Search

In this work, a detailed comprehensive investigation explores the impact of processing methods and chemical activation parameters on the textural properties of a series of activated carbons (ACs) derived from Argan Nut Shell (ANS) residue using KOH. This study aims to develop and optimize high-quality ACs for use as solid-state supercapacitors with superior performance in a 1 M H2SO4 electrolyte. The two-step process (involving ANS carbonization followed by activation) is found to significantly enhance specific surface area and total pore volume compared to a one-step process, reaching values of 2334 m2·g−1 and 1.21 cm3·g−1 versus 1394 m2·g−1 and 0.83 cm3·g−1, respectively. The superior surface area and pore volume results are achieved at a KOH/char mass ratio of 4:1, carbonization temperature of 500 °C, and an activation temperature of 800 °C, reaching 3091 m2·g−1 and 1.52 m3·g−1, respectively. Evaluation of electrochemical properties in a two-electrode system with 1 M H2SO4 electrolyte demonstrates outstanding capacitance and maximum specific energy at a current density of 125 mA·g−1, reaching up to 416 F·g−1 and 14.5 Wh·kg−1 at a specific power of ~120 Wkg−1. This superior performance is attributed to the ACs' high surface area, well-developed microporosity, and favorable surface chemistry. Additionally, the ACs demonstrate exceptional cycling stability, as they retain 99 % of their specific capacitance even after undergoing 2500 charge and discharge cycles at a rate of 1 A·g−1. This work offers a novel strategy for reclaiming Argan Nutshell, offering a sustainable approach for electrode materials in solid-state supercapacitors.

Microporous activated carbons with remarkably high CO2 uptake have been synthesized using polyurethane (PU) commercial foams as precursors. The PU samples were pyrolyzed at different temperatures in N2 atmosphere and chemically activated with KOH. Adsorption isotherms at 0 °C and 25 °C and immersion calorimetry were applied to provide a deeper knowledge about the most relevant factors implied in CO2 capture process explaining, hence, the values of adsorption obtained. CO2 adsorption uptakes reached values of 7.4 mmol/g at 0 °C, 5.0 mmol/g at 25 °C and 2.7 mmol/g at 50 °C. The remarkably high values of CO2 uptake are closely related with the BET specific surface area of the carbon foams but also with the high volume of narrow micropores all the samples present a high volume of micropores (between 0.59 and 0.71 cm3/g) as also confirmed by immersion calorimetry.

Although high-surface area activated carbons used as supercapacitor (SC) electrodes are frequently produced by KOH activation, this study shows that, when aqueous electrolytes are used, CO2 activation is a better choice from the point of view of SC performance, environment and economy. Ordered mesoporous carbons (OMCs) produced by a mechanochemical synthesis method from mimosa tannin are activated with KOH to use these materials as electrodes for SCs. A comparative analysis of the same OMCs but activated with CO2 is presented to examine the effect of the activation process on materials performance. KOH-activated materials exhibit good electrochemical performance at low charging rates, reaching specific cell capacitance values of 49 F g−1 at 0.5 Ag−1, however, restricted access to microporosity and low water affinity water reduces their performance at high charging rates. In contrast, the best performing CO2-activated material can retain 81% of capacitance at 20 A g−1, compared to 25% for a KOH-activated OMC with similar properties and tested under the same conditions. A thorough review of the open literature suggests that CO2 activation would produce materials with a suitable combination of pore network connectivity and water affinity, resulting in SCs with high rate capability in an aqueous electrolyte. These conclusions were drawn by judiciously integrating the analysis of: (i) the hysteresis loop scanning of N2 adsorption-desorption isotherms; and (ii) water adsorption isotherms as tools to more accurately assess the pore network connectivity and water affinity of the materials, which are not generally considered when studying SC performance.

Biomass from agricultural by-products is gaining increasing interest as cheap and abundant precursor in the development of active materials for efficient and environmentally friendly devices like fuel cells. Herein, we investigated iron and nitrogen co-doped nanoporous carbons derived from aronia, peach stones and coal tar pitch/furfural as electrocatalysts for the electrochemical oxygen reduction reaction (ORR) in alkaline media. Urea was used as nitrogen precursor and two annealing steps with intermediate acid leaching served to activate the catalysts. Within the series, the peach stone-derived catalyst exhibited a catalytic activity for the ORR close to the benchmark Pt/C, with a 60 mV dec−1 Tafel slope upon the incorporation of 0.57 wt% Fe and proper combination of N-Fe species (20%) with pyridinic/pyridonic moieties (49%). We concluded that the microporosity and a certain content of meso/macro-pores of the activated carbon, together with the creation of graphitic domains result in a high relative amount of Fe-N4 and nitrogen functionalities, which determine the electrocatalytic performance.

In this paper, lignin-activated carbon xerogels from the black liquor of the paper industry were synthesized by the sol-gel polycondensation of resorcinol and lignin with formaldehyde, NaOH, and water. The sol-gel was prepared at three Lignin/(Lignin+Resorcinol) mass ratios of 0, 13, and 27%. A portion of sol-gel was functionalized with a no-common crosslinking agent, cyanuric chloride (CC) saturated in 1,4-dioxane, to introduce some nitrogen functional groups on the surface. The sol-gels non- and functionalized were dried and activated with H3PO4 at a mass ratio of 1:1 at 450 °C. After the functionalization with the CC and activation process, the superficial nitrogen content percentage increased to 7 wt%, where the most prevalent functionality was N-pyrrole. The sample with a high percentage of nitrogen groups did not show the best electrochemical behavior (112 F g−1 at 0.125 A g−1). However, increasing lignin content up to 27% without functionalization improved the gravimetric capacitance value of 149.5 F −1. These results are mainly attributed to electrostatic effects and the high presence of phosphorus functionalities introduced after the activation process that enhance the surface wettability of the electrodes. Furthermore, the analysis of the variance confirmed that the CC impregnation process did not favor gravimetric capacitance. Lignin can be considered as an excellent substitute for resorcinol partially to produce bio-based activated carbon xerogels and use them as electrodes for supercapacitors and reduce production costs.

The mechanisms behind enhanced charge storage of P-functionalized carbons are unraveled for the first time using non-porous graphene oxide treated with phosphoric acid and annealed at either 400 or 800 degrees C. The electrochemical study in 1 M H2SO4 reveals that phosphorus groups boost charge storage and electrochemical stability, with more effect for the higher annealing temperature. Annealing at 800 degrees C also leads to the material withstanding 60,000 charge-discharge cycles with no capacitance loss at 1.5 V. The improvement in the electrochemical performance is shown to be mainly governed by the change in surface chemistry comprehensively studied with NMR, FTIR and XPS characterization techniques. The collective analysis of electrochemical response and surface chemistry demonstrates that enhanced charge storage by phosphorus-functionalized graphene materials is made possible due to the following synergistic mechanisms: i) non-Faradaic charging; ii) nascent hydrogen storage in the interlayer; iii) benzoquinoneto-hydroquinone redox processes; iv) phosphate-to-phosphonate like transformation. From the practical perspective, the stored charge can be boosted due to the higher capacitance upon prior electrochemical activation in the vicinity of oxygen evolution potential and the wider usable electrochemical window enabled by phosphorus-related groups. (C) 2020 The Author(s). Published by Elsevier Ltd.

The effect of heteroatoms on hydrogen adsorption properties of activated and hybrid carbon materials is critically described. For that purpose, olive stones were activated chemically with KOH, and subsequently washed or not, and oxidised with ozone or not. Olive stones were also activated physically with CO2. A series of activated carbons prepared by chemical activation of sucrose was also investigated for comparison. As a result, many activated carbons with different pore-size distributions, surface areas, average micropore widths, oxygen contents and amounts of mineral matter could be compared. All were thoroughly characterised by adsorption of N2, CO2 and H2O, elemental analysis, XPS, thermogravimetry, and adsorption of H2 at different pressures. Many correlations between textural parameters, composition and adsorption properties could be evidenced, and were critically discussed. We show that the hydrogen uptake at 77 K is controlled by the following parameters, listed by decreasing order of importance: specific surface area, average micropore size, surface chemistry and shape of the pore size distribution. At room temperature (i.e., at 298 K), the adsorbed hydrogen uptake was in the range of 0.19–0.42 wt %; the presence of large amounts of alkali metals can further improve the hydrogen adsorption properties, but surface chemistry still has a major influence, especially through the acidic surface functions.

A high-surface area activated carbon material (PPAC) prepared from a petroleum residue has been used as a host structure to promote the nucleation and growth of confined methane hydrates after the incorporation of additives. Sodium dodecyl sulfate (SDS), Leucine and Tetrahydrofuran (THF) have been evaluated either dissolved in the pre-impregnation media (ultrapure water) or incorporated in the carbon surface using a mechanochemical approach. High-pressure methane adsorption isotherms show that the mechanochemical approach does not improve the storage performance of the original carbon material due to the steric effects by the bulkier organic functionalities. Furthermore, the incorporation of the additives shifts the nucleation in narrow pores to higher pressures. On the contrary, when the additives are dissolved in water, high-pressure methane isotherms anticipate a modification in the kinetics and/or thermodynamics of the nucleation process. Whereas gas hydrates grown in large pores and/or the external surface exhibits a significant reduction in the induction time, the nucleation process in narrow micropores becomes more uniform and occurs in a narrower pressure window, slightly shifted to higher pressures. However, no changes in the total uptake at 10 MPa could be found. The situation is different when THF is dissolved in water. In this case, the nucleation takes place at pressures below the natural process (<3–4 MPa), the total amount of methane stored at 10 MPa being slightly limited. In-situ synchrotron X-ray diffraction studies anticipate a sI structure for the pure water methane hydrates, while a combination of sI and sII is identified for the THF-based system.

Hydrothermal carbonization (HTC) is considered an attractive and constantly expanding eco-friendly methodology for thermochemical processing of different types of biomass in energy, carbonaceous materials, structured hybrids, and other chemical products. However, some challenges for the technique to be adequate in clean production need to be overcome. In this way, a comprehensive review is presented regarding the process variables, fundamentals, properties, possible uses of the products obtained and the challenges for cleaner production. This paper stands out in relation to those available in the literature when gathering exclusive information on hydrothermal carbonization with a focus on lignocellulosic biomass, the possibility of reusing the wastewater from HTC in recirculation processes and the challenges of the technique aiming at the sustainability of the process. Initially, we discuss the general concepts and reactional mechanisms involved in the methodology and then it explored the physical-chemical properties and possible applications of hydrochar, the modification methodologies and activation processes to obtain activated carbon in an eco-friendly approach. Current knowledge gaps and the overcoming of challenges such as the recirculation of the aqueous phase of hydrothermal carbonization are also discussed together with new perspectives and alternatives for the technique based on a biorefinery system, in order to enable a wider exploration of hydrothermal carbonization of its great potential in a cleaner production concept.

Activated carbons, with different surface chemistry and porous textures, were used to study the mechanism of electrochemical hydrogen and oxygen evolution in supercapacitor devices. Cellulose precursor materials were activated with different potassium hydroxide (KOH) ratios, and the electrochemical behaviour was studied in 6 M KOH electrolyte. In situ Raman spectra were collected to obtain the structural changes of the activated carbons under severe electrochemical oxidation and reduction conditions, and the obtained data were correlated to the cyclic voltammograms obtained at high anodic and cathodic potentials. Carbon-hydrogen bonds were detected for the materials activated at high KOH ratios, which form reversibly under cathodic conditions. The influence of the specific surface area, narrow microporosity and functional groups in the carbon electrodes on their chemical stability and hydrogen capture mechanism in supercapacitor applications has been revealed.

This work addresses two environmental issues of major concern: hydrogen storage for hydrogen economy implementation and CO2 capture to reduce greenhouse gas emissions. For these purposes, two granular activated carbons were synthesized through chemical activation of olive stones by means of potassium salts (KOH and K2CO3). The porosity characterizations reveal typical ultramicroporous carbons with average pore sizes of about 0.53 and 0.69 nm for K2CO3 and KOH-activated carbons, respectively. The volumetric measurements of cryogenic hydrogen adsorption show monolayer process. At sub-atmospheric pressures the narrower micropores show stronger binding energy to hydrogen molecules. However, at higher pressures this porosity range saturates and KOH-activated carbon exhibits a H2 storage capacity of 3 wt%, 70% of which is achievable at only 1 bar. CO2 shows a similar behavior than H2 when it was adsorbed purely at 0 °C, and AC_KOH retains its excellence with a capacity of 5.6 mmol g−1at 1 bar. Finally, the two carbons were tested as CO2 adsorbents in conditions representative of post combustion capture applications (10% CO2 at atmospheric pressure and at 50 °C). Both carbons show fast adsorption-desorption kinetics, perfectly described by pseudo-first order model. At these conditions, it was proven that only narrow micropores are essential for CO2 adsorption.

Coconut shell-based activated carbons intended for CH4 storage were prepared by chemical activation with H3PO4 or ZnCl2 and/or physical activation with CO2. Efforts were focused to establish relationships among the activation procedure, the textural and physical-chemical properties of the resulting materials and their volumetric CH4 uptake. The best results were achieved by using the combination of a relatively soft chemical activation with H3PO4 or ZnCl2, just the sufficient to minimize the presence of macropores originated from the botanical structure of the precursor, followed by physical activation with CO2 to develop a narrow pore size distribution mainly located around the optimum pore diameter/width for CH4 storage. This combined methodology permitted to obtain excess volumetric CH4 adsorption capacities as high as 94, 148 and 145 V/V for granular, powdered/compressed and monolithic carbons, respectively. The corresponding total CH4 storing capacities were estimated to be 102, 165 and 163 V/V. It is worthy to highlight that the precursor was a low cost and abundant agricultural by-product, which is very important for large-scale applications such as gas transportation and gas storage in vehicular fuel tanks.

Nitrogen functionalization (ca. 4 at. % NXPS) of a highly microporous activated carbon (SBET > 3000 m2/g) has been achieved by two different approaches at mild conditions: (i) oxidation and post-reaction with nitrogen reactants, and (ii) direct reaction of pristine carbon material with nitrogen reactants. Interestingly, the introduction of nitrogen functionalities allows full preservation of the microporosity when pathway (ii) is followed. The electrochemical performance of the carbon materials as electrodes for supercapacitors was evaluated by using symmetric and asymmetric configuration in 1 M H2SO4. Both nitrogen-functionalized carbon materials showed larger stability and energy efficiency than the pristine carbon material when working at 1.4 V. The non-oxidized and functionalized activated carbon evidences the best performance as electrode for supercapacitor, providing energy and power density of 14.5 Wh/kg and 61.2 kW/kg and keeping 83% of original capacitance after 5000 charge–discharge cycles. This improvement is related to the presence of surface nitrogen functionalities that provide a higher electrochemical stability, avoiding the formation of detrimental oxygen groups during the operation of the supercapacitor.

Two petroleum residues were pyrolyzed under two different conditions to obtain pitches with low or high mesophase content. The effect of the KOH: precursor ratio and the activation temperature on the packing density and porous texture of the carbons have been studied and optimized. Activated carbons combining high micropore volume (>1 cm3/g) and high packing density (0.7 g/cm3) have been successfully prepared. Regarding excess methane adsorption capacities, the best results (160 cm3 (STP)/cm3 at 25 °C and 3.5 MPa) were obtained using the pitch with the higher content of the more organized mesophase, activated at relatively low temperature (700 °C), with a medium KOH: precursor ratio (3:1). Some of the activated carbons exhibit enhanced adsorption capacity at high pressure, giving values as high as 175 cm3 (STP)/cm3 at 25 °C and 5 MPa and 200 cm3 (STP)/cm3 at 25 °C and 10 MPa (the same amount as in an empty cylinder but at half of the pressure), indicating a contribution of large micropores and narrow mesopores to adsorption at high pressure. The density of methane in pores between 1 and 2.5 nm at pressure up to 10 MPa was estimated to understand their contribution to the total adsorption capacity.

This paper compares the importance of the texture and surface chemistry of waste tyre activated carbons in the adsorption of commercial dyes. The adsorption of two commercial dyes, Basic Astrazon Yellow 7GLL and Reactive Rifafix Red 3BN on activated carbons made up of reinforcing fibres from tyre waste and low-rank bituminous coal was studied. The surface chemistry of activated carbons was modified by means of HCl–HNO3 treatment in order to increase the number of functional groups. Moreover, the influence of the pH on the process was also studied, this factor being of great importance due to the amphoteric characteristics of activated carbons. The activated carbons made with reinforcing fibre and coal had the highest SBET, but the reinforcing fibre activated carbon samples had the highest mesopore volume. The texture of the activated carbons was not modified upon acid oxidation treatment, unlike their surface chemistry which underwent considerable modification. The activated carbons made with a mixture of reinforcing fibre and coal experienced the largest degree of oxidation, and so had more acid surface groups. The adsorption of reactive dye was governed by the mesoporous volume, whilst surface chemistry played only a secondary role. However, the surface chemistry of the activated carbons and dispersive interactions played a key role in the adsorption of the basic dye. The adsorption of the reactive dye was more favored in a solution of pH 2, whereas the basic dye was adsorbed more easily in a solution of pH 12.

Tyre wastes and their blends with coal and a bituminous waste material obtained from the benzol distillation column of the by-product section of a coking plant were employed as a precursor for the production of activated carbons (ACs). Pyrolysis up to 850 °C followed by physical activation with CO2 yielded mesoporous carbons with different pore size distributions and surface areas depending on the degree of burn-off. ACs with surface areas of 475 and 390 m2/g were obtained for the two tyre wastes. The inclusion of coal in the blend gave rise to surface areas of up to 1120 m2/g due to an increase in the microporosity. The time needed to obtain the desired degree of burn-off depended on the reactivity of the char. The coal-containing materials required the longest amount of time. The surface chemistry of the samples was studied by Infrared spectroscopy (FTIR) and X-Ray photoelectron spectroscopy (XPS). The principal oxygenated groups found were quinones, lactones and carboxylic acids.

Tyre wastes and their blends with coal and a bituminous waste material obtained from the benzol distillation column of a by-product section of a coking plant were employed as a precursor for the production of activated carbons (ACs). Pyrolysis up to 850 °C followed by physical activation with CO2 produced mesoporous carbons with different pore size distributions and surface areas. The surface chemistry of the samples was studied by measuring the point of zero charge (pHpzc) and by temperature programmed desorption (TPD). The activated carbons obtained contained higher amounts of basic functional groups. Their textural and surface chemistry characteristics make them highly suitable for adsorbing anionic dyes of large molecular size, such as Congo red. The adsorption kinetics was found to conform closely to the pseudo-second-order kinetic model. To determine the adsorption mechanism, the kinetic data were also analyzed using the Weber and Morris intraparticle diffusion model and the Boyd model to distinguish between the pore and film diffusion steps. The equilibrium isotherms were of the Langmuir isotherm type. The efficiency of the low-cost ACs prepared for the removal of Congo red dye was similar to that reported in the literature for coal-based ACs and greater than that of other low-cost ACs.

The aim of this study is to understand the different sorption behaviors of mercury species on activated carbons in the oxy-fuel combustion of coal and the effect of high quantities of water vapor on the retention process. The work evaluates the interactions between the mercury species and a series of activated carbons prepared from a macroalgae waste (algae meal) from the agar–agar industry in oxy-combustion atmospheres, focussing on the role that the high concentration of water in the flue gases plays in mercury retention. Two novel aspects are considered in this work (i) the impact of oxy-combustion gases on the retention of mercury by activated carbons and (ii) the performance of activated carbons prepared from biomass algae wastes for this application. The results obtained at laboratory scale indicate that the effect of the chemical and textural characteristics of the activated carbons on mercury capture is not as important as that of reactive gases, such as the SOx and water vapor present in the flue gas. Mercury retention was found to be much lower in the oxy-combustion atmosphere than in the O2 + N2 (12.6% O2) atmosphere. However, the oxidation of elemental mercury (Hg0) to form oxidized mercury (Hg2+) amounted to 60%, resulting in an enhancement of mercury retention in the flue gas desulfurization units and a reduction in the amalgamation of Hg0 in the CO2 compression unit. This result is of considerable importance for the development of technologies based on activated carbon sorbents for mercury control in oxy-combustion processes.

The main environmental problem caused by the production of energy from coal combustion is the emission of CO2. One emerging technology designed for CO2 capture is oxy-combustion. Among other issues to be solved in oxy-combustion power plants is the presence of mercury as this may damage the CO2 compression unit. Hence the study of the behavior of mercury in oxy-combustion is of great interest both from an environmental and a technological point of view. The present study performed at laboratory scale evaluates the retention of mercury in a CO2-enriched flue gas using the same activated carbon before and after it has been impregnated with sulfur and proposes a mechanism to explain the interactions between mercury and activated carbons. The results show that carbonyl and quinone groups are responsible for mercury oxidation and retention in the carbons. Although the contact time between mercury and the activated carbon surface limits the amount of mercury that can be captured, high retention capacities can be achieved in an oxy-combustion atmosphere. The presence of water, in high concentrations in oxy-combustion, may compete for the same active sites (carbonyl groups) as mercury, thereby inhibiting mercury adsorption on the surface of the activated carbons. Moreover, the presence of sulfur in the impregnated material, which is the key to mercury capture in other atmospheres, does not modify mercury capture in oxy-combustion.

The long term performance of semiconductor-free activated carbons showing photochemical activity was explored by monitoring the photodegradation of phenol from aqueous solution along 20 h of illumination in consecutive photocatalytic cycles. The efficiency of the process was evaluated in terms of phenol conversion, mineralization degree and evaluation of degradation intermediates upon cycling. Data showed a strong dependence of the photooxidation efficiency on the hydrophobic/hydrophilic nature of the carbons. The outstanding role of dissolved oxygen as a promoter of phenol photodegradation through the formation of O-radicals upon illumination of the carbons was also demonstrated. The excess of oxygen not only improved phenol conversion and mineralization, but delayed the clogging of the carbon’s porosity upon cycling. This is important since a fraction of the photooxidation reaction also takes place inside the porous network of the carbon materials. Overall, the performance of the activated carbons, especially in conditions of excess of oxygen, is comparable to that of commercial titania.