Facile Synthesis of Mesoporous and Thin-Walled NiâCo Sulfide

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Facile Synthesis of Mesoporous and Thin-walled Ni-Co Sulfide Nanotubes as Efficient Electrocatalysts for Oxygen Evolution Reaction Jing-chao Zhang, Daojun Zhang, Ren-Chun Zhang, Nana Zhang, Cancan Cui, Jingru Zhang, Bei Jiang, Baiqing Yuan, Tanyuan Wang, Huan Xie, and Qing Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00099 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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ACS Applied Energy Materials

Facile Synthesis of Mesoporous and Thin-walled NiCo Sulfide Nanotubes as Efficient Electrocatalysts for Oxygen Evolution Reaction

Jingchao Zhang,a, ⊥Daojun Zhang,a, ⊥Renchun Zhang,a Nana Zhang,a Cancan Cui,a Jingru Zhang,a Bei Jiang,a Baiqing Yuan,a Tanyuan Wang,b Huan Xie,b Qing Li*, b

a Henan Province Key Laboratory of New Opto-electronic Functional Materials, College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang 455000, Henan, China b State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

ACS Paragon Plus Environment

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ABSTRACT

Development of high-performance and inexpensive electrocatalysts for oxygen evolution reaction (OER) is of important significance for sustainable energy conversion technologies. In this work, mesoporous Co and Ni-Co sulfide nanotubes with ultrathin nanowalls are designed and fabricated by a facile and template-free solvothermal method. The obtained CoS1.097 nanotubes can be used as an OER electrocatalyst, and the incorporation of Ni into CoS1.097 lattice could further enhance the catalytic activity of the catalysts. The best-performing Ni0.13Co0.87S1.097 nanotubes exhibit high performance for OER with a small overpotential of 316 mV to achieve a current density of 10 mA cm-2 and excellent stability, which outperforms that of commercial IrO2 and most of the studied Co-based OER catalysts. Our work demonstrates a new strategy to design highly efficient non-previous metal OER electrocatalysts with unique structure and can be extended to other transition metal based systems.

KEYWORDS: oxygen evolution reaction, nanotubes, water splitting, metal sulfides, electrocatalysis


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Introduction The increasing environmental pollution and energy consumption pose an urgent challenge for developing alternative and sustainable electrochemical energy sources1-8. Electrochemical energy storage and conversion can be achieved via forming or breaking chemical bonds in the molecules 9-12

. For instance, water can be electrolyzed to produce hydrogen and oxygen, which are

important and renewable energy carriers13-20. Oxygen evolution reaction (OER) is the anode reaction of a water electrolyzer but suffers from sluggish kinetics, which requires efficient electrocatalysts to overcome the kinetic limitations and reduce the overpotential21, 22. Precious metal oxides, i.e., IrO2 or RuO2, are commercially available OER catalysts but the prohibitive costs hinder their wide applications. In recent years, 3d transition metal based oxides23-30, hydroxides31,32, phosphides33-35, selenides36-40, and sulfides41 have been explored as non-precious metal catalysts for OER. In particular, cobalt and nickel chalcogenides with high electrical conductivity, intrinsic corrosion tolerance in alkaline electrolyte and variable valence states were regarded as encouraging non-noble metal electrocatalysts towards OER. Although cobalt chalcogenides can catalyze hydrogen evolution reaction (HER) in alkaline solution, the development of nanostructured cobalt chalcogenides as OER catalysts are still less explored36-41. Since most Co oxides and chalcogenides based electrodes suffer from the fast activity and stability degradation due to the large volume changes during long time cycling42, how to design OER catalysts with excellent activity and durability based on transition metal chalcogenides is still an active research area and of practical importance. From the structural point of view, hollow and porous nanostructures may offer great promise to circumvent the difficulties43-46. Especially, one-dimensional (1-D) mesoporous nanotubes can not only improve the charge transfer along their longitudinal channels, but also decrease inter-crystalline contacts and enable


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the ion diffusion from interior to surface via their porous ultrathin walls, thereby enhancing their catalytical performance47-51. However, the controllable synthesis of high-quality and uniform 1D porous metal sulfides nanotubes with the absence of template is still a challenge. Therefore, it is highly desirable to develop a convenient method to synthesize ultrathin mesoporous nanotubes as excellent materials for activating OER with high performance. Herein, we reported a simple solvothermal strategy for the synthesis of mesoporous CoS1.097 nanotubes and employed a subsequent anion-exchange process to form Ni-doped, as illustrated in Scheme 1. The synthesis of nanotubes was achieved in a mixture of N,N-dimethylformamide (DMF), water, isopropanol and cyclohexane at 180°C, in the presence of organic amine (thioacetamide and propylamine) which assisted the anisotropic crystal growth and stabilized a bubble template-based ripening process for the formation of 1-D CoS1.097 nanotubes (details see Experimental Section). The as-prepared CoS1.097 nanotubes can be chemically transformed into Ni0.13Co0.87S1.097 mesoporous nanotubes with well-maintained structures, precisely-controlled Ni/Co molar ratio, and high surface area. The developed Ni0.13Co0.87S1.097 nanotubes demonstrate superior OER activity to commercial IrO2 and most of previously published Co-based catalysts, which are attributable to the synergistic effect between Ni and Co, unique nanotube structure, and increased electronic conductivity and surface area.

Scheme 1. Schematic illustration of the formation of CoS1.097 nanotubes and in-situ formation of Ni-doped CoS1.097 nanotubes by anion exchange process.


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Experimental section Synthesis of hierarchical mesoporous CoS1.097 nanotubes In a typical procedure, a mixture of cobalt (II) acetylacetonate (0.1 mmol), sodium citrate (0.11 mmol) and polyvinyl pyrrolidone (PVP-K30, 0.1 mmol) were dispersed in the mixture of DMF (4.0 mL), H2O (4.0 mL), isopropanol (2.0 mL), cyclohexane (0.75 mL) and propylamine (0.3 mL), followed by adding a small amount of thioacetamide (0.3 mM) to the above solution at vigorous stirring. The system was stirred for a few minutes, sealed and maintained at 180 oC for 12 h. The obtained products were separated by centrifugation with ethanol and distilled water and finally dried in vacuum at 60 oC for 1 h. The synthetic procedure of CoS1.097 nanoclusters is similar as mentioned above without the addition of propylamine. Synthesis of porous Ni0.13Co0.87S1.097 nanotubes. The Ni0.13Co0.87S1.097 nanotubes were prepared by anion exchange method for partial substitution of cobalt ions by nickel ions. The prepared CoS1.097 mesoporous nanotubes was redispersed in the mixed solution of H2O (6.0 mL) and isopropanol (2.0 mL). Subsequently, 200


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µL of nickel acetate tetrahydrate (0.0125 mM) aqueous solution was injected to the system and fully stirred. After that, the mixture was heated to 140 oC and held for 1 h. The products were treated as described above. Materials characterization The crystalline phase of the mesoporous CoS1.097 and Ni0.13Co0.87S1.097 nanotubes was determined by X-ray diffraction (XRD) on a Rigaku-Ultima III with CuKa radiation (λ = 1.5418Å). The surface morphology and composition was measured by field scanning electron microscopy (FSEM) on a Hitachi SU8010 instrument equipped with energy-dispersive X-ray spectroscope (EDX). TEM and HAADF-STEM images were taken by a FEI Tecnai G2 instrument. The specific surface areas of the samples were analyzed by N2 adsorption measured using Gemini VII 2390 Analyzer at 77 K by using the volumetric method. The specific surface area was obtained from the N2 adsorption-desorption isotherms and was calculated by using the Brunauer-Emmett-Teller (BET) method. The valence states of surface elements were analyzed by ESCALAB 250 X-ray photoelectron spectrometer. I-V analysis was performed on the pellets using a two-probe method with a Keithley 4200 semiconductor parameter analyzer (Keithley Instruments, Cleveland). Raman spectra were collected from a LabRAM HR800 (Horiba JobinYvon) using a wavelength of 532 nm laser. Electrochemical Characterization A homogenous catalytic ink, which was comprised of 5 mg catalyst powder, 0.95 mL of water and isopropanol (v/v ~3:1) and 50 µL Nafion solution, was dispersed under continuous ultrasonication for at least 30 min. Subsequently, a polished rotating disk electrode (RDE, 5 mm diameter, 0.196 cm2) was covered by 5 µL of the catalyst ink with a mass loading of 0.128 mg


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cm-2, and allowed to dry naturally. Before OER test, the electrode was recovered by 2 µL dilute Nafion solution (10%). All the electrochemical tests were performed on a CHI 760E electrochemical workstation at room temperature and 1.0 M KOH solution was used as the electrolyte. A Pt electrode and double salt bridge Ag/AgCl electrodes were used as the counter and reference electrodes, respectively. The linear sweep voltammetry (LSV) curves were obtained on a rotating disk electrode (RDE) at 1600 rpm with a sweep rate of 1 mV s-1. All the OER catalytic measurements for the CoS1.097 and Ni0.13Co0.87S1.097 samples were conducted with iR compensation enabled. All potentials were later converted to the reversible hydrogen electrode (RHE) scale. EIS measurements were conducted at applying an open circuit voltage with 5 mV amplitude in a frequency range from 0.01 to 100000 Hz. The electrochemical surface area (ECSA) of the nanotube electrodes was measured by CV curves in a small potential range of 1.12-1.22 V and calculated from the liner slope of the current density (1.17 V) vs. scan rates. To measure the Faraday efficiency, the empirical collection efficiency (N) of the RRDE was determined by the average value of the ratio of ring and disk current obtained at different rotation rates in a mixed solution of K3Fe(CN)6 (10 mM) and KOH (0.1 M). The disk electrode was swept from -0.6 to 0.4 V (vs. Ag/AgCl) at 20 mV s-1, while altering the rotation rates from 100 to 2500 rpm as maintaining the ring electrode potential at 0.595 V (vs. Ag/AgCl), as shown in Fig.S9. The calculated N is ~0.4165. Then, the Ni0.13Co0.87S1.097 nanotubes coated RRDE was placed in a N2 saturated electrolyte (1.0 M KOH) and blanketed with N2 throughout the measurements. The ring potential was held at 0.425 V (vs. RHE) as a series of potential (1.496, 1.504,1.516,1.528, 1.538 V vs. RHE) was applied on the disk, which corresponding to the current density of 1.5, 2.0, 3.0, 4.5 and 6.0 mA cm-1, and recorded the ring (ir) and disk (id) current, simultaneously.


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Results and discussion

Fig. 1 Characterizations of the CoS1.097 mesoporous nanotubes. (a) The typical SEM image for as-prepared CoS1.097 nanotubes. (b) The HAADF-STEM image of the CoS1.097 nanotubes. (c, d) TEM images with different magnifications of CoS1.097 nanotubes. (e) HRTEM image of the marked area in d. The scanning electron microscope (SEM) image reveals that the CoS1.097 nanotubes have welldefined tubular structure with average diameter and length of approximately 280 nm and 4.8 µm, respectively (Fig. 1a). The high angle annular dark field-scanning transmission electron microscope (HAADF-STEM) (Fig. 1b) and TEM (Fig. 1c) images further demonstrate the


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hollow structure and ultrathin thickness (~19.5 nm) of the tubular walls. The magnification image on the tip of a nanotube reveals a rough surface consist of numerous nanoparticles (Fig. 1d). The HRTEM image in the defining area of Fig.1d (Fig. 1e) demonstrate a series of distinct lattice fringes with a distance of 0.252 nm, corresponding to the (220) plane of CoS1.097. The scanning TEM energy dispersive X-ray spectroscopy (STEM-EDS) quantitative elemental mapping (QMap) of an individual CoS1.097 nanotube (Fig.2a) shows the relative element concentration (at%) distribution with different colours. The Co signal exhibits very similar pattern with that of S, indicating the uniform distributions of Co and S over the entire nanotube. Line-scan image in Fig. 2b further suggests that the Co and S are overlapped along the scanning direction with stronger signals at the edges, revealing a hollow and thin-walled tubular structure. In order to understand the morphology evolution of CoS1.097 nanotubes, a series of control experiments were conducted. Only irregular nanosheets can be obtained in the absence of thioacetamide (Fig.S1a). With the concentration of thioacetamide increases from 0.1 to 0.3 mM, the yield of nanotubes is gradually enhanced (Fig.S1b-d). The role of propylamine exhibits similar trend with thioacetamide as further increasing the adding amount of propylamine during the synthesis from 0 to 300 µL would significantly tune the dominant morphology from featureless nanoclusters to nanotubes (Fig.S2). Fig.S3 indicated that the employments of sodium citrate (Na3Cit) and PVP also influenced the morphology of nanotubes. In the absence of Na3Cit the mixture of nanoparticles and nanotubes were obtained, while the nanotubes became short without adding PVP. When both Na3Cit and PVP were absent, not only the purity but also the quality of CoS1.097 nanotubes were compromised.


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Fig. 2 (a) The EDX QMap and (b) the line-scan analysis of an individual CoS1.097 nanotube. Ni doping was successfully induced via anion exchange and the morphology of the resulting Ni0.13Co0.87S1.097 nanotubes with Ni/Co molar ratio of 1:7 was examined by FESEM and TEM (Fig.3). The average lengths of the CoS1.097 nanotubes decreased to ~1.4 µm after the Ni doping, likely due to the corrosion effect during the anion exchange process. The EDX analysis revealed the successful incorporation of Ni ions into the CoS1.097 lattices without destroying the parent crystal structure of CoS1.097 (Fig.S4). The EDX-line scan and mapping presented in Fig.3b and 3c respectively further implied the hollow construction and uniform distributions of Co, Ni and S elements. The nitrogen sorption isotherm of Ni0.13Co0.87S1.097 nanotubes in Fig. 3d showed a higher BET surface area of 28.98 m2 g-1 than that of the CoS1.097 nanotubes (15.28 m2 g-1, Fig.S5). The majority of the pores of the Ni0.13Co0.87S1.097 and CoS1.097 nanotubes are in 23.48 nm and 23.70 nm, respectively, as calculated from the BJH method.


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Fig. 3 (a) The typical SEM image for as-prepared Ni0.13Co0.87S1.097 nanotubes, (b) the line-scan analysis of an individual Ni0.13Co0.87S1.097 nanotube, (c) the EDX mapping of an individual Ni0.13Co0.87S1.097 nanotube, (d) the N2 adsorption-desorption isotherm and BJH pore size distribution plot (inset) of the Ni0.13Co0.87S1.097 nanotubes. The XRD patterns in Fig. 4a indicated the phase purity of the CoS1.097 (JCPDS No.19-0366) and Ni0.13Co0.87S1.097 nanotubes with the similar broad peaks of low intensity, which further confirmed the successful incorporation of Ni ions into the CoS1.097 lattice. Furthermore, the XPS survey and the high revolution XPS spectra of Co 2p, Ni 2p, and S 2p have been compared and shown in Fig. S6 and Fig. 4b-d. For CoS1.097 sample, the doublet peaks located at 776.57 and 791.64 eV could be divided into four peaks, which corresponded to the 2p3/2 and 2p1/2 electronic level of Co(III) at 776.56 eV, 791.7 eV and Co(II) at ~ 777.33 eV and 793.3 eV, respectively


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(Fig. 4b). For Ni0.13Co0.87S1.097 nanotubes, significant positive shift of Co(III) related peaks to 776.91 and 791.84 eV were observed, which proved that the introduction of Ni would remarkably affect the electronic structure of Co. The Ni doping in CoS1.097 nanotubes was further verified by the appearance of Ni 2p3/2 and Ni 2p1/2 characteristic peaks at 854.88 and 872.52 eV (Fig. 4c), respectively, including the coexistence of Ni(II) (851.48, 872.22 eV) and Ni(III) (854.96, 873.5 eV) 52. The S 2p spectra of the two samples (Fig. 4d) could be deconvoluted to S 2p3/2, S 2p1/2 and satellite peaks, attributing to metal-sulfur bond and partial surface oxidation of the exposed sulfide in air.

Fig. 4 (a) XRD patterns and high-resolution (b) Co 2p, (c) Ni 2p, and (d) S 2p XPS spectra of CoS1.097 and Ni0.13Co0.87S1.097 mesoporous nanotubes.


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The homogenous incorporation of Ni into CoS1.097 nanotubes is able to tune the surface electronic structure and provides a model for investigating the dependence of metal composition on their electrocatalytic activity for OER. The cyclic voltammetry in the range of 1.165 ~1.265 V vs. RHE with the different scan rate of 5~25 mV s-1 was used to determine the electrochemical specific surface area (ECSA) of these nanotubes. The ECSA values of Ni0.13Co0.87S1.097 and CoS1.097 mesoporous nanotubes calculated from the slopes of the linear plots of scan rate vs. the variation of charging current density (∆j) at 1.21 V (Fig. 5c and 5d) are 26.42 and 14.02 mF/cm2, respectively, which indicates a larger catalytically accessible surface area of Ni0.13Co0.87S1.097 mesoporous nanotubes.


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Fig. 5 Cyclic voltammograms of (a) CoS1.097 and (b) Ni0.13Co0.87S1.097 mesoporous nanotubes at different scan rates in 1.0 M KOH. The linear plots of the scan rates vs. current density variation (∆j = ja - jc) at 1.21 V of (c) CoS1.097 and (d) Ni0.13Co0.87S1.097 mesoporous nanotubes. The OER activity of the developed catalysts was evaluated by linear sweep voltammetry (LSV) polarization curves in 1.0 M KOH solution (Fig. 6a). Among four studied catalysts, the bimetallic Ni0.13Co0.87S1.097 nanotube catalyst requires the lowest onset potential (measured at 1 mA cm-2) of 262 mV and yields a current density of 10 mA cm-2 at the smallest overpotential of 316 mV. The CoS1.097 nanotubes require 280 and 331 mV to deliver the current densities of 1 mA cm-2 and 10 mA cm-2, respectively, attesting to the promotional effect of Ni to OER activity enhancement. In the contrast, CoS1.097 nanocluster catalyst (as shown in Fig. S2a and Fig. S7-S9) requires an overpotential of 354 mV to generate the current density of 10 mA cm−2, demonstrating the benefits of mesoporous nanotube structure relative to nanocluster due to the improved charge and mass transport. Importantly, the performance of Ni0.13Co0.87S1.097 nanotube catalyst is significantly higher than that of commercial IrO2, which requires 343 mV to achieve 10 mA cm-2. The kinetic processes of OER on these electrocatalysts were further evaluated by Tafel plots (Fig. 6b). The Ni0.13Co0.87S1.097 sample shows a Tafel slope of 54.72 mV dec-1, which is smaller than those of CoS1.097 nanotubes (55.54 mV dec-1), CoS1.097 nanoclusters (61.60 mV dec-1), and precious metal catalyst IrO2 (90.27 mV dec-1), suggesting an accelerated OER rate via Ni doping. The enhanced OER activity measured on Ni0.13Co0.87S1.097 could be mainly attributed to the synergistic effect between Ni and Co53-57, unique ultrathin-walled mesoporous nanotube structure and increased surface area. Moreover, Ni0.13Co0.87S1.097 nanotubes exhibit better catalytic performance compared to most of the reported transition-metal sulfides, oxides, layered double hydroxides, and phosphides catalysts, and a detailed comparison of the performance of


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various catalysts for OER is shown in Table S1. Furthermore, the electrical impendence spectra of CoS1.097 and Ni0.13Co0.87S1.097 nanotubes were conducted at open circuit potential and exhibited in Fig. 6c. The inset of Fig. 6c is the equivalent circuit used to fit the impedance spectra, which includes the electrolyte resistance (Rs), the electron transfer resistance (Rct), Warburg impendance and constant phase angle (CPE). Compared to the Rct of CoS1.097 nanotubes (69.7 ohm), a smaller Rct is measured with Ni0.13Co0.87S1.097 nanotubes (24.7 ohm), which is constant with the I-V behavior of the two nanotubes (Fig.S10). Raman spectra of CoS1.097 and Ni0.13Co0.87S1.097 nanotubes are characterized (Fig. S11). Both of the D band (~1593 cm-1) and G band (~1350 cm-1) features for carbon are missing, suggesting that there might be no carbon materials in the synthesized CoS1.097 and Ni0.13Co0.87S1.097 samples. The electrocatalytic stability of the two samples was characterized by galvanostatic method. On the contrary to the increased overpotential observed on the CoS1.097 nanotubes, the Ni0.13Co0.87S1.097 nanotubes present a decrease of overpotential, attesting to the outstanding durability of Ni0.13Co0.87S1.097 after 2 h.


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Fig.6 (a) LSV polarization curves and (b) Tafel plots of CoS1.097 nanotubes, Ni0.13Co0.87S1.097 nanotubes, CoS1.097 nanoclusters, and IrO2 catalysts. (c) Nyquist plots (inset is the equivalent circuit) and (d) chronopotentiometry curves obtained at the current density of 10 mA cm-2 of the CoS1.097 and Ni0.13Co0.87S1.097 nanotubes. To investigate the stability of the catalyst, the structure, morphology, and composition of the CoS1.097 after the OER stability test at the current density of 10 mA cm-2 for 2 h were determined by XRD, SEM, and EDX, respectively. The catalyst after OER stability test was collected from indium-tin oxide (ITO) electrode (Fig. S12a) and washed and centrifuged by water and ethanol, then re-dispersed in ethanol. As shown in Fig. S12c-e, the morphology of CoS1.097 exhibited no distinct changes and maintained nanotube structure and the elemental distribution kept relatively uniform after OER stability test. The corresponding atomic molar ratio of Co:S measured from


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EDX spectrum (Fig. S12f) is also close to the original composition of CoS1.097. The Faradaic efficiency (FE) of the Ni0.13Co0.87S1.097 nanotubes was estimated without iR compensation by using a rotating ring disk electrode (RRDE, Fig. S13) 39,58. The FE’s calculated at the current densities of 1.5, 2.0, 3.0, 4.5 and 6.0 mA cm-1 were 94.2%, 81.9%, 70.5%, 60.6%, and 52.8%, respectively. The gradual reduction of the FE can be ascribed to the hindrance of generated oxygen produced at high potentials which are difficult to be removed from the electrode surface (Fig.7).

Fig. 7 Faradaic efficiency of the Ni0.13Co0.87S1.097 nanotubes. Conclusions In conclusion, mesoporous Ni-Co sulfide nanotubes with ultrathin walls are synthesized by a simple solvothermal method and employed as an excellent platform to promote electrochemical water oxidation. The developed Ni0.13Co0.87S1.097 nanotubes exhibit excellent OER activity with an overpotential of 316 mV to achieve a current density of 10 mA cm-2, a small Tafel slope of 54.72 mV dec-1 and outstanding durability, which is superior to that of commercial IrO2 and most


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of the reported cobalt-based OER catalysts. The enhanced OER performance can be ascribed to unique mesoporous nanotube structure, synergistic effect between Ni and Co, and increased electrochemically accessible surface area and conductivity. This study may open new opportunities for designing highly efficient and structurally robust OER catalysts based on earthabundant elements.

ASSOCIATED CONTENT Supporting Information. The SEM images of morphology evolution of CoS1.097 nanotubes, the N2 adsorption-desorption isotherm of CoS1.097, EDX of Ni0.13Co0.87S1.097, XRD pattern of the CoS1.097 nanoclusters, RRDE voltammograms at various rotation rates, the XPS survey spectra, I-V curves, Raman spectra of CoS1.097 and Ni0.13Co0.87S1.097, the XRD, SEM and EDX of CoS1.097 after the OER stability test and Table S1 are available in ESI. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ⊥

These authors contributed equally to this work.

ACKNOWLEDGMENTS


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J. C. Zhang thanks for the financial support from National Natural Science Foundation of China (21501006) and the National Undergraduate Training Program for Innovation and Entrepreneurship (201710479002). D. J. Zhang acknowledges the financial supports by National Natural Science Foundation of China (21603004, U1604119) and Program for Innovative Research Team of Science and Technology in the University of Henan Province (18IRTSTHN006), China. Q. Li thanks for financial supports from National 1000 Young Talents Program of China, National Natural Science Foundation of China (21603078) and National Materials Genome Project (2016YFB0700600). REFERENCES (1) Larcher, D.; Tarascon, J-M., Towards Greener and More Sustainable Batteries for Electrical Energy Storage. Nat. Chem. 2015, 7, 19–29. (2) Hu, X. L.; Zhang, W.; Liu, X. X.; Mei, Y. N.; Huang,Y. H., Nanostructured Mo-based Electrode Materials for Electrochemical Energy Storage. Chem. Soc. Rev. 2015, 44, 2376–2404. (3) Li, L.; Wu, Z.; Yuan, S.; Zhang, X. B., Advances and Challenges for Flexible Energy Storage and Conversion Devices and Systems. Energy Environ. Sci. 2014, 7, 2101–2122. (4) Zhang, K.; Han, X. P.; Hu, Z.; Zhang, X. L.; Tao, Z. L.; Chen, J., Nanostructured Mn-based oxides for Electrochemical Energy Storage and Conversion. Chem. Soc. Rev. 2015, 44, 699–728. (5) Wang, T.; Xie, H.; Chen, M.; D’Aloia, A.; Cho, J.; Wu, G.; Li, Q., Precious Metal-free Approach to Hydrogen Electrocatalysis for Energy Conversion: From Mechanism Understanding to Catalyst Design. Nano Energy. 2017, 42, 69–89.


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Facile Synthesis of Mesoporous and Thin-Walled NiâCo Sulfide

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