For the widespread commercialization of Polymer Electrolyte Fuel Cell (PEFC), reducing the amount of platinum used for cathode catalyst material is important by improving the oxygen reduction reaction (ORR) activity. 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 catalysis (T.K.K.) from 25oC 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.
Experimental
Electrochemical parameters obtained by electrochemical characterization using platinum-supported carbon (Pt/C) as a cathode catalyst and precisely temperature-controlled RDE and temperature variable Operand XAS on RDE surface. 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 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 HClO 4 aq for the electrolyte. Electrochemical surface area was calculated by CV measurement under N 2 saturation 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 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.
Result
According to RDE results, the reaction rate of ORR specific activity was determined for Pt/C from 25 oC to 60 oC. 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 was more easily oxidized at 60℃ than 25 oC.
Fig. 1. Schematic view of operando XAS cell for measuring ORR activity of Pt/C catalyst casted on RDE under controlling temperature. |
