Carbon Supported Pt Nanocatalysts Decorated with Titanium Oxide By Coaxial Pulse Arc Plasma Deposition

Abstract

Fuel cells are power sources converting chemical energy into electrical energy through electrochemical reactions occurring on catalyst surfaces, in general, on Pt or Pt alloy surfaces. Hydrogen is mostly and widely used as the fuel, but other fuels such as methanol, ethanol and formic acid are investigated as alternatives because of their higher energy density as well as easy transport in our society. Among these alternative systems, direct methanol fuel cells (DMFCs) have been developed and considered for applications in electronics because of their high theoretical energy density, simple systems and ease of handling. In contrast to these merits even over the hydrogen-based systems, the complicated and slow electrooxidation reactions of methanol have hampered their commercialization. In this light, a variety of electrocatalysts for methanol oxidation reactions have been synthesized and evaluated in order to enhance the sluggish reactions, in other words, to reduce a fuel cell stack cost by decreasing the amount of Pt usage (e.g., alloying with other metals, making Pt monolayer on other metals, making composites with other substances to have synergy effects, and controlling the shapes to utilize more active surface structures). The carbon supported Pt nanoparticles (Pt/C) have been recognized as the most reasonable starting point for each fundamental research, though alloys are widely used in the research and commercial products for achieving better performance. Pt and its alloys have the best activities in methanol oxidation reactions (MOR) and oxygen reduction reactions (ORR), and, in order to utilize Pt surface effectively, Pt/C catalysts have been generally prepared, e.g., by an impregnation method involving a wet - chemical reduction ( i.e. , impregnation of the carbon supports with solutions containing the metal salts to be deposited). This method is quite simple and is easy for large-scale production. Very recently, we have proposed a new approach for preparation of Pt/C catalysts by arc plasma deposition (APD) method, where Pt nanoparticles can be deposited on carbon in vacuum and has been applied for various fields other than fuel cell catalyst. Herein, we focus on a preparation of new MOR and ORR catalysts by using our coaxial pulse APD method, where two different metal sources (Pt and Ti) can be deposited on carbon support through vacuum processes. The morphology of the obtained sample was characterized by the scanning electron microscope (SEM) and the transmission electron microscopy (TEM). The SEM images show a typical morphology of an activated carbon, and the TEM images show that the deposited nanoparticles are well dispersed on the carbon support and have an average size of ~3 nm. The elemental mapping data show that the Pt and Ti are homogeneously distributed on the entire of carbon support. X-ray photoelectron spectroscopy (XPS) measurement is carried out to ascertain the electronic state of Pt and Ti, and the Pt 4f7/2 and Pt 4f5/2 doublet peaks are observed at ~71.5 and ~74.5 eV, which mean the Pt is pure metallic Pt. Regarding core-level Ti XPS, the peaks located at 458.9 eV and 464.8 eV can be assigned to Ti(IV) 2p 3/2 and Ti(IV) 2p 1/2, respectively. Another peak located at 455 eV is probably attributed to the Ti3+. Considering the fact that the XRD pattern does not show any crystalline TiOx phases, the Ti should form amorphous TiOx phases. Thus, the materials are found to be Pt nanoparticles (~3 nm) and amorphous TiOx phases on carbon black. Compared with other Pt-based catalysts, our catalysts show superior electrochemical performances in MOR and ORR, probably owing to the presence of the secondary metal oxide phase. The cyclic voltammograms (CVs) of methanol oxidation reaction in 0.5 M H2SO4 containing 0.5 M methanol solution were scanned for the synthesized catalyst, compared with pure Pt catalyst on carbon support prepared by the APD method, commercially available Pt/C catalyst, and Pt black. All the current densities were normalized by Pt mass (i.e., Pt mass activity), and our catalyst exhibits higher activities than the other catalysts, probably because of bifunctional mechanism as found in PtRu or other systems, where the metal oxide surface provides active oxygen for removal of intermediates, such as CO, on the Pt surface effectively. The ORR performance of our catalysts were preliminarily investigated by linear sweep voltammetry (LSV) using a rotating disk electrode in O2-saturated 0.1 M HClO4 solution at a scan rate of 10 mV s-1 and 1600 rpm. The synthesized PtTi/C and the synthesized Pt/C show almost the same onset potentials, and show the better mass activity than the commercially available products. More details will be presented on the day.