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Gram-Scale Preparation of 2D Transition Metal Hydroxide/Oxide Assembled Structures for Oxygen Evolution and Zn-Air Battery Wenxian Liu, Ruilian Yin, Wenhui Shi, Xilian Xu, Xuhai Shen, Qichen Yin, Lixin Xu, and Xiehong Cao ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01613 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 9, 2018

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ACS Applied Energy Materials

Gram-Scale Preparation of 2D Transition Metal Hydroxide/Oxide Assembled Structures for Oxygen Evolution and Zn-Air Battery Wenxian Liu,†,‡ Ruilian Yin,†,‡ Wenhui Shi,§,± Xilian Xu,† Xuhai Shen,† Qichen Yin,† Lixin Xu,† Xiehong Cao*,†,# †College

of Materials Science and Engineering, Zhejiang University of Technology, 18 Chaowang

Road, Hangzhou 310014, China §Center

for Membrane Separation and Water Science & Technology, Ocean College, Zhejiang

University of Technology, 18 Chaowang Road, Hangzhou 310014, China ±Huzhou

Institute of Collaborative Innovation Center for Membrane Separation and Water

Treatment, Zhejiang University of Technology, 313000, Huzhou, P. R. China #State

Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, Zhejiang

University of Technology, 18 Chaowang Road, Hangzhou 310032, China.

ABSTRACT: Construction of 2D materials into a hierarchical structure cannot only effectively avoid restacking of individual nanosheets but also endows them with improved catalytic efficiency, which have generated extensive interest in recent years. Nevertheless, the scalable and effective preparation of 2D materials constructed hierarchical structures, such as unique 2D/1D

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structure, is rarely achieved. Herein, we report a facile alkali-soaking strategy for the preparation of 2D transition metal hydroxide/oxide nanosheets-assembled hierarchical structures, in which the rational designed bimetallic metal-organic frameworks (MOFs) are annealed followed by a simple alkali soaking. Our method is capable of preparing various hierarchical structures based on 2D materials, including Co(OH)2, Ni(OH)2, and Mn3O4 nanosheets. Moreover, a high yield of ~2.2 g was achieved for a batch of Co(OH)2 nanosheet-assembled structure. Impressively, the assynthesized Co(OH)2 hierarchical structure shows excellent electrocatalytic performances towards the oxygen evolution reaction (OER), which achieves a current density of 10 mA·cm-2 at a low overpotential of 267 mV with a small Tafel slope of 62.0 mV dec-1. Furthermore, it delivers a small charging/discharging voltage gap of 2.2 V at 75 mA·cm-2, and high stability for over 240 h, when used as air cathode for rechargeable Zn-air batteries.

KEYWORDS: 2D materials, hierarchical structures, metal-organic frameworks, oxygen evolution reaction, Zn-air batteries

1．INTRODUCTION 2D nanomaterials, such as graphene, MXenes, transition metal dichalcogenides, layered metal oxides and hydroxides, have attracted extensive interest due to their unique physical and chemical properties.1-4 These distinctive properties make them promising for a variety of applications, such as catalysis, energy storage and conversion, sensing, and gas separation.5-9 In particular, 2D materials based on layered transition metal oxides/hydroxides are extremely attractive, due to the tunability of metal ions and considerable theoretical activity.8,

10-13

For instance, Co3O4

nanosheets,14 ultrathin α-Co(OH)2 nanosheets,15 and porous Ni(OH)2 nanosheets10,16 have been


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reported possessing outstanding oxygen evolution reaction (OER) activity. However, the practical applications of 2D metal oxides and/or hydroxides are hindered by their highly tendency towards restacking, which results in decreasing of surface area and blocking of active sites. Construction of 2D metal hydroxide/oxide into a hierarchical structure is able to overcome drawbacks of individual nanosheets. Especially, the smart assembly of 2D nanosheets into 1D structure (2D/1D structure) can achieve the combination of high surface-to-volume ratio of 2D materials and the superior charge-transport ability of 1D structure.17-21 In this regard, great effort has been devoted to the exploration of hierarchical metal hydroxide/oxide structures built by their 2D counterparts. For example, a hierarchical Co(OH)F assembled by building blocks of 2D nanoﬂake and 1D rod was reported by Cao et al18 showing excellent activity for OER. Qi et al. developed a diffusion-mediated hydrothermal method to produce hierarchical nanosheet-based NiMoO4 nanotubes with a large specific surface area, which exhibited superior electrochemical performances when used as electrode materials for supercapacitors.22 Nevertheless, current methods are generally complicated with poor universality and inefficient. Thus, it is highly desirable to develop a general and scalable method to prepare hierarchical metal hydroxide/oxide nanosheets with highly ordered nanostructure. Recently, metal-organic frameworks (MOFs), assembled by metal ions/clusters and organic ligands, have been demonstrated to be promising precursors for porous nanomaterials.23-29 Huang et al. synthesized a porous Co3O4 polyhedron by annealing the Co-based zeolitic imidazolate framework (ZIF-67).30 Wang et al. fabricated a mesoporous NixCo3-xO4 rod by the calcination of bimetallic Co/Ni-MOF-74.31 Our group recently developed a series of nanostructures, such as reduced graphene oxide (rGO)-wrapped MoO3 rod,32 porous Co3O4/rGO, Fe2O3/rGO composite


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fibers, by using MOFs as precursors.33-35 However, the scalable construction of 2D/1D hierarchical structure through a facile way is still highly demanded. Here, we report a facile, general, and scalable alkali-soaking method for the preparation of various hierarchical structures based on 2D transition metal hydroxide/oxide nanosheets, including Co(OH)2, Ni(OH)2, and Mn3O4 nanosheets. The rational designed bimetallic MOF rods obtained in a facile refluxing method were served as precursors for the preparation of metal molybdate rods, which were then immersed into alkaline solution at ambient condition, resulting in the formation of 2D metal hydroxide/oxide nanosheets-assembled 1D structures. Remarkably, the obtained Co(OH)2 hierarchical structure manifested superior catalytic activity and stability, when evaluated as an electrocatalyst for oxygen evolution reaction (OER). Moreover, rechargeable Zn-air battery assembled by the prepared Co(OH)2 hierarchical structure and Zn plate exhibited high open-circuit voltage of 1.564 V and low charging/discharging voltage gap, further revealing the feasibility of our preparation strategy for practical application. 2．EXPERIMENTAL METHODS 2.1 Synthesis of bimetallic Co/Mo-based metal-organic framework (Co/Mo-MOF) rods. In a typical synthesis, 3.0 g of Co(NO3)2·6H2O, 5.0 g MoO3, and 4.8 g of 2-methylimidazole (Hmim) were added to a round bottom flask containing 500 mL deionized water. Then the mixture was refluxed at 120 °C for 12 h. After naturally cooling to room temperature, the products were collected and washed by deionized water for three times. After drying in vacuum at 60 °C for 12 h, the purple Co/Mo-MOF crystals were obtained. 2.2 Synthesis of Co(OH)2 hierarchical structure.


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Co(OH)2 hierarchical structure were synthesized through a calcination process followed by an alkali soaking approach by using the as-synthesized Co/Mo-MOF as precursor. Briefly, the asobtained Co/Mo-MOF crystals were placed in a tube furnace, and annealed in air at 500 °C for 3 h at a ramp rate of 10 °C min-1 to obtain Co/Mo-MOF derived CoMoO4 rods. Afterwards, 1.8 g of the as-synthesized CoMoO4 rods were added into a KOH aqueous solution (0.1 M, 500 mL) and further stirred for 6 h. Co(OH)2 hierarchical structure were harvested by centrifugation, followed by washing with deionized water for three times, and drying in vacuum at 60 °C for 12 h. 2.3 Synthesis of other bimetallic MOF rods. The samples were prepared by a procedure similar to that of Co/Mo-MOF, except for 3.0 g of Ni(NO3)2·6H2O or 2.6 g of Mn(NO3)2·4H2O was used instead for the preparation of Ni/Mo-MOF rods, and Mn/Mo-MOF rods, respectively. 2.4 Synthesis of other transition metal hydroxide/oxide nanosheets-assembled hierarchical structure. Ni(OH)2 and Mn3O4 hierarchical structure were prepared by a procedure similar to that of Co(OH)2 hierarchical structure, except for Ni/Mo-MOF and Mn/Mo-MOF precursors were used, respectively. 2.5 Material characterizations. The morphologies of the as-prepared samples were examined using field emission scanning electron microscopy (FESEM, FEI Nova NanoSEM 450), transmission electron microscopy (TEM, JEM-100CX II), and high-resolution TEM (HRTEM, FEI Talos S-FEG). The crystal-line structures of the samples were identified by X-ray diffraction (XRD) on an X’Pert PRO


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(PNAlytical) with Cu Kα radiation (1.54 Å). The element contents of the samples were determined by inductively coupled plasma mass spectrometry (ICP-MS, NexlON 300X, PerkinElmer). The N2 adsorption-desorption isotherms and Brunauer Emmett Teller (BET) surface area were determined using a TriStar II 3020 surface area analyzer. The samples were degassed at 353 K for 6 h prior to analysis. 2.6 Electrochemical measurements. OER tests were carried out in a standard three-electrode system on a CHI 760E electrochemical station (CH Instruments, Shanghai, China) in 1.0 M KOH aqueous electrolyte. To prepare the catalyst ink, 5.0 mg of catalysts were dispersed into a mixed solution of 768 µL deionized water, 200 µL absolute ethanol, and 32 µL Nafion (5%, DuPont) under sonication for 40 min. Then, 200 uL of the catalyst ink was coated onto the carbon paper (1×1 cm2) electrode and dried at room temperature to obtain the working electrode (loading amount of 1 mg·cm-2). Saturated calomel electrode (SCE) and carbon rod was applied as the reference and counter electrode, respectively. All the measured potentials were calibrated to the reversible hydrogen electrode (RHE) using the following equation: E(RHE) = E(SCE) + 0.241 + (0.059 × pH). The polarization curve measurements were performed by linear sweep voltammetry (LSV) at a scan rate of 5 mV·s-1 with iR corrected. The electrochemical impedance spectroscopy (EIS) tests were measured at the polarization potential (1.5 V vs. RHE) in the frequency between 0.01 Hz and 100 kHz. The stability of the catalyst was tested by using chronopotentiometry at 10 mA·cm-2 for 12 h. For home-made rechargeable Zn-air battery assembly, a polished Zn plate and an aqueous solution containing 6 M KOH and 0.2 M zinc acetate were used as anode and electrolyte, respectively. The air-cathode was prepared by coating Co(OH)2 hierarchical structure coupled with


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20 wt.% Pt/C (mass ratio of 1:1) on Nafion-coated nickel foam with mass loading of 2.0 mg·cm-2. The polarization curves were measured by LSV at 10 mV·s-1 using a CHI 760E electrochemical station. The cycling performance of the Zn-air battery was examined on a NEWARE battery test system at a current density of 15 mA·cm-2. Each cycle period was set to be 70 min, i.e. discharge for 30 min, charge for 30 min and stand for 10 min. 3．RESULTS AND DISCUSSION Scheme 1 illustrates the synthetic process of metal hydroxide/oxide nanosheets-assembled hierarchical structures based on alkali-soaking strategy. First, bimetallic MOFs (M/Mo-MOF, M=Co, Ni, Mn) were synthesized through refluxing of commercial MoO3 (Figure S1), transition metal nitrates, and 2-methylimidazole (Hmim). Subsequently, the resultant bimetallic M/Mo-MOF rods were subjected to calcination at 500 °C in air to form rod-like metal molybdates (MMoO4). After that, MMoO4 rods were immersed in an aqueous solution of KOH. By carefully regulation the metathesis reaction between MMoO4 and OH- (equation 1, 2), metal hydroxides and/or oxides with hierarchical structure were obtained. It is worth mentioning that the simple alkali treatment allows this synthetic method to be scaled up to gram level (Figure S2). MMoO4 +2OH ― ⇄M(OH)2 +MoO24 ―

(1)

MMoO4 +2OH ― ⇄MO + MoO24 ― + H2O

(2)


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Scheme 1. Schematic illustration of the synthetic process for the 2D transition metal hydroxide/oxide nanosheet-assembled hierarchical structure. Figure 1a and b are the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of Co/Mo-MOF rods obtained by the refluxing process, in which the as-synthesized purple bimetallic Co/Mo-MOF with smooth surface exhibit a rod-like morphology with diameters of 500 - 800 nm and lengths of 5 - 10 µm. The single crystalline nature of the bimetallic Co/Mo-MOF was confirmed by the high-resolution TEM (HRTEM) image and the selected area electron diffraction (SAED) pattern (Figure 1c). Figure 1d shows a high-angle annular dark-field (HAADF) scanning TEM image and the corresponding energy dispersive X-ray (EDX) mapping images of the rod-like bimetallic MOF, in which Co, Mo, C, N elements are homogeneously distributed. After the subsequent calcination of Co/Mo-MOF in air, CoMoO4 rods with gray-purple color were obtained (Figure 1e-h). As shown in Figure 1e and f, CoMoO4-derived from Co/Mo-MOF maintains the 1D rod-like structure well, and no significant changes in size


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were observed as well. HRTEM image in Figure 1g displays distinct lattice fringes with interplanar spacings of 0.67 nm, which is in agreement with the (001) lattice plane of CoMoO4. It is worth mentioning that CoMoO4 rod obtained by calcining MOF maintains a single crystalline nature, as indicated by the SEAD pattern (inset of Figure 1g). In addition, the EDX mapping in Figure 1h reveals the homogeneous distribution of Co, Mo, O elements in the MOF-derived CoMoO4 rods. Impressively, after soaking the gray-purple CoMoO4 rods in the KOH solution, a brown color product was obtained. Representative SEM and TEM images in Figure 1i and j show the sample changed from a rod-like structure with smooth surface to a 2D nanosheets-assembled hierarchical structure after soaking in a KOH solution. The interconnected nanosheets with a diameter of ~400 nm and a thickness of ~10 nm are uniformly and orderly assembled on the rods (inset of Figure 1i and j). HRTEM image of a typical nanosheet shows a lattice spacing of 0.47 nm (Figure 1k), which can be indexed to the (001) facets of Co(OH)2. Noticeably, the HAADF TEM-EDX characterizations indicate that the rod is composed of Co and there is no Mo (Figure 1l), which may be due to the dissolution of the generated MoO42- in the reaction supernatant. To gain further insight into the formation process of Co(OH)2 hierarchical structure, we carried out the time-dependent experiments to investigate the reaction intermediate states. As shown in Figure S3a, a single-crystalline CoMoO4 rod presents a smooth surface with a diameter of ~500 nm. After soaking CoMoO4 rods in a KOH solution for 1 min, a small amount of nanosheets appeared on the surface of the rod (Figure S3b). As the reaction time prolongs, the size and quantity of Co(OH)2 nanosheets gradually increased, finally forming Co(OH)2 hierarchical structures (Figure S3c, d).


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Figure 1. (a, e, i) SEM images, (b, f, j) TEM images, (c, g, k) HRTEM images, and (d, h, l) EDX mappings of Co/Mo-MOF rods (a-d), MOF derived CoMoO4 rods (e-h), and Co(OH)2 hierarchical structures (i-l). Insets: corresponding photographs, high-magnification SEM images and SEAD patterns of the samples. Inductively coupled plasma mass spectrometry (ICP-MS) measurements display that there was a large number of Mo species in the supernatant fluid after the alkali treatment (c ≈ 3.4 × 10-3 mol·L-1), while no Co species was detected (lower than the detection limit of 1.7 × 10-4 mol·L1),

which further confirms that Mo in the CoMoO4 rod was etched during the alkali treatment.

Furthermore, in the comparative experiments, it was found that both monometallic Co-Hmim MOF and Mo-Hmim MOF could not be converted into hierarchical nanostructures after the aforementioned calcination-alkali soaking process (Figure S4 and 5), indicating that the rational


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designed Co/Mo bimetallic MOF plays an important role in the preparation of Co(OH)2 hierarchical structure. The XRD patterns of the bimetallic Co/Mo-MOF, MOF-derived CoMoO4, and Co(OH)2 hierarchical structure are shown in Figure 2a and S6. The sharp and strong diffraction peaks of Co/Mo-MOF reveal its good crystallinity (Figure S6). After the calcination process, the XRD diffraction peaks appearing in the sample can be well-indexed to CoMoO4 (JCPDS No. 21-0868). Subsequently, after the alkali soaking process, the diffraction peaks of CoMoO4 crystals were disappeared, and a new set of diffraction peaks appeared at 19°, 32°, 38°, 51°, and 58°, which corresponds to the (001), (100), (101), (102), and (110) crystal planes of hexagonal Co(OH)2 (JCPDS No. 45-0031), respectively.36 To further investigate the chemical composition and chemical environment of the samples, X-ray photoelectron spectroscopy (XPS) measurements were performed. The survey spectra in Figure 2b indicate that Mo was etched after the alkali treatment, which is consistent with the observation from EDX mapping (Figure 1h and i). Figure 2c shows the high-resolution XPS spectrum of Co 2p. The deconvoluted peaks at 780.6 and 782.1 eV in Co 2p3/2 spectra, as well as two satellite peaks observed at 786.2 and 802.9 eV indicate that Co exists in the formed Co(OH)2.5, 7 In O 1s spectrum (Figure 2d), the binding energies of 531.1 and 532.8 eV correspond to Co(OH)2 and H2O, respectively, which is consistent with previously reported result of the O 1s spectrum.7, 37, 38


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Figure 2. (a) XRD patterns of MOF-derived CoMoO4 rods, and Co(OH)2 hierarchical structure. (b) Full-survey XPS spectra of Co/Mo-MOF rods, MOF-derived CoMoO4 rods and Co(OH)2 hierarchical structure. (c-d) High-resolution Co 2p (c) and O 1s (d) spectra of Co(OH)2 hierarchical structure. In order to demonstrate the universality of our developed method, other metal hydroxides and/or oxides with 2D nanosheets-assembled hierarchical structures, i.e. Ni(OH)2 and Mn3O4, were prepared by replacing Co(NO3)2·6H2O with the corresponding metal salts (Figure 3 and S7-S12). The bimetallic Ni/Mo-MOF (Figure S7) exhibits a 1D rod-like structure with a length of ~8 µm and a diameter of ~500 nm. TEM image in Figure 3a further reveals its single-crystalline nature. After the calcination process, Ni/Mo-MOF crystals converted into NiMoO4 rods with a smooth surface (Figure S8). Subsequently, after a simple alkali soaking process, rods assembled by Ni(OH)2 nanosheets are formed, in which the thickness of the nanosheets was about 10 nm and the diameter was about 200 nm (Figure 3b). Their corresponding XRD patterns (Figure S9)


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demonstrate the crystal phase transformation of the samples, and indicate the formation of hexagonal-phased β-Ni(OH)2 structures (JCPDS No. 14-0117). As another example, bimetallic Mn/Mo-MOF crystals (Figure 3c and S10) were chosen as precursor for the preparation of Mn3O4 nanosheets-assembled hierarchical structure. The rod-like MnMoO4 intermediate is obtained by calcining Mn/Mo-MOF in air (Figure S11a and b). The lattice spacing of 0.69 nm corresponds to the (001) lattice plane of MnMoO4 (Figure S11c). The Mn3O4 nanosheets-assembled hierarchical structure was obtained after the alkali treatment process (Figure 3d). Their corresponding XRD patterns further confirm the formation of tetragonal Mn3O4 (JCPDS No. 24-0734, Figure S12).

Figure 3. (a) TEM image of a Ni/Mo-MOF rod. (a1) HRTEM image and (a2) corresponding SEAD pattern of the Ni/Mo-MOF rod in (a). (b) SEM images of Ni(OH)2 hierarchical structure. Inset: high-magnification SEM image. (c) TEM image of a Mn/Mo-MOF rod. (c1) HRTEM image and (c2) corresponding SEAD pattern of the Mn/Mo-MOF rod in (c). (d) SEM images of Mn3O4 hierarchical structure. Inset: high-magnification SEM image.


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To demonstrate the structural advantage of the hierarchical 2D/1D materials in electrocatalysis, the OER activity of the as-obtained Co(OH)2 hierarchical structure was evaluated in 1.0 M KOH using a standard three-electrode cell. For comparison, bare carbon paper, commercial IrO2, and the stacked Co(OH)2 nanosheets prepared by a hydrothermal method (Figure S13)36,39 were also investigated. Figure 4a shows linear sweep voltammetry (LSV) curves of the catalysts with iR compensation. In sharp contrast, the Co(OH)2 hierarchical structure exhibited a relatively low overpotential of 267 mV to achieve a current density of 10 mA·cm-2, which is much lower than that of stacked Co(OH)2 nanosheets (340 mV) and commercial IrO2 (371 mV). In addition, compared to stacked Co(OH)2 nanosheets, both BET surface area and electrochemically active surface area (ECSA) of Co(OH)2 hierarchical structure are high (Figure S14-15). Furthermore, the as-prepared Co(OH)2 hierarchical structure shows comparable and even better OER performance to those of previously reported hydroxide/oxide nanosheets (Figure 4c, S16),18,36,40-43 due to their unique 2D nanosheets-assembled structure. It is worth mentioning that the bare carbon paper displayed negligible current within the potential rang of 1.1-1.6 V (blue line, Figure 4a). Moreover, the Tafel plots of the prepared catalysts were used to further evaluate the kinetics of the electrocatalytic OER. As shown in Figure 4b, the Tafel slope of Co(OH)2 hierarchical structure is 62.0 mV dec-1, which is smaller than those of commercial IrO2 (79.3 mV dec-1) and stacked Co(OH)2 nanosheets (78.0 mV dec-1), indicates a faster OER kinetic of Co(OH)2 hierarchical structure. The electrochemical impedance spectroscopy (EIS) was used to further study the OER kinetics (Figure S17). As shown in the Nyquist plots, the charge transfer resistance (Rct) in Co(OH)2 hierarchical structure is much smaller than those of stacked Co(OH)2 nanosheets, indicating that charge transfer process within Co(OH)2 hierarchical structure is more effective and thus resulting in better performance. It is worth noting


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Figure 4. (a) Linear sweep voltammetry (LSV) curves, and (b) corresponding Tafel plots of Co(OH)2 hierarchical structure (brown), stacked Co(OH)2 nanosheets (green), commercial IrO2 (purple), and bare carbon paper (blue). (c) Comparison of OER performance of our prepared Co(OH)2 hierarchical structure and other previously reported OER electrocatalysts in 1.0 M KOH at 10 mA·cm-2. Abbreviation: PI = polyimide; CNT = carbon nanotube. (d) Galvanostatic measurement of OER of Co(OH)2 hierarchical structure (brown), stacked Co(OH)2 nanosheets (green), and commercial IrO2 (purple) at a constant current density of 10 mA·cm-2. that, some of Co2+ species in Co(OH)2 hierarchical structure was oxidized to Co3+ species in the electrochemical process, as evidenced by high-resolution XPS spectra of Co 2p region in Figure S18a, which is consistent with the previous reports.41-43 Remarkably, Co(OH)2 hierarchical structure showed excellent stability in alkaline media. As shown in Figure 4d, the Co(OH)2 hierarchical structure exhibited a nearly constant operating potential at the current density of 10 mA cm-2 for 12 h (~15 mV increase), while the operating potential of stacked Co(OH)2 nanosheets


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and IrO2 increased considerably within 6 h (~25 mV for stacked Co(OH)2, and ~63 mV for IrO2). Figure S19 shows the 2D nanosheets-assembled structure of the examined electrocatalyst remained after the cycling test, indicating its long-term structural stability during OER process.

Figure 5. (a) Schematic illustration of configuration of the rechargeable Zn-air battery using Co(OH)2 hierarchical structure-Pt/C as the air-cathode. (b) Photograph of a fabricated Zn-air battery showing an open-circuit voltage of 1.564 V. (c) Photograph of a red LED lighted by two Zn-air batteries connected in series. (d) Charge and discharge polarization curves of the fabricated Zn-air batteries. (e) Discharge and charge voltage profiles of Zn-air batteries using Co(OH)2 hierarchical structure-Pt/C as air-cathode at the current density of 15 mA·cm-2.

To further investigate the practical application of the Co(OH)2 hierarchical structure, a homemade rechargeable Zn-air battery was built,44-49 in which the prepared Co(OH)2 hierarchical


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structure coupled with 20 wt% Pt/C with mass ratio of 1:1 was used as the air-cathode (Figure 5a). The open-circuit voltage of the assembled Zn-air battery is measured to be as high as 1.564 V (Figure 5b). Moreover, two batteries connected in series can light up a red light-emitting diode (LED, Figure 5c). As shown in the charge and discharge polarization curves (Figure 5d), the charging voltage of the Zn-air battery based on prepared Co(OH)2 hierarchical structure-Pt/C is lower than that of commercial IrO2-Pt/C,50-53 e.g. 2.2 V versus 2.3 V at 75 mA·cm-2, indicating better rechargeability of our prepared sample. Furthermore, the fabricated Zn-air battery based on Co(OH)2 hierarchical structure-Pt/C exhibited remarkable cycling stability at a constant current density of 15 mA·cm-2 (Figure 5e). Specifically, after a long-term cycling of 207 cycles (over 240 h), the Zn-air battery based on Co(OH)2 hierarchical structure-Pt/C exhibited a negligible change in the voltage gap between charge and discharge, which is identical to IrO2-Pt/C. 4．CONCLUSIONS In summary, we have developed a facile and general method for scalable fabrication of transition metal hydroxide/oxide nanosheets-assembled hierarchical structures through a calcination process followed by an alkali soaking. In the developed approach, the transition metal doped Mo-MOFs were rational designed as the precursors to construct the hierarchical 2D/1D structure. The asobtained Co(OH)2 hierarchical structure showed a remarkable catalytic performance for OER, affording a current density of 10 mA·cm-2 at a low overpotential of 267 mV. In addition, the hierarchical Co(OH)2 nanosheets catalysts exhibited high performance and durability when used as an air-cathode for rechargeable Zn-air battery. Our novel synthetic strategy opens up an avenue for the scalable fabrication of nanosheets with hierarchical 2D/1D structure for energy conversion and storage applications.


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ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website or from the authors. Additional information about the SEM, TEM, XRD, photograph, and electrochemical performance (PDF) AUTHOR INFORMATION Corresponding Author *(X. Cao) E-mail: [email protected] or [email protected]. Author Contributions ‡ W. Liu and R. Yin contributed equally to this paper. ORCID Xiehong Cao: 0000-0002-3004-7518 Wenxian Liu: 0000-0002-2808-1864 Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT This work was supported by the financial support from the Nation-al Natural Science Foundation of China (51602284, 51702286), Zhejiang Provincial Natural Science Foundation of China


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Table of Contents


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Oxide

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