Interconnected Molybdenum Carbide-Based Nanoribbons for Highly

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Interconnected Molybdenum Carbide Based Nanoribbons for Highly Efficient and Ultrastable Hydrogen Evolution Zhihua Cheng, Jian Gao, Qiang Fu, Changxia Li, Xiaopeng Wang, Yukun Xiao, Yang Zhao, Zhipan Zhang, and Liangti Qu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 15, 2017

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ACS Applied Materials & Interfaces

Interconnected

Molybdenum

Carbide

Based

Nanoribbons for Highly Efficient and Ultrastable Hydrogen Evolution Zhihua Cheng, Jian Gao, Qiang Fu, Changxia Li, Xiaopeng Wang, Yukun Xiao, Yang Zhao*, Zhipan Zhang, Liangti Qu * Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R, China. KEYWORDS: molybdenum carbide nanoribbon assembly, exposure active sites, hydrogen evolution reaction, hydrogen bubble release, ultrastable performance

ABSTRACT

Electrocatalytic hydrogen evolution reaction (HER) is of great significance to produce clean, sustainable and cost-effective hydrogen. However, the development of low-cost and high efficient non-noble-metal catalysts with a combination of superior catalytic activity and longtime stability still remains a challenge. Herein, we demonstrate a rationally designed three dimensional (3D) architecture assembled from 1D molybdenum carbide (MoC) based nanoribbons where the MoC nanoparticles are embedded within nitrogen-doped crystallized

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carbon nanolayers (MoC@NC nanoribbon). Such unique architecture of MoC@NC nanoribbon not only provides abundant edge active sites and multi-electron pathways for efficient mass/charge transport, but also greatly accelerates the hydrogen release from the reaction surface, thus boosting its electrocatalytic performances for HER either in acid or alkaline aqueous solution. This advance provides a promising candidate towards replacing the noblemetal based catalysts for highly stable and efficient HER electrocatalysis.

1. INTRODUCTION Electrocatalytic hydrogen evolution is regarded as an efficient way to produce high energy density, clean and sustainable hydrogen which can be an alternative for next-generation energy. 1-3

Usually, the electrocatalytic hydrogen evolution reaction (HER) involves in multi-pathway

processes at the interfaces between the electrolyte and catalyst.4 In the acidic solution, the hydrated proton can be reduced after receiving an electron to from catalyst-hydrogen (Cat-H) bond at the surface of the catalyst (H3O+ + e- + Cat → Cat-H + H2O, Volmer reaction), where Cat-H bond denotes an active site on the catalyst binding to hydrogen.5 It indicates that the catalyst with excellent conductivity and abundant active sites will be benefit to achieve high electron mobility and catalytic activity. The subsequent process is the release of molecular hydrogen through one of the two processes:6 the indirect reactions between the adsorbed H, hydrated proton and electron (H3O+ + e- + Cat-H → H2 + H2O, Heyrovsky reaction) or the direct combination of two Cat-H bonds (Cat-H + Cat-H → H2, Tafel reaction), which directly affects the effective active sites and stability of catalyst because of the aggregations of hydrogen bubbles on the surface of catalyst, indicating the importance of the nanostructure for a superior catalyst. Therefore, designing and developing a favorable catalyst with high catalytic activities and


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stabilities for HER is imperative to realize fast thermodynamic and kinetics for practical application. Platinum (Pt) is regarded as the most state-of-the-art electrocatalytic benchmark for HER due to the special d-orbit electron configurations, which affords high current at a very low overpotential4,6,7 Considering the high cost, low abundance and poor durability of Pt that limit its widespread utilization in practice8, the development of low cost and efficient alternatives is of great significance.4,6,9 In this regard, molybdenum carbide with similar electron configuration of Pt can act as a new promising platinum-alternative catalyst for HER.8,10-12 Since the first investigation of molybdenum carbide microparticles in the application of electrocatalytic hydrogen evolution,13 it has triggered intensive efforts to fabricate various molybdenum carbide14,15 based electrocatalysts due to the advantages of low cost,16,17 excellent conductivity,18 high catalytic activity and stability.8,11 It is believed that the number of active sites increases with decreasing the crystalline size or increasing surface roughness of catalyst, thus leading to a high performance.19 Many efforts have been devoted to designing the various specific nanostructures including hierarchical nanotubes,20 nanooctahedrons,21 nanoflower,22 nanosphere23 and etc., or introducing atomic dopants such as nitrogen,24 nickel,25 iron,18 in order to increase the active sites and promote the activities of the catalyst with much lower activation energy during the HER process. Apart from the intrinsic catalytic activities, the kinetic process of effective hydrogen departure from the catalyst surface also plays important role to guarantee the persistence of catalytic activity and the stability of nanostructures, which effectively prevents the inhibition generated by the produced hydrogen bubbles that occupy the active sites.26 Nevertheless, it receives few attentions. Therefore, to satisfy the increasing practical demands for high active and durable catalyst, the effort on


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exploring and researching new structures both in nanoscale and microscale which not only possess high electrocatalytic activity but also promote hydrogen release during HER process is urgent. In this work, we have developed an efficient HER electrocatalyst based on a rational assembly of 1D topological molybdenum carbide (MoC) based nanoribbons into 3D interconnected framework, where the MoC based nanoribbons are composed of the MoC nanoparticles embedded in nitrogen-doped crystallized carbon nanolayers (named as MoC@NC nanoribbon). By the means of a novel controllable vapor deposition strategy at solid-gas interface, the resultant MoC@NC nanoribbon is formed with an ultralow density of 3.8 mg cm-3, the lowest density achieved to date for MoC based architectures. With favorable abundant exposed surface active sites, short diffusion distances, and multiple electron/electrolyte/gas separation and transfer pathways, this specially designed MoC@NC nanoribbon greatly facilitates the hydrogen production and release, which exhibits low onset potential of -24 mV and -36 mV vs. RHE, small Tafel slope of 54 mV dec-1 and 51 mV dec-1 in acidic and alkaline solution, respectively, accompanied by extremely large cathodic current density and superior stability. This work will open a new horizon for high efficient hydrogen production by synergistically nano- and microstructural modulations.

2. EXPERIEMENTAL SECTION 2.1 Materials and synthesis Reagents: Ammonium molybdate tetrahydrate ((NH4)6Mo7O24▪4H2O), commercial Pt/C (20 wt %) and Nafion solution (5 wt %) were purchased from Sigma-Aldrich. Dicyanodiamide (DCA) was purchased from Tianjin Fuchen Chem. Reagents Ltd. The nitrite acid and sulfuric acid were


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purchased from Beijing Reagents Ltd., and the commercial MoO3 and active carbon were purchased from Aladdin. All reagents were used as received without further purification. Preparation of α-MoO3 nanoribbon monolith: The α-MoO3 nanoribbon was synthesized in the acidic solutions by using a modified hydrothermal method27. In a typical procedure, the ammonium molybdate tetrahydrate (1.236 g) was dissolved into 35 mL deionized water, followed by dropping 5 mL HNO3 (67 wt %) into the solution. The obtained solution was then sealed into a 50 mL Teflon-lined autoclave and maintained at 180 oC for 16 h. After the autoclave was naturally cooled down to room temperature, the white flocculent α-MoO3 nanoribbon was prepared by washing the residue several times with deionized water and then dialyzed for 3 days to remove possible remnants. Finally, the α-MoO3 nanoribbon monolith was obtained after freeze-drying treatment for 2 days. Preparation of MoC@NC nanoribbon: For preparation of MoC@NC nanoribbon, the α-MoO3 nanoribbon monolith (30 mg) and DCA (4.8 g) were put in separated ceramic boats, followed by thermal treatment at 800 oC for 6 h with a heating speed of 3 oC min-1 under the Ar/H2 atmosphere. Finally, the interconnected MoC@NC nanoribbon was obtained. Preparation of MoC@NC bundle: The MoC@NC bundle was obtained by thermal treating the naturally dried α-MoO3 nanoribbon (30 mg) and DCA (4.8 g) in separated ceramic boats at 800 o

C for 6 h with a heating speed of 3 oC min-1 under the Ar/H2 atmosphere.

Preparation of MoC@NC sheet: The MoC@NC sheet was directly obtained by thermal treating the mixture of α-MoO3 nanoribbon monolith (30 mg) and DCA (4.8 g) at 800 oC for 6 h with a heating speed of 3 oC min-1 under the Ar/H2 atmosphere.


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Preparation of MoC@NC bulk: The MoC@NC bulk was prepared by thermal treating the commercial MoO3 (30 mg) and DCA (4.8 g) in separated ceramic boats at 800 oC for 6 h with a heating speed of 3 oC min-1 under the Ar/H2 atmosphere. Preparation of MoC nanoparticles: The MoC nanoparticle was directly obtained by thermal treating the mixture of commercial MoO3 (30 mg) and DCA (60 mg) at 800 oC for 6 h with a heating speed of 3 oC min-1 under the Ar/H2 atmosphere. Preparation of nitrogen-doped carbon: The nitrogen-doped carbon was directly obtained by thermal treating active carbon (30 mg) and DCA (4.8 g) in separated ceramic boats at 800 oC for 6 h with a heating speed of 3 oC min-1 under the Ar/H2 atmosphere. 2.2 Characterization The morphology of the prepared samples was investigated by scanning (SEM, JSM-7500F) and transmission (TEM, 7650B, Hitachi) electron microscope. The elemental mappings were performed on a scanning transmission electron microscope (STEM) unit with high-angle annular dark-field (HAADF) detector (HITACHI S-5500) operating at 30kV. X-ray diffraction (XRD) patterns were obtained by using a Netherlands 1710 diffractometer with a Cu Kα irradiation source (λ= 1.54 Å), and a self-calibration process was performed with a SiO2 internal standard sample prior to target measurement. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300W AlKα radiation. 2.3 Electrocatalytic measurements


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Preparation of electrode materials: Typically, 4 mg of sample was first ultrasonically dispersed in the mixture of water (400 µL), ethanol (400 µL) and 5% Nafion solution (40 µL), then the mixed ink (~20 µl) was attached onto a glass carbon (GC) electrode with the diameter of 5.6 mm (loading mass 0.385 mg cm-2) as working electrode for electrochemical measurements. Electrochemical HER Measurements: All of the electrochemical measurements were performed on CHI760D electrochemical workstation in three electrode systems using a rotating disk electrode (PINE Research Instrumentation, the rotation speed is 1600 rpm.) at ambient temperature. A graphite rod (Alfa Aesar, 99.9995%) and Ag/AgCl (in 3 M KCl solution) electrode were used as counter and reference electrodes, respectively. 0.5 M H2SO4 (pH =0.3) and 0.1 M KOH (pH =13) solution were degassed with N2 and used as the electrolyte. All polarization curves were corrected for the iR losses and all of the potentials were calibrated to a reversible hydrogen electrode (RHE) based on the equation: Evs.RHE = Evs.Ag/AgCl +0.209 +0.059 pH, and the current density (J) was normalized to the geometrical area of the working electrode. Linear sweep voltammetry (LSV) was carried out with a scan rate of 5 mV s−1 and cyclic voltammetry curves (CVs) were conducted between 0.05 and 0.35 V (vs. RHE, in 0.5 M H2SO4) from 20 to 200 mV s−1 to investigate the effective surface areas of sample. The double-layer capacitive (Cdl) can be estimated by plotting the ∆J =Ja-Jc at 0.2 V vs. RHE against the scan rate, where the slope is twice to Cdl. The electrochemical impedance spectroscopy (EIS) was measured with frequencies ranging from 0.01 to 100 kHz at various overpotentials from -130 mV to -250 mV (vs. RHE, 0.5 M H2SO4). Electrodes were cycled with a scan rate of 100 mV s−1 at least 100 cycles prior to any measurements. 2.4 Calculation of the MoC content of the catalyst


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The MoC content of the MoC@NC nanomesh was determined by the inductively coupled plasma mass spectrometry (ICP-MS). Three parallel experiments were carried out, which reveal that the contents of the molybdenum are 64.51at%, 64.47at% and 64.89 at%, respectively. The equal value of molybdenum (64.62 at%) was used to calculate the MoC content. According to Law of Conservation of Atoms, the MoC content is turned out to be 72.7 at%. 3

RESULTS AND DISCUSSION

The formation process of MoC@NC nanoribbon is illustrated in Figure 1a–c. We first produced molybdenum trioxide (α-MoO3) nanoribbon with the size of ca. 200–300 nm in width by using a simple hydrothermal method and freeze-drying treatment (Figure 1a, Figure S1 and S2). To form the MoC@NC nanoribbon, the obtained α-MoO3 nanoribbon monolith and dicyanodiamide (DCA) powder were put in different ceramic boats separately, followed by thermal treatment in Ar/H2 atmosphere at 800 oC for 6 h. During the thermal treatment process, the DCA powder was turned into vapor which was then immediately absorbed on the surface of α-MoO3 nanoribbon as the increase of temperature (stage I). When the temperature was up to 450 oC, the as-synthesized α-MoO3 would transform into MoO2, and meanwhile the DCA coated on the surface of MoO2 nanoribbon was spontaneously self-condensed into g-C3N4 which could efficiently prevent the collapse of the ribbon-like structures and result in the formation of a core-shell structure of MoO2@g-C3N4 nanoribbon (Figure 1b, Figure S3 and S4). As increasing the carbonization temperature to 600 oC, the MoC nanoparticles were gradually formed and the remaining g-C3N4 shells began to decompose and convert into nitrogen-doped amorphous nanocarbons (stage II, Figure S5 and S6). The nitrogen-doped amorphous nanocarbons were finally transformed into nitrogen-doped crystallized nanocarbons at 800 oC for 6 h, leading to the in-situ formation of MoC@NC nanoribbon framework (Figure 1c). However, a higher treating temperature (such as


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~900 oC) could lead to the aggregation and excess growth of the MoC nanoparticles, as well as the completely consumption and decomposition of nitrogen-doped carbon species, thus resulting in the collapse of the ribbon-like structure (Figure S7 and S8). The key step to MoC@NC nanoribbon is to introduce DCA vapor into the α-MoO3 nanoribbon monolith system during thermal treatment, in which DCA vapor not only acts as precursor to synthesize the MoC species (Figure S9), but also can effective prevent the selfaggregated behavior of MoC nanoparticles and maintain the interconnected ribbon-like nanostructures. In contrast, when α-MoO3 nanoribbon monolith is directly mixed with DCA under the same conditions, the normal sheet-like structure is formed instead of ribbon-like structure (named as MoC@NC sheet, Figure S10 and S11), which is probably due to the molten DCA results in the collapse of the ribbon-like structure during the heat treatment. Similarly, when the original α-MoO3 nanoribbon monolith is treated under the same conditions without DCA, the aggregated Mo particles are obtained (Figure S12). The as-prepared MoC@NC nanoribbon has an ultralow density of 3.8 mg cm-3, which is comparable to that of the lightest silica aerogels (~3 mg cm-3),28 and to the best of our knowledge, is the lightest MoC based material. The ultralow weight even allows a ca. 8 cm3 MoC@NC nanoribbon standing stably on a soft green fern-like foliage of asparagus setaceus (Figure 1d). The MoC@NC nanoribbon exhibits an interconnected and loose 3D porous framework (Figure 1e) with a specific surface area of 33 m2 g-1 (Figure S13), which also possesses excellent wettability (supporting information, Figure S14 and movie S1). The enlarged scanning electron microscope (SEM) image reveals that MoC@NC nanoribbons with a uniform width size of ca. 100–300 nm are randomly cross linking together (Figure 1f), as also verified by electron transmission microscope (TEM) in Figure 1g. The high-resolution TEM (HRTEM)


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image of the as-prepared MoC@NC nanoribbon shows the well-distributed MoC nanoparticles (ca. 5–10 nm) encapsulated in few nitrogen-doped carbon nanolayers (Figure 1h). The corresponding lattice spacing on MoC nanoparticle belongs to (100) crystal plane of γ-MoC phase. Besides, the carbon matrixes around the MoC nanoparticles with short-range ordered layers structure have an interplanar spacing of 0.335 nm, which are ascribed to graphitic carbon (004) crystal plane. The typical STEM images in Figure 1i and 1j reveal a topological mosaiclike nanoribbon with a high roughness surface, which would benefit the electrocatalytic process. The corresponding elemental mapping reveals the coexistent and uniform signals of C, N and Mo elements which are distributed all over the entire plane of MoC@NC nanoribbon (Figure S15), corresponding to that TEM observation (Figure 1g). The high resolution elemental mappings (Figure 1j) of the sample further confirm the uniform distributions of C and N atoms. More importantly, the signal of Mo element is mainly concentrated in the MoC nanoparticles of the prepared nanoribbon, suggesting that the MoC nanoparticles are indeed well encapsulated within the nitrogen-doped carbons species.


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Figure 1. a–c) Schematic illustration of the procedures to synthesize the MoC@NC nanoribbon: a) The α-MoO3 nanoribbon monolith produced by using a simple hydrothermal method and freeze-drying treatment; b) The MoO2@g-C3N4 nanoribbon monolith with core-shell structure formed during the carbonization process; c) The topological MoC@NC nanoribbon obtained after thermal treatment; d) Digital image of the 3D MoC@NC nanoribbon standing on a soft green fern-like foliage of asparagus setaceus; e, f) SEM images of the interconnected MoC@NC nanoribbon; g) TEM and h) HRTEM images of the MoC@NC nanoribbon; i, j) STEM images of a single nanoribbon and the corresponding elements mapping: C-K; N-K and Mo-L.


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Figure 2a shows the X-ray diffraction (XRD) pattern of MoC@NC nanoribbon. The diffraction peak at 26.5o suggests the formation of graphitic-like carbon during the thermal treatment. As can be seen, the typical main diffraction peaks at 35.7o, and 48.5o can be ascribed to γ-MoC species, according to the standard card of γ-MoC (JCPDS NO. 65-8765). The X-ray photoelectron spectroscopy (XPS) measurements further verify the existence of a pronounced Mo 3d peak, along with obvious C 1s, N 1s peaks and a weak O 1s peak without any impurities (Figure 2b). The four oxidation states of Mo, such as Mo0, Mo3+, Mo4+, and Mo6+ in the MoC@NC nanoribbon are observed in the high-resolution Mo 3d spectrum (Figure 2c), which is consistent with the previously reports.29 The existence of dominant Mo0 peak along with small peaks of low oxidation state of Mo3+ can be attributed to the Mo-Mo and Mo-C bonds in MoC materials. It is believed that the high oxidation states (Mo4+ and Mo6+) on the surface of MoC species stem from the exposure of MoC to the ambient air.13 Similar to most of nitrogen-doped carbon based materials,30-32 the high-resolution N 1s spectrum reveals the presence of both quaternary N (401.1 eV) and pyridinic N (398.1 eV) (Figure 2d), suggesting the N atoms have been successfully incorporated into the carbon-carbon bonds of the carbon nanolayers.


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Figure 2. a) XRD patterns of the MoC@NC nanoribbon and the standard cards of γ–MoC (bottom, JCPDS NO. 65-8765); b) The XPS survey spectrum; c, d) The high-resolution XPS spectra of Mo 3d and N 1s, respectively. The HER performance of the MoC@NC nanoribbon is evaluated in 0.5 M sulfuric acid electrolyte using a standard three-electrode setup with a sample mass loading of 0.385 mg cm-2 on glassy carbon electrode (GCE) (See the Preparations). The MoC@NC bulk and MoC@NC bundle synthesized by thermal treating the commercial MoO3 with DCA vapor (Figure S16, S17 and S18) and the naturally dried α-MoO3 nanoribbon with DCA vapor (Figure S19 and S20) under the same condition, respectively, the MoC@NC sheets and state-of-the-art 20 wt % Pt/C are studied for comparison. Besides, the controllable electrodes prepared by different annealing temperature and time are also investigated in Figure S21. As demonstrated in Figure 3a, the onset potential of MoC@NC nanoribbon which can be defined here as the potential when the Tafel plot starts to deviate from the liner region (Figure S22)21 is about -24 mV, approaching to


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Pt/C electrode (0 mV), lower than that of MoC@NC bundle (-190 mV), Mo@NC bulk (-200 mV), MoC@NC sheet (-210 mV), MoC nanoparticles (-215 mV, Figure S23a and b), N-doped carbon (-230 mV, Figure S23b) and most of reported Mo-based compounds (Figure S24), suggesting a superior electrocatalytic activity. It is noteworthy that the MoC@NC nanoribbon with unique interconnected architecture plays a key role in its outstanding HER performance, which achieves an extremely high current density of -300 mA cm-2 at the potential of -270 mV, far larger than the MoC@NC bundle, MoC@NC bulk and MoC@NC sheet electrodes, indicating the fast mass and charge transport. Moreover, the Tafel slope and exchange current density (J0) are also investigated in acidic solution (η= blogJ + a, where b is Tafel slope, J is the current density, and a is the intercept relative to J0), as listed in Table S1. As shown in Figure 3b, the Tafel slope of MoC@NC nanoribbon is 54 mV dec-1, indicating that the release of molecular hydrogen is the rate-limiting step (Heyrovsky reaction step).4 Impressively, the as-prepared MoC@NC nanoribbon also exhibits favorable HER activity in alkaline solution. As expected, the MoC@NC nanoribbon possesses high HER activities with a much low onset potential of ca. -36 mV, a large J0 of ca. 20.4×10-3 mA cm-2 and small Tafel slop of 51 mV dec-1 in alkaline solution (Figure S25), which follows the same trend of that in the acidic solution. The electrochemical double layer capacitance (Cdl) is carried out to estimate the electrochemical surface areas33-34 (ECSA) of the solid-liquid interface for MoC@NC nanoribbon by using a simple CV method with a potential range of 0.05 ~ 0.35 V vs. RHE (Figure S26 a–c). In Figure 3c, the MoC@NC nanoribbon exhibits a very high Cdl of 6.7 mF cm-2, 3 times that of MoC@NC bulk (2.3 mF cm-2) and MoC@NC sheet (2.1 mF cm-2), indicating the highly exposed active sites and excellent HER activity. Furthermore, the electrochemical impedance spectroscopy (EIS) collected at a bias voltage from -130 mV to -250 mV (vs. RHE, in 0.5 M


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H2SO4) is also investigated to analyze the charge-transfer process at low frequency19 for HER process (Figure S26 d–f). Figure 3d shows the Nyquist plots of MoC@NC nanoribbon, MoC@NC bulk and MoC@NC sheet at overpotential 250 mV vs. RHE. The MoC@NC nanoribbon exhibits the minimum charge transfer resistance (Rct) compared with other counterparts, indicating the fast electron/mass transfer between catalyst and electrolyte and thus leading to an acceleration of HER kinetics.

Figure 3. a) Polarization curves of as-prepared MoC@NC nanoribbon, MoC@NC bundle, MoC@NC bulk, MoC@NC sheet, and 20 wt % commercial Pt/C in 0.5 M H2SO4 at a scan rate of 5 mV s-1 (All the curves have been corrected with iR compensation and background current); b) The Tafel plots of MoC@NC nanoribbon, MoC@NC bundle, MoC@NC bulk, and MoC@NC sheet and Pt/C, respectively; c) Plots showing the extraction of the double layer capacitance (Cdl) for MoC@NC nanoribbon, MoC@NC bulk and MoC@NC sheet at 0.2 V vs. RHE, respectively; d) The Nyquist plots collected at a bias voltage of -250 mV vs. RHE for MoC@NC nanoribbon, MoC@NC bulk and MoC@NC sheet, respectively.


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As an efficient HER electrocatalyst for practical application, it is expected to use a small overpotential to afford high-yielding hydrogen at a high speed. As the above mentioned, the MoC@NC nanoribbon reaches a large cathodic current density compared with other MoC@NC morphologies under the same potential, indicating the outstanding hydrogen production speed. Apart from the intrinsic activity and high exposed active sites, we propose that the enhanced HER performance when passing high current densities derives from the nano-/micro-structures of the MoC@NC nanoribbon. The higher applied current density leads to a corresponding more intense generation of the hydrogen bubbles. In this regard, the MoC@NC nanoribbon with unique topological rough 1D nanostructure and interconnected network microstructure can effectively facilitate hydrogen bubbles depart from the electrode surface, thus preventing inhibition of active sites caused by the produced hydrogen bubble on catalyst and retaining the intimate contact between electron and electrolyte (Figure 4a). As demonstrated in Figure 4 b–d, the MoC@NC bulk and MoC@NC sheet on GCE are collapsed shrinkage (Figure 4b) and severely aggregated (Figure 4c), which usually lead to the accumulation of large bubbles at the electrode surfaces, thus reducing the effective catalyst surface area. In contrast, the interconnected MoC@NC nanoribbon on GCE can provide a high surface roughness and hierarchy porous loose structure (Figure 4d), which tends to puncture hydrogen bubbles into smaller ones and wick them away from the catalyst surface.35 The corresponding photo of MoC@NC nanoribbon electrode with no obvious hydrogen bubbles when operating under a current density of -35 mA cm-2 (Inset of Figure 4d) also confirms the superior hydrogen departure capacity to the MoC@NC sheet and MoC@NC bulk electrodes with large hydrogen bubbles pinned on the reaction surface (Inset of Figure 4b and 4c, respectively). Such unique nano- and microstructures of MoC@NC nanoribbon can greatly enhance the catalytic activities


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by virtue of promoted hydrogen release with applying high current density, which guarantee the expressions of high cyclability and long-term stability.

Figure 4. a) The schematic illustrations of the catalytic surfaces for different morphological MoC@NC structures, from left to right are MoC@NC sheet, MoC@NC bulk and MoC@NC nanoribbon; The corresponding SEM images (side and top views) of the catalytic surface of b) MoC@NC sheet, c) MoC@NC bulk, and d) MoC@NC nanoribbon; Insert images of b, c) demonstrating the hydrogen bubble aggregated on electrode during the HER process, and the insert image of d) demonstrating the hydrogen bubble released from electrode during the HER process.


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To further investigate the stability and durability of the as-prepared MoC@NC nanoribbon, long-time continuous hydrogen production testing at current density of ca. -35 mA cm-2 is conducted in acidic environment. As shown in Figure 5a, a negligible degradation of HER current density is observed after continuously testing for 70 h, indicating the excellent catalytic activity for the MoC@NC nanoribbon electrode. Although the graphite rod as counter electrode is unfortunately deteriorated after a long period of 60 h that hinders the HER process, the SEM morphology of MoC@NC nanoribbon still exhibits no noticeable changes (Figure S27 and Figure S28a–c), which is also confirmed by XRD and XPS spectra (Figure S28d–f). Notably, the well-designed MoC@NC nanoribbon on GCE also possesses excellent hydrogen release ability in comparison of MoC@NC bulk and MoC@NC sheet. The typical serrate shapes in Figure 5b and c for MoC@NC bulk and MoC@NC sheet electrodes reveal the common behavior of alternate accumulation and release processes of H2 (g) bubbles, which is unfavorable for the sustainable hydrogen production. The good durability of MoC@NC nanoribbon can be attributed to three aspects: 1) The existence of γ-MoC which has been reported as the most stable type of MoC species in both experiment12,29 and theoretical studies36-37 may greatly enhance its intrinsic stability; 2) The unique roughness nanostructure of 1D nanoribbon with the MoC nanoparticles embedded in nitrogen-doped graphitic carbon nanolayers can prevent the degradation of catalyst during the long-time continuous testing; 3) The interconnected microstructure would efficiently prevent the mechanical damage of the catalyst from the vigorous hydrogen production at high applied current density. Therefore, it is noteworthy to emphasize the as-prepared MoC@NC nanoribbon possesses a favorable advantage of hydrogen generation in ongoing utilize because of the


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synergistic effects of the high electrochemical surface area and the interconnected microstructure for the fast hydrogen bubble release.

Figure 5. The long-time durability testing of MoC@NC nanoribbon a), MoC@NC bulk b) and MoC@NC sheet c), respectively. The hydrogen bubble releasing and aggregation processes are indicated with yellow and red arrows, respectively. 4

CONCLUSIONS


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In summary, the rationally designed 3D MoC@NC nanoribbon architecture has been achieved by using a controllable vapor deposition strategy at solid-gas interface. With the merits of the increased active sites on the unique 1D rough ribbon-like structure and the enhanced hygrogen bubble release from the interconnected networks, the MoC@NC nanoribbon exhibits excellent HER activities with small onset potential of 24 and 36 mV, Tafel slope of 54 and 51 mV dec-1, as well as high exchange current density of 7 and 20.4 µA cm-2 in acid and alkaline media, respectively, which is comparable to or even better than most of transitional metal based materials. This work paves the way of constructing new type of molybdenum based materials and designing specific interface structure for a variety of renewable energy applications beyond the highly efficient HER catalytic abilities demonstrated in this study. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at… Morphological characterization of the α-MoO3 nanoribbon monolith (Figure S1) and corresponding XRD diagram (Figure S2); Morphological characterization of the intermedia products at 450 oC (Figure S3) and 600 oC (Figure S3) with corresponding XRD diagrams (Figure S4 and S6); the products that annealed at 900 oC (Figure S7) and corresponding XRD diagram (Figure S8); optical photos of the α-MoO3 nanoribbon monolith and MoC@NC nanoribbon framework (Figure S9); the morphologies of the MoC@NC sheet (Figure S10) and corresponding XRD (Figure S11); the controlled products without DCA vapor (Figure S12); the wetting ability of the MoC@NC nanoribbon (Figure S13); STEM images of the MoC@NC nanoribbon (Figure S14); characterization of the commercial MoO3 (Figure S15) and MoC@NC


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bulk (Figure S16) and corresponding XRD (Figure S17); morphological characterization of the MoC@NC bundle (Figure S18) and corresponding XRD (Figure S19); the control experiments and performance evaluations (Figure S20); the overall Tafel slope of the samples (Figure S21); the characterized MoC nanoparticles and performance (Figure S22); the comparison of HER (Figure S23); the table of the kinetic parameters (Table S1); the HER performance in alkaline solution (Figure S24); the electrochemical surface area and impedance (Figure S25); optical images after long-time testing (Figure S26); the morphology of the MoC@NC nanoribbon (Figure S27). AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (L. Qu), Tel./fax: +86 010 68918608 T-mail: [email protected] (Y. Zhao) Notes The authors declare no competing financial interest ACKNOWLEDGMENT We thank the financial support from Beijing Natural Science Foundation (2164070, 2152028), the NSFC (21325415, 51673026, 21174019, 21604003), Excellent young scholars Research Fund of Beijing Institute of Technology. REFERENCES 1.

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Table of Content


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Interconnected Molybdenum Carbide-Based Nanoribbons for Highly

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