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 (CF 3 SO 3 H) 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 CF 3 SO 3 H 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≤Δ ec−UPD )≤+20 J mol −1 K −1 ), and enthalpy (−8≤Δ ec − ads H°(-H UPD )≤−43 kJ mol −1 ) of electro-adsorption, and the Pt-H UPD surface bond energy (+225≤E Pt−H UPD ≤+261 kJ mol −1 ). The lateral interactions between the H UPD adatoms are repulsive; the energy of lateral interactions is T-dependent (ω(H UPD )= a − b T) and is in the +14≤ω(H UPD )≤+22 kJ mol −1 range. The values of Δ ec − ads G°(H UPD ), Δ ec − ads S°(H UPD ), Δ ec − ads H°(H UPD ), and E Pt−H UPD for UPD H in CF 3 SO 3 H are very similar to the analogous values obtained in aqueous H 2 SO 4 and HClO 4 solutions. The anion present in the electrolyte has a small impact on UPD H and influences the values of Δ ec− ads G°(H UPD ) only over a narrow H UPD coverage range. The anion nature has practically no impact on the values of Δ ec− ads S°(H UPD ), Δ ec−ads H°(H UPD ), or E Pt−H UPD .