Resumen de Microstructural tailoring of nanocomposite electrodes for solid oxide fuel cells
The high demand for electrical energy induced by the rapid population growth has arisen the necessity to develop sustainable and environmentally friendly energy sources. In this context, fuel cells are one of the most promising technologies to obtain electrical energy from a wide variety of fuels with good efficiencies and lower emission of pollutants. In particular, Solid Oxide Fuel Cells (SOFCs) have attracted great attention in recent years due to their fuel flexibility, good tolerance to impurities in the fuel and higher efficiencies.
However, the high operating temperatures of SOFCs (600-800 ºC) needed to achieve a good electrode performance and a sufficient ionic conductivity for the electrolyte, negatively affect the long-term stability of these devices. For this reason, decreasing the operating temperature is one of the main goals for the wide implementation of SOFCs. It is well known that the crystal structure and composition of the electrodes play a key role in the electrochemical performance; nevertheless, the microstructural optimization of the electrodes has demonstrated to be crucial to boost the electrochemical properties at low operating temperatures in both oxidizing and reducing conditions.
In this PhD thesis, different nanostructured and nanocomposite electrode layers based on the combination of perovskite-type electrodes, i.e. LaCrO3, SrTiO3 or LaFeO3 and the ionic conductor Ce0.9Gd0.1O1.95 (CGO) with fluorite-type structure have been prepared and tested for their implementation in SSOFCs. The electrode layers were prepared directly onto Zr0.84Y0.16O1.92 (YSZ) or La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) electrolytes by spray-pyrolysis. Additionally, pulsed laser deposition (PLD) was employed for the preparation of active layers. For comparison purposes, the same electrode compositions were prepared as powders from freeze-dried precursors and then deposited onto the electrolyte by screen-printing method.
The results revealed that La0.98Cr0.75Mn0.25O3-δ-CGO and (La0.8Sr0.2)0.95Fe0.8Ti0.2O3-δ-CGO are promising electrodes for symmetrical SOFCs with good electrochemical properties and durability in both oxidizing and reducing atmospheres. In particular, La0.98Cr0.75Mn0.25O3-δ-CGO exhibits polarization resistance of 0.09 Ω cm2 at 700 ºC in H2, comparable to the state-of-the-art Ni-YSZ anode.
Nanocomposite active layers were also prepared by spray-pyrolysis deposition to improve the ORR activity of a LSM cathode. Among the different compositions, LSM-CGO layers showed improved adherence and electrical properties. The incorporation of this active layer enhances the ion transfer at the cathode/electrode interface and also extended the ionic/electronic conducting paths for electrochemical reactions. A Ni-YSZ/YSZ/LSM-CGO/LSM anode-supported cell showed a maximum power density of 1200 mW cm-2 at 800 ºC compared to 790 mW cm-2 for the same cell without an active layer.
Vertically Aligned Nanostructures (VANs) of (La0.8Sr0.2)0.98Fe0.8Ti0.2O3−δ-CGO and (Sr0.7Pr0.3)0.95Ti0.9Ni0.1O3−δ-CGO were prepared by PLD for their implementation as redox stable active layers for SOFCs. The heteroepitaxial films exhibited long-range columnar architecture of 5 nm width. The VAN films showed higher conductivity than that observed for the polycrystalline samples.
In conclusion, this thesis has demonstrated the great influence of tailoring the electrode microstructure to improve the electrochemical properties of SOFC electrodes.
