Porous ZnO-Coated Co3O4 Nanorod as a High ... - ACS Publications

Subscriber access provided by TUFTS UNIV

Energy, Environmental, and Catalysis Applications

Porous ZnO-Coated Co3O4 Nanorod as a High-Energy-Density Supercapacitor Material Miao Gao, Wei-Kang Wang, Qing Rong, Jun Jiang, Ying-Jie Zhang, and Han-Qing Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07082 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Porous ZnO-Coated Co3O4 Nanorod as a High-Energy-Density Supercapacitor Material

Miao Gao, Wei-Kang Wang, Qing Rong, Jun Jiang∗, Ying-Jie Zhang, Han-Qing Yu∗ CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science & Technology of China, Hefei, 230026, China

*

Corresponding authors: Dr. Jun Jiang, Fax: +86-551-63602449; E-mail: [email protected] Prof. Han-Qing Yu, Fax: +86-551-63601592; E-mail: [email protected]

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Co3O4 with a high theoretical capacitance has been widely recognized as a promising electrode material for supercapacitor, but its poor electrical conductivity and stability limit its practical applications. Here, we developed an effective synthetic route to synthesize one-dimensional (1D) porous ZnO/Co3O4 heterojunction composites. Benefiting from the heterostructure to promote the charge transfer and protect Co3O4 from corrosion, and the 1D porous structure to improve ion diffusion and prevent structural collapse in charge and discharge process, the as-prepared ZnO/Co3O4 composites exhibited an excellent capacitive performance and good cycling stability. The specific capacitance of the ZnO/Co3O4-450 (1135 F g-1 at 1 A g-1) was 1.4 times higher than that of Co3O4 (814 F g-1), and the high-rate performance for ZnO/Co3O4-450 was 4.9 times better than that of Co3O4. Also, approximately 83% of its specific capacitance was retained after 5000 cycles at 10 A g-1. Most importantly, the as-fabricated asymmetric supercapacitor, with a ZnO/Co3O4-450 positive electrode and an activated carbon (AC) negative electrode, delivered a prominent energy density of 47.7 W h kg-1 and a high power density of 7500 W kg-1. Thus, the ZnO/Co3O4 composites could serve as a high-activity material for supercapacitor, and the preparation method also offers an attractive strategy to enhance the capacitive performance of Co3O4.

KEYWORDS: heterojunction; high energy density; porous structure; supercapacitor; ZnO/Co3O4 composites 2


Page 2 of 37

Page 3 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION

Supercapacitors as a promising energy storage platform have attracted increasing interests in the past few decades.1 They possess advantages of higher power density and longer cycling lifetime compared with batteries,2,3 and can be divided into double-layer capacitor4,5 and pseudocapacitor.6-8 With the participation of faradaic reactions, the intercalation pseudocapacitors are expected to exhibit a higher specific capacitance. Recently, transition metal oxides were widely studied for pseudocapacitor with the rich valence states and variable electronic structures.9 Among them, Co3O4 with a high theoretical capacitance (≈ 3560 F g-1), low cost and natural abundance has attracted great interests.10-15 However, such a high theoretical capacitance has not been fully achieved experimentally because of its poor electrical conductivity, low available surface area and unsatisfactory stability.16,17 To solve these problems, it is essential to enhance the kinetics of charge transport in electrodes and electrode-electrolyte interfaces with the fabrication of superior nanostructures.18 1D hierarchically nanostructured materials, such as nanorods, nanobelts and nanotubes, exhibit enhanced capacitive properties compared to their bulk counterparts because of low diffusion resistance to ionic species, which leads to high charge/discharge rates.18-20 Especially, 1D porous structures are regarded as promising material platforms for supercapacitors that combine the advantages of 1D nano-architectures and porous structures.21,22 3



The fabrication of hierarchical heterojunction structures such as ZnO-NiO, Ni3S3@CdS and Co3S4/CoMo2S4 has also been recognized as an effective approach to accelerate the electrochemical activity.23-26 Among them, the p-n heterojunctions can enhance the surface reaction kinetics and promote charge transport because of the formed internal electric field at heterointerfaces.24,27 ZnO has a good electrical conductivity, low price and high chemical stability.28 ZnO/Co3O4 can be constructed as a p-n heterojunction as ZnO is an n-type semiconductor, while Co3O4 is a p-type one. Thus, design and fabrication of porous and heterojunction structured ZnO/Co3O4 nanorods could promote the charge transfer and protect Co3O4 from corrosion during charge/discharge process of supercapacitors. In most previous studies, ZnO and Co3O4 were physically mixed, or formed a core shell structure with Co3O4 on the ZnO surface was prepared.29,30 However, a core shell structure with ZnO on the Co3O4 surface in which porous and heterojunction structure coexist has not been reported yet. With the aforementioned considerations, in this work, 1D porous ZnO-coated Co3O4 composites were synthesized via a hydrothermal reaction, followed with reflux and annealing processes. The morphology, composition, and porous and heterojunction structures of the as-obtained products were characterized. Furthermore, the capacitance performance and stability of the as-prepared ZnO/Co3O4 composites were measured in 1.0 M KOH, and mechanisms for enhanced capacitor performance of the composites over the Co3O4 were elucidated. In this way, a high-performance

4


Page 4 of 37

Page 5 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrode material for supercapacitor was fabricated and an effective strategy to enhance the pseudocapacitance of Co3O4 was established.

EXPERIMENTAL SECTION

Preparation of the Materials. Urea ((NH2)2CO), ammonium fluoride (NH4F), Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), and zinc chloride (ZnCl2) were analytical grade. In a typical procedure, 0.291 g of Co(NO3)2·6H2O, 0.300 g of (NH2)2CO and 0.111 g of NH4F were dissolved in 35 mL of deionized water to form mixed solution. Subsequently, the above mixed solution was stirred for 30 min to form homogeneous solution and then transferred into a 50 mL Teflon-lined stainless steel autoclave followed by keeping at 120 oC for 5 h. After cooling to room temperature (approximately 25 oC), the product was separated by centrifugation, washed with deionized water and ethanol for three times. The the samples were dried at 60 oC for 12 h. The obtained Co(CO3)0.5(OH)·0.11H2O (CoCH) (0.02 g) was put into the round bottomed flask containing 0.04 g of ZnCl2 and 25 mL of deionized water. Then the mixture was refluxed for 1 h under stirring at 100 oC. To prepare the ZnO/Co3O4 composites, ZnO/CoCH was annealed in air in an electrical furnace from room temperature to 350, 450, 550 and 650 oC, respectively, at a heating rate of 3 oC min-1, and maintained for 90 min. The products were collected 5



Page 6 of 37

after cooling to room temperature (approximately 25 oC). The products annealed at 350, 450, 550 and 650

o

C were named as ZnO/Co3O4-350, ZnO/Co3O4-450,

ZnO/Co3O4-550 and ZnO/Co3O4-650, respectively. For comparison, pure Co3O4 was prepared with a similar way except for replacing ZnO/CoCH with CoCH. Pure ZnO was also prepared in a round bottomed flask containing 0.04 g of ZnCl2 and 25 mL of deionized water with refluxing for 1 h under stirring at 100 oC. Physicochemical Characterizations. This section is given in Supporting Information. Electrochemical Measurements. All electrochemical measurements were carried out with an electrochemical workstation (CHI 660C, Chenhua Instrument Co., China) in a three-electrode configuration in 1.0 M KOH. The samples were mixed with

polyvinylidene

fluoride

(PVDF)

and

acetylene

black

in

N-methyl-2-pyrrolidinone at a mass ratio of 8:1:1. The mixture was then coated onto the clean Ni foam substrate with 1×1 cm2 effective geometric area and dried under vacuum at 60 oC for 12 h. Then it was used directly as the working electrode, along with a platinum wire as a counter electrode and a saturated Ag/AgCl (Ag/AgCl in saturated KCl solution) as reference electrode. Galvanostatic charge/discharge (GCD) and cycle voltammetry (CV) and tests were conducted to evaluate the obtained samples. Electrochemical impedance spectroscopy (EIS) were conducted in the frequency range from 0.01 Hz to 100 kHz. According to the GCD curves, the specific capacitance was calculated with the following equation: 6


Page 7 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


I ∆t

(1)

C = ms ∆V

where I (A) is the constant discharge current, ∆t (t) is the discharge time, ms (g) is the weight of the active material on the working electrode and ∆V is the voltage window. The weight of active material was approximately 1.0 mg per 1×1 cm2 of Ni foam. Fabrication of Asymmetric Supercapacitor Device. For practical application, the asymmetric capacitor with a ZnO/Co3O4-450 positive electrode and an AC negative electrode was assembled. The negative electrode was prepared by mixing AC with PVDF in N-methyl-2-pyrrolidinone with a mass ratio of 9:1. The charge balance follows the equation Q+ = Q−, and Q = C × V × M. Thus, the mass balance of the positive and negative electrodes can be expressed as m+ / m- = (C- × ∆E-) / (C+ × ∆E+).31 CV curves of the two electrodes were tested via three-electrode system at 50 mV s-1. With the calculation of analytical results in CVs, the optimization mass ratio was estimated to be 5 between negative and positive electrodes. The total loading mass of ZnO/Co3O4-450//AC was estimated to be 6.0 mg. The ZnO/Co3O4-450 and AC electrodes were pressed individually with one piece of cellulose paper as the separator. Then, the supercapacitor with a pair of Pt foils was wrapped with the parafilm. For the two-electrode system, the specific capacitance (C), energy density (E) and power density (P) were calculated according to the following equations:32

7



I ∆t

Page 8 of 37

(2)

C= mt ∆V E =

C ( ∆V )

2

(3)

2 E

(4)

P= t

where I (A), t (s), ∆V (V) and mt (g) are the discharge current, discharging time, cell voltage and total mass of the active materials on two electrodes, respectively.

RESULTS AND DISCUSSION

Structures of ZnO/CoCH Precursors and ZnO/Co3O4 Composites. The morphologies of CoCH and ZnO/CoCH precursors were imaged by SEM. Both of these precursors (Figure S1a and c) were nanorod structures with lengths about tens of micrometers. The high-magnification SEMs in Figure S1b and d reveal that these nanorods had smooth surface without obvious porous structure, further verifying by the TEM image (Figure S1f). Phase information of CoCH and ZnO/CoCH precursors was determined by XRD (Figure S1e). All diffraction peaks from CoCH were indexed to orthorhombic Co(CO3)0.5(OH)·0.11H2O (JCPDS No. 48-0083), while the ZnO/CoCH were composed of CoCH and hexagonal ZnO (JCPDS No. 36-1451). After a pyrolytic reaction with these precursors, the phase and structure of the product (ZnO/Co3O4-450) were analyzed. All diffraction peaks (Figure 1a) of the ZnO/Co3O4-450 could be indexed into the mixture of cubic Co3O4 (JCPDS No. 43-1003) and hexagonal ZnO (JCPDS No. 36-1451) without any impurity peaks. For 8


Page 9 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


comparison, the ZnO/CoCH was annealed in an Ar ambience. As shown in Figure S16, the obtained composite were composed of cubic CoO (JCPDS No. 48-1719), cubic Co3O4 (JCPDS No. 43-1003) and hexagonal ZnO (JCPDS No. 36-1451). Meanwhile, the nanorod morphology (Figure 1b-c) was integrated retained after annealing except the formation of rough surface. The TEM image (Figure 1d) further reveals the rough surface, which could be attributed to the presence of abundant pores on these nanorods. These pores were formed via the decomposition of ZnO/CoCH to release CO2 in annealing process as described in equation (5):

ZnO/Co(CO3)0.5(OH) ⋅ 0.11H2O → ZnO/Co3O4 + CO2 + H2O

(5)

The microstructure of ZnO/Co3O4-450 was further characterized by HRTEM (Figure 1f) on the marked region in Figure 1e. The composite showed a high crystallinity with clear lattice fringes. In details, these continuous lattice fringes with an interplanar lattice spacing of 0.29 and 0.14 nm matched well with the (220) and (440) planes of the Co3O4 phase, respectively, while lattice spacing of 0.15 nm could be indexed to the (103) planes of the ZnO phase. These results demonstrate that the heterojunction between Co3O4 and ZnO was successfully fabricated with small lattice mismatch of 6.90% (Table S1), which might facilitate charge transfer in the GCD process of supercapacitors.23 The diffraction rings in the SAED pattern (Figure S2) were corresponded to polycrystalline feature. The HAADF-STEM and EDS mapping images of the ZnO/Co3O4-450 could be visualized the location distribution of Co, Zn and O on the nanorods (Figure 1g). These images confirmed that the Zn, Co and O elements were distributed on the nanorod. As shown in Figure S3, the HAADF line 9



scan profiles of a nanorod were consistent with the ZnO-enriched surface feature. However, it should be mentioned that the formed ZnO/Co3O4 nanorods were not complete core-shell structures because of the migration of ZnO into the pores of the nanorods in the reflux and annealing processes. Moreover, the corresponding EDX analytical results (Figure S4) with a combination of the ICP results show that the Co/Zn atomic ratio was around 5:1 in the products. The surface elemental composition and chemical states of the ZnO/Co3O4-450 (Figure 2) were revealed by XPS analysis, where all binding energies were corrected by referencing them to the C 1s peak (set at 284.6 eV). From the survey spectrum (Figure 2a), peaks corresponding to Co, Zn and O were obviously observed. In Co 2p spectrum (Figure 2b), the deconvoluted Co 2p1/2 spectrum shows two peaks with binding energies at 796.8 and 795.2 eV, which were assigned to Co(II) and Co(III), respectively. The deconvoluted Co 2p3/2 spectrum shows two peaks with binding energies at 781.5 and 780.0 eV.33 These results confirmed the formation of Co3O4. Moreover, absence of protruding satellite peaks further verified the existence of Co3O4 phase.34,35 In the Zn 2p spectrum (Figure 2c), the peaks at 1021.4 and 1044.3 eV corresponded to the Zn 2p3/2 and 2p1/2 spin-orbital peaks, respectively.36 The three O 1s spectral peaks at 530.0, 531.2 and 532.4 eV (Figure 2d), respectively matched with the Co-O and Zn-O bonds in the Co3O4 and ZnO phase, the oxygen in the form of hydroxide ions, and the oxygen of surface-adsorbed carbonate anions.35 This XPS analysis further confirmed the coexistence of Co3O4 and ZnO in the products.

10


Page 10 of 37

Page 11 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Formation of Porous ZnO/Co3O4 Composites. Formation process of the ZnO/Co3O4 composites is illustrated in Scheme 1, and there were three steps involved in. First, the CoCH nanorods were synthesized with Co2+, NH4F and urea via a hydrothermal reaction. Then, with CoCH nanorods as the scaffolds, ZnO was epitaxially grown onto the nanorods by a reflux reaction. In this process, ZnCl2 with different dosages was added to adjust the ZnO contents. Finally, the porous structures were formed via the decomposition of the ZnO/CoCH precursor in the annealing treatments through the release of CO2 gas at different annealing temperatures (T = 350, 450, 550, and 650 oC). To explore the impacts of annealing temperature on the product, the thermal behavior of ZnO/CoCH was firstly investigated using TGA (Figure S5). The TGA plot shows that a main thermal event occurred at around 274 oC in the heating process. In the subsequent experiments, ZnO/CoCH was annealed at various temperatures above 274 oC to form the ZnO/Co3O4 composites. All ZnO/Co3O4 composites annealed at 350, 450, 550 and 650 oC exhibited good crystallinity with sharp peaks in the XRD patterns (Figure 3a) and rough surfaces in the TEM images (Figure 3c-e). However, the products became deformed when the annealing temperature was higher than 550 oC (Figure 3e). In addition, the porous structures of the ZnO/Co3O4 composites could be tuned by adjusting the annealing temperature. As shown in Figure 3f, all of the N2 adsorption/desorption isotherms could be categorized as type IV with hysteresis loops. The BET surface-areas of the ZnO/CoCH and ZnO/Co3O4 composites annealed at 350, 450, 550 and 650 oC were calculated to be approximately 11



59.64, 49.74, 53.66, 22.55 and 15.14 m2 g-1, respectively. The pore-size distributions of the ZnO/Co3O4-350 and ZnO/Co3O4-450 composites (Figure 3g) show hierarchical porous structures compared with the ZnO/CoCH. Such hierarchical porous structures are beneficial for enhancing the capacitive performance because they could adjust the ionic transportation path.37 For the ZnO/Co3O4-550 and ZnO/Co3O4-650 composites, both pores volumes and BET surface-areas reduced because of the partial destruction of the porous structure at relatively high temperatures. Electrochemical Properties of ZnO/Co3O4 Composites. CV curves of the ZnO/Co3O4 composites electrodes at a scan rate of 5 mV s-1 is shown in Figure 4a. The different redox peak potentials were attributed to the diverse available sites annealed at different temperatures. The ZnO/Co3O4-450 composites exhibited the highest specific capacitance among the composites with the largest integral area of CV curve, which was further confirmed by the galvanostatic discharge results (Figure 4b). With the galvanostatic discharge curves, the calculated specific capacitances of these composites were in the order of: ZnO/Co3O4-450 > ZnO/Co3O4-550 > ZnO/Co3O4-650 > ZnO/Co3O4-350. To further investigate the effect of ZnO contents on the capacitive performance of the composites, different amounts of ZnCl2 were dosed in the formation of ZnO/Co3O4-450. The products were noted as ZnO/Co3O4-x, where x stands for the feeding amount of ZnCl2. The ICP-AES results indicate that the Co/Zn atomic ratios of ZnO/Co3O4-0.02 and ZnO/Co3O4-0.08 were 7:1 and 1:1, respectively. With the variation of the ZnO contents in the ZnO/Co3O4-450, the CV integral areas (Figure 4c) 12


Page 12 of 37

Page 13 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of the ZnO/Co3O4-0.04 were much larger than those of both ZnO/Co3O4-0.02 and ZnO/Co3O4-0.08, suggesting that the optimum value of ZnO content was in the ZnO/Co3O4-0.04. This result showed that the high content of ZnO might block the reactive sites of Co3O4, leading to the decrease in capacitance. A further evaluation on the performance of the samples by the GCD tests (Figure 4d) showed the specific capacitances of Co3O4, ZnO/Co3O4-0.02, ZnO/Co3O4-0.04 and ZnO/Co3O4-0.08 were 814, 948, 1135 and 738 F g-1, respectively. The specific capacitances of the samples are summarized in Figure 4e. CV and GCD tests revealed electrochemical properties of the ZnO/Co3O4-450. For comparison, Co3O4 and ZnO electrodes were prepared and tested under the idential conditions. Figure 5a shows CV curves of composite, Co3O4 and ZnO electrodes in a potential range of 0 to +0.55 V at a scan rate of 5 mV s−1. Each curve contained two or more redox peaks. These results demonstrated that the capacitive process were governed by the Faradaic reactions. In addition, the CV integral areas of the materials were in the order of: ZnO < Co3O4 < ZnO/Co3O4-450, demonstrating that the composite electrode exhibited the best capacitance performance among these electrodes. The nonlinear galvanostatic discharge curves of ZnO/Co3O4-450, Co3O4 and ZnO electrodes (Figure 5b) at 1 A g-1 also show their faradaic behavior.2,38 The calculated capacitance performance of the ZnO/Co3O4-450 (1135 F g-1) was better than those of Co3O4 (814 F g-1) and ZnO (72 F g-1), which was consistent with the CV results. Moreover, the performance of the ZnO/Co3O4-450 was also better than that of 13



the sum of ZnO and Co3O4, confirming that the Co3O4 and ZnO exhibited a synergistic effect in the composite structure. The coulombic efficiency of ZnO/Co3O4-450 obtained from the GCD curve at 1 A g-1 was approximately to 89%. Such a high coulombic efficiency indicated a good reversible pseudocapacitive behavior of the material. Moreover, the capacitance performance of Ni foam (22 F g-1) was also tested for comparison (Figure S6). In sharp contrast, Ni foam only showed a negligible capacitance activity, indicating that the current collector had little contribution on the capacitance performance of the samples. Hence, as reported previously,19,39 the pseudocapacitive energy storage mechanisms of the composites were attributed mainly to the reversible redox reactions among the different oxidation states, Co3O4/CoOOH/CoO2. Figure 5c shows the CV curves of the ZnO/Co3O4-450 electrode at different scan rates from 5 to 100 mV s-1. The current densities and the CV integral areas increased gradually with a raised scan rate. The redox peaks could be obviously observed even at a high scan rate, suggesting a good capacitive behavior. Figure 5d displays the GCD curves of the ZnO/Co3O4-450 electrode at different current densities from 1 to 10 A g-1. These approaching symmetric curves indicate a good electrochemical capacitive characteristic of ZnO/Co3O4-450.40 The ZnO/Co3O4-450 delivered capacitances of 1135, 960, 823, 670, 550 and 476 F g-1 at the current densities of 1, 2, 3, 5, 8 and 10 A g-1, respectively. In Figure 5e, all of the specific capacitances of ZnO/Co3O4-450 and Co3O4 decreased with increasing current density. The specific capacitance drop was due to the presence of the inner active sites, which 14


Page 14 of 37

Page 15 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


failed to sustain the redox transitions completely with faster scan rates.37 Importantly, at each current density, the specific capacitance of ZnO/Co3O4-450 was larger than that of Co3O4. Furthermore, about 42% of the capacitance was retained for ZnO/Co3O4-450 from 1 to 10 A g-1, while only 9% was retained for Co3O4. This result proves that the charge could be promoted to transfer from one to the other species inside the hetero-structure of ZnO/Co3O4. Thus, the decoration with ZnO could enhance the high-rate performance of Co3O4 for practical applications. The electrode kinetics of ZnO/Co3O4-450 was further investigated by EIS analysis (Figure 5e), and the equivalent circuits of the ZnO/Co3O4-450 and Co3O4 electrodes with several simulated electrochemical parameters were fitted (Figure S11). The equivalent series resistance (Rs) for the ZnO/Co3O4-450 (1.4 Ω) was estimated to be lower than that of Co3O4 (3.3 Ω). These results indicated a higher conductivity of the ZnO/Co3O4-450 than that of Co3O4, consistent with the high-rate performance of the former. The charge transfer resistance (Rct) of the ZnO/Co3O4-450 was estimated to be around 87.9 Ω, much lower than that of Co3O4 (288.8 Ω), indicating the rapid charge transfer rate on the ZnO/Co3O4-450 electrode, consistent with their CV and GCD behaviors. Cycling stability was another important factor for practical application. To estimate cycling life of ZnO/Co3O4-450, repeated GCD tests were conducted at 10 A g-1 for 5000 cycles (Figure 5f). ZnO/Co3O4-450 exhibited an excellent cycling stability and the specific capacitance could retain around 83% of its original capacitance. For comparison, the cycling stabilities of blank Co3O4, ZnO/Co3O4-350, ZnO/Co3O4-550, and ZnO/Co3O4-650 at 10 A g-1 were measured. As shown in Figure 15



S12, the specific capacitance of Co3O4 retained approximately 75% of its original capacitance, suggesting that ZnO could protect Co3O4 from corrosion in the GCD cycles. Among the composites annealed at different temperatures, ZnO/Co3O4-450 showed the best cycling stability. The good cycling stability of the ZnO/Co3O4-450 was due to the porous structure, which could avoid the large volume change during the insertion and desertion cycles of OH-, and the ZnO could protect Co3O4 from corrosion in the GCD cycles.22 On the contrary, the specific capacitance of the ZnO/Co3O4-650 retained only 67% of its original capacitance due to the partial destruction of the porous structure formed at relatively high temperatures. The SEM image (Figure S7a) of the ZnO/Co3O4-450 shows that the nanorod morphology was retained after cycling, suggesting the stability of its structure. Nanoparticle aggregates in Figure S7a were from the acetylene black nanoparticles, as illustrated by EDX analysis (Figure S13). Moreover, the binding energies of the ZnO/Co3O4-450 in XPS spectra (Figure S7b) after cycling were nearly identical to those in the XPS spectra of Co, Zn, and O elements before cycling. For the ZnO/Co3O4-450, capacitive and diffusion processes were involved in the total charge stored. Rate-controlling factors contained diffusion and capacitive processes, and their separate contributions could be estimated based on the CV curves at different scan rates (Figure 5c).41,42 The CV data were analyzed based on the equation i = aνb with b = 0.5 for the diffusion-controlled process and b = 1 for the capacitive process. The b-values calculated from the cathodic and anodic currents of the CV curves at different scan rates were plotted against the potential (Figure S8b). 16


Page 16 of 37

Page 17 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Near the peak potentials, b-values were close to 0.5, implying that diffusion process was prominent at these potentials. Moreover, to further estimate the separate contributions from diffusive and capacitive processes, the Trasatti's method was adopted. In this method, the total charge storage is complied by the diffusion law q (v) = qc + k × v-1/2, where k × v-1/2 stands for the charge during diffusion process and qc is the capacitive charge. When v → ∞, the charge storage was completely governed by capacitive process, which is shown in Figure S8c by plotting capacity as a function of v-1/2. The calculated capacitive charge was 205.0 C g-1. The specific contributions of diffusive and capacitive processes at different scan rates are shown in Figure S8d. At a low scan rate of 10 mV s-1, the diffusion capacity contributed approximately 68% of the total capacity, while at a high scan rate of 100 mV s-1, the diffusion capacity accounted for 34% of the total capacity. The main contribution of diffusive process could be ascribed to the hierarchical porous and heterojunction structures of the obtained products by promoting rapid ion intercalation/deintercalation processes. Electrochemical

Performance

of

ZnO/Co3O4-450//AC

Asymmetric

Supercapacitor. Figure 6a displays the fabricated ZnO/Co3O4-450//AC asymmetric supercapacitor. To obtain appropriate operating potential of this device, CV curves were measured at different potential window at 50 mV s-1. In Figure 6b, when the potential was extended to 1.6 V, evolution of oxygen occurred clearly. Thus, 1.5 V was selected to be the operating potential. Figure 6c shows the CV curves of the device at different scan rates. The specific capacitances of the ZnO/Co3O4-450//AC calculated from the galvanostatic discharge curves (Figure 6d) were 153, 144, 142 17



and 105 F g-1 at current densities of 1, 2, 5, and 10 A g-1, respectively. The coulombic efficiencies at various current densities (Figure S14b) were obtained from the complete charge-discharge profiles of ASC. The corresponding power densities and energy densities are shown in Ragone plot (Figure 6e). The energy density declined with increasing power density, and the maximum energy density and power density were 47.7 W h kg-1 and 7500 W kg-1, respectively. The energy and power densities of ZnO/Co3O4-450//AC asymmetric supercapacitor are comparable to those of Co3O4-based materials reported previously.43-47 Figure 6f shows a possible mechanism for enhanced capacitance.27 It is well known that creating heterostructures by combining nanomaterials with different band gaps can accelerate charge transport and surface reaction kinetics.24,48 The promoted charge transfer is attributed to the formed internal electric field formed at heterointerfaces. In our system, Co3O4 is a p-type semiconductor with a narrow band gap of 1.2 eV, while ZnO works as wide-band-gap (3.2 eV) n-type semiconductor. The formed ZnO/Co3O4 p-n heterojunction can induce an internal electric field in the heterointerfaces, which could raise the charge-transfer rate and improve the performance of the composites. As shown in Figure S8, the obtained composite was mainly controlled by diffusive process, and intercalation of ions was involved in the bulk of material.22 Under the formed internal electric field at heterointerfaces, OHdiffusion would become much easier in the composites, promoting the reaction rate between Co3O4 and OH- and performance of ZnO/Co3O4 composites. In the charge process, the electric field induced by the p-n heterojunction was in the direction from 18


Page 18 of 37

Page 19 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co3O4 to ZnO. Under this electric field, OH- diffusion was facilitated from ZnO to Co3O4.27 Figure S15 illustrated the band alignment at the heterojunction interface together with possible OH- diffusion process. There are some unique features contributed to the high capacitive performance. First, it could be seen from the EIS plots and capacitive performances that the hetero-structures facilitated charge transfer within the electrode and at the electrode/electrolyte interface. Secondly, ZnO protected Co3O4 from corrosion in the GCD process, promoting the stability of the ZnO/Co3O4-450. Finally, 1D porous structure of the composites could promote electrolyte diffusion and avoid structural damage to some extent during GCD process.

CONCLUSIONS

In summary, a well-designed synthetic route, including hydrothermal, reflux and annealing process, was developed to synthesize porous and heterojunction structured ZnO/Co3O4 nanorods. Especially, the annealing temperatures and Zn/Co ratios were found to be critical factors to affect the formation of porous and heterojunction structure, and further govern the distinct electrode activity for supercapacitor. With the well-defined porous and heterojunction structure in the ZnO/Co3O4 nanorods, ZnO/Co3O4-450 displayed the highest capacitance, energy density and good cycling stability in comparison to the other prepared products. The porous structure could improve the ion diffusion and prevent structural collapse in the GCD process, while heterojunction promoted charge transfer, further leading to high-rate performance. 19



Therefore, ZnO/Co3O4-450 could be considered as an appealing electrode material for supercapacitors. Moreover, the synthetic routes and strategies of the composites could also be applied to improve the capacitance and stability of electrode materials for supercapacitor.

ACKNOWLEDGEMENTS The authors thank the National Natural Science Foundation of China (21590812 and 51538011) and the Program for Changjiang Scholars and Innovative Research Team in University and the Collaborative Innovation Center of Suzhou Nano Science and Technology of the Ministry of Education of China for supporting this work.

SUPPORTING INFORMATION AVAILABLE Details of SEM, TEM images and XRD patterns of CoCH and ZnO/CoCH, related calculation of lattice mismatch between Co3O4 and ZnO, SAED pattern of ZnO/Co3O4-450, STEM-EDX line scan of the ZnO/Co3O4-450, EDX spectrum of the ZnO/Co3O4-450, TGA and DrTGA curves of ZnO/CoCH, GCD curves of the Ni foam at 1 A g-1, SEM image and XPS spectra for ZnO/Co3O4-450 after 5000 cycles, the distribution of capacitive and diffusion-controlled processes towards the total capacity of the ZnO/Co3O4-450, CV curves of AC and ZnO/Co3O4-450 at 50 mV s-1, IR drop of ZnO/Co3O4-450, the equivalent circuit diagram of EIS, cycling stability of the ZnO/Co3O4 composites and blank Co3O4, EDX spectrum of nanoparticles and nanorods in ZnO/Co3O4-450 after 5000 cycles, complete GCD curves of the 20


Page 20 of 37

Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


asymmetric supercapacitor and the corresponding coulombic efficiencies, and band diagram and mechanism of electric field for the ZnO/Co3O4 p–n heterojunctions. This information is available free of charge via the Internet at http://pubs.acs.org/.

REFERENCES

(1) Wang, G.; Zhang, L.; Zhang, J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797-828. (2) Zhang, S.; Pan, N. Supercapacitors Performance Evaluation. Adv. Energy Mater. 2015, 5, 1401401-1401420. (3) Zhou, G.; Xiong, T.; He, S.; Li, Y.; Zhu, Y.; Hou, H. Asymmetric Supercapacitor Based on Flexible TiC/CNF Felt Supported Interwoven Nickel-Cobalt Binary Hydroxide Nanosheets. J. Power Sources 2016, 317, 57-64. (4) Lin, T.; Chen, I. W.; Liu, F.; Yang, C.; Bi, H.; Xu, F.; Huang, F. Nitrogen-Doped

Mesoporous

Carbon

of

Extraordinary

Capacitance

for

Electrochemical Energy Storage. Science 2015, 350, 1508-1513. (5) Chen, L. F.; Huang, Z. H.; Liang, H. W.; Gao, H. L.; Yu, S. H. Three-Dimensional Heteroatom-Doped Carbon Nanofiber Networks Derived from Bacterial Cellulose for Supercapacitors. Adv. Funct. Mater. 2014, 24, 5104-5111. (6) Wu, Z. S.; Zhou, G.; Yin, L. C.; Ren, W.; Li, F.; Cheng, H. M. Graphene/Metal Oxide Composite Electrode Materials for Energy Storage. Nano Energy 2012, 1, 107-131. 21



Page 22 of 37

(7) Chen, L. F.; Yu, Z. Y.; Wang, J. J.; Li, Q. X.; Tan, Z. Q.; Zhu, Y. W.; Yu, S. H. Metal-Like Fluorine-Doped β-FeOOH Nanorods Grown on Carbon Cloth for Scalable High-Performance Supercapacitors. Nano Energy 2015, 11, 119-128. (8) Chen, L. F.; Yu, Z. Y.; Ma, X.; Li, Z. Y.; Yu, S. H. In Situ Hydrothermal Growth of Ferric Oxides on Carbon Cloth for Low-Cost and Scalable High-Energy-Density Supercapacitors. Nano Energy 2014, 9, 345-354. (9) Wang, H.; Guo, J.; Qing, C.; Sun, D.; Wang, B.; Tang, Y. Novel Topotactically

Transformed

Carbon-CoO-NiO-NiCo2O4

Nanosheet

Hybrid

Hetero-Structured Arrays as Ultrahigh Performance Supercapacitors. Chem. Commun. 2014, 50, 8697-8700. (10) Yan, J.; Wei, T.; Qiao, W. M.; Shao, B.; Zhao, Q. K.; Zhang, L. J.; Fan, Z. J. Rapid Microwave-Assisted Synthesis of Graphene Nanosheet/Co3O4 Composite for Supercapacitors. Electrochim. Acta 2010, 55, 6973-6978. (11) Xiao, Y.; Liu, S.; Li, F.; Zhang, A.; Zhao, J.; Fang, S.; Jia, D. 3D Hierarchical Co3O4 Twin-Spheres with an Urchin-Like Structure: Large-Scale Synthesis, Multistep-Splitting Growth, and Electrochemical Pseudocapacitors. Adv. Funct. Mater. 2012, 22, 4052-4059. (12) Liu, W.; Li, X.; Zhu, M.; He, X. High-Performance All-Solid State Asymmetric Supercapacitor Based on Co3O4 Nanowires and Carbon Aerogel. J. Power Sources 2015, 282, 179-186.

22


Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13) Zheng, X.; Han, Z.; Yang, W.; Qu, F.; Liu, B.; Wu, X. 3D Co3O4@MnO2 Heterostructures Grown on a Flexible Substrate and Their Applications in Super-Capacitor Electrodes and Photocatalysts. Dalton T. 2016, 45, 16850-16858. (14) Wang, J.; Zhang, X.; Wei, Q.; Lv, H.; Tian, Y.; Tong, Z.; Liu, X.; Hao, J.; Qu, H.; Zhao, J.; Li, Y.; Mai, L. 3D Self-Supported Nanopine Forest-Like Co3O4@CoMoO4

Core-Shell

Architectures

for

High-Energy

Solid

State

Supercapacitors. Nano Energy 2016, 19, 222-233. (15) Chen, L. F.; Xu, Q. Converting MOFs into Amination Catalysts. Science 2017, 358, 304-305. (16) Zhang, Y. R.; Yang, Y. F.; Mao, L. X.; Cheng, D.; Zhan, Z. Y.; Xiong, J. Growth of Three-Dimensional Hierarchical Co3O4@NiMoO4 Core-Shell Nanoflowers on Ni foam as Electrode Materials for Hybrid Supercapacitors. Mater. Lett. 2016, 182, 298-301. (17) Xiong, S. L.; Yuan, C. Z.; Zhang, M. F.; Xi, B. J.; Qian, Y. T. Controllable Synthesis of Mesoporous Co3O4 Nanostructures with Tunable Morphology for Application in Supercapacitors. Chem. Eur. J. 2009, 15, 5320-5326. (18) Yuan, C. Z.; Yang, L.; Hou, L. R.; Shen, L. F.; Zhang, X. G.; Lou, X. W. Growth of Ultrathin Mesoporous Co3O4 Nanosheet Arrays on Ni Foam for High-Performance Electrochemical Capacitors. Energ. Environ. Sci. 2012, 5, 7883-7887.

23



Page 24 of 37

(19) Xia, X. H.; Tu, J. P.; Mai, Y. J.; Wang, X. L.; Gu, C. D.; Zhao, X. B. Self-Supported Hydrothermal Synthesized Hollow Co3O4 Nanowire Arrays with High Supercapacitor Capacitance. J. Mater. Chem. 2011, 21, 9319-9325. (20) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845-854. (21) Davis, M. E. Ordered Porous Materials for Emerging Applications. Nature 2002, 417, 813-821. (22) Wei, Q.; Xiong, F.; Tan, S.; Huang, L.; Lan, E. H.; Dunn, B.; Mai, L. Porous One-Dimensional

Nanomaterials:

Design,

Fabrication

and

Applications

in

Electrochemical Energy Storage. Adv. Mater. 2017, 29, 1602300-1602338. (23) Pang, H.; Ma, Y. H.; Li, G. C.; Chen, J.; Zhang, J. S.; Zheng, H. H.; Du, W. M. Facile Synthesis of Porous ZnO-NiO Composite Micropolyhedrons and Their Application for High Power Supercapacitor Electrode Materials. Dalton T. 2012, 41, 13284-13291. (24) Wang, X.; Shi, B.; Fang, Y.; Rong, F.; Huang, F.; Que, R.; Shao, M. High Capacitance and Rate Capability of a Ni3S2@CdS Core-Shell Nanostructure Supercapacitor. J. Mater. Chem. A 2017, 5, 7165-7172. (25) Yang, X.; Sun, H.; Zan, P.; Zhao, L.; Lian, J. Growth of Vertically Aligned Co3S4/CoMo2S4 Ultrathin Nanosheets on Reduced Graphene Oxide as a High-Performance Supercapacitor Electrode. J. Mater. Chem. A 2016, 4, 18857-18867.

24


Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26) Liu, Q.; Nayfeh, O.; Nayfeh, M. H.; Yau, S. T. Flexible Supercapacitor Sheets Based on Hybrid Nanocomposite Materials. Nano Energy 2013, 2, 133-137. (27) Zheng, Y.; Zhou, T.; Zhang, C.; Mao, J.; Liu, H.; Guo, Z. Boosted Charge Transfer in SnS/SnO2 Heterostructures: Toward High Rate Capability for Sodium-Ion Batteries. Angew. Chem. Int. Ed. 2016, 55, 3408-3413. (28) Li, G. C.; Liu, P. F.; Liu, R.; Liu, M. M.; Tao, K.; Zhu, S. R.; Wu, M. K.; Yi, F. Y.; Han, L. MOF-Derived Hierarchical Double-Shelled NiO/ZnO Hollow Spheres for High-Performance Supercapacitors. Dalton T. 2016, 45, 13311-13316. (29) Cai, D.; Huang, H.; Wang, D.; Liu, B.; Wang, L.; Liu, Y.; Li, Q.; Wang, T. High-Performance Supercapacitor Electrode Based on the Unique ZnO@Co3O4 Core/Shell Heterostructures on Nickel Foam. ACS Appl. Mater. Inter. 2014, 6, 15905-15912. (30) Borhani,

S.;

Moradi,

M.;

Kiani,

M.

A.;

Hajati,

S.;

Toth,

J.

CoxZn1−xZIF-Derived Binary Co3O4/ZnO Wrapped by 3D Reduced Graphene Oxide for Asymmetric Supercapacitor: Comparison of Pure and Heat-Treated Bimetallic MOF. Ceram. Int. 2017, 43, 14413-14425. (31) Wang, T.; Zhang, S.; Yan, X.; Lyu, M.; Wang, L.; Bell, J.; Wang, H. 2-Methylimidazole-Derived Ni-Co Layered Double Hydroxide Nanosheets as High Rate Capability and High Energy Density Storage Material in Hybrid Supercapacitors. ACS Appl. Mater. Inter. 2017, 9, 15510-15524.

25



Page 26 of 37

(32) Sennu, P.; Aravindan, V.; Lee, Y.-S. High Energy Asymmetric Supercapacitor with 1D@2D Structured NiCo2O4@Co3O4 and Jackfruit Derived High Surface Area Porous Carbon. J. Power Sources 2016, 306, 248-257. (33) Zhang, M.; Wang, Y.; Pan, D.; Li, Y.; Yan, Z.; Xie, J. Nitrogen-Doped 3D Graphene/MWNTs Nanoframework-Embedded Co3O4 for High Electrochemical Performance Supercapacitors. ACS Sustain. Chem. Eng. 2017, 5, 5099-5107. (34) Bekermann, D.; Gasparotto, A.; Barreca, D.; Maccato, C.; Comini, E.; Sada, C.; Sberveglieri, G.; Devi, A.; Fischer, R. A. Co3O4/ZnO Nanocomposites: From Plasma Synthesis to Gas Sensing Applications. ACS Appl. Mater. Inter. 2012, 4, 928-934. (35) Xiong, S. L.; Chen, J. S.; Lou, X. W.; Zeng, H. C. Mesoporous Co3O4 and CoO@C

Topotactically

Transformed

from

Chrysanthemum-Like

Co(CO3)0.5(OH)•0.11H2O and Their Lithium-Storage Properties. Adv. Funct. Mater 2012, 22, 861-871. (36)Zheng, X.; Yan, X.; Sun, Y.; Bai, Z.; Zhang, G.; Shen, Y.; Liang, Q.; Zhang, Y. Au-Embedded ZnO/NiO Hybrid with Excellent Electrochemical Performance as Advanced Electrode Materials for Supercapacitor. ACS Appl. Mater. Inter. 2015, 7, 2480-2485. (37) Rong, Q.; Long, L. L.; Zhang, X.; Huang, Y. X.; Yu, H. Q. Layered Cobalt Nickel Silicate Hollow Spheres as a Highly-Stable Supercapacitor Material. Appl. Energy (Netherlands) 2015, 153, 63-69.

26


Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Brousse, T.; Belanger, D.; Long, J. W. To Be or Not To Be Pseudocapacitive? J. Electrochem. Soc. 2015, 162, A5185-A5189. (39) Meher, S. K.; Rao, G. R. Ultralayered Co3O4 for High-Performance Supercapacitor Applications. J. Phys. Chem. C 2011, 115, 15646-15654. (40) Tang, Y.; Liu, Z.; Guo, W.; Chen, T.; Qiao, Y.; Mu, S.; Zhao, Y.; Gao, F. Honeycomb-Like Mesoporous Cobalt Nickel Phosphate Nanospheres as Novel Materials for High Performance Supercapacitor. Electrochim. Acta 2016, 190, 118-125. (41) Singh, A. K.; Sarkar, D. Substrate-Integrated Core–Shell Co3O4@Au@CuO Hybrid Nanowires as Efficient Cathode Materials for High-Herformance Asymmetric Supercapacitors with Excellent Cycle Life. J. Mater. Chem. A 2017, 5, 21715-21725. (42) Yang, P.; Sun, P.; Du, L.; Liang, Z.; Xie, W.; Cai, X.; Huang, L.; Tan, S.; Mai, W. Quantitative Analysis of Charge Storage Process of Tungsten Oxide that Combines Pseudocapacitive and Electrochromic Properties. J. Phys. Chem. C 2015, 119, 16483-16489. (43) Salunkhe, R. R.; Tang, J.; Kamachi, Y.; Nakato, T.; Kim, J. H.; Yamauchi, Y. Asymmetric Supercapacitors Using 3D Nanoporous Carbon and Cobalt Oxide Electrodes Synthesized from a Single Metal-Organic Framework. ACS Nano 2015, 9, 6288-6296. (44)Zhang, J. F.; Lin, J. M.; Wu, J. H.; Xu, R.; Lai, M.; Gong, C.; Chen, X.; Zhou, P. Excellent Electrochemical Performance Hierarchical Co3O4@Ni3S2 Core/Shell

27



Nanowire Arrays for Asymmetric Supercapacitors. Electrochim. Acta 2016, 207, 87-96. (45) Wu, C.; Cai, J. J.; Zhang, Q. B.; Zhou, X.; Zhu, Y.; Shen, P. K.; Zhang, K. L. Hierarchical Mesoporous Zinc-Nickel-Cobalt Ternary Oxide Nanowire Arrays on Nickel Foam as High-Performance Electrodes for Supercapacitors. ACS Appl. Mater. Inter. 2015, 7, 26512-26521. (46) Sun, G. L.; Ma, L. Y.; Ran, J. B.; Shen, X. Y.; Tong, H. Incorporation of Homogeneous Co3O4 into a Nitrogen-Doped Carbon Aerogel via a Facile in situ Synthesis Method: Implications for High Performance Asymmetric Supercapacitors. J. Mater. Chem. A 2016, 4, 9542-9554. (47) Guan, C.; Liu, X. M.; Ren, W. N.; Li, X.; Cheng, C. W.; Wang, J. Rational Design of Metal-Organic Framework Derived Hollow NiCo2O4 Arrays for Flexible Supercapacitor and Electrocatalysis. Adv. Energy Mater. 2017, 7, 1602391-1602398. (48) Chang, X.; Wang, T.; Zhang, P.; Zhang, J.; Li, A.; Gong, J. Enhanced Surface Reaction Kinetics and Charge Separation of p-n Heterojunction Co3O4/BiVO4 Photoanodes. J. Am. Chem. Soc. 2015, 137, 8356-8359.

28


Page 28 of 37

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Formation process of the ZnO/Co3O4 composites using the hydrothermal method with a combination with reflux and annealing process.

29



Figure legends Figure 1. (a) XRD pattern, (b) low- and (c) high-magnification SEM images, (d, e) low- and (f) high-resolution TEM images, and (g) HAADF-STEM image and the corresponding EDS mapping images of the ZnO/Co3O4-450 composite. Figure 2. XPS spectra of (a) survey spectrum, (b) Co 2p, (c) Zn 2p and (d) O 1s of the ZnO/Co3O4-450. Figure 3. (a) XRD patterns, TEM images of (b) ZnO/CoCH, (c) ZnO/Co3O4-350, (d) ZnO/Co3O4-550 and (e) ZnO/Co3O4-650, Scale bar: 90 nm, (f) N2 adsorption/desorption isothermals, and (g) the corresponding pore-size distribution curves of ZnO/CoCH and ZnO/Co3O4 composites. Figure 4. (a) CV curves at the scan rate of 5 mV s-1, (b) galvanostatic discharge curves at 1 A g-1 of ZnO/CoCH and ZnO/Co3O4 composites, (c) CV curves at the scan rate of 5 mV s-1, (d) galvanostatic discharge curves at 1 A g-1 of ZnO/Co3O4-450 with different ZnO contents, and (e) the calculated specific capacitances of the samples. Figure 5. (a) CV curves at the scan rate of 5 mV s-1, and (b) galvanostatic dischrge curves at the current density of 1 A g-1 of ZnO/Co3O4-450, Co3O4 and ZnO electrodes, (c) CV curves, and (d) galvanostatic charge/dischrge curves of the ZnO/Co3O4-450, (e) specific capacitances as a function of current densities, and EIS plots (inset) of ZnO/Co3O4-450 and Co3O4, and (f) cycling stability of the composite at the current density of 10 A g-1. The inset of (f) galvanostatic dischrge curves before and after cycling at 1 A g-1. Figure 6. (a) Structure schematic diagram of the ZnO/Co3O4-450//AC asymmetric supercapacitor, (b) CV curves measured at different potential window (at 50 mV s-1), and (c) at different scan rates, (d) galvanostatic dischrge curves of the asymmetric supercapacitor at different current densities, (e) Ragone plot of the ZnO/Co3O4-450//AC asymmetric supercapacitor with comparison to some reports in the literature, and (f) diagram of the charge transfer in the ZnO/Co3O4-450 hetero-structure.

30


Page 30 of 37

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1

31



Figure 2

32


Page 32 of 37

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3

33



Figure 4

34


Page 34 of 37

Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5

35



Figure 6

36


Page 36 of 37

Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents

37


Porous ZnO-Coated Co3O4 Nanorod as a High ... - ACS Publications

Recommend Documents