Historical Overview and Fundamental Aspects of Molecular Catalysts for Energy Conversion

In this chapter we focus on the historical background of the electrocatalysts especially of molecular catalysts that are considered as key technology for energy conversion systems. The energy conversion is a basic process with which humans can utilize natural energy by converting into useful forms of energy such as heat, electricity, or other secondary energies. The most important process to be established in this century will be the usage of renewable energy, which has least impact on the global environment. The central technologies for this process will be solar cells, photosynthesis, and fuel cells. Hydrogen energy society would be the most probable choice interconnecting these technologies, and toward this goal the establishment of efficient catalysts is indispensable. The designing of molecular catalysts is an important issue for solving the energy conversion yields and efficiency. Through biomimetic approaches many good candidates of catalysts for energy conversion have been studied. Porphyrins from cytochrome analogs have been studied since late 1960s as oxygen reduction center or oxygen carrier with variety of modifications. Also reduction of H

+

is part of an artificial photosynthesis, and many supra-molecular and hybrid complexes are studied since 1970s. The chapter starts with the history and design concepts of oxygen reduction catalysts and fuel oxidation catalysts in fuel cells, to cope with the control of multi-electron transfer reactions. The state-of-the-art molecular catalysts are characterized as metal-nitrogen ligand complex or metal-nitrogen-oxygen conjugates on carbon support. Photochemical reduction of H

+

is reviewed which is coupled to water oxidation, where historically metallophthalocyanines or polypyridyl complexes are studied intensively since mid-1980s. Charge separation antenna chlorophylls are models of dye-sensitizers for photoreductive H

2

evolution, and these are incorporated in Graetzel cell for electrochemical solar cells. Design and application of molecular catalysts for these cells are reviewed.