Redefining the roles of alkali activators for porous carbon

Abstract

Alkali activation is a common method to prepare commercial porous carbon. In a mixed alkali activation system, the role of each individual alkali has generally been assumed to be the same as in a single alkali activation system, and the low corrosiveness of weak alkalis has mainly been emphasized. However, the intrinsic roles of the individual alkalis should be understood in detail and redefined to illuminate the activation pathways from the perspective of internal chemical reactions rather than corrosiveness. Herein, by combining in situ TG-MS analysis, DFT calculation and other characterizations, the activation processes were precisely tracked, and activation pathways were proposed. In the mixed alkali activation system, the strong alkali KOH served as the activation promoter, first decomposing into K₂O, which then attacked the C–C bonds to form active reaction sites defined as pore seeds. The weak alkali K₂CO₃ acted as the activation pathway modifier; CO₃²⁻ preferentially etched the pore seeds over K₂O due to the lower reaction barrier of CO₃²⁻ interacting with the pore seeds. Consequently, the rough etching reaction of KOH was replaced and suppressed by the gentler action of CO₃²⁻, forming more micropores. When the ratio of strong to weak alkali was 1 : 1, the obtained CK₁K₂-122 exhibited the highest microporosity (82.61%) and a high specific surface area (1962.18 m² g⁻¹). It exhibited a high specific capacitance of 296.7 F g⁻¹ and excellent cycling stability with 98.3% retention after 10 000 cycles. The supercapacitor demonstrated a high energy density of 114.4 W h kg⁻¹ at a power density of 17.5 kW kg⁻¹, with a broad potential window of 3.5 V.