Nanocomposite Membranes Based on PBI and ZrO2 for HT-PEMFCs

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, and do not produce greenhouse gas emissions. 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, HTPEMFCs do not require external humidification. 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. In this work, a new family of hybrid inorganic-organic PEM is developed, based on PBI and nanometric ZrO2 with formula PBI/(ZrO2)x with x ranging from 0.7 to 16 wt%. ZrO2 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. 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 membrane is studied by FT-MIR ATR vibrational spectroscopy; the electric behavior is characterized by broadband electrical spectroscopy in the 5 – 190°C and 1 – 106 Hz 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. Acknowledgements. The authors thank the Strategic Project “From materials for Membrane electrode Assemblies to electric Energy conversion and SToRAge devices” (MAESTRA) of the University of Padova for funding this activity.