In Situ Preparation of Mo2C Nanoparticles Embedded in Ketjenblack

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In situ Preparation of Mo2C Nanoparticles Embedded in Ketjenblack Carbon as Highly Efficient Electrocatalysts for Hydrogen Evolution Dezhi Wang, Junchao Wang, Xiaonan Luo, Zhuangzhi Wu, and Lei Ye ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03317 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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ACS Sustainable Chemistry & Engineering

In situ Preparation of Mo2C Nanoparticles Embedded in Ketjenblack Carbon as Highly Efficient Electrocatalysts for Hydrogen Evolution Dezhi Wanga, b, Junchao Wanga, Xiaonan Luoa, Zhuangzhi Wua,b*, Lei Ye c* a

School of Materials Science and Engineering, Central South University, Changsha 410083, China

b

Key Laboratory of Ministry of Education for Non-ferrous Materials Science and Engineering, Changsha 410083, China

c

School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China

*

Correspondence: Z Wu, Fax: +86-731-88830202; Tel: +86-731-88830202; E-mail address: [email protected]; L Ye, Fax: +86-27-87792461; Tel: +86-27-87792461; E-mail address: [email protected]

KEYWORDS: molybdenum carbide, Ketjenblack carbon, in situ carbonization, electrocatalysis, hydrogen evolution reaction

1 Environment ACS Paragon Plus


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ABSTRACT

Recently, to enhance the catalytic activity of molybdenum carbide (Mo2C) electrocatalysts for the hydrogen evolution reaction (HER), the conductive carbon-based materials with different structures have been used to support Mo2C particles for providing sufficient catalytic hydrogen production sites. Nevertheless, it is always hard to use a simple method to ensure both uniform distribution of Mo2C particles and good charge transfer between Mo2C and carbon matrix. Herein, we used a low-cost carbonaceous material as ingredient via a facile method of in situ carbonization to design the structure of Mo2C nanoparticles embedded in chainlike Ketjenblack carbon (KB) with strong chemical link, to achieve the Mo2C/KB hybrid catalyst with uniform distribution of active Mo2C nanocrystals on KB for high density of catalytic sites and excellent charge-transfer ability. Moreover, the effects of carbonization temperature and carbon content on the HER activity were investigated to optimize the Mo2C/KB catalyst. The optimized Mo2C/KB catalyst exhibits outstanding HER activity in both acidic and alkaline media with small Tafel slopes of 49 mV dec-1 and 48 mV dec-1, low overpotentials and remarkable stability. The enhanced HER activity of Mo2C/KB catalyst could be ascribed to its unique chainlike structure with a large specific surface area of 580.3 m2 g-1, the high electronic conductivity and active Mo2C nanocrystals protected by robust carbon matrix.

INTRODUCTION

With the intensification of the shortage of traditional fossil fuels and the environmental pollution, it is extremely critical to exploit a sustainable and clean energy source to alleviate the imminent energy crisis and environmental issues.1 Hydrogen, due to its high gravimetric energy density and zero emission of global warming gases as a renewable energy carrier, is believed to be one 2 Environment ACS Paragon Plus

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of the most promising replacements of carbon-based fuels.2,3 However, current methods for H2 production are mainly based on fossil fuels, limited by their disadvantages of high energy consumption and low efficiency.4 Therefore, the rise of electrolysis of water into hydrogen has provided a promising technique for the large-scale production of hydrogen, based on its sustainable and environmentally friendly superiority.5 At present, despite of their good catalytic properties for HER, traditional Pt-group noble metals are limited for large-scale applications by the high cost and scarcity.6,7 Therefore, to explore alternatives with high HER activity and low cost, plays a very important role in developing hydrogen energy source. Nowadays, many alternatives, including transition metal dichalcogenides,8,9 nitrides,10 phosphides,11,12 carbides,1317

etc, have demonstrated their potential, but among them, molybdenum carbide (Mo2C) might be

the most promising HER catalytic material on account of its well thermal stability, good mechanical hardness, excellent conductivity and outstanding catalytic performance originating from the unique Mo-C bonds and noble-metal-like d-state density around the Fermi level.18-20 Great efforts have been made on the development of the molybdenum carbide based catalysts, such as constructing nanostructure,21,22 controlling morphology23,24 or doping a second transition metal.25 Especially in recent years, to further improve the HER activity, molybdenum carbide has been supported on certain carbon matrix to not only prevent the aggregation of Mo-based compounds but also accelerate the rate of charge transfer during HER.26-28 Youn et al. synthesized the molybdenum carbide on carbon nanotude-graphene hybrid support, which exhibited an excellent catalytic activity with a Tafel slope of 58 mV dec-1 in 0.5 M H2SO4.26 Pan et al. reported enhanced HER activity of molybdenum carbide supported on reduced grapheme oxide (RGO) with a low onset overpotential (~70 mV) and a Tafel slope of 57.3 mV dec-1.27 Huang et al. developed Mo2C nanoparticles dispersed on hierarchical carbon microflowers as a



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highly active HER catalyst with small Tafel slopes of 55 mV dec-1 and 65 mV dec-1 in acidic and alkaline media respectively.28 Despite of great efforts, there are more or less the entanglement or aggregation of these one-dimensional (1D) or two-dimensional (2D) carbonaceous supports under high reaction temperature during the preparation process, to hinder the integration of nanoparticles with carbonaceous supports, leading to low catalytic surface areas.28,29 More importantly, the complexity of synthesis processes and the high cost also limit the application of these carbonaceous supports. As one of the carbonaceous supports, Ketjenblack carbon (KB) is considered as the most promising supporting material due to its unique chainlike structure and outstanding properties, leading to high electrochemical stability, good conductivity and large specific surface area (~1300 m2 g-1). More importantly, during the preparation process of electrocatalysts, KB serves as not only the conductive support without entanglement or aggregation, but also the carbon source to prepare Mo2C nanoparticles with the structure in situ embedded in KB matrix, exhibiting large catalytic surface areas. Moreover, the strong chemical link between Mo2C nanoparticles and KB provides good charge-transfer ability and uniform distribution of Mo2C nanoparticles, to achieve high catalytic activity toward HER. Herein, for the first time, we described a facile in situ synthetic route to prepare Mo2C nanoparticles embedded in Ketjenblack carbon (Mo2C/KB) as the electrocatalyst for HER, which possessed the unique chainlike structure with fine Mo2C nanocrystals uniformly anchored into the carbon matrix via strong MoC chemical bond. And the optimized Mo2C/KB catalyst showed good charge-transfer ability and a large specific surface area, resulting in an excellent HER performance in both acidic and alkaline conditions with small Tafel slopes of 49 mV dec-1 and 48 mV dec-1 as well as


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pronounced stability. We believe that our work provides a novel strategy for the development of new-generation electrocatalysts for HER based on transition metal carbide.

EXPERIMENTAL SECTION

Preparation of Mo2C/KB electrocatalysts. The typical preparation process of the Mo2C/KB catalyst is schematically described in Figure 1. Specifically, 1 g ammonium heptamolybdate ((NH4)6Mo7O24·4H2O) and 0.6 g Ketjenblack carbon (EC-600JD) were well dispersed in a beaker with 50 mL ethanol. Next, the black mixture was evaporated under vigorous stirring at 80 o

C until the ethanol was completely dried and then moved to an oven kept at 60 oC for 6 h.

Finally, after grounded into powder, the black precursor was first annealed at 550 oC for 1 h in a tube furnace with a heating rate of 10 oC min-1 and then carbonized at 800 oC for 3 h in a mixture of H2/Ar gas with 10 vol. % of H2. Afterwards, the product in the furnace (denoted as Mo2C/KB800) was allowed to cool down inside the furnace to room temperature. For contrast, different carbonization temperatures (700 oC and 900 oC) were scheduled to investigate the effect of carbonization temperature on HER performance of the catalysts (denoted as MA/KB-700 and Mo2C/KB-900, respectively). Characterizations. The structures and phases of the obtained catalysts were examined with a D/Max 2500 X-ray diffractometer with a Cu Kα irradiation source (λ=0.154 nm). The Raman spectroscopy was collected using a LabRAMHR-800 instrument from HORIBA with an excitation wavelength of 633 nm. The morphology and microstructure were characterized by the scanning electron microscope (SEM, FEI Sirion 200) and transmission electron microscope (TEM, JOEL 2100F). The specific surface areas (BET) and corresponding pore size distribution were determined by N2 adsorption at 77 K with a volumetric unit (Quadrasorb SI-3MP).



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Moreover, X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) was used to verify the surface compositions.

Figure 1. Schematic illustration of the formation procedure of the Mo2C/KB catalyst. Electrode preparation. 3 mg of as-prepared catalysts were dispersed in a mixed solution of 5 wt % Nafion (80 µL), ethanol (200 µL) and deionized water (800 µL) under ultrasonic irradiation for 30 min. Then, 5 µL of the catalyst slurry was dropped on a smooth glassy carbon electrode with a diameter of 3 mm and dried in air. The mass loading was ∼0.213 mg cm-2.

Electrochemical measurements. All electrochemical measurements were performed on an

electrochemical workstation (CHI 660E). A three-electrode cell was employed to characterize the HER activity of catalysts, with the saturated calomel electrode (SCE) as the reference electrode and glassy carbon as the counter electrode. Aqueous solutions of 0.5 M H2SO4 and 1 M KOH were used as the electrolytes for electrochemical tests. The linear sweep voltammetry (LSV) measurements were conducted from 0.1 to -0.4 V with a scan rate of 2 mV s-1. Besides, the working electrode was cycled at least 20 cycles to obtain stable polarization curves.


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Electrochemical impedance spectroscopy (EIS) measurements were carried out from 1 MHz to 1 Hz at -200 mV with an amplitude of 5 mV. All potentials were quoted with respect to a reversible hydrogen electrode (RHE) and without iR correction. Electrochemically active surface area and turnover frequency calculations (TOF)

12,30

The capacitance of the double layer Cdl can be used to estimate the electrochemically active surface area (AECSA) of catalysts, assuming that the two quantities are linearly proportional. Cyclic voltammetry can be used as a simple method to determine the Cdl. Thus, the CV measurements were carried out in the region of 0.1~0.3 V at different scan rates (20, 40, 60, 80, 100, 120, 140 mV s-1). The capacitance was calculated from the scan rate dependence of charging current density at a potential in which no obvious faradic processes were observed, i.e. at 0.2 V vs. RHE, where the slope of the ∆j vs. scan rate curve is twice of the Cdl. The areaaveraged capacitance depends on the electrode materials in the range of 20~60 µF cm-2. Because the surface area-normalized capacitance associated with double layer charging is expected to be similar (i.e. within an order of magnitude) for the catalyst materials in the same aqueous electrolyte, we assumed 40 µF cm-2 as a moderate value for the following calculations of AECSA. The calculation of electrochemically active surface area: AECSA =

the capacitance of the double layer Cdl 40 µF cm-2 per cm2ECSA

We used the following formula to calculate the per-site TOF as intrinsic activity: =

no. of total hydrogen turnovers/c of geometric area no. of active sites/c of geometric area

The total number of hydrogen turnovers was calculated from the current density according to: no. of H2 = j

mA 1Cs-1 1mol of e- 1mol of H2 6.022×1023 H2

cm2 1000mA 96485.3C 2mol of e-1 2mol of e-1



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=3.12×1015

H2 /s mA per 2 2 cm cm

Because the exact hydrogen binding site is not known, we estimated the number of active sites as the number of surface sites (including both Mo and C atoms as possible active site) from the roughness factor together with the unit cell (volume of 37.2 Å3) of the Mo2C crystal structure in all cases of catalysts in this work. The number of active sites per real surface area: atoms 2 unit cell no. of active sites= Å 37.2 unit cell

2/3

= 1.42×1015 atoms cm$ !"#

Finally, the plot of current density can be converted into a TOF plot according to: H2 /s mA 2 per cm2 ×|j|) cm = (1.42×1015 atoms cm$ !"# )×AECSA (3.12×1015

RESULTS AND DISCUSSION

The crystalline phases of the catalysts obtained at different carbonization temperatures were characterized using XRD. In Figure 2(a), the typical peaks (except those from pristine KB) of the catalyst prepared at 800 oC (Mo2C/KB-800), located at 34.6, 38.0 and 39.4º, are well indexed to the (100), (002) and (101) planes of β-Mo2C (JCPDS Card NO. 35-0787),14,30 which is the most active phase of molybdenum carbides for HER.31 But for the catalyst prepared at 700 oC (MA/KB-700), only a small amount of β-Mo2C is obtained while molybdenum metal and molybdenum oxides coexist as the major phases (Supporting Information, Figure S1), indicating that the carbonization is incomplete due to the low temperature. The crystalline phase of the catalyst prepared at 900 oC (Mo2C/KB-900) was also studied, as show in Figure S1(a). Although


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the typical peaks of β-Mo2C are still observed in Mo2C/KB-900, the crystallite size of Mo2C (~26.1 nm, calculated by the Scherrer equation32) in Mo2C/KB-900 is larger than that of Mo2C (~20.2 nm) in Mo2C/KB-800, demonstrating that excessively high carbonization temperature leads to the coarsening of the Mo2C crystal. To further investigate the phases of Mo2C/KB-800, the results of Raman spectrum analysis are shown in Figure 2(b). The characteristic peaks of Mo2C at 660, 812, and 987 cm-1 are observed, as well as the D and G bands of graphitic carbon at 1300 and 1600 cm-1 which correspond to the disordered graphitic carbon and the Eg vibration of the sp2-bonded carbon atoms, respectively.27,28 Moreover, the intensity ration of the D and G band (ID/IG=2.28) of pristine KB, is larger than that of the D and G band (ID/IG=1.37) of Mo2C/KB-800, implying that more disorder carbons of pristine KB are transformed into ordered graphene of Mo2C/KB-800 through high carbonization temperature for better electron transfer ability. Besides, as shown in Figure S2, the amount of Mo2C nanoparticles embedded on the final hybrid catalyst (Mo2C/KB-800) is estimated to be ~54.7 wt% according to the thermogravimetric analysis (TGA),33 which is close to the theoretical value.

Figure 2. (a) XRD patterns and (b) Raman spectra of Mo2C/KB-800 and KB.



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Figure 3(a) and 3(b) show the SEM images of pristine KB and Mo2C/KB-800. Despite of high carbonization temperature to perfectly embed Mo2C nanoparticles into KB carbon matrix, the Mo2C/KB-800 catalyst still retains the branch morphology and rough surface, similar to the microstructure of pristine KB. The BET specific surface areas and the corresponding pore size distribution of pristine KB and Mo2C/KB-800 were evaluated by the N2 adsorption-desorption curves to reveal the porous structure as shown in Figure S3. Based on the results, the Mo2C/KB800 catalyst possesses the similar mesoporous structure of KB (3~18 nm) and exhibits a large BET value of 580.3 m2 g-1, contributing to sufficient exposure of active Mo2C sites for HER. In addition, the Energy Dispersive X-ray Spectroscopy (EDX) elemental mapping images (Figure 3(c)) indicate the uniform distribution of Mo and C elements on the KB matrix. And the TEM image (Figure 3(d)) suggests Mo2C nanoparticles are well-dispersed on the carbon matrix, agreeing on the EDX results. The high-resolution TEM (HRTEM) image (Figure 3(e)) demonstrates that the well-crystallized Mo2C nanoparticle, with a size of ca. 10 nm, is firmly inlaid or anchored on the KB matrix. The clear lattice fringes with interplanar distances of 0.23, 0.24 and 0.26 nm can be observed, which are respectively corresponding to the (101), (002) and (100) crystal planes of β-Mo2C,30 to further prove the previous XRD analysis.


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Figure 3. SEM images of (a) pristine KB and (b) Mo2C/KB-800. (c) EDX mapping, (d) TEM and (e) HRTEM images of Mo2C/KB-800. X-ray photoelectron spectroscopy (XPS) analyses were carried out to investigate the valence states and surface compositions of Mo2C/KB-800. As shown in Figure S4, the XPS survey reveals the presence of C, O and Mo elements in the catalyst. The C 1s spectrum (Figure 4(a)) presents a main peak at the binding energy of 284.8 eV, which proves that the graphite carbon



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(sp2 C=C) is the main species.34 Moreover, there are other weak peaks at 283.7, 285.5 and 286.3 eV, corresponding to C-Mo, C-C and C-O bonding, respectively.14,34 As observed in Mo 3d spectrum (Figure 4(b)), the Mo 3d5/2 and Mo 3d3/2 peaks located at 228.6 and 231.8 eV, with a spin energy separation of 3.2 eV, demonstrate the characteristic doublets of the Mo2+ state of βMo2C.32,33 However, due to the unavoidable oxidation on the surface, characteristic doublets of MoO3 (232.5 and 235.7 eV) and MoO2 (229.2 and 232.4 eV) can also be observed in the Mo 3d spectrum.14,33

Figure 4. XPS spectra of the Mo2C/KB-800: (a) C 1s, (b) Mo 3d. Electrochemical measurements were performed to verify the HER activity of the hybrid catalysts synthesized at different carbonization temperature in 0.5 M H2SO4 with pristine KB and commercial 20 wt% Pt/C for comparison. The polarization curves (V-i plot, without iR correction) are shown in Figure 5(a). Obviously, pristine KB matrix exhibits negligible activity, which excludes the contribution of KB to the total HER activity, and 20 wt% Pt/C performs the best as expected. Among the synthesized hybrid catalysts, Mo2C/KB-800 catalyst shows the optimal catalytic activity toward HER as indicated by the highest current density. To achieve a current density of 10 mA cm-2, it only needs an overpotential of 180 mV (η10) for Mo2C/KB-800, which


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is the lowest in the hybrid catalysts. For the inferior HER activity of MA/KB-700 catalyst, the reason is that the carbonization to produce Mo2C is incomplete due to the low carbonization temperature. While, in the Mo2C/KB-900 catalyst, the Mo2C crystalline grain is coarsened to reduce the density of active sites because of excessively high carbonization temperature, resulting in its relatively worse HER activity with a higher η10 of 225 mV. All electrochemical dates are listed in Table 1. Table 1. Summary of properties of catalysts prepared at different carbonization temperatures.

Catalysts

Tafel slope

η10

Rct

AECSA

BET

(mV)

(mV)

(Ω)

(m2 g-1)

(m2 g-1)

Main phases

MA/KB-700

MoO2, Mo

60

263

240

272.5

603.1

Mo2C/KB-800

Mo2C

49

180

33

170.0

580.3

Mo2C/KB-900

Mo2C

53

221

45

149.7

414.2

To explore the HER mechanism, Tafel slopes of the catalysts are fitted by the Tafel equation (η = b log j + a, where j is the current density and b is the Tafel slope) as shown in Figure 5(b). As a rule, the Tafel slope is an inherent property of the catalyst determined by the rate-limiting step of HER. If Volmer step is the rate-limiting step, the Tafel slope can be up to 116 mV dec-1. Otherwise, the Heyrovsky and Tafel step is relevant to the Tafel slope of 40 and 30 mV dec-1, respectively.35 As observed, the Mo2C/KB-800 catalyst exhibits the smallest Tafel slope of 49 mV dec-1 among the as-prepared hybrid catalysts, which is also superior to the majority of reported Mo2C-based catalysts as listed in Table S1, suggesting the intimate advantageous coupling between Mo2C active sites and KB carbon via the in situ carbonization. The value of 49 13 Environment ACS Paragon Plus


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mV dec-1 is close to the theoretical value of 40 mV dec-1 when the Volmer-Heyrovsky mechanism is rate-limiting step. As another inherent measure to evaluate HER activity, the exchange current density (j0) is calculated by using extrapolation methods from Tafel plots. Mo2C/KB-800 catalyst displays the largest j0 of 3×10-3 mA cm-2 among hybrid catalysts, which is quite a satisfactory value for non-noble metal HER catalysts,36-38 further to prove its enhanced HER activity.

Figure 5. Electrochemical measurements of the catalysts for HER: (a) Polarization curves and (b) Tafel plots of KB (Ⅰ), MA/KB-700 (Ⅱ), Mo2C/KB-800 (Ⅲ), Mo2C/KB-900 (Ⅳ) and 20 wt% Pt/C (Ⅴ) in 0.5 M H2SO4. In addition, electrochemical impedance spectroscopy (EIS) was carried out to further investigate the HER kinetics of the hybrid catalysts as shown in Figure 6. Obviously, only one semicircle can be observed in all Nyquist plots (Figure 6(a)), suggesting that the equivalent circuit for HER can be characterized by one time constant with a resistor and capacitor in parallel. Moreover, the Bode plots (Figure 6(b)) show an additional resistor element in series with the above units. Thus, it is perceived that these hybrid catalysts possess a similar catalytic system which is described as a simple equivalent electrical circuit (Figure S5). In the electrical circuit, Rs


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is attributed to the uncompensated solution resistance and Rct is the charge-transfer resistance, related to the HER kinetics.35 Compared to the other two catalysts, the Mo2C/KB-800 catalyst shows a much smaller semicircle with a lower charge-transfer impedance (Rct) of 33 Ω at -200 mV (Table 1), indicating the best charge-transfer ability to facilitate the HER process. We further tested the electrochemically double layer capacitance (Cdl) (Figure S6) to calculate the values of the electrochemically active surface area (AECSA) and TOF as also summarized in Table 1. The slopes of ∆j vs. scan rate curves are shown in Figure 6(c), and the MA/KB-700 catalyst possesses the largest Cdl of 10.9 mF cm-2 and AECSA of 272.5 m2 g-1, which possibly benefits from those of KB (11.3 mF cm-2 and 282.3 m2 g-1, as observed in Figure S6(a)) without participating in the carbonization reaction, but the poor intrinsic activity as inferred by small values of TOF (Figure 6(d)) for MA/KB-700 catalyst claims the lowest HER activity. The Mo2C/KB-800 catalyst exhibits a preferable AECSA of 170 m2 g-1 and TOF curve than these of Mo2C/KB-900, which should be attributed to the reduction of specific surface areas and the coarsening of Mo2C crystalline grain in Mo2C/KB-900 under excessively high carbonization temperature. Obviously, the KB support can provide a large electrochemically active surface area due to the extremely large surface area (~1300 m2 g-1), which will be reduced by the subsequent carbonization. Higher temperatures lead to more molybdenum carbide with better crystallinity, leading to the damage of the original mesoporous structures associated with smaller surface areas. Therefore, a lower synthesis temperature means a better HER activity on condition that the βMo2C can be completely converted in the presence of KB. And more strategies can be considered to decrease the carbonization temperature of β-Mo2C, so that better HER activity can be expected, which is also in progress and will be reported in future.



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Figure 6. Electrochemical measurements of the obtained Mo2C/KB catalysts: (a) Nyquist plots, (b) Bode plots, (c) Measured capacitive currents plotted as a function of scan rate and (d) TOF. Meanwhile, we also investigated the electrocatalytic activity of the Mo2C/KB-800 catalyst towards HER in alkaline solution. Figure S7(a) and S7(b) show the polarization curve and Tafel plot of Mo2C/KB-800 in 1 M KOH, respectively. As expected, the excellent HER activity is comparable to that in acidic media with a low η10 of 210 mV and a small Tafel slope of 48 mV dec-1, which manifests the outstanding electrocatalytic property of Mo2C/KB-800 hybrid catalyst in both extreme pH conditions.


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Long-term stability is another critical factor to evaluate HER catalysts. Herein, to probe the durability of the Mo2C/KB-800 catalyst, two different evaluation methods were adopted in both 0.5 M H2SO4 and 1 M KOH. In acidic solution, as depicted in Figure 7(a), the polarization curve shows a very slight shift after 2000 cycles between -0.4 and 0.1V with a scan rate of 50 mV s-1. As for the amperometric (i-t) stability, the Mo2C/KB-800 catalyst is examined at a static overpotential of 180 mV, and the current density shows a negligible loss at 10 mA cm-2 for 10 hours (Figure 7(b)), indicating the remarkable stability in acidic media. In alkaline solution, the Mo2C/KB-800 catalyst still exhibits robust operation stability but with a little larger degradation of cathodic current density (Figure 7(a)), which might be related to the gradual corrosion of Mo2C in KOH.21,28

Figure 7. Long-term stability tests of the Mo2C/KB-800 catalyst in both acidic and alkaline conditions: (a) cycling stability and (b) i-t curves of the hybrid catalyst biased at η=180 mV in 0.5 M H2SO4 or η=210 mV in 1 M KOH. In addition, to examine the effect of the KB content on HER activity of the Mo2C/KB catalysts, other two catalysts with different mass ratio of KB and ammonium heptamolybdate (0.2 and 1.0) were synthesized at the same carbonization temperature of 800 oC (denoted as Mo2C/KB-0.2 and Mo2C/KB-1.0, respectively, to compare with above Mo2C/KB-800 with a mass ratio of 0.6, 17 Environment ACS Paragon Plus


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denoted as Mo2C/KB-0.6 here). The XRD patterns of obtained Mo2C/KB catalysts are shown in Figure S8, in which similar typical peaks of β-Mo2C can be observed for all the samples.14 However, focusing on the peak of graphite carbon at 2θ=26º,28 the peak intensity is strengthened gradually with the increase of the KB content, indicating a continuous increase of graphite carbon. The HER activity in 0.5 M H2SO4 of these catalysts was also evaluated under same conditions (Table S2). As observed in Figure S9-S11, the Mo2C/KB-0.6 catalyst demonstrates the best HER activity while the Mo2C/KB-0.2 catalyst possesses a much smaller electrochemically active surface area, a larger grain size (22.5 nm) and a higher charge-transfer impedance than those of Mo2C/KB-0.6 due to less restriction and support of KB matrix, leading to worse HER activity. But for the Mo2C/KB-1 catalyst, the exposure of Mo2C active sites is hindered due to too much amorphous carbon, also resulting in worse HER activity than the Mo2C/KB-0.6 catalyst. Hence, the suitable KB content leads to the optimal HER activity of Mo2C/KB catalysts. The excellent HER performance of Mo2C/KB hybrid catalysts should be ascribed to the intrinsic coupling mechanism between Mo2C and KB matrix. Decorated with ultrafine Mo2C grains, the hybrid catalyst still exhibits a large specific surface area, leading to a high density of active sites for HER. Meanwhile, the ultrafine Mo2C grains are tightly integrated onto the carbon support, achieving atomic affinity to KB matrix and ensuring good charge-transfer for catalytic sites. In addition, confined and protected by robust carbon in KB, the catalytic sites possess excellent structural and electrochemical stability. All of these unique features ensure the superior HER performance of the Mo2C/KB electrocatalysts to most of Mo2C-based catalysts as listed in Table S1.


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CONCLUSIONS

We designed a novel HER catalyst of Mo2C nanoparticles embedded in KB via a simple method of in situ carbonization. The optimized Mo2C/KB catalyst exhibits a unique chainlike structure and excellent HER activity in both acidic and alkaline conditions with small Tafel slopes of 49 mV dec-1 and 48 mV dec-1, which are superior to most of the Mo2C-based electrocatalysts. The intrinsic coupling mechanism of Mo2C/KB attributed to the strong chemical link between Mo2C nanoparticles and KB, limits the maldistribution of Mo2C nanoparticles caused by their aggregation and flaking at high carbonization temperatures, as well provides good chargetransfer ability for HER catalytic sites, to achieve superior HER activity. Obviously, this strategy can also be extended to other applications, which require large catalytic surface areas and excellent electrical conductivity, such as oxygen evolution reaction (OER), oxygen reduction reaction (ORR), etc.

ACKNOWLEDGMENT

Financial supports from the National Natural Science Foundation of China (Grants 51302326 and 51572301), the Hunan Provincial Natural Science Foundation of China (Grants2016JJ3153) and the Fundamental Research Funds for the Central Universities of Central South University are gratefully acknowledged.

SUPPORTING INFORMATION



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XRD patterns, TGA curve, N2 sorption isotherms and pore size distributions, XPS survey, equivalent circuit model and electrochemical measurements of as-prepared samples; Summaries of HER activity of Mo2C-based catalysts.

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TABLE OF CONTENTS

Synopsis Highly active electrocatalysts of Mo2C/KB nanoparticles have been in situ prepared, enabling a cheap and large-scale production of the sustainable hydrogen energy.


In Situ Preparation of Mo2C Nanoparticles Embedded in Ketjenblack

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