Operando X-Ray Absorption Spectroscopy Measurement Studies on Oxygen Reduction Reaction of Ionic-Liquid-Encapsulated Pd/Pt Core Shell Catalysts

Abstract

For the widespread use of Polymer Electrolyte Fuel Cell (PEFC), it is needed to reduce the amount of platinum used for electrode catalyst materials from the viewpoint of improving oxygen reduction reaction (ORR) activity of platinum. Recently, it is reported by Erlebacher1), 2) that the ORR activity of nanoporous NiPt alloy catalysts encapsulated by ionic liquid (IL) is higher than that of this material not encapsulated by. Based on the report3) that promoting the oxygen solubility at the interface between catalysts and electrolyte improves the ORR activity, this result implies that the high oxygen solubility of IL mainly contributes to high ORR activity. However, the reason of improvement of the ORR activity has not been clarified yet. In this study, we prepared Pd/Pt core shell catalyst, which is one of the most notable nanomaterials showing substantially high ORR activity, and encapsulated the surface of this catalyst with IL. Then we studied on the ORR of IL-encapsulated Pd/Pt core shell nanoparticles (NPs) by operando X-ray absorption spectroscopy (XAS) method. Pd/C (particle diameter of Pd: 3.6 nm) was applied in this study. Cu monolayer has been deposited on Pd NPs using under potential deposition (UPD) of Cu in 10 mM CuSO4 solution. A single UPD Cu monolayer replacement with Pt in K2PtCl4 solution formed Pt monolayer film on Pd NPs (Pt/Pd/C). The surface of Pt/Pd/C was coated with [MTBD][NTf2] 2), 3) (IL). Convection voltammetry was performed at 25 ˚C in O2-saturated 0.1 M HClO4 aq and H2SO4 aq using rotating disc electrode (RDE). RDE was rotated at 300, 500, 700, 900, 1200, 1600, and 2500 rpm. Electrochemical activity value at 0.9 V vs. Reversible Hydrogen Electrode (RHE) was calculated from Koutecky-Levich plot. Operando XAS measurements for Pt L Ⅲ -edge and L Ⅱ -edge of Pt/Pd/C catalysts were carried out by using the beamline BL14B2 in SPring-8 (Japan). These measurements were performed by fluorescence method in O2-saturated 0.1 M HClO4 aq and H2SO4 aq. Potentials were maintained at 0.5, 0.85, and 1.15 V vs. RHE. Specific activity value of Pt/Pd/C in 0.1 M H2SO4 aq was much lower than that of Pt/Pd/C in 0.1 M HClO4 aq. Pt-Pt bond distances were calculated by analyzing EXAFS spectra of Pt L Ⅲ -edge. At each potentials, there are no significant differences between Pt-Pt bond distances of Pt/Pd/C in 0.1 M HClO4 aq and H2SO4 aq. It implied that the irreversible specific adsorption of SO4 2- on the surface of Pd/Pt NPs decreased the number of active site of catalysts. The specific activity value of IL-encapsulated Pt/Pd/C was 1.31 times higher than that of non-encapsulated Pt/Pd/C in 0.1 M HClO4 aq. Pt-Pt bond distance estimated by EXAFS spectra of Pt L Ⅲ -edge of IL-encapsulated Pt/Pd/C was definitely longer than that of non-encapsulated Pt/Pd/C. These results suggested that coating with IL on the surface of Pt/Pd/C have lattice strain. The quantitative lattice strain establishes a direct correlation to monolayer Pt shell ORR activity and the strain effect induced d-band shift regulates the adsorption properties of rate-limiting intermediates in catalytic processes.4)