Browsing by Author "Pogrebnoi, Alexander"

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Biogas-slurry derived mesoporous carbon for supercapacitor applications
(Elsevier Ltd., 2017-09) Enock, Talam; King’ondu, Cecil; Pogrebnoi, Alexander; Jande, Yusufu
This study reports on the transformation of biogas slurry into mesoporous carbon for supercapacitor electrodes. Pore structures have been modified by altering activation time, temperature and KOH/carbon mass ratio. The mesoporous carbons are successively developed as evidenced by type IV isotherms obtained in nitrogen sorption studies. BET, micropore and mesopore surface area of 515, 350, and 165 m2 g−1, respectively as well as a narrow pore width distribution of 3–4.5 nm are obtained. X-ray photoelectron results have confirmed the presence of functional groups of oxygen and nitrogen in the samples which facilitates the pseudocapacitance. The electrochemical measurements in 6 M KOH using a three electrode cell with Ag/AgCl as reference electrode and platinum as counter electrode has been performed. The materials activated at 700 °C, 3:1 KOH to carbon mass ratio, and for 120 min exhibit high specific capacitance of 289 F g−1 at a scan rate of 5 mV s−1. Shortening activation time to 30 and 60 min reduces specific capacitance to 163 and 182 F g−1, in that order. Additionally, at 3:1 KOH to carbon mass ratio and 60 min activation time, specific capacitances of 170 and 210 F g−1 at 600 and 800 °C, respectively are obtained. Moreover, specific capacitance increases with increasing the KOH to carbon mass ratio from 148 F g−1 for 1:1–163 F g−1 for 3:1 at 700 °C. Electrochemical impedance spectroscopy studies demonstrate that material has high conductivity. In addition; capacity retention of 96% after 20,000 cycles is shown at scan rate of 30 mV s−1. The study shows that high performance electrodes can be designed from biogas slurry derived porous carbon.
Design of a Cooling System Integrated with Ultraviolet Light for Preservation of Fruits and Vegetables at Variable Tropical Weather Conditions: A Case Study of Arusha, Tanzania
(AJOL, 2023-09-30) Gunda, John; Pogrebnoi, Alexander; Kichonge, Baraka
Post-harvest losses of fruits and vegetables pose significant challenges, especially in tropical climates. This study introduces the development and performance evaluation of a solar-powered evaporative cooling storage system integrated with ultraviolet light (UV) designed for preservation of perishable fruits and vegetables. The cooling chamber supplied with ultraviolet lamp was developed using locally available materials such as sisal, sponge, and bricks. The performance of the system was evaluated in terms of air temperature decrease, relative humidity increase, and evaporative cooling power capacity both for sunny and cloudy tropical weather conditions. The study reveals that activating the UV light considerably extends the shelf life of fruits and vegetables. The system was able to extend the shelf life of perishable products by up to 21 days when exposed to ultraviolet light and by 9 days when not exposed to ultraviolet light. On sunny days, active system operation leads to an average temperature reduction of 5.0 °C, along with a relative humidity increase of 23%. On the contrary, on cloudy days, the cooling impact diminishes slightly, resulting in temperature decrease of approximately 3.5 °C and relative humidity increase of 18%. These findings emphasize the potential of the solar-powered evaporative cooling system, combined with UV light treatment, as a viable approach to combat post-harvest losses in tropical environments.
Dynamics of Kaolinite-Urea Nanocomposites via Coupled DMSO-Hydroxyaluminum Oligomeric Intermediates
(Hindawi Publishing Corporation Indian Journal of Materials Science, 2015) Sempeho, Siafu Ibahati; Kim, Hee Taik; Mubofu, Egid; Pogrebnoi, Alexander; Shao, Godlisten; Hilonga, Askwar
Kaolinite-urea nanocompositeswere prepared via intercalation reactions in an attempt to investigate the dynamic nature of kaolinite morphology for advanced applications in controlled release systems (CRS). Characterization was done using SEM-EDX, XRF, ATR-FTIR, XRD, and DT/DTG; Andreasen pipette sedimentation technique was used to determine the grain size distribution of the raw kaolinite. The X-ray diffraction pattern revealed the existence of an FCC Bravais lattice where the intercalation ratios attained were 51.2%, 32.4%, 7.0%, and 38.4% for hydroxyaluminum oligomeric intercalated kaolinite, substituted urea intercalated kaolinite, calcined DMSO intercalated kaolinite, and hydroxyaluminum reintercalated kaolinite, respectively, along with their respective crystallite sizes of 33.51–31.73 nm, 41.92–39.69 nm, 22.31–21.13 nm, and 41.86–39.63 nm. The outcomes demonstrated that the employed intercalation routes require improvements as the intercalation reactions were in average only ≈32.3%. The observations unveiled that it is possible to manipulate kaolinite structure into various morphologies including dense-tightly packed overlapping euhedral pseudo hexagonal platelets, stacked vermiform morphologies, postulated forms, and unique patterns exhibiting self-assembled curled glomeruli-like morphologies. Such a diversity of kaolinite morphologies expedites its advanced applications in the controlled release systems (CRS) such as drug delivery systems and controlled release fertilizers (CRFs).
Effect of Electron Donor Groups on Optoelectronic Properties of Betalain Dyes: A DFT Study
(wiley, 2025-05) Tsere, Melkizedeck; Costa, Rene; Deogratias, Geradius; Pogrebnaya, Tatiana; Pogrebnoi, Alexander; Machunda, Revocatus; Al-Qurashi, Ohoud; Wazzan, Nuha; Babu, Nambury
A sensitizer is a vital component of dye-sensitized solar cells(DSSCs); it absorbs incident photons, excites electrons, and facil-itates charge transfer to the semiconductor. In the present work,modification of betalain dyes through grafting of electron donorgroups has been performed. The reported optoelectronic properties of the investigated dyes are determined using densityfunctional theory (DFT) and time-dependent DFT methods. The investigated sensitizers exhibit maximum absorption between400 and 442 nm with light-harvesting efficiencies exceeding 93%.Favorable interactions are observed between the dyes and the hydrogenated TiO2 cluster, (TiO2)6H3. The obtained binding energies range from 1.39 to 0.97 eV in the gas phase and 0.31 to 0.03 eV in water. The electronic spectra of the dye@TiO2 complexes show broader and intensive bands with bathochromicshifts when compared to the individual dyes. The charge density distribution in the complexes indicates appropriate ability of thedyes for charge injection to the semiconductor. Among the considered dyes, the most promising candidates for use in DSSCs have been selected.
Encapsulated Urea-Kaolinite Nanocomposite for Controlled Release Fertilizer Formulations
(Hindawi Publishing Corporation Journal of Chemistry, 2015) Sempeho, Siafu Ibahati; Kim, Hee Taik; Mubofu, Egid; Pogrebnoi, Alexander; Shao, Godlisten; Hilonga, Askwar
Urea controlled release fertilizer (CRF) was prepared via kaolinite intercalation followed by gum arabic encapsulation in an attempt to reduce its severe losses associated with dissolution, hydrolysis, and diffusion. Following the beneficiation, the nonkaolinite fraction decreased from 39.58% to 0.36% whereas the kaolinite fraction increased from 60.42% to 99.64%. The X-ray diffractions showed that kaolinite was a major phase with FCC Bravais crystal lattice with particle sizes ranging between 14.6nm and 92.5 nm. Theparticle size varied with intercalation ratioswithmethanol intercalated kaolinite >DMSO-kaolinite > urea-kaolinite (KPDMU). Following intercalation, SEM analysis revealed a change of order from thick compact overlapping euhedral pseudohexagonal platelets to irregular booklets which later transformed to vermiform morphology and dispersed euhedral pseudohexagonal platelets. Besides, dispersed euhedral pseudohexagonal platelets were seen to coexist with blocky-vermicular booklets. In addition, a unique brain-form agglomeration which transformed into roundish particles mart was observed after encapsulation. The nanocomposites decomposed between 48 and 600∘C. Release profiles showed that 100% of urea was released in 97 hours from KPDMU while 87% was released in 150 hours from the encapsulated nanocomposite.The findings established that it is possible to use Pugu kaolinite and gum arabic biopolymer to prepare urea CRF formulations.
Enhancing the optoelectronic properties of blended triphenylamine-betalain based dyes through tailoring the anchoring unit: a theoretical investigation
(Taylor & Francis, 2024-11-25) Tsere, Melkizedeck; Costa, Rene; Deogratias, Geradius; Al-Qurashi, Ohoud; Wazzan, Nuha; Pogrebnaya, Tatiana; Pogrebnoi, Alexander; Machunda, Revocatus
A series triphenyl-betalain organic dyes featuring carboxylic acid and nitro anchoring groups CH = C(X)COOH for the A1-X dyes and -CH = C(X)NO2 for the A2-X dyes, respectively, where X = CN, CH3, CCl3 and CF3 was evaluated for dye sensitised solar cells application. The geometrical structures, molecular orbitals and energies, light absorption patterns, free energies of electron injection and dye regeneration and binding to the semiconductor have been explored using DFT/TD-DFT methods. The nitro-based anchoring group resulted in pronounced red-shift in absorption spectra between 111 and 317 nm compared to carboxylic acid-based dyes. Attachment of the dyes to the semiconductor was modelled via binding to (TiO2)6H3 cluster; A2-X dyes exhibited more stable Dye@TiO2 complexes with binding energies (BEs) ranging between −4.08 and −2.88 eV compared to A1-X dyes with BEs range of −1.11 to −0.05 eV. The results evince that the dyes with CH = C(X)NO2 anchoring groups could be promising materials for light harvesting application.
Glycerol Carbonate Synthesis Using a Catalyst Derived from Oldoinyo Lengai Volcanic Ash: Porosity, Process Optimization, Kinetic and Thermodynamic Analysis
(Wiley-VCH GmbH, 2025-11-08) Kandola, Isack; Tsere, Melkizedeck; Kichonge, Baraka; Jacob, Fortunatus; Bakari, Ramadhani; Pogrebnoi, Alexander; Mpumi, Nelson; Ekeoma, MO; Okoye, Patrick; Hilonga, Askwar; Kivevele, Thomas
This study investigates the use of natural catalysts from volcanic lava ash (VLA) to promote the conversion of glycerol (GL) to glycerol carbonate (GC). The surface basicity was characterized by temperature-programmed desorption of carbon dioxide (TPD-CO2), and textural properties by Brunauer–Emmett–Teller (BET). Kinetic modeling and thermodynamic calculations were performed to understand the reaction system. The VLA catalyst exhibited a surface area of 32.16 m2 g−1 with a total pore volume of 0.076 cm3 g−1 and an average pore diameter of 18.60 nm. Textural properties of volcanic ash lava revealed abundant active sites for adsorption and a mesoporous structure, which propagated diffusions of reactants, reactions at the active sites, and desorption of the products. High yield of glycerol carbonate and conversion of glycerol was attributed to high surface basicity of 32.40 mmol g−1 based on temperature-programmed desorption of carbon dioxide analysis. Process optimization based on response surface methodology using the Box-Behnken design revealed that 97.0 ± 0.1% of glycerol and 91.00 ± 0.15% of glycerol carbonate can be achieved at optimal conditions of 80 °C, 4.5 wt% catalyst loading, dimethyl carbonate to glycerol molar ratio of 3:1, under 90 min reaction time. The regression models exhibited R2 = 0.98 for glycerol conversion and R2 = 0.99 for glycerol carbonate yield. Kinetic studies revealed pseudo-first-order behavior with a rate constant of 0.1 min−1 at 348.15 K, and an activation energy of ∼33 kJ mol−1. Thermodynamic parameters (ΔG* = 92 kJ mol−1, ΔH* = 31 kJ mol−1, ΔS* = −174 J mol−1 K−1) suggest an endergonic and nonspontaneous reaction. The characterization results revealed that the catalyst possesses a basic and mesoporous structure. These properties played a crucial role in enhancing its catalytic efficiency and sustainable route for optimized glycerol carbonate synthesis with defined kinetics.
Lawsone isomers, lawsone ether and bilawsone for dye-sensitized solar cells applications: DFT and UV-Vis studies.
(Elsevier Inc., 2020-01-01) Makoye, Amosi; Pogrebnoi, Alexander; Pogrebnaya, Tatiana
Structural and optoelectronic properties of lawsone (L), lawsone ether (LE) and bilawsone (BL) were studied theoretically using the DFT and time-dependent DFT methods with hybrid functional B3LYP5 and 6-311G (2d,p) basis set. For the monomer lawsone molecule, isomerization reaction between two rotational isomers was analyzed based on a thermodynamic approach. The electronic spectra of the dyes molecules in a vacuum and solvents (DMSO and CHCl) were computed. The maximum wavelengths were found at 355-408 nm for the LE and 350-448 nm for BL that indicated bands shift to visible range compared to L (340 nm). The UV-Vis spectra of the L and BL were measured experimentally in DMSO solution. For the BL, a broad and intensive band was observed in a visible region at 452 nm that apparently would favour sensitizing ability of the dye. The optoelectronic properties of the LE and BL showed them as more promising candidates for DSSCs applications compared to the individual lawsone dye.
Status of Biomass Derived Carbon Materials for Supercapacitor Application
(Hindawi, 2017-01-31) Enock, Talam; King’ondu, Cecil; Pogrebnoi, Alexander; Jande, Yusufu
Environmental concerns and energy security uncertainties associated with fossil fuels have driven the world to shift to renewable energy sources. However, most renewable energy sources with exception of hydropower are intermittent in nature and thus need storage systems. Amongst various storage systems, supercapacitors are the promising candidates for energy storage not only in renewable energies but also in hybrid vehicles and portable devices due to their high power density. Supercapacitor electrodes are almost invariably made of carbon derived from biomass. Several reviews had been focused on general carbon materials for supercapacitor electrode. This review is focused on understanding the extent to which different types of biomasses have been used as porous carbon materials for supercapacitor electrodes. It also details hydrothermal microwave assisted, ionothermal, and molten salts carbonization as techniques of synthesizing activated carbon from biomasses as well as their characteristics and their impacts on electrochemical performance.
Theoretical Design of Complex Molecule via Combination of Natural Lawsone and Synthetic Indoline D131 Dyes for Dye Sensitized Solar Cells Application
(Scientific Research Publishing Inc., 2018-10-31) Madili, Nyanda; Pogrebnoi, Alexander; Pogrebnaya, Tatiana
The dye sensitized solar cells (DSSCs) have been extensively studied due to their low production cost and simple fabrication process. Dye co-sensitization broadens the absorption spectrum of the sensitizer; thus enhances light harvesting efficiency; and contributes to the improvement of the DSSCs overall efficiency. In this study we performed theoretical design of complex molecule (C45H32N2O4) through combination (esterification reaction) of the natural dye lawsone and synthetic metal free indoline dye D131. The excitation energies, vibration spectra, molecular structures, electronic absorption spectra and electron transitions in individual dyes and complex molecule were investigated using density functional theory (DFT) and time dependent density functional theory (TD-DFT) B3LYP5 methods, with 3-21G, 6-31G and 6-31G(d,p) basis sets. The UV-Vis absorption spectra of the individual dyes and their mixture in chloroform solution were measured using spectrophotometer. For the complex formation reaction, enthalpy, entropy and Gibbs free energy were calculated and the results indicated the reaction was endothermic and non-spontaneous. Electron density distribution of the frontier and adjacent molecular orbitals and energy levels alignment were used for analysis of the electronic spectra and mechanism of transitions. The results indicated that the designed complex molecule satisfied the requirements for good photosensitizer of DSSCs.
Tuning optoelectronic properties of triphenylamine based dyes through variation of pi-conjugated units and anchoring groups: A DFT/TD-DFT investigation.
(Elsevier Inc., 2020-01-01) Deogratias, Geradius; Seriani, Nicola; Pogrebnaya, Tatiana; Pogrebnoi, Alexander
Dye-sensitized solar cells (DSSCs) have attracted widespread attention due to their unique features. In the present work, molecular engineered triphenylamine based dyes featuring donor-bridge-acceptor architecture have been considered and investigated for suitable properties for DSSCs applications. Hydantoin anchoring group has been introduced replacing the commonly used cyanoacrylic acid to improve the long-term stability of the device. Results on the effects of varied anchoring groups and pi-spacers have been interpreted from the viewpoint of DFT/TD-DFT calculations. Designed sensitizers exhibit suitable light-harvesting efficiencies, excited-state lifetimes, electron injection and regeneration abilities. Red-shifted electronic spectra are observed for three hydantoin dyes compared to others in the same family. Further analysis of chemical descriptors and observation from full-electron donor-acceptor map reveal that the three dyes among nine are potential materials with promising properties towards improving DSSCs performance.