Self-Templating Synthesis of Cobalt Hexacyanoferrate Hollow

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Energy, Environmental, and Catalysis Applications

Self-Templating Synthesis of Cobalt Hexacyanoferrate Hollow Structures with Superior Performance for Na-Ion Hybrid Supercapacitors Xuemin Yin, Hejun Li, Haiqi Wang, Zhiyong Zhang, Ruimei Yuan, Jinhua Lu, Qiang Song, Jian-Gan Wang, Leilei Zhang, and Qiangang Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08455 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Applied Materials & Interfaces

Self-Templating Synthesis of Cobalt Hexacyanoferrate Hollow Structures with Superior Performance for Na-Ion Hybrid Supercapacitors

Xuemin Yina, Hejun Lia,*, Haiqi Wanga, Zhiyong Zhangb, Ruimei Yuana, Jinhua Lua,*, Qiang Songb, Jian-Gan Wangb,*, Leilei Zhanga, Qiangang Fua a

State Key Laboratory of Solidification Processing, Carbon/Carbon Composites Research Center,

Northwestern Polytechnical University, Xi’an 710072, China b

State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School

of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Lab of Graphene (NPU), Xi’an 710072, China

KEYWORDS: self-templating synthesis; hybrid supercapacitors; cobalt hexacyanoferrate; hollow structures; prussian blue analogues

ACS Paragon Plus Environment

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ABSTRACT

Prussian blue (PB) and its analogues (PBA), especially with hollow-structures, have attracted growing attention from the researchers of energy storage field. Herein, we have developed a facile self-templating method to synthesize hollow-structured cobalt hexacyanoferrate (CoHCF) with controllable morphologies by using water-soluble precursors as templates. The method is versatile and can be extended to synthesize various PB/PBA hollow-structures with tunable composition and morphology. Profiting from the unique hollow structure, the CoHCF hollow prisms manifest exceptional electrochemical performance in Na2SO4 aqueous electrolyte, including a high specific capacitance (284 F g-1 at 1 A g-1), high rate capability and excellent cycling stability (92% retention after 5000 cycles). A hybrid supercapacitor device assembled with the CoHCF hollow prisms and activated carbon (AC) shows a high specific density of 47 Wh kg-1 at a specific power of 1000 W kg-1 and stable cycling performance.

1. INTRODUCTION

In recent years, with the rapid development of economy and the increasing demand of quality life, energy crisis and environmental pollution have become a growing concern all over the world, and have necessitated the development of better electrical energy storage systems.1 Among the energy storage systems, supercapacitors show great advantage over other alternatives, due to their high power density, long cycling life and fast charging/discharging ability.2,3 As is known to all, the electrode materials play a significant role in improving the electrochemical performance of supercapacitors. Accordingly, an enormous amount of effort has been paid to exploiting new electrode materials with proper structures.4-7


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Designing and synthesizing electrode materials with proper structures have been demonstrated to be a reliable method to improve the electrochemical properties.8-10 It is worth to note that hollow structures with tunable composition and morphology have triggered considerable research interest in recent years for the use in different potential application fields, such as energy storage/conversion, biosensors, catalysts and adsorption.11-14 Compared with solid materials, hollow materials have great advantages in energy related fields because of their unique structural characteristics. The reasons are summarized in detail as follows: (1) hollow structures with nanoscale building blocks of nanoparticles and holes could shorten the transfer pathway of ions/electrons; (2) high surface area could provide large electrode/electrolyte contact area for more effective electrochemical active sites; (3) the intrinsic internal void space of the hollowstructured materials can buffer structural changes caused by ion insertion/extraction during the electrochemical processes.14-17 Taking into account the advantages of hollow structures mentioned above, it is highly desirable to broaden the application of hollow structures in energy related areas. Prussian blue (PB), as one of the oldest synthetic compounds with a three-dimensional network of zeolitic feature, was usually used as dark blue pigment in paints, owing to its environmental compatibility, low cost, and chemical stability.18-21 In virtue of high surface area, good electrochemical reversibility and uniform porosity, PB and its analogues (PBA) have drawn increasing attention in the areas of energy devices, electrochemical sensors, catalysts and gas/ion adsorption in the recent years.22-28 Among the above-mentioned applications, the energy storage (e.g., supercapacitor, battery) systems based on PB/PBA have been widely investigated.29-49 Although much work on the application of PB/PBA in energy storage have been published, the electrochemical performance could not meet the requirements. In order to improve the properties


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of PBA, designing unique nanostructures, especially hollow structures of PBA, is an important strategy to take full advantage of nanomaterials to enhance the properties. As far as we know, hollow-structured PB/PBAs with different designed structures for energy storage have barely been reported.24,29 The common established synthesis of hollow PB/PBAs is based on selective etching of PB/PBAs templates, which is limited by the shape and structure of the PBAs itself, thus basically resulting in hollow cubes in shape. In addition, the composition of PBAs after etched is relatively difficult to control. Therefore, much more research effort is still needed to explore new approaches for preparing hollow-structured PB/PBA with tailored composition and architecture. Herein, we develop a facile yet effective self-templating method for rational synthesis of cobalt hexacyanoferrate (CoHCF) hollow structures using water-soluble precursors at room temperature. More notably, the self-templating strategy shows significant versatility in rationally synthesizing the hollow-structured PB/PBA with tunable morphology and composition. Due to the unique architectures, the CoHCF hollow structures are examined as supercapacitor electrode materials, which exhibit enhanced electrochemical properties with high specific capacitance, good rate performance and excellent cycling durability. Moreover, when the CoHCF hollow structures and AC are assembled into a hybrid supercapacitor device, the device could achieve superior electrochemical performance in terms of good cycling stability and enhanced specific energy/power. 2. EXPERIMENTAL SECTION All chemical reagents in this experiment were of analytical grade and used without further purification.

Cobalt

acetate

tetrahydrate

(Co(Ac)2·4H2O),


cobalt

nitrate

hexahydrate

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(Co(NO3)2·6H2O), 2-Methylimidazole (C4H6N2, 98%) and polyvinylpyrrolidone (PVP, MW ~58000) were supplied by Aladdin Industrial Corporation. Glycerol, isopropanol, methanol, ethanol and potassium hexacyanoferrate (K3[Fe(CN)6]) were acquired from Sinopharm Chemical Agent, Co. Ltd.. Deionized water used in this work (18.2 MΩ) was purified by Mili-Q water purification system (TGI Pure Water Systems, USA). 2.1 Synthesis of Co-precursors with different morphologies In this work, the Co-precursors with different morphologies were prepared through the reported method.50-52 The cobalt acetate hydroxide prism precursors were synthesized by a solution reaction, as reported in previous work.50 The ZIF-67 polyhedrons precursors were prepared by a previously reported literature of other research group using Co(NO3)2·6H2O and 2Methylimidazole.51 Co-glycerate spheres precursors were fabricated by a solvothermal method.52 2.2 Synthesis of Cobalt hexacyanoferrate (CoHCF) hollow structures with different morphologies The CoHCF hollow structures were obtained by a simple one-step solution reaction. The Coprecursors with different morphologies (20 mg) were well dispersed in 20 mL of ethanol using ultrasonic treatment, and 20 mg of K3[Fe(CN)6] was dissolved in 20 mL of water. Then, the K3[Fe(CN)6] solution was pulled into the Co-precursors solution without stirring for different time. Finally, the product with tan color was collected by centrifugation, washed with water and ethanol for five times, and dried at room temperature for further use. 2.3 Materials characterization


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The microstructures and morphologies of the samples were investigated using transmission electron microscope (TEM; FEI Tecnai G2 F30) and field-emission scanning electron microscope (FESEM; FEI Nova Nano SEM450), equipped with energy dispersive X-ray spectroscopy (EDX) for determining the component composition. X-ray diffraction (XRD) patterns were performed on Philips X’Pert Pro with Cu Kα radiation to investigate the crystal structure and phase composition. X-ray photoelectron spectroscopy spectra (XPS; Kratos Axis Ultra DLD, Al Kα hυ=1486.6 eV) was applied to detect the surface chemistry of the samples. Fourier-transform infrared (FT-IR) spectra was conducted on iS 50 spectrometer (Thermo Scientific). BET surface area of the products was collected by Micromeritics ASAP 2020 analyzer. 2.4 Electrochemical measurements All electrochemical measurements were carried out on a CHI660D electrochemical workstation with a three-electrode system in 0.5 M Na2SO4 electrolyte. A saturated calomel electrode (SCE) and a Pt foil electrode were used as the reference electrode and the counter electrode, respectively. The working electrodes were prepared by mixing the samples, carbon black and polytetrafluoroethylene (PTFE) with a mass ration of 7:2:1. The slurry was casted on the nickel foam and dried at 80 oC for 12 h. The mass loading of active materials was about ~1.6-2.0 mg cm-2. Cyclic voltammetry (CV) experiments and galvanostatic charge-discharge (GCD) techniques were tested in a potential window from -0.1 to 1.0 V with a potential amplitude of 5 mV. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 10 kHz to 0.01 Hz. The specific capacitance of electrodes was calculated from the discharge GCD curves according to the following equation：


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C=I△t/m△V

(1)

where I, △t, m, △V are the discharge current density, the discharge time, the mass of active materials and the voltage range, respectively. The hybrid supercapacitors were assembled by utilizing CoHCF as the positive electrode and the activated carbon (AC) as the negative electrode. The electrolyte was 0.5 M Na2SO4 aqueous solution. Based on the charge balance equation (q+=q-), the mass balance of positive and negative electrodes can be expressed by the following equation: m+/m-=C-△V-/C+△V+

(2)

where m, C, and △V are mass, the specific capacitance and the voltage range of the positive (+) and negative (-) electrodes, respectively. The specific capacitance (Ccell), specific energy (E) and specific power (P) of hybrid supercapacitors were calculated according to the following Equation: Ccell=I△t/m+-△V

(3)

E=1/2 Ccell△V2

(4)

P=E/△t

(5)

where I, △t, △V, m+- are the discharge current density, the discharge time, the voltage range and the total mass of active materials on both electrodes, respectively.


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3. RESULTS AND DISCUSSION The self-templating synthesis strategy of CoHCF hollow structures with different morphologies is schematically illustrated in Figure 1. Firstly, solid Co-based precursors with different morphologies (e.g., prisms, polyhedrons, spheres) were purposely prepared by solution reaction in organic solvents according to the previously-reported studies.53 The Co-based precursors are prone to be dissolved slowly in the presence of water solvent, thereby giving rise to soluable Co2+ ions at the solid/liquid interface. Accordingly, a subsequent addition of an aqueous solution containing [Fe(CN)6]3- would results in the typical coordination reaction with Co2+ ions, forming a heterogeneous solid CoHCF layer on the surface of precursors. A prolonged reaction time facilitates a gradual evacuation of the precursor core template and a simultaneous growth of a thick CoHCF shell, resulting in the morphology evolution from yolk-shell to hollow structure.


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Figure 1. Schematic illustration of the construction of CoHCF hollow structures with different morphologies.


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Figure 2. (a) FESEM image, (b-d) TEM images, (e) HRTEM image and (f) the corresponding SAED pattern of CoHCF hollow prisms.

In order to better understand the formation mechanism of CoHCF hollow structures, the CoHCF hollow prisms are selected as a typical example for a systematic study. As shown in Figure S1a and b, the prism-like Co-precursors were firstly synthesized by a solution reaction route, the products of which display a high-aspect-ratio, smooth surface and solid structure. No peaks of impurities were detected in the X-ray diffraction (XRD) pattern of the Co-precursors (Figure S1c), in which all peaks can be assigned to tetragonal cobalt acetate hydroxide phase, indicating a high purity. The successful preparation of cobalt acetate hydroxide precursors can also be confirmed by Fourier transform infrared spectroscopy (FT-IR) result (Figure S1d). After


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the addition of an aqueous [Fe(CN)6]3- solution, the prism-like Co-precursors were then served as sacrificial templates to react with [Fe(CN)6]3-. Figure S2 shows the digital photographs of the solution during the reaction process. It is observed that the solution color becomes dark brown after a short time of 10 min and maintains upon an extended reaction time. The brown products were collected and firstly characterized by field-emission scanning electron microscopy (FESEM). It can be easily seen from Figure 2a that the CoHCF inherits the parent precursor prism morphology with rough surfaces and distinct hollow interiors. The hollow structures are confirmed by the transmission electron microscopy (TEM) image (Figure 2b), and the assynthesized hollow prisms show a length of 2.5 μm and a width of 500 nm. The magnified TEM images (Figure 2c, d) show that the shell of hollow prism, with a uniform thickness of 100 nm, is composed of numerous nanoparticles. The high resolution TEM (HRTEM) image (Figure 2e) of hollow prisms shows an interplanar space of 0.209 and 0.231 nm, corresponding to the (422) and (420) planes of the cubic CoHCF, respectively. Furthermore, there are some nanoholes (diameter < 3 nm) observed in the HRTEM image, which could enlarge the specific surface area. The formation of CoHCF phase can be further confirmed by the selected area electron diffraction (SEAD) pattern (Figure 2f). The crystalline structure of the hollow prisms was determined by XRD. From the XRD results (Figure 3a), the main diffraction peaks at 17.5o, 25o, 35.5o are detected, which can be indexed to the (200), (220) and (400) planes of the face-centered cubic CoHCF (JCPDS No. 460907), respectively. In addition, the absence of the precursor peaks suggests the complete dissolution of Co-precursor yolks. The FT-IR analysis was carried out to validate the formation of the CoHCF phase. The peaks at 2126 and 594 cm-1 in the FT-IR spectrum (Figure 3b) are characteristic of the PBA materials, which can be attributed to the stretching vibrations and the


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bending modes of Co-C≡N-Fe, respectively.24 Furthermore, the energy dispersive X-ray spectroscopy (EDX) spectrum (Figure S3) and the X-ray photoelectron spectroscopy (XPS) survey scan (Figure 3c) manifest the presence of Co, Fe, C, and N elements. Additionally, the peaks at 720.8 and 708.1 eV in the high-resolution Fe 2p spectrum (Figure 3d) can be assigned to the Fe 2p1/2 and Fe 2p3/2 in [Fe(CN)6]3+/[Fe(CN)6]4+, respectively. Additionally, the highresolution spectrum of Co 2p (Figure 4d) illustrates two peaks at 781.9 eV and 797.5 eV, corresponding to Co 2p3/2 and Co 2p1/2, respectively. As determined by N2 sorption measurement (Figure 3e, f), the CoHCF hollow prisms show a Brunauer-Emmett-Teller (BET) specific surface areas of 166.18 m2 g-1 with a narrow pore size distribution at 3.66 nm, which shows a great potential in the area of electrochemical energy storage. Based on the above analysis and discussions, it is found that two important factors, i.e., the consumption rate of Co-based precursors and the formation rate of CoHCF layer, could affect the reaction result during the experiment process. Hence, the morphologies of prism-like products are investigated in detail by controlling the experimental parameters, such as the reactant concentration, temperature, solvent, and reaction time. Concentration of [Fe(CN)6]3- is an important factor affecting the above two processes. As displayed in Figure S4, hollow prisms can not be obtained at an extremely low concentration of [Fe(CN)6]3-, which may presumably be ascribed to the insufficient formation of solid CoHCF on the precursor surface that results in the collapse of hollow structures. Hollow structures are yielded at a proper concentration level. However, increasing to a higher concentration would lead to the destruction of hollow structure. As the concentration of K3[Fe(CN)6] increases, the pH value of solution drops, which could accelerate the dissolution of precursors template at low pH value and there is no time for formation of CoHCF shell. In addition, a higher reaction temperature (e.g., 60 and 100 oC) is


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unfavorable for the formation of hollow structures (Figure S5), which may destroy the balance of the reaction. Furthermore, the precursors solution using other organic solutions as solvents, such as methanol and isopropyl alcohol, can be suitable for preparation of CoHCF hollow prisms (Figure S6). To demonstrate the versatility of our self-templating strategy, Co-precursors with different morphologies are employed for the synthesis of hollow-structured CoHCF. In addition to the prisms mentioned above, Co-glycerate solid spheres (Figure S7) and ZIF-67 solid polyhedrons (Figure S8) with smooth surface and uniform size have been successfully synthesized. When the [Fe(CN)6]3--containing aqueous solution is mixed with the Co-precursors (Figure S9, S10), the coordination reaction occurs instantly. As shown in Figures S11 and S12, the hollow-structured CoHCF with sphere and polyhedron shapes were also successfully obtained. Figure 4 presents the typical morphology evolution of hollow-structured CoHCF with different morphologies during the formation process. At the initial stage, a solid thin CoHCF layer is formed surrounding the parent matrix, yielding a yolk-shell structure. The CoHCF layer is growing via the consumption of the Co-precursor cores, and finally, hollow structures are generated. It is important to note that the hollow structures perfectly inherit their respective parent morphology of prism/sphere/polyhedron, revealing the present strategy is effective for fabricating complex hollow morphology. Moreover, our self-templating strategy could be further extended to fabricate other types of PB/PBA (My[M’(CN)6]z, M=Ni, Co, Mn; M’=Ni, Co, Fe) materials. For example, hollow-structured cobalt hexacyanocobaltate (CoHCC, Figure S13) can be prepared in a similar growth manner by simply using [Co(CN)6]3- species instead of [Fe(CN)6]3-. The encouraging results demonstrate the versatility of the self-templating strategy for the synthesis of hollow-structured PB/PBA materials with tailored composition and unique interior architecture.


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Figure 3. (a) XRD pattern, (b) FT-IR spectrum, (c) XPS survey spectrum, (d) high-resolution XPS spectrum of Fe 2p and Co 2p, (e) BET nitrogen adsorption and desorption isotherms and (f) pore size distribution of CoHCF hollow prisms.


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Figure 4. TEM images of (a-c) CoHCF hollow prisms, (d-f) hollow spheres and (g-i) hollow polyhedrons at different reaction time: (a, d, g) 0 min; (b) 1 min; (c) 10 min; (e, h) 1 h; (f, i) 6 h.


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Figure 5. (a) CV plots of the CoHCF hollow prisms with different scan rates. (b) GCD curves at different specific currents. (c) The specific capacitance of discharge process as a function of specific current (d) Cycling performance at 10 A g-1 specific current.

The electrochemical properties of the as-prepared CoHCF hollow structures were further investigated as electrode materials for supercapacitor by using a three-electrode cell in 0.5 M Na2SO4 solution. Figure 5a displays the CV curves of the CoHCF hollow prisms at different scan rates ranging from 2 to 50 mV s-1, which shows typical pseudocapacitive behavior. One pair of redox peaks are detected at a voltage of 0.4 V, which corresponds to the Faradaic redox reaction of Fe2+/3+ in CoHCF, resulting from the intercalation/deintercalation of Na+ ions into/out the


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metallic framework.44, 54 To better understand the Na+-insertion reaction mechanism in CoHCF, the Ex-situ XPS analysis was applied in Figure S14. When in charge state, the peaks of Fe 2p1/2 and 2p3/2 are shifted to higher binding energy and no valance change for Co ions is detected, confirming the charge storage mechanism is Faradaic redox reaction of Fe2+/3+ in CoHCF during the intercalation/deintercalation of Na+ ions.54 Additionally, the peak current increased with the increase of scan rate and the shape of CV curves were also well maintained, indicating that the electrochemical reactions showed good reversibility and fast reaction rate. Generally, the relationship between the CV peak current density (Ip) and scan rate (v) is related to that the redox reaction is diffusion-controlled or surface-controlled, which is in consistent with the formula (Ip=a vb). Whereas a b-value of 0.5 would indicate that the current is controlled by semi-infinite linear diffusion, while a value of 1 indicates that the current is surface-controlled.55, 56 As shown in the Figure S15, the b value is calculated from the slop of linear plot of log (i) versus log (v) at different potentials, and are all above 0.8 (close to 1), indicating that the electrochemical process in this experiment predominantly is a surface-controlled process. The GCD curves at various specific currents from 1 to 20 A g-1 were exhibited in Figure 5b. The slopped voltage plateaus at around 0.3-0.5 V are corresponding to the Faradaic redox reaction observed in the CV plots, confirming the pseudocapacitive behavior. Additionally, the curves are well symmetrical between charge and discharge parts, verifying the good reversibility of electrode. Figure 5c plots the specific capacitance as a function of specific current. Notably, the CoHCF hollow prism electrode shows high specific capacitances of 284, 258.6, 235, 213 and 181 F g-1 (corresponding to a specific capacity of 78.9, 71.8, 65.3, 59.2 and 50.3 mA h g-1) at 1, 2, 5, 10, and 20 A g-1 specific current, respectively. The high capacitance retention of 63.7% from 1 to 20 A g-1 indicates good rate capability of the electrode. The cycling stability is an important factor that


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should be considered for practical application, which was evaluated by repeated GCD test at 10 A g-1 specific current. As displayed in Figure 5d, the CoHCF electrode manifests excellent cycling stability with 92% of its initial capacitance even after 5000 cycles. The CV curves of the CoHCF electrode before and after cycles are almost overlapped (Figure S16). After the long charging-discharging cycles, the hollow structure of CoHCF can still maintain the electrode integrity (Figure S17). Compared with the hollow spheres and polyhedrons (Figures S18 and S19), the CoHCF hollow prisms show higher specific capacitance, superior rate capability and better cycle life. Electrochemical impedance spectrum (EIS) results of CoHCF electrodes are shown in Figure S20. It can be observed that the CoHCF hollow prisms electrode manifests a lower charge transfer resistance than other electrodes, suggesting a faster electron transfer rate to achieving better electrochemical properties. The outstanding electrochemical performance of CoHCF hollow structures in this work is also superior or at least comparable to most of the previously-reported CoHCF-based materials (Table S1). To further evaluate the electrochemical performance of CoHCF hollow prisms in a twoelectrode full cell, a hybrid supercapacitor was assembled using the CoHCF hollow prism as the positive electrode and AC as the negative electrode in 0.5 M Na2SO4 solution. From the CV plots of the two electrodes at 50 mV s-1 (Figure 6a), it is noted that the potential window of the AC electrode (-1.0~0 V) could match well with that of the CoHCF electrode (-0.1~1.0 V), which indicated that the operating voltage window could be expected to extend up to 2.0 V. CV curves of the hybrid supercapacitor with different operating potential windows at 20mV s-1 are shown in Figure S20. It can be easily observed that no obvious polarization was found in the voltage window of 0~2.0V, testifying that 2.0 V is a reasonable voltage window in this work. Figure 6b shows the CV curves of the hybrid device at different scan rates with the potential range of 0~2.0


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V. The shape of CV curves could be well maintained at a high scan rate of 100 mV s-1, indicating good rate capability. In addition, the CV curves show both pseudocapacitive and electrical double-layer capacitive characteristics. GCD curves (Figure 6c) exhibit good symmetry between charge and discharge parts, which verifies the good reversibility of electrode. Figure 6d exhibits the discharge specific capacitance of the CoHCF//AC device. The device can achieve a high specific capacitance of 81 F g-1 at 1 A g-1 specific current. Additionally, the capacitance could be retained in about 68% at 20 A g-1 specific current, indicating outstanding rate performance. The cycling performance of the device evaluated by GCD measurements at 5 A g-1 specific current was shown in Figure 6e. The hybrid device shows outstanding cycling performance with a capacitance loss of 7.55% after 3000 cycles. To better show the superior performance of CoHCF in the practical application, a hybrid supercapacitor device could light a red light-emitting-diode (LED) (inset in Figure 6e), indicating the practical feasibility. Figure 6f shows the Ragone plots of CoHCF//AC hybrid device and some previously reported PB based hybrid systems. Impressively, the hybrid device in this report delivers a high specific energy of 47 Wh kg-1 at a specific power of 1000 W kg-1. Moreover, the device can retain 30.56 Wh Kg-1 even at a high specific power of 20000 W kg-1. The CoHCF//AC hybrid supercapacitor in this work exhibits better electrochemical properties than many reported PB/PBA-based hybrid systems, such as CoHCF//mRGO,44 CoHCF/rGO//AC,57 MnHCF//Fe3O4/rGO,43 CuHCF/CFs//[email protected]


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Figure 6. (a) CV curves of the CoHCF and AC electrodes at 50 mV s-1 in the Na2SO4 electrolyte. (b) CV curves of the CoHCF//AC hybrid devices at different scan rates. (c) GCD curves at different specific currents. (d) The corresponding specific discharge capacitance at different specific currents. (e) Cycling stability of CoHCF//AC hybrid devices (Inset: the digital image of a hybrid cell powering a red LED) and (f) Ragone plots of the CoHCF//AC hybrid devices and


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other PB-based hybrid cells.

The CoHCF hollow prisms exhibit improved electrochemical properties, which benefit from their unique hierarchically porous hollow structure. Firstly, the porous hollow structure can not only produce more ion migration channels to promote the electrolyte penetration/diffusion, but also shorten the ionic/electronic transport pathways for enhanced reaction kinetics. These are why CoHCF hollow prisms can exhibit high specific capacitance and good rate property. Additionally, the stable hollow structure, which can buffer the structural strain and alleviate volume change caused by ion insertion/extraction, is important for long-term cycling stability of supercapacitors.

CONCLUSIONS In summary, different morphologies of CoHCF hollow structures were synthesized by a selftemplating method using water-soluble precursors as templates. This synthesis strategy was versatile and feasible in preparing PBA-based hollow materials with different morphologies and compositions using water-soluble precursors. Benefiting from the unique hollow structure, the CoHCF hollow structures, especially CoHCF hollow prisms, exhibit excellent electrochemical performance in neutral electrolyte. CoHCF hollow prisms deliver a high specific capacitance of 284 F g-1 at 1 A g-1 with good rate capability (181 F g-1 at 20 A g-1) and outstanding cycling performance (8% loss after 5000 cycles). When assembling the CoHCF and AC into hybrid supercapacitors, the devices show high specific energy and power and long cycling life.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. SEM images, TEM images, XRD patterns, EDX spectrums and FT-IR spectrums of the cobaltbased precursors and CoHCF products; SEM images of CoHCF product with different reaction temperatures/solvents/reaction time; digital photos of the reaction process; TEM images of CoHCC; electrochemical performances of CoHCF hollow polyhedrons and hollow spheres; CV curve and SEM images of CoHCF hollow prisms after cycling; comparison of electrochemical performance of CoHCF based electrodes.

AUTHOR INFORMATION Corresponding Author *E-mail for H.J.L.: [email protected] *E-mail for J.H.L.: [email protected] *E-mail for J.-G.W.: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT The authors acknowledge the financial supports of this work by the National Natural Science Foundation of China (51432008, 51521061), the Natural Science Foundation of Shaanxi Province (2018JM5044), the Program of Introducing Talents of Discipline to Universities (B08040), and the Research Fund of the State Key Laboratory of Solidification Processing (142TZ-2016).

REFERENCES [1] Dunn, B.; Kamath, H.; Tarascon, J. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928-935. [2] Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845854. [3] Simon, P.; Gogotsi, Y.; Dunn, B. Where Do Batteries End and Supercapacitors Begin? Science 2014, 343, 1210-1211. [4] Jiang, J.; Li, Y.; Liu, J.; Huang, X.; Yuan, C.; Lou, X. W. D. Recent Advances in Metal Oxide-Based Electrode Architecture Design for Electrochemical Energy Storage. Adv. Mater. 2012, 24, 5166-5180. [5] Wang, M.; Zhao, Y. P.; Zhang, X. J.; Qi, R. J.; Shi, S. S.; Li, Z. P.; Wang, Q. J.; Zhao, Y. F. Interface-Rich Core-Shell Ammonium Nickel Cobalt Phosphate for High-Performance Aqueous hybrid Energy Storage Device Without a Depressed Power Density. Electrochim. Acta 2018, 272, 184-191.


23


Page 24 of 31

[6] Wang, M.; Jin, F. D.; Zhang, X. J.; Wang, J.; Huang, S. F.; Zhang, X. Y.; Mu, S. C.; Zhao, Y, P.; Zhao, Y. F. Multihierarchical Structure of Hybridized Phosphates Anchored on Reduced Graphene Oxide for High Power Hybrid Energy Storage Devices. . ACS Sustainable Chem. Eng. 2017, 5, 5679-5685. [7] Zhao, Y. F.; Ma, H. G.; Huang, S. F.; Zhang, X. J.; Xia, M. R.; Tang, Y. F.; Ma, Z. F. Monolayer Nickel Cobalt Hydroxyl Carbonate for High Performance All-Solid-State Asymmetric Supercapacitors. ACS Appl. Mater. Inter. 2016, 8, 22997-23005. [8] Wang, X.; Feng, J.; Bai, Y.; Zhang, Q.; Yin, Y. Synthesis, Properties, and Applications of Hollow Micro-/Nanostructures. Chem. Rev. 2016, 116, 10983-11060. [9] Lai, X.; Halpert, J. E.; Wang, D. Recent Advances in Micro-/Nano-Structured Hollow Spheres for Energy Applications: From Simple to Complex Systems. Energ. Environ. Sci. 2012, 5, 5604-5618. [10] Qi, J.; Lai, X.; Wang, J.; Tang, H.; Ren, H.; Yang, Y.; Jin, Q.; Zhang, L.; Yu, R.; Ma, G.; Su, Z.; Zhao, H.; Wang, D. Multi-Shelled Hollow Micro-/Nanostructures. Chem. Soc. Rev. 2015, 44, 6749-6773. [11] Yu, L.; Wu, H. B.; Lou, X. W. D. Self-Templated Formation of Hollow Structures for Electrochemical Energy Applications. Acc. Chem. Res. 2017, 50, 293-301. [12] Yu, L.; Hu, H.; Wu, H. B.; Lou, X. W. D. Complex Hollow Nanostructures: Synthesis and Energy-Related Applications. Adv. Mater. 2017, 29, 1604563. [13] Yu, X.; Yu, L.; Lou, X. W. D. Metal Sulfide Hollow Nanostructures for Electrochemical Energy Storage. Adv. Energy Mater. 2016, 6, 1501333. [14] Lou, X. W. D.; Archer, L. A.; Yang, Z. Hollow Micro-/Nanostructures: Synthesis and Applications. Adv. Mater. 2008, 20, 3987-4019.


24

Page 25 of 31 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


[15] Hong, Y. J.; Son, M. Y.; Kang, Y. C. One-Pot Facile Synthesis of Double-Shelled SnO2 Yolk-Shell-Structured Powders by Continuous Process as Anode Materials for Li-Ion Batteries. Adv. Mater. 2013, 25, 2279-2283. [16] Peng, S.; Li, L.; Hu, Y.; Srinivasan, M.; Cheng, F.; Chen, J.; Ramakrishna, S. Fabrication of Spinel One-Dimensional Architectures by Single-Spinneret Electrospinning for Energy Storage Applications. ACS Nano 2015, 9, 1945-1954. [17] Liu, J.; Qiao, S. Z.; Budi Hartono, S.; Lu, G. Q. M. Monodisperse Yolk-Shell Nanoparticles with a Hierarchical Porous Structure for Delivery Vehicles and Nanoreactors. Angew. Chem. 2010, 122, 5101-5105. [18] Kong, B.; Selomulya, C.; Zheng, G.; Zhao, D. New Faces of Porous Prussian Blue: Interfacial Assembly of Integrated Hetero-Structures for Sensing Applications. Chem. Soc. Rev. 2015, 44, 7997-8018. [19] Hostert, L.; Alvarenga, G. D.; Marchesi, L. F.; Soares, A. L.; Vidotti, M. One-Pot Sonoelectrodeposition of Poly(Pyrrole)/Prussian Blue Nanocomposites: Effects of the Ultrasound Amplitude in the Electrode Interface and Electrocatalytical Properties. Electrochim. Acta 2016, 213, 822-830. [21] Zhang, L.; Wu, H. B.; Madhavi, S.; Hng, H. H.; Lou, X. W. D. Formation of Fe2O3 Microboxes with Hierarchical Shell Structures From Metal-Organic Frameworks and their Lithium Storage Properties. J. Am. Chem. Soc. 2012, 134, 17388-17391. [22] Hu, M.; Belik, A. A.; Imura, M.; Mibu, K.; Tsujimoto, Y.; Yamauchi, Y. Synthesis of Superparamagnetic Nanoporous Iron Oxide Particles with Hollow Interiors by Using Prussian Blue Coordination Polymers. Chem. Mater. 2012, 24, 2698-2707.


25


Page 26 of 31

[23] Nossol, E.; Zarbin, A. J. G. A Simple and Innovative Route to Prepare a Novel Carbon Nanotube/Prussian Blue Electrode and its Utilization as a Highly Sensitive H2O2 Amperometric Sensor. Adv. Funct. Mater. 2009, 19, 3980-3986. [24] Zhang, M.; Hou, C.; Halder, A.; Ulstrup, J.; Chi, Q. Interlocked Graphene-Prussian Blue Hybrid Composites Enable Multifunctional Electrochemical Applications. Biosens. Bioelectron. 2017, 89, 570-577. [25] Wang, J.; Zhang, Z.; Zhang, X.; Yin, X.; Li, X.; Liu, X.; Kang, F.; Wei, B. Cation Exchange Formation of Prussian Blue Analogue Submicroboxes for High-Performance Na-Ion Hybrid Supercapacitors. Nano Energy 2017, 39, 647-653. [26] Xu, Y.; Zheng, S.; Tang, H.; Guo, X.; Xue, H.; Pang, H. Prussian Blue and its Derivatives as Electrode Materials for Electrochemical Energy Storage. Energy Storage Materials 2017, 9, 11-30. [27] Han, L.; Yu, X.; Lou, X. W. D. Formation of Prussian-Blue-Analog Nanocages Via a Direct Etching Method and their Conversion Into Ni-Co-Mixed Oxide for Enhanced Oxygen Evolution. Adv. Mater. 2016, 28, 4601-4605. [28] Vázquez-González, M.; Torrente-Rodríguez, R. M.; Kozell, A.; Liao, W.; Cecconello, A.; Campuzano, S.; Pingarrón, J. M.; Willner, I. Mimicking Peroxidase Activities with Prussian Blue Nanoparticles and their Cyanometalate Structural Analogues. Nano Lett. 2017, 17, 49584963. [29] Bu, F.; Hu, M.; Zhang, W.; Meng, Q.; Xu, L.; Jiang, D.; Jiang, J. Three-Dimensional Hierarchical Prussian Blue Composed of Ultrathin Nanosheets: Enhanced Hetero-Catalytic and Adsorption Properties. Chem. Commun. 2015, 51, 17568-17571.


26

Page 27 of 31 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


[30] Zhang, W.; Zhao, Y.; Malgras, V.; Ji, Q.; Jiang, D.; Qi, R.; Ariga, K.; Yamauchi, Y.; Liu, J.; Jiang, J.; Hu, M. Synthesis of Monocrystalline Nanoframes of Prussian Blue Analogues by Controlled Preferential Etching. Angew. Chem. 2016, 128, 8368-8374. [31] Lee, H.; Wang, R. Y.; Pasta, M.; Woo Lee, S.; Liu, N.; Cui, Y. Manganese Hexacyanomanganate Open Framework as a High-Capacity Positive Electrode Material for Sodium-Ion Batteries. Nat. Commun. 2014, 5, 5280. [32] Jiang, Y.; Yu, S.; Wang, B.; Li, Y.; Sun, W.; Lu, Y.; Yan, M.; Song, B.; Dou, S. Prussian Blue@C Composite as an Ultrahigh-Rate and Long-Life Sodium-Ion Battery Cathode. Adv. Funct. Mater. 2016, 26, 5315-5321. [33] Pasta, M.; Wessells, C. D.; Liu, N.; Nelson, J.; McDowell, M. T.; Huggins, R. A.; Toney, M. F.; Cui, Y. Full Open-Framework Batteries for Stationary Energy Storage. Nat. Commun. 2014, 5, 3007. [34] Su, D.; McDonagh, A.; Qiao, S.; Wang, G., High-Capacity Aqueous Potassium-Ion Batteries for Large-Scale Energy Storage. Adv. Mater. 2017, 29, 1604007. [35] Wang, L.; Lu, Y.; Liu, J.; Xu, M.; Cheng, J.; Zhang, D.; Goodenough, J. B. A Superior Low-Cost Cathode for a Na-Ion Battery. Angew. Chem. Int. Ed. 2013, 52, 1964-1967. [36] Wang, R. Y.; Wessells, C. D.; Huggins, R. A.; Cui, Y. Highly Reversible Open Framework Nanoscale Electrodes for Divalent Ion Batteries. Nano Lett. 2013, 13, 5748-5752. [33] You, Y.; Wu, X.; Yin, Y.; Guo, Y. High-Quality Prussian Blue Crystals as Superior Cathode Materials for Room-Temperature Sodium-Ion Batteries. Energ. Environ. Sci. 2014, 7, 1643-1647.


27


Page 28 of 31

[37] You, Y.; Yao, H.; Xin, S.; Yin, Y.; Zuo, T.; Yang, C.; Guo, Y.; Cui, Y.; Wan, L.; Goodenough, J. B. Subzero-Temperature Cathode for a Sodium-Ion Battery. Adv. Mater. 2016, 28, 7243-7248. [38] Wessells, C. D.; Huggins, R. A.; Cui, Y. Copper Hexacyanoferrate Battery Electrodes with Long Cycle Life and High Power. Nat. Commun. 2011, 2, 550. [39] Wessells, C. D.; Peddada, S. V.; Huggins, R. A.; Cui, Y. Nickel Hexacyanoferrate Nanoparticle Electrodes for Aqueous Sodium and Potassium Ion Batteries. Nano Lett. 2011, 11, 5421-5425. [40] Wessells, C. D.; McDowell, M. T.; Peddada, S. V.; Pasta, M.; Huggins, R. A.; Cui, Y. Tunable Reaction Potentials in Open Framework Nanoparticle Battery Electrodes for Grid-Scale Energy Storage. ACS Nano 2012, 6, 1688-1694. [41] Yue, Y.; Binder, A. J.; Guo, B.; Zhang, Z.; Qiao, Z.; Tian, C.; Dai, S. Mesoporous Prussian Blue Analogues: Template-Free Synthesis and Sodium-Ion Battery Applications. Angew. Chem. 2014, 126, 3198-3201. [42] Xue, L.; Li, Y.; Gao, H.; Zhou, W.; Lü, X.; Kaveevivitchai, W.; Manthiram, A.; Goodenough, J. B. Low-Cost High-Energy Potassium Cathode. J. Am. Chem. Soc. 2017, 139, 2164-2167. [43] Lu, K.; Li, D.; Gao, X.; Dai, H.; Wang, N.; Ma, H. An Advanced Aqueous Sodium-Ion Supercapacitor with a Manganous Hexacyanoferrate Cathode and a Fe3O4/RGO Anode. J. Mater. Chem. A 2015, 3, 16013-16019. [44] Lu, K.; Song, B.; Gao, X.; Dai, H.; Zhang, J.; Ma, H. High-Energy Cobalt Hexacyanoferrate and Carbon Micro-Spheres Aqueous Sodium-Ion Capacitors. J. Power Sources 2016, 303, 347353.


28

Page 29 of 31 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


[45] Wang, Y.; Chen, Q. Dual-Layer-Structured Nickel Hexacyanoferrate/MnO2 Composite as a High-Energy Supercapacitive Material Based On the Complementarity and Interlayer Concentration Enhancement Effect. ACS Appl. Mater. Inter. 2014, 6, 6196-6201. [46] Pang, H.; Zhang, Y.; Cheng, T.; Lai, W.; Huang, W. Uniform Manganese Hexacyanoferrate Hydrate Nanocubes Featuring Superior Performance for Low-Cost Supercapacitors and Nonenzymatic Electrochemical Sensors. Nanoscale 2015, 7, 16012-16019. [47] Li, Z.; Xiang, K.; Xing, W.; Carter, W. C.; Chiang, Y. Reversible Aluminum-Ion Intercalation in Prussian Blue Analogs and Demonstration of a High-Power Aluminum-Ion Asymmetric Capacitor. Adv. Energy Mater. 2015, 5, 1401410. [48] Huang, Y.; Xie, M.; Zhang, J.; Wang, Z.; Jiang, Y.; Xiao, G.; Li, S.; Li, L.; Wu, F.; Chen, R. A Novel Border-Rich Prussian Blue Synthetized by Inhibitor Control as Cathode for Sodium Ion Batteries. Nano Energy 2017, 39, 273-283. [49] Ren, W.; Qin, M.; Zhu, Z.; Yan, M.; Li, Q.; Zhang, L.; Liu, D.; Mai, L. Activation of Sodium Storage Sites in Prussian Blue Analogues Via Surface Etching. Nano Lett. 2017, 17, 4713-4718. [50] Yu, L.; Zhang, L.; Wu, H. B.; Lou, X. W. D. Formation of NixCo3-xS4 Hollow Nanoprisms with Enhanced Pseudocapacitive Properties. Angew. Chem. 2014, 126, 3785-3788. [51] Hu, H.; Han, L.; Yu, M.; Wang, Z.; Lou, X. W. D. Metal-Organic-Framework-Engaged Formation of Co Nanoparticle-Embedded Carbon@Co9S8 Double-Shelled Nanocages for Efficient Oxygen Reduction. Energ. Environ. Sci. 2016, 9, 107-111.


29


Page 30 of 31

[52] Shen, L.; Yu, L.; Yu, X. Y.; Zhang, X.; Lou, X. W. D. Self-Templated Formation of Uniform NiCo2O4 Hollow Spheres with Complex Interior Structures for Lithium-Ion Batteries and Supercapacitors. Angew. Chem. Int. Ed. 2015, 54, 1868-1872. [53] Yu, L.; Xia, B. Y.; Wang, X.; Lou, X. W. D. General Formation of M-MoS3 (M = Co, Ni) Hollow Structures with Enhanced Electrocatalytic Activity for Hydrogen Evolution. Adv. Mater. 2016, 28, 92-97. [54] Lu, K.; Song, B.; Li, K.; Zhang, J. T.; Ma, H. Y. Cobalt Hexacyanoferrate Nanoparticles and MoO3 Thin Films Grown on Carbon Fiber Cloth for Efficient Flexible Hybrid Supercapacitor. J. Power Sources 2017, 370, 98-105. [55] Augustyn, V.; Come, J.; Lowe, M. A.; Kim, J. W.; Taberna, P.; Tolbert, S. H.; Abruña, H. D.; Simon, P.; Dunn, B. High-Rate Electrochemical Energy Storage through Li+ Intercalation Pseudocapacitance. Nat. Mater. 2013, 12, 518-522. [56] Augustyn, V.; Simon, P.; Dunn, B. Pseudocapacitive Oxide Materials for High-Rate Electrochemical Energy Storage. Energ. Environ. Sci. 2014, 7, 1597-1614. [57]

Wang,

J.;

Zhang,

Z.;

Liu,

X.;

Wei,

B.

Facile

Synthesis

of

Cobalt

Hexacyanoferrate/Graphene Nanocomposites for High-Performance Supercapacitor. Electrochim. Acta 2017, 235, 114-121. [58] Senthilkumar, S. T.; Kim, J.; Wang, Y.; Huang, H.; Kim, Y. Flexible and Wearable Fiber Shaped High Voltage Supercapacitors Based On Copper Hexacyanoferrate and Porous Carbon Coated Carbon Fiber Electrodes. J. Mater. Chem. A 2016, 4, 4934-4940.


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Self-Templating Synthesis of Cobalt Hexacyanoferrate Hollow

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