rGO composites for asymmetric supercapacitor applications

Accepted Manuscript Investigation of band structure and electrochemical properties of h-BN/rGO composites for asymmetric supercapacitor applications Sanjit Saha, Milan Jana, Pranab Samanta, Naresh C. Murmu, Nam H. Kim, Tapas Kuila, Joong H. Lee PII:

S0254-0584(17)30051-2

DOI:

10.1016/j.matchemphys.2017.01.025

Reference:

MAC 19432

To appear in:

Materials Chemistry and Physics

Received Date: 11 August 2016 Revised Date:

25 November 2016

Accepted Date: 8 January 2017

Please cite this article as: S. Saha, M. Jana, P. Samanta, N.C. Murmu, N.H. Kim, T. Kuila, J.H. Lee, Investigation of band structure and electrochemical properties of h-BN/rGO composites for asymmetric supercapacitor applications, Materials Chemistry and Physics (2017), doi: 10.1016/ j.matchemphys.2017.01.025. 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.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

Investigation of band structure and electrochemical properties of hBN/rGO composites for asymmetric supercapacitor applications

RI PT

Sanjit Saha,a,b Milan Jana, a,b Pranab Samanta, a,b Naresh C. Murmu, a,b Nam H. Kim,c Tapas Kuila a,b * and Joong H. Lee c,d*

Surface Engineering & Tribology Division, CSIR-Central Mechanical Engineering Research

SC

a

Institute, Durgapur -713209, India

Academy of Scientific and Innovative Research (AcSIR), CSIR-CMERI Campus, Durgapur-

M AN U

b

713209, India

c

Advanced Materials Institute of BIN Convergence Technology (BK21 plus Global) & Dept. of

BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, Republic

d

TE D

of Korea

Carbon Composite Research Centre, Department of Polymer & Nanoscience and Technology,

AC C

EP

Chonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea

-------------------------------------------------------------------------------------------------------*Correspondence to Tapas Kuila. Tel.: +91-9647205077; Fax: 91-343-2548204 E-mail address: [email protected] (Tapas Kuila) and Joong Hee Lee ([email protected] ) --------------------------------------------------------------------------------------------------------

1

ACCEPTED MANUSCRIPT

ABSTRACT: The effect of different content of graphene oxide (GO) on the electrical and electrochemical property of h-BN/reduced GO (rGO) hetero-structure is investigated elaborately. The increasing amount of rGO within the h-BN moiety plays fascinating role by reducing the

RI PT

electronic work function while increasing the density of state of the electrode. Furthermore, different h-BN/rGO architecture shows different potential window and the transition from pseudocapacitance to electrochemical double layer capacitance (EDLC) is observed with

SC

increasing π-conjugation of C atoms. The rod like h-BN is aligned as sheet while forming superlattice with rGO. Transmission electron microscopy images show crystalline morphology of the

M AN U

hetero-structure super-lattice. The valance band and Mott-Shotky relationship determined from Mott-Shotky X-ray photoelectron spectroscopy shows that the electronic band structure of superlattice is improved as compared to the insulating h-BN. The h-BN/rGO super-lattice provides high specific capacitance of ~960 F g-1. An asymmetric device configured with h-BN/rGO super-

TE D

lattice and B, N doped rGO shows very high energy and power density of 73 W h kg-1 and 14000 W kg-1, respectively. Furthermore, very low relaxation time constant of ~1.6 ms and high stability (~80%) after 10,000 charge-discharge cycles ensure the h-BN/rGO super-lattice as

EP

potential materials for the next generation energy storage applications.

AC C

KEYWORDS: Specific capacitance; Super-lattice; Supercapacitor; Voltametric charge; Energy density

1. Introduction

The exciting and unique physical properties such as electrical conductivity, carrier mobility and electrochemical activity of the two dimensional (2D) materials arise from the lowering of dimensionality [1-5]. Alloying of the different 2D materials of neighbouring elements in the

2

ACCEPTED MANUSCRIPT

periodic table may generate another interesting class of materials due to the formation of heterostructure and super-lattice [1]. The existence of different band gaps within the hetero junctions set up new selection rules of each quantum well that affect the conditions for charges flow

RI PT

through the structure [6-9]. Furthermore, the hetero-structure promotes the defects which in turn tune up the mobility of the charge carriers and enhances the electrochemical activity due to the availability of the free electrons [7,10-12]. Extremely high surface area and electron mobility of

SC

graphene established it as the state of art for electrochemical applications. Electrons in graphene behave like mass less Dirac fermions and the relativistic nature of carriers shows 100%

M AN U

tunnelling through a potential barrier by changing its chirality [1]. The hexagonal boron nitride (h-BN) has similar type of sp2 structure like graphene but shows insulating properties due to the presence of two different chemical species at the two different sub-lattices breaking the inversion symmetry [1,9,13,14]. The substitution of C in graphene by B and N can give an alloyed BCN

TE D

configuration and different types of alloying are possible depending on the different growth process [1]. The charge transfer in different kinds of BCN structures are different and possess varied electronic properties due to the unlike electro negativities of B, C, and N (2.0, 2.5 and 3.0)

EP

[1,9].

AC C

Different charge storage mechanisms in the hetero-structure materials depend on the unoccupied state above the Fermi level and work function of the adjacent materials [15]. The alteration in the coordination of the atoms and band structure change the potential window of the current response when same electrolyte is used [15]. The carbon doping reduces the band gap of the insulating h-BN and introduces π-conjugation of the carbon pathway enhancing its electrical conductivity and the electrochemical activity [9,16,17]. The charge storage mechanism of the hBN is also influenced by the insertion of the carbon atoms due to the increased electrical

3

ACCEPTED MANUSCRIPT

conductivity and elevated charge transfer within the different electronegative species of the h-BN moiety [9]. Graphene shows electrochemical double layer capacitance (EDLC) and vastly used as the negative electrode materials in supercapacitor applications. The insertion of the controlled

RI PT

amount of B or N atoms enhances the current response of graphene without affecting the potential window [18,19]. Doping increases the porosity as well as overall conductivity of the electrode materials [9,11,20,21]. On the other hand, the presence separate phase of graphene and

SC

h-BN alters the band structure as well as the electronic work function of the electrode materials [1,9]. The presence of separate phase of the h-BN and graphene introduces the redox charge

M AN U

storage mechanism possessing pseudo peaks with different potential window as compared to the bare graphene or reduced graphene oxide (rGO) electrode [9]. Furthermore, the controlled growth process results in the formation of super-lattice ensuing enhanced electrochemical activity due to the synergistic effect of h-BN /graphene hetero-structure and carbon doped h-BN

TE D

[9]. Another advantage of the existence of separate phase within the nano dimension is that all Chexagons are connected to each other increases the stability of C-C and B-N bonds as well as helps in the π-conjugation [1].

EP

On the basis of above discussion, the relative position and the ratio of B, N and C atoms are

AC C

the key factors that control the electrochemical properties. The practical applications of supercapacitor demands high energy density and the extension of potential window is the desired solution in this regards [22,23]. However, the use of aqueous electrolyte provides the potential window of ~ 1-1.2 V. The combination of the operating voltage window of positive and negative electrodes may achieve by developing the asymmetric supercapacitor (ASC) device [9,22,23]. B and N doped graphene or rGO is the established negative electrode materials with elevated specific capacitance as compared to the pristine graphene [18,19]. Saha et al. reported high

4

ACCEPTED MANUSCRIPT

specific capacitance of h-BN/rGO hetero-structure as the positive electrode materials due to the formation of super-lattice [9]. Herein, in depth investigation of the electrochemical properties of different microstructure consisting of B/N/rGO were carried out. It was highly interesting to see

RI PT

diverse morphology as well as different electrical and electrochemical properties with the variation of B, N and rGO composition. The increasing rGO concentration changes the band structure of the composite and the offset Fermi energy decreases simultaneously. Furthermore,

SC

the electrochemical nature of the hetero-structure electrode changes from faradic to EDLC due to the formation of different microstructure. The optimized concentration of B, N and GO form

M AN U

super-lattice structure and significantly increase the total active surface area of the electrode materials is recorded. Furthermore, an asymmetric supercapacitor has been fabricated with the hBN/rGO super-lattice and B, N doped rGO as the positive and negative electrode materials, respectively. The long life stability and huge power deliver capacity with high energy density

2. Experimental

TE D

ensures the application of ASC in future generation energy storage device.

EP

2.1 Reagents. Natural graphite flakes were purchased from Sigma-Aldrich. Hydrochloric acid, sulphuric acid, hydrogen peroxide, potassium permanganate, boric acid, N, N-dimethyl

AC C

formamide (DMF) and potassium hydroxide were purchased from Merck, Mumbai, India. Urea was purchased from SRL, Mumbai, India. Conducting carbon black (EC-600JD, purity: >95%) and PVDF were purchased from MTI Corporation, USA. Nickel foam was purchased from Shanghai Winfay New Material Co., Ltd., China 2.2 Preparation of h-BN/rGO composite. GO was prepared from natural graphite flakes according to the modified Hummer’s method. For h-BN/rGO composite preparation, about 0.3 g

5

ACCEPTED MANUSCRIPT

boric acid, 0.3 g urea and 0.1 g GO was ground into homogeneous form and transfer in to a porcelain crucible. The crucible was kept at 600 oC for 5 h under Ar atmosphere. The product was cooled at room temperature and washed several times with distilled water until the pH was

RI PT

neutral. Finally, the product (hBNG1) was dried at 100 oC inside a vacuum oven. The quantity of GO was varied to synthesize other three composites. The amount of GO was 0.2, 0.3 and 0.4 g

SC

for hBNG2, hBNG3 and hBNG4, respectively.

2.3 Structural and morphological characterization. X-ray diffraction (XRD) studies of the

M AN U

composite were carried out at room temperature on D/Max 2500 V/PC (Rigaku Corporation, Tokyo, Japan) at a scan rate of 1º min-1(Cu Kα radiation, λ= 0.15418 nm). Crystalline size of the nanoparticles was calculated according to the (311) peak line width using the Scherrer equation, D = 0 .9 λ

BCosθ

, where, D is the dimension of the nanoparticles, λ is the wave length of the Kα

TE D

radiation; B is the ‘Full Width Half Maximum’ (FWHM) of the XRD peak, and θ is the corresponding angle. Field emission scanning electron microscopy (FE-SEM) images were recorded with Ʃigma HD, Carl Zeiss, Germany. Transmission electron microscopy (TEM) was

EP

recorded using JEOL JEM-2200 FS. For sample preparation, the composite sample was dispersed in ethanol-water mixture (~ 0.1 mg ml-1) by 20 min ultrasonication followed by drop

AC C

casting onto a fresh lacey carbon copper grid. Raman spectra of the samples were obtained on a Nanofinder 30 (Tokyo Instruments Co., Osaka, Japan) using Laser wavelength of 514 nm and 100 µm spot size. X-ray photoelectron spectroscopy (XPS) (Axis-Nova, Kratos Analytical Ltd., Manchester, UK) was carried out using a monochromatic Al-Kα X-ray source (1486.6 eV) from KBSI Jeonju Centre. The base pressure and dwell time were 5.2 × 10-9 T and 100 ms, respectively. The pass energy of the hemisphere analyzer was set at 40 and 160 eV for narrow and wide scan, respectively. The XPS results were fitted with Casa XPS 2.3.17 software. The

6

ACCEPTED MANUSCRIPT

UV-vis spectroscopy was measured by an Agilent Cary 60 spectrophotometer. The electrical conductivity was measured using four probe set up with KEITHLEY delta system consisting of

RI PT

AC & DC current source, model: 6221 and Nano voltmeter model: 2182A. 2.3 Supercapacitor electrode preparation and electrochemical characterization. The supercapacitor electrodes were prepared using the rGO/h-BN composite of ∼80 wt % active

SC

materials, 10 wt % PVDF, and 10 wt % carbon black dispersed in 10 mL of DMF. The electrochemical tests of the individual electrode were carried out in a three-electrode cell, where

M AN U

platinum wire and Ag/AgCl acted as the counter and reference electrodes, respectively. A cleaned Ni foam substrate (5 cm × 0.2 cm) was used as the working electrode and the above said composite was deposited (∼100 mg) on to it (area of 0.5 cm × 0.2 cm). The working electrode was dried inside a vacuum oven at 60 °C for ∼24 h. For asymmetric supercapacitor (ASCs), two pieces of nickel foam (0.66 cm diameter) were taken and about 15 drops (equivalent to 160 mg)

TE D

of the above-mentioned composite were drop-casted onto the positive electrode. Similarly, the negative electrode material was prepared by mixing 80 wt % TRGO, 10 wt % PVDF, and 10 wt

EP

% carbon black, dispersed in 10 mL of DMF. About 25 drops (equivalent to 340 mg) of this composite were casted on to the negative electrode. The electrodes were dried under vacuum for

AC C

24 h at 60 °C. The two electrodes were placed inside a split test cell (EQ-STC, MTI Corporation, Richmond, CA) to design the ASC. The electrodes were separated by Whatman 42 filter paper impregnated with 6 M aqueous KOH. The electrochemical properties were recorded with PARSTAT 4000 (Princeton Applied Research, USA) electrochemical workstation using 1 M Na2SO4 as the electrolyte. Specific capacitance was calculated from the CV curves using the equation: CCV =

(∫ ΙdV ) mvV , where C

CV

is the specific capacitance (F g-1 ) determined from the

CV curve, I is the response current density (A g-1 ), V is the potential window, υ is the scan rate

7

ACCEPTED MANUSCRIPT

(mV s−1 ), and m is the deposited mass on the electrode. The specific capacitance can be calculated in terms of discharge time according to the equation: CCD = Ι∆ t

mV

, where CCD is the

RI PT

specific capacitance (F g-1 ) determined from the charge-discharge curve, I is the discharging current (A g-1 ), V is the potential window (V) and m is the deposited mass on the electrode. Volumetric capacitance (FV) was calculated as FV = Fg × d m , where Fg and dm are the

SC

gravimetric capacitance and electrode density, respectively.The mass balance of the electrodes m+ C− × ∆Ε − = , where m− C+ × ∆Ε +

M AN U

for asymmetric configuration, was carried out according to the relation:

m+ and m- are the mass of the positive and negative electrode materials, C+ and ∆E+ are the specific capacitance and potential window (calculated from the three electrode measurement) for positive electrode, C- and ∆E- are the specific capacitance and potential window of the negative electrode measured at the same scan rate. The energy density (ED) and power density (PD) can be 2

TE D

calculated according to the formula, E D = CV

2

and PD =

ED

∆Τ

, where, C is the specific

EP

capacitance, V is the operating voltage, ∆T is the discharge time.

AC C

3. Results and discussion

The XRD pattern of the composites shows broad and intense peaks at 2θ~26.8o (Fig. S1 of the supporting information). The h-BN has (002) characteristic peak at 2θ~26.7o and the graphitic peak of rGO corresponding to (002) plane appears at the same position (Fig. S2 of the supporting information) [9]. Therefore, it is very difficult to identify the peaks corresponding to h-BN and rGO separately from the XRD pattern of the composite materials [9]. The peak broadening is noticed with increasing GO concentration in the precursor. The broadening (full width half

8

ACCEPTED MANUSCRIPT

maxima of the peak) of the (002) peaks of hBNG1, hBNG2, hBNG3 and hBNG4 was measured as 0.267, 0.317, 0.388 and 0.341o, respectively. The formation of the nano domain of the hBN/rGO hetero-structure may be the reason of the peak broadening. The decreased peak width of

RI PT

hBNG4 as compared to hBNG3 suggests increase in the crystalline dimension due to the presence of larger amount of rGO domain in the composites. Fig. S3 of the supporting information shows the Raman spectra of composite samples. The G peak of pure graphite or

SC

pristine graphene appears due to the bond stretching of sp2 carbons [9,16,24]. Graphene shows another characteristic 2D peak resulting from the two phonon double resonance process within

M AN U

the band structure [9,16,24]. On the other hand, the presence of defects or disorders in the graphene moiety generates D band spectra [9,16,24]. Fig. S4 of the supporting information shows the Raman spectra of rGO. The D and G band of rGO are appeared at 1345 and 1569 cm1

, respectively. The intensity of G band of rGO is higher than that of the D band. In addition, the

TE D

sharp 2D peak of rGO is also clearly visible at 2690 cm-1. h-BN has similar structure to that of graphite but the stacking sequence is different. h-BN shows its characteristic Raman-active highenergy phonon due to the E2g symmetry similar to the D band of the graphene [25]. The ID/IG

EP

ratio (ID = intensity of the D band, IG = intensity of the G band of Raman spectra) can provide the change in the microstructure and composition of rGO and h-BN within the composites [9,16,24].

AC C

The ID/IG ratio shows consistency with the variation of rGO concentration in the composites. The ID/IG ratio was calculated as 3.69, 1.52, 1.05 and 0.38 for hBNG1, hBNG2, hBNG3 and hBNG4, respectively. The low ID/IG ratio and sharp 2D peaks of hBNG4 is due to the higher concentration of rGO. On the other hand, high ID/IG ratio and disappearance of 2D peak suggests the presence of h-BN dominated phase in hBNG1. Although it is expected that the rGO concentration increases in hBNG2 but the D band intensity was still very high and no prominent

9

ACCEPTED MANUSCRIPT

2D peak was present. The generation of defects and lack of homogeneity in the composite may be the reason of large D band intensity and invisible 2D peak of hBNG2. However, the appearance of sharp 2D peak was noticed in hBNG3 and the D band intensity also decreases as

RI PT

compared to hBNG1 and hBNG2. It is expected that the proper concentration of B, N and GO in the precursor creates super-lattice structure where the carbon hexagons are surrounded by BN hexagons and π-conjugation is allowed through the carbon path way. The formation of stable

SC

super-lattice structure decreased the defect level and G band intensity increases. The appearance

and rGO phase within the super-lattice.

M AN U

of 2D peak also supports the decrease in domain size due to the proper incorporation of the h-BN

Fig. 1 (a-d) represents the XPS survey spectra of hBNG1, hBNG2, hBNG3 and hBNG4. Prominent C1s (~285 eV), B1s (~190 eV), N1s (~400 eV) and O1s (~533 eV) peaks are present in all the composite samples [9,16, 26-28]. The atomic percentage (At%) of C is lower (~17%)

TE D

for hBNG1 and continuously increases for hBNG2 (~41%), hBNG3 (~51%) and hBNG4 (~68%), respectively. The increasing C1s At% indicates continuous increase in the amount of rGO phase within the microstructure of the h-BN/rGO hetero-structure. A continuous decrease in

EP

At% of B1s and N1s is noticed analogous to the increase in rGO concentration. Fig. 2 (a-d)

AC C

shows the high resolution C1s spectra of hBNG1, hBNG2, hBNG3 and hBNG4. The deconvoluted C1s spectra of the h-BN/rGO hetero-structures show the presence of C-C (~284.4 eV), C-N (~286.1 eV), C-B (~283.8 eV) and C-O (~285.8 eV) respectively [9,16,24,27, 28]. However, the intensity of the C-C deconvoluted peaks do not decreases consistently as was expected from the decreasing C1s At% from the survey analysis. The concentrations of C-C initially increases followed by decreases and again increase with increasing At% of C (Table S1

10

ACCEPTED MANUSCRIPT

of the supporting information). This discrepancy of C1s spectra and survey analysis may be due

AC C

EP

TE D

M AN U

SC

RI PT

to the change in micro structure of the B/N/C domain.

Fig. 1. XPS survey spectra of (a) hBNG1, (b) hBNG2, (c) hBNG3 and (d) hBNG4. The inset of the figures shows the position and the At% of the XPS peaks. The study of high resolution B1s spectra may lighten the bond formation and change in micro structure within the composite with varying rGO concentration. The B1s spectra can be deconvoluted in two separate peaks due to the B-C (~189.5 eV) and B-N (~190.5 eV) bond

11

ACCEPTED MANUSCRIPT

[9,16,26] . The concentrations of the B-C bond in the B1s spectra were measured as 32, 23, 41and 70% in the hBNG1, hBNG2, hBNG3 and hBNG4, respectively (Table S2 of the supporting information). On the other hand, the concentrations of the B-N bond in the B1s

RI PT

spectra were calculated as 68, 59, 77 and 30% in the hBNG1, hBNG2, hBNG3 and hBNG4, respectively. The detail analysis of B1s spectra suggested that with lower amount of rGO (hBNG1) the ratio of the B-N bond concentration is much larger than the B-C bond (Fig. S5 of

SC

the supporting information). The B-C bond concentration decreases and the B-N bond concentration increases in hBNG2. The formation of h-BN microstructure with higher amount of

M AN U

B and N may be the reason of this kind of variation of bonding ratio in the composite samples. However, the increase in B-C bond concentration and decrease in B-N bond concentration were noticed with further increase in rGO content in the composite (hBNG3). It is expected that with lower C1s At%, small amount of rGO was doped in h-BN grain (hBNG1). Separate rGO and h-

TE D

BN grain were formed with increasing concentration of rGO (hBNG2) resulting decreased B-C and increased C-C bonding concentration. The formation of super-lattice may be expected with further increasing C1s At% (hBNG3). The h-BN micro structure was surrounded by C atoms

EP

(rGO) and vice versa in the super-lattice of rGO/h-BN and as a result the B-C bonding strength increases in hBNG3 as compared to the hBNG2. The increase in B-C bond strength in hBNG4 as

AC C

compared to hBNG3 is expected due to the increase in C1s At% and decrease in B1s and N1s At% as a whole. Furthermore, the detail study of high resolution N1s spectra strengthens the previous explanation provided by the B1s and C1s spectra. The deconvoluted N1s spectra shows two separate peaks of B-N (~398.5 eV) and B-N-C (~399 eV) (Fig. S6 of the supporting information) [9,16,26]. The B-C-N concentration decreases in hBNG2 as compared to hBNG1 indicating presence of separate rGO and h-BN domain in the composite. The B-C-N bond

12

ACCEPTED MANUSCRIPT

formation in hBNG3 increases due to the formation of super-lattice. Very high B-C-N

AC C

EP

TE D

M AN U

SC

RI PT

concentration in hBNG4 may be attributed to the high C1s At%.

Fig. 2. High resolution C1s spectra of (a) hBNG1, (b) hBNG2, (c) hBNG3 and (d) hBNG4.

13

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 3. FE-SEM image of (a) hBNG2, (b) hBNG3. The atomic frame work of (c) hBNG2 and (d)

EP

hBNG3.

AC C

Fig. 3(a&b) shows the FE-SEM images of hBNG2 and hBNG3. The morphology of hBNG2 and hBNG3 are totally different. The formation of sheet like structure is more prominent in hBNG3 as compared to the hBNG2. Two different structures corresponding to h-BN and rGO in hBNG2 agreed well with the XPS analysis. The higher concentration of rGO plays the role of a two dimensional template and the h-BN aligned as sheets. The similar type of dimensionality and architecture increase the intercalation between rGO and h-BN forming super-lattice heterostructure. The formation of super-lattice with increasing rGO concentration was further approved

14

ACCEPTED MANUSCRIPT

by the FE-SEM images of hBNG2 and hBNG4 (Fig. 3a & 3b). The covalent bonding is not formed due to the lower At% of C in hBNG2. The probable crystal structures of hBNG2 is given in the Fig. 3c (showing different layer of rGO and h-BN) where the layers of rGO and h-BN are

RI PT

non-covalently attached. However, only sheets are visible in hBNG3. The appearance of sheet like morphology suggested the increasing rGO concentration helped to form rGO/h-BN superlattice where h-BN also aligned with the rGO sheets. The probable atomic frame work of h-

AC C

EP

TE D

M AN U

structure is expected during h-BN and rGO stacking.

SC

BN/rGO super-lattice (hBNG3) is given in the Fig. 3d where the formation of intermediate

Fig. 4. (a) TEM and (b) HR-TEM image of hBNG3. The inset view shows the magnifying image and (c) SAED pattern of hBNG3

15

ACCEPTED MANUSCRIPT

TEM image of hBNG3 shows that the h-BN layers are aligned as graphene like sheet as shown in Fig. 4a. The high resolution TEM image of the h-BN/rGO super-lattice (Fig. 4b) shows clear orientation of rGO and h-BN nano sheets. The inset view of the Fig. 4b shows the repeated

RI PT

growth of sandwiched rGO layers in between the h-BN nano sheets. The width of the superlattice grain was measured as ~15 nm. The SAED pattern image (Fig. 4c) was captured corresponding to the h-BN/rGO layer as shown in Fig. 4b. The SAED pattern image confirmed

SC

the both the h-BN and rGO has hexagonal structure and their periodic growth maintain the

AC C

EP

TE D

M AN U

crystallinity as was expected from the super-lattice structure.

Fig. 5. (a) UV-visible absorption spectra (b) (αhν)2 vs. hν plot, (c) XPS valance band spectra and (d) XPS valance band offset of hBNG1, hBNG2, hBNG3 and hBNG4.

16

ACCEPTED MANUSCRIPT

The formation of different micro structure was further re-investigated using UV-vis spectroscopy, optical and electrochemical band gap analysis. Fig. 5a represents the UV-vis absorption plot of the composites. The intense and single peak at 261 nm of hBNG1 suggests the

RI PT

presence of h-BN. It is expected that the rGO were completely doped in the h-BN moiety. The broadening of peaks (258-297 nm) was noticed for hBNG2. The formation of incomplete stacking or inhomogeneous structure results in the peak broadening. The presence of two very

SC

close peaks corresponding to the separate region of rGO and h-BN may be another reason for peak broadening. Two broad peaks are seen in hBNG3 at 261 and 339 nm. The peak at lower

M AN U

wave length region is in between that of the pure rGO and h-BN and may be attributed to the insertion of the C atoms in the h-BN hexagon [9,16]. The peak at higher wave length is corresponding to the stacking of h-BN and rGO layers [9,16]. This result is quite similar to the XPS analysis where the concentration of bonding between C-B/N increases suggesting

TE D

homogeneous stacking of h-BN and rGO microstructure. The single absorption peak at ~292 nm is attributed to the resonant excitonic effects of rGO corresponding to the electron-hole interactions in the π→π* transition [9,16]. The absence of separate peaks related to the B, N or h-

EP

BN confirms the B, N atoms are doped within the rGO (hBNG4). The electronic and electrochemical properties are directly related to the band structure of the materials. Band gap

AC C

was calculated by extrapolation of the (αhν)2 vs. hν plot to ν = 0 as shown in Fig. 5b. The presence of small amount of rGO reduces the band gap of the hBNG4 (3.19 eV) as compared to the pure h-BN (5.3 eV) [9]. However, the accurate calculation of the band gap energy of hBNG2 is difficult due to the highly broadened absorption edge. Furthermore, the XPS analysis assumed the presence of separate rGO and h-BN grain in hBNG2. The broadening of the absorbance peak may be due to the presence of two separate grains or phase in hBNG2. The band gap was

17

ACCEPTED MANUSCRIPT

estimated to be 2.8 eV for hBNG2. Two band gaps were generated corresponding to the two different absorption edge of hBNG3 (2.02 and 3.15 eV) [9,16]. The appearance of the absorption edge at higher wave length and corresponding low band gap energy may consider as the

RI PT

evidence of h-BN/rGO stacking or formation of super-lattice structure [9,16]. The opening of the small band gap of 1.6 eV of hBNG4 may be attributed to the B, N doping within the rGO moiety [9,16]. It is evident that the band gap of the composite is reduces with increasing the

SC

concentration of rGO. However, the UV-vis spectroscopy does not give any information about the position of the valance and conduction band in the h-BN/rGO microstructure. The XPS

M AN U

valance band analysis was carried out for proper understanding of the band alignment in the super-lattices. The spectra have four distinct peaks at 2.8, 5, 7 and 9 eV corresponding to graphitic π bonding, B2p, C2p and N2p, respectively (Fig. 5c) [29,30]. The C2p is related to the σ bonding in graphite. The peaks related to the π and σ bonding is absent in the hBNG1. Other

TE D

samples show intense π bonding peaks as compared to the peaks due to σ bonding. The presence of π bond peak also influences the valance band offset of the samples (Fig. 5d). The increase in the density of state (hBNG2, hBNG3 and hBNG4) near the Fermi level can be ascribed to the

EP

graphitization or the stacking of the sp2 rGO and h-BN.27 The valance band offset was measured as 2.07, 0.89, 0.85, and 0.66 eV for hBNG1, hBNG2, hBNG3 and hBNG4, respectively. The low

AC C

density of state or large valance band offset of hBNG1 is due to the lower concentration of rGO. The flat band potential related to the alignment of the conduction band can be evaluated from the electrochemical Mott–Schottky plots (Fig. 6a) [30,31]. The extrapolation of the MottSchottky plots yielding the flat band potential as -1.16, -1.14, -1.11 and -0.91 eV for hBNG1, hBNG2, hBNG3 and hBNG4, respectively. The delocalization of the C atoms increases the πconjugation and the donor level of the samples induces below the conduction band with

18

ACCEPTED MANUSCRIPT

increasing the rGO concentration. The band gap energy was calculated from the difference between the valance band offset and the conduction band offset measured from XPS and MottSchottky analysis, respectively. The measured band gap energy shows good similarity (except

RI PT

hBNG2) with the previously calculated optical band gap energy from the UV-vis spectroscopy (TableS3 of supporting information). The discrepancy in the band gap energy of hBNG2 may be due to the broad UV-vis absorption peaks as the result of inhomogeneous structure. The above

SC

discussion on the band structure shows that the entire four h-BN/rGO composite with different microstructure are semiconducting materials. The electrical conductivity of the semiconducting

M AN U

materials depends on the band gap energy. On the other hand, the electrochemical activity of the electrode materials depends on the electrical conductivity and the electronic work function [32,33]. The electronic work function can be measured from the variation of the electrical conductivity with temperature. Fig. 6b shows the temperature vs. electrical conductivity plot of

TE D

the samples. The electrical conductivity of all the samples increases with temperature showing typical semiconducting behaviour. The electrical conductivity of the hBNG1 (0.2 S m-1) is very low and almost insulating in nature. Electrical conductivity increases with increasing rGO

EP

concentration. The electrical conductivity of the hBNG2 and hBNG3 were measured as 288 and 529 S m-1, respectively. Very high electrical conductivity (714 S m-1) of hBNG4 is due to the

AC C

small band gap energy. UV-vis analysis shows that hBNG2 has broad absorption edge and hBNG3 has two band gaps corresponding to the C doped h-BN and h-BN/rGO interface. The XPS analysis predicted the formation of super-lattice in hBNG3 where the hBNG2 consists of separate layer of rGO and h-BN. The high electrical conductivity of the hBNG3 may be ascribed to the carrier confinement within the “graphene paths” embedded in the h-BN domains and enhanced π conjugation due to the formation of super-lattice structure. The electronic work

19

ACCEPTED MANUSCRIPT

function was calculated from the slope of the log ρ vs. 103/T plot (Fig. 6c). The h-BN rich hBNG1 shows large work function of 296 meV analogous to its large band gap energy and low electrical conductivity. The electronic work function also decreases with increasing rGO

RI PT

concentration. The electronic work function of hBNG2 (89 meV) is quite larger as compared to the hBNG3 (22 meV). The presence of separate domains of h-BN makes the availability of the electron less in hBNG2 but the formation of h-BN/rGO super-lattice in hBNG3, electro

AC C

EP

TE D

M AN U

expected due to the rGO rich environment.

SC

accessibility increases. The electronic work function of hBNG4 (9 meV) is very small as

Fig. 6. (a) Mott-Schottky plots (b) Temperature vs. conductivity plot and (c) log ρ vs. 103/T plot of hBNG1, hBNG2, hBNG3 and hBNG4.

20

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 7. CV of (a) hBNG1, (b) hBNG2, (c) hBNG3 and (d) hBNG4 at different scan rates.

EP

The CV analysis was carried out to correlate the effect of different micro structure, band gap and work function on the electrochemical properties of the samples. Fig. 7 (a-d) shows the CV

AC C

profiles of hBNG1, hBNG2, hBNG3 and hBNG4, respectively. The potential window and the nature of the CV curves were differed abruptly with the variation of microstructure of the hBNG composites. Generally, rGO shows rectangle shaped CV (i.e. EDLC) curve at the negative potential region (Fig. S7 of the supporting information). No redox peak is observed and the specific capacitance was calculated as 280 F g-1 at 10 mV s-1 scan rate. Fig. S8 of the supporting information shows the retention of the specific capacitance of rGO. The retention was very high (~68%) even at a high scan rate of 200 mV s-1. The near rectangle shape CV curves of hBNG4 at

21

ACCEPTED MANUSCRIPT

the negative potential region (-1 to 0.4 V) suggests the presence of rGO dominated phase. The specific capacitance of hBNG4 was calculated as 320 F g-1 at a scan rate of 10 mV s-1. On the other hand, the potential window of hBNG1 is shifted to the positive region (-0.1 to 0.5 V) and

RI PT

prominent pseudo peaks (cathodic~0.25V and anodic~ 0.18 V) was observed. The oxygen evolution is started after ~0.36 V. Furthermore, the low specific capacitance of hBNG1 (218 F g-1 at a scan rate of 10 mV s-1) may be attributed to the presence of h-BN dominated regions. It

SC

was found that the potential window of hBNG2 lies in both the positive and negative region (-0.5 to 0.5) and the CV curves was neither EDLC nor pseudocapacitive but showing a distorted

1

M AN U

nature. The specific capacitance of hBNG2 was calculated as 481 F g-1 at a scan rate of 10 mV s. On the other hand, the nature of the CV curves of hBNG3 is quite similar to that of hBNG1and

prominent pseudo peaks were observed at the same position. However, the current response of hBNG3 is much better as compared to the hBNG1. In addition, the oxygen evolution of hBNG3

TE D

is started after ~0.42 V which is better as compared to the hBNG1. The specific capacitance of hBNG3 was calculated as 960 F g-1 at a scan rate of 10 mV s-1. Poor intercalation between the rGO and h-BN layer may be the reason of the distorted CV nature of hBNG2. The wide potential

EP

window of hBNG2 also supports the presence of two separate grains of rGO and h-BN. On the other hand, the synergistic effect of the EDLC and pseudocapacitance suggest the good

AC C

incorporation of rGO and h-BN layer due to the formation of the super-lattice. Fig. S9 of the supporting information presents the variation of the specific capacitance with scan rate. The specific capacitance of hBNG1, hBNG2, hBNG3 and hBNG4at high scan rate of 200 mV s-1 were calculated as 109, 269, 609 and 215 F g-1, respectively. The good rate capability of hBNG3 as compared to the hBNG2 also supports the facile electrochemical activity due to the formation of super-lattice. The retention of the specific capacitance was 50, 55, 63 and 67 % for hBNG1,

22

ACCEPTED MANUSCRIPT

hBNG2, hBNG3 and hBNG4, respectively (Fig. S10 of the supporting information). The improvement in the rate capability of the rGO/h-BN electrodes is observed with increasing rGO concentration. Finally, the rate capability of hBNG4 is almost similar to the pure rGO samples.

RI PT

The specific capacitance and the rate capability of the samples shows good consistency with the elemental structural analysis, band gap and work function study. Furthermore, the electrochemical performance of the h-BN/rGO super-lattice electrode was compared with other

SC

graphene or rGO based electrode as shown in Table S9 of the supporting information. The specific capacitance of h-BN/rGO is comparable or some times better than other reported

M AN U

electrode materials. The volumetric capacitance of h-BN/rGO composites was measured to verify the utility of the electrode materials for the practical applications. The thickness of the single electrode was measured as 0.152 cm. The volume of the electrode was measured as ~0.0152 cm3. The electrode material’s density (dm) in working electrode was calculated as 6.4

TE D

mg cm-3. Finally, the volumetric capacitance of hBNG1, hBNG2, hBNG3 and hBNG4 was measured as 1395, 3087, 6144 and 2048 mF cm-3, respectively. However, the charge storage mechanism is deeply interrelated with the available active site as

EP

well as the microstructure of the electrode materials. The proportion of the EDLC and pseudocapacitance was estimated from the power law obeyed by current (i) and the scan rate (υ):

AC C

i = aυ b , where, a and b has appropriate values [34,35]. The value of b close to 0.5 indicating the diffusion controlled relation of i and υ or redox charge storage mechanism of the electrode [34,35]. On the other hand, if the value of b is close to 1 then EDLC charge storage mechanism is dominated [34,35]. The slope of log (i) versus log(υ) plot is the measurement of b (Fig. 8a). The b was calculated as 0.69, 0.79, 0.81, 0.83 and for hBNG1, hBNG2, hBNG3 and hBNG4, respectively. The CV nature shifted from pseudocapacitance to EDLC with increasing the rGO

23

ACCEPTED MANUSCRIPT

concentration. Although, the CV plot shows prominent redox peak in hBNG3 than the hBNG2, the larger b value of hBNG3 as compared to the hBNG2 may be attributed to the increasing πconjugation due to the formation of super-lattice. The charge storage capacity increases with

RI PT

increasing the accessibility of the total active site of the supercapacitor electrodes. The accessibility of the electrochemical active surface area is the measurement of voltametric charge (q*). The total voltametric charge of an electrode arises from the contribution of the outer surface

SC

voltametric charge (q*out) and the inner surface voltametric charge (q*in). The q*out and q*in were measured from the extrapolation of q* to V= 0 and V=∞, respectively (Fig. 8b & 8c) [36-38].

M AN U

The easily accessible q*out is smaller than q*in for all the samples. The q*out was calculated as 75.5, 213.9, 515.9 and 197.8 C g-1 for hBNG1, hBNG2, hBNG3 and hBNG4, respectively. On the other hand, the measured value of q*in was 154.8, 561.8, 649.4, and 505C g-1 forhBNG1, hBNG2, hBNG3 and hBNG4, respectively. Fig. 8d shows the percentages of the inner and outer

TE D

active surface of the samples. The q*out percentage of hBNG1 and hBNG3 (48 and 79%) are quite higher than that of hBNG2 and hBNG4 (37 and 39%). It may be attributed to the increased diffusion control reaction and the creation of defect. It is expected that the diffusion controlled

EP

capacitance dominates and generation of defects at the h-BN/ rGO hetero-junction also elevated with increasing the concentration of h-BN. The generation of active sites and defects is improved

AC C

with increasing hetero-junction. The lack of intercalation prevents the formation of heterojunction in hBNG2. The presence of separate grain of h-BN and rGO reduces the percentage q*out in hBNG2.The percentage q*out increases when the super-lattice is formed in hBNG3.However, very low content of rGO in hBNG1 reduces the surface area and the percentage of q*out decreases as compared to the hBNG3.

24

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 8. (a) log (i) versus log(υ) plots (b) extrapolation of q* to υ =0 from the 1/q* vs. the υ1/2 plot, 1/2

plot, (d) variation of q*out/q*in vs. q*in of

EP

(c) extrapolation of q* to υ =∞ from the q* vs 1/υ hBNG1, hBNG2, hBNG3 and hBNG4.

AC C

The electrochemical impedance spectroscopy (EIS) was further analysed to investigate the changes occurring within intercalation compounds due to the presence of different micro structure [39]. Equivalent circuit was generated by curve fitting and simulation of the Nyquist plot. The poor capacitive performance of hBNG1 is clearly visible from the Nyquist plot and the generated equivalent circuit (Fig. S11 of the supporting information). The fitted results are summarized in Table S4 of the supporting information. The charge storage mechanism was dominated by the diffusion controlled mechanism of h-BN. The presence of rGO provides little

25

ACCEPTED MANUSCRIPT

contribution to the capacitance. High solution resistance and very large internal resistances of hBNG1 may be attributed to the poor electrical conductivity of the h-BN domain. Furthermore, very high impedance of hBNG1 at low frequency region (~10,000 ohm) suggests its poor

RI PT

electrochemical activity (Fig. S12 of the supporting information). The Nyquist fitting and the equivalent circuit of hBNG2 is shown in Fig. S13 of the supporting information and the fitted result are summarized in the Table S5 of the supporting information. The solution resistance of

SC

hBNG2 decreases as compared to hBNG1. The charge storage mechanism of hBNG2 was also dominated by diffusion controlled mechanism due to the large concentration of h-BN. Two

M AN U

EDLC elements were in parallel connection. The appearance of two EDLC elements may be due to the presence of rGO in the h-BN domain and π-conjugation between C and h-BN structure. The existence of phase separated rGO and h-BN grain was identified from the XPS and optical analysis. The appearance of another EDLC element (in series connection) may be due to the

TE D

poorly intercalated separate rGO grains of hBNG2. The internal resistance of the hBNG2 was found to be very high even when the rGO concentration increases as compared to the hBNG1. Poor intercalation between the different grains of rGO and h-BN generates huge internal

EP

resistance. The large band gap of the hBNG2 was another reason of poor internal resistance. The increased knee frequency of hBNG2 may be attributed to the increase in diffusion controlled

AC C

reaction during the charge storage (Fig. S14 of the supporting information). However, the impedance at the low frequency region is very large (~2000 Ω) indicating poor capacitive performance of hBNG2. The formation of super-lattice as well as incorporation of the rGO and h-BN layer was confirmed by the Nyquist plot and equivalent circuit of hBNG3 (Fig. S15 of the supporting information). Table S6 presents the detailed of the parameters of the equivalent circuit. Very low solution resistance of hBNG3 was due to the increasing rGO concentration.

26

ACCEPTED MANUSCRIPT

Apart from the hBNG1 and hBNG2, the charge storage mechanism of hBNG3 was dominated by EDLC. The parallel connected Q and second EDLC element may be ascribed to the h-BN and C doped h-BN domain of the hBNG3 super-lattice. The increase in the π-conjugation may be the

RI PT

reason of the high capacitance of the secondary EDLC element. Electron flow through the hetero-junction of a super-lattice is increased due to the tunnelling effect. The low internal resistances of hBNG3 (~0.2, 16 and 0 Ω) may be attributed to the enriched intercalation of the

SC

different domain in the super-lattice structure. The impedance of hBNG3 at the lower frequency region also very small (13 ohm) and the knee frequency was measured as 100 Hz indicating the

M AN U

formation of shorter diffusion path within the super-lattice structure (Fig. S16 of the supporting information). The capacitance behaviour of the hBNG4 is dominated by the formation of double layer capacitance (Cdl) due to the presence of large concentration of rGO (Fig.S17 of the supporting information). Very low series resistance may be attributed to the high electrical

TE D

conductivity of hBNG4. The simulated results of hBNG4 are given in the Table S7 of the supporting information. The constant phase element and EDLC (parallel connected) arises due to the diffusion caused by the doped B, N within the rGO network and the π-conjugation of B, N

EP

and rGO microstructure. Very low internal resistances between the different domains (element) indicate homogeneous structure of the electrode materials. Fig. S18 of the supporting

AC C

information presents the Bode plot of hBNG4. The impedance is very low (0.6 ohm) at high frequency and reach ~5 ohm at the low frequency region. The low knee frequency ~ 1Hz suggests the charge storage mechanism of hBNG4 is dominated by EDLC. On the basis of above discussion, hBNG3 and hBNG4 were selected as the positive and negative electrode materials to form an asymmetric device. The CV of hBNG3 and hBNG4 is compared (Fig. 9a) for mass balance and the asymmetric (ASC) device was scanned in the range

27

ACCEPTED MANUSCRIPT

of 0-1.5 V (Fig. 9b). However, the CV of ASC shows quite distortion above 1.4 V indicating an IR drop. The IR drop can be minimised by setting the potential window up to 1.4 V during CD analysis (Fig. 9c). The CD was performed within the range of low (1 A g-1) to very high (20 A g) current density. High specific capacitance of 270 and 128 F g-1 were recorded at current

RI PT

1

density of 1 and 20 A g-1, respectively. Fig. 9d shows the Ragone plot of the ASC. The maximum energy density was recorded as 73 W h kg-1 corresponding to a power density of 700

SC

W kg-1. The energy density remains 35 W h kg-1 at a very high power density of 14000 W kg-1. In order to investigate the practical utility, volumetric energy and power density of the ASC were

M AN U

also calculated [40,41]. The thickness of the asymmetric supercapacitor was the summation of two working electrode (nickel foam current collector of 1.144 cm diameter) and the separator (0.02 cm). The total thickness is ~0.324 cm. The volume of the asymmetric supercapacitor was calculated as ~0.333 cm3. The electrode density was calculated by dividing the total deposited

TE D

mass by the volume of the electrode (Electrode density (ED) = Mass/Volume). The electrode density was measured as ~1.5 mg cm-3. The volumetric capacitance was measured as 405 and 192 mF cm-3 were calculated at current density of 1.5 and 30 mA cm-3, respectively. The energy

EP

density was recorded as 109 and 52.5 W h L-1 corresponding to a power density of 1050 and 21000 W L-1, respectively. Fig. S19 of the supporting information shows the ZSimpWin fitted

AC C

Nyquist plot of ASC and the fitted results are summarized in the Table S8 of the supporting information. The impedance analysis shows the proper balance of mass or charge of the individual electrode materials while forming the ASC device (Detailed analysis was discussed in the supporting information). The supercapacitor performance (i.e. the specific capacitance, energy and power density) of the ASC device was compared with other asymmetric device in Table S10 of the supporting information. The comparison revelled that the ASC device based on

28

ACCEPTED MANUSCRIPT

hBNG3 and hBNG4 is comparable or sometimes superior to the other reported supercapacitor

EP

TE D

M AN U

SC

RI PT

device.

Fig. 9. Comparison of the potential window of hBNG3 and hBNG4, (b) CV of ASC (c) CD

AC C

plots of ASC at different current density, and (d) Ragone plot of the ASC. Furthermore, the fast charge delivery capability or low relaxation time constant (τ0) is another indication of the efficient supercapacitor device. The τ0 was calculated from the power dissipation analysis of the system [23]. Fig. S20 of the supporting information represents the variation of real P (ω) and imaginary part Q (ω) of the normalized complex power S (ω) as a function of frequency. The detailed calculation of the P (ω), Q (ω), S (ω), relaxation frequency

29

ACCEPTED MANUSCRIPT

(f0) and τ0has been given in the supporting information. The f0 was measured as ~99 Hz from the convergence of the P (ω) and Q (ω) of the normalized complex power (Fig. S20 of the supporting information). At high frequency region, the behaviour of supercapacitor is pure

RI PT

resistive and all the power (~100 %) is dissipated into the system. On the other hand, power dissipation is almost zero for a pure capacitance system at low frequency region. The f0 is the indication of the suppressed resistive behaviour of the supercapacitor by the capacitor below this

SC

frequency range. The low vale of τ0 (~1.6ms) indicates extremely fast energy delivery capability of the ASC. The fast frequency response of the ASC may be attributed to the proper charge

M AN U

balance of the electrodes with homogeneous intercalated structure. The stability was recorded at very high current density of 10 A g-1and the ASC shows high stability of ~80 % even after 10,000 CD cycles (Fig. S21 of the supporting information). The initial increase of the discharging time up to 1000 cycles may be due to the proper wetting of the electrode materials

4. Conclusions

TE D

by electrolyte.

EP

The increasing concentration of rGO within the hBNG composite results in different microstructure of C doped h-BN, h-BN/rGO hetero-structure, h-BN/rGO super-lattice and B, N

AC C

doped rGO, respectively. The band gap energy and the position of the conduction and valance band of hBNG composites changed with increasing rGO concentration. The electrical conductivity and the density of state increased while the activation energy decreased consistently with increasing rGO in the hBNG composites. Two different grain structures of rod and sheet were found in hBNG2 while hBNG3 showed sheet like morphology confirming the formation of super-lattice. Different microstructures showed different charge storage mechanism and divers potential window. The EDLC nature of the electrode increased in presence of higher amount of

30

ACCEPTED MANUSCRIPT

rGO. The Nyquist plots of the hBNG samples were explained by different equivalent circuit consisting of series and parallel combination of capacitance and resistance originated from the rGO, h-BN, π-conjugation of B, N and C moiety and available free electron due to the doping.

RI PT

Combination of EDLC and diffusion controlled charge storage mechanism in hBNG3 provide high specific capacitance of ~960 F g-1. On the other hand, hBNG4 showed good EDLC nature with very high rate capability as the negative electrode materials. ASC device was formed by

SC

using hBNG3 and hBNG4 which showed high energy and power density of 73 W h kg-1 and 14000 W kg-1, respectively. Furthermore the ASC showed very small relaxation time (1.6 ms)

M AN U

and high stability (~80%) even after 10000 CD cycles suggesting the potential applications of hBN/rGO super-lattice in next generation energy storage device.

Acknowledgment

Authors are thankful to the Director of CSIR-CMERI. Authors are also thankful to the

TE D

Department of Science and Technology, New Delhi, India, for the financial support from the DST-INSPIRE Faculty Scheme - INSPIRE Programme (IFA12CH-47) and Council of Scientific

AC C

(ESC0112/RP-II).

for funding MEGA Institutional project

EP

and Industrial Research, New Delhi, India,

References

[1] N. Hu, Composites and Their Properties, InTech, 2012. [2] L. L. Zhang, R. Zhou, X. S. Zhao, Graphene-Based Materials as Supercapacitor Electrodes X. S. J. Mater. Chem. 20 (2010) 5983-5992. [3] H. Liu, A. T. Neal, Z. Zhu, Z. Luo, X. Xu, D. Toma´nek, P. D. Ye, An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 8 (2014) 4033-4041.

31

ACCEPTED MANUSCRIPT

[4] F. H. L. Koppens, T. Mueller, P. Avouris, A. C. Ferrari, M. S. Vitiello, M. Polini, Photodetectors Based on Graphene, Other Two-Dimensional Materials and Hybrid Systems. Nat. Nanotechnol 9 (2014) 780-793.

graphene and beyond. Nanoscale 3 (2011) 20-30.

RI PT

[5] R. Mas-Balleste, C. Gomez-Navarro, J. Gomez-Herrero, F. Zamora, 2D materials: to

[6] H. Li, X. Duan, X. Wu, X. Zhuang, H. Zhou, Q. Zhang, X. Zhu, W.Hu, P. Ren, P.

SC

Guo, L. Ma, X. Fan, X. Wang, J. Xu, A. Pan, X. Duan, Growth of Alloy MoS2xSe2(1−x) Nanosheets with Fully Tunable Chemical Compositions and Optical

M AN U

Properties. J. Am. Chem. Soc. 136 (2014) 3756-3759.

[7] K. I. Seetharam, Undergraduate Thesis, Duke University, 2011. [8] V. K. Kononenko, L. Manak, D.V. Ushakov, Optoelectronic Properties and Characteristics of Doping Superlattices. Photoconversion: Sci. Technol. 3580 (1998)

TE D

10-27.

[9] S. Saha, M. Jana, P. Khanra, P. Samanta, H. Koo, N. C. Murmu, T. Kuila, Band gap engineering of boron nitride by graphene and its application as positive electrode

EP

material in asymmetric supercapacitor device. ACS Appl. Mater. Interfaces 7 (2015) 14211-14222.

AC C

[10] S. Saha, S. Chhetria, P. Khanra, P. Samanta, H. Koo, N. C. Murmu, T. Kuila, In-Situ Hydrothermal synthesis of MnO2/NiO@Ni hetero structure electrode for hydrogen

evolution reaction and high energy asymmetric supercapacitor applications. J. Energ. Storage 6 (2016) 22-31.

32

ACCEPTED MANUSCRIPT

[11] S. Saha, M. Jana, P. Khanra, P. Samanta, H. Koo, N. C. Murmu, T. Kuila, T. Band gap modified boron doped NiO/Fe3O4 nanostructure as the positive electrode for high energy asymmetric supercapacitors. RSC Adv. 6 (2016) 1380-1387.

RI PT

[12] R. Wang, X. Yan, Superior asymmetric supercapacitor based on Ni-Co oxide nanosheets and carbon nanorods. Sci. Rep. 4 (2014) 3712.

[13] D. P. Dubal, D. Aradilla, G. Bidan, P. Gentile, S. T. J. Schubert, J. Wimberg, S.

SC

Sadki, G. P. Romero, 3D hierarchical assembly of ultrathin MnO2 nanoflakes on silicon nanowires for high performance micro-supercapacitors in li- doped ionic

M AN U

liquid. Sci. Rep. 5 (2015) 09771.

[14] D. Golberg, Y. Bando, Y. Huang, T. Terao, M. Mitome, C. Tang, C. Zhi, Boron nitride nanotubes and nanosheets. ACS Nano 4 (2010) 2979-2993. [15] W. Zhang, C. Cheng, P. Fang, B. Tang, J. Zhang, G. Huang, X. Cong, B. Zhang, X.

TE D

Ji, L. Miao, The role of terminations and coordination atoms on the pseudocapacitance of titanium carbonitride monolayers. Phys. Chem. Chem. Phys. 18 (2016) 4376-4384.

EP

[16] L. Ci, L. Song, C. Jin, D. Jariwala, D. Wu, Y. Li, A. Srivastava, Z. F. Wang, K. Storr, L. Balicas, F. Liu, P. M. Ajayan, Atomic layers of hybridized boron nitride

AC C

and graphene domains. Nat. Matter. 9 (2010) 430-435.

[17] C. Chang, S. Kataria, C. Kuo, A. Ganguly, B. Wang, J. Hwang, K. Huang, W.Yang, S. Wang, C. Chuang, M. Chen, C. Huang, W. Pong, K. Song, S. Chang, J. Guo, Y. Tai, M. Tsujimoto, S. Isoda, C. Chen, L. Chen, K. Chen, Band gap engineering of chemical vapor deposited graphene by in situ bn doping. ACS Nano 7 (2013) 13331341.

33

ACCEPTED MANUSCRIPT

[18] Z. Wu, A. Winter, L. Chen, Y. Sun, A. Turchanin, X. Feng, K. Müllen, Threedimensional nitrogen and boron co-doped graphene for high-performance all-solidstate supercapacitors. Adv. Mater. 24 (2012) 5130-5135.

RI PT

[19] J. Han, L. L. Zhang, S. Lee, J. Oh, K. Lee, J. R. Potts, J. Ji, X. Zhao, R. S. Ruoff, S. Par, Generation of b‑doped graphene nanoplatelets using a solution process and their supercapacitor applications. ACS Nano 7 (2013) 19-26.

SC

[20] U. Alver, H. Yaykash¸ S. Kerli, A. Tanrıverdi, synthesis and characterization of

Mater. 20 (2013) 1097-1101.

M AN U

boron-doped NiO thin films produced by spray pyrolysis. Int. J. Miner. Metall.

[21] H. Z. Chi, Y. Li, Y. Xin, H. Qin,

Boron-doped manganese dioxide for

supercapacitors. Chem. Commun. 50 (2014) 13349-13352. [22] J. Chang, M. Jin, F. Yao, T. H. Kim, V. T. Le, H. Yue, F. Gunes, B. Li, A. Ghosh, S.

TE D

Xie, Y. H. Lee, Asymmetric supercapacitors based on graphene/MnO2 nanospheres and graphene/MoO3 nanosheets with high energy density. Adv. Funct. Mater. 23 (2013) 5074-5083.

EP

[23] A. Singh, A. Chandra, Significant performance enhancement in asymmetric supercapacitors based on metal oxides, carbon nanotubes and neutral aqueous

AC C

electrolyte. Sci. Reports 5 (2015) 15551.

[24] S. Saha, M. Jana, P. Samanta, N. C. Murmu, T. Kuila, In situ preparation of a SACRGO@Ni electrode by electrochemical functionalization of reduced graphene oxide using sulfanilic acid azocromotrop and its application in asymmetric supercapacitors. J. Mater. Chem. A 3 (2015) 19461-19468.

34

ACCEPTED MANUSCRIPT

[25] S. Reich, A. C. Ferrari, R. Arenal, A. Loiseau, I. Bello, J. Robertson, Resonant raman scattering in cubic and hexagonal boron nitride. Phys. Rev, 71 (2005) 205201. [26] M. A. Mannan, Y. Baba, N. Hirao, T. Kida, M. Nagano, H. Noguchi, Hexagonal

spectroscopy. Mater. Sci. Appl. 4 (2013) 11-19.

RI PT

nano-crystalline BCN films grown on Si (100) substrate studied by X-ray absorption

[27] X. Feng, Z. Yan, N. Chen, Y. Zhang, X. Liu, Y. Ma, X. Yang, W. Hou, Synthesis of graphene/polyaniline/MCM-41nanocomposite

and

its

application

SC

a

as

a

supercapacitor. New J. Chem. 37 (2013) 2203-2209.

M AN U

[28] X. Feng, N. Chen, Y. Zhang, Z. Yan, X. Liu, Y. Ma, Q. Shen, L. Wang, W. Huang, The self-assembly of shape controlled functionalized graphene-MnO2 composites for application as supercapacitors. J. Mater. Chem. A 2 (2014) 9178-9184. [29] Y. V. Butenko, S. Krishnamurthy, A. K. Chakraborty, V. L. Kuznetsov, V. R.

TE D

Dhanak, M. R. C. Hunt, L. Šiller. Photoemission study of onion like carbons produced by annealing nano diamonds. Phys. Rev. B 71 (2005) 075420. [30] J. Fang, H. Fan, M. Li, C. Long. Nitrogen self-doped graphitic carbon nitride as

EP

efficient visible light photocatalyst for hydrogen evolution. J. Mater. Chem. A 3 (2015) 13819-13826.

AC C

[31] R. V. Krol, A. Goossens, J. Schoonman, Mott-Schottky Analysis of nanometer-scale thin-film anatase TiO2. J. Electrochem. J. Electrochem. Soc 144 (1997) 1723-1726.

[32] A. M. Kuznetsov, The role of the electronic work function in electrochemical kinetics. Ekctrochim. Acta 13 (1968) 1293-1297.

[33] B. E. Conway, Electrochemical supercapacitors: scientific fundamentals and technological applications, Springer Science, New York.

35

ACCEPTED MANUSCRIPT

[34] R. Wang, J. Lang, P. Zhang, Z. Lin, X. Yan, Fast and large lithium storage in 3D porous VN nanowires–graphene composite as a superior anode toward highperformance hybrid supercapacitors. Adv. Funct. Mater. 25 (2015) 2270-2278.

RI PT

[35] J. Wang, J. Polleux, J. Lim, B. Dunn, Pseudocapacitive contributions to electrochemical energy storage in TiO2 (Anatase) nanoparticles. J. Phys. Chem. C 111 (2007) 14925-14931.

SC

[36] W. Ya-qiong, T. Hong-yang, X. Wen-lin, Electrocatalytic activity of Ti/TiO2

Eng 3 (2003) 356-360.

M AN U

electrodes in H2SO4 solution. The Chinese Journal of Process Engineering. J. Process

[37] S. Ardizzone, G. Fregonara, S. Trasatti, Inner” and “outer” active surface of RuO2 electrodes. Electrochem. Acta 35 (1990) 263-267.

[38] M. Toupin, T. Brousse, D.Belanger, Influence of microstucture on the charge storage

TE D

properties of chemically synthesized manganese dioxide. Chem. Mater. 14 (2002) 3946-3952.

[39] J. Bisquert, G. Garcia-Belmonte, P. Bueno, E. Longo, L. O. S. Bulho˜es. Impedance

EP

of constant phase element (CPE)-blocked diffusion in film electrodes. J. Electroanal. Chem. 452 (1998) 229-234.

AC C

[40] Q. Wang, J. Yan, Z. Fan, Carbon materials for high volumetric performance supercapacitors: design, progress, challenges and opportunities. Energy Environ. Sci. 9 (2016) 729-762.

[41] Y. Gogotsi, P. Simon, True Performance Metrics in Electrochemical Energy Storage. Science Magazine 334 (2011) 917-918.

36

ACCEPTED MANUSCRIPT

Research Highlights Band gap energy of boron nitride decreased with increasing graphene oxide content

•

Graphene oxide effectively affected the charge storage mechanism of the composite

•

Morphology of boron nitride changed from rod to sheet while forming superlattice

•

Highly conducting superlattice showed excellent supercapacitor performance

•

Asymmetric device exhibited long stability with high energy and power density

AC C

EP

TE D

M AN U

SC

RI PT

•