Research On The Performance Of Gas Diffusion Electrodes For Zinc-air Fuel Cells

With the advantages of high energy density, large power, low costs and environment harmlessness etc, zinc air batteries distinguish themselves from others batteries to be the primary choice of dynamic power. Gas diffusion electrode (GDE) is the energy convertor of zinc air battery, so the performance of it has a great impact on the development of zinc air battery. The nature of GDE consists of the capability to convert chemical energy to electric energy and the cycle life of the electrode. The targets of this dissertation include developing methods to enlarge the output power and to prolong the cycle life of GDE. Also the working mechanisms of GDE was studied by a mathematical model.The main content of this dissertation comprise:first, methods to improve the discharge performance of GDE, including testing the effect of different catalyst layer position, functions of component materials of catalyst layer, optimization of the mixture ratio of the component, increasing the specific area of GDE and improving the catalysis ability of catalysts. Second, research into the decline of GDE's performance was conducted. The main factor responsible for it and the mechanism of the decline were confirmed. Third, a mathematical model considering the mechanism of oxygen reduction reaction (ORR) on manganese oxide-catalyzed GDE and mass transport process was developed. The main progresses are as follows:1) Measurements of the position of catalyst layer on the GDE discharge performance was made. And also that of the addition of absorbent charcoal, acetylene black, graphite, MnO2, polytetrafluoroethylene (PTFE) on the performance respectively. The mixture ratio that consists the catalyst layer was confirmed.2) The output power of GDE was enlarged by increasing the specific area of electrodes and depressing the active overpotential. The details comprised pretreatment of PTFE with ethanol, treatment of GDEs under different temperature, addition of pore-former and a new method to fabricate a dual-catalyst GDE.3) A series experiments were conducted to explore the factors that cause the decline of GDE performance. The results indicated that the migration of electrolyte was the primary cause. Further investigations were carried out on the motivation of migration and the mechanism of decline cause by migration.4) Simplification was made to describe the porous structure of GDE mathematically. A model considering the ORR mechanism and the mass transport process of species was built to simulate the operation of GDE. The numerical results showed the impact of oxygen dissolved boundary condition on the evolution of functions such as concentrations and transfer current and the effect of environmental conditions and the variation of parameters on the electrode performance. These results can be used as references to understand the chemical and physical processes occurred inside the electrode and the optimization of design.