Influence of Minor Electronic Conduction in the Electrolyte on the Impedance Response of Proton Conducting Ceramic Fuel Cell Cathode

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Proton conducting ceramic fuel cell (PCFC) is expected as a next generation fuel cell operating with high energy conversion efficiency at intermediate temperatures. In order to realize the practical application of PCFC, one of big challenges is to reduce the polarization resistance at the cathode [1]. So far, a various kinds of cathode materials have been examined. For investigating the electrode reaction of PCFC cathode, electrochemical impedance spectroscopy (EIS) is very often utilized. In most cases, the polarization resistance is evaluated from the EIS response by assuming the transport number of proton in the electrolyte as unity. However, typical PCFC electrolytes shows electron hole conduction in addition to protonic conduction, particularly under higher oxygen partial pressures and higher temperatures [2]. Thus the transport number of proton cannot be always regarded as unity even under fuel cell operation conditions [3]. The existence of such minor electronic conduction in the electrolyte may result in underestimation of the polarization resistance by conventional electrochemical analyses including EIS [4], although little attention has been paid to how significant electronic conduction in the electrolyte is to the electrochemical response. In this study, we tried to experimentally understand influence of minor electronic conduction in the electrolyte on the impedance response of PCFC cathode.For the purpose mentioned above, EIS measurements were applied to investigate the reaction at a porous Pt electrode on a typical proton-conducting electrolyte, BaZr0.8Yb0.2O3-δ (BZYb), as a model PCFC cathode. A three terminal electrochemical cell was prepared on the sintered electrolyte pellet with 1 mm of thickness. Porous Pt electrodes as a working electrode (WE) and a counter electrode (CE) were placed on both sides of the pellet by painting the Pt paste and firing them at 1273 K. A porous Pt electrode as a reference electrode (RE) was set on the lateral side of the pellet. EIS measurements (AC amplitude: 30 mV, frequency; 106 to 10-3 Hz) were carried out at 773-973 K under 2% humidified 10-4-1 bar O2 for the WE and 2% humidified 10-2-1 bar H2 or 10-4-1 bar O2 for the CE/RE.Typical impedance spectra obtained at 973 K under various p(O2) for WE and a fixed atmosphere for CE/RE is presented in Fig. 1. As seen in this figure, some characteristic responses were observed; (i) the shape of the Nyquist plots was not typical semi-circular and a linear rise in the high frequency region was seen, (ii) the frequency corresponding to the impedance response was much slower than that for a typical gas reaction at a Pt electrode, (iii) the impedance response, which may correspond to polarization resistance, was relatively smaller than that of a typical gas reaction at a Pt gas electrode, and (iv) the x-intercept of the Nyquist plot, which is generally interpreted as the ohmic resistance of the electrolyte, shifted toward higher resistance side as p(O2) decreased. Moreover, when EIS measurements were performed under a fixed atmosphere for WE and various p(H2) or p(H2) for CE/RE, the impedance response of WE became drastically smaller with increasing p(O2) or decreasing p(H2) despite not changing the atmosphere of WE. These characteristic EIS responses were all considered due to influence of minor electronic hole conduction associated with the change in oxygen nonstoichiometry of the electrolyte. For instance, the slow frequency response could be well explained by assuming the oxygen nonstoichiometry change of the electrolyte due to the electrode polarization. These results tell us that EIS measurements tend to underestimate the polarization resistance of PCFC cathode because of the influence of minor electronic conduction in the electrolyte, and, in most cases, the estimation of the polarization resistance by fitting the impedance spectra assuming a conventional simple R//C or R//CPE circuit is not appropriate.