Surface oxide growth on platinum electrode in aqueous trifluoromethanesulfonic acid

Abstract

Platinum in the form of nanoparticles is the key and most expensive component of polymer electrolyte membrane fuel cells, while trifluoromethanesulfonic acid (CF3SO3H) is the smallest fluorinated sulfonic acid. Nafion, which acts as both electrolyte and separator in fuel cells, contains -CF2SO3H groups. Consequently, research on the electrochemical behaviour of Pt in aqueous CF3SO3H solutions creates important background knowledge that can benefit fuel cell development. In this contribution, Pt electro-oxidation is studied in 0.1 M aqueous CF3SO3H as a function of the polarization potential (E(p), 1.10 ≤ E(p) ≤ 1.50 V), polarization time (t(p), 10(0) ≤ t(p) ≤ 10(4) s), and temperature (T, 278 ≤ T ≤ 333 K). The critical thicknesses (X1), which determines the applicability of oxide growth theories, is determined and related to the oxide thickness (d(ox)). Because X1 > d(ox) for the entire range of E(p), t(p), and T values, the formation of Pt surface oxide follows the interfacial place-exchange or the metal cation escape mechanism. The mechanism of Pt electro-oxidation is revised and expanded by taking into account possible interactions of cations, anions, and water molecules with Pt. A modified kinetic equation for the interfacial place exchange is proposed. The application of the interfacial place-exchange and metal cation escape mechanisms leads to an estimation of the Pt(δ+)-O(δ-) surface dipole (μ(PtO)), and the potential drop (V(ox)) and electric field (E(ox)) within the oxide. The Pt-anion interactions affect the oxidation kinetics by indirectly influencing the electric field within the double layer and the surface oxide.

Atomistic Mechanism of Pt Extraction at Oxidized Surfaces: Insights from DFT

“…The calculated dipole moments are consistent with values reported in [17]. Upon coordination of Pt45 with three O ads , a strong local dipole moment (>3.5 D) directed toward the surface occurs, which destabilizes Pt45 and leads to its extraction from the surface.…”

“…In spite of tremendous efforts in both experiment and theory focusing on understanding degradation issues and improving catalyst stability [14][15][16], the understanding of materials properties that control Pt stability and mechanisms involved in catalyst dissolution remains a controversial topic [17,18]. Especially, the comprehension of the elementary surface processes involved is still elusive.…”

“…The adsorbed surface species at Pt that is formed by water oxidation is usually identified as chemisorbed oxygen (O ads at 0.85 < E < 1.10 V RHE ), which forms up to half of a monolayer (ML) coverage [5]. As the potential increases, further discharge of water molecules forms additional O ads that is susceptible to undergoing interfacial exchange with Pt surface atoms; this so-called place-exchange is a basic process involved in irreversible oxide growth [17,[25][26][27]. In situ Xray scattering [28], as well as STM studies under ultrahigh vacuum conditions [29] confirmed the existence of interfacial place-exchange at Pt(111).…”

Formation of Pt Skin Layer on Ordered and Disordered Pt-Co Alloys and Corrosion Resistance in Sulfuric Acid

“…Energetically unstable O atoms at the surface enter the subsurface site of Pt, which is energetically favorable. This is a socalled place exchange [52,[78][79][80]. The place exchange between Pt and O atoms was confirmed by an X-ray absorption fine structure (XAFS) spectroscopy measurement [54] and a scanning tunneling microscopy (STM) observation [81].…”

Platinum oxidation in alkaline electrolyte under potentiostatic conditions

“…Dotted line is extrapolated for procedure I. of the oxidised layer is described as PtO [7,22] (at lower E p a contribution from PtOH/Pt(OH) 2 is considered), there might be potential influence on e.g., non-stoichiometry, interaction with ions or water. [8,9,31,32]. Presented results indicate that for a polycrystalline Pt electrode the layers formed at a certain potential maintain some of their properties determined by the potential of their formation also during subsequent surface oxidation at a different potential.…”