Hierarchically porous carbon by activation of shiitake mushroom for capacitive energy storage

We present a facile yet effective two-step activation method to prepare a hierarchically porous carbon with natural shiitake mushroom as the starting materials. The first step involves the activation of shiitake mushroom with H₃PO₄, while the second step is to further activate the product with KOH. The resulting carbon is comprised of abundant micro-, mesopores and interconnected macropores that has a specific surface area up to 2988 m² g⁻¹ and pore volume of 1.76 cm³ g⁻¹. With the unique porous nature, the carbon exhibited a specific capacitance of 306 and 149 F g⁻¹ in aqueous and organic electrolyte, respectively. Moreover, this carbon also shows a high capacitance retention of 77% at large current density of 30 A g⁻¹ and exhibited an outstanding cycling stability with 95.7% capacitance preservation after 15,000 cycles in 6 M KOH electrolyte. The far superior performance as compared with those of the commercially most used activated carbon RP20 in both aqueous and non-aqueous electrolyte demonstrates its great potential as high-performance supercapacitor electrode. The two-step method developed herein also represents a very attractive approach for scalable production of various functional carbon materials using diverse biomasses as starting materials.     

Nitrogen-doped porous carbons through KOH activation with superior performance in supercapacitors

A series of nitrogen-doped porous carbons are prepared through KOH activation of a nonporous nitrogen-enriched carbon which is synthesized by pyrolysis of the polymerized ethylenediamine and carbon tetrachloride. The porosity and nitrogen content of the nitrogen-doped porous carbons depend strongly on the weight ratio of KOH/carbon. As the weight ratio of KOH/carbon increases from 0.5 to 2, the specific surface area increases from 521 to 1913 m² g⁻¹, while the nitrogen content decreases from 10.8 to 1.1 wt.%. The nitrogen-doped porous carbon prepared with a moderate KOH/carbon weight ratio of 1, which possesses a balanced specific surface area (1463 m² g⁻¹) and nitrogen content (3.3 wt.%), exhibits the largest specific capacitance of 363 F g⁻¹ at a current density of 0.1 A g⁻¹ in 1 M H₂SO₄ aqueous electrolyte, attributed to the co-contribution of double-layer capacitance and pseudocapacitance. Moreover, it shows excellent rate capability (182 F g⁻¹ remained at 20 A g⁻¹) and good cycling stability (97% capacitance retention over 5000 cycles), making it a promising electrode material for supercapacitors.     

Synthesis of 2-D nanostructured BiVO4:Ag hybrid as an efficient electrode material for supercapacitors

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     

Polystyrene-based carbon spheres as electrode for electrochemical capacitors

A series of carbon spheres with various porous texture parameters were prepared from polystyrene-based macroreticular resin spheres by carbonization and activation. The as-prepared carbon spheres had a maximum specific surface area of 996 m² g^?1, total pore volume of 1.34 cm³ g^?1 and average pore size of 5.39 nm. Moreover, these carbon spheres showed a mesopore size distributed mainly in about 40 nm. A high specific capacitance of 153 F g^?1 for carbon sphere by carbonization, 164 F g^?1 for carbon sphere by activation for 1 h and 182 F g^?1 for carbon sphere by activation for 2 h can be obtained. Moreover, a specific energy between 2.3 and 5.1 Wh kg^?1 for these carbon spheres can be obtained in 6 mol L^?1 KOH electrolyte.     

The effect of reduction time on the surface functional groups and supercapacitive performance of graphene nanosheets

Graphene nanosheets were prepared by reducing graphite oxide with hydrazine hydrate. The effects of reduction time on the structure and morphology of graphene nanosheets have been investigated. Their electrochemical performance in aqueous and organic electrolytes was also analyzed. With an increase of reduction time, the C and N contents of graphene nanosheets increased, while the specific surface areas and the specific capacitances decreased. Changes in reduction time produced a significant effect on the numbers as well as the types of oxygen and nitrogen functionalities. The graphene nanosheets, prepared by using a reduction time of 30 min have the highest specific capacitance of 192 F g^?1 in a 6 mol L^?1 KOH electrolyte. All prepared graphene nanosheets have a good rate performance and cycle stability.     

Electrochemical performance of graphitized carbide-derived-carbon with hierarchical micro- and meso-pores in alkaline electrolyte

Graphitized carbide-derived-carbon (CDC) with hierarchical micro- and meso-pores is synthesized by chlorination of titanium carbide powder at 1000 °C. The produced CDC has many bilayer graphenes and some narrow graphite ribbons, which contributes a large amount of micropores (∼1.35 nm) and some mesopores. Although hierarchical pore is an attractive structure for supercapacitor, the low hydrophilicity of the graphitized CDC leads to poor electrochemical performance in alkaline electrolyte. The specific capacitance of the CDC in KOH aqueous electrolyte is only 5 F g⁻¹. A strategy that adding ethanol to alkaline electrolyte is presented to improve its surface wettability. The specific capacitance of the graphitized CDC in KOH aqueous electrolyte with addition of ethanol increases to 60 F g⁻¹ at a scan rate of 20 mV s⁻¹. The optimal content of ethanol in KOH electrolyte is 10 wt.%. In addition, cyclic voltammogram curve can maintain a quasi-rectangular shape well even at a scan rate of 500 mV s⁻¹ and the retention rate of the specific capacitance is about 70%. The specific capacitance is stable at high current density (e.g. 1 A g⁻¹), and almost no performance degradation is observed after 8000 consecutive cycles.     

Composites of MnO2 nanocrystals and partially graphitized hierarchically porous carbon spheres with improved rate capability for high-performance supercapacitors

Manganese dioxide/carbon nanocomposites with partially graphitized hierarchical porous structure have been designed and synthesized. A high specific capacitance of 412 F g⁻¹ and excellent rate capability of these composites can be achieved owing to the interconnected meso- and micro-porous structure and the graphitic pore walls facilitating the ion diffusion and electron transportation, respectively, which is highly demanded for high-performance supercapacitor electrodes materials. Even at a high scan rate of 100 mV s⁻¹, a specific capacitance of 251 F g⁻¹ can be obtained, corresponding to 61% capacitance retention. Moreover, a long cycling stability with initial capacitance retention of ∼88% is obtained after over 4000 cycles at a current density of 1.0 A g⁻¹. This work presents an efficient electrode materials design and a novel composite which holds great promise in high-performance supercapacitor applications.     

Electrodeposited hybrid films of polyaniline and manganese oxide in nanofibrous structures for electrochemical supercapacitor

Hybrid films of polyaniline (PANI) and manganese oxide (MnO_x) were obtained through potentiodynamic deposition from solutions of aniline and MnSO₄ at pH 5.6. The hybrid films demonstrated characteristic redox behaviors of PANI in acidic aqueous solution. Characterization of the hybrid films by XRD indicated the amorphous nature of MnO_x in the films in which manganese existed in oxidation states of +2, +3 and +4, based on XPS measurement. Hybrid film of PANI and MnO_x, PM120 obtained from the solution of 0.1 M aniline and 120 mM Mn²⁺ displayed a well opened nanofibrous structure which showed an 44% increase in specific capacitance from that of PANI (408 F g^?1) to 588 F g^?1, measured at 1.0 mA cm^?2 in 1 M NaNO₃ (pH 1). The hybrid film kept more than 90% of its capacitance after 1000 charging-discharging cycles, with a coulombic efficiency of 98%. The specific capacitance of a symmetric capacitor using PM120 as the electrodes is 112 F g^?1.     

Easy synthesis of porous graphene nanosheets and their use in supercapacitors

We report the easy synthesis of porous graphene nanosheets (PGNs) using the etching of graphene sheets by MnO₂. An electrode made from PGNs exhibits a specific capacitance of 154 F g^?1 at 500 mV s^?1 in 6 M KOH compared to a value of 67 F g^?1 for graphene nanosheets, and a low capacitance loss of 12% after 5000 cycles. Interestingly, PGN electrode material shows an excellent rate capability due to its open layered and mesopore structures that facilitate the efficient access of electrolytes to the electrode material and shorten the ion diffusion pathway through the porous sheets. This approach offers the potential for cost-effective, environmentally friendly and large-scale production of PGNs.     

High-performance supercapacitor electrodes based on highly corrugated graphene sheets

Highly corrugated graphene sheets (HCGS) have been prepared by a rapid, low cost and scalable approach through the thermal reduction of graphite oxide at 900 °C followed by rapid cooling using liquid nitrogen. The wrinkling of the graphene sheets can significantly prevent them from agglomerating and restacking with one another face to face and thus increase the electrolyte-accessible surface area. The maximum specific capacitance of 349 F g^?1 at 2 mV s^?1 is obtained for the HCGS electrode in 6 M KOH aqueous solution. Additionally, the electrode shows excellent electrochemical stability along with an approximately 8.0% increase of the initial specific capacitance after 5000 cycle tests. These features make the present HCGS material a quite promising alternative for next generation of high-performance supercapacitors.     

Mesoporous carbons synthesized by direct carbonization of citrate salts for use as high-performance capacitors

A simple one-step synthesis methodology for the fabrication of mesoporous carbons with an excellent performance as supercapacitor electrodes is presented. The procedure is based on the carbonization of non-alkali organic salts such as citrate salts of iron, zinc or calcium. The carbonized products contain numerous inorganic nanoparticles (i.e. Fe, ZnO or CaO) embedded within a carbonaceous matrix. These nanoparticles act as endotemplate, which when removed, leaves a mesoporous network. The resulting carbon samples have a large specific surface area up to ∼1600 m² g⁻¹ and a porosity made up almost exclusively of mesopores. An appropriate heat-treatment of these materials with melamine allows the synthesis of N-doped carbons which have a high nitrogen content (∼8–9 wt.%), a large specific surface area and retain the mesoporous structure. The mesoporous carbon samples were employed as electrode materials in supercapacitors. They exhibit specific capacitances of 200–240 F g⁻¹ in 1 M H₂SO₄ and 100–130 F g⁻¹ in EMImTFSI/AN. More importantly, the carbon samples possess a good capacitance retention in both electrolytes (>50% in H₂SO₄ and >80% in EMImTFSI/AN at 100 A g⁻¹) owing to their mesoporous structure which facilitates the penetration and transportation of ions.     

All solid supercapacitor based on activated carbon and poly [2,5-benzimidazole] for high temperature application

In the present work, we report high temperature performance of solid electrolyte supercapacitor based on activated carbon (AC) and phosphoric acid doped poly [2,5 benzimidazole] (ABPBI). Supercapacitors with varying concentrations of solid electrolyte in the electrode were fabricated and unit cells were analyzed over a wide temperature range of 27–120 °C. Supercapacitor with AC/ABPBI wt ratio 1.0:0.25 exhibited a specific capacitance of 197 F g^?1 at room temperature. To the best of our knowledge, the value reported here is one of the highest for electric double layer supercapacitor with a solid electrolyte. The specific capacitance of supercapacitors having various compositions increased with temperature. The specific capacitance for AC/ABPBI wt ratio 1.0:0.25, capacitance increased to 248 F g^?1 at 120 °C. The performance of supercapacitors was also analyzed by electrochemical impedance spectroscopy. The Nyquist plots at room temperature and 100 °C were studied by fitting them using Randles equivalent circuit. Supercapacitor with AC/ABPBI wt ratio 1.0:0.25 showed phase angle of 86.8° at low frequency which indicated excellent capacitive behavior at room temperature. The supercapacitor was found to have good stability during galvanostatic charge–discharge cycling even after repeated heating and cooling.     

Synthesis and supercapacitor performance studies of N-doped graphene materials using o-phenylenediamine as the double-N precursor

N-doped graphene (NG) materials have been prepared through a one-step solvothermal reaction by using o-phenylenediamine as a double-N precursor. N-doping and reduction of graphene oxide (GO) are both achieved simultaneously during the solvothermal reaction. The results of scanning electron microscopy and high resolution transmission electron microscopy measurements indicate that NG is highly crumpled. And the N-doping is confirmed by elemental analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transformed infrared spectroscopy and ultraviolet–visible spectroscopy. The doping level of nitrogen reaches up to 7.7 atom% and the types in NG are benzimidazole-N and phenazine-N. The NG materials exhibit excellent electrochemical performance for symmetric supercapacitors with a high specific capacitance of 301 F g⁻¹ at a current density of 0.1 A g⁻¹ in 6 M KOH electrolyte, which is remarkably higher than the solvothermal products of pristine GO (210 F g⁻¹ at 0.1 A g⁻¹). The NG materials also exhibit superior cycling stability (97.1% retention) and coulombic efficiency (99.2%) after 4000 cycles, due to the high content of nitrogen atoms, unique types of nitrogen and improved electronic conductivity.     

Carbonization temperature dependence of pore structure of silicon carbide spheres and their electrochemical capacitive properties as supercapacitors

Three-dimensional silicon carbide-based frameworks with hierarchical micro- and mesoporous structures (2MSiC) are prepared by employing the template method and carbonization reaction using aerosol-spray drying. The mesopores are generated by the self-assembly of a structure-directing agent, whereas the micropores originate from the partial evaporation of Si atoms during the carbonization process. During the carbonization process, the proportion of micro- and mesopores in 2MSiC can be controlled by the carbonization temperature by controlling the amount of partial evaporation of Si atoms. The 2MSiC electrode prepared using a Brij56 structure-directing agent as the mesopore template and carbonized at 1250 °C exhibits a high charge storage capacity with a specific capacitance of 259.9 F g⁻¹ at a scan rate of 5 mV s⁻¹ with 88.1% rate performance from 5 to 500 mV s⁻¹ in 1 M KCl aqueous electrolyte. This outstanding electrochemical performance can be attributed to the synergistic effect of both the enhanced electric double layer properties caused by micropores and reduced resistant pathways for ion diffusion in the pores as well as a large accessible surface area for ion transport/charge storage caused by mesopores. These encouraging results demonstrate that the 2MSiC electrode prepared with Brij56 and carbonized at 1250 °C is a promising candidate for high-performance electrode materials for supercapacitors.     

High-surface area carbons from renewable sources with a bimodal micro-mesoporosity for high-performance ionic liquid-based supercapacitors

Highly porous materials with a bimodal pore size distribution in the micro-mesopore range have been produced from biomass by adding melamine to the hydrochar/KOH mixture used in the activation process. These carbons are characterized by BET surface areas in excess of ∼3300 m² g⁻¹ and a porosity equally distributed between micropores and mesopores. The use of melamine in the synthesis process not only extends the pore size distribution into the mesopore region, but leads to the incorporation of a certain amount of nitrogen atoms into the carbon framework. These materials combine high ion adsorption capacities (micropores) and enhanced ion-transport kinetics (mesopores) leading to an outstanding capacitive performance in ionic liquid-based supercapacitors. Thus, they have specific capacitances >160 F g⁻¹ at 1 A g⁻¹ and >140 F g⁻¹ at 60 A g⁻¹ in both pure ionic liquid and in acetonitrile-diluted ionic liquid, enabling these materials to store up to a maximum of ca. 60 W h kg⁻¹ in both kinds of electrolytes and deliver ca. 20 W h kg⁻¹ at ∼42 kW kg⁻¹ (discharge time ca. 2 s) in pure ionic liquid and ∼25–30 W h kg⁻¹ at ∼97–100 kW kg⁻¹ (discharge time ∼1 s) in acetonitrile-diluted ionic liquid.     

Facile synthesis of sandwich-like polyaniline/boron-doped graphene nano hybrid for supercapacitors

The physicochemical property of chemically prepared graphene can be significantly changed due to the incorporating of heteroatoms into graphene. In this article, boron-doped graphene sheets are used as carbon substrates instead of graphene for loading polyaniline by in situ polymerization. Compared with the individual component and polyaniline/non-doped graphene, the sandwich-like polyaniline/boron-doped graphene exhibits remarkably enhanced electrochemical specific capacitance in both acid and alkaline electrolytes. In a three-electrode configuration, the hybrid has a specific capacitance about 406 F g⁻¹ in 1 M H₂SO₄ and 318 F g⁻¹ in 6 M KOH at 1 mV s⁻¹. In the two-electrode system of a symmetric supercapacitor, this hybrid achieves a specific capacitance about 241 and 189 F g⁻¹ at 0.5 A g⁻¹ with a specific energy density around 19.9 and 30.1 Wh kg⁻¹, in the acid and alkaline electrolytes, respectively. The as-obtained polyaniline/boron-doped graphene hybrid shows good rate performance. Notably, the obtained electrode materials exhibit long cycle stability in both acid and alkaline electrolytes (∼100% and 83% after 5000 cycles, respectively). The improved electrochemical performance of the hybrid is mainly attributed to the introduction of additional p-type carriers in carbon systems by boron-doping and the well combination of pseudocapacitive conducting polyaniline.     

Dense carbon monoliths for supercapacitors with outstanding volumetric capacitances

A commercially available dense carbon monolith (CM) and four carbon monoliths obtained from it have been studied as electrochemical capacitor electrodes in a two-electrode cell. CM has: (i) very high density (1.17 g cm⁻³), (ii) high electrical conductivity (9.3 S cm⁻¹), (iii) well-compacted and interconnected carbon spheres, (iv) homogeneous microporous structure and (v) apparent BET surface area of 957 m²g⁻¹. It presents interesting electrochemical behaviors (e.g., excellent gravimetric capacitance and outstanding volumetric capacitance). The textural characteristics of CM (porosity and surface chemistry) have been modified by means of different treatments. The electrochemical performances of the starting and treated monoliths have been analyzed as a function of their porous textures and surface chemistry, both on gravimetric and volumetric basis. The monoliths present high specific and volumetric capacitances (292 F g⁻¹ and 342 F cm⁻³), high energy densities (38 Wh kg⁻¹ and 44 Wh L⁻¹), and high power densities (176 W kg⁻¹ and 183 W L⁻¹). The specific and volumetric capacitances, especially the volumetric capacitance, are the highest ever reported for carbon monoliths. The high values are achieved due to a suitable combination of density, electrical conductivity, porosity and oxygen surface content.     

Simultaneous reduction,exfoliation, and nitrogen doping of graphene oxide via a hydrothermal reaction for energy storage electrode materials

Nitrogen (N)-doped graphene (NG) sheets were prepared using (NH₄)₂CO₃ and an aqueous dispersion of graphene oxide (GO) by an eco-friendly hydrothermal reaction. The in situ produced ammonia played an important role in the simultaneous nitrogen doping, the reduction and exfoliation of GO. The (NH₄)₂CO₃/GO mass ratio and reaction temperature were varied to investigate the effects on the N doping level. The elemental analysis determined from the X-ray photoelectron spectroscopy showed that the nitrogen content of the NG was about 10.1 at.% and the oxygen content decreased significantly due to the hydrothermal reduction of GO. The electrochemical performances of the NG sheets increased with increasing doped N content. The highest specific capacitance of 295 F g⁻¹ at a current density of 5 A g⁻¹ and the highest specific surface area of 412 m² g⁻¹ were observed with the sample processed at 130 °C. The retention of the specific capacitance was maintained at ∼89.8% after 5000 charge–discharge cycles. These results imply that NG sheets obtained by this simple eco-friendly approach are suitable for use in high performance energy storage electrode materials.     

Electrodeposition and characterization of ordered mesoporous cobalt hydroxide films on different substrates for supercapacitors

Novel ordered mesoporous cobalt hydroxide films have been successfully electrodeposited on different substrates, i.e., foamed nickel mesh and titanium plate, from cobalt nitrate dissolved in the aqueous domains of the hexagonal lyotropic liquid crystalline phase of Brij 56. Field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM) studies present that the as-deposited films have an interlaced nanosheet-like surface morphology and possess a regular nanostructure with hexagonal arrays of pores of nanometer dimension and extended periodicity. Various electrochemical test results show that the films on both substrates exhibit excellent electrochemical capacitive behavior due to the special ordered mesoporous nanostructure. Furthermore, it is obvious that the ordered mesoporous cobalt hydroxide film on foamed Ni mesh has much higher specific capacitance (maximum: 2646 F g^?1) than that of the film on Ti plate (maximum: 1018 F g^?1), which is mainly because that the foamed Ni mesh substrate with much larger surface area than Ti plate could enhance the utilization and the capacitance of Co(OH)₂ film greatly.     

Impregnation assisted synthesis of 3D nitrogen-doped porous carbon with high capacitance

Three-dimensional (3D) porous carbons with controlled mesopore and micropore structures were prepared through a simple and low-cost ultrasonic and impregnation assisted method from waste air-laid paper. The ammonia management was used to dope the 3D porous carbons with different types of nitrogen heteroatoms in a way that replaced carbon atoms. The N₂ adsorption–desorption characterization suggested that the nitrogen-doped carbons have a high surface area of 1470 m² g⁻¹ with the average pore diameter of 4.2 nm, which are conducive to form electric double layer under high current density. The resulting 3D carbon exhibited a higher capacitance at 296 F g⁻¹ in comparison with the nitrogen-free one at 252 F g⁻¹ in 6 M KOH electrolyte. Moreover, a high power density ca. 0.313 kW kg⁻¹ and energy density ca. 34.3 Wh kg⁻¹ were achieved in the ionic liquid ([EMIm]BF₄). The findings will open a new avenue to use waste materials for high-performance energy-storage devices.