Investigations Of Multi-Physics Coupled Simulation Of Solid Oxide Fuel Cells And The Mechanism Of Short Circuit Inside Electrolyte

Solid oxide fuel cell(SOFC)has the advantages of high power generation efficiency,fast reaction rate,all-solid structure and wide adaptability of fuel gases.It has attracted more and more attention from all over the world in solving the current energy crisis and environmental pollution problems.The SOFC multi-physics coupled simulation technology is helpful for understanding the electrochemical reaction process,species migration process and physical field distribution inside the fuel cell,which plays an important role in guiding experimental research and improving the comprehensive performance of SOFCs.On the basis of Loss Voltage Model and CFD model,this work introduces the solution steps for the SOFC multi-physics coupled simulation technology.Loss Voltage Model is one of the most widely used electrochemical models in the SOFC multiphysics coupled mathematical model.The differences between Ohm's law in the Loss Voltage Model and Nernst-Planck equation are discussed in the second chapter of this work.Additionally,methods for determining parameters in continuity equation,momentum equation and concentration equation of computer fluid dynamics(CFD)model are illustrated.This work also introduces the transient simulation program of SOFC sunder constant initial global current density.This work analyzes and corrects the source term problem of the continuity equation in the CFD model.After reviewing quantities of literatures,authors found that there are two types of continuity equations,including conservation form and non-conservative form,when researchers attempted to apply the unified flow field method to simulate the flow field inside solid oxide fuel cell.Electrochemical reactions would introduce source terms at the electrode and electrolyte interface,and the influence of the source term on the continuity equations has not yet been solved.We found that the conservation form ignores the difference in diffusion effects at different interfaces,leading to wrongly introduce source terms into the continuity equations.The non-conservation form completely ignores the influence of electrochemical reaction source terms upon continuity equations,and this form can not capture the volume changes of the fluid field.Based on perfect gas assumption,the volume conservation form of continuity equation is proposed,which overcomes the above problems and obtains the simulation solutions which are consistent with the actual situations.Additionally,this work establishes a complete and multiphysics coupledmathematical model of proton-conducting SOFCs considering the existence of short current in electrolytes.Using the four electrode half-reactions as key conjunctions in the internal circuit cycling and the external circuit cycling,charge transport forms of the H-SOFC system is constructed.Current leakage is achieved by electron holes passing through the electrolyte and combined with electrons on the anode side.The Nernst-Planck equation is used to describe the transport process of protons and electron holes inside the electrolyte,and the analytical solution of the leakage current is obtained based on the gas chemical potential.Then,by means of Loss Voltage Model,considering the transmission process of the mixed gas in the porous electrode and the electrochemical polarization process in the three-phase region(TPB,)a complete mathematical model for H-SOFCs under the short circuit state is constructed.In addition,BZY-NiO/BZY/LSCF anode-supported coin fuel cells are prepared and tested.Good compatibilities are observed between experimental data and model prediction results.The assumption of quasi-equilibrium is adopted for the charged defects at electrode/electrolyte interface in the current SOFC multiphysics coupled mathematical model.This work also found that the step potential at interfaces had a significant impact on the defect equilibria.The electrolyte-supported SFM-GDC/LSGM/SFM-GDC button-symmetric fuel cells are prepared and tested.The electrochemical characteristics of the anode and cathode of the fuel cells are separated by the three-electrode method with a reference electrode.Comparisons between the changes of measured voltages and overpotentials obtained from the EIS data are conducted.Good compatibility between these potential losses indicates the rationality and correctness of the actual potential frameworks.Based on the potential frameworks,it is found that the existence of the step potential would seriously damage the defect balance.One part of Gibbs energy of the half reaction is converted into electrical energy and stored as a form of potential steps.Therefore,the equilibrium constant of the electrode half reaction needs to be corrected by the step potential.The actual potential framework for SOFC with short circuit is still an unsolved problem in the SOFC multi-physics coupled mathematical model.So far,the equivalent circuit method is widely implemented to indirectly represent the potential distribution inside SOFC with short circuit.The equivalent circuit method is used to study the mixed ion conducting SOFCs in this work.The independent variable,output current density,is introduced to update the current equivalent circuit method.Thegeneral expressions of actual polarization and ohmic resistances are proposed.NiO-GDC/GDC/LSCF electrolyte-supported button type single cells are prepared and tested.The electrical properties and electrochemical impedance spectroscopy under different working conditions are measured.The Ohmic resistances,polarization resistances and electron hole resistances of the fuel cells under different fuel gas supplies and different operating temperatures are discussed.The experimental results show that,from the high current density discharge state to the high current density electrolysis state,the electrode polarization resistance,Ohmic resistance and electron hole resistance all monotonously decrease.The only difference is that the polarization resistances exhibit an order of magnitude change,while the others are relatively stable.In this paper,simple theoretical and experimental explorations of the short circuit problem are carried out,which have certain guiding significance for the subsequent exploration on this problem.