Insufficient durability of commercial catalysts is a persistent issue for PEFC applications.¹ Herein, Pt on Nb-doped-TiO₂ is reported as a highly durable electrocatalyst on which the direct 4-electron reduction of oxygen to water is more facile compared to Pt/C. All performance metrics are reported comparing Pt/Nb-TiO₂ with a 15% Pt loading, against a commercial Pt/C catalyst (46.5% Pt loading, Tanaka, K. K.). Nb-doped-TiO₂ with high surface area and high electronic conductivity was synthesized using the supercritical fluid method. Initially, the durability of the catalyst was characterized using accelerated stability tests (ASTs) involving 10,000 high potential cycles (DOE/FCCJ protocol) and the Pt/Nb-TiO₂ was found to retain 78% of its initial electrochemically active surface area (ECSA) compared to the 57.6 % retained by Pt/C. These observations were in excellent agreement with previous reports that the Pt particle size of Pt/C grew from 2nm to 8nm during the AST protocol along with severe corrosion and amorphization of the carbon surface.^{2, 3} TEM and XPS studies of the Pt-Nb-doped-TiO₂ catalyst showed that the Pt particle size grew from 3nm to 6nm and the Nb(IV) and Ti(III) in the support were oxidized to Nb(V) and Ti(IV) after the durability test. Thus, the improvement in Pt/Nb-TiO₂ ECSA retention was attributed to the lower extent of particle growth and lack of oxidative support loss upon oxidation as compared to Pt/C. The oxygen reduction reaction (ORR) performance was characterized by linear polarization using a rotating disk electrode (RDE). The electrochemical surface areas of Pt/Nb-TiO₂ and Pt/C were found to be 48m²·g^-1 and 83m²·g^-1 respectively, and the mass activity for the ORR at 0.9V vs. RHE were found to be 150 mA·mg^-1_Pt and 124 mA·mg^-1_Pt respectively. The improved mass activity on Pt/Nb-TiO₂ was attributed to strong metal support interaction (SMSI) between the Nb-TiO₂ support and the Pt catalyst based on the 625 meV decrease in the binding energy of the Pt4f x-ray photo-electron spectroscopy (XPS) peaks of Pt/Nb-TiO₂ compared to Pt/C. To quantify the impact of the SMSI, a kinetic model was applied to calculate the elementary reaction rate constants for the various steps of the ORR on both catalysts. The reaction rate constant (k₁) for the direct 4-electron transfer pathway to produce H₂O was significantly larger in Pt/Nb-TiO₂ as compared to Pt/C. Thus, the reduction in the electron binding energy as observed in the XPS was found to aid the facile filling of the higher energy 2p orbitals of O₂ thereby leading to improved 4-electron transfer kinetics and improved overall activity.

References:

1. R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J. E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K. Kimijima and N. Iwashita, Chem. Rev., 107, 3904 (2007).
2. Y.-C. Park, K. Kakinuma, M. Uchida, D. A. Tryk, T. Kamino, H. Uchida and M. Watanabe, Electrochim. Acta, 91, 195 (2013).
3. A. Kumar and V. Ramani, ACS Catalysis, 4, 1516 (2014).