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Nanostructured nickel phosphide supported on carbon nanospheres: Synthesis and application as an efficient electrocatalyst for hydrogen evolution
Journal of Power Sources 285 (2015) 169e177
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Nanostructured nickel phosphide supported on carbon nanospheres: Synthesis and application as an efficient electrocatalyst for hydrogen evolution Yuan Pan, Yunqi Liu*, Chenguang Liu* State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, China National Petroleum Corporation (CNPC), China University of Petroleum, 66 West Changjiang Road, Qingdao, Shandong 266580, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Nanostructured nickel phosphides supported on carbon nanospheres (CNSs) have been synthesized for the first time. The Ni2P/CNSs-x hybrids exhibit excellent activity and stability for the HER. The Ni2P/CNSs-x hybrids exhibit higher catalytic activity than the Ni/ CNSs hybrid. The Ni2P/CNSs-x hybrids can be a promising candidate for substituting noble metal catalyst.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 January 2015 Received in revised form 11 March 2015 Accepted 15 March 2015 Available online 17 March 2015
New electrocatalysts to replace noble metal catalysts for the hydrogen evolution reaction (HER) are highly desired to produce renewable and environmentally-friendly energy. In this work, nanostructured nickel phosphides supported on carbon nanospheres (CNSs) with different carbon content (Ni2P/CNSs-x, x ¼ 10, 20, 40, 60) are synthesized by thermal decomposition using nickel acetylacetonate as nickel source and trioctylphosphine as phosphorus source in an oleylamine solution containing CNSs for the first time. The structure and morphology are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and N2 adsorption-desorption. Then the electrocatalytic properties of as-synthesized Ni2P/CNSs-x for the HER are studied. In addition, the Ni/CNSs-20 hybrid is synthesized and the electrocatalytic properties are studied. The results show that all the Ni2P/CNSs hybrids exhibit higher catalytic activity than the Ni/CNSs-20 hybrid. The catalytic activity of the as-synthesized Ni2P/CNSs hybrid can be enhanced by changing the carbon content. The superior catalytic activity is attributed to the coupling effect between the Ni2P nanoparticles and CNSs, the electronic effect of Ni, the ensemble effect of P, the large surface area, and the high electron conductivity of CNSs. This study paves the way for the design of HER electrocatalysts with high performance and low-cost that can be employed under acid conditions. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructured Nickel phosphide Carbon nanospheres Electrocatalyst Hydrogen evolution reaction
1. Introduction * Corresponding authors. E-mail addresses: [email protected] (Y. Liu), [email protected] (C. Liu). http://dx.doi.org/10.1016/j.jpowsour.2015.03.097 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Hydrogen is considered to be an ideal energy carrier that can meet the increasing energy demand and environmental pollution
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[1] and therefore increasing attention has been paid to the production of hydrogen [2]. The development of efficient and sustainable hydrogen production technologies to satisfy global energy demand in an economical and environmentally friendly manner is required. Nowadays, electrolysis of water is regarded as a clean and renewable method to produce hydrogen [3]. Generally, the electrochemical hydrogen evolution reaction (HER) is catalyzed by Ptbased noble metal catalysts [4] but large scale applications of these noble metal catalysts are limited due to the high price and low abundance [5]. Therefore, the development of efficient, stable and inexpensive non-noble metal electrocatalysts to replace Ptbased noble metal catalysts for the HER is highly desired, although many difficulties are still ahead. Over the past years, several kinds of Mo-based materials have been used as effective HER catalysts, such as molybdenum sulfide [2], molybdenum boride [6], molybdenum nitride [7], molybdenum selenium [8], and molybdenum carbide [9]. Recently, transitionmetal phosphides (TMPs), such as nickel phosphide [10], molybdenum phosphide [11], iron phosphide [12], and cobalt phosphide [13], have emerged as attractive HER electrocatalysts. The electrical conductivity and surface area of electrocatalysts are important factors which influence the electrocatalytic efficiency. Therefore, carbon materials have been adopted as ideal supports to enhance the electrocatalytic activity due to their high electrical conductivity and large surface area [14] and, as a consequence, carbon nanospheres (CNSs) are good candidates due to their excellent physicochemical properties [15]. For example, Bian et al. [16] synthesized an efficient electrocatalyst for hydrogen evolution based on MoS2 on ordered mesoporous carbon nanospheres. The as-synthesized nanocomposites exhibited high catalytic activity for the HER with a low overpotential and a very high current density. Sun et al. [17] synthesized a nanohybrid electrocatalyst, which consisted of carbon nanospheres decorated with single-crystal Pt nanowires, via a simple chemical route. However, reports are rare on the design and electrocatalytic properties of novel electrocatalysts based on nanostructured nickel phosphide supported on carbon nanospheres. In this work, we report the first synthesis of nanostructured nickel phosphide supported on carbon nanospheres with different carbon content (Ni2P/CNSs-x, x ¼ 10, 20, 40, 60) prepared by thermal decomposition using nickel acetylacetonate [Ni(acac)2] as nickel source and trioctylphosphine (TOP) as phosphorus source in an oleylamine (OAm) solution containing CNSs. The structure and morphology of the as-synthesized Ni2P/CNSs-x were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and N2 adsorption-desorption. We further studied the electrocatalytic properties of the as-synthesized Ni2P/ CNSs-x for the HER. Additionally, a Ni/CNSs-20 hybrid was synthesized and its electrocatalytic properties were studied. The results show that the Ni2P/CNSs-x hybrids exhibit excellent electrocatalytic activity for the HER with a low overpotential, a high current density, a small Tafel slope, and good stability.
further purification. All reactions were carried out under an argon atmosphere using standard air-free techniques. 2.2. Synthesis of Ni2P/CNSs-x hybrids In a typical synthesis, Ni(acac)2 (0.256 g, 1 mmol), OAm (7 mL, 21.3 mmol) and x mg CNSs (x ¼ 10, 20, 40, 60) were placed in a fourneck flask and stirred magnetically under a flow of argon. The mixture was heated to 120 C with a heating rate of 10 C min1 and kept at this temperature for 30 min to remove moisture and dissolved oxygen. After TOP (3.4 mL, 7.5 mmol) was quickly injected into the solution, the mixture was rapidly heated to 320 C and maintained for 2 h. After cooling to room temperature, the black precipitate was washed three times with a mixture of hexane and ethanol by centrifugation (4000 rpm, 10 min). Then the Ni2P/CNSsx hybrids with different carbon content were obtained by drying in vacuum at 60 C for 24 h. 2.3. Synthesis of Ni/CNSs-20 hybrid Ni(acac)2 (0.256 g, 1 mmol), OAm (7 mL, 21.3 mmol) and CNSs (20 mg) were placed in a four-neck flask and stirred magnetically under a flow of argon. The mixture was heated to 120 C with a heating rate of 10 C min1 and kept at this temperature for 30 min to remove moisture and dissolved oxygen. After TOP (1 mL, 2.2 mmol) was quickly injected into the solution, the mixture was rapidly heated to 200 C and maintained for 30 min. After cooling to room temperature, the black precipitate was washed three times with a mixture of hexane and ethanol by centrifugation (4000 rpm, 10 min). The Ni/CNSs-20 hybrid was obtained by drying in vacuum at 60 C for 24 h. 2.4. Characterization X-ray diffraction (XRD) was performed on a panalytical X'pert PROX-ray diffractometer with Cu Ka monochromatized radiation (l ¼ 1.54 Å) and operated at 45 kV and 40 mA. Transmission electron microscopy (TEM) was performed on a JEM-2100 UHR microscope (JEOL, Japan) at an accelerating voltage of 200 kV. An energy dispersive X-ray (EDX) instrument was attached to the TEM system. X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALABMK II spectrometer using an Al Ka (1486.6 eV) photon source. N2 adsorption-desorption experiments were carried out on a ChemBET 3000 (Quantachrome, USA) instrument.
2. Experimental 2.1. Materials Nickel(II) acetylacetonate (Ni(acac)2, 95%), trioctylphosphine (TOP, 90%), oleylamine (OAm, 95%) and carbon nanospheres were obtained from Aladdin Chemistry Co. Ltd. Hexane (99.5%), ethanol (99.7%), and sulfuric acid (H2SO4, 98%) were obtained from Sinopharm Chemical Reagent Co. Ltd.. Nafion solution (5% in a mixture of lower aliphatic alcohols and water) was purchased from SigmaeAldrich. All chemicals were used as received without
Fig. 1. XRD patterns of the as-synthesized Ni2P/CNSs-x and Ni/CNSs-20 hybrids.
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Fig. 2. TEM images of (a) Ni2P/CNSs-10, (b) Ni2P/CNSs-20, (c) Ni2P/CNSs-40, (d) Ni2P/CNSs-60 and (e) Ni/CNSs-20 hybrids. SAED image of (f) Ni2P/CNSs-20 hybrid. HRTEM images of (g) Ni2P/CNSs-20 and (h) Ni/CNSs-20 hybrids, respectively.
2.5. Electrochemical measurements All electrochemical measurements were carried out using a Reference 600 instrument (Gamry Instruments, USA) in a standard three-electrode setup. A saturated calomel electrode (SCE) was used as reference electrode (Ag/AgCl) and a Pt electrode as counter electrode. The electrocatalytic activity of the sample in the HER was examined by measuring polarization curves using linear sweep voltammetry (LSV) with a scan rate of 5 mV s1 at room temperature in 0.5 M H2SO4 solutions. A durability test was carried out by cyclic voltammetry (CV) scanning of 500 cycles with a scan rate of 100 mV s1 in 0.5 M H2SO4. Electrochemical impedance
spectroscopy (EIS) measurements were carried out in 0.5 M H2SO4 at various overpotentials from 50 to 100 mV (vs. the reversible hydrogen electrode (RHE)) in the frequency range of 100 kHz to 0.1 Hz with a single modulated AC potential of 5 mV. Experimental Table 1 Textural properties of the as-synthesized Ni2P/CNSs-x hybrids. Catalyst
BET surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
Ni2P/CNSs-10 Ni2P/CNSs-20 Ni2P/CNSs-40 Ni2P/CNSs-60
40.7 41.4 74.3 84.1
0.08 0.09 0.15 0.17
12.2 8.5 7.5 7.8
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EIS data were analyzed and fitted with the software of Zsimpwin. All the potentials reported in our work are relative to RHE. E(RHE) ¼ E(SCE) þ (0.222 þ 0.059 pH). 2.6. Preparation of working electrodes Five mg of catalyst and 80 mL Nafion solution (5 wt. %) were dispersed in 1 mL ethanol and sonicated for 30 min to form a slurry. Then 5 mL of the slurry was loaded onto the surface of a glassy carbon electrode (GCE, 4 mm in diameter) and the electrode was dried at room temperature. 2.7. Calculation of the number of active sites The number of active sites (n) was determined using cycle voltammograms (CVs) collected from 0.2 to þ0.6 V vs. RHE in 1.0 M phosphate buffer solution (PBS, pH ¼ 7) with a scan rate of 20 mV s1. After obtaining the number of voltammetric charges (Q) by subtraction of the blank value, n (mol) was calculated with the following equation:
n¼
Q 2F
where F is the Faraday constant (96485C mol1).
Fig. 3. Nitrogen adsorptionedesorption isotherm (a) and the BJH pore-size distribution curve (b) of the as-synthesized Ni2P/CNSs-x hybrids.
3. Results and discussion The structures of the crystalline phases of the as-synthesized Ni2P/CNSs-x (x ¼ 10, 20, 40, 60), Ni/CNSs-20, and CNSs hybrids were characterized by XRD (Fig. 1). The CNSs sample showed two diffraction peaks at 25.2 and 42.8 , which are due to the (002) and (101) planes of hexagonal graphite [18]. The Ni/CNSs-20 hybrid showed a weak and broad diffraction peak at 44.5 , which can be assigned to the (111) plane of face-centered cubic nickel (PDF#01087-0712), indicating that the nickel nanoparticles (NPs) are highly dispersed on the CNSs. Ni2P/CNSs-x hybrids show diffraction peaks at 40.7, 44.6 , 47.4 , 54.2 , 66.6 , 72.8 , 74.9 , 80.5 , and 88.9 , which can be attributed to the (111), (201), (210), (300), (310), (311), (400), (401), and (321) planes, respectively, of the hexagonal structure of Ni2P (PDF # 03-065-3544). The Ni2P/CNSs-x hybrids show an additional diffraction peak at 25.2 , which supports the formation of CNSs decorated with Ni2P NPs. The morphologies of the as-synthesized Ni2P/CNSs-x (x ¼ 10, 20, 40, 60), Ni/CNSs-20 and CNSs hybrids were characterized by TEM. The TEM images (Fig. S1) illustrate that the CNSs exhibit sphere-like morphology with an average particle size of about 40 nm. Fig. 2aed show the morphology images of the Ni2P/CNSs-10, Ni2P/CNSs-20, Ni2P/CNSs-40, and Ni2P/CNSs-60 hybrids. A large number of Ni2P NPs is supported on the CNSs, which indicates that the formed Ni2P NPs decorate the CNSs. The Ni2P NPs typically exhibited a spherical shape with high density and good distribution. No aggregated NPs can be observed, which indicates that the Ni2P NPs are well dispersed. The average sizes of the Ni2P NPs (Fig. S2aed) for the Ni2P/CNSs-10, Ni2P/CNSs-20, Ni2P/CNSs-40 and Ni2P/CNSs-60 hybrids are 4.22 ± 0.91, 4.15 ± 0.93, 3.83 ± 0.85 and 3.73 ± 0.93 nm, respectively. The SAED pattern (Fig. 2f) indicates that the major diffraction rings match well with the as-synthesized Ni2P/CNSs-20 hybrid. The HRTEM image of the Ni2P/CNSs-20 hybrid (Fig. 2g) reveals fringe spacings of about 2.06 Å and 1.94 Å, corresponding to the (201) and (210) lattice planes of hexagonal Ni2P, respectively. EDX analysis (Fig. S3) of the Ni2P/CNSs-20 hybrid confirmed the presence of C, Ni, and P elements. The measured atomic Ni: P ratio (1.8) is very close to the stoichiometric ratio of 2 in Ni2P. Fig. 2e shows the morphology of the Ni/CNSs-20 hybrid, also indicating the formation of Ni NPs decorated on the CNSs with high density and well distributed. The average size of the Ni NPs (Fig. S2e) for the Ni/CNSs-20 hybrid is 4.74 ± 0.96 nm. The HRTEM image of the Ni/ CNS-20 hybrid (Fig. 2h) reveals that the fringe spacing is about 2.01 Å, corresponding to the (111) lattice planes of face-centered cubic nickel. All these results strongly support the successful synthesis of Ni2P/CNSs-x and Ni/CNSs-20 hybrids. N2 adsorption-desorption measurements were carried out to further study the textural properties (BET surface area, pore volume, and pore size) of the as-synthesized Ni2P/CNSs-x hybrids (Table 1). The BET surface area and pore volume increased gradually with increasing carbon content. N2 sorption (Fig. 3a) on the Ni2P/CNSs-x hybrids revealed type IV isotherms with a distinct hysteresis loop in the P/P0 region from 0.4 to 1.0, which is a typical characteristic of mesoporous material. The BarretteJoynereHalenda (BJH) pore-size distribution (Fig. 3b) of the Ni2P/CNSs-10, Ni2P/CNSs-20, Ni2P/CNSs40, and Ni2P/CNSs-60 hybrids showed a narrow peak at 3.8, 3.3, 3.2, and 3.3 nm and the average pore-sizes were 12.2, 8.5, 7.5, and 7.8 nm respectively, further indicating the nanoporous nature of the Ni2P/ CNSs-x hybrids. The formation of Ni2P in the hybrids was further studied by XPS analysis. Fig. 4 shows the XPS results of the as-synthesized Ni2P/ CNSs-20 hybrid. The presence of C, O, P, and Ni elements in the XPS survey (Fig. 4a) indicates the formation of Ni2P NPs on the CNSs. That the element O was observed can be attributed to the residual oxygen-containing functionalities in the CNSs and the surface
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Fig. 4. XPS spectra of (a) survey spectrum, (b) C 1s, (c) Ni 2p and (d) P 2p regions for the Ni2P/CNSs-20 hybrid.
oxidation of Ni2P NPs due to air contact [19]. C 1s peaks (Fig. 4b) at binding energies of 284.5, 285.3, and 288.4 eV can be assigned to graphite carbon atoms, hybridized carbon atoms, and carbon atoms bonded to oxygen atoms [20], respectively. For the Ni 2p region (Fig. 4c), the peaks located at binding energy 852.7 and 870 eV are assigned to the Ni 2p3/2 and Ni 2p1/2 energy levels of Ni2P. The Ni 2p binding energies for Ni2P are very close to those of Ni metal, which indicates that the Ni atoms in Ni2P have a very small positive charge (Nidþ, 0
catalytic activity with nearly zero overpotential. All the Ni2P/CNSs hybrids exhibited excellent HER activity with nearly the same onset overpotential of 40 mV. Video S1 (see the supporting information) presents the obvious hydrogen bubbles generated on the Ni2P/ CNSs-20 hybrid modified GCE during the LSV scanning. However, the Ni/CNSs-20 hybrid exhibited a very small HER activity with high onset overpotential of 196 mV. These results indicate that all the Ni2P/CNSs hybrids exhibit higher catalytic activity than the Ni/ CNSs-20 hybrid. In addition, for achieving a current density of 10 and 20 mA cm1, the Ni2P/CNSs-40 hybrid only needed an overpotential of 92 and 108 mV, respectively. However, the Ni2P/CNSs20 hybrid needed an overpotential of 107 and 123 mV and the Ni2P/ CNSs-10 hybrid an overpotential of 118 and 142 mV, respectively. These results indicate that the electrocatalytic activity of the assynthesized Ni2P/CNSs-x hybrids can be enhanced by increasing the carbon content. However, when the content of CNSs increased once again, the catalytic activity decreased gradually. The Ni2P/ CNSs-60 hybrid needed an overpotential of 99 and 115 mV, respectively. The difference in catalytic activity among the Ni2P/ CNSs-x hybrids may be correlated to the active surface area of Ni2P on the CNSs surface. Higher carbon content provides greater surface area and electrical conductivity, but lower total available sites since the fraction of Ni2P is decreasing. When these two functions reached a balance, the HER catalytic activity could not be increased.
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Fig. 5. (a) LSV curves (iR corrected) of the Ni2P/CNSs-x, Ni/CNSs-20 hybrids and Pt/C catalyst in 0.5 M H2SO4 with a scan rate of 5 mV s1. (b) Tafel plots of Ni2P/CNSs-x, Ni/CNSs-20 hybrids and Pt/C catalyst derived from the polarization curves. (c) CVs of Ni2P/CNSs-x, Ni/CNSs-20 and bare GCE recorded at pH ¼ 7 with a scan rate of 20 mV s1 (d) CV curves of the Ni2P/CNSs-x and Ni/CNSs-20 hybrids 0.5 M H2SO4 before (solid) and after (short dash) long-term 500 cycles. (e) Time-dependent current density curve of Ni2P/CNSs-20 hybrid under static overpotential of 150 mV. (f) Nyquist plots of Ni2P/CNSs-x and Ni/CNSs-20 hybrids in 0.5 M H2SO4 with an overpotential of 150 mV.
Table S1 compares the HER activity of the Ni2P/CNSs-x hybrids with some reported transition metal phosphide (TMPs) catalysts, such as FeP nanosheets [12], FeP/GSs [19], FeP NA/Ti [30], FeP NAs/CC [34], Co2P nanorods [35], CoP/CNT [36], CoP/Ti [37], CoP/CC [38], Ni12P5 NPs [39], Ni2P NPs [10], Ni5P4 NPs [32], NiP2 NS/CC [40], MoP NPs [11b], MoP/CF [28], MoP-CA2 [41], and Cu3P NW/CF [29]. The
potentials of the Ni2P/CNSs-x hybrids compare favorably to the behavior of previously reported TMPs HER electrocatalysts in 0.5 M H2SO4. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2015.03.097. Fig. 5b shows that the Tafel plots of the Ni2P/CNSs-x, Ni/CNSs-20
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Fig. 6. (a) Nyquist plots and (b) Bode plots of the Ni2P/CNSs-20 hybrid in 0.5 M H2SO4. (c) Equivalent electrical circuit used to model the HER kinetics process. Rs is the solution resistance, Q and Rct are the element and charge transfer resistance. (d) Tafel slope of the Ni2P/CNSs-20 hybrid fitted from EIS data.
hybrids and Pt/C catalyst derived from the polarization curves fitted well with the Tafel equation (h ¼ a þ b log j, where b is the Tafel slope and j is the current density). The Tafel slope for the Pt/C, Ni2P/ CNSs-60, Ni2P/CNSs-40, Ni2P/CNSs-20, Ni2P/CNSs-10, and Ni/CNSs20 hybrids is 30, 47, 46, 49, 67, and 146 mV dec1, respectively, which indicates that the HER rate of the Ni2P/CNSs-x hybrids is faster than that of the Ni/CNSs-20 hybrid. The Pt/C catalyst exhibits the fastest HER rate. In addition, the Ni2P/CNSs-40 hybrid exhibited the fastest HER rate of all the Ni2P/CNSs-x hybrids. The smaller Tafel slope of Ni2P/CNSs-40 indicates that the catalytic activity is higher than that of the other hybrids. The Tafel slopes of the Ni2P/CNSs-x hybrids also reveal that the HER occurs via a Volmer-Heyrovsky mechanism [25]. The exchange current density of the Pt/C catalyst, Ni2P/CNSs-x and Ni/CNSs-20 hybrids was also derived from Tafel plots by applying the extrapolation method (Fig. S4). As shown in Table S2, the Ni2P/CNSs-40 hybrid displayed an exchange current density of 0.4898 mA cm2, which further indicates that the catalytic activity of the Ni2P/CNSs-40 hybrid is better than that of the other Ni2P/ CNSs-x and Ni/CNSs-20 hybrids. The number of active sites for the Ni2P/CNSs-10, Ni2P/CNSs-20, Ni2P/CNSs-40, Ni2P/CNSs-60 and Ni/CNSs-20 hybrids was determined by CV sweeps (Fig. 5c) to be 9.59 109, 6.51 108, 7.67 108, 6.98 108, and 4.56 109 mol, respectively, based on a reported method [26]. All these results demonstrate the superior HER performance of the Ni2P/CNSs-x hybrids over the Ni/
CNSs-20 hybrid. Fig. 5d shows the stability of the as-synthesized Ni2P/CNSs-x and Ni/CNSs-20 hybrids using CV sweeps for scanning 500 cycles in 0.5 M H2SO4 with a scan rate of 100 mV s1. The observation of negligible current loss indicates that all the Ni2P/CNSs-x hybrids exhibit excellent stability in acidic solution. However, a large current loss of the Ni/CNSs-20 hybrid was observed, which indicates that the Ni/CNSs-20 hybrid shows poor stability in acidic solution. Fig. 5e shows the time-dependent current density curve of the Ni2P/CNSs-20 hybrid under a static overpotential of 150 mV. The result indicates that the Ni2P/CNSs-20 hybrid maintains its high catalytic activity for at least 60’000 s. All the above results imply that the Ni2P/CNSs-x hybrids show excellent durability and thus are promising for practical application. In addition, EIS experiments were carried out to study the electronic transport behavior of as-synthesized Ni2P/CNSs-x and Ni/CNSs-20 hybrids at the same potential of 150 mV (Fig. 5f). The Ni2P/CNSs-40 hybrid has the smallest diameter and the Ni/CNSs-20 hybrid has the largest diameter, which further indicates that the Ni2P/CNSs-x hybrids have higher conductivity and, thus, better electron transfer ability than the Ni/CNSs-20 hybrid. It can also be observed that the conductivity can be improved with increasing carbon content for the Ni2P/CNSs-x hybrids. The high activity and stability of the as-synthesized Ni2P/CNSs-x can be attributed to the following factors. First, the coupling effect between the Ni2P NPs and CNSs facilitates the electron transfer and
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decreases the hydrogen binding energy during the HER process. Second, the XPS results indicate that the Ni and P atoms in Ni2P have a small positive charge and negative charge, respectively. Previously it was reported that TMPs complex HER electrocatalyst, such as CoP [27], MoP [28], Cu3P [29], and FeP [30], share the catalytic mechanism with hydrogenase [31]. Therefore, in our present work, the role of Ni and P should be similar as in previously reported TMPs HER electrocatalysts [27e32]. The Ni and P atoms act as the hydride acceptor and proton acceptor [10], respectively, to enhance the HER process. In addition, the P atoms may also facilitate the formation of Ni-hydride for subsequent hydrogen evolution via electrochemical desorption [33]. Finally, the electrical conductivity and surface area also significantly influence the electrochemical performance. The small size of the Ni2P NPs in the hybrids favor the exposure of more active sites for the HER. The introduction of CNSs could further increase the surface area of the Ni2P/CNSs-x hybrids. In addition, the excellent electrical conductivity of the CNSs support facilitates charge transfer in the hybrids. In all, the coupling effect between the Ni2P NPs and CNSs, the electronic effect of Ni, the ensemble effect of P, the large surface area as well as the high electron conductivity of CNSs contribute to the superior catalytic activity of Ni2P/CNSs-x hybrids. The kinetics of the Ni2P/CNSs-20 hybrid was further studied by EIS experiments. The Nyquist plots of the Ni2P/CNSs-20 hybrid at various potentials are shown in Fig. 6a. With the increase of potential from 50 to 150 mV, the diameter of the semicircles decreased gradually, which indicates that the Ni2P/CNSs-20 hybrid has good electron transfer ability at high potential. The corresponding Bode plots (Fig. 6b) show only one time constant. Thus, as shown in Fig. 6c, the HER process can be described by a simple equivalent electrical circuit, the corresponding parameters of which are listed in Table S3. The solution resistance (Rs) is approximately 8 U. The value of the charge transfer resistance (Rct) is potential-dependent, and the lower Rct reflects the superior electrocatalytic activity of the Ni2P/CNSs-20 nanohybrid. The Tafel slope of the Ni2P/CNSs-20 hybrid can be obtained by fitting the log(1/Rct)-h plot, as shown in Fig. 6d. The Tafel slope of the Ni2P/ CNSs-20 hybrid is 52 mV$dec1, and this value is nearly in accordance with the Tafel slope (49 mV$dec1) obtained from the polarization curve. 4. Conclusions Nanostructured Ni2P/CNSs-x hybrids were successfully synthesized by a thermal decomposition using Ni(acac)2 as nickel source and TOP as phosphorus source in an OAm solution with dispersed CNSs. As novel HER electrocatalysts, the as-synthesized Ni2P/CNSsx hybrids exhibit high efficiency and excellent stability in acidic media. In addition, it is observed that the catalytic activity of the Ni2P/CNSs-40 hybrid is better than that of the other Ni2P/CNSs-x hybrids, which indicates that the catalytic activity of the Ni2P/CNSs hybrids can be enhanced by changing the carbon content. The excellent activity and stability is attributed to the coupling effect between Ni2P NPs and CNSs, the electronic effect of Ni, the ensemble effect of P, the large surface area, and the high electron conductivity of CNSs. This work provides a general strategy for the synthesis of other electrocatalysts based on carbon materials decorated with transition metal phosphides for HER applications. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grants No. 21006128, 21176258, U1162203), China University of Petroleum for Postgraduate Technology Innovation Project (Grants No. YCX2014033), the
Fundamental Research Funds for the Central Universities (Grants No. 15CX06039A), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20110133110002). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.03.097. References [1] (a) M.S. Dresselhaus, I.L. Thomas, Nature 414 (2001) 332e337; (b) J.A. Turner, Science 285 (1999) 687e689; (c) N. Liu, L. Yang, S. Wang, Z. Zhong, S. He, X. Yang, Q. Gao, Y. Tang, J. Power Sources 275 (2015) 588e594. [2] B. Hinnemann, P.G. Moses, J. Bonde, K.P. Jǿrgensen, J.H. Nielsen, S. Horch, I. Chorkendorff, J.K. Nǿrskov, J. Am. Chem. Soc. 127 (2005) 5308e5309. [3] (a) J.A. Turner, Science 305 (2004) 972e974; (b) L. Zhang, K. Xiong, S. Chen, L. Li, Z. Deng, Z. Wei, J. Power Sources 274 (2015) 114e120. [4] (a) E. Antolini, Energy Environ. Sci. 2 (2009) 915e931; (b) B.E. Conway, B.V. Tilak, Electrochim. Acta 47 (2002) 3571e3594. [5] (a) D. Merki, X.L. Hu, Energ. Environ. Sci. 4 (2011) 3878e3888; (b) B.Z. Fang, J.H. Kim, J.S. Yu, Electrochem. Commun. 10 (2008) 659e662. [6] H. Vrubel, X.L. Hu, Angew. Chem. Int. Ed. 51 (2012) 12703e12706. [7] W.F. Chen, S. Iyer, S. Iyer, K. Sasaki, C.H. Wang, Y.M. Zhu, J.T. Muckerman, E. Fujita, Energy Environ. Sci. 6 (2013) 1818e1826. [8] H.T. Wang, D.S. Kong, P. Johanes, J.J. Cha, G.Y. Zheng, K. Yan, N.A. Liu, Y. Cui, Nano Lett. 13 (2013) 3426e3433. [9] W.F. Chen, C.H. Wang, K. Sasaki, N. Marinkovic, W. Xu, J.T. Muckerman, Y. Zhu, R.R. Adzic, Energy Environ. Sci. 6 (2013) 943e951. [10] E.J. Popczun, J.R. McKone, C.G. Read, A.J. Biacchi, A.M. Wiltrout, N.S. Lewis, R.E. Schaak, J. Am. Chem. Soc. 135 (2013) 9267e9270. [11] (a) P. Xiao, M.A. Sk, L. Thia, X.M. Ge, R.J. Lim, J.Y. Wang, K.H. Lim, X. Wang, Energy Environ. Sci. 7 (2014) 2624e2629; (b) J.M. McEnaney, J.C. Crompton, J.F. Callejas, E.J. Popczun, A.J. Biacchi, N.S. Lewis, R.E. Schaak, Chem. Mater. 26 (2014) 4826e4831. [12] Y. Xu, R. Wu, J.F. Zhang, Y.M. Shi, B. Zhang, Chem. Commun. 49 (2013) 6656e6658. [13] (a) E.J. Popczun, C.G. Read, C.W. Roske, N.S. Lewis, R.E. Schaak, Angew. Chem. Int. Ed. 53 (2014) 5427e5430; (b) P. Jiang, Q. Liu, C. Ge, W. Cui, Z. Pu, A.M. Asiribc, X.P. Sun, J. Mater. Chem. A 2 (2014) 14634e14640. [14] Z. Lei, N. Christov, L.L. Zhang, X.S. Zhao, J. Mater. Chem. 21 (2011) 2274e2281. [15] M. Liu, L. Gan, W. Xiong, Z. Xu, D. Zhu, L. Chen, J. Mater. Chem. A 2 (2014) 2555e2562. [16] X. Bian, J. Zhu, L. Liao, M.D. Scanlon, P. Ge, C. Ji, H.H. Girault, B. Liu, Electrochem. Commun. 22 (2012) 128e132. [17] S. Sun, F. Jaouen, J.P. Dodelet, Adv. Mater. 20 (2008) 3900e3904. [18] (a) Q.G. Zhu, A.N.A. Sujari, S.A. Ghani, J. Electrochem. Soc. 160 (2013) B23eB29; (b) Z.Z. Lu, H.K. Wang, CrystEngComm 16 (2014) 550e555. [19] Z. Zhang, B. Lu, J. Hao, W. Yang, J. Tang, Chem. Commun. 50 (2014) 11554e11557. [20] Y. Yan, X. Ge, Z. Liu, J.Y. Wang, J.M. Leea, X. Wang, Nanoscale 5 (2013) 7768e7771. n, A. Jime nez-Lo pez, [21] J.A. Cecilia, A. Infantes-Molina, E. Rodríguez-Castello J. Catal. 263 (2009) 4e15. [22] Y. Lu, X. Wang, Y. Mai, J. Xiang, H. Zhang, L. Li, C. Gu, J. Tu, S.X. Mao, J. Phys. Chem. C 116 (2012) 22217e22225. [23] P.E.R. Blanchard, A.P. Grosvenor, R.G. Cavell, A. Mar, Chem. Mater. 20 (2008) 7081e7088. [24] (a) J.L. Wang, Q. Yang, Z.D. Zhang, S.H. Sun, Chem. Eur. J. 16 (2010) 7916e7924; (b) J. Bai, X. Li, A.J. Wang, R. Prins, Y. Wang, J. Catal. 287 (2012) 161e169. [25] Y.F. Xu, M.R. Gao, Y.R. Zheng, J.J. Jiang, S.H. Yu, Angew. Chem. Int. Ed. 52 (2013) 8546e8550. [26] D. Merki, S. Fierro, H. Vrubel, X.L. Hu, Chem. Sci. 2 (2011) 1262e1267. [27] H.F. Du, Q. Liu, N.Y. Cheng, A.M. Asiri, X.P. Sun, C.M. Li, J. Mater. Chem. A 2 (2014) 53e14816. [28] W. Cui, Q. Liu, Z.C. Xing, A.M. Asiri, K.A. Alamry, X.P. Sun, Appl. Catal. B 164 (2005) 144e150. [29] J.Q. Tian, Q. Liu, N.Y. Cheng, A.M. Asiri, X.P. Sun, Angew. Chem. Int. Ed. 53 (2014) 9577e9581. [30] P. Jiang, Q. Liu, Y.H. Liang, J.Q. Tian, A.M. Asiri, X.P. Sun, Angew. Chem. Int. Ed. 53 (2014) 1e6. [31] P. Liu, J.A. Rodriguez, J. Am. Chem. Soc. 127 (2005) 14871e14878. [32] Y. Pan, Y. Liu, J. Zhao, K. Yang, J. Liang, D. Liu, W. Hu, D. Liu, Y. Liu, C. Liu, J. Mater. Chem. A 3 (2015) 1656e1665. [33] W. Zhang, J. Hong, J. Zheng, Z. Huang, J. Zhou, R. Xu, J. Am. Chem. Soc. 133
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Nanostructured nickel phosphide supported on carbon nanospheres: Synthesis and application as an efficient electrocatalyst for hydrogen evolution Yuan Pan, Yunqi Liu*, Chenguang Liu* State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, China National Petroleum Corporation (CNPC), China University of Petroleum, 66 West Changjiang Road, Qingdao, Shandong 266580, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Nanostructured nickel phosphides supported on carbon nanospheres (CNSs) have been synthesized for the first time. The Ni2P/CNSs-x hybrids exhibit excellent activity and stability for the HER. The Ni2P/CNSs-x hybrids exhibit higher catalytic activity than the Ni/ CNSs hybrid. The Ni2P/CNSs-x hybrids can be a promising candidate for substituting noble metal catalyst.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 January 2015 Received in revised form 11 March 2015 Accepted 15 March 2015 Available online 17 March 2015
New electrocatalysts to replace noble metal catalysts for the hydrogen evolution reaction (HER) are highly desired to produce renewable and environmentally-friendly energy. In this work, nanostructured nickel phosphides supported on carbon nanospheres (CNSs) with different carbon content (Ni2P/CNSs-x, x ¼ 10, 20, 40, 60) are synthesized by thermal decomposition using nickel acetylacetonate as nickel source and trioctylphosphine as phosphorus source in an oleylamine solution containing CNSs for the first time. The structure and morphology are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and N2 adsorption-desorption. Then the electrocatalytic properties of as-synthesized Ni2P/CNSs-x for the HER are studied. In addition, the Ni/CNSs-20 hybrid is synthesized and the electrocatalytic properties are studied. The results show that all the Ni2P/CNSs hybrids exhibit higher catalytic activity than the Ni/CNSs-20 hybrid. The catalytic activity of the as-synthesized Ni2P/CNSs hybrid can be enhanced by changing the carbon content. The superior catalytic activity is attributed to the coupling effect between the Ni2P nanoparticles and CNSs, the electronic effect of Ni, the ensemble effect of P, the large surface area, and the high electron conductivity of CNSs. This study paves the way for the design of HER electrocatalysts with high performance and low-cost that can be employed under acid conditions. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructured Nickel phosphide Carbon nanospheres Electrocatalyst Hydrogen evolution reaction
1. Introduction * Corresponding authors. E-mail addresses: [email protected] (Y. Liu), [email protected] (C. Liu). http://dx.doi.org/10.1016/j.jpowsour.2015.03.097 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Hydrogen is considered to be an ideal energy carrier that can meet the increasing energy demand and environmental pollution
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[1] and therefore increasing attention has been paid to the production of hydrogen [2]. The development of efficient and sustainable hydrogen production technologies to satisfy global energy demand in an economical and environmentally friendly manner is required. Nowadays, electrolysis of water is regarded as a clean and renewable method to produce hydrogen [3]. Generally, the electrochemical hydrogen evolution reaction (HER) is catalyzed by Ptbased noble metal catalysts [4] but large scale applications of these noble metal catalysts are limited due to the high price and low abundance [5]. Therefore, the development of efficient, stable and inexpensive non-noble metal electrocatalysts to replace Ptbased noble metal catalysts for the HER is highly desired, although many difficulties are still ahead. Over the past years, several kinds of Mo-based materials have been used as effective HER catalysts, such as molybdenum sulfide [2], molybdenum boride [6], molybdenum nitride [7], molybdenum selenium [8], and molybdenum carbide [9]. Recently, transitionmetal phosphides (TMPs), such as nickel phosphide [10], molybdenum phosphide [11], iron phosphide [12], and cobalt phosphide [13], have emerged as attractive HER electrocatalysts. The electrical conductivity and surface area of electrocatalysts are important factors which influence the electrocatalytic efficiency. Therefore, carbon materials have been adopted as ideal supports to enhance the electrocatalytic activity due to their high electrical conductivity and large surface area [14] and, as a consequence, carbon nanospheres (CNSs) are good candidates due to their excellent physicochemical properties [15]. For example, Bian et al. [16] synthesized an efficient electrocatalyst for hydrogen evolution based on MoS2 on ordered mesoporous carbon nanospheres. The as-synthesized nanocomposites exhibited high catalytic activity for the HER with a low overpotential and a very high current density. Sun et al. [17] synthesized a nanohybrid electrocatalyst, which consisted of carbon nanospheres decorated with single-crystal Pt nanowires, via a simple chemical route. However, reports are rare on the design and electrocatalytic properties of novel electrocatalysts based on nanostructured nickel phosphide supported on carbon nanospheres. In this work, we report the first synthesis of nanostructured nickel phosphide supported on carbon nanospheres with different carbon content (Ni2P/CNSs-x, x ¼ 10, 20, 40, 60) prepared by thermal decomposition using nickel acetylacetonate [Ni(acac)2] as nickel source and trioctylphosphine (TOP) as phosphorus source in an oleylamine (OAm) solution containing CNSs. The structure and morphology of the as-synthesized Ni2P/CNSs-x were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and N2 adsorption-desorption. We further studied the electrocatalytic properties of the as-synthesized Ni2P/ CNSs-x for the HER. Additionally, a Ni/CNSs-20 hybrid was synthesized and its electrocatalytic properties were studied. The results show that the Ni2P/CNSs-x hybrids exhibit excellent electrocatalytic activity for the HER with a low overpotential, a high current density, a small Tafel slope, and good stability.
further purification. All reactions were carried out under an argon atmosphere using standard air-free techniques. 2.2. Synthesis of Ni2P/CNSs-x hybrids In a typical synthesis, Ni(acac)2 (0.256 g, 1 mmol), OAm (7 mL, 21.3 mmol) and x mg CNSs (x ¼ 10, 20, 40, 60) were placed in a fourneck flask and stirred magnetically under a flow of argon. The mixture was heated to 120 C with a heating rate of 10 C min1 and kept at this temperature for 30 min to remove moisture and dissolved oxygen. After TOP (3.4 mL, 7.5 mmol) was quickly injected into the solution, the mixture was rapidly heated to 320 C and maintained for 2 h. After cooling to room temperature, the black precipitate was washed three times with a mixture of hexane and ethanol by centrifugation (4000 rpm, 10 min). Then the Ni2P/CNSsx hybrids with different carbon content were obtained by drying in vacuum at 60 C for 24 h. 2.3. Synthesis of Ni/CNSs-20 hybrid Ni(acac)2 (0.256 g, 1 mmol), OAm (7 mL, 21.3 mmol) and CNSs (20 mg) were placed in a four-neck flask and stirred magnetically under a flow of argon. The mixture was heated to 120 C with a heating rate of 10 C min1 and kept at this temperature for 30 min to remove moisture and dissolved oxygen. After TOP (1 mL, 2.2 mmol) was quickly injected into the solution, the mixture was rapidly heated to 200 C and maintained for 30 min. After cooling to room temperature, the black precipitate was washed three times with a mixture of hexane and ethanol by centrifugation (4000 rpm, 10 min). The Ni/CNSs-20 hybrid was obtained by drying in vacuum at 60 C for 24 h. 2.4. Characterization X-ray diffraction (XRD) was performed on a panalytical X'pert PROX-ray diffractometer with Cu Ka monochromatized radiation (l ¼ 1.54 Å) and operated at 45 kV and 40 mA. Transmission electron microscopy (TEM) was performed on a JEM-2100 UHR microscope (JEOL, Japan) at an accelerating voltage of 200 kV. An energy dispersive X-ray (EDX) instrument was attached to the TEM system. X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALABMK II spectrometer using an Al Ka (1486.6 eV) photon source. N2 adsorption-desorption experiments were carried out on a ChemBET 3000 (Quantachrome, USA) instrument.
2. Experimental 2.1. Materials Nickel(II) acetylacetonate (Ni(acac)2, 95%), trioctylphosphine (TOP, 90%), oleylamine (OAm, 95%) and carbon nanospheres were obtained from Aladdin Chemistry Co. Ltd. Hexane (99.5%), ethanol (99.7%), and sulfuric acid (H2SO4, 98%) were obtained from Sinopharm Chemical Reagent Co. Ltd.. Nafion solution (5% in a mixture of lower aliphatic alcohols and water) was purchased from SigmaeAldrich. All chemicals were used as received without
Fig. 1. XRD patterns of the as-synthesized Ni2P/CNSs-x and Ni/CNSs-20 hybrids.
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Fig. 2. TEM images of (a) Ni2P/CNSs-10, (b) Ni2P/CNSs-20, (c) Ni2P/CNSs-40, (d) Ni2P/CNSs-60 and (e) Ni/CNSs-20 hybrids. SAED image of (f) Ni2P/CNSs-20 hybrid. HRTEM images of (g) Ni2P/CNSs-20 and (h) Ni/CNSs-20 hybrids, respectively.
2.5. Electrochemical measurements All electrochemical measurements were carried out using a Reference 600 instrument (Gamry Instruments, USA) in a standard three-electrode setup. A saturated calomel electrode (SCE) was used as reference electrode (Ag/AgCl) and a Pt electrode as counter electrode. The electrocatalytic activity of the sample in the HER was examined by measuring polarization curves using linear sweep voltammetry (LSV) with a scan rate of 5 mV s1 at room temperature in 0.5 M H2SO4 solutions. A durability test was carried out by cyclic voltammetry (CV) scanning of 500 cycles with a scan rate of 100 mV s1 in 0.5 M H2SO4. Electrochemical impedance
spectroscopy (EIS) measurements were carried out in 0.5 M H2SO4 at various overpotentials from 50 to 100 mV (vs. the reversible hydrogen electrode (RHE)) in the frequency range of 100 kHz to 0.1 Hz with a single modulated AC potential of 5 mV. Experimental Table 1 Textural properties of the as-synthesized Ni2P/CNSs-x hybrids. Catalyst
BET surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
Ni2P/CNSs-10 Ni2P/CNSs-20 Ni2P/CNSs-40 Ni2P/CNSs-60
40.7 41.4 74.3 84.1
0.08 0.09 0.15 0.17
12.2 8.5 7.5 7.8
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EIS data were analyzed and fitted with the software of Zsimpwin. All the potentials reported in our work are relative to RHE. E(RHE) ¼ E(SCE) þ (0.222 þ 0.059 pH). 2.6. Preparation of working electrodes Five mg of catalyst and 80 mL Nafion solution (5 wt. %) were dispersed in 1 mL ethanol and sonicated for 30 min to form a slurry. Then 5 mL of the slurry was loaded onto the surface of a glassy carbon electrode (GCE, 4 mm in diameter) and the electrode was dried at room temperature. 2.7. Calculation of the number of active sites The number of active sites (n) was determined using cycle voltammograms (CVs) collected from 0.2 to þ0.6 V vs. RHE in 1.0 M phosphate buffer solution (PBS, pH ¼ 7) with a scan rate of 20 mV s1. After obtaining the number of voltammetric charges (Q) by subtraction of the blank value, n (mol) was calculated with the following equation:
n¼
Q 2F
where F is the Faraday constant (96485C mol1).
Fig. 3. Nitrogen adsorptionedesorption isotherm (a) and the BJH pore-size distribution curve (b) of the as-synthesized Ni2P/CNSs-x hybrids.
3. Results and discussion The structures of the crystalline phases of the as-synthesized Ni2P/CNSs-x (x ¼ 10, 20, 40, 60), Ni/CNSs-20, and CNSs hybrids were characterized by XRD (Fig. 1). The CNSs sample showed two diffraction peaks at 25.2 and 42.8 , which are due to the (002) and (101) planes of hexagonal graphite [18]. The Ni/CNSs-20 hybrid showed a weak and broad diffraction peak at 44.5 , which can be assigned to the (111) plane of face-centered cubic nickel (PDF#01087-0712), indicating that the nickel nanoparticles (NPs) are highly dispersed on the CNSs. Ni2P/CNSs-x hybrids show diffraction peaks at 40.7, 44.6 , 47.4 , 54.2 , 66.6 , 72.8 , 74.9 , 80.5 , and 88.9 , which can be attributed to the (111), (201), (210), (300), (310), (311), (400), (401), and (321) planes, respectively, of the hexagonal structure of Ni2P (PDF # 03-065-3544). The Ni2P/CNSs-x hybrids show an additional diffraction peak at 25.2 , which supports the formation of CNSs decorated with Ni2P NPs. The morphologies of the as-synthesized Ni2P/CNSs-x (x ¼ 10, 20, 40, 60), Ni/CNSs-20 and CNSs hybrids were characterized by TEM. The TEM images (Fig. S1) illustrate that the CNSs exhibit sphere-like morphology with an average particle size of about 40 nm. Fig. 2aed show the morphology images of the Ni2P/CNSs-10, Ni2P/CNSs-20, Ni2P/CNSs-40, and Ni2P/CNSs-60 hybrids. A large number of Ni2P NPs is supported on the CNSs, which indicates that the formed Ni2P NPs decorate the CNSs. The Ni2P NPs typically exhibited a spherical shape with high density and good distribution. No aggregated NPs can be observed, which indicates that the Ni2P NPs are well dispersed. The average sizes of the Ni2P NPs (Fig. S2aed) for the Ni2P/CNSs-10, Ni2P/CNSs-20, Ni2P/CNSs-40 and Ni2P/CNSs-60 hybrids are 4.22 ± 0.91, 4.15 ± 0.93, 3.83 ± 0.85 and 3.73 ± 0.93 nm, respectively. The SAED pattern (Fig. 2f) indicates that the major diffraction rings match well with the as-synthesized Ni2P/CNSs-20 hybrid. The HRTEM image of the Ni2P/CNSs-20 hybrid (Fig. 2g) reveals fringe spacings of about 2.06 Å and 1.94 Å, corresponding to the (201) and (210) lattice planes of hexagonal Ni2P, respectively. EDX analysis (Fig. S3) of the Ni2P/CNSs-20 hybrid confirmed the presence of C, Ni, and P elements. The measured atomic Ni: P ratio (1.8) is very close to the stoichiometric ratio of 2 in Ni2P. Fig. 2e shows the morphology of the Ni/CNSs-20 hybrid, also indicating the formation of Ni NPs decorated on the CNSs with high density and well distributed. The average size of the Ni NPs (Fig. S2e) for the Ni/CNSs-20 hybrid is 4.74 ± 0.96 nm. The HRTEM image of the Ni/ CNS-20 hybrid (Fig. 2h) reveals that the fringe spacing is about 2.01 Å, corresponding to the (111) lattice planes of face-centered cubic nickel. All these results strongly support the successful synthesis of Ni2P/CNSs-x and Ni/CNSs-20 hybrids. N2 adsorption-desorption measurements were carried out to further study the textural properties (BET surface area, pore volume, and pore size) of the as-synthesized Ni2P/CNSs-x hybrids (Table 1). The BET surface area and pore volume increased gradually with increasing carbon content. N2 sorption (Fig. 3a) on the Ni2P/CNSs-x hybrids revealed type IV isotherms with a distinct hysteresis loop in the P/P0 region from 0.4 to 1.0, which is a typical characteristic of mesoporous material. The BarretteJoynereHalenda (BJH) pore-size distribution (Fig. 3b) of the Ni2P/CNSs-10, Ni2P/CNSs-20, Ni2P/CNSs40, and Ni2P/CNSs-60 hybrids showed a narrow peak at 3.8, 3.3, 3.2, and 3.3 nm and the average pore-sizes were 12.2, 8.5, 7.5, and 7.8 nm respectively, further indicating the nanoporous nature of the Ni2P/ CNSs-x hybrids. The formation of Ni2P in the hybrids was further studied by XPS analysis. Fig. 4 shows the XPS results of the as-synthesized Ni2P/ CNSs-20 hybrid. The presence of C, O, P, and Ni elements in the XPS survey (Fig. 4a) indicates the formation of Ni2P NPs on the CNSs. That the element O was observed can be attributed to the residual oxygen-containing functionalities in the CNSs and the surface
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Fig. 4. XPS spectra of (a) survey spectrum, (b) C 1s, (c) Ni 2p and (d) P 2p regions for the Ni2P/CNSs-20 hybrid.
oxidation of Ni2P NPs due to air contact [19]. C 1s peaks (Fig. 4b) at binding energies of 284.5, 285.3, and 288.4 eV can be assigned to graphite carbon atoms, hybridized carbon atoms, and carbon atoms bonded to oxygen atoms [20], respectively. For the Ni 2p region (Fig. 4c), the peaks located at binding energy 852.7 and 870 eV are assigned to the Ni 2p3/2 and Ni 2p1/2 energy levels of Ni2P. The Ni 2p binding energies for Ni2P are very close to those of Ni metal, which indicates that the Ni atoms in Ni2P have a very small positive charge (Nidþ, 0
catalytic activity with nearly zero overpotential. All the Ni2P/CNSs hybrids exhibited excellent HER activity with nearly the same onset overpotential of 40 mV. Video S1 (see the supporting information) presents the obvious hydrogen bubbles generated on the Ni2P/ CNSs-20 hybrid modified GCE during the LSV scanning. However, the Ni/CNSs-20 hybrid exhibited a very small HER activity with high onset overpotential of 196 mV. These results indicate that all the Ni2P/CNSs hybrids exhibit higher catalytic activity than the Ni/ CNSs-20 hybrid. In addition, for achieving a current density of 10 and 20 mA cm1, the Ni2P/CNSs-40 hybrid only needed an overpotential of 92 and 108 mV, respectively. However, the Ni2P/CNSs20 hybrid needed an overpotential of 107 and 123 mV and the Ni2P/ CNSs-10 hybrid an overpotential of 118 and 142 mV, respectively. These results indicate that the electrocatalytic activity of the assynthesized Ni2P/CNSs-x hybrids can be enhanced by increasing the carbon content. However, when the content of CNSs increased once again, the catalytic activity decreased gradually. The Ni2P/ CNSs-60 hybrid needed an overpotential of 99 and 115 mV, respectively. The difference in catalytic activity among the Ni2P/ CNSs-x hybrids may be correlated to the active surface area of Ni2P on the CNSs surface. Higher carbon content provides greater surface area and electrical conductivity, but lower total available sites since the fraction of Ni2P is decreasing. When these two functions reached a balance, the HER catalytic activity could not be increased.
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Fig. 5. (a) LSV curves (iR corrected) of the Ni2P/CNSs-x, Ni/CNSs-20 hybrids and Pt/C catalyst in 0.5 M H2SO4 with a scan rate of 5 mV s1. (b) Tafel plots of Ni2P/CNSs-x, Ni/CNSs-20 hybrids and Pt/C catalyst derived from the polarization curves. (c) CVs of Ni2P/CNSs-x, Ni/CNSs-20 and bare GCE recorded at pH ¼ 7 with a scan rate of 20 mV s1 (d) CV curves of the Ni2P/CNSs-x and Ni/CNSs-20 hybrids 0.5 M H2SO4 before (solid) and after (short dash) long-term 500 cycles. (e) Time-dependent current density curve of Ni2P/CNSs-20 hybrid under static overpotential of 150 mV. (f) Nyquist plots of Ni2P/CNSs-x and Ni/CNSs-20 hybrids in 0.5 M H2SO4 with an overpotential of 150 mV.
Table S1 compares the HER activity of the Ni2P/CNSs-x hybrids with some reported transition metal phosphide (TMPs) catalysts, such as FeP nanosheets [12], FeP/GSs [19], FeP NA/Ti [30], FeP NAs/CC [34], Co2P nanorods [35], CoP/CNT [36], CoP/Ti [37], CoP/CC [38], Ni12P5 NPs [39], Ni2P NPs [10], Ni5P4 NPs [32], NiP2 NS/CC [40], MoP NPs [11b], MoP/CF [28], MoP-CA2 [41], and Cu3P NW/CF [29]. The
potentials of the Ni2P/CNSs-x hybrids compare favorably to the behavior of previously reported TMPs HER electrocatalysts in 0.5 M H2SO4. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2015.03.097. Fig. 5b shows that the Tafel plots of the Ni2P/CNSs-x, Ni/CNSs-20
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Fig. 6. (a) Nyquist plots and (b) Bode plots of the Ni2P/CNSs-20 hybrid in 0.5 M H2SO4. (c) Equivalent electrical circuit used to model the HER kinetics process. Rs is the solution resistance, Q and Rct are the element and charge transfer resistance. (d) Tafel slope of the Ni2P/CNSs-20 hybrid fitted from EIS data.
hybrids and Pt/C catalyst derived from the polarization curves fitted well with the Tafel equation (h ¼ a þ b log j, where b is the Tafel slope and j is the current density). The Tafel slope for the Pt/C, Ni2P/ CNSs-60, Ni2P/CNSs-40, Ni2P/CNSs-20, Ni2P/CNSs-10, and Ni/CNSs20 hybrids is 30, 47, 46, 49, 67, and 146 mV dec1, respectively, which indicates that the HER rate of the Ni2P/CNSs-x hybrids is faster than that of the Ni/CNSs-20 hybrid. The Pt/C catalyst exhibits the fastest HER rate. In addition, the Ni2P/CNSs-40 hybrid exhibited the fastest HER rate of all the Ni2P/CNSs-x hybrids. The smaller Tafel slope of Ni2P/CNSs-40 indicates that the catalytic activity is higher than that of the other hybrids. The Tafel slopes of the Ni2P/CNSs-x hybrids also reveal that the HER occurs via a Volmer-Heyrovsky mechanism [25]. The exchange current density of the Pt/C catalyst, Ni2P/CNSs-x and Ni/CNSs-20 hybrids was also derived from Tafel plots by applying the extrapolation method (Fig. S4). As shown in Table S2, the Ni2P/CNSs-40 hybrid displayed an exchange current density of 0.4898 mA cm2, which further indicates that the catalytic activity of the Ni2P/CNSs-40 hybrid is better than that of the other Ni2P/ CNSs-x and Ni/CNSs-20 hybrids. The number of active sites for the Ni2P/CNSs-10, Ni2P/CNSs-20, Ni2P/CNSs-40, Ni2P/CNSs-60 and Ni/CNSs-20 hybrids was determined by CV sweeps (Fig. 5c) to be 9.59 109, 6.51 108, 7.67 108, 6.98 108, and 4.56 109 mol, respectively, based on a reported method [26]. All these results demonstrate the superior HER performance of the Ni2P/CNSs-x hybrids over the Ni/
CNSs-20 hybrid. Fig. 5d shows the stability of the as-synthesized Ni2P/CNSs-x and Ni/CNSs-20 hybrids using CV sweeps for scanning 500 cycles in 0.5 M H2SO4 with a scan rate of 100 mV s1. The observation of negligible current loss indicates that all the Ni2P/CNSs-x hybrids exhibit excellent stability in acidic solution. However, a large current loss of the Ni/CNSs-20 hybrid was observed, which indicates that the Ni/CNSs-20 hybrid shows poor stability in acidic solution. Fig. 5e shows the time-dependent current density curve of the Ni2P/CNSs-20 hybrid under a static overpotential of 150 mV. The result indicates that the Ni2P/CNSs-20 hybrid maintains its high catalytic activity for at least 60’000 s. All the above results imply that the Ni2P/CNSs-x hybrids show excellent durability and thus are promising for practical application. In addition, EIS experiments were carried out to study the electronic transport behavior of as-synthesized Ni2P/CNSs-x and Ni/CNSs-20 hybrids at the same potential of 150 mV (Fig. 5f). The Ni2P/CNSs-40 hybrid has the smallest diameter and the Ni/CNSs-20 hybrid has the largest diameter, which further indicates that the Ni2P/CNSs-x hybrids have higher conductivity and, thus, better electron transfer ability than the Ni/CNSs-20 hybrid. It can also be observed that the conductivity can be improved with increasing carbon content for the Ni2P/CNSs-x hybrids. The high activity and stability of the as-synthesized Ni2P/CNSs-x can be attributed to the following factors. First, the coupling effect between the Ni2P NPs and CNSs facilitates the electron transfer and
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decreases the hydrogen binding energy during the HER process. Second, the XPS results indicate that the Ni and P atoms in Ni2P have a small positive charge and negative charge, respectively. Previously it was reported that TMPs complex HER electrocatalyst, such as CoP [27], MoP [28], Cu3P [29], and FeP [30], share the catalytic mechanism with hydrogenase [31]. Therefore, in our present work, the role of Ni and P should be similar as in previously reported TMPs HER electrocatalysts [27e32]. The Ni and P atoms act as the hydride acceptor and proton acceptor [10], respectively, to enhance the HER process. In addition, the P atoms may also facilitate the formation of Ni-hydride for subsequent hydrogen evolution via electrochemical desorption [33]. Finally, the electrical conductivity and surface area also significantly influence the electrochemical performance. The small size of the Ni2P NPs in the hybrids favor the exposure of more active sites for the HER. The introduction of CNSs could further increase the surface area of the Ni2P/CNSs-x hybrids. In addition, the excellent electrical conductivity of the CNSs support facilitates charge transfer in the hybrids. In all, the coupling effect between the Ni2P NPs and CNSs, the electronic effect of Ni, the ensemble effect of P, the large surface area as well as the high electron conductivity of CNSs contribute to the superior catalytic activity of Ni2P/CNSs-x hybrids. The kinetics of the Ni2P/CNSs-20 hybrid was further studied by EIS experiments. The Nyquist plots of the Ni2P/CNSs-20 hybrid at various potentials are shown in Fig. 6a. With the increase of potential from 50 to 150 mV, the diameter of the semicircles decreased gradually, which indicates that the Ni2P/CNSs-20 hybrid has good electron transfer ability at high potential. The corresponding Bode plots (Fig. 6b) show only one time constant. Thus, as shown in Fig. 6c, the HER process can be described by a simple equivalent electrical circuit, the corresponding parameters of which are listed in Table S3. The solution resistance (Rs) is approximately 8 U. The value of the charge transfer resistance (Rct) is potential-dependent, and the lower Rct reflects the superior electrocatalytic activity of the Ni2P/CNSs-20 nanohybrid. The Tafel slope of the Ni2P/CNSs-20 hybrid can be obtained by fitting the log(1/Rct)-h plot, as shown in Fig. 6d. The Tafel slope of the Ni2P/ CNSs-20 hybrid is 52 mV$dec1, and this value is nearly in accordance with the Tafel slope (49 mV$dec1) obtained from the polarization curve. 4. Conclusions Nanostructured Ni2P/CNSs-x hybrids were successfully synthesized by a thermal decomposition using Ni(acac)2 as nickel source and TOP as phosphorus source in an OAm solution with dispersed CNSs. As novel HER electrocatalysts, the as-synthesized Ni2P/CNSsx hybrids exhibit high efficiency and excellent stability in acidic media. In addition, it is observed that the catalytic activity of the Ni2P/CNSs-40 hybrid is better than that of the other Ni2P/CNSs-x hybrids, which indicates that the catalytic activity of the Ni2P/CNSs hybrids can be enhanced by changing the carbon content. The excellent activity and stability is attributed to the coupling effect between Ni2P NPs and CNSs, the electronic effect of Ni, the ensemble effect of P, the large surface area, and the high electron conductivity of CNSs. This work provides a general strategy for the synthesis of other electrocatalysts based on carbon materials decorated with transition metal phosphides for HER applications. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grants No. 21006128, 21176258, U1162203), China University of Petroleum for Postgraduate Technology Innovation Project (Grants No. YCX2014033), the
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