Activity of Pd-Ni Alloys Towards Ethanol Electro-Oxidation in Alkali: Is There any Synergy between Pd and Ni?

Pd-based materials are attracting much interest as electrocatalysts in alkaline DAFC, due to intrinsic high activity and good stability of Pd in alcohol electro-oxidation. Many articles propose anodes consisting of finely dispersed Pd and Pd-alloy particles supported on nanostructured carbonaceous materials to obtain high Pd area and low noble metal content, thereby ensuring high mass activity and low cost. Few cases are reported of Pd-Ni alloy anodes obtained by electrodeposition. It is shown that alloying with Ni may warrant the catalyst stability in an alkaline environment, but the relation between composition and performances has received limited attention. In this contribution, we present the preparation of several compact and porous Pd-Ni anodes, either homogeneous or heterogeneus, and their application to ethanol oxidation in alkaline media. Different morphologies and structures (obtained by several routes) are compared. The dimensionless parameter fr,Pd is defined as the ratio between the estimated areas APd and Ageom, the former being measured by PdO reduction charge (for each sample) in CV experiments in alkali, and the latter by the PdO reduction charge for 1cm2 of mirror-flat Pd. This fr,Pd parameter expresses the equivalent Pd surface per unit area, reducing to a traditional roughness factor fr for pure Pd samples. Oxidation peak currents jp, recorded in anodic part of CV of ethanol oxidation, are plotted against the respective fr,Pd values. All the considered Pd-Ni samples show, despite different morphologies and compositions, an electrode activity proportional to fr,Pd, namely to the Pd surface exposed to the electrolyte. This finding indicates that only the exposed Pd surface sustains the catalytic activity, whereas Ni atoms present both in the substrate and admixtures are essentially inert.