Synthesis of O-doped coal-based carbon electrode materials by ultrasound-assisted bimetallic activation for application in supercapacitors

[Display omitted] •Ultrasound-assisted is proposed to modified coal-based carbon electrode material.•AC-20 show high C of 309 F g−1, brilliant rate performance and cycle stability.•AC-20//AC-20 exhibits ultra-high energy density of 9.6 Wh kg−1 at 250 W kg−1.•Carbonyl, hydroxyl, carboxyl and lactone on carbon surface promote K+ adsorption. Abundantly available low-grade coal is used to prepare activated carbon (AC) for supercapacitor (SCs) electrode materials. Aiming to transform low-grade coal into pore structure, microcrystalline, and surface chemistry multi-scale coordinated AC, a new ultrasound-assisted bimetallic activation strategy is proposed. The introduction of ultrasonic waves in the FeCl3/MgCl2 co-impregnation process synergistically controls the pore and microcrystalline structure of the precursor. Meanwhile, the surface modification is performed in a CO2 atmosphere. Comparative experiments and density functional theory (DFT) calculations show that the AC-20 possesses abundant micropores (0.92 cm3 g−1), appropriate mesopores (0.40 cm3 g−1), high specific surface area (SSA, 2329 cm2 g−1), low impurity content, and is highly O-doped (13.65 at.%), which are essential for enhancing carbon-based electrochemical processes. Meanwhile, the graphitization degree improves (ID/IG is down to 0.89), stability enhances (TGA curves shift right), and electrical resistivity decreases (Re is 0.15 Ω cm−1). Therefore, AC-20 has a specific capacitance (C) of 309 F g−1 at 0.5 A g−1. Furthermore, AC-20//AC-20 can reach an energy density (E) of 9.6 Wh kg−1 when the power density (P) is 250 W kg−1. Combined with low price, abundant reserves, a simple preparation method, and good practicability, the obtained AC-20 has great potential to expand production.