Structures of Pt Ultra-Thin Films on Ni Substrate Prepared By Galvanic Replacement
Authors: Yoshioka Risa , Sakai Sayumi , Kondo Toshihiro 
Published: February 2020

Introduction

Polymer electrolyte fuel cell (PEFC) has been expected as a high-power and clean electric source to solve the social problems such as environmental pollution and energy crisis [1]. In order to operate the PEFC at room temperature, it is necessary to promote the reaction rate of the oxygen reduction reaction (ORR), which is the rate determining step of the cathode reaction, by Pt as a cathode electro-catalyst. However, there are some problems with Pt catalyst for actual operation and wide general spread of PEFC; Pt is the expensive and precious metal and electro-catalytic activity of Pt for ORR is insufficient. In order to widely spread PEFC, therefore, it is indispensable to reduce the amount of Pt catalyst and to increase its ORR activity.
It is well known, on the other hand, that ultra-thin noble metal layers formed on foreign metal surface often have higher electro-catalytic activity, because they have different surface energy from their bulk crystal [2]. Pt ultra-thin film covered with a low cost metal surface is one of the candidate as a cheaper electro-catalyst with higher electro-catalytic activity for ORR in PEFC. We have succeeded to electrochemically prepare Ni core Pt shell (Ni@Pt) nanoparticles using Ni as a cheaper metal core and demonstrated that its electro-catalytic activity for ORR is higher than Pt itself [3,4]. In these studies, such Pt thin layer coated Ni nanoparticles were prepared by a galvanic replacement technique.
There are several methods to form the Pt ultra-thin film on the foreign metal surface. Among them, the galvanic replacement technique, in which such Pt ultra-thin film can be prepared by a simple operation, just dipping a substrate into a solution containing Pt ion, is one of the best candidates. However, there are few reports to structurally investigate the galvanic replacement process with an atomic dimension. In this report, the galvanic replacement process of Ni substrate surface with Pt was structurally investigated by the electrochemical and angle-resolved X-ray photoelectron spectroscopy (ARXPS) measurements.

Experiments

After several pre-treatments of Ni substrate, several droplets of the solution containing Pt2+ ion were added to the 0.05 M Na2SO4 solutions with pHs of 3.0, 4.0, and 5.0 as the Ni substrate was immersed into each solution during monitoring the open circuit potential (OCP). After the OCP was stabilized to a constant value, the substrate was taken out from the solution and rinsed with ultrapure water by a sonication for ca. 5 min. After drying the substrate, cyclic voltammogram (CV) and ARXPS were measured to obtain the Pt surface area and Pt film thickness, respectively.

Results and Discussion

In all the solutions, OCP was positively shifted and Pt existed onto the Ni substrate, although Ni oxide also existed of all the substrates. Thickness value of Pt thin film was dependent on pH value of solution. Pt thin film prepared from the solution with pH 3.0 was thinnest but its surface was quite rough. In addition, Pt black was also formed on this substrate surface and it was easy to wash it off with ultrapure water from the surface. Details of the Galvanic replacement are now under investigation.

References

[1] M. Watanabe, Surf. Sci., 27 (2006) 604.
[2] G. A. Somorjai, Surface Chemistry and Catalysis, John Wiley & Sons (1990).
[3] H. Nagai, H. Aso, M. Kawabuchi, and T. Kondo, ECS Trans., 58 (2014) 27.
[4] M. Ueda and T. Kondo, ECS Trans., 75 (2017) 43.