Nitrogen and Oxygen-Doped Hierarchical Porous Carbons from Algae Biomass: Direct Carbonization and Excellent Electrochemical Properties

Herein, the algae pollutants of Enteromorpha prolifera were applied as precursors to obtain nitrogen and oxygen co-doped hierarchical carbons with major pores at around 1.5 and 5.0 nm through direct carbonization at 700 and 900 °C after freeze-drying treatment. Physical properties of porous carbons including morphology, elemental analysis, and pore size distribution were evaluate by transmission electron microscopy, X-ray photoelectron spectroscopy and N2 sorption. In both cases, surface areas were only close to 450 m2 g−1 with 70% areas inherited from mesopores, resulting in high density materials. The carbon derived at 700 °C had the highest capacitance of 234 F g−1 at a current density of 0.5 A g−1 in 6 M KOH electrolyte and also had excellent cycling stability with 95% of capacitance after 2000 cycles, owing to the naturally heteroatoms and unique hierarchical nanopores contributed pseudo-capacitance and provided active sites for facilitating electrolyte ions diffusion. In addition, the carbon derived at 900 °C had CO2 uptake of 6.48 mmol g−1 at 20 bar and 25 °C.