Universal Strategy to Fabricate a Two-Dimensional Layered

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A Universal Strategy to Fabricate Two-dimensional Layered Mesoporous Mo2C Electrocatalyst Hybridized on Graphene Sheets with High Activity and Durability for Hydrogen Generation Lili Huo, Baocang Liu, Geng Zhang, and Jun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 Jun 2016 Downloaded from http://pubs.acs.org on June 23, 2016

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ACS Applied Materials & Interfaces

A Universal Strategy to Fabricate Two-dimensional Layered Mesoporous Mo2C Electrocatalyst Hybridized on Graphene Sheets with High Activity and Durability for Hydrogen Generation Lili Huo,a,b Baocang Liu,a,b Geng Zhang,a,b and Jun Zhang*a,b a

College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, P.R. China b

Inner Mongolia Key Lab of Nanoscience and Nanotechnology, Inner Mongolia University, Hohhot 010021, P.R. China

ABSTRACT ： A universal strategy was developed for fabrication of highly active and durable precious-metal-free mesoporous Mo2C/graphene (m-Mo2C/G) electrocatalyst with a two-dimensional layered structural feature via a nanocasting method using glucose as a carbon source and an in-stiu assembled mesoporous KIT-6/graphene (KIT-6/G) as a template. The m-Mo2C/G catalyst exhibits high catalytic activity and excellent durability for hydrogen evolution reaction (HER) over a wide PH range, which displays a small onset potential of 8 mV, owerpotential (η10) for driving a cathodic current density of 10 mA•cm-2 of 135mV, a Tafel slope of 58 mV•dec-1, and an exchange current density of 6.31 ×10-2 mA•cm-2 in acidic media, and onset potential of of 41 mV, η10 of 128 mV, Tafel slope of 56 mV•dec-1, and an exchange current density of 4.09 ×10-2 mA•cm-2 in alkaline media, respectively. Furthermore, such m-Mo2C/G electrocatalyst also gives about 100% Faradaic yield , and shows excellent durability during 3000 cycles long-term test and the catalytic current remains stable over 20 h at fixed overpotentials, making it has great potential application prospect for energy issues. KEYWORDS: : Nanocasting method, Mesoporous Mo2C Nanocrystals, Precious-metal-free electrocatalyst, Two-dimensional layered structure, Hydrogen evolution reaction

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1. INTRODUCTION As the drying up of traditional fossil fuels and ever-increasing global pollution, it is imperative to exploit sustainable and cleaner energy source to alleviate the imminent intractable energy and environment issues.1-2 Hydrogen, as a renewable energy carrier with a high gravimetric energy density and zero emission of global warming gases, is considered to be one of the most promising replacements of carbon-based fuels because its only emission is water.3-5 Electrochemical water splitting has attracted a great deal of attention as one of the efficient ways to produce hydrogen due to its superiority of being a sustainable and environmental-friendly method.6,7 It is wellknown that Pt-group noble metalshave excellent activity for the HER,8,9 but their low abundance and high price limit their widespread application. Therefore, it has become crucial and urgent to seek and develop non-noble metal based catalysts with excellent catalytic activity, long-term stability, high abundance and low cost for practical largescale hydrogen production.10,11 Several

non-noble

metal

based

materials,

including

transition-metal

chalcogenides,12,13 nitrides,14,15 phosphides,16,17 carbides18-20 and complexes,21-23have attracted a wide range of interest as alternative catalysts for the HER owing to their similar electronic structure to that of noble metals.24,25 Among them, transition metal carbides, particularly molybdenum carbide (Mo2C), exhibited remarkable HER activity and good stability to be regarded as a promising HER electrocatalyst on account of its low cost, high melting point, and good conductivity.26-30 Many outstanding strategies, embracing constructing nanostructure,18,31 controlling morphology,32,33 doping a second transition metal,34,35 and exploiting hybrid structures,31,36 have been made to develop Mo2C-based catalysts with high-performance and excellent durability. Furthermore, various carbon-supported Mo2C nanocomposites have been designed to enhance their activity and stability.20,37,38 Graphene is one of the excellent carriers 2 ACS Paragon Plus Environment

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because of its superior electro-conductivity, large surface area, and high chemical stability. So far, Mo2C-based catalysts, such as np-Mo2C nanowires,37 porous Mo2C nanorods,35 Mo2-xFexC,34 Mo2C/CNT-GR20 and etc, have been demonstrated to exhibit high activity towards hydrogen evolution. Meanwhile, considerable studies suggested that the HER performance of molybdenum carbide-based materials depended on their small grain size of molybdenum carbides, large surface area, and highly dispersed distribution of molybdenum carbides nanoparticles, which were closely associated with their synthesis method.31,38 Generally speaking, chemical vapor deposition (CVD) and high temperature pyrolysis of precursors are most common preparation routes.39-42 However, during these processes, some harmful gases such as CO, C2H6 or CH4 are commonly used in the synthesis, which are harmful to the environment and the residual chars may cover the catalytic active sites.30,31 In addition, high-temperature calcination generally produces larger and coarsening size of molybdenum carbides with low specific surface area and subsequently lowers the density of catalytic active sites.33 Up to now, very limited researches about the controllable synthesis of well-defined molybdenum carbide nanomaterials with small crystalline size and high distribution of molybdenum carbides nanoparticles have been reported.18,19 Herein, we developed a cost-efficient and environmental-benign nanocasting method to

synthesize

highly

active

and

durable

precious-metal-free

mesoporous

Mo2C/graphene (m-Mo2C/G) electrocatalyst with a two-dimensional layered structural feature using glucose as a carbon source and in-situ assembled mesoporous KIT-6/G as a template. The obtained two-dimensional layered m-Mo2C/G electrocatalyst possesses unique well-organized mesoporous structure (~3.6 nm) of ultrasmall Mo2C nanocrystals duplicated from mesoporous KIT-6 template hybridizing on surface of graphene sheets during the synthetic process. The two-dimensional layered m-Mo2C/G 3 ACS Paragon Plus Environment


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electrocatalyst shows larger surface area (up to 294 m2·g-1), higher mass and charge transfer

efficiency,

and

expose

more

active

sites

than

other

Mo2C-based

electrocatalysts without mesoporous structural features, which is believed to be conducive to improve the electrocatalytic performance for the HER. As anticipated, the m-Mo2C/G electrocatalyst exhibits high activity and excellent durability for the HER over a wide PH range. The unique mesoporous structural feature of Mo2C and the compact integration of Mo2C nanocrystals with graphene sheets may result in much faster charge transfer rate and larger electroactive area, accounting for the promotion of its catalytic performance for the HER. The highly active and durable m-Mo2C/G electrocatalyst may be regarded as a promising HER catalyst in practical application. 2. EXPERIMENTAL SECTION 2.1 Materials preparation. Graphene was prepared by a modified hummer’s method. Typically, 1 g of graphite power was slowly added into a mixed solution of concentrated sulfuric acid (130 mL) and phosphoric acid (30 mL) containing 6 g of KMnO4 in a 250 mL ground flask at an ice bath. Then, the flask was transferred to an oil bath and kept at 50 °C for over 12 h under mechanical stirring. After naturally cooled to ambient temperature, the turbid liquid was slowly poured into a 500 mL of beaker containing 15 mL H2O2 and 100 mL deionized water with continually stirring, and the solution in beaker immediately turned into light yellow, indicating the formation of graphene oxide (GO). After the turbid liquid was completely added, the solution was further stirred 2 h to ensure the complete oxidation of graphite. Subsequently, the obtained yellow turbid was centrifuged at 2000 rpm in a 10 mL of tube, and the black particles (unreacted graphite) were discarded. The turbid in the top of tube was washed with concentrated HCl, deionized water, and ethanol for three times, respectively. Then, the products were freeze dried overnight to obtain the GO. KIT-6/G template was prepared according to a modified method reported previously.43 Briefly, 6.0 g of triblock copolymer pluronic of P123, 1 g of GO, and 6 g of n-butanol, and 4 ACS Paragon Plus Environment

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11.8 g of concentrated HCl were together added into 150 mL aqueous solution and continuously stirred at 45 °C for 24 h. Then, 12 g of TEOS was added to above solution under vigorous stirring at the speed of 0.5 mL/min. Then the mixture solution was kept under stirring condition at 45 °C for another 24 h followed with the treatment in a refluxing treatment at 80 °C for 6 h. Finally, the dark yellow solid products were collected by filtration and washed with water and ethanol before calcining at 350 °C for 4 h in air. The m-Mo2C/G electrocatalysts were prepared via a cost efficient and environmentalbenign nanocasting method. For the synthesis of typical meso-Mo2C/G electrocatalyst, 0.5 g of glucose and 1.0 g of (NH4)6Mo7O24·4H2O were dissolved in 25 mL distilled water. Then, 1.0 g of KIT-6/G template was added into the above solution. The mixture was stirred vigorously for 12 h at room temperature, and then the water was evaporated in vacuum at 50 °C until the water was completely dried. The nanocomposite was calcined at 450 °C for 2 h at a heating ramp of 2 °C/min and then heated at 2 °C /min up to 900 °C for 3 h in a highpurity nitrogen atmosphere. The calcination temperature can be adjusted from 800 to 1000 °C depending on the demands. Finally, after cooled to room temperature naturally, the resulting black solid was washed with 2 M NaOH aqueous solution at 80 °C to remove KIT-6 template to achieve the m-Mo2C/G electrocatalyst. For the synthesis of this m-Mo2C/G electrocatalyst, the mass ratio of (NH4)6Mo7O24·4H2O (1.0 g, Mo precursor) and glucose (0.5 g, carbon source) was fixed at 2:1 and the electrocatalyst was thus denoted as m-Mo2C/G(2:1). Similarly, the m-Mo2C/G electrocatalysts with the mass ratios of (NH4)6Mo7O24·4H2O and glucose at 1:1 (0.5 g Mo precursor and 0.5 g glucose) and 3:1 (1.5 Mo precursor and 0.5 g glucose) were denoted as meso-Mo2C/G(1:1) and m-Mo2C/G(3:1), respectively. Synthesis of meso-Mo2C and Mo2C/G electrocatalysts followed the similar process as that of meso-Mo2C/G electrocatalyst except that the assembled KIT-6/G template was replaced by individual KIT-6 template or graphene sheets during the synthetic process. Bulk Mo2C electrocatalyst was prepared following the similar process but in the absence of graphene 5 ACS Paragon Plus Environment


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sheets or KIT-6 template. Platinum, nominally 20wt% on carbon black, HiSPECTM 3000 (Pt/C) was obtained from Alfa Aesar (China). 2.2 Materials characterization. XRD was performed on a PANalytical Empyrean diffractometer with Cu Kalpha1 radiation (λ = 1.5405Å) in the Bragg angle ranging between 20° and 80°. Scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM) characterizations were performed on a FEI Tecnai F20 field-emission transmission electron microscope (FE-TEM). Surface area measurements were performed on an ASAP 2020 Brunauer–Emmett–Teller (BET) analyzer. Raman spectra were acquired using a LabRAMHR800 spectrographer (Horiba). The excitation source was a helium–neon laser with a wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) measurements were performed in a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh vacuum (UHV) chambers. Thermogravimetry (TG) of the samples was carried out on a thermal analyzer (STA 449/F3, Netzsch) and the samples were heated at a rate of 5 ℃/min in an air flow. 2.3 Electrochemical measurements. Electrochemical measurements were carried out on a Zennium electrochemical workstation (Zahner, Germany) assembled with a modulated speed rotator (RRDE-3A) in a standard three electrode system with a platinum wire as the counter electrode, a catalysts-modified glassy carbon electrode (GCE) with an area of 0.0707 cm2 as the working electrode, and a saturated calomel electrode (SCE, 0.241 V vs RHE) in acid media or Ag/AgCl (3M KCl) electrode (0.209 V vs RHE) in alkaline and neutral media as the reference electrode. All potentials were referenced to a reversible hydrogen electrode (RHE) by adding a value of (0.241 + 0.059 × pH) or (0.209 + 0.059 × pH) V. The catalyst ink was prepared by ultrasonically mixing 5.0 mg of the as-prepared catalyst with 25 µL of Nafion® (5 %) solution, 0.25 mL of water, and 0.25 mL of ethanol for 30 min to form a homogeneous suspension. A total of 2 µL of well-dispersed catalyst ink was pipetted and spread onto 6 ACS Paragon Plus Environment

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the surface of a pre-polished rotation disk electrode (RDE) (d = 3 mm) and dried in air for 30 min before measurement, corresponding to a catalyst loading of 269 µg·cm-2. Linear sweep voltammetry (LSV) polarization curves in the potential range of -0.6 to 0 or -1.5 to -0.6 or -1.6 to -0.8 V (vs. SCE) for the HER were acquired in N2 saturated 0.5 M H2SO4, 0.1M PBS, and 1.0 M KOH electrolyte with a sweep rate of 2 mV·s-1 at 1600 rpm. Before each HER activity measurement, the electrodes were pre-treated by cycling the potential between -0.6 and 0 V at a sweep rate of 100 mV·s-1 for 30 cycles without rotation to activate the catalysts, remove surface contamination, and stabilize the current. The longer-term (3000 cycles) stability was also tested and the amperometric current density–time (i–t) curves were measured for 20 h in N2 saturated 0.5 M H2SO4 , 1.0 M KOH and 0.1 M PBS solution under controlled potentials. Electrochemical impedance spectroscopy (EIS) measurements were carried out from 100 mHz to 10K Hz in 0.5 M H2SO4, 1.0 M KOH and 0.1 M PBS solution at different bias voltages. 3. RESULTS AND DISCUSSION The synthetic process towards m-Mo2C/G electrocatalyst is illustrated in Scheme 1. Initially, order mesoporous KIT-6 was in-situ assembled on surface of graphene oxide (GO) sheets to form an assembly of KIT-6/GO hybrid using tetraethyl orthosilicate as a silicon source and P123 as a soft template. The KIT-6/GO hybrid was further calcined under different temperatures to remove the P123 and transfer the GO to graphene (G).43 The assembling process may ensure the formation of two-dimensional layered mesoporous KIT-6 on surface of graphene sheets with compact interaction. Then, ammonium heptamolybdatetetrahydrate ((NH4)6Mo7O24·4H2O) and glucose with different mass ratios were filled into the mesopore channel of KIT-6/G hybrid via a wet-impregnation method. The obtained product was further annealed at 450 °C for 2h, then heated up to 900 °C for 3 h to convert the precursors into Mo2C via a typical high temperature solid-state reaction under a N2 atmosphere. After 7 ACS Paragon Plus Environment


removing KIT-6 template with NaOH solution, the two-dimensional layered m-Mo2C/G electrocatalyst was achieved. For convenience, the m-Mo2C/G electrocatalysts obtained with different mass ratios of (NH4)6Mo7O24·4H2O and glucose at 1:1, 2:1, and 3:1 during the synthetic process are nominated as m-Mo2C/G(1:1), m-Mo2C/G(2:1), and m-Mo2C/G(3:1), respectively, in the following discussion. TEOS

Calcination

Precursor

P123 GO

KIT-6/G

P123/KIT-6/GO

Precursor/KIT-6/G

P123/KIT-6

KIT-6

Precursor/KIT-6

Calcination

NaOH

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Mo2C NPs m-Mo2C/G

Scheme 1. Schematic illustration shows the synthetic process of m-Mo2C/G electrocatalyst. Figure 1a and b displays the TEM images of KIT-6/G template. It is certain that the KIT6/G template possesses highly ordered mesoporous structure. This is further confirmed by the SAXRD displayed in Figure 1c, in which two diffraction peaks corresponding to the (211) and (332) crystal planes of KIT-6 are observed, indicative of the existence of mesoporous structure in KIT-6/G template. This suggests that the KIT-6 template can be in-situ assembled on surface of graphene sheets with compact interaction during the formation of KIT-6 in the presence of GO. Using this mesoporous KIT-6/G as template, the m-MoC2/G electrocatalyst can be easily synthesized via a nanocasting strategy. Figure 1d-m systematically illustrates the TEM, HRTEM, STEM, and elemental mapping images of m-Mo2C/G(3:1) and mMo2C/G(2:1) electrocatalysts. The low-magnification TEM images shown in Figure 1d, e, g, and h reveal that the m-Mo2C/G(3:1) and m-Mo2C/G(2:1) electrocatalysts are composed of graphene sheets (marked by yellow arrows) with ultrasmall Mo2C nanocrystals uniformly dispersed on. It can be clearly seen that the average diameter of Mo2C nanocrystals is about 8 ACS Paragon Plus Environment

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~4.5 nm without any larger and coarsening size of molybdenum carbides observed due to the presence of KIT-6/G template. The alternate dark and light images suggest the presence of mesoporous structure in m-Mo2C/G(3:1) and m-Mo2C/G(2:1) electrocatalysts. In order to verify the existed confinement effect of KIT-6 template during the formation of mesoporous m-Mo2C/G electrocatalyst, we also synthesize the Mo2C/G electrocatalyst via directly depositing (NH4)6Mo7O24·4H2O and glucose precursors on GO sheets followed with the carbonization under N2 atmosphere. The TEM images show that the direct deposition method can also result in Mo2C/G electrocatalyst but without mesoporous structural feature (Figure S1). Instead, serve aggregation of Mo2C nanocrystals with large particle size on graphene sheets is observed, indicating that our in-situ assembled KIT-6/G hybrid may be used as a very effective template for fabricating two-dimensional layered mesoporous Mo2C/G electrocatalyst. The HRTEM images of m-Mo2C/G(3:1) and m-Mo2C/G(2:1) electrocatalysts shown in Figure 1f and I further confirm the ultrasmall particle sizes of Mo2C nanocrystals distributed on graphene sheets and the lattice spacing of 0.229 and 0.238 nm are consistent with the (101) and (002) crystal planes of β-Mo2C, respectively, which is in agreement with the corresponding fast Fourier transform (FFT) image placed in the inset of Figure 1f and i. The HAADF-STEM and elemental mapping images further confirm the elemental composition and distribution of Mo and C in m-Mo2C/G electrocatalyst in micro-scale. The HAADF-STEM images shown in Figure 1j and k further reveal that the m-Mo2C/G(2:1) is composed of graphene sheets (marked by yellow arrows) with ultrasmall Mo2C nanocrystals and abundant mesopores in the interparticles uniformly dispersed on. The corresponding EDX elemental mapping images given in Figure 1l and m clearly indicate that that C (red) and Mo (blue) elements are uniformly distributed on the surface of whole two-dimensional layered mMo2C/G(2:1) electrocatalyst. The actual content of Mo2C in m-Mo2C/G catalysts was determined by ICP-MS (Table 1). As reported in previous literatures, the residual carbon on the Mo2C nanocrystals surface may largely influence its HER activity.44 Therefore, the 9 ACS Paragon Plus Environment


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additional HRTEM and TGA analysis for the m-Mo2C/G catalysts were shown in Figure S2. The HRTEM images exhibits that the ultrasmall Mo2C nanocrystals (as indicated by white

Figure 1. (a and b) TEM images and (c) SAXRD pattern of KIT-6/G template; (d and e) TEM and (f) HRTEM images of m-Mo2C/G(3:1) electrocatalyst; (g and h) TEM, (i) HRTEM, (j and k) STEM, and (l and m) elemental mapping images of m-Mo2C/G(2:1) electrocatalyst.


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as marked by red arrows). The TGA curves also confirm that the different amount of residual carbon present in various m-Mo2C/G catalysts and appropriate residual carbon may result in high catalytic activity and stability to some extent.44 On the one hand, the lamellar graphitized carbon shell coated on Mo2C nanoparticles not only could suppress the dissolution or agglomeration, but also allows the rapid charge transfer kinetics during the electrochemical process. On the other hand, the existence of residual carbon can effectively increase the surface area of m-Mo2C/G catalysts, which is conductive to exposes more special catalytic sites as well as facilitate electrolyte access and diffusion of generated hydrogen.44 Figure 2a displays the XRD patterns of different m-Mo2C/G catalysts. The sharp diffraction peaks located at 34.6, 37.7, 39.3, 52.2, 62, and 74.8° are attributed to the typical hexagonal structure of Mo2C (PDF#35-0787) and the broad diffraction peaks located at 20~30° are assigned to the amorphous carbon. No diffraction peaks corresponding to Mo metal or MoOx are detected, indicating that our synthetic approach is very effective for fabricating pure-phase two-dimensional layered mesoporous Mo2C. Furthermore, we also observe that the diffraction peaks corresponding to the hexagonal Mo2C gradually become weaker and boarder with the increase of glucose amount for the synthesis, implying that excess glucose may inhibit the enlargement of Mo2C nanocrystals and lead to the generation of Mo2C nanocrystals with ultrasmall particle sizes. According to the Scherrer equation, the particle sizes of Mo2C nanocrystals in m-Mo2C/G(1:1), m-Mo2C/G(2:1), and m-Mo2C/G(3:1) electrocatalysts are estimated to be 4.2, 4.5, and 6.7 nm, respectively. Whereas the broad diffraction peaks are gradually weaken and even vanished with the decrease of glucose amount. The XRD patterns of bulk Mo2C, m-Mo2C, and Mo2C/G catalysts also were found to be in accordance with hexagonal phase structure of Mo2C (Figure S3a). Figure 2b shows the Raman spectra of m-Mo2C/G electrocatalyst. The characteristic peaks of Mo2C45 and two expected peaks at 1350 and 1590 cm-1 corresponding to the D and G band of graphitic carbon are monitored and assigned to sp3 defect and 11 ACS Paragon Plus Environment


disordered graphitic carbon and sp2-hybridized graphitic carbon layer, respectively.46 Obviously, the intensity ratio of the characteristic peaks belonged to Mo2C and graphitic carbon are gradually strengthened with the decrease of glucose amount for the synthesis, causing the Mo2C nanocrystals distributed uniformly on two dimensional layered m-Mo2C/G electrocatalysts. (a )

(b )

M o 2C

In te n s ity / a .u .

In te n s ity / a .u .

m -M o 2 C /G (1 :1 ) m -M o 2 C /G (2 :1 ) m -M o 2 C /G (3 :1 )

M o 2C

m -M o 2 C /G (3 :1 )

m -M o 2 C /G (2 :1 ) m -M o 2 C /G (1 :1 )

D

G

M o 2C

P D F # 3 5 -0 7 8 7

20

30 40 50 60 2 T h e ta / d e g .

-1

10

70

4

6 D / nm

8

10

m-Mo2C/G(1:1) m-Mo2C/G(2:1) m-Mo2C/G(3:1)

0.2

0.4 0.6 P/P0

0.8

800 1200 1600 R a m a n S h ift / c m -1

(d) Intensity / CPS

2

0.0

400

80

(c)

dV / dD / cm3 ⋅g-1⋅nm-1

3

Adsorbed Quantity / cm ⋅ g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Mo(VI)

Mo(IV)

Mo(II)/Mo = 26.1 at%



1.0

238

236

2000

Mo(II) Mo(II) Mo(IV)

m-Mo2C/G(3:1)

m-Mo2C/G(1:1)

m-Mo2C/G(2:1)

234 232 230 Binding Energy

228

Figure 2. (a) XRD patterns, (b) Raman spectra, (c) N2 adsorption/desorption with an inset graph showing pore size distribution, and (d) XPS spectra of m-Mo2C/G(1:1), mMo2C/G(2:1), and m-Mo2C/G(3:1) electrocatalysts. The surface area and pore feature of m-Mo2C/G electrocatalysts are explored by N2 sorption/desorption

technique

as

illustrated

in

Figure

2c.

All

m-Mo2C/G

electrocatalysts exhibit a typical IV isotherm in the curve at relative pressures (P/P0) from 0.42 to 1.0,47 suggesting the presence of mesopores. The BJH pore size distribution curves displayed in Figure 2d further confirm the presence of highly uniform mesopores exsiting in m-Mo2C/G electrocatalysts with a pore size centered at ~3.6 nm. The H3-type hysteresis loop at P/P0 between 0.8 and 1.0 indicates that the 12 ACS Paragon Plus Environment

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presence of mesopores derived from the stack of two-dimensional layered m-Mo2C/G electrocatalysts. The surface area of m-Mo2C/G(1:1), m-Mo2C/G(2:1), and mMo2C/G(3:1) electrocatalysts are relatively high and are determined to be 376, 294, and 287 m2·g-1, respectively. As control, the nitrogen sorption isotherms and BJH pore size distribution curves of bulk Mo2C, m-Mo2C, and Mo2C/G catalysts were also tested (FigureS3b). The bulk Mo2C and Mo2C/G exhibit a typical type II isotherm with H2 hysteresis loop, whereas the m-Mo2C displays a typical type VI isotherm with H2 hysteresis loop. Thus, it indicates that the bulk Mo2C and Mo2C/G catalysts are nonporous, whereas the m-Mo2C catalyst possesses mesoporous structure. The BJH pore size distribution curves further confirm the presence of highly uniform mesopores exsiting in m-Mo2C electrocatalysts with a pore size centered at ~3.6 nm. The surface area of a bulk Mo2C, Mo2C/G, and meso-Mo2C eletrocatalysts are determined to be 25.6, 50.4, and 148.2 m2•g-1, respectively. Obviously, although the meso-Mo2C eletrocatalyst possesses the highest specific surface area among these control samples, which is still smaller than that of meso-Mo2C/G eletrocatalyst. Thus, our porous Mo2C/G composite electrocatalysts possesses superior HER activity to the control catalysts may root in its large specific surface area and unique graphene hybrid mesoporous structure. The HER activity of m-Mo2C/G catalysts may largely rely on the element composition and valance of m-Mo2C/G catalysts surface. Hence, we carried out the XPS spectra of the as-prepared m-Mo2C/G with different amount of Mo2C. Figure 2d shows that the XPS spectra of Mo 3d in m-Mo2C/G catalysts can be fitted three couples peaks, which can be attributed to Mo2+ (228.9 eV and 231.9 eV), Mo4+ (229.7 eV and 232.8 eV), and Mo6+ (233.1 eV and 236.2 eV), respectively.35 Obviously, the peak intensity attributed to different valence states of Mo was distinguishing in various m-Mo2C/G catalysts and the content of Mo2+ of m-Mo2C/G (2:1) is relatively higher 13 ACS Paragon Plus Environment


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than other two m-Mo2C/G catalysts, implying that the moderate amount of residual carbon may effectively hinder the surface oxidation of Mo2C nanocrystals and stabilize the whole hybrid catalysts. Such results suggest that the m-Mo2C/G (2:1) catalyst may possess the highest HER activity. The

electrocatalytic

performance

of

two-dimensional

layered

m-Mo2C

electrocatalysts for HER are studied in acidic (0.5 M H2SO4), alkaline (1 M KOH), and neutral [0.1 M phosphate buffer solution (PBS) ] conditions using a typical threeelectrode system with a catalyst loading of 0.269 mg•cm-2. All the polarization curves and Tafel plots are acquired without iR correction with a sweep rate of 2 mV•s-1. For comparison, the HER performance of bare graphene, Mo2C/G, meso-Mo2C, bulk Mo2C, and Pt/C (20 wt%) electrocatalysts are also evaluated. Figure 3a display the polarization curves of different meso-Mo2C electrocatalysts and other reference electrocatalysts in acidic media. The Pt/C electrocatalyst exhibits the highest HER activity with a near zero onset overpotential (η1, the overpotential for driving a cathodic current density of 1 mA·cm-2),48 whereas Mo2C/G，meso-Mo2C, and bulk Mo2C present relatively poor HER activity with the order of Mo2C/G > meso-Mo2C > bulk Mo2C in acidic, alkaline and neutral medium. However, the graphene displays a negligible HER activity. By a sharp contrast, in 0.5 M H2SO4 solution, all the mesoMo2C/G electrocatalysts show low onset overpotentials, which are comparable with benchmark Pt/C. For a driving cathodic current density of 10 mA·cm-2, the mesoMo2C/G(2:1), meso-Mo2C/G(1:1), and meso-Mo2C/G(3:1) electrocatalysts only need overpotentials of 135, 145, and 180 mV, respectively, which are much smaller than those of Mo2C/G (262 mV), meso-Mo2C (300 mV), and bulk Mo2C (412 mV) electrocatalysts (Table 1). To our best knowledge, The HER activity (η10 of 135 mV) of m-Mo2C/G(2:1) is superior to most of reported MoP/CF (~200 mV),49 β-Mo2C nanotubes (172 mV),33 Mo2C/CNT(152 mV),31 Mo2C/RGO (150 mV),38 Mo2C/N14 ACS Paragon Plus Environment

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doped CNT (147 mV),50 and Mo2C/CC (140 mV),51 but slightly inferior to nanoporous Mo2C NWs (130 mV)19, MoC–Mo2C heteronanowires (126 mV)52, and MoP-CA2(125 mV).53 Similarly, in 1.0 M KOH solution, the meso-Mo2C/G(2:1), meso-Mo2C/G(1:1), and meso-Mo2C/G(3:1) electrocatalysts exhibit smaller onset overpotentials of 41, 79 and 91 mV, which are much more positive than Mo2C/G (184 mV)，meso-Mo2C (368 mV) and bulk Mo2C (453 mV). Furthermore, for a driving cathodic current density of 10 mA·cm-2,

the

meso-Mo2C/G(2:1),

meso-Mo2C/G(1:1),

and

meso-Mo2C/G(3:1)

electrocatalysts only need overpotentials of 128, 168, and 206 mV, respectively (Figure S4a and Table 1). Extraordinarily, the m-Mo2C/G electrocatalysts also show preferable catalytic activity in 0.1 M PBS (Figure S5a). The turnover frequency (TOF) for each active site in m-Mo2C/G catalyst was also calculated based on the methods reported in previous literature to reveal the intrinsic activity of various m-Mo2C/G catalysts (the detail calculation process see ESI).54 As seen in Figure S6, it can be clearly seen that the m-Mo2C/G (2:1) catalysts possesses the highest TOF value at the same overpotential among various m-Mo2C/G catalysts, and the overpotentials of ∼128 and 121 mV were needed to achieve a TOF of 0.8 s−1 in acidic and alkaline media, which is comparable to the TOF values of Pt obtained at overpotential of ∼0 mV,55 but much smaller than that required for defect-rich MoS2 (300 mV).56 Meanwhile, to reach a TOF of 4 s−1, this m-Mo2C/G electrode needs an overpotential of 233 mV in in acidic media, 39 mV smaller than that used by core−shell MoO3−MoS2 nanowires on FTO (272 mV).57 The mass-normalized HER activity in acidic, alkaline and neutral conditions were also provided (Figure S7), in which the mMo2C/G (2:1) catalyst displays the highest mass-normalized HER activity, expect for Pt/C catalyst among our obtained Mo2C based catalysts in a wide pH range. The result 15 ACS Paragon Plus Environment


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Page 16 of 33

suggests that the m-Mo2C/G (2:1) catalyst exactly possesses the highest intrinsic HER activity, which also can be proved by its highest TOF values. Overall, the mesoMo2C/G electrocatalysts display higher current density than other reference electrocatalysts apart from Pt/C in whole potential region in all PH range. These results demonstrate that the excellent electrocatalytic performance of meso-Mo2C/G electrocatalysts for the HER may root in their unique mesoporous structure hybridized on graphene sheets. The mesoporous structure of Mo2C nanocrystals may enlarge the numbers of exposed active site and improve mass-transport efficiency, and the graphene can enhance the electron transfer property, which are all beneficial for improving electrocatalytic performance. In addition, according to the HER of different Mo2C based catalysts, we induced that the improvement of HER activity of m-Mo2C/G catalysts mainly roots in Mo2C nanocrystals and mesoporous structure rather than bare graphene. Furthermore, the time dependent generated hydrogen amount was further examined by gas chromatography (GC) analysis (Figure S8). The nearly 100% Faradaic efficiency in both acidic and alkaline media indicates that all of the electrons were used for H2 evolution. Table 1 Comparison of HER catalysts. η: overpotential required to achieve the stated current density, b: Tafel slope, J0: the exchange current density. Catalysts

Mo2C Content/wt%

Electrolyte

η1 / mV

η10 / mV

b / mV·dec-1

J0 / mA·cm-2

m-Mo2C/G(2:1)

17

0.5 M H2SO4 1.0 M KOH

8 41

135 128

58 56

6.31 × 10-2 4.09 × 10-2

m-Mo2C/G(1:1)

11

0.5 M H2SO4

10

145

79

5.01 × 10-2

m-Mo2C/G(3:1)

34

1.0 M KOH 0.5 M H2SO4 1.0 M KOH

79 21 91

168 180 204

68 81 83

2.60 × 10-2 4.79 × 10-2 2.7 × 10-2

Mo2C/G

60

0.5 M H2SO4 1.0 M KOH

51 184

262 329

81 93

1.1 × 10-2 0.72 × 10-2

m-Mo2C

65

0.5 M H2SO4

155

300

97

1.54 × 10-2

1.0 M KOH

368

489

95

0.69 × 10-2

bulk Mo2C

68

0.5 M H2SO4 1.0 M KOH

200 453

412 599

123 117

1.80 × 10-2 0.25 × 10-2


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To further study the HER reaction mechanism, the Tafel plots are fitted using the Tafel equation (η = a + b log |j|), where j is the current density and b is the Tafel slope. Figure 3b gives the Tafel plots of different electrocatalysts in 0.5 M H2SO4 solution. In accordance with the results of polarization curves, Pt/C electrocatalyst displays the smallest Tafel slope of 30.0 mV·dec-1. The Tafel slopes of m-Mo2C/G(1:1), mMo2C/G(2:1), and m-Mo2C/G(3:1) electrocatalyst are 79, 58, and 81 mV·dec-1, respectively, much lower than those of Mo2C/G (82 mV), meso- Mo2C (97 mV), and bulk Mo2C (123 mV), suggesting the m-Mo2C/G electrocatalysts might be based on a Volmer-Heyrovsky mechanism with the Heyrovsky step as the rate determining step.6 Furthermore, the exchange current density (j0), as an important kinetic parameter for the HER, represents the intrinsic activity of electrocatalysts. In acidic media, the j0 of m-Mo2C/G(1:1), m-Mo2C/G(2:1), and m-Mo2C/G(3:1) are 5.01×10-2, 6.31×10-2, and 4.79×10-2 mA·cm-2, respectively, which are higher than those of Mo2C/G (1.1×10-2 mA·cm-2), meso-Mo2C (1.54×10-2 mA·cm-2), and bulk Mo2C (1.8×10-2 mA·cm-2), suggesting that the proton discharge kinetics on m-Mo2C/G electrocatalysts is much faster than that on the other three control Mo2C-based electrocatalysts. The similar results are also observed for m-Mo2C/G electrocatalysts in alkaline media (Figure S4b, Table1). Obviously, the HER activity of m-Mo2C/G electrocatalysts is superior to Mo2C/G, meso-Mo2C, and bulk Mo2C electrocatalysts, which may be mainly attributed to the following factors: (i) graphene sheets as supports for uniform distribution of mesoporous Mo2C nanocrystals can improve the conductivity, increase the charge transfer rate, and provide high surface area to contact with the electrolyte; (ii) the presence of mesopores in meso-Mo2C/G electrocatalysts may not only increase the numbers of exposed active sites but also enhance the accessibility of active sites because of the outstanding mass transfer efficiency.


0 -20

Pt/C

-40

m-Mo2C/G(2:1) m-Mo2C/G(3:1)

-60 -80

m-Mo2C/G(1:1)

-2

Mo2C/G

-4

m-Mo2C

-6

bulk Mo2C

-8

G

-100

-0.6

-0.5

-10 -0.3 -0.2 -0.1 0.0

-0.4

-0.3

-0.2

-0.1

0.0

0.3 0.2

(b)

-1

0.1

58 mV

⋅dec

c-1 31 mV⋅de

0.0 0.0

0.1

Page 18 of 33

Pt/C -1 m-Mo2C/G(2:1) c -1 V⋅de c m-Mo2C/G(1:1) 123 m V⋅de -1 m 97 m-Mo2C/G(3:1) V⋅dec 82 m Mo2C/G -1 meso-Mo2C c V⋅de bulk Mo2C 81 m -1 c V⋅de 79 m

0.4

(a)

Overpotential / V

Current density / mA⋅ cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-2


E vs. RHE / V

0.5 1.0 1.5 -2 Log [j/(mA⋅cm )]

2.0

Figure 3. (a) Polarization curves and (b) Tafel plots of m-Mo2C/G(1:1), mMo2C/G(2:1),

m-Mo2C/G(3:1),

Mo2C/G,

m-Mo2C,

bulk

Mo2C,

and

Pt/C

electrocatalysts with a scan rate of 2 mV/s under a rotating speed of 1600 rmp/min in 0.5 M H2SO4 solution Electrochemical impedance spectroscopy (EIS) is also carried out from 10 kHz to 0.1 Hz at various negative potentials in 0.5 M H2SO4 to study the activity of electrocatalysts for the HER. Figure 4a-f displays the Nyquist and corresponding Bode plots of m-Mo2 C/G electrocatalysts. It is found that all the m-Mo2C/G electrocatalysts exhibit two time-constant behaviors. The equivalent circuit obtained by fitting experimental data is shown in the inset of Figure S9. It consists of a resistance Rs, which contains components arising from the resistance in the wiring (Rwiring) and carbon support (Rcarbon), a resistance due to Mo2C (Rcarbide), and a solution resistance (Rsolution) and two parallel branches, one is related to the charge-transfer process (Cd1-Rct) that depends strongly on potentials at the lower frequencies, the other is related to the surface porosity (Cd2-R2) that is constant at different overpotentials at the higher frequencies.31 The Bode plots with two relaxation times at different frequencies further confirm these results. To explore the charge transfer mechanism, the Tafel slopes are acquired from the plot of log (Rct-1) versus potential via fitting the impedance data of various m-Mo2C/G electrocatalysts to the above corresponding equivalent circuit as shown in Figure


140

100

300

80 60

40

-0.12 V -0.14 V -0.16 V -0.18 V -0.20 V -0.22 V

400

Z' / Ω

35 30 25 20

200

15 10

40

100

5

20

0

0 100

200

300

0 -1 10

400

0

1

10

10

Z' / Ω (c)

80

10

4

10

5

10

(d)

400

10

-0.12 V 30 -0.14 V -0.16 V 25 -0.18 V -0.20 V 20 -0.22 V 15

300

Z/Ω

-Z'' / Ω

100

3

Frequency / HZ

-0.12 V -0.14 V -0.16 V -0.18 V -0.20 V -0.22 V

120

2

60

200

10

40 100

20

5 0

0 100

200

300

400

0 -1 10

500

0

1

10

Z' / Ω

300

(e)

4

10

(f)

1000

10

10

200 150 100 50

600

40 30

400

20

200

10

0

0 0

200

400

600

0 -1

10

800

0

1

10

( g)

10

c -1

d

V

3 00

Z '' / Ω

m

ec

2 00

0 .1 4 m -M o 2 C /G (1 :1 ) m -M o 2 C /G (2 :1 ) m -M o 2 C /G (3 :1 )

0 .1 2 0 .0 0 8

0 .0 1 2 -1

10

5

10

10

0 .0 1 6

L o g (R ) / Ω

10 0 80 60 40 20

1 00

0

0

0 .0 0 4

4

(h )

Z '' / Ω

c -1 de

de

V

V

m m 79

83

4 00 -1

58

3

10

m -M o 2 C /G (1 :1 ) m -M o 2 C /G (2 :1 ) m -M o 2 C /G (3 :1 ) M o 2 C /G m -M o 2 C b u lk M o 2 C

5 00

0 .1 6

2

Frequency / HZ

Z' / Ω

0 .1 8

5

50

-0.12 V -0.14 V -0.16 V -0.18V -0.20 V -0.22 V

800

Z/Ω

-Z'' / Ω

250

3

10

Frequency / HZ

-0.12 V -0.14 V -0.16 V -0.18 V -0.20 V -0.22 V

350

2

10

°

0

0 .2 0

-Phase / °

0

-Phase /

-Z'' / Ω

120

(b)

(a)

-0.12 V -0.14 V -0.16 V -0.18 V -0.20 V -0.22 V

160

η / V vs RHE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-Phase / °

Page 19 of 33

0 .0 2 0

-1

0

10 0

2 00

3 00

50

4 00

10 0

150

Z' / Ω

500

60 0

200

70 0

250

80 0

Z' / Ω

Figure 4. Nyquist and Bode plots of (a and b) m-Mo2C/G(1:1), (c and d) m-Mo2C/G(2:1), and (e and f) m-Mo2C/G(3:1) electrocatalysts at the selected overpotentials in N2-saturated 0.5 M H2SO4 solution; (g) Plots of Log(Rct-1) vs. overpotential form of Mo2C/G electrocatalysts. (h) Comparison of the Nyquist plots of different electrocatalysts at an overpotential of -0.22V. 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4g. The Tafel slopes of m-Mo2C/G(1:1), m-Mo2C/G(2:1), and m-Mo2C/G(3:1) are 79, 58, and 83 mV·dec-1, respectively, which are consistent with those by fitting the LSV plots in Figure 3b. The results suggest that the mass-transfer resistance is much less than charge-transfer resistance and might be negligible. Moreover, it also suggests that according to the VolmerHeyrovsky mechanism for the HER over m-Mo2C/G electrocatalyst, the proton discharge reaction is fast and the electrochemical desorption of Hads and H3O+ is the rate determining step. Figure 4h compares the Nyquist plots of various Mo2C-based electrodes at the assigned potential of -0.22 V. Notably, the Mo2C/G electrode with the smaller semicircle indicates much lower Faradaic impedances and faster electron transfer rates than the m-Mo2C/G electrodes, which is contrary to the LSV results and imply that the presence of mesopores in m-Mo2C/G electrocatalysts can increase the numbers and accessibility of exposed active sites towards the HER. Furthermore, the m-Mo2C and bulk Mo2C electrodes with the larger semicircles further imply that graphene support can both improve the conductivity of the electrocatalysts and increase the charge transfer rate. We also performed the EIS spectra of porous Mo2C/G composite electrocatalysts in alkaline media and neutral media (Figure S10). It can be seen clearly that the obtained EIS data in neutral and alkaline media are the similar as the results obtained in acidic media. These results imply that graphene support can both improve the conductivity of the m-Mo2C/G electrocatalysts and increase the charge transfer rate during HER in a wide pH range (0-14). It is a remarkable fact that m-Mo2C/G(2:1) electrocatalyst exhibits the best HER activity among three as-synthesized m-Mo2C/G electrocatalysts. The high HER activity of m-Mo2C/G electrocatalyst might be attributed to its faster electron transfer rate as evidenced by the EIS measurements. To further elucidate the reason why mMo2C/G(2:1) electrocatalyst displays superior HER activity to m-Mo2C/G(1:1) and mMo2C/G(3:1) electrocatalysts, effective electrochemical active surface areas are estimated from the double-layer capacitances (Cdl) in proportion to the contact area 20 ACS Paragon Plus Environment

Page 20 of 33

Page 21 of 33

between the electrocatalysts and electrolyte measured by the cyclic voltammetry (CV). As it can be seen in Figure 5, the m-Mo2C/G(2:1) electrocatalyst possesses a higher capacitance value (46.6 mF·cm-2) compared to m-Mo2C/G(1:1) (42.5 mF·cm-2) and mMo2C/G(3:1) (20.5 mF·cm-2), thus resulting in its superior performance for the HER. m-Mo2C/G(1:1)

(a)

20 40

J / mA cm-2

5

0

-5

200

J / mA cm-2

10

mV S-1

-10 0.15

0.20

0.25

0.30

0.35

12 9 6 3 0 -3 -6 -9 -12 -15 -18

20 40

220 mV S-1

0.15

0.20

4 -2

(c)

20

20 40

15

m-Mo2C/G(3:1)

2 0 -2 -4

200

-6 0.25

0.35

0.30

. 46

10

F 6m

cm

m .5 42

5

0

0.35

(d)

-2

-2

F

cm

m 20.5

mV S

-8 0.20

0.30

m-Mo2C/G(1:1) m-Mo2C/G(2:1) m-Mo2C/G(3:1)

-1

0.15

0.25

E. vs RHE

∆J / mA cm-2

6

(b)

m-Mo2C/G(2:1)

E. vs RHE

J / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

100

-2

F cm

150

200

Scan rate / mV s-1

E .vs RHE

Figure 5. CVs performed at various scan rates in the region of 0.1–0.35 V vs. RHE for (a) mMo2C/G(1:1), (b) m-Mo2C/G(2:1), and (c) m-Mo2C/G(3:1) electrocatalysts; (d) The differences in current density at 0.25 V vs. RHE plotted against the scan rate and fitted to a linear regression allows for the estimation of Cdl. High catalytic durability is also a significant criterion in practical application for the HER. Long-term stability of m-Mo2C/G(2:1) electrocatalyst is tested by CV for scanning 3000 cycles and continuous HER at fixed overpotentials. As it is shown in Figure 6a, the two polarization curves nearly overlap before and after 3000 cycles for m-Mo2C/G(2:1) electrocatalyst in 0.5M H2SO4 solution, the corresponding inset in Figure 6a indicates that the catalytic currents remain stable at around 12 mA·cm-2 over 20 h. While as shown in Figure 6b and Figure S3b, there are only a slight decay of 21 ACS Paragon Plus Environment


cathodic current after 3000 cycles and at an overpotential of 140 and 600 mV over 20 h for m-Mo2C/G(2:1) electrocatalyst in 1.0 M KOH and 0.1 M PBS solution, respectively. Such a good stability indicates that m-Mo2C/G electrocatalyst has potential application prospect for the HER. We further characterized the morphology and structure of m-Mo2C/G catalysts after stability test using TEM, and the results are shown in Figure S11. It can be seen that the morphology and structure of m-Mo2C/G catalysts have no obvious change and also maintain 2D layered mesoporous structure. This result indicates that the m-Mo2C/G catalysts also possess outstanding structure

Current density / mA⋅ cm-2

-20 -40 -60 -80

-100

-0.3

-0.2

0 η = 140 mv without IR correction

-10 -20 -30

0

5

10 15 Time / h

-0.1

E vs. RHE / V

0.0

20

0.1

-2

(a)

Initial After 3000 cycles

0 -10

(b)

Initial After 3000 Cycles

-20 Current density / mA⋅ cm-2

0


-2

stability. Current density / mA⋅ cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

-30 -40 -50

0

η =140 mv without IR correction

-10

-60

-20

-70

-30

-80 -90 -0.4

10

-0.3

-0.2

0

-0.1

5

10 Time (h)

0.0

15

0.1

20

0.2

E vs. RHE / V

Figure 6. Stability tests of the m-Mo2C/G(2:1) electrocatalyst via a CV scanning for 3000 cycles with an inset graph showing time-dependent current density curve in (a) 0.5 M H2SO4 and (b) and 1.0 M KOH solution. In order to reveal the influence of the pyrolysis temperature on the electrocatalytic activity, we further synthesize other two m-Mo2C/G(2:1) electrocatalysts under 800 and 1000 °C. The XRD patterns of m-Mo2C/G electrocatalysts obtained under different calcination temperatures are also consistent with the reference XRD patterns of hexagonal Mo2C (PDF#35-0787, Figure S12). The similar polarization curves and Tafel plots are inspected in acidic (Figure 7a and b) and alkaline conditions (Figure S13). Among the as-prepared electrocatalysts, mMo2C/G(2:1) electrocatalyst obtained under 900 °C exhibits the best HER activity in both acidic and alkaline conditions since it exhibits much lower Faradaic impedance (Figure 7c) and higher capacitance (Figure 7d). It clearly turns out that the pyrolysis temperature has 22 ACS Paragon Plus Environment

Page 23 of 33

important effect on the HER activity of meso-Mo2C/G electrocatalysts and the optimal

m-Mo2C/G (900 °C) m-Mo2C/G (800 °C) m-Mo2C/G (1000 °C)

0

-10

(a)

0 -2

-20

-4 -6

-30

-8 -10

-0.2 -0.1 0.0

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

-1

⋅dec 67 mV

0.16

6 2 mV

⋅dec 58 mV

0.3

E vs. RHE / V

0.6

(c)


24 20

40

20

50

100

150 Z'(Ω)

1.2

200

250

(d)


-2

m

F ⋅c

16

m 46

-2

12

2

8 4

0

0.9 -2

Log [j(mA⋅cm )]

∆ J / mA cm-2

60

-1

⋅dec

-1

0.12

0.08 0.0

0.0

(b)

m-Mo2C/G(900°C) m-Mo2C/G(800°C) m-Mo2C/G(1000°C)

0.20

Overpotential (V)


-2

calcination temperature could be fixed around 900 °C.

Z''(Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

F ⋅cm 5m

-2

3.8 mF⋅cm 40

80

120

160

-1

200

240

Scan rate / mV s

Figure 7. (a) Polarization curves and (b) Tafel plots of the m-Mo2C/G electrocatalysts obtained with the mass ratio of (NH4)6Mo7O24 ·4H2O and glucose at 2:1 under 800, 900, and 1000°C with a scan rate of 2 mV/s under a rotating speed of 1600 rpm in 0.5 M H2SO4 solution; (c) Nyquist plots of the various catalysts at η = -0.22 V in 0.5 M H2SO4 solution; (d) The differences in current density at 0.25 V vs. RHE plotted against the scan rate and fitted to a linear regression allows for the estimation of Cdl in 0.5 M H2SO4 solution. Based on the above results, we propose a synergetic mechanism to explain the superior HER performance of m-Mo2C/G electrocatalysts.As illustrated in Scheme 2, the enhanced electrocatalytic activity and the improved durability of m-Mo2C/G electrocatalysts may be attributed to the unique two-dimensional layered mesoporous structure and the compact integration of Mo2C nanocrystals with graphene sheets. Firstly, the existed graphene sheets can effectively enhance the electron transport performance during the process of electrocatalysis. Secondly, the unique mesoporous structure is not only beneficial for improving the mass transport properties of H+ and H2, but also helpful to increase the 23 ACS Paragon Plus Environment


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numbers of exposed active sites. Meanwhile, the mesoporous structure also can prevent unwanted agglomeration of active sites, which may greatly contribute to the enhancement of electrocatalytic activity and stability. Thirdly, the thin two-dimensional layered structure of mMo2C/G electrocatalysts can ensure the accessibility of the reactant molecules due to the short diffusion distance to active sites, leading to a further promoted HER performance. Finally, the synergy effect between ultrafine Mo2C nanocrystals and graphene can further enhance the HER activity due to the strong electronic coupling.58, 59 Thus, as summarized in Table S1, our two-dimensional layered

m-Mo2C/G

electrocatalyst

shows superior electrocatalytic

performance for the HER over most of the representative Mo2C-based electrocatalysts recently reported. The HER may follow the Vomler-Heyrovsky mechanism. In the first step, a proton-coupled electron transfer at the catalyst surface yields an intermediate adsorbed hydrogen atom (H(aq)+ + e- → Hads); Then, the adsorbed hydrogen atom can react with another proton from the solution accompanied by a second electron transfer to form molecular hydrogen (H(aq)+ + Hads + e- → H2(g)).60 H+ H

Abundant Active Sites Effective Mass-transfer

eCathode

+

e-

H2

H H2

H2

H+

Excellent Conductibility

eVolmer-Heyrovsky mechanism

Scheme 2. The proposed synergetic mechanism for the enhanced catalytic performance of mMo2C/G electrocatalysts for the HER. 4. CONCLUSIONS


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In summary, we successfully synthesized a series of precious-metal-free HER electrocatalysts containing mesoporous Mo2C nanocrystals decorated on graphene sheets using inexpensive glucose as a carbon source and in-situ assembled mesoporous KIT-6/G as a hard template. The obtained meso-Mo2C/G electrocatalysts possess unique two-dimensional layered structural feature with well distributed mesopore (~3.6 nm), high surface area (up to 297 m2·g-1) and ultrasmall Mo2C nanocrystals (as small as 4.5 nm). As a consequence of the unique mesoporous structure of Mo2C nanocrystals and the compact integration of Mo2C nanocrystals with graphene sheets, the obtained m-Mo2C/G electrocatalysts display excellent HER activity and stability in a wide PH range due to the much faster charge transfer rate and larger electroactive area, demonstrating that it can be regarded as a promising HER catalyst in practical application. ASSOCIATED CONTENT Supporting Information TEM and HRTEM images of Mo2C/G electrocatalyst; Polarization curves and Tafel plots of m-Mo2C/G catalysts and control samples in 1.0 M KOH and 0.1 M PBS solutions; EIS plots of m-Mo2C/G catalysts; XRD of m-Mo2C/G catalysts obtained at different calcination temperature. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Tel.: +86 4995400. E-mail address: [email protected] (Jun Zhang) Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS



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This work was supported by NSFC (21261011), Application Program from Inner Mongolia Science and Technology Department (2011401), Program for New Century Excellent Talents in University (NCET-10-0907). REFERENCES 1. Chow, J.; Kopp, R. J.; Portney, P. R. Energy Resources and Global Development. Science 2003, 302, 1528-1531. 2. Dresselhaus, M. S.; Thomas, I. L. Alternative Energy Technologies. Nature 2001, 414, 332-337. 3. Turner, J. A. Sustainable Hydrogen Production. Science 2004, 305, 972-974. 4. Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S.W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473.

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Table of Contents Graphic

TEOS

Calcination

Precursor

P123 GO

KIT-6/G

P123/KIT-6/GO

Precursor/KIT-6/G NaOH

P123/KIT-6

KIT-6

Precursor/KIT-6

Calcination

Mo2C NPs m-Mo2C/G

+ Vex e-

_

H2 e-

H2

H2

Meso-Mo2C/G

H2O Pt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H2

eeH3O+

H3O+


H3O+

Universal Strategy to Fabricate a Two-Dimensional Layered

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