Hybrid inorganic-organic nanocomposite membranes for HT-PEMFCs based on PBI and HfO2: Preparation, characterization and study of electrical properties and conduction mechanism

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) are an innovative family of highly efficient and environmentally-friendly energy conversion devices operating at 120 < T < 250°C. HT-PEMFCs are able to address several of the drawbacks of conventional PEMFCs based on traditional perfluorinated ionomers (e.g., Nafion™) and functioning at lower temperatures. In particular, HT-PEMFCs do not require very pure reactants, nor humidified reagent streams; furthermore, the thermal management of the power plant is relatively easy. Consequently, HT-PEMFCs are compact and their engineering is generally straightforward; thus, they are suitable for a variety of applications, with a particular reference to the automotive sector. HT-PEMFCs use as the electrolyte a polymeric membrane characterized by a high thermal and chemical stability (e.g., polybenzimidazole, PBI). The latter is swollen with a suitable proton-conducting medium, usually H3PO4. The resulting system is capable of efficient proton transport in the typical operating conditions of HT-PEMFCs. In this work, innovative hybrid inorganic-organic membranes for application in HT-PEMFCs are developed. The membranes consist of PBI including between 0 and 35 wt% of nanometric HfO2 and are obtained by solvent-casting process. HfO2 is used as the nanofiller for its remarkable chemical and electrochemical stability and for the basic character of its surface. In particular, this latter feature is expected to play a crucial role in the development of interactions with the other components of the hybrid membranes (i.e., PBI and H3PO4), leading to an improved thermal stability and proton conductivity. The essay of Hf in the hybrid membranes is determined by ICP-AES; as the concentration of HfO2 is raised beyond 10 wt%, the two faces of the membranes become different since the nanofiller decants to the bottom side during the solvent-casting. The thermal stability and transitions of the samples are investigated by HR-TG and DSC, respectively; FT-MIR ATR vibrational spectroscopy is carried out on both sides of the membranes to probe the structure of the nanocomposites. Finally, the electric response is measured by broadband electrical spectroscopy (BES) in the 5 - 190°C and 1 – 107 temperature and frequency ranges, respectively. All the studies are carried out both on completely dry membranes, which are manipulated under inert atmosphere to prevent contamination from atmospheric moisture, and on membranes swollen with H3PO4. Results witness that, with respect to the pristine PBI reference, the introduction of the HfO2 nanofiller: (a) improves the thermal stability of the hybrid membranes and inhibits the high-temperature condensation of H3PO4 to form H4P2O7; and (b) increases the electrical conductivity by a factor of ca. 1.5-2, up to a value of 6.6•10-2 S/cm for the membrane including 11 wt% of filler. These features make the proposed hybrid membranes promising candidates for application in HT-PEMFCs.