One-step and low-temperature KOH-K2CO3 synergistic activation of residual carbon from coal gasification slag for supercapacitor electrode material

The cheap and massive residual carbon (RC) from coal gasification slags (CGSs) has been transformed into the carbon electrode material in supercapacitors through one-step low-temperature KOH-K₂CO₃ molten salt synergistic activation. Under the conditions of RC to the molten salt mass ratio of 1:2, an activation temperature of 400 °C, a series of activated products (ARCX) have been prepared by tuning the activation time (X = 1, 2, 3, 4 or 5 h). The electrochemical measurement results showed that the ARC3 exhibited the most excellent performance among the activated samples. At a current density of 0.2 A/g, the specific capacitance of ARC3 reached 249.5 F/g. By using the ARC3 as electrode active materials to assemble a symmetrical supercapacitor (ARC3//ARC3), its energy density was 7.3 Wh/kg at a power density of 250 W/kg, with the capacitance retention rate of ARC3//ARC3 remaining at 99% even after 10,000 charge–discharge cycles. For the ARC3, the unique structures formed from the coordinated pore-forming effects of KOH-K₂CO₃ eutectic salt induced the excellent performance, including high specific surface areas (715.08m²/g), appropriate hierarchical pore structure, and abundant surface oxygen-containing functional groups on the carbon surfaces. This work provides a completely new strategy to construct carbon electrode materials using CGSs or the other carbonate solid wastes as raw materials.