| 1. |
- Banerjee, Amitava
(författare)
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Materials Modelling for Energy Harvesting : From Conversion to Application through Storage
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this Ph.D. thesis, ab initio density functional theory along with molecular dynamics and global optimization methods are used to unveil and understand the structures and properties of energy relevant materials. In this connection, the following applications are considered: i. electrocatalyst for solar fuel production through water splitting, ii. hybrid perovskite solar cell for generation of electrical energy and iii. Battery materials to store the electrical energy. The water splitting mechanism in terms of hydrogen evolution and oxygen evolution reactions (HER and OER) on the catalytic surfaces has been envisaged based on the free energy diagram, named reaction coordinate, of the reaction intermediates. The Ti-functionalized two-dimensional (2D) borophene monolayer has been emerged as a promising material for HER and OER mechanisms as compared to the pristine borophene sheet. Further investigation in the series of this noble metal free monolayer catalyst is 2D Al2C monolayer both in form of pristine and functionalized with nitrogen (N), phosphorous (P), boron (B), and sulphur (S). It has been observed that only B substituted Al2C shows very close to thermoneutral, that could be the most promising candidate for HER on functionalized Al2C monolayer. The adsorption of O* intermediate is stronger in S-substituted Al2C, whereas it is less strongly adsorbed on N-substituted Al2C. The subsequent consideration is being the case of n-type doping (W) along with Ti codoped in BiVO4 to enhance the efficiency of BiVO4 photoanode for water splitting. The determined adsorption energy and corresponding Gibbs free energies depict that the Ti site is energetically more favorable for water splitting. Moreover, the Ti site possesses a lower overpotential in the W–Ti codoped sample as compared to the mono-W doped sample. We have also explored the effect of mixed cation and mixed anion substitution in the hybrid perovskite in terms of structural stability, electronic properties and optical response of hybrid perovskite crystal structures. It has been found that the insertion of bromine (Br) into the system could modulate the stability of the Guanidinium lead iodide (GAPbI3) hybrid perovskite. Moreover, the band gap of the mixed hybrid perovskite is increased with the inclusion of smaller Br anion while replacing partially the larger iodine (I) anion. Finally the electrochemical storage mechanism for Sodium (Na) and lithium (Li) ion insertion has been envisaged in inorganic electrode (eldfellite, NaFe(SO4)2) as well as in more sustainable organic electrode (di-lithium terephthalate, Li2TP). The full desodiation capability of the eldfellite enhances the capacity while the activation energies (higher than 1 eV) for the Na+ ion diffusion for the charged state lower the ionic insertion rate. The key factor as the variation of Li-O coordination in the terephthalate, for the disproportionation redox reaction in Li2TP is also identified.
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| 2. |
- Irunde, Regina Filemon
(författare)
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Treatment of arsenic contaminated drinking water from the sources around the gold mining areas of Geita and Mara, Tanzania : Removal efficiency of locally available materials, bauxite, gypsum and magnesite
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In recent years, high arsenic (As) levels of about 300 µg/L have been reported around the gold mining areas of Geita and Mara regions within the Lake Victoria basin (LVB) in Tanzania. During a sampling campaign at Geita and Mara, the groundwater and surface water samples revealed the presence of high As concentrations as well as Fe and sulfide. Several wells are abandoned because of odor due to high content of sulfide, as well as red color, due to high iron content. About 53% of the analyzed As water samples exceeded the WHO guideline for drinking water. The release of As is primarily attributed to the weathering of sulfide minerals like arsenopyrite related to gold mining activities. In some parts of the LVB, an increasing number of cancer cases are being reported, and clinical investigations are now on the national agenda to identify the possible causes. Water treatment using bauxite, gypsum, and magnesite shows promising results, especially bauxite and magnesite, which could lower As concentrations to below 0.1 µg/L. Both bauxite and magnesite worked efficiently on As removal even at higher concentrations above 5 mg/L, while gypsum is preferable for treatment of low As concentrations. Furthermore, magnesite has a unique chemical character of influencing other materials to have high efficiency of As removal; however, it raises the pH of the water up to 10. Addition of 5 g/L magnesite to water containing 5 mg/L As, could lower the As concentration to below 10 µg/L within 30 min. The As removal increased with dosage and contact time up to 98 % in 4 hours, which is in agreement with Visual MINTEQ simulation. The performance of calcined magnesite, and gypsum fitted well with Freundlich adsorption isotherm, which indicates the presence of chemical reaction as controlling factor for As removal, while bauxite fitted Langmuir isotherm indicates monolayer surface coverage. The kinetic reactions were observed to follow pseudo-second-order. The statistic obeys linear regression with R2 ranging between 0.7 and 0.9. The artificial neural network revealed pH as a most influencing parameter for As removal from water. The mini-scale column revealed that a flow rate of 0.5 – 1 mL/min for 30 min gave an adsorption capacity ranging between 0.07 and 0.14 µg/g, which follows Thomas linear model with rate constant of kTH of 29.48 to 211.25 mL/min µg. The release of elements from spent magnesite, gypsum, and bauxite, such as magnesium (Mg), aluminum (Al), iron (Fe), and calcium (Ca) were found to be below WHO standards after water treatment. However, the desorption process of As from spent magnesite and gypsum was a challenge, which means there was formation of strong bond between Mg-O-As and Ca-O-As. This study is based on 5 papers that provide significant insights to the scientific community, policymakers, and the community living around As contaminated areas to learn about the occurrences of As and simple remediation techniques evaluated in this study.
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| 3. |
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| 4. |
- Johansson, Malin B, 1972-, et al.
(författare)
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Cesium Bismuth Iodide Solar Cells from Systematic Molar Ratio Variation of CsI and BiI3
- 2019
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Ingår i: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 58:18, s. 12040-12052
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Tidskriftsartikel (refereegranskat)abstract
- Metal halide compounds with photovoltaic properties prepared from solution have received increased attention for utilization in solar cells. In this work, low-toxicity cesium bismuth iodides are synthesized from solution, and their photovoltaic and, optical properties as well as electronic and crystal structures are investigated. The X-ray diffraction patterns reveal that a CsI/BiI3 precursor ratio of 1.5:1 can convert pure rhombohedral BiI3 to pure hexagonal Cs3Bi2I9, but any ratio intermediate of this stoichiometry and pure BiI3 yields a mixture containing the two crystalline phases Cs3Bi2I9 and BiI3, with their relative fraction depending on the CsI/BiI3 ratio. Solar cells from the series of compounds are characterized, showing the highest efficiency for the compounds with a mixture of the two structures. The energies of the valence band edge were estimated using hard and soft X-ray photoelectron spectroscopy for more bulk and surface electronic properties, respectively. On the basis of these measurements, together with UV-vis-near-IR spectrophotometry, measuring the band gap, and Kelvin probe measurements for estimating the work function, an approximate energy diagram has been compiled clarifying the relationship between the positions of the valence and conduction band edges and the Fermi level.
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| 5. |
- Philippe, Bertrand, Dr. 1986-, et al.
(författare)
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Valence Level Character in a Mixed Perovskite Material and Determination of the Valence Band Maximum from Photoelectron Spectroscopy : Variation with Photon Energy
- 2017
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:48, s. 26655-26666
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Tidskriftsartikel (refereegranskat)abstract
- A better understanding of the electronic structure of perovskite materials used in photovoltaic devices is essential for their development and optimization. In this investigation, synchrotron-based photoelectron spectroscopy (PES) was used to experimentally delineate the character and energy position of the valence band structures of a mixed perovskite. The valence band was measured using PES with photon energies ranging from ultraviolet photoelectron spectroscopy (21.2 eV) to hard X-rays (up to 4000 eV), and by taking the variation of the photoionization cross sections into account, we could experimentally determine the inorganic and organic contributions. The experiments were compared to theoretical calculations to further distinguish the role of the different anions in the electronic structure. This work also includes a thorough study of the valence band maximum and its position in relation to the Fermi level, which is crucial for the design and optimization of complete solar cells and their functional properties.
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| 6. |
- Shukla, Vivekanand
(författare)
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Computational Studies of 2D Materials : Application to Energy Storage and Electron Transport in Nanoscale Devices
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The field of two-dimensional (2D) layered materials provides a new platform for studying diverse physical phenomena that are scientifically interesting and relevant for technological applications. Novel applications in electronics and energy storage harness the unique electronic, optical, and mechanical properties of 2D materials for design of crucial components. Atomically thin, with large surface to volume ratio, these materials are attractive for broad applications for hydrogen storage, sensing, batteries and photo-catalysis. Theoretical predictions from atomically resolved computational simulations of 2D materials play a pivotal role in designing and advancing these developments.The central topic of this thesis is 2D materials studied using density functional theory and non-equilibrium Green’s function. The electronic structure and transport properties are discussed for several synthesized and predicted 2D materials, with diverse potential applications in nanoscale electronic devices, gas sensing, and electrodes for rechargeable batteries. Lateral and vertical heterostructures have been studied for applications in nanoscale devices such as graphene/hBN heterostructure nanogap for a potential DNA sequencing device, while in case of twisted bilayer black phosphorus nanojunction, where electronic and transport properties have been explored for diode-like characteristics device. We also have addressed the structural, electronic and transport properties of the recently synthesized polymorphs of 2D borons known as borophenes. We have explored the conventional methods of tuning the material’s properties such as strain in borophene and substitutional doping in black phosphorus with the further investigation of their gas sensing application.A significant portion of this thesis is also dedicated to the energy storage applications of different 2D materials. Energy storage technologies arise with vital importance in providing effective ways to transport and commercialize the produced energy, aiming at rechargeable batteries with high energy and power density. In this context, first-principles simulations have been applied together with other theoretical tools to evaluate structural properties, ion intercalation kinetics, specific capacity and open circuit voltage of selected 2D materials at the atomic level. The simulation study supports the understanding while improving the properties of the materials to increase their efficiency in battery operation.
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| 7. |
- Watcharatharapong, Teeraphat
(författare)
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Defect Thermodynamics and Kinetics in Polyanionic Cathodes : A Theoretical Roadmap for Na-ion based Batteries and Hybrid Supercapacitors
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, the framework of the density functional theory is employed to study and predict properties of polyanionic cathodes for Na-ion batteries and hybrid supercapacitors. It consists of three main parts as follows:The first part is primarily dedicated to kröhnkite-type Na2Fe(SO4)22H2O cathode. The major goal is to probe diffusion mechanisms of Na+ ions. The chemical potentials diagrams for the pentrary compound are determined under thermodynamic equilibrium and are used to calculate pH value for solution-based synthesis. We find that the presence of NaFe facilitates a faster migration and reduces the channel blockage issue. Moreover, the defect concentration can be tuned by controlling the pH condition. We conclude that defects and small hole polarons play a role in ionic and electronic conductivity.The second part focuses on alluaudite-type Na2+2δFe2-δ(SO4)3 (NFSδ). We unveil the effect of the non-stoichiometry on the thermodynamics, defect nature, and voltage profiles NFSδ with δ = 0, 0.25 and 0.5. The relation between Na ion distribution and energetics is studied and reveals the necessity of using a supercell model. Chemical potential diagrams indicate an inevitable impurity precipitation in all cases, but can be reduced at low δ. Defect formation analysis shows an unlikely formation of channel blockage and can explain the impurity precipitation in experiment. Two types of phase transition are observed after half-desodiation. A higher degree of non-stoichiometry offers an improvement in specific capacity and structural reversibility for NFS0.25 and NFS0.5. The voltage profiles and formation energy reveal the Na intercalation mechanism and strategy to enhance the specific capacity.The third part is associated with battery-type cathodes used in hybrid supercapacitors, namely the NaMPO4 and MMoO4 (where M is a transition metal). We find that triphylite NaNiPO4 shows a better electrochemical performance as compared to maricite phase due to the merit of intercalation mechanism. A mixed-NaMn1/3Co1/3Ni1/3PO4 is predicted to show faradaic behavior, mainly contributed from the Ni and Mn redox reactions, along with an improved electronic conductivity. Moreover, the effect of M substitution on phase stability, electronic properties and charge transfer is also studied in MMoO4 with M = Mn, Co and Ni. The highest capacitance is predicted for NiMoO4 amongst the others and is attributed to the higher active surface area. To compromise the capacitance and cycling stability, Mn1/3Co1/3Ni1/3MoO4 is synthesized. We predict its crystal structure by using the SQS method. Based on electronic structure, we can identify a source of the improved cycling efficiency and specific capacitance of this mixed compound.
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