Committee Members:
Prof. Meilin Liu, Advisor, MSE
Prof. Preet Singh, MSE
Prof. Hamid Garmestani, MSE
Prof. Matthew McDowell, ME/MSE
Prof. Angus Wilkinson, ChBE
Development of Proton Conducting Electrolytes with Enhanced Performance and Stability for Reversible Solid Oxide Cells
Abstract:
Reversible solid oxide cells (RSOCs) that efficiently operate under both fuel cell (fuel to energy) and electrolysis (energy to fuel) modes in a switchable manner are a promising technological solution for energy storage and conversion. Proton-conducting electrolytes, especially doped barium cerates are receiving increasing attention due to their promising conductivity at intermediate temperatures, which could enable operation of solid oxide fuel cells (SOFCs) and electrolysis cells (SOECs) with high efficiency. However, one of the reasons that they have not been widely adopted is the difficulty in finding an electrolyte material that possesses both high ionic conductivity and sufficient stability, especially against high concentrations of water and carbon dioxide. This work focuses on improving the electrochemical performance and chemical stability of RSOCs through the development of novel ionically conducting electrolyte materials.
The objectives of this work will include the optimization of ionic conductivity in proton conducting electrolytes by manipulating defect chemistry, development of electrolyte materials with enhanced chemical stability using novel dopants, improvement of single cell performance with the deployment of electrode compatible electrolytes, and fundamental understanding for the rational design of highly conductive and durable perovskite oxide-based electrolytes for RSOCs.
To complete the stated objectives, several hypotheses will be tested. First, a thorough study on the new BaHf0.1Ce0.7R0.2O3-? (BHCR172, R = Yb, Er, Y, Gd, Sm) series will be performed to uncover the complex correlations between R3+ and conductivity, ionic transference number, chemical stability, electrode (NiO) compatibility, and cell performance. Y is the most widely used acceptor dopant in barium cerate system, which gives high conductivity. However, Y may not be a good dopant for chemical stability, as predicted from DFT-based calculations. Also, the introduction of Y is known to cause reaction with NiO during cell fabrication, due likely to preferential cation segregation at elevated temperatures. Thus, replacing Y with another NiO compatible dopant might be a viable strategy to improve cell performance. The second hypothesis is that donor elements (Nb/Ta)-doped electrolytes will offer improved chemical stability against contaminants due to lower basicity than Zr/Ce-doped counterparts. While it is reported that Nb doping has negative impact on conductivity, it is proposed that excess acceptor doping can compensate the introduction of donor elements by creating more oxygen vacancies to improve charge carrier concentration.
The research objectives and hypotheses will be completed and tested using several scientific and technical approaches. Electrolyte materials with controlled composition will be fabricated and screened for conductivity, ionic transference number, compatibility with NiO, and short-term stability. Conductivity will be tested using electrochemical impedance spectroscopy (EIS) on dense electrolyte pellets, and ionic transference number will be determined using concentration cell method. NiO compatibility and short-term stability will be tested by checking the phase stability of electrolyte materials before and after exposure to contaminants (NiO and CO2) using X-ray diffraction (XRD). Materials which show adequate conductivity, ionic transference number, NiO compatibility, and short-term stability will go through long-term stability tests against H2O and CO2. The degree of degradation will be determined with XRD and Raman spectroscopy analysis. In addition, thermogravimetric analysis (TGA) and electrochemical performance testing under different atmospheres will be used to understand the hydration capability and transport properties of the candidate electrolytes, respectively. Finally, the best performing candidates will be used as electrolyte in single cells to demonstrate improvement in cell performance and durability in aggressive testing conditions.