Strong Metal-Support Interactions Enhance the Activity and Durability of Platinum Supported on Tantalum-Modified Titanium Dioxide Electrocatalysts

摘要

Rutile phase tantalum-modified titanium oxide (Ta0.3Ti0.7O2) was synthesized and studied using electrochemical and spectroscopic methods to evaluate its efficacy as a corrosion-resistant electrocatalyst support material. A 20 wt % Pt supported on Ta0.3Ti0.7O2 catalyst was prepared and compared in terms of activity and stability against a 20 wt % Pt supported on Vulcan XC-72R carbon catalyst (20% Pt/C; synthesized in-house) and a 46 wt % Pt/C commercially sourced catalyst (Tanaka KK). Catalysts 20% Pt/Ta0.3Ti0.7O2, 20% Pt/C, and 46% Pt/C possessed electrochemically active surface areas (ECSAs) of 60, 57, and 65 m2 g-1, respectively, and mass activities for the oxygen reduction reaction (at 0.9 V vs RHE) of 185, 148, and 224 mA mg-1 Pt, respectively, as evaluated in an operating polymer electrolyte fuel cell. Accelerated stability tests (ASTs) were performed on membrane electrode assemblies (MEAs) in an operating fuel cell to investigate both support and platinum catalyst stability. The loss in voltage at a current density of 0.4 Acm-2 after 10 000 support stability AST cycles was only 23 mV for 20% Pt/Ta0.3Ti0.7O2, over an order of magnitude lower than the losses observed in 20% Pt/C and 46% Pt/C (～330 mV). Although the latter loss would correspond to catastrophic fuel cell and stack failure, the former is well within the limits of system tolerance. Post-mortem transmission electron microscopy (TEM) analyses of the electrocatalyst recovered from cycled MEAs confirmed the excellent stability of Pt nanoparticles supported on Ta0.3Ti0.7O2. The average Pt particle size increased by ～20% in 20% Pt/ Ta0.3Ti0.7O2, as compared with a doubling in size in the case of 20% Pt/C and a near tripling in size in 46% Pt/C. The existence of strong metal-support interactions in 20% Pt/Ta0.3Ti0.7O2 was ascertained from the X-ray absorption near edge structure analysis. The number of unfilled d states in 20% Pt/Ta0.3Ti0.7O2 was found to be ～1.47, which was lower than the value of ～1.60 found in both the carbon-supported Pt catalysts. The decrease in the number of unfilled d states confirmed electron donation from the Ta0.3Ti0.7O2 support to the Pt atoms, resulting in an increased electron density on Pt. This interaction enhanced both electrocatalytic activity and catalyst stability, as evidenced by the results above.