(Invited) Effect of Electrode Polarization on Oxide Ion-Conducting and Proton-Conducting Electrolytes

Polarization at the electrode/electrolyte interface has been a central issue in electrochemistry, which is also true for high-temperature ion conducting devices. Charge transfer limitation does not simply apply to such interfaces where electrons make direct exchange. Thus, overpotentials have been discussed in terms of a shift of local oxygen activity from equilibrium with the gas phase [1]. This approach has been successful in explaining phenomena at high temperature electrodes, such as large chemical capacitance of mixed-conducting film electrodes and appearance of Gerischer-type impedance with porous electrodes [2,3]. As well as the effect on the electrode properties, the interface potential shift can also affect the electrolyte. The Hebb-Wagner polarization technique utilizes the polarization at ion-blocking electrode to control the chemical potential within the electrolyte. The enhanced n-type or p-type conduction of ceria-based or proton conducting oxide electrolyte under water electrolysis operations is recognized as a practical problem of reduced energy conversion efficiency. However, the effect of overpotential at practical semi-blocking electrodes (or incompletely reversible electrodes) on the electrolyte is not well understood. The aims of this study is to elucidate whether the overpotential directly defines the chemical potential shift in the electrolyte near the interface. Two kinds of approaches were taken: i.e. direct measurement of the chemical potential in the electrolyte and non-linear equivalent circuit analysis of electrochemical responses. The direct measurements were made by using embedded probes in an oxide ion conductor. Yttria stabilized zirconia powder was pressed and sintered with five platinum wires of 0.1~0.2 mm thick. Porous platinum electrodes were pasted on the both ends of the sample and direct or alternating current was applied. Electric fields between the probes/electrodes were estimated by high frequency response of ac impedance, and subtracted from the electrical potential of the probes under dc. The distribution of chemical potential of electron, and thus, of oxygen was obtained assuming constant chemical potential of oxide ion. When oxygen exchange on the cathode was blocked using a glass paste, the oxygen potential at each probe shifted to the negative direction, and the change propagated slowly from the cathode side to the anode side. The result was in close agreement with the estimation from the mobility and carrier concentration of electrons in YSZ, suggesting that the probes successfully detected the local oxygen potential. Evaluation of chemical potential shifts with half-blocking electrode is in progress. For the equivalent circuit approach, a model circuit was developed to simulate the linear and nonlinear responses of electrode/electrolyte systems. The circuit consisted of charged carrier transport lines connected by capacitors exhibiting the local defect equilibrium, as proposed by Jamnik and Maier[4]. Unlike an equivalent circuit for a regular impedance analysis, the circuit elements were defined using parameters that varied with local chemical potentials. The responses of the circuit were calculated by using a general-purpose circuit analyzer LT-SPICE that enabled analyses of transient response as well as linear impedance. It also enabled higher harmonic analyses which is useful for detecting electrolyte polarization. The developed model was tested with a simple symmetric cell with mixed conducting electrode. Qualitative correspondence was obtained regarding the oxygen potential dependence in the linear impedance and the second harmonic responses. In addition to the oxide ion conductors, proton conductor with hole and oxide ion as minor carriers were modeled. Calculations with different assumptions of the oxygen (and hydrogen) potential shift will be compared with the ongoing experiments to find the effect of polarization of a half-blocking electrode. References [1] J. Mizusaki, K. Amano, S. Yamauchi, K. Fueki, Solid State Ionics, 22(4), 313-322 (1987) [2] T. Kawada, J. Suzuki, M. Sase, A. Kaimai, K. Yashiro, Y. Nigara, J. Mizusaki, K. Kawamura, H. Yugami, J. Electrochem. Soc., 149(7), E252-E259 (2002) [3] T. Kawada, Current Opinion in Electrochemistry, 21, 274-282 (2020) [4] J. Jamnik and J. Maier, Phys. Chem. Chem. Phys. 2001, 3, 1668-1678 (2001)