Tailoring biomass-derived carbon for high-performance supercapacitors from controllably cultivated algae microspheres

A high-performance "green" carbon-based supercapacitor electrode material is synthesized from selected algae microspheres, which are grown under controlled cultivation conditions. The best-performing sample possesses a high specific surface area of 1337.9 m² g^-1 with a hierarchically porous structure and naturally intrinsic nitrogen dopant. This leads to an excellent specific capacitance of 353 F g^-1 at 1 A g^-1 and 92% capacitance retention even after 10 000 charge-discharge cycles at 20 A g^-1, which makes it superior to many recently reported biomass- and synthetically derived carbon electrode materials. It is found that residual nitrogen and metal contents in algae-derived carbon are highly influenced by biomass components, such as proteins. The content of these components can be controlled by adjusting the concentrations of nutrients in cultivation media. However, nitrogen in algae proteins is not pyrolytically stable, and studies indicate that excessive residual metal content plays the role of "dead mass" and results in a less-developed porous structure. Therefore, this suggests that both biomass selection and cultivation should aim for proteins with a more stable nitrogen content and minimized content of electrochemically inactive metals such as Mg and Ca. Hence, this study does not just demonstrate a green candidate electrode material for high-performance supercapacitors, but also provides an innovative selection and cultivation strategy to improve the capacitive performance of biomass-derived carbon.