Influence of cationic structures on oxygen reduction reaction at Pt electrode in fluorohydrogenate ionic liquids

Various parameters related to the oxygen reduction reaction (ORR) on a Pt electrode such as kinetically limited current density (j(k)), yield of H2O2 (XH2O2), and solubility (C) and diffusion coefficient (D) of oxygen were evaluated at 298 K in fluorohydrogenate ionic liquids (FHILs) using the rotating ring-disk electrode (RRDE) method. The FHILs investigated in this study were 1-ethyl-3-methylimidazolium fluorohydrogenates (EMIm(FH)(1.3)F and EMIm(FH)(2.3)F), N-ethyl-N-methyl pyrrolidinium fluorohydrogenates (EMPyr(FH)(1.7)F and EMPyr(FH)(2.3)F), trimethylsulfonium fluorohydrogenate (S-111(FH)(1.9)F), triethyl-n-pentylphosphonium fluorohydrogenate (P-2225(FH)(2.1)F), and 5-azoniaspiro[4.4]nonane fluorohydrogenate (AS[4.4](FH)(2.0)F). Among them, EMPyr(FH)(1.7)F showed the largest jk value (-1.5 mA cm(-2)) at 0.7 V vs. RHE. EMPyr(FH)(1.7)F, EMPyr(FH)(2.3)F and P-2225(FH)(2.1)F showed small X-H2O2 values around 1.5% at a disk electrode potential of 0.2 V vs. RHE. The C and D values obtained for the FHILs were in the ranges of 0.23-1.3 mmol dm(-3) and (1.1-3.2) x 10(-5) cm(2) s(-1), respectively. The crossover currents of oxygen in the FHILs were estimated using the obtained C and D values, which were of the same order of magnitude as that for a 0.5 M H2SO4 aqueous solution. (C) 2014 Elsevier B.V. All rights reserved.