Department of Physics

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Browsing Department of Physics by Author "Maluta, E. N."

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    Multiscale modeling of sodium-iron battery materials
    (2025-05-16) Dima, Ratshilumela Steve; Maphanga, R. R.; Maluta, E. N.
    In recent years, there has been a growing interest in alternative energy storage technologies as a result of the diminishing reserves of fossil fuels. The development of these technologies requires a careful evaluation of factors such as energy storage and conversion, implementation costs, and environmental impact. Rechargeable batteries are expected to become crucial energy storage devices and promote a more sustainable energy ecosystem. Battery technology has the potential to become cost competitive, especially for portable applications, and exhibits exceptional efficiency, exceeding 90% in electrical efficiency. Sodium ion batteries are considered to be cost-effective and economically feasible alternatives. This work used multiscale computer modelling techniques to understand, control, and improve the intrinsic properties of NaxMnPO4, an electrode material that undergoes Na intercalation and de-intercalation processes. This work aims to promote a more sustainable energy ecosystem. Firstly, we examine the structural and electrochemical performance of NaxMnPO4 using the first-principle density functional theory method. Comparison of the exchange correlation functionals PBE, PBEsol, and PBE+U was conducted, and the results showed that the PBE+U replicated the structural parameters and the energy band gap values well and was used to further analyse the electrochemical performance of the de-intercalated systems. The effect of Na atom de-intercalation on the structural, electronic, mechanical, and thermodynamic properties of both maricite and olivine polymorphs of NaMnPO4 has been investigated by first-principle calculations. The calculated values for the formation energy were found to be negative for all NaMnPO4 systems, hence the solid solution is predicted for states of de-intercalation. The analysis of the electronic density of states indicated that, during the Na removal stages, the material exhibited a rise in its metallic properties between the first and third stages. On the contrary, in the fourth stage, the material displayed semiconductor behaviour, characterised by a band gap of 0.194 eV. A voltage range of 3.997 to 3.848 V was observed, and the computed formation energy values of the de-intercalated systems were determined to be negative, indicating the anticipated presence of a solid in the material. Secondly, the ab initio molecular dynamics method was used to simulate the dynamic properties of NaxMnPO4 materials at different temperatures. The results showed an increasing mean-square displacement gradient as the number of de-intercalated Na atoms increased. The Na-ion diffusion coefficients for olivine and maricite NaMnPO4 were calculated at 100 K and 300 K. Both polymorphs had low diffusion rates at 100 K but increased at 300 K, suggesting faster ion movement. These findings are crucial for understanding the behavior of NaxMnPO4 materials and their potential applications, as diffusion rates can affect processes such as charge / discharge rates in batteries and ion transport in solid-state electrolytes. Controlling temperature and understanding its influence on diffusion coefficients can optimize the performance of NaxMnPO4 materials. Lastly, the cluster expansion (CE) method was introduced as a multiscale pipelining method, establishing a connection between first-principles calculation and large-scale atomistic simulations, as well as Monte Carlo simulation. CE was used to examine the phase stabilities of Na concentrations in relation to vacancies. The stability of the predicted structures on the isotopically optimized volume binary diagram was assessed by calculating their mechanical, electronic, and dynamic properties. Structures that underwent isotropic volume optimisation yielded a cross-validation score of 1.1 meV. This score suggests that the cluster expansion is of good quality, as it falls below the threshold of 5 meV per active position. Based on the analysis of the electronic structure, it is observed that both parent structures (MnPO4 and NaMnPO4) exhibit semiconducting behaviour, while the remaining structures (Na1MnPO4, Na0.825MnPO4, Na0.75MnPO4, Na0.625MnPO4, and Na0.25MnPO4) have semi-metallic characteristics. The mechanical stability of NaMnPO4 was shown by the estimated elastic constants, since the stability conditions were met for all intercalated systems, except for the parent structure MnPO4. Based on the Pugh criterion pertaining to the properties of ductility and brittleness, the structures of Na1MnPO4, Na0.825MnPO4, Na0.75MnPO4, Na0.625MnPO4, and Na0.25MnPO4 exhibit ductile characteristics, while the structures of Na0.5MnPO4 and MnPO4 display brittleness. In addition, MD simulations were performed, revealing that the mean square displacement slope is influenced by the concentration of sodium ions, whereas the diffusion coefficients of sodium ions are influenced by the temperature. These findings suggest that the addition of sodium ions improves the ductility of Na1-xMnPO4 structures. The higher concentration of sodium ions leads to increased ductility, as evidenced by the ductile characteristics observed in Na1MnPO4 and Na0.825MnPO4. However, as the concentration of sodium ions decreases, the structures become more brittle, as seen in Na0.5MnPO4 and MnPO4. Furthermore, the MD simulations indicate that the movement of sodium ions within the structures is influenced by both the concentration of sodium ions and the temperature, highlighting the complex relationship between the composition and mechanical properties in these materials.
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    Studies of interaction of dye molecules with TiO2 Brookite clusters for application in dye sensitized solar cells
    (2019-09-20) Elegbeleye, Ife Fortunate; Maluta, E. N.; Maphanga, R. R.
    Dye sensitized solar cells (DSSCs) have attracted rapid interest over the recent years with prospect of emerging as a viable alternative to conventional silicon based solar cells. The photoanode of DSSCs comprises of dye molecules anchored to the surface of semiconductors such as TiO2. However, the major drawback of Titanium dioxide (TiO2) is its wide band gap (3.0 eV to 3.2 eV) which limits its photocatalytic activities to the ultraviolet region of the electromagnetic spectrum. Understanding the interaction of dye molecules with the surfaces of TiO2 is crucial for optimizing light-harvesting, photoconversion function and photocurrent densities in DSSCs. The three polymorphs of TiO2 are anatase, brookite and rutile. The optical properties of brookite semiconductor have not been much studied although brookite has been reported to have good photocatalytic properties. In this work, Density functional theory (DFT) computational approach was used through various computational softwares which are CASTEP, GAUSSIAN, GAUSSUM, GPAW, ASE, and AVOGADRO with B3LYP, LANL2DZ, PBE, and GGA functional to explore the photocatalytic properties of the typical ruthenium N3 complex, polyenediphenyl-aniline dye moiety, croconate dye molecules and three modelled surfaces of brookite which are (TiO2)5, (TiO2)8 and (TiO2) 68 for application in DSSCs. We also studied the absorption of the corresponding dye molecules on the three surfaces of brookite TiO2. Our findings showed strong binding ability, good electronic coupling, efficient charge separation, spontaneous electron injection and good spectral properties upon adsorption of the dye molecules to brookite TiO2 semiconductor clusters. Our findings on the optical absorption spectra of ruthenium N3 dye, croconate dye and polyenediphenyl-aniline dye molecule absorbed on (TiO2)5 and (TiO2)8 brookite cluster shows bathocromatic shift of the absorption maxima to higher wavelength and improve optical response of TiO2 brookite cluster. A red spectra shift and absorption over a wide range of the solar spectrum in the visible and near infra-red region of the solar spectrum was achieved upon absorption of the ruthenium N3 complex and polyenediphenyl-aniline dye molecules on (TiO2)5 and (TiO2)8 brookite cluster. The results generally suggest that the absorption of dye molecules on TiO2 brookite cluster improves its spectra responsivity in the UV region and makes it possible to absorb over the whole spectrum range, that is, the UV, visible and near infra – red region of the solar spectrum. Our findings also showed good electron injection kinetics from the dye to TiO2 brookite clusters, which suggests higher photocurrents density and open circuit voltage in DSSCs.