Impact of Surface Poisoning and Electrocatalysis on the Durability of Pt based Catalysts for Fuel cells

Vivek Murthi (Northeastern University)

Direct Methanol Fuel Cells (DMFC) promises many advantages such as high efficiency, portability, low temperature operation and compact design in comparison to batteries and other types of fuel cells such as PEMFC, SOFC, PAFC etc. Unsupported PtRu alloy nanoparticles are currently considered as state-of-the-art DMFC anode electrocatalysts due to the ability of surface Ru to remove CO intermediates from the electrode surface even at lower potentials. However, they fall short of meeting durability expectations.[1] The focus of research traditionally has been to understand fundamental anode catalysts, alternative membranes, MEA structure and design of cell components to improve the performance of DMFC. Recently, the focus of research has shifted towards long term durability by identifying and understanding the reasons behind performance losses in DMFC MEAs composed of PtRu black catalyst at the anode. Such studies have resulted in identifying one of the main reasons for DMFC performance degradation. Namely, the leaching of Ru from the anode both at open circuit potentials and at applied load conditions and migrating to the cathode catalyst layer thereby causing severe performance losses.[2] Here, the consequences of anodic dissolution of Ru from state-of-the-art PtRu electrocatalysts and its profound effect on a DMFC cathode will be elucidated. Consequently, the right choice of cathode electrocatalyst for the electroreduction of oxygen is invaluable to the overall performance of a PEMFC or a DMFC. Oxygen reduction reaction at the cathode continues to be a challenge because of its complex kinetics and the need for better electrocatalysts. Improved performance is shown by many Pt based transition metal alloys due to their ability to inhibit the formation of a Pt-OH layer (a poison for ORR formed due to water activation on Pt). Changes in short range atomic order, particle size, Pt d-band vacancy, Pt skin effects and lateral effects are some reasons attributed for the enhancement offered by these alloys.[3-8] Recent work has focused on improved understanding of the structure and composition of the electrocatalyst using synchrotron X-ray Absorption Spectroscopy (XAS). This presentation will discuss the effect of water activation combined with the use of novel in situ XANES (X-ray Absorption Near Edge Structure) region analysis to provide detailed information on site specific adsorption of oxygen on electrocatalyst surfaces.[9-12] Since these effects cause profound changes in the reactivity of a fuel cell, such studies ultimately help in understanding the fundamental reactions occurring in an operating fuel cell and designing electrocatalysts suitable for future applications.

References [1] P. Piela, C. Eickes, E. Brosha and P. Zelenay. in Book of Abstracts, 204th Meeting of The Electrochemical Society. 2003. Orlando, FL. [2] P. Piela, C. Eickes, E. Brosha, F. Garzon and P. Zelenay, J. Electrochem. Soc., 151, A2053 (2004) [3] S. Mukerjee, In-Situ X-Ray Absorption of Carbon Supported Pt and Pt Alloy Electrocatalysts: Correlation of Electrocatalytic Activity with particle Size and Alloying, in Catalysis and Electrocatalysis at Nanoparticle surfaces, A. Weickowski, E. Savinova, and C. G. Vayenas, eds., Marcel Dekker, Inc. (2003). [4] R. Parsons and T. Vandernoot, J. Electroanal. Chem., 257, 9 (1988) [5] V. Jalan and E. J. Taylor, J. Electrochem. Soc., 130(11), 2299 (1983) [6] K. Kinoshita, J. Electrochem. Soc., 137, 845 (1990) [7] M. T. Paffett, J. G. Beery and S. Gottesfeld, J. Electrochem. Soc., 135(6), 1431 (1988) [8] V. S. Murthi, R. C. Urian and S. Mukerjee, J. Phys. Chem. B., 108(30), 11011 (2004) [9] M. Teliska, W. E. O'Grady and D. E. Ramaker, J. Phys. Chem. B, 108(7), 2333 (2004) [10] D. E. Ramaker and D. C. Koningsberger, Phys. Rev. Lett., 89 (13)(13), 139701 (2002) [11] M. Teliska, V. S. Murthi, S. Mukerjee and D. E. Ramaker, J. Electrochem. Soc., 152, A2159 (2005) [12] M. Teliska, V. S. Murthi, S. Mukerjee and D. E. Ramaker, J. Phys. Chem. B., (in press)

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