Molybdenum Polysulfide Anchored on Porous Zr-Metal Organic

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Molybdenum Polysulfide Anchored on Porous Zr–Metal Organic Framework to Enhance the Performance of Hydrogen Evolution Reaction Xiaoping Dai, Mengzhao Liu, Zhanzhao Li, Axiang Jin, Yangde Ma, Xingliang Huang, Hui Sun, Hai Wang, and Xin Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02818 • Publication Date (Web): 27 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016

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The Journal of Physical Chemistry

Molybdenum Polysulfide Anchored on Porous Zr–Metal Organic Framework to Enhance the Performance of Hydrogen Evolution Reaction

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Xiaoping Dai,*, ,§ Mengzhao Liu, ,§ Zhanzhao Li, Axiang Jin, Yangde Ma, Xingliang Huang, Hui Sun, Hai Wang, Xin Zhang*, †

a

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State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249,

China b

National Institute of Metrology, Beijing 100013, China

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These authors contributed equally to this work.

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Corresponding authors: Prof X. P. Dai: State Key Laboratory of Heavy Oil Processing China University of Petroleum, Beijing 102249, PR China Tel.: +86 10 89734979; Fax: +86 10 89734979. E–mail address: [email protected] Prof X. Zhang: State Key Laboratory of Heavy Oil Processing China University of Petroleum, Beijing 102249, PR China Tel.: +86 10 89734979; Fax: +86 10 89734979. Email: [email protected]

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Abstract: Replacement of precious platinum with efficient and low−cost catalysts for electrocatalytic hydrogen evolution reaction (HER) at low overpotentials holds tremendous promise for clean energy devices. Herein, molybdenum polysulfide (MoSx) anchored on porous Zr–metal organic framework (Zr−MOF, UiO–66–NH2) by chemical interactions is fabricated by a facile and one−pot solvothermal method for HER application. The distinctive design of the Zr–MOF stabilized MoSx composite enables remarkable electrochemical HER activity with a Tafel slope of 59 mV·dec−1, lower onset potential of nearly 125 mV, and a cathode current of 10 mA·cm–2 at an overpotential of 200 mV, which also exhibits excellent durability in acid medium. The superior HER performance should ascribe to the fast electron transport from the less conducting MoSx nanosheets to the electrode, high effective surface area and number of active sites, as well as the favourable delivery for local proton in the porous Zr–MOF structure during the electrocatalytic reaction. Thus, this work paves a potential pathway for designing efficient Mo–based HER electrocatalysts by the combination of molybdenum polysulfide and versatile proton–conductive MOFs.


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1. INTRODUCTION Hydrogen as a promising source for sustainable energy applications has attracted great interest because of its potential to reduce the heavy dependence on fossil fuels and excessive concerns about CO2 emissions. In view of the energy and environmental issues, water electrolysis is an efficient and clean technology to generate highly pure hydrogen.1,2 As one half reaction of water electrolysis, hydrogen evolution reaction (HER) requires highly active and stable catalysts to reduce the overpotential and consequently increase the efficiency.3 Currently, although Pt–group metals, along with its alloys, are the benchmark electrocatalysts that require very small overpotentials to drive the HER, they suffer from scarcity and high cost, hindering their widespread use for H2 production.4 Motivated by this challenge, the extensive search for cost−effective, earth−abundant materials with high HER activity and excellent stability has recently attracted significant interests. Toward this end, earth−abundant catalysts,5–16 such as transition–metal chalcogenides, carbides, phosphides, and nitrides have been identified as potential electrocatalysts for HER. Among these materials, molybdenum chalcogenides have been already emerged as promising alternative due to the close free energy of adsorbed atomic hydrogen with that of Pt–group metals (i.e., ∆GH*≈0).17 Subsequent studies have suggested that HER activity correlates linearly with the total length of the exposed edges of the MoS2.18 Since then, tremendous efforts have been made to obtain abundant exposed edge sites by increasing sulfur active edge sites, and to improve the conductivity properties by using the conductive substrates. In addition to MoS2, Some other molybdenum sulfide structures, such as [Mo3S4]4+,19 [Mo3S13]2–,20 amorphous MoSx,21–24 and molecular analogues that contain MoS2 edge sites,25–27 could also promote the evolution of hydrogen. Metal−organic frameworks (MOFs) are a new class of porous materials with extremely large surface areas, which also possess the regularity of the void spaces, the tunability of the pore size, and the adjustability of the pore surface properties.28–31 However, the stability of MOFs has long been ACS Paragon Plus Environment


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considered as a major obstacle in their electrochemical applications.29,32 Recently, some MOFs and MOF−based composites were demonstrated for oxygen reduction reaction (ORR),33,34 hydrogen evolution reaction (HER),35–37 electro−reduction of CO2,38,39 electro−oxidation of methanol and ethanol.40,41 Qin et al.35 proposed novel polyoxometalate−based MOFs for HER, which showed the onset potential of 180 mV and overpotential of 237 mV at current density of 10 mA·cm−2 with a Tafel slope of 96 mV·dec−1 in 0.5 M H2SO4, suggesting that both POM units and MOFs porosity are essential for high HER activity. Hod et al.36 fabricated a high porosity, MOF films as scaffolds for the deposition of Ni−S electrocatalyst, which exhibited significantly enhanced HER performance in acid medium (0.1 M HCl), decreasing the kinetic overpotential by more than 238 mV at a benchmark current density of 10 mA·cm−2 due to the favourable proton transport. On the other hand, Zr−metal organic framework (Zr−MOF, UiO−66) consists of a cubic framework of cationic Zr6O4(OH)4 nodes and 1,4−benzenedicarboxylate linkers (BDC). Zr−MOFs have been proved to be excellently stable under a wide range of thermal and chemical conditions, as well as their substantial internal surface area,42–45 and shows good wettability in water.46 The porous Zr−MOF loaded with guest molecules displays proton−conducting properties (10−2−10−6 S·cm−1), which can be further improved by self−assembly of the corresponding functionalized ligands, or post−synthetic modifications of the MOF particles.47–51 Moreover, the proton−conductive Zr−MOF is favorable to modify the immediate chemical environment of the sulfide−based catalyst in HER.36 Controllable integration of MOFs and functional materials is leading to the creation of new multifunctional composites, which exhibit new properties that are superior to those of the individual components. Inspired by those prominent works, we herein demonstrated a facile strategy for fabrication of the MoSx anchored on Zr−metal organic framework (UiO−66−NH2) by one−pot solvothermal method. The optimized components showed an outstanding HER performance with low overpotential and small Tafel slope by the effectively reducing the charge−transfer impedance, ACS Paragon Plus Environment

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increasing the active surface area and number of active sites, as well as the favourable delivery for local proton in the porous MOF structure, which indicated a promising cathode catalyst candidate. 2. EXPERIMENTAL SECTION 2.1 Material Preparation. Chemicals. Zirconium tetrachloride (ZrCl4), 2–aminaterephthalic acid (ATP), N,N–dimethylformamide (DMF) were supplied from Sinopharm Chemical Reagent Co., Ltd. Sulphuric acid and ethanol were purchased from Beijing Chemical Reagent Company. Ammonium tetrathiomolybdate ((NH4)2MoS4), commercial Pt black were purchased from Alfa Aesar. All chemicals were obtained commercially and used without further purification. Deionized (DI) water was used in the experiment. Preparation of Molybdenum Polysulfide Anchored UiO–66–NH2. Molybdenum Polysulfide Anchored UiO–66–NH2 was synthesized according to a modified solvothermal synthesis reported previously.52 In a typical synthesis, ZrCl4 (149 mg, 0.64 mmol), ATP (116 mg, 0.64 mmol) and (NH4)2MoS4 were thoroughly dissolved in DMF (18 mL), and were further sonicated for 10 min. It was transferred into a 40 mL Teflon–lined stainless steel autoclave, which was sealed in a preheated oven at 80 o C for 12 h and then held at 100 oC for 24 h. After cooling, the resultant precipitate was collected by centrifugation, and repeatedly washed with absolute ethanol for 3 days at 60 oC in a water bath, followed by filtered, and dried under vacuum at 80 oC for 10 h. The obtained samples were denoted as UiO–66–NH2–Mo–x with different mole ratio of Mo to Zr, such as Mo/Zr=0.1, 0.3, 0.5 and 0.6 corresponding to x=1, 3, 5 and 6, respectively. Preparation of UiO–66–NH2. The synthetic procedure of UiO–66–NH2 as control sample was similar to the original UiO–66–NH2, except for the addition an appropriate amount of (NH4)2MoS4 before solvothermal treatment. 2.2 Catalyst Characterization. FT−IR spectra were obtained using KBr pellets on a Scimitar 2000 FT–IR spectrophotometer (Varian Inc.). The Powder X−ray diffraction (XRD) was carried out using ACS Paragon Plus Environment


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a Brüker D8 Advance diffractometer at 40 kV with Cu Kα radiation. Scanning electron microscopy (SEM) images were made by FEI XL30 Sirion SEM, and Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) images were obtained with FEI Tecnai G2 F20 electron microscope. The Langmuir surface area and Brunauer−Emmett−Teller (BET) surface areas of the samples were measured by nitrogen adsorption/desorption at −196 oC on Quantachrome Autosorb. The micropore distribution and volume were determined by SF method, and the mesopore distribution and volume were calculated according to BJH method. To determine the amount of Mo or Mo/Zr in the sample, the thermogravimetric analysis (TGA) was performed using a thermogravimetric analyzer (Mettler TGA/DSC) from 30 to 900 °C at a heating rate of 10 oC min−1 in air. Elemental analyses for C, H, N and S were determined on an Elementar Vario ELIII analyzer. The metal amount of Zr and Mo was detected by inductively coupled plasma mass spectrometry (ICP−MS) with Thermo ELEMENT 2. The Raman spectra were obtained with a Renishaw Micro−Raman System 2000 spectrometer with a 20 mW air−cooled argon ion laser (532 nm) as the exciting source. X−ray photoelectron spectroscopy (XPS) was carried out on a PHI 5000 Versaprobe system using monochromatic Al Kα radiation. All binding energies were calibrated using the C 1s peak (284.6 eV). For the measurement of the electrical resistivity, the films with same composition as electrochemical measurement (except solvent and nafion) were directly transferred to individual glass substrate, and examined by using a four−point probe system (4 Probes Tech S−2A). 2.3 Electrochemical Measurement. All of the electrocatalytic properties were performed on a three–electrode system (CHI660E), which include a saturated calomel reference electrode (SCE), a Pt slice counter electrode and modified glassy carbon working electrode. The working electrode was prepared as follows: 2 mg catalyst, 1 mg carbon black and 500 µL 4:1 (v/v) water–ethanol were homogeneously mixed under sonication for 30 min. Then 5 µL of this ink was drop–casted onto a GCE (3 mm in diameter) to obtain catalyst loading of 0.285 mg/cm2, followed by dropping 3 µL of ACS Paragon Plus Environment

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0.05 wt. % Nafion (Alfa Aesar) for protection. Liner sweep voltammetry (LSV) polarization curves were conducted in N2–saturated 0.5 M H2SO4 at a scan rate of 5 mV·s–1. For the stability study, a graphite rod was used as the counter electrode to avoid the possible contribution of dissolved Pt species to the HER. After cyclic voltammetry between –0.2 to 0.2 (vs. SCE) at a sweep rate of 100 mV·s–1 for a given number of cycles, the LSV polarization curves were conducted to compare with the initial LSV curve. The electrochemical impedance spectroscopy (EIS) was measured in the same configuration at an overpotential of 150 mV within the frequencies range of 105–0.01 Hz with perturbation voltage amplitude of 5 mV. Capacitance measurements were carried out at various scan rate (20, 40, 60, 80, 100, 120, 140, 160 and 180 mV·s–1) in the region of 0.1–0.2 V vs. RHE. The double–layer capacitance was estimated by plotting the △J (Ja–Jc) at 0.15 V vs. RHE under different sweep rate, while the half of the slop was Cdl.53 During the experiments, a flow of nitrogen was maintained over the electrolyte to eliminate dissolved oxygen. All results were calibrated with respect to reversible hydrogen electrode (RHE) by E(RHE) = E(SCE) + 0.273 V. Cyclic voltammetry at a scan rate of 50 mV·s–1 were performed in pH=7 phosphate buffer to determine the total number of the active sites according to the previous methods.10,20,22

n=

Q 1 1 i ⋅ t 1 1 i ⋅ V / u 1 1 10 ⋅ S ⋅ ⋅ = ⋅ ⋅ = ⋅ ⋅ = F 2 m F 2 m F 2 m F⋅m

(eq. 1)

Where N is the total number of active sites (mol/g catalyst), Q is the integrated charge from cyclic voltammograms, F is the Faraday constant (96485 C·mol−1), i, V, t, u, m and S are the current (A), potential (V), sweep time (s), sweep rate (V·s–1), catalyst mass (g) and integrated effective area in cyclic voltammograms recorded in pH=7 phosphate buffer after deduction of the blank value for bare GCE under the same condition, respectively. Moreover, the per–site turnover frequencies (TOF, s–1) can be calculated with the following equations.10,20,22 ACS Paragon Plus Environment


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Q 1 i ⋅ t 1 i ⋅ V / u 1 10 ⋅ S ⋅ = ⋅ = ⋅ = F 2 F 2 F 2 F j 1 TOF = ⋅ F⋅ N 2 N=

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(eq. 2) (eq. 3)

Where j was the current (in A) during the linear sweep measurement, N was the total number of active sites (mol).

3. RESULTS AND DISCUSSION 3.1 Structural and Compositional Characterization The crystallinity and phase purity of as–synthesized samples were examined by powder X–ray diffraction (XRD) (Figure 1A). The UiO–66–NH2 exhibits a typical Fm3m symmetric space group. The peaks at 7.3

o

and 8.5

o

are attributed to the crystal plane (111) and (200) of UiO–66–NH2.54

Although the crystallinity decrease gradually with increasing Mo loading, all of the as–prepared UiO–66–NH2–Mo–x exhibit the characteristic diffraction peaks of UiO–66–NH2, indicating that the framework integrity of the parent MOFs are well preserved after anchored MoSx. Notably, the crystallinity and structure on UiO–66–NH2–Mo–5 after a long treatment in ethanol at 60 ⁰C is well preserved, suggesting no effects on structure by ethanol washing (Figure S1, bottom), which agrees well with the results on UiO–66 and UiO–66–NH2 treated with various solvents from the previous reports.45,52 Furthermore, no characteristic peaks of MoSx are observed in UiO–66–NH2–Mo–x. Nitrogen adsorption/desorption isotherms of the UiO–66–NH2 show a typical type I shape with characteristic of microporous materials in Figure 1B. The UiO–66–NH2 possesses a Langmuir surface area of ca. 1339 m2/g (BET, 1021 m2/g) and pore volume of 0.422 cm3/g via SF method with micropore size of 1.34 nm. With the addition of (NH4)2MoS4, the as–prepared UiO–66–NH2–Mo–5 presents the mixed type of I and IV of adsorption isotherms with the Langmuir surface area of ca. 451 m2/g (BET surface, 335 m2/g) and pore volume of 0.131 cm3/g with micropore size of 1.25 nm. Notably, the UiO–66–NH2–Mo–5 also shows the narrow mesopore size distribution plots calculated by BJH method with 3.3 to 4.2 nm (inset of Figure 1B). The surface area and pore size distribution ACS Paragon Plus Environment

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data support the occupation of the cages and the channel surface modification of UiO–66–NH2 by MoSx nanoparticles, implying the incorporation of MoSx into the UiO–66–NH2. To investigate the interaction between MoSx and UiO–66–NH2, FTIR spectra of UiO–66–NH2 and UiO–66–NH2–Mo–5 are displayed in Figure 1C–D, which indicate C=C ring vibration peak at 1388 cm–1. No obvious changes are found in the hydroxyl, carboxyl groups (1550–1660 cm–1) and amino group (2800~3500 cm–1) after anchored MoSx. Notably, compared with (NH4)2MoS4, UiO–66–NH2, and physical mixture of UiO–66–NH2+MoSx, the new peak at 1046 cm−1 in the UiO–66–NH2–Mo–5 and UiO–66–Mo–5 illustrates that the chemical interactions could occur between the carbon atom on the benzene ring of the organic ligand (ATP) and sulfur atoms of the MoSx by π–π stacking (Figure S2, Figure 1D and Figure S2).51,55 The interaction between MoSx and UiO–66–NH2 should be similar as the incorporation of organometallic Fe2 complex into Zr–MOF.56 We also perform the same synthetic procedure as UiO–66–NH2–Mo–5 without Zr4+ to verify the roles of Mo, but we don’t find any crystal in the end. The results indicate that the Mo ions cannot be the nodes for MOF under the present condition. Based on the SEM observations (Figure S3 and S4), the sphere particles are observed, and the UiO–66–NH2 shows that the particles are of nanometer size with a distribution in the range of 135 nm. After anchored MoSx, the much smaller particles with 33 nm are observed. The TEM and HRTEM images of the UiO–66–NH2 and UiO–66–NH2–Mo–5 are displayed in Figure 2. Notably, the lattice fringes of UiO–66–NH2 are not observed in high–resolution TEM (HRTEM) image of UiO–66–NH2 are not attainable owing to the fact that it tends to be damaged under high energy electron beam irradiation.57 For UiO–66–NH2–Mo–5, lattice fringes (~0.64 nm) of MoS2 have not been observed, indicating the amorphous nature of MoSx (Figure 2E–F).58 The Zr and Mo/Zr content are evaluated through thermogravimetric analysis (TGA) in UiO–66–NH2 and UiO–66–NH2–Mo–5 (Figure 3A), which shows two main weight–loss stages. For ACS Paragon Plus Environment


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UiO–66–NH2, the weight loss of 8.9 % in the first stage (~150 oC) is attributed to the release of H2O and DMF molecules, while high weight loss of 20.8% for UiO–66–NH2–Mo–5 may ascribed to the release of H2O and DMF molecules, as well as partial oxidation of MoSx in the air. During the second stage (~800 oC), the weight loss of 49.9 % and 47.3 % is attributed to the decomposition of the ligands in the frameworks and total oxidation of MoSx. The remaining weight of 31.9 % and 41.2 % should be corresponding to the percentage of ZrO2 and the hybrids of ZrO2/MoO3 for UiO–66–NH2 and UiO–66–NH2–Mo–x,59,60 respectively. Raman spectrum confirms the presence of bridging S22− ligands in the UiO–66–NH2–Mo–5 in Figure 3B. No peak is observed over the UiO–66–NH2 in the range of 200–700 cm−1, while the UiO–66–NH2–Mo–5 displays broad bands at 330, 450 and 552 cm−1. The broad bands at 330 and 450 cm−1 should ascribe to Mo–S vibration with the presence of bridging S22− ligands, while the band at 552 cm−1 confirms the characteristic S–S vibration of bridging S22− ligands,21,61 further demonstrating the amorphous MoSx. The element compositions of the as–prepared materials are estimated according to elemental and ICP−MS analyses (Table S1). Without the anchored MoSx, the formula of the material thus obtained is estimated to be [Zr6O4(OH)4(BDC–NH2)6]·14H2O (Found: C 29.8 N 4.0 H4.1 %; Calculated: C 28.7, N 4.4, H 3.1%). ICP−MS analyses further confirm that the MoSx are successfully loaded on the Zr–MOF with high S/Mo ratio, which are much higher than commonly reported MoSx. Those results suggest that an extremely S–rich molybdenum sulfide structure has been formed in the UiO–66–NH2–Mo–5. The S–rich molybdenum sulfide structure is further confirmed in Figure 4A by the high–resolution XPS of S 2p due to the presence of bridging S22− and/or apical S2− and terminal S22− and/or S2−.21,22 The deconvolution of high–resolution XPS of Mo 3d–S 2s in Figure 4B exhibits seven peaks, and the two at 228.2 and 226.7 eV corresponding to S 2s of MoSx in the UiO–66–NH2–Mo–5. Compared with the Mo 3d–S 2s of (NH4)2MoS4, two new peaks at 229.4 and 232.2 eV are nearly the same binding energies as those of MoS2 nanoparticles growth on graphene or ACS Paragon Plus Environment

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mesoporous carbon,9 indicating the existence of mixed value with Mo (IV) and Mo (VI) in the UiO–66–NH2–Mo–5. The shoulder at 236.0 eV can be assigned to MoO3, which may be formed during the preparation of the materials.10,62 No changes are observed in XPS analyses for C 1s, Zr 3d, O1s and N 1s (Figure S5A–D). Although the functional groups are attached to the S atoms, the modulation on the electronic structure mainly happens to the Mo 4d states.63 It is worth noting that, compared with pure (NH4)2MoS4, the peaks for S2− (2p3/2 and 2p1/2) shift from low binging energy toward high binging energy with about 0.4 eV, further verifying the chemical interaction between MoSx and UiO–66–NH2.

3.2 Electrocatalytic Activity and Stability toward HER The HER polarization curves of as–prepared samples and commercial Pt/C in 0.5 M H2SO4 are shown in Figure 5A. As expected, Pt/C exhibits the highest HER electrocatalytic activity with near–zero onset potential. To investigate the effect of MoS2 and NH2–functionalized MOF on the HER activity, control experiments of bulk MoS2 and pure UiO–66–NH2 were performed under the same conditions (Figure 5A and Figure S6), which demonstrates the negligible HER activity in UiO–66–NH2 and bulk MoS2, respectively. After anchored MoSx, the UiO–66–NH2–Mo–x are highly active toward HER. The UiO–66–NH2–Mo–5 exhibits the best performance with lower onset potential of nearly 120 mV, beyond which cathodic current rises rapidly to 10 mA·cm–2 at 200 mV. In contrast, it needs an overpotential of 235, 223 and 213 mV to obtain a current density of 10 mA cm−2 for UiO–66–NH2–Mo–x with x= 0.1, 0.3 and 0.6, respectively. More importantly, the UiO–66–NH2–Mo–5 in acidic medium shows comparable current density (10 mA·cm–2) at the overpotential of 200 mV with most of non–precious HER electrocatalysts reported up to date (Table S2), such as amorphous MoSx,64 MoS3/CNT,62 MoS2/3D-NPC,65 Ni–MoS2,66 NiMoNx/C,16 Co−NRCNTs,67 Polymolybdate−Based MOF,35 Ni-S/NU−100036 and

Cu−MOF/ 8 wt.% GO.37 We

also investigate the HER performance on the UiO–66–Mo–5 as control sample (Figure S7), which ACS Paragon Plus Environment


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shows inferior HER performance than that of UiO–66–NH2–Mo–5, exhibiting higher onset potential (~160 mV) and higher potential (~230 mV) to reach 10 mA·cm–2. Although we lack specific insight into about how –NH2 group might assist MoSx in catalyzing the HER by improving the electron and proton transfer, we reason that the –NH2 group in UiO–66–NH2 plays an important role in facilitating protons transport from bulk solution toward the edge active sites of MoSx particles, which may significantly improves the HER activity.36,68 Cao et al.15 proposed that a large number of N−H moieties should be available to participate in the HER reaction mechanism. The Grotthuss mechanism should be followed during the proton transfer in NH2−functionalized UiO−66 by the protons hopping forward along the hydrogen bonding networks formed by the functional groups, adsorbed water molecules, and oxygen−rich Zr6O4(OH)4(CO2)12 clusters in aqueous acid solutions.69–71 Stability is another significant criterion to evaluate an advanced electrocatalyst. The accelerated degradation measurements are performed by continuous cyclic voltammetry (CV) sweeps between −0.3 and +0.2 V vs RHE at 50 mV·s−1 in acidic environment. As shown in Figure 5B, negligible difference can be observed between the curves measured at the initial cycle and after 5000 CV sweeps. The long−term stability of this electrode is also assessed by prolonged electrolysis at constant overpotential of 200 mV (inset of Figure 5B). The current density generally remains stable over 7 h with small degradation, corroborating the good stability in acidic environment, which was also observed on NU−1000_Ni−S for HER in 0.5 M H2SO4.36 The UiO–66–NH2–Mo–5 also contains −COOH and −NH2 functional groups as NU−1000_Ni−S electrocatalysts, which could play same roles as NU−1000_Ni−S in HER. The high stability of the present UiO–66–NH2–Mo–5 in the electrocatalyitc experiment could hint no significant change of the electronic structure, although in absent of the direct evidence of XPS and XRD, which are difficult to carry out because of the tiny amount of the catalyst after reaction. The morphology of the UiO–66–NH2–Mo–5 after the reaction ACS Paragon Plus Environment

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is generally retained as indicated by HRTEM measurement (Figure S9), which further confirms the superior stability. The excellent activity and high stability in acid solution make it promising candidates for HER.

3.3 The Mechanism of HER at the UiO–66–NH2–Mo–x Composites To get further insight into the activity of as–synthesized UiO–66–NH2–Mo–5 toward HER, electrochemical impedance spectroscopy (EIS) analysis is also performed in Figure 6A. The EIS indicates one semicircle at η=150 mV for each EIS nyquist plots, which can be fitted to an equivalent circuit (inset of Figure 6A). The charge–transfer resistance (Rct) is 29.6 Ω·cm2 for UiO–66–NH2–Mo–5, which is much lower than those of the as–prepared catalysts, suggesting a highly efficient electron transport and favorable HER kinetics at the UiO–66–NH2–Mo–5 modified electrolyte interface. The results also suggests that the anchored MoSx has slightly effect on the electronic properties of UiO–66–NH2 with internal resistance (Rs) in Table 1, which could support the charge hopping regime as leading conduction mechanism for UiO–66–NH2–Mo–x.72 Also, It is found that the electrical resistivity of UiO–66–NH2 is about 3.7 × 10-3 Ω·m, which is slightly lower than that of UiO–66–NH2–Mo–5 (~ 4.7 × 10-3 Ω·m). The HER kinetics of the above–mentioned catalysts are further evaluated by Tafel slope (Figure 6B and Table 1). The Tafel slope on Pt/C is 32 mV·dec–1, indicating a combination of Volmer reaction and Tafel reaction. The as–prepared UiO–66–NH2–Mo–x exhibit Tafel slops of 52, 54, 59 and 51 mV·dec–1 with increasing Mo content, respectively, which is much lower than that of UiO–66–NH2. Those Tafel slopes for UiO–66–NH2–Mo–x suggest that the Volmer reaction for converting protons into absorbed hydrogen atoms (H3O+ + e = H* + H2O) is the rate limiting step in the HER process, which occurs through a Volmer–Heyrovsky mechanism. By applying extrapolation method to the Tafel plots, exchange current densities are obtained (Figure S10). The UiO–66–NH2–Mo–5 exhibits remarkable exchange current density (j0) of 6.46 µA·cm−2 (Table 1), which is 4~22 times than those of ACS Paragon Plus Environment


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UiO–66–NH2–Mo–x (x=1, 3 and 6), suggesting the superior activity for HER catalysis.

3.4 The Origin of High Performance toward HER The high HER activity in the UiO–66–NH2–Mo–5 can be ascribed to the high effective surface area (Aeff), which can be obtained by cyclic voltammetry at various scan rates (20, 40, 60 mV·s–1, etc.) in 0.1‒0.2 V vs. RHE region (Figure 7A and Figure S11). From the cyclic voltammograms, the Aeff can be calculated by plotting the △J at 0.15 V vs. RHE in CV against the scan rate (Figure 7B), where the slope is twice the Cdl, generally also considered as Aeff in HER.53,61 The calculated values of the Aeff are 2.8, 3.4, 3.8 and 3.2 mF with the increasing Mo content, respectively, which are in accordance with the order of the HER activity on the UiO–66–NH2–Mo–x. The turnover frequency (TOF) is then calculated by quantifying the active sites through integrated charges from cyclic voltammograms in pH = 7 phosphate buffer in the region of −0.1 to 0.6 V.10,20,22,73 The total number of active sites can be calculated according to the integrated charges by deduction of the blank value for bare GCE (Figure 7C and Figure S12), showing the highest value (0.140 mmol/g) for UiO–66–NH2–Mo–5 in as–prepared catalysts (Table 1), which provides strong evidences for the high HER activity of UiO–66–NH2–Mo–5. Furthermore, the calculated Turnover frequency (TOF) for UiO–66–NH2–Mo–5 reaches 1.276 s–1 at 200 mV, which is 2.78, 1.41 and 1.95 times more than those of the UiO–66–NH2–Mo–x (x=1, 3 and 6), respectively (Figure 7D and Table 1), indicating the better intrinsic HER activity of UiO–66–NH2–Mo–5.

4. CONCLUSION In summary, we first demonstrate that molybdenum polysulfide anchored on Zr–MOF is an efficient electrocatalysts for the hydrogen evolution reaction. The optimized UiO–66–NH2–Mo–5 exhibits relatively low overpotential of just 200 mV to reach cathode current of 10 mA·cm–2 as well as a small Tafel slope of 59 mV·dec−1, and also shows excellent durability. The superior HER performance should ascribe to the fast electron transport, high effective surface area and number of ACS Paragon Plus Environment

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active sites, as well as the favourable delivery for local proton in the porous MOF structure. This facile solvothermal method could provide a promising strategy for the combination of molybdenum polysulfide and versatile proton–conductive MOF as an effectively alternative catalyst for platinum–based HER electrocatalysts.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: Supporting figures and table as described in the text (PDF)

AUTHOR INFORMATION Corresponding Authors *

(X.P.D.) State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing

102249, PR China. Tel.: +86 10 89734979; Fax: +86 10 89734979; E-mail address: [email protected]. *

(X.Z.) State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing

102249, PR China. Tel.: +86 10 89734979; Fax: +86 10 89734979; E-mail address: [email protected].

Author Contributions §

These authors contributed equally to this work. The manuscript was written through contributions

of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial supports from the NSFC (No. 21576288, 21573286, and 21173269), Ministry of Science and Technology of China (No. 2011BAK15B05), and Specialized Research Fund for Doctoral Program of Higher Education (20130007110003). The authors also ACS Paragon Plus Environment


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gratefully acknowledge Prof. Yongfeng Li and Mr. Wang Yang, China University of Petroleum (Beijing, China), for the characterization about the electrical resistivity.

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[68] Creţu, R.; Kellenberger, A.; Vaszilcsin, N. Enhancement of Hydrogen Evolution Reaction on Platinum Cathode by Proton Carriers. Int. J Hydrogen. Energy 2013, 38, 11685–11694 [69] [Ponomareva, V. G.; Kovalenko, K. A.; Chupakhin, A. P.; Dybtsev, D. N.; Shutova, E. S.; Fedin, V. P. Imparting High Proton Conductivity to a Metal−Organic Framework Material by Controlled Acid Impregnation, J. Am. Chem. Soc. 2012, 134, 15640−15643 [70] Yang, F.; Huang, H. L.; Wang, X. Y.; Li, F.; Gong, Y. H.; Zhong, C. L.; Li, J. R. Proton Conductivities in Functionalized UiO−66: Tuned Properties, Thermogravimetry Mass, and Molecular Simulation Analyses, Cryst. Growth Des. 2015, 15, 5827−5833 [71] Borges, D. D.; Vinot, S. D.; Jobic, H.; Ollivier, J.; Nouar, F.; Semino, R.; Devic, T.; Serre, C.; Paesani, F.; Maurin G. Proton Transport in a Highly Conductive Porous Zirconium-Based Metal–Organic Framework: Molecular Insight, Angew. Chem. 2016, 128, 3987 –3992 [72] Liu, J. X.; Wachter, T.; Irmler, A.; Weidler, P. G.; Gliemann, H.; Pauly, F.; Mugnaini, V.; Zharnikov, M.; Wöll, C. Electric Transport Properties of Surface-Anchored Metal−Organic Frameworks and the Effect of Ferrocene Loading, ACS Appl. Mater. Interfaces 2015, 7, 9824−9830 [73] Dai, X. P.; Du, K. L.; Li, Z. Z.; Liu, M.; Z, Ma, Y. D.; Sun. H.; Zhang, X.; Yang, Y. Co–doped MoS2 Nanosheets with the Dominant CoMoS Phase Coated on Carbon as an Excellent Electrocatalyst for Hydrogen Evolution. ACS. Appl. Mater. Interfaces 2015, 7, 27242−27253


Page 24 of 33

Page 25 of 33 FIGURE CAPTIONS

7

8

9

10

UiO-66-NH2 UiO-66-NH2 -Mo-1 UiO-66-NH2-Mo-3 UiO-66-NH2-Mo-5 UiO-66-NH2-Mo-6 5

10

15

20

25

30

35

40

800 700 600

0.09 0.3 0.06

0.2

3.8 nm

a

a

0.1

0.03

b*2

b

0.0 0

500

Mesopore

Micropore

0.4

0.00 2

4

0

2

4

6

8

250

200

150

10

Pore size (nm)

400

UiO-66-NH2-5

300 a

200

100

UiO-66-NH2 50

Volume Adsorbed (cm3/g)

6

B

Pore Volume (cc/nm/g)

5

Volume Adsorbed (cm3/g)

Intensity (a.u.)

900

Pore Volume (cc/nm/g)

1000

A

100

45

b

0

0.0

0

0.2

0.4

o

2θ ( )

0.6

0.8

1.0

Relative Pressure (p/p0)

D

C

a

a

Transmittance (%)

Transmittance (%)

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


b H-N-H

c

2926 2973

1496

b

1383

1572

c

1046

d

d 3472

3600

1658

3357

3200

2800

2400

2000

2000

-1

Wavenumber (cm )

1434 1388

1500

1258

768 663

1000

500

-1

Wavenumber (cm )

Figure 1 (A) XRD patterns of UiO–66–NH2–Mo–x with different atomic ratio of Mo to Zr, (B) Nitrogen adsorption–desorption isotherms and pore size distributions of (a) UiO–66–NH2 and (b) UiO–66–NH2–Mo–5; FTIR spectra (C) in the 2000–400 cm–1 range, and (D) in the 3600–2000 cm–1 range of (a) (NH4)2MoS4, (b) UiO–66–NH2, (c) UiO–66–NH2–Mo–5, (d) UiO–66–NH2+MoSx.



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

A

B

200 nm

200 nm

C

D

20 nm

10 nm

E

F

20 nm

5 nm

Figure 2 TEM images of (A,C,D) UiO–66–NH2 and (B,E,F) UiO–66–NH2–Mo–5.


Page 26 of 33

Page 27 of 33

100

b

A

8.9%

B

90

20.8%

b

a Intensity (a.u.)

80

Weight loss (%)

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


70

49.9% 60 50 40

a

47.3%

30 20 200

400

600

Temperature (oC)

800

200

300

400

500

600

700

Wavenumber (cm-1)

Figure 3 (A) Thermogravimetric analysis in air, (B) Raman spectra of (a) UiO–66–NH2 and (b) UiO–66–NH2–Mo–5.



A

S2p

22

S 2p3/2

B

S2- 2p3/2 S2- 2p1/2

c

Mo3d-S2s

II Mo 3d3/2 I Mo 3d5/2

Intensity (a.u.)

S22p1/2 2

Intensity (a.u.)

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

Page 28 of 33

I Mo 3d3/2

II Mo 3d5/2

II S 2s I S 2s

6+

Mo

c

b b

a 168

165

162

159

Binding Energy (eV)

240

235

230

225

Binding Energy (eV)

Figure 4 High–resolution scans of (A) S 2p, (B) Mo 3d–S 2s for (a) UiO–66–NH2, (b) (NH4)2MoS4 and (c) UiO–66–NH2–Mo–5.


0

B

2

-10 -20 -30

UiO-66-NH2-Mo-1 UiO-66-NH2-Mo-6

-40


-50 -0.4

0

2

2

UiO-66-NH2

Current density (mA/cm )

A Current density(mA/cm )

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



Page 29 of 33

-0.3

-0.2

Pt/C

-0.1

0.0

0.1

-10 -20

0

?=-0.2 V vs RHE -4

-8

-12

-16

-30

0

5000

10000

15000

20000

25000

Time (s) Initial 1000 cycles 5000 cycles

-40 -50 -0.4

-0.3

Potential (V vs. RHE)

-0.2

-0.1

0.0

0.1

Potential (V vs. RHE)

Figure 5 (A) Polarization curves, (B) Durability for the UiO–66–NH2–Mo–5 in 0.5 M H2SO4 and time–dependent current density curve under static overpotential of 200 mV.



0.4

3000 CPE

Rs

2500 UiO-66-NH2

2000

Rct

1500 UiO-66-NH2-Mo-1

1000

UiO-66-NH2-Mo-3 UiO-66-NH2-Mo-0.5 UiO-66-NH2-Mo-0.6

500 0 0

400

800

1200 1600 2000 2400 2800

B Potential (V Vs. RHE)

A

-Z'' (ohm)

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

Page 30 of 33

UiO-66-NH2

UiO-66-NH2-Mo-1

0.3

UiO-66-NH2-Mo-6

0.2 UiO-66-NH2-Mo-5 UiO-66-NH2-Mo-3

0.1 Pt/C

0.0 0.1

1

10

100 2


Z' (ohm)

Figure 6 (A) EIS nyquist plots (symbol) at 150 mV, ﬁtted data (solid line) by equivalent electrical circuit diagrams (inset); (B) Tafel plots of the as–prepared samples with various mole ratio of Mo to Zr.


Page 31 of 33

A

Scan rate

0.8

1.5

B


1.2

j0.15V vs RHE (mA/cm )

-2


0.6 -2

0.4 0.2 0.0 -0.2 -0.4 -0.6

0.9 UiO-66-NH2-Mo-6

0.6

UiO-66-NH2-Mo-1

0.3

-0.8

0.0

0.10

0.12

0.14

0.16

0.18

0

0.20

20

40

60

C

80

100 120 140 160 180 200

Scan rate (mV/s)

Potential (V vs. RHE) 0.04

D

0

UiO-66-NH2-Mo-5

0.03

UiO-66-NH2-Mo-3

2

0.02 0.01 0.00

TOF (s )


4


UiO-66-NH2-Mo-3

-1

Current (mA)

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


-0.01 -0.02

6

UiO-66-NH2-Mo-5

8

-0.03 -0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

10 -0.4

Potential (V)

-0.3

-0.2

-0.1

0.0

Potential (V)

Figure 7 (A) Cyclic voltammograms (0.1‒0.2 V) recorded in 0.5 M H2SO4 for UiO–66–NH2–Mo–x, (B) The differences in current density (∆J=Ja–Jc) at 0.15 V plotted against scan rate fitted to a linear regression allows for the estimation of Cdl, (C) Cyclic voltammograms (–0.10~0.6 V) of UiO–66–NH2–Mo–x recorded at pH =7 phosphate buffer with a scan rate of 50 mV· s–1, (D) calculated turnover frequencies (TOFs) of UiO–66–NH2–Mo–x.



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

Page 32 of 33

Table

Table 1. Electrochemical parameters of the UiO–66–NH2–Mo–x Samples

Effective surface

Number of active

Rs

Rct 2

Tafel, 2

Exchange current –1

–2

TOF at 200 mV, s–1

area, Aeff/mF

sites, mmol/g catalyst

Ω·cm

Ω·cm

–

–

0.65

15542.3

280

–

–

UiO-66-NH2-Mo-1

2.8

0.089

0.68

200.9

52

0.29

0.458

UiO-66-NH2-Mo-3

3.4

0.118

0.66

41.4

54

1.62

0.903

UiO-66-NH2-Mo-5

3.8

0.140

0.78

29.6

59

6.46

1.276

UiO-66-NH2-Mo-6

3.2

0.096

0.71

77.0

51

0.93

0.654

UiO-66-NH2

mV·dec


density, j0/µA·cm

Page 33 of 33

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


Table of contents graphic

MoSx OH

H+ NH2

MoS42- + ZrCl4 +

One-pot Solvothermal

OH


H2

Molybdenum Polysulfide Anchored on Porous Zr-Metal Organic

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