Unveiling proton transfer dynamics at the triple phase boundary of fuel cells via Ab Initio molecular dynamics

Abstract

This study investigates the proton transfer (PT) mechanisms at the triple phase boundary (TPB) within the catalyst layers (CLs) of proton exchange membrane fuel cells using ab initio molecular dynamics simulations. Despite extensive research into chemical reactions, the complete PT process at the TPB is still unclear, making it difficult to improve CLs by designing structures with higher PT conductivity and better stabilities. Therefore, this work focuses on three critical parameters affecting PT conductivity: the distance between Pt and ionomers (h), the distance between ionomer side chains (d), and the crystal surface. As the h increases, water condensation on the Pt surface intensifies, forming a one-way water channel that hinders proton detachment from SO₃H groups while facilitating PT between ${\text{SO}}_3^{-}$ groups. The diffusion coefficient of water and hydronium ions decreases with d, indicating that narrow water channels caused by excessive ionomers surrounding the Pt catalysts can reduce proton conductivity. In addition, the Pt(111) facet exhibits the highest PT frequency of 3.2 times per ps, owing to its superior water condensation and complete solvation structures. This is followed by Pt(110) and Pt(100),with PT frequencies of 1.85 and 1.1 times per ps, respectively. This study also proves that the PT at the TPB is primarily via water-mediated surface migration, with the H₃O⁺ migration accounting for most of the contribution, particularly in water-deficient environments, which far exceeds the proton hopping.