Search

The important role of mesopores has been investigated in electric double-layer capacitors (EDLCs) operating from 24 °C down to − 40 °C by using two in-house synthesized carbons with hierarchical porosity. These carbons were prepared from colloidal nanoparticles of SiO2 as the template and d-glucose as the carbon source. A decrease in the average diameter of the nanoparticles from 12 to 8 nm results in increased surface area and offers a perfect match between ions of binary mixture of imidazolium-based fluorinated ionic liquids and the pores of carbon. Short-range graphene layers produced with 8-nm silica nanoparticles lead to the creation of transport channels which better accommodate ions. We explain these findings per coulombic interactions among the ions and between the pore wall and the ionic species under confinement and electrochemical polarization conditions. Further, it is shown that a microporous carbon (another in-house produced rice-husk carbon SBET = 1800 m2∙g−1) performs better than hierarchical carbons at room temperature; however, thanks to the large fraction of mesopores, the latter exhibit far higher capacitance down to − 40 °C. While the ordering of ions in confinement is more critical at room temperature and dictated by the micropores, low temperature performance of supercapacitors is determined by the mesopores that provide channels for facile ion movement and keep the bulk ionic liquid–like properties. Graphical abstract

For the Republic of Kazakhstan, like for any country with a developed mining and processing sector, it is important to develop knowledge to improve methods and technologies for the complex processing of raw materials, including those for the more complete extraction of precious metals. This is needed due to the high loss of precious metals during their processing and separation and the need to improve the process of their concentration. Advances in the field of carbon nanomaterials offer great prospects for improving existing technologies for the extraction of precious metals from waste solutions and pulps. The goal of the work is to conduct comprehensive studies on the influence of the flow rate of solutions, pH, and the presence of ions of other metals on the extraction of gold on a carbon nanostructured material from rice husk (with its further regeneration and reuse). The effect of the pH of the solution on the degree of extraction of gold(III) ions is studied, it is found that the highest recovery for gold ions is observed at pH ~ 2. The selectivity of gold extraction is established in the combined presence of copper, nickel, and silver. The dependence of the electrochemical reduction sorption of gold on the flow rate of solutions is investigated. It is found that the optimal flow rate of solutions is 10 mL/min. The sorption capacity of the sorbent based on carbonized rice husk is calculated. The investigation into the electrochemical sorption/desorption of gold(III) ions showed that the desorption process proceeds better in an acetone/water/NaOH mixture; the degree of desorption is 96%, which demonstrates the possibility of regeneration of the carbon material of the electrode for reuse. The results can be applied to optimize the processes of extraction of precious metals from their solutions.

Controlling the porosity of carbon-based electrodes is key toward performance improvement of charge storage devices, e.g., supercapacitors, which deliver high power via fast charge/discharge of ions at the electrical double layer (EDL). Here, eco-friendly preparation of carbons with adaptable nanopores from polymers obtained via microwave-assisted cross-linking of poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP) is reported. The polymeric hydrogels possess porous and foam-like structures, giving excellent control of porosity at the precursor level, which are then subjected to activation at high temperatures of 700–900 °C to prepare carbons with a surface area of 1846 m2 g–1 and uniform distribution of micro-, meso-, and macropores. Then, graphene as an additive to hydrogel precursor improves the surface characteristics and elaborates porous texture, giving composite materials with a surface area of 3107 m2 g–1. These carbons show an interconnected porous structure and bimodal pore size distribution suitable for facile ionic transport. When implemented in symmetric supercapacitor configuration with aqueous 5 mol L–1 NaNO3 electrolyte, a capacitance of 163 F g–1 (per average mass of one electrode) and stable evolution of capacitance, coulombic, and energy efficiency during 10 000 galvanostatic charge/discharge up to 1.6 V at 1.0 A g–1 have been achieved.

Nanoporous carbon materials are well recognized as the main components of electrodes in capacitive deionization. Herein, the activated carbons were produced based on rice husk which is an abundant waste material in southern regions of Kazakhstan. The resulting carbons were characterized electrochemically by comparing their performance with well-known brands of commercial porous carbons (i.e. Norit DLC Super 30, Kuraray YP 50F). The features of carbon/ carbon electrochemical cells were analyzed using the means of galvanostatic cycling with potential limitation and cyclic voltammetry. Whilst the surface morphology and elemental composition of carbons were observed using scanning electron microscopy combined with energy dispersive X-ray spectroscopy. Using the method of low-temperature nitrogen adsorption it has been established that the specific surface of home-made carbon produced based on rice husk is equal to 2290 m2g-1. The salt adsorption analysis has been performed using different concentrations of inlet solutions of sodium chloride. Our study has shown that the manufacturing and application of activated carbons based on rice husk can be highly efficient because the resulting electrode materials exhibit a high electrosorption capacity of 20.02 mg g-1, which exceeds similar values obtained in the case of application of commercial porous carbons.

The effect of ionic liquids (ILs) confinement in the nanoporosity of carbons on their low temperature transitions has been studied by differential scanning calorimetry. The ILs consisted of the [EMIm+] cation and [FSI−], [TFSI−] or [BF4−] anions. The carbons were strictly microporous (Maxsorb) and mesoporous with a small amount of micropores, i.e. a MgO templated carbon TC-1 (Lmeso = 3.4 nm), a silica templated carbon TC-2 (Lmeso = 8.7 nm), and a high surface area carbon black SC2A (Lmeso = 9.6 nm). All ILs encapsulated in the microporous Maxsorb demonstrated “flat” low temperature thermograms typical of an adsorbed state. In the narrow mesopores of TC-1, the interactions between the ions and the pore walls constrained the molecular motions, leading to a glass transition upon heating. The three confined ILs in SC2A displayed a melting peak at lower temperature (ΔTm ∼ -30 °C) than their neat counterpart. The ΔTm shift was comparable for TC-2 soaked with [EMIm+] [FSI−], whereas only a glass transition was given by [EMIm+] [TFSI−] and [EMIm+] [BF4−] confined in this carbon. Overall, the work shows that the liquid state of ILs can be extended to lower temperatures by their confinement in carbons possessing relatively wide mesopores.

This paper reports a study of the kinetics of the initial stages of electroreduction of AuCl 4 - in the case of gold electrosorption using activated carbon. The activated carbon was synthesized from rice husk by the chemical activation. Obtained sorption materials were characterized by Raman Spectroscopy, Transmission electron microscopy, Scanning electron microscopy and SORBTOMETR-M techniques. The kinetics of electrodeposition of gold (III) on platinum from chloride electrolytes by the cyclic voltammetry and chronoamperometry methods have been studied. Chronoamperometric measurements were performed at the potential of +0.2 V vs. Ag/AgCl with varying gold concentration and temperature. Diffusion coefficient of Au3+ ions determined by the cyclic voltammetry method on the basis of the Randles-Sevcik equation is in good agreement with the value determined by the chronoamperometry using the Cottrell law.

The adsorption behavior of Au3+ ions on metal electrodes has been studied using an electrochemical quartz crystal microbalance combined with the cyclic voltammetry technique. The experiments were carried out for HAuCl4 using 0.1 mol·L-1 HCl (pH~1) as a background electrolyte solution. The kinetics of electroreduction of Au3+ ions on the rice husk based activated carbon and gold electrodes in chloride electrolytes by the cyclic voltammetry and the electrochemical quartz crystal microbalance with a variation of the scan rate in the range of 5‒50 mV·s-1 has been studied. The diffusion coefficient of Au3+ ions for the tested solution on gold and carbon electrodes was determined by the cyclic voltammetry method on the basis of the Randles-Ševčik equation. It is found that electroreduction of gold goes via the discharge of AuCl4- complexes to the formation of metallic gold with a current efficiency of 97‒99%. The scanning electron microscopic images of the gold adsorbed carbon surface was taken to see gold particles and their morphology. In SEM images, it is clearly seen that the surface of carbon has a relief structure and gold has grown in the form of clusters. The smallest gold nanoparticles that could be examined were 100‒250 nm in diameter on the surface of the c arbon electrode.

Considering the cost-effectiveness, safety, and environmental friendliness for energy storage and delivery at high rates, hybrid electrochemical capacitors in aqueous electrolytes containing redox-active species are attractive alternatives to expensive organic electrolyte based electric double-layer capacitors (EDLCs). Here, the influence of electrode mass-balancing on the equilibrium potential of hybrid cells in aqueous sodium nitrate + sodium iodide (5 mol L−1 NaNO3 + 0.5 mol L−1 NaI) has been investigated. The shift of equilibrium potential determines, whether the positive electrode behaves fully battery-like (charge/discharge strictly in the iodide redox potential range) or shows a mixed battery-like and EDL capacitive behavior. With an appropriate mass-balancing of the positive and negative electrodes (mass ratio = 1:2), the equilibrium potential shows a negligible shift during galvanostatic charge/discharge cycles at 0.5 A g−1, which results in full battery-like behavior of the positive electrode. Consequently, the hybrid cell exhibits stable electrochemical performance. By contrast, an equal or higher mass of the positive compared to the negative electrode, leads to a shift of the equilibrium potential resulting in two different charge storage mechanisms at the positive electrode. As a result, the overall performance of the hybrid cell deteriorates. We show by thermogravimetric analysis and Raman spectroscopy that the formation of polyiodides (I3− and I5−) is controlled by the oxidation of iodide (I−) anions to molecular iodine in nanoporous carbon based positive electrode, and that more polyiodides are produced, if the positive electrode operates strictly within the iodide/iodine redox potential range.

Activated carbons (ACs) have been prepared from chemical treatment of walnut shells (WS) precursor at various temperatures (400‒800 °C) by using phosphoric acid (H3PO4) as activating agent. Influence of activation temperature on the porosity development and capacitive properties of resulting carbons was investigated. Thermal post-treatment of carbons previously activated at moderate temperature, e.g. 400 °C allowed further structural and porosity modification. Then, these carbons were investigated by scanning electron microscopy, Raman spectroscopy, energy-dispersive X-ray spectroscopy, electrochemical techniques and low temperature nitrogen adsorption exhibiting high BET specific surface area of approximately 2100 m2 g-1 and a total pore volume up to 1.3 cm3 g-1. Carbon material obtained through activation by H3PO4 at 400 °C and post-treated at 800 °C was used to make electrodes which were implemented to realize AC/AC capacitor using 1 mol L-1 Li2SO4. The electrochemical capacitor demonstrated high capacitance of 123 F g-1 per mass of one electrode, reduced cell resistance and stable capacitance for 5000 galvanostatic charge/discharge cycles at 1.0 A g-1.

We report on electrical double-layer capacitors (EDLCs) operating effectively at low temperature (down to −40 °C) while implementing nanoporous carbon electrodes and an ionic liquid (IL) electrolyte. For this purpose, the binary mixture of [EMIm][FSI] and [EMIm][BF4] in 1:1 mol ratio has been selected, since it has been previously shown that it remains liquid down to its vitrification at −97 °C, unlike the parent ILs with melting points of −13 °C and 14 °C, respectively. To enhance the mass transport of the IL ions, especially at a low temperature, where the IL electrolyte exhibits a high viscosity, carbon electrode materials with a substantial share of mesopores serving as passageways for bulky ions were selected. These were a carbon black (SC2A) with a broad range of interparticle mesopores in addition to a reasonable amount of micropores, as well as a home-made templated carbon (MP98B) with hierarchical porous texture made of interconnected micropores and well-defined mesopores. From 20 to −40 °C, the capacitor with MP98B electrodes displayed greater specific capacitance, energy and power as well as better charge propagation than the cell with SC2A, whereas on volumetric basis its performance was inferior due to lower electrode density. Advanced SCMP composite electrodes of intermediate density were formulated by mixing these two carbons in the 1:1 mass ratio and harnessed to realize EDLCs with enhanced both specific and volumetric energy. Owing to the adjusted electrode properties, the performance of SCMP-based EDLCs surpassed literature data obtained with analogous constructions based on IL electrolytes and lightweight mesoporous materials, which suffered from poor volumetric metrics.

The kinetics of a strongly nonequilibrium system of electrons and holes excited in QDs by a ~30 ps laser pulse are studied by a time-resolved photoluminescence method. The excitation of electrons near the 1Pe states and holes near the 1P3/2 states is used. Three bands of the transient photoluminescence spectra were identified, and the rise and decay of each of them was investigated. The complex picture of the time evolution of the bands is explained by the fast processes of cascade intraband relaxation and the Pauli exclusion principle. The durations of formation and decay for each of the photoluminescence bands are determined.

Results of the study of micro-mesoporous carbonaceous materials with characteristic turbostratic carbon structure and specific surface area of 1000 - 2000 m 2 /g were shown. These materials were derived by use the method of chemical activation of plant fiber represented by apricot and walnut shells, and also by the rice husk. Derived carbon-containing samples were used to manufacture composite electrode materials, which were subsequently subjected to electrochemical testing in the composition of capacitors with a double electric layer. The possibility of using these electrode materials assembled as a part of composite electrodes of supercapacitors was studied; the maximum specific electric capacity of which amounted to 160 F/g.

Results of applying the method of thermo-oxidative modification of fiber, based on the shell of apricot along with producing on its base microporous activated carbons that have high specific surface area and a significant amount of micropores were presented. The paper contains analysis and interpretation data of changes in the structure and composition of samples, which occurring as a result of thermal degradation of lignocellulosic materials. Morphological features of the surface of produced activated carbons were studied by using SEM microscopy; the pore structure and specific surface area were investigated using the method of low-temperature nitrogen adsorption.

This article presents the results of the synthesis of carbon nanomaterials: Nanoscale materials obtained by carbonization of waste agricultural products (apricot kernel, Greek walnut (GW), rice husk). The results of physico-chemical characteristics of the obtained nanomaterials. Physico-chemical parameters of the synthesis of carbonized sorbents based on plant raw material are investigated along with the properties of these sorbents. The data of FTIR, ESR spectroscopy & BET-method, as well as electron microscopy are reported. It is stated that carbonized sorbents possess high specific surface area and porosity. Carboxylic, carbonyl, hydroxyl groups are detected on the surface of the synthesized sorbents. Separate fusicoccine and similar biostimulators effectively; remove LPS-endotoxines from blood plasma selectively. They may be used as carriers to introduce probiotics into intestine thanks to formation of stable colonies on their developed surface. A method of preparation of honeycomb monoliths from carbonized rice husk with developed mesoporous structure via modification of the porous structure by silica leaching has been developed.