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Mechanical Characterization of Solid Polymer Electrolyte Membranes for Energy Applications
No full textElectrolysis is a key technology for the production of clean hydrogen that will enable the hydrogen economy and expand the renewable energy landscape. Among the most viable candidates for low-temperature electrolysis is the proton-exchange membrane (PEM) water-splitting electrolyzer (PEMWE), which uses an ion-conductive polymer solid-electrolyte. To be commercially viable, PEMs in these devices must perform over a long operational time in liquid operating environments under compression, which makes stability an important aspect of design considerations. While a hydrated environment is an intrinsic result of the operation and key to the membrane’s conductive function, it undermines PEM stability. Hydration and pressure are used as design parameters for optimizing electrolyzer performance; however, their role in durability is not well established. This creates a gap between the performance and lifetime assessment of electrolyzer membranes. Thus, there is a need for new metrics that could capture ion-exchange membrane durability by accounting for performance operators. This dissertation shows the oft-used tensile testing of membrane samples cannot correspond to compression behavior and identifies compression creep as a potential material stability metric that can account for operation-dependent variables such as pressure and hydration. This work guides a better assessment of life-limiting issues in electrolysis and structurally analogous energy and environmental devices wherein the components are under compressive stress and/or pressurized. The mechanical stability of PEMs is commonly characterized by tensile testing, and the applicability to electrolyzers wherein the membrane is held under compression is unclear. Chapter 2 introduces the custom-designed compression setup used for all compression tests within this dissertation, including monotonic, cyclic, and creep of dry and hydrated samples while temperature is controlled. Our results show compressive response significantly differs from tensile behavior in both dry and hydrated states. Chapter 3 aims to improve the understanding of the mechanical interactions between cell components by exploring the monotonic compression response of the various electrolyzer cell components, such as the porous transport layers and gaskets, and compares them with the PEM response under quasistatic conditions at room temperature and 80 C. The compressive stress-strain responses of all electrolyzer cell components have a degree of nonlinearity owing to each component's unique morphologies and deformation mechanisms. The acquired mechanical properties are used to analyze the compression of the cell components under applied assembly stress with a simplified 1-D mechanical spring network representation.Chapter 4 expands on the monotonic compression of the PEM. The chapter presents a comprehensive analysis of the influence of membrane hydration on the mechanical behavior of PFSA membranes under compression and at different temperatures, offering valuable insights for electrochemical devices. The study confirms the crucial role of membrane hydration in mechanical response by quantifying the amount of water in hydrated membranes and examining the relationship between volumetric change and swelling strain. The research highlights the differences in the mechanical responses of dry and wet PFSA membranes, attributing the nonlinear response of the latter to various phenomena, including the plasticizing effect of water, free water squeezing out of the membrane under high compression, and softening of the polymer matrix. In addition, the study delves into the cyclic response of PFSA under both dry and hydrated conditions, revealing significant strain ratcheting and time-dependent residual strain, thus contributing to a deeper understanding of polymer membrane mechanical response and properties.
In electrolyzer systems, state-of-the-art industry trends are to increase operational pressure while also reducing membrane thickness to minimize the need for additional pumps to pressurize the generated hydrogen and reduce Ohmic resistance and cost. This poses a challenge as higher operational pressure requires higher sealing pressure of the cell, thereby increasing the internal compressive stress experienced by the active area of the membrane. This higher stress, maintained for long time periods on thin membranes, makes compressive creep and thinning a concern since thinning could impact the interfacial resistance between the membrane and electrodes. Chapter 5 explores the compression creep of membranes under various environmental conditions. Chapter 5 shows that PEMs exhibit creep response under compression with a continuous decrease in thickness over 24 hours, with a dependence on the applied pressure, hydration state, pretreatment or thermal history, among other things.Chapter 6 investigates the influence of geometry on material behavior under compression using two finite element analysis (FEA) models: a simple uniaxial compression model and a complex model with non-uniform surfaces. The simpler model, incorporating a Mooney-Rivlin hyperelastic model, demonstrated uniform stress and deformation in a frictionless scenario, while friction introduced non-uniform stress distribution. The complex model, which accounted for surface roughness and varying thicknesses, revealed that smaller surface features led to higher average stress but a more uniform distribution, highlighting the critical role of surface quality and membrane thickness in material response. The findings emphasize the importance of considering these factors in engineering design and material selection, particularly for applications requiring compressive strength and durability, such as electrolyzer cells. This research provides valuable insights into the complex interactions between surface features, friction, and material deformation, which are difficult to capture through experimental methods alone.This dissertation demonstrates the importance of studying the mechanical properties of PEMs under compression, which is more relevant to the membrane's stress states during operation in electrolysis and similar electrochemical technologies
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Phenomena Governing Ionomer Thin Film Structure-Property Relationships
No full textIn the push to decarbonize the energy sector, hydrogen technologies have garnered significant interest. Fueled by (clean) hydrogen, proton-exchange-membrane fuel cells (PEMFCs) have potential to replace internal combustion engines and batteries in vehicles, especially for heavy-duty applications. These devices rely on ion-conducting polymers (ionomers) to facilitate mass transport of reactive species, which directly influences device performance and efficiency. However, PEMFC performance is limited by transport resistances through ionomer thin films in the device electrodes (catalyst layers, CLs, comprised of catalyst and ionomer). To better understand and design ionomers in PEMFCs and other electrochemical energy conversion devices, this dissertation focuses on ionomer thin films and the phenomena governing their structure-property relationships.In CLs, ionomers act as the catalyst-binding electrolyte and are confined to a nanoscale thin film surrounding the catalyst particles. Reactive species must move through the ionomer to reach catalyst sites. However, due to confinement, CL ionomer properties deviate from bulk properties (e.g., that of membranes), including greater mass transport resistances. PEMFC performance is limited by low water uptake at low humidities, ionomer adsorption to catalyst sites, and poor oxygen permeability in the CL ionomer. To add further complications, CL ionomer properties also depend on the processing conditions during CL fabrication and device assembly. Supported thin films have been widely used as a model system to gain insight into CL ionomer behavior.The most widely used ionomer is perfluoro sulfonic acid (PFSA) ionomer. PFSA is a random copolymer with a hydrophobic backbone and pendant sidechains terminating in hydrophilic sulfonic acid groups. The backbone lends chemical and mechanical integrity while the acid groups enable ionic conductivity. Due to the dissimilarity between the hydrophobic backbone and hydrophilic sulfonic acid groups, the ionomer exhibits nanophase separation into hydrophobic and hydrophilic domains. Upon hydration, the hydrophilic domain coalesces into a network forming pathways for ion conduction.While recent studies provided insights into the origins of performance limitations caused by the CL ionomer, limited improvements demonstrate the need for new ionomers and integration strategies designed to improve film function. Rational design requires a deeper understanding of how chemistry impacts ionomer thin film structure-property relationships. Furthermore, ionomers are sensitive to processing conditions, and consideration for processing effects is necessary when evaluating potential CL ionomers. This dissertation focuses on two central ideas to control CL ionomer function: rational ionomer design and structure-property modulation via processing effects. PFSA sidechain and backbone modification strategies are investigated through systematic thin film characterization to elucidate chemistry-structure-property relationships. The impacts of dispersion solvent and thermal treatment temperature are explored to assess ionomer response to processing conditions.To begin, two sidechain modifications to PFSA with potential to improve water uptake and/or mitigate sulfonate adsorption are explored. Application-relevant properties (i.e., water uptake and thermal transition temperature) are characterized via in situ spectroscopic ellipsometry (SE), and nanomorphology is probed using environmentally-controlled grazing-incidence small- and wide-angle x-ray scattering (GISAXS/GIWAXS). In addition, film function is assessed via proton conductivity measurements. Introducing multiple acid groups to sidechains increases water uptake and improves phase separation while reducing chain mobility. Replacing sulfonic acid end groups with increasingly hydrophobic end groups leads to the opposite trends. Moreover, results indicated that ionomer chemistry has more impact on morphology and water uptake than ionomer-substrate interactions. Importantly, proton conductivity—measured via potentiostatic electrochemical impedance spectroscopy (PEIS)—is strongly correlated with film water volume fraction. These findings provide a deeper understanding of how two ionomer modification strategies impact thin film behavior.With an understanding of the structure-property relationships for sidechain-modified PFSA-based ionomers, the impact of dispersion solvent on ionomer structure is investigated next, using select ionomers from the two modification strategies explored. Using solvents relevant to CL fabrication, water:n-propanol (nPA) mixtures are used to disperse ionomers and cast thin films on supports. Ionomer structure in dispersion is characterized via pH measurements and transmission small-angle x-ray scattering (SAXS), and thin film structure is characterized via GISAXS. Additionally, the impact of dispersion water content on proton conductivity is assessed using PEIS. Importantly, the relationship between dispersions (ionomer + solvent) and thin films is also discussed. Ionomer aggregation/conformation in dispersion is primarily driven by ionomer chemistry, but all ionomers exhibit similar structural evolution with increasing dispersion water content. These differences in dispersion translate to differences in thin film hydrophilic domain spacing and orientation, ultimately impacting proton conductivity. Thus, while ionomer design has more influence on ionomer structure and properties, dispersion water content offers a method to modulate ionomer behavior.In the final study, these findings are extended to a backbone-modified PFSA-based ionomer designed to improve thin film oxygen permeability, referred to as a high-oxygen-permeability ionomer (HOPI). HOPI membrane and thin film morphology, water uptake, and proton conductivity are characterized to assess confinement effects and compared to PFSA. Then, two processing effects are evaluated and discussed: processing solvent and thermal treatment temperature. When cast from water-rich dispersion, HOPI thin films exhibit greater proton conductivity despite reduced water uptake and a lack of observable morphological differences in the hydrophilic domain. Elevating thermal treatment temperature after film casting leads to lower water uptake and proton conductivity. Importantly, HOPI is more sensitive to confinement effects and processing conditions than PFSA. Careful control of processing conditions can enable water uptake and proton conductivity in HOPI that is comparable or greater than that of PFSA. Thus, processing conditions are important to consider when evaluating potential CL ionomers. Moreover, thin film properties can be modulated via processing.The chemistry-structure-property relationships elucidated in this dissertation guide ionomer design and integration strategies to overcome ionomer functional limitations. Additionally, these findings inform processing conditions used in device fabrication for performance and efficiency improvements. The insight gained into the relationship between chemistry, processing conditions, and ionomer functionality enables modulation of ionomer behavior for PEMFCs and can be extended to other energy conversion devices utilizing electrochemically-active interfaces (e.g., water splitting electrolyzers, carbon dioxide reduction technologies)
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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EFFECT OF COMPRESSION ON CONDUCTIVITY AND MORPHOLOGY OF PFSA MEMBRANES
Get PDFkoamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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