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Alloys for Fuel Cell Interconnects

#3385


Development of Ti-Based Alloys with Intermetallic Coatings for SOFC Interconnect

Tech Area / Field

  • MAT-ALL/High Performance Metals and Alloys/Materials
  • NNE-BCM/Batteries and Components/Non-Nuclear Energy

Status
3 Approved without Funding

Registration date
31.10.2005

Leading Institute
VNIIEF, Russia, N. Novgorod reg., Sarov

Supporting institutes

  • A.N.Frumkin Institute of Physical Chemistry and Electrochemistry, Russia, Moscow\nHigh Temperature Electrochemistry Institute, Russia, Sverdlovsk reg., Ekaterinburg\nTsNIIChermet (Ferrous Metallurgy), Russia, Moscow

Collaborators

  • International Association for Hydrogen Energy, USA, FL, Coral Gables\nUniversity of Connecticut / School of Engineering, USA, CT, Storrs

Project summary

Planar solid oxide fuel cells are a new type of alternative electrochemical devices, which, if commercialized successfully, will provide environmentally clean noiseless and renewable energy for backup and primary power sources.

The primary task is development of economically efficient technologies and materials for fuel cell (FC) components, methods of improvement of FC performance characteristics. One of key components of planar SOFC determining its lifetime characteristics is a separator/interconnect that performs the functions of a principal construction unit providing distribution of fuel/oxidizer gas flows and electric commutation of fuel cells within the stack.

The main requirements towards separators/interconnects for operation with various electrolyte types are as follows: a low value of thermal expansion coefficient (TEC) - (10-12)*10-6C-1 within the temperature range of (700-800)oC; good electric conductivity within the temperature range of (700800)C; high corrosion resistance in cathode and anode mediums within the temperature range of (700-800)oC.

The main factor of construction stability of planar SOFC is the match of TEC dependence on the separator/interconnect temperature with the electrolyte’s TEC.

Mismatch of TEC dependencies in the elastic deformation area and impossibility of relaxation processes at temperatures up to 600oC while thermal cycles leads to mechanical deformation and destruction of the electrolyte, and, as a result, to operability fault of the whole FC. Ferrite Fe-Cr steels are, thanks to low cost thereof, most commercially attractive materials for separators/interconnects of planar SOFC. The TEC of various types of commercial ferrite steels makes up (12-12,8)*10-6C-1 at temperatures of ~800oC. Oxidation stability of ferrite Fe-Cr steels is provided via formation of an oxide film on the surface during operation; the said film being formed, mainly, by Cr2O3 that has a low electric resistance level at 800C. RFNC-VNIIEF in cooperation with a number of Institutes conducted research of corrosion resistance of such steel type Fe-25Cr, including at contact with cathode materials of MLS type (La0,6Sr0,4MnO3).

For ferrite steel Fe-25Cr, Cr diffusion into MLS samples to the depth of up to 20 µm at contact with steel without coating was revealed. X-ray analysis has shown that the oxide film on the steel surface is formed, basically, with Cr2O3 and 3(Cr2O3)Fe2O3 compound. In addition, γ-Fe and Ni and Fe-containing composite compounds were found out on the surface. Thus, due to Cr diffusion into cathode materials the destruction/change of the oxide film based on Cr oxide takes place. Essential doping of Fe-Cr steel with components improving oxidation resistance via Cr binding leads to ferrite structure change and TEC rise. One of possible ways of oxidation stability improvement is to apply coatings based on composite oxides of rare-earth metals while using, for example, plasma spraying or magnetron sputtering techniques. But these coatings are porous, and Cr diffuses easily through them. Oxygen also diffuses but in reverse direction and penetrates into the support while oxidizing it and changing its structure.

The Project goal is to develop Ti-based alloys for SOFC interconnects with intermetallic coatings that provide heat resistance and electric conductivity under anode and cathode conditions.

We propose to apply a new approach towards creation of alternative materials for separators/interconnects, whereat the primary task is creation of a metal substrate from an alloy with the TEC being maximally close to the TEC of electrolytes of various types, especially within the low-temperature area, where stress relaxation is not possible. The most suitable material for the substrate are alloys based on Ti with Nb, V, Mo, which allow to obtain TEC within a preset range. Preliminary studies conducted at RFNC-VNIIEF jointly with a number of Institutes have shown that the TEC values within the range of (7-12)*10-6 C-1 can be attained via combining the content of α''- and
β
-phases in titan-niobium and titan-vanadium alloys. In particular, for Ti-32%V and Ti-36%Nb alloys the TEC values (*10-6 C-1) 10,2 and 10,8, respectively, were obtained at the temperature of 800oC. Ti-based alloys have a low level of contact electric conductivity. We propose to use intermetallic coatings for modifying composition and structure of the alloy surface in order to provide corrosion resistance and electric conductivity.

Most promising is NiAl intermetallide that preserves long-term stability up to 1200oC. At this temperature NiAl has a low corrosion rate (at most 0,07 g/(m2h)). The specific resistance of NiAl makes up only 8,5*10-6Ωcm. The specific resistance of TiAl intermetallide is higher, i.e. 33*10-6Ωcm, but is has a higher ability to diffuse into the Ti-based alloy.

The work results will be of scientific and practical value and will serve as a step towards commercialization of one of most fast-evolving energy areas. The Project will be fulfilled by a team of highly qualified specialists, the most of which are involved in development of defence products. The team includes Doctors and Candidates of science, specialists in chemistry, physics, material study, automation of experimental studies, mathematical simulation.

The labor effort of weapon scientists makes up 4250 man/days (54 % from the total scope of works). Technical issues will be constantly discussed with Project Collaborators in order to achieve some practical result available for commercial application just after the Project implementation. Joint experiments and calculations are possible.

The proposed Project meets the ISTC goals and objectives and allows to support applied research in the area of development of fuel cell-based power plants (FC PP), which are promising devices converting chemical energy of hydrocarbon fuel into electric and heat energy and meeting current environmental requirements.


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