Modeling Study On The Effect Of Electrode Overpotentials On The Electrical Conduction Behavior Of Oxide-type Mixed Conducting Electrolytes

Oxide-type mixed ionic and electronic conducting（MIEC）electrolytes are a class of oxide electrolytes with both ionic and electronic conductivities.They are widely used in various electrochemical devices,such as solid oxide fuel cells（SOFCs）,solid oxide electrolysis cells（SOECs）,proton-conducting electrochemical reactors（PCERs）,gas separation membranes,and so on.Since the performance and efficiency of electrochemical devices are directly affected by the electrical conduction behavior of MIEC electrolytes,it is necessary to study the electrical conduction behavior of MIEC electrolytes and its influencing factors.There are lots of factors affecting the electrical conduction behavior of MIEC electrolytes,such as the chemical composition,crystal structure and thickness of electrolytes.Apart from the above internal factors,the interfacial effect,induced by electrode overpotentials,is also one of the important influencing factors.Hence,to investigate the influence of the electrode overpotentials,theoretical models,with considering the interfacial effect,are built for investigation of the electrical conduction behavior of MIEC electrolytes in this thesis.The main research contents and conclusions are as follows:In chapter 2,taking the mixed oxygen ionic and electronic conducting electrolyte as the object of study,a charge transport model of mixed oxygen ionic and electronic conducting electrolyte is built based on the Nernst-Planck equation.Meanwhile,analytical solutions of oxygen ionic current and electronic current are modified based on the interfacial effects.Finally,the theoretical models of SOFCs and SOECs are developed by combining the charge transport model and the electrochemical model.In chapter 3,applying the above theoretical model,the impact of electrode overpotentials on the electrical conduction behavior of electrolyte in SOFC and SOEC modes are investigated.The influence of electrolyte thickness,exchange current density of air and fuel electrodes(J_0,air、J_0,fuel)on the oxygen partial pressure and electron conductivity distribution,average electron conductivity/ionic transport number and leakage current in the electrolyte layer are analyzed.The results indicate that the impact of air electrode overpotential on the electrical conduction behavior of electrolyte is not noticeable,while the fuel electrode overpotential could lead to a significant change in oxygen partial pressure distribution within the electrolyte layer,then resulting in the changes of the electronic conductivity and leakage current.This finally affects the electrical conduction behavior of electrolyte.In both modes,the influence of the fuel electrode overpotential on the electrical conduction behavior of electrolyte becomes insignificant with the increase of electrolyte thickness.In chapter 4,taking a PCER with mixed protonic and electronic conducting electrolyte as the object of study,a thermodynamic model of PCER for ammonia synthesis is proposed.In the development of model,a complete and clear expression of interfacial effect for mixed protonic and electronic conducting electrolytes is derived,and the mathematical relationship between the thermodynamic feasibility of electrochemical synthesis and electrode overpotential is also established.In chapter 5,the thermodynamic feasibility of ammonia synthesis in PCER is evaluated by using the thermodynamic model.The hydrogen partial pressure distribution within the electrolyte layer,hydrogen partial pressure at the end of the electrolyte layer on the ammonia electrode side and Gibbs free energy at different protonic current densities and ammonia electrode overpotentials are analyzed.The results show that under the applied voltage/current,the energy of electrode overpotential can transform into the chemical potential of hydrogen at the end of the electrolyte layer,leading to significant growth of the chemical potential/partial pressure of hydrogen,then resulting in a positive to negative conversion of the Gibbs free energy,demonstrating the thermodynamic feasibility of the electrochemical synthesis of ammonia at atmospheric pressure.In all,this thesis can provide theoretical guidelines for the application of MIEC electrolytes in SOFCs/SOECs,as well as basic thermodynamic principles for ammonia synthesis or hydrogenation in solid-state PCERs.