A recent article in ACS Applied Polymer Materials explored polymer electrolyte membranes (PEMs) designed for next-generation polymer electrolyte fuel cells (PEFCs). The membranes are based on poly(4-(p-styryl)-1-butanephosphonic acid) (sbPA), featuring phosphonic acid groups on side chains connected via a hydrophobic alkylene spacer. These PEMs were synthesized through monomer polymerization followed by deprotection of phosphonate esters.
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Background
Fuel cells are a promising clean energy technology that generate electrical power through the electrochemical reaction of hydrogen and oxygen. Water is the only byproduct, making them an environmentally friendly option. Polymer electrolyte membranes (PEMs) are a key component in polymer electrolyte fuel cells (PEFCs), where they act as the medium for ion conduction.
Nafion, a commonly used PEM made of perfluorosulfonic acid, has significant environmental concerns. Its high chemical stability prevents degradation, leading to long-term persistence in the environment and bioaccumulation. Furthermore, fuel cells using sulfonic acid-based PEMs require low operating temperatures (below 100 °C) and high humidity to maintain consistent performance, which increases system size and cost.
Polymers containing phosphonic acid groups offer an alternative due to their greater chemical stability and durability. They can operate effectively at higher temperatures and under low-humidity conditions. This study investigates sbPA membranes, which incorporate phosphonic acid groups on side chains, as a potential solution for next-generation PEMs.
Methods
p-(4-Bromobutyl)styrene, a precursor of diethyl 4-(p-styryl)-1-butanephosphonate, was synthesized using a halogen-lithium exchange reaction of p-bromostyrene, followed by nucleophilic substitution. The diethyl 4-(p-styryl)-1-butanephosphonate monomer was then synthesized via the Michaelis–Arbuzov reaction between a phosphite and an alkyl halide.
Poly(diethyl 4-(p-styryl)-1-butanephosphonate) (poly(4-(p-styryl)-1-butanephosphonic acid diethyl ester) (sbPAdE) was synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization of diethyl 4-(p-styryl)-1-butanephosphonate. Finally, sbPA was synthesized by reacting sbPAdE with bromotrimethylsilane, followed by a reaction with methanol to deprotect the ethyl groups from the phosphonate ester.
Synthesized poly(p-styrenephosphonic acid) (sPA) and commercial Selemion HSFN were used as the control samples. The synthesized monomers and polymers were identified using nuclear magnetic resonance (NMR) spectroscopy, while gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) were used for further characterization.
sbPA and sPA membranes were prepared via solution casting into a polypropylene container. The insolubility of these PEMs in water was determined through immersion tests at 60 °C for three hours. Their microscopic structures were analyzed using transmission electron microscopy (TEM) and X-ray scattering. The proton conductivities of the PEMs were measured using alternating current (AC) impedance spectroscopy.
Results and Discussion
The sbPA membrane formed a phase-separated structure, with the hydrophobic regions comprising phenylene groups and alkylene spacers in the polymer side chains and the hydrophilic regions comprising phosphonic acid groups. The glass transition temperature (Tg) of sbPA was 172 °C, compared to 218 °C for sPA. This lower Tg for sbPA reflects higher segmental mobility due to the flexible alkylene spacers.
TGA measurements showed that sbPA exhibited minimal degradation below 180 °C, a temperature well above the working range of next-generation PEFCs (100–150 °C under low humidity). sbPA remained stable up to 400 °C without significant weight loss, indicating resistance to phosphonic acid elimination or decomposition of the polystyrene backbone.
Immersion tests revealed that the sPA membrane completely dissolved in water, whereas the sbPA membrane showed minimal dissolution. Despite a lower density of acid groups compared to sPA, the sbPA membrane demonstrated higher proton conductivities (σDCs), particularly at elevated temperatures and low humidity.
For example, at 120 °C and 20 % relative humidity, sbPA achieved a conductivity of 1.1 mS/cm, which was two orders of magnitude higher than that of the sPA membrane (0.027 mS/cm) and four times higher than the Selemion HSFN membrane (0.26 mS/cm).
The higher σDCs of the sbPA membrane were attributed to its distinct proton conduction mechanism. The flexible alkylene spacers allowed greater freedom of movement for the phosphonic acid groups, enabling protons to travel more efficiently between them, even under low-humidity conditions.
Conclusion
The researchers successfully synthesized a diethyl 4-(p-styryl)-1-The researchers successfully synthesized diethyl 4-(p-styryl)-1-butanephosphonate monomer, which was polymerized into the sbPA polymer featuring phosphonic acid groups on side chains connected via alkylene spacers. Deprotection of the alkyl groups on the phosphonic acid ester completed the synthesis.
The sbPA membrane remained stable, showing no dissolution after immersion in water at 60 °C for three hours. It also exhibited higher proton conductivities than the sPA membrane at 80 and 120 °C, particularly under low-humidity conditions. This improved conductivity was attributed to the efficient proton transport between phosphonic acid groups, facilitated by their direct connection to the polymer backbone through flexible alkylene spacers.
Future work will focus on evaluating the mechanical properties of sbPA membranes. The researchers also plan to blend sbPA with sulfonic acid ionomers to enhance conductivity by preventing dehydration condensation between phosphonic acid groups.
Journal Reference
Nakayama, T., et al. (2024). Polymer Electrolyte Membranes of Polystyrene with Directly Bonded Alkylenephosphonate Groups on the Side Chains. ACS Applied Polymer Materials. DOI: 10.1021/acsapm.4c02688, https://pubs.acs.org/doi/10.1021/acsapm.4c02688
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