Electronic Effect On Oxidation of Formic Acid On Supported Pd-Cu Bimetallic Surface

Electronic Effect on Oxidation of Formic Acid on Supported Pd-Cu Bimetallic Surface

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

Formic acid is the energy carrier in fuel cells that produces carbon-neutral electric power when it is used with regenerative direct formic acid fuel cells (DFAFCs). In regenerative DFAFCs, formic acid is oxidized at the anode to produce electricity and carbon dioxide.  Carbon dioxide then reacts with water in another reactor to form formic acid through an electrochemical reduction process using renewable energy sources, such as solar energy, wind energy, or tidal energy. Therefore, CO₂-derived fuels hold interesting prospects for future energy systems based on non-fossil energy sources.

Direct formic acid fuel cells (DFAFCs) have many advantages over H₂ proton exchange membrane (PEM) fuel cells and direct methanol fuel cells due to: 1) non-toxicity, 2) easy to store and transport, and 3) low crossover flux through membrane. Among the anode catalysts for DFAFC, palladium (Pd) offers the best electrochemical activity towards the formic acid oxidation reaction. However, it is expensive and shows a poor long-term stability due to surface poisoning caused by intermediate species, such as CH₂(OH)₂, that form during the reaction. To minimize poisoning of the anode and lower the cost, bimetallic surfaces of Pd using inexpensive transition metals as supporting materials are investigated. In this study, the layered configuration of thin film samples was chosen versus nanoparticles to minimize contributions from uneven particle size, polycrystallinity of particles, and surface component effects that are typically associated with bimetallic nanoparticles. Furthermore, to avoid possible contaminations from precursors commonly used in wet chemistry, thermal vapor deposition method was used to grow Pd films on Cu or other transition metal (M) thick films over glassy carbon surfaces to facilitate X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and chronoamperometry (CA) measurements. A positive binding energy shift of the Pd peaks and an opposite shift for M = Cu peaks were measured. These opposite binding energy shift indicate that there is charge transfer from Pd to Cu. Similarly, a change of the valence band shape and shift of the d-band center away from the Fermi level as Pd layer decreased were observed and measured by XPS. This electronic perturbation of the Pd-Cu systems results in a lower bond energy between Pd and adsorbate. As a result, a drastic increase in current density for a thin layered Pd-Cu bimetallic sample compare to bulk Pd, and an improved stability for formic acid electro-oxidation were measured by both CV and CA. These findings indicate that Pd-Cu bimetallic surfaces are better electrocatalytic materials than bulk Pd and could be use to enhance direct formic acid fuel cell (DFAFC) performance.