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Functionalization of MoO3-NiMoO4 Nanocomposite using Organic Template for Energy
Storage Application
Irum Shaheena, Khuram Shahzad Ahmada*, Camila Zequineb, Ram Guptab, Andrew
Thomasc and Mohammad Azad Malikc
aDepartment of Environmental Sciences, Fatima Jinnah Women University, Rawalpindi,
PakistanbDepartment of Chemistry, Pittsburg State University, 1701 South Broadway Street
Pittsburg, KS 66762, USAcDepartment of Materials, Photon Science Institute and Sir Henry Royce Institute, Alan Turing
Building The University of Manchester, Oxford Road, Manchester M13 9PL, U.K
*Email: [email protected]; [email protected]
Abstract:
Over the recent times, sustainable advances in the metal oxides nanomaterials to develop
effective and efficient supercapacitor electrode is critically investigated. In this regard, we have
tailored the surface chemistry and nano scaled morphology of MoO3-NiMoO4 nanocomposite via
organic functional groups of E. cognata and scrutinized it as an electrode for supercapacitor.
MoO3-NiMoO4 nanocomposite was synthesized by the sol gel synthesis route using bioactive
compounds of E. cognata. The phase formation of nanocomposite was confirmed by X-ray
diffraction and energy dispersive spectroscopy while the morphology was examined by field
emission scanning electron microscopy. The organic functional groups were revealed by Fourier
transform infrared spectroscopy and X-ray photoelectron spectroscopy. Moreover, Gas
chromatography-mass Spectroscopy (GC-MS) affirmed the presences of organic compounds in
the synthesized nanocomposite. The optical band gap energy of functionalized MoO3-NiMoO4
was 3.34 eV, demonstrated by Tauc plot. The organic framework derived MoO3-NiMoO4
1
revealed specific capacitance of 204 Fg-1 and maximum energy density of 9.4 Whkg−1, calculated
by galvanostatic charge-discharge measurements. Consequently, the nano-scale and organic
species of E. cognata were found to enhance the electrochemical behavior of MoO3-NiMoO4
electrode towards supercapacitor.
Key Words: Nickel Molybdenum Oxide; Nanostructures; Bio-template; Stabilizing agents;
Supercapacitor
2
1. Introduction
The ever increasing dependency on renewable energies due to depleting fossil fuels has greatly
increased the demand of energy storage systems.1-6 Among energy storage devices,
supercapacitors (SCs) are the most potential candidates with significantly high power density,
quick charging-discharging process, safe and environmental friendly operation.5-7 Recently,
various electrode materials have been intensively investigated to enhance its energy density 7-10,15-
18 Based on storage mechanisms, there are two types of supercapcitor, electrical double layer
capacitor (EDLCs) and pseudo-capacitor. In EDLCs, carbonaceous materials are used as
electrode while metal oxides based electrodes are used in pseudo-capacitor.6, 9,15-18 Transition
metal oxides nanomaterials like NiO,72-74, ZnO,39, 40,51 ,55 Mo3O4, MoO36, 62, 66 are highly studied
and most attractive pseudo-capacitor electrode materials owing to their low cost,
environmentally friendly nature and earth abundance.3, 6, 9-18
Transitional metal oxides based pseudo-capacitor store energy faradaically based on
redox reactions of electrode. There have been huge efforts to enhance capacitance of electrode
by generating more redox reactions such as by use of extra redox additives in electrolyte83 and
combination of more than one metal oxides as electrode material.79-82 Combination of binary and
ternary metal oxides result in introduction of oxygen vacancies into the surface of combined
metal oxides, as a result electronic structure and morphology will be changed, leading to efficient
electrochemical behavior.61,63,65,69, Additional oxygen vacancies of metal oxides are also capable
of providing more active sites for superior redox behavior and energy storage.45,79-82 Lin et al.,
2019 synthesized ternary metal oxides based ( Ni-Co-Mo-O and the Ni-Co-Cu-O ) electrodes by
hydrothermal method and revealed huge energy storage potential of 21.9 Wkg-1 due to
combination of metal oxides and incorporation of metal into metal oxides.82
3
Among different binary and ternary oxides, oxides of molybdate are most significant
class of electrochemically active compounds because of its multiple valence states, sustainable
nature and different oxidation states. However, nanoscale and morphology of binary metal
oxides can provide considerable higher surface area for efficient redox behavior to achieve much
higher specific capacitance.10, 53, 58, 65 Chavan et al., 2018 demonstrated higher capacitance of
1853 Fg -1 at 1 Ag -1 due nano scale morphology of NiMoO4 nanoflake thin film prepared by
SILAR (successive ionic layer adsorption and reaction) technique.75 Similarly, Hong and Lin,
2019 reported superior electrochemical results for NiCo2O4@NiMoO4 on the Ni foam substrate
for supercapcitor.79 In another study by Chavan et al., 2018 extraordinary high capacitance (1180
Fg-1 at 1 Ag-1 ) of Ultrathin films of porous Ni-Mo oxide nanoflakes was reported.77 Moreover,
Hong et al., 2016 investigated one-dimensional (1-D) NiMoO4 nano structures as supercapacitor
electrode with outstanding charge storage potential.76 However, despite of this extensive research
on the composite fabrication of molybdate, synthesis of binary and ternary metal oxides
nanomaterial with modified surface chemistry have certain limitations of large scale production,
sophisticated techniques and use of hazardous and toxic chemicals and reagents. Thus,
sustainable surface modification of mixed metal oxides for charge storage is still challengeable
among scientific community.
In constructing and modifying the nano-morphologies and surface chemistry of metal
oxides composites, the toxicity as well as environmental and economic costs exploited are
critically important.8-15 Furthermore, it was illustrated that efficient active material and its
enhanced charge storage potential are depending on the synthesizing parameters.79-81 In this
regard, here we have tailored the surface chemistry of MoO3-NiMoO4 to introduce oxygen and
carbon related functional groups via organic species of E. cognata. We have developed an
4
effective, efficient and environmentally friendly sol gel synthesis route to fabricate
nanostructures of MoO3-NiMoO4 via inorganic-organic framework. Euphorbia cognata Boiss
was used as a source of organic compounds for the first time. E. cognata have significant organic
compounds 21, 22 like octodrine, alanine, cyclobutanol , etc which can be used as fuel for the
synthesis and functionailzation of metal oxides nanomaterials.59, 60 Besides, the bio organic
compounds are mainly composed of C, N, O and H atoms. By introducing C, N, O functional
groups additional active sites and pathways will be created which will enhance charge storage
and electronic paths of electrodes.98,99 Previously metal oxides with carbon nanotubes, graphene,
carbon black etc have been investigated as an outstanding approach for acquiring carbon and
oxygen containing surface functional groups to improve conductivity and capacitance of
capacitor.11, 10, 16, 15, 19, 20 Moreover, functionalization of MoO3-NiMoO4 nanocomposite does not
involve any additional chemical or reagents. Therefore, we believe that when such functional
groups will be introduced in metal oxides nanomaterials, their electronic structure as well as
morphology will be changed (by the introduction of C, N, O functional group of organic
compounds), which will create more active sites and quick diffusion pathways leading to better
electrical conductivity and enhance redox behavior of electrode.
2. Material and Methods
Molybdenum(II) acetate (Mo₂(O₂CCH₃)₄), nickel(II) acetate tetrahydrate (Ni(CH₃CO₂)₂·4 H₂O),
ethanol and methanol were purchased from Merck chemicals Ltd. Double distilled and deionized
water was used throughout the experiment. Phytochemicals extract of E. cognata plant leaves
was used as reducing and stabilizing agents in the synthesis of MoO3-NiMoO4 and was collected
from Rawalakot AJK Pakistan. Acetylene black, polyvinylidene (PVDF) and N-methyl
pyrrolidinone (NMP) were used in the fabrication of the electrode.
5
2.1. Synthesis of MoO3-NiMoO4
The organic framework based sol gel strategy to synthesize MoO3-NiMoO4
nanocomposite (figure 1) was developed by modifying reported methodologies particularly
relating to metal-organic frameworks (MOFs) and sol gel.19, 20, 23-28 40 mM solution of
Ni(CH₃CO₂)₂·4 H₂O and Mo₂(O₂CCH₃)₄ were prepared separately in deionized water. Each
solution was subjected to constant magnetic stirring at room temperature till complete
dissolution, afterward both sols were mixed together. In another beaker organic compounds were
extracted by treating 2 g of dried powdered leaves of E. cognata with deionized water on
vigorous stirring for 30 minutes at 60 oC. Afterwards, 20 ml cooled and filtered extract of
organic compounds was added into mixed solution of precursors on continuous stirring at 70 oC
for 2 hours and then incubated at room temperature in dark conditions for 24 hours to complete
gelation and phyto-functionalization process. Hereafter, the mixture of inorganic metal
precursors and organic species of E. cognata, was first dried at 95 oC for overnight and then
dried powder was annealed at 450 oC for 4 hours to procure MoO3-NiMoO4 nanocomposite.
6
Figure 1: Schematic diagram of modified sol gel method to synthesize MoO3-NiMoO4
nanocomposite via bio organic template and fabrication of its electrode
The figure 1 is further depicting the mechanism of MOF derived synthesis. According to
schematic diagram the bio active compounds reduce metal salts into pure metal, before
calcinations; during this process they incorporated into metal to stabilize them at nano scale to
avoid agglomeration. Upon the calcinations of this phyto metal complex, respective metal oxides
were synthesized.
Thermally annealed MoO3-NiMoO4 nanocomposite was mixed with acetylene black and
polyvinylidene (PVDF) in 8:1:1 in the presence of N-methyl pyrrolidinone (NMP) following
reported methodologies. 29-32 As prepared slurry was then consistently pasted onto the porous Ni
foam and dried at 60 °C under vacuum for 10 hours (figure1). The mass loading was accurately
7
measured by weighing the nickel foam before and after electrode preparation using an analytical
balance. Same nanomaterial synthesis methodology and electrode fabrication procedure (figure
1) was repeated for the synthesis of MoO3 NPs in order to study the electrochemical properties of
MoO3-NiMoO4 in comparison with organic template assisted MoO3 electrode.
3. Characterization
The functionalized nanomaterial was well characterized by ultraviolet-visible spectroscopy (UV-
Vis.) 1602, Biomedical services, Spain for band gap and optical properties. Phyto-stabilizing
agents were individualized by gas chromatography-mass spectroscopy-GC-MS-QP5050
(SHIMADZU) (GC-MS) and Fourier transforms infrared spectroscopy (FTIR) 8400, Shimadzu,
Japan. Phase identification and crystallinity were examined by XRD5 PANaytical X’Pert Pro
(XRD). Quanta 250-FEG scanning electron microscope (FE-SEM) with Gatan 3View attachment
was used to study morphology and Energy-dispersive X-ray spectroscopy (EDX) was used to
study chemical composition of synthesized MoO3-NiMoO4. Surface chemistry was investigated
by X-ray photoelectron spectroscopy (XPS) using a Kratos Axis Ultra spectrometer with a
monochromated Al ka X-ray source.
3.1. Electrochemical Measurements
Electrochemical characterizations of organic compound incorporated MoO3-NiMoO4 were
carried out using three electrode system, where fabricated nickel foam with MoO3-NiMoO4 was
used as working electrode, a platinum wire was used as a counter electrode and saturated
calomel electrode was used as a reference electrode. All experiments were conducted in 3M
KOH aqueous solution. The electrochemical properties of MoO3-NiMoO4 were investigated by
cyclicvoltammetry (CV) at various scan rates (2 to 300 mVs-1), galvanostatic charge-discharge
8
(GCD) at various current densities (0.5–30 Ag-1) and electrochemical impedance spectroscopy
(EIS) with a frequency range of 50 mHz to 10 kHz.
4. RESULTS AND DISCUSSION
4.1. Functionalized MoO3-NiMoO4 Nanomaterial
In order to identify the bioactive organic compounds of the plant extract, the spectroscopic
characterization of prepared extract of E. cognata leaves was carried out and revealed the
distinctive peaks of phenolic compounds as shown in figure 1bS1 and table 1S2 in
supplementary data. The identified peaks in figure 1bS1 and table 1S2 were corresponding to
plant phenols, flavonoids, and flavonols of E. cognata.
Figure 2: Identification of organic stabilizing agents of MoO3-NiMoO4 (a) FTIR of MoO3-
NiMoO4 , (b) GC-MS spectra of MoO3-NiMoO4 presenting the incorporated organic compound.
The dried powder leaves of E. cognata was subjected to scan at the full wavelength (400
to 4000 cm-1) of FTIR (figure 1a S1 and table 1S2). FTIR illustrated vibration modes associated
with phytochemicals at particular frequencies (cm-1) corresponding to O-H stretching and H
bond, C-H, C-H, N-H, -C꞊C-, C-C medium stretch (in ring), C-H wag (-CH2X) bond, C-N and C-
9
O, C-N, C-N stretch, ꞊C-H, N-H wag and C-H oop , illustrating the alcohol, phenol, aromatic,
alkanes, 1o and 2o amines, aliphatic amines, carboxylic acid, esters and ethers groups as organic
constituents of the investigated plant leaves. The detailed GC-MS profile of bioactive
compounds of E. cognata has been provided in supplementary data (figure 1c S1 and table 2 S3).
GC-MS profile of E. cognata demonstrated different phenolic compounds such as decanoic acid,
octodrine, cyclobutanol, d-alanine, cyclohexylethylamine. The synthetic d-alanine, decanoic
acid, and cylcobutonal have been reported as efficient fuel and reducing agents in the synthesis
of nanomaterials in numerous studies.59-60 Therefore, the current investigation revealed that the
investigated plant have several organic compounds which have potential to used as fuel and
reducing agents in the synthesis of nano material.
To confirm the possible role of organic compounds of E. cognata in the synthesis of
nanomaterial, annealed powder of MoO3-NiMoO4 was probed to FTIR and revealed the peaks
frequencies as shown in figure 2a. These vibrational peaks were corresponding to O-H stretch
(phenol, alcohol) at 3431.9 cm-1, N-H bend (1o amines) at 1608.44 cm-1, C-O stretch at 1069.55
cm-1 ( carboxylic acid, ester, ether) and C-H oop was observed at two peaks 869.3 and 790.35
cm-1 representing the aromatics compounds. The insert in figure 2 is showing M-C, and M-O (M
= Ni, Mo) bonds in the frequency range of 600 – 400 cm -1. The peaks at 498.2, 481.22, 465.36,
444.03, and 421.9 cm-1 are indicating metal oxides of Ni73,74 and Mo.6 Therefore, after annealing
carboxylic acid, aromatics and 1o amines were found to be present as stabilizing agents in
synthesized nanocomposite of metal oxides
10
The incorporated organic stabilizing agents were further individualized by GC-MS
profile of NIST library at their respective retention times as shown in figure 2b. The identified
peak at 20.4 is corresponding to benzenemethanol (C7H8O), also known as Benzyl alcohol, as
part of MoO3-NiMoO4. Thus, speculated stabilizing agent of MoO3-NiMoO4 was identified as
Benzyl alcohol by NIST library. The analytical grade synthetic benzyl alcohol has been reported
as efficient stabilizing agent in the synthesis of nanomaterial.60, 100, 101 However, in the present
study the identified stabilizing agent was extracted from a cost effective, efficient and greener
source for the stabilization and functionalization of MoO3-NiMoO4 nanomaterial.
11
Figure 3: (a) XRD patterns of MoO3-NiMoO4 nanocomposite, and (b) Raman scattering of
MoO3-NiMoO4 nanocomposite
The XRD analysis of bio-organic compounds derived MoO3-NiMoO4 nanocomposite and
MoO3 NPs was carried out in order to identify phase purity and crystallinity of synthesized
material. The figure 3a presents XRD patterns of MoO3-NiMoO4 nanocomposite while figure 3a
S5 (in supplementary data) demonstrated X Ray diffractogram of MoO3 along with stick pattern.
The figure 3a S5 illustrated the prominent sharp (crystalline) peaks of MoO3 in agreement with
the standard JCPDS file (00-005-0508). However, the diffraction patterns in figure 3a shows
well defined prominent peaks of Molybdenum Oxide -MoO3 (ICSD 00-005-0508) as well as
Nickel Molybdenum Oxide- NiMoO4 (ICSD 00-033-0948) (Stick pattern has been provided in
supplementary data Figure 2 S4). The observed XRD patterns revealed the growth of monoclinic
shaped NiMoO4 with space group of I2/m and unit cell parameters of a: 9.509, b: 8.759, c:
7.667(Å). NiMoO4 produce peaks in figure 3 (●) at 2 theta (θ) = 14.244, 16.088, 18.875, 28.832,
32.74, 43.91, 45.92, 47.66, 49.36 and 52.75corresponding to (110), (011), (101), (220), (-312),
(330), (141), (-204), (-512), and (-521) hkl planes respectively. The X-ray diffratogram revealed
MoO3 peaks (*) at 2 theta (Ɵ) = 12.775, 23.41, 25.67, 27.38, 33.81, 35.48, 38.95, 46.39, 49.36,
52.75, 54.19, 56.44, 57.75, and 58.79 corresponding to hkl planes of (020), (110), (040), (021),
(111), (041), (060), (210), (002), (211), (112), (042), (171), (081), (062), and (190) respectively.
Moreover, MoO3 exhibited the space group Pbnm with Orthorhombic shape and cell parameters
12
of a: 3.962, b: 13.858, c: 3.697 (Å). However, minor shift (less than 0.1%) of diffraction peaks
towards lower diffraction angle can be observed for some peaks for example 23.41o, 35.48o,
52.78o of MoO3 and 16.08o, 18.87o, 32.47o of NiMoO4. This indicates compression of crystal
lattice due to stress of organic stabilizing agents. The crystallite size of MoO3-NiMoO4 was
calculated from the full width half maximum (FWHM) of the peaks using Debye Scherrer's
equation (1) 71 and found to be 21.1 nm. Therefore, according to XRD final product was
composite of two electrochemically active oxides; MoO3 and NiMoO4 as MoO3-NiMoO4
nanocomposite. The raman scattering vibration modes of MoO3-NiMoO4 are shown in figure 3b,
corresponding to MoO3 and NiMoO4 phase at 100 to 1400 cm -1 wavelength range. In agreement
with Wong et al 2014 93 the peaks at 710.7 cm-1 and 817.63 cm-1 are indication of carbon
containing organic compounds. Therefore, raman spectrum of MoO3-NiMoO4 has considerably
changed in the range of 100–1400 cm-1 due to the addition of organic compounds.
13
Figure 4: (a) EDX elemental map depicting the distribution of Ni, Mo, C and O in MoO3-
NiMoO4 nanocomposite, (b, c, d and e) Elemental Mapping of Ni, Mo, O and C respectively,
and (f) Elemental analysis of synthesized nanomaterial by Energy dispersive X-ray
spectroscopy.
14
EDX spectrum (figure 4) describes the chemical composition of fabricated material to be
consisted with Ni, Mo, O and C. According to atomic rations of Ni, Mo and O, nanomaterial is
classified as
MoO3- NiMoO4. In
agreement
with XRD phase
analysis,
EDX demonstrated
the empirical
formula of
synthesized
material as
MoO3- NiMoO4.
Nevertheless,
EDX depicted the presence of significant atomic percentage of carbon because of organic
functional groups of stabilizing agents. Continuing the investigation of chemical composition,
MoO3-NiMoO4 was probed to XPS analysis to examine the local bonding environment of
synthesized material and to speculate the functional groups related to bioactive compounds of E.
cognata. The survey scan in figure 5 (a-d) shows the composition of surface to be consisted with
C, O, Ni and Mo. The presences of C, O, Ni and Mo as major elements of surface chemistry of
the sample is in well agreement with the EDX analysis.
15
Figure 5: XPS spectra recorded from MoO3-NiMoO4 (a) Spectra of Ni 2p , (b) XPS spectra of
Mo 3d, (c) spectrum of C 1s, and (d) XPS spectrum of O1s
Two prominent peaks can be observed in Ni 2p spectra (figure 5a) corresponding to Ni
2p 3/2 and Ni 2p ½ at their respective binding energies as shown in figure 5a. Furthered Ni 2p
spectra revealed the presences of Ni2+ and Ni3+ along with two satellites having Ni2+/Ni3+. 72-74 In
figure 5b, two wide peaks of Mo 3d5/2 and Mo 3d3/2 are depicted corresponding to MO4+ and
MO6+. 62 The O1s region is shown in figure 5d can be fitted with a peak at binding energies of
529.16 eV corresponding to metal oxides bonds (M-O). The O 1s is also showing the presences
of organic (mainly C=O, C-O).29,42, The presences of organic functional groups were further
revealed by C1s spectra of MoO3-NiMoO4 in figure 5c. In agreement with Furlan et al., 2014,
Oswald et al 2018 and Gervas et al 2019, the C 1s spectrum is showing surface functional
groups of C-O, COO, O-C=O, C=O, C-C, C-H and C=C.29,42, 64 According to Furlan et al., 2014
16
the peak at 295 eV is representing ϭ* states Sp3 bonding. They furthered reported that the peak
at 283.4 eV is indication of Ni-C bond.64 Hence, the peak at 283.4 eV is suggestion of M-C bond
(M=Metal) in the synthesized material.94 Therefore, XPS successfully demonstrated the
functionalization of MoO3-NiMoO4 by organic species, indicating phenyl groups on the surface
of nanocomposite and proposing the some stabilization by benzenemethanol (C7H8O) of E.
cognata.
Figure
6: (a-d)
FE- SEM
images of organic template assisted MoO3-NiMoO4 at different magnifications and (e and f) FE-
SEM images of MoO3-NiMoO4 without organic template at 5 µm and 2 µm respectively.
The FE-SEM images of bio-organic assisted MoO3-NiMoO4 were presented in Figure 6 (a-d)
which were then compared with FE-SEM images of MoO3-NiMoO4 which was synthesized
17
without organic template (figure 6 e and f). MoO3-NiMoO4 nanomaterial was examined at
different magnifications to illustrate particle size range, particles distribution/agglomeration and
surface morphology. The morphology of electrode material is considered as key parameter for
effective electrochemical behaviour.79-81 The morphology varied greatly when organic template
was used for synthesis of MoO3-NiMoO4 nanomaterial. The well defined spherical nano particles
were obtained at each magnification with bioorganic template as annotated in figure 6(a-d).
While figure 6 (e-f) revealed larger and non uniform sized particles of MoO3-NiMoO4
synthesized (without bioactive compounds) using ethylene glycol and citric acid in the ethanol
solvent (detailed methodology has been given in supplementary data). Not only size and shape of
particles of both samples were varied greatly, but it can be also observed that more
agglomeration was found in the ethylene glycol assisted nanomaterial as compared to organic
framework derived MoO3-NiMoO4. As discussed before, the bioactive compounds while
reducing the metal salts also stabilized them in nanoscale with minimum agglomeration.
Whereas, the chemically synthesized particles (Figure 6 e and f) have demonstrated the mixture
of larger and smaller size particles with different morphology and more agglomeration.
Therefore, the present study in consistence with reported literature23-26 concluded that
phytochemicals of the plant leaves have not only reduced the particle size but also prevented
them from intense agglomeration. Moreover, the uniform spherical nano shaped particles of
organic framework derived MoO3-NiMoO4 can provide larger surface area for charge storage
while less particles agglomeration can facilitate the transport of ions and electrons.
4.2. Band Gap Energy
18
Figure 7. (a) U-Vis. absorption spectra of MoO3-NiMoO4, and (b) band gap energy of MoO3-
NiMoO4 via Tauc,s plot
Figure 7(a) shows the absorption spectrum of phyto-fabricated MoO3-NiMoO4. From figure 7a
wide range absorption can be observed which gradually increases towards lower wavelength
with maximum absorption peak at 237.85 nm due to incorporation of carbon based organic
compounds in the synthesized nanomaterial. Bio active compounds particularly phenols are
indicated by UV in range of 200-350 nm while absorption range for metal oxides nanomaterial is
200-400 nm. In the figure 7a UV Vis absorption band range from 200-400 with the maximum
band edge absorption at 400 nm after that absorption was started decreasing. Absorption spectra
can be further related to dispersion of synthesized nanomaterials on the bases of shape (width) of
surface plasmon peak (SPR) of absorption spectra.90-92 Synthesized Nanomaterial was sonicated
for 10-15 minutes on ultra sonicator and then was subjected to UV Vis. Upon sonication particles
were uniformly and homogenous disperse in aqueous medium. In consistence with reported
literature 90-92 the shape of SPR bands in figure 7a is the indication of uniformly dispersed NPs,
which can also be observed in figure 6(a-d) as less agglomerated nanoparticles.
19
The figure 7b is the representation of Tauc plot to determine the optical band gap energy
(Eg) of synthesized material using following relation.
(αhv)2= A1 (hv _ Eg)…..(1)
where α is absorption coefficient, hv and Eg is defined as photon and band gap energy
respectively. The corresponding band gap energy was 3.34 eV for organic framework derived
MoO3-NiMoO4 composite from Tauc plot. The band gap value was changed from band gap of
pure NiO and Mo oxides due to synergistic effects of combination of different metal oxides.
Therefore, the band gap studies, in agreement with XRD and XPS, is demonstrating the
successfully organic-inorganic framework derived synthesis of electrochemical active oxides.
4.3. Supercapacitive Studies
The electrochemical properties of the organic compound derived nano structured MoO3-NiMoO4
were characterized by CV, GCD and EIS. Figure 8a shows typical voltammograms of the
fabricated electrode at scan rates ranged from 2-300 mVs-1. In consistence with the previous
studies24, 29, 30, Ni-foam was just used as mechanical support and current collector in the current
investigation as it has no contribution to the capacitance of the electrode because of the
negligible integrated area of CV curves. However, the synthesized MoO3-NiMoO4 electrode was
revealed characteristics redox peaks, as shown in figure 8a. In the figure 8a the redox peaks
(0.42V) at positive current density are the indication of oxidation process, whereas the cathodic
peaks (0.34V) at the negative current density are the process of reduction. The redox behavior of
MoO3-NiMoO4 was attributed to the Ni2+/Ni3+ and Mo4+/Mo6+ based on the following reaction
(M= Mo/Ni).8,53
MO + OH- MOOH + e-
20
However, it was well indicated by XPS that organic functional groups like C=O, C-H, OH, H-
O-C, C-C-R etc of stabilizing agent (C7H8O) were present on the surface of electrode, which
were expected to play their role in the electrochemical reactions of MoO3-NiMoO4 to enhance
the SCs performance.
In the current study, the redox peaks of MoO3-NiMoO4 were found to exhibit the direct
positive correlation with the current density, oxidation reduction peaks were found to be
increased with the increasing the current density, as observed in figure 8a. Whereas, the current
of anodic and cathodic peaks was found to be increased with the increasing scan rate from 2 to
300 mVs-1. The redox peaks were potentially shifted in the more positive and negative directions
respectively with the increasing scan rate, creating a wider potential difference (Ec-Ea) at higher
potential, under quasi-reversible kinetics.49,34, 32, 53 This is possibly attributed to the limited ion
diffusion rate during the faradic reactions to satisfy electronic neutralization, at higher scan rates.
13, 14, 32 . This phenomenon of electrode is associated with typical battery type electrode. 13, 14, 34, 32 It
is further noted that the separation between the redox peaks was increasing with the increasing
scan rate, indicating the fast faradic reactions because of nano porous structures of MoO3-
NiMoO4. According to Zhang et al., 2019 and Bhagwan et al., 2019 such redox behavior is due
to Ohmic resistance of electrode.4,41 Therefore, cyclic voltammetry indicated the functionalized
MoO3-NiMoO4 as pseudocapacitor which was frequently relying on the its redox behavior of
battery type electrode . This is because of the presence of Mo, Ni, O , and C at the surface of
electrode as major constituents of active material as shown by XPS in figure 5 and by EDX in
figure 4. Consequently, the present study revealed that electrochemical behavior of MoO3-
NiMoO4 is depending on its surface composition and surface properties. These results are in well
agreement with Hong and Lin, 2019 where they investigated the surface properties of solid-state
21
hybrid supercapacitor of NiCo2O4@NiMoO4 nanostructures and concluded that the
electrochemical properties are greatly depend on the surface properties of electrode material as
electrochemical reactions takes place at interface between active materials and electrolyte. 81
Figure 8: (a) Cyclic voltammograms of MoO3-NiMoO4 electrode at various scan rates, (b)
Galvanostatic charge-discharge characteristics of MoO3-NiMoO4 at various current densities, (c)
Variation of specific capacitance as a function of scan rate, and (d) variation of specific
capacitance as a function of applied current density.
The larger integrated area of CV curve is significantly important as it reflects the
capacitance of the electrode by the following relation: 87-89
C sp=∫ IdV
v .∆ V . A……….(2)
Where, ∫ IdV is integral area of CV curve, v is scan rate, ΔV is the potential window and A is
active area of electrode material on the electrode (or mass of active material on electrode).
22
Greater the area under redox peaks, higher will be the capacitance.1, 29, 30 As shown in figure 8c
fabricated electrode revealed higher capacitance of 171.25 Fg-1 at 2 mVs-1, while the specific
capacitance was observed to be decreased with the increased scan rates because of the
inaccessibility of OH- ions of electrolyte to some parts of electrode at higher scan rates. Despite
of this limitation of electrolyte, a capacitance of 45.3 Fg-1 was still achievable at the scan rates of
50. This indicated the good rate capability (figure 8c) of MoO3-NiMoO4 because of large number
of active sites provided by Mo, Ni and functional groups of organic species as well as by the
nano morphology. It is worth observing that the calculated capacitance of MoO3-NiMoO4
electrode which is much higher than 63.5 Fg-1 of bio template MoO3 electrode at 2 mVs-1. The
figure 3b S5 in supplementary data depicts the cyclic voltammograms of MoO3 electrode at same
potential window and scan rate. It can be observed from CV analysis that MoO 3-NiMoO4 was
revealed much higher specific capacitance than MoO3. This is attributed to the additional oxygen
vacancies (as discussed in introduction) provided by NiMoO4. In the synthesized binary MoO3-
NiMoO4 the additional metal and metal oxides efficiently enhanced electrochemical behavior
leading to higher capacitance of electrode as compared to MoO3 NPs. These results are in
consistence with Chen and Lin, 2018, where they fabricated the metal oxides (nickel, cobalt,
molybdenum and copper) based nickel foam electrode showed excellent electrochemical
properties due to mixed metal oxides.78
Moreover, the specific capacitance calculated for MoO3-NiMoO4 in the current study is
much higher than capacitance of 36 Fg-1 of MoO3 nanorodes,67, 66 and 110 Fg-1 of NiO73 at the scan
rate of 5 mVs-1 and 1 mVs-1 respectively. Moreover, a literature survey was conducted to
compare capacitance of MoO3-NiMoO4 with different fabricated electrodes. The survey has
shown in table 1 and is vividly demonstrating that bio-organic compound derived electrode
23
revealed much higher potential for supercapacitor because of combine effects of surface
chemistry analyzed by XPS and nano morphology revealed by FE- SEM.
Table 1: Comparison of supercapacitive behavior of fabricated nanomaterial with reported
investigations by cyclicvoltammetry
Electrode Electrolyte Specific
capacitance
(Fg-1)
Scan rate
(mVs-1)
References
Molybdenum trioxide
(MoO3) nanowire
Na2SO4 23 5 69
6 100
Nickel vanadium oxide
(Ni3V2O8) nanocomposite
KOH 118 5 74
Carbon cloth nickel oxide-
polyaniline (EC-NiP)
PANI 110 1 73
Molybdenum trioxide
(MoO3) coated Titanium
dioxide (TiO2) nanotubes
with four MoO3 deposition
cycles
KCl 51.3 5 68
MoO3-TiO2 nanotubes with
MoO3 deposition cycles
KCl 44.9 5 68
MoO3-NiMoO4 KOH 171.3 2 Present work
50 45.3Na
2SO
4=Sodium sulfate, KOH= Potassium hydroxide, PANI= Polyaniline, KCl= Potassium Chloride
The specific capacitance was further calculated from the charge-discharge curve (figure 8b)
using the following equation:
C sp=I × ∆ t
∆ V ×m…….(3)
24
Where, I is the discharge current (A), Δt is the discharge time (s), ΔV is the potential window
(V), and m is the mass (g) of MoO3-NiMoO4 nanocomposites on the electrode. The capacitance
of MoO3-NiMoO4 at various current densities was studied as shown in figure 8b and 8d. The
GCD investigations of the fabricated electrode revealed the highest capacitance of 204 Fg-1 at 0.5
Ag-1 and the lowest capacitance of 57 Fg-1 at the highest current density of 30 Ag-1. The
calculated capacitance is quite higher than 81.67 Fg-1 capacitance of NiO nanosheet at 0.5 Ag-1,72
20 Fg-1 of MoO270 and 23 Fg-1 of MoO3 nanowires69 at lower current density of 1 Ag-1.56 In
agreement with CV, GCD measurements revealed enhanced rate capability of MoO3-NiMoO4
due to increased active sites offered by Mo, Ni, and organic compounds. Table 2 revealed the
comparison of capacitance of inorganic-organic framework synthesized MoO3-NiMoO4 with
previous investigations and revealed comparatively excellent potential of fabricated electrode for
supercapacitor. Therefore, in consistence with the literature,1, 29, 30, 55, 69, 70 the current investigations
revealed the higher capacitance of electrode at lower current densities and lower capacitance at
higher current rates which is the indication of battery type electrode. Moreover , MoO3-NiMoO4
electrode revealed fast charge and discharge times because of carbon based organic stabilizing
agents (C=O, C-H) in the sample as revealed by XPS. From the figure 8b , it can be further
observed that area under the charging and discharging curve is larger at lower current densities
and vice versa. This is the indication of fast charging and discharging of MoO3-NiMoO4 at 30
Ag-1 while slowest charging as well as discharging of the electrode at 0.5 Ag -1. Such inverse
correlation between charge-discharge time and the applied current density is proposing the
insufficient faradic reactions due to poor utilization of the active mass at higher current
density.29-32
25
Table 2: Comparison of supercapacitive behavior of MoO3-NiMoO4 nanomaterial with
reported investigations Via galvanostatic charge-discharge measurements
KOH =
Potassium hydroxide, NaOH = Sodium hydroxide, PANI= Polyaniline, Na2SO
4= Sodium sulfate
Nevertheless, the presented results from GCD measurements are well in agreement with CV
results; the fabricated electrode depicted pseudocapacitance behavior with outstanding redox
peaks and excellent rate capability.
To understand the conducting behavior of functionalized MoO3-NiMoO4-based
electrodes, EIS measurements were performed for the estimation of internal resistance and
charge transfer resistance. Impedance results of MoO3-NiMoO4 are shown in figure 9 (a-b) while
Nyquist plot of fabricated MoO3 electrode has been given in Figure 3c S5 in the supplementary
data. The insert of in figure 9a presented a semicircle arc in low-frequency region (R ct) and the
intercept at real part in the high frequency range stands for the internal resistance (Ri) as well as
Equivalent series resistance (Rs). While figure 9a shows a line in the low-frequency region called
Warburg element (Zw).1, 29, 30 It is observed in insert of figure 9a that organic stabilized MoO3-
26
Electrode Electrolyte Specific
capacitance
(Fg-1)
Current
density
(Ag-1)
References
Mo-doped ZnO nanoflakes KOH 46.2 10 50
MnMoO4 NaOH 9.7 1 65
carbon
cloth nickel oxide-
polyaniline (EC-NiP)
PANI 192.31 0.5 73
CoMoO4 NaOH 62.8 1 65
MoO3 nanowire Na2SO4 23 0.5 69
MoO3-NiMoO4 KOH 204 0.5 Present
Work 118 5
47 10
NiMoO4 has smaller intercept (of 0.2 Ω) at real axis in the high frequency region so it has smaller
Ri/Rs. However, Rs value of MoO3-NiMoO4 electrodes is much smaller than Rs value of MoO3
which is 0.6 Ω as depicted in figure 3c S5 in supplementary data. This illustrated that NiMoO 4
showed greatly reduced internal resistance due to additional electron pathways of Ni, Mo and O.
This is due to the fact that surface composition of electrode material is responsible for the
diffusion/transfer of ions in electrolyte as reported by Hong et al 2019.77 Moreover, the internal
resistance of MoO3-NiMoO4 electrodes is smaller than numerous fabricated metal oxides based
electrodes.8-10, 16,32,49 This is strongly attributed to the fact that in the current study pathways for
electron transport were not only provided by Ni, Mo and O but also by the incorporated organic
stabilizing agents (C4H8O) of synthesized material as indicated by XPS (figure 5 and MS spectra
(figure 2 b). Gul and Ahmad, 2019 described the involvement of organic compounds for electron
flow and current generation by following equations:38
At anode: Organic substrate + H2O CO2 + H++ e-
At cathode: O2 + 4H+ + 4e- 2H2O
However, in the current study carbon also take part in above electrochemical reaction as follow:36
At anode: C + 2H2O CO2 + 4H+ + 4e-
At cathode : 4H+ + 4e- 2 H
The fabricated electrode has low charge-transfer resistance (Rct) according to minor
semicircle arc in high frequency region (figure 9a). This is furthered due to the faradaic reactions
of Ni-Mo-O as stated by Ujjain et al., 2015; Xiao et al., 2016; Zequine et al., 2019,16,32,49 thus Rct
is confirming the enhanced pseudocapacitance of the electrode. It can be further observed in the
plot 9a that there is a clear separation between minor semicircle and vertical line of Zw. The plot
sharply increased in figure 9a and nearly became vertical, more close to 45°, corresponding to
27
the Zreal axis indicating the smaller Warburg impedance. Therefore, lower Zw is confirming the
rapid diffusion of ion into the electrolyte and fast adsorption onto the electrode surface due to
surface chemistry and less aggregated nano particles (figure 6) of MoO3-NiMoO4.
Figure 9: (a) Nyquist plot of MoO3-NiMoO4 in low frequency region, (b) Nyquist plot
(semicircle arc) in high-frequency region (c) variation of impedance as a function of frequency,
and (d) Ragone plot of MoO3-NiMoO4
28
The figure 9b presented the total impedance as the function frequency. The total
impedance of the electrode was observed to be decreased in the present investigation which is
consistent to Gervas et al., 2019, and Bhoyate et al., 2018.29,30 Thus, the lower total impedance
furthered designated the better conductive characteristics of MoO3-NiMoO4. These results
indicated good electrical conductivity of fabricated electrode with Faradic reactions.
Futhermore, the storage potential of MoO3 and MoO3-NiMoO4 electrodes was estimated
by the energy density (E) and power density (P) using discharge voltage via following
equations and expressed as W h kg-1 and W kg-1;
Energy density ( E )=C × ∆ V 27.2 ….(4)
Power Density ( P )= E ×3600t …….(5)
Where C (Fg-1) is the capacitance via galvanostatic charge discharge, ∆V (V) is the potential
window and t (s) is the discharge time.29,30,56 The Ragone plot (presenting the higher energy
density) of MoO3-NiMoO4 is given in figure 9c while Ragone plot of MoO3 is given the
supplementary data Figure 3d S5. In the present research, maximum calculated energy density of
MoO3-NiMoO4 was 9.4 against power density of 144 which is remarkably higher than 1.8 Whkg-
1 energy density of synthesized MoO3 against the same power density. This indicated that
enhanced energy density 9.4 Whkg-1 of MoO3-NiMoO4 was due the synergetic effects of combine
metal oxides as reported by Lin et al., 2019.83 Moreover, the phytofabricated MoO3-NiMoO4
exhibited enhanced energy storage potential than several reported devices.39, 40, 49, 56, 65-70 On the
other hand, maximum recorded power density of MoO3-NiMoO4 was 6339.1 Wkg-1 (6.3 KW kg-
1), which was extensively higher than 4.7 KW kg-1 power density of MoO3 NPs (figure 3d S5).
The Ragone plots (figure 9c and figure 3dS5 ) exhibited that when power density increased, the
29
energy density was decreased and vice versa. Negative correlation between energy density and
powder density is a general phenomena associated with numerous other fabricated
supercapcitors.29-32 Consequently, energy density and power density, in comparison with reported
investigation, were strongly suggesting MoO3-NiMoO4 potential for energy storage device. More,
the energy density and power density, in agreement with low resistance, and efficient redox
behavior, demonstrated fabricated MoO3-NiMoO4 electrode as potential candidate for
supercapcitor.
5. Conclusion
In conclusion, we have successfully synthesized and functionalized of nanocomposites of binary-
metal oxides using bio-active compounds of E. cognata. The synthesized MoO3-NiMoO4
composite revealed notable uniformity and regularity in particles arrangements with nano-
structures of spherical shapes. XPS well demonstrated the surface functionalization by phenolic
compound (benzenemethanol (C7H8O)) which was identified by GCMS NIST library. The
obtained bio template-assisted MoO3-NiMoO4 was revealed enhanced electro-active sites for
energy storage devices with the maximum energy density of 9.4 Whkg-1 and maximum power
density of 6339.1 Wkg-1. The significant capacitance was found at both lower and higher scan
rates and current densities. Whereas, the charge transfer and ion diffusion were guaranteed by the
uniform and connected particle arrangements of binary metal oxides. Therefore, the present
investigations reported the significant potential of organic template derived MoO3-NiMoO4 for
supercapacitor. The current work presented the cost effective and sustainable fabrication of metal
oxides nanocomposite with organic-inorganic framework and projected its practical application
for energy storage device like supercapacitor.
ACKNOWLEDGEMENTS
30
The authors acknowledge the Higher Education Commission of Pakistan, Department of
Environmental Sciences, Fatima Jinnah Women University Rawalpindi Pakistan and the
department of Materials, Photon Science Institute and Sir Henry Royce Institute, The University
of Manchester U.K. The authors express their sincere acknowledgment to the Polymer Chemistry
Program and the Kansas Polymer Research Center, Pittsburg State University for providing
financial and research support to complete electrochemical part of this project.
Conflict of interest statement
The authors declare that they have no known conflict of interest that could have appeared to
influence the work reported in this paper.
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