| Popis: |
Quaternized Polybenzimidazole-Cross-Linked Poly(vinylbenzyl chloride) Membranes and Their Performance in HT-PEMFCs Keywords: Proton-exchange membrane, ion pair, high-temperature, phosphoric acid, quaternary ammonium, hydrogen crossover High temperature proton-exchange membrane fuel cells (HT-PEMFCs) are promising electrochemical energy conversion devices for the hydrogen economy. In this fuel cell type, phosphoric acid is immobilized as an electrolyte within a polybenzimidazole (PBI) membrane acting as a matrix. These membrane systems allow operating temperatures up to 200 °C, which is significantly higher than for sulfonated polymers that are used in low temperatures PEMFCs at around 80 °C. Operating PEMFCs above 100 °C harbors advantages such as faster reaction kinetics, higher tolerances against fuel impurities, and easier cooling. Nonetheless, phosphoric acid doped membranes also is the main challenge and drawback of these systems due to leaching of the dopant over time. A high acid-oping level is desired since it ensures high proton conductivity. However, the mechanical properties of PBI-based materials generally deteriorate upon increasing acid doping levels. In this regard, cross-linking PBI with another polymer is a promising route to enhance the mechanical properties of acid-doped membranes. Further, polymers with specific functional groups, such as quaternary ammonium (QA), can be used as cross-linkers to enhance the retention of phosphoric acid by forming strong interactions with biphosphate anions. Here, we present a new ion-pair-coordinated membrane (IPM) system decorated with QA groups. Poly(vinylbenzyl chloride) is used as a macromolecular cross-linker for PBI, and three different amines (Quinuclidine, Quinuclidinol, DABCO) are used as quaternizing agents. The performance of these membranes is evaluated ex-situ as well as electrochemically within HT-PEMFC operation and compared to a commercial m-PBI membrane (Dapazol). The IPMs show reduced swelling and better mechanical properties upon doping. Further, the commercial reference can be outperformed within HT-PEMFC operation at less acid doping than conventional PBI membranes. The best-performing IPM led to a 25% improved fuel cell performance. The peak power density of an HT-PEMFC incorporating a Dapazol membrane was 430 mW cm–2 at 180 °C under H2/air conditions and at ambient pressure, while the HT-PEMFC with the best-performing IPM yielded 530 cm–2 at equal parameters. Further, the hydrogen gas crossover of the IPMs is similar or less than that of the commercial reference even at lower membrane thicknesses, which renders these membranes as promising candidates for application in HT-PEMFC. |
