Antioxidant strategies for non-fluorinated aromatic proton exchange membranes for hydrogen fuel cells

Abstract

Proton exchange membranes are used in low temperature fuel cells and serve three important functions: conducting protons, providing electrical insulation, and serving as a gas barrier. Currently, most commercial fuel cell membranes are made from perfluorinated polymers such as perfluoroalkylsulfonic acid (PFSA) ionomer. These polymers work well in fuel cells due to their high chemical stability and good performance. However, the EU has announced that non-essential fluorinated materials may be phased out, which could affect fuel cell membrane production. Therefore, there is a need for alternative, environmentally friendly materials. An alternative group of membranes, aromatic hydrocarbon-based membranes, have shown encouraging fuel cell performance. A major drawback of these membranes is that they are susceptible to oxidative degradation during fuel cell operation. Of the reactive species formed in the fuel cell, HO• was identified to be the most harmful. Radical attacks lead to deleterious reactions that ultimately cause reduced mechanical strength and premature cell failure. Previous kinetics studies, performed at PSI and ETH, have shown that utilising Ce(III) as a repair agent may mitigate membrane damage by preventing irreversible degradation. This aim of this thesis is to promote the understanding of radical-induced chemical degradation of aromatic hydrocarbon-based membranes. This is achieved by studying model compounds and several differently modified membranes. The importance of developing a targeted antioxidant strategy is highlighted by contrasting to strategies used for perfluorinated materials. The final objective of this work is to develop a repair mechanism that could be applied to aromatic hydrocarbon ionomers generically. This would allow for the production of hydrocarbon membranes that could compete with perfluorinated membranes. The investigation began by building a kinetics framework measuring the temperature dependence of degradation and repair reactions. For this study a model compound, poly(α-methylstyrenesulfonate), and Ce(III) were used. Pulse radiolysis experiments were conducted where radical species, mainly HO•, are generated and reactions are followed with time-resolved spectroscopy. Furthermore, Ce(III) regeneration reactions were investigated using stopped-flow experiments. Results from kinetics measurements were integrated into a model of the molecular reaction mechanisms. Upon reaction of HO• with the model compound, aromatic cation radicals are formed, which are relatively long-lived radical intermediates. It was found that repair of the radical cation intermediate by Ce(III) was faster than self-decay of the intermediate, both at room temperature and 80°C. This allows for an effective repair strategy where the complete regeneration of Ce(III) is feasible through the reduction of formed Ce(IV) by H2O2. Furthermore, the effect of molecular weight and ionic strength on the formation and decay of the cation radical was investigated. At higher molecular weights and ionic strengths, the oligomers are more confined and both the formation and decay reactions are faster. Mn(II) was also investigated as a catalytic repair agent. Results proved that Mn(II) can also reduce the cation radical, however, the rate of repair was approximately one order of magnitude slower than for Ce(III). To test whether this repair mechanism is viable in fuel cell conditions, Ce(III) crown ether complexes were covalently attached to the polymer backbone of a grafted membrane. Cerium-containing membranes showed significantly slower degradation than reference membranes in fuel cell accelerated stress tests. This result was confirmed by post-test titration and FTIR analysis, indicating that Ce(III) could be utilised as a repair agent in fuel cell membranes. Further kinetics and fuel cell testing with the same experimental framework as for the Ce(III) investigation were performed on Cu(II)-porphyrin containing membranes. The use of Cu(II)-porphyrins was inspired by hemes. Hemes are a group of chemicals found in biological systems capable of handling highly oxidizing species and thus, are resistant to irreversible oxidative damage. It was found from kinetics experiments that damage can likely be transferred from the cation radical to Cu(II)-porphyrin complexes. A model compound, t-butylmethoxyphenyl sulfonate, was irradiated in the presence and absence of Cu(II)-porphyrins. When Cu(II)-porphyrins were present, there was a systematically lower rate of degradation. Fuel cell tests and post-test analysis of Cu(II)-porphyrin-containing membranes and an analogous membrane without Cu(II) were performed. Cu(II) containing membranes showed increased resistance to radical-induced degradation over a few hundred hours of testing. Post-test results revealed the turnover number of Cu(II)-porphyrin to be at least 35, indicating that the repair mechanism is likely catalytic.