Preparation And Electrochemical Performance Of Carbon-based Nanostructrued Electrode Materials

Traditional carbon materials used as electrode materials in supercapacitor usually possess relatively high Brunauer-Emmett-Teller?BET?specific surface area?SSA?and developed porous structure.Some reports reveal that biomass or nitrogen-containing polymer derived carbons with low BET SSA(less than 100 m² g^-1)exhibit abnormal high areal capacitance(100¹000?F cm^-2).However,the well-accepted pseudocapacitance introduced by heteroatoms could not explain this phenomenon undoubtedly.In order to explore the nature of the energy storage mechanism for these low surface area carbons,three carbon precursors?laver,melamine formaldehyde resin and polyaniline?were used to prepare low surface carbons,which were further heated under hydrogen atmosphere to regulate the heteroatom contents.The results show that there are many ultramicropores?<0.4 nm?located in the pore entrance,which will restrict the entry of N₂ or Ar with large kinetic diameters,thus obtaining a low BET specific surface area.However,carbon dioxide could penetrate into those ultramicropore due to its smaller kinetic dimater and higher test temperature and pressure,resulting in larger micropore surface area.Micropore surface area probed by carbon dioxide is more close to the actural electrolyte/electrode interface where electrochemical energy storage process occurs by comparing the detected pore sizes using different adsorbates with electrolyte ion size.Therefore,the energy storage mechanism for this type of carbon is mainly ultramicropore-dominated electric double layer capacitance and the pseudocapacitance introduced by heteroatoms is less than 15%.A new method using dissoluble inorganic salts such as soudium chloride,sodium sulphate and sodium phosphate as templates and combining freeze-drying technology is developped to prepare porous carbons with microporous and macroporous structure.The templates could be removed using deionized water instead of hazardous chemicals such as HF or NaOH,therefore,such a process is sustainable.Carbon materials with the composition of C and O could be synthesized with the templates of sodium chloride or sodium phosphate.In comparison,there is a sulfur dopping for the final porous carbon when sodium sulphate is used as a template and sodium sulphate-templated carbon possesses a more developed porous structure compared with sodium chloride or sodium phosphate templated carbons.Besides,nitrogen-doped carbons could also be prepared through two strategies:1)Ammonium chloride could be introduced into sodium chloride/glucose system as a nitrogen source;2)Nitrogen-containing carbon precursor such as dopamine could be used as carbon source and nitrogen source at the same time.Sulphur and nitrogen codoped carbons are prepared using sodium sulphate and dopamine or glucosamine as template and sulphur source,and carbon precursor and nitrogen sources,respectively.Systematic porous structure analysis indicates that dopamine-derived carbon materials prepared by direct carbonization process under 600 ^oC possess a relatively high BET surface area.There is a further increase of surface area under800 ^oC heat treatment when oxygen and sulphur are escaped.The assembled supercapacitor exhibits a moderate supercapacitive performance when the sulphur and nitrogen codopped carbon is used as electrode material.