An aerogel is an open-celled, mesoporous, and solid foam composed of a network of interconnected nanostructures with porosity of over 50%. The term mesoporous or mesoporous material is commonly defined as a material that contains pores ranging from 2 to 50 nm in diameter. The term aerogel does not refer to a specific substance, but rather to a network structure, which a substance can take on. Aerogels can be made of a wide variety of substances, including silica, most of the transition metal oxides, lanthanide and actinide metal oxides, several main group metal oxides, organic polymers, semiconductor nanostructures, metals, and carbon-based nanomaterials (carbon powder, CNTs, and graphene).