A combined theoretical and experimental approach of a new ternary metal oxide in molybdate composite for hybrid energy storage capacitors

Sustainable energy sources require an efficient energy storage system possessing excellent electrochemical properties. The better understanding of possible crystal configurations and the developmen ...

The energy density of supercapacitors can be improved by increasing the voltage and capacitance.This can be achieved through selecting the appropriate transition metal cations, as the energy storage properties are linked to the electrodes and its structure and morphology, without affecting the cost and safety issues. 13][4] The manganese oxide is one of the most promising electrode materials with a high specific capacitance of 68 F g 1 examined in the three-electrode configuration. 5However, its poor electrical conductivity and cycling stability have limited its application.Nickel oxide (NiO) materials have generated great interest due to their low cost, low toxicity, and environmental friendliness.Unfortunately, NiO possesses high resistivity, which is a serious setback to apply for a practical application to supercapacitors. 6Whereas, the phosphate framework (LiMPO 4 ) is environmentally friendly and have long cycle life with relatively low cost but their low conductivity limits their energy density and found not suitable for high energy applications. 7][10][11] Nickel molybdate (NiMoO 4 ) possessing a redox couple (Ni 2+ /Ni 4+ ) exhibits a high capacitance, low toxicity and is cost-effective. 12][10][11] In recent years, mixed metal cations in oxide frameworks were introduced and found to be more effective for a hybrid supercapacitor electrode in the KOH electrolyte than their single layered oxides. 13,14In an effort to achieve a high performance hybrid supercapacitor, a unique and rationale approach have been developed to a new ternary composite material based on Mn-Co-Ni molybdate then both the computational and electrochemical results are presented in this work.
To the best of our knowledge, ternary transition metals in metal molybdate have not been reported yet for hybrid supercapacitor devices.Moreover, in this work, the reported ternary metal oxide exhibits a wide voltage window that led to improved energy storage performance.The objective of the current study is the development of a new ternary material with a different crystal configuration crystallized in the pristine MMoO 4 (M = Mn, Co, or Ni) framework and to achieve improved electrochemical performance.The unique role of Co is to prevent a cation disorder, possessing excellent catalytic characteristics and improving rate capability, 14 Mn is believed to support the composite structure 13 and Ni redox couple contributes obtaining high capacitance. 15The benefits and rationale of choosing each metal cation in the ternary metal oxide in molybdate have been shown in this work both computationally and experimentally to improve the electrochemical properties due to the synergetic effects of multiple cations.
In order to have a profound understanding of the experimentally synthesized ternary metal oxide in molybdate systems, the spin-polarized density functional theory (DFT) calculation is performed as implemented in the Vienna Ab-initio Simulation Package (VASP). 16,17The aim is to investigate possible configurations for Mn 1/3 Co 1/3 Ni 1/3 molybdate termed as "MCNMoO 4 " via the geometry optimization and further study their electronic ground state properties.The projector-augmented wave (PAW) 18 is employed to describe pseudopotentials of each constituent element with a reasonable energy cutoff as 500 eV for a plane wave basis set.The structural relaxation is converged when the total energy of the configuration is less than 10 5 eV and the Hellmann-Feynman forces are below 0.01 eV/Å.
To evaluate the MCNMoO 4 structure, the 48-atom NiMoO 4 in αand β-phases are fully optimized within generalized gradient approximation (GGA) exchange correlation functional proposed by Perdew, Burke, and Ernzerhof (PBE). 19Later, they are extended in the c-direction by the repetition of 1 × 1 × 3 in order to preserve the desired mixing ratio of the transition metal before used as initial models in the sampling procedure.In this process, a generalized special quasi-random structure (SQS) method 20 in the evolutionary algorithm is employed to explore the alloy structure as implemented in USPEX. 21A candidate structure is randomly generated by applying a permutation operator to one individual.According to our experimental results of the MCNMoO 4 crystallinity, only transition metal atoms are being exchanged under the permutation.The population size is here designated at 60.In each generation, the structural order of each candidate is minimized/maximized in order to find the most disordered/ordered.The selected candidate is thereafter permutated to create a new set of structures for next generation, and these structures will subsequently go through the structural order optimization.Such procedures are iterated around 30-60 times in each simulation (the number of generations), and the simulation is repeatedly performed 10 times in this work.
After the sampling process, all obtained structures are fully relaxed using DFT again.Because of a large system size, the k-point mesh of 1 × 1 × 1 is high enough to relatively compare their computed structures and energetics and will be used in this work.To avoid the self-interaction error and have a more accurate electronic structure description, HSE06 functional 22,23 has been also used with the integration over the Brillouin zone approximated at Γ point.It is applied in the case of the most stable configuration in each phase.To perceive the electronic structure of this ternary material, the density of states (DOS) is then investigated.Since we are not focusing on magnetic property, the only ferromagnetic configuration is assumedly considered in our calculations.
MCNMoO 4 was synthesized using cobalt, manganese, and nickel nitrates in the presence of ammonium molybdate tetrahydrate and urea as a fuel.All the chemicals were supplied by Sigma Aldrich.Accurately measured chemicals were dissolved in 20 ml D.I water, and the pH of the solution was adjusted by drop wise addition of ammonia solution.Subsequently, the solution was evaporated on a heater and moved to an oven with 150 was transferred into a furnace at 300 • C for 3 h.All the physicochemical characterization of the synthesized ternary metal oxide in molybdate composite is given in Figs.S1-S3 and Table S1 of the supplementary material.The XRD analysis (in Fig. S1) is used to determine the crystal phase of MCNMoO 4 .The morphology of the obtained product observed by the TEM images is shown in Fig. S2.X-ray photoelectron spectra (XPS) acquired from this compound showed the oxidation states of the cations present in the ternary composite (Fig. S3) along with the binding energies tabulated in Table S1.
An aqueous solution of 2M NaOH was employed as an electrolyte for all electrochemical measurements.The hybrid supercapacitor device (shown in Fig. S4 of the supplementary material) was constructed with MCNMoO 4 as the positive electrode and activated carbon (AC) as the negative electrode.Galvanostatic charge-discharge experiments and electrochemical impedance spectroscopy were performed using SP-150, Bio-Logic instrument.Charge-discharge studies were carried out at a current density of 2 A g 1 .Specific capacitance and energy density of the hybrid device were calculated at the end of each charge-discharge test.Electrochemical impedance spectroscopy (EIS) was carried out with an amplitude of 5 mV over a frequency range of 10 MHz-700 kHz at open circuit potential.The optimal mass ratio between the two electrodes, AC and CoMoO 4 , was determined to be 1.6 for the fabricated asymmetric supercapacitor.Therefore, the mass of the AC (negative) and MMoO 4 (positive) electrode was 24.0 and 15.0 mg, respectively.The specific capacitance and energy density of the device have been calculated using the following equations: where SC is the specific capacitance (F g 1 ), I is the current (A) imposed to the cell for charge/discharge, ∆t is the time taken to discharge in seconds (calculated from the discharge curves), m is the weight of the active electrodes (CoMoO 4 and AC) in g, and ∆V is the voltage window (V).
The monoclinic structures of αand β-NiMoO 4 in a C2/m space group were optimized and illustrated in Fig. 1(a).Basically, their structures look quite similar, but the primary difference is related to the coordination number of Mo 6+ ion and the β angle of unit cells. 24Because of the size and electronegativity of the transition metal (X M ) where X Mn < X Co < X Ni , the CoMoO 4 and NiMoO 4 are likely to have more packed structure leading to the formation of octahedral MoO 6 (α-phase) whereas the crystal cell with a higher volume containing tetrahedral MoO 4 ( β-phase) is preferable to occur in MnMoO 4 . 24However, a phase transition from αto β-phase was experimentally reported to occur in CoMoO 4 and NiMoO 4 systems. 25For this reason, the MCNMoO 4 ternary composite is possibly formed in either one of them.
According to the sampling results from the evolutionary algorithm, all obtained structures can be distinguished by the mixing composition of 4 binding octahedral MO 6 unit as shown in Fig. 1(b).This group of 4 octahedrons is connected by sharing their edges with each other and all ligands (oxygen) have a bonding with Mo 6+ ions.Each 4MO 6 unit is bridged by MoO x components where x = 4 and 6 in αand β-phases, respectively.Owing to the permutation operator, all binding octahedral units of obtained configurations are randomly mixed in various types.The representations of a 4MO 6 mixing type are here defined by (n 1 M 1 :n 2 M 2 :. . . ) where n denotes the number of a center transition atom M i and i n i = 4.For example, among many possibilities, a 4MO 6 unit might be composed of 2 different M 2+ ions such as 1 octahedral CoO 6 connected with 3 octahedral NiO 6 or 2 octahedral CoO 6 with 2 octahedral NiO 6 .The former and latter will be then denoted as (1Co:3Ni) and (2Co:2Ni), respectively.
In Table I, crystal lattice parameters and total energies of chosen configurations obtained from PBE and HSE06 based geometric relaxations calculations are reported.It is noted that the total energy is referenced with that of B1 (∆E = E E B1 ).Moreover, the structures forming in αand β-phases are represented as Ai and Bi (where i is a structure number), respectively, while the BA configuration is originally an α-phase structure that transforms into β-phases after geometry optimization.From the results, the β-phase structure with a number of diverse 4MO 6 units can possibly undergo a phase transformation or even become an unstable structure.For instance, the B2 configuration which uniformly contains 4MO 6 units with (4Mn) and (2Co:2Ni) bases is crystallized in the β-phase.However, the corresponding geometry of BA is significantly changed to the α-phase with the total energy increment during the coexistence of (3Co:Ni) and (4Ni) along with (4Mn) and (2Co:2Ni).This could be attributed to a strong distortion on tetrahedral MoO 4 induced by an inhomogeneous mixing of 4MO 6 units.When the MoO x structural unit is too much distorted, it would lead to a broken confirmation around that particular site.
The stability can be relatively determined from the total energy of the structure.Comparing the total energies of each identical phase structure, we found that A1 and B1 configurations (in Fig. 2) prefer to form in MCNMoO 4 rather than the one with mixing M in the 4MO 6 unit.It is worth mentioning that they both have the identical mixing pattern of 4MO 6 unit where only (4M) basis is orderly arranged as depicted in Fig. 2. Nevertheless, B1 is the most stable configuration we have found in this work.The relative energies between A1 and B1 structures are 0.21 and 1.19 eV based on PBE and HSE06 functional, respectively.This indicates that when the localization of d-electron is correctly described by more accurate exchange correlation, the relative stability between αand β-phases are more obviously varied in our MCNMoO 4 system.Based on the lowest total energy of B1 and B2 configurations, the β-phase structure should highly tend to be observed in MCNMoO 4 .
A comparative plot of the total energy is depicted in Fig. 1(c).Here, the relative total energy is plotted with respect to cell volume, and it is noticed that the MCNMoO 4 structure becomes more energetically favorable while its cell volume gradually decreases.The calculated volume per formula (V ) of α-NiMoO 4 and α-MnMoO 4 are shown by dashed-lines in this plot.One can observe that the volume of the ternary metal oxide in the molybdate composite is less than that of α-MnMoO 4 .This is due to the smaller atomic size as well as the higher X of Ni and Co atoms.Moreover, it could further imply that a surface area of MCNMoO 4 should be relatively higher compared with the pristine α-MnMoO 4 .Since we know that the surface area is significantly proportional to the specific capacitance of a supercapacitor, our results can explain why an initial specific capacitance for the ternary metal oxide in molybdate is less than the Ni-based system but still more as compared with the Mn-based molybdate as reflected in Fig. 4(a) (discussed in the electrochemical experimental results).This is also in agreement with the work of Park et al. who found that the surface area of Ni x Co 1x MoO 4 increased with the increase of x Ni content. 26In one of our earlier works, we showed that the pristine α-MnMoO 4 exhibited a stable electrochemical performance 27 as well as the most stable structure among the group of Mn, Co, and Ni molybdate studied. 28Therefore, the cycling stability of the MCNMoO 4 system should be undoubtedly improved when it has been crystallized in β-phase.The predictions of ternary metal oxides in the molybdate structure and cycling stability are all in good agreement with our experimental data, discussed later in the electrochemical experimental results.
The Density of States (DOS) calculations have been performed within PBE and HSE06 functional.Due to the well-known underestimation of the bandgap in PBE, MCNMoO 4 is found to incorrectly show a metallic character.From the HSE06, the predicted bandgap of A1 and B1 configurations are 2.87 and 3.40 eV at Γ point, respectively.Their projected DOS (PDOS) can be calculated as demonstrated in Fig. 3.The electron contributions of Mo 4d -O 2s hybridization are mostly dominant in the valence band region between 8 and 4.5 eV, for both A1 and B1.However, in the conduction band, the clear feature of Mo-O hybridization is observed in a range of 2.2-4 and 5.2-7.3eV for A1, but different in a range of 2.9-7 eV for B1.The similarity of their DOS indicates that the structural phase transformation does not affect the electronic structure of MCNMoO 4 much.Additionally, to clearly capture the electron contributions of M, the areas under the corresponding curves are highlighted by the black, blue, and green colors for the PDOS of Mn, Co, and Ni, respectively.According to crystal field theory, all M 2+ ions in two cases are both found in the high-spin state under the octahedral crystal field of MO 6 , and they all undergo the Jahn-Teller distortion.Their electron configuration in d-orbital can be then written as Ni 2 + : The strong hybridization of Mn 3d-O 2s obviously shows near the Fermi level (E F ), whereas that of Co 3d-O 2s and Ni 3d-O 2s are observed in-depth energy range.In our earlier work on the pristine molybdate systems, the hybridization of M 3d-O 2p was also observed near the E F , but the intensity of oxygen part was higher than M, especially for α-phase.It means when an electron is removed from the bulk MCNMoO 4 , the redox reaction of M 2+ ion would occur easier in the way that the transition of Mn 2+ into Mn 3+ is more likely to take place than in the case of other M cations, such as Co or Ni.Therefore, we can infer that MCNMoO 4 has emerged as a molybdate based new ternary metal oxide system which could possess improved electrochemical characteristics to that of pristine counterparts with single metal oxide.Electrochemical studies have been carried out on this ternary composite electrode to validate its suitability for energy storage applications.
In order to explore the practical application of the ternary composite, the AC || MCNMoO 4 device is fabricated and its sketch is shown in Fig. S4 of the supplementary material.Potentiostatic (charge-discharge) experiments have been performed in the device, and the results are shown for extended voltage windows in Figs.4(a) and 4(b).The symmetrical shape of the charge and discharge curves is maintained even at a higher voltage window, exhibiting two plateaus in regions 1.4 and 0.5 V, implying the process is reversible.0][31][32][33] For the cell tested at 1.4 V cutoff, the coulombic efficiency of the charge to discharge capacitance is found to be 95% [Fig.4(b)], but this efficiency decreases to 80% for the cell tested at an extended voltage window 1.85 V. Nevertheless, the available capacitance for the device doubled (∼18 to ∼35 C g 1 ) for the voltage window from 1.4 to 1.85 V.The specific capacitance for the various cut-off voltage windows, i.e., 1.2, 1.4, 1.6, and 1.8, corresponds to 7, 18, 23, and 35 C g 1 , respectively.A discrepancy in coulombic efficiency is due to the ohmic drop of 0.15 V seen at a higher cut-off voltage window [Fig.4(a)].Despite this, a large voltage window is of great importance for practical storage applications and to have larger power and energy densities.The calculated specific capacitance values at a current density of 0.5, 1, 1.1, and 1.5 A g 1 correspond to 58, 52, 50, and 45 F g 1 , respectively.Even at a high current density of 1.5 A g 1 , 77% of the capacitance is retained.The decreasing capacitance at a higher current density is due to low utilization of active materials.Long term cycling stability of the hybrid device [in Figs.4(c) and 4(d)] with a cut-off voltage of 1.85 V was tested over 2000 cycles at a current density of 0.5 A g 1 .In comparison to that of single cation molybdate (Mn, Co, and Ni), 28 the available specific capacitance of 58 F g 1 for ternary (MCNMoO 4 ) composite is higher and found to be quite stable with excellent capacity retention of 90% at the end of 2000 cycles.The role of Mn cations acts as a stabilizer to retain the molybdate framework for reversibility and hence the improved capacitance after multiple cycles. 27The role of Ni and Co enhanced the redox potential and specific capacitance. 11,15The presence of nanorods in the composite can provide active sites for electron transfer to occur and enabling OH ions access to APL Mater.6, 047701 (2018) the structure reversibly that resulted in enhanced capacitance and retention rate.Among the various criteria to quantify the performance of a hybrid device, coulombic efficiency is crucial.This indicates the amount of charge that is involved in a device facilitating an electrochemical reaction.The ratio of the time taken to discharge and charge the device within the cut-off voltage is defined as "coulombic efficiency."The hybrid device [in Fig. 4(d)] with a cut-off voltage of 1.85 V tested over 2000 cycles showed 80% with an excellent capacitance retention of 90%.The optimal energy density (E) of the device, using Eq. ( 2), is calculated to be 27 Wh kg 1 at a power density of 500 W kg 1 , as shown in the Ragone plot [in Fig. 4(e)].The obtained energy density is superior to those reported in the literature 8,11,15,[29][30][31][32][33] for pristine (single metal oxide) molybdate as electrodes for supercapacitor applications in the aqueous system.On the basis of the above discussion, the synergistic effect of three metal oxides and its morphology through combustion process 34 in the molybdate framework paved the way for excellent stability through dynamics of electrons and ions movements within the bulk of the molybdate particles enhancing the energy density of the ternary molybdate with a wide voltage window.
Electrochemical impedance spectroscopy (EIS) was also conducted and the corresponding Nyquist plot is shown in Fig. 4(f).It can be observed that the plot has a very small semi circle in the high-frequency region and a sloping line in the low-frequency region indicating a low charge transfer resistance.The Rs value was obtained 4.6 Ω from the intercept of the Z axis.The straight line at the lower frequency region is the Warburg impedance.The deviation of the 90 • slope of this line [inset of Fig. 4(f)] indicates the pseudocapacitive behavior of the material. 27,29The results indicate that MCNMoO 4 material has low internal resistance and small interfacial charge resistance suitable for supercapacitor properties.
In conclusion, the ternary metal oxide in molybdate composite [(Mn 1/3 Co 1/3 Ni 1/3 )MoO 4 ] with an appropriate stoichiometric ratio of a metal oxide was successfully synthesized using the combustion route.A hybrid supercapacitor was fabricated based on MCNMoO 4 nanorods as a positive electrode and activated carbon (AC) as a negative electrode in the 2M NaOH electrolyte.The ternary composite was found to be crystallized β form with α-MnMoO 4 type in agreement with our density functional theory (DFT) calculations, revealing the energetically favorable structure.The nanorods possess a specific capacitance of 58 F g 1 at a current density of 0.5 A g 1 and good reversibility with cycling efficiency of 90% after 2000 cycles.The obtained capacitance is contributed from both electrochemical double layer capacitance and redox pseudocapacitance that led to high capacitance with excellent capacitance retention.This new ternary metal oxide in molybdate would be cost effective with high energy density, and high voltage represents major progress towards energy storage alternatives comparable to that of conventional electrochemical double layer capacitance (EDLC) and other reported hybrid capacitors.
See supplementary material for the complete physicochemical characterization of the ternary metal oxide in molybdate composite including the schematic diagram of the supercapacitor device.
M.M. would like to acknowledge the AINSE Research (Grant No. ALNGRA15051) to carry out the microscopy work at ANSTO.T.W. would like to acknowledge DPST administered by IPST, Thailand.SNIC and HPC2N are also acknowledged for computational supports.S.C. and R.A. would like to acknowledge the Carl Tryggers Stiftelse for Vetenskaplig Forskning (CTS), Swedish Research Council (VR).

FIG. 1 .
FIG. 1.(a) Illustration of permutated structures for ternary metal oxide (Mn 1/3 Co 1/3 Ni 1/3 ) in molybdate composite.(b) A group of 4 binding octahedral MO 6 components (M = Mn, Co, and Ni) in metal molybdate structure.(c) A comparative plot showing a relative total energy for each considered configurations against their corresponding volumes per formula unit calculated within PBE functional.

FIG. 3 .
FIG. 3. Projected DOS of ternary metal oxide in molybdate composite structures with A1 and B1 configurations.The electron contributions of Mn, Co, Ni, Mo, and O are presented by black, blue, green, brown, and red lines, respectively.The dashed-line denotes the Fermi level aligned at 0 eV.

TABLE I .
The geometric details of all optimized Mn 1/3 Co 1/3 Ni 1/3 MoO 4 structures calculated within PBE and HSE06 functionals.The relative total energy (∆E) is referenced to the total energy of the B1 configuration.