Model electrodes for understanding solid oxide fuel cell reactions

Technical solid oxide fuel cell electrodes are complicated by a porous geometry, and as such model electrodes are commonly used in studies focussing on elucidating mechanistic detail. However, similar studies report contrasting electrode performances. Inaccuracy in measuring the electrochemically active area – the triple phase boundary – has been suggested in the literature to account for unexpected results. This inaccuracy has been proposed to be caused by defects introduced during electrode fabrication, which are not accounted for in triple phase boundary length measurement. The hypothesis investigated in the thesis is that microstructural changes taking place in model electrodes during electrochemical testing, are also contributing to the triple phase boundary length inaccuracy, and causing a variation in electrode performance between similar studies.

Microstructural changes occurring during electrochemical testing at open circuit voltage in both H2 and CO based atmospheres were identified for a range of pattern electrode thicknesses, and the impact of these microstructural changes on the electrochemically active area was assessed. Electrodes 300 nm and thinner were found to be susceptible to solid-state dewetting during testing, whilst 1 µm thick electrodes were found to degrade differently in H2 and CO based atmospheres. Pores developing in the electrode microstructure were found to affect the line specific resistance values by two orders of magnitude in both H2 and CO atmospheres.

Other causes of electrode performance variation are also investigated, including an additional area-scaling pathway for hydrogen oxidation, which is evaluated by comparing the performance of identical nickel and platinum pattern electrodes. An area-scaling pathway in which hydrogen diffuses through the bulk of the metal electrode was found to be plausible based on the hydrogen permeability of the metals, whilst an oxygen diffusion pathway does not correlate with the results. Magnifications used to measure triple phase boundary length and the model electrode substrate are also shown to influence electrode performance and the activation energies.

Finally, several methods for limiting the solid-state dewetting process in nickel pattern electrodes were investigated. Nano-additives and the use of polymer spheres under nickel pattern electrodes were found to slow the rate of dewetting compared to analogous unmodified electrodes and provide a route for further research.