Pt-TiO2 Nanocomposite PEFC Electrocatalysts for High Potential Cycle Durability

Abstract

Introduction Pt/C is widely used as the electrocatalysts for polymer electrolyte fuel cells (PEFCs). However, Pt/C has a difficulty in durability due to the thermochemical instability of carbon support and the weak binding energy between Pt and carbon support, which cause the detachment and aggregation of Pt nanocatalysts. In our group, we have developed electrocatalysts using oxides such as (SnO2) as catalyst supports, realizing high potential cycle durability[1-3].The use of oxide support suppresses the detachment and aggregation of Pt nanocatalysts due to the strong bonding between Pt and such oxides [4]. With increasing the contact area between Pt and oxides, such interaction could be further pronounced. Here, we prepare “nanocomposite electrocatalysts” maximizing the contact area between Pt and oxides. TiO2 is used as a model oxide. The purpose of this study is to develop a highly durable electrocatalysts and to establish a new nanocomposite concept for PEFCs by optimizing the ratio of Pt to TiO2 and the heat-treatment conditions. E xperimental Pt and TiO2 were co-loaded on the graphitized carbon black (GCB200 from Cabot) by the acetylacetonate (acac) method[5]. In this acac method, a one-step preparation procedure was applied where precursors of Pt and TiO2 (Pt(acac)2 and Ti(acac)2OiPr2, both from Sigma Aldrich) could be simultaneously dissolved in acetone. The volume ratio of Pt to TiO2 was set to be 1:1, 1:2, and 1:4. The loading of the Pt-TiO2 nanocomposite was set to be 20wt.%. In the heat-treatment for the Pt and TiO2 co-loading, an additional heat-treatment was made in a humidified N2 at a higher temperature for promoting the crystallization of TiO2, just after the typical heat-treatment at 210°C for 3 h and then 240°C for 3 h in the Pt-acac method. Pt loading and the ratio of Pt to TiO2 were quantitatively evaluated by inductively-coupled plasma (ICP) spectroscopy and thermogravimetry (TG) analysis. The degree of crystallization and the microstructure of nanocomposite electrocatalysts were evaluated by XRD and STEM, respectively. Half-cell and MEA tests were conducted to evaluate the electrochemical activity and durability of these electrocatalysts prepared. Results and Discussion From the results of the ICP and TG analysis, the loading and the ratio of Pt to TiO2 were derived. After the XRD measurements shown in Figure 1, it was confirmed that there was no change in Pt peak width and thus Pt particle size, even though the preparation conditions were varied. We also confirmed that the crystallization of TiO2 was promoted by the heat-treatment at higher temperature up to e.g. 500°C. STEM micrograph and EDS map in high-resolution STEM, shown in Figure 2, reveal that Pt nanocatalysts of about 1 to 2 nm in diameter were supported with high dispersion, and TiO2 was distributed between Pt nanocatalysts, indicating a successful preparation of Pt-TiO2 nanocomposite electrocatalysts. To mention a few, the electrochemical results of load potential cycle tests between 0.6 and 1.0 VRHE confirmed that sufficient crystallization of TiO2 actually improves the potential cycle durability of these nanocomposite electrocatalysts. R eferences A. Masao, S. Noda, F. Takasaki, K. Ito, and K. Sasaki,Electrochem . Solid-State Lett., 12 (9), B119-B122 (2009). F. Takasaki, S.Matsuie, Y. Takabatake, Z. Noda, A. Hayashi, Y. Shiratori, K. Ito, and K. Sasaki, J. Electrochem . Soc., 158 (10), B1270-B1275 (2011). D.Horiguchi, T. Tsukatsune, Z. Noda, A. Hayashi, and K. Sasaki, ECS Trans., 64 (3), 215-220 (2014). T. Daio, A.Staykov, L. Guo, J. Liu, M. Tanaka, S. M. Lyth, and K. Sasaki, Sci. Rep., 5, 13126 (2015). A. Hayashi, H.Notsu, K. Kimijima, J. Miyamoto, and I. Yagi, Electrochim . Acta, 53, 6117 (2008). Figure 1