A method of forming one or more composite structures is provided wherein one or more carbon structures is formed from a carbon source via a hydrothermal process. Said carbon source is a biomass material including glucose and glucosamine hydrochloride. In particular, the method further comprises introducing a seeding additive of potassium or phosphate salt, preferably monopotassium phosphate to the carbon source. The method includes introducing iron onto the carbon structures to form the one or more composite structures including carbon and iron.