Controlled Synthesis of Hollow Co–Mo Mixed Oxide Nanostructures

Mar 22, 2016 - Xun Xu , Hanfeng Liang , Fangwang Ming , Zhengbing Qi , Yaqiang Xie ... Xiangjun Lu , An Xie , Yong Zhang , Haichang Zhong , Xuecheng X...
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Controlled Synthesis of Hollow Co–Mo Mixed Oxide Nanostructures and Their Electrocatalytic and Lithium Storage Properties Yong Yang, Shitong Wang, Caihua Jiang, Qichen Lu, Zilong Tang, and Xun Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00761 • Publication Date (Web): 22 Mar 2016 Downloaded from http://pubs.acs.org on March 22, 2016

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Controlled synthesis of Hollow Co–Mo Mixed Oxide Nanostructures and Their Electrocatalytic and Lithium Storage Properties Yong Yang, Shitong Wang, Caihua Jiang, Qichen Lu, Zilong Tang, Xun Wang* Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing, 100084, China State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China ABSTRACT: Hollow mixed metal oxides have received much attention owing to their great performance for wide potential

applications. Here, we have successfully prepared hollow Co–Mo mixed oxide nanostructures with controlled structure and compositions, including hollow Co3O4/CoMoO4 heterostructures and ball-in-ball CoMoO4 nanospheres. Uniform CoMohybrid precursors are prepared through one-pot solvothermal method and then transformed into hollow structures after thermal treatment. Importantly, this strategy can be used to prepare other ternary Mo-based oxides. In view of the unique heterostructure with hollow structure, the Co3O4/CoMoO4 heterostructures exhibit much better electrocatalytic activity for the oxygen evolution reaction than CoMoO4. Moreover, the as-synthesized carbon-coated Co3O4/CoMoO4 nanospheres show excellent lithium storage properties. Our result described here provides a method to fabricate other mixed metal oxides with complicated structure.

Introduction Hollow structures with tunable composition have become a hotspot in nanosceince because of their distinctive chemical features in biosensing, catalysis and electrochemical energy storage.[1-5] Until now, a lot of methods have been used to prepare well-defined hollow architectures, such as hollow spheres, boxes, tubes and polyhedrals. The synthetic strategies usually involve template route,[6-7] galvanic replacement,[1,8] Kirkendall effect,[9-10] and Ostwald ripening process.[11-12] Very recently, metal–organic frameworks (MOF) and coordination polymers have been regarded as a promising precursor for the preparation of hollow structures after thermal decomposition.[13-17] During the annealing process, recrystallization and phase change would take place. These precursors will transform into a hollow structure with high porosity. For example, Yan have synthesized porous NiFe2O4 hollow structures using Prussian blue analogues.[18] Moonhyun developed a cation exchange method to prepare multiple yolk-shell hybrid metal oxides with controllable composition.[19] Even though the great progresses have been achieved, the fabrication of controllable novel mixed metal oxides with well-defined hollow micro/nano-architectures remains a big challenge. Mixed metal oxides are found to be very promising electrode materials for electrocatalysts and energy conversion and storage.[20-21] Among of them, Co–Mo based mixed oxides, especially the cobalt molybdates (CoMoO4), have been widely used in supercapacitors and lithium ion batteries (LIBs).[22-26] However, its low electronic conductivity and large volume changes usually hamper their applications. It was reported that

their poor cycle performance could potentially be relieved or solved by constructing heterostructure [27-28]. Up to now, a variety of composites with different nanoparticles have been synthesized and showed enhanced Li-storage performance. [2931] For example, Chen reported the synthesis of MoO2/Mo2C heteronanotubes. The combination of MoO2 and Mo2C can provide excellent cycling stability.[32] Compared to the corresponding monometal oxides, hetero-structures exhibit enhanced electrical/ionic conductivity and mechanical stability because of the specific synergy. In virtue of these advantages, extensive efforts have been made to design and fabricate various hetero-structures. [33-35] Lou have synthesized Co3O4/NiCo2O4 double-shelled nanocages and found that such unique multicompositional structure exhibited superior performance in pseudocapacitors and electrocatalysts for the oxygen evolution reaction.[36] Huang reported the synthesis of porous ZnO/ZnFe2O4/C octahedral using a MOF as selfsacrificing template, which shows excellent electrochemical lithium-storage performance.[37] The fabrication of hollow structure with different compositions would exhibit enhanced performance because of their specific synergetic effect between different materials. Therefore, it would be desirable to synthesize novel hollow structure with multicompositions, especially ternary Mo-based oxides. Herein, we develop a facile solvothermal approach combined with calcination treatment to synthesize hollow Co–Mo mixed oxide nanostructures with controlled structure and compositions, including hollow Co3O4/CoMoO4 heterostructures and ball-in-ball CoMoO4 nanospheres. The synthesis involves the fabrication of CoMo-hybrid precursor spheres

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and thermal treatment to yield the hollow structures. Importantly, this synthetic method described here is applicable to prepare other Mo-based oxides with controllable structure. The as-prepared hollow Co3O4/CoMoO4 nanospheres show better oxygen evolution reaction (OER) electrocatalytic performance than the CoMoO4 catalyst. In addition, the resulting carbon-coated Co3O4/CoMoO4 composite also exhibits an excellent performance as an anode material in lithium storage. Experimental Section Synthesis of Co3O4/CoMoO4 hollow spheres and CoMoO4 hollow spheres. For example, 0.1 mmol of Co(NO3)2·6H2O and 0.1mmol of molybdenyl acetylacetonate were dispersed in a mixture of (6 mL) glycerol (3 mL) and isopropanol (12 mL) under intense stirring to form a transparent pink solution. The mixed solution was transferred and sealed in the autoclave. Then the autoclave was heated at 180 °C for 12h. After reaction, the brown product was separated by centrifugation, washed with alcohol and dried in air. Co3O4/CoMoO4 hollow spheres were obtained by the thermal treatment of as-obtained CoMo-hybrid precursor in air for 2 h. The sample was heated to 500 ºC with a heating rate of 1 ºC min-1. For the synthesis of ball-in-ball CoMoO4 nanospheres, only 2 mL of glycerol was added into the mixutre solution while keeping other experimental conditions the same. Synthesis of ball-in-ball NiMoO4 and Zn-doped CoMoO4 nanospheres. For the preparation of NiMo-hybrid solid spheres, 0.1 mmol of Ni(NO3)2 · 6H2O and 0.1mmol of molybdenyl acetylacetonate were dispersed in a mixture of glycerol (3 mL) and isopropanol (12 mL) under intense stirring to form a homogeneous solution. Then the mixture solution was transferred into Teflon-lined autoclave (40 mL) and heated at 180 °C for 12h. After cooling down to room temperature, the yellow product was collected and dried in air at 80 °C. The ball-in-ball NiMoO4 nanospheres were obtained through the thermal treatment of NiMo-hybrid solid spheres in air for 2 h. The sample was heated to 500 ºC with a heating rate of 1 ºC min-1. For Zn-doped CoMoO4 nanospheres, 0.1 mmol of Co(NO3)2·6H2O and 0.1mmol of molybdenyl acetylacetonate and a certain amount of Zn(CH3COO)2 ·2H2O were dispersed in a mixture of glycerol (2 mL) and isopropanol (12 mL). Then the solution was transferred into Teflon-lined autoclave (40 mL) and heated at 180 °C for 12 h. After reaction, the product was collected and dried in air. Zn-doped CoMoO4 nanospheres were synthesized by the thermal treatment of the as-obtained precursor in air for 2 h. The sample was heated to 500 ºC with a heating rate of 1 ºC min-1.

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ization curves were recorded with 100% iR compensation by using the CHI software. Galvanostatic measurement (j = 10 mA/cm2) was carried out to evaluate the long-term stability of the sample. All the measured potentials were modified to RHE by following calculations: E (RHE) =E (SCE) + 0.059pH +0.244V. Lithium storage measurements: The Li-storage performances of the sample were tested on a LAND 2001A system using CR2032 cointype half cells. The working electrode includes 75 Wt% of active material, 15 Wt% of conductive carbon black (Super-P), and 10 Wt% of poly (vinyldifluoride) (PVDF). After stirring, the slurry was transferred on Cu foil and pasted using a blade. Before pressing, the electrodes were heated in a vacuum oven for 12h in order to remove the organic solvent. The active material loading is about 0.7～0.9 mg/cm2 . Lithium plate was used as the counter electrodes. The electrolyte consists of 1 M LiPF6 in a mixture of ethylene carbonate (50 Wt %) and diethyl carbonate (50 Wt %). All half-cell was assembled in a glove box under Ar atmospheres with low moisture and oxygen concentrations. The cycling and rate performances were operated at room temperature with a voltage of 0.01-3 V. Materials Characterization: X-ray diffraction (XRD) patterns of the sample were collected by a Rigaku D/max-2400 X-ray diffractometer using CuK radiation. Operating conditions of XRD and current are 40 kV and 40 mA, respectively. The morphology and structure of the sample were characterized by using TEM (HITACHI H-7700), HRTEM (FEI Tecnai G2 at a voltage of 200 kV) and SEM (LEO 1530). The surface chemical state and composition of samples were conducted on a Quantera SXM with monochromated Al Kα radiation at 250 kV. Quadrasorb SI-MP instrument was used to investigate the Brunauer-Emmett-Teller (BET) surface area of the sample. Results and Discussion

Uniform CoMo-hybrid precursor spheres are first prepared via a one-pot solvothermal treatment and the detailed explanation of the procedures can be found in the experimental section. From the scanning electron microscopy (SEM, Figure 1a and Figure S1, Supporting Information) and transmission electron microscopy (TEM, Figure 1c) images, the morphology of CoMo-hybrid precursor is uniform with a diameter of around 200 nm and the surfaces of the spheres are not very smooth. X-ray diffraction (XRD, Figure S2a in Supporting Information) pattern is similar with the metal glycolates. Energydispersive X-ray (EDX) result shows that the as-obtained CoMo–glycerate precursor is composed of Co, Mo, C, and O element (Figure S2b in Supporting Information).

Synthesis of carbon-coated Co3O4/CoMoO4 hollow spheres. To obtain carbon-coated Co3O4/CoMoO4, the Co3O4/CoMoO4 hollow spheres (50 mg) and Tris (15 mg) were dispersed in deionized water (50 mL) under ultrasonic treatment for several minutes. After that, 25 mg of dopamine hydrochloride was added and the solution was stirred for another 12 h. After coating, the sample was collected and dried in air. Finally, the carbon-coated Co3O4/CoMoO4 nanocomposites were obtained after carbonization under N2 atmospheres. The sample was heated to 500 ºC with a heating rate of 2 ºC min-1. Oxygen evolution reaction measurements: All the electrochemical measurements were tested in 1.0 M KOH electrolyte by a CHI660E electrochemical workstation (Chenhua, China) using a standard threeelectrode system. A Pt plate and a saturated calomel electrode (SCE) were employed as the counter electrode and reference electrode, respectively. For preparation of catalyst, 5 mg of sample was dispersed 0.8 mL of water and 0.2 mL of ethanol, then 50 µL of Nafion solution (5 wt%) was added under sonication for 1 h to obtain a homogeneous emulsion. 10 µL of the solution was dropped on the glassy carbon electrode. The surface area of the electrode is 0.196 cm2 and the loading density is calculated to be ~0.24 mg/cm2. Cyclic voltammetry (CV) was tested at scan rate of 50 mV s−1 to stabilize the sample. After that, Linear sweep voltammetry (LSV) was conducted in O2 presaturated 1.0 M KOH. The scan rate was of 5 mV s−1. All polar-

Figure 1. (a-b) SEM images of CoMo-hybrid precursor; (c) TEM image of CoMo-hybrid precursor; (d-e) SEM images of Co3O4/CoMoO4 hollow spheres. (f) TEM image of Co3O4/CoMoO4 hollow spheres.

Hollow Co3O4/CoMoO4 nanospheres were obtained by annealing treatment of CoMo-hybrid precursor in air at 500 °C. The crystallorghic structure of the product was characterized

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by XRD, as shown in Figure S3. It can be found that the XRD pattern of the as-obtained sample can be assigned to the monoclinic CoMoO4 (JCPDS 21-0868, space group: C2/m). Besides, several sharp peaks around 7.5°, 53.2°, 63.0° and 75.6° are very visible. These peaks can be allocated to the diffractions of (111), (200), (220) and (311) planes of cubic Co3O4 (JCPDS 42-1467, space group: Fd-3m). The Co/Mo atomic rate is about 3.3 by ion coupled plasma atomic emission spectrometry (ICP-AES), which is different from the atomic ratio of CoMoO4 phase. The corresponding EDX result also reveals that the as-obtained sample belongs to Co-rich oxide (Figure S4, Supporting Information). It is proposed that the sample must contain other Co-based structures. Raman spectrum was carried out to further investigate the structure. As shown in Figure S5, three peaks located at 483 cm−1, 520 cm−1 and 683 cm−1 can be ascribed to the Eg, F2g and A1g raman modes of the Co3O4, [38] further confirming the formation of Co3O4. The peak about 921 cm−1 can be attributed to the Mo-O-Co stretching vibrations in cobalt molybdate. [33,48]. All of these results manifest that the thermal annealing of CoMo-hybrid spheres results in the formation of hollow Co3O4/CoMoO4 heterostructures.

Figure 2. (a) A typical TEM image of Co3O4/CoMoO4 hollow spheres; (bc) HRTEM images of as-prepared Co3O4/CoMoO4 hollow spheres; (d) Energy-dispersive X-ray spectroscopy spectrums of Co3O4/CoMoO4 hollow spheres.

Both SEM and TEM were used to further reveal the morphology and structure of the obtained hollow Co3O4/CoMoO4 nanospheres. After the annealing treatment, the morphology of spheres can be well retained. The SEM images in Figure 1d-e show a typical broken region. TEM observation also shows a well-defined void structure with a shell thickness of around 20 nm. Figure 2a shows a typical high-resolution Transmission electron microscopy (HRTEM) image of hollow structure. The obvious lattice fringes are about 0.336 nm and 0.466 nm, which is close to the (002) planes of the CoMoO4 phase and the (111) plane of Co3O4, respectively (Figure 2b-c). The corresponding selected area electron diffraction (SAED) pattern in Figure S6 shows the polycrystalline nature of the sample. The element mapping of hollow Co3O4/CoMoO4 nanospheres are further investigated by the high-angle annular dark-field

scanning TEM (HAADF-STEM). As shown in Figure 2d, the homogeneous distribution of cobalt, molybdenum and oxygen element can be clearly observed. X-ray photoelectron spectroscopy (XPS) was further carried out to analyze the surface chemical state and composition of the product. The result shows three typical peaks Mo 3d，Co 2p and O 1s (Figure S7a, Supporting Information). Figure S7b shows the high-resolution XPS spectra of Mo 3d. The obvious two peaks at 232.13 and 235.27 eV can be ascribed to Mo 3d3/2 and Mo 3d1/2, respectively. And the binding energy is in good agreement with those reported for Mo6+ in CoMoO4. [39] On the other hand, the high-resolution XPS spectra of Co 2p exhibits two peaks at 796.1 and 780.1 eV, which are attributed to the Co2+ of CoMoO4 (Figure S7c, Supporting Information). In addition, the peak at around 781.5 eV in Co 2p spectra indicates the presence of Co3+ in the sample,[40] which is consistent with the mixed phases analyzed by XRD and ICP results. As shown in Figure S8, the specific surface area of the hollow Co3O4/CoMoO4 nanospheres was calculated to be 55.8 m2 g−1 based on the nitrogen adsorption-desorption isotherms. In addition, the IV isotherm indicate the presence of mesoporous structure, which is ascribed to the collapse of sphere structure during the thermal decomposition. In view of the hollow heterostructures with mesoporous, the as-prepared Co3O4/CoMoO4 nanospheres may facilitate the electro exchange as well as the transport of ion in electrochemical reactions. Controlled experiments were performed to reveal the evolution of CoMo-hybrid solid spheres during the different solvothermal stages. Figure S9a shows that CoMo-hybrid spheres were formed after solvothermal treatment for 2 h. The size and morphology of spheres will not change but the yields increase over time. It was found that the size and composition of final hollow structure are closely related to the amounts of glycerol. For example, CoMo-hybrid solid spheres with larger diameter can be fabricated through decreasing the amounts of glycerol (Figure S10, Supporting Information). After calcination, CoMoO4 nanospheres with a diameter of about 500 nm can be synthesized in large scale. The corresponding XRD pattern shows that the peaks can be well assigned to the pure monoclinic CoMoO4 phase (JCPDS 21-0868) without any impurities (Figure S11, Supporting Information). The ICP result also shows the Co/Mo atomic ratio is about 1, indicating the purity phase of CoMoO4. As shown in Figure 3a-c, the assynthesized sample exhibits a unique yolk–shell structure. The formation process of this special structure is mainly attributed to the heterogeneous contraction in the thermal treatment process. Similar structural evolution can be observed for other mixed metal precursors. [13, 16, 25, 28] Therefore, the composition of the final product can be easily changed by adjusting the amounts of glycerol. Obviously, it is very efficient way to prepare Mo-based oxides with controllable compositions. What's more, the present strategy is applicable to synthesize other Mo-based mixed metal oxides with a hollow strucutre. For instance, the ball-in-ball NiMoO4 hollow structure can be also obtained by the thermal treatment of NiMo-hybrid solid spheres (Figure S12, Supporting Information). NiMo-hybrid solid spheres can be prepared through the solvothermal treatment mentioned above. Then the as-synthesized NiMo-hybrid precursor can transform into NiMoO4 hollow structure after calculation treatment. As shown in Figure S13, the XRD analysis can be well indexed to

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Figure 3. (a-b) SEM images of ball-in-ball CoMoO4 hollow strucutures; (c) TEM image of ball-in-ball CoMoO4 hollow strucutures; (d-e) SEM images of ball-in-ball NiMoO4 hollow strucutures; The scale bar is 300 nm; (f) TEM image of ball-in-ball NiMoO4 hollow strucutures; (g-h) SEM images of ball-in-ball Zn-doped CoMoO4 hollow strucutures; (i) TEM image of ball-in-ball Zn-doped CoMoO4 hollow strucutures.

the monoclinic NiMoO4 (JCPDS 45-0142, space group: C2/m(12)), which is consistent with the EDX result (Figure S14, Supporting Information). As can be seen (Figure 3d-f), similar ball-in-ball structures with a spherical shape can be observed and most of the hollow structure can be well maintained. It is worth to mention that cation-doped CoMoO4 nanospheres with ball-in-ball structure can be also prepared through the same approach. The ball-in-ball architecture of Zn-doped CoMoO4 nanospheres is clearly observed by TEM observations (Figure 3g-i). The elemental mapping results in Figure S15 show that the element Zn, Co, Mo and O are uniformly distributed throughout the structures, indicating the incorporation of Zn. We believe that this s trategy is a general approach to prepare various hollow architeture with multicompositions. Many mixed metal oxides show excellent electrocatalytic performance in various applications.[36,39] We first investigate the electrocatalytic activity of the-obtained Co3O4/CoMoO4 and CoMoO4 nanostructures for OER. The OER catalytic activity of the sample was evaluated in O2 presaturated 1.0 M KOH solution. As shown in Figure 4a, the linear sweep voltammetry (LSV) of the samples were performed to evaluate

their catalytic activity. The benchmark to evaluate the OER performance is the operation potential at a current density of 10 mA cm-2.[41] For Co3O4/CoMoO4 hollow nanospheres, the potential for a current density of 10 mA cm−2 is 1.60 V (vs ERHE, η= 370 mV), whereas the ball-in-ball CoMoO4 spheres show comparatively poor OER activity with 1.64 V. Moreover, the Tafel slopes are also investigated to evaluate OER kinetics. The Tafel slopes can be calculated from the linear fitting of the polarization curves based on the formula (η = b log j + a). [42] The Tafel plot of Co3O4/CoMoO4 hollow nanospheres is 59 mV dec−1, as shown in Figure 4b. Such a small Tafel slope means its favorable kinetics for the water-splitting reaction compared to 84 mV dec−1 for CoMoO4 nanospheres. What is more, Co3O4/CoMoO4 hollow nanospheres exhibited higher electrochemically active surface area than CoMoO4 nanospheres (Figure S16, Supporting Information), which is consistent with the LSV results. In addition, the resulting Co3O4/CoMoO4 hollow nanospheres exhibited considerable stability in 1.0 M KOH solution. In addition, the catalyst has a nearly constant operating potential at 10 mA cm-2 after 5h (Figure S17, Supporting Information). The structure of Co3O4/CoMoO4 hollow nanospheres can be well retained based on TEM observation (Figure S18, Supporting Information), indicating a good durability performance.

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Figure 4. Electrocatalytic activity of Co3O4/CoMoO4 and CoMoO4 nanostructures catalysts in O2-saturated 1.0 M KOH for OER. (a) Polarization curves and (b) Tafel plots.

We further evaluate the electrochemical storage lithium performance of carbon-coated Co3O4/CoMoO4 nanospheres. Carbon-coated Co3O4/CoMoO4 nanospheres were prepared using the dopamine hydrochloride method reported before. [43] As shown in Figure S19, the surface of Co3O4/CoMoO4 nanospheres is well covered by a uniform layer. The content of carbon coating is about 25 Wt % (Figure S20, Supporting Information). The cyclic voltammograms (CVs) of carboncoated Co3O4/CoMoO4 electrode was first investigated. Figure 5a displays the CVs of the first, second, and third cycle at a scan rate of 0.1 mV s-1. It is easy to observe two cathodic/anodic peaks around at 0.4/1.1 V and 1.3/2.0 V for carbon coated Co3O4/CoMoO4 sample. In the first cycle, two peaks located at 0.4 and 1.1 V can be identified. This two cathodic should be ascribed to the reduction reaction of CoMoO4 and Co3O4 to metallic Co and Mo, respectively. On the contrary, the two anodic peaks located at 1.3 and 2.0 V represent the oxidation of Co0 and Mo0. During the second cycles, the appearance of new cathodic peak at ~0.92 V indicates the existence of some irreversible reaction for carbon-coated Co3O4/CoMoO4 electrode. The charge-discharge curves of carbon-coated Co3O4/CoMoO4 electrode for the first three circles are presented in Figure 5b. In the first cycle, two voltages plateau ~ 0.6 V in the discharging and ~ 2.0 V in the charging can be observed, which corresponds to the lithium insertion/deinsertion process based on the following reaction: CoMoO4+8Li+⇄Co+Mo+4Li2O and Co3O4+8Li+⇄3Co+4Li2O. The initial irreversible capacity loss can be easily identified from the second cycle. This loss may be associated with the occurrence of solid–electrolyte interface (SEI) layer, which results from the electrolyte decomposition or some side reaction. [44-45] In the subsequent cycles, the voltage profiles are mostly overlapping, showing good charge-discharge performance. Figure 5c shows the stability performance of carbon-coated Co3O4/CoMoO4 composite. A current of 200 mA g-1 was chosen to investigate the cycling performance of the sample. After 100 cycles, the capacity of carbon coated Co3O4/CoMoO4 composite electrode can maintain about 918 mAh g-1. The corresponding coulombic efficiency of carbon-coated Co3O4/CoMoO4 in the first cycle is 76.1%, and is nearly 100% during the subsequent cycles. The carbon-coated Co3O4/CoMoO4 electrode showed better cycle performance compared to previously reported results about CoMoO4-based electrodes (Table S1, Supporting Information).[23,46-51] Moreover, the rate capability of carbon-coated Co3O4/CoMoO4 composite electrode was examined. As shown in Figure S21, the electrode was tested at different charge/discharge current densities from 100 to 600 mA g−1. The carbon-coated

Co3O4/CoMoO4 composite sample shows a capacity of 700 mA h g−1 at a current densities of 600 mA g−1. Apparently, as the current decreases from 600 to 100 mA g−1, the capacity of the sample can restore to above 900 mA h g−1. The improved electrochemical storage performance of carbon-coated Co3O4/CoMoO4 composite could be summarized as several points. First, the unique hetero-structure with large specific surface area will increase the contact area between the electrode and electrolyte. Second, the hollow structure can relieve the pressure caused by the volume change during charging and discharging process. The surface coating layer can well maintain the integrity of the hollow structure (Figure S22 and Figure S23, Supporting Information); on the other hand, this carbon coated layer will improve the electrical conductivity of electrode material, leading to enhance the electrochemical storage lithium performance.

Figure 5. Electrochemical tests of carbon-coated Co3O4/CoMoO4 electrode between 0.01-3.0 V. (a) CVs for the first three circles; (b) Dischargecharge curves for the first three circles; (c) Cycling test and coulombic efficiency; All the tests are operated at a current density of 200 mA g-1.

Conclusion

In conclusion, Hollow Co–Mo mixed oxides with controlled composition have been prepared through the thermal decomposition of uniform CoMo-hybrid precursor spheres. Importantly, other mixed metal oxides with ball-in-ball structure have been successfully synthesized through the same technique. The resultant Co3O4/CoMoO4 nanospheres show improved electrocatalytic OER activity compared to CoMoO4 nanospheres. What is more, the as-obtained carbon coated Co3O4/CoMoO4 composite exhibit enhanced lithium storage properties. Our result described here provides a strategy to synthesize other complicated and hollow mixed metal oxides for catalysis and energy storage.

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ASSOCIATED CONTENT Supporting Information XRD; Raman spectra; X-ray photoelectron spectra; BrunauerEmmett-Teller; high-angle annular dark-field scanning TEM; The selected-area electron diffraction pattern；Cyclic voltammetry; Stability measurements; TGA; Rate and cycling performance. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Funding Sources This work was supported by NSFC (21431003, 21521091).

Notes The authors declare no competing financial interest.

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