Scalable synthesis of heterogeneous W-W2C nanoparticles

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Scalable synthesis of heterogeneous W-W2C nanoparticles embedded CNT networks for boosted hydrogen evolution reaction in both acidic and alkaline media Yang Hu, Bo Yu, Manigandan Ramadoss, Wenxin Li, Dongxu Yang, Bin Wang, and Yuanfu Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01199 • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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ACS Sustainable Chemistry & Engineering

Scalable synthesis of heterogeneous W-W2C nanoparticles embedded CNT networks for boosted hydrogen evolution reaction in both acidic and alkaline media Yang Hu, Bo Yu, Manigandan Ramadoss, Wenxin Li, Dongxu Yang, Bin Wang,Yuanfu Chen*

School of Electronic Science and Engineering, and State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, No. 4, Section 2, North Jianshe Road, Chengdu 610054, P. R. China

*Corresponding author, E-mail: [email protected], Tel.: +86 028 83202710.

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ABSTRACT Practical hydrogen production via hydrogen evolution reaction (HER) is reported as a clean and sustainable strategy for future energy. Tungsten (W)-based compounds are reported as promising alternatives to Pt-based electrocatalyst for HER. However, inefficient chargetransfer, high onset overpotential, and particularly lacking of reliable synthetic method still restricts its widespread application. Herein, for the first time, W-W2C nanoparticles embedded CNT (W-W2C/CNT) composite, constructed by heterogeneous ultrafine W-W2C nanoparticles uniformly embedded into highly conductive CNT networks, was prepared via spray-drying process followed carbonization method. The optimized W-W2C/CNT electrocatalyst exhibits excellent HER performance in both acidic and alkaline media: it shows a small onset overpotential of only 40 (or 20) mV, small Tafel slope of 56 (or 51) mV dec-1 in 0.5 M H2SO4 (or 1 M KOH); moreover, it simultaneously shows remarkable long-term stability, particularly over 50 hours under alkaline medium. The boosted HER performance in acid or alkaline solution is mainly attributed to the ligand effect of metallic W and W2C and the synergistic effect of the unique porous nanoarchitecture, which affords abundant active catalytic sites, enhance the transfer ability of electrons and ions thus significantly improve its HER activity. This work presents a scalable synthesis approach to synthesize noble-metal-free electrocatalysts with controllable nanoarchitecture and boosted HER performance. KEYWORDS: Hydrogen evolution reaction; W-W2C/CNT; Spray drying; Scalable synthesis; Electrocatalyst

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INTRODUCTION With the growing of global fossil energy consumption and greenhouse effect, hydrogen as one of the most clean and renewable energy source for replacing fossil energy, has drawn more and more attention by scientists in recent years.1-3 Compared to the methane steam reforming and gasification of coal, electrocatalytic water splitting is a cost-efficient strategy via hydrogen evolution reaction (HER) to generate hydrogen gas with high purity, which is more sustainable and produces less environmental contaminants.3,4 Platinum (Pt)-based noble metals have been investigated to be the most active electrocatalysts for HER, but the high priced, scarcity supply and poor stability greatly limits the large-scale application.4,5 Therefore, the economical nonnoble transition metal compounds (e.g., selenides,6-9 sulfides,10,11, borides,12 and carbides5) are widely researched for hydrogen evolution reaction to take place of the Pt-based materials. Recently, the transition-metal carbides (TMCs) including WC,13-15 Mo2C,16-18 Ni3C,19 V8C720 etc., have been explored for HER electrocatalysis because of their earth-abundant and inexpensive. It has been corroborated that the existence of carbon atoms into the transitionmetals (Mo and W) lattice could result in higher d-band electronic density of states (DOS).14,21,22 Among them, tungsten carbide has been proved to be a promising candidate for HER electrocatalysis.21 Unfortunately, high onset overpotential and inefficient charge-transfer make the tungsten carbide still far away from the practical production, although tungsten carbide had been first reported as a promising Pt-like catalyst in 1964.23 The following three challenges seriously hinder the practical application.21,24-27 First of all, the conventional synthetic method for tungsten carbide is using high-temperature carbonization with gaseous carbon source (such as CH4, C2H6 or CO), which cause the inevitable aggregation and

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uncontrollable particle size. Secondly, excess gaseous carbon sources may cover the active site, seriously affects the catalytic performance. Last but most importantly, few existing methods are reported on the tungsten carbide catalyst via novel morphology design to expose more reactive sites or to enhance electronic conductivity of tungsten carbide by tunable composition design. Recent reports show that the introduction of metal phase is an effective method to modify the electronic structure of metal carbide and boost the HER performance. Chen et al. demonstrated that metal-to-ligand charge transfer on carbide materials.16 Dong et al. reported that the ligand effect in Mo rich Mo2C electrocatalyst can optimize electronic structure and change the electron distribution, thus enhanced HER activity in acidic media.18 Ma et al. reported a novel multi-interfacial nickel/tungsten carbide loaded on N-doped carbon sheets. In their report, they elucidated that unique multi-interfacial feature of Ni/WC electrocatalyst leads to feasible charge transfer path through WC to Ni, which generates the Ni element with an increase electron-rich state, thus boosting the HER performance of the composite catalyst.13 Furthermore, compared to the WC, the W2C has attracted far less attention for the HER in recent years. So far, it is still challenging to design a novel structure with W-W2C ligand effect and develop large-scale synthetic method to enhance the HER performance of W2C-based catalyst. In this study, for the first time, heterogeneous W-W2C ultrafine nanoparticles homogenously embedded into conductive CNT networks has been synthesized via an inexpensive, simple and large-scale spray-drying process followed carbonization. The optimized catalyst (W-W2C/CNT-6) exhibits outstanding HER activity with a small onset

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overpotential of only 40 mV and 20 mV, small Tafel slopes of 56 mV dec-1 and 51 mV dec-1 in 0.5 M H2SO4 and 1 M KOH, respectively. Moreover, it shows high durability over 25 hours and 50 hours in acidic and alkaline media, respectively. The excellent HER property of the asprepared sample can be summarized to the following reasons: spray-drying process can build CNT frameworks for supporting W2C to stop its aggregation and boost conductivity. The ligand effect of optimal content of metallic W in W-W2C/CNT hybrid and synergistic effect of porous nanoarchitecture can improve electron-transfer ability and maximal conductivity for HER under both acidic and alkaline electrolytes.

EXPERIMENTAL SECTION Synthesis of W-W2C/CNT electrocatalysts All the chemicals are analytical grade and no further purification. W-W2C/CNT were synthesized via a spray-drying process and followed by carbonization. In a typical procedure, Ammonium metatungstate hydrate (AMT, (NH4)6H2W12O40·xH2O) and water-based carbon nanotube conductive paste (CNT, 5 wt. %) were used as tungsten source and carbon source, respectively. AMT (1 mmoL) was dissolved in 1000 mL CNT solution (containing various amounts of CNT), and obtained black homogeneous suspension after ultrasonic treatment for 60 min. The precursor powders were prepared using the as-obtained suspension via a commercial spray-drying process. The temperature at inter of the spray dryer was 180 oC and the fluid moved at a speed of 1000 mL h-1. Later, the final product (W-W2C/CNT) was obtained after carbonizing the spray-dried powders in a tube furnace at 500 oC for 60 min, and at 800 oC for 180 min with a temperature ramping speed of 5 oC min-1 under an argon atmosphere. Notably, the spray-drying method followed by carbonization is resulting around 8 g product at

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a time (Figure S1), which suggests that this method is scalable for massive production. In order to investigate influence of W content presence in the W-W2C/CNT on HER activity, we have prepared the sample with different molar ratios of C/W (4, 6, and 8), which were denoted as W-W2C/CNT-4, W-W2C/CNT-6, and W-W2C/CNT-8, respectively. For comparison, the W-W2C/CNT-6 sample was annealed at different carbonization temperatures such as 700 oC and 900 oC. The W2C/CNT was prepared using the similar procedure described to W-W2C/CNT, except the change in molar ratio of C/W (10) and carbonization time (from 3 h to 5 h). Characterization The phases and crystallinity of W-W2C/CNT powders were determined on a Rigaku D/MAXrA diffractometer (XRD) by using Cu !I radiation ;JK/*1+ Å). The XPS experiments for characterizing the chemical compositions of sample were recorded on a Escalab 250Xi from ThermoFisher Scientific. The morphology and nanostructure of the products were performed using a FEI Inspect F50 (SEM) and a FEI Tecnai G2 F20 (TEM) with an accelerating voltage of 200 kV. The elemental mappings of the tungsten and carbon were obtained by the Energy Dispersive X-ray Spectroscopy. The pore-size distribution and specific surface area (BET) of samples were examined through N2 adsorption–desorption isotherms (TriStar II Plus). Electrochemical measurements The electrochemical measurements of the W-W2C/CNT for HER were tested on the CHI660D electrochemical station. A standard three-electrode setup under room temperature was used, where the graphite rod, glassy carbon electrode (GCE) and a Hg/Hg2Cl2 electrode (SCE, in 0.5 M H2SO4) (or Hg/HgO electrode in 1 M KOH) were used as the counter electrode, work

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diffraction (XRD) (Figure 2a). It is clearly observed that the characteristic diffraction peaks at 34.5°, 37.9° and 39.4° are corresponding to the (100), (002) and (101) crystal planes of W2C (PDF#35-0776), respectively. Three ,S peaks at 40.2°, 58.2° and 73.1° are indexed to the simulated cubic W peaks (PDF#893728), respectively. As shown in Figure 2b, when the carbonizing temperature decreases to the 700 oC, the main phases are WO2, metallic W, and no diffraction peak of W2C is observed. Besides, when the carbonizing temperature elevates to the 900 oC, the characteristic diffraction peak of WC (PDF#51-0939) is detected. According to the results are shown in Figure S2, the diffraction peaks corresponds to the metallic tungsten increases upon increase of annealing temperature from 800 oC to 900 oC, meanwhile—WC is generated. These results provide the support information of a solid-state reaction between CNT and (NH4)6H2W12O40·xH2O under 800 oC, and the metallic tungsten particles are being generated on the surface of W2C due to the decomposition of some W2C particles at high annealing temperature. These results concerning the synthesis of tungsten carbide can be described in the following sequences:

U

2

U

W2 U 7

(under the high

temperature).14,28 For comparison, the W-W2C particle without CNT-wrapping has synthesized was in Figure S3. What’s more, the Rietveld refinement method29,30 has been further put forward to determine the proportion of metallic W and W2C in the W-W2C/CNT catalysts. The mass ratio of metallic W to W2C were calculated to be 0.2568, 0.1325 and 0.0716 for WW2C/CNT-4, W-W2C/CNT-6 and W-W2C/CNT-8, respectively, indicating that tuning the content of CNT in starting materials are directly influences the content of metallic tungsten in the final product.

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a

b

C/W=6:1

W W2C

W-W2C/CNT-8

700 900

WC WO2

Intensity (a.u.)

(211) (200)

(103)

(110)

(200)

W-W2C/CNT-6 (102)

(100)

(002) (101) (110)

W-W2C/CNT-4

Intensity (a.u.)

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


W2C/CNT WO2(PDF#32-1393) WC (PDF#51-0939)

W2C (PDF#35-0776) W (PDF#89-3728)

20

40

60

2

80

20

degree

40

60

2

80

degree

Figure 2. (a) XRD patterns of the W-W2C/CNT catalysts; (b) XRD patterns of the samples (molar ratio of C/W =6) at different carbonization temperatures (700 oC and 900 oC).

The chemical environment and valence electron of the sample were characterized by X-ray photoelectron spectroscopy (XPS). According to the experimental results, the optimized WW2C/CNT-6 electrocatalyst was considered to be further characterized and discussed as it has the best HER performance. From the survey spectrum in Figure S4, it is obviously identified the existence of W and C elements. The W 4f spectrum of W-W2C/CNT-6 can be deconvoluted into six peaks (Figure 3a). The peaks at 31.1 and 33.2 eV are assigned to metallic tungsten (W0 4f7/2, W0 4f5/2).31,32 The binding energy of 32.0 and 34.1 eV are assigned to W-C bonds from W2C (W 4f7/2, W 4f5/2), which is known to be demonstrated as the active sites toward HER.21,22,33,34 The Peaks at high binding energy (35.9 eV and 38.0 eV) are attributed to tungsten oxide, resulting from the inevitable surface oxidation of tungsten carbide in the air.13 The Raman spectra (Figure S5) shows that no WO3 characteristic peaks can be observed and electrochemical tests (Figure S6) demonstrated that the tungsten oxide shows negligible active toward HER, which is similar to previous report on carbides when exposed to air.15,21 Moreover, 9



The C 1s spectrum of W-W2C/CNT-6 can be fitted into four peaks (Figure 3b): C-W (283.5 eV), C-C/C=C (284.5 eV), C-O (286.0 eV) and O=C-O (289.0 eV), and it is similar to previous reports.35-37

(a)

W 4f

C 1s

WOX

C=C/C-C

Intensity (a.u.)

WOX

(b)

W-C 4f7/2

W-C 4f5/2

Intensity (a.u.)

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

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W0 4f5/2

C-O

40

38

36

34

32

C-W

O=C-O

W0 4f7/2

30

292

290

Binding Energy (eV)

288

286

284

282

Binding Energy (eV)

Figure 3. The XPS high resolution spectrum of W 4f (a), and C 1s (b) of the W-W2C/CNT-6 electrocatalyst.

The scanning electron microscope (SEM) was carried out to examine the morphology and microstructure of the W-W2C/CNT-6 electrocatalyst. As displayed in Figure 4a and 4b, the WW2C/CNT-6 composites display the uniform distribution of microspheres without obvious particles aggregation. Moreover, the microspheres show the uneven size of porous texture due to the unique 3D CNT networks. The pure spray dried CNT has been examined from the images of SEM (Figure S7). According to the results, it can be inferred that the porous architecture, formed through spray-drying process, not only can expose much more active sites but also provide conductive path for charge transfer. The TEM images (Figure 4c and Figure S8) show that the ultrafine nanoparticles are embedded into the porous CNT networks. In addition, the N2 adsorption-desorption isotherm was provide to verify the existence of pores. Figure S9 10


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shows that existence of three different pore sizes (3 nm, 18 nm and 100 nm) in the WW2C/CNT-6 with a specific surface area of 44.2 m2 g-1, which is much larger than that of WW2C (9.4 m2 g-1, Figure S10). The HRTEM images (Figure 4d-4f) show that W-W2C ultrafine nanoparticles have grown on the walls of carbon nanotube. The chemical inertness of carbon nanotube is responsible for the formation of carbide ultrafine nanoparticles, and it can effectively prevent excessive growth of W2C particles.38,39 In addition, these particular small particles have been reported to exhibit excellent properties in the field of electrochemical energy storage.38-40 The clear lattice fringe around the nanoparticles with a distance of 3.4 Å are correspond to the (002) planes of CNT. The apparent lattice fringes of 2.2 Å and 2.3 Å are assigned to (110) plane of cubic metallic W and (101) plane of hexagonal W2C, respectively, which are consistent with the result of the XRD patterns. The EDS spectrum of W-W2C/CNT-6 (Figure S11) shows that the percentage of W and C is 46.01 wt. % and 45.22 wt. %, respectively. The existence of O elemental (8.70 wt. %) results from the inevitable oxygen absorption from air and little surface oxidation of tungsten carbide.15 Furthermore, the distribution of W and C elements in W-W2C/CNT-6 electrocatalyst has been performed on an energy dispersive X-ray (EDS) spectroscopy. Figure 4g-4i shows that W and C elementals are homogeneously dispersed in the carbonized electrocatalyst.

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of W-W2C particles has shown in Figure S14 and S15. And the mixture of commercial CNT and W (W/CNT) also shows the small current density in both acidic and alkaline solutions, which suggests that the mechanical mixture of commercial W and CNT has very poor catalytic activity (Figure S16). The commercial Pt/C (20 wt. %), W2C/CNT, pure CNT, and bare GCE were investigated for comparison. The LSV of all of samples were displayed in Figure 5a (0.5 M H2SO4) and Figure 5b (1 M KOH) without iR-correction. As expected, the Pt/C shows the excellent performance (onset overpotential of ~ 0 mV) for HER under both acidic and alkaline condition, while bare GCE displays inactive HER catalytic activity, which was consistent with previous reports.17,41 It is obviously found that the W2C/CNT, W-W2C/CNT-4, W-W2C/CNT-6 and W-W2C/CNT-8 all exhibit superior HER activity. Among them, the W-W2C/CNT-6 electrocatalyst displays the best HER performance with the smallest onset overpotential of 40 and 20 mV (vs. RHE), achieving the current density of 10 mV cm-2 at an overpotential ;X10) of 155 mV and 147 mV in acidic and alkaline solutions, respectively, which were lower than W2C/CNT, W-W2C/CNT-4 and W-W2C/CNT-8. Moreover, as shown in Figure S17, compared to commercial CNT, W, W/CNT and W2C/CNT, the W-W2C/CNT synthesized by spraydrying followed carbonization, shows the best HER performance in both acidic and alkaline solutions, which is mainly attributed to the unique nanoarchitecture and the synergistic effect of W-W2C/CNT electrocatalyst.

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To further study the intrinsic catalytic activities of HER, the Tafel slope is evaluated from the Tafel equation. As shown in Figure 5c and 5d, the Tafel slopes of the commercial Pt/C, W2C/CNT, W-W2C/CNT-4, W-W2C/CNT-6 and W-W2C/CNT-8 is 33 mV dec-1, 63 mV dec1,

61 mV dec-1, 56 mV dec-1, 58 mV dec-1 in 0.5 M H2SO4 solution, and 31 mV dec-1, 64 mV

dec-1, 59 mV dec-1, 51 mV dec-1, 56 mV dec-1 in 1 M KOH solution, respectively. It’s obviously found that the Tafel slope of W-W2C/CNT-6 electrocatalyst is the closest to the value of commercial Pt/C in 0.5 M H2SO4 and 1 M KOH solution. In addition, there are two widely reported HER mechanisms in acidic and alkaline media, which proceed by a primary discharge of H3O+ and the generation of Hads.35,42 In acidic media, the HER mechanism can be explained through the following three steps: Volmer-reaction

H3O+ + eYU Hads + H2O

(1)

Heyrovsky-reaction

H3O+ + eY + Hads U H2 Z + H2O

(2)

Tafel-reaction

2* Hads U H2 Z

(3)

In alkaline media, the HER mechanism can be explained through the following three steps: Volmer-reaction

H2O + eY U Hads + OHY

(4)

Heyrovsky-reaction

H2O + eY + Hads U H2 Z + OHY

(5)

Tafel-reaction

2* Hads U H2 Z

(6)

The small Tafel slopes of W-W2C/CNT-6 (the values are between 40 and 120 mV dec-1) indicate that the kinetics of HER carried out on W-W2C/CNT electrocatalyst are based on a Volmer–Heyrovsky process in both acidic and alkaline media, illuminating that the rate determining step is electrochemical desorption reaction. According to the Figure 5e and 5f, it is clearly found that the trend of performance indicators (the X10 and Tafel slope) for those

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catalysts, W-W2C/CNT-6 < W-W2C/CNT-8 < W-W2C/CNT-4 < W2C/CNT, suggests that the existence and weight of metallic tungsten play an important role in catalytic activity. The asprepared W-W2C/CNT-6 electrocatalyst exhibits the best HER activity and efficient kinetics of HER process in both acid and alkaline solutions. Encouragingly, the activities of those catalysts are comparable to most previous reported tungsten-based electrocatalysts (Table. S1).

Stability of HER electrocatalyst is a significant parameter in practical operation. The polarization curves of the W-W2C/CNT-6 electrocatalyst display negligible loss of currents after 2000 CV sweeps in both 0.5 M H2SO4 and 1 M KOH (Figure 6a and 6b), indicating its perfect stability during the long-term cycling test. Meanwhile, the chronoamperometric curves (I-t) of W-W2C/CNT-6 electrocatalyst are measured at overpotential of -230 mV (acidic electrolyte) and -180 mV (alkaline electrolyte), respectively. I-t plot (Figure 6c) shows that catalytic current density exhibits constant with a decrease of ~ 10% after 25 hours of testing in acidic solution. The loss of current density due to the bubbles gather and a little catalyst fall off from the surface of electrode. Additionally, Figure 6d depicts that current density shows negligible decay (from 16.13 to 14.12 mA cm-2) in alkaline over 50 hours. To further confirm the HER stability of electrocatalyst, the morphology and material composition of WW2C/CNT-6 after long-term HER stability test in acidic and alkaline media have been characterized by SEM and XRD. As shown in Figure S18-S20, the porous networks structure of W-W2C/CNT-6 hybrid keeps well-retained and no change in chemical composition, which confirms the excellent stability of W-W2C/CNT-6 during the long-term electrochemical process. The exceptional durability of W-W2C/CNT-6 in both acidic and alkaline condition is

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attributed to the specific structure of CNT network and the double-phases (metallic W and W2C), which exposed more active sites and prevent the particles from being aggregated.

0

-20

-40

initial after 2000 cycles

-60

-0.5

c

b

0.5M H2SO4

Current density (mA/cm2)


a

-0.4

-0.3

-0.2

-0.1

0

1M KOH

-20

-40

-60 initial after 2000 cycles

-80

0.0

-0.5

-0.4

Potential (V vs RHE)

d

0


0.5 M H2SO4


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


-20

-40

-60

-80 0

5

10

15

20

25

Time (h)

-0.3

-0.2

-0.1

0.0

Potential (V vs RHE) 0

1M KOH

-20

-40

-60

-80 0

10

20

30

40

50

Time (h)

Figure 6. (a, b) LSV curves for W-W2C/CNT-6 after 2000 CV cycles under air atmosphere in 0.5 M H2SO4 and 1 M KOH, respectively. Chronoamperometry curves (I-t) of W-W2C/CNT-6 in (c) 0.5 M H2SO4 and (d) 1 M KOH.

To estimate the electrochemical surface area (ECSA), the Cdl of the catalysts should be investigated. As shown in Figure S21 and S22, the cyclic voltammograms (CV) are measured in the range from 0.205-0.305 V (vs. RHE) in 0.5 M H2SO4 and 0.03-0.13 V (vs. RHE) in 1 M KOH. The Cdl values of the W-W2C/CNT-6 electrocatalyst are 18.15 mF cm-2 and 24.08 mF cm-2 in 0.5 M H2SO4 and 1 M KOH (Figure 7a and 7b), which are higher than the W-W2C/CNT8 (13.68 mF cm-2 and 11.29 mF cm-2), W-W2C/CNT-4 (10.04 mF cm-2 and 9.19 mF cm-2), and 17



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W2C/CNT (7.22 mF cm-2 and 5.59 mF cm-2), respectively. This result indicates that the WW2C/CNT-6 has the largest ECSA and much more active sites. What’s more, in order to further investigate the HER catalytic kinetics of the electrocatalyst, the electrochemical impedance spectra (EIS) measurement was performed with the frequency range of 105 to 0.01 Hz. Figure 7c and 7d depict the Nyquist plot semicircles of the four samples at -215 mV (vs. RHE) in 0.5 M H2SO4 and -200 mV (vs. RHE) in 1 M KOH. The charge-transfer resistance (Rct) of WW2C/CNT-6 (29.06

in 0.5 M H2SO4) is much smaller than those of W-W2C/CNT-8 (40.14

), W-W2C/CNT-4 (60.76 ), and W2C/CNT (72.98 ). Meanwhile, in the 1 M KOH solution, the Rct of W-W2C/CNT-6 (36.32 W2C/CNT-4 (56.94

) is also smallest than W-W2C/CNT-8 (41.5

), and W2C/CNT (69.87

), W-

). The small charge-transfer resistance

indicates that there is faster electron transfer between W-W2C/CNT catalyst and the electrode. The W/W2C dependence of Rct in acidic and alkaline solutions is shown in Figure S23. With increasing W/W2C from 0 to 13.25%, Rct obviously decreases due to the increase of conductive metallic W phase; when the W/W2C is further increased up to 25.68%, Rct increases because of the excess W content causes the decrease of the carbon contents, which is not beneficial to form a perfect conductive network, leading to the increase in Rct. As per results, with the rising of W content from 0 to 13.25%, both the increase (decrease) in conductivity (Rct) and the ligand effect of W and W2C, resulting in enhanced HER performance. When the ratio increases up to 25.68%, the content of active material W2C and the carbon in conductive network is decreased due to the excess W content, which leads to degradation of HER performance. Our research results are consistent with previous reports on similar metal phase/metal carbide material system.30,31,43

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4.0

a

b

W-W2C/CNT-4 W-W2C/CNT-6 W-W2C/CNT-8

2

j/2 (mA/cm2)

W2C/CNT 5 .1 18

2.4

m

1.6

cm F/

2

j/2 (mA/cm2)

3.2

/cm mF 2 8 .6 m 13 F/c m 04 2 10. cm mF/ 7.22

0.8

W-W2C/CNT-6 W-W2C/CNT-8

4

W2C/CNT

2

8 .0 24

3

m

cm F/

2 2 /cm /cm 9.19 mF mF 9 2 . 2 11 F/cm 5.59 m

2

0 0

c

W-W2C/CNT-4

5

1

0.0 40

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-Z" ( )

W2C/CNT

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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


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Figure 7. (a, b) Estimated Cdl and ECSAs for the catalysts in 0.5 M H2SO4 and 1 M KOH. (c, d) Nyquist plot semicircles at -215 mV in 0.5 M H2SO4 and -200 mV in 1 M KOH. (inset: fitting equivalent circuit).

Additionally, Figure 8 schematically illustrates the electrocatalytic process of WW2C/CNT in both acidic and alkaline media. The electrons can transfer through the carbon nanotubes to heterogeneous W-W2C nanoparticles attached to the conductive CNT networks. Hydronium ions or water molecule reduced by the transferred electrons at the exposed active sites of W-W2C nanoparticles are adsorbed on the surface (Hads), which is released as hydrogen gas (H2). Taking into account of this investigation, the excellent HER performance of the W-

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outstanding HER activity with a small onset overpotentials, a low Tafel slopes and excellent long-term durability in acidic and alkaline media. The superior HER activity of the WW2C/CNT electrocatalyst can be attribute to the unique porous structure, conductive CNT skeleton, the optimal content of metallic W ligand effect for the electronic structure and the synergistic effect in W-W2C/CNT. Furthermore, this work provides a facile and large-scale approach to synthesis the non-noble metal-based electrocatalysts through spray-drying process for low-cost hydrogen production.

ASSOCIATED CONTENT Supporting information Digital image, XRD, XPS, TEM, EDS, BET and pore size distribution of the catalyst; SEM images of CNT networks; LSV curves and corresponding Tafel plots of the samples at different carbonization, cyclic voltammetry curves of control groups, SEM and XRD after durability test, relationship between the metallic W content and the Rct of W-W2C/CNT electrocatalysts in both acidic and alkaline media; Table S1. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.C.). ORCID Yang Hu: 0000-0003-2731-335X Bo Yu: 0000-0003-3438-0761

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Manigandan Ramadoss: 0000-0001-9633-0220 Wenxin Li: 0000-0002-7061-6754 Dongxu Yang: 0000-0002-4753-1154 Bin Wang: 0000-0001-5455-1834 Yuanfu Chen: 0000-0002-6561-1684 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The research was supported by the National Natural Science Foundation of China (Grant Nos. 21773024, 51372033), and National High Technology Research and Development Program of China (Grant No. 2015AA034202).

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Scalable synthesis of heterogeneous W-W2C nanoparticles

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