Aquivion®-based composite proton exchange membranes containing cerium oxide nanoparticles as radical scavengers with perfluoroalkylic-silanes as surface decorations.

Polymeric proton exchange membrane fuel cells offer a feasible solution to decarbonise transportation, especially in heavy duty vehicles. The main factors hindering their commercialisation are identified in the cost, the durability and the performance. To tackle these issues various approaches are being investigated[1]. A great effort has gone into limiting the degradation of the catalyst layer and the membrane[2] during operation. The latter is degraded both mechanically and chemically. Mechanical degradation is caused by the combined effect of the compression the membrane electrode assembly is subjected to in the cell, and the membrane dimensional changes caused by its swelling and shrinking during operation[3]. On the other hand, radical species are known to chemically degrade the polymer[4]. Mitigation strategies for both issues generally revolve around the introduction of active fillers or reinforcement materials[5],[6],[7]. In this work we propose to improve on the compatibility between a well known and widely used inorganic filler species that acts as radical scavenger, CeO2, and the organic matrix of the short side chain perluorosulforic polymer used, Aquivion® to fabricate beyond state-of-the-art nanocomposite PEMs. To do so we decorated the surface of the ceria nanoparticles with different fluoroalkaly chains using a silane grafting reaction. This results in an increased affinity between the now partially hydrophobic surface of the oxide and the polymer’s backbone, which helps in obtaining a homogeneous dispersion in the final membrane; as shown by SEM-EDX imaging. Furthermore, the filler is now expected to be more likely forced into the hydrophobic domain of the ionomer, resulting in a lower detrimental effect on the interconnection of the ionic domains. In turn, this causes a lower reduction of the membrane’s conductivity, which is expected when introducing a non-conductive species in the membrane, especially at higher loading; as is demonstrated with the conductivity measurements and in fuel cell measurements (high frequency resistance electrochemical impedance spectroscopy and polarization curve). Last but not least, the mechanical properties of the nanocomposite membranes are superior to the pristine Aqiuvion® as is indirectly proved by the water uptake and swelling ratio studies and measured with the stress-strain curves. At the same time the radical scavenging efficacy of cerium oxide is maintained, despite the partial surface coverage with the silanes; as is validated both via Fenton test and as a post-mortem fluoride emission rate analysis on a wet-dry accelerated stress test.