Cobalt Phosphide Hollow Polyhedron as Efficient Bifunctional

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Cobalt Phosphide Hollow Polyhedron as Efficient Bifunctional Electrocatalysts for the Evolution Reaction of Hydrogen and Oxygen Mengjia Liu and Jinghong Li* Beijing Key Laboratory for Microanalytical Methods and Instrumentation, Department of Chemistry, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: The development of efficient and low-cost hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts for renewable-energy conversion techniques is highly desired. A kind of hollow polyhedral cobalt phosphide (CoP hollow polyhedron) is developed as efficient bifunctional electrocatalysts for HER and OER templated by Cocentered metal−organic frameworks. The as-prepared CoP hollow polyhedron, which have large specific surface area and high porosity providing rich catalytic active sites, show excellent electrocatalytic performances for both HER and OER in acidic and alkaline media, respectively, with onset overpotentials of 35 and 300 mV, Tafel slopes of 59 and 57 mV dec−1, and a current density of 10 mA cm−2 at overpotentials of 159 and 400 mV for HER and OER, respectively, which are remarkably superior to those of particulate CoP (CoP particles) and comparable to those of commercial noble-metal catalysts. In addition, the CoP hollow polyhedron also show good durability after long-term operations. KEYWORDS: cobalt phosphide, hollow, polyhedron, hydrogen evolution reaction, oxygen evolution reaction solar cells.21 Among them, transition-metal phosphides have been widely used as industrial catalysts for hydrodesulfurization and hydrodenitrification involving the adsorption and desorption of H2.22,23 Accordingly, it can be inferred that phosphides have high affinities for H2 and they could be suitable for HER electrocatalysis. In the last years, great effort has been devoted to developing efficient nanostructured transition-metal phosphides for HER,24−26 among which cobalt (Co)-based phosphides have emerged much with different morphologies such as nanowires,27 nanorods,28 nanosheets,29 and nanoparticles30 as well as ternary phosphides.31 Nevertheless, previously reported OER catalysts mainly covered transitionmetal oxides32,33 and hydroxides34 with very little transitionmetal phosphides. Recently, a kind of electrodeposited Co−Pderived film was developed as a competent bifunctional catalyst for an overall water splitting with good performances; however, this catalyst was amorphous without regular morphology,35 while morphology would play an important part in optimization of the catalytic performances. Compared to previous zero-, one-, and two-dimensional catalysts, three-

1. INTRODUCTION Hydrogen (H2) is a green energy of high energy density without carbon dioxide release in the energy conversion process so it has attracted much attention.1,2 Electrolysis is an efficient reliable technology for hydrogen evolution reaction (HER) with a simple process, little pollution, and high product purity.3 However, the high overpotential in the cathode leads to an increase of the electrolysis cell voltage and power consumption, which has limited large-scale industrial application.4,5 On the other hand, oxygen evolution reaction (OER) is a key step in many renewable-energy conversion techniques such as regenerative fuel cells and rechargeable metal−air batteries.6−8 However, the sluggish kinetics of OER at the anode imposes considerable electrochemical overpotential requirements.9,10 Platinum and ruthenium/iridium oxides have been recognized as the most efficient electrocatalysts for HER and OER, respectively, but their limited reserves and high costs are great hinders.11−13 Therefore, the development of highly efficient non-noble-metal electrocatalysts is desirable and also urgent. Transition-metal compounds such as chalcogenides, nitrides, and carbides have high stability, corrosion resistance, melting point, and mechanical properties as well as low cost and thus have been good candidates in many electrochemistry fields including electrocatalysis,14−18 lithium-ion batteries,19,20 and © 2015 American Chemical Society

Received: November 6, 2015 Accepted: December 29, 2015 Published: December 29, 2015 2158

DOI: 10.1021/acsami.5b10727 ACS Appl. Mater. Interfaces 2016, 8, 2158−2165

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ACS Applied Materials & Interfaces

Scheme 1. Schematic Diagram To Illustrate the HER and OER Catalytic Principles on CoP Hollow Polyhedra

Electrode Preparation. The glassy carbon electrode (GCE, 5 mm in diameter) was polished by a 0.3 μm alumina slurry to obtain a mirror-like surface, then washed with ethanol and distilled water with the assistance of sonication, and blow-dried by N2 airflow prior to use. Samples (2 mg) were dissolved in a solvent mixture (1:9, v/v) of Nafion (5%) and water (2 mL) by sonication to form a suspension (1 mg mL−1). Subsequently, the catalyst (20 μL) was dropped onto the surface of the prepolished GCE. Then the electrode was allowed to dry at room temperature for 12 h before measurements (loading ∼0.102 mg cm−2). Characterization. Powder X-ray diffraction (XRD) patterns were conducted on a Bruker D8-Advance using Cu Kα radiation (λ = 1.5418 Å). Scanning electron microscopy (SEM) images were recorded on a Hitachi SU8010 scanning electron microscope. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), scanning TEM (STEM), and energy-dispersive X-ray (EDX) elemental mapping images were all recorded on a JEM 2010 (120 kV) high-resolution transmission electron microscope. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantera scanning X-ray microprobe using a monochromic Al Kα (λ = 1486.7 eV; the binding energy is calibrated with C 1s, 284.8 eV). The Brunauer−Emmett−Teller (BET) surface area, pore volume, and pore size were measured on a Quantachrome NOVA 1000 system at liquid-N2 temperature. Electrochemical Measurements. Electrochemical measurements were conducted using a computer-controlled potentionstation (CHI 1030B, CH Instruments, Inc., USA). The electrochemical cell was assembled with a conventional three-electrode system: a Pt wire electrode as the counter electrode, an Ag/AgCl-saturated KCl electrode as the reference electrode, and the sample modified GCE as the working electrode. For HER measurements, electrolyte solutions were saturated with nitrogen by bubbling N2 prior to the start of each measurement, and a flow of N2 was maintained over the electrolyte during the scanning process to exclude O2. Potentials were referenced to a reversible hydrogen electrode (RHE) by adding a value of (0.197 + 0.059pH) V. All data are presented without iR compensation.

dimensional porous polyhedral morphology could provide larger specific surface area, more catalytic sites, and more interconnected electron-transfer access, thus enhancing the electrocatalytic activities. To realize porous polyhedral morphology, a proper template is needed. Metal−organic frameworks (MOFs) are an intriguing class of porous crystalline inorganic−organic hybrid materials, and they have been widely used in gas storage or separation,36 catalysis,37 drug delivery,38 and imaging fields.39 Recently, taking them as sacrificial templates to synthesis porous and electroactive materials for electrocatalysis has been springing up. Among them, however, the regualr morphologies of many MOFs could not be maintained well, so the advantages of MOFs, such as large specific surface area and high porosity, could not be fully utilized.40,41 Herein we developed regular hollow polyhedral cobalt phosphide (CoP hollow polyhedron) electrocatalysts for HER and OER templated by Co-centered MOFs through simple oxidation and phosphorization calcinations. The CoP hollow polyhedron showed excellent electrocatalytic performances for both HER and OER in acidic and alkaline media, respectively, with onset overpotentials of 35 and 300 mV, Tafel slopes of 59 and 57 mV dec−1, and a current density of 10 mA cm−2 at overpotentials of 159 and 400 mV, respectively, which were remarkably superior to those of particulate CoP (CoP particles) and comparable to those of commercial noble-metal catalysts. Scheme 1 shows a schematic diagram to illustrate the HER and OER catalytic principles on CoP hollow polyhedron.

2. EXPERIMENTAL SECTION Materials Synthesis. ZIF-67 MOFs were synthesized by a modified reported method.42 Typically, Co(NO3)2·6H2O (0.996 g) and 2-methylimidazole (1.312 g) were dissolved in methanol (100 mL), respectively. Then the latter ligand solution was poured into the former salt solution under vigorous stirring. The mixture was stirred for 10 min and then kept for 24 h at room temperature. The solid product was separated by centrifugation and washed with methanol three times, followed by vacuum drying at 60 °C for 8 h. As-produced ZIF-67 MOFs were put into a tube furnace and annealed at 300 °C for 2 h in air to evolve into Co3O4 polyhedron. Then Co3O4 polyhedron and NaH2PO2·H2O were placed at two separate positions in a porcelain boat with NaH2PO2·H2O at the upstream side of the furnace. The molar ratio of Co-to-phosphorus (P) is 1:20. Subsequently, the samples were heated at 250 °C for 2 h in a static N2 atmosphere and then naturally cooled to ambient temperature to obtain CoP hollow polyhedron. CoP particles were synthesized from commercial Co3O4 particles through the same procedures as those above. Commercial platinum/carbon (Pt/C) catalysts (20% Pt on Vulcan XC-72R) were purchased from E-TEK Division, PEMEAS Fuel Cell Technologies, and all other chemicals were purchased from SigmaAldrich and used without any further purification.

3. RESULTS AND DISCUSSION 3.1. Material Preparation and Characterization. A Cocentered MOF material ZIF-67 was chosen as the template to synthesize CoP hollow polyhedron. We synthesized ZIF-67 according to the method previously reported,42 and XRD results show that the ZIF-67 synthesized had good crystallinity identical with the reference (Figure S1). Then the Co3O4 polyhedron were synthesized from ZIF-67 after oxidation in air, and CoP hollow polyhedron were further obtained from Co3O4 polyhedron after phosphorization calcination (see the Experimental Section for details). Figure 1 shows XRD patterns of Co3O4 polyhedron and CoP hollow polyhedron. The Co3O4 polyhedron show nine strong diffraction peaks corresponding to the (111), (220), (311), (222), (400), (422), (511), (440), and (533) planes of Co3O4, respectively (JCPDS 42-1467). After phosphorization, only the diffraction peaks of the CoP phase can be observed, which are indexed to the (011), (111), 2159


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To further examine the microscopic structures of CoP hollow polyhedron, TEM characterizations were performed. In Figure 2D, we can see a regular polyhedral morphology with some cracks and pores on it. From the HRTEM image (Figure 2E), a clear crystal lattice fringe can be observed with a spacing of 0.283 nm corresponding to the (011) plane of CoP. Figure 2F is the selected-area electron diffraction (SAED) pattern of CoP hollow polyhedron, where a series of well-defined rings can be assigned to the (011), (111), (211), (103), and (301) planes of orthorhombic CoP. Besides, STEM was applied to investigate the structures of CoP hollow polyhedron, and regular shapes and large numbers of pores could be observed. We further made EDX elemental mapping for CoP hollow polyhedron showing a uniform distribution of the Co and P elements around the whole polyhedron (Figure 2G). To compare with CoP hollow polyhedron, we synthesized CoP particles by phosphorization of the corresponding Co3O4 particles. Figure S2A shows the XRD patterns of Co3O4 particles and CoP particles in which both are crystalline. Their morphologies are both particulate aggregations (Figure S2B,C). There is a significant difference between CoP hollow polyhedron and CoP particles in their specific surface area and porous nature from the N2 adsorption/desorption isotherms. CoP hollow polyhedron had a BET surface area of about 46.9 m2 g−1, while for CoP particles, it only had 8.0 m2 g−1 (Figure S3A). The Barrett−Joyner−Halenda pore-size-distribution curve of CoP hollow polyhedron showed a broad peak ranging from 5 to 30 nm and a narrow peak at about 3 nm with relatively high pore volumes to CoP particles (Figure S3B). The above results suggest that the wrinkles and cracks on CoP hollow polyhedron would provide large specific surface area and high porosity, thus promoting the transfer of electrons and masses in the electrocatalysis process. XPS measurements were further conducted to characterize CoP hollow polyhedron and CoP particles. From XPS survey spectra, we can see that there are Co, P, oxygen (O), and some C elements, among which O and C should arise from superficial oxidation of CoP because of air contact43,44 and binding energy calibration, respectively (Figure 3A). High-

Figure 1. XRD patterns of Co3O4 polyhedron and CoP hollow polyhedron.

(112), (211), (103), (020), (301), and (222) planes of CoP (JCPDS 29-0497), respectively. These observations suggest the successful conversion of Co3O4 into CoP. SEM characterizations were carried out to investigate the morphology evolution of ZIF-67, Co3O4 polyhedron, and CoP hollow polyhedron. As shown in Figure 2A, ZIF-67 shows a regular rhombic dodecahedral shape with a size of about 700 nm, and from the inset, we can observe a smooth surface. After calcination in air, the smooth surface became depressed and wrinkled, while the polyhedral morphology remained basically unchanged for Co3O4 polyhedron (Figure 2B and inset). After further phosphorization calcination to CoP hollow polyhedron, they still showed regular polyhedral morphology with some wrinkles and cracks on the surface (Figure 2C and inset). The above results suggest that the oxidation and phosphorization calcinations could successfully transform ZIF-67 templates into CoP hollow polyhedron without morphology damage.

Figure 2. Low- and (inset) high-magnification SEM images of (A) ZIF-67, (B) Co3O4 polyhedron, and (C) CoP hollow polyhedron. Inset scale bar: 300 nm. (D) TEM image, (E) HRTEM image, and (F) SAED patterns of CoP hollow polyhedron. (G) STEM image and EDX elemental mapping of Co and P for CoP hollow polyhedron. 2160


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Figure 3. (A) XPS survey spectra and (B) high-magnification XPS spectra in the Co 2p region of CoP hollow polyhedron and CoP particles. Highmagnification XPS spectra in the P 2p region of (C) CoP hollow polyhedron and (D) CoP particles.

than those in CoP hollow polyhedron (Figure 3B,D), implying that the superficial oxidation took place more on the surface of CoP particles than CoP hollow polyhedron. Maybe the contacts and aggregations in particles would make the oxidation process spread and thus influence the electrocatalytic activities of CoP particles. To investigate the electrical conductivities of CoP hollow polyhedron and CoP particles, electrochemical impedance spectroscopy (EIS) spectra were conducted in a 5 mM Fe(CN)63−/4− solution containing 0.5 M KCl from 0.1 Hz to 100 kHz with a signal amplitude of 10 mV (Figure S4). Through fitting to the equivalent circuit (Figure S4, inset), electron-transfer resistance (Rct) could be obtained. CoP hollow polyhedron showed better electrical conductivity with Rct of 65.03 Ω than CoP particles (Rct = 94.55 Ω), suggesting that the porous polyhedral structure could enhance charge transfer in favor of the proceedings of the electrocatalysis process. In addition, the electrocatalytic activity of a material often depends on its specific surface area. Therefore, the double-layer capacitances (Cdl) of the two CoP catalysts, proportional to their electrochemical specific surface areas, were measured in a 1 M KCl solution, as shown in Figure S5. The Cdl values of CoP hollow polyhedron and CoP particles are calculated to be 112.12 and 37.03 μF cm−2, respectively, suggesting the large specific surface area of CoP hollow polyhedron, which would have an effect on their electrocatalytic activities.

magnification XPS spectra in the Co 2p and P 2p regions of CoP hollow polyhedron are shown in parts B and C of Figure 3, respectively. The existence of the Co−O and P−O components agrees with that in the survey spectra as oxidized Co and P species. Co 2p3/2 (778.6 eV) and P 2p3/2 (129.1 eV) are considered to stand for the binding energies of Co and P in CoP,45 and it can be seen that they are positively and negatively shifted from the binding energies of metallic Co (778.1−778.2 eV) and elemental P (130.2 eV), respectively,46 suggesting that the Co atom in CoP had a partial positive charge and the P atom had a partial negative charge.45,47 Therefore, electron transfer should exist from Co to P, and it would promote adsorption and desorption of reactant and product molecules, respectively, in the electrocatalysis process.48 For HER, the positively charged Co centers and the negatively charged basic P centers could act as the hydride-acceptor and proton-acceptor centers, respectively, and the P centers could promote the formation of cobalt hydride, which could facilitate the following H2 evolution by electrochemical desorption,49 while for OER, it should be the same principle that the positively charged Co centers could act as hydroxyl acceptors and the negatively charged P centers could facilitate the adsorption of hydroxyl on Co centers then in favor of oxygen evolution through discharging and desorption. As is the case with CoP particles, there is also a charge transfer from Co (778.9 eV) to P (129.3 eV). On the other hand, it can be observed that the ratios of Co−O and P−O relative to Co 2p3/2 and P 2p3/2, respectively, are much higher 2161


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Figure 4. (A) Polarization curves of bare GCE, ZIF-67, Co3O4 polyhedron, CoP hollow polyhedron, CoP particles, and Pt/C in a N2-saturated 0.5 M H2SO4 solution. Scan rate: 2 mV s−1. (B) Tafel plots of CoP particles, CoP hollow polyhedron, and Pt/C. Polarization curves of (C) CoP hollow polyhedron and (D) CoP particles before and after potential sweeps (+0.1 to −0.6 V vs RHE) at a scan rate of 50 mV s−1 for 3000 cycles in a N2saturated 0.5 M H2SO4 solution. Scan rate: 2 mV s−1. Inset: Time dependence of the current density for (C) CoP hollow polyhedron and (D) CoP particles at a static overpotential of 159 mV for 20 h. Catalyst loading: 0.102 mg cm−2.

coverage.50,51 The Tafel slope for CoP hollow polyhedron was 59 mV dec−1 lower than that for CoP particles of 77 mV dec−1, revealing the more efficient HER process on CoP hollow polyhedron, and both of them catalyzed the proceedings of HER through the Volmer−Heyrovsky mechanism with a desorption process as the rate-determining step.50,51 The exchange current density of CoP hollow polyhedron was calculated to be 0.037 mA cm−2, significantly superior to that of CoP particles (0.005 mA cm−2). Compared to other reported phosphide HER catalysts,24−31 our CoP hollow polyhedron could rank ahead given the good catalytic performances and low electrode loading. In addition, the CoP hollow polyhedron also showed high durability. After 3000 potential sweeps, the polarization curve of CoP hollow polyhedron exhibited negligible loss compared to the initial one (Figure 4C), while for CoP particles, a significant loss of the current density appeared after 3000 cycles (Figure 4D). A 20 h static overpotential (η = 159 mV) electrolysis showed a stable current density over the entire time for CoP hollow polyhedron (Figure 4C, inset). For CoP particles, however, only 81% of the initial current density remained after the same period of time (Figure 4D, inset). Figure S6 shows SEM images of CoP hollow polyhedron and CoP particles after 3000 cyclic voltammetry (CV) cycles. It can be seen that CoP hollow polyhedron still kept the polyhedral morphology (Figure S6A)

3.2. Electrocatalytic Performances of HER and OER. We first evaluated the HER activities of CoP hollow polyhedron and CoP particles with the same loading of approximately 0.102 mg cm−2 in an N2-saturated 0.5 M H2SO4 solution at a scan rate of 2 mV s−1. For comparison, bare GCE, ZIF-67, Co3O4 polyhedron, and commercial Pt/C catalysts were also examined. Figure 4A shows the polarization curves for the above electrodes without iR compensation. We can see that both bare GCE and ZIF-67 showed barely HER performances. Co3O4 polyhedron had some catalytic activity only under very high overpotentials. After phosphorization, CoP hollow polyhedron showed remarkably enhanced electrocatalytic performances close to Pt/C with an onset overpotential of 35 mV and could achieve current densities of 1 and 10 mA cm−2 at overpotentials of 70 and 159 mV, respectively, whereas CoP particles had an onset overpotential of 75 mV and needed overpotentials of 187 and 355 mV to achieve the same current densities. Through fitting to the Tafel equation (η = b log j + a, where η is the overpotential, b is the Tafel slope, j is the current density, and a is the exchange current density), the Tafel slopes and exchange current densities could be calculated for CoP hollow polyhedron, CoP particles, and commercial Pt/ C catalysts (Figure 4B). The Tafel slope for Pt/C was close to the reported value as 30 mV dec−1, implying the Volmer−Tafel catalysis mechanism with a recombination process as the ratedetermining step because of high adsorption hydrogen (Hads) 2162


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Figure 5. (A) Polarization curves of bare GCE, ZIF-67, Co3O4 polyhedron, CoP hollow polyhedron, and CoP particles in a 1 M KOH solution. Scan rate: 2 mV s−1. (B) Tafel plots of CoP particles and CoP hollow polyhedron. Polarization curves of (C) CoP hollow polyhedron and (D) CoP particles before and after potential sweeps (1.3−1.8 V vs RHE) at a scan rate of 50 mV s−1 for 3000 cycles in a 1 M KOH solution. Scan rate: 2 mV s−1. Inset: Time dependence of the current density for (C) CoP hollow polyhedron and (D) CoP particles at a static overpotential of 400 mV for 20 h. Catalyst loading: 0.102 mg cm−2.

loss than CoP particles after 3000 potential sweeps as well as 20 h of potentiostatic electrolysis at η = 400 mV (Figure 5C,D). The morphology changes after long-term operations were also similar to those in HER. These results suggest that the structures of CoP hollow polyhedron are quite stable, making them promising for practical applications. From the above results, we can get an outline of the origins for the superior electrocatalytic performances of CoP hollow polyhedron over CoP particles. At first, the polyhedral morphology provides large specific surface area and high porosity, which could facilitate the exposure of more catalytic active sites and transfer of electrons and masses. Second, there are more surface-oxidized species on CoP particles than on CoP hollow polyhedron. A too thick surface oxidation layer would cover catalytic sites and hinder the diffusion of reactants and products, thus decreasing the catalytic activities. Last, resulting from the porous polyhedral morphology and less surface oxidation, CoP hollow polyhedron have higher electrical conductivities and electrochemical specific surface areas than CoP particles in favor of fast electron and mass transport around the whole polyhedral structure.

while CoP particles aggregated to bulk shape, thus decreasing the activities (Figure S6B). We next assessed the catalytic activities of CoP hollow polyhedron and CoP particles for OER in a 1 M KOH solution. As the same in HER, bare GCE and ZIF-67 showed hardly any catalytic performance for OER. Co3O4 polyhedron had some performances, but they did not show an appreciable anodic current until ∼1.65 V vs RHE. CoP hollow polyhedron exhibited excellent catalytic performances with an onset overpotential of 300 mV and achieved current densities of 10 and 50 mA cm−2 at overpotentials of 400 and 497 mV, respectively, while CoP particles showed lower performances than polyhedron with an onset overpotential of 340 mV and required an overpotential of 427 mV to reach the current density of 10 mA cm−2 (Figure 5A). Linear fitting of their Tafel plots resulted in Tafel slopes of 57 and 59 mV dec−1 for CoP hollow polyhedron and CoP particles, respectively (Figure 5B). The above results are comparable to those of many previously reported non-noble-metal OER elecrocatalysts32−34,52,53 and even higher than those of commercial RuO2 catalysts,13 although they are a little lower than those of some nickel phosphide catalysts.54,55 This further suggests the advantage of polyhedral structure that could provide more active sites and more fluent electron-transfer accesses to promote the proceedings of electrocatalysis. Besides the high OER activity, the CoP hollow polyhedron also featured excellent long-term durability in OER catalysis, showing up as less current density

4. CONCLUSIONS CoP hollow polyhedron were developed as efficient bifunctional electrocatalysts for both HER and OER through oxidation and phosphorization calcinations of Co-centered MOFs. CoP hollow polyhedron exhibited excellent catalytic 2163


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(10) Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, P. The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis. ChemCatChem 2010, 2, 724− 761. (11) Merki, D.; Hu, X. L. Recent Developments of Molybdenum and Tungsten Sulfides as Hydrogen Evolution Catalysts. Energy Environ. Sci. 2011, 4, 3878−3888. (12) Chen, W. F.; Muckerman, J. T.; Fujita, E. Recent Developments in Transition Metal Carbides and Nitrides as Hydrogen Evolution Electrocatalysts. Chem. Commun. 2013, 49, 8896−8909. (13) Li, L. L.; Tian, T.; Jiang, J.; Ai, L. H. Hierarchically Porous Co3O4 Architectures with Honeycomb-Like Structures for Efficient Oxygen Generation from Electrochemical Water Splitting. J. Power Sources 2015, 294, 103−111. (14) Cao, B. M.; Veith, G. M.; Diaz, R. E.; Liu, J.; Stach, E. A.; Adzic, R. R.; Khalifah, P. G. Cobalt Molybdenum Oxynitrides: Synthesis, Structural Characterization, and Catalytic Activity for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2013, 52, 10753−10757. (15) Zhang, L.; Zhang, Q.; Lu, X. B.; Li, J. H. Direct Electrochemistry and Electrocatalysis Based on Film of Horseradish Peroxidase Intercalated into Layered Titanate Nano-Sheets. Biosens. Bioelectron. 2007, 23, 102−106. (16) Jin, Q.; Pei, L. K.; Hu, Y. X.; Du, J.; Han, X. P.; Cheng, F. Y.; Chen, J. Solvo/Hydrothermal Preparation of MnOx@rGO Nanocomposites for Electrocatalytic Oxygen Reduction. Huaxue Xuebao 2014, 72, 920−926. (17) Wu, L. F.; Wang, X. S.; Sun, Y. P.; Liu, Y.; Li, J. H. Flawed MoO2 Belts on Graphene Template Transformated from MoO3 towards Hydrogen Evolution Reaction. Nanoscale 2015, 7, 7040− 7044. (18) Yan, H. J.; Tian, C. G.; Wang, L.; Wu, A. P.; Meng, M. C.; Zhao, L.; Fu, H. G. Phosphorus-Modified Tungsten Nitride/Reduced Graphene Oxide as a High-Performance, Non-Noble-Metal Electrocatalyst for the Hydrogen Evolution Reaction. Angew. Chem., Int. Ed. 2015, 54, 6325−6329. (19) Xu, H. T.; Zhang, H. J.; Fang, L.; Yang, J.; Wu, K.; Wang, Y. Hierarchical Molybdenum Nitride Nanochexes by a Textured SelfAssembly in Gas−Solid Phase for the Enhanced Application in Lithium Ion Batteries. ACS Nano 2015, 9, 6817−6825. (20) Wang, H.; Feng, H. B.; Li, J. H. Graphene and Graphene-like Layered Transition Metal Dichalcogenides in Energy Conversion and Storage. Small 2014, 10, 2165−2181. (21) Azarpira, A.; Lublow, M.; Steigert, A.; Bogdanoff, P.; Greiner, D.; Kaufmann, C. A.; Kruger, M.; Gernert, U.; van de Krol, R.; Fischer, A.; Schedel-Niedrig, T. Efficient and Stable TiO2: Pt−Cu(In,Ga)Se2 Composite Photoelectrodes for Visible Light Driven Hydrogen Evolution. Adv. Energy Mater. 2015, 5, 1402148. (22) Silva-Rodrigo, R.; Jimenez, H. C.; Guevara-Lara, A.; MeloBanda, J. A.; Sarabia, A. O.; de la Torre, A. I. R.; Flores, F. M.; Mares, A. C. Synthesis, Characterization and Catalytic Properties of NiMoP/ MCM41-γAl2O3 Catalysts for DBT Hydrodesulfurization. Catal. Today 2015, 250, 2−11. (23) Badari, C. A.; Lonyi, F.; Dobe, S.; Hancsok, J.; Valyon, J. Catalytic Hydrodenitrogenation of Propionitrile over Supported Nickel Phosphide Catalysts as a Model Reaction for the Transformation of Pyrolysis Oil Obtained from Animal By-Products. React. Kinet., Mech. Catal. 2015, 115, 217−230. (24) Jiang, P.; Liu, Q.; Liang, Y. H.; Tian, J. Q.; Asiri, A. M.; Sun, X. P. 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. (25) Wang, X. G.; Kolen’ko, Y. V.; Bao, X. Q.; Kovnir, K.; Liu, L. F. One-Step Synthesis of Self-Supported Nickel Phosphide Nanosheet Array Cathodes for Efficient Electrocatalytic Hydrogen Generation. Angew. Chem., Int. Ed. 2015, 54, 8188−8192. (26) Xiao, P.; Sk, M. A.; Thia, L.; Ge, X. M.; 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.

performances with low overpotentials of 159 and 400 mV to reach a current density of 10 mA cm−2 and Tafel slopes of 59 and 57 mV dec−1 for HER and OER, respectively, as well as strong durability, which were remarkably superior to those of CoP particles and comparable to those of noble-metal catalysts. In addition, the raw materials to prepare CoP hollow polyhedron are of low cost and the process is simple, making them promising for practical applications. This synthesis method based on MOF templates can be extended to other materials with different structures and properties.

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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.5b10727. XRD patterns of ZIF-67, XRD patterns and SEM images of Co3O4 particles and CoP particles, N2 adsorption/ desorption isotherms, EIS spectra, Cdl measurements, and SEM images after CV cycles of CoP hollow polyhedron and CoP particles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 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 (Grant 51572139) and National Basic Research Program of China (Grant 2013CB934004).

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REFERENCES

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Cobalt Phosphide Hollow Polyhedron as Efficient Bifunctional

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