Highly Dispersed TaOx Nanoparticles Prepared by Electrodeposition as Oxygen Reduction Electrocatalysts for Polymer Electrolyte Fuel Cells

Based on the chemical stability of group IV and V elements in acidic solutions, TaOx nanoparticles prepared by electrodeposition in an ethanol-based Ta plating bath at room temperature were investigated as novel nonplatinum electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs). Electrodeposition conditions of Ta complexes and subsequent various heat treatments for the deposited TaOx were examined for the best performance of the ORR. TaOx particles on carbon black (CB), electrodeposited at a constant potential of -0.5 VA(g/AgCl) for 10 s and then heat-treated by pure H-2 flow at 523 K for 1 h, showed excellent catalytic activity with an onset potential of 0.93 V-RHE (for 2 mu A cm(-2)) for the ORR Surface characterizations of the catalysts were performed by scanning transmission electron microscopy (STEM), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). The loading amounts of the electrodeposited material on the CB were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). All the physical results suggested that high dispersion of TaOx particles on the CB surface with 2-3 nm size was critical and key for high activity. The chemical identity and modified surface structure for the deposited TaOx catalysts before and after H-2 heat treatment were analyzed by X-ray photoelectron spectroscopy (XPS). The formation of more exposed active sites on the electrode surface and enhanced electroconductivity of the tantalum oxide promoted from the H-2 treatment greatly improved the ORR performance of the electrodeposited TaOx nanoparticles on CB. Finally, the highly retained ORR activity after an accelerated durability test in an acidic solution confirmed and proved the chemical stability of the oxide nanoparticles. The high utilization of the electrodeposited TaOx nanoparticles uniformly dispersed on CB for the ORR was comparable to that of commercial Pt/CB catalysts, visually demonstrating their catalytic potential for PEFC electrocatalysts.