MoS2-Coated Ni3S2 Nanorods with Exposed {110} High-Index Facets

Apr 30, 2019 - Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People's. Republic of China, School of Chemistr...
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Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11101−11109

MoS2‑Coated Ni3S2 Nanorods with Exposed {110} High-Index Facets As Excellent CO-Tolerant Cocatalysts for Pt: Ultradurable Catalytic Activity for Methanol Oxidation Bo Tang,† Yuan Lv,† Jiannan Du,† Ying Dai,‡ Siyu Pan,† Ying Xie,*,† and Jinlong Zou*,† †

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Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China ‡ School of Civil Engineering, Heilongjiang Institute of Technology, Harbin 150050, China S Supporting Information *

ABSTRACT: Methanol oxidation reaction (MOR) efficiency is lowered by poor COads-tolerance and structural stability of Pt-based catalysts. Herein, single crystal Ni3S2 nanorods (with exposed {110} high-index facets) coated with MoS2 particles are grown on nickel foam (MoS2/Ni3S2-nrs/ NF) as a low-loading Pt support/cocatalyst (Pt, 0.5 wt %), which not only enhances anti-COads poisoning capacity but also extremely improves the structure stability of the catalyst. Pt/MoS2/ Ni3S2-nrs/NF catalyst exhibits a mass activity of 805.4 mA mgPt−1, which is 1.97 times higher than that of commercial Pt/C (10 wt %). It also shows an excellent cyclic durability with 4.6% decline (Pt/C, 40.2%) after 28 h. Electrochemcial tests and theoretical calculations (DFT) reveal that the excellent MOR activity and durability of Pt/MoS2/Ni3S2-nrs/NF are primarily attributed to the close binding effects among Pt, MoS2, and Ni3S2-nrs. Theoretical calculations show that, when Pt nanoparticles deposit on the preconstructed MoS2/Ni3S2-nrs hybrid, they preferentially attach to the single crystal Ni3S2-nrs rather than MoS2 particles. These heterostructures can offer sufficient active sites in radial direction, which energetically promote the charge transfer along axial dimension. Sufficient Mo−Sx edge (interface) sites facilitate OHads generation from H2O decomposition. Meanwhile, OHads can fast react with/eliminate CO-species on MoS2/Ni3S2-nrs attracted from Pt active-sites. Therefore, MoS2/Ni3S2-nrs/NF as a promising MOR support/cocatalyst provides unique perspectives for high-efficient utilization of Pt. KEYWORDS: Cocatalyst, CO tolerance, Density functional theory, Ni3S2 {110} high-index facets, Methanol oxidation

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INTRODUCTION

at a high potential to result in material corrosion and low durability.8

Direct methanol fuel cells (DMFCs) are generally regarded as encouraging power sources for portable devices because of their small size, lightweight, and high energy density.1,2 However, the practical application of DMFCs still has not been well achieved even if decades of researches have been conducted.3−5 This is mainly attributed to the lack of highly active and durable methanol oxidation reaction (MOR) catalysts for the anode. For DMFCs, platinum (Pt)-based catalysts generally show the high electrocatalytic MOR activity on the anode.3−5 However, conventional Pt-based catalysts (≥10 wt %) still suffered from the high loading and price, sluggish kinetics, and poor stability.6 Pt surface can easily adsorb COads and other carbonaceous species formed in MOR, resulting in the block of catalytic active sites to lower electrocatalytic activity.6 In MOR, the dehydrogenation of methanol first happened to form COads (eq 1), and meanwhile, the removal of COads by oxidation, which can be considered as the important step for MOR, is promoted by the presence of OH− species on (or adjacent to) Pt active sites (eqs 2 and 3).7 Although the OHads is benefit for removal of COads adjacent to and/or on Pt sites, the excessive OHads will oxidize Pt to PtO © 2019 American Chemical Society

CH3OH → COads + 4H+ + 4e−

(1)

COads + 2OH− → CO2 + H 2O + 2e−

(2)

COads + 2OHads → CO2 + H 2O

(3)

Under alkaline conditions, the free OH− may eliminate COads from the catalyst surface by the Eley−Rideal (E-R) mechanism (eq 2).7 However, Lu et al. report that the COads-removal reaction is primarily performed via the Langmuir−Hinshelwood (L-H) mechanism (eq 3) rather than the E-R mechanism (eq 2).9 Moreover, it also indicates that the bifunctional catalysts with both CO and OH− adsorption sites are more desirable for MOR.9 Therefore, to construct a heterostructured catalyst, which is conducive to provide the binding affinities to OHads and O-containing intermediates (CO, etc.), is energetically favorable for promoting the MOR Received: December 29, 2018 Revised: April 9, 2019 Published: April 30, 2019 11101

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[(NH4)6Mo7O24·4H2O (AHM)], KSCN and CTAB were dissolved in 20 mL of deionized (DI) water to form a mixed solution, and a piece of dried NF was immersed in this solution and sonicated for 30 min. Then, this NF-immersed solution was poured into a 50 mL Teflon-lined stainless steel autoclave to place in an oven at 200 °C for 24 h. After the autoclave was cooled to 25 °C, the MoS2/Ni3S2-nrs composites were washed with DI water and absolute ethanol for several times and then dried in an oven at 60 °C for 12 h. The Ni3S2/ NF was prepared using the similar procedures without AHM addition. The schematic diagram of the preparation of the Pt/MoS2/Ni3S2-nrs/ NF is shown in Scheme 1a.

activity. In summary, it can be concluded from the above analyses that at present, the major challenges of MOR anode catalyst are to reduce the Pt loading without losing its activity and durability, and to improve the antipoisoning (COads) property in catalytic process. Recently, several transition-metal semiconductors (such as MoS2, Ni3S2, CoP,10 Fe2P,11 Fe2N,12 VN,13,14 etc.) are widely used in fuel cells, lithium-ion batteries (LIBs), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR) and electrochemical capacitors, because of their unique physical-chemical properties.15−19 Especially, semiconducting MoS2 has been considered as a up-and-coming support due to its low cost, environmental friendliness, and high stability.15−19 In our previous study, the MoS2 combined with nitrogen doped carbon (CNX) has been used as a cocatalyst/support for MOR, which can extremely improve the catalytic performance.15 In addition, several studies have also demonstrated that MoS2 can be used as an excellent Pt cocatalyst for MOR.15,20−23 Ni3S2 as an excellent metallic conductor of layer transition metal semiconductors (LTMDs) with outstanding chemisorption capacity for OHads and oxygencontaining intermediates has widely been used for HER, OER, LIBs, supercapacitor, etc.17,24,25 Therefore, it is of great significance to investigate the electrochemical behaviors by using the composites of semiconducting MoS2 and metallic Ni3S2 as cocatalysts for MOR, which has not been reported previously. Moreover, it is also important to reasonably construct a special and stable structure to ensure the durability of the catalyst during the MOR process. Therefore, to facilitate the exposure of the highly active crystalline facets on the surfaces of catalysts, which have a strong binding capacity for similar materials, is the promising route to construct a stably structured catalyst. Herein, the MoS2-coated Ni3S2 nanorods (with exposed {110} high-index facets) are firmly grown on nickel foam (MoS2/Ni3S2-nrs/NF) as Pt (0.5 wt %) cocatalysts for MOR. In the heterostructures, MoS2 nanoparticles are uniformly and closely decorated on the outer surface of Ni3S2-nrs because the single crystal Ni3S2-nrs can offer a strong binding capacity for MoS2. This stable structure may be of benefit for generating abundant Mo−Sx interface sites to efficiently decompose H2O to form OHads. Meanwhile, the CO-species on Pt surfaces can be fast absorbed by the MoS2/Ni3S2-nrs heterostructure to react with these generated OHads. In addition, the heterostructure (MoS2/Ni3S2-nrs) with a rough surface can offer enough sites to anchor Pt nanoparticles, and their growth on the conducting NF provides good electrical conductivity for smooth charge transfer during MOR. As expected, the Modoped Ni3S2-nrs heterostructures can be well used as the cocatalysts for Pt nanoparticles, which may exhibit higher mass activity, CO tolerance, and durability for MOR than those of commercial Pt/C catalyst (10 wt %) in alkaline electrolyte (1 M KOH + 1 M CH3OH). Therefore, this study provides a promising route to design reasonable cocatalyst for Pt to improve the MOR catalytic activity.

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Scheme 1. Schematic Diagram of the Preparation of Pt/ MoS2/Ni3S2-nrs Nanostructure on NFs (a); Schematic Illustration of Methanol Oxidation by Pt/MoS2/Ni3S2-nrs/ NF Catalyst (b)

Synthesis of Pt/MoS2/Ni3S2-nrs/NF. The Pt/MoS2/Ni3S2-nrs/ NF catalysts were obtained by using microwave-assisted polyol method. Then, 5 mL of chloroplatinic acid ethylene glycol solution (H2PtCl6-EG) with Pt2+ concentration of 0.2 mM was added in a 10 mL beaker, and the pH of the solution was adjusted by adding 1 M NaOH-EG solution until the pH was maintained at 12 under vigorous stirring. Subsequently, a piece of MoS2/Ni3S2-nrs/NF was immersed in the above solution and the beaker was placed in a microwave oven (2450 MHz, 800W) with consecutive heating time for 15 s for the complete reduction of the Pt compound. Before the heating, argon gas was used to eliminate oxygen from the solution for 15 min. After stillstanding overnight, Pt/MoS2/Ni3S2-nrs/NF was washed with DI water and absolute ethanol three times to eliminate residual impurities. Following, the Pt/MoS2/Ni3S2-nrs/NF heterostructures were dried in a vacuum oven at 60 °C for 8 h. Meanwhile, Pt/Ni3S2/ NF was also synthesized as the reference sample using the same method. The theoretical loading of Pt on MoS2/Ni3S2-nrs/NF or Ni3S2/NF support was 0.5 wt %, while 10 wt % Pt/C (commercial) was used as the reference sample, which was provided by Hesen Electric Co., Ltd. (Shanghai, China), and the carbon support was “VXC-72R carbon black”. The information for X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive

EXPERIMENTAL SECTION

Catalyst Preparation. The MoS2/Ni3S2-nrs heterostructures, Ni3S2 and MoS2 were prepared by a simple one-step hydrothermal reduction method according to our previous study. Briefly, commercial Ni foam (NF) (1.0 cm × 1.0 cm, ∼ 42 mg) was sequentially sonicated in water, ethanol, 0.5 M HCl solution to remove the surface oxides. Heptamolybdate tetrahydrate 11102

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Figure 1. Field emission SEM images of MoS2/Ni3S2-nrs/NF (a−c); Mapping images of Pt (d), Ni (e), Mo (f), O (g), and S (h) elements and the EDX spectrum (i) of the Pt/MoS2/Ni3S2-nrs/NF heterostructures.

Figure 2. Survey spectrum (a) of the Pt/MoS2/Ni3S2-nrs/NF and Pt/Ni3S2/NF; panels b−e are the Pt 4f, the Mo 3d, the Ni 2p, and the S 2p spectrum of Pt/MoS2/Ni3S2-nrs/NF, respectively; and the XRD patterns (f) of Pt/MoS2/Ni3S2-nrs/NF and Pt/Ni3S2/NF. X-ray spectrometry (EDS), field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), cyclic

voltammetric (CV), amperometric (i-t), and electrochemical impedance spectroscopy (EIS) measurements and density functional 11103

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Figure 3. TEM image (a) of Pt/MoS2/Ni3S2-nrs/NF and HRTEM images (b and c) of Ni3S2-nrs; the corresponding SAED pattern (d) of Pt/ MoS2/Ni3S2-nrs/NF; HRTEM images (e and f) of a single hybrid nanostructure. theory (DFT) model and parameters were introduced in the Supporting Information.

Ni3S2-nrs/NF, Pt/Ni3S2/NF. As shown in Figure 2a, the characteristic peaks of Pt 4f, Mo 3d, Ni 2p, and S 2p are detected in the XPS spectra of Pt/MoS2/Ni3S2-nrs/NF and Pt/Ni3S2/NF. The Pt peaks (Figures 2b and S4a) can be deconvoluted into two pairs of doublets, which represent the metallic Pt (around 71.2 and 74.5 eV) and Pt (II) in PtO or Pt(OH)2 species (around 72.4 and 75.7 eV).29,30 Note that the Pt 4f peaks of Pt/MoS2/Ni3S2-nrs/NF are located at higher binding energies than those of Pt/Ni3S2/NF in Figure S4f. The positive shift in the Pt 4f binding energies of Pt/MoS2/Ni3S2nrs/NF indicates that the electronic structure of depositing Pt is influenced by alloying with MoS2 and/or Ni3S2.9,15,31 As shown in Figure 2c, the presence of Mo 3d3/2 (around 231.7 eV) and Mo 3d5/2 (around 235.1 eV) peaks indicates the successful doping of Mo4+ in Pt/MoS2/Ni3S2-nrs/NF.32 As shown in Figures 2d and S4b, the peaks with binding energies at around 873.0 (satellite peak at around 861.3 eV) and 855.5 eV (satellite peak at around 879.6 eV) are assigned to Ni 2p3/2 and Ni 2p1/2, respectively, corresponding to the shakeup-type peaks of Ni.33 Note that the Ni 2p3/2 signal for Pt/MoS2/ Ni3S2-nrs/NF heterostructures exhibits a positive shift of approximately 0.3 eV as compared with that of Pt/Ni3S2/NF (Figure S 4d). The high resolution spectra of S 2p can be decomposed into two main peaks, which correspond to S 2p3/2 (around l61.9 eV) and S 2p1/2 (around 163.5 eV) ligands, respectively (Figures 2e and S4c).34,35 A broad peak at around 167.5 eV is ascribed to the characteristic peak of the S−O vibration.33 These results mean that the strong electronic interactions between Ni3S2 and MoS2 should facilitate the formation of desirable coupling interfaces in Pt/MoS2/Ni3S2nrs/NF. In the XRD patterns of Pt/Ni3S2/NF and Pt/MoS2/Ni3S2nrs/NF (Figure 2f), the peaks at 44.8°, 52.3°, and 76.8° are the characteristic peaks of metallic Ni (NF). The diffraction peaks located at 22.1°, 31.5°, 38.2°, 50.1°, 55.5°, 73.4°, and 78.2° are ascribed to the (101), (110), (003), (113), (122), (214), and (401) planes of Ni3S2 (JCPDS no. 44-1418), respectively, and

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RESULTS AND DISCUSSION The crystalline phases of the pure NF, Ni3S2/NF, and MoS2/ Ni3S2-nrs/NF heterostructures are investigated through XRD (Figure S1a) and Raman spectroscopy (Figure S1b) analyses. Commercial NF is used as the substrate, which shows an intricate reticular structure and very smooth surface on the limb (SEM images, Figure S2a and 2b). By contrast (Figure S2c-2d), the surface of NF becomes rougher by the formation of Ni3S2 originated from the reaction of NF with KSCN, as confirmed in Figure S2. After the addition of AHM, some clumps are formed on the surface of NF (Figure S2e-S 2f). As shown in Figure S3a-3b, some irregular triangle bulges with a rough surface formed on the surface of NF (without Mo source) can be offered as base points to grow Ni3S2-nrs. When an appropriate amount of AHM is introduced, the heterostructure of Ni3S2-nrs covered by MoS2 particles is obtained. The obtained MoS2/Ni3S2-nrs with a diameter of 100−250 nm and a length of 1−6 μm are grown uniformly and nearly vertically on NF (Figure 1a-1c). The EDX-mapping image further shows the uniform coating of MoS2 nanoparticles on Ni3S2-nrs (Figure 1f). The MoS2 nanoparticles shells are tightly connected with the Ni3S2-nrs cores by robust physicochemical adhesion. The heterostructures can offer sufficient active sites in the radial direction, which should contribute to the smooth charge transfer along the axial direction in the interconnecting structure.26−28 The EDS is used to analysis the elements composition of Pt/Ni3S2/NF and Pt/MoS2/Ni3S2-nrs/NF. The content percentages of Pt, Ni (Mo), O, and S in the two catalysts are shown in Figures 1i and S3i. As shown in Figures 1d−h and S3e-3h, the elemental mappings show the uniform distribution of Pt, Ni (Mo), O, and S in the structure of Pt/ Ni3S2/NF and Pt/MoS2/Ni3S2-nrs/NF, indicating the even contact of Pt with Ni3S2 or MoS2/Ni3S2-nrs. XPS measurements are carried out to investigate the electronic properties and surface components of Pt/MoS2/ 11104

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Figure 4. CV curves (a) of Pt/MoS 2/Ni3S2-nrs/NF, Pt/Ni3S2/NF and 10 wt % Pt/C catalysts in 1.0 M KOH solution, CV curves (b) and COstripping curves (c) of the electrodes modified with different catalysts in N2-saturated 1.0 M KOH M solution, and the forward peak current densities of the Pt/MoS2/Ni3S2-nrs/NF (d), Pt/Ni3S2/NF (e), and commercial Pt/C (f) catalysts as a function of the number of potential scanning cycles, revealing the best cycling stability of Pt/MoS2/Ni3S2-nrs/NF.

shows the highest ESAPt, indicating that more Pt surface sites are available on the special structure of MoS2/Ni3S2-nrs/NF. Generally, the catalyst with higher ESApt can offer more active sites for MOR, suggesting that the MoS2/Ni3S2-nrs/NF can be considered as a promising cocatalyst for Pt. The electrocatalytic activity (in methanolic alkaline electrolyte) of Pt/ MoS2/Ni3S2-nrs/NF, Pt/Ni3S2/NF and commercial Pt/C is given in Figure 4b. Before each test, the electrode covered with catalyst is scanned for several cycles until the stable CV curves are gained. Typically, the methanol oxidation current peak appears between +0.14 and +0.23 V in the positive scan direction, while the oxidation peak between −0.46 and −0.33 V in the negative scan direction corresponds to the oxidization of the residual intermediates including COads and CHOads. In general, current density of the forward peak is an indicator of the electrocatalytic performance of the catalysts.36 As shown in Figure 4b, the mass current densities of Pt/MoS2/Ni3S2-nrs/ NF and Pt/Ni3S2/NF are 805.4 and 465.1 mA mgpt−1, respectively. Both Pt/MoS2/Ni3S2-nrs/NF and Pt/Ni3S2/NF catalysts display higher MOR activities than that of commercial Pt/C (405.4 mA mgpt−1). CO-stripping (CO tolerance) tests are further conducted in 1.0 M KOH (Figure 4c). Pt/MoS2/Ni3S2-nrs/NF has the lowest onset potential of −0.184 V, which is more negative than those of Pt/Ni3S2/NF (−0.095 V, 87 mV shift) and Pt/C (−0.011 V, 171 mV shift), indicating that the CO oxidation reactions on Pt/MoS2/Ni3S2-nrs/NF are more efficient than those of others, which correspondingly contributes to the higher MOR performance.37 The excellent anti-COads poisoning function of Pt/MoS2/Ni3S2-nrs/NF should be mainly related to the synergies between MoS2 and Ni3S2. Further-

the intensity of Pt/MoS2/Ni3S2-nrs/NF is lower than that of Pt/Ni3S2/NF, which is caused by the intensive interactions between MoS2 and Ni3S2, and the epitaxial grown of MoS2 on Ni3S2-nrs due to their similar electronic structures. Moreover, the peaks located at 40.4°, 46.9°, and 68.8° can be referred to the (111), (200), and (220) planes of the face-centered cubic (fcc) structured Pt (JCPDS, no. 65-2868), respectively. However, the diffraction peaks of MoS2 are invisible in the XRD patterns, which may be covered by these stronger peaks of Ni and/or Ni3S2. Figure 3a shows the typical TEM image of an individual Ni3S2-nrs coated by MoS2 particles. As shown in the HRTEM images (Figure 3b,c), the lattice spacing of 0.287 nm corresponds to the (110) plane of Ni3S2 (hexagonal, a = b = 0.574 nm, c = 0.714 nm). The SAED patterns (inset of Figure 3d is the schematic diagram of the relative orientation of (110), (003), and (021) facets in hexagonal Ni3S2 structure, as shown in Figure S7) further confirm the formation of the single crystalline Ni3S2-nrs. The lattice distances of 0.226 and 0.273 nm in Figure 3e,f correspond to the (111) facet of Pt and (100) facet of MoS2, respectively, confirming the good construction of Pt/MoS2/Ni3S2-nrs/NF hierarchical structure. Significantly, the Pt NPs are grown near MoS2 NPs, which may have a subtle contribution to the synergistic effects between them. In addition, the TEM images of Ni3S2/NF and MoS2/ Ni3S2/NF are provided in Figure S6. CV tests for Pt/MoS2/Ni3S2-nrs/NF, Pt/Ni3S2/NF, and Pt/ C are performed from −0.8 to +0.4 V in a N2-saturated 1.0 M KOH solution (Figure 4a). The ESAPt’s of Pt/MoS2/Ni3S2nrs/NF, Pt/Ni3S2/NF and Pt/C are 76.7, 43.5, and 30.3 m2 gpt−1, respectively. Among them, the Pt/MoS2/Ni3S2-nrs/NF 11105

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Figure 5. EIS spectra of 10 wt % commercial Pt/C (a), Pt/Ni3S2/NF (b), and Pt/MoS2/Ni3S2-nrs/NF (c) electrode in 1.0 M CH3OH + 1.0 M KOH solution at different potentials.

appeared in the second quadrant at potentials of −0.20 V is due to the adsorption of reaction intermediates, indicating a heavy poisoning of the Pt surface.42−45 The negative impedance (in the second quadrant) should be ascribed to the desirable elimination of CO intermediates because the functional OHads is formed in this potential range.46 A further increase of potential (−0.2 to +0.1 V) actually lowers the MOR rate, because the strong OH− adsorption may hinder the Pt-catalyzed methanol oxidation at high potentials, consistent with the peak potential of CV tests (around 0.1 V, Figure 4b).46 When the potential is above 0.3 V, the diameter of the semicircles decreases because of the methanol oxidation at high potentials, consistent with the current increase between 0.2 and 0.4 V in CV curves (Figure 4b). It shows that the impedance profiles of Pt/Ni3S2/NF and Pt/MoS2/Ni3S2-nrs/ NF electrodes are different from that of Pt/C (Figure 5b,c). The EIS curves for both Pt/Ni3S2/NF and Pt/MoS2/Ni3S2nrs/NF are placed in the first quadrant, and the arc diameters decrease as the potential increases. Note that the diameters of the impedance arcs of the Pt/Ni3S2/NF and Pt/MoS2/Ni3S2nrs/NF are smaller than those of Pt/C. Among them, the Pt/ MoS2/Ni3S2-nrs/NF electrode shows the smallest arc diameter, suggesting the faster charge-transfer kinetics. The efficient charge-transfer on Pt/MoS2/Ni3S2-nrs/NF electrode contributes to the much better MOR activity than that of Pt/C. The EIS results are well consistent with the CV results. To gain insight into the possible origination of excellent cycling stability and catalytic activity of Pt/MoS2/Ni3S2-nrs/ NF, DFT calculations are performed. As the interfacial interactions among Pt, MoS2 and Ni3S2 have significant impact on the stability of the catalyst, the bonding energies (BE) are calculated by the following (eq 4):

more, the durabilities of Pt/MoS2/Ni3S2-nrs/NF, Pt/Ni3S2/ NF, and Pt/C are investigated in the continuous CV tests with 1000 potential cycles (Figure S5a-5c). The initial forward peak current density of Pt/MoS2/Ni3S2-nrs/NF and Pt/Ni3S2/NF catalyst decreases approximately 4.6 and 12.4% after 1000 cycles, respectively, while Pt/C catalyst declines approximately 40.2%, which fully confirms the excellent durability of Pt/ MoS2/Ni3S2-nrs/NF. Amperometric i-t curve is also performed to study the catalytic durability of these catalysts. As shown in Figure 4d-4f, the normalized current declines (ratio of the current at 100000 s to initial current (I100000s/Iinitial)) for Pt/ MoS2/Ni3S2-nrs/NF, Pt/Ni3S2/NF, and Pt/C are 1.0, 0.87, and 0.62, respectively. A performance comparison of Pt/ MoS2/Ni3S2-nrs/NF and other recently reported catalysts is shown in Table S1. The promising durability should be related to the uniform dispersion of Pt nanoparticles on MoS2/Ni3S2-nrs/NF and the befitting interactions among Pt, MoS2, and Ni3S2-nrs that avoid the fast inactivation of Pt active sites during MOR. Moreover, the hierarchical nanostructures of MoS2/Ni3S2-nrs/NF can hinder the aggregation of Pt nanoparticles and increase the mass/charge-transfer ability. Significantly, the intralayer S and Mo atoms are covalently bonded and the quantity of Mo−Sx facets is quite large, which are combined by the weak van der Waals forces. The dangling bonds are formed at the edges of the layer, which facilitate the charge transfer through the nonvertical transitions.38 The coordinated Mo−Sx sites along the edges of MoS2 own high dissociation capability for H2O, which is helpful to enhance catalytic performance and CO tolerance of Pt.39,40 Furthermore, due to S sulfidation and Mo modification, the enriched empty d-orbitals on Ni (Ni3S2-nrs/ NF) should strengthen the binding with CO-species to improve the MOR activity.41 Thus, the synergistic effects among MoS2, Ni3S2-nrs, and NF can endow the MoS2/Ni3S2nrs/NF support with high cocatalytic activity and CO cooxidation capacity. These synergistic effects are vividly illustrated in Scheme 1b and the corresponding discussion can be found in the Supporting Information. The Nyquist plots (EIS tests) for commercial Pt/C, Pt/ Ni3S2/NF, and Pt/MoS2/Ni3S2-nrs/NF are shown in Figure 5a−c. In agreement with the CV results, the reaction does not occurs below −0.6 V, and so only straight lines are detected in the EIS spectra below −0.6 V. As the electrode potential is higher than −0.6 V, the diameter of the arcs gradually decreases, indicating the faster electron-transfer kinetics at higher potentials. As shown in Figure 5a, the impedance arc

BE = EA/B − EA − E B

(4)

where EA/B is the total energy of the composite, and EA and EB are the ones of relevant components (Pt, MoS2, and Ni3S2). The computational models are shown in Figure 6a−c. The calculations show that the BE between Pt and Ni3S2 is −2.343 J m−2, while the value between MoS2 and Ni3S2 is −2.613 J m−2. Therefore, strong chemical bonds should be formed between Pt (MoS2) and Ni3S2. Such strong interfacial interactions are very helpful for anchoring Pt (MoS2) particles on the surface of Ni3S2-nrs to obtain an excellent structural stability of the catalyst. Thus, the particles (Pt and MoS2) are firmly fixed along the surface of Ni3S2-nrs to avoid migration, which also suppresses the nanoparticles aggregation to obtain a 11106

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favorable for triggering the synergistic effects among Pt, MoS2 and Ni3S2. To further reveal the bonding characteristics of the interfaces, densities of states (DOSs) are then calculated (Figure 7a-7c). Figure 7a shows that after Pt attaches to the MoS2 surface, the total densities of states (TDOSs) of S move to a lower energy position with slight changes in its shape at [−4.0 eV, − 2.0 eV], whereas the TDOSs of Pt are also changed slightly. Therefore, a weak interaction between the two components is confirmed, consistent with the calculated BE above. For the Pt/Ni3S2 system in Figure 7b, the covalent interactions between Pt and Ni3S2 surface not only change the distributions of the Pt5d states obviously, but also lead to the leftward shift of the S3p. Similar bonding characteristics are also observed at the interface of MoS2/Ni3S2 (Figure 7c). Therefore, the strong interactions between Pt5d and S3p states in the Pt/Ni3S2 system and the one between Ni3d and S3p states in MoS2/Ni3S2 are both identified. These interactions are important for anchoring the Pt and MoS2 particles on the Ni3S2-nrs surface, which are responsible for obtaining the excellent structural stability of the catalyst to contribute to the excellent electrocatalysis cycling durability. To reveal the performance of CO tolerance of the catalysts, the adsorption energy (AE) of CO molecule on the catalyst surface is calculated by the following eq 5: Figure 6. Calculation models of MoS2/Ni3S2 (a), Pt/Ni3S2 (b), and Pt/MoS2 (c) systems; the most stable adsorbed schematic models of CO adsorbed on Pt (111) (d), Pt/MoS2 (111) (e), and Pt/Ni3S2 (111) (f) slabs yellow, blue, purple, and green spheres represent S, Mo, Ni, and Pt atoms, respectively.

AE = ECO − A/B − ECO − EA/B

(5)

The corresponding CO absorption models are depicted in Figure 6d−f. The calculations suggest that the AE of CO on Pt surface is approximately −3.12 eV, indicating that pure Pt catalyst easily suffers from CO poisoning. The same calculation result (−3.12 eV) has been also reported by Sharma et al., demonstrating that the calculation results and models in this study are reasonable.47 When Pt particles are combined with Ni3S2 (or MoS2), the AE is 0.96 eV (or −2.86 eV), which should energetically suppress the CO−Pt bonding during MOR process. Therefore, the resistance of the Pt/MoS2/ Ni3S2-nrs/NF catalyst against CO poisoning is significantly enhanced to improve MOR activity, well consistent with the experimental observations.

good dispersibility, consistent with the HRTEM results. Because of the strong interfacial interactions, the active components will retain their shape and size during the repeated electrochemical cycles, which contributes to the excellent cycling durability of catalyst. Further calculations show that the BE between Pt and MoS2 is −0.274 J m−2, which is lower than that of Pt/Ni3S2 (−2.343 J m−2). It implies that the interactions between Pt and MoS2 will make the growth of Pt close to the MoS2 particles. This growth pattern should be

Figure 7. Densities of states of different contacting atoms for (a) Pt/MoS2, (b) Pt/Ni3S2, and (c) MoS2/Ni3S2 systems. 11107

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passivated Molybdenum Phosphide/carbon Supported Pt Catalysts. J. Mater. Chem. A 2016, 4, 7674−7682. (4) Zhang, J.; Sui, X.; Huang, G.; Gu, D.; Wang, Z. Hierarchical Carbon Coated Molybdenum Dioxide Nanotubes as a Highly Active and Durable Electrocatalytic Support for Methanol Oxidation. J. Mater. Chem. A 2017, 55 (8), 4067−4074. (5) Huang, H.; Zhu, J.; Li, D.; Shen, C.; Li, M.; Zhang, X.; Jiang, Q.; Zhang, J.; Wu, Y. Pt Nanoparticles Grown on 3D RuO2-modified Graphene Architectures for Highly Efficient Methanol Oxidation. J. Mater. Chem. A 2017, 5 (9), 4560−4567. (6) Shi, X.; Wen, Y.; Guo, X.; Pan, Y.; Ji, Y.; Ying, Y.; Yang, H. Dentritic CuPtPd Catalyst for Enhanced Electrochemical Oxidation of Methanol. ACS Appl. Mater. Interfaces 2017, 9 (31), 25995−26000. (7) Spendelow, J. S.; Lu, G. Q.; Kenis, P. J. A.; Wieckowski, A. Electrooxidation of Adsorbed CO on Pt(111) and Pt(111)/Ru in Alkaline Media and Comparison with Results from Acidic Media. J. Electroanal. Chem. 2004, 568, 215−224. (8) Wang, Z.-B.; Yin, G.-P.; Shao, Y.-Y.; Yang, B.-Q.; Shi, P.-F.; Feng, P.-X. Electrochemical Impedance Studies on Carbon Supported PtRuNi and PtRu Anode Catalysts in Acid Medium for Direct Methanol Fuel Cell. J. Power Sources 2007, 165 (1), 9−15. (9) Lu, S.-Q.; Li, H.-M.; Sun, J.-Y.; Zhuang, Z.-B. Promoting the Methanol Oxidation Catalytic Activity by Introducing Surface Nickel on Platinum Nanoparticles. Nano Res. 2018, 11 (4), 2058−2068. (10) Chang, J.-F; Feng, L.-G.; Jiang, K.; Xue, H.-G.; Cai, W.-B.; Liu, C.-P.; Xing, W. Pt−CoP/C as an Alternative PtRu/C Catalyst for Direct Methanol Fuel Cells. J. Mater. Chem. A 2016, 4, 18607−18613. (11) Wang, F.-L.; Xue, H.-G.; Tian, Z.-Q.; Xing, W.; Feng, L.-G. Fe2P as a Novel Efficient Catalyst Promoter in Pd/C System for Formic Acid Electro-oxidation in Fuel Cells Reaction. J. Power Sources 2018, 375, 37−42. (12) Yu, P.; Wang, L.; Xie, Y.; Tian, C.-G.; Sun, F.-F.; Ma, J.-Y.; Tong, M.-M.; Zhou, W.; Li, J.-H.; Fu, H.-G. High-efficient, Stable Electrocatalytic Hydrogen Evolution in Acid Media by Amorphous FexP Coating Fe2N Supported on Reduced Graphene Oxide. Small 2018, 14, 1801717. (13) Wang, Y.; Wang, L.; Tong, M.-M.; Zhao, X.-J.; Gao, Y.-T.; Fu, H.-G. Co-VN Encapsulated in Bamboo-like N-doped Carbon Nanotubes for Ultrahigh-stability of Oxygen Reduction Reaction. Nanoscale 2018, 10 (9), 4311−4319. (14) Liu, X.; Wang, L.; Yu, P.; Tian, C.-G.; Sun, F.-F.; Ma, J.-Y.; Li, W.; Fu, H.-G. A Stable Bifunctional Catalyst for Rechargeable ZincAir Batteries: Iron-Cobalt Nanoparticles Embedded in a NitrogenDoped 3D Carbon Matrix. Angew. Chem., Int. Ed. 2018, 57, 16166− 16170. (15) Tang, B.; Lin, Y.-H.; Xing, Z.-P.; Duan, Y.-Q.; Pan, S.-Y.; Dai, Y.; Yu, J.; Zou, J.-L. Porous Coral Reefs-like MoS2/nitrogen-doped Bio-carbon as an Excellent Pt Support/co-catalyst with Promising Catalytic Activity and CO-tolerance for Methanol Oxidation Reaction. Electrochim. Acta 2017, 246, 517−527. (16) Zhang, J.; Wang, T.; Pohl, D.; Rellinghaus, B.; Dong, R.; Liu, S.; Zhuang, X.; Feng, X. Interface Engineering of MoS 2 /Ni 3 S 2 Heterostructures for Highly Enhanced Electrochemical OverallWater-Splitting Activity. Angew. Chem., Int. Ed. 2016, 55 (23), 6702−6707. (17) Wei, W.; Mi, L.; Gao, Y.; Zheng, Z.; Chen, W.-H.; Guan, X.-X. Partial Ion-Exchange of Nickel-Sulfide-Derived Electrodes for High Performance Supercapacitors. Chem. Mater. 2014, 26 (11), 3418− 3426. (18) Wang, J.; Chao, D.-L.; Liu, J.-L.; Li, L.-L.; Lai, L.-F.; Lin, J.-Y.; Shen, Z.-X. Ni3S2@MoS2, Core/shell Nanorod Arrays on Ni Foam for High-performance Electrochemical Energy Storage. Nano Energy 2014, 7 (7), 151−106. (19) Lv, J.-J.; Zhao, J.; Fang, H.; Jiang, L.-P.; Li, L.-L.; Ma, J.; Zhu, J.J. Incorporating Nitrogen-Doped Graphene Quantum Dots and Ni3S2 Nanosheets: A Synergistic Electrocatalyst with Highly Enhanced Activity for Overall Water Splitting. Small 2017, 13 (24), 1700264. (20) Patil, S. H.; Anothumakkool, B.; Sathaye, S. D.; Patil, K. R. Architecturally Designed Pt−MoS2 and Pt−graphene Composites for

CONCLUSIONS In summary, MoS2/Ni3S2-nrs/NF as a promising Pt support/ cocatalyst for MOR is prepared. Nickel foam (NF) is used as the substrate to guide the growth of hierarchial MoS2/Ni3S2nrs heterostructures, which facilitates the uniform deposition of Pt nanoparticles. MoS2/Ni3S2-nrs/NF can offer sufficient binding sites for depositing Pt with extremely low-loading of 0.5 wt %, which energetically obtains the high activity and durability of Pt/MoS2/Ni3S2-nrs/NF for MOR. Especially, the ESApt and mass activity of Pt/MoS2/Ni3S2-nrs/NF are as high as 76.7 m2 gpt−1 and 805.4 mA mgpt−1, respectively, which are much better than those of Pt/C (10 wt %). According to the DFT calculations, the promising MOR activity of Pt/MoS2/ Ni3S2-nrs/NF is mainly derived from the strong chemical bonds between Pt (MoS2) and Ni3S2. Moreover, the MoS2/ Ni3S2-nrs/NF can not only provide more Mo−Sx sites (OHads from water decomposition) and empty d-orbitals on Ni3S2-nrs/ NF (CO-species binding) to enhance CO removal via the bifunctional mechanism but also facilitates the electron transfer from Pt to MoS2/Ni3S2-nrs/NF to boost the MOR kinetics. Therefore, the MoS2/Ni3S2-nrs/NF heterostructures as an excellent MOR support/cocatalyst should be extremely meaningful for further cost reduction via a low Pt-loading.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b06855.

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Experimental details, Figures S1−S7, and Table S1 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-451-86608549. Fax: +86-451-86608549. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jinlong Zou: 0000-0003-0651-761X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support by National Natural Science Foundation of China (51578218, 21806031, and 51108162), Research and development projects of scientific and technological achievements in Heilongjiang Provincial Universities (TSTAU-R2018021), and Scientific and technological innovation talents of Harbin (2016RQQXJ119).

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REFERENCES

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