KOH Activation Process Research Articles

19F Ex Situ Solid-State NMR Study on Structural Differences in Pores of Activated Carbon Series Derived from Chemical and Physical Activation Processes for EDLCs

This study demonstrates the differences in the capacitive behaviors between the pore structures generated by physical and chemical activation and infiltrated electrolyte ions in the pores. Three paired samples were successfully synthesized with similar specific surface areas and average pore widths through steam activation (physical) and KOH activation (chemical) processes. Their capacitive behaviors were investigated with their effective charge (β) values through 19F ex situ solid-state NMR and electrochemical methods. Despite their similar pore characteristics, their capacitive behaviors were totally different because of the differences in their surface structures such as the edge-rich for KOH activation and basal-rich for steam activation. The KOH-activated series displayed higher capacitances and β values than those of the steam-activated series due to the existence of an edge-rich surface structure, which induced more effective charging under electrochemical polarization. Additionally, the pore development model (domain theory) through the KOH and steam activation approaches was discussed by using the β values and chemical shifts by using this 19F ex situ solid-state NMR technique.

A Novel Method to Produce Cost Effective, Environment Friendly Superabsorbent from Eichhornia crassipes for Improved Water Retention in Soil

Eichhornia crassipes is considered the worst aquatic weed in the world as it has become a serious threat to the environment and biodiversity hence eco-friendly utilization of this hydrophyte is needed and important. Enhancement of agricultural production to cater for increased food demand is a global challenge which is aggravated by climate change and scarcity of water. Superabsorbent materials have been developed to improve water retention in soil but majority of superabsorbent are synthetic based and non-biodegradable. Therefore, this study is aimed at developing an eco-friendly superabsorbent from E. crassipes in a cost-effective manner and assess its efficacy on improved water retention in soil. E. crassipes was collected from tank nearby Faculty of Applied Sciences, Wayamba University of Sri Lanka and petiole was treated with potassium hydroxide (KOH) followed by microwave irradiation in which reaction conditions were optimized to obtain maximum water absorption and swelling capacity. Prepared superabsorbent was characterized using Fourier Transform Infrared Spectroscopy (FTIR), XRD and scanning electron microscopy (SEM) techniques. Efficacy of the superabsorbent on improved water retention was assessed using normal loam soil. The superabsorbent showed a maximum swelling index of 1276% at KOH concentration of 0.1 moles/l which is attributed to highly porous structure, presence of hydrophilic functional groups in cellulose and hemicellulose, increased number of surface hydrophilic functional groups during the KOH activation process and carboxymethyl cellulose created during microwave irradiation. Water absorption capacity of the superabsorbent is greatly influenced by KOH concentration, reaction time, microwave power and exposure time. Water retention studies in soil showed that superabsorbent has capacity to retain water for 27 days with a slow rate of water evaporation whereas soil samples without superabsorbent showed a high rate of water evaporation retaining water only for 15 days. Findings of this study disclose an innovative method for development of an eco-friendly superabsorbent from E. crassipes in a cost-effective manner excluding toxic chemical reagents which can be used for improved water retention in soil effectively for climate change resilient sustainable agriculture.

Electrochemical performance of polyacrylamide hydrogel based nitrogen-doped porous carbon for supercapacitor

Nitrogen-doped porous carbon materials for supercapacitor was successfully fabricated from precursor of polyacrylamide gel through freeze drying, carbonization and KOH activation process. The precursor of polyacrylamide gel was prepared using acrylamide and N,N-methylene diacrylamide as monomers and cross-linking agents, respectively. The nitrogen-doped porous carbon was tested by field emission scanning electron microscope, transmission electron microscope, Fourier transform infrared spectroscopy, Raman spectrum, X-ray electron spectrum and specific surface analysis, respectively. Further, the electrochemical performance of nitrogen-doped porous carbon was tested by using a three-electrode system, which exhibited large specific capacitance of 302.1 F/g at 1 A/g, good rate property (71.8%, 217 F/g at 20 A/g) and cycle property (96% after 8000 cycles). Symmetrical supercapacitor was also assembled with an extended operating voltage window of 1.8 V in 1 M Na2SO4 electrolyte. More significantly, the nitrogen-doped porous carbon maintains an energy density of 23.27 Wh/kg in the case of a power density of 450 W/kg.

Versatile bifunctional nitrogen-doped porous carbon derived from biomass in catalytic reduction of 4-nitrophenol and oxidation of styrene

The scarcity and weak durability of metal, especially precious metal catalysts are big obstacles for their large-scale application in many reactions. The state-of-the-art of the catalytic science prefers such type of catalysts, which can replace metal-based catalysts to alleviate energy and environmental crises and exhibit catalytic performance comparable to or even exceeding these metal catalysts. Herein, we report that N-doped porous carbon (NKC) derived from cheap and abundant radish can be employed as versatile and efficient bifunctional catalysts in both the catalytic reduction of 4-nitrophenol (NRR) and oxidation of styrene (SOR). The series of NKC catalysts were prepared with a simple and facile one-pot strategy by coupling the N-doping, carbonization and KOH activation processes. These catalysts show hierarchical porosity, with the specific surface area, total pore volume and N-doping content ranging from 918.9–3062.7 m2 g−1, 1.01–2.04 cm3 g−1 and 1.29–15.3 at%, respectively. Interestingly, our finding suggests that the catalytic performance is not directly related to these parameters but correlates positively with the content of graphitic N dopants, which is the dominant contributor for impelling both the NRR and SOR. Another intriguing finding is that for both reactions, the optimal catalyst was found to be the NKC-3-800 which possesses the highest graphitic N content of 3.13 at%. In addition, to gain insight into the catalytic behavior, analyses of kinetics and thermodynamics were performed, and the catalytic mechanisms were postulated. This work paves the way for the construction of biomass-derived N-doped carbon catalysts for bi- or even multi-functional applications in various organic reactions.

Non‐metallic printed circuit board waste derived carbon for the supercapacitor electrode material

Highly specific surface area carbon materials derived from the waste printed circuit board (WPCB) for supercapacitor electrode material are feasibly prepared via microwave carbonisation and KOH activation process with controlled activation temperature. The effect of activation temperature on the pore structure and the electrochemical performance of the products are studied. The results demonstrate that the appropriate pore structure plays an important role in the application of supercapacitor. The electrochemical performance tests show that the specific capacitance of WPCB-800 can reach 134.2 F g−1 at current density of 200 mA g−1, and the capacity retention rate is 96.2% after 2000 cycles.

Oxygen-rich porous carbons derived from alfalfa flowers for high performance supercapacitors

Alfalfa flowers derived oxygen-rich porous carbons are fabricated via carbonization, acidic washing and KOH activation processes. Supercapacitor electrodes designed from these porous carbons are characterized in both three- and two-electrode measurement systems with 6 M KOH as electrolyte at room temperature by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. The experimental results show that the acidic washing for carbonized carbons and activation temperature perform profound effects on the capacitance performance of the porous carbons. Acidic washing treatment removes most of residues, which do not contribute capacitance for porous carbons. High temperature activation leads to large specific surface area and thus increases the capacitance of porous carbons. In three-electrode system, the electrode designed from carbons activated at 800 °C after acidic washing shows the highest specific capacitance of 350.1 F g−1 at current density of 0.5 A g−1 and 297 F g−1 is kept at current density of 50 A g−1. The specific capacitance of the symmetrical supercapacitor is 201.4 F g−1 at 0.1 A g−1 with 82.7% capacitance retention (166.6 F g−1) at 10 A g−1. The energy density of the symmetrical supercapacitor increases from 23.2 Wh kg−1 to 28 Wh kg−1 with decreasing power density from 10.3 kW kg−1 to 100 W kg−1.

Ecklonia cava based mesoporous activated carbon for high-rate energy storage devices

Activated carbon has received enormous global attention as an electrode material for electrical double layer capacitors (EDLCs) due to its superior electrochemical performance. But while manufacturing high-quality activated carbon as an electrode material, overcoming limitations such as depletion of raw material resources, poor high-rate performance, and cycle stability is still a significant challenge. To address these limitations, we firstly suggest a novel mesoporous activated carbon derived from ecklonia cava (Meso-ACDE) using an acid-base reaction and KOH activation processes. The optimized Meso-ACDE electrode showed a superior specific capacitance of 182Fg−1 at the current density of 0.2Ag−1 and a high-rate capacitance of 154Fg−1 at the current density of 20.0Ag−1 with an excellent cycling stability up to 2000 cycles. The enhanced electrochemical performance of the Meso-ACDE electrode was mainly attributed to the well-developed mesoporous structure with a high concentration of oxygen related functional groups, which were acquired during the activation process. Thus, we believe that the Meso-ACDE could be a promising alternative to conventional activated carbon for high-performance EDLCs.

KOH activation of solid residue of Japanese citron after extraction by microwave process and property as EDLC electrode

The solid residue of Japanese citron after its compression to obtain juice and then the extraction of oil was used as the raw material of activated carbon in order to substantiate the cascading use of Citrease for several high value products. Most of the inorganic compounds in the carbonized samples were K4H2(CO3)3·1.5 H2O and CaCO3. Although the amount of these inorganic compounds was independent of the carbonization temperature, most of these inorganic compounds were removed by the KOH activation process. The BET specific surface area of the sample after the KOH activation increased with the increasing amount of KOH added for activation, and the value was greater than 1900 m2 g−1 when the amount of KOH was greater than 67 wt%. The pore sizes and pore volumes were calculated by several methods, such as the Langmuir, t method, αs method, MP method, BJH method, and INNES method, in order to analyze in detail the surface condition related to the pores. The capacitance value of the sample activated at KOH 75 wt% was 157 F g−1 at 1 mV s−1.

Microwave Assisted KOH Activation of Salacca Peel Dericed Activated Carbons as Adsorbents for Methylene Blue Removal from Aqueous Phase

In this study, salacca peel derived activated carbons (SPAC) were prepared by microwave assisted- KOH activation process. In the preparation process, the fixed mass ratio between salacca peel and KOH of 1:2 was used. Next, the effects of activation time (5, 15, 20 and 25 minutes) and microwave power (30, 380 and 700 W) on the characteristics of SPAC were then investigated. The BET surface area of SPAC was about 1491 m2/g for activation time of 25 minutes and microwave power of 700 W. The as-prepared SPAC was then then as adsorbents for methylene blue (MB) removal from aqueous phase. The adsorption data of SPAC were analyzed by Langmuir and Freundlich adsorption isotherms and the best correlation was shown by the Langmuir isotherm with maximum capacity of 327.9 mg/g. The analyses of kinetic data showed that the adsorption of MB on SPAC follow the pseudo-second order kinetic model.

Walnut shell-derived hierarchical porous carbon with high performances for electrocatalytic hydrogen evolution and symmetry supercapacitors

Walnut Shell-derived hierarchical porous carbon has been successfully synthesized by the efficient KOH activation process. The hierarchical porous carbon material activated at 600 °C, has the specific micropore area of 1037.31 m2 g−1 and micropore volume of 0.51 cm3 g−1, which leads to have electrochemical performances of the hydrogen evolution reaction (HER) and supercapacitors. Specifically, as the hydrogen evolution reaction electrocatalyst, the walnut shell-derived carbon material activated at 600 °C exhibits a lower onset potential of 6.00 mV, a smaller Tafel slope of 69.76 mV dec−1 and outstanding stability above long-term cycling. As a supercapacitor electrode material, the sample possesses specific capacitance of 262.74 F g−1 at 0.5 A g−1, the remarkable rate capability of 224.60 F g−1 at even 10 A g−1 and good long-term stability. A symmetric supercapacitor shows the highly energy density of 7.97 Wh kg−1 at a power density of 180.80 W kg−1. This novel and low-cost biomass material is very promising for the electrocatalytic water splitting and supercapacitors.

Hierarchically porous carbon from waste coffee grounds for high-performance Li–Se batteries

Lithium-selenium (Li–Se) batteries represent a promising energy storage system due to the relatively high electronic conductivity and high volumetric energy density of Se as a cathode. The design of porous carbon with tunable structure and low cost is a key to enabling Se cathodes for high-performance Li–Se batteries. In this study, hierarchically microporous activated carbon (AC) was fabricated from waste coffee grounds through a carbonization and KOH-activation process. Despite the simple synthesis process, the optimized AC (AC-700) had a high surface area of 1355 m2 g−1 and a large microspore volume of 0.52 cm³ g−1. The Se/AC-700 cathode showed a reversible capacity of 655 mAh g−1 after 100 cycles at 0.1C in Li–Se batteries based on a carbonate electrolyte. Moreover, the Se/AC-700 cathode demonstrated excellent cyclic performance over 400 cycles without appreciable capacity decay. The main reason for the good battery performance was attributed to fast electron transfer and Li-ion diffusion in Se confined in the microporous carbon of AC-700. It is expected that this work will shed light on the development of low-cost and stable Se cathodes for high-energy Li–Se batteries.

Optimized sulfur-loading in nitrogen-doped porous carbon for high-capacity cathode of lithium–sulfur batteries

Among the candidates to the next-generation energy storage system (EES), lithium–sulfur (LiS) batteries represent an attractive option due to high theoretical specific energy. Intensively studied carbon/sulfur (C/S) composite cathodes are the key elements to determine the performance of LiS batteries. Herein, a novel in-situ nitrogen-doped porous carbon possessing ultrahigh surface area (2163.776 m2/ g) and total pore volume (1.048 cm3/ g) was obtained from hickory-shell via a transitional KOH activation process. Numerous pores in micro-scale offer substantial active sites for sulfur adsorption and accommodate the volume expansion during the charge/discharge cycles. Meanwhile, the nitrogen-doped carbon promotes polysulfides adsorption and improves the conductivity of the carbon framework. C/S composite cathodes of different sulfur loadings were investigated to exhibit that the composite with 55% sulfur delivers the highest initial discharge capacities of 1446.1 and 991.7 mAh g−1 at 0.1 and 1.0C rate, respectively, with a tiny capacity fading rate of 0.095% per cycle up to 1000 cycles at 1.0C. Considering the low cost, environmental friendliness and excellent electrochemical performances, this nitrogen-doped porous carbon exhibits great potential in applications of high performance LiS batteries.

Effect of removing silica in rice husk for the preparation of activated carbon for supercapacitor applications

Renewable biomass-derived activated carbons have attracted attentions for supercapacitor applications. Rice husk is a kind of abundant biomass, which has been studied for the preparation of hierarchical activated carbons. Rice husk is rich in silica. Silica in rice husk affects the formation of pore structure in the KOH activation process, which further affects the electrochemical performance of the resultant activated carbon for supercapacitors. In this paper, the effects of silica in rice husk on the activation process were investigated. When the silica in rice husk is not removed, KOH preferentially reacts with silica to produce silicate, which hinders the formation and enlargement of mesopores; the obtained activated carbon exhibits high microporous ratio, high specific area (up to 3263m2/g), high specific capacitance (315F/g at 0.5A/g), but poor rate capability (51.7% capacitance retention with increasing the current density from 0.5A/g to 20A/g). When the silica in rice husk is removed, the pores produced by the removal of silica are etched to mesopores in activation process; the obtained activated carbon exhibits high mesoporous ratio, relative high specific surface area of 2804m2/g and relative high capacitance of 278F/g at 0.5A/g with excellent rate capability (76.6% capacitance retention with increasing the current density from 0.5A/g to 20A/g).

Carbon felt cathodes for electro-Fenton process to remove tetracycline via synergistic adsorption and degradation

The adsorbability on the carbonaceous cathode plays an important role in electro-Fenton systems but does not obtain enough attention. In this work, various carbon felts were obtained with different adsorption property by using the method of KOH activation at different temperatures to explore the influence of adsorption on the degradation efficiency of tetracycline (TC). The results of morphology characterization, nitrogen adsorption-desorption tests, XPS and FTIR analysis revealed that the surface area of carbon felt was improved, and the oxygen-containing functional groups on the surface were increased via KOH method with significantly enhanced adsorption capacity. In the batch experiment of TC electro-Fenton degradation, the carbon felt with better adsorption capacity tended to get higher degradation and mineralization efficiency. With the KOH activation process, CF-900 (carbon felt at 900 °C for 1 h) exhibited the best TC adsorption property for the removal of TC within 30 min (initial concentration: 80 mg/L). Meanwhile, the constant of reaction rate for TC degradation at the CF-900 is 0.0648 min−1. It is higher than that (0.012 min−1) obtained at the raw carbon felt (∼5 times of enhancement). There are also synergistic effects between adsorption and degradation performance on some other organic pollutants. In addition, the degradation pathway was also studied by the methods of solid-phase extraction and high performance liquid chromatography-mass spectrometry (HPLC). From the obtained results, it is shown that good adsorbability is favorable for degradation in the homogeneous electro-Fenton system.

Preparation of Hierarchical Porous Carbon Aerogels by Microwave Assisted Sol-Gel Process for Supercapacitors.

Low-cost resorcinol formaldehyde (RF) organic aerogels were prepared by using resorcinol and formaldehyde as precursors, and sodium hydroxide as a catalyst through a single-mode microwave radiation-assisted sol-gel method and ambient temperature drying. Because of the ring focusing and power-max technology, the fabrication procedure of carbon aerogels (CAs) are much easier, faster, and cheaper than traditional methods. The RF aerogels were then pyrolysized at 900 °C, and the KOH activation process was used to further dredge micropores in the carbon aerogels. The CAs were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption/desorption, and a series of electrochemical tests. The KOH activated carbon aerogels with 3D-nano-network structure exhibited a high specific surface area of 2230 m2 g−1 with appropriate pore volumes of micro-, meso-, and macropores. The specific capacitance of CAs activated by KOH measured in a two-electrode cell was 170 F g−1 at 0.5 A g−1 with excellent rate capability and cycle stability in 6 M KOH electrolyte.

Constructed nitrogen and sulfur codoped multilevel porous carbon from lignin for high-performance supercapacitors

In this work, the lignin by-products are converted into the three-dimensional interconnected nitrogen and sulfur codoped porous carbon materials by the carbonization and KOH activation process. The as-obtained N, S-HPC-1 shows the multilevel pore structure (macro-, meso-, and micropores) with a high specific surface area of 1454.7 m2 g−1, high nitrogen and sulfur content up to 2.1 and 4.3 wt%. As a result, the N, S-HPC-1-based supercapacitor shows a high specific capacitance of 269 F g−1 at 0.5 A g−1, excellent rate performance of 62% capacitance retention at 50 A g−1, and favorable capacitance retention of 98.4% after 10 000 cycles at 5 A g−1. Furthermore, it can yield a remarkable energy density of 37.4 Wh kg−1 at a power density of 62 W kg−1. These results exhibit a low-cost and environmental strategy of the lignin utilization for electrodes of supercapacitors.

Biomass-derived ultrathin mesoporous graphitic carbon nanoflakes as stable electrode material for high-performance supercapacitors

With the motivation of materializing a high-performance electrode material for the high-energy supercapacitor, ultrathin mesoporous graphitic‑carbon was synthesized from biomass green-tea wastes via the KOH activation process combined with either of the water or the hydrochloric acid treatment. The water-treated graphitic‑carbon showed an interconnected ultrathin-nanoflake structure with a high porosity, while the hydrochloric acid-treated graphitic carbon exhibited an aggregated structure of irregular nanoparticles. The supercapacitor with an electrode of water-treated graphitic‑carbon nanoflakes displayed an enhanced specific capacitance of 162F/g at 0.5 A/g. Furthermore, the device revealed an excellent cycle stability after multiple cyclic charge-discharge operations (i.e., 121% cyclic capacitance retention over 5000cycles). These may open up a new avenue toward the recycling of biomass carbonaceous resources (e.g., green tea wastes) for inexpensive high-performance electrochemical energy-storage devices such as high-energy supercapacitors.

A Facile Synthesis of Nitrogen‐Doped Porous Carbon Materials for High‐Performance Supercapacitors

AbstractA simple way for N‐doped porous carbon materials (ANCs) was prepared in N2 atmosphere through the KOH‐assisted carbonization of commercial melamine foam (MF) attached glucose. Adding glucose and KOH activation process have great effects on improving the electrochemical properties of ANCs materials. Compared with other materials, the optimal ANCs‐800, which is obtained with a KOH/glucose/MF weight ratio of 5:20:1 at 800 °C, exhibits the best capacitance performance with the specific surface area (SSA) of 1481 m2 g−1 and high nitrogen content of 3.7 at. %. It possesses prominent specific capacitance (387 F g−1 at 0.5 A g−1) and excellent cycling stability (98.6% retention after 10,000 cycles). Furthermore, in the symmetric supercapacitor, the ANCs‐800 electrode material maintains high energy density (24.2 W h kg−1 at 701.07 W kg−1) and great cycling performance (96.3% after 10,000 cycles). Accordingly, the facile synthetic approach and attractive electrochemical properties make ANCs‐800 a popular capacitor material.

Physical and Chemical Properties of the Rice Straw Activated Carbon Produced from Carbonization and KOH Activation Processes

In this study, highly porous activated carbon was produced from rice straw by carbonization and followed by activation using potassium hydroxide (KOH). Activated carbon samples were prepared under different activation temperatures, i.e., 650, 750 and 850°C, and their physical and chemical properties were characterized accordingly. The BET surface area of the activated carbon samples was increased from 520 to 1048 m2/g with the increase of activation temperature from 650 to 850°C. These values were much higher than the non-activated rice straw carbon i.e., 1.16 m2/g. The pore sizes of the rice straw activated carbon were found to be mainly in mesopore size range of 2-50 nm. Total carbon content of the AC sample was increased from 8.35% to 31.73% with the increase of activation temperature from 650 to 850°C. XRD and Raman spectroscopy confirmed the graphite properties of the activated carbons produced. SEM images proved high porosity of the AC after KOH activation.

Nitrogen-doped nanoporous carbons derived from lignin for high CO2 capacity

In this paper, nitrogen (N)-doped ultra-porous carbon derived from lignin is synthesized through hydrothermal carbonization, KOH activation, and post-doping process for CO2 adsorption. The specific surface areas of obtained N-doped porous carbons range from 247 to 3064 m2/g due to a successful KOH activation. N-containing groups of 0.62–1.17 wt% including pyridinic N, pyridone N, pyridine-N-oxide are found on the surface of porous carbon. N-doped porous carbon achieves the maximum CO2 adsorption capacity of 13.6 mmol/g at 25 °C up to 10 atm and high stability over 10 adsorption/desorption cycles. As confirmed by enthalpy calculation with the Clausius–Clapeyron equation, an adsorption heat of N-doped porous carbon is higher than non-doped porous carbon, indicating a role of N functionalities for enhanced CO2 adsorption capability. The overall results suggest that this carbon has high CO2 capture capacity and can be easily regenerated and reused without any clear loss of CO2 adsorption capacity.