Effect of pH on the Activity of Hydrogen Oxidation Reaction/Hydrogen Evolution Reaction over PtRu Bimetallic Catalysts

Abstract

Hydrogen proton exchange membrane fuel cell (PEMFC) powered automobiles have become commercially available, but are still too expensive. Hydroxide exchange membrane fuel cells (HEMFCs) offer the possibility of cheaper non-precious metal catalysts. However, the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) are significantly slower in base than in acid for Pt, Pd, Ir, and Rh1-4. PtRu shows higher HOR/HER activity than Pt5, 6, so understanding the mechanism of the HOR/HER on PtRu is key to developing non-precious metal catalysts for HEMFCs. Using the rotating disk electrode, bimetallic PtRu showed a minimum in the HOR/HER activity, pH relationship (Figure 1). The hydrogen binding energy and the hydroxide binding energy measured from the hydrogen underpotential desorption peak and the CO stripping peak, respectively, did not change significantly with pH. Leaching Ru from the catalyst, it was found that the HOR/HER activity of PtRu remains approximately constant while the Ru content is reduced. When a small amount of Ru is leached from the surface, the activity increases and only after a significant amount of Ru has been removed from the surface/near surface of the catalyst does the activity change. This suggests that the HOR mechanism is not the bifunctional mechanism. The charge transfer coefficient, α, decreases with the Ru content going from 0.8 to 0.5. Fitting the kinetic data with the dual pathway model7 shows that below pH 11, the Heyrovsky and Volmer reaction barriers are comparable and above pH 11, the Volmer step becomes rate determining. It is proposed that Ru can align more oxygen species on the surface giving water a more favorable orientation for the Volmer step to proceed, which results in an increase in activity.