Polymer electrolyte Fuel Cells (PEFCs) have been developed for the electric device applications such as residential fuel cell cogeneration system and fuel cell vehicles. PEFC cathode catalyst requires an excess amount of platinum due to the degradation of oxygen reduction reaction (ORR) activity at acidic environment and slow reaction kinetics at low temperature. Therefore, it is important to reduce the use of platinum in the catalyst and improve its catalytic activity as well as durability. Core-shell structure is one of the solutions to achieve a higher catalytic activity, higher durability and less use of platinum. However, the factor of controlling ORR activity in core-shell catalyst still has not well understood such as potential/temperature-dependent surface structures and oxidation states of platinum.
Convection voltammetry using rotating disc electrode (RDE) is one of the usable ways for evaluating ORR activity of cathode catalyst materials for PEFCs. However, the correlation between ORR activity and the electronic as well as local structure of these materials casted on RDE is not fully clear yet. In this paper, we report the temperature dependency of ORR activity of TEC10V30E Pt/C catalysts (T.K.K.) and Pd-core/Pt-shell/C catalysts from 25 oC to 60 oC, combining RDE measurements and Operando X-ray absorption spectroscopic measurements (XAS). We investigated the correlation with the electronic structure / local structural parameters of the Pt/C catalyst and investigated the factors that govern the oxygen reduction reaction activity on the RDE surface from the viewpoint of temperature dependency.
In this research, we synthesized the 1 mono-layer platinum shell on the carbon-supported palladium (Pt-Pd/C) by copper under potential deposition method and investigated its ORR activity at different temperatures and the state of platinum by rotating disk electrode (RDE) measurements and operando X-ray absorption fine structure (XAFS) respectively. The effect of oxygen coverage at Pt surface on ORR activity was mainly discussed.
Experimental
A model electrode was prepared by applying a catalyst ink prepared from TEC 10 V 30 E Pt / C (TKK) to the surface of glassy carbon RDE. The 1 mono-layer platinum shell on the carbon-supported palladium (Pt-Pd/C) by copper under potential deposition method. The electrochemical cell was constructed using a model electrode fabricated in the working electrode, a Pt mesh for the counter electrode, a reversible hydrogen electrode for the reference electrode, and 0.1 M HClO4 aq for the electrolyte. Electrochemical surface area was calculated by CV measurement under N2 saturated condition and area specific activity value was determined by LSV measurement under N2 and O2 saturated conditions. Measurements were carried out at 10, 25, 40, 50, 60 ºC. For this model electrode, operand XAS measurement of Pt LIII-edge and Pt LII-edge were carried out by using the beamline BL37XU/BL01B1 in SPring-8 (Japan). The measurements were performed by a fluorescent method using a temperature variable type Operand cell that can directly measure the electrocatalyst applied to RDE (Fig. 1). The number of Pt 5d orbital vacancies was calculated from the obtained XANES spectrum, and the local structure of Pt was analyzed from the EXAFS spectrum. The oxygen reduction activity performance in the electrochemical measurement was evaluated from the area specific activity value at 0.9 V of the Pt / C catalyst at 10, 25, 40, 50, 60 ºC. The oxygen reduction activity of the model electrode is determined by the electrochemical parameters related to the oxygen reduction reaction such as the oxygen coverage.
Results and discussion
According to RDE results, Pt-Pd/C has a potentially higher ORR specific activity than the conventional Pt/C at low temperature (< 50℃) while its catalytic activity has suddenly deteriorated over 60℃. Linear sweep voltammetry showed the coverage of oxygen species at Pt surface dramatically increased at high temperature in 1.1 V (vs. RHE). This oxygen species could reduce the ORR by blocking active sites for additional O2 adsorption. We also found the rapid increase of Pt 5d orbital vacancy by X-ray absorption spectra with the temperature, indicating that Pt on Pd/C was more easily oxidized over 60℃ than Pt/C.