Thesis Title: New Materials for Intermediate-temperature Solid Oxide Fuel Cells to be Powered by Carbon- and Sulfur- Containing Fuels

Abstract:  The demand for clean, secure, and renewable energy has stimulated great interest in fuel cells. Among all types of fuel cells, solid oxide fuel cells (SOFCs) offer great promise for the most efficient and cost-effective utilization of a wide variety of fuels such as hydrocarbons, coal gas and gasified biomass. The critical technical barrier to fuel flexibility is the vulnerability of the state-of-the-art Ni-YSZ (yttria-stabilized-zirconia) anode materials to coking and sulfur poisoning. In addition, the high operating temperatures of SOFCs, stemming from the low ionic conductivity of the electrolyte materials and the poor performance of the cathode materials at lower temperatures, increase costs and reduce the system operation life. Therefore, the main objective of the research is to develop new electrolyte and electrode materials with high electrical conductivity and electrocatalytic activity at low temperatures and to gain fundamental understanding of the interrelationships between lattice structure, local atomic environment, bulk transport, surface property and electrocatalytic activity.

Four research thrusts will be detailed in this presentation. First, a new electrolyte was shown to have the highest ionic conductivity below 750oC of all known electrolyte materials for SOFCs applications. Synchrotron-based X-ray diffraction and Extended X-ray Absorption Fine Structure (EXAFS) were employed to investigate the lattice structure and local atomic environment. Second, when used in combination with Ni as a composite anode, it was shown to provide excellent tolerance to carbon buildup (coking) and deactivation (poisoning) by contaminants commonly encountered in readily available fuels. The mechanism responsible for the enhanced electrocatalytic activity was unraveled by analyzing the anode surfaces using Raman spectroscopy and Scanning Auger Nanoprobe. Third, a simple, cheap surface modification of state-of-the-art Ni-YSZ anode was developed that could be more readily adopted in the latest fuel cell systems, demonstrating direct utilization of hydrocarbons, CO and gasified carbon fuels. Advanced electron microscopy and spectroscopy, Raman spectroscopy and Density functional theory (DFT) calculations were used to understand the anodic reactions occurring on nano-islands and nanostructured metal/oxide interface. Last, a new composite cathode with simultaneous transport of proton, oxygen vacancies and electronic defects was developed for low-temperature SOFCs based on oxide proton conductors.

In conclusion, this report represents a critical step toward an economically feasible fuel cell for utilization of a wide variety of readily available fuels as well as a unique mechanistic investigation of structure-property relationship and surface- and interfaces-involved chemical and electrochemical reactions.