3Cl-MnO2 Spheres Synthesized by Microwave ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Self-Assembled Three-Dimensional Macroporous Co2(OH)3Cl-MnO2 Spheres Synthesized by Microwave-Assisted Method: A New Hybrid for High-Performance Asymmetric Supercapacitors Weize Yang,† Jianzhong Zheng,‡ Shirong Hu,*,† Wuxiang Zhang,† Chan Wei,† Peihui Dong,† Yaru Yan,† and Haixia Hu† †

College of Chemistry and Environment, Minnan Normal University, Zhangzhou, Fujian 363000, P. R. China Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, P. R. China

‡

S Supporting Information *

ABSTRACT: Three-dimensional (3D) Co2(OH)3Cl-MnO2 hybrid sphere was first obtained through a facile solvothermal microwave-assisted synthesis method. Because of the 3D sphere structure with a flowerlike morphology built up by nanosheets, this type of hybrid accelerated ion transfer as well as the charge and simultaneously maximized the electroactivity of Co2(OH)3Cl or MnO2 with a high capacitance of 942.2 F g−1, which was greater than that of the single Co2(OH)3Cl or MnO2. Meanwhile, an asymmetric supercapacitor device was assembled with the hybrid and active carbon (AC) as the positive and negative electrodes, which yielded a specific capacitance of 49.26 F g−1 and a high energy density of 12.62 Wh kg−1 at a power density of 679.10 W kg−1. In addition, the capacitance of this device retained 83.77% after 5000 cycles, which showed a well-defined, long cycle life. KEYWORDS: Co2(OH)3Cl-MnO2, Three-dimensional (3D) macroporous spheres, Hybrid material, Asymmetric supercapacitors

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INTRODUCTION With the ever-increasing demand for energy and consumption of fossil fuels around the globe, it requires human society to develop clean, renewable, and sustainable resources. For meeting the needs of contemporary society and solving emerging environment problems, advanced and eco-friendly energy conversion and storage systems have constantly been explored by researchers around the world.1,2 Among multifarious energy conversion and storage devices, one effective and practical technology is supercapacitors.3 Because of their fast charge−discharge properties, much higher power densities, and more durable cycling lifespans than those of traditional secondary batteries, supercapacitors have gained significant attention in recent years. Some waste products, such as newspaper, wood, coffee grounds, and tires, are recycled to © 2017 American Chemical Society

synthesize the carbon-based materials that are applied as electrode materials in the field of supercapacitors.4,5 Nevertheless, some weaknesses of carbon-based supercapacitors undermine their widespread application in energy storage systems; in particular, one of the most deadly is low energy density.6−8 Thus, many researchers have paid much more attention to developing new electrode materials and ingenious device designs. A hybrid electrochemical capacitor (so-called asymmetric supercapacitor (ASC)) is assembled with two kinds of different electrode materials (one is a carbon-based material, the other is a pseudocapacitive material) in the same cell. For Received: November 27, 2016 Revised: February 27, 2017 Published: May 2, 2017 4563

DOI: 10.1021/acssuschemeng.6b02864 ACS Sustainable Chem. Eng. 2017, 5, 4563−4572

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ACS Sustainable Chemistry & Engineering

compounds that possibly leads to superior performance in electrochemistry, such as high rate, cycling capabilities, and so forth.23,24 Recently, Co2(OH)3Cl has been investigated as an anode material in the field of Li-ion batteries, which showed superior electrochemical performance. There has been a literature report on Co2(OH)3Cl as an electrochemical material for supercapacitors. Ranganatha et al.25 fabricated interconnected mesoporous Co2(OH)3Cl nanostructured xerogel by a one pot sol−gel method. Owing to the unique structure of the as-obtained material, it exhibits superior electrochemical performance and shows a high capacitance of 450 F g−1. Nonetheless, for optimizing the electrochemical performance of Co hydroxide chloride electrode material, one tactic is to excogitate reasonably large specific surface area Co2(OH)3Cl together with porous structure, which can decrease the volume change and stress, provide more room for electrolyte infiltration, and narrow the ion diffusion gap between the electrode/electrolyte. Moreover, it was duly demonstrated that MnO2-based composites combined with other metal oxides/ hydroxides showed advanced capacitance performance compared to that of the individual components. Although the individual capacitive property of these two materials has been extensively demonstrated, no study on their composite’s electrochemical capacitance has been reported. Therefore, we present a novel hybrid structure of three-dimensional (3D) macroporous Co2(OH)3Cl-MnO2 spheres by a microwaveassisted synthesis method for supercapacitor applications. Microwave irradiation, as a simple, fast, inexpensive, efficient energy-saving and uniform heating method, has been widely applied in industry and academia in recent years.26 In addition, it was duly demonstrated that the microwave technique provides a valid way to control the distribution of particle size and macroscopic morphology in the synthesis.27 In this work, we have reported how to achieve 3D macroporous Co2(OH)3Cl-MnO2 spheres built up by nanosheets dozens of nanometers thick using the one-step solvothermal microwave-assisted synthesis process without any surfactant. The as-fabricated Co2(OH)3Cl-MnO2 sample as a novel electrode material shows a high capacitance of 914.3 F g−1 at 1 A g−1. Meanwhile, for evaluating the possibility of the as-obtained Co2(OH)3Cl-MnO2 hybrid used in a practical supercapacitor, an asymmetric supercapacitor device was constructed. The electrochemical performance of this device was evaluated by different electrochemical techniques. As a result, the assembled Co2(OH)3Cl-MnO2//AC ASC exhibits outstanding electrochemical properties.

both the supercapacitors (higher rate capability, longer cycling lifespans) and advanced batteries (higher energy density), the ASC holds significant advantages.9 Significant efforts have gone into developing different ACS devices, that is, LiMn2O4//AC,10 Ni(OH) 2//AC,11 MnO2//AC,12 KxMnO 2//AC,13 PPy@ MoO3//AC,14 and so forth. It is extensively accepted that the electrode materials exert a significant impact on determining the electrochemical performance of the supercapacitor.15 Pseudocapacitive electrode materials, the main contributors of the supercapacitor, have received significant recent attention, particularly based on transitional metal oxides/hydroxides.16 Hybrid materials composed of two components have gained significant attention because of enhanced properties compared to their individual components, especially hybrid transitional metal oxides/hydroxides widely investigated in the field of supercapacitors. For instance, MnO2-based nanocomposites have been widely researched for supercapacitor applications. However, poor electrical conductivity as well as low specific surface area of manganese oxide remains a major issue.17 For this drawback to be overcome, further efforts have focused on incorporating MnO2 nanostructures with other metal oxides or hydroxides and the preparation of an integrated smart architecture of a MnO2-based hybrid. Zhang et al.18 designed and synthesized hierarchical MnCo2O4.5@MnO2 core−shell nanowires hybridized with mesoporous MnCo2O4.5 nanowires serving as the core and δ-MnO2 nanosheets as the shell layer, which hence endowed the MnO2 with a very high specific capacitance with this configuration. Wu et al.19 fabricated ultrathin porous Ni(OH)2-MnO2 hybrid nanosheets through a hydrothermal codeposition process. It was also observed that the Ni(OH)2-MnO2 hybrid can inherit the advantages of both MnO2 and Ni(OH)2 and has superiority over either of the single ones. Rao et al.20 successfully synthesized ZnO@MnO2 core−shell nanofibers by modification of high-aspect-ratio electrospinning. Because of the high conductivity of the ZnO nanofibers and the MnO2 nanoflakes coating, the hybrid delivers a high capacitance of 907 F g−1. Kim et al.21 fabricated heterogeneous NiCo2O4−MnO2 arrays. Because of the synergy between the mesoporous NiCo2O4 nanowire and the crosslinked MnO2 nanosheet grown on a freestanding graphene foam, the attained nanocomposite electrode exhibits a high capacitance (2577 F g−1 at 1 A g−1), outstanding cycle stability (94.3% capacitance retention after 5000 cycles), and good rate capability. These studies strongly supported the enhancement of the hybrids over single ones. On the basis of the foregoing results, we can conclude that the hybrids showed outstanding charge-storage performance over that of single ones, which could be considered the outcome of the synergy between different components and a peculiar nanostructure, indicating that the significance of the selection of chemical components and an integrated smart architecture in the hybrid design can result in better electrochemical characteristics, such as high specific capacitance and benign electrochemical stability.22 However, the synthesis methods of these hybrids require complicated fabrication procedures, long reaction times, and high energy wasting, which are not practical for commercial production. Thus, improvement of facile, mild, and effective methods to synthesize hybrid materials for supercapacitors still remains a challenge. Co2(OH)3Cl, a cobalt-based composite with the simultaneous incorporation of Cl and OH groups, exhibits a similar octahedral structure and is composed of slightly distorted corner-sharing tetrahedrons. Furthermore, it is one of the

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EXPERIMENTAL SECTION

Materials. Methanol, acetone, ethanol, cobalt nitrate (Co(NO3)2· 6H2O), manganese chloride (MnCl2·4H2O), N-methyl pyrrolidone (NMP), potassium hydroxide (KOH), and hydrochloric acid (37%, HCl) were purchased from Xilong Chemical Co., Ltd. (Guangdong, China). Synthesis of Materials. The 3D macroporous Co2(OH)3ClMnO2 spheres were prepared via a solvothermal method with methanol as the solvent. In a typical procedure, 0.1 g of Co(NO3)2· 6H2O and 0.1 g of MnCl2·4H2O were put into 15 mL methanol solution. The as-obtained solution was poured slowly into a 30 mL microwave reaction vial. Then, the vial was placed in a microwave reactor (Monowave 300, Anton Paar, Austria). The mixture was heated for 30 min at 165 °C (1200 rpm stirring speed); afterward, it was cooled to 55 °C by pressurized air. The product was collected by filtration and followed by washing with ethanol and water three times. Then, the as-obtained product was dried at 60 °C for 12 h. For the 4564



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sake of comparison, pure MnO2 and Co2(OH)3Cl were also fabricated by the previously reported methods, which is discussed in the Supporting Information in detail. Materials Characterization. Scanning electron microscopy (SEM, JSM-6010LA, JEOL) and high-resolution transmission electron microscopy (HRTEM, FEI F20, JEOL) were employed to observe morphological characteristics of the as-obtained sample. The crystalline structure was identified by XRD pattern recorded in a Rigaku Ultima IV X-ray diffractometer. The composition of the samples was analyzed by energy-dispersive X-ray spectroscopy (EDX, GENESIS, EDAX), X-ray photoelectron spectrum (XPS, Thermo ESCALAB 250Xi), inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700ce), and Fourier transform infrared (FT-IR, NICOLET iS10) spectroscopy. The specific surface area of Co2(OH)3Cl-MnO2 powder was measured by the N2 adsorption− desorption isotherms of a Quadrasorb SI apparatus. Electrode Fabrication. To ensure a clean surface of nick foam (NF), it was first washed sequentially with acetone, 2 M HCl, ethanol, and deionized water for 15 min, respectively. The active carbon (AC) was purchased from Wuxi Graphene Industrial Development Demonstration Zone Test Center (Jiangsu, China). The negative electrode was made by mixing 10 wt % of polytetrafluoroethylene (PTFE, Aladdin) and 90 wt % of AC in NMP. The obtained slurry was coated onto the cleansed 1.0 cm2 NF and dried in a 60 °C vacuum oven overnight. Electrochemical Measurements. Electrochemical experiments, including galvanostatic charge−discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) measurements, were carried out on an electrochemical workstation (CHI 660E, Chenghua, Shanghai, China) with a three-electrode cell and two-electrode configuration. The three-electrode test system was equipped with the hybrid-modified working electrode, reference electrode (saturated calomel electrode (SCE)), and counter electrode (Pt film (1.5 × 1.5 cm2)). The two-electrode device was configured with AC and as-obtained hybrid powder as the negative and positive electrodes, respectively. The LANHE CT2001A batteries testing system was employed to test the cycle life of the device. The working electrode was made by mixing 10 wt % PTFE binder, 15 wt % acetylene black (commercially available), and 75 wt % active material. The proper amount of NMP was dropped into the mixture powder followed by stirring to generate a slurry. This slurry was uniformly coated onto the cleansed NF and subsequently dried in a 60 °C vacuum drying oven overnight. After that, the Co2(OH)3Cl-MnO2modified electrode was weighed and pressed at 15 MPa. The load mass of electroactive material was approximately 5.5 mg. The electrode was immersed in 1 M KOH for 12 h before the measurement. The Cs (F g−1) of Co2(OH)3Cl-MnO2 nanocomposite and AC electrode were calculated from GCD curves as Cs = I Δt /(mΔV )

RESULTS AND DISCUSSION Characterization of the Co2(OH)3Cl-MnO2 Nanocomposite. In Figure 1, the product was detected by powder XRD,

Figure 1. XRD pattern of the Co2(OH)3Cl-MnO2 hybrid material.

showing the XRD pattern of the as-fabricated composite. The sharp diffraction peaks appearing at 16.0°, 18.02°, 19.04°, 25.78°, 30.54°, 31.84°, 32.66°, 38.96°, 40.56°, 43.22°, 45.92°, 49.5°, 53.22°, 59.0°, and 60.52° are clearly corresponding to the rhombohedral Co2(OH)3Cl crystal planes (PDF Card no.732134) of (101), (003), (012), (110), (021), (113), (202), (024), (211), (205), (107), (033), (220), (208), and (217), respectively. Other peaks at 19.02°, 37.06°, 45.2°, and 59.8° can be assigned to the (111), (311), (400), and (511) planes of MnO2 (PDF Card no.42-1169), respectively. Compared with the strong peaks of Co2(OH)3Cl, the peak intensities of MnO2 were relatively weak, probably due to the small average crystallite domain size of MnO2, which was also consistent with previous reports.29,30 The SEM images of the as-fabricated hybrid powders, shown in the inset of Figure 2a, reveal a spherical feature whose diameters are 4−6 μm. From Figure 2a, the 3D structure with flowerlike morphology built up by nanosheets was clearly observed. The nanosheets are organized together and support each other with almost all of the surfaces exposed. The mean thickness of these nanosheet building blocks is approximately 50 nm (Figure S1). The nanosheets are wrinkled and interconnected in a disorderly manner, which make the hybrid highly porous. The porous flowerlike hierarchical structure not only contributes to a large surface area but can also serve as an electrolyte diffusion channel, which is beneficial for pseudocapacitor electrode applications.31 The microstructure of Co2(OH)3Cl-MnO2 was also confirmed by TEM and HRTEM. Figure 2b is a TEM image of a nonorganized nanosheet from the broken structure, and it further shows an unsmooth and transparent edge, indicating an ultrathin nature. A higher-

(1)

where I (A), Δt (s), ΔV (V), and m (g) are the charge/discharge current, discharge time, charge/discharge potential range excluding IR drop, and loading mass of the active material in the electrode, respectively. Cs′ (F g−1) of the assembled ACS was calculated from GCD curves as

Cs′ = I Δt /(m′ΔV )

(2)

where m′ (g) is the total load mass of negative and positive electrode materials. The energy density (E, Wh kg−1) and power densities (P, W kg−1) of the composite-based electrodes as well as the assembled ACS were calculated as28

E = 5/36 × C × V 2

(3)

Pave = 3600E /Δt

(4)

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where C is the Cs of Co2(OH)3Cl-MnO2 composite electrode or the Cs′ of the assembled ACS, and V (V) and Δt (s) are the cell potential range excluding IR drop and discharge time, respectively. 4565


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Figure 2. Typical (a) SEM, (b, c) TEM, and (d) HR-TEM images of the Co2(OH)3Cl-MnO2 hybrid material. (e) EDX spectrum of the Co2(OH)3Cl-MnO2 hybrid material with the corresponding SEM image in the inset. (f) O, Cl, Co, and Mn elemental mapping images reveal that all species are uniformly distributed among the area of the inset.

Figure 3. XPS spectra of Co2(OH)3Cl-MnO2 hybrid material: (a) Co 2p and (b) Mn 2p spectrum.

magnification TEM image further clearly shows that these hybrid spheres consist of ultrathin nanosheets (Figure 2c). Moreover, the lattice fringes of approximately 0.2807 and

0.2305 nm can be assigned to the (024) and (113) planes of Co2(OH)3Cl, respectively. The other lattice fringe with 0.2456 nm corresponds well to the (311) plane of MnO2 (Figure 2d). 4566


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Co2(OH)3Cl-MnO2 spheres. Figure 4 displays SEM images of the intermediate products at the following times: (a) 5, (b) 10,

For comparison, the SEM and TEM images of the single Co2(OH)3Cl and MnO2 are shown in Figure S2. Panels a and b in Figure S2 demonstrate that Co2(OH)3Cl is composed of irregular particles approximately 30−80 nm in diameter. Panels c and d in Figure S2 display that MnO2 is composed of many small nanoparticles that are tightly compacted with each other. The EDX spectrum (Figure 2e) reveals that the hybrid powder contains C, Co, Mn, O, and Cl elements, which should be offered by Co2(OH)3Cl and MnO2. Furthermore, elemental mapping analysis of the resulting powder reveals the uniform distribution of O, Cl, Co, and Mn elements throughout the asprepared hybrid powder sample in Figure 2f. In addition, the ICP-MS indicates that the Co/Mn atomic ratio in the product is around 1:0.16 (Table S1), which is nearly consistent with the outcome of EDX elemental analysis. For further confirming the chemical constitution and valence states of various elements in the hierarchical porous Co2(OH)3Cl-MnO2 hybrid material, XPS and FT-IR analyses were performed. The survey XPS spectroscopy (Figure S3) indicates the presence of C, Cl, O, Co, and Mn elements. The C peak is ascribed to the residual organic solvent. In light of the XPS handbook, the peak at 531.68 eV for O 1s can be principally attributed to the bonding energy of the O−H group (Figure S4).32 In Figure S5, a peak at 198.73 eV for Cl 2p can be associated with the bonding energy of the (OH)3-Cl group.33 As shown in Figure 3a, two peaks at around 781.58 and 797.58 eV are the characteristic peaks of Co 2p3/2 and Co 2p1/2, respectively. Furthermore, two other weak peaks at around 786.88 and 803.03 eV correspond to the shakeup satellite peaks of Co 2p3/2 and Co 2p1/2, which demonstrates the presence of Co2+.34 Figure 3b exhibits two strong peaks appearing near 654.23 and 642.53 eV with a binding energy difference of 11.7 eV, which can be referred to as Mn 2p1/2 and Mn 2p3/2 of Mn4+ in MnO2, respectively.35,36 More characteristics of Co2(OH)3Cl-MnO2 are also observed in its FT-IR spectrum (Figure S6a). The spectra for the sample has the seven primary vibration bands at around 3553.48, 1619.96, 1383.40, 840.13, 732.93, 587.24, and 424.26 cm−1. The absorption detected at around 3553.48 cm−1 is attributed to the O−H bond stretching vibration, which confirms the existence of OH− with respect to the metal−OH layer in the crystalloid. The bands detected at around 1619.96 and 1383.40 cm−1 are normally attributed to the OH group bending vibrations related to the water molecules located in the interlamellar space of the sample. The intense bands observed at 587.24 and 424.26 cm−1 can be assigned to Mn−O and Co− OH bending vibrations, respectively.37 The other sharp bands at 840.13, and 732.93 cm−1 are assigned to in-plane bending deformation of Co−O−H. The peak at 701.96 cm−1 is the bending deformation of O−H···Cl.38 These results reveal that the as-fabricated sample is composed of Co2(OH)3Cl and MnO2. In addition, the BET specific surface area of the Co2(OH)3Cl-MnO2 hybrid was found to be approximately 113.76 m2 g−1, which is greater than that of unitary Co2(OH)3Cl (22.30 m2 g−1) and approximately one-half that of unitary MnO2 (232.30 m2 g−1), as shown in Figure S6b−d. The result can be attributed to the incorporation of Mn2+ that plays its part in adjusting the morphology of Co2(OH)3Cl and improving the surface area of this hybrid material. For investigating the intermediate products of the formation of 3D macroporous Co2(OH)3Cl-MnO2 sphere architecture, SEM was used to characterize the obtained samples at different times during the fabrication process of 3D macroporous

Figure 4. SEM images of the morphological evolution of Co2(OH)3Cl-MnO2 formed after the reaction at 165 °C for (a) 5, (b) 10, (c) 20, and (d) 30 min.

(c) 20, and (d) 30 min. In the early stage of the reaction, spheres made up of several particles with uneven surface morphologies were observed (Figure 4a), and there were several nanocrystalline particles distributed in these sphere surfaces. As the reaction progressed, the morphologies of these spheres were not significantly changed. Multiple crisscrossed cracks on the surface of these spheres were observed (Figure 4b). At extended reaction times, hierarchical surface morphologies built up by thick flakes interconnected with each other were observed (Figure 4c). Subsequently, these thick flakes were further evolved into thinner and more flexible nanosheets (Figure 4d). From these experimental outcomes, a possible formation mechanism can be proposed, as illustrated in Scheme 1. At the very beginning, NO3− reduced quickly by methanol under microwave heating and produced OH−: 4CH3OH + NO3− → Scheme 1. Possible Formation Mechanism of 3D Macroporous Co2(OH)3Cl-MnO2 Spheres

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ACS Sustainable Chemistry & Engineering 4HCHO + OH− + NH3 + 2H2O; then, Co2+ ions from Co(NO3)2 and Cl− ions from MnCl2 react with OH− ions to form Co2(OH)3Cl nuclei.39,40 A great number of nuclei further grow to particles of larger sizes. The primary particles start to crystallize and then gradually form primary spheres after they persistently agglomerate. Soon thereafter, because of the high surface energy endowed by continuous microwave radiation, these primary spheres burst and further form multiple crisscrossed cracks on the surface by microwave etching. At this time, Mn2+ ions from the MnCl2 react with OH− to generate Mn(OH)2 nuclei. Then, it is converted to MnO2 by reacting with dissolved oxygen at high temperatures.41 Owing to van der Waals interactions and crystal-face attraction, the MnO2 attached onto and loaded into the interlayer spacing of the surfaces of Co2(OH)3Cl spheres.42,43 As the reaction progresses, these spheres can be divided gradually to form several thick flakes that were interconnected with each other by microwave etching.44 Subsequently, these thick flakes are epitaxially grown and further evolved into thinner and more flexible nanosheets with the reaction time extension. The MnO2 primary particles may also continue via aggregation, anisotropic growth, and crystallization to gradually become primary nanosheets. MnO2 nanosheets may stack intricately with Co2(OH)3Cl nanosheets by the H-bond interaction between the OH groups of adjacent nanosheets and grow intertwined to form hybrid nanosheets.45 Significantly, microwave heating technology can help to accelerate the chemical reaction process and the mass diffusion, which benefits stable growth of the 3D macroporous Co2(OH)3Cl-MnO2 sphere architecture.46 Electrochemical Characterizations of the Co2(OH)3ClMnO2 Nanocomposite Electrode. The electrochemical performance of the Co2(OH)3Cl-MnO2 electrode was analyzed by conducting cyclic voltammetry and galvanostatic charge− discharge measurements in a conventional three-electrode system. CV measurements were carried out from −0.2 to 0.6 V (vs SCE). The comparison of integrated areas of the CV curves, in Figure 5, demonstrate that the Co2(OH)3Cl-MnO2 electrode holds a higher capacitance than that of pure Co2(OH)3Cl or the MnO2 electrode because the capacitance is a direct ratio with the integrated area of a CV curve. Figure 6a reveals the CV curves of the Co2(OH)3Cl-MnO2 electrode at 5 to 100 mV s−1 within a potential range of −0.2 to 0.6 V. A pair of obvious redox peaks is shown. Pseudocapacitance rather than

double layer capacitance behavior can be considered as the mean of charge storage of the electrode, which is illustrated by the well-defined redox peaks. Compositionally, Co2(OH)3Cl is a sosoloid of CoCl2 and Co(OH)2.47 However, when the electrodes were immersed into KOH solution, CoCl2 could react with electrolyte to generate Co(OH)2 because of the strong chemoaffinity ability of Co2+ and OH−.48 Thus, the redox reaction mechanism for the faradaic pseudocapacitance of the Co2(OH)3Cl-MnO2 composite is described by eqs 5−8.49,50 Co(OH)2 + OH− ⇌ CoOOH + H 2O + e−

(5)

CoOOH + OH− ⇌ CoO2 + H 2O + e−

(6)

MnO2 + K+ + e− ⇌ MnOOK

(7)

MnOOH + OH− ⇌ MnO2 + H 2O + e−

(8)

With increasing scan rates, the shape of CV remains basically unvaried, whereas the peak shifts slightly because of weak electrode polarization.51 Figure 6b displays the GCD curves of the Co2(OH)3Cl-MnO2 electrode at various current densities. The observation points out that GCD curves are unsymmetric at various current densities. Additionally, the discharge curve reveals an approximate platform, which further indicates that the as-obtained hybrid displays strong pseudocapacitive behavior. The IR drop is ascribed to the ohmic resistance of this electrode. The specific capacitances calculated from the GCD curves are 942.2, 914.3, 622.7, 425.9, 329.2, and 284.4 F g−1 at 0.5, 1, 2, 5, 8, and 10 A g−1, respectively (Table S2). The Cs (914.3 F g−1 at 1 A g−1) of the Co2(OH)3Cl-MnO2 hybrid is greater than the specific capacitances of those previously reported (Table S4). For further understanding the electrochemical behavior of asfabricated Co2(OH)3Cl-MnO2, electrochemical impendance spectroscopy measurements were conducted at an open circuit voltage of 0.275 V. The frequency range investigated was from 1 mHz to 100 kHz. Compared with unitary MnO2 or Co2(OH)3Cl, macroporous Co2(OH)3Cl-MnO2 spheres with nanosheet structure facilitate electron transport between active materials and the current collector, which can be verified by the decreasing equivalent series resistance (RESR) in the highfrequency range,52 as shown in Figure 6c. From the data shown in the inset, the RESR of the Co2(OH)3Cl-MnO2 electrode is approximatley 0.57 Ω. The semicircle, because of the redox reaction of the electrode involved in the exchange of OH−, corresponds to the charge-transfer resistance (Rct). The inset also shows a very low Rct of 0.06 Ω. The 45° inclined curve in the middle frequencies, resulting from the frequency response of ion diffusion and transfer in electrolyte, is the Warburg impedance (ZW). The slope of the curve higher than 45° in the low-frequency region demonstrates satisfactory frequency response characteristics of the Co2(OH)3Cl-MnO2 nanocomposite.53 The relationship between current density and specific capacitance of the unitary Co2(OH)3Cl, MnO2, and hybrid spheres is shown in Figure 6d. For comparison, the specific capacitance of the Co2(OH)3Cl-MnO2 electrode is obviously greater than those of the single Co2(OH)3Cl or MnO2 electrodes at the same current density (Figure S7). The specific capacitance is inversely related to the current density resulting from the increase in the IR drop, polarization effect of the electrode, and insufficient contact between electrolyte ions

Figure 5. Comparison of CV curves of pure Co2(OH)3Cl, pure MnO2, and Co2(OH)3Cl-MnO2 hybrid material electrode at 20 mV s−1. 4568


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Figure 6. (a) CV curves of the Co2(OH)3Cl-MnO2 hybrid material electrode at different scan rates. (b) Galvanostatic charge−discharge curves of the Co2(OH)3Cl-MnO2 hybrid material electrode at different current densities. (c) EIS spectra of the Co2(OH)3Cl-MnO2 hybrid material electrode. (d) Specific capacitance vs current increase curves of pure Co2(OH)3Cl, pure MnO2, and Co2(OH)3Cl-MnO2 hybrid material.

and active material under higher current density during the redox reaction.54 On the basis of the obtainable specific capacitance values, the energy densities of the Co2(OH)3ClMnO2 electrode were calculated to be 22.54, 22.4, 14.26, 8.14, 5.6, and 4.41 Wh kg−1 at power densities of 103.76, 210, 406.14, 927.34, 1400, and 1671.16 W kg−1, respectively (Table S2). Electrochemical Properties of Co2(OH)3Cl-MnO2//AC Asymmetric Supercapacitors. For evaluating the possibility of constructing a practical supercapacitor device with the asobtained Co2(OH)3Cl-MnO2 composite, an asymmetric supercapacitor with AC and Co2(OH)3Cl-MnO2 hybrids as the negative and positive electrode materials, respectively, was further constructed. Figure S8a displays the CV curve of the AC electrode at 10−100 mV s−1 within a potential range of −1.0 to 0.0 V. Furthermore, Figure S8b displays the GCD curves of the AC electrode at multifarious current densities ranging from 0.5 to 15 A g−1. The specific capacitance of the AC electrode calculated from the GCD curves is 118.6 F g−1 at 1 A g−1. For the construction of an ASC, the electric quantity of active and negative electrodes should follow the charge balance equation q + = q−

q = CmΔV

Thus, the optimized mass ratio of the two electrodes should be m(AC)/m(Co2(OH)3Cl-MnO2) = 3.31 from the specific capacitance calculated by GCD curves. For this work, the loading masses of electroactive substances are approximately 3.7 mg for Co2(OH)3Cl-MnO2 and 12.3 mg for AC. Figure 7a shows CV curves of the Co2(OH)3Cl-MnO2//AC capacitor at different scan rates. It exhibits the dual property of double-layer capacitance and faradaic pseudocapacitance. The nearly symmetric GCD curves (Figure 7b) with various current densities exhibit typical triangular shapes, testifying to wellbalanced charge storage and good reversibility. Its Cs values can be calculated to be 57.43, 54.07, 52.44, 49.26, 45.45, and 31.77 F g−1 at 0.19, 0.31, 0.5, 1.0, 2.0, and 8.0 A g−1, respectively, as shown in Figure 7c. According to these Cs values, the energy densities of the Co2(OH)3Cl-MnO2//AC can further calculated to be 15.99, 14.87, 14.15, 12.62, 10.42, and 3.23 Wh kg−1 at power densities of 134.50, 218.14, 348.43, 679.10, 1284.66, and 3420 W kg−1, respectively. The Ragone plot related to the corresponding P and E for the Co2(OH)3Cl-MnO2//AC asymmetric supercapacitor is shown in Figure 7d. The result exhibits a deeply improved energy density value at a good power density compared to those of previously reported systems, such as a symmetrical CNT//CNT supercapacitor (6.1 Wh kg−1 at 195 W kg−1),55 MnO2//MnO2 symmetric cell (3.75 Wh kg−1 at 250 W kg−1),56 and other ACS devices, for example, Cu/CuOx@NiCo2O4//AG (12.6 Wh kg−1 at 344 W kg−1),57 Co0.85Se//AC (14.2 Wh kg−1 at 400 W kg−1),53 LiMn2O4//MnFe2O4 (5.5 Wh kg−1 at 1080 W kg−1),58 TiO2− CNT//CNT (4.47 Wh kg−1 at 50 W kg−1),59 and Ni−Co oxide//AC (12 Wh kg−1 at 95 W kg−1).60 Therefore, the

(9) (10)

where C, m, ΔV and are the specific capacitance, load mass, and voltage window of each electrode, respectively. Therefore, the mass balance will follow the relationship m+ /m− = (C −ΔV −)/(C+ΔV+)

(11) 4569


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Figure 7. (a) CV curves of the asymmetric supercapacitor at different scan rates. (b) GCD curves of the asymmetric supercapacitor at different current densities. (c) Specific capacitances of the asymmetric supercapacitor at various current densities. (d) Comparison of energy density vs power density curves of Co2(OH)3Cl-MnO2//AC asymmetric supercapacitors and previously reported devices in the literature.

cycles, comparable to those in the literature.62−64 Moreover, it is noteworthy that the Coulombic efficiency of the ASC can be maintained at almost 100%. The results mentioned above reveal that our asymmetric supercapacitor based on Co2(OH)3Cl-MnO2//AC possesses good electrochemical stability.

considerable energy density obtained from this simple Co2(OH)3Cl-MnO2//AC asymmetric supercapacitor seems important for industrial applications in the area of supercapacitors. For supercapacitors, a long cycling life is a prerequisite requirement. The durability of the Co2(OH)3Cl-MnO2//ACbased ACS device upon charge/discharge cycling within a potential range of 0 to 1.43 V at 2 A g−1 and Coulombic efficiency are illustrated in Figure 8. The capacitance of this device increased in the first 200 cycles because of an electroactivation process of electrodes.61 The asymmetric capacitor remains 83.77% of its initial capacitance after 5000

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CONCLUSIONS In summary, we successfully synthesized 3D macroporous Co2(OH)3Cl-MnO2 spheres via a facile one-step low-temperature solvothermal microwave-assisted synthesis method without any surfactant. Because of the unique architecture of Co2(OH)3Cl-MnO2, the as-obtained hybrid material showed a superior pseudocapacitive behavior with a high specific capacitance of 914.3 F g−1 at 1 A g−1, which was greater than that of pure Co2(OH)3Cl or MnO2. Moreover, an assembled ASC exhibited satisfactory electrochemical properties with a high energy density of 12.62 Wh kg−1 at 679.10 W kg−1. Additionally, this ASC shows outstanding cycling stability along with 83.77% capacitance retention at 2 A g−1 after 5000 cycles. Furthermore, the Coulombic efficiency of the ASC can be maintained at almost 100%. All of these excellent capacitive performances make the Co2(OH)3Cl-MnO2 hybrid a potential candidate for the supercapacitor electrode.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02864.

Figure 8. Cycling performance of the asymmetric supercapacitor at a current density of 2 A g−1. 4570


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Preparation method, SEM and TEM images, CV and GCD curves of MnO2 and Co2(OH)3Cl; SEM images at high magnification, XPS spectra, FT-IR spectrum, ICPMS data of Co2(OH)3Cl-MnO2; nitrogen adsorption− desorption plots of Co2(OH)3Cl-MnO2, Co2(OH)3Cl, and MnO2, CV and GCD curves of active carbon, the Cs, E, and P data calculated by including/excluding IR drop, comparison of the specific capacitance of this work and previous reports, and discussion of the influence of the estimated values to the real ones by neglecting the IR drop (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 596 2528075. Fax: +86 596 2528075. E-mail: [email protected]. ORCID

Shirong Hu: 0000-0002-6952-1125 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (No. 21507054), the Technology Foundation of Fujian Provincial Bureau Quality and Technical Supervision (FJQI2013108), the Science and Technology Program Fujuan Educational Bureau (JA15310), and Zhangzhou City Natural Science Foundation (ZZ2014J02).

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