| For most polymer electrolyte membrane (PEM) fuel cells, water facilitates the proton transport that is a key process in the fuel cell's operation. Therefore, understanding how water interacts with the membrane can guide material improvements for better fuel cell performance. Presently, optimal PEM fuel cell conditions are at approximately 80°C and 100% relative humidity. It is desired, however, to operate at temperatures above 120°C and consequently lower relative humidity (as low as 25%). Such operating conditions present a number of advantages, including a higher tolerance for CO poisoning of the commonly Pt catalyst, greater heat rejection (smaller radiator requirements) and higher rates of activation for the respective electrochemical half cell reactions. However, at conditions above 80oC and below 100% relative humidity (without pressurization of the system), fuel cell performance suffers greatly primarily due to the dehydration of the membrane.; The question arises, then, how can a sufficient amount of water critical for proton transport be retained in the fuel cell without pressurization of the system? More fundamentally, what is the nature of proton transport in the membrane and the role of water or a similar "proton transport facilitator"? Additionally, what chemical and structural properties of the membrane are beneficial for operation at the desired conditions? To answer these questions, this work studied the thermodynamics of hydration of hydrophilic ionic acid groups for different membranes and model systems. Membranes were equilibrated at different activities of water corresponding to different levels of membrane humidity. Isopiestic data was collected from the equilibrated membranes. Differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) were employed to couple the heat release or gain and weight loss, both with respect to temperature, in order to arrive at an average change in enthalpy. Such enthalpic information gives insight to the condition of water in the membrane, either free (absorbed) or bound (chemically attached) and thereby can assist in the tailoring of materials that will retain water at temperatures above 100°C. Conductivity was measured by two-point electrical impedance spectroscopy (EIS) to investigate proton conduction. Diffusion and relaxation was measured by nuclear magnetic resonance spectroscopy (NMR) for insights into water transport in the membrane materials. |
