Preparation and Electrochemical Activities of Pt-TiO2 Nanocomposite Electrocatalysts for PEFCs

Abstract

Pt/C is widely used as the electrocatalyst for polymer electrolyte fuel cells (PEFCs). However, the durability is one of the important issues for Pt/C because the detachment and aggregation of Pt catalysts are caused by the oxidative corrosion of carbon support at a higher potential of the cathode up to 1.5 VRHE. In our group, we have developed electrocatalysts using conductive oxide (SnO2) on conductive fillers (VGCF-H), realizing both high catalytic activity and high voltage cycle durability[1-3]. As such high durability is obtained by the application of stable SnO2, where the movement and aggregation of Pt nanoparticles are suppressed by the strong bonding between Pt and SnO2 [4]. With increasing the contact area between Pt and the oxide, such interaction may be further emphasized. Here, by using this strong interaction, we prepare the “nanocomposite electrocatalysts” consisting of Pt catalysts and TiO2 nanoparticles. TiO2 is known as one of the metal oxides thermochemically stable under the PEFC cathode condition, and TiO2 has also been considered as a support material. The purpose of this study is to develop a highly durable electrocatalyst and to establish a new nanocomposite concept for PEFCs by optimizing preparation and heat treatment conditions. We co-loaded Pt and TiO2 on graphitized carbon black (GCB200 from Cabot) by the acetylacetonate (acac) method[5]. In this acac method, a 1-step preparation procedure was applied where Pt(acac)2 and Ti(acac)2OiPr2 were simultaneously dissolved in acetone. The molar ratio of Pt to Ti was set to be 1:1, and the loading ratio of the Pt-TiO2 composites was set to be 20wt.%. In the heat treatment for Pt and TiO2 loading, we performed additional heat treatment in a humidified N2 or at a higher temperature, just after the typical heat treatment at 210 ºC for 3h and then 240 ºC for 3h, to fully decompose and oxidize Ti(acac)2OiPr2 to TiO2. We analyzed Pt loading quantitatively by ICP emission analysis, and particle size and crystalline phases by XRD and STEM. In addition, we conducted half-cell measurements to evaluate the electrochemical activity of the electrocatalysts prepared. The electrochemical surface area (ECSA) of Pt was measured by cyclic voltammetry (CV), and oxygen reduction reaction (ORR) activity by rotating electrode (RDE) method. Furthermore, along with the procedures recommended by Fuel Cell Commercialization Conference of Japan (FCCJ), we evaluated load and start/stop potential cycle durability of the prepared electrocatalysts, by applying 400,000 load potential cycles between 0.6 and 1.0 VRHE simulating acceleration and deceleration, and 60,000 start/stop potential cycles between 1.0 and 1.5 VRHE simulating start/stop operation of FCVs. After the ICP measurements, it was confirmed that designated amount of Pt could be loaded by the acac method. From the XRD results, there was no clear peak of Pt-Ti alloys such as PtTi and Pt3Ti, indicating that Pt and TiO2 existed separately as a composite. EDS analysis in high resolution STEM revealed that Pt nanoparticles of about 1-2 nmφ are supported with high dispersion, and TiO2 nanoparticles are distributed between the Pt nanoparticles, as shown in Figure 1. Nanocomposite electrocatalysts of Pt and TiO2 could be prepared, exhibiting a high electrocatalyst surface area and ORR activities. Figure 1