ReaxFF reveals why hydrogen radicals accelerate PFSA membrane failure

Perfluorosulfonic acid (PFSA) membranes are central to proton exchange membrane fuel cells, but their long-term durability is limited by radical-driven chemical degradation. Experiments can identify fluoride release and degraded fragments, but the atomistic sequence of bond breaking, radical conversion, and membrane failure is difficult to resolve directly.

A recent study used ReaxFF molecular dynamics in the Amsterdam Modeling Suite to follow reactive degradation of hydrated PFSA membrane models under hydroxyl radical, hydrogen radical, mixed radical, and radical-free conditions. The simulations captured bond breaking and formation in large polymer systems, while ADF calculations provided energetic support for key elementary reactions.

The simulations show that H· radicals are especially destructive because they abstract fluorine from –CF₂– backbone units and attack –COOH end groups, initiating unzipping, defluorination, HF formation, and chain scission. In contrast, ·OH radicals mainly attack –SO₃H groups and tertiary carbon sites, causing side-chain damage with much less backbone disruption.

A key insight is the synergy in mixed radical environments. ·OH can help regenerate H· radicals, sustaining backbone attack and accelerating failure beyond what isolated radical pathways would suggest. ReaxFF-MD made it possible to connect this chemistry to morphological changes, from early microvoids to percolating damage zones, rather than only observing final degradation products.

The work also highlights a design trade-off. Polar groups such as –SO₃H and –COOH support proton transport, but they also create vulnerable sites for radical attack. Selective passivation of –COOH end groups reduced HF formation while largely retaining proton conductivity, suggesting a practical route toward more durable PFSA membranes for fuel cell R&D.