Influence of Fuels and pH on the Activity and Dissolution Stability of Bifunctional Alloy Electrocatalysts

Abstract

After two decades of slumber, research on organic fuel cells is getting increasing interest. Organic fuels, such as alcohols provide an attractive avenue to overcome the drawbacks of hydrogen as an energy carrier however, catalysis is still a bottleneck for their efficient oxidation. In most studies almost exclusively primary alcohols (e.g., methanol and ethanol) are investigated as fuels. The major downside of this approach is that these are not “zero emission” systems since CO2 forms as a final product. Moreover, CO evolves as an intermediate, which poisons the catalyst surface leading to deteriorated device performance. A possible solution is to use secondary alcohols, such as isopropanol in which case the product is acetone. A prominent member of the bifunctional electrocatalyst family is PtRu, which proven to be the state-of-the-art electrocatalyst in alcohol oxidation. However, in the case of isopropanol the forming acetone poisons its surface, thus there is an inevitable need for the development of novel catalyst materials circumventing these issues. In general, most research efforts so far have addressed the activity and mechanism of oxidation reactions on bifunctional electrocatalysts, but less is known about the influence of alcohols on their stability during operation. On-line techniques allow an elegant option to study such processes besides activity and to track the formed intermediates in real time.In this study we employ on-line ICP-MS to map the activity and stability of Pt-rich PtRu and PtIr alloy catalysts towards the electrocatalytic oxidation methanol, ethanol and isopropanol. Material libraries were synthesized by combinatorial reactive magnetron sputtering that allowed precise control over the metal/metal ratio in each sample. To determine both the surface and bulk composition of PtRu and PtIr material libraries XRD, XPS and EDX measurements were performed. Activity and stability of the electrocatalyst samples were studied in both acidic (HClO4) and alkaline (KOH) media and in the presence of the alcohols. The observed trends in the activity and dissolution characteristics of both systems are going to be discussed in detail along with the identification of the possible underlying processes.