The phlogiston theory is a chemical hypothesis that was supported in the 18th century. According to this theory, all flammable materials contain an element called phlogiston and, when a substance is burned, its phlogiston is released and the remaining ash is held to be its true form. The background of the phlogiston theory was the desire for catharsis exemplified in witch-hunts and bloodletting in the early modern era. As to the former people thought burning witches at the stake could expel Satanism from them, lighten their sins as well as their physical weight and reduces them to their true forms. As to the latter they thought bloodletting could expel the cause of diseases from the body, lighten their symptom and reduce patients to their true healthy forms.

Fuel cells were invented in the 19th century, but their practical application has been extremely limited to date, as thermal power generation has become the mainstream method of power generation. Today, with decarbonization and the hydrogen economy, fuel cells are again attracting attention as a method of power generation using hydrogen. Innovations are necessary for fuel cells to become more practical than they are today, not only in the fuel cells themselves but also in the way of storing and transporting their fuel, as well as in the method of producing it.