Comprehensive Study of the Electrochemical Behavior of Polycrystalline Platinum Electrodes in Aqueous Solutions of Trifluoromethanesulfonic Acid
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Trifluoromethanesulfonic acid (CF3SO3H) is the smallest fluorinated sulfonic acid and serves as a model imitating the Nafion® ionomer of catalyst layers of polymer electrolyte membrane fuel cells (PEMFCs). The difference in the electrochemical behavior of Pt in CF3SO3H, as compared to H2SO4 or HClO4, originates from the different anion nature. Because PEMFCs operate in the potential range in which electrochemical reactions involving O and H occur, the thesis focuses on: (i) Pt electro-oxidation; (ii) H electro-adsorption; and (iii) electrochemical and chemical Pt dissolution. Platinum electro-oxidation in 0.1 M CF3SO3H is studied at various polarization potentials (Ep), polarization times (tp) and temperatures (T). The reaction mechanism 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 the metal cation escape mechanisms results in the determination of the Pt–O surface dipole moment, as well as the potential drop (Vox) and electric field (Eox) within the oxide. The platinum-anion interactions indirectly affect the surface electro-oxidation kinetics. The under-potential deposition of H (UPD H) on Pt in CF3SO3H is investigated over a broad T range using cyclic voltammetry. The general electrochemical adsorption isotherm is used to determine standard Gibbs energy, entropy, and enthalpy of electro-adsorption, and energy of the Pt‒HUPD surface bond. The lateral interactions between HUPD adatoms are repulsive. Platinum electro-dissolution in 0.1 and 0.5 M CF3SO3H, H2SO4, and HClO4 solutions is studied using potential cycling and inductively coupled plasma mass spectrometry. The results demonstrate that the anion nature has no or negligible impact on Pt electro-dissolution; however, pH significantly affects the process and the higher the pH value the greater the electro-dissolution of Pt. An analysis of potential versus pH diagrams (Pourbaix diagrams) for acid solutions of different concentrations demonstrates that dissolved Pt (present as Pt2+ and Pt4+) can form through anodic dissolution of metallic Pt, as well as through anodic electrochemical and chemical dissolution of PtO.