Abstract

Proton conducting ceramic fuel cell (PCFC) is a next generation fuel cell that can operate with high efficiency. One of challenges for the commercialization of PCFC is to reduce the polarization resistance of the cathode ^[1], and a variety of cathode materials have been investigated. Electrochemical impedance spectroscopy (EIS) with alternating current (AC) is often used to evaluate the polarization resistance of PCFC cathode. In most cases of the analysis, the ionic transport number of the electrolyte is assumed to be 1. However, in typical PCFC electrolytes, the ionic transport number often cannot be assumed as 1 because the electrolyte shows the mixed conduction of proton and electron hole, especially under high temperature and high oxygen partial pressure conditions.^[2] Such mixed conduction in the electrolyte may lead to underestimation of the polarization resistance. In this study, using a porous Pt electrode on a typical PCFC electrolyte, BaZr_0.8Yb_0.2O_3-δ (BZYb), as a model case, we aimed to experimentally clarify effect of mixed conduction in the electrolyte on the impedance responses of PCFC cathode. And, we attempted to establish the precise method for evaluating the polarization resistance of PCFC cathode by the fitting with appropriate equivalent circuits considering mixed conduction in the electrolyte.

Experimental

An electrochemical cell was prepared using BaZr_0.8Yb_0.2O_3-δ (BZYb) as a proton conducting electrolyte. On both sides of the electrolyte pellet, porous Pt electrodes were sintered at 1273 K as a working electrode (WE) and a counter electrode (CE). A reference electrode (RE) of porous Pt was set around the side of the electrolyte. The cell was placed in two-chamber configuration, and EIS measurements (AC amplitude: 30 mV, frequency; 10⁶ to 10^-3 Hz, under open circuit) were performed with the three-terminal method. Temperature was 773-973 K and the atmospheres were 2% humidified 0.01-100% O₂ for the WE, and 2% humidified 5-100% H₂ or 0.01-100% O₂ for the CE/RE.

Results and discussion

EIS measurements were carried out under various p(O₂), while the atmosphere of CE/RE was fixed at wet 5% H₂. As results, unique responses were observed. Firstly, the Nyquist plot showed a linear rise in the high frequency region. Secondly, the frequency corresponding to the impedance response was much slower than the general fuel cell cathodic reaction on a Pt electrode. Finally, 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. It was considered that these responses were caused by the change in electronic hole conduction associated with the change in oxygen nonstoichiometry of the electrolyte. In fact, the calculated capacitance ^[3] assuming the oxygen nonstoichiometry change of the electrolyte due to the electrode polarization was in good agreement with the observed capacitance. In order to confirm effect of hole conduction to the impedance response, EIS measurements were performed under the constant WE atmosphere of wet 100% O₂, while varying the CE/RE atmosphere from wet 100% H₂ to wet 100% O₂. The impedance response decreased drastically when the CE/RE atmosphere changed from H₂ to O₂, even though the WE atmosphere was constant. This indicated 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 the spectrum fitting assuming a conventional simple R//C circuit is not appropriate.