PERFLUOROALKYL PHOSPHONIC AND PHOSPHINIC ACID ELECTROLYTES FOR PROTON EXCHANGE MEMBRANE FUEL CELLS

Perfluorinated sulfonic acid polymers have been considered as the state-of-art membrane materials for proton exchange membrane fuel cells. A major technical issue with these polymers is that they do not function well at temperatures above 80 °C and at low humidities. Therefore, research has been focused on developing PEM materials that can operate independent of hydration requirements. The operation of fuel cells at high temperature (e.g. 120 oC) increases fuel cell system efficiency due to faster electrode kinetics and better CO tolerance. This dissertation will describe a study of proton-transport rates and mechanisms under anhydrous and aqueous conditions using a series of acid model compounds analogous to comb-branch perfluorinated ionomers functionalized with phosphonic, phosphinic, sulfonic and carboxylic acid protogenic groups. Model compounds were synthesized and characterized with respect to proton conductivity, viscosity, proton and anion (conjugate base) self-diffusion coefficients, and Hammett acidity and as a function of increasing perfluoroalkyl chain length. The highest conductivities and also the highest viscosities were typically observed for the phosphonic and phosphinic acid model compounds. The results of the study collectively supported the hypothesis that anhydrous proton transport in the phosphonic and phosphinic acid model compounds occurs primarily by a structure-diffusion, hopping-based mechanism rather than a vehicle mechanism. Further analysis of ionic conductivity and ion self-diffusion rates using the Nernst-Einstein equation reveals that the phosphonic and phosphinic acid model compounds are relatively highly dissociated even under anhydrous conditions. In contrast, sulfonic and carboxylic acid-based systems exhibit relatively low degrees of dissociation under anhydrous conditions. Investigations of these model acids under low hydration levels (3 mols of acid per a mole of acid) indicate that the proton conductivity of these phosphonic and phosphinic acids can be improved by more than an order of magnitude relative to the water-free conditions. These findings suggest that fluoroalkyl phosphonic and phosphinic acids are good candidates for further development as anhydrous, high temperature proton conductors. The chapter 5 describes the synthesis, characterization and ion transport of lithium polymer electrolytes that resist concentration polarization. Investigation of the purity of the ionic-melt by HPLC analysis and electrospray ionization mass spectrometry indicated that the ionic-melt is free of non-ionic impurities. The highest ionic conductivity of 7.1 × 10−6 S/cm at 30 °C was obtained for the sample consisting of a lithium salt of an arylfluorosulfonimide anion attached to a polyether oligomer with an ethyleneoxide (EO) to lithium ratio of 12. The conductivity order of various ionic melts having different polyether chain lengths suggests that at higher EO:Li ratios the conductivity of the electrolytes at room temperature is determined in part by the amount of crystallization of the polyether portion of the ionic melt. The chapter 6 describes the synthesis and characterization of a new room-temperature ionic liquid based on a alkylimidazolium cation and new fluoro anion [(CF3)2PO2-]. Investigation of its thermal stability, viscosity, voltage window, and conductivity suggests that it may be a useful as an electrolyte in batteries