Analysis of fuel oxidation reaction steps in Ni/GDC anode electrode of solid oxide fuel cells by using palladium nanoparticles

Abstract

Fuel oxidation reaction in anode electrode of solid oxide fuel cells (SOFC) is a multi-step reaction. The oxidation reaction begins with dissociative adsorption of fuel molecule on the surface of the electrode and subsequently diffusion of hydrogen atoms to the triple phase boundary sites, where the hydrogen atom combines with the oxygen ion and forms water; the final product of the fuel cell cycle. The electrons that release during this reaction will be transferred to the electrode materials and finally will be collected by current collector layer and transmitted to the external circuit. What people normally measure as the impedance of this circuit is summation of all the resistances in the circuit. By using palladium catalyst nanoparticles we will be able to alter the resistance against adsorption and diffusion step of the reaction and estimate the share of each step of the reaction in the whole electrode resistance. Our results reveal that presence of Pd catalyst nanoparticles cause a sharp decrease in the activation energy of the adsorption and diffusion step of the reaction, while the activation energy for charge transfer step does not change. Presence of Pd nanoparticles causes a significant decrease in anode impedance value and also separates the impedance spectra into two split portion. Incremental application of bias current on the anode electrode leads to gradual decrease in the resistance against both adsorption/diffusion and charge transfer step of the reaction. The reduction in the resistance is almost equal in percentage for both reaction steps. Studying impedance spectra for pure and Pd impregnated Ni/GDC anode at open circuit and under bias potential reveal that the impedance spectra for hydrogen oxidation reaction over pure Ni/GDC anode is mostly formed by resistance against adsorption and diffusion of the hydrogen species. Thus the main effort for enhancing the performance of the anode electrode should be focused on increasing the affinity of the electrode materials for adsorbing hydrogen species.