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Hierarchical Hetero-structured Mo2C/Mo3Co3C Bouquet-like Nanowire Arrays: an Efficient Electrocatalyst for Hydrogen Evolution Reaction Yu-Qing Wang, Ying Xie, Lei Zhao, Xu-Lei Sui, Da-Ming Gu, and Zhen-Bo Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00358 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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ACS Sustainable Chemistry & Engineering

Hierarchical Hetero-structured Mo2C/Mo3Co3C Bouquet-like Nanowire Arrays: an Efficient Electrocatalyst for Hydrogen Evolution Reaction Yu-Qing Wang,† Ying Xie,‡ Lei Zhao,† Xu-Lei Sui,† Da-Ming Gu*,†, and Zhen-Bo Wang*,† †MIIT

Key Laboratory of Critical Materials Technology for New Energy

Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92 West-Da Zhi Street, Harbin, 150001 China ‡Key

Laboratory of Functional Inorganic Material Chemistry, Ministry of Education,

School of Chemistry and Materials Science, Heilongjiang University, No. 74 Xue Fu Road, Harbin, 150080, PR China * Corresponding author. Tel.: +86-451-86417853; Fax: +86-451-86418616. Email: [email protected] (D. M. Gu), [email protected] (Z. B. Wang) Abstract: Multiple active constituents of hetero-structured catalysts could form moderate hydrogen binding strength on the surface, which is expected to solve the inefficient issue of hydrogen evolution reaction (HER). Herein, hydrothermal followed by carbon deposition treatment is introduced to design hierarchical hetero-structured Mo2C/Mo3Co3C bouquet-like nanowire arrays on Ni foam (Mo2C/Mo3Co3C-NF), and bring abundant Mo2C-Mo3Co3C interfaces for synergetic electrocatalysis. The addition of Co modifies the electronic structure of the resultant catalysts, and the HER intrinsic activity have been remarkably promoted. Additionally, the 3D Ni foam act as a current collector facilitates

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the directional growth of the active phases, and the exposure of more active sites for hydrogen evolution reaction. As expected, the Mo2C/Mo3Co3C-NF-2 (the molar ratio of Co/Mo is 2/4) catalyst electrode exhibits a low onset overpotential of 24 mV and achieves an overpotential of 87 mV at the current density of 10 mA cm-2 in 1M KOH. Density functional theory (DFT) calculations have clarified the effect of Co substitution on the HER activities of the electrode materials and relevant mechanisms, which will be beneficial for further design of the cost-effective materials. Keywords: molybdenum carbide; Mo-Co bimetallic carbide; DFT calculations; hierarchical heterostructure; binder-free electrode Introduction Concerns about environmental pollution and fossil fuels depletion have prompted a pressing requirement to exploit alternative energy resources1. Hydrogen is a sustainable and environmentally benign energy resource with high combustion value and zero-emission. The production and utilization of hydrogen energy is believed as the most prospective method to conquer future energy anxieties2-3. Electrochemical water splitting, an emerging clean-energy technology for interconversion between water and hydrogen, is the most efficient strategy to convert electrical energy into a high purity storable chemical energy4. Electrocatalysts are essential to lower the unavoidable overpotential that primarily leads to excessive energy depletion. At present, Pt-based materials are outstanding HER catalysts, but the insufficient Pt reserves and high expense restrict their widespread application5-6. Hence, intensive efforts into the exploration of profitable and efficient non-noble metal catalysts for the sluggish hydrogen evolution reaction are


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crucial7. Transition metal compounds are highly desired as candidates for non-precious metal HER electrocatalysts, such as transition metal chalcogenides8-12, nitrides13-15, phosphides16-19, and carbides20-23. Particularly, molybdenum carbides have aroused ever-growing interest because of their stability, good corrosion resistance and similar electronic configuration with Pt-group metals24-26. Noticeably, the control of morphology (such as nanowires, nanoflakes or 3D structure) and surface electronic structure (such as current collector supports or heteroatoms doping) has been paid more attention, and has been demonstrated to be effective in decreasing the overpotential for HER. For example, Zhu et al. prepared mesoporous Mo2C@GC core−shell nanowires via a combined hard-templating growth and in-situ carburization process27. Zou et al. developed a 3D self-supported porous electrode constructed by nano-porous Mo2C nanoflakes in-situ grafted on a commercial graphite felt through a carburization approach28. Jing et al. synthesized N-doped porous Mo2C nanobelts with an environmentally-friendly and template-free method29. However, molybdenum carbides still suffer from insufficient alkaline performance and durability despite the devotion of tremendous efforts, which hamper their applicability toward industrial hydrogen production. Besides, research on the electronic structure and the active sites density distribution are needed at both theoretical and experimental levels. Therefore, it is desirable to achieve highly active molybdenum carbides-based electrocatalysts through effective strategy. Several strategies to optimize the conductivity and activity of molybdenum carbides



in hydrogen evolution reaction have been designed as follows: (1) The combination of electrocatalysts and various current collectors (e.g., NF, Ti foil, and rGO) increases the electrical conductivity30; (2) The uniform growth of directional active phases on the substrates enlarges the specific surface area and increase the amount of active sites; (3) The addition of a second transition metal to the carbide to adjust the electronic structure and improve the stability and activity of monometallic carbides31. Guided by the relationship between metal-hydrogen (M-H) bond strength and HER exchange current density, we can devise an efficient catalyst by combing weak M-H binding metals (such as Fe, Co, Ni etc.) with strong M-H binding molybdenum carbides materials32-34. It might lead to a relatively moderate balance between adsorbing and desorbing of the hydrogen atoms, and favor the overall reaction of HER35. Increasing the density of exposed active sites and controlling the surface morphology of the electrocatalyst is also the key to enhancing HER catalytic activity. Usually, the polymer binders are used to bond the electrocatalysts with the electrode surfaces in the electrochemical test, but they may hinder diffusion, cover the active sites and increase the electron transfer resistance36. Therefore, making the active phases directly grow on a current-collecting carrier is deliberated to be a decent strategy to expose active sites and guarantee the effective dispersion of the electrocatalyst. Herein, we devote our effort to constructing novel hierarchical hetero-structured Mo2C/Mo3Co3C bouquet-like nanowire arrays on Ni foam as a HER electrode by combined hydrothermal and carbon deposition treatment for the first time. The electrode


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exhibits remarkable HER catalytic activity and durability in alkaline electrolytes, which could be attributed to the exposure of more active sites and larger surface area provided by the unique hetero-structure. The Mo2C/Mo3Co3C catalyst is superior in catalytic performance and durability which can attain a low onset overpotential of 24mV and achieve a current density of 10 mA cm-2 at 87 mV in 1M KOH, and remain steady performance after 5000 cycles. Furthermore, to reveal the HER mechanisms, DFT calculations are performed. Our calculations not only clarify the effect of Co substitution on the HER activities of the electrode materials but also demonstrate the underlying reasons for their distinct performance, which would be helpful for further optimization of the electrode materials. Results and discussion Physical characterization of various catalysts Figure 1 exhibits a schematic representation of the in-situ growth of hierarchical Mo2C/Mo3Co3C nanowire arrays on a Ni foam substrate using a combination of hydrothermal and carbon deposition treatment. The Ni foam was selected as a substrate because of its 3D structure, outstanding electrical conductivity, and various electrochemical features toward HER (Figure 1(a)). First, the Mo-Co hydroxide nanowire arrays were grown on a 3D Ni foam (Figure 1(b)) through hydrothermal treatment of Mo6+ and Co2+ (different Co/Mo molar ratios of 1/4, 2/4 and 3/4 labeled as Mo2C/Mo3Co3C-NF-1, Mo2C/Mo3Co3C-NF-2 and Mo2C/Mo3Co3C-NF-3) with water at 180 °C for 4 h in a homogeneous reactor. Next, the as synthesized precursor was



converted into hierarchical Mo2C/Mo3Co3C nanowire arrays through a carbon deposition process under the mixture gas flow (C2H2/H2/Ar) at 700 °C for 2 h. The FESEM image (Figure 1(c)) shows that Ni foam acts as a skeleton that grow the Mo2C/Mo3Co3C nanowire arrays, and confirms the formation of bouquet-like nanowire arrays. Structure information of the samples with variety of Co/Mo molar ratios is confirmed by XRD patterns. As displayed in Figure 2, besides the diffraction peaks from Ni foam (JCPDS No. 65-2865), all the samples reveal characteristic peaks of β-Mo2C (JCPDS No. 65-8766). The same characteristic peaks are shown at 34.5°, 38.0°, 39.6°, 52.3°, 61.9°, 69.8°, 72.8°, 75.0° and 76.0°, which could be assigned to the (100), (002), (101), (102), (110), (103), (200), (112) and (201) planes of molybdenum carbide, respectively. In addition, the pattern for Mo2C/Mo3Co3C-NF-2 sample showed diffraction peaks at 32.4°, 35.4°, 40.0°, 42.5°, 46.5°, 49.4°, 59.7°, 64.7° and 72.5°, which matched well with the standard data of Mo3Co3C (JCPDS No. 65-7128). It was found that the addition of Co encouraged the formation and growth of β-Mo2C and Mo3Co3C37. The diffraction peaks for Mo3Co3C became sharper with the increasing Co/Mo molar ratio till Co/Mo = 2/4, suggesting that the formation of Mo3Co3C was ascribed to the strong synergy effect between Co and Mo38-39. The strengths of peaks for Mo3Co3C decreased when Co/Mo molar ratio became large (Co/Mo = 3/4), which indicated that the increasing proportion of Co atoms led to the possibility reduction of this strong synergy effect generation37. The SEM images illustrate the microstructure and composition of the Mo2C/Mo3Co3C-NF and Mo2C-NF (Figure 3(a, b), S1 in Supporting Information). The


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Ni foam was used as a catalyst support, and a representative SEM image is shown in Figure S1(b). As revealed, the Ni foam has a 3D network structure, which is beneficial for the direct loading of the active components and excellent electrical connection between the active species and substrate. Figure S1(a) shows that the nanosheets composed by Mo2C particles are loaded on the skeletons of Ni foam. As observed in Figure 3(a, b) and Figure S1(c-f), the addition of cobalt element promotes the formation of the hetero-structured Mo2C/Mo3Co3C nanowire arrays. The β-Mo2C and Mo3Co3C active phases almost grow bouquet-like and perpendicularly on the substrates without obvious agglomeration, which could enlarge the specific surface area and increase the amount of exposed active sites. The nanowire arrays of Mo2C/Mo3Co3C-NF-1 are thinner and more broken (Figure S1(c, d)). In comparison, the nanowire arrays of Mo2C/Mo3Co3C-NF-3

are

the

thickest,

and

more

obvious

than

that

of

Mo2C/Mo3Co3C-NF-1 (Figure S1(e, f)). Particularly, the Mo2C/Mo3Co3C-NF-2 has the most obvious nanowire arrays structure with a cross section of quadrilateral. Hence, the formation of this unique structure may attribute to the moderate synergy effect between Co and Mo caused by appropriate Co/Mo molar ratio (Co/Mo = 2/4). The SEM results illustrated the generation of exceptional bouquet-like nanowire arrays, which are beneficial to the contact between catalysts and electrolyte, and favor the electrolyte transport. To achieve an in-depth understanding of the crystalline structure of the Mo2C-NF and Mo2C/Mo3Co3C-NF-2, high-resolution TEM (HRTEM) images are provided in



Figure S2 and Figure 3(c, d). Figure S2(a) shows that the Mo2C-NF has a general crystallinity, and the inset selected-area electron diffraction (SAED) pattern indicates the (101) and (110) planes of the Mo2C. Figure 3(c) shows distinct lattice fringes with interplanar distances of 0.259 and 0.335 nm, corresponding to the (100) plane of Mo2C and the (311) plane of Mo3Co3C, respectively. Figure 3(d) exhibits the (111) plane of Mo3Co3C with interplanar distances of 0.629 nm. The HRTEM results indicated that the hetero-structured Mo2C/Mo3Co3C-NF-2 catalytic electrode is composed of homogeneous β-Mo2C and Mo3Co3C active phases, which is matched with the XRD results. Furthermore, the SAED patterns in the inset (Figure 3(c, d)) display diffraction spots, indicating the crystalline nature of the Mo2C/Mo3Co3C-NF-2. Figure 3(e) presents the EDX elemental mapping images of Mo2C/Mo3Co3C-NF-2, revealing that the elements are distributed uniformly in the nanowire arrays with diameters of about 500 ~ 600 nm. The XPS spectra can be applied to illustrate the element composition and valence state. High-resolution C 1s spectrum is shown in Figure 4(a), the peak at 284.6 eV demonstrates the graphite-like sp2-hybridized carbon40. The C-Mo, C-O and O-C=O bonds at 284, 286.4 and 286.2 eV are also shown in C 1s spectrum41-43. After the addition of Co into Mo2C, the percentage of C-Mo was increased, proving that cobalt element promotes the generation of hetero-structured Mo2C/Mo3Co3C (Table S1). Meanwhile, the C-Mo percentage of Mo2C/Mo3Co3C-NF-2 is the highest about 48%, indicating the existence of large amount of β-Mo2C and Mo3Co3C active phases in the catalyst. The increase of C-Mo content leads to an ascent in activity, revealing that the appropriate


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addition of Co is favorable for the C-Mo bond generation and catalytic activity improvement. Figure 4(b) displays the high-resolution XPS of Mo 3d region. The predominant pair of peaks at binding energy of 228.1 and 231.3 eV corresponding to typical Mo 3d5/2 and Mo 3d3/2 peaks of Mo(Ⅱ) in Mo2C40, 44-45. Mo2C/Mo3Co3C-NF-2 has about 69.7% content of Mo(Ⅱ), which is the highest among these catalysts (Table S2). Other binding energies at 229.1 and 232.3 eV are associated with Mo(Ⅳ) in MoO2, while the other two peaks at 233.1 and 236.3 eV can be assigned to Mo(Ⅵ) in MoO341-43, 46.

The presence of molybdenum oxides is presumably caused by partial oxidation on the

Mo2C surface through the air contact. Figure 4(c) shows that Co 2p spectrum presents two peaks at 778.5 and 793.1 eV denoting to the binding energy of Co2+, and the peaks at 785.0 eV and 799.6 eV stand for satellite peaks47. Electrochemical performance of various catalysts Figure 5(a) illustrates the polarization curves of the Mo2C/Mo3Co3C-NF in a H2-saturated alkaline medium and their HER properties are summarized in Table 1. The catalytic behavior of 20 wt% Pt/C-NF (with Pt: 60 μg cm-2), Mo2C-NF and NF were studied for contrast. The 20 wt% Pt/C-NF shows an exceptional HER activity with insignificant overpotential, while NF shows quite limited HER activity with retarded catalytic onset. As observed from Figure 5(a), Mo2C/Mo3Co3C-NF-2 electrode is extremely active toward HER which can approach a low onset overpotential (at the current density of 2.5 mA cm-2) of 24 mV and shows a overpotential of 87 mV at the current density of 10 mA cm-2, which is considerably superior than that of pure Mo2C-NF (with an onset



overpotential of 121 mV). In addition, the catalytic activities of the electrode materials with lower and higher Co content (Mo2C/Mo3Co3C-NF-1 and Mo2C/Mo3Co3C-NF-3) require larger overpotentials of 151 and 153 mV to deliver a current density of 10 mA cm−2 (Table 1). We speculate that the high catalytic activity of Mo2C/Mo3Co3C-NF-2 is likely interrelated to the uniform growth of β-Mo2C and Mo3Co3C active phases; the unique hierarchical hetero-structure on the substrate which increase the number of active sites; the appropriate Co/Mo molar ratio (Co/Mo = 2/4) leads to a relatively moderate strength of M-H bonds that favor the hydrogen evolution reaction. These results further indicate the significant impact of the amount of Co on HER activities. In fact, the catalytic performance of Mo2C/Mo3Co3C-NF-2 is comparable or superior to the recently reported Mo2C-based HER catalysts (Table S3, Figure 6)48-63. Relevant Tafel slopes and exchange current density j0 derived from linear-sweep voltammetry data are used to elucidate the electrocatalytic kinetics of HER. Figure 5(b) indicates the Tafel plots for the Mo2C/Mo3Co3C-NF and Mo2C-NF with 20 wt% Pt/C-NF as a comparison, the linear parts of the Tafel plots are matched with the Tafel equation (η = a + b log j, ‘a’ stands for the intercept, ‘b’ stands for the Tafel slope and ‘j’ stands for the current density). 20 wt% Pt/C-NF has a Tafel slope of 35.1 mV dec-1, which is consisted with the published value. The Tafel slopes of Mo2C/Mo3Co3C-NF-1, Mo2C/Mo3Co3C-NF-2 and Mo2C/Mo3Co3C-NF-3 are 76.1, 50.7 and 74.3 mV dec-1, respectively, which are evidently lower than that of the Mo2C-NF (109.3 mV dec-1). As reported, the hydrogen evolution reaction mechanism on the catalyst surface in an


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alkaline medium consists of the following two steps: (1): Butler–Volmer reaction (Discharge reaction): H2O + e- + M (active site)→MHads + OH(2): Heyrovsky process (Ion and atom reactions): H2O + e- + MHads→M + OH- + H2 or Tafel process (combination reaction): 2 MHads→2 M + H2 The Butler–Volmer reaction with a Tafel slope of 118 mV dec-1 and the Tafel process that corresponds to the reaction of atom combination at a Tafel slope of 30 mV dec-1 or the Heyrovsky process at a Tafel slope of 40 mV dec-1 for the ions or atoms reactions64-65. Consequently, the Tafel slope for the Mo2C/Mo3Co3C-NF-2 sample reveals that the HER pathway probably through a Volmer-Heyrovsky mechanism, which is correlate with the determining step of electrochemical desorption rate in the HER64, 66. Exchange current density (j0), the intrinsic measure of HER performance, is calculated by extrapolation methods and represents the significant kinetic influence of the Mo2C/Mo3Co3C-NF electrochemical reaction rate under equilibrium state65. Based on Tafel equation, j0 for Mo2C/Mo3Co3C-NF-2 is calculated to be about 2.36 mA cm-2, higher than the Mo2C/Mo3Co3C-NF-1 and Mo2C/Mo3Co3C-NF-3 (0.81 and 0.60 mA cm-2), which is approximately 4 times that of Mo2C-NF (0.55 mA cm-2) (Table 1). The EIS technique is used under alkaline conditions to demonstrate the kinetic behavior of Mo2C/Mo3Co3C-NF and Mo2C-NF. We simulate a circuit that ohmic



resistance (Rs) in series with a unit, where the charge-transfer resistance (Rct) is in parallel with a constant phase element (CPE). The Rct is analyzed by the diameter of high frequencies semicircle in a Nyquist plot. As presented in Figure 5(c), Nyquist plots and data indicate that the charge-transfer resistance value for Mo2C/Mo3Co3C-NF-2 (2.6 Ω cm2) is lower than Mo2C/Mo3Co3C-NF-1 (3.5 Ω cm2), Mo2C/Mo3Co3C-NF-3 (3.6 Ω cm2) and Mo2C-NF (19.5 Ω cm2). Furthermore, the Mo2C/Mo3Co3C-NF-2 has an extremely low ohmic resistance value about 2.6 Ω cm2, which shows that the electrode material has perfect conductivity. These results suggest that Mo2C/Mo3Co3C-NF have the most efficient charge transport during the electrochemical HER process and superior electronic conductivity, which cause favorable HER kinetics. In general, we have found that the addition of Co into Mo2C can promote the HER activity. The Co addition can accelerate electron transfer between Mo2C/Mo3Co3C-NF and the underlying supporting Ni foam. It can be verified by Figure 5(c), the Mo2C/Mo3Co3C-NF electrodes show lower impedance than a pure Mo2C-NF electrode. The impedance reduction brings about faster electron transfer between Mo2C/Mo3Co3C-NF and the Ni foam, leading to a fast hydrogen evolution reaction rate. The synergistic effect and the optimized electronic structure of Mo3Co3C and Mo2C can change the electron distribution on Mo2C/Mo3Co3C-NF electrodes. The alkaline stability of the Mo2C/Mo3Co3C-NF-2 is evaluated by accelerated degradation studies using 5000 cyclic voltammetry (CV) sweeps ranging from -0.24 to 0.26 V vs. RHE at a scan rate of 100 mV s-1. After 5000 cycles, only a slight loss (about 7


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mV) in catalytic activity is observed at the region of low current density (Figure 5(d)). The result demonstrates that Mo2C/Mo3Co3C-NF-2 has respectable stability under strong alkaline medium. The electrochemically active surface area (ECSA) is studied by the electrochemical double-layer capacitance (Cdl), which is measured via cyclic voltammetry (CV) at different scan rates between 0.1 and 0.2 V versus reversible hydrogen electrode (RHE). Figure 7 shows cyclic voltammograms of NF, Mo2C-NF, Mo2C/Mo3Co3C-NF-1, Mo2C/Mo3Co3C-NF-2 and Mo2C/Mo3Co3C-NF-3 with different scanning rates. In Figure 7(f), the current density at 0.15 V is plotted against the scan rate, the Cdl values of 1.27, 22.24, 46.34, 62.75 and 47.67 mF cm-2 are derived from the different NF, Mo2C-NF, Mo2C/Mo3Co3C-NF-1, Mo2C/Mo3Co3C-NF-2 and Mo2C/Mo3Co3C-NF-3 samples, respectively (Table 1). These values indicate larger ECSA and abundant catalytic active sites on Mo2C/Mo3Co3C-NF, which partially account for the higher HER activity of the electrode materials. This demonstrates that Mo2C/Mo3Co3C nanowire arrays grown on the NF are more efficient in expansion of the catalytic active surface area compared to bare NF. Therefore, enhanced utilization and better exposure of the active sites on Mo2C/Mo3Co3C-NF significantly promoted its extraordinary HER activity. Then the turnover frequency (TOF) for each active site is quantified by an electrochemical method to provide insight into the intrinsic catalytic activity (Supporting Information)67-68. The number of active catalytic sites for the Mo2C/Mo3Co3C-NF-2 electrode is about 2.5 times that of the Mo2C-NF electrode. Accordingly, the TOF of the Mo2C/Mo3Co3C-NF-1,



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Mo2C/Mo3Co3C-NF-2 and Mo2C/Mo3Co3C-NF-3 are calculated to be 0.648, 0.732 and 0.707 s−1 at overpotential of 200 mV, which is higher than that of Mo2C-NF electrode (0.509 s−1). The results suggest that the addition of Co into Mo2C contributes much to the improvement of the HER activities. Table 1. Comparison of the catalytic performances of materials got from different Co incorporations for HER in 1 mol L-1 KOH solution Onset potentiala

Tafel slope

j0

Rct

Cdl

Catalysts

Relative surface area (mV)

(mV

dec-1)

(mA

cm-2)

(ohm)

(mF

cm-2)

NF

~350

--

--

--

1.27

1.00

Mo2C-NF

121

109.3

0.55

1.95

22.24

17.51

Mo2C/Mo3Co3C-NF-1

72

76.1

0.81

0.35

46.34

36.49

Mo2C/Mo3Co3C-NF-2

24

50.7

2.36

0.26

62.75

49.41

Mo2C/Mo3Co3C-NF-3

80

74.3

0.60

0.36

47.67

37.54

a

Onset potential at a current density of 2.5 mA cm-2.

Density functional theory (DFT) calculations The

electrochemical

data

indicate

the

superior

intrinsic

activity

of

Mo2C/Mo3Co3C-NF-2 over Mo2C-NF. As confirmed by XRD, SEM and TEM, the Co is commendably added into the Mo2C. Under this circumstance, a synergistic catalysis of Co and Mo2C for improving the HER activity may occur over the Mo2C/Mo3Co3C-NF catalysts. In order to figure out the factors conducing to the superior HER activity, we have performed relevant DFT calculations. Based on the experimental results, two typical


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surfaces for Mo2C and Co-substituted Mo2C materials are taken into considerations, as depicted in Figure 8(a-d). Figure 8(e) depicts the HER activities of different surfaces for Mo2C and Co-substituted Mo2C. In alkaline media, the HER process can be described by two steps, i.e. water dissociation for generating H* intermediates and the desorption of OH* intermediates from catalyst surface (the Volmer step); the electrochemical desorption of H* and/or the chemical desorption by the recombination of the H* to form H236, 69. For the (100) and (111) surfaces of Mo2C, the activation of water is energetically favorable, and the energies required for desorption of OH* from relevant surfaces are 0.312 and 0.004 eV (Table 2), respectively. However, the energy barriers for the evolution of H2 on Mo2C surface are calculated to be 1.029 and 0.773 eV, suggesting that it is the rate-determine step. When Co was introduced into Mo2C, the HER mechanism will be changed. In comparison to the (100) and (111) surfaces of Mo2C, the energy barriers for the evolution of H2 on the (100) and (111) surface of Co-substituted Mo2C are much lower and calculated to be 0.676 and 0.135 eV. As a result, (100) and (111) surfaces of Co-substituted Mo2C will exhibit a better HER activity than that of Mo2C. Furthermore, our calculations also clearly showed that the energy barrier of the generating H* for the (111) surface of Co-substituted Mo2C are only 0.336 eV, confirming that Co-substituted Mo2C does have an excellent HER activity in alkaline environment. The theoretical results are well consistent with the experimental observations. Table 2. Energy barriers of different steps for Mo2C and Co-substituted Mo2C



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Energy Barriers (eV) OH*+H*

OH-+H*

OH-+1/2H2

Mo2C (100) surface

-1.342

0.312

1.029

Co-substituted Mo2C (100) surface

-1.539

0.864

0.676

Mo2C (111) surface

-0.778

0.004

0.773

Co-substituted Mo2C (111) surface

0.336

-0.471

0.135

Due to the dissolution of Ni, it is difficult for us to test the HER activities of the materials in acidic environment. However, our calculations have also revealed their HER performance in such a condition. It was reported that HER pathway in acidic condition can be described as three stages, an initial state (H+ + e-), an intermediate state (adsorbed H denoted as H*), and the final state (1/2H2)70-71. A good catalyst for hydrogen evolution in acidic condition usually requires that the free energy of adsorbed H (GH*) should be approximately zero, which is very helpful for fast proton/electron-transfer and fast hydrogen release process72. Our results suggest that when hydrogen is adsorbed on the C sites on the (100) and (111) surfaces of pure Mo2C, GH* are calculated to be -0.125 eV and -0.601 eV, respectively (Table 3). For the Mo sites on the (100) and (111) surfaces of pure Mo2C, relevant values (-0.859 and -0.603 eV) are also very negative (Table 3, Figure 8(f)). Therefore, it can be concluded that C sites on the Mo2C (100) surface are the active centers for the HER in acidic condition. When Co was incorporated into Mo2C, the adsorption energies are changed significantly. The data in Figure 8(f) and Table 3 clearly shows that GH* for the Co sites on (100) and (111) surfaces of Co-substituted Mo2C are -0.506 and 0.035 eV, indicating that the HER activities of the materials in acidic


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environment will be improved obviously. In addition, all the ΔGH* values of the Co site on different Co-substituted Mo2C interfaces are closer to 0 (Figure S3 and Table S4). Our calculations have confirmed that the introduction of Co species into Mo2C does have a synergetic effect on the improvement of the HER performances of the materials either in acidic or in alkaline conditions. Table 3. DFT calculated Gibbs free energies (GH*) on Mo2C and Co-substituted Mo2C surfaces Surface

Catalyst

Site

 GH* (eV)

(100)

Mo2C

C

-0.125

Mo

-0.859

C

-0.180

Co

-0.506

C

-0.601

Mo

-0.603

C

-0.631

Co

0.035

Co-substituted Mo2C

(111)

Mo2C

Co-substituted Mo2C

Conclusions In summary, the novel hierarchical hetero-structured Mo2C/Mo3Co3C bouquet-like nanowire arrays on Ni foam were successfully synthesized through the combined hydrothermal reaction and the chemical vapor deposition process. This binder-free electrode was developed to address the performance insufficient of the Mo-based



catalysts, especially around the alkaline environment. Superior activity and outstanding stability in challenging alkaline media could be attributed to the large specific surface area and extraordinary intrinsic activity of Mo2C/Mo3Co3C, which also make the Mo2C/Mo3Co3C-NF a promising binder-free electrode for hydrogen evolution. The two phases of the β-Mo2C and Mo3Co3C were studied for the first time. The Co/Mo ratios were discussed and prove effective to increase the density of active sites. The DFT calculations evidently indicate that the addition of Co into the Mo2C adjusts the electronic structure on the catalyst surface, which leads to a synergistic effect of Mo2C/Mo3Co3C. Consequently, our study provides an economical HER electrocatalyst with high activity and stability and offers a unique methodology for designing Mo2C based composite materials for wide-ranging applications. Supporting Information Experimental, computational methods and models, characterization data Acknowledgements We acknowledge the National Natural Science Foundation of China (Grant No. 51802059, 21273058 and 21673064), China Postdoctoral Science Foundation (Grant No. 2017M621284, 2018M631938 and 2018T110307), Heilongjiang Postdoctoral Fund (Grant No. LBH-Z17074), and Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 2019040 and 2019041). References 1.

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For Table of Contents Use Only

Schematic illustration of the formation of Mo2C/Mo3Co3C-NF and computational models.


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Figure 1. Schematic illustration of the formation of Mo2C/Mo3Co3C nanowire arrays on Ni foam and their corresponding morphology. (a) Bare Ni foam, (b) in-situ growth of the as synthesized precursor, (c) Mo2C/Mo3Co3C nanowire arrays supported on Ni foam by carburization treatment.

Figure 2. XRD patterns of Mo2C-NF, Mo2C/Mo3Co3C-NF-1, Mo2C/Mo3Co3C-NF-2 and Mo2C/Mo3Co3C-NF-3.



(a)

(c)

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(e) Mo2C (100) Mo3Co3C (311)

d = 0.259 nm

C

Mo

Co

Ni

d = 0.335 nm

(b)

(d) Mo3Co3C (111)

d = 0.629 nm

Figure 3. SEM (a, b), HRTEM (c, d) and SAED (e) images of Mo2C/Mo3Co3C-NF-2.

Figure 4. XPS analysis to elucidate the influence of different Co incorporations. High-resolution C 1s (a), Mo 3d (b), and Co 2p (c) spectrum of Mo2C –NF, Mo2C/Mo3Co3C-NF-1, Mo2C/Mo3Co3C-NF-2 and Mo2C/Mo3Co3C-NF-3.


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Figure 5. Polarization curves of Mo2C-NF, Mo2C/Mo3Co3C-NF-1, Mo2C/Mo3Co3C-NF-2, Mo2C/Mo3Co3C-NF-3, NF and 20% Pt/C-NF in 1 mol L-1 KOH (a). The relevant Tafel plots of different catalysts (b). Nyquist plots of different catalytic electrodes recorded at 190 mV vs. RHE in 1 mol L-1 KOH (c). Polarization curves for Mo2C/Mo3Co3C-NF-2 before and after 5000 CV sweeps between −0.24 to 0.26 V vs RHE in 1 mol L-1 KOH (d).



Figure 6. Comparison of the Tafel slope and η10 (overpotential required to drive a current density of 10 mA cm-2) of Mo2C/Mo3Co3C-NF-2 with other recently reported electrocatalysts.

Figure 7. Cyclic voltammograms of (a)NF, (b) Mo2C-NF, (c) Mo2C/Mo3Co3C-NF-1, (d) Mo2C/Mo3Co3C-NF-2 and (e) Mo2C/Mo3Co3C-NF-3 with scanning rates from 20 to 120 mV s-1 and the potential range from 0.10 - 0.20 V vs RHE in a 1 M KOH solution. (f) Estimated the double-layer capacitances (Cdl) of the different catalytic electrodes at 0.15 V vs. RHE.


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C site

Mo site

(e)

(a)

(b) (f)

Co site

(c)

(d)

Figure 8. Computational models of the (100) (a, c) and (111) (b,d) surfaces for Mo2C and Co-substituted Mo2C. Predicted HER activities of different surfaces for Mo2C and Co-substituted Mo2C in alkaline (e) and acidic (f) conditions.


Hierarchical Heterostructured Mo - ACS Publications - American

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