Electrodeposited Ultrafine TaOx/CB Catalysts for PEFC Cathode Application: Their Oxygen Reduction Reaction Kinetics.

Ultrafine TaO x nanoparticles were electrodeposited on carbon black (CB) powder in a nonaqueous Ta complex solution at room temperature, and the resultant TaO x /CB catalysts were assessed as oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte fuel cell (PEFC) cathodes. The Ta electrodeposition process was scaled up using a newly designed working electrode containing a CB dense layer, without introducing any binder such as the ionomer Nafion in the electrode for electrodeposition. The electrodeposited TaO x /CB powders were removed from the deposition electrode and subsequent H 2 treatment at varying temperatures between 523 and 1073 K was attempted to increase the ORR performance. The TaO x /CB samples were characterized by SEM, STEM, XPS, and EELS measurements. XPS and EELS results indicated the reduced nature of the Ta species caused by the high-temperature treatment in H 2 , while STEM images clearly revealed that the TaO x particles aggregated as the treatment temperature increased. When the TaO x /CB catalyst, which was treated at 873 K for 2 h, was deposited on a glassy carbon substrate with Nafion ionomer, it resulted in the highest activity among the samples investigated, giving an onset potential of 0.95 V RHE at -2 μA cm −2 in a 0.1 M H 2 SO 4 solution. Moreover, the long-term stability test with 10,000 cycles of the voltammetry only led to a 6% loss in the ORR currents, demonstrating the high stability of the TaO x /CB catalysts. Kinetic analysis by R(R)DE indicated that the four-electron transfer pathway in the ORR process was dominant for this TaO x /CB catalyst, and Tafel plots showed a slope corresponding to a one-electron reaction for the rate-determining step. [ABSTRACT FROM AUTHOR]