Enhanced Pseudocapacitance of MoO3-Reduced Graphene Oxide

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Enhanced Pseudocapacitance of MoO-Reduced Graphene Oxide Hybrids with Insight from Density Functional Theory Investigations Alok Pathak, Abhijeet Sadashiv Gangan, Satyajit Ratha, Brahmananda Chakraborty, and Chandra Sekhar Rout J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04478 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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The Journal of Physical Chemistry

Enhanced Pseudocapacitance of MoO3-Reduced Graphene Oxide Hybrids with Insight from Density Functional Theory Investigations Alok Pathak,a Abhijeet Sadashiv Gangan,b Satyajit Ratha,a Brahmananda Chakraborty,b* and Chandra Sekhar Rout a ,** a

School of Basic Sciences, Indian Institute of Technology, Bhubaneswar, Odisha, India-751013

b

High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre,

Trombay, Mumbai-400085, India Email: **[email protected], [email protected] (C.S.R) * [email protected] (B.C)

Abstract: Hydrothermally obtained MoO3/reduced graphene oxide (RGO) hybrid registered a specific capacitance of 724 Fg-1 at 1Ag-1, superior to the supercapacitor performance obtained from similar hybrid structures. Density Functional Theory (DFT) simulations further corroborated our claimin terms of both enhanced quantum capacitance and relevant insight from the electronic Density of States (DOS) for MoO3/RGO. Maximum capacitance is achieved for 12 wt% of RGO and then it reduces as observed in the experiment. The appearance of additional Density of states from C pz orbital in the band gap region near Fermi level on introduction of RGO in MoO3 is responsible for the enhanced capacitance in MoO3/RGO.

ACS Paragon Plus Environment

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1. Introduction Rapid depletion of fossil fuels due to their ever increasing consumption, have been putting enormous pressure on the research and development (R&D) sector to push for sustainable and renewable sources of energy.However, the current development in the field of sustainable energy sources such as solar power, wind power, hydro power etc. is still incipient. Effective generation of energy from these renewable sources, and implementation of compatible storage mechanism for the controlled utilization of those energy is rather constricted. The current storage technology is limited to Li-ion battery and carbon based supercapacitors only. Though the Li-ion storage has gone under intense commercialization over the years, its sluggish charge uptake and limited life cycle (~1000 charge-discharge cycles) have established supercapacitors, with their ultra fast charging property, as a potential alternative. Most of these supercapacitors make use of the electrodes fabricated from carbonaceous materials (primarily graphene) and store charge via Helmholtz double layer technique. However, they possess low energy densities due to fast discharging property. In order to achieve higher energy densities from these carbon based electrode materials, they are often hybridized with compounds having excellent reductionoxidation (redox/faradic) property. The redox based charge storage (pseudocapacitance and/or faradic capacitance) is comparatively on the slower side than the double layer mechanism which is why a desired balance between power and energy densities might be achieved if both faradic and double layer techniques can be implemented together.Considering both the cost effectiveness and

overall

environmental

impact,

most

of

the

research

has

been

focused

on

binary/ternary/mixed metal (primarily of transition group) oxides due to their broad range of oxidation states, outstanding structural stability/flexibility and high pseudocapacitance. Till now, a vast range of transition metal oxides have been studied for their possible application as


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supercapacitor electrodes, e.g. RuO2, V2O5, MnO2, NiO etc.1–7 Hydrous RuO2 is considered as the best electrode material due to its high theoretical specific capacitance range (1400Fg-1 – 2200 Fg-1) , high cyclic stability, having intrinsically fast, high and reversible intercalation properties and high electron conductivity.5 However, practical implementation and commercialization of RuO2 on an industrial scale is restricted due to its high toxicity and high cost associated with the extraction of Ru. MoO3 is one of the promising transition metal oxides which have gained lots of interest due to its high work function, wide range of oxidation states i.e. +2 to +6. In orthorhombic phase, it acts as an n-type semiconductor and has been investigated vigorously in recent past due to distinctive properties like electrochromism, thermochromism, and photochromism. It also has yielded promising results in gas sensing devices, catalytic reactions and can be an effective host material for intercalation processes.8–13 The major drawback that MoO3 possesses is its inherent low conductivity and structure degradation issue resulting in poor kinetic reaction rate and thus shows significant capacity decrement.14–16 To overcome these issues, MoO3 is often hybridised with carbonaceous materials, e.g. carbon nanotube (CNT), reduced graphene oxide etc.17,18 Such hybridization of MoO3 with graphene (which is already known for its high surface area,19,20 high electrical and thermal conductivity and high mechanical strength,21,22) provides greater stability and efficiency, yielding much improved performances. There are a significant number of reports depicting a stark difference between the electrochemical activities of transition metal oxides in pristine form and their hybrids with aforementioned carbon materials.2,23–25 Graphene/reduced graphene oxide (RGO) facilitates fast charge transportation and forms many active sites which enhance both the electrochemical and physical properties of such hybridized materials.19 Though a sizable amount of reports are available exploring the supercapacitor performance of bare


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MoO3,26–28 reports on the capacitive performance of MoO3/RGO are rather scarce.17 Here we report the detailed supercapacitor performance of MoO3/RGO hybrid synthesized via hydrothermal method. A thorough comparison has been carried out between bare MoO3 and MoO3/RGO in terms of their supercapacitor performances. Also, Density Functional Theory (DFT) simulations have been performed to compute the quantum capacitance and to provide theoretical insights for the enhanced capacitance in theMoO3/RGO hybrid. 2. Methods 2.1. Material Synthesis. All the chemicals were of analytical grade and used as supplied. Sodium molybdate dihydrate was obtained from Merck specialities Pvt. Ltd. and graphene oxide was obtained from Reinste nano ventures Pvt. Ltd. 2.1.1. Synthesis of MoO3Nanorods. MoO3 nanorods were synthesized by a typical hydrothermal method. First, 0.29 gm of Na2MoO4.2H2O (sodium molybdate dihydrate) was dissolved in 40ml of DI water and stirred up to 30 minutes to make a clear solution which recorded a pH value of 7. Thereafter, the solution pH was adjusted to 2 by controlled mixing of 2M HCl to it under constant stirring condition. The final mixture solution was then transferred to a Teflon lined stainless steel autoclave of 50 ml capacity and kept inside a hot air oven at 200° C for 48 hrs. A white coloured precipitate was collected post hydrothermal treatment which was then washed repeatedly with DI water and acetone and dried at 100° C. After drying, the sample was kept inside a vacuum desiccator for further characterization and measurement. 2.1.2. Synthesis of MoO3/RGO Nanorods .MoO3/RGO hybrids were synthesized by the same hydrothermal technique adopted for bare MoO3, the only exception is the addition of graphene oxide (GO) to the reaction mixture. 0.29 gm of Na2MoO4.2H2O (sodium molybdate dihydrate)


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was dissolved in 40ml of DI water and ultra-sonicated to obtain a transparent solution. 20 mg of GO was added to this solution and kept under vigorous sonication for 10-15 minutes for uniform dispersion. Then 2M HCl was added to the solution under continuous stirring till the PH reaches a value of 2. The final mixture solution was transferred into 50 ml Teflon autoclave and kept at 200° C for 48 hrs. The obtained black precipitate was washed with DI water and acetone several times and dried at 100° C. MoO3/RGO hybrids with 80 mg and 120 mg of RGO were synthesized by following similar procedure. 2.2. Structural and Morphological Analyses. Characterization of MoO3 and MoO3/RGO hybrid has been done with the help of X-ray diffraction technique (Bruker D8 advanced diffractometer, 40 kV, 40 mA having Ni filtered Cu-Kα radiation with a wavelength, λ = 1.54184 Å). Morphology of the samples were investigated by FESEM (Merlin compact with GEMINI-I column, Zeiss Pvt. Ltd., Germany). Compositional investigationhas been carried out with the help of energy dispersive X-ray spectroscopy (EDS) and elemental mapping. 2.3. Supercapacitor Electrode Fabrication. 2 mg of the finely ground sample was dispersed thoroughly in 2-Propanol by sonicating for 15 minutes and the resultant slurry was drop casted on a pre-cleaned Ni-foam (treated with dilute HCl) so as to cover an area of ~1 cm2. The modified Ni-foam was kept in a vacuum desiccator to dry at ambient conditions for approximately 2-3 hours. After drying, the modified Ni-foam was crimped with a nominal pressure of 1MPa using a hydraulic crimping machine to achieve better adhesion between the sample and the nickel foam.The mass loading of the sample in the case of the dry and crimped modified electrode was measured to be ~2 mg.


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2.4. Electrochemical Measurements and Capacitance Evaluation. All the electrochemical measurements were carried out in a typical three-electrode electrochemical setup. Pt wire was used as the counter electrode, Ag/AgCl electrode was used as the reference electrode and the modified Ni-foam was taken as the working electrode. 3M aqueous solution of KOH was used as the electrolyte for all the measurements. The measurement of specific capacitance from CV curve has been carried out usingthe following equation;

(1)

where the integral portion in the numerator represents the area under the cyclic voltammetry curve, m is the mass of the sample, dV is the small change in potential,

’ is the working

potential window. The specific capacitance from the discharge curve is measured by the formula:

(2)

where, i is the applied constant current, m is the mass of the sample and dV/dt is the slope of the discharge curve. The energy density E (Wh kg-1) and Power density P (W kg-1) can be calculated by using the following set of equations: (3)

(4)

where, t is the discharge time obtained from the charge-discharge experiment. 2.5. Computational Details.First Principles simulations have been performed using the Density Functional Theory based projector augmented wave (PAW) method as implemented in the


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VASP code.29,3031 PAW based pseudo-potentials have been used for Mo, O and C with PW91 as the exchange-correlation functional and the semi-core p states are included for Mo.31 We have considered α-MoO3 having an orthorhombic unit cell (space group Pbnm), which is thermodynamically stable phase of the molybdenum trioxide in ambient conditions.32 The cut off energy is taken to be 500 eV and the Brillouin zone is sampled using a Monkhorst pack mesh of 16x4x16 for bulk geometry and 11x11x1 k-points for (010) surface of bulk for supercapacitor study.33 The convergence criterion of 0.01 eV/Ǻ for Hellmann-Feynman forces and 10-6eV for total energy was used. As orthorhombic phase of α-MoO3 has a layered structure there are Van der Waals interactions between the MoO3 layers as well as between MoO3 and RGO. In order to account for the weak Van der Waals forces we have considered Grimme DFT-D2 dispersion correction which uses a pair-wise force field for describing the Van der Waals interactions.33 3. Results and Discussion 3.1. Structure and Morphology. Henceforth, MoO3/RGOhybrids containing 20mg, 80mg and 120mg of RGO are labeled as MoO3/RGO_20, MoO3/RGO_80, and MoO3/RGO_120 for convenience. FESEM images of bare MoO3 have been depicted in Figure 1a,b. Figure 1c,d show the FESEM images of MoO3/RGO_80 possessing nanorod structures with an average diameter of ~200 nm. Similarly, the FESEM images of MoO3/RGO_20 and MoO3/RGO_120 have been provided in Figure S1a, b and S1c, d, respectively (see supporting information). Elemental mapping for MoO3/RGO_80 (see Figure S2 in the supporting information) shows the uniform distribution of the elements over the mapping region. Fig. 2 shows the X-ray diffraction spectrafor bare MoO3 and all the MoO3/RGO hybrids, analyzed with the help of Xpert High score software (reference number: 00-005-0507) confirmingthe formation of an orthorhombic MoO3(JCPDS file no: 75-0912). The samples produced sharp interference peaks suggesting the


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highly crystalline MoO3. In the diffraction spectra for the hybrid samples, absence of peak at ~11° confirms the complete reduction of GO during hydrothermal treatment. Also, due to the high crystalline nature of MoO3, the characteristic peak corresponding to RGO at 2θ value, ~26° has been suppressed. 3.2. Electrochemical Analyses 3.2.1. Supercapacitor Properties of Samples. The electrochemical measurements of MoO3 and MoO3/RGO hybrids have been carried out in detail. Fig. 3a shows the comparison of cyclic voltammetry curves of MoO3, MoO3/RGO_20, MoO3/RGO_80, and MoO3/RGO_120 at a scan rate of 5mV/s. It can be observed that the redox activity in the case of MoO3 is the lowest and shows gradual improvement in an ascending manner for MoO3/RGO_20 and MoO3/RGO_80. In comparison to bare MoO3, a significant increase in the capacitance value can be seen in the case of the hybrid which can be ascribed to the much faster charge transportation and superior stability provided by the additional RGO content.16 For MoO3/RGO_80, the electrochemical activity was found to be the highest. However, in the case of MoO3/RGO_120, a significant drop in the redox activity can be observed which may be ascribed to the agglomeration effect of RGO (in an excess concentration) resulting in degraded effective surface areawhich significantly hinders the surface kinetics (resulting in poor redox activity) and results in poor electrostatic charge adsorption process. Fig. 3b and c, respectively show cyclic voltammetry curves of MoO3 and MoO3/RGO_80 at different scan rates. At any fixed scan rate, the redox activity of the MoO3/RGO_80 is found to be approximately twice the value that was obtained from bare MoO3 (considering the values of the peak current). Cyclic voltammograms for the samples MoO3/RGO_20 and MoO3/RGO_120 have been provided in Figure S3a and b, respectively (see supporting information), clearly demonstrating almost similar electrochemical activities. Fig.4a


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and b, respectively show charge-discharge curves of MoO3 and MoO3/RGO_80 at different current densities. The specific capacitance of MoO3 at current densities of 1, 2, 3, 4 and 6 A/g was calculated to be 515, 440, 418, 336 and 186F/g, respectively. For better evaluation, a detailed comparison has been drawn between the supercapacitor performance of MoO3 being reported in this work and performances of few similar metal oxides including MoO3 reported in previous literatures which has been elucidated in Table 1. Clearly, the MoO3 obtained in our report shows comparable faradic capacitance. Furthermore, an exceptional specific capacitance of 724 F/g was obtained in the case of MoO3/RGO_80 at a current density of 1A/g. The impressive capacitance of the MoO3/RGO_80 hybrid exceeds the performance of bare MoO3 and similar hybrid structures reported in literatures as illustrated in Table 2. Charge-discharge curves for MoO3/RGO_20 and MoO3/RGO_120 have been illustrated in Figure S4a and b (see supporting information), respectively. The specific capacitance vs. current density plot (Figure 4c) elucidates detailed comparison of the supercapacitor performance of bare MoO3 with MoO3/RGO hybrids. Towards higher discharge current values, rapid degradation in the capacitance can be observed in the case of bare MoO3. As can be seen, MoO3/RGO_80 shows better stability and superior charge storage than bare MoO3 and the other hybrids (i.e. MoO3/RGO_20 and MoO3/RGO_120) as well. Calculations from the long cyclic chargedischarge measurement suggest that the hybrid, MoO3/RGO_80 shows capacitance retention of 50% even after 800 charge-discharge cycles as illustrated in Figure 4d. 3.2.2. Electrochemical Impedance Spectroscopy. In order to investigate the role of electrode/electrolyte interaction during electrochemical measurement, both bare MoO3 and MoO3/RGO_80 were subjected to electrochemical impedance spectroscopy (EIS) in a threeelectrode electrochemical cell configuration, and the corresponding impedance (real and


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imaginary) values have been obtained. Fig. 5a and b, respectively represent the Nyquist plot alongside the Randles equivalent circuit (inset) for MoO3and MoO3/RGO_80. A charge-transfer resistance (Rp) value of 1.46 kΩ was obtained in the case of bare MoO3, whereas MoO3/RGO_80 recorded an Rp value of 0.19 Ω. Lower value of charge-transfer resistance suggests better electrochemical activity for MoO3/RGO_80 which is evident from its enhanced supercapacitor performance. 3.3. Post-measurement Sample Characterization. To investigate whether the enhanced specific capacitance is explicitly due to redox activity or there is/are any additional contribution(s), the hybrid sample has been characterized (using X-ray diffraction technique) after carrying out all the requisite electrochemical measurements. Figure S5 in the supporting information shows three peaks with phase (021), (060), and (112). The peak intensities in the case of both (021) and (112) phases decrease significantly due to intercalation of potassium ion into the layered structure of MoO3. Furthermore, it can be observed that the peak intensity of (060) is higher than the other two peaks and appears deviated from its standard position (JCPDS file no. 75-0912). This peak shift might be due to the aforementioned intercalation process.15 So the enhanced specific capacitance of the MoO3/RGO_80 might be due to the combined effect of, 1. faradaic diffusion process involving K+ ion34, 2. faradaic charge transfer processes with the surface atoms, 3. possibly, from the atoms located in the interlayer lattice planes (referred to as pseudocapacitance)34 , and 4. the non-faradaic double-layer capacitance. 4. Density Functional Theory Investigations Density Functional Theory (DFT) simulations have been carried out to qualitatively support our experimental data and the conclusion drawn thereof.


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4.1. Structure andDensity of States for Bulk MoO3. Figure6a displays the relaxed structure of bulk MoO3 with optimized lattice parameters of a= 3.88, b=12.66 and c=3.71 Å, matches reasonably well with the experimental value of a= 3.96, b=13.85 and c=3.69 Å.35 The orthorhombic phase is layered structure consisting of double layers of MoO6 octahedral held together by covalent forces in the (100) and (001) directions and by Van der Waals forces in the (010) direction. The computed M-O bond length in MoO6 in bulk are 1.95, 1.95, 2.31, 2.14, 1.75 and 1.70 Å, thus in good agreement with the reported DFT simulated value as well as experimental value.35,36 The Density of States (DOS) for bulk MoO3 using GGA and GGA+U method is shown in Figure 6b. The computed band gap using GGA and GGA+U method comes out to be 1.72 and 2.12 eV, respectively, matching nicely with simulated value of 1.9 eV from GGA and 2.12 eV from GGA+U.35–37It should be noted that the DFT predicted band gap cannot produce the experimental band gap of 3.3 eV,38 as DFT underestimates the band gap and employing the U corrections also did not produce any significant improvement as there are not much d states in the valence band. 4.2. Computation of Quantum Capacitance for MoO3 and MoO3/RGO. For computing the quantum capacitance we have put a layer of RGO, 3 Å above the (010) surface of MoO3 and allowed the system to relax. The relaxed structure of MoO3 and MoO3/RGO are shown in Figure 7a and b, respectively. For MoO3 surface, the computed M-O bond lengths in MoO6 are 1.96, 1.96, 2.39, 2.18, 1.74 and 1.70 which are in good agreement with the reported DFT simulated value.35 Figure 8a and b display the band structure of (010) surface of α-MoO3 and MoO3+RGO (12 wt%), respectively. For MoO3+RGO, we can see the presence of additional bands due to the presence of carbon content of RGO in the band gap region. We have computed the quantum capacitance using Density of States from the formula;38


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∞

CQ = e 2 ∫ D( E ) FT ( E − eϕ G )dE

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(5)

∞

where D(E) is the Density of states, ϕG is the electrode potential and the thermal broadening function, FT (E) is expressed as,

FT ( E ) = (4 K BT ) −1 sec h 2 ( E / 2 K BT )

(6)

Figure 8c depicts the total Density of States of MoO3 (lower panel), MoO3+RGO (20 wt%) and MoO3+RGO (12 wt%). It can be clearly observed that the intensity of DOS near Fermi level shows depletion while going from 12 wt% of RGO to 20 wt% of RGO resulting in substantial decrease in the capacitance. Also, the Fermi energy EF, shifts toward lower energy when RGO wt% increases from 12 wt% to 20 wt%. So the magnitude of the Fermi velocity increases when RGO wt% is increased from 12 wt% to 20 wt%. Figure 9 displays the plot of computed quantum capacitance C Q against the electrode potential ϕG for pure MoO3, MoO3 with 12 wt% RGO and , MoO3 with 20 wt% RGO. With the increase in the RGO concentrationin the hybrid, the capacitance increases and attains amaximum value for 12 wt% of RGO showing decreasing trend afterwards in the case of higher wt% of RGO as observed in the experiment. So our DFT data qualitatively support the experimental observations. At approximately 2 V of electrode potential, the quantum capacitance for pure MoO3, MoO3 with 12 wt% RGO and MoO3 with 20 wt% RGO comes out to be 466, 1003 and 615 µF/cm2, respectively. Thus, for the hybrid with 12 wt% RGO, the quantum capacitance is enhanced by ~2.15 times. Here we mention that in experiment we measure total capacitance. For low dimensional electrode, the total capacitance is given by50

1 1 1 = + CT CQ CEDL

(7)


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Where CEDL is the electric double-layer capacitance which depends on electrode-electrolyte interfacial interaction.

4.3. Insight from Electronic Structure for Enhanced Capacitance in MoO3/RGO Hybrids. In order to explain the enhanced capacitance in MoO3-RGO with the insight from the electronic structure, the Partial Density of States (PDOS) of Mo d and p orbital, O p orbital and C p orbital of MoO3-RGO (12 wt%) have been plotted in Figure 10a. The DOS plot reveals that the conduction band is mostly contributed by the Mo d orbital, whereas the valence band is dominated by the O p orbital. There are empty 4d states in Mo available for storing charges which might be the reason for supercapacitor performance of MoO3. When small amount of RGO is introduced in MoO3, there appear additional states from C pz orbital in the band gap region near Fermi level enhancing the capacitance. Thus, when the RGO content is increased beyond 12 wt%, the capacitance decreases. As stated in the section 4.2 (Figure 8c), with the increase in the RGO content beyond 12 wt%, the Fermi velocity increases. And, as quantum capacitance is inversely proportional to the Fermi velocity,14 therefore it is ought to decrease when RGO content is increased beyond 12 wt%. Fermi velocity is the highest for bare MoO3 as compared to MoO3+RGO, leading to lower value of capacitance in pure MoO3. The interaction between MoO3 and RGO layer is mostly of Van der Waals nature and there is neither any charge transfer nor involvement of chemical bonding. Figure 10b-g present the electron density corresponding to the KS orbitals of the valence band maxima (VBM) and conduction band minima (CBM) at Γ point of α-MoO3 bulk (Figure 10b,c), (010) surface of α-MoO3 (Figure 10 d,e) and RGO (12 wt%) with (010) surface of α-MoO3 (Figure 10 f,g). Here the upper figures correspond to the VBM states while the lower ones to CBM states and the iso-surface value is set to 5x10-3 for all the cases.One can see that the states at VBM in the case of α-MoO3 and its (010) surface are mostly contributed by oxygen p states while the Mo-d states being mostly empty as


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seen in Figure 10b and d. On the other hand, the states near CBM are mostly dominated by Mo-d states with a slight contribution from the O-p states as observed in the Figure 10c and e.Things change slightly when RGO is added, now the VBM states are dominated by carbon pz states as can be expected because of graphene’s semi-metal character which is seen in Figure 10f. On the contrary, the CBM is still dominated by Mo-d states, which is consistent with the PDOS plot of Figure 10a.

5. Conclusion Detailed supercapacitor performance of hydrothermally obtained MoO3 and its graphene hybrid (MoO3/RGO_80) have been carried out in a three electrode electrochemical cell. The graphene hybrid showed exceptional charge storage capacity in terms of specific capacitance which is of the order of ~724 F/g (at a mass normalized current of 1 A/g) and found to be superior to bare MoO3, earlier reported graphene hybrids of MoO3 and several other binary metal oxides. Quantum capacitance computed from Density Functional Theory (DFT) simulations predicts that when RGO is mixed with MoO3, the capacitance increases and the maximum capacitance is obtained around 12 wt% of RGO in agreement with the experimental observations. With the introduction of small amount of RGO (up to 12 wt%) there appear additional states from C pz orbital in the band gap region near Fermi level which is responsible for enhanced capacitance in in MoO3/RGO hybrid. Even though the sample has shown moderate cyclic stability, further optimization in the electrode fabrication would bring more stability and efficiency. AUTHOR INFORMATION

Corresponding Author


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*[email protected], [email protected] * [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgement Dr. B. Chakraborty would like to thank Dr. N.K.Sahoo for support and encouragement. Dr. Chakraborty would also like to thank the staff of BARC computer division for supercomputing facility. Dr. C.S. Rout would like to thank DST (Government of India) for the Ramanujan fellowship (Grant No. SR/S2/RJN-21/2012). This work was supported by the DST-SERB Fasttrack Young scientist (Grant No. SB/FTP/PS-065/2013), UGC-UKIERI thematic awards (Grant No. UGC-2013-14/005) and BRNS-DAE, (Grant No. 37(3)/14/48/2014-BRNS/1502). Also, part of this work is supported by the Indo-US Science and Technology Forum (IUSSTF) through a joint INDO-US centre grant and Ministry of Human Resources Development (MHRD), India through a center of excellence grant.


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Figures captions: Figure 1: (a) High and (b) low magnified FESEM images of bare MoO3. (c) high and (d) low magnified FESEM images of MoO3/RGO_80. (e) EDS spectra, atomic and weight percentages of the elements of (e) MoO3 and (f) MoO3/RGO_80.

Figure 2:

X-ray diffraction spectra of MoO3, MoO3/RGO_20, MoO3/RGO_80, and

MoO3/RGO_120

Figure 3: Cyclic voltammetry curves obtained at a scan rate of 5 mV/s for (a) MoO3, MoO3/RGO_20, MoO3/RGO_80, and MoO3/RGO_120 showing graphical comparison of their respective electrochemical activity in terms of oxidation-reduction peaks. Cyclic voltammograms at different scan rates for (b) MoO3 and (c) MoO3/RGO_80.

Figure 4: Constant current charge-discharge curves at different current densities for (a) MoO3 and (b) MoO3/RGO_80, (c) variation of capacitance with applied current density valuesas observed in the case of MoO3, MoO3/RGO_20, MoO3/RGO_80 and MoO3/RGO_120. (d) capacity retentionof MoO3/RGO_80 in percentage showing moderate stability after 800 cycles.

Figure 5: Nyquist plot and corresponding Randles equivalent circuit of (a) MoO3 and (b) MoO3/RGO_80

Figure 6.DFT optimized structures of α-MoO3 (a) and the total density of states of the following with and without Ueff (b). Here the red spheres indicate oxygen and the green spheres indicate Mo atoms. Mo in the octahedral environment with co-ordination +6 can be seen.


16

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Figure 7. DFT optimized structures of (010) MoO3 surface (a), (010) MoO3 surface with RGO; the red spheres indicate oxygen atoms, the green spheres indicate Mo atoms and purple spheres C atoms.

Figure 8. Band structure along the high symmetry k-points with Fermi level is set to 0 of (a) (010) surface of α-MoO3, (b) (010) surface of α-MoO3 with 12 wt% RGO, (c) Total Density of States of MoO3 (lower panel), MoO3+RGO (20 wt%) and MoO3+RGO (12 wt%); different Fermi levels are shown in red dotted line.

Figure 9. Variation of quantum capacitance (QC) of (010) surface of α-MoO3, (010) surface of αMoO3 with 12 wt% of RGO and (010) surface of α-MoO3 with 20 wt% of RGO.

Figure 10. Partial Density of States of (010) surface with RGO (12 wt%) (a) and the electron density corresponding to the KS orbitals of the VBM and CBM at Γ point of α-MoO3 bulk (b,c), (010) surface of α-MoO3 (d,e) and RGO (12 wt%) with (010) surface of α-MoO3 (f,g). Here the upper figures correspond to the VBM states while the lower to CBM states and the iso-surface value is set to to 5x10-3 for all the cases. The atoms are represented in the same manner as Figure 2.


17


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Figure 1


18

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Figure 2


19


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Figure 3


20

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Figure 4


21


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Figure 5


22

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Figure 6


23


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Figure 7


24

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Figure 8


25


Figure 9

2 Quantum Capacitance (µF/cm )

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

Page 26 of 37

MoO3-RGO(12 wt%)

1200

MoO3 MoO3-RGO(20 wt%)

900 600 300 0 -6

-3 0 3 Electrode Potential (V)


6

26

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Figure 10


27


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Page 28 of 37

Table 1. Comparison of supercapacitor performance of MoO3 with other reported Metal Oxides Sample

Specific Capacitance

Reference

(Fg-1) MoO3 (nanorods)

214 (0.1 A g-1)

MoO3 (Microspheres)

168 (0.1 A g-1)

MoO3 (Microbelts)

94 (0.1 A g-1)

MoO3 (thin film)

12.7 (10 mV s-1)

18

MoO3 (Nanobelts)

302 (0.1 A g-1)

39

H-MoO3 NHs

144 (10 mV s-1)

40

Co3O4 nanotubes

574 (0.1 A g-1)

41

α-Fe2O3 nanotubes

138 (1.3 A g-1)

42

CuOnanosheets

43 (10 mV s-1)

43

NiO

137.7 (0.2 A g-1)

44

MoO3 (Nanorods)

512.82 (1A g-1)

28

Present work


28

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Table 2. Comparison of supercapacitor performance of Metal Oxide-graphene hybrids with present work. Specific Capacitance

Sample

Reference

(Fg-1) MoO3/CNT

70 (10 mVs-1)

18

MoO3/RGO

22.83 (0.3 A g-1)

17

MoO3/RGO

360 (0.2 A g-1)

33

MnO2 nanowire/RGO

211.2 (0.15 A g-1)

45

Co3O4 nanoparticle-RGO

415 (3 A g-1)

46

Monolayer rGO

NiO– 525 (0.2 A g-1)

47

176 (5 mV s-1)

48

MnO2 nanowire– rGO

MoO3 nanodots on 103 (0.42 A g-1) MWCNT MoO3/RGO

724.63 (1 A g-1)

49

Present work


29


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Page 30 of 37

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TOC Figure:


37

Enhanced Pseudocapacitance of MoO3-Reduced Graphene Oxide

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