Phosphoric Acid-Doped Biphenyl-Backbone Ion-Pair Coordinated Pems with Broad Relative Humidity Tolerance

Abstract

Fuel cell has been recognized as a promising electrochemical device attributed to its high energy efficiency and zero CO2 emission. As an essential component of fuel cell, currently the most widely used proton exchange membrane (PEM), Nafion®, operates only at low temperature (<100 ˚C) and high relative humidity (>80% RH). Although phosphoric acid (PA)-doped polybenzimidazole membranes have been reported to offer good proton conductivity at temperatures 140–180 ˚C, they suffer from severe performance degradation due to acid leaching when moisture is present. For wider adoption of PEM fuel cell technology, the development of PEM that is capable of working over a broad operating condition window is highly desirable. In this presentation, three PA-doped biphenyl-backbone ion-pair coordinated PEMs with different quaternary ammonium structures (alkyltrimethylammonium, piperidinium, and 1,2-dimethylimidazolium) were reported. The resulting PA-doped PEMs were comprehensively characterized in terms of PA doping level, proton conductivity, RH tolerance, thermal stability, and mechanical properties. Acid titration result indicates the PA doping levels of three membrane samples were between six and eight, and the size and structure of cation groups of the ion-pair polymers were found to affect their PA doping levels. Proton conductivity was studied as a function of RH over a wide range of 5% to 95% RH. Stable conductivity was observed up to 70% RH at 80 ˚C for 10 h. A combination of high proton conductivity at low RH conditions and good humidity tolerance makes this new class of PEMs great potential candidates for use in electrochemical devices such as PEM fuel cell and electrochemical hydrogen compressor.