The capacitive properties of nitrogen/phosphorus co-doped nonporous carbon nanofibers and nitrogen doped nonporous carbon nanofibers are comprehensively and comparatively investigated in different aqueous electrolytes in order to identify the role of phosphorus groups in improving the capacitive performance of carbon. The introduction of phosphorus groups is favourable for the adsorption of electrolyte ions onto the carbon surface, especially protons, and thus greatly enhances the electric double layer capacitance.