Synthesis and capacitance properties of N-doped porous carbon/NixCoyOz/carbon micro–nanotubes composites using coal-based polyaniline as a carbon and nitrogen source

Using coal-based polyaniline as a carbon and nitrogen source, N-doped porous carbon (NPC)/nickel cobalt oxides (NixCoyOz)/carbon micro–nanotubes (CMNT) composites possessing rich nanoscale meso–macropores, spinel NiCo2O4 sheets and multi-walled CMNT are successfully synthesized by the first pyrolysis and the second wet oxidation process. Therein, CMNT owning a diameter of 10–200 nm and a length of several micrometers is grown by a two-stage furnace process in a hydrogen-free atmosphere, using nickel acetate as a growth catalyst precursor. CMNT exhibits polymorphic features, including most curved tubes, a few vertical tubes, some bamboo-like tubes and some Chinese-sugar-gourd-skewer-like tubes, resulting from the diversity and low hydrogen content of gaseous cracking products from coal-based polyaniline. Carbon in CMNT mainly exists in the graphitic state while carbon in NPC mainly presents in the amorphous state. Nitrogen in CMNT mainly exists in the form of graphitic N while nitrogen in NPC (0.89 wt% in composites) mainly presents in the form of pyrrolic N (74.4 at.%) and oxidized N (25.6 at.%). The intercalated Ni and Co impurity in NPC and CMNT are transformed to most NiCo2O4 and a few NiO. The BET specific surface area and average pore width of composites are 169.3 m2/g and 8.4 nm. CMNT incorporation obviously improves capacitance properties of NPC/NixCoyOz/CMNT composites. The composites demonstrate a higher specific capacitance of 598.4 F/g at 1 A/g, and a good cycling stability retaining a high capacity of 190.1 F/g (81.1% retention) at 5 A/g after 5000 charge–discharge cycles. It is attributed to the nitrogen incorporation of porous carbon, high conductivity and large specific surface area of CMNT, and high electrochemical activity of NixCoyOz, especially, the synergistic effects of NPC, CMNT and NixCoyOz. This study has developed a polygeneration process for coal pyrolysis, which also enables the combined utilization of coal pyrolysis products.