The need for affordable, effective, and sustainable energy production is growing globally. Functional material applications are growing in significance due to recent advancements and discoveries in materials science and technology. An indispensable manual for creating and using these materials in producing sustainable energy is provided by functional materials for sustainable energy applications. The first section examines functional materials for solar energy, such as thermophotovoltaic device modeling, photoelectrochemical cells, and silicon-based, thin-film, and dye-sensitized photovoltaic solar cells. Functional materials for the synthesis and storage of hydrogen are the subject of part two. In part three, functional materials for fuel cells are further examined. Part two discusses functional materials for hydrogen synthesis and storage.