Ni₁₂P₅ nanoparticles decorated on carbon nanotubes with enhanced electrocatalytic and lithium storage properties.

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DOI: 10.1039/C5NR05432J

Ni12P5 Nanoparticles Decorated on Carbon Nanotubes with Enhanced Electrocatalytic and Lithium Storage Properties

Hefei National Laboratory of Physical Sciences at the Microscale (HFNL), Department of Chemistry, Laboratory of Nanomaterials for Energy Conversion (LNEC) and Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China (USTC), Hefei, Anhui 230026, The People’s Republic of China *

E-mail: [email protected]; Fax: +86-551-63606266; Tel: +86-551-63600243

Abstract: Transition-metal phosphides (TMPs) have been proved to be of great potential in electrochemical energy conversion and Li-ion storage. In this work, we have designed a useful one-pot hot-solution colloidal synthetic route to synthesize a new kind of unique hybrid nanostructures (the Ni12P5/CNTs nanohybrids) by directly in situ growing Ni12P5 nanocrystals onto oxidized multi-wall carbon nanotubes (CNTs). The CNTs can improve the conductivity of the hybrids and effectively prevent the aggregation of Ni12P5 nanoparticles in the cycle process. When they are evaluated as a novel non-noble-metal hydrogen evolution reaction (HER) catalyst operating in acidic electrolytes, the Ni12P5/CNT nanohybrids exhibit an onset overpotential of as low as 52 mV, a Tafel slope of 56 mV dec-1 and only require overpotentials of 65 and 129 mV to attain current densities of 2 and 10 mA cm-2, respectively. Moreover, they also exhibit enhanced electrochemical performance for lithium-ion batteries serving as an anode material, the Ni12P5/CNT nanohybrids show a high capacity, excellent cycling stability and good rate performance. The unusual property arises from a synergetic effect between Ni12P5 and CNTs. Keywords: nickel phosphide, carbon nanotubes, nanocomposites, hydrogen evolution reaction, lithium ion battery 1. Introduction: Over the last few decades, quasi-zero-dimensional (0D) nanostructures have drawn much attention due to a wide variety of potential applications in biomedical, optical, catalytic and electronic fields.[1-6] The property of these 0D nanoparticles can be controlled by adjusting a set of physical parameters, including elemental composition, size, morphology and material structure,[7-11] and it will be also optimized by combining these parameters selectively. Up to now, there have been several methods typically including the ones of pyrolysis, hydrothermal, solvothermal and solution synthesis for creating various nanoparticles.[12-15] It is interestingly noted that transition-metal phosphides (TMPs), as a sort of important functional material, have incurred great interests in recent years.[16-22] However, the synthesis and the synthetic procedures are too tough and expensive to be performed at a relatively mild temperature as compared to those of transition-metal oxides and chalcogenides. For example, we have just obtained uniform nanostructured TMPs in evacuated and sealed quartz tube at 380-390 °C via reaction of metal or ferrocene with triphenylphosphine.[23,24] Schaak and co-workers have synthesized several kinds of metal phosphides hollow nanoparticles at a relatively lower


Published on 20 October 2015. Downloaded by Université Laval on 20/10/2015 14:35:53.

Chunde Wang, Tao Ding, Yuan Sun, Xiaoli Zhou, Yun Liu, Qing Yang*

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temperature (~ 320 °C) and investigated their interesting properties in electrochemical water splitting into hydrogen.[25,26] Stevenson’s group have found that amorphous FeP2 nanoparticles demonstrate remarkable performance toward lithium-ion batteries (LIBs).[27] Despite the great advances, however, the current energy materials are still far from meeting the requirements of combined high activity, strong durability and low cost, which severely limits their practical application in energy conversion and storage devices. Therefore, it is still a challenge and will be desirable to develop efficient energy materials with multifunctionalities. Recently, several kinds of carbon materials including carbon cloth,[28-30] carbon nanotubes (CNTs),[31-34] reduced graphene oxide (RGO)[35-37] in addition to amorphous carbon[38,39] have been used as active material supports for non-noble metals to improve the electrical conductivity of these materials and increase the dispersion of the active site. As compared to the other carbon materials, multiwalled CNTs may show some merits to support active inorganic nanocrystals since their inner walls remain intact to offer highly open electronic channel while the outer walls can be oxidized and functionalized to afford chemical reactivity for the hybrid or composites.[40,41] Therefore, it is highly interesting to design effective energy materials by synergistic coupling of non-noble functional materials, which is highly required for energy storage and conversion technologies. Herein, we present our recent efforts in developing a nanohybrid that consists of CNTs decorated with Ni12P5 nanocrystals (Ni12P5/CNT nanohybrids, shortened as Ni12P5/CNT) by in situ one-pot hot-solution methods at a relatively mild temperature for the first time. As expected, when evaluated as an electrocatalyst for hydrogen evolution reaction (HER) in acidic media, the Ni12P5/CNT nanohybrids are highly active with an onset overpotential (ƞ) of 52 mV, a Tafel slope of 56 mV dec-1, and good stability. An overpotential of 65 and 129 mV is needed to afford a catalytic current density of 2 and 10 mA cm-2, respectively. Moreover, when evaluated as an anode material for LIBs, the Ni12P5/CNT nanohybrids exhibit enhanced electrochemical performance relative to the corresponding pure-phase Ni12P5 nanoparticles. 2. Experimental sections 2.1. Chemicals Nickel(II) acetylacetonate hydrate (Ni(acac)2·xH2O, 95%) was purchased from TCI, and tri-n-octylphosphine (TOP, P(C8H16)3, tech. 90%) from Alfa-Aesar. Oleylamine (OAm, C18H37N, tech. 70%), 1-octadecene (ODE, C18H36, tech. 90%) and titanium foil (99.99%, 0.1 mm thickness) were purchased from Sigma-Aldrich. Carbon nanotube (CNT) was purchased from Beijing Dk Nano technology Co. LTD. Absolute ethyl alcohol, toluene and sulfuric acid of analytical grade were purchased from Shanghai Chemical Reagent Company. All reagents were used as received without further purification. 2.2. Synthesis of Materials 2.2.1 Synthesis of mildly oxidezed carbon nanotubes The pretreated multiwalled carbon nanotubes (CNTs) were applied as catalyst carbon support. The CNTs were first treated in the mixture of sulfuric acid and nitric acid (H2SO4-HNO3) at 80 °C by stirring for 2 h to purify and add some surface functional groups, then filtered, washed with deionized water till pH is around 7, and dried at 80 °C for 12 h in vacuum. 2.2.2 Synthesis of Ni12P5 nanoparticles In a typical procedure, 0.5 mmol (0.128 g) of Ni(acac)2 was dispersed in 3.0 mL of OAm, 2.0 mL of



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ODE and 1.0 mL of TOP. Next, all chemicals were added into a 100 mL three-neck flask at room temperature and magnetically stirred under argon flow. Then, the mixed solution was heated to 140 °C for 30 min to remove the moisture and dissolved oxygen. Subsequently, the equipment was heated to 270 °C at a rate of 10 °C min-1 and kept at this temperature for 1 hour. Finally, the solution was cooled down to room temperature naturally; the black precipitate was washed several times with a mixture of toluene and ethanol by centrifugation, and then dried at 50 °C under vacuum for further characterizations. 2.2.3 Synthesis of Ni12P5/CNT nanohybrids 0.25 mmol (0.064 g) of Ni(acac)2, 3.0 mL of OAm, 2.0 mL of ODE, 1.0 mL of TOP and CNT (10 mg) were placed in a three-neck flask (100 mL) and stirred magnetically under a flow of argon. The temperature was raised to 140 °C with a heating rate of 10 °C min-1 and kept at this temperature for 30 min to remove moisture and dissolved oxygen. Next, the mixture was rapidly heated to 270 °C and maintained for 60 min. After cooling to room temperature, the black precipitate was washed several times with a mixture of toluene and ethanol by centrifugation. Then the Ni12P5/CNT nanohybrids were obtained by drying in vacuum at 50 °C for further characterizations. 2.3. Characterization The products were characterized by X-ray diffractometer (XRD, Philips X’pert PRO X-ray diffractometer, Cu Kα, 1.54178 Å), scanning electronic microscope (SEM, JEOL-JSM-6700F), transmission electronic microscope (TEM, Hitachi H7650). The high resolution TEM (HRTEM), selected area electron diffraction (SAED) patterns, high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and the corresponding energy-dispersive X-ray spectroscope (EDX) mapping analyses were performed on a JEOL JEM-ARF200F TEM/STEM with a spherical aberration corrector. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VGESCA-LAB MKII X-ray photoelectron spectrometer using Mg as the exciting source. The weight percentage of carbon was characterized by elemental analysis (EA, Elemental vario E cube, Thermal Conductivity Detector) in pure oxygen atmosphere. 2.4. Electrochemical measurements The HER activity was performed with a CHI 660E electrochemical analyzer (CH Instruments, Inc., Shanghai) in a standard three-electrode system, using Ni12P5/CNT decorated Ti foils as the working electrode, a Pt foil electrode was used as counter electrode, an Ag/AgCl (in saturated KCl solution) electrode was used as reference electrode. In all measurements, the Ag/AgCl reference electrode was calibrated with respect to RHE. In 0.5 M H2SO4, E (RHE) = E (Ag/AgCl) + 0.059 pH + 0.197. Before the HER electrochemical measurement, the electrolyte (0.5 M H2SO4) was degassed by bubbling argon for 30 min. All linear sweep voltammetry curves were recorded at a sweep rate of 5 mV s-1, and all cyclic voltammograms (CV) curves were recorded at a sweep rate of 100 mV s-1. The preparation method of the working electrodes can be found as follows. In short, 5 mg of catalyst powder was dispersed in 1 mL of hexanes to generate a homogeneous solution. Next, 30 μL of the catalyst ink was transferred onto the 0.2 cm2 pieces of Ti foil (loading ~0.75 mg cm-2). Following drying under ambient conditions, finally, the Ni12P5/CNT-decorated Ti foils were annealed at 450 °C for 30 min under 5% H2/N2. The battery tests were measured with coin-type half cells which were assembled under an argon-filled glovebox (H2O, O2 < 1 ppm). A working electrode was composed of active material, super P carbon black, and sodium carboxymethylcellulose (Na-CMC) at a weight ratio of 80:10:10.



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3. Result and discussion: The monodisperse Ni12P5 nanoparticles and the Ni12P5/CNT nanohybrids on large scale were obtained by a facile one-pot hot-solution colloidal synthetic route as seen in Scheme 1. When the CNTs were removed from source materials, the monodisperse Ni12P5 nanoparticles were achieved instead of the Ni12P5/CNT nanohybrids, as observed in the transmission electron microscopy (TEM) images (Figure 1a, b). The images clearly demonstrate that the Ni12P5 nanoparticles with a uniform size were successfully synthesized in high yield. The composition, size and structure of the nanoparticles were further determined, and the nanoparticles are about 9 nm in diameter with tetragonal structure (JCPDS No. 74-1381) on the basis of energy-dispersive X-ray spectroscopy (EDX) spectrum (Figure S1), particle size distribution histogram (Figure S2), typical X-ray diffraction (XRD) pattern, typical high-resolution TEM (HRTEM) and high-scanning TEM energy dispersive X-ray spectroscopic (STEM-EDX) elemental mappings (Figure S3). Figure 1c, d is the scanning electron microscopy (SEM) and TEM images for the shape and structure of the Ni12P5/CNT nanohybrids obtained with the addition of CNTs to the source materials, and the

Scheme 1. Illustration of synthetic process for the monodisperse Ni12P5 nanoparticles (a) and the Ni12P5/CNT nanohybrids (b). images clearly showed the formation of the nanoparticles with average size of ～9 nm on the CNTs, without any singular nanoparticles detached from the CNTs. The corresponding EDX spectrum (Figure S4) and the selected area electron diffraction (SAED) pattern (Figure S5)



The active mass loading on the electrode is about 1.0 mg cm-2. A metallic Li sheet was used as counter electrode, and 1 M LiPF6 in a mixture of ethylene carbonate/dimethylcarbonate (EC/DMC; 1:1 by volume) and 5 wt % fluoroethylene carbonate (FEC) were used as the electrolyte. The CV curves were obtained on a CHI 660E electrochemical workstation. The AC impedance spectra were recorded with a CHI 660E electrochemical station by applying an AC voltage of 5 mV in amplitude in the frequency range of 0.01 Hz to 100 kHz at room temperature. Galvanostatic measurements were made using a LAND-CT2001A instrument.

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Figure 1. (a) Low-, and (b) high-magnified TEM image for the as-prepared monodisperse Ni12P5 nanoparticles in large area, (c) low-magnification SEM, (d) TEM images, and (e) HRTEM images of the Ni12P5/CNT nanohybrids (Ni12P5/CNT), and (f) STEM image and STEM-EDX elemental mapping images of Ni, P and C for a typical individual Ni12P5/CNT. The Ni12P5/CNT nanohybrids were further characterized by the power XRD, and the patterns are in the Figure 2. The CNTs (black curve) show two peaks at 26.23° and 44.37°, which were indexed to the (002) and (101) reflections of hexagonal graphite, respectively.[42] In contrast, the Ni12P5/CNT nanohybrids (red curve) show several additional peaks at 32.73°, 35.81°, 38.41°, 41.76°, 44.42°, 46.96°, 48.96°, 56.16°, 68.59° and 79.61°, which are indexed to the (310), (301), (112), (400), (330), (240), (312), (501), (620), and (262) planes of tetragonal Ni12P5, respectively.[43] In order to investigate the chemical composition of the products, X-ray photoelectron spectra (XPS) was carried out, and the spectra demonstrate that the Ni12P5/CNT nanohybrids contain Ni, P, C and O. As detailed shown in Figure S6a, the peaks at 284.7 eV can be attributed to a sp2 hybridized graphite-like carbon atom.[44] For the Ni 2p3/2 energy level, three peaks locating at 853.7, 856.7 and 862.2 eV (Figure S6b), which can correspond to Ni in



indicate that the composite consists of CNTs and Ni12P5 nanoparticles. HRTEM image in Figure 1f clearly reveals that lattice fringes spaces of 0.203 and 0.216 nm are consisted with the (330) and (-231) plane of Ni12P5, respectively. The TEM and HRTEM images also reveal that the inner walls of the CNTs were not destroyed by the oxidation and reactions during the hybrid synthesis. STEM-EDX elemental mapping was employed to obtained elemental distribution of Ni, P and C in the hybrids (Fig. 1f), verifying the uniform dispersion of Ni and P elements on the outer walls of the CNTs.

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Ni12P5, oxidized Ni species and the satellite of the Ni 2p1/2 peak, respectively.[45] A high-resolution image of the P 2p region (Figure S6c) shows two peaks at 130.7 and 129.8 eV, reflecting the binding energy (BE) of P 2p1/2 and P 2p3/2, respectively, and the peak at 134.6 eV is assigned to oxidized phosphorus species, which arise from superficial oxidation of Ni12P5/CNT because of air contact.[46,47] An XPS survey spectrum (Figure S6d) also confirms the presence of oxygen in the sample. The Ni 2p3/2 BE of 853.7 eV is higher than that of metallic Ni0 (852.3 eV), while the BE of P 2p (129.8 eV) is lower than that of elemental P (130.2 eV),[48] which can be concluded that there is an electron transfer from Ni to P. In addition, the composition of carbon in the hybrids was detected to be 32.08 wt% by elemental analysis.

Figure 2. XRD patterns for both CNTs (down) and Ni12P5/CNT nanohybrids (top). Via careful investigation, it was found that the key step is to select suitable media for the growth of the monodisperse Ni12P5 nanoparticles in addition to the corresponding Ni12P5/CNT nanohybrids. In the current work, both OAm and ODE were used as reaction media, and research indicated that OAm employed is critical to the shape and uniformity of the products, as displayed in Figure 3. When ODE (5 mL) is used as solvent, the product is only hexagonal Ni by XRD characterization (Figure 3a), in good agreement with the standard JCPDS card No. 45-1027, and these Ni nanoparticles aggregate to form microscale spheres as seen in SEM and TEM images (Figure 3b, c). When 1 mL among total 5 mL of ODE is replaced by OAm, Ni12P5 nanoparticles other than Ni can be obtained on the basis of XRD detection (Figure 3d), and the Ni12P5 nanoparticles are partly merged into large irregular spheres (Figure 3e, f). It is interesting that the Ni12P5 nanocrystals demonstrated better monodispersity when the amount of OAm was increased in the reaction system (Figure 1b), as compared to those without OAm. The above investigations suggest that OAm plays an important role in the formation of tetragonal Ni12P5 nanoparticles in addition to preventing the aggregation of the nanocrystals during the growth, and at the same time, OAm can promote the reaction activity of Ni with phosphorus source to form Ni12P5 in the synthetic procedures. The optimal synthetic condition is also available for the fabrication of the Ni12P5/CNT nanohybrids.



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Figure 3. XRD patterns, and the corresponding SEM, TEM images of the samples synthesized at 270 °C for different reaction solvent: (a-c) in ODE (5.0 mL), and (d-f) in OAm (1.0 mL) with ODE (4.0 mL). TMPs have received great attention as earth-abundant active catalysts for water splitting,[49-52] and recently investigations demonstrate that the synergistic interaction between compounds and carbon materials can help to weak the adsorption energies of some catalytic species, decreasing the overpotential of the electro-catalysis reaction.[38,53] To this point, the Ni12P5/CNT nanohybrids fabricated in the current work would be served as an ideal catalyst for the study of HER. Typically, the HER catalysis of Ni12P5/CNT was evaluated in argon-saturated 0.5 M H2SO4 solution with a scan rate of 5 mV s-1 using a three-electrode setup. For comparison, bare Ti plate, Ni12P5 and commercial Pt/C (20 wt %) were also examined. Figure 4a displays the polarization curves, all potentials were reported with respect to reversible hydrogen electrode (RHE). As expected, Pt/C shows excellent activity while bare Ti plate has almost no HER activity. It is surprising that the Ni12P5 nanoparticles show high catalytic activity with a very low onset overpotential of 108 mV for HER, suggesting that the Ni12P5 nanoparticles are a very active HER catalyst. The Ni12P5/CNT hybrids exhibited much smaller onset overpotential of 52 mV and achieved current densities of 2 and 10 mA cm-2 at overpotentials of 65 and 129 mV, respectively. These overpotentials compare favorably to the behavior of other non-noble-metal HER catalysts in 0.5 M H2SO4 like defect-rich MoS2,[54] Mo2C nanoparticles deposited on carbon nanotube,[55] CoP/CNT,[56] and CoSe2 nanoparticles grown on carbon fiber paper,[57] and etc (Table S1), implying the superior HER activity of the Ni12P5/CNT nanohybrids. To obtain further insight into the catalytic activity of the Ni12P5/CNT nanohybrids, the linear portions of the Tafel plots were fitted to the Tafel equation (η = blog j + a, where j is the current density and b is the Tafel slope), yielding Tafel slopes of ~30, ~56, and ~80 mV decade-1 for Pt/C, Ni12P5/CNT nanohybrids, and Ni12P5 nanoparticles, respectively (Figure 4b). The small Tafel slope of the Ni12P5/CNT nanohybrids is advantageous for practical applications since it will lead to a faster increment of HER rate with increasing overpotential. By applying the extrapolation method to the Tafel plots, exchange current density values of various samples were also obtained (Figure S7). The Ni12P5/CNT nanohybrids display the larger exchange current density of 0.071 mA cm−2



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Figure 4. (a) Polarization curves of Ni12P5/CNT, Ni12P5, Pt/C, and bare Ti foil electrodes, (b) corresponding Tafel plots, (c) stablity of the Ni12P5/CNT-modified electrode with an initial polarization curve and after 1000 cycles in 0.5 M H2SO4, and (d) cartoon of Ni12P5/CNT nanohybrid for hydrogen evolution reaction catalysis. Another important criterion for a good electrocatalyst is high durability. To assess this, continuous cyclic voltammetry (CV) between -0.2 and 0.2 V vs. RHE at a scan rate of 100 mV s-1 was conducted. As shown in Figure 4c, the polarization curve remains largely unchanged compared with the initial one. The long-term stability of Ni12P5/CNT was also examined under a static overpotential of 160 mV (Figure S8), after a long period (10 h), the current density only shows slight degradation. It is worth noting that TEM (Figure S9a) analysis of Ni12P5/CNT after 1000 cycling shows a well-retained intact original morphology. Further XRD (Figure S9b) and XPS (Figure S9c, d) analysis indicates there is no obviously change in the crystalline structure and chemical composition for the Ni12P5/CNT. These observations clearly confirm the excellent stability of Ni12P5/CNT in acidic media. In addition, the stability of the Ni12P5 catalyst in HER was also tested (Figure S10), the current density keeps almost constant without detective degradation. The superior catalytic performance and stablity of the Ni12P5/CNT cathode could be attributed to the synergetic chemical coupling effects between Ni12P5 and carbon nanotubes, high conductive and enlarge surfaced area, and the HER catalysis of the Ni12P5/CNT is schematically shown in Figure 4d. The anodic performance of the Ni12P5/CNT nanohybrids was tested in a Li cell. Figure 5 shows CV plots of the Ni12P5/CNT electrode performed over the potential range of 0.01–3.0 V (versus Li/Li+) at a scanning rate of 0.2 mV s−1. In the first cycle, the strong cathodic peak at 1.40 V can be attributed to the Li+ insertion, and the peaks located at about 0.65 V correspond to the diffusion and conversion processes, namely the full decomposition of Ni12P5 into metallic Ni, and the



than Ni12P5 nanoparticles (0.025 mA cm−2). These results indicate that the catalytic activity of the Ni12P5/CNT nanohybrids is better than that of the Ni12P5 nanoparticles.

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Figure 5. Cyclic voltammograms of the Ni12P5/CNT nanohybrids at a scanning rate of 0.2 mV s-1 in the potential range 0.01–3 V (vs. Li/Li+). Figure 6a presents the representative diacharge-charge voltage profiles of the Ni12P5/CNT nanohybrids at a current density of 100 mA g-1. The initial discharge and charge specific capacities of Ni12P5/CNT electrode can reach 1170.6 and 711.3 mA h g-1, respectively, indicating irreversible losses of about 39.2%, which may be ascribed to the irreversible formation of SEI and lithium ion insertion into carbon during the first cycle.[62,63] The discharge capacity of 732.4 mA h g-1 is obtained in the second cycle, and the corresponding charge capacity of 671.6 mA h g-1, giving rise to a high coulombic efficiency (CE) of about 92%. Remarkably, the discharge capacity remains at about 681.3 mA h g-1 after 50 cycles at a current rate of 100 mA g-1 and its CE maintains consistently at 95 %. Figure 6b illustrates the cycling performance of Ni12P5/CNT, Ni12P5 and CNT with a voltage window from 0.01 V to 3 V at a current density of 100 mA g-1, the discharge capacity remains at about 665, 365 and 296 mA h g-1 after 100 cycles, respectively. Moreover, the Ni12P5/CNT nanohybrids can be cycled with high stability at higher current density of 2 A g-1 (Figure S11). After 100 cycles , the SEM and TEM images of Ni12P5/CNT nanohybrids are shown in Figure S12a and Figure S12b. From the SEM and TEM images, no significant aggregation is observed. We can conclude that these composites still remain the same structure.



formation of amorphous Li3P and the solid electrolyte interphase (SEI) layers.[58,59] During the charge process, the anodic peak located between 1.0 and 1.3 V are related to the decomposition of the SEI and Li3P,[60,61] and to the reversible reaction at about 2.4 V. After the initial cycle, the former two cathodic peaks, which shifted to near 1.7 V could still be observed, indicating the existence again of the insertion process. However, the oxidation peaks had no shift and weak attenuation, suggesting good reversibility of the elecrode.

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Figure 6. Electrochemical lithium-storage properties of annealed Ni12P5/CNT nanohybrids: (a) discharge–charge voltage profiles for the 1st, 2nd, and 50th cycles at a current density of 100 mA g-1, (b) cycling performance at a current density of 100 mA g-1 between 0.01 and 3 V, and (c) rate capability at various current densities between 0.01 and 3 V and the corresponding Coulombic efficiency. Besides the high capacity and good cycling stability, the rate performance (Figure 6c) is also an important characteristic for high performance LIBs. As can be seen, the reversible capacity of Ni12P5/CNT decreases from 646 to 584, 490, 445, 378 to 325 mA h g-1 when the current density increases from 100 to 200, 500, 1000, 2000, and 3000 mA g-1, respectively. After the current density is reduced back to 100 mA g-1, the discharge capacity of the Ni12P5/CNT electrode recovers to as high as 668 mA h g-1, and normally attributed to the presence of a possible activation process in the electrode, indicating the good kinetic properties of Ni12P5/CNT. As a comparison, the rate performance of the randomly Ni12P5 nanoparticles were also tested, as shown in Figure S13, which we can see the Ni12P5/CNT nanohybrids are superior to bare Ni12P5 nanoparticles. Figure S14 shows the comparision of the electrochemical impedance spectra (EIS) of Ni12P5/CNT nanohybrids and Ni12P5 nanoparticles, which shows that the Ni12P5/CNT nanohybrids exhibit a smaller diameter of the high-frequency semicircle than bare Ni12P5,



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4. Conclusion In summary, we have developed a simple one-pot hot-solution strategy to synthesize the unique Ni12P5/CNT nanohybrids by in situ growth of Ni12P5 nanocrystals onto mildly oxidized CNTs at a relatively modest temperature. Within the Ni12P5/CNT nanohybrids, the mildly oxidized CNTs provided functional groups on the outer walls to nucleate and anchor nanocrystals, while retaining intact inner walls for highly conducting network, which is advantageous in both charge transfer and active catalytic sites. All these merits leading to high cathodic current and low Tafel slope for water splitting, and high capacity, long cycle life, superior rate performance for LIBs. The enhanced electrocatalytic and lithium storage properties of these well-defined Ni12P5/CNT nanohybrids would be attributed to synergetic effect between Ni12P5 and CNTs. This nanostructure design provides new candidates as low-cost and effective materials for energy storage and conversion. Acknowledgments This work was supported by National Nature Science Foundation of China (51271173, 21571166,21071136) and National Basic Research Program of China (2012CB922001). Reference: [1] R. Hao, R. J. Xing, Z. C. Xu, Y. L. Hou, S. Gao, S. H. Sun, Adv. Mater. 2010, 22, 2729–2742. [2] A. P. Alivisatos, Science 1996, 271, 933–937. [3] Y. Liu, C. Wang, Y. J. Wei, L. Y. Zhu, D. G. Li, J. S. Jiang, N. M. Markovic, V. R. Stamenkovic, S. H. Sun, Nano Lett. 2011, 11, 1614–1617. [4] B. Ritz, H. Heller, A. Myalitsin, A. Kornowski, F. J. Martin-Martinez, S. Melchor, J. A. Dobado, B. H. Juarez, H. Weller, C. Klinke, ACS Nano 2010, 4, 2438–2444. [5] K. A. Ritter, J. W. Lyding, Nat. Mater. 2009, 8, 235–242. [6] J. Deng, P. J. Ren, D. H. Deng, X. H. Bao, Angew. Chem. Int. Ed. 2015, 54, 2100–2104. [7] X. H. Xia, Y. Wang, A. Ruditskiy, Y. N. Xia, Adv. Mater. 2013, 25, 6313–6333. [8] C. J. Murphy, T. K. Sau, A. M. Gole, C. J. Orendorff, J. Gao, L. Gou, S. E. Hunyadi, T. Li, J. Phys. Chem. B 2005 , 109 , 13857–13870. [9] J. Park, J. Joo, S. G. Kwon, Y. Jang, T. Hyeon, Angew. Chem. Int. Ed. 2007, 46, 4630–4660. [10] Y. N. Xia, Y. J. Xiong, B. Lim, S. E. Skrabalak, Angew. Chem. Int. Ed. 2009, 48, 60–103. [11] W. S. Wang, M. Dahl, Y. D. Yin, Chem. Mater. 2013, 25, 1179–1189. [12] Z. J. Wang, H. M. Zhang, L. G. Zhang, J. S. Yuan, S. G. Yan, C. Y. Wang, Nanotechnology 2003, 14, 11–15. [13] H. Imagawa, A. Suda, K. Yamamura, S. H. Sun, J. Phys. Chem. C 2011, 115, 1740–1745. [14] Y. S. Yu, W. W. Yang, X. L. Sun, W. L. Zhu, X. Z. Li, D. J. Sellmyer, S. H. Sun, Nano Lett. 2014, 14,



indicating smaller charge-transfer resistance. In addition, we also investigate the electrical behaviors of the Ni12P5/CNT composites, CNTs and pure Ni12P5 nanoparticles. As shown in Figure S15, it shows the typical I-V curves of these samples, respectively. According to the R = ρL/A, the electrical resistivity of the Ni12P5/CNT composites at room temperature is about 25 Ω·cm and pure CNTs is about 12 Ω·cm, while pure Ni12P5 nanoparticles is about 162 Ω·cm. The result confirms that the use of CNTs improves the conductive property of Ni12P5, which further verified the synergic effect between Ni12P5 and CNTs.

Nanoscale


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