Pt-Ni nanourchins as electrocatalysts for oxygen reduction reaction

Abstract

Pt-Ni bimetallic alloys with various nanostructures have shown excellent activity toward oxygen reduction reaction (ORR). The ORR activity is highly dependent on the structure of the catalyst. In this paper, Pt-Ni nanourchins were synthesized with an average size of 50 nm consisting of 10–20 nanorods and nanooctahedra by adjusting the synthesis condition. The formation of Pt-Ni nanourchins is mainly dependent on the adding order of solvents (benzyl ether, oleylamine and oleic acid). Pt-Ni nanourchins present a reasonable high ORR activity (0.81 A/mg at 0.9 V).

Keywords

Pt-Ni alloys / nanourchins / oxygen reduction reaction / shape control / fuel cells

Introduction

Pt-Ni bimetallic alloys have shown a high activity toward oxygen reduction reaction (ORR) [1–14]. Since the ORR activity is highly dependent on the morphology of nanocatalysts, Pt-Ni nanocatalysts with various morphologies have been synthesized and characterized including nano-octahedra [10,13,15–20], nano-cubes [18–20], nanorods [21,22], nanowires [23–25], porous structure [26–29], and nanourchins [30]. In those synthesis, the solvents and capping agents played key roles in controlling the morphology of nanostructures [13, 16]. For example, Pt-Ni octahedra could be obtained by heating the mixture of platinum acetylacetonate (Pt(acac)₂), nickel acetylacetonate(Ni(acac)₂), oleylamine (OAm), and oleic acid (OAc) in the presence of CO or W(CO)₆ at 200ºC–230ºC. Zhang et al. [18] proposed that W⁰ formed during the decomposition of W(CO)₆ could reduce Pt²⁺ and accelerate the formation of Pt nuclei in the early stage of the synthesis. Wu et al. [13] proposed that CO generated from the same source could adsorb preferentially on the Pt-Ni(111) surface, resulting in the formation of an octahedral shape. In contrast, in the absence of Ni precursors, CO preferentially adsorbed on the Pt(100) surface, forming cubic Pt. Choi et al. [16] also found the importance of CO in the morphology control of Pt-bimetallic NCs. Shen et al. [30] argued that different heating rates could result in different morphologies of nanocrystals. They prepared Pt-Ni nanourchins with a heating rate of 7ºC/min when W(CO)₆ solid powder was added. When the heating rate was changed to 3.5ºC/min, Pt-Ni nanopolyhedra was acquired.

In this paper, the preparation and morphological investigation of the Pt-Ni nanocatalysts as well as their ORR activities were presented. Pt-Ni nanourchins with the size of 50 nm were prepared and the effect of synthesis conditions on the morphology was systematically studied.

Experimental

Materials

Pt(acac)₂(99.99 %), Ni(acac)₂(95 %), OAm (70 %), OAc (90 %), benzyl ether (BE, 98 %), W(CO)₆(99.99 %) were obtained from Sigma-Aldrich and used as received.

Preparation of Pt-Ni nanourchins and Pt-Ni octahedra

In the synthesis of Pt-Ni nanourchins, 20 mg of Pt(acac)₂ and 10 mg of Ni(acac)₂ were put in a three-neck flask. The solvents were added following the order of 6 mL of benzyl ether, 1 mL of OAc and 2 mL of OAm. The mixture was ultrasonically mixed and heated to 130°C at a rate of 10°C/min. 1 mL of BE (with or without 50 mg of W(CO)₆) was added to the solution when the temperature reached 130°C. Then, the solution was heated to 230°C with a heating rate of 10°C/min. The Pt-Ni nanourchins were obtained after 40 min of reaction at this temperature. The product was centrifuged and washed by 5 mL of toluene and 15 mL of ethanol. After centrifugation, 12 mg of carbon black and Pt-Ni were ultrasonically dispersed in 10 mL of toluene for 3 h. The Pt-Ni nanourchins/C were collected and dried at 80°C for 1 h. In the synthesis of Pt-Ni octahedra, 20 mg of Pt(acac)₂ and 10 mg of Ni(acac)₂ were weighted into a three-neck flask. Then the solvents were added following the order of 7 mL of benzyl ether, 2 mL of OAm and 1 mL of OAc. After mixing, the solution was heated to 130°C at a rate of 10°C/min and 50 mg of W(CO)₆was added at the same time. The solution was heated to 230°C with a heating rate of 10°C/min. The Pt-Ni octahedra were obtained after 40 min.

Physical characterization

Transmission electron microscopy (TEM) images were taken using a JEOL 2010 microscope at 200 kV. The Pt loadings of Pt-Ni/C were examined by ICP-MS (Optima 2000, VARIAN). XRD patterns were obtained by a Philips PW1830 powder X-ray diffractometer equipped with a Cu Ka radiation source (l=1.5406 Å) and a graphite monochromator. XPS data were required by a Kratos Axis Ultra DLD multi-technique surface analysis system.

Electrochemical evaluation

A three-electrode system with an electrochemical workstation (CHI 627) was used to conduct the electrochemical measurements. A 0.1 mol/L HClO₄ solution (double distilled, 70%, GFS chemicals) was used as the electrolyte. The counter and reference electrode were a piece of Pt flag and an Ag/AgCl, respectively. All the potentials were calibrated to a reversible hydrogen electrode (RHE) using a hydrogen reference electrode. The electrochemical performance was evaluated using the thin film-rotating disk electrode (TF-RDE) technique. 2.5 mg of catalysts were ultrasonically dispersed in a 2.0 mL of water, 0.5 mL of isopropanol and 10 µL of Nafion. 10 µL of the uniform ink was deposited on a pre-cleaned glassy carbon RDE tip. CV measurements were conducted in an Ar-saturated 0.1 mol/L HClO₄ solution. A stabilized curve was obtained with a scan rate of 50 mV/s after 20 cycles in the range of 0.02–1.20 V vs. RHE at a scan rate of 100 mV/s. The ORR polarization curves were recorded in an O₂-saturated 0.1 mol/L HClO₄ solution at a scan rate of 10 mV/s with a rotating speed of 1600 r/min.

Results and discussion

The morphology of Pt-Ni nanourchins synthesized without the assistance of W(CO)₆ was characterized by TEM, as shown in Fig.1(a). The size of Pt-Ni nanourchins is approximately 50 nm. Each nanourchin consists of 10-20 nanorods with an average length and width of 10 and 5 nm, respectively. The composition of Pt-Ni nanourchins from EDS (Fig. 1(c)) is in agreement with the result of ICP, which reveals the atomic ratio of Pt to Ni= 2.2:1. Figure 1(b) shows the XRD pattern of Pt-Ni/C nanourchins with a typical fcc feature. Comparing with pure Pt, all the peaks shifted to higher 2q angles, suggesting the formation of a Pt-Ni bimetallic alloy. According to Scherrer’s equation, the average crystallite size of Pt-Ni nanourchins was calculated to be 4 nm based on the full width at half maximum of the (111) peak.

The chemical status of as-synthesized Pt-Ni nanourchins was determined by XPS. Compared with the commercial Pt/C (TKK, 48%(wt)), the Pt 4f_7/2 and Pt 4f_5/2 peaks were positively shifted by 0.4 eV and 0.3 eV, respectively, demonstrating the alloying effect from Ni (Fig. 2(a)). The Ni 2P spectra can be assigned to metallic Ni at 852.6 eV and Ni(OH)₂ at 855.5 eV. The Ni(OH)₂ may exist on the surface of the nanostructure.

To gain more insight into the formation mechanism of Pt-Ni nanourchins, different synthesis conditions were explored. Shen et al. [30] found that heating rates could influence the morphology of Pt-Ni in a similar synthesis with the assistance of W(CO)₆. They demonstrated that Pt-Ni nanourchins could be obtained when the heating rate was 7°C/min, while polyhedra were formed when the heating rate was reduced to 3.5°C/min. In the present work, the effect of heating rate on the morphology was also assessed. After adding 1 mL of BE containing W(CO)₆, the temperature was raised from 130°C to 230°C with a heating rate of 10°C/min or 20°C/min. TEM images of Pt-Ni nanostructures at different heating rates are compared in Fig. 3(a) and (b). Uniform Pt-Ni nanourchins could be synthesized regardless of heating rates. Therefore, heating rates did not change the morphology of Pt-Ni significantly (listed in Table 1).

Figure 1 indicates that W(CO)₆ is not necessary in the formation of nanourchins. When adding W(CO)₆ by dissolving it in 1 mL of BE, Pt-Ni nanourchins, instead of octahedra, were formed. One of possible reasons could be the decomposition of W(CO)₆ before it was added into the reactor. During dissolving of W(CO)₆ in 1 mL of BE, the mixture was heated to 60°C in order to completely dissolve the former. The high temperature may cause the decomposition of W(CO)₆ [31,32]. In the synthesis of Pt-Ni octahedra [13,16], W(CO)₆ powder was added directly into the reactor at 130°C. In this case, the decomposition of W(CO)₆ was avoided before being added into the reactor. The decomposition of W(CO)₆ generates CO, which is believed to be the main cause of the formation of octahedral shape. With the decreased amount of W(CO)₆, the formation of Pt-Ni octahedra is less favorable.

Interestingly, it is found that the order of adding different solvents plays an important role in controlling the morphologies of Pt-Ni. Specifically, under the same condition that W(CO)₆ was dissolved in 1 mL of BE, Pt-Ni nanourchins were formed when OAm was added into the flask after BE and OAc (Fig.3(a)), while a mixture of Pt-Ni octahedra and branch-like nanostructures were obtained when OAc was added last (Fig. 3(c)). Moreover, when W(CO)₆ powder was added directly into the flask at 130°C, Pt-Ni octahedral were formed with a yield of nearly one hundred percentage (Fig. 3d).

Figure 4(a) shows the CV of Pt-Ni /C nanourchin (denoted as Pt_2.2Ni/C) synthesized without W(CO)₆ in an N₂-saturated 0.1 mol/L HClO₄ solution after electrochemical cleaning. The typical hydrogen adsorption/desorption and surface oxidation/reduction features of Pt suggest that the catalyst surface is free of surfactants, such as OAm and OAc. The reduction peak Pt oxide is positively shifted by 40 mV compared to Pt/C, suggesting a much faster recovering of Pt surface from oxides. This observation is consistent with the XPS results as the electronic properties of Pt are modified by Ni. The electrochemical surface area (ECSA) of Pt calculated from the charge associated with hydrogen underpotential deposition (H_UPD) is 38.1 m²/g_Pt. Figure 4(b) presents the positive-going ORR polarization curves of Pt_2.2Ni/C and Pt/C. The Pt mass activity at 0.9 V (vs. RHE) of Pt_2.2Ni/C is 0.81 A/mg, which is 4-fold higher than that of Pt/C (Table 2). The specific activity of Pt_2.2Ni/C (2.13 mA/cm²) is 9-fold higher than Pt/C (0.24 mA/cm²). The activity of Pt-Ni nanourchin is lower than that of 10 nm octahedron (3.3 A/mg) [16]. The activity difference is mainly caused by the structure of the catalysts. The octahedron consists of eight {111} facets, which are believed to have the highest activity toward ORR.

Conclusions

Pt-Ni nanourchins with an average size of 50 nm were successfully prepared. These nanostructures consist of 10–20 nanorods with an average length and width of 10 and 5 nm, respectively. The morphologies of the Pt-Ni bimetallic alloys are highly dependent on the synthesis conditions. It was found that the heating rate does not significantly affect the morphology of the product. In addition, W(CO)₆ is not necessary during the synthesis of Pt-Ni nanourchins. Interestingly, the order of adding solvents (BE, OAm and OAc) into the reactor was found to influence the morphology of Pt-Ni. When OAm was added last, Pt-Ni nanourchins were formed. While a mixture of Pt-Ni octahedra and branch-like nanostructures were obtained when OAc was added last. If OAc was the last added solvent, well-controlled Pt-Ni octahedra with a very high yield could be produced when W(CO)₆ was added into the reactor as the powder forms. Pt-Ni nanourchins present a high ORR activity (0.81 A/mg) at 0.9 V, which is 4-times higher than that of Pt/C. However, it is somewhat lower than that of Pt-Ni octahedra (3.3 A/mg), which is consistent with the fact that {111}facets of Pt alloys are more active than other low-index ones toward ORR.

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