Co-pyrolysis of algae and lignocellulosic biomass in molten salts to produce N-doped carbon for supercapacitor application

Conventional pyrolysis strategies present challenges in maintaining the nitrogen fraction and improving the electrical conductivity of the product carbon at the same time. Co-pyrolysis of algae and lignocellulosic biomass with different temperature-controlled procedures in molten Na2CO3–K2CO3 is innovatively proposed to produce N-doped capacitive carbon. Results show that interactions between algae and bamboo in the presence of molten salt incorporate more nitrogen species into the carbon matrix and increase carbon yield. Co-pyrolysis with an optimal switching temperature of two-stage pyrolysis improves the nitrogen amount in carbon and the carbon yield by up to 108.7 % and 34.7 %, respectively. Through diffusion and infiltration, the liquid molten carbonates increase the contact area between the salt molecules and the carbon matrix, resulting in superior and uniform capacitive carbon. The resulting carbon shows a high specific capacitance of 306.2 F/g at 0.25 A/g with a cycling stability of 91.3 % after 5000 cycles, due to its hierarchical porous structure, high specific surface area (1326.58 m2/g) and abundant pyridine-N/pyrrolidine-N (81 %). Therefore, applying co-pyrolysis in molten carbonates with two-stage pyrolysis to produce high-yield N-doped carbon for supercapacitor application is proved to be promising.