Rozprawy doktorskie na temat „Electrodics and Electrocatalysis”

Dans ce travail des électrodes macro- et mesoporeuses hautement organisées ont été fabriquées grâce à l' électrodéposition dans différents types de template. Des cristaux colloïdaux obtenus par la technique de Langmuir-Blodgett ont été infiltrés par des métaux ou des polymères conducteurs en utilisant l'électrodéposition potentiostatique suivi par la dissolution du template. La taille des pores, ainsi que l'épaisseur du film macroporeux pouvaient être contrôlée respectivement par le diamètre des billes de silice et par des oscillations temporelles du courant. Différentes superstructures colloïdales ont également été produites menant à des électrodes avec des défauts artificiels ou des gradients bien définis en termes de taille des pores. Des couches alternantes de différents métaux ont été déposées avec grande précision dans une monocouche de particules entrainant une modification des propriétés optiques du matériau. La miniaturisation a pu être démontrée par l'élaboration des microcylindres d'or macroporeux qui disposent non seulement d'une plus grande surface active mais aussi d'une plus grande activité catalytique envers la réduction de l'oxygène en comparaison avec leurs homologues non poreux. Dans ce même contexte une cellule électrochimique miniaturisée composé de deux électrodes macroporeuses a été proposée. Par ailleurs du platine mesoporeux a été électrodéposé en présence d`un template de type cristaux liquides lyotropes sur des réseaux de microélectrodes. Grâce à une plus grande surface active par rapport à leurs homologues non poreux des microélectrodes mesoporeuses ont montré une meilleure performance dans l'enregistrement de l' activité neuronale due à un niveau de bruit plus faible
In the present work template-assisted electrodeposition was used to produce highly ordered macro- and mesoporous electrodes. Colloidal crystals obtained by the Langmuir-Blodgett (LB) technique were infiltrated using potentiostatic electrodeposition of metals and conducting polymers followed by removal of the inorganic template. In the resulting macroporous electrodes, the pore diameter was controlled by the size of the silica spheres, while the thickness could be controlled by temporal current oscillations caused by a periodic change of the electroactive area in the template. Various colloidal superstructures were produced in this way leading to electrodes with on purpose integrated planar defects or well-defined gradients in terms of pore size. Furthermore we showed that alternating multilayers of different metals could be deposited with high accuracy into a colloidal monolayer altering the optical properties of the material. Successful miniaturization of the process was demonstrated by elaborating macroporous gold microcylinders showing besides higher active surface areas also increased catalytic activity towards the reduction of oxygen compared to their flat homologues. In this context a miniaturized electrochemical cell composed of two macroporous gold electrodes was also proposed. Finally, mesoporous platinum films were deposited on microelectrode arrays (MEAs) using lyotropic liquid crystals as templates. The increased surface area of mesoporous compared to smooth electrodes led to improved performance in the recording of neuronal activity with MEAs owing to a reduced noise level

An electrochemical sensor based on a glassy carbon electrode (GCE) modified by a thin film of hybrid copper cobalt hexacyanoferrate (Cu-CoHCF) was prepared and tested for the determination of three thiols: L-cysteine (CySH), L-glutathione (GSH) and 1,4-butanedithiol (BdSH). Cyclic voltammetry (CV) measurements were carried out with the as prepared and thermally treated chemically modified electrode (CME) in phosphate buffer solution from pH 2 to 7. CV results showed that at pH higher than 5, the Cu-CoHCF layer was unstable and underwent significant fouling. Then, chronoamperometric measurements were carried out to develop an analytical method for the determination of thiols. Cysteine showed the lowest limit of detection (7.5 × 10-7 M), but GSH and BdSH also showed good results. The above sensor was also employed for the indirect determination of Hg2+. It exploits the formation of a redox inactive complex with thiols. CySH and BdSH were used, with the former giving more sensitive results. Interference studies led to Cu2+ being the major interferent. The interference from Cu2+ was avoided by exploiting the faster reaction kinetics between CysH and Hg2+. GCE were also modified by carbon nanomaterials, graphene oxide, GO, and multi-walled carbon nanotubes alone, mixed together (Composite) or in the form of bi-layers. The reduction of GO was carried out by means of a green approach using electroreduction. Catechol and dopamine, which are representative of polyphenols class were investigated to find which of them allows the best electron transfer kinetics. To this aim the fouling effects of the electrode surface were also taken into account. The electrochemically active areas were estimated by using two approaches in order to highlight the different phenomena that could affect the redox processes of the two analytes at the different CME. The Composite configuration displayed the best compromise in terms of sensitivity and resistance to fouling.

The main objective of this work it to produce membrane electrode assemblies (MEAs) that have improved performance over MEAs produced by the conventional manner, by producing highly efficient, electroactive, uniform catalyst layers with lower quantities of platinum electrocatalyst. The catalyst coated membrane (CCM) method was used to prepare the MEAs for the PEM fuel cell as it has been reported that this method of MEA fabrication can improve the performance of PEM fuel cells. The MEAs performances were evaluated using polarisation studies on a single cell. A comparison of polarisation curves between CCM MEAs and MEAs produced in the conventional manner illustrated that CCM MEAs have improved performance at high current densities (>
800 mA/cm2).

The investigation of the kinetics of fuel cell (FC) molecules such as methanol (MeOH), ethylene glycol (EG) and formic acid (FA) on platinum (Pt), platinum/ruthenium (PtRu) and platinum based metal complexes (ruthenium tetrakis(diaquaplatinum)octacarboxy-phthalocyanine (RuOcPcPt) modified basal plane pyrolytic graphite electrode (BPPGE) was carried out. One of the major limitations of FC molecules is that Pt undergoes surface poisoning by strongly adsorbed reaction intermediates, carbon monoxide (CO) that eventually decreases the fuel cell efficiency. Thus, the integration of Pt and or Pt/Ru with functionalized multi-walled carbon nanotubes (fMWCNTs) and some N4-macrocycles such as ruthenium phthalocyanine complexes on the BPPGE towards these FC molecules have been studied in this work. However, this study focused mainly on Pt, Pt/Ru, ruthenium octacarboxy-phthalocyanine (RuOcPc) and RuOcPcPt nanoparticles. The MWCNTs, metal and N4-macrocycles provided the needed platform for the efficient electrooxidation of FC molecules with minimum or no poisoning. The first part of the thesis deals with electrocatalytic oxidation of the FC molecules using electrodes prepared by electrodeposition techniques. The section describes the comparative electrocatalytic behaviour of MeOH, EG and FA at MWCNT-Pt/Ru immobilized on BPPGE. The Pt/Ru nanoparticles were deposited on the substrate using the electrodeposition technique. The second part of this work deals with electrocatalysis of the FC molecules using BPPG electrode modified with chemically synthesized Pt nanoparticles integrated with RuOcPc. In both cases, successful modification of the electrodes with the metal nanoparticle/carbon nanotube or metal nanocomplex nanocomposite was established using the field emission /high resolution scanning electron microscopy (FESEM/HRSEM), high resolution transmission electron microscopy (HRTEM), x-ray diffraction (XRD) spectroscopy and electron dispersive x-ray spectroscopy (EDS). The average particle size for the synthesised Pt nanoparticles is 1.4 nm. The electrocatalytic behaviour of the modified electrodes was investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results of the electrodeposition study showed that the presence of Pt nanoparticles, together with Ru nanoparticles, gave better performance with the FA showing the least tolerance to electrode poisoning. The impedance spectra of the MWCNT-Pt/Ru hybrids in all the FC materials studied showed some dependence on the oxidation potential. These spectra were somewhat complicated but generally followed electrical equivalent circuit models characteristic of adsorption-controlled charge transfer kinetics. EG and MeOH showed conventional positive Faradaic impedance spectra, irrespective of the applied oxidation potential. FA impedance spectra exhibited an inductive loop only at the extreme forward anodic peak potential, characteristic of Faradaic current being governed by the occupation of an intermediate state. On the other hand, the presence of phthalocyanine with the synthesized Pt-Ru nanocatalysts showed an improvement on the tolerance to CO poisoning during MeOH oxidation and therefore its application in the direct fuel cell oxidation is encouraged. The synthesized Pt-based nanoparticles gave better performance compared to the electrodeposited Pt-based nanoparticles. The comparative electrocatalytic behaviour of the chemically synthesized nanocatalysts indicated that the BPPGE-fMWCNT/RuOcPcPt electrode gives the best performance towards MeOH oxidation compared to other electrodes studied, while FA oxidation was favoured on the BPPGE-RuOcPcPt electrode without CNTs support. However, EG oxidation was not successful at the electrodes at all. The oxidation of these FC molecules are characterized by both diffusion (forward) and adsorption-controlled (reverse) processes. The two electrodes (BPPGE-fMWCNT/RuOcPcPt and BPPGE-RuOcPcPt) gave better tolerance to oxidation poison with the ratio of the current density of the forward anodic peak to the reverse anodic peak (Jfa/Jra) and (Jfa1/Jfa2) of 4.0 and 1.0 respectively. The electrodeposited and chemical synthesized nanocatalysts results shown in this work have for the first time provided some useful insights into the electrocatalytic response of FC molecules (MeOH, FA, EG) for potential application in fuel cell technology. The third part of the thesis describes the electrocatalytic reduction of molecular oxygen in alkaline solution using a novel ruthenium tetrakis (diaquaplatinum)octacarboxyphthalocyanine (RuOcPcPt) electrocatalyst supported on MWCNTs. The results revealed that the MWCNT-RuOcPcPt electrode is electro-catalytically active than MWCNT, MWCNT-RuOcPc, RuOcPc and RuOcPcPt electrodes towards oxygen reduction reaction. The study shows that the oxygen reduction activity follows a direct 4-electron transfer process with high kinetic rate constant, 3.57 x 10-2 cm s-1. The results obtained imply that more energy has been achieved and therefore the electrode is a promising candidate as a catalyst in the cathodic reaction of fuel cell.
Thesis (PhD)--University of Pretoria, 2012.
Chemistry
Unrestricted

L’immobilisation fonctionnelle des enzymes redox sur un support solide conducteur, en termes de densités de courant catalytique et de stabilité, est l’un des défis les plus importants à relever avant la commercialisation de systèmes tels que les piles à combustible enzymatiques et les biocapteurs. Cette thèse vise à la compréhension des facteurs moléculaires qui contrôlent le processus de transfert d'électrons interfacial et l'efficacité catalytique des enzymes redox immobilisées, en considérant l'effet de la quantité, de l'orientation et de la conformation des biomolécules à l’interface électrochimique. L'objectif ultime est d'obtenir une rationalisation des bioélectrodes. Des enzymes redox d’origine et de propriétés différentes, et des surfaces d’électrodes de chimie et structuration variées sont explorées pour moduler la connexion électrique de l’enzyme, et donc le processus de transfert d'électrons. Les propriétés requises de surface de l'électrode en tant que matrice hôte pour les enzymes sont tout d’abord déterminées sur électrodes planes puis étendues à des électrodes nanostructurées. Afin de déduire un modèle d'adsorption, un couplage sans précédent de l'électrochimie à des techniques de surface telles que la résonance plasmonique de surface (SPR), la spectroscopie d'absorption par réflectance infrarouge à modulation de polarisation (PMIRRAS) et l'ellipsométrie. Enfin, de nouvelles électrodes nanostructurées obtenues par ablation au laser sont explorées comme de nouvelles plates-formes stables capables de contrôler le taux de recouvrement surfacique enzymatique, ouvrant la voie à l'électrochimie d'une molécule unique
Functional immobilization of redox enzymes on conductive solid support, which must result in high current densities and operation stabilities, is one of the most significant challenge before the commercialization of biodevices like biofuel cells and biosensors. In order to get and to maintain the enzymatic activity in the immobilized state, this thesis aims to develop the molecular understanding that controls the efficiency of immobilized redox enzymes while considering the effect of loading, orientation and conformation as a function of various parameters like pH, electric field, ionic strength and covalent connections. The overall goal is to get a full rationalization of bioelectrodes.In this thesis, varieties of combinations that include properties of redox enzymes and/or electrode surfaces are explored to optimize immobilization, electrical wiring and thereby the ET process and its viability for bioelectronics. In order to deduce an adsorption model based on electrostatic interaction, an unprecedented coupling of electrochemistry to surface sensitive techniques like surface plasmon resonance (SPR), Polarization Modulation Infrared Reflectance Absorption Spectroscopy (PMIRRAS) and Ellipsometry is discussed which will ultimately help in understanding the combined effect of loading, orientation, and conformation on bioelectrocatalysis. Additionally, role of nanomaterials for the optimization of bioelectrocatalytic process is also explored. The desired properties of the electrode surface as a host matrix for enzymes are put forward accordingly while comparing the merit of planar and nanostructured electrodes

In this work a modified polyol method was developed to synthesize in-house catalysts. The method was modified for maximum delivery of product and proved to be quick and efficient as well as cost effective. The series of IH catalysts were characterized using techniques such as UV-vis and FT-IR spectroscopy, TEM, XRD, ICP and CV.

Le mécanisme de la réaction d’oxydation de l’hydrogène (HOR) dans Ni (111) est bien connu et se fait par étapes de Volmer-Heyrovsky, en milieu alcalin. Il a été proposé que la formation d'eau puisse jouer un rôle important. Dans cette thèse, j'ai étudié les surfaces de nickel et de nickel bimétallique en utilisant la théorie de la densité fonctionnelle (DFT). J'ai calculé des magnitudes thermodynamiques (comme les énergies libres d'adsorption de Gibbs) et des propriétés cinétiques (comme des barrières d'activation pour la formation d'eau). Plusieurs surfaces Ni / Cu ont été analysées. Celle qui contient 25% de Cu (sur la couche supérieure) a les meilleures performances: 1) l’énergie d’activation est de 0,2 eV, et 2) OH et H ne doivent pas être fortement adsorbés dans la plage de potentiel HOR
The mechanism of hydrogen oxidation reaction (HOR) in Ni(111) is well-known and it happens through Volmer-Heyrovsky steps, in alkaline media. However it was proposed that water formation could play an important role. In this thesis, I have studied nickel and bimetallic nickel surfaces using density functional theory (DFT). I calculated thermodynamical magnitudes (like Gibbs energies of adsorption) and kinetic properties (like activation barriers for water formation). Several Ni/Cu surfaces were analyzed. The one with 25% of Cu (on top layer) has the best performance because: 1) the activation energy is 0.2 eV, and 2) OH and H are not to strongly adsorbed on the HOR potential range

Dans ce mémoire nous discutons le développement de l’électrode de travail d’un bioréacteur électrochimique, dispositif permettant de synthétiser suivant un procédé dit de « Chimie Verte » des substances chimiques à haute valeur ajoutée. L’électrode de travail étant le siège de la synthèse électrocatalytique en jeu, l’optimisation de sa structure a été étudiée dans le but de maximiser l’aire de sa surface active. L’élaboration d’électrodes macroporeuses hautement organisées et de taille définie par les dimensions du prototype du réacteur pilote, a pu être obtenue en utilisant la méthode de Langmuir-Blodgett pour assembler le cristal colloïdal servant de template. La formation de ce dépôt organisé de colloïdes est suivie de l’électrodéposition du matériau d’électrode puis de la dissolution du template afin de révéler la structure macroporeuse. L’immobilisation de l’intégralité du matériel bio-électrocatalytique à l’intérieur des pores a été investiguée dans le but de prévenir la pollution du milieu contenant le produit final d’électrosynthèse par un des constituants redox et d’augmenter la durée de vie du dispositif. Ainsi, des couches ultra-minces de silice électrogénérée et des matrices de polymère électrodéposé ont été étudiées dans le but de préserver et d’optimiser l’activité enzymatique du système qu’elles encapsulent. Une attention particulière a été portée sur la qualité des dépôts au sein des structures poreuses. La procédure d’immobilisation des protéines rédox dans les matrices de silice et de polymère a été en outre associée à un jeu de construction moléculaire qui a permis par l’instauration de diverses interactions électrostatiques, de retenir toutes les espèces responsables de la catalyse à la surface de l’électrode. Enfin, dans le but d’intensifier les réactions catalytiques responsables de la synthèse à réaliser, des nano-particules d’ormodifiées par une couche monomoléculaire d’un médiateur redox ont été incorporées aux différents matériaux d’immobilisation permettant de ce fait d’augmenter les interfaces d’échanges électrochimiques entre matériau conducteur et biomolécules. L’insertion de ces nano-objectscombinée à la nanostructuration du matériau d’électrode a permis de multiplier par plus de 170 fois l’intensité des réactions enregistrées
The present work deals with the development of the working electrode of an electrochemicalbioreactor. This device enables the green synthesis of high added value chemical compounds. As theelectrochemical synthesis is located at the interface of the working electrode, structural optimizationof this reactor key component is required in order to maximize the available active surface area.Elaboration of highly organized macroporous gold electrodes with a size required by the pilot reactordimensions were obtained with the Langmuir-Blodgett method that was used to assemble a colloidalcrystal as a template. The elaboration of the organized colloidal deposit is first followed by theelectrodeposition of the electrode material, then by the dissolution of the template. The immobilization of the complete bio-electrochemical system inside the electrode pores was investigated in order to prevent pollution of the final product medium by one of the catalytic chaincomponent. This also improves the device life time. Subsequently electrogenerated ultra-thin silicalayers and electrodeposited polymer matrices were studied in order to preserve and optimize the catalytic activity of the redox proteins. In order to enhance the electrocatalytic synthesis, mediatormodified gold nanoparticules were incorporated in the different immobilization matrices. This allowed to increase the area of the electrochemical interface. The combination of the nano-objectincorporation and electrode nano-structuring intensified by a factor of 170 the catalytic process
Die vorliegende Arbeit beschäftigt sich mit der Entwicklung einer Arbeitselektrode für einenelektrochemischen Bioreaktor, der die umweltfreundliche Synthese von wertvollen chemischenKomponenten ermöglicht. Da die elektrochemische Synthese an der Oberfläche der Arbeitselektrodestattfindet, ist es nötig, den strukturellen Aufbau der Schlüsselkomponente des Reaktors zuoptimieren und die aktive Oberfläche der Elektrode zu erhöhen. Mit Hilfe der Langmuir-BlodgettTechnik wurden kolloidale Kristalle erzeugt, die als Template dienten, um hochgeordnetemakroporöse Goldelektroden, deren Dimensionen von dem Pilotreaktor bestimmt wurden,herzustellen. Nach dem Erzeugen von geordneten kolloidalen Filmen wurde der Zwischenraumzwischen den Partikeln mittels elektrochemischer Abscheidung gefüllt und das Templateanschließend chemisch aufgelöst. In der Folge wurde die Immobilisierung des komplettenbioelektrochemischen Systems im Poreninnenraum untersucht, mit dem Ziel eine Verunreinigung desReaktionsmediums durch eine der katalytischen Komponenten zu verhindern. Die Lebensdauer derElektrode kann so zusätzlich erhöht werden. Es wurde untersucht, inwieweit durch elektrogenerierteultra-dünne Silikaschichten oder durch Elektroabscheidung erzeugte Polymerfilme die katalytischeAktivität der Redoxproteine erhalten und weiter optimiert werden kann. Goldnanopartikel, die miteinem Mediator modifiziert wurden, wurden in die jeweilige Immobilisationsschicht integriert, mitdem Ziel die Effizienz der elektrokatalytischen Synthese zu erhöhen. Auf diese Weise konnte dieaktive elektrochemische Oberfläche der Elektrode weiter erhöht werden. Die Kombination aus einernanostrukturierten Elektrode und Nanoobjekten die in die Immobilisationsschicht eingebettetwurden, führte zu einer Signalerhöhung des katalytischen Prozesses um mehr als eineGrössenordnung

Hydrogen evolution via proton reduction occurs at a high rate at the surface of Pt than at Au electrodes. Using cyclic voltammetry, chemically modified nanometer-sized Au electrodes, prepared by the Laser-Assisted Puller Method, were employed to examine current amplification by electrocalysis at Pt nanoparticles adsorbed on the modified Au electrode surfaces. The electrodes were modified with Self-Assembled Monolayers (SAMs) of cysteamine and soaked in Pt colloid solutions overnight. Monitoring the decrements of the characteristic steady-state catalytic current for proton reduction indicated that aggregates of Pt nanoparticles are adsorbed on the cysteamine monolayers and desorb from them particle by particle. The results also indicate that some particles are strongly attached to the modified electrode surface and do not deplete even after thorough rinsing.

Three types of electrodes: carbon paste electrodes modified with nanoparticles of metallophthalocyanines (MPcNP-CPEs, M = Mn, Fe, Ni, Co), basal plane pyrolytic graphite electrodes modified with iron or nickel phthalocyanine nanoparticles and multiwalled carbon nanotube composites (FePcNP/MWCNT-BPPGE or NiPcNP/MWCNT-BPPGE),and basal plane pyrolytic graphite electrodes modified with multiwalled carbon nanotubes and electropolymerized metal tetra-aminophthalocyanines (poly-MTAPc-MWCNT-BPPGE), where M is Mn, Fe, Ni or Co, were prepared. Electrochemical characterizations showed that faster electron transfer kinetics occurred at the NiPcNP/MWCNT-BPPGE than at the FePcNP/MWCNT-BPPGE surface. SEM and electrochemical characterizations of poly-MTAPc-MWCNT-BPPGE showed that MTAPc had been deposited on the MWCNTBPPGE surface, and that the poly-CoTAPc-MWCNT-BPPGE exhibited the fastest electron transfer kinetics of all the poly-MTAPc-MWCNT-BPPGEs. Using amitrole and asulam as test analytes, electrochemical experiments showed that, amongst the CPEs, the FePcNP-CPE and NiPcNP-CPE displayed the most electrocatalytic behavior towards amitrole and asulam oxidation, respectively, and further experiments were done to obtain the electrochemical parameters associated with these electrodes and the corresponding analytes. Although, the FePcNP/MWCNT- BPPGE displayed electrocatalytic behavior towards amitrole oxidation in comparison with the bare BPPGE, it was less electrocatalytic than the FePcNP-CPE in terms of detection potential. The NiPcNP/MWCNT-BPPGE displayed the same detection potential as the NiPcNP-CPE. The poly-FeTAPc-MWCNT-BPPGE exhibited the most electrocatalytic behavior towards amitrole, of all the electrodes investigated, and the poly-CoTAPc-MWCNT-BPPGE displayed the best electrocatalytic behavior towards asulam, amongst the poly-MTAPc-MWCNT-BPPGEs.

Studies of single, isolated nanoparticles provide better understanding of the structure-function relationship of nanoparticles since they avoid complications like interparticle distance and nanoparticle loading that are typically associated with collections of nanoparticles distributed on electrode supports. However, interpretation of results obtained from single nanoparticle immobilization studies can be difficult to interpret since the underlying nanoelectrode platform can contribute to the measured current, or the immobilization technique can adversely affect electron transfer. Here, we immobilized ligand-free gold nanoparticles on relatively electrocatalytically inert nitrogen-doped carbon ultramicroelectrodes that were prepared via a soft nitriding method. Sizes of the particles were estimated by a recently reported electrochemical method and were found to vary linearly with deposition time. The particles also exhibited electrocatalytic activity toward methanol oxidation. This immobilization strategy shows promise and may be translated to smaller nanoelectrodes in order to study electrocatalytic properties of single nanoparticles.

In pursuit of solving the foreseeable depletion of fossil energies and environmental pollution caused by combustion of them, great efforts have been devoted to exploring renewable and clean energies, like the solar energy, nuclear energy, and geothermal energy, etc. as well as the technologies in converting these new energies into the form of usable electricity. In this regard, (rechargeable) zinc-air batteries and fuel cells have demonstrated promising potentials due to their large output energy density, power density and more importantly, their environmental compatibility. To consume oxygen molecules at cathode, these devices suffer greatly from the large overpotential and sluggish kinetics from the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Platinum, iridium and other noble metal-based electrocatalysts (NMCs) are conventionally used at the cathode of zinc-air batteries and fuel cells. However, the NMCs are subjected to high cost and insufficient durability. Thus, substituting the NMCs with other earth-abundant elements is currently imperative for the large-scale commercialization of the zinc-air batteries and fuel cells. Within this framework, this thesis attempts to utilize graphene as the building blocks to couple with other active elements, e.g. transition metal ions and nitrogen doped disordered carbon to fabricate advanced electrocatalysts for OER and ORR. A series of synthesizing methods have been developed to synthesize the graphene-based nanocomposites, including room-temperature coordination adsorption, hydrothermal treatment, and high-temperature calcination, etc. The physical features of the resultant nanocomposites have been thoroughly investigated by using XRD, SEM, and TEM. Meanwhile, their electrochemical performances were explored in terms of the potential-current response and the corresponding working durability. Besides, the associated origin of their intrinsic activity has been investigated and discussed. Molecular Ni–/Co–porphyrin multilayers were spontaneously adsorbed on the surface of graphene sheets layer-by-layer via non-covalent forces such as Van der Waals’ force and π-π interactions. It was observed that the electrochemical performance of the nanocomposite could be tuned by controlling the number of the Ni–/Co–porphyrin layers on the surface of graphene. This is ascribed to the counterbalance between the steric hindrance and the content of the active species. Such research work manifested the controllability of the OER/ORR performance at the molecular level and revealed the essential influence between the content of the active sites and the steric hindrance caused by their spatial accommodation. To implement a low-cost and scalable synthesis strategy, carbon black NPs and amorphous CoBi nanoplates were assembled with graphene to build a sandwich-like nanocomposite by use of amphipathicity of graphene oxide. The obtained sandwich-like nanocomposite exhibited excellent ORR/OER performance, which was comparable to the state-of-the-art materials. The performance enhancement towards ORR was assigned to the enlarged accessible active surface area of the nanocomposite catalyst. Without changing the chemical composition of the active species, this work highlighted the significance of the rational design of the geometrical configuration by means of the non-covalent force in an electrocatalyst. The resultant nanocomposite was further assembled in a rechargeable zinc-air battery to demonstrate its practicability. The lack of the high-efficiency and noble metal-free electrocatalyst in the acid media has been an intractable problem for years. To address this issue, a disordered carbon layer impregnated with Co-N on the surface of graphene sheet was fabricated by pyrolysing the hydrothermal product of graphene oxide and cobalt gluconate. The resultant nanocomposite exhibited remarkable activity to ORR in both alkaline and acid media, which was due to the high dispersion of abundant active sites. Moreover, different active working sites in alkaline and acid condition for the obtained material were suggested. This inspired us to investigate different roles of the metal species in the ORR electrocatalysts. For the nitrogen-doped carbon materials, the pyridinic nitrogen doping is believed to possess the highest activity for ORR in alkaline environment. To verify that theory and further enhance the activity of the nitrogen-doped carbon materials, an ultrathin holey carbon layer coupled with graphene nanosheets was prepared. The edge enriched feature makes it easier to form pyridinic nitrogen during the nitrogen doping process. The obtained composite displayed the expected outstanding ORR performance in alkaline media and even surprisingly high activity in acid solution. The rationality of the design of this material was manifested by solving the commonly encountered insufficient charge transfer ability and stability of the holey graphene materials while preserving the high activity in the holey carbon sites. In a nutshell, this thesis contributes to the exploration of the graphene-based OER/ORR electrocatalyst in the aspects of i) tuning the electrochemical activity of the transition metal based electrocatalyst at the molecular level; ii) isolating and highlighting the significance in geometrical configuration of the ORR electrocatalyst with respect to kinetic process; iii) suggesting and verifying the different active sites of the same electrocatalyst tested under different pH values; iv) selectively inducing the formation of the active pyridinic nitrogen species in the ultrathin holey carbon layer coupled on the surface of graphene nanosheet.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
Full Text

Metal nanoparticles have been widely used for many catalytic and electrocatalytic applications due to their larger surface area-to-volume ratios and higher densities of active sites compared to bulk materials. This has resulted in much interest in understanding the electrocatalytic behavior of metal nanoparticles with respect to their structure. However, most research on this topic has employed collections of nanoparticles. Due to difficulties in controlling and characterizing particle loading and interparticle distance in nanoparticle ensembles, single nanoparticles studies have recently become a topic of great interest. In this study, a soft nitriding technique was applied to chemical vapor-deposited carbon ultramicroelectrodes (UMEs) in order to immobilize ligand-free AuNPs onto the carbon substrate. The feasibility of this method is geared toward studying the properties of single AuNPs immobilized onto carbon nanoelectrodes. The ligand-free AuNPs immobilized onto the nitrided carbon UMEs were highly electrocatalytic toward methanol oxidation.

The main purpose of this work - to investigate patterns of electrochemical oxidation of ascorbic acid on polyaniline and poly(N-methylaniline) modified electrodes, in order to develop ascorbate-sensitive sensors. A detailed study of various factors affecting the aniline and N-methylaniline electrochemical polymerization and the resulting properties of PANI and PNMA layers was carried out for this purpose. Comparative study of modified electrodes in solutions of different acidity was performed and it was shown that PNMA had a better redox activity in slightly acidic and neutral solutions compared to polyaniline. The nature of amperometric response of modified electrodes to ascorbate was investigated and autocatalytic mechanism of ascorbate electrooxidation on PANI modified electrode was suggested. Using PANI and PNMA modified electrodes, prototypes of amperometric ascorbate sensors have been developed and their comparative studies were carried out.
Darbo tikslas - ištirti askorbo rūgšties elektrocheminės oksidacijos ant elektrodų, modifikuotų polianilinu ir poli(N-metilanilinu), dėsningumus, siekiant sukurti jautrius askorbatui jutiklius. Ištirta įvairių faktorių įtaka anilino ir N-metilanilino elektrocheminei polimerizacijai bei gautų PANI ir PNMA sluoksnių savybėms. Atlikti palyginamieji modifikuotų elektrodų tyrimai skirtingo pH tirpaluose ir parodyta, kad PNMA lyginant su polianilinu pasižymi geresniu aktyvumu silpnai rūgščiuose ir neutraliuose tirpaluose. Ištirtas modifikuotų elektrodų amperometrinio atsako į askorbatą pobūdis, ir pasiūlytas autokatalizinis askorbato elektrooksidacijos mechanizmas ant PANI modifikuoto elektrodo. Panaudojus PANI ir PNMA modifikuotus elektrodus, sukurti amperometrinių askorbato jutiklių prototipai ir atlikti jų palyginamieji tyrimai.

Alkaline water electrolysis need a catalyst with low overpotential and high current densitiy for oxygen evolution in order to be a commercial viable hydrogen source in the future. Finding and establishing a correlation between electrocatalytic activity and charge carrier density will help towards finding an optimum catalyst for this purpose. Such comparisons have been made using theoretical values for charge carrier density, but the aim of this work is to use charge carrier data from experimental values.Five powders of La_1-xSr_xCoO₃ (with compositions x = 0, 0.25, 0.5, 0.75, 1) were synthesized by solid-state synthesis and sintered to pellets. The pellet surfaces were investigated in alkaline solution (pH = 13) by cyclic voltammetry, polarization and impedance measurements. Polariza- tion curves with Tafel lines and Mott-Schottky plots were established. The powders and pellet surfaces were investigated by XRD, SEM, EDS, AFM and light microscope.The polarization curves revealed a volcanic behavior with an increase in catalytic activity from x = 0 up to x = 0.75 and then decreasing. The charge carrier density increased with increasing strontium doping. The resulting comparison gave figure 34. Surface investigation revealed much porosity. Because of corrosion, the surface area increased with measuring, and finding the real surface area and the roughness proved to be problematic.A volcanic behavior of the charge carrier density and electrocatalytic activity relationship were observed. Finding roughness factor values by measuring double layer capacitance measured by the cyclic voltammetry method and dividing by the nominal capacitance for a flat surface proved to be unsuccessful. Better synthesis and sintering procedures of pellets are needed to increase the density of the samples in order to decrease the roughness and the effect of corrosion.

Nickel-based catalysts in aqueous alkaline media are low-cost electrode materials for electrolytic hydrogen generation, a renewable method of producing fuel and industrial feedstock. However, further work is necessary to develop inexpensive electro-catalyst materials with high activity and long-term stability. This thesis employs spectroscopic and electrochemical methods to directly address specific research problems for the development of improved materials and devices with commercial or industrial value. The first chapter reviews the applications of nickel electrodes; the structures of nickel, nickel hydroxides, and nickel hydrides; and techniques for measuring the electrochemically active surface area (AECSA) of nickel. In the second chapter, electrochemically precipitated nickel hydroxide materials are fully characterized by Raman spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). This work unifies and simplifies the large body of literature on the topic by considering two fundamental phases, α- and β-Ni(OH)2, and various types and extents of structural disorder. The third chapter examines and demonstrates the potential applications of in situ Raman spectroscopy by monitoring the spontaneous ageing of α-Ni(OH)2 to β-Ni(OH)2 in pure water at room temperature. The fourth chapter considers the longstanding problem of electrode deactivation, the gradual decrease in nickel electro-catalyst activity during prolonged hydrogen production. Voltammetric and XRD evidence demonstrates that hydrogen atoms can incorporate into the electrode material and cause structural disorder or the formation of α-NiHx and β-NiHx at the surface. The voltammetric formation of NiOx, α-Ni(OH)2, β-Ni(OH)2, and β-NiOOH surface species are examined by electrochemical and XPS measurements. The fifth chapter of this thesis presents a new method to measure the AECSA by adsorption of oxalate to the (001) surface of the surface Ni(OH)2, as evidenced by voltammetric and attenuated total reflectance (ATR) FT-IR spectroscopy measurements. The adsorbed oxalate limits the surface hydroxide to a single layer. The surface NiOOH/Ni(OH)2 reduction peak during the reverse scan may be used to accurately and precisely measure the AECSA. The error of this method is estimated at < 10 %.

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Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Synthesis and functionalization of large-area graphene and its structural, electrical and electrochemical properties has been investigated. First, the graphene films, grown by thermal chemical vapor deposition (CVD), contain three to five atomic layers of graphene, as confirmed by Raman spectroscopy and high-resolution transmission electron microscopy. Furthermore, the graphene film is treated with CF4 reactive-ion plasma to dope fluorine ions into graphene lattice as confirmed by X-ray photoelectron spectroscopy (XPS) and UV-photoemission spectroscopy (UPS). Electrochemical characterization reveals that the catalytic activity of graphene for iodine reduction enhanced with increasing plasma treatment time, which is attributed to increase in catalytic sites of graphene for charge transfer. The fluorinated graphene is characterized as a counter-electrode (CE) in a dye-sensitized solar cell (DSSC) which shows ~ 2.56% photon to electron conversion efficiency with ~11 mAcm−2 current density. Second, the large scale graphene film is covalently functionalized with HNO3 for high efficiency electro-catalytic electrode for DSSC. The XPS and UPS confirm the covalent attachment of C-OH, C(O)OH and NO3- moieties with carbon atoms through sp2-sp3 hybridization and Fermi level shift of graphene occurs under different doping concentrations, respectively. Finally, CoS-implanted graphene (G-CoS) film was prepared using CVD followed by SILAR method. The G-CoS electro-catalytic electrodes are characterized in a DSSC CE and is found to be highly electro-catalytic towards iodine reduction with low charge transfer resistance (Rct ~5.05 Wcm2) and high exchange current density (J0~2.50 mAcm-2). The improved performance compared to the pristine graphene is attributed to the increased number of active catalytic sites of G-CoS and highly conducting path of graphene. We also studied the synthesis and characterization of graphene-carbon nanotube (CNT) hybrid film consisting of graphene supported by vertical CNTs on a Si substrate. The hybrid film is inverted and transferred to flexible substrates for its application in flexible electronics, demonstrating a distinguishable variation of electrical conductivity for both tension and compression. Furthermore, both turn-on field and total emission current was found to depend strongly on the bending radius of the film and were found to vary in ranges of 0.8 – 3.1 V/μm and 4.2 – 0.4 mA, respectively.

La décomposition électrolytique de l’eau en hydrogène et oxygène à l’aide d’électricité renouvelable générée par les courants marins ou à partir d’énergie éolienne ou solaire, constitue l’une des voies les plus propres et directes pour produire de l’hydrogène. Toutefois, la production de grands volumes d’hydrogène par décomposition électrolytique de l’eau comporte un verrou technologique qui réside dans la forte surtension à vaincre à l’anode où de l’oxygène est dégagé. Ce travail de thèse s’est attaché donc à mettre au point des matériaux d’électrodes capables de catalyser de l’eau en oxygène de façon efficace et stable, en utilisant des éléments chimiques suffisamment abondants sur terre. Pour cela nous avons exploré des composés présentant des porosités à structures hiérarchiques et des procédés de préparation efficaces, aisées à mettre en œuvre et susceptibles d’un usage à l’échelle industrielle. Nous avons développé deux types d’électrocatalyseurs d’oxydation de l’eau en oxygène en mettant au point deux voies de préparation impliquant chacune une phase d’activation in situ. Le premier type est une mousse de nickel dopée à la fois avec des cristaux de nickel, des nanoparticules de tétroxyde de tricobalt et des nanofeuilles d’oxyde de graphène via nickelage électrolytique, suivi d’une activation électrochimique in situ pour former de l’hydroxyde de nickel et des nano-plaques d’oxy-hydroxyde du même métal. Ce catalyseur hybride s’est avéré avoir des performances électrocatalytiques de bon niveau, comparables à celles des électrodes à base de métaux nobles qui sont disponibles dans l’état actuel de la technique ; il a en outre fait preuve d’une excellente stabilité en fonctionnement. Ces propriétés remarquables semblent liées à la fois aux dépôts formés sur la mousse de nickel par les différentes phases actives citées, aux nanoparticules d’oxy-hydroxyde de nickel, ainsi qu’aux effets de synergie qu’elles y induisent. Le second type d’électrocatalyseurs a été obtenu en combinant la projection thermique (HVOF) et un processus d’activation chimique puis électrochimique. Le matériau résultant possède de nanocouches du type jamborite formée in situ, sur la matrice poreuse à structure hiérarchique. Le catalyseur développé dans ce travail présente non seulement une surtension et une pente de Tafel exceptionnellement faibles, mais également une stabilité remarquable. Ces performances sont dues à un puissant effet de synergie dans laquelle interviennent la grande activité intrinsèque des nanofeuilles de jamborite et la grande rapidité des transports d’électrons et d’ions assurée par l'architecture poreuse hiérarchique. Il convient de noter que cette nouvelle méthodologie a le potentiel de produire des électrodes de grandes tailles apte à l’électrolyse alcaline de l'eau et crée ainsi de nouvelles perspectives dans le cadre de la conception d'électrocatalyseurs à la fois très actifs et stables. Nous avons également développé, initialement, des électrocatalyseurs destinés à la réduction de l’eau en hydrogène, qui impliquent également une activation électrochimique in situ. Ces électrodes peuvent être ainsi couplées aux électrodes précitées d’oxydation de l’eau en oxygène pour former des cellules électrochimiques complètes à deux électrodes, dont les performances rivalisent avec celles développées par le couple dioxyde de ruthénium/platine qui représente le meilleur état de la technique dans le cadre de la production d’hydrogène et d’oxygène par électrolyse de l’eau. En résumé, en combinant des techniques conventionnelles de revêtement et d’activation électrochimique in situ, ce travail a permis de développer une méthodologie de préparation d'électrodes catalytiques offrant de hautes performances et susceptibles de commercialisation. La technique d’activation électrochimique in situ exploite un comportement d'auto-optimisation dynamique qui est aisé à mettre en œuvre, facilement adaptable, efficace et respectueux de l'environnement
Splitting water into hydrogen and oxygen by electrolysis using electricity from intermittent ocean current, wind, or solar energies is one of the easiest and cleanest routes for high-purity hydrogen production and an effective way to store the excess electrical power without leaving any carbon footprints. The key dilemma for efficient large-scale production of hydrogen by splitting of water via the hydrogen and oxygen evolution reactions is the high overpotential required, especially for the oxygen evolution reaction. Hence, engineering highly active and stable earth-abundant oxygen evolution electrocatalysts with three-dimensional hierarchical porous architecture via facile, effective and commercial means is the main objective of the present PhD study. Finally, we developed two kinds of good performance oxygen evolution electrocatalysts through two different way combined with in situ electrochemical activation.For the first oxygen evolution electrocatalyst, we report a codoped nickel foam by nickel crystals, tricobalt tetroxide nanoparticles, graphene oxide nanosheets, and in situ generated nickel hydroxide and nickel oxyhydroxide nanoflakes via facile electrolytic codeposition in combination with in situ electrochemical activation as a promising electrocatalyst for oxygen evolution reaction. Notably, this hybrid catalyst shows good electrocatalytic performance, which is comparable to the state-of-the-art noble catalysts. The hybrid catalyst as an electrocatalytically-active and robust oxygen evolution electrocatalyst also exhibits strong long-term electrochemical durability. Such a remarkable performance can be benefiting from the introduced active materials deposited on nickel foam, in situ generated nickel oxyhydroxide nanoflakes and their synergistic effects. It could potentially be implemented in large-scale water electrolysis systems.For the second oxygen evolution electrocatalyst, a facile and efficient means of combining high-velocity oxy-fuel spraying followed by chemical activation, and in situ electrochemical activation based on oxygen evolution reaction has been developed to obtain a promising self-supported oxygen evolution electrocatalyst with lattice-distorted Jamborite nanosheets in situ generated on the three-dimensional hierarchical porous framework. The catalyst developed in this work exhibits not only exceptionally low overpotential and Tafel slope, but also remarkable stability. Such a remarkable feature of this catalyst lies in the synergistic effect of the high intrinsic activity arising from the lattice-dislocated Jamborite nanosheets as the highly active substance, and the accelerated electron/ion transport associated with the hierarchical porous architecture. Notably, this novel methodology has the potential to produce large-size-electrode for alkaline water electrolyzer, which can provide new dimensions in design of highly active and stable self-supported electrocatalysts.Furthermore, we have also initially developed good hydrogen evolution electrocatalysts upon in situ electrochemical activation, coupled with the obtained superior oxygen evolution electrocatalysts forming two-electrode configurations, respectively, both of which rivalled the integrated state-of-the-art ruthenium dioxide-platinum electrode in alkaline overall water splitting.In summary, a methodology of fabricating easy-to-commercial, high performance catalytic electrodes by combining general coating processes with in situ electrochemical activation has been realized and well developed. The in situ electrochemical activation mentioned above is a dynamic self-optimization behavior which is facile, flexible, effective and eco-friendly, as a strategy of fabricating self-supported electrodes for efficient and durable overall water splitting. We hope our work can promote advanced development toward large-scale hydrogen production using excess electrical power whenever and wherever available

Vätgas kan framställas från förnybara energikällor genom vattenelektrolys med anjonbytande membran (AEMWE). AEMWE har vissa fördelar jämfört med traditionell alkalisk vattenelektrolys och elektrolysmed protonledande membran. Till exempel finns det möjlighet att använda alkalisk elektrolyt (även rent vatten) och billiga platinagruppsmetallfria katalysatorer tillsammans med ett anjonbytesmembran. Den största utmaningen med tekniken är att uppnå utmärkt och stabil prestanda för membran och elektroder. AemionTM anjonbytande membran (AEMs) av olika tjocklek, vattenupptag och kapacitet undersöktes i ett AEMWE system med 5 cm2 elektrodarea. Elektrokemisk prestanda hos dessa kommersiella AEM studerades med hjälp av porösa nickel elektroder. Bland de undersökta membranen visade AF2-HWP8-75-X stabil prestanda med en högfrekvent resistans (HFR) på 90 mΩ•cm2 och kunde nå en strömtäthet på 0,8 A/cm2 vid 2,38 V med 1 M KOH vid 60 ˚C.  AEMWE med AF2-HWP8-75-X och olika elektrodkombinationer undersöktes under samma driftsförhållanden. En elektrodkombination med Raney-Ni och NiFeO som katod respektive anod visade bäst prestanda under utvärderingen och gav en strömtäthet på 1,06 och 3,08 A/cm2 vid 2,00 respektive 2,32 V. KOH-lösningens temperatur och koncentration sänktes till 45 ˚C respektive 0,1 M för att undersöka effekten av driftsparametrar på flödescellens prestanda. Flödescellen uppvisade god stabilitet under de nya driftsförhållandena, men dess prestanda minskade avsevärt. Den nådde en strömtäthet på 0,8 A/cm2 vid 2,25 V.
Hydrogen can be produced from renewable energy sources using a novel anion exchange membrane water electrolysis (AEMWE) system. AEMWE has some benefits over the currently used state-of-the-art alkaline and proton exchange membrane water electrolysis systems. For instance, there is a possibility of using alkaline electrolytes (even pure water) and low-cost platinum-group-metal free catalysts together with an ion exchange membrane. However, the main challenge is that the AEMWE system should show excellent and stable performance, depending on the stability of the membrane and the electrodes. AemionTM anion exchange membranes (AEMs) of different thickness and water uptake capacity were investigated using a 5 cm2 AEMWE system. The electrochemical behaviour of these commercial AEMs was studied using nickel (Ni) felt electrodes. Among the investigated AEMs, the AF2-HWP8-75-X showed stable performance with a high frequency resistance (HFR) of 90 mΩ•cm2 and was able to reach a current density of 0.8 A/cm2 at 2.38 V using 1 M KOH at 60 ˚C.  AEMWE systems based on AF2-HWP8-75-X and different electrode combinations were examined under the same operating conditions. An electrode combination with Raney-Ni and NiFeO as cathode and anode, respectively, showed the best performance during the degradation test and provided a current density of 1.06 and 3.08 A/cm2 at 2.00 and 2.32 V, respectively. The operating temperature and concentration of the KOH solution were reduced to 45 ˚C and 0.1 M, respectively, to study the effect of operating parameters on the flow cell performance. The flow cell showed good stability under the new operating conditions, but its performance was reduced significantly. It reached a current density of 0.8 A/cm2 at 2.25 V.

Estudos com eletrodos modificados foram conduzidos utilizando dois sistemas porfirínicos supramoleculares diferentes. O primeiro foi baseado na modificação de eletrodo de carbono vítreo com uma porfirina de níquel tetrarrutenada, [NiIITPyP{RuII(bipy)2Cl}4]4+. A modificação do eletrodo foi realizada por meio de sucessivos ciclos voltamétricos em meio alcalino (pH 13), gerando um eletrodo com característica similar a eletrodos modificados com α-Ni(OH)2. A caracterização química do filme formado foi realizada através das técnicas de voltametria cíclica, ressonância paramagnética eletrônica, espectroscopia eletrônica por reflectância e espectroscopia Raman com ensaio espectro-eletroquímico. Os resultados sugerem a formação de um polímero de coordenação, [µ-O2-NiIITPyP{RuII(bipy)2Cl}4]n, composto por subunidades porfirínicas ligadas entre si por pontes µ-peroxo axialmente coordenadas aos átomos de níquel (Ni-O-O-Ni). O crescimento do filme apresentou dependência da alcalinidade do meio pela formação do precursor octaédrico [Ni(OH)2TRPyP]2+ em solução, pela coordenação de OH- nas posições axiais do átomo de níquel. O processo de eletropolimerização indicou a participação de radical hidroxil, gerado por oxidação eletrocatalítica da água nos sítios periféricos da porfirina contendo o complexo de rutênio. O mesmo eletrodo foi aplicado como sensor eletroquímico para análise amperométrica de ácido fólico em comprimidos farmacêuticos. O sensor foi associado a um sistema de Batch Injection Analysis (BIA) alcançando considerável rapidez e baixo limite de detecção. Para as análises das amostras também foi proposto um método para a remoção da lactose, que agia como interferente. O segundo estudo envolveu a modificação de eletrodos de carbono vítreo com diferentes hemoglobinas, naturais (HbA0, HbA2 e HbS) e sintéticas (Hb-PEG5K2, αα-Hb-PEG5K2 e BT-PEG5K4), para a avaliação da eficiência na redução eletrocatalítica de nitrito mediada por FeI-heme. Os filmes foram produzidos pela mistura de soluções das hemoglobinas com brometo de didodecildimetiltrimetilamônio (DDAB), aplicados nas superfícies com consecutiva evaporação, formando filmes estáveis. Os valores de potencial redox para os processos do grupo heme e a sua associação com a disponibilidade do grupo na proteína foram avaliados por voltametria cíclica. Os valores das constantes de velocidade, k, para redução de nitrito foram obtidos por cronoamperometria em -1,1 V (vs Ag/AgCl(KCl 3M)) que foram utilizados para estudo comparativo entre as espécies sintéticas para eventual aplicação clínica.
Studies with modified electrodos were conducted using two different supramolecular porphyrin systems. The first one was based on the modification of glassy carbon electrode with a tetraruthenated nickel porphyrin, [NiIITPyP{RuII(bipy)2Cl}4]4+. The electrode modification was carried out through successive voltemmetric cycles in alkaline media (pH 13), generating an electrode with feature similar to α-Ni(OH)2 modified electrodes. The chemical characterization of this film was performed by cyclic voltammetry, electronic paramagnetic resonance, reflectance electronic spectroscopy and Raman spectroscopy with spectroelectrochemistry assay. The results suggested the formation of a coordination polymer, [µ-O2-NiIITPyP{RuII(bipy)2Cl}4]n, composed by porphyrin subunits linked by µ-peroxo bridges axially coordinated to nickel atoms (Ni-O-O-Ni). The film growth showed dependence of the alkaline media by the formation of octahedral precursor [Ni(OH)2TRPyP]2+ in solution by way of axial coordination of OH- to the nickel atoms. The electropolymerization process showed to have a contribution from hydroxyl radicals, generated by electrocatalytic oxidation of water on the peripheral sites containing the ruthenium complexes. The same electrode was applied as an electrochemical sensor for amperometric analysis of folic acid in pharmaceutical tablets. The sensor was associated to a Bath Injection Analysis (BIA) system, achieving good sampling frequency and low detection limit. For the samples analysis, it was also proposed a method for lactose removal. The second study comprises the modification of glassy carbon electrodes with different hemoglobin species, of natural occurrence (HbA0, HbA2 e HbS) and synthetics (Hb-PEG5K2, αα-Hb-PEG5K2 e BT-PEG5K4) for evaluation of efficiency on electrocatalytic reduction of nitrite mediated by FeI-heme. The films were produced by mixing solutions of the hemoglobins with didecyldimethylammonium bromide (DDAB), applied on the surfaces with following solvent evaporation, forming stable films. The redox potential values for the heme group processes and the heme availability in the protein were evaluated by cyclic voltammetry. The reaction rate constants, k, for nitrite reduction were obtained by chronoamperometry at -1,1 V, which were used for comparative study between the synthetic species for further clinical applications.

Dans ce travail, nous décrivons la méthode de synthèse, la structure, les propriétés et quelques applications en catalyse de différentes formes du carbone, en particulier les nanostructures carbonées (Chapitre I). La technique de dépôt chimique en phase vapeur en réacteur à lit fluidisé a été utilisée pour le dépôt de métaux ou d’oxydes de métaux sur des supports comme l’alumine ou la silice. Le matériau résultant est utilisé comme catalyseur pour la synthèse de diverses nanostructures carbones par dépôt chimique en phase vapeur catalytique : nanotubes de carbone mono- et multi-feuillets (SWCNTs, MWCNTs), nanofibres de carbone (CNFs), et des nanotubes de carbone (N-MWCNTs) ou nanofibres (N-CNFs) dopés en azote (Chapitre II). Après dissolution du catalyseur par un traitement a l'acide sulfurique ou par la soude, suivit dans le cas des MWCNTs et CNFs, par un traitement à l'acide nitrique pour générer des fonctions carboxyliques de surface, les nanostructures carbonées ont été utilisées comme supports de catalyseurs. L’hydrogénation du cinnamaldehyde a été choisit comme réaction modèle pour comparer les performances de différents catalyseurs bimétalliques de Pt-Ru en fonction de la nature du support. Une étude paramétrique détaillée ainsi que l'étude de l'effet d'un traitement thermique sur l'amélioration des performances du catalyseur de Pt-Ru/MWCNT sont présentes. Une explication de l'augmentation des performances catalytiques sera proposée après analyses du catalyseur par HREM, EDX, EXAFS et WAXS (Chapitre III). Les nanostructures carbonées préparées seront également testées comme supports conducteurs d'électrocatalyseurs pour l'élaboration d'électrodes de "polyelectrolyte membrane fuel cells" (PEMFC)
In this work, we describe the synthesis, structure, physical properties and some applications in catalysis of previously known carbon allotropes, and recently discovered carbon nanostructure (Chapter I). First, FB-OM-CVD deposition was used for metal or metal oxide deposition on metal oxide supports like alumina or silica, leading to the production of supported catalysts. The resulting material was used as catalyst for catalytic chemical vapor deposition of carbonaceous nanostructures i.e single- and multi-walled carbon nanotubes (SWCNTs, MWCNTs), carbon nanofibers (CNFs), and nitrogen doped carbon nanotubes (N-MWCNTs) and nanofibers (N-CNFs) (Chapter II). After catalyst removal by a H2SO4 or NaOH treatments and carboxylic surface group generation by a HNO3 treatment in the case of MWCNTs and CNFs, the carbon nanostructures were used as supports for heterogeneous catalysis. The hydrogenation of cinnamaldehyde was used as a model reaction to compare the performance of different bimetallic Pt-Ru catalysts as a function of the nature of the support. Detailed parametric studies as well as the effect of a heat treatment on the performance improvement of the Pt-Ru/MWCNT catalyst are presented. An explanation for the increase of performances upon heat treatment will be proposed after HREM, EDX, EXAFS and WAXS characterization of the catalyst (Chapter III). The prepared carbon nanostructures were also tested as supports for Pd based electrocatalysts for direct alkaline fuel cells applications in both cathodes for the ORR reaction and anodes for alcohols oxidation

Etude par voltammetrie lineaire et par coulometrie, du mecanisme d'oxydoreduction des halogenures de trifluoromethyl sur une electrode de carbone dans le dmf. La deuxieme partie decrit les essais de trifluoromethylation de differents substrats organiques (aldehyde, styrene)

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O presente trabalho descreve a preparação, caracterização e aplicação eletroanalítica de um metalosilsesquioxano ligado em uma sílica mesoporosa (MCM-41) e em uma zeólita (H-FAU-Si/Al 40) com hexacianoferrato de níquel adsorvido em sua superfície. Algumas técnicas espectroscópicas como: Espectroscopia no infravermelho por transformada de Fourier (FTIR), Microscopia eletrônica de varredura (MEV), Espectroscopia por Energia Dispersiva de Raios X (EDS) e Voltametria cíclica, foram utilizadas na caracterização dos materiais formados. Os espectros na região do Infravermelho para o MTTiPNiH e ZTTiPNiH apresentaram as bandas de seus precursores (TTiP, MCM-41 e H-FAU-Si/Al 40), apenas duas absorções diferiram dos demais espectros, uma em, aproximadamente, 2168 cm-1 e outra a 2098 cm-1, tais resultados foram atribuídos ao estiramento ν (C≡N). Para o MTTIPNiH, através das micrografias pode-se observar partículas com tamanhos variados, entre 0,30 µm a 0,60 µm. Para o ZTTiPNiH as micrografias apresentaram um pequeno aumento no tamanho de partículas, aproximadamente de 50 nm, quando comparado a seus precursores. O EDS determinou a composição química semi-quantitativa dos elementos carbono, oxigênio, silício, titânio, fósforo, níquel e ferro, presentes em ambas as amostras. O eletrodo de pasta de grafite exibiu um par redox bem definido com potencial de médio padrão (E°’) de +0,53 V e +0,51 V para o MTTiPNiH e ZTTiPNiH, respectivamente, atribuído ao processo redox FeII(CN)6 / FeIII(CN)6 em presença de níquel. O eletrodo de pasta de grafite modificado com MTTiPNiH apresentou atividade eletrocatalítica para dipirona e para o sulfito, já o eletrodo de pasta de grafite modificado com ZTTiPNiH apresentou atividade eletrocatalítica apenas para o sulfito. Em uma segunda etapa, prepararam-se eletrodos impressos via screen-printed para detecção e quantificação de pindolol utilizando a técnica de voltametria de onda quadrada em uma faixa de concentração de 0,10 µmol L-1 – 10,0 µmol L-1. Após a detecção de pindolol, o protocolo de detecção foi utilizado na avaliação e recuperação desta substância a partir de urina humana. Eletrodos impressos via screen-printed foram empregados pela primeira vez na detecção da substância psicoativa Synthacaine. A detecção eletroquímica indireta para MPA/2-AI simultaneamente, utilizando eletrodos impressos apresentou resultados satisfatórios. Após a detecção simultânea de MPA e 2-AI, amostras reais de Synthacaine foram utilizadas para quantificar, através de técnicas eletroquímicas, estes analitos por meio do protocolo de detecção anteriormente proposto.
This research describes the preparation, characterization and electroanalytical application of a metalosilsesquioxane bonded in mesoporous silica (MCM-41) and in zeolite (H-FAU-Si/Al 40) with nickel hexacyanoferrate adsorbed on their surface. Techniques as fourier transform infra-red spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and cyclic voltammetry were used in the characterization of the synthesized materials. The spectra in the infrared region for MTTiPNiH and ZTTiPNiH showed the bands of their precursors (TTiP, MCM-41 and H-FAU Si/Al 40), only two absorption differed from the others spectra, one at, approximately, 2168 cm-1 and another at 2098 cm-1 , which were attributed to stretching ν(C≡N). For MTTIPNiH through the micrographs can be observed particles with varied sizes between 0.30 µM to 0.60 µm. For ZTTiPNiH the micrographs showed a small increase in particle size, approximately, 50 nm, as compared to their precursors. EDS determined the semi-quantitative chemical composition of carbon, oxygen, silicon, titanium, phosphorus, nickel and iron present in both samples. The carbon paste electrode exhibited a well-defined redox couple with E°' = +0.53 V and +0.51 V for MTTiPNiH and ZTTiPNiH, respectively, attributed to the redox process FeII(CN)6 / FeIII(CN)6 in presence of nickel. Carbon paste electrode modified with MTTiPNiH showed electrocatalytic activity to dipyrone and sulfite. However the carbon paste electrode modified with ZTTiPNiH showed electrocatalytic activity only for sulfite. In a second step, screenprinted electrodes were produced for the detection and quantification of pindolol using square wave voltammetry technique at a concentration range of 0.10 µmol L-1 to 10.0 µmol L-1 . After pindolol detection, the protocol of detection was used in the evaluation and recovery of pindolol from human urine. Screen-printed electrodes were used for the first time in the detection of psychoactive substance Synthacaine. Indirect electrochemical detection for MPA/2-AI simultaneously, using Screen-printed electrodes showed satisfactory results. After the simultaneous detection of MPA and 2-AI, real samples of Synthacaine were used to quantify through electrochemical techniques, these analytes by protocol of detection previously proposed.

Orientador: Devaney Ribeiro do Carmo
Resumo: O presente trabalho descreve a preparação, caracterização e aplicação eletroanalítica de um metalosilsesquioxano ligado em uma sílica mesoporosa (MCM-41) e em uma zeólita (H-FAU-Si/Al 40) com hexacianoferrato de níquel adsorvido em sua superfície. Algumas técnicas espectroscópicas como: Espectroscopia no infravermelho por transformada de Fourier (FTIR), Microscopia eletrônica de varredura (MEV), Espectroscopia por Energia Dispersiva de Raios X (EDS) e Voltametria cíclica, foram utilizadas na caracterização dos materiais formados. Os espectros na região do Infravermelho para o MTTiPNiH e ZTTiPNiH apresentaram as bandas de seus precursores (TTiP, MCM-41 e H-FAU-Si/Al 40), apenas duas absorções diferiram dos demais espectros, uma em, aproximadamente, 2168 cm-1 e outra a 2098 cm-1, tais resultados foram atribuídos ao estiramento ν (C≡N). Para o MTTIPNiH, através das micrografias pode-se observar partículas com tamanhos variados, entre 0,30 µm a 0,60 µm. Para o ZTTiPNiH as micrografias apresentaram um pequeno aumento no tamanho de partículas, aproximadamente de 50 nm, quando comparado a seus precursores. O EDS determinou a composição química semi-quantitativa dos elementos carbono, oxigênio, silício, titânio, fósforo, níquel e ferro, presentes em ambas as amostras. O eletrodo de pasta de grafite exibiu um par redox bem definido com potencial de médio padrão (E°’) de +0,53 V e +0,51 V para o MTTiPNiH e ZTTiPNiH, respectivamente, atribuído ao processo redox FeII(CN)6 / FeIII(CN)6 em... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: This research describes the preparation, characterization and electroanalytical application of a metalosilsesquioxane bonded in mesoporous silica (MCM-41) and in zeolite (H-FAU-Si/Al 40) with nickel hexacyanoferrate adsorbed on their surface. Techniques as fourier transform infra-red spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and cyclic voltammetry were used in the characterization of the synthesized materials. The spectra in the infrared region for MTTiPNiH and ZTTiPNiH showed the bands of their precursors (TTiP, MCM-41 and H-FAU Si/Al 40), only two absorption differed from the others spectra, one at, approximately, 2168 cm-1 and another at 2098 cm-1 , which were attributed to stretching ν(C≡N). For MTTIPNiH through the micrographs can be observed particles with varied sizes between 0.30 µM to 0.60 µm. For ZTTiPNiH the micrographs showed a small increase in particle size, approximately, 50 nm, as compared to their precursors. EDS determined the semi-quantitative chemical composition of carbon, oxygen, silicon, titanium, phosphorus, nickel and iron present in both samples. The carbon paste electrode exhibited a well-defined redox couple with E°' = +0.53 V and +0.51 V for MTTiPNiH and ZTTiPNiH, respectively, attributed to the redox process FeII(CN)6 / FeIII(CN)6 in presence of nickel. Carbon paste electrode modified with MTTiPNiH showed electrocatalytic activity to dipyrone and sulfite. However the carbon paste electrode... (Complete abstract click electronic access below)
Doutor

Fuel cells are appealing alternatives to combustion engines for efficient conversion of chemical energy to electrical energy, with the potential to meet substantial energy demands with a small carbon footprint. Intermediate temperature fuel cells (200-300 °C) combine the kinetic benefits and fuel flexibility of higher operating temperatures along with the flexibility in material choices that lower operating temperatures allow. Solid acid fuel cells (SAFCs) offer the unique benefit amongst intermediate temperature fuel cells of a truly solid electrolyte, specifically, CsH₂PO₄, which in turn, provides significant system simplifications relative to phosphoric acid or alkaline fuel cells. However, the power output of even the most advanced SAFCs has not yet reached levels typical of conventional polymer electrolyte or solid oxide fuel cells. This is largely due to poor activity of the cathodes. That is, while it has been possible to limit electrolyte voltage losses in SAFCs through fabrication of thin-membrane fuel cells (with electrolyte thicknesses of 25–50 μm), it has not been possible to attain high activity cathodes or to limit Pt loadings to competitive levels. In this thesis, the efficacy of non-precious metal catalysts in the solid acid electrochemical system is evaluated. In addition, an attractive synthesis route (specifically, the electrospray method) to fabricating high surface area electrodes with high catalyst utilization is presented.

Elimination of Pt was pursued by the evaluation of carbon nanostructures as potential oxygen reduction reaction (ORR) catalysts in the solid acid electrochemical system. Multi-walled carbon nanotubes were the most consistently catalytically active in comparison with nano-graphite. It is demonstrated that the a) precursor partial pressure, b) seed catalyst size, c) growth temperature and d) chemical functionalization can be used to control the defect density and atomic composition of multi-walled carbon nanotubes (MWCNTs), all of which play a significant role on the measured ORR activity. Increasing the precursor partial pressure, decreasing the seed catalyst size, and decreasing the growth temperature increases the density of ORR active defects. In addition, the oxygen reduction reaction (ORR) electrochemical activity evaluated by symmetric cell AC impedance spectroscopy and fuel cell measurements, were significantly enhanced by chemical functionalization with oxygen containing functional groups. Area normalized impedance responses as low as 7 Ω cm² were measured on symmetric MWCNT/ CsH₂PO₄ cells. However, it was discovered that these reactive MWCNTs also catalyze and are slightly consumed by steam reforming. Moreover, the orders of magnitude improvement with functionalization measured in impedance measurements is not replicated in fuel cell power output as a result of a decrease in open circuit voltage relative to standard cells. It is proposed that the loss in voltage results from hydrogen production at the cathode via the steam reforming reaction, although formation of hydrogen peroxide rather than water as the oxygen reduction product cannot be ruled out. This work has a significant contribution to catalysis, it demonstrates how carbon nanostructures can be designed by synthesis routes and chemical functionalization processes, to create active precious-metal-free ORR catalysts. It is also important that we have demonstrated potential ORR catalysts in acidic media. These catalysts have potential applications in phosphoric acid fuel cells and PEMFCs.

In addition to the study of carbon nanostructures, oxides were evaluated as potential ORR catalysts. Specifically, TiO_x nanoparticles were studied. Analysis shows that the activity is controlled by the oxidation state of Ti. The active site seems to be on or near slightly reduced Ti sites. In this study we have outlined synthesis routes to tune the oxidation state of Ti and enhance ORR activity in the solid acid fuel cell.

Finally, the fundamentals of the electrospray process are explored to understand how the particle size ultimately resulting from electrospray synthesis depends on both solution properties and process parameters. This analysis presents a systematic way to control the fabrication of high surface area SAFC electrodes with increased throughput, catalyst utilization and consequently power density.

Advances in the application of bipolar electrodes (BPEs) for screening of electrocatalysts, localized activation of a single conductive electrode, the optical tracking of single particles interacting with an active electrode, and the introduction of microwires in paper-based analytical devices are described. In an original proof of concept study arrays of BPEs were used to determine the relative activity of model nanoparticle systems for the oxygen reduction reaction (ORR) by a simple optical readout: the electrodissolution of Ag microbands. The number of bands that dissolved during the screening procedure determined the relative activity of the materials. These screening results for model nanoparticle systems were related to traditional electrochemical experiments and showed a strong correlation. Building on that initial study, the BPE platform for screening ORR electrocatalyst candidates was improved so that more materials could be evaluated simultaneously by increasing the density of electrodes in the array, controlled compositional variations were prepared with the implementation of piezodispensing, and a different reporter, Cr, replaced Ag at the BPE anodes which reduced the risk of contamination and improved reliability of screening experiments. Further studies into the versatility of the screening platform have been carried out using non-noble metal systems for the hydrogen evolution reaction (HER), which has a long history of interest for electrochemists. A single conductive electrode material can be made to act as an array of electrodes by confining it at the intersection of two orthogonal microfluidic channels. By manipulating the direction and magnitude of the electric field in the device, faradaic reactions can be selectively localized on the BPE. An approach for optically tracking individual, insulating microparticles interacting with an active UME has been achieved. This approach brings new insight and understanding of single particle electrochemical studies. Finally, a method for incorporating microwires and mesh electrodes into paper-based electroanalytical devices is reported. This has many advantages over traditional screen-printed carbon electrodes that are traditionally used in paper-based devices.
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The kinetics of the reduction of O₂ by Ru(NH₃)₆⁺² as catalyzed by cobalt(II) tetrakis(4-N-methylpyridyl)porphyrin are described both in homogeneous solution and when the reactants are confined to Nafion coatings on graphite electrodes. The catalytic mechanism is determined and the factors that can control the total reduction currents at Nafion-coated electrodes are specified. A kinetic zone diagram for analyzing the behavior of catalyst-mediator-substrate systems at polymer coated electrodes is presented and utilized in identifying the current-limiting processes. Good agreement is demonstrated between calculated and measured reduction currents at rotating disk electrodes. The experimental conditions that will yield the optimum performance of coated electrodes are discussed, and a relationship is derived for the optimal coating thickness.

The relation between the reduction potentials of adsorbed and unadsorbed cobalt(III) tetrakis(4-N-methylpyridyl)porphyrin and those where it catalyzes the electroreduction of dioxygen is described. There is an unusually large change in the formal potential of the Co(III) couple upon the adsorption of the porphyrin on the graphite electrode surface. The mechanism in which the (inevitably) adsorbed porphyrin catalyzes the reduction of O₂ is in accord with a general mechanistic scheme proposed for most monomeric cobalt porphyrins.

Four new dimeric metalloporphyrins (prepared in the laboratory of Professor C. K. Chang) have the two porphyrin rings linked by an anthracene bridge attached to meso positions. The electrocatalytic behavior of the diporphyrins towards the reduction of O₂ at graphite electrodes has been examined for the following combination of metal centers: Co-Cu, Co-Fe, Fe-Fe, Fe-H₂. The Co-Cu diporphyrin catalyzes the reduction of O₂ to H₂O₂ but no further. The other three catalysts all exhibit mixed reduction pathways leading to both H₂O₂ and H₂O. However, the pathways that lead to H₂O do not involve H₂O₂ as an intermediate. A possible mechanistic scheme is offered to account for the observed behavior.

This work focuses on tracking formic acid adsorption as formate onto polycrystalline gold and its subsequent catalyzed oxidation to carbon dioxide. Formic acid oxidation is notoriously dependent on supporting electrolyte composition, a dependency that is little characterized. Additionally, the mechanism of oxidation is in disagreement in the literature. As such, the two preceding topics are the primary focus of this work, and are studied in HClO4 and H2SO4 solutions. Cyclic voltammetry experiments supplemented by mathematical modelling and fitting of data were used. Solution pH and adsorption of supporting electrolyte anions onto Au(poly) were very influential factors in determining formate coverages on Au(poly). This alone explains the effect of supporting electrolyte on this reaction. The coverage of adsorbed formate was found to be singularly responsible for determining the rate of formic acid oxidation. This implies a chemical rate limiting step for oxidation, leaving the oxidation rate constant independent of potential. Another segment of this work focuses on the statistical mechanics of lattice gases, namely the role of sites available for adsorption on the activity. This topic is central to the modelling of multiple adsorbing species in competition for the same adsorption sites. Activity for interaction-free lattice gases in the thermodynamic limit was found to be coverage of adsorbates over coverage of sites available for adsorption. This relationship was exploited to simulate coadsorption of two species, the first obeying the Langmuir isotherm and the second following the hard hexagon isotherm. This system was originally considered as a possible model for coadsorption of formate and sulfate in H2SO4 solutions, but did not match with data.
Graduate
0494
jstrobl@uvic.ca

碩士
國立暨南國際大學
應用化學系
96
We prepapred a cobalt porphyrin-modified electrode, showing characteristics of electrocatalytic reduction of oxygen and carbon tetrachloride in aqueous solution. The water-soluble cobalt(III) porphyrin was modified onto gold disk surface through 4-mercaptopyridine as a bridge. The modified electrode showed excellent catalytic activity for the oxygen reduction in phosphate buffer solutions at pH 2. The electrochemical behavior and stability of the modified electrode were investigated using cyclic voltammetry and rotating disk electrode methods. The heterogeneous rate constant for the reduction of O2 at the surface of the modified electrode and the diffusion coefficient of oxygen were determined. Organohalides are an important source of environmental pollutants. The obtained modified electrode was used for detection of carbon tetrachloride. Cyclic voltammetric results showed that the modified thin film can facilitate electron transfer, lower the overpotential required and improve electrochemical behavior of carbon tetrachloride reduction, as compared to the bare gold electrode. The modified electrode permitted the detection of carbon tetrachloride in mixed solvents or aqueous solution with ease and good reproducibility.

There are many problems that need to be overcome if solar energy is to be viable on a global scale. Photons must be harvested and stored in a usable to allow efficient use of energy throughout the day. The functionalization of electrode surfaces with molecular catalysts is an attractive route for assembling (photo)electrochemical devices that convert renewable energy into chemical fuels. This work focuses on one method of noncovalently attaching molecular catalysts to graphitic surfaces. The first part describes the synthesis of a pyrene-appended bipyridine ligand that serves as the linker between the catalysts and the surface. Using this ligand, a rhodium proton-reduction catalyst and a rhenium CO2-reduction catalyst were synthesized in order to study the electrochemistry of the surface-attached species. Electrochemical and spectroscopic analysis confirm catalyst immobilization and electrocatalytically active assemblies. Bulk electrolysis of the surface-attached complexes confirm catalytic turnover formation of H2 for the rhodium complex and CO for the rhenium complex. The second part describes three new complexes utilizing the same pyrene-appended bipyridine ligand. These are [Ru(P)(4,4’-dimethyl-2,2’-bipyridine)2]Cl2, [Cp*Ir(P)Cl]Cl, and [Mn(P)(CO)3Br]. Once again, spectroscopic and electrochemical analyses confirmed successful immobilization of these complexes on high surface area carbon electrodes. The iridium complex was found to be unstable with respect to redox cycling due to ligand exchange. The ruthenium complex exhibited very high stability over long periods of redox cycling. The manganese complex was found to catalytically produce CO during bulk electrolysis.

碩士
國立暨南國際大學
應用化學系
95
The study investigates the preparation of water-soluble cobalt（III） tetrakis（N-methyl-4-pyridyl）porphyrin[　CoIII(4-TMPyP)5+ ] and platinum particle complexes-modified electrode through layer-by-layer self-assembly method. The electrochemical oxidation of sodium 4-aminobenzenesulfonate in potassium chloride aqueous solution leads to formation of amine cation radical, which subsequently formed 4-ABS-modified SPE. CoIII(4-TMPyP)5+　and K2PtCl6 were alternately deposited on a 4-ABS-modified SPE based on electrostatic interaction, and the multilayer films modified electrodes were fabricated. The CoIII(4-TMPyP)5+/Pt particle films was fabricated by electrochemical reduction. The complexes-modified electrode exhibits electrocatalytic activity for the oxidation of cysteine. At the electrooxidation of 10 mM cysteine in 0.5 M NaOH solution, reducing the overpotential by about 0.4 V, and the maximum catalytic current and the catalytic potential obtained from cyclic voltammetry were 66 μA and -0.08 V, respectively.

碩士
國立交通大學
應用化學系所
96
Tin-Doped indium oxide (ITO) film is a good photoelectric material for its good conductivity and photo penetrability. It is a new type of microchip substrate in some research. However, the electro-catalytic activity of ITO is relative low versus most kind of detector. In order to increase the electro-catalytic activity, modification of surface of ITO glass was needed. The electronic property, chemical and mechanical stability of carbon nanotubes (CNTs) make them attractive for electrochemical and biochemical sensor. In addition, high surface area, high conductivity of CNTs increases the catalytic current and sometimes decreases catalytic voltage of cyclic voltammetry. Ionic polymer would help for binding nanotubes on ITO electrode under the allowable of catalytic current lost. In this research, multi-walled carbon nanotubes (MWNTs)/Nafion(R) composite films were made. The surface of ITO glass was modified with such nano composite films for the measurement of pyrocatechol. The functional groups beside CNTs make them more attractive and selective under electrocatalysis. We use CNTs matrix as none-oriented films spin-coated on ITO electrode for specific analyte. The oxidation current of modified-electrode measuring with Catechol and Dopamine is 100 fold and 20 fold larger than bare ITO respective. The reduction current is 100 fold and 1000 fold lager than bare ITO respective.

L'azoto gioca un ruolo indispensabile per la vita sulla terra e per lo sviluppo degli esseri umani. Industrialmente, è necessario convertire l'azoto gassoso in ammoniaca (NH3) per la produzione di fertilizzanti, in modo da integrare la quantità di azoto fissata spontaneamente in natura. Attualmente, l'unica tecnologia di sintesi dell'ammoniaca su scala industriale è il processo sviluppato da Haber e Bosch all'inizio del XX secolo che utilizza N2 e H2 come gas di alimentazione. Tuttavia, il processo Haber-Bosch richiede condizioni molto drastiche, apparecchiature complesse e porta ad un elevato consumo energetico, operando inoltre a bassi tassi di conversione che non sono coerenti con le esigenze sempre crescenti di sviluppo economico e sociale. In alternativa al metodo Haber-Bosch, l'elettrocatalisi rappresenta una delle vie più promettenti che possono integrare l'elettricità prodotta da tecnologie di energia rinnovabile con la produzione di ammoniaca a temperatura ambiente e a pressione atmosferica. Una sfida specifica è legata allo sviluppo di nuovi elettrocatalizzatori/elettrodi con l'obiettivo di ottenere una produzione di ammoniaca a basso costo, su larga scala e delocalizzata sul territorio. Alla luce delle suddette questioni scientifiche fondamentali, questo lavoro di dottorato si concentra su tre aspetti principali legati alla reazione elettrocatalitica di riduzione dell'azoto (NRR): i) l’ingegneria e la progettazione dell'elettrocatalizzatore, ii) la progettazione dell'elettrodo e del dispositivo elettrochimico e iii) il miglioramento e l’ottimizzazione delle condizioni di reazione, per migliorarne le prestazioni nella sintesi dell'ammoniaca. La maggior parte delle attività di ricerca di questo dottorato, dalla sintesi e caratterizzazione dei materiali elettrocatalitici all'assemblaggio/collaudo degli elettrodi in dispositivi elettrochimici non convenzionali, sono state svolte presso il laboratorio CASPE (Laboratorio di Catalisi per la Produzione e l'Energia Sostenibile) dell'Università di Messina. Durante i tre anni, un periodo di 12 mesi è stato inoltre trascorso in cotutela con l'École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), dove sono state studiate tecniche di sintesi avanzate per la preparazione di elettrocatalizzatori a base organometallica da utilizzare come elettrodi cataliticamente attivi nella NRR. La tesi di dottorato è organizzata in cinque capitoli principali. Il capitolo 1 si concentra sulle questioni di fissazione dell’azoto e sulla descrizione del processo industriale Haber-Bosch, con una panoramica sulle implicazioni generali relative al suo elevato fabbisogno energetico. Vengono poi presentati i metodi alternativi per la fissazione elettrochimica dell'azoto, con un'ampia descrizione dei vantaggi, legati alle condizioni più favorevoli (cioè temperatura ambiente e pressione atmosferica) e degli svantaggi, e discutendo gli elementi da sviluppare per una futura implementazione di questa tecnologia, includendo anche una descrizione del possibile meccanismo di reazione, ancora non del tutto chiaro in letteratura. Il capitolo 2, invece, si riferisce alla descrizione dei materiali elettrocatalitici sviluppati in questo lavoro di dottorato per la preparazione degli elettrodi: 1) i “Metal-Organic Frameworks” (MOF), una classe di materiali porosi molto promettenti per le loro caratteristiche peculiari di elevata superficie, proprietà adattabili, funzionalità organica e porosità, oltre che per la possibilità di creare specifici siti attivi catalitici grazie sia ai gruppi funzionali che ai centri ionici metallici; 2) i MXeni, una classe di materiali a base di carburi o nitruri metallici con struttura bidimensionale (2D), che hanno recentemente attirato un grande interesse per una vasta gamma di applicazioni, tra cui la catalisi e la fissazione di N2, per le loro proprietà uniche di conducibilità metallica e la natura idrofila delle superfici con terminali idrossilici o di ossigeno. Nei capitoli 3-5 vengono presentati e discussi i risultati sperimentali. Il capitolo 3 riguarda la preparazione di una serie di elettrodi di MOF a base di Fe (Fe@Zn/SIM-1) e il loro test nella NRR utilizzando un reattore trifasico avanzato, che lavora in fase gassosa. Questo nuovo dispositivo funziona a temperatura ambiente e a pressione atmosferica, e possiede il controelettrodo e l’elettrodo di riferimento immersi in una semicella anodica (dove avviene l'ossidazione di H2O a O2) contenente un elettrolita liquido (l'anolita), mentre la semicella catodica per la NRR opera in fase gassosa senza elettrolita liquido. Questo tipo di reattore elettrocatalitico è quindi molto diverso dai reattori elettrocatalitici convenzionali che operano in fase liquida, con il grande vantaggio di evitare problematiche legate alla bassa solubilità e al trasporto di N2 nell'elettrolita, e di permettere inoltre un più facile recupero dell'ammoniaca prodotta. I risultati ottenuti da questi test elettrocatalitici in fase gassosa sono stati molto utili per migliorare la progettazione degli elettrodi a base di MOF, evidenziando i limiti di questo tipo di materiali in termini di contenuto di N, stabilità e possibilità di preparare elettrocatalizzatori più avanzati mediante carbonizzazione. Un'ampia parte di questo capitolo è stata dedicata allo sviluppo di nuove strategie sperimentali per evitare i falsi positivi nella rilevazione dell'ammoniaca, che è uno degli argomenti più investigati negli ultimi due anni dai ricercatori che lavorano sulla NRR. Mentre in letteratura sono stati recentemente proposti protocolli molto accurati che utilizzano tecniche analitiche avanzate (basati sull’azoto marcato 15N), in questo lavoro viene invece suggerita una metodologia più semplice basata sull'analisi spettrofotometrica UV-visibile (accoppiata a test in bianco con gas inerti in luogo dell’azoto) che hanno permesso con successo di evitare contaminazioni da ammoniaca e identificare i falsi positivi, anche se tecniche analitiche più sofisticate sono sicuramente necessarie per confermare definitivamente la vera fonte di ammoniaca. Nel capitolo 4 viene presentata una serie di materiali MOF migliorati (MOF UiO-66-(COOH)2 a base di Fe o Fe e metalli alcalini), sintetizzati con la tecnica di reazione a scambio cationico per sostituire il protone dell'acido carbossilico con un catione di ferro. Rispetto ai materiali Fe@Zn/SIM-1, questa nuova classe di MOF è più stabile in acqua e non contiene atomi di azoto nella sua struttura. I risultati hanno dimostrato che il Fe@UiO-66-(COOH)2 ottenuto mediante l'80% di scambio cationico (con un contenuto effettivo di Fe di circa 8% in peso) è stato il miglior elettrocatalizzatore testato tra i vari materiali MOF a base di Fe sintetizzati. Le prestazioni nella NRR dipendono fortemente dal design della cella e dell'elettrodo. Più in dettaglio, è stato ottenuto un rendimento di ammoniaca di 1.19 μg•h-1•mgcat-2 con una configurazione di strati assemblati ed ordinati nel modo seguente: i) Nafion (la membrana), ii) MOF a base di Fe (l'elettrocatalizzatore), iii) il GDL (lo strato di diffusione gassosa a base di carbonio) e iv) un ulteriore strato di Fe-MOF. È stato anche esplorato l'effetto del voltaggio applicato, con un potenziale ottimale di -0.5 V vs RHE per massimizzare l'attività nella NRR e limitare la reazione collaterale di evoluzione dell'idrogeno. Inoltre, come attualmente utilizzato nei catalizzatori industriali per il processo Haber-Bosh, è stata studiata anche l'introduzione del potassio negli elettrocatalizzatori, al fine di facilitare il trasferimento di carica dagli ioni K- verso la superficie del catalizzatore a base di ferro, bilanciando il chemisorbimento dissociativo tra H2 e N2, e sopprimendo le reazioni collaterali, migliorandone così sia l'attività che la stabilità. I risultati ottenuti sono molto promettenti, anche se sono necessari ulteriori studi per migliorare le loro prestazioni nella NRR, per superare le limitazioni legate ai materiali MOF stessi, soprattutto a causa della loro bassa conducibilità e stabilità. Infine, il capitolo 5 si riferisce all'esplorazione di materiali avanzati, i MXeni (Ti3C2 MXeni), e al tentativo di sintetizzare una nanoarchitettura 3D partendo dalla loro forma bidimensionale. Per comprendere il ruolo della nanostruttura dei materiali MXeni nella NRR, “nanoribbons” (nano-nastri) di Ti3C2 sono stati trattati con KOH per ottenere una forma finale di strutture porose tridimensionali (3D). In particolare, l'obiettivo di questa parte di lavoro è stato quello di indagare come la conversione dei “nanoribbons” di Ti3C2 in strutture tridimensionali influenzi la reattività nella NRR condotta nel dispositivo elettrochimico in fase gassosa. È stata anche effettuata una caratterizzazione completa dei “nanoribbons” di MXeni (SEM, TEM, HRTEM, XRD, XPS e EDX). I risultati hanno mostrato che la nanostruttura tridimensionale porta ad un significativo miglioramento dell'attività di fissazione di N2 a causa della formazione di siti esposti di Ti-OH. È stata anche osservata una relazione lineare tra il tasso di formazione di ammoniaca e la quantità di ossigeno sulla superficie dei Ti3C2 MXeni.
Nitrogen plays an indispensable role for all life on earth and for the development of human beings. Industrially, nitrogen gas is converted to ammonia (NH3) and nitrogen-rich fertilisers to supplement the amount of nitrogen fixed spontaneously by nature. At present, the only industrial-scale ammonia synthesis technology is the process developed by Haber and Bosch in the early 20th century using gas phase N2 and H2 as the feeding gases. However, the Haber-Bosch process requires harsh conditions, complex equipment and high energy consumption, and operates with low conversion rates, which are inconsistent with economic and social growing development requirements. Compared to the Haber-Bosch method, electrocatalysis is one of the promising routes that can integrate electricity produced from renewable energy technologies for the production of ammonia at room temperature and ambient pressure. A specific challenge is related to the development of novel electrocatalysts/electrodes with the aim to achieve a low-cost, large-scale and delocalized production of ammonia. In view of the above key scientific issues, this PhD work focuses on three main aspects of the electrocatalytic nitrogen reduction reaction (NRR): i) engineering and design of the electrocatalyst, ii) electrode and cell design of the electrochemical device and iii) improvement and optimization of the reaction conditions, to enhance the performances of ammonia synthesis. Most of the research activities of this PhD work about synthesis and characterization of the electrocatalytic materials and assembling/testing of the electrodes in unconventional electrochemical devices were carried out at the laboratory CASPE (Laboratory of Catalysis for Sustainable Production and Energy) of the University of Messina. Moreover, during the three years, a period of 12 months was spent in cotutelle with the École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), where advanced synthesis routes were explored for the preparation of organometallic-based electrocatalysts to be used as more active electrodes in NRR. The PhD thesis is organized in five main chapters. Chapter 1 focuses on N2 fixation issues and on describing the industrial Haber-Bosch process, with an overview of the general implications related to its high energy requirements. The alternative methods based on the electrochemical nitrogen fixation are then presented, with a wide description of pros and cons related to the milder conditions (i.e., room temperature and atmospheric pressure) and by discussing the elements to be developed for a future implementation of this technology, including a description of the possible reaction mechanism, which is still unclear in literature. Chapter 2, instead, refers to the electrocatalytic materials developed in this PhD work for the preparation of the electrodes: 1) the Metal-organic Frameworks (MOFs), a class of porous materials very promising for their peculiar characteristics of high surface area, tunable properties, organic functionality and porosity, as well as for the possibility of creating specific catalytic active sites thanks to both the functional groups and the metal ion centres; 2) the MXenes, a class of metal carbide or nitride materials with a two-dimensional (2D) structure, which have recently attracted a large interest for a broad range of applications, including catalysis and N2 fixation, for their unique properties of metallic conductivity and hydrophilic nature of the hydroxyl or oxygen terminated surfaces. In Chapters 3-5, the experimental results are presented and discussed. Chapter 3 concerns the preparation of a series of Fe-MOF-based (Fe@Zn/SIM-1) electrodes and their testing in NRR by using an advanced engineered three-phase reactor, working in gas-phase. This novel device operates at room temperature and atmospheric pressure, with counter and reference electrodes immersed into an anode half-cell (where the oxidation of H2O to O2 occurs) containing a liquid electrolyte (the anolyte), while the cathode half-cell for NRR operates in gas phase without a liquid electrolyte (electrolyte-less conditions). This type of electrocatalytic reactor is thus quite different from the conventional electrocatalytic reactors operating in liquid phase, with the main advantages of avoiding issues related to the low N2 solubility and transport in the electrolyte, and allowing an easier recovery of ammonia. The results obtained from these electrocatalytic tests in gas-phase were very useful to improve the design of the MOFs-based electrodes, evidencing the limits of these kinds of materials in terms of N content, stability and possibility to prepare more advanced electrocatalysts by carbonization. A wide part of this chapter was dedicated to the development of new experimental strategies for avoiding false positive in the detection of ammonia, which is one of the topics most studied from scientists working in NRR in the last two years. As accurate protocols were recently suggested in literature, also using advanced analytical techniques (i.e. using 15N labelled nitrogen), an easier methodology based on UV-visible spectrophotometric analysis (coupled with blank tests with inert gases) was suggested in this work to avoid ammonia contaminations and false positives, although more sophisticated analytical techniques may definitely confirm the real source of ammonia. In Chapter 4, a series of improved Fe-MOF-based materials (Fe-based and Fe-alkali metal-based MOF UiO-66-(COOH)2), synthesized by cation exchange reaction technique to replace the proton of carboxylic acid with an iron cation, are presented. With respect to Fe@Zn/SIM-1, this new class of MOFs are more stable in water and do not contain nitrogen atoms in their structure. Results evidenced that 80% cation exchange Fe@UiO-66-(COOH)2 (with an effective Fe content of around 8 wt.%) was the best electrocatalyst among the tested Fe-based MOF synthesized materials. The performances in NRR highly depended on cell and electrode design. More in detail, an ammonia yield of 1.19 μg•h-1•mgcat-2 was obtained with an assembling configuration of layers ordered as i) Nafion (the membrane), ii) Fe-based MOF (the electrocatalyst), iii) GDL (the carbon gas diffusion layer) and iv) a further layer of Fe-MOF. The effect of applied voltage was also explored, indicating an optimal voltage of -0.5 V vs. RHE to maximize activity in NRR and limiting the side hydrogen evolution reaction. Moreover, as currently used in the industrial catalysts for Haber-Bosh process, the introduction of potassium in the electrocatalysts was also investigated, in order to facilitate charge transfer from K- ions to the iron-based catalyst surface, balancing the dissociative chemisorption between H2 and N2, and suppressing side reactions, thus improving both activity and stability. These results were very promising, although a further experimentation is needed to improve their performances in NRR, to overcome limitations related to MOF materials themselves, majorly due to their low conductivity and stability. Finally, Chapter 5 refers to the exploration of advanced MXene materials (Ti3C2 MXene) and to the attempt of synthesizing a 3D nanoarchitecture starting from 2D-dimensional MXene-based catalysts. To understand the role of the nanostructure of MXene materials in NRR, Ti3C2 nanosheets were treated with KOH to obtain a final shape of three-dimensional (3D) porous frameworks nanoribbons. Specifically, the objective of this research was to investigate how the conversion of Ti3C2 nanosheets to 3D-like nanoribbons influence the NRR reactivity in the gas-phase electrochemical device. A full characterization of MXenes nanoribbons (SEM, TEM, HRTEM, XRD, XPS and EDX) was also presented. Results showed that the 3D-type nanostructure (nanoribbons) leads to a significant enhancement of the N2 fixation activity due to the formation of exposed Ti-OH sites. A linear relationship was observed between ammonia formation rate and amount of oxygen on the surface of Ti3C2 MXene.
L'azote joue un rôle indispensable pour toute vie sur terre et pour le développement des êtres humains. Industriellement, l'azote gazeux est converti en ammoniac (NH3) et en engrais riches en azote pour compléter la quantité d'azote fixée spontanément par la nature. À l'heure actuelle, la seule technologie de synthèse de l'ammoniac à l'échelle industrielle est le procédé mis au point par Haber et Bosch au début du XXe siècle, qui utilise les phases gazeuses N2 et H2. Cependant, le procédé Haber-Bosch nécessite des conditions difficiles, des équipements complexes et une consommation d'énergie élevée, et fonctionne avec de faibles taux de conversion, ce qui est incompatible avec les exigences d’un développement durable. Par rapport à la méthode Haber-Bosch, l'électrocatalyse est l'une des voies prometteuses qui permet d'intégrer l'électricité produite à partir de technologies d'énergies renouvelables pour la production d'ammoniac à température ambiante et à pression ambiante. Un défi spécifique est lié au développement de nouveaux électrocatalyseurs/électrodes dans le but de parvenir à une production d'ammoniac à faible coût, à grande échelle et délocalisée. Compte tenu ces défis scientifiques, ce travail de doctorat se concentre sur trois aspects principaux de la réaction électrocatalytique de réduction de l'azote (NRR) : i) ingénierie et conception de l'électrocatalyseur, ii) conception de l'électrode et de la cellule du dispositif électrochimique et iii) amélioration et optimisation des conditions de réaction, afin d'améliorer les performances de la synthèse de l'ammoniac. La plupart des activités de recherche de ce travail de doctorat sur la synthèse et la caractérisation des matériaux électrocatalytiques et l'assemblage/le test des électrodes dans des dispositifs électrochimiques non conventionnels ont été menées au laboratoire CASPE (Laboratory of Catalysis for Sustainable Production and Energy) de l'université de Messine. En outre, une période de 12 mois a été passée en cotutelle avec l'École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), où des voies de synthèse avancées ont été explorées pour la préparation d'électrocatalyseurs à base de composés organométalliques qui ont été utilisés comme électrodes plus actives dans la RRN. Cette thèse de doctorat est organisée en cinq grands chapitres. Le chapitre 1 se concentre sur les questions de fixation de l'azote et sur la description du processus industriel de Haber-Bosch, avec un aperçu des implications générales liées à ses besoins élevés en énergie. Les méthodes alternatives basées sur la fixation électrochimique de l'azote sont ensuite présentées, avec une large description des avantages et des inconvénients liés aux conditions plus douces (c'est-à-dire la température ambiante et la pression atmosphérique) et en discutant des éléments à développer pour une future mise en œuvre de cette technologie, y compris une description du mécanisme de réaction possible, encore débattu dans la littérature. Le chapitre 2 fait référence aux matériaux électrocatalytiques développés pour la préparation des électrodes : 1) les matériaux hybrides organiques-inorganiques de type MOF, une classe de matériaux poreux très prometteurs pour leurs caractéristiques particulières de surface spécifique élevée et leurs propriétés ajustables ainsi que pour la possibilité de créer des sites catalytiques actifs spécifiques grâce aux groupes fonctionnels et aux centres d'ions métalliques ; 2) les MXènes, une classe de matériaux en carbure ou nitrure de métal à structure bidimensionnelle (2D), qui ont récemment suscité un grand intérêt pour un large éventail d'applications, notamment la catalyse et la fixation de N2, pour leurs propriétés uniques de conductivité métallique et de nature hydrophile des surfaces terminées par un hydroxyle ou un oxygène. Les chapitres 3 à 5 présentent et analysent les résultats expérimentaux. Le chapitre 3 concerne la préparation d'une série d'électrodes à base de Fe-MOF (Fe@Zn/SIM-1) et leur test dans la réaction NRR en utilisant un réacteur triphasé de pointe, fonctionnant en phase gazeuse. Ce nouveau dispositif fonctionne à température ambiante et à la pression atmosphérique, avec des électrodes de comptage et de référence immergées dans une demi-cellule anodique (où se produit l'oxydation de H2O en O2) contenant un électrolyte liquide (l'anolyte), tandis que la demi-cellule cathodique pour le NRR fonctionne en phase gazeuse sans électrolyte liquide. Ce type de réacteur électrocatalytique est donc très différent des réacteurs électrocatalytiques classiques fonctionnant en phase liquide, avec les principaux avantages d'éviter les problèmes liés à la faible solubilité et au transport de N2 dans l'électrolyte, et de permettre une récupération plus facile de l'ammoniac. Les résultats obtenus lors de ces essais électrocatalytiques en phase gazeuse ont été très utiles pour améliorer la conception des électrodes à base de MOFs, mettant en évidence les limites de ce type de matériaux en termes de teneur en N, de stabilité et de possibilité de préparer des électrocatalyseurs plus avancés par carbonisation. Une grande partie du chapitre 3 a été consacrée au développement de nouvelles stratégies expérimentales pour éviter les faux positifs dans la détection de l'ammoniac, qui est l'un des sujets les plus étudiés par les scientifiques travaillant dans la NRR ces deux dernières années. Comme des protocoles précis ont été récemment suggérés dans la littérature, utilisant également des techniques analytiques avancées (c'est-à-dire utilisant de l'azote marqué à 15N), une méthodologie plus facile basée sur l'analyse spectrophotométrique UV-visible (couplée à des essais à blanc avec des gaz inertes) a été suggérée dans ce travail pour éviter les contaminations par l'ammoniac et les faux positifs, bien que des techniques analytiques plus sophistiquées puissent définitivement confirmer la source réelle d'ammoniac. Dans le chapitre 4, une série de matériaux améliorés à base de Fe-MOF (incluant un dopage additionel par un métal alcalin du MOF UiO-66-(COOH)2), synthétisés par une technique de réaction d'échange de cations pour remplacer le proton de l'acide carboxylique par un cation de fer, sont présentés. En ce qui concerne le Fe@Zn/SIM-1, cette nouvelle classe de MOF est plus stable dans l'eau et ne contient pas d'atomes d'azote dans sa structure. Les résultats ont montré que l'échange cationique à 80 % Fe@UiO-66-(COOH)2 (avec une teneur effective en Fe d'environ 8 % en poids) était le meilleur électrocatalyseur parmi les matériaux synthétisés de MOF à base de Fe testés. Les performances du NRR dépendaient fortement de la conception de la cellule et de l'électrode. Plus en détail, un rendement en ammoniac de 1.19 μg•h-1•mgcat-2 a été obtenu avec une configuration d'assemblage de couches ordonnées comme i) Nafion (la membrane), ii) MOF à base de Fe (l'électrocatalyseur), iii) GDL (la couche de diffusion de gaz carbonique) et iv) une autre couche de Fe-MOF. L'effet de la tension appliquée a également été exploré, indiquant une tension optimale de -0,5 V par rapport à la RHE pour maximiser l'activité dans le NRR et limiter la réaction latérale d'évolution de l'hydrogène. En outre, comme c'est le cas actuellement dans les catalyseurs industriels pour le procédé Haber-Bosh, l'introduction de potassium dans les électrocatalyseurs a également été étudiée, afin de faciliter le transfert de charge des ions K- à la surface du catalyseur à base de fer, en équilibrant la chimisorption dissociative entre H2 et N2, et en supprimant les réactions secondaires, ce qui améliore à la fois l'activité et la stabilité. Ces résultats étaient très prometteurs, bien qu'une nouvelle expérimentation soit nécessaire pour améliorer leurs performances dans les NRR, afin de surmonter les limitations liées aux matériaux MOF eux-mêmes, principalement en raison de leur faible conductivité et de leur stabilité. Enfin, le chapitre 5 fait référence à l'exploration des matériaux avancés à base de MXène (Ti3C2 MXène) et à la tentative de synthèse d'une nanoarchitecture 3D à partir de catalyseurs à base de MXène en 2D. Pour comprendre le rôle de la nanostructure des matériaux à base de MXène dans la NRR, des nanofeuilles de Ti3C2 ont été traitées au KOH pour obtenir une forme finale de nanorubans à armature poreuse tridimensionnelle (3D). Plus précisément, l'objectif de cette recherche était d'étudier comment la conversion des nanofeuilles de Ti3C2 en nanorubans tridimensionnels influençait la réactivité du NRR dans le dispositif électrochimique en phase gazeuse. Une caractérisation complète des nanorubans MXenes (SEM, TEM, HRTEM, XRD, XPS et EDX) a également été présentée. Les résultats ont montré que la nanostructure de type 3D (nanorubans) conduit à une amélioration significative de l'activité de fixation du N2 en raison de la formation de sites Ti-OH exposés. Une relation linéaire a été observée entre le taux de formation d'ammoniac et la quantité d'oxygène à la surface du Ti3C2 MXene.