(583eq) Pd-Based Bimetallic Core-Shell Catalysts for Direct Formic Acid Fuel Cells

Pd-based Bimetallic Core-Shell Catalysts for Direct Formic Acid Fuel Cells

Shuozhen Hu^a, Louis Scudiero^b, Su Ha^a

^a The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164

^b Chemistry Department and Materials Science and Engineering Program, Washington State University, Pullman, WA 99164

The potential use of CO₂-derived fuels in so-called regenerative direct formic acid fuel cell (DFAFC) system holds interesting prospects for future energy systems based on non-fossil energy sources. In the regenerative DFAFC system, formic acid is used as an energy carrier and it is important to efficiently oxidize formic acid at the fuel cell’s anode to achieve a high system performance. Among all catalysts, Pd has attracted a lot of attention due to its superior catalytic activity on formic acid oxidation. However, it is expensive and shows a poor long-term stability due to surface poisoning caused by intermediate species that form during the reaction. To minimize poisoning of the anode catalyst and lower the cost, Pd-based bimetallic catalysts are investigated.

From previous work we found that a change in the electronic structure of the Pd surface significantly alters the activity and stability of Pd towards formic acid electro-oxidation. In this study, about 10 nm M-Pd (M = Au, Ru, Cu, etc.) bimetallic core-shell nanoparticles with Pd as the shell are prepared. The particle size and shape are measured by TEM. X-ray photoelectron spectroscopy (XPS) results show a binding energy (BE) shift of Pd peaks to a higher binding energy, while M peaks shift in an opposite direction compared to their pure metal states. Those opposite BE shifts indicate that there is charge transfer from Pd to M (M = Au, Ru, Cu, etc.). In addition, a change of the valence band shape and shift of the d-band center away from the Fermi level are also observed in the XPS valence bands for the M-Pd samples with different core M metals. This electronic perturbation of the Pd surface results in a surface chemical alteration between the intermediate species and the Pd surface. As a result, changes in current density and stability of formic acid electro-oxidation are observed by cyclic voltammetry (CV) and chronoamperometry (CA). A correlation between XPS findings (BE shift and d-band center shift) and electrochemical activity and stability toward formic acid electro-oxidation is demonstrated for M-Pd core-shell nanoparticles.