Preparation of oxygen-enriched hierarchically porous carbon by KMnO4 one-pot oxidation and activation: Mechanism and capacitive energy storage

Reduced graphene oxide-wrapped ZnS-SnS2 heterojunction bimetallic hollow cubic boxes as high-magnification and long lifespan supercapacitor anode materials

Metal sulfides have attracted extensive attention due to their excellent electrochemical performance. However, issues such as poor conductivity and severe volume expansion during charge and discharge processes affect the applications of sulfides as electrode materials. Here, a combination of coprecipitation and high-temperature sulfidation methods are employed to synthesize a ZnS-SnS2 composite with a hollow cubic structure, which is further composited with reduced graphene oxide (RGO) to form ZnS-SnS2 hollow cubic boxes encapsulated in a conductive framework of reduced graphene oxide (RGO) (denoted as ZnS-SnS2@RGO) for electrode materials. The hollow structure effectively alleviates the pulverization of ZnS-SnS2@RGO caused by volume expansion during charge and discharge processes. The heterogeneous structure formed by ZnS and SnS2 effectively reduces the electron transfer resistance of the material. The use of RGO wrapping enhances the conductivity of the ZnS-SnS2 hollow cubic boxes, and RGO's dispersion effect on the ZnS-SnS2 cubes improves particle agglomeration, further mitigating volume expansion of the material. These results indicate the outstanding electrochemical performance of heterostructural ZnS-SnS2 hollow cubic electrodes encapsulated with reduced graphene oxide as a conductive framework. The fabrication process provides a novel approach for addressing volume expansion and poor conductivity issues in other pseudocapacitive materials.

Oxygen-enriched pitch-derived hierarchically porous carbon toward boosted zinc-ion storage performance

The lack of cathode materials with satisfactory Zn2+ storage capability substantially hinders the realization of high-performance aqueous zinc-ion hybrid capacitors (ZHCs). Herein, we propose a facile KMnO4 template-assisted KOH activation strategy to prepare a novel oxygen-enriched hierarchically porous carbon (HPC-1-4). This strategy efficiently converts coal tar pitch (CTP) into a well-tuned carbon material with a large specific surface area of 3019 m2 g-1 and a high oxygen content of 9.20 at%, which is conducive to providing rich active sites, rapid charge transport, and appreciable pseudocapacitance for Zn-ion storage. Thus, the as-fabricated HPC-1-4-based aqueous ZHC exhibits prominent performance, including a high gravimetric capacity (206.7 mAh g-1 at 0.25 A g-1), a remarkable energy density (153.4 Wh kg-1 at 184.2 W kg-1), and an impressive power output (15240 W kg-1 at 63.5 Wh kg-1). In-depth ex-situ characterizations indicate that the excellent electrochemical properties of ZHCs are due to the synergistic effect of the Zn2+ adsorption mechanism and reversible chemisorption. In addition, the assembled quasi-solid-state device demonstrates excellent electrochemical stability of up to 100% capacity retention over 50000 cycles, accompanied with a desirable energy density of 115.6 Wh kg-1. The facile preparation method of converting CTP into carbonaceous functional materials has advanced the development of efficient and eco-friendly energy storage technologies.

Crystallization-induced formation of two-dimensional carbon nanosheets derived from sodium lignosulfonate for fast lithium storage

Sodium lignosulfonate, an abundant natural resource, is regarded as an ideal precursor for the synthesis of hard carbon. The development of high-performance, low-cost and sustainable anode materials is a significant challenge facing lithium-ion batteries (LIBs). The modulation of morphology and defect structure during thermal transformation is crucial to improve Li+ storage behavior. Synthesized using sodium lignosulfonate as a precursor, two-dimensional carbon nanosheets with a high density of defects were produced. The synergistic influence of ice templates and KCl was leveraged, where the ice prevented clumping of potassium chloride during drying, and the latter served as a skeletal support during pyrolysis. This resulted in the formation of an interconnected two-dimensional nanosheet structure through the combined action of both templates. The optimized sample has a charging capacity of 712.4 mA h g-1 at 0.1 A g-1, which is contributed by the slope region. After 200 cycles at 0.2 A g-1, the specific charge capacity remains 514.4 mA h g-1, and a high specific charge capacity of 333.8 mA h g-1 after 800 cycles at 2 A g-1. The proposed investigation offers a promising approach for developing high-performance, low-cost carbon-based anode materials that could be used in advanced lithium-ion batteries.

Hybrid nanoarchitectonics of coal-derived carbon with oxidation-induced morphology-selectivity for high-performance supercapacitor

Coal-derived porous carbon with a large specific surface area is a common electrode material for supercapacitors. Its deep and branched micropores, dense bulk morphology and amorphous structure have greatly limited its practical applications. Herein, hybrid carbon materials were obtained from coal through oxidation followed by activation. The method allows tuning the morphology, porosity, structure, and the degree of graphitization. The pre-oxidation with KMnO₄ can break raw coal into small hydrocarbon fragments, which deposit and grow on the surface of generated MnO during pyrolysis leading to hybrid carbon with mesoporous and graphitic nanostructures. Meanwhile, homogeneous etching of the carbon skeleton by the reaction intermediate of K₂CO₃ led to the formation of abundant active sites. Hence, the optimized sample exhibited a high capacitance of 333 F g^-1 at 1 A g^-1, an excellent rate capability with 58% capacitance retention at 100 A g^-1 and superior cycle durability in a three-electrode system. Besides, an assembled symmetric two-electrode device displayed a high energy density of 8.9 Wh·kg^-1 at 250 W·kg^-1. This work proposed a facile and rational synthesis strategy by balancing the tradeoff between active sites and intrinsic conductivity and thus provided a new avenue for the value-added utilization of coal.

The Influence of Physical Mixing and Impregnation on the Physicochemical Properties of Pine Wood Activated Carbon Produced by One-Step ZnCl2 Activation

In this study, two different sample preparation methods to synthesize activated carbon from pine wood were compared. The pine wood activated carbon was prepared by mixing ZnCl₂ by physical mixing, i.e., "dry mixing" and impregnation, i.e., "wet mixing" before high temperature carbonization. The influence of these methods on the physicochemical properties of activated carbons was examined. The activated carbon was analyzed using nitrogen sorption (surface area, pore volume and pore size distribution), XPS, density, Raman spectroscopy, and electrochemistry. Physical mixing led to a slightly higher density carbon (1.83 g/cm³) than wet impregnation (1.78 g/cm³). Raman spectroscopy analysis also showed that impregnation led to activated carbon with a much higher degree of defects than physical mixing, i.e., I_D/I_G = 0.86 and 0.89, respectively. The wet impregnated samples also had better overall textural properties. For example, for samples activated with 1:1 ratio, the total pore volume was 0.664 vs. 0.637 cm³/g and the surface area was 1191 vs. 1263 m²/g for dry and wet mixed samples, respectively. In the electrochemical application, specifically in supercapacitors, impregnated samples showed a much better capacitance at low current densities, i.e., 247 vs. 146 F/g at the current density of 0.1 A/g. However, the physically mixed samples were more stable after 5000 cycles: 97.8% versus 94.4% capacitance retention for the wet impregnated samples.

Reduced cadmium (Cd) accumulation in lettuce plants by applying KMnO4 modified water hyacinth biochar

In this study, water hyacinth was adopted to prepare biochar followed by modification using KMnO₄. And the modified biochars were applied in Cd contaminated soil, exploring the effects of water hyacinth biochar on lettuce growth, Cd enrichment, soil enzyme activities and microbial changes by pot experiments. Modified biochar application significantly reduced the Cd accumulation in lettuce shoots and roots. Compared to the control, the application of water hyacinth biochar at 1% rate resulted in significant reduction of Cd contents by 40.7% and 33.7% in the shoots and roots of lettuce. Also, the reduction was 33.3% and 20.8% compared with the application rate of unmodified biochar. With the increase of biochar application, the amount of Cd was absorbed by lettuce shoots and roots showing significant reduction of plant Cd accumulation in response to the biochar application rate. Additionally, the lowest available Cd concentration in soil (1.34 mg kg⁻¹) was obtained with the application of modified biochar at 1% rate, which might be the main reason for the lower Cd concentration in lettuce shoot and root parts. Furthermore, structural analysis showed that Cd was fixed on the modified biochar, in a passivated state, by larger specific surface area, more active sites and more stable covalent binding complexes leading to a strong decrease of the available Cd in the soil. Moreover, it was concluded that the increment of the enzyme activities in the soil was up to 2.51 times significantly following the application of modified water hyacinth biochar with 3% amount. Lastly, 16sRNA sequencing showed that biochar addition may lead to changes of microbial structure and abundance in soil.

Advances in Micro-/Mesopore Regulation Methods for Plant-Derived Carbon Materials

In recent years, renewable and clean energy has become increasingly important due to energy shortage and environmental pollution. Selecting plants as the carbon precursors to replace costly non-renewable energy sources causing severe pollution is a good choice. In addition, owing to their diverse microstructure and the rich chemical composition, plant-based carbon materials are widely used in many fields. However, some of the plant-based carbon materials have the disadvantage of possessing a large percentage of macroporosity, limiting their functionality. In this paper, we first introduce two characteristics of plant-derived carbon materials: diverse microstructure and rich chemical composition. Then, we propose improvement measures to cope with a high proportion of macropores of plant-derived carbon materials. Emphatically, size regulation methods are summarized for micropores (KOH activation, foam activation, physical activation, freezing treatment, and fungal treatment) and mesopores (H3PO4 activation, enzymolysis, molten salt activation, and template method). Their advantages and disadvantages are also compared and analyzed. Finally, the paper makes suggestions on the pore structure improvement of plant-derived carbon materials.

Molten Salt Self-Template Synthesis Strategy of Oxygen-Rich Porous Carbon Cathodes for Zinc Ion Hybrid Capacitors

Porous carbon materials are widely used in capacitive energy storage devices because of their chemical stability, low cost, and controllable textures. Molten salt self-template methods are powerful and sustainable synthesis strategies for preparing porous carbons with tunable pore textures and surface chemistries. Herein, we propose a self-template synthesis strategy for preparing oxygen-rich porous carbons (ORC) by directly carbonizing potassium chloroacetate (ClCH₂COOK) as the single carbon source. The potassium chloride salts generated in the carbonization play the roles of the template and etchant agent in the pore formation process. The as-prepared ORC samples feature abundant mesopores (average pore sizes of 1.95-2.19 nm and mesopore ratio of 36.4%), high specific surface areas (1410-1886 m² g^-1), and high oxygen doping levels (4.3-8.2 atom %). The zinc ion hybrid capacitors with an ORC cathode exhibited an ultrahigh capacitance of 308 F g^-1 at 0.5 A g^-1 and a high energy density of 136.5 Wh kg^-1 at a power density of 570 W kg^-1. Density functional theory demonstrates that oxygen-containing functional groups are conducive to the adsorption of Zn ions. Our work proposes a general synthesis methodology for the synthesis of oxygen-rich porous carbons for a variety of electrochemical energy storage devices.

Nanoporous carbon materials as a sustainable alternative for the remediation of toxic impurities and environmental contaminants: A review

Due to rapidly deteriorating water resources, the world is looking forward to a sustainable alternative for the remediation of noxious pollutants such as heavy metals and organic and gaseous contaminants. To address this global issue of environmental pollution, nanoporous carbon materials (NPCMs) can be used as a one-stop solution. They are widely applied as adsorbents for many toxic impurities and environmental contaminants. The present review provides a detailed overview of the role of different synthesis factors on the porous characteristics of carbon materials, activating agents, reagent-precursor ratio and their potential application in the remediation. Findings revealed that synthetic parameters result in the formation of microporous NPCMs (S_BET: >4000 m³/g; V_Total (cm³/g) ≥ 2; V_Micro (cm³/g) ≥ 1), micromesoporous (S_BET: >2500 m³/g; V_Total (cm³/g) ≥ 1.5; V_Micro (cm³/g) ≥ 0.7) and mesoporous (S_BET: >2500 m³/g; V_Total (cm³/g) ≥ 1.5; V_Micro (cm³/g) ≥ 0.5) NPCMs. Moreover, it was observed that a narrow pore size distribution (0.5-2.0 nm) yields excellent results in the remediation of noxious contaminants. Further, chemical activating agents such as NaOH, KOH, ZnCl₂, and H₃PO₄ were compared. It was observed that activating agents KОН, H₃PO₄, and ZnCl₂ were generally used and played a significant role in the possible large-scale production and commercialization of NPCMs. Thus, it can be interpreted that with a well-planned strategy for the synthesis, NPCMs with a "tuned" porosity for a specific application, in particular, microporosity for the accumulation and adsorption of energetically important gases (CO₂, CH₄, H₂), micro-mesoporosity and mesoporosity for high adsorption capacity for towards metal ions and a large number of dyes, respectively.

Improved adsorption properties of tetracycline on KOH/KMnO4 modified biochar derived from wheat straw

Modification of pristine biochars has received increasing attentions due to the significant potential in enhancing adsorption performance. In this work, the co-modification of KOH and KMnO₄ on biochar (K-Mn-BC) was performed, with the effect of KOH/KMnO₄ modification on biochar properties and their adsorption toward tetracycline (TC) being extensively explored. Results showed that KOH/KMnO₄ modification can significantly regulate biochars to form hierarchical structure. The obtained K-Mn-BC was characterized with a high specific surface area (1524.6 m² g^-1) and total pore volume (0.85 cm³ g^-1). In addition, the K-Mn-BC exhibited a high adsorption capacity of 584.19 mg g^-1 toward TC at 318 K, and pseudo-second-order (R²:0.993～0.998) and Langmuir (R²: 0.834～0.874) models can fit well with the adsorption behavior. Moreover, the obtained K-Mn-BC can efficiently adsorb TC within a wide pH range (3.0-10.0), and were not affected by the co-existing ions. The possible mechanisms for the high adsorption capacity were ascribed to the pore filling and π-π interaction, following by hydrogen bonding and metal complexation. The obtained K-Mn-BC is a suitable adsorbent for TC removal from water due to the hierarchical structure, high adsorption capacity, and stable adsorption effect.

Enzymatic Hydrolysis Lignin-Derived Porous Carbons through Ammonia Activation: Activation Mechanism and Charge Storage Mechanism

The low energy density and low cost performance of electrochemical capacitors (ECs) are the principal factors that limit the wide applications of ECs. In this work, we used enzymatic hydrolysis lignin as the carbon source and an ammonia activation methodology to prepare nitrogen-doped lignin-derived porous carbon (NLPC) electrode materials with high specific surface areas. We elucidated the free radical mechanism of ammonia activation and the relationship between nitrogen doping configurations, doping levels, and preparation temperatures. Furthermore, we assembled NLPC∥NLPC symmetric ECs and NLPC∥Zn asymmetric ECs using aqueous sulfate electrolytes. Compared with the ECs using KOH aqueous electrolyte, the energy densities of NLPC∥NLPC and NLPC∥Zn ECs were significantly improved. The divergence of charge storage characteristics in KOH, Na₂SO₄, and ZnSO₄ electrolytes were compared by analyzing their area surface capacitance. This work provides a strategy for the sustainable preparation of lignin-derived porous carbons toward ECs with high energy densities.

N-doped graphitized porous carbon derived from N-rich polymer for improved supercapacitor performance

Porous carbons with the large specific surface area, high electrical conductivity as well as abundant heteroatom doping are regarded as a promising candidate for supercapacitor applications. In this report, a...

Preparation of Lignin Carbon/Zinc Oxide Electrode Material and Its Application in Supercapacitors

In this paper, carbon/zinc oxide (LC/ZnO) composites were successfully synthesized and characterized by X-ray powder diffraction, field emission scanning electron microscopy, Fourier transform infrared spectroscopy, Raman, thermogravimetry, and N₂ adsorption-desorption, and tested by electrochemical performance. Studies have shown that the morphology of LC/ZnO composites is that lignin pellets are embedded in ZnO microplates. The lignin carbon in the composites mainly exists in an amorphous structure, and the specific surface area and pore channels of metal oxides are increased by the presence of lignin carbon. The electrochemical performance test shows that the carbonization temperature of LC/ZnO with the highest specific capacitance is 550 °C, and the capacitance retention rate reaches 96.74% after 1000 cycles of testing, indicating that the composite material has good cycle stability. Compared with the control group, it is found that the specific capacitance of LC/ZnO-550 °C is 2.3 times and 1.8 times that of ZnO-550 °C and LC-550 °C, respectively. This shows that during the electrochemical test, the lignin carbon and the metal oxide promote each other and act synergistically. In addition, the composite material exhibits the characteristics of a pseudo-capacitance capacitor, indicating that the redox reaction occurred in the electrochemical performance test.

High Energy Density Heteroatom (O, N and S) Enriched Activated Carbon for Rational Design of Symmetric Supercapacitors

Carbon-based symmetric supercapacitors (SCs) are known for their high power density and long cyclability, making them an ideal candidate for power sources in new-generation electronic devices. To boost their electrochemical performances, deriving activated carbon doped with heteroatoms such as N, O, and S are highly desirable for increasing the specific capacitance. In this regard, activated carbon (AC) self-doped with heteroatoms is directly derived from bio-waste (lima-bean shell) using different KOH activation processes. The heteroatom-enriched AC synthesized using a pretreated carbon-to-KOH ratio of 1:2 (ONS@AC-2) shows excellent surface morphology with a large surface area of 1508 m²  g^-1 . As an SC electrode material, the presence of heteroatoms (N and S) reduces the interfacial charge-transfer resistance and increases the ion-accessible surface area, which inherently provides additional pseudocapacitance. The ONS@AC-2 electrode attains a maximum specific capacitance (C_sp ) of 342 F g^-1 at a specific current of 1 Ag^-1 in 1 m NaClO₄ electrolyte at the wide potential window of 1.8 V. Moreover, as symmetric SCs the ONS@AC-2 electrode delivers a maximum specific capacitance (C_sc ) of 191 F g^-1 with a maximum specific energy of 21.48 Wh kg^-1 and high specific power of 14 000 W kg^-1 and excellent retention of its initial capacitance (98 %) even after 10000 charge/discharge cycles. In addition, a flexible supercapacitor fabricated utilizing ONS@AC-2 electrodes and a LiCl/polyvinyl alcohol (PVA)-based polymer electrolyte shows a maximum C_sc of 119 F g^-1 with considerable specific energy and power.

Insight into activated carbon from different kinds of chemical activating agents: A review

Activated carbon (AC) is an important material in various fields owing to its low cost, well-developed porosity, and favorable chemical stability. Key factors for the optimal synthesis of AC are the carbon precursors, activation pathways, activating agents, and design of the procedure parameters. So far, no case studies have reviewed the activating agents used during the chemical activation process. Accordingly, the present review provides a summary of recent research, highlighting the development of activating agents during the process of AC. Detailed lists of pore-forming mechanisms by various activating agents, including alkaline, acidic, neutral, and self-activating agents, have been systematically summarized. Furthermore, the effects of activating agents on the experimental procedures have also been established. Finally, a comprehensive discussion about the influences of activating agents on the physical and chemical properties of the resultant AC is included. The objective of this study is to reveal and distinguish the individual roles of different activating agents during AC synthesis.

Simple pyrolysis of alginate-based hydrogel cross-linked by bivalent ions into highly porous carbons for energy storage

Searching for a sustainable precursor that is capable of absorbing microwave energy is crucial for its rapid conversion into porous carbon via microwave heating. Here, alginate-based hydrogel beads cross-linked by bivalent ions (Cu2+, Zn2+, and Ca2+) are converted into porous carbons (SPCs) in 10 min by applying simple microwave-assisted pyrolysis. Water wrapped in hydrogel beads and bivalent ions serve as initial microwave absorbers to convert alginate into char which acts as a good microwave absorber in the following stages. Additionally, bivalent ions also act as a gelling agent to generate hydrogel beads, as a porogen to obtain a highly porous structure, and as a metal donor to form the SPC/Cu composite. The resultant SPC has a high specific surface area of 1336 m2 g-1, a large total pore volume of 0.56 cm3 g-1, interconnected macropores, as well as a high oxygen content of up to 13.2%. These attractive characteristics give SPC a remarkable rate capability of 72% at 50 A g-1. Interestingly, the interconnected macropores in SPC offer sufficient space for Cu growth via the redox reactions of Cu2+; thus, the SPC/Cu composite without acid wash shows a specific capacitance as high as 804 F g-1 at 0.5 A g-1.

Sakura-based activated carbon preparation and its performance in supercapacitor applications

3D porous carbonaceous materials were prepared by combining pre-carbonization and KOH activation with sakura petals as raw materials. The prepared porous sakura carbon (SAC-4) exhibits a high specific surface area, a suitable pore size distribution, a low proportion of oxygen-rich groups and N functional groups, and a partially graphitized phase, which are very beneficial for the electrochemical performance of the material as a supercapacitor electrode. In the activation step, when the mass ratio of KOH to sakura carbon (SC) is 4, a supercapacitor is prepared. A maximal specific capacitance of 265.8 F g⁻¹ is obtained when the current density is 0.2 A g⁻¹. When the current density is 1 A g⁻¹, after 2000 cycles in succession, the capacitance retention rate is excellent and the cycling stability can reach as high as 90.2%. The obtained results indicate that porous carbon prepared with sakura blossom as the raw material is an effective and environmentally friendly electrode material for energy storage.

3D porous carbonaceous materials were prepared by combining pre-carbonization and KOH activation with sakura petals as raw materials.