The co-doping of graphene with nitrogen and sulfur was investigated aiming at understanding their interactions with the presence of oxygen in graphene. The co-doped graphene (NS-G) was synthesized via a one-pot hydrothermal route using graphene oxide as starting material and L-cysteine, an amino acid containing both N and S, as the doping agent. The obtained NS-G with a three-dimensional hierarchical structure containing both macropores and mesopores exhibited excellent mechanical stabilities under both wet and dry conditions. As compared to N or S singly doped graphene, the co-doped sample contains significantly higher concentrations of N and S species especially pyrollic N groups. The co-doped sample considerably outperformed the singly doped samples when used as free-standing electrode in supercapacitors due to enhanced pseudocapacitance. The simultaneous incorporation of S and N species with the presence of oxygen significantly modified the surface chemistry of carbon leading to considerably higher doping levels, although directly bonding between N and S is neither likely nor detected. Hence, the synergetic effect between N and S occurred through carbon atoms in neighboring hexagonal rings in a graphene sheet.

This work presents an efficient strategy for preparing chitosan (CS)-derived boron (B), nitrogen (N)-codoped porous carbon (C) nanosheets by using three amino acids with different acidities and the help of boric acid. Amino acids act not only as a N sources but also as the structure-directing agents through the interaction with CS to induce the formation of special morphology and structure. Boric acid serves both as the B source and as the reactive template, improving activition efficiency to creat more pores. When amino acids with different acidities are added, the morphology of the prepared samples changes from large bulks to thin nanosheets. In particular, the as-prepared carbon formed by a CS–aspartic acid gel precursor shows thin curved nanosheets. After adding KOH and boric acid, the samples possess loose and cross-linked morphologies with porous structure, which is favorable for ion transport and has benefit for the supercapacitor (SC) performance. As a result, the obtained B/N-codoped porous carbons show enhanced electrochemical performance. The sample CS/His-B prepared with basic amino acid shows the superior capacitance of 478 F g–1. Meanwhile, the assembled symmetric SC achieves a maximum energy density of 30.1 Wh kg–1 when the power density is 225.1 W kg–1 and demonstrates an ultralong cycling life for which the retention of capacitance is approximately 100% after 100 000 cycles. The maximum area capacitance is up to 12.7 F cm–1 with 50 mg of active material loaded. For an all-solid-state SC using KOH-polyvinyl alchydroohol (PVA) electrolyte, the device owns a wide potential range of 1.4 V, which shows an excellent maximum energy density of 14.4 Wh kg–1 and still remains 4.4 Wh kg–1 at the power density of 1049.5 W kg–1. B/N-codoped carbon nanosheets derived from the self-assembly of chitosan-amino acid gels represent a promising route for preparing carbon materials for high-performance SCs.

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The organic xerogel was synthesized from the polymerization reaction of a kind of amino acid, tyrosine, and pyrrole-2-carbaldehyde using phosphoric acid as the catalyst and phosphorus source. Nitrogen (N) and phosphorus (P) co-doped carbon (NPC) aerogels were obtained via the cabonization of the synthesized organic xerogel and subsequently being activated by KOH. The influences of the activation agent ratio and activation temperature on the structure, morphology, and the final electrochemical property of NPC materials have been investigated. Compared to the nonactivated sample, the obtained activated N/P-co-doped hierarchical porous carbon exhibits superior electrochemical performance. The optimized sample shows a high specific capacitance of 302 F g–1 at 0.5 A g–1 and good rate capability with capacitance remaining of 214 F g–1 at 20 A g–1, as well as excellent cycling stability with 100% retention after 20,000 cycles in 6 M KOH electrolyte. Considering the practical application, when 60 mg of active material is loaded, the mass specific capacitance of the prepared high mass loading electrode can reach to 240 F g–1 at 0.5 A g–1, and the maximum area capacitance is up to 14.4 F cm–2. Furthermore, the constructed symmetric supercapacitor displays a remarkable maximum energy density of 21.5 W h kg–1 at a power density of 250 W kg–1 and ultralong cycling life with no attenuation in the specific acpacitance even after 100,000 cycles.

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