Molybdenum Carbide-Embedded Nitrogen-Doped Porous Carbon

Molybdenum Carbide-Embedded NitrogenDoped Porous Carbon Nanosheets as Electrocatalysts for Water Splitting in Alkaline Media ́ Chenbao Lu,†,‡ Diana Tranca,§ Jian Zhang,§ Fermıń Rodrıguez Hernández,§,⊥,# Yuezeng Su,‡ ,† ,† § Xiaodong Zhuang,* Fan Zhang,* Gotthard Seifert, and Xinliang Feng*,†,§ †

School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, 200240 Shanghai, P. R. China ‡ School of Electronic Information and Electrical Engineering, School of Aeronautics and Astronautics, Shanghai Jiao Tong University, 200240 Shanghai, P. R. China § Center for Advancing Electronics Dresden (cfaed) & Department of Chemistry and Food Chemistry, Technische Universität Dresden, Mommsenstrasse 4, 01062 Dresden, Germany ⊥ Theoretical Chemistry, Technische Universität Dresden, Mommsenstrasse 13, 01062 Dresden, Germany # DynAMoS (Dynamical processes in Atomic and Molecular Systems), Facultad de Fı ́sica, Universidad de La Habana, San Lázaro y L, 10040 La Habana, Cuba S Supporting Information *

ABSTRACT: Molybdenum carbide (Mo2C) based catalysts were found to be one of the most promising electrocatalysts for hydrogen evolution reaction (HER) in acid media in comparison with Pt-based catalysts but were seldom investigated in alkaline media, probably due to the limited active sites, poor conductivity, and high energy barrier for water dissociation. In this work, Mo2C-embedded nitrogendoped porous carbon nanosheets (Mo2C@2D-NPCs) were successfully achieved with the help of a convenient interfacial strategy. As a HER electrocatalyst in alkaline solution, Mo2C@2D-NPC exhibited an extremely low onset potential of ∼0 mV and a current density of 10 mA cm−2 at an overpotential of ∼45 mV, which is much lower than the values of most reported HER electrocatalysts and comparable to the noble metal catalyst Pt. In addition, the Tafel slope and the exchange current density of Mo2C@2D-NPC were 46 mV decade−1 and 1.14 × 10−3 A cm−2, respectively, outperforming the state-of-the-art metal-carbide-based electrocatalysts in alkaline media. Such excellent HER activity was attributed to the rich Mo2C/NPC heterostructures and synergistic contribution of nitrogen doping, outstanding conductivity of graphene, and abundant active sites at the heterostructures. KEYWORDS: hydrogen evolution reaction, alkaline media, molybdenum carbide, interface, N-doped carbon nanosheet implementation of water splitting technologies.4 Although platinum-based electrocatalysts have been proven to be the most active and stable HER electrocatalysts, widespread applications of Pt are critically hampered by its scarcity and high cost.5 Therefore, development of high-performance costeffective HER electrocatalysts based on earth-abundant elements is highly imperative.

B

ecause of growing concerns about environmental pollution and energy crises, considerable efforts have been devoted to exploiting clean and sustainable energy sources and carriers. Hydrogen (H2) is an abundant and renewable clean fuel that is regarded as a promising energy carrier to replace fossil fuels in the future.1 Electrocatalytic water splitting to produce H2 seems to be one of the cleanest and most sustainable methods for large-scale H2 production.2 The water splitting reaction is divided into two half-reactions: the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).3 The development of efficient HER electrocatalysts with a low overpotential is crucial for the successful © 2017 American Chemical Society

Received: January 17, 2017 Accepted: March 14, 2017 Published: March 14, 2017 3933

DOI: 10.1021/acsnano.7b00365 ACS Nano 2017, 11, 3933−3942

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ACS Nano To date, numerous inorganic materials containing nonprecious transition metals (Fe, Co, Ni, Mo, and W), including phosphides,6−8 sulfides,9−13 carbides,14−17 and other materials,18−22 have been widely explored to replace Pt as HER electrocatalysts in acid media. However, as we know, highly efficient HER electrocatalysts in alkaline solutions are still rarely documented. So far, molybdenum-contained materials, such as molybdenum boride (MoB), 23 molybdenum disulfide (MoS2),24,25 molybdenum carbides (Mo2C),26−28 and other materials,29,30 have already shown relative high HER activities in basic media. Among these electrocatalysts, Mo2C has attracted great attention because its d-band electronic structure is similar to that of Pt.17 Therefore, extensive effort has been devoted to improving the HER catalytic activity of Mo2C by enhancing electron conduction through phase control,31 nanostructure and heterostructure engineering,26,27 intercalation,32 and doping.33,34 However, in most reported procedures, the aggregation or excessive growth of Mo2C nanoparticles often occurs, resulting in serious decrease of exposed active sites35,36 and therefore poor electrocatalytic HER performance. It is still a considerable challenge to simultaneously provide high conductivity and abundant active sites for Mo2C-based electrocatalysts for HER in alkaline media. In this study, we report a liquid−liquid interfacial approach for design and synthesis of polyaniline nanosheets with controlled components, which can be further used as precursor for preparation of Mo2C-embedded N-doped porous carbon nanosheets (denoted as Mo 2C@2D-NPC). As-prepared Mo2C@2D-NPC featured evenly distributed Mo2C nanoparticles with diameters down to 5 nm, apparent Mo2C/NPC interfaces, and high specific surface areas, indicating the presence of abundant exposed active sites for electrocatalysis. As the catalyst for electrocatalytic HER, Mo2C@2D-NPC exhibited an ultralow onset overpotential of 0 mV, an overpotential of only 45 mV at 10 mA cm−2, a small Tafel slope of 46 mV dec−1, and excellent stability in 1 M KOH electrolyte, which are superior to all of the previously reported Mo2C-based catalysts, much better than the values of most reported HER electrocatalysts in alkaline media, and even comparable to the noble metal catalyst Pt.

Scheme 1. Schematic of the Procedure for Preparation of Mo2C-Anchored N-Doped Porous Carbon Nanosheetsa

a (i) In situ polymerization of aniline on graphene oxide surfaces at the toluene/water interface in the presence of Mo4O132−: top, aniline in toluene; bottom, graphene oxide, (NH4 )2 Mo 4O13, HCl, and ammonium persulfate in water. (ii) Pyrolysis at 700−1100 °C under 5% H2/Ar atmosphere for 2 h. (a−f) Digital photos showing interfacial polymerization (from a to f: 0, 5 min, 30 min, 2 h, 4 h, 6 h) of aniline in a water/tolune system. The top layer is aniline dissolved in the toluene; the bottom layer is the aqueous solution of GO, transition metal salt, acid, and ammonium peroxydisulfate. In the beginning, the aqueous layer and organic layer form an ideal aqueous/organic interface. After 5 min, green polyaniline appears at the interface; as the reaction proceeds, the GO gradually suspends onto the interface, the volume of polyaniline spreads, and the color becomes darker. Finally the volume of products stop changing, indicating reaction completion.

diffuse through the interface and adsorb on GO surface, and therefore cause the partially hydrophobic property of the graphene based nanosheets due to the covered aniline. The scanning TEM (STEM) image and elemental mapping reveal the uniform distribution of Mo in the PANI nanosheets (Figure S3), indicating the strong interaction between Mo4O132− and PANI, which ensures the homogeneous distribution of Mobased nanoparticles in carbon materials after pyrolysis. Thermogravimetric analysis revealed that Mo@2D-PANI and Mo@PANI only showed 33% and 37% weight losses up to 800 °C, respectively, indicating the carbon-rich features of such materials (Figure S4). As shown in Figure 1a,b, the 2D morphologies of Mo2C@ 2D-NPC were similar to that of Mo@2D-PANI after pyrolysis, indicating graphene to be one highly efficient morphologycontrolled 2D template. The TEM images in Figure 1c,d reveal Mo2C nanoparticles uniformly embedded in carbon matrices. The high-resolution TEM (HR-TEM) image depicted clear lattice fringes with interplanar distance of 0.23 and 0.26 nm, corresponding to the (101) and (100) crystal planes of the Mo2C nanoparticles, respectively (Figure 1e). The X-ray diffraction (XRD) pattern of Mo2C@2D-NPC exhibited obvious peaks at 34.4°, 37.9°, 39.4°, 52.1°, 61.5°, 69.6°,

RESULTS AND DISCUSSION The synthesis procedure for Mo2C@2D-NPC is illustrated in Scheme 1. First, the Mo4O132−-anchored PANI nanosheet, denoted as Mo@2D-PANI, was fabricated through in situ polymerization at room temperature using GO as a 2D template, aniline as a monomer, and Mo4O132− as Mo source at the toluene/water interface. The obtained Mo@2D-PANI was then pyrolyzed under hydrogen/argon (5%) atmosphere at 700−1100 °C for 2 h. Unless noted otherwise, the as-produced Mo2C-anchored nitrogen-doped carbon nanosheet at 900 °C is denoted as Mo2C@2D-NPC. As control experiments, PANI, Mo@PANI, and 2D-PANI were also prepared by similar interfacial in situ polymerization methods without using GO/ (NH4)2Mo4O13, GO, or (NH4)2Mo4O13, respectively. Asproduced pyrolyzed products for PANI, Mo@PANI, and 2DPANI were denoted as NPC, Mo2C@NPC, and 2D-NPC, respectively. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of Mo@2DPANI revealed that numerous free-standing nanosheets were covered by polyaniline conical fibers (Figure S1), and selfpolymerization of aniline (Figure S2) was prevented at the interface. During the interfacial polymerization, aniline can 3934


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74.6°, and 75.5°, which can be ascribed to the 100, 002, 101, 102, 110, 103, 112, and 201 planes of Mo2C (JCPDS No. 350787), respectively (Figure 2a). The XRD patterns of Mo@2DPANI in Figure 2a indicated two diffraction peaks at 14.4° and 27.4°, corresponding to the typical planes of pure PANI. Furthermore, an obvious diffraction peak was observed at 6.5°, which represents notable linearization of the polymeric chain and the periodic distance between the dopants and the N atom on the adjacent chain, demonstrating the ordering due to chain to chain stacking in the PANI nanofibers.37−39 The high-angle annular dark-field imaging (HAADF) scanning transmission electron microscopy (STEM) image and elemental mapping further demonstrated the uniform distribution of C, Mo, and N elements in Mo2C@2D-NPC (Figure 1f). All these results reveal that Mo2C@2D-NPC possesses uniform 2D morphology and evenly distributed Mo2C. For comparison, the morphological and structural analyses for Mo2C@NPC are illustrated in Supporting Information (Figures S5−S7). To reveal the nitrogen physisorption properties of the asprepared materials, the Brunauer−Emmett−Teller (BET) specific surface area of Mo2C@2D-NPC is calculated as 110.2 m2 g−1, which is larger than that of 72.7 m2 g−1 for Mo2C@ NPC (Figure S8a and Table S1), suggesting that both the graphene template and conical shapes derived from polyaniline parts could contribute to the high surface areas. The pore size distributions were calculated through the nonlocal density functional theory (NLDFT) method, revealing typical mesoporous (2−50 nm) character with the pore diameters mainly centered between 2 and 10 nm (Figure S8b). The degrees of graphitization of the as-prepared materials were further studied by comparing the relative intensities of the D and G bands from Raman spectroscopy. We observed two peaks centered at 1334 and 1590 cm−1 (Figure S9), corresponding to the asymmetrical breakdown at the edges of the graphene sheets (D band) and the E2g vibrational mode of the graphite layers (G band),

Figure 1. (a, b) SEM, (c, d) TEM, and (e) HR-TEM images of Mo2C@2D-NPCs, (f) HAADF STEM image and elemental mapping of C, Mo, and N for Mo2C@2D-NPC.

Figure 2. (a) XRD patterns of Mo2C@2D-NPC and Mo@2D-PANI. Asterisk (*) indicates the presence of amorphous carbon. (b) C 1s, (c) Mo 3d, and (d) N 1s XPS spectra of Mo2C@2D-NPC. 3935


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Figure 3. HER performance of Mo2C@2D-NPC in alkaline media. (a) Polarization curves and (b) corresponding Tafel plots of as-prepared materials in 1 M KOH. (c) Capacitive current at 0.35 V as a function of scan rate for Mo2C@2D-NPC. (d) Nyquist plots of different samples over the frequency range from 1000 kHz to 0.02 Hz at the open-circuit voltage with an AC voltage of 10 mV. Inset shows enlarged Nyquist plots in high frequency zone.

Table 1. Comparison of Catalytic Parameters in Alkaline Media of Different HER Catalysts catalysts

onset potential (mV vs RHE)

overpotential at 10 mA cm−2 (mV vs RHE)

j0 (mA cm−2)

Tafel slope (mV dec−1)

com-Mo2C Mo2C@NPC Mo2C@2D-NPC Pt/C

53 5 0 0

170 72 45 33

0.028 0.45 1.14 0.78

67 52 46 31

respectively.40 The ID/IG of Mo2C@2D-NPC was determined to be 1.21, which is higher than those of 2D-NPC (1.01) and Mo2C@NPC (1.10), suggesting the presence of more active defects in Mo2C@2D-NPC. Therefore, this 2D sandwich-like nanostructure, with a relatively large surface area and more defects could be ideal for energy storage and conversion. X-ray photoelectron spectroscopy (XPS) spectra were measured to confirm the chemical composition and valence states of Mo2C@2D-NPC and Mo2C@NPC (Figures 2, S10, and S11). The XPS survey spectrum of Mo2C@2D-NPC indicated the existence of C, Mo, N, and O elements (Figure S10). The C 1s XPS peak can be fitted into three peaks centered at 284.6, 285.6, and 288.8 eV (Figure 2b), which can be attributed to C−C/CC, C−N, and CO species,41 respectively. However, no carbidic peak (approximately 282.7 eV) was observed. This can be ascribed to the carbon on the surface, where the graphitic carbon covered the signal of the carbidic carbon in a similar binding energy region.17 The Mo 3d spectrum (Figure 2c) displayed two pairs of peaks (Mo 3d5/2/ 3d3/2) at binding energies of 232.6/235.6 and 228.6/231.6 eV, which can be ascribed to surface-oxidized MoOx and Mo2C, respectively. The two peaks of the N 1s spectrum (Figure 2d) can be ascribed to pyridinic N (398.5 eV) and graphitic N (401.3 eV).42,43 It can be observed that pyridinic N is the main nitrogen species in Mo2C@2D-NPC, which is beneficial to the HER caused by the lone electron pair in the plane of the carbon

matrix, and can withdraw electrons and active hydrogen.8 Moreover, the elemental content of Mo2C@2D-NPC was calculated through XPS analysis and summarized in Table S2. All these results together with the HR-TEM image in Figure 1e indicate that Mo2C@2D-NPC is composed of typical Mo2C and NPC, and Mo2C is enwrapped by carbon walls. The electrocatalytic HER activities of the as-prepared samples were examined in 1 M KOH aqueous solution. First, the influence of carbonization temperature was studied (Figures S12−S15, Table S2). The sample derived by carbonization at 900 °C exhibited optimal electrocatalytic HER performance, which possibly is ascribed to the poor conductivity (Figure S16 and Table S3) of the catalyst pyrolyzed at 700 °C (Mo2C@2DNPC/700) and the aggregation of Mo2C nanoparticles in the catalyst pyrolyzed at 1100 °C (Mo2C@2D-NPC/1100). Hereafter, the catalyst pyrolyzed at 900 °C (Mo2C@2DNPC) will be discussed. As shown in Figure 3a, Mo2C@2DNPC showed the lowest onset overpotential of 0 mV, and produced a current density (j) of 10 mA cm−2 at an overpotential (η) of 45 mV. The Tafel slope is one of the experimental kinetic parameters often used for the characterization of HER. The linear regions of Tafel plots can be fitted by Tafel equation η = a + b log(j), where η is the overpotential, a is the Tafel constant, b is the Tafel slope, and j is the current density.44 The Tafel slope of Mo2C@2D-NPC was calculated to be only 46 mV dec−1 (Figure 3b), which is much lower than 3936


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Figure 4. HER performance of Mo2C@2D-NPC in acid media. (a) Polarization curves and (b) corresponding Tafel plots of as-prepared materials in 0.5 M H2SO4.

Figure 5. Current density−time (I−t) curve of Mo2C@2D-NPCs: (a) in 1 M KOH, (b) in 0.5 M H2SO4.

those of Mo2C@NPC (52 mV dec−1), commercial Mo2C (com-Mo2C, 67 mV dec−1), and all of the reported metal carbide-based electrocatalysts (Table S4), suggesting that the kinetics of the water molecule dissociation step were efficiently facilitated on the surfaces of the Mo2C@2D-NPC catalysts. The Tafel slope of Mo2C@2D-NPC indicates that HER occurs through a Volmer-Heyrovsky mechanism, and electrochemical recombination with an additional proton is the rate-limiting step.26,28 Based on the Tafel analysis, the exchange current density (j0) of the Mo2C@2D-NPC was determined to be approximately 1.14 × 10−3 A cm−2 (Table 1), which is even higher than that of Pt/C (0.78 × 10−3 A cm−2), further revealing the exceptional H2 evolution efficiency of Mo2C@2DNPC. All of these results demonstrated that the as-prepared Mo2C@2D-NPC exhibited the best electrochemical performance for catalyzing HER in 1 M KOH among the transition metal carbide-based electrocatalysts (Table S4). Metal carbides, especially Mo2C, have been long proved to be promising candidates for HER. However, the low exposed active sites have long hindered the performance optimization. In Mo2C@2D-NPC, Mo2C nanoparticles possess small sizes down to 5 nm (Figure 2e). In addition, the atomic content of Mo in Mo2C@2D-NPC is up to 7.9% according to the XPS analysis (Table S2). Both of these two factors combined with high surface area (Table S1) ensure the maximized exposure of the active sites for boosting the HER performance.36 To assess the effective surface active areas of these electrocatalysts, a series of cyclic voltammetry measurements were performed at scan rates varying from 20 to 200 mV s−1 in the region from 0.30 to 0.4 V (Figure S17). The double-layer capacitance (Cdl, Figure 3c) of Mo2C@2D-NPC was 15.1 mF cm−2, which is

approximately 3 times and 75 times higher than those of Mo2C@NPC (5.7 mF cm−2) and com-Mo2C (0.2 mF cm−2), respectively. In addition, electrochemical impedance spectroscopy was used to reveal the HER kinetic process. Figure 3d displays the obtained Nyquist plots, which can be fitted by an equivalent circuit (Figure S18, and Table S3). The charge transfer resistance (Rct) at the materials/electrolyte interface is usually used to probe the electrocatalytic activity. The Rct value of 94 Ω for Mo2C@2D-NPC was much lower than those for Mo2C@NPC (1412 Ω) and com-Mo2C (18740 Ω), which could be attributed to the long-distance conductivity of the graphene layers. The HER performance of the as-prepared materials in acid media (0.5 M H2SO4) was also studied (Figure 4). Com-Mo2C and Mo2C@NPC require overpotential (η) of about 302 and 207 mV, respectively, to reach current density of 20 mA cm−2, which are both much better than those for the obtained 2DNPC and NPC. In sharp contrast, the Mo2C@2D-NPC produces current density of 10 and 20 mA cm−2 at overpotentials of 86 and 105 mV, respectively. Such impressive overpotential and ultralow Tafel slope of 62 mV dec−1 (Figure 4b) indicated that Mo2C@2D-NPC is also a promising electrocatalysts for HER even in acid media in comparison with most recently developed metal carbide-based electrocatalysts (Table S5). Stability is another crucial parameter to reflect the durable operation of HER electrocatalysts. For the current density− time (i-t) test, the working electrode was performed at η = 70 mV (1 M KOH) or η = 90 mV (0.5 M H2SO4) for 20 h, while the current density was continuously monitored (Figure 5), demonstrating the excellent stability of Mo2C@2D-NPC. The 3937


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Figure 6. Theoretical analysis of the mechanism for Mo2C@2D-NPC catalyzed HER in alkaline media. (a) The overall pathway for the HER in alkaline media. (b) Heterostructure of Mo2C and pristine graphene (G@Mo2C). (c) Heterostructure of Mo2C and pyridinic N-doped graphene (two vacancies: Pyr.2VN3@Mo2C). (d) Calculated free energy diagram for HER in alkaline media on various studied systems.

too strong, while a large positive free-energy indicates a very weak H* adsorption and easy product desorption.50 Both of them are unfavorable for HER. According to the results, pristine graphene (denoted as G in Figure 6) has an endothermic free-energy of 1.73 eV, implying an energetically unfavorable interaction with hydrogen (Figure 6d). Regarding chemical modification, doping in graphene offers the possibility to enlarge the wide range of applicability of the material. For this reason, big efforts are made, both theoretically and experimentally, to prepare, characterize, and understand doped graphene. The literature values show that B and N doped graphene can be synthesized to exhibit p and n type semiconducting properties that can be systematically tuned with the concentration of the doped material. From our theoretical results, for the graphene structure doped with graphitic N atoms (denoted as GN in Figure 6) a free energy barrier of 2.02 eV has been obtained. The HER results for the graphene and graphitic structures show that the reaction mechanism can barely proceed because of the slow proton− electron transfer. Our next step, doping graphene simultaneously with vacancies, constitutes a large step toward understanding the behavior of improving the HER mechanism. The free energies of pyridinic-N-doped graphene with one vacancy (denoted as Pyr.N3) or two vacancies (denoted as Pyr.2VN3) were calculated to be −2.04 and −1.37 eV, respectively, indicating low catalytic activities for the HER. The HER values for Pyr.N3 and Pyr.2VN3 show that the H is too strongly adsorbed to the structure and much energy is required for removing the H from the structure and forming the H2 molecule. The Mo2C surface has been adsorbed on top of the graphene structures. The distance between the Mo and C atoms is approximately 2.7 Å. The adsorption of the Mo2C on top of the graphene structures is helping in reducing the free-energy barrier for the HER mechanism. Remarkably, the free-energy barriers for G@Mo2C

excellent stability of Mo2C@2D-NPC in both alkaline and acidic media can be attributed to the Mo2C components being enwrapped and protected by NPC walls, therefore preventing the gradual corrosion of Mo2C in electrolytes.45 Based on the above results and discussion, the superior HER activities of Mo2C@2D-NPC can be better understood from following aspects: (1) first of all, the small size of Mo2C ensures a large amount of exposed active sites; (2) the carbon matrix derived from PANI provides good conductivity; (3) graphene layers in such sandwich structure offer long distance conductivity; (4) the strong interaction between Mo2C and NPC reduces the energy barrior for water splitting. All in all, graphene, carbon matrix, dopant, and Mo2C nanoparticles synergistically contribute to the state-of-the-art Mo2C-based HER performance in alkaline media. To gain further insight into the high electrocatalytic activity of the Mo2C@2D-NPC material, we additionally performed density functional theory (DFT) calculations. A qualitative scheme for the evolution of the free energy along the reaction coordinate is shown in Figure 6a. Theoretically, the HER pathway in alkaline media can be described by the following two steps: (1) electrocouple water dissociation (Volmer step) and (2) subsequent combination of the formed H* into molecular H2, whereas OH− is released to the solution.46,47 For a wide variety of inorganic HER electrocatalysts, the Gibbs free energy of the intermediates has been considered as an important descriptor for assessing the HER activity.48,49 The optimum value of Gibbs free energy of the adsorbed hydrogen should be zero, which well benefits a fast formation of adsorbed hydrogen and a rapid concomitant hydrogen release. In order to get profound insights into such an electrocatalytic HER performance of Mo2C@2D-NPC, DFT calculations were further employed to study the Gibbs free energies of the intermediates (Figures 6b,c and S21−28). A large negative freeenergy indicates that chemical adsorption of H* on its surface is 3938


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milling balls (Φ 10 mm) and 16 small milling balls (Φ 6 mm) were added (ball to powder weight ratio ∼10:1), then the milling was performed at 400 rpm for 24 h. Characterization. SEM measurements were performed on a FEI Sirion-200 field emission scanning electron microscope. Transmission electron microscopy (TEM) images were acquired using a Tecnai G2 F20 S-TWIN transmission electron microscope (FEI) operated at 200 kV. XRD analysis was performed on a RigakuD/Max 2500 X-ray diffractometer with Cu Kα radiation (k = 1.54 Å) at a generator voltage of 40 kV and a generator current of 50 mA with a scanning speed of 6° min−1 over the range 5−80° (2θ). The Raman spectra of samples were obtained on Lab-RAM HR800 with excitation by an argon ion laser (532 nm). TGA of the samples was performed using a Q5000IR (TA Instruments, USA) thermogravimetric analyzer at a heating rate of 20 °C min−1 under nitrogen flow. X-ray photoemission spectroscopy (XPS) measurements were performed on a PHI-5000C ESCA system; the C 1s value was set at 284.6 eV for charge corrections. The gas sorption isotherms were measured via an Autosorb-iQA3200-4 sorption analyzer (Quantatech Co., USA) based on N2 adsorption/desorption. Electrochemical Measurements. The electrochemical experiments for HER were carried out in a conventional three-electrode cell using a CH Instrument (model 660D) at room temperature. Ag/AgCl (3 M KCl) and platinum wire were used as reference and counter electrodes, respectively. For working electrodes, 5 mg of catalyst was blended with 500 μL of Nafion solution (0.05 wt %) and sonicated for 2 h, producing catalyst ink; then 18 μL of catalyst ink was pipetted onto the glassy carbon surface (0.2471 cm−2). The electrodes were dried at room temperature before measurement. The polarization curves were obtained in 0.5 M H2SO4 and 1 M KOH with a scan rate of 5 mV s−1 at room temperature. All potentials in this study were iRcompensated and referred to a reversible hydrogen electrode (RHE) via calibration measurement in N2-saturated electrolyte. EIS measurements were carried out from 1000 kHz to 0.02 Hz with an amplitude of 10 mV at the open-circuit voltage. The electrochemical double-layer capacitances (Cdl) of catalysts were calculated from CV curves. The CV curves were performed at scan rates varying from 20 to 200 mV s−1 in the region from 0.30 to 0.40 V vs RHE. The capacitive currents of ΔJ (Janodic − Jcathodic) are plotted as a function of the CV against the scan rate. The slope of the fitting line is equal to twice the Cdl, which is linearly proportional to the electrochemically active surface area of the electrode. DFT Calculation. For the periodic structures, DFT calculations were performed using the program VASP (Vienna Ab Initio Simulation Package),54−57 where the electronic wave functions have been expanded into plane-waves up to an energy cutoff of 400 eV, and a projected-augumented-wave (PAW)58 scheme has been used to describe the interactions between the valence electrons and the nuclei (ions). The exchange−correlation interactions between electrons were treated within the generalized gradient approximation (GGA) as implemented by Perdew, Burke, and Ernzerhof (PBE).59 The periodic systems have been modeled using a supercell for graphene of 9.872 × 17.153 × 14.00 Å3 and for Mo2C/graphene 9.872 × 17.153 × 25.00 Å3. Only one K-point, the Γ point, was considered. For the periodic structures, sufficient vacuum space in Z-direction has been considered to separate the interaction between periodic images. The minimum energy configurations were considered to be converged when the forces on each atom of the molecules were less than 0.01 eV/Å. The graphene model has been constructed as a 4 × 4 periodic supercell comprising 64 atoms. The Mo2C (001) has been constructed as a 2 × 3 periodic supercell comprising 72 atoms. Since there is a lattice mismatch between the Mo2C (001) and graphene, the Mo2C (001) structure has been streched in the a direction by approximately 4.1% and in the b direction by approximately 5.3%. After it was streached, the structure was reproduced in x and y directions by 2 × 2. All the atoms have been allowed to freely relax. The total amount of atoms that have been relaxed is approximately 210 atoms. The free energy of adsorbed H (ΔGH) on different surfaces is calculated as

(0.44 eV) and GN@Mo2C (0.65 eV) are substantially lower than the free-energy barriers for the suspended G and GN. Moreover, the free-energy barriers for Pyr.N3@Mo2C and Pyr.2VN3@Mo2C heterostructures were calculated to be −0.33 and −0.04 eV, respectively, presenting a favorable pathway for the adsorption of H* and desorption of molecular hydrogen. As reported, the (000l) planes of Mo2C are the electrocatalysis active sites for the HER and the caculated free-energy barriers for Mo2CTx is 0.048 eV.51−53 The free-energy barriers for Pyr.2VN3@Mo2C in this work is similar to Mo2CTx, which can be attributed to the synergistic effect between Mo2C and pyridinic N-doping features with largely accelerated sluggish HER kinetics. The particle size of Mo2C in the Mo2C@2DNPC domain is ∼7.3 nm according the full width at half maxima (fwhm) of the XRD peak, which is consistent with TEM images and benefits the electrocatalysis. Such small size of Mo2C ensures the exposure of the basal plane and therefore more electrochemically active surface area (Figures S19 and S20). Overall, theoretical results accord well with the experiments, which found a low onset potential of approximately 0 mV and a current density of 10 mA cm−2 at an overpotential of approximately 45 mV, which is much lower than the values of most reported HER electrocatalysts and comparable to the noble metal catalyst Pt.

CONCLUSIONS In summary, we report an easy and highly efficient interfacial strategy toward Mo2C-embedded N-doped porous carbon nanosheets. The outstanding HER performance in alkaline media of the as-prepared 2D hybrid can be attributed to the 2D sandwich-like nanostructure, high nitrogen doping content, abundance of active sites, and strong interaction between Mo2C and the carbon matrix. Such interfacial approach offers opportunities for design and preparation of 2D superstructures for versatile applications. EXPERIMENTAL SECTION Chemicals and Materials. Aniline (C6H5NH2, AR), ammonium molybdate ((NH4)2Mo4O13), graphite, and ammonium peroxydisulfate (APS) were purchased from Aladdin Reagent. Sodium nitrate (NaNO3), hydrogen peroxide (H2O2), potassium permanganate (KMnO4), potassium hydroxide (KOH), sulfuric acid (H2SO4, 98 wt %), and hydrochloric acid (HCl, 36 wt %) were bought from Sinopharm Chemical Reagent. Mo2C and Pt/C (20 wt %) were purchased from Sigma-Aldrich. Nafion solution (0.5 wt %) was purchased from DuPont, Ltd. Synthesis of Mo2C@2D-NPC. An amount of 245 mg of APS and 2 g of (NH4)2Mo4O13 were dissolved in 8 mL of 1 M hydrochloric acid solution. After stirring for a while, 2 mL of GO (5 mg mL−1) solution was injected into the above solution, and the mixture was treated with ultrasonication for 30 min to form a homogeneous suspension, as the aqueous layer. Then, 100 μL of aniline was dissolved in 10 mL of toluene, as the organic layer. The organic phase was added slowly onto the aqueous phase, forming an organic/aqueous interface. Finally, the Mo4O132− coupled PANI nanosheets were obtained after repeated washing steps with water and ethanol. After pyrolysis at 900 °C for 2 h under hydrogen/argon (5%) atmosphere, Mo2C@2D-NPC can be readily obtained. For comparison, the Mo2C@NPC, 2D-NPC, and NPC were prepared by the same process, without adding GO, (NH4)2Mo4O13, and GO/(NH4)2Mo4O13. Synthesis of Ball Milled Commercial Mo2C. The ball milled commercial Mo2C (bm-com Mo2C) was prepared by ball-milling in a high-energy planetary mill with stainless steel balls as milling media. Five grams of commercial Mo2C was placed in the grinding bowl, 8 big 3939


Article

ACS Nano ΔG = ΔE + ΔZPE + ΔCorrOH − T ΔS

(5) Xu, S.; Li, D.; Wu, P. One-Pot, Facile, and Versatile Synthesis of Monolayer MoS2/WS2 Quantum Dots as Bioimaging Probes and Efficient Electrocatalysts for Hydrogen Evolution Reaction. Adv. Funct. Mater. 2015, 25, 1127−1136. (6) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267−9270. (7) Han, S.; Feng, Y. L.; Zhang, F.; Yang, C. Q.; Yao, Z. Q.; Zhao, W. X.; Qiu, F.; Yao, Y. F.; Zhuang, X. D.; Feng, X. L.; et al. MetalPhosphide-Containing Porous Carbons Derived from an IonicPolymer Framework and Applied as Highly Efficient Electrochemical Catalysts for Water Splitting. Adv. Funct. Mater. 2015, 25, 3899−3906. (8) Callejas, J. F.; Read, C. G.; Popczun, E. J.; McEnaney, J. M.; Schaak, R. E. Nanostructured Co2P Electrocatalyst for the Hydrogen Evolution Reaction and Direct Comparison with Morphologically Equivalent CoP. Chem. Mater. 2015, 27, 3769−3774. (9) Wu, Y.; Li, G. D.; Liu, Y.; Yang, L.; Lian, X.; Asefa, T.; Zou, X. Overall Water Splitting Catalyzed Efficiently by an Ultrathin Nanosheet-Built, Hollow Ni3S2-Based Electrocatalyst. Adv. Funct. Mater. 2016, 26, 4839−4847. (10) Tang, Y. J.; Wang, Y.; Wang, X. L.; Li, S. L.; Huang, W.; Dong, L. Z.; Liu, C. H.; Li, Y. F.; Lan, Y. Q. Molybdenum Disulfide/ Nitrogen-Doped Reduced Graphene Oxide Nanocomposite with Enlarged Interlayer Spacing for Electrocatalytic Hydrogen Evolution. Adv. Energy Mater. 2016, 6, 1600116. (11) Voiry, D.; Yamaguchi, H.; Li, J.; Silva, R.; Alves, D. C.; Fujita, T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; Chhowalla, M. Enhanced Catalytic Activity in Strained Chemically Exfoliated WS2 Nanosheets for Hydrogen Evolution. Nat. Mater. 2013, 12, 850−855. (12) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100−102. (13) Wang, D.-Y.; Gong, M.; Chou, H.-L.; Pan, C.-J.; Chen, H.-A.; Wu, Y.; Lin, M.-C.; Guan, M.; Yang, J.; Chen, C.-W.; et al. Highly Active and Stable Hybrid Catalyst of Cobalt-Doped FeS2 NanosheetsCarbon Nanotubes for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2015, 137, 1587−1592. (14) Liu, Y.; Li, G.-D.; Yuan, L.; Ge, L.; Ding, H.; Wang, D.; Zou, X. Carbon-Protected Bimetallic Carbide Nanoparticles for a Highly Efficient Alkaline Hydrogen Evolution Reaction. Nanoscale 2015, 7, 3130−3136. (15) Fan, X.; Peng, Z.; Ye, R.; Zhou, H.; Guo, X. M3C (M: Fe, Co, Ni) Nanocrystals Encased in Graphene Nanoribbons: An Active and Stable Bifunctional Electrocatalyst for Oxygen Reduction and Hydrogen Evolution Reactions. ACS Nano 2015, 9, 7407−7418. (16) Wang, Z.-L.; Hao, X.-F.; Jiang, Z.; Sun, X.-P.; Xu, D.; Wang, J.; Zhong, H.-X.; Meng, F.-L.; Zhang, X.-B. C and N Hybrid Coordination Derived Co-C-N Complex as a Highly Efficient Electrocatalyst for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2015, 137, 15070−15073. (17) Xiao, P.; Ge, X.; Wang, H.; Liu, Z.; Fisher, A.; Wang, X. Novel Molybdenum Carbide-Tungsten Carbide Composite Nanowires and their Electrochemical Activation for Efficient and Stable Hydrogen Evolution. Adv. Funct. Mater. 2015, 25, 1520−1526. (18) Gao, S.; Li, G.-D.; Liu, Y.; Chen, H.; Feng, L.-L.; Wang, Y.; Yang, M.; Wang, D.; Wang, S.; Zou, X. Electrocatalytic H2 Production from Seawater over Co, N-Codoped Nanocarbons. Nanoscale 2015, 7, 2306−2316. (19) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B. R.; Mikmeková, E.; Asefa, T. Cobalt-Embedded Nitrogen-Rich Carbon Nanotubes Efficiently Catalyze Hydrogen Evolution Reaction at All pH Values. Angew. Chem. 2014, 126, 4461−4465. (20) Wang, H.; Kong, D.; Johanes, P.; Cha, J. J.; Zheng, G.; Yan, K.; Liu, N.; Cui, Y. MoSe2 and WSe2 Nanofilms with Vertically Aligned Molecular Layers on Curved and Rough Surfaces. Nano Lett. 2013, 13, 3426−3433.

(1)

where ΔE is the hydrogen adsorption energy, ΔZPE is the zero point vibrational energy, ΔCorrOH is the correction term for the OH, and TΔS is the entropy value. The free energy equation is applied at standard conditions (T = 298.15 K, P = 1 bar, pH = 0). The free energy change of the protons relative to the specified electrode at non-zero pH is represented by the Nernst equation60

ΔG(pH) = − kBT ln(10) × pH

(2)

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00365. SEM, TEM, and TGA of Mo@PANI and Mo@2DPANI; SEM, TEM, and XRD of Mo2C@NPC; Raman and BET of Mo2C@NPC and Mo2C@2D-NPC; XPS of Mo2C@NPC and Mo2C@2D-NPC; influence of carbonization temperature; and theoretical model description of G, GN, Pyr.N3, Pyr.2VN3, G@Mo2C, GN@Mo2C, Pyr.N3@Mo2C, and Pyr.2VN3@Mo2C (PDF)

AUTHOR INFORMATION Corresponding Authors

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

Fan Zhang: 0000-0003-2319-6133 Author Contributions

C.L. and D.T. contributed equally to this work. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the 973 Programs of China (2013CBA01602), National Natural Science Foundation of China (51403126, 21574080 and 61306018), Shanghai Committee of Science and Technology (15JC1490500, 16JC1400703), German Research Foundation (DFG) within the Cluster of Excellence “Center for Advancing Electronics Dresden” (cfaed), ERC Grant on 2DMATER, UP-GREEN, and EU Graphene Flagship, and Open Project Program of the State Key Laboratory of Photocatalysis on Energy and Environment (SKLPEE-KF201702), Fuzhou University, and the International Max Planck Research School (IMPRS) “Dynamical Processes in Atoms, Molecules and Solids” at the Max Planck Institute for the Physics of Complex Systems, Dresden, Germany. REFERENCES (1) Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305, 972−974. (2) Kibsgaard, J.; Jaramillo, T. F. Molybdenum Phosphosulfide: An Active, Acid-Stable, Earth-Abundant Catalyst for the Hydrogen Evolution Reaction. Angew. Chem., Int. Ed. 2014, 53, 14433−14437. (3) Zou, X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148−5180. (4) Liang, H. W.; Bruller, S.; Dong, R.; Zhang, J.; Feng, X.; Mullen, K. Molecular Metal-Nx Centres in Porous Carbon for Electrocatalytic Hydrogen Evolution. Nat. Commun. 2015, 6, 7992. 3940


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Molybdenum Carbide-Embedded Nitrogen-Doped Porous Carbon

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