Ni-Decorated Molybdenum Carbide Hollow Structure Derived from

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Ni-decorated Molybdenum Carbide Hollow Structure Derived From Carbon-coated Metal-Organic Framework for Electrocatalytic Hydrogen Evolution Reaction Xiaobin Xu, Farhat Nosheen, and Xun Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02586 • Publication Date (Web): 16 Aug 2016 Downloaded from http://pubs.acs.org on August 18, 2016

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Chemistry of Materials

Ni-decorated Molybdenum Carbide Hollow Structure Derived From Carbon-coated Metal-Organic Framework for Electrocatalytic Hydrogen Evolution Reaction Xiaobin Xu, Farhat Nosheen, Xun Wang* Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing, 100084, China ABSTRACT: To enhance the intrinsic properties and realize novel functionalities toward desired applications, it’s of great significance to construct molybdenum carbide with unique architecture but still remains highly challenging. Herein, we demonstrate a simple synthetic method. Initially, Mo-based polyoxometalate anion incorporated Ni-MOF (Metal-Organic Framework) hollow structure is prepared through dissolution-regrowth process. Then carbon-containing polymer is coated on the surface of hollow precursor. After annealed at high temperature, carbon layer coated Ni-decorated hollow molybdenum carbide structures are obtained successfully. Due to the unique composition and configuration, these hollow structures exhibit remarkable hydrogen evolution reaction properties and stabilities.

INTRODUCTION As a clean, secure and renewable energy source hydrogen can meet the rapid growing global energy consumption.1-3 Electrochemically splitting water to produce hydrogen, or the hydrogen evolution reaction (HER), is one of the most economical, sustainable and attractive method.4-8 To date, platinum (Pt) or Pt-based materials give the best HER electrocatalytic performance but their applications are severely limited by the low abundance and high cost.9-10 It is urgently needed to develop noble-metal free catalysts with low cost, high activity and good stability. Molybdenum carbide, one of the most important early transition metal carbides, has been widely studied as high-performance HER electrocatalyst because of its Ptlike electronic structure.11-12 To enhance the intrinsic properties and realize novel functionalities toward desired applications, it’s of great significance to construct molybdenum carbide with uniform size and unique architecture.13-15 As a class of special morphology, hollow structure possesses fascinating properties such as large surface area, low density, short diffusion paths and rapid masstransfer kinetics.16-19 In particular, mass loss existed in solid structure due to large number of unexposed internal active sites can be minimized in hollow counterpart.20 However, until now the as-prepared molybdenum carbide nanomaterials are mostly irregular particles in shape.12, 21-22 Construction of molybdenum carbide nanomaterials with well-defined hollow structures still remains highly challenging. This is mainly because the synthesis always needs high-temperature annealing of precursors containing molybdenum and carbon.14 Such extreme conditions always make the original shape of precursors destroyed or col-

lapsed. To overcome these shortcomings, appropriate precursors should meet these requirements: (i) Mo and carbon uniformly dispersed in well-organized way, leading to confined and uniform carbonization, (ii) a high degree of robustness, thus the configuration of precursor could be retained even after harsh annealing and electrocatalytic conditions. Recently, polyoxometalate23-24 (POM, a class of early transition metal oxygen anion clusters) incorporated metal-organic framework25-27 (MOF, porous material constructed by metal ions/clusters and organic ligands) has emerged as a promising precursor in which POMs are evenly distributed in ordered carbon-containing host matrix.28-29 Host-guest supramolecular interactions are widely present between metal and POM ions, such as electrostatic, van der Waals, coordination interactions and hydrogen bonding.30-31 Generally, driven by this interaction, POM anions act as templates during the self-assembly of metal ions/clusters and organic ligands, which prevent the polymer from self-interpenetrating.30, 32 In the asachieved product, POM ions are resided within the pores of MOFs as charge-compensating anions.32 Another common synthetic method is through post-synthetic modification such as impregnating the MOF with a POM solution, by which “POM-loaded MOF material” is prepared.28 (Please refer to the literatures for more details of POM based MOFs composites.28, 30) Utilizing POM-based MOF as reactant, in situ and homogeneous carburization reaction confined within organic species (carbon precursor) and POMs (Mo precursor) is achieved.5 At the same time, the transition metal (TM) ions in MOF will convert into TM/TM oxide

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Scheme 1. Schematic illustration for the preparation of carbon coated Ni-decorated molybdenum carbide hollow spheres. after pyrolysis, which may also act as active catalytic sites and bring potential synergistic effect in HER.33-34 Nonetheless, as compared with the rich variety of morphologies of parent MOF, the synthesis of POM-based MOF nanocrystals is still in its infancy.28 As a consequence, very limited progress is made in this field and the products are mainly nano-octahedrons and solid nanoparticles (NPs).5, 35 On the other hand, it is necessary to increase the mechanical reinforcement of hollow precursor before pyrolytic process. A promising method is coating the precursor with a thin layer of carbon, by which the coalescence and collapsion of hollow structure can be avoided in calcination process. 36-37 Herein, we design a simple method to prepare robust molybdenum carbide-based hollow structure. The typical preparation route is illustrated in Scheme 1. Initially, Mobased POM anions incorporated Ni-MOF hollow structure (H1, here “H” represented “hollow”) is prepared through dissolution-regrowth process. Then carbon nitride polymer is coated on the surface of hollow precursor, forming H2. After annealing H2 at high temperature, carbon layer coated Ni-decorated hollow molybdenum carbide structures (HC-T) are obtained successfully. Due to the unique composition and configuration, these hollow structures exhibit remarkable HER properties and stabilities.

EXPERIMENTAL SECTION Preparation of H1: Nickel nitrate hexahydrate (Ni(NO3)2•6H2O, 28.8 mg), hexaammonium molybdate tetrahydrate ((NH4)6Mo7O24•4H2O , 18.0 mg) and 1, 3, 5benzenetricarboxylic acid (H3BTC, 80 mg) were added in DMF solution (32 ml). After stirring for 10 minutes, the solution was transferred to a 40 ml Teflon-lined stainlesssteel autoclave and heated at 180 ℃ for 28 h. After cooling to room temperature, the products were harvested via centrifugation at 5000 rpm for 4 minutes and further washed with methanol for 3 times. H0 was obtained when the reaction stopped at 0.5 h. Preparation of H2: 20.0 mg of H1 was dispersed in 30.0 ml of hexamethylenetetramine (HTM) ethanol solution (20.0 mg/ml) by ultrasonication, followed by hydro-

thermal treatment for 9 hours at 170℃. The products were centrifuged at 8000 rpm for 6 minutes and further purified with methanol for 5 times. Preparation of HC800: H2 was annealed under N2 flow at 800℃ for 6 h with the heating rate of 2 ℃ min-1, the obtained sample was denoted by HC800. Other control samples were synthetized according to same procedure but different conditions (Annealing parameters are shown in Table S1). Electrochemical measurements. All HER performance was conducted on CHI electrochemical workstation (Shanghai Chenhua Co., China) using a threeelectrode configuration in 0.5 M H2SO4, phosphate buffer, 1 M KOH. A glassy carbon electrode (5 mm in diameter), a saturated calomel electrode (SCE), and a graphite rod were used as the working, reference and counter electrode, respectively. The catalyst (5 mg) was dispersed in 1 ml of mixture solution containing 0.95 ml of ethanol and 50μl of 0.5 wt% Nafion solution by ultrasonication for 60 min. Then 5μl of catalyst suspension was pipetted on the GC surface. All potentials were converted to a reversible hydrogen electrode (RHE): E(RHE) = E(SCE) + (0.242 + 0.059 pH) V. Linear sweep voltammetry (LSV) was recorded from -270 to 0 mv (vs RHE, in 0.5 M H2SO4), from 290 to 0 mv (vs RHE, in phosphate buffer) and from -430 to 0 mv (vs RHE, in 1 M KOH) at a scan rate of 5 mV s-1. EIS measurements were performed with frequency from 0.1 to 100,000 Hz at an overpotential of 180 mV (in 1 M KOH). The long-term stability tests were carried out using Chronoamperometry technique at an overpotential of 222 mV (in 0.5 M H2SO4), 290 mV (in phosphate buffer) and 230 mV (in 1 M KOH).

RESULTS AND DISCUSSION Sample of H1 was solvothermally prepared using nickel ions as metallic nodes, 1, 3, 5-benzenetricarboxylic acid (H3BTC) as organic linkers, hexaammonium molybdate ((NH4)6Mo7O24•4H2O) as the Mo source, N,NDimethylformamide (DMF) as solvent. At 0.5h, as shown by the transmission electron microscopy images (TEM, Figure S1a), products of solid nanospheres (denoted by H0) were formed. The energy-dispersive X-ray spectroscopy (EDX, Figure S1b) showed no Mo-Containing signal was detected at this time. The chemical formula of Ni6(HBTC)5(OH)2(DMF)•15H2O was confirmed by induc-


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tively coupled plasma optical emission spectrometry (ICP-OES) and C, H, N elemental analysis (Table S2). Fourier Transform Infrared spectrum (FT-IR, Figure S2) verified the coordination between Ni ions and carboxylic acid group of H3BTC by a red shift in the -COO stretching frequency.38 The powder X-ray diffraction (XRD) pattern (Figure S3) could be assigned to Ni-MOF (NiHBTC).39 In the following stage (2h), evidenced by STEM and mapping images (Figure 1a, S4), the solid core Ni-MOF’s surface was covered by a layer of shell materials containing Ni, Mo, C, O. As time was prolonged further to 15h (Figure 1a, S5), hollow voids were clearly visible between the inner core and outer shell, accompanied by the increasing content of Mo. Eventually, totally hollow spheres (H1) were produced after complete dissolution of the Ni-MOF nanospheres at a reaction time of 28h. Figure 1b, c showed TEM and SEM images of uniform H1 with a submicrometer size of ~470 nm and shell thickness of about ~75 nm. Elemental mapping by EDX (Figure 1d) showed that the elements Mo, Ni, C and O were evenly distributed over the whole particle. ICP-OES and elemental analysis (Table S2) confirmed a formula of Ni10(HBTC)9Mo6O19(C2H7N)4(CH3OH)9•19H2O. On the one hand, FT-IR (Figure S2) in the -COO stretching frequency region and XRD results (Figure S3) of H1 was analogous to Ni-MOF, which confirmed the skeleton structure of NiMOF was maintained in H1. On the other hand, X-ray photoelectron spectroscopy (XPS, Figure S6) indicated that the oxidative states of Ni and Mo were +2, +6 respectively. The Mo6+ might be originated from molybdate ion. In accordance with this assumption, molybdate ion vibrations were observed in both FT-IR (Figure S2) and Raman spectra (Figure 1e) of H1, where new bands appeared at 967.9 cm-1 (FT-IR) and 940.4, 892.3 cm-1 (Raman) respectively compared with those of Ni-MOF.40-41 In summary, it could be concluded that H1 was a kind of Ni-based MOF with molybdate ion encapsulated. The formation of the hollow structures (H1) underwent an interesting dissolution-regrowth process.42-43 Firstly, in basic environment of DMF solution, organic ligand (H3BTC) was deprotonated quickly. Because of the high concentration of Ni2+ and carboxylate anions and the strong coordination interactions between them, Ni-MOF solid spheres (H0) were formed rapidly through control of reaction kinetics.43 Secondly, as described before, between Ni2+ and POM ions there were host-guest supramolecular interactions.30 Specially, the oxygen atoms on the surface of POM anions could act as electron donors for Ni ions, thus generating coordination interactions between them.30 In addition, the anionic character of POM endowed electrostatic interaction between Ni2+ and POM ions.44 Due to lack single-crystal data of H1, it was difficult to identify the bond distance and energy. But no question that the supramolecular interactions did exist. Driven by these interactions, POM ions had the tendency to substitute organic ligands to form Ni-POM supramolecules. Therefore, the Ni-MOF precursors reacted with molybdate anions to form shell containing Ni ions, organic ligands and molybdate. As the fast formed

Ni-MOF nanosphere was unstable thermodynamically, the inner sphere was dissolved gradually and the outer shell regrew simultaneously.43 As the core was consumed, well-defined hollow interior was formed and only shell of POM encapsulated Ni-MOF was left.

Figure 1. (a) EDX mapping images of products collected at different reaction time. (b) SEM, (c) TEM , (d) HAADFSTEM and EDX mapping images of H1. (e) Raman spectra of H0 and H1. (f) TEM image of H2. In order to prepare molybdenum carbide hollow structure, H1 was carbonized in N2 atmosphere. Regretfully, no molybdenum carbide formed after annealing H1 at 500℃ for 4 h. As TEM (Figure 2a, b) and EDX mapping images (Figure 2c) showed, metallic Ni nanocrystals modified hollow structures were obtained [denoted by H500 (here “500” corresponded to calcination temperature); See SI for XRD pattern (Figure S7a)]. Further increased temperature to 800℃, the morphology was destroyed totally with serious agglomeration (denoted by H800, here “800” also corresponded to calcination temperature; Figure S7b). Distinct from the solid structure,5 in this system the sole hollow MOF precursor couldn’t survive the extreme annealing environment. Therefore, the hollow structure should be reinforced before annealing process. Based on this consideration, the self-polymerization reaction of HTM was selected to construct carbon nitride polymer on the surface of H1.45 In a typical synthesis, H1 was firstly dispersed in HTM solution, followed by solvothermal



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treatment at 170℃ for 9h. The resulting product was denoted by H2. As shown by TEM image (Figure 1f) and XRD pattern (Figure S8), the morphology and structure of H2 was almost the same with H1. The XPS spectra (Figure S9) of Ni and Mo indicated that their oxidative states kept the same. Careful observation by the high-magnification TEM image (Figure S10) showed that there might be an amorphous thin layer on the surface of hollow sphere. As revealed by elemental analysis (Table S2), the content of N (C) increased from 1.390% (28.853%) in H1 to 6.674% (36.416%) in H2, unambiguously confirming that substance containing C and N elements formed in H2. The ICP-OES and elemental analysis (Table S2) gave a formula of Ni8(HBTC)10[(CH2)12N8]3Mo5O13(CH3OH)24•3H2O.

Figure 2. (a, b) TEM, (c) HAADF-STEM image and EDX mapping images of H500. Then H2 was annealed in an N2 atmosphere at 500℃ for 4 h (800℃ for 6h) and the product was denoted by HC500 (HC800) [here “HC” represented “hollow” and “Carbon coated”, “500” and “800” corresponded to calcination temperature]. The specific Brunauer-EmmettTeller (BET) surface areas of HC800 and HC500 were determined by Nitrogen adsorption-desorption isotherms (Figure S11). Evidenced by the type IV isotherms with hysteresis loops, both HC800 and HC500 exhibited mesoporous structures, which were inherited from the uniform porous structures of the POM-MOFs precursors.5, 46 Interestingly, HC800 possessed the largest specific surface area of 81.8 m2 g-1. Such porous structures with high specific surface areas would expose more active sites and thus promote the HER properties.14 The crystal structure and composition of HC500 and HC800 were investigated by Raman, XRD and XPS spectra. Firstly, the Raman spectra (Figure S12) verified the presence of amorphous carbon after carbonization process. For both samples, two distinct peaks were observed at about ~1345 and ~1580 cm-1, corresponding to the D and G band of graphitic carbon, respectively.14 Secondly, XRD patterns further confirmed the carbonization process of MOF precursor occurred in both conditions. As shown in Figure 3a, for both HC500 and HC800, the strong peak assigned to Ni-MOF at 10.4° was no longer observed while

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new and sharp peaks appeared. For HC800, it was wellcrystalline and the diffraction peaks could be satisfactorily assigned to hexagonal β-Mo2C phase (JCPDS card no. 350787), η-MoC (JCPDS card no.08-0384) and metallic Ni (JCPDS card no.65-0380).

Figure 3. (a) XRD patterns of HC500 and HC800. (b) Mo 3d XPS profile of HC800. (c, d) SEM, (e) TEM, (f) HAADF-STEM and EDX mapping images of HC500. As for HC500, only weak peaks indexed to metallic Ni and η-MoC appeared and no peaks belonging to β-Mo2C phase. The poor-crystallized nature of HC500 was possibly resulted from the incomplete carbonization. The composition information could be further provided by XPS spectra. As shown in Figure 3b, the peak fitting of Mo 3d high-resolution XPS of HC800 suggested that there were four oxidation states for Mo (Mo0, Mo3+, Mo4+, and Mo6+) on the surface of the carbides. Among them, the dominant Mo0 peak along with small peaks of low oxidation states of Mo (Mo3+) stemmed from β-Mo2C while the high oxidation states (+4, +6) were resulted from surface oxidation.47-48 For HC500 (Figure S13a), the peak fitting of Mo 3d profiles also suggested four states (0, +3, +4 and +6), while the species percentage was quite different (Table S3). The proportion of Mo6+ was only 6.4% in HC800 while that reached up to 39.8% in HC500. On the other hand, in the C1s spectra (Figure S13c, S13C) three peaks (284.6, 286.1 and 288.6eV) were correspond to the chemical bonding of C-C, C-O, -COO respectively.49 Similarly, the peak intensity of –COO stemmed from unreacted MOF precursor in HC500 was much higher than HC800. Based on the above XRD patterns and XPS spectra, it was


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concluded that the MOF precursor almost converted into molybdenum carbide and metallic Ni entirely in HC800. Nonetheless, the carbonization reaction was incomplete in HC500, accompanied by the formation of intermediate products.

Figure 4. (a, b) SEM, (c) TEM, (d) HAADF-STEM and EDX mapping images, (e, f) HRTEM images of HC800. Then SEM, TEM and EDX mapping characterizations were performed to observe morphology and elemental distribution of HC500 and HC800. For HC500, SEM (Figure 3c) showed the product consisted of spheres with the intact hollow structure which could be directly observed from an individual broken particle. Careful examination by HRSEM (Figure 3d) revealed that the surfaces of these hollow spheres were tough with small particles attached. TEM (Figure 3e) and high-angle annular darkfield scanning TEM (HAADF-STEM) images (Figure 3f) further confirmed the small nanoparticles with a small size of ~8 nm were uniformly decorated throughout the hollow structure. Judging from EDX mapping images (Figure 3f), Mo and C elements were distributed over the hollow structures while Ni element mostly located in the isolated embedded particles. But the accurate elemental distribution information couldn’t be provided, which might be caused by the resolution limit of the instrument, small particle sizes of Ni and the residual unreacted MOF precursor. Similar to HC500 in shape, HC800 were also the hollow structure with small particles embedded, as evi-

denced in Figure 4a-c. EDX mapping measurements (Figure 4d) were carried out to investigate the distributions of the existing elements. Unambiguously, Mo and C elements were uniformly distributed over the hollow structures while Ni element was located only in the separated particles. Distinct from the uniform Ni particles in HC500, here they were slightly aggregated, possibly caused by higher carbonization temperature and longer calcination time. HRTEM clearly demonstrated that the surfaces of the spherical particles were coated with carbon layers (Figure 4e, labeled by white circles and arrows).50 Due to the well-crystallinity, the lattice fringes could also be observed in the HRTEM images (Figure 4f), in which the 2.04 Å could be assigned to the (111) inter plane space of metallic Ni and the 2.36 Å /2.08 Å belonged to (002)/(101) plane of β-Mo2C. The electrocatalytic HER performance of the molybdenum carbide based hollow structures were evaluated in basic aqueous solution (1 M KOH). All the samples were deposited on a glassy carbon (GC) electrode with same loading of 0.12 mg cm−2 and all potentials (overpotentials) were reported vs RHE. The corresponding polarization curves were shown in Figure 5a. Impressively, an interesting trend could be observed: samples with carbon layers coated possessed more excellent HER performance (HC800>H800, HC500>H500). Specially, to achieve the current density of 10 mA cm-2, H800 required overpotential of 203 mv. However, for HC800 it required only 123 mV. Strikingly, this low overpotential made HC800 one of the best HER electrocatalysts in basic solution when compared with many other representative noble-metal free catalysts (Table S4). Furthermore, the difference in performance between HC800 and H800 was probably resulted from the difference in morphology between intact hollow structure and irregular solid shapes, thus demonstrating the significance of constructing hollow structures. Likewise, H500 showed very poor HER activity, i.e., even at a high overpotential of 430 mV current density was below 2.8 mA cm−2. While for HC500, small overpotential of ~73 mV was required to drive the same current density (2.8 mA cm−2). Besides, Tafel plots depicted in Figure 5b exhibited a similar trend, HC800 showed a Tafel slope of 83 mV/decade, lower than all other catalysts [HC500 (121 mV/decade), H800 (99 mV/decade) and H500 (161 mV/decade, Figure S14a)]. The remarkable electrocatalytic HER property of HC800 might be resulted from faster electron transfer. To better understand the underlying origin, EIS analysis at selected overpotential (η= 180mv) was performed on these catalysts. Consistent with the previous reports, two time-constant model (Figure S15) was used to fitted the EIS Nyquist plots.51 The model consisted of a series resistance, Rs, in series with two parallel branches; one at low frequency reflected the charge-transfer process (Rct); the other at high frequency was related to the surface porosity (Rp). As the Rct showed strong correlation with the electrochemical performance, the Nyquist plots (Figure 5c) were fitted and compared. It could be seen that the Rct value for HC800 (4.0 Ω) was much smaller than that for HC500 (6.3 Ω), H800 (19.4Ω)



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and H500 (> 50Ω) at the same overpotential, in agreement well with their different HER activities. The durability was another important catalytic performance aspect for HER. To evaluate their stability, the asprepared hollow structures and commercial Pt/C were tested at high overpotential (η=230 mv) to obtain the time-dependent current density curves. As shown in Figure 5d, both samples with carbon coating (HC500 and HC800) demonstrated excellent stability. Interestingly, there was minor current drop at the beginning of HC500. But after ~400s, the current density returned to almost the initial value and stabilized afterwards. As a control, the current density of Pt/C decreased dramatically (Figure 5d) and the samples without carbon coating (H500 and H800) also showed poor stability (Figure S14b-d).

Figure 5. (a) Polarization curves and (b) Tafel plots measured in 1 M KOH. (c) Nyquist plots measured at an overpotential of 180 mv over the frequency range 100 kHz to 0.01 Hz. (d) The time dependence of current density under a static overpotential of 230 mV over 7200 s for samples HC500, HC800 and commercial Pt/C. Based on above results the role of carbon coating could be summarized as follows: (1) the existence of carboncontaining polymer layer avoided aggregation or detachment of particles during pyrolysis process and thus retain the structural integrity; (2) similarly, the as-obtained carbon layers after pyrolysis acted as mechanical reinforcement of the hollow structure, by which the corrosion, morphological collapsion and oxidation of the catalyst under extreme alkaline electrocatalytic environment could be efficiently reduced; (3) The unique porous structure ensured the easy contact between catalyst and electrolyte, facilitating the charge and mass transfer during the HER; 5 (4) the carbon layer could enhance the electronic conductivity, which facilitated charge transfer during HER process.52-53 Combined with the merits of hollow structures, HC800 proved to be a robust efficient HER electrocatalyst. Furthermore, the role of metallic Ni NPs was also investigated. By etching the metallic Ni NPs in HC800 with 0.5 M H2SO4, molybdenum carbide hollow structures (denoted by HC800-Etching) was obtained. Both XRD pattern

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(Figure S16a) and TEM image (Figure S16b) confirmed that most of Ni NPs were etched and hollow structure consisting of η-MoC and β-Mo2C NPs were left. The polarization curve (Figure S16c) suggested that HC800Etching revealed much poorer HER activity than sample HC800, corresponded with the EIS measurements. As shown in Figure S16d, the Rct of sample HC800-Ethcing was ~18.9 Ω, much larger than the Rct of sample HC800 (~4.0 Ω ). Above results indicated that the addition of Ni NPs efficiently promoted HER performance of molybdenum carbide catalyst, which was in good agreement with previous report.33 In fact, pure Ni-based NPs were also efficient HER catalysts in alkaline media. 54 Introduction of Ni would not reduce the relative content of active ingredient for HER catalysis. The real role of Ni was it brought synergistic function between molybdenum carbide and Ni NPs.15, 55 Generally, in alkaline media, the HER reaction proceeded through the Volmer-Heyrovsky or Volmer-Tafel process. Both pathways involved the intermediate state of adsorbed H atom (Hads) on electrode surface.34 An ideal HER catalyst should possess proper hydrogen binding energy: with too weak hydrogen binding the intermediate state (Hads) couldn’t be stabilized. Thus the rate of both initial Volmer step and overall reactions were slowed down. On the other hand, strong binding hydrogen atoms blocked the active site and made bond breaking and the generation of gaseous H2 difficult.10 Unfortunately, pure molybdenum carbide possessed strong hydrogen binding energy. However, after the incorporation of Ni, the strength of Hads binded to Mo atoms could be reduced, thus benefiting the Hads desorption. This change was attributed to the electron transfer from molybdenum carbide to neighboring Ni atoms. As the Ni could gain electrons from the adjacent Mo atoms easily, the closer distance of Mo and Ni atoms was, the more positive charge Mo atoms possessed. This synergistic effect that molybdenum carbide served as electron donor while Ni as electron acceptor could decrease the adsorption Gibbs free energy of H (|∆GH*|) and in turn facilitate the desorptions of Hads and H2 release.15, 55 Therefore, the Ni modification was a key factor to promote HER activity of molybdenum carbide. We further tested the HER performance of HC800 in phosphate buffer (pH=7) and 0.5 M H2SO4 solution (pH=0) to examine its efficiency over wide pH values. As shown in Figure S17a, in neutral media, to drive a current density of 10 mA cm-2, HC800 only needed an overpotential of 212 mV, which was comparable to those reported values of noble metal-free HER electrocatalysts under neutral-pH condition (Table S5). Both the small Tafel slope of 69 mV/decade (Figure S17b) and high stability compared with Pt/C (Figure S17c) further confirmed its remarkable activity and great durability. Likewise, HC800 exhibited good activity in acid solution: it produced a current density of 10 mA cm-2 at an overpotential of 192 mV (Figure S17A) and gave a Tafel slope of 98 mv/decade (Figure S17B). This result was also comparable with many Mo2C based HER electrocatalysts (Table S6). Unfortunately, the electrocatalytic activity decreased obviously with exten-


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sion of reaction time in acid solution (Figure S17C). The instability of HC800 was probably caused by the etching of Ni NPs. As described above, the synthetic effect between Mo2C and Ni NPs played an important role in promoting the HER activity of Mo2C.55 With etching of Ni NPs, the adsorbed H atoms were binded to residual Mo2C NPs tightly and thus desorptions of Hads and H2 release became very difficult. To sum up, HC800 proved to be an efficient HER electrocatalyst over a wide pH range and could work stably in neutral and alkaline media.

*E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by NSFC (21431003, 21521091).

REFERENCES

(1) Dresselhaus, M. S.; Thomas, I. L., Alternative energy technologies. Nature 2001, 414, 332-337.

Conclusion

(2) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.;

In summary, we developed a simple approach for robust molybdenum carbide-based hollow structure as excellent HER electrocatalyst. The unique precursor consisting of Ni-based MOFs host and Mo-based POMs guest enabled the confined and uniform carburization reaction while the outer layer of carbon-containing polymer avoided the particles coalescence or detachment effectively during annealing process. As a consequence, the combination of compositional advantages (both molybdenum carbide and Ni NPs could act as active sites for HER) and unique architecture (i.e. hollow nature and carbon layer coating) made the product highly efficient and stable HER catalyst. Such facile strategy could be extended to prepare other high-performance composites with novel architecture. (i) Owing to the adjustable compositions and rich structural versatilities of POM-MOF crystal materials,28 it is possible to prepare diverse POM-MOFs nanomaterials with novel morphologies and uniform sizes by rationally designing synthetic systems. That is, the choice of precursor is quite general. In fact, many POMMOF nanocomposites, for example, NENU-5,46 [Zn(bimbp)2]3[PMo12O40]2•2H2O (PECP-1),21 POM@MIL100 (Fe)35 have been employed as precursors to prepare MoO2@ phosphorus-doped nanoporous carbon, MoxC@graphene shells with nitrogen dopants, Fe3C/Mo2C@ N, P co-doped graphitic carbon respectively. (ii) This carbon coating method is applicable to various nanomaterials. There is no specific requirement for the composition, morphology and surface functionalization of precursor. (iii) Due to its high stability, the carbon-coated precursor may suffer from harsher annealing conditions, such as under flow of NH3 or on-site PH3, thus it open up exciting opportunities for preparing not only metal carbides but metal nitrides/phosphides.11, 56

Mi, Q.; Santori, E. A.; Lewis, N. S., Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. (3) Hao, R.; Jiang, B.; Li, M.; Xie, Y.; Fu, H., Fabrication of mixed-crystalline-phase spindle-like TiO2 for enhanced photocatalytic hydrogen production. Sci. China Mater. 2015, 58, 363-369. (4) Xu, Y.-F.; Gao, M.-R.; Zheng, Y.-R.; Jiang, J.; Yu, S.-H., Nickel/Nickel(II) Oxide Nanoparticles Anchored onto Cobalt(IV) Diselenide Nanobelts for the Electrochemical Production of Hydrogen. Angew. Chem. Int. Ed. 2013, 52, 8546-8550. (5) Wu, H. B.; Xia, B. Y.; Yu, L.; Yu, X.-Y.; Lou, X. W., Porous molybdenum carbide nano-octahedrons synthesized via confined carburization in metal-organic frameworks for efficient hydrogen production. Nat. Commun. 2015, 6, 6512. (6) Chen, J.; Wu, X.-J.; Yin, L.; Li, B.; Hong, X.; Fan, Z.; Chen, B.; Xue, C.; Zhang, H., One-pot Synthesis of CdS Nanocrystals Hybridized with Single-Layer Transition-Metal Dichalcogenide Nanosheets for Efficient Photocatalytic Hydrogen Evolution. Angew. Chem. Int. Ed. 2015, 54, 1210-1214. (7) Gao, M.-R.; Liang, J.-X.; Zheng, Y.-R.; Xu, Y.-F.; Jiang, J.; Gao, Q.; Li, J.; Yu, S.-H., An efficient molybdenum disulfide/cobalt diselenide hybrid catalyst for electrochemical hydrogen generation.

ASSOCIATED CONTENT

Nat. Commun. 2015, 6, 5982.

Supporting Information. The Supporting Information available free of charge via the Internet at http://pubs.acs.org. Experimental details and characterization data (TEM, EDX mapping images, XRD, XPS, FT-IR spectra, electrochemical measurement tests, et al.).

(8) Yang, Y.; Xu, X.; Wang, X., Synthesis of Mo-based nanostructures from organic-inorganic hybrid with enhanced electrochemical for water splitting. Sci. China Mater. 2015, 58, 775784.

AUTHOR INFORMATION

(9) Zou, X.; Zhang, Y., Noble metal-free hydrogen evolution

Corresponding Author catalysts for water splitting. Chem. Soc. Rev. 2015, 44, 5148-5180.


Chemistry of Materials (10) Zheng, Y.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z., Advancing the

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

Electrochemistry of the Hydrogen-Evolution Reaction through Combining Experiment and Theory. Angew. Chem. Int. Ed. 2015, 54,

Page 8 of 12

surfaces: recyclable hard templates for engineering hollow nanocatalysts. Sci. China Mater. 2014, 57, 7-12. (20) Mahmood, A.; Guo, W.; Tabassum, H.; Zou, R., MetalOrganic Framework-Based Nanomaterials for Electrocatalysis. Adv.

52-65. (11) Zhong, Y.; Xia, X.; Shi, F.; Zhan, J.; Tu, J.; Fan, H. J., Transition Metal Carbides and Nitrides in Energy Storage and

Energy Mater. 2016, 1600423. (21) Yang, X.; Feng, X.; Tan, H.; Zang, H.; Wang, X.; Wang, Y.; Wang, E.; Li, Y., N-Doped graphene-coated molybdenum carbide

Conversion. Adv. Sci. 2016, 3, 1500286. (12) Liao, L.; Wang, S.; Xiao, J.; Bian, X.; Zhang, Y.; Scanlon, M. D.; Hu, X.; Tang, Y.; Liu, B.; Girault, H. H., A nanoporous molybdenum carbide nanowire as an electrocatalyst for hydrogen evolution reaction. Energy Environ. Sci. 2014, 7, 387-392. (13) Ma, R.; Zhou, Y.; Chen, Y.; Li, P.; Liu, Q.; Wang, J., Ultrafine Molybdenum Carbide Nanoparticles Composited with

nanoparticles as highly efficient electrocatalysts for the hydrogen evolution reaction. J. Mater. Chem. A 2016, 4, 3947-3954. (22) Lin, H.; Shi, Z.; He, S.; Yu, X.; Wang, S.; Gao, Q.; Tang, Y., Heteronanowires of MoC-Mo2C as efficient electrocatalysts for hydrogen evolution reaction. Chem. Sci. 2016, 7, 3399-3405. (23) Hill, C. L., Introduction: PolyoxometalatesMulticomponent

Carbon as a Highly Active Hydrogen-Evolution Electrocatalyst.

Molecular Vehicles To Probe Fundamental Issues and Practical

Angew. Chem. Int. Ed. 2015, 54, 14723-14727.

Problems. Chem. Rev. 1998, 98, 1-2.

(14) Ma, F.-X.; Wu, H. B.; Xia, B. Y.; Xu, C.-Y.; Lou, X. W. D.,

(24) Long, D.-L.; Tsunashima, R.; Cronin, L., Polyoxometalates:

Hierarchical β-Mo2C Nanotubes Organized by Ultrathin Nanosheets

Building Blocks for Functional Nanoscale Systems. Angew. Chem. Int.

as a Highly Efficient Electrocatalyst for Hydrogen Production. Angew.

Ed. 2010, 49, 1736-1758.

Chem. Int. Ed. 2015, 54, 15395-15399.

(25) Furukawa, H.; Cordova, K. E.; O'Keeffe, M.; Yaghi, O. M.,

(15) Liu, Y.; Yu, G.; Li, G.-D.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X., Coupling Mo2C with Nitrogen-Rich Nanocarbon Leads to Efficient Hydrogen-Evolution Electrocatalytic Sites. Angew. Chem. Int. Ed. 2015, 54, 10752-10757.

The Chemistry and Applications of Metal-Organic Frameworks. Science 2013, 341, 1230444-1230444. (26) Li, J.-R.; Sculley, J.; Zhou, H.-C., Metal–Organic Frameworks for Separations. Chem. Rev. 2011, 112, 869-932.

(16) Zhang, Z.; Chen, Y.; Xu, X.; Zhang, J.; Xiang, G.; He, W.;

(27) Murdock, C. R.; Jenkins, D. M., Isostructural Synthesis of

Wang, X., Well-Defined Metal–Organic Framework Hollow

Porous Metal–Organic Nanotubes. J. Am. Chem. Soc. 2014, 136,

Nanocages. Angew. Chem. Int. Ed. 2014, 53, 429-433.

10983-10988.

(17) Lou, X. W.; Archer, L. A.; Yang, Z., Hollow Micro-

(28) Du, D.-Y.; Qin, J.-S.; Li, S.-L.; Su, Z.-M.; Lan, Y.-Q.,

/Nanostructures: Synthesis and Applications. Adv. Mater. 2008, 20,

Recent advances in porous polyoxometalate-based metal–organic

3987-4019.

framework materials. Chem. Soc. Rev. 2014, 43, 4615.

(18) Ferrero, G. A.; Preuss, K.; Marinovic, A.; Jorge, A. B.;

(29) Zhang, Z.-M.; Zhang, T.; Wang, C.; Lin, Z.; Long, L.-S.; Lin,

Mansor, N.; Brett, D. J. L.; Fuertes, A. B.; Sevilla, M.; Titirici, M.-M.,

W., Photosensitizing Metal–Organic Framework Enabling Visible-

Fe–N-Doped Carbon Capsules with Outstanding Electrochemical

Light-Driven Proton Reduction by a Wells–Dawson-Type

Performance and Stability for the Oxygen Reduction Reaction in Both

Polyoxometalate. J. Am. Chem. Soc. 2015, 137, 3197-3200.

Acid and Alkaline Conditions. ACS Nano 2016, 10, 5922-5932. (19) Han, L.-N.; Ye, T.-N.; Lv, L.-B.; Wang, K.-X.; Wei, X.; Li,

(30) Yu, R.; Kuang, X.-F.; Wu, X.-Y.; Lu, C.-Z.; Donahue, J. P., Stabilization and immobilization of polyoxometalates in porous

X.-H.; Chen, J.-S., Supramolecular nano-assemblies with tailorable


Page 9 of 12


coordination polymers through host–guest interactions. Coord. Chem.

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

(39) Gao, C.; Liu, S.; Xie, L.; Ren, Y.; Cao, J.; Sun, C., Design and construction of a microporous metal–organic framework based on

Rev. 2009, 253, 2872-2890. (31) Xu, X.; Lu, Y.; Yang, Y.; Nosheen, F.; Wang, X., Tuning the growth of metal-organic framework nanocrystals by using

the pillared-layer motif. CrystEngComm 2007, 9, 545-547. (40) Masteri-Farahani, M.; Mahdavi, S.; Rafizadeh, M.,

polyoxometalates as coordination modulators. Sci. China Mater. 2015,

Microemulsion-mediated synthesis and characterization of

58, 370-377.

monodispersed nickel molybdate nanocrystals. Ceram. Int. 2013, 39,

(32) Sun, C.-Y.; Liu, S.-X.; Liang, D.-D.; Shao, K.-Z.; Ren, Y.-H.; Su, Z.-M., Highly Stable Crystalline Catalysts Based on a

4619-4625. (41) Murase, K.; Ando, H.; Matsubara, E.; Hirato, T.; Awakura,

Microporous Metal−Organic Framework and Polyoxometalates. J.

Y., Determination of Mo(VI) Species and Composition in Ni-Mo

Am. Chem. Soc. 2009, 131, 1883-1888.

Alloy Plating Baths by Raman Spectra Factor Analysis. J.

(33) Wang, S.; Wang, J.; Zhu, M.; Bao, X.; Xiao, B.; Su, D.; Li, H.; Wang, Y., Molybdenum-Carbide-Modified Nitrogen-Doped

Electrochem. Soc. 2000, 147, 2210-2217. (42) Zhang, T.; Ge, J.; Hu, Y.; Zhang, Q.; Aloni, S.; Yin, Y.,

Carbon Vesicle Encapsulating Nickel Nanoparticles: A Highly

Formation of Hollow Silica Colloids through a Spontaneous

Efficient, Low-Cost Catalyst for Hydrogen Evolution Reaction. J. Am.

Dissolution–Regrowth Process. Angew. Chem. Int. Ed. 2008, 47,

Chem. Soc. 2015, 137, 15753-15759.

5806-5811.

(34) Gong, M.; Zhou, W.; Tsai, M.-C.; Zhou, J.; Guan, M.; Lin,

(43) Zhang, Z.; Chen, Y.; He, S.; Zhang, J.; Xu, X.; Yang, Y.;

M.-C.; Zhang, B.; Hu, Y.; Wang, D.-Y.; Yang, J.; Pennycook, S. J.;

Nosheen, F.; Saleem, F.; He, W.; Wang, X., Hierarchical Zn/Ni-

Hwang, B.-J.; Dai, H., Nanoscale nickel oxide/nickel heterostructures

MOF-2 Nanosheet-Assembled Hollow Nanocubes for

for active hydrogen evolution electrocatalysis. Nat. Commun. 2014, 5,

Multicomponent Catalytic Reactions. Angew. Chem. Int. Ed. 2014, 53,

4695.

12517-12521.

(35) Li, J.-S.; Tang, Y.-J.; Liu, C.-H.; Li, S.-L.; Li, R.-H.; Dong,

(44) Dolbecq, A.; Dumas, E.; Mayer, C. R.; Mialane, P., Hybrid

L.-Z.; Dai, Z.-H.; Bao, J.-C.; Lan, Y.-Q., Polyoxometalate-based

Organic−Inorganic Polyoxometalate Compounds: From Structural

metal-organic framework-derived hybrid electrocatalysts for highly

Diversity to Applications. Chem. Rev. 2010, 110, 6009-6048.

efficient hydrogen evolution reaction. J. Mater. Chem. A 2016, 4,

(45) Zhao, Z.; Dai, Y.; Lin, J.; Wang, G., Highly-Ordered Mesoporous Carbon Nitride with Ultrahigh Surface Area and Pore

1202-1207. (36) Liu, H.; Li, W.; Shen, D.; Zhao, D.; Wang, G., Graphitic Carbon Conformal Coating of Mesoporous TiO2 Hollow Spheres for High-Performance Lithium Ion Battery Anodes. J. Am. Chem. Soc.

Volume as a Superior Dehydrogenation Catalyst. Chem. Mater. 2014, 26, 3151-3161. (46) Tang, Y.-J.; Gao, M.-R.; Liu, C.-H.; Li, S.-L.; Jiang, H.-L.; Lan, Y.-Q.; Han, M.; Yu, S.-H., Porous Molybdenum-Based Hybrid

2015, 137, 13161-13166. (37) Zhang, L.; Zhang, G.; Wu, H. B.; Yu, L.; Lou, X. W. D., Hierarchical Tubular Structures Constructed by Carbon-Coated SnO2 Nanoplates for Highly Reversible Lithium Storage. Adv. Mater. 2013,

Catalysts for Highly Efficient Hydrogen Evolution. Angew. Chem. Int. Ed. 2015, 54, 12928-12932. (47) Wan, C.; Regmi, Y. N.; Leonard, B. M., Multiple Phases of Molybdenum Carbide as Electrocatalysts for the Hydrogen Evolution

25, 2589-2593. (38) Yaghi, O. M.; Li, H.; Groy, T. L., Construction of Porous

Reaction. Angew. Chem. Int. Ed. 2014, 53, 6407-6410.

Solids from Hydrogen-Bonded Metal Complexes of 1,3,5Benzenetricarboxylic Acid. J. Am. Chem. Soc. 1996, 118, 9096-9101.


Chemistry of Materials (48) Vrubel, H.; Hu, X., Molybdenum Boride and Carbide

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

Catalyze Hydrogen Evolution in both Acidic and Basic Solutions. Angew. Chem. Int. Ed. 2012, 51, 12703-12706. (49) Zhang, H.; Ma, Z.; Duan, J.; Liu, H.; Liu, G.; Wang, T.; Chang, K.; Li, M.; Shi, L.; Meng, X.; Wu, K.; Ye, J., Active Sites Implanted Carbon Cages in Core–Shell Architecture: Highly Active and Durable Electrocatalyst for Hydrogen Evolution Reaction. ACS Nano 2016, 10, 684-694. (50) Zhang, H.; Liang, C.; Liu, J.; Tian, Z.; Shao, G., The formation of onion-like carbon-encapsulated cobalt carbide core/shell nanoparticles by the laser ablation of metallic cobalt in acetone. Carbon 2013, 55, 108-115. (51) Chen, W. F.; Wang, C. H.; Sasaki, K.; Marinkovic, N.; Xu, W.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R., Highly active and durable nanostructured molybdenum carbide electrocatalysts for hydrogen production. Energy Environ. Sci. 2013, 6, 943-951. (52) Li, J.-S.; Wang, Y.; Liu, C.-H.; Li, S.-L.; Wang, Y.-G.; Dong, L.-Z.; Dai, Z.-H.; Li, Y.-F.; Lan, Y.-Q., Coupled molybdenum carbide and reduced graphene oxide electrocatalysts for efficient hydrogen evolution. Nat. Commun. 2016, 7, 11204. (53) Zhang, G.; Xu, Y.; Wang, L.; Wang, J.; Kang, Y.; Sun, X., Rational design of graphene oxide and its hollow CoO composite for superior oxygen reduction reaction. Sci. China Mater. 2015, 58, 534542. (54) Gong, M.; Wang, D.-Y.; Chen, C.-C.; Hwang, B.-J.; Dai, H., A mini review on nickel-based electrocatalysts for alkaline hydrogen evolution reaction. Nano Res. 2015, 9, 28-46. (55) Xiong, K.; Li, L.; Zhang, L.; Ding, W.; Peng, L.; Wang, Y.; Chen, S.; Tan, S.; Wei, Z., Ni-doped Mo2C nanowires supported on Ni foam as a binder-free electrode for enhancing the hydrogen evolution performance. J. Mater. Chem. A 2015, 3, 1863-1867. (56) Xiao, P.; Chen, W.; Wang, X., A Review of PhosphideBased Materials for Electrocatalytic Hydrogen Evolution. Adv. Energy Mater. 2015, 5, 1500985.


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Ni-Decorated Molybdenum Carbide Hollow Structure Derived from

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