High-voltage rectifier diode

Magistrsko delo obravnava izdelavo visokonapetostne diode za namen laboratorijskih meritev visokonapetostne opreme pod vplivom enosmerne napetosti. Poudarek je na lastno konstruirani ter izdelani diodi, ki je bila testirana v visokonapetostnem laboratoriju Elektroinštituta Milana Vidmarja. V prvem delu so opisani osnovni pojmi, ki so bili pri izdelavi modela izrednega pomena. Tu gre predvsem za specifične parametre kot so električno polje, lastnosti izolacij, električna zdržnost materialov, itd. V drugem delu magistrskega dela sem se nekoliko bolj osredotočil na fizikalne osnove pn-spoja oziroma diode, ki je bila ključni element praktičnega dela, ter na že obstoječe rešitve, ki se uporabljajo danes. Za konec je predstavljenih več praktičnih preizkusov, ki so bili opravljeni tekom celotne raziskave ter izdelave visokonapetostne diode. Prišel sem do zaključka, da je kljub uspešno zaključenemu projektu vpliv električnega polja pri napetostih višjih od 300 kV tolikšen, da je konstrukcijska zasnova usmerniške diode z zaporednim sestavljanjem izredno kompleksna in na način kot je opisan v nadaljevanju naloge ni primerna za resnejšo uporabo.The master\u27s thesis deals with the manufacture of high-voltage diode for the purpose of laboratory measurements of high-voltage equipment under the influence of the DC voltage. The emphasis is on own designed and manufactured diodes which have been tested in high-voltage laboratory of Milan Vidmar Electric Power Institute. The first section describes the basic concepts that have been of the utmost importance for model production. These were primarily specific parameters such as electric field, properties of insulation, electrical materials etc. In the second part of the master thesis I focused more on the physical basis of pn-junction diode which has been a key element of practical work as well as on existing solutions which are used today. In the end, number of practical tests that were carried out during all the research and manufacture of high-voltage diodes are presented. I came to the conclusion that despite the successful completion of the project the influence of the electric field at voltages higher than 300 kV is such, that the structural design of the rectifier diode by successive assembling is extremely complex and in the manner as described in the thesis is not suitable for serious use

Electronic Origin of the Surface Reactivity of Transition-Metal-Doped TiO₂(110)

We investigate the surface reactivity of doped rutile M-TiO₂(110) (M = V, Cr, Mo, W, Mn, Fe, Ru, Co, Ir, and Ni) using density functional theory (DFT) and Hubbard-<i>U</i> corrected DFT calculations (DFT+<i>U</i> method). The oxygen adsorption bond, used as the surface reactivity measure, is stronger on the doped TiO₂ surfaces as compared with that on the undoped TiO₂ surface. We relate this increase in reactivity of the doped TiO₂ surfaces to the presence of localized surface resonances and surface states in the vicinity of the Fermi level. We find that the center of these localized states on doped TiO₂ is a good descriptor for the oxygen adsorption energy. The inclusion of the Hubbard-<i>U</i> correction to DFT barely modifies the oxygen adsorption energy on undoped TiO₂, whereas it destabilizes the oxygen adsorption energies on doped TiO₂ when compared with results from standard DFT. Nevertheless, we find that the oxygen adsorption energy trends predicted by a standard GGA-DFT functional are reproduced when the Hubbard-<i>U</i> correction is applied

Ni Ingress and Egress in SrTiO₃ Single Crystals of Different Facets

Metal-ion surface interactions and/or doping in host perovskite-oxides are techniques that are widely employed for electronic structure tuning purposes and in developing novel heterogeneous catalysts; however, an in-depth understanding of the different elementary steps and factors involved in these processes is lacking. Herein, we use Atomic Force Microscopy (AFM), Scanning Transmission Electron Microscopy (STEM), and ab initio thermodynamics through density functional theory (DFT) to specifically investigate Ni surface adsorption, ingress, migration, segregation, and egress processes across different SrTiO3 (STO) single-crystal facets and terminations, specifically the (001), (110), and the (111). Under oxidizing and reducing conditions at different temperatures, Ni egress is observed on (110) STO samples, but not the (001). DFT results demonstrate Ni to have a higher thermodynamic egress propensity, specifically through an oxygen-terminated (110) facet in comparison to other (001) terminations, whereas for the (111)-Ti terminated facet, Ni is likely to remain in the bulk post ingress. We suggest that the observed uniqueness of the (110) surface facet toward Ni egress is possibly a consequence of a surface phase transition. These results can help guide design interests with regard to Ni surface stabilization, ingress/egress suppression, or facilitation in STO by elucidating the nuances involved across different facets

Theoretical Investigation of the Activity of Cobalt Oxides for the Electrochemical Oxidation of Water

The presence of layered cobalt oxides has been identified experimentally in Co-based anodes under oxygen-evolving conditions. In this work, we report the results of theoretical investigations of the relative stability of layered and spinel bulk phases of Co oxides, as well as the stability of selected surfaces as a function of applied potential and pH. We then study the oxygen evolution reaction (OER) on these surfaces and obtain activity trends at experimentally relevant electro-chemical conditions. Our calculated volume Pourbaix diagram shows that β-CoOOH is the active phase where the OER occurs in alkaline media. We calculate relative surface stabilities and adsorbate coverages of the most stable low-index surfaces of β-CoOOH: (0001), (011̅2), and (101̅4). We find that at low applied potentials, the (101̅4) surface is the most stable, while the (011̅2) surface is the more stable at higher potentials. Next, we compare the theoretical overpotentials for all three surfaces and find that the (101̅4) surface is the most active one as characterized by an overpotential of η = 0.48 V. The high activity of the (101̅4) surface can be attributed to the observation that the resting state of Co in the active site is Co³⁺ during the OER, whereas Co is in the Co⁴⁺ state in the less active surfaces. Lastly, we demonstrate that the overpotential of the (101̅4) surface can be lowered further by surface substitution of Co by Ni. This finding could explain the experimentally observed enhancement in the OER activity of Ni_<i>y</i>Co_1–<i>y</i>O_<i>x</i> thin films with increasing Ni content. All energetics in this work were obtained from density functional theory using the Hubbard-U correction

Water Dissociative Adsorption on NiO(111): Energetics and Structure of the Hydroxylated Surface

The energetics of the reactions of water with metal oxide surfaces are of tremendous interest for catalysis, electrocatalysis, and geochemistry, yet the energy for the dissociative adsorption of water was only previously measured on one well-defined oxide surface, iron oxide. In the present paper, the enthalpy of the dissociative adsorption of water is measured on NiO(111)-2 × 2 at 300 K using single-crystal adsorption calorimetry. The differential heat of dissociative adsorption decreases with coverage from 170 to 117 kJ/mol in the first 0.25 ML of coverage. Water adsorbs molecularly on top of that, with a heat of ∼92 kJ/mol. Density functional theory (DFT) calculations reproduce the measured energies well (all within 17 kJ/mol) and provide insight into the atomic-level structure of the surfaces studied experimentally. They show that the oxygen-terminated O-octo(2 × 2) structure is the most stable NiO(111)-2 × 2 termination and gives reaction energies with water that are more consistent with the calorimetry results than the metal-terminated surface. They show that water adsorbs dissociatively on this (2 × 2)-O-octo surface to produce a hydroxyl-covered surface with a heat of adsorption of 171 ± 5 kJ/mol in the low-coverage limit (very close to 170 kJ/mol experimentally) and an integral heat that decreases by 14 kJ/mol up to saturation (compared to ∼30 kJ/mol experimentally). Sensitivity of this reaction’s energy to choice of DFT method is tested using a variety of different exchange correlation functionals, including HSE06, and found to be quite weak

Importance of Correlation in Determining Electrocatalytic Oxygen Evolution Activity on Cobalt Oxides

Co-based oxides are suitable electrode materials for the electrocatalytic oxygen evolution reaction (OER) with promising activity and stability, in addition to being widely available and relatively cheap. We investigate OER on Co₃O₄(001) and β-CoOOH (011̅2) surfaces using density functional theory calculations (DFT). We construct surface Pourbaix diagrams and investigate the theoretical overpotential for the elementary steps involved in OER on these surfaces. We show that inclusion of the Hubbard-<i>U</i> correction to DFT (DFT+<i>U</i>) is necessary to recover experimentally observed trends in the activity for the strongly correlated cobalt oxides. We find that the inclusion of the Hubbard-<i>U</i> correction lowers the activity of both Co₃O₄(001) and β-CoOOH(011̅2) when compared to results from pure DFT. In addition, the Hubbard-<i>U</i> correction shifts the location of Co₃O₄ and β-CoOOH from the strong binding leg to the weak binding leg of the OER volcano plot. The calculations also suggest that the theoretical overpotentials for Co₃O₄ and β-CoOOH are very nearly the same. We ascribe this to a similar local coordination environment of the active Co site in Co₃O₄ and CoOOH under OER conditions

Computational Design of Active Site Structures with Improved Transition-State Scaling for Ammonia Synthesis

The Haber–Bosch process for the reduction of atmospheric nitrogen to ammonia is one of the most optimized heterogeneous catalytic reactions, but there are aspects of the industrial process that remain less than ideal. It has been shown that the activity of metal catalysts is limited by a Brønsted–Evans–Polanyi (BEP) scaling relationship between the reaction and transition-state energies for N₂ dissociation, leading to a negligible production rate at ambient conditions and a modest rate under harsh conditions. In this study, we use density functional theory (DFT) calculations in conjunction with mean-field microkinetic modeling to study the rate of NH₃ synthesis on model active sites that require the singly coordinated dissociative adsorption of N atoms onto transition metal atoms. Our results demonstrate that this ”on-top” binding of nitrogen exhibits significantly improved scaling behavior, which can be rationalized in terms of transition-state geometries and leads to considerably higher predicted activity. While synthesis of these model systems is likely challenging, the stabilization of such an active site could enable thermochemical ammonia synthesis under more benign conditions

Tuning the Basal Plane Functionalization of Two-Dimensional Metal Carbides (MXenes) To Control Hydrogen Evolution Activity

Hydrogen evolution reaction (HER) via electrocatalysis is one method of enabling sustainable production of molecular hydrogen as a clean and promising energy carrier. Previous theoretical and experimental results have shown that some two-dimensional (2D) transition metal carbides (MXenes) can be effective electrocatalysts for the HER, based on the assumption that they are functionalized entirely with oxygen or hydroxyl groups on the basal plane. However, it is known that MXenes can contain other basal plane functionalities, e.g., fluorine, due to the synthesis process, yet the influence of fluorine termination on their HER activity remains unexplored. In this paper, we investigate the role and effect of basal plane functionalization (T_<i>x</i>) on the HER activity of 5 different MXenes using a combination of experimental and theoretical approaches. We first studied Ti₃C₂T_<i>x</i> produced by different fluorine-containing etchants and found that those with higher fluorine coverage on the basal plane exhibited lower HER activity. We then controllably prepared Mo₂CT_<i>x</i> with very low basal plane fluorine coverage, achieving a geometric current density of −10 mA cm^–2 at 189 mV overpotential in acid. More importantly, our results indicate that the oxygen groups on the basal planes of Mo₂CT_<i>x</i> are catalytically active toward the HER, unlike in the case of widely studied 2H-phase transition metal dichalcogenides such as MoS₂, in which only the edge sites are active. These results pave the way for the rational design of 2D materials for either the HER, when minimal overpotential is desired, or for energy storage, when maximum voltage window is needed

Two-Dimensional Molybdenum Carbide (MXene) as an Efficient Electrocatalyst for Hydrogen Evolution

The hydrogen evolution reaction (HER) is an important energy conversion process that underpins many clean energy technologies including water splitting. Herein, we report for the first time the application of two-dimensional (2D) layered transition metal carbides, MXenes, as electrocatalysts for the HER. Our computational screening study of 2D layered M₂XT_<i>x</i> (M = metal; X = (C, N); and T_<i>x</i> = surface functional groups) predicts Mo₂CT_<i>x</i> to be an active catalyst candidate for the HER. We synthesized both Mo₂CT_<i>x</i> and Ti₂CT_<i>x</i> MXenes, and in agreement with our theoretical predictions, Mo₂CT_<i>x</i> was found to exhibit far higher HER activity than Ti₂CT_<i>x</i>. Theory suggests that the basal planes of Mo₂CT_<i>x</i> are catalytically active toward the HER, unlike in the case of widely studied MoS₂, in which only the edge sites of the 2H phase are active. This work paves the way for the development of novel 2D layered materials that can be applied in a multitude of other clean energy reactions for a sustainable energy future

Enhancing Catalytic CO Oxidation over Co₃O₄ Nanowires by Substituting Co²⁺ with Cu²⁺

Co₃O₄ is an attractive earth-abundant catalyst for CO oxidation, and its high catalytic activity has been attributed to Co³⁺ cations surrounded by Co²⁺ ions. Hence, the majority of efforts for enhancing the activity of Co₃O₄ have been focused on exposing more Co³⁺ cations on the surface. Herein, we enhance the catalytic activity of Co₃O₄ by replacing the Co²⁺ ions in the lattice with Cu²⁺. Polycrystalline Co₃O₄ nanowires for which Co²⁺ is substituted with Cu²⁺ are synthesized using a modified hydrothermal method. The Cu-substituted Co₃O₄_Cu<i>x</i> polycrystalline nanowires exhibit much higher catalytic activity for CO oxidation than pure Co₃O₄ polycrystalline nanowires and catalytic activity similar to those single crystalline Co₃O₄ nanobelts with predominantly exposed most active {110} planes. Our computational simulations reveal that Cu²⁺ substitution for Co²⁺ is preferred over Co³⁺ both in the Co₃O₄ bulk and at the surface. The presence of Cu dopants changes the CO adsorption on the Co³⁺ surface sites only slightly, but the oxygen vacancy is more favorably formed in the bonding of Co³⁺–O–Cu²⁺ than in Co³⁺–O–Co²⁺. This study provides a general approach for rational optimization of nanostructured metal oxide catalysts by substituting inactive cations near the active sites and thereby increasing the overall activity of the exposed surfaces