Electrocatalytic Activity for Oxygen Reduction Reaction of Electrochemically Prepared Ni Core - Pt Shell Nanoparticles
Authors: Ueda Maya , Kondo Toshihiro 
Published: February 2020

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. PEFC has been intensively developed for applications to fuel cell vehicle (FCV) and residential cogeneration systems [1].
In order to operate PEFC at room temperature, it requires platinum (Pt) as a cathode catalyst for oxygen reduction reaction (ORR) due to its slow kinetics. However, electrocatalytic activity of polycrystalline Pt for ORR is not enough for actual operation of PEFC, and moreover, Pt is the expensive and precious metal, and then the commercially available PEFC is too expensive, unfortunately. For a goal of the widely spread of the PEFC, it is essential to decrease the loading amount of the Pt catalyst and to increase its ORR activity.
We previously reported that Pt ultra-thin layers deposited on the foreign metal surface, which have different surface energy from the polycrystalline Pt, have higher electrocatalytic activity [2]. Moreover, using Ni as a low cost core material can lead to the cost reduction. Thus, Ni@Pt is expected as a key cathode material for the PEFC.
From a viewpoint of finding a clue for the design of high performance and low cost cathode catalysts, in this report, the Ni core – Pt shell (Ni@Pt) nanoparticles were electrochemically prepared and their electrocatalytic activities for ORR were investigated.

Experimentals.

In the preparation of the Ni@Pt nanoparticles, the simple potential step chronoamperometry and galvanic replacement were employed [3,4]. From the cathodic peak potential values in the cyclic voltammogram measured in the electrolyte solutions containing Ni2+, the deposition potentials were determined.
The Ni nanoparticles were electrodeposited on the glassy carbon electrode (GCE) by applying the determined deposition potentials for several periods. Just after the GCE was disconnected, for the galvanic replacement of Pt with Ni on the Ni nanoparticle surface, a few drops of concentrated Pt2+ solution were added or the GCE was immersed in the solution containing Pt2+, for the certain periods.
After ultrasonic cleaning, electrocatalytic activity of the prepared Ni@Pt nanoparticles for ORR was qualified with the rotating ring-disk electrode (RRDE) system. The size and structure of the prepared Ni@Pt nanoparticles were evaluated by scanning electron microscopy (SEM).

Results and Discussion.

From the linear sweep voltammograms of the GCE modified with the Ni@Pt nanoparticles, which were measured in the oxygen saturated 0.1 M HClO4, all Ni@Pt nanoparticles, which were prepared in the several Ni2+ solutions for several Ni deposition periods, indicated that cathodic current due to ORR was observed at the potentials more negative than ca. 1.00 V. Moreover, the typical results of them had higher electrocatalytic activity for ORR than that of polycrystalline Pt.
SEM image of the Ni nanoparticles showed that the shapes were spherical and uniformly distributed. In addition, the size was strongly dependent on the deposition time in preparation. But, the size and also the shape of nanoparticles were much dependent both on the period in Galvanic replacement and pH of the Pt2+ solution.
More optimum conditions and the details of the relationship between the ORR activity and the characteristics of the Ni@Pt nanoparticle, such as the size, the shape, and the composition, are now under investigation.

References.

[1] R. R. Adzic, et al., Top. Catal., 45 (2007) 249.
[2] T. Kondo, et al., Chem. Lett., 40 (2011) 1235.
[3] H. Nagai, et al., ECS Trans., 58 (2014) 27.
[4] M. Ueda et al., Pacifichem 2015, Honolulu (2015) #333.