Thermodynamics of the Under-Potential Deposition of Hydrogen on Polycrystalline Platinum in Aqueous Trifluoromethanesulfonic Acid Solution

Abstract

Understanding of the electrocatalytic behavior of the Pt/Nafion® ionomer interface is of great importance to fuel cell technology. Trifluoromethanesulfonic acid (CF3SO3H) is used as an electrolyte because it is the smallest fluorinated sulfonic acid and serves as a suitable molecular model mimicking the Nafion® ionomer. The under-potential deposition of H (UPD H) on polycrystalline Pt electrode in CF3SO3H is investigated using cyclic voltammetry in the 278–333 K temperature (T) range. The general electrochemical adsorption isotherm is used to determine the Gibbs energy (−13 ≤ Δec − adsG°(HUPD) ≤ −27 kJ mol−1), entropy (−59 ≤ Δec − adsS°(HUPD) ≤ +20 J mol−1 K−1), and enthalpy (−8 ≤ Δec − adsH°(HUPD) ≤ −43 kJ mol−1) of electro-adsorption, and the Pt–HUPD surface bond energy (+225 ≤ \( {E}_{\mathrm{Pt}-{\mathrm{H}}_{\mathrm{UPD}}} \) ≤ +261 kJ mol−1). The lateral interactions between the HUPD adatoms are repulsive; the energy of lateral interactions is T-dependent (ω(HUPD) = a − b T) and is in the +14 ≤ ω(HUPD) ≤ +22 kJ mol−1 range. The values of Δec − adsG°(HUPD), Δec − adsS°(HUPD), Δec − adsH°(HUPD), and \( {E}_{\mathrm{Pt}-{\mathrm{H}}_{\mathrm{UPD}}} \) for UPD H in CF3SO3H are very similar to the analogous values obtained in aqueous H2SO4 and HClO4 solutions. The anion present in the electrolyte has a small impact on UPD H and influences the values of Δec − adsG°(HUPD) only over a narrow HUPD coverage range. The anion nature has practically no impact on the values of Δec − adsS°(HUPD), Δec − adsH°(HUPD), or \( {E}_{\mathrm{Pt}-{\mathrm{H}}_{\mathrm{UPD}}} \).