Supercapacitors (SCs) are known as one of the most promising energy storage devices due to their high-power density and excellent cycle stability. The performance of SCs is closely related to the electrochemical performance of electrode materials. The widely-used electrodes are carbon-based materials and their derivatives. Carbon materials contribute energy storage based on double layer capacitance associated with ion adsorption and desorption, achieving excellent power density and cycle stability. However, they lack in high capacitance because of limitation of surface area. Instead, Fe-based Prussian blue (PB) is a promising pseudocapacitive material with open frame structure and high theoretical capacitance, but the poor electrical conductivity limits its rate performance and cycle stability. Combining PB with carbon materials is considered as an effective solution to develop advanced electrode composites. Herein, nanoporous carbon (NPC) derived from lignite has been prepared with high specific surface area and good electrical conductivity. Then, Fe-Prussian blue was successfully loaded on NPC via co-precipitation method to obtain Fe-Prussian blue@nanoporous carbon composite (PB@NPC). The fabricated PB@NPC composite possesses high specific surface area, high specific capacitance and excellent cycle stability.

The lignite powder was obtained by crushing the lignite with 100 mesh and sifting, and then mixed in 4 mol/L HNO₃ and stirred for 8-10 h. After washing and drying, the collection was dispersed in 5 mol/L NaOH for 8-10 h with stirring, following by washing with deionized water and drying to obtained pre-treated lignite. Then, the pre-treated lignite was mixed with KOH at a mass ratio of 1∶3 and heat treated at 800 ℃ in Ar atmosphere for 1 h, and then washed and dried to obtain nanoporous carbon (NPC).

0.32　g FeCl₃·6H₂O was dissolved in 100 mL deionized water. 0.5 g NPC and 0.37 g K₄Fe(CN)₆·3H₂O were mixed with 100 mL deionized water and ultrasonicated to obtain a black suspension. Then, ferric chloride solution was added into the suspension under stirring for 4 h and aging for 2 h. The precipitate formed was collected by centrifugation and then dried at 80 ℃ for 12 h. The as-prepared sample was labeled as PB@NPC. The preparation method of PB was similar to that of PB@NPC but without NPC addition in the preparation process.

The phase structure, valence state, bonding mode, specific surface area and pore size distribution of NPC, PB and PB@NPC were investigated. The mechanism of improved properties of PB@NPC was discussed. The electrochemical performance of both PB and PB@NPC symmetrical capacitor were conducted.

From XRD and Raman tests, the characteristic peaks of PB and NPC can be observed, which proves the successful synthesis of PB@NPC. The XPS spectra show that C-N bond binding energy in PB@NPC is significantly shifted compared with PB. It is concluded that the shift of binding energy of C and N in the composite is related to the change of electronegativity, which is the result of the interface binding effects between PB and NPC. The specific surface area of PB@NPC reached to 2122 m²/g with a large number of micropores and mesopores by N₂ adsorption/desorption analysis.

The electrochemical performance was tested in a three-electrode system. The CV curves of PB@NPC have clear redox peaks, revealing typical pseudocapacitive behavior. The curve near the high potential and low potential is approximately rectangular, also indicating the characteristics of double layer capacitance. The specific capacitance of PB@NPC is 309.35 F/g at a current density of 0.2 A/g and even maintains 213.09 F/g at 1.0 A/g, which is much higher than PB (112.20 F/g), showing remarkable rate performance of PB@NPC. The EIS results demonstrate that the internal resistance R_s and diffusion resistance R_ct of PB@NPC are both lower than that of PB, showing better electrical conductivity and ions diffusion rate. It is proposed that the porous structure of NPC provides more ion transportation channels for PB and accelerates ion transfer. At a sweep speed of 5 mV/s, PB@NPC yields a specific capacitance of 364.14 F/g after 500 cycles. The capacitance retention rate is 75%, which is higher than that of PB (65%).

Compared with PB capacitors, PB@NPC capacitors have higher capacitance under different current density and can still return to the initial capacitance after high current density, reflecting the good structural stability. As shown in Ragone plot, PB@NPC capacitor has much better performance than PB capacitor, providing 22.4 Wh/kg energy density at the power density of 800 W/kg. Moreover, the capacitance retention of the capacitor after 1000 cycles is 91%, much higher than that of PB capacitor (34%). It is proposed that the porous structure of NPC in the composite can delay the lattice distortion of PB and promote the penetration and diffusion of electrolyte.

The PB@NPC composite was prepared via co-precipitation method. It possesses high specific surface area and excellent electrical conductivity. The fabricated porous structure provides abundant ion transport paths to promote the penetration and diffusion of electrolyte. PB@NPC demonstrates high capacitance due to synergistic effect of double-layer capacitance and pseudo-capacitance. The specific capacitance of PB@NPC electrode is 483.45 F/g at a scan rate of 5 mV/s. The specific capacitance is 309.35 F/g at a current density of 0.2 A/g. The symmetrical capacitor assembled with PB@NPC electrodes exhibits capacitance retention of 91% after 1 000 cycles, revealing excellent cycle stability. PB@NPC is a new type of supercapacitor electrode material with excellent performance and a potential electrode material to be commercialized.