Synthesis of RGO-Supported Molybdenum Carbide (Mo2C-RGO) for

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Kinetics, Catalysis, and Reaction Engineering

Synthesis of RGO-supported molybdenum carbide (Mo2C-RGO) for hydrogen evolution reaction under the function of poly (ionic liquid) Yongli Sun, Baoli Wang, Na Yang, Xiaowei Tantai, Xiaoming Xiao, Haozhen Dou, Luhong Zhang, Bin Jiang, and Deqiang Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00209 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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Industrial & Engineering Chemistry Research

Synthesis of RGO-supported molybdenum carbide (Mo2C-RGO) for hydrogen evolution reaction under the function of poly (ionic liquid) Yongli Sun,

†,‡

Baoli Wang,

†

Na Yang, *,† Xiaowei Tantai,

†

Xiaoming Xiao,

†

Haozhen Dou, † Luhong Zhang, †,‡ Bin Jiang, † Deqiang Wang§ † School ‡

of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University,

Tianjin 300072, China §

Shenglong

chemical

co.

LTD

，

Tengzhou

1

ACS Paragon Plus Environment

，

277500

，

China

Industrial & Engineering Chemistry Research 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

ABSTRACT: Electrochemical water splitting, which is economical and sustainable, has been considered as one of the most potential methods to produce large amount of hydrogen with high purity. However, the development of uniformly dispersed electrocatalysts with robust catalytic activity and high stability for electrochemical water splitting still faces a great challenge. In this study, poly(ionic liquid) (PIL) was used as a bridge between the polyoxometallate and the reduced graphene oxide (RGO) to anchor Mo uniformly on the surface of the RGO. As a result, the Mo2C-RGO exhibits excellent activity in hydrogen evolution reactions in an acidic media with a small overpotential of 99 mV to drive 10 mA cm-2 and a low tafel slope (54.6 mV dec-1) in acidic solution. And no significant degradation was observed after 20 h operation. This facile strategy offers new opportunities to accurately design functional 2D materials with unique properties. Keywords: Hydrogen evolution, Molybdenum carbide, Poly(ionic liquid), Reduced graphene oxide

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

INTRODUCTION To alleviate energy crisis and environmental pollution, hydrogen has been regarded

as a promising alternative to fossil fuels, due to its environmentally friendly, renewable and abundant characteristics. However, the main methods of achieving industrialization of hydrogen production at present are methane steam reforming and coal gasification, which require huge amounts of fuels and emit a large amount of dangerous and environmentally harmful gases.1-3 Compared with those traditional methods, electrochemical water splitting strategy for hydrogen generation is considered to be more economical, sustainable and efficient hydrogen generation strategy. Although the best electrocatalysts for hydrogen evolution reactions are Pt or Pt based materials, their high cost and low reserve are the biggest challenges limiting their industrial applications.4-6 Thus, the development of efficient and inexpensive HER electrocatalysts has become a significant and urgent need.7 Mo-based compounds, such as Mo2C,8-11 MoN2,12-14 MoS2,15-18 and others19-21 have been investigated as a new class of electrocatalysts, owing to their Pt-like catalytic behaviours.22 Among them, Mo2C has attracted great interest due to its excellent electrocatalytic performance.20, 23, 24 However, Mo2C particles are highly susceptible to sintering and aggregation during high-temperature carbonization, which greatly reduce the activity of the catalyst.9, 20 In order to enhance the HER activity of Mo2C, it is a good choice to combine Mo2C particles with conductive supports such as carbon nanosheets (NSs) 25-27 and carbon nanotubes (CNT) 12, 28, 29. Among these conductive

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supports, reduced graphene oxide (RGO) has been extensively studied due to its excellent electron chemical stability and transport properties.30,

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Therefore,

RGO-supported Mo2C is expected to be a highly active and stable electrocatalyst for HER.12, 29, 32-34 However, obtaining RGO-supported Mo2C with a thin layer and high dispersion requires a very complicated and delicate design. Poly (ionic liquids) (PILs) has attracted tremendous attention as a special and large family. PIL has numerous advantages, including designable structure, adjustable solubility, low toxicities, chemical and thermal stability, making it suitable for the catalytic applications.35, 36 Moreover, ion exchange allows PIL to be a proponent of polyoxometallates (POMs), and the introduction of monomers with different functional groups will change the functionality. Based on the above excellent properties, PIL was expected as a bridge to combine Mo2C and RGO NSs. In the family of PIL, the extensively studied imidazolium-based polymer ionic liquids (IPILs) have drawn our attention. On the one hand, since the imidazole ring contains two N atoms, it is expected that the appropriate amount of N and C will be introduced through the structural designability of PIL. On the other hand, IPIL can not only easily bind to POMs, but also non-covalently interact with the conjugated graphene surface via π-π/ non-covalent interaction or cation-π stack.37 It is possible that the presence of PIL can effectively resist the sintering and agglomeration of Mo2C particles during the high temperature carbonization, and the irreversible stacking of RGO NSs. At the same time, the changes in the molecular structure can be achieved by the changes in the side chains of the IPIL, which may cause differences in the 4


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properties of the final product. Herein, we chose the simplest IPILs (the molecular structure shown in figure.1) was chosen as the research object.

Figure 1. Schematic illustration of the synthesis process of PILs. In this work, we first proposed to use PIL as a bridge connecting Mo2C and RGO to synthesize RGO-supported Mo2C. PILs were coated on the surface of the RGO via π-π/ non-covalent interaction or cation-π stack to form PIL modified RGO (PIL-RGO). Then Mo was anchored on the surface RGO by ion exchange reaction between PIL-RGO and POM, followed by high-temperature carburization. Since different functional groups will change the functionality of PIL，the effect of the length of the side chain of PIL on the microstructure and catalytic effect of the final product was investigated when the side chains were ethyl, n-butyl, n-octyl and n-hexadecyl, respectively (named PIL2, PIL4, PIL8 and PIL16). The electrocatalytic behavior of the prepared Mo2C-RGO (synthesized by PIL2, PIL4, PIL8 and PIL16, respectively named Mo2C-RGO@2, Mo2C-RGO@4, Mo2C-RGO@8 and Mo2C-RGO@16) for hydrogen evolution was systemically studied and compared with Mo2C and Pt/C catalysts, indicating that it has excellent catalytic activity and high stability in acidic solution, especially Mo2C-RGO@8 .

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

EXPERIMENTAL SECTION Materials. Ethyl chloride, n-butyl chloride, n-octyl chloride and n-hexadecyl

chloride were purchased from Aladdin Reagent Co. Ltd. Nafion solution (5 wt %) was purchased from Sigma-Aldrich Co. LLC. Argon gases (99.999%) were purchased from Tianjin DongXiang Chemical Technology Co., Ltd. Ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O), 2, 2-Azobis (2-methylpropionitrile) (AIBN), 1-Methyl-2-pyrrolidone (NMP) and ethyl acetate (EA) were purchased from Tianjin Jiangtian Chemical Technology Co. Ltd.

Preparation. The Mo2C-RGO nano hybrid catalysts were synthesized by the following two steps (as shown in Figure. 2): (1) preparation of precursor Mo8O26@PIL-RGO (Mo7O246- isopolyanion was assumed to reassemble into Mo8O264during preparation.)38, 39 precipitates; (2) annealing of the Mo8O26@PIL-RGO powder to obtain Mo2C-RGO. In the first step, GO nano sheets were synthesized by Hummers method40 and PILs was synthesized in our previous work38. Then, PIL-RGO was synthesized based on the reported method with a slight modification.41, 42 In a typical procedure, GO (0.02 g) was dispersed in 40 mL of anhydrous NMP after ultrasound for 30 min with a bath solicitor, followed by an addition of 75 mg PIL with stirring. Then a uniform brown suspension was obtained and heated to 150°C for 1 h in an oil bath. Solvent thermal reduction occurred during this process and the brown suspension became black. After cooling to the room temperature, 10 ml of an aqueous solution of (NH4)6Mo7O24·4H2O (0.165 g) was slowly dropped into the black

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suspension, and then stirred at room temperature for 2 h in order to react sufficiently. Then precursor (Mo8O26-PIL-RGO) was obtained, after centrifugation and freeze-drying.

Figure 2. Schematic illustration of the synthetic process of Mo2C-RGO. The as-synthesized Mo8O26-PIL-RGO was transferred into a tube furnace. The sample was annealed from the ambient temperature to 800°C at a heating rate of 2°C min-1 for 2 h, under an argon atmosphere. Mo2C-RGO was obtained, after the furnace was naturally cooled to room temperature. For comparison, Mo2C, Mo2C-RGO@0 (synthesized without using PIL) and Mo2C-RGO (catalysts, synthesized with PIL2, PIL4, PIL8 or PIL12, were named Mo2C-RGO@2, Mo2C-RGO@4, Mo2C-RGO@8 and Mo2C-RGO@16, rapidly) with various PILs were synthesized in a similar manner.

Characterization methods. X-ray powder diffraction (XRD) measurements were carried out using a Smart Lab X-ray diffract meter (Panalytical, Holland) 7



equipped with Cu-Kα radiation (λ = 0.15406 nm) source. The data was collected at 40 k V and 40 mA with 2θ angular regions between 10° and 80°, with a scan rate of 8° min−1. The FT-IR of the catalyst was record on a Bio-Rad FTS 6000 FT-IR spectrometer. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a spectrometer Ecolab 250Xi (Thermo Fisher Scientific, USA) with an Al X-ray source operated at 150 W. Scanning electron microscopy (SEM) measurements were carried out by using a FESEM S-4800 (Hitachi Corp, Japan) to get the information of morphology and structure of the as-prepared catalysts. Field emission transmission electron microscope (FETEM) and energy dispersive X-ray spectroscopy (EDX) investigations were carried out by using a FETEM JEM-2100F (Rigaku Corp., Japan) at an accelerating voltage of 200 k V to get the information of particle size distribution and the composition of the as-prepared catalysts. FT-IR spectra of monomer, precursor were recorded on a Bio-Rad FTS 6000 FT-IR spectrometer using a KBr pellet technique. AFM measurements were performedon freshly cleaved micaby using a AFM NTEGRA Spectra (NT-MDT, Russia). The procedure to assemble on a quartz slide plateor freshly cleaved mica was the same as that on ITO.The Brunauer-Emmett-Teller surface areas and porosity were evaluated by nitrogen absorption-desorption isotherm measured at 77   K using an ASAP 2020 accelerated surface area and porosimetry system (Micromeritics, USA). The pore size distribution plot was analyzed with the desorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) model.

Electrochemical measurements. Electrochemical measurements were 8


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obtained through a computer controlled CHI 660e (Shanghai Chenhua Co., China)) three-electrode system electrochemical workstation. A saturated calomel electrode (SCE) and carbon rods43, 44 were used as a reference electrode and counter electrode, respectively. A glassy carbon disk 3.0 mm in diameter, coated by catalysts films, was used as the working electrode. The catalysts films were prepared as follows: 5.0 mg of catalyst was dispersed in 1.0 mL solution containing 20.0 μL of 5 wt% Nafion solution and 980.0 μL of isopropanol. After applying ultra-sonication for about 40 min, a homogeneous ink was obtained, then 5.0 μL the as-prepared ink was dropped on the electrode by using a micropipette, and then dried in air; the calculated catalysts loading was 0.50 mg cm−2. A Pt/C catalyst (20 wt% Pt/C, Johnson Matthey), used as reference material, was prepared by same way, and the mass loading was 0.25 mg cm−2. After initial stabilization of activities, the linear sweep voltammetry (LSV) curves were recorded with a scan rate of 5.0 mV·s−1 in 0.5 M H2SO4 aqueous solution that purged with N2. Cyclic voltammograms were obtained from 0.13V to 0.28V (in 0.5 M H2SO4) at the scan rate of 20-200 mV s-1. Electrochemical impedance spectroscopy (EIS) was measured at an open circuit potential with a frequency range of 0.1 to 100M Hz and an AC amplitude of 10 mV. For the electrochemical stability tests, a glassy carbon disk, prepared by above method, was used as the working electrode. The HER activities of all the as-prepared catalysts were recorded during a long time period. All potentials were recorded relative to the reversible hydrogen electrode (RHE) (ERHE = ESCE + (0.242 + 0.591PH) V). The ohmic resistance (R) caused by the electrolyte/contact resistance, set and measured by EIS. The current

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density (I) is normalized to the geometric area of the working electrode.

 RESULTS AND DISCUSSION Physicochemical characterization. The XRD patterns of GO, RGO and PIL-RGO are exhibited in Figure 3a. A sharp diffraction peak at 2θ = 10.9° was observed in XRD pattern of GO, and the corresponding interlayer space was calculated to be 0.81 nm according to the Bragg's law. The dispersion peak of RGO appeared at about 24.6° and the interlayer spacing was reduced to 0.36 nm due to the removal of the oxygen-containing groups.37 Compared with RGO, the PIL-RGO diffraction peak intensity weakened and moved to 21° with a layer spacing of 0.42 nm, indicating that PIL played a role in effectively preventing re-stacking of RGO nanosheets.35 After heating treatment at 800°C, the characteristic peaks at 34.6, 38.0, and 39.68, corresponding to the (100), (002), and (101) planes of hexagonal β-Mo2C (PDF #35–0787) observed in Figure. 3b, were broader and exhibited lower intensity than those of Mo2C because of the smaller crystallites of Mo2C or Mo2C coated with amorphous carbon. Additionally, no impurities were detected from the XRD pattern. Figure 3c and d showed XRD diffraction patterns of the products obtained at different carbonization temperatures and using different PILs, respectively. Although the XRD characteristic peaks of Mo2C became more and more obvious as the carbonization temperature increased, the presence of metallic molybdenum could be clearly observed when the temperature raised to 900°C at 2θ = 42.3°. And we found that Mo2C-RGO@8 had stronger peaks than others. 10


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Figure 3. (a) XRD patterns of GO, RGO, and PIL-RGO. ; (b) XRD Patterns of Mo2C-RGO@8

and

Mo2C.

(c)

XRD

patterns

of

Mo2C-RGO@700°C,

Mo2C-RGO@800°C and Mo2C-RGO@900°C. (d) XRD Patterns of Mo2C-RGO@2, Mo2C-RGO@4 and Mo2C-RGO@16.

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1

2 3 Size (nm)

4

5

Figure 4. (a) SEM image of RGO, (b) SEM image of Mo2C and (c) SEM image of Mo2C-RGO@8, (d-g) EDX elemental mapping of C, N and Mo of Mo2C-RGO@8, (h-k) SEM images of Mo2C-RGO@2, Mo2C-RGO@4, Mo2C-RGO@8 and Mo2C-RGO@16, respectively. (l, m) TEM and (n) HRTEM image of Mo2C-RGO@8. Scale bar: l (50 nm); m (10 nm) and n (2 nm). Inset: distribution of particle size.

Figure 5. PIL2, PIL4, PIL8 and PIL16 dissolved in NMP solution (under the same conditions). Figure 4a shows a SEM image of RGO. It can be observed that the RGO microplates were irreversibly aggregated after the solvothermal reduction process. For 12


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comparison, Mo2C was also synthesized by a similar preparation procedure without GO. Figure 4b reveals that Mo2C nanoparticles tended to agglomerate during the heat treatment to form large nanoparticles, which decreased the exposed active surface. After annealing at 800°C, Mo2C-RGO maintained well 2D sheet-like structure (Figure 4c), which increased the number of exposed active sites compared to Mo2C. The ion self-assembly of polymeric cation and polymolybdate anion might hinder the aggregation of molybdenum carbide during high temperature carbonization. And the sheets of Mo2C-RGO are thinner than RGO, probably because the PIL coated on the surface of RGO hindered its restacking. The AFM pattern indicates that Mo2C-RGO@8 has a typical lamellar structure and the thickness of the sheet is about 3 nm (as shown in Figure S6). The structure had a large BET surface area of 212.5 m2 g−1 (as shown in Figure S4). Figures 4d-g show the SEM and corresponding energy dispersive X-ray spectroscopy (EDX) elemental mapping images, which confirmed that C, Mo and N were uniformly distributed on the surface of Mo2C-RGO. In order to investigate the effect of PIL side chain length on morphology, the microscopic morphology of four different side chain lengths were observed by SEM, which are shown in Figure 4h-4k. Obviously, when the side chain is an n-octyl group, the product has the best microscopic morphology. This phenomenon might be due to the fact that the change of side chain has a great influence on the solubility of PIL in NMP solution. To investigate the solubility in NMP, an equal amount of PILs (PIL2, PIL4, PIL8 and PIL16) were added into 10 ml of NMP and stirred for 30 min. As shown in Figure 5, only PIL8 completely dissolved. Moreover, the solubility of PIL in

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NMP increased first and then decreased as the length of the side chain increased, demonstrating that the structure of PIL substantially influenced the HER performance. A transmission electron microscopy (TEM) image of Mo2C-RGO@8 revealed that a large amount of Mo2C nanoparticles (NPs) were uniformly assembled on the RGO NSs at a high density and that voids were present (Figure 4l and 4m). This can be attributed to a clear interconnected mesoporous structure, which was formed during the ion self-assembly of the polymeric cation and polymolybdate anion.38 For comparison, transmission electron microscopy (TEM) images of several other catalysts were also obtained, as shown in Figure S2. The molybdenum carbide nanoparticles of Mo2C-RGO@8 were most uniformly distributed and the smallest size. The high-resolution TEM (HRTEM) image exhibited clear lattice fringes with a planar distance of 0.23 nm, corresponding to the (101) planes of Mo2C (Figure 4n). Notably, the Mo2C NPs were embedded in the carbon shells, which can control the Mo2C particles in smaller sizes.26 These results demonstrated the successful synthesis of the Mo2C-RGO nano composite. Hence, the above results confirm that the presence of PIL plays an important role in the generation of highly dispersed and nano-sized Mo2C NPs coated on RGO NSs.

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Figure 6. High-resolution XPS spectra of (a) N 1s, (b) Mo 3d electrons of Mo2C and (c) N 1s, (d) Mo 3d electrons of Mo2C-RGO@8. X-ray photoelectron spectroscopy (XPS) analyses of Mo2C-RGO@8 and Mo2C catalysts were carried out to elucidate their valence states and compositions. As observed, the XPS spectrum indicated the presence of C, N, O and Mo in the catalyst. The N1s spectra of Mo2C exhibited the peaks at 398.6 and 394.8 e V, which were assigned to pyridine-N and Mo-N bond formed by the carbonization treatment (Figure 6a),45 respectively. High-resolution Mo 3d XPS spectrum can be deconvoluted into five peaks, corresponding to Mo-C (228.4 e V), Mo3+ (229.1 and 232.5 e V), Mo4+

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(231.8 e V), and Mo6+ (235.2 e V) (Figure 6b).9, 25,46 XPS analyses of Mo2C-RGO are shown in Figure 6c and d. Compared with the deconvolution of N1s for Mo2C, the intensity of N-Mo bond in Mo2C-RGO is higher, as a result of a large number of N-Mo-C bonds existed in Mo2C-RGO, which were the active sites for hydrogen evolution reaction.47 Peak deconvolution of Mo3d verified the existence of Mo2C (Mo2+ at 228.4 eV), MoO2 (Mo4+ at 231.8 eV), MoO3 (Mo6+ at 235.2 eV) and Mo3+ (229.1 and 232.5 e V). Moreover, a significant increase in the content of Mo-C and Mo3+ in Mo2C-RGO was observed, while the content of MoO2 and MoO3 were substantially reduced. The decrease in aberration of Mo-O from MoO3 and MoO2 after doping with reduced graphene oxide may be due to changes in the type and number of exposed crystal faces of the crystal (Figure S2 f and g).48-50

Electrocatalytic HER performance. A three-electrode system was adopted to evaluate the electrocatalytic activities of Mo2C-RGO toward the HER in 0.5 M H2SO4. In comparison, the electrocatalytic activities of Mo2C and commercial Pt-C (20 wt% Pt on carbon black from Johnson Matthey) were also assessed. The corresponding polarization curves are shown in Figure 7a. The commercial Pt-C displayed the highest electrocatalytic activity, with an onset over potential of nearly zero. Impressively, Mo2C-RGO@8 exhibited a low onset over potential of 40 m V, which was almost a quarter of Mo2C or Mo2C-RGO@0. Meanwhile, the poor electrocatalytic performance of RGO was clarified in Figure 7a. To achieve the current density 10 mA cm-2, Mo2C and Mo2C-RGO@0 required overpotential of 320 mV and 270 mV, while Mo2C-RGO required only about 99 mV. By analyzing the 16


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XRD and SEM images of Mo2C-RGO@0 (Figure S3), it was found that the graphene sheets were severely stacked when PIL was absent. PIL plays an important role in the formation of 2D structures. This structure may provide more exposed active sites and contribute to the high activity of the catalyst. To elucidate the HER mechanism, tafel slopes were fitted to tafel equation (that is, η=blog (j) + a, where b is the tafel slope, j is the current density and a is the overpotential value when the current density is a unit value (1A/cm2)), as shown in Figure 7b. The tafel slope of commercial Pt-C was 30.6 mV dec-1, which was in agreement with the reported value.34 The tafel slope of Mo2C-RGO@8 was 54.6 m V dec-1, indicating a higher performance than that of Mo2C (183.5 mV dec-1). The tafel slope of the catalyst modified electrode embodies the inherent properties of the rate limiting step of HER. The steps that may occur during the occurrence of HER on the electrode surface can be distinguished by the tafel slope analysis. In an acidic solution, three separate reaction steps are likely to cause hydrogen detaches from the electrode surface.51 Meanwhile, the tafel slope of Mo2C-RGO suggested that hydrogen evolution on the Mo2C-RGO electrode was probably via a Volmer-Heyrovsky mechanism. In contrast, Mo2C exhibited a high tafel slope, suggesting that may arise from various reaction pathways depending on the surface coverage of adsorbed hydrogen.52

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Figure 7. (a) Polarization curves of Mo2C, RGO, Mo2C-RGO@0, Mo2C-RGO@8 and Pt-C. (b) Tafel slopes of Mo2C, Mo2C-RGO@8 and Pt-C. (c, d) Polarization curves and Tafel slopes of Mo2C-RGO with different kind of PILs (PIL2, PIL4, PIL8, PIL16). (e, f) Polarization curves and Tafel slopes of Mo2C-RGO at different carbonization temperature. The effect of the carbon chain length of PILs on electrocatalytic performance was

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also investigated as shown in Figure 7c and 7d. The HER activities of the corresponding Mo2C-RGO@2, Mo2C-RGO@4, Mo2C-RGO@8 and Mo2C-RGO@16 were also evaluated. As seen from Figure 7c, 7d, Mo2C-RGO@8 showed the lowest onset overpotential and the smallest tafel slope among three samples. It is speculated that the worse solubility of PIL in NMP, the dispersibility of Mo2C on RGO NSs was poorer. Subsequently, the influence of carbonization temperature on catalytic effect was also studied. Figure 7e, 7f show the polarization curves and tafel slopes of three samples carbonized at 700°C, 800°C and 900°C (defined as Mo2C-RGO@700°C, Mo2C-RGO@800°C and Mo2C-RGO@900°C). The onset overpotentials of Mo2C-RGO@700°C, Mo2C-RGO@800°C and Mo2C-RGO@900°C were 150, 99 and 200 mV, and the tafel slopes were 66.9, 54.6 and 97.8 mV dec-1, respectively. Among these catalysts, the Mo2C-RGO@800°C exhibited the optimal HER activity, possibly because few active sites of Mo2C were produced when Mo2C-RGO@700°C was carbonized at 700°C. Meanwhile, the content of metallic molybdenum increased with the increment of carbonization temperature (Figure 3), resulting in the decline of active sites at 900°C. All of these results were consistent with the SEM, XRD characterizations (Figures 3-4). Therefore, appropriate selection of PIL structure and carbonization temperature was essential for forming high-HER active sites.

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Figure 8. (a) Nyquist plots measured over the frequency range of 100 MHz to 0.1 Hz at overpotential of 200 mV in 0.5M H2SO4. (b) Time-dependent current density curve of Mo2C-RGO@8 under a static overpotential of 100 mV for 20 h. The electrochemical impendence (EIS) was carried out to test the charge transfer efficiency of the obtained electrocatalysts. It was obvious that Mo2C-RGO@8 exhibited the lowest charge transfer resistance RCT (55 Ω) compared with Mo2C (>1000 Ω) and Mo2C-RGO@4 (375 Ω), which represented faster charge transfer (Figure 8a). To probe the durability of the Mo2C-RGO, the durability of Mo2C-RGO@8 was examined by electrolysis at astatic overpotential of 100 m V. The Figure 8b shows that the current density experienced a negligible loss at ~10 mA cm-2 for 20 h, confirming that Mo2C-RGO@8 was a stable electrocatalyst in acidic solutions. As shown in Figure S5, the high Cdll of Mo2C-RGO@8 (17.8 mF cm-2) was comparable to those in the previous literature. 53 However, for carbon composites, a portion of the Cdll is provided by the carbon material. Therefore, the catalytically active region of this material can be indirectly determined by the solid-liquid interface electric double layer capacitor (Cdll), while other parameters need to be compared. The larger the Cdll value, the more surface sites are exposed to be exposed.54-57 20


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

CONCLUSIONS In summary, we used PIL as a linker to construct RGO-supported Mo2C with

uniform dispersion and typical 2D structure. PIL is rich in N, which (especially pyridine N content) could effectively increase the number of active sites. Besides, PIL can uniformly disperse Mo2C NPs on the surface of RGO NSs by ion exchange, thereby increasing the accessibility of active substances. Furthermore, the addition of PIL effectively prevented irreversible packing of RGO NSs during the reduction process and increased the specific surface area. The results showed, the structure of the PIL has a great influence on the performance of the final product, so that the performance can be adjusted by changing the structure of the PIL. The Mo2C-RGO@8 obtained by this method exhibited excellent HER activity and high stability. Therefore, the simple and easy-to-regulate strategy may enrich the rational design of 2D functional materials for a variety of applications.



AUTHOR INFORMATION

Corresponding Author *E-mail:

[email protected] Tel./Fax: +86 2227400199.

ORCID Na Yang: 0000-0003-4888-5971 Luhong Zhang: 0000-0001-5190-2918

Notes The authors declare no competing financial interest. 

ACKNOWLEDGEMENTS 21



Page 22 of 31

This work is supported by National Key R&D Program of China (Nos. 2016YFC0400406 and 2017B0602702).

 ASSOCIATED CONTENT Supporting Information Figure S1. FT-IR spectra of precursors; Figure S2. (a-e) The HRTEM images of catalysts, (f) FFT patterns of Mo2C-RGO@8 and (g) Mo2C; Figure S3. (a) XRD patterns

and

(b)

SEM

image

of

Mo2C-RGO@0;

Figure

S4.

(a)

N2

adsorption-desorption isotherm curves with calculated BET surface areas and (b) BJH pore size distribution plots; Figure S5. CVs of Mo2C-RGO@8; Figure S6. AFM images of Mo2C-RGO@8.

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ABSTRACT GRAPHIC

31


Synthesis of RGO-Supported Molybdenum Carbide (Mo2C-RGO) for

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