Resumo: No presente trabalho estudou-se a construção, caracterização e aplicação físico-química do eletrodo Pt|Hg|Hg2Ox|Grafite, sensível ao íon oxalato (Ox), em meio de NaNO3 a (25,0 ± 0,1) ºC, no intervalo de força iônica compreendido entre 0,300 e 1,200 mol L-1, cujos experimentos foram conduzidos no menor intervalo de tempo possível. Os dados potenciométricos foram adquiridos a cada 30s, onde o coeficiente angular médio das curvas analíticas foi de 28,4 mV/dec, com um intervalo útil de concentração de 0,0348 a 0,0669 mol L-1. Os valores de coeficiente molar de atividade do íon oxalato (yOx) em meio de NaNO3 foram maiores do que os respectivos valores em meio de NaClO4 (obtidos em trabalho anterior), para as mesmas forças iônicas devido às características intrínsecas de cada eletrólito. O eletrodo apresenta resposta repetitiva, em função do tempo de uso, em meio de NaClO4 por um período superior a oito meses, enquanto que em meio de NaNO3, apresenta diminuição na sua sensibilidade de cerca de 50%, para 0,500 mol L-1 (NaNO3), em 75 dias. As micrografias das superfícies das pastilhas sensoras utilizadas em meio de NaClO4 não apresentaram alteração. Todavia as utilizadas em meio de NaNO3 mostraram alterações significativas após cerca de trinta horas consecutivas de imersão em solução contendo Na2Ox, entretanto, tais alterações não foram observadas até cerca de quatro horas de imersão. / Abstract: An oxalate sensitive electrode based on Pt|Hg|Hg2Ox|Grafite matrix was studied in conditions as follows: 0.300 - 1.200 mol L-1 NaNO3 interval of ionic strength, at (25.0 ± 0.1) ºC. All experiments were carried out in the shortest time interval. The potenciometric data was acquired at each 30s, where the medium angular coefficient of the analytical curves was of 28.4 mV/dec, with an useful interval of concentration of 0.0348 to 0.0669 mol L-1. The values of single molar ion activity coefficients associated to oxalate ion (yOx) in NaNO3 media were higher than those obtained in previous work in NaClO4 media, in the same ionic strength due to the intrinsic characteristics of each electrolyte. The electrode presents repetitive response in NaClO4 media during eight months. In sodium nitrate media, it presents decrease in sensibility of about 50%, for 0.500 mol L-1 NaNO3 during 75 days. The surfaces of the tablet sensors used in NaClO4 media did not present any alteration. On the other way, the tablets showed significant alterations on surface after thirty hours of continuous immersion in solution containing Na2Ox and NaNO3. However, there are no significant alterations after fours hours under immersion. / Orientador: Mercedes de Moraes / Coorientador: Leonardo Pezza / Banca: Assis Vicente Benedetti / Banca: João Roberto Fernandes / Mestre

The effect of elevated temperatures on the phase transformation mechanisms in electrodes for lithium-ion batteries (LIBs) is an important but – to date – only less studied subject in battery research. In real-life applications, LIBs usually function at non-ambient conditions and especially increased temperatures give rise to safety concerns. This thesis focuses to gain deeper insights into the phase transformations at high temperatures (HTs) by tackling both the challenging hardware development of a HT in situ synchrotron X-ray diffraction (XRD) battery testing system as well as its application to study two important cathode materials: LiFePO4 and V6O13. This allows unprecedented insights into the structural changes and its influence on electrochemical performance at variable temperatures (VTs). LiFePO4 was investigated for various battery cycling rates and temperatures. Electrochemical cycling of LiFePO4 in the newly designed in situ XRD setup proved that the in situ XRD cells work from low to high cycling rates between 25 to 150oC. The current induced non-equilibrium solid solution metastable LiFePO4 phase, present at room temperature during high rate cycling, was found to be less pronounced at temperatures above 125oC. This is possibly due to faster Li-ion diffusion at HT, leading to faster phase separations in the solid solution phases. In a next step, V6O13, a promising cathode material for HT applications, especially for oil field applications, was tested using the in situ HT XRD setup. The material exhibits a very high capacity with a complex voltage profile. The underlying asymmetric discharge and charge phase transition mechanisms, which include a six-step discharge and five-step charge process, are unravelled by in situ XRD. The LixV6O13 unit cell expands sequentially in c, b, and a directions during discharge and reversibly contracts back during charge. The process is associated with a change of occupied lithium sites as well as charge ordering in LixV6O13. Density functional theory (DFT) calculations and nuclear magnetic resonance spectroscopy gave further insight into the electronic structures and preferred Li positions in the different structures formed upon cycling, particularly at high lithium contents. At HT, V6O13 exhibits an even greater capacity, as well as a more symmetric discharge and charge profile. Combining the results from the HT in situ XRD study and the DFT calculation, the most Li puckered phase was found to be able to open further along the b axis, with a new Li site getting (partially) occupied. The new Li site corresponds to more Li intercalation into the LixV6O13 structure and, therefore, more electrode charge storage capacity. The more symmetric discharge and charge process was attributed to the disappearance of phase 2 (present at room temperature for 1.7 < x ≤ 2.1 in LixV6O13) at HT.

Chemical and electrochemical methods for depositing platinum microparticles at the membrane-electrode interface of solid polymer fuel cell oxygen reduction electrodes have been developed and studied.

Magister Scientiae - MSc / In this study, the main objective was to investigate the tolerance of platinum based binary anode catalysts for CO poisoning from 10ppm up to1000ppm and to identify the best anode catalysts for PEMFCs that tolerates the CO fed with reformed hydrogen. / South Africa

The method of fabrication of electrodes by using a laser-puller is a commonly used method to produce small electrodes. Simulation shows that the taper-shaped insulation layer of a puller-made electrode has an obvious effect on its voltammetric response, especially when the electrode has a small RG (the ratio of the insulating sheath radius to the electrode radius). The effect of low-pass filtering on the voltammetric response of a very small puller-made electrode has also been studied.

The method of fabrication of electrodes by using a laser-puller is a commonly used method to produce small electrodes. Simulation shows that the taper-shaped insulation layer of a puller-made electrode has an obvious effect on its voltammetric response, especially when the electrode has a small RG (the ratio of the insulating sheath radius to the electrode radius). The effect of low-pass filtering on the voltammetric response of a very small puller-made electrode has also been studied.

Supercapacitors (SCs) have generated significant interest due to their advantages including lightweight, rapid charge-discharge, good rate capability and high cyclic stability. Electrodes are one of the most important factors influencing the performance of SCs. MXene is a promising candidate for supercapacitor electrodes, which is a relatively new material with formula Mn+1XnTx, where M is a transitional metal, X stands for C or N, and Tx is surface terminations. Due to its multi-layered structure, high surface area and rich redox chemistry, good electrochemical performance can be expected. To further enhance the conductivity of the MXene electrodes, multi-walled carbon nanotubes (MCNT) were applied as the conducting additive. The as-fabricated composite electrodes showed reduced resistance and enhanced electrochemical performance. Advanced co-dispersants such as cationic celestine blue (CCB) and anionic catechol violet (ACV) were employed to improve the dispersion of components. CCB and ACV can adsorb strongly on the MXene and MCNT surface to form a homogenous suspension and thus improve the mixing between them. Another advanced dispersant 3,4,5-trihydroxybenzamide (THB) also showed adsorption on both MXene and MCNT particles, favored their dispersive mixing and improved electrochemical performance. Iron oxides are promising materials for negative electrodes for supercapacitors. The attempt to combine highly capacitive Fe3O4 with MXene-MCNT composites proved the synergistic effect of individual components. Investigation of Zn-doped FeOOH as high active mass loading anode with MCNT as conducting additive allowed for enhanced performance. Zn-Fe double hydroxide materials are promising for the fabrication of advanced supercapacitor electrodes. A safe and neutral Na2SO4 electrolyte was was beneficial for the development of asymmetric devices with enlarged voltage window. For cathodes working in an overlapping window with Zn-FeOOH anode, polypyrrole coated carbon nanotube electrode was fabricated with a comparable capacitance. The advanced dopant eriochrome cyanine R (ECR) allowed for the uniform thickness of PPy coating on MCNT and enhanced charge transfer between PPy and MCNT was achieved. Enhanced capacitive properties of cathodes and anodes at high active mass loading working in complimentary voltage windows allowed for fabrication of high-performance supercapacitor, which was a promising device for practical applications. / Thesis / Doctor of Philosophy (PhD) / To reduce the consumption of fossil fuel and meet the surging demand of electric energy, intensive attention has been drawn to new energy storage device, such as capacitors, batteries and supercapacitors. Owing to their higher energy density compared with conventional capacitors and higher power density compared with batteries, supercapacitors are attracting tremendous research interest. The advantages of supercapacitors are fast charge-discharge rate, high power and energy density and excellent cyclic stability. The objective of this work was to fabricate high-performance supercapacitor devices based on the development of advanced electrode materials. MXene and Fe-based composite materials were synthesized by conceptually new colloidal approach and some efficient dispersants were developed during the process. The enlarged voltage window and superior performance were recorded for asymmetric supercapacitors. The results presented in this work showed much more promising performance compared with that reported in the literature and paved the way for future research.

There is considerable interest in ESs as they have huge potential in energy storage devices, play a key role in advanced power systems, and have the potential to revolutionize hybrid vehicles and electronics. SCs are known for their hybrid power and energy density, fast charge and discharge rates, and long-term cycling stability. The performance of SCs depend largely on the specific capacitance of their electrodes. Among various cathode materials, unitary transition metal oxides (TMOs), especially manganese oxide, are favored due to their multiple oxidation states, excellent redox properties, abundant availability, simple synthesis, and cost-effectiveness. The low intrinsic conductivity of manganese oxide can be significantly enhanced by adding conductive additives such as multi-walled carbon nanotubes (CNTs). We are developing a novel colloid processing technique to synthesize MnOx-CNT nanocomposites with enhanced electronic conductivity. Our research involves the use of advanced capping agents and co-dispersants to fabricate MnOx-CNTs nanocomposite electrodes that exhibit superior performance and bypass the lengthy activation process commonly cited in our previous results. Testing results indicate that functional catechol-based molecules, including quercetin (QC), rhamnolipid (RL), tetrahydroxy-1,4-quinone (TQ), catechin (CT) and gallocyanine (GA), have excellent dispersion properties for MnOx and CNTs. These compounds form uniform and stable suspensions that improve the nanostructure and electrochemical performance of the electrodes. They also serve as capping agents for Mn3O4 synthesis, reducing agglomeration and improving morphology. Additionally, murexide was tested as a co-dispersant and capping agent due to its chelating properties, forming a tridentate bond with Mn atoms and adsorbing onto the carbon rings of CNTs. As a capping agent, murexide can promote electrostatic dispersion by forming strong tridentate bonds with Mn3O4 particle surfaces, thereby reducing agglomeration and improving composite morphology. In addition, binary (MnFe2O4) or ternary (La0.65Sr0.35MnO3(LSM)) metal oxides can overcome the limitations of single metal oxides through the synergistic effect between metal ions, improve capacitive performance and expand the potential window. These compounds are promising candidates as ES electrode materials. High-energy ball milling (HEBM) helps reduce particle size, enhance electrolyte contact with active material surfaces, achieve high capacitance at high active mass loading, and produce high-performance supercapacitors (SCs). / Thesis / Candidate in Philosophy / There is considerable interest in ESs as they have huge potential in energy storage devices, play a key role in advanced power systems, and have the potential to revolutionize hybrid vehicles and electronics. SCs are known for their hybrid power and energy density, fast charge and discharge rates, and long-term cycling stability. The performance of SCs depend largely on the specific capacitance of their electrodes. Among various cathode materials, unitary transition metal oxides (TMOs), especially manganese oxide, are favored due to their multiple oxidation states, excellent redox properties, abundant availability, simple synthesis, and cost-effectiveness. The low intrinsic conductivity of manganese oxide can be significantly enhanced by adding conductive additives such as multi-walled carbon nanotubes (CNTs). We are developing a novel colloid processing technique to synthesize MnOx-CNT nanocomposites with enhanced electronic conductivity. Our research involves the use of advanced capping agents and co-dispersants to fabricate MnOx-CNTs nanocomposite electrodes that exhibit superior performance and bypass the lengthy activation process commonly cited in our previous results. Testing results indicate that functional catechol-based molecules, including quercetin (QC), rhamnolipid (RL), tetrahydroxy-1,4-quinone (TQ), catechin (CT) and gallocyanine (GA), have excellent dispersion properties for MnOx and CNTs. These compounds form uniform and stable suspensions that improve the nanostructure and electrochemical performance of the electrodes. They also serve as capping agents for Mn3O4 synthesis, reducing agglomeration and improving morphology. Additionally, murexide was tested as a co-dispersant and capping agent due to its chelating properties, forming a tridentate bond with Mn atoms and adsorbing onto the carbon rings of CNTs. As a capping agent, murexide can promote electrostatic dispersion by forming strong tridentate bonds with Mn3O4 particle surfaces, thereby reducing agglomeration and improving composite morphology. In addition, binary (MnFe2O4) or ternary (La0.65Sr0.35MnO3(LSM)) metal oxides can overcome the limitations of single metal oxides through the synergistic effect between metal ions, improve capacitive performance and expand the potential window. These compounds are promising candidates as ES electrode materials. High-energy ball milling (HEBM) helps reduce particle size, enhance electrolyte contact with active material surfaces, achieve high capacitance at high active mass loading, and produce high-performance supercapacitors (SCs).

L'utilisation de l'énergie des micro-sources de production d'électricité est un concept prometteur qui consiste à récolter des sources d'énergie faible et diffuse présent dans notre environnement pour l’alimentation de systèmes autonomes. Le nombre en croissance de nouveaux appareils miniaturisés et communicants dans les domaines civils et militaires devrait accentuer le phénomène de dépendance énergétique et ouvre de nouveaux marché.Parmi les éventuelles sources d’énergies renouvelables, l’énergie solaire est la source la plus prometteuse car elle est potentiellement la plus puissante et la mieux répartie. Le développement de ces systèmes de récupération des micro-sources d’énergie passe par de faibles coûts avec substrat souple (papier,polymère) et des matériaux facilement exploitables. Après la récupération de l’énergie, il est nécessaire pour les systèmes autonomes de stocker l'électricité.Dans cet objectif, les supercondensateurs sont les candidats idéaux. En effet, Le principal avantage des supercondensateurs par rapport aux batteries est leur haute densité de puissance (la collecte rapide de l’énergie) ainsi qu'une longue durée de vie. La thèse concerne donc la fabrication d’un supercondensateur et in fine le couplage avec une cellule solaire. Les travaux concernent spécifiquement l’étude de l'oxyde de graphène (GO) synthétisé par la méthode Hummers et Marcano, de sa réduction en oxyde de graphène réduit (RGO) par les voies chimique et électrochimique et de réalisation du supercondensateur. Dans ce projet, les propriétés de l'oxyde de graphène réduit (RGO) seront optimisées lors de l'étape de réduction et le matériau sera mis en forme dans une structure sandwich (RGO/ électrolyte /RGO) ou interdigité Mots clés: Graphène,supercondensateur, oxyde de graphène, micro-source d'énergie / The use of micro-power generation energy is a promising concept that consists in harvesting low and diffuse energy sources present in our environment for the supply of autonomous systems. The growing number of new miniaturized and communicating devices in civil and military fields should accentuate the phenomenon of energy dependence and open up new markets.Among possible sources of renewable energy, solar energy is the most promising source because it is potentially the most powerful and best distributed. The development of these micro-energy recovery systems involves low costs with flexible substrate (paper, polymer) and easily exploitable materials. After energy recovery, it is necessary for the autonomous systems to store electricity.For this purpose, supercapacitors are ideal candidates. Indeed, the main advantage of supercapacitors over batteries is their high power density (fast energy collection) as well as a long cycle life. The thesis concerns the manufacture of a supercapacitor and ultimately coupling with a solar cell. The work specifically concerns the study of graphene oxide (GO) synthesized by the Hummers and Marcano methods, its reduction in reduced graphene oxide (RGO) by chemical and electrochemical routes and the realization of supercapacitor. In this project, the properties of reduced graphene oxide (RGO) will be optimized during the reduction step and the material will be shaped into a sandwich structure (RGO / electrolyte / RGO) or interdigitated.Keywords: Graphene,supercapacitor, graphene oxide,energy micro-source

In this study, an electrolytic micro-machining tester is employed to investigate the effects of supply voltage,electrode gap and initial machining position on the geometry of the tungsten needle. The tungsten needle to be electrolyzed is dipped in an aqueous electrolyte of 2wt % sodium hydroxide as the anode, and the stainless steel needle with a diameter of 50 £gm as the cathode(tool electrode), and the electrode gap is set to be 30 £gm.Morever,the tungsten needle can be allowed to be fixed or in the reciprocating motion. Experimental results show that when the tip diameter of stainless steel needle is less than 50 £gm, a great quantity of current density and power are generated to cause the temperature failure of the tool electrode. At a certain gap between the electrodes, the electric field is focused on the narrow range to improve its processing efficiency. Under the higher supply voltage condition, it is difficult to control the size because the diameter of tungsten rod is reduced very quickly. Under the lower supply voltage condition, the diameter of tungsten rod can be processed to smaller sizes, but it takes a long time. Therefore, in the beginning of the experiment, the tungsten rod is electrolyzed under a higher voltage to the small size, and then the supply vltage is switched to lower value to conduct the electrolysis process. Consequently, a finer and more even tungsten needle can be obtained. Experimental results show that a more even tungsten needle with the length of 600 £gm and the diameter of 4 £gm can be manufactured.