Role of Hydrous Ruthenium Oxide in Pt−Ru Direct Methanol Fuel Cell Anode Electrocatalysts:  The Importance of Mixed Electron/Proton Conductivity

Abstract

Pt−Ru is the favored anode catalyst for the oxidation of methanol in direct methanol fuel cells (DMFCs). The nanoscale Pt−Ru blacks are accepted to be bimetallic alloys as based on their X-ray diffraction patterns. Our bulk and surface analyses show that although practical Pt−Ru blacks have diffraction patterns consistent with an alloy assignment, they are primarily a mix of Pt metal and Ru oxides plus some Pt oxides and only small amounts of Ru metal. Thermogravimetric analysis and X-ray photoelectron spectroscopy of as-received Pt−Ru electrocatalysts indicate that DMFC materials contain substantial amounts of hydrous ruthenium oxide (RuO x H y ). A potential misidentification of nanoscale Pt−Ru blacks arises because RuO x H y is amorphous and cannot be discerned by X-ray diffraction. Hydrous ruthenium oxide is a mixed proton and electron conductor and innately expresses Ru−OH speciation. These properties are of key importance in the mechanism of methanol oxidation, in particular, Ru−OH is a critical component of the bifunctional mechanism proposed for direct methanol oxidation in that it is the oxygen-transfer species that oxidatively dissociates −C⋮O fragments from the Pt surface. The catalysts and membrane-electrode assemblies of DMFCs should not be processed at or exposed to temperatures >150 °C, as such conditions deleteriously lower the proton conductivity of hydrous ruthenium oxide and thus affect the ability of the Ru component of the electrocatalyst to dissociate water. With this analytical understanding of the true nature of practical nanoscale Pt−Ru electrocatalysts, we can now recommend that hydrous ruthenium oxide, rather than Ru metal or anhydrous RuO2, is the preferred Ru speciation in these catalysts.

Rapid Microwave Synthesis of CO Tolerant Reduced Graphene Oxide-Supported Platinum Electrocatalysts for Oxidation of Methanol

“…Hence, the oxygen defect sites play a major role in removing the carbonaceous species and contribute to the low catalyst poisoning on the Pt/RGO hybrids, in a way similar to that found in Pt/RuO 2 and Pt/Ru/MoO x systems. 14,31,32 Figure S6 in the Supporting Information presents the same dependence as that presented in Figure 5a for a complete series of scan rates ranging from 20 to 200 mV/s. It is clear that, at all scan rates ( Figure S6 in the Supporting Information), the three Pt/RGOs exhibit the same general trend electrodes, displaying much higher I f /I b ratios compared to that of Pt/C, indicating superior antipoisoning behavior.…”

Systematic Study of Pt-Ru/C Catalysts Prepared by Chemical Deposition for Direct Methanol Fuel Cells

“…The conducting properties of Pt-RuO x H y are key in performance the for methanol electro-oxidation as they promote the formation of Ru-OH. Ru-OH aids in CO tolerance on Pt surfaces by the bifunctional mechanism [24], and since Ru metal and anhydrous RuO 2 do not have these capabilities, they are not as active for methanol electro-oxidation [25].…”

Metamorphosis of the mixed phase PtRu anode catalyst for direct methanol fuel cells after exposure of methanol: In situ and ex situ characterizations

“…While an overwhelming majority of the researchers believe that oxides are harmful, according to Rolison et al the presence of oxides enhances the performance of DMFC anode. 2 However, it is well known that both the physical and chemical properties of the anode catalysts changes with the exposure of methanol and during operation. Keeping this in mind, we studied the possible metamorphosis of the mixed phase anode catalyst in-situ by cyclic voltammetry(CV) and impedance spectroscopy.…”