A 3-D covalently crosslinked N-doped porous carbon/holey graphene composite for quasi-solid-state supercapacitors

A 3-D covalently crosslinked N-doped porous carbon/holey graphene composite (NPC/HG) was synthesized by carbonizing the precursor mixture of casein, graphene oxide (GO) and KOH. The polypeptide and amino acid produced by hydrolysis of casein in KOH solution could react with GO, which was conducive to the uniform coating of the NPC precursor on GO and the mutual crosslinking of GO. After carbonization, NPC with a high specific surface area was uniformly coated in the two-dimensional plane of graphene, and graphene was bonded into a 3-D structure by the “solder”-NPC, moreover, the KOH was an activator of NPC and also reacted with carbon atoms of GO to form holes in the carbonization process. The NPC/HG-2 showed the high specific surface area of 1135.3 m2 g−1, the rich hierarchical pore structure, and the high nitrogen doping amount of 6 at%. Used in the symmetric supercapacitor, the NPC/HG-2 exhibited the energy density of 29.33 Wh kg−1 at a power density of 42.75 kW kg−1, lower internal resistance and the excellent rate performance due to its unique internal pore structure. Moreover, the voltage of the NPC/HG-2 symmetric quasi-solid-state supercapacitor was up to 1.4 V. [Display omitted] •The 3-D N-doped porous carbon/holey graphene (NPC/HG) was designed and synthesized.•The reaction of GO and casein hydrolysate was the key to the synthesis of NPC/HG.•Graphene achieved covalently cross-linking through the “solder” —casein-derived NPC.•The NPC/HG showed a good 3-D cross-linked structure and a high specific surface area.•The NPC/HG symmetric quasi-solid-state supercapacitor showed high voltage and rate performance.