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Molybdenum carbides embedded on carbon nanotubes for efficient hydrogen evolution reaction
Journal of Electroanalytical Chemistry 801 (2017) 7–13
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Molybdenum carbides embedded on carbon nanotubes for efficient hydrogen evolution reaction
MARK
Junpo Guo1, Jie Wang1, Cuijuan Xuan, Zexing Wu, Wen Lei, Jing Zhu, Weiping Xiao, Deli Wang⁎ Key laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hydrogen evolution reaction Electrocatalysts Molybdenum carbide Carbon nanotubes Embedded structure
Molybdenum carbides embedded on carbon nanotubes are prepared via a simple hydrothermal method with subsequent post-treatment at high temperature. In order to optimize the electrocatalytic activity for hydrogen evolution reaction (HER), effects of hydrothermal and post-annealing temperature are systematically investigated. Electrochemical testing results indicated that Mo2C/CNT prepared at a solvothermal temperature of 200 °C and 800 °C post heat-treatment exhibits the best HER activity with an overpotential of 125 mV in acid media and 93 mV in alkaline media at a current density of 10 mA cm− 2. Such enhanced catalytic activity may originate from high electro-conductivity and ECSA value, together with ultra-small particle size, which accelerates the electron transfer rate and provides large surface area and active sites, respectively.
1. Introduction Hydrogen, a green and renewable energy source, has been intensely investigated as a promising alternative to conventional fossil fuels [1–3]. Recently, electrochemical water splitting to produce hydrogen has attracted tremendous attention [4–6]. Although Pt-group precious metals or alloy are mostly active and stable catalysts for the hydrogen evolution reaction (HER) [7,8], their scarcity and high cost made them impractical for global-scale applications. Much efforts have been made to develop cost-effective and earth-abundant 3d–transition metal (TMs) based HER electrocatalysts, including carbides [9–11], nitrides [12,13], sulfides [14–17] and phosphides [18–21], et al. Among these catalysts, molybdenum carbides are considered as the effective non-Pt electrocatalysts for HER in both acidic and basic conditions, owing to their similar d-band electronic density with Pt [22,23], lower hydrogen-adsorption properties [10,24,25] and relative high chemical stability. However, although molybdenum carbides exhibit good electronic conductivity [26,27], the HER rate is restricted by the electron transfer rate. Therefore, a combination of carbon material is usually adopted to enhance the electronic conductivity. Recently, Wang and co-workers reported a facile, one-step synthetic route to obtain molybdenum carbide (Mo2C) embedded in nitrogen-rich carbon layers by using ammonium molybdate and dicyandiamide as precursor [28]. Mo2C nanoparticles decorated graphitic carbon sheets (Mo2C/GCSs) were prepared via a one-step solid-state method, utilizing sodium alginate (ALG) as a low cost environmental friendly carbon source [29]. Latterly, graphene supported
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Corresponding author. E-mail address: [email protected] (D. Wang). These authors contributed equally.
http://dx.doi.org/10.1016/j.jelechem.2017.07.020 Received 23 March 2017; Received in revised form 6 June 2017; Accepted 12 July 2017 Available online 13 July 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved.
molybdenum carbide octahedral nanoparticles with porous and small size was derived via an in situ carburization of metal organic framework structure (MOFs) [30]. The HER activity was obviously improved via the incorporation of Mo2C and carbon support. In addition, further improvement of HER activity would be acquired through a more intimate incorporation between molybdenum compounds and carbon support [31–32]. Herein, we report a facile, two-step synthetic route for the synthesis of molybdenum carbides embedded on carbon nanotubes. Pure phase molybdenum carbide compound (Mo2C/CNT) are obtained via the solvothermal (within 180–240 °C) and post-annealing process (within 700–900 °C). Mo2C/CNT synthesized via a 200 °C solvothermal with post annealing of 800 °C (Mo2C/CNTS200–800) exhibits the best HER activity compared with other catalysts prepared at different temperature. Such enhanced catalytic activity may originate from the low hydrogen binding energy, high electro-conductivity and the ultra-small particle size, which decrease the free-energy, accelerate the electron transfer rate and increase active sites, respectively. 2. Experimental part 2.1. Materials synthesis 2.1.1. Synthesis of MoO2/CNT composites 180 mg of (NH4)6Mo7O24·4H2O was fully dissolved in a mixture solvent of 10 mL distilled water and 20 mL ethylene glycol by
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Fig. 1. XRD, Raman and XPS spectra of Mo2C/CNT catalysts calcined at different temperature after 200 °C solvothermal process. (a) Powder XRD patterns of Mo2C/CNTS200–700, Mo2C/ CNTS200–800 and Mo2C/CNTS200–900; (b) Raman spectrum of Mo2C/CNTS200–700, Mo2C/CNTS200–800 and Mo2C/CNTS200–900; XPS spectra of Mo2C/CNTS200–800 (c) and corresponding high-resolution spectra of Mo 3d (d).
2.3. Electrochemical measurements
sonication. Then 120 mg of CNT was added into the mixed solution. The mixture was stirred at room temperature for approximately 20 min until a homogeneous solution was achieved. Then the solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and heated in an oven at 200 °C for 10 h. The solvothermal product was collected after being washed with DI water and ethanol for several times, and then dried via the freeze dryer.
5 mg of catalyst and 1 mL of Nafion solution (1 wt‰) were sonicated to form a homogeneous ink. Then 16.5 μL of the catalyst ink was loaded onto a glassy carbon electrode (5 mm in diameter), the loading is 420 μg/cm2. For comparison, the electrodes were also modified with Pt/C. All electrochemical measurements were performed using a Autolab PGSTAT302N electrochemical workstation in a three-electrode setup with modified glass carbon working electrodes, reversible hydrogen electrode (RHE) as reference, carbon rod as a counter electrode. 0.5 M H2SO4, 1 M KOH and 1 M phosphate buffer (pH = 7) solution were used as electrolyte and are deaerated with nitrogen. The electrocatalytic activity of the catalysts towards HER was evaluated by using linear sweep voltammetry (LSV) in 0.5 M H2SO4 and 1 M KOH at a scan rate of 5 mV s− 1. Electrochemical impedance spectroscopy (EIS) measurements were conducted at overpotential of 100 mV from 105 Hz to 0.01 Hz.
2.1.2. Synthesis of Mo2C/CNT composites The solvothermal product was annealed in H2 (5%)/Ar (95%) atmosphere at 700 °C, 800 °C and 900 °C for 2 h at a heating rate of 10 °C min− 1, respectively. The obtained molybdenum compounds were denoted as Mo2C/CNTS200–700, Mo2C/CNTS200–800 and Mo2C/ CNTS200–900. In order to explore the difference of the obtained materials in solvothermal process, the solvothermal temperature was investigated at 180 °C, 200 °C, 220 °C and 240 °C for 10 h, and then annealed in H2/Ar atmosphere up to 800 °C for 2 h with a heating rate of 10 °C min− 1, respectively, The resulting powder was denoted as Mo2C/ CNTS180–800, Mo2C/CNTS200–800, Mo2C/CNTS220–800 and Mo2C/ CNTS240–800, respectively. The obtained materials were kept in 5 mL centrifuge tube immediately, which were filled with nitrogen before sealed.
3. Results and discussion The crystal structure of molybdenum-based materials was confirmed by the XRD measurement. MoO2 phase was first obtained via solvothermal process (XRD patterns in Fig. S1). After the subsequent annealing treatment at 700 °C, 800 °C and 900 °C, Mo2C/CNT was obtained which can be seen in Fig. 1a. The small diffraction peak located at 26° is ascribed to (002) plane of the carbon nanotubes, while the diffraction peaks located at 34.4°, 37.8°, 39.4°, 52.1°, 61.5°, 69.6°, 72.4°, and 74.6°, are indexed to (100), (002), (101), (102), (110), (103), (200) and (112) lattice planes of Mo2C (JCPDS NO. 01-035-0787) [33]. Typical for Mo2C/CNT solvothermal in 200 °C, annealing at 800 °C (denoted as Mo2C/CNTS200–800), the additional structural information was obtained from Raman spectroscopy measurements. The full-range Raman spectra showed characteristic peaks of molybdenum carbide (Fig. 1b) located at 670 cm− 1, 840 cm− 1 and 990 cm− 1, which is consistent with the XRD pattern in Fig. 1a [30]. Moreover, it can be seen that D band located at 1350 ± 20 cm− 1 and G band located at 1575 ± 20 cm− 1, which are corresponding to the disordered graphitic
2.2. Material characterization The crystal structure was determined using X-ray diffraction, and diffraction patterns were collected using Cu Kα (λ = 1.5406 Å) radiation at a scanning rate of 4° min− 1. Raman spectra were collected by a LabRam HR800 spectrometer with a 532 nm laser excitation. The morphologies were measured by scanning electron microscopy (SEM, Sirion200), Transmission electron microscopy (TEM) images were recorded by a JSM-2100 transmission electron microscopy (JEOL, Japan) at an acceleration voltage of 200 kV. Further, the chemical states of the elements in catalysts were studied by XPS using an AXIS-ULTRA DLD600 W Instrument, and the binding energy of the C 1s peak at 284.6 eV was taken as an internal reference. 8
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carbon and the Eg vibration of the sp2-bonded carbon atoms, respectively. The ID/IG intensity value of Mo2C/CNTS200–700, Mo2C/ CNTS200–800 and Mo2C/CNTS200–900 is calculated to be 0.92, 0.86 and 0.81, respectively (Fig. 1b), indicating the increase of graphitization degree with increasing the temperature, which may enhance the overall electrical conductivity and facilitate the charge transfer during electrochemical testing [34–36]. X-ray photoelectron spectroscopy (XPS) was performed to probe the electronic structures on the surface of the catalysts, and all binding energy (BE) values were calibrated using the C 1s peak at 284.6 eV as a reference. As shown in Fig. 1c, the survey spectrum shows apparent signals located at 231.5, 285.1, 395.2, 413.6, and 531.5 eV, which are ascribed to the existence of Mo 3d, C 1s, Mo 3p, and O 1 s, respectively. In the high resolution XPS spectrum of Mo 3d (Fig. 1d), two deconvoluted peaks at 228.5 and 231.6 eV can be assigned to Mo2C, respectively, Peaks located at the binding energies of 229.2 and 232.3 eV could be attributed to MoO2, and another two peaks at 232.4 and 235.5 eV could be attributed to MoO3, both of them are the result of surface oxidation of Mo2C [37–39]. The high-resolution C1s XPS spectrum (Fig. S2) shows the asymmetric peak of MoeC (at 283.8 eV), CeO (at 286.0 eV), C]O (at 287.0 eV) and OeC]O (at 290.7 eV) groups which indicated the defects in the structure of CNT support and the existence of Mo2C in the Mo2C/CNTS200–800 catalyst [40–42]. The morphology of the molybdenum based catalysts was characterized by scanning electron microscopy (SEM) and transition electron microscopy (TEM). As depicted in Fig. S3a, the CNTs were covered by MoO2 on the surface, indicating the intimate combination between MoO2 and CNT. The SEM images (Fig. S3b–d) display a morphological transformation when the annealing temperature elevated from 700 °C to 900 °C. Mo2C nanoparticles were separated out at high temperature of 700 °C, and uniformly distributed when annealed at 800 °C. However, the high temperature of 900 °C resulted in aggregation of large particle size which can be obviously seen in Fig. S3d. Therefore, 800 °C was chosen as the optimized annealing temperature while lower or higher the temperature will result in inadequate transformation or particle aggregation. In addition, the uniformly distribution of Mo2C nanoparticles on CNTs would increase the active sites which contribute to the enhanced HER activity. Fig. 2 shows the typical TEM images for Mo2C/CNTS200–800 catalyst. Carbon nanotubes acts as the stabilizing agent, carbon precursor and reducing agent of the obtained MoO2. After forming Mo2C nanoparticles, part of the CNT would scarify some carbon source and leave some defects where particles located. Thus, the final particles would be embedded on the CNT. And, when combined with the full range TEM image in Fig. 2a, the main particle size is 7.4 nm when calculated from more than 300 nanoparticles (inset of Fig. 2a). The tight fix between Mo2C nanoparticles and CNTs would prevent the particle fall off during electrochemical measurement and enhance the electronic conductivity towards one dimensional direction. The obvious inter-planar spacing in Fig. 2b are measured to be 0.23 nm and 0.25 nm from high resolution transmission electron microscopy (HRTEM) image of an individual particle, corresponding to (101) and (100) plane [43–45]. The value of measured inter-planar spacing is consistent with SAED pattern (inset) and the theoretical inter-planar spacing from the XRD patterns (Fig. 1a). Moreover, the EDX mappings were performed to investigate the distributions of the C and Mo in Mo2C particles. As depicted in Fig. 2c, there are some vacancies on the CNTs and moreover, Mo2C particles filled in the vacancies (Fig. 2e) which in all confirm the embedding of Mo2C particles. Fig. 2d shows that Mo element is centered on the particle, indicating the adequate transformation of MoO2 to Mo2C. The electrocatalytic performance of as-prepared catalysts for HER was investigated in 0.5 M H2SO4 solution together with Pt/C for comparison. As shown from the polarization curves in Fig. 3a, Mo2C/ CNTS200–800 exhibited the highest HER activity compared with the other two molybdenum catalysts calcined at 700 °C and 900 °C, while Pt/C exhibited extremely high catalytic activity with nearly zero onset
overpotential. As reported, the overpotential for driving a current density of 10 mA cm− 2 is a reference parameter for comparing catalysts in solar hydrogen production [46]. And, the overpotential for Mo2C/CNTS200–700, Mo2C/CNTS200–800 and Mo2C/CNTS200–900 are 164 mV, 125 mV and 193 mV, respectively, indicating a faster hydrogen evolution rates on the catalyst calcined at 800 °C. In order to study the mechanism and the inherent properties for the as-prepared Mo2C/CNT catalysts, Tafel plots were obtained by plotting the logarithm of the kinetic current density derived from the polarization curves. As shown in Fig. 3b, the Tafel slopes of Mo2C/CNTS200–700, Mo2C/ CNTS200–800 and Mo2C/CNTS200–900 at low overpotential were calculated to be 74 mV dec− 1, 58 mV dec− 1, 90 mV dec− 1, respectively, lower than 120 mV dec− 1, indicating a Volmer-Heyrovsky mechanism for HER where the HER rate is determined by the discharge reaction or the electrochemical desorption of H and H+ from the catalyst surface to form hydrogen. The exchange current densities (j0) is another factor to reflect the intrinsic rate of HER activity, Which was obtained from Tafel curves by using extrapolation methods [47,48]. As described in Fig. S4, the j0 value of Mo2C/CNTS200–800 is around 0.16 mA cm− 2, which is about 2.5 and 4 times higher than Mo2C/CNTS200–700 (0.064 mA cm− 2) and Mo2C/CNTS200–900 (0.04 mA cm− 2), respectively (Table S1). The high j0 value on Mo2C/CNTS200–800 could be attributed to the highly dispersion of nanoparticles, which affords large amounts of active sites [49]. The electrochemical surface area (ECSA) provides insight into the Mo2C electrocatalysts. Although the accurate measurement of ECSA is difficult, it can be visualized through calculating the double-layer capacitances (Cdl) which are proportional to the ECSA values. The estimated values of Cdl using the cyclic voltammograms (CV, Fig. S5) in 0.5 M H2SO4 were alternatively utilized to provide a relative comparison. As shown, the measured Cdl is 19, 20 and 7 mF cm− 2 for Mo2C/CNTS200–700, Mo2C/CNTS200–800 and Mo2C/ CNTS200–900, respectively. It can be clearly seen that the Cdl value on Mo2C/CNTS200–800 is 2.8 times higher than Mo2C/CNTS200–900 and larger than Mo2C/CNTS200–700, which may provide more active sites to achieve prominent HER activity. Besides, to further optimize the experimental conditions, the solvothermal temperature was also investigated for the influence of HER performance. The solvothermal temperature was set to be 180 °C, 200 °C, 220 °C and 240 °C, respectively. It is worth noting that when the temperature was set below 180 °C, there would be no solvothermal products. After subsequent calcination at 800 °C under Ar/H2 atmosphere, the as-prepared products were first measured via the XRD analysis. As described in Fig. S6, the diffraction peaks located at 34.4°, 37.8°, 39.4°, 52.1°, 61.5°, 69.6°, 72.4°, and 74.6° are corresponding to the characteristic (100), (002), (101), (102), (110), (103), (200) and (112) lattice planes of Mo2C, respectively. Thus, negligible structure transformation can be detected from the characteristic diffraction peaks of Mo2C. However, the diffraction peak located at 26° (characteristic peaks of CNTs) decreased when the temperature rise from 180 °C to 240 °C, which may be attributed to the deterioration of CNTs. The HER polarization performance of the as-prepared four catalysts are investigated in 0.5 M H2SO4 (Fig. 3c), where Mo2C/CNTS200–800 exhibits the smallest overpotential of 125 mV at current density of 10 mA cm− 2 compared with Mo2C/CNTS180–800 (184 mV), Mo2C/CNTS220–800 (154 mV) and Mo2C/CNTS240–800 (197 mV). The corresponding Tafel slope value (Fig. 3d) at low overpotential of the as-prepared catalysts derived from the polarization curves in Fig. S7a are 58 mV dec− 1, 89 mV dec− 1, 69 mV dec− 1 and 92 mV dec− 1, respectively, also indicating a Volmer-Heyrovsky mechanism where Mo2C/CNTS200–800 deliver a higher H2 evolution rate [50,51]. The much difference of activities may be ascribed to the different interaction capability between molybdenum based compounds and carbon nanotubes after solvothermal process. Meanwhile, the fast kinetic of Mo2C/ CNTS200–800 for HER was verified by the smaller Tafel slope than most of the reported materials (Table S2). The j0 value at overpotential of 0 V 9
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Fig. 2. Typical TEM images for Mo2C/CNTS200–800. (a) Full range TEM image and the corresponding particle size distribution (inset); (b) HRTEM image and the selected area electron diffraction (SAED) pattern (inset); (c–e) EDX mapping for C, Mo and their composites, respectively.
Fig. 3. Electrochemical performance of Mo2C/CNT catalysts. (a) Polarization curves of Mo2C/CNTS200–700, Mo2C/ CNTS200–800, Mo2C/CNTS200–900 and Pt/C in 0.5 M H2SO4 solution; (b) Tafel plots of the corresponding catalysts in (a); Electrochemical performance of Mo2C/CNT catalysts prepared at different solvothermal temperature and annealed at 200 °C. (c) Polarization curves of Mo2C/ CNTS180–800, Mo2C/CNTS200–800, Mo2C/CNTS220–800 and Mo2C/CNTS240–800 catalysts in 0.5 M H2SO4 solution; (d) Tafel plots of the corresponding catalysts in (c).
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Fig. 4. (a) Nyquist plots of catalysts recorded at 100 mV in 0.5 M H2SO4 solution; (b) HER polarization curves of Mo2C/ CNTS200–800 and Pt/C before and after 5000 potential cycles between −0.3 V to 0.2 V at a scan rate of 100 mV s− 1 in 0.5 M H2SO4 solution.
Fig. 5. (a) Polarization curves of Mo2C/CNTS200–800 catalyst and Pt/C in 1 M KOH solution before and after 5000 potential cycles between − 0.3 V to 0.2 V at a scan rate of 100 mV s− 1; (b) Tafel plots of Mo2C/CNTS200–800 catalyst and Pt/C.
From the above analysis, it can be summarized that the proper temperature of 200 °C in solvothermal process and 800 °C in post-annealing process resulted in the highest HER activity. The enhanced electrocatalytic HER performance on Mo2C/CNTS200–800 could be ascribed to the following aspects: i) Small nanocrystallites increased the active sites and enable rapid charge transfer for HER; ii) The dimensional structure of the as-prepared catalysts facilitate the electrons transfer rate during the electrochemical reactions [56]. Besides the HER activity, the stability performance for the electrode reaction is also an integral part for their potential practical application. The stability of the high performance Mo2C/CNTS200–800 catalyst and Pt/C was measured by potential cycling for 5000 cycles between −0.3 V to 0.2 V at a scan rate of 100 mV s− 1 in 0.5 M H2SO4 solution (Fig. 4b), promisingly, there is negligible current decay for the two catalysts even after 5000 potential cycling test, indicating excellent electrochemical stability for the as-prepared catalysts in acid solution [57,58]. The as-prepared Mo2C/CNTS200–800 catalyst has been proved to the best catalyst among the catalysts synthesized at different solvothermal and post-annealing temperatures in acid media, while in order to expand the application range of the catalysts, the HER activity was also investigated in basic media. As shown in Fig. 5a, Mo2C/CNTS200–800 catalyst exhibited excellent electrocatalytic activity towards HER in 1 M KOH solution, with an overpotential of 93 mV to drive a current density of 10 mA cm− 2, although slightly lower than Pt/C (50 mV), indicating a faster hydrogen evolution rate. In addition, the Tafel value of Mo2C/ CNTS200–800 catalyst shown in Fig. 4b is 65 mV dec− 1, when compared with Pt/C, which indicating a Volmer-Heyrovsky mechanism. Furthermore, the HER performance of the Mo2C/CNTS200–800 is the best compared with the reported Mo2C-based HER catalysts both in acid and alkaline media [59] (Table S2). The stability of Mo2C/CNTS200–800 and Pt/C were also tested in 1 M KOH solution. After a 5000 potential cycles between − 0.3 V to 0.2 V at a scan rate of 100 mV s− 1, Mo2C/ CNTS200–800 and Pt/C suffer slightly current decay compared with the initial polarization curves (Fig. 5a). The polarization curves after the cycling stability still possess excellent HER activity. The relative lessstable of the catalysts in alkaline solution compared with the stability in
in Fig. S7 for the different solvothermal temperature derived catalysts also showed the highest j0 value of Mo2C/CNTS200–800 (specific j0 value can be seen from Table S1). A summary of the electrocatalytic performance comparison of the as-prepared materials and reported Mo2C-based HER catalysts for HER are listed in Table S2, as expected, the j0 on Mo2C/CNTS200–800 is higher than other catalysts. R. R. Adzic et al. have recently obtained the products of molybdenum carbide (Mo2C) anchored to carbon nanotubes by similar method, with the particle size of range from 7 to 15 nm and the j0 value of 0.014 mA cm− 2. Therefore, the enhanced HER activities of Mo2C/ CNTS200–800 catalyst could be originated from ultra-small particle size and higher j0 value which provides large surface area and active sites. Electrochemical impedance spectroscopy (EIS) measurements were carried out to further elucidate the mechanism of charge transfer on different catalysts. Representative Nyquist plots of Mo2C/CNTS200–700, Mo2C/CNTS200–800 and Mo2C/CNTS200–900 in 0.5 M H2SO4 electrolyte are shown in Fig. 4a, where the as-prepared three catalysts exhibited an obvious semicircle at overpotential of 100 mV, indicating that the corresponding equivalent circuit for the HER was characterized by onetime constant as fitted from the experimental data (inset of Fig. S8) [46,52]. In addition, Nyquist plots of Mo2C/CNTS180–800, Mo2C/ CNTS200–800, Mo2C/CNTS220–800 and Mo2C/CNTS240–800 are shown in Fig. S10a. Among the above catalysts, Mo2C/CNT S200–800 catalyst exhibited the lower charge-transfer resistance (49 Ω) than Mo2C/ CNTS200–700 (108 Ω), Mo2C/CNT S200–900 (253 Ω), Mo2C/ CNTS180–800 (297 Ω), Mo2C/CNTS220–800 (153 Ω) and Mo2C/ CNTS240–800 (364 Ω), indicating the excellent electron transfer rate of Mo2C/CNTS200–800, which could be attributed to the enhanced electronic conductivity via uniformly distribution on CNTs and partially embedment into the carbon nanotubes [53–55]. Thus, in accordance with the XRD measurements in Fig. S6, it is concluded that solvothermal temperature of 200 °C and annealing temperature of 800 °C is the proper synthetic condition for Mo2C embedded on CNTs, while lower or higher the temperature would result in incomplete contact between Mo2C and CNTs or severe CNTs deterioration, which would furtherly reduce the HER activity. 11
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Qiao, Advancing the electrochemistry of the hydrogen-evolution reaction through combining experiment and theory, Angew. Chem. Int. Ed. 54 (2015) 52–65. [25] Y. Jiao, Y. Zheng, M. Jaroniec, S.Z. Qiao, Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions, Chem. Soc. Rev. 44 (2015) 2060–2086. [26] Q. Zhai, X. Zhang, J. Li, E. Wang, Molybdenum carbide nanotubes: a novel multifunctional material for label-free electrochemical immunosensing, Nano 8 (2016) 15303–15308. [27] M. Shi, L. Zhao, X. Song, J. Liu, P. Zhang, L. Gao, Highly conductive Mo2C nanofibers encapsulated in ultrathin MnO2 nanosheets as a self-supported electrode for high-performance capacitive energy storage, ACS Appl. Mater. Interfaces 8 (2016) 32460–32467. [28] R. Ma, Y. Zhou, Y. Chen, P. Li, Q. Liu, J. Wang, Ultrafine molybdenum carbide nanoparticles composited with carbon as a highly active hydrogen-evolution electrocatalyst, Angew. Chem. Int. Ed. 54 (2015) 14723–14727. [29] W. 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acid could attributed to the fact that etching of carbon support in the presence of high concentration KOH solution, which has been reported that active sites would be suspected to detach from the support in case of the carbon corrosion. Additionally, i–t chronoamperometric curves (Fig. S10) for Mo2C/ CNTS200–800 in both 0.5 M H2SO4 and 1 M KOH solution was recorded at constant potentials of 125 and 93 mV for 10 h, respectively. It can be seen that there is negligible current decay in 0.5 M H2SO4 and slight current decrease in 1 M KOH during 10 h stability testing, indicating excellent durability of Mo2C/CNTS200–800. The current decrease of Mo2C/CNTS200–800 in alkaline media may be caused by the carbon corrosion effect [60]. Meanwhile, in order to investigate the stability change of Mo2C/CNTS200–800 in 0.5 M H2SO4 and 1 M KOH solution, TEM images after 5000 potential cycles were also characterized in Fig. S11. The particle size and distribution of Mo2C was maintained well in 0.5 M H2SO4 solution (Fig. S11a), while obvious corrosion of carbon and particle growth was occurred in 1 M KOH solution, results in lower electro-conductivity and reduced catalytic performance, which is consistent with the result of i–t chronoamperometric curves. In addition, the HER activity of Mo2C/CNTS200–800 in neutral solution was measured in 1 M phosphate buffer solution (PH = 7). As can be seen from Fig. S12a and b, an overpotentials of 276 mV and Tafel slope of 114 mV dec− 1 in neutral solution were obtained, which were lower than the results obtained in 0.5 M H2SO4 and 1 M KOH solution. Moreover, from Fig. S12c and d, the charge-transfer resistance (Rct) and electrochemical double layer capacitance (Cdl) were measured to be 864 Ω and 6.83 mF cm− 2, respectively. The lower catalytic activity in neutral solution probably attributed to the limited electronic conductivity and lower electrochemically active surface area. 4. Conclusions Molybdenum carbides embedded on carbon nanotubes are prepared via simple hydrothermal method with subsequent post-treatment at high temperature. Hydrothermal and post-annealing temperatures are investigated in detail to optimize the experimental condition for better hydrogen evolution reaction (HER) activity. Results show that Mo2C/ CNTS200–800 with uniform particle distribution and ultra-small particle size (~ 8 nm) exhibits the best HER activity. Such enhanced catalytic activity may originate from the low hydrogen binding energy, high electro-conductivity and the ultra-small particle size, which decrease the free-energy, accelerate the electron transfer rate and increase active sites, respectively. The present work also appeals the research who emphasizes on the characterization on one dimensional direction. Acknowledgements This work was supported by the National Natural Science Foundation (21573083), the Program for New Century Excellent Talents in Universities of China (NCET-13-0237), the Doctoral Fund of Ministry of Education of China (20130142120039), 1000 Young Talent (to Deli Wang), and initiatory financial support from Huazhong University of Science and Technology (HUST). The authors thank the Analytical and Testing Center of HUST for allowing use its facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jelechem.2017.07.020. References [1] J. Ge, B. Zhang, L. Lv, H. Wang, T. Ye, X. Wei, J. Su, K. Wang, X. Li, J. Chen, Constructing holey graphene monoliths via supramolecular assembly: enriching nitrogen heteroatoms up to the theoretical limit for hydrogen evolution reaction, Nano Energy 15 (2015) 567–575.
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[30] Z. Shi, Y. Wang, H. Lin, H. Zhang, M. Shen, S. Xie, Y. Zhang, Q. Gao, Y. Tang, Porous nanoMoC@graphite shell derived from a MOFs-directed strategy: an efficient electrocatalyst for the hydrogen evolution reaction, J. Mater. Chem. A 4 (2016) 6006–6013. [31] B. Šljukić, M. Vujković, L. Amaral, D. Santos, R. Rocha, C. Sequeira, J. Figueiredo, Carbon-supported Mo2C electrocatalysts for hydrogen evolution reaction, J. Mater. Chem. A 3 (2015) 15505–15512. [32] L.F. Pan, Y.H. Li, S. Yang, P.F. Liu, M.Q. Yu, H.G. Yang, Molybdenum carbide stabilized on graphene with high electrocatalytic activity for hydrogen evolution reaction, Chem. Commun. 50 (2014) 13135–13137. [33] C. Ge, P. Jiang, W. Cui, Z. Pu, Z. Xing, A.M. Asiri, A.Y. Obaid, X. Sun, J. Tian, Shapecontrollable synthesis of Mo2C nanostructures as hydrogen evolution reaction electrocatalysts with high activity, Electrochim. Acta 134 (2014) 182–186. [34] R.D. Nikam, A.-Y. Lu, P.A. Sonawane, U.R. Kumar, K. Yadav, L.-J. Li, Y.-T. Chen, Three-dimensional heterostructures of MoS2 nanosheets on conducting MoO2 as an efficient electrocatalyst to enhance hydrogen evolution reaction, ACS Appl. Mater. Interfaces 7 (2015) 23328–23335. [35] Z. Wu, R. Liu, J. Wang, J. Zhu, W. Xiao, C. Xuan, W. Lei, D. Wang, Nitrogen and sulfur co-doping of 3D hollow-structured carbon spheres as an efficient and stable metal free catalyst for the oxygen reduction reaction, Nano 8 (2016) 19086–19092. [36] Z. Wu, R. Liu, J. Wang, J. Zhu, W. Xiao, C. Xuan, W. Lei, D. Wang, Activated carbon derived from non-metallic printed circuit board waste for supercapacitor application, Electrochim. Acta 211 (2016) 488–498. [37] C. Wan, Y.N. Regmi, B.M. Leonard, Multiple phases of molybdenum carbide as electrocatalysts for the hydrogen evolution reaction, Angew. Chem. Int. Ed. 126 (2014) 6525–6528. [38] Y.Y. Chen, Y. Zhang, W.J. Jiang, X. Zhang, Z.H. Dai, L.J. Wan, J.S. Hu, Pomegranate-like N,P-doped Mo2C@C nanospheres as highly active electrocatalysts for alkaline hydrogen evolution, ACS Nano 10 (2016) 8851–8860. [39] F. Yang, K. Sliozberg, H. Antoni, W. Xia, M. Muhler, MoxC/CNT composites as active electrocatalysts for the hydrogen evolution reaction under alkaline conditions, Electroanalysis 28 (2016) 2293–2296. [40] K. Zhang, Y. Zhao, S. Zhang, H. Yu, Y. Chen, P. Gao, C. Zhu, MoS2 nanosheet/Mo2Cembedded N-doped carbon nanotubes: synthesis and electrocatalytic hydrogen evolution performance, J. Mater. Chem. A 2 (2014) 18715–18719. [41] Q. Gao, X. Zhao, Y. Xiao, D. Zhao, M. Cao, A mild route to mesoporous Mo2C–C hybrid nanospheres for high performance lithium-ion batteries, Nanoscale 6 (2014) 6151–6157. [42] J. Wang, H. Xia, Z. Peng, C. Lv, L. Jin, Y. Zhao, Z. Huang, C. Zhang, Graphene porous foam loaded with molybdenum carbide nanoparticulate electrocatalyst for effective hydrogen generation, ChemSusChem 9 (2016) 855–862. [43] C. Tang, A. Sun, Y. Xu, Z. Wu, D. Wang, High specific surface area Mo2C nanoparticles as an efficient electrocatalyst for hydrogen evolution, J. Power Sources 296 (2015) 18–22. [44] M. Fan, H. Chen, Y. Wu, L.-L. Feng, Y. Liu, G.-D. Li, X. Zou, Growth of molybdenum carbide micro-islands on carbon cloth toward binder-free cathodes for efficient hydrogen evolution reaction, J. Mater. Chem. A 3 (2015) 16320–16326. [45] K. Zhang, C. Li, Y. Zhao, X. Yu, Y. Chen, Porous one-dimensional Mo2C–amorphous carbon composites: high-efficient and durable electrocatalysts for hydrogen
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Molybdenum carbides embedded on carbon nanotubes for efficient hydrogen evolution reaction
MARK
Junpo Guo1, Jie Wang1, Cuijuan Xuan, Zexing Wu, Wen Lei, Jing Zhu, Weiping Xiao, Deli Wang⁎ Key laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hydrogen evolution reaction Electrocatalysts Molybdenum carbide Carbon nanotubes Embedded structure
Molybdenum carbides embedded on carbon nanotubes are prepared via a simple hydrothermal method with subsequent post-treatment at high temperature. In order to optimize the electrocatalytic activity for hydrogen evolution reaction (HER), effects of hydrothermal and post-annealing temperature are systematically investigated. Electrochemical testing results indicated that Mo2C/CNT prepared at a solvothermal temperature of 200 °C and 800 °C post heat-treatment exhibits the best HER activity with an overpotential of 125 mV in acid media and 93 mV in alkaline media at a current density of 10 mA cm− 2. Such enhanced catalytic activity may originate from high electro-conductivity and ECSA value, together with ultra-small particle size, which accelerates the electron transfer rate and provides large surface area and active sites, respectively.
1. Introduction Hydrogen, a green and renewable energy source, has been intensely investigated as a promising alternative to conventional fossil fuels [1–3]. Recently, electrochemical water splitting to produce hydrogen has attracted tremendous attention [4–6]. Although Pt-group precious metals or alloy are mostly active and stable catalysts for the hydrogen evolution reaction (HER) [7,8], their scarcity and high cost made them impractical for global-scale applications. Much efforts have been made to develop cost-effective and earth-abundant 3d–transition metal (TMs) based HER electrocatalysts, including carbides [9–11], nitrides [12,13], sulfides [14–17] and phosphides [18–21], et al. Among these catalysts, molybdenum carbides are considered as the effective non-Pt electrocatalysts for HER in both acidic and basic conditions, owing to their similar d-band electronic density with Pt [22,23], lower hydrogen-adsorption properties [10,24,25] and relative high chemical stability. However, although molybdenum carbides exhibit good electronic conductivity [26,27], the HER rate is restricted by the electron transfer rate. Therefore, a combination of carbon material is usually adopted to enhance the electronic conductivity. Recently, Wang and co-workers reported a facile, one-step synthetic route to obtain molybdenum carbide (Mo2C) embedded in nitrogen-rich carbon layers by using ammonium molybdate and dicyandiamide as precursor [28]. Mo2C nanoparticles decorated graphitic carbon sheets (Mo2C/GCSs) were prepared via a one-step solid-state method, utilizing sodium alginate (ALG) as a low cost environmental friendly carbon source [29]. Latterly, graphene supported
⁎
1
Corresponding author. E-mail address: [email protected] (D. Wang). These authors contributed equally.
http://dx.doi.org/10.1016/j.jelechem.2017.07.020 Received 23 March 2017; Received in revised form 6 June 2017; Accepted 12 July 2017 Available online 13 July 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved.
molybdenum carbide octahedral nanoparticles with porous and small size was derived via an in situ carburization of metal organic framework structure (MOFs) [30]. The HER activity was obviously improved via the incorporation of Mo2C and carbon support. In addition, further improvement of HER activity would be acquired through a more intimate incorporation between molybdenum compounds and carbon support [31–32]. Herein, we report a facile, two-step synthetic route for the synthesis of molybdenum carbides embedded on carbon nanotubes. Pure phase molybdenum carbide compound (Mo2C/CNT) are obtained via the solvothermal (within 180–240 °C) and post-annealing process (within 700–900 °C). Mo2C/CNT synthesized via a 200 °C solvothermal with post annealing of 800 °C (Mo2C/CNTS200–800) exhibits the best HER activity compared with other catalysts prepared at different temperature. Such enhanced catalytic activity may originate from the low hydrogen binding energy, high electro-conductivity and the ultra-small particle size, which decrease the free-energy, accelerate the electron transfer rate and increase active sites, respectively. 2. Experimental part 2.1. Materials synthesis 2.1.1. Synthesis of MoO2/CNT composites 180 mg of (NH4)6Mo7O24·4H2O was fully dissolved in a mixture solvent of 10 mL distilled water and 20 mL ethylene glycol by
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Fig. 1. XRD, Raman and XPS spectra of Mo2C/CNT catalysts calcined at different temperature after 200 °C solvothermal process. (a) Powder XRD patterns of Mo2C/CNTS200–700, Mo2C/ CNTS200–800 and Mo2C/CNTS200–900; (b) Raman spectrum of Mo2C/CNTS200–700, Mo2C/CNTS200–800 and Mo2C/CNTS200–900; XPS spectra of Mo2C/CNTS200–800 (c) and corresponding high-resolution spectra of Mo 3d (d).
2.3. Electrochemical measurements
sonication. Then 120 mg of CNT was added into the mixed solution. The mixture was stirred at room temperature for approximately 20 min until a homogeneous solution was achieved. Then the solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and heated in an oven at 200 °C for 10 h. The solvothermal product was collected after being washed with DI water and ethanol for several times, and then dried via the freeze dryer.
5 mg of catalyst and 1 mL of Nafion solution (1 wt‰) were sonicated to form a homogeneous ink. Then 16.5 μL of the catalyst ink was loaded onto a glassy carbon electrode (5 mm in diameter), the loading is 420 μg/cm2. For comparison, the electrodes were also modified with Pt/C. All electrochemical measurements were performed using a Autolab PGSTAT302N electrochemical workstation in a three-electrode setup with modified glass carbon working electrodes, reversible hydrogen electrode (RHE) as reference, carbon rod as a counter electrode. 0.5 M H2SO4, 1 M KOH and 1 M phosphate buffer (pH = 7) solution were used as electrolyte and are deaerated with nitrogen. The electrocatalytic activity of the catalysts towards HER was evaluated by using linear sweep voltammetry (LSV) in 0.5 M H2SO4 and 1 M KOH at a scan rate of 5 mV s− 1. Electrochemical impedance spectroscopy (EIS) measurements were conducted at overpotential of 100 mV from 105 Hz to 0.01 Hz.
2.1.2. Synthesis of Mo2C/CNT composites The solvothermal product was annealed in H2 (5%)/Ar (95%) atmosphere at 700 °C, 800 °C and 900 °C for 2 h at a heating rate of 10 °C min− 1, respectively. The obtained molybdenum compounds were denoted as Mo2C/CNTS200–700, Mo2C/CNTS200–800 and Mo2C/ CNTS200–900. In order to explore the difference of the obtained materials in solvothermal process, the solvothermal temperature was investigated at 180 °C, 200 °C, 220 °C and 240 °C for 10 h, and then annealed in H2/Ar atmosphere up to 800 °C for 2 h with a heating rate of 10 °C min− 1, respectively, The resulting powder was denoted as Mo2C/ CNTS180–800, Mo2C/CNTS200–800, Mo2C/CNTS220–800 and Mo2C/ CNTS240–800, respectively. The obtained materials were kept in 5 mL centrifuge tube immediately, which were filled with nitrogen before sealed.
3. Results and discussion The crystal structure of molybdenum-based materials was confirmed by the XRD measurement. MoO2 phase was first obtained via solvothermal process (XRD patterns in Fig. S1). After the subsequent annealing treatment at 700 °C, 800 °C and 900 °C, Mo2C/CNT was obtained which can be seen in Fig. 1a. The small diffraction peak located at 26° is ascribed to (002) plane of the carbon nanotubes, while the diffraction peaks located at 34.4°, 37.8°, 39.4°, 52.1°, 61.5°, 69.6°, 72.4°, and 74.6°, are indexed to (100), (002), (101), (102), (110), (103), (200) and (112) lattice planes of Mo2C (JCPDS NO. 01-035-0787) [33]. Typical for Mo2C/CNT solvothermal in 200 °C, annealing at 800 °C (denoted as Mo2C/CNTS200–800), the additional structural information was obtained from Raman spectroscopy measurements. The full-range Raman spectra showed characteristic peaks of molybdenum carbide (Fig. 1b) located at 670 cm− 1, 840 cm− 1 and 990 cm− 1, which is consistent with the XRD pattern in Fig. 1a [30]. Moreover, it can be seen that D band located at 1350 ± 20 cm− 1 and G band located at 1575 ± 20 cm− 1, which are corresponding to the disordered graphitic
2.2. Material characterization The crystal structure was determined using X-ray diffraction, and diffraction patterns were collected using Cu Kα (λ = 1.5406 Å) radiation at a scanning rate of 4° min− 1. Raman spectra were collected by a LabRam HR800 spectrometer with a 532 nm laser excitation. The morphologies were measured by scanning electron microscopy (SEM, Sirion200), Transmission electron microscopy (TEM) images were recorded by a JSM-2100 transmission electron microscopy (JEOL, Japan) at an acceleration voltage of 200 kV. Further, the chemical states of the elements in catalysts were studied by XPS using an AXIS-ULTRA DLD600 W Instrument, and the binding energy of the C 1s peak at 284.6 eV was taken as an internal reference. 8
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carbon and the Eg vibration of the sp2-bonded carbon atoms, respectively. The ID/IG intensity value of Mo2C/CNTS200–700, Mo2C/ CNTS200–800 and Mo2C/CNTS200–900 is calculated to be 0.92, 0.86 and 0.81, respectively (Fig. 1b), indicating the increase of graphitization degree with increasing the temperature, which may enhance the overall electrical conductivity and facilitate the charge transfer during electrochemical testing [34–36]. X-ray photoelectron spectroscopy (XPS) was performed to probe the electronic structures on the surface of the catalysts, and all binding energy (BE) values were calibrated using the C 1s peak at 284.6 eV as a reference. As shown in Fig. 1c, the survey spectrum shows apparent signals located at 231.5, 285.1, 395.2, 413.6, and 531.5 eV, which are ascribed to the existence of Mo 3d, C 1s, Mo 3p, and O 1 s, respectively. In the high resolution XPS spectrum of Mo 3d (Fig. 1d), two deconvoluted peaks at 228.5 and 231.6 eV can be assigned to Mo2C, respectively, Peaks located at the binding energies of 229.2 and 232.3 eV could be attributed to MoO2, and another two peaks at 232.4 and 235.5 eV could be attributed to MoO3, both of them are the result of surface oxidation of Mo2C [37–39]. The high-resolution C1s XPS spectrum (Fig. S2) shows the asymmetric peak of MoeC (at 283.8 eV), CeO (at 286.0 eV), C]O (at 287.0 eV) and OeC]O (at 290.7 eV) groups which indicated the defects in the structure of CNT support and the existence of Mo2C in the Mo2C/CNTS200–800 catalyst [40–42]. The morphology of the molybdenum based catalysts was characterized by scanning electron microscopy (SEM) and transition electron microscopy (TEM). As depicted in Fig. S3a, the CNTs were covered by MoO2 on the surface, indicating the intimate combination between MoO2 and CNT. The SEM images (Fig. S3b–d) display a morphological transformation when the annealing temperature elevated from 700 °C to 900 °C. Mo2C nanoparticles were separated out at high temperature of 700 °C, and uniformly distributed when annealed at 800 °C. However, the high temperature of 900 °C resulted in aggregation of large particle size which can be obviously seen in Fig. S3d. Therefore, 800 °C was chosen as the optimized annealing temperature while lower or higher the temperature will result in inadequate transformation or particle aggregation. In addition, the uniformly distribution of Mo2C nanoparticles on CNTs would increase the active sites which contribute to the enhanced HER activity. Fig. 2 shows the typical TEM images for Mo2C/CNTS200–800 catalyst. Carbon nanotubes acts as the stabilizing agent, carbon precursor and reducing agent of the obtained MoO2. After forming Mo2C nanoparticles, part of the CNT would scarify some carbon source and leave some defects where particles located. Thus, the final particles would be embedded on the CNT. And, when combined with the full range TEM image in Fig. 2a, the main particle size is 7.4 nm when calculated from more than 300 nanoparticles (inset of Fig. 2a). The tight fix between Mo2C nanoparticles and CNTs would prevent the particle fall off during electrochemical measurement and enhance the electronic conductivity towards one dimensional direction. The obvious inter-planar spacing in Fig. 2b are measured to be 0.23 nm and 0.25 nm from high resolution transmission electron microscopy (HRTEM) image of an individual particle, corresponding to (101) and (100) plane [43–45]. The value of measured inter-planar spacing is consistent with SAED pattern (inset) and the theoretical inter-planar spacing from the XRD patterns (Fig. 1a). Moreover, the EDX mappings were performed to investigate the distributions of the C and Mo in Mo2C particles. As depicted in Fig. 2c, there are some vacancies on the CNTs and moreover, Mo2C particles filled in the vacancies (Fig. 2e) which in all confirm the embedding of Mo2C particles. Fig. 2d shows that Mo element is centered on the particle, indicating the adequate transformation of MoO2 to Mo2C. The electrocatalytic performance of as-prepared catalysts for HER was investigated in 0.5 M H2SO4 solution together with Pt/C for comparison. As shown from the polarization curves in Fig. 3a, Mo2C/ CNTS200–800 exhibited the highest HER activity compared with the other two molybdenum catalysts calcined at 700 °C and 900 °C, while Pt/C exhibited extremely high catalytic activity with nearly zero onset
overpotential. As reported, the overpotential for driving a current density of 10 mA cm− 2 is a reference parameter for comparing catalysts in solar hydrogen production [46]. And, the overpotential for Mo2C/CNTS200–700, Mo2C/CNTS200–800 and Mo2C/CNTS200–900 are 164 mV, 125 mV and 193 mV, respectively, indicating a faster hydrogen evolution rates on the catalyst calcined at 800 °C. In order to study the mechanism and the inherent properties for the as-prepared Mo2C/CNT catalysts, Tafel plots were obtained by plotting the logarithm of the kinetic current density derived from the polarization curves. As shown in Fig. 3b, the Tafel slopes of Mo2C/CNTS200–700, Mo2C/ CNTS200–800 and Mo2C/CNTS200–900 at low overpotential were calculated to be 74 mV dec− 1, 58 mV dec− 1, 90 mV dec− 1, respectively, lower than 120 mV dec− 1, indicating a Volmer-Heyrovsky mechanism for HER where the HER rate is determined by the discharge reaction or the electrochemical desorption of H and H+ from the catalyst surface to form hydrogen. The exchange current densities (j0) is another factor to reflect the intrinsic rate of HER activity, Which was obtained from Tafel curves by using extrapolation methods [47,48]. As described in Fig. S4, the j0 value of Mo2C/CNTS200–800 is around 0.16 mA cm− 2, which is about 2.5 and 4 times higher than Mo2C/CNTS200–700 (0.064 mA cm− 2) and Mo2C/CNTS200–900 (0.04 mA cm− 2), respectively (Table S1). The high j0 value on Mo2C/CNTS200–800 could be attributed to the highly dispersion of nanoparticles, which affords large amounts of active sites [49]. The electrochemical surface area (ECSA) provides insight into the Mo2C electrocatalysts. Although the accurate measurement of ECSA is difficult, it can be visualized through calculating the double-layer capacitances (Cdl) which are proportional to the ECSA values. The estimated values of Cdl using the cyclic voltammograms (CV, Fig. S5) in 0.5 M H2SO4 were alternatively utilized to provide a relative comparison. As shown, the measured Cdl is 19, 20 and 7 mF cm− 2 for Mo2C/CNTS200–700, Mo2C/CNTS200–800 and Mo2C/ CNTS200–900, respectively. It can be clearly seen that the Cdl value on Mo2C/CNTS200–800 is 2.8 times higher than Mo2C/CNTS200–900 and larger than Mo2C/CNTS200–700, which may provide more active sites to achieve prominent HER activity. Besides, to further optimize the experimental conditions, the solvothermal temperature was also investigated for the influence of HER performance. The solvothermal temperature was set to be 180 °C, 200 °C, 220 °C and 240 °C, respectively. It is worth noting that when the temperature was set below 180 °C, there would be no solvothermal products. After subsequent calcination at 800 °C under Ar/H2 atmosphere, the as-prepared products were first measured via the XRD analysis. As described in Fig. S6, the diffraction peaks located at 34.4°, 37.8°, 39.4°, 52.1°, 61.5°, 69.6°, 72.4°, and 74.6° are corresponding to the characteristic (100), (002), (101), (102), (110), (103), (200) and (112) lattice planes of Mo2C, respectively. Thus, negligible structure transformation can be detected from the characteristic diffraction peaks of Mo2C. However, the diffraction peak located at 26° (characteristic peaks of CNTs) decreased when the temperature rise from 180 °C to 240 °C, which may be attributed to the deterioration of CNTs. The HER polarization performance of the as-prepared four catalysts are investigated in 0.5 M H2SO4 (Fig. 3c), where Mo2C/CNTS200–800 exhibits the smallest overpotential of 125 mV at current density of 10 mA cm− 2 compared with Mo2C/CNTS180–800 (184 mV), Mo2C/CNTS220–800 (154 mV) and Mo2C/CNTS240–800 (197 mV). The corresponding Tafel slope value (Fig. 3d) at low overpotential of the as-prepared catalysts derived from the polarization curves in Fig. S7a are 58 mV dec− 1, 89 mV dec− 1, 69 mV dec− 1 and 92 mV dec− 1, respectively, also indicating a Volmer-Heyrovsky mechanism where Mo2C/CNTS200–800 deliver a higher H2 evolution rate [50,51]. The much difference of activities may be ascribed to the different interaction capability between molybdenum based compounds and carbon nanotubes after solvothermal process. Meanwhile, the fast kinetic of Mo2C/ CNTS200–800 for HER was verified by the smaller Tafel slope than most of the reported materials (Table S2). The j0 value at overpotential of 0 V 9
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Fig. 2. Typical TEM images for Mo2C/CNTS200–800. (a) Full range TEM image and the corresponding particle size distribution (inset); (b) HRTEM image and the selected area electron diffraction (SAED) pattern (inset); (c–e) EDX mapping for C, Mo and their composites, respectively.
Fig. 3. Electrochemical performance of Mo2C/CNT catalysts. (a) Polarization curves of Mo2C/CNTS200–700, Mo2C/ CNTS200–800, Mo2C/CNTS200–900 and Pt/C in 0.5 M H2SO4 solution; (b) Tafel plots of the corresponding catalysts in (a); Electrochemical performance of Mo2C/CNT catalysts prepared at different solvothermal temperature and annealed at 200 °C. (c) Polarization curves of Mo2C/ CNTS180–800, Mo2C/CNTS200–800, Mo2C/CNTS220–800 and Mo2C/CNTS240–800 catalysts in 0.5 M H2SO4 solution; (d) Tafel plots of the corresponding catalysts in (c).
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Fig. 4. (a) Nyquist plots of catalysts recorded at 100 mV in 0.5 M H2SO4 solution; (b) HER polarization curves of Mo2C/ CNTS200–800 and Pt/C before and after 5000 potential cycles between −0.3 V to 0.2 V at a scan rate of 100 mV s− 1 in 0.5 M H2SO4 solution.
Fig. 5. (a) Polarization curves of Mo2C/CNTS200–800 catalyst and Pt/C in 1 M KOH solution before and after 5000 potential cycles between − 0.3 V to 0.2 V at a scan rate of 100 mV s− 1; (b) Tafel plots of Mo2C/CNTS200–800 catalyst and Pt/C.
From the above analysis, it can be summarized that the proper temperature of 200 °C in solvothermal process and 800 °C in post-annealing process resulted in the highest HER activity. The enhanced electrocatalytic HER performance on Mo2C/CNTS200–800 could be ascribed to the following aspects: i) Small nanocrystallites increased the active sites and enable rapid charge transfer for HER; ii) The dimensional structure of the as-prepared catalysts facilitate the electrons transfer rate during the electrochemical reactions [56]. Besides the HER activity, the stability performance for the electrode reaction is also an integral part for their potential practical application. The stability of the high performance Mo2C/CNTS200–800 catalyst and Pt/C was measured by potential cycling for 5000 cycles between −0.3 V to 0.2 V at a scan rate of 100 mV s− 1 in 0.5 M H2SO4 solution (Fig. 4b), promisingly, there is negligible current decay for the two catalysts even after 5000 potential cycling test, indicating excellent electrochemical stability for the as-prepared catalysts in acid solution [57,58]. The as-prepared Mo2C/CNTS200–800 catalyst has been proved to the best catalyst among the catalysts synthesized at different solvothermal and post-annealing temperatures in acid media, while in order to expand the application range of the catalysts, the HER activity was also investigated in basic media. As shown in Fig. 5a, Mo2C/CNTS200–800 catalyst exhibited excellent electrocatalytic activity towards HER in 1 M KOH solution, with an overpotential of 93 mV to drive a current density of 10 mA cm− 2, although slightly lower than Pt/C (50 mV), indicating a faster hydrogen evolution rate. In addition, the Tafel value of Mo2C/ CNTS200–800 catalyst shown in Fig. 4b is 65 mV dec− 1, when compared with Pt/C, which indicating a Volmer-Heyrovsky mechanism. Furthermore, the HER performance of the Mo2C/CNTS200–800 is the best compared with the reported Mo2C-based HER catalysts both in acid and alkaline media [59] (Table S2). The stability of Mo2C/CNTS200–800 and Pt/C were also tested in 1 M KOH solution. After a 5000 potential cycles between − 0.3 V to 0.2 V at a scan rate of 100 mV s− 1, Mo2C/ CNTS200–800 and Pt/C suffer slightly current decay compared with the initial polarization curves (Fig. 5a). The polarization curves after the cycling stability still possess excellent HER activity. The relative lessstable of the catalysts in alkaline solution compared with the stability in
in Fig. S7 for the different solvothermal temperature derived catalysts also showed the highest j0 value of Mo2C/CNTS200–800 (specific j0 value can be seen from Table S1). A summary of the electrocatalytic performance comparison of the as-prepared materials and reported Mo2C-based HER catalysts for HER are listed in Table S2, as expected, the j0 on Mo2C/CNTS200–800 is higher than other catalysts. R. R. Adzic et al. have recently obtained the products of molybdenum carbide (Mo2C) anchored to carbon nanotubes by similar method, with the particle size of range from 7 to 15 nm and the j0 value of 0.014 mA cm− 2. Therefore, the enhanced HER activities of Mo2C/ CNTS200–800 catalyst could be originated from ultra-small particle size and higher j0 value which provides large surface area and active sites. Electrochemical impedance spectroscopy (EIS) measurements were carried out to further elucidate the mechanism of charge transfer on different catalysts. Representative Nyquist plots of Mo2C/CNTS200–700, Mo2C/CNTS200–800 and Mo2C/CNTS200–900 in 0.5 M H2SO4 electrolyte are shown in Fig. 4a, where the as-prepared three catalysts exhibited an obvious semicircle at overpotential of 100 mV, indicating that the corresponding equivalent circuit for the HER was characterized by onetime constant as fitted from the experimental data (inset of Fig. S8) [46,52]. In addition, Nyquist plots of Mo2C/CNTS180–800, Mo2C/ CNTS200–800, Mo2C/CNTS220–800 and Mo2C/CNTS240–800 are shown in Fig. S10a. Among the above catalysts, Mo2C/CNT S200–800 catalyst exhibited the lower charge-transfer resistance (49 Ω) than Mo2C/ CNTS200–700 (108 Ω), Mo2C/CNT S200–900 (253 Ω), Mo2C/ CNTS180–800 (297 Ω), Mo2C/CNTS220–800 (153 Ω) and Mo2C/ CNTS240–800 (364 Ω), indicating the excellent electron transfer rate of Mo2C/CNTS200–800, which could be attributed to the enhanced electronic conductivity via uniformly distribution on CNTs and partially embedment into the carbon nanotubes [53–55]. Thus, in accordance with the XRD measurements in Fig. S6, it is concluded that solvothermal temperature of 200 °C and annealing temperature of 800 °C is the proper synthetic condition for Mo2C embedded on CNTs, while lower or higher the temperature would result in incomplete contact between Mo2C and CNTs or severe CNTs deterioration, which would furtherly reduce the HER activity. 11
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acid could attributed to the fact that etching of carbon support in the presence of high concentration KOH solution, which has been reported that active sites would be suspected to detach from the support in case of the carbon corrosion. Additionally, i–t chronoamperometric curves (Fig. S10) for Mo2C/ CNTS200–800 in both 0.5 M H2SO4 and 1 M KOH solution was recorded at constant potentials of 125 and 93 mV for 10 h, respectively. It can be seen that there is negligible current decay in 0.5 M H2SO4 and slight current decrease in 1 M KOH during 10 h stability testing, indicating excellent durability of Mo2C/CNTS200–800. The current decrease of Mo2C/CNTS200–800 in alkaline media may be caused by the carbon corrosion effect [60]. Meanwhile, in order to investigate the stability change of Mo2C/CNTS200–800 in 0.5 M H2SO4 and 1 M KOH solution, TEM images after 5000 potential cycles were also characterized in Fig. S11. The particle size and distribution of Mo2C was maintained well in 0.5 M H2SO4 solution (Fig. S11a), while obvious corrosion of carbon and particle growth was occurred in 1 M KOH solution, results in lower electro-conductivity and reduced catalytic performance, which is consistent with the result of i–t chronoamperometric curves. In addition, the HER activity of Mo2C/CNTS200–800 in neutral solution was measured in 1 M phosphate buffer solution (PH = 7). As can be seen from Fig. S12a and b, an overpotentials of 276 mV and Tafel slope of 114 mV dec− 1 in neutral solution were obtained, which were lower than the results obtained in 0.5 M H2SO4 and 1 M KOH solution. Moreover, from Fig. S12c and d, the charge-transfer resistance (Rct) and electrochemical double layer capacitance (Cdl) were measured to be 864 Ω and 6.83 mF cm− 2, respectively. The lower catalytic activity in neutral solution probably attributed to the limited electronic conductivity and lower electrochemically active surface area. 4. Conclusions Molybdenum carbides embedded on carbon nanotubes are prepared via simple hydrothermal method with subsequent post-treatment at high temperature. Hydrothermal and post-annealing temperatures are investigated in detail to optimize the experimental condition for better hydrogen evolution reaction (HER) activity. Results show that Mo2C/ CNTS200–800 with uniform particle distribution and ultra-small particle size (~ 8 nm) exhibits the best HER activity. Such enhanced catalytic activity may originate from the low hydrogen binding energy, high electro-conductivity and the ultra-small particle size, which decrease the free-energy, accelerate the electron transfer rate and increase active sites, respectively. The present work also appeals the research who emphasizes on the characterization on one dimensional direction. Acknowledgements This work was supported by the National Natural Science Foundation (21573083), the Program for New Century Excellent Talents in Universities of China (NCET-13-0237), the Doctoral Fund of Ministry of Education of China (20130142120039), 1000 Young Talent (to Deli Wang), and initiatory financial support from Huazhong University of Science and Technology (HUST). The authors thank the Analytical and Testing Center of HUST for allowing use its facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jelechem.2017.07.020. References [1] J. Ge, B. Zhang, L. Lv, H. Wang, T. Ye, X. Wei, J. Su, K. Wang, X. Li, J. Chen, Constructing holey graphene monoliths via supramolecular assembly: enriching nitrogen heteroatoms up to the theoretical limit for hydrogen evolution reaction, Nano Energy 15 (2015) 567–575.
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