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Achieving a High Areal Capacity with a Binder-Free Copper Molybdate Nanocone Array-Based Positive Electrode for Hybrid Supercapacitors Sung Min Cha, S. Chandra Sekhar, Ramulu Bhimanaboina, and Jae Su Yu* Department of Electronic Engineering, Institute for Wearable Convergence Electronics, Kyung Hee University, 1 Seocheon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Korea

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ABSTRACT: Herein, we develop a binder-free copper molybdate nanocone array with a prism-like morphology on nickel foam (Cu3Mo2O9 NCAs/Ni foam) using a single-step hydrothermal method. With an optimal growth time (10 h) under hydrothermal conditions, the prism-like Cu3Mo2O9 NCAs are uniformly decorated on Ni foam with good adhesion and crystallinity. The prepared Cu3Mo2O9 NCAs/Ni foam has been directly used as a binder-free electrode to examine its suitability as a positive electrode in hybrid supercapacitors. In an aqueous 1 M KOH electrolyte, the binder-free Cu3Mo2O9 NCAs/Ni foam showed battery-type behavior with a high areal capacity of 449.5 μAh cm−2 at a discharge current density of 2 mA cm−2 and also exhibited a good cycling stability. In addition, the pouch-type hybrid supercapacitor is assembled using the prism-like Cu3Mo2O9 NCAs/Ni foam as a positive electrode and the activated carbon as a negative electrode in a 1 M KOH electrolyte. The hybrid supercapacitor achieves a maximum cell potential of 1.6 V with superior energy storage properties, including a high areal capacitance of 609.7 mF cm−2 at 3.5 mA cm−2, a high areal energy (0.21 mWh cm−2), and a high power density (2.73 mW cm−2). The obtained results suggest that the facilely synthesized Cu3Mo2O9 NCAs/Ni foam electrode has great potential in high-performance energy storage devices.

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INTRODUCTION With the concern of environmental pollution and everincreasing green energy needs, the development of eco-friendly and sustainable energy storage devices has recently become a significant research interest.1−5 Among the miscellaneous energy storage devices, supercapacitors are considered as the most promising energy storage device because of their rapid charge−discharge rate, high power density, long cycling durability, and safe operation.5−10 Accordingly, they have been used in various electronic applications such as military devices, regenerative braking systems, high-power electric vehicles, memory devices, mobile electronics, etc.11−13 However, they are used as only a backup energy source for conventional batteries because of their low energy density, which is one of their main disadvantages in supercapacitors.14,15 This restricts their further application in the rapidly growing modern electronic industry.16 Generally, an improvement in energy density could be ascribed to the development © XXXX American Chemical Society

of advanced electroactive materials, which play a key role in monitoring the energy density of supercapacitors, enabling the cell potential and stable cycling life.16,17 On the basis of their charge storage mechanisms, a wide range of electroactive materials have been employed in supercapacitors, such as capacitive-type materials (activated carbon, reduced graphene oxide, etc.), pseudocapacitive materials (RuO2, MnO2, etc.), and battery-type/prominent redox materials (Co 3 O 4 , NiCo2O4, NiMoO4, NiMn LDH, etc.).2,5,18−23 A fundamental improvement in energy storage performance was obtained when the electroactive materials could exhibit the following merits: novel materials with versatile morphologies, good electrochemical activity with an electrolyte, binder-less synthesis, robust adhesion to a current collector, large surface area, and unique porous architecture for rapid electron transport Received: April 23, 2018

A

DOI: 10.1021/acs.inorgchem.8b01119 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Schematic illustration demonstrating the synthesis of prism-like Cu3Mo2O9 NCAs on Ni foam by a hydrothermal method.

demonstrated a maximum areal capacity of ∼175 μAh cm−2 at 1 mA cm−2 (1517 F g−1).35 Recently, copper molybdate (CuMoO4 or Cu3Mo2O9) has been considered as an another type of metal molybdate-based material, which can be widely used in various fields such as photocatalysis,36 Li-ion batteries,37 magnetic properties,38 etc. Moreover, the copper molybdate material has also begun to be frequently used in energy storage devices because of its low cost, good electrochemical conductivity, facile synthesis, and eco-friendly nature.39,40 It can effectively serve as an electrode material for energy storage applications because of the existence of a metal and metal oxide moiety, which facilitates the improvement in electrochemical performance.41 However, only limited reports about the Cu3Mo2O9-based nanomaterials as supercapacitor electrodes are available, and this material has not yet been studied as a positive electrode in hybrid supercapacitors. Accordingly, we have prepared the vertically aligned prism-like Cu3Mo2O9 nanocone array on a highly conductive nickel foam substrate (Cu3Mo2O9 NCAs/Ni foam) and studied its electrochemical properties in this work. Additionally, we also investigated the electrochemical behavior of Cu3Mo2O9 NCAs/Ni foam as a new class of a battery-type electrode in hybrid supercapacitors. Here, we employed the hydrothermal method to prepare the vertically aligned Cu3Mo2O9 NCAs/Ni foam, which is directly used as a binder-free electrode for supercapacitors. This process has advantages of a low fabrication cost and less preparation time and also eliminates the use of polymer binders for growing nanomaterials vertically on the conductive substrates.42 The electrochemical properties of Cu3Mo2O9 NCAs/Ni foam investigated in an aqueous alkaline electrolyte confirmed that the as-prepared material is a suitable candidate for the fabrication of hybrid supercapacitors with an improved energy density. Compared to the state-of-

with a low fabrication cost.24 With the benefit of the desired advantages of capacitive-type (as the power density source) and pseudocapacitive/battery-type materials (as the energy density source), fabrication of asymmetric/hybrid supercapacitors is a promising approach for overcoming the shortfalls of a low energy density in supercapacitors.23,25−28 This unique cell synergistically improves the electrochemical performance, including cell potential, energy density, and power density. Therefore, the design of functional nanostructured materials with versatile morphologies on highly porous and conductive substrates for high-performance hybrid supercapacitors is an urgent need.28 Among the various forms of nanostructures (nanoparticles, nanorods, nanosheets, nanoflowers, etc.), vertically aligned one-dimensional (1D) nanorods (NRs) on conductive substrates have become a significant research interest because of their fundamental merits of a large surface area and because they provide efficient channels for electrolyte diffusion and fast electron transfer kinetics during the electrochemical measurements.29−31 On the other hand, metal molybdate-based 1D nanomaterials have been extensively studied for hybrid supercapacitors because of their versatile oxidation states and superior electrical conductivity. For instance, honeycomb-like NiMoO4 nanosheets were prepared by Kang et al. using an electrodeposition technique, and they achieved an areal capacity of ∼67.7 μAh cm−2 at 20 mA cm−2 (1694 F g−1).32 Mao et al. synthesized CoMoO4−NiMoO4·xH2O bundles by a co-precipitation method and obtained an areal capacity of ∼461.1 μAh cm−2 at 2.5 mA cm−2 (1039 F g−1).33 Daoping et al. reported an areal capacity of ∼405.8 μAh cm−2 at 3 mA cm−2 (974.4 F g−1) for hydrothermally synthesized NiMoO4 nanospheres.34 The α-NiMoO4 nanoparticles synthesized by Baskar et al. via a solution combustion synthesis technique B

DOI: 10.1021/acs.inorgchem.8b01119 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the-art monometallic molybdate-based materials, our electrode showed excellent electrochemical properties, including a high capacity and a high energy density with a large potential window.

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METHODS

Chemicals. Copper acetate monohydrate [Cu(CO2CH3)2·H2O] and ammonium molybdate tetrahydrate [(NH4)6Mo7O24·4H2O] were purchased from Sigma-Aldrich (Yongin, South Korea). Potassium hydroxide (KOH) was purchased from Daejung Chemicals Ltd. Ni foam (1.6 mm thickness) was purchased from MTI Korea. All the chemicals were of analytical grade purity and used as received without further purification. Synthesis of Prism-like Cu3Mo2O9 NCAs/Ni Foam. The Cu3Mo2O9 NCAs on a 3D Ni foam were facilely prepared by a simple hydrothermal method as shown in Figure 1. Prior to the growth process, the Ni foam (1 cm × 3 cm) was ultrasonically cleaned twice in deionized (DI) water and isopropanol and dried under a nitrogen gas flow at room temperature (RT). Meanwhile, the growth solution was prepared by mixing 70 mM Cu(CO2CH3)2·H2O and 10 mM (NH4)6Mo7O24·4H2O that were dissolved in 80 mL of DI water at RT. After being stirred for 30 min, the growth solution was transferred into the Teflon-lined autoclave, and well-cleaned Ni foam was dipped into the liner and then heated at 180 °C for 10 h. After cooling to RT, the Ni foam was removed from the autoclave and washed with DI water several times, followed by oven drying at 80 °C for 3 h. To remove the hydrated species and improve the crystallinity in Cu3Mo2O9, the as-grown sample [i.e., Cu3(OH)2(MoO4)2] on Ni foam was calcined at 300 °C for 2 h under an air atmosphere with a tubular furnace. Ultimately, the prism-like Cu3Mo2O9 NCAs were successfully formed on Ni foam with a mass loading of ∼4 mg cm−2. To compare the structural and electrochemical properties, the Cu3Mo2O9 NCAs on Ni foam were prepared with different growth times of 8 and 12 h, respectively. Characterization Techniques. The structural and morphological properties of the prepared samples were analyzed by using a fieldemission scanning electron microscope (FE-SEM, Carl Zeiss, LEOSUPRA 55) and a transmission electron microscope (TEM, JEM 200CX, JEOL) equipped with an energy dispersive X-ray spectroscopy (EDX) instrument. An X-ray diffraction (XRD) instrument with Cu Kα radiation was used to examine the crystallinity and phase purity of the prepared sample. The electronic states of elements were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Electron MultiLab2000). An Iviumstat (Ivium Technologies, Eindhoven, The Netherlands) electrochemical workstation was used to investigate the electrochemical properties of the synthesized samples. Electrochemical Measurements. Three-Electrode System. Prior to the electrochemical measurements, the 1 cm × 3 cm size of the binder-free Cu3Mo2O9 NCAs/Ni foam (used as a positive electrode) was cut into two parts with each part (0.5 cm × 3 cm) holding a 0.5 cm × 2 cm portion for synthesized material, and the leftover 0.5 cm × 1 cm portion was employed for an electrode contact. Meanwhile, the negative electrode was prepared by mixing commercially available activated carbon (AC, 80 mg), super P carbon black (10 mg), and polyvinylidene difluoride (PVdF, 10 mg) with a few drops of the N-methyl 2-pyrrolidine (NMP) solvent to obtain a homogeneous slurry. The slurry was then cast on Ni foam and dried at 80 °C for 4 h to obtain the negative electrode (i.e., AC/Ni foam). The electrochemical properties [cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and electrochemical impedance (EIS)] were examined in a beaker-type three-electrode cell, which was composed of Cu3Mo2O9 NCAs/Ni foam or AC/Ni foam as a working electrode, platinum mesh as a counter electrode, and a Ag/ AgCl electrode as a reference electrode, with a 1 M KOH electrolyte solution at RT. The areal capacity, areal capacitance, and specific capacitance of Cu3Mo2O9 NCAs/Ni foam and AC/Ni foam samples were calculated from their respective GCD curves using the following equations:43−46

Q ac =

I Δt a

(1)

Cac =

I Δt aΔV

(2)

Cs =

I Δt m

(3) −2

where Qac is the areal capacity (Ah cm ), Cac is the areal capacitance (F cm−2), Cs is the specific capacity (Ah g−1), I is the discharge current (A), Δt is the discharge time (s), ΔV is the potential window (V), and a is the area of the electrode (cm2). Two-Electrode System (hybrid supercapacitor). The hybrid supercapacitor was fabricated with Cu3Mo2O9 NCAs/Ni foam and AC/Ni foam as the positive and negative electrodes, respectively, with a piece of cellulose paper as a separator. Before the device was assembled, both electrodes and a separator were soaked in a 1 M KOH electrolyte for 30 min. Afterward, as-soaked device components (where the cellulose paper was placed between the two electrodes) were sealed properly using a polyethylene bag with a few milliliters of a KOH electrolyte. Herein, the total working area of the device was 1 cm × 1 cm. To achieve high performance, the charge balance between the electrodes is important. According to the mass balancing equation, the ratio between the positive and negative electrode was fixed to be 1.33. The areal energy and power densities of the fabricated hybrid supercapacitor were estimated using the following equations:31,43 Ed =

1 CacΔV 2 7.2

Pd =

Ed × 3600 Δt

(4) (5) −2

where Cac is the areal capacitance (F cm ) calculated from eq 2, Ed is the areal energy density (Wh cm−2), ΔV is the potential window (V), Pd is the areal power density (W cm−2), and Δt is the discharge time (s).

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RESULTS AND DISCUSSION The phase purity and crystallinity of the hydrothermally synthesized Cu3Mo2O9 sample were studied by XRD analysis, as shown in Figure 2a. For XRD measurement, the powder

Figure 2. (a) XRD pattern of the prism-like Cu3Mo2O9 NCAs powder. (b) Magnified view of the XRD spectrum. C

DOI: 10.1021/acs.inorgchem.8b01119 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Low- and high-magnification FE-SEM images of (a and b) pristine Ni foam and (c−f) prism-like Cu3Mo2O9 NCAs on Ni foam.

Figure 4. (a−c) TEM image, SAED pattern, and HR-TEM images of prism-like Cu3Mo2O9 NCAs, respectively, which were scraped out of Ni foam. (d−g) Elemental mapping images and EDX spectrum of the corresponding sample.

D

DOI: 10.1021/acs.inorgchem.8b01119 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. XPS analysis of the prism-like Cu3Mo2O9 NCAs. (a) Total survey scan spectrum. High-resolution XPS spectra of (b) Cu 2p, (c) Mo 3d, and (d) O 1s.

interconnected with each other and created nanoarray prismlike structures (Figure 3e). Such unique structures and binderfree growth of Cu3Mo2O9 NCAs/Ni foam could increase the surface area and give an easy pathway for electrolyte penetration, resulting in rapid electrochemical reactions. Therefore, the improved electrochemical performance of prism-like Cu3Mo2O9 NCAs/Ni foam could be expected. The reaction time and temperature play an important role in forming the unique morphology and structure of various metal oxide composites. To explore the influence of growth time on the hydrothermal process, the prism-like Cu3Mo2O9 NCAs were also synthesized at different times of 8 and 12 h at the same temperature of 180 °C, and the corresponding FE-SEM images are presented in Figure S1. The morphological properties of prism-like Cu3Mo2O9 NCAs were further investigated by TEM measurement. To prepare the sample for TEM analysis, the prism-like Cu3Mo2O9 NCAs were collected from the Ni foam and dispersed in a small vial containing ethanol. The solution was dropped on a Ni-based TEM grid, dried in the air, and inserted into the TEM chamber. The TEM image presented in Figure 4a also confirms the Cu3Mo2O9 NC with a prism-shaped morphology, which has an inner width of ∼700 nm and a hieght of ∼1.2 μm. The SAED pattern (Figure 4b) indicates the single-crystalline characteristic of the as-prepared sample. Additionally, the high-resolution TEM (HR-TEM) image taken from the edge part of the prism-like Cu3Mo2O9 NC showed a lattice distance of 0.226 nm, which is in good agreement with the (232) plane of the Cu3Mo2O9 NCA (Figure 4c). Elemental mapping and EDX spectroscopy were used to analyze the elemental distributions in the Cu3Mo2O9 NC. The elemental mapping images displayed in Figure 4d−f revealed that the Cu, Mo, and O elements were uniformly distributed on the surface of the Cu3Mo2O9 NC. Moreover,

sample was used to eliminate the peaks of substrates. All the diffraction peaks of Cu3Mo2O9 can be perfectly matched to the pure orthorhombic crystal structure with a Pna21 (33) space group (JCPDS Card 24-0055).47 Precisely, as shown in the magnified XRD pattern in Figure 2b, the diffraction peaks at 2θ values of 12.1, 21.6, 22.4, 23.2, 24, 24.3, 25.2, 25.8, 26.6, 27.0, 25.9, 29.1, 29.6, 32.7, 33.8, 35.5, 35.8, 36.3, 37.7, 38.7, 39.7, 43.1, 45.3, 46.1, 49.7, 51.5, and 53.3° correspond to the lattice planes of (020), (130), (031), (200), (210), (040), (131), (002), (201), (140), (022), (112), (230), (150), (240), (202), (042), (241), (142), (160), (232), (340), (071), (203), (411), (271), and (004), respectively. Moreover, no other phase or additional impurity reflection peaks were noticed, indicating that the as-prepared Cu3Mo2O9 was successfully synthesized without any impurities. The morphological properties of pristine Ni foam and prismlike Cu3Mo2O9 NCAs/Ni foam were investigated by FE-SEM observations as displayed in Figure 3. Panels a and b of Figure 3 show the low- and high-magnification FE-SEM images of the pristine Ni foam substrate, respectively. These images indicate that the Ni foam exhibits a 3D porous conductive skeleton with a smooth surface. The conductive and porous texture of the substrate is favorable for high mass loading and also provides compact adhesion of an active material with the substrate. As shown in low-magnification FE-SEM images in panels c and d of Figure 3, the Cu3Mo2O9 NCAs densely covered an entire surface of Ni foam, including its inner pores. From the increased-magnification FE-SEM image (Figure 3e), the Cu3Mo2O9 NCAs were uniformly grown on the stems of Ni foam with a vertical alignment. The as-grown NCAs look to have prism-like shapes. It can be clearly seen from Figure 3f that the Cu3Mo2O9 NCAs exhibit rough surfaces (with inner thicknesses and heights of 600−800 nm and 1.1−1.3 μm, respectively). In addition, the Cu3Mo2O9 NCAs were E

DOI: 10.1021/acs.inorgchem.8b01119 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Electrochemical properties of the prism-like Cu3Mo2O9 NCAs/Ni foam in a 1 M KOH electrolyte. (a) Comparative CV curves of the pristine Ni foam and prism-like Cu3Mo2O9 NCAs/Ni foam at a scan rate of 10 mV s−1. (b) CV curves and (c) GCD curves of the prism-like Cu3Mo2O9 NCAs/Ni foam at different scan rates and current densities, respectively. (d) Areal capacity values as a function of current density and (e) cycling stability of the corresponding material. The inset of panel e shows the EIS curves of the sample before and after cycling stability.

529.5 eV is mainly due to the O2− ions. These results confirm that the composition of the material is quite consistent with the result of XRD without any impurities. The energy storage performance of the binder-free Cu3Mo2O9 NCAs/Ni foam using a beaker-type three-electrode cell in a 1 M KOH electrolyte was used to investigate the effect of morphology and crystallinity on the energy storage properties. Prior to the electrochemical measurement, the Cu3Mo2O9 NCAs/Ni foam was activated by repititive CV cycles (100) at a scan rate of 10 mV s−1. The comparative CV curves of the pristine Ni foam and binder-free Cu3Mo2O9 NCAs/Ni foam measured at a constant scan rate of 10 mV s−1 with a potential window of 0 to 0.6 V are shown in Figure 6a. From the CV curves, it is evident that the area under the CV curve of pristine Ni foam is small and the current response is too low in an aqueous electrolyte. This means that the contribution of pristine Ni foam to energy storage is negligible. Meanwhile, the Cu3Mo2O9 NCAs/Ni foam electrode exhibited a significantly larger CV area under the given potential window. Additionally, it showed strong redox peaks with larger peak current density values, indicating the battery-type behavior of the material. The redox peaks of the prism-like Cu3Mo2O9 NCAs/Ni foam are mainly due to the reversible

the EDX spectrum of the prism-like Cu3Mo2O9 NCAs further confirms the existence of Cu, Mo, and O elements, as shown in Figure 4g. The detailed electronic states of as-prepared Cu3Mo2O9 NCAs/Ni foam were further examined by XPS analysis, as presented in Figure 5. The total survey scan XPS spectrum of Cu3Mo2O9 NCAs in Figure 5a shows that the prepared sample is comprised of Cu, Mo, and O elements with a molar ratio of approximately 3:2:9, which is consistent with the chemical formula of the as-prepared material. The high-resolution XPS spectra of Cu, Mo, and O were well fitted using the Gaussian method, and the corresponding images are shown in Figure. 5b−d. The Cu 2p spectrum consisted of two major peaks at binding energy values of 933.6 and 953.5 eV, which correspond to Cu 2p3/2 and Cu 2p1/2 of Cu3Mo2O9, respectively, as shown in the high-resolution XPS spectrum of Figure 5b. Meanwhile, the high-resolution XPS spectrum of Mo 3d in Figure 5c also shows the spin−orbit splitting of Mo 3d into Mo 3d5/2 and Mo 3d3/2 at binding energy values of 231.7 and 234.9 eV, respectively. This agrees with the fact that the Mo has a +6 oxidation state in the prepared material. The high-resolution XPS spectrum of O 1s is shown in Figure 5d. The characteristic peak of O 1s at a binding energy value of F

DOI: 10.1021/acs.inorgchem.8b01119 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) Schematic representation of the fabricated hybrid supercapacitor with prism-like Cu3Mo2O9 NCAs/Ni foam and activated carbon as the positive and negative electrodes, respectively. (b) CV curves of both positive and negative electrodes measured in a three-electrode system at a scan rate of 10 mV s−1. (c) CV curves and (d) GCD curves of the hybrid supercapacitor tested at different scan rates and current densities with a potential window of 0−1.6 V. (e) Areal capacitance value as a function of current density and (f) Ragone plot of the hybrid device. (g) Schematic representation of two hybrid supercapacitors connected in series and a photographic image showing the glowing of three LED bulbs by serially connected devices. (h) Cycling stability of the device. The inset in panel h shows the EIS of the device.

redox reaction of Cu(II) to Cu(I).48 Herein, the contribution of Mo in the prepared material is to increase the electrochemical conductivity rather than performing the redox reactions during electrochemical measurements. Thus, the redox process of Mo makes no contribution to electrochemical energy storage, which is well consistent with the previously reported metal molybdate-based materials for supercapacitors.40 In addition, the sample grown for 10 h showed peak current densities higher than those of the samples with two different reaction times (i.e., 8 and 12 h), as shown in Figure S2. This is because the sample under optimal growth conditions (i.e., 10 h) shows uniform growth without any surface cracks and strong adhesion, leading to the high rate of electrochemical utilization and good charge storage ability of the Cu3Mo2O 9 NCAs/Ni foam sample. As such, the Cu3Mo2O9 NCAs/Ni foam sample synthesized for 10 h was utilized for the remaining electrochemical properties. Figure 6b shows the CV curves of an optimal Cu3Mo2O9 NCAs/Ni foam sample (i.e., 10 h) at different scan rates of 1−30 mV s−1. The mirror image shapes of all these CV plots indicate that these Cu3Mo2O9 NCAs/Ni foams employ highly reversible Faradaic

redox reactions. Meanwhile, it is noticeable that the shapes of CV curves display negligible changes except the shifting of redox peaks to higher and lower potentials at the increased scan rate, which indicates the good quasi-reversible nature of the material. Moreover, the peak current also increases with an increase in the scan rate, signifying the fast charge transfer. The symmetric CV shapes with redox peaks and the increase in peak current with an increase in scan rate may be caused by the slow electron transfer kinetics and low ionic diffusivity of battery-type materials. Figure 6c displays the GCD profiles of Cu3Mo2O9 NCAs/Ni foam (i.e., synthesized with a 10 h growth time) measured at different current densities of 2−40 mA cm−2. The nonlinear charge−discharge plateaus of Cu3Mo2O9 NCAs/Ni foam further indicate the presence of the redox process, confirming the battery-type characteristic behavior of the prepared material. Also, the symmetric GCD shapes with almost equal time responses of charge−discharge curves at all current densities imply the high Coulombic efficiency of the material, which is mainly governed by the fast redox reactions in Cu3Mo2O9 NCAs with an aqueous KOH electrolyte. Because of the battery-type mechanism, the areal G

DOI: 10.1021/acs.inorgchem.8b01119 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry capacity in terms of Ah cm−2 was calculated using eq 1 based on GCD curves. The areal capacity variation as a function of discharge current density is plotted in Figure 6d. The areal capacity values were estimated to be 449.5, 367.4, 308.5, 274.96, 251.1, and 197.4 μAh cm−2 at discharge current densities of 2, 4, 8, 12, 20, and 30 mA cm−2, respectively. Additionally, the specific capacity values were also estimated using eq 3, and the values are 112.3, 91.8, 77.1, 68.7, 62.7, and 49.3 mAh g−1 obtained at the same respective current densities. As shown in Figure 6d, though the current density increased 6-fold (i.e., at 12 mA cm−2), prism-like Cu3Mo2O9 NCAs still maintained a rate capability of 61% of its initial value. Likewise, at the high discharge current of 30 mA cm−2, the material exhibited a maximum capacity retention of 43.9%, indicating the moderate rate capability of the prism-like Cu3Mo2O9 NCAs in an aqueous electrolyte with extended current densities. Furthermore, the calculated areal capacity values of the prism-like Cu3Mo2O9 NCAs are comparable to or even higher than the previously reported values from the literature based on metal oxide/metal molybdates (Table S1). The stability of Cu3Mo2O9 NCAs/Ni foam was investigated by a long-term cycling process of ≤2000 cycles at a current density of 20 mA cm−2 in a 1 M KOH aqueous solution. In Figure 6e, the gradual increment in capacity retention during initial cycles was observed, which is attributed to an electrochemical activation of the material. Here, the activation denotes that all Cu3Mo2O9 NCAs take a while to wet completely by continuous diffusion of electrolyte ions. This leads to the generation of a large number of electroactive sites, resulting in the rapid redox reactions. Consequently, the capacity retention was increased progressively and saturated after 200 cycles, suggesting that the maximum areal capacity of the material was achieved. After that, the retention started to fade gradually until the end of 2000 cycles, and 89% of its initial areal capacity was still retained, suggesting the excellent stability of the Cu3Mo2O9 NCAs/Ni foam. The FE-SEM images in Figure S4 reveal that the Cu3Mo2O9 NCAs are strongly attached to the Ni foam without detachment and the shape of the NCAs remained unchanged, suggesting the robust adherence of NCAs to the current collector even after the cycling test. The EIS test was performed before and after the cycling test of the Cu3Mo2O9 NCAs/Ni foam electrode at frequencies between 0.01 Hz and 100 kHz in an open circuit potential with an amplitude of 5 mV. From the Nyquist plot of the Cu3Mo2O9 NCAs electrode presented in the inset of Figure 6e, the EIS curve exhibits a semicircle shape and a sloped line in high- and low-frequency regions, respectively, as displayed in the magnified Nyquist plot. The internal resistance (Rs) can be calculated from the first X-axis intercept of the EIS curve and measured to be 0.7 and 0.42 Ω before and after the cycling test, respectively, while the charge transfer resistance (Rct) is calculated from the diameter of the semicircle shape of the curve and the estimated values are 1.68 and 2.24 Ω before and after cycling, respectively. The lower Rct values of the Cu3Mo2O9 NCAs further indicate the good electrochemical conductivity of the material. The higher capacity and cycling stability of prism-like Cu3Mo2O9 NCAs/Ni foam could mainly be attributed to the following. The highly conductive Ni foam provides fast electron transfer throughout its skeleton during the electrochemical measurements. The porous and homogeneous void space between the prism-like Cu3Mo2O9 NCAs/Ni foam allows the electrolyte efficiently into the interior of the foam,

resulting in the complete utilization of the active material for redox reactions. The binder-free growth of Cu3Mo2O9 NCAs/ Ni foam provides good adhesion and greatly decreases the charge transfer resistance, which leads to good cycling stability of the material. The combined advantages of capacitive (for a high power density) and Faradaic (for a high energy density) mechanisms in one supercapacitor device have led to the coining of the term “hybrid supercapacitor” in recent years. To obtain highenergy storage properties for practical applications, we fabricated the pouch-type hybrid supercapacitor in this study with prism-like Cu3Mo2O9 NCAs/Ni foam as the positive electrode and activated carbon-coated Ni foam (AC/Ni foam) as the negative electrode in a 1 M KOH electrolyte. The corresponding schematic diagram displayed in Figure 7a clearly demonstrates the fabricated hybrid supercapacitor. Prior to the assembly of the device, the electrochemical properties of the negative electrode (i.e., AC/Ni foam) were characterized (Figure S3 and Section 2) to balance the charge storage in positive and negative electrodes. The CV profiles of prism-like Cu3Mo2O9 NCAs/Ni foam and AC at 10 mV s−1 implemented in a three-electrode cell are presented in Figure 7b. Here, the Cu3Mo2O9 NCAs and AC work at 0−0.6 and −1.0 to 0 V, respectively, indicating that both electrodes in the hybrid supercapacitor operate up to a potential of 1.6 V. As displayed in Figure 7c, the CV curves of the prism-like Cu3Mo2O9 NCAs//AC hybrid device were measured at various scan rates ranging from 5 to 100 mV s−1 within the potential range of 0− 1.6 V. Even at high scan rates, the CV curves do not show obvious polarization, confirming that the cell potential of 1.6 V is reasonable. Also, both CV curves exhibited battery-type behavior as well as capacitive behavior because of the effect of two different charge storage mechanisms involved in the materials. On the other hand, the device performance was evaluated by GCD analysis at various current densities as shown in Figure 7d. Subsequently, the symmetrical charge−discharge plateaus indicate the good electrochemical charge storage and high Coulombic efficiency of the hybrid device. With the increased current density, a small GCD profile can be seen, which is due to the battery-type behavior of the positive electrode. The calculated areal capacitance values with respect to the applied current density are plotted in Figure 7e. The device showed a maximum areal capacitance of 609.7 mF cm−2 at a current density of 3.5 mA cm−2 with a rate capability of 57.6% (351.5 mF cm−2) at a high current density of 35 mA cm−2. The decrease in capacitance at a higher applied current is mainly due to limited diffusion of ions into both active materials. Any electrochemical hybrid supercapacitor must offer a high energy density without much of a drop in power density, which is crucial for practical applications. The areal energy and power densities of the assembled hybrid supercapacitor were calculated on the basis of the GCD curves. A Ragone plot in Figure 7f shows the relationship between areal energy and power densities of the Cu3Mo2O9 NCAs//AC hybrid device, which were calculated using eqs 3 and 4, respectively. At a current density of 3.5 mA cm−2, the hybrid device achieved a high areal energy density of 0.21 mWh cm−2 with an areal power density of 2.73 mW cm−2, which are mainly attributed to the high areal capacitance and potential window of the device. Furthermore, the obtained areal energy and power density values of the Cu3Mo2O9 NCAs//AC hybrid device were comparable to and even higher than the values previously H

DOI: 10.1021/acs.inorgchem.8b01119 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

excellent cycling stability. Furthermore, the Cu3Mo2O9 NCAs/ Ni foam has been effectively utilized as a positive electrode for hybrid supercapacitors with an AC-based negative electrode. The as-assembled device demonstrated a high areal capacitance of 609.7 mF cm−2 at 3.5 mA cm−2 with a maximum areal energy density of 0.21 mWh cm−2 and an areal power density of 2.73 mW cm−2. Utilizing the maximum areal energy and power densities, the as-assembled hybrid supercapacitors (connected in series) were also employed to power various light-emitting diodes.

reported in the literature. Values for Ag@Ni−Co LDHs//AC (0.078 mWh cm−2 and 0.785 mW cm−2),49 CuO@rGO// CuO@rGO (0.036 mWh cm−2 and 0.12 mW cm−2),50 Mn0.4Ni0.6Co2O4//AC (0.148 mWh cm−2 and 3.7 mW cm−2),51 NiMoO4//AC (0.221 mWh cm−2 and 3.102 mW cm−2),52 CoMoO4//AC (0.125 mWh cm−2 and 1.5 mW cm−2),53 CuO//AC (0.082 mWh cm−2 and 1.48 mW cm−2),25 CuO@MnO2//MEGO (0.0398 mWh cm−2 and 0.405 mW cm−2),54 Cu1.79Co0.21CH//GO (0.112 mWh cm−2 and 1.04 mW cm−2),55 Co(OH)2@CoMoO4//AC (0.167 mWh cm−2 and 1.5 mW cm−2),56 and NiO//rGO (0.099 mWh cm−2 and 0.78 mW cm−2)57 devices have been reported. The unit conversion equations of areal energy and power densities of the reports mentioned above are described in Section 1 of the Supporting Information. Furthermore, the areal energy density of our device maintained an Ed of ∼0.07 mWh cm−2 at a maximum Pd of 20.9 mW cm−2 at a higher current density of 35 mA cm−2. To demonstrate the potential usage of the prismlike Cu3Mo2O9 NCAs//AC hybrid device, the two hybrid supercapacitors were connected in series as shown in the schematic diagram in Figure 7g. After the devices had been charged for 30 s, they powered the yellow, green, and red lightemitting diodes (LEDs) for 2 min as displayed in the photographic image of Figure 7g. A long cycle life is one of the most essential factors for practical application of supercapacitor devices. Accordingly, the cycling stability test was conducted with the as-constructed pouch-type hybrid supercapacitor device at a current density of 14 mA cm−2, and the obtained plot is displayed in Figure 7h. The hybrid supercapacitor demonstrated an excellent cycling stability with a capacitance retention of 93.7% after 2000 continuous charge−discharge cycles. In addition, the EIS of hybrid supercapacitor was also measured (inset of Figure 7h) by applying an open circuit potential of 5 mV in the frequency range of 0.01−100 kHz, as illustrated in Figure 7h-i. The magnified Nyquist plot in the inset of Figure 7h-ii displays a semicircle in the high-frequency region and a sloped line in the low-frequency region, corresponding to charge transfer resistance (Rct) and Warburg resistance, respectively.58 The value of the internal resistance (Rs) is 4.6 Ω, which is measured from the initial X-intercept of the curve, and the Rct value of ∼0.97 Ω is calculated from the diameter of the semicircle shape. The low values of Rs and Rct of the hybrid supercapacitor suggest its superior electrochemical conductivity and fast electron transportation, respectively. All the remarkable results described above confirm that the prismlike Cu3Mo2O9 NCAs//AC hybrid device can definitely be a potential candidate for stable electrochemical energy storage devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01119. FE-SEM images of Cu3Mo2O9 NCAs/Ni foam prepared with different growth times and Cu3Mo2O9 NCAs/Ni foam after cycling stability, comparative CV curves of synthesized samples with different growth times, conversion equations of specific energy and power density to areal energy and power density, electrochemical characterization of AC/Ni foam, and comparison of the areal capacity value of Cu3Mo2O9 NCAs/Ni foam with those from previously repoted works in a three-electrode system (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +82-31-201-3820. Fax: +82-31-206-2820. ORCID

S. Chandra Sekhar: 0000-0002-9826-5036 Jae Su Yu: 0000-0003-0258-7936 Author Contributions

S.M.C. and S.C.S. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2017R1A2B4011998).

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REFERENCES

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CONCLUSION In summary, we have fabricated the binder-free prism-like Cu3Mo2O9 NCAs/Ni foam via a facile hydrothermal method for a battery-type electrode in hybrid supercapacitors. FE-SEM and XRD analyses confirmed the as-synthesized Cu3Mo2O9 material has a prism-like NCA morphology and high crystallinity, respectively. In an aqueous 1 M KOH electrolyte, the vertically aligned Cu3Mo2O9 NCAs on Ni foam provide uniform pathways for accessibility of electrolyte ions, resulting in superior electrochemical properties. At a discharge current density of 2 mA cm−2, Cu3Mo2O9 NCAs/Ni foam showed a maximum areal capacity of 449.5 μAh cm−2 with a rate capability of 43.9% at 30 mA cm−2 and also demonstrated an I

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DOI: 10.1021/acs.inorgchem.8b01119 Inorg. Chem. XXXX, XXX, XXX−XXX