Strontium-Diffusion in Cer-Gadolinium-Oxid als Degradationsmechanismus der Festoxid-Brennstoffzelle
Jülich / Forschungszentrum Jülich, Zentralbibliothek (2015, 2016) [Book, Dissertation / PhD Thesis]
Page(s): III, 160 S. : Ill., graph. Darst.
Solid oxide fuel cells possess the potential to be used in stationary applications, especially in combined heat and power plants, where the produced heat and power can be used simultaneously. For such applications long lifetimes of about 10 years and a power degradation lower than 10 % are required. In this work solid oxide fuel cells were characterised concerning their long time degradation behaviour. Focus was laid on the investigation of cell-intrinsic degradation phenomena. A literature review exposed several degradation phenomena, that influence the solid oxide fuel cell’s behaviour during the life time of about 10 years. State of the art cells show a very low power degradation of 0.2 %/1.000 h. However, due to the choice of materials an undesired formation of strontium zirconate occurs on top of the electrolyte. This leads to an increase in cell resistance. Strontium zirconate is formed due to the decomposition of the cathode material (La0,58Sr0,4Co0,2Fe0,8O3−δ), leading to the release of strontium. Strontium diffuses through the CGO (Ce0,8Gd0,2O2−δ)-barrier and the zirconate is formed by the reaction with the zirconia from the YSZ (Zr0,84Y0,16O2−δ). The literature review additionally showed no alternative materials, which exhibit a competitive power output and comparable low degradation with a lower tendency for second phase formation. In this work a model was proposed to estimate, how the cell resistance increases with increasing amount of strontium zirconate. Additionally, preparation conditions to reduce the formation of strontium zirconate were deduced. The diffusion of strontium through CGO-layers was characterised by model experiments. CGO-layers containing different grain sizes and different amounts of fast diffusion path were prepared. Hereby it was possible to differentiate between the diffusion along grain boundaries and the diffusion along inner surfaces. According to the proposed model the influence of the strontium zirconate on the cell resistance was estimated. Aditionally, the solid oxide fuel cell degradation was characterized by electrochemical examination. The resistance model showed, that the formation of strontium zirconate is reduced at low temperatures, and by employing a thicker and denser CGO-layer. At 700 ◦C the strontium zirconate formation does not limit the fuel cell operation over a lifetime of 10 years. The electrochemical experiments showed, that the degradation can be reduced by a decrease in current density as well as an operation at high partial pressures of oxygen and hydrogen respectively. In general, the formation of strontium zirconate is a subordinary degradation mechanism, which is not recognizable for a long run time. Starting from a electrolytes surface coverage of 90 % with strontium zirconate, a resistance increase is noticeable. From this point on the resistance increases disproportionaly strong. This critical surface coverage corresponds to the end of the cells life time.