Final report on the characterization of the film on inert anodes

Abstract

_2_ r • SUI_RY Results of post-test microscopic and elementdl analysis of the reaction zone on polarized cermet inert anodes, over a range of current densities and alumina concentrations, suggest that an alumina film does not form to protect the anode from dissolution. Rather, significant morphological and compositional changes occur at or near the anode surface. These changes and the • chemical reactions that cause them involve the cermet material itself and appear to be responsiblefor propertiesthat were previouslyassigned to an " alumina film. In particular,a reaction layer formed from the cermet material may have protectiveproperties,while changes in roughness and porosity may contributeto the electrochemicalimpedance. iii ACKNOWLEDGMENTS The authorsacknowledge the assistance of the staffat the PacificNorthwest Laboratory (PNL),in particular the managerial guidanceprovidedby L. G. Morgan,the technical contributions of D. M. Strachan, the microscopy work done by N. T. Saenzand J. M. McCarthy, and the secretarial supportby D. L. Schneider.We are also gratefulfor the programmatic assistance pro--videdby M. J. McMonigle, Officeof .Conservation and Renewable Energy_,CE), U. S. Department of Energy(DOE),Washington, DC, and the RichlandOperations -Office,DOE.

Results from a pilot cell test of cermet anodes

“…Another type of reaction that was observed in the reaction layer was the apparent substitution of Al for Fe in the ferrite phase. This type of reaction was observed in numerous other studies (Weyand et al 1986;Windisch and Stice 1991) and it is unlikely that it can be completely eliminated. Pre-• vious work (Windisch and Stice 1991) indicated that the substitution might involve (at least initially) the small number of excess Fe ions that exist in 6.5 the ferrite phase.…”

“…This type of reaction was observed in numerous other studies (Weyand et al 1986;Windisch and Stice 1991) and it is unlikely that it can be completely eliminated. Pre-• vious work (Windisch and Stice 1991) indicated that the substitution might involve (at least initially) the small number of excess Fe ions that exist in 6.5 the ferrite phase. These Fe ions probably exist as Fe 2 + and are more likely to be oxidized or react under anodic polarization {because, unlike the other cations in the ferrite, the divalent Fe has a higher oxidation state available).…”

Final report on the PNL program to develop an alumina sensor. Sensors Development Program

“…The results of the studies performed in FY 1989 and FY 1990 indicated that the principal source of the anode impedance was not a film but rather the oxygen gas bubbles produced during electrolysis at the anode. This explanation was provided after numerous laboratory experiments that were discussed in three separate PNL reports Windisch and Stice 1991a;Windisch and Stice 1991b) and also in one literature publication . In these studies, the anode impedance was also found to depend strongly on anode surface structure and on cell geometry.…”