No abstract

Oxygen reduction reactions on Pt, Ag, and Pt-Ag alloys were investigated at temperatures between 300oC and 500oC with the help of electrochemical impedance spectroscopy. The rate-determining step in oxygen reduction reactions was found to depend on the choice of material as well as porosity and thickness. On 600nm dense Ag, dissociative adsorption/reduction of oxygen was dominant. On porous Pt charge transfer reaction in the vicinity of triple phase boundary is rate determining. Slow oxygen diffusion was rate dominant with 600nm dense Ag3Pt. I-V characterization was also performed at 350oC on fuel cells consisting of different metal cathodes. Silver-based alloys were found not suitable as cathodic catalysts for low temperature solid oxide fuel cells due to formation of silver oxide, in turn leading to a rapid decline in voltage with increasing current densities. Among the investigated metals, thin porous Pt was identified as the best cathode.

One of the crucial factors for improving intermediate-temperature solid oxide fuel cell (SOFC) performance relies on the reduction in the activation loss originating from limited electrode reaction kinetics. We investigated the properties and functions of the nanocrystalline interlayer via quantum simulation and electrochemical impedance analyses. Electrode impedances were found to decrease several folds as a result of introducing a nanocrystalline interlayer and this positive impact was the most significant when the interlayer was a highly ionic-conducting nanocrystalline material. Both exchange current density and maximum power density were highest in the ultrathin SOFCs (fabricated with microelectromechanical systems (MEMS) compatible technologies) consisting of a 50 nm thick nano-gadolinia doped ceria (GDC) interlayer. Oxygen vacancy formation energies both at the surface and in the bulk of pure zirconia, ceria, yttria-stabilized zirconia, and GDC were computed from density functional theory, which provided insight on surface oxygen vacancy densities.

The kinetic Monte Carlo technique was employed to simulate an entire solid oxide fuel cell (SOFC) during operation to gain insight into the electrode kinetics and rate-limiting steps in the intermediate temperature range. By combining the quantum simulation studies of oxide ion migration in the fuel cell electrolyte with the experimental studies of the cathode and anode reaction rates, a complete SOFC can be modeled. To study the effect of triple phase boundaries and the size of the catalyst, simulations were performed for different sizes of Pt clusters on the electrolyte surface. The results confirm that the charge-transfer reaction rates depend on the catalyst size. The fuel cell with smaller catalyst particles produces higher power density as expected. The reaction rates of each process were recorded as a function of time. The overpotentials were subsequently determined as a function of catalyst size. The results show that oxygen adsorption is the slowest step on the cathode, while water formation is the slowest step on the anode. The methodology can be used to optimize the catalyst size on both electrodes to reduce the activation loss in intermediate temperature SOFCs.

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