(123b) Understanding CO and Methanol Electro-Oxidation Activity on AgxPd1-X/C Alloys Under Alkaline Conditions

CO electro-oxidation (CO-ox) is one of the most widely studied reactions in electrocatalysis for both fundamental and technological reasons. CO* is a strongly-binding intermediate which poisons the most active catalysts (Pt,Pd) during the electro-oxidation of many small organic molecules (e.g. methanol, formic acid, ethanol). For this reason, there has been much interest in developing CO-tolerant materials to mitigate this poisoning effect and improve electro-oxidation activity, which would improve the efficiency of direct methanol/ethanol/formic acid fuel cells.

We recently published work in which we developed a descriptor-based microkinetic model for the CO-ox reaction, which predicted that the optimal material should bind CO* weaker and OH* stronger than Pd(111). Ag_xPd_1-x alloys are a class of materials which have been shown (using first-principles calculations) to exhibit these properties. It has been proposed that Ag_xPd_1-x alloys exhibit weaker CO* binding on Pd sites (relative to pure Pd) and stronger OH* binding on Ag sites (relative to pure Pd) due to a combination of electronic and geometric effects, making them promising materials for the CO-ox reaction.

In this work, we tested a series of Ag_xPd_1-x/C alloys (TEM and XRD in Figs. (a) and (b)) for their CO-ox activities under alkaline conditions to test the hypothesis that they would be more active than pure Pd for the CO-ox reaction. It was found that each of the Ag_xPd_1-x/C alloys were more active than pure Pd for the CO-ox reaction at low overpotentials (CO stripping, Fig. (c)), likely due to the aforementioned electronic/geometric effects produced upon alloying. We also tested these materials for their methanol electro-oxidation (Me-ox) activities (Fig. (d)) and also found improvements relative to pure Pd at low potentials. This work validates the prediction of the microkinetic model that Ag_xPd_1-x/C alloys are active CO-ox materials, which in turn yielded improved activity for the Me-ox reaction.