Carbon Nanotubes for

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

Ultralow Pt Loaded Molybdenum Dioxide/Carbon Nanotubes for Highly Efficient and Durable Hydrogen Evolution Reaction Xiao Xie,†,‡ Yi-Fan Jiang,† Cheng-Zong Yuan,† Nan Jiang,† Sheng-Jie Zhao,† Li Jia,§ and An-Wu Xu*,† †

Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China ‡ Metals and Chemistry Research Institute, China Academy of Railway Sciences, Beijing 100081, P. R. China § Hefei Midea Refrigerator Co., Ltd., Hefei 230601, P. R. China S Supporting Information *

ABSTRACT: Efficient electrocatalytic hydrogen evolution reaction (HER) is one of the most promising ways to lowcost hydrogen production in the future. However, it is still a challenge to develop efficient, durable, and affordable catalysts. In this paper, we choose ultrasmall Pt nanoclusters, metallic MoO2 semiconductor, and conductive material with ultralow Pt loadings as an active and durable catalyst for HER in acid media. The obtained 0.5 wt % Pt−MoO2/MWCNTs catalyst with ultralow Pt loadings exhibits high-efficiency electrocatalytic activity and excellent stability. Benefiting from the space confinement effect and good electric conductivity of metallic MoO2 and MWCNTs, the HER activity of as-made 0.5 wt % Pt−MoO2/MWCNTs catalyst is comparable to that of commercial 20 wt % Pt/C catalyst. Remarkably, the strong interaction between metallic Pt and MoO2 support can prevent the aggregation of Pt species; the 0.5 wt % Pt−MoO2/MWCNTs sample shows better stability than 0.5 wt % Pt−MWCNTs and commercial 20 wt % Pt/C. Consequently, highly catalytic activity and long-term stability toward HER have been achieved for the composite catalyst with only 0.5 wt % Pt loading. Our work opens up a new track to explore high-efficiency catalysts without heavy consumption of noble metal Pt, while maintaining the superior catalytic activity and durability.

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INTRODUCTION The excessive consumption of fossil fuels and incremental environmental problems that come along with it have become the driving force for developing clean energy conversion technologies.1 Electrolyzers and fuel cells are the most significant renewable energy storage and conversion devices.2 The hydrogen evolution reaction (HER) in aqueous medium is a fundamental reaction for the development of nonfossil energy storage and conversion devices.3 The HER is used in electrolyzers, which stores the energy in chemical form by liberating hydrogen. Usually, cathodic HER is catalyzed by noble metal Pt.4,5 Generally, large-scale electrochemical hydrogen production is restricted by two main problems: dependence on the precious metal platinum and the lack of long-term stability of the electrode materials under the strong acidic conditions.6,7 Scientists are searching for suitable HER catalysts based on more economical metals such as Ni, Mo, W, and other transition-metal-based electrocatalysts. Expensive Pt-based catalysts may be replaced by these metals and their intermetallic compounds,8,9 oxide,10 carbide,11,12 sulfide,13,14 phosphide,15,16 and selenide.17 However, according to the literature reports, few alternative materials exhibit excellent HER activity as the commercial Pt/C catalyst. What is more, the fabrication of efficient electrodes for H2 evolution usually involves multiple © XXXX American Chemical Society

steps including precursor synthesis and high-temperature treatment, which increases the cost of hydrogen production.18,19 To understand the apparent reaction mechanism of HER, it is significative to investigate the kinetic parameters of electrochemistry such as the Tafel slope and exchange current density (J0) at a particular temperature and pressure. The HER mechanism determined by the Tafel slope involves the following three steps: Volmer: Tafel:

H+ + e− ↔ Had (discharge) 2Had ↔ H 2 (combination)

(1) (2)

Heyrovsky: H+ + e− + Had ↔ H 2 (ion + atom combination)

(3)

The Tafel slope is an inherent property of a catalyst, and it plays an important role in the rate-determining step through the whole HER process.20,21 According to previous reports, a Tafel slope of around 30 mV per decade indicates the limit of Received: August 18, 2017 Revised: October 18, 2017

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DOI: 10.1021/acs.jpcc.7b08283 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

100 mg of MWCNTs, 7 mL of 98% H2SO4, and 3 mL of 30% H2O2 into a 50 mL round-bottom flask. The mixture was stirred gently at 50 °C for 12 h and diluted with 200 mL of deionized (DI) water. The as-prepared MWCNTs were then collected by filtration and washed thoroughly with DI water and ethanol. The obtained black product was dried under vacuum overnight. Preparation of MoO2/MWCNTs Composite. In a typical procedure, 30 mg of Mo powder was added in 2 mL of 30% H2O2; then a mixture of 30 mL of ethanol and 50 mg of oxidized MWCNTs was added into the solution after ultrasonic dispersion. The resulting solution was stirred for 0.5 h, then transferred into a 50 mL Teflon-lined stainless steel autoclave, heated at 160 °C for 15 h, and then naturally cooled down to room temperature. The black MoO2/MWCNTs precipitate was collected by centrifugation, washed for several times with DI water, and dried at 60 °C for 12 h. Preparation of Pt−MoO2/MWCNTs Catalyst. The Pt− MoO2/MWCNTs catalyst was obtained using a photoreduction method. 50 mg of as-made MoO2/MWCNTs was dispersed in 50 mL of DI water; then a certain amount of K2PtCl6 (the nominal weight content of Pt: 1 wt %) was added into the solution. The mixture was irradiated under a UV light source (Mejiro Precision Inc., SHG-200, 260 W) for 4 h. The final product was collected by filtration, washed, and dried under vacuum for 12 h. Characterization. The powder X-ray diffraction (XRD) patterns of the products were recorded on a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ = 0.154 178 nm); the operation voltage was maintained at 40 kV and current at 200 mA. Transmission electron microscopic (TEM) images were recorded on a JEM-2100 electron microscope at an accelerating voltage of 200 kV. High-resolution TEM (HRTEM) images, scanning transmission electron microscopy (STEM), and energy-dispersive X-ray spectroscopy (EDX) elemental mapping analyses were taken on a FEI Talos F200X. The field emission scanning electron microscopy (FE-SEM) images were taken on a JEOL JSM-6700F SEM. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements were carried out on a PerkinElmer Optima 8000 ICP-AES/ICP-OES spectrometer. The X-ray photoelectron spectroscopy (XPS) measurements were performed in the National Synchrotron Radiation Laboratory (NSRL, Hefei, P. R. China) on a VG ESCALAB MK II X-ray photoelectron spectrometer with an exciting source of Mg Kα = 1253.6 eV. The binding energies obtained in the XPS spectral analysis were corrected by referencing C 1s to 284.5 eV. Electrochemical Measurements. 4 mg of the sample was dispersed in 1 mL of ethanol by sonication for 2 h, and then 10 μL (5 wt %) of Nafion solution was added to form a homogeneous slurry. Then, 20 μL of the slurry was loaded onto the surface of a round glassy carbon electrode (GCE, 5 mm in diameter) with a catalyst loading of 0.47 mg cm−2. 0.5 wt % Pt−MoO2/MWCNTs catalyst modified GCE was then dried at room temperature. For comparison, GCEs were also modified with other samples as well as commercial 20 wt % Pt/C under the same conditions. All electrochemical tests were performed using a CHI660E potentiostat (CH Instruments, China) in a standard three-electrode setup with a modified GCE as a working electrode, a Ag/AgCl electrode as a reference, and platinum wire as a counter electrode. The electrocatalytic activity of the samples toward the HER was examined by obtaining polarization curves using linear sweep voltammetry

reaction rate is the mechanism of the HER proceeds through the Volmer−Tafel mechanism in which the combination step. A Tafel slope of 40 mV per decade suggests that electrochemical hydrogen production follows the Volmer−Heyrovsky route, and the electrochemical desorption step is the ratelimiting step. Distinct from these, a Tafel slope of 120 mV per decade may result from various reaction pathways which depend on the surface coverage of adsorbed hydrogen.22,23 Conventionally, the rotating disk electrode (RDE) method is usually used to measure the electrochemical kinetic parameters of different crystalline and amorphous electrodes. Pt metal shows the highest HER activity among different monometallic systems in acidic medium.24 Several groups have reported that the HER exchange current density of Pt metal in acidic medium is on the order of 1 mA cm−2 measured by RDE method.25 Carbon materials such as graphene and carbon nanotubes (CNTs) are widely utilized in heterogeneous catalysis including organic synthesis and electrochemical reactions. The size, structure, morphology, and nature of carbon materials as conductive support materials for active transition metals generally boost the activities of these catalysts.26,27 The large surface area, outstanding electronic conductivity, and strong metal−support interactions (SMSI) are probably responsible for the significant improvement in the catalytic performances of supported catalysts.10 Surface functional groups,28 oxygen,29 carbon vacancies, and defects generated from partial oxidation of carbon materials may introduce chemically active sites and also act as anchoring sites for the deposition of metal nanoparticles.30,31 It is beneficial to prevent metal nanoparticles from aggregation, thus leading to highly dispersed, high density, ultrasmall metal nanoparticles on carbon support.32 Here, we report the design and synthesis of a highly active and stable HER electrocatalyst material consisting of ultralow amount of Pt loaded MoO2 nanoparticles on multiwalled carbon nanotubes (MWCNTs). The material architecture is built by a two-step chemical synthesis: First, strong interactions with CNTs and particle size control are established by the restricted growth of MoO2 nanoparticles on CNTs. Then, high catalytic activity for HER is achieved by decorating Pt clusters onto MoO2 nanoparticles via a simple redox reaction under UV irradiation. The unique material structure directly endows superior HER catalytic performance which is comparable to commercial 20 wt % Pt/C catalyst. In 0.5 M H2SO4, 0.5 wt % Pt−MoO2/MWCNTs hybrid catalyst exhibits a negligible onset overpotential and a Tafel slop of 43 mV per decade. At a mass loading of 0.47 mg cm−2, the obtained material requires overpotentials of only 60 mV and 84 mV to reach stable catalytic current densities of 10 and 20 mA cm−2, respectively. Our study demonstrates that the low-cost and easy-tofabricate Pt−MoO2/MWCNTs hybrid material is a promising candidate to be a highly efficient and durable electrochemical hydrogen evolution catalyst.

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EXPERIMENTAL SECTION Chemicals. Molybdenum powder (Mo, 99%), concentrated sulfuric acid (H2SO4, 98%), hydrogen peroxide (H2O2, 30%), and potassium hexachloroplatinate (K2PtCl6) were all purchased from Sinopharm Chemical Reagent Co. Ltd.; multiwalled carbon nanotubes (MWCNTs) were purchased from Aldrich. All reagents were used directly without further purification. Preparation of Surface Partially Oxidized MWCNTs. Surface partially oxidized MWCNTs were prepared by adding B


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The Journal of Physical Chemistry C with a scan rate of 5 mV s−1 in 0.5 M H2-saturated H2SO4 aqueous solution at room temperature. The stability of the Pt− MoO2/MWCNTs electrocatalyst was studied by a linear sweep voltammetry polarization curve initial and after 2000 cycles in 0.5 M H2SO4 at a scan rate of 5 mV s−1. The electrochemical impedance spectroscopy (EIS) measurements were carried out in the same configuration at an overpotential of 50 mV from 106 to 0.05 Hz with an ac voltage of 5 mV. Other samples were all tested under the same conditions. All the potentials reported in this work were against the reversible hydrogen electrode (RHE).

Pt loading does not influence the morphology of the composite. Energy dispersive spectrometer (EDS) analysis indicates that about 0.4 wt % Pt exists in the sample, which is in good agreement with the result of ICP-AES. Figure S1b shows TEM image of 0.5 wt % Pt directly reduced onto MWCNTs by UV irradiation; large Pt nanoparticles are clearly seen. From highresolution TEM (HRTEM) image (Figure 2c), it can be observed that 0.5 wt % Pt−MoO2/MWCNTs composite catalyst consists of MoO2 NPs with a clear lattice spacing of 0.350 nm for the (110) plane and that of 0.238 nm for (020) plane, in line with XRD result (Figure 1). By closer examination, no obvious Pt nanoparticles can be found. However, as shown in Figure 2d, high angle annular dark field scanning transmission electron microscopy (HAADFSTEM) image and corresponding energy dispersive X-ray (EDX) elemental mapping images reveal the homogeneous distribution of Pt atoms on MoO2 NPs, thus suggesting Pt most likely exists in the form of either nanoclusters or single atoms. From these results, it concluded that Pt atoms are intimately anchored on the surface of MoO2 NPs with strong metal and support interaction (SMSI), thus preventing the aggregation of Pt atoms, in contrast to 0.5 wt % Pt−MWCNTs sample (Figure S1b). Notably, the SMSI between Pt and MoO2 can modify the structural and electronic properties of Pt species; this contributes to the enhanced catalytic activity and durability of Pt. The atomic valence states and composition of MoO2/ MWCNTs and 0.5 wt % Pt−MoO2/MWCNTs samples were further characterized by X-ray photoelectron spectroscopy (XPS) (Figure 3 and Figure S2). As presented in Figure 3a, the asymmetric C 1s spectrum of 0.5 wt % Pt−MoO2/MWCNTs can be separated into four peaks at 284.6, 286.6, 287.7, and 289.0 eV, which are assigned to sp2-hybridized C, C in C−O bonds, carbonyl C, and carboxylate C (OC−O), respectively.33 The sp2-hybridized C group is the most common (79.1%) in the composite; other oxidation species such as the C−O functional group and carbonyl C have a low content (20.9%). Figure 3b exhibits the XPS spectrum of Pt 4f for 0.5 wt % Pt−MoO2/MWCNTs sample. Deconvolution of the Pt 4f region exhibits the presence of a pair of peaks. The peaks appearing at 72.2 eV (Pt 4f7/2) and 75.6 eV (Pt 4f5/2) are assigned to Pt2+, indicating strong Pt−O−Mo bond interactions,34 in line with HAADF-STEM results. Figures 3c and 3d display high-resolution XPS scan of Mo 3d regions of MoO2/ MWCNTs and 0.5 wt % Pt−MoO2/MWCNTs samples, respectively. As an intermediate valence compound, exposed surfaces of MoO2 are easily oxidized by oxygen, leading to a series of peaks in XPS spectrum.10 The two main peaks at 233.1 and 236.1 eV in the Mo 3d spectrum of MoO2/MWCNTs composite are characteristic of Mo(VI) due to the surface oxidation. The Mo 3d peaks at 230.4 and 232.8 eV suggest Mo(IV) species also exist, and peaks at 231.2 and 235.3 eV indicate Mo(V) also consist in MoO2/MWCNTs sample. However, after loading Pt on MoO2/MWCNTs by photodeposition, the peak of Mo(V) disappears. This phenomenon suggests that during the preparation process PtCl62− was reduced to Pt2+ by Mo5+ and Mo4+ in MoO2 semiconductor photoexcited by UV irradiation. The actual weight percentage of MoO2 in 0.5 wt % Pt−MoO2/MWCNTs was estimated to be ca. 20.6 wt % from XPS data. The electrocatalytic activity of hydrogen evolution reaction (HER) over 0.5 wt % Pt−MoO2/MWCNTs catalyst was assessed in 0.5 M H2SO4 aqueous solution on a glassy-carbon

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RESULTS AND DISCUSSION In this study, metallic MoO2 nanoparticles coating on multiwalled carbon nanotubes (MWCNTs) were first prepared by oxidizing metal molybdenum powder with hydrogen peroxide under solvothermal condition. 0.5 wt % Pt−MoO2/ MWCNTs hybrid catalyst was then obtained by a simple redox reaction between MoO2 and K2PtCl6 under UV irradiation (see Experimental Section for details). The collected black products can be well dispersed in water. X-ray diffraction (XRD) patterns of MoO2/MWCNTs composite and 0.5 wt % Pt−MoO2/MWCNTs catalyst are shown in Figure 1. The diffraction peaks of MoO2/MWCNTs

Figure 1. XRD patterns of (a) MoO2/MWCNTs sample and (b) 0.5 wt % Pt−MoO2/MWCNTs catalyst. Standard PDF card of the MoO2 (JCPDS No. 65-5787; space group: P21/c) was given as reference.

located at 26.2°, 36.8°, 41.7°, 53.9°, and 60.9° can be well assigned to the (110), (020), (120), (220), (031), and (311) planes of monoclinic MoO2 (JCPDS No. 65-5787; space group: P21/c), respectively. The absence of the peak around 26° for MWCNTs in the composite can be ascribed to overlap by the strong peaks of high crystalline MoO2 semiconductor. 0.5 wt % Pt−MoO2/MWCNTs sample exhibits a similar XRD pattern to MoO2/MWCNTs composite. The diffraction peaks of Pt could not be observed in the hybrid nanocatalyst because of its ultralow content (0.5 wt %) based on the inductively coupled plasma−atomic emission spectroscopy (ICP-AES) analysis. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images provided in Figure 2 reveal general microstructure and morphology of 0.5 wt % Pt−MoO2/ MWCNTs sample. Figures 2a and 2b show that carbon nanotubes are uniform with the diameter of around 40 nm and the length of micrometers. From Figure 2b it can be seen that MoO2 nanoparticles (NPs) have a size of about 5 nm and are well dispersed on the surface of MWCNTs. MoO2/MWCNTs sample exhibits a similar morphology (Figure S1a), indicating C


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Figure 2. (a) SEM image, (b) TEM image (inset, EDS spectrum), and (c) HRTEM image of as-made 0.5 wt % Pt−MoO2/MWCNTs catalyst. (d) HAADF-STEM image taken from 0.5 wt % Pt−MoO2/MWCNTs sample and corresponding elemental mappings of C, Mo, O, and Pt atoms.

Figure 3. XPS spectra of two samples. (a) C 1s spectrum; (b) Pt 4f scan of 0.5 wt % Pt−MoO2/MWCNTs sample; (c) Mo 3d spectrum of MoO2− MWCNTs sample; and (d) Mo 3d spectrum of 0.5 wt % Pt−MoO2/MWCNTs hybrid catalyst.

electrode (GCE) with a mass loading of about 0.47 mg cm−2. Figure 4a shows the polarization curves of the MoO2/ MWCNTs and 0.5 wt % Pt−MoO2/MWCNTs hybrid material in comparison with commercial 20 wt % Pt/C catalyst at a scan rate of 5 mV s−1. In spite of the low mass loading of 0.47 mg

cm−2, 0.5 wt % Pt−MoO2/MWCNTs electrode displays a near zero onset overpotential vs the reversible hydrogen electrode (RHE). The catalytic current density increased rapidly with further cathodic polarization to 10 and 20 mA cm−2 at overpotentials of 60 and 84 mV, respectively (Figure 4a). D


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Figure 4. Electrochemical measurements of the samples. (a) Polarization curves of MoO2/MWCNTs, 0.5 wt % Pt−MoO2/MWCNTs hybrid material and commercial 20 wt % Pt/C catalyst at the same mass loading of 0.47 mg cm−2. (b) Tafel slopes of 0.5 wt % Pt−MoO2/MWCNTs nanocatalyst and commercial 20 wt % Pt/C modified electrode.

potentials than commercial 20 wt % Pt/C in large-scale industrial applications. The interface reactions and electrode kinetics of the catalysts in the hydrogen generation can be further manifested by the electrochemical impedance spectroscopy (EIS) method.36 The Nyquist plots of four samples at −50 mV vs RHE are given in Figure 5, where the ohmic series resistance (Rs) is assigned to

However, MoO2/MWCNTs sample hardly exhibits any HER activity in the range of 0 to −0.4 V vs RHE under the same conditions. The polarization curve recorded for commercial 20 wt % Pt/C is also given in Figure 4a, which reveals a negligible onset potential vs RHE and achieves 10 mA cm−2 at a small overpotential of 39 mV, which is similar to previous reports.35 The huge improvement of HER performance of 0.5 wt % Pt− MoO2/MWCNTs catalyst indicates that a tiny quantity of Pt loadings exhibits highly efficient HER activity, which is comparable to commercial 20 wt % Pt/C. It is noted that the polarization curve of 0.5 wt % Pt loaded MWCNTs without MoO2 shows an overpotential less than 5 mV and a current density of 10 mA cm−2 at an overpotential of 68 mV (Figure S3a). This result indicates that with MoO2 functioning as binding agent the hydrogen evolution activity of Pt−MoO2/ MWCNTs catalyst boosts dramatically. The improvement can be contributed to the strong metal−support interaction, enhanced charge transfer between Pt and MoO2/MWCNTs, and the reduced size of active Pt species. A Tafel slope of 43 mV per decade is derived from the Tafel slope curve (Figure 4b), thus suggesting the HER mechanism of 0.5 wt % Pt−MoO2/MWCNTs catalyst follows a Volmer− Heyrovsky mechanism with a rate-limited step of electrochemical desorption.22 While commercial 20 wt % Pt/C shows a Tafel slope of 31 mV per decade, indicating a Volmer−Tafel mechanism and the combination step limits the generation rate of hydrogen.23 0.5 wt % Pt/MWCNTs catalyst displays a Tafel slope of 68 mV per decade (Figure S3b), demonstrating a Volmer−Heyrovsky process, and the rate-limiting step in the process might be the Volmer step.11 Another important kinetics parameter for electrocatalytic HER measurement is exchange current density (J0), which is associated with the rate of electron transfer under reversible conditions. Conventional ideal catalysts have low Tafel slopes and high exchange current densities (J0). Our obtained 0.5 wt % Pt−MoO2/MWCNTs catalyst has an exchange current density of 0.93 mA cm−2, which is comparable to that of commercial 20 wt % Pt/C (1.05 mA cm−2) (Figure 4b) but much higher than 0.5 wt % Pt− MWCNTs (0.54 mA cm−2) (Figure S3b). It is obviously seen that 0.5 wt % Pt−MoO2/MWCNTs hybrid catalyst exhibits higher catalytic activity than 0.5 wt % Pt−MWCNTs and is comparable to commercial 20 wt % Pt/C toward HER activity. Notably, by taking into consideration of ultralow Pt loadings, our 0.5 wt % Pt−MoO2/MWCNTs catalyst will find more

Figure 5. Electrochemical impedance spectra of 0.5 wt % Pt−MoO2/ MWCNTs sample, 0.5 wt % Pt−MWCNTs composite, MoO2/ MWCNTs composite, and commercial 20 wt % Pt/C at −50 mV vs RHE.

the intercept of the semicircle on the real axis, and the charge transfer resistance (Rct) was fitted by the semicircle of the Nyquist plot. The Rs value of four samples locates at about 9 ohm, indicating the total resistance of the circuit. As seen from Figure 5, 0.5 wt % Pt−MoO2/MWCNTs catalyst presents a lower Rct (22 ohm) than 0.5 wt % Pt/MWCNTs (35 ohm), MoO2/MWCNTs sample (>2000 ohm), and commercial 20 wt % Pt/C achieving the lowest Rct value of 12 ohm under the same conditions. These results are in line with the Tafel slopes. It has been well established that the Rct is related to the electrocatalytic kinetics and that a lower Rct value leads to a faster reaction rate. In this case, Pt−MoO2/MWCNTs hybrid catalyst exhibits both improved charge separation and the kinetics of charge transfer at the electrolyte interface as compared to Pt−MWCNTs; this results from good electrical conductivity of MoO2 metallic semiconductor and MWCNTs, E


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MWCNTs catalyst are accessible to the electrolyte. In that case, the densities of active sites of the commercial 20 wt % Pt/C were estimated to be about 3.44 × 1016 sites per cm2 based on EASA results. Accordingly, the normalized TOF values per Pt site of the catalysts were calculated and plotted against the overpotential, as shown in Figure 7. In particular, the TOF

the strong interaction between metallic Pt and MoO2 support, and ultrasmall active Pt catalyst. Another important criterion for a good electrocatalyst is high durability. To examine the stability of catalysts, accelerated linear potential sweeps were conducted continuously for 2000 cycles at 0.5 wt % Pt−MoO2/MWCNTs modified electrode. At the end of cycling, the catalyst exhibits approximate polarization curve (Figure 6). Although 0.5 wt % Pt−MWCNTs hybrid also

Figure 7. TOF plots of 0.5 wt % Pt−MoO2/MWCNTs composite, 0.5 wt % Pt−MWCNTs, and commercial 20 wt % Pt/C catalyst. Figure 6. Linear sweep voltammograms of 0.5 wt % Pt−MoO2/ MWCNTs modified electrode before and after 2000 cycles in 0.5 M H2SO4 aqueous solution at a scan rate of 5 mV s−1.

values of 0.5 wt % Pt−MoO2/MWCNTs are 2.8 and 11.5 s−1 at overpotentials of 50 and 100 mV, respectively. For direct comparison, Figure 7 also includes the normalized TOF values of commercial 20 wt % Pt/C and 0.5 wt % Pt−MWCNTs catalysts. The TOF values of 0.5 wt % Pt−MoO2/MWCNTs are much higher than those of 0.5 wt % Pt−MWCNTs and commercial 20 wt % Pt/C, indicating Pt clusters in 0.5 wt % Pt−MoO2/MWCNTs display a much higher HER activity than other Pt NPs including commercial 20 wt % Pt/C. The superior activity can be attributed to strong interactions between metal and MoO2 support, which not only prevents the agglomeration of Pt atoms38 but also accelerates the electron transfer due to its metallic MoO2 semiconductor.39

shows a satisfied HER activity, the long-term durability is less satisfactory. Figure S4 displays the polarization curve of 0.5 wt % Pt−MWCNTs for 2000 cycles; it is clearly seen that the current density significantly decreases. The excellent stability of 0.5 wt % Pt−MoO2/MWCNTs can be attributed to the reason that Pt atoms are tightly anchored on the surface of MoO2 via strong Pt−O−Mo bond interaction, consequently preventing the aggregation of Pt atoms. From TEM images shown in Figure S5, no obvious Pt particles are observed for 0.5 wt % Pt−MoO2/MWCNTs sample after 2000 cycles; however, large Pt particles are clearly found for 0.5 wt % Pt−MWCNTs and commercial 20 wt % Pt/C after 2000 cycles. Taken together, these results demonstrate that metallic MoO2 support plays a key role in boosting HER activity and durability. To further understand the enhanced HER performance of 0.5 wt % Pt−MoO2/MWCNTs catalyst, the electrochemically active surface area (EASA) was measured by calculating the underpotential deposition hydrogen (Hupd). As seen from Figure S6, cyclic voltammetry curves were tested to measure the Hupd, and the EASA of 0.5 wt % Pt−MoO2/MWCNTs sample and commercial 20 wt % Pt/C was calculated to be 19 and 87 cm2/mg of sample, respectively. This indicates under the same mass loading condition, the numbers of active sites in 0.5 wt % Pt−MoO2/MWCNTs are about 21% of those in 20 wt % Pt/C. By comparing the HER polarization curves of the 0.5 wt % Pt−MoO 2/MWCNTs and MoO 2 /MWCNTs catalysts in the range of 0 to −0.3 V vs RHE (Figure 4a), a sufficient improvement in HER activity after Pt loading is observed. Such observation clearly confirms the contribution of Pt atoms to the HER. After assigning the active sites of the catalysts to the Pt-containing centers, the normalized TOF values of per active site were estimated, which indicates the intrinsic per site activity of a catalyst and makes it competitive among different catalysts.37 We determined the upper limit of the active sites to be about 7.22 × 1015 sites per cm2, based on the hypothesis that all Pt species in 0.5 wt % Pt−MoO2/

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CONCLUSIONS In summary, we have developed a facile method for the synthesis of 0.5 wt % Pt−MoO2/MWCNTs hybrid catalyst for excellent hydrogen evolution reaction. Owing to the space confinement and good electric conductivity of MoO2, the superior HER catalytic performance of 0.5 wt % Pt−MoO2/ MWCNTs in acid medium and excellent long-term stability have been demonstrated. More importantly, the Pt atoms in 0.5 wt % Pt−MoO2/MWCNTs display a much higher HER activity than other Pt-based catalysts, including commercial 20 wt % Pt/C. A synergistic effect from strong metal−support interaction, increased electric conductivity, and active ultrasmall Pt catalyst has been well established, which is responsible for the superior catalytic performance with an ultralow content of Pt loadings. The excellent HER activity of ultralow Pt-loaded MoO2/MWCNTs composite catalyst with outstanding durability at a low mass loading makes it promising low-cost electrocatalysts for water splitting and other electrochemical applications.

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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/acs.jpcc.7b08283. F


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TEM images; XPS survey; polarization curve and Tafel slope of 0.5 wt % Pt/MWCNTs; linear sweep voltammograms of the 0.5 wt % Pt/MWCNTs modified electrode before and after 2000 cycles in 0.5 M H2SO4 aqueous solution; TEM images of catalysts after 2000 cycles; cyclic voltammetry curves for 0.5 wt % Pt− MoO2/MWCNTs hybrid material and commercial 20 wt % Pt/C; a set of EDS analysis and linear sweep voltammograms of Pt−MoO2/MWCNTs with different Pt content (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86 0551 63602346 (A.W.X.). ORCID

An-Wu Xu: 0000-0002-4950-0490 Author Contributions

X.X. and Y.-F.J. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 51572253, 21771171, and 51561135011), Scientific Research Grant of Hefei Science Center of CAS (Grant 2015SRG-HSC048), and cooperation between NSFC and Netherlands Organization for Scientific Research (Grant 51561135011).

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