Research Article pubs.acs.org/acscatalysis
MoS2−Ni3S2 Heteronanorods as Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting Yaqing Yang,† Kai Zhang,‡ Huanlei Lin,† Xiang Li,‡ Hang Cheong Chan,† Lichun Yang,*,‡ and Qingsheng Gao*,† †
Department of Chemistry, College of Chemistry and Materials Science, Jinan University, Guangzhou 510632, P. R. China School of Materials Science and Engineering, Guangdong Provincial Key Laboratory of Advanced Energy Storage Materials, South China University of Technology, Guangzhou 510641, P. R. China
ACS Catal. 2017.7:2357-2366. Downloaded from pubs.acs.org by WESTERN SYDNEY UNIV on 01/09/19. For personal use only.
‡
S Supporting Information *
ABSTRACT: Exploring noble-metal-free electrocatalysts with high efficiency for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) holds promise for advancing the production of H2 fuel through water splitting. Herein, one-pot synthesis was introduced for MoS2− Ni3S2 heteronanorods supported by Ni foam (MoS2−Ni3S2 HNRs/NF), in which the Ni3S2 nanorods were hierarchically integrated with MoS2 nanosheets. The hierarchical MoS2− Ni3S2 heteronanorods allow not only the good exposure of highly active heterointerfaces but also the facilitated charge transport along Ni3S2 nanorods anchored on conducting nickel foam, accomplishing the promoted kinetics and activity for HER, OER, and overall water splitting. The optimal MoS2− Ni3S2 HNRs/NF presents low overpotentials (η10) of 98 and 249 mV to reach a current density of 10 mA cm−2 in 1.0 M KOH for HER and OER, respectively. Assembled as an electrolyzer for overall water splitting, such heteronanorods show a quite low cell voltage of 1.50 V at 10 mA cm−2 and remarkable stability for more than 48 h, which are among the best values of current noble-metal-free electrocatalysts. This work elucidates a rational design of heterostructures as efficient electrocatalysts, shedding some light on the development of functional materials in energy chemistry. KEYWORDS: heterointerfaces, metal sulfides, electrocatalysis, hydrogen evolution reaction, oxygen evolution reaction, overall water splitting
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INTRODUCTION To address the rapid growth of energy consumption and the associated environmental issues, renewable and clean energy sources have attracted immense research interest.1,2 Hydrogen is viewed as a promising energy carrier because of its cleanliness, high efficiency, and renewability.3 For sustainable hydrogen production, water electrolysis using renewable energy sources (e.g., sunlight and wind) is a promising method,4 which requires a thermodynamic potential of 1.23 V for the splitting into hydrogen and oxygen.5,6 However, because of the inevitable dynamic overpotentials in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER),5,7 the electrolysis efficiency is severely limited. Electrocatalysts are significant for decreasing the overpotentials. So far, platinumbased materials are the most effective HER electrocatalysts, while iridium and ruthenium oxides hold the benchmark for OER.5 Unfortunately, the application of these noble-metalbased catalysts is impeded by their high cost and low abundance.8,9 Recent advances have been made by developing the alloys, oxides, sulfides, phosphides, carbides, and nitrides of Earth-abundant transition metals.7,10−15 However, most studies © 2017 American Chemical Society
focus on how to improve the electrocatalytic activity for HER or OER but ignore the development of a bifunctional electrocatalyst with both high HER and OER activity. The good integration of HER and OER electrocatalysts into a single nanostructure has become one of the hottest issues in this field, holding the promise for efficient overall water splitting.16,17 Heterogeneous nanostructures, showing the synergistically promoted kinetics on varied active sites and electronreconfigured interfaces, emerge as electrocatalysts that are superior to their single-component counterparts for HER or OER.18−22 Compatibly integrating HER and OER activity materials in a designed manner, they can serve as bifunctional electrocatalysts for overall water splitting. For example, nickel sulfides (e.g., Ni3S2, NiS2, NiS, etc.), being highly active for OER,23,24 unfortunately exhibit a restricted performance for overall water splitting because of unsatisfactory HER activity.24−27 Their HER and the consequent overall water Received: November 9, 2016 Revised: January 17, 2017 Published: February 16, 2017 2357
DOI: 10.1021/acscatal.6b03192 ACS Catal. 2017, 7, 2357−2366
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ACS Catalysis Scheme 1. Illustration of the Fabrication of Heterostructured MoS2−Ni3S2 HNRs/NF Compositesa
a
AHM denotes ammonium heptamolybdate tetrahydrate and NF nickel foam.
Figure 1. SEM images of (a) bare NF and (b) Ni3S2/NF. (c and e) SEM and (d and f) TEM images of the products obtained in the presence of AHM for (c and d) 8 h and (e and f) 24 h. Insets of panels d and f are the corresponding SAED patterns obtained on a single nanorod.
sional (1D) morphology exhibit highly efficient electrocatalysis, because they can provide abundant active sites in a radial direction, and facilitate charge transfer along the axial dimension in interconnecting networks.22,33−37 In this regard, constructing 1D Mo−Ni−S heterostructures is a key innovation for improved HER, OER, and overall water splitting. However, their fabrication and engineering are still prohibited by the harsh and multiple-step synthesis, which usually employs
splitting activity can be promoted by integrating with nanosized MoS2,18,28 which shows efficient HER on undercoordinated Mo−S edges.29,30 To fulfill the synergy in heterostructures, rational engineering is necessary, in which the heterointerfaces of every component should be fully manifested, and the heterostructures can guarantee the fast ion/electron transportation throughout all electrocatalysts.31,32 As highlighted in very recent reports, the heterostructures with a one-dimen2358
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Figure 2. (a) Energy dispersive spectrum and (b) corresponding elemental mapping of MoS2−Ni3S2 HNRs/NF. The inset of panel a shows the atomic (Ni, S, and Mo) contents of MoS2−Ni3S2 HNRs.
feeding ratio of 0.07 (in weight) was adopted for the fabrication. As the preparation is conducted for a shortened period of time (e.g., 8 h), coarse nanorods are produced (Figure 1c). In the corresponding transmission electron microscopy (TEM) investigation (Figure 1d), these nanorods clearly display (22̅0) and (311̅) lattice fringes of α-NiMoO4 (JCPDS Card No. 32-0692), between which the interplanar angle of 68.5° is in accordance with the theoretical value (68.8°). This α-NiMoO4 1D nanostructure should originate from the anisotropic arrangement of molybdate anions.38,39 Meanwhile, the typical lattice fringe of MoS2(002) is also detected on the surface of NiMoO4 (Figure 1d), indicating the formation of MoS2 nanolayers in these hierarchical nanorods. With a prolonged reaction time of 24 h, the MoS2−NiMoO4 mentioned above further evolves to MoS2−Ni3S2 because of sufficient reaction with TAA, as confirmed by its XRD pattern (Figure S2), and the core nanorods decorated with ultrathin nanosheets are clearly observed in SEM and TEM images (Figure 1e,f). The corresponding SAED pattern detected on a single nanorod is well indexed by Ni3S2 (JCPDS Card No. 080126), in which the interplanar angle between Ni3S2(012)̅ and Ni3S2(011) is 89.3°, close to the theoretical value of 89.2°. Obviously, the 1D nanostructure is well-preserved as NiMoO4 evolves to Ni3S2. On the Ni3S2 nanorod surface, MoS2 nanosheets displaying the typical (002) lattice (0.62 nm) are clearly observed. Interestingly, the MoS2−Ni3S2 interfaces are visible in such hierarchical structures, which are constituted by MoS2(002) with neighboring Ni3S2(012̅) and Ni3S2(011). Furthermore, energy dispersive spectroscopy and corresponding elemental mapping are conducted to analyze the composition of the MoS2−Ni3S2 HNRs peeled off from NF by ultrasonic force. As displayed in Figure 2a, the energy dispersive spectra reveal the main composition of Ni, Mo, and S in MoS2−Ni3S2 HNRs. The molar ratio of Ni and S is determined to be 1.46 (inset of Figure 2a), in accordance with the molecular formula of Ni3S2. With regard to the high Ni/Mo ratio of 34.48, Ni3S2 is believed to be dominant in the HNRs, and MoS2 serves as a modification on the surface. This is consistent with the low MoS2 content (3.09 wt %) determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Furthermore, the corresponding elemental mapping shows the uniform distribution of Ni, S, and Mo in the
either chemical vapor deposition or a surfactant-mediated solvothermal procedure.31,32 Herein, we propose a one-pot and facile fabrication of MoS2−Ni3S2 heteronanorods supported by Ni foam (MoS2− Ni3S2 HNRs/NF) via environmentally friendly hydrothermal processes at a mild temperature, accomplishing synergistically enhanced activity for HER, OER, and overall water splitting. As shown in Scheme 1, 1D NiMoO4 is first generated on Ni foam (NF) because of the anisotropic growth of molybdates38,39 and further evolves to MoS2−Ni3S2 as it reacts with thioacetamide (TAA) for prolonged times, resulting in the inner Ni3S2 nanorod surface being decorated by MoS2 nanosheets. Such hierarchical nanostructures evenly composed of 1D Ni3S2 and two-dimensional (2D) MoS2 would allow the well-exposed active sites of Ni3S2 and MoS2, and more importantly their heterointerfaces. In addition, the 1D heteronanorods with a coarse surface and their direct attachment to the conducting NF provide pathways for efficient charge transport during electrolysis and promote the evolution and release of gas bubbles from the catalyst surface. As expected, the optimal MoS2−Ni3S2 HNRs/NF presents low overpotentials (η10) of 98 and 249 mV to reach a |j| of 10 mA cm−2 in 1.0 M KOH for HER and OER, respectively. The superior activity toward overall water splitting is featured by a quite low cell voltage of 1.50 V at 10 mA cm−2 and remarkable stability for more than 48 h, which outperforms that of commercial IrO2/C−Pt/C coupled electrocatalysts and those of most of the current noblemetal-free electrocatalysts.
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RESULTS AND DISCUSSION To fabricate Ni3S2-based nanostructures, commercial nickel foam (NF) after the removal of surface oxides with 3 M HCl was employed as the substrate (Figure S1), which shows a smooth surface in a scanning electron microscopy (SEM) investigation (Figure 1a). Bulky micrometer-sized products are produced by reaction of NF with TAA (Figure 1b), being confirmed as Ni3S2 (JCPDS Card No. 08-0126) by X-ray diffraction (XRD) (Figure S2). By contrast, as ammonium heptamolybdate tetrahydrate (AHM) is introduced, hierarchical nanostructures are obtained. With an increase in the rate of AHM feeding, the product evolves from nanorods to nanoparticles (Figure S3), and thus, an optimal AHM/NF 2359
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unsaturated S atoms on Ni−S and Mo−S sites.42,43 The peak at 168.7 eV is associated with residual SO42− species,24 and its high intensity in MoS2−Ni3S2 suggests the oxidation of surface S by oxidative molybdates. In addition, MoS2−Ni3S2 exhibits visible peaks for Mo4+ and Mo6+,42 which are absent in the bare Ni3S2 sample (Figure 3c). The existence of Mo6+ is caused by the surface oxidation of MoS2 as it is exposed to air. To evaluate the electrocatalytic activity for HER, MoS2− Ni3S2 HNRs/NF was tested in a 1.0 M KOH solution using a typical three-electrode configuration, in which the foam with dimensions of 0.5 cm × 0.5 cm was directly used as the working electrode. For comparison, bare NF, Ni3S2/NF, MoS2/NF, and commercial Pt/C supported by NF with the same mass loading were also tested. Figure 4a displays their polarization curves with iR correction. Undoubtedly, Pt/C on NF exhibits the best performance. As-prepared MoS2−Ni3S2 HNRs/NF affords remarkable HER activity, in which the cathodic current increases sharply with the applied potential. The overpotentials required for j values of −10 and −100 mA cm−2 are as low as 98 and 191 mV, respectively, superior to those of NF (η10 = 308 mV), MoS2/NF (η10 = 235 mV), and Ni3S2/NF (η10 = 183 mV). The obvious improvement in HER activity indicates the synergy between MoS2 nanosheets and Ni3S2 nanorods. As further compared with the current noble-metal-free catalysts (Table S1), our MoS2−Ni3S2 HNRs/NF performs as one of the best. In the case of nickel-based electrocatalysts, the η10 of MoS2−Ni3S2 HNRs/NF of 98 mV is lower than those of the reported MoS2−Ni3S2 nanoparticles/NF (110 mV),28 NiCo-P nanocubes (150 mV),44 NiSe2 nanobelts (184 mV),45 Ni−Fe/ C (219 mV),46 and Ni3S2 nanosheets/NF (223 mV)24 and comparable to those of NiSe nanowires/NF (96 mV)33 and MoS2@Ni/carbon cloth (91 mV).47 Typically, the alloys combining Mo with other transition metals (e.g., Ni, Co, and Fe) hold the benchmark of HER on noble-metal-free electrocatalysts.48,49 Showing comparable or slightly lower activity, our MoS2−Ni3S2 HNRs/NF can open new opportunities for Ni-based catalysts. Via further comparison with other noble-metal-free electrocatalysts, the activity of MoS2− Ni3S2 HNRs/NF is better than those of MoC−Mo2C heteronanowires (120 mV),22 CoSe/Ti (121 mV),50 a Zn0.76Co0.24S/CoS2 nanowire array (175 mV),51 1D Ni/ Mo2C/C (179 mV),52 and Co−S/carbon paper (190 mV).53 Accordingly, their Tafel plots show the consistent enhancement of the HER kinetics of MoS2−Ni3S2 (Figure 4b). MoS2− Ni3S2 HNRs/NF delivers a low onset overpotential (ηonset) of 31 mV and a low Tafel slope (b) of 61 mV dec−1, superior to those of NF (ηonset = 250 mV, and b = 149 mV dec−1), MoS2/ NF (ηonset = 176 mV, and b = 121 mV dec−1), and Ni3S2/NF (ηonset = 140 mV, and b = 108 mV dec−1). The small Tafel slope of MoS2−Ni3S2 HNRs/NF indicates a fast increase in the hydrogen generation rate with applied overpotential, corresponding to the high activity presented in the polarization curves. According to the classic theory,54 HER in alkaline aqueous media proceeds in two steps (eqs 1−3), where the asterisk indicates the active site for HER and H* a hydrogen atom bound to an active site. The first step is a discharge step (Volmer reaction) with a Tafel slope of 118 mV dec−1 (eq 1), and the second step is either the ion and atom reaction (Heyrovsky reaction) with a slope of 40 mV dec−1 (eq 2) or the atom combination reaction (Tafel reaction) with a slope of 30 mV dec−1 (eq 3).54 Although the Tafel slope alone is insufficient to determine the specific mechanism, the evidently reduced slope of MoS2−Ni3S2 HNRs/NF (61 mV dec−1), as
HNRs (Figure 2b), indicating the even integration of Ni3S2 nanorods with MoS2 nanosheets. XPS analysis was employed to investigate the chemical states of elements in MoS2−Ni3S2 HNRs, along with that of bare Ni3S2 free from MoS2 decoration. The deconvoluted Ni 2p profiles clearly show the strong Ni 2p3/2 and 2p1/2 peaks at 855.2 and 873.2 eV,40 respectively, as well as their satellite peaks, on both MoS2−Ni3S2 and bare Ni3S2 (Figure 3a). This is
Figure 3. High-resolution XPS profiles of (a) Ni 2p, (b) S 2p, and (c) Mo 3d in MoS2−Ni3S2 HNRs and Ni3S2.
consistent with the previously reported Ni3S2 growing on the NF substrate.24,28 MoS2−Ni3S2 also exhibits the peaks attributed to Ni3S2 species at 852.9 and 871.4 eV,41 which are blue-shifted in comparison with those of bare Ni3S2 (852.6 and 871.0 eV, respectively). It indicates that the electronic interactions between Ni3S2 and MoS2 lead to the redistribution of charge on their interfaces.28 Meanwhile, MoS2−Ni3S2 exhibits coincident binding energies of 162.5 eV (2p3/2) and 163.9 eV (2p1/2) for bridging S22− (Figure 3b), implying 2360
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Figure 4. (a) Polarization curves and (b) Tafel plots of NF, Ni3S2/NF, MoS2−Ni3S2 HNRs/NF, Pt/C on NF, and MoS2/NF. (c) Estimation of Cdl by plotting the current density variation [Δj = (ja − jc)/2, at 150 mV vs the reversible hydrogen electrode; data obtained from the cyclic voltammogram in Figure S4] vs scan rate to fit a linear regression. (d) Nyquist plots (at η = 200 mV). (e) η10 and Cdl values of various MoS2−Ni3S2/ NF samples derived with different feeding ratios. (f) Stability of MoS2−Ni3S2 HNRs/NF and Pt/C on NF with an initial polarization curve and after 10000 cycles in 1.0 M KOH. The inset of panel f shows the long-term durability tests at η = 150 mV (MoS2−Ni3S2 HNRs/NF) and 120 mV (Pt/C on NF) for HER in a 1.0 M KOH electrolyte.
compared with that of Ni3S2/NF (108 mV dec−1), confirms the promoted Volmer step in HER kinetics. Feng et al.28 have demonstrated the enhanced H binding on MoS2−Ni3S2 interfaces by density functional theory calculation. Our XPS analysis shows an obvious blue-shift of the Ni 2p binding energy in heterostructured MoS2−Ni3S2 (Figure 3a), in comparison with that in Ni3S2. The reduced electron density around Ni is reasonably suggested, which can provide sufficient
empty d orbitals to enhance binding with the H atom.55 Such strengthened H binding will facilitate the kinetics of the Volmer step, contributing to the obviously decreasing Tafel slope and the improved HER activity on MoS2−Ni3S2. H 2O(l) + e− + * ⇌ H* + OH−(aq) Vomer reaction (1) 2361
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Figure 5. (a) CV curves. (b) Tafel plots of NF, Ni3S2/NF, MoS2−Ni3S2 HNRs/NF, MoS2/NF, and IrO2/C supported by NF and (c) corresponding Nyquist plots (η = 300 mV). (d) Long-term durability of MoS2−Ni3S2 HNRs/NF for OER at 1.57 V vs the reversible hydrogen electrode in 1.0 M KOH.
H* + H 2O(l) + e− Heyrovsky reaction ⇌ H 2(g) + OH−(aq) + *
(2)
H* + H* ⇌ H 2(g) + 2* Tafel reaction
(3)
by Cdl, where Cdl represents the amount of active sites. As displayed in Figure S5, the obviously high value of MoS2− Ni3S2/NF confirms the intrinsic optimization of active sites on heterointerfaces, which is contributed by the enhanced empty d orbitals on Ni3S2 and the consequently promoted chemisorption of H* after MoS2 decoration. Meanwhile, electrochemical impedance spectroscopy (EIS) (Figure 4d) displays the varied charge-transfer resistance in MoS2−Ni3S2 HNRs/NF, Ni3S2/NF, and bare NF. Correspondingly with respect to the order of HER activity, MoS2−Ni3S2 HNRs/NF delivers an Rct obviously lower than those of Ni3S2/ NF and bare NF. This suggests the rapid electron transport for hydrogen evolution on the 1D nanostructures directly anchored on conducting NF. In addition, the impact of pH on HER performance was studied in the pH range of 7.4−14.0 (Figure S6). It is clear that the higher pH level leads to better electrocatalytic activity, which is possibly due to the solution conductivity being improved at high electrolyte concentrations. The hierarchical nanostructures integrating MoS2 nanosheets on Ni3S2 nanorods would improve the exposure of active MoS2−Ni3S2 interfaces, as indicated by the evolving HER activity associated with varied morphology. With a variation in the AHM/NF feeding ratio in weight (RAHM/NF), the MoS2− Ni3S2 heterostructures evolve from nanorods to nanoparticles (Figure S3), which show different HER activity (Figure S7). As
We further analyzed the surface area of the composite materials described above, as well as their contribution to electrocatalysis. At first, the BET surface is taken into account. As displayed in Table S2, the MoS2−Ni3S2 HNRs/NF presents a BET surface area (∼4.0 m2 g−1) smaller than that of Ni3S2/ NF (10.1 m2 g−1). By contrast, the former delivers a HER activity much higher than that of the latter. Because the BET surface represents only the total surface area of materials, including those that are inactive for electrocatalysis, the enhanced HER activity on MoS2 −Ni3 S2 HNRs/NF is reasonably independent of the BET surface. Therefore, another rational measurement of double-layer capacitances (Cdl) is herein employed, which is proportional to the electrochemical surface area (ECSA) and can provide a relative comparison.56 Derived from the cyclic voltammograms (CVs) versus the scan rate in 1.0 M KOH (Figure S4), the Cdl of 121.3 mF cm−2 on MoS2−Ni3S2/NF is much higher than that on Ni3S2/NF (30.3 mF cm−2), as shown in Figure 4c. The high Cdl value implies the enriched active sites on sulfides for HER. To further access the intrinsic activity, the electrocatalytic currents are normalized 2362
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Figure 6. (a) Polarization curves and (b) long-term durability tests at 1.53 V of MoS2−Ni3S2 HNRs/NF for overall water splitting in a 1.0 M KOH solution. The inset of panel b shows the SEM image of MoS2−Ni3S2 HNRs/NF after water electrolysis for 48 h.
displayed in Figure 4e, the η10 values of MoS2−Ni3S2 are 109 and 98 mV as low RAHM/NF values of 0.02 and 0.07 are adopted, respectively, in which the HNRs are dominant. With RAHM/NF increasing to 0.12 and 0.19, irregular particles emerge and are accompanied by MoS2−Ni3S2 HNRs, and η10 increases to 118 and 122 mV, respectively. As excessive AHM is employed in the synthesis (RAHM/NF = 0.28), only the products of nanoparticles are produced, resulting in the high η10 of 165 mV for HER. Accordingly, the measurement of Cdl for these MoS2−Ni3S2/NF nanoparticles shows consistent trends (Figure 4e); i.e., the maximal Cdl of 121.3 mF cm−2 is achieved on MoS2−Ni3S2 HNRs/NF with an optimal feeding RAHM/NF of 0.07. This clearly indicates the promoted exposure of MoS2− Ni3S2 active interfaces on hierarchical nanorods. Operational stability is another important criterion for a HER catalyst. The long-term stability of our MoS2−Ni3S2 HNRs/NF was examined using 10000 cycles continuously and chronoamperometry in 1.0 M KOH, with the reference being commerical Pt/C supported by NF (Figure 4f). Although Pt/C delivers a high initial HER activity, it suffers an obvious loss of HER activity in tests. By contrast, MoS2−Ni3S2 HNRs/NF affords j−V curves similar to that of the initial cycle with a negligible loss of the cathodic current after 10000 cycles, confirming the satisfactory durability. As has been further evaluated by prolonged electrolysis at 150 mV (inset of Figure 4f), MoS2−Ni3S2 HNRs/NF exhibits a high j of ∼40 mA cm−2 over 48 h. MoS2−Ni3S2 HNRs/NF was further explored with respect to the electrocatalytic OER in 1.0 M KOH. Figure 5a displays the CV curves for MoS2−Ni3S2 HNRs/NF, Ni3S2/NF, MoS2/NF, and bare NF at a scan rate of 1 mV s−1, along with that of the benchmark IrO2/C supported by NF with the same mass loading. In comparison with bare NF, the slightly enhanced activity is observed on MoS2/NF, which indicates the low OER activity of MoS2, in accordance with previous reports.57 Ni3S2/ NF presents an η10 of 314 mV. By contrast, MoS2−Ni3S2 HNRs/NF shows remarkably improved activity with a low η10 of 249 mV and an η100 of 341 mV, which outperforms that of conventional IrO2/C (20 wt % IrO2) on NF (η10 = 308 mV). As seen in the analysis for HER, the OER currents normalized by Cdl can reflect the intrinsic activity of the composite electrocatalysts described above. As shown in Figure S8, the obviously higher value of MoS2−Ni3S2/NF, in comparison with
that of Ni3S2/NF, easily confirms the intrinsically enhanced activity associated with MoS2−Ni3S2 synergy. Such synergistic effects have been indicated by the blue-shift of Ni 2p in the XPS profiles of MoS2−Ni3S2 HNRs (Figure 3a). The consequently enriched empty d orbitals on Ni after MoS2 modification will contribute to the strengthened binding with the OH* intermediate and thus the improved OER activity. Furthermore, the anodic (1.39 V) and cathodic (1.29 V) peaks on MoS2− Ni3S2 HNRs/NF correspond to the redox reaction of surface Ni species,28,58 generating in situ efficient OER active sites. Accordingly, the Raman and TEM investigations clearly identify the presence of oxidized species on the surface of MoS2−Ni3S2 after OER cycles (Figure S9). These peaks are more intense than Ni3S2/NF peaks, suggesting the promoted exposure of active sites on hierarchical MoS2−Ni3S2. Remarkably, the low overpotential (η10 = 249 mV) presented by MoS2−Ni3S2 HNRs/NF confirms the OER activity is superior to that of current noble-metal-free electrocatalysts (Table S3), e.g., Ni3S2 nanorods/NF (∼420 mV),59 NiMo nanorods/Ti mesh (310 mV),60 CoP nanowires/Ti mesh (310 mV),61 exfoliated NiFe LDHs (305 mV),62 FeOOH/Co/FeOOH nanotubes (245 mV),63 1D Ni/Mo2C-porous carbon (368 mV),52 Au@Co3O4 (390 mV),64 etc. Such OER activity is comparable to that of benchmark (NiCo)0.85Se/carbon cloth (255 mV),65 CoNiLDH/Fe-porphyrin film (264 mV),66 and Co-phytate nanoplates/Cu (265 mV).67 Figure 5b illustrates that the Tafel slope of MoS2−Ni3S2 HNRs/NF is approximately 57 mV dec−1, lower than those of Ni3S2/NF (82 mV dec−1), MoS2/NF (80 mV dec−1), NF (91 mV dec−1), and even the benchmark IrO2/C (59 mV dec−1). Its ηonset is 195 mV, outperforming Ni3S2/NF (281 mV), MoS2/NF (301 mV), NF (308 mV), and IrO2/C (280 mV). This result strongly suggests the kinetic merit of the MoS2− Ni3S2 HNRs for water oxidation. Coincident with the OER activity, MoS2−Ni3S2 HNRs/NF exhibits an Rct smaller than those of Ni3S2/NF and NF (Figure 5c), indicating the fast Faradaic process on MoS2−Ni3S2 heterostructures. To access the stability of MoS2−Ni3S2 HNRs/NF for OER, a long-term water oxidation is conducted at 340 mV (Figure 5d), in which steady OER activity is observed for 48 h. The MoS2−Ni3S2 HNRs/NF is further utilized as a bifunctional electrocatalysts for overall water splitting in a two-electrode system (Figure 6). Remarkably, it affords a 2363
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current density of 10 mA cm−2 in 1.0 M KOH at a cell voltage of 1.50 V, i.e., a combined overpotential of ∼270 mV. Such activity is superior to those of NF, MoS2/NF, and Ni3S2/NF, confirming that the synergy between MoS2 and Ni3S2 is efficient for overall water splitting. MoS2−Ni3S2 HNRs/NF outperforms the commercial 20 wt % IrO2/C−40 wt % Pt/C couple (1.52 V at 10 mA cm−2) and the recently reported bifunctional electrocatalysts (Table S4), e.g., MoS2−Ni3S2 nanoparticles/NF (1.56 V),28 Ni1−xFex/C (1.58 V),46 Fedoped CoP nanoarray/Ti mesh (1.60 V),68 Ni3Se2/NF (1.61 V),69 Co-doped NiSe2/Ti mesh (1.62 V),70 NiS/NF (1.64 V),71 Co0.85Se/NiFe-LDH (1.67 V),72 NiCo2S4 nanowires/ carbon cloth (1.68 V),73 V/NiF (1.74 V),74 etc. In particular, our MoS2−Ni3S2 HNRs/NF shows a lower OER activity (η10 = 249 mV) but a higher HER activity (η10 = 98 mV) compared to those of the recently reported MoS2−Ni3S2 nanoparticles/NF (η10 values of 218 and 110 mV for OER and HER, respectively). For overall water splitting at 10 mA cm−2, MoS2−Ni3S2 HNRs/NF requires a cell voltage (1.50 V) lower than that of MoS2−Ni3S2 nanoparticles/NF (1.56 V),28 indicating the compatible integration of HER and OER associated with the hierarchical and heterogeneous structures. In addition, the Faradaic efficiencies for H2 and O2 evolution were calculated by comparing the amount of experimentally quantified gas with the amount of theoretically calculated gas.68 The good agreement of both values (Figure S10) identifies the efficiencies that approximate 100% for both HER and OER, which can eliminate O2 reduction at the cathode and H2 oxidation at the anode. Moreover, the electrolyzer gives a smooth line with a stable current density of ∼17 mA cm−2 for 48 h, when the applied voltage is set to 1.53 V (Figure 6b). The hierarchical nanostructures of MoS2−Ni3S2 remain even after overall water splitting for 48 h (inset of Figure 6b), confirming the satisfactory long-term durability of MoS2−Ni3S2 HNRs under the strong alkaline electrolyte.
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Research Article
EXPERIMENTAL SECTION
Materials. All reagents were purchased from commercial sources (analytical or reagent grade when possible). Ammonium heptamolybdate tetrahydrate (AHM), urea, ethanol, thioacetamide, potassium hydroxide, sodium hydroxide, trisodium citrate dehydrate, and chloroiridic acid were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Hydrochloric acid was purchased from Guangzhou Chemical Reagent Factory. Ni foam was purchased from Shenzhen Green and Creative Environmental Science and Technology Co. Ltd. (Shenzhen, China). Commercial Pt/C (40 wt %) and titanium wire were provided by Alfa Aesar. Phosphate-buffered saline (PBS) was purchased from Invitrogen. All aqueous solutions were prepared using ultrapure water (>18 MΩ). Catalyst Preparation. Synthesis of MoS2−Ni3S2 HNRs/NF. Commercial NF (1 cm × 1 cm, ∼42 g) was first immersed in acetone for 1 h and then in an aqueous HCl solution (3 M) for 2 h to remove the surface oxides. Afterward, the NF was placed in a Teflon-lined stainless steel autoclave, in which a varied amount of ammonium heptamolybdate tetrahydrate [(NH4)6Mo7O24·4H2O (AHM)] and fixed amounts of TAA (27 mg), urea (25 mg), ethanol (1.7 mL), and deionized water (1.3 mL) had been already loaded. After reactions at 240 °C for 24 h, MoS2−Ni3S2 HNRs/NF was produced. The AHM/NF feeding ratio in weight was varied in the range of 0.02−0.28 to tailor the MoS2−Ni3S2 heterostrutures, and Ni3S2/NF was fabricated via similar procedures without adding AHM. The loading mass on such NF was kept at 13 mg cm−2, which was calculated from the weight increment after catalyst preparation. Synthesis of IrO2/C. According to previous reports,75,76 11.4 g of a H2IrCl6 solution (0.35 wt %) and 0.19 g of trisodium citrate dehydrate were added to a 100 mL beaker with 50 mL of distilled water; the pH was then adjusted to 7.5 with a NaOH solution (1 M) and the sample heated to 95 °C for 30 min while being stirred. The solution was transferred to a flask, and 0.184 g of carbon black (Vulcan XC72R) was added while the mixture was being stirred. The mixture was refluxed at 95 °C for 2 h under an O2 flow. After being dried at 70 °C under vacuum, the solid was heated at 300 °C for 30 min under an air flow to remove organic ligands, and 20% IrO2/C was finally harvested. Physical Characterization. SEM was performed on a ZEISS ULTRA55 instrument. TEM, HRTEM, energy dispersive spectroscopy, and elemental mapping investigations were performed with a JEOL JEM 2100F instrument. XRD analysis was performed on a Bruker D8 diffractometer using Cu Kα radiation (λ = 1.54056 Å). XPS was processed on a PerkinElmer PHI X-tool XPS instrument, using C 1s (binding energy of 284.6 eV) as a reference. Mo elemental analysis was performed by ICP-AES. N2 sorption isotherms were recorded on a Quantachrome Autosorb-iQ-MP adsorption analyzer at −196 °C (77 K). The Brunauer−Emmett−Teller (BET) specific surface area was calculated from adsorption data. The Raman investigation was performed on a laser confocal Raman microspectrometer (XploRA, Horiba Jobin Yvon, Ltd.). The evolving H2 and O2 were analyzed by gas chromatography (FuLi GC9790II instrument) with a TCD detector. Electrochemical Measurements. The electrochemical measurements of HER and OER were performed using an electrochemical workstation (CHI 760, CH Instruments, Inc., Shanghai, China) in a three-electrode system. MoS2−Ni3S2 HNRs/NF can be used directly as a working electrode, with
CONCLUSION
In summary, MoS2−Ni3S2 HNRs/NF was successfully achieved via facile hydrothermal processes, utilizing anisotropic molybdate intermediates to direct the growth of hierarchical MoS2−Ni3S2 heterostructures. In the composite electrocatalysts, Ni3S2 nanorods are evenly integrated with ultrathin MoS2 nanosheets, allowing the well-exposed heterointerfaces. Such MoS2−Ni3S2 electrocatalysts display a superior performance for HER and OER, which is evidenced by the low η10 values of 98 and 249 mV, respectively. As for overall water splitting, a cell voltage of only 1.50 V is required for a j of 10 mA cm−2, outperforming most current noble-metal-free electrocatalysts and even the IrO2/C−Pt/C couple. Such superior catalytic performance should be ascribed to the MoS2−Ni3S2 interfaces with favored chemisorption of H- and O-containing intermediates, the effective exposure of active interfaces in hierarchical nanostructures, and the facilitated electron transport along 1D Ni3S2 that is anchored on the NF substrate. Exploring the efficient water splitting over wellorganized metal−sulfide heterointerfaces, this work will open up new opportunities to develop promising electrocatalysts via rational engineering on interfaces and nanostructures. 2364
DOI: 10.1021/acscatal.6b03192 ACS Catal. 2017, 7, 2357−2366
Research Article
ACS Catalysis dimensions of 0.5 cm × 0.5 cm. As for MoS2, Pt/C, and IrO2/ C, the catalysts (4 mg) were first dispersed in 1 mL of a 4:1 (v/ v) water/ethanol mixture and 10 μL of a 5% PVDF solution prior to a ≥30 min sonication to form a homogeneous ink and then loaded onto a preprocessed 0.25 cm2 NF by drop-coating. A graphite rod and a saturated calomel electrode were used as the counter and reference electrodes, respectively. Linear sweep voltammetry (LSV) for HER and cyclic voltammetry for OER were conducted with a scan rate of 1 mV s−1 in 1.0 M KOH (pH 14.0). Electrochemical impedance spectra were recorded at corresponding potentials from 0.01 to 1000000 Hz for HER and OER, with an amplitude of 5 mV. For overall water splitting, MoS2−Ni3S2 HNRs/NF was integrated as both the anode and the cathode in a two-electrode cell with a onechamber setup (Figure S11). Polarization curves were recorded using LSV with a scan rate of 1 mV s−1. All the potentials reported here were referenced to a reversible hydrogen electrode by adding a value of (0.241 + 0.059 pH) V. iR correction was manually performed for every electrode, using potentio-electrochemical impedance spectroscopy. The compensated potential was corrected by the equation Ecompensated = Emeasured − iRs, where Rs is the series resistance determined by electrochemical impedance spectroscopy.
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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/acscatal.6b03192. Photographs of the MoS2−Ni3S2 HNRs/NF and the setup for overall water splitting, XRD patterns, SEM images, CV curves, j normalized by Cdl (HER and OER), pH dependent HER activity of the MoS2−Ni3S2 HNRs/ NF, the Raman and TEM investigation of after OER test, and the comparison of activity for HER, OER and overall water splitting (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Qingsheng Gao: 0000-0002-4273-8500 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We appreciate the financial support from the National Natural Science Foundation of China (21373102, 21433002, 51671089, and 51402110), the Guangdong Natural Science Funds for Distinguished Young Scholar (2015A030306014), and the Guangdong Program for Support of Top-notch Young Professionals (2014TQ01N036).
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