Shell NiO@Co3O4

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Materials and Interfaces 3

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Surface-Restructuring Engineered Core/Shell NiO@CoO Nanocomposites as Efficient Catalysts for Oxygen Evolution Reaction Fengru Cheng, Xiaoming Fan, Xikui Chen, Cheng Huang, Zeheng Yang, Fei Chen, Mengqiu Huang, Shuai Cao, and Weixin Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02626 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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Industrial & Engineering Chemistry Research

Surface-Restructuring NiO@Co3O4

Engineered

Nanocomposites

Core/Shell

as

Efficient

Catalysts for Oxygen Evolution Reaction Fengru Cheng,†§ Xiaoming Fan,†§ Xikui Chen, † Cheng Huang, † Zeheng Yang,† Fei Chen,† Mengqiu Huang,† Shuai Cao,† and Weixin Zhang*† † School of Chemistry and Chemical Engineering, Hefei University of Technology, Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, Hefei 230009, China. Email: [email protected] ABSTRACT: Developing electrocatalysts with rich active sites is highly demanded for oxygen evolution reaction (OER) with the virtues of high efficiency and cheap cost. Herein, we prepared unique oxygen vacancy-rich core/shell NiO@Co3O4 nanocomposites decorated with Co3O4 nanosheets as efficient OER electrocatalyst. First, NiO@Co3O4 nanocomposites were synthesized through a rationally-designed stepwise co-precipitation process followed by controlled heating treatment. With a facile NaBH4 reduction treatment, Co3O4 nanosheets were in situ formed on the surface of NiO@Co3O4 nanocomposites, introducing more oxygen vacancies and edge sites simultaneously. This surface-restructured core/shell NiO@Co3O4 nanocomposites exhibited significantly improved activity as an OER electrocatalyst possessing a low overpotential (290 mV at 10 mA cm-2) and a small Tafel

ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 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

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slope (68 mV dec-1) in 1 M KOH media. Our work offers an effective strategy to prepare core/shell

metal

oxide

nanocomposites

with

adjustable

surface

structures

for

electrocatalysis. KEYWORDS: transition metal oxides, core/shell, nanosheets, oxygen vacancies, oxygen evolution reaction INTRODUCTION Electrocatalytic water splitting has been one of the highly potential candidates to meet the demands of solutions to energy production and consumption.1,2 However, efficient electrocatalysts for oxygen evolution reaction (OER), the half-cell anodic reaction in electrocatalytic water splitting, are crucial for practical use owing to its intrinsic sluggish four-electron transfer kinetics.3,4 Noble metal oxides including RuO2 and IrO2 have been recognized as efficient OER electrocatalysts, presenting relatively low overpotentials.5,6 Nevertheless, their high cost and scarcity limit commercial applications.7,8 Therefore, it urgently needs to develop novel cost-effective and earth-rich materials for efficient OER electrocatalysis. Recently, transition metal oxide (TMO) electrocatalysts have drawn substantial interests in OER due to their cheap cost and abundant reserves.9,10 However, the OER performances of single-component TMO, such as NiO, CoxOy, MnxOy, are not satisfactory in terms of their activity and durability.11,12 Therefore, many researchers focus on developing new strategies by component optimization and structure regulation in order to improve their OER performances. On one hand, TMOs composites or multi-component TMOs, such as NiCo2O4, NiFe2O4, CoFe2O4, NiO/Co3O4@NC, NiCo@NiCoO2/C13-17 are found to be


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effective to activate OER at lower overpotentials by optimizing the adsorption energies of oxygen intermediates when foreign metals are introduced. On the other hand, design of nanostructured electrocatalysts could realize the electronic configuration and surface geometric

construction,

which

influences

the

enhancement

of

intrinsic

OER

performance.18,19 Moreover, the electronic configuration of electrocatalysts, correlated with their electrical conductivity and water adsorption capability, has been widely adopted to enhance their OER activities.20 Defect engineering, especially for the creation of oxygen vacancies, has also been regarded as an effective way to achieve the delocalization of electrons to promote the electrochemical behaviour of catalysts.21,22 Therefore, a surfacerestructuring strategy for defects creation and nanostructure construction, would endow TMOs composites with high-performance for OER electrocatalysis. To date, Co3O4 and NiO bimetal oxide composites are considered as good candidates for active TMOs electrocatalysts.23-27 However, developing a simple wet-chemical method to modulate simultaneously the surface/geometric structure and defects of Co3O4/NiO composites remains a big challenge. Herein, we report unique oxygen vacancy-rich core/shell NiO@Co3O4 nanocomposites decorated with Co3O4 nanosheets as efficient OER electrocatalyst. Core/shell NiO@Co3O4 nanocomposites (denoted as NiO@Co3O4) composed of both nanoparticles were first prepared by stepwise co-precipitation of nickel carbonate and cobalt carbonate followed by controlled heating treatment. Then, the product was reduced by NaBH4 aqueous solution and changed into modified NiO@Co3O4 (denoted as M-NiO@Co3O4), which led to forming Co3O4 nanosheets in situ on the M-NiO@Co3O4 surface. With this facile post-treatment, enriched edge sites and oxygen vacancies could be produced, which make the as-prepared M-NiO@Co3O4 display significantly improved electrocatalytic performances for OER.


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EXPERIMENTAL SECTION Synthesis and Modification of NiO@Co3O4 Core/shell NiO@Co3O4 nanocomposites were prepared in a simple stepwise coprecipitation process. In a typical experiment, 50 mL of NaHCO3 aqueous solution (0.75 M) was added to 50 mL of NiSO4 aqueous solution (0.025 M) and a green precipitate was formed under magnetic stirring at room temperature. Then 50 mL of CoSO4 aqueous solution (0.05 M) was added to the suspension. After 7 h, the precipitate was obtained by using centrifugation and washed with deionized (DI) water for several times, and further annealed at 450°C for 2 h under air atmosphere to gain core/shell NiO@Co3O4 nanocomposites. The nanocomposites were further treated in 1 M NaBH4 aqueous solution (40 mL) for 1 h under magnetic stirring, and then the solid powder was obtained by using centrifugation, and then washed with DI water for several times and dried at 70 °C for 12 h in vacuum (denoted as M-NiO@Co3O4). Material Characterizations The as-prepared samples were tested by X-ray powder diffraction (XRD) on an X-ray diffractometer by using a Cu Kα radiation source (λ= 0.154178 nm) performed at 40 kV and 80 mA (X’PERT PRO, Panalytical). The 2θ ranged from 10o to 80o and scan rate was 10o min-1. Fieldemission scanning electron microscope (FESEM) (SU8020, Hitachi Limited Corporation) measurements were carried out at an acceleration voltage of 15 kV and 5 kV, respectively. The samples were dispersed in absolute ethyl alcohol under ultrasonic treatment, and then dropped on a silicon substrate. The morphologies of catalysts for stability test were observed by casting catalyst powders on the nickel foam. Transmission electron microscope (TEM) images were measured by LaB6 transmission electron microscope performed at 120 kV (JEM-1400flash, JEOL


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Limited Corporation). High-resolution TEM images (HRTEM), energy dispersive spectroscopy (EDS) and select area electron diffraction (SAED) patterns of the samples were carried out with a field-emission TEM (FETEM) performed at 200 or 100 kV (JEM-2100F, JEOL Limited Corporation). The samples were dispersed in absolute ethyl alcohol under ultrasonic treatment, and then dropped on the copper grid. X-ray photoelectron spectroscopy (XPS) was recorded in an XPS system with a Al Kα source operated at 150 W (AXIS ULTRA, KRATOS). All the XPS data were calibrated according to the C 1s peak which is centered at 284.6 eV representing C-C bonding. The nitrogen sorption isotherms were measured on a physisorption apparatus at liquid nitrogen temperature (NOVA 2200e, Quantachrome). The specific surface area for each sample, which was degassed at 120 oC overnight before the measurement, was calculated based on multipoint Brunauer-Emmett-Teller (BET) procedure. OER Catalytic Measurements The catalyst ink for OER studies was prepared by adding 10 mg of catalyst in 1 mL of absolute ethyl alcohol with the presence of 100 μl Nafion solution (5 wt.%), which were homogenized in an ultrasonic bath for 30 min. The electrochemical measurements were conducted by using an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Co. Ltd.) and a rotating disk electrode (RDE) system (Pine Instruments Co. Ltd) in a three-electrode cell in which 1 M O2saturated KOH solution was used as electrolyte. The as-prepared catalyst ink (4 µL) was then casted on a freshly polished glassy carbon rotating disk electrode (GC-RDE, 5 mm in diameter) via a pipette with a constant mass loading (0.186 mg cm-2), which was used as the working electrode. As a contrast, commercial RuO2 suspension with the same mass loading was dropped on the GC-RDE surface. A Pt wire was applied as the counter electrode, and Hg/HgO (1 M KOH) electrode was chosen as the reference electrode. All the potentials in this work were presented


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regarding to a reversible hydrogen electrode (RHE). The GC-RDE was rotated at 1600 rpm by a PINE speed regulator. The electrochemical experiments were carried out at about 25 °C with iRcompensation. All the potentials in this work were iR-corrected ones, which were calculated according to the following equation: ERHE (iR-compensation) = EHg|HgO + 0.098+0.059*pH, and overpotential η = ERHE1.23 V. The linear sweep voltammograms (LSVs) curves were conducted at 10 mV s-1 for OER in 1.0 M KOH electrolyte. Cyclic voltammetry method was used to determine Cdl. Tafel slopes were calculated in the fitted linear range of the Tafel plots, which was derived from the corresponding polarization curves, following the equation η = a + blog|j| where η was the overpotential, b represented the Tafel slope, and j referred to the current density. Electrochemical impedance spectroscopy (EIS) was also tested to gain the OER kinetics. The frequency ranged from 0.01 Hz to100 kHz, and the corresponding amplitude was 5 mV. Catalyst powder was casted on nickel foam for stability test. The chronopotentiometry was conducted to evaluate the overpotential at 10 mA cm-2 during the long-term measurement. Accelerated stability was tested in O2-saturated 1.0 M KOH electrolyte at 50 mV s-1 for 1000 cycles at room temperature. At the end of cycles, the OER polarization curve was tested for the resulting electrode.


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RESULTS AND DISCUSSION

Figure 1. Schematic illustration for the preparation of core/shell NiO@Co3O4 nanocomposites as electrocatalysts for OER (the size of the particles doesn't represent the real size). The schematic synthetic process of core/shell M-NiO@Co3O4 (Figure 1) shows that unique core/shell NiO@Co3O4 has been prepared by stepwise co-precipitation followed by controlled heating treatment. Nickel carbonate was first precipitated by mixing sodium bicarbonate and nickel sulfate solution, and then with the addition of cobalt sulfate, cobalt carbonate was then precipitated and attached on the pre-formed nickel carbonate to form core/shell precursors. By controlling the post-heating treatment temperature, core/shell NiO@Co3O4 nanocomposites was formed. The product was further treated with NaBH4 solution to form core/shell M-NiO@Co3O4, which was covered by thin nanosheets on the external surface.


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▼

*▼

(422) (511) (220) (440)

▼

(220)

(111)

(311) (111) (200) (400)

* ▼

Intensity (a.u.)

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▼

▼

▼

*

M-NiO@Co3O4

▼Co3O4 PDF # 42-1467

NiO@Co3O4

* NiO PDF # 47-1049 10

20

30

40 50 60 2Theta (degree)

70

80

Figure 2. XRD patterns of the NiO@Co3O4 and M-NiO@Co3O4 samples. The compositions and microstructures of NiO@Co3O4 nanocomposite oxides before and after NaBH4 reduction have been investigated, respectively. Their XRD patterns (Figure 2) could be both indexed to spinel Co3O4 (JCPDS No. 42-1467) and cubic NiO (JCPDS No. 47-1049), respectively. SEM and TEM images (Figure 3) indicate that both of NiO@Co3O4 and M-NiO@Co3O4 are composed of nanospheres with average diameters of about 100 nm, whereas NiO@Co3O4 particles have relatively smooth surfaces (Figure 3a) and MNiO@Co3O4 possesses rough surfaces (Figure 3b). Interestingly, the rough surface of MNiO@Co3O4 results from the formation of thin nanosheets which could be clearly observed on the external surface (inset in Figure 3b, Figure S1), demonstrating a surfacerestructuring process from nanoparticle to nanosheet during NaBH4 reduction reaction of NiO@Co3O4.


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Figure 3. SEM images of (a) NiO@Co3O4 and (b) M-NiO@Co3O4 (inset: TEM images). HRTEM image and SAED pattern of (c, d) M-NiO@Co3O4 in the shell region (cycle A marked in red in Figure 3b) and (e, f) M-NiO@Co3O4 in the core region. (g) TEM image and EDS images of MNiO@Co3O4. (inset: EDS line-scanning profiles of each element). HRTEM images have been used to further identify the microstructures of core/shell MNiO@Co3O4 nanocomposites. Figure 3c shows that the lattice fringes with a distance of 0.202 nm and 0.246 nm for the thin nanosheet in the shell region could be assigned to (400) and (311) facets of spinel Co3O4. A set of rings in SAED pattern (Figure 3d) originated


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from (511), (311), (222) and (422) facets are in agreement with those of Co3O4 in the XRD pattern (Figure 2). Furthermore, a visible lattice spacing of 0.241 nm in the core region of M-NiO@Co3O4 (Figure 3e) is in accordance with the (111) plane of the cubic NiO. The SAED pattern (Figure 3f) also proves the nanocrystalline nature of the sample, and the rings derived from (111), (200) and (200) facets of NiO can be observed (Figure 2). So Co3O4 nanosheets have been confirmed to be formed and in situ coated on the surface of core/shell M-NiO@Co3O4 with NaBH4 treatment, which would provide additional active sites in edge planes for improved performance in electrocatalytic oxygen evolution.28 EDS has been performed to further identify the core/shell structure. Obviously, for the MNiO@Co3O4, Ni element mainly distributes in the smaller core region, while Co element distributes over a whole particle (Figure 3g). TEM-EDS line concentration profiles further demonstrate the core/shell structures for M-NiO@Co3O4 (inset images in Figure 3g), in which Ni signal mainly concentrates in the central part while Co signal mainly distributes on both sides. Figure S2 also confirms a similar core/shell structure of the NiO@Co3O4 without NaBH4 reduction. The formation of unique core/shell structure for NiO@Co3O4 oxides could be attributed to a rationally designed stepwise co-precipitation process in our method, in which nickel carbonate was precipitated first and subsequently-precipitated cobalt carbonate attached on the pre-formed nickel carbonate (Figure S3 and S4). Furthermore, a facile NaBH4 reduction strategy induced the partial transformation of Co3O4 nanoparticles (outside shell) into Co3O4 nanosheets, leading to the enrichment of active edge sites for electrocatalysis reaction. Interestingly, if the precipitation sequence was reversed, with cobalt carbonate precipitated first and nickel carbonate next, to generate carbonate precursors, which were then calcined and reduced to form the M-Co3O4@NiO,


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no obvious core/shell structure and nanosheets could be observed (Figure S5). Furthermore, for comparison, no nanosheets could be observed to form on the NiO particles reduced by NaBH4 (MNiO), while abundant nanosheets could be found around the Co3O4 particles treated by NaBH4 (M-Co3O4) under the same condition (Figure S6 and S7). These results further confirm that Co3O4 nanosheets secondary structure could be only developed on the surface of the unique metal oxide composites, composed of NiO core encapsulated in Co3O4 shell, by a facile NaBH4 reduction strategy. Inspired by the results mentioned above, the transformation from nanoparticles to nanosheets may be resulted from the partial valence states transformation in Co3O4 (Co2+/Co3+). Nitrogen adsorption/desorption results reveal that the M-NiO@Co3O4 with derived Co3O4 nanosheets possesses higher specific surface area (93 m2 g-1) and larger volume of meso/macro-pores (0.30 cm3 g-1) in comparison with NiO@Co3O4 (39 m2 g-1 and 0.15 cm3 g-1) (Figure 4a, b). The increase of surface area and meso/macro-pore volume may result from the decomposition of carbonate precursors and formation of Co3O4 nanosheets on the surface of M-NiO@Co3O4, which would lead to more active sites and rapid charge transport pathway. XPS measurement was conducted in order to investigate the chemical states on the surface of M-NiO@Co3O4, which was also an important factor for the electrocatalyst. Figure 4c shows the presence of the elements Ni, Co, and O for NiO@Co3O4 and M-NiO@Co3O4. High-resolution spectra for Ni 2p, Co 2p and O1s demonstrate the difference of oxidation states of metal atoms on the surface. In the case of Co 2p spectrum (Figure 4e), the binding energies of peaks at 781.7 eV and 796.5 eV could be indexed to Co2+ and the peaks fitted at 794.9 eV and 780.2 eV could be assigned to Co3+, confirming the existence of Co2+ and Co3+ species.29 The M-NiO@Co3O4 possesses slightly more Co2+ than the NiO@Co3O4,


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revealing the change of valence distribution for Co element after NaBH4 modification. Due to partial conversion from Co3+ to Co2+ via NaBH4 reduction, oxygen vacancies are in situ generated in the surface lattice of M-NiO@Co3O4, resulting in the increased unsaturated coordination for cobalt.30 High-resolution O1s spectrum also proves the generation of oxygen vacancies (Figure 4f). The higher ratio of O2-type fitting peak at 531.3 eV, related to defect sites that originated from low oxygen coordination,30 indicates that the MNiO@Co3O4 possessed more oxygen vacancies. All these results demonstrate that with a simple NaBH4 reduction post-treatment, surface-restructured core/shell NiO@Co3O4 could be achieved, featuring Co3O4 nanosheets decorated on the surface of M-NiO@Co3O4 with enriched edge sites and oxygen vacancies, which would be beneficial for electrocatalytic oxygen evolution.


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Figure 4. N2 adsorption/desorption isotherms of (a) NiO@Co3O4 and (b) M-NiO@Co3O4. (c) Survey XPS spectra of and (d-f) high-resolution XPS spectra of NiO@Co3O4 and MNiO@Co3O4. OER experiments were conducted by casting the catalysts on glassy carbon electrode to investigate their electrocatalytic performance (1.0 M KOH solution). In contrast, M-


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NiO@Co3O4, NiO@Co3O4 and commercial RuO2 catalyst were tested under the same condition. The LSV polarization curve (Figure 5a) indicates that the M-NiO@Co3O4 with abundant oxygen vacancies and edge sites exhibits a low overpotential of 290 mV at 10 mA cm-2, being 77 and 20 mV lower than NiO@Co3O4 and commercial RuO2, respectively. This unique core/shell M-NiO@Co3O4 oxides also present remarkable OER performance in comparison with most other reported metal oxides catalysts (Table S1). Figure 5b shows that the M-NiO@Co3O4 possessed a low Tafel slope of 68 mV dec-1, being lower than that of NiO@Co3O4 and commercial RuO2 (73 and 82 mV dec-1). Obviously, the MNiO@Co3O4 exhibits more favorable OER kinetics. The lower Tafel slope may result from the M-NiO@Co3O4 with Co3O4 nanosheets covered on the surface, possessing higher active surface area and larger mesopore volume, which could provide a shortened charge transfer pathway at the interface of catalysts/electrolyte.31 EIS measurement was further performed to explain the excellent OER activity of M- NiO@Co3O4 (Figure S10). Notably, MNiO@Co3O4 has a smaller semicircle in the low frequency domain of Nyquist plot, related to lower charge transfer resistance compared with NiO@Co3O4, demonstrating the structure advantages of M-NiO@Co3O4 with opened nanosheets and larger pore volume to provide easily accessible active sites and fast charge transfer pathway. Furthermore, electrochemical active surface area (ECSA) test demonstrates the advantages of the unique structure of M-NiO@Co3O4. The double layer capacitance (Cdl) is considered as a useful tool to determine the ECSA32. As presented in Figure 5c, the M-NiO@Co3O4 has a Cdl value of 49 mF cm-2, being almost twice as high as that of the NiO@Co3O4 (25 mF cm-2) (Figure S8). The higher ECSA for the M-NiO@Co3O4 could offer not only more active sites but also higher contact areas at electrode/electrolyte interface during the


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electrolyte ion diffusion process.33 In addition to the improved OER properties mentioned above, the stability is also a key factor to evaluate the electrocatalytic performance. Stability test shows that the as-prepared M-NiO@Co3O4 owns an overpotential of 308 mV for the first cycle, which only increases to 318 mV after tested for 1000 cycles (Figure 5d). Moreover, the M-NiO@Co3O4 almost maintains a constant potential value (~1.55 V vs. RHE) at 10 mA cm-2 for 50 h, revealing its remarkable durability. SEM images and XPS spectra of M-NiO@Co3O4 after long-term stability test are shown in Figure S11 and S12. The M-NiO@Co3O4 catalyst casted on nickel foam exhibits typical granular structure with abundant nanosheets covered on the surface before stability test. After OER process for 50 h, the M-NiO@Co3O4 catalyst still stays on the nickel foam and maintains its original structure (Figure S11), demonstrating its good structure stability. The surface chemical states of M-NiO@Co3O4 after long-term stability test were also identified by XPS spectra (Figure S12). All the XPS spectra (Ni 2p, Co 2p and O1s) could be deconvoluted to the similar species compared with M-NiO@Co3O4 before stability test. We also find that MNiO@Co3O4 after a long-term OER process possesses significantly increased highvalence-state Ni3+ and slightly increased Co3+ compared with the raw M-NiO@Co3O4 (Figure S13), revealing that Ni2+ and Co2+ are partially oxidized to high-valence-state Ni3+ and Co3+ which act as active catalytic sites during OER 34, 35. This result suggests that both NiO core and Co3O4 shell in the core/shell M-NiO@Co3O4 play the crucial role in boosting the OER activity.


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Figure 5. (a) LSV curves, (b) Tafel plots, (c) the capacitive currents plotted as a function of scan rate of M-NiO@Co3O4, NiO@Co3O4 and RuO2.(d) LSV curves of the MNiO@Co3O4 (pasted on the nickel foam) for the first and 1000th cycle (inset: time dependent potential curve at 10 mA cm-2). (e) LSV curves of Co3O4, NiO and NiO@Co3O4


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nanocomposites before and after NaBH4 treatment. (f) LSV curves of the M-NiO@Co3O4, M-Co3O4@NiO and p-M-NiO@Co3O4 (400 oC under air). This work demonstrates the advantages of component optimization and structure regulation strategy to produce the unique core/shell structure, in which NiO core is encapsulated in Co3O4 shell coated with Co3O4 ultrathin nanosheets. As shown in Figure 5e and S9, among all these available materials, the composite oxides M-NiO@Co3O4 present a low overpotential of 290 mV at 10 mA cm-2, being 52 and 44 mV lower than that of singlecomponent oxides (M-NiO and M-Co3O4), respectively. XPS peak fitting results (Figure 4d) show that the M-NiO@Co3O4 possesses a certain amount of Ni3+ in NiO, which is larger than that in pure M-NiO (Figure S14), suggesting partial electron transfer from nickel to cobalt. Such an interaction between NiO and Co3O4 via bridging O2- was beneficial for the enhancement of OER performance.36 Moreover, the M-NiO@Co3O4 also exhibits a small Tafel slope of 68 mV dec-1, being smaller than that of M-NiO (81 mV dec-1) and comparable to that of M-Co3O4 (68 mV dec-1) (Figure S9), which may be due to that both M-NiO@Co3O4 and M-Co3O4 have a similar structure with opened nanosheets on the surface (Figure 3b and Figure S7). This result demonstrates the advantage of structure regulation strategy by a facile NaBH4 reduction method in our work. In addition, the oxygen vacancies are very important in the modified electrocatalysts. To check the contribution of oxygen vacancies in the M-NiO@Co3O4 to OER, an oxygen vacancies-poor sample (denoted as p-M-NiO@Co3O4) was prepared through annealing the MNiO@Co3O4 at 400 oC under air, because high temperature treatment could release the strain, resulting in the concentration decrease of oxygen vacancies. Comparison experiments prove that the p-M-NiO@Co3O4 needs larger overpotentials to launch oxygen


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evolution reaction (348 mV at 10 mA cm-2, Figure 5f). To further confirm the uniqueness of the designed core/shell structure, an inverted composite (marked as M-Co3O4@NiO) was prepared by calcining and reducing the precursors obtained via precipitation of cobalt carbonate first and nickel carbonate next (Figure S5). It is found that the M-NiO@Co3O4 covered by nanosheets obviously outperforms the M-Co3O4@NiO without nanosheets as electrocatalysts to generate oxygen in alkaline solution (Figure 5f). All the above results manifest the advantages of rationally designed core/shell NiO@Co3O4 composites with enriched edge sites and oxygen vacancies as electrocatalysts in our method. In conclusion, a simple, yet effective strategy has been successfully developed to prepare nanostructured OER electrocatalyst (M-NiO@Co3O4) by component optimization and structure tailoring, in which stepwise co-precipitation method is adopted to prepare core/shell NiO@Co3O4 nanocomposites and NaBH4 reduction treatment is further used for the introduction of more active sites including edges sites and oxygen vacancies through surface restructuring. The resulting MNiO@Co3O4 presents a low overpotential of 290 mV at 10 mA cm-2, a small Tafel slope of 68 mV dec-1 and a long-term stability for 50 h. The OER activity is significantly improved, mainly owing to the increased active sites in the M-NiO@Co3O4. Moreover, this structure-design and surfacerestructuring strategy is expected to be useful for the synthesis of other core/shell composites for various applications including energy storage and catalysis. ASSOCIATED CONTENT Supporting Information Experimental details, characterization data and electrochemical measurement data.


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AUTHOR INFORMATION Corresponding Author *E-mail

address: [email protected]

Author Contributions §F.

Cheng and X. Fan contribute equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China [NSFC grant numbers 91834301, 21808046, 91534102] and Anhui Provincial Science and Technology Department Foundation [grant numbers 17030901067, 1608085QB28].


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REFERENCES (1) Bergmann, A.; Zaharieva, I.; Dau H.; Strasser P. Electrochemical water splitting by layered and 3D crosslinked manganese oxides: correlating structural motifs and catalytic activity. Energy Environ. Sci. 2013, 6, 2745-2755. (2) Silva, U. D.; Masud, J.; Zhang, N.; Hong, Y.; Liyanage W. P. R. Nickel telluride as a bifunctional electrocatalyst for efficient water splitting in alkaline medium. J. Mater. Chem. A 2018, 6, 7608-7622. (3) Ping, J. F.; Wang, Y. X.; Lu, Q. P.; Chen, B.; Chen, J.; Huang, Y.; Ma, Q. L.; Tan, C. L.; Yang, J.; Cao, X. H.; Wang, Z. J.;J. Wu, Ying Y. B.; Zhang, H. Self-assembly of single-Layer CoAllayered double hydroxide nanosheets on 3D graphene network used as highly efficient electrocatalyst for oxygen evolution reaction. Adv. Mater. 2016, 28, 7640-7645.

(4) Ma, X. Z.; Wen, J.; Zhang, S.; Yuan, H. R.; Li, K. Y.; Yan, F.; Zhang X. T.; Chen, Y. J. Crystal CoxB (x =1-3) synthesized by a ball-milling method as high-performance electrocatalysts for the oxygen evolution reaction. ACS Sustainable Chem. Eng. 2017, 5, 10266-10274. (5) Hu, J. M.; Zhang, J. Q.; Cao, C. N. Oxygen evolution reaction on IrO2-based DSAtype electrodes: kinetics analysis of Tafel lines and EIS. Int. J. Hydrogen Energy 2004, 29, 791-797. (6) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Yang S.-H. Synthesis and activities of rutile IrO2, and RuO2, nanoparticles for oxygen evolution in acid and alkaline solutions. J. Phys. Chem. Lett. 2012, 3, 399-404.


20

Page 21 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


(7) Nong, H. N.; Oh, H.; Reier, T.; Willinger, E.; Willinger, M.; Petkov, V.; Teschner D.; Strasser P. Oxide-supported irniox core-shell particles as efficient, cost-effective, and stable catalysts for electrochemical water splitting. Angew. Chem. Int. Ed. 2015, 54, 2975-2979. (8) Huang, H.; Yu, C.; Han, X. T.; Li, S. F.; Cui, S.; Zhao, C. T.; Huang, H. L.; Qiu J. S. Interface engineering of Ni3N@Fe3N heterostructure supported on carbon fiber for enhanced water oxidation. Ind. Eng. Chem. Res. 2017, 56, 14245-14251. (9) Zhuang, L.; Ge, L.; Yang, Y.; Li, M.; Jia, Y.; Yao, X.; Zhu, Z. H. Ultrathin iron-cobalt oxide nanosheets with abundant oxygen vacancies for the oxygen evolution reaction. Adv. Mater. 2017, 29, 1606793. (10) Han, L.; Dong, S.; Wang, E. Transition-metal (Co, Ni, and Fe)-based electrocatalysts for the water oxidation reaction. Adv. Mater. 2016, 28, 9266-9291. (11) Jung, S.; Mccrory, C. C. L.; Ferrer, I. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking nanoparticulate metal oxide electrocatalysts for the alkaline water oxidation reaction. J. Mater. Chem. A 2016, 4, 3068-3076. (12) Masa, J.; Xia, W.; Sinev, I.; Zhao, A.; Sun, Z.; Grützke, S.; Weide, P.; Muhler M.; Schuhmann, W. G. MnxOy/NC and CoxOy/NC nanocomposites embedded in a nitrogen-doped carbon matrix for high-performance bifunctional oxygen electrodes. Angew Chem Int. Ed. 2014, 53, 8508-8512. (13) Kim, J. S.; Kim, B.; Kim, H.; Kang, K. Recent progress on multimetal oxide catalysts for the oxygen evolution reaction. Adv. Energy Mater. 2018, 8, 1702774.


21


Page 22 of 31

(14) Fominykh, K.; Tok, G. C.; Zeller, P.; Hajiyani, H.; Miller, T.; Döblinger, M.; Pentcheva, R.; Bein, T.; Dina F.-R. Rock salt Ni/Co oxides with unusual nanoscale-stabilized composition as water splitting electrocatalysts. Adv. Funct. Mater. 2017, 27, 1605121. (15) Cui, B.; Lin, H.; Li, J. B.; Li, X.; Yang, J.; Tao, J. Core–ring structured NiCo2O4 nanoplatelets: synthesis, characterization, and electrocatalytic applications. Adv. Funct. Mater. 2008, 18, 14401447. (16) Li, M.; Xiong, Y.; Liu, X.; Bo, X.; Zhang, Y.; Han, C.; Guo, L. P. Facile synthesis of electrospun MFe2O4 (M = Co, Ni, Cu, Mn) spinel nanofibers with excellent electrocatalytic properties for oxygen evolution and hydrogen peroxide reduction. Nanoscale 2015, 7, 8920-8930. (17) Xu, H.; Shi, Z. X.; Tong, Y. X.; Li, G. R. Porous microrod arrays constructed by carbonconfined NiCo@NiCoO2 core@shell nanocomposites as efficient electrocatalysts for oxygen evolution. Adv. Mater. 2018, 30, 1705442. (18) Gao, X.; Zhang, H.; Li, Q.; Yu, X.; Hong, Z.; Zhang, X.; Liang C. D.; Lin, Z. Hierarchical NiCo2O4 hollow microcuboids as bifunctional electrocatalysts for overall water-splitting. Angew. Chem. Int. Ed. 2016, 55, 6290-6294. (19) Xu, J.; Cui, J.; Guo, C.; Zhao, Z.; Jiang, R.; Xu, S.; Zhuang, Z. B.; Huang, Y.; Wang, L. Y.; Li, Y. D. Ultrasmall Cu7S4@MoS2 hetero-nanoframes with abundant active edge sites for ultrahigh-performance hydrogen evolution. Angew. Chem. Int. Ed. 2016, 55, 6502-6505. (20) Zhao, J.; Wang, X.-R.; Wang, X.-J.; Li, Y.-P.; Yang, X.-D.; Li, G.-D.; Li, F.-T. Ultrathin porous nanosheet-assembled hollow cobalt nickel oxides microspheres with optimized compositions for efficient oxygen evolution reaction. Inorg. Chem. Front. 2018, 5, 1886-1893.


22

Page 23 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


(21) Tang, C.; Wang, B.; Wang, H. F.; Zhang, Q. Defect engineering toward atomic Co-Nx-C in hierarchical graphene for rechargeable flexible solid Zn-air batteries. Adv. Mater. 2017, 29, 1703185. (22) Zhang, R.; Zhang, Y. C.; Pan, L.; Shen, G. Q.; Mahmood, N.; Ma, Y. H.; Shi, Y.; Jia, W. Y.; Wang, L.; Zhang, X. W.; Xu W.; Zou, J.-J. Engineering cobalt defects in cobalt oxide for highly efficient electrocatalytic oxygen evolution. ACS Catal. 2018, 8, 3803-3811. (23) Xia, X.; Tu, J.; Zhang, Y.; Wang, X.; Gu, C.; Zhao, X.-B.; Fan, H. J. High-quality metal oxide core/shell nanowire arrays on conductive substrates for electrochemical energy storage. ACS Nano. 2012, 6, 5531-5538. (24) Tahir, M.; Pan, L.; Zhang, R.; Wang, Y.-C.; Shen, G.; Aslam, I.; Qadeer, M. A.; Mahmood, N.; Xu, W.; Wang, L.; Zhang, X.; Zou, J.-J. High-valence-state NiO/Co3O4 nanoparticles on nitrogen-doped carbon for oxygen evolution at low overpotential. ACS Energy Lett. 2017, 2, 21772182. (25) Wang, X.W.; Zheng, D.L.; Yang, P.Z.; Wang, X.E.; Zhu, Q.Q.; Ma, P.F.; Sun, L.Y. Preparation and electrochemical properties of NiO-Co3O4 composite as electrode materials for supercapacitors. Chem. Phys. Lett. 2016, 667, 260-266. (26) QayoomMugheri, A.; Tahira, A.; Aftab, U.; IshaqAbro, M.; Chaudhry, S. R.; Amaral, L.; Ibupoto, Z. H. Co3O4/NiO bifunctional electrocatalyst for water splitting. Electrochim. Acta 2019, 306, 9-17. (27) Xu, H.; Zhuang, J.; Chen, Y.; Wu, J.; Zhang, J. Preparation and performance of Co3O4-NiO composite electrode material for supercapacitors. RSC Adv. 2014, 4, 15511-15517.


23


Page 24 of 31

(28) Wu, H.; Sun, W.; Shen, J.; Rooney, D. A.; Wang, Z.; Sun, K. Role of flower-like ultrathin Co3O4 nanosheets in water splitting and non-aqueous Li-O2 batteries. Nanoscale 2018, 10, 1022110231.

(29) Chen, Z.; Zhao, B.; He, Y.-C.; Wen, H.-R.; Fu, X.-Z.; Sun, R.; Wong, C.-P. NiCo2O4 nanoframes with a nanosheet surface as efficient electrocatalysts for the oxygen evolution reaction. Mater. Chem. Front. 2018, 2, 1155-1164.

(30) Hu, C.; Zhang, L.; Zhao, Z. J.; Luo, J.; Shi, J.; Huang Z. Q.; Gong, J. Edge sites with unsaturated coordination on core-shell Mn3O4@MnxCo3-xO4 nanostructures for electrocatalytic water oxidation. Adv. Mater. 2017, 29, 1701820. (31) Zhang, G.; Yang, J.; Wang, H.; Chen, H.; Yang, J.; Pan, F. Co3O4-δ quantum dots as a highly efficient oxygen evolution reaction catalyst for water splitting. ACS Appl. Mater. Inter. 2017, 9, 16159-16167. (32) Zhong, D. Z.; Zhang, L.; Li, C. C.; Li, D. D.; Wei, C. C.; Zhao, Q.; Li, J. P.; Gong, J. L. Nanostructured NiFe (oxy)hydroxide with easily oxidized Ni towards efficient oxygen evolution reaction. J. Mater. Chem. A 2018, 6, 16810-16817. (33) Han, X.; Yu, C.; Zhou, S.; Zhao, C.; Huang, H.; Yang, J.; Liu, Z. B.; Zhao, J. J.; Qiu, J. S. Ultrasensitive iron-triggered nanosized Fe-CoOOH integrated with graphene for highly efficient oxygen evolution. Adv. Energy Mater. 2017, 7, 1602148.


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(34) Li S.; Peng S.; Huang L.; Cui X.; Al-Enizi A. M.; Zheng, G. Carbon-coated Co3+-rich cobalt selenide derived from ZIF-67 for efficient electrochemical water oxidation. ACS Appl. Mater. Interfaces 2016, 8, 20534-20539. (35) Chen R., Wang H.-Y., Miao J., Yang H.; Liu B. A flexible high-performance oxygen evolution electrode with three-dimensional NiCo2O4 core-shell nanowires. Nano Energy 2015, 11, 333-340. (36) Zhao, S.; Wang, Y.; Dong, J.; He, C. T.; Yin, H.; An, P.; Zhao, K.; Zhang, X.; Gao, C.; Zhang, L.; Lv, J.; Wang, J.; Zhang, J.; Khattak, A. M.; Khan, N. A.; Wei, Z.; Zhang, J.; Liu, S.; Zhao, H.; Tang, Z. Ultrathin metal-organic framework nanosheets for electrocatalytic oxygen evolution. Nat. Energy. 2016, 1, 16184-16194.


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Figure 1. Schematic illustration for the preparation of core/shell NiO@Co3O4 nanocomposites as electrocatalysts for OER (the size of the particles doesn't represent the real size). 160x87mm (300 x 300 DPI)


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Figure 2. XRD patterns of the NiO@Co3O4 and M-NiO@Co3O4 samples. 83x67mm (300 x 300 DPI)



Figure 3. SEM images of (a) NiO@Co3O4 and (b) M-NiO@Co3O4 (inset: TEM images). HRTEM image and SAED pattern of (c, d) M-NiO@Co3O4 in the shell region (cycle A marked in red in Figure 3b) and (e, f) MNiO@Co3O4 in the core region. (g) TEM image and EDS images of M-NiO@Co3O4. (inset: EDS line-scanning profiles of each element). 91x75mm (300 x 300 DPI)


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Figure 4. N2 adsorption/desorption isotherms of (a) NiO@Co3O4 and (b) M-NiO@Co3O4. (c) Survey XPS spectra of and (d-f) high-resolution XPS spectra of NiO@Co3O4 and M-NiO@Co3O4. 102x113mm (300 x 300 DPI)



Figure 5. (a) LSV curves, (b) Tafel plots, (c) the capacitive currents plotted as a function of scan rate of MNiO@Co3O4, NiO@Co3O4 and RuO2.(d) LSV curves of the M-NiO@Co3O4 (pasted on the nickel foam) for the first and 1000th cycle (inset: time dependent potential curve at 10 mA cm-2). (e) LSV curves of Co3O4, NiO

and NiO@Co3O4 nanocomposites before and after NaBH4 treatment. (f) LSV curves of the M-NiO@Co3O4, MCo3O4@NiO and p-M-NiO@Co3O4 (400 oC under air).


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Abstract Graphics 84x47mm (300 x 300 DPI)


Shell NiO@Co3O4

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