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Accepted Article Title: Cuboid Ni2P as a bifunctional catalyst for efficient hydrogen generation from hydrolysis of ammonia borane and electrocatalytic hydrogen evolution Authors: Wei Luo, Yeshaung Du, Chao Liu, and Gongzhen Cheng This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Chem. Asian J. 10.1002/asia.201701302 Link to VoR: http://dx.doi.org/10.1002/asia.201701302
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10.1002/asia.201701302
Chemistry - An Asian Journal
Cuboid Ni2P as a bifunctional catalyst for efficient hydrogen generation from hydrolysis of ammonia borane and electrocatalytic hydrogen evolution Yeshuang Du,a Chao Liu,a Gongzhen Chenga and Wei Luoa,b* a
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei
430072, P. R. China, Tel.: +86-027-68752366 *Corresponding author. E-mail addresses: [email protected]. b
Key laboratory of Advanced Energy Materials Chemistry (Ministry of Education),
Nankai University, Tianjin 300071, P. R. China
Abstract The design of high-performance catalysts for hydrogen generation is highly desirable for the upcoming hydrogen economy. Herein, we report the colloidal synthesis of nanocuboid Ni2P via thermal decomposition of nickel chloride hexahydrate (NiCl2∙6H2O) and trioctylphosphine (TOP). The obtained nanocuboid Ni2P is characterized by powder X-ray diffraction (XRD), transmission electron microscope (TEM), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). For the first time, the as-synthesized nanocuboid Ni2P is used as bifunctional catalyst for hydrogen generation from hydrolysis of ammonia borane and electrocatalytic hydrogen evolution. Thanks to the strong synergistic electronic effect between Ni and P, the as-synthesized Ni2P exhibits superior catalytic performance in comparison to its counterpart without P 1
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doping. Keywords: Transition metal phosphides; ammonia borane; electrocatalytic hydrogen evolution; cuboid Ni2P Introduction Hydrogen has received significant attention as a clean energy carrier alternative to fossil fuels, in consideration of its high energy density, non-toxicity, and environmental benignity.1-4 However, safe, efficient storage and release of hydrogen under ambient conditions are still the major hurdles for the prospective hydrogen economy.5-8 Water electrolysis is considered as one of the easiest and most efficient way to produce highly pure hydrogen through the hydrogen evolution reaction (HER).9-11 On the other hand, among various hydrogen storage approaches, including metal hydrides, sorbent materials, and chemical hydride materials, ammonia borane (NH3-BH3, AB) with high gravimetric hydrogen densities (19.6 wt%) and favorable kinetics of hydrogen release, has recently received great attention.12-18 Currently, noble metal catalysts are considered as the state-of-the-art HER and AB hydrolysis catalysts, nevertheless, their widespread applications have been severely hindered by the high cost and scarcity.19,20 Therefore, developing efficient earth-abundant catalysts toward HER and hydrolysis of AB as alternatives to noble metals is highly desirable, but still remains huge challenge.21-23 Transition metal phosphides (TMPs) with unique charged natures (positive charge in transition metal and negative charge in phosphorus), as well as hydrogenase-like catalytic mechanism, have been widely studied as Pt alternatives toward electrocatalytic HER.24-30 For example, Liu’s group reported Ni2P on 3D materials (Ni 2
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foam or carbon fiber paper) for HER by direct phosphorization of commercially available nickel foam using red phosphorus (P) as the precursor. 31-33 On the other hand, Fu and coworkers first reported the synthesis of Ni2P by phosphorization of Ni(OH)2 powders using NaH2PO2 as P source at relatively high temperature in argon, and its superior catalytic activity toward hydrolysis of AB.34 Sun’s group developed some metal phosphides for the hydrolytic dehydrogenation of AB or NaBH4.35-37 Our recently study has shown that RhNiP supported on reduced graphene oxide (RhNiP/rGO) is an active catalyst toward catalytic hydrogen generation from alkaline solution of hydrazine. 38
Despite the significant progresses, in the view of practical application, developing
highly efficient bifunctional TMP-based catalysts toward HER and AB hydrolysis is highly desirable, but still a great challenge. Here, we report the colloidal synthesis of nanocuboid Ni2P through thermal decomposition approach by using nickel chloride hexahydrate as nickel source, trioctylphosphine (TOP) as the phosphorus source, oleylamine (OAm) as reluctant, and 1-octadecene (ODE) as solvent. The as-prepared catalyst exhibits exceptional catalytic performance and high stability toward hydrolysis of AB under ambient temperature, with the initial turnover frequency (TOFinitial) value of 13.5 min-1. Moreover, as a bifunctional catalyst, the Ni2P exhibits high activity and good durable toward HER with an overpotential of 138 mV at a current density of 10 mA cm−2 in 0.5 M H2SO4.
Experimental section Materials 3
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All reagents were commercially available and used without further purification. Ammonia-borane (NH3BH3, AB, Aldrich, 90%), nickel chloride hexahydrate (NiCl2∙6H2O, Sinopharm Chemical Reagent Co., Ltd., ≥98%), oleylamine (C18H37N, oAm, Aladdin, 80~90%), trioctylphosphine (C24H51P, TOP, Aldrich, 90%), 1octadecene (C12H24, ODE, Aladdin, ≥90%), polyvinyl pyrrolidone ((C6H9NO)n, PVP, Aldrich), ethanol (C2H5OH, Sinopharm Chemical Reagent Co., Ltd., >99.8%) ，hexane (n-C6H12, Sinopharm Chemical Reagent Co., Ltd., ≥ 97.0%), Trichloromethane (CHCl3, Sinopharm Chemical Reagent Co., Ltd., ≥99.0%), acetone (CH3COCH3, Sinopharm Chemical Reagent Co., Ltd., ≥ 99.5%), potassium chloride (KCl, Sinopharm Chemical Reagent Co., Ltd., ≥99.5%), isopropyl alcohol(CH3CHOHCH3, Sinopharm Chemical Reagent Co., Ltd., ≥ 99%), Nafion (Sinopharm Chemical Reagent Co., Ltd., ≥5%)
The synthesis of cuboid Ni2P Typically, the reaction was carried out under a nitrogen atmosphere. 0.2 mmol NiCl2, 1.0 mL TOP (2 mmol), 2 mL OAm and 2 mL ODE were added into a three-neck flask and stirred magnetically. Then, the mixture was heated to 120 oC with the heating rate of 10 oC/min and kept at this temperature for 30 min to remove moisture and dissolved oxygen. Finally, the solution was rapidly heated to 320 oC and maintained for 2 h. After cooling to room temperature, the black precipitate was obtained by adding ethanol and separated by centrifugation (8500 r/min). The final product was obtained by washing with a mixture of hexane and ethanol for three times to remove excess surfactant and 4
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residuary organic solvent, followed by drying in vacuum. Then, the obtained Ni2P and 200 mg PVP was dissolved in 15 mL CHCl3. The solution was heated to 65 oC for 10 h under a nitrogen atmosphere. After the mixture was cooled to room temperature, the precipitate was gained by adding acetone to cause flocculation and separated by centrifugation (8500 r/min). Finally, the received product was dried in an oven under vacuum. Sphere Ni2P nanoparticles were prepared in the same way excepted that NiCl2 was replaced by Ni(acac)2. Ni was synthesized through the similar method without adding TOP.
The synthesis of cuboid Ni2P/C 10 mg cuboid Ni2P catalyst and the same mass of XC-72 were added into 15 mL hexane. Then the mixture was stirred for 10 h under nitrogen flow at room temperature. Finally, the product was obtained by centrifugation and dried in an oven under vacuum. Other catalysts like spherical Ni2P and Ni are loaded on XC-72 with about 50 wt% mass loading.
Decomposition of AB The activity of Ni2P toward hydrolysis of AB was determined by measuring the rate of hydrogen generated from AB using the water filled gas burette system. 20 mg asprepared catalyst was dispersed in 4 mL H2O in a two-neck round-bottom flask. One neck was connected to a gas burette to monitor the volume of the gas evolution, and the 5
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other was plugged by a rubber plug. The flask was kept in the water bath with the temperature maintained 25 ±0.5 oC. Next, 1.0 mmol AB dissolved in 1.0 mL H2O was injected into the catalyst solution under magnetically stirring. In order to measure the activation energy (Ea), the catalytic reaction was carried out at different temperatures (298 K, 303 K, 313 K and 323 K). The value of initial turnover frequency (TOF initial) can be calculated using following equation. TOFinitial = (Patm•VH2/RT)/ (nmetal•t) TOFinitial is initial turnover frequency, Patm is the atmospheric pressure, VH2 is the volume of the generated gas when the conversion reached 50% (while 1.5 equivalents of n (H2)/n (NH3BH3) are obtained) , R is the universal gas constant, T is room temperature (298 K), nmetal is the mole amount of metal, and t is the reaction time.
Stability test For testing the reusability of catalyst, the catalytic reaction was repeated 5 times by injecting another same amount of AB (1.0 mmol) into catalyst solution after the previous cycle.
Preparation of working electrodes To make working electrodes, 5 mg Ni2P/C (or Ni/C) was dissolved into 1 mL Nafion/isopropyl alcohol (0.1%) with sonication for 30 min to obtain uniform catalyst solution. Then 18 µL of catalyst ink was deposited on a glass carbon electrode (area of 0.196 cm2) to achieve about 0.45 mg/cm2 mass loading. 6
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Electrochemical measurement All the electrochemical tests were performed by an electrochemical analyzer in a threeelectrode system. Electrochemical measurements were performed in 0.5 M H2SO4 using a rotation disk electrode with a rotation speed of 1600 rpm. The glass carbon electrode deposited with catalyst, a carbon rod and Hg/Hg2Cl2 served as working electrode, counter electrode and reference electrode, respectively. All potential data were calibrated by measuring RHE potential with a Pt/C modified GC electrode in saturated H2 atmosphere. The catalytic activity was measured by liner sweep voltammetry from 0.005 V to -0.44 V vs RHE at a scan rate of 5 mV/s. Final data were adjusted by iR correction (caused by resistance) through electrochemical impedance spectroscope. Accelerated degradation tests were performed by cyclic voltammograms sweeps between 0.005 V and -0.16 V vs RHE at a scan rate of 100 mV/s. The stability of catalyst was performed by maintaining for 70000 s at a current density of 10 mA/cm2
Characterizations Powder X-ray diffraction (XRD) patterns were measured by a Bruker D8-Advance Xray diffractometer using Cu Kα radiation source (λ = 0.154178 nm) with a velocity of 6°/min, X-ray photoelectron spectroscopy (XPS) measurement was performed with a Kratos XSAM 800 spectrophotometer. The morphologies and sizes of the samples were obtained by using a Tecnai G20 U-Twin transmission electron microscope (TEM) equipped with an energy dispersive X-ray (EDX) spectrometer at an acceleration voltage of 200 kV. Inductively coupled plasma-atomic emission spectroscopy (ICP7
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AES) was performed on IRIS Intrepid II XSP. Result and discussion The cuboid Ni2P nanocatalysts were prepared through thermal decomposition of NiCl2 and TOP in OAm and ODE at 320 oC for 2 h. For comparison, Ni NPs were synthesized through the similar method without adding TOP and sphere Ni2P NPs were fabricated using Ni(acac)2 as Ni precursor. Fig. 1a and Fig. S1 show the powder X-ray diffraction (XRD) patterns of cuboid Ni2P, sphere Ni2P and Ni. The diffraction peaks at 40.8o, 44.7o, 47.5o, 54.2o and 54.9o are attributed to the (111), (201), (210), (300) and (211) planes of Ni2P (PDF #03-0953), respectively.39 Spherical Ni2P has similar hexagonal crystal structure to cuboid Ni2P. In addition, the XRD pattern of Ni in Fig. S1a shows four prominent peaks at 39.1o, 41.5o, 44.5o and 58.4o, corresponding to the (010), (002), (011) and (012) crystal planes (PDF #45-1027), respectively.40 The morphologies of Ni, cuboid Ni2P and sphere Ni2P were characterized by transmission electron microscope (TEM). As shown in Fig. 1b, well dispersed Ni2P nanoparticles with cuboid morphology are observed. High resolution TEM image in Fig. 1c displayed crystal lattice fringes with the lattice plane distance of 0.336 nm, corresponding to (001) plane of Ni2P. Without the addition of TOP, aggregated Ni nanoparticles are obtained, as shown in Fig. S2a. Fig. S2b displayed the monodisperse spherical Ni2P NPs when Ni(acac)2 was used as nickel source, which is consistent with literature.41 This result indicates the key fact of Ni precursor in the formation of unique cuboid morphology. The energy dispersive X-ray spectrum (EDX) of Ni2P nanocuboid showed in Fig. 1d further confirms the co-existence of Ni and P. The atomic ratio of Ni/P is measured to 8
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be 67:33, which is in accordance with the stoichiometric ratios of Ni2P. In addition, the composition was further confirmed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), which agreed well with the EDX result as shown in Table S1. The surface structure of Ni2P nanocuboid was analyzed by X-ray photoelectron spectroscopy (XPS) measurement. Fig. 2a and 2b show the XPS spectra of the Ni 2p3/2 and P 2p regions for Ni2P. It can be seen from Fig. 2a that the peak located at binding energy of 852.4 eV is assigned to Niδ+, which is close to Ni metal with a small positive charge (Niδ+, 0< δ< 2).42 The peaks at 855.8 and 860.1 eV are attributed to oxidized Ni species and satellite peak, respectively.43 Fig. 2b shows the XPS spectra of P 2p, the peak at 129.8 eV corresponding to Pδ- is very close to that of zero valence state P, indicating P atom in Ni2P exhibits a marginal negative charge (Pδ-, 0< δ< 1). The other binding energy peak located at 133.4 eV is assigned to small amount of oxidized P species formed on the surface of catalyst due to the exposure to air.42 These results indicate the strong electron transfer between Ni and P, which is the key fact for the enhanced catalytic activity.30,44 It has been reported that hydrogenase is a high active HER catalyst and its active sites are pendant bases close to the metal centers.45,46 Ni2P has similar electronic structure to hydrogenase with a metal center Ni (δ+) and a pendant base P (δ-) close to it. Thus, the high catalytic activity of Ni2P might be derived from its hydrogenase-like catalytic mechanisms. The catalytic activities of cuboid Ni2P/C, spherical Ni2P/C and Ni/C catalysts toward hydrolysis of AB were first studied. As shown Fig. 3a, cuboid Ni2P/C exhibits superior activity with the turnover frequency (TOF) value of 13.5 min-1, which is 27 times higher 9
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than that of Ni/C (TOF = 0.5 min-1). However, spherical Ni2P showed inferior activity with induction period and incomplete hydrogen generation. It is noteworthy that, the catalytic activity of cuboid Ni2P/C is higher than those of most of the reported non noble metal-based catalysts and even some noble metal-based catalysts as indicated in Table S2. The durability of as-prepared cuboid Ni2P/C up to fifth run for hydrolysis of AB is shown in Fig. 3b. There is no obvious deactivation even after five cycles, indicating its superior durability. To obtain the activation energy (Ea) of hydrolysis of AB catalyzed by cuboid Ni2P/C, the reactions were set at different temperatures, varying from 298 K to 323 K. It can be seen from Fig. 3c that the achieving time of decomposing AB are 1.67 min, 2.5 min, 5 min and 7.5 min, with the TOF values of 67.8, 39.6, 20.8 and 13.5 min-1, respectively. Fig. 3d shows that the activation energy is calculated to be 43.4 kJ/mol, which is also close to the reported Ea for hydrolysis of AB as indicated in Table S2. The electrocatalytic activities of cuboid Ni2P/C, spherical Ni2P and Ni/C toward HER were evaluated under 0.5 M H2SO4 with a scan rate of 5 mV s-1 using a three-electrode setup with the working electrode prepared by the deposition of the catalyst on a glassy carbon electrode. Fig. 4a shows the linear sweep voltammetry (LSV) curves of cuboid Ni2P/C, spherical Ni2P/C, Ni/C and commercial Pt/C (20 wt%) vs the reversible hydrogen electrode (RHE) scale. It can be seen that Pt/C exhibits the highest catalytic activity with overpotential of 22 mV to afford a current density of 10 mA cm−2. However, the spherical Ni2P/C and Ni/C required 187 and 397 mV to reach 10 mA cm−2, respectively. As expected, Ni2P/C exhibits high activity with overpotential of 138 mV 10
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to achieve 10 mA cm−2, which is superior to most of the reported NiP-based catalysts, including Ni2P nanoparticles/graphene (η10mAcm-2 = 264 mV), 47 Ni2P nanoparticles/Ti (η10 mA cm-2 = 130 mV),41 Ni12P5/CNT (η10 mA cm-2 = 240 mV),48 monodispersed Ni12P5 nanoparticles/GCE (η10 mA cm-2 = 208 mV).49 Fig. 4b exhibits the Tafel plots of the catalysts. As expected, Pt/C presents the smallest Tafel slope of 30.2 mV dec-1. The nanocuboid Ni2P/C shows a Tafel slope of 75.5 mV dec-1, much smaller than those of spherical Ni2P (93 mV dec-1) and Ni/C (137 mV dec-1). Thus, the incorporation of phosphorus to nickel can greatly increase the HER electrocatalytic activity. The reason that Ni2P with different shape exhibited activity imparity may be exposed lattice plane. It has been reported that Ni2P (001) surface has higher HER activity41,50 and the cuboid Ni2P was exposed with (001) crystal plane. The stability is another critical criterion for judging an efficient HER electrocatalysts. The chronopotentiometric curve in Fig. 4c indicates the overpotential slightly increased after 70000 s, from an initial of 138 mV to 143 mV, at a constant current density of 10 mA cm-2. In addition, as shown in Fig. 4d, after long-term cycling, Ni2P/C exhibits a similar polarization curve as before. The morphology of cuboid Ni2P after cycling was characterized by TEM. As shown in Fig. S3, Ni2P maintained its morphology and dispersion without obvious change. These results indicate the superior stability of cuboid Ni2P/C toward HER. We also calculate the Faradaic efficiency of cuboid Ni2P by measuring the volume of generated hydrogen. As shown in Fig. S4, the amount of experimentally generated hydrogen is in accordance with theoretical value, indicating the 100% Faradaic efficiency.
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Conclusion In summary, we have reported a colloidal method to synthesize nanocuboid Ni2P with superior bifunctional catalytic performance toward hydrolysis of AB and electrocatalytic HER. When used as catalyst for hydrolysis of AB, the as-synthesized cuboid Ni2P/C exhibits superior catalytic activity in comparison to spherical Ni2P and Ni/C, with the TOF value of 13.5 min-1 and good durable stability. When used as electrocatalyst for HER, it exhibits HER activity with a low overpotential (138 mV), a small Tafel slope (75.5 mV dec-1), 100% Faraday efficiency and superior stability. This study may open new avenue in designing and utilizing TMPs for more applications.
Acknowledgements This article was financially supported by the National Natural Science Foundation of China (21571145), the Fundamental Research Funds for the Central Universities (410500194) and Large-scale Instrument and Equipment Sharing Foundation of Wuhan University.
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Fig. 1 (a) Powder XRD patterns of cuboid Ni2P, (b, c) TEM images of cuboid Ni2P and (d) EDX spectrum of cuboid Ni2P.
Fig. 2 XPS spectra of (a) Ni 2p3/2 and (b) P 2p for cuboid Ni2P. 17
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Fig. 3 (a) Hydrogen generation from the hydrolysis of aqueous NH3BH3 (1mmol) catalyzed by cuboid Ni2P/C, spherical Ni2P/C and Ni/C. (b) Cycle stability test of cuboid Ni2P/C for hydrogen generation from the hydrolysis of aqueous NH3BH3 (1mmol) under an ambient atmosphere at 298 K. (c) Time course plots for hydrogen generation from the decomposition of AB by cuboid Ni2P/C at 298 K, 303 K, 313 K and 323 K. (d) Plot of lnk versus 1/T during the AB decomposition over cuboid Ni2P at different temperatures. (catalyst/NH3BH3 = 0.037).
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Fig. 4 (a) LSV curves for cuboid Ni2P/C, sphere Ni2P/C, Ni/C and Pt/C with a scan rate of 5 mV/s. (b) Tafel plots for cuboid Ni2P, sphere Ni2P/C, Ni/C and Pt/C. (c) Time dependent potential curve of cubiod Ni2P at a fixed current density of 10 mA/cm2. (d) LSV curves for cuboid Ni2P before and after 1000 cycles. All experiments were done in 0.5 M H2SO4.
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TOC
Nanocuboid Ni2P has been synthesized through a facile colloidal method and further used as bifunctional catalyst for efficient hydrogen generation from hydrolysis of ammonia borane and electrocatalytic hydrogen evolution
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Accepted Article Title: Cuboid Ni2P as a bifunctional catalyst for efficient hydrogen generation from hydrolysis of ammonia borane and electrocatalytic hydrogen evolution Authors: Wei Luo, Yeshaung Du, Chao Liu, and Gongzhen Cheng This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Chem. Asian J. 10.1002/asia.201701302 Link to VoR: http://dx.doi.org/10.1002/asia.201701302
A Journal of
A sister journal of Angewandte Chemie and Chemistry – A European Journal
10.1002/asia.201701302
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Cuboid Ni2P as a bifunctional catalyst for efficient hydrogen generation from hydrolysis of ammonia borane and electrocatalytic hydrogen evolution Yeshuang Du,a Chao Liu,a Gongzhen Chenga and Wei Luoa,b* a
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei
430072, P. R. China, Tel.: +86-027-68752366 *Corresponding author. E-mail addresses: [email protected]. b
Key laboratory of Advanced Energy Materials Chemistry (Ministry of Education),
Nankai University, Tianjin 300071, P. R. China
Abstract The design of high-performance catalysts for hydrogen generation is highly desirable for the upcoming hydrogen economy. Herein, we report the colloidal synthesis of nanocuboid Ni2P via thermal decomposition of nickel chloride hexahydrate (NiCl2∙6H2O) and trioctylphosphine (TOP). The obtained nanocuboid Ni2P is characterized by powder X-ray diffraction (XRD), transmission electron microscope (TEM), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). For the first time, the as-synthesized nanocuboid Ni2P is used as bifunctional catalyst for hydrogen generation from hydrolysis of ammonia borane and electrocatalytic hydrogen evolution. Thanks to the strong synergistic electronic effect between Ni and P, the as-synthesized Ni2P exhibits superior catalytic performance in comparison to its counterpart without P 1
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doping. Keywords: Transition metal phosphides; ammonia borane; electrocatalytic hydrogen evolution; cuboid Ni2P Introduction Hydrogen has received significant attention as a clean energy carrier alternative to fossil fuels, in consideration of its high energy density, non-toxicity, and environmental benignity.1-4 However, safe, efficient storage and release of hydrogen under ambient conditions are still the major hurdles for the prospective hydrogen economy.5-8 Water electrolysis is considered as one of the easiest and most efficient way to produce highly pure hydrogen through the hydrogen evolution reaction (HER).9-11 On the other hand, among various hydrogen storage approaches, including metal hydrides, sorbent materials, and chemical hydride materials, ammonia borane (NH3-BH3, AB) with high gravimetric hydrogen densities (19.6 wt%) and favorable kinetics of hydrogen release, has recently received great attention.12-18 Currently, noble metal catalysts are considered as the state-of-the-art HER and AB hydrolysis catalysts, nevertheless, their widespread applications have been severely hindered by the high cost and scarcity.19,20 Therefore, developing efficient earth-abundant catalysts toward HER and hydrolysis of AB as alternatives to noble metals is highly desirable, but still remains huge challenge.21-23 Transition metal phosphides (TMPs) with unique charged natures (positive charge in transition metal and negative charge in phosphorus), as well as hydrogenase-like catalytic mechanism, have been widely studied as Pt alternatives toward electrocatalytic HER.24-30 For example, Liu’s group reported Ni2P on 3D materials (Ni 2
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foam or carbon fiber paper) for HER by direct phosphorization of commercially available nickel foam using red phosphorus (P) as the precursor. 31-33 On the other hand, Fu and coworkers first reported the synthesis of Ni2P by phosphorization of Ni(OH)2 powders using NaH2PO2 as P source at relatively high temperature in argon, and its superior catalytic activity toward hydrolysis of AB.34 Sun’s group developed some metal phosphides for the hydrolytic dehydrogenation of AB or NaBH4.35-37 Our recently study has shown that RhNiP supported on reduced graphene oxide (RhNiP/rGO) is an active catalyst toward catalytic hydrogen generation from alkaline solution of hydrazine. 38
Despite the significant progresses, in the view of practical application, developing
highly efficient bifunctional TMP-based catalysts toward HER and AB hydrolysis is highly desirable, but still a great challenge. Here, we report the colloidal synthesis of nanocuboid Ni2P through thermal decomposition approach by using nickel chloride hexahydrate as nickel source, trioctylphosphine (TOP) as the phosphorus source, oleylamine (OAm) as reluctant, and 1-octadecene (ODE) as solvent. The as-prepared catalyst exhibits exceptional catalytic performance and high stability toward hydrolysis of AB under ambient temperature, with the initial turnover frequency (TOFinitial) value of 13.5 min-1. Moreover, as a bifunctional catalyst, the Ni2P exhibits high activity and good durable toward HER with an overpotential of 138 mV at a current density of 10 mA cm−2 in 0.5 M H2SO4.
Experimental section Materials 3
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All reagents were commercially available and used without further purification. Ammonia-borane (NH3BH3, AB, Aldrich, 90%), nickel chloride hexahydrate (NiCl2∙6H2O, Sinopharm Chemical Reagent Co., Ltd., ≥98%), oleylamine (C18H37N, oAm, Aladdin, 80~90%), trioctylphosphine (C24H51P, TOP, Aldrich, 90%), 1octadecene (C12H24, ODE, Aladdin, ≥90%), polyvinyl pyrrolidone ((C6H9NO)n, PVP, Aldrich), ethanol (C2H5OH, Sinopharm Chemical Reagent Co., Ltd., >99.8%) ，hexane (n-C6H12, Sinopharm Chemical Reagent Co., Ltd., ≥ 97.0%), Trichloromethane (CHCl3, Sinopharm Chemical Reagent Co., Ltd., ≥99.0%), acetone (CH3COCH3, Sinopharm Chemical Reagent Co., Ltd., ≥ 99.5%), potassium chloride (KCl, Sinopharm Chemical Reagent Co., Ltd., ≥99.5%), isopropyl alcohol(CH3CHOHCH3, Sinopharm Chemical Reagent Co., Ltd., ≥ 99%), Nafion (Sinopharm Chemical Reagent Co., Ltd., ≥5%)
The synthesis of cuboid Ni2P Typically, the reaction was carried out under a nitrogen atmosphere. 0.2 mmol NiCl2, 1.0 mL TOP (2 mmol), 2 mL OAm and 2 mL ODE were added into a three-neck flask and stirred magnetically. Then, the mixture was heated to 120 oC with the heating rate of 10 oC/min and kept at this temperature for 30 min to remove moisture and dissolved oxygen. Finally, the solution was rapidly heated to 320 oC and maintained for 2 h. After cooling to room temperature, the black precipitate was obtained by adding ethanol and separated by centrifugation (8500 r/min). The final product was obtained by washing with a mixture of hexane and ethanol for three times to remove excess surfactant and 4
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residuary organic solvent, followed by drying in vacuum. Then, the obtained Ni2P and 200 mg PVP was dissolved in 15 mL CHCl3. The solution was heated to 65 oC for 10 h under a nitrogen atmosphere. After the mixture was cooled to room temperature, the precipitate was gained by adding acetone to cause flocculation and separated by centrifugation (8500 r/min). Finally, the received product was dried in an oven under vacuum. Sphere Ni2P nanoparticles were prepared in the same way excepted that NiCl2 was replaced by Ni(acac)2. Ni was synthesized through the similar method without adding TOP.
The synthesis of cuboid Ni2P/C 10 mg cuboid Ni2P catalyst and the same mass of XC-72 were added into 15 mL hexane. Then the mixture was stirred for 10 h under nitrogen flow at room temperature. Finally, the product was obtained by centrifugation and dried in an oven under vacuum. Other catalysts like spherical Ni2P and Ni are loaded on XC-72 with about 50 wt% mass loading.
Decomposition of AB The activity of Ni2P toward hydrolysis of AB was determined by measuring the rate of hydrogen generated from AB using the water filled gas burette system. 20 mg asprepared catalyst was dispersed in 4 mL H2O in a two-neck round-bottom flask. One neck was connected to a gas burette to monitor the volume of the gas evolution, and the 5
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other was plugged by a rubber plug. The flask was kept in the water bath with the temperature maintained 25 ±0.5 oC. Next, 1.0 mmol AB dissolved in 1.0 mL H2O was injected into the catalyst solution under magnetically stirring. In order to measure the activation energy (Ea), the catalytic reaction was carried out at different temperatures (298 K, 303 K, 313 K and 323 K). The value of initial turnover frequency (TOF initial) can be calculated using following equation. TOFinitial = (Patm•VH2/RT)/ (nmetal•t) TOFinitial is initial turnover frequency, Patm is the atmospheric pressure, VH2 is the volume of the generated gas when the conversion reached 50% (while 1.5 equivalents of n (H2)/n (NH3BH3) are obtained) , R is the universal gas constant, T is room temperature (298 K), nmetal is the mole amount of metal, and t is the reaction time.
Stability test For testing the reusability of catalyst, the catalytic reaction was repeated 5 times by injecting another same amount of AB (1.0 mmol) into catalyst solution after the previous cycle.
Preparation of working electrodes To make working electrodes, 5 mg Ni2P/C (or Ni/C) was dissolved into 1 mL Nafion/isopropyl alcohol (0.1%) with sonication for 30 min to obtain uniform catalyst solution. Then 18 µL of catalyst ink was deposited on a glass carbon electrode (area of 0.196 cm2) to achieve about 0.45 mg/cm2 mass loading. 6
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Electrochemical measurement All the electrochemical tests were performed by an electrochemical analyzer in a threeelectrode system. Electrochemical measurements were performed in 0.5 M H2SO4 using a rotation disk electrode with a rotation speed of 1600 rpm. The glass carbon electrode deposited with catalyst, a carbon rod and Hg/Hg2Cl2 served as working electrode, counter electrode and reference electrode, respectively. All potential data were calibrated by measuring RHE potential with a Pt/C modified GC electrode in saturated H2 atmosphere. The catalytic activity was measured by liner sweep voltammetry from 0.005 V to -0.44 V vs RHE at a scan rate of 5 mV/s. Final data were adjusted by iR correction (caused by resistance) through electrochemical impedance spectroscope. Accelerated degradation tests were performed by cyclic voltammograms sweeps between 0.005 V and -0.16 V vs RHE at a scan rate of 100 mV/s. The stability of catalyst was performed by maintaining for 70000 s at a current density of 10 mA/cm2
Characterizations Powder X-ray diffraction (XRD) patterns were measured by a Bruker D8-Advance Xray diffractometer using Cu Kα radiation source (λ = 0.154178 nm) with a velocity of 6°/min, X-ray photoelectron spectroscopy (XPS) measurement was performed with a Kratos XSAM 800 spectrophotometer. The morphologies and sizes of the samples were obtained by using a Tecnai G20 U-Twin transmission electron microscope (TEM) equipped with an energy dispersive X-ray (EDX) spectrometer at an acceleration voltage of 200 kV. Inductively coupled plasma-atomic emission spectroscopy (ICP7
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AES) was performed on IRIS Intrepid II XSP. Result and discussion The cuboid Ni2P nanocatalysts were prepared through thermal decomposition of NiCl2 and TOP in OAm and ODE at 320 oC for 2 h. For comparison, Ni NPs were synthesized through the similar method without adding TOP and sphere Ni2P NPs were fabricated using Ni(acac)2 as Ni precursor. Fig. 1a and Fig. S1 show the powder X-ray diffraction (XRD) patterns of cuboid Ni2P, sphere Ni2P and Ni. The diffraction peaks at 40.8o, 44.7o, 47.5o, 54.2o and 54.9o are attributed to the (111), (201), (210), (300) and (211) planes of Ni2P (PDF #03-0953), respectively.39 Spherical Ni2P has similar hexagonal crystal structure to cuboid Ni2P. In addition, the XRD pattern of Ni in Fig. S1a shows four prominent peaks at 39.1o, 41.5o, 44.5o and 58.4o, corresponding to the (010), (002), (011) and (012) crystal planes (PDF #45-1027), respectively.40 The morphologies of Ni, cuboid Ni2P and sphere Ni2P were characterized by transmission electron microscope (TEM). As shown in Fig. 1b, well dispersed Ni2P nanoparticles with cuboid morphology are observed. High resolution TEM image in Fig. 1c displayed crystal lattice fringes with the lattice plane distance of 0.336 nm, corresponding to (001) plane of Ni2P. Without the addition of TOP, aggregated Ni nanoparticles are obtained, as shown in Fig. S2a. Fig. S2b displayed the monodisperse spherical Ni2P NPs when Ni(acac)2 was used as nickel source, which is consistent with literature.41 This result indicates the key fact of Ni precursor in the formation of unique cuboid morphology. The energy dispersive X-ray spectrum (EDX) of Ni2P nanocuboid showed in Fig. 1d further confirms the co-existence of Ni and P. The atomic ratio of Ni/P is measured to 8
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be 67:33, which is in accordance with the stoichiometric ratios of Ni2P. In addition, the composition was further confirmed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), which agreed well with the EDX result as shown in Table S1. The surface structure of Ni2P nanocuboid was analyzed by X-ray photoelectron spectroscopy (XPS) measurement. Fig. 2a and 2b show the XPS spectra of the Ni 2p3/2 and P 2p regions for Ni2P. It can be seen from Fig. 2a that the peak located at binding energy of 852.4 eV is assigned to Niδ+, which is close to Ni metal with a small positive charge (Niδ+, 0< δ< 2).42 The peaks at 855.8 and 860.1 eV are attributed to oxidized Ni species and satellite peak, respectively.43 Fig. 2b shows the XPS spectra of P 2p, the peak at 129.8 eV corresponding to Pδ- is very close to that of zero valence state P, indicating P atom in Ni2P exhibits a marginal negative charge (Pδ-, 0< δ< 1). The other binding energy peak located at 133.4 eV is assigned to small amount of oxidized P species formed on the surface of catalyst due to the exposure to air.42 These results indicate the strong electron transfer between Ni and P, which is the key fact for the enhanced catalytic activity.30,44 It has been reported that hydrogenase is a high active HER catalyst and its active sites are pendant bases close to the metal centers.45,46 Ni2P has similar electronic structure to hydrogenase with a metal center Ni (δ+) and a pendant base P (δ-) close to it. Thus, the high catalytic activity of Ni2P might be derived from its hydrogenase-like catalytic mechanisms. The catalytic activities of cuboid Ni2P/C, spherical Ni2P/C and Ni/C catalysts toward hydrolysis of AB were first studied. As shown Fig. 3a, cuboid Ni2P/C exhibits superior activity with the turnover frequency (TOF) value of 13.5 min-1, which is 27 times higher 9
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than that of Ni/C (TOF = 0.5 min-1). However, spherical Ni2P showed inferior activity with induction period and incomplete hydrogen generation. It is noteworthy that, the catalytic activity of cuboid Ni2P/C is higher than those of most of the reported non noble metal-based catalysts and even some noble metal-based catalysts as indicated in Table S2. The durability of as-prepared cuboid Ni2P/C up to fifth run for hydrolysis of AB is shown in Fig. 3b. There is no obvious deactivation even after five cycles, indicating its superior durability. To obtain the activation energy (Ea) of hydrolysis of AB catalyzed by cuboid Ni2P/C, the reactions were set at different temperatures, varying from 298 K to 323 K. It can be seen from Fig. 3c that the achieving time of decomposing AB are 1.67 min, 2.5 min, 5 min and 7.5 min, with the TOF values of 67.8, 39.6, 20.8 and 13.5 min-1, respectively. Fig. 3d shows that the activation energy is calculated to be 43.4 kJ/mol, which is also close to the reported Ea for hydrolysis of AB as indicated in Table S2. The electrocatalytic activities of cuboid Ni2P/C, spherical Ni2P and Ni/C toward HER were evaluated under 0.5 M H2SO4 with a scan rate of 5 mV s-1 using a three-electrode setup with the working electrode prepared by the deposition of the catalyst on a glassy carbon electrode. Fig. 4a shows the linear sweep voltammetry (LSV) curves of cuboid Ni2P/C, spherical Ni2P/C, Ni/C and commercial Pt/C (20 wt%) vs the reversible hydrogen electrode (RHE) scale. It can be seen that Pt/C exhibits the highest catalytic activity with overpotential of 22 mV to afford a current density of 10 mA cm−2. However, the spherical Ni2P/C and Ni/C required 187 and 397 mV to reach 10 mA cm−2, respectively. As expected, Ni2P/C exhibits high activity with overpotential of 138 mV 10
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to achieve 10 mA cm−2, which is superior to most of the reported NiP-based catalysts, including Ni2P nanoparticles/graphene (η10mAcm-2 = 264 mV), 47 Ni2P nanoparticles/Ti (η10 mA cm-2 = 130 mV),41 Ni12P5/CNT (η10 mA cm-2 = 240 mV),48 monodispersed Ni12P5 nanoparticles/GCE (η10 mA cm-2 = 208 mV).49 Fig. 4b exhibits the Tafel plots of the catalysts. As expected, Pt/C presents the smallest Tafel slope of 30.2 mV dec-1. The nanocuboid Ni2P/C shows a Tafel slope of 75.5 mV dec-1, much smaller than those of spherical Ni2P (93 mV dec-1) and Ni/C (137 mV dec-1). Thus, the incorporation of phosphorus to nickel can greatly increase the HER electrocatalytic activity. The reason that Ni2P with different shape exhibited activity imparity may be exposed lattice plane. It has been reported that Ni2P (001) surface has higher HER activity41,50 and the cuboid Ni2P was exposed with (001) crystal plane. The stability is another critical criterion for judging an efficient HER electrocatalysts. The chronopotentiometric curve in Fig. 4c indicates the overpotential slightly increased after 70000 s, from an initial of 138 mV to 143 mV, at a constant current density of 10 mA cm-2. In addition, as shown in Fig. 4d, after long-term cycling, Ni2P/C exhibits a similar polarization curve as before. The morphology of cuboid Ni2P after cycling was characterized by TEM. As shown in Fig. S3, Ni2P maintained its morphology and dispersion without obvious change. These results indicate the superior stability of cuboid Ni2P/C toward HER. We also calculate the Faradaic efficiency of cuboid Ni2P by measuring the volume of generated hydrogen. As shown in Fig. S4, the amount of experimentally generated hydrogen is in accordance with theoretical value, indicating the 100% Faradaic efficiency.
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Conclusion In summary, we have reported a colloidal method to synthesize nanocuboid Ni2P with superior bifunctional catalytic performance toward hydrolysis of AB and electrocatalytic HER. When used as catalyst for hydrolysis of AB, the as-synthesized cuboid Ni2P/C exhibits superior catalytic activity in comparison to spherical Ni2P and Ni/C, with the TOF value of 13.5 min-1 and good durable stability. When used as electrocatalyst for HER, it exhibits HER activity with a low overpotential (138 mV), a small Tafel slope (75.5 mV dec-1), 100% Faraday efficiency and superior stability. This study may open new avenue in designing and utilizing TMPs for more applications.
Acknowledgements This article was financially supported by the National Natural Science Foundation of China (21571145), the Fundamental Research Funds for the Central Universities (410500194) and Large-scale Instrument and Equipment Sharing Foundation of Wuhan University.
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Fig. 1 (a) Powder XRD patterns of cuboid Ni2P, (b, c) TEM images of cuboid Ni2P and (d) EDX spectrum of cuboid Ni2P.
Fig. 2 XPS spectra of (a) Ni 2p3/2 and (b) P 2p for cuboid Ni2P. 17
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10.1002/asia.201701302
Chemistry - An Asian Journal
Fig. 3 (a) Hydrogen generation from the hydrolysis of aqueous NH3BH3 (1mmol) catalyzed by cuboid Ni2P/C, spherical Ni2P/C and Ni/C. (b) Cycle stability test of cuboid Ni2P/C for hydrogen generation from the hydrolysis of aqueous NH3BH3 (1mmol) under an ambient atmosphere at 298 K. (c) Time course plots for hydrogen generation from the decomposition of AB by cuboid Ni2P/C at 298 K, 303 K, 313 K and 323 K. (d) Plot of lnk versus 1/T during the AB decomposition over cuboid Ni2P at different temperatures. (catalyst/NH3BH3 = 0.037).
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This article is protected by copyright. All rights reserved.
10.1002/asia.201701302
Chemistry - An Asian Journal
Fig. 4 (a) LSV curves for cuboid Ni2P/C, sphere Ni2P/C, Ni/C and Pt/C with a scan rate of 5 mV/s. (b) Tafel plots for cuboid Ni2P, sphere Ni2P/C, Ni/C and Pt/C. (c) Time dependent potential curve of cubiod Ni2P at a fixed current density of 10 mA/cm2. (d) LSV curves for cuboid Ni2P before and after 1000 cycles. All experiments were done in 0.5 M H2SO4.
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This article is protected by copyright. All rights reserved.
10.1002/asia.201701302
Chemistry - An Asian Journal
TOC
Nanocuboid Ni2P has been synthesized through a facile colloidal method and further used as bifunctional catalyst for efficient hydrogen generation from hydrolysis of ammonia borane and electrocatalytic hydrogen evolution
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