Two-dimensional carbon nanosheets codoped with N and P species have been successfully synthesized by a template carbonization method coupled with nitrogenization and phosphorylation processes using trisodium citrate dihydrate, melamine, and NH4H2PO4 as C, N, and P sources, respectively. Dopants of N and P species play crucial roles in the determination of carbon porosities and electrochemical performance; notably, increasing the P content can lead to a decrease in the BET surface area together with a corresponding decrease in the electrochemical performance. For instance, regulating the mass ratio between the C source and the N and P sources to 2:1 results in the maximum BET surface area of 1340 m2 g–1, whereas a ratio of 1:2 results in a decreased value of only 47 m2 g–1. Moreover, the mass ratio of 1:1 results in superior electrochemical behaviors, with a maximum energy density that can reach up to 13.3 Wh kg–1. The present synthesis method provides an alternative route for producing N- and P-containing carbon nanostructures with two-dimensional features, serving as excellent electrode materials for energy propagation and storage.

2-D BiVO4 nanosheets with monoclinic phase were synthesized at room temperature, and incorporated with Ag to form BiVO4:Ag hybrid materials. The experiments demonstrated that doping Ag has largely increased the electrochemical performances of supercapacitor. Furthermore, the specific capacitance can reach up to 109 F g–1 at 1 A g–1 (the undoped one is of 27 F g–1); energy density has enhanced to 15.2 Wh kg–1 compared with the pristine one without Ag (3.8 Wh kg–1). Therefore, doping Ag into bismuth-based compound provides us an alternative approach for the synthesis of 2-D nanostructured hybrid as an efficient electrode material for supercapacitors

We demonstrate a synchronous carbonization and graphitization process for producing nanoporous graphitic carbon materials, using ammonium iron (III) citrate as sole starting material. The temperature has a crucial role in the determination of carbon structure, graphitization as well as porosity. The carbon sample obtained at 800 °C has delivered the largest capacitance, whereas the one at 1000 °C is the lowest, primarily due to its smallest porosity incurred by the structure collapse. More importantly, 4-(4-nitrophenylazo) resorcinol serving as redox additive has been incorporated into KOH electrolyte. As a result, when designating the redox additive concentration as 3, 6, and 9 mmol L−1, the corresponding capacitances can reach up to 59.2, 105.8, and 141.0 F g−1, which are almost the 1.78, 3.17, and 4.23 times than the one without redox additive. Thereby, the present 4-(4-nitrophenylazo) resorcinol can serve as simple but effective redox additive for remarkably improving the capacitive performance.

•Nanoporous carbon material is achieved by a template carbonization method.•Pyrocatechol violet can serve as effective redox additive for elevating the capacitance.•The redox additive delivers superior behaviors in H2SO4, KOH and Na2SO4 electrolytes.•The increasing fold of redox additive can reach up to 2.06 in the case of H2SO4.In present work, we demonstrate a simple but effective redox additive of pyrocatechol violet (abbr. PCV) to largely promote the capacitive performances especially when carried out in three different kinds of electrolytes (H2SO4, Na2SO4 and KOH), mostly due to its fast electron and proton transfer occurring in the electrode/electrolyte interface. It reveals that the PCV dosage incorporated into electrolyte plays a crucial role in the determination of capacitive performance. When conducted in a two-electrode system, incorporating 0.06 mol L−1 PCV into 1 mol L−1 H2SO4 can achieve large capacitance up to 200 F g−1, which is almost 2.06 times than the one without the addition of PCV; besides, the corresponding energy density is of 28 Wh kg−1 (also increasing 2 times). What’s more, PCV has been extended as the redox additive in other electrolytes such as Na2SO4 and KOH, and remarkable promotion in capacitance and energy density also occur, well evincing the high efficiency and universal applicability of PCV for the large promotion of supercapacitors’ performances due to its prominent electrochemical reversibility and high solubility.

•N-doped carbon materials are derived from diphenylcarbazide.•Diphenylcarbazide can also serve as novel redox additive.•Incorporating redox additive into electrode is simple but highly effective.•The specific capacitance has been largely improved up to 497.8 F g−1.Nitrogen-doped nanoporous carbon materials have been prepared by a template carbonization method, in which diphenylcarbazide serves as carbon/nitrogen source and Mg(NO3)2·6H2O powder as hard template, respectively. The mass ratio of diphenylcarbazide and Mg(NO3)2·6H2O powder plays a crucial role in determining the pore structures and electrochemical performances. The resulting carbon-2:1 sample displays large BET surface area of 1366 m2 g−1, and high pore volume of 2.18 cm3 g−1. It also exhibits good cycling stability and superior electrochemical behaviors, including high specific capacitance of 323.5 F g−1 at 1 mA cm−2 in a three-electrode system using 6 mol L−1 KOH as electrolyte. More importantly, to further improve the electrochemical performance, different amounts (8, 16 and 24 mg) diphenylcarbazide herein serving as novel redox additive is introduced into the carbon-2:1 system to form the carbon-2:1-8/16/24 electrodes. As a result, the specific capacitances of the carbon-2:1-8/16/24 electrodes at 2 mA cm−2 have been improved up to be 232.7, 344.4 and 497.8 F g−1, respectively, which are 1.02, 1.51, and 2.18 times than that of the pristine system (∼228.8 F g−1). Furthermore, the carbon-2:1-24 also retains high cycling stability as 77.5% after 5000 cycles. The present method of incorporating diphenylcarbazide (redox additive) into carbon system is simple but efficient for large improvement of supercapacitor performances.

In this work, a series of porous carbon materials with hierarchical porosities have been synthesized via a template carbonization method, in which cheap CaCO3 serves as a template and glucose as a carbon precursor. During the carbonization process, CO2 produced by the decomposition of the CaCO3 template can act as an internal activating agent, significantly improving microporosity and mesoporosity. All the carbon materials obtained by regulating the ratio of glucose to CaCO3 exhibit the amorphous features with a low graphitization degree. Among them, the carbon-1:2 sample shows a high BET surface area of up to 818.5 m2 g−1 and a large total pore volume of 1.78 cm3 g−1 as well as a specific capacitance of 107.0 F g−1 at 1 A g−1. In addition, a series of hydroquinone (HQ), p-aminophenol (PAP) and p-nitrophenol (PNP) as novel redox additives that can produce pseudo-capacitances have been added into the KOH electrolyte for promoting the total capacitive performances via redox reactions at the electrode–electrolyte interface. As expected, a 2.5-fold increase in the galvanostatic capacitance of 240.0 F g−1 in the HQ-0.5 electrolyte occurs, compared with the conventional KOH electrolyte. Similarly, the PAP-0.5 electrolyte and the PNP-0.5 electrolyte also show a high specific capacitance of 184.0 F g−1 at 2 A g−1 (156.6 F g−1 at 3 A g−1) and 153.0 F g−1 at 3 A g−1, respectively. Additionally, the three kinds of electrolytes exhibit excellent cyclic stability. The remarkable improvement of supercapacitors is attributed to the quick reversible Faradaic reactions of amine and hydroxyl groups adhering to the phenyl rings, which largely accelerates electron migration and brings additional pseudocapacitive contribution for carbon-based supercapacitors.

In present work, we demonstrate a simple but effective strategy for high-performance supercapacitors by adding the p-nitroaniline (PNA) into an alkaline electrolyte of KOH. PNA possesses a unique molecular structure with the functional groups of –NH2 and –NO2. Besides, both the product of nitro-reduction (–NH2) and intrinsic –NH2 on the benzene ring can lead to the occurrence of Faradaic redox reactions accompanied by the electron/proton transfer in the mixed electrolytes, whose pseudocapacitance can greatly enhance the total capacitance. Furthermore, another effective additive of the dimethylglyoxime (DMG) has been incorporated into carbon materials for further improving the performances of supercapacitors with a PNA + KOH electrolyte. As for the DMG + PNA + KOH system, a galvanostatic capacitance up to 386.1 F g−1 of the DMG-0.15–PNA-0.15 sample at 3 A g−1, which is nearly two times higher than that of the PNA-0.15 sample (183.6 F g−1) in the PNA + KOH system and nearly three-fold capacitance of the carbon-blank (132.3 F g−1) in the KOH system at the same current density. Furthermore, the specific capacitance still can reach up to 260.0 F g−1 even at 40 A g−1 with a 67.4% capacitance retention ratio. Besides, the DMG-0.15–PNA-0.15 sample exhibits an exceptional capacitance retention of 113% after 5000 charge/discharge cycles by virtue of the potential activated process, which clearly reveals the excellent cycling stability. These remarkable enhancements are ascribed to the synergistic effects of novel additives of PNA and DMG.

•PAP substance serves as effective redox additive both in KOH and H2SO4 solution.•Incorporating PAP can drastically improve the capacitance and energy density.•Two different redox reaction mechanisms exist for KOH and H2SO4 solution.•The incorporation of PAP into solution is simply operated but highly effective.As a dual functional redox additive, p-aminophenol (abbr. PAP) simultaneously possessing hydroxyl and amine groups has been incorporated into KOH or H2SO4 electrolyte. It reveals that the PAP concentration and cell configuration play crucial roles in the determination of capacitive behaviors. PAP substance can release two protons and two electrons in KOH solution, whereas one proton and one electron occur in H2SO4 solution. The incorporation of PAP into KOH or H2SO4 solution can markedly elevate the specific capacitance, energy density and both of which remain high cycling stabilities. When designating 15 mmol L− 1 PAP in KOH or H2SO4 solution, the increase folds in capacitance are of 2. 2 and 1.4, respectively, and the correlative energy densities can reach up to 7.24, and 8.44 Wh kg− 1, respectively. What's more, the redox process in KOH solution is controlled by the surface reaction, while the one in H2SO4 solution indicates the semi-infinite diffusion mechanism.

•Both of PPD and PNA serve as effective redox additives.•PPD releases two protons/electrons, while PNA gives one proton/electron.•Nitro group can be partially reduced into amine.•The potential window strongly affects the capacitive performances.Using p-phenylenediamine (PPD) or p-nitroaniline (PNA) as effective redox additive can largely improve the capacitive performances of supercapacitors when incorporated into KOH solution. And how to vividly differentiate the roles of amine and nitro groups adherent to phenyl ring really is an interesting scientific issue. Herein, with the help of a series of capacitive techniques, it is discerned that the PPD can release two protons and two electrons, while the PNA realizes the gain/loss of one proton and one electron, in particular accompanied with the partial reduction of nitro group. Clearly, introducing PPD or PNA substances into KOH solution can largely improve the specific capacitance as well as the resultant energy densities, but slightly deteriorates the energy efficiency. Besides, the incorporation of PNA leads to higher cycling stability than that of the PPD, which is primarily incurred by the fact that the nitro group adherent to phenyl ring delivers better stability. Furthermore, the potential window of 0–1 V or − 0.5–0.5 V strongly affects the capacitive performances. The present research results are expected to provide us the guidance for the usage of PPD or PNA when incorporated into KOH solution for elevating the supercapacitors' performances.

•Folic acid serves as excellent carbon/nitrogen sources.•Mg(OAc)2·4H2O has been utilized as template for pore formation.•p-Phenylenediamine acts as effective redox additive.•Redox reaction occurs at the electrode–electrolyte interface.In the present work, we have demonstrated a simple but effective template carbonization approach to convert folic acid into highly nitrogenated nanoporous carbon materials, using Mg(OAc)2·4H2O as template. For the C-800/900/1000 samples, the nitrogen contents can reach up to 9.03%, 6.81%, and 6.65%, respectively, and they also deliver large BET surface areas of 1242.2, 1559.1, and 1302.8 m2 g− 1, and high pore volumes of 1.36, 2.26, and 1.44 cm3 g− 1, respectively. On the other hand, it is revealed that the incorporation of p-phenylenediamine as a redox additive into KOH electrolyte has greatly improved the capacitances, mostly due to the occurrence of pseudo-capacitance derived from the redox reaction of p-phenylenediamine at the electrode/electrolyte interface. For instance, the capacitances of the C-900-6/9/12 samples can reach up to 105.1, 232.8, and 317.6 F g− 1 at 5 A g− 1, respectively, and all of which are much higher than that of the pristine C-900 sample (42. F g− 1). Obviously, we have provided a simple but highly efficient approach for the improvement of the supercapacitor capacitance by introducing the p-phenylenediamine into KOH electrolyte, which exhibits the advantages of low cost, easy operation, high efficiency, etc.

•Sb2MoO6, Bi2MoO6, Sb2WO6, Bi2WO6 are controllably prepared.•Dosage of NaOH in solution plays crucial role in determining the products.•Orthorhombic Sb2O3 and tetragonal Bi12O17Cl2 also occur.•Bi2WO6 and Bi2MoO6 are doped with Eu3+ ions to prepare red phosphors.Under hydrothermal conditions, a series of flake-like Sb2MoO6, Bi2MoO6, Sb2WO6, Bi2WO6 with the Aurivillius structure have been prepared controllably. It reveals that the initial molar ratios of SbCl3-to-NaOH (or BiCl3-to-NaOH) in the reaction system (SbCl3-Na2MoO4, BiCl3-Na2MoO4, SbCl3-Na2WO4, and BiCl3-Na2WO4) play important roles in the determination of product phases. Besides, properly changing the content of NaOH involved can produce some unexpected phases such as orthorhombic Sb2O3 and tetragonal Bi12O17Cl2. Moreover, substituting Bi3+ with Eu3+ at the A site is readily carried out because of their same valence states together with the similar ion radii. Consequently, the as-prepared Bi2WO6 and Bi2MoO6 samples have been doped with Eu3+ ions also under hydrothermal conditions to prepare the phosphors, which possess excellent red characteristics in terms of excitation and emission measurement. The present synthesis protocol has opened up an intriguing but effective avenue for producing antimony/bismuth-based materials, also exhibiting the potential application of red phosphors.

•Magnesium citrate and nickel nitrate as carbon source and graphitization catalyst.•Synchronous carbonization and graphitization method is simple and effective.•Adding redox additive of p-nitroaniline can highly improve the performance.•Redox additive into KOH electrolyte can be operated at ambient condition.Highly nanoporous carbon materials have been produced by a synchronous carbonization/graphitization process, using magnesium citrate serves as the carbon source and nickel nitrate as graphitization catalyst. The carbonization temperature plays a crucial role in determining the porosity and graphitization. The lower temperature favors for the formation of larger porosity, whilst higher temperature for better crystallinity. Resultantly, a high BET surface area of 2587.13 m2 g−1 and large total pore volume of 4.64 cm3 g−1 appear, the case of C-800 sample, thereby resulting in a large specific capacitance of 305.3 F g−1 at 1 A g−1 from the contribution of electric double layer capacitances. More importantly, we demonstrate a novel redox active additive of p-nitroaniline (PNA) into the 6 mol L−1 KOH electrolyte to largely improve the capacitance by the quick self-discharge redox reaction of H+/e−. The C-800-2 sample with the PNA concentration of 2 mmol delivers largely improved capacitance of 502.1 F g−1 at 1 A g−1, which is almost 1.65 fold increase. Apparently, the present PNA is commercially available, and highly effective for elevating the specific capacitance and might be implemented for the wide supercapacitor application.

•PVC is converted into nanoporous carbon by a template carbonization method in large scale.•Incorporating MnOx into carbon can greatly improve the electrochemical performance.•The carbon material exhibits high pore volume and hierarchical pore size distribution.•A high specific capacitance as 751.5 F g–1 has been achieved in a three-electrode system.We herein demonstrate a rational template carbonization approach to convert waste polyvinyl chloride into nanoporous carbon, in which inexpensive Mg(OH)2 serves as hard template. The carbon-blank sample that is obtained by designating the mass ratio of polyvinyl chloride and Mg(OH)2 as 1:2 at the carbonization temperature of 700 °C is amorphous and highly porous in essence. It also exhibits large BET surface area of 958.6 m2 g−1, high pore volume of 3.56 cm3 g−1, and hierarchical pore size distribution. To further improve the electrochemical performance, various amounts of MnOx nanoparticles are incorporated into the nanoporous carbon by direct redox reaction between carbon-blank sample and KMnO4 solution at 70 °C. Therein, carbon-Mn2 sample (the mass ratio of carbon-blank sample and KMnO4 is 1:1) behaves the optimal electrochemical performance. Though its porosity to some extent decreases, its specific capacitance has greatly elevated up to 751.5 F g−1 at 1.0 A g−1, compared with that of the carbon-blank sample (∼47.8 F g−1). The incremental capacitance of the carbon-Mn2 sample is mostly attributed to the contribution of pseudocapacitance incurred by faradic reaction of MnOx material. The present synthesis method opens up an avenue to properly dispose waste polyvinyl chloride into nanoporous carbon, especially with the promise in supercapacitor application.

•Phenidone act as carbon/nitrogen sources for producing nanoporous carbon materials;•Mg(OH)2 template effectively enlarges the porosities of carbon materials;•Introducing azobisformamide can further improve the nitrogen content of carbon;•The N-doped carbon materials deliver superior electrochemical behaviors for supercapacitor applications.In this study, we present a simple but efficient template carbonization method to prepare nitrogen-doped nanoporous carbon material, in which phenidone acts as carbon/nitrogen sources and Mg(OH)2 as hard template. The results indicate that the carbon-1:1 sample is highly disordered with large BET surface area of 1513 m2 g−1, high pore volume of 2.2 cm3 g−1 and nitrogen content of 3.78%. As a result, it exhibits decent electrochemical behaviors, whose specific capacitance reaches up to 202.0 F g−1 when measured at 1  A g−1 in a three-electrode system. Moreover, azodicarbonamide has been introduced in the process of carbonization to further tailor the porosity of nanoporous carbon, named as the carbon-1:1:1 sample. In consequence, its BET surface area has decreased to be 1261 m2 g−1 but the pore volume increased up to 2.8 cm3 g−1, together with the large enhancement of nitrogen content up to 7.05%. Besides, it thus delivers a higher specific capacitance of 281.0 F g−1 at 1 A g−1, mostly due to the incremental content of nitrogen species. The proposed Mg(OH)2-assisted template carbonization method has provided an intriguing synthesis approach for N-doped nanoporous carbons, especially the nitrogen improvement simply by the addition of azodicarbonamide.

Nitrogen-doped nanoporous carbon materials with large BET surface area (1322.5 m2 g−1) and pore volume (0.87 cm3 g−1) have been achieved by a synchronous carbonization/nitridation process, simply using potassium biphthalate and azodicarbonamide as carbon/nitrogen sources, respectively. The above carbon materials have been further impregnated with MnOx nanocrystallites that comes from the thermal decomposition of Mn(NO3)2. Taking the carbon-2:1-Mn sample as an excellent example, it also has large BET surface area (1160.1 m2 g−1) and pore volume (0.77 cm3 g−1) and exhibits high nitrogen/manganese contents of 4.13%, and 3.30%, respectively. The carbon-2:1-Mn sample delivers excellent capacitances of 564.5 and 496.8 F g−1 at the current density of 0.5 and 1.0 A g−1, respectively, as well as superior cycling stability of 96.10% even after charging/discharging for 5000 times. The present method of incorporating cheap MnOx substance into carbon matrix is efficient and also easy for large scale production of carbon nanocomposites, especially possessing large BET surface area and high pore volume.

•Diphenylcarbazide acts as both carbon source and redox additive.•Electron and proton transfers occur in the redox additive–carbon material system.•Introducing redox additive has highly improved the supercapacitor performance.•Redox additive into KOH electrolyte can be realized at ambient condition.In this work, we demonstrate two kinds of novel redox additives, diphenylcarbazide (abbr. DC) and phenylazoformic acid 2-phenylhydrazide (abbr. PP), with quick and reversible redox reaction incorporated into carbon-based supercapacitors. It is revealed that the 4-electron and 4-proton transfer occurring in the redox additive–carbon material system within the electrode can result in additional capacitance. When introducing 24 mg DC or PP substance into 24 mg carbon system, largely improved specific capacitances of 544.3 or 427.6 F g− 1 are achieved, which are almost 4.31 and 3.39 times, respectively, than that of the pristine carbon (126.1 F g− 1) at the current density of 2 A g− 1. Besides, their corresponding capacitance retentions can reach up to 78.9% and 75.6%, respectively, after 5000 charge–discharge cycles, and both of them are somewhat smaller than that of the pristine carbon (96.8%) due to the incremental electronic resistances. The present commercially available, low-cost and highly effective redox additives of DC and PP are quite promising and expected to be implemented for high performance supercapacitors.

•Azodicarbonamide serves as efficient nitrogen source for N-doped carbon.•A template carbonization method is implemented to produce N-doped porous carbon.•The carbon materials exhibit large surface areas and pore volumes.•The N-doped carbon materials deliver superior electrochemical behaviors for supercapacitor applications.A series of N-doped nanoporous carbons are prepared via a synchronous template carbonization and nitridation method, in which glucose serves as carbon precursor, azodicarbonamide (ADC), urea and melamine as nitridation agents and templates. Results indicate that all the obtained carbon materials deliver the amorphous features with low graphitization degree. The Carbon-G-A sample prepared by heating glucose and ADC at 800 °C exhibits high surface area of 624.8 m2 g−1, large total pore volume of 0.53 cm3 g−1 and fairly high nitrogen content of 10.35%. The Carbon-G-A sample delivers superior electrochemical performance, whose specific capacitance can reach up to 202.7 F g−1 at a current density of 1 A g−1. What’s more, it exhibits long-term cycling ability of 92.3% retention even after 5000 cycles. Besides, in the cases of urea and melamine as nitrogen sources, the resultant nitrogen-containing carbons deliver smaller porosities and lower nitrogen contents, but also acceptable specific capacitances of 148.7 and 126.1 F g−1 at 1 A g−1, respectively. It opens a straightforward and easy protocol to convert inexpensive glucose into highly capacitive nitrogen-doped nanoporous carbon materials, using ADC, urea and melamine as nitrogen sources.

•A direct carbonization method has been implemented to produce nanoporous carbon.•Commercially available Mg(OH)2 or Ca(OH)2 powder serves as template, and thiocarbanilide as carbon/nitrogen source.•The mass ratio and the carbonization temperature play crucial roles in determining the pore structures.•Three-electrode system has been adopted to measure the electrochemical behaviors.•The carbon materials deliver superior capacitive performance for supercapacitor applications.A valid and facile template carbonization route for producing nanoporous carbons with hierarchical porosities has been implemented by using thiocarbanilide as carbon/nitrogen precursor, and commercially available Mg(OH)2 or Ca(OH)2 powder as template. This work is to clarify the significance and difference between Mg(OH)2 and Ca(OH)2 being as templates. The mass ratio of thiocarbanilide and Mg(OH)2 or Ca(OH)2, as well as the carbonization temperature plays crucial role in determining the pore structures and the resultant capacitive behaviors. It reveals that carbon-Mg sample whose template is Mg(OH)2 (with a mass ratio of 1:2 at 700 °C) exhibits the amorphous feature with low graphitization degree. Similar result also occurs in the case of the carbon-Ca sample. The carbon-Mg sample presents high specific surface area (1018.48 m2 g−1), large pore volume (5.29 cm3 g−1), while those of the carbon-Ca sample are 429.11 m2 g−1 and 2.52 cm3 g−1, respectively. The carbon-Mg sample delivers a high specific capacitance of 327.4 F g−1 at a current density of 1.0 A g−1, as well as a large energy density of 45.47 Wh kg−1 at a power density of 0.5 kW kg−1 in comparison with carbon-Ca sample of (260.0 F g−1) and (36.11 Wh kg−1). What’s more, the carbon-Mg sample exhibits higher capacitance retention of 95.45% after 10,000 charge/discharge cycles than carbon-Ca sample of 91.62% in 6 mol L−1 KOH electrolyte. Using Ca(OH)2 or Mg(OH)2 as template by a template carbonization route provides a simple but feasible protocol to achieve nanoporous carbons with precisely controlled architecture and hierarchical porosities.

•The zinc citrate serves both as carbon source and template.•Adding ZnCl2 into zinc citrate can further increase the carbon porosity.•The capacitive performance has been largely enhanced by introducing PPD additives.•The specific capacitance can reach up to 934.6 F g−1.In this work, we demonstrate a simple template carbonization method to prepare nanoporous carbon materials, in which zinc citrate serves as carbon source and the ZnO substance together with some gases in situ thermally produced as templates. The resultant carbon-800 sample has a large surface area of 740.5 m2 g−1, and high pore volume of 1.96 cm3 g−1. To further increase the porosity of the above carbon materials, ZnCl2 that can act as activation agent and template has been added into the zinc citrate precursor. As a result, the surface area of the carbon-ZnCl2-800 sample has largely increased up to 1244.2 m2 g−1, and pore volume up to 2.21 cm3 g−1. In a conventional 6 mol L−1 KOH electrolyte, the specific capacitances of the carbon-800 and carbon-ZnCl2-800 samples can reach up to 194.8–335.6 F g−1, respectively, at 2 A g−1. Next, a novel redox active electrolyte of p-phenylenediamine (PPD) with quick reversible Faradaic process at the electrode–electrolyte interface has been introduced into the 6 mol L−1 KOH electrolyte. Remarkably, the specific capacitance of the carbon-ZnCl2-800 sample have reached up to 934.6 F g−1 at 2 A g−1 when designating the PPD concentration as 14 mmol L−1 in 6 mol L−1 KOH electrolyte, which is nearly 2.8 fold higher than the pristine one, clearly indicating the pseudo-capacitive effect of the PPD additive.

The nanoporous graphitic carbon materials (NGCM) have been prepared by a synchronous carbonization and graphitization process, using waste polyvinylidene fluoride (PVDF) as carbon precursor and Ni(NO3)2·6H2O as the graphitic catalyst. It reveals that the carbonization temperature plays a crucial role in determining the pore structures as well as their electrochemical performances. Increasing the carbonization temperature from 800 to 1200 °C, the corresponding porosity has slightly decreased, accompanied by an increase of graphitization degree. Next, to further improve the electrochemical performance of the sample prepared at 800 °C, a novel redox additive of 4-(4-nitrophenylazo)-1-naphthol (NPN) with different amounts has been introduced in 2 mol L–1 KOH electrolyte. Therein, the specific capacitance by adding 4 mmol L–1 of NPN can reach 2.98 times higher than the pristine value. Apparently, the mixed electrolytes have largely enhanced the electrochemical performance, which is expected to be applied in the field of high performance supercapacitors.

How to effectively improve capacitances is still challenging in the field of supercapacitors. In this work, a simple but efficient redox additive of sodium p-aminobenzenesulfonate has been incorporated into KOH electrolyte, which can largely elevate the capacitances of carbon-based supercapacitors (produced by the template carbonization of polyacrylamide and zinc metal). It clearly indicates that the p-aminobenzenesulfonate concentration in KOH electrolyte has exerted a crucial role in the determination of final capacitances as well as energy efficiency. Owing to efficient electron/proton transfer of the p-aminobenzenesulfonate, the C-900-10 sample with the concentration of 10 mmol L−1 can deliver a much higher capacitance of 681.5 F g−1, when measured at 2 A g−1, which is ca. 2.86 times increase than that of pristine one (238.2 F g−1). The present p-aminobenzenesulfonate is anticipated to be employed in KOH electrolyte as efficient redox additive for simply producing high performance supercapacitors.

Nanoporous carbon materials with hierarchical porosities have been produced via a template carbonization method, in which potassium citrate (or gelatin) serves as the carbon precursor and Mg(OH)2 powder as the hard template. The P-3:1 sample derived from potassium citrate and Mg(OH)2 (with a mass ratio of 3:1 at 800 °C) possesses high BET surface area of 1894.7 m2 g−1 and large total pore volume of 2.27 cm3 g−1. To further improve the electrochemical performance, p-phenylenediamine (PPD, as redox-additive) of 5, 10, and 15 mmol L−1 is introduced into the 6 mol L−1 KOH as the mixed electrolyte, forming P-3:1-5/10/15 samples. Interestingly, the specific capacitances toward the P-3:1-5/10/15 samples have been greatly enhanced up to 579.2, 712.8 and 852.3 F g−1 at 2 A g−1, respectively, which are greatly higher than that of 325 F g−1 for the case of the pristine P-3:1 sample when measured at 6 mol L−1 KOH electrolyte. Furthermore, the P-3:1-15 sample delivers high capacitance retention of 70.5% even after 5000 charge–discharge cycles. What's more, the synthesis method has been readily extended for the case of gelatin and Mg(OH)2, and a similar electrochemical trend in the cases of the P-3:1-5/10/15 samples also occurs.

Producing nanoporous carbons that possess high porosity and superior electrochemical performance is a challenge for scientists. In this work, a simple and efficient template carbonization method, without any physical/chemical activation treatment, has been implemented to produce nanoporous carbons doped with nitrogen species. 4-(4-Nitrophenylazo)resorcinol serves as a carbon/nitrogen source and Mg(OH)2 as a hard template. All resultant carbon samples are amorphous, highly porous in nature and display sheet-like nanostructures. The carbon-1:3-L sample whose precursor is obtained by solution method exhibits better porosity, and larger nitrogen content, than those obtained by solid state method (the carbon-1:3-S sample). The carbon-1:3-L sample exhibits large specific surface area (SBET) of 1427.7 m2 g−1, and high total pore volume (VT) of 5.91 cm3 g−1, whereas those of the carbon-1:3-S sample are of 1036.6 m2 g−1 and 4.76 cm3 g−1, respectively. Notably, the present pore volumes are much higher than most of the previously reported for nanoporous carbons. As a consequence, the carbon-1:3-L sample delivers superior electrochemical behaviors, whose specific capacitance reaches up to 378.5 F g−1 when measured at 1 A g−1 in a three-electrode system, compared with that of 263.4 F g−1 of the carbon-1:3-S sample. The present Mg(OH)2-assisted template carbonization method is simple and easy to operate, indicating its potential application for producing nanoporous carbons.

•A direct carbonization method has been developed to prepare nanoporous carbon.•Potassium biphthalate, zinc, magnesium, and aluminum metals are commercial available.•The carbons have large BET surface areas and high total pore volumes.•Two-electrode system and three-electrode system were implemented.A simple but efficient template carbonization method has been implemented for the production of highly nanoporous carbon by the reaction of potassium biphthalate with zinc, magnesium, or aluminum metal. The mass ratio and carbonization temperature are pivotal factors influencing the structures of the carbons. It explicitly reveals that the carbons produced in the potassium biphthalate-zinc metal system exhibit superior porosities and better electrochemical performances. Taking the Zn-3:1-900 sample as an example, it has a large BET surface area of 1605.1 m2 g−1 and a high total pore volume of 1.18 cm3 g−1. A large specific capacitance of 338.2 F g−1 is achieved at a current density of 1 A g−1 and it can retain a 100.5 F g−1 even at a high current density up to 100 A g−1 when measured in a three-electrode system. It also exhibits a high energy density of 76.7 Wh kg−1 obtained in a two-electrode system. Furthermore, in the two kinds of electrode systems, all of the carbons display excellent cycling stabilities.

In this work, a simple template carbonization method has been developed to produce nitrogen-containing nanoporous carbon from diphenylcarbazide, using Mg(OH)2 as hard template. The carbonization temperature has a crucial role in determining the carbon structure. The carbon-3:1-800 sample obtained with the mass ratio of diphenylcarbazide and Mg(OH)2 as 3:1 at 800 °C exhibits the optimum pore structure as well as the resultant best electrochemical performance. It has a large BET surface area of 1538.0 m2 g−1, high pore volume of 3.48 cm3 g−1, and hierarchical pore size distribution. As a result, it delivers superior electrochemical behaviors in a three-electrode system using 6 mol L−1 KOH as electrolyte, whose specific capacitance calculated from galvanostatic charge-discharge curve can reach up to 517.4 F g−1 at a current density of 1 A g−1, which is much larger than most of the nanocarbons ever reported in the literature. The carbon-3:1-800 sample also exhibits good cycling stability within 10000 cycles. Comparatively, the electrochemical test has also been carried out in a two-electrode system using [EMIm]BF4/AN as electrolyte. More importantly, the operation temperatures of 25/50/80 °C can greatly broaden the application scope of nanoporous carbon in the supercapacitor.

A rational template carbonization method has been implemented to produce nanoporous carbon as high performance supercapacitor electrode material. Sodium carboxymethyl cellulose (NaCMC) acts as carbon source, and inexpensive Mg(OAc)2·4H2O and Zn(OAc)2·2H2O as templates. It reveals that the carbonization temperature and the mass ratio of NaCMC, Mg(OAc)2·4H2O and Zn(OAc)2·2H2O are crucial for determining the carbon structure. The NaCMC-Mg-Zn-1:5:0.5 sample displays highly porous structure with large surface area (1596 m2 g−1), pore volume (5.93 cm3 g−1) and hierarchical pore size distribution. The carbon sample is measured in a two/three-electrode system, respectively. It has a high specific capacitance of 428.4 F g−1 at 1 A g−1, together with nice cycling durability. A high energy density up to 68.6 Wh kg−1 can be achieved as the power density of 1.5 kW kg−1. The templates used in this work are cheap, commercially available and this template carbonization method, especially without any activation process, can be readily extended to prepare other kinds of nanoporous carbon.

•A direct carbonization method has been adopted to produce porous carbon.•Flake-like aluminium salicylate coordination polymer is used as precursor.•The surface areas and pore structures of porous carbon can be simply tuned.•The carbon sample exhibits excellent electrochemical performance.Flake-like nanoporous carbon has been synthesized by the direct carbonization of aluminium salicylate coordination polymer that serves as hard templates and carbon sources. It reveals that the carbonization temperature plays a crucial role in the formation of nanoporous carbon. The correlation between specific surface areas, pore structures, surface functionalities and capacitive performances are investigated. The nanoporous carbon synthesized at 900 °C, named as carbon-900, exhibits high surface area of 1162 m2 g−1 and large total pore volume of 0.80 cm3 g−1. The electrochemical behaviors are measured in a three-electrode system. In details, high specific capacitance of 220.0 F g−1 is achieved at a current density of 0.5 A g−1, whereas it also delivers high energy density of 30.5 W h kg−1, and long cycle stability (∼88.57% retention after 3000 cycles).

Nitrogen-doped nanoporous carbon has been produced by the carbonization of a 5-[(4-nitrophenyl)azo]salicylate–zinc complex, which is prepared by mixing Zn2+ ion and sodium 5-[(4-nitrophenyl)azo]salicylate in aqueous solution. Furthermore, zinc metal was added to the complex before carbonization to improve the electrochemical performance. It was found that the zinc metal-2:1-900 sample exhibits the best capacitive behavior. It has amorphous features and a porous structure, and displays a large Brunauer, Emmett and Teller surface area of 1177.2 m2 g−1, a total pore volume of 0.89 cm3 g−1, and a nitrogen content of 3.63%. Based on the three-electrode system measurement, the zinc metal-2:1-900 sample delivers a large specific capacitance 266.2 F g−1 at 1 A g−1 and has good cycling stability. It also has a large energy density of 33.4 W h kg−1 for a power density of 0.5 kW kg−1 when measured in a two-electrode system. More importantly, the superior electrochemical performances obtained at the operational temperatures of 25/50/80 °C in a two-electrode system can greatly increase their application as a practical supercapacitor. The present synthesis protocol can be extended to prepare other complexes such as 5-[(4-nitrophenyl)azo]salicylate–magnesium/calcium/aluminium, which also readily yield nanoporous carbons.

Halogen-containing plastic materials have been converted into nanoporous carbon by a template carbonization method, using zinc powder as an efficient hard template. The mass ratio between plastics and zinc powder as well as carbonization temperature plays a crucial role in determining the carbon structures and resultant electrochemical performances. The PTFE-1:3-700 sample that is obtained by carbonizing polytetrafluoroethene and zinc powder (the mass ratio of 1:3) at 700 °C has a large BET surface area of 800.5 m2 g–1 and a high total pore volume of 1.59 cm3 g–1, also delivering excellent specific capacitance of 313.7 F g–1 at 0.5 A g–1. Moreover, it exhibits a superior cycling stability with high capacitance retention of 93.10% after cycling for 5000 times. More importantly, it can be extended to produce nanoporous carbon derived from other halogen-containing plastic materials such as poly(vinylidene fluoride) and poly(vinyl chloride), revealing the generality of the synthesis method.

In this work, we demonstrate a novel and general synthetic approach for producing nanoporous carbon materials, using adipic acid and zinc powder as raw materials. The mass ratio and carbonization temperature have crucial effects on the structure and electrochemical behavior of the carbon samples. The optimum sample is carbon-1:2-700; it is amorphous in nature and has a high BET surface area of 1426 m2 g−1 and a very large pore volume of 5.92 cm3 g−1. What's more, the sample takes on sheet-like structures entirely composed of nanopores. The electrochemical performance is measured in a three-electrode system using 6 mol L−1 KOH as the electrolyte, and a two-electrode system using [EMIm]BF4/AN as the electrolyte, respectively. In the three-electrode system, it delivers a high specific capacitance of 373.3 F g−1 at a current density of 2 A g−1. Furthermore, it displays a good cycling durability of 93.9% after 10000 cycles. In the two-electrode system, the voltage window has been largely broadened and a series of temperature-dependent measurements are adopted. More importantly, the present synthetic method can be extended to other chemical substances as carbon precursors to produce porous carbon, which can greatly enrich the field of porous carbon synthesis as well as their application as supercapacitors.

•A direct carbonization method was developed for nitrogen-doped porous carbon.•Tartrazine can serve as not only carbon source but also as nitrogen source.•High surface areas and pore volumes are achieved with Ca(OAc)2·H2O.•The carbon samples exhibit excellent capacitive behaviors.Nitrogen-doped porous carbons possessing high surface areas and large pore volumes have been prepared by directly heating the mixture of tartrazine and Ca(OAc)2·H2O at 800 °C especially without further physical or chemical activation, where Ca(OAc)2·H2O serves as the hard template to regulate the surface area and pore structures. It reveals that the addition of Ca(OAc)2·H2O can remarkably improve the surface area and total pore volume. The T-Ca-800-3:1 sample displays the highest BET surface area as 1669 m2 g−1 and largest total pore volume 0.85 cm3 g−1, which is much larger than those without adding Ca(OAc)2·H2O. Furthermore, it exhibits excellent capacitive performances, including high specific capacitance (ca. 224.3 F g−1 at 0.5 A g−1), good rate capability (the retention of 42.6% at 60 A g−1) and good cycling stability (the retention of 92.3% within 5000 cycles).High-performance nitrogen-doped porous carbons have been prepared by directly heating the mixture of tartrazine and Ca(OAc)2·H2O at 800 °C, and the Ca(OAc)2·H2O serves as the hard template.

High-performance porous carbons have been prepared as supercapacitor electrode materials by co-doped with nitrogen and MnOx via a direct carbonization method, using sodium butyl naphthalene sulfonate (abbr. BNS–Na) as carbon source. It is believed that the in situ formed Na6(SO4)2(CO3) in the product would probably serve as temporary template for producing porous structures. The impacts of nitrogen/MnOx contents as well as the structures upon the capacitive performances were emphatically discussed. It indicates that introducing nitrogen and/or MnOx into the carbon matrix can remarkably improve their capacitive performances based on the cyclic voltammetry and galvanostatic charge–discharge measurements in 6 mol L−1 KOH aqueous solution. The specific capacitances of doped carbons can reach up to ca. 167.0–241.8 F g−1 compared with that of the undoped carbon of ca. 105.6 F g−1. Of these samples, the carbon–Mn-1:30-N-1:15 sample co-doped with nitrogen and MnOx exhibits the highest specific capacitance and energy density up to 241.8 F g−1 and 33.6 Wh kg−1, respectively. In particular, these carbons also exhibit high intrinsic capacitances (i.e., capacitance per surface area) up to ca. 0.66–1.92 F m−2.Graphical abstractHighlights► A simple carbonization method was developed to prepare porous carbon. ► The doping processes for nitrogen/MnOx are simple and reproducible. ► Doping nitrogen/MnOx into carbons can greatly improve capacitive performances.

•A simple carbonization method was developed to prepare porous carbon.•The precursor of CMS-Na is cheap and sustainable.•The doping process for nitrogen is simple and reproducible.•Doping nitrogen into carbons can greatly improve capacitive performances.Solid-state method as well as hydrothermal treatment and postannealing method were applied to prepare porous carbons, using sodium carboxymethyl starch (abbr. CMS-Na) as carbon source. The as-prepared carbons were further heated with urea to produce nitrogen-doped carbons. The nitrogen content within carbons has crucial impact on the surface areas, pore structures and capacitive properties. The experimental results reveal that the capacitive performances of nitrogen-doped carbons by solid-state method are much better than those by hydrothermal treatment and postannealing method. Specific capacitance at the current density of 1 A g− 1 of the carbon-S-N-1:20 sample has improved up to 176.0 F g− 1 from 147.2 F g− 1 (carbon-S-blank); the carbon-H-N-1:20 sample has dramatically improved up to 170.3 F g− 1 from 68.3 F g− 1 (carbon-H-blank). Furthermore, the carbon-S-N-1:20 sample exhibits better rate capability, cycling stability, and power density.Solid-state method as well as hydrothermal treatment and postannealing method have been applied to prepare a series of porous carbons doped with nitrogen in a comparative manner, using sodium carboxymethyl starch (abbr. CMS-Na) as carbon source.

In this work, a series of porous carbon materials with hierarchical porosities have been synthesized via a template carbonization method, in which cheap CaCO3 serves as a template and glucose as a carbon precursor. During the carbonization process, CO2 produced by the decomposition of the CaCO3 template can act as an internal activating agent, significantly improving microporosity and mesoporosity. All the carbon materials obtained by regulating the ratio of glucose to CaCO3 exhibit the amorphous features with a low graphitization degree. Among them, the carbon-1:2 sample shows a high BET surface area of up to 818.5 m2 g−1 and a large total pore volume of 1.78 cm3 g−1 as well as a specific capacitance of 107.0 F g−1 at 1 A g−1. In addition, a series of hydroquinone (HQ), p-aminophenol (PAP) and p-nitrophenol (PNP) as novel redox additives that can produce pseudo-capacitances have been added into the KOH electrolyte for promoting the total capacitive performances via redox reactions at the electrode–electrolyte interface. As expected, a 2.5-fold increase in the galvanostatic capacitance of 240.0 F g−1 in the HQ-0.5 electrolyte occurs, compared with the conventional KOH electrolyte. Similarly, the PAP-0.5 electrolyte and the PNP-0.5 electrolyte also show a high specific capacitance of 184.0 F g−1 at 2 A g−1 (156.6 F g−1 at 3 A g−1) and 153.0 F g−1 at 3 A g−1, respectively. Additionally, the three kinds of electrolytes exhibit excellent cyclic stability. The remarkable improvement of supercapacitors is attributed to the quick reversible Faradaic reactions of amine and hydroxyl groups adhering to the phenyl rings, which largely accelerates electron migration and brings additional pseudocapacitive contribution for carbon-based supercapacitors.

In present work, we demonstrate a simple but effective strategy for high-performance supercapacitors by adding the p-nitroaniline (PNA) into an alkaline electrolyte of KOH. PNA possesses a unique molecular structure with the functional groups of –NH2 and –NO2. Besides, both the product of nitro-reduction (–NH2) and intrinsic –NH2 on the benzene ring can lead to the occurrence of Faradaic redox reactions accompanied by the electron/proton transfer in the mixed electrolytes, whose pseudocapacitance can greatly enhance the total capacitance. Furthermore, another effective additive of the dimethylglyoxime (DMG) has been incorporated into carbon materials for further improving the performances of supercapacitors with a PNA + KOH electrolyte. As for the DMG + PNA + KOH system, a galvanostatic capacitance up to 386.1 F g−1 of the DMG-0.15–PNA-0.15 sample at 3 A g−1, which is nearly two times higher than that of the PNA-0.15 sample (183.6 F g−1) in the PNA + KOH system and nearly three-fold capacitance of the carbon-blank (132.3 F g−1) in the KOH system at the same current density. Furthermore, the specific capacitance still can reach up to 260.0 F g−1 even at 40 A g−1 with a 67.4% capacitance retention ratio. Besides, the DMG-0.15–PNA-0.15 sample exhibits an exceptional capacitance retention of 113% after 5000 charge/discharge cycles by virtue of the potential activated process, which clearly reveals the excellent cycling stability. These remarkable enhancements are ascribed to the synergistic effects of novel additives of PNA and DMG.

In this work, we demonstrate a novel and general synthetic approach for producing nanoporous carbon materials, using adipic acid and zinc powder as raw materials. The mass ratio and carbonization temperature have crucial effects on the structure and electrochemical behavior of the carbon samples. The optimum sample is carbon-1:2-700; it is amorphous in nature and has a high BET surface area of 1426 m2 g−1 and a very large pore volume of 5.92 cm3 g−1. What's more, the sample takes on sheet-like structures entirely composed of nanopores. The electrochemical performance is measured in a three-electrode system using 6 mol L−1 KOH as the electrolyte, and a two-electrode system using [EMIm]BF4/AN as the electrolyte, respectively. In the three-electrode system, it delivers a high specific capacitance of 373.3 F g−1 at a current density of 2 A g−1. Furthermore, it displays a good cycling durability of 93.9% after 10000 cycles. In the two-electrode system, the voltage window has been largely broadened and a series of temperature-dependent measurements are adopted. More importantly, the present synthetic method can be extended to other chemical substances as carbon precursors to produce porous carbon, which can greatly enrich the field of porous carbon synthesis as well as their application as supercapacitors.

Producing nanoporous carbons that possess high porosity and superior electrochemical performance is a challenge for scientists. In this work, a simple and efficient template carbonization method, without any physical/chemical activation treatment, has been implemented to produce nanoporous carbons doped with nitrogen species. 4-(4-Nitrophenylazo)resorcinol serves as a carbon/nitrogen source and Mg(OH)2 as a hard template. All resultant carbon samples are amorphous, highly porous in nature and display sheet-like nanostructures. The carbon-1:3-L sample whose precursor is obtained by solution method exhibits better porosity, and larger nitrogen content, than those obtained by solid state method (the carbon-1:3-S sample). The carbon-1:3-L sample exhibits large specific surface area (SBET) of 1427.7 m2 g−1, and high total pore volume (VT) of 5.91 cm3 g−1, whereas those of the carbon-1:3-S sample are of 1036.6 m2 g−1 and 4.76 cm3 g−1, respectively. Notably, the present pore volumes are much higher than most of the previously reported for nanoporous carbons. As a consequence, the carbon-1:3-L sample delivers superior electrochemical behaviors, whose specific capacitance reaches up to 378.5 F g−1 when measured at 1 A g−1 in a three-electrode system, compared with that of 263.4 F g−1 of the carbon-1:3-S sample. The present Mg(OH)2-assisted template carbonization method is simple and easy to operate, indicating its potential application for producing nanoporous carbons.