Thermodynamic Stability and Interfacial Impedance of Solid-Electrolyte Cells with Noble-Metal Electrodes

Abstract

In solid-electrolyte cells, the electrode-electrolyte interfacial stability and impedance are found to be dependent on temperature, atmosphere, current density, microstructure and the process history of the cell. The modifications induced by temperature and oxygen pressure on the impedance spectra of Pt/Yttria-stabilized zirconia (YSZ) and Pd/YSZ interfaces have been studied. The interfacial impedance was controlled by adsorption/desorption of oxygen with a Langmuir-type dependency. When the surface coverage was small, the interfacial impedance decreased with increase in temperature and \(P_{O_2 }\). In certain temperature and \(P_{O_2 }\) regimes and depending on the process history, the metal electrode formed stable oxygen-containing species. In this region, the interfacial impedance increased markedly and its \(P_{O_2 }\) dependence also changed. Anodic and cathodic currents altered the local thermodynamic conditions at the charge-transfer sites and accordingly influenced the interfacial impedance. The concentration of oxygen-containing species and the interfacial microstructure are shown to influence the shape of the impedance response. Pt was found to form a neck at the YSZ electrolyte and Pd did not. The electrode polarization in the case of Pt/YSZ interface corresponded to one impedance-response arc signifying charge-transfer resistance at the three-phase boundary (TPB), gas/Pt/YSZ interface. For the Pd/YSZ interface, the electrode polarization corresponded to two impedance-response arcs at low \(P_{O_2 }\). The high-frequency response is related to charge transfer at the TPB and the low frequency to the gas-phase mass transfer.