Electrical double-layer capacitance of micro- and mesoporous activated carbon prepared from rice husk and beet sugar

Progress on Separation and Hydrothermal Carbonization of Rice Husk Toward Environmental Applications

Owing to the increasing global demand for carbon resources, pressure on finite materials, including petroleum and inorganic resources, is expected to increase in the future. Efficient utilization of waste resources has become crucial for sustainable resource acquisition for creating the next generation of industries. Rice husks, which are abundant worldwide as agricultural waste, are a rich carbon source with a high silica content and have the potential to be an effective raw material for energy-related and environmental purification materials such as battery, catalyst, and adsorbent. Converting these into valuable resources often requires separation and carbonization; however, these processes incur significant energy losses, which may offset the benefits of using biomass resources in the process steps. This review summarizes and discusses the high value of RHs, which are abundant as agricultural waste. Technologies for separating and converting RHs into valuable resources by hydrothermal carbonization are summarized based on the energy efficiency of the process.

Recent advancements in lignocellulose biomass-based carbon fiber: Synthesis, properties, and applications

A growing need to reduce the global carbon footprint has prompted all sectors to make significant efforts in this direction. For example, there has been much focus on green carbon fiber sustainability. For example, it was found that the polyaromatic heteropolymer lignin might act as an intermediary in synthesising carbon fiber. Biomass is seen as a potential carbon accommodated solid natural sources that protects the nature and has a big overall supply and widespread distribution. With growing environmental concern in recent years, biomass has gained appeal as a raw material for production of carbon fibers. Especially, the positives of lignin material include its reasonable budget, sustainability, and higher carbon content, which makes it a dominating precursor. This review has examined a variety of bio precursors that help produce lignin and have higher lignin concentrations. In addition, there has been much research on plant sources, lignin types, factors affecting carbon fiber synthesis, spinning methods, stabilization, carbonization, and activation the characterisation techniques used for the lignin carbon fiber to comprehend the structure and features. In addition, an overview of the applications that use lignin carbon fiber has been provided.

Ultrahigh Content Boron and Nitrogen Codoped Hierarchically Porous Carbon Obtained from Biomass Byproduct Okara for Capacitive Deionization

Capacitive deionization (CDI) is an environmentally friendly, energy efficient, and low cost water purification technique in comparison with other conventional techniques, and it has attracted considerable attention in recent years. Here, we use biomass byproduct okara as the starting material to fabricate a boron and nitrogen codoped hierarchically porous carbon (BNC) with ultrahigh heteroatom contents and abundant in-plane nanoholes for CDI application. With the interconnected hierarchical porous structure, the BNC not only exhibits a large surface area (647.0 m³ g^-1) for the adsorption of ions but also offers abundant ion transport channels to access the entire internal surface. Meanwhile, the ultrahigh dopants' content of B (11.9 at%) and N (14.8 at%) further gives rise to the increased surface polarity and enhanced capacitance for BNC. Owing to these favorable properties, BNC exhibits top-level salt adsorption capacity (21.5 mg g^-1) and charge efficiency (59.5%) at the initial NaCl concentration of ∼500 mg L^-1. Moreover, we performed first-principle simulations to explore the different effects between N-doping and N,B-codoping on the capacitive property, which indicate that the boron and nitrogen codoping of carbon can largely increase the quantum capacitance over the double layer capacitance. The results of this work suggest a promising prospect for the BNC material in practical CDI application.

Influence of sequential HTC pre-treatment and pyrolysis on wet food-industry wastes: Optimisation toward nitrogen-rich hierarchical carbonaceous materials intended for use in energy storage solutions

Due to elevated protein content, the food-industry bio-wastes are promising feedstock to produce hierarchical (micro-mesoporous) carbonaceous materials with the intended use as electrodes in the energy storage solutions. However, the high initial water content, makes their direct activation through high-temperature processes costineffective due to significant heat requirements. In this study, the influence of pretreatment with hydrothermal carbonization (HTC) on wet food-industry bio-wastes, further pyrolysed, was investigated. Selected wastes (brewer's spent grains, spent coffee grains and spent sugar beets) were pre-treated by HTC at 180 °C or 240 °C, and then pyrolysed at 500 °C or 700 °C. Obtained materials were examined using elemental analysis, gas adsorption (N₂ and CO₂) and FT-IR. Besides minor differences caused by the bio-composition of wastes, the general trends were similar for feedstock. The pre-treatment had a beneficial influence on the properties of all wastes. The HTC at 180 °C and pyrolysis at 700 °C for all wastes show the most promising total specific surface area 560 ± 10 m²/g and accessible specific surface area 96 m²/g. Those conditions simultaneously did not reduce the total solid yield in comparison to the one-step process. The pre-treatment at 240 °C led to elevated nitrogen incorporation in the carbonaceous structure compared to HTC at 180 °C. However, it formed a hierarchical structure that was not stable for the thermal treatment. Study proves the HTC pre-treatment at 180 °C is beneficial for the conversion of food-industry bio-wastes into hierarchical carbonaceous material for their use in the energy storage systems application.

Role of SiOx in rice-husk-derived anodes for Li-ion batteries

The present study investigated the role of SiO_x in a rice-husk-derived C/SiO_x anode on the rate and cycling performance of a Li-ion battery. C/SiO_x active materials with different SiO_x contents (45, 24, and 5 mass%) were prepared from rice husk by heat treatment and immersion in NaOH solution. The C and SiO_x specific capacities were 375 and 475 mAh g⁻¹, respectively. A stable anodic operation was achieved by pre-lithiating the C/SiO_x anode. Full-cells consisting of this anode and a Li(Ni_0.5Co_0.2Mn_0.3)O₂ cathode displayed high initial Coulombic efficiency (~ 85%) and high discharge specific capacity, indicating the maximum performance of the cathode (~ 150 mAh g⁻¹). At increased current density, the higher the SiO_x content, the higher the specific capacity retention, suggesting that the time response of the reversible reaction of SiO_x with Li ions is faster than that of the C component. The full-cell with the highest SiO_x content exhibited the largest decrease in cell specific capacity during the cycle test. The structural decay caused by the volume expansion of SiO_x during Li-ion uptake and release degraded the cycling performance. Based on its high production yield and electrochemical benefits, degree of cycling performance degradation, and disadvantages of its removal, SiO_x is preferably retained for Li-ion battery anode applications.

Electrochemical Capacitance of Activated Carbons Regenerated using Thermal and Chemical Activation

Spent activated carbons (SACs) collected from a water treatment plant were regenerated and then adopted as electrochemical material in capacitors. The SACs used in this study were regenerated via two steps, namely thermal and chemical activation. However, during the activation process, the adsorbates were converted into ashes, which caused pore blockage and decreased specific surface area. The regenerated SACs were washed with acid solutions with different levels of acidity (strong: HCl, mild: H3PO4, and weak: H2O2) to remove the ashes. The regenerated SACs washed with HCl exhibited the highest specific surface area, although their capacitance was not the highest. Conversely, the specific surface area of regenerated SACs washed using H3PO4 was slightly lower than that of HCl, but exhibited higher capacitance and electrochemical stability. Although the strong acid removed the generated ashes in the pores efficiently, it could adversely affect their structural stability, which would lead to lower capacitance.

Discarded antibiotic mycelial residues derived nitrogen-doped porous carbon for electrochemical energy storage and simultaneous reduction of antibiotic resistance genes(ARGs)

The question of how to reasonably dispose and recycle antibiotic mycelial residues (AMRs), a hazardous waste, is a critical issue. The AMRs containing nitrogen-rich organic matters shows a promising alternative feedstock of nitrogen-doped porous carbons (NPCs). Here, the NPCs with the ultrahigh surface area (2574.9 m² g^-1) were prepared by using the discarded oxytetracycline mycelial residues (OMRs) and further used as an electrode for supercapacitor. A series of experiments including scanning/transmission electron microscope, Brunauer-Emmett-Teller measurement, and electrochemical impedance spectrum revealed that the NPC-2-900 exhibited a high N content, large surface area, and high electrical conductivity. The electrochemical performance of the NPC was tested by cyclic voltammetry, galvanostatic charge/discharge cycling, and rate capability test. The optimized NPC-2-900 displayed distinguish specific capacitance (307 F g^-1), cycling stability (over 95% capacitance retention after 2000 cycles even at a high current density of 20 A g^-1) and superior rate performance. Of particular interest, the qPCR test indicates the ARGs were reduced in the conversion process from OMRs to NPCs.

Modified Activation Process for Supercapacitor Electrode Materials from African Maize Cob

In this work, African maize cobs (AMC) were used as a rich biomass precursor to synthesize carbon material through a chemical activation process for application in electrochemical energy storage devices. The carbonization and activation were carried out with concentrated Sulphuric acid at three different temperatures of 600, 700 and 800 °C, respectively. The activated carbon exhibited excellent microporous and mesoporous structure with a specific surface area that ranges between 30 and 254 m²·g^-1 as measured by BET analysis. The morphology and structure of the produced materials are analyzed through Field Emission Scanning Electron Microscopy (FESEM), Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD), Boehm titration, X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy. X-ray photoelectron spectroscopy indicates that a considerable amount of oxygen is present in the materials. The functional groups in the activated carbon enhanced the electrochemical performance and improved the material's double-layer capacitance. The carbonized composite activated at 700 °C exhibited excellent capacitance of 456 F g^-1 at a specific current of 0.25 A g^-1 in 6 M KOH electrolyte and showed excellent stability after 10,000 cycles. Besides being a low cost, the produced materials offer good stability and electrochemical properties, making them suitable for supercapacitor applications.

Significant enhancement of electron transfer from Shewanella oneidensis using a porous N-doped carbon cloth in a bioelectrochemical system

Modifying the surface of an anode can improve electron transfer, thus enhancing the performance of the associated bioelectrochemical system. In this study, a porous N-doped carbon cloth electrode was obtained via a simple thermal reduction and etching treatment, and then used as the anode in a bioelectrochemical system. The electrode has a high nitrogen-to‑carbon (N/C) ratio (~3.9%) and a large electrochemically active surface area (145.4 cm², about 4.4 times higher than that of the original carbon cloth), which increases the bacterial attachment and provides more active sites for extracellular electron transfer. Electrochemical characterization reveals that the peak anodic current (0.71 mA) of the porous N-doped carbon cloth electrode in riboflavin is 18 times higher than that of the original carbon cloth electrode (0.04 mA), confirming the presence of more electroactive sites for the redox reaction. We also obtained a maximum current density of 0.29 mA/cm² during operation of a bioelectrochemical system featuring the porous N-doped carbon cloth electrode, which is 14.5 times higher than that of the original carbon cloth electrode. This result demonstrates that the adoption of our new electrode is a viable strategy for boosting the performance of bioelectrochemical systems.

Oxygen/phosphorus co-doped porous carbon from cicada slough as high-performance electrode material for supercapacitors

Synthesizing high-performance electrode materials plays a vital role in fabricating advanced supercapacitors. Heteroatom doping has been proved to be effective in enhancing the electrochemical properties of carbon-based electrodes. Herein, we report an O, P co-doped porous carbon (PC) originated from waste biomaterials, cicada sloughs. The PC possesses meso-/microporous structure with a large specific surface area (1945 m²·g^-1) and a high O, P co-doping ratio of 18 wt.%. These superior factors together enable it to deliver high specific capacitance (295 F·g^-1 in 6 M KOH and 291 F·g^-1 in 1 M H₂SO₄), good cycling stability (100% capacitance retention after 10000 cycles in 1 M H₂SO₄) and rate performance. Therefore, from the respects of environment friendliness and cost effectivity, obtaining heteroatom doped carbons from the nature might be better compared to pyrolyzing heteroatom-containing chemicals.

Recent Progress in Some Amorphous Materials for Supercapacitors

A breakthrough in technologies having "green" and sustainable energy storage conversion is urgent, and supercapacitors play a crucial role in this area of research. Owing to their unique porous structure, amorphous materials are considered one of the best active materials for high-performance supercapacitors due to their high specific capacity, excellent cycling stability, and fast charging rate. This Review summarizes the synthesis of amorphous materials (transition metal oxides, carbon-based materials, transition metal sulfides, phosphates, hydroxides, and their complexes) to highlight their electrochemical performance in supercapacitors.

High-performance solid state supercapacitors assembling graphene interconnected networks in porous silicon electrode by electrochemical methods using 2,6-dihydroxynaphthalen

The challenge for conformal modification of the ultra-high internal surface of nanoporous silicon was tackled by electrochemical polymerisation of 2,6-dihydroxynaphthalene using cyclic voltammetry or potentiometry and, notably, after the thermal treatment (800 °C, N₂, 4 h) an assembly of interconnected networks of graphene strongly adhering to nanoporous silicon matrix resulted. Herein we demonstrate the achievement of an easy scalable technology for solid state supercapacitors on silicon, with excellent electrochemical properties. Accordingly, our symmetric supercapacitors (SSC) showed remarkable performance characteristics, comparable to many of the best high-power and/or high-energy carbon-based supercapacitors, their figures of merit matching under battery-like supercapacitor behaviour. Furthermore, the devices displayed high specific capacity values along with enhanced capacity retention even at ultra-high rates for voltage sweep, 5 V/s, or discharge current density, 100 A/g, respectively. The cycling stability tests performed at relatively high discharge current density of 10 A/g indicated good capacity retention, with a superior performance demonstrated for the electrodes obtained under cyclic voltammetry approach, which may be ascribed on the one hand to a better coverage of the porous silicon substrate and, on the other hand, to an improved resilience of the hybrid electrode to pore clogging.

Porous carbons derived from waste printing paper for high rate performance supercapacitors in alkaline, acidic and neutral electrolytes

Waste printing paper is used as a carbon precursor for the preparation of high performance carbon materials in supercapacitor applications.

Eco-Friendly and High Performance Supercapacitors for Elevated Temperature Applications Using Recycled Tea Leaves

Used tea leaves are utilized for preparation of carbon with high surface area and electrochemical properties. Surface area and pore size of tea leaves derived carbon are controlled by varying the amount of KOH as activating agent. The maximum surface area of 2532 m2 g-1 is observed, which is much higher than unactivated tea leaves (3.6 m2 g-1). It is observed that the size of the electrolyte ions has a profound effect on the energy storage capacity. The maximum specific capacitance of 292 F g-1 is observed in 3 m KOH electrolyte with outstanding cyclic stability, while the lowest specific capacitance of 246 F g-1 is obtained in 3 m LiOH electrolyte at 2 mV s-1. The tea leaves derived electrode shows almost 100% capacitance retention up to 5000 cycles of study. The symmetrical supercapacitor device shows a maximum specific capacitance of 0.64 F cm-2 at 1 mA cm-2 and about 95% of specific capacitance is retained after increasing current density to 12 mA cm-2, confirming the high rate stability of the device. An improvement over 35% in the charge storage capacity is seen when increasing device temperature from 10 to 80 °C. The study suggests that used tea leaves can be used for the fabrication of environment friendly high performance supercapacitor devices at a low cost.

The ideal porous structure of EDLC carbon electrodes with extremely high capacitance

We propose an ideal porous structure of carbon electrodes for electric double-layer capacitors (EDLCs). The porous carbon successfully improved the gravimetric capacitance above ∼200 F g^-1 even in an organic electrolyte by utilizing the carbon nanopore surface more effectively. High-resolution transmission electron microscopy images and X-ray diffraction patterns classified 15 different porous carbon electrodes into slit-shape and worm-like-shape, and the pore size distributions of the carbons were carefully determined applying the grand canonical Monte Carlo method to N₂ adsorption isotherms at 77 K. The ratio of pores where solvated ions and/or desolvated ions can penetrate also has a significant effect on the EDL capacitance as well as the pore shape. The detailed study on the effect of porous morphologies on the EDLC performance indicates that a hierarchical porous structure with a worm-like shaped surface and a pore size ranging from a solvated ion to a solvent molecule is an ideal electrode structure.

High-Performance Flexible Supercapacitors obtained via Recycled Jute: Bio-Waste to Energy Storage Approach

In search of affordable, flexible, lightweight, efficient and stable supercapacitors, metal oxides have been shown to provide high charge storage capacity but with poor cyclic stability due to structural damage occurring during the redox process. Here, we develop an efficient flexible supercapacitor obtained by carbonizing abundantly available and recyclable jute. The active material was synthesized from jute by a facile hydrothermal method and its electrochemical performance was further enhanced by chemical activation. Specific capacitance of 408 F/g at 1 mV/s using CV and 185 F/g at 500 mA/g using charge-discharge measurements with excellent flexibility (~100% retention in charge storage capacity on bending) were observed. The cyclic stability test confirmed no loss in the charge storage capacity of the electrode even after 5,000 charge-discharge measurements. In addition, a supercapacitor device fabricated using this carbonized jute showed promising specific capacitance of about 51 F/g, and improvement of over 60% in the charge storage capacity on increasing temperature from 5 to 75 °C. Based on these results, we propose that recycled jute should be considered for fabrication of high-performance flexible energy storage devices at extremely low cost.

Promising biomass-derived hierarchical porous carbon material for high performance supercapacitor

High surface area, porous carbon materials were obtained from rice husk ash and exhibited good charge storage capacity.

Composites of hierarchical metal–organic framework derived nitrogen-doped porous carbon and interpenetrating 3D hollow carbon spheres from lotus pollen for high-performance supercapacitors

Nitrogen-doped porous carbon prepared via facile carbonization which displays high specific capacitance, is an excellent potential material for supercapacitors.

Large and porous carbon sheets derived from water hyacinth for high-performance supercapacitors

Large and porous carbon sheets derived from water hyacinths owns high specific surface and desirable microstructures ensuring large specific capacitance, excellent rate capability and superior cyclic stability for high performance supercapacitors.

Renewable pine cone biomass derived carbon materials for supercapacitor application

We report on the transformation of pine cone biomass into porous carbon via KOH activation and carbonization at 800 °C as electrode materials for supercapacitors.

Hierarchical porous carbon materials with high capacitance derived from Schiff-base networks

A type of hierarchical porous carbon material was prepared using a Schiff-base network as the precursor and ZnCl2 as the activation agent, and their electrochemical performances were investigated in acid and alkaline aqueous solutions, respectively. The as-prepared materials have high surface areas, appropriate distributions of hierarchical pore sizes, and various forms of nitrogen/oxygen derivatives. These structural advantages guarantee the outstanding performances of such carbon materials as electrodes for supercapacitors, which include high specific capacitances, fast current responses, and high cycling stabilities.

High surface area and oxygen-enriched activated carbon synthesized from animal cellulose and evaluated in electric double-layer capacitors

Mechanism diagram for the synthesis of activated carbons from crab shell wastes.

Effect of removing potassium ions from activated carbon on its electrochemical performance for supercapacitors

Oxidation-ultrasound process is effective to deeply remove K⁺ and make ACs with high performance for supercapacitor applications.

Activated carbon nanospheres derived from bio-waste materials for supercapacitor applications – a review

Supercapacitors are perfect energy storage devices; they can be charged almost instantly and release energy over a long time.

MnO2-doped, polyaniline-grafted rice husk ash nanocomposites and their electrochemical capacitor applications

The utilization of renewable rice husk ash (RHA) in an energy storage system without any activation is described.