Catalytic Activity of Pt/TaB2(0001) for the Oxygen Reduction Reaction

摘要

Proton-exchange-membrane fuel cells (PEMFCs) are a promising power source for automobiles. For their wide application, however, there still remain several problems. One problem is the limited mass activity (reaction rate per mass) of cathode electrocatalysts for the oxygen reduction reaction (ORR). Bulk Pt has a high specific activity (reaction rate per surface area), and the specific activity can be further increased by alloying the subsurfaces with several nonpre-cious metals, such as Fe, Co, Ni, Cu, Sc, or Y, or by replacing subsurfaces with Pd. However, the specific areas (surface area per mass of the precious metal) of bulk materials are small, and therefore, the mass activities (specific activity multiplied by specific area) are also small. To increase the mass activity, the specific surface area should be increased by decreasing the catalyst size to the nanometer scale. Although Pt nanoparticles supported on carbon (Pt/C) are used practically in PEMFCs, the mass activity is not sufficiently high because the decrease in the size of the catalyst leads to a decrease in the specific activity as a result of the so-called particle-size effect. To avoid the particle-size effect, the specific surface area must be increased while maintaining the extended bulklike surface morphology. This new approach was employed by the company 3M in the development of nanostructured thin-film (NSTF) catalysts, in which Pt films with a thickness of a few tens of nanometers are deposited on organic nanostructured whiskerlike supports. The discovery of these new electrocatalysts inspired a number of studies on the fabrication of electrocatalysts with an extended Pt surface and high specific surface area with the aim of further increasing the mass activity.