Abstract
Introduction Since conventional PEFC electrocatalysts have a difficulty in durability against start-stop and load potential cycling, development of high-performance and high-durability cells for next-generation fuel cell vehicles is desired. SnO2 is known to be stable at a high potential under the PEFC cathode conditions, so that SnO2 is considered as an alternative support material for PEFC cathodes [1-4]. Here in this study, electrocatalysts are prepared using highly graphitized carbon black (GCB), which is a carbon material with an increased degree of graphitization, acting as a conductive framework in the electrocatalyst layers. Nanocrystalline SnO2 particles of several nm in diameter were deposited on the GCB. The use of nanocrystalline SnO2 enables to shorten the electron pathway between the catalyst and the GCB-based conductive framework, down to a few nm. Platinum catalysts were then deposited on SnO2 nanoparticles. The ultimate objective of this study is to develop high-performance cells with high durability by optimizing preparation conditions for nanostructuring. Drying procedure is mainly examined in this study. Experimental Nanocrystalline SnO2 was deposited on GCB (GCD200, Cabot) via the evaporation-to-dryness and the freeze drying. Pt deposition on SnO2 was made using platinum acetylacetonate in order to prepare Pt/SnO2/GCB electrocatalysts. Nanostructure of the electrocatalysts obtained was observed by field-emission scanning electron microscopy (FE-SEM) and scanning transmission electron microscopy (STEM). Thermogravimetry (TG) measurement was applied to quantify SnO2 loading. ICP analysis was made to quantify Pt loading. Average crystallite size of SnO2 and Pt in Pt/SnO2/GCB was measured by XRD. Electrochemical characterization of these electrocatalysts was carried out by half-cell measurements. Electrochemical surface area (ECSA) was measured by cyclic voltammetry (CV), and oxygen reduction reaction (ORR) activity was derived from kinetically controlled current density (ik ) after the rotating disk electrode (RDE) measurements. Results and discussion Figures 1 and 2 show FESEM images of Pt/SnO2/GCB prepared by using the evaporation-to-dryness and the freeze drying, respectively. Figure 1 reveals aggregation of platinum particles, and inhomogeneous distribution was confirmed for platinum particles. On the other hand, as shown in Fig. 2, the platinum particles were homogeneously supported in higher dispersion. The catalytic activity of these electrocatalysts was evaluated by the half-cell measurements. Pt/SnO2/GCB prepared by the evaporation-to-dryness process exhibited ECSA of 68.4 m2g-1, and mass activity of 82.9 Ag-1 at 0.9 VRHE. The electrocatalyst prepared by the freeze drying process exhibited larger ECSA of 80.8 m2g-1, and higher mass activity of 152.5 Ag-1 at 0.9 VRHE. From the results of microstructural observation and the half-cell measurements, the freeze drying process, which enables uniform dispersion of oxide nanoparticles, is suitable for the preparation of the nanocrystalline oxide support, leading to higher electrochemical properties. Acknowledgment Financial support by the Center-of-Innovation program, JST Japan, is gratefully acknowledged.
Published Version
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