Preparation of N-doped porous carbon from municipal sludge-agricultural wastes and research of its energy storage property

To improve the efficiency of collaborative high-value utilization of municipal sludge and agricultural waste, nitrogen-doped porous carbon materials are prepared via a two-step method. Municipal sludge organics are first extracted using low-temperature NaOH/urea aqueous solution to obtain a mixed solution of sludge organics with NaOH/urea (MSO-NU). Subsequently, wheat straw (WS) or corn cob (CC) is mixed with MSO-NU, and carbon materials are prepared through two processes: hydrothermal carbonization–pyrolytic activation and direct freeze-drying–pyrolytic activation. The effects of agricultural waste/MSO-NU mass ratio, hydrothermal temperature, and the two processing methods on material structure and electrochemical performance are investigated. The results indicate that products from the hydrothermal carbonization group exhibit a three-dimensional interpenetrating hierarchical porous structure. When the agricultural waste/MSO-NU mass ratio is 2% and the hydrothermal temperature is 230 ℃, samples HT-WS-2-230-700 and HT-CC-2-230-700 achieve the maximum specific surface areas of 1 189.4 m²/g and 1 008.9 m²/g, respectively, both superior to those of the direct pyrolytic activation group (<800 m²/g). At a current density of 0.5 A/g, their specific capacitances are 330.0 F/g and 327.0 F/g, respectively, representing an improvement of 18% to 25% compared with the direct pyrolytic activation group. Compared with the direct pyrolytic activation group, hydrothermal carbonization facilitates the reconstruction of the carbon skeleton, enhances the degree of graphitization (with graphitic nitrogen proportion increased by 1.0–1.6 times), stabilized surface oxygen-containing functional groups (with C—O proportion increased by approximately 0.73 times), and improved pore interconnectivity, and shortened ion diffusion pathways (with time constant τ₀ decreased by approximately 70%).