Electrochemical Analysis of Oxygen Reduction in Phosphonium Ionic Liquids Based on Rotating Ring-Disk Electrode Method

Yamada, Hirohisa

Ionic liquids (ILs) have characteristic physicochemical properties such as high thermal stability, low volatility, low melting point, and flame resistance. Moreover, ILs exhibit also excellent electrochemical properties represented by a wide potential window.[1] Hence, they have a great potential for electrochemical applications such as electrolytes for batteries, capacitors, fuel-cells and so on. Phosphonium ILs have unique physical characters such as lower viscosity and higher ionic conductivity compared with corresponding structured ammonium ILs.[2] Recently, we have evaluated the diffusion coefficient and the solubility of oxygen molecules in phosphonium ILs and have clarified that such oxygen behaviors strongly depends on the chemical structure of cation rather than that of anions. In the present study, we demonstrated the proton effect for the ORR in phosphonium ILs using rotating ring disk electrode (RRDE) technique.[3] Tetraalkyl-phosphonium(PXXXY⁺ and bis(trifluoromethane sulfone) imide (TFSA^-)) ILs were used for electrolytes.

Cyclic voltammetry (CV) and convective voltammogram were performed using a three electrode cell at 25^oC. The RRDE constructed a glassy carbon (GC) or platinum (Pt) disk, and Pt ring electrodes were used as the working electrodes. The ring current was measured at a potential of 1.0 V vs. Ag/Ag⁺ reference electrode, where the intermediate oxidation reaction enough attached to diffusion limit.

In the non-protonic media, ORR currents were observed from –0.6 V on GC disk electrode and –0.5 V on Pt disk electrode respectively, which were corresponding to the one-electron reduction reaction in ORR. On the other hand, the on-set potential of ORR in protonic media was higher than that in non-protonic media on both electrodes. Such potential shifts were strongly appeared on the Pt disk electrode than that on the GC disk electrode. Moreover, the diffusion limiting current was observed on Pt disk electrode at higher potential region in protonic media, and the current increased with increasing H-TFSA content. Therefore, the limiting current originated from not the transport of oxygen molecules, but that of proton, while the ring current oppositely decreased with increasing H-TFSA content. The ring current originated from intermediate formed on Pt disk electrode was smaller than that on GC disk electrode. These results indicated that the number of electrons in ORR increased by the existence of proton, and the effect of proton was promoted on Pt surface better than the GC surface.

[1] A. Khan, C. Zhao, ACS sustainable Chem. Eng. 4, 506 (2016)

[2] K. Tsunashima, and M. Sugiya, Electrochemistry Communications, 9, 2353- 2358(2007)

[3] M. Hildera, et al, Electrochimica Acta, 202, 100-109(2016).