Walnut-like Transition Metal Carbides with Three-Dimensional

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Walnut-Like Transition Metal Carbides with 3D Networks by a Versatile Electropolymerization-Assisted Method for Efficient Hydrogen Evolution Lixia Guo, Lvlv Ji, Jianying Wang, Shangshang Zuo, and Zuofeng Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07127 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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ACS Applied Materials & Interfaces

Walnut-Like Transition Metal Carbides with 3D Networks by a Versatile Electropolymerization-Assisted Method for Efficient Hydrogen Evolution Lixia Guo, Lvlv Ji, Jianying Wang, Shangshang Zuo and Zuofeng Chen*

Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China *[email protected] (Z.-F. C.)

KEYWORDS molybdenum carbides; tungsten carbides; electropolymerization; electrocatalysis; hydrogen evolution reaction

ABSTRACT Mo2C@NPC (N,P-doped carbon) electrocatalysts are developed on carbon cloth (CC) as binder-free cathodes for efficient hydrogen evolution through a facile route of electropolymerization followed by pyrolysis. Electropolymerization of pyrrole to form polypyrrole occurs with the homogeneous incorporation of PMo12 driven by Columbic force between the positively charged polymer backbone and PMo12 anions. This electrochemical synthesis is easily scaled up, requiring neither complex instrumentation nor intentionally added electrolyte (PMo12 also acts as electrolyte). After pyrolysis, the resultant Mo2C@NPC/CC electrode exhibits a unique interconnected walnut-like porous structure, which ensures strong adhesion between the active material and the substrate and favors electrolyte penetration into the

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electrocatalyst. This method is effective with other monomers such as aniline and is readily extended to fabricate other metal carbide electrodes such as WC@NPC/CC. These carbide electrodes exhibit high catalytic performance for hydrogen production, e.g., WC@NPC/CC can deliver an unprecedented current density of 600 mA cm‒2 at an overpotential of only 200 mV either in an acidic or an alkaline solution. Considering the simplicity, scalability and versatility of the synthetic method, the unique electrode structure and the excellent catalysis performance, this study opens up new avenues for the design of various novel binder-free metal carbide cathodes based on electropolymerization.

INTRODUCTION

Hydrogen, as a sustainable energy source and one of the most important industrial feed-stocks, has been intensively investigated to cope with the imminent intractable energy issues at present.1-3 A promising way to produce hydrogen is to split water electrochemically with efficient electrocatalysts, which could be coupled with renewable energy sources such as solar energy or wind energy.4-7 Platinum (Pt)-based materials are the most effective electrocatalysts toward the cathodic hydrogen evolution reaction (HER), but they are not applicable on a global-scale because of their scarcity and high cost.8 Therefore, the development of nonprecious metal-based electrocatalysts is urgently needed for practical applications.9 Transition metal carbides (TMCs) such as molybdenum carbides10-14, tungsten carbides15-19, and even bimetallic (e.g. NiMo) carbides20 have attracted extensive

2


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attention as high-performance HER electrocatalysts both in acidic and alkaline media due to their similar electronic structure and catalytic properties to the Pt-group metals. TMCs

are

usually

prepared

by

carbothermal

reduction,

in

which

the

high-oxidation-state metal sources such as metal oxides are reduced into metal carbides at high temperatures in the presence the carbon sources acting as the reducing agents.21-24 In this process, parts of the carbon are converted to CO or CO2 and the release of these gases creates a porous structure.11,25 Unfortunately, the high reaction temperature needed for the preparation of TMCs often induces extensive particle sintering, which leads to electrocatalysts with low surface areas.26 To enhance the HER activity, these materials have been combined with conductive supports, such as carbon nanotubes (CNTs),27-28 graphene,9,

29-31

and carbon black,32 which are

expected to prevent the catalyst materials from aggregating and thus increase the dispersion of catalyst active sites. However, these low-dimensional carbonaceous supports themselves are prone to entanglement or aggregation, which may risk negating all the advantages associated with the small nanoparticle size. In addition, the carbide materials reported previously were usually in a powdered form, which had to be immobilized on a conductive substrate (e.g., glassy carbon) by using binding agents.29,33-34 The binding agents such as Nafion may hamper the transport of electrons or protons, block the catalytically active sites, and thus decrease the overall activity. Polymers, such as polypyrrole (PPy)11 and polyaniline (PANI)35 are expected to be used as N-containing carbon sources, which can leave high contents of carbonaceous 3



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residues after pyrolysis. On the other hand, as early transition-metal oxygen anionic clusters with a polynuclear metal-oxo structure, polyoxometalates (POMs) are widely used as inorganic building blocks for the preparation of various hybrid materials.36-37 By using these two materials, we here report an electropolymerization-assisted method to synthesize well-defined walnut-like Mo2C on carbon cloth (CC) as highly active electrocatalysts for the HER. Carbon cloth is a fascinating substrate candidate,38-39 which has high conductivity, fine flexibility, and can be used in acidic media in comparison with other three-dimensional substrates, such as nickel foam.40-42 The polymeric hybrid precursors were first prepared by electropolymerization with a two-electrode setup in a beaker that was powered simply by a direct-current (DC) electrical source with no need to access complex instrumentation. Driven by the Columbic force, the PMo12 anions were in situ incorporated into the positively charged PPy framework during electropolymerization of Py. The pyrolysis of polymeric hybrid precursors with the assistance of oxygen species from the decomposition of PMo12 created porous walnut-like Mo2C dispersed in the N,P-doped carbon matrix (NPC) with high specific surface areas and sufficient catalytic sites. The binder-free Mo2C@NPC/CC electrode exhibited excellent electrocatalytic activity and stability for the HER and is superior to most MoxC-based HER electrocatalysts reported to date. The electropolymerization-assisted method is applicable to other monomers such as aniline and is readily extended to fabricate other metal carbide electrodes such as WC@NPC/CC. Thus, this study thus may open up a significant and exciting opportunity for fabricating various highly efficient metal 4


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carbide-based materials to replace Pt or Pt-based catalysts.

RESULTS AND DISCUSSION Electropolymerization-assisted preparation of molybdenum carbides. Mo2C materials on

CC were prepared by a facile two-step route, as demonstrated in Scheme 1 and detailed in the Experimental section in the Supporting Information. Briefly, a polymeric hybrid of PMo12-PPy was first synthesized on a CC substrate (PMo12-PPy/CC) through electropolymerization of pyrrole (Py) in the presence of added PMo12. The merits of this method are remarkable in several aspects. First, electropolymerization can be carried out by a simple two-electrode setup with a DC power supply, which greatly simplifies the instrumentation of electrocatalyst preparation. Second, in the presence of PMo12 anions as charge carriers, electropolymerization can be carried out without adding extra supporting electrolyte. This solution condition is also essential for the incorporation of PMo12 anions into the positively charged polymer backbone since the anions of the added electrolyte will cause the competitive incorporation of PMo12 anions. Third, the in situ doping of PMo12 by the Columbic force guarantees the uniform distribution (ideally at a molecular scale) of PMo12 in the polymer matrix and thus prevents Mo2C aggregation during pyrolysis. Last, the electropolymerization method has the advantage of integrating the synthesis of catalyst or catalyst precursor materials and subsequent film formation into one facile electrochemical step without the need for binding agents. 5



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Scheme 1. (A) Schematic illustration of the preparation of molybdenum and tungsten carbides via a two-step route for HER (EP: electropolymerization). (B) Photos of PMo12-PPy/CC (left) and Mo2C@NPC/CC (right) prepared by different electropolymerization times.

The as-prepared polymeric hybrid electrode was carburized under a flow of ultrapure argon at 900 °C for 3 h with a ramping rate of 5 °C min–1 to form the Mo2C@NPC/CC electrode. As shown in Scheme 1 and demonstrated later, the strategy can be applied to other monomers such as aniline (ANI); in addition, it can be readily extended to the preparation of WC@NPC/CC with PW12 anions as the tungsten source. The composition and structure of the as-prepared electrocatalysts were investigated by a series of techniques. The FT-IR spectrum of the PMo12-PPy polymeric hybrid precursor (Figure 1A) features a set of characteristic peaks of PMo12 located at 1049, 964, and 809 cm–1, indicating that PMo12 anions were in situ doped into the positively 6


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charged

PPy

matrix

during

electropolymerization.35

After

pyrolysis,

the

energy-dispersive X-ray (EDX) spectrum (Figure 1B) confirms the co-existence of Mo, C, N, P and O in the resultant material electrode. The XRD pattern of the electrode (Figure 1C) displays diffraction peaks at 34.4°, 38.1° and 39.6°, which correspond to (100), (002) and (101) crystal planes of hexagonal β-Mo2C (JCPDS No. 35-0787), respectively. There are no discernible impurity peaks for the molybdenum metal and the oxides. The Raman spectrum (Figure 1D) shows the fingerprint bands of β-Mo2C at 657, 816, and 989 cm–1, while the two distinct peaks at 1361 and 1583 cm–1 are attributed to the D-band and G-band of the carbon-based matrix.34, 40 X-ray photoelectron spectroscopy (XPS) measurements were carried out to probe the surface electronic state and composition. The survey XPS spectrum (Figure S1) shows the obvious signals of elemental Mo, C, N, P and O, which are consistent with the EDX spectrum. The high-resolution Mo 3d XPS spectrum (Figure 1E) was deconvoluted into six peaks, corresponding to Mo2+ (230.7 and 228.5 eV), Mo4+ (233.6 and 229.8 eV), and Mo6+ (232.6 and 236.0 eV) species, respectively. Mo2+ is related to the molybdenum carbides, which are known to serve as active sites for the HER,21, 43 while the presence of high-oxidation-state Mo4+ and Mo6+ is attributed to the molybdenum oxides due to the superficial oxidation of the Mo species.44 The main peak of the deconvoluted C 1s spectrum at 284.6 eV (Figure 1F) implies that graphite carbon is the main species.9,

45

The high-resolution N 1s XPS spectrum was

deconvoluted into peaks at 397.8, 399.5 and 401.0 eV (Figure 1G), which correspond to N in pyridinic N, and pyrrolic N and quaternary N in the carbon-based matrix, 7



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respectively.5,46-47 For P 2p XPS spectrum (Figure 1H), the signal at 133.5 eV is ascribed to P-C bonding states,48 and the peak at 134.7 eV is associated with P-O bonding states, arising from the oxidation of P species through air contact.49 The results of N 1s and P 2p XPS spectra of Mo2C@NPC/CC indicate that the carbon matrix is doped with nitrogen and phosphorus atoms, which may accordingly improve its surface properties, such as electric conductivity,35 surface polarity,50 and electron-donor affinity,51 leading to an enhanced HER activity. Especially, the N-doped carbon matrix has been reported to enhance the HER performance by decreasing the free energy of hydrogen adsorption (ǀ∆GH*ǀ) on MoxC through theoretical calculation.9,45 Together, all these results suggest the successful fabrication of β-Mo2C@NPC on a CC substrate by the electropolymerization-assisted strategy.

Figure 1. (A) FT-IR spectra of PMo12, PPy, and PMo12-PPy. (B) EDX spectrum of Mo2C@NPC/CC. (C) XRD patterns of CC and Mo2C@NPC/CC. (D) Raman spectra of CC and Mo2C@NPC/CC. High-resolution XPS of (E) Mo 3d, (F) C 1s, (G) N 1s (overlapped with Mo 3p), and (H) P 2p of Mo2C@NPC/CC. The surface morphology of the electrode was characterized by scanning electron 8


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microscopy (SEM) and transmission electron microscopy (TEM). The SEM image of the bare CC shows a 3D intricate network structure with woven carbon fibers (Figure 2A), which render the high electrical conductivity and a large surface area of the substrate electrode. After electropolymerization, the CC substrate was covered by a uniform and compact layer of PPy with incorporated PMo12 (Figure 2B). By pyrolysis at 900 °C, the polymeric hybrid precursors were transformed to nanosized walnut-like Mo2C on the CC (Figure 2C). There is no evidence of disconnection and desquamation of Mo2C on the CC, implying the high mechanical stability of the coating. The existence of PMo12 at a molecular scale in the polymer matrix allows the formation of the small opening well-proportioned walnut-like Mo2C material and effectively prevents Mo2C aggregation during pyrolysis.11,

49

The EDX elemental

mapping images of Mo2C@NPC/CC show the homogeneous distribution of the Mo, C, N and P elements within the electrode material (Figure 2D). The catalyst material was further evaluated by TEM, following the removal from the surface (Figure 2E). The TEM image exhibits interconnected nanoparticles dispersed in the carbon matrix, consistent with the SEM images. The high-resolution TEM (HRTEM) image reveals a lattice fringe with an interplanar distance of 0.23 nm, which corresponds well to the (101) crystal plane of Mo2C.45,52 In addition, the selected-area electron diffraction (SAED) pattern in the inset shows diffraction rings, which are consistent with the polycrystalline structure of the Mo2C nanoparticles.

9



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Figure 2. SEM images of (A) bare CC, (B) PMo12-PPy/CC, (C) Mo2C@NPC/CC; (D) EDX elemental mapping images of Mo, C, N and P of Mo2C@NPC/CC. (E) TEM and HRTEM images of Mo2C scraped from Mo2C@NPC/CC (Inset: SAED pattern). Electrocatalytic

HER

performance.

The electrocatalytic

performance of

Mo2C@NPC/CC toward the HER was measured in both acidic (0.5 M H2SO4) and alkaline (1 M KOH) solutions using a typical three-electrode configuration. As observed from the linear sweep voltammetric (LSV) curves in the acidic solution (Figure 3A), the commercial 20% Pt/C-CC electrode reveals a nearly zero onset overpotential consistent with the expected high HER activity,53 while the CC substrate exhibits a severely retarded catalytic onset consistent with the poor HER activity. By 10


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contrast, the Mo2C@NPC/CC electrode features the catalytic onset occurring at an overpotential of 80 mV, which is followed by a sharply increased catalytic wave. From the LSV curve, Mo2C@NPC/CC can deliver a current density of 420 mA cm–2 at an overpotential of 200 mV. The high HER performance with increasing current density at high overpotentials is attributed to efficient charge transfer and mass diffusion across the Mo2C@NPC/CC electrode. As a contrast electrode sample, the Mo2C@NPC composite was removed from the CC substrate and reloaded onto CC with Nafion as binding agent (denoted as Mo2C@NPC-CC). In Figure S2, it is clear that Mo2C@NPC-CC of the same mass loading exhibits inferior HER catalytic activity over Mo2C@NPC/CC, because Nafion may block the catalytically active sites and hamper the transport of electrons or protons. In the Tafel plot (Figure 3B), the linear portion was fitted to the Tafel equation (η = b logj + a, where η is the overpotential, b is the Tafel slope and j is the current density), yielding a small Tafel slope of 50.9 mV dec–1 for Mo2C@NPC/CC. For the parallel test of electrocatalysis in 1 M KOH (Figures 3C and 3D), Mo2C@NPC/CC features an onset overpotential of 170 mV and a Tafel slope of 70.2 mV dec–1. The low overpotentials and small Tafel slopes of Mo2C@NPC/CC in both acidic and alkaline media signify its superior HER activity. To probe the stability of Mo2C@NPC/CC, constant potential electrolysis (CPE) experiments were carried out at η = 180 mV in an acidic solution (Figure 3E) and at η = 230 mV in an alkaline solution (Figure 3F). The catalytic currents were sustained over 20 h and the catalyst degradation after electrolysis was very slight which may be 11



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due to the hindrance of the reaction by hydrogen bubbles remaining on the electrode.29 These results indicate that Mo2C@NPC/CC is a highly stable HER electrocatalyst in both acidic and alkaline solutions. To explain the high catalytic activity, the electrochemical double-layer capacitance (Cdl) was measured to investigate the electrochemically active surface area (ECSA) by cyclic voltammetry (CV) (note details in the Supporting Information). Based on CV curves and the plots of the double-layer charging current densities versus scan rates (Figure S3), the Cdl of Mo2C@NPC/CC was determined to be 926.5 mF cm–2. This value indicates a large ECSA and abundant catalytically active sites, which partially accounts for the high HER activity of Mo2C@NPC/CC. To study the conductivity of Mo2C@NPC/CC, electrochemical impedance spectroscopy (EIS) measurements were conducted at various overpotentials in 0.5 M H2SO4 (Figure S4). The EIS Nyquist plots exhibit two time-constants at all applied potentials.34 The first time constant at the high frequency domain is associated with the Warburg impedance, indicating the porous property of the 3D structure, while the second one at the low frequency domain signifies the charge transfer of the reaction. As seen, the charge transfer resistance is decreased with increasing the overpotential and the small charge transfer resistance of Mo2C@NPC/CC can be attributed to the high conductivity of the composite catalyst.

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0

B

CC 20% Pt/C-CC Mo2C@NPC/CC

20% Pt/C-CC Mo2C@NPC/CC

0.2

V 50.9 m

-200 η (V)

-2

j (mA cm )

A

-400

-1

dec

0.1 -1

29.4 mV dec

0.0

-600 -0.6

0

-0.4 -0.2 E (V vs. RHE)

-0.1 0.0

0.0

D

CC 20% Pt/C-CC Mo2C@NPC/CC

20% Pt/C-CC Mo2C@NPC/CC

0.3

0.1

m 70.2

V de

2.0

-1

c

-1

-400

32.4 mV dec

0.0 -600 -0.6

-0.4 -0.2 E (V vs. RHE)

0

-0.1 0.0

0.0

F

initial after CPE

0.5

0

-200

-200

-90 -120 -150

0

5

10 15 t (h)

20

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 E (V vs. RHE)

-60

-600

-2

-400

j (mA cm )

j (mA cm )

-2

-600

2.5

initial

-2

-2

-60

-400

1.0 1.5 2.0 -2 log (| j (mA cm )|)

after CPE

j (mA cm )

E

0.5 1.0 1.5 -2 log (|j (mA cm )|)

0.2

-200

η (V)

-2

j (mA cm )

C

j (mA cm )

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


-80 -100 -120

0

5

10 15 t (h)

20

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 E (V vs. RHE)

Figure 3. LSV curves (A, C) and Tafel plots (B, D) of the electrodes indicated in the figures in 0.5 M H2SO4 (A, B) or in 1 M KOH (C, D). LSV curves of Mo2C@NPC/CC before and after electrolysis in 0.5 M H2SO4 (E) or in 1 M KOH (F). Insets show long-term electrolysis curves of Mo2C@NPC/CC in 0.5 M H2SO4 under an overpotential of 180 mV (E) or in 1 M KOH under an overpotential of 230 mV (F). The effects of the fabrication conditions including polymerization time (tp) and applied current density (jp) on the catalyst performance were investigated. A series of material electrodes were prepared by applying a jp of ~ 10 mA cm–1 for different tp values and then applying different jp values for 10 min. In both cases, the loading masses of the PMo12-PPy precursor and the resultant Mo2C@NPC increase 13



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monotonously with increasing tp or jp (Figure S5). In contrast, the catalytic activity of the Mo2C electrode increases rapidly when tp or jp increases initially, and then it gradually decreases by further increasing tp or jp (Figures 4A-4D). With an optimized tp = 10 min and jp = 10 mA cm–2, the catalytic current density at the resultant Mo2C electrode is maximized. The decreasing HER performance of Mo2C@NPC/CC prepared with high tp and jp can be rationalized by the excessive loading of Mo2C@NPC on the CC that may block rapid electron transfer from/to the underlying CC substrate.9, 24 This assumption is supported by the results of the EIS measurements (Figures 4E and 4F), in which the changes in the semi-circle diameter of Mo2C@NPC/CC prepared by different tp and jp values follow the same trend as their catalytic performances.

14


B

0 -200

-2

-200

CC 20% Pt/C-CC 5 min 7 min 10 min 15 min

j (mA cm )

0

-2

A

-400

-0.6 0.3

-400

-0.6

0.0

D 0.3

20% Pt/C-CC -1 dec 5 min mV -1 117 dec 7 min mV 99.8 10 min -1 c 15 min 50.8 mV de -1

0.2 η (V)

-0.4 -0.2 E (V vs. RHE)

mV 76.3

0.1

dec

-0.4 -0.2 E (V vs. RHE)

0.1

-1

V 50.8 m

-1

E

0.5 1.0 1.5 -2 log (| j (mA cm )|)

2.0

0.0

F

5 min

m

0.5 1.0 1.5 -2 log (| j (mA cm )|)

2.0

-2

5 mA cm

-2

7 min

10 mA cm

200

-2

20 mA cm

-Z'' (ohm)

10 min

400

68.4

29.4 mV dec

0.0

600

dec -1

29.4 mV dec

0.0 0.0

0.0

20% Pt/C-CC -2 5 mA cm -1 -2 c 10 mA cm V de -1 4.0 m -2 ec 20 mA cm 8 mV d 75.6 -1 -2 c 30 mA cm V de

0.2 η (V)

C

CC 20% Pt/C-CC -2 5 mA cm -2 10 mA cm -2 20 mA cm -2 30 mA cm

-600

-600

-Z'' (ohm)

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


j (mA cm )

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15 min

200

-2

30 mA cm

100

0

0 0

100

200 Z' (ohm)

300

400

0

100

200 300 Z' (ohm)

400

Figure 4. (A, C, E) LSV curves, Tafel plots and Nyquist plots of Mo2C@NPC/CC prepared by different electropolymerization times (applied current density, 10 mA cm– 1

). (B, D, F) LSV curves, Tafel plots and Nyquist plots of Mo2C@NPC/CC prepared

by different applied current densities (electropolymerization time, 10 min). Electrolyte solution, 0.5 M H2SO4.

Electropolymerization with other monomers. To demonstrate the versatility of the electropolymerization-assisted method as a way to produce high-performance binder-free molybdenum carbide electrocatalysts, we prepared the polymeric hybrid precursor electrode with Py substituted by ANI monomers. The resultant precursor electrode was denoted as PMo12-PANI/CC. 15



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The comparison of FT-IR spectra of PMo12, PANI and PMo12-PANI (Figure S6A) indicates that PMo12 was in situ doped into the PANI framework during electropolymerization. After pyrolysis, the formation of Mo2C@NPC/CC is indicated by the EDX spectrum (Figure S6B), XRD pattern (Figure S6C), Raman spectrum (Figure S6D) and XPS data (Figure S7). Similarly, the Mo2C@NPC/CC electrode by PANI also exhibits a walnut-like structure as evidenced by the SEM images; in addition, the related EDX elemental mapping images reflect the uniform distribution of the Mo, C, N and P elements (Figure S8). The TEM image reveals interconnected nanoparticles dispersed in the carbon-based matrix (Figure S9A), and the HRTEM image further confirms the crystalline nature of Mo2C (Figure S9B), which features a lattice fringe with an interplanar distance of 0.23 nm for the (101) crystal plane of Mo2C. As expected, the Mo2C@NPC/CC electrode by PANI also exhibits an impressive HER activity with a low onset overpotential, small Tafel slope and long-time durability in both acidic and alkaline solutions (Figure S10). Similarly, the high catalytic performance of this Mo2C@NPC/CC electrode can be attributed to the large catalytically active surface areas as indicated by a large Cdl at ~ 805 mF cm–2 (Figure S11A) and the small charge transfer resistance as indicated by the EIS measurement (Figure S11B), both of which result from the interconnected walnut-like structure of Mo2C on the CC substrate.

Extension to the WC@NPC/CC electrode. Tungsten carbide is another important member of the TMCs, which has attracted extensive research attention in catalysis 16


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due to its d-band electronic structure which is similar to that of the Pt-group metals. We demonstrate here that tungsten carbide could also be prepared by the procedure similar to that for Mo2C@NPC/CC. Figure 5A shows the FT-IR spectrum of the PW12-PPy polymeric hybrid precursor prepared through electropolymerization of Py in the presence of added PW12. This spectrum features a set of characteristic peaks of PW12 located at 1080, 985, and 889 cm–1, indicating that the PW12 anions were in situ doped into the positively charged PPy matrix during electropolymerization. In Figure 5B, the XRD pattern of the material after being pyrolyzed reveals characteristic diffraction peaks of hexagonal WC located at approximately 31.5°, 35.6°, 48.3° and 64.0° (JCPDS No. 51-0939), which are consistent with those reported in the literature. Figure 5C shows the fingerprint bands of WC@NPC/CC at 800.6, 698.5, 321.2, 250.5 and 132.5 cm–1, which are consistent with that of commercially available WC powder. The two distinct peaks at 1361 and 1583 cm–1 are attributed to the D-band and G-band of the carbon-based matrix, respectively. XPS (Figure S12) measurements were also carried out to investigate the composition and structure of the as-prepared WC@NPC/CC electrode. All these data suggest that WC@NPC/CC was successfully synthesized by the electropolymerization-assisted method.

17



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Figure 5. (A) FT-IR spectra of PW12, PPy and PW12-PPy. (B) XRD pattern of WC@NPC/CC. (C) Raman spectra of CC, commercially available WC powder and WC@NPC/CC. (D) SEM image of PW12-PPy/CC; (E) SEM image and (F) HRTEM image (inset: SAED pattern) of WC@NPC/CC.

The SEM images (Figures 5D and 5E) show that PW12-PPy/CC with a compact film was converted to WC@NPC/CC consisting of interconnected walnut-like nanoparticles by pyrolysis, which is similar to that of Mo2C@NPC/CC. The corresponding EDX mapping images (Figure S13) demonstrate that elemental W, C, N and P were uniformly distributed within the WC@NPC/CC electrode. A HRTEM image of the material (Figure 5F) shows an interplanar spacing of 0.28 nm, which is identical to the (001) lattice plane of WC. As an efficient HER electrocatalyst, WC@NPC/CC exhibits impressive HER performance with low onset overpotentials, small Tafel slopes and long-time durability in both acidic and alkaline solutions (Figure 6A-D). For example, in acidic solution, the as-prepared WC@NPC/CC electrode exhibits a very low onset 18


Page 19 of 31

overpotential of 37.2 mV (10 mA cm–2) and a Tafel slope of 59.2 mV dec–1. From the LSV curves in both media, WC@NPC/CC can deliver a current density of 600 mA cm–2 at an overpotential of 200 mV. This performance is superior to most tungsten-based materials for the HER (Figure 7).16-17,24,31,54-59 For its unique walnut-like porous morphology, WC@NPC/CC has a large electrocatalytic active surface area with the Cdl of 615 mF cm–2 and a small charge transfer resistance (Figure 6E-F), which accounts for its high catalytic performance. A

B

CC

0

20% Pt/C-CC

CC

0

20% Pt/C-CC

-100 -150 -200

-600

0

5

10 15 t (h)

0.0

0.2

-200 0

5

10 15 t (h)

20

20% Pt/C-CC WC@NPC/CC

0.2 V dec

-1

29.4 mV dec

0.0

0.3

-1

-1

59.2 m

η (V)

η (V)

-150

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 E (V vs. RHE)

D

20% Pt/C-CC WC@NPC/CC

0.0

-100

-600

E (V vs. RHE)

0.1

-50

-250

20

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2

C

-400

-250 -300

WC@NPC/CC (after CPE)

-200 -2

-50

j (mA cm )

-2

j (mA cm )

-400

WC@NPC/CC (initial)

WC@NPC/CC (after CPE)

-200 j (mA cm-2)

-2

j (mA cm )

WC@NPC/CC (initial)

87.3

mV

dec

0.1 -1

c 32.4 mV de

0.0 -0.1 0.0

-0.1 0.5

2 - 30 mV s

1.5

F

-1

30

-20

20

e op Sl

=

-Z'' (ohm)

40

1.0

1.5

2.0

-2

20 0

0.5

log (| j (mA cm )|)

-2

-2

40

1.0 -2 log (| j (mA cm )|)

∆ j0.1(mA cm )

E j (mA cm )

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


23 1.

10

100 mV 110 mV 120 mV 130 mV 140 mV 150 mV

25 20 15 10 5

-1

Scan rate (mV s )

-40

0

0.0

0

10

20

0.1 0.2 0.3 E (V vs.RHE)

30

0.4

0 20

30

40 50 Z' (ohm)

60

70

Figure 6. (A, B) LSV curves of CC, WC@NPC/CC and 20% Pt/C-CC in 0.5 M H2SO4 and 1 M KOH, Insets: electrolysis curves of WC@NPC/CC under a static overpotential of 180 mV for 20 h. (C, D) Tafel plots of WC@NPC/CC and 20% 19



Page 20 of 31

Pt/C-CC in 0.5 M H2SO4 and 1 M KOH. (E) CVs of WC@NPC/CC between 0 - 0.2 V at scan rates from 2 - 30 mV s–1 in 0.5 M H2SO4, and capacitive currents at 0.1 V as a function of scan rates. (F) Nyquist plots of WC@NPC/CC at various overpotentials in 0.5 M H2SO4. -1

0

100

Tafel slope (mV dec ) 200 300 400

WN NW/CC

WSoyGnP

WS2/rGO

(Ref. 57)

(Ref. 55) (Ref. 54)

(Ref. 53)

WS2 nanoflake

(Ref. 52)

Tafel slop -2 η @10 mA cm

P-WN/rGO (Ref. 31) This work

0

100

500

(Ref. 56)

WS2/SNCF-3

W 2C/MWNT

WS2/CC

(Ref. 58)

200 300 400 -2 Overpotential at 10 mA cm

500

Figure 7. Comparison of the HER performance of our WC@NPC/CC electrode with recently reported W-based electrocatalysts in acidic solution.

CONCLUSIONS In summary, Mo2C@NPC electrodes were prepared on a flexible carbon cloth (CC) via

a

facile

route

of

electropolymerization

for

electropolymerization preparation

of

the

followed polymeric

by

pyrolysis. hybrid

The

precursor

PMo12-PPy/CC takes the advantage of the Columbic force between the positively charged polymer backbone and the PMo12 anions, which allows for the incorporation of PMo12 at a molecular scale. The electropolymerization also ensures the strong adhesion between the deposited material and the substrate, which facilitates the interfacial electron transfer. The resultant Mo2C@NPC/CC electrode exhibits a unique interconnected walnut-like porous structure, favoring electrolyte penetration 20


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into the hybrid electrode. As a result, the binder-free Mo2C electrocatalyst exhibits remarkable electrocatalytic performance in both acidic and alkaline solutions, making it among the best MoxC HER electrocatalysts. The strategy can be applied to other monomers such as aniline and can be readily extended for the preparation of other metal carbides such as WC for the efficient HER (see Tables S1 and S2). The simplicity and versatility of the synthetic method and the high performance of the electrocatalysts are appealing. The present electropolymerization-assisted strategy may open up new avenues for the design of various novel binder-free metal carbide electrocatalysts

for

the

high-performance

HER

and

other electrochemical

applications.

ASSOCIATED CONTENT Supporting Information. Experimental section and additional information as noted in the text.

AUTHOR INFORMATION Corresponding Author [email protected] (Z.-F. C.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

21



Page 22 of 31

This work was supported by the National Natural Science Foundation of China (21573160, 21872105), the Fundamental Research Funds for the Central Universities, and

the

Science

&

Technology

Commission

of

Shanghai

Municipality

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Entry for the Table of Contents

31


Walnut-like Transition Metal Carbides with Three-Dimensional

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