Abstract
Fundamental understanding of the oxygen reduction reaction (ORR) mechanism in electrochemical cells (e.g., solid oxide fuel cells (SOFCs)) is rather challenging because of several processes, which occur with similar time constants. Also, it is very difficult to elucidate ORR reaction steps using a conventional equivalent circuit modeling in ac impedance spectroscopy. There is no unique model to fully explain the ORR mechanism, especially in high temperature SOFCs. In this study, attempt has been made using impedance spectroscopy genetic programming (ISGP) technique to describe SOFC cathode ORR processes. Using ISGP, we analyzed the electrochemical performance of Ba0.5Sr0.5Fe0.91Al0.09O3-δ (BSFAl) and Ba0.5Sr0.5Fe0.8Cu0.2O3-δ (BSFCu) cathodes with oxide ion conducting La0.8Sr0.2Ga0.8Mg0.2O3-δ(LSGM) and proton conducting Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3-δ (BSCZGY) electrolytes. The ORR mechanism is explained by finding the distribution function of relaxation time (DFRT) with the help of ISGP. By monitoring the changes in the DFRT models at different temperatures and using both oxide ion and proton conducting electrolytes, we are able to deconvolute the ORR to its several polarization subprocesses. Using the present approach, it is possible to gain additional information, which may be convoluted and therefore undetected in conventional impedance analysis. The analysis procedure results in a direct and unambiguous DFRT model, with a distinct physical meaning. Therefore, making it especially beneficial in comparative studies, as demonstrated in this work, where it was found that BSFCu-LSGM cathode showed better charge-transfer properties than BSFAl-LSGM cathode, due to higher conductivity of BSFCu phase. In both BSCZGY-BSFCu and BSCZGY-BSFAl cathodes, the rate-limiting step was found to be the charge-transfer process, owing to low electrical (ionic) conductivity of BSCZGY.