Shell Hierarchical Nanospheres with Enhanced

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Energy, Environmental, and Catalysis Applications

Ni/Ni3C Core/Shell Hierarchical Nanospheres with Enhanced Electrocatalytic Activity for Water Oxidation Qing Qin, Jing Hao, and Wenjun Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00716 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Applied Materials & Interfaces

Ni/Ni3C Core/Shell Hierarchical Nanospheres with Enhanced

Electrocatalytic

Activity

for

Water

Oxidation †

†

Qing Qin, Jing Hao, and Wenjun Zheng*, †

† ‡

Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE),

TKL of Metal and Molecule-based Materials Chemistry, College of Chemistry, Nankai University, Tianjin 300071, P. R. China ‡

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai

University, Tianjin 300071, P. R. China KEYWORDS: core-shell nanoarchitecture, carbides, low temperature-synthesizing, growth mechanism, OER performance

ABSTRACT: Developing efficient and low-cost catalysts with high activity and excellent electrochemical and structural stability toward oxygen evolution reaction (OER) is of great significance both for energy and environment sustainability. Herein, Ni/Ni3C core/shell hierarchical nanospheres have been in situ synthesized via an ionic liquid-assisted hydrothermal method at relatively low temperature. Ionic liquid 1-butyl-3-methylimidazolium acetate ([Bmim]Ac) has played multiple roles in the whole synthesis process. Benefiting from the high

ACS Paragon Plus Environment

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electrical conductivity, more exposed active sites and core-shell interface effect, the obtained Ni/Ni3C core/shell hierarchical nanospheres exhibit outstanding OER performance with lower overpotential, small Tafel slope and excellent stability. This fundamental method and insight with in situ coupling high conductivity metal support and metal carbide in a core/shell nanoarchitecture by an ionic liquid-assisted hydrothermal method would be open up a new pathway to achieve high performance electrocatalysts toward OER.

1. INTRODUCTION The clean and sustainable hydrogen energy produced by electrochemical water splitting is an important method to alleviate the global energy crisis.1,2 Oxygen evolution reaction (OER), as the oxidative half reaction of water splitting process, is kinetically sluggish,3-5 which requires highly active electrocatalysts to boost this process. The state-of-the-art OER electrocatalysts, such as RuO2, IrO2, can effectively reduce the energy barrier of electrochemical process and thus, greatly improve the OER efficiency.6 However, the scarcity and exorbitant price severely hamper their large scale commercial application.7 Therefore, to develop alternative non-noble-metal catalysts with lower-cost and earth-abundant is urgently needed for improving OER efficiency. Recently, transition metal carbides (TMC) are widely used in the field of electrocatalysis due to their outstanding physicochemical properties, such as high electrical conductivity, high chemical stability to resist electrochemical corrosion, high mechanical strength and hardness.8-10 However, it is challenging to develop the TMCs as scalable electrocatalysts. The required relatively high reaction temperature and harsh reducing conditions make the synthesis of TMCs considerable difficult.11-14 Although many effective routes have been developed to prepare TMCs nanomaterials, such as, high-energy ball milling,15 pyrolysis of polymer precursors,6,16,17 sputter deposition technique,11 and sonolysis,18 the obtained products are usually nanoparticles and


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agglomerates that greatly lower the electrocatalytic active. Therefore, developing a new method to realize the large-scale synthesis of TMCs with high electrocatalytic activity and long durability under a mild reaction conditions is full of scientific significance. Core-shell hierarchical nanoarchitectures are extremely attractive for obtaining high performance electrocatalysts, due to their large surface area with more exposed active sites and more favorable electron transfer leading by the core induced electronic effects.19-21 Recently, a serious of metal@metal oxides/phosphides/borides/bimetals layered double hydroxide (LDH) core-shell nanostructures with enhanced electrocatalytic activities have been widely reported. Strickler’s group reports that Au@MxOy (where M = Ni, Co, Fe, and CoFe) core-shell nanoparticles exhibit higher OER performance than their corresponding pure metal oxides in alkaline media.21 By an in situ growth strategy, Co@CoB core-shell nanosheets were prepared by Xie et al., which delivered excellent OER activity both in alkaline and neutral electrolytes.22 The electrocatalytic OER activity of Ag@CoxP core-shell heterogeneous nanostructures is greatly improved than their Co2P monomers.23 Cu@CoFe LDH hierarchical core-shell nanosheets synthesized by Yu’s group is proved to be an efficient bifunctional electrocatalysts for overall water splitting.19 These typical examples intensively prove that strategically coupling high conductivity metal support and metallic compound in a core-shell nanoarchitecture is a promising way to achieve high performance OER catalysts. Herein, we present the in situ synthesis of Ni/Ni3C core/shell hierarchical nanospheres via an ionic liquid-assisted one-step hydrothermal method at relatively low temperature. The growth mechanism of the Ni/Ni3C core/shell hierarchical nanospheres has been proposed, which experienced three important stages: (Ⅰ) Ni2+ transformed into elemental Ni under the assistance of ionic liquid 1-butyl-3-methylimidazolium acetate ([Bmim]Ac), (Ⅱ) carbonization of ionic


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liquid [Bmim]Ac driven by the catalysis effect of elemental Ni, (Ⅲ) the new formed active carbons diffuse into Ni nanospheres to form Ni3C nanosheets induced by the structure-directing effect of [Bmim]Ac. It is worth noting that [Bmim]Ac plays critical role in the whole process of forming the Ni/Ni3C core/shell hierarchical structure. Benefiting from the high electrical conductivity, more exposed active sites and core-shell interface effect, the obtained Ni/Ni3C core/shell hierarchical nanospheres exhibited outstanding OER electrocatalytic performance in terms of a low overpotential (η: 350 mV at j = 10 mA cm-2), small Tafel slope (57.6 mV dec-1) and excellent stability (24 h of i-t measurement without obvious current loss). We expected this synthesis strategy with ionic liquid-assisted in situ construct core-shell structure can be expanded to prepare other carbide-based hybrid nanomaterials. 2. EXPERIMENTAL SECTION 2.1 Synthesis of Ni/Ni3C core/shell hierarchical nanospheres. In a typical procedure, 0.08 g of Ni(NO3)2•6H2O was added into 5 mL of [Bmim]Ac and 1 mL of deionized water to form a homogenous solution under continuous ultrasonic blending. Then, the homogenous solution was transferred into a 25 mL stainless-steel autoclave, sealed and heated to 180 oC. After 24 h, the reaction was completed. When the autoclave was cooled to room temperature, the black products were collected and fully washed with anhydrous ethanol and deionized water. The final product was vacuum dried at 60 oC for 8 h and stored in a glove box, denoted as Ni/Ni3C. Using the [Omim]Ac to replace [Bmim]Ac, the obtained products denoted as Ni/Ni3C-C. 2.2 Electrocatalytic study. All the electrocatalytic performance was carried out on Zahner Zennium

Electrochemical

Workstation

(Germany)

in

a

conventional

three-electrode

electrochemical testing setup. The 0.1 M KOH solution was used as electrolyte. Glassy Carbon (GC; 3 mm in diameter, 0.071 cm2) loaded with catalysts was used as working electrode,


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Ag/AgCl (in 3 M KCl solution) electrode and a Pt wire were used as reference and counter electrodes, respectively. The potential was calibrated to the RHE, according to the formula ERHE = EAg/AgCl + 0.196 V + 0.0591pH V. Furthermore, the overpotentials (η) are obtained by a simple calculation on the base of the formula η = EAg/AgCl + 0.196 V + 0.0591pH – 1.23 V. The working electrode was prepared according to the steps below: Firstly, the as-prepared catalyst powder (2 mg) was added into 0.97 mL of deionized water and absolute ethyl alcohol (vdeionized water: vethyl alcohol = 3 : 1) with 30 µL of 5 wt % Nafion solution. Then, the mixture was ultrasonicated for 20 min to get a homogeneous ink. Finally, 7µL of the ink was pipetted onto the GC disk to form a catalyst layer and dried naturally. 3. RESULTS AND DISCUSSION 3.1 Structure and Phase of the as-prepared Ni/Ni3C and Ni/Ni3C-C hierarchical nanospheres

Figure 1. XRD pattern of the as-prepared Ni/Ni3C core/shell hierarchical nanospheres.


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The phase and structure of the as-prepared Ni/Ni3C was characterized by XRD measurement, as demonstrated in Figure 1. The diffraction peaks marked with red rectangle can be easily assigned to (110), (006), (113), (116), (300), (119) planes of Rhombohedra (RH) Ni3C (JCPDS No. 06-0697), while the diffraction peaks marked with black circular correspond to the Cubic Ni with lattice parameter a = 0.3524 nm (JCPDS No. 04-0850). EDS spectrum in Figure S1 further confirms the existing of C and Ni element in products and the atom ratio of C to Ni is 18.32 % to 81.68 %. Figure 2a is the representative panoramic SEM image of the Ni/Ni3C, which suggests the large-scale synthesis of spheres with 400-700 nm in diameter. High-magnified SEM image in Figure 2b shows the hierarchical structure of Ni/Ni3C nanospheres with three-dimensional network surface constructed by numerous crisscrossed ultrathin Ni3C nanosheets. Figure 2c shows the low-magnified TEM image of Ni/Ni3C nanospheres, further confirming the hierarchical structure. The typical TEM image of a single Ni/Ni3C nanosphere is shown in Figure 2d. HRTEM images in Figure 2e and f are corresponding to the regions in Figure 2d marked with yellow ellipses, demonstrating the core-shell structure. The lattice spacing of 0.203 nm is assigned to (111) plane of elemental Ni and the 0.201 nm corresponds to the (113) plane of RH Ni3C, intensively indicating the Ni/Ni3C nanosphere is constituted of a Ni core and a Ni3C shell with 4-5 nm in thickness. In addition, elemental mapping image in Figure 2g-j further demonstrate the composition and core/shell structure of the as-prepared products. As mentioned above, Ni/Ni3C core/shell hierarchical nanospheres were successfully synthesized via an ionic liquid-assisted hydrothermal route.


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Figure 2. Representative structure and morphology of as-prepared Ni/Ni3C core/shell hierarchical nanospheres with an ionic liquid assisted route: (a) Low- and (b) high-magnified SEM images, (c) TEM image, (d) TEM image of single Ni/Ni3C nanosphere, (e) and (f) HRTEM images corresponding to the regions in Figure 2d marked with yellow ellipses, (g-j) elemental mapping images recorded on an individual Ni/Ni3C nanosphere, confirming the core/shell structure. Changing the length of alkyl chain located at the 1-position of imidazole ring will affect the composition of the final products. Typically, Ni/Ni3C core/shell hierarchical nanospheres with a thin quasi-graphitic carbon layer covered on the surface of Ni3C nanosheets were obtained by using 1-octyl-3-methylimidazolium acetate ([Omim]Ac) to replace 1-butyl-3-methylimidazolium acetate ([Bmim]Ac). As the XRD pattern shown in Figure S2a, except for the peaks belonging to


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Ni and Ni3C, there is a broad and low density peak at 26.6 degree, which is close to graphitic carbon.24 The corresponding EDS spectrum (Figure S2b) demonstrated that the material was composed of C and Ni, and compared with Ni/Ni3C nanospheres, the content of C obviously increased. On the account of the SEM image and magnified SEM image in Figure 3a and b, the products are nanospheres and the surface layer is interspersed by numerous small Ni3C nanosheets. TEM and HRTEM images in Figure 3c-f further confirmed the core/shell structure of the obtained nanocomposites. The clear layer spacing of 0.34 nm in the inset of Figure 3f is close to the interlamellar spacing value of graphite (0.335 nm),25 intensively indicating the coverage of a thin quasi-graphitic carbon layer on the surface of Ni3C nanosheets.26,27 Elemental mapping images were shown in Figure 3g-j, demonstrating the nearly uniform distribution of C across the nanosphere, again verifying the cover of carbon layer on the surface of Ni3C nanosheets.


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Figure 3. Representative structure and morphology of as-prepared Ni/Ni3C-C nanospheres: (a) Low- and (b) high-magnified SEM images, (c) TEM image, (d) TEM image of single Ni/Ni3C-C nanosphere, (e) Magnified TEM image, corresponding to the region in Figure 3d marked with yellow ellipse, (f) HRTEM image, corresponding to the region in Figure 3e marked with yellow ellipses, (g-j) elemental mapping images recorded on an individual Ni/Ni3C-C nanosphere. The elemental chemical states of Ni/Ni3C and Ni/Ni3C-C nanocomposites were further investigated by X-ray photoelectron spectroscopy (XPS). The XPS survey spectra of Ni/Ni3C (Figure S3a) and Ni/Ni3C-C (Figure S3b) reveals that all two kinds of products are composed of C, O, and Ni, without any other elements. The main peak at 283.8 eV in the C 1s of Ni/Ni3C


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(Figure 4a) can be assigned to Ni3C,28,29 while a small peak at higher binding energy of 285.1 eV belongs to disordered graphitic C,28 indicating the possible existence of little residual carbon. Nevertheless, the intensity and area of peak at 283.7 eV are obviously weaker than that of the peak at 285.2 eV in C 1s of Ni/Ni3C-C nanocomposites (Figure 4b), strongly demonstrating that relatively more carbon exits in the Ni/Ni3C-C nanocomposites. High resolution of XPS of Ni 2p for Ni/Ni3C (Figure 4c) exhibits a low-energy band (Ni 2p3/2) and a high-energy band (Ni 2p1/2) at 855.6 and 873.3 eV with two satellites at 861.1 and 879.7 eV, respectively, which all belong to Ni2+. The shoulder peak at 852.7 eV in Ni/Ni3C indicates the presence of metallic Ni.30,31 Moreover, the Ni2+ also was clearly identiﬁed by the Ni 2p3/2 (855.9 eV) and Ni 2p1/2 (873.6 eV) peaks for Ni 2p of Ni/Ni3C-C with two satellites at 861.2 and 879.6 eV (Figure 4d). The shoulder at 852.8 eV also indicated the existence of metallic Ni in Ni/Ni3C-C nanospheres. Benefitting from the coexisting various valence states of Ni0 and Ni2+, the nanocomposites composed of metallic element and their carbides is conducive to enhance the electrochemical OER performance. Specifically, Ni0 is beneficial to induce electronic effects due to its high electronic conductivity, providing a lower barrier mechanistic pathway for OER process,21 while Ni2+ would be attributed to OER activity.6 As is well-known, graphitic carbon owns highly inherent electrical conductivity, which will be a significant help for accelerating the electrochemical reaction. Therefore, those nanocomposites consisting of Ni0, Ni2+ and/ or quasi-graphitic carbon layer would achieve high electrocatalytic OER properties due to the synergetic effect of each component.


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Figure 4. XPS spectra of C 1s for Ni/Ni3C (a) and Ni/Ni3C-C (b) core/shell hierarchical nanospheres in the range of 280-292 eV; XPS spectra of Ni 2p for Ni/Ni3C (c) and Ni/Ni3C-C (d) core/shell hierarchical nanospheres in the range of 848-886 eV, further confirming the compositions of the as-prepared products. 3.2 Possible growth mechanism of Ni/Ni3C and Ni/Ni3C-C core/shell hierarchical nanospheres Time-dependent experiments were carried out to detailed investigate the phases and morphologies evolution process of Ni/Ni3C nanospheres. As shown in Figure 5a, the sheet-like products are obtained with maintaining the temperature at 180 oC for 2 h. No obvious XRD diffraction peaks are observed in Figure 5f(A), demonstrating the products collected at 2 h adopt an amorphous phase. When the reaction continued to 4 h (Figure 5b), numerous monodispersed nanospheres with 200-400 nm in diameter were found to coexist with some blocks. XRD pattern


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in Figure 5f(B) further confirm that there are two phases consisted in the products, i.e., Ni(OH)2 and Ni(CH3COO)2•4H2O. When the [Bmim]Ac was added into the reaction system, part of Acwould hydrolyze and produce OH-, which forms the precipitate with Ni2+. Further prolonging the reaction time to 6 h, Ni nanospheres can be observed, as demonstrated by Figure 5c and 5f(C). Notably, the surface of Ni nanospheres is dirty and seems covered by a thin organic film. This phenomenon may be produced by the adsorption of [Bmim]Ac through the strong electrostatic interaction.32 Continuous to increase reaction time to 8 h, the adsorbed organic molecules decomposed to form the active carbon, driven by the catalytic action of small Ni particles.33-36 Consequently, numerous small nanoparticles covered on the surface of Ni nanospheres were clearly observed and the diameter of nanospheres obviously increased, as evidenced by Figure 5d. XRD pattern in Figure 5f(D) and the magnified profile inset further confirm the existing of carbon species; the diffraction peak at 26.6o corresponding to the (002) plane of graphitic carbon. Driven by the concentration gradient of the carbon, the active carbon atoms would gradually diffusion from the surface into the inside of Ni nanospheres and thus form the Ni3C interstitial compound (Figure 5e and 5f(E)).37,38 The newly formed Ni3C shell tends to prevent the further formation of more Ni3C by interdicting the contact of active carbon atoms and Ni atoms.37,38 Those processes led to the intact Ni core in the center and finally, formed the core/shell structure, which can be perfectly illuminated by Figure 5g. It is worth noting, the self-assembled ability of ionic liquids that affected by the length of alkyl chain can produce prominent influence on the morphologies of nanomaterials.39-41 Thus, more ionic liquid molecules will adsorb on the newly formed Ni core when [Bmim]Ac was replaced by [Omim]Ac and produce more active carbon driven by the catalytic action of element Ni. Consequently, a thin quasi-graphitic carbon layer remained and covered on the surface of Ni3C nanosheets to form the Ni/Ni3C-C nanocomposites.


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Figure 5. SEM images (a-e) and XRD patterns (f) of products collected at different time intervals with [Bmim]Ac assisted route: (a, A) 2 h, (b, B) 4 h, (c, C) 6 h, (d, D) 8 h, (e, E) 12 h; (g) Schematic illustration of the formation mechanism for core-shell hierarchical nanostructure. Which part of ionic liquids decomposes to produce the active carbon atoms, imidazole cations or Ac-? This is a question of great significance. To clarify the source of carbon atoms, a serious of contrast experiments were carried out. There were no products obtained when the [Bmim]Ac was replaced by equimolar of [Bmim]Cl. In order to eliminate the difference of pH value caused by the hydrolysis of Ac- ions of ionic liquids, little of NH3•H2O was dropped into the reaction system. As shown in Figure S4, sheet-like Ni(OH)2 was obtained, indicating that active carbon


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atoms were not from the decomposition of imidazole ring cations. Furthermore, [Bmim]Ac were replaced by equimolar NaAc and the products were pure Ni(OH)2 nanoplates, as evidenced by XRD pattern in Figure S5a and SEM image in Figure S5b. Because of the short chain length, it is not benefit for adsorption of Ac- on the surface of Ni clusters and at this case, the role played by Ac- is to adjust the pH value of reaction system. All those mentioned above demonstrated that it is the synergistic effect of imidazole cations and Ac- anions to produce the active carbon atoms on the surface of Ni nanospheres. As reported by previous studies, the newly formed Ni nanoparticles are a good kind of catalyst to catalyze the organic molecules decompose on their surfaces and thus, produce the active carbon. In 1981, Kenneth J. Klabund has reported the lowtemperature cleavage of alkanes by Ni nanoparticles.42 After that, the organic molecules decompose with the catalysis effect of Ni particles for the formation of active carbon have been reported in succession, such as sucrose,43 oleic acid and oleylamine,44 trioctylphosphine oxide (TOPO),32 propanol or ethanold36 and tetraethylene glycol.45 In our present case, it may experience the same process, i.e., Ac- would prefer adsorb on the surface of Ni clusters and decompose to produce carbon atoms driven by Ni catalysis effect. For elucidating the influence of reaction temperature on the phase and morphology of the final products, temperature-dependent experiments were conducted with other conditions unchanging. When the reaction proceeded at 150 oC, the products were pure Ni(OH)2 nanosheets (Figure S6). If increased the temperature to 165 oC, Ni/Ni3C core/shell nanospheres were obtained (Figure S7). However, the main component of nanocomposites obtained at this temperature is elemental Ni. Thinnish Ni3C shell layer is composed of numerous nanoparticles with smaller diameter and loosely dotted on the Ni core. Further increasing the reaction temperature to 180 oC, perfectly hierarchical Ni/Ni3C core/shell nanospheres with high Ni3C content were obtained. Therefore,


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the appropriate temperature to construct the Ni/Ni3C core/shell structure is 180 oC, which, to the best of our knowledge, is the relatively lower temperature used to synthesize inorganic carbide up to now.11-14,32 It will meet the industrialization concept of low cost and environmental-friendly by dramatically reduce the reaction temperature, thus lower the requirement for equipment. Moreover, the preparation of graphitic carbon usually needs high temperature (> 600 oC) through a calcination process in the tube furnace.24,26,46 However, it may be obtained at a relatively lower temperature via the ionic liquid-assisted hydrothermal method，which will be another very important finding. We expect that those findings will help to design new synthesis method to prepare the inorganic carbide or graphitic carbon at a low temperature. 3.3 Electrocatalytic performance of Ni/Ni3C and Ni/Ni3C-C core/shell hierarchical nanospheres


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Figure 6. Electrocatalytic performance of Ni/Ni3C, Ni/Ni3C-C and commercial IrO2 catalyst toward OER in 0.1 M KOH solution. (a) Polarization curves of Ni/Ni3C, Ni/Ni3C-C, IrO2 and GC (Glassy Carbon) recorded at a scan rate of 5 mV s-1. (b) Tafel plots. (c) Summary of Tafel slope and overpotantial at j = 10 mA cm-1. (d) Polarization curves of initial and 2000th cycles of Ni/Ni3C core/shell hierarchical nanospheres. The electrocatalytic performance of the as-synthesized nickel supported nickel carbide hybrid materials were performed in a conventional three-electrode system with O2-saturated 0.1 M KOH as the electrolyte. The electrocatalytic performance of commercial IrO2 was also recorded as the reference. In order to improve the OER performance, cyclic voltammetry at 100 mV s-1 for 20 scan cycles were proceeded for all three samples as electroactivation. As shown in Figure 6a, the polarization curve referred to Ni/Ni3C shows a dramatically small OER onset potential47,48 of 1.48 V, beyond which the anodic current rises fast at a smaller overpotential. While, the approximated polarization curve with Ni/Ni3C nanospheres is observed in the Ni/Ni3C-C hierarchical nanospheres along with rapidly increasing anodic current. The peaks appeared before the onset potential for two curves attribute to the oxidation of Ni2+ to Ni3+.49-51 Tafel plots of Ni/Ni3C and Ni/Ni3C-C in Figure 6b show good linearity, similarity, and afford lower Tafel slopes of 57.6 mV dec-1 for Ni/Ni3C and 58.2 mV dec-1 for Ni/Ni3C-C. Such small Tafel slope is effective evidence to confirm the highly efficient kinetics of those hybrid catalysts for water oxidation. The overpotential (η) at j = 10 mA cm-2 for Ni/Ni3C is 350 mV, preferable to that of commercial IrO2 (412 mV, 70.5 mV dec-1) (Figure 6c). Further, compared to the behavior of most nonmetal catalysts52-55 and even some metal catalysts, such as Co2+/birnessite,56 NiCo LDH,57 CoZn-NC-700,58 (listed in the Table S1) in alkaline electrolytes, those overpotentials exhibited by as-prepared Ni/Ni3C and Ni/Ni3C-C are favorable. In addition, electrochemical


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impedance spectroscopies (EIS) of Ni/Ni3C and Ni/Ni3C-C nanocomposites were shown in the Figure S8. Each of them contains two smaller semicircles, indicating the typical Gerischer impedance and the smaller diameter of semicircles reconfirmed the high conductivity in the OER process.59 The long-term durability and performance is another crucial criterion to evaluate an electrocatalysts able to be used for commercial application. Two methods of continuous cyclic voltammetric and chronoamperometry test were used to evaluate the catalytic stability. First, long-term electrocatalysis was conducted by using Ni/Ni3C as catalyst with the overpotential of 350 mV (Figure S10). Evidently, high current density of 10 mA cm-2 has been obtained, and the electrocatalyst exhibits good electrochemical stability with negligible current lost in 24 h. As demonstrated by Figure 6d, continuous CV scanning at 100 mV s-1 for 2000 cycles was also conducted for Ni/Ni3C hierarchical nanospheres. At the end of the scanning cycles, it shows similar polarization curve with a small shift of about 0.016 V at j = 10 mA cm-2, indicating excellent durability. After the continuous CV cycles, the sample was collected and detected by XRD and SEM measurements. The phase and morphology of sample are almost maintained as evidenced by Figure S11(a) and (b), respectively, demonstrating excellent structural stability. Accordingly, all those mentioned above strongly indicate that the as-prepared nickel supported nickel carbide hybrid materials possess superior electrocatalytic activity and durability in longterm electrochemical process. The enhanced electrocatalytic properties are attributed to the advanced structure of Ni/Ni3C, which possesses numerous superiorities: (ⅰ) core-shell interface effects:21 the metal core can serve as electron acceptor, accelerating the electron transfer and thus to stabilize the high oxidation metal, leading to high electrocatalytic activity; (ⅱ) low dimensional effect of Ni3C


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shell layer: 2D structure endows nanomaterial with large specific surface area and more exposed surface atoms, which can serve as catalytic active sites; (ⅲ) interconnected three-dimensional network surface also accelerate electron transfer. In a word, it is a synergistic effect of every component to tune the electrocatalytic activity, providing a promising way to achieve enhanced OER kinetics. 4. CONCLUSIONS In summary, we demonstrated an ionic liquid-assisted hydrothermal method to in situ synthesize Ni/Ni3C core/shell hierarchical nanospheres at relatively low temperature. The growth mechanism of Ni/Ni3C core/shell hierarchical nanospheres has been proposed and the roles played by ionic liquid [Bmim]Ac have been systematically studied. When used as electrocatalyst, the obtained Ni/Ni3C core/shell hierarchical nanospheres exhibit an outstanding OER performance in terms of a low overpotential (η: 350 mV at j = 10 mA cm-2), small Tafel slope (57.6 mV dec-1) and excellent stability (24 h of i-t measurement without obvious current loss). We hope that this fundamental approach and insight with in situ coupling high conductivity metal support and metal carbide in a core-shell nanoarchitecture by an ionic liquid-assisted onestep hydrothermal method would provide a certain reference to the other materials for realizing the optimal OER performance. ASSOCIATED CONTENT Supporting Information. Details on chemicals and sample characterization AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Shell Hierarchical Nanospheres with Enhanced

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