Enhancing the lifetime of SOFC stacks for combined heat and power applications

• Forschungsstelle: BFE
• Projektnummer: 102290

Schlüsselwörter (Deutsch)

TP0076;F-Brennstoffzellen

Kurzbeschreibung (Deutsch)

Enhancing the lifetime of SOFC stacks for combined heat and power applications

Publikationen / Ergebnisse (Englisch)

On the foundation of a general literature survey the fundamental physical phenomena that dominate the degradation of Ni based anodes for solid oxide fuel cells have been identified. Microstructural changes under ideal operating conditions are due to (i) the coarsening of the metal grains in the metal ceramic composite. Because the metallic phase is a three dimensionally percolating network of the metal phase in a 3 D percolating ceramic matrix intermingled with a 3 D percolating pore structure. This system is thermodynamically highly unstable due to its large interfacial area under reducing conditions. The interface between Ni and the ceramic phase is weak due to (ii) bad wetting of metallic Ni on the ceramic. Third factor for degradation under ideal operating conditions is attributed to (iii) the local fuel utilization that might cause surface reactions (carbon deposition), Ni(OH)2 formation or segregation and diffusion of C into the Ni. All this factors lead to a constant decrease in properties of the anode. Under real operating conditions the disruption of fuel supply due to thermal cycling or system failures can affect the anode`s lifetime and performance drastically. From a thermodynamic point of view a disruption of fuel supply means a strong deviation of system`s equilibrium conditions. Once the anode is exposed to a critical oxygen partial pressure, the oxidation process forms Ni-oxides on the metal surfaces and might change in the case of a nonstoichiometric ceramic matrix its stoichiometry. The process of oxidation of Ni metal to NiO is accompanied by a volume change of 39.9%. This volume change causes additional tensile stresses on the ceramic network and might locally disrupt its mechanical integrity. The similar applies when reducing again the anode back to normal operation status and the resulting thickness of oxide layers. There are two strategies to improve anodes. On the one side one can try to reduce oxidation by doping the metal part of the composite by alloying. On the other hand during operation under H2 one can try to extend the catalytic active surface area by lowering the conduction band of ceramic matrix via dopants. This means to introduce a mixed electronic/ionic conducting ceramic phase in the cermet. In this case the transition probability for electrons from the Ni-conduction to the ceramics conduction band should be lowered, so that the active surface is not anymore determined by the triple phase boundary (TPB) line but by an area and its adjacent sites.

Auftragnehmer/Contractant/Contraente/Contractor:
ETH-Zürich

Autorschaft/Auteurs/Autori/Authors:
Galinski,Henning

Zugehörige Dokumente

• Enhancing the Lifetime of SOFC Stacks for Combined Heat and Power Applications | SOF-CH | WP 1: Enhancing the Redox Stability of NI-Based Anodes for SOFC - Zwischenbericht [PDF] 216 kB
• Enhancing the Lifetime of SOFC Stacks for Combined Heat and Power Applications | SOF-CH | WP 2 Cathode - Rapport annuel 2007 [PDF] 1'011 kB

Schlussbericht (Englisch)

The solid oxide fuel cell (SOFC) is a direct conversion process, which allows the production of electricity with high efficiency while maintaining pollutant emissions at low level. This technology is now far beyond the theoretical status, but extensive commercialization is not yet attractive. The main challenges to face are cost reduction, lifetime, reliability and volumetric power densities. Typical issues are the degradation rate of the performance during steady operation and the risk of failure during controlled or emergency shut-ups/start-ups. Post mortem analysis shows that the mechanical integrity of the cells is often not ensured. Modelling of the thermo-electro-chemical behaviour of an intact or partially damaged SOFC stack is required to predict and reduce the degradation. The influence of small-scale defects and microstructural changes increases with respect to time and the number of thermal and loading cycles. A multiscale approach is suited to understand and characterize the phenomena at the membrane electrode assembly (MEA) level and study their impact and propagation at the repeat element (RE) level. Once critical conditions are identified, a control strategy is required to ensure the safe operation of an aging stack during both steady and transient operation.

Auftragnehmer/Contractant/Contraente/Contractor:


Autorschaft/Auteurs/Autori/Authors:
Nakajo,Arata

Zugehörige Dokumente

• Enhancing the Lifetime of SOFC Stacks for Combined Heat and Power Applications | SOF-CH | WP 5.1: Thermomechanics - Final Report ZHAW-ICP 2007 [PDF] 1'019 kB
• Enhancing the Lifetime of SOFC Stacks for Combined Heat and Power Applications | SOF-CH | WP 5.1: Thermomechanics - Final Report EPFL-LENI 2007 [PDF] 441 kB