Mo2C@C - ACS Publications - American Chemical Society

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MoP/Mo2C@C: A New Combination of Electrocatalysts for Highly Efficient Hydrogen Evolution over the Entire pH Range Lu-Nan Zhang,† Si-Heng Li,† Hua-Qiao Tan,*,† Shifa Ullah Khan,† Yuan-Yuan Ma,† Hong-Ying Zang,*,† Yong-Hui Wang,† and Yang-Guang Li*,† †

Key Laboratory of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, P.R. China S Supporting Information *

ABSTRACT: During the exploration of highly efficient noble-metalfree electrocatalysts for the hydrogen evolution reaction (HER), a promising and challenging strategy is to fabricate composite nanocatalysts by finely tuning metal and/or nonmetal element components. Herein, we report a new HER electrocatalyst, which is composed of molybdenum phosphide and molybdenum carbide composite nanoparticles (NPs) coated by few-layer N-doped graphitic carbon shells (denoted as MoP/Mo2C@C). Such a new combination mode of electrocatalysts is realized by a one-step annealing route with the mixture of a Mo/P-based polyoxometalate (POM) and dicyandiamide. On the basis of this method, the simultaneous phosphorization and carbonization in a nanoscale confined space can be easily achieved by the use of POM as the molecular-elementregulating platform. MoP/Mo2C@C exhibits more remarkable HER performance over the whole pH range than those of MoP, Mo2C, and the physical mixture of MoP and Mo2C. The low overpotentials of 89, 136, and 75 mV were obtained at a current density of 10 mA cm−2 in the media of pH = 0, 7, and 14, respectively. Furthermore, MoP/Mo2C@C shows a long-term durability for 14 h over the entire pH range (0−14). Because of the protection of carbon shells, such composite electrocatalyst also possesses better transition-metal tolerance exemplified by Fe2+, Co2+, and Ni2+ than that of 20% commercial Pt/C. This work demonstrates the advantage of POM precursors in adjusting the component and properties of nanoscale composite electrocatalysts for HER, which may suggest new options for the fabrication of highly efficient composite electrocatalysts. KEYWORDS: polyoxometalates, molybdenum phosphide, molybdenum carbide, electrocatalysis, hydrogen evolution reaction

1. INTRODUCTION With the continuous fossil fuel depletion and environmental contamination, hydrogen (H2) as a renewable, clean, and highenergy-density fuel has been an promising alternative energy resource.1,2 Water electrolysis is an economical method for the production of highly pure hydrogen.3−7 The hydrogen evolution reaction (HER) is a pivotal half reaction of the water-splitting reaction, which needs highly active electrocatalysts that can decrease the overpotential (η) to obtain high efficiency.8−11 Pt-based catalysts have been identified as the most effective HER electrocatalysts, which could produce large cathodic current densities at nearly zero overpotential.12−15 However, the low crustal abundance and high cost severely restrict their global-scale applications.16,17 Motivated by this challenge, enormous efforts have been devoted to searching low-cost and earth-abundant transition metal (TM)-based alternatives including carbides,18−23 phosphides,16,24−27 nitrides,28−31 and sulfides32−35 as well as a series of nonmetal N-doped carbon materials.20,36 Tuning chemical composition is a paramount way to fabricate effective HER electrocatalysts. Most of the current © 2017 American Chemical Society

work in this area has focused on the regulation of metal composition.37 A series of bimetallic hybrid electrocatalysts, such as Ni−Mo−S,38 Co−Mo−N,39 Co−Mo−P,3 and Fe− Co−P,40,41 have been explored. It is impressive that the synergistic effect between two metal components can tune the electronic properties of the composite materials.42 Another alternative strategy is the regulation of nonmetal composition such as N, C, P, and S. Especially considering the catalytically active metal phosphides43,44 and carbides,45,46 it is interesting to fabricate a new composite material composed of both metal phosphide and carbide with a synergistically enhanced electrocatalytic activity. However, such combination is still a realistic challenge because the simultaneous phosphorization and carbonization in a confined space is always difficult to carry on. Polyoxometalates (POMs), as one type of inorganic nanoscale metal-oxo clusters, are mainly composed of early transition metals (TMs) such as Mo, W, V, and Nb.47 Because Received: March 17, 2017 Accepted: April 26, 2017 Published: April 26, 2017 16270

DOI: 10.1021/acsami.7b03823 ACS Appl. Mater. Interfaces 2017, 9, 16270−16279

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ACS Applied Materials & Interfaces

2. EXPERIMENTAL SECTION

of their unique chemical composition and structural diversity, POMs can act as one of the ideal precursors for the preparation of small and uniform Mo/W-based HER electrocatalysts.48−52 Our previous work has proved that small-sized Mo/W carbides covered with few-layer graphitic carbon shells can be easily obtained by using POM precursors.48,53 Furthermore, a considerable number of POMs contain multiple nonmetals such as P, C, and S,47 meaning that POMs can act as not only the single Mo/W sources, but also the potential nonmetal sources. Therefore, we attempt to utilize POMs as a molecularelement-regulating platform to design and fabricate new composite HER electrocatalysts composed of both metal carbide and phosphide. Herein, we report the first composite HER electrocatalyst which is composed of molybdenum phosphide and molybdenum carbide nanoparticles (NPs) coated by few-layer N-doped graphitic carbon shells (denoted as MoP/Mo2C@C). MoP/ Mo2C@C is prepared by annealing the mixture of Mo/P-based POM (NH4)6{Mo2VO4[(Mo2VIO6)NH3CH2CH2 CH2C(O) (PO3)2]2}·10H2O {abbr. P4Mo6} (Figure S1a)54 and dicyandiamide (DCA) with a large-scale yield (Scheme 1). Such

2.1. Chemicals and Reagents. All chemicals were purchased and used as received without further purification. Nafion solution (5 wt %) was purchased from Alfa Aesar. Platinum on graphitized carbon (20 wt % Pt/C) were purchased from Aldrich. Hydrazine monohydrate was purchased from Aladdin, dicyandiamide (DCA), (NH4)6Mo7O24· 4H2O, and alendronic acid were purchased from Sigma-Aldrich. The water used throughout all experiments was purified through a Millipore system. P4Mo6 was synthesized according to a method previously described by the Wang’s group54 and characterized by powder X-ray diffraction (PXRD) (Figure S1b). 2.2. Preparation of MoP/Mo2C@C. The preparation process for different samples was basically the same. In a typical preparation of MoP/Mo2C@C, 0.2 g of P4Mo6 and 0.4 g of DCA were dissolved in deionized water at 100 °C under magnetic stirring until a transparent solution was formed. The solution was heated to boiling until dry. Then the as-prepared powder was placed in a porcelain boat and heated to 500 °C in a tube furnace for 30 min at a heating rate of 2 °C min−1. The temperature in the furnace is further raised to 800 °C at a ramp rate of 5 °C min−1 and kept for 6 h. The furnace is cooled to room temperature subsequently. During the pyrolysis process, the furnace is under N2 flow. Then, MoP/Mo2C@C is obtained as black powder form (which is also denoted as S-800-1/2 in the Supporting Information). On the basis of this method, a series of samples were prepared by identical condition except that the mass ratio of P4Mo6 and DCA is 1:1, 1:3, and 1:4 denoted as S-800-1/1, S-800-1/3, and S-800-1/4, respectively. Likewise, several additional samples were also prepared by similar condition except that the annealed temperature is 700, 750, 850, and 900 °C, denoted as S-700-1/2 and S-750-1/2, S-850-1/2, and S-900-1/2, respectively. 2.3. Preparation of MoP/MoCx Aggregate. For comparison, the P4Mo6 precursor was also directly annealed without adding DCA under the same condition. The obtained sample was labeled as MoP/ MoCx. 2.4. Preparation of MoP/Mo2C@C′. MoP/Mo2C@C′ was prepared according to the same method of MoP/Mo2C@C except that DCA was replaced by glucose, which contains no N element. 2.5. Preparation of Mo2C and MoP. Mo2C was prepared according to the same method of MoP/Mo2C@C except that P4Mo6 was replaced by (NH4)6Mo7O24·4H2O. MoP was fabricated by the calcination of a mixture of (NH4)6Mo7O24·4H2O, DCA and phosphoric acid with the mass ratio of 1:2:7 at 800 °C for 6 h under a nitrogen atmosphere. In addition, the physical mixture of MoP and Mo2C (denoted as MoP/Mo2C-mix) was used as a control sample by mixing MoP and Mo2C uniformly. 2.6. Preparation of the Working Electrodes. The working electrode was fabricated as follows: 4 mg of catalyst was dispersed in 500 μL of 0.5 wt % Nafion solution. After ultrasonication for 1 h, 4 μL of the homogeneous ink was drop-casted onto a glassy carbon electrode (GCE) with a diameter of 3 mm. The loading of catalyst is about 0.453 mg cm−2. The electrode was then dried in air. 2.7. Preparation of Electrolytes. 0.5 M H2SO4 (pH = 0.30), 0.05 M H2SO4 + 0.45 M Na2SO4 (pH = 1.35), 5 mM H2SO4 + 0.49 M Na2SO4 (pH = 2.40), 0.5 mM H2SO4 + 0.49 M Na2SO4 (pH = 3.39), 0.05 mM H2SO4 + 0.49 M Na2SO4 (pH = 4.44), 5 μM H2SO4 + 0.49 M Na2SO4 (pH = 5.30), 0.5 μM H2SO4 + 0.49 M Na2SO4 (pH = 6.11), 0.5 M Na2SO4 (pH = 6.67), 1 M KOH (pH = 14.00), 0.1 M KOH + 0.6 M K2SO4 (pH = 13.3), 0.01 M KOH + 0.66 M K2SO4 (pH = 12.05), 1 mM KOH + 0.66 M K2SO4 (pH = 11.42), 0.1 mM KOH + 0.66 M K2SO4 (pH = 9.95), 0.01 mM KOH + 0.66 M K2SO4 (pH = 9.10), 1 μM KOH + 0.66 M K2SO4 (pH = 8.00). One M PBS was prepared by dissolving 13.61 g KH2PO4 in 100 mL deionized water, and the pH of the mixture was adjusted to 7.00 with 1 M KOH.

Scheme 1. Illustration of the Preparation of MoP/Mo2C@ Ca

a The precursor P4Mo6 is well dispersed by DCA, then the as-prepared solid is annealed in N2, leading to final MoP/Mo2C@C composite materials. DCA: dicyandiamide; P4Mo6: (NH4)6{Mo2VO4[(Mo2VIO6)NH3CH2CH2 CH2C(O) (PO3)2]2}·10H2O. Blue and yellow shells represent the N-doped carbon layers. The central core with red and blue colors represent MoP/Mo2C composite nanoparticles.

composite electrocatalyst exhibits more excellent HER performance over the whole pH range of 0−14 than those of MoP, Mo2C, and the physical mixture of MoP and Mo2C. The low overpotentials of 89, 136, and 75 mV were achieved at a current density of 10 mA cm−2 in the pH of 0, 7, and 14, respectively. Furthermore, MoP/Mo2C@C shows a long-term durability for 14 h over the whole pH range (0−14). Moreover, such composite electrocatalyst also possesses better transitionmetal-tolerance exemplified by Fe2+, Co2+, and Ni2+ than that of 20% commercial Pt/C.

3. RESULTS AND DISCUSSION 3.1. Preparation of MoP/Mo2C@C. In the preparation of MoP/Mo2C@C, a Mo/P-based POM (P4Mo6) was chosen as 16271


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Figure 1. (a) TEM images of MoP/Mo2C@C; (b,c) HRTEM images of MoP/Mo2C@C; (d−g) corresponding TEM elemental mapping of P (e), Mo (f), and C (g) in MoP/Mo2C@C; (h) Powder XRD patterns of MoP/Mo2C@C, and the standard MoP and Mo2C; (i) Raman spectra of MoP/ Mo2C@C. The ID/IG of MoP/Mo2C@C is 1.89; (j) EDX spectra of MoP/Mo2C@C.

shown in Figure 1c, high-resolution TEM (HRTEM) image of MoP/Mo2C@C shows that MoP/Mo2C NPs are coated by few-layer graphitic carbon shells. The lattice plane distance of the MoP is 0.21 nm, which is consistent with the lattice plane (101) of MoP. The lattice fringe with interplanar distance of 0.23 nm is associated with the (002) crystallographic planes of Mo2C. Moreover, a lattice spacing of ca. 0.34 nm observed in Figure 1c corresponds to the typical layer spacing of highquality graphitic carbon. Additional SEM images of MoP/ Mo2C@C were shown in Figure S2. It is worth mentioning that the graphitic carbon layers dramatically restrain the agglomeration of MoP/Mo2C NPs, because the direct annealing of P4Mo6 precursor just led to hybrid NPs aggregated together (Figure S3). Elemental mapping exhibits the elemental distribution of P, Mo, C in MoP/Mo2C@C (Figure1d−g), revealing that MoP/Mo2C particles reside in the carbon matrix. Figure 1h shows the powder X-ray diffraction (PXRD) pattern of MoP/Mo2C@C. The PXRD peaks can be attributed to the mixture of MoP (JCPDS, No. 24-0771) and Mo2C (JCPDS, No. 45-1013). The peaks located at 32.17°, 43.14°, and 57.48° are clearly observed, indexing to (100), (101), and (110) facets of MoP (JCPDS, No. 24-0771), respectively. There are also four characteristic peaks located at 34.40°, 38.12°, 39.47°, and 41.58°, which are attributed to (400), (002), (401), and (231) facets of Mo2C (JCPDS, NO. 451013), respectively. The additional peak observed at 24° should be ascribed to graphitic carbon. Besides, the degree of

the precursor. Such a diphosphonate-functionalized POM (Figure S1a) consists of P and Mo elements with a ratio of 4:6, which may furnish substantial P and Mo sources to produce MoP and Mo2C during the annealing process. Before heating at 800 °C under N2 atmosphere, the precursors P4Mo6 and DCA with the mass ratio of 1:2 were well mixed by dissolving in water first and then dried by evaporation. Such pretreatment solid can provide a simultaneous phosphorization and carbonization in a nanoscale confined space during the annealing treatment, leading to the formation of uniform composite MoP/Mo2C NPs. The use of DCA is crucial in the fabrication of MoP/Mo2C@C, because DCA can not only disperse the POM units but also in situ generate the graphitic carbon species coupled with MoP/Mo2C NPs. The formation of graphitic carbon shells can prevent the coalescence of composite MoP/Mo2C NPs and protect the catalyst from etching, thus improving the electrocatalytic stability over the whole pH range and the TMs-tolerance.48 Meanwhile, the introduction of DCA facilitates the deoxygenation of P4Mo6 precursor and provides enough carbon source. The synthetic strategy for MoP/Mo2C@C is shown in Scheme 1. 3.2. Characterization of MoP/Mo2C@C. Figure 1a,b show the transmission electron microscopy (TEM) of MoP/Mo2C@ C, indicating that the superstructure of catalyst is assembled from the composite MoP/Mo2C@C NPs, and these NPs possess a relatively narrow size distribution (in the range of ca. 4−10 nm) with an average particle diameter of ca.7.5 nm. As 16272


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Figure 2. XPS high-resolution scans of (a) C 1s, (b) P 2p, (c) Mo 3d, and (d) N 1s electrons of MoP/Mo2C@C.

confirm the heteroatom N-doping in the graphitic carbon shells. The specific surface area of the obtained electrocatalyst was measured using nitrogen sorption technique (Figure S4). The Brunauer−Emmett−Teller (BET) surface area of MoP/ Mo2C@C is 136 m2 g−1. The N2 sorption isotherm of catalyst exhibits a typical IV hysteresis loop, indicating that the catalyst possesses a mesoporous structure. Such a structural feature may expose more active sites and facilitate the penetration of electrolyte.18 3.3. HER Performance of MoP/Mo2C@C. The HER electrocatalytic activity of MoP/Mo2C@C was first investigated by linear sweep voltammetry (LSV) in acidic (0.5 M H2SO4) aqueous solution. Simultaneously, the HER activities of the commercial Pt/C (20 wt % Pt on carbon black), bare glassy carbon electrode (Bare GCE), MoP, Mo2C, and the physical mixture of MoP and Mo2C (labeled as MoP/Mo2C-mix) were also measured under the same conditions (Figure 3a). All the tested samples were deposited on a glassy carbon electrode with the same loading of 0.453 mg cm−2. As shown in Figure 3a, the Pt/C catalyst exhibits active performance toward HER with a nearly zero onset potential and a high current density as expected, but the bare glassy carbon electrode possesses very poor catalytic activity. Impressively, MoP/Mo2C@C shows dramatically enhanced performance compared with MoP, Mo2C, and the physical mixture of MoP and Mo2C, implying a synergistic effect between MoP and Mo2C in MoP/Mo2C@ C. The overpotential (η) at j = 10 mA cm−2 for the MoP/ Mo2C@C is 89 mV (after iR correction), which is lower than that observed on the other contrast samples and most of the recently reported Mo-based non-noble metal HER electrocatalysts (Table S2). PXRD patterns of MoP and Mo2C are shown in Figure S5. In order to optimize the experiment, a series of catalysts prepared at various temperatures (700, 750,

graphitization of MoP/Mo2C@C is further confirmed by Raman spectrum (Figure 1i). The two peaks at 1350 and 1580 cm−1 correspond to the D and G bands of the graphitic carbon. The value of ID/IG was 1.89, implying the partial graphitization, which can endow the good electron transferring for the hybrid catalyst.55 EDX results also confirmed the composition of MoP/Mo2C@C (Figure 1j and Table S1). To further determine the elemental compositions and valence states of MoP/Mo2C@C, X-ray photoelectron spectroscopy (XPS) was performed. As depicted in Figure 2, the elements of P, Mo, C, and N can be obviously identified. The high-resolution C 1s spectrum (Figure 2a) can be fitted into four different signals at 284.47, 285.07, 286.27, and 289.64 eV, which are attributed to CC, C−P, C−N, and O−CO, respectively.28,56,57 The existence of C−N demonstrates the heteroatom N-doping in the graphitic carbon shells, and the C−P might originate from MoP bonded to Mo2C or the graphitic carbon shell. The doublet in the P 2p spectrum (129.40, 130.30 eV) (Figure 2b) can be assigned to P bonded to Mo in the molybdenum phosphide.58 The peak at 133.40 eV is ascribed to P−C,57 further indicating the connection between MoP and Mo2C or carbon layer. An additional broad peak at 134.10 eV is assigned to the surface P−O species,57 which is due to the exposure of the catalyst to air. The peak fitting of the Mo 3d region (Figure 2c) suggests that there are three oxidation states (+δ, + 4, and +6) for Mo element in MoP/ Mo2C@C. The doublet 228.10 and 231.20 eV can be attributed to Moδ+ (0 ≤ δ ≤ 4) species, confirming the existence of MoP and Mo2C.59 The high oxidation state of Mo4+ (228.8 and 231.6 eV) and Mo6+ (233.00 and 236.11 eV) in MoP/Mo2C@ C may arise from the surface oxidation due to air contact.60 High-resolution N 1s spectrum (Figure 2d) reveals that the existence of the pyridinic N (398.20 eV), pyrrolic N (399.20 eV), and graphitic N (401.58 eV).61 This results further 16273


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Figure 3. (a) Polarization curves of MoP/Mo2C@C, MoP, Mo2C, physical mixture of MoP and Mo2C(MoP/Mo2C-mix), Pt/C and bare GCE in 0.5 M H2SO4; (b) Tafel plots of MoP/Mo2C@C, MoP, Mo2C, MoP/Mo2C-mix and Pt/C in 0.5 M H2SO4; (c) CVs of MoP/Mo2C@C with different rates from 25 to 200 mV s−1 in 0.5 M H2SO4; (d) The capacitive current at 0.25 V as a function of scan rate for MoP/Mo2C@C in 0.5 M H2SO4; (e) Nyquist plots of electrochemical impedance spectra (EIS) of MoP/Mo2C@C recorded in 0.5 M H2SO4. Inset: Two-time-constant model equivalent circuit used for data fitting of EIS spectra (Rs is the overall series resistance, CPE1 and CPE2 are the constant-phase element and resistance related to surface porosity. Rp and Rct are the charge-transfer resistance related to HER process); (f) Polarization curves of MoP/Mo2C@C initially and after 1000 cycles in 0.5 M H2SO4. Inset: Time-dependent current density curve of MoP/Mo2C@C under a static overpotential of 75 mV for 14 h.

800, 850, and 900 °C) and with different weight ratio of starting materials (1:1, 1:2, 1:3, 1:4 wt) have also been tested by LSV measurements (Figure S6). The optimal annealing temperature is 800 °C, and the weight ratio (P4Mo6/DCA wt) is 1:2. PXRD patterns of these a series of control samples are shown in Figure S7. The HER performance of MoP/Mo2C@C electrocatalyst is also compared with those of P4Mo6 precursor and the contrast sample obtained by directly annealing P4Mo6 without the addition of DCA. As shown in Figure S8, the HER activity of P4Mo6 precursor is very poor. While the P4Mo6 annealed without DCA shows an overpotential of 320 mV at a current density of 10 mA cm−2, which is much larger than the one of

MoP/Mo2C@C (89 mV). This result suggests that the fewlayer graphitic carbon shells play important roles in promoting the electrocatalytic activity of MoP/Mo2C@C for HER. For comparison, another contrast sample was also prepared by annealing the mixture of P4Mo6 and glucose (without N element) (denoted by MoP/Mo2C@C′). The corresponding PXRD pattern of MoP/Mo2C@C′ is shown in Figure S10. The HER performance of MoP/Mo2C@C′ shows obviously inferior activity in contrast to MoP/Mo2C@C (Figure S9). The result indicates that the N dopant on the carbon layer can efficiently improve the HER activity of the electrocatalyst. The Tafel slope is an inherent parameter of electrocatalysts determined by the mechanism, and a small Tafel slope leads to 16274


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Figure 4. (a) HER polarization curves of MoP/Mo2C@C in 0.5 M H2SO4 with 10 mM NiSO4; (b) HER polarization of 20% Pt/C in 0.5 M H2SO4 with 10 mM NiSO4; (c) HER polarization curves of MoP/Mo2C@C in 0.5 M H2SO4 with 10 mM CoSO4; (d) HER polarization of 20% Pt/C in 0.5 M H2SO4 with 10 mM CoSO4; (e) HER polarization curves of MoP/Mo2C@C in 0.5 M H2SO4 with 10 mM FeSO4; (f) HER polarization of 20% Pt/C in 0.5 M H2SO4 with 10 mM FeSO4.

good reaction kinetics for HER.62 In order to indicate the kinetics of HER processes promoted by as-prepared catalysts, the Tafel slopes of Pt/C, MoP, Mo2C, physical mixture of MoP and Mo2C, and MoP/Mo2C@C in 0.5 M H2SO4 have been obtained by linear fitting of the polarization curves (Figure 3b) according to the Tafel equation η = b log j + a, where b is the Tafel slope and j is the current density. Commercial Pt/C shows the Tafel slope of 32 mV dec−1 in 0.5 M H2SO4, which is in agreement with the reported results, proving the validity of our electrochemical measurements. The Tafel slope of MoP/ Mo2C@C obtained from the Tafel plots is 45 mV dec−1, suggesting that the release of molecular hydrogen is the ratedetermining step. This hydrogen evolution process follows the Volmer−Heyrovsky mechanism (slope between 40 and 120 mV dec−1).28 The exchange current density (j0) was gained by extrapolating the Tafel plot. The calculated j0 of MoP/Mo2C@ C was 0.215 mA cm−2. These results suggest that MoP/ Mo2C@C possesses favorable HER activity with low over-

potential, small Tafel slope, and high exchange current density in acidic media. To evaluate the effective active surface area of the solid− liquid interface for MoP/Mo2C@C, the electrochemical double-layer capacitance (Cdl) was carried out using a simple cyclic voltammetry (CV) method (Figure 3c). The CV curve was performed at various scan rates (25, 50, 75, 100, 125, 150, 175, and 200 mV s−1), and the potential range of 0.16−0.36 V (vs RHE) in acidic electrolyte was selected owing to negligible Faradaic current features in this region. The value of Cdl is estimated by plotting the ΔJ(Ja − Jc) at 0.25 V (vs RHE) against the scan rate (Figure 3d), where the slope is twice Cdl.63 The Cdl was calculated to be 80 mF cm−2. Such high Cdl value indicates the highly exposed active sites, which can promote the electrochemical process. For comparison, the effective active surface area of MoP/Mo2C@C and MoP/Mo2C-mix were also measured by cyclic voltammetry (CV) method. MoP/Mo2C@ C possesses a CV loop with a larger area at a scan rate of 150 16275


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Figure 5. (a) Polarization curves of MoP/Mo2C@C, Pt/C and bare GCE in 1 M KOH. Inset: Tafel plots of Pt/C, MoP/Mo2C@C; (b) Polarization curves of MoP/Mo2C@C initially and after 1000 cycles in 1 M KOH. Inset: Time-dependent current density curve of MoP/Mo2C@C under a static overpotential of 60 mV for 14 h; (c) Polarization curves of MoP/Mo2C@C, Pt/C, and bare GCE in 1 M PBS. Inset: Tafel plots of Pt/C, MoP/ Mo2C@C; (d) Polarization curves of MoP/Mo2C@C initially and after 1000 cycles in 1 M PBS. Inset: Time-dependent current density curve of MoP/Mo2C@C under a static overpotential of 130 mV for 14 h.

mV s−1 than that of physical mixture of MoP and Mo2C (Figure S11). Electroconductivity is another crucial parameter for a HER electrocatalyst, and electrochemical impedance spectroscopy (EIS) measurements at overpotentials from 50 to 250 mV in 0.5 M H2SO4 were performed to evaluate such property. The Nyquist plots of the EIS response are shown in Figure 3e. The semicircles at low frequency region sharply decrease with the increasing overpotentials, reflecting the charge transfer resistances (Rct). Furthermore, the data were fitted to a classical two time constants circuit and the resultant parameters are listed in Table S3.61 Generally speaking, MoP/Mo2C@C exhibits a small charge transfer impedance (Rct = 7.08 Ω) at overpotential of 250 mV, implying good electron transport ability for HER, which might be related to the graphitic carbon shells that decrease charge-transfer resistance at the catalyst/ electrolyte interface and increase the electrochemical conductivity. Durability is another key factor in practical applications. The long-term cycling test of MoP/Mo2C@C in acidic media was probed by measuring 1000 continuous cyclic voltammetry sweeps between +0.2 V and −0.2 V (vs RHE) at 100 mV s−1 in 0.5 M H2SO4. As shown in Figure 3f, the MoP/Mo2C@C catalyst exhibits no measurable loss of HER activity after 1000 sweeps at the current density of 10 mA cm −2. The chronoamperometric curve for MoP/Mo2C@C (insert Figure 3f) suggests that such nanoscale composite electrocatalyst maintains its current density for at least 14 h. Both results demonstrate that MoP/Mo2C@C has superior stability in a

long-term electrochemical process under strongly acidic conditions. The stability of MoP/Mo2C@C electrocatalyst was further confirmed by exploring the influence of TMs ions on the electrocatalytic activity. Figure 4a,c,e show that the HER activity of MoP/Mo2C@C catalyst almost remains unchanged in the presence of TM ions (0.5 M H2SO4 with 10 mM NiSO4, CoSO4, FeSO4). While the activity of 20% Pt/C has partial reduction after 3 cycles in 0.5 M H2SO4 with the same TM ions (Figure 4b,d,f). This fact demonstrates that MoP/Mo2C@C exhibits better TMs-tolerance than that of commercial Pt/C. The remarkable stability of the catalyst may also be attributed to the graphitic carbon shells on the surface of the nanoparticles, which efficiently prevent the etching and agglomeration of MoP/Mo2C cores during the HER. The electrocatalytic activities of MoP/Mo2C@C for HER over the all pH range were also evaluated. Notably, this composite electrocatalyst requires 75 mV for a current density of 10 mA cm−2 in pH = 14 aqueous solution (1 M KOH) (Figure 5a). This is one of the best results in currently reported Mo-based catalysts for HER in alkaline media (Table S4). More interestingly, MoP/Mo2C@C even gives higher current density than the one of Pt/C at high overpotential (>240 mV). The Tafel slope of MoP/Mo2C@C is 58 mV dec−1 in pH = 14 aqueous media, suggesting that the electrocatalyst follows the Volmer−Heyrovsky mechanism. Furthermore, MoP/Mo2C@C retains a stable electrocatalytic property in 1 M KOH electrolyte during the long-term electrochemical process (Figure 5b). All the results prove that MoP/Mo2C@C possesses superior HER activity in alkaline media. In the 16276


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ACS Applied Materials & Interfaces neutral media (phosphate buffered saline, 1 M PBS), MoP/ Mo2C@C requires 136 mV to drive a current density of 10 mA cm−2, and the Tafel slope is 93 mV dec−1 (Figure 5c). In the neutral solution, MoP/Mo2C@C also keeps favorable longtime stability as shown in Figure 5d. Finally, the overpotentials (j = 10 mA cm−2) and Tafel slopes of MoP/Mo2C@C in electrolytes with different pH values (1−13) were determined one-by-one (Figure S12−S14), and all these results are listed in Table S5. These comparatively small overpotentials and Tafel slopes demonstrate that MoP/Mo2C@C is a favorable electrocatalyst over the whole pH range. The above measurements confirm the promising HER performance of MoP/Mo2C@C over the entire pH range. Such remarkable electrocatalytic properties may be attributed to the following reasons: (i) The regulation of nonmetal composition to obtain composite MoP/Mo2C NPs can tune the intrinsic electronic properties of the Mo-based electrocatalysts and improve its HER activity; (ii) The synergistic effect between composite MoP/Mo2C NPs and graphitic carbon shells further enhances the HER performance; (iii) The graphitic carbon coat may not only improve the electroconductivity of the composite catalyst but also prevent the corrosion of MoP/Mo2C NPs during the electrocatalytic process; (iv) The presence of N dopants also increases the electron density in the graphitic carbon shells, prompting the HER activity.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yang-Guang Li: 0000-0002-9696-8192 Author Contributions

L.-N. Zhang and S.-H. Li contributed equally. 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.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (grant no. 21671036, 21401131, and 21301166), Fundamental Research Funds for the Central Universities (grant no. 2412016KJ018) and the Opening Project of Key Laboratory of Polyoxometalate Science of Ministry of Education (grant no. 130014556) for their financial support.

4. CONCLUSIONS In summary, we achieve the regulation of nonmetal composition in the Mo-based HER electrocatalyst and report the first composite molybdenum phosphide and carbide nanoparticles coated by few-layer N-doped graphitic carbon shells (MoP/Mo2C@C). Such hybrid material is prepared from the mixture of Mo/P-based POM and DCA by virtue of a facile annealing process with a large scale of yield. MoP/Mo2C@C exhibits highly efficient electrocatalytic activity and long-term durability for HER over the entire pH range, which can be attributed to the synergistic effects among highly dispersive nanoscale MoP/Mo2C NPs, graphitic carbon shells, and the Ndopants. This work may suggest a feasible route to design efficient Mo/W-based HER electrocatalysts by modulating the nonmetal composition exemplified by C, P, and N based on the POMs as the molecular element-regulating platform.

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MoP/Mo2C-mix, the values of Rct and Rs for MoP/ Mo2C@C with overpotential from 50 to 250 mV in 0.5 M H2SO4, comparison of HER performance in alkaline media for MoP/Mo2C@C for other HER electrocatalysts, the HER polarization plots and Tafel plots of MoP/Mo2C@C in pH = 1−14 one by one, comparison of catalytic parameters of MoP/Mo2C@C in different electrolytes (PDF)

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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/acsami.7b03823. Physical characterization, electrochemical measurements, the polyhedral and ball-and-stick representation of polyoxoanion in P4Mo6 precursor, the PXRD patterns of P4Mo6, SEM images of MoP/Mo2C@C, TEM image of P4Mo6 annealed without DCA (MoP/MoCx), EDX data for MoP/Mo2C@C, N2 sorption isotherm of MoP/ Mo2C@C, comparison of HER performance in acidic media for MoP/Mo2C@C with other HER electrocatalysts, PXRD patterns of MoP and Mo2C, polarization curves of control samples (S-700-1/2, S-800-1/2, S-9001/2, S-800-1/1, S-800-1/3, P4Mo6 precursor, MoP/ MoCx, MoP/Mo2C@C′), PXRD patterns of control samples (S-700-1/2, S-800-1/2, S-900-1/2, S-800-1/1, S800-1/3, MoP/Mo2C@C′), CVs of MoP/Mo2C@C and 16277


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