(Invited) A Practical Model of Oxygen Exchange Reaction on a Mixed Conducting Electrode for SOFC and SOEC
Authors: Kawada Tatsuya , Namigawara Takayuki , Amano Yuki , Wang Xingwei , Yashiro Keiji 
Published: December 2020

Oxygen exchange on mixed conducting electrodes for SOFC/SOEC have been investigated by many researchers, and the reaction mechanism has been made clearer in these decades. In a practical modeling works, however, a Butler-Volmer equation with unproven parameters is still used to describe current-voltage (I-V) relationships due to the complexity of realistic reaction models. Recently, we have collaborated with Muramatsu and Terada to develop a simulation code “SIMUDEL” that can calculate oxygen potential distribution and the resulting chemical strain in SOFC/SOEC components [1]. In this code, porous electrode is described as a homogeneous medium in which oxygen exchange reaction is given as the source term in the continuity equation. In this code, the surface reaction rate is given as a function of oxygen potentials in the gas phase and at the solid surface. This work aimed to provide a simple but plausible model of oxygen exchange at a mixed conducting electrode surface.
The work started with modeling I-V characteristics of a dense film electrode. Steady state dc measurements were made with La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) films deposited by pulsed laser deposition (PLD) on doped ceria substrates. Oxygen vacancy concentration of the film was estimated from the chemical capacitance by ac impedance measurements and was used as a parameter to represent the I-V relationships. The reaction rate on a dense electrode was compared with the volumetric surface reaction rate in a porous electrode assuming one-dimensional transport approximation. Several porous electrodes with micro- and nano- meter-sized particles were fabricated by slurry coating or metal organic decomposition. A plausible reaction equation obtained for a film electrode was applied to estimate I-V curves of a porous electrodes, and the result was compared with the experiments to test the model.
The surface reaction resistance on a dense film electrode showed an activation energy in the range of 1.5 to 2.0 eV. It was much larger than that of a porous electrode when it was obtained as the volumetric reaction resistance of the porous medium and normalized to the actual surface area. The magnitude of the reaction resistance on a porous electrode was close to that observed for a sample powder by pulse isotope exchange experiments. No remarkable difference was observed between the micro and nano porous electrodes. The large activation energy on the dense film electrode is partly due to the reduced oxygen vacancy concentration in a PLD film electrode. As many other researchers pointed out, oxygen exchange reaction on a mixed conductor is strongly affected by the concentration and mobility of oxygen vacancy. The apparent activation energy of surface reaction is the sum of activation energies for vacancy formation and reaction. The difference between the dense and porous electrode, however, was not fully explained only with the difference in vacancy concentration. Furthermore, the reaction rate on a dense film electrode was found to be strongly affected by the coexisting chemicals on the surface. As we reported earlier, applying LaSrCoO4 (R-P phase) particles on the surface dramatically enhanced the surface reaction rate. It also decreased the activation energy. Existence of R-P phase was found to clean the Sr enriched segregates on the surface. On the contrary, significant degradation was observed for reaction on a sintered ceramic surface due to the segregation of Sr rich layer. These observations suggested the existence of two series reaction barriers for oxygen exchange; i.e. the surface reaction and sub-surface transport. The I-V curve of a dense film electrode was modelled on the basis of the above consideration. The surface process was described to be proportional to the oxygen vacancy concentration, and the sub-surface process was given to overcome some energy barrier. For a porous electrode, the sub-surface process was assumed to be negligible. The difference in oxygen vacancy concentration and lack of sub-surface process explained fairly well the oxygen partial pressure dependence of I-V curves of the porous electrodes. The obtained model is to be applied to the description of the surface reaction in SIMDEL code.

Acknowledgements

This work was partly supported by Japan Science and Technology Agency for the financial support during JST-CREST program (JPMJCR11C1, 2011-2016). Development of SIMDEL was made under the support by NEDO.

References

[1] M. Muramatsu et al. Engineering Computations, 34(6), 1956 (2017)