(590e) Designing Ion-Conducting Materials for High-Temperature Fuel Cell Applications Using Low-Cost, Biorenewable Lignin

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) have many benefits, such as faster electrode kinetics, high tolerance to fuel impurities, and simplified water management. Acid-doped polybenzimidazole (PBI) membranes are promising candidates for HT-PEMFCs due to high proton conductivity at elevated temperatures and good chemical and thermal stability. However, PEMFCs operated at high temperature suffer from leaching of doped acid due to weak acid-base interaction between PBI and phosphoric acid (PA) affecting the membrane performance. Several approaches have been adopted to improve PA retention within the PBI membrane matrices. In these efforts, quaternary ammonium cation-containing species appear to be ideal candidates for their favorable interactions with biphosphate anions (of PA) preventing the leaching of PA. This talk will show our efforts in designing cationic version of lignin to improve the PA capture and holding within the PBI membranes. Lignin is the second most naturally abundant polymer (next to cellulose). Lignin is also a by-product of pulp and paper industries, cellulosic biorefineries, and agricultural farms. However, most cases, this biomass remains unvalorized and thus wasted, whereas the three-dimensional, hyperbranched structure of lignin can favor the formation of unique ion conduction pathways. By functionalizing neutral lignin with cationic quaternary ammonium groups and incorporating cationic lignin within PBI membrane matrices, we were able to improve the PA retention by the composite PBI membranes which in turn improved the proton conductivity of the PBI membranes even at high temperature. Additionally, the cycling experiments showed that the cationic lignin-PBI-based composite membranes retained the high protonic conductivity at high temperature (140 °C) over an extended period of time (240 h). The stable and high conductivity thus suggested the effectiveness of cationic lignin in boosting the ion transport performance of high temperature PEMFCs in a green manner.