Structural Optimization And Interfacial Strength Analysis Of Solid Oxide Fuel Cells Based On Thermal Stress

Solid oxide fuel cell(SOFC)is an electrochemical device that converts chemical energy into electrical energy directly.It has the advantages of high efficiency,low pollution and low noise and so on,so SOFC is one of the most promising clean energy sources in the future.However,in the process of thermal cycling,because of the large temperature change and the difference of thermal expansion coefficients between different layers,the thermal stress appears in the cell and leads to the cracking and delamination.Then it directly affects the mechanical stability and electrochemical performance of SOFC.Based on the existing research,the thermal stress,interface strength and crack growth of SOFC during thermal cycling are studied by theoretical derivation and finite element simulation.The main work is as follows:(1)Based on excessive thermal stress exists in anode-supported planar SOFC under thermal load,hierarchical model with material nonlinearity and geometric nonlinearity of anode functional layer in SOFC is established and nonlinear parameters of anode functional layer are optimized.By controlling the sub-layer number of sublayers,composition gradient index and thickness gradient index of anode functional layer,the thermal stress distribution of the cell under different structural parameters is studied,and the optimization scheme of anode functional layer to minimize the thermal stress of SOFC is obtained.The results show that thermal stress of SOFC can be significantly reduced by introducing anode functional layer.By analyzing the influence of different anode functional layer structure parameters on the maximum axial stress of anode,the maximum compressive stress of electrolyte and the interfacial stress,the optimization scheme for minimizing the thermal stress of SOFC is obtained.(2)At present,the effect of electrode material interface layer on thermal stress of SOFC is not clear.According to the material diffusion mechanism and composition linear distribution law,the material property of interface layer is determined and the analytical expression of interface thermal stress is established.Based on Timoshenko beam theory,the thermal stress of half-cell system is analyzed and the interface thermal stress distribution with integrated physical significance is obtained.In addition,the feasibility of theoretical model is verified by comparing the numerical results and theoretical results of thermal stress at the interface.By comparing the thermal stress between anode-electrolyte interface and cathode-electrolyte interface of SOFC,it is found that the thermal stress of anode-electrolyte interface is higher than that of cathode-electrolyte interface,so anode-electrolyte interface is more prone to delamination even failure.When introducing single anode functional layer into SOFC,the interfacial thermal stress increases with the increment of composition gradient exponent of anode functional layer.In addition,the effects of three different thickness interfacial layers on interfacial thermal stress are compared.It can be concluded that the interfacial shear stress and interfacial peeling stress decrease with the increase of interfacial layer thickness.(3)Based on the potential energy principle and first variational principle of half-cell system,the relationship between interface morphology of electrode material and SOFC performance is optimized.The analytical expression of peel force of half-cell system is obtained by establishing the parabolic interface shape function.The interface strength between electrolyte film and anode substrate is characterized by the peel force acting on electrolyte film,so as to determine the range of peel angle and amplitude-wavelength ratio of shape function when the interface strength is maximum.It is found that the maximum peel force of the cell with corrugated interface is larger than that with planar interface,so introducing corrugated interface into the half-cell can improve its interface strength.The extremum of peel forces corresponding to parabolic function and sinusoidal function at different peel angles are compared,and the turning points of ultimate peel strength and allowable peel strength based on amplitude-wavelength ratio are further determined.When the amplitude-wavelength ratio is less than turning point,the ultimate peel strength corresponding to parabolic function is higher,and the allowable peel strength corresponding to sinusoidal function is higher.When amplitude-wavelength ratio is larger than turning point,the allowable peel strength corresponding to sinusoidal function is higher,and the ultimate peel strength corresponding to parabolic function is higher.In addition,the corresponding peel angle and amplitude-wavelength ratio are also determined and discussed for when peel angle is less than inclination angle of corrugated film.(4)The stress distribution and interface crack propagation of corrugated SOFC during cooling process is analyzed.Compared with plalar SOFC,the corrugated SOFC could significantly reduce the interface crack propagation and improve the mechanical stability.As the shear stress and normal stress at the interface between electrodes and electrolyte can be considered as the driving forces of interface crack propagation,the normal stress and shear stress at the anode-electrolyte interface and cathode-electrolyte interface of two types of SOFC are analyzed and compared respectively.The results show that crack is more prone to initiate at the free end of SOFC because of the higher stress at the interface.And the stress of anode-electrolyte interface is higher than that of cathode-electrolyte interface,so the crack is easier to initiate at free end and anode-electrolyte interface.The stress distribution of planar SOFC is more uniform,while the stress of corrugated SOFC fluctuates with the change of interfacial morphology away from the free end of electrode-electrolyte interface in the cell.Based on the results of normal stress and shear stress analyses,the interface edge crack and middle crack are introduced into anode-electrolyte interface to analyze and compare the crack growth of two SOFCs.The results of crack propagation and the analysis of interfacial energy release rate show that corrugated SOFC can inhibit the propagation of interface cracks and reduce the possibility of interfacial delamination compared with planar SOFC.