Development of composite or reinforced proton exchange membranes for improved durability in fuel cells

Proton exchange membrane fuel cells (PEMFC) are deeply investigated as an integral part of the so-called hydrogen economy: a proposed energetic model centred around hydrogen rather than fossil fuels. Among the drawbacks of this technology much research has gone into understanding the degradation mechanisms shortening the device’s lifetime. The two main causes were identified in the chemical degradation of the polymeric electrolyte caused by radical species, and its mechanical deterioration in the harsh conditions of a fuel cell. To improve on the first issue, the current state-of-the-art solution revolves around the introduction of radical scavenging species in the membrane electrode assembly (MEA) to increase their durability. In this work we propose the use of cerium oxide nanoparticles as radical scavengers coupled with a short side chain ionomer as the polymeric matrix of the membrane: Aquivion®. The novelty lays in the use of organo-silanes bearing perfluorinated chains as grafting agents for the surface of CeO2 to improve the compatibility between the inorganic oxide and the polymeric matrix. We prepared and thoroughly characterised several differently decorated nanoparticles and nanocomposite membranes that were compared to the reference Aquivion® material. The most promising results show a uniform dispersion of the filler in the nanocomposite membranes, a reduction of 50% in the swelling ratio and of the fluoride ions emission from ex situ AST, whilst maintaining conductivity values above 150 mS cm-1 at 80 °C, 100% RH. Most importantly, these samples showed fuel cell properties comparable to, or even above, the reference material; with an almost negligible degradation after >100 h in wet-dry cycling in situ AST OCV-hold measurement were observed. Regarding the mechanical degradation Aquivion® is subjected to, a promising active reinforcement material was studied in the form of triazole-grafted poly(phenilen-oxide). Thie success of the grafting procedure was proven with spectroscopic techniques and the material was electrospun to create a porous reinforcement mat that was thoroughly investigated in its morphology and interaction with the conductive polymer. Different composite membranes were fabricated using two equivalent weight dispersions of Aquivion® and the electrospun PPO-triazole. Their functional properties and fuel cell performances were assesses and compared to the respective non-reinforced counterparts. The result showed an improvement in mechanical response with a decrease of swelling ratio and water uptake. That came at the expenses of slightly lower performances in fuel cell polarization, caused by a decrease in conductivity and a higher high frequency resistance: Nonetheless the reinforced material presented a far lower gas crossover and, most importantly, a reduced degradation when subjected to combined mechanical and chemical in situ AST in the form of wet-dry cycling at OCV.