Understanding Phosphoric Acid Dynamics in High-Temperature Pemfcs: Mechanisms and Durability of PA-PBI and Ion-Pair-Based Systems.

High-Temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) can operate at > 100 o C under anhydrous conditions, providing benefits such as system simplification and superior contaminant tolerance. Most researches on HT-PEMFC have focused on PA-PBI-based system since 1995, however the durability of this system is still on controversial. Some insisted the stable operation of PA-PBI HT-PEMFC, which showed the stability under mild conditions such as low current density about 0.2 A cm -2 , while others pointed out the instability specifically under accelerated stress conditions with higher current densities above 0.6 A cm -2 , lower ( 180 oC) operating temperatures, or dynamic protocols like start-up/shut-down cycles, etc. This limited durability of PA-PBI HT-PEMFC is known to be related to acid loss from the membrane-electrode-assemblies (MEAs). Several mechanisms were proposed to explain the linear and nonlinear acid loss behaviors. PA evaporation and steam-distillation are known to explain the constant PA loss. Despite the extensive efforts on mechanisms and acid loss mitigation strategies, current PA-PBI systems still struggle against the limited durability due to PA loss. To improve the durability of HT-PEMFC, a novel system was introduced based on the quaternary ammonium-biphosphate ion-pair coordinated PEM 1. Due to a stronger interaction in the ion-pair PEMs, the durability was also improved by alleviating the PA loss during PEMFC operation. Nonetheless, the acid retention mechanism of this novel system is not explored yet. Here, we investigate the mechanisms of PA loss and PA retention within electrodes of both PA-PBI-based and ion-pair-based HT-PEMFC systems. Through PA energetics studies and experimental investigations under controlled stress conditions, we elucidate distinct PA loss/retention mechanisms in these systems, leading to different durability behaviors. Our findings offer insights for enhancing the durability of both HT-PEMFC systems 2. 1 Lim, K. H.; Lee, A. S.; Atanasov, V.; Kerres, J.; Park, E. J.; Adhikari, S.; Maurya, S.; Manriquez, L. D.; Jung, J.; Fujimoto, C.; Matanovic, I.; Jankovic, J.; Hu, Z.; Jia, H.; Kim, Y. S. Protonated phosphonic acid electrodes for high power heavy-duty vehicle fuel cells, Nature Energy , 7, 248 (2022). 2 Lim, K. H.; Matanovic, I.; Maurya, S.; Kim, Y.; Castro, E. S.; Jang, J-H.; Park, H.; Kim, Y. S. High temperature polymer electrolyte membrane fuel cells with high phosphoric acid retention, ACS Energy Letters , 8, 529 (2023).