The surface properties, pore size distribution and specific surface area of porous carbon are the main factors affecting its electrochemical performance. To further improve the electrochemical properties of porous carbon materials, two commonly used strategies are activation and doping with non-metallic heteroatoms. In this paper, a hierarchical porous carbon (HPC) was prepared by a one-step carbonation-activation method with poly(vinylpyrrolidone) as a carbon source, fibrous brucite as a template, and trispotassium phosphate as an activator to achieve the passive regulation of the pore structure and surface properties of porous carbon. The pore structure and specific surface area of hierarchical porous carbon were modulated, and the in-situ doping of nitrogen atoms were achieved. In addition, the influence of tripotassium phosphate addition on the pore size distribution, specific surface area, pore structure and electrochemical properties of the porous carbon was also investigated.

A certain amount of polyvinylpyrrolidone (PVP), tripotassium phosphate and fibrous brucite were mixed with deionised water under ultrasonication and stirring, obtaining a homogeneously dispersed mixture. The mixture as a precursor was sintered in a tube furnace via high-temperature carbonization to obtain a template/carbon composite. Afterwards, the composite was acid washed to remove the template and dried, obtaining a hierarchical porous carbon. The physical phase, pore structure, morphology and elemental composition of the hierarchical porous carbon were analysed by X-ray difffraction (XRD), scanning electron microscopy (SEM), specific surface area measurement (BET), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS). The electrochemical properties of hierarchical porous carbon were determined by galvanostatic charge-discharge (GCD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a three-electrode system using Hg/HgO, Pt sheet and the obtained hierarchical porous carbon as reference, counter and working electrodes, respectively. For the preparation process of the working electrode, the synthesised hierarchical porous carbon, conductive carbon black and PTFE emulsion were mixed in a mortar at a mass ratio of 8∶1∶1, and the mixture with an appropriate amount of ethanol solution was ground into a homogeneous slurr. The slurry was coated on the nickel foam collector, dried, and then pressed in a manual hydraulic press for 5 min to obtain the working electrode.

The results of XRD and EDS show that the characteristic diffraction peaks of potassium fluorosilicate appear in the synthesised hierarchical porous carbon in addition to the amorphous carbon, but their contrast capacitance hardly contributes. The hierarchical porous carbon has a one-dimensional hollow carbon tube stacked reticular structure, which is conducive to the rapid transfer of electrons. Meanwhile, the hierarchical porous carbon obtained after the activation of tripotassium phosphate contains abundant pores, which can provide a large number of active sites for the electrolyte ions and a convenient channel for the rapid diffusion and transport of ions. In addition, the microporosity and specific surface area of the activated hierarchical porous carbon increase significantly, compared with those of the unactivated porous carbon. The specific surface area and pore volume are 707.9 m²/g and 1.65 cm³/g, respectively. The large specific surface area and pore volume can lead to more active sites and diffusion channels. The results of XPS demonstrate that element N also appears in the hierarchical porous carbon, indicating that nitrogen in-situ doping is realised during the carbonisation process. The introduction of nitrogen improves the electrical conductivity and wettability of the carbon material, and also has active sites and pseudocapacitance. Compared with the unactivated porous carbon, the activated hierarchical porous carbon exhibits superior electrochemical properties due to its rich pore structure, suitable heteroatom doping and hierarchical porous structure.

Nitrogen-doped hierarchical porous carbon was synthesized by a one-step carbonation-activation method with fibrous brucite as a template and tripotassium phosphate as an activator. Meanwhile, the different additions of tripotassium phosphate could achieve the regulation of the morphology, specific surface area, pore structure and electrochemical properties of the hierarchical porous carbon. The hierarchical porous carbon with an appropriate addition of tripotassium phosphate exhibited high specific surface area and superior electrochemical properties. The results showed that the specific capacitance of HPC/K₃ activated by tripotassium phosphate could reach 281.94 F/g at a current density of 0.5 A/g, which was greater than that of non-activated HPC/K₀ (i.e., 200.31 F/g). The capacity retention rate after 8 000 charge-discharge cycles reached 84.7%. This study demonstrated that tripotassium phosphate activation could improve the electrochemical performance of porous carbon. In addition, this study could also provide some insights for the high value-added application of natural mineral fibrous brucite.