg-1) is the specific power density, C (F/g) is the specific capacitance. ΔV (V) is the cell voltage for charging and discharging and t (h) is the corresponding discharge time.

3. Results and Discussions Fig. 1a presents the X-ray diffraction (XRD) patterns of the graphene oxide (GO), reduced graphene oxide (rGO), pristine V2O5 particles, and V2O5@rGO composite. The diffraction peak obtained at 2θ = 10.56o indicates that the formation of GO phase, and this peak is shifted to 2θ = 25.15o for rGO [37] gives an evidence that the graphene oxide was reduced by sodium borohydride. Meanwhile, it can also observe that the peaks of graphene

oxide become broadened after reduction, which is a typical pattern for amorphous carbon structure, revealing that the stacking of graphene sheets is substantially disordered. It can be found that seven strongest diffraction peaks appear at 15.39, 20.29, 21.73, 26.16, 31.03, 32.38 and 34.34° corresponding to the characteristic planes of (200), (001), (101), (110), (301), (011) and (310) planes for V2O5. The XRD pattern of V2O5 matches well with the ICSD data base (01-077-2418) with a space group of Pmmn (59). No impurity peaks were observed in the sample. XRD pattern confirms the crystalline nature of the material with orthorhombic structure with the lattice parameters a = 11.513, b = 3.566 and c = 4.376Å, respectively [38, 39]. The crystallite size obtained from Debye-Scherrer method is 43.17 nm. It is important to note that the stacking peak of reduced graphene nano sheets usually located at 2θ = 26o, which was not observed in V2O5@rGO composite indicating that V2O5 particles uniformly dispersed on rGO sheets which is also confirmed by FESEM and TEM image and moreover, the intensity of V2O5 peaks diminish the rGO diffraction. Fig. 1b shows the X-ray diffraction pattern of honey derived carbon. The XRD pattern exhibits a highly broadened, low intense Bragg reflection (002) and (100) plane of graphite at 2θ = 23.3° and 43.5°, which is a typical non graphitic carbon material with disordered structure. The formation of a higher degree of interlayer condensation which has positive impact on electrical conductivity is confirmed by the presence of peak at 2θ = 43.5°. The d spacing value of (002) plane is 0.38 nm. The obtained peak-to-background ratio of (002) plane (R parameter) yields a value of 1.52, revealed that the existence of chaotically oriented graphene sheets (Fig. 1c). Therefore it is expected that this carbonaceous material should have prominent consequences on the electrochemical behaviour [40, 41].

Raman spectroscopy is a typical technique for evaluating carbon based materials. Generally KOH activation induces the disorderness in the carbon matrix. From the ID/IG ratio

value, it is possible to know the information about the degree of graphitization in which ID/IG value is inversely proportional to the degree of graphitization [42]. Defect and disorderness in the carbon material can be confirmed by the presence of D band at 1358 cm-1 and graphitic structure can be verified by the presence of G band at 1583 cm-1. The ID/IG value of activated carbon is 1.08 (Fig. 2a) indicating that material has well disordered structure and a lower degree of graphitization.

Fig. 2b shows raman spectra of rGO. The D and G bands are observed for rGO at 1338 and 1593 cm-1, respectively. The ratio of ID/IG value of rGO is 1.13 which suggests that a high degree of disorder in the graphene layers results more defects. This kind of increase in intensity and broadening of the D band creates more defects in the rGO, which generally increases the conductivity. The 2D and D+G peak at 2670 and 2902 cm-1 in rGO is associated with good electronic property as well as the number of graphene layers. The Raman spectra of V2O5@rGO is shown in Fig. 2c. The peak at 139 cm-1 is assigned to the skeleton bending vibration of V-O-V bond. The peak at 529 and 680 cm-1 is due to the bending vibration of bridging doubly coordinated oxygen (V-O-V) and triply coordinated oxygen bond respectively [43, 44]. The bending and stretching vibration mode of V=O bond is confirmed by the presence of peaks at 276 and 985 cm-1 [45] and the characteristic peaks of rGO were also observed at 1342 and 1588 cm-1. The weight percentage of rGO in the composite is low, therefore the intensity of D and G band becomes low and the intensity of V2O5 has been dominated. The N2 adsorption isotherm of activated carbon is shown in Fig. 3a. BEL Master-data evaluation software is used to calculate the surface area and pore volume in BET analysis. The BET surface area of the material is found to be 1554 m2g-1 with the pore volume of 0.71cm3 g-1. The as prepared activated carbon from honey exhibits type I isotherm according

to IUPAC nomenclature. The pore size distribution estimated by the Barret-Joyner-Halenda (BJH) method is shown in Fig. 3b which suggests the presence of micro (<2 nm) as well as mesopores (2-5 nm). The steep raise at relatively low pressure is due to the existence of micropores which can also be confirmed by the presence of intense peak (002) at 23.3° in Xray diffraction pattern [26]. The small hysteresis loop at relatively high pressure is the evidence for the existence of mesopores. The occurrence of both micropores and mesopores will be beneficial for supercapacitor application [46]. XPS is a distinctive tool to explore the elemental composition which is exposed in the Fig. 4. The deconvolution of C1s spectra (Fig. 4a) reveals the existence of sp2 C and sp3 C, moieties at 283.9, 285.2 eV [47]. In O1s spectra (Fig. 4b), two peaks are observed at 531.5, 532.6 eV that corresponds to the C=O and C−O [48]. Although honey has minor quantity of enzyme and aminoacid naturally, XPS analysis clearly shows that honey derived activated carbon does not contain nitrogen because of chemical activation at high temperature. Fig. 5&6 depicts the FESEM & HRTEM images of rGO, V2O5, V2O5@rGO composite and honey derived activated carbon. rGO shows sheet like morphology (Fig. 5a). The prepared V2O5 exhibits sand stone wall like morphology (Fig. 5b). In higher magnification (Fig. S1, see Supporting Information), it clearly shows that V2O5 has interconnected layer structure and looks like a nanorods. In the case of V2O5@rGO composite, the probability of finding rGO is very less, since V2O5 nanorods are consistently anchored on reduced graphene oxide sheets (Fig. 5c). Interestingly honey comb like structure has retained in the FESEM image of honey derived carbon (Fig. 5d). It is also substantiation for porous nature of this activated carbon which is essential for ion adsorption (charge storage). Scrambled sheet like structure was found on HRTEM image of rGO (Fig. 6a), which is consistent with FESEM analysis. Fig. 6b further confirmed the interconnected layer structure of V2O5 where in V2O5@rGO composite, V2O5 is interwined well with rGO sheet (Fig. 6c). It looks like nano

rods (V2O5) resided on the surface of rGO sheets. Porous network of honey carbon is further confirmed by HRTEM image (Fig. 6d) which clearly shows that pores are interconnected. The morphology of interconnected assembly of both anode and cathode will facilitate the electrochemical energy storage mechanism. Bright spots are observed in the SAED pattern (Fig. S2, see Supporting Information) reveal the crystalline nature of V2O5 and diffuse rings (Fig. S3, see Supporting Information) confirm the amorphous nature of activated carbon. Fig. 7 shows the electrochemical behaviour of V2O5 nanorod@rGO composite anode. Before making NIC device, both cathode and anode was investigated as half cell where metallic sodium was used as reference electrode. The galvanostatic charge-discharge tests were carried out to evaluate the half cell performance with the potential range of 0.01-3 V. The distorted linear curve (Fig. 7a) confirms the pseudocapacitive nature of V2O5@rGO composite. The reaction of V2O5 with sodium can be described by the following equation.

𝑉2 𝑂5 + 𝑥𝑁𝑎+ + 𝑥𝑒 − ↔ 𝑁𝑎𝑥 𝑉2 𝑂5

(4)

where ‘x’ is the mole fraction of inserted sodium ions [16]. Fig. 7b describes the relation between specific capacitance and current density (from 0.01 to 1.77 A g-1). The V2O5@rGO composite delivered maximum capacitance of 289 Fg-1 at 0.01 A g-1 which is comparable with previous reported values. Zhen chen et al, developed CNT/V2O5 nanocomposites [49] and V2O5/CNT nanowire composites [16]. They obtained a higher capacity of 220 mAh g-1 at C/4 rate and ~400 C g-1 at 1 mVs-1 respectively. When current density increased from 0.03 to 1.77 A g-1, the capacitance decreased from 289 to 48 F g-1 respectively. The decrease in capacitance at higher current densities is due to polarization effects. Furthermore, the long term cycling stability of V2O5@rGO composite electrode was investigated using galvanostatic charge-discharge measurement at a current density of 0.06 A g-1 (Fig. 7c). After

1000 cycles, it retains 85% of the initial capacitance (112.2 F g-1). The rGO can serve as a backbone to combine V2O5 and enhance the electrical conductivity and chemical stability which results very good cycling stability. The combination of V2O5 nanorods and rGO can contribute to the total capacitance of the NIC device by an added pseudocapacitance to the double layer capacitance from rGO. The cyclic voltammogram (CV) of V2O5@rGO composite anode is shown in Fig. 7d. It was recorded at the different scan rate from 5-100 mV s-1 with the potential range of 0.01-3 V vs Na+/Na. The capacitance value obtained from CV is well matches with galvanostatic charge discharge studies. Scan rate is directly proportional to the current density and inversely proportional to the capacitance. Therefore, at the higher scan rate, there is insufficient time for ions to intercalate and deintercalate, which leads to a decrease in capacitance value. The excellent electrochemical performance of V2O5@rGO anode is mainly due to the Na+ ion diffusion into the layers of V2O5, and the electrical conductivity provided by the rGO network, which enhances the mass transport and cycling stability. Further the electrochemical performance of V2O5@rGO is investigated using electrochemical impedance spectroscopy (EIS) which is shown in (Fig. S4, see Supporting Information). The semi-circle in the Nyquist plot corresponds to the charge-transfer resistance (Rct) and SEI resistance (RSEI) between electrode and electrolyte. The line slanted at 45° angle is the Warburg region correlated with sodium ion diffusion process in the electrode. The diffusion coefficient (D) of the V2O5@rGO composite has been derived using the following formula [50, 51].

𝑅2𝑇 2 𝐷= 2 4 4 2 2 2𝐴 𝑛 𝐹 𝐶 𝜎

(5)

Where R is the gas constant (JK-1mol-1), T is the absolute temperature (K), A is the area of the electrode (cm2), n is the number of electrons transferred, F (coulomb/mol) is the faraday

constant, C is concentration (mol/cm3), σ is the coefficient of Warburg impedance, It (σ) is obtained from the extrapolation of straight line (low frequency region) from the semicircle to real axis and is equal to (Rs + Rct − 2σ2Cdl). Rs is the solution resistance in the bulk electrolyte, Rct, Cdl is the charge transfer resistance and double layer capacitance in the electrode-electrolyte interface respectively. The calculated diffusion coefficient of V2O5@ rGO composite is 9.65 x 10-11 cm2 s-1 which is attributed to the enhanced electrochemical activity of V2O5@rGO composite. The half cell performance of honey derived activated carbon cathode is shown in Fig. 8. The non-faradaic nature (electrical double layer capacitance) of carbon is confirmed from the typical linear charge-discharge curves (Fig. 8a) at the potential range of 1.5-4.3 V. Fig. 8b reveals the obtained capacitances of carbon at different current densities. The maximum capacitance of 224 F g-1 was attained at the current density of 0.01 A g-1. Then capacitance decreased from 224 to 32 F g-1, when the current density increased from 0.01 to 2.2 A g-1. The cycling stability of cathode was observed at the current density of 0.07 A g-1 (Fig. 8c). After 1000 cycles, it retains 88% of the initial capacitance (201.6 F g-1). CV tests were carried out at same potential range with the scan rate of 5-100 mV s-1 vs Na/Na+ (Fig. 8d). The typical rectangular curve of ideal capacitor was obtained at the slow scan rate of 0.1mV s-1 is the evidence for non-faradaic nature (Fig. S5, see Supporting Information) and deviation of curve at the high scan rate is due to the fast ion adsorption on the surface of the cathode. Jia Ding et al., attained the capacity of 161 mAh g-1 at 0.1 A g-1 with peanut shell nanosheet carbon [21]. As Compared to previous work [21], we achieved more capacitance of 201.6 F g-1 at the same current density of 0.1 A g-1. Since surface adsorption is the mechanism for energy storage at the cathode part, it is vital for carbon to possess high surface area. The higher capacitance value of honey derived carbon is due to the higher surface area (1554 m2 g-1) and also the pore size distribution (upto 4 nm). The co-existence of both micropores and

mesopores enhance the accessibility and transportation of electrolyte ions and electrical conductivity. The high surface area enhances the electrochemically active sites [46]. Finally NIC device was fabricated using V2O5 nanorods@rGO composite anode and honey derived activated carbon cathode. The electrochemical studies of this full cell were done at the potential range of 0.01-3 V. The galvanostatic charge-discharge profiles obtained at various current densities are shown in Fig. 9a. Since, both faradaic nature of anode and non-faradaic nature of cathode involved in the charge storage mechanism and the linearity nature of ideal super capacitor has disturbed slightly which results distorted charge-discharge profile. The device delivered maximum capacitance of 62 F g-1 at the current density of 0.01 A g-1. Fig. 9b shows the calculated specific capacitance of NIC device as a function of the current density. The capacity retention of 74% was attained after 1000 cycles at the current density of 0.06 A g-1 (Fig. 9c). In the case of full cell configuration, faradaic temperament dominates non-faradaic temperament which has confirmed from the cyclic voltammogram of NIC device is shown in Fig. 9d. The energy density and power density was calculated using equation 2 and 3. The NIC device consists of V2O5 nanorods@rGO composite anode and honey derived activated carbon cathode delivers the maximum energy density of ≈65 Wh kg-1 and power density of ≈72 W kg-1 at 0.03 A g-1. The assembled NIC device in the present work shows comparable electrochemical performance with the earlier literatures (AC/V2O5CNT, AC/Nb2O5@Carbon-rGO, Peanut shell derived carbon, AC/Na2Ti3O7 nanotube, AC/NaMn1/3Co1/3Ni1/3PO4, AC/NaTi2(PO4)3-rGO with the energy density values of ~38, 76, 201, 36, 50, 53 Wh kg-1 respectively [52,53].

4. Conclusions To summarize, we developed hybrid sodium ion capacitor with high surface area, activated porous carbon derived from naturally occurring viscous liquid honey as a cathode

and sol-gel derived V2O5 nanorod@rGO nanocomposite as an anode. Both the electrodes showed excellent electrochemical performances in half cell as well as in full cell configurations. The perceptible performance of honey derived carbon cathode is mainly due to the high surface area and porous nature (micro and meso) that results in higher specific capacitance of 224 F g-1 at 0.01 A g-1. In the case of anode, the enhanced performance of V2O5@rGO composite is mainly attributed by the rGO sheets leads to the excellent cycling stability (85% retention after 1000 cycles) and high specific capacitance of 289 Fg-1 achieved at 0.01 Ag-1. The assembled sodium-ion capacitor (NIC) using honey derived activated carbon (AC) and V2O5@rGO anode delivers the energy density of ≈65 Wh kg-1 and power density of ≈72 W kg-1 at 0.03 A g-1. Our results suggest that the assembled NIC device is a promising energy storage device for next generation electrochemical energy storage systems with high power and energy density. Acknowledgments We acknowledge Department of Science and Technology (DST), India for support of the work under DST-INSPIRE Faculty Award Project.

Appendix A. Supporting information FESEM image of V2O5 at higher magnification, SAED pattern of V2O5 and honey derived carbon, Nyquist plot of V2O5@rGO composite, Cyclic voltammogram of honey derived carbon at 0.1 mV s-1.

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List of Figures

1. Fig. 1. (a) XRD pattern of GO, rGO, V2O5, V2O5@rGO composite (b) XRD pattern of honey derived activated carbon (c) Representation of ‘R’ value calculation 2. Fig. 2. (a) Raman spectra of honey derived activated carbon (b) Raman spectra of rGO (c) Raman spectra of V2O5@rGO composite 3. Fig. 3. (a) N2 adsorption-desorption isotherm of honey derived carbon (b) Pore size distribution of honey derived carbon 4. Fig. 4. (a) C 1s spectra of honey derived activated carbon (b) O 1s spectra of honey derived activated carbon 5. Fig. 5. FESEM- Images of (a) rGO, (b) V2O5, (c) V2O5@rGO composite, (d) honey derived activated carbon 6. Fig. 6. HRTEM-Images of (a) rGO, (b) V2O5, (c) V2O5@rGO composite, (d) honey derived activated carbon 7. Fig. 7. Electrochemical performance of V2O5@rGO composite vs Na/Na+ (half cell) (a) Galvanostatic charge-discharge curves of V2O5@rGO composite (b) Specific capacitances of V2O5@rGO composite at different current densities (c) Cycling performances of V2O5@rGO composite (d) Cyclic voltammograms of V2O5@rGO composite at different scan rate 8. Fig. 8. Electrochemical performance of Honey derived carbon vs Na/Na+ (half cell) (a) Galvanostatic charge-discharge curves of honey derived carbon (b) Specific capacitances of honey derived carbon at different current densities (c) Cycling performance of honey derived carbon (d) Cyclic voltammograms of honey derived carbon different scan rate 9. Fig. 9. Electrochemical performance of AC//V2O5@rGO (full cell) (a) Galvanostatic charge-discharge curves of AC//V2O5@rGO (full cell) (b) Specific capacitances of full cell at different current densities (c) Cycling performance of AC//V2O5@rGO (full cell) (d) Cyclic voltammograms of full cell (AC//V2O5@rGO) at different scan rate

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