Electrode/Electrolyte Interphase Characterization in Solid Oxide Fuel Cells

ANALÍA SOLDATI; LAURA BAQUÉ; HORACIO TROIANI; ADRIANA SERQUIS

Hydrogen Energy - Challenges and Perspectives

In Tech

Lugar: Rijeka; Año: 2012; p. 279 - 304

Fuel cells (FC) have reached a recognized place within the potencially efficient devices to convert chemical energy into electricity. First developed by William Grove in 1835,  FC had a profound impact in the aerospatial industry and more rencently gained special attention for wider applications, because they are considered as environmental friendly devices. Indeed, when Hydrogen is used as fuel, the device produces only water as residue, with no impact on the undesidered CO2 emissions. Between the different FC types, those based of solid oxides (SOFC) found a place in mobile and stationary applications. These cells are operated between high (1000ºC) and intermediate (700ºC) temperature ranges and the principal characteristic is that their fundamental parts consist on ceramic materials. The reduction of the oxygen molecule takes place in the cathode, liberating two electrons; afterwards the O2- ion travels through the electrolyte to the anode material, where it oxidizes the hydrogen molecule, releasing a  water molecule and closing the circuit.  Good gas permeability, high electron (and oxygen ion) conductivities and low inter-material reactivities are a material requirement for a sucessfull operation, independent of the electrode/electrolyte nature. On one hand, the combination of mixed conductors such as rare earth doped perovskites with Ceria or Ytria are considered good electrode/electrolyte choices . On the other hand, reducing cathode particle size has shown to be a good alternative to decrease the cathode area specific resistance (ASR) and improve the general efficiency of the device.  Other important point to optimize is the assembly electrode/electrolyte interface.,  In situ observations of microstructure, morphology and composition at the atomic scale at the interfacial area are needed in order to understand the phenomena that occur in this region. In this chapter we present a review of current techniques that can be used to select and study this challenging region, based on literature data and our own research in that field.  One of the most versatile tools to prepare samples of selected areas for in situ microscopy and spectroscopy analyses is the Focused Ion Beam (FIB). With help of an electron microscope a desired area can be selected with micrometric precision and a Ga-ion beam is used to extract a thin sample of the bulk. This foil, with only some nm thickness, can be after analyzed with scanning (SEM) or transmission (TEM) microscopes and related techniques. Z-contrast, Energy dispersive spectroscopy (EDS) and Energy Loss Spectroscopy (EELS) are commonly choices for composition analyses. Electron Backscatter Diffraction (EBSD) is used for texture analysis. FIB-SEM micro-tomography allows finding structure related parameters as pore concentration, tortuosity or contact area. We will present results of these kinds of analyses in two cathode/electrolyte interfaces of intermediate temperature (IT-) cells. The two assemblies studied comprise a nanostructured cathode of La0.4Sr0.6Co0.8Fe0.2O3-d (LSCF) in the first case and La2NiO4+d (LNO) in the second case, deposited over Ce0.9Gd0.1O2-d (CGO) electrolytes by spin coating. The LSCF/CGO half cell presents a semi-coherent interface at the atomic level, with very high percentage of contact area. These structural properties contribute to the good conductivity and lower ASR values measured in this assembly. Besides, the LNO/CGO interface shows high interfacial reactivity, with zones of change of composition, formation of other phases and interfacial porosity. In this latter assembly, structural, composition and morphological observations could be related to the low general efficiency and high degradation rates under operation conditions.