Doped-ceria interlayer for solid oxide fuel cells [electronic resource]
Keywords: Materials Science and Engineering

Solid oxide fuel cells (SOFCs) have attracted widespread research attention for their high energy conversion efficiency and pollution-free characteristics. However, traditional SOFCs require high operating temperatures (> 800°C) to maintain sufficient power output. This temperature limitation poses significant challenges for fuel cells in terms of components compatibility and thermal stability. Many researchers have expended great effort trying to lower the operating temperature of SOFCs to under 500 °C. One of the biggest challenges for SOFCs working at a low temperature regime is that oxide ion incorporation kinetics are greatly reduced, causing a large activation loss at the cathode/electrolyte interface. The research described in this dissertation focuses on decreasing this activation loss and therefore increasing fuel cell performance by utilizing a ceria-doped interlayer. The first part of the dissertation is a detailed explanation of the rationale of adding a cathodic interlayer to solid oxide fuel cells. The purposes and expected functionalities of this interlayer are presented in detail. Then, we discuss potential candidates for interlayer materials and the fabrication process of the chosen material. Yttria-doped ceria (YDC) was selected as the interlayer material because of its high ionic conductivity and superior oxide ion incorporation kinetics. In regard to process choice, we investigated a wide range of film fabrication methods and picked atomic layer deposition (ALD) as the growth method for YDC thin films. Experimental details, such as the ALD chamber design, precursor selection, temperature settings, etc., were probed and explicated. Various characterization methods were employed to study the deposited film properties. The second part of the dissertation discusses the doping effects of the YDC interlayer. A thin YDC layer was utilized as a cathodic interlayer for SOFCs, and its Y2O3 doping level was finely tuned by ALD to maximize fuel cell performance. According to the performance enhancement factor, the optimal doping recipe for YDC was identified as 6Ce1Y, which yields a 14 M% Y2O3 doping concentration; and 6 cycles of ceria deposition with one cycle of yttria deposition complete one ALD super cycle of YDC deposition. Impedance analysis suggests that the enhanced power densities of the interlayered fuel cells are due to enhanced oxide ion incorporation kinetics at the Pt- YDC interface. The third part of the dissertation describes the thickness effects of the interlayered YDC films. The minimum number of YDC super cycles needed to form a complete film was determined to be 10. Auger elemental mapping and X-ray photoelectron spectroscopy (XPS) analysis were employed to confirm our hypothesis. Inserting this YDC layer with the minimum required thickness can avoid unnecessary Ohmic losses of the given fuel cells, and can therefore further enhance fuel cell performance at low operating temperatures. By optimizing both the concentration and thickness of the inserted YDC layer, we determined its optimal ALD recipe as (6Ce1Y) × 10. The optimal YDC interlayer identified in this research has significant implications in interfacial engineering. The last part of the dissertation describes how, with the help of this optimal YDC interlayer, the performance of a nanoscale fuel cell can be enhanced by a factor of 1.4 at low operating temperatures (< 500 °C). This performance could potentially be even further enhanced by utilizing more advanced fuel cell fabrication techniques, such as employing fuel cells with corrugated membranes.