Carbon Hybrid Nanotubes Synthesized by a Dual

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Mesoporous Mo2C/Carbon Hybrid Nanotubes Synthesized by A Dual-Template Self-Assembly Approach for Efficient Hydrogen Production Electrocatalyst Dan Hou, Jiacheng Zhang, Qian Li, Peng-Fei Zhang, Chuanshuang Chen, Deyue Yan, and Yiyong Mai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02530 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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Mesoporous Mo2C/Carbon Hybrid Nanotubes Synthesized by A Dual-Template Self-Assembly Approach for Efficient Hydrogen Production Electrocatalyst Dan Hou, Jiacheng Zhang, Qian Li, Pengfei Zhang, Chuanshuang Chen, Deyue Yan, Yiyong Mai* School of Chemistry and Chemical Engineering, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China

Abstract: Molybdenum carbide-containing nanomaterials have drawn considerable attention in the application of hydrogen production electrocatalysts, in light of the high abundance, low cost and Pt-like electronic structure of molybdenum carbide. In this paper, we report the synthesis of one-dimensional (1D) Mo2C/carbon mesoporous nanotubes (Mo2C/C PNTs) through a dual-template self-assembly approach, which employs 1D MoO3 nanobelts as the structure-directing template as well as one of the Mo2C precursors, and block copolymer (BCP) micelles as the pore-forming template. In aqueous solution, the interface self-assembly of the micelles with pyrrole (Py) molecules absorbed in the PEO domains leads to the tight arrangement of the micelles on the surfaces of the MoO3 nanobelts. The polymerization of Py and the subsequent pyrolysis at 800 oC under nitrogen atmosphere yield Mo2C/C PNTs with well-defined mesopores. Among the resultant Mo2C/C PNT samples, Mo2C/C PNTs with a specific surface area of 69 m2/g, a N atom percentage of 5.5 at.% and an optimum Mo2C content of 40 wt.% exhibit the highest HER catalytic performance in 0.5 M H2SO4 electrolyte, with a low onset potential of 34 mV, a satisfied overpotential of 140 mV at 10 mA/cm2 and excellent cycling stability. This study not only opens an avenue toward new Mo2C-containing nanomaterials, but also provides a new system for the fundamental study on 1D porous nanohybrids with potential application as hydrogen production electrocatalysts.

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Introduction Hydrogen has been considered as one of the promising sources for clean and sustainable energy. Among a number of techniques towards hydrogen production, water electrolysis shows unique advantages with zero carbon emission and great convenience.1 To date, Pt-based materials have been known as the state-of-the-art catalysts for hydrogen evolution reaction (HER). However, their costliness and scarcity impede the wide practical applications.2 Thereby, the synthesis of economic cost and efficient hydrogen catalysts that can substitute Pt-based materials has attracted great interest, although it remains a challenge.6-12 Recently, molybdenum carbides (e.g. Mo2C) have been demonstrated as ideal HER electrocatalysts, which combine advantages of high abundance, low cost and Pt-like electronic structure, thus being a competitive candidate to replace Pt-based catalysts.13-23 However, Mo2C also exhibits a number of obvious drawbacks. (1) Their electrical conductivity is low; (2) the synthesis requires high temperature calcination of molybdenum and carbon precursors, which generally yields large Mo2C particles or agglomerates with low density of catalytic active sites; (3) Mo2C nanoparticles aggregate seriously during the process of hydrogen production and thus reduce their specific surface areas and cyclic stability.24-25 To solve these problems, much effort have been devoted to the design and fabrication of nanostructured Mo2C integrating with carbon matrix, which may effectively prevent the aggregation of Mo2C and also enhance its electrical conductivity.6, 17-18, 26-27 Moreover, the introduction of mesopores, in the carbon matrix, can not only reserve the electrolytes and thus improve the mass transfer efficiency, but also facilitate the release of produced hydrogen 2 ACS Paragon Plus Environment

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gas.18, 28-29 For example, Zhu et al. employed SBA-15 as a hard template for the fabrication of mesoporous Mo2C@carbon nanowire arrays; the resultant catalyst showed high HER activity with a 125 mV overpotential at 10mA/cm2.30 Liu et al. utilized SiO2 nanospheres as a template for the synthesis of ultrafine Mo2C particles confined in carbon foams, yielding 3D mesoporous Mo2C@carbon nanohybrids for efficient HER electrocatalysts.31 In addition to the compositions and mesoporous structures, the morphology of the resultant catalysts may also play a crucial role in their performance. In this regard, one-dimensional (1D) nanomaterials show unique advantages and thus has proven to be ideal candidates for electrocatalysts.32-34 For example, 1D nanomaterials can provide straight channels for fast charge transport with decreased scattering. Moreover, the surfaces of 1D nanomaterials facilitate the formation and release of bubbles, thus hindering the bubble blocking in the process of hydrogen production.37 To our knowledge, however, the synthesis of Mo2C/carbon hybrid materials combining the advantages of 1D and mesoporous structures has been rarely reported so far, due to the difficulties in finding suitable 1D and pore-creating templates. Here, we report the first synthesis, by a facile dual-template method, of 1D Mo2C/carbon porous nanotubes (denoted as Mo2C/C PNTs) with Mo2C nanoparticles (NPs) embedded in mesoporous tubular carbon matrix (Figure 1). The approach utilizes 1D MoO3 nanobelts as the structure-directing template, which also serves as one of the Mo2C precursors, and spherical micelles formed by polystyrene-b-poly(ethylene oxide) (PS-b-PEO) as the soft pore-forming template. In aqueous solution, the co-assembly of MoO3, the micelles and pyrrole (Py) that acts as the carbon source leads to the close packing of the micelles on the surface of MoO3, while the Py 3 ACS Paragon Plus Environment

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molecules are absorbed in the hydrophilic PEO domains. The polymerization of Py and the subsequent pyrolysis at 800 oC under nitrogen atmosphere produce Mo2C/C PNTs with well-defined mesopores. The resultant Mo2C/C heterotubes possess specific surface areas (SSA) of 38-77 m2/g, N contents of ca. 2.7-8.3 at.% and Mo2C contents of 37.7%-59.4 wt.%. With an optimum Mo2C content of 40 wt% and a high SSA of 69 m2/g, Mo2C/C PNTs-3 exhibits the best HER catalytic performance under acidic condition (0.5 M H2SO4), with a low starting potential (ηonset) of 34 mV, a satisfied 140 mV overpotential (η10) at 10 mA/cm2 and excellent long-term durability. Such performance surpasses those of many reported molybdenum carbide-containing carbon electrocatalysts lack of mesopores. Results and Discussion The MoO3 nanobelts were synthesized by following a reported procedure, 24 the details are described in the experimental section of Supporting Information (SI). Scan electron microscopy (SEM) images reveal a 1D belt-like structure for MoO3 (Figure S1a, b). Based on the statistics of 500 objects in the SEM images, the nanobelts have lengths of 5-15 µm and an average width of 500 nm. AFM topological profile suggests a mean thickness of ca. 75 nm for the nanobelts (Figure S1c, d). Transmission electron microscopy (TEM) micrographs indicate the solid feature of the MoO3 nanobelts (Figure S1e). X-ray diffraction (XRD) plots of the as-prepared MoO3 shows apparent signals attributed to MoO3 crystals, which agree well with the Joint Committee on Powder Diffraction Standards (JCPDS) No.05-0508 (Figure S1f).24 On the other hand, the PS160-b-PEO114 copolymer (the subscripts express the number of the repeating units) with a polydispersity index of 1.1 was synthesized by ATRP (see details in the SI).38 The BCP spherical micelles were prepared according to a reported cosolvent method 4 ACS Paragon Plus Environment

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(see details in the SI).39-40 It has been well known than the micelle cores and the coronae around the cores are constructed by the hydrophobic PS blocks and the hydrophilic PEO chains,

respectively.39-41

TEM

images

display

that

the

spherical

micelles

are

narrowly-dispersed and their average diameter are determined to be 18 ± 1 nm (Figure S4). The synergistic self-assembly of the micelles, Py molecules and MoO3 nanobelts were induced by mixing their aqueous solutions, which resulted in a tight micelle packing on the surfaces of the MoO3 nanobelts (Figure 1 and SEM image in Figure S5). Two driving forces, namely H-bonding and Mo-N coordination interaction, are considered to contribute to the surface self-assembly; the H-bonding induced the adhesion of Py molecules in the PEO domain along with the micelle interconnection, 42-43 while the Mo-N coordination led to the micelle adsorption on the MoO3 surface.40 The polymerization of Py, initiated by ammonium persulfate, yielded polypyrrole (PPy) network around the micelle cores. Previous work has demonstrated that the polymerization may trigger the migration of Py molecules from the top and bottom of the micelles into the vacancy of neighboring micelles, leading to the lack of the PPy moiety on the micelle top and bottom.42-43 After the polymerization, ammonia was added to convert MoO3 to MoO42-, which then dissolved in the aqueous solution. Thereby, the addition of different amounts of ammonia may control over the mass of MoO3 and the consequent content and size of Mo2C particles in the final hybrid nanotubes. Subsequently, the resultant MoO3/BCP/PPy hybrids were treated at 800 oC under nitrogen atmosphere, by which the BCP template was removed, while MoO3 and PPy were converted to Mo2C and carbon matrix, respectively, thus yielding Mo2C/C PNTs with different Mo2C contents (named as Mo2C/C PNTs-1 to 4, see Table 1). Thermogravimetric analysis (TGA) under air 5 ACS Paragon Plus Environment

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atmosphere revealed that the Mo2C/C PNTs underwent a slight weight increase at ∼350 oC and subsequently a weight loss at ∼450 oC; then different residual weight percentages in the range of 41.8∼83.8 wt% retained over 650 oC (Figure S6a). The weight increase at ∼350 oC was ascribed to the conversion of Mo2C to MoO3, while the sharp weight loss at ∼450 oC was caused by the combustion of carbon.28,44 Based on the TGA curves, the contents of Mo2C were estimated to be in the range of 30-60 wt.% for the different Mo2C/C PNTs samples (see Table 1 and the calculation in Page S12), which was confirmed by inductively coupled plasma (ICP) spectrometry analyses (Figure S6b). Moreover, based on the TGA results, the weight ratios of Mo2C to C were estimated to be in the range of 0.4-1.5 for different samples (Table 1). The higher the Mo2C content, the larger is the weight ratio of Mo2C to C. The structure of the resultant Mo2C/C PNTs was characterized by SEM and TEM (Figure 2 and Table 1). Typical SEM micrographs display a belt-like and mesoporous structure for Mo2C/C PNTs, on the surfaces of which closely-packed mesopores are clearly observed (Figure 2a, b and Figure S7). Based on the SEM images, the average pore sizes were measured to be 16~20 nm for different samples (Table 1). Moreover, the SEM images also demonstrate the tubular structure of the nanohybrids; in some of which open ends are clearly seen (Figure 2b). The mesoporous and tubular structure are confirmed by TEM observations (Figure 2c-f). TEM images also show that some of the nanotubes are end-capped. Measured from the TEM images, the average width (∼500 nm) of the tube cavities is quite close to that of the MoO3 nanobelt template. The average wall thickness is ca. 18 nm, approaching the mean diameter of the micelles, indicating that the walls are composed of monolayer mesoporous PPy that were formed through the templating of the closely-packing micelles on 6 ACS Paragon Plus Environment

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the MoO3 nanobelts. Crystal lattice of the incorporated particles are clearly seen in the high-resolution TEM (HRTEM) micrographs (Figure 2g), which are demonstrated to be Mo2C crystals by electronic diffraction (ED) patterns (inset of Figure 2g) coupled with XRD spectra (Figure 3a). TEM images reveal the presence of Mo2C agglomerates (high contrast area) in Mo2C/C PNTs-1&2 with relatively higher contents of Mo2C (Figure 2c,d), while no apparent aggregation of Mo2C particles is observed in Mo2C/C PNTs-3&4, in which the average diameters of the Mo2C nanoparticles are ca. 5 nm (Figure 2e,f and Figure S8). Elemental

mapping

images

from

high-angle

annular

dark-field

scanning

TEM

(HAADF-STEM) show homogeneous dispersion of C, N and Mo elements in Mo2C/C PNTs-3&4 (Figure 2h I-IV), confirming no serious aggregation of Mo2C particles in these two samples. Raman spectra of Mo2C/C PNTs reflects the information of the inherent state of the carbons, with the signal of the disordered carbons located at 1345 cm−1 (D band) and that of graphitic carbons appeared at 1585 cm−1 (G band), respectively (Figure 3b). The high intensity ratios (IG/D≈1) of G to D band suggest high degrees of graphitization for Mo2C/C PNTs,

which

facilitate

the

charge

transfer

in

the

carbon

matrix.45

Nitrogen

adsorption–desorption measurement was employed to investigate the mesoporous structure of Mo2C/C PNTs. The adsorption–desorption isotherms exhibit typical features of mesoporous structure (Figure 3c, d). Employing the Barrett-Joyner-Halenda (BJH) protocol, the specific surface areas of the different samples are calculated to be in the range of 38-77 m2/g and the average pore sizes are ca. 18 nm (Table 1). The pore sizes agree well with the SEM results. The low specific surface areas of Mo2C/C PNTs-1&2 are probably due to the presence of the 7 ACS Paragon Plus Environment

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Mo2C agglomerates, which not only provide much weight but also block some of the mesopores. To elucidate the surface composition and chemical state, Mo2C/C PNTs were analyzed by X-ray photoelectron spectroscopy (XPS). Take the XPS spectra of Mo2C/C PNTs-3 as an example (those of the other samples are given in Figures S9−S11), Figure 4a displays the characteristic signals of the existing elements. In the C1s energy level signal (Figure 4b), three peaks can be fitted, which are centered at 288.8 eV, 285.8eV, and 284.6 eV, attributable to the carbon atoms in C=O, C−N, and C−C/C=C binding species, respectively.46 However, the signal of carbides (at 282.7 eV) is absent, which is possibly owing to the surface depositing of carbon, where the signal of the graphitic carbons covers that of the carbides in an analogous binding energy window.47 In the Mo3d spectrum (Figure 4c), the relatively weak peak at 228.4 eV originates from Mo2C,48 indicating a low loading content of Mo2C in Mo2C/C PNTs-3. The intensity of this peak varies in the Mo3d spectra of the different samples (Figure S9-S11), confirming their different Mo2C contents. The peaks at 232.1 and 236.0 eV belong to MoO3, while the peak at 233.0 eV can be assigned to MoO2.44, 49 It should be noted here that XPS detection can only reflect the surface information of samples. Although the spectra indicate the presence of the molybdenum oxides, it is attributable to the fact that Mo2C on materials surface is easily oxidized in the air.47,50 Precious studies has reported that a small amount of molybdenum oxides in Mo2C-based HER electrocatalysts do not significantly affect their catalytic performance.11,24,48 The deconvolution of N1s signal (Figure 4d) reveals the existence of pyridinic N (398.7 eV) and graphitic N (401.0 eV).51 The pyridinic N dominates the nitrogen species; which has proven to be beneficial to HER 8 ACS Paragon Plus Environment

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performance.48 The atom percentages of C and N elements in the different Mo2C/C PNTs samples are listed in Table 1. As the STEM elemental mapping results have proved the homogeneous distribution of C and N elements in the hybrids, their atom percentages on the surface (detected by XPS) can represent their overall contents. The HER performance of the Mo2C/C PNTs under acidic condition were assessed by using a three electrode system (see experimental section in SI). As a control sample, Pt/C (20wt% Pt on carbon black) was also evaluated. The polarization curves without ohmic potential drop compensation are presented in Figure 5a, and their HER catalytic performance is summarized in Table 2. Comparably, Pt/C displays the highest efficiency with a small initial potential (ηonset) of nearly zero and an ultralow overpotential (η10) of 30 mV at 10 mA/cm2. In contrast, Mo2C/C PNTs-3 needs a potential of ca. 34 mV and ca. 140 mV to initiate the production of hydrogen and reach a current density of 10 mA/cm2, respectively, which are much smaller than those of the other three Mo2C/C PNTs samples (Table 2), indicating a better electrocatalytic activity. To our knowledge, such performance is superior to those of many reported molybdenum carbide-containing HER electrocatalysts without mesopores (see Table S1). To understand the mechanism of Mo2C/C PNTs serving as HER catalysts, Tafel plots (Figure 5b) were calculated based on the formula: η = b log (j) + a, in which η represents the overpotential at the current density of j, b stands for the Tafel slope, and a denotes the overpotential at 1 mA/cm2. Typically, the HER process involves three primary reactions in acid condition, which refer to the Vomer reaction (H3O+ + e- → Hads + H2O), the Heyrovsky reaction (Hads + H3O+ + e- → H2↑ + H2O), and the Tafel reactions (Hads + Hads → H2↑).6, 9 ACS Paragon Plus Environment

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In these reactions, the H atoms absorbed on the active site are denoted as Hads. The

kinetic model reveals that the Tafel slope will be totally different when the rate-controlling step changes, being around 120, 40 or 30 mV/decade govern by the Volmer, Heyrovsky or Tafel reaction, respectively.53 In our work, the Tafel slope of Mo2C/C PNTs-3 was determined to be 92.6 mV/dec, lower than those of Mo2C/C PNTs-1 (102.7 mV dec-1), Mo2C/C PNTs-2 (93.7 mV dec-1), and Mo2C/C PNTs-4 (105.7 mV dec-1). This result suggests a higher efficiency of water molecule dissociation on the surfaces of Mo2C/C PNTs-3.6,52 The Tafel slopes of Mo2C/C PNTs catalysts stay in the range of 38 to 116 mV dec-1, indicating that the hydrogen production in the catalysts probably involves the Volmer–Heyrovsky mechanism.54-55 By extrapolating the Tafel plot (Figure 5b), the exchange current density (j0), an important parameter of the inherent electrochemical reaction rate, was obtained. Notably, the Mo2C/C PNTs-3 exhibits the largest j0 of 0.32 uA/cm2, which was higher than Mo2C/C PNTs-2 (0.20 uA/cm2), Mo2C/C PNTs-1 (0.13 uA/cm2), and Mo2C/C PNTs-4 (0.06 µA/cm2), respectively, further proving that the Mo2C/C PNTs-3 is more active in HER.11 Additionally, j0 has been proved to be proportional to the electrochemically effective surface area (ECSA) that relates to the number of HER active sites.56-57 To estimate ECSA of the as-prepared samples, their electric double layer capacitances (EDLC) were measured via employing cyclic voltammetry (CV) testing within the potential window of 0.06 to 0.16 V (Figure S12).58-60 The calculation of the straight-line slope of the current density against the scan speed (Figure 5c) gives an EDLC of 24.2 mF/cm2 for Mo2C/C PNTs-3, much larger than those of Mo2C/C PNTs-2 (9.9 mF/cm2), Mo2C/C PNTs-1 (7.7 mF/cm2) and Mo2C/C PNTs-4 (6.3 mF/cm2). The 10 ACS Paragon Plus Environment

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larger j0 of Mo2C/C PNTs-3 can be ascribed to the higher EDLC. The result also indicates the highest ECSA of Mo2C/C PNTs-3 for electrocatalytic hydrogen production, which surely contributes to its best HER performance.61 Electrochemical impedance spectroscopy (EIS) measured at the applied potential gave a deeper insight into the kinetic process of HER. Figure 5d displayed the Nyquist plots of the as-prepared samples and Figure S13 showed the corresponding equivalent circuit diagram for these electrodes. The equivalent circuit was constructed according to literature (see Page S17).62 After fitting the EIS spectra using the equivalent circuit diagram, the Mo2C/C PNTs-3 shows the lowest Rct (0.44 Ω); while Mo2C/C PNTs-2 (0.67 Ω) and Mo2C/C PNTs-1(0.69 Ω) have similar Rct, and Mo2C/C PNTs-4 exhibits the highest Rct (0.75 Ω). The lower intrinsic impedance of Mo2C/C PNTs-3 can be associated with its higher content of carbon, which is surely beneficial to electron transfer. The reason that Mo2C/C PNTs-3 displays the highest HER efficiency among all of the samples can be ascribed to its appropriate Mo2C content and Mo2C/C ratio. When the Mo2C content is too high, such as those in Mo2C/C PNTs-1&2, large Mo2C agglomerates form, which significantly reduce SSA and active sites of the nanotubes, leading to the low HER catalytic activity. On the other side, the too low content of Mo2C NPs in Mo2C/C PNTs-4 results in insufficient Mo2C active sites for hydrogen production, also leading to the lower catalytic activity although its SSA is higher than that of Mo2C/C PNTs-3. Finally, we tested the durability of Mo2C/C PNTs-3 as the long-term stability is crucial for the wide utilizations of HER catalysts. Continuous CV was measured in the voltage range of -0.3-0V at a scan rate of 50 mV/s. The polarization curve remained almost unchanged after 11 ACS Paragon Plus Environment

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5,000 cycles (Figure 5e), demonstrating the excellent stability of Mo2C/C PNTs-3. In addition, the durability of Mo2C/C PNTs-3 was also evaluated through a long-term HER test in 0.5M H2SO4 solution. The i-t curve tested under an overpotential of 170 mV exhibits that the current density retained at ca. 85% (from 20 to 17 mA/cm2) after continuous electrolysis for 45 hours (Figure 5f). The excellent cycling stability of the HER catalyst is associated with the strong coupling of Mo2C particles in the N-doped carbon skeleton. Conclusion In summary, this work synthesized 1D mesoporous Mo2C/C hybrid nanotubes through a dual-template self-assembly strategy, which utilizes 1D MoO3 nanobelts as the structure-directing template and BCP micelles as the pore-forming template. The content of Mo2C particles, which is controllable by adjusting the amount of ammonia that can convert MoO3 to MoO42- during the synthesis, significantly affects the HER catalytic performance of the hybrid nanotubes. The nanotubes with an optimum Mo2C content of 40.0 wt%, which results in an optimal balance of SSA and homogeneous incorporation of Mo2C NPs, exhibit the highest HER catalytic activity under acidic condition, with a low onset potential of 34 mV, a 140 mV overpotential at 10 mA cm-2 and excellent cycling stability. The dual-template method can be extended for the synthesis of other porous materials containing metallic carbides with potential applications as hydrogen production catalysts.


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ASSOCIATED CONTENT Supporting Information Experiments, additional figures, tables and calculations, etc. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] NOTES The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors appreciate the financial support from the National Natural Science Foundation of China (51573091, 21774076, 21320102006 and 91527304) and Program of the Shanghai Committee of Science and Technology (17JC1403200). The authors also thank the Instrumental Analysis Center at Shanghai Jiao Tong University for measurements.


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(62). Xiong, S. Q.; Fan, J. C.; Wang, Y.; Zhu, J.; Yu, J. R.; Hu, Z. M. A facile template approach to nitrogen-doped hierarchical porous carbon nanospheres from polydopamine for high-performance supercapacitors. J. Mater. Chem. A. 2017, 5 (34), 18242-18252.


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Figure 1. Schematic diagram of the synthesis of Mo2C/C mesoporous nanotubes by a dual-template self-assembly approach.


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Figure 2. Structure characterizations of Mo2C/C PNTs. (a,b) Representative SEM images of Mo2C/C PNTs-3. The yellow arrow highlights the open end. (c-f) Typical TEM images of Mo2C/C PNTs-1 (c), Mo2C/C PNTs-2 (d), Mo2C/C PNTs-3 (e), and Mo2C/C PNTs-4 (f), respectively. (g) HRTEM image of Mo2C particles embedded in the carbon domain; the inset shows the corresponding ED pattern. (h) STEM image (I) of Mo2C/C PNTs-3 along with the conrrespond C (II), N (III) and Mo (IV) elemental mapping images.


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Figure 3. (a) XRD spectra. (b) Raman spectra. (c) Nitrogen adsorption-desorption isotherms, where the isotherms of Mo2C/C PNTs-4, Mo2C/C PNTs-3 and Mo2C/C PNTs-2 are vertically offset by 300, 200, and 100 cm3/g STP, respectively. (d) Pore size distribution curves.


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Figure 4. XPS spectra of Mo2C/C PNTs-3. (a) The whole spectrum; (b) C 1s; (c) Mo 3d and (d) N 1s.


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Figure 5. (a) Polarization curves (inset: the production of H2 bubbles on the surface of Mo2C/C PNTs). (b) Tafe plots. (c) Estimation of Cdl by plotting the current density variation at 0.11V vs RHE; data are obtained from CV curves against scan rate from 20 mV s-1 to 200 mV s-1 (Figure S12). (d) Nyquist plots measured at 200 mV. (e) Polarization curves of Mo2C/C PNTs-3 measured at a scan speed of 5 mV s-1 in the potential window of -0.5V-0 V. (f) Time-dependent current density (i-t) curve of Mo2C/C PNTs-3 under an overpotential of 170 mV for 45 hours.


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Table 1. Structural parameters of Mo2C/C PNTs a

c

b

P

d

P

f

C

[nm]

[nm]

[wt%]

Mo2C/C PNTs-1

18 ± 2

38

17

59.4

Mo2C/C PNTs-2

17 ± 2

57

19

Mo2C/C PNTs-3

19 ± 1

69

Mo2C/C PNTs-4

19 ± 1

77

Samples

a

SEM

BJH

Mo2C

RMo2C/C

f

C

e

SSA [m2/g]

C

N

C

[at%]

[at%]

1.5

2.7

83.3

53.2

1.1

4.9

82.3

19

40.0

0.7

5.5

80.6

18

29.6

0.4

8.3

78.6

b

c

Average pore sizes obtained from SEM images. Specific surface area. Average pore sizes calculated from the

nitrogen adsorption-desorption curves by the BJH method.

d

The content of Mo2C measured by TGA, which is

e

supported by ICP (see Figure S6). The weight ratio of Mo2C to C estimated from TGA. f The atom percentages of N and C elements measured by XPS.

Table 2. Typical parameters of HER electrocatalytic performance of Mo2C/C PNTs

a

a

b

c

Samples

ηonset （mV）

η10 (mV)

Tafel (mV/dec)

(µA/cm )

(mF/cm )

Pt/C

0

-30

36.8

3.16

-

Mo2C/C PNTs-1

-79

-182

102.7

0.13

7.7

0.69

Mo2C/C PNTs-2

-69

-159

93.7

0.20

9.9

0.67

Mo2C/C PNTs-3

-34

-140

92.6

0.32

24.2

0.44

Mo2C/C PNTs-4

-204

-278

105.7

0.06

6.3

0.75

J0

2

Exchange current densities (j0) were obtained by extrapolating Tafel curves.

b

Cdl

2

Rct (Ω)

-

Electric double layer

c

capacitances (Cdl) were calculated from the CV results (Figure S12). Charge transfer resistence (Rct) was measured by fitting Nyquist plots.


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Mesoporous Mo2C/Carbon Hybrid Nanotubes Synthesized by A Dual-Template Self-Assembly Approach for Efficient Hydrogen Production Electrocatalyst

TOC graph


Carbon Hybrid Nanotubes Synthesized by a Dual

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