(367h) Development of the Hydrocarbon Polymer Electrolyte Membranes and the Modeling Analysis By Using Molecular Dynamics

As environmental pollution and fossil fuel using problems increase, researches on alternative energy have been actively conducting worldwide. Fuel cells are systems that produce electrical energy through electrochemical reactions, and have the advantages of having no emission of pollutants and higher output than other alternative energies. Fuel cells are currently used in various places, including automobiles, aircraft, ships, buildings, and power generation. However, since the elements that make up the fuel cell each have a complex structure and play an important role in determining the efficiency of the fuel cell, various and intensive studies on the subcomponents of the fuel cells have been conducted [1].

In this research, we focus on the membrane electrodes assembly (MEA) among the subcomponents of the fuel cells. MEA is a critically important in the electrochemical systems in which reactions occur directly, and a gas diffusion electrode (GDE) composing a gas diffusion layer (GDL) and catalyst layer (CL) is attached to both sides of the Polymer Electrolyte Membrane (PEM) in a sandwich form. PEM transfers proton from anode to cathode and acts as a supporter of the electrode and prevents fuel crossover, so it is important to improve the performance of the PEM. As the efficiency of PEM varies greatly depending on the material and internal microstructure, the molecular dynamics simulation of the PEM is studied through a theoretical approach rather than an experiment, and the correlation is performed with the experiment. With the same research paradigm as Fig. 1, we focus on developing membranes that bring more accessible fabrication processes with reasonable prices than currently commercialized PEM while maintaining high conductivity and durability.

Currently, the most commonly used PEM is Nafion, which reportedly has high durability and conductivity. On the other hand, it has several drawbacks such as difficult to synthesize and less durable when the operating temperature exceeds 80℃. Therefore, the development of alternative membrane has been actively conducted, and one of the most possible candidate is a sulfonated poly (ether ether ketone) (SPEEK) membrane in which is hydrocarbon polymers with sulfonated functional groups. Although SPEEK membrane and its derivatives have been actively researched due to their durability, simple synthetic process, and low cost, their thermal and electrical performances are not satisfactory comparing to Nafion.

In this dissertation, we studied the microstructure of PEM as a function of the molecular structures as well as provide the recipes to experimentally synthesize the hydrocarbon electrolytes by hybridizing the theoretical and experimental aspects of PEM and MEA. Experiments were conducted to compare the performance of the SPEEK membranes with Nafion after comparing the performance of the SPEEK according to the degree of

sulfonation (DS) and the use of solvent for making dispersion. DS values were compared by varying sulfonation times. In addition, the performance of the membranes produced by using different types of solvents (e.g., NMP and Ethanol) was compared to figure out whether the dispersion of SPEEK in solvents affect the performance of the PEM including the proton conduction as well as the fuel cross-over.

As a theoretical study, we simulated the PEM by using coarse-grained molecular dynamics to building a model to explain the proton conduction through the membrane based on its microstructural mutations when the water contents changes. Previous phenomenological or mathematical models roughly explained the water clusters and proton conductions, while the three dimensional tortuous paths and the mechanisms of proton conductions have barely been elucidated. Here, we built the fundamental models of PEM, which can extensively show the proton conduction mechanisms, which are correlated to our experimental results.