Influence of CeO₂ Nanoparticle Morphology on the Electrocatalytic Activity of Palladium toward the Formate Electrooxidation Reaction

This study investigates the electrocatalytic activity of palladium (Pd) nanocatalysts combined with nanoparticles (NPs) of cerium oxide (CeO₂) polyhedra (Pd/CeO₂/C poly) and with morphologies of cube (Pd/CeO₂/C NC), hexagonal sheet (Pd/CeO₂/C NS), and nanorod (Pd/CeO₂/C NR) for the formate electrooxidation reaction (FER) in an alkaline medium, a key process in direct formate fuel cells (DFFCs). X-ray diffraction (XRD) patterns indicate that the CeO₂ NP dislocation density follows the decreasing order of Pd/CeO₂/C NR > Pd/CeO₂/C NS > Pd/CeO₂/C NC > Pd/CeO₂/C poly. This order corresponds to the Pd⁰ concentration observed in X-ray photoelectron spectroscopy (XPS) data. Pd/CeO₂/C NR showed CeO₂ NPs with compressive microstrain values, while the other materials presented tensile microstrain. Pd/CeO₂/C NR showed a higher amount of oxygen vacancies in XPS analysis. Transmission electron microscopy (TEM) micrographs showed Pd NPs surrounding and in close contact with CeO₂ NPs. The average Pd NPS size was 4-10 nm, while CeO₂ NC, CeO₂ NS, and CeO₂ poly showed 30, 15, and 50 nm, respectively. The length and width of CeO₂ NR were 100 and 10 nm, respectively. Cyclic voltammetry (CV) demonstrates that Pd/CeO₂/C NR exhibits higher catalytic activity toward FER than the other materials studied (Pd/C, Pd/CeO₂/C NC, Pd/CeO₂/C NS, Pd/CeO₂/C poly). Chronoamperometric analysis (CA) shows that the current density from FER on Pd/CeO₂/C NR at the end of the experiment was 2.65 times higher than on Pd/C, while for Pd/CeO₂/C NC and Pd/CeO₂/C NS it was 1.45 and 1.13 times higher than on Pd/C. The enhanced catalytic activity of Pd/CeO₂/C NR results from the interaction between palladium and ceria, involving microstrain, oxygen vacancies, and metal-support interaction, facilitating polar adsorbate adsorption. This makes Pd/CeO₂/C NR a promising catalyst for direct formate fuel cell applications.