Thermal stability in air of Pt/C catalysts and PEM fuel cell catalyst layers

FUEL 227

Kenneth J. Wynne1, Olga A. Baturina, oabaturina@vcu.edu1, and Steven R. Aubuchon, saubuchon@tainst.com2. (1) Department of Chemical and Life Science Engineering, Virginia Commonwealth University, 601 West Main Street, Richmond, VA 23284, (2) TA Instruments-Waters, LLC, 109 Lukens Drive, New Castle, DE 19720
Durability of PEM fuel cell materials is essential to applications longevity. This research explores the limits of thermal stability of platinum/Vulcan XC 72 catalysts and a 46wt% Pt/Vulcan XC 72/Nafion„¥ catalyst layer. The thermal stability of Pt/Vulcan XC 72 catalysts and the PEM fuel cell catalyst layer is studied by thermal gravimetric analysis (TGA) in air. The products of decomposition are analyzed with TGA coupled with mass spectrometry (TGA-MS). Low temperature (100-200„aC) carbon combustion in the presence of platinum is confirmed. The high precision and sensitivity of TGA allows differentiation of two oxidative / mass loss regimes for 46% Pt/C. The presence of surface protective groups raises the activation energy for the low temperature/ low conversion („T 5%) oxidation of 46% Pt/Vulcan XC 72 (197 „b 13 kJ/mole) compared to a higher temperature / higher conversion level (10-30%) process (140 „b 10 kJ/mole). In PEM fuel cell catalyst layers the thermal decomposition temperature of Nafion is lowered by about 100„aC to 300„aC in the presence of Pt/C catalysts. As a result of the above studies it was found that TGA is convenient for the determination of Pt wt% in catalyst-coated membranes. The low temperature kinetic data for carbon support oxidation may be used to estimate carbon mass loss for a PEM fuel cell at 80„aC, a typical operating temperature for automotive application. For a cell using a 46%Pt/C catalyst such as the one employed in this study about 10% loss of carbon support is predicted after one year of continuous operation. This percent mass loss may be a lower limit as more rapid oxidation occurs at the cathode under electrochemical conditions.