Perfluoroalkylsulfonic acid (PFSA) ionomers are commonly used as electrolyte membrane in low temperature fuel cells and membrane water electrolyzers. The quest for alternative membranes based on hydrocarbon chemistry has, however, not subsided and these materials continue to be of interest. Their versatile chemistry provides design flexibility and offers the prospect of tailoring membrane chemistry and architecture to specific requirements. Typical hydrocarbon ionomers are thermally stable polymers that contain aromatic units in the main chain, which may be linked by a variety of functional groups (e.g., polysulfones, polyphenylenes). They are of high interest because of their expected lower cost, intrinsically lower gas permeability, and higher glass transition temperatures compared to PFSA membranes [1]. The lower reactant permeability in particular is extremely attractive: it enables the use of thinner membranes, which leads to cost reduction and increased power density. Furthermore, the lower gas permeability leads to a lower radical formation rate through reaction of H₂ and O₂ on the catalyst surface. However, polyaromatic membranes are generally and intrinsically more susceptible to attack and degradation induced by oxidizing radicals [2]. In particular, aromatic units present in most of these polymers react readily and rapidly with radical intermediates, e.g. HO^·, which are formed in the fuel cell in the presence of H₂, O₂ and the noble metal catalyst. Radical attack triggers chain scission and polymer decomposition.

The objective of this study is to investigate, implement and assess strategies to mitigate radical induced degradation of aromatic hydrocarbon based proton exchange membranes for fuel cells. Other than in PFSA membranes, where HO^· radicals have a lifetime on the order of microseconds, their lifetime is expected to be significantly shorter, i.e. ~1 ns, in polyaromatic membranes due to the fast reaction of HO^· with the aromatic units. Therefore, scavenging of HO^· using cerium-ions or ceria nanoparticles is not likely to be an effective approach. Alternative radical scavengers, such as hindered phenols, which are also used as antioxidants in plastics, may achieve a higher scavenging yield, yet in order to sustain stabilization over thousands of hours, the radical quenching needs to be a catalytic process, as it is the case with the redox cycling ability of Ce and Mn ions used as antioxidants in PFSA ionomers [3].

An alternative approach is to target the intermediates formed upon attack of the ionomer by HO^·. Some intermediates, such as aromatic radical cations, may be sufficiently long lived for suitable repair chemistry to take effect [4]. In this study, we present a kinetic study of possible pathways for damage transfer and ionomer repair reactions and highlight requirements for polymer intermediate lifetimes and kinetics of repair and regeneration reactions of additives. One challenge is to stabilize the ionomer intermediates sufficiently long (milliseconds range) to allow repair reactions to take place. Another challenge is to ensure the kinetics of repair reactions are sufficiently fast and the additive can be repeatedly regenerated. Evidently, the chemistry of the polymer and the additive needs to be adequately tuned and side-reactions minimized. Furthermore, additives incorporated into the membrane merely by doping may be leached out over time, calling for strategies to tether the functional motifs to the polymer chain or an inorganic filler for immobilization.

(Figure: Schematic representation of an antioxidant strategy for hydrocarbon based proton exchange membranes based on the ‘damage transfer’ and ‘repair’ concept. the ionomer P is attacked by HO^·, the damage inflicted to the polymer is then transferred to an additive (M, mediator), which undergoes a subsequent repair reaction with H₂O₂.)

References:

1. F. Wang, M. Hickner, Y. S. Kim, T. A. Zawodzinski, J. E. McGrath, J. Membr. Sci., 197, 231 (2002).
2. L. Gubler, W.H. Koppenol, in: The Chemistry of Membranes Used in Fuel Cells: Degradation and Stabilization, S. Schlick (Ed.), 107-138, John Wiley & Sons (2018).
3. F. D. Coms, H. Liu, J. E. Owejan, ECS Trans., 16, 1735 (2008).
4. L. Gubler, T. Nauser, F.D. Coms, Y.H. Lai, C.S. Gittleman, J. Electrochem. Soc., 165(6), F3100 (2018).