(Digital Presentation) ORR Activities on Hydrophobic Phosphonium Ionic Liquid Modified Pt/C Catalysts
Authors: Ando Uta , Okada Takuya , Matsumoto Mitsuhiro , Inoue Yohtaro , Katakura Katsumi , Tsunashima Katsuhiko , Yamada Hirohisa 
Published: November 2022

PEFCs have been attracted as one of the clean and efficient energy conversion systems. Platinum nanoparticles are commonly used as cathode catalysts in Polymer electrolyte fuel cells (PEFCs) due to their high activity for oxygen reduction reaction (ORR). However, platinum is rare and expensive, therefore it needs to improve the mass activity and durability for widespread fuel cells. Recently, ionic liquids (ILs) have been studied to enhance the ORR activity of platinum catalysts1). We try to improve the catalytic activity of Pt/C catalysts by using Ionic Liquids (ILs). RTILs are salts that melt at temperatures below 100 oC and generally consist of bulky cations and anions. It has been studied in a wide range of fields, including the energy sector because of its highly conductive and nonvolatile. Recently, it has been reported that ILs layer enhance the ORR catalytic activity on Pt surface, due to the suppress the coverage of OHad from water molecules1). However, the correlation between increased ORR activity and the structure of the ionic liquid has not been clarified. In this study, we focused on ILs quaternary phosphonium cations and a bis(trifluoromethylsulfonyl)amide (TFSA) anion. The phosphonium cations based ILs have been reported to have higher thermal stability than imidazolium or ammonium cations2). In addition, the phosphonium ILs show higher electrochemical stability, conductivity, and hydrophobicity than the ammonium ILs.

The ORR activity was analyzed using the rotating disk electrode (RDE) method3). Commercially available 20 wt% Pt/C (Cabot, Vulcan XC-72R🄬) catalyst was used for bare Pt/C modified electrodes. The Pt/C electrodes were modified with phosphonium ILs (PXXXY+TFSA- [alkyl chain X= 4 Y= 1, 12, 16]). The thickness of the ILs layer was calculated by using the BET surface area of carbon supports. The electrochemical measurements were conducted under Ar or O2 saturated conditions, respectively. Electrochemical surface area (ECSA) was estimated from hydrogen adsorption wave in cyclic voltammograms (CVs).

Mass activity (MA) and surface area specific activity (SA) of ORR at 0.9 V on Pt nanoparticles were calculated from the Koutecky-Levich equation. The ILs layer enhanced ORR activity. However, ORR activity decreased when the ILs layer was excessively thick. These results suggest that the ORR activity decreases in a thicker ILs layer due to increased resistance to oxygen mass transport or proton conductivity. Moreover, the ORR activity of the ILs modified Pt/C catalyst increased as the asymmetric part alkyl chains of the ILs became longer. Our previous reports show that as the size -is larger, both the molar volume and oxygen solubility increases. Our results show that these physicochemical properties originating from the structure of the cation affect ORR activities.

[Reference]

1) Gui-Rong Zhang, et al., ACS Catal., 8, 8244-8254 (2018).

2) K. Tsunashima and M. Sugiya, Electrochem. Commun., 9, 2353, (2007).

3) H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner, Appl. Catal. B Environ. 56, 9. (2005).