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

• ORR on Pt electrode was evaluated in various fluorohydrogenate ionic liquids.
• The largest kinetically limited current was observed in EMPyr(FH)1.7F.
• Low yield of H2O2 (about 1.9%) was observed in EMPyr(FH)1.7F.
• Diffusion coefficients and solubilities of oxygen in various ionic liquids were determined.
• The estimated crossover currents of oxygen were small enough for practical use.

Various parameters related to the oxygen reduction reaction (ORR) on a Pt electrode such as kinetically limited current density (jk), yield of H2O2 (XH2O2XH2O2), 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)1.3F and EMIm(FH)2.3F), N-ethyl-N-methyl pyrrolidinium fluorohydrogenates (EMPyr(FH)1.7F and EMPyr(FH)2.3F), trimethylsulfonium fluorohydrogenate (S111(FH)1.9F), triethyl-n-pentylphosphonium fluorohydrogenate (P2225(FH)2.1F), and 5-azoniaspiro[4.4]nonane fluorohydrogenate (AS[4.4](FH)2.0F). Among them, EMPyr(FH)1.7F showed the largest jk value (−1.5 mA cm−2) at 0.7 V vs. RHE. EMPyr(FH)1.7F, EMPyr(FH)2.3F, and P2225(FH)2.1F showed small XH2O2XH2O2 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) × 10−5 cm2 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.