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 m² g⁻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.

The search for a low-cost metal-free cathode material with excellent mass transfer structure and catalytic activity in oxygen reduction reaction (ORR) is one of the most challenging issues in fuel cells. In this work, nitrogen-rich m-phenylenediamine is introduced into the synthesis of porous carbon spheres to tune the pore structure and nitrogen-doped active sites. As a result, more pyridinic N and pyrrolic N functional species were observed at the interior and surface of the carbon spheres. The introduction of m-phenylenediamine also regulated the nucleating of precursors, an urchin-like mesoporous surface structure ensures point contact and less agglomeration between each particle was obtained. With optimized proportion of micropores/mesopores and improved nitrogen-contained functional species, the ORR activity can be remarkably improved. The half-wave potential of this catalyst could achieve to 0.81 V (versus RHE) which is only 42 mV lower than commercial Pt/C catalyst. Furthermore, the optimized cathode catalyst achieved a 69 mW cm⁻² maximum power density when operated in direct methanol fuel cells at room temperature.