Advancing ionomer design to boost interfacial and thin-film proton conductivity via styrene-calix[4]arene-based ionomers

Sub-micrometer-thick ion-conducting polymer (ionomer) layers often suffer from poor ionic conductivity at the substrate/catalyst interface. The weak proton conductivity makes the electrochemical reaction at the cathode of proton-exchange-membrane fuel cells sluggish. To address this, here we report on a class of polysty-rene-based ionomers having sub-nanometer-sized, sulfonated macrocyclic calix[4]arene-based pendants (PS-calix). In films with thickness comparable to that of ionomer-based binder layers, the conductivity of PS-calix film (-41 mS/cm) is X13 times higher than that of the current state-of-the-art ionomer, Nafion. We observe a similar improvement in proton conductivity when PS-calix interfaces with Pt nanoparticles, demonstrating the potential of PS-calix in catalyst ink. Leveraging a favorable interfacial chem-ical composition, PS-calix enhances proton conduction at the film -substrate interface, a shortcoming of Nafion. Moreover, the water in PS-calix films diffuses faster than bulk water and the water confined in Nafion films, suggesting an important role played by sub-nanometer-sized calix[4]arene cavities in creating unique water/ion transport pathways.

Automated summary

In this study, a new class of polystyrene-based ionomers with sub-nanometer-sized, sulfonated macrocyclic calix[4]arene-based pendants (PS-calix) was reported. The conductivity of the PS-calix film was found to be 13 times higher than that of the state-of-the-art ionomer, Nafion. PS-calix also showed improved proton conductivity when interfacing with Pt nanoparticles, indicating its potential in catalyst ink. The sub-nanometer-sized calix[4]arene cavities in PS-calix created unique water/ion transport pathways, resulting in faster water diffusion compared to bulk water and water confined in Nafion films.