Application of Chemical Doping and Architectural Design Principles

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Application of chemical doping and architectural design principles to fabricate nanowire Co2Ni3ZnO8 arrays for aqueous asymmetric supercapacitors Qi Liu, Bin Yang, Jingyuan Liu, Yi Yuan, Hongsen Zhang, Lianhe Liu, Jun Wang, and Rumin Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01872 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 20, 2016

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ACS Applied Materials & Interfaces

Application of Chemical Doping and Architectural Design Principles to Fabricate Nanowire Co2Ni3ZnO8 Arrays for Aqueous Asymmetric Supercapacitors Qi Liu †‡, Bin Yang†, Jingyuan Liu†, Yi Yuan†, Hongsen Zhang†, Lianhe Liu†,‡, Jun Wang*,†,‡, Rumin Li*†,‡, † Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, 150001, P. R. China. ‡ Institute of Advanced Marine Materials, Harbin Engineering University, 150001, P. R. China. * Corresponding author: Tel: +86 451 8253 3026; Fax: +86 451 8253 3026. E-mail: [email protected]. KEYWORDS: Co2Ni3ZnO8 nanowire arrays, chemical doping, asymmetric supercapacitor, architecture design, electron transfer rate, ions replace

ABSTRACT: Electrode materials derived from transition metal oxides have a serious problem of low electron transfer rate, which restricts their practical application. However, chemically doped graphene transforms the chemical bonding configuration to enhance electron transfer rate and, therefore, facilitates the successful fabrication of Co2Ni3ZnO8 nanowire arrays. In addition,

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the Co2Ni3ZnO8 electrode materials, considered as Ni and Zn ions doped into Co3O4, have a high electron transfer rate and electrochemical response capability, because the doping increases the degree of crystal defect and reaction of Co/Ni ions

with the electrolyte. Hence, the

Co2Ni3ZnO8 electrode exhibits a high rate property and excellent electrochemical cycle stability, as determined by electrochemical analysis of the relationship between specific capacitance, IR drop, Coulomb efficiency and different current densities. From the results of three-electrode system of electrochemical measurement, the Co2Ni3ZnO8 electrode demonstrates a specific capacitance of 1115 F g-1 and retains 89.9% capacitance after 2000 cycles at a current density of 4 A g-1. The energy density of asymmetric supercapacitor (AC//Co2Ni3ZnO8) is 54.04 W h kg-1 at the power density of 3200 W kg-1.

1. INTRODUCTION:

Electrode materials derived from transition metal oxides combine the properties of electric double-layer capacitors and batteries, attract increasing attention because of their performance complementation; they provide a higher power and energy densities.1-5 Nevertheless, it is imperative for the electrode materials to overcome the weaknesses of batteries and electric double-layer capacitors, which have low rates of electrochemical reactions, cycle stability uand energy density for practical application.6-9 These problems have been solved by the architectural design of electrode materials to facilitate the transfer rate of electrons and ions.10-15 The architectural design of electrode materials, is divided into three parts: (1) electrode-electrolyte interface (2) crystal structure of electrode materials (3) the connection between electrode materials and current collector.16 With regard to the electrode materials of transition metal


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oxides, the architectural design of crystal structure is crucial to overcome large internal resistance and low electron transfer rate.12,15,17 Chemical doping is a highly efficient way of boosting the electron transfer rate for transition metal oxide materials. Usually, the chemically doping is divided into two parts: (1) the adsorption of metal or organic molecules onto the surface of materials, (2) substitutional doping, which brings in heteroatoms or heteroions, including nitrogen atoms that is non-metal atoms or metal ions into the lattice of the electroactive substance.18 For example, N-doping graphene (as shown in Fig. 1a) which has three common bonding configurations, namely, pyridinic N, porrolic N and quaternary N, has been extensively investigated in energy storage devices, because N-containing functionalities have electron-donor properties that widen the band gap from the conduction band to the valence band, resulting in improvement of electrical conductivity and capacitance performance.19-22 In addition, some reports have identified the research potential for metal-doped metal-organic frameworks or metal oxide materials, such as involving solar water splitting, photocatalyst and hydrogen uptake, because the doping metal atoms form a new chemical bond and modify the band gap width.23-28 Therefore, it is imperative to investigate chemical doping for electrode materials derived from transition metal oxides. Recently, transition bimetal oxide materials have been extensively investigated in electrochemical energy storage/conversion devices, because bimetal elements exhibit multivalency which aggravates the degree of crystal defect and speeds up electron transfer rate.29-31 The chemical doping principle explains this phenomenon.


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Fig 1. (a) Bonding configurations for nitrogen atoms in N-graphene. (b) the crystal structure of Co3O4.

In combining architectural design of electrode materials and chemical doping principle, the Co-Ni-Zn oxides (Co2Ni3ZnO8) electrode materials have been successfully synthesized by a twostep process onto Ni foam which is shown in Fig. 2. It gives full expression to the characteristic of the electroactive materials as follows: (1) On the basis of the chemical doping principle, Co2Ni3ZnO8 can be considered as Ni and Zn ions replacing Co ions and the crystal structure of Co2Ni3ZnO8 is the same as Co3O4, as shown from the XRD data analysis, resulting in an increase of the degree of crystal defect and electron transfer rate (as shown in Fig. 1b). (2) According to the architectural design of electrode material process, the Co2Ni3ZnO8 electrode materials exhibit a high electrochemical response capability (Co and Ni ions react with hydroxyl ions in the electrolyte) with a harmonious electron/ion transfer rate, leading to a low degree of polarization, low IR drop, high Coulomb efficiency and high rate performance. At the same time, the complex multi-valency of the ternary transition metal oxides offer more sites to absorb/desorb electrolyte ions. Therefore, the Co2Ni3ZnO8 electrode sufficiently embodies the both advantage between electric double-layer capacitors and batteries while lessening their disadvantages. The specific capacitance of Co2Ni3ZnO8 electrode is 1115 F g-1 and retains an overall capacitance of 89.9%


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after 2000 cycles at a current density of 4 A g-1 after the electrochemical measurement of threeelectrode system. The asymmetric supercapacitor (AC//Co2Ni3ZnO8) shows a high energy density 54.04 W h kg-1 at an average power density of 3200 W kg-1 at a current density of 4 A g1

. At the same time, it retains an overall capacitance of 89.6% after 2000 cycles.

2. EXPERIMENTIAL: 2.1 Fabrication of the precursors and Co2Ni3ZnO8 nanowire arrays on the Ni foam: All chemical reagents are of analytical grade. The Ni foam was pretreated with 3M HCl, acetone and ethanol by ultrasonic washing for 20 min, respectively. The Co, Ni and Zn nitrates aqueous solution, 20 mmol Co(NO3)2•6H2O, 5mmol Ni(NO3)2•6H2O and 5 mmol Zn(NO3)2•6H2O, were dissolved in 100mL deionized water added into 100mL of 15mmol Triton X-100 aqueous solution and stirred for 30 min. Then 60mmol urea was added to the solution and stirred for 3 h. Finally, the above solution and Ni foam were transferred into a 30 mL Teflonlined stainless and reacted at 120 °C for 24 h. After the reaction, the Ni foam which loads the electroactive materials was washed with deionized water and alcohol to remove surface ions and vacuum dried in 60 °C for 24 h. The as-grown precursors were annealed in air at a relatively low temperature of 350 °C for 3 h. After the reaction, the electrochemical materials were scraped onto Ni foam and dissolved into the 0.1mmol L-1 HCl solution, which was transformed Co, Ni and Zn ions. Then, the transition metal ions solution were measured by ICP. The results shown in the Table S1. Meanwhile, the mass ratio of Co, Ni and Zn is approximately 2: 3: 1. The simple chemical formula is (Co, Ni, Zn)3O4, following the calculation, the final chemical formula is Co2Ni3ZnO8. The weight of the original Ni foam was measured and the results was labeled as m1. After the reaction, the weight of Ni foam which loaded the electroactive materials was labeled as


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m2. The weight of active materials Co2Ni3ZnO8 is 4.7 mg cm-3, which was calculated by equation (1) : mactive materials=m2-m1

(1)

2.2 Fabrication of the NiCo2O4, ZnCo2O4 and Co3O4 on the Ni foam: Fabrication of the NiCo2O4 on the Ni foam, is the same as synthesis of Co2Ni3ZnO8 onto Ni foam, except that the aqueous solution contained 20mmol Co(NO3)2•6H2O, 10mmol Ni(NO3)2•6H2O. The fabrication of the ZnCo2O4 on the Ni foam, is the same as synthesis of Co2Ni3ZnO8 onto Ni foam, except that the aqueous solution contained 20mmol Co(NO3)2•6H2O, 10mmol Zn(NO3)2•6H2O. The fabrication of the Co3O4 on the Ni foam, is the same as synthesis of Co2Ni3ZnO8 onto Ni foam, except that the aqueous solution contained 30mmol Co(NO3)2•6H2O.

Fig. 2. Schematic illustration of synthetic Co2Ni3ZnO8 nanowire arrays on the Ni foam.

2.3 Fabrication of the Actived Carbon (AC) electrode: Fabrication of the Actived Carbon electrode has been reported in our earlier work.16 2.4 Characterization:


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The characterization of electroactive materials are measured by the Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), X-ray-diffraction system (XRD), X-ray photoelectron spectroscopy (XPS), Energy-dispersive spectroscopy (EDS) and Inductively Coupled Plasma Spectrometer (ICP). The detail device type has been reported our earlier work. 16

2.5 Electrochemistry measurement: Electrochemical measurement was carried out on Electrochemistry Lab (VMP3/Z, Princeton Applied Research, American) and CHI 660D electrochemistry workstation. The Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) were studied on a three-electrode system that the electrolyte is 2 M KOH at room temperature. The CV and GCD were measured by the CHI 660D electrochemistry workstation and EIS was measured by the Electrochemistry Lab measurement. Co2Ni3ZnO8 nanowire arrays onto Ni foam as positive electrode and AC as negative electrode assembled asymmetric supercapacitor with 6 M KOH electrolyte. The quality ratio of positive and negative electrodes was calculated by equation (2) : m+ m-

C- ×∆E-

=C

+ ×∆E+

(2)

Where C (F g-1) is the specific capacitance which is the same current density of positive and negative electrodes, ∆E is the potential change during the discharge in the GCD electrochemical measurement. The specific capacitance of each electrode was calculated from the GCD curves for the following equation (3): I ×t

‫ܥ‬௦௣ = m × ∆E

(3)

Where I (A) is a current density at the process of discharge, t (s) is a discharge time, m (g) is a mass of electroactive materials.


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3. RESULTS AND DISCUSSION: 3.1. Materials characterization As shown in Fig. S1, the morphology of Co2Ni3ZnO8 is the same with the NiCo2O4, ZnCo2O4 and Co3O4. Fig. 3a demonstrates that Co2Ni3ZnO8 uniformly grow onto the Ni foam scaffold. Fig. 3b, with enlarged scale of Fig. 3a, shows that the morphology of Co2Ni3ZnO8 is nanowire arrays.


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Fig. 3. (a) and (b) the SEM images of Co2Ni3ZnO8, (c) and (d) the TEM images of Co2Ni3ZnO8, (e) the HRTEM image of Co2Ni3ZnO8, (f) the SEAD pattern of Co2Ni3ZnO8,

We note that the morphology of the precursors is the same as Co2Ni3ZnO8 from Fig. S2a and b, which indicates that by controlling the synthesis route of precursors we are able to develop a


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highly efficient means to fabricate an elaborate nanostructure of transition metal oxides. Fig. 3c and d show the TEM images of the Co2Ni3ZnO8 nanowire arrays. Compared with the TEM images of the precursors in Fig. S1 c and d, the Co2Ni3ZnO8 nanowire arrays show numerous mesopores on the surface material, which enhances the land area between electrode and electrolyte. This increases the electrochemical performance of the Co2Ni3ZnO8 nanowire arrays electrode. Fig. 3e shows a high resolution image (HRTEM) which obtained from the transmission electron microscope, which reveals a clear lattice fringe and interplanar spacing of 0.20 nm and the results accord with (400) lattice planes of the Co3O4. The diffraction rings of the selected area of electron diffraction (SAED) pattern is readily indexed to the (311), (220) and (511) planes of Co3O4 in Fig. 3f, which is consistent with the above analysis. At the same time, the HRTEM and SEAD images of Co2Ni3ZnO8 samples demonstrate the same crystal face exponent and similar interplanar spacing. The crystal structure of Co2Ni3ZnO8 was further investigated by X-ray diffraction (XRD) pattern measurement. The XRD curves of the precursors and Co2Ni3ZnO8 samples on the Ni foam in the Fig. 4a and b. The diffraction peaks of the curve are well indexed with the mixture of Co(CO3)0.5OH (JCPDS Card no. 48-0083), Ni5(CO3)4(OH)2 (JCPDS Card no. 46-1398), Zn(OH)2 (JCPDS Card no. 38-0385) and Co(OH)2 (JCPDS Card no. 30-0443), as shown in Fig. 4a. After thermal treatment in air, the Co2Ni3ZnO8 which the ions ratio of Co, Ni and Zn has been calculated by the ICP measurement (as shown in Table S1) nanowire arrays are obtained and accord with the Co3O4 (JCPDS Card no. 42-1467) from Fig. 4b, indicating that Ni and Zn ions partially replace Co ions in the crystal structure of Co3O4; the XRD curves demonstrate that these crystal structures doesn’t change. The valence of Co, Ni, Zn and O elements was further investigated by X-ray photoelectron spectroscopy (XPS) measurements.


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Fig. 4 (a) XRD patterns of precursors, (b) XRD patterns of Co2Ni3ZnO8, XPS spectra of (c) Co 2p, (d) Ni 2p (e) Zn 2p (f) O 1s for the Co2Ni3ZnO8.

X-ray photoelectron spectroscopy (XPS) analysis is an important technique to research the chemical valence state of elemental composition for the materials. The Co 2p and Ni 2p spectra


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(Fig. 4c and d) had been divided into two spin−orbit doublets and two shakeup satellites by Gaussian fitting method. The strong peaks at 779.2 eV and 781.2eV for Co 2p3/2 are demonstrated in Fig. 4c, indicating that the Co element exists both Co3+ and Co2+ in the Co2Ni3ZnO8 sample. As shown in Fig 4d, the strong peaks at 855.6 eV and 853.8 eV for Ni 2p3/2 indicate the existence of both Ni3+ and Ni2+.32-34 The peaks at 1045.1 and 1022.2 eV are attributed to Zn 2p1/2 and Zn 2p3/2, respectively (Fig. 4e). The core level spectra of O1s region show two main peaks at 529.3 and 532.0 eV (Fig. 4f). The peak at 529.3 eV is well index with metal–oxygen bonds. The peak at 532.0 eV is attributed to oxygen ions in low coordination at the surface.35 It demonstrates, therefore, that the metal elements of Co2Ni3ZnO8 exhibit multivalency, which aggravates the degree of crystal defect and improves electron transfer rate.36 Obviously, the surface area and pore size distribution has been shown that it plays a key role in energy storage devices. The N2 adsorption /desorption isotherms and the corresponding pore size distributions of Co2Ni3ZnO8 nanowire arrays are shown in Fig. S3. An H3-type hysteresis loop (P/P0 > 0.4) were shown in Fig. S 3a, indicating that the sample exists mesopores nanostructure. It can be seen that the measured pore size is within 2–12 nm from the Fig. S 3b. According to several reports, a pore size distribution within 2–5 nm is optimal for the behaviour of ions adsorption/desorption on the surface of electroactive materials. 37-39 3.2 Electrochemical measurement: Fig. 5a shows the mass of electroactive materials on the Ni foam, which is 4.7, 6.1, 2.7 and 3.7 mg cm-3 for the Co2Ni3ZnO8, NiCo2O4, ZnCo2O4 and Co3O4 electrode, respectively. Fig. 5b exhibits the CV curves of Co2Ni3ZnO8, NiCo2O4, ZnCo2O4 and Co3O4 electrode at the scan of 5 mV s-1. We note that the CV curve of Co2Ni3ZnO8 exhibits a larger


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Fig.5 (a) the mass of electroactive materials for the Co2Ni3ZnO8, NiCo2O4, ZnCo2O4 and Co3O4, (b) the comparison of CV curves for the electrodes, (c) the GCD curves of electrodes, (d) the IR drop of these electrodes, (e) the coulombic efficiency of these electrodes, (f) the relationship between specific capacitance and current densities.

area than any other CV curves and quasi-rectangular, redox peaks in the Fig. 5b, indicating that it exhibits an excellent electrochemical performance and combines the battery and electric doublelayer properties. It demonstrates, therefore, that Co and Ni elements of Co2Ni3ZnO8 exhibit multi-valency, which contribute to the high electrochemical response capacity and battery-type characteristic. From the appearance of the electric double-layer characteristic in the CV curves, more defect sites of anion or cations form by replacement of Co and Ni ions adsorb hydroxyl ions for Zn ions. In addition, the way of storage charges is mainly redox reaction for NiCo2O4 and Co3O4 electrode. ZnCo2O4 electrode mainly stores charges by ion absorption on the surface of electroactive materials. Fig. 5c demonstrates the GCD curves of Co2Ni3ZnO8, NiCo2O4, ZnCo2O4 and Co3O4 electrode at the current density of 1 A g-1 and it exhibits the longest discharge time. The relationship between Co2Ni3ZnO8, NiCo2O4, ZnCo2O4, Co3O4 electrode and current density (Fig. 5d). It shows that the Co3O4 electrode has the largest IR drop and


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Co2Ni3ZnO8 has the smallest IR drop, indicating that the ternary transition metal oxides has a faster electron transfer rate and smaller internal resistance than binary and single transition metal oxides. Therefore, doping of transition metal ions reduce the wide bandgaps of semiconductors materials. Fig. 5e exhibits the relationship between coulombic efficiency and current density for the different electrode materials. The coulombic efficiency is calculated by the equation (4): t

η= tc ×100%

(4)

d

where tc and td are the charge, discharge time, and η is coulombic efficiency. With a current density of 1 A g-1, Coulomb efficiency of Co2Ni3ZnO8 is 0.89 and increasing to 0.9744 with a current density of 8 A g-1.Therefore, the ternary transition metal oxides exhibits a higher degree of electrochemical reversible reaction than the electrode of binary and single transition metal oxides, implying that the these metal oxides has a more excellent cycle stability than the other electrodes. Fig. 5f shows that the relationship between specific capacitance and current density have been demonstrated and it exhibits a higher rate performance and specific capacitance than the other electrodes, the battery property of which provides a high specific capacitance and the harmonious electron/ion transfer rate leads to a high rate performance. From the Fig. 5b, the way of storage charges are redox reaction and ion absorption for the Co2Ni3ZnO8 electrode and the ZnCo2O4 electrode mainly stores charges by ion absorption on the surface of electroactive materials. The reason is that Zn ions doped into the microstructure to form more defect sites of anion or cation to adsorb hydroxyl ions. Obviously, the pseudocapacitance specific capacitance decreases with the discharge current densities increasing. The reason is that IR drop increases and the electroactive materials do not take sufficiently part in redox reactions. Meanwhile, the IR drop of ZnCo2O4 is larger than that of the Co2Ni3ZnO8 electrode, because the Co2Ni3ZnO8


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electrode has a higher degree of crystal defect than ZnCo2O4. As for the electrode materials of the pseudocapacitor, the rate

Fig. 6 (a) and (b) the cycling performance of different electrodes at current of 4 A g-1

of electrochemical reaction is the determinative factor rather than IR drop. Therefore, Co2Ni3ZnO8 exhibits a higher ratio of capacitance decrement than ZnCo2O4 with increasing the current density, which exhibits a higher degree of battery-type characteristic than ZnCo2O4 electrode and NiCo2O4 shows a lower rate performance than Co2Ni3ZnO8 electrode. The cycle stability of the electrodes have been demonstrated from 2000 cycles of GCD measurement in the Fig. 6a and b. Fig. 7a shows the CV curves of Co2Ni3ZnO8 nanowire arrays in the potential range of 0-0.5 V. The CV curves exhibit quasi-rectangular at low scan rates, indicating that the materials have a mixture of properties, the values of which lie between the double electric layer capacitor and battery.40 Meanwhile, the CV curves of the Co2Ni3ZnO8 nanowire arrays electrode demonstrate oxide/redox peaks at low scan rates, because the faradaic redox reaction related to M(OH)2/MOOH (M represents Ni or Co).41 With the scan rate increasing, the polarization phenomenon intensifies and the materials insufficiently


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Fig.7. (a) the CV curves of Co2Ni3ZnO8 at different scan rate, (b) the CV curves of Co2Ni3ZnO8 and precursors at scan rate of 5 mV s-1, (c) the single charge and discharge curves of Co2Ni3ZnO8 at different current densities, (D) the IR drop of Co2Ni3ZnO8 and precursors, (e) the coulombic efficiency of Co2Ni3ZnO8 and precursors, (f) the relationship between area specific capacitance (specific capacitance) and current densities.

react with ions in the electrolyte, resulting in deformation of the curves. At a scan rate of 5 mV s1

, a comparison of CV curves of Co2Ni3ZnO8 and precursors nanowire arrays electrode materials

is shown in Fig. 7b. Clearly, the CV curve of Co2Ni3ZnO8 exhibits a larger area than the precursors, implying that the Co2Ni3ZnO8 electrode exhibits a higher electrochemical response capacity than that of precursors. The main reason for this is that the Ni and Co elements are multi-valent and electrode materials present complex chemical compositions, leading to aggravation of degree of crystal defect and acceleration of electron transfer rate.29, 44, 45 Apart from that, the mesoporous surface of Co2Ni3ZnO8 nanowire arrays electrode contributes as another key factor in enhancing the electrochemical response capacity, as a result of increasing the area of electrode-electrolyte interface. The GCD curves (Fig. S4 a) split into the single charge and discharge curves of Co2Ni3ZnO8 electrode at different current densities, as shown in


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Fig. 7c. It shows that the charge curves are high symmetric to the discharge counterparts, and the charge or discharge curves are similarly aligned. This indicates that the materials exhibit an ideal charge-discharge process which adequately embodies the characteristics of electric double-layer capacitors.42 The GCD curves of the precursor electrode are shown in Fig. S4 b at the current densities of 1, 2, 4, 8 A g-1. Fig. 7d demonstrates the relationship between IR drop and different current densities for precursors and Co2Ni3ZnO8 nanowire arrays electrode materials. The precursor electrode materials exhibit a larger value of IR drop than Co2Ni3ZnO8 at the same current density, because Co2Ni3ZnO8 has a higher electron transfer rate and smaller internal resistance than the precursor electrode materials. Furthermore, the value of IR drop increases with an increase in current densities for the precursors or Co2Ni3ZnO8 electrode, implying that the degree of energy loss rises with an increase in current density.

Fig.8. (a) The EIS plots of Co2Ni3ZnO8 and precursors, (b) cycling performance of different electrodes at current of 4 A g1

.

The coulombic efficiency reflects the degree of electrochemical quasi-reversible reaction for the pseudocapacitive electrode materials, indicating that it determines the electrochemical cycle stability which is critical in assessing the performance for their potential application. The


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coulombic efficiency is calculated by equation (3). We see from Fig. 7e that coulombic efficiency increases with an increase in current density, due, perhaps, to promotion of the mass ratio of effective electrode materials in the discharge-charge process with increase of current density, resulting in improvement of the degree of electrochemical reversible reaction. At the same time, the coulombic efficiency of Co2Ni3ZnO8 is higher than precursor nanowire arrays electrode, implying that the Co2Ni3ZnO8 electrode materials exhibit excellent cycle stability. According to equations (3) and (5): ‫=ܥ‬

I ×t ∆E

(5)

the mass specific capacitance (Csp) and area specific capacitance (C) are calculated at different densities, as shown in Fig. 7f. The specific capacitances of Co2Ni3ZnO8 nanowire arrays electrode are 1222.2, 1191, 1115, 856 F g-1 (the area specific capacitances are: 5.74, 5.6, 5.24, 4.02 F. cm-3) from 1 to 8 A. g-1, respectively. Meanwhile, the mass capacitance and area capacitance of Co2Ni3ZnO8 are larger than the precursors electrode at the same current density, because Co2Ni3ZnO8 exhibits a higher electrochemical response capacity and faster electron transfer rate.43, 44 In addition, Co2Ni3ZnO8 exhibits a higher rate performance (as a percentage of the initial capacitance) than the precursors electrode, which is a key factor in evaluating the performance of the electroactive materials. Fig. 8a gives the electrochemical impedance spectroscopy (EIS) plots of Co2Ni3ZnO8 and precursors nanowire arrays electrodes at open circuit potential in a frequency range from 0.01 to 106 Hz. The EIS plots conclude a linear part and a semicircle parts which represent the Warburg impedance (Zw) and the charge transfer resistance (Rct) in the low frequency and frequency region, respectively. Fig. 8a shows that the Co2Ni3ZnO8 electrode material has a smaller intersection at the real part (Z´) than the precursors electrode, indicating that Co2Ni3ZnO8 has a


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lower equivalent series resistance than the precursors electrode. At the same time, the EIS plots of Co2Ni3ZnO8 demonstrate a smaller Rct than the precursors electrode, because the complex crystal structure and multi-valence of Co, Ni, Zn ions increases electron transfer rate for Co2Ni3ZnO8 nanowire arrays electrode. A steeper slope of line for Co2Ni3ZnO8 electrode indicates a faster charge transfer and interface ion absorption-desorption rate than the precursors electrode. The cycle of GCD measurement is of great important performance in practical application. Fig. 8b shows the relationship between cycle numbers and specific capacitances for the Co2Ni3ZnO8 and precursors nanowire arrays electrodes at a current density of 4 A g-1. The specific capacitance gradually reduces with an increase in cycle numbers because of the depletion of electroactive sites. Meanwhile, the Co2Ni3ZnO8 electrode has a higher percentage of capacitance retained than the precursors electrode after 2000 cycles, which demonstrates the Co2Ni3ZnO8 electrode has excellent cycle stability (as shown in Fig. S 5). Based on the three-electrode electrochemical performance analysis of above-mentioned, we conclude that the advantages in using the electrode materials are: (1) The Co2Ni3ZnO8 nanowire arrays was synthesized via a precursors transformation route and the precursors was obtained by a one-step fabrication route of the doped transition metal ions. In addition, the ternary transition oxides show a high degree of harmony between the ion absorption/desorption and electron transfer rate, resulting in a reduction of the degree of polarization process and energy loss, because the potential windows of CV and GCD curves are the same (0-0.5V) which is an ideal condition and the Co2Ni3ZnO8 nanowire arrays electrode exhibit a lower IR drop and coulombic efficiency than precursors electrode in the three-electrode electrochemistry measurement;44, 45 (2) The Co2Ni3ZnO8 nanowire arrays electrode exhibits the characteristics of battery and electric


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double-layer capacitors, indicating a high energy density, rate performance and excellent cycle stability;46-49 (3) The high load mass of the electroactive materials indicates that asymmetric supercapacitor, with the Co2Ni3ZnO8 nanowire arrays as positive electrode materials, exhibits a higher areal energy and power density, which are important performance features for practical application.

Fig. 9. Schematic illustration of the asymmetric supercapacitor configuration.

The two-electrode electrochemical measurement has been recommended as a standard to evaluate the electrochemical performance for practical application.31, Accordingly, the asymmetric supercapacitor, with the Co2Ni3ZnO8 nanowire arrays and activated carbon


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(AC//Co2Ni3ZnO8) is assembled, as shown in Fig. 9. Fig. S6 a and b shows that the electrochemical performance which was the negative electrode with a three-electrode system in 2M KOH electrolyte. Fig. S6 a shows the typical properties of electric double-layer capacitors, displaying a similar rectangle at different scan rates for the CV curves. The specific capacitance was calculated by the GCD curve (Fig. S6 b) at a current density of 1 A g-1 to be 162.7 F g-1. The mass ratio of the positive and negative electrode is 0.267 from the equation of mass distribution ratio (equation 4) calculation and the real mass ratio is 0.2, because excess negative materials increase the use ratio of positive materials. Therefore, the mass of electrode materials is 28.2 mg cm-3 and the mass of electrodes is 63.8 mg cm-3.


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Fig. 10. (a) the CV curves of (AC//Co2Ni3ZnO8) at different scan rate, (b) the single charge and discharge curves of (AC//Co2Ni3ZnO8) at different current densities, (c) Ragone plots of the (AC//Co2Ni3ZnO8), (d) cycling performance of the (Co2Ni3ZnO8//AC) device at current density of 4 A g-1 (inset shows the charge-discharge curves of the first 20 cycles.

The CV curves of asymmetric supercapacitors with different scan rates (Fig. 10a) demonstrate that an ideal mix of capacitor facilitate an electron absorption/desorption process for the quasi-rectangular curve from 0-0.6 V and a redox reaction process with the redox peak from 0.6-1.6V at low scan rate.50, 51 However, the CV curves change with an increase in scan rate, because the degree of polarization increases for the electroactive materials. Fig. 10b (split by the GCD curves, as shown in Fig. S7) shows the single charge/discharge curves which are high symmetry at different current densities, indicating a high electrochemical reversibility.

52

According to equation (3), the specific capacitances of asymmetric supercapacitor are 175, 162.5, 152, 119.5 F g-1 from 1 to 8 A g-1, respectively The energy densities and power densities at different current densities are shown in Fig. 10c which are based on the total mass of active materials and electrodes. According to the mass of active materials, the highest energy density (62.22 W. h kg-1 at 800 W kg-1) greatly surpasses those of most nickel, cobalt or bimetal oxides and other typical materials based on asymmetric supercapacitors (Table S1, Electronic Supporting Information). Nevertheless, the energy and power densities are not sufficient (energy density of 27.5 W h kg-1 at a power density of 353.6 W kg-1), because the Ni foam increases the total mass of electrodes without providing the desired electrochemical performance. Fig. 10d demonstrates the cycle stability of the asymmetric supercapacitor, which retains an initial specific capacitance of 89.6% after 2000 cycles at a current density of 4 A g-1. And Fig. 10d inset displays a similar sharp in the first 20 cycles, leading to an excellent cycle stability of the devices. 4. CONCLUSION:


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Based on the principles of chemical doping and architectural design of electrode materials, we have successfully synthesized the Co2Ni3ZnO8 nanowires arrays by a hydrothermal process and annealed procedure in air. Electrochemical measurement of Co2Ni3ZnO8 electrode materials exhibits a mixed characteristic of battery and electric double-layer in the CV curves. At the same time, smaller IR drop, higher Coulomb efficiency and improved rate performance are demonstrated, as compared with the precursor electrode by the GCD curves. In addition, the Co2Ni3ZnO8 electrode shows a lower equivalent series resistance and Rct than the precursor electrode in the EIS plots. The asymmetric supercapacitor (AC//Co2Ni3ZnO8) is fabricated for practical application and exhibits high energy density (54.04 W h kg-1 at 3600 W kg-1) and cycling stability (retaining 89.6% of overall capacitance after 2000 cycles at current density of 4 A g-1). Therefore, it sufficiently embodies pseudocapacitive characteristics, that is, the power density of the pseudocapacitor is higher than the battery, and the energy density is higher than the electric double-layer capacitor. 5. ACKNOWLEDGEMENTS: This work was supported by National Natural Science Foundation of China (51402065), Heilongjiang Province Natural Science Funds for Distinguished Young Scholar (JC201404), Special Innovation Talents of Harbin Science and Technology for Distinguished Young Scholar (2014RFYXJ005), Fundamental Research Funds of the Central University (HEUCFZ), Natural Science Foundation of Heilongjiang Province （ B201404 ） , Program of International S&T Cooperation special project (2013DFR50060), Special Innovation Talents of Harbin Science and Technology (2013RFQXJ145, 2014RFQXJ013), and the fund for Transformation of Scientific and Technological Achievements of Harbin (2013DB4BG011), Natural Science Foundation of


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Application of Chemical Doping and Architectural Design Principles

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