Study of Ruthenium Contamination Effect on Oxygen Reduction Activity of Platinum-Based PEMFC and DMFC Cathode Catalyst

Abstract

Proton exchange membrane fuel cells (PEMFC) and PEM-based direct methanol fuel cells (DMFC) are promising energy sources for vehicular, stationary and portable applications. While PEMFC-based power systems have already reached a limited commercialization level, durability remains to be one of the main obstacles on the path towards their broad commercialization. One of the degradation mechanisms reducing durability of reformate-based PEMFC and DMFC is ruthenium contamination of the cathode catalyst [i] following ruthenium dissolution from the anode PtRu catalyst and its cross-over to the cathode [ii].The aim of our research is to quantitatively measure the effect of ruthenium contamination on the oxygen reduction reaction (ORR) activity of a typical cathode catalyst. For this purpose, various and precisely known amounts of ruthenium (equivalent to 0.15-4 ruthenium monolayers) were deposited on a commercial 50%Pt/C catalyst (Johnson Matthey HiSPEC8000) using electroless reduction method. The obtained Ru-contaminated Pt/C catalysts were examined using TGA, SEM-EDS, XPS, HR-TEM and STEM-EDS to determine their composition, particles size distribution and structure. Cyclic-voltammetry with rotating–disk–electrode was used to quantitatively determine the effect of ruthenium contamination on the ORR specific activity.It was found that ORR activity exhibits a pseudo-exponential decay with increased ruthenium coverage. At a contamination level of 0.15 equivalent ruthenium monolayer (EDS %at Ru3Pt97, XPS %at Ru7Pt93) a massive, ~45%, decrease in ORR specific activity was measured. At a contamination level of 1 equivalent ruthenium monolayer (EDS %at Ru27Pt73, XPS %at Pt50Ru50) almost a complete inhibition of ORR activity was observed. Ruthenium deposition on platinum in the form of bi-layer islands and 3d-clusters and deactivation of platinum atoms surrounding ruthenium deposition sites were proposed as a possible explanation for the pseudo-exponential decay of the ORR activity. The obtained results allowed us to estimate the decrease in power density of reformate-based PEMFC and DMFC due to ruthenium contamination by 0.15 equivalent monolayer at roughly 10%.Assuming that the decrease in ORR activity is originating from the strong O and OH binding to ruthenium, DFT-calculated O and OH binding energies to ruthenium and platinum were used to estimate ORR activity on model Run/Pt(111) surfaces that represent Ru-contaminated Pt/C catalyst. The estimated ORR activities have shown a dependence on ruthenium contamination that was qualitatively similar to experimental results, indicating that the strong O and OH binding to ruthenium have indeed a negative effect on the ORR activity.[i] L. Gancs, B. N. Hult, N. Hakim, S. Mukerjee, Electrochem. Solid-State Lett., 2007, 10, 9, B150[ii] P. Piela, C. Eickes, E. Brosha, F. Garzon, P. Zelenay, Journal of The Electrochemical Society, 2004, 151 (12), A2053–A2059