Solubility and Diffusion Coefficient of Oxygen in Protic Ionic Liquids with Different Fluoroalkyl Chain Lengths

A series of protic ionic liquids (PILs) were synthesized from N,N-diethylmethylamine (dema) and three kinds of fluoroalkylsulfonic acids having different chain lengths (H-SO3(CF2)(n)F, n = 1 similar to 3) as electrolytes for fuel cells. Solubility and diffusion coefficient of oxygen in those PILs were determined from 30 degrees C to 120 degrees C by the combination measurement of linear-sweep voltammetry and hydrodynamic chrono-coulometry with a rotating disk electrode (RDE). The solubility and diffusion coefficient of oxygen in N,N-diethylmethylammonium heptafluoropropanesulfonate ([dema][HfO]) was 10.5 mmol dm(-3) and 1.48 x 10(-6) cm(2) s(-1) at 120 degrees C, respectively. With increasing the fluoroalkyl chain length of anion, the solubility increased. In contrast, the diffusion coefficient decreased. The activation energies of oxygen diffusion in N,N-diethylmethylammonium pentafluoroethanesulfonate ([dema][PfO]) and [dema][HfO] were estimated to be 35.5 kJ mol(-1) and 38.4 kJ mol(-1) from the Arrhenius plots, respectively. The permeabilities of oxygen in the synthesized PILs were also estimated and compared with Nafion (R) in water to discuss oxygen cross-over in fuel cell application. (C) 2014 Elsevier Ltd. All rights reserved.