Academic literature on the topic 'Cathodic reaction mechanism'

In this research, 3-Mercaptopropionic acid (MPA) as corrosion inhibitor of carbon steel in CO2 aerated 1% NaCl solution with buffer pH adjustment has been studied by means of electrochemical impedance (EIS) and polarization (Tafel plot). MPA was found to be an effective carbon steel inhibitor. Percentage inhibition efficiency (IE %) calculated by both Tafel plot and EIS, ranged from 85% to 90%. MPA was found to affect the cathodic processes and act as cathodic-type inhibitors. Mechanism of inhibit corrosion by adsorption mechanism leads to the formation of a protective chemisorbed film on the metal surface film which suppresses the dissolution reaction and the hydrogen evolution reaction is activation controlled.

The corrosion inhibition mechanism of three inhibitors for hot dipped coating steels in HCl acid was studied by coulostatic method. The results showed that the anodic Tafel slope ba and cathodic Tafel slope bc significantly increased with increasing of corrosion inhibitor concentration. While the corrosion current density Icorr decreases with the corrosion inhibition concentration increasing. And the inhibition efficiency increased with the inhibitor concentration. The inhibiting action of these compounds were attributed to blocking of the electrode surface by adsorption through its active centers. The three inhibitors were both mixed inhibitors, and reacted as good inhibition by the adsorption of the active sites in the cathodic reaction and the anodic reaction of corrosion process.

The kinetics of hydrogen evolution reaction on FeSi2-electrode in 1.0 M NaOH solution has been studied using methods of polarization and impedance measurements. With the help of diagnostic criteria for the hydrogen evolution reaction mechanisms based on the analysis of the dependence of the parameters of the equivalent electric circuit on overvoltage, it was established that the reaction of hydrogen evolution on iron disilicide in the alkaline electrolyte proceeds along the discharge - electrochemical desorption route, where desorption is the rate-determining stage. Both stages are irreversible, the transfer coefficients of the stages are equal (α1 = α2 = α), simultaneously the hydrogen absorption reaction by the electrode material proceeds in the diffusion mode (in the whole investigated range of potentials). It was found that the adsorption of atomic hydrogen is described by the equation of the Langmuir isotherm. The influence of various methods of modifying of the surface of FeSi2-electrode on the kinetics and mechanism of the cathodic process has been studied. It was found that the modification of the disilicide surface by hydrogenation at a current density of i = 30 mA/cm2, an anodic etching in 0.5 M H2SO4 at the potential E = 0.4 V relative to the standard hydrogen electrode, an anodic etching in 1.0 M NaOH at the potential E = 0.1 V, chemical etching in 5.0 M NaOH at 70 °C reduce the overvoltage of hydrogen evolution, but the mechanism of the cathodic process does not change as a result of the modification. Reduction of the overvoltage of hydrogen evolution on iron disilicide is due to the action of two factors: the development of the surface and the change in the composition of the surface layer of the electrode. It has been concluded that FeSi2 in the alkaline electrolyte is a promising electrode material that exhibits activity in the electrolytic hydrogen evolution reaction.

The remediation of nitrate-contaminated water using electrochemical reduction at a tin cathode has previously been shown to give almost quantitative denitrification (removal of dissolved nitrogen species) under highly cathodic polarization. A particular focus of this project was to identify specific role(s) for tin in the reaction in the context of the previous literature. The current efficiency for denitrification was enhanced in alkaline solution, and the reaction was accelerated by the presence of small concentrations of Sn(II) salts, which are in a dynamic exchange between cathodic deposition and corrosion of the cathode. Literature precedent indicates that Sn(II) salts promote the “dimerization” pathway of NO to hyponitrite in preference to reduction to ammonia. Hyponitrite is a known intermediate in the electrochemical reduction of nitrate, but its spontaneous decomposition gives predominantly N2O, which does not reduce further to N2. We have shown that hyponitrite is reduced electrochemically in competition with its thermal decomposition, which provides a pathway to N2 via the spontaneous dehydration of HO−NH−NH−OH. The possible role of surface-bound Sn−H species in the reduction mechanism is discussed, but further work is needed to substantiate this proposal.

The O-terminated octopolar NiO(111) is predicted to facilitate cathodic transformation of CO₂ to CO₃²⁻ through sequential Mars-van Krevelen and Eley-Rideal mechanisms.

The kinetics of hydrogen evolution reaction on FeSi2-electrode in 0.5 M H2SO4 solution has been studied using methods of polarization and impedance measurements. With the help of diagnostic criteria for the hydrogen evolution reaction mechanisms based on the analysis of the dependence of the parameters of the equivalent electric circuit on overvoltage, it was established that the reaction of hydrogen evolution on iron disilicide in the sulfuric acid solution proceeds along the discharge - electrochemical desorption route, where desorption is the rate-determining stage. Both stages are irreversible, the transfer coefficients α of the stages are equal, simultaneously the hydrogen absorption reaction by the electrode material proceeds in the kinetic mode (in the whole investigated range of potentials). It was found that the adsorption of atomic hydrogen is described by the equation of the Langmuir isotherm. The influence of thin oxide film on the hydrogen evolution kinetics is noted. The influence of various methods of modifying of the surface of FeSi2-electrode on the kinetics and mechanism of the cathodic process has been studied. It was found that the modification of the disilicide surface by hydrogenation at a current density of i = 30 mA/cm2, an anodic etching in 0.5 M H2SO4 at the potential E = 0.4 V relative to the standard hydrogen electrode, an anodic etching in 1.0 M NaOH at the potential E = 0.1 V, chemical etching in 5.0 M NaOH at 70 °C reduce the overvoltage of hydrogen evolution, but the mechanism of the cathodic process does not change as a result of the electrode modification. Reduction of the overvoltage of hydrogen evolution on iron disilicide is due to the action of two factors: the development of the surface and the change in the composition of the surface layer of the electrode. It has been concluded that FeSi2 in the sulfuric acid solution is a promising electrode material that exhibits activity in the electrolytic hydrogen evolution reaction.

Ni-diamond composite coatings are conducted by electrodepositon. The crystal structure and surface morphology of the composite coatings were examined with X-ray diffraction (XRD) and scanning electron microscopy (SEM) and the mechanism of Ni-diamond co-deposition is conducted by catholic polarization and cyclic voltammetry method. The result reveals that diamond particles are successfully embed in Ni matrix and the coatings have an amorphous structure. Cyclic voltammetry indicate that nickel deposition process is an irreversible electrode reaction and cathode polarization curve shift towards positive with the increase of scan rates. Cathodic polarization curve shows that reduction potential of nickel to shift to more negatives with the addition of complexing agent and the slope of the polarization curve is decrease; The addition of brightener and dispersant hinder the deposition of Ni2+ and promote the growth of crystal nucleus; The join of wetting agents and diamond particles have no big influence on the deposition of nickel.

The electrochemical activity of gold nanoparticles at graphite electrodes by the method of cyclic voltammetry is studied. In this article the nature of the supporting electrolyte, modification time of graphite electrode by gold nanoparticles and the potential range on the value of the “inverse” cathodic peak are investigated. The “inverse” cathodic peak of gold nanoparticles formed in the reaction mixture HAuCl4:Na3C6H5O7:NaBH4=1:1:4 is observed on the cathodic branch of cyclic voltamperegram at Ec = 0.05 V at graphite electrode. The mechanism of stepwise electrochemical oxidation and reduction of the phase structure of gold on the surface of the graphite electrode in 0.1 M NaOH is offered. The “inverse” cathodic peak of gold nanoparticles on the cathodic branch of cyclic voltammogram at graphite electrode is caused by oxidation of Au2O to Au2O3.<br /><br />

Perfluorooctanoic acid (PFOA), C7F15COOH, has been widely employed over the past fifty years, causing an environmental problem because of its dispersion and low biodegradability. Furthermore, the high stability of this molecule, conferred by the high strength of the C-F bond makes it very difficult to remove. In this work, electrochemical techniques are applied for PFOA degradation in order to study the influence of the cathode on defluorination. For this purpose, boron-doped diamond (BDD), Pt, Zr, and stainless steel have been tested as cathodes working with BDD anode at low electrolyte concentration (3.5 mM) to degrade PFOA at 100 mg/L. Among these cathodic materials, Pt improves the defluorination reaction. The electro-degradation of a PFOA molecule starts by a direct exchange of one electron at the anode and then follows a complex mechanism involving reaction with hydroxyl radicals and adsorbed hydrogen on the cathode. It is assumed that Pt acts as an electrocatalyst, enhancing PFOA defluorination by the reduction reaction of perfluorinated carbonyl intermediates on the cathode. The defluorinated intermediates are then more easily oxidized by HO• radicals. Hence, high mineralization (xTOC: 76.1%) and defluorination degrees (xF−: 58.6%) were reached with Pt working at current density j = 7.9 mA/cm2. This BDD-Pt system reaches a higher efficiency in terms of defluorination for a given electrical charge than previous works reported in literature. Influence of the electrolyte composition and initial pH are also explored.

Kyoto University (京都大学)
0048
新制・課程博士
博士(人間・環境学)
甲第16947号
人博第590号
新制||人||141(附属図書館)
23||人博||590(吉田南総合図書館)
29622
京都大学大学院人間・環境学研究科相関環境学専攻
(主査)教授 内本 喜晴, 教授 杉山 雅人, 教授 田部 勢津久, 准教授 藤原 直樹, 准教授 雨澤 浩史
学位規則第4条第1項該当

Cyclopropyl-containing substrates have been frequently utilized as "probes" for the detection of SET pathways in organic and biorganic systems. These reactions are based on the cyclorpropylcarbinyl - homoallyl rearrangement, which is fast and essentially irreversible. The implicit assumption in such studies is that if a "radical" species is produced, it will undergo ring opening. We have found that there are two important factors to consider in the design of SET probes: 1) ring strain, the thermodynamic driving force for the rearrangement, and 2) resonance energy, which may help or hinder rearrangement, depending on the specific system. Delocalization of spin and charge were found to be important factors pertaining to substituent effects on the rates of radical anion rearrangements. Previous studies from our lab have centered on highly conjugated phenyl cyclopropyl ketones. This work considers a series of compounds varying in their conjugative components from a highly conjugated spiro[2.5]octa-4,7-dien-6-one and derivatives to simple aliphatic ketones. Utilizing cyclic, linear sweep voltammetry, and preparative electrolysis techniques, it was discovered that all substrates yielded ring opened products with rates and selectivities that will prove useful and informative in the design of mechanistic probes based on the cyclorpropylcarbinyl - homoallyl rearrangement. Rates of homogeneous electron transfer from a series of hydrocarbon mediators to substrates were measured using homogeneous catalysis techniques. Standard reduction potentials and reorganization energies of substrates were derived using Marcus theory. Conjugative interactions with the cyclopropyl group are discussed.
Ph. D.

In this study, we have developed a dense La0.85Sr0.15MnO3-δ (LSM) Ce0.9Gd0.1O1.95 (GDC) composite electrode system for studying the surface modification of cathodes. The LSM and GDC grains in the composite were well defined and distinguished using energy dispersive x-ray (EDX) analysis. The specific three-phase boundary (TPB) length per unit electrode surface area was systematically controlled by adjusting the LSM to GDC volume ratio of the composite from 40% up to 70%. The TPB length for each tested sample was determined through stereological techniques and used to correlate the cell performance and degradation with the specific TPB length per unit surface area. An overlapping spheres percolation model was developed to estimate the activity of the TPB lines on the surface of the dense composite electrodes developed. The model suggested that the majority of the TPB lines would be active and the length of those lines maximized if the volume percent of the electrolyte material was kept in the range of 47 57%. Additionally, other insights into the processing conditions to maximize the amount of active TPB length were garnered from both the stereology calculations and the percolation simulations. Steady-state current voltage measurements as well as electrochemical impedance measurements on numerous samples under various environmental conditions were completed. The apparent activation energy for the reduction reaction was found to lie somewhere between 31 kJ/mol and 41 kJ/mol depending upon the experimental conditions. The exchange current density was found to vary with the partial pressure of oxygen differently over two separate regions. At relatively low partial pressures, i0 had an approximately dependence and at relatively high partial pressures, i0 had an approximately dependence. This led to the conclusion that a change in the rate limiting step occurs over this range. A method for deriving the electrochemical properties from proposed reaction mechanisms was also presented. State-space modeling was used as it is a robust approach to addressing these particular types of problems due to its relative ease of implementation and ability to efficiently handle large systems of differential algebraic equations. This method combined theoretical development with experimental results obtained previously to predict the electrochemical performance data. The simulations agreed well the experimental data and allowed for testing of operating conditions not easily reproducible in the lab (e.g. precise control and differentiation of low oxygen partial pressures).

京都大学
新制・課程博士
博士(人間・環境学)
甲第23286号
人博第1001号
京都大学大学院人間・環境学研究科相関環境学専攻
(主査)教授 内本 喜晴, 教授 田部 勢津久, 教授 高木 紀明
学位規則第4条第1項該当
Doctor of Human and Environmental Studies
Kyoto University
DFAM

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.17.03 – Технічна електрохімія. – Національний технічний уні-верситет “Харківський політехнічний інститут”, Харків, 2019. Дисертаційну роботу присвячено розробці технології електроосадження функціональних покривів сплавами заліза з кобальтом і молібденом з комплек-сних цитратних електролітів. За результатами аналізу іонних рівноваг і кінетичних закономірностей встановлено, що молібден відновлюється у сплав з ферумом та кобальтом до металевого стану з гетероядерних комплексів через утворення проміжних сполук як внаслідок катодної поляризації, так і ад-атомами водню за рахунок реалізації спілловер-ефекту. Варіювання режимів і параметрів електролізу дозволяє формувати композитні металоксидні покриви в системі ферум-кобальт-молібден інкорпорацію до складу металевої матриці оксидів молібдену, як інтермедіатів електродних реакцій. Обґрунтовано кількісний склад електроліту та режими нанесення покривів із заданим вмістом компонентів, морфологією, структурою та експлуатаційними характеристиками. Визначено оптимальні режими поляризації, застосування яких дозволяє отримувати бездефектні покриви. Корозійний опір покривів системи Fe–Co–Mo(МоОₓ) перевищує значення для сплавотвірних компонентів, а мікротвердість майже втричі вища за мікротвердість матеріалу основи та індивідуальних компонентів тернарної системи. Високу електрокаталітичну активність покривів виявлено в катодних реакціях виділення водню, яка внаслідок реалізації синергетичного ефекту вища порівняно із індивідуальними металами і зростає з вмістом молібдену, а активність покривів Fe–Co–Mo(МоОₓ) в анодних реакціях окиснення низькомолекулярних спиртів за густиною струмів анодних і катодних піків навіть вища, ніж на платині. Покриви є “магнітом’якими матеріали”, які можна застосовувати у виробництві магніто-оптичних інформаційних накопичувачів, а сенсорні властивості щодо окремих компонентів газових середовищ використано для створення чутливого елемента сенсора. Запропоновано технологічну схему електроосадження покривів Fe–Co–Mo(МоОₓ) залежно від їх практичного призначення.
Thesis for the degree of Candidate of Technical Sciences in the speciality 05.17.03 – Technical еlectrochemistry. – National Technical University “Kharkiv Polytechnic Institute” Kharkiv, 2019. The dissertation is devoted to the development of technology for electrodeposition of functional coatings by alloys of iron with cobalt and molybdenum from complex citrate electrolytes. Based on the analysis of ionic equilibria and kinetic laws, it was found that molybdenum is converted into an alloy with iron and cobalt to a metallic state from heteronuclear complexes through the formation of intermediate spokes both as a result of cathodic polarization and as a result of the formation of hydrogen and hydrogen atoms. realize overflow effect. Changing the modes and parameters of electrolysis allows the formation of composite metal oxide coatings in iron-cobalt-molybdenum system by including a metal matrix of molybdenum oxide as an intermediate link of electrode reactions. The quantitative composition of the electrolyte and the modes of coating with a given content of components, morphology, structure and operational characteristics are justified. The optimal polarization modes are determined, the use of which allows one to obtain defect-free coatings. The corrosion resistance of the coatings of the Fe-Co-Mo(MoOₓ) system exceeds the value for the alloy components, and the microhardness is three times higher than the microhardness for steel and individual components of the ternary system. High electrocatalytic activity of the coatings was found in cathodic hydrogen evolution reactions, which, as a result of the synergistic effect, is higher than for individual metals, and grows with the molybdenum content and the activity of Fe-Co-Mo (MoOₓ). Coatings in the reactions of anodic oxidation of low molecular weight alcohols at a current density of the anodic and cathodic peaks are even higher than on a platinum electrode. The coatings turned out to be "soft magnetic materials" that can be used in the manufacture of magneto-optical information storage devices, and the sensory properties of individual components of gaseous media were used to create a sensitive element of the sensor. The technological scheme of electrodeposition of Fe-Co-Mo (MoOₓ) coatings is proposed, depending on their practical purpose.

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.17.03 – Технічна електрохімія. – Національний технічний університет «Харківський політехнічний інститут», Харків, 2019. Дисертаційну роботу присвячено розробці гальванохімічної технології покривів тернарними сплавами і композитами системи ферум-кобальт-молібден з підвищеним рівнем функціональних властивостей на підставі гіпотези про інкорпорацію оксидів тугоплавких компонентів як інтермедіатів електродних реакцій, до складу металевої матриці. Обґрунтовано компонентний склад електроліту та співвідношення концентрацій сплавотвірних компонентів в системі ферум-кобальт-молібден і закономірності комплексоутворення в присутності цитрату, які склали підгрунтя до розробки електролітів для нанесення металевих і металооксидних покривів. Встановлено, що бездефектні покриви високої якості з вмістом молібдену понад 30 ат.% формуються з електролітів із концентрацією натрій цитрату 0,4−0,5 М та оксометалату 0,2 М. Доведено, що утворення гетероядерних комплексів є передумовою для гнучкого керування іонними рівновагами в розчині, а відтак, і механізмом та перенапругою електродних реакцій, перебіг яких підпорядковується закономірностям змішаної кінетики, про що свідчать і визначена енергія активації процесу. Відновлення молібдат-іону до металевої фази відбувається через утворення поверхневих оксидів проміжного ступеню окиснення. Залежно від повноти перебігу цього процесу створюються умови до формування металевого покриву тернарним сплавом або металоксидного композиту, друга фаза якого складається з оксидів молібдену в проміжному ступені окиснення, тобто утворюється безпосередньо в електродному процесі. Встановлено, що відновлення оксометалату може перебігати в декілька стадій − як за електрохімічним, так і хімічним механізмом за участю ад-атомів гідрогену, які утворюються в катодній реакції. Саме така особливість забезпечує варіативність катодного процесу та надає можливість гнучкого керування перебігом окремих стадій і складом та властивостями цільового продукту технологічного процесу. Головними чинниками, що забезпечують варіативність складу покривів, є режим поляризації – гальваностатичний або імпульсний, та амплітудні і часові параметри струму. За однакових густин струму застосування імпульсного електролізу дозволяє формувати покриви із значно вищим вмістом молібдену. Зокрема, за сталих тривалості імпульсу 10-20 мс та паузи 5–20 мс склад покривів збагачується молібденом до 30 ат. % при суттєво нижчому вмісті оксигену. Такі зміни у складі покриву порівняно з стаціонарним режимом зумовлені перебігом впродовж паузи хімічної реакції відновлення проміжних оксидів молібдену ад-атомами водню внаслідок реалізації спілловер-ефекту. Вища кількість фази оксидів в складі покривів тернарним сплавом, сформованих в гальваностатичному режимі, дає підстави класифікувати їх як композити. За однакового типу поляризації струмозалежними виявились не тільки вміст компонентів сплаву/композиту, а й морфологія поверхні осадів та вихід за струмом. В умовах стаціонарного електролізу вихід за струмом сплаву знаходиться в межах 56−62 %, а при застосуванні імпульсного електролізу ефективність процесу зростає до 61−70 % за рахунок внеску хімічної реакції відновлення оксидів молібдену ад–атомами водню. Розсіювальна здатність електроліту також залежить від густини струму і має екстремальний характер із максимумом у 62 % при і=2,5 А/дм². Отримані результати розсіювальної здатності узгоджуються із значеннями для відомих електролітів. Композитні Fe−Co−MoОₓ і металеві Fe−Co−Mo покриви мають дрібноглобулярну структуру поверхні, розвиненість якої зростає зі збільшенням густини струму, а характер і розмір кристалітів залежить від складу покривів і режимів електролізу. Так, для покривів Fe48Сo40Mo12, отриманих постійним струмом, середній розмір кристалітів становить 63 Ǻ, а для покриву Fe43Сo39Mo18, отриманому в імпульсному режимі, середній розмір кристалітів складає 56 Ǻ. Залежно від режимів електроосадження різниться і шорсткість поверхні – в гальваностатичному та імпульсному режимах параметр Ra для сплавів становить 0,15 і 0,11, відповідно, що характерно для 9–10 класів шорсткості. Синтезовані покриви мають широкий спектр фізико-хімічних і фізико-механічних властивостей з високим рівнем споживчих характеристик. Так, тестуванням корозійної тривкості встановлено, що за глибинним показником (0,018 – 0,02 мм/рік) покриви мають 4 бал стійкості за десятибальною шкалою, а ранжовані за густиною струму корозії є "стійкими" в кислому середовищі та "вельми стійкими" у нейтральному та лужному. Корозійну стійкість в кислому середовищі підвищує наявність молібдену через кислотний характер його оксидів, а в нейтральному і лужному середовищах покриви проявляють стійкість внаслідок пасивації феруму і кобальту. Вільна енергія поверхні металевих покривів і композитів в межах 118−128 мДж/м², що майже на порядок величини нижча за сплавотвірні компоненти, а поверхні композитів Fe−Co−MoОₓ нижча ніж сплаву Fe−Co−Mo завдяки вищому вмісту кисню в його структурі, внаслідок чого покриви композитами є хімічно стійкішими. Мікротвердість гальванічних покривів знаходиться в межах 595 – 630 кгс/мм² і є вищою, порівняно із сплавотвірними компонентами, а також в 2,5–3 рази більшою за сталеву основу. Мікротвердість осадів симбатно змінюється із вмістом молібдену і в інтервалі досліджених густин струму також зростає з підвищенням цього параметра. Результатами комплексних випробувань механічних характеристик доведено високу адгезію покривів до поверхні підкладки, стійкість до полірування, нагріву і зламу. Встановлено високу електрокаталітичну активність тернарного сплаву в анодних реакціях окиснення низькомолекулярних спиртів, а значення анодних і катодних піків струму на циклічних вольтамперограмах навіть вищі за платиновий електрод, тому гальванічні покриви сплавом Fe−Co−Mo можна розглядати як перспективні каталітичні матеріали паливних елементів. Високу електрокаталітичну активність покривів виявлено і в катодних реакціях виділення водню з лужних та кислих середовищ, яка внаслідок реалізації синергетичного ефекту вища порівняно із індивідуальними металами. Встановлено залежність між складом сплаву і каталітичними властивостями – більший вміст молібдену в цілому покращує якість покривів. Водночас, густина струму обміну реакції виділення водню на композитних покривах в усіх модельних розчинах вище, ніж для металевих, що узгоджується з результатами визначення виходу за струмом. Покривам притаманні магнітні властивості, а значення коерцитивної сили для покривів Fe−Co−Mo знаходиться в інтервалі 7−10 Ое, що перевищує значення для бінарного Fe−Co сплаву (6,5–7,2 Ое). Сплави Fe−Co−Mo, як "магнітом’які матеріали", можна застосовувати і у виробництві елементів магнітних інформаційних накопичувачів. Означений сплав виявляє сенсорні властивості щодо окремих компонентів газових середовищ та може бути використаний, зокрема, як матеріал чутливого елемента сенсора для визначення граничної концентрації водню. На підставі визначених кінетичних характеристик і технологічних струмозалежних параметрів створено програмний і технологічний модулі і запропоновано варіативну технологічну схему нанесення покривів Fe−Co−Мо(МоОₓ) керованого складу та прогнозованими фізико-механічними і фізико-хімічними властивостями. За результатами дослідно-промислових випробувань виробів та елементів обладнання з покривами тернарними сплавами на ПАТ "Укрндіхіммаш" та в Метрологічному центрі військових еталонів Збройних Сил України доведено високий рівень експлуатаційних характеристик синтезованих покривів та ефективність технології їх нанесення. Результати досліджень впроваджені в навчальний процес кафедри фізичної хімії НТУ "ХПІ" і Військового інституту танкових військ НТУ "ХПІ".
Thesis for the degree of Candidate of Technical Sciences in the speciality 05.17.03 – Technical Electrochemistry. – National Technical University «Kharkiv Polytechnic Institute» Kharkiv, 2019. The component composition of the electrolyte and the ratio of the concentrations of the alloys forming components in the ferum-cobalt-molybdenum system and the regularities of the complex formation in the presence of citrate, which became the basis for the development of electrolytes for metal deposition and metal oxide coatings are substantiated. It was found that high-quality coatings with a molybdenum content of more than 30 at.% Are formed from electrolytes with a concentration of sodium citrate of 0,4 – 0,5 М and oxometalate of 0,2 М. It is proved that the formation of heteronuclear complexes is a prerequisite for the flexible control of ionic equilibria in solution, the mechanism and overvoltage of electrode reactions, the course of which obeys the laws of mixed kinetics, which is confirmed and determined by the activation energy of the process. The reduction of the molybdate ion to the metal phase occurs by the formation of surface oxides of an intermediate oxidation state. Depending on the completeness of the course of this process, conditions are created for the formation of a metal coating of a ternary alloy or a metal oxide composite, the second phase of which consists of molybdenum oxides in an intermediate oxidation state, that is, is formed directly in the electrolysis process. The reduction of oxometalate can occur in several stages using both the electrochemical and chemical mechanisms, which include hydrogen ad-atoms and atoms that are formed in the cathodic reaction. It is this feature that provides the variability of the cathode process and allows flexible control of the stages, as well as the composition and properties of the product of the technological process. The main factors ensuring variability of the coating composition are polarization modes — galvanostatic and pulsed modes, and amplitude and time parameters of the current. At the same current densities, the use of pulsed electrolysis allows the formation of coatings with a significantly higher molybdenum content. In particular, with a constant pulse duration of 10–20 ms and pauses of 5–20 ms, the composition of the shells is enriched in molybdenum to 30 at.% With a significantly lower oxide content. Such changes in the composition of the coating compared with the stationary regime are due to the chemical reaction of the reduction of intermediate molybdenum oxides by hydrogen atoms as a result of the overflow effect. The higher content of the oxide phase in the composition of tournament alloys formed in the galvanostatic mode allows us to classify them as composites. With the same polarization mode, the parameters depending on the current are determined not only by the content of the components of the alloy or composite, but also by the morphology of the coating surface and the current efficiency. Under the conditions of stationary electrolysis, the efficiency of the alloy is in the range 56−62 %, and when using pulsed electrolysis, the efficiency of the process increases to 61–70 % due to the chemical reaction of the reduction of molybdenum oxides. hydrogen atoms of hydrogen. The dissipated ability of the electrolyte also depends on the current density and is extreme in nature with a maximum of 62% at i = 2.5 A/dm². Dissipation results are consistent with known electrolytes. Composite coatings Fe−Co−MoOₓ and metallic coatings Fe−Co−Mo have a fine-crystalline structure, surface development increases with increasing current density, and the nature and size of crystallites depends on the composition of the coatings and electrolysis conditions. So for Fe48Co40Mo12 coatings obtained by direct current, the average crystallite size is 63 Ǻ, and for Fe43Co39Mo18 coatings obtained in a pulsed mode, the average crystallite size is 56 Ǻ. Depending on the electrodeposition modes, the surface roughness also varies - in the galvanostatic and pulsed modes, the parameter Ra for the alloys is 0,15 and 0,11, respectively, which corresponds to grades 9-10. The synthesized coatings have a range of physico-chemical and physico-mechanical properties with a high level of performance. Thus, corrosion resistance testing shows that the depth of the index (0,018 – 0,02 mm/year) coatings are characterized as 4 points of resistance on a ten-point scale, and ranked according to the density of the corrosion current is "stable" in acidic solutions and "very stable" in neutral and alkaline solutions. Corrosion resistance to the acid solutions increases the presence of molybdenum through the acidic nature of its oxides, and in neutral and alkaline solutions the covers exhibit resistance due to passivation of iron and cobalt. The free energy of the surface of metal coatings and composites is in the range of 118-128 mJ/m², which is almost an order of magnitude lower than the alloys of the component and the surfaces of the Fe−Co−MoOₓ composites lower than the Fe−Co−Mo alloy due to the higher oxygen content in its structure. , causing the composites to be chemically stable. The microhardness of galvanic coatings is in the range of 595–630 kgf/mm² depending on the individual components and is 2,5–3 times higher than for steel. The microhardness of the coatings increases symbatically with an increase in the amount of molybdenum in the alloy and also increases with an increase in this parameter in the integral of current densities. The high adhesion of the coatings to the surface of the steel, resistance to polishing, heating and kink is established. The high electrocatalytic activity of ternary alloys in the reactions of anodic oxidation of low molecular weight alcohols was established, and the magnitude of the peaks of the anodic and cathodic currents in the cyclic voltammogram is even higher than that of the platinum electrode, so galvanic coatings with Fe−Co−Mo alloy can be considered a promising catalytic material for fuel cells. High electrocatalytic activity of the skin was also detected in cathodic reactions of hydrogen evolution from alkaline and acidic media, which is higher as a result of the synergistic effect compared to individual metals. A connection was established between the alloy composition and catalytic properties – a higher molybdenum content usually improves the quality of coatings. At the same time, the exchange current density of the hydrogen evolution reaction on composite coatings in all model solutions is higher than for metal coatings, which is consistent with the results of determining the current efficiency. The coatings have magnetic properties, and the value of the coercive force for Fe—Co−Mo coatings is in the range of 7-10 Oe, which is higher than the value for the Fe−Co alloy (6,5-7,2 Oe). Fe−Co−Mo alloys are "Magnetic materials" and can be used in the production of magnetic information storage elements. The alloy has sensory properties on the individual components of the gas environment and can be used, in particular, as a sensor material of the sensor to determine the maximum hydrogen concentration. Based on kinetic characteristics and technological parameters, software and technological module have been created and a variable technological scheme for applying Fe−Co−Mo(MoOₓ) coatings of controlled composition and predicted physicomechanical and physicochemical properties has been proposed. According to the results of tests and elements of equipment coated with ternary alloys at PJSC "Ukrndikhimmash" and at the Metrological center of military standards of the Armed Forces of Ukraine, a high level of operational characteristics of the synthesized coatings and the effectiveness of the technology for their synthesis have been proved. The research results were introduced into the educational process of the Department of Physical Chemistry NTU "KhPI" and the Military Institute of Tank Troops NTU "KhPI".

京都大学
新制・課程博士
博士(人間・環境学)
甲第23288号
人博第1003号
京都大学大学院人間・環境学研究科相関環境学専攻
(主査)教授 内本 喜晴, 教授 高木 紀明, 教授 中村 敏浩
学位規則第4条第1項該当
Doctor of Human and Environmental Studies
Kyoto University
DFAM

Kyoto University (京都大学)
0048
新制・課程博士
博士(人間・環境学)
甲第21876号
人博第905号
新制||人||216(附属図書館)
2018||人博||905(吉田南総合図書館)
京都大学大学院人間・環境学研究科相関環境学専攻
(主査)教授 内本 喜晴, 教授 田部 勢津久, 教授 吉田 鉄平
学位規則第4条第1項該当

New methods for analysis of current-potential curves in terms of their derivatives are presented for studying various types of electrode processes – such as simple electron transfer reactions (reversible, quasi-reversible, and irreversible electron transfer) as well as chemically coupled electron transfer reactions along with a diagnostic scheme for differentiating these various types of electrochemical reaction mechanisms. Expressions for first- and higher order derivatives are derived from theoretical analytical solutions for currents for the different types of electrode mechanisms. The derivative curves are analyzed in terms of various parameters which characterize peak shape or peak symmetry with an emphasis on the second derivatives with well-defined anodic and cathodic peaks. Second derivatives can yield, in a simpler manner, the symmetry ratios; i.e., a ratio of anodic to cathodic peak-currents (ipa/ipc), and a ratio of anodic to cathodic peak-widths (Wpa/wpc) and a ratio of anodic to cathodic peak potential differences (ΔEpa/ΔEpc) or a peak separation (Epa-Epc) are evaluated, and these ratio can be related to kinetic parameters associated with a particular types of electrode mechanisms. Peaks are found to be symmetrical for a simple reversible electron transfer process (Er). However, peaks become asymmetrical when the electron transfer become slower (namely, irreversible, Eirr) or e− transfer reaction is coupled with homogeneous chemical reactions such as a prior reaction (CEr) or a follower-up reaction (ECr). From measured values of such symmetry ratios above, one can gain insight to the nature of the electrochemical systems enabling us to determine various kinetic parameters associated with a system. A diagnostic criteria for assigning an electrode mechanism is devised based on the values of asymmetry parameters measured, which are unity for a simple reversible electron transfer process.

Selective and efficient electrochemical methods to characterize aluminum are necessary. Current methods are based on potentiodynamic polarization, recurrent potential double pulses, chronopotentiometry, open-circuit chronopotentiometry, and potentiostatic electrolysis, but have not been used to characterize the deposition of aluminum in Na3AlF6-Al2O3-KF molten salts. The control processes of the formation of aluminum-tungsten inter-metallic compounds, and the deposition of aluminum have been investigated by using steady-state potentiodynamic cathodic polarization curves. The dissolution loss rate of aluminum was determined with an increase in KF concentration by the analysis of recurrent potential double pulses. Using chronopotentiometry, it was confirmed that the deposition potential of aluminum shifted more negative as the KF concentration increased, and a higher KF concentrations induced a higher cathodic overpotential. From open-circuit potential measurements and scanning electron micrographs, it was concluded that aluminum(III) ions react with tungsten substrates to form an aluminum-tungsten compound, and the reaction mechanism of aluminum was determined. These electrochemical methods applied with aluminum electrolysis were accurate, efficient, and reliable.

The hydrogen production by SOECs coupled with renewable energy sources is a promising route for the sustainability hydrogen economy. Multiphysics computing simulations appear to be the most efficient approaches to analyze the coupled mechanisms of SOEC operation. Using a relevant model, it is possible to predict the electrical behavior of solid oxide electrodes considering the current collector design. The influences of diffusion and grain diameter on cell performances can be investigated through 2D simulations, current–voltage characteristics, and current source distribution through electrodes. The simulation results emphasize that diffusion is linked to a relocation of the reaction away from the interface electrolyte/electrode, in the volume of the cathode. Furthermore, the current collector proves itself to be a great obstacle to gas access, inducing underneath it a shortage of steam. Inducing gradients of grain diameters in both anode and cathode drives the current sources to occur close to the electrode/electrolyte interface, thus decreasing ohmic losses and facilitating gas access. This approach shows the crucial importance of cathode microstructure as this electrode controls the cell response.

In spite of industries’ effort over the last 40 years, corrosion-related issues continue to be one of the largest unresolved problems for nuclear power plants worldwide. There are several types of strange corrosion phenomena from the point of view of our current understanding of corrosion science established in other fields. Some of these are IGSCC, PWSCC, AOA, and FAC (Erosion-Corrosion). Through studying and coping with diverse corrosion phenomena, the author believes that they share a common basis with respect to the assumed corrosion mechanism (e.g., ‘local cell action’ hypothesis). In general, local cell action is rarely severe since it produces a fairly uniform corrosion. The ‘long cell action’ that transports electrons through structures far beyond the region of local cell corrosion activities has been identified as a basic mechanism in soil corrosion. If this mechanism is assumed in nuclear power plants, the structure becomes anodic in the area where the potential is less positive and cathodic where this potential is more positive. Metallic ions generated at anodic corrosion sites are transported to remote cathodic sites through the circulation of water and deposits as corrosion products. The SCC, FAC (E-C) and PWSCC occur in the anodic sites as the structure itself acts as a short-circuiting conductor between the two sites, the action is similar to a galvanic cell but in a very large scale. This situation is the same as a battery that has been short-circuited at the terminals. No apparent external potential difference exists between the two electrodes, but an electrochemical reaction is still taking place inside the battery cell with a large internal short current. In this example what is important is the potential difference between the local coolant and the surface of the structural material. Long cell action corrosion is likely enhancing the local cell action’s anodic corrosion activities, such as SCC, FAC/E-C, and PWSCC. It tends to be more hazardous because of its localized nature compared with the local cell action corrosion. There exist various mechanisms (electrochemical cell configurations) that induce such potential differences, including: ionic concentration, aeration, temperature, flow velocity, radiation and corrosion potentials. In this paper, the author will discuss these potential differences and their relevance to the un-resolved corrosion issues in nuclear power plants. Due to the importance of this potential mechanism the author is calling for further verification experiments as a joint international project.

Principle of Gas Tungsten Arc Welding is quite simple. Electron, which is emitted from Tungsten electrode, hits into the metal, and the heat from this impingement produce enough heat for fusion welding. However, actual behaviors are quite complicated. Electron emission is much influenced by physical condition of cathode and ambient circumstance. Considerable electron emits from unbelievable location of the cathode, especially at the stage of the cathode is not enough hot. The anode spot, where main electrons hit into the metal, is also much influenced by surface condition. The anode spot is usually appeared at the center of molten pool where the temperature seems to be maximum value. But, it sometimes moves irregularly or hits on the solid surface. This situation is mainly caused by oxidized layer on the solid metal or molten metal. Some minor elements in/on the metal act as a kind of catalytic agent. It sometimes drastically changes weld geometry. Fundamental experiment to observe arc instability was carried out from vacuum to high pressure, where contents of gas are controlled. Effects of oxygen and minor elements on anode reactions were measured by high speed digital video cameras. Measured molten metal flow, plasma distribution and anode behavior during GTA welding will be discussed in detail.

In the previous papers, the author has established various ‘long cell’ corrosion configurations that should exist in nuclear power plants. With these corrosion mechanisms in place, the plant can be characterized as an assembly of gigantic short-circuited electrical batteries, inducing electrochemical corrosion at localized anodic sites. If these corrosion cells are involved at nuclear power plants, macroscopic electrochemical potential differences must be demonstrated between anodic sites where dissolution of metal (i.e. corrosion) is taking place and cathodic sites where deposition (also called sedimentation) of corrosion products are often observed. Among these, the radiation-induced corrosion cell is an important mechanism of corrosion issues among nuclear power plants, since it plays a major role in the corrosion problems found in primary water, including PWSCC and AOA in PWRs and IGSCC in the BWRs. There is numerous experimental evidence indicating a potential difference induced by radiation, however, the exact mechanism of such phenomena has not been investigated from the ‘long cell action’ corrosion hypothesis point of view. The author investigated the basic mechanism by combining radiation chemistry, electrochemistry and corrosion science to confirm the existence of radiation-induced ‘long-cell’ action (macro) corrosion cell. By performing a competition kinetic study, which is a simplified approach to determine which of several competing reactions will predominate, the hydrated electrons, e−aq, reacting mainly with stable molecules, are found responsible for inducing a large portion of the potential difference both in the PWR and BWR water chemistry environment. The hydrated electrons react with a cathodic half-cell included in the stable solutes thereby inducing redox reactions in the mixed cell configuration with both reducing and oxidizing actions. This method reproduces the reported experimentally observed ECP variation to a certain extent (observed in the INCA Test Loop in Sweden and NRI-Rez BWR-2 Loop in Czech Republic) which was measured by widely changing the solute concentrations, such as dissolved hydrogen and oxygen. The author believes the results support the assumed major reactions acting in the redox process.

Under cathodic conditions, rubber/steel adhesive bonded joints have been documented to ‘weaken’ due to attack by the generated alkali. If this were to occur under the action of cleavage mechanical loads, the bonds are likely to completely ‘delaminate’ causing the bonded constituents to physically separate. These two modes of disbondment are referred to as ‘weakening’ and ‘delamination’, respectively. Previously, Hamade and coworkers have implemented empirical and semi-empirical approaches to modeling cathodic disbondment of adhesive joints. Here, a method is presented to simulate bond weakening progress via numerical solutions. Bond degradation is modeled as a liquid-solid chemical reactor due to the attack by the alkaline medium. Specifically, the diffusion and chemical reaction processes involved in weakening are mathematically represented via a simplified, 2 partial differential equations (p.d.e.) boundary value problem (BVP). This is a reduced version of the more complex electrochemical formulation needed to fully describe the chemistry at the bondline under cathodic conditions. The weakening model is capable of simulating weakened bond lengths vs. time as function of electrolyte type (artificial sweater, ASW, or 1N NaOH), cathodic potential, and temperature. Furthermore and to model bond delamination, a mechano-chemical failure criterion is incorporated into the weakening formulation effectively coupling fracture mechanics principles with those of cathodic degradation. A fracture mechanics parameter, applied strain energy release rate, G, is used to represent the effect of externally applied loads. The failure criterion stipulates that the bond will delaminate if the applied G exceeds that of the degraded bond’s residual resistance. Both, the weakening and delamination formulations are validated against experimental data of bond weakening and delamination under a variety of conditions. As such, the numerical simulations developed in this work may be used to provide first order estimates of the life of rubber/steel bonded joints (weakened or delaminated lengths vs. time) as function of cathodic parameters and applied G (if the joint is loaded in the case of delamination).

Decomposed hydrogen peroxide (H2O2) contains excess moisture and stabilizers that can deteriorate SOFC performance and durability markedly in the cathode. A numerical study is performed for a strontium-doped lanthanum manganate and yttria stabilized zirconia (LSM-YSZ) composite cathode using an oxidant stream consisting of a 20% O2 and 80% H2O mixture to study the detailed reaction mechanism and local transport and polarization phenomena. Specifically the 1-D cathode model couples multi-component gas and charge transport with an oxygen reduction mechanism. The model is validated with data from the literature and used to study the transport effect only of H2O in the cathode. Future work will consider the effect of H2O on the electrochemical reaction mechanism.

The oxygen reduction mechanism at porous Sm0.5Sr0.5CoO3−δ/Sm0.2Ce0.8O1.9 composite cathode, in which Sm0.5Sr0.5CoO3−δ is a perovskite type mixed ionic and electronic conductor (MIEC), was studied with respect to the oxygen partial pressure and temperature. Symmetric half cells with Sm0.2Ce0.8O1.9 electrolyte were prepared, and cathode behavior was measured by using electrochemical impedance spectroscopy at frequency range of 0.1Hz∼5MHz and temperature range of 400∼900°C. Oxygen partial pressure range for the measurement was from 0.0002 to 1atm. In present research, reaction model based on the empirical equivalent circuit was established. Three elementary reaction steps were considered to describe the oxygen reduction reaction at Sm0.5Sr0.5CoO3−δ/Sm0.2Ce0.8O1.9 composite cathode. Electrode resistances corresponding to the high and low frequency seem to represent the oxygen ion transfer at the interface of electrolyte and gas phase diffusion of oxygen, respectively, from electrochemical impedance analyses as functions of oxygen partial pressure and temperature. The medium frequency process is expected to correspond to the oxygen ion conduction in the bulk cathode from this study.

Corrosion caused by the discharge of 60 Hz AC current from a pipeline in a high voltage AC (HVAC) corridor has been discussed and studied over the past 20 or more years. More recent studies in Europe have specifically addressed these corrosion issues following several failures attributed to the presence of AC discharge from the pipeline. Very few corrosion failures in North America have been specifically attributed to what is termed AC-enhanced corrosion (ACEC). One missing area of research is well-controlled laboratory experiments in soil environments. This study proposed a mechanism of ACEC that is based on conventional electrochemistry using the same equivalent analog circuits used to discuss other corrosion processes. It was shown that only a small amount of the 60 Hz AC current discharge passes through the resistive component of the equivalent circuit, which results in corrosion (metal loss) reactions. The AC current passing through this resistive component produces both anodic and cathodic polarization shift (sine wave dependent) resulting in a net increase in the average corrosion rate as compared to the free-corrosion rate. The proposed model for ACEC does not invoke any new electrochemical concepts and is based on the conventional (DC) treatment of the corrosion processes; the model excludes treatment of cases with imposed cathodic protection current. The amount of ACEC is dependent on the magnitude of AC current that passes through the resistive component of the parallel resistive-capacitive electrochemical interface. ACEC is characterized by the rapid formation of a diffusion controlled (Warburg) process for corrosion in soils. Although diffusion controlled, the overall impedance decreases as the total AC current increases. The model suggests that AC currents (60Hz) cause anodic (positive) polarization shifts during the positive portion of the imposed AC sinewave along with cathodic polarization shifts in the negative portion of the AC sinewave; the net result is an increase in the average oxidation (metal loss) current as compared to the free-corrosion condition. The proposed model for the ACEC mechanism showed excellent correlation with the experimental results. The research work was made possible by the funding from PRCInternational.

High surface area nanometal particles of nano-cobalt (n-Co) (approx 8 nm particles), produced at QuantumSphere Inc., were blended in various ratios with Pt and Nafion® ionomer, and investigated for their electrocatalytic activity in the oxygen reduction reaction (ORR). The ORR was evaluated by voltammetry using Pt/n-Co blended catalyst on glassy carbon to determine both kinetic activity and as an indicator of direct methanol fuel cell (DMFC) cathode performance. Kinetic enhancement was observed for Pt:n-Co where n-Co is 30–50% (by weight) of the catalyst mixture, including a minimum of 10 mV improvement in the open circuit voltage (OCV). By Tafel slope measurements, it is clear that the mechanism for ORR does not change, however the reaction rate is enhanced by addition of n-Co to Pt in the catalytic ink. For ink compositions similar to those used for standard DMFC cathodes, eliminating 50% of the Pt black resulting in 50% higher energy density while reducing total catalyst cost by roughly 44%.

The correct prediction of the temperature distribution is a prerequisite for the reliable determination of species and current distributions in any solid oxide fuel cell (SOFC) model. It is even more crucial if the model is intended for the analysis of thermo-mechanical stresses. This paper addresses the different mechanisms of heat generation and absorption in the fuel cell. Particular attention is paid to the heating associated with the oxidation of hydrogen, which is commonly assigned to the interface between electrolyte and anode in SOFC modeling. But for a detailed determination of the temperature profile in the fuel cell solid components the separate consideration of the cathodic and anodic half-reactions is required. A method for determining the specific entropy change of the half-reactions based on Seebeck-coefficient data is adopted from the literature and applied to the SOFC. In order to exemplarily demonstrate the contribution of the various heat sources to the overall heat generation as well as the influence of their location, a spatially discretized model of a tubular SOFC is used. Temperature profiles obtained with and without separate consideration of the electrode reactions are compared. The comparison shows that the spatially descretized reaction model is indeed necessary for the reliable assessment of temperature gradients in the ceramic SOFC components.

Analysis and design of flow fields for proton exchange membrane fuel cell (PEMFC) require coupled solution of the flow fields, gas transport and electrochemical reaction kinetics in the anode and the cathode. Computational cost prohibits the widespread use of three-dimensional models of the anode and cathode flow fields, gas diffusion layers (GDL), catalyst layers (CL) and the membrane for fluid flow and mass transport. On the other-hand, detailed cross-sectional two-dimensional models cannot resolve the effects of the anode and cathode flow field designs. Here, a two-dimensional in-plane model is developed for the resolution of the effects of anode and cathode flow channels and GDLs, catalyst layers are treated as thin-layers of reaction interfaces and the membrane is considered as a thin-layer that resist the transfer of species and the ionic current. Brinkman equations are used to model the in-plane flow distribution in the channels and the GDLs to account for the momentum transport in the channels and the porous GDLs. Fick’s law equations are used to model transport of gas species in the channels and GDLs by advection and diffusion mechanisms, and electrochemical reactions in the CL interfaces are modeled by Butler-Volmer equations. Complete features of the flow in the channels and inlet and outlet manifolds are included in the model using resistance relationships in the through-plane direction. The model is applied to a small cell having an active area of 1.3 cm2 and consisting of 8 parallel channels in the anode and a double serpentine in the cathode. Effects of the anode and cathode stoichiometric ratios on the cell performance and hydrogen utilization are investigated. Results demonstrate that for a sufficiently high cathode stoichiometric ratio enough, anode stoichiometric ratio can be lowered to unity to obtain very high hydrogen utilization and output power.