Synthesis of Pt Catalysts Supported on W/Ta-Doped TiO2 As a Highly Durable Electrocatalyst for PEFCs

Abstract

One of the technical problems impeding commercial use of polymer electrolyte fuel cells (PEFCs) is the degradation of the carbon support under startup/shutdown (SU/SD) conditions, which leads to severe and eventually drastic reduction of the performance due to agglomeration, dissolution, or detachment of Pt nanoparticles. Thus, nanoceramic supports (NCSs) with both high electrical conductivity and durability under SU/SD operating conditions have been actively developed to replace the commercial carbon support.[1-6] Titanium dioxide (TiO2) doped with aliovalent cations is also a candidate as a high durability cathode NCS. We synthesized Pt catalysts supported on tantalum and tungsten-doped TiO2with unique microstructure, involving a fused-aggregated network, and evaluated the oxygen reduction reaction (ORR) activity and durability under SU/SD operation conditions. We prepared the NCSs by a flame spray synthesis method. The morphology of the NCSs, which was investigated by use of transmission electron microscopy (TEM), had nanoparticles with a partially fused aggregated network structure, similar to that of carbon black (Fig. 1(a)(b)). The crystal structures of the NCSs included rutile and anatase. The surface areas, which were estimated using the Brunauer-Emmett-Teller (BET) technique, were greater than 30 m2g-1. The electrical conductivities, which were measured by use of the two-probe method, were as high as 10-4 S/cm at room temperature under a pressure of 19 MPa (Fig. 2). The NCSs were confirmed to be extremely stable in 1.0 M H2SO4 at 80 oC under air atmosphere. Pt nanoparticles, which were loaded on the NCSs by a colloidal method, were uniformly dispersed on the surface (Fig. 1(c)(d)). The Pt loadings were 19 wt% (Pt/Ta-TiO2) and 19.4 wt% (Pt/W-TiO2), determined by ICP-MS. Pt particle sizes were 4 nm (Pt/Ta-TiO2) and 10 nm (Pt/W-TiO2), determined by TEM. The ORR activity increased with increasing heat treatment temperature and reached that of a commercial carbon-supported Pt catalyst (c-Pt/CB). The durability of Pt/Ta-TiO2 under SU/SD conditions was superior to that of the c- Pt/CB. Figure 1