In Situ Synthesis Strategy for Hierarchically Porous Ni2P Polyhedrons

Mar 14, 2017 - Herein, a practical in situ synthesis strategy for a three-dimensional Ni2P .... hydrogen evolution reaction electrocatalyst in alkalin...
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In Situ Synthesis Strategy for Hierarchically Porous Ni2P Polyhedrons from MOFs Templates with Enhanced Electrochemical Properties for Hydrogen Evolution Liting Yan,†,‡ Pengcheng Dai,† Ying Wang,†,‡ Xin Gu,† Liangjun Li,† Lei Cao,† and Xuebo Zhao*,† †

Research Centre of New Energy Science and Technology, Research Institute of Unconventional Oil & Gas and Renewable Energy, and ‡College of Science, China University of Petroleum (East China), Qingdao 266580, P. R. China S Supporting Information *

ABSTRACT: The development of highly active and stable noble metal-free electrocatalysts of hydrogen evolution reaction (HER) under both acidic and basic conditions for renewableenergy 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 toward 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. KEYWORDS: nickel phosphide, hierarchical pores, metal−organic frameworks, hydrogen evolution reaction, electrocatalyst

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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 as a consequence of their noble metal free properties and high catalytic activities in hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydroprocessing (HPC), photocatalytic degradation, lithium ion batteries, and HER.7−12 Recent research has demonstrated experimentally the excellent electrocatalytic activity and superior durability toward the HER of various TMPs, such as MoP,13−15 WP,16 FeP,17,18 CoP,19−22 Co2P,23 Cu3P,24 Ni2P,25,26 and Ni12P5.27,28 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 catalyst,29 suggesting the synthesis of nickel phosphide with a © 2017 American Chemical Society

single phase of Ni2P is of great significance in both of theory and practice. The high activity of Ni2P is a 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 moderately bond to reaction intermediates and create a surface with proton and hydride acceptor sites.29,30 Combining high thermostability and catalytic activity, Ni2P could be one of the most practical catalysts to use for HER.29 Recently, Ni 2 P with different morphologies has been extensively investigated and found to be efficient electrocatalysts for HER.10 Compared to other morphologies, porous materials, particularly hierarchically porous materials, frequently show unique and fascinating catalytic performances across many fields.19,21,31,32 Hierarchically porous materials are an intriguing class of porous materials, which possess more than one length levels of pores and structure from micropores, mesopores, to macropores.33−35 Due to their alterable chemical ingredients, low density, high porosity, large surface area, and interconnecting hierarchically porous structure at multiple length levels,34 hierarchically porous materials have a wide potential applied Received: January 20, 2017 Accepted: March 14, 2017 Published: March 14, 2017 11642

DOI: 10.1021/acsami.7b01037 ACS Appl. Mater. Interfaces 2017, 9, 11642−11650

Research Article

ACS Applied Materials & Interfaces

samples was increased from ambient temperature to 1150 °C with a heating rate of 10 °C per min with a constant flow of argon at 100 mL per min. Synthesis of MOF-74-Ni Nanocrystals. MOF-74-Ni was prepared according to the previous report with a slight modification.56 DHTA (3 mmol, 0.594g) was dispersed in 30 mL of THF and nickel acetate tetrahydrate (3 mmol, 0.747g) was dispersed in 30 mL of deionized water (DIW). Subsequently, the two solutions were mixed in a 100 mL autoclave (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 h. After the solution cooled 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 soxheltextraction device and extracted at 120 °C for 48 h, using methanol as the extraction agent. In the end, MOF-74-Ni nanocrystals were obtained by drying in a vacuum drying chamber at 120 °C for 12 h. One-Step Synthesis of Porous Ni2P Polyhedrons. The asprepared 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) with sodium hypophosphite at the upstream positions of the tube furnace. The samples were heated at 275 °C for specific hours with a heating rate of 5 °C·per min under a constant flow of argon at 100 mL·min−1. After naturally cooling 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. A total of 5 mg of as-prepared Ni2P polyhedrons and 100 μL of Nafion solution (5 wt %) were dissolved in a mixture of 950 μL of DIW and 950 μL of ethanol. Then, the mixed solution formed 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 particle size) and rinsed with DIW several times, therewith by ultrasonic treatment in ethyl alcohol and DIW. A total of 3.5 μL of the catalysis slurry was dropped onto the surface of the bare GCE and dried at 60 °C, and then the working electrode was obtained. 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 of 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: EvsRHE = EvsAg/AgCl + 0.197 + 0.059 pH. The activity of Ni2P polyhedrons toward 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.

prospect in energy storage and conversion, catalysis (heterogeneous catalysis, photocatalysis and electrochemical catalysis), adsorption and separation, gas sensing, and biomedicine.34,36−40Metal−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 materials,19,21,41−44 such as porous carbon materials,45−48 nanoporous metal oxides,49−51 and composite materials.52−54 In 2015, Jiang et al., synthesized nickel phosphide nanoparticles by directly phosphorization calcination of a Ni-based metal− organic 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 well inherited.55 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 nanostructures.31 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 three-dimensional 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−Emmett−Teller 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.

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EXPERIMENTAL SECTION

Materials and Characterization. Nickel(II) acetate tetrahydrate (Ni(CH3COO)2·4H2O, 98 wt %), methanol (≥99.5 wt %), sulfuric acid (H2SO4, 95.0−98.0 wt %), and tetrahydrofuran (THF, ≥ 99 wt %) were obtained from Sinopharm Chemical Reagent Co. Ltd. (SCRC). 2,5-Dihydroxyterephthalic acid (DHTA, 98 wt %) was obtained from Chemsoon Co. Ltd.. Sodium hypophosphite (NaH2PO2·H2O, 99 wt %) was obtained from Aladdin. The platinum on carbon(Pt/C, 10 wt %) and Nafion solution (5 wt %) were obtained from Sigma-Aldrich Co. LLC. All of 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, energydispersive X-ray (EDX) spectra, 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 (U.S.A.) spectrometer. N2 adsorption− desorption experiments were performed on an Autosorb-iQ2 (U.S.A.) instrument. Thermogravimetric analysis was performed on a ZRT-A thermogravimetric analyzer (China), and the temperature of the

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RESULTS AND DISCUSSION Material Preparation and Characterization. A Nicentered MOF material (MOF-74-Ni, or CPO-27-Ni) was chosen as the precursor to synthesize porous Ni2P polyhedrons. MOF-74-Ni was obtained according to previous literature with small modifications,56 which is shown in detail in the Experimental Section. PXRD patterns indicated that the synthesized MOF-74-Ni had a good crystal structure identical to the reference (Figure S1). The Ni2P polyhedrons were obtained from the synthesized MOF-74-Ni after phosphoriza11643

DOI: 10.1021/acsami.7b01037 ACS Appl. Mater. Interfaces 2017, 9, 11642−11650

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ACS Applied Materials & Interfaces

elemental mapping images (Figures 2a−d and S3) and EDS line scan analysis (Figure 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 the 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. Figure 3a,b indicates that its regular polyhedral morphology is the same as that of its MOF precursor. After phosphorization calcinations, the morphology of MOF-74-Ni was well maintained while the surface of the polyhedron becomes coarse and porous. Comparison between the high-resolution FESEM images (inset of Figure 3a,b) reveals the highly porous properties of the Ni2P Ps-3. To further examine the microscopic structure of the Ni2P Ps-3, TEM and HRTEM characterizations were carried out. In Figure 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 Figure 3c). From the HRTEM image (Figure 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 (Figures 3b and 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. Figure 4a shows that the Ni2P Ps-3 and MOF74-Ni have similar thermal decomposition temperatures, ranging from 400 to 500 °C, which is accordant with values reported in the previous literature.60 However, Ni2P Ps-3 loses about 18% of its weight, including solvent, water, and small molecules adsorbed from air, when the temperature is at around 500 °C, and loses 3% (wt) further at a temperature up to 780 °C. On the contrary, MOF-74-Ni loses 67% of its weight when the temperature is higher than 500 °C, and there is no obvious lose of weight at 780−850 °C. FESEM images of asprepared samples after thermal treatment were obtained; Figure 4c shows that the Ni2P Ps-3 could maintain its polyhedron morphology after 600 °C thermal treatment, whereas MOF-74Ni (Figure 4b) decomposed after thermal treatment at the same temperature. XRD analysis of MOF-74-Ni and Ni2P Ps-3 after thermal treatment was performed. Figure 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 confirming the stability of Ni2P Ps-3. In addition, to verify the weight loss at 780 °C observed by TG analysis, the 850 °C thermal treatments of MOF-74-Ni (Figure S6) and Ni2P Ps-3 (Figure S7) were carried out. Results indicate that the Ni2P Ps-

tion calcination. The Ni2P polyhedrons with different phosphorization times, such as 2 and 3 h, were termed as Ni2P Ps-2 and Ni2P Ps-3. Figure 1 shows PXRD patterns of the

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

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-74Ni at 7°, 12°, and 49° were also detected, indicating the sample consists of a mixed phase of Ni2P and MOF-74-Ni. After increasing the phosphorization time to 3 h, all of 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 (Figure 2e,f), which is consistent with the XRD patterns. However, the EDX spectrum of Ni2P Ps-2 (Figure S3e,f) showed an atomicratio of Ni−P to be only 18:6, further confirming the part transformation of the nickel in the MOF structures. In addition,

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 bar 10 μm. 11644

DOI: 10.1021/acsami.7b01037 ACS Appl. Mater. Interfaces 2017, 9, 11642−11650

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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, and (d) HRTEM images of the Ni2P Ps-3.

graphic data (∼1.6 nm). In the Ni2P Ps-3, the pore size becomes larger during the phosphorization calcination process with a predominant size of 16.4 nm (Figure 4d, inset). Correspondingly, the BET special surface area of the Ni2P 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 Figure 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 Ni 2P 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; Figure 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 Figures 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 (Figure S13) further confirm the low porosity of the Ni2P particles obtained from nonporous precursor. XPS analysis was performed to get further characteristics of the Ni2P Ps-3 and the MOF-74 precursor. The XPS survey spectra of the Ni2P Ps-3 are shown in Figure 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

Scheme 1. Scheme for the Formation Procedure of Porous Ni2P Polyhedrons from MOF-74-Ni

3 decomposed after 850 °C 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. Figure 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 (Figure 3c). The DFT pore-size distribution of MOF-74-Ni (Figure S8) shows that the pore distribution of the MOF precursor is primarily concentrated at approximately 1.7 nm, which is accordant with the crystallo11645

DOI: 10.1021/acsami.7b01037 ACS Appl. Mater. Interfaces 2017, 9, 11642−11650

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ACS Applied Materials & Interfaces

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 (600 °C), (c) FESEM image of the Ni2P Ps-3 after 3 h of thermal treatment (600 °C), and (d) nitrogen adsorption−desorption isotherm of the Ni2P Ps-3 (inset: the corresponding DFT pore diameter distribution).

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 MOF-74-Ni, and (d) high magnification XPS spectra in the P 2p region of the Ni2P Ps-3.

which are attributed to the Ni2+ in surface NiO and satellite, respectively.61 It can be seen that the binding energy of Ni 2p3/2 negatively shifted from 856.8 eV (Ni2+ in MOF-74-Ni, Figure 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 (Figure 5d), which is corresponding to Pδ‑ in Ni2P and P5+ in some superficial

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 shown in Figure 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, respectively.61 The peak around 861.5 eV is the satellite of Ni 2p3/2.62 Similarly, two peaks are observed around 875.0 and 880.7 eV for the Ni 2p1/2 level, 11646

DOI: 10.1021/acsami.7b01037 ACS Appl. Mater. Interfaces 2017, 9, 11642−11650

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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 and 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.

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 toward the HER. The electrocatalytic performance of the as-prepared Ni2P Ps-3 toward 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 as-prepared 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 the as-prepared catalyst were shown in Figure 6a, which were obtained by LSV measurements. As expected, both the bare GCE and MOF-74Ni show no significant HER activity from 0.1 V to −0.5 V (vs RHE), while the Pt/C catalyst shows superior HER activity with negligible overpotential. 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 efficiency.34,68 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 = b log j + α, η for overpotential, b for Tafel slope, j for current density, and α for Tafel constant), and the Tafel slopes and exchange current densities were calculated for Ni2P Ps-3, Ni2P particles, and Pt/ C catalyst (Figure 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 Volmer−Heyrovsky mechanism with a desorption process being the rate-determining step.69,70 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 Figure 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 11647

DOI: 10.1021/acsami.7b01037 ACS Appl. Mater. Interfaces 2017, 9, 11642−11650

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ACS Applied Materials & Interfaces 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 Figure 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 possesses 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 the Supporting Information).71 The TOF values of Ni2P Ps-3 and Ni2P particles determined at the potential of −200 mV (vs RHE) were 0.35 s−1 and 0.06 s−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-3.20,72,73 The Ni2P Ps-3 catalyst performs similar catalytic activity in both of neutral and alkaline solutions (see the LSV and Tafel plots in Figure S15). Figure 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 the practical applications depend strongly on the stability of catalysts, the long-term durability of the Ni2P Ps-3 toward HER was further examined using carbon rod as the 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 (Figure 6e), suggesting the superior stability of the Ni2P Ps-3 in the long term electrochemical process. From the FESEM images (Figure S16), TEM images (Figure S17), HRTEM images (Figure S18), XRD, and XPS analysis (Figure 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 (Figure 6f) of the Ni2P Ps-3 was tested. The current density had no significant decrease (96.8%, 94.2%, and 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, further 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.

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 h in acid, neutral, and alkaline solution. These findings not only promote a new insight for preparing hierarchically porous transition-metal phosphides but also highlight the potential application of metal−organic frameworks to synthesize hierarchically porous materials.

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 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 center 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

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01037. 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 after 3 h of thermal treatment (850 °C), 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, 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. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Liting Yan: 0000-0002-3600-0323 Notes

The authors declare no competing financial interest.

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

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REFERENCES

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