Baukje de Boer

Ph.D. thesis

Thesis title:

SOFC Anode - Hydrogen oxidation at porous nickel and nickel/yttria-stabilised zirconia cermet electrodes

[thesis in pdf format]




Prof. Dr. Ir. H. Verweij

Assistant promotor:

Dr. H.J.M. Bouwmeester


Nickel/yttria-stabilised zirconia cermet is the state-of-the-art anode material for SOFCs. This thesis describes a number of experimental studies (impedance and I-h measurements) on different types of anodes, with emphasis on the kinetics of the hydrogen oxidation reaction and the effect of the microstructure on the electrochemical performance.

Porous nickel electrodes are studied because of their relative simple microstructure, which is characterised with image analysis in term of the Triple Phase Boundary (electrolyte/electrode/gasphase) length. A linear relation is obtained between the TPB length and the electrode conductivity. pH2 and pH2O dependent data indicate that a simple description in terms of a Butler-Volmer formalism is excluded. A strong variation in the fractional coverage of adsorbed intermediates with overpotential h is suggested.

Modification of porous nickel electrode with fine YSZ resulted in a significant improvement (total electrode resistance decreases with more than 50%), which is ascribed to an increase in reaction sites.

The microstructure of nickel/YSZ cermet electrodes was modified by using different fractions coarse and fine YSZ. The microstructures was quantified in terms of porosity, nickel particle size and surface coverage of the electrode /electrolyte interface. A ladder network model was used to obtain more insight in the spatial extension of the TPB perimeter in the electrode. The microstructure of cermets does not change the requirements regarding the thickness of the electrodes. The kinetics of the hydrogen oxidation reaction at the cermet electrodes is studied. The impedance spectra show a complex behaviour at the low frequency side, which is not completely understood.

Concluding: The linear relationship between the TPB length (related with the active area for the electrode reaction) and the total electrode conductivity for the nickel electrodes suggests that for cermet electrodes part of the bulk is active in the electrode reaction. The results of reaction kinetic studies at the different types of electrode support the conclusion that charge transfer governs electrode processes.