Composite Nafion® Membranes with Differently Doped Nano-crystalline Yttria-Stabilised-Zirconia (YSZ) For Proton Exchange Membranes Fuel Cells (PEMFCs) Applications in Drastic Conditions

The European Strategy plan for the Energy Technologies foresees that at least 65% of the electric energy should derive from renewable energy sources and CO2 emissions reduced by 50% within 2050. In this context, Polymer Electrolyte Fuel Cells (PEFCs) play an important role, above-all in terms of clean, safe and efficient electric power generation with low environment impact and, hence, independence from fossil fuels. As known, Direct H2-PEFCs (DH-PEFCs) convert the chemical energy of the reaction between H2 and air, respectively fed at anode and cathode sides, in electrical energy, generating water and heat. This occurs thanks to electrochemical reactions at the electrodes and the protons transportation from anode to cathode through the polymeric membrane, considered the core of the PEFC. In order to expand the application field, the research interest has been addressed towards development of High-Temperature PEFCs (HT-PEFCs). The increase of working temperature produces several important benefits, such as the improvement of the catalyst activity, the impurities tolerance and the simplification of the thermal and water management of the system. In drastic conditions, the most commonly used perfluoro-sulphonic membranes (in particular, Nafion®) are subjected to swelling and de-swelling processes that lower its particularly high proton conductivity and mechanical strength with a consequent performance failure. Different solutions have been considered to overcome these obstacles such as the development of membranes based on: i) short-side chain perfluorinated sulphonic acid (SSCA-PFSA) thermo-stable polyaromatic polymers reinforced membranes; iv) organic/inorganic polymer matrices containing hygroscopic and/or proton conductor fillers to develop composite membranes. The aim consists in the enhancing of the water retention and the mechanical properties of the membranes when hydrophilic inorganic oxides, such as SiO2, TiO2, ZrO2 are added and in the improvement of the proton conductivity when proton conductors, such as heteropolyacids (PWA, PMoA, SiWA, etc.) are used to modify the polymer matrix. In particular, inorganic sulphated or phosphated zirconia nanoparticles fillers with acidic functionalities can contribute to the proton conductivity of the membranes, while nano-sized Yttria- Stabilized-Zirconia (YSZ) fillers seem to provide hybrid membranes stable at high temperature with a reduced methanol crossover. It was demonstrated that nano-YSZ becomes a proton conductor below 120°C in water-saturated air and this low-temperature proton conductivity is greatly dependent on the grain size of nanostructured YSZ. YSZ has been investigated as a possible additive to mitigate the degradation by free radicals of the membranes and the interest in such YSZ based-materials as filler for Proton Exchange Membranes (PEMs) has been increased to work at intermediate temperatures <150°C. Hence, composite Nafion® membranes represent a possible solution to operate at temperatures higher than 100°C and low RH in DH-PEFCs and YSZ based-materials have got important properties to be investigated for the aim. For these reasons, in our previous works, a commercial Zirconia stabilised with an 8 mol.% of Yttria, usually used as conductor for Solide Oxide Fuel Cells (SOFCs) at extremely high cell temperature, was used as a filler for Nafion® composite membranes in a single HT-PEFC and also in self-made stack configuration. A 10 wt.% of this commercial filler was found to be the optimal content in terms of performance at high temperature. Starting from evidence that a protonic conduction at low-temperature in nano-YSZ was hypothesized in the actual work, a YSZ filler containing an 8 mol. % of yttrium oxide was synthesised and its ideal content was investig