SECM Study for Surface-Grain-Resolved Hydrogen Oxidation Reaction Activity of Polycrystalline Pt Electrode
Authors: Sampei Masaki , Noda Disuke , Hayashi Kenta , Todoroki Naoto , Wadayama Toshimasa 
Published: December 2024

Introduction

Carbon-supported Pt nanoparticles (Pt/C) are used as anode as well as cathode catalyst for polymer electrolyte fuel cell (PEFC). In general, electrochemical reactions that proceed at electrocatalyst surfaces are strongly depended upon their surface atomic arrangements. Therefore, to develop novel catalyst materials, guidelines of microstructural surface design of the catalysts are indispensable through clarifying various surface properties of the catalysts, such as surface atomic arrangements, size, and shapes and comprehensive understandings of the surface microstructures and catalytic properties should be required. From the above perspectives, fundamental studies for the electrocatalysis by using "well-defined" single crystal surfaces of Pt (Pt(hkl)) have been conducted to date [1]. However, gaps of material’s sizes between the surfaces of millimeter-sized single crystal and nano-sized particles make it difficult to understand practical catalytic properties of the nano-sized particles. To bridge the forementioned gaps, this study aims to clarify the dependences of hydrogen oxidation reaction (HOR) properties on micrometer-sized surface grains of a polycrystalline Pt electrode, through comparing two-dimensional (2D) surface mappings with micrometer resolution of surface grain orientations analyzed by an electron backscatter diffraction (EBSD) method and of hydrogen oxidation reaction (HOR) activities evaluated by a scanning electrochemical microscope (SECM) [2]. We successfully correlate the micrometer-sized surface grains and corresponding HOR properties.

Experimental

Polycrystalline Pt substrate (10mm × 10mm, 1mm thickness) was pre-annealed at 1000°C for 15 hours in ultra-high vacuum (UHV: ~10-7 Pa). The surface grain orientations of the polycrystalline electrode were analyzed using EBSD in air. For comparison of 2D mapping images of EBSD-analyzed surface grain orientations and SECM-evaluated HOR activities, ten-nanometer-size triangle shapes were marked by a nano indenter (TTX-NHT, Anton Paar) on the Pt substrate surface as signs of the positions. The polycrystalline Pt substrate surface thus prepared was cleaned by three cycles of Ar+ sputtering and annealing at 1000°C for 10 minutes alternately in UHV and, after that, HOR activities of the electrode was performed in 0.01M HCIO4 + 0.1M NaClO4 mixed electrolyte by SECM. 2D mapping image of micrometer-resolved HOR activities was collected by a scanning Pt-made microelectrode (φ = 10μm) with a distance of ca. 7μm between the microelectrode and polycrystalline electrode. Because a possible scanning region of the microelectrode was 200 µm square, a whole 2D mapping image of HOR activities (350µm × 650μm; Fig. 1(a)) was constructed by eight pieces of the 200 µm square images. Finally, by referring to the position markings made by the nano indenter, 2D images of surface grain orientations and HOR activities were combined with micrometer resolution.

Results

Fig. 1 shows 2D mapping images of (a) the surface grain orientations and of (b) HOR activities for the polycrystalline Pt electrode surface, respectively, where the grains having surface orientations close to (111), (110), and (100) are distinguished in blue, green, and red and purple in (a; right-hand-side invers pole figure) and the iT
values of the scanning microelectrode currents that induced by HOR are presented by colors (b; of a right-hand-side indicator). Close correlations are identified between the surface grain orientations and corresponding HOR activities with micrometer resolution. From detailed comparisons of the two 2D mappings, the order of HOR activities is (111) oriented surface grains > (110) > (100). Notably, in the vicinities of specific grain boundaries, HOR activities changed within the same surface grains. In the presentation, we will discuss dependences of the orientations of surface grains, grain boundaries on corresponding HOR activities for the polycrystalline Pt and Ir electrodes.

Acknowledgement

This study was supported by new energy and industrial technology development organization (NEDO) of Japan and Japan society for the promotion of science (JSPS).
Reference
[1] K. Hayashi, et al., Phys. Chem. Chem. Phys., 25, 2770-2775, (2023)
[2] O. Wipf, et al., J. Electrochem. Soc. 167, 146502, (2020)

Figure 1