N,P-Doped Molybdenum Carbide Nanofibers for Efficient Hydrogen

Subscriber access provided by University of Chicago Library

Energy, Environmental, and Catalysis Applications

N,P-Doped Molybdenum Carbide Nanofibers for Efficient Hydrogen Production Lvlv Ji, Jianying Wang, Xue Teng, Huan Dong, Xiaoming He, and Zuofeng Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00363 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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

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

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

ACS Applied Materials & Interfaces

N,P-Doped Molybdenum Carbide Nanofibers for Efficient Hydrogen Production Lvlv Ji, Jianying Wang, Xue Teng, Huan Dong, Xiaoming He and Zuofeng Chen* Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China ABSTRACT Molybdenum (Mo) carbides-based electrocatalysts are considered promising candidates to replace Pt-based materials toward the hydrogen evolution reaction (HER). Among different crystal phases of Mo carbides, although Mo2C exhibits the highest catalytic performance, the activity is still restricted by the strong Mo-H bonding. To weaken the strong Mo-H bonding, creating abundant Mo2C-MoC interfaces and/or doping a proper amount of electron-rich (such as N and P) dopants into Mo2C crystal lattice are effective due to the electron transfer from Mo to surrounding C in carbides and/or N/P dopants. In addition, Mo carbides with well-defined nanostructures, such as one-dimensional nanostructure, are desirable to achieve abundant catalytic active sites. Herein, well-defined N,P-co-doped Mo2C/MoC nanofibers (N,P-MoxC NF) were prepared by pyrolysis of phosphomolybdic ([PMo12O40]3–, PMo12) acid doped polyaniline (PANI) nanofibers at 900 oC under an Ar atmosphere, in which the hybrid polymeric precursor was synthesized via a facile interfacial polymerization method. The experimental results indicate that the judicious choice of pyrolysis temperature is essential for creating abundant Mo2C-MoC interfaces and regulating the N,P-doping level in both Mo carbides and carbon matrixes, which leads to optimized electronic properties for accelerating HER kinetics. As a result, N,P-MoxC NF exhibits excellent HER catalytic activity in both acidic and alkaline media. It requires an overpotential of only 107 and 135 mV to reach a current density of 10 mA cm–2 in 0.5 M H2SO4 and 1 M KOH, respectively, which is comparable and even superior to the best of Mo carbide-based electrocatalysts and other noble-metal-free electrocatalysts. KEYWORDS molybdenum carbide, hydrogen evolution reaction, nanofibers, electrocatalysis, polyaniline

INTRODUCTION Hydrogen has been long regarded as a clean and sustainable alternative to traditional fossil fuels.1 The most promising pathway for hydrogen production is electrochemical water splitting, preferably when it is driven by renewable resources (such as solar, nuclear and wind) derived electricity.2 To lower the overpotential and save the energy cost, electrocatalysts are indispensable for the two half reactions of water splitting hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).3 Although Pt-based materials show highly efficient HER catalytic activities, their industrial applications are severely hindered due to the resource scarcity and high cost of Pt.4 Therefore, intensive efforts have been motivated to design

and synthesize efficient HER electrocatalysts based on earth-abundant elements.5-7 Molybdenum (Mo) carbides, an important member of early transition metal carbides (TMCs), have received ever-increasing research interests as a kind of high-performance HER electrocatalysts because their d-band electronic structures are similar to Pt-group metals.8 Synthesis of highly crystallized Mo carbides usually involves a step of the high temperature pyrolysis (usually ≥ 700 oC) of the hybrid precursor containing Mo- and carbon-based sources under an inert atmosphere.8 Because the carbothermal reduction often causes sintering of nanoparticles, most of reported Mo carbides are irregular in shape and very limited research progress has been made in obtaining Mo carbides with well-defined nanostructures, such as nanowires9, nanotubes10, nano-octahedrons11 and nanospheres12, etc. These Mo carbides are advantageous they could not only provide a large surface area to expose abundant catalytic active sites, but also facilitate electrolyte penetration and mass transport. The controllable synthesis of Mo carbides with optimized nanostructures is largely dependent on the design of the hybrid precursor of Mo- and carbon-based sources.8 Polyaniline (PANI), a kind of N-containing conducting polymer, is endowed with many advantages, such as facile synthesis route, low cost and environmental stability, etc.13 One-dimensional (1D) PANI can be successfully synthesized by interfacial polymerization13, electrochemical polymerization14 and template-assisted method15. Phosphomolybdic 3– ([PMo12O40] , PMo12) acid, a Keggin-type polyoxometalate (POM), exhibits both acidic and redox properties, which could protonate PANI during the polymerization process.16 By taking these advantages, PMo12 anions could be uniformly doped into the positively charged PANI matrix via coulombic interactions.16 It is thus becoming very appealing to synthesize PANI-PMo12 hybrid with 1D nanostructure as the precursor of 1D nanostructured Mo carbides. Among different crystal phases of Mo carbides, Mo2C exhibits the highest HER catalytic activity according to previous experimental reports, presumably because of the Pt-like Fermi level (Ef) energy in Mo2C.8,17 Density functional theory (DFT) calculations suggest that the free energy of hydrogen adsorption (∆GH*) on Mo2C is negative, indicating a strong Mo-H bonding on the Mo2C surface.9,18-20 It means that the initial proton reduction step (Volmer step) on Mo2C surface is promoted while the Hads desorption step (Heyrovsky/Tafel step) is contrarily restricted.18,21 In this regard, weakening the strong Mo-H bonding via the strategy of electron density regulation could

1

ACS Paragon Plus Environment


RESULTS AND DISCUSSION Preparation of hybrid polymeric precursor and Mo carbide electrocatalysts. The typical preparation procedure of N,P-MoxC NF is described in Scheme 1: (1) PANI NF-PMo12 precursor was first synthesized via a facile interfacial polymerization method (see Supporting Information for details).16 Aniline (ANI) polymerization was triggered on the organic-aqueous interface with the resultant PANI NF migrated to the aqueous phase. Simultaneously, PMo12 anions acting as counter ions were doped into the positively charged PANI matrix via Columbic interactions. Interfacial polymerization method is remarkable because it is a very general route to produce PANI NF with high yield and high quality over a broad range of temperatures, independent on any specific template or instrumentation.13 (2) The as-prepared PANI NF-PMo12 was then pyrolyzed under an Ar atmosphere at 900 oC for 2 h at a ramping rate of 5 oC min–1.

Scheme 1. Schematic synthesis route of N,P-MoxC NF. Figure 1A shows the FT-IR spectra of PANI NF, PANI NF-PMo12 and PMo12. In general, the FT-IR spectrum of PANI NF-PMo12 is dominated by the adsorption peaks of PANI. Moreover, a series of characteristic peaks of PMo12 are observed which are located at 1063, 953 and 882 cm–1, indicating that PMo12 are successfully doped into the positively charged PANI matrix as counter anions.16,24 Figures 1B and 1C show scanning electron microscopic (SEM) images of PANI NF and PANI NF-PMo12. Both PANI NF and PANI NF-PMo12 exhibit nanofiber structure with diameters of approximately 50 and 70 nm, respectively. EDX spectra reveal that PANI NF is doped with Cl– derived from HCl, while PANI NF-PMo12 is predominantly doped with PMo12 with only a trace amount of Cl– (Figure S1). Therefore, the larger diameter for PANI NF-PMo12 is probably attributable to the larger size of PMo12 compared with Cl–. In order to determine the doping level, UV-vis spectra of various PANI samples were recorded (Figure 1D). As expected, PANI NF of emeraldine base form (EB-PANI NF, undoped PANI), which was obtained by treating PANI NF in ammonia solution, shows characteristic absorption peaks of π-π* transition of benzenoid rings at 330 nm and n-π* transition of quinonoid rings at 624 nm.25 For PANI NF, the peak intensity of n-π* transition is greatly reduced and a small peak of polaron-π* transition appears at 455 nm, indicating that PANI is partly protonated by HCl during synthesis. For PANI NF-PMo12, the peak of n-π* transition disappears completely and the peak intensity of polaron-π* transition of acid-doped PANI is further increased. In addition, the peak of π-polaron transition with a long tail at 850 nm can be obviously observed because of the acid doping.26,27 These results indicate that PANI is fully protonated by PMo12 and PMo12 exhibits stronger doping ability than HCl, which may be due to the stronger Coulombic interactions between PANI and PMo12 (PMo12 anion has three negative charges and Cl– has a single negative charge). The corresponding formulas of these samples were shown in Figure 1E. Given the unique acid-doping mechanism of PANI, it can be speculated that PMo12 is doped into PANI matrix at a molecular level in the PANI NF-PMo12 sample.16 A

B

C

PANI NF

Transmittance (a.u.)

promote the HER kinetics.22,23 For example, the electron density around the Mo sites is closely relied on the surrounding C atoms in the crystal lattice and increasing C content would lead to a reduced electron density of Mo due to the electron transfer from Mo to C.21 As a result, MoC, with higher C content, usually has a weak Mo-H bonding and thus exhibits a promoted Heyrovsky/Tafel step but a hindered Volmer step.21 Considering the individual advantages of Mo2C and MoC on H adsorption and desorption, it is promising to achieve synergistically-enhanced Mo sites on the Mo2C-MoC interfaces.21 In addition, introducing a proper amount of electron-rich (such as N and P) dopants into Mo2C crystal lattice may also decrease the density of empty d-bands in Mo2C, thus weakening the strong Mo-H bonding of Mo2C.9,19 Taking these aspects together, we have rationally designed and synthesized novel PMo12-doped PANI nanofiber (PANI NF-PMo12) precursor, which led to Mo carbide nanofibers by pyrolysis under an Ar atmosphere. The strong Columbic interaction between PMo12 and PANI ensures the homogeneous distribution of PMo12 in the polymer matrix and prevents the aggregation of the resultant Mo carbide nanoparticles.24 Remarkably, the pyrolysis temperature could not only alter the crystal phases of Mo carbides, but also regulate the N,P-doping level in Mo carbides. By virtue of the delicate design of the hybrid precursor and judicious choose of the pyrolysis temperature, N,P-MoxC NF was successfully synthesized, which consists of numerous N,P-doped Mo2C/MoC nanoparticles with abundant Mo2C-MoC interfaces, embedded uniformly in the N,P-doped carbon matrix. The nanocomposite exhibits excellent HER catalytic activity in both acidic and alkaline media, outperforming most of the reported Mo carbide-based and other noble-metal-free HER electrocatalysts.

PANI NF-PMo12

♣

♣♣

PMo12 1600 1400 1200 1000 800

600

-1

Wavenumber (cm )

D ∗

Absorbance (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

Page 2 of 10

π-π

∗

n-π

EB-PANI NF PANI NF PANI NF-PMo12

E

∗

polaron-π

400

600

π-polaron

800

1000

Wavelength (nm)

Figure 1. (A) FT-IR spectra of PANI NF, PANI NF-PMo12 and PMo12. SEM images of (B) PANI NF and (C) PANI NF-PMo12. (D) UV-vis adsorption spectra of EB-PANI NF, PANI NF and PANI NF-PMo12. (E) General formulas of EB-PANI, PANI doped with

2


Page 3 of 10

HCl and PANI doped with PMo12. A

B NC NF

Intensity (a.u.)

Intensity (a.u.)

NC NF N,P-MoxC NF Mo2C

N,P-MoxC NF

MoC 20

C

30

2+

3+

Mo

70

80

500

1000

1500

4+

D

5+

Mo

Mo

Mo 3p N-Mo Pyridinic N Pyrrolic N Graphitic N

Intensity (a.u.)

238

236

234

232

230

228

404 402 400 398 396 394 392 Binding Energy (eV)

Binding Energy (eV)

P-Mo 2p3/2 P-Mo 2p1/2 P-C P-O

136

134

132

130

Binding Energy (eV)

128

F

C-P C=C C=N

Intensity (a.u.)

E

2000 -1

Raman Shift (cm )

Intensity (a.u.)

Mo 6+ Mo

40 50 60 2θ (degree)

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


290

288

286

284

(+2, +3, +4, +5 and +6) for Mo on the sample surface. Among them, the high-oxidation-state Mo4+/5+/6+ species are resulted from surface oxidation,32 while the low-oxidation-state Mo2+/3+ species with peaks at 228.0-229.0 eV (Mo 3d5/2) and 231.0-232.0 eV (Mo 3d3/2) are consistent with the presence of Mo in the carbide phase.21 In addition, a portion of Mo3+ species is presumably related to the formation of Mo-N or Mo-P bonding states.12,33 In Figure 2D, the high-resolution N 1s XPS spectrum can be deconvoluted into four peaks. The peaks located at 398.7, 400.0 and 401.7 eV are corresponding to the pyridinic N, pyrrolic N and graphitic N in carbon-based matrix, respectively; while the peak located at 396.5 eV is associated with the N-Mo bonding state.12 In Figure 2E, the main peaks of P 2p XPS spectrum are located at 133.5 and 134.3 eV, which are indexed to the bonding states of P-C and P-O, respectively. In addition, the doublet in the P 2p region (129.3 and 130.4 eV) can be assigned to P-Mo bonding state.12 In Figure 2F, the high-resolution C 1s XPS spectrum can be deconvoluted into three peaks located at 283.7, 284.6 and 285.8 eV, which can be assigned to the bonding states of C-P, C=C and C=N, respectively.34,35 Together, the XPS analysis of N,P-MoxC NF reveals that N and P are co-doped in both Mo carbides and carbon matrix.

282

Binding Energy (eV)

Figure 2. (A) XRD patterns and (B) Raman spectra of N,P-MoxC NF and NC NF. High-resolution XPS spectra of (C) Mo 3d, (D) N 1s (overlapped with Mo 3p), (E) P 2p and (F) C 1s of N,P-MoxC NF. Figure 2A shows the powder X-ray diffraction (XRD) patterns of the pyrolyzed samples. The NC NF sample obtained by pyrolysis of PANI NF at 900 oC exhibits two broad peaks located at approximately 24o and 44o, which are associated with the amorphous carbon-based materials.24 For the XRD pattern of N,P-MoxC NF, it consists of two kinds of Mo carbides - Mo2C of hexagonal phase (JCPDS No. 35-0787) and MoC of cubic phase (JCPDS No. 65-0280), respectively, with their crystal structures shown in Figure S2.24,28-31 The two pyrolyzed samples were further characterized by Raman spectroscopy, Figure 2B. Both samples exhibit two distinct peaks located at around 1348 and 1592 cm–1, attributed to the D-band and G-band of the carbon-based matrix, respectively.24 For N,P-MoxC NF, three additional peaks are observed at around 660, 816 and 991 cm–1, which are ascribed to the formation of Mo carbides.24 Assuming that Mo2C and MoC exist equally in N,P-MoxC NF, the weight percentage of Mo carbides in N,P-MoxC NF is estimated to be around 59.4% based on the thermogravimetric analysis (TGA), Figure S3. To probe the elemental compositions and oxidation states of N,P-MoxC NF, X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) measurements were conducted. The XPS survey spectrum of N,P-MoxC NF in Figure S4A exhibits the presence of P, Mo, C, N and O in this sample, consistent with the result of EDX in Figure S4B. Figure 2C shows the peak fitting of high-resolution Mo 3d XPS spectrum of N,P-MoxC NF, which suggests five oxidation states

Figure 3. (A) SEM, (B) TEM and (C) HRTEM image of N,P-MoxC NF. (D) SAED pattern of the nanoparticles in N,P-MoxC NF. (E) TEM-EDX elemental mapping images of C, Mo, N and P on N,P-MoxC NF. The morphology of N,P-MoxC NF was investigated by SEM and transmission electron microscopy (TEM). As shown in Figure 3A, the SEM image of N,P-MoxC NF exhibits that the morphology of nanofibers is well preserved after pyrolysis of PANI NF-PMo12 hybrid precursor at 900 oC, although the

3



surface becomes slightly roughened with the diameter slightly shrunken. These nanofibers stacked randomly to form a porous and loose structure, which could not only provide abundant catalytic sites but also facilitate the electrolyte penetration and hydrogen emission. The Brunauer-Emmett-Teller (BET) specific surface area of N,P-MoxC NF is 32.1 m2 g–1 as determined by the N2 sorption isotherms (Figure S5A). A major pore size distribution at around 2.5 nm is observed for N,P-MoxC NF (Figure S5B). The TEM image of N,P-MoxC NF in Figure 3B further confirms the nanofiber morphology. In addition, it can be observed that numerous nanoparticles are uniformly dispersed within the nanofibers. In Figure 3C, the high-resolution TEM (HRTEM) image of N,P-MoxC NF reveals the lattice fringes with d-spacings of 0.23 and 0.247 nm, which can be identified to the (101) and (111) crystal planes of Mo2C and MoC, respectively.24,28 These Mo2C/MoC nanoparticles are well embedded in the carbon matrix with an average size of 3.21 nm. The ultra-small particle size of Mo2C/MoC may be related to the unique acid-doping mechanism of PANI, which ensures the dispersion of PMo12 in PANI matrix at a molecular level and hence effectively prevents the aggregation and/or excessive growth of nanoparticles during the pyrolysis process. These ultra-small Mo2C/MoC nanoparticles are essential to the high catalysis performance by exposing more catalytic sites. The selected area electron diffraction (SAED) pattern in Figure 3D shows several bright rings that consist of discrete spots, which can be ascribed to crystal planes of Mo2C and MoC. Furthermore, TEM-EDX elemental mapping measurements were investigated to probe the elemental composition and distribution of N,P-MoxC NF (Figure 3E), illustrating that C, Mo, N and P are uniformly distributed on the sample surface. These results confirm further the successful synthesis of N,P-MoxC NF.

B

0 -20

0.6

122 mV dec

-40 20% Pt/C -60

20% Pt/C N,P-MoxC NF

0.2

31.9 mV dec

-0.3

-0.2

-0.1

0.0

0.0

0.5

D

0

8 6 4 2

-20 0.2

0.3

0.4

-4 -2

10

-20 -40 -60

20 40 60 80 100 -1 Scan rate (mV s )

0.5

E (V vs. RHE)

0.6

0.7

j (mA cm )

-2 -2

-2

∆ j0.3 (mA cm )

-15

-25

j (mA cm )

0

20-100 mV s

2.0

Initial After 1000 cycles

5

-1

1.5 -2

10

-5

1.0

log(|j (mA cm )|)

E (V vs. RHE)

-10

-1

-1

0.0

NC NF -0.4

65.1 mV dec

NC NF

N,P-MoxC NF

-80 -0.5

C

-1

0.4

η (V)

-2

j (mA cm )

A

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

-8 -12 -16 0

2

4

6 t (h)

8 10

-80 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 E (V vs. RHE)

Figure 4. (A) LSV curves and (B) Tafel plots of N,P-MoxC NF, NC NF and 20% Pt/C in 0.5 M H2SO4. (C) CVs of N,P-MoxC NF with different scan rates from 20 to 100 mV s–1 within the potential window of 0.2 - 0.4 V vs. RHE. Inset: the capacitive current at 0.3 V as a function of scan rate for N,P-MoxC NF. (D) LSV curves of N,P-MoxC NF before and after 1000 CV cycles in 0.5 M H2SO4. Inset: long-term electrolysis of N,P-MoxC NF in 0.5 M H2SO4 under an overpotential of 120 mV.

Electrocatalytic HER performance. A three-electrode system was adopted to evaluate the HER catalytic activities of N,P-MoxC NF with a mass loading of 0.265 mg cm–2 in 0.5 M

Page 4 of 10

H2SO4 at a scan rate of 2 mV s–1. For comparison, NC NF and commercial 20% Pt/C were examined with the same mass loading under identical experimental conditions. The linear sweep voltammetry (LSV) curves of the electrocatalysts are shown in Figure 4A. As expected, 20% Pt/C shows the highest HER catalytic activity with almost zero onset overpotential, whereas NC NF displays a far inferior catalytic activity with an onset overpotential of more than 350 mV. In comparison, N,P-MoxC NF exhibits a low onset overpotential of around 40 mV and an overpotential of 107 and 168 mV to obtain a current density of 10 and 80 mA cm–2, respectively. To our best knowledge, this performance is competitive or superior to the best of the previously reported Mo carbides-based HER electrocatalysts, such as β-Mo2C nanotubes,10 MoCx nano-octahedrons,11 nanoporous Mo2C NWs36 and Mo2C@C nanospheres,12 etc, in the same media (Table S1). In line with LSV results, Tafel plots were fitted by using Tafel equation (η = b log(j) + a, where η is the overpotential, b is the Tafel slope, j is the current density and a is the intercept) to elucidate the HER kinetics, Figure 4B. The Tafel slope of N,P-MoxC NF is 65.1 mV dec–1, which suggests that the HER process on the surface of N,P-MoxC NF probably proceeds via a Volmer-Heyrovsky mechanism.2 The corresponding exchange current density (j0) of N,P-MoxC NF is calculated to be 0.25 mA cm–2 by extrapolating from the Tafel plots, which is larger than most Mo carbides-based electrocatalysts (Table S1). To estimate the electrochemically active surface area (ECSA) of N,P-MoxC NF under the working conditions, the electrochemical double-layer capacitance (Cdl) at the solid-liquid interface was measured. Cyclic voltammetries (CVs) were performed in the potential range from 0.2 to 0.4 V vs. RHE at scan rates from 20 to 100 mV s–1 in 0.5 M H2SO4, Figure 4C. The Cdl of N,P-MoxC NF is calculated to be 38.3 mF cm–2, which is larger than those of Mo2C@NPC/NPRGO,18 Mo2C nanoparticles33 and Mo2C@C nanospheres12, etc, indicating a large ECSA and thus abundant catalytic active sites. To gain further insight into the HER catalytic activity of N,P-MoxC NF, electrochemical impedance spectroscopy (EIS) measurement was performed at various overpotentials in 0.5 M H2SO4 and the Nyquist plots of the EIS response are shown in Figure S6. These spectra display a typical two-time-constant behavior. The semi-circles in low-frequency region are associated with the HER charge transfer process on the surface of N,P-MoxC NF, which depends on the overpotentials.24,37 The diameters of semi-circles in low-frequency region decrease with an increase of overpotential, in accordance with the faster HER kinetics at larger overpotentials. To assess the durability of N,P-MoxC NF, continuous CV scans were conducted between -0.2 and 0.2 V vs. RHE at a scan rate of 100 mV s–1 in 0.5 M H2SO4. As shown in Figure 4D, the LSV curve of N,P-MoxC NF remains with a negligible activity degradation after 1000 cycles. The EDX spectra and XRD patterns of N,P-MoxC NF before and after the CV cycling test are shown in Figures S7A and 7B, which show no composition or phase change for the electrode sample. In addition, the durability of N,P-MoxC NF was further tested by long-term electrolysis under an overpotential of 120 mV. The inset in Figure 4D shows that the HER catalytic current is sustainable with a negligible current loss for more than 10 h. These results

4


rapid HER kinetics.21 For N,P-MoxC NF in this study, sufficient Mo2C-MoC interfaces should be present in the electrocatalyst because it is synthesized by in situ carbothermal reaction rather than by simply mixing Mo2C and MoC. Therefore, the Mo sites on Mo2C-MoC interfaces may play a crucial role to the excellent catalytic performance.

suggest that N,P-MoxC NF is a highly stable HER electrocatalyst in acidic media. Based on the above characterizations, the excellent HER catalytic activity of N,P-MoxC NF can be attributable to the following remarkable features. (i) The well-defined N,P-MoxC NF of nanofiber morphology could provide abundant catalytic active sites. (ii) A portion of N and P atoms co-doped into the Mo2C/MoC nanocomposite may enhance the catalytic activities of the adjacent Mo sites.12 Since no XRD characteristic peaks of Mo nitride or phosphide can be observed for N,P-MoxC NF, it can be inferred that these N and P atoms may occupy part of C sites in Mo2C/MoC crystal lattices.12 In addition, both N-Mo (396.5 eV) and P-Mo (129.3 and 130.4 eV) peaks of N,P-MoxC NF shifted negatively to the lower binding energy regions in comparison with those of N-Mo (397 eV) and P-Mo (129.8 and 130.6 eV) in previous reports.38,39 This observation indicates the presence of the electron transfer from Mo to N and P.12 For Mo2C, this is beneficial to moderate the strong adsorption of H on the adjacent Mo sites. For MoC, the negatively charged N and P are believed to act as bases to trap positively charged protons, which can serve as proton transport mediators to the vicinity of Mo sites that accelerate the HER.40,41 Moreover, the N and P co-doped carbon with high conductivity and hydrophilicity favors the electron transfer and electrolyte contact.24,42 (iii) There may exist a synergic enhancement effect between Mo2C and MoC. The Tang group’s study indicated that the catalytic activity of Mo2C is restricted by the strong Mo-H bonding, while the catalytic activity of MoC is contrarily prohibited by the weak Mo-H bonding.21 They suggest that a well-defined composition with sufficient Mo2C-MoC interfaces could provide Mo sites with optimized Mo-H bonding for the

700 C o

600 C Mo2C

(d)

Intensity (a.u.)

Intensity (a.u.)

o

(c)

MoO2 30

40 50 60 2θ (degree)

70

(a) 408

80

404

400

396

392

Binding Energy (eV)

C

MoO2 MoC

80 60 40 20

PANI NF PANI NF-PMo12

200

400

D

C=N

F

0 -10 -20

N-MoC NF N,P-MoC NF

-30 -40

C-P

N,P-MoxC NF N,P-Mo2C NF -0.5

-0.4

(d)

(d) Intensity (a.u.)

G

(b)

(a)

(a)

136 132 128 Binding Energy (eV)

124

292

288 284 Binding Energy (eV)

-0.2

-0.1

0.0

0.3 -1

79.6 mV dec

(c)

(b)

-0.3

E (V vs. RHE)

-1

77.9 mV dec

-1

67.1 mV dec -1 65.1 mV dec N-MoC NF N,P-MoC NF N,P-MoxC NF

0.2

0.1

140

800 1000 1200

T ( C)

C=C

P-C P-Mo 2p1/2 P-O P-Mo 2p3/2

(c)

600 o

(b)

MoC

20

100

MoC MoC & Mo2C Mo2C

o

800 C

E Weight (%)

o

900 C

Pyridinic N Pyrrolic N N-Mo Mo 3p Graphitic N

-2

B

o

1000 C

Effects of pyrolysis temperature and PMo12 content. The pyrolysis temperature is an important factor that can regulate the composition of electrocatalysts. Figure 5A shows the XRD patterns of samples obtained by pyrolyzing PANI NF-PMo12 precursor at 600-1000 oC. Obviously, the pyrolysis temperature could effectively alter the composition and crystal phase of the electrocatalysts. At 600 oC, the product is HER inert material MoO2 (JCPDS No. 32-0671) with a poor crystallinity.24 By elevating the temperature to 700 oC, a pure crystal phase of MoC appears which remains at 800 oC. When the temperature is further elevated to 900 oC, the optimized temperature adopted above, an additional crystal phase of Mo2C appears along with MoC. Finally, at a higher temperature of 1000 oC, the crystal phase of MoC disappears and only a pure crystal phase of Mo2C is present. In Figures 5B-D, XPS spectra of the pyrolyzed samples suggest that some N atoms were doped into MoC matrix at 700 oC (denoted as N-MoC NF), while both N and P atoms were doped into MoC at 800 oC (N,P-MoC NF) and Mo2C at 1000 oC (N,P-Mo2C NF). Moreover, the content of N and P in MoC/Mo2C matrixes is increased with increasing the pyrolysis temperature, while the content of N and P doped in the carbon matrix is contrarily decreasing. The Mo 3d XPS spectra of the samples pyrolyzed at different temperatures are shown in Figure S8.

η (V)

A

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


j (mA cm )

Page 5 of 10

280

N,P-Mo2C NF 0.5

1.0

1.5

2.0

2.5

3.0

-2

log(|j (mA cm )|)

Figure 5. (A) XRD patterns of the samples obtained by pyrolyzing PANI NF-PMo12 at different temperatures. (B) N 1s (overlap with Mo

5


ACS Applied Materials & Interfaces 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

Page 6 of 10

3p), (C) P 2p and (D) C 1s XPS of (a) N-MoC NF (700 oC), (b) N,P-MoC NF (800 oC), (c) N,P-MoxC NF (900 oC) and (d) N,P-Mo2C NF (1000 oC). (E) TGA curves of PANI NF and PANI NF-PMo12 under a N2 atmosphere. (F) LSV curves and (G) Tafel plots of the samples indicated in 0.5 M H2SO4. The nanofiber morphologies of the samples prepared at different temperatures are all well preserved, as evidenced by SEM images in Figure S9. However, in Figures S10 and S11, it is noted that the graphitization degree of carbon matrix and the average particle size of carbon-encapsulated MoC/Mo2C nanoparticles are found to increase in accompany with the increase of pyrolysis temperature. In Figure S12A, Raman spectra of these samples confirm further the increase of the graphitization degree for these samples. Besides, TGA curves in Figure S12B indicate that the MoC/Mo2C contents are increasing with an increase in the pyrolysis temperature, which is consistent with the atomic percentage of the electrocatalysts by EDX analysis, Table S2. The BET surface areas of the pyrolyzed samples are found to decrease with increasing the pyrolysis temperature (Figure S13A) but they all display a similar major pore size distribution at around 2.5 nm (Figure S13B). To make a clear comparison, Table S3 lists the characterization information of the pyrolyzed materials at different temperatures. In order to provide more insight into the relationship between the pyrolysis temperature and the formation process of Mo carbides, TGA measurements of PANI NF and PANI NF-PMo12 were conducted under a N2 atmosphere. As shown in Figure 5E, for PANI NF-PMo12, the initial weight loss below 150 oC is due to the loss of the absorbed water, and the gradual weight loss between 150 and 600 oC is due to the degradation of doped PMo12 to MoO2. A sharp weight loss due to the polymer chain decomposition and carbonation occurred between 650 - 750 oC. As discussed above, the process within this temperature range is accompanied with the formation of MoC crystal phase. Compared with PANI NF, a comparably high temperature for the decomposition and carbonation of polymer chains in the polymeric hybrid may be related to the strong interaction between PMo12 and PANI matrix.25 Beyond 750 oC, the gradual weight loss is attributable to the loss of N and P dopants in the carbon matrix, which is accompanied with a gradual graphitization process of residue carbon. Therefore, it is reasonable to speculate that the transformation from MoC to Mo2C may be closely related to the graphitization process of the residue carbon. The graphitization process at a high pyrolysis temperature (> 800 oC) may create a migration force to drag carbon atoms from MoC crystal lattice to the surrounding residue carbon, thus forming Mo2C crystal phase.

Table 1. Summary of the HER catalytic activity of N-MoC NF, N,P-MoC NF, N,P-MoxC NF and N,P-Mo2C NF in 0.5 M H2SO4. Sample

ηonset

η10

b

(mV)

(mV)

(mV dec )

(mA cm )

(mF cm ) 57.8

j0 –1

0.5 M H2SO4. As shown in Figure 5F, to obtain a current density of 10 mA cm–2, overpotentials of 204, 178, 107 and 139 mV are needed for N-MoC NF, N,P-MoC NF, N,P-MoxC NF and N,P-Mo2C NF, respectively. Accordingly, Tafel plots of these Mo carbides show that the catalytic activities follow the order of N,P-MoxC NF > N,P-Mo2C NF > N,P-MoC NF > N-MoC NF, Figure 5G. In addition, j0 values of the samples also follow the same order. Additional Cdl and EIS measurements of the samples are provided in Figures S14 and S15. A summary of the HER catalytic activities of these electrocatalysts is listed in Table 1. Comparing the HER catalytic activity of N-MoC NF with N,P-MoC NF, we find that N,P co-doped MoC shows higher catalytic activity than only N doped MoC. In addition, N,P-Mo2C NF exhibits superior catalytic activity to N,P-MoC NF. These results indicate that higher N,P-doping level and the crystal phase of Mo2C could lead to a higher catalytic activity. It is also interesting that N,P-MoxC NF shows an even higher catalytic activity than N,P-Mo2C NF, although its N,P-doping level are lower than the latter. These results indicate that a synergic enhancement effect between Mo2C and MoC may exist in N,P-MoxC NF, which constitute the optimized electronic properties of Mo sites on Mo2C-MoC interfaces and plays a predominant role on the excellent catalysis performance. The effect of the PMo12 feeding content on the nature of Mo carbides and related HER catalytic activities was also investigated. The contrast precursor sample PANI NF-PMo12# was synthesized under the identical conditions but with less amount of PMo12 (4 g). Similarly, this hybrid polymeric precursor was then pyrolyzed at different temperatures from 600 - 1000 oC. As shown in Figure S16, the XRD patterns exhibit that the pyrolyzed materials consist of MoO2 (600 oC), MoC (700 and 800 oC), MoC/Mo2C composite (900 oC), and Mo2C (1000 oC), respectively. These observations are consistent with those of high PMo12 feeding content (8 g) discussed above. Therefore, we conclude that the pyrolysis temperature is the predominant factor to regulate the composition and crystal phase of the pyrolyzed samples, while the loading content of PMo12 in PANI matrix has insignificant effect. A more detailed characterization and electrocatalytic test of N,P-MoxC# NF obtained at 900 o C was provided in Figures S17 and S18. We note although the higher PMo12 feeding content is in favor of the higher HER catalytic activity, the addition of excessive PMo12 (such as 12 g) during the precursor synthesis process is restricted by the solubility of PMo12 and saturation of PMo12 doping.

Cdl –2

–2

N-MoC NF

98

204

79.6

0.014

N,P-Mo2C NF

80

178

77.9

0.04

51.4

N,P-MoxC NF

40

107

65.1

0.25

38.3

N,P-Mo2C NF

63

139

67.1

0.081

21.4

The HER catalytic activities of the pyrolyzed samples at different temperatures were investigated by LSV technique in

6


Page 7 of 10

A

B

0

144.6 mV dec

η (V)

-40 20% Pt/C

-60

N,P-MoxC NF

-0.4

-0.3

-0.2

0.4

20% Pt/C N,P-MoxC NF

0.2

0.0

0.0

0.5

D

0

Initial After 1000 cycles -2

-40 -60

50

20 j (mA cm )

100

-20

-2

(d)

j (mA cm )

(e) (c)

150

0 -20 -40 0

0 Electrocatalysts

1.5 -2

(a) (b)

1.0

log(|j (mA cm )|)

300

200

-1

-1

0.0 -0.1

E (V vs. RHE)

250

57.1 mV dec

NC NF 33 mV dec

NC NF

-80 -0.5

C

-1

-20

-2

j (mA cm )

0.6

η 10 (mV)

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


2

4

6 t (h)

8 10

-80 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 E (V vs. RHE)

Figure 6. (A) LSV curves and (B) Tafel plots of N,P-MoxC NF, NC NF and 20% Pt/C in 1 M KOH. (C) Comparison of overpotentials required at 10 mA cm–2 (η10) for (a) N-MoC NF, (b) N,P-MoC NF, (c) N,P-MoxC NF, (d) N,P-Mo2C NF and (e) N,P-MoxC# NF in 1 M KOH. (D) LSV curves of N,P-MoxC NF before and after 1000 CV cycles in 1 M KOH. Inset: long-term electrolysis of N,P-MoxC NF in 1 M KOH under an overpotential of 150 mV.

Electrocatalysis in alkaline media. Because the state of the art OER electrocatalysts, such as NiFe-based hydroxides/oxyhydroxides,43,44 function efficiently and stably only in alkaline media, HER electrocatalysts that can operate in alkaline media is highly desirable to realize efficient water splitting.45 As shown in Figure 6A, 20% Pt/C still shows the highest HER catalytic activity and NC NF displays a far retarded catalytic activity in 1 M KOH. In contrast, N,P-MoxC NF exhibits a low onset overpotential of around 60 mV and an overpotential of 135 mV to reach a catalytic current density of 10 mA cm–2. Figure 6B shows that the Tafel slope of N,P-MoxC NF is 57.1 mV dec–1 and the corresponding j0 value is calculated to be 0.047 mA cm–2. As shown in Figure 6C, the catalytic performance of N,P-MoxC NF, as evidenced by their corresponding overpotentials at 10 mA cm–2, is superior to those of N-MoC NF, N,P-MoC NF N,P-Mo2C NF and N,P-MoxC# NF. The catalytic performance of N,P-MoxC NF is also outstanding compared to those of other Mo carbides-based electrocatalysts and recently reported noble-metal-free electrocatalysts in 1 M KOH, as listed in Table S4. In addition, the durability tests show that N,P-MoxC NF is also highly stable in 1 M KOH, Figure 6D.

CONCLUSION In summary, we have reported fabrication of N,P-MoxC NF via pyrolysis of the PANI NF-PMo12 precursor that was prepared by a facile interfacial polymerization method. The uniform dispersion of PMo12 in the PANI matrix by the Columbic interaction allows the formation of ultra-small Mo2C/MoC nanoparticles that are well-dispersed in the carbon matrix. The pyrolysis temperature plays a key role in regulating the composition and crystalline structure of Mo carbides, and the N,P-doping level in both Mo carbides and carbon matrix. The optimized exectrocatalyst N,P-MoxC NF obtained at 900 oC

holds several merits for HER applications: (i) the ultra-small N,P-doped Mo2C/MoC nanoparticles can provide abundant catalytic active sites; (ii) the N,P-doping in Mo carbides can optimize the electronic structure of Mo carbides toward the HER, while the N,P-doping in the carbon matrix can ensure efficient electron transfer and sufficient electrolyte contact; (iii) the synergistic effect between Mo2C and MoC contributes significantly to the high catalytic activity. As a result, N,P-MoxC NF exhibits excellent catalytic performance with low overpotentials of 107 and 135 mV for achieving a current density of 10 mA cm–2 in 0.5 M H2SO4 and 1 M KOH, respectively, which makes it among the best of Mo carbide-based electrocatalysts reported so far. The present strategy may open up new opportunities for fabrication and electronic structure regulation of other TMCs with unique nanofiber morphology for various catalysis applications.

Supporting Information. Detailed experimental procedure; EDX spectra and HRTEM images of PANI NF and PANI NF-PMo12; TGA curve, XPS survey spectrum, EDX spectrum, N2 adsorption/desorption isotherms, pore size distribution plot and Nyquist plots of N,P-MoxC NF; Mo 3d XPS spectra, SEM images, HRTEM images, N2 adsorption/desorption isotherms, particle size distribution plot, Raman spectra, TGA curves and electrocatalysis performance comparison of N-MoC NF, N,P-MoC NF, N,P-MoxC NF and N,P-Mo2C NF; XRD pattern, TGA curve and electrochemical data of N,P-MoxC# NF. AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21573160), the Fundamental Research Funds for the Central Universities, and Science & Technology Commission of Shanghai Municipality (14DZ2261100).

REFERENCES (1) Roger, I.; Shipman, M. A.; Symes, M. D., Earth-Abundant Catalysts for Electrochemical and Photoelectrochemical Water Splitting. Nat. Rev. Chem. 2017, 1, 0003. (2) Zou, X.; Zhang, Y., Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. (3) Li, X.; Hao, X.; Abudula, A.; Guan, G., Nanostructured Catalysts for Electrochemical Water Splitting: Current State and Prospects. J. Mater. Chem. A 2016, 4, 11973-12000. (4) Ji, L.; Wang, J.; Zuo, S.; Chen, Z., In Situ Preparation of Pt Nanoparticles Supported on N-Doped Carbon as Highly Efficient Electrocatalysts for Hydrogen Production. J. Phys. Chem. C 2017, 121, 8923-8930. (5) Yu, X. Y.; Feng, Y.; Jeon, Y.; Guan, B.; Lou, X. W.; Paik, U., Formation of Ni-Co-MoS2 Nanoboxes with Enhanced Electrocatalytic Activity for Hydrogen Evolution. Adv. Mater. 2016, 28, 9006-9011.

7



(6) Yang, X.; Feng, X.; Tan, H.; Zang, H.; Wang, X.; Wang, Y.; Wang, E.; Li, Y., N-Doped Graphene-Coated Molybdenum Carbide Nanoparticles as Highly Efficient Electrocatalysts for the Hydrogen Evolution Reaction. J. Mater. Chem. A 2016, 4, 3947-3954. (7) Lv, C.; Huang, Z.; Yang, Q.; Wei, G.; Chen, Z.; Humphrey, M. G.; Zhang, C., Ultrafast Synthesis of Molybdenum Carbide Nanoparticles for Efficient Hydrogen Generation. J. Mater. Chem. A 2017, 5, 22805-22812. (8) Miao, M.; Pan, J.; He, T.; Yan, Y.; Xia, B. Y.; Wang, X., Molybdenum Carbide-Based Electrocatalysts for Hydrogen Evolution Reaction. Chem. Eur. J. 2017, 23, 10947-10961. (9) Shi, Z.; Nie, K.; Shao, Z.-J.; Gao, B.; Lin, H.; Zhang, H.; Liu, B.; Wang, Y.; Zhang, Y.; Sun, X.; Cao, X.-M.; Hu, P.; Gao, Q.; Tang, Y., Phosphorus-Mo2C@Carbon Nanowires Toward Efficient Electrochemical Hydrogen Evolution: Composition, Structural and Electronic Regulation. Energy Environ. Sci. 2017, 10, 1262-1271. (10) Ma, F. X.; Wu, H. B.; Xia, B. Y.; Xu, C. Y.; Lou, X. W., Hierarchical β-Mo2C Nanotubes Organized by Ultrathin Nanosheets as a Highly Efficient Electrocatalyst for Hydrogen Production. Angew. Chem. Int. Ed. 2015, 54, 15395-15399. (11) Wu, H. B.; Xia, B. Y.; Yu, L.; Yu, X. Y.; Lou, X. W., Porous Molybdenum Carbide Nano-Octahedrons Synthesized via Confined Carburization in Metal-Organic Frameworks for Efficient Hydrogen Production. Nat. Commun. 2015, 6, 6512. (12) Chen, Y.-Y.; Zhang, Y.; Jiang, W.-J.; Zhang, X.; Dai, Z.; Wan, L.-J.; Hu, J.-S., Pomegranate-like N,P-Doped Mo2C@C Nanospheres as Highly Active Electrocatalysts for Alkaline Hydrogen Evolution. ACS Nano 2016, 10, 8851-8860. (13) Huang, J.; Kaner, R. B., A General Chemical Route to Polyaniline Nanofibers. J. Am. Chem. Soc. 2004, 126, 851-855. (14) Guo, Y.; Zhou, Y., Polyaniline Nanofibers Fabricated by Electrochemical Polymerization: A Mechanistic Study. Eur. Polym. J. 2007, 43, 2292-2297. (15) Li, X.; Tian, S.; Ping, Y.; Kim, D. H.; Knoll, W., One-Step Route to the Fabrication of Highly Porous Polyaniline Nanofiber Films by Using PS-b-PVP Diblock Copolymers as Templates. Langmuir 2005, 21, 9393-9397. (16) Yang, H.; Song, T.; Liu, L.; Devadoss, A.; Xia, F.; Han, H.; Park, H.; Sigmund, W.; Kwon, K.; Paik, U., Polyaniline/Polyoxometalate Hybrid Nanofibers as Cathode for Lithium Ion Batteries with Improved Lithium Storage Capacity. J. Phys. Chem. C 2013, 117, 17376-17381. (17) Wan, C.; Regmi, Y. N.; Leonard, B. M., Multiple Phases of Molybdenum Carbide as Electrocatalysts for the Hydrogen Evolution Reaction. Angew. Chem. Int. Ed. 2014, 126, 6525-6528. (18) Li, J. S.; Wang, Y.; Liu, C. H.; Li, S. L.; Wang, Y. G.; Dong, L. Z.; Dai, Z. H.; Li, Y. F.; Lan, Y. Q., Coupled Molybdenum Carbide and Reduced Graphene Oxide Electrocatalysts for Efficient Hydrogen Evolution. Nat. Commun. 2016, 7, 11204. (19) Liu, Y.; Yu, G.; Li, G. D.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X., Coupling Mo2C with Nitrogen-Rich Nanocarbon Leads to Efficient Hydrogen-Evolution Electrocatalytic Sites. Angew. Chem. Int. Ed. 2015, 54, 10752-10757. (20) Yang, T. T.; Saidi, W. A., Tuning the Hydrogen Evolution Activity of Beta-Mo2C Nanoparticles via Control of Their Growth Conditions. Nanoscale 2017, 9, 3252-3260.

Page 8 of 10

(21) Lin, H.; Shi, Z.; He, S.; Yu, X.; Wang, S.; Gao, Q.; Tang, Y., Heteronanowires of MoC–Mo2C as Efficient Electrocatalysts for Hydrogen Evolution Reaction. Chem. Sci. 2016, 7, 3399-3405. (22) Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I. B.; Norskov, J. K., Computational High-Throughput Screening of Electrocatalytic Materials for Hydrogen Evolution. Nat. Mater. 2006, 5, 909-913. (23) Michalsky, R.; Zhang, Y.-J.; Peterson, A. A., Trends in the Hydrogen Evolution Activity of Metal Carbide Catalysts. ACS Catal. 2014, 4, 1274-1278. (24) Ji, L.; Wang, J.; Guo, L.; Chen, Z., In Situ O2-Emission Assisted Synthesis of Molybdenum Carbide Nanomaterials as an Efficient Electrocatalyst for Hydrogen Production in Both Acidic and Alkaline Media. J. Mater. Chem. A 2017, 5, 5178-5186. (25) Su, C.; Ji, L.; Xu, L.; Zhu, X.; He, H.; Lv, Y.; Ouyang, M.; Zhang, C., A Novel Ferrocene-Containing Aniline Copolymer: Its Synthesis and Electrochemical Performance. RSC Adv. 2015, 5, 14053-14060. (26) Abdiryim, T.; Xiao-Gang, Z.; Jamal, R., Comparative Studies of Solid-State Synthesized Polyaniline Doped with Inorganic Acids. Mater. Chem. Phys. 2005, 90, 367-372. (27) Lu, X.; Zhang, W.; Wang, C.; Wen, T.-C.; Wei, Y., One-Dimensional Conducting Polymer Nanocomposites: Synthesis, Properties and Applications. Prog. Polym. Sci. 2011, 36, 671-712. (28) He, C.; Tao, J., Synthesis of Nanostructured Clean Surface Molybdenum Carbides on Graphene Sheets as Efficient and Stable Hydrogen Evolution Reaction Catalysts. Chem. Commun. 2015, 51, 8323-8325. (29) Guo, J.; Wang, J.; Wu, Z.; Lei, W.; Zhu, J.; Xia, K.; Wang, D., Controllable Synthesis of Molybdenum-Based Electrocatalysts for a Hydrogen Evolution Reaction. J. Mater. Chem. A 2017, 5, 4879-4885. (30) Anjum, M. A. R.; Lee, M. H.; Lee, J. S., BCN Network-Encapsulated Multiple Phases of Molybdenum Carbide for Efficient Hydrogen Evolution Reactions in Acidic and Alkaline Media. J. Mater. Chem. A 2017, 5, 13122-13129. (31) Youn, D. H.; Han, S.; Kim, J. Y.; Kim, J. Y.; Park, H.; Choi, S. H.; Lee, J. S., Highly Active and Stable Hydrogen Evolution Electrocatalysts Based on Molybdenum Compounds on Carbon Nanotube-Graphene Hybrid Support. ACS Nano 2014, 8, 5164-5173. (32) Cui, W.; Cheng, N.; Liu, Q.; Ge, C.; Asiri, A. M.; Sun, X., Mo2C Nanoparticles Decorated Graphitic Carbon Sheets: Biopolymer-Derived Solid-State Synthesis and Application as an Efficient Electrocatalyst for Hydrogen Generation. ACS Catal. 2014, 4, 2658-2661. (33) Ma, R.; Zhou, Y.; Chen, Y.; Li, P.; Liu, Q.; Wang, J., Ultrafine Molybdenum Carbide Nanoparticles Composited with Carbon as a Highly Active Hydrogen-Evolution Electrocatalyst. Angew. Chem. Int. Ed. 2015, 54, 14723-14727. (34) Zhang, W.; Huang, H.; Li, F.; Deng, K.; Wang, X., Palladium Nanoparticles Supported on Graphitic Carbon Nitride-Modified Reduced Graphene Oxide as Highly Efficient Catalysts for Formic Acid and Methanol Electrooxidation. J. Mater. Chem. A 2014, 2, 19084-19094. (35) Li, W.-J.; Chou, S.-L.; Wang, J.-Z.; Liu, H.-K.; Dou, S.-X., Significant Enhancement of the Cycling Performance and Rate

8


Page 9 of 10 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


Capability of the P/C Composite via Chemical Bonding (P–C). J. Mater. Chem. A 2016, 4, 505-511. (36) Liao, L.; Wang, S.; Xiao, J.; Bian, X.; Zhang, Y.; Scanlon, M. D.; Hu, X.; Tang, Y.; Liu, B.; Girault, H. H., A Nanoporous Molybdenum Carbide Nanowire as an Electrocatalyst for Hydrogen Evolution Reaction. Energy Environ. Sci. 2014, 7, 387-392. (37) Huang, Z.; Chen, Z.; Chen, Z.; Lv, C.; Meng, H.; Zhang, C., Ni12P5 Nanoparticles as an Efficient Catalyst for Hydrogen Generation via Electrolysis and Photoelectrolysis. ACS Nano 2014, 8, 8121-8129. (38) Cao, B.; Veith, G. M.; Neuefeind, J. C.; Adzic, R. R.; Khalifah, P. G., Mixed Close-Packed Cobalt Molybdenum Nitrides as Non-Noble Metal Electrocatalysts for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 19186-19192. (39) Xing, Z.; Liu, Q.; Asiri, A. M.; Sun, X., Closely Interconnected Network of Molybdenum Phosphide Nanoparticles: a Highly Efficient Electrocatalyst for Generating Hydrogen from Water. Adv. Mater. 2014, 26, 5702-5707. (40) Liu, P.; Rodriguez, J. A., Catalysts for Hydrogen Evolution from the [NiFe] Hydrogenase to the Ni2P(001) Surface: the Importance of Ensemble Effect. J. Am. Chem. Soc. 2005, 127, 14871-14878. (41) Yan, H.; Tian, C.; Wang, L.; Wu, A.; Meng, M.; Zhao, L.; Fu, H., Phosphorus-Modified Tungsten Nitride/Reduced Graphene Oxide as a High-Performance, Non-Noble-Metal Electrocatalyst for the Hydrogen Evolution Reaction. Angew. Chem. Int. Ed. 2015, 54, 6325-6329. (42) Zheng, Y.; Jiao, Y.; Li, L. H.; Xing, T.; Chen, Y.; Jaroniec, M.; Qiao, S. Z., Toward Design of Synergistically Active Carbon-Based Catalysts for Electrocatalytic Hydrogen Evolution. ACS Nano 2014, 8, 5290-5296. (43) Wang, J.; Ji, L.; Chen, Z., In Situ Rapid Formation of a Nickel–Iron-Based Electrocatalyst for Water Oxidation. ACS Catal. 2016, 6, 6987-6992. (44) Wang, J.; Ji, L.; Zuo, S.; Chen, Z., Hierarchically Structured 3D Integrated Electrodes by Galvanic Replacement Reaction for Highly Efficient Water Splitting. Adv. Energy Mater. 2017, 7, 1700107. (45) Mahmood, N.; Yao, Y.; Zhang, J.-W.; Pan, L.; Zhang, X.; Zou, J.-J., Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes: Mechanisms, Challenges, and Prospective Solutions. Adv. Sci. 2017, 1700464.

9



Page 10 of 10

Graphical Abstract

10


N,P-Doped Molybdenum Carbide Nanofibers for Efficient Hydrogen

Recommend Documents