Proton exchange membrane fuel cells (PEMFCs) are one of the most promising environmentally friendly energy conversion devices. The high temperature PEMFC system requires simplified heat and water management, which would provide substantial cost benefits and high potential for commercialization. Phosphonated polyaromatic electrolytes are promising candidates as proton conductors at high temperatures due to their ability to conduct protons under the full range of humidity levels and their covalently bound phosphonic acid structure, which eliminates the risk of leaching of the acid group at high humidity conditions. Despite their synthesis of research for more than decades, only few successful examples of phosphonated polymers in the PEMFCs have been reported.

Here we present the new synthesis of phosphonic acid-functionalized polyaromatic electrolytes as electrode ionomers of high temperature PEMFCs. The polyaromatic electrolytes were synthesized by acid-catalyzed Friedel-Crafts polycondensation and followed by the S_N^Ar Michaelis-Arbuzov reaction of aryl pentafluorides. The electrolytes with the ion exchange capacity ranged 1.9-2.2 meq/g have good solubility in polar aprotic solvents, which eases the fabrication of materials as electrode binders. We could obtain the peak power density up to 700 mW/cm² at current density of 2.1 A/cm² at 200 °C with the membrane electrode assembly using the phosphonic acid functionalized ionomer. We will compare the fuel cell performance to that of ion-pair coordinated electrode binders¹ and discuss the high applicability of the phosphonated polymer electrolytes in high temperature systems.

Reference

1. K.-S. Lee, J.S. Spendelow, Y.K. Choe, C. Fujimoto, Y. S. Kim, Nature Energy, 2016, 1, 16120-16124.