Free-standing boron and oxygen co-doped carbon nanofiber (BO-CNF) films are prepared by a very simple casting and pyrolysis process.

The BO-CNF films exhibit high gravimetric and volumetric capacitances (192.8 F g⁻¹ and 179.3 F cm⁻³ at 1 A g⁻¹).

A volumetric capacitance of 140.7 F cm⁻³ at 100 A g⁻¹ can be maintained, delivering an excellent rate capability with 78.5% from 1 A g⁻¹ to 100 A g⁻¹.

Polyaniline nanoparticles can be anchored on the BO-CNF film to obtain a higher volumetric capacitance. The recycling of boron source is highly possible with a recovery ratio of about 50%.