Imaging Channel Connectivity in Proton and Hydroxide Conducting Membranes for Fuel Cells

Proton exchange membrane (PEM) fuel cells offer an alternative as an efficient power source with low environmental impact. The heart of the fuel cell is the membrane, which conducts protons through an aqueous channel network. Proton transport is critically tied to the channel connectivity – disconnected channels do not participate in the overall electrochemical activity of the cell. Nafion, the current benchmark PEM, is a random statistical copolymer, characterized by a percolating network of cylindrical channels. In previous work, conductive-probe atomic force microscopy (cp-AFM) was used to image the conductance of Nafion. Although cp-AFM provides relevant information on which channels are connected, it provides no information on the disconnected “dead-end” channels at the surface. Electrostatic Force Microscopy (EFM) was used to analyze the structure and frequency of the “dead-end” channels. Applying a simple parallel plate model allowed us to assign differences in the EFM signal to particular channel shapes: connected cylindrical channels, “dead-end” cylinder channels, and bottle-neck channels. Anion exchange membranes (AEMs), which conduct hydroxide, have attracted recent interest due to improved reaction kinetics in alkaline media, yet suffer from low conductivity and easily degrade. To this end, our AFM methodology was applied to analyze the channel connectivity of a commercial FumaTech AEM was investigated in its hydroxide form over a wide range of relative humidity (RH) by combining phase imaging and cp-AFM. At high RH, our AFM data indicates significant surface swelling. Lastly, we investigated a class of phosphonium-containing diblock copolymer AEMs that formed ordered morphologies. Although channels were observed to be well-connected in the bulk by TEM, channels aligned parallel at the surface leading to many “dead-end” channels shown by EFM. Correlating these findings with bulk measurements could offer insight toward AEMs with improved conductivity and chemical stability

Ion Transport & Electrokinetics in Colloidal Particle Networks

Currently around 10 percent of the people in the world are living under conditions of extreme water scarcity and it is expected that this will only become worse in the near future for various reasons such as climate change. For this reason, it is important that we find alternative water resources to reduce the current and future water scarcity. One option to reduce this problem is to remove the salt of brackish (slightly saline) waters. This could be done with a technique called electrodialysis, which removes salt from water with electricity and cat- and anion selective membrane pairs. However, to implement this technique it is important to optimize the ion transport to make this technique more attractive for industrial applications. In this thesis the ion-transport a fundamental study is conducted for electrodialysis applications using a model system composed of colloidal particle networks. Colloidal particle networks are porous materials composed of close-packed nano- or microparticles and could be useful to study a wide variety of different electro-driven processesIn Chapter 1, some background information on various types of electrodialysis processes and their ion transport limitations is provided as well as for instance the difference between ion-exchange membranes and colloidal networks. Next to this the origin of several relevant electrokinetic phenomena are discussed. In Chapter 2, the use of alternating conductive and non-conductive patched model membranes is explored to optimize the transport of ions for electrodialysis. In Chapter 3 the influence of charge regulation on the performance of shock electrodialysis was numerically studied. Chapter 4 discusses the water splitting, water transport and temperature development characteristics for a thick model bipolar membrane that is composed of anion and cation ion exchange resin particles. In Chapter 5 a commercial anion exchange membrane is coated with MOF film via a novel synthesis method. In Chapter 6 a particle focusing technique based on AC-EOF and the induced dipole force is explored both by an experimental and numerical study. In Chapter 7 the main conclusions of the thesis are presented and further ideas for follow-up studies are provided<br/

In-situ cross-linked porous anion exchange membranes with high performance for efficient acid recovery

Diffusion dialysis (DD) has high economic competitiveness for acid recovery; however, the fabrication of highly acid-permeable and salt-rejecting anion exchange membranes (AEMs) for DD is still a grand challenge. This paper presents in-situ cross-linked porous AEMs with tunable microstructures and high DD performance. The AEMs were fabricated based on chloromethyl polyethersulfone substrate using N, N, N′, N″, N″-pentamethyldiethylenetriamine as a bifunctional agent for cross-linking and quaternization. The prepared porous AEMs showed significantly superior DD performance over conventional dense AEMs due to the high free volume and cross-linked networks within our membranes. The acid dialysis coefficient (UH+) and acid/salt separation factor (S) of the optimal AEM were 2.6 and 255.4 times as high as those of the commercial DF-120 AEM, respectively. Therefore, our low-cost, high-performance in-situ cross-linked porous AEMs may pave the way for large-scale acid recovery applications.</p

Structure-Property Relationships of Imidazolium-Containing Polymer Systems: Homopolymers, Block Copolymers, and Block Copolymer/Ionic Liquid Mixtures

Advanced solid-state polymer electrolytes for electrochemical and energy storage applications are needed to replace conventional liquid electrolytes that are unstable, flammable, and volatile. In particular, a fundamental understanding of morphology-ionic conductivity relationships is necessary to improve the ionic conductivity of ion-containing polymer systems. To this end, we investigate the structure-property relationships of homopolymer and block copolymer systems containing imidazolium-based ionic liquids (ILs). We first explore the effects of anion type and pendant alkyl chain length on the morphologies and properties of polymerized ionic liquid (PIL) homopolymers with bound cations. In both acrylate-based and vinylimidazolium PIL homopolymers, nanoscale segregation of polar and non-polar moieties is detrimental for ionic conductivity. Increasing the length of alkyl pendant groups increases ion aggregation and decreases ion mobility, while increasing the size of mobile anions decreases ionic aggregate formation and the glass transition temperature, leading to increased ionic conductivity. The incorporation of hydroxyl-terminated alkyl pendant groups in vinylimidazolium homopolymers further decreases compositional heterogeneity of polar and non-polar moieties and increases ionic conductivity by one order of magnitude. We then examine the morphology-ionic conductivity relationships of IL-containing block copolymers and show that microdomain orientation, chain length, and confinement of polymer segments strongly impact ionic conductivity. Non-ionic diblock copolymer/IL mixtures, with the same block copolymer composition, display anisotropic lamellar morphologies, and morphology factors describing the extent of anisotropy are determined from X-ray scattering data. Ionic conductivities increase when lamellar microdomains are aligned parallel to the ion transport measurement direction and with increasing molecular weight. Self-consistent field theory calculations predict a more uniform IL distribution within microdomains when molecular weight is increased, suggesting that composition and dynamic gradients are detrimental to ionic conductivity. In hydrophilic PIL copolymers, ion transport is heavily dependent on water content and monomeric sequence. The conductivity of block copolymers is ten to fifteen times greater than that of random copolymers due to the microphase separation of ion-conducting and insulating blocks and local enhancement in ion concentration. At 90% relative humidity, several PIL block copolymers exhibit ionic conductivities that exceed their analogous homopolymers, suggesting that confinement of PIL chains and water in microdomains accelerates ion transport

Structure and dynamics of nanoconfined water and aqueous solutions

This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid's structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed

Hidróxidos Duplos Lamelares (HDL) como nanoreservatórios versáteis para aplicação em revestimentos multifuncionais

The objective of the present work is synthesis and characterization of inorganic nanoreservoirs based on layered double hydroxides (LDH) loaded with active species, namely corrosion inhibitors and pH indicators. The most attractive feature of LDH is the anion-exchange ability. Despite the countless studies that describe the use of LDH for applications in protective coatings for anti-corrosion applications, the study related to immobilization and consequent release of anionic species is somewhat limited. Besides, there is still a lack of systematic studies correlating the structure of nanocontainers with properties (release profiles, triggering conditions) and the corresponding effect in coatings. In this work, several methodologies were used for preparation of LDH. Structure, morphology, colloidal and textural properties of resulting nanocontainers have been characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), dynamic light scattering (DLS), thermogravimetric analysis (TG) and nitrogen adsorption-desorption isotherms. Particular attention has been paid to the release studies of active species immobilized in LDH, determination of triggering conditions, establishment of kinetic models and definition of release mechanisms by UV-Visible spectrophotometry. Additionally, the adsorption of active species in LDH was investigated to understand how the active species are immobilized during the synthesis, as well as for evaluation of its impact on the application of nanoreservoirs in multi-functional coatings. In general, the release of active species is rate-limited by diffusion with possibility of anion-exchange reaction when anionic species are presenting in the environment. However, the release profiles, extent of release and adsorption of active species are strongly dependent on the method of synthesis and on the structure and properties of the active species. In addition, the LDH compositions studied were found to release the active species in response to the presence of anions in solution (e.g. chlorides, hydroxides, carbonates), even when the active species are not intercalated into the LDH galleries, but adsorbed onto the external surface of the particles. Besides, the propensity of LDH to dissolution or reaction under extreme pH conditions, in combination with changes in the surface properties, can play a role in the agglomeration and/or aggregation of the particles, thereby affecting adsorption and release of different species. Taking into account the properties of the materials obtained, it is possible to conclude that several LDH compositions studied in this work are prospecting additives for application in multi-functional coatings. Moreover, the present work can be used as some sort of library of experimental data to support the building and validation of computational models, to aid on the prediction of the uptake and release of active species from LDH.O objetivo do presente trabalho consistiu na síntese e caracterização de nanoreservatórios inorgânicos baseados em hidróxidos duplos lamelares (HDL) com diferentes espécies ativas imobilizadas, nomeadamente inibidores de corrosão e indicadores de pH. Uma das características mais relevantes associada com o HDL é a sua capacidade de permuta aniónica. Apesar dos inúmeros trabalhos que descrevem a utilização do HDL para aplicações em revestimentos protetores contra a corrosão, o estudo relacionado com a imobilização e consequente libertação das espécies aniónicas é algo limitado. Além disso, não existem estudos sistemáticos na literatura que correlacionem a estrutura dos nanocontentores com as propriedades (perfis e condições de libertação controlada) e o efeito correspondente nos revestimentos. Neste trabalho, várias metodologias foram aplicadas para a preparação dos HDL. A estrutura, morfologia, propriedades coloidais e texturais dos nanocontentores resultantes foram caracterizadas por difração de raios-X (DRX), espectroscopia no infravermelho por transformada de Fourier (FTIR), microscopia eletrónica de varrimento (SEM), dispersão dinâmica de luz (DLS), análise termogravimétrica (TG) e isotérmicas de adsorção-desadsorção de azoto. Atenção especial foi prestada aos estudos de libertação de diferentes espécies ativas imobilizadas em HDL, nomeadamente as condições experimentais que podem levar à libertação das espécies ativas (ex.: pH, presença de sais), estabelecimento de modelos cinéticos e definição de mecanismos de libertação, recorrendo à espetrofotometria de UV-Visível. Além disso, o processo de adsorção de espécies ativas foi investigado para tentar perceber o que ocorre durante a síntese de diferentes composições de HDL bem como avaliar o impacto na aplicação prática de nanoreservatórios em revestimentos multifuncionais. Os resultados obtidos permitem concluir que, de forma geral, a libertação de espécies ativas é determinada pela difusão, com possibilidade de reação de permuta aniónica, quando espécies aniónicas estão presentes no ambiente. No entanto, os perfis de libertação, extensão da mesma e adsorção de espécies ativas em HDL dependem do método de síntese, bem como da estrutura e propriedades das espécies ativas. Igualmente relevante, é que as diferentes composições de HDL apresentam capacidade de libertar as espécies ativas imobilizadas em resposta à presença de aniões na solução (ex.: cloretos, hidróxidos, carbonatos), mesmo quando não intercaladas nas galerias do HDL, encontrando-se apenas adsorvidas na superfície externa do HDL. Além disso, a tendência dos materiais de HDL à dissolução ou reação sob condições extremas de pH, em combinação com alterações nas propriedades da superfície, tem influência no maior ou menor grau de aglomeração e/ou agregação das partículas, podendo afetar a adsorção e libertação das diferentes espécies. Atendendo às propriedades dos materiais obtidos, concluiu-se que vários HDL estudados são candidatos promissores para aplicação em revestimentos multifuncionais. Para além disso o presente trabalho pode ser usado como suporte na elaboração e validação de modelos computacionais que visam prever a captação e libertação de espécies ativas do HDL.III Quadro Comunitário de ApoioPrograma Doutoral em Ciência e Engenharia de Materiai

A study of a divalent cation-selective electrode

The ion-selective membrane electrode, recently introduced commercially, has shown excellent promise as a tool for determining ionic activities and concentrations in solution. It was the intent of the author to study the Orion divalent cation-selective electrode with two objectives. The first objective of this research was to characterize the response of the divalent cation-selective electrode to calcium and magnesium ions in aqueous solutions. The effect of anions, monovalent cations; and pH upon this response was investigated. An examination was made of the response of this electrode to other divalent cations and of the effect of organic solvents on the membrane material. The second object was to ascertain if this electrode could be used for the quantitative determination of calcium and magnesium in the presence of high levels of sodium by the standard additions technique. A cursory examination of this electrode as an end-point detector in the titration of calcium and magnesium with disodium ethylenediaminetctraacetate and sodium fluoride was also made