A simple, cost-effective, and environmentally friendly strategy for the preparation of porous carbons for supercapacitors via direct carbonization of potassium humate is presented. The porous carbons obtained from leonardite potassium humate (denoted as LPC) and biotechnology potassium humate (denoted as BPC) showed macro-meso-micro hierarchical porous structure, moderate surface area (668 m2 g−1 for LPC and 604 m2 g−1 for BPC) and were enriched in oxygen-containing functional groups on the surface. These porous carbons applied as electrode materials for supercapacitors exhibited an excellent capacitive behavior in basic, acid, and neutral aqueous electrolytes. The respective specific capacitances for LPC and BPC were 223 and 200 F g−1 at current density of 50 mA g−1, and 175 and 151 F g−1 at current density of 2.5 A g−1 in a 3 M KOH electrolyte. Moreover, the porous carbons had high area specific capacitance (up to 33.4 μF cm−2), superior cycling performance, and low resistance. This work demonstrates a promising preparation route for large-scale production of hierarchical porous carbons for high-performance supercapacitors.

A hierarchically porous activated carbon (HPAC) with high surface area (3064 m2/g) and large pore volume (2.319 cm3/g) was prepared from lignite using KOH as activation agent by microwave heating. Nitrogen adsorption–desorption at −196 °C, X-ray diffraction, scanning electron microscope, transmission electron microscope and X-ray photoelectron spectroscopy were used to characterize the HPAC. Because of its high surface area, macro–meso–micro hierarchically porous structure and oxygen-enriched surface, the HPAC exhibits excellent electrochemical performance in terms of specific capacitance, energy density and cycling stability as electrode material for supercapacitors. The HPAC showed a high specific capacitance of 390 F/g in aqueous electrolyte at a current density of 50 mA/g and 94.1 % of the initial specific capacitance was retained after 2000 cycles. Furthermore, this HPAC displayed a high specific capacitance of 198 F/g in an organic electrolyte.

•By contrast with template methods, HPCs are obtained by activation of humic acid, which is simple and cost-effective.•HPCs are rich in oxygen-containing functional groups, which allow the formation and uniform anchoring of fine MnO2 via strong chemical interactions between the functional groups of HPCs and the nanomaterials.A growing manganese dioxide (MnO2) nanoparticles on hierarchical porous carbons (HPCs) is conducted via a simple route starting with KMnO4 and ethanol aimed to enhance the electrochemically active surface area of MnO2. It is found that these MnO2 nanoparticles are uniformly grown on the external surface of the HPCs and still maintain hierarchical porous structure, yielding a composite electrode showing good electron transport, rapid ion penetration, fast and reversible Faradic reaction when used as supercapacitor electrode materials. HPCs–MnO2 composite displays the specific capacitance as high as 167 F g−1 and 192 F cm−3 in 3 M KOH aqueous electrolyte and 94 F g−1 and 113 F cm−3 in 1 M tetraethylammonium tetrafluoroborate/propylene carbonate (Et4NBF4/PC) organic electrolyte. Furthermore, it also exhibits a superior cycling stability with 96% retention of the initial specific capacitance after 1000 cycles and stable Coulombic efficiency of 99% in 3 M KOH measured using the galvanostatic charge–discharge technique.

•Mesoporous ACs were successfully prepared from lignite by KOH activation.•Prepared ACs have high specific surface area and large pore volume.•Mesoporous ACs possess a hierarchical pore structure and oxygen-enriched surface.•Electrochemical capacitors with ACs as electrode show superior capacitive behavior.Mesoporous activated carbons (ACs) were successfully prepared from lignite using KOH as activation agent at the temperature above 700 °C. The pore structure and surface chemistry of the as-prepared ACs were characterized by means of nitrogen adsorption–desorption, X-ray diffraction, scanning electron microscope, transmission electron microscope and X-ray photoelectron spectroscopy. The results show that such prepared mesoporous ACs have a high specific surface area (~ 3036 m2·g− 1) with a hierarchical macro–meso–micro-pore structure and oxygen-enriched surface. The electrochemical performances of the ACs as electrode materials for electrochemical capacitors (ECs) were assessed by galvanostatic charge–discharge, cyclic voltammetry and cycling durability tests. It was demonstrated that the mesoporous ACs produced in this study possessed a maximum specific capacitance of 355 F·g− 1 and 196 F·g− 1 in 3 M KOH aqueous and 1 M (C2H5)4NBF4/PC organic electrolytes, respectively, at a current density of 50 mA·g− 1, and exhibited a desirable energy and power density with a superior cycling performance. The excellent capacitive behavior of the prepared mesoporous ACs in aqueous system is attributed to their unique macro–meso–micro-hierarchical pore structure with high surface area and oxygen-containing surface. Their superb electrochemical performance in the organic electrolyte is attributed to their well-developed mesoporous structure.

•Hierarchical porous carbons (HPCs) are prepared by activation of humic acid.•The preparation of HPCs is simple comparing with template methods.•HPCs show unique pore size distribution: pores with the size of 1–2 nm and of 3–5 nm.•The textural properties can be tailored by changing the activation conditions.Hierarchical porous carbons (HPCs) are prepared from humic acid by a simple KOH activation. HPCs show unique pore size distribution: abundant micropores with the size of 1–2 nm, mesopores with the size of 3–5 nm. Of special interest is the fact that the mesopore content with the size of 3–5 nm can be fine-tuned easily. HPCs show high performance for supercapacitors possessing of the optimized capacitance of 189 F g−1 in 3 M KOH with high capacitive retention. Furthermore, HPCs also exhibit a superior cycling stability, which can be attributed to the unique pore size distribution.

Nitrogen-containing carbons were prepared by modification of activated carbons. The modified carbons were used as electrode materials with improved electrochemical performance. Precursor anthracite was activated by KOH (KOH: anthracite= 1:1), modified by melamine or urea and then treated at 1173 K to obtain the modified carbons. The porous structure, the chemical composition and the electrochemical characteristics of the carbons were investigated by nitrogen sorption, XPS and electrochemical methods respectively. Electrochemical experiments were performed in an organic electrolytic solution of 1 M (C2H5)4NBF4/PC. The samples modified by the different methods showed differences in chemical composition that introduced varying degrees of electrochemical performance enhancement. The presence of nitrogen enhanced the electron donor properties and the surface wettability of the activated carbons: this ensured a sufficient utilization of the exposed surface for charge storage.