Understanding mechanisms that affect degradation and durability of solid oxide fuel cell (SOFC) stacks and systems is becoming increasingly important as systems are being deployed around the world. A novel simulation approach accounting for state-of-the-art understanding of degradation mechanisms in the context of an operating SOFC system has been developed. Investigative tools simulating lifetime degradation have been applied to a physical SOFC system model designed and controlled to allow dynamic dispatch. The system was operated in two modes (a) constant power output mode, and (b) diurnal dynamic dispatch mode. Results show that on a time basis the SOFC system with dynamic dispatch proved more durable and less degraded than the system operated in constant full-power output. The net energy production was roughly equivalent between the two modes, with the dynamically dispatched fuel cell degrading slightly less and producing a greater portion of power during peak demand. ©The Electrochemical Society.

Solid oxide fuel cell (SOFC) stack design must yield the highest performance, reliability and durability to achieve the lowest cost of electricity delivered to end-users. Existing modelling tools can cope with the first aim, but cannot yet provide sufficient quantitative guidance in the two others. Repeating unit models, with as degradation processes the decrease in ionic conductivity of the electrolyte, metallic interconnect corrosion, anode nickel particles coarsening and cathode chromium contamination are used to investigate their distribution, evolution and interactions in a stack. The spatial distribution of the degradation is studied for the operating conditions optimised in Part I for the highest system electrical efficiency during long-term operation under constant system power output. Current-voltage characterisations performed at different times underestimate the degradation. In the present conditions, the degradation of the cathode dominates. The lower and more uniform cathode overpotential in counter-flow configuration, combined with the beneficial effect of internal reforming on reducing the air-fuel ratio yields the highest lifetime, because it alleviates chromium contamination and interactions between the degradation processes. Increasing the operating temperature alleviates cathode chromium contamination. The beneficial decreases of the cathode overpotential exceed the detrimental higher release rate of chromium species from the metallic interconnect. © 2012 Elsevier B.V.

Electrochemical and mechanical aspects in solid oxide fuel cell stack must be understood to meet the reliability targets for market implementation. This study presents a stack modelling framework that combines thermo-electrochemical models, including degradation and a contact finite-element thermo-mechanical model. It considers rate-independent plasticity and creep of the component materials and proposes periodic boundary conditions to model the stacking of repeating units. This Part I focuses on the effects of the operating conditions and design alternatives. In the present conditions, the stresses in both the anode and the cathode contribute to the probability of failure (Pf), which can be lowered by adjusting the operating conditions. The requirements for mechanical reliability are here opposite to those that alleviate electrochemical degradation. Gas-diffusion layers (GDL) and interconnect design alternatives and stacking have a lower impact on the Pf, but affect the contact pressure on the GDLs, which can cause electrical contacting challenges. © 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Intricate relationships between mechanical and electrochemical degradation aspects likely affect the durability of solid oxide fuel cell stacks. This study presents a modelling framework that combines thermo-electrochemical models including degradation and a contact thermo-mechanical model that considers rate-independent plasticity and creep of the components materials and the shrinkage of the nickel-based anode during thermal cycling. This Part II investigates separately or together the contributions of mechanical and electrochemical degradation on the behaviour during long-term operation and thermal cycling. Electrochemical degradation modifies the temperature profile under constant system power conditions and consequently the risks of cell failure. Irreversible deformation of the stack components causes losses of contact pressure during thermal cycling and variation of the electrical load and changes the risks of anode and cathode cracking. Critical tensile stress progressively develops in the cell compatibility layer (GDC between YSZ and LSCF) during thermal cycling depending on the temperature profile in operation. © 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Understanding of mechanisms affecting degradation, durability and performance of solid oxide fuel cell (SOFC) stacks and integrated systems has become important due to increasing worldwide deployment. A novel simulation approach accounting for state-of-the-art understanding of degradation mechanisms in the context of an operating SOFC system has been developed. Investigative tools simulating lifetime degradation have been applied to a physical SOFC system model designed and controlled to allow dynamic dispatch. The system was operated in two modes (a) constant power output mode, and (b) diurnal dynamic dispatch mode. Results show that on a time basis the dynamically dispatched SOFC system proved more durable and degraded less than a system operated at constant full-power output. The net energy production over the stack lifetime was roughly equivalent between the two modes (1.5 to 2 kWh cm-2), with the dynamically dispatched fuel cell operating longer, degrading less and producing more power during highly valued peak demand periods. © 2011 The Electrochemical Society.

The degradation of solid oxide fuel cells (SOFC) depends on stack and system design and operation. A methodology to evaluate synergistically these aspects to achieve the lowest production cost of electricity has not yet been developed. A repeating unit model, with as degradation processes the decrease in ionic conductivity of the electrolyte, metallic interconnect corrosion, anode nickel particles coarsening and cathode chromium contamination, is used to investigate the impact of the operating conditions on the lifetime of an SOFC system. It predicts acceleration of the degradation due to the sequential activation of multiple processes. The requirements for the highest system efficiency at start and at long-term differ. Among the selected degradation processes, those on the cathode side here dominate. Simulations suggest that operation at lower system specific power and higher stack temperature can extend the lifetime by a factor up to 10, because the beneficial decrease in cathode overpotential prevails over the higher release of volatile chromium species, faster metallic interconnect corrosion and higher thermodynamic risks of zirconate formation, for maximum SRU temperature below 1150 K. The counter-flow configuration, combined with the beneficial effect of internal reforming on lowering the parasitic air blower consumption, similarly yields longer lifetime than co-flow. © 2012 Elsevier B.V.