Hierarchical Membrane-Electrode Assembly and Metal-Free Cathode Catalysts for Polymer Electrolyte Membrane Fuel Cells

Abstract

Fuel cells have come to occupy an important position in power sources of the next generation. They have risen as potential alternatives to alleviate our dependence on fossil fuels because of their high efficiency and low/no pollutant emissions. Among the various kinds of fuel cells, polymer electrolyte membrane fuel cells (PEMFCs) are the most encouraging for commercial applications due to their high efficiency, low operation temperature, and rapid start-up. Also PEMFC have been considered as the most effective solution of stealthy power sources for underwater vehicles. However, PEMFCs have not been completely commercialized yet

owing to their high cost-components and low durability. In particular, the use of expensive/rare Pt metal as a catalyst becomes a matter of concern. Therefore it would be of great interest to investigate a more effective electrode structure for the higher performance through a careful design of membrane-electrode assembly (MEA), which is the heart of the PEMFC. Also, lots of significant efforts have been devoted to replacing Pt-based catalysts with inexpensive, more abundant nonprecious metal catalysts. The main theme of this thesis is the realization of high performance MEA in PEMFC: 1) by adopting a new catalyst layer structure, such as a three-dimensional ordered macroporous assembly, and 2) by applying novel metal-free catalyst to both acidic and alkaline polymer membrane electrolyte fuel cell. As mentioned above, a sophisticated design of the electrode in MEA must be needed and this subject in the previous work has not been sufficiently investigated

more detailed study was necessary. Yet most studies for the new catalyst layer design have been confined to the half-cell data and only demonstrated the potential for practical use, however the half-cell is not a practical fuel cell device. Therefore in this thesis new approach for the electrode in MEA has presented and verified a realistic practical use in single-cell, MEA. Chapter 1 briefly describes the fundamental of fuel cell such as a principle, history, type and challenge. That includes a brief report about the application of PEMFC to silent power source for underwater platforms, like submarine. In chapter 2, this section introduces a large-area, hierarchical macroporous Pt electrode for use in practical devices such as MEA in PEMFCs, and this electrode has shown 85% higher performance than that of a conventional catalyst slurry ink based electrode with a similar Pt loading. These three-dimensional ordered macroporous materials could be attractive materials in electrochemical device because of the benefits from the periodic structure. Owing to their open and interconnected pore architecture, these electrodes maintained a good effective porosity, effective catalyst utilization and mass transfer, and satisfactory water management, while the concentration loss was minimized. This chapter provides useful information on development of attractive materials for electrochemical device, not restricted to the fuel cell electrode. In chapter 3, this part introduces a facile and gram-scale synthesis of graphitic carbon nitride hybrid as a metal-free hybrid catalyst for both acidic (proton as conducting reactants) and alkaline (hydroxide ion as conducting reactants) fuel cells, and this metal-free cathode electrode has exhibited an outstanding performance, i.e., 69% and 80% of commercial Pt/C performance in actual fuel cell devices using MEA with acidic and alkaline polymer electrolytes, respectively. Although numerous reports have been published on nonprecious metal catalysts for the oxygen reduction reaction of fuel cell cathode, few studies have demonstrated a realistic practical use of these catalysts in fuel cells, and furthermore, the reported performances are inferior to the performance obtained in this chapter. The fabrication method and remarkable performance of the single cell in this chapter are progresses toward realistic applications of metal-free materials in commercialized fuel cells.