Molecular insights into mass transport in proton exchange membrane fuel cell cathode catalyst layers incorporating covalent organic frameworks

Efficient water management and oxygen transport through cathode catalyst-layer ionomer films are essential to proton exchange membrane fuel cell (PEMFC) performance. Here, fully atomistic molecular dynamics simulations were used to examine sulfonated DhaTab covalent organic framework (SDT-COF) as a porous additive to Nafion. Three limiting interfacial configurations were compared over hydration levels λ = 2–15: pure Nafion on Pt (N-Pt), a COF domain adjacent to Pt (NC-Pt), and a Nafion domain adjacent to Pt (CN-Pt). COF incorporation lowered the average ionomer packing density and reorganized water from irregular Nafion-rich aggregates into the ordered COF nanochannels. At low hydration, water was preferentially localized near sulfonate-rich pore walls and its mobility was suppressed; progressive pore filling at intermediate hydration connected the hydrated regions and increased the fitted through-plane water mobility in these finite-film models. Conditional oxygen-displacement and permeation analyses showed that the COF-containing systems generally provided greater oxygen accessibility than pure Nafion. However, the relative accessibility of the Pt-proximal region in NC-Pt and CN-Pt reversed with hydration as water progressively occupied the COF domain. These results show that COF incorporation does not uniformly enhance every transport process; instead, mass transport is jointly governed by hydration and the local COF/Nafion arrangement near Pt.