Nano-nitride Cathode Catalysts of Ti, Ta, and Nb for Polymer Electrolyte Fuel Cells: Temperature-Programmed Desorption Investigation of Molecularly Adsorbed Oxygen at Low Temperature

TiN, NbN, TaN, and Ta 3 N 5 nanoparticles synthesized using mesoporous graphitic (mpg)-C 3 N 4 templates were investigated for the oxygen reduction reaction (ORR) as cathode catalysts for polymer electrolyte fuel cells. The temperature-programmed desorption (TPD) of molecularly adsorbed O 2 at 120–170 K from these nanoparticles was examined, and the resulting amount and temperature of desorption were key factors determining the ORR activity. The size-dependent TiN nanoparticles (5–8 and 100 nm) were then examined. With decreasing particle size, the density of molecularly adsorbed O 2 per unit of surface area increased, indicating that a decrease in particle size increases the number of active sites. It is hard to determine the electrochemical active surface area for nonmetal electrocatalysts (such as oxides or nitrides), because of the absence of proton adsorption/desorption peaks in the voltammograms. In this study, O 2 -TPD for molecularly adsorbed O 2 at low temperature demonstrated that the amount and strength of adsorbed O 2 were key factors determining the ORR activity. The properties of molecularly adsorbed O 2 on cathode catalysts are discussed against the ORR activity.