General Synthesis of N-Doped Macroporous Graphene-Encapsulated Mesoporous Metal Oxides and Their Application as New Anode Materials for Sodium-Ion Hybrid Supercapacitors

A general method to synthesize mesoporous metal oxide@N-doped macroporous graphene composite by heat-treatment of electrostatically co-assembled amine-functionalized mesoporous silica/metal oxide composite and graphene oxide, and subsequent silica removal to produce mesoporous metal oxide and N-doped macroporous graphene simultaneously is reported. Four mesoporous metal oxides (WO_{3− x}, Co₃O₄, Mn₂O₃, and Fe₃O₄) are encapsulated in N-doped macroporous graphene. Used as an anode material for sodium-ion hybrid supercapacitors (Na-HSCs), mesoporous reduced tungsten oxide@N-doped macroporous graphene (m-WO_{3− x} @NM-rGO) gives outstanding rate capability and stable cycle life. Ex situ analyses suggest that the electrochemical reaction mechanism of m-WO_{3− x} @NM-rGO is based on Na⁺ intercalation/de-intercalation. To the best of knowledge, this is the first report on Na⁺ intercalation/de-intercalation properties of WO_{3− x} and its application to Na-HSCs.