Phosphoric Acid Loss Mechanism of Acid-Base and Ion-Pair Coordinated Proton Exchange Membranes

Acid retention of phosphoric acid-doped proton exchange membranes (PEMs) is one of the most critical factors that determine the performance and durability of high-temperature PEM fuel cells. However, the mechanism of phosphoric acid loss in the PEMs in the presence of water has not been discussed in terms of the energetics of phosphoric acid. In this study, we investigate the interactions of the phosphoric acid cluster by 31P NMR experiments and density functional theory calculations to propose new phosphoric acid loss mechanism of the phosphoric acid-polybenzimidazole PEM and phosphate-quaternary ammonium ion-pair PEM. The small molecule study suggests that the removal of the phosphoric acid from the phosphoric acid cluster does not occur due to the stronger interaction of phosphoric acid and water. On the contrary, adding water molecules to the PA cluster increases the cluster interaction energy by creating hydrogen bonding. In the phosphoric acid incorporated PEMs, we observed a consistent result that adding water increases the PA cluster volume rather than losing phosphoric acid. However, with excessive water activity, the polymer loses phosphoric acid due to the incapability to hold the water and phosphoric acid beyond a certain level.1 In the phosphoric acid loss process, the equilibrium phosphoric acid composition in the cluster plays a critical role. In the latter part of this presentation, we explain how the acid-base interaction and ion-pair interaction impact the equilibrium phosphoric acid composition the phosphoric acid cluster. We also explain how the equilibrium phosphoric acid composition affects the proton conductivity at different RH level. Finally, we demonstrate the high-temperature membrane fuel cell durability in the presence of water to prove the newly proposed acid loss mechanism. This presentation provides critical insights to design high-temperature proton exchange membranes. Reference 1. A. S. Lee, Y. -K. Choe, I. Matanovic, Y. S. Kim, The energetic of phosphoric acid interactions reveals a new acid loss mechanism, J. Mater. Chem. A. DOI: 10.1039/C9TA01756A (2019).