Interface-Synergistically Enhanced Acidic, Neutral and Alkaline

Subscriber access provided by University of Sunderland

Article

Interface-Synergistically Enhanced Acidic, Neutral and Alkaline Hydrogen Evolution Reaction over Mo2C/MoO2 Heteronanorods Mengzhao Liu, Yang Yang, Xuebin Luan, Xiaoping Dai, Xin Zhang, Jiaxi Yong, Hongyan Qiao, Huihui Zhao, Weiyu Song, and Xingliang Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

ACS Sustainable Chemistry & Engineering

Interface-Synergistically Enhanced Acidic, Neutral and Alkaline Hydrogen Evolution Reaction over Mo2C/MoO2 Heteronanorods

Mengzhao Liu,# Yang Yang,# Xuebin Luan, Xiaoping Dai,* Xin Zhang, Jiaxi Yong, Hongyan Qiao, Huihui Zhao, Weiyu Song and Xingliang Huang

State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing, 18 Fuxue Road, Changping, Beijing 102249, China

* CORRESPONDING AUTHOR.

Prof X. P. Dai: State Key Laboratory of Heavy Oil Processing

China University of Petroleum, Beijing 102249, PR China

Tel.: +86 10 89734979; Fax: +86 10 89734979.

E–mail address: [email protected]

ACS Paragon Plus Environment

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

ABSTRACT: Water electrolysis provides a promising way to achieve sustainable hydrogen production, which strongly depends on highly effective electrocatalysts for industrial application. Herein, Mo2C/MoO2 heteronanorods have been synthesized by means of a simple self-templated carbothermal reduction of polymolybdate-melamine precursor. Through manipulating the pyrolysis temperature, the optimal Mo2C/MoO2-650 heteronanorods shows a low overpotential (168, 290 and 204 mV) to achieve current density of 10 mA cm-2 in 0.5 M H2SO4, 0.1 M PBS and 1.0 KOH, respectively, which also presents a small Tafel slope (~58 and 87 mV dec-1), high mass activity (115.4 and 33.8 A g-1 at η=200 mA) and outstanding stability in 0.5 M H2SO4 and 1.0 KOH. The superior performance is mainly ascribed to the hetero-interfaces between Mo2C and MoO2 promoting synergistic effects, quasi 1D nanostructures facilitating fast charge/mass transfer, and high electrochemical active surface area (ECSA) providing a large number of available sites. This work elucidates a feasible way by heteronanostructure engineering to explore and optimize electrocatalysts in energy chemistry.

KEYWORDS: Mo2C/MoO2; Heteronanorods; Electrocatalyst; Hydrogen evolution reaction; Interface-synergistic effect.

ACS Paragon Plus Environment

Page 2 of 31

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


INTRODUCTION Water splitting to generate hydrogen with high purity (~100%) has made considerable attentions due to its environmentally friendly and sustainable advantages. Non-noble metal catalysts have been one of the crucial role to drive the industrial application in order to replace expensive platinum-based catalysts in hydrogen evolution reaction (HER).1,2 Over the past decade, robust electrocatalysts derived from earth-abundant elements has undergone rapid development for HER, such as non-noble metal chalcogenides,3-10 carbides11-17 and phosphides.18-22 Among them, molybdenum carbides have been considered as one of the most promising candidates in HER because of the similar d-band electronic characteristics to the Pt-group metals.13,23 Since the commercial Mo2C has been reported as HER catalyst in both acidic and basic solutions, many efforts have focused on designing the nano-structure Mo2C with enriched active sites in order to improve the HER performance, which strongly depended on the morphology and composition of catalysts, as well as the synthetic protocol. To date, a number of nanostructured Mo2C, including nanodots,24 nanotubes,11 nanowires15, hollow spheres,16,25 and nano-octahedrons,17 have been reported with increased electrochemical performance. Particularly, one-dimensional Mo2C can provide abundant active sites in the radial direction, and facilitate charge transfer along the axial dimension in their interconnecting networks.11,15 However, the more negative hydrogen binding energy (△GH*) could limits the Hads desorption to generate H2 (i.e., the Heyrovsky/Tafel step) in HER because of the empty d-orbitals with high density in Mo2C.13,14,26 The introducing various dopants such as Fe, Co, Ni, N, S, and P into Mo2C have successfully tune the unoccupied d orbitals of Mo, and thus promote the HER kinetics.14,27-33 Recently, constructing heteronanostructures by the introduction of heteroatom/second component is a powerful ways to tune the electronic properties and achieve desirable performance in HER.34-40 The heterostructured catalysts drawn much attention because the large variety of elements ACS Paragon Plus Environment


Page 4 of 31

can be used for engineering the structure/interface by the precise control. Gao and co-workers fabricated a novel MoC-Mo2C heteronanorods by controlled carbonization of MoOx-amine nanowires precursors, which showed prominent HER performance by the synergistic effects between the interfaces.39 It is notable that the introduction of MoC into Mo2C to form heterostructures appropriately

weaken

hydrogen

binging,

and

facilitated

Heyrovsky/Tafel

step.

A

Fe3C-Mo2C/nitrogen-doped carbon hetero-nanofibers was also designed for HER, and the hetero-interfaces of Fe3C-Mo2C remarkably promoted HER kinetics and intrinsic activity owning to the strong H binding on Mo2C and the relatively weak binding on Fe3C, a conductive and porous N-doped carbon matrix, and hierarchical 1 D nanostructures.40 The synergistic performance of heterogeneous catalysis is directly affected by compositions, morphology and hetero-interfaces.41 Molybdenum dioxide (MoO2), as a semiconductor, has received much attention to be used for building heteronanostructures with Mo2C due to the facile in-situ method.38,42,43 However, traditionally, MoO2/Mo2C have been prepared by the reduction of MoO3 or MoO2, suffering from inevitably agglomeration and irregular shape during carburization under high reaction temperatures (e.g., >800 oC). The realization of uniform carburization of Mo-based nanoparticles without agglomeration is promising to enhance their catalytic performance. Particularly, the HER kinetics are determined through a subtle balance between the water dissociation (Volmer step) and the subsequent chemisorption of the water-splitting intermediates (Heyrovsky/Tafel step) on the surface of electrocatalyst. Thus, the interface and composition engineering of MoO2/Mo2C with well-defined morphology is highly desirable to achieve a synergistically-enhanced HER performance. Herein, Mo2C/MoO2 heteronanorods with ultrasmall building block and tunable composition were constructed by a facile and controllable low-temperature pyrolysis strategy with clean and truly single-source precursor without extra additives. Owing to the synergistic effects between the Mo2C-MoO2 interfaces, optimal composition and quasi one-dimensional (1D) nanostructures, ACS Paragon Plus Environment

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


Mo2C/MoO2-650 heteronanorods shows excellent HER activity, a low Tafel slope, high mass activity and outstanding stability in 0.5 M H2SO4 and 1.0 KOH, respectively, outperforming most of the current Mo2C-based electrocatalysts.

Experimental section Material preparation. Chemicals. Ammonium heptamolybdate ((NH4)6Mo7O24), melamine, ethylene glycol (C6H6O2, EG), and acetone were supplied from Sinopharm Chemical Reagent Co., Ltd. Sulfuric acid and ethanol were both obtained from Beijing Chemical Reagent Company. Commercial Pt black were purchased from Alfa Aesar. Deionized water was used in all experiments. All of the chemicals used in this experiment were analytical grade and used as received. Synthesis of molybdenum-melamine (MoMA) precursors. 1.0 g (NH4)6Mo7O24 was fully dissolved in 13.5 mL EG solution at 75 oC for 8 h (Note: ammonia will release in this step), and 1.0 g melamine (C3H6N6) was also completely dissolved in the mixed aqueous solution (20.7 mL EG+9.3 mL H2O) under 75 oC for 8 h. Then, the two solutions were slowly cooled down to room temperature without stirring, respectively. When the above two solutions were mixed, a white precipitate immediately formed, and further stirred another 2 hours at 75 oC. After cooled down to ambient temperature, the white solid was collected and dried at 80 oC for 3 hours after washed with ethanol and acetone to obtain molybdenum-melamine (MoMA) precursors. The composition of MoMA precursors by Elemental analysis and Thermogravimetry is about 27.2 wt.% N, 12.3 wt.% C, 2.7 wt.% H, and 33.8 wt.% Mo, which is very close to the Mo19O66(C3H7N6)18·12H2O. Synthesis of Mo2C/MoO2 heteronanorods. The as-synthesized MoMA precursors was pyrolyzed in a quartz boat for 2 h under N2 (50 mL/min) to synthesize Mo2C/MoO2 heteronanorods under different temperature of 600, 650, 700 and 800 oC with the heating rate of 3 oC/min, which were denoted as Mo2C/MoO2-X (X=600, 650, 700 and 800 oC). Synthesis of Mo2C nanorods and MoO2 nanorods. Mo2C nanorods was obtained by the ACS Paragon Plus Environment


pyrolysis of the as-synthesized MoMA precursors at 650 oC for 2 h under H2/Ar atmosphere (10 vol. % in Ar, 50 mL/min). MoO2 nanorods was also prepared by the pyrolysis of the as-synthesized MoMA precursors at 500 oC for 2 h under N2 (50 mL/min). Characterization. Powder X-ray diffraction (XRD) patterns were collected on a Bruker AXS D8 Advance with a Cu Kα source (λ= 0.15406 nm). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were acquired on a FEI Quanta 200F and FEI Tecnai G2 F20 electron microscope operating at 200 keV, respectively. X-ray photoelectron spectroscopy (XPS) measurement were taken on a PHI 5000 Versaprobe system using C 1s peak (284.6 eV) as reference for calibration. Raman spectra were recorded on a Renishaw Micro-Raman System 2000 spectrometer with an excitation laser of 532 nm. N2 adsorption–desorption isotherms were recorded on an automatic gas adsorption analyzer (Quantachrome Autosorb-iQ-MP). To determine the Mo2C and MoO2 ratio in the Mo2C/MoO2-650, the thermogravimetric analysis (TGA) was performed on a thermogravimetric analyzer (Mettler TGA/DSC1) from 30 to 700 °C at a heating rate of 10 oC min−1 in air. Electrochemical measures. Electrochemical measurements were carried out on a three– electrode system (CHI 660E) with a platinum wire as the counter electrode, a saturated calomel electrode (SCE) in acid media or Hg/HgO electrode in alkaline as the reference electrode. The reference electrode was calibrated according to the method published by Asefa and co-workers.44 The potential, measured against the reference electrode, was converted to the potential versus the reversible hydrogen electrode (RHE) according to ERHE = ESCE +EθSCE+0.059·pH or ERHE = EHg/HgO +EθHg/HgO +0.059·pH, respectively. The working electrodes were fabricated as follows. The catalyst ink was prepared by homogeneous dispersion of 2 mg catalyst and 1 mg carbon black into 500 µL of 4:1 (v/v) water-ethanol and 40 µL Nafion (5 wt%). Then, 5 µL ink was drop-casted onto a glassy carbon electrode (GCE, 3 mm in diameter), and obtained catalyst loading of 0.285 mg cm-2. ACS Paragon Plus Environment

Page 6 of 31

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


Linear Sweep Voltammetry (LSV) polarization curves in N2-saturated 0.5 M H2SO4 and 1.0 M KOH were acquired at a scan rate of 5 mV/s, respectively. Before each HER activity measurement, the electrode were pretreated with cyclic scan about 20 cycles at a sweep rate of 100 mV s-1 to activate the catalysts, remove surface contamination, and stabilize the current. For the long-term (5000 cycles) stability test, a graphite rod was used as the counter electrode to avoid the possible contribution of dissolved Pt species to the HER. The amperometric current density-time (i-t) curves were also measured in N2-saturated 0.5 M H2SO4 and 1.0 M KOH solution under controlled potentials, respectively. The electrochemical impedance spectroscopy (EIS) was carried out at an overpotential of 150 mV from 100 MHz to 10 KHz with a perturbation voltage amplitude of 5 mV in 0.5 M H2SO4. The estimation of the electrochemically active surface area (EASA) was carried out based on the measured double-layer capacitance (Cdl) from a series of CV measurements at various scan rates (20, 40, 60 and 80 mV s-1, etc) in 0.1-0.2 V vs RHE region. The density of active sites were calculated by electrochemical approach through cyclic voltammetry measurements in pH=7 phosphate buffer at a scan rate of 50 mV s-1.45,46 The per-site turnover frequencies (TOFs) can be evaluated according to the previous methods.47

Results and discussion The synthesis scheme of Mo2C/MoO2 heteronanorods were presented in Figure 1A. Firstly, the EG-MoO4 were fabricated by the strong electrostatic interactions between the protonated EG and MoO4- in solution A (Figure S1A).48 The chain of EG-MoO4 should array as in Figure S1B. After the mixed solution A and B, the protonated melamine will gradually replace the EG in EG-MoO4 to form MoMA (MoO42--Melamine) precursors with nanorods morphology (Figure S1B-C).48 The chelation of molybdate ions with the periodically spaced amine groups also resulted in the homogeneous distribution of the MoO42- in the precursor matrix. During carbonization process, the pristine morphologies are inherited to form rough Mo2C/MoO2-X nanorods with several ACS Paragon Plus Environment


micrometers long (less than 5 µm) and few tens of nanometers wide (less than 80 nm) (Figure 1B-E and Figure S2). The melamine/ethylene glycol serve as both reducing agent and carbon resource. The nanorods are were further characterized by XRD, TGA, Raman and (HR)TEM. The XRD patterns in Figure 2A and Figure S3 display that the diffraction peaks at 2θ=26.0o, 37.1o and 53.6o can be ascribed to the (011), (020) and (022) planes of monoclinic MoO2 (JCPDS no. 65-1273, a=3.012, b=3.012, c=4.735 Å), while the peaks at 2θ=34.6°, 37.8°, 39.5° and 61.9 ° are indexed to the (100), (002), (101) and (110) of β-Mo2C (JCPDS no. 35-0787, a=5.608, b=4.843, c=5.626 Å) over Mo2C/MoO2-650. As the pyrolysis temperature increased from 600 to 800 oC, the intensity ratio between (002) plane of Mo2C and (020) plane of MoO2 significantly increases (Figure S3), indicating that MoO2 phase gradually weaken. Moreover, the (002) plane of β-Mo2C gradually becomes the strongest peak, and the new peak at 2θ=42.4o presents over Mo2C/MoO2-800, which are actually contributed to the (200) reflection of cubic α-MoC1-x with fcc crystal structure (JCPDS no. 65-0280). Compared with the pure β-Mo2C and MoO2, the shifts toward high degree about 0.3o at 2θ=37.8 and low degree about 0.15 o at 2θ=26.0 are observed, respectively, indicating the formation of Mo2C/MoO2 heterostructures. Notably, Compared with standard Mo2C (JCPDS no. 35-0787), the slight shift of characteristic peaks toward high degree on Mo2C indicate the possible substitution of C sites with N atoms in Mo-C-Mo units, which indicates that N is doped into the carbides.49,50 TGA was used to analyze the composition of Mo2C/MoO2-650 by assuming MoO3 as the final product,51 which shows a Mo2C to MoO2 ratio of 68.1:31.9 (wt. %) by combining the reactions MoO2 + 0.5O2 = MoO3 and Mo2C + 4O2 = MoO3+ CO2 (Figure S4). The Raman spectra unveil dominant characteristic peaks at 990, 817, 660, 375, 334, and 281 cm-1, which can be characterized to the surface oxides of Mo2C or MoO2.52 Notably, the peaks at 660, 817 and 990 cm-1 can also be identified as characteristic band Mo2C.31,53 The absence of D- and G- bands of carbon at about 1350 and 1590 cm-1 indicates the negligible contents of free carbon in the as-prepared Mo2C/MoO2-X, ACS Paragon Plus Environment

Page 8 of 31

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


Mo2C and MoO2 nanorods. TEM image of Mo2C/MoO2-650 presents a single nanorod, which is composed with ultra-small particles (Figure 3a). The ultra-small size and high dispersion of Mo2C/MoO2 should be ascribed to the in-situ carbothermal reduction of Mo species via the confining effect of the carbon matrix, which prohibits their excessive growth and/or further aggregation. Selected area electron diffraction (SAED, inset of Figure 3a) verifies the diffraction spots that might be assigned to (121) and (221) of Mo2C, and (011) and (020) of MoO2, respectively, indicating that the as-prepared nanocomposites are indeed composed of monocrystalline Mo2C and MoO2 on Mo2C/MoO2-650. HRTEM image demonstrates further confirms that the nanorods are composed of both Mo2C and MoO2 (Figure 3b), where the lattice fringes with 0.335 nm for the (011) plane of MoO2 and 0.226 nm for the (101) plane of Mo2C can be clearly observed, indicating the establishment of coupling interfaces in Mo2C/MoO2 heterostructures. The MoO2 and Mo2C are well bound together, decreasing the grain boundary resistance in electrocatalysis. The overlay of Mo, C and O signals in elemental mapping images (Figure 3c) clearly discloses the uniform distribution throughout the nanorods. To further elucidate the composition and valence states of these materials, X-ray photoelectron spectroscopy (XPS) was performed (Figure 4, Figure S5 and Table S1). From the survey spectra, the elements of Mo, C, N and O can be clearly identified. The high-resolution Mo 3d XPS spectrum in Mo2C/MoO2-650 can be fitted into three coupled peaks, which should be attributed to Mo6+ (232.9 and 236.0 eV), Mo4+ (229.7 and 233.5 eV), and Mo2+ (228.6 and 231.8 eV),54,55 respectively. Among them, the peaks located at 228.6 and 231.8 eV exhibit a spin energy separation of 3.2 eV, demonstrating the characteristic doublet of Mo2+ state of β-Mo2C.56 Meanwhile, the binding energies at 229.7 and 233.5 eV are the typical values for Mo 3d5/2 and 3d3/2 peaks of Mo4+ in MoO2.57 The Mo6

+

species are commonly observed as carbides and MoO2 are exposed to air. These results

indicate that Mo2C and MoO2 coexisted in the Mo2C/MoO2-650. From the view of the crystal-field ACS Paragon Plus Environment


theory, Mo (II) and Mo (IV) are in the electronic states of Mo-4d3(↑)4d1(↓) and Mo-4d2(↑)4d0(↓), respectively, where the partially filled eg orbit and the partially filled t2g spin-up channel can endow Mo (II/IV) with electronic conductivity. Notably, compared with the peak position of Mo4+ in MoO2, Mo2+ in Mo2C, the peaks at 229.7 (Mo4+) and 228.6 (Mo2+) in the Mo2C/MoO2-650 exhibit negative and positive shift (~0.15 eV), respectively, implying the existence of strong electronic interactions and the presence of coupling interfaces.34 Mo6+ also presents slight shift toward high binding energy, which could be relevant with the oxidation about Mo2C and MoO2 of different composition samples. Figure S5B compares the O1s spectra with two fitting peaks at 530.5 eV and 532.5 eV, corresponding to Mo-O bond in MoOx and loosely chemisorbed oxygen (such as H2O or CO32-), respectively. The C1s peak is fitted into four peaks in Figure 4B, where the binding energies at 284.6, 286.2 eV and 288.4 eV corresponds to the C-C/C=C, C-O and O-C=O, respectively. Compared with bare MoO2, the appearance of C1s signal at 283.3 eV over Mo2C and Mo2C/MoO2-650 is attributed to the presence of Mo-C bonds.58 The N 1s in Figure S5C present the can be deconvoluted into four peaks at 397.3 eV, 398.7 eV, and 400.5 eV corresponded to pyridinic, pyrrolic, and quaternary type N atoms, respectively, while the weak peak at 395.15 eV was ascribed to the Mo 3p with N-Mo bonding, which proves the possible N-doping into the Mo-based material.59 In addition, N2 isothermal sorption reveals the large surface of the Mo2C/MoO2-650 (Figure S6). Particularly, Mo2C/MoO2-650 presents a specific surface area of 104.6 m2 g-1, larger than that of MoO2 (63.1 m2 g-1) and Mo2C (49.2 m2 g-1). The HER performance of Mo2C/MoO2-X (X=600, 650, 700, 800) was firstly investigated in 0.5 M H2SO4 solution with a typical three-electrode system. For comparison, Mo2C, MoO2 and Pt/C catalysts were also examined. Figure 5A and Figure S7 display the polarization curves with a sweep rate of 5 mV s-1, which shows that Pt/C catalyst has the highest activity with an onset overpotential (η0) nearly 0 mV at 1 mA cm-2, whereas MoO2 and Mo2C exhibit inferior HER activity. In contrast, ACS Paragon Plus Environment

Page 10 of 31

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


Mo2C/MoO2-X exhibit superior HER activity. Among them, Mo2C/MoO2-650 heteronanorods exhibit a significant smaller onset overpotential (η1=108 mV) and overpotential of 168 mV to achieve 10 mA cm-2 catalytic current density (η10) than those of Mo2C (143 mV, 202 mV), MoO2 (201 mV, 283 mV) and physically mixed sample (denoted as Mo2C+MoO2, 125 mV, 190 mV) (Figure 5A and Figure S8), respectively. Thus, it is inferred that the superior performance of Mo2C/MoO2-650 heteronanorods originate from the well-formed Mo2C and MoO2 interfaces, which greatly promotes the electron transfer and dissociation of water molecules through the synergistic effects between Mo2C and MoO2 in the nanodomains. The N species in the composites could also make some contribution to the improvement in HER performance due to the formation of Mo-N-Mo. Compared with the reported Mo2C-based catalysts, such as β-Mo2C nanotube,12 Mo2C nanowire,60 Mo2C nanoparticles,61 Mo2C QDs/NGCLs,62 Mo2C@NCNT,63 Mo2C@NPC,64 MoSx@Mo2C,65 MoO2-α-Mo2C,38 MoO2-Mo2C@NC and Mo2C@NC@MoSx,66 the optimal Mo2C/MoO2-650 nanorods exhibits comparable overpotential (168 mV) to achieve current density of 10 mA cm-2 with higher mass activity (115.4 A g-1) at overpotential η=200 mV in acidic condition (Table S2). Electrochemical impedance spectroscopy (EIS) tests and corresponding equivalent circuit in Figure 5B were conducted to investigate the kinetics in HER at overpotential of 170 mV in 0.5 M H2SO4 solution. Mo2C/MoO2-650 heteronanorods exhibits a greatly reduced semicircle in the low frequency region as compared to Mo2C and MoO2, where the resistant charge-transfer (Rct) is as low as 7.2 Ω cm-2 by Mo2C/MoO2-650. The Mo2C attached tightly with MoO2 significantly increase the electronic conductivity of the heteronanorods and reduce the grain boundary resistance, indicating that fast charge transfer and favorable kinetics. The reaction mechanism are further unveiled by Tafel slopes in Figure 5C, which shows 58.0, 60.8 and 80.8 mV·dec-1 for Mo2C, MoO2 and Mo2C/MoO2-650, respectively, suggesting a HER route following the Volmer-Heyrovsky mechanism (Volmer: H3O++e-→Hads+H2O, and Heyrovsky: H2O+Hads+e-→H2↑+ OH−), and the ACS Paragon Plus Environment


Volmer reaction is most likely rate-determining step. Generally, to ensure fast kinetics, strong H binding on active-sites is required for the electrochemical H+ reduction (Volmer step), whereas a weak binding is preferred during Hads desorption (Heyrovsky or Tafel step).40 The integration of the positive △GH* value of MoO2 and the negative value of Mo2C in Mo2C/MoO2 heteronanorods should be helpful in weakening the binding strength of hydrogen on MoO2 while strengthening on the Mo2C surface.13,24,40,67 The Mo2C/MoO2 heterostructures promote the elementary reactions in HER to achieve a synergistically-enhanced activity on Mo2C-MoO2 interfaces by improving the migration of Hads to the MoO2 surface, allowing the further combination to generate H2. By extrapolating the Tafel plot, the exchange current density of Mo2C/MoO2-650 can be calculated as 0.019 mA cm-2 (Figure S9), which is higher than those of the Mo2C and MoO2, suggesting the highest inherent activity on the heterostructure. The superior performance over Mo2C/MoO2-650 should be also relevant with the largest electrochemical active surface area (ECSA, 22.6 mF·cm-2), implying more rough and accessible surface toward HER, which can be attributed to the synthesis of two different types of nanostructures of Mo2C and MoO2 (Figure 5D and 5E). The density of active sites gives consistent results with ECSA, where 0.459 mmol g−1 on Mo2C/MoO2-650 is 1.4 and 3.6 times than those of Mo2C and MoO2 (Figure 5F and 5G), respectively. Furthermore, the calculated TOF of Mo2C/MoO2-650 reaches 1.28 s-1 at 200 mV, which is 1.45 and 9.13 times more than those of Mo2C and MoO2 (Figure S10), respectively. Thereby, the greatly enhanced intrinsic HER activities of Mo2C/MoO2-650 heterostructures should be mainly attributed to the integration of the constructed interfaces and the active surface areas. Another parameter to evaluate the hydrogen evolution activity of the Mo2C/MoO2-650 catalyst is stability. Figure 5H shows the polarization curves before and after 5000 potential cycles of Mo2C/MoO2-650, which only gives a small potential increase (

Interface-Synergistically Enhanced Acidic, Neutral and Alkaline

Recommend Documents