Nitrogen-containing ultramicroporous carbon nanospheres (N-UCNs) are fabricated.
Hexamethylenetetramine is used as both a catalyst and nitrogen source to prepare N-UCNs.
Introduction of phloroglucinol/terephthalaldehyde endows ultramicropores for N-UCNs.
N-UCN electrode exhibits a high specific capacitance and excellent cycle stability.