Two-Stage Chemical Degradation of PTFE with H- and HO-Radicals: A DFT Study Under PEMFC Operating Conditions

Abstract

Polytetrafluoroethylene (PTFE) plays a fundamental role in proton exchange membrane fuel cells (PEMFCs) as it enhances the structural integrity of the gas diffusion and microporous layers and, more importantly, helps with water management. However, it is also vulnerable to different kinds of degradation under PEMFC operating conditions. One of the principal degradation mechanisms is chemical degradation involving H- andHO-radicals. In this study, we propose a two-stage degradation mechanism in which the process is initiated by H-radical attack on PTFE and subsequently propagated through reactions with the HO-radicals. To this end, a comparative density functional theory (DFT) study was conducted to investigate six chemical degradation pathways of PTFE via HO-radicals at the ab initio PBE0/def2-TZVPP level. The solvation model based on density (SMD) was employed as an implicit solvation model with water as the solvent to model PEMFC conditions. Reaction enthalpies, Gibbs free energies, and activation free energies were calculated for all of the elementary steps. Among the competing routes, pathways initiated at secondary carbon sites are both thermodynamically and kinetically favored, ultimately resulting in PTFE chain scission and the formation of CO2 and water. These findings provide atomistic insight into the chemical degradation of PTFE and establish a mechanistic foundation for the development of more chemically durable components of PEMFCs while also providing thermodynamic and kinetic data that can support future multiscale degradation models for fuel cell durability.