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

摘要

Based on the chemical stability of group IV and V elements in acidic solutions, TaO_x nanoparticles prepared by electrodeposition in an ethanol-based Ta plating bath at room temperature were investigated as novel nonplatinum electro-catalysts 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 TaO_x were examined for the best performance of the ORR. TaO_x. particles on carbon black (CB), electro-deposited at a constant potential of —0.5 V_(Ag/AgCl) for 10 s and then heat-treated by pure H2 flow at 523 K for 1 h, showed excellent catalytic activity with an onset potential of 0.93 V_(rhe)(for 2 μA 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 TaO_x 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 TaO_x catalysts before and after H2 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 H2 treatment greatly improved the ORR performance of the electrodeposited TaO_x 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 TaO_x nanoparticles uniformly dispersed on CB for the ORR was comparable to that of commercial Pt/CB catalysts, visually demonstrating their catalytic potential for PEFC electrocataiysts.