Stainless Steel Mesh-Supported NiS Nanosheet Array as Highly

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Stainless Steel Mesh-supported NiS nanosheet Array as Highly Efficient Catalyst for Oxygen Evolution Reaction Jun Song Chen, Jiawen Ren, Menny Shalom, Tim Fellinger, and Markus Antonietti ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10099 • Publication Date (Web): 05 Feb 2016 Downloaded from http://pubs.acs.org on February 5, 2016

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ACS Applied Materials & Interfaces

Stainless Steel Mesh-supported NiS Nanosheet Array as Highly Efficient Catalyst for Oxygen Evolution Reaction Jun Song Chenab*, Jiawen Renac, Menny Shaloma, Tim Fellingera and Markus Antoniettia a

Max-Planck-Institute of Colloids and Interfaces, Department of Colloid Chemistry, Am Mühlenberg 1, 14476 Potsdam-Golm, Germany. b

Department of Materials Science and Engineering, National University of Singapore, Singapore 117574. c

Exploration Hall, 20101 Academic Way, George Washington University, Ashburn, Virginia, United States. Zip code: 20147.

KEYWORDS: NiS, nanosheets, stainless steel mesh, electrochemical catalyst, oxygen evolution

1 Environment ACS Paragon Plus


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ABSTRACT Nickel (II) sulfide (NiS) nanosheets with a thickness of 10 nm and a size of 200 nm were facilely grown on stainless steel (SLS) meshes via a one-pot hydrothermal method. This unique construction renders an excellent electrical contact between the porous film of active NiS sheets and the highly conductive substrate, which exhibits a superior catalytic activity towards oxygen evolution reaction (OER). The NiS@SLS electrocatalyst exhibits an unusually low overpotential of 297 mV (i.e., 1.524 V vs. RHE) at a current density of 11 mA·cm-2, and an extra small Tafel slope of only 47 mV·dec-1 proves an even more competitive performance at high to very high current densities. This performance compares very favorably to other Ni-based catalysts and even to the precious state-of-the-art IrO2 or RuO2 catalyst.

Introduction Searching for new solutions to master the global energy cycles is one of the major concerns of nowadays material science community. Oxygen evolution reaction (OER) is an important electrochemical pathway to convert renewable energy, e.g. from solar or windmills, to storable chemical energy through the water splitting process.1 This reaction is intrinsically sluggish, because it involves a complex transfer of four electrons.2-3 Even though materials like RuO2 and IrO2 have been identified as very efficient catalysts with an OER-overpotential of only about 100 to 200 mV,4-8 their high cost and scarcity hinder the practical large-scale application of these noble metal-based electrolyzers. First-row transition metals appear to possess the properties which could overcome this drawback: they are abundant, have low costs and at the same time they also demonstrate promising catalytic activities towards OER.9 Fe, Co, Ni, and Mn are probably the most studied transition metal-based catalysts for this


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purpose and in many different forms, including as bare metal,10-12 oxides,13-18 or hydroxides.19-25 In addition to oxides and hydroxides, metal sulfides are highly interesting and have recently gained attention.26 For example, molybdenum sulfide (MoS2) and many other transition metal sulfides have demonstrated promising catalytic activity in hydrogen evolution reaction (HER),27-31 and in energy storage devices such as lithium-ion batteries and supercapacitors.32-36 However, there is plenty of room for the investigation of metal sulfides for OER, as only little information about the catalytic activity in this reaction is so far reported.37-38 Inspired by the MoS2 success case, we therefore aimed to study the catalytic properties of metal sulfide for OER. Here we show the development of a simple hydrothermal method39-42 to grow NiS nanosheets directly on the stainless steel (SLS) mesh substrates (denoted as NiS@SLS). NiS was selected as the oxygen evolution catalyst, because of the high catalytic activity of pure Ni among the different transition metals towards both OER9 and HER.43 In the current work, SLS was used as the substrate, since it is a common chemical engineering part, has a high physical robustness and a chemical resistance in both basic and acidic environments. Additionally, it has a relatively low electrical resistivity of 74 mΩ·cm. These physical properties suggest that SLS can be applied as a good support material for electrochemical processes like OER. The structure and chemical composition of the as-prepared NiS@SLS were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffractometer (XRD). The catalytic properties were evaluated by linear sweep voltammograms (LSV) and chronoamperometric measurements. The mechanism was investigated from Tafel plots.



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Results and Discussion NiS nanosheets were deposited onto SLS using the hydrothermal technique as described in the experimental part. Electron microscopy was utilized to investigate the morphology of the as-prepared NiS@SLS (Figure 1). At a low magnification (Figure 1A) we can conclude that the SLS mesh structure is perfectly retained after the hydrothermal treatment. From an optical photograph (Figure 1A, upper inset), it is observed that the color of the SLS mesh changed from silvery white to black, suggesting the successful deposition of a uniform NiS layer. At higher magnification it is also obvious (Figure 1B and inset) that the surface of the SLS substrate is indeed completely covered with vertically aligned sheetlike structures with a thickness of about 10 nm and a sheet diameter of ~200 nm. A cross-section image (Figure 1A, lower inset) shows that the nanosheets form a homogeneous, house˗of˗cards type film with a thickness of ~280 nm with interstitial porosity. Figure 1C depicts the TEM image of two overlapping NiS nanosheets, and it can be clearly seen that pores in the lower nanometer range are present in these nanosheets (Figure 1C, inset). The high-resolution (HR) TEM image (Figure 1D) depicts two sets of visible lattice fringes from a single nanosheet, with interlattice spacings of 0.29 and 0.25 nm, corresponding to the (100) and (101) planes of α-NiS, respectively. The crystallographic information of the sample was collected using X-ray diffraction (XRD), and all the identified diffraction peaks in the XRD pattern (Figure 2A) can be assigned to hexagonal α-NiS (JCPDS Card No. 02-1280, S.G.: P63/mmc, ao = bo = 3.41 Å, co = 5.317 Å),44 confirming the phase purity of the as-prepared nanosheet structure on the SLS substrate. At the same time, broadening of the (100), (102) and (110) peaks can be observed, which could probably be attributed to lattice imperfections in the crystal structure.45 The valence state of Ni was investigated by X-ray photoelectron spectroscopy (XPS), and the spectrum of Ni 2p is shown in Figure 2B. Two main peaks can be identified at binding energies of 853.6 and 871.8 eV, which could be assigned to Ni 2p3/2 and Ni 2p1/2,


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respectively. The other two smaller peaks at 858.7 and 878.1 eV are the satellite peaks associated with the corresponding main peak.46 The Raman spectrum of the sample is shown in Figure 2C, with 4 distinct peaks at Raman shift of 222, 285, 335, and 376 cm−1, which are consistent with previous results.47-48 Nitrogen sorption results (Table 1) suggest that the NiS@SLS possess an increased surface area of 79.5 m2 g-1 and an enhanced total pore volume of 0.088 cm3 g-1 compared to the pristine SLS mesh with values of 15.6 m2 g-1 and 0.019 cm3 g-1, respectively. NiS@SLS exhibits a surface area which is almost 5 times that of bare SLS, which could be attributed to the highly porous NiS nanosheets (as indicated by the above TEM analysis) giving rise to a much larger total pore volume, as well as the NiS film itself, which significantly increases the surface roughness and in turn increases the surface area. Considering the high densities of SLS and NiS (7.9 and 5.5 g cm˗3, respectively), these values are unexpectedly high. It is important to note that the current hydrothermal system is quite versatile in producing nickel sulfide materials on different substrate, and a distinct sheet-like structure can also be synthesized on the surface of nickel foam, with the results shown in Figure 3. It is very clear that the sample formed on the surface of Ni foam is very different from NiS@SLS. It consists of very fine but interconnected nanosheets with a more random orientation. As confirmed by XRD (Figure 3C), the obtained sample is phase-pure Ni3S2 (JCPDS No. 44-1418, S. G. R32, ao = bo = 5.7454 Å, co = 7.1350 Å),37 and thus denoted as Ni3S2@Ni. Since Ni is one of the important alloy compounds present in SLS, elemental analysis was performed to study the compositional changes before and after the hydrothermal treatment. As shown in Table S1, when the substrate was hydrothermally treated only with the sulfur source, thioacetamide (TAA), in the absence of the Ni salt, the relative composition of different elements of SLS remains constant, and no sulfur was introduced. This observation implies that under the current hydrothermal conditions, TAA alone is not able to convert the metallic Ni present in the SLS to NiS. As a result, also



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no change of the surface morphology (Figure S1) and crystal phase (Figure S2) can be observed after this process. However, the Ni and S contents increase significantly after the successful growth of the NiS nanosheets on SLS, suggesting that the as-formed NiS originates entirely from the added S and Ni precursors. It is also apparent that the content of all the metal elements present in the SLS mesh has decreased significantly after the hydrothermal treatment. It is thus valid to assume that the content of Ni from SLS also decreases to about 3 at%. As a result, the atomic ratio of Ni to S in the nickel sulfide film formed on the SLS is about 1:1.28, suggesting that the as-prepared sample is NiS. The NiS@SLS was subsequently applied as electrocatalyst in OER in an alkaline electrolyte. The performances of reference materials, including 20 wt% Pt on carbon and free-standing NiS nanosheets (morphology shown in Figure S3) pasted on SLS (denoted as Pt/C on SLS and NiS on SLS, respectively), commercial IrO2 powder, and Ni3S2@Ni were added for comparison. The samples were first cycled in the potential range of 1.064 V – 1.764 V (vs. RHE) to get a constant voltammogram, and a typical CV profile of NiS@SLS was given in Figure S4A. Apparently, two current peaks at ~ 1.5 V (vs. RHE, anodic sweep) and ~1.3 V (vs. RHE, cathodic sweep) can be identified. This pair of peaks is commonly observed in Ni-based compounds,17, 19, 49-50 and it can be explained using the following redox reaction:51 NiS + OH− ↔ NiSOH + e−

(1)

Subsequently, LSVs were recorded for all samples, as shown in Figure 4A. The current densities of the samples rapidly increased with more positive polarization, indicating water splitting activity in the alkaline electrolyte. For the NiS@SLS sample, the anodic peak at ~1.42 V (vs. RHE) can be clearly identified which is also present in the Ni3S2@Ni sample, and the current densities were compared to the NiS on SLS sample over the entire potential range. By considering that the NiS sheets in both samples were synthesized under the same hydrothermal conditions, the reason for NiS@SLS to have a much


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better activity is attributed to its better contact of the active NiS nanosheets to the SLS substrate, as the pure NiS synthesized without SLS consists of much larger aggregates of sheet-like subunits (Figure S3). It is worth mentioning that this contact between the NiS nanosheets and the SLS substrate is so intimate that these nanosheets cannot be detached from the substrate even by ultrasonication, i.e. it forms a real heterojunction. This renders potentially also the best possible electron transfer pathway from the catalytically active NiS to the conductive SLS substrate. Interestingly, no apparent oxidation peak can be identified for the NiS on SLS sample. This might be due to the poor contact between the NiS nanosheets and the SLS substrate, and the poorly exposed Ni-based active centers (as a result of the severely aggregated NiS nanosheets), which make reaction 1 very sluggish, and thus lead to a much lower catalytic activity. NiS@SLS exhibits higher current densities than Pt/C on SLS, as Pt itself is known to be not a very active catalyst towards OER.9 However, it still demonstrated much better performance compare to the state-of-the-art IrO2 catalyst over the entire potential range. It is worth mentioning that even though the pristine SLS substrate shows a performance with a much lower activity than the NiS@SLS sample, it is still better compared to the SLS plate,52 reflecting the possibly higher surface area of the SLS mesh and that it can serve as a better substrate for electrochemical reactions. Furthermore, when the scan rates increases from 5 mV·s-1 to 10 mV·s-1 (Figure S4B), no significant differences in the LSV pattern can be observed during the oxygen evolution. This implies that the scan rates have a little influence on the anodic currents of NiS@SLS, yet they greatly affect the charge stored in the sample, which decreases rapidly upon increasing scan rates (Figure S4C). At the same time, the dependence of peak current on the scan rate is quite linear, as depicted in Figure S4C. In order to further analyze the catalytic activities, Tafel plots were constructed (Figure 4B). The linear portions of these curves were fitted to the Tafel equation in the form of η = b log j + a, where η is the overpotential, j is the current density, and b is the Tafel slope. The as-prepared NiS@SLS clearly



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demonstrates the lowest Tafel slope of 47 mV·dec-1 among all the samples. The detailed mechanism of the NiS@SLS in OER is not well understood at the current stage, yet we believe that it is similar to that of nickel oxide (NiO)-based catalyst, which has been intensively studied.13, 53-55 In the case of NiO, considering the Tafel slope with a value of ~40 mV·dec-1 and a reaction order of 1, the transfer of the second electron could be regarded to be the rate-determining step (RDS) for the OER in alkaline medium.53 Similar to the oxide counterpart, NiS also undergoes a transition of Ni(II) to Ni(III) (refer to Equation 1), and Ni(III) could probably act as the metal center of the catalytic site on the surface of the catalyst. Furthermore, the adsorbed H2O molecules on the surface of NiS would probably lead to the formation of hydrated NiS, which could also serve as additional active sites.37 Nevertheless, more investigation is required in order to fully understand the mechanism of NiS-based catalysts for OER. We then studied the overpotential at a current density of about 10 mA·cm-2, like recently suggested for comparable benchmarking electrolyzers by Jamarillo et al..50 To exclude the contribution of e.g. faradaic currents originating from the electrooxidation of the electrode, like commonly observed for Ni electrodes,19-20, 49, 56 chronoamperometric measurements were performed at an overpotential of 297 mV (i.e., 1.524 V vs. RHE). As shown in Figure 4C, a current density of about 11 mA·cm-2 can be reached at this relatively low overpotential. The prolonged test up to 10 h showed no significant decay, demonstrating a superior long-term stability. Such an overpotential is significantly lower than that of the other samples: Pt/C on SLS sample has an overpotential of 325 mV (1.552 V vs. RHE), 327 mV (1.554 V vs. RHE) for NiS on SLS, 350 mV (1.577 V vs. RHE) for Ni3S2@Ni, and 353 mV (1.58 V vs. RHE) for SLS (please refer to Table 2 for a more detailed comparison). This value is also much lower than the IrO2 noble metal-based catalyst tested in the present work, which shows an overpotential of 343 mV (1.57 V vs. RHE), and is also only slightly higher than the values for IrO2 and RuO2 reported in literatures generally with an overpotential of 270‒280 mV (1.497‒1.507 V vs. RHE).57


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We subsequently calculated the specific current density per catalyst surface area (js) at this overpotential (detailed calculation procedures are shown in the supporting information), and a value of 1 mA·cm-2 was obtained for NiS@SLS, 0.018 mA·cm-2 for Pt/C on SLS, 0.089 mA·cm-2 for NiS on SLS, 0.01 mA·cm-2 for Ni3S2@Ni, and 0.097 mA·cm-2 for bare SLS. All the important electrochemical parameters are calculated and summarized in Table 2. Apparently, the as-prepared NiS@SLS sample demonstrates much higher js compared to other samples. The electrical impedance spectra (EIS) of the samples were obtained in order to understand the charge transport process inside the electrodes. As shown in Figure 5, all 4 samples exhibit similar Nyquist pattern, with an up-sloping near-linear line in the high frequency range, followed by a semicircle bending towards the x-axis in the low frequency domain. These patterns can be fitted using an equivalent circuit shown in Figure 5, inset: Rs represents the series resistance usually consisting of contact resistance and electrolyte resistance. RF implies the resistance of film and QPEDL stands for the modified double-layer capacitance arising from the accumulation of electron charges on the surface of the film. RCT represents the charge transfer resistance associated with the OER process,58 along with the QPECT corresponding to the interface between the electrolyte and the film. It is important to note that using constant phase element is more suitable than a simple capacitor for EIS modeling in practical situations as the former considers surface roughness, inhomogeneous distribution of the catalytic active sites.58 Ws denotes the finite length Warburg impedance (short circuit) which represents the diffusion of ions through the film. By calculating all the fitted data (Table S2), it is apparent that NiS@SLS exhibits the lowest charge transfer resistance among all five samples, suggesting a highly facilitated OER process. At the same time it also shows the highest Q2 value, implying that it probably has the highest number of active sites. These data unravel the reason for the as-prepared NiS@SLS to have the best electrocatalytic activity. It is interesting to find that Pt/C on SLS shows the lowest Rs value, and this



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might be due to the presence of a large amount of highly conductive graphitic carbon located inside the mesh structure, which could probably create additional charge transport pathways and thus lead to a resistance that appears to be smaller than the others. When compared to other Ni- or noble metal-based catalysts reported in previous works (Table S3), the current NiS@SLS exhibits outstanding OER catalytic activities. All the above results indicate that the as-prepared NiS@SLS is a highly active, cheap and scalable OER catalyst. We have also conducted detailed post-mortem analysis for NiS@SLS after the 10-h chronoamperometry test. From the EDX data shown in Table S1, it is apparent that the sample demonstrates a significant decrease in the S content. Such a finding could be explained using the Pourbaix diagram for sulphur-water system under electrochemical conditions.59 The diagram depicts that at the potential of 1.524 V (vs. RHE) with a pH of 13, the sulphur usually exists in the form of SO42− at equilibrium, and this ion is highly soluble in water. As expected, the dissolution of S leads to the decrease in sulphur content which is also companied by changes in the sample’s morphology as well as crystal structure. The ordered film structure consisting of NiS nanosheets was indeed disrupted after the test as evidenced by the SEM images (Figure S6A−B). Yet, the individual sheet-like subunits can still be identified under TEM (Figure S6C−D), consisting of nanocrystallites with visible lattice fringes. It is interesting to find that these crystallites show an interlattice spacing of 0.2 nm, corresponding to the (102) crystal plane of α-NiS. The broad peak in the XRD pattern (Figure S6E) also suggests the disappearance of long-range orderness in the crystal structure and the presence of small crystallites, consistent with the above SEM and TEM observations. The XPS spectrum of Ni 2p (Figure S6F) demonstrates no noticeable differences after the test, implying that the valence state of Ni(II) remained largely unchanged. We can thus conclude that even though the morphology and crystal structure of the


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sample were altered, the activity of the Ni(II)-based catalyst sites was not affected by the loss of the sulphur content, suggesting a relatively robust electrocatalyst system.

Conclusions In conclusion, a facile hydrothermal method to directly grow NiS nanosheets on stainless steel mesh substrates was presented. These nanosheets have a thickness of ~10 nm and a size of ~200 nm, and they form a homogeneous film on the steel support with a thickness of ~280 nm. The as-prepared NiS@SLS demonstrates superior catalytic activities for OER with a low overpotential of only 297 mV (1.524 V vs. RHE) and an ultra-low Tafel slope of 47 mV·dec-1. In the closely related NiO case, such a Tafel slope value suggests that the transfer of the second electron can be regarded as the RDS for OER in alkaline medium. This promising performance was attributed to unique architecture providing direct contact between the active NiS and the highly conductive stainless steel substrate, which grants a highly efficient electron transfer pathway, and also to the NiS sheet-like nanostructure which gives a correct exposure of catalytic nickel centers. Such a result is also supported by EIS study, which suggests that the as-prepared NiS@SLS exhibits lowest charge transfer resistance and highest number of active sites associated with the OER process when compared to other reference samples. Considering the structural flexibility/robustness and chemical stability of the current SLS-supported NiS catalyst, such a material is suitable for a wide range of practical scenarios with different engineering conditions. We believe that this work stimulates further work on the future design of metal sulfide-based catalyst for electrochemical applications like OER and HER, based on a novel category of inexpensive, highly active electrocatalysts.



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MATERIALS AND METHODS Material synthesis. NiS@SLS was synthesized via a one-pot hydrothermal method. Briefly, 0.8 g thioacetamide (TAA) was first dissolved in 20 ml of H2O, followed by the addition of 0.25 mmol NiCl2. The solution was then transferred into a Teflon-lined stainless steel autoclave. A piece of cleaned stainless steel (SLS) mesh (316L, 300 mesh) with a size of 2 cm × 2 cm was immersed into the solution. The autoclave was put inside an electric oven at 120 oC for 20 h and then cooled naturally to room temperature. The mesh was subsequently flushed with copious amount of DI water and tried in a vacuum oven at 60 oC overnight. After this method, a catalyst loading of ~1 mg·cm-2 can be achieved on the stainless steel substrate. Free-standing NiS nanosheets were synthesized following the same procedure without placing the SLS substrate, and Ni3S2@Ni was synthesized by replacing SLS substrate with Ni foam. Material Characterization. The morphology of the sample was studied by field-emission scanning electron microscopy (FESEM; JOEL, JEM-6700F, 5 kV) and transmission electron microscopy (TEM; Zeiss EM 912 Ω). The elemental compositions of the samples were analyzed with energy-dispersive Xray spectroscopy (EDX) attached to the FESEM instrument. The chemical composition of the sample was determined by a D8 Diffractometer (Bruker Instrument, Cu Kα, λ = 0.154 nm). The surface characterization was performed using a Quantachrome Autosorb-1 at liquid nitrogen temperature. The valence state of the sample was investigated by an X-ray photoelectron spectrometer equipped with an Al Kα X-ray source (1486.6 eV). Raman spectrum was obtained using a LabRam HR Evolution Raman microscope with an Ar ion laser at 514 nm excitation wavelength. Electrochemical Measurements. The OER tests were conducted using a standard three-electrode setup on a Gamry Reference 600 instrument. 0.1 M KOH was used as electrolyte, and Pt wire and Ag/AgCl/KCl (saturated) as counter and reference electrode, respectively. The electrolyte was bubbled


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with pure O2 prior to measurements in order to reach O2 saturation. Linear scan voltammograms (LSV) were conducted at scan rates of 2 mV s-1 to 10 mV s-1. The Tafel slope was obtained by conducting chronoamperometric tests at different potentials. Electrochemical impedance spectroscopy (EIS) was performed with a frequency range of 105 to 0.1 Hz. The long-term stability test was performed by chronoamperometry at a constant potential of 1.524 V (vs. RHE). Cyclic voltammetry (CV) was carried at a scan rate of 25 mV s-1. 20 wt% Pt on carbon and free-standing NiS nanosheets were pasted on SLS substrate with a catalyst loading of ~1 mg cm-2 to serve as controls. The reference samples were prepared by mixing the catalysts, e.g., free-standing NiS nanosheets or Pt/C, with conductive agent carbon black, the polymeric binder poly(vinylidene fluoride) at a weight ratio of 8:1:1, and then mixture was pasted on the SLS substrate. Current interruption was selected for iR compensation during the measurements.

Supporting Information. EDX data, SEM images, XRD pattern, electrochemical measurements, detailed calculation of ECSA, comparison of the OER performance between the current NiS and other Ni- and noble metal-based catalysts, and post-mortem analysis data. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT The authors are grateful to the Alexander von Humboldt-Stiftung for the financial support.

AUTHOR INFORMATION *Corresponding author. Email: [email protected].



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(26) Du, Y.; Yin, Z.; Zhu, J.; Huang, X.; Wu, X. J.; Zeng, Z.; Yan, Q.; Zhang, H. A General Method for the Large-scale Synthesis of Uniform Ultrathin Metal Sulphide Nanocrystals Nat. Commun. 2012. (27) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y. Defect-Rich MoS2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution Adv. Mater. 2013, 25, 5807-5807. (28) Chang, Y. H.; Lin, C. T.; Chen, T. Y.; Hsu , C. L.; Lee , Y. H.; Zhang , W.; Wei , K. H.; Li, L. J. Highly Effi cient Electrocatalytic Hydrogen Production by MoSx Grown on Graphene-Protected 3D Ni Foams Adv. Mater. 2013, 25, 756-760. (29) Xiang, Q.; Yu, J.; Jaroniec, M. Synergetic Effect of MoS2 and Graphene as Cocatalysts for Enhanced Photocatalytic H2 Production Activity of TiO2 Nanoparticles J. Am. Chem. Soc. 2012, 134, 6575-6578. (30) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction J. Am. Chem. Soc. 2011, 133, 7296-7299. (31) Yu, X.-Y.; Hu, H.; Wang, Y.; Chen, H.; Lou, X. W. Ultrathin MoS2 Nanosheets Supported on Ndoped Carbon Nanoboxes with Enhanced Lithium Storage and Electrocatalytic Properties Angew. Chem. Int. Ed. 2015, 54, 7395-7398. (32) Shen, L.; Yu, L.; Wu, H. B.; Yu, X.-Y.; Zhang, X.; Lou, X. W. Formation of Nickel Cobalt Sulfide Ball-in-Ball Hollow Spheres with Enhanced Electrochemical Pseudocapacitive Properties Nat. Commun. 2015, 6, 10.1038/ncomms7694. (33) Stephenson, T.; Li, Z.; Olsenab, B.; Mitlin, D. Lithium Ion Battery Applications of Molybdenum Disulfide (MoS2) Nanocomposites Energy Environ. Sci. 2014, 7, 209-231. (34) Yang, L.; Wang, S.; Mao, J.; Deng, J.; Gao, Q.; Tang, Y.; Schmidt, O. G. Hierarchical MoS2/Polyaniline Nanowires with Excellent Electrochemical Performance for Lithium-Ion Batteries Adv. Mater. 2013, 25, 1180-1184. (35) Ding, S. J.; Chen, J. S.; Lou, X. W. Glucose-Assisted Growth of MoS2 Nanosheets on CNT Backbone for Improved Lithium Storage Properties Chem. Euro. J. 2011, 17, 13142-13145. (36) Masa, J.; Xia, W.; Sinev, I.; Zhao, A.; Sun, Z.; Grützke, S.; Weide, P.; Muhler, M.; Schuhmann, W. MnxOy/NC and CoxOy/NC Nanoparticles Embedded in a Nitrogen-Doped Carbon Matrix for HighPerformance Bifunctional Oxygen Electrodes Angew. Chem., Int. Ed. 2014, 53, 8508-8512. (37) Zhou, W.; Wu, X.-J.; Cao, X.; Huang, X.; Tan, C.; Tian, J.; Liu, H.; Wang, J.; Zhang, H. Ni3S2 Nanorods/Ni Foam Composite Electrode with Low Overpotential for Electrocatalytic Oxygen Evolution Energy Environ. Sci. 2013, 6, 2921-2924. (38) Huang, L.; Chen, D.; Ding, Y.; Wang, Z. L.; Zeng, Z.; Liu, M. Hybrid Composite Ni(OH)2@NiCo2O4 Grown on Carbon Fiber Paper for High-Performance Supercapacitors ACS Appl. Mater. Interfaces 2013, 5, 11159-11162.


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(39) Chen, N.; Zhang, W.; Yu, W.; Qian, Y. Synthesis of Nanocrystalline NiS with Different Morphologies Mater. Lett. 2002, 55, 230-233. (40) Chen, X.; Fan, R. Low-Temperature Hydrothermal Synthesis of Transition Metal Dichalcogenides Chem. Mater. 2001, 13, 802-805. (41) Jiang, J.; Yu, R.; Yi, R.; Qin, W.; Qiu, G.; Liu, X. Biomolecule-assisted Synthesis of Flower-like NiS Microcrystals via A Hydrothermal Process J. Alloys Compd. 2010, 493, 529-534. (42) Liu, X. Hydrothermal Synthesis and Characterization of Nickel and Cobalt Sulfides Nanocrystallines Mater. Sci. Eng., B 2005, 119, 19-24. (43) Jiang, H.; Yao, Y.; Zhu, Y.; Liu, Y.; Su, Y.; Yang, X.; Li, C. Iron Carbide Nanoparticles Encapsulated in Mesoporous Fe–N-Doped Graphene-Like Carbon Hybrids as Efficient Bifunctional Oxygen Electrocatalysts ACS Appl. Mater. Interfaces 2015, 7, 21511-21520. (44) Wang, Y.; Zhu, Q.; Tao, L.; Su, X. Controlled-synthesis of NiS Hierarchical Hollow Microspheres with Different Building Blocks and Their Application in Lithium Batteries J. Mater. Chem. 2011, 21, 9248-9254. (45) Ungár, T. Microstructural Parameters from X-Ray Diffraction Peak Broadening Scr. Mater. 2004, 51, 777-781. (46) Li, N.; Zhou, B.; Guo, P.; Zhou, J.; Jing, D. Fabrication of Noble-Metal-Free Cd0.5Zn0.5S/NiS Hybrid Photocatalyst for Efficient Solar Hydrogen Evolution Int. J. Hydrogen Energy 2013, 38, 1126811277. (47) Bishop, D. W.; Thomas, P. S.; Ray, A. S. Raman Spectra of Nickel(II) Sulfide Mater. Res. Bull. 1998, 33, 1303-1306. (48) Salavati-Niasari, M.; Davar, F.; Emadi, H. Hierarchical Nanostructured Nickel Sulfide Architectures through Simple Hydrothermal Method in the Presence of Thioglycolic Acid Chalcogenide Lett. 2010, 7, 647-655. (49) Louie, M. W.; Bell, A. T. An Investigation of Thin-Film Ni–Fe Oxide Catalysts for the Electrochemical Evolution of Oxygen J. Am. Chem. Soc. 2013, 135, 12329-12337. (50) McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction J. Am. Chem. Soc. 2013, 135, 16977-16987. (51) Zhu, B. T.; Wang, Z.; Ding, S.; Chen, J. S.; Lou, X. W. Hierarchical Nickel Sulfide Hollow Spheres for High Performance Supercapacitors RSC Adv. 2011, 1, 397-400. (52) Moureaux, F.; Stevens, P.; Toussaint, G.; Chatenet, M. Development of An Oxygen-Evolution Electrode from 316L Stainless Steel: Application to the Oxygen Evolution Reaction in Aqueous Lithium–Air Batteries J. Power Sources 2013, 229, 123-132.



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(53) Doyle, R. L.; Godwin, I. J.; Brandon, M. P.; Lyons, M. E. G. Redox and Electrochemical Water Splitting Catalytic Properties of Hydrated Metal Oxide Modified Electrodes Phys. Chem. Chem. Phys. 2013, 15, 13737-13783. (54) Lyons, M. E. G.; Brandon, M. P. The Significance of Electrochemical Impedance Spectra Recorded During Active Oxygen Evolution for Oxide Covered Ni, Co and Fe Electrodes in Alkaline Solution J. Electroanal. Chem. 2009, 631, 62-70. (55) Lyons, M. E. G.; Brandon, M. P. A Comparative Study of the Oxygen Evolution Reaction on Oxidised Nickel, Cobalt and Iron Electrodes in Base J. Electroanal. Chem. 2010, 641, 119-130. (56) Corrigan, D. A. The Catalysis of the Oxygen Evolution Reaction by Iron Impurities in Thin Film Nickel Oxide Electrodes J. Electrochem. Soc. 1987, 134, 377-384. (57) 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. (58) Jin, C.; Lu, F.; Cao, X.; Yang, Z.; Yang, R. Facile Synthesis and Excellent Electrochemical Properties of NiCo2O4 Spinel Nanowire Arrays as A Bifunctional Catalyst for the Oxygen Reduction and Evolution Reaction J. Mater. Chem. A 2013, 1, 12170-12177. (59) Bouroushian, M., Electrochemistry of the Chalcogens. In Electrochemistry of Metal Chalcogenides, 1st ed.; Scholz, F., Ed. Springer Verlag: Berlin Heidelberg, 2010; Chapter 2, pp 57-75.


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Figures and Captions

Figure 1. Scanning Electron Microscopy (SEM) images of the sample at different magnifications (A – B). Transmission Electron Microscopy (TEM) image of two nanosheets overlapping one another (C), and high-resolution (HR) TEM image showing visible lattice fringes from a single nanosheet (D). The upper inset in A shows a photograph that compares the stainless steel (SLS) substrate before and after the hydrothermal treatment, and the lower inset shows a cross-section image of the film structure built from NiS nanosheets. The insets in B and C show a magnified image of the corresponding sheet structure.

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Figure 2. Characterization results: X-ray diffraction patterns (XRD; A); X-ray photoelectron spectrum (XPS) of Ni 2p (B); Raman spectrum (C).


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Table 1. Surface characterization data of pristine SLS and NiS@SLS.

2

-1

BET surface area (m g ) 3

-1

Total pore volume (cm g ) Average pore size (nm)

SLS

NiS@SLS

15.6

79.5

0.019 2.4

0.088 2.2

Figure 3. SEM images (A−B) and XRD pattern (C) of Ni 3S2@Ni synthesized by replacing SLS with Ni foam. The asterisks in C mark the diffraction peaks due to the Ni foam substrate.



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Figure 4. OER performance of pristine SLS, NiS on SLS, 20 wt% Pt on carbon, NiS@SLS, and Ni3S2@Ni: Polarization curves at a scan rate of 2 mV s-1 (A). Tafel plot (overpotential vs. log current density) derived from Chronoamperometry (B). Long-term stability tests of the samples at 1.524 V (vs. RHE) (C). All measurements were conducted in 0.1 M KOH electrolyte, using a Ag/AgCl electrode (saturated KCl) as the reference electrode, and Pt wire as the counter electrode. Current interruption was selected for iR compensation for the measurements when necessary.


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Table 2. Electrochemical parameters of the samples. NiS@SLS

PtC on SLS

NiS on SLS

Ni3S2@Ni

SLS

0.6

2.2

0.9

10.65

0.24

EASA (cm ) RFc

15 11.03

55 110

22.5 11.25

266.25 190.18

6 4.32

VRHE @ jg = 10 mA·cm-2 (V)d

1.524

1.552

1.554

1.577

1.58

jg @ VRHE = 1.524 V (mA·cm-2)

10

2

1

1.87

0.42

1

0.018

0.089

0.01

0.097

CDL (mF)

a

2 b

-2 e

js (mA·cm )

Electrochemical double-layer capacitance. b Electrochemically active surface area. c Roughness factor. d VRHE: potential vs. RHE; jg: current density per geometric area. e js: current density per catalyst surface area. Detailed calculation procedures can be found in SI. a

Figure 5. Electrochemical impedance spectra of different samples. The dotted lines represent the modeled data using the equivalent circuit shown in the inset.



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Stainless Steel Mesh-Supported NiS Nanosheet Array as Highly

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