Integration Model And Optimization Of (Fe,Ni) Foam Supported Solid Oxide Fuel Cell

Solid oxide fuel cel s（SOFC）have received widespread attention as a clean and efficient energy conversion device.It directly converts the chemical energy in the fuel into electrical energy.Compared with other power supply technologies,SOFC has a higher energy density.The National Aeronautics and Space Administration（NASA）has developed SOFC as a technical reserve to provide high-energy-density energy storage and power transmission systems for future space missions.However,the mass-specific power density（MSPD）of SOFC still needs to be improved.NASA successful y developed a commercial SOFC stack with an MSPD of 2.5 k W/kg in 2018.In the laboratory,the MSPD of the SOFC single cell with gadolinium oxide doped cerium oxide(Gd_0.1Ce_0.9O_1.95,GDC)as the electrolyte reached to 3 k W/kg.The optimization of the single-cell structure is one of the most important means to improve the MSPD of SOFC.By optimizing the support from a random microporous structure to a hierarchical y oriented macroporous structure,the areal power density of SOFC significantly increased,but the MSPD has not been substantially improved.In this paper,commercial SOFC materials are used as the integrated cell unit:GDC-Ni is used as the anode,GDC is used as the electrolyte,and GDC-LSCF is used as the cathode.A high-power density SOFC structure is designed through theoretical models,and metal foam is used as support.Compared with the existing support,the metal foam has a higher porosity（>0.9）and a larger pore size（≈100μm）,which can not only increase the areal power density of the SOFC but also has the advantage of lightweight and improves MSPD.Through process optimization,metal foam supported SOFC（MF-SOFC）with high power density was successfully prepared.Firstly,an integrated SOFC model is established based on irreversible thermodynamics,which considers the coupling effects of electronic conductance and electrochemical reactions.The integrated model is used to study the relationship between the support structure parameters（porosity,tortuosity factor,and pore size）and the SOFC output performance.According to the performance requirements（MSPD>6 k W/kg）,the designed support structure parameters are:porosity>0.85,tortuosity factor<1.5,pore size>50μm.Accordingly,the iron-nickel foam is selected as the SOFC support.In order to break through the preparation problem of MF-SOFC,a process scheme of connecting the integrated cell unit and the iron-nickel foam support by sintering the iron-nickel mixed powder was designed.In order to achieve a good interface connection of the bonding layer,the phase structure of the bonding layer is designed as an iron-nickel solid solution according to the phase diagram of the iron-nickel binary system,and the Ni content is greater than 20 wt.%.Secondly,the structure and performance of the electrolyte,anode,and cathode in the integrated cell unit are optimized.Based on the sintering density,the optimal sintering temperature of the electrolyte is selected to be 1450 ℃,and the density reaches 98%,which meets the design requirements.The distance correlation function method is used for the anode functional layer（AFL）to reconstruct the two-dimensional microscopic morphology in three dimensions.The effective three-phase boundaries（TPBs）length is obtained according to the three-dimensional structure.The results show that when the mass ratio of Ni O and GDC is 1,and the volume fraction of pore former is 35%,the effective TPBs length is the highest,reaching 1.56μm·μm^-3,which meets the design requirements.For the cathode,the electrochemical activity of the GDC-LSCF composite cathode after sintering in an Ar is studied.Based on electrochemical impedance spectroscopy（EIS）characterization and relaxation time distribution（DRT）analysis,it is found that the polarization resistance increases with the increase of the sintering temperature,but the electrochemical process has not changed significantly.Choose the temperature corresponding to the minimum polarization resistance,950 ℃,as the best sintering temperature for the cathode.After structural optimization,the battery integrated unit meets the design index.Moreover,the iron-nickel mixed powder connects the foamed iron-nickel support and the integrated cell unit through sintering.Characterization of the sintered phases of iron-nickel mixed powders with different proportions shows that when the Ni content is greater than 30 wt.%,the phases are all iron-nickel solid solutions.With the increase of Ni content,the lattice parameters of the iron-nickel solid solution decrease,the coefficient of thermal expansion increases.According to the coefficient of thermal expansion,the mass ratio of iron and nickel is determined to be 1:1.The impact value of the single-cell is 1.23 J/cm²,which is an order of magnitude higher than that of the Ni-GDC anode supported SOFC.After the diffusion bonding,the iron element diffuses into the AFL by about 30μm,and the bonding layer and the AFL form a diffusion bonding interface with an element concentration gradient.The bonding layer and the Fe-Ni foam also form a diffusion connection interface,and the interface composition is uniform.To study the formation mechanism of the diffusion interface,a Fe-Ni phase-field dynamic model was established,and the diffusion behavior of Fe and Ni during sintering at 950 ℃ was simulated.It was revealed that Fe and Ni formed a diffusion bonding interface through interdiffusion under the concentration gradient.The high-strength connection between the iron-nickel foam support and the integrated cell unit is realized through the sintering process of the iron-nickel mixed powder.Finally,the MF-SOFC is successful y prepared.At 650 ℃,the areal power density of MF-SOFC reaches 1.01 W/cm²,and the MSPD reaches 6.56 k W/kg,which is more than ten times the highest level of the existing single cell of the same material.The resistance of MF-SOFC is 0.1943Ωcm² when the anode environment hydrogen partial pressure is 0.2.It is much lower than the Ni-GDC anode-supported SOFC.The 3D-CT characterization results show that the porosity of the MF support is 0.9,and the pore size distribution is 50～150μm.The gas diffusion coefficient in the MF support tends to the limit predicted by the Bruggeman theory.DRT analysis shows that although the gas concentration polarization in the support is smal,it significantly impacts the anode activation polarization.There is little concentration polarization in the MF support,thereby improving the electrochemical performance of MF-SOFC.An analytical y theoretical model is established to characterize the electrochemical process of SOFC.The analysis shows that the high porosity and low tortuosity factor of the iron-nickel foam support are critical in improving the single cel’s electrochemical performance.