Novel Dual-Ion Hybrid Supercapacitor Based on a NiCo2O4 Nanowire

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A Novel Dual-Ion Hybrid Supercapacitor Based on NiCo2O4 Nanowire Cathode and MoO2-C Nanofilm Anode Yuanyuan Li, Fan Tang, Renjie Wang, Chong Wang, and Jinping Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10249 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

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ACS Applied Materials & Interfaces

A Novel Dual-Ion Hybrid Supercapacitor Based on NiCo2O4 Nanowire Cathode and MoO2-C Nanofilm Anode Yuanyuan Li *,†, Fan Tang†, Renjie Wang†, Chong Wang‡, and Jinping Liu*,‡

†

School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, P. R. China

‡

School of Chemistry, Chemical Engineering and Life Sciences, and State Key Laboratory of

Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, Hubei 430070, China

*E-mail: [email protected]; [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT: Cobalt/nickel-based compounds have been extensively used as cathode (positive electrode) materials in alkaline electrolyte for hybrid supercapacitors (HSC). In these HSCs, however, the anodes (negative electrodes) are almost carbon-based materials that exhibit limited capacitance, leading to relatively low energy density of the device. Herein, we report a novel dual-ion HSC concept, that is, utilizing anion and cation in the electrolyte respectively by the two electrodes for charge storage, to promote the device's performance. Based on this, it is possible to exploit cation-consumed metal oxide as capacitive anode to couple with cobalt/nickel oxide cathode. As a demonstration, a 1.8 V MoO2-C/LiOH electrolyte/NiCo2O4 HSC device is established. In such a design, NiCo2O4 cathode and MoO2-C anode react with OH- and Li+ respectively to store energy. With the benefits from enhanced kinetics in NiCo2O4 nanowire array (direct electron transport pathway and sufficient electrolyte/ion penetration) and increased stability and electrical conductivity in carbon-encapsulated MoO2 nanofilm, our device delivers a high capacitance (94.9 F g-1), high energy density and power density (41.8 Wh kg-1 and 19922.2 W kg-1), long cycling stability >3000 times and good rate capability (~3.3 s charging/discharging with 43.6% capacitance retention). The dual ion charge storage concept will stimulate great interest in the design of high-performing all-oxide hybrid electric energy storage systems.

KEYWORDS: nickel cobaltite; molybdenum dioxide; nanostructured electrode; hybrid supercapacitor; dual-ion charge storage

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1. INTRODUCTION Supercapacitors, also defined as electrochemical capacitors, are well-known as an important class of energy storage system due to their exceptionally high charge and discharge rate, high power density and long lifetime.1-3 With the growing demands of both high energy density and power density for future applications, traditional supercapacitors based on carbonaceous electrode materials cannot fulfill the performance requirements. In comparison with carbonaceous materials, most transition metal oxides (TMDs) could offer much superior capacitance/capacity and thus higher energy density of the devices, thanks to their abundant Faradaic reactions.4-5 Therefore, tremendous research efforts have been devoted to fabricating a wide spectrum of TMD materials such as RuO2, MnO2, NiO, CoOx, V2O5, VN and Fe2O3, etc. as positive (cathode) or negative (anode) electrodes in the past decade.6-21 In addition to the materials exploration, asymmetric hybrid supercapacitor (HSC) architecture has been further proposed as an attractive way to realize obvious increase in energy density, benefiting from its much wider operating potential window. A HSC device typically consists of the energy source of a battery-like electrode and the power source of a capacitive electrode.22 Some Faradaic positive electrodes such as cobalt (Co) and nickel(Ni)-based oxides work well in alkaline electrolyte,23 but to construct effective HSC devices, few capacitive oxide electrodes are available to pair with them; metal oxide negative electrodes that can store charges in basic solutions with excellent capacitive behavior (rather than battery type) are limited.24-26 Carbonaceous materials are the most popular choice for negative electrode in these HSCs based on electric double-layer charge storage.27-28 However, the typical specific capacitance of pristine nanocarbon is still too low (~ 100 F g-1), far beyond metal oxides positive electrode. This results in a huge mass/volume difference between positive and negative electrodes after charge balance, 3



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not optimal for real device applications. Therefore, it is highly necessary to search for capacitive oxide negative electrode coupled with Co, Ni-oxide electrodes and develop new kinds of device configuration to boost the energy density of HSC. In recent years, spinel nickel cobaltite (NiCo2O4) has received great interest because of its higher electrochemical activity and better electrical conductivity than single metal oxide counterparts. For HSC applications, the electrochemical energy storage of NiCo2O4 electrodes were generally investigated either in a three-electrode cell or by pairing with activated carbon, graphene, etc.29-37 Herein, for the first time, we utilize conductive MoO2 as the capacitive negative electrode to couple with NiCo2O4 positive electrode in aqueous LiOH electrolyte and construct a novel HSC device that works via “dual ion utilization”, as shown in Figure 1 (OH- for redox reaction with NiCo2O4; while Li+ for surface adsorption/desorption with MoO2 and charge accumulation on decorated carbon). When charged, OH- in the electrolyte goes to the NiCo2O4 positive electrode and Li+ diffuse to the surface of MoO2 negative electrode; during the discharging process, OH- and Li+ are both return back into the electrolyte. To enhance electrochemical performance of the device, NiCo2O4 is designed with a three-dimensional (3D) nanowire array structure for improving the charge (electron and ion) transport/diffusion while MoO2 is hybridized with continuous carbon matrix to ensure high stability and electrical conductivity. The unique HSC device exhibits 1.8 V maximum voltage, a high capacitance (94.9 F g-1), high energy density and power density (41.8 Wh kg-1 and 19922.2 W kg-1), excellent rate capability and long cyclic stability. Our work provides a great opportunity for the construction of high-performing all-oxide HSC.

2. EXPERIMENTAL SECTION 4


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2.1 Synthesis of NiCo2O4 Nanowire Array Positive Electrode To fabricate the NiCo2O4 nanowire array, carbon cloth was firstly washed by ultrasonication in ethanol and distilled water for 5 minutes, then dried in oven, and finally immersed into an autoclave containing a 16.5 mL aqueous solution of 0.5214 g CoCl2·6H2O, 0.2607 g NiCl2·6H2O and 0.198 g CO(NH2)2 (this mixed solution was prepared under magnetic stirring for 30 min in air). After a simple hydrothermal process at 120 oC for 6.5 h, the carbon cloth with precursor grown on was taken out and subjected to heat treatment at 400 oC in Argon gas for 3 h. 2.2 Fabrication of MoO2-C Nanofilm Negative Electrode The MoO2-C film electrode was synthesized via electrodeposition on carbon cloth by electrochemical workstation with subsequent carbonization. Typically, the carbon cloth was pretreated by the same way reported above. The treated carbon cloth was immersed in the bath electrolyte consisting of 3.7 g (NH4)6Mo7O24·4H2O, 0.38 g glucose and 60 mL deionized water. After cycling 20-60 times within -1~0 V at 20 mV s-1, the carbon cloth was taken out and then post-treated at 550 oC in Argon gas for 1 h. 2.3 Materials Characterizations and Electrochemical Measurements The morphology was characterized by a scanning electron microscope (SEM-6700F, 5 kV). The crystallographic information was obtained by an X-ray diffractometer (XRD, Bruker D-8 Avance) and a transmission electron microscope (TEM, JEM-2010FEF, 200 kV). Specific surface area was attained on a Micromeritics ASAP 2010 analyzer (accelerated surface area and porosimetry system). The mass of active materials was recorded by an AX/MX/UMX Balance (METTLER TOLEDO, maximum=5.1 g; d=0.001 mg). All the performance tests were conducted using an electrochemical workstation (CHI 760). The electrochemical investigation of individual electrodes was conducted in a three-electrode cell, with the directly-grown array or 5



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nanofilm as working electrode, Pt as counter electrode and a saturated calomel electrode (SCE) as reference electrode. For the full-cell testing, NiCo2O4 array (0.5±0.035 mg cm-2; specific surface area: ~51 m2 g-1) and MoO2-C film (1.3±0.065 mg cm-2; specific surface area: ~43 m2 g-1) were arranged face to face and used as positive electrode and negative electrode, respectively. The cellulose separator was TF4030, NKK and the electrolyte was 1M LiOH aqueous solution. Electrochemical impedance spectroscopy (EIS) measurements were carried out by applying an alternating current voltage with 5 mV amplitude and frequency ranging from 0.05 Hz to 100 kHz. Self-discharge curve of the HSC device was tested by firstly charging the voltage to Vmax and then measuring the open-circuit voltage of the supercapacitor between Vmax and 1/2Vmax versus the time.2 The specific capacitance of electrodes and the HSC device was obtained by using the formula C= I×△t/ (m×△V), 2 where I is the discharge current, △t is the discharge time, △V is the potential window excluding the IR drop, and m is the mass of active material. The gravimetric energy density and power density were estimated according to: 2 E=

I ∫ V (t )dt

; P = E / ∆t m Where V(t) is discharge voltage at t and dt is time differential.

3. RESULTS AND DISCUSSION

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Positive electrode NiCo2O4

Separator Charge

Negative electrode e MoO2-C -

OHLi+

Discharge e-

Figure 1. Schematic demonstration of the configuration and working mechanism of the HSC device.

Figure 2a shows the typical SEM image of the metal oxide positive electrode. It is observed that NiCo2O4 nanowires are grown vertically and uniformly on carbon cloth fiber with average length of several micrometers. NiCo2O4 nanowire array was attained by annealing the hydrothermally obtained precursor; this process led to the lattice shrinkage and rearrangement of the nanowires, with carbon dioxide and water vapor released. Therefore, the resulting NiCo2O4 nanowires are typically porous, as evidenced by TEM results (Figure 2b and its inset). The nanowires consist of interconnected nanoparticles of 15-30 nm with mean mesopores of ~5 nm. High-resolution TEM image reveals obvious interplanar spacings of 0.23 and 0.28 nm, which correspond to (222) and (220) planes of spinel NiCo2O4. The composition of the nanowires was further proven by XRD result (Figure 2c). All the diffraction peaks can be well assigned to spinel NiCo2O4 phase (JCPDS card No. 73-1702) except for that from the carbon cloth current collector. 7



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Figure 2. NiCo2O4 nanowire array positive electrode: (a) The typical SEM morphology; (b) the TEM and HRTEM images. The scale bar in the inset picture is 50 nm; (c) XRD pattern; (d) CV curves with various scan rates; (e) The specific capacitance versus current density plot. Inset shows the related charge-discharge curves.

Figure 2d illustrates the representative cyclic voltammetry (CV) curves of the NiCo2O4 electrode at scan rates in the range of 3 to 100 mV s-1, which clearly exhibit the Faradaic characteristics (battery-like). Well-defined redox peaks are centered at around 0.34 and 0.23 V at 3 mV s-1, which can be ascribed to the redox reaction between M-O/M-O-OH(M stands for Co, Ni ions) and OH-.38 It is found that the redox current increases with the increase of the scan rate; 8


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at the same time, the oxidation and reduction peaks shift towards higher and lower potential, respectively, with an obvious potential separation due to the weak polarization of the electrode.39 In the galvanostatic charge-discharge profiles (inset in Figure 2e), the charge and discharge voltage plateaus are detected at ~0.30 and 0.22 V, respectively, consistent with CV results and previous reports.40 The specific capacitance is determined to be ~571.5, 569.2, 517.4, 501.9, 464.6, 433.3, and 269.1 F g-1 at current density of 0.5, 1, 2, 5, 10, 20, and 50 mA cm-2, respectively (Figure 2e). The coulombic efficiency is always around 100% at each current, indicative of good electrochemical reversibility. In particular, with the current increasing 100 times from 0.5 to 50 mA cm-2, ~64.6 % capacitance can be retained, revealing good rate capability.

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Figure 3. MoO2-C negative electrode: (a) The general SEM image; the scale bar is 20 µm. Inset shows the crack in the nanofilm; (b) HRTEM image; (c) XRD pattern; (d) CV curves with various scan rates; (e) Rate capability plot. The inset displays the corresponding charge-discharge curves.

The general morphology of MoO2-C negative electrode is shown in Figure 3a. After the electrodeposition and post-calcination, MoO2-C film has been tightly coated on the surface of carbon fibers. Cracks exist at some areas, as evidenced in the inset of Figure 3a, but it is found that this does not affect the overall electrochemical properties. The formation of MoO2-C is due to the carbonization of glucose at high temperature and the subsequent reduction of the directly electrodeposited MoO3 by carbon. As is well-known, the presence of carbon would increase the electrical conductivity and help to stabilize the electrode for long-term cycling. HRTEM image clearly demonstrates (Figure 3b) that the electrode film is composed of interconnected MoO2 nanoparticles encapsulated with thin-layer carbon that was partially crystallized. An interplanar spacing of 0.34 nm is detected, which is indexed to the (110) facet of MoO2. XRD result further confirms the composition of MoO2 (Figure 3c; JCPDS card No. 65-5787); the carbon (002) peak is probably merged with the (110) peak from MoO2. The CV curves of optimized MoO2-C electrode at different scan rates are further displayed in Figure 3d, from which only a couple of very small redox peaks are observed (-0.6 V/-0.9 V at 100 mV s-1). This is quite different from the NiCo2O4 positive electrode and reflects the pseudocapacitive behavior of MoO2. The energy storage mechanism was reported to be based on adsorption/desorption of Li+ onto/from the surface

of

MoO2 (contributes

to

the

rectangular

part

in

CV)

5,22,41

and

the

intercalation/deintercalation of Li+ into/from MoO2 (the small redox peaks).24 From the galvanostatic charge-discharge profiles (Figure 3e), quasi-linear potential-time plots are observed 10


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and the discharge time increases with current density decreasing. The capacitance is further determined as 208.4, 173.6, 154.9, 122.08, 97.6, 83.1, and 70.9 F g-1 at discharge current densities of 3, 4, 5, 10, 20, 30 and 40 mA cm-2, respectively. To match with the NiCo2O4 positive electrode for full-cell application, the total charge stored in the negative electrode should be equal to that of the positive electrode. Electrodeposition is a well-known technique for precisely controlling the loading mass of active materials; with this, the charge storage of MoO2-C negative electrode can be readily tuned. Figure 4a presents the CV curves of MoO2-C electrodes attained with different deposition conditions (28, 32, 45 cycles) at 20 mV s-1. The CV datum for NiCo2O4 positive electrode was also included for comparison. Based on the calculation, the MoO2-C electrode obtained with 32 cycles of electrodeposition can store comparable charges to NiCo2O4 nanowire electrode, placing it the most matching negative electrode. We therefore constructed a full-cell HSC for device performance investigation using 32 cycles-MoO2-C negative electrode. Nyquist plot of the MoO2-C/LiOH electrolyte/NiCo2O4 HSC device is displayed in Figure 4b, in which EIS data from individual positive and negative electrodes are also presented. It is found that the solution resistance (Rs) and charge-transfer resistance (Rct) of the device are ~10 and 1.5 Ω, respectively, which are both larger than the values of the NiCo2O4 positive electrode and MoO2-C negative electrode. This is reasonable as the device’s resistance must count in the whole energy storage system containing not only electrolyte but also both positive and negative electrodes and their interfaces. Nevertheless, these values are relatively small for HSC systems. The slope of the spike at the low frequency is also relatively high, indicating that the diffusion resistance is not high; this would be beneficial to ion diffusion within the electrode.

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Figure 4. (a) CV curves of MoO2-C negative electrode attained from different electrodeposition cycles. CV of NiCo2O4 is also provided for comparison; (b) Nyquist plots of the NiCo2O4 positive electrode, MoO2-C negative electrode and the full cell.

Figure 5a illustrates the typical CV curves of the HSC device at various scan rates in the range of 5-200 mV s-1. As expected, the HSC device can work in a wider potential window of 0-1.8 V, much larger than traditional symmetric SCs (typically within 1.0 V). This is due to the asymmetric device configuration that ensures positive and negative electrodes be operated in separated potential range. In addition, the device displays a rectangular-like CV geometry; no sharp redox peaks can be observed. The charge-discharge curves at constant current densities are further displayed in Figure 5b, from which the capacitance of our HSC can be estimated to be 94.9, 86.4, 74.4, 60.9, 53.0 and 40.7 F g-1 at current density of 3, 5, 10, 20, 30 and 40 mA cm-2,

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respectively. Note that at low current densities, it is not easy to charge the device to 1.8 V, and in these cases water may be slightly decomposed, resulting in relatively low coulombic efficiencies. With the cut-off voltage decreasing to 1.6 V, the coulombic efficiency can be improved (Supporting Information, Figure S1). As demonstrated in the rate capability plot in Figure 5c, 43.6 % of initial capacitance could be retained with the current density increased from 3 to 40 mA cm-2, at which the charging and discharging are finished within a very short time (~3.3 s). To further highlight the performance improvement of our device in electric energy storage, gravimetric energy density versus power density plot (Ragone plot) is provided; comparison with previous data is also included (Figure 5d). Our HSC device delivers a highest energy density of 41.8 Wh kg-1, at which the power density is 2493.1 W kg-1. The overall performance (red line) is apparently better than that of the reported activated carbon (AC)/NaOH electrolyte/CoO@PPy (11.57 Wh kg-1 at 5500 W kg-1),2 AC/KOH electrolyte/NiCo2O4 microspheres (19.1 Wh kg-1 at 1830 W kg-1),37 AC/KOH electrolyte/NiCo2O4 nanosheets (15.42 Wh kg-1 at 750 W kg-1),42 AC/KOH electrolyte/Co3O4-MWCNT (31.0 Wh kg-1 at 3000 W kg-1),43 AC/KOH electrolyte/Ni3S2-CNTs (19.8 Wh kg-1 at 798 W kg-1)44 and AC/KOH electrolyte/CoMoO4-G (7.1 Wh kg-1 at 11545.4 W kg-1) 45 devices (all the data are calculated on the basis of active materials), etc. This provides direct evidence that utilizing metal oxide as negative electrode instead of carbon-based materials significantly enhances the HSC device’s energy density. The device can be charged very fast within 3.3 s (exhibiting the highest power density of 19922.2 W kg-1); under this condition, its energy density is still high (23.4 Wh kg-1). The HSC device also demonstrates a relatively long self-discharge time (∼6.9 h), as can be seen in Supporting Information Figure S2. Cycling stability of our supercapacitor was further investigated by continuously testing the

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charge-discharge curves at constant current of 10 mA cm-2 (Figure 5e). It is noted that after the activation for ~750 cycles, the capacitance can be readily maintained up to ~3000 cycles without apparent fading; this datum is comparable to that of many carbonaceous negative electrode-based HSCs37,42,43 and indicative of good cycling performance. We measured the microscopic images (SEM and TEM) of cathode and anode after cycling, and found that the NiCo2O4 nanowire array and MoO2-C nanofilm still adhere to the carbon cloth tightly without obvious peeling off (Figure S3, Supporting Information). The good cycle life directly reflects the effectiveness of the dual-ion utilization mechanism in our new HSC configuration, which in combination with the high performance enables our HSC device a promising electric energy storage system.

4. CONCLUSIONS In summary, we have proposed a new type of dual-ion HSC device. As a model example, NiCo2O4 nanowire array and MoO2-C nanofilm were chosen as the electrodes. The MoO2-C/LiOH electrolyte/NiCo2O4 HSC device was further constructed, in which the positive electrode utilizes OH- for battery-type energy storage while the negative electrode uses Li+ for capacitive energy storage. Our device could achieve a maximum capacitance of 94.9 F g-1, high energy and power delivery ability (41.8 Wh kg-1 and 19922.2 W kg-1), good rate performance and long cycling performance of ~ 3000 times. The present work paves an avenue for developing new kinds of high-performance electric energy storage systems on the basis of traditional materials.

AUTHOR INFORMATION Corresponding Author 15



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*E-mail: [email protected]; [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by grants from the National Key Research Program of China (No. 2016YFA0202602), the National Natural Science Foundation of China (No. 51672205, 51102105, 11104088), the Youth Chenguang Project of Science and Technology of Wuhan City (Grant No.2014070404010206), the Science Fund for Distinguished Young Scholars of Hubei Province (Grant No. 2013CFA023), the Fundamental Research Funds for the Central Universities (WUT: 2016IVA083), and the Research Start-Up Fund from Wuhan University of Technology.

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TOC graphic

Positive electrode NiCo2O4

Separator Charge

Negative electrode e MoO2-C -

OHLi+

Discharge

e-

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Novel Dual-Ion Hybrid Supercapacitor Based on a NiCo2O4 Nanowire

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