Surface Sulfurization of NiCo-Layered Double Hydroxide Nanosheets

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Surface Sulfurization of NiCo Layered Double Hydroxide Nanosheets Enable Superior and Durable Oxygen Evolution Electrocatalysis Kun Xiang, Jia Guo, Jia Xu, Tingting Qu, Yu Zhang, Shanyong Chen, Panpan Hao, Muhong Li, Mingjiang Xie, Xuefeng Guo, and Weiping Ding ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00723 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Applied Energy Materials

Surface Sulfurization of NiCo Layered Double Hydroxide Nanosheets Enable Superior and Durable Oxygen Evolution Electrocatalysis Kun Xiang,† Jia Guo,† Jia Xu,† Tingting Qu,† Yu Zhang,† Shanyong Chen,† Panpan Hao,† Muhong Li,† Mingjiang Xie,‡ Xuefeng Guo*,† and Weiping Ding† †

Key Lab of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing

University, Nanjing 210023, P. R. China ‡

Hubei Key Laboratory for Processing and Application of Catalytic Materials, Huanggang

Normal University, Huanggang 348000, P. R. China KEYWORDS: surface sulfurization, NiCo-layered double hydroxide, electrocatalyst, oxygen evolution reaction, durability ABSTRACT: Developing earth-abundant, highly active, and durable electrocatalysts for the oxygen evolution reaction (OER) is very important for many renewable energy conversion processes. Herein, we report a novel OER electrocatalyst of NiCo layered double hydroxide@NiCo-hydroxysulfide (NiCo-LDH@HOS) nanosheet arrays, which are prepared by a rapid room-temperature sulfurization of the surface of NiCo-LDH nanosheets grown on Ni foam. The surface sulfurization exerts important influences/changes on the structure, composition, surface properties and chemistry of NiCo-LDH. After surface sulfurization, the resulted NiCo-

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hydroxysulfide layer armor improved electrical conductivity and chemical resistance to alkaline electrolyte, delivers a stable current density of 10.0 mA cm–2 at a low overpotential of 293 mV in 0.1 M KOH solution, maintaining high stability during a 62 h test. The achieved enhanced oxygen evolution activity and improved durability are superior to those of NiCo-LDH nanosheets and benchmark commercial RuO2. This example of NiCo-LDH@HOS obtained via surface sulfurization with enhanced OER electrocatalysis performance, highlights an important strategy to fabricate high-performance metal hydroxide/hydroxysulfides heterostructured catalysts for OER and other electrochemical storage and conversion progress.

1. Introduction The rapid depletion of carbonaceous fossil fuels and the growing environmental issues have triggered increasing research interest in the development of renewable and clean energy conversion and storage technologies.1-4 Oxygen evolution reaction (OER) hold a critical role in these sustainable energy systems such as water splitting, metal-air batteries and fuel cells.5-8 However, the complex four proton-coupled electron transfers and the formation of oxygen– oxygen bond lead to the sluggish kinetics of OER and consequently demand a high overpotential to drive these electrochemical processes.9 The currently used commercial OER catalysts are precious metal oxides such as ruthenium dioxide (RuO2) and iridium oxide (IrO2), accompanying with scarcity and high cost as well as poor durability, limiting their practical large-scale application.10 Therefore, great efforts have been dedicated to designing the alternative OER catalysts based on earth-abundant and cost-efficient 3d metals and their derivatives.11-19 Recently, two dimensional layered double hydroxides (2D-LDH) for energy storage and conversion have attracted extensive attention due to their tunability in component metal cations,


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sheet−like structure with low cost and high surface area.20-30 Among them, NiCo-LDH nanosheets have shown great potential for OER catalysts owing to their abundance on earth, theoretically high catalytic activities and synergistic effect of Ni and Co.31-36 Although significant achievements have been made, the catalytic OER performance of 2D-LDH materials is mainly limited by two tough obstacles: unavoidable aggregation and restacking of nanosheets, as well as the relatively low electrical conductivity and stability.9,33,37,38 For the first issue, it is of great importance to develop binder-free electrode by directly in-situ growing the active materials on current collectors in comparison with LDH powdered products.39,40 For example, Liu and coworkers reported the controlled synthesis of Ni–Co-layered double hydroxide nanosheet arrays on nickel foam with high catalytic activity toward the OER.31 To alleviate the second issue, an important strategy is to combine LDH materials with conducting substances such as carbon, 38,41,42

metals43 and polymer44. Nevertheless, the low intrinsic electrical conductivity and

relatively poor chemical stability of LDH in alkaline (such as KOH) electrolyte still overshadow their practical application to some degree.33 Recently, transition metal sulfides, featuring the enhanced intrinsic electroconductivity, chemical resistance against alkaline electrolyte, low cost, and environmental sustainability, are highly concerned in energy storage/conversion devices.5,45 However, the metal sulfides are thermodynamically unstable in the aqueous and strongly oxidizing environments of OER, and can be easily oxidized to the corresponding metal oxides/hydroxides.46-48 Very recently, doping S2‒ in transition metal-based hydroxides (Co3FeS1.5(OH)6, NiCo2(SOH)x and so on) has been reported to enhance the electronic conductivity and electrocatalytic activity.8,46,49 These sulfurization treatments generally involve the high-temperature/time-consuming procedure, to vulcanize the bulk phase of metal hydroxides entirely. Since the electrocatalytic reaction mainly occurs on the surface of electrode


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materials, the surface sulfurization of NiCo-LDH and its OER performance are very intriguing. To the best of our knowledge, however, the related studies have never been reported up to date. Herein, we fabricate novel two-dimensional NiCo-LDH@NiCo-hydroxysulfide (NiCoLDH@HOS) heterostructure nanosheets on Ni foam as OER catalyst by rapid (30 s) in-situ surface sulfurization of the NiCo-LDH at room temperature in Na2S solution. The surfacesulfurized hybrid nanosheets afford a stable current density of 10 mA cm–2 at a low overpotential of 293 mV in 0.1 M KOH solution, maintaining improved stability during a 62 h test. The highefficiency and durable catalytic activity of the obtained OER electrocatalysts are attributed to their overall advantageous structural and electronic features derived from the surface sulfurization, including the highly conductive and stable hydroxysulfide armor, the homogeneously dispersed nanosheets on the Ni foam, and enhanced reactivity of the active sites. 2. Experimental 2.1 Synthesis of NiCo-LDH@HOS nanosheets arrays/Ni foams In a typical synthesis, 0.3201 g Co(NO3)2·6H2O, 0.1599 g Ni(NO3)2·6H2O and 0.9345 g hexamethylenetetramine (HMT) are dissolved into a solution of 40 mL of ethylene glycol/water mixture (v/v=7/1) to form a clear pink solution after violent stirring for 30 min. Then a piece of surface-cleaned Ni foam (diameter of 15 mm) is immersed into the mixture solution, and transferred into a Teflon-lined stainless steel autoclave for hydrothermal treatment at 120°C for 4 hours. The prepared Ni foam with NiCo-LDH nanosheet arrays is taken out from the solution and cleaned by ultrasonication to remove the loosely attached products on the surface. After washing three times with DI water and ethanol, the NiCo-LDH/Ni foams are immersed into 0.7 M Na2S solution for 30 s. After surface sulfurization reaction, the obtained NiCo-LDH@HOS nanosheets arrays grown on Ni foam are washed three times with DI water and ethanol, followed


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by drying at room temperature in the vacuum oven. The mass loading of the NiCo-LDH@HOS catalyst is about 0.45 mg cm‒2. 2.2 Materials Characterization Powder XRD patterns are obtained on a Bruker D8 Advance X-ray diffractometer system with Co Kα radiation of 1.7902 Å. The SEM and TEM images of materials are performed by field emission scanning electron microscopy (FESEM, Hitachi, S-4800) and transmission electron microscopy (TEM, JEOL, JEM-1101), respectively. The scanning transmission electron microscopy (STEM) images and element mapping are used to analyse surface properties of the samples by high resolution transmission electron microscopy (FEI, Talos F-200). The specific surface area and pore size distribution of the products are evaluated by the ASAP 3020 instrument. An X-ray photoelectron spectroscopy (XPS) survey is conducted using a PHI 5000 VersaProbe with an Al Kα excitation source. The atomic ratios of different elements in the products are measured using inductively coupled plasma atomic emission spectrometer (ICPOES, Optima 5300DV). The electrical conductivity is measured by a four-wire method using a source measure unit (SMU, Keithley 6430). The Raman spectrum is obtained using a Renishaw in Via Reflex Raman system with the laser power of 1 mW and the excitation wavelength of 532 nm. 2.3 Electrochemical measurements The products grown on Ni foams are directly used as the working electrode with a mass loading of 0.45 mg cm–2. The electrochemical measurements are performed in a three-electrode system in an oxygen gas saturated 0.1 M KOH aqueous solution on an electrochemical workstation (CHI 660E, CH Instruments, Inc., Shanghai, China). The counter and reference electrodes are the platinum wire and the Hg/HgO electrode, respectively. Before test, the


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working electrode is activated by cyclic voltammetry (CV) range from 0 to 0.7 V (vs. Hg/HgO) at 50 mV s–1 until the oxygen evolution currents showed negligible change. For oxygen evolution reaction measurement, the LSV curves are carried out at a scan rate of 5 mV s–1 for the polarization curves and 1 mV s–1 for Tafel plots from 0-1 V (vs. Hg/HgO). The electrochemical double-layer capacitance (Cdl) is determined from the CV curves at 1.04 V (vs. RHE) with different scan rates (1, 3, 5, 7 and 9 mV s–1) according to the following equation: Cdl = Ic/ν, where Cdl, Ic, and ν are the double-layer capacitance (F cm–2) of the electroactive materials, charging current (mA cm–2), and scan rate (mV s–1). The TOF value is calculated from the equation: TOF = (J×A)/(4×F×n). Where J is the current density at overpotential of 300 mV. A is the surface area of the electrode, F is the Faraday constant, and n is the number of moles of metal on the electrode. The stability test of catalysts are performed by Chronoamperometric (CP) measurement which is operated at corresponding potentials to drive the current density of 10 mA cm–2. The potentials recorded are calibrated to a reversible hydrogen electrode (RHE) scale according to the following Nernst equation: ERHE=EHg/HgO + 0.059 pH + 0.098, where pH = 13 in 0.1 M KOH solution. Overpotential (η) is calculated using the following formula: η (V) = ERHE – 1.23 V. All polarization curves are corrected with 95% iR-compensation. The internal resistance R is determined by electrochemical impedance spectroscopy. EIS is performed at overpotentials of 350 mV with frequency from 0.01 to 100,000 Hz and an amplitude of 5 mV. 3. Results and discussion The morphological and structural characterizations of the products are performed with scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM). The low-magnification close-up SEM images of NiCo-LDH/Ni foam in Figure 1a and 1b show a coating of NiCo-LDH nanosheets have been


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uniformly grown on the whole skeleton of Ni foam. Typically, as shown in Figure 1c and d, the dense vertical nanosheets with lateral size of several hundred nanometers and thickness of ca. 20 nm homogeneously align and uniformly distribute on Ni foam, forming a porous and open walllike structure, which can facilitate the electrolyte transportation and oxygen gas release. Similarly, the SEM images of NiCo-LDH@HOS NSs on Ni foam in Figure 1e-h clearly present almost same shape and size as the NiCo-LDH NSs on Ni foam. This result demonstrates that surface sulfurization treatment will not cause obvious morphologic changes of the sheet-like framework.

Figure 1. (a-d) Low- and high-magnification SEM images of NiCo-LDH NSs on Ni foam; (e-h) Low- and high-magnification SEM images of NiCo-LDH@HOS NSs on Ni foam. The phase information of NiCo-LDH NSs and NiCo-LDH@HOS NSs is characterized by Xray powder diffraction (XRD). In order to eliminate the effect of nickel substrate, the powder sample are scratched from Ni foams by strong ultrasonication for the XRD analysis. As shown in Figure 2, the characteristic peaks at low 2θ angle are indexed as (003) and (006) reflections, which match well with the hydrotalcite-like structure, confirming the NiCo-LDH phase.36 The two diffraction patterns present the same peak positions corresponding to NiCo-LDH before and


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after sulfurization. However, the intensity of every diffraction peak of NiCo-LDH@HOS NSs is weaker than that of NiCo-LDH NSs, indicating that the sulfurization only occurs on the surface of NiCo-LDH nanosheets and does not destroy the inner LDH bulk structure.

Figure 2. XRD patterns of NiCo-LDH and NiCo-LDH@HOS NSs. As shown in Figure 3a and 3c, the TEM and STEM image of NiCo-LDH@HOS NSs presents the

nearly

transparent

ultrathin

characteristic

of

the

NiCo-LDH@HOS

nanosheets

interconnecting with each other. The high-resolution TEM (HRTEM) image in Figure 3b of the NiCo-LDH@HOS nanosheets shows the presence of crystalline NiCo-LDH possessing lattice fringes of 0.266 nm, which correspond to the (012) crystallographic plane.33 It is noteworthy that the crystalline NiCo-LDH is tightly wrapped by amorphous NiCo-hydroxysulfide shell whose thickness ranges from 5 to 6 nm. The HRTEM results clearly demonstrate that the sulfurization reaction only occurs on the surface of NiCo-LDH NSs. The resultant NiCo-hydroxysulfide shell may exert important influence on the electrochemical performance of NiCo-LDH@HOS NSs. Furthermore, the corresponding elemental mapping demonstrates that Ni, Co, O and S elements


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in the sample are homogeneously distributed on the surface, which is consistent with the surface sulfurization of NiCo-LDH (diagrammatic sketch as shown in Figure 3d) indicated by the XRD

Figure 3. (a) TEM and (b) HRTEM images of NiCo-LDH@HOS NSs; (c) The typical STEM image of NiCo-LDH@HOS NSs and corresponding elemental mappings of Ni, Co, O, and S; (d) Diagrammatic sketch of NiCo-LDH@HOS NSs.


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results. The N2 adsorption and desorption isotherms at 77 K show a type IV curve, with a hysteresis loop at relative pressure between 0.4 to 0.8, indicating the existence of mesopores in the NiCo-LDH@HOS nanosheets (Figure S1). The Brunauer-Emmett-Teller (BET) surface area is about 94.3 m2 g–1, the mesopores and big surface area of the nanosheets are beneficial to the access and transportation of electrolyte. XPS and ICP-OES are performed to investigate the sulfurization degree and examine the elemental composition of the products. The analysis results are presented in Figure 4 and the relative contents of all the other elements are based on the content of Ni. Compared with the XPS spectra of NiCo-LDH NSs, the obvious S peak emerges in the XPS spectra of NiCo-LDH@HOS NSs, suggesting that the successful sulfurization of the NiCo-LDH through the rapid sulfurization treatment for 30 s at room temperature. Furthermore, the atomic ratios of Ni:Co in NiCo-LDH@HOS in Figure 4b calculated from the XPS and ICP-OES analysis results are roughly the same (about 1:2), very close to the feed ratio. From the XPS data, the relative content of sulfur species increases from 0 to 5.17% after sulfurization treatment, which suggests that part of OH− on the surface of NiCo-LDH have been replaced by S2−. The quantified XPS analysis presents the Ni:Co:S ratio of 1:1.92:0.93 for NiCo-LDH@HOS NSs, while ICP-OES analysis gives the value of 1:1.95:0.56. Since XPS signals generally refer to the surface composition of the sample, while ICP-OES results come from the whole sample, the higher S content given by XPS than that of ICP-OES implies that the sulfurization mainly occurred on the surface of NiCoLDH, not the bulk, which is in accordance with the HRTEM and XRD results. To clarify the point further, the in-situ XPS technique with an argon (Ar) ion bombardment is conducted (Figure 4). The surface of the NiCo-LDH@HOS NSs is bombarded by the Ar ion for 30 s, and the Ni:Co:S ratio of NiCo-LDH@HOS NSs changes to be 1:1.97:0.18 (vs. 1:1.92:0.93 before Ar


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ion bombardment). The S content has dramatically reduced along with the peeling off of the amorphous sulfurized layer on the surface of NiCo-LDH@HOS NSs by Ar ion bombardment, which confirms the existence of NiCo-hydroxysulfide layer on the NiCo-LDH@HOS NSs again. The S2− ions in the Na2S solution may exchange partial OH− ions of the NiCo-LDH surface rapidly to form a thin hydroxysulfide layer as an armor on the surface of NiCo-LDH nanosheet arrays due to the huge difference of solubility between the metal hydroxide and its corresponding metal sulfide. Furthermore, Raman spectroscopy is also used to characterize the surface sulfurized sample (Figure S2). Due to the amorphous characteristic of the NiCo-hydroxysulfide layer, the Raman peaks of M–S (M = Ni and Co) between 100 to 350 cm–1 are weak and broad.50,51 The two distinct peaks at Raman shift of 443 cm−1 and 522 cm−1 correspond to Ni-O and Co-O vibrational modes of the crystalline NiCo-hydroxide, respectively.52 The Raman characterization results is consistent with the heterostructure of NiCo-hydroxysulfide layer on the NiCo-LDH surface. These results of HRTEM, in-situ XPS, ICP-OES and Raman analysis, combined with the XRD results, clearly confirm the NiCo-hydroxysulfide-armored NiCo-LDH nanosheets (NiCo-LDH@HOS, diagrammatic sketch is shown in Figure 3d).

Figure 4. (a) Full XPS spectra of α) NiCo-LDH NSs, γ) and β) NiCo-LDH@HOS NSs before and after Ar ions bombardment, respectively; (b) The relative contents of Ni, Co and S in NiCo-


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LDH@HOS NSs by I) XPS and II) ICP-OES analysis, III) the relative contents of Ni, Co and S of NiCo-LDH@HOS NSs after Ar ions bombardment by XPS analysis. As shown in Figure 5a, the high-solution XPS analysis is carried out to investigate the changes of electronic structures of metal active sites. In the Co2p XPS spectrum of NiCo-LDH, the binding energies are 794.5 eV for Co2p1/2 and 779.5 eV for Co2p3/2 with a spin–orbit level energy spacing difference of 15.0 eV, indicating that the Co species are mainly Co (III) in the near-surface of the material. In the case of Co 2p XPS spectrum of NiCo-LDH@HOS, the spinorbit splitting value of Co 2p1/2 and Co 2p3/2 is calculated as 15.8 eV, indicative of the coexistence of Co2+ and Co3+ species.53-55 These changes suggest that a portion of Co (III) in NiCoLDH is transformed into Co (II) by surface sulfurization treatment. In contrast, the spin-orbit splitting values of Ni 2p spectra (Figure 5b) of both two samples are almost the same (18 eV for NiCo-LDH and 17.9 eV for NiCo-LDH@HOS), indicating that the nickel ions maintain the divalent state during the surface sulfurization reaction. This result demonstrates that the Co species in the near-surface of NiCo-LDH@HOS after surface sulfurization have lower oxidative state because the sulfur anions with lower electronegativity are easily polarized and can adjust the electrical structure of adjacent metal ions to be more dispersed.49 In addition, the change of the O 1s XPS peaks of the NiCo-LDH and NiCo-LDH@HOS NSs is also investigated. As shown in Figure 5c, due to the relatively strong polarization effect of S species, the binding energy of O in NiCo-LDH@HOS NSs is shifted about 0.4 eV to lower value compared to the NiCo-LDH NSs. In the case of S 2p XPS spectrum of NiCo-LDH@HOS NSs (Figure 5d), the peak centered at 168 eV can be attributed to the sulfur species bonded to oxygen. The two peaks from 160 eV to 165 eV correspond to the binding energies of M–S (M = Ni and Co).56 These results further confirmed the formation of NiCo-hydroxysulfide layer on the NiCo-LDH surface. The resistance


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(R) of the obtained materials are measured by a four-wire method using a source measure unit and shown in Figure S3.57,58 The electrical conductivity (κ) of the NiCo-LDH@HOS is 5.15×10‒4 S m‒1, which is twice as high as that of the pristine NiCo-LDH nanosheets (2.64×10‒4 S m‒1), indicating that the formed thin hydroxysulfide layer armor enhances the electrical conductivity. All the results of characterizations demonstrate that a rapid and mild surface sulfurization reaction affords a well-defined NiCo-LDH@HOS to study the relationship between the structure, chemistry and its electrocatalytic performances.

Figure 5. High-resolution (a) Co 2p, (b) Ni 2p XPS and (c) O 1s spectra of NiCo-LDH and NiCo-LDH@HOS NSs, respectively; (d) S 2p spectrum NiCo-LDH@HOS NSs.


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As shown in Figure 6, the electrocatalytic performance of NiCo-LDH@HOS for OER is assessed in 0.1 M KOH electrolyte using a typical three-electrode system. For comparison, NiCo-LDH, bare Ni foam and commercial benchmark catalyst RuO2 are also measured under the same conditions. According to the iR-corrected LSV polarization curves in Figure 6a, the NiCoLDH@HOS possesses higher current density and more negative OER onset potential than NiCoLDH, RuO2 and bare Ni foam. To achieve a current density of 10 mA cm–2, considering a metric related to solar fuel synthesis,59 a low overpotential of 293 mV is needed for NiCo-LDH@HOS catalyst, considerably smaller than those of NiCo-LDH (346 mV) and bare Ni foam (477 mV). Remarkably, even commercial noble metal oxides RuO2 catalysts need an overpotential of 326 mV at 10 mA cm–2, significantly inferior to the NiCo-LDH@HOS catalyst in this work. The low overpotential at 10 mA cm–2 of NiCo-LDH@HOS catalyst is also among the best values of the state-of-the-art NiCo-LDH-based electrocatalysts in alkaline condition reported recently (Table S1).31-36,46,60 More importantly, the anodic current density of OER on the NiCo-LDH@HOS catalyst is 86.5 mA cm–2 at 1.59 V (vs. RHE), which is 5.1, 4.5, and 34.5 times larger than that of NiCo-LDH nanosheets, commercial RuO2 and bare Ni foam electrodes, respectively, showing improved OER electrocatalytic activity. In Figure 6a, the distinct oxidation peaks between 1.2 V and 1.5 V (vs. RHE) in the LSV polarization curves of NiCo-LDH and NiCo-LDH@HOS before the sharp increase in current density caused by the oxygen evolution are corresponding to the oxidation of Ni and Co species.61 It is noteworthy that in comparison with NiCo-LDH, the anodic peak of NiCo-LDH@HOS shifts in the negative direction from 1.349 V to 1.325 V (vs. RHE). This result indicates that the relatively lower oxidation states of Co species in NiCoLDH@HOS (compared to those in NiCo-LDH), originating from surface sulfurization process, are more easily oxidized/activated to high-valence Co species at lower potential, which may


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contribute to the improved OER electrocatalytic activity. In addition, the synergy between S2‒ and OH‒ of NiCo-LDH@HOS is beneficial for adsorbing OER intermediates onto the surface of the electrocatalysts, thus contributing to superior electrocatalytic performance.46,56 The catalytic kinetics of the samples are assessed by Tafel slope. Generally, a smaller Tafel slope is more favorable for practical applications as it leads to a remarkably increased OER rate with an increase in overpotential. In Figure 6b, the Tafel plots of the NiCo-LDH@HOS electrode is 72 mV dec–1, which is smaller than NiCo-LDH electrode with 87 mV dec–1 and RuO2 with 102 mV dec–1. This result indicates that the surface sulfurization treatment significantly facilitates the reaction kinetics for NiCo-LDH@HOS nanosheets and there is favorable change in the intrinsic electrocatalytic performance. To further investigate the kinetics of the electrocatalytic process, electrical impedance spectroscopy (EIS) is employed. Figure 6c exhibits the resulting Nyquist plots of the NiCo-LDH and NiCo-LDH@HOS NSs at the overpotential of 350 mV. The semicircles in the EIS curves correspond to the charge transfer resistance (Rct), which is related to the kinetic of OER occurring at the electrode/electrolyte interface. The Rct (the diameter of the semicircle) of NiCoLDH@HOS NSs is 39 Ω, while that of NiCo-LDH is 70 Ω. Meanwhile, the point intersecting with the real axis exhibits the internal resistance (Rs). Similarly, the Rs of the NiCo-LDH@HOS is evaluated to be approximately 3.53 Ω, which is also smaller than that of pristine-NiCo-LDH (5.38 Ω). The markedly smaller Rct and Rs of NiCo-LDH@HOS NSs demonstrate that NiCoLDH@HOS NSs have relatively low charge transfer resistance and superior charge transport kinetics caused by the enhanced electrical conductivity, originating from the surface sulfurization treatment.


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The improved electrocatalytic behavior of NiCo-LDH@HOS for OER are contributed to the surface sulfurization, which can be also confirmed by the changes of electrochemical active surface area (ECSA). The ECSA is in proportion to the number of effective active sites and can

Figure 6. (a) Polarization curves of Ni foam, NiCo-LDH, commercial RuO2, and NiCoLDH@HOS in 0.1 M KOH. (b) Tafel plots of NiCo-LDH, commercial RuO2, and NiCo-


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LDH@HOS. (c) Electrochemical impedance spectra of different electrodes. (d) Plot of the current density vs. the scan rate. (e) Durability tests at a constant current density of 10 mA cm–2.

Figure 7. CV curves of (a) NiCo-LDH and (b) NiCo-LDH@HOS in 0.1 M KOH at different scan rates from1 to 9 mV s‒1. be estimated from the electrochemical double-layer capacitance (Cdl) at the solid-liquid interface by using a simple cyclic voltammetry (CV) method. As shown in Figure 7 and Figure 6d, the Cdl of the NiCo-LDH@HOS is measured and found to be 5.38 mF cm–2, apparently larger than that of NiCo-LDH electrocatalysts (3.24 mF cm–2), corresponding to a 66% increase of the electrochemical active surface area. This result indicates that surface sulfurization of NiCo-LDH can significantly enhance the ECSA, and lead to enhanced electrocatalytic OER activity. Furthermore, the activity of the obtained catalysts can be also expressed in terms of turnover frequency (TOF), assuming that all the metal ions (Ni and Co) in the nanosheets are active. The TOF of NiCo-LDH@HOS is 0.008 s‒1 at the overpotential of 300 mV, about twice as high as that of NiCo-LDH (0.0039 s‒1).


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Apart from the electrocatalytic activity, in view of practical application, the durability of the electrocatalyst for OER is another major concern. The durability of NiCo-LDH@HOS is assessed by using chronopotentiometric held at 10 mA cm–2 in a test procedure. Remarkably, the overpotential of NiCo-LDH@HOS increased by only 2.4% after 62 hours of durability test, in sharp contrast to 7% after 20 hours for NiCo-LDH nanosheets and 3% after 10 hours for commercial RuO2, respectively, as shown in Figure 6e. To further evaluate the stability of the NiCo-LDH@HOS nanosheets, successive CV scanning is carried out for 1000 cycles at a scan rate of 100 mV s‒1. As shown in Figure S4, the changes of LSV curves can be neglected at the end of cycling, which indicates the high cycling stability of NiCo-LDH@HOS nanosheets. The chemically stable hydroxysulfide shell in alkaline electrolyte plays a vital role of armor, leading to the largely enhanced durability for prolonged OER operation. In addition, as shown in Figure S5, the morphology of the NiCo-LDH@HOS catalyst after 62 h stability test is almost the same as that of initial NiCo-LDH@HOS NSs. The robust structural stability of the NiCo-LDH@HOS ensures its remarkable electrochemical durability. In contrast to NiCo-LDH@HOS, the morphology/structure of NiCo-LDH array after 62 h OER durability test was seriously destroyed and showed an irreparable collapse leading to a severe activity decay (Figure S6). In the case of NiCo-LDH@HOS catalyst, the OH‒ confined in NiCo hydroxysulfide layer has stronger electronegativity compared with S2‒, which attracts electrons from the antibonding orbital of metal−S bonds to metal−O bonds and thus making the metal−S bond become shorter and more stable.46 The enhanced binding energy between metal and S, consequently strengthens the immobilization of the sulfur ions and therefore renders the NiCo-LDH@HOS catalysts a superior durability.46 Although the NiCo-LDH@HOS catalyst exhibits excellent durability, according to the ICP-OES result, the Ni:Co:S ratio of the NiCo-LDH@HOS NSs after 62 hours


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of durability test has changed to be 1:2.04:0.43 (vs. 1:1.95:0.56 of initial state). That indicates only a small part of S species have lost during the stability tests.

Figure 8. (a) High resolution TEM image and (b) SAED pattern of the sample obtained after twelve hours’ sulfurization of NiCo-LDH NSs. Based on the present sulfurization approach, the sulfurization degree of NiCo-LDH NSs can be easily adjusted by prolonging the sulfurization time, a series of electrocatalysts with different sulfurization degrees are prepared to explore their OER performances. As shown in Figure S7, the diffraction intensities of the obtained samples with the sulfurization time of 30 s, 5 min, 1 h, 2 h and 12 h become weakened along with the increase of sulfurization times due to the resulted amorphous NiCo-hydroxysulfide on the surface of NiCo-LDH NSs, indicating that the crystal structure has changed during the sulfurization process. Especially, the XRD pattern (Figure S 7f) of the sample obtained after twelve hours’ sulfurization of NiCo-LDH NSs is a nearly straight line without obvious peaks, which indicates the bulk sulfurization. The HRTEM image of the bulk sulfurization sample is shown in Figure 8a. The whole structure is amorphous and homogeneous,

and

the

hydroxysulfide-hydroxide

heterojunction

is

disappeared.

The

corresponding selected area electron diffraction (SAED) pattern in Figure 8b shows no diffraction ring, again indicating amorphous structure, in accordance to the results of XRD


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analysis. The OER performances of the samples with increased sulfurization degree are recorded in Figure 9. The LSV curves of all the sulfurized catalysts are similar. The overpotentials of obtained materials with the sulfurization time of 30 s, 5 min, 1 h, 2 h and 12 h are 293, 301, 300, 301 and 302 mV at 10 mA cm‒2, respectively. The overpotentials of the samples would be slightly increased by prolonging the sulfurization time. Meanwhile, the potential of the sulfurized catalyst with the sulfurization time of 12 h increased by 3% after 62 hours of durability test, which is higher than that of NiCo-LDH@HOS catalyst (2.4%) (Figure S8).

Figure 9. (a) The LSV curves of the samples with various sulfurization times at 5 mV s‒1; (b) the overpotentials of the samples with various sulfurization times at 10 mA cm‒2. These results indicate that surface sulfurization catalyst (NiCo-LDH@HOS obtained with sulfurization time of 30 s) delivers higher activity and stability in comparison with the bulk sulfurization catalyst, suggesting that the heterojunction between hydroxysulfide and hydroxide on the surface of NiCo-LDH@HOS NSs is beneficial to the OER activity and stability.

Conclusions


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In this work, we have introduced a simple and rapid (30 s) surface sulfurization treatment at room temperature to fabricate NiCo-hydroxysulfides layer-armored NiCo layered double hydroxide grown on Ni foam as a highly efficient and durable electrocatalyst for OER. Benefiting from the special structure, the obtained novel electrocatalyst exhibits high electrocatalytic activity and enhanced electrocatalytic stability as long as 62 hours. The impressive OER performance can be mainly attributed to the NiCo-hydroxysulfides layer, which can significantly increase the electrical conductivity and enhance reactivity of active sites as well as offer anti-erosion ability in alkaline electrolyte. This present work of NiCo-hydroxysulfidearmored NiCo-LDH provides a facile, rapid and effective surface sulfurization strategy to fabricate high-performance metal hydroxide/hydroxysulfides heterostructured catalysts for OER and other electrochemical storage and conversion progress.

ASSOCIATED CONTENT Supporting Information N2 adsorption-desorption isotherms of NiCo-LDH@HOS NSs (Figure S1); The Raman spectrum of the NiCo-LDH@HOS NSs (Figure S2); The resistances of NiCo-LDH NSs and NiCoLDH@HOS NSs (Figure S3); LSV polarization curves of NiCo-LDH@HOS before and after 1000 CV cycles (Figure S4); SEM images of NiCo-LDH@HOS NSs after long-time test (Figure S5); SEM images of NiCo-LDH NSs after long-time test (Figure S6); The XRD patterns of NiCo-LDH with different sulfurization times (Figure S7); Durability tests of the catalysts with surface sulfurization and bulk sulfurization at a constant current density of 10 mA cm–2 (Figure S8); Comparisons of the OER performance between NiCo-LDH@HOS NSs and other previously


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reported materials (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Kun Xiang: 0000-0003-0303-5703 Yu Zhang: 0000-0002-5421-9409 Xuefeng Guo: 0000-0002-5492-5899 Weiping Ding: 0000-0002-8034-5740 Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This work was financially supported by the National Key Technology R&D Program of China (2017YFB0310704), the National Natural Science Foundation of China (21773112 and 21273109) and the Fundamental Research Funds for the Central Universities. The authors would like to thank Prof. Qiang Wu and Dr. Hao Fan for the assistance in electrical conductivity measurements. REFERENCES


22

Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(1)

Anantharaj, S.; Ede, S. R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S. Recent

Trends and Perspectives in Electrochemical Water Splitting with an Emphasis on Sulfide, Selenide, and Phosphide Catalysts of Fe, Co, and Ni: A Review. ACS Catal. 2016, 6, 8069-8097. (2)

Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Norskov, J. K.; Jaramillo, T.

F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, eaad4998. (3)

Liang, J.; Wang, Y.-Z.; Wang, C.-C.; Lu, S.-Y. In Situ Formation of NiO on Ni Foam

Prepared with a Novel Leaven Dough Method as an Outstanding Electrocatalyst for Oxygen Evolution Reactions. J. Mater. Chem. A 2016, 4, 9797-9806. (4) Guan, B. Y.; Yu, L.; Wang, X.; Song, S.; Lou, X. W. Formation of Onion-Like NiCo2S4 Particles via Sequential Ion-Exchange for Hybrid Supercapacitors. Adv. Mater. 2017, 29, 1605051. (5)

Suen, N. T.; Hung, S. F.; Quan, Q.; Zhang, N.; Xu, Y. J.; Chen, H. M. Electrocatalysis

for the Oxygen Evolution Reaction: Recent Development and Future Perspectives. Chem. Soc. Rev. 2017, 46, 337-365. (6)

Zhao, S.; Wang, Y.; Dong, J.; He, C.-T.; Yin, H.; An, P.; Zhao, K.; Zhang, X.; Gao, C.;

Zhang, L.; Lv, J.; Wang, J.; Zhang, J.; Khattak, A. M.; Khan, N. A.; Wei, Z.; Zhang, J.; Liu, S.; Zhao, H.; Tang, Z. Ultrathin Metal–Organic Framework Nanosheets for Electrocatalytic Oxygen Evolution. Nat. Energy 2016, 1, 16184.


23


Page 24 of 32

(7) Wu, L.-K.; Wu, W.-Y.; Xia, J.; Cao, H.-Z.; Hou, G.-Y.; Tang, Y.-P.; Zheng, G.-Q. Nanostructured NiCo@NiCoOx Core-Shell Layer as Efficient and Robust Electrocatalyst for Oxygen Evolution Reaction. Electrochim. Acta 2017, 254, 337-347. (8)

Wang, H. F.; Tang, C.; Wang, B.; Li, B. Q.; Zhang, Q. Bifunctional Transition Metal

Hydroxysulfides: Room-Temperature Sulfurization and Their Applications in Zn-Air Batteries. Adv. Mater. 2017, 29, 1702327. (9)

Gong, M.; Dai, H. A Mini Review of NiFe-based Materials as Highly Active Oxygen

Evolution Reaction Electrocatalysts. Nano Res. 2014, 8, 23-39. (10) Hong, W. T.; Risch, M.; Stoerzinger, K. A.; Grimaud, A.; Suntivich, J.; Shao-Horn, Y. Toward the Rational Design of Non-Precious Transition Metal Oxides for Oxygen Electrocatalysis. Energy Environ. Sci. 2015, 8, 1404-1427. (11) Hunter, B. M.; Gray, H. B.; Muller, A. M. Earth-Abundant Heterogeneous Water Oxidation Catalysts. Chem. Rev. 2016, 116, 14120-14136. (12) Balogun, M.-S.; Huang, Y.; Qiu, W.; Yang, H.; Ji, H.; Tong, Y. Updates on the Development of Nanostructured Transition Metal Nitrides for Electrochemical Energy Storage and Water Splitting. Mater. Today 2017, 20, 425-451. (13) Han, L.; Dong, S.; Wang, E. Transition-Metal (Co, Ni, and Fe)-Based Electrocatalysts for the Water Oxidation Reaction. Adv. Mater. 2016, 28, 9266-9291. (14) Zhang, B.; Zheng, X.; Voznyy, O.; Comin, R.; Bajdich, M.; García-Melchor, M.; Han, L.; Xu, J.; Liu, M.; Zheng, L.; de Arquer, F. P. G.; Dinh, C. T.; Fan, F.; Yuan, M.; Yassitepe, E.; Chen, N.; Regier, T.; Liu, P.; Li, Y.; Luna, P. D.; Janmohamed, A.; Xin, H. L.; Yang, H.;


24

Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Vojvodic A.; Sargent, E. H. Homogeneously Dispersed, Multimetal Oxygen-Evolving Catalysts. Science 2016, 352, 333-337. (15) Chang, C.; Zhang, L.; Hsu, C.-W.; Chuah, X.-F.; Lu, S.-Y. Mixed NiO/NiCo2O4 Nanocrystals Grown from the Skeleton of a 3D Porous Nickel Network as Efficient Electrocatalysts for Oxygen Evolution Reactions. ACS Appl. Mater. Interfaces 2018, 10, 417426. (16) Wu, L,-K.; Wu, W.-Y.; Xia, J.; Cao, H.-Z.; Hou, G.-Y.; Tang Y.-P.; Zheng, G.-Q. A Nanostructured Nickel–Cobalt Alloy with an Oxide Layer for an Efficient Oxygen Evolution Reaction. J. Mater. Chem. A 2017, 5, 10669-10677. (17) Xiao, K.; Zhou, L.; Shao, M.; Wei, M. Fabrication of (Ni,Co)0.85Se Nanosheet Arrays Derived from Layered Double Hydroxides toward Largely Enhanced Overall Water Splitting. J. Mater. Chem. A 2018, 6, 7585-7591. (18) Guan, B. Y.; Yu, L.; Lou, X. W. General Synthesis of Multishell Mixed-Metal Oxyphosphide Particles with Enhanced Electrocatalytic Activity in the Oxygen Evolution Reaction. Angew. Chem. Int. Ed. 2017, 56, 2386-2389. (19) Xu, L.; Zhang, F.-T.; Chen, J.-H.; Fu, X.-Z.; Sun, R.; Wong, C.-P. Amorphous NiFe Nanotube Arrays Bifunctional Electrocatalysts for Efficient Electrochemical Overall Water Splitting, ACS Appl. Energy Mater. 2018, 1, 1210-1217. (20) Long, X.; Wang, Z.; Xiao, S.; An, Y.; Yang, S. Transition Metal Based Layered Double Hydroxides Tailored for Energy Conversion and Storage. Mater. Today 2016, 19, 213-226.


25


Page 26 of 32

(21) Shao, M.; Zhang, R.; Li, Z.; Wei, M.; Evans, D. G.; Duan, X. Layered Double Hydroxides toward Electrochemical Energy Storage and Conversion: Design, Synthesis and Applications. Chem. Commun. 2015, 51, 15880-15893. (22) Yu, J.; Wang, Q.; O'Hare, D.; Sun, L. Preparation of Two Dimensional Layered Double Hydroxide Nanosheets and Their Applications. Chem. Soc. Rev. 2017, 46, 5950-5974. (23) Anantharaj, S.; Karthick, K.; Kundu, S. Evolution of Layered Double Hydroxides (LDH) as High Performance Water Oxidation Electrocatalysts: A Review with Insights on Structure, Activity and Mechanism. Mater. Today Energy 2017, 6, 1-26. (24) Thenuwara, A. C.; Attanayake, N. H.; Yu, J.; Perdew, J. P.; Elzinga, E. J.; Yan, Q.; Strongin, D. R. Cobalt Intercalated Layered NiFe Double Hydroxides for the Oxygen Evolution Reaction. J. Phys. Chem. B 2018, 122, 847-854. (25) Wang, Y.; Zhang, Y.; Liu, Z.; Xie, C.; Feng, S.; Liu, D.; Shao, M.; Wang, S. Layered Double Hydroxide Nanosheets with Multiple Vacancies Obtained by Dry Exfoliation as Highly Efficient Oxygen Evolution Electrocatalysts. Angew. Chem. Int. Ed. 2017, 56, 5867-5871. (26) Wang, Y.; Xie, C.; Zhang, Z.; Liu, D.; Chen, R.; Wang, S. In Situ Exfoliated, N-Doped, and Edge-Rich Ultrathin Layered Double Hydroxides Nanosheets for Oxygen Evolution Reaction. Adv. Funct. Mater. 2017, 1703363. (27) Liu, R.; Wang, Y.; Liu, D.; Zou, Y.; Wang, S. Water-Plasma-Enabled Exfoliation of Ultrathin Layered Double Hydroxide Nanosheets with Multivacancies for Water Oxidation. Adv. Mater. 2017, 29, 1701546.


26

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Yu, L; Yang, J. F.; Guan, B. Y.; Lu, Y.; Lou, X. W. Hierarchical Hollow Nanoprisms Based on Ultrathin Ni-Fe Layered Double Hydroxide Nanosheets with Enhanced Electrocatalytic Activity towards Oxygen Evolution. Angew. Chem. Int. Ed. 2018, 57,172-176. (29) Bi, Y.; Cai, Z.; Zhou, D.; Tian, Y.; Zhang, Q.; Zhang, Q.; Kuang, Y.; Li, Y.; Sun, X.; Duan, X. Understanding the Incorporating Effect of Co2+/Co3+ in NiFe-Layered Double Hydroxide for Electrocatalytic Oxygen Evolution Reaction. J. Catal. 2018, 35, 100-107. (30) Xiong, X.; Cai, Z.; Zhou, D.; Zhang, G.; Zhang, Q.; Jia, Y.; Duan, X.; Xie, Q.; Lai, S.; Xie, T.; Li, Y.; Sun, X.; Duan, X. A Highly-Efficient Oxygen Evolution Electrode Based on Defective Nickel-Iron Layered Double Hydroxide. Sci. China Mater. 2018, 61, 939-947. (31) Liu, W.; Bao, J.; Guan, M.; Zhao, Y.; Lian, J.; Qiu, J.; Xu, L.; Huang, Y.; Qian, J.; Li, H. Nickel-Cobalt-Layered Double Hydroxide Nanosheet Arrays on Ni Foam as a Bifunctional Electrocatalyst for Overall Water Splitting. Dalton Trans. 2017, 46, 8372-8376. (32) Li, J.; Xu, W.; Li, R.; Luo, J.; Zhou, D.; Li, S.; Cheng, P.; Yuan, D. A. Tremella-Like Ni76Co24 Layered Double Hydroxides Nanosheets as an Efficient Catalyst for Oxygen Evolution Reaction. J. Mater. Sci. 2016, 51, 9287-9295. (33) Yu, C.; Liu, Z.; Han, X.; Huang, H.; Zhao, C.; Yang, J.; Qiu, J. NiCo-Layered Double Hydroxides Vertically Assembled on Carbon Fiber Papers as Binder-Free High-Active Electrocatalysts for Water Oxidation. Carbon 2016, 110, 1-7. (34) Liang, H.; Meng, F.; Caban-Acevedo, M.; Li, L.; Forticaux, A.; Xiu, L.; Wang, Z.; Jin, S. Hydrothermal Continuous Flow Synthesis and Exfoliation of NiCo Layered Double Hydroxide Nanosheets for Enhanced Oxygen Evolution Catalysis. Nano Lett. 2015, 15, 1421-1427.


27


Page 28 of 32

(35) Jiang, J.; Zhang, A.; Li, L.; Ai, L. Nickel–Cobalt Layered Double Hydroxide Nanosheets as High-Performance Electrocatalyst for Oxygen Evolution Reaction. J. Power Sources 2015, 278, 445-451. (36) Song, F.; Hu, X. Exfoliation of Layered Double Hydroxides for Enhanced Oxygen Evolution Catalysis. Nat. Commun. 2014, 5, 4477. (37) Ping, J.; Wang, Y.; Lu, Q.; Chen, B.; Chen, J.; Huang, Y.; Ma, Q.; Tan, C.; Yang, J.; Cao, X.; Wang, Z.; Wu, J.; Ying, Y.; Zhang, H. Self-Assembly of Single-Layer CoAl-Layered Double Hydroxide Nanosheets on 3D Graphene Network Used as Highly Efficient Electrocatalyst for Oxygen Evolution Reaction. Adv. Mater. 2016, 28, 7640-7645. (38) Cao, Y.; Li, G.; Li, X. Graphene/Layered Double Hydroxide Nanocomposite: Properties, Synthesis, and Applications. Chem. Eng. J. 2016, 292, 207-223. (39) Chaudhari, N. K.; Jin, H.; Kim, B.; Lee, K. Nanostructured Materials on 3D Nickel Foam as Electrocatalysts for Water Splitting. Nanoscale 2017, 9, 12231-12247. (40) Li, Y.; Zhao, M.; Zhao, Y.; Song, L.; Zhang, Z. FeNi Layered Double-Hydroxide Nanosheets on a 3D Carbon Network as an Efficient Electrocatalyst for the Oxygen Evolution Reaction. Part. Part. Syst. Charact. 2016, 33, 158-166. (41) Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H. An Advanced Ni-Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation. J. Am. Chem. Soc. 2013, 135, 8452-8455.


28

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(42) Daud, M.; Kamal, M. S.; Shehzad, F.; Al-Harthi, M. A. Graphene/Layered Double Hydroxides Nanocomposites: A Review of Recent Progress in Synthesis and Applications. Carbon 2016, 104, 241-252. (43) Zhu, W.; Liu, L.; Yue, Z.; Zhang, W.; Yue, X.; Wang, J.; Yu, S.; Wang, L.; Wang, J. Au Promoted Nickel-Iron Layered Double Hydroxide Nanoarrays: A Modular Catalyst Enabling High-Performance Oxygen Evolution. ACS Appl. Mater. Interfaces 2017, 9, 19807-19814. (44) Jia, X.; Gao, S.; Liu, T.; Li, D.; Tang, P.; Feng, Y. Fabrication and Bifunctional Electrocatalytic

Performance

of

Ternary

CoNiMn

Layered

Double

Hydroxides/Polypyrrole/Reduced Graphene Oxide Composite for Oxygen Reduction and Evolution Reactions. Electrochim. Acta 2017, 245, 59-68. (45) Zhou, L.; Shao, M.; Zhang, C.; Zhao, J.; He, S.; Rao, D.; Wei, M.; Evans, D. G.; Duan, X. Hierarchical CoNi-Sulfide Nanosheet Arrays Derived from Layered Double Hydroxides toward Efficient Hydrazine Electrooxidation. Adv. Mater. 2017, 29, 1604080. (46) Peng, L.; Wang, J.; Nie, Y.; Xiong, K.; Wang, Y.; Zhang, L.; Chen, K.; Ding, W.; Li, L.; Wei, Z. Dual-Ligand Synergistic Modulation: A Satisfactory Strategy for Simultaneously Improving the Activity and Stability of Oxygen Evolution Electrocatalysts. ACS Catal. 2017, 7, 8184-8191. (47) 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.


29


Page 30 of 32

(48) Jin, S. Are Metal Chalcogenides, Nitrides, and Phosphides Oxygen Evolution Catalysts or Bifunctional Catalysts? ACS Energy Lett. 2017, 2, 1937-1938. (49) Li, B. Q.; Zhang, S. Y.; Tang, C.; Cui, X.; Zhang, Q. Anionic Regulated NiFe (Oxy)Sulfide Electrocatalysts for Water Oxidation. Small 2017, 13, 1700610. (50) Akin, I.; Aslan, E.; Patir, I. H. Enhanced Hydrogen Evolution Catalysis at the Liquid/Liquid Interface by NixSy and NixSy/Carbon Nanotube Catalysts. Eur. J. Inorg. Chem. 2017, 3961-3966. (51) Yang, Y.; Zhang, K.; Lin, H.; Li, X.; Chan, H. C.; Yang, L.; Gao, Q. MoS2−Ni3S2 Heteronanorods as Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting. ACS Catal. 2017, 7, 2357-2366. (52) Cai, X.; Shen, X.; Ma, L.; Ji, Z.; Xu, C.; Yuan, A. Solvothermal Synthesis of NiCoLayered Double Hydroxide Nanosheets Decorated on RGO Sheets for High Performance Supercapacitor. Chem. Eng. J. 2015, 268, 251-259. (53) Liang, J.; Ma, R.; Iyi, N.; Ebina, Y.; Takada, K.; Sasaki, T. Topochemical Synthesis, Anion Exchange, and Exfoliation of Co-Ni Layered Double Hydroxides: A Route to Positively Charged Co-Ni Hydroxide Nanosheets with Tunable Composition. Chem. Mater. 2010, 22, 371378. (54) Menezes, P. W.; Indra, A.; Bergmann, A.; Chernev, P.; Walter, C.; Dau, H.; Strasser, P.; Driess, M. Uncovering the Prominent Role of Metal Ions in Octahedral versus Tetrahedral Sites of Cobalt–Zinc Oxide Catalysts for Efficient Oxidation of Water. J. Mater. Chem. A 2016, 4, 10014-10022.


30

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(55) Oku M.; Hirokawa K. X-ray Photoelectron Spectroscopy of Co3O4, Fe3O4, Mn3O4 and Related Compounds. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 475-481. (56) Cai, P.; Huang, J.; Chen, J.; Wen, Z. Oxygen-Containing Amorphous Cobalt Sulfide Porous Nanocubes as High-Activity Electrocatalysts for the Oxygen Evolution Reaction in an Alkaline/Neutral Medium, Angew. Chem. Int. Ed. 2017, 56, 4858-4861. (57) Lyu, Z.; Xu, D.; Yang, L.; Che, R.; Feng, R.; Zhao, J.; Li, Y.; Wu, Q.; Wang, X.; Hu, Z. Hierarchical Carbon Nanocages Confining High-Loading Sulfur for High-Rate Lithium-Sulfur Batteries. Nano Energy 2015, 12, 657-665. (58) Xiang, K.; Xu, Z.; Qu, T.; Tian, Z.; Zhang, Y.; Wang, Y.; Xie, M.; Guo, X.; Ding, W.; Guo, X. Two Dimensional Oxygen-Vacancy-Rich Co3O4 Nanosheets with Excellent Supercapacitor Performances. Chem. Commun. 2017, 53, 12410-12413. (59) McCrory, C. C.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices. J. Am. Chem. Soc. 2015, 137, 4347-4357. (60) Nai, J.; Yin, H.; You, T.; Zheng, L.; Zhang, J.; Wang, P.; Jin, Z.; Tian, Y.; Liu, J.; Tang, Z.; Guo, L. Efficient Electrocatalytic Water Oxidation by Using Amorphous Ni-Co Double Hydroxides Nanocages. Adv. Energy Mater. 2015, 5, 1401880. (61) Zhang, Y.; Cui, B.; Zhao, C.; Lin, H.; Li, J. Co-Ni Layered Double Hydroxides for Water Oxidation in Neutral Electrolyte. Phys. Chem. Chem. Phys. 2013, 15, 7363-7369.


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Surface Sulfurization of NiCo-Layered Double Hydroxide Nanosheets

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