MODELING THE INTERDEPENDENCE OF ELECTROCHEMICAL AND MECHANICAL PROPERTIES IN PER SULFONATE ACID PROTON EXCHANGE MEMBRANES

Abstract

Proton exchange membrane fuel cells (PEMFC’s) offer an attractive alternative energy resource over traditional fossil fuels. The advantages such as high power density, relatively quick start-up, rapid response to varying loads and low operating temperatures make it a preferred technology option compared to other alternative energy sources. Nafion® by DuPont plays an integral role in the success of PEM fuel cells due to its high proton conductivity and high chemical and thermal stability. This research project aims to study the effect of mechanical and hygro-thermal stresses on the mechanical performance and proton conductivity of the membrane by subjecting it to realistic operating conditions such as those encountered in an automobile. In this thesis, the time-dependent behavior of the membrane has been modeled using a Prony series and the change in the conductivity due to mechanical loading was experimentally measured. The modeling of both electrochemical and mechanical properties can further be used in studying the degradation properties of the membrane and should guide the development of better membrane materials. Visco-elastic stress relaxation theory has been used in modeling the time-dependent behavior of the specimen. The EIS spectrum has been analyzed using a non-linear least squares method and an equivalent circuit method was also used to fit the spectra. This project was conducted in three phases. In the first phase a novel test facility was built to perform the experiments. A conductivity measurement test cell that measured the proton conductivity of a membrane was modeled and manufactured. The second phase included the design of different experiments that helped in modeling the interdependence of electrochemical and mechanical properties of the membrane. In this process, three series of experiments that tested the electrochemical and mechanical properties of the specimen were conducted. The membrane was held at constant strain and the through plane impedance was measured at different times during the test, specifically before and after stretching at ambient and varying environmental conditions. The membrane was also subjected to both mechanical and hygro-thermal loading conditions during the test. In the third phase, time-dependant mathematical model for the changes in the material properties were developed. The experimental apparatus thus tested the mechanical and electrochemical properties of the membrane simultaneously while the specimen was being subjected to constant mechanical and varying hygro-thermal conditions. Since the testing method is a novel procedure, the reliability and repeatability of the experimental facility has been verified before conducting the experiments. The experimental apparatus can further be used to test the membrane at varying strain rates and different hygro-thermal loading conditions in a consistent manner. The model developed can be used to analyze the degradation behavior of membrane and also to build better fabrication methods and membrane materials in future.