Electrochemical Accesebility of Platinum Nanoparticles Supported on Anatase TiO2 Using Conductive Carbon Additives

Abstract

Degradation of carbon supports at the cathode of polymer electrolyte fuel cells is a critical issue leading to a decrease in activity of the platinum electrocatalysts. Metal oxides based on Ti and Sn have been studied as alternative supports, as they show high stability under severe cathodic conditions.1 However, these metal oxides are typically poorly conductive when compared with carbon support and needs to be either reduced or doped with other metals in order to be able to function as a conducting support. Making an electronically conductive path among nanosized metal oxides with conducting carbon-based materials may offer an alternative approach towards utilization of such non-conducting supports. In this study, we pursued the possibility of utilizing carbon nanotubes (CNTs) as a conducting additive to Pt nanoparticles deposited on non-conducing anatase-type TiO2. Pt nanoparticles were photo-deposited on anatase-type TiO2 (P25, average particle diameter=25 nm). The amount of platinum on TiO2 was adjusted to be 2 mass%. CNTs were added to a dispersion of Pt/TiO2, adjusted to 20 mass% CNT with respect to Pt/TiO2, and dried to prepare the CNT-Pt/TiO2 composites. The catalyst ink was dropped on a glassy carbon disk electrode (φ= 6 mm, 3.5 µg-Pt cm–2) and dried at room temperature. This electrode was used as the working electrode, and electrochemical studies were conducted with a three-electrode electrochemical cell setup. CO stripping voltammetry in 0.1 M HClO4electrolyte (25°C) was performed in order to estimate the electrochemically active surface area (ECSA) of the platinum nanoparticles. Figure (a) shows a typical transmission electron microscope (TEM) image of CNT-Pt/TiO2. CNTs form a network-like morphology with Pt/TiO2 particles. The average diameter of platinum nanoparticles determined from 150 particles was 2.9 nm. The specific surface area of the platinum nanoparticles based on the particle size was estimated to be 96 m2 g–1. ECSA was estimated from the oxidation charge of pre-adsorbed CO on platinum surface. The ECSA of CNT-Pt/TiO2 was 63 m2 (g-Pt)–1. Thus, the utilization of the platinum surface was 66% compared with the expected value, i.e. 96 m2 g–1. Although the ECSA of CNT-Pt/TiO2 was lower than Pt/C (83 m2 (g-Pt)–1, platinum particle size of 2.1 nm), the utilization of platinum surface was comparable with that of commercial Pt/C (62%). From these result, it is concluded that an electronically conductive path among platinum supported on TiO2particles was successfully established using a CNT network. 1) K. Sasaki, F. Takasaki, Z. Noda, S. Hayashi, Y. Shiratori, and K. Ito, ECS Trans., 33, 473 (2010). Figure 1