Onsite Substitution Synthesis of Ultrathin Ni Nanofilms Loading

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Onsite Substitution Synthesis of Ultrathin Ni Nanofilms Loading Ultrafine Pt Nanoparticles for Hydrogen Evolution Mingshu Xiao, Rui Cheng, Meifeng Hao, Mao Zhou, and Yuqing Miao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07472 • Publication Date (Web): 09 Nov 2015 Downloaded from http://pubs.acs.org on November 15, 2015

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ACS Applied Materials & Interfaces

Abstract graphic

j (mA/cm2)

0

-50 Pt

-100

-150 -0.6

Ni

GCE GCE-Ni GCE-Ni/Pt

-0.4

-0.2

Potential (V vs RHE)

ACS Paragon Plus Environment

0.0


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Onsite Substitution Synthesis of Ultrathin Ni Nanofilms Loading Ultrafine Pt Nanoparticles for Hydrogen Evolution Mingshu Xiao, Rui Cheng, Meifeng Hao, Mao Zhou, Yuqing Miao* University of Shanghai for Science and Technology, Shanghai 200093, P. R. China

Abstract: Here, the ultrathin Ni nanofilms loading ultrafine Pt nanoparticles (Ni/Pt nanocomposites) were synthesized by a simple substitution method for the electrocatalysis of hydrogen evolution reaction (HER). Firstly, the ultrathin Ni nanofilms were prepared by using NaBH4 to reduce Ni salt. Then the ultrafine Pt nanoparticles attached on the surface of the ultrathin Ni nanofilms through the onsite substitution

reaction

between

PtCl62-

and

Ni

element.

X-ray photoelectron spectroscopy (XPS) experiment confirmed that Ni in Ni/Pt nanocomposites exists in the form of Ni(OH)2. Transmission electro microscope (TEM) study showed that the ultrafine Pt nanoparticles were sufficiently dispersed and loaded at Ni ultrathin nanofilms. The obtained Ni/Pt nanocomposites exhibited superior activity of HER and good stability in acidic media. It obtained 10 and 100 mA/cm2 with overpotential of only 36 and 115 mV, respectively. The stability experiment of 20000 s gave nearly negligible current decrease. On the one hand, the ultrathin Ni nanofilms help to disperse and form the ultrafine Pt nanoparticles. On the other hand, the ultrathin Ni nanofilms help to load the ultrafine Pt nanoparticles as catalyst support and immobilize both of them onto the electrode surface due to the high surface free energy of ultrathin nanofilm and the leading high adsorption ability. In addition, Ni itself exhibited somewhat electrocatalytic activity of HER, which contributed to the whole HER

electrocatalysis of Ni/Pt nanocomposites．The

excellent electrocatalysis may lead to the decreased consumption of expensive Pt and open up new opportunities for applications in hydrogen energy. Keywords:

Ni/Pt; ultrathin nanofilm; ultrafine nanoparticle; hydrogen evolution

reaction; electrocatalysts

*Corresponding author. Email addresses: [email protected] (Y. Miao). 1


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1. Introduction The rapidly increasing consumption of fossil fuels and the resulted environmental issues have triggered an urgent demand for seeking abundant and clean energy sources.1-5 Hydrogen, a zero-emission and renewable energy carrier, has been proposed as a promising candidate to replace fossil fuels in the future.6-11 Of all various techniques to generate hydrogen, electrochemical splitting water provides a simple and promising way to produce highly pure hydrogen.12, 13 But the key of hydrogen production requires the high efficient electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with high current density at low overpotential.14-17 Platinum, compared with other metals, exhibits the superior catalytic activity with lower overpotential.18-20 Therefore, it is still important choice for the electrocatalysts of HER. However, it is also expensive due to the limited global reserves. As a result, it is necessary to lower the amount of Pt usage in the industrial HER by increasing its electrolytic efficiency.21-23 Various nanomaterials of carbon, such as carbon nanospheres,24 carbon nanotube,25 and mesoporous carbon,26 have been applied as catalyst supports to disperse and load more catalysts as possible, which helps to increase the efficiency of catalysts and thus decrease the usage of expensive catalysts. In order to reduce the usage of expensive Pt, it’s acceptable to enhance the electrocatalytic activity of Pt-based catalysts. Usually, the ultrafine Pt nanoparticles mean better catalytic activity due to their high specific surface area and active free energy. Also, these catalysts should be dispersed adequately with the help of catalyst supports to reduce the contact hindrance between catalysts and target molecules. Consequently, it’s reported that these highly active Pt nanocatalysts were supported by non-noble Ni substrates to develop the composite electrodes for HER catalysis.27, 28 Moffat et al. reported the self-terminating electrodeposition of ultrathin Pt overlayers on electrodeposited Ni films on Au for H2 production.27 Musiani et al. developed the spontaneous deposition of Pt on electrodeposited porous Ni layers and commercial Ni foams to produce Pt-modified 3D Ni electrodes for HER.20 Jerkiewicz et al. deposited platinum on nickel foams via chemical reduction of Pt cations and studied the Pt/Ni foams as electrode materials for hydrogen evolution, hydrogen reduction, oxygen reduction, and oxygen evolution reactions in an aqueous alkaline electrolyte.28 Herein, the ultrathin Ni nanofilms loading ultrafine Pt nanoparticles (Ni/Pt nanocomposites) were prepared as electrocatalysts for HER using a simple onsite substitution method. It’s expected that the Ni ultrathin nanofilms work as catalyst support to enhance the electrocatalytic activity of Ni/Pt nanocomposites.

2. Experiments 2.1 Reagents Nickel sulfate heptahydrate (NiSO47H2O), polyvinyl pyrrolidone (PVP, MW 10000) and sodium tartaric were purchased from Aladdin Reagent Database Inc. (Shanghai, China). Potassium platinic chloride (K2PtCl6), sodium borohydride (NaBH4) and sulfuric acid were from Sinopharm Chemical Reagent Co. Ltd, Nafion (5 wt%) from Sigma-Aldrich (China) and Pt/C (5 wt% Pt) from J&K Scientific Ltd. (Beijing, China). All chemicals were of analytical grade and used without further purification. The ultra-purified water purified by a Milli-Q system (Millipore, Milford, 2



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MA) was used in all experiments. All experiments were performed at ambient temperature. 2.2 Preparation of Ni/Pt nanocomposite catalysts by onsite substitution reaction In a typical procedure, the catalysts of Ni/Pt nanocomposites were prepared as followed. 18.5 mL ultra-purified water, 0.4 mL 0.1 M NiSO4, 0.4 mL 0.1 M sodium tartaric and 0.7 mL 5 mM PVP were subsequently mixed under violently stirring, where sodium tartaric works as complex agent and PVP as protectant agent. After half an hour, 0.5 mL 1% NaBH4 was added into the above mentioned solution drop by drop. As nickel ion was reduced to be elemental state by NaBH4, the black colloid solution was obtained immediately and the reaction was allowed to last 5 minutes. The pH values before and after NaBH4 added were monitored to be pH 5.5 and 8.5 respectively. Then the colloid solution was instantly centrifuged and washed with ultra-purified water and ethanol respectively for several times to remove the unreacted chemicals. The yielded Ni precipitates were dispersed with 10 mL ultra-purified water, and then 0.5 mL 10 mM K2PtCl6 was added drop by drop under stirring with a final pH value about 7. The onsite substitution reaction was allowed to last 2 h. After centrifuged and washed several times using ultra-purified water and ethanol to remove the unreacted chemicals, the obtained precipitates were dried at 40° C for 8 h and the final product was denoted as Ni/Pt nanocomposites. 2.3 Instruments and methods The morphology of the Ni/Pt nanocomposites was examined at 200 KV by a JEOL JEM-2010F transmission electron microscope (TEM). X-ray photoelectron spectroscopy (XPS) experiments were carried out on an RBD up graded PHI-5000C ESCA system (PerkinElmer) with Mg Kα radiation (hγ 1256.3 eV) or Al Kα radiation (hγ 1486.6 eV). All electrochemical measurements were performed using a CHI 660D electrochemical analyzer (CH Instruments, Shanghai, China) with a conventional three-electrode setup with a glassy carbon electrode (GCE) with the inner diameter of 3 mm as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a platinum wire as the counter electrode. The bare GCEs were firstly polished to be a mirror-like surface on polishing cloth using alumina of 30 nm diameter, then rinsed thoroughly with ultra-purified water, and sonicated sequentially in ethanol, 1:1 HNO3 solution and ultra-purified water, each for 3 min. In order to immobilize the Ni/Pt nanocomposites on electrode surface, they are firstly dispersed in 2 mL ultra-purified water and then 40 µL was taken to drop onto the GCE. A centrifuge tube was covered on the GCE to make the Ni/Pt nanocomposites colloid solution dry slowly and even at room temperature. The fabricated electrode was referred as GCE-Ni/Pt. As a comparison, the yielded Ni precipitates without Pt were modified onto the GCE surface and denoted as GCE-Ni. The commercial Pt/C catalysts were ultrasonically dispersed in distilled water containing 0.1 wt% of Nafion and was then transferred onto the GCE surface to obtain the GCE-Pt/C. Unless stated otherwise, the electrochemical studies were performed in 0.5 M H2SO4 at room temperature. For the HER test, the cyclic 3


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voltammograms (CVs) were performed with a scan rate of 100 mV s−1 and the polarization curves using linear sweep voltammograms (LSVs) with a scan rate of 2 mV s−1. The current density (mA/cm2) was referred to geometric surface area of electrode. The potentials reported in the CVs and the polarization curves were against the reversible hydrogen electrode (RHE).29-31 For conversion of the obtained potentail vs. SCE to RHE, the following equation was used: ERHE = ESCE + 0.059pH + E0SCE, E0SCE = 0.25 V. Durability test for time-dependent current curve was also carried out in the −0.3 V vs. SCE. The Tafel plots are fit into the Tafel equation (η= b log(j) + a, where b is the Tafel slope). The electrochemical impedance spectroscopy (EIS) were carried out in 0.1 M KCl containing 5 mM Fe(CN)63-/4- at open circuit potential from 0.01 to 106 Hz with an AC voltage of 5 mV.

3. Results and discussion 3.1. TEM and XPS Characterizations of Ni/Pt nanocomposites The morphologies of the yielded Ni/Pt nanocomposites were determined by TEM. As shown in Fig. 1A and B, the Pt nanoparticles (black) are well dispersed on the matrix of Ni nanofilms (grey). The further magnified TEM images in the Fig.1C and D show that the Pt nanoparticles are extremly small with a size smaller than about 5 nm and the Ni nanofilm ultrathin with a thickness lower than about 2 nm. It’s observed that the ultrafine Pt nanoparticles were well dispersed and loaded on the surface of ultrathin Ni nanofilms. High resolution TEM (HRTEM) image (Fig.1E) and selected area electron diffraction (SAED) pattern (Fig.1F) of Ni region in Ni/Pt nanocomposite show non-crystalline or amorphous nanostructure. Fig.1G showed the crystalline structure of Pt with the interplanar distance of 0.227 nm between the adjacent lattice fringes, corresponding to the interplanar distance of the {111} plane of Pt metal.32 The SAED pattern obtained from Pt region is shown in Fig. 1H. The observed three rings with intense spots are assigned to the {111}, {200}, {220} and {311} diffraction planes of Pt face-centered cubic (fcc) structure.33, 34 In order to analyze the valence state of elements, the XPS of Ni/Pt nanocomposites was examined. The overall XPS spectrum of the surveyed sample in Fig.2A shows the typical signals of Ni, Pt and C elements where the latter is mainly caused by the adsorbed carbon on the sample surface due to exposure to air. In Fig. 2B, the Pt 4f spectrum exhibits two peaks at 71.8 and 74.5 eV which are assigned to Pt 4f7/2 and Pt 4f5/2, respectively, suggesting the presence of elementary substance Pt.35, 36 Fig 2C exhibits four peaks. Two main peaks at 856.1 and 873.4 eV with significantly split spin-energy of 17.3 eV are ascribed to Ni 2p3/2 and Ni 2p1/2 respectively. Two satellite peaks are also observed at 861.9 for Ni 2p3/2 and 879.4 for Ni 2p1/2. These XPS results are characteristic of Ni(OH)2, consistent with the literatures reported before.37-40 For comparison with the previous reports, Ni and NiO typically exhibit the binding energy of Ni 2p3/2 with a main peaks at 852.6 and 853.7 eV,41, 42 respectively. The synthesis mechanism of Ni/Pt nanocomposites by the onsite substitution reaction could be described as followed:20 2Ni + PtCl62− + 4H2O → Pt + 2Ni(OH)2 + 6Cl− + 4H+ 3.2. Electrochemical study on the HER of Ni/Pt nanocomposites The electrocatalytic HER of bare GCE, GCE-Ni, GCE-Ni/Pt and GCE-Pt/C was investigated in 0.5 M H2SO4 using CVs with a scan rate of 100 mV/s. As shown in 4



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Fig. 3A, the bare GCE doesn’t exhibit obvious HER activity within the applied potential range. The GCE-Ni exhibits an onset potential of HER at about −0.05 V and a current density of about 20 mA/cm2 for the overpotential of 0.15 V. The GCE-Ni/Pt exhibits better HER electrocatalysis with an onset potential at about −0.01 V and a current density of about 150 mA/cm2 for the overpotential of 0.15 V. The HER performance of Ni/Pt nanocomposites approachs that of a commercially available Pt/C electrocatalyst with an onset potential at about 0 V and a current density of about 130 mA/cm2 for the overpotential of 0.15 V. In order to further study the HER properties of the electrocatalysts, the polarization curves of the bare GCE, GCE-Ni, GCE-Ni/Pt and GCE-Pt/C were measured in a typical three-electrode system with 0.5 M H2SO4 electrolyte at a scan rate of 2 mV/s, which is shown in Fig. 3(B). Similarly, the bare GCE don’t show any HER activity. The GCE-Ni shows electrocatalytic activity toward the HER with an onset potential about −0.05V. With the overpotential 0.15 V, the current density of 21 mA/cm2 is obtained for GCE-Ni. Observed from the inset of Fig. 3B, the GCE-Ni/Pt presents higher HER activity with a near zero overpotential as expected. The GCE-Ni/Pt shows the current density of 10, 100 and 150 mA/cm2 for the overpotential of 0.036, 0.115 and 0.147 V respectively. For the commercial Pt/C catalysts, the current densities of 10 and 100 mA/cm2 agree with the overpotential of 0.036 and 0.115 respectively, while the HER current is only 121 mA/cm2 for the overpotential of 0.147 V. The Tafel slope, which is an inherent property of electrocatalysts, reveals the HER proceeds by a Volmer-Heyrovsky mechanism.43, 44 Furthermore, Tafel plots were fitted in to the Tafel equation within the linear regions, and a small Tafel slope value suggested a strong HER rate.45, 46 Seen in Fig. 3C, the Tafel slopes are 57, 43 and 31 mV/dec for GCE-Ni, GCE-Ni/Pt and GCE-Pt/C respectively, indicating the good electrocatalytic performance for HER of GCE-Ni/Pt. The ultrafine Pt NPs dispersed and loaded on the ultrathin Ni nanofilms may contribute to superior catalytic activity of GCE-Ni/Pt. 3.3. The stability of Ni/Pt toward HER electrocatalysis Stability is an important part to evaluate catalysts for practical application. As shown in Fig. 4A, the polarization curve shows negligible loss of cathodic current density compared with the initial one after continuous CV scanning for 5000 cycles in 0.5 M H2SO4 between 0.1 and −0.1 V at a scan rate of 100 mV/s, which suggests good stability of GCE-Ni/Pt in the acidic environment. Fig.4 B exhibits the time-dependent current density curve for GCE-Ni/Pt and GCE-Pt/C under a static overpotential of 50 mV. The current density of GCE-Ni/Pt shows little degradation after a long period of 20000 s. With the help of Nafion, the commercial Pt/C catalysts were immobilized onto the electrode surface but their electrocatalytic current drop down obviously with the increase of time. To make matters worse, the immobilization of Pt/C catalysts in the absence of Nafion didn’t work well. The Pt/C catalysts fall off quickly and their electrocatalytic current dispears soon. The ultrathin Ni nanofilms help to immobilize the Ni/Pt nanocomposites onto the electrode surface due to their high adsorption ability. 3.4. EIS and capacitance characterization The electrochemical impedance spectroscopy (EIS) is an efficient tool for studying the interface properties of the surface-modified electrodes.5 The electron transfer kinetics and diffusion characteristics can be determined based on the shape of an 5


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impedance spectrum, where the value of electron transfer resistance (Ret) at the electrode surface is equal to the semicircle diameter of Nyquist plot. The EIS were subsequently performed in 0.1 M KCl with 5 mM Fe(CN)63-/4- to investigate the electrochemical activity of the bare GCE, GCE-Ni and GCE-Ni/Pt, shown in Fig. 5A. It was found that the bare GCE exhibits the smallest semicircle and it means the good electron transfer of Fe(CN)63-/4- at GCE. The semicircle diameter of the GCE-Ni was remarkably increased due to the low conductivity of Ni(OH)2 and its high Ret for Fe(CN)63-/4- redox reaction. Compared with the GCE-Ni, a slightly decreased semicircle arc was observed for the GCE-Ni/Pt, which is due to the higher conductivity of Pt and its lower Ret for Fe(CN)63-/4- redox reaction than Ni(OH)2. For further confirmation, the CVs of the bare GCE, GCE-Ni and GCE-Ni/Pt were carried out in 0.1 M KCl containing 5 mM Fe(CN)63-/4-. The redox signals of Fe(CN)63-/4presented in Fig. 5B are consistent with the EIS results mentioned above. The effective surface area can be estimated by measuring the double layer capacitance at the solid-liquid interface of GCE-Ni/Pt with CV.48 The CVs were collected in the region of 0.1 to 0.2 V, shown in Fig. 4C, where the current response should only be the charging of the double layer. The capacitance of Ni/Pt in Fig. 4D is calculated to be 28.6 mF/cm2. It is larger than 14.1 mF/cm2, that of reported CoSe2 nanoparticle modified electrode, meaning a higher surface roughness.47, 49 The electrochemical active surface area per unit mass (EAS-M) was often employed to characterize the electrocatalytic peroformance of catalysts by a simple CV method. The coulombic charge associated with hydrogen-adatom desorption (QH) of electrode surface in H2SO4 was used to calculate the active Pt sites at electrodes by integrating the CV in the hydrogen desorption region (not shown). The EAS-M values of the catalysts were obtained from the following formulation:50-52 EAS-M =

୉୅ୗ ୛ౌ౪

=

୕ౄ ୕ౄ౥ ୛ౌ౪

where EAS is the electrochemical active surface area (cm2), WPt the weight (mg/cm2) of loaded Pt fraction in the catalyst and QHo the amount of electricity per unit surface area of Pt, corresponding to the full coverage of Pt surface by one monolayer of hydrogen (for polycrystalline platinum QHo = 0.21 mC/cm2). The value of EAS-M for the obtained Ni/Pt catalysts was calculted to be 36.93 m2/g, which is higher than that for the reported Ni2Pt nanocatalysts (7.84)52 and slightly lower than those for Pt40/Vulcan XC-72 (40±4),51 Pt (47.0) and Ptcommercial (52.01).52 The catalytic performance of Ni/Pt nanocomposites will be improved in the future study.

4. Conclusions The nanocomposites with the ultrafine Pt NPs loaded on the ultrathin Ni nanofilms were sucessfully prepared by a simple onsite substitution reaction where PtCl62- was employed to oxidize the previously obtained Ni elements. The nanocomposites of Ni/Pt nanocomposites were modified onto the surface of electrode and exhibited excellent electrocatalytic activity of HER and stability. Due to the presence of Ni, the ultrafine Pt nanoparticles were obtained, dispersed and anchored on the surface of ultrathin Ni nanofilms. Both the ultrafine Pt NPs and the ultrathin Ni nanofilms contributed to the HER electrocatalysis of Ni/Pt nanocomposites. The electrocatalysts will be developed further for possible applications in hydrogen production for energy 6



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crisis.

Acknowledgments The authors greatly appreciate the support from Innovation Program of Shanghai Municipal Education Commission (14ZZ139) and National Natural Science Foundation of China (21305090).

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27. Liu, Y.; Hangarter, C. M.; Garcia, D.; Moffat, T. P. Surf. Sci. 2015, 631, 141-154. 28. van Drunen, J.; Pilapil, B. K.; Makonnen, Y.; Beauchemin, D.; Gates, B. D.; Jerkiewicz, G. ACS Appl. Mater. Interfaces 2014, 6, (15), 12046-12061. 29. Yu, Y.; Huang, S. Y.; Li, Y.; Steinmann, S. N.; Yang, W.; Cao, L. Nano Letters 2013, 14, (2), 553-558. 30. Gupta, S.; Patel, N.; Miotello, A.; Kothari, D. C. J. Power Sources 2015, 279, 620–625. 31. Li, F.; Zhang, L.; Li, J.; Lin, X.; Li, X.; Fang, Y.; Huang, J.; Li, W.; Tian, M.; Jin, J. J. Power Sources 2015, 292, 15–22. 32. Nguyen Viet, L.; Nguyen Duc, C.; Tomokatsu, H.; Hirohito, H.; Gandham, L.; Masayuki, N. Nanotechnology 2010, 21, (3), 035605. 33. Venu, R.; Ramulu, T. S.; Anandakumar, S.; Rani, V. S.; Kim, C. G. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2011, 384, (1–3), 733-738. 34. Yang, T.; Du, M.; Zhu, H.; Zhang, M.; Zou, M. Electrochim. Acta 2015, 167, 48-54. 35. Zhao, Y.; E, Y.; Fan, L.; Qiu, Y.; Yang, S. Electrochim. Acta 2007, 52, (19), 5873–5878. 36. Duke, A. S.; Galhenage, R. P.; Tenney, S. A.; Sutter, P.; Chen, D. A. J. Phy. Chem. C 2014, 119, (1), 381-391. 37. Zhang, F.; Zhu, D.; Chen, X. a.; Xu, X.; Yang, Z.; Zou, C.; Yang, K.; Huang, S. Phy. Chem. Chem. Phy. 2014, 16, (9), 4186-4192. 38. Lien, C. H.; Chen, J. C.; Hu, C. C.; Wong, S. H. J. Taiwan Inst. Chem. E. 2013, 45, (3), 846–851. 39. Kung, C. W.; Cheng, Y. H.; Ho, K. C. Sensor Actuat. B-Chem. 2014, 204, 159–166. 40. Miao, Y.; Ouyang, L.; Zhou, S.; Xu, L.; Yang, Z.; Xiao, M.; Ouyang, R. Biosens. Bioelectron. 2014, 53, (9), 428-439. 41. Grosvenor, A. P.; Biesinger, M. C.; Smart, R. S. C.; McIntyre, N. S. Surf. Sci. 2006, 600, (9), 1771-1779. 42. Venezia, A.; Bertoncello, R.; Deganello, G. Surf. Interface Anal. 1995, 23, (4), 239-247. 43. Bhardwaj, M.; Balasubramaniam, R. Int. J. Hydrogen Energ. 2008, 33, (9), 2178–2188. 44. Conway, B.; Tilak, B. Electrochim. Acta 2002, 47, (22), 3571-3594. 45. Merki, D.; Hu, X. Energ. Environ. Sci. 2011, 4, (10), 3878-3888. 46. John O'Mara Bockris, S. U. M. K., Surface Electrochemistry: A Molecular Level Approach. Plenum Press: NewYork, 1993. 47. Tian, J.; Liu, Q.; Cheng, N.; Asiri, A. M.; Sun, X. Angew. Chem 2014, 126, (36), 9731-9735. 48. Trasatti, S.; Petrii, O. A. J. Electroanal. Chem. 1991, 327, (5), 353–376. 49. Kong, D.; Wang, H.; Lu, Z.; Cui, Y. J. Am. Chem. Soc. 2014, 136, (13), 4897-4900. 50. Pozio, A.; De Francesco, M.; Cemmi, A.; Cardellini, F.; Giorgi, L. J. Power Sources 2002, 105, (1), 13-19. 51. Grigoriev, S. A.; Millet, P.; Fateev, V. N. J. Power Sources 2008, 177, (2), 281-285. 52. Domínguez-Crespo, M. A.; Ramírez-Meneses, E.; Torres-Huerta, A. M.; Garibay-Febles, V.; Philippot, K. Int. J. Hydrogen Energy 2012, 37, (6), 4798-4811.

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Page 10 of 15

Captions: Fig. 1 (A, B, C and D) TEM images of Ni/Pt nanocomposites. HRTEM images of Ni region (E) and Pt region (F) in Ni/Pt nanocomposites. SAED patterns of Ni region (G) and Pt region (H) in Ni/Pt nanocomposites. Fig. 2 (A) Overall XPS of Ni/Pt nanocomposites. (B and C) XPS spectrum of Ni/Pt nanocomposites. Fig. 3 (A) CVs of the GCE, GCE-Ni, GCE-Ni/Pt and GCE-Pt/C in 0.5 M H2SO4 at the scan rate of 100 mV/s. (B) Polarization curves of the GCE, GCE-Ni, GCE-Ni/Pt and GCE-Pt/C in 0.5 M H2SO4 at the scan rate of 2 mV/s. Inset: the partial amplification for the polarization curve of the GCE-Ni/Pt. (C) Tafel plots of the GCE-Ni, GCE-Ni/Pt and GCE-Pt/C. Fig. 4 (A) Polarization curves of the GCE-Ni/Pt initially and after 5000 CV scans between −0.1 to 0.1 V (vs. RHE). (B) Time-dependent current density curve for GCE-Ni/Pt and GCE-Pt/C under an overpotential 50 mV for 20000 s. Fig. 5 (A) EIS measurement and (B) CVs of 5 mM Fe(CN)63-/4- in 0.1 M KCl at the bare GCE, GCE-Ni and GCE-Ni/Pt. (C) CVs of the GCE-Ni/Pt in 0.5 M H2SO4. (D) The capacitive currents at 0.15 V as a function of scan rate for GCE-Ni/Pt (∆j0 = ja − jc).

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(A) (B)

Pt Ni

(C)

(D)

(E)

(F)

(G) 311 220 200 111

(H) Fig. 1 10



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N(E) (A)

C1s Ni2p

Pt4f

1200

900

600

300

0

Binding Energy (eV)

N(E)

N(E) Pt4f7/2

(B)

Ni2p3/2

(C)

Pt4f5/2

Ni2p1/2

Sat.

Sat.

77.5

75.0

72.5

70.0 885

Binding Energy (eV)

880

875

870

865

860

855

Binding Energy (eV)

Fig. 2

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j (mA/cm2)

j (mA/cm2)

0

GCE GCE-Ni GCE-Ni/Pt GCE-Pt/C

0 (A)

-40

0

-50

(B)

-50

-80 -100

-100

GCE GCE-Ni GCE-Ni/Pt GCE-Pt/C -150

-120 -160 -0.1

0.0

0.1

0.2

-150 -0.1

-0.6

0.0

-0.4


0.1

-0.2

0.0


Overpotential (V) 0.12

GCE-Ni GCE-Ni/Pt GCE-Pt/C

0.08 57 mV dec

-1

43 mV dec

0.04

-1

31 mV dec

-1

0.00 (C) -0.04 -0.6

0.0

0.6

1.2

1.8

Log [j (mA/cm2)]

Fig. 3

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j (mA/cm2) Initial 5000th

0

-50 (A) -100

-150 -0.45

-0.30

-0.15

0.00

Potential (V vs RHE) j (mA/cm2) 20 GCE-Ni/Pt GCE-Pt/C

(B)

0

-20

-40

-60 0

5000

10000

15000

20000

T (s)

Fig. 4

13


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j (mA/cm2) 2 (B)

(A)

GCE GCE-Ni GCE-Ni/Pt

1

0

-1

-2 -0.2

0.0

0.2

0.4

0.6

Potential (V vs SCE) ∆ j0.15 (mA/cm2)

j (mA/cm2) 4

5 (C)

(D) 4

2

3 0 2 -2

1

-4

0 0.10

0.15

0.20

0


20

40

60

80

100 120 140 160

Scan rate (mV/s)

Fig. 5

14


Onsite Substitution Synthesis of Ultrathin Ni Nanofilms Loading

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