Electrocatalytic Activities of Pt Monolayer Covered Ni and Co Core-Shell Nanoparticles for Oxygen Reduction Reaction

Introduction.

       Because of modern social problems such as an environmental pollution and energy crisis, polymer electrolyte fuel cell (PEFC) has been expected as one of the clean and high power sources for next generation [1]. PEFC requires platinum (Pt) as a cathode catalyst for oxygen reduction reaction (ORR). However, Pt is expensive and precious metal, and moreover, electrocatalytic activity of polycrystalline Pt for ORR is not enough for actual operation, unfortunately. Thus, many studies such as construction of Pt nanoparticles, Pt ultra-thin layers, and alloys with/without Pt, have been carried out in order to increase the electrocatalytic activity for ORR and to reduce the loading amount of Pt, i.e., cost of catalysts, for PEFC [1].

       On the other hand, ultrathin metal films formed on the foreign metal substrates have different properties from the deposited and substrate metals, especially they often have higher electrocatalytic activities because of the unique surface atomic arrangements and their induced surface electronic energy change [2]. Previously, we succeeded in construction of Pt ultrathin layer covered Ni or Co nanoparticles (Pt@Ni or Pt@Co) and they showed higher electrocatalytic activity than Pt polycrystalline [3].

       In this study, preparation conditions of these core-shell type nanoparticles on glassy carbon electrode (GCE) were optimized to increase electrocatalytic activity for ORR. Size and size distribution of these nanoparticles were investigated by atomic force microscopy (AFM) and thickness of the ultrathin Pt layers was determined by X-ray photoelectron spectroscopy (XPS).

Experimentals.

       Potential of the GCE was stepped from open circuit potential (OCP) to several deposition potentials in a deaerated 1.0 M Na₂SO₄ electrolyte solution containing 10 mM Ni²⁺ or Co²⁺ [3-5]. Time dependence of transient current during Ni or Co deposition showed that it was appropriate to prepare Ni or Co nanoparticles in the instantaneous mode [3,6] in order to control the size of nanoparticles with a narrow size distribution. After several deposition periods, the potential was back to OCP, namely the GCE was disconnected. As keeping potential monitoring, a few drops of conc. PtCl₄^2- solution was added in the electrolyte solution in order to replace the outermost Ni or Co layers of the nanoparticles with ultrathin Pt layers by Galvanic replacement [3,7,8]. After thoroughly rinsing with conc. H₂SO₄, linear sweep voltammograms (LSVs) were measured in an oxygen saturated 0.1 M HClO₄ to estimate electrocatalytic activity for ORR. Structure and size of these Pt@Ni and Pt@Co were investigated by XPS and AFM.

Results and Discussion.

       At the all GCEs modified with Pt@Ni and Pt@Co, cathodic current for ORR was observed at the potentials more negative than ca. 1.0 V (vs. RHE). Best ORR activity showed Pt@Ni, which was prepared at the deposition potential of -1.68 V (vs. MSE) for the deposition period of 5000 s, and Pt@Co, which was prepared at the deposition potential of -1.57 V (vs. MSE) for the deposition period of 3000 s. Under these preparation conditions, current transients showed that growth mode of these nanoparticles was instantaneous. Values of kinetic current density at 0.9 V, which were obtained from Koutecky-Levich plot, were 3.1 mA cm^-2 at Pt@Ni and 4.8 mA cm^-2 at Pt@Co, which were higher than that of polycrystalline Pt of 1.6 mA cm^-2. Diameter of Pt@Ni and Pt@Co was ca. 20 nm determined by AFM. XPS results showed that thickness of Pt layers was ca. 3 nm, suggesting that Pt monolayer was covered with the surface of Ni and Co nanoparticles. Durability and longevity of electrocatalytic activities of these Pt@Ni and Pt@Co nanoparticles for ORR are now under investigation.

References.

[1]   R. R. Adzic, et al., Top. Catal., 46 (2007) 249 and references there in.

[2]   G. A. Somorjai, Surface Chemistry and Catalysis, John Wiley & Sons, New York (1990).

[3]   H. Nagai, et al., 224th ECS Meeting, San Francisco, #45 (2013).

[4]   C. H. Rios-Reyes, et al., J. Solid State Electrochem., 14 (2010) 659.

[5]   E. Gomez, et al., J. Appl. Electrochem., 22 (1992) 872.

[6]   B. Scharifker and G. Hills, Electrochim. Acta, 28 (1983) 879.

[7]   J. Zhang, et al., J. Phys. Chem. B, 108 (2004) 10955.

[8]   M. Fayette, et al., Langmuir, 27 (2011) 5650.