The effect of phosphorus doping on the composition and capacitance properties of mesoporous graphitic carbon nitride (MGCN) is investigated systemically and reported in this article. Pristine and phosphorous doped MGCN are synthesized by using a bio-compatible template, carboxymethyl cellulose, and orthophosphoric acid as a dopant. Powder X-ray diffraction and transmission electron microscopic studies confirm the formation of stacked layers of MGCN. Vibrational and X-ray photoelectron spectroscopic studies reveal that phosphorus is doped preferentially on the sites of carbon and, thereby induces defects. The defect and surface area are found to increase on increasing the dopant concentration. Owing to the presence of functionalities, such as P-O and C-P, P-doped MGCN delivers higher specific capacitance than pristine MGCN at all current rates. A symmetrical supercapacitor device fabricated using the electrodes of MGCN synthesized using 0.5 g of dopant delivers an energy density of 17.22 Wh kg(-1) at a power density of 500 W kg(-1). (C) 2019 The Electrochemical Society.