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