Nanocomposite Membranes Based on PBI and ZrO2 for H T-P E M F C s

As of today, one of the most important challenges faced by mankind is the extensive restructuring of the entire energy system at the global level. In particular, the emissions of greenhouse gases must be reduced to mitigate the climate change. The exploitation of hydrogen as an energy vector represents a very promising approach to address these issues. Fuel cells (FCs) are able to convert the chemical energy stored in hydrogen into electrical energy with a very high efficiency, up to two-three times higher in comparison with traditional internal combustion engines (ICEs), and when fed with hydrogen do not produce greenhouse gas emissions at the point of operation. Despite these attractive features, FCs do not experience a widespread market penetration yet owing to a variety of drawbacks including expensive functional materials, complex and/or bulky power plants and an insufficient durability.

Among the FC families, high-temperature proton exchange membrane fuel cells (HT-PEMFCs) show great promise to provide a viable solution to the shortcomings mentioned above. HT-PEMFCs operate at a high temperature, 120 < T < 250°C; in these conditions, the electrocatalysts are not poisoned easily by the most common contaminants found in the reactant streams (e.g., CO in the H₂ fuel). Furthermore, HT-PEMFCs do not require external humidification. Consequently, HT-PEMFC power plants are relatively simple to engineer and do not require bulky and expensive heat and water management modules. In summary, HT-PEMFCs can be very compact, resulting particularly suitable for application in the automotive sector.

The state of the art of electrolyte membranes for application in HT-PEMFCs consists in a polymer characterized by a high thermal and chemical stability such as polybenzimidaziole (PBI), which is doped with H₃PO₄ to bestow to the membrane a high proton conductivity at high temperatures and in anhydrous conditions. In this work, a new family of hybrid inorganic-organic proton-exchange membranes is developed, based on PBI and nanometric ZrO₂ with formula PBI/(ZrO₂)_x with x ranging from 0.7 to 16 wt%. ZrO₂ nanoparticles (NPs) are chosen as the filler for their high chemical stability in an acid environment and for the ZrO₂ – PBI interactions in membranes. This feature is expected to give rise to strong interactions between the different components constituting the final hybrid inorganic-organic membranes (i.e., PBI, H₃PO₄ and ZrO₂), thus improving their conductivity, thermal and mechanical properties.

The membranes are obtained by solvent-casting processes, and undergo an extensive characterization both in a completely dry state and after doping with H₃PO₄. ICP-AES and microanalysis are used to determine the chemical composition of the membranes; HR-TG is adopted to study their thermal stability, while the thermal transitions are investigated by DSC. The structure of the proposed hybrid inorganic-organic nanocomposites is studied by FT-MIR ATR vibrational spectroscopy; the electric behavior of the samples is characterized in detail by broadband electrical spectroscopy (BES) in the 5 – 190°C and 1 – 10⁷ temperature and frequency ranges, respectively.

It is observed that, with respect to pristine PBI, in the hybrid membranes the condensation of H₃PO₄ to H₄P₂O₇ is brought to higher temperatures. Furthermore, the conductivity at 190°C of the membrane including 10 wt% of ZrO₂ is higher in comparison with pristine PBI (4.65·10^-2 S/cm and 4.46·10^-2 S/cm, respectively). The integration of the results allows to shed light on the complex interplay between the structural features, the thermal properties and the electrical response of this family of hybrid inorganic-organic proton conducting membranes.