Preparation of NiCoP Hollow Quasi-Polyhedra and Their

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Preparation of NiCoP Hollow Quasi-Polyhedra and Their Electrocatalytic Properties for Hydrogen Evolution in Alkaline Solution Yapeng Li, Jindou Liu, Chen Chen, Xiaohua Zhang, and Jinhua Chen* State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, P.R. China S Supporting Information *

ABSTRACT: Double metal phosphide (NiCoP) with hollow quasi-polyhedron structure was prepared by acidic etching and precipitation of ZIF-67 polyhedra and further phosphorization treatment with NaH2PO2. The morphology and microstructure of NiCoP quasi-polyhedron and its precursors were investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and a micropore and chemisorption analyzer. Electrocatalytic properties were examined by typical electrochemical methods, such as linear sweep voltammetry, cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy in 1.0 M KOH aqueous solution. Results reveal that, compared with CoP hollow polyhedra, NiCoP hollow quasi-polyhedra exhibit better electrochemical properties for hydrogen evolution with a low onset overpotential of 74 mV and a small Tafel slope of 42 mV dec−1. When the current density is 10 mA cm−2, the corresponding overpotential is merely 124 mV, and 93% of its electrocatalytic activity can be maintained for 12 h. This indicates that NiCoP with hollow quasi-polyhedron structure, bimetallic merit, and low cost may be a good candidate as electrocatalyst in the practical application of hydrogen evolution. KEYWORDS: double metal phosphide, hollow structure, quasi-polyhedron, ZIF-67, hydrogen evolution reaction, alkaline solution WP2,15 and FeP.16 As a typical TMP, CoP is highly effective for catalyzing HER. By exposing and stabilizing more active sites, the catalytic activity of cobalt phosphide-based HER catalysts could be further improved. As a result, different morphologies such as zero-dimensional (0D) nanoparticles,10 one-dimensional (1D) nanowire arrays,17 two-dimensional (2D) nanosheets,11 threedimensional (3D) urchinlike spheres,18 as well as a hollow frame19,20 have been reported. These cobalt phosphide-based catalysts exhibit the transfer of electron density from transition metal to phosphorus. The basic P functioning as proton-acceptor center can promote the formation of M-hydride for subsequent hydrogen evolution by electrochemical desorption in HER.13,15,17,21 Nevertheless, the transformation of the valence electrons merely generates one anionic active site, which leads to a less efficient proton discharge process in HER. The introduction of binary transition metal results in redistributions of the valence electrons and offers two electron donating active sites leading to an increase of HER activity.22 However, metal phosphides with two metal species for HER are still limited in their reporting.23−25

1. INTRODUCTION Hydrogen (H2) is regarded as a renewable and clean energy carrier alternative to fossil fuels.1,2 Water electrolysis is a CO2free route to generate H23 and ordinarily implemented in alkaline or neutral solution. Hence, catalysts for the hydrogen evolution reaction with excellent electrocatalytic performance are highly needed. Pt and Pt-based materials have excellent electronic structure and have emerged as the hydrogen evolution reaction (HER) electrocatalysts with the greatest promise so far.4−7 Nevertheless, high cost and low terrestrial abundance of Pt-group metals prevent their further large scale applications as HER electrocatalysts for hydrogen production. Therefore, studies on nonprecious metal or metal-free electrocatalysts8 are being performed with more attention and interest. A transition metal containing unoccupied d orbitals and unpaired d electrons could be a candidate for replacing the noble metal. Now, transition metal compound catalysts for hydrogen evolution mainly include transition metal disulfides (TMS), transition metal nitride (TMN), transition metal boride (TMB), transition metal carbide (TMC), and transition metal phosphide (TMP). Among these non-noble metal catalysts, TMP has a good electronic structure effect. Simultaneously, owing to their exceedingly low price and expected durability in the operating environments with wide pH range, TMP has attracted increasing recent attention, including Ni2P,9 CoP,10,11 Cu3P,12 MoP,13,14 © 2017 American Chemical Society

Received: November 4, 2016 Accepted: January 25, 2017 Published: January 25, 2017 5982

DOI: 10.1021/acsami.6b14127 ACS Appl. Mater. Interfaces 2017, 9, 5982−5991

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ACS Applied Materials & Interfaces

polyhedra exhibited excellent properties as the HER catalyst with an onset overpotential of 74 mV (versus RHE), a small Tafel slope of 42 mV dec−1, a low overpotential of 124 mV at a current density of 10 mA cm−2 in 1 M KOH with mass loading of 0.283 mg cm−2, and 93% of its catalytic activity maintained for 12 h in alkaline media.

Furthermore, the optimization of morphology is an effective strategy for improving the superiority of nanomaterials. Among various nanoarchitectures, because of their remarkable features such as well-defined interior voids, large surface area, light weight, and short diffusion lengths for the transport of both active species and charge, hollow micro/nanostructures have aroused tremendous interest.26−30 However, there are few reports regarding metal phosphides with hollow nanostructure, which are expected to further improve the HER performance.19,20,31,32 Hollow micro/nanostructures can be produced by using the template method including hard, soft, and sacrificial templates.33−35 Specifically, metal−organic frameworks (MOFs) are a kind of promising sacrificial template or self-template to construct hollow or porous structures.36,37 As a classic example of a metal−organic framework (MOF), the zeolitic imidazolate framework (ZIF-67) contains cobalt ions linked by coordinated dimethyl imidazole ligands into an infinite array.38 Because of its size-tunable polyhedron structure, high carbon content, and uniform distribution of catalytic activity of cobalt, ZIF-67 can be utilized as a perfect template to prepare electrocatalysts with hollow nanostructure.23,39−42 In addition, layered double hydroxides (LDHs) are typical inorganic layered materials that can usually be denoted as the general formula [MII1−xMIIIx(OH)2]z+(An−)z/n·yH2O. Taking advantage of the high dispersion of active species in a layered matrix and 2D nanostructures, LDH could serve as a template to synthesis different catalysts.43,44 Meanwhile, combining MOF and LDH to construct a unique 3D structure is also a promising strategy to obtain materials with excellent properties.45,46 In light of the attractive properties of porous hollow double metal phosphides, we sought to develop a new strategy for their synthesis. Herein, we demonstrated a facile and economical twostep preparation of porous hollow double metal phosphide (NiCoP polyhedra) (Scheme 1). First, a rhombic dodecahedral

2. EXPERIMENTAL SECTION 2.1. Reagents. Cobalt nitrate hexahydrate, nickel nitrate hexahydrate, 2-methylimidazole, methanol, ethanol, and sodium hypophosphite were bought from Sinopharm Chemical Reagent Co. Ltd., China. All chemicals were of analytical grade and used without further purification. 2.2. Preparation of ZIF-67 Polyhedra. ZIF-67 was prepared according to the literature.47 Typically, two solutions were first prepared by dissolving 1 mmol Co(NO3)2·6H2O and 4 mmol 2-methylimidazole in 25 mL of methanol, respectively. Then, the 2-methylimidazole solution was quickly poured into the Co(NO3)2 solution under agitation. After 10 min, the resultant mixed solution was aged for 24 h at room temperature. Finally, a purple precipitate (ZIF-67) was obtained by centrifugation collection, washing with absolute methanol, and drying overnight in vacuum at 60 °C. 2.3. Preparation of NiCo LDH and Co3O4 Polyhedra. First, a certain quantity (250 mg) of Ni(NO3)2·6H2O was added into a 100 mL round bottomed flask containing 50 mL of ethanol. Then, 100 mg of asprepared ZIF-67 powder was transferred into the above solution. The mixture was refluxed for 1 h under stirring at 90 °C. A greenish precipitate denoted as NiCo LDH-2.5 (the mass ratio of Ni(NO3)2· 6H2O to ZIF-67 was 2.5:1) was obtained and harvested by centrifugation, following by washing with anhydrous methanol and vacuum-drying overnight. Similarly, by changing the mass ratio of Ni(NO3)2·6H2O to ZIF-67 to 2:1, 3:1, 3.5:1, and 4:1, the prepared materials were denoted as NiCo LDH-2, NiCo LDH-3, NiCo LDH-3.5, and NiCo LDH-4, respectively (see Table S1). Co3O4 hollow polyhedra were obtained according to the reported method.48 First, 200 mg of ZIF-67 was put in a tube furnace and heated with a rate of 5 °C min−1 and maintained at 350 °C for 0.5 h under Ar gas flow. After switching off the Ar gas and maintaining this temperature for another 0.5 h in air, black products (Co3O4 hollow polyhedra) were obtained. 2.4. Preparation of NiCoP Hollow Quasi-Polyhedra and CoP Polyhedra. The resulting NiCo LDH-n was put in a tube furnace, and 1.0 g of NaH2PO2·H2O was placed at the upstream side. Under Ar gas, the temperature of the furnace was elevated to 300 °C at a rate of 2 °C min−1 and maintained for 2 h. After the system was cooled to ambient temperature under Ar, the final products were denoted as NiCoP-n. On the other hand, the NiCo LDH-2.5 sample was annealed at different temperatures for 2 h under Ar, and the obtained sample was defined as NiCoP-2.5-T. The details for NiCoP-n and NiCoP-2.5-T polyhedra are listed in Table S2. CoP hollow polyhedra were synthesized with the same procedure except the precursor was Co3O4 hollow polyhedron. 2.5. Characterization. The morphology and structure of NiCoP and its precursors were characterized by field-emission scanning electron microscopy (FESEM, JSM-6700F, Japan) with energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN, Holland), powder X-ray diffraction (XRD, D/MAX-RA, Japan), a micropore and chemisorption analyzer (ASAP 2020, USA), Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700, USA) and X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, USA). 2.6. Electrochemical Measurements. The electrochemical measurements were carried out with a typical three-electrode system. A modified glassy carbon (GC) electrode (diameter = 3 mm) was used as the working electrode. A platinum foil (1 × 2 cm2) and a Hg/Hg2Cl2 (saturated KCl) electrode were used as the auxiliary and reference electrodes, respectively. Five milligrams of NiCoP quasi-polyhedra and 20 μL of 5 wt % Nafion ethanol solution were dispersed in 0.98 mL of water and ethanol solution with a volume ratio of 1:1 by sonication to

Scheme 1. Schematic Diagram for the Preparation of Porous NiCoP Hollow Quasi-Polyhedra

ZIF-67 particle was fabricated as a sacrificial template. By the acidic etching and precipitation route, hollow NiCo LDHs with nanosheets were synthesized. Then, via low temperature phosphorization treatment of the NiCo LDH with NaH2PO2 in Ar atmosphere, porous NiCoP was obtained and almost maintained the polyhedron morphology of ZIF-67 and the hollow structure of NiCo LDH. The as-prepared NiCoP 5983


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Figure 1. SEM images of ZIF-67 (A, B), NiCo LDH-2.5 (C, D), and NiCoP-2.5-300 (E, F). Insets of B, D, and F are the corresponding TEM images. form a homogeneous ink. The GC electrode was carefully polished before the preparation of working electrode with 500 and 50 nm Al2O3 slurries, thoroughly rinsed with alcohol and ultrapure water, and dried at room temperature. Then, 4 μL of the above slurries (containing 20 μg of NiCoP quasi-polyhedra) was dropped onto the surface of the polished GC electrode. Linear sweep voltammetry and cyclic voltammetry were performed with a CHI 660D electrochemical workstation (CH Instrument, Chenhua Co., Shanghai, China). Chronoamperometry was executed on an Autolab PGSTAT12 electrochemical workstation (Metrohm, Switzerland). Electrochemical impedance spectroscopy (EIS) results were gained on a Zahner Electrochemical Workstation (Im6ex, Zahner-Elektrik Gmbh & Co. KG, Germany). All of the electrochemical experiments were carried out at room temperature. All of the potentials reported in this work were with IR correction and given versus reversible hydrogen electrode (RHE). The current density shown in this paper is the apparent current density based on the geometric area of the electrode (∼7.07 mm2). For comparison, the electrochemical properties of CoP polyhedra and commercial E-TEK Pt/C catalysts (20 wt %) were also investigated according to the same procedure.

sacrificial templates and reacted with Ni(NO3)2 in alcohol solution, the NiCo LDH-2.5 polyhedra were successfully formed with similar diameter and polyhedral morphology as those of ZIF-67 (Figure 1C). The SEM image recorded at high magnification (Figure 1D) clearly shows that many nanosheets (∼10 nm thick) are interconnected. The hollow structure of NiCo LDH-2.5 with ∼17 nm of shell is clearly observed from the related TEM images (inset plot in Figure 1D). After further phosphorization, it is noted that the obtained NiCoP-2.5-300 particles have quasi-polyhedron morphology with a particle size of ∼500 nm (Figure 1E). The magnified SEM (Figure 1F) and TEM images (inset plot in Figure 1F) further reveal that the assynthesized NiCoP-2.5-300 polyhedra are hollow with ∼13 nm of shell, and many wrinkled nanosheets are attached on their outer shells. The high-resolution TEM (HRTEM) image of NiCoP-2.5-300 (Figure S1) reveals two lattice fringes with space of 2.16 and 2.43 Å corresponding to the (111) crystalline plane of Ni2P/Co2P and (102) crystalline plane of CoP, which indicates the coexistence of Ni2P, Co2P, and CoP. Figure 2 shows the X-ray diffraction (XRD) results of ZIF-67, NiCo LDH-2.5, and NiCoP2.5-300. In Figure 2A, all the diffraction peaks of ZIF-67 match well with the simulated as well as the published results.41 After the solvothermal process, the diffraction peaks of ZIF-67 are totally absent (Figure 2B), indicating the complete disintegration of ZIF-67 during the acidic etching process. However, four new peaks, corresponding to (003), (006), (009), and (110) peaks,

3. RESULTS AND DISCUSSION 3.1. Morphologies and Microstructures of NiCoP Quasi-Polyhedra and Their Precursors. The SEM and TEM images of ZIF-67, NiCo LDH-2.5, and NiCoP-2.5-300 are shown in Figure 1. As is shown, the particles of ZIF-67 (Figure 1A and B) have rhombic dodecahedron morphology with a particle size of ∼550 nm. When ZIF-67 particles were used as the 5984


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can be observed at 11.7°, 22.5°, 33.7°, and 60.2° (Figure 2B), respectively, which are unambiguously ascribed to a typical LDH structure.47,49−51 This indicates clearly that NiCo LDH-2.5 is formed. Furthermore, the FT-IR and XPS results suggest that nitrate ions should be present in the intercalated layers (Figure S2). The possible formula of NiCo LDH-2.5 may be written as [(Ni0.86Co0.14)0.47(Ni0.75Co0.25)0.53(OH)2]0.53+(NO3−)0.53·H2O according to the XPS results. From Figure 2C, there are a number of diffraction peaks of the phosphorization product, which can be well-indexed to Co2P (JCPDS no. 54-0413), CoP (JCPDS no. 65-2593), and Ni2P (JCPDS no. 65-1989), demonstrating that NiCoP-2.5-300 is composed of Co2P, CoP, and Ni2P. However, for the phosphorization of Co3O4, the EDS results of the obtained product (Figure S3A) indicate that Co, P, and O elements exist (the O element may result from exposure of the product to air), and the XRD results further indicate that the obtained product is CoP (JCPDS no. 65-2593) (Figure S3B). The N2 adsorption−desorption investigation was further performed to characterize the porous texture of NiCoP-2.5300. Figure 3A shows the nitrogen adsorption/desorption isotherm plot of NiCoP-2.5-300 hollow polyhedra, and the Brunauer−Emmett−Teller (BET) specific surface area (SSA) of NiCoP-2.5-300 is determined to be 11.5 m2 g−1. The corresponding Barrett−Joyner−Halenda (BJH) pore size distribution curve (Figure 3B) indicates that NiCoP-2.5-300 contains mainly two types of pores:52 the mesopores with a pore size distribution of 2−8 nm, which mainly result from the shell of NiCoP-2.5-300, and the big pores centered at ∼50 nm, which originate from the hollow structure. The hierarchical hollow structure of NiCoP-2.5-300 with macro-/mesopores may provide abundant catalytic active sites and facilitates the transport of electron and active species during the catalysis process.53 For CoP hollow polyhedra, the BET specific surface area is 27.2 m2 g−1, and the pore size distribution mainly ranges from 10 to 100 nm (Figure S3C). The bigger BET SSA of CoP compared to that of NiCoP may result from their smaller diameter. As shown in Figure S3E, after calcination in air, the particle size of Co3O4 polyhedra diminishes to half of that of the original ZIF-67. After further phosphorization calcination, the prepared CoP hollow polyhedra are almost 250 nm (Figure S3F). However, there is mainly one type of pore centered at ∼35 nm in CoP hollow polyhedra (Figure S3D). On the other hand, the electrochemically active surface area (ECSA) of NiCoP-2.5300 hollow polyhedron is still larger than that of the CoP hollow polyhedron (Figure S4). These imply that NiCoP-2.5-300

Figure 2. XRD patterns of ZIF-67 (A), NiCo LDH-2.5 (B), and NiCoP2.5-300 (C).

Figure 3. (A) Nitrogen adsorption−desorption isotherms of NiCoP-2.5-300. (B) Corresponding pore size distribution of NiCoP-2.5-300. 5985


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Figure 4. XPS survey spectrum (A) of NiCoP-2.5-300 hollow quasi-polyhedra and the related high-resolution spectra of Co 2p (B), Ni 2p (C), and P 2p (D).

Figure 5. (A) Polarization curves of NiCoP-2.5-300, CoP, and commercial E-TEK Pt/C in 1 M KOH with a scan rate of 5 mV s−1. The inset presents enlarged plots below a current density of 10 mA cm2. (B) Tafel plots of NiCoP-2.5-300, CoP, and commercial E-TEK Pt/C.

satellite peaks of Ni 2p3/2 and Ni 2p1/2 are centered at 861.9 and 880.5 eV, respectively.32,57 Moreover, the peaks at 130.7 and 129.7 eV (Figure 2D) are close to the binding energies of P 2p1/2 and P 2p3/2 of P in NiCoP-2.5-300, and the peak of P 2p3/2 shows a negative shift from that of the elemental P (130.2 eV).13 Additionally, the broad peak at 133.9 eV is assigned to the oxidized phosphorus species due to exposure of the product to air.25 The XPS data shown in Figure 4 and Figure S2 demonstrate the successful chemical conversion from NiCo LDH-2.5 to NiCoP-2.5-300 via the phosphorization process. Meanwhile, it is noted that both Ni and Co carry a partial positive charge (δ+) and P carries a partial negative charge (δ−), which suggests a small electron density transfer from Ni and Co to P.58 According to the previous report,59 a metal complex HER catalyst incorporates proton relays from pendant acid−base groups positioned close to the metal center where hydrogen evolution occurs. Hence, it is expected that NiCoP-2.5-300 will have good HER activity. 3.2. Electrochemical Properties of NiCoP Hollow Quasi-Polyhedra. Figure 5A shows the polarization curves of NiCoP-2.5-300 hollow quasi-polyhedra, CoP hollow polyhedra,

hollow polyhedra may have better electrocatalytic properties than CoP hollow polyhedra. X-ray photoelectron spectroscopy (XPS) was also used to evaluate the compositions and the valence states of elements in NiCoP-2.5-300. Figure 4A reveals the coexistence of Co, Ni, P and O elements (O element may result from exposure of the product to air), matching well with the XRD results presented above. Figure 4B shows the high-resolution Co 2p spectrum. The peaks at 793.6 and 778.7 eV are assigned to the Co 2p1/2 and Co 2p3/2 of Co species in Co−P,25,54 which have a partial positive from that of Co metal (777.9 eV).55 The peaks at 798.1 and 782.0 eV are assigned to the Co 2p1/2 and Co 2p3/2 of the oxidized cobalt species, respectively, along with two satellite peaks at 802.9 and 785.7 eV.54,56 Similarly, as shown in Figure 4C, the peak at 853.5 eV for Ni 2p3/2 can be ascribed to Niδ+ in Ni−P compound, which is a partial positive from that of metallic Ni (852.3 eV),55 and the peak at 857.0 eV is assigned to Ni2+ in nickel oxide.24 For Ni 2p1/2, the corresponding peak attributed to Niδ+ in the Ni−P compound is located at 870.7 eV, and the peak attributed to Ni2+ is situated at 874.9 eV. The corresponding 5986


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ACS Applied Materials & Interfaces and commercial E-TEK Pt/C obtained by linear sweep voltammetry (LSV) at a scan rate of 5 mV s−1 in 1 M KOH aqueous solution. Argon gas was employed to purge the electrolyte during the hydrogen evolution test to eliminate the dissolved oxygen and to remove hydrogen gas bubbles formed at the electrode surface. On the other hand, an IR correction is applied to all HER data to eliminate the effect of ohmic resistance and to reflect the inherent catalytic behavior. As expected, the commercial E-TEK Pt/C catalyst exhibits excellent HER activity with ultrasmall onset overpotential of 6 mV (overpotential at current density of 1 mA cm−2, which is actually a poorly defined term60) and achieves current density of 10 mA cm−2 (which is the current density expected for a 12.3% efficient solar water-splitting device60) at overpotential of only 32 mV. However, it is surprising that the NiCoP-2.5-300 hollow quasi-polyhedra show a large cathodic current density with a small onset overpotential of 74 mV and achieves current density of 10 mA cm−2 at overpotential of 124 mV, whereas CoP hollow polyhedra have an onset overpotential of 128 mV and needs overpotential of 182 mV to achieve the same current density. These suggest that NiCoP-2.5-300 hollow quasi-polyhedra have much better electrocatalytic performance for HER than CoP hollow polyhedra, although the electrocatalytic activities of NiCoP2.5-300 hollow quasi-polyhedra are still lower than those of the commercial E-TEK Pt/C catalysts. These data obtained from the NiCoP-2.5-300 hollow quasi-polyhedra are also better than those from most of the reported non-noble HER catalysts (Table S3). Figure 5B shows the Tafel plots of NiCoP-2.5-300 hollow quasi-polyhedra, CoP hollow polyhedra, and commercial Pt/C catalysts. It can be obtained that the Tafel slopes in linear portions are 33, 42, and 57 mV dec−1 for the commercial E-TEK Pt/C catalyst, NiCoP-2.5-300 hollow quasi-polyhedra, and CoP hollow polyhedra, respectively. The Tafel slope (33 mV dec−1) of E-TEK Pt/C is consistent with the reported value.61−63 The Tafel slopes of NiCoP-2.5-300 hollow quasi-polyhedra (42 mV dec−1) and CoP hollow polyhedra (57 mV dec−1) imply that the HER may proceed via a Volmer−Heyrovsky mechanism.43,64,65 Obviously, the Tafel slope of NiCoP-2.5-300 hollow quasipolyhedra is smaller than that of CoP hollow polyhedra, suggesting its excellent intrinsic electrocatalytic activity for actuating a large catalytic current density at small overpotential.66 Moreover, the value of exchange current density of NiCoP-2.5300 hollow quasi-polyhedra is calculated to be 0.016 mA cm−2, which is 2.6× larger than that of the CoP hollow polyhedron (0.006 mA cm−2), although it is still much smaller than that of the commercial E-TEK Pt/C catalyst (1.046 mA cm−2). Furthermore, it is noted from Figure 6 that NiCoP-2.5-300 hollow quasipolyhedra have a much smaller semicircle domain than the CoP hollow polyhedra, and the charge transfer resistance (Rct) obtained from the semicircles are 13.5 and 96 Ω for NiCoP-2.5300 hollow quasi-polyhedra and CoP hollow polyhedra, respectively. These results clearly indicate that NiCoP-2.5-300 hollow quasi-polyhedra have much superior catalytic performance over CoP hollow polyhedra due to their higher ECSA (Figure S4) for exposing more catalytic active sites and the unique hollow quasi-polyhedron structure with macro-/mesopores to facilitate both fast mass transport and charge transfer. 3.3. Optimization of the Preparation Parameters of NiCoP Hollow Quasi-Polyhedra. Keeping the phosphorization temperature at 300 °C, the effect of the mass ratio between Ni(NO3)2·6H2O and ZIF-67 on the electrocatalytic properties of NiCoP hollow quasi-polyhedra has been investigated. Among all NiCoP hollow quasi-polyhedra, a sharp increase of the cathodic

Figure 6. Nyquist plots of NiCoP-2.5-300 and CoP at open circuit potential. The inset is the equivalent circuit used to fit the impedance spectra of which Rs, Rct, Zw, and CPE represent electrolyte resistance, electron transfer resistance, Warburg impedance, and the constant phase angle element, respectively.

current at smaller negative potential can be observed on the NiCoP-2.5-300 catalyst (Figure 7A). The onset overpotentials (overpotentials at current densities of 10 mA cm−2) for HER on NiCoP-2-300, NiCoP-2.5-300, NiCoP-3-300, NiCoP-3.5-300, and NiCoP-4-300 catalysts are 130 (216), 74 (124), 67 (129), 124 (192), and 145 (230) mV, respectively (Figure 7B). The Tafel slope values on NiCoP-2-300, NiCoP-2.5-300, NiCoP-3300, NiCoP-3.5-300, and NiCoP-4-300 catalysts are 63, 42, 51, 57, and 62 mV dec−1, respectively (Figure 7C). These results suggest that the HER activity of NiCoP-n-300 increases with the increase of the mass ratio between Ni(NO3)2·6H2O and ZIF-67 and then decreases when the mass ratio is more than 2.5. NiCoP2.5-300 hollow quasi-polyhedra have the best electrocatalytic activity for HER. Therefore, the optimization mass ratio between Ni(NO3)2·6H2O and ZIF-67 is 2.5. The phosphorization temperature is the another important parameter for the electrocatalytic properties of NiCoP hollow quasi-polyhedra. From Figure 8A, it is noted that the onset overpotentials (overpotentials at current densities of 10 mA cm−2) on NiCoP-2.5 hollow quasi-polyhedra at phosphorization temperatures of 260, 280, 300, 320, and 340 °C are 148 (221), 124 (196), 74 (124), 102 (149), and 125 (198) mV, respectively. The corresponding Tafel slope values on NiCoP-2.5 hollow quasi-polyhedra at phosphorization temperatures of 260, 280, 300, 320, and 340 °C are 68, 64, 42, 47, and 62 mV dec−1, respectively (Figure 8B). The results show that the HER activity of NiCoP-2.5-T increases with the increase of phosphorization temperature and then decreases when the temperature is higher than 300 °C. Therefore, 300 °C is selected as the optimal phosphorization temperature in the preparation of NiCoP hollow quasi-polyhedra. 3.4. Long-Term Stability of NiCoP Hollow QuasiPolyhedra. Long-term stability is also a crucial criterion for the practical application of a HER catalyst. The stability of the NiCoP-2.5-300 hollow quasi-polyhedra was first evaluated using cyclic voltammetry by cycling 1000 times between 0 and −124 mV vs RHE. After cycling, the LSV curve of the NiCoP-2.5-300 catalyst is almost the same compared with that of the initial cycle (Figure 9A). However, the related polarization curve of the CoP catalyst shifts obviously in the negative direction (Figure 9B). These results imply that the NiCoP-2.5-300 catalyst has much better long-term stability than that of the CoP catalyst in an alkaline environment. The excellent cycling stability is probably related to the good structural stability of NiCoP-2.5-300. The 5987


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Figure 7. (A) Polarization curves of the as-synthesized NiCoP-n-300. The corresponding histogram (B) of overpotentials (η) at j = 1 and 10 mA cm−2 and the Tafel plots (C). The phosphorization temperature was 300 °C.

Figure 8. (A) Polarization curves of NiCoP-2.5-T obtained at 260, 280, 300, 320, and 340 °C in 1 M KOH and (B) the corresponding Tafel plots.

Figure 9. (A) Polarization curves of NiCoP-2.5-300 before and after 1000 cycles at a scan rate of 100 mV s−1. The insert is the current−time curve of NiCoP-2.5-300 measured at an overpotential of 125 mV in 1 M KOH. (B) Polarization curves of CoP before and after 1000 cycles at a scan rate of 100 mV s−1.

hollow quasi-polyhedra show high activity toward HER with an onset overpotential of 74 mV, a Tafel slope of 42 mV dec−1, and an exchange current density of 0.016 mA cm−2 in alkaline aqueous solution. It requires an overpotential of 124 mV to support a current density of 10 mA cm−2, and its catalytic activity can be maintained at 93% of the initial value at 12 h. NiCoP-2.5300 hollow quasi-polyhedra have superior catalytic performance over that of CoP hollow polyhedra due to their better intrinsic catalytic activity, higher ECSA for exposing more catalytic active sites, and the unique hollow quasi-polyhedron structure with macro-/mesopores to facilitate both fast mass transport and charge transfer. The preparation strategy shown in this paper provides a general methodology to explore the design of double

SEM and EDS results indicate that the post-test NiCoP-2.5-300 after 1000 cycles maintains similar morphology and compositions (Figure S5 and Table S4). The excellent long-term stability of the NiCoP-2.5-300 catalyst is further confirmed by chronoamperometric investigation at an overpotential of 125 mV in 1 M KOH. From the inset plot in Figure 9A, the current density at 12 h is ∼93% of the initial value.

4. CONCLUSIONS NiCoP hollow quasi-polyhedra were prepared by acidic etching and precipitation of ZIF-67 polyhedra and further phosphorization treatment with NaH2PO2. By optimizing the mass ratio of reactants and the phosphating temperature, the NiCoP-2.5-300 5988


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or ternary transition metal phosphides with unique hollow nanostructure for HER and other applications.

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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.6b14127. HRTEM of NiCoP-2.5-300, FTIR and XPS spectrum of NiCo LDH-250, SEM images and XRD patterns of Co3O4 and CoP, BET and EDS results of CoP, cyclic voltammograms of NiCoP and CoP, tables regarding the nomenclature of NiCo LDH-n, NiCoP-n-300, and NiCoP-2.5-T, SEM image and EDS results of NiCoP2.5-300 after HER experiments in 1 M KOH for 1000 cycles, and a comparison of the HER performance (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-731-88821848. E-mail: [email protected]. ORCID

Jinhua Chen: 0000-0001-5089-0927 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC) (21475035, 21235002, 21275041), the Foundation for Innovative Research Groups of NSFC (21521063), and the Program for Changjiang Scholars and Innovative Research Team (IRT1238).

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REFERENCES

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Preparation of NiCoP Hollow Quasi-Polyhedra and Their

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