Preparation of porous carbons by hydrothermal carbonization and KOH activation of lignite and their performance for electric double layer capacitor

KHCO3 Chemical-Activated Hydrothermal Porous Carbon Derived from Sugarcane bagasse for Supercapacitor Applications

The reuse of waste biomass resources had become a hot topic in the sustainable development of human society. Biomass was an ideal precursor for preparing porous carbon. However, due to the complexity of biomass composition and microstructure, the quality reproducibility of biomass porous carbon was poor. Therefore, it was of great significance to develop a reliable method for preparing porous carbon from biomass. In this paper, the activated hydrothermal porous carbon was prepared by a combination of hydrothermal carbonization treatment and KHCO3 mild activation. The hydrothermal carbonization treatment could complete the morphology adjustment and iron doping of the carbon in one step, and the mild activation of KHCO3 could activate the porous carbon while maintaining the spherical morphology. Fe-modified porous carbon with carbon ball/nanosheet structure prepared from bagasse exhibited a high surface area (2169.8 m2/g), which facilitated ion/electrolyte diffusion and increased accessibility between surface area and electrolyte ions. Therefore, bagasse derived activated porous carbon had good specific capacitance (315.2 F/g at 1 A/g) and good cycle stability, with a capacitance loss of only 5.8 % after 5000 charge-discharge cycles, and the Na2SO4-based device showed the maximum energy density of 13.02 Wh/kg. This study showed that the combination of hydrothermal treatment and mild activation provided an effective way for the conversion of waste biomass into high-performance electrode materials.

Activity and product distribution in Ni-Co and Ni-Cu catalyst-mediated lignin depolymerization into phenolic substances with isopropanol H-donating solvent

Lignin, a vital renewable biopolymer, serves as the Earth's primary source of aromatics and carbon. Its depolymerization presents significant potential for producing phenolic fine chemicals. This study assesses promoted Ni-based bimetallic catalysts (Ni-Co/C and Ni-Cu/C) supported on activated carbon in isopropanol for lignin depolymerization, compared to monometallic counterparts. BET, SEM, EDX, and XPS analyses highlight their physicochemical properties and promotional effects, enhancing hydrogenolysis activity and hydrogen transformation. Reaction parameter exploration elucidates the influence on lignin depolymerization, with cobalt and copper as promoters notably increasing conversion and monomer yield. Ni-Co/C exhibits the highest lignin conversion (94.2%) and maximum monomer yield (53.1 wt%) under specified conditions, with lower activation energy (36.1 kJ/mol) and higher turnover frequency (31.6 h-1) compared to Ni/C. FT-IR, GPC, GC-FID, and GC-MS analyses confirm effective depolymerization, identifying 20 monomer products. Proposed reaction mechanisms underscore the potential of Ni-based bimetallic catalysts for lignin valorization, offering insights into developing efficient catalytic systems for lignin hydrogenolysis. This research enhances understanding and facilitates the development of selective catalytic processes for lignin valorization.

The Adsorption of Pb(II) from Aqueous Solution Using KOH-Modified Banana Peel Hydrothermal Carbon: Adsorption Properties and Mechanistic Studies

Adopting banana peel as a raw material, the adsorption properties of banana peel hydrothermal carbon modified with a KOH solution for lead ions in aqueous solution were studied. The surface structure and functional groups of the modified hydrothermal carbon were analyzed by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared (FT-IR) spectroscopy, the Brunner-Emmet-Teller (BET) method, element analysis, and Raman spectroscopy. The results showed that an adsorption capacity of 42.92 mg/g and a removal rate of 86.84% were achieved when the banana peel hydrothermal carbon was modified with a KOH solution of 0.5 mol/L, with a pH of 6 and a solid-liquid ratio of 1 g/L. The equilibrium adsorption time for lead ions in solution being adsorbed using KOH-modified hydrothermal carbon was 240 min, the adsorption process satisfied the quasi-second-order kinetic model and the Redlich-Peterson isotherm equation, and the equilibrium removal efficiency was 88.62%. The adsorption of lead ions using KOH-modified hydrothermal carbon is mainly chemical-physical adsorption.

Porosity-Induced Improvement in KOH Activation of Chitin Nanofiber-Based Porous Carbon Leading to Ultrahigh Specific Capacitance

The applicability of chitin-based carbon as a supercapacitor electrode material was investigated by adjusting its pore structure through polystyrene latex templating, without significant N doping. 2,2,6,6-tetramethylpiperidinyloxy (TEMPO)-oxidized chitin nanofibers were mixed with polystyrene latex, hydrothermally treated at 220 °C, carbonized, and activated using KOH at 800 °C, yielding activated hierarchical porous carbon. The variation of both polystyrene latex amount and carbonization temperature resulted in changes in the surface area and pore structure, which dictated the degree of pore uniformity and activation efficiency. The pore structure affected activation by allowing the selective removal of amorphous carbon, exposing the basal plane carbon, resulting in higher specific capacitance. By making activated hierarchical porous carbon more conducive to activation, specific capacitance of 567 F g^-1 at 0.5 A g^-1 was achieved, with no loss in performance after 10000 charge-discharge cycles.

Preparation and characterization of three-dimensional hierarchical porous carbon from low-rank coal by hydrothermal carbonization for efficient iodine removal

Low-rank coal, such as Shengli lignite (SL) and Datong bitumite (DT), has abundant reserves and is low in cost. Due to its high moisture content, abundant oxygen-containing groups, high ash content and low calorific value, low-rank coal is mainly used in a low-cost method of direct combustion. For better value-added utilization of SL and DT, a novel strategy has been developed for the preparation of oxygen-rich hierarchical porous carbons (HPCs) by hydrothermal carbonization (HTC), followed by steam activation. In this paper, firstly, the physical and chemical properties of SL and DT were improved by HTC pretreatment, bringing them closer to high rank coal. Then, the effects of HTC pretreatment and activation temperature on the properties of the HPCs were investigated in detail. The results show that the HPCs have mainly microporous structures (the microporosity of 200-SLHPC-900 is 79.58%) based on the N2 adsorption-desorption isotherm analysis and exhibit a higher specific surface area (SSA) and larger pore volume (25.02% and 2.69% improvement for 200-SLHPC-900; 4.93% and 14.25% increase for 200-DTHPC-900, respectively) after HTC pretreatment. The two types of HPCs also present good adsorption performance. The iodine adsorption value of lignite-based HPC presents an increase of 13.72% from 503 mg g-1 to 572 mg g-1, while the value of bitumite-based HPC increases up to 924 mg g-1. A preliminary additional HTC step is therefore an effective method by which to promote the performance of low-rank coal based porous carbon. The process of hydrothermal carbonization and steam activation is a cost-effective and environmentally-friendly preparation method, which omits the use of a chemical activator and reduces the step of alkaline waste liquid discharge compared with the route of carbonization and chemical activation.

Rationally tuning ratio of micro- to meso-pores of biomass-derived ultrathin carbon sheets toward supercapacitors with high energy and high power density

The carbon pore structure could have a significant effect on supercapacitor performance; however, this effect has not yet been systematically studied. A facile approach for synthesizing porous, ultrathin carbon sheets while rationally tuning the ratio of micro-to meso-pores via partial corrosion has been developed for the fabrication of high-performance devices. The prepared carbon from biomass with an optimal ratio of micro- to meso-pores has a large specific surface area of 1785 m² g ^-1, a high specific capacitance of 447F g ^-1 at 0.5 A g^-1, a high energy density of 15.5-9.7 Wh kg^-1, and an excellent power density of 0.062-6.24 kW kg^-1. After 10,000 charge-discharge cycles, the capacitance retention was maintained at 95%, which exceeded most of the biomass-carbon-based capacitors. Volcano relationships were found to exist through plots of both specific surface area and specific capacitance versus the micro-to meso-pore ratio. An enhancement mechanism with a rational pore structure is proposed, which not only networks micropores to remove died-end micropores to achieve the largest specific active surface area and high specific capacitance but also realizes fast mass-transport channels, resulting in high power density. This work provides an effective approach based on waste re-use by tuning a rational pore structure for achieving high energy/power density toward green energy applications with universal significance.

Preparation of porous biochar and its application in supercapacitors

In this study, economical porous biochar was prepared from an apricot shell and used as an electrode material for a supercapacitor, showing excellent capacitance, cycling stability and rate performance.

Acetic acid-mediated cellulose-based carbons: Influence of activation conditions on textural features and carbon dioxide uptakes

In this work, we developed a simple methodology for producing highly porous carbons. Herein, we combined the hydrothermal method with chemical activation to fabricate cellulose-based, melamine modified porous carbons, using acetic acid as an additive. The preparation conditions including activation temperature, activation time, and melamine ratio were varied to obtain an optimized adsorbent exhibiting efficient textural features and maximized carbon dioxide (CO₂) adsorption uptake. By varying the preparation conditions, high specific surface area (SSA) (1260-3019 m² g^-1), microporosity in the range of 0.21-1.13 cm³ g^-1, and a well-developed porous structure was obtained. The optimized adsorbent exhibits an excellent CO₂ adsorption uptake of 297.05 mg g^-1 (6.75 mmol g^-1) and 174.4 mg g^-1 (3.96 mmol g^-1) at 273 K and 298 K at 1 bar, respectively, due to the existence of ultra-micropores (<0.68 nm, < 0.81 nm), high SSA (3019 m² g^-1), and high nitrogen content (8%). Furthermore, the role of micropores in the CO₂ adsorption process suggests that micropores between 0.68 nm and 1 nm exhibit high CO₂ adsorption potential. Additionally, all synthesized carbons exhibited a high isosteric heat of adsorption (45 kJ mol^-1) and a greater affinity for adsorbed CO₂ species than nitrogen (N₂) molecules. Thus, as-fabricated porous carbon adsorbents are an effective competitor for CO₂ uptake applications to mitigate global warming.

Single Alkali Metal Ion-Activated Porous Carbon With Heteroatom Doping for Supercapacitor Electrode

A single alkali metal ion activation method was used to prepare sulfur-doped microporous carbons. A series of alkali metal ions such as Li⁺, Na⁺, K⁺, and Cs⁺ was used in the polymerization process of 3-hydroxythiophenol and formaldehyde to obtain metal ion anchored in the sulfur-containing resin, which was further treated to obtain xerogel and carbonized to obtain microporous carbon with sulfur doping. In this case, the monodispersed alkali metal ions could realize highly effective activation with low activating agent dosage. Intensive material characterizations show that the alkali metal ions determine the pore structure and surface properties of as-prepared carbons. C-Cs prepared by Cs⁺ ion possesses a high Brunauer-Emmett-Teller specific surface area of 1,037 m² g^-1 with interconnected microporosity and sulfur doping. The specific capacitance of C-Cs can reach up to 270.9 F g^-1 in a two-cell electrode measurement system, whereas C-Cs-based supercapacitors can deliver an energy density of 7.6 Wh kg^-1, which is much larger than that of other samples due to its surface functionalities and well-interconnected porosities.

Supermagnetic Sugarcane Bagasse Hydrochar for Enhanced Osteoconduction in Human Adipose Tissue-Derived Mesenchymal Stem Cells

Hydrothermally carbonized sugarcane bagasse (SCB) has exceptional surface properties. Looking at the huge amount of SCB produced, its biocompatible nature, cheap-cost for carbonization, and its easy functionalization can give impeccable nano-biomaterials for tissue engineering applications. Herein, sugarcane bagasse was converted into hydrochar (SCB-H) by hydrothermal carbonation. The SCB-H produced was further modified with iron oxide (Fe₃O₄) nanoparticles (denoted as SCB-H@Fe₃O₄). Facile synthesized nano-bio-composites were characterized by SEM, HR-TEM, XRD, FT-IR, XPS, TGA, and VSM analysis. Bare Fe₃O₄ nanoparticles (NPs), SCB-H, and SCB-H@Fe₃O₄ were tested for cytocompatibility and osteoconduction enhancement of human adipose tissue-derived mesenchymal stem cells (hADMSCs). The results confirmed the cytocompatible and nontoxic nature of SCB-H@Fe₃O₄. SCB-H did not show enhancement in osteoconduction, whilst on the other hand, Fe₃O₄ NPs exhibited a 0.5-fold increase in the osteoconduction of hADMSCs. However, SCB-H@Fe₃O₄ demonstrated an excellent enhancement in osteoconduction of a 3-fold increase over the control, and a 2.5-fold increase over the bare Fe₃O₄ NPs. Correspondingly, the expression patterns assessment of osteoconduction marker genes (ALP, OCN, and RUNX2) confirmed the osteoconductive enhancement by SCB-H@Fe₃O₄. In the proposed mechanism, the surface of SCB-H@Fe₃O₄ might provide a unique topology, and anchoring to receptors of hADMSCs leads to accelerated osteogenesis. In conclusion, agriculture waste-derived sustainable materials like “SCB-H@Fe₃O₄₄” can be potentially applied in highly valued medicinal applications of stem cell differentiation.

Activated hydrochar produced from brewer's spent grain and its application in the removal of acetaminophen

Acetaminophen has shown a gradual increase in detection in surface waters. Although present in low concentrations, it should be removed to prevent deleterious effects. Thus, adsorption onto activated carbon is emphasized. Adsorbents may be produced by hydrothermal carbonization (HTC), an environmental-friendly process. Therefore, this work aimed to investigate the use of HTC, verifying its application in acetaminophen removal. Brewer's spent grain (BSG), its hydrochar (HC-BSG) and its activated hydrochar (AHC-BSG) were characterized. HTC provided material with high carbon content. Lignocellulosic breakdown has been demonstrated in HC-BSG and AHC-BSG, but in the latter it was more intense as a result of activation with KOH. Also, a high surface area was found in AHC-BSG (1512.83 m² g^-1), resulting in an adsorption of 318.00 mg g^-1. The pseudo-second-order and Langmuir models were fitted to the experimental data. Therefore, HTC was effective as a pretreatment for AHC-BSG, resulting in significant acetaminophen removals.

Preparation of a Hierarchically Porous Lead/Carbon Composite and Its Application in Lead-Carbon Batteries

A novel and scalable lead-modified phenolic resin-based carbon material (Pb/PRC) has been successfully prepared by using lead-modified phenolic resin (Pb/PR) as a precursor, toluene as a pore-forming agent, and KOH as an activating agent. The Pb/PRC composite presents a hierarchically porous nanosphere structure, and this structure contributes to prolong its cycling life under high-rate partial state-of-charge (HRPSoC) operation. Pb/PRC with nano-lead can effectively inhibit hydrogen evolution and provide pseudocapacitance. Compared with a blank negative plate, a lead-carbon battery with Pb/PRC displays 18 000 cycles under HRPSoC operation and exhibits a capacity of 100 mA h g^-1 at 2 C discharge rate.

Adsorption and regeneration of leaf-based biochar for p-nitrophenol adsorption from aqueous solution

As an environmentally friendly and low-cost adsorbent, biochar has great potential in wastewater treatment. This study investigated biochar derived from Platanus orientalis L. leaves (PLB) activated by KOH in terms of its capacity and reusability to adsorb p-nitrophenol (PNP). PLB had a large specific surface area and total pore volume, and exhibits good PNP removal with a maximal adsorption capacity of 622.73 mg g-1 at 298 K. Batch experiments showed that PLB had a high PNP adsorption capacity under acidic conditions. Experimental results were well described by the pseudo-second-order kinetic model and the Langmuir adsorption isotherm model. The thermodynamic study showed that PNP adsorption was a spontaneously exothermic process, and increasing temperature was not conducive to adsorption. In addition, PNP adsorption was mainly attributed to hydrophobic interaction. The regeneration experiment showed that PLB had good reusability. After the fifth regeneration, the adsorption capacity of PLB still reached 557.05 mg g-1. The deactivation of oxygen-containing functional groups and pore blockage were the causes for the decrease in adsorption capacity of the recycled PLB. Moreover, the biochar showed good adsorption efficiency and reusability, thereby suggesting its potential to serve as an efficient PNP adsorbent for wastewater treatment.

Pyrolysis of Chinese chestnut shells: Effects of temperature and Fe presence on product composition

To understand the role of Fe on biomass pyrolysis, Fe-catalyzed biomass pyrolysis in a fixed-bed reactor was investigated. It was found that the introduction of Fe increased the yields of gases and solid char while decreasing the yield of liquid oil. With increasing temperature, Hydrogen content in gaseous products obtained in the presence of Fe increased, while that of CH₄ decreased. In the case of liquid oil, the introduction of Fe promoted the formation of ketones and acids at 400-600 °C, and these species became dominant (67.51%) at 700-800 °C. Finally, solid char obtained in the presence of Fe at 700-800 °C featured a larger pore volume, specific surface area, and graphitization degree, and was characterized by a mesoporous structure with narrow pores size distribution (∼5.3 nm).

Template-free preparation of anthracite-based nitrogen-doped porous carbons for high-performance supercapacitors and efficient electrocatalysts for the oxygen reduction reaction

The conversion of coal into high-performance electrochemical energy materials, exemplified by electrodes and electrocatalysts for supercapacitors and fuel cells, is currently crucial to the advancement of high value-added, clean and non-fuel utilization of coal resources. In this work, anthracite-based nitrogen-doped porous carbon (ANPC) materials with well-defined pore architectures and adjustable nitrogen concentrations were prepared without any template: ANPC-1 by a one-step activation/doping process and ANPC-2 by a two-step process. The specific capacitance value of the ANPC-1 materials could attain a maximum of 346.0 F g-1 at the current density of 0.5 A g-1 in 6 M KOH. Supercapacitors composed of the ANPC-1 electrodes were able to achieve high energy densities up to 10.3 W h kg-1 and 20.8 W h kg-1, together with good charge/discharge stabilities of 95.4% and 91.3% after 5000 cycles, in KOH and Na2SO4 aqueous electrolytes, respectively. The ANPC-2 materials are more associated with the oxygen reduction reaction (ORR): one possessed a comparable ORR electrocatalytic activity to the commercial JM Pt/C (20% Pt) catalyst, and, moreover, its onset potential (0.96 V vs. RHE), half-wave potential (0.85 V vs. RHE), catalyst durability (95.9% activity retained after 40 000 s) and methanol tolerance were all superior to the benchmark electrocatalyst. This study provides a feasible route to rational design of coal-based multifunctional materials towards electrochemical energy storage and conversion.

A novel route for preparation of chemically activated carbon from pistachio wood for highly efficient Pb(II) sorption

Pistachio wood-derived activated carbon prepared by a two-stage process (PWAC-2), conducting two consecutive chemical activation processes with NH₄NO₃ and NaOH, respectively. The results showed that explosive characteristic of NH₄NO₃ can primarily be employed to produce a char, with a large surface area and a highly-ordered pore structure, which can be subjected to a second activation process with NaOH to prepare a more suitable activated carbon, with a highly porous structure and useful functional groups, for removal of lead ions from aqueous media. An L₂₅ Taguchi experimental design was used by varying impregnation ratio, activation time and temperature in both pre- and post-activation stages, and the results showed that, in both stages, a small activating agent/precursor and a proportional low activation time suffice for preparation of an advantageous activated carbon for Pb(II) adsorption. A comprehensive study was performed on the equilibrium, kinetic and thermodynamic aspects of Pb(II) adsorption by the new activated carbon. The results exhibited that, having had a high lead adsorption capacity (190.2 mg g^-1), a high adsorption rapidness, and thermodynamic favorability, PWAC-2 is a beneficial alternative for utilization in full-scale plants of lead removal from waters and wastewaters.

Lignite-derived carbon quantum dot/TiO2 heterostructure nanocomposites: photoinduced charge transfer properties and enhanced visible light photocatalytic activity

A facile and green route to cleanly utilize lignite coal as a carbon source for preparing CQDs/TiO₂ catalysts.

Correlations between hydrochar properties and chemical constitution of orange peel waste during hydrothermal carbonization

For efficient hydrothermal treatment of biomass, this study aims to figure out the correlations between complex chemical constitution of orange peel (OraPeel) as typical bio-waste and the physicochemical structure of its derived hydrochar, which could be utilized to adjust hydrochar properties for specific applications (e.g., adsorbent, fuel) by regulating respective proportions of each component in bio-waste. Cellulose, hemicellulose and lignin were used as the control variables of feedstocks composition in this work. After hydrothermal process, lignin added feedstock produced more hydrochar, which contained rougher surface with nearly doubled BET areas and more benzene rings. Hemicellulose-aided hydrochar possessed higher density of carbonaceous microspheres and richer hydroxyl. This char was simultaneously covered by more esters or lactones with more aromatic oxygen-containing groups inside. Similar to hemicellulose, cellulose improved the formation of diverse oxygenous groups but reduced the size of microspheres on hydrochar.