Nitrogen Doped Hydrothermal Carbon for Supercapacitors

Although pure carbon materials have been readily adopted for energy storage applications, recent years have seen an increase in studies incorporating heteroatoms into the carbon matrix for energy storage applications. The doping of carbon materials with heteroatoms increases performance significantly in energy storage applications, specifically supercapacitors. This increase in performance is due to heteroatoms providing electrochemically active sites in which highly reversible redox reactions can occur (pseudocapacitance), boosting the overall performance of the electrode. This has led to the widespread development of doped carbon materials incorporating nitrogen, phosphorus, sulphur and various transition metals into almost every allotrope of carbon.

Currently, a number of technologies have been utilized to dope nitrogen into carbon structures, such as electric arc, pyrolysis, hydrothermal and pre/post treatments with nitrogen sources. Of these, hydrothermal carbonization (HTC) is one of the most promising techniques in carbon materials research. In comparison to other carbonization techniques, HTC offers an environmentally friendly approach to the formation of highly functionalized, doped carbon structures from renewable sources as carbonization occurs under water at substantially lower temperatures (180°-300°C). Furthermore, heteroatoms incorporation, chemical and structural morphology as well as surface functionality can be easily controlled through the manipulation of the treatment conditions, solution pH and addition of soluble additives (i.e., (NH₄)₂SO₄.

In this study, we have investigated the chemical structure via NEXAFS, morphology and electrochemical performance of hydrothermal carbons produced from sucrose doped with different ammonium salts. This produced nitrogen doped carbons with a spherical morphology with a wide variety of oxygen and nitrogen functionalities spread across the surface. Electrochemically, these carbons displayed a wide range of psudocapacitive behaviour as well as electrical double layer capacitance, delivering up to 200 F/g in three electrode cells with 1 M KOH as the electrolyte.