Concentration and mobility of protons in H+-SOFC cathode materials

G. Raimondi, R. Merkle

Cathodic materials based on barium ferrite are prepared by doping the barium and iron sites [1] in order to obtain the desired electrode properties. In particular, my project is focussed on the effect of doping the iron site with dopants of different valence and size. So, I will synthesize BaFeO3 doped with Zn, In, Mg and Zr, using wet-chemical synthesis.

The characterization of the materials will include different techniques in order to probe their structural and electrical properties:

1. Measurement of the hole conductivity and discrimination of ionic and electronic conductivity as a function of temperature and oxygen partial pressure, using Electrochemical Impedance Spectroscopy (EIS).

2. Determination of the hole mobility by Seebeck or Hall effect measurements.

3. Investigation of local lattice distortions caused by the dopant (e.g. a tilting of the BO6 octahedra of the perovskite) using Extended X-ray Absorption Fine Structure (EXAFS) and Pair Distribution Function (PDF) analysis, in order to assess the local environment of iron, barium and of the dopant elements.

4. Measurements of proton uptake using Thermo Gravimetry (TG): this technique will allow to determine proton concentrations as a function of temperature and water vapour pressure.

Expected results

- Tuning the most suitable preparation route for a controlled synthesis of barium ferrite-based H+-SOFC cathode materials.

- Structural characterization, by both local (EXAFS and PDF) and long-range (XRD) techniques, of the different B-site doped barium ferrite oxides.

- Functional characterization of the differently doped barium ferrite cathodes, involving: capability of proton uptake; conductivity, ionic and electronic; compatibility with barium zirconate-based electrolytes.

-Establishment of structure-properties correlation for the barium ferrite compounds under investigation. 

Fig. 1: (a) possible reaction paths for oxygen reduction on proton-conducting electrolyte; (b) thermogravimetry for Ba0.5Sr0.5Fe0.8Zn0.2O3-d, showing absolute weight changes (-> proton concentration) as well as the transient behavior (-> proton diffusivity)

The proton concentration and proton diffusivity extracted from the thermogravimetry measurements on dense BSFZ pellets, a proton conductivity of about 10-3 S/cm is obtained at 400 °C. For 100 nm thick electrodes, this value is expected to suffice for enabling oxygen reduction on the whole electrode surface. The diameter dependence measured for dense thin-film BSFZ and BSCF microelectrodes confirms that oxygen reduction indeed proceeds via the bulk path.

Publications:

F. He, T. Wu, R. Peng and C. Xia,
J. Power Sources, 2009, 194, 263. 

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