A fuel cell is an electrochemical conversion device that converts chemical energy directly into electrical energy using hydrogen and oxygen as fuel, which has high thermal efficiency, low/zero pollution and noise, and high operating efficiency. Among the many fuel cells, the polymer electrolyte membrane fuel cells (PEMFCs) have been applied to a various of applications and attracted the many attention in the research fields due its simple design, high power density, and quick start-up. The core of the PEMFC is a polymer electrolyte membrane (PEM), which must have high ionic conductivity, low fuel permeability, physical stability and chemical stability.
The development issues for the polymer electrolyte membranes to be developed in this study are as follows: i) high electrochemical characteristics (ionic conductivity, single cell performance) at low humidification conditions, ii) development of low-priced and high heat resistant polymer electrolyte membranes, and iii) good durability for long-term operation.
The main purpose of this study is to develop a PEM, which has excellent electrochemical properties under low RH conditions for the commercialization of PEMFC.
Chapter 1 presents the need of fuel cells, and briefly summarizes the basic principles and components of PEMFC.
Chapter 2 shows the various structures for the development of polymer electrolyte. In this chapter, the requirements to meet physicochemical properties of PEM are introduced based on the characteristics for commercialization of PEMFC, and the advantages and disadvantages according to structure of PEM are presented.
Chapter 3 reports that prepared PEM that minimizes the dimensional change of the membrane, provides a hydrophilic/hydrophobic separation by controlling the molar ratio of the hydrophilic oligomer containing the thiophene unit and the perfluorinated hydrophobic oligomer. The prepared PEM exhibited a distinct phase separation as expected and demonstrated excellent electrochemical performance by exhibiting a similar ionic conductivity (129 mS cm?1) and maximum power density (333.2 mW cm-2) to the commercial membrane Nafion.
Chapter 4 introduces a grafted pendant sulfonic acid group to develop a PEM with excellent electrochemical properties through the formation of high ion clusters with improved physical properties through appropriate dimensional stability. In this study, the hydrophilic polymers according to the repeat unit using the newly prepared pendant type monomer are prepared, which showed very good dimensional stability and high ion conductivity of 131-154 mS cm-1. It has been confirmed that the hydrophilic/hydrophobic micro-phase separation leads to local concentration, which greatly influences the electrochemical properties through morphological control.
To improve ionic conductivity and fuel cell performance even under low RH, an organic/inorganic composite membrane was prepared using SPAEK having pendant groups and sulfonated SiO2. The water content of the prepared composite membranes was increased even though there was almost no dimensional change even at 90 ℃ compared with the pristine SPAEK membranes. In addition, all prepared membranes showed very high ionic conductivity values of 159.8-199.8 mS cm-1 under 100% RH at 90 ℃, and especially, composite membranes containing f-SiO2 inorganic nanofillers (8.5-10.5 mS cm-1) showed improved ionic conductivity values at 50% RH compared to the initial membrane (1.0 mS cm-1) due to hydrogen bonding between the polymer matrix and f-SiO2.
In conclusion, the physical properties and electrochemical performance of the PEM developed by controlling the molar ratio or the repeating unit between the hydrophilic polymer and the hydrophobic polymer were studied. When the sulfonic acid group was introduced in the form of pendant, it exhibited better characteristics than the direct sulfonated acid group into polymer backbone.
In addition, introduction of the functionalized inorganic nano fillers proved that not only the physical properties but also the electrochemical performance can be improved under the low RH condition. Therefore, the prepared PEMs are very promising for the commercialization of PEMFC.