Study Of Gas Diffusion In Porous Electrodes Of Reversible Solid Oxide Cells Based On Proton Conducting Electrolyte

Reversible solid oxide cell based on proton conducting electrolyte（H-RSOC）is a new type of high-efficiency energy conversion device with two modes of operation:fuel cell and electrolysis mode.It can be used to store,convert and reuse electrical energy.During the operation of H-RSOC,the gas transport phenomenon occurs in the porous electrode.However,the existed diffusion resistance leads to uneven gas distribution in the electrode,generating concentration polarization and reducing the performance of the cells.Therefore,it is important to correctly characterize the gas diffusion in the electrodes and reduce the concentration polarization for improving the H-RSOC performance.Among various gas transport models,the dust-gas model（DGM）is considered to be one of the most accurate models for predicting gas diffusion transport phenomena.However,its complexity in modeling multi-component systems and the difficulty in obtaining analytical expressions are detrimental to its application in modeling and commercial software.In this thesis,with the H-RSOC as the research object and neglecting the role of the viscosity term,an analytical solution calculation method for binary and ternary systems based on the DGM is proposed.The accuracy and applicability conditions of the analytical calculation method are studied by comparing with the numerical solution method.The main contents of this thesis are as follows:In chapter 1,the research background and the working principles of H-RSOC are introduced firstly;then the gas diffusion in the electrodes,the status of domestic and foreign research about gas transport model and the calculation method of DGM are presented;finally,the motivation and purpose of this thesis and the chapter arrangement are briefly described.In chapter 2,the gas transport model for the binary system is constructed based on the DGM.For the numerical method solution,the pressure gradient and concentration gradient expressions are obtained by eliminating Maxwell’s diffusion term through the iterative summation method,and then the numerical solutions for the pressure and gas concentration are calculated by the fourth-order Runge-Kutta algorithm.For the analytical solution,neglecting the viscosity term the expressions for the pressure and gas concentration are derived by solving the first-order differential equations,which leads to the analytical solution.In chapter 3,firstly,when H-RSOC is operated under typical operating conditions,the gas distribution and pressure distribution in the H-RSOC fuel electrode（H₂-N₂ mixture）are calculated by using the above numerical and analytical methods respectively,and compared.Then,using the numerical solution method as a benchmark,the influence of the conditions of the working environment,such as current density and working temperature,and the electrode structure,such as electrode thickness and pore radius,on the calculation accuracy of the analytical solution method for the DGM of the binary system are investigated.The results show that since the working environment and electrode structure change the gas viscosity,diffusion coefficient and the size of gas transport channels in the system,the influence of the viscosity term on the accuracy of the analytical solution is affected.In all,the relative error range of the analytical solution is 0-30%.In chapter 4,the construction of the gas transport model for the ternary system on the basis of the above study is carried out.The solution of the numerical method of DGM for the ternary system is similar to that for the binary system,and the same numerical iterative method and the Runge-Kutta algorithm are used to solve it.Different from the binary system,the matrix decoupling is required to solve the first-order system of non-simultaneous differential equations with constant coefficients,resulting in the analytical expression of gas concentration distribution.In chapter 5,the gas and pressure distributions in the air electrode（O₂-H₂O-N₂mixture）of H-RSOC under typical operating conditions are predicted using the analytical and numerical solutions of the above ternary system,respectively.The analytical solution is verified by comparing it with the numerical solution.Then the accuracy and applicability of the analytical solution of ternary system of DGM under different operating conditions and electrode structures are systematically investigated.The results show that the accuracy of the analytical solution method of DGM for the ternary system is relatively high,with a relative error range of-7.0%to 0.6%,except at high current density,high curvature and large electrode thickness.