A study of the stability of bicontinuous nanoporous metals is presented. Atomic scale simulations are used to probe the dominant mechanisms of geometric relaxation in these materials. A method is presented for generating model bicontinuous metal / void structures for use in atomistic simulations of bicontinuous nanoporous solids. The structures are generated with periodic boundary conditions using a phase-field model to simulate the spinodal decomposition of an ideal system. One phase in the model is then associated with the pore volume while the other phase is associated with the metal ligaments. Small angle neutron scattering was used to quantitatively compare experimental samples to those generated by the phase field method. EAM results using model structures with experimentally accessible length scales are presented which demonstrate the potential of such simulations in understanding the behavior of nanoporous metals. Simulated relaxations of these structures, as well as the relaxation of model spherical clusters, indicate that the surface relaxation effect dominates the overall dimensional relaxation of np-metals post processing. Capillary effects play a secondary role in the overall relaxation. The simulation results presented also identify a maximum surface area to volume ratio necessary to maintain mechanical stability beyond which the pore structure collapses. / Ph. D.

L’or, habituellement considéré comme catalytiquement inactif, fait preuve d’une activité étonnante pour diverses réactions d’oxydation pourvu qu’il soit supporté sur un oxyde approprié. Ces dix dernières années, des méthodes de synthèse par dissolution sélective du composé le moins noble d’un alliage métallique (dealloying) ont permis l’obtention de catalyseurs d’or nanoporeux. Ces catalyseurs font preuve d’une très forte activité catalytique vis-à-vis de la réaction d’oxydation du monoxyde de carbone. Cependant, des études plus récentes semblent montrer que cette activité est due aux impuretés présentes dans les catalyseurs, qui sont imputables aux limitations de la méthode de synthèse utilisée. Dans cette étude, un catalyseur nanoporeux d’or pur a été obtenu par oxydation spontanée d’un alliage AuZr à température ambiante puis dissolution sélective totale de ZrO2 dans HF. Ce catalyseur démontre des caractéristiques structurales et morphologiques similaires à celles des échantillons obtenus par dealloying. Leur évaluation catalytique a été réalisée par réaction d’oxydation du CO et en PrOx : les résultats montrent que l’or pur nanoporeux n’est pas catalytiquement actif. La préparation de catalyseurs AgAu selon la même méthode a permis l’obtention de catalyseurs de différentes teneurs en argent, proches des résidus obtenus par dealloying. L’impact de la présence de l’impureté d’argent sur la catalyse est avéré : elle permet d’exacerber l’activité de l’or à température ambiante par synergie des deux éléments. Cependant, l’effet promoteur de l’hydrogène disparaît en PrOx et l’impact de la concentration d’argent est faible lors de l’oxydation du CO. Une étude exploratoire sur les nanomousses NiPd a été menée en parallèle. Le palladium, qui présente le meilleur compromis activité/sélectivité pour les hydrogénations sélectives, voit son activité exacerbée lorsqu’il est déposé à la surface d’un monocristal de Nickel. Cet effet n’existe pas pour des nanoparticules Pd/Ni supportées. Un catalyseur NiPd a donc été préparé dans cette étude selon la méthode des nickels de Raney® afin de combiner les propriétés des monocristaux et des nanoparticules / Gold, generally considered as catalytically inactive, demonstrates a surprising activity toward several oxidation reactions when supported on a proper oxide. New synthesis ways have been developed for ten years to obtain nanoporous gold catalysts based on selective dissolution of the less noble component of a metallic alloy (dealloying). These catalysts exhibit very high activity towards the carbon monoxide oxidation reaction. However recent studies seem to reveal that this activity could be due to impurities inherent to dealloying. In this study a very pure nanoporous catalyst was obtained by spontaneous oxidation of a AuZr alloy at room temperature; a total selective dissolution of ZrO2 was then carried out in HF. Its structural and morphological characteristics proved to be similar to the dealloyed catalysts ones. The evaluation of its catalytic properties by CO oxidation showed that pure nanoporous gold was not catalytically active. Besides bimetallic AgAu catalysts were prepared following the same preparation method with three silver concentrations chosen close to the residual impurities concentrations obtained by dealloying. Their catalytic properties proved to be impacted by silver impurities: gold activity was emphasized at room temperature by synergy between the two elements. However, the promotional effect of hydrogen disappeared in PrOx and the role of silver concentration was low for CO oxidation. In parallel an exploratory study was carried out on NiPd nanofoams. The catalysts were prepared following the Raney® nickel method to improve the palladium activity towards the selective hydrogenation reaction. The results showed a slight increase of the catalytic activity

The structure of nanoporous gold (np-Au) provides a very limited volume for deformation to occur, and thus offers an opportunity to study the role of defects such as dislocations in nanoscale metal volumes. A practical goal is to understand mechanical properties of np-Au so that it can be can produced in stable form, for use in applications that require some mechanical integrity. Bulk and thin film np-Au have been fabricated and studied here. Bulk np-Au was prepared by electrochemically dealloying Au-Ag alloys with 25 and 30 at.% Au. In the lower Au content material, cracks always formed during dealloying. When Au content increased to 30 at.% and a two-step electrochemical dealloying method was used (first using diluted electrolyte and then concentrated acid), bulk np-Au with no volume change and minimal cracking was successfully fabricated. Thermal and mechanical behavior of np-Au was studied by heat treatment and microindentation. During annealing in air, Ostwald ripening governed ligament coarsening, while annealing of ligaments in vacuum was more likely a sintering process. Nanoporous Au thin films were produced by dealloying sputtered Au-Ag alloy films. Residual stresses in np-Au films were measured with wafer curvature. Similar to bulk materials, np-Au thin films made from 25 at.% Au alloy films exhibited extensive cracking during dealloying, whereas films from 30 at.% Au precursor alloys were completely crack-free. 25 at.% Au np-Au films carried almost no stress because of extensive cracking, whereas stress in 30 at.% Au np-Au films was up to ~230 MPa. Ligament coarsening followed a t1/8 time dependence for stress-free films, versus t1/4 in films under stress. It was proposed that bulk diffusion was responsible for formation of larger pits at grain centers during the incipient stages of dealloying. In situ nanoindentation experiments inside the transmission electron microscope were performed to investigate deformation of np-Au films and dislocation motion within ligaments. Dislocations were generated easily and moved along ligament axes, after which they interacted with other dislocations in the nodes of the porous network. It was found that slower displacement rates caused load drops to occur at shorter distance intervals and longer time intervals.

FABRICATION OF NANOPOROUS GOLD AND BIOLOGICAL APPLICATIONS By Badharinadh Uppalapati A Dissertation submitted in partial fulfillment of the requirements for the degree of Master of Science at Virginia Commonwealth University. Virginia Commonwealth University, 2014 Major Director: Maryanne M. Collinson, Professor, Department of Chemistry Fabrication of nanoporous gold electrodes by dealloying Au:Ag alloys has attracted much attention in sensing applications. In the first part of this work, the electrochemical response of the redox active molecule, potassium ferricyanide, in a solution of bovine serum albumin in buffer, serum or blood was studied using nanoporous gold and comparisons made to planar gold. Nanoporous gold electrodes with different surface areas and porosity were prepared by dealloying Au:Ag alloy in nitric acid for different dealloying times, specifically, 7.5, 10, 12.5, 20 minutes. Characterization was done using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), and cyclic voltammetry (CV). Using cyclic voltammetry, planar gold electrodes exposed to bovine serum albumin in buffer showed a decrease in Faradaic peak current and an increase in peak splitting for potassium ferricyanide. The time required for the peak Faradaic current to drop to one-half of its original value was 3 minutes. At nanoporous gold electrodes, however, no significant reduction in Faradaic peak current or increase in peak splitting was observed. Nanoporous gold electrodes having the smallest pore size and largest surface area showed ideal results to biofouling. These electrodes are believed to impede the mass transport of large biomolecules while allowing small redox molecules to exchange electrons effectively with the electrode. In the second part of this work, the open circuit potential (OCP) of biologic solutions (e.g., blood) was measured using nanoporous gold electrodes. Historically, the measurement of blood redox potential has been hindered due to significant fouling and surface passivation of the metal electrodes. As nanoporous gold electrodes retained electrochemical activity of redox probes like potassium ferricyanide in human serum and rabbit blood, they were used to measure the OCP of blood and plasma from various animals like pig, rabbit, rat, monkey and humans. Comparisons were made to planar gold electrodes. The OCP values at both the planar gold and nanoporous gold electrodes were different from each other and there was variability due to different constituents present in blood and plasma. The OCP of rabbit blood and crashed rabbit blood was measured and the values were found to be different from each other indicating that ORP helps in measuring the animal condition. Ascorbic acid was added to rabbit and sheep blood and OCP measured at the nanoporous electrodes. Addition of reducing agent to blood at different intervals and different concentrations showed a change in potential with concentration.

Les métaux micro ou nanoporeux sont très attrayants notamment pour leur grande surface spécifique. Le désalliage dans un bain de métal liquide permet une dissolution sélective d'une espèce chimique (l'élément soluble) à partir d'un alliage d'origine (le précurseur) composé de l'élément soluble et d'un élément cible (qui deviendra nano/micro poreux) non soluble dans le bain de métal liquide. Quand le précurseur est plongé dans le bain de métal liquide, à son contact, l'élément soluble va se dissoudre dans le bain tandis que l'élément cible va en parallèle se réorganiser spontanément afin de former une structure poreuse. Quand l'échantillon est retiré du bain, il est sous la forme d'une structure bi-continue composée de deux phases : l'une étant la structure poreuse composée de l'élément cible et l'autre est une phase dans laquelle est présente l'élément du bain avec l'élément sacrificiel en solution solide. Cette phase peut être dissoute par une attaque chimique afin d’obtenir le métal nano/micro poreux. Les objectifs principaux de cette thèse sont l'élaboration et la caractérisation microstructurale et mécanique de 3 différents types de matériaux par désalliage dans un bain de métal liquide : des composites métal-métal (FeCr-Mg), des métaux poreux (FeCr) et des composites métal-polymère (FeCr-matrice époxy). Le dernier objectif est l'évaluation des possibilités d'utiliser la technique de désalliage dans un bain de métal liquide dans un contexte industriel. L'étude de la microstructure est basée sur des observations 3D faites par tomographie aux rayons X et des analyses 2D réalisées en microscopie électronique (SEM, EDX, EBSD). Pour mieux comprendre le désalliage, le procédé a été suivi in situ en tomographie aux rayons X et diffraction. Enfin, les propriétés mécaniques ont été évaluées par nanoindentation et compression. / Nanoporous metals have attracted considerable attention for their excellent functional properties. The first developed technique used to prepare such nanoporous noble metals is dealloying in aqueous solution. Porous structures with less noble metals such as Ti or Fe are highly desired for various applications including energy-harvesting devices. The less noble metals, unstable in aqueous solution, are oxidized immediately when they contact water at a given potential so aqueous dealloying is only possible for noble metals. To overcome this limitation, a new dealloying method using a metallic melt instead of aqueous solution was developed. Liquid metal dealloying is a selective dissolution phenomenon of a mono-phase alloy solid precursor: one component (referred as soluble component) being soluble in the metallic melt while the other (referred as targeted component) is not. When the solid precursor contacts the metallic melt, only atoms of the soluble component dissolve into the melt inducing a spontaneously organized bi-continuous structure (targeted+sacrificial phases), at a microstructure level. This sacrificial phase can finally be removed by chemical etching to obtain the final nanoporous materials. Because this is a water-free process, it has enabled the preparation of nanoporous structures in less noble metals such as Ti, Si, Fe, Nb, Co and Cr. The objectives of this study are the fabrication and the microstructure and mechanical characterization of 3 different types of materials by dealloying process : (i) metal/metal composites (FeCr-Mg), (ii) porous metal (FeCr) (iii) metal/polymer composites (FeCr-epoxy resin). The last objective is the evaluation of the possibilities to apply liquid metal dealloying in an industrial context. The microstructure study was based on 3D observation by X-ray tomography and 2D analysis with electron microscopy (SEM, SEM-EDX, SEM-EBSD). To have a better understanding of the dealloying, the process was followed in situ by X-ray tomography and X-ray diffraction. Finally the mechanical properties were evaluated by nanoindentation and compression.

De nouveaux matériaux métalliques nanoporeux à base d’éléments n’appartenant pas à la famille du Pt ont été synthétisés par le "dealloying" (ou dissolution sélective) d’alliages rapidement solidifiés. L’objectif est d’examiner les propriétés catalytiques en vue d’utilisation dans des piles à combustible alcalines directes ou en tant que substrats actifs pour la spectroscopie SERS. Des surfaces et des matrices nanostructurées de cuivre de morphologie très fines et une forte surface spécifique ont été obtenues respectivement par le dealloying aqueux de rubans CuxCa100-x amorphe (35 <x< 80 % at.) et par le dellaoying chimique d’échantillons massifs de Cu90(HfZr)10. Des substrats nanoporeux d’Ag et de Co ont été produits par dealloying en retirant les phases riches en Cu et Si de rubans M38,75Cu38,75Si22,5 cristallin (avec M = Co ou Ag). En complément des techniques de caractérisations conventionnelles, toutes les structures nanoporeuses ont pu être reconstruites par nanotomographie à partir de découpes FIB. Des outils numériques spécifiques à la nanotomographie de visualisation et de cartographie en 3 dimensions ont permis de révéler la diversité morphologique des trois systèmes avec la porosité traversante. Ces matériaux ont pour la première fois été étudiés pour leur utilisation pratique en tant que catalyseurs anodiques auto-supportés. Cette étude suggére qu’ils constituent une alternative sérieuse aux composites commerciaux instables à base de Pt et supportés par du C. Des essais électrochimiques en demi-cellule ont montré une excellente activité catalytique vis-à-vis de l’oxydation d’un combustible en borane ainsi qu’une stabilité de fonctionnement supérieure dans un environnement alcalin en comparaison d’un catalyseur Pt/C. Le Co nanoporeux a montré dans des conditions similaires une meilleure efficacité mais une stabilité moindre, attribuée à la composition chimique complexe de son réseau poreux. Le Cu nanoporeux n’a pas été étudié pour les applications mentionnées précédemment en raison de sa grande fragilité. Il est suggéré d’améliorer la voie de synthèse de son précurseur pour augmenter sa tenue mécanique. Enfin le comportement mécanique de ces nouveaux matériaux métalliques nanoporeux a été abordé par des mesures de nanoindentation sur des substrats nanoporeux d’Ag. L’étude a permis de proposer un modèle phénoménologique de dépendance entre la charge et le déplacement pour cette classe de matériaux métalliques. / New nanoporous metallic materials based on non-Pt group metals have been synthesized via dealloying of rapidly solidified alloys and aimed to demonstrate competitive catalytic performance in the field of direct alkaline fuel cells and SERS-active substrates. Nanostructured copper surface and nanoporous copper matrix with very fine morphology and specific surface area were obtained by chemical dealloying of bulk Cu90(HfZr)10 and melt-spun amorphous CuxCa100-x (x ranging from 35 to 80 at.%) family of alloys accordingly. Nanoporous silver and cobalt substrates were produced by dealloying of M38.75Cu38.75Si22.5 crystalline ribbons (M = Co and Ag) as a result of the removal of Cu and Si-rich phases. In addition to conventional characterization methods, all nanoporous structures have been reconstructed by FIB-nanotomography, clearly exposing the morphological diversity of the three systems with transversal porosity when visualized and color-mapped in 3D by a special numerical tomography tool. It is for the first time that a practical significance of these materials has been explored in the scope of self-supported anodic catalysts, suggested throughout this study as an alternative to the unstable Pt-based carbon-supported commercial composites. Half-cell electrochemical tests demonstrated an excellent catalytic activity towards the oxidation of a borane fuel and superior stability of functioning in the alkaline environment compared to Pt/C catalyst. In similar conditions, nanoporous Co showed higher efficiency but lower stability, attributed to the complex chemical composition of its porous scaffold. Nanoporous Cu has not been exploited for the mentioned applications due to its high brittleness and is suggested to go through improvements on the step of precursor synthesis. Lastly, while exploring the mechanical behavior of the NPMs by instrumented nanoindentation of different nanoporous Ag substrates, a load-displacement dependence phenomenological model has been suggested for this class of metallic materials

abstract: Dealloying induced stress corrosion cracking is particularly relevant in energy conversion systems (both nuclear and fossil fuel) as many failures in alloys such as austenitic stainless steel and nickel-based systems result directly from dealloying. This study provides evidence of the role of unstable dynamic fracture processes in dealloying induced stress-corrosion cracking of face-centered cubic alloys. Corrosion of such alloys often results in the formation of a brittle nanoporous layer which we hypothesize serves to nucleate a crack that owing to dynamic effects penetrates into the un-dealloyed parent phase alloy. Thus, since there is essentially a purely mechanical component of cracking, stress corrosion crack propagation rates can be significantly larger than that predicted from electrochemical parameters. The main objective of this work is to examine and test this hypothesis under conditions relevant to stress corrosion cracking. Silver-gold alloys serve as a model system for this study since hydrogen effects can be neglected on a thermodynamic basis, which allows us to focus on a single cracking mechanism. In order to study various aspects of this problem, the dynamic fracture properties of monolithic nanoporous gold (NPG) were examined in air and under electrochemical conditions relevant to stress corrosion cracking. The detailed processes associated with the crack injection phenomenon were also examined by forming dealloyed nanoporous layers of prescribed properties on un-dealloyed parent phase structures and measuring crack penetration distances. Dynamic fracture in monolithic NPG and in crack injection experiments was examined using high-speed (106 frames s-1) digital photography. The tunable set of experimental parameters included the NPG length scale (20-40 nm), thickness of the dealloyed layer (10-3000 nm) and the electrochemical potential (0.5-1.5 V). The results of crack injection experiments were characterized using the dual-beam focused ion beam/scanning electron microscopy. Together these tools allow us to very accurately examine the detailed structure and composition of dealloyed grain boundaries and compare crack injection distances to the depth of dealloying. The results of this work should provide a basis for new mathematical modeling of dealloying induced stress corrosion cracking while providing a sound physical basis for the design of new alloys that may not be susceptible to this form of cracking. Additionally, the obtained results should be of broad interest to researchers interested in the fracture properties of nano-structured materials. The findings will open up new avenues of research apart from any implications the study may have for stress corrosion cracking. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2012

Los materiales porosos están atrayendo mucha atención tanto en la Academia como en la industria por su amplia gama de aplicaciones y potenciales de desarrollo. Debido a sus superiores propiedades intrínsecas y funcionales, se han incrementado los esfuerzos para desarrollar materiales con diferente estructura, tamaño de poro y distribución, con el fin de satisfacer los requisitos en numerosas áreas de aplicación. Se pueden enunciar como ejemplos la gestión del calor, la biomedicina, la filtración, los procesos de separación, el catalizador o el soporte, el almacenamiento, el control acústico, el tratamiento del agua, entre otros. Sin embargo, una parte considerable de la investigación se ha centrado en materiales con una distribución uniforme del tamaño de los poros. No obstante, los estudios actuales sobre materiales multi-porosos indican que, con un diseño metodológico adecuado, es posible adaptar y controlar la microestructura en función de los fines de la aplicación y dar diferentes funciones a cada nivel de porosidad introduciendo diferentes tamaños de poros en la estructura unimodal. Dado que estos materiales multi-porosos bien diseñados (es decir, con distribución bimodal, trimodal y jerárquica) reducen las limitaciones debidas a la difusión con sus distintos niveles de porosidad, son excepcionalmente preferidos en aplicaciones en las que se requiere contener el flujo de fluidos y una elevada superficie. Aunque la atención en el diseño de estructuras multiporosas ha aumentado en los últimos años, los estudios sobre estos materiales multifuncionales ajustables han sido limitados. Esto se debe a que la relación entre la microestructura y las propiedades funcionales es difícil de comprender y controlar. Así pues, el diseño de materiales multi-porosos sigue siendo una tarea difícil en el mundo científico. Teniendo en cuenta la insuficiencia de estudios sobre la síntesis y caracterización de materiales multi-porosos en la literatura, se necesita especialmente la investigación más profunda y detallada de la influencia de los diferentes tamaños de poros en una misma estructura sobre las propiedades funcionales. De acuerdo con este propósito en esta tesis se sintetizaron materiales con porosidad única y múltiple siguiendo métodos de fabricación tanto novedosos como convencionales. Comparando el rendimiento de los materiales con porosidad simple y múltiple, se investigó la influencia de la microestructura en las propiedades y su relación desde diferentes perspectivas. En conclusión, los objetivos específicos de esta tesis son: i) diseñar dos nuevos enfoques experimentales para fabricar espumas metálicas de celdas-abiertas interconectadas multi-porosas utilizando metodologías conocidas y sus combinaciones. ii) demostrar el efecto de la multi-porosidad en un solo cuerpo sobre las propiedades de las espumas fabricadas y demostrar que superan, junto con los nuevos enfoques, a sus homólogos convencionales. iii) presentar nuevos hallazgos científicos y nuevos enfoques sobre la relación entre las propiedades funcionales y estructurales (porosidad, tamaño y distribución de los poros, etc.) mediante una amplia gama de caracterizaciones. iv) Ofrecer metodologías reproducibles de vanguardia para la síntesis de materiales multi-porosos que puedan aplicarse a diferentes tipos de materiales e inspirar futuros trabajos en la comunidad científica. Este estudio de tesis consta de 6 capítulos. El Capítulo 1 explica brevemente la motivación y los objetivos de la tesis. En este capítulo se expone la información sobre los materiales multi-porosos que se pretenden fabricar y el alcance de la tesis. El Capítulo 2 ofrece una introducción a los materiales multi-porosos y sus métodos de fabricación en perspectiva. Se ha realizado un amplio estudio bibliográfico y desde el pasado hasta el presente se presentan los avances y trabajos recientes realizados en la fabricación de estos materiales. En este capítulo se ha prestado especial atención a la producción de espumas metálicas monomodales, bimodales y jerárquicas mediante métodos de replicación, dealeación, plantillas, sinterización y sus combinaciones. En el Capítulo 3 se explican todas las metodologías, montajes experimentales, materiales, parámetros de funcionamiento y maquinaria/dispositivos seguidos en esta tesis sobre la fabricación y caracterización de materiales y se presenta la información teórica relevante. El Capítulo 4, en primer lugar, resume la literatura necesaria sobre la fabricación de espumas metálicas, la infiltración con el método de replicación, las espumas de aluminio y sus áreas de aplicación, y los métodos de caracterización. También se presenta en este capítulo la extensa revisión de la literatura sobre la relación entre las características microestructurales y las propiedades funcionales de los materiales porosos. Se expone cómo, mediante el método de replicación, se fabricaron espumas de aluminio interconectadas de célula abierta con una distribución monomodal y bimodal del tamaño de los poros. Después de la fabricación, se caracterizaron en detalle las propiedades estructurales (densidad, porosidad, distribución del tamaño de los poros, etc.), el comportamiento del flujo de fluidos (caída de presión, permeabilidad, coeficiente de arrastre, regímenes de flujo, etc.) y el comportamiento térmico (coeficiente de transferencia de calor, conductividad térmica) de las espumas. (coeficiente de transferencia de calor, conductividad térmica) de las espumas. El Capítulo 5 presenta un método novedoso para fabricar espumas de plata con eficacia antibacteriana y distribución jerárquica del tamaño de los poros. Para ello, se ha utilizado la combinación de métodos de templado, replicación y dealeación. Con esta combinación y el diseño adecuado de las metodologías se introdujeron micro-, meso- y macroporos en las estructuras de las espumas. Para comparar las propiedades y el rendimiento de las espumas jerárquicas, se fabricaron espumas de plata con porosidad uniforme mediante el método de replicación y el proceso de sinterización, y espumas de plata con solamente nanoporos mediante el método de dealeación. Se llevaron a cabo estudios de caracterización para todas las espumas fabricadas en el sentido de la distribución del tamaño de los poros, la porosidad, la superficie específica, la eficacia antibacteriana, además de la modelización y la estimación de la superficie específica y la distribución del tamaño de los poros. Además de los pasos de las estrategias experimentales, se presentó una información de fondo detallada sobre los principios rectores de los métodos utilizados en este capítulo. En el Capítulo 6 se evaluaron todos los estudios realizados, se resumieron los resultados y las conclusiones, y se presentaron recomendaciones y estrategias para futuros estudios. / Financial support from the Spanish “Agencia Estatal de Investigación” (AEI) and European Union (FEDER funds) through grant MAT2016-77742-C2-2-P. “Santiago Grisolía” grant (GRISOLIA/2017/187) financed by Generalitat Valenciana.

Developing highly active and durable oxygen reduction reaction (ORR) catalysts is crucial to reduce the cost of polymer electrolyte fuel cells (PEFCs). To meet those requirements, unsupported Pt-Cu alloy nanochains (aerogels) were synthesized by a simple co-reduction route in aqueous solution and their structure was characterized by X-ray absorption spectroscopy and STEM-EDX. These catalysts exceeded the ORR activity of commercial Pt/C catalysts by more than 100 % in RDE experiments and met the US DOE targets, thereby qualifying as very promising materials. The behavior of Pt-Cu aerogels under PEFC operation conditions was mimicked by acid washing experiments which showed that the Cu content in the alloy phase and ORR activity decrease through this step. Comparing composition, structure and ORR activity for various specimens, the Cu content in the alloy phase was identified as the main descriptor of ORR activity. An almost linear correlation was found between those two parameters and complemented by supporting data from the literature.

Selective dissolution also known as dealloying is a corrosion process in which one component of a binary alloy system is selectively removed through an electrochemically controlled process which leads to the formation of a porous metal "sponge" with a porosity that is completely interconnected and random in direction. Nanoporous metals are desirable since they have larger surface areas than an equal volume of non-porous material. Because of their enormous surface area per volume, these highly porous metal electrodes are superior materials for high surface area applications such as in biomedical devices, microfilters and catalysts. Understanding the kinetic processes governing the development of porosity during dealloying and having ability to change the electrochemical conditions will allow us to better control over the average ligament size and distribution in porosity. The basic kinetic processes involved in the formation of these structures are related to such issues as environmental effects and electrochemical conditions on diffusion, microscopic coarsening phenomenon at room temperature and elevated temperatures, alloy passivation, and Gibbs-Thomson effects. The average pore size and distribution was found to depend on the electrolyte composition, dealloying rate, applied potential and time. The porosity was found to significantly coarsen at room temperature during the dealloying process and this coarsening was highly dependent on the applied potential. It is showed that the commonly accepted measurement of the critical potential for alloy dissolution calculated based on extrapolation of anodic polarization data results in an overestimation of this quantity. A series of constant applied potential experiments prove to be a more accurate method for critical potential determination. / Ph. D.