Fundamental Aspects of Spontaneous Cathodic Deposition of Ru onto Pt/C Electrocatalysts and Membranes under Direct Methanol Fuel Cell Operating Conditions: An in Situ X-ray Absorption Spectroscopy and Electron Spin Resonance Study

Abstract

In situ X-ray absorption spectroscopy (XAS), using both EXAFS and the Δμ-XANES analysis procedures, is utilized to examine Ru deposition onto Pt/C cathodes at millimolar concentration of Ru in a 1 M HClO4 electrolyte. Also, electron spin resonance (ESR) spectroscopy is utilized to examine the effects of Ru3+ ion exchanged Nafion membranes. The Δμ-XANES analysis of the data allows a determination of the coverage of Ru with time (minutes to hours) and also to identify the binding site of the deposited Ru species (atop/bridged at low coverage and 3-fold at higher coverage) apparently onto the corners and edges initially and at higher coverage onto the faces of the cubooctahedral clusters when exposed to Run+ at OCP. The deposition of Ru appears to be inhibited at potentials where adsorbates (such as H and O(H)) usually adsorb, and Coulomb enhanced at OCP when substantial O exists on the surface. The ESR analysis of Ru3+ in the Nafion membrane indicates significant detrimental changes to the membrane in the presence of Ru ions; a decrease in the water uptake and an increase in the microviscosity of the fluid regions were noticed. Together these data indicate the critical nature of keeping the fuel cell under potential control and avoiding an uncontrolled shut down under direct methanol operating conditions.