Source: {"pile_set_name": "USPTO Backgrounds"}

Electrochemical energy conversion devices include fuel cell systems as well as hydrogen generators and other electrolysers, such as forco-electrolysing water and CO2.
Fuel cells convert gaseous fuels (such as hydrogen, natural gas and gasified coal) via an electrochemical process directly into electricity. A fuel cell continuously produces power when supplied with fuel and oxidant, normally air. A typical fuel cell consists of an electrolyte (ionic conductor, H+, O2−, CO32− etc.) in contact with two electrodes (mainly electronic conductors). On shorting the cell through an external load, fuel oxidises at the negative electrode resulting in the release of electrons which flow through the external load and reduce oxygen at the positive electrode. The charge flow in the external circuit is balanced by ionic current flows within the electrolyte. Thus, at the positive electrode oxygen from the air or other oxidant is dissociated and converted to oxygen ions which migrate through the electrolyte material and react with the fuel at the negative electrode/electrolyte interface. The voltage from a single cell under load conditions is in the vicinity of 0.6 to 1.0 V DC, and current densities in the range of 100 to 1000 mAcm−2 can be achieved. In addition to the electricity, water is a product of the fuel cell reaction.
Hydrogen generators and other electrolysers may be considered as fuel cell systems operating in reverse. Thus, a hydrogen generator produces hydrogen and oxygen when electricity and water are applied to the electrochemical cell.
A fuel cell system capable of producing electricity may be designed to run in reverse in order to produce hydrogen, for example producing electricity during the day and hydrogen at night, with the hydrogen optionally being stored for use the next day to produce more electricity. However, it may be advantageous from the efficiency perspective to design separate fuel cell systems and hydrogen generators. While the invention is concerned with electrochemical energy conversion devices generally, for convenience only it will be described hereinafter primarily with reference to electricity generating fuel cell systems and cells for them.
Several different types of fuel cells have been proposed. Amongst these, solid oxide fuel cell systems (SOFC) are regarded as the most efficient and versatile power generation system, in particular for dispersed power generation, with low pollution, high efficiency, high power density and fuel flexibility, and the invention is particularly concerned with solid oxide electrochemical energy conversion cells and with devices using them. Numerous SOFC configurations are under development, including tubular, monolithic and planar designs, and are now in production. The planar or flat plate design is perhaps the most widely investigated and now in commercial use, and the invention is particularly concerned in one aspect with electrochemical energy conversion devices comprising a stack of such solid oxide electrochemical cells. However, in another aspect, the invention also extends to solid oxide electrochemical energy conversion cells generally, that is it is concerned with tubular cells and monolithic cells, as well as with planar cells.
For convenience only, the invention will be further described solely with respect to planar or flat plate design solid oxide electrochemical energy conversion cells, and devices using them. In these devices, individual planar SOFCs comprising electrolyte/electrode laminates alternate with gas separators, called interconnects when the gas separators convey electricity from one SOFC′ to the next, to form multi-cell unit or stacks. Gas flow paths are provided between the gas separators and respective electrodes of the SOFCs, for example by providing gas flow channels in the gas separators, and the gas separators maintain separation between the gases on each side. Apart from having good mechanical and thermal properties, as well as good electrical properties in the case of interconnects and good electrochemical properties in the case of the fuel cells themselves, the individual fuel cell device components must be stable to demanding fuel cell operating environments. SOFCs operate in the vicinity of 600° C.-1000° C. and, for devices using them to be economical, typical lifetimes of 5-6 years or more of continuous operation are desired. Thus, long term stability of the various device components is essential. Only a few materials fulfil all the requirements. In general, the high operating temperature of the SOFCs, the multi-component nature of the devices and the required life expectancy of several years severely restricts the choice of materials for the fuel cells, gas separators and other components such as seals, spacer plates and the like.
A variety of different materials have been proposed for SOFC gas separators, including ceramic, cermet and alloys. For electrically conductive gas separators, that is interconnects, metallic materials have the advantage generally of high electrical and thermal conductivities and of being easier to fabricate. However, stability in a fuel cell environment that is high temperatures in both reducing and oxidising atmospheres, limits the number of available metals that can be used in interconnects. Most high temperature oxidation resistant alloys have some kind of built-in protection mechanism, usually forming oxidation resistant surface layers. Metallic materials commonly proposed for high temperature applications include, usually as alloys, Cr, Al and Si, all of which form protective layers. For the material to be useful as an interconnect in SOFC devices, any protective layer which may be formed by the material in use must be at least a reasonable electronic conductor. However, oxides of Al and Si are poor conductors. Therefore, alloys which appear most suitable for use as metallic interconnects in SOFCs, whether in cermet or alloy form, contain Cr in varying quantities.
Cr containing alloys form a layer of Cr2O3 at the external surface which provides oxidation resistance to the alloy. The formation of a Cr2O3 layer for most electrical applications is not a problem as it has acceptable electrical conductivity. However, for SOFC applications, a major problem is the high vapour pressure and therefore evaporation of oxides and oxyhydroxides of Cr (Cr6+) on the positive electrode side of the fuel cell at the high operating temperatures. At high temperatures, oxides and oxyhydroxides of Cr (Cr6+) are stable only in the gas phase and have been found to react with positive electrode materials leading to the formation of new phases such as chromates, which destroy the electrode material and make it electrically resistive, as well as to deposits of Cr2O3 on the electrolyte. These reactions very quickly reduce electrode activity to the oxygen reduction reaction at and adjacent the positive electrode/electrolyte interface, and thereby considerably degrade the electrochemical performance of the cell.
It has been attempted to alleviate this problem of degraded electrochemical performance by coating the positive electrode side of the interconnect with a perovskite barrier layer such as strontium-doped lanthanum manganite (LaMnO3) (LSM), which may also be the material of the positive electrode, but while short term performance was maintained there continued to be an unacceptable long term degradation in performance.
The problem of degradation due to evaporation of oxides and oxyhydroxides of Cr from chromium-containing materials on the positive electrode side of the fuel cell was greatly relieved by the invention described in the applicant's WO96/28855, that is forming a self-repairing coating on the positive electrode side of a chromium-containing interconnect, the coating comprising an oxide surface layer comprising at least one metal M selected from the group Mn, Fe, Co and Ni and a M, Cr spinel layer intermediate the chromium-containing substrate of the interconnect and the oxide surface layer. Such a coating may also be formed on other chromium-containing heat resistant steel surfaces that are on the positive electrode side of the plant. However, it remains a challenge to ensure the coating remains full dense to prevent the release of the chromium species in the demanding fuel cell operating conditions.
Other solutions have also been proposed for alleviating the degradation in fuel cell performance due to evaporation of oxides and oxyhydroxides of Cr on the positive electrode side of the fuel cell. For example, a low (or no) chromium steel is proposed in the applicant's WO00/75389, in which an alumina coating is formed on oxidation of the surface rather than chromium oxide. However, due to the low electrical conductivity of alumina, this heat resistant steel composition is not suitable for gas separators that are intended to act as interconnects conducting electricity from one side to the other.
In a further effort to limit the problem of degradation due to evaporation of oxides and oxyhydroxides of Cr on the positive electrode side of the fuel cell, it has been proposed to introduce another layer (referred to hereinafter as “shield layer”) on the positive electrode layer to absorb chromium before it reaches the positive electrode layer.
Positive electrode material for SOFCs are generally perovskites or oxides having perovskite-type structures (refined to herein as “perovskites”), such as lanthanum strontium manganite or LSM (La1-xSrxMnO3-δ), lanthanum strontium cobaltite or LSCo (La1-xSrxCoO3-δ), lanthanum strontium ferrite or LSF (La1-xSrxFeO3-δ), La1-xSrxCo1-yFeyO3-δ (LSCF), LaNixFe1-xO3-δ (LNF), and Ba1-xSrxCo1-yFeyO3-δ (BSCF) where 0≦δ<1 depending on the dopant. Other examples include SmxSr1-xCoO3-δ (SSC), LaxSr1-xMnyCo1-yO3-δ (LSMC), PrxSr1-xFeO3-δ (PSF), SrxCe1-xFeyNi1-yO3-δ (SCFN), SrxCe1-xFeyCo1-y03-δ, PrxCe1-xCOyFe1-yO3-δ and Pr