In situ formation of N-doped carbon-coated porous MoP nanowires: a highly efficient electrocatalyst for hydrogen evolution reaction in a wide pH range

Journal Pre-proof In Situ Formation of N-Doped Carbon-Coated Porous MoP Nanowires: A Highly Efficient Electrocatalyst for Hydrogen Evolution Reaction in a Wide pH Range Chaoran Pi, Chao Huang, Yixuan Yang, Hao Song, Xuming Zhang, Yang Zheng, Biao Gao, Jijiang fu, Paul K Chu, Kaifu Huo

PII:

S0926-3373(19)31104-X

DOI:

https://doi.org/10.1016/j.apcatb.2019.118358

Reference:

APCATB 118358

To appear in:

Applied Catalysis B: Environmental

Received Date:

20 July 2019

Revised Date:

28 October 2019

Accepted Date:

29 October 2019

Please cite this article as: Pi C, Huang C, Yang Y, Song H, Zhang X, Zheng Y, Gao B, fu J, Chu PK, Huo K, In Situ Formation of N-Doped Carbon-Coated Porous MoP Nanowires: A Highly Efficient Electrocatalyst for Hydrogen Evolution Reaction in a Wide pH Range, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118358

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In Situ Formation of N-Doped Carbon-Coated Porous MoP Nanowires: A Highly Efficient Electrocatalyst for Hydrogen Evolution Reaction in a Wide pH Range

Chaoran Pi,a,1 Chao Huang,b,1 Yixuan Yang,a Hao Song,a Xuming Zhang,a* Yang Zheng,a* Biao Gao,a,b Jijiang fua, Paul K Chub* and Kaifu Huoa

The State Key Laboratory of Refractories and Metallurgy, Institute of Advanced

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a

Materials and Nanotechnology, Wuhan University of Science and Technology, Wuhan 430081, China. b

Department of Physics, Department of Materials Science & Engineering, and

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Department of Biomedical Engineering, City University of Hong Kong, Tat Chee

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Avenue, Kowloon, Hong Kong, China

*

Corresponding authors:

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E-mail: [email protected] (X.M. Zhang); [email protected] (Y. Zheng);

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[email protected] (P.K. Chu)

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Graphical Abstract

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Highlights： ●

Porous MoP@NC NWs was prepared by phosphorization of organic-inorganic

hybrid NWs. The in-situ formed NC with lots of micropores facilitates the H+ absorption.

●

MoP@NC owns fast ion/electron transport, more active sites, and synergistic effects.

●

MoP@NC exhibits excellent HER activity and stability in wide pH range media.

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●

Abstract: Molybdenum phosphide (MoP) is a promising non-noble-metal

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electrocatalyst for hydrogen evolution reaction (HER), but coordination of high HER Herein, we proposed a highly efficient

lP

activity and fast HER kinetics is a challenge.

HER electrocatalyst composed of N-doped carbon-coated porous MoP nanowires

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(MoP@NC) by phosphorization of organic-inorganic Mo3O10(C6H8N)2·2H2O NWs. The MoP@NC with mesopores MoP framework and micropores N-doped carbon-

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coating facilitates the electrolyte/bubbles diffusion and ion/electron transport, owns

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abundant active sites and smaller adsorption free energy of H (ΔGH*) at the interface boosting the HER performance. Small overpotentials (η10) of 96, 191 and 149 mV with Tafel slopes of 49.2, 95.0 and 61.7 mV dec−1 in acidic, natural and alkaline solutions were achieved in addition to excellent stability, which is better than that of bare MoP NWs and other MoP-based electocatalysts.

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The results reveal a simple and

effective approach to produce inexpensive electrocatalysts with excellent HER properties in a wide pH range.

Keywords: Molybdenum phosphide; Hydrogen evolution reaction; Nitrogen-doped

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lP

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carbon; Inorganic-organic hybrid nanowire; Wide pH range;

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1. Introduction Hydrogen with high energy density and environmentally friendly is a potential substitute for traditional fossil fuels to combat global warming and pollution [1, 2] and electrochemical splitting of water via the hydrogen evolution reaction (HER) is an attractive technique to mass produce hydrogen [3]. Commercially, Pt and Pt-group metals with a nearly zero overpotential and high electrocatalytic activity are widely

widespread adoption [4].

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used as electrocatalysts in HER, but the high cost and natural scarcity limit more Hence, development of non-noble metal electrocatalysts

such as metal chalcogenides, carbides, borides, and nitrides, as well as non-metal

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catalysts is critical [5] and economical HER electrocatalysts with high efficiency, low

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overpotential, superior stability are vital to renewable energy generation, albeit

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challenging.

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Transition metal phosphides (TMPs) with different nanostructure have attracted tremendous interests due to their excellent catalytic performance toward HER [6-13].

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Among them, molybdenum phosphide (MoP) was regarded as the promising candidates because of their natural abundance, efficient activity and feasibility in a wide pH range

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electrolyte [7, 9].

The electrocatalytic reaction occurring on TMPs relies on the

trapping effect of negatively-charged P atoms towards positive protons to lower the HER barrier [14-16] and TMPs with a large P concentration can elevate the HER activity [17, 18].

However, the electrical conductivity of TMPs depends on the

concentration of P.

A larger P content converts the transition metal phosphide from 4

being metallic to semi-conducting or non-conducting resulting in sluggish HER kinetics [15].

To overcome this hurdle, reducing the size of the TMPs crystals to create more

active sites and increase the electron transfer ability are possible strategies [6]. Combining nanosized TMPs with a carbon substrate such as carbon nanotubes, graphene, and active carbon, is a viable technique to enhance the conductivity and electrochemical stability while maintaining the large number of active sites [19].

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However, the pure carbon matrix exhibits inert activity and poor wettability in an

aqueous electrolyte [20]. Heteroatom dopants such as N, S, and P introduced into the carbon matrix can induce an uneven spin density and charge distribution around

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neighboring carbon atoms thereby producing not only enhanced H+ absorption, but also

[21-25].

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superior wettability between the carbon sites and hydronium ions for enhanced HER For example, ultra-fine TMPs with small phosphorus concentrations (Ni2P,

lP

Co2P, Cu3P, Fe2P) in N-doped carbon nanofibers were prepared by electrospinning and

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subsequent pyrolysis in H2 and exhibits good HER performance at all pH values [26]. Bimetallic Ni2-xCoxP embedded N-doped carbon nanofibers via electrospinning

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followed by pyrolysis under an inert flow afford low overpotentials for water splitting in a wide pH range [27].

OsP2 nanoparticles (NPs) dispersed over N, P co-doped

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carbon film using a combination of template and pyrolysis methods, shows superior HER performance in acid, neutral and alkaline solutions as a result of the strong interaction between OsP2 and carbon matrix [28] and other nanosized TMPs decorated non-metal-doped carbon substrates have been proposed [29-32].

These non-metal

doped carbon supported TMPs electrocatalysts exhibit superior HER performance, but 5

some fabrication procedures for phosphides/carbon composites invariably involve multiple steps and furthermore, these conventional composites of TMPs NPs embedded in the one-dimensional carbon or two-dimensional carbon could be confronted with reduced effective active area or loose combination thus undermining the electrocatalytic efficiency and durability, meanwhile, a too thick carbon covered on the TMPs NPs could go against the fast electron transfer between TMPs and carbon matrix Previously, our group

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and subsequently obstruct the synergistic effects [33, 34].

reported hybrid electrocatalyst composed of MoP nanoflakes dispersed between

nitrogen-doped graphene, the sandwiched structure provides abundant edge sites of

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MoP and enables fast electron transport delivering excellent electroactivity and

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durability for HER [19]. However, the rapid diffusion of electrolyte and evolved gas is another important factor for an advanced electrocatalyst.

To overcome the

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drawbacks mentioned above, it could be an alternative strategy to create a porous TMPs

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framework with thin non-metal-doped carbon coating, the novel catalyst could provide robust structure for long term stability, more active sites and fast electrolyte and

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electron migration and enhanced overall conductivity and corrosion resistance resulting from the N-doped carbon shell as well as strong electron interaction between the TMPs

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core and thin carbon shell boding well highly efficient HER.

Herein, we proposed a facile method to prepare thin N-doped carbon-coated porous MoP nanowires (MoP@NC NWs) by phosphating the inorganic-organic hybrid precursor NWs (Mo3O10(C6H8N)2·2H2O). 6

The synthesis involves a mild

solvothermal reaction in (NH4)6Mo7O24·4H2O and aniline in an acidic solution (pH 45) [35].

After phosphating, the inorganic component is converted into mesoporous

MoP NWs composed of connected NPs and the organic component in the precursor is in situ carbonized into N-doped carbon (NC) with lots of micropores and a thickness of about 2 nm uniformly coated on the porous MoP NWs.

The robust MoP@NC hybrids

owns rapid ion/electron transport, abundant active sites and smaller ΔGH* at the

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interface than the bare NC and MoP boosting the HER performance, which deliver

excellent HER performance in acidic, natural and alkaline solutions as examples, small overpotentials of 96, 191 and 149 at 10 mA cm-2, low Tafel slopes of 49.2, 95.0 and

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2. Experimental Section

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61.7 mV dec-1 as well as robust stability.

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2.1 Synthesis of porous MoP@NC NWs

All the chemicals were purchased from Sigma-Aldrich and put to use without The DI water was used throughout the experiment.

The

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purification.

Mo3O10(C6H8N)2·2H2O NWs were synthesized by a hydrothermal reaction described

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previously [36]. Firstly, 1.24 g of (NH4)6Mo7O24·4H2O were dissolved into 15 mL of deionized water and 1.67 g of aniline were added.

Afterwards, 1.0 M HCl was added

dropwise to the solution under magnetic stirring at room temperature until a white precipitate was formed (pH = 4-5). After keeping at 50 °C for 2 h, the product was filtered, thoroughly washed with ethanol, and dried at 60 7

o

C.

The

Mo3O10(C6H8N)2·2H2O NWs

were then placed downstream

from

hypophosphite (NaH2PO2) at a distance of 20 cm in the furnace tube.

sodium After

phosphating at 900 °C for 2 h under Ar flow, the porous MoP@NC NWs were obtained after cooling down naturally to room temperature.

2.2 Synthesis of MoP NWs and NC NWs In brief, 10 mL of

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The MoO3 NWs were synthesized by hydrothermal method.

DI water and 20 mL of 30% (wt%) H2O2 were mixed uniformly in a beaker, and then 2 g of molybdenum power was slowly added into the solution until the solution change

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to yellow. The solution was stirred for 30 min to dissolve completely and transferred

After cooling to

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to Teflon-lined stainless-steel autoclave and kept at 220 °C for 3 h.

room temperature, the MoO3 NWs was obtained. After phosphating the MoO3 NWs

lP

under the same conditions, MoP NWs was obtained.

To obtain the NC NWs, the as-

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prepared MoP@NC was dissolved in 6 M KOH solution at 50 oC for 24 h,the product

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was filtered and washed with deionized water several times and dried at 60 oC in air.

2.3 Materials Characterization

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The morphology, structure and chemical composition was characterized by the

scanning electron microscopy (SEM, FEI nanoSEM 450), transmission electron microscopy (TEM, FEI Titan 60-300 Cs), X-ray diffraction equipment (XRD, Philips X’ Pert Pro, λ = 1.5418 Å), X-ray photoelectron spectroscopy (XPS, ESCALB MK-II), Fourier transform infrared (FTIR) spectroscopy (IFS-85 (Bruker) spectrometer) and 8

Raman spectra with 514.5 nm argon laser (HR RamLab Raman Microscope). The content of N-doped carbon coating in MoP@NC was determined by thermogravimetric analysis (TG, STA449) coupling with differential thermal analysis (DTA, STA449). The specific area was measured by Brunauer-Element-Teller (Micromeritics SASP 2020) at 77 K.

The composition of evolved gas collected at working electrode was

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determined by gas chromatographic analysis (Techcomp GC7900).

2.4 Electrochemical Measurements

The commercial 20% Pt/C catalyst purchased from Sigma-Aldrich was used as

The electrochemical measurement was performed in a three-

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reference catalyst.

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electrode system in 0.5 M H2SO4, 1 M PBS and 1 M KOH, respectively (CHI 760e, Shanghai CHI Company, China). A graphite sheet and a saturated calomel electrode

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(SCE) were used as the counter electrode and reference electrode, and the To prepare

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electrocatalyst-modified GCE was the working electrode, respectively.

working electrode, 24 mg of precursor was dispersed in 6 mL ultrapure water under

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fierce sonication for 30 min, and then took out of 5 L of solution dropping on the surface of GCE to obtain 3 mm in diameter with a mass loading density of 0.28 mg The modified GCE dried in air for 12 h and then 5 L diluted Nafion was loaded

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cm−2.

on GCE to fix the catalyst.

All potentials were iR corrected and converted to standard

reversible hydrogen electrode (RHE) by ERHE = ESCE + 0.0591 × pH [1, 37]. The SCE was also referenced to the RHE, as shown in Figure S1. (LSV) was carried out at a scanning rate of 5 mV s−1. 9

Linear sweep voltammetry

The Tafel slopes were calculated

by η = a + blog j (η is the overpotential, b is the slope and j is the current density). Electrochemical impedance spectroscopy (EIS) was conducted at an overpotential of 100 mV in a frequency range from 100 kHz to 0.1 Hz with an AC perturbation of 5 mV. The electrochemical active surface area (ECSA) was measured by cyclic voltammetry (CV) at various scanning rates from 20 to 100 mV s-1 in the non-faradaic region between 0.1 and 0.2 V (vs. RHE).

The durability of the catalyst was assessed by cyclic

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voltammetry at a scanning rate of 200 mV s−1 and galvanostatic method at a current

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density of 50 mA cm-2.

2.5 Computational Method

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The density-functional theory (DFT) calculation was carried out with the Vienna

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Ab initio Simulation Package (VASP) code [38] and Perdew-Burke-Eznerhof (PBE) function [39]. The projector augmented wave (PAW) method was adopted to describe A supercell consisting of 4  4 graphene

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the interaction of ions and electrons [40].

unit cells was selected to model the clean carbon slab and one carbon atom was replaced

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by an N atom to construct the N-doped carbon slab. Moreover, the 4  4 monolayer

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graphene unit and 2  3 unit of MoP (101) surface consisting of three layers were employed to construct the MoP@NC heterojunction.

A vacuum space of 20 Å was

employed to avoid the periodic image interactions in the c axis.

The plane-wave

cutoff energy of 500 eV and 3 × 3 × 1 k-points mesh were employed for structural relaxation [41] until the total force and energy converged within 0.02 eV/Å and 10-5 eV per atom, respectively.

The adsorption energy of hydrogen on the catalyst was 10

evaluated by the following equation: 1

𝐸𝑎𝑑𝑠 = 𝐸𝑠𝑢𝑏+𝐻∗ − 𝐸𝑠𝑢𝑏 − 2 𝐸𝐻2

(1)

where Esub+H* and Esub are the total energies of the catalyst substrate with and without adsorption of H* and EH2 denotes the total energy of a gaseous H2 molecule. The Gibbs free energy is defined as follows: ∆𝐺𝐻∗ = 𝐸𝑎𝑑𝑠 + ∆𝑍𝑃𝐸 − 𝑇∆𝑆

(2)

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where △ZPE and △S are the zero-point energy and entropy change between the

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adsorbed state and gas phase and T is the temperature (298 K).

3. Results and Discussion

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Figure 1 illustrates the synthetic process of the porous MoP@NC NWs.

The

lP

organic-inorganic hybrid Mo3O10(C6H8N)2·2H2O NWs are produced by stirring the solution containing ammonium molybdate and aniline in a water bath to form C6H8N+ The

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and Mo3O102- followed by precipitation based on electrostatic interactions [35].

Fourier transform infrared spectroscopy (FTIR) data acquired from the product in

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Figure S2 confirm the existence of benzenoid at 1493 cm-1, quinoid ring at 1590 cm-1,

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as well as the CH stretching and out-of-plane deformation modes at around 2927 cm-1 and 884 cm-1, respectively [36].

The X-ray diffraction (XRD) patterns in Figure 2a show that the crystal phase of pristine product can be indexed to Mo3O10(C6H8N)2·2H2O (JCPDS 50-2402) [36]. After thermal treatment in the presence of NaH2PO2 at 900 oC, the new formed 11

diffraction peaks correspond to the hexagonal MoP (JCPDS 24-0771) without impurities [18].

When the phosphating temperature gradually increased from 600 oC

to 900 oC, the inorganic component first form MoO2 NPs (JCPDS 65-5787) [42], and further converted to MoP phases, as shown in Figure S3.

During the conversion, the

connected MoP NPs shaped porous structure by the sintering effect and the organic component in the precursor is in-situ carbonized into N-doped carbon (NC) which The SEM images in Figures 2b and 2c

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prevent the further aggregation of MoP NPs.

disclose that the pristine product has a nanowire structure with a length of 50 μm and width of 200 nm (Figure S4). After phosphating, the smooth topography becomes

Figure 2e presents the HR-TEM image of the

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structure as shown in Figure 2d.

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rough and transmission electron microscopy (TEM) confirms the formation of porous

porous framework containing interconnected NPs with a diameter of 12 nm and a thin

lP

carbon coating about 2 nm thick are observed. The selected-area electron diffraction

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(SAED) pattern reveals the polycrystalline nature derived from the crystalline MoP NPs and the rings are consistent with the crystal plane of MoP observed by XRD.

The HR-

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TEM image in Figure 2f reveals a lattice fringe of 0.21 nm corresponding to the (101) plane of MoP [43] encapsulated by a thin amorphous carbon layer, and many small

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defects can be clearly observed from the thin carbon layer.

The Raman spectra in

Figure S5 shows two intense peaks at 1380 cm-1 and 1572 cm-1 corresponding to the D and G bands of carbon [44] indicating conversion of the organic precursor to carbon, and the high intensity D-band further demonstrates the defects and disorder-induced structures in the carbon layer.

The Brunauer-Element-Teller (BET) analysis of porous 12

MoP@NC hybrids reveals lots of micropores with a size of 2-4 nm, mesopores of 20 nm and the specific surface area of 10.1 m2/g (Figure S6). The mesopores can be attributed to the porous framework of NWs and a lot of micropores could come from the N-doped carbon shell, as confirmed by the HR-TEM and Raman spectra.

The

elemental maps in Figure 2g reveal that the Mo, P, N, and C elements are uniformly distributed in the whole NWs, suggesting the N-doped carbon encapsulates the MoP The content of N-doped carbon

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framework forming the porous MoP@NC hybrids.

coating in the MoP@NC hybrids determined by thermogravimetric analysis (TGA) and

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differential thermal analysis (DTA) is about 16.1 wt% and shown in Figure S7.

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The X-ray photoelectron spectroscopy (XPS) is performed to study the composition and chemical states of the MoP@NC hybrids. The high-resolution spectra of Mo 3d

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in Figure 3a show two typictal peaks at 228.2 eV and 231.2 eV which can be assigned

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to Mo-P bond. The two doublets peaks at 228.8 eV/231.9 eV and 233.4 eV/236.1 eV

after phosphating [10, 16].

The peaks of P 2p at 129.4 and 130.4 eV can be assigned

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can be attribute to Mo4+-O bond and Mo6+-O bond, due to the residual oxygen species

The P-C bond located at 133.6 eV can be

to low-valence P in MoP [12] and the larger binding energy coressponding to the P-O

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bond due to the residual oxygen [45, 46].

clearly fitted and identified (Figure 3b), implying the strong electron interaction between the NC shell and MoP core and it is also confirmed by the C1s spectra (Figure 3c) [47, 48].

The C signal in the composite can be fiitted with the C-N, C-P, and C-

O in addition to C-C [49].

Figure 3d shows the N 1s spectra in the NC and the three 13

fitted peaks at 398.4, 400.1, and 401.1 eV are assigned to the pyridinic-N, pyrrolic-N and graphitic-N, respectively [47, 50, 51].

The characterization results confirm

successful synthesis of N-doped carbon coated porous MoP NWs.

The HER electrocatalytic activity of the electrocatalysts with mass loading of 0.28 mg cm-2 is evaluated by linear sweep voltammetry (LSV) at a scanning rate of 5 mV sin 0.5 M H2SO4, 1.0 M PBS and 1.0 M KOH, respectively.

curves are iR corrected (Figure S8).

All the polarization

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Figures 4a-c show the LSV curves of the porous

MoP@NC hybrids with a small overpotential η10 (defined as the overpotential at a

The control samples of NC and bare MoP NWs

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and alkaline medium, respectively.

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current density of 10 mA cm-2) of 96 mV, 191 mV and 149 mV in the acidic, neutral

was prepared for comparison (Figures S9 and S10).

The η10 of MoP@NC hybrids

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are much smaller than those of the bare MoP and NC electrocatalysts, and better than We

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the samples thermally treated at different phosphating temperature (Figures S11).

also collected the evolved gas from the working electrode and analyzed the composition

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by gas chromatogram (Figure S12), only component of H2 gas can be determined in comparison with the standard hydrogen gas chromatography.

In addition, the Faradic

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efficiency was calculated by measuring the volume of evolved gases at a constant current (10 mA) at different time (Figure S13), the amount of evolved H2 agrees with the theoretical value, suggesting almost 100% Faradic efficiency.

The corresponding

Tafel plots of commercial 20% Pt/C, MoP and MoP@NC catalysts are also investigated to reveal the HER kinetics.

As shown in Figures 4d-f, the Pt/C catalyst shows a low 14

overpotential and small Tafel slopes of 31.4 mV dec-1, 39mV dec-1 and 55.9 mV dec-1 in the acidic, neutral and alkaline solutions, respectively, consistent with previous reports [1, 52, 53].

The Tafel slope of the porous MoP@NC hybrid electrocatalyst is

as low as 49.2 mV dec-1 in the acidic medium, 95.0 mV dec-1 in the neutral medium and 61.7 mV dec-1 in the alkaline medium, which are almost half smaller than the bare MoP (85.8, 152.2 and 134.1 mV dec-1).

The polarization curves in Figures 4g-i disclose

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electrocatalysts are assessed.

The cycling stability of the MoP@NC hybrid

negligible decline in the overpotential in different pH solutions after 5,000 CV cycles corroborating the superior stability in HER.

The galvanostatic method was performed

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at a current density of 50 mA cm-2 in 0.5 M H2SO4, the potential remained at about 200

electrocatalyst (Figure S14).

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mV for over 45 h, indicating a good long-term stability of the porous MoP@NC hybrid The excellent durability can be attributed to the The XPS results show that the Mo-P, P-O,

lP

protection rendered by the carbon layer.

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and P-C bonds in MoP@NC have no change after cycling, on the other hand, without the carbon layer, the Mo-P bond diminishes dramatically also confirming the high The HER activity of the porous

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oxidation resistance of carbon coating (Figure S15).

MoP@NC hybrid electrocatalyst is investigated at different temperature from 5 oC to

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70 oC (Figure S16) and satisfactory HER activity is obtained at above 10 oC indicating a wide applicability.

To understand the superior catalytic performance of the porous MoP@NC hybrids, the electrochemical active surface area (ECSA) is calculated by cyclic voltammetry. 15

The double-layer capacitance (Cd1) measured in the non-faradic range is expected to be linearly proportional to the effective active surface area.

The rectangular shape of the

CV curves of the MoP@NC NWs suggests the representative double-layer capacitance (Figures 5a and 5b) with Cd1 being 14.15 mF cm-2 that is 6.5 times larger than that of the bare MoP NWs (2.21 mF cm-2) (Figure S17), suggesting that there are more exposed active sites on the porous MoP@NC hybrids and is consistent with the

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improvement of catalytic activity. The turnover frequency (TOF) calculation shows that porous MoP@NC hybrid electrocatalyst has significantly higher catalytic

efficiency than bare MoP (Figure S18), which could because of the NC coating with

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abundant micropores facilitating the H+ absorption. Electrochemical impedance

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spectroscopy (EIS) conducted under acidic, neutral and alkaline conditions at an overpotential of 100 mV and as shown in Figure S19, the porous MoP@NC hybrid

lP

catalyst has low charge transfer resistance of 20 ± 10 Ω cm2 in different pH solutions

ability.

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smaller than the pristine MoP NWs (100-250 Ω cm2), indicating the fast charge transfer Density-functional theory calculation is also carried out to provide insights

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into the high electrocatalytic activity of the MoP@NC hybrid electrocatalyst (Figure S20), where the H+ adsorption occurred on MoP at the interface due to the lots of

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micropores in NC coating in favor of H+ permeation.

It is well known that if the

adsorption free energy of H (ΔGH*) associated with proton/electron transfer and hydrogen release is closer to zero, the HER kinetics is better. As shown in Figure 5c the MoP@NCs hybrids exhibits a small ΔGH* of -0.18 eV which is much smaller than those of the NC (0.69 eV) and bare MoP (-0.54 eV). 16

It is due to the strong electronic

interaction between the MoP and N-doped carbon and the HER activity of the MoP@NC hybrids is better than that of most previously reported MoP-based catalysts as well as carbide and sulfide-based electrocatalysts (Figure 5d and Table S1) [6, 5463].

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4. Conclusion A highly active and stable porous MoP@NC hybrid electrocatalyst is prepared via one-step phosphating of the inorganic-organic Mo3O10(C6H8N)2·2H2O NWs.

The in

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situ formed porous MoP NWs are uniformly coated with thin N-doped carbon layer,

delivering outstanding HER performance in wide pH range solutions on account of the

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low adsorption free energy of H at the interface, more active sites and fast ions/gas

lP

bubbles migration due to the porous structure and enhanced overall conductivity and corrosion resistance resulting from the micropores N-doped carbon shell boding well Small overpotentials of 96

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excellent HER activity and kinetics and superior stability.

mV, 191 mV and 149 mV at the current density of 10 mA cm−2 and Tafel slopes of 49.2

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mV dec−1, 95.0 mV dec−1 and 61.7 mV dec−1 in a 0.5 M H2SO4, 1.0 M PBS and 1.0 M Our results not only demonstrate an attractive MoP-

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KOH are achieved respectively.

based hybrid electrocatalyst with excellent HER activity in a wide pH range, but also provide deep insights into the design and fabrication of cost-effective and efficient nonnoble-metal electrocatalysts for electrocatalytic water splitting.

Author contributions: 17

Chaoran Pi and Chao Huang contributed equally to this work.

Acknowledgments This work was financially supported by National Natural Science Foundation of China (No. 51572100, 51504171 and 61434001), Major project of Technology Innovation of Hubei Province (2018AAA011), Wuhan Yellow Crane Talents Program,

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and City University of Hong Kong Strategic Research Grant (SRG) No. 7005105.

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Appendix A. Supplementary materials

The supplementary data associated with this article can be found in the online

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version at http://dx.doi.org/xx.xxxx/j.apcatb.xx.xx.xx.

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Figure captions: Figure 1. Schematic illustration of the porous MoP@NC hybrid NWs.

Figure 2. (a) XRD patterns of the MoP@NC NWs and Mo3O10(C6H8N)2·4H2O NWs; SEM images of (b) Mo3O10(C6H8N)2·4H2O NWs and (c) MoP@NC NWs; (d-f) TEM and HRTEM images of porous MoP@NC NWs; (g) Elemental maps of the porous The inset image in (d) is the SEAD pattern.

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MoP@NC NWs.

Figure 3. High-resolution XPS spectra of (a) Mo 3d, (b) P 2p, (c) C1s and (d) N 1s in

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porous MoP@NC NWs.

Figure 4. (a-c) Polarization curves of porous MoP@NC, bare MoP, Pt/C and N-doped

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carbon; (d-f) Corresponding Tafel plots; (g-i) Polarization curves of porous MoP@NC elecctrcatalyst before and after 5000 cycles. Where (a) (d) (g) in 0.5 M H2SO4, (b) (e)

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(h) in 1 M PBS and (c) (f) (i) in 1 M KOH.

Figure 5. (a) CV curves of porous MoP@NC elecctrcatalyst at different scan rates from

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10 mV s-1 to 200 mV s-1; (b) Corresponding Cdl of porous MoP@NC and bare MoP; (c) Hydrogen adsorption binding energy configuration at the interfaces between MoP (101) and NC, MoP (101), NC and Pt at the equilibrium potential. (d) Comparison of the HER performance of Mo-based electrocatalysts in acidic solution.

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