Pd-doped perovskite oxide for symmetric solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are alternative energy devices with relevant advantages such as: high efficiency, fuel flexibility, low emissions and relatively low cost. Perovskite oxides represent the state of art of cathode materials because of their high structural stability and high catalytic activity toward oxygen reduction reaction (ORR). Specifically, Sr-doped lanthanum ferrites, such as La1-xSrxFe1-yMyO3-with M= Mn, Co or Ni (LSFM), have also shown the proper thermal-mechanical stability and mixed ionic-electronic conductivity (MIEC) for cathode application. More recently, LSFM were explored as anodes too, to overcome the well-known limitations of conventional Ni-YSZ anode and guarantee the cell stability in presence of alternative fuels such as bio-fuels and hydrocarbons. However, the main drawbacks of LSFM in reducing conditions are: structure instability and low catalytic activity. B-site doping with a small amount of a noble metal cation canbe a strategy to significantly improve the catalytic activity and the structural stability. In my thesis, La0.6Sr0.4Fe1-xMxO3- perovskite oxides doped with low (5 mol% and 10 mol%) molar percentages of Pd and labeled as LSFPd are investigated as electrodes for intermediate temperature SOFCs. The structural stability and reversibility are investigated switching between oxidizing and reducing conditions. The compounds are used both as cathode and anode in a symmetric solid oxide fuel cell (SSOFCs) configuration. The catalytic and electrocatalytic activities are also explored in presence of methane-based biogas mixtures and fuel cells tests are performed by using different fuels. Part A of my thesis is dedicated to the electrochemical description of SOFC and to the state-of-the-art of materials for IT-SOFCs. The focus is on materials with perovskite structure and their performance for cathodic and anodic applications. The structural, conductivity and electrochemical properties are described in detail. In particular the behavior in reducing conditions is discussed. The catalytic properties of perovskite oxide toward dry reforming reaction and partial oxidation of methane are also reported. Part B contains a first chapter with the description of the experimental procedures for the oxides synthesis and the techniques used for structural and microstructural characterizations. The experimental setup for the catalytic activity measurements, the electrical and electrochemical characterizations are also described. Finally, single cell fabrication and fuel cell tests are also detailed. In the following chapters, three published manuscripts are reported. In the first one, the structure reversibility of La1-xSrxFe0.95Pd0.05O3- oxide is investigated switching between oxidizing and reducing atmosphere. By exposure to reducing atmosphere, the presence of exsolved metallic nanoparticles is monitored. In the second manuscript, the electrocatalytic activity of LSFPd oxides toward the oxygen reduction reaction and hydrogen oxidation are evaluated as function of Pd doping. Electrolyte supported symmetric cells based on La0.8Sr0.2Ga0.8Mg0.2O3- (LSGM) electrolyte are prepared and tested using hydrogen as fuel. The third manuscript is focused on the investigation of the catalytic activity of LSFPd oxides toward the dry reforming of methane reaction (DRM) and partial oxidation reaction of methane (POM). Fuel cell tests are reported by using different methane mixtures. The effect of (5mol%) Ni addition to the anode is also explored. The stability toward carbon deposition is investigated.