Design, synthesis, and impedance spectroscopy of triple ionic-electronic conducting thin film electrodes

Abstract

Triple ionic-electronic conducting oxides (TIECs) conduct electrons alongside at least two ionic species and are of interest for use in intermediate temperature (300°C-600°C) electrochemical devices. Transport properties of these materials are dependent on defect concentrations, which can be moderated by cation substitution and by operating conditions. Thus, effective improvement and development of these materials for targeted device applications necessitates comprehensive understanding of how these variables affect materials properties. In pursuit of this understanding, new tools were developed and applied to a promising TIEC material system. To begin, a review of TIEC materials is presented to summarize current understanding of defect formation and conduction mechanisms in single-phase TIECs. To build on this understanding, combinatorial materials synthesis and characterization methods were utilized and advanced. An instrument was developed for spatially resolved measurements of thin-film impedance at temperatures up to 300°C under dry or humidified gas (air or nitrogen). The probe arm was uniquely designed to direct localized gas flow toward the sample point being measured, enabling benchtop operation and allowing time-dependent measurement of impedance as the sample hydrates. The stage and probe assembly are also used to collect I-V curves to determine DC conductivity. Custom data processing procedures, including loading and visualization routines, were developed within a combinatorial data analysis software package. Lastly, the instrument was applied to a study of electronic properties of combinatorial thin film libraries of Ba(Co,Fe,Zr,Y)O3 (BCFZY), a material that is being explored for electrocatalytic applications in ceramic fuel cells and electrolyzers. The libraries were deposited by pulsed laser deposition (PLD). Device performance, as represented by polarization resistance of half fuel cells comprising BCFZY thin films atop an electrolyte layer, was evaluated as a function of temperature and gas atmosphere. The dependence of polarization resistance on chemical composition, temperature, and hydration are reported.