Committee Members: 

• Prof. Meilin Liu, Advisor, MSE
• Prof. Thomas Fuller, CHBE
• Prof. Mark Losego, MSE
• Prof. Matthew McDowell, ME/MSE
• Prof. Preet Singh, MSE

Enhancing the Stability and Performance of Solid Oxide Cells by Tailoring Surfaces and Interfaces through Surface Modification

Abstract:

Reversible solid oxide cells (RSOCs) are an extremely promising solution for efficient electric grid storage. However, breakthroughs in materials innovation are required for RSOCs to be implemented on a large scale, as several challenges remain to be fully resolved. Wide spread use is limited by energy loss due to sluggish electrode reactions and inadequate durability of key materials for prolonged operation, causing increased system costs. The main goal of this work is to enhance the stability and performance of RSOCs through surface or interface modification within the cell.

The air electrode is one area of focus, as the kinetics of oxygen reduction and evolution reactions are notoriously sluggish, resulting in large overpotentials and low energy efficiency. Further, the problem is often exacerbated by reactions with contaminates commonly encountered in ambient air (e.g., H2O and CO2) and from other cell components (e.g., Cr), leading to degradation in performance over time. To combat these problems, a surface sol-gel (SSG) process was developed to achieve layer-by-layer deposition of catalytically active catalysts (e.g., PrOx and BaO) on the surface of a porous air electrode, decreasing the polarization resistance and increasing the stability of the electrode. The advantages of the SSG process over conventional surface modification methods include excellent control of the composition, thickness, and uniformity of the coatings. In this dissertation, multiple different SSG coatings were investigated, including PrOx, CeO2, BaO, and CoOx. The deposition was validated with a quartz crystal microbalance to verify the linear addition of catalyst as a function of deposition cycles. The morphology of the coatings typically resembled numerous evenly dispersed particles across the electrode, as verified by SEM. Finally, the electrochemical performance of optimized coatings were investigated with electrochemical impedance spectroscopy and distribution of relaxation times. For PrOx and BaO modifications, the polarization resistance was greatly reduced by the surface modification, as the catalysts effectively decreased the impedance of the ORR reactions. Ultimately, the PrOx surface modification is demonstrated in YSZ-based single cells, increasing the peak power density from 0.32 to 0.46 W cm-2 at 650 °C.

The interface between the electrolyte and the air electrode is the other area of focus, where the electrolyte experiences degradation due to exposure to high concentrations of water during water electrolysis. Here, a dense and highly stable electrolyte composition is deposited on the surface of a more conductive electrolyte prior to the application of the electrode, creating an electrolyte protection layer or bilayer electrolyte. BaHf0.8Yb0.2O3 (BHYb) is shown to be much more stable than the more conductive BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb). Thus, a BHYb electrolyte protection layer was fabricated using co-sputtering, creating a BHYb/BZCYYb bilayer electrolyte that offers significantly enhanced stability with little to no impact on electrochemical performance. An epitaxial, dense, and uniform BHYb layer is shown effective in preventing electrolyte degradation against high concentrations of steam and  CO2 present in the air electrode, as confirmed by XRD, Raman spectroscopy, and SEM/TEM analyses (no formation of a significant amount of barium carbonate). Finally BHYb/BZCYYb bilayer-based single cells demonstrate state-of-the-art peak power densities of 1.64 W cm-2 at 650 °C, which is among the highest ever reported for proton-conducting RSOCs. Additionally, the cells demonstrate excellent stability in the fuel cell, electrolysis, and reversible modes.

Overall, this work demonstrated the power of surface modification in enhancing both performance and durability of reversible solid oxide cells. Modification of the air electrode surface improved the electrocatalytic activity for ORR while protecting the electrode from degradation. Similarly, modification of the electrolyte/electrode interface protected the electrolyte from degradation in high concentrations of H2O and CO2. Thus, the performance and stability of RSOCs was improved by tailoring the properties of the surfaces and interfaces within the cell.