Oxidation Behavior,Cr Evaporation Feature And Surface Modification Of Metallic Interconnect For Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cell?SOFC?is considered to be an energy conversion device that efficiently and environmental friendly converts the chemical energy into power.Usually,cells are assembled into a stack to meet voltage and power requirement.For a plannar stack,interconnect is an important component,which prvides electrical contact and separates air and fuel.As SOFC operating temperature decreases from high-temperature?¹000??to the intermediate-temperature?600⁸00??,metallic alloys,especialls Fe-Cr alloys,are consider as the most sutable materials for the interconnect application.At high temperature,a continiu and dense Cr₂O₃ oxide scale,formed on the Fe-Cr alloy surface,could protect alloy from further oxidizing.However,anomalous oxidation hehavior of Fe-Cr alloys have been found under SOFC interconnect exposure conditions.The thick oxide scale would cause the increase of resistance and deteriorate the stack performance.Besides,Cr species would evaporate from Fe-Cr alloy surface under high temperature and humidity condition and then deposit on the cathode.Cr deposits reduce the amount of active sits for oxygen reduction or react with the cathde materials to form secondary high electrical resistance phases and furher destroy the cathode structure,which leads to the degradation of cathode performance.In conclusion,both anomalous oxidizing and Cr evaporation are the significant challenges for Fe-Cr alloys application.In order to clarify the mechanism of anomalous oxidizing and Cr deposition,commercial SUS430 alloy is chosen as research object.Firstly,the effects of dual-atmosphere,electric current,temperature distribution and gas composition on oxidation behavior of SUS430 alloy are investigated.Secondely,the Cr vaporization rates of SUS430alloy under different temperature are measured.And then the Cr deposition phenomenon on(La_0.8Sr_0.2)_0.95MnO_3-??LSM?cathode is investigated by SEM and TEM technique.The fudanmental reason for Cr deposition is also clarified.Thirdly,the effect of MnCu_0.5Co_1.5O₄?MCC?spinel coating on the oxidation behavior and Cr vaporization of SUS430 alloy is investigated.Besides,the LaCo_0.6Ni_0.4O_3-??LCN?contact layer,applied between the SUS430 alloy and LSM-YSZ cathode,is used to capture the volatile Cr species.More details are as follows:?1?The oxidation behavior of SUS430 alloy in dual-atmosphere and real stack.The oxidation behavior under dual-atmosphere differs from that in a single air atmosphere.A thicker Mn-Cr spinel layer is formed on CH₄/air dual atmosphere sample while Fe₂O₃nodules are found on 5%H₂/air sample,and a continual Fe₂O₃ layer is formed for H₂/air sample.It can be concluded that the anomalous oxidation behavior of SUS430 alloy is caused by hydrogen diffusion from the fuel side to the air side across the alloy,which results in the formation of cationic vacancies and accelerate the diffusivity of iron cations.The growth of the oxide scale thickness on SUS430 is influenced by the electric current resulting in a thick oxide scale at negative side and a thin oxide scale at positive side.The oxidation rates of alloy in real stack vary greatly depending on its local environment,with different thermal,electrical density,as well as gas composition conditions.The area specific resistance?ASR?of AIFO region is reach to 306m?cm²,which means commercial SUS430alloy cann't satisfy the usage request and needs to be modified.?2?The measurement of Cr vaporization rate and the mechanism of Cr deposition.The formation of MnCr₂O₄ spinel layer on top of the chromia scale leads to a decrease in the Cr vaporization rate of SUS430 alloy.The presences of SUS430 alloys cause rapid degradation in LSM cathode performance,which implies that the volatile Cr has a poison effect on LSM cathode.The deposition of Cr species is driven by chemical reaction,not electrochemical reduction.Cr species are induced to deposit on LSM cathode surface by chemical reaction with Mn²⁺ions,which generated under cathodic current on LSM surface and spread out to the YSZ surface.And TEM result indicates that the deposited particles on LSM cathode are MnCr₂O₄ spinels,which not only occupy the catalytic sites,but also lead to the increase of electrical resistance of cathode.With the polarization current density increased,the width of Cr deposition areas is extended from LSM/YSZ interface to LSM surface,and the size and amount of Cr deposits also increase accordingly,which demonstrates that high polarization current accelerates Cr deposition.?3?The surface modification of SUS430 alloy.MCC spinel coating is successfully applied on SUS430 alloy by dip-coating process.The dense coating is proposed to improve oxidation resistance of SUS430 alloy in dual-atmosphere by inhibite the diffusion of hydrogen and oxygen.Besides,MCC coating serves as a barrier to Cr species evaporation from SUS430 alloy.The Cr vaporization rate is much lower than that of the uncoated alloy,resulting in less deposition of Cr on LSM cathde.Applying LCN contact layer between SUS430 alloy and LSM-YSZ cathode is another way to minimize Cr poisoning effect.LCN powders would react with Cr₂O₃ powders at high temperature and the spinels can be formed.Therefor,it can be speculated that LCN contact layer also serve as a“Cr getter”,which captures the volatile Cr species by chemical reaction.