The supercapacitor performance of hierarchical porous activated carbon electrodes synthesised from demineralised (waste) cumin plant by microwave pretreatment

Waste paper-derived porous carbon via microwave-assisted activation for energy storage and water purification

The reuse of waste papers by conversion into valuable carbon materials has received considerable attention for diverse applications such as energy storage and water purification. However, traditional methods for converting waste papers into materials with suitable properties for specific applications are often complex and ineffective, involving consecutive carbonization and activation steps. Herein, we propose a simple one-step microwave (MW)-assisted synthesis for preparing waste paper-derived porous carbons (WPCs) for energy storage and water purification. Through a 30-min synthesis, WPCs with graphitic structure and high specific surface area were successfully produced. The fabricated WPCs exhibited outstanding charge storage capability with a maximum specific capacitance of 237.7 F g-1. Additionally, the WPC demonstrates a high removal efficiency for various dyes, achieving a maximum removal efficiency of 95.0% for methylene blue. The developed one-step MW synthesis not only enables the production of porous carbon from waste paper, but also offers a viable approach to address solid waste management challenges while simultaneously yielding valuable materials.

Fomes fomentarius as a Bio-Template for Heteroatom-Doped Carbon Fibers for Symmetrical Supercapacitors

Nowadays, commercial electric double-layer supercapacitors mainly use porous activated carbons due to their high specific surface area, electrical conductivity, and chemical stability. A feature of carbon materials is the possibility of obtaining them from renewable plant biomass. In this study, fungi (Fomes fomentarius) were used as a bio-template for the preparation of carbon fibers via a combination of thermochemical conversion approaches, including a general hydrothermal pre-carbonization step, as well as subsequent carbonization, physical, or chemical activation. The relationships between the preparation conditions and the structural and electrochemical properties of the obtained carbon materials were determined using SEM, TEM, EDAX, XPS, cyclic voltammetry, galvanostatic measurements, and EIS. It was shown that hydrothermal pretreatment in the presence of phosphoric acid ensured the complete removal of inorganic impurities of raw fungus hyphae, but at the same time, saved some heteroatoms, such as O, N, and P. Chemical activation using H3PO4 increased the amount of phosphorus in the carbon material and saved the natural fungus’s structure. The combination of a hierarchical pore structure with O, N, and P heteroatom doping made it possible to achieve good electrochemical properties (specific capacitance values of 220 F/g) and excellent stability after 25,000 charge/discharge cycles in a three-electrode cell. The electrochemical performance in both three- and two-electrode cells exceeded or was comparable to other biomass-derived porous carbons, making it a prospective candidate as an electrode material in symmetrical supercapacitors.

Regeneration and usage of commercial activated carbon from the waste electrodes for the application of supercapacitors

Currently, efficient and cost-effective recycling of waste capacitors is a pressing issue. In this study, the recovery of electrode powder from waste supercapacitors and enabling the reuse of the prepared samples are reported. The recovered powder is directly activated by mixing it with KOH using chemical activation to regenerate the waste-activated carbon. The regenerated activated carbon's specific surface area could be restored to a level similar to that of the original commercial powder, reaching 1803.15 m²/g. The regenerated activated carbon has a high proportion of microporous, which played a crucial role in its electrochemical performance. The samples' capacity in the organic system reached 125.96 F/g at 0.2 A/g and 111.77 F/g at 20 A/g, with a retention rate of 88.74%. Furthermore, the capacitance was maintained at 91.18% after 10,000 cycles, showing good cycling performance. Additionally, the supercapacitor assembled from the regenerated activated carbon delivered a high energy density of 31.83 Wh/kg and a power density of 269.76 W/kg, indicating great application potential. Overall, this study offers a useful and low-cost approach for recycling activated carbon from waste electrodes, which would be possible for supercapacitors recycling.

Metal-Free Carbon-Based Supercapacitors—A Comprehensive Review

Herein, metal-free heteroatom doped carbon-based materials are being reviewed for supercapacitor and energy applications. Most of these low-cost materials considered are also derived from renewable resources. Various forms of carbon that have been employed for supercapacitor applications are described in detail, and advantages as well as disadvantages of each form are presented. Different methodologies that are being used to develop these materials are also discussed. To increase the specific capacitance, carbon-based materials are often doped with different elements. The role of doping elements on the performance of supercapacitors has been critically reviewed. It has been demonstrated that a higher content of doping elements significantly improves the supercapacitor behavior of carbon compounds. In order to attain a high percentage of elemental doping, precursors with variable ratios as well as simple modifications in the syntheses scheme have been employed. Significance of carbon-based materials doped with one and more than one heteroatom have also been presented. In addition to doping elements, other factors which play a key role in enhancing the specific capacitance values such as surface area, morphology, pore size electrolyte, and presence of functional groups on the surface of carbon-based supercapacitor materials have also been summarized.

Comparison of porous carbons derived from sodium alginate and calcium alginate and their electrochemical properties

Here, sodium alginate and calcium alginate which have the same carbon-forming component (alginic acid) and different regulation component (sodium/calcium) were used to prepare porous carbons, and comparisons were made of the microstructures and electrochemical properties of the obtained charcoals. The morphology was characterized by Scanning electron microscopy (SEM), and the results show that porous carbons can inherit plane or concave structures from their corresponding carbonized samples. The Horvath–Kawazoe (HK) method was used to analyze micropore size distributions, and the results show that, under the same mass ratio of potassium hydroxide to carbonized sample (KOH/C), the positions of extreme points on the two curves are similar, but the extreme values are different, and new extreme points appear at larger pore sizes with increases in the KOH/C ratio. The results of cyclic voltammetry (CV) and galvanostatic charge and discharge (GCD) tests show that the capacitance of sodium alginate-derived porous carbon is greater than that of porous carbon derived from calcium alginate when the KOH/C ratios are 2 and 4, and the size relationship is reversed when the KOH/C ratio is 3. The results of cycling performance tests show that the cycle numbers corresponding to the three stages on the two curves are similar under the same KOH/C ratio, but the cycle numbers at the same stage are significantly different under different KOH/C ratios.

Comparisons of the microstructures and electrochemical properties of porous carbons derived from the carbon-forming component alginic acid under the action of Na/Ca.

Fabrication of resorcinol-based porous resin carbon material and its application in aqueous symmetric supercapacitors

Carbon materials with porous structures with their unique surface area and charge transport properties have been attracting significant attention as electrode materials in renewable energy storage devices. The rapid agglomeration of layered materials during electrochemical processes reduces their shelf life and specific capacitance, which can be prevented by the introduction of suitable pores between the layers. In this study, resorcinol-based porous resin carbon was facilely prepared via a simple carbonization of the potassium salts of resorcinol-potassium resin. The morphology, structure and surface properties of the carbon materials were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption and energy dispersive spectroscopy (EDS). It is proposed that the fast nucleophilic addition between the phenols and formaldehyde produces nano-sized gel particles, followed by carbonization into carbon particles, finally packing to the mesopores. Due to the synergistic effects of the tailored porosity and O-doping, the prepared carbon materials show a high specific capacitance (198 F g⁻¹ for RC700), good capacitance retention (96.5% for RC700) at 2 A g⁻¹ in 6 M KOH and the specific area of RC700 is 540 m² g⁻¹.

Activated preparation of environmentally friendly and sustainable carbon materials and their successful application in supercapacitor devices.

The performance of sulphur doped activated carbon supercapacitors prepared from waste tea

The pore structure, high surface area and good conductivity are the key properties for the electrochemical double layer based supercapacitors. The activated carbons were produced from the waste tea, utilising microwave pretreatment with H₃PO₄ and activation at 450°C. Sodium thiosulfate pentahydrate (Na₂S₂O₃·5H₂O) was used as sulphur doping agent at 800°C to enhance conductivity of the activated carbons. Supercapacitor electrodes were prepared from both the activated carbon (WTAC) and sulphur doped activated carbon (WTAC-S) samples and the electrochemical performances were tested in the presence of 6 M KOH and 1 M H₂SO₄ as electrolytes. The activated carbon samples were characterised by Brunauer-Emmett-Teller (BET) surface area, Scanning Electron Microscopy/Energy Dispersive X-Ray Spectroscopy (SEM/EDS) and Fourier Transform Infrared Spectroscopy (FTIR) analysis techniques. The electrochemical performance analyses were performed by galvanostatic charge-discharge (GCD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods. The specific capacitance values of the WTAC and WTAC-S samples under the 1 A g^-1 current density were found to be 89.3, 144.7 F g^-1 for KOH electrolyte and 73.8 and 101.9 F g^-1 for H₂SO₄ electrolyte, respectively. The results show that the sulphur doping process enhances the electrochemical performance of activated carbon samples.

Effect of porosity enhancing agents on the electrochemical performance of high-energy ultracapacitor electrodes derived from peanut shell waste

In this study, the synthesis of porous activated carbon nanostructures from peanut (Arachis hypogea) shell waste (PSW) was described using different porosity enhancing agents (PEA) at various mass concentrations via a two-step process. The textural properties obtained were depicted with relatively high specific surface area values of 1457 m² g⁻¹, 1625 m² g⁻¹ and 2547 m² g⁻¹ for KHCO_3, K₂CO₃ and KOH respectively at a mass concentration of 1 to 4 which were complemented by the presence of a blend of micropores, mesopores and macropores. The structural analyses confirmed the successful transformation of the carbon-containing waste into an amorphous and disordered carbonaceous material. The electrochemical performance of the material electrodes was tested in a 2.5 M KNO₃ aqueous electrolyte depicted its ability to operate reversibly in both negative and positive potential ranges of 0.90 V. The activated carbon obtained from the carbonized CPSW:PEA with a mass ratio of 1:4 yielded the best electrode performance for all featured PEAs. The porous carbons obtained using KOH activation displayed a higher specific capacitance and the lower equivalent series resistance as compared to others. The remarkable performance further corroborated the findings linked to the textural and structural properties of the material. The assembled device operated in a neutral electrolyte (2.5 M KNO₃) at a cell potential of 1.80 V, yielded a ca. 224.3 F g⁻¹ specific capacitance at a specific current of 1 A g⁻¹ with a corresponding specific energy of 25.2 Wh kg⁻¹ and 0.9 kW kg⁻¹ of specific power. This device energy was retained at 17.7 Wh kg⁻¹ when the specific current was quadrupled signifying an excellent supercapacitive retention with a corresponding specific power of 3.6 kW kg⁻¹. These results suggested that peanut shell waste derived activated carbons are promising candidates for high-performance supercapacitors.

Modifying the microstructure of algae-based active carbon and modelling supercapacitors using artificial neural networks

An improved activated carbon material is synthesized from nostoc flagelliforme algae (NF) using an acid immersing method. The material has more pores and lower internal resistance compared with raw NF. Hydrofluoric acid can effectively decompose cellulose fibers and remove inorganic impurities, giving the carbon materials high mesopore volumes, which makes electrolyte ions rapidly transfer to the active site on the electrode surface. The specific capacitance of the sample was increased from 200 to 283 F g-1 after immersing in hydrofluoric acid. In addition, the symmetric supercapacitor shows an excellent energy density of 22 W h kg-1 at a power density of 80 W kg-1. The capacitance remains at 101.7% after 10 000 cycles. Furthermore, in order to find the relationship between the biochar structure and electrochemical performance in supercapacitors, an artificial neural network (ANN) method is used for studying the complex synergy mechanism. The specific capacitance is modelled using various input factors like aspect ratio (r L/D), cellulose ratio (CL(%)), specific surface area (S BET), pore volume (V tot), internal resistance (R s) and so on. The Levenberg-Marquart back propagation algorithm with sigmoid and ReLu active function is adopted to train the model. Random forest is used to analyse the relative importance of every input factor on specific capacitance. Results show that the model can estimate the energy storage with a mean squared error of 4.39 for materials with specific structure. Importance analyses indicate the first three significant variables are S BET, R s and V por. The ANN model can accurately predict the electrical properties of biomass-based carbon materials, and also provide guidance for the selection of energy storage materials in the future.