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)nF, n = 1∼3) as electrolytes for fuel cells. Solubility and diffusion coefficient of oxygen in those PILs were determined from 30 °C to 120 °C by the combination measurement of linear-sweep voltammetry and hydrodynamic chronocoulometry 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 × 10−6 cm2 s−1 at 120 °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® in water to discuss oxygen cross-over in fuel cell application.