Solid Oxide Fuel Cell
One of the leading solid oxide fuel cell (SOFC) technologies employs the Ni-YSZ anode as mechanical support structure, where the issue of “RedOx” stability is a major challenge. In the present situation, fine nickel particles in conventional nickel-zirconia anodes exposed to high oxygen partial pressures at operating temperature (>700°C) will rapidly reoxidize to nickel oxides, thereby expand and possibly lead to mechanical fracture (usually of the thin electrolyte).
The aim of this project is to understand better the mechanisms behind expansion of anode-support during “RedOx” cycles and then to find a new microstructure that can bear such cycles.
Accelerated Testing Of SOFC Compounds
In the present context of global warming and increased energy demand, efficiency in energy conversion with minimal environmental impact is a major issue. Direct conversion of chemical energy into electrical energy with a high efficiency can be achieved using Solid Oxide Fuel Cells (SOFCs), which are devices that produce electricity via the electrochemical combining reactions of fuel and oxidant gases across an ion-conducting ceramic at high temperature. Combined heat and power generation (CHP) at high efficiencies can be obtained by stationary SOFC-CHP-units.
Although operation of SOFC-CHP plants have been successfully conducted, high cost and duration issues of these systems make it difficult to develop commercially viable industrial products. Especially, lifetime requirements exceeding 40’000 h are not fulfilled by current systems. On this time-scale it becomes unreasonable to test devices and systems in the laboratory in order to ensure system longevity. Rather, methods have to be found to accelerate the degradation of the SOFC devices over time in order to be able to predict the durability from much shorter testing periods.
Cathode Degradation By Chromium Poisoning
During SOFC operation, volatile chromium (Cr) species, stemming from Cr-containing stack and system compounds, principally metallic interconnects (MICs) and balance-of-plant (BoP) components, tend to deposit at the cathode/electrolyte/air triple-phase-boundary (TPB) blocking the active sites for oxygen reduction, known as Cr-poisoning. Already small quantities of Cr, at ppm level, can lead to a severe decrease of performance of standard state-of-the-art cathodes, which are generally composed of yttria-stabilized zirconia (YSZ) and strontium doped lanthanum manganite ((La,Sr)MnO3 abbrv. LSM).
Simultaneously, other degradation phenomena lead to additional SOFC stack/cell/compound performance decrease: among others, conductivity decrease of YSZ electrolyte, formation of insulating zirconates phases upon reaction between electrolyte YSZ and cathode LSM materials, resistance increase by the oxide layers growth on MICs; looking only on the air-side of a SOFC.
On the way to understanding and accelerated testing of Cr-poisoning, the latter degradation effect has to be isolated from the, above-mentioned, other processes. If this is not possible, these effects have to be understood to enable the deconvolution of all involved degradation signatures. Similarly, the tested system has to be protected against exogenous contaminants, stemming from test-rig and gas-lines, by dedicated testing setup and conditions.
Only in this conditions a satisfying understanding of Cr-poisoning is expected to be achieved. As no accelerated testing should be undertaken before understanding the underlying degradation phenomena, this project will turn more towards understanding.