N-P-O co-doped high performance 3D graphene prepared through red phosphorous-assisted "cutting-thin" technique: A universal synthesis and multifunctional applications

Highlights: • An N-P-O co-doped 3D graphene is produced through a novel "cutting-thin" strategy. • This "cutting-thin" strategy is applicable to variable carbon sources. • The 3D graphene exhibits ultrahigh specific capacitance of 426 F g⁻¹ (424 F cm⁻³). • The 3D graphene can be directly utilized as an excellent metal free ORR catalyst. Large scale production of three dimensional (3D) graphene materials with high density and low degree of defects stands for the main challenge hindering their practical applications. Herein, we report a universal and readily scalable strategy to produce an N-P-O co-doped free standing 3D graphene through a one-pot red phosphorus-assisted "cutting-thin" technique. The solid carbon precursor is gradually exfoliated through the slowly released gases (e.g. pH₃, H₂, CO₂) and metallic K during the reaction, which allows the formation of dominant amount nanopores, and ensures the high density of the products. The as-produced graphene exhibits continuously 3D hierarchical porous (3D-HPG) structure with good quality (I_D/I_G=0.4, I_2D/I_G=0.65). Density functional theory (DFT) calculations indicate the N-P-O co-doping can significantly enhance the charge delocalization with benefited electrochemical activity. The 3D-HPG is directly utilized as the supercapacitor electrode and a metal free catalyst for oxygen reduction reaction (ORR), offering ultrahigh specific capacitance of 426 F g⁻¹(424 F cm⁻³), as well as excellent catalytic performance. The assembled all-solid-state cell exhibits both high gravimetric (25.3 W h kg⁻¹) and volumetric (25.2 W h L⁻¹) energy density, which are among the highest values of the state-of-art carbon only supercapacitors. Remarkably, this "cutting-thin" strategy is applicable to variable carbon sources.