Tubular nanostructured Co3O4@N-containing carbon nanofibers to tune the surface structure for efficient electrochemical capacitors

•Tubular nanostructured Co3O4@N-containing CNF is prepared by coaxial electrospinning.•The surface structure of PPMCoU is tuned by controlling the amount of urea.•The micro/mesopores balance aids in adsorption capacity and electron transport.•Many oxygen vacancies provide the number of active sites for the redox reactions.•N-functional groups of the carbon electrode enhance the electrical conductivity.