11 research outputs found
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 TiO<sub>2</sub>(110)
We investigate the surface reactivity of doped rutile
M-TiO<sub>2</sub>(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
TiO<sub>2</sub> surfaces as compared with that on the undoped TiO<sub>2</sub> surface. We relate this increase in reactivity of the doped
TiO<sub>2</sub> 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 TiO<sub>2</sub> 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 TiO<sub>2</sub>, whereas it destabilizes
the oxygen adsorption energies on doped TiO<sub>2</sub> 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 SrTiO<sub>3</sub> 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 Co<sup>3+</sup> during the
OER, whereas Co is in the Co<sup>4+</sup> 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<sub><i>y</i></sub>Co<sub>1–<i>y</i></sub>O<sub><i>x</i></sub> 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 Co<sub>3</sub>O<sub>4</sub>(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 Co<sub>3</sub>O<sub>4</sub>(001) and β-CoOOH(011̅2)
when compared to results from pure DFT. In addition, the Hubbard-<i>U</i> correction shifts the location of Co<sub>3</sub>O<sub>4</sub> 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 Co<sub>3</sub>O<sub>4</sub> and β-CoOOH are very nearly the same. We ascribe this to a
similar local coordination environment of the active Co site in Co<sub>3</sub>O<sub>4</sub> 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
N<sub>2</sub> 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 NH<sub>3</sub> 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<sub><i>x</i></sub>)
on the HER activity of 5 different MXenes using a combination of experimental
and theoretical approaches. We first studied Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> 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
Mo<sub>2</sub>CT<sub><i>x</i></sub> with very low basal
plane fluorine coverage, achieving a geometric current density of
−10 mA cm<sup>–2</sup> at 189 mV overpotential in acid.
More importantly, our results indicate that the oxygen groups on the
basal planes of Mo<sub>2</sub>CT<sub><i>x</i></sub> are
catalytically active toward the HER, unlike in the case of widely
studied 2H-phase transition metal dichalcogenides such as MoS<sub>2</sub>, 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 M<sub>2</sub>XT<sub><i>x</i></sub> (M = metal;
X = (C, N); and T<sub><i>x</i></sub> = surface functional
groups) predicts Mo<sub>2</sub>CT<sub><i>x</i></sub> to
be an active catalyst candidate for the HER. We synthesized both Mo<sub>2</sub>CT<sub><i>x</i></sub> and Ti<sub>2</sub>CT<sub><i>x</i></sub> MXenes, and in agreement with our theoretical predictions,
Mo<sub>2</sub>CT<sub><i>x</i></sub> was found to exhibit
far higher HER activity than Ti<sub>2</sub>CT<sub><i>x</i></sub>. Theory suggests that the basal planes of Mo<sub>2</sub>CT<sub><i>x</i></sub> are catalytically active toward the HER,
unlike in the case of widely studied MoS<sub>2</sub>, 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 Co<sub>3</sub>O<sub>4</sub> Nanowires by Substituting Co<sup>2+</sup> with Cu<sup>2+</sup>
Co<sub>3</sub>O<sub>4</sub> is an
attractive earth-abundant catalyst for CO oxidation, and its high
catalytic activity has been attributed to Co<sup>3+</sup> cations
surrounded by Co<sup>2+</sup> ions. Hence, the majority of efforts
for enhancing the activity of Co<sub>3</sub>O<sub>4</sub> have been
focused on exposing more Co<sup>3+</sup> cations on the surface. Herein,
we enhance the catalytic activity of Co<sub>3</sub>O<sub>4</sub> by
replacing the Co<sup>2+</sup> ions in the lattice with Cu<sup>2+</sup>. Polycrystalline Co<sub>3</sub>O<sub>4</sub> nanowires for which
Co<sup>2+</sup> is substituted with Cu<sup>2+</sup> are synthesized
using a modified hydrothermal method. The Cu-substituted Co<sub>3</sub>O<sub>4</sub>_Cu<i>x</i> polycrystalline nanowires exhibit
much higher catalytic activity for CO oxidation than pure Co<sub>3</sub>O<sub>4</sub> polycrystalline nanowires and catalytic activity similar
to those single crystalline Co<sub>3</sub>O<sub>4</sub> nanobelts
with predominantly exposed most active {110} planes. Our computational
simulations reveal that Cu<sup>2+</sup> substitution for Co<sup>2+</sup> is preferred over Co<sup>3+</sup> both in the Co<sub>3</sub>O<sub>4</sub> bulk and at the surface. The presence of Cu dopants changes
the CO adsorption on the Co<sup>3+</sup> surface sites only slightly,
but the oxygen vacancy is more favorably formed in the bonding of
Co<sup>3+</sup>–O–Cu<sup>2+</sup> than in Co<sup>3+</sup>–O–Co<sup>2+</sup>. 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