A Universal Strategy For N-Doped 2D Carbon Nanosheets With Sub-Nanometer Micropore For High-Performance Supercapacitor

Preparing carbon nanosheets with precise control of open porous morphology via universal process and understanding the relationship between structure and capacitive performance are very urgent for achieving advanced supercapacitors. Herein, we propose a simple yet effective additive-free method to transform a bulk layered potassium phthalimide salt to novel nitrogen-doped two-dimensional carbon sheets by self-activation during calcination. The obtained samples showed large-sized and flat structure with lateral size around 10 µm, uniform sub-nanometer micropore size distribution of about 0.65 nm dimension, large specific surface area up to 2276.7 m²g⁻¹, and suitable nitrogen doping. Benefited from these merits, the optimized sample delivers a high specific capacitance of 345 F g⁻¹ at 1 A g⁻¹ and retains 270 F g⁻¹ even at 50 A g⁻¹ in 6.0 M KOH electrolyte. Remarkably, the symmetric supercapacitor shows maximum energy densities of 16.43Wh kg⁻¹ and 23.6 Wh kg⁻¹ in 6.0 M KOH and 1.0 M Na₂SO₄ electrolytes, respectively. Importantly, on account the universality and simplicity of this method, the undoped as-prepared carbon sheet with uniform sub-nanometer micropore distribution can be synthesized from different potassium-containing salts with layered structure, which can be employed as a model for a deep understanding the effect of sub-nanometer micropores on capacitive performances. We find the number of micropores centered at 0.65 nm can be applied as one indicator to clarify the correlation between capacitance and critical pore size below 1 nm.