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Synthesis of Ternary Metal Oxides for Battery-Supercapacitor Hybrid Devices: Influences of Metal Species on Redox Reaction and Electrical Conductivity Ying-Yu Huang, and LuYin Lin ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00781 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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ACS Applied Energy Materials

Synthesis of Ternary Metal Oxides for Battery-Supercapacitor Hybrid Devices: Influences of Metal Species on Redox Reaction and Electrical Conductivity

Ying-Yu Huanga1 and Lu-Yin Lina1*

a

Department of Chemical Engineering and Biotechnology, National Taipei University of

Technology, 1 Sec. 3, Zhongxiao E. Rd., Taipei 10608, Taiwan 1

The authors contributed equally.

*

Corresponding author: Tel: +886–2–2771–2171 ext. 2535; Fax: +886–2–2731–7117

E-mail (L. Y. Lin): [email protected]

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Synthesis of Ternary Metal Oxides for Battery-Supercapacitor Hybrid Devices: Influences of Metal Species on Redox Reaction and Electrical Conductivity Abstract Designing battery-type materials with good electrocapacitive performance and high electrical conductivity is necessary to improve the energy-storage capability of an battery-supercapacitor hybrid devices (BSH). Ternary metal oxides are synthesized by using a hydrothermal reaction with an extra metal of Mo, Fe, Cu, Zn, or Al incorporated in the nickel cobalt oxide as the battery-type material to enhance the electrical conductivity and generate numerous Faradaic reactions via the multiple oxidation state of transition metals. Due to the larger surface area of the nanosheet structure and the smaller charge transfer resistance with the participation of molybdenum, the best electrocapacitive performance among the ternary metal oxide electrodes is attained for the NixCoyMozO electrode, which is further optimized by tuning the Mo ratios in the precursor solution. An optimized NixCoyMozO electrode is prepared by using the Ni:Co:Mo ratio of 1:2:2. This electrode achieves an areal capacitance (CF) of 2.94 F/cm2, which is higher than those for the binary metal oxide electrodes of NixMoyO (1.11 F/cm2), CoxMoyO (1.63 F/cm2), and NixCoyO (1.45 F/cm2), inferring the success to improve the energy-storage ability of the electrode by incorporating more transition metals in the oxide as the electrocapacitive material. An BSH based on the NixCoyMozO positive electrode and an activated carbon negative electrode shows a CF value of 126 mF/cm2 at 10 mA/cm2, a potential window of 1.8 V, and a maximum energy density of 22.02 Wh/kg at a power density of 3.50 W/kg. This result provides new blueprints for constructing multiple metal oxides as the battery-type material for achieving more Faradaic reactions and higher electrical conductivity, and hence for enhancing the energy-storage capability of an BSH. Keywords:

battery-supercapacitor

hybrid

device;

cyclic

voltammetry;

hydrothermal reaction; nickel cobalt oxides; nickel cobalt molybdenum oxide

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charge/discharge;

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1. Introduction The eager demands for clean and sustainable power sources have triggered great academic and industrial interests to exploit efficient and environmentally friendly energy storage devices. Developing energy storage devices with high energy capacities, long lifetime and high rate capability is indispensable to overcome the impending exhaustion of fossil fuel reserves and allay the environmental concerns among the available clean renewable energy technologies. The supercapacitor (SC) is one of the most promising energy-storage devices because of its long lifespan comparing to that for the secondary batteries as well as the high capacitance and excellent reliability comparing to those for the conventional dielectric capacitors.1-6 According to the charge storage mechanism, the SC is classified into the electrical double-layer capacitor (EDLC) and the pseudocapacitor. The charges are adsorbed at the electrode/electrolyte interface electrostatically for the EDLC, and the energy is stored via the redox reactions on electrode materials for the pseudocapacitor.7 Recently, much interest has been paid on constructing the battery-supercapacitor hybrid device (BSH),8 because of its higher energy density than that of the EDLC and higher power density than that of the rechargeable batteries. The BSH is expected to be used in driving electric vehicles (EVs) and hybrid electric vehicles (HEVs) in the near future.9 Combining a battery-type positive electrode and a carbon negative electrode is one of the feasible ways to assemble the BSH. The hybridization of two kinds of electrodes is able to enhance the energy and power density of the system via providing wider operating potential windows and larger specific capacitances.10-11 Numerous attempts have been conducted to develop transition metal oxides as the electrocapacitive materials for the battery-type positive electrode in BSH, due to the low price, abundance in resources, as well as facile and scalable preparation properties.12-14 Recently, binary metal oxides have acquired much attention because of their better electrical conductivity, feasible oxidation states, as well as the good chemical and thermal stabilities to get superior electrochemical performance.15-16 Nickel and cobalt are regarded as the most promising species to comprise the



binary metal oxides due to their high redox activities.17-19 The nickel cobalt binary metal oxide is widely reported to have better electrocapacitive performance than those for the corresponding single metal oxides, the nickel oxide and the cobalt oxide, primarily due to the more Faradaic redox reactions occurring at the interface of the electrocapacitive material and the electrolyte for the former case.7 Chang et al. synthesized nickel cobalt oxides with various Ni/Co ratios using a template-free approach. The nickel cobalt oxide electrode possesses a specific capacitance (CF) value of 867.3 F/g at 1 A/g and a CF retention of 92.3% at 10 A/g, outperforming the pure NiO and Co3O4 electrodes.20 Li et al. prepared a sandwich-like NiCo2O4/rGO/NiO heterostructure composite on Ni foam via a three-step hydrothermal method. A better electrochemical performance was obtained for the binary NiCo2O4 electrode comparing to that for the NiO electrode.21 Obviously, the most advantageous property for the binary metal compounds comparing to the single metal compounds as the electrocapacitive material is the more Faradaic redox reactions conducting on the electrode materials. Regarding to this concept, it is interesting to investigate that if more Faradaic redox reactions could be generated and hence higher specific capacitances could be achieved when more transition metals are participated in the electrocapacitive material. Only a few literatures studied the application of the ternary metal oxides as the electrocapacitive material on the energy storage devices. Wu et al. synthesized Zn−Ni−Co ternary oxide (ZNCO) nanowire arrays on Ni foam by using a hydrothermal method and a calcination process. The ZNCO nanowire electrode showed a CF value of 2481.8 F/g at 1 A/g and rate capability of 91.9% for the CF retention at 5 A/g.12 Hu et al. synthesized flower-like nickel-zinc-cobalt oxide nanowire arrays on Ni foam by using a hydrothermal method and a thermal treatment. The flower-like Ni-Zn-Co oxide electrode showed a CF value of 776 F/g at 2 A/g and cycling stability of 88.9% after 10,000 charge/discharge cycles.22 Zate et al. synthesized thin films of sprayed nickel–manganese ferrite onto stainless-steel substrate for the energy storage application.23 A CF value of 185 F/g at 5 mV/s was obtained for the Ni0.8Mn0.2Fe2O4 film electrode. However, the reports for applying metal oxides as the electrocapacitive material for BSH usually focused on


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proposing a new electrocapacitive material and analysing their physical and electrochemical properties. Systematic studies for comparing the properties of different ternary metal oxides and investigating their electrochemical performances are limited in the previous literatures. In this work, a battery-type electrode and a capacitor-type electrode were assembled to fabricate the BSH. The main studied target is the energy-storage material on the battery-type electrode. The ternary metal oxides based on nickel and cobalt were synthesized by incorporating the additional metals of molybdenum, iron, copper, zinc, and aluminium. Molybdenum, iron, copper, zinc, and aluminium are chosen as the third metal to incorporate in the nickel cobalt oxide due to the following reasons. Firstly, these metals are commonly used in the electrocapacitive material for energy storage. Secondly, the sizes of these metals are similar to those for the nickel and cobalt, so the combination of these metals with nickel and cobalt in the oxide may be more possible and easier. Last, the electrical conductivity for these metals is high, so the charge transfer may be enhanced with the incorporation of these metals in the oxide. The morphology variations and the electrochemical performances for the ternary metal oxide electrodes were carefully studied. Among the ternary metal oxide electrodes, the best electrochemical performance was obtained for the nickel cobalt molybdenum oxide electrode, which further presented an optimized CF value of 2.94 F/cm2 with the optimized Mo ratio in the oxide. Its corresponding binary metal oxide electrodes of nickel molybdenum oxide, cobalt molybdenum oxide, and nickel cobalt oxide electrodes respectively show smaller CF values of 1.11, 1.63, and 1.45 F/cm2. Moreover, a BSH composed of the nickel cobalt molybdenum oxide positive electrode and the activated carbon negative electrode achieved a CF value of 126 mF/cm2 based on the whole mass of the device at 10 mA/cm2 and the maximum energy density of 22.02 Wh/kg at a power density of 3.50 W/kg. 2. Experimental 2.1 Materials Aluminum nitrate nonahydrate (Al(NO3)2 · 9H2O, 98.0%), copper nitrate trihydrate (Cu(NO3)2 · 5 ACS Paragon Plus Environment


3H2O, 99.0%), nickel nitrate hexahydrate (Ni(NO3)2 · 6H2O, 99.0%), sodium molybdate dihydrate (Na2MoO4 · 2H2O, ≥ 99%), and zinc nitrate nonahydrate (Zn(NO3)2 · 9H2O, 98.0%) were obtained from Acros. Potassium hydroxide (KOH, analytical reagent grade) was brought from Fisher. Hydrochloric acid (HCl, 37%) was purchased from Sigma−Aldrich. Cobalt nitrate hexahydrate (Co(NO3)2 · 6H2O, 99.0%), charcoal activated carbon (99.99%), iron nitrate nonahydrate (Fe(NO3)2 · 9H2O, 99.0%), and urea (CH4N2O, 99.0%) were obtained from Showa. Acetylene carbon black (CB, 99.9900%) and N-methyl-2-pyrrolidinone (NMP, 99%) were obtained from Echo Chemical CO., LTD. Poly(vinylidene fluoride) (PVDF) was purchased from Scientific Polymer Products, INC. 2.2 Preparation of nickel and cobalt-based metal oxides on Ni foam The Ni foam (110PPI, thickness = 1.05 mm, Innovation Materials Co., Ltd, Taiwan) was cut in the size of 1 cm × 3 cm and cleaned prior to be applied in the hydrothermal reaction. The Ni foam was soaked in 6 M HCl for 10 min under ultrasonic vibration and sequentially washed by using deionized water (DIW). The cleaned Ni foam was then dried in a vacuum oven overnight. The nickel and cobalt-based ternary metal oxides were synthesized on Ni foam by using a hydrothermal reaction. An aqueous solution was prepared by dissolving 1 mmol Ni(NO3)2 · 6H2O, 2 mmol Co(NO3)2 · 6H2O, 5 mmol urea, and 1 mmol of the precursor for the third metal incorporated in the metal oxide in 40 ml DIW under stirring for 10 min at the room temperature. The precursors of Na2MoO4 · 6H2O, Fe(NO3)2 · 6H2O, Cu(NO3)2 · 6H2O, Zn(NO3)2 · 6H2O, and Al(NO3)2 · 9H2O were respectively used for synthesizing the nickel cobalt molybdenum oxide (NixCoyMozO), nickel cobalt iron oxide (NixCoyFezO), nickel cobalt copper oxide (NixCoyCuzO), nickel cobalt zinc oxide (NixCoyZnzO), and nickel cobalt aluminium oxide (NixCoyAlzO). Then 15 ml of the as-prepared solution and the cleaned Ni foam were then transferred to a 100 mL Teflon-lined autoclave which was heated at 140 oC for 4 h. The autoclave was cooled to the room temperature after the reaction, and the ternary metal oxide-coated nickel foam was rinsed by using DIW for several times and then


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dried at 60 oC overnight. Finally, the ternary metal oxide-coated nickel foam was calcinated at 350 o

C in air for 2 h. The binary metal oxides were also made based on the best-performed ternary metal oxide. The

ternary metal oxide was made by adding the precursors for the three metals in the hydrothermal solution. The corresponding binary metal oxides were made by simply adding two of the precursors for the three metals in the hydrothermal solution. Then the same experimental process as that for synthesizing the ternary metal oxide was applied for synthesizing the binary metal oxides. 2.3 Fabrication of the battery-supercapacitor hybrid device Prior to constructing the BSH, the mass balance of the positive and negative materials was conducted to achieve the charge balance in the positive and negative electrodes according to Equation 1 as follows.

=

× ×

(1)

where m is mass, I is the discharge current for positive (+) and negative (-), and t is the discharge time for positive (+) and negative (-) electrodes. An activated carbon electrode was made by using a doctor-blade method and used as the negative electrode for a BSH. The paste for fabricating the activated carbon electrode was prepared by mixing 80% activated carbon, 10% CB and 10% PVDF binder in NMP solvent. The Ni foam was used as the support and current collector, with a geometric area of 2.83 cm2 (a circle with the diameter of 19 mm). The BSH was assembled by using a ternary metal oxide electrode as the positive electrode with the geometric area of 1.33 cm2 (a circle with the diameter of 13 mm), an activated carbon electrode as the negative electrode, and 1 M KOH as the electrolyte. The electrode separator was filter paper with a geometric area of 2.83 cm2 (a circle with the diameter of 19 mm) which was soaked in the electrolyte for 30 min before use. 2.4 Material characterization and electrochemical measurements The surface morphologies were investigated using the field-emission scanning electron microscopy (FE-SEM, Nova NanoSEM 230, FEI, Oregon, USA), and the composition of atoms was 7 ACS Paragon Plus Environment


analyzed by using the energy-dispersive X-ray spectroscopy (EDX) in the FE-SEM equipment. The transmission electron microscopy (TEM, JEM-1230, JEOL, Tokyo, Japan) was applied for observing the inner morphologies of the materials. The XRD were recorded at SP8 (Japan) 12B2 Taiwan beamline of National Synchrotron Radiation Research Center (NSRRC), the electron storage ring was operated at 8.0 GeV with a constant current of ~100 mA. The diffraction angles were calibrated by utilizing Bragg positions of CeO2 standard, and integrating a cake-type pattern through program of GSAS II was used to obtain corresponding one-dimensional powder diffraction profile. The cyclic voltammetry (CV) and Galvanostatic charge/discharge (GC/D) curves were obtained by using a potentiostat/galvanostat (PGSTAT 204, Autolab, Eco–Chemie, the Netherlands) carried out with a three-electrode electrochemical system, where the sample was used as the working electrode, a Pt wire was used as the counter electrode, and an Ag/AgCl/saturated KCl electrode was used as the reference electrode in a 1 M KOH solution. The EIS was carried out using a potentiostat/galvanostat (PGSTAT 204, Autolab, Eco–Chemie, the Netherlands) equipped with an FRA2 module, and the frequency range explored was 0.01 Hz to 100 kHz. The applied bias voltage was set at the opencircuit potential. 3. Results and discussion 3.1 Material characterizations and electrochemical analysis for nickel cobalt oxide and ternary metal oxides To attain more Faradaic redox reactions at the electrocapacitive material/electrolyte interface, the ternary metal oxides were made as the electrocapacitive material on the positive electrode for a BSH. The extra precursors for Mo, Fe, Cu, Zn, and Al were incorporated in the hydrothermal reaction along with the salts for Ni and Co to fabricate the corresponding ternary metal oxides. The morphology is reported to play significant roles on the electrocapacitive performance of the electrodes. The structures for nickel cobalt oxide (NixCoyO) and the ternary metal oxides were observed by using the SEM images. Figure S1(a)-(f) in the supplementary information (SI) shows 8 ACS Paragon Plus Environment

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the SEM images for NixCoyO, NixCoyMozO, NixCoyFezO, NixCoyCuzO, NixCoyZnzO and NixCoyAlzO, respectively. The binary NixCoyO presents a well-defined nanowire structure (Figure S1(a)), while the ternary metal oxides show different morphologies of nanowires or nanosheets (Figure S1(b)-(f)). The NixCoyMozO shows a smooth sheet-like structure with the size larger than 1 µm, and the sheets were grown in random directions (Figure S1(b)). The wires with the diameter smaller than 100 nm grew in a parallel way and lay on the nickel foam substrate for NixCoyFezO (Figure S1(c)). The NixCoyCuzO presents the similar wires structure as NixCoyFezO but grew in a more vertical direction (Figure S1(d)). The NixCoyZnzO shows a structure like the transformation between the nanosheet and nanowires (Figure S1(e)). The NixCoyAlzO presents folded nanosheet structure grown in vertical directions (Figure S1(f)). Since the size, conductivity, electronic configuration, electrocapacitive ability, and the compatibility to Ni and Co are varied for metal to metal, different morphologies for the ternary metal oxides containing Mo, Fe, Cu, Zn or Al in NixCoyO is reasonable to obtain. By referring to the periodic table, the most probably factor to influence the morphology for the ternary metal oxide is their sizes and electronic configurations. The sizes and electronic configurations for Fe and Cu is more similar to those for Ni and Co. Hence the similar nanowire structures as that for NixCoyO were obtained for NixCoyCuzO and NixCoyFezO. The NixCoyZnzO possessing less similar size and electronic configuration to those for Ni and Co comparing to those for Fe and Cu presents the morphology like the transform state of the wire and the sheet. The NixCoyMozO and NixCoyAlzO present the nanosheet structures, which are the most different morphology comparing to that for NixCoyO with the nanowire structure. This phenomenon is probably due to the most different sizes and electronic configurations of Mo and Al, as compared with those for Ni and Co among the ternary metal oxides. Furthermore, the compositions of NixCoyO and ternary metal oxides were examined by using the EDS spectra. Figure S2(a)-(f) in the SI shows the EDS spectra for NixCoyO, NixCoyMozO, NixCoyFezO, NixCoyCuzO, NixCoyZnzO and NixCoyAlzO, respectively. The Ni and Co signals were 9 ACS Paragon Plus Environment


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clearly detected in all the spectra, proving the successful synthesis of the nickel cobalt-based oxides. Extra signals could be found for the ternary metal oxides. The Mo, Fe, Cu, Zn and Al signals were respectively found in the EDS spectra for the NixCoyMozO, NixCoyFezO, NixCoyCuzO, NixCoyZnzO and NixCoyAlzO (Figure S2(b)-(f)). The results suggest that using a simple one-step hydrothermal reaction process is feasible to fabricate multiple metal oxides by merely adding the precursors for the required metals in the hydrothermal reaction. After investigating the physical properties of the nickel cobalt-based oxides, the electrochemical performance for the electrodes were further studied by using the CV and GC/D measurements. Figure S3(a) in the SI shows the CV plots for NixCoyO and the ternary metal oxide electrodes, measured at the scan rate of 10 mV/s. The CF value was calculated by using CV plots according to Equation 2 as follows.24-25

=

∙ ∙

(2)

where I is the current density, is the integrated area of the CV curve, is the scan rate, Δ is the potential window, and is the active area of the electroactive material in the electrode. The NixCoyO electrode shows two couples of the redox peaks in the CV curves at the potentials of around 0.25 and 0.40 V, respectively resulting from the reversible redox reactions of Ni2+/Ni3+ and Co2+/Co3+ as well as Co3+/Co4+ transitions based on Equation 3 and 4 as follows.16 NiCo2O4 + OH- + H2O↔ NiOOH + 2CoOOH + e-

(3)

CoOOH + OH- ↔ CoOO + H2O + e-

(4)

However, the redox peaks in the CV curves for the NixCoyO electrode cannot be observed in those for the ternary metal oxide electrodes. The redox behaviours for the ternary metal oxide electrodes are varied from that for the NixCoyO electrode, inferring to the successful incorporation of the third metal in the ternary metal oxides. Nevertheless, the redox peaks can still be observed for the ternary metal oxide electrodes. The NixCoyMozO electrode shows one couple of the redox peaks at around 10 ACS Paragon Plus Environment

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0.10 and 0.35 V for the reduction and oxidation, respectively. Other ternary metal oxide electrodes also present one couple of the redox peaks, but unclear redox peaks were observed for the NixCoyFezO, NixCoyCuzO and NixCoyZnzO electrodes. Only an oxidation peak at the potential of around 0.40 V was observed for these electrodes. The contribution on the redox behaviours of the third metals may be too small to be obviously presented in their CV curves. The NixCoyAlzO electrode shows a reduction peak at around 0.35 V and an oxidation peak at around 0.5 V, the different potentials for the redox peaks comparing to those for the NixCoyO electrode may be attributed from the different redox behaviour for the Al ions in the alkaline.26 On the other hand, the CF values of 1.45, 2.22, 0.79, 1.05, 1.26, and 1.75 F/cm2 corresponding to the capacity of 0.80, 1.23, 0.43, 0.59, 0.70 and 0.97 mAh/cm2 were obtained for the NixCoyO, NixCoyMozO, NixCoyFezO, NixCoyCuzO, NixCoyZnzO and NixCoyAlzO electrodes, respectively. Only the NixCoyMozO and NixCoyAlzO electrodes show higher CF values comparing to that for the binary NixCoyO electrode. It is inferred that Ni, Co and Mo ions may probably fully conduct the redox reactions in their corresponding potential regions, and the only one couple of the redox peaks in the CV curve for the NixCoyMozO electrode may be formed by the overlapping of the redox reactions for the three metals.27 The larger peak currents for the NixCoyAlzO electrode comparing to those for the NixCoyO electrode may be caused by the dominated contribution of the redox reactions from the Al ion. In addition, the results also indicate that the higher CF values are not definitely obtained when more metals involved in the metal oxides. The smaller CF values for the NixCoyFezO, NixCoyCuzO, and NixCoyZnzO electrodes comparing to that for the NixCoyO electrode may be due to the antagonistic combination of the three metals in the oxides, leading to the disappearance of the original redox peaks for the nickel and cobalt as well as the formation of the unapparent redox peaks for iron, copper, and zinc in their corresponding CV curves. Furthermore, the electrocapacitive performances for the NixCoyO, NixCoyMozO, NixCoyFezO, NixCoyCuzO, NixCoyZnzO and NixCoyAlzO electrodes were also estimated by using the GC/D plots



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measured at a current density of 5 mA/cm2, as shown in Figure S3(b) in the SI. The CF value was calculated from the GC/D plots based on Equation 5 as follows.28 ∆

= ∙∆

(5)

where ∆t is the discharge time. All the plots show high symmetries for the charge and discharge plots, suggesting the high reversibility for NixCoyO and ternary metal oxide electrodes. The CF values of 1.08, 1.94, 0.42, 0.73, 0.87 and 1.69 F/cm2 corresponding to the capacity of 0.60, 1.08, 0.23, 0.41, 0.48 and 0.94 mAh/cm2 were respectively achieved for the NixCoyO, NixCoyMozO, NixCoyFezO, NixCoyCuzO, NixCoyZnzO and NixCoyAlzO electrodes, again suggesting the best electrocapacitive performance of the NixCoyMozO electrode. Also, the smallest potential for the redox platform in the GC/D curves for the NixCoyMozO electrode is consistent with its smallest potentials for the redox peaks in the CV curve, while other electrodes also show similar potentials for the platforms in the corresponding GC/D plots. Therefore, the best-performed NixCoyMozO electrode was further studied and optimized regarding to the relative ratio of Ni, Co and Mo salts in the precursor solution to achieve a better electrocapacitive performance for the positive electrode of a BSH. 3.2 Physical and electrochemical characterizations for the single metal oxide electrodes It is worthy to mention that not only the capacitance enhancement benefitted from more Faradaic redox reactions but also the improvement on the electronic conductivity and charge transfer resistance could be attained by incorporating more metals in the metal oxides. However, the contributions of the third metals in the ternary metal oxides on the electrocapacitive ability and the electrical conductivity have never been discussed and compared in the previous literatures. Herein, single metal oxides were prepared by merely adding the precursors of the extra metals like Mo, Fe, Cu, Zn and Al in the hydrothermal reaction and keeping other experimental parameters the same as those for making the binary and ternary metal oxides to estimate the capacitance and electrical conductivity contributions of the extra metals incorporated in the nickel cobalt oxides.


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Firstly, the morphology and the electrochemical performance for the single metal oxide electrode were examined. The SEM images for MoOx, FeOx, CuOx, ZnOx and AlOx were respectively shown in Figure S4(a)-(e) in the SI. The MoOx shows very tiny particles deposited on the Ni foam, while the nickel cobalt molybdenum oxide presents well-defined nanosheet configuration (Figure S1(b)). The FeOx shows sheet-like debris structure, while the nickel cobalt iron oxide presents nanowire configuration (Figure S1(c)). The CuOx shows aggregated nanowire structure with some tiny nanoparticles on the top, and the nickel cobalt copper oxide also shows a similar nanowire configuration (Figure S1(d)). Moreover, the ZnOx presents nanosheet morphology with several big holes on the surface, while the nickel cobalt zinc oxide shows the configuration like the transformation between wire and sheet (Figure S1(e)). The AlOx shows small nanosheet morphology folded on each other. However, the NixCoyAlzO shows well-defined and vertically grown nanosheet configuration (Figure S1(f)). It is thus inferred that the morphology relation between the single metal oxide and its corresponding nickel cobalt-based ternary metal oxide is very limited. Since the electronic configuration of Ni and Co is similar, it is inferred that the structure of the ternary metal oxide is constructed based on the nickel cobalt oxide. The incorporation of the extra metal of Mo, Fe, Cu, Zn and Al may probably influence the original size and hybrid electronic configuration of the nickel cobalt oxide. However, for making the single metal oxide, the materials were grown on the substrate independently and the incorporation of Mo, Fe, Cu, Zn and Al is not to change a certain configuration but to dominate the growth of the metal oxide. Since the morphology relation between single metal oxides and ternary metal oxides is limited, the electrochemical performance for the single metal oxide electrodes were further examined, and the electrochemical performance relation between single metal oxides and ternary metal oxides were further investigated. Figure S4(f) in the SI shows the CV curves for the single metal oxide electrodes. The redox peaks in the CV plots located at very different potentials for the single metal oxide electrodes, due to different intrinsic properties for conducting redox reactions. The similar phenomenon was also observed in the



GC/D curves (Figure S4(g) in the SI) that the platforms were formed at different potentials. The CF values of 0.27, 0.09, 0.33, 0.35, and 1.05 F/cm2 corresponding to the capacity of 0.15, 0.05, 0.18, 0.19, and 0.58 mAh/cm2 were respectively obtained for the MoOx, FeOx, CuOx, ZnOx and AlOx electrodes. Except for MoOx, the trend of the CF value for the single metal electrodes is the same as that for their corresponding ternary metal oxide electrodes, suggesting that adding the extra metals of Fe, Cu, Zn, and Al in the oxide could be benefit for enhancing the electrocapacitive performance of the corresponding ternary metal oxides. However, the NixCoyMozO electrode presented the best performance but its corresponding single metal oxide, MoOx, does not show the best performance among the single metal oxide electrodes. This result suggests that the contribution of Mo in the NixCoyMozO electrode is not mainly on the electrochemical performance. To further understand the role of Mo in the NixCoyMozO electrode in considering of the electrocapacitive performance, the EIS was applied to measure the charge transfer resistance of the electrodes. Figure S4(h) in the SI presents the Nyquist plots for the single metal oxide electrodes. The charge transfer resistance at the interface between the electrocapacitive material and the electrolyte (Rct) is fitted by using the equivalent circuit inserted. The Rct value of 0.52, 2.90, 93.40, 16.30, and 13.40 Ω were respectively obtained for the MoOx, FeOx, CuOx, ZnOx and AlOx electrodes. The much smaller Rct value for the MoOx electrode suggests that Mo plays the role of reducing the charge transfer resistance in the electrode instead of improving the electrochemical performance of the NixCoyMozO electrode via the good electrocapacitive performance of the MoOx electrode. 3.3 Optimization of the nickel cobalt molybdenum oxide electrodes The nickel cobalt molybdenum oxide electrode was further optimized by tuning the ratio of molybdenum in the precursor solution for the hydrothermal reaction. Most of the literatures studying the nickel cobalt oxides or sulfides considered 1 to 2 to be the optimized ratio for nickel to cobalt.16 Hence the ratio of nickel to cobalt in the hydrothermal solution was kept at 1 to 2 for synthesizing NixCoyMozO, and the ratios of 0.5, 1.0, 1.5, 2.0, and 2.5 were applied for the molybdenum salt 14 ACS Paragon Plus Environment

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precursor. The morphology variation was first investigated by using the SEM and TEM images. The SEM images of NixCoyMozO prepared using the precursor ratio of 1:2:0.5, 1:2:1.0, 1:2:1.5, 1:2:2.0, and 1:2:2.5 for Ni:Co:Mo were respectively shown in Figure 1(a,f), 1(b,g), 1(c,h), 1(d,i), and 1(e,j). As observed from the low magnification images (Figure 1(a)-(e)), the NixCoyMozO grew uniformly on the nickel foam and there is almost no bare nickel foam exposed to the electrolyte. However, the aggregations increase on the upper layer when higher ratio of the molybdenum precursor added in the hydrothermal solution. The aggregations on the upper layer may provide higher surface area for conducting redox reactions with the electrolyte, but the surface area of the bottom layer may be sacrificed if the aggregation fully covered the bottom layer, as found for electrode prepared using the Ni:Co:Mo ratio of 1:2:2.5 (Figure 1(e)). A closer observation was made in the high magnification SEM images with the side-view images inserted (Figure 1(f)-(j)). Only the NixCoyMozO made with the Ni:Co:Mo ratio of 1:2:0.5 presents nanowire structure, while those prepared using higher Mo ratios present perfect nanosheet morphologies. As discussed in the previous text, the nanowires could only be obtained for the ternary metal oxides composed of nickel, cobalt and the third metal with similar properties as nickel and cobalt, i.e., iron and copper. The similar phenomenon could be explained by using the same reason. The nanosheet structure is formed by adding the molybdenum precursor in the hydrothermal solution. Hence if less molybdenum precursors were participated in the hydrothermal solution, the nanowire structure would be formed instead of the nanosheet morphology. Furthermore, the size of the nanosheets is similar for the NixCoyMozO electrode synthesized using the Mo ratio of higher than 1.0. The only difference for the sheet-like NixCoyMozO electrode is the density of the aggregations on the upper layer. The thickness of the nanoflakes is not easy to control. Usually the thickness of the materials was varied when new materials were added in the synthesis.29 Moreover, the inner structure of NixCoyMozO was observed by using the TEM images. Figure 2(a)-(e) shows the TEM images for the nickel cobalt molybdenum oxides prepared using the



precursor ratio (Ni:Co:Mo) of 1:2:0.5, 1:2:1.0, 1:2:1.5, 1:2:2.0, and 1:2:2.5, respectively. The NixCoyMozO prepared using the Ni:Co:Mo ratio of 1:2:0.5 shows pure nanowire structures with several voids inside, which may probably be generated by the evaporation of the gases formed during the hydrothermal reaction. The small amounts of nanowires and numerous nanosheets structure were observed for the NixCoyMozO prepared using the Ni:Co:Mo ratio of 1:2:1, while the pure nanosheet structures were obtained when higher Mo ratio was applied for the synthesis. As stated in the previous section, the size and the electronic configuration for molybdenum are very different from those for nickel and cobalt. Therefore, adding more molybdenum in the nickel cobalt oxide is reasonable to make the morphology transformed from nanowires to nanosheets. Also, the small voids can be observed in the nanosheet structures, and the voids become more obvious when higher ratios of Mo were incorporated in the oxide. The mechanical stability is good for the nickel cobalt molybdenum oxides. After repeatedly bending materials were still remained on the substrate and no cracks were observed on the surface of the electrode. Furthermore, the composition of the nickel cobalt molybdenum oxides prepared using the Ni:Co:Mo precursor ratio of 1:2:0.5, 1:2:1, 1:2:1.5, 1:2:2, and 1:2:2.5 were confirmed by using the EDX spectra, as shown in Figure 3(a)-(e), respectively. All the spectra show the signals for Ni, Co, Mo, and O, suggesting the successful fabrication of the nickel cobalt molybdenum oxide on the nickel foam no matter the amounts of the molybdenum in the hydrothermal solution. Also, the XRD patterns were also measured to examine the composition of the samples, as shown in Figure 4. As indicated using the black arrows at 2θ values of around 33o, 39o and 55o, a spinel structure (space group Fd-3m) isostructural to the well-crystallized cubic spinel Co3O4 (PDF #43-1003)30 can be indexed for the samples prepared using the Ni:Co:Mo precursor ratio of 1:2:0.5 and 1:2:1. The large reduction on the intensity for all the peaks were observed for the sample prepared using the Ni:Co:Mo precursor ratio of 1:2:1.5. As for the samples prepared using the Ni:Co:Mo precursor ratio of 1:2:2, and 1:2:2.5, almost no obvious peaks can be found. It is suggested that the ternary metal 16 ACS Paragon Plus Environment

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oxides based on the nickel cobalt oxide is constructed by the partial replacement of cobalt ions by the third metal,30 i.e., molybdenum for our case, so the more molybdenum synthesized in the ternary metal oxide may replace more cobalt ions in the structure. Hence the peaks become more and more unobvious when more Mo was added in the nickel cobalt molybdenum oxide, primarily due to the destruction of the original cobalt oxide structure. Also, since the peaks remained almost unchanged even different ratios of Mo was used in the synthesis, the molybdenum is inferred to be no influence on the crystal structure of spinel Co3O4. The electrochemical performances of the NixCoyMozO electrodes prepared using the precursor ratio of 1:2:0.5 (NiCoMo-0.5), 1:2:1 (NiCoMo-1), 1:2:1.5 (NiCoMo-1.5), 1:2:2 (NiCoMo-2), and 1:2:2.5 (NiCoMo-2.5) for Ni:Co:Mo were investigated by using the CV curves measured at the scan rate of 10 mV/s, as shown in Figure 5(a). Only one couple of the redox peaks were obtained for all the cases at the similar potentials of 0.1 and 0.37 V for the reduction and oxidation, respectively. This phenomenon may be caused by two probabilities. One is that the contribution of molybdenum in the ternary metal oxide is dominated even only 0.5 for the Mo ratio was added in the hydrothermal solution. Since if the molybdenum is not dominated in ternary metal oxide especially for the case with less Mo ratio (Ni:Co:Mo = 1:2:0.5), increasing the molybdenum salt ratio in the hydrothermal solution may enhance the redox behavior of molybdenum and shift the potentials toward the redox reaction for molybdenum from those for nickel and cobalt. The other possibility is that the function of molybdenum is not to enhance the electrocapacitive performance but to reduce the charge transfer resistance of the ternary metal oxide. Hence regardless of the amount for molybdenum added in the oxide the redox behavior remained nearly the same. Furthermore, the redox peaks are located at different potentials for NiCoMo-0.5 comparing to those for other materials. The most possible reason is due to the least amount of Mo in the oxide for this case. The small amount of Mo may have limited influence on the redox behavior of the electrocapacitive material, while the redox behavior for those with higher amounts of Mo in the oxides may be affected in larger extent. This inference can be



verified from the morphology for the oxide with different amounts of Mo. The morphology of nanowires can only be observed for NiCoMo-0.5, while other samples present the nanosheet structures. From the unique morphology for NiCoMo-0.5 it is inferred that the influence of Mo is different for this sample from other cases not only on the morphology but also on the redox behavior, as found in the different potentials for the redox peaks for NiCoMo-0.5. Also, the electrochemical performances for the nickel cobalt molybdenum oxide electrodes were measured by using the GC/D plots at the current density of 5 mA/cm2, as shown in Figure 5(b). The CF values of 1.35, 1.94, 2.30, 2.94, and 2.21 F/cm2 corresponding to the capacity of 0.75, 1.08, 1.28, 1.63 and 1.23 mAh/cm2 were respectively obtained for the NiCoMo-0.5, NiCoMo-1.0, NiCoMo-1.5, NiCoMo-2.0, and NiCoMo2.5 electrodes. The CF value and the capacity increase for the electrodes made using higher amounts of the molybdenum salt in the hydrothermal reaction, and the parameters reached optimized values of 2.94 F/cm2 and 1.63 mAh/cm2 for the NiCoMo-2.0 electrode. In turn, the CF value and the capacity decrease for the sample prepared using the Ni:Co:Mo ratio of 1:2:2.5 in the hydrothermal solution. As observed in the SEM images, only the NiCoMo-0.5 electrode shows the nanowire structure, which may possess smaller surface area than those for the sheet-like structures for other samples. Hence the smallest CF value and capacity for the NiCoMo-0.5 electrode is thus reasonable due to its smallest surface area. In addition, among the NixCoyMozO electrodes with sheet-like structures, the aggregations on the upper layer may play an important role on the electrocapacitive performance. The samples synthesized using higher ratios of molybdenum show more aggregations on the upper layer. The aggregations may provide extra surface area for conducting the redox reactions and therefore enhance the CF value and the capacity for the electrodes. However, the large amounts of the aggregation lead to the full coverage of the bottom layer for the NiCoMo-2.5 electrode. The surface area of the bottom layer may be sacrificed, and the CF value and the capacity are hence reduced for this case. Moreover, the Nyquist plots for the NixCoyMozO electrodes and the corresponding equivalent circuit were respectively shown in Figure 5(c) and 5(d). To more clearly


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compare the resistances of the series resistance (Rs) and the charge transfer resistance (Rct) between the samples containing different Mo contents, Figure S5 in the SI presents the relation of the Rs and Rct values for the sample to the Mo ratio in the NixCoyMozO. For the electrode containing the Mo ratio of 0.5, the high Rs value of 1.9 Ω was obtained, suggesting the smallest electric conductivity for this case. When more Mo was added in the synthesis, i.e., the samples containing the Mo ratio of 1.0 and 1.5, the Rs values were gradually reduced. The result suggests that the addition of Mo in the ternary metal oxide can enhance the electric conductivity for the electrode. On the other hand, the smallest Rct value of about 1.2 Ω was found for the NiCoMo-2.0. This charge transfer resistance is inferred to be highly related to the morphology of the nickel cobalt molybdenum oxide. The NiCoMo-2.0 electrode presents larger surface area comparing to those with less Mo in the electrode, and shows less aggregations on the top layer comparing to the NiCoMo-2.5 electrode. The results for the trend of Rs and Rct values strongly support the electrochemical analysis that the optimized condition is obtained for the NiCoMo-2.0 electrode. Not only the capacitance but also the high-rate charge/discharge ability is important for an efficient electrocapacitive electrode. To evaluate the high-rate charge/discharge capacity of the optimized NixCoyMozO electrode (NiCoMo-2.0), the CV plots were measured using the scan rates of 5, 10, 20, 30 and 40 mV/s, as shown in Figure 6(a). The shapes of the CV curves measured at even higher scan rates are similar. In a fixed potential window, the oxidation peaks in the curves especially measured by using the scan rates higher than 30 mV/s are disappeared due to the larger peak separation. The oxidation peaks would be incomplete for the curves measured at higher scan rates since the potential for this measurement is not positive enough. The high-rate charge/discharge capability of the NiCoMo-2.0 electrode was also examined by using the GC/D curves measured by using the current densities of 5, 10, 20, 30, and 40 mA/cm2 (Figure 6(b)). The discharge time increases when smaller current densities were applied on the measurement. All of the curves measured at different current densities maintained highly symmetric shapes and there is nearly no 19 ACS Paragon Plus Environment


distortions between them. The results again suggest the excellent high-rate capacity for the optimized NiCoMo-2.0 electrode. To more quantitatively evaluate the high-rate capacity, the relation between the CF value obtained from the CV curves and the corresponding scan rate, as well as the CF value obtained from the GC/D curves and the corresponding current density were respectively shown in Figure 6(c) and (d). The CF retention of 34% was obtained when 8-fold increases on the scan rate were applied on the measurement. Similarly, the CF retentions of 57% was achieved when the electrode was measured using an 8-fold current density. The results suggest the excellent high-rate reversibility for the NiCoMo-2.0 electrode. 3.4 Investigation of the physical properties and electrochemical performances for the ternary nickel cobalt molybdenum oxide and the corresponding binary metal oxides In the final part, the influences of the metal species participated in the oxides on the physical and the electrochemical properties are discussed. Based on the experimental parameters for making the optimized NixCoyMozO electrode, the corresponding binary metal oxides were synthesized. The morphology, composition, and the electrocapacitive parameters for the binary metal oxides were investigated, and these results were compared with those for the ternary metal oxide, NixCoyMozO. The SEM images for the nickel cobalt oxide (NixCoyO), nickel molybdenum oxide (NixMoyO) and cobalt molybdenum oxide (CoxMoyO) were respectively shown in Figure S6(a,d), (b,e) and (c,f) in the SI. All of the binary metal oxides present large aggregations on the top of the nanowire array or nanosheet array. The aggregations are composed of the nanospheres assembled by the well-defined nanowires for the NixCoyO electrode, while the NixMoyO and CoxMoyO electrodes show irregular aggregations distributed randomly on the edges of the Ni foam. On the other hand, the bottom layer of the NixCoyO electrode presents the nanowire array configuration, while the Mo-based NixMoyO and CoxMoyO electrodes show the nanosheet arrays in the bottom layer. It is inferred that the nanosheet morphology and irregular aggregations would prefer to be formed for the molybdenumbased oxides. The size of the nanosheets is larger for the CoxMoyO electrode comparing to that for 20 ACS Paragon Plus Environment

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the NixMoyO electrode. Moreover, the ternary metal oxide, NixCoyMozO, shows even larger and well-defined nanosheet morphology (Figure 1(i)) comparing to those for the NixMoyO and CoxMoyO electrodes. The larger and well-defined nanosheet morphology for NixCoyMozO is benefit for enhancing the electrochemical performance. Furthermore, the EDX spectra were applied to examine the composition of the binary metal oxides. Figure S7(a)-(c) in the ESI show the EDX spectra for NixCoyO, NixMoyO and CoxMoyO electrodes, respectively. All the spectra present O signals, suggesting the formation of oxides for all the cases. The peaks for Ni and Co, Ni and Mo, Co and Mo are respectively found in the spectra for NixCoyO, NixMoyO and CoxMoyO electrodes, indicating the successful synthesis of the binary metal oxides by using the simple hydrothermal reaction. The electrochemical performances for the binary metal oxides were further examined by using the CV and GC/D techniques, as respectively shown in Figure 7(a) and 7(b), and the curves for the ternary metal oxide were also shown in the corresponding figures for comparison. From the observation for the CV curves in Figure 7(a), the redox peaks are located at the similar potentials for NixCoyO and NixMoyO electrodes, inferring that the redox reaction is dominated by the nickel ions. Also, the CoxMoyO electrode presents unclear redox peaks in its CV curve, again suggesting the primary contribution of the redox behavior is from the nickel. The CF values of 2.94, 1.11, 1.63 and 1.45 F/cm2 corresponding to the capacity of 1.63, 0.62, 0.91 and 0.80 mAh/cm2 were respectively obtained for the NixCoyMozO, NixMoyO, CoxMoyO, and NixCoyO electrodes. The CF value and the capacity for the ternary metal oxide electrode are almost two-fold comparing to those for the binary metal oxide electrodes, owing to the more metals in the oxide for enhancing the Faradaic redox reactions and improving the electric conductivity for the NixCoyMozO electrode. On the other hand, the high-rate capacity for the ternary metal oxide and binary metal oxide electrodes was also investigated. The CV curves at various scan rates, the GC/D plots at different current densities, and the relation between the scan rate and the corresponding the CF value as well as the relation between the current density and the corresponding CF value for the NixMoyO electrode were respectively



shown in Figure S8(a)-(d) in the SI. Similarly, the same measurements were conducted for the CoxMoyO and NixCoyO electrode, as respectively shown in Figure S9(a)-(d) and Figure S10(a)-(d) in the ESI. To more clearly compare the high-rate charge/discharge capability for the ternary and binary metal oxide electrodes, the CF value in the relation of the current density applied for measuring the GC/D curves for NixCoyMozO, NixMoyO, CoxMoyO and NixCoyO electrodes was shown in Figure 7(c). The CF retentions of 57%, 55%, 47% and 50% for 8-fold enhancement on the current density were respectively obtained for the NixCoyMozO, NixMoyO, CoxMoyO and NixCoyO electrodes. All the electrodes show excellent high-rate charge/discharge capacities. The best highrate charge/discharge capacity was achieved for the ternary metal oxide NixCoyMozO electrode, probably due to the high electrical conductivity attributed to the three metals participated in the oxide. In all, for the nickel, cobalt, and molybdenum-based oxides, the ternary metal oxide electrode present better electrocapacitive performances as compared with those for the binary metal oxides. It is inferred that the role of nickel, cobalt, and molybdenum is to enhance the capacitive ability, increase charge/discharge stability, and improve the electrical conductivity, respectively. Hence by combining the metals with different electrochemical functions in the oxide, it is expected to create more efficient electrocapacitive materials for battery-supercapacitor hybrid devices. 3.5 Electrochemical performance of the battery-supercapacitor hybrid device using the optimized nickel cobalt molybdenum oxide on the positive electrode For achieving a more practical application, an BSH composed of the NixCoyMozO positive electrode and an activated carbon negative electrode was made. Firstly, the optimized potential window for operating the BSH was found by measuring the suitable potential window for the positive and negative electrodes. The CV curves for the positive and negative electrodes were measured, as shown in Figure 8(a). The suitable potential window for the positive electrode is between -0.1 and 0.6 V, while that for the negative electrode is between -1.0 and 0 V. Hence the potential window slightly higher than 1.6 V may be the suitable potential window for this BSH. To


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more precisely decide the applied potential window, the CV curves with the potential window of 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 V for the BSH were measured as shown in Figure 8(b). The CV curves maintained almost no distortion for the cases with the potential window of 1.0 to 1.8 V, but the great increases on the current was found when the potential higher than 1.8 V was applied for the measurement. Hence, subsequently different scan rates of 5, 10, 15, 20, and 25 mV s-1 were used for measuring the CV curves for the BSH with a potential window of 1.8 V, as presented in Figure 8(c). The high reversibility was obtained for this BSH with the highly similar shapes for the CV curve measured using various scan rates. In addition, the GC/D plots for the BSH with the potential window of 1.2, 1.4, 1.6, and 1.8 V were attained, as shown in Figure 8(d). The highly similar shape for the GC/D plots again suggests the suitable potential window of 1.8 V for this BSH. Figure 8(e) shows the GC/D plots for the BSH measured by using various current densities of 10, 20, 30, and 40 mA/cm2. Also, the GC/D curves are almost symmetric even at higher current density, again suggesting the high reversibility for this case. Based on the GC/D curve measured with the potential window of 1.8 V and the current density of 10 mA/cm2, a CF value of 126 mF/cm2 was obtained for the BSH. Last, the correlation between power and energy densities of the BSH was investigated by using the Ragone plot, as shown in Figure 8(f). When the power density increases, the corresponding energy density decreases. A maximum energy density of 22.02 Wh/kg was achieved at the power density of 3.50 W/kg. The valuable highlight of this work is based on the simple synthesis of ternary metal oxides and the application of these ternary metal oxides on the energy storage device. To achieve more Faradaic reactions and attain better energy storage ability, it is inferred that more metals in the oxide as the electrocapacitive material may be better for generating more redox reactions and attending higher electrical conductivity. This work was designed based on this concept and the results surprisingly suggested that adding more metals in the oxide as the electrocapacitive material cannot always enhance the electrocapacitive performance. The functions of the metal in the oxide are to enhance the



electrical conductivity and redox behavior, but incorporating more metals may influence the arrangement of the original nickel and cobalt in the oxide and may reduce the electrochemical performance. Choosing the suitable metals incorporating in the oxide is thus of the great importance to attain efficient energy storage. 4. Conclusions The simple hydrothermal reaction was used to synthesize the single, binary, and ternary metal oxides on Ni foam as the electrocapacitive electrodes for battery-supercapacitor hybrid devices. The nickel cobalt-based ternary metal oxides were synthesized using molybdenum, iron, copper, zinc and aluminium. Due to the different similarities of the electronic configuration for the third metals to those for the nickel and cobalt, the 2D nanosheet structures with larger surface area were only obtained for the NixCoyMozO and NixCoyAlzO electrodes, leading to the better electrochemical performances than those for the binary nickel cobalt oxide for these two cases. The ratios of Mo in the NixCoyMozO electrode were tuned to optimize the electrocapacitive performance. The sheet-like morphology and aggregations formed increasingly for the NixCoyMozO electrode prepared using more Mo precursor for the synthesis. The optimized Ni:Co:Mo ratio of 1:2:2.0 was found for the NixCoyMozO electrode with the CF value of 2.94 F/cm2, which is higher than those for the corresponding binary metal oxide, i.e., NixMoyO (1.11 F/cm2), CoxMoyO (1.63 F/cm2), and NixCoyO (1.45 F/cm2) electrodes, due to the enhanced electrocapacitive ability and improved electrical conductivity for the ternary metal oxide. A BSH was made by using the optimized NixCoyMozO as the electrocapacitive material for the positive electrode and an activated carbon negative electrode. A CF value of 126 mF/cm2 at the current density of 10 mA/cm2 and a maximum energy density of 22.02 Wh/kg at a power density of 3.5 W/kg were obtained. Acknowledgements This work was supported in part by the Ministry of Science and Technology of Taiwan, under grant numbers: MOST 106-2221-E-027-108- and MOST 106-2119-M-027-001-. We are 24 ACS Paragon Plus Environment

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grateful to Dr. Yen-Fa Liao of the National Synchrotron Radiation Research Center for his assistance during XRD experiments. ASSOCIATED CONTENT Supporting Information Available: Additional information about the physical properties and electrochemical analysis for metal oxide electrodes is included here.



Figure 1 The SEM images for the nickel cobalt molybdenum oxides prepared using the precursor ratio (Ni:Co:Mo) of (a,f) 1:2:0.5, (b,g) 1:2:1.0, (c,h) 1:2:1.5, (d,i) 1:2:2.0, and (e,j) 1:2:2.5.


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Figure 2 The TEM images for the nickel cobalt molybdenum oxides prepared using the precursor ratio (Ni:Co:Mo) of (a) 1:2:0.5, (b) 1:2:1, (c) 1:2:1.5, (d) 1:2:2, and (e) 1:2:2.5.



Figure 3 The EDX spectra for NixCoyMozO prepared using the precursor ratio (Ni:Co:Mo) of (a) 1:2:0.5, (b) 1:2:1, (c) 1:2:1.5, (d) 1:2:2, and (e) 1:2:2.5.


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Figure 4 The XRD patterns for NixCoyMozO prepared using the precursor ratio (Ni:Co:Mo) of 1:2:0.5 (NiCoMo-0.5), 1:2:1.0 (NiCoMo-1.0), 1:2:1.5 (NiCoMo-1.5), 1:2:2.0 (NiCoMo-2.0), and 1:2:2.5 (NiCoMo-2.5).



Figure 5 (a) The CV plots measured at 10 mV/s, (b) GC/D plots measured at 5 mA/cm2, (c) the Nyquist plots and (d) the corresponding equivalent circuit for the NixCoyMozO electrodes prepared with different precursor ratios.


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Figure 6 (a) The CV curves measured using different scan rates, (b) the GCD plots obtained using various current densities, (c) the relation between CF values obtained from CV plots and corresponding scan rates, and (d) the relation between CF values obtained from GC/D curves and corresponding current density for the NiCoMo-2.0 electrode.



Figure 7 (a) The CV curves measured at 10 mV/s, (b) the GC/D plots obtained at 5 mA/cm2, and (c) the CF value obtained from the GC/D plots in the relation of the current density for the ternary metal oxide and the binary metal oxide electrodes.


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Figure 8 (a) The CV curves for the positive and negative electrodes, the CV curves measured by using (b) different potential windows and (c) different scan rates, and the GC/D plots measured by using (d) different potential windows and (e) different current densities, and (f) the Ragone plot of the BSH.



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Ternary metal oxide based on nickel and cobalt is systematically studied considering morphology and electrocapacitive performance for asymmetric supercapacitor application.

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Synthesis of Ternary Metal Oxides for Battery ... - ACS Publications

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