Impact of Nonuniform Potential in SOFC Composite Cathodes on the Determination of Electrochemical Kinetic Parameters

Abstract

Engineering of porous solid oxide fuel cell ͑SOFC͒ composite cathodes comprising a mixture of electrocatalyst and ionic conductor requires the knowledge of electrochemical kinetic parameters such as the reaction order and the charge-transfer coefficients. Conventional dc techniques are commonly employed in electrochemical measurements for composite cathodes. The results are often analyzed in terms of the low-or high-field approximations of the Butler-Volmer equation, which is based on the assumption that no potential or overpotential gradients exist within the electrode. In this study, numerical simulation of lanthanum strontium manganate-yttria-stabilized zirconia composite cathode was performed with an assumed oxygen reduction reaction mechanism and corresponding electrochemical kinetic parameters. The simulation results indicated that for composite cathodes significant gradients in the overpotential exist. The apparent reaction order and apparent charge-transfer coefficients, derived on the basis of nominal overpotential and net current, the two quantities that are typically accessible experimentally, were significantly different than the actual kinetic parameters used in the simulation. The extent of the variation between the actual and apparent electrochemical kinetics parameters was found to be dependent on the thickness and microstructure of the composite cathode. Simulation of conventional cathodes, which had no potential gradients within the bulk of the cathode, showed no errors.

Short-stack modeling of degradation in solid oxide fuel cells

“…A similar situation occurs with the observed charge transfer coefficients, as recently shown in [23] and [18]. Patterned electrode experiments are useful in trying to measure electrochemical parameters on a simple geometry [24], and provide evidence of the inverse relationship between polarization resistance and triple-phase-boundary length, reflected in this model as the electroactive surface area.…”

“…The approach used in this work is the one used by Kenney and Karan [18], approximating the probability of percolation with the volume fraction of the corresponding phase, as presented in detail in [19]:…”

Characterization of La[sub 0.5]Ba[sub 0.5]CoO[sub 3−δ] as a SOFC Cathode Material

“…For example, a plot of the logarithm of HF and LF resistances vs the inverse of temperature yields activation energy. However, a plot of the logarithm of HF and LF resistances vs the logarithm of pO 2 yields a parameter, n, [35][36][37] that can be thought of as the reaction order for the process. The combined knowledge of activation energies and reaction orders provides a stronger basis for the identification of the process responsible for the HF and LF responses.…”

Effective Transport Coefficients for Porous Microstructures in Solid Oxide Fuel Cells

“…In this paper, we present numerical frameworks to compute the effective transport coefficients efficiently and accurately for realistic porous electrode microstructures. These properties can directly be used to obtain performance calculations for transport phenomena in porous electrodes (4,5,6,8,11,13,14).…”