MetalâOrganic-Framework-Derived YolkâShell-Structured Cobalt

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Metal organic frameworks-derived yolk-shell structured cobalt-based bimetallic oxide polyhedron with high activity for electrocatalytic oxygen evolution Zhou Yu, Yu Bai, Yuxuan Liu, Shimin Zhang, Dandan Chen, Naiqing Zhang, and Kening Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07000 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 24, 2017

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ACS Applied Materials & Interfaces

Metal organic frameworks-derived yolk-shell structured cobalt-based bimetallic oxide polyhedron with high activity for electrocatalytic oxygen evolution Zhou Yu,†, ‡ Yu Bai, *, ‡, # Yuxuan Liu, † Shimin Zhang, † Dandan Chen, † Naiqing Zhang, ‡, § and Kening Sun, *, ‡, § †

School of Chemistry and Chemical Engineering, Harbin Institute of Technology,

Harbin, 150090, P. R. China ‡

Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of

Technology, Harbin, 150090, P. R. China #

Advanced Research Institute for Multidisciplinary Science, Beijing Institute of

Technology, Beijing, 100081, P. R. China §

State Key laboratory of Urban Water Resource and Environment, Harbin Institute of

Technology, Harbin, 150090, P. R. China ABSTRACT: The development of inexpensive, efficient and environmentally friendly catalysts for oxygen evolution reaction (OER) is of great significant for green energy utilization. Herein, binary metal oxides (MxCo3-xO4, M = Zn, Ni, and Cu) with yolk-shell polyhedron (YSP) structure were fabricated by facile pyrolysis of bimetallic zeolitic imidazolate frameworks (MCo-ZIFs). Benefiting from the synergistic effects of metal ions and the unique yolk-shell structure, MxCo3-xO4 YSP displays good OER catalytic activity in alkaline media. Impressively, ZnxCo3-xO4 YSP shows a comparable overpotential of 337 mV at 10 mA cm-2 to commercial RuO2 and exhibits superior long-term durability. The high activity and good stability reveals its promising application. 1

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KEYWORDS: oxygen evolution reaction, yolk-shell, binary metal oxide, bimetallic zeolitic imidazolate frameworks, alkaline media.

1.INTRODUCTION Converting water into oxygen and hydrogen by electrolysis or solar driven photo-electrochemical reaction is one of the most appealing strategies for energy research.1-5 However, the electrolysis of water is significantly hindered by the sluggish oxygen evolution reaction (OER) process.6, 7 Despite the high OER catalytic activity of precious metal oxides, the relatively high cost and rarity restrict their practical application.8,

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Therefore, a lot of effort has been made to develop

cost-effective catalysts with high OER catalytic activity and durability. In virtue of their earth-abundance, cost effective, and high activity, transition metal oxides have been regarded as promising alternative to traditional precious metal oxides.10-13 Among them, Co3O4 has been considered as one of the most promising candidates due to its environmental benignity, excellent catalytic property and good stability.14-16 Moreover, partial substitution of Co ions in Co3O4 with cheaper and more eco-friendly alternative metal ions is believed to manipulate its inherent electronic and surface properties, thus improving the activity and stability.17-21 For example, mesoporous NixCo3-xO4 nanowire arrays have been prepared on Ti substrate.22 Doping Co3O4 nanowires arrays with Ni ions was found to enhance the electrochemical performance through modifying their physical properties. Driess et al. showed that spinel ZnCo2O4 is more active and stable than Co3O4 for OER. The improved OER activity can be attributed to the introduction of Zn into the spinel Co3O4, which leads 2


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to the generation of cationic vacancies at the surface.23 The nano-materials’ properties are greatly rely on the morphological features, the synthesis approach and their phase purity. Therefore, tremendous effort has been devoted to fabricating binary transition-metal oxide nanomaterials with desired size and architecture through various methods. Metal-organic frameworks (MOFs), formed by metal ions and organic ligands, have gained extensive consideration owing to the unique architectures and versatile functionalities.24-28 MOFs have recently been demonstrated as useful precursors for the synthesis of various porous nanostructured metal oxides with good catalytic activity.29-31 Lou et al. constructed hollow Fe2O3 microboxes with hierarchical multishelled structure via a template-engaged strategy using Prussian blue template and different alkaline substance.32 Ai et al. showed the porous Co3O4 derived from zeolitic imidazolate frameworks (ZIFs) exhibited promising OER properties. 33

Nevertheless, the fabrication of bimetallic metal oxides with yolk-shell structure for

OER has been rarely explored so far. Herein, we demonstrate a facile method to prepare MxCo3-xO4 (M = Zn, Ni and Cu) polyhedron with yolk-shell structure (MxCo3-xO4 YSP). The synthesis involves the fabrication of bimetallic ZIFs and the subsequent thermal annealing of the as prepared template to form MxCo3-xO4 YSP. Importantly, the bimetallic oxides with yolk-shell structure exhibit better OER activity than pristine Co3O4. The ZnxCo3-xO4 YSP exhibits an overpotential of 337 mV to drive 10 mA cm-2. Furthermore, the Tafel slope of ZnxCo3-xO4 YSP is only 59.3 mV dec-1, which is better than most noble-metal-free 3



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OER catalysts. Meanwhile, the unique yolk-shell structure endows the ZnxCo3-xO4 with high long-term durability as compared with hollow structure. We believe this work provides a valuable route to obtain cost-effective and viable OER electrocatalysts.

2. EXPERIMENTAL SECTION 2.1 Synthesis of MCo-ZIFs (M = Zn, Ni and Cu) and ZIF-67. In a typical preparation, 4 mmol Co(NO3)2·6H2O and 2 mmol M(NO3)2·6H2O were dissolved into 40 mL of methanol (MeOH) and solution was then added into 30 mL of MeOH containing 24 mmol 2-methylimidazole (MeIM) under string. The mixed solution was placed at 25 °C for 24 h. The as-obtained powders were centrifuged with ethanol and dried in vacuum overnight to obtain the MCo-ZIFs. The preparation of ZIF-67 was performed with a similar process to MCo-ZIFs in the absence of M(NO3)2·6H2O. 2.2 Synthesis of MxCo3-xO4 YSP (0 < x ≤ 1). MxCo3-xO4 YSP was prepared by annealing MCo-ZIFs in a tube-oven at 400 °C for 1 h with a heat ramp of 1 °C/min in a nitrogen flow. Then the sample was kept in air at 400 °C for another 1h to finally generate MxCo3-xO4 YSP. 2.3 ZnxCo3-xO4 hollow polyhedron (ZnxCo3-xO4 HP). ZnxCo3-xO4 HP was synthesized by pyrolyzing ZnCo-ZIFs at 400 °C for 2 h with a heat ramp of 2 °C/min in air. 2.4 Characterization. Field emission scanning electron microscopy (FESEM) images were investigated on a Hitachi S-8010. Energy-dispersive X-ray (EDX) elemental mapping images and transmission electron microscopy (TEM) were all 4


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measured on a FEI Tecnai G2 F30 (200 kV). X-ray diffraction (XRD) characterization was obtained by using a PANalytical X’Pert PRO with Cu Ka radiation. X-ray photoelectron spectra (XPS) were recorded with a K-Alpha electron spectrometer. ASAP2020 instrument was utilized to investigate the Brunauer−Emmett−Teller (BET) surface area of the product. Thermogravimetric analysis (TGA) was performed on NETZSCH, STA 449F3 with a temperature ramp of 10 °C/min. 2.5 Electrochemical Characterization. Electrocatalytic activity evaluations were performed in a O2-purged within the solution of 1 M KOH with a three-electrode system. The glassy carbon (GC) electrode (3 mm diameter) was utilized as the working electrode. A Pt mesh and Ag/AgCl electrode were served as counter electrode and reference electrode, respectively. 2 mg sample was dispersed in 500 µL Nafion (5 wt%) -water- isopropyl alcohol mixture solution with a volume ratio of 0.05 : 4 : 1 by sonicating. Then, 6 µL of the dispersion was deposited onto a GC and dried in air overnight (loading ~0.21 mg cm-2). The linear sweep voltammetry (LSV) was measured at 5 mV s-1 scanning rate. All the applied potentials were referenced to a reversible hydrogen electrode (RHE) scale. The current-time (I-t) tests were performed at 1.56 V. Cyclic voltammograms (CVs) were measured from 0.25 to 0.30 V vs. Ag/AgCl at scanning rates of 5, 10, 20, 40, 60, 80 and 100 mV s-1. Electrochemical impedance spectroscopy (EIS) was recorded at 1.65 V (vs RHE) over a frequency range from 0.1 Hz to 100 kHz with a 5 mV AC signal amplitude on a PAR-STAT 2273 test system.

3. Result and discussion 5



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The synthesis of MxCo3-xO4 YSP is schematically illustrated in Fig. 1. Well-defined MCo-ZIFs were synthesized via room-temperature co-precipitation reaction of MeIM with Co(NO3)2 and M(NO3)2 in methanol. The fabrication of yolk-shell structure can be ascribed to the heterogeneous contraction arises from the non-equilibrium heat treatment.34-37 During the initial calcination period, there is a high temperature gradient (∆T) along the radical direction for MCo-ZIFs, which leads to the formation of a layer of rigid MxCo3-xO4 around the MCo-ZIFs. Along with the sintering, the interface of the MxCo3-xO4 shell and MCo-ZIFs core receives two opposite forces. One is the contraction force (Fc), which comes from the weight loss induced by the degradation of the organic species, and the other is the opposite adhesive force (Fa), which is related with the formation of MxCo3-xO4 and the release of gas due to the decomposition of organic species. Obviously, the Fc makes the inner MCo-ZIFs precursor shrink inwards, while Fa prevents the inward contraction. At large ∆T, the Fc exceeds the Fa and the core contracts inward and separates from the shell. Since the ∆T decreases from the shell to the inner core, the ∆T at the new interface is too small to induce the formation of another shell for the separated inner core. As a result, the inner core decomposes into a porous structure, and the yolk-shell structured MxCo3-xO4 is obtained. MCo-ZIFs and ZIF-67 were first characterized by powder X-ray diffraction (XRD). The as-prepared MCo-ZIFs exhibit strong diffraction peaks at identical positions as ZIF-67, confirming that the MCo-ZIFs show good crystallinity and phase-pure ZIF-67 structure (Fig. 2a). In principle, M2+ has similar ionic radius and electronegativity to 6


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Co2+ and thus can homogeneously replace Co2+ sites in the ZIF-67 (Fig. 2b). Field-emission scanning electron microscopy (FESEM) (Fig. 2c-f) reveals that the as-synthesized MCo-ZIFs are composed of uniform rhombic polyhedral with size of ~600 nm and their surface is smooth. The Thermogravimetric analysis (TGA) results of bimetallic zeolitic imidazolate frameworks (Fig. S1) reveals a dramatic weight loss at the temperature of about 400 °C in air, indicating the complete decomposition of the organic molecules in precursors. It was remarkable that the calcination in nitrogen atmosphere is helpful for preserving the precursor as a result of the fact that the carbon formed during the annealing process could serve as a temporal buffer, preventing further contraction of products.38, 39 The calcined samples are examined by XRD to confirm the crystallographic phase (Fig. 3a). When ZIF-67 was used as precursor, all of the diffraction peaks correspond to cubic spinel Co3O4. The characteristic peaks of MxCo3-xO4 YSP are similar to the XRD pattern of Co3O4, indicating that the incorporation of second metals ions (Zn, Ni, or Cu) does not affect the spinel structure.40, 41 Since there is no new diffraction peak originating from residues or impurities of other metal oxides, all of the metal ions are confirmed to be well incorporated into the lattice of Co3O4 by replacing the sites of Co ions (Fig. 3b-c). The cubic cell parameter (a0) was calculated to be 8.082 Å, 8.095 Å, 8.106 Å, and 8.102 Å for Co3O4, ZnxCo3-xO4, NixCo3-xO4 and CuxCo3-xO4, respectively. This result is in accord with the expanded spinel cell parameters which caused by doping Co3O4 with Zn, Ni, or Cu elements.

42, 43

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X-ray (EDX) spectroscopy (Fig. S2) indicates that the molar ratio of M: Co: O is about 1: 2: 4 and no other element exists within MxCo3-xO4 YSP. The detailed morphologies of ZnxCo3-xO4 YSP are examined by FESEM and transmission electron microscopy (TEM). As shown in Fig. 4a, the ZnxCo3-xO4 YSP has a well-defined polyhedral feature with a uniform diameter of ~500 nm. Furthermore, the inner spherical core can be well identified from the particles with broken outer shells and as shown in Fig. 4b. In addition, many pores are distributed in the inner cores. The TEM image (Fig. 4c, d) further confirms the as-prepared ZnxCo3-xO4 YSP, since there is a sharp contrast between the core with dark and shell with grey areas of the polyhedral. The surface of polyhedral is rough and the differentia of the contrast grade reveals that the as-prepared ZnxCo3-xO4 YSP is highly porous (inset in Fig 4d). Furthermore, the ZnxCo3-xO4 YSP is composed of numerous primary crystallites with size of ∼20 nm. As illustrated in Fig. 4e, the high-resolution TEM (HRTEM) images indicate the lattice spacing of 0.24 and 0.47 nm, which can be indexed to the (311) and (111) crystal planes in the XRD, respectively. The selected area electron diffraction (SAED) pattern further reveals that the ZnxCo3-xO4 YSP is polycrystalline (Fig. 4f). The elemental mapping results (Fig 4g) further manifest that the O, Co and Zn elements are uniformly distributed in the ZnxCo3-xO4 YSP. As indicated by the FESEM and TEM images of NixCo3-xO4 YSP (Fig. S3 a-c), CuxCo3-xO4 YSP (Fig. S3 d-f) and Co3O4 YSP (Fig. S3 g-i), the obtained bimetallic oxides and Co3O4 show similar structures with ZnxCo3-xO4 YSP in shape, size and uniformity. Importantly, this strategy can be extended to fabricate various binary and 8


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monometal oxides with complex interior structures by employing the corresponding MOFs as the precursor. X-ray photoelectron spectroscopy (XPS) measurements were further investigated the composition of ZnxCo3-xO4 YSP (Fig. 5). The survey spectrum indicates the existence of Zn, Co, O and C elements in Fig. 5a, which matches well with the XRD results. As illustrated in Fig. 5b, the two strong peaks at 1020.8 and 1043.1 eV can be assigned to the Zn 2p3/2 and Zn 2p1/2 of Zn species, respectively.44 The Co 2p spectrum (Fig. 5c) can be deconvoluted into two strong peaks at 778.4 eV and 793.6 eV, which are assigned to the Co 2p3/2 and Co 2p1/2 peaks, respectively, and two shakeup satellites.45 The spine orbit splitting of the two major peaks is above 15 eV, revealing the existence of both Co2+ and Co3+.46, 47 The O 1s spectrum (Fig. 5d) exhibits two bands centered at 528.2 and 530.5 eV, which are assigned to the oxygen species of the Co-O and Zn-O bonds, and OH-, respectively.44, 48 The OER activities of different metal oxides were investigated in O2-saturated 1 M KOH solution with a system of three-electrode. As shown in Fig. 6a, the bare GC electrode shows little activity toward oxygen evolution. Obviously, the MxCo3-xO4 YSPs feature enhance OER activity compared with Co3O4 YSP, with the overpotential at 10 mA cm-2 decreasing from 422 mV for Co3O4 YSP to 337 mV for ZnxCo3-xO4 YSP, 363 mV for NixCo3-xO4 YSP, and 395 mV for CuxCo3-xO4 YSP (Fig. 6b). This indicates that incorporating metal ions into Co3O4 could enhance the catalytic activity toward OER in alkaline media. As previously reported,17, 49 the OER activity of the MxCo3-xO4 YSP improvement was attributed to the incorporation of M ions into the 9



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Co3O4 lattice, which leads to enlarge the Co3+-O bonds, promotes the OH- to adsorb onto Co3+ and facilitate the oxidization of Co4+ species. The creation of Co4+ species as active centers is favorable for enhanced OER performance. Moreover, the overpotential of ZnxCo3-xO4 YSP at 10 mA cm-2 is comparable to commercial RuO2 (332 mV). Furthermore, ZnxCo3-xO4 YSP generates a current density of 30.47 mA cm-2 at 1.60 V (vs. RHE), which is superior to the current densities obtained by other metal oxides (Fig. 6b). The Tafel plots for all samples were further explored to assess the OER kinetics (Fig. 6c) and the obtained values were summarized in Table 1. The Tafel slope of ZnxCo3-xO4 YSP (59.3 mV dec-1) is lower than other studied catalysts including RuO2 (79.5 mV dec-1), confirming its favorable kinetics for OER. It is worth noting that the overpotential and Tafel slope of ZnxCo3-xO4 YSP is even smaller than those for most reported noble-metal-free OER catalysts (Table S1). Double-layer capacitance (Cdl) determined by cyclic voltammetry (CV) measurements was employed to evaluate the electrochemically active surface area (ECSA) of the various samples at different scanning rates (ranging from 5 to 100 mV s-1) in a non-Faradaic region (Fig S4 a-d). The values of Cdl are derived by calculating the linear slope from the plots of current density versus scan rate (Fig. 6d).21 It can be seen in Table 1 that ZnxCo3-xO4 YSP is 1.6, 2.7, and 8.3 times larger than that of NixCo3-xO4 YSP, CuxCo3-xO4 YSP, and Co3O4 YSP, respectively. This indicates that the ZnxCo3-xO4 YSP features large accessible surface area and more active sites, which could contribute to the enhanced OER activity.50 The electrochemical impendence spectroscopy (EIS) was further used to study the electron transfer kinetics of various catalysts toward 10


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OER. The charge transfer resistance (Rct) can be derived by fitting the Nyquist plots (Fig. 6e, f). As listed in Table 1, the Rct of MxCo3-xO4 YSP is much smaller than that of Co3O4 YSP, suggesting faster electron transfer rate for MxCo3-xO4 YSP.51 The smaller Rct might be attributed to the incorporation of additional metal ions into spinel Co3O4.52,

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Compared with other bimetallic oxides, ZnxCo3-xO4 YSP shows the

smallest Rct., which is consistent with the OER polarization and Tafel results. To study the advantage of yolk-shell structure, the ZnxCo3-xO4 hollow polyhedral (denoted as ZnxCo3-xO4 HP) was further synthesized for comparison. As revealed by the FESEM images presented in Fig. 7a and b, the ZnxCo3-xO4 HP possesses a homogeneous rhombic polyhedral morphology with the diameter of ~500 nm. After pyrolysis, the smooth surface becomes concave and wrinkled, while the obtained ZnxCo3-xO4 HP remains the morphology of the ZnCo-ZIFs template. As characterized by TEM (Fig. 7c and d), ZnxCo3-xO4 HP features hollow structure and is assembled by individual nanoparticles. The XRD pattern, EDS spectrum and elemental mapping results (Fig. S5 and S6) further confirm the high purity of ZnxCo3-xO4 HP derived from the pyrolysis of ZnCo-ZIFs. N2 adsorption-desorption measurements were performed to investigate the porosity and the specific surface area of the ZnxCo3-xO4 YSP and ZnxCo3-xO4 HP (Fig. S7). Both samples exhibit a typical type-IV isotherm, indicating the mesoporous structures of the sample. The specific surface area of ZnxCo3-xO4 YSP is 184.5 m2 g−1 with the pore size centered at ~25 nm. By virtue of the unique yolk-shell structure, the ZnxCo3-xO4 YSP shows larger surface area than ZnxCo3-xO4 HP (146.5 m2 g−1). The 11



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large surface area is favorable for improving the catalytic activity since more active sites could be provided. 54, 55 The electrochemical performances of ZnxCo3-xO4 YSP and ZnxCo3-xO4 HP was further investigated by the LSV and Tafel characterization (Fig. 8a and b). As compared with ZnxCo3-xO4 YSP, ZnxCo3-xO4 HP features relatively low activity with an overpotential of ~360 mV at 10 mA cm-2 current density and a Tafel slope of 73.5 mV/dec-1. As revealed by EIS (Fig. 8c), the Rct of ZnxCo3-xO4 HP (5.0 Ω) is larger than that for ZnxCo3-xO4 YSP (2.2 Ω), confirming that ZnxCo3-xO4 YSP exhibits favorable reaction kinetics and thus better OER electrocatalytic performance as compared with ZnxCo3-xO4 HP. Meanwhile, the ZnxCo3-xO4 YSP possesses higher catalytic active surface area for OER, as confirmed by its larger Cdl (15.2 mF cm−2 for ZnxCo3-xO4 HP as shown in Fig. S8). The excellent stability is also a pivotal property for electrocatalyst selection. Therefore, chronoamperometric measurement was performed to study the stability of ZnxCo3-xO4 YSP. As shown in Fig. 8d, the ZnxCo3-xO4 YSP electrode exhibits stable OER current density and retains over 90% of its original current density after 12h testing, whereas ZnxCo3-xO4 HP shows fast current loss of ~35% after testing. The stability of the ZnxCo3-xO4 YSP electrode was also assessed by recording the LSV curves before and after performing 1000 cycles of CV. As shown in Fig. S9, the LSV curve is nearly invariable after 1000 cycles of CV, indicating the outstanding stability of ZnxCo3-xO4 YSP. In contrast, the related LSV curve of the ZnxCo3-xO4 HP shifts markedly in the negative direction. Compare to conventional hollow shells, the ZnxCo3-xO4 YSP is believed to be more robust due to 12


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its unique yolk-shell structure and the enhanced electrochemical stability of ZnxCo3-xO4 is related with its unique structure.56 Besides, the porous structure and large surface area of ZnxCo3-xO4 YSP could promote the detachment of O2 from the electrode surface, thus hindering O2 bubbles from aggregating and harming the catalysts.57 The structural characteristics and intrinsic properties manifest that ZnxCo3-xO4 YSP is an efficient electrocatalyst for OER.

4.CONCLUSIONS In summary, cobalt-based bimetallic oxides with yolk-shell structure were fabricated by a facile strategy through direct pyrolysis of MCo-ZIFs. The unique mesoporous yolk-shell structure combined with appropriate metal ions selection endows MxCo3-xO4 YSP with excellent OER activity. Additionally, the ZnxCo3-xO4 YSP outperforms the commercial RuO2 and exhibits favorable long-term stability. The facile synthesis procedure and excellent performance of MxCo3-xO4 YSP offers a promising strategy to develop low-cost catalysts with high catalytic activity and long-term stability for electrochemical oxygen evolution. ASSOCIATED CONTENT Supporting Information TG curves of ZnCo-ZIFs under air and nitrogen atmosphere. EDS patterns of ZnxCo3-xO4 YSP, NixCo3-xO4 YSP and CuxCo3-xO4 YSP. FESEM and TEM images of NixCo3-xO4 YSP, CuxCo3-xO4 YSP and Co3O4 YSP. CV in the double-layer region of the electrodes loaded with ZnxCo3-xO4 YSP, NixCo3-xO4 YSP, CuxCo3-xO4 YSP, and Co3O4 YSP. XRD and EDS patterns of ZnxCo3-xO4 HP. TEM image of an individual 13



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ZnxCo3-xO4 HP and EDX mapping with of Co, Zn and O elements. N2 adsorption-desorption isotherms and pore size distribution of the ZnxCo3-xO4 YSP and ZnxCo3-xO4 HP. CV curves of the electrodes loaded with ZnxCo3-xO4 HP and scan rate dependence of the current density. LSV curves of ZnxCo3-xO4 YSP and ZnxCo3-xO4 HP before and after 1000 cycles. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y Bai) *E-mail: [email protected] (KN Sun) Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgements This project is supported by the National Natural Science Foundation of China (Grant No. 51203036), the Postdoctoral Science Special Foundation of China (Grant No. 2013T60380), the Postdoctoral Science Foundation of China (Grant No. 2012M520748), and the Scientific Research Foundation of Beijing Institute of Technology.

REFERENCES (1) Armand, M.; Tarascon, J. M. Building better batteries. Nature 2008, 451, 652-657. (2) Kim, T. W.; Choi, K. S. Nanoporous BiVO4 Photoanodes with Dual-Layer 14


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Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990-994. (3) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. ChemInform Abstract: Design of Electrocatalysts for Oxygen-and Hydrogen-Involving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. (4) Zhang, M.; De, R. M.; Frei, H. Time-Resolved Observations of Water Oxidation Intermediates on a Cobalt Oxide Nanoparticle Catalyst. Nat. Chem. 2014, 6, 362-367. (5) Jiang,

N.;

You,

B.;

Sheng,

M.;

Sun,

Y.

Electrodeposited

Cobalt-Phosphorous-Derived Films as Competent Bifunctional Catalysts for Overall Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 6251-6254. (6) Yu, M. Q.; Jiang, L. X.; Yang, H. G. Ultrathin Nanosheets Constructed CoMoO4 Porous Flowers with High Activity for Electrocatalytic Oxygen Evolution. Chem. Commun. 2015, 51, 14361-14364. (7) Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Metal-Organic Framework Derived Hybrid Co3O4-Carbon Porous Nanowire Arrays as Reversible Oxygen Evolution Electrodes. J. Am. Chem. Soc. 2014, 136, 13925-13931. (8) Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W.; Wang, X. A Metal-Organic Framework-Derived Bifunctional Oxygen Electrocatalyst. Nat. Energy 2016, 1, 15006-15014. (9) Yang, Y.; Fei, H.; Ruan, G.; Tour, J. M. Porous Cobalt-Based Thin Film as a Bifunctional Catalyst for Hydrogen Generation and Oxygen Generation. Adv. Mater. 2015, 27, 3175-3180. (10) Robinson, D. M.; Yong, B. G.; Greenblatt, M.; Dismukes, G. C. Water Oxidation by λ-MnO2: Catalysis by the Cubical Mn4O4 Subcluster Obtained by Delithiation of Spinel LiMn2O4. J. Am. Chem. Soc. 2010, 132, 11467-11469. (11) Han, S.; Liu, S.; Yin, S.; Chen, L.; He, Z. Electrodeposited Co-Doped Fe3O4 Thin Films as Efficient Catalysts for the Oxygen Evolution Reaction. Electrochim. Acta 2016, 210, 942-949. (12) Shi, Y.; Guo, B.; Corr, S. A.; Shi, Q.; Hu, Y. S.; Heier, K. R.; Chen, L.; Seshadri, R.; Stucky, G. D. Ordered Mesoporous Metallic MoO2 Materials with Highly Reversible Lithium Storage Capacity. Nano Lett. 2009, 9, 4215-4220. 15



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

(13) Jin, Y.; Wang, H.; Li, J.; Yue, X.; Han, Y.; Shen, P. K.; Cui, Y. Porous MoO2 Nanosheets as Non-noble Bifunctional Electrocatalysts for Overall Water Splitting. Adv. Mater. 2016, 28, 3785-3790. (14) Wu, Z. S.; Ren, W.; Wen, L.; Gao, L.; Zhao, J.; Chen, Z.; Zhou, G.; Li, F.; Cheng, H. M. Graphene Anchored with Co3O4 Nanoparticles as Anode of Lithium Ion Batteries with Enhanced Reversible Capacity and Cyclic Performance. ACS Nano 2010, 4, 3187-3194. (15) Hou, Y.; Li, J.; Wen, Z.; Cui, S.; Yuan, C.; Chen, J. Co3O4 Nanoparticles Embedded in Nitrogen-Doped Porous Carbon Dodecahedrons with Enhanced Electrochemical Properties for Lithium Storage and Water Splitting. Nano Energy 2015, 12, 1-8. (16) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780. (17) Liu, X.; Chang, Z.; Luo, L.; Xu, T.; Lei, X.; Liu, J.; Sun, X. Hierarchical ZnxCo3-xO4 Nanoarrays with High Activity for Electrocatalytic Oxygen Evolution. Chem. Mater. 2014, 26, 1889-1895. (18) Peng, Z.; Jia, D.; Al‐Enizi, A. M.; Elzatahry, A. A.; Zheng, G. From Water Oxidation to Reduction: Homologous Ni-Co Based Nanowires as Complementary Water Splitting Electrocatalysts Adv. Energy Mater. 2015, 5, 17-37. (19) Yang, Y.; Fei, H.; Ruan, G.; Xiang, C.; Tour, J. M. Efficient Electrocatalytic Oxygen Evolution on Amorphous Nickel-Cobalt Binary Oxide Nanoporous Layers. ACS Nano 2014, 8, 9518-9523. (20) Wang, H.; Lee, H. W.; Deng, Y.; Lu, Z.; Hsu, P. C.; Liu, Y.; Lin, D.; Cui, Y. Bifunctional Non-Noble Metal Oxide Nanoparticle Electrocatalysts through Lithium-Induced Conversion for Overall Water Splitting. Nat. Commun. 2015, 6, 7261. (21) Kuang, M.; Han, P.; Wang, Q.; Li, J.; Zheng, G. CuCo Hybrid Oxides as Bifunctional Electrocatalyst for Efficient Water Splitting. Adv. Funct. Mater. 2016, 26, 8551-8561. 16


Page 16 of 31

Page 17 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


(22) Li, Y.; Hasin, P.; Wu, Y. NixCo3-xO4 Nanowire Arrays for Electrocatalytic Oxygen Evolution. Adv. Mater. 2010, 22, 1926-1929. (23) Driess, M.; Menezes, P.; Indra, A.; Bergmann, A.; Chernev, P.; Walter, C.; Dau, H.; Strasser, P. Uncovering the Prominent Role of Metal Ions in Octahedral Versus Tetrahedral Sites of Cobalt-Zinc Oxide Catalysts for Efficient Oxidation of Water. J. Mater. Chem. A 2016, 4, 10014-10022. (24) Cho, K.; Han, S. H.; Suh, M. P. Copper-Organic Framework Fabricated with CuS Nanoparticles: Synthesis, Electrical Conductivity, and Electrocatalytic Activities for Oxygen Reduction Reaction. Angew. Chem. 2016, 5, 15301-15305. (25) Chughtai, A. H.; Ahmad, N.; Younus, H. A.; Laypkov, A.; Verpoort, F. Metal-Organic Frameworks: Versatile Heterogeneous Catalysts for Efficient Catalytic Organic Transformations. Chem. Soc. Rev. 2015, 44, 6804-6849. (26) Müller, G. F. J.; Stürzel, M.; Mülhaupt, R. Core/Shell and Hollow Ultra High Molecular Weight Polyethylene Nanofibers and Nanoporous Polyethylene Prepared by Mesoscopic Shape Replication Catalysis. Adv. Funct. Mater. 2014, 24, 2860-2864. (27) Huang, Z. F.; Song, J.; Li, K.; Tahir, M.; Wang, Y. T.; Pan, L.; Wang, L.; Zhang, X.; Zou, J. J. Hollow Cobalt-based Bimetallic Sulfide Polyhedra for Efficient All-pH-Value Electrochemical and Photocatalytic Hydrogen Evolution. J. Am. Chem. Soc. 2016, 138, 1359-1365. (28) Zhang, G.; Lou, X. W. General Synthesis of Multi-Shelled Mixed Metal Oxide Hollow Spheres with Superior Lithium Storage Properties. Angew. Chem. 2014, 53, 9041-9044. (29) Yu, Y.; Yin, X.; Kvit, A.; Wang, X. Evolution of Hollow TiO2 Nanostructures via

the

Kirkendall

Effect

Driven

by

Cation

Exchange

with

Enhanced

Photoelectrochemical Performance. Nano Lett. 2014, 14, 2528-2535. (30) Guo, W.; Sun, W.; Wang, Y. Multilayer CuO@NiO Hollow Spheres: Microwave-Assisted Metal-Organic-Framework Derivation and Highly Reversible Structure-Matched Stepwise Lithium Storage. ACS Nano 2015, 9, 11462-11471. (31) Xia, W.; Zou, R.; An, L.; Xia, D.; Guo, S. A Metal-Organic Framework Route to in Situ Encapsulation of Co@Co3O4@C Core@Bishell Nanoparticles into a Highly 17



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

Ordered Porous Carbon Matrix for Oxygen Reduction. Energy Environ. Sci. 2014, 8, 568-576. (32) Lei, Z.; Hao, B. W.; Xiong, W. L. Metal-Organic-Frameworks-Derived General Formation of Hollow Structures with High Complexity. J. Am. Chem. Soc. 2013, 135, 10664-10672. (33) Li, L.; Tian, T.; Jiang, J.; Ai, L. Hierarchically Porous Co3O4 Architectures with Honeycomb-Like Structures for Efficient Oxygen Generation from Electrochemical Water Splitting. J. Power Sources 2015, 294, 103-111. (34) Zhou, L.; Zhao, D.; Lou, X. W. Double-Shelled CoMn2O4 Hollow Microcubes as High-Capacity Anodes for Lithium-Ion Batteries. Adv. Mater. 2012, 24, 745-748. (35) Wu, Z.; Sun, L. P.; Yang, M.; Huo, L. H.; Zhao, H.; Grenier, J. C. Organic Frameworks-Derived Co3O4 Hollow Dodecahedrons with Controllable Interiors as Outstanding Anodes for Li Storage. J. Mater. Chem. A 2014, 2, 12194-12200. (36) Li, J.; Wang, J.; Liang, X.; Zhang, Z.; Liu, H.; Qian, Y.; Xiong, S. Hollow MnCo2O4 Submicrospheres with Multilevel Interiors: From Mesoporous Spheres to Yolk-In-Double-Shell Structures. ACS Appl. Mater. Interfaces 2014, 6, 24-30. (37) Guan, J.; Mou, F.; Sun, Z.; Shi, W. Preparation of Hollow Spheres with Controllable Interior Structures by Heterogeneous Contraction. Chem. Commun. 2010, 46, 6605-6607. (38) Xu, X.; Cao, R.; Jeong, S.; Cho, J. Spindle-like Mesoporous α-Fe₂O₃ Anode Material Prepared from MOF Template for High-Rate Lithium Batteries. Nano Lett. 2012, 12, 4988-4991. (39) Wu, R.; Qian, X.; Rui, X.; Liu, H.; Yadian, B.; Zhou, K.; Wei, J.; Yan, Q.; Feng, X. Q.; Long, Y. Zeolitic Imidazolate Framework 67-Derived High Symmetric Porous Co3O4 Hollow Dodecahedra with Highly Enhanced Lithium Storage Capability. Small 2014, 10, 1932-1938. (40) Mohamed, S. G.; Tsai, Y. Q.; Chen, C. J.; Tsai, Y. T.; Hung, T. F.; Chang, W. S.; Liu, R. S. Ternary Spinel MCo2O4 (M=Mn, Fe, Ni, and Zn) Porous Nanorods as Bifunctional Cathode Materials for Lithium-O2 Batteries. ACS Appl. Mater. Interfaces 2015, 7, 12038-12046. 18


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(41) Xu, W.; Scott, K. CuxCo3-xO4 (0≤ x

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