Vis sammendrag
The main topics of this thesis are the surface kinetics and bulk diffusion of oxygen in Ba(1-x)Gd0.8La(0.2+x)Co2O(6-d) (BGLC). Previous work has identified BGLC as very promising electrode material for SOFC’s and electrolyzers. With conductivities in the range of ~1000 S/cm and polarization resistances in the order of 10^-2 Ocm2 around operating temperatures (i.e. 500-700 ºC). BGLC is in line to surpass current state of the art materials. Yet in order to introduce a new electrode, thorough characterization needs to be performed on its physicochemical properties. BGLC has been identified as double perovkite (double on the c-axis) by x-ray diffractometry (XRD), and synchotron measurements, in the form of AA'B2O(6-d). Cobalt occupies the B sites at the cell’s corners, with octahedral coordination of oxygen atoms, while barium and the lanthanides occupy the center of each square cell. Such a structure is highly defective; the heterovalency between barium and the lanthanides introduces oxygen vacancies in the material. The perfect structure is defined at BaGd0.8La0.2Co2O6 in order to establish a defect model. It is suggested that cobalt ions of differing valence (+2, +3 and +4) are evenly distributed through the material, and that the oxygen sites are ordered in layers surrounding the different cations. From this starting point, effects of temperature and atmosphere on the structure and properties of BGLC is studied. High temperature in-situ XRD in the temperature range from 25 to 800 ºC shows little change in the structure. As opposed to other perovskites (ex.: GdBaCo2O6-d), BGLC remains stable through operating temperatures. Lanthanum can be used to dope the barium sites without compromising stability, leading to more compositions. Synchotron data shows the majority of oxygen vacancies aggregated in the lanthanide layer (O1 site), a reduced amount in the cobalt layer (O2 site), and no vacancies present in the barium layer (O3 site). Such a flexible structure can be reduced or oxidized easily. Changes in temperature using a thermal balance revealed the oxygen non-stoichiometry as function of temperature down to [O] ~4.9. The same experiments were performed isothermally with changing pO2. The defect model proposed is able to model the data closely for BGLC and related double perovskites. These oxygen vacancies allow for ionic diffusion through the material. The transport regimes were divided in two: surface exchange of oxigen, and bulk diffusion (self-, chemical, and tracer). Thermogravimetry (TG), isotope exchange gas phase analysis (GPA) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) were employed to measure the diffusion coefficients (D) and surface coefficient (k) of BGLC at various temperatures between 350 to 800 ºC. Dchem was measured to 10^-6 to 10^-7 cm2/s at relatively low temperatures of 600 – 500 ºC with an equally low activation energy of ~0.74 eV. This leads to an oxide ion conductivity of 10^-2 to 10^-3 S/cm at operating conditions. The values were found to be competitive with state-of-the-art materials in literature. The pre-exponential values and activation energies are in close agreement to related double perovskites such as GdBaCo2O6-d and PrBaCo2O6-d. The oxygen surface exchange was studied using GPA, where a mass spectrometer tracks the concentration of species in the gas phase of a reaction chamber. Various reaction sequences are proposed for the oxygen exchange process of adsorption-dissociation-association-desorption. The desorption/adsorption steps were found to be rate limiting for this material. The surface exchange coefficient, k, is characterized by 6.0*10^-2 Exp(-66570/RT), a high value for k0 and low Ea for perovskites, but on par with novel double-perovskites. An imporant of feature of BGLC is the relatively high proton content (~1 mol%) under wet atmosphere. The hydration properties are included and modeled in the defect structure. The different basicity in the oxygen sites defined (O1, O2 and O3) is used to elucidate the hydration of the material. O3, the always-filled oxygen site is considered the main site for protonation due to its basicity (relative to the other oxygen sites). The adjacent vacancies in O2, a unique feature to BGLC, then allow for hydration. TGA hydration data resulted in an extracted entropy and enthalpy of hydration of ~ -120 J/molK and -46 kJ/mol respectively for all compositions of BGLC. Thourough agreement between the techniques used allow the characterization of the material properties, and the development of BGLC as future cathode material.
