In Situ Synthesis Strategy for Hierarchically Porous Ni2P Polyhedrons

Subscriber access provided by University of Newcastle, Australia

Article

An in-situ synthesis strategy for hierarchically porous NiP polyhedrons from MOFs templates with enhanced electrochemical properties for hydrogen evolution 2

Liting Yan, Pengcheng Dai, Ying Wang, Xin Gu, Liangjun Li, Lei Cao, and Xuebo Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01037 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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 free 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 accessible to all readers and 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.

ACS Applied Materials & Interfaces 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 30

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

An In-situ Synthesis Strategy for Hierarchically Porous Ni2P Polyhedrons from MOFs Templates with Enhanced Electrochemical Properties for Hydrogen Evolution Liting Yan,a,b Pengcheng Dai,a Ying Wang,a,b Xin Gu,a Liangjun Li,a Lei Cao,a and Xuebo Zhao*a a

Research Centre of New Energy Science and Technology, Research Institute of Unconventional

Oil & Gas and Renewable Energy, China University of Petroleum (East China),Qingdao 266580, P. R. b

College of Science, China University of Petroleum (East China), Qingdao 266580, P. R. China.

E-mail: [email protected].

Keywords: nickel phosphide, hierarchical pores, metal-organic frameworks, hydrogen evolution reaction, electrocatalyst

ACS Paragon Plus Environment

1


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 30

ABSTRACT

The development of highly active and stable noble metal-free electrocatalysts of hydrogen evolution reaction (HER) under both acidic and basic conditions for renewable-energy conversion techniques is of great significance. Herein, a practical in-situ synthesis strategy for a three-dimensional Ni2P polyhedron with a hierarchically porous structure was presented, which was efficiently obtained from a nickel centered metal-organic frameworks (MOF-74-Ni) by direct low-temperature phosphorization. The as-prepared Ni2P polyhedron showed a high BET surface area (175.0 m2·g-1), hierarchically porous property, and outstanding metal dispersion， which well inherited the morphology and porosity of its MOF precursor. Compared with Ni2P particles obtained from a nonporous precursor, the as-prepared Ni2P polyhedron used as electrocatalyst exhibited excellent electrocatalytic performance towards the HER, with a low overpotential of 158 mV to produce the cathodic current density of 10 mA cm-2 . A small Tafel slope of 73 mV per decade is obtained for Ni2P polyhedron, which revealed a VolmerHeyrovsky mechanism during the HER. In addition, benefiting from the structural stability, the porous Ni2P polyhedron used as a electrocatalyst showed satisfactory long term durability for the HER in acidic media.


2

Page 3 of 30

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


Introduction Due to environmental problems resulting from the overuse of conventional fossil fuels, attention is increasingly focusing on the benefits of hydrogen production, being environment friendly, sustainable, renewable and efficient.1-4 The hydrogen evolution reaction (HER) is an essential strategy for the production of hydrogen using specific electrocatalysts 5. Among HER catalysts, precious metals like platinum (Pt) show the best catalytic performance,6 but the high cost and scarce resources of noble metals confine their large-scale commercial applications. Consequently, it is of great commercial significance to develop HER catalysts free of noble metals but maintain excellent electrocatalytic performance and long-term stability. Transition-metal phosphides (TMPs) are broadly used as catalysts in consequence of their noble metal free properties and high catalytic activities in hydrodesulfurization (HDS),

hydrodenitrogenation

(HDN),

hydroprocessing

(HPC),

photocatalytic

degradation, lithium ion batteries, and HER7-12. Recent researches have demonstrated experimentally the excellent electrocatalytic activity and superior durability towards the HER of various TMPs, such as MoP13-15, WP16, FeP17, 18, CoP19-22, Co2P23, Cu3P24, Ni2P25, 26

, and Ni12P527,

28

. Among these, the HER behavior of Ni2P is of particular interest.

Density functional theory (DFT) calculations indicate that Ni2P (001) shows excellent activity over the conventional platinum catalyst29, suggesting the synthesis of nickel phosphide with a single phase of Ni2P is of great significance in both of theory and practice. The high activity of Ni2P is in consequence of the remarkable cooperativity between the phosphorus and nickel: the metal sites on the surface bond with hydrogen strongly enough to prevent the remove of H2 on the surface, while P sites could


3


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 4 of 30

moderately bond to reaction intermediates and create a surface with proton and hydride acceptor sites29, 30. Combining high thermostability and catalytic activity, Ni2P could be one of the most practical catalysts to use for HER29. Recently, Ni2P with different morphologies has been extensively investigated and found to be efficient electrocatalysts for HER10. Compared to other morphologies, porous materials, particularly hierarchically porous materials, frequently show unique and fascinating catalytic performances across many fields19, 21, 31, 32. Hierarchically porous materials is an intriguing class of porous materials, which possess more than one length levels of pores and structure from micropores, mesopores, to macropores33-35. Due to their alterable chemical ingredients, low density, high porosity, large surface area, and interconnecting hierarchically porous structure at multiple length levels34, hierarchically porous materials have widely potential applied prospect in energy storage

and

conversion,

catalysis

(heterogeneous

catalysis,

photocatalysis

and

electrochemical catalysis), adsorption and separation, gas sensing, and biomedicine.34, 3640

Metal-organic frameworks (MOFs), or known as porous coordination polymers (PCPs),

are a family of porous crystallographic organic-inorganic hybrid materials. Their unique microstructure and high porosity make MOFs the perfect template on which to synthesize a variety of porous materials19, 21, 41-44, such as porous carbon materials45-48, nanoporous metal oxides49-51, and composite materials52-54. In 2015, Jiang et al., synthesized nickel phosphides nanoparticles by directly phosphorization calcination of a Ni-based metalorganic framework, Ni-BTC. Even though Ni2P and Ni12P5 nanoparticles were obtained from a MOF precursor, however, the advantages of the MOF precursor, such as regular morphology, large specific surface area and high porosity, were not be well inherited.55


4

Page 5 of 30

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


Most recently, Lou et al., showed that porous nickel phosphides could be obtained by directly phosphorization of a MOF precursor with similarly porous plate-like nanostructures31. However, the Ni-P porous nanoplates they obtained were the mixed phases of Ni5P4 and Ni2P. As far as we know, there has been no related report in the literature of single phase transition-metal phosphides materials with a hierarchically porous structure and regular morphology synthesized from a MOF precursor. In this work, using MOF-74-Ni as the precursor, we synthesized a regular threedimensional hierarchically porous nickel phosphide polyhedron with a single phase of Ni2P (termed as Ni2P polyhedron) using an in-situ conversion strategy of a one-step calcination at low temperature. In this strategy, the Ni2P polyhedron well inherited the regular morphology and porosity of MOF-74-Ni, which had a high Brunauer-EmmettTeller surface area and hierarchical pores properties. Besides, benefiting from the high metal contents and uniform distribution of the metal center in MOF-74-Ni morphology, the as-prepared Ni2P polyhedron exhibited perfect dispersity and the active sites were highly exposed. With all these properties, the MOF-derived hierarchically porous Ni2P polyhedron showed excellent electrocatalytic performances and durability for HER in acidic media, promising it to be a potential noble metal-free electrocatalyst for HER.

Experimental Materials and Characterization Nickel(II) acetate tetrahydrate (Ni(CH3COO)2·4H2O, 98wt%), methanol (≥99.5wt%), sulfuric acid (H2SO4, 95.0-98.0wt%), and tetrahydrofuran (THF, ≥99wt%) were obtained from Sinopharm Chemical Reagent Co. Ltd. (SCRC). 2,5-dihydroxyterephthalic acid (DHTA, 98wt%) was obtained from Chemsoon Co. Ltd.. Sodium hypophosphite


5


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 30

(NaH2PO2·H2O, 99wt%) was obtained from Aladdin. The platinum on carbon(Pt/C, 10wt%) and Nafion solution (5wt%) were obtained from Sigma-Aldrich Co. LLC.. All the reagents were put to use without any pretreatment. Powder X-ray diffraction (PXRD) patterns were obtained on PANalytical X'pert PRO (Holland) diffractometer. The field emission scanning electron microscopy (FESEM) photographs, energy-dispersive X-ray (EDX) spectrums, and elemental mapping images were recorded on JEOL JSM-7500F (Japan). Low and high-resolution transmission electron microscopy (TEM and HRTEM) photographs were all recorded on a JEOL JEM2100F (Japan) Transmission Electron Microscope. An X-ray photoelectron spectra (XPS) was conducted on a Thermo ESCALAB 250Xi (USA) spectrometer. N2 adsorption-desorption experiments were performed on an Autosorb-iQ2 (USA) instrument. Thermogravimetric analysis was performed on a ZRT-A thermogravimetric analyzer (China) , the temperature of the samples was increased from ambient temperature to 1150 oC with a heating rate of 10 oC per minute with a constant flow of argon at 100 mL per minute. Synthesis of MOF-74-Ni nanocrystals MOF-74-Ni was prepared according to the previous report with tiny modification.56 DHTA (3 mmol, 0.594g) was dispersed in 30 mL THF and nickel acetate tetrahydrate (3 mmol, 0.747g) was dispersed in 30 mL deionized water (DIW). Subsequently, the two solutions were mixed in a 100 mL autoclave (with teflon lined). After 10 min of vigorous stirring at ambient temperature, the autoclave was capped and transferred to an oven at 110 °C for 24 hours. After the solution cooled down to ambient temperature naturally, the fine yellow nanocrystals were collected by centrifugation and washed several times with THF, DIW and methanol. Then the synthesized sample was loaded in the soxhelt-


6

Page 7 of 30

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


extraction device and extracted at 120 oC for 48 hours, using methanol as the extraction agent. In the end, MOF-74-Ni nanocrystals were obtained by drying in a vacuum drying chamber at 120 oC for 12 h. One-step synthesis of porous Ni2P polyhedrons The as-prepared MOF-74-Ni and sodium hypophosphite were placed in two separate positions in a typical corundum porcelain boat (The molar ratio of Ni-to-P is 1:20) boat with sodium hypophosphite at the upstream positions of the tube furnace. The samples were heated at 275 oC for specific hours with a heating rate of 5 oC·per minute under a constant flow of argon at 100 ml· min-1. After naturally cooling down to ambient temperature, the hierarchically porous Ni2P polyhedrons were collected. Synthesis of Ni2P particles Ni2P particles were synthesized using a similar procedure to that described above, except for the use of nickel acetate tetrahydrate as the metal source. Electrochemical measurements 5 mg of as-prepared Ni2P polyhedrons and 100 µL Nafion solution (5wt%) were dissolved in a mixture of 950 µL DIW and 950 µL ethanol. Then, the mixed solution form a homogeneous catalysis slurry with at least 30 min ultrasonic treatment. The glassy carbon electrode (GCE, 3 mm in diameter, 0.071 cm-2 of area) was carefully polished with alumina powder(1.0, 0.3 and 0.05 mm of particle size), and rinsed with DIW several times, therewith by ultrasonic treatment in ethyl alcohol and DIW. 3.5 µL of the catalysis slurry was dropped onto the surface of the bare GCE and dried at 60oC, then the working electrode was obtained.


7


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 8 of 30

The electrochemical measurement was performed with a CHI 760E electrochemical workstation in a typical three-electrode system at ambient temperature, with Ag/AgCl electrode as reference electrode and Pt wire as counter electrode. All the electrochemical measurements were performed using 0.5 M H2SO4 as the electrolyte solution. All potentials measured were calibrated with respect to the reversible hydrogen electrode (RHE) on the basis of the Nernst equation without iR correction: Evs.RHE = Evs.Ag/AgCl + 0.197 + 0.059 pH. The activity of Ni2P polyhedrons towards the HER was examined by linear sweep voltammetry (LSV) with a scan rate of 5 mV·s-1 at ambient temperature in 0.5 M H2SO4, 1.0 M PBS, and 1.0 M KOH solution, respectively. The stability test was obtained by cyclic voltammetry (CV) scanning of 2000 cycles with a scan rate of 50 mV·s-1. In addition, to avoid the potential interference of the platinum wire , a carbon rod electrode was employed as counter electrode.57 The time-dependent current density curve was obtained in 0.5 M H2SO4, 1.0 M PBS, and 1.0 M KOH solution, respectively.

Results and discussion Material Preparation and Characterization A Ni-centered MOF materials (MOF-74-Ni, or CPO-27-Ni) was chosen as the precursor to synthesize porous Ni2P polyhedrons. MOF-74-Ni was obtained according to a previous literature with small modifications,56 which is show in details in the experimental section. PXRD patterns indicated that the synthesized MOF-74-Ni had good crystal structure identical to the reference (Fig. S1). The Ni2P polyhedrons were obtained from the synthesized MOF-74-Ni after phosphorization calcination. The Ni2P polyhedrons with different phosphorization times, such as 2 and 3 hours were termed as


8

Page 9 of 30

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


Ni2P Ps-2 and Ni2P Ps-3. Fig. 1 shows PXRD patterns of the porous Ni2P polyhedron with different phosphorization times. The Ni2P Ps-2 shows six strong diffraction peaks at 40.8°, 44.6°, 47.3°, 54.2°, 54.9° and 74.6°, corresponding to (111), (201), (210) , (300), (211), and (400) planes of hexagonal Ni2P (JPCDS No. 03-0953). However, the typical peaks of MOF-74-Ni at 7°, 12°, and 49° were also detected, indicating the sample is consist of a mixed phase of Ni2P and MOF-74-Ni. After increasing the phosphorization time to 3 h, all the peaks of MOF-74-Ni disappeared and a pure Ni2P phase was obtained. To further confirm the composition of the Ni2P Ps-3, the EDX analysis was conducted. The EDX spectrum of Ni2P Ps-3 showed an atomic-ratio of Ni-P to be around 19:9 (Fig. 2e and 2f), which is consistent with the XRD patterns. However, the EDX spectrum of Ni2P Ps-2 (Fig. S3e and S3f) showed an atomic-ratio of Ni-P to be only 18:6, further confirm the part transformation of the nickel in the MOF structures. In addition, elemental mapping images (Fig. 2a, 2b, 2c, 2d, and Fig. S3) and EDS line scan analysis (Fig. S4) further demonstrated that the elements of Ni and P were uniformly distributed throughout the Ni2P Ps-2 and Ni2P Ps-3. Taken together, the experimental results prove that the metal-center of MOF precursor had been successfully converted to Ni2P via a low temperature phosphorization reaction. FESEM was employed to study the morphological evolution of MOF-74-Ni and the Ni2P Ps-3. Fig. 3a and 3b indicate that its regular polyhedral morphology is the same as that of its MOF precursor. After phosphorization calcinations, the morphology of MOF74-Ni was well maintained while the surface of the polyhedron become coarse and porous. Comparison between the high-resolution FESEM images (inset of Fig. 3a and b) reveals the highly porous properties of the Ni2P Ps-3. To further examine the microscopic


9


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 10 of 30

structure of the Ni2P Ps-3, TEM and HRTEM characterizations were carried out. In Fig. 3c, many Ni2P nanocrystals and pores in it can be observed. The Ni2P nanocrystals show outstanding metal dispersion with an average particle diameter of 7.9 ± 2.6 nm (see details in the inset of Fig. 3c). From the HRTEM image (Fig. 3d), clear crystal lattice fringes can be observed with a distance of approximately 0.203 and 0.221 nm, which is corresponding to the (201) plane and (111) plane of Ni2P. Due to the similar morphology of the Ni2P polyhedron (Fig. 3b and Fig. S2)with its MOF precursor, an in-situ transformation process may occur at low temperature phosphorization calcinations (Scheme 1). Phosphine,58,

59

which is generated by the

decomposition of sodium hypophosphite in a furnace, enters the channels of MOF structures via free diffusion and the metal center of the MOF which is in contact with phosphine may be reduced to Ni2P gradually with the diffusion process of PH3. The oxygen atoms coordinated with nickel atoms60 in the MOF-74 nanocrystals are probably replaced by phosphorus atoms to form the Ni2P crystal lattice, which then gathers into Ni2P clusters. The pore size of the MOF structure is increased by the formation of Ni2P clusters. The overall stability of the Ni2P Ps-3 increased due to the thermostability of the nickel phosphide crystal. Thermogravimetric (TG) analysis and thermal treatment of the samples were carried out to characterize the stability of the Ni2P Ps-3 and MOF-74-Ni. Fig. 4a shows that the Ni2P Ps-3 and MOF-74-Ni have similar thermal decomposition temperatures, ranging from 400oC to 500oC, which is accordant with values reported in the previous literature.60 However, the Ni2P Ps-3 lose about 18% of their weight, included solvent, water, and small molecules adsorbed from air, when the temperature is at around 500oC, and lose 3% (wt) further at temperature up to 780oC. On the contrary, MOF-74-Ni


10

Page 11 of 30

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


loses 67% of its weight when the temperature is higher than 500oC, and there is no obvious lose of weight at 780-850oC. FESEM images of as-prepared samples after thermal treatment were obtained, Fig. 4c shows that the Ni2P Ps-3 could maintain its polyhedron morphology after 600oC thermal treatment, whereas MOF-74-Ni (Fig. 4b) decomposed after thermal treatment at the same temperature. XRD analysis of MOF-74Ni and Ni2P Ps-3 after thermal treatment was performed. Fig. S5 shows that the phase of Ni2P was well preserved in Ni2P Ps-3 while the phase of MOF-74-Ni was transformed to Ni/C, further confirmed the stability of Ni2P Ps-3. In addition, to verify the weight-loss at 780oC observed by TG analysis, the 850oC thermal treatment of MOF-74-Ni (Fig. S6) and Ni2P Ps-3 (Fig. S7) were carried out. Results indicate that the Ni2P Ps-3 decomposed after 850oC thermal treatment, which is in agreement with the TG analysis results. To investigate the transformation process of porous structures during the phase transition of MOF polyhedrons, N2 adsorption-desorption experiments for the catalysts were obtained. Fig. 4d shows the N2 isotherms and pore size distribution (inset) for the Ni2P Ps-3 that was calculated from N2 isotherms using the density functional theory model. The isotherms were identified as a typical type IV, which is one of the features of mesoporous materials. The pore-size distribution shows a series of pores varying from 1.7 to 32.1 nm in the sample, which is in agreement with the hierarchical pore structures of the TEM images (Fig. 3c). The DFT pore-size distribution of MOF-74-Ni (Fig. S8) shows that the pore distribution of the MOF precursor is primarily concentrated at approximately 1.7 nm, which is accordant with the crystallographic data (≈ 1.6 nm). In the Ni2P Ps-3, the pore size become larger during phosphorization calcination process with a predominant size of 16.4 nm (Fig. 4d inset). Correspondingly, the BET special surface area of the Ni2P


11


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 12 of 30

Ps-3 was calculated to be 175.0 m2·g-1, which is lower than 1159.7 m2·g-1 for MOF-74-Ni, the latter was calculated from the isotherms shown in Fig. S9. The increasing of pore size and decreasing of BET surface area of the Ni2P Ps-3 compared to the MOF precursor may contribute to the increased number of Ni2P nanocrystals during the phosphorization calcination process, which then results in several pores becoming blocked or collapsing. The micropores (1.7 nm) in the Ni2P Ps-3 were thought to be inherited from the MOF precursor. The sharp distribution of the pore diameter 1.7, 4.2, 12.7, 16.7, 22.8 and 32.1 nm suggests that the hierarchically porous Ni2P polyhedron has high monodispersity. To gain insights into the formation mechanism of these hierarchical pore structures, a nonporous precursor (acetate tetrahydrate) was selected to synthesize a type of nickel phosphide compound (termed as Ni2P particles) (Fig. S10). The BET special surface area and cumulative pore volume of the Ni2P particles were calculated to be 18.7 m2·g-1 and 0.17 cm3·g-1, respectively, which is far less than that of the Ni2P Ps-3 from the MOF precursor. The raw data of the N2 isotherms and pore size distribution for Ni2P particles is shown in Fig. S11 and S12. The hierarchical pore structures will be maintained during the transformation process from the MOF precursor to Ni2P. In addition, the TEM images (Fig. S13) further confirm the low porosity of the Ni2P particles obtained from nonporous precursor. XPS analysis was performed to get further characteristic of the Ni2P Ps-3 and the MOF-74 precursor. The XPS survey spectra of the Ni2P Ps-3 was shown in Fig. 5a. It can be seen from the XPS spectra that there are Ni, P, C, and some O elements, among which the C should come from the MOF precursor and the O elements should be inherited from the organic. High-magnification XPS spectra in the Ni 2p regions of the Ni2P Ps-3 are


12

Page 13 of 30

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


shown in Fig. 5b. The two peaks around 853.4 and 857.1 eV for Ni 2p3/2 are corresponding to Niδ+ in Ni2P and Ni2+ in surface NiO, respectively61. The peak around 861.5 eV is the satellite of Ni 2p3/262. Similarly, two peaks are observed around 875.0 and 880.7 eV for the Ni 2p1/2 level, which are attributed to the Ni2+ in surface NiO and satellite, respectively61. It can be seen that the binding energy of Ni 2p3/2 negatively shifted from 856.8 ev (Ni2+ in MOF-74-Ni, Fig. 5c63) to 853.4 eV, indicating that the Niδ+ in Ni2P originates from the MOF precursor. There are two peaks around 129.8 and 134.1 eV in the P 2p spectrum ( Fig. 5d), which is corresponding to Pδ- in Ni2P and P5+ in some superficial passivation of phosphide clusters (e.g. PO43- or P2O5 )61, 64, 65. Ni 2p3/2 (853.4 eV) and P 2p3/2 (129.8 eV) are considered to be the binding energies of Ni and P in Ni2P, and the positive and negative shifts of Ni (852.4 eV)66 and elemental P (130.2 eV)67 in the Ni2P Ps-3 suggest a charge transfer from Ni to P. Therefore, electron transfer should exist from Ni to P, and it would promote adsorption and desorption of reactant and product molecules, respectively, in the electrocatalytic process. Electrocatalytic performances towards the HER The electrocatalytic performance of the as-prepared Ni2P Ps-3 towards HER was evaluated in a 0.5 M H2SO4 solution using a standard three-electrode mode with a scan rete of 5 mV s-1. Catalyst-based working electrode was obtained through loading the asprepared samples onto the surface of the GCE. For comparison, a bare GCE, MOF-74-Ni, Ni2P particles, and commercial Pt/C catalyst were investigated under identical conditions. The polarization curves of as-prepared catalyst were shown in Fig. 6a, which were obtained by LSV measurements. As expected, both the bare GCE and MOF-74-Ni show no significant HER activity from 0.1 V to -0.5 V (vs. RHE), while the Pt/C catalyst shows


13


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

superior

HER

activity

with

negligible

overpotential.

Page 14 of 30

After

low

temperature

phosphorization calcinations, the Ni2P Ps-3 showed markedly enhanced electrocatalytic performance close to the Pt/C catalyst with a low onset overpotential of 29 mV and could achieve the current density of 10 mA cm-2 ( j10 ) at an overpotential of 158 mV. Whereas Ni2P particles had an onset overpotential of 85 mV and needed an overpotential of 310 mV to achieve the same current density. The Ni2P Ps-3 has better electrocatalytic performance than the Ni2P particles. Using MOFs as precursors, the Ni2P Ps-3 has a hierarchical pore structure, which is beneficial in exposing more active metal centers which could enhance the activity of the catalyst. The existence of the hierarchical pore structure can make the electrolyte diffuse faster and promote charge transfer through the interface of the working electrode, thus improving the current density and conversion efficiency34, 68. Furthermore, the electrocatalytic performance of the Ni2P Ps-3 is superior to most of the reported TMPs catalysts with a lower loading toward HER (Table S1). The HER kinetics of the as-synthesized samples were obtained using corresponding Tafel plots by fitting the linear regions to the Tafel equation (η = blog j + α , η for overpotential, b for Tafel slope, j for current density, and α for Tafel constant), the Tafel slopes and exchange current densities were calculated for Ni2P Ps-3, Ni2P particles, and Pt/C catalyst (Fig. 6b). The HER on Pt/C with a low Tafel slope of 32 mV dec-1 is close to precious reported values, suggesting that the well known mechanism of Volmer-Tafel has a recombination process as the rate determining step because of the high adsorbed hydrogen (Hads) coverage.69, 70 The Tafel slope for the Ni2P Ps-3 is calculated to be 73 mV dec-1, which is lower than that for Ni2P particles of 109 mV dec-1, suggesting the more efficient HER process by the Ni2P Ps-3, and both of them catalyzed the HER through the


14

Page 15 of 30

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


Volmer-Heyrovsky mechanism with a desorption process being the rate-determining step69, 70. Electrochemical impedance spectroscopy (EIS) measurement was performed on Ni2P Ps-3 and Ni2P particles at overpotential of 160 mV to further investigate the HER kinetics at the interface of electrode/electrolyte. The value of charge-transfer resistance (Rct) is related to the electrocatalytic kinetics of the catalyst, and the lower value of Rct implies a faster reaction rate, which could be calculated from the semicircle diameter in the Nyquist plot.57 As shown in Fig. 6d, the Rct of Ni2P Ps-3 (42 Ω) is much lower than that of Ni2P particles (135 Ω), indicating that the hierarchically porous structure could enhance charge transfer in favor of the kinetics of Ni2P Ps-3 during the HER process.19 In addition, the electrochemical surface areas (ECSAs) was next estimated for further insight into the different catalytic performances, which is proportional to double-layer capacitance (Cdl).46, 71 The Cdl was determined by measuring cyclic voltammograms (CVs) of Ni2P Ps-3 and Ni2P particles with multiple scan rates in nonfaradaic potential region as shown in Fig. S14. The Cdl-derived ECSAs for Ni2P Ps-3 and Ni2P particles were 85.1 and 57.7 cm2, respectively, indicating that Ni2P Ps-3 possess more surface sites available for HER. To obtain the activity data of more fundamental relevance, using the value of ECSA, TOF of Ni2P Ps-3 and Ni2P particles was calculated based on the assumption of a cubic unit cell (see details in supporting information).71 The TOF values of Ni2P Ps-3 and Ni2P particles determined at the potential of -200 mV (vs. RHE) were 0.35s-1 and 0.06s-1, respectively, further confirmed higher intrinsic catalytic activity of Ni2P Ps-3. Using 1.0 M PBS (PH=7) and 1.0 M KOH (PH=14) solutions as electrolyte, the HER activity of the as-prepared Ni2P Ps-3 in neutral and alkaline was evaluated to confirm the intrinsic catalytic activity of Ni2P Ps-320, 72, 73. The Ni2P Ps-3 catalyst performs similar catalytic


15


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 16 of 30

activity in both of neutral and alkaline solutions (see the LSV and Tafel plots in Fig. S15). Fig 6c shows that the Ni2P Ps-3 catalyst could achieve j10 at low overpotential of 164 and 146 mV, with low Tafel slope of 81 and 65 mV dec-1. Because of the practical applications depend strongly on the stability of catalysts, the long-term durability of the Ni2P Ps-3 towards HER was further examined using carbon rod as counter electrode. After continuous CV scanning for 2000 cycles in 0.5 M H2SO4 with a scan rate of 50 mV s-1, the polarization curve showed negligible loss of current density compared with the original one (Fig. 6e), suggesting the superior stability of the Ni2P Ps-3 in the long-term electrochemical process. From the FESEM images (Fig. S16),TEM images (Fig. S17) HRTEM images (Fig. S18), XRD and XPS analysis (Fig. S19), it can be observed that the structure and chemical states of the Ni2P Ps-3 were well preserved after 2000 CV cycles in acid media. In addition, the time-dependent current density curve in 0.5 M H2SO4, 1.0 M PBS, and 1.0 M KOH (Fig. 6f) of the Ni2P Ps-3 was tested. The current density had no significant decrease (96.8%, 94.2%, 94.7% of the initial current density, respectively) after 20 h testing, suggesting the superior long-term durability of the Ni2P Ps-3 in acid, neutral and alkaline solution, futher confirmed the intrinsic catalytic activity of Ni2P Ps-3. The high electrocatalytic activity and structural stability of the Ni2P Ps-3 make it a promising material for practical/commercial applications.

Conclusions In summary, we developed a feasible low temperature in-situ synthesis strategy for a regular hierarchically porous nickel phosphide polyhedron with a single phase of Ni2P from MOFs templates. The obtained Ni2P Ps-3 well inherited the morphology and


16

Page 17 of 30

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


porosity of the MOF precursor, which had a high Brunauer-Emmett-Teller surface area (175.0 m2·g-1) and hierarchically porous properties. On one hand, benefiting from the high metal contents and uniform distribution of the metal centre in MOF morphology, the as-prepared Ni2P polyhedron exhibited perfect dispersity and the active sites were highly exposed. On the other hand, due to the stability of the nickel phosphide crystal, the overall stability of the Ni2P Ps-3 accordingly increased. With the synergistic effect of all these favorable factors, the as-prepared Ni2P Ps-3 exhibited enhanced electrocatalytic activity toward HER with a low overpotential of 158 mV to reach a current density of 10 mA cm-2 and Tafel slopes of 73 mV dec-1, which is superior to most of the reported transition-metal phosphides catalysts with a lower loading (0.124 mg cm-2). Furthermore, the as-prepared Ni2P Ps-3 had a superior long-term durability and their electrocatalytic activity was able to last for at least 20 hours in acid, neutral and alkaline solution. These findings not only promote a new insight for preparing hierarchically porous transitionmetal phosphides, but also highlight the potential application of metal-organic frameworks to synthesize hierarchically porous materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21473254 and 21401215), and the Special Project Fund of “Taishan Scholar” of Shandong Province (No.ts201511017).

Supporting Information XRD patterns of MOF-74-Ni, FESEM images, elemental mapping and Ni/P contents of Ni2P Ps-2, EDS line scan analysis of Ni2P Ps-3, FESEM images of MOF-74-Ni and Ni2P Ps-3


17


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 18 of 30

after 3h thermal treatment (850 oC), N2 isotherm and pore diameter distribution of MOF-74-Ni, XRD patterns of Ni2P particles, N2 isotherm, pore diameter distribution and TEM image of Ni2P particles, CVs with different scan rate and corresponding linear plot of Ni2P Ps-3 and Ni2P particles, Polarization curves and Tafel plots of the Ni2P Ps-3 in an acid, or neutral), or alkaline solution, Comparison of the HER activity for several recently reported TMPs based electrocatalysts in acid solution, FESEM and TEM image of the Ni2P Ps-3 after 2000 CV cycles, XRD patterns, XPS survey spectra, Ni 2p, and P 2p before and after 2000 CV cycles in acidic media of the Ni2P Ps-3.

REFERENCE (1) Pan, Y.; Liu, Y.; Zhao, J.; Yang, K.; Liang, J.; Liu, D.; Hu, W.; Liu, D.; Liu, Y.; Liu, C. Monodispersed Nickel Phosphide Nanocrystals with Different Phases: Synthesis, Characterization and Electrocatalytic Properties for Hydrogen Evolution. J. Mater. Chem. A 2015, 3, 1656-1665. (2) Pan, Y.; Hu, W.; Liu, D.; Liu, Y.; Liu, C. Carbon Nanotubes Decorated with Nickel Phosphide Nanoparticles as Efficient Nanohybrid Electrocatalysts for the Hydrogen Evolution Reaction. J. Mater. Chem. A 2015, 3, 13087-13094. (3) Liu, P.; Rodriguez, J. A.; Takahashi, Y.; Nakamura, K. Water-gas-shift Reaction on a Ni2P(001) Catalyst: Formation of Oxy-phosphides and Highly Active Reaction Sites. J. Catal. 2009, 262, 294-303. (4) Bai, Y.; Zhang, H.; Li, X.; Liu, L.; Xu, H.; Qiu, H.; Wang, Y. Novel Peapod-like Ni2P Nanoparticles with Improved Electrochemical Properties for Hydrogen Evolution and Lithium Storage. Nanoscale 2015, 7, 1446-1453. (5) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267-9270. (6) Jiang, P.; Liu, Q.; Sun, X. NiP2 Nanosheet Arrays Supported on Carbon Cloth: An Efficient 3D Hydrogen Evolution Cathode in both Acidic and Alkaline solutions. Nanoscale 2014, 6, 13440-13445. (7) Sun, M.; Liu, H.; Qu, J.; Li, J. Earth-Rich Transition Metal Phosphide for Energy Conversion and Storage. Adv. Energy Mater. 2016, 6, 1600087-n/a. (8) Wang, H.; Feng, H.; Li, J. Graphene and Graphene-like Layered Transition Metal Dichalcogenides in Energy Conversion and Storage. Small 2014, 10, 2165-2181. (9) Wu, L.; Wang, X.; Sun, Y.; Liu, Y.; Li, J. Flawed MoO2 Belts Transformed from MoO3 on a Graphene Template for the Hydrogen Evolution Reaction. Nanoscale 2015, 7, 7040-7044.


18

Page 19 of 30

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


(10) Shi, Y.; Zhang, B. Recent Advances in Transition Metal Phosphide Nanomaterials: Synthesis and Applications in Hydrogen Evolution Reaction. Chem. Soc. Rev. 2016, 45, 15291541. (11) Wang, X.; Kolen'ko, Y. V.; Bao, X. Q.; Kovnir, K.; Liu, L. One-Step Synthesis of SelfSupported Nickel Phosphide Nanosheet Array Cathodes for Efficient Electrocatalytic Hydrogen Generation. Angew. Chem., Int. Ed. 2015, 54, 8188-8192. (12) Li, X.; Feng, J.; Guo, J.; Wang, A.; Prins, R.; Duan, X.; Chen, Y. Preparation of Ni2P/Al2O3 by Temperature-programmed Reduction of a Phosphate Precursor with a Low P/Ni Ratio. J. Catal. 2016, 334, 116-119. (13) 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. (14) Chen, X.; Wang, D.; Wang, Z.; Zhou, P.; Wu, Z.; Jiang, F. Molybdenum Phosphide: a New Highly efficient Catalyst for the Electrochemical Hydrogen Evolution Reaction. Chem. Commun. 2014, 50, 11683-11685. (15) Xiao, P.; Sk, M. A.; Thia, L.; Ge, X.; Lim, R. J.; Wang, J.-Y.; Lim, K. H.; Wang, X. Molybdenum Phosphide as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction. Energy Environ. Sci. 2014, 7, 2624-2629. (16) McEnaney, J. M.; Crompton, J. C.; Callejas, J. F.; Popczun, E. J.; Read, C. G.; Lewis, N. S.; Schaak, R. E. Electrocatalytic Hydrogen Evolution Using Amorphous Tungsten Phosphide Nanoparticles. Chem. Commun. 2014, 50, 11026-11028. (17) Zhu, X.; Liu, M.; Liu, Y.; Chen, R.; Nie, Z.; Li, J.; Yao, S. Carbon-coated Hollow Mesoporous FeP Microcubes: An Efficient and Stable Electrocatalyst for Hydrogen Evolution. J. Mater. Chem. A 2016, 4, 8974-8977. (18) Jiang, P.; Liu, Q.; Liang, Y.; Tian, J.; Asiri, A. M.; Sun, X. A Cost‐Effective 3D Hydrogen Evolution Cathode with High Catalytic Activity: FeP Nanowire Array as the Active Phase. Angew. Chem., Int. Ed. 2014, 53, 12855-12859. (19) Liu, M.; Li, J. Cobalt Phosphide Hollow Polyhedron as Efficient Bifunctional Electrocatalysts for the Evolution Reaction of Hydrogen and Oxygen. ACS Appl. Mater. Interfaces 2016, 8, 2158-2165. (20) Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X. Self-supported Nanoporous Cobalt Phosphide Nanowire Arrays: An Efficient 3D Hydrogen-evolving Cathode over the Wide Range of pH 0– 14. J. Am. Chem. Soc. 2014, 136, 7587-7590. (21) Xu, M.; Han, L.; Han, Y.; Yu, Y.; Zhai, J.; Dong, S. Porous CoP Concave Polyhedron Electrocatalysts Synthesized from Metal-organic Frameworks with Enhanced Electrochemical Properties for Hydrogen Evolution. J. Mater. Chem. A 2015, 3, 21471-21477. (22) Jiao, L.; Zhou, Y.-X.; Jiang, H.-L. Metal-organic Framework-based CoP/reduced Graphene Oxide: High-performance Bifunctional Electrocatalyst for Overall Water Splitting. Chem. Sci. 2016, 7, 1690-1695. (23) Zhuang, M.; Ou, X.; Dou, Y.; Zhang, L.; Zhang, Q.; Wu, R.; Ding, Y.; Shao, M.; Luo, Z. Polymer-Embedded Fabrication of Co2P Nanoparticles Encapsulated in N,P-Doped Graphene for Hydrogen Generation. Nano Lett. 2016, 16, 4691-4698. (24) Tian, J.; Liu, Q.; Cheng, N.; Asiri, A. M.; Sun, X. Self‐Supported Cu3P Nanowire Arrays as an Integrated High‐Performance Three‐Dimensional Cathode for Generating Hydrogen from Water. Angew. Chem., Int. Ed. 2014, 53, 9577-9581.


19


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 20 of 30

(25) Panneerselvam, A.; Malik, M. A.; Afzaal, M.; O'Brien, P.; Helliwellt, M. The Chemical Vapor Deposition of Nickel Phosphide or Selenide Thin Films from a Single Precursor. J. Am. Chem. Soc. 2008, 130, 2420-2421. (26) Pan, Y.; Yang, N.; Chen, Y.; Lin, Y.; Li, Y.; Liu, Y.; Liu, C. Nickel phosphide Nanoparticles-nitrogen-doped Graphene Hybrid as an Efficient Catalyst for Enhanced Hydrogen Evolution Activity. J. Power Sources 2015, 297, 45-52. (27) Wang, C.; Ding, T.; Sun, Y.; Zhou, X.; Liu, Y.; Yang, Q. Ni12P5 Nanoparticles Decorated on Carbon Nanotubes with Enhanced Electrocatalytic and Lithium Storage Properties. Nanoscale 2015, 7, 19241-19249. (28) 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. (29) 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. (30) Ledendecker, M.; Calderon, S. K.; Papp, C.; Steinrueck, H.-P.; Antonietti, M.; Shalom, M. The Synthesis of Nanostructured Ni5P4 Films and their Use as a Non-Noble Bifunctional Electrocatalyst for Full Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 12361-12365. (31) Yu, X.-Y.; Feng, Y.; Guan, B.; Lou, X. W.; Paik, U. Carbon Coated Porous Nickel Phosphides Nanoplates for Highly Efficient Oxygen Evolution Reaction. Energy Environ. Sci. 2016, 9, 1246-1250. (32) You, B.; Jiang, N.; Sheng, M.; Bhushan, M. W.; Sun, Y. Hierarchically Porous Urchin-Like Ni2P Superstructures Supported on Nickel Foam as Efficient Bifunctional Electrocatalysts for Overall Water Splitting. ACS Catal. 2016, 6, 714-721. (33) Seo, M.; Kim, S.; Oh, J.; Kim, S. J.; Hillmyer, M. A. Hierarchically Porous Polymers from Hyper-cross-linked Block Polymer Precursors. J. Am. Chem. Soc. 2015, 137, 600-603. (34) Sun, M.-H.; Huang, S.-Z.; Chen, L.-H.; Li, Y.; Yang, X.-Y.; Yuan, Z.-Y.; Su, B.-L. Applications of Hierarchically Structured Porous Materials from Energy Storage and Conversion, Catalysis, Photocatalysis, Adsorption, Separation, and Sensing to Biomedicine. Chem. Soc. Rev. 2016, 45, 3479-3563. (35) Saba, S. A.; Mousavi, M. P. S.; Buhlmann, P.; Hillmyer, M. A. Hierarchically Porous Polymer Monoliths by Combining Controlled Macro- and Microphase Separation. J. Am. Chem. Soc. 2015, 137, 8896-8899. (36) Yuan, Z.-Y.; Su, B.-L. Insights into Hierarchically Meso-macroporous Structured Materials. J. Mater. Chem. 2006, 16, 663-677. (37) Pérez-Ramírez, J.; Christensen, C. H.; Egeblad, K.; Christensen, C. H.; Groen, J. C. Hierarchical Zeolites: Enhanced Utilisation of Microporous Crystals in Catalysis by Advances in Materials Design. Chem. Soc. Rev. 2008, 37, 2530-2542. (38) Kanamori, K.; Nakanishi, K. Controlled Pore Formation in Organotrialkoxysilane-derived Hybrids: from Aerogels to Hierarchically Porous Monoliths. Chem. Soc. Rev. 2011, 40, 754-770. (39) Parlett, C. M. A.; Wilson, K.; Lee, A. F. Hierarchical Porous Materials: Catalytic Applications. Chem. Soc. Rev. 2013, 42, 3876-3893. (40) Sun, Q.; Dai, Z.; Meng, X.; Xiao, F.-S. Porous Polymer Catalysts with Hierarchical Structures. Chem. Soc. Rev. 2015, 44, 6018-6034.


20

Page 21 of 30

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


(41) Srinivas, G.; Krungleviciute, V.; Guo, Z.-X.; Yildirim, T. Exceptional CO2 Capture in a Hierarchically Porous Carbon with Simultaneous High Surface Area and Pore Volume. Energy Environ. Sci. 2014, 7, 335-342. (42) Hall, A. S.; Kondo, A.; Maeda, K.; Mallouk, T. E. Microporous Brookite-Phase Titania Made by Replication of a Metal-Organic Framework. J. Am. Chem. Soc. 2013, 135, 1627616279. (43) Zhang, W.; Wu, Z.-Y.; Jiang, H.-L.; Yu, S.-H. Nanowire-Directed Templating Synthesis of Metal-Organic Framework Nanofibers and Their Derived Porous Doped Carbon Nanofibers for Enhanced Electrocatalysis. J. Am. Chem. Soc. 2014, 136, 14385-14388. (44) Zhao, X.; Xiao, B.; Fletcher, A. J.; Thomas, K. M.; Bradshaw, D.; Rosseinsky, M. J. Hysteretic Adsorption and Desorption of Hydrogen by Nanoporous Metal-organic Frameworks. Science 2004, 306, 1012-1015. (45) Xi, K.; Cao, S.; Peng, X.; Ducati, C.; Vasant Kumar, R.; Cheetham, A. K. Carbon with Hierarchical Pores from Carbonized Metal-organic Frameworks for Lithium Sulphur Batteries. Chem. Commun. 2013, 49, 2192-2194. (46) Xu, G.; Ding, B.; Shen, L.; Nie, P.; Han, J.; Zhang, X. Sulfur Embedded in Metal Organic Framework-derived Hierarchically Porous Carbon Nanoplates for High Performance Lithium– sulfur Battery. J. Mater. Chem. A 2013, 1, 4490-4496. (47) Li, Z.; Yin, L. MOF-derived, N-doped, Hierarchically Porous Carbon Sponges as Immobilizers to Confine Selenium as Cathodes for Li–Se Batteries with Superior Storage Capacity and Perfect Cycling Stability. Nanoscale 2015, 7, 9597-9606. (48) Li, L.; Dai, P.; Gu, X.; Wang, Y.; Yan, L.; Zhao, X. High oxygen Reduction Activity on a Metal–organic Framework Derived Carbon Combined with High Degree of Graphitization and Pyridinic-N Dopants. J. Mater. Chem. A 2017, 5, 789-795. (49) Kim, T. K.; Lee, K. J.; Cheon, J. Y.; Lee, J. H.; Joo, S. H.; Moon, H. R. Nanoporous Metal Oxides with Tunable and Nanocrystalline Frameworks via Conversion of Metal-Organic Frameworks. J. Am. Chem. Soc. 2013, 135, 8940-8946. (50) Zhang, X.; Tu, K. N. Preparation of Hierarchically Porous Nickel from Macroporous Silicon. J. Am. Chem. Soc. 2006, 128, 15036-15037. (51) Zhang, Y.; Lan, D.; Wang, Y.; Cao, H.; Jiang, H. MOF-5 Decorated Hierarchical ZnO Nanorod Arrays and its Photoluminescence. Physica E 2011, 43, 1219-1223. (52) Yang, J.; Zhang, F.; Wang, X.; He, D.; Wu, G.; Yang, Q.; Hong, X.; Wu, Y.; Li, Y. Porous Molybdenum Phosphide Nano-Octahedrons Derived from Confined Phosphorization in UIO-66 for Efficient Hydrogen Evolution. Angew. Chem., Int. Ed. 2016, 55, 12854-12858. (53) Ma, B.; Guan, P. Y.; Li, Q. Y.; Zhang, M.; Zang, S. Q. MOF-Derived Flower-like MoS2@TiO2 Nanohybrids with Enhanced Activity for Hydrogen Evolution. ACS Appl. Mater. Interfaces 2016, 8, 26794-26800. (54) Praveen Kumar, D.; Choi, J.; Hong, S.; Reddy, D. A.; Lee, S.; Kim, T. K. Rational Synthesis of MOF-Derived Noble Metal-Free Nickel Phosphide Nanoparticles as a Highly Efficient Co-catalyst for Photocatalytic Hydrogen Evolution. ACS Sustainable Chem. Eng. 2016, 4, 7158-7166. (55) Tian, T.; Ai, L.; Jiang, J. Metal-organic Framework-derived Nickel Phosphides as Efficient Electrocatalysts toward Sustainable Hydrogen Generation from Water Splitting. RSC Adv. 2015, 5, 10290-10295.


21


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 22 of 30

(56) Dietzel, P. D. C.; Besikiotis, V.; Blom, R. Application of Metal-organic Frameworks with Coordinatively Unsaturated Metal Sites in Storage and Separation of Methane and Carbon Dioxide. J. Mater. Chem. 2009, 19, 7362-7370. (57) Wu, C.; Yang, Y.; Dong, D.; Zhang, Y.; Li, J. In Situ Coupling of CoP Polyhedrons and Carbon Nanotubes as Highly Efficient Hydrogen Evolution Reaction Electrocatalyst. Small 2017, 1602873-n/a. (58) Guan, Q.; Li, W. A novel Synthetic Approach to Synthesizing Bulk and Supported Metal Phosphides. J. Catal. 2010, 271, 413-415. (59) Guan, Q.; Li, W.; Zhang, M.; Tao, K. Alternative Synthesis of Bulk and Supported Nickel Phosphide from the Thermal Decomposition of Hypophosphites. J. Catal. 2009, 263, 1-3. (60) Dietzel, P. D. C.; Panella, B.; Hirscher, M.; Blom, R.; Fjellvag, H. Hydrogen adsorption in a nickel based coordination polymer with open metal sites in the cylindrical cavities of the desolvated framework. Chem. Commun. 2006, 959-961. (61) Zhao, Y.; Zhao, Y.; Feng, H.; Shen, J. Synthesis of Nickel Phosphide Nano-particles in a Eutectic Mixture for Hydrotreating Reactions. J. Mater. Chem. 2011, 21, 8137-8145. (62) Mansour, A. Characterization of NiO by XPS. Surf. Sci. Spectra 1994, 3, 231-238. (63) Klein, J. C.; Hercules, D. M. Surface Characterization of Model Urushibara Catalysts. J. Catal. 1983, 82, 424-441. (64) Chen, Y.; She, H.; Luo, X.; Yue, G.-H.; Peng, D.-L. Solution-phase Synthesis of Nickel Phosphide Single-crystalline Nanowires. J. Cryst. Growth 2009, 311, 1229-1233. (65) Zhou, K.; Zhou, W.; Yang, L.; Lu, J.; Cheng, S.; Mai, W.; Tang, Z.; Li, L.; Chen, S. Ultrahigh-Performance Pseudocapacitor Electrodes Based on Transition Metal Phosphide Nanosheets Array via Phosphorization: A General and Effective Approach. Adv. Funct. Mater. 2015, 25, 7530-7538. (66) Schreifels, J.; Maybury, P.; Swartz, W. X-Ray Photoelectron Spectroscopy of Nickel Boride Catalysts: Correlation of Surface States with Reaction Products in the Hydrogenation of Acrylonitrile. J. Catal. 1980, 65, 195-206. (67) Liu, Q.; Tian, J. Q.; Cui, W.; Jiang, P.; Cheng, N. Y.; Asiri, A. M.; Sun, X. P. Carbon Nanotubes Decorated with CoP Nanocrystals: A Highly Active Non-Noble-Metal Nanohybrid Electrocatalyst for Hydrogen Evolution. Angew. Chem. 2014, 126, 6828-6832. (68) Qiu, B. C.; Xing, M. Y.; Zhang, J. L. Mesoporous TiO2 Nanocrystals Grown in Situ on Graphene Aerogels for High Photocatalysis and Lithium-Ion Batteries. J. Am. Chem. Soc. 2014, 136, 5852-5855. (69) Conway, B.; Tilak, B. Interfacial Processes Involving Electrocatalytic Evolution and Oxidation of H2, and the Role of Chemisorbed H. Electrochim. Acta 2002, 47, 3571-3594. (70) Pentland, N.; Bockris, J. M.; Sheldon, E. Hydrogen Evolution Reaction on Copper, Gold, Molybdenum, Palladium, Rhodium, and Iron Mechanism and Measurement Technique under High Purity Conditions. J. Electrochem. Soc. 1957, 104, 182-194. (71) Seo, B.; Baek, D. S.; Sa, Y. J.; Joo, S. H. Shape Effects of Nickel Phosphide Nanocrystals on Hydrogen Evolution Reaction. CrystEngComm 2016, 18, 6083-6089. (72) Yin, J.; Fan, Q. H.; Li, Y. X.; Cheng, F. Y.; Zhou, P. P.; Xi, P. X.; Sun, S. H. Ni-C-N Nanosheets as Catalyst for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2016, 138, 1454614549. (73) Oh, A.; Sa, Y. J.; Hwang, H.; Baik, H.; Kim, J.; Kim, B.; Joo, S. H.; Lee, K. Rational Design of Pt-Ni-Co Ternary Alloy Nanoframe Crystals as Highly Efficient Catalysts toward the Alkaline Hydrogen Evolution Reaction. Nanoscale 2016, 8, 16379-16386.


22

Page 23 of 30

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


Scheme 1 Scheme for the formation procedure of porous Ni2P polyhedrons from MOF-74-Ni.

Figure 1 XRD pattern of MOF-74-Ni, Ni2P Ps-2, and Ni2P Ps-3


23


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 24 of 30

Figure 2 Elemental mapping of C (b), Ni (c), P (d), EDX spectrum (e), and Ni/P contents (f) of Ni2P Ps-3 (a). Scale bar10 µm.


24

Page 25 of 30

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


Figure 3 (a) Low and high (inset)-resolution FESEM images of MOF-74-Ni, (b) Low and high (inset)-resolution FESEM images of the Ni2P Ps-3, (c) TEM (inset: the size distribution pattern of Ni2P nanoparticles ) images of the Ni2P Ps-3, (d) HRTEM images of the Ni2P Ps-3.


25


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 26 of 30

Figure 4 (a) Thermogravimetric measurements of MOF-74-Ni and the Ni2P Ps-3, (b) FESEM image of MOF-74-Ni after 3h thermal treatment (600oC), (c) FESEM image of the Ni2P Ps-3 after 3h thermal treatment (600oC), (d) Nitrogen adsorption-desorption isotherm of the Ni2P Ps-3 (inset: the corresponding DFT pore diameter distribution).


26

Page 27 of 30

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


Figure 5 (a) XPS survey spectra of the Ni2P Ps-3, (b) high magnification XPS spectra in the Ni 2p region of the Ni2P Ps-3, (c) high magnification XPS spectra in the Ni 2p region of the MOF74-Ni, (d) high magnification XPS spectra in the P 2p region of the Ni2P Ps-3.


27


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 28 of 30

Figure 6 (a) Polarization curves of the bare GCE, MOF-74-Ni, Ni2P particles, the Ni2P Ps-3, and Pt/C in 0.5 M H2SO4 solution at a scan rate of 5 mV s−1, (b) Tafel plots of the Ni2P Ps-3, Ni2P particles, and Pt/C, (c) Summary of onsetpotential, Tafel slope and overpotential at j = 10 mA cm-2 for the HER catalyzed by Ni2P Ps-3 in an acid (0.5 M H2SO4), neutral (1.0 M PBS), and alkaline (1.0 M KOH) solution. (d) Electrochemical impedance spectra of Ni2P Ps-3 and Ni2P particles. Rs and Rct represent the electrolyte, charge transfer resistance, respectively. (e) Polarization curves of the Ni2P Ps-3 before and after 2000 cycles of potential sweeps (0.1 to -0.6 V vs RHE) at a scan rate of 100 mV s−1, (f) The current-time curve of the Ni2P Ps-3 measured in 0.5 M H2SO4, 1.0 M PBS, and 1.0 M KOH .


28

Page 29 of 30

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


TOC:


29


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

82x25mm (300 x 300 DPI)


Page 30 of 30

In Situ Synthesis Strategy for Hierarchically Porous Ni2P Polyhedrons

Recommend Documents