Chemical Stability Enhancement of Aromatic Proton Exchange Membranes Using a Damage Repair Mechanism

Abstract

Hydrocarbon-based fuel cell membranes are susceptible to radical-induced degradation. Such polymers commonly contain aromatic units, which react rapidly with HO·. Antioxidant strategies therefore need to focus on repairing intermediates formed upon radical attack to mitigate irreversible degradation. We summarize our recent ex situ g-irradiation studies on an aromatic model compound and in situ fuel cell tests of radiation grafted membranes with a polymer-bound crown ether-cerium complex to elucidate its antioxidant mechanism. Cerium is effective in reducing aromatic hydrocarbon membrane degradation and likely follows a catalytic mode of action.

A Nature‐Inspired Antioxidant Strategy based on Porphyrin for Aromatic Hydrocarbon Containing Fuel Cell Membranes**

“…A high TON indicates catalytic repair. In comparison to our previous work, [40] where a TON of ~25 was estimated for the repair by Ce(III), the comparatively higher TON found in the present study suggests a higher efficiency for the Cu(II)−porphyrin as an antioxidant. The higher TON and improved stability, i. e., better retention of the antioxidant during FC operation, are apparent merits of the porphyrin‐based approach.…”

“…The fuel cell experiments reported here were carried out under accelerated chemical degradation conditions, for which we previously estimated a rate of HO⋅ formation of 2.6 ⋅ 10 −6 M ⋅ s −1 [40] . Under nominal fuel cell operating conditions, the HO⋅ formation rate is estimated to be 10 −8 to 10 −7 M ⋅ s −1 (see Supporting Information).…”

“…The fuel cell experiments reported here were carried out under accelerated chemical degradation conditions, for which we previously estimated a rate of HO * formation of 2.6 • 10 À 6 M • s À 1 . [40] Under nominal fuel cell operating conditions, the HO * formation rate is estimated to be 10 À 8 to 10 À 7 M • s À 1 (see Supporting Information). If we target a membrane-lifetime of 25'000 h, [41] and assume a concentration of the repair agent of 1 %, i. e., ~0.01 M, then a TON of 80 to 800 would be required.…”

Radical Attack and Damage Mitigation in Hydrocarbon-Based Ionomers

“…Two different types of repair agent were introduced into radiation grafted proton exchange membranes based on ETFE film of 25 µm thickness, prepared using a monomer mixture of α-methylstyrene (AMS), 2-methyleneglutaronitrile (MGN), and vinylbenzyl chloride (VBC) (Figure 5). In the first case, an 1-aza-15-crown-5 ether Ce(III) complex was chemically attached to the grafted chain through reaction with the VBC, leading to a Ce(III) content of 2.3% with respect to the content of sulfonic acid groups (10,13). In the second case a Cu(II)-porphyrin was covalently attached to the VBC unit of the polymer (24).…”

“…These compounds are intermediates and undergo further reactions, leading to irreversible damage of a sulfonated oligomer or polymer, such as chain scission, crosslinking, ring hydroxylation, ring opening, or desulfonation (9). • Ar-OH and Ar •+ are in rapid equilibrium, and the concentration ratio of the two intermediates depends on the electron density of the ring and the pH (10). A high electron density and a low pH pushes the equilibrium towards Ar •+ .…”