Preparation of Monocrystalline Platinum Electrodes And Their Structural Transformation Upon Electro-Oxidation and Electro-Dissolution in Aqueous Acidic Media

Abstract

Atomic-level understanding of the degradation of platinum (Pt) materials is important for rational design of nanoscopic Pt electrocatalysts for fuel cells. Studies employing Pt monocrystalline (single crystal) electrodes can lead to the understanding of atomic-level degradation. Electrochemistry studies on monocrystalline electrodes require well-ordered and highly reproducible surfaces. Therefore, it is of vital importance to understand, optimize, and control each step of single-crystal growth, orientation, and final preparation in order to obtain accurate and reproducible experimental results. Firstly, in-three parts, we report on the development of experimental methodology that allow one to prepare Pt single crystal surfaces and acquire cyclic voltammetry (CV) profiles for hemispherical Pt(111), Pt(110), and Pt(100) as well as polyoriented monocrystalline Pt (Pt(spherical)) electrodes. Then, we report on the dissolution and structural transformation of Pt(spherical) electrode in 0.50 M aqueous H2SO4 upon potential cycling in the surface oxide formation-reduction region. The potential cycling is performed in the lower potential (EL) – upper potential (EU) range (EL = 0.07 V and 0.90 ≤ EU ≤ 1.50 V) to correlate dissolution and morphology data to EU. The amount of dissolved Pt is monitored using flow injection coupled to inductively coupled plasma mass spectrometry (ICPMS) and structural changes, which modify cyclic voltammetry profiles, are examined using scanning electron microscopy (SEM). In the case of EU ≤ 1.20 V, there is minor dissolution of the (100) and (110) facets, while the (111) one remains stable. In the case of EU ≥ 1.30 V, all facets undergo significant dissolution. Changes in the surface morphology of Pt(spherical) upon repetitive potential cycling in the 0.07–1.50 V range were examined in relation to the number of transients (1 ≤ n ≤ 30000). The SEM images reveal the (111) facet develops pits, the (100) facet uniformly distributed hillocks (pyramids), and the (110) facet columns. We report structural changes for twenty-five basal, stepped, and kinked facets. Their analysis demonstrates that the (531) facet is the least roughened, thus the most stable. The original results reported in this article represent a major contribution to the current understanding of the interfacial electrochemistry and electrocatalysis of Pt materials.