Sanja Krstić

Flexible and lightweight supercapacitors are optimal power supplies for portable electronic devices. One of their key components is the flexible current collector, which should meet several requirements: high conductivity to minimize internal resistance while remaining light and improved mass specific capacitance. Carbon non-woven fabric (CNW) is a promising flexible current collector due to its exceptional flexibility, mechanical stability, light weight, ease of fabrication and cost efficiency. In this work, graphene, as widely recognized supercapacitive material, was deposited on CNW and the charge storage properties of the resulting composite were investigated. The electrochemical properties of neat CNW and the CNW/graphene composite were investigated in a 0.5 M Na2SO4 electrolyte in a standard three-electrode cell, using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV measurements were performed at sweep rates of 200, 100, 50, and 10 mV s⁻¹, while EIS was carried out at open circuit potential over a frequency range of 100 kHz to 10 mHz. The results obtained are characteristic of capacitive materials in which charge storage occurs through the electric double-layer capacitance mechanism. The enhanced capacitive response of the CNW/graphene composite, compared to neat CNW, is attributed to the high electrical conductivity and large specific surface area of graphene, which facilitate more efficient charge accumulation at the electrode/electrolyte interface. These findings demonstrate that CNW is a promising flexible current collector for graphene-based capacitive applications.

Brassinosteroids (BRs) are a class of plant hormones that play important roles in regulating various physiological and developmental processes in plants. One of the most effective BRs involved in modulating crop growth is 24-epibrassinolide (24-EBL). The effects of different concentrations of 24-EBL on various biochemical and biophysical parameters critical to early growth stages and seedling development were investigated using two maize hybrids, ‘ZP 434’ (a new-generation hybrid) and ‘ZP 704’ (an older-generation hybrid). The evaluation of results is based on measurements of germination percentage, morphometric parameters, redox status, comparative analysis of thermodynamic parameters (such as Gibbs free energy, enthalpy, entropy), and the concentration of specific sugars in different parts of maize seedlings. The results indicate that the germination and initial growth of maize seedlings are influenced by the flow of crucial sugars from the remaining seed (as a source of nutrients) towards the plumule and radicle (as sink organs). Furthermore, alterations in Gibbs free energy play a significant role in these sugar transfers within the maize seedlings. The seed germination was most affected by the highest concentrations of 24-EBL, showing inhibitory effects, whereas lower and moderate concentrations of exogenously added 24-EBL exhibited a beneficial influence on the initial phases of seedling growth. The mentioned approach gives new insights into source–sink relationships and can be used as a quantitative measure of the germination energy, which until now has been a qualitative criterion in seed science.

Thermo-chemical conversion of cigarette butt filters (CBFs) waste was investigated using various thermal analysis techniques (simultaneous TG-DTG-DTA and DSC methods) at different heating rates in an inert atmosphere. Thermo- and thermophysical properties of waste material were discussed, from the point of view of chemical structure and the influence of experimental parameters on the conversion process. It was established that acetyl groups of plasticizer (triacetin, TAC) interact with cellulose acetate (CAc) through dipolar interactions and hydrogen bonding’s. Influence of these polar interactions can affect the position of glass transition temperature (Tg) of CBF. Based on estimated value of Tg from DSC analysis, it was found that CAc present in CBF has degree of substitution (DS) equals to 2.8 (DS = 2.8), where the presence of cellulose triacetate (CTA) was confirmed. It was assumed that an increase of DS leads to decline in the crystallinity. A decline of crystallinity causes the reduction of hydroxyl groups, leading to less organized chains, and whereby decreasing of inter-molecular interactions through hydrogen bonding. Based on the examination of thermophysical characteristics of the tested material, it was found that both, the heat capacity and the thermal inertia (TI) of material linearly increase with temperature, during pyrolysis progression. It was concluded that the type of bio-char produced in this process would have a large capacity to store the heat, which may depend on the formed particles size diameter and porosity. Furthermore, it was inferred that magnitude drops of thermal conductivity (κ) after Tg depends on the material fibrillation.

In this work, the coefficient of fluid-wall mass transfer in an inverse fluidized bed was determined using the adsorption method. The experiments were carried out in a column with a diameter of 45 mm with spherical and non-spherical particles of polypropylene and polyethylene with a diameter of 3.3?4.9 mm and a density of about 930 kg m-3. A diluted solution of methylene blue was used as a fluidization medium, which was adsorbed on part of the surface of the column on silica gel. The obtained results showed that the presence of particles during inverse fluidization does not contribute significantly to mass transfer compared to the influence of particles on transfer in conventional fluidized beds. Therefore, the pseudo-fluid concept was introduced into the analysis and an empirical correlation was performed to determine the mass transfer coefficient. The obtained results were compared with literature correlations for inverse and conventional fluidized beds.

In the presented paper, activated carbons were prepared from fructose and glucose, and activating agents (KOH, NaOH, LiOH) by hydrothermal treatment (HTC) treatment. After preparation, samples were characterized in details. Different techniques were used: x-ray powder diffraction analysis, Raman spectral analysis, elemental analysis, and determination of textural and morphological properties. Obtained results showed dependence of investigated properties and the nature of precursors (glucose or fructose) as well as the type of hydroxides used as activating agents. After characterization, samples were tested as materials for heavy metals (Pb²⁺, Cd²⁺ and Zn²⁺) and methylene blue removal. Also, adsorption experiments were performed on wastewaters taken from tailings of the lead and zinc mine and kinetic of the methylene blue removal was studied. The factors which distinguished the KOH activated samples were high yield (~14%), content of organic carbon (63–74%), porosity and specific surface area (S_BET ~700–1360 m²/g), a low degree of the crystal phase, indications that potassium ions may be included in heavy metals removal, good removal of the heavy metal ions (~47–59 mg/g for Pb²⁺, ~21–27 mg/g for Cd²⁺ and ~6–10 mg/g for Zn²⁺) and fast (~10–30 min) and good methylene blue (~60–200 mg/g) removal.