Quantifying the Effect of Ruthenium Contamination on the ORR Activity of DMFC Cathode Catalyst

Abstract

Direct methanol fuel cell (DMFC) technology is a promising energy source for portable, stationary and light transportation applications in the range of several watts and up to a few kW. Currently, one of the main drawbacks preventing widespread commercialization of DMFCs is their relatively short lifetime, up to a few thousands of hours at best. Usually a DMFC lifetime is not defined by a complete failure of the cell but by a slow and continuous degradation of the performance, up to the point when the delivered power is too low for the intended application. Several different degradation mechanisms contribute to this degradation [i], one of them is ruthenium contamination of the cathode catalyst [ii] following ruthenium dissolution from the anode PtRu catalyst and its cross-over to the cathode [iii]. The aim of our research is to quantitatively measure the effect of ruthenium contamination on the ORR performance of the cathode catalyst. For this purpose, different amounts of ruthenium (equivalent of 0.15-3 monolayers) were deposited on commercial Pt/C nanopowder to simulate ruthenium deposition on the cathode during a DMFC operation. The ruthenium-contaminated Pt/C powders were prepared using electroless polyol reduction method with methanol as the reducing agent. Johnson Matthey HiSPEC 8000 50%Pt/C was chosen to simulate a DMFC cathode catalyst. The composition, particles size and structure of these RuPt/C powders were verified by TGA, SEM-EDS, XPS, HR-TEM and STEM-EDS. Electrochemical characterization was performed with the use of cyclic-voltammetry and rotating–disk–electrode (RDE) techniques and compared to that of HiSPEC 8000. It was found that already at 0.25 equivalent monolayer of ruthenium (EDS %At Pt92Ru8, XPS %At Pt90Ru10) there is a massive, more that 25%, decrease in ORR specific activity of the commercial Pt/C catalyst. This decrease grows rapidly up to a complete inhibition of ORR at approximately 1 equivalent monolayer of ruthenium (EDS %At Pt64Ru36, XPS %At Pt50Ru50). The obtained results quantitatively show that ruthenium contamination indeed has a major role in the performance degradation of DMFCs. [i] A. Mehmood, M. A. Scibioh, J. Prabhuram, M. G. An, H.Y. Ha, Journal of Power Sources, 2015, 297,224–241 [ii] L. Gancs, B. N. Hult, N. Hakim, S. Mukerjee, Electrochem. Solid-State Lett., 2007, 10, 9, B150 [iii] P. Piela, C. Eickes, E. Brosha, F. Garzon, P. Zelenay, Journal of The Electrochemical Society, 2004, 151 (12), A2053–A2059