Dissertations / Theses on the topic 'Energy storage device'

The work in this thesis focuses on the characterization, modeling and emulation of both the supercapacitor and the new supercapattery energy storage device. The characterization involves the selection of dynamic models and experimental methodologies to derive model parameters. The characterizing processes focus on predicting short-term device dynamics, energy retention (self-discharging) and losses and round-trip efficiency. A methodology involving a pulse current method is applied for the first time to identify a model parameter to give fast device dynamic characteristics and a new constant power cycling method is used for evaluating round-trip efficiency. Experimental results are shown for a number of supercapacitor and supercapattery devices and good results are obtained. The derived models from the characterization results are implemented into the emulator system and the emulator system is used to mimic the dynamic characteristics of a scaled-up 1kW supercapattery device. The thesis also addresses voltage equalizing circuits and reports a study that investigates efficiency, a cell voltage deviation and voltage equalizing time for different control methods.

Master of Science
Department of Electrical and Computer Engineering
Noel Schulz
Many situations can cause a fault on a shipboard power system, especially in naval battleships. Batteries and ultra-capacitors are simulated to be backup energy storage devices (ESDs) to power the shipboard power system when an outage or damage occurs. Because ESDs have advantages such as guaranteed load leveling, good transient operation, and energy recovery during braking operation, they are commonly used for electrical ship applications. To fulfill these requirements, an energy management subsystem (EMS) with a specific control algorithm must connect ESDs to the dc link of the motor drive system. In this research, the real-time simulation of shipboard power system (SPS), bidirectional DC-DC converter, EMS, and ESDs are designed, implemented, and controlled on OPAL-RT system to test SPS survivability and ESD performance in various speed operations.

This study describes a simulation-based approach for informing the incorporation of Phase Change Materials (PCMs) in buildings designed to the "Passive House" standard. PCMs provide a minimally invasive method of adding thermal mass to a building, thus mitigating overheating events. Phase change transition temperature, quantity, and location of PCM were all considered while incrementally adding PCM to Passive House simulation models in multiple climate zones across the United States. Whole building energy simulations were performed using EnergyPlus from the US Department of Energy. A prototypical Passive House with a 1500 Watt electric heater and no mechanical cooling was modeled. The effectiveness of the PCM was determined by comparing the zone-hours and zone-degree-hours outside the ASHRAE defined comfort zone for all PCM cases against a control simulation without PCM. Results show that adding PCM to Passive Houses can significantly increase thermal comfort so long as the house is in a dry or marine climate. The addition of PCM in moist climates will not significantly increase occupant comfort because the majority of discomfort in these climates arises due to latent load. For dry or marine climates, PCM has the most significant impact in climates with lower cooling degree-days, reducing by 93% the number of zone-hours outside of thermal comfort and by 98% the number of zone-degree-hours uncomfortable in Portland, Oregon. However, the application of PCM is not as well suited for very hot climates because the PCM becomes overcharged. Only single digit reductions in discomfort were realized when modeling PCM in a Passive House in Phoenix, Arizona. It was found that regardless of the climate PCM should be placed in the top floor, focusing on zones with large southern glazing areas. Also, selecting PCM with a melt temperature of 25°C resulted in the most significant increases in thermal comfort for the majority of climates studied.

Exploring uses of two-phase mixtures as a way to store peak solar energy for off-peak usage is a novel approach that has been gaining attention in recent years to address the issues tied to solid fuel dependence. This research explores a “solar salt” mixture (40%wt KNO3 and 60%wt NaNO3) in an aluminum enclosure under two test conditions: conduction enhancement and no conduction enhancement. The central aim is to develop an understanding of thermal distributions and melt developments as the system moves from room temperature to 300 oC. Thermal pattern development is explored by experimentally observing a 2-D temperature field at 8 co-planar points, comprised of 3 radial positions with complementary circumferential measurements, using thermocouples. The instrument array is traversed to three different axial positions where collected data is compared with results from a numerical solver. Results find three important details. First, the melt pattern of the fin experiments show quicker rates of melting after the onset of melt at the bottom of the enclosure. Second, the spatial effects of the instrumentation influence the presence of thermal phenomena. Lastly, approximations of the salts behavior using numerical simulations are supported in identifying phases of melt development.

Thermal energy storage (TES) has a crucial role to play in conserving and efficiently utilising energy, dealing with mismatch between demand and supply, and enhancing the performance and reliability of our current energy systems. This thesis concerns TES materials and devices with an aim to establish a relationship between TES device level performance to materials properties. This is a multiscale problem. The work focuses on the use of carbonate-salt-based composite phase change materials (CPCMs) for medium and high temperature applications. A CPCM consists of a carbonate salt based phase change material (PCM), a thermal conductivity enhancement material (TCEM, graphite flake in this work) and a ceramic skeleton material (CSM, MgO in this work). Both mathematical modelling and experiments were carried out to address the multiscale problem. The wettability of carbonate salt and MgO system is first studied, followed by exploring the CPCMs microstructure characteristics and formation mechanism, and then the effective thermal conductivity of the CPCMs is carried out based on the developed microstructures. At the last part, heat transfer behaviour of CPCMs based TES at component and device levels is investigated.

A worldwide conversion towards renewable energy sources has to be implemented in order to hopefully avoid the irreversible consequences of the global temperature increment caused by the greenhouse gases production. In addition, the current need to benefit from electricity in every moment of daily life, mainly in case of limited access to the electric grid, is forcing the scientific community to an intensive effort towards the production of integrated energy harvesting and storage devices. The topic of this PhD thesis is to investigate and propose innovative solutions for the integration of third generation photovoltaic (PV) cells and electrochemical double layer capacitors (EDLCs), the so-called photo-capacitors. Different photo-capacitor structures have been studied and experimentally fabricated. At first, flexibility was explored, as it is a mandatory requirement to cover non-planar or bendable surfaces, which are more and more common in nowadays portable electronics. Easily scalable fabrication processes have been used for both the harvester and the storage units, employing photopolymer membranes as electrolytes and metallic grids as current collectors and electrodes substrates. For this configuration, the best overall conversion and storage efficiency ever reported for a flexible Dye sensitized solar cell (DSSC)-based photo-capacitor was demonstrated. Subsequently, observing in the literature an evident lack in the exploitation of high voltage photo-capacitors, EDLC electrolytes with broad voltage windows have been examined. These electrolytes allowed to fabricate stable and reliable devices integrating the EDLC with a PV module and not only with a single solar cell, as normally is done. High voltage values, up to 2.5 V, have been obtained employing an ionic liquid electrolyte (Pyr14TFSI) or –alternatively- a solid state electrolyte (PEO-Pyr14TFSI) for storage section fabrication. Moreover, novel electrolyte mixtures of organic solvents and ionic liquids with good physical and electrochemical properties have been employed with the aim to increase energy density and voltage with respect to commercial EDLCs. Finally, a novel polymer-based platform has been suggested for the fabrication of an innovative “two-electrodes” self-powered device. The multifunctional polymeric layer, made of two poly(ethylene glycol)-based sections separated by a perfluorinated barrier, was obtained by oxygen-inhibited UV-light crosslinking procedure. For the energy harvesting section, one side of the polymeric layer was adapted to enable iodide/triiodide diffusion in a DSSC, while the other side empowered sodium/chloride ions diffusion and was used for on-board charge storage. The resulting photo-capacitor results in a planar architecture appreciably simplified with respect to other recently proposed solutions and is definitely more easily exploitable in low power electronics.

The last couple of decades have seen the rise of the hybrid electric vehicle as a compromise between the outstanding specific energy of petrol fuels and its low-cost technology, and the zero tail-gate emissions of the electric vehicle. Despite this, considerable reductions in cost and further increases in fuel economy are needed for their widespread adoption. An alternative low-cost energy storage technology for vehicles is the high-speed flywheel. The flywheel has important limitations that exclude it from being used as a primary energy source for vehicles, but its power characteristics and low-cost materials make it a powerful complement to a vehicle's primary propulsion system. This thesis presents an analysis on the integration of a high-speed flywheel for use as a secondary energy storage device in hybrid vehicles. Unlike other energy storage technologies, the energy content of the flywheel has a direct impact on the velocity of transmission. This presents an important challenge, as it means that the flywheel must be able to rotate at a speed independent of the vehicle's velocity and therefore it must be coupled via a variable speed transmission. This thesis presents some practical ways in which to accomplish this in conventional road vehicles, namely with the use of a variator, a planetary gear set or with the use of a power-split continuously variable transmission. Fundamental analyses on the kinematic behaviour of these transmissions particularly as they pertain to flywheel powertrains are presented. Computer simulations were carried out to compare the performance of various transmissions, and the models developed are presented as well. Finally the thesis also contains an investigation on the driving and road conditions that have the most beneficial effect on hybrid vehicle performance, with a particular emphasis on the effect that the road topography has on fuel economy and the significance of this.

Die Menschheit steht vor der großen Herausforderung der Energieversorgung des 21. Jahrhundert. Nirgendwo ist diese noch dringlicher geworden als im Bereich der Energiespeicherung und Umwandlung. Konventionelle Energie kommt hauptsächlich aus fossilen Brennstoffen, die auf der Erde nur begrenzt vorhanden sind, und hat zu einer starken Belastung der Umwelt geführt. Zusätzlich nimmt der Energieverbrauch weiter zu, insbesondere durch die rasante Verbreitung von Fahrzeugen und verschiedener Kundenelektronik wie PCs und Mobiltelefone. Alternative Energiequellen sollten vor einer Energiekrise entwickelt werden. Die Gewinnung erneuerbarer Energie aus Sonne und Wind sind auf jeden Fall sehr wichtig, aber diese Energien sind oft nicht gleichmäßig und andauernd vorhanden. Energiespeichervorrichtungen sind daher von großer Bedeutung, weil sie für eine Stabilisierung der umgewandelten Energie sorgen. Darüber hinaus ist es eine enttäuschende Tatsache, dass der Akku eines Smartphones jeglichen Herstellers heute gerade einen Tag lang ausreicht, und die Nutzer einen zusätzlichen Akku zur Hand haben müssen. Die tragbare Elektronik benötigt dringend Hochleistungsenergiespeicher mit höherer Energiedichte. Der erste Teil der vorliegenden Arbeit beinhaltet Lithium-Ionen-Batterien unter Verwendung von einzelnen aufgerollten Siliziumstrukturen als Anoden, die durch nanotechnologische Methoden hergestellt werden. Eine Lab-on-Chip-Plattform wird für die Untersuchung der elektrochemischen Kinetik, der elektrischen Eigenschaften und die von dem Lithium verursachten strukturellen Veränderungen von einzelnen Siliziumrohrchen als Anoden in einer Lithium-Ionen-Batterie vorgestellt. In dem zweiten Teil wird ein neues Design und die Herstellung von flexiblen on-Chip, Festkörper Mikrosuperkondensatoren auf Basis von MnOx/Au-Multischichten vorgestellt, die mit aktueller Mikroelektronik kompatibel sind. Der Mikrosuperkondensator erzielt eine maximale Energiedichte von 1,75 mW h cm-3 und eine maximale Leistungsdichte von 3,44 W cm-3. Weiterhin wird ein flexibler und faserartig verwebter Superkondensator mit einem Cu-Draht als Substrat vorgestellt. Diese Dissertation wurde im Rahmen des Forschungsprojekts GRK 1215 "Rolled-up Nanotechnologie für on-Chip Energiespeicherung" 2010-2013, finanziell unterstützt von der International Research Training Group (IRTG), und dem PAKT Projekt "Elektrochemische Energiespeicherung in autonomen Systemen, no. 49004401" 2013-2014, angefertigt. Das Ziel der Projekte war die Entwicklung von fortschrittlichen Energiespeichermaterialien für die nächste Generation von Akkus und von flexiblen Superkondensatoren, um das Problem der Energiespeicherung zu addressieren. Hier bedanke ich mich sehr, dass IRTG mir die Möglichkeit angebotet hat, die Forschung in Deutschland stattzufinden
Human beings are facing the grand energy challenge in the 21st century. Nowhere has this become more urgent than in the area of energy storage and conversion. Conventional energy is based on fossil fuels which are limited on the earth, and has caused extensive environmental pollutions. Additionally, the consumptions of energy are still increasing, especially with the rapid proliferation of vehicles and various consumer electronics like PCs and cell phones. We cannot rely on the earth’s limited legacy forever. Alternative energy resources should be developed before an energy crisis. The developments of renewable conversion energy from solar and wind are very important but these energies are often not even and continuous. Therefore, energy storage devices are of significant importance since they are the one stabilizing the converted energy. In addition, it is a disappointing fact that nowadays a smart phone, no matter of which brand, runs out of power in one day, and users have to carry an extra mobile power pack. Portable electronics demands urgently high-performance energy storage devices with higher energy density. The first part of this work involves lithium-ion micro-batteries utilizing single silicon rolled-up tubes as anodes, which are fabricated by the rolled-up nanotechnology approach. A lab-on-chip electrochemical device platform is presented for probing the electrochemical kinetics, electrical properties and lithium-driven structural changes of a single silicon rolled-up tube as an anode in lithium ion batteries. The second part introduces the new design and fabrication of on chip, all solid-state and flexible micro-supercapacitors based on MnOx/Au multilayers, which are compatible with current microelectronics. The micro-supercapacitor exhibits a maximum energy density of 1.75 mW h cm-3 and a maximum power density of 3.44 W cm-3. Furthermore, a flexible and weavable fiber-like supercapacitor is also demonstrated using Cu wire as substrate. This dissertation was written based on the research project supported by the International Research Training Group (IRTG) GRK 1215 "Rolled-up nanotech for on-chip energy storage" from the year 2010 to 2013 and PAKT project "Electrochemical energy storage in autonomous systems, no. 49004401" from 2013 to 2014. The aim of the projects was to design advanced energy storage materials for next-generation rechargeable batteries and flexible supercapacitors in order to address the energy issue. Here, I am deeply indebted to IRTG for giving me an opportunity to carry out the research project in Germany. September 2014, IFW Dresden, Germany Wenping Si

Supercapacitors, also known as electrochemical capacitors, have shown great potential as energy storage devices; and 3D printing likewise as a manufacturing technique. This research progressively investigates combining these two technologies to fabricate 3D-printed, electrochemical double-layer capacitors (EDLCs). Small EDLCs were designed in a sandwich structure with an FDM-printed plastic frame and carbon electrodes. Inkjet printing was initially combined with FDM printing to produce a pilot sample with a silver ink current collector, however this performed poorly (Cs = 6 mF/g). Henceforth a paste extrusion system was added to the FDM printer to deposit the current collectors and electrodes, fabricating the entire device in a single continuous process. This process was progressively developed and tested, ultimately attaining specific capacitances of 200 mF/g. The fully integrated 3D printing process used to manufacture the EDLCs was a novel approach. Combining the FDM printer with a paste extruder allowed for a high degree of dimensional accuracy, as well as simplifying the production process. This aspect of the design functioned successfully, without significant faults, and proved a reliable fabrication method. The later designs used in this study provided the EDLCs extendable by incorporating connection jacks. This was to create the possibility to increase capacitance simply by connecting multiple EDLCs together. Tests of this feature showed that it worked well, with the extendable EDLCs delivering outputs very close to the theoretical maximum efficiency of the unit. Carbon conductive paint was applied as a current collector and electrode for the 3D printed EDLCs in an exploration of metal-free 3D printed supercapacitors. These metal-free EDLCs were found to provide around 60% of the specific capacitance of the best performing EDLC variant produced (silver paint current collectors with activated carbon and carbon paint mixture electrodes). Although considerable improvement is required to produce EDLC samples with comparable capacitances to existing commercial manufacturing techniques, this study lays important groundwork in this area, and has introduces effective and innovative design ideas for supercapacitors and integrated 3D printing processes.

Описывается система тягового привода электроподвижного состава с накопителем энергии, рассмотрены режимы ее работы, предложена система управления процессами обмена энергией между тяговыми электродвигателями, накопителем энергии и контактной сетью, а также определена энергетическая эффективность предлагаемой системы.
A system of a traction unit with an energy storage device was described, operation modes of this system were considered, energy management system which controls the power flow between traction engines, storage device and trolley line was proposed, it's energy efficiency was calculated.

Bio-derived materials have attracted increased attention recently due to not only the sustainability, care-for-the-environment concerns, but also their naturally possessed unique structures, interesting mechanical properties, and abundant functional groups. These features endow them to potentially solve the issues that the next-generation high-capacity conversiontype lithium-ion batteries (LIBs) are facing, including but not limited to 1) great volume variation during charge/discharge for most conversion-type active materials (AMs), causing electrode pulverization, a phenomenon that active material particles are disassociated with the electrode and 2) serious shuttle effect brought by the dissolution of poly-intermediates into the electrolyte, leading to AMs loss, self-discharging, capacity fading, and shortened battery life. Bio-derived materials could provide strong binding forces and excellent mechanical strength to maintain the electrode integrity for high-capacity anodes, such as aluminum (Al) and silicon (Si) anodes. Meanwhile, Bio-derived materials possess abundant functional groups which could suppress the shuttle effect for high-capacity cathodes, such as lithium-iodine (Li-I2) batteries. These unique chemical, physical and mechanical properties of bio-derived materials make them promising in developing next-generation high-capacity LIBs. In the first study, aluminum with a high specific capacity, abundance, and electrical conductivity had been used as an active material to react with lithium ions and the electrical current collector simultaneously to save cost in the battery manufacturing process. However, its almost 100% volume variation will lead to serious electrode pulverization during lithiation/delithiation and reduce the cycle life of Al anode. Herein, a novel and robust biomassderived poly(furfuryl alcohol)/carbon black binder composite is prepared and applied on the surface of aluminum foil, and this hybrid Al anode had shown a superior 150 cycle life than unprotected Al anode that only can last for 25 cycles under a cut-off capacity loading of 400 mAhꞏg-1. This fast, quick, green, and low-cost method is potentially capable of solving the pulverization issue of high-capacity LIBs with minimal alternation on the existing battery manufacturing process. Instead of adopting complex and costly electrode structural modification methods, applying functional binders has the least impact on the existing LIBs fabrication process. Silicon has the highest theoretical capacity of 4200 mAhꞏg-1 but with an even more significant volume variation (~300%) than that of Al. Natural binders extracted from abundant plants provide effective strategies to solve the pulverization of silicon particles during charge/discharge processes. Green and abundant glutinous rice that had been used to build the Great Wall inspired the use of its main ingredient inside: amylopectin (AP). AP is a long and highly branched bio-polymer rich with carboxylic groups (-COOH) and hydroxyl groups (-OH), which can covalently bond with the SiOx on the Si nanoparticles. Also, its viscoelastic property allows it to accommodate the drastic volume variation of Si during charge/discharge. The asprepared Si-AP can uphold a high discharge capacity of 1517.9 mAhꞏg-1 at a rate of 0.1 C after 100 cycles, in which cycling stability is much higher than that of using traditional polyvinylidene fluoride (PVDF) and aqueous carboxymethylcellulose (CMC) binders. Okra, popularly known as Lady’s finger, is another commonly cultivated crop with a thick and slimy mucilage because of rich polysaccharides in it. It can be extracted and separated from the pods with a facile method and used as a binder, named okra gum (OG), showing great potential in addressing the volume variation of Si during lithiation/delithiation. Benefiting from its complex compositions and highly branched structure with rich hydroxyl groups, and viscoelastic properties, OG is able to form an interconnected network that bonds and holds the Si nanoparticles, conductive carbon, and the current collector. The as-prepared Si-OG electrode exhibits a discharge capacity of 1434 mAhꞏg-1 at a rate of 0.1 C after 50 cycles and is about 1.5 times greater than that of the Si-CMC electrode. These two studies suggest that bio-derived binder materials are able to construct more reliable LIBs with high-capacity, and both the extractions of AP and OG are greener, quicker, and cheaper than refined CMC binder. In contrast to famous lithium-sulfur (Li-S) batteries, lithium-iodine (Li-I2) batteries have also drawn great attention recently due to their high energy and power density, and iodine is low in cost and abundant. However, like Li-S batteries, Li-I2 batteries also suffer from the notorious shuttle effect, in which the dissolved iodine will leak from the cathode and diffuse to the lithium anodic side and cause self-discharge and eventually capacity fades. Here, an instant coffeederived heteroatom-rich honeycomb-like carbon filter is prepared to confine the dissolved iodine on the cathode region. In addition, this as-prepared bio-derived interlayer can bring Li-I2 battery with additional surface pseudo-capacity and results in a robust and highly reversible capacity of 224.5 mAhꞏg-1 at a rate of 10 C, and great capacity retention of 120.2 mAhꞏg-1 after 4,000 cycles. In summary, the explored bio-derived materials demonstrate the potential of solving the challenges that next-generation high-capacity lithium-ion batteries possess. Besides the facts that these materials are sustainable, green, and low cost, the extraction and electrode preparation processes are also beneficial to the environment and the operators. More importantly, these materials with environmentally friendly fabrication processes can be further developed into industrialized products for future high-capacity LIBs.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
Full Text

With increasing interest in energy storage and conversion devices for automobile applications, the necessity to understand and predict life behavior of rechargeable batteries, PEM fuel cells and super capacitors is paramount. These electrochemical devices are most beneficial when used in hybrid configurations rather than as individual components because no single device can meet both range and power requirements to effectively replace internal combustion engines for automobile applications. A system model helps us to understand the interactions between components and enables us to determine the response of the system as a whole. However, system models that are available predict just the performance and neglect degradation. In the first part of the thesis, a framework is provided to account for the durability phenomena that are prevalent in fuel cells and batteries in a hybrid system. Toward this end, the methodology for development of surrogate models is provided, and Pt catalyst dissolution in PEMFCs is used as an example to demonstrate the approach. Surrogate models are more easily integrated into higher level system models than the detailed physics-based models. As an illustration, the effects of changes in control strategies and power management approaches in mitigating platinum instability in fuel cells are reported. A system model that includes a fuel cell stack, a storage battery, power-sharing algorithm, and dc/dc converter has been developed; and preliminary results have been presented. These results show that platinum stability can be improved with only a small impact on system efficiency. Thus, this research will elucidate the importance of degradation issues in system design and optimization as opposed to just initial performance metrics. In the second part of the thesis, modeling of silicon negative electrodes for lithium ion batteries is done at both particle level and cell level. The dependence of the open-circuit potential curve on the state of charge in lithium insertion electrodes is usually measured at equilibrium conditions. Firstly, for modeling of lithium-silicon electrodes at room temperature, the use of a pseudo-thermodynamic potential vs. composition curve based on metastable amorphous phase transitions with path dependence is proposed. Volume changes during lithium insertion/de-insertion in single silicon electrode particle under potentiodynamic control are modeled and compared with experimental data to provide justification for the same. This work stresses the need for experiments for accurate determination of transfer coefficients and the exchange current density before reasoning kinetic hysteresis for the potential gap in Li-Si system. The silicon electrode particle model enables one to analyze the influence of diffusion in the solid phase, particle size, and kinetic parameters without interference from other components in a practical porous electrode. Concentration profiles within the silicon electrode particle under galvanostatic control are investigated. Sluggish kinetics is established from cyclic voltammograms at different scan rates. Need for accurate determination of exchange current density for lithium insertion in silicon nanoparticles is discussed. This model and knowledge thereof can be used in cell-sandwich model for the design of practical lithium ion cells with composite silicon negative electrodes. Secondly, galvanostatic charge and discharge of a silicon composite electrode/separator/ lithium foil is modeled using porous electrode theory and concentrated solution theory. Porosity changes arising due to large volume changes in the silicon electrode with lithium insertion and de-insertion are included and analyzed. The concept of reservoir is introduced for lithium ion cells to accommodate the displaced electrolyte. Influence of initial porosity and thickness of the electrode on utilization at different rates is quantitatively discussed. Knowledge from these studies will guide design of better silicon negative electrodes to be used in dual lithium insertion cells for practical applications.

This thesis aims at formulating and validating models for electrochemical energy storage devices. More specifically, the devices under consideration are lithium ion batteries and polymer electrolyte fuel cells.

A model is formulated to describe an experimental cell setup consisting of a Li_xNi_0.8Co_0.15Al_0.05O₂ composite porous electrode with three porous separators and a reference electrode between a current collector and a pure Li planar electrode. The purpose of the study being the identification of possible degradation mechanisms in the cell, the model contains contact resistances between the electronic conductor and the intercalation particles of the porous electrode and between the current collector and the porous electrode. On the basis of this model formulation, an analytical solution is derived for the impedances between each pair of electrodes in the cell. The impedance formulation is used to analyse experimental data obtained for fresh and aged Li_xNi_0.8Co_0.15Al_0.05O₂ composite porous electrodes. Ageing scenarios are formulated based on experimental observations and related published electrochemical and material characterisation studies. A hybrid genetic optimisation technique is used to simultaneously fit the model to the impedance spectra of the fresh, and subsequently also to the aged, electrode at three states of charge. The parameter fitting results in good representations of the experimental impedance spectra by the fitted ones, with the fitted parameter values comparing well to literature values and supporting the assumed ageing scenario.

Furthermore, a steady state model for a polymer electrolyte fuel cell is studied under idealised conditions. The cell is assumed to be fed with reactant gases at sufficiently high stoichiometric rates to ensure uniform conditions everywhere in the flow fields such that only the physical phenomena in the porous backings, the porous electrodes and the polymer electrolyte membrane need to be considered. Emphasis is put on how spatially resolved porous electrodes and nonequilibrium water transport across the interface between the gas phase and the ionic conductor affect the model results for the performance of the cell. The future use of the model in higher dimensions and necessary steps towards its validation are briefly discussed.

Large scale storage of electricity is a vital requirement for the realization of a carbon-neutral electricity grid. This thesis provides a study of integrated solar energy conversion and storage systems in order to increase the efficiency and reduce the utilization cost of solar energy. The efficient performance of photogalvanic cells relies on high dye solubility and selective electrodes with fast electron transfer kinetics. A new configuration is proposed for photogalvanic cells that removes these impractical requirements. Instead of illuminating the device through the electrode a new vertical configuration is employed with light coming between the two electrodes. This way, the light absorption and hence electron generation is spread through the depth of the device which can be adjusted according to the concentration of the dyes to absorb all the incoming photons even with low solubility dyes and slow electrode kinetics. The proposed configuration is mathematically studied and a numerical model is built for detailed analysis that gives practical guidelines for working towards device parameters with high power conversion efficiency. The analysis suggests that upon the realization of highly selective electrodes and an improved dye/mediator couple, an efficiency higher than 13 % should be achievable from the new configuration compared to 3.7 % at best using the conventional approach. Storage however in this system will be challenging due to the characteristic recombination times of dyes and mediators in the same phase. For significant and long-lived storage we designed and demonstrated an integrated solar-battery structure based on two relatively well established technologies of the redox flow battery and the dye-sensitized solar cell. The cell consists of a sensitized electrode in a redox flow battery structure. The design enables independent scaling of power and energy rating of the system thus it is applicable for large scale storage purposes. An areal energy capacity of 52 μWhcm−², charge capacity of 1.2 mAhL−¹, energy efficiency of 78 % and almost perfect Coulombic efficiency are observed for the integrated cell. These values show a 35 times increase in charge capacity and 13 times improvement in areal energy density compared to similar devices.
Applied Science, Faculty of
Electrical and Computer Engineering, Department of
Graduate

Multifunctional composites have attracted a great deal of attention as they offer a way to cut down the parasitic weight in vehicles which not only reduces the operational costs but also reduces the fuel consumption in vehicles. Current engineering design is increasingly sophisticated, requiring more efficient material utilisation; sub-system mass and volume are crucial application determinants. This dissertation contributes to the fabrication of composites that can store electrical energy and are known as structural supercapacitors. The key in the fabrication of structural supercapacitors was not simply to bind two disparate components together, but to produce a single coherent material that inherently performed both roles of a structural composite and a supercapacitor. This design approach is at a relatively early stage, and faces significant design and material synthesis challenges. Disparate material requirements, such as structural and electrochemical properties, have to be engineered and optimised simultaneously. This study investigates on structural supercapacitors fabricated by using as-received as well as activated carbon fibre cloths as reinforcement and electrodes; multifunctional resin as electrolyte and matrix; and glass fibre cloths, filter papers or polymer membranes as insulators. Such a system should deliver electrical energy storage capacity as well as bear mechanical loads. Different liquid electrolytes, such as ionic liquids and salts based on Li+ and NH4+, were studied in order to optimise the multifunctionality of polymer electrolyte. Mesoporous silica particles were also introduced into polymer electrolytes in order to enhance the mechanical and electrochemical performance of polymer electrolytes. Nanostructured/ multifunctional resin blends were cured in cylindrical form and were examined by compression testing as well as impedance spectroscopy. An ionic conductivity of 0.8 mS/cm and a compression modulus of 62 MPa have been synthesised for the polymer electrolyte in the current study. By varying the separators, multifunctional resins and the electrodes, different structural supercapacitor configurations were manufactured using a resin infusion under flexible tooling (RIFT) method and were characterised to study the electrochemical performance by using charge/discharge method and mechanical performance by using ±45° laminate shear testing. The improved structural supercapacitors showed an energy density of 0.1 Wh/kg, a power density of 36 W/kg and a shear modulus of 1.7 GPa.

In this study, the electrochemical properties and sample characterization of activated carbon have been analysed to gain a clear understanding of its behaviour reflecting the energy storage performance. When tested in a three-electrode configuration for electrochemical characterization working with 2M NaOH electrolyte solution, the activated carbon exhibits excellent capacitance and energy density value which is 124.436 F g-1 and 62.218 Wh kg-1, respectively. The activated carbon showed large BET surface area and large pores area, which are 1133.8038 m2 g-1 and 891.909 m2 g-1, respectively, ascribed to micropores. Besides, 2h holding time at 200oC by hydrothermal carbonisation of solid hydrochar from Chlorella algae species was used as precursor material. It is for the preparation of microalgae derived carbon by chemical activation method with KOH. An approach has been made to test the influences of the activation temperature on the pore structure and the energy storage performance. The macroalgal hydrochar is synthesized at two different temperatures, 800oC and 900oC, for 60 min under same impregnation ratio of KOH/hydrochar is being 4:1. Two samples thus synthesized present low capacitance and thus low energy density value which sample 1 (AC1-4:1-800) exhibited 10.679 F g-1 and 5.34 Wh kg-1, respectively, while sample 2 (AC2-4:1-900) exhibit 0.939 F g-1 and 0.47 Wh kg-1, respectively. From the BET and porosity analysis, it is concluded that both samples have low BET surface area, pores volume and low pores area. The conclusion from this experiment is that a high activation temperature, (i.e., 900oC), may degrade the pore structure of electrode materials.

Thesis (M.S. in electrical engineering)--Washington State University, May 2010.
Title from PDF title page (viewed on July 15, 2010). "School of Electrical Engineering and Computer Science." Includes bibliographical references (p. 71-75).

Thermal energy storage via the latent heat of suitable phase change materials has the advantages of higher energy storage density and relatively isothermal behaviour compared to sensible heat storage systems. Glauber's salt (Na₂S0₄∙10H₂0) is one of the most extensively studied phase change materials for solar energy systems because of its low price, suitable phase change temperature and high latent heat. However, segregation due to incongruent melting behaviour leading to loss in the heat storage efficiency upon repeated melting-freezing cycling is a serious problem which has severely limited application of Glauber's salt. In this study Glauber's salt was encapsulated in 25 mm diameter hollow spheres and agitated in different systems including a liquid fluidized bed, rotating drum and rotating tube to reduce or eliminate the Toss in its heat storage efficiency. The encapsulated mixture consisted of 96% Glauber's salt and 4% borax by weight with 5% by volume air space in the capsules. Some capsules containing 25%, 15% and 5% by weight excess sodium sulfate and 10% by weight excess water were also prepared, to test the effect of sodium sulfate concentration under different agitation conditions. The heat storage capacity of 5756 capsules, agitated by fluidizing with water in a pilot plant size (0.34 m diameter) column, showed a decrease over the first three cycles to about 60% of that theoretically possible, but there was no further decrease over the next 93 cycles under fluidization conditions. The heat storage efficiency was found to be improved by increasing the superficial water velocity and by decreasing the cooling rate. Heating rate had little or no effect. The fluidized capsules provide enhanced heat transfer rates to or from the heat storage medium, enabling the energy to be charged or discharged in about one hour with realistic inlet and outlet temperatures. The high heat transfer rates are an important advantage for the system and may open new areas of applications for thermal energy storage by encapsulated phase change material. Economic analysis of the liquid fluidized bed heat storage system shows that operating costs are almost negligible compared to fixed capital costs. The heat storage efficiency of capsules decreased to 38.4% of the theoretical capacity or 67% of the corresponding agitated (fluidized) system in only 7 cycles under fixed bed conditions, and the efficiency decreased with further cycling. 97.5% of the original heat storage-capacity was recovered within three cycles when these capsules were refluidized. Performances of the regular and different composition capsules were tested in the rotating tube, with rotation around a fixed horizontal axis passing through the capsules' centers, and in the rotating drum, with impact due to collisions in addition to rotation. The results showed that full rotation of a capsule around a horizontal axis improves the heat storage efficiency. However, full recovery of the theoretical capacity was not possible, even under vigorous mixing conditions. The efficiencies in the rotating tube were similar to those in the rotating drum for capsules subject to the same number of rotations around a horizontal axis. At high rotation speeds centrifugal force had a negative influence, especially in the rotating tube. On the basis of heat storage capacity per unit volume or weight of phase change material, 47% by weight sodium sulfate concentration was found to be optimal for the rotating drum and the rotating tube cases. Some small scale experiments were performed to determine the relative importance of different factors in the loss of heat storage capacity. Sodium sulfate concentration gradients in the capsules with different thermal cycling histories were found by thermogravimetric analysis. The results showed that bulk segregation of anhydrous sodium sulfate is not the only reason for the loss of heat storage capacity in systems using Glauber's salt. Microencapsulation of anhydrous sodium sulfate beneath a layer of Glauber's salt crystals is at least as important. Experiments to determine the degree of subcooling, believed to be another factor in the loss of heat storage capacity, showed that a mixture of 96% Glauber's salt and 4% borax by weight undergoes subcooling of about 5 K in gently agitated capsules. Nucleation and crystallization temperatures both increase with increased agitation.
Applied Science, Faculty of
Chemical and Biological Engineering, Department of
Graduate

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.
Includes bibliographical references (p. 143-154).
The Nanostructured (TM) 3D Fabrication and Assembly Process was developed as a novel method of creating three-dimensional (3D) nanostructured devices using two- dimensional micro- and nanopatterning tools and techniques. The origami method of fabrication is a two-part process in which two-dimensional (2D) membranes are first patterned and then folded into the desired 3D configuration. This thesis presents an origami fabrication method based on the use of SU-8 membranes and elastic gold hinges. Magnetic actuation, stress-induced folding, vertical spacing, and lateral alignment of the membranes are discussed. This thesis also reports on the used of the Nanostructured OrigamiTM process to create a functional electrochemical energy storage device. An electrochemical capacitor, or a supercapacitor, is selected because its performance can be readily improved by the addition of 3D geometry and nanoarchitecture. In addition to improved performance, the origami fabrication method allows such devices to be integrated into preexisting MEMS and IC processes, thus enabling the fabrication of complete micro- and nanosystems with an integrated power supply. The supercapacitors were created by selectively depositing carbon-based electrode materials on the SU-8 membrane and then folding the structure so that oppositely-charged electrode regions face each other in a 3D arrangement. The fabrication process, electrochemical testing procedure, and analysis of the results are presented.
by Hyun Jin In.
S.M.

Materials engineering and nano-manipulation play a key role in the development of advanced Lithium-ion batteries (LIBs) in terms of energy and power density (both gravimetric and volumetric), stability, rate capability, safety and the cost of production. In this thesis, two strategies are used to address the demands, i.e., the use of low cost and environmentally benign carbonaceous nanostructured materials (CNMs) and the use of hydrogenation technology. In the first strategy, CNMs including carbon nanotubes (CNTs) and graphene are incorporated with anode materials (such as metal oxide and carbon) to synthesize corresponding CNM composites that possess improved electrochemical performance because it can not only provide highly conductive matrix but also prevent the aggregation of the nanostructured electrode materials and the CNMs. TiO2-reduced graphene oxide (TiO2-RGO) was prepared for LIBs using photocatalysis method. TiO2 nanoparticles can be anchored on the GO sheets via the abundant oxygen-containing functional groups. Using the TiO2 photocatalyst, the GO was photocatalytically reduced under UV illumination, leading to the production of TiO2-RGO nanocomposite. The resultant LIBs of the TiO2-RGO nanocomposite possess more stable cyclic performances, larger reversible capacities, and better rate capability, compared with those of the pure TiO2 and TiO2-GO samples.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Griffith School of Environment
Science, Environment, Engineering and Technology
Full Text

The present thesis focuses on the development of a new generation of miniature electronic devices by employing nano-scale materials. Specifically, ZnO nanowire arrays were investigated to increase the conversion efficiency of energy harvesting devices and graphene nano-platelets employed to enhance supercapacitors’ energy storage capability. The results obtained in this work pave the way to the possibility of conceiving novel autonomous devices integrating both energy units. The present thesis has been structured in five chapters. A first introduction chapter reviews the pros and cons of renewable energies against the conventional ones produced from fossil fuels as well as their impact on the modern societies. The theoretical background on vibration energy harvesting and electrochemical energy storage is provided. Vibration energy harvesting mechanism relies on piezoelectric phenomena, where a pressure applied on a piezoelectric material turns ultimately into energy. Instead, supercapacitors store large quantity of energy for time unit by high surface material dielectric polarization. In this chapter, the reasons why ZnO nanowire arrays and graphene nano-platelets were considered are introduced. The second chapter presents promising methods to synthesize piezoelectric ZnO nano-materials prior their integration into energy harvesting devices. Since the highest piezoelectric properties of the ZnO-crystal are along its c-axis, the most suitable growing methods were selected to tailor the crystal’s unit-cell best orientation. In this chapter physical and chemical growing methods are reported. Physical vapor deposition (PVD) was used to grow ZnO thin film, then employed as a seed layer for the growth of 1D-ZnO nanowires by chemical methods in a second step. ZnO nanowires were synthesized either with or without a nanoporous template by: i) electrochemical deposition (ECD), and ii) hydrothermal technique. The fundamental process parameters to tailor the chemical growth are reported as well as the morphological and microstructural characterization of the structures fabricated. In the third chapter, the characteristics of the energy harvesting device fabricated from the piezoelectric ZnO nanostructures are reported. Piezoresponse force microscopy was initially used to measure the d33 piezoelectric coefficient of the ZnO nanostructures fabricated fairly matching the theoretical expectations. Finally, this chapter reports the energy harvested by the devices fabricated, measured by connecting an external resistive load to it: a maximum energy harvested equal to 2 μJ/cm2 was found. The fourth chapter focuses on nano-scale graphene based materials for supercapacitors’ electrodes. Specifically, the synthesis and the characterization of the graphene nano-platelets used in this work is described. XRD and Raman spectroscopy were used to distinguish pure graphite from graphene, BET and SEM to measure its specific surface area and morphology. To determine the graphene’s properties functional to the application thermogravimetric analysis (TGA) was carried out. To identify the types of oxygen groups present in the graphene materials, the corresponding Fourier Transform Infrared spectra (FTIR) were recorded and their contribution in rGO was examined by X-Ray photoelectron spectroscopy (XPS) analysis. Overall this chapter reviews the relevant analysis to be performed in candidate materials for fabrication of supercapacitor electrodes. The fifth chapter discusses the fabrication of supercapacitor electrodes made with the graphene nano-platelets previously described as well as the methods for their electrochemical characterization. As being the standard of the energy storage industry, cyclic voltammetry (CV) and constant current charge and discharge experiments were carried out for capacitance estimation. The electrochemical characteristics of the device were then linked to the properties of the graphene nano-materials employed. All measurements were done in a full-scale electrochemical cell mimicking a real supercapacitor device. The results suggest that mechanically exfoliated graphene nano-platelets (GNP) best perform among the variety of materials investigated.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references.
Artificial muscles (i.e., stimuli-responsive materials) are muscle-like materials and devices that mimic muscle's functionality (e.g., contraction, rotation, and bending) in different aspects. Some of the common performance metrics used for evaluating artificial muscles are cycle life, gravimetric/volumetric energy and/or power density, efficiency, cost, and controllability of muscle. Having a good combination of these performance metrics is very desirable and an active field of research. Many of the state-of-the-art designs are made from some exotic materials such as carbon nanotubes and metal nanowires which are not yet commercially available; here, new designs are proposed which their performance favorably compares to those of the rival materials and yet made of readily available materials. In addition to artificial muscles, designs for fast charging micro-supercapacitors are also proposed. Fast charging energy storage devices such as supercapacitors have applications in different industries ranging from automobile to telecommunication. Cellphones, for example, use fast charging micro-supercapacitors in their GSM/GPRS modulus to generate high current pulses for signal transmission purposes. The current technologies, such as tantalum/niobium oxide micro-supercapacitors are evolving around enhancing the energy and power density by increasing the specific capacitance and operating voltage. Yet, increasing the specific capacitance is still a major challenge. In this thesis, aside from discrete component geometry, flexible (e.g., yam-based) supercapacitors have various applications from flexible circuits to wearable devices. Design and fabrication of high performance supercapacitors by utilizing metal nanowires (e.g., niobium nanowires) in both forms (i.e., flexible and solid/rigid devices) are investigated as well.
by Seyed M. Mirvakili.
Ph. D.

A spray deposition manufacturing route has been developed for the fabrication of carbon nano-structured and micro-structured energy storage devices in a thin film format, with controlled film thickness, homogeneous film surface morphology and high electrochemical performance for both supercapacitors and lithium ion battery anodes. Three types of low cost commercially available carbon materials (graphite, activated carbon and carbon black) have been investigated, and electrodes characterised in terms of surface morphology, surface chemistry, microstructure and electrochemical properties. By using ball milling, CO₂ activation and adding suitable carbon conductive additives, nano-graphite-based film electrodes (one meter long and ~ 3 µm thickness) have been fabricated, with excellent ion transport and low electrical resistance (< 1.8 Ω). Specific capacitance of 110 F/g at a scan rate of 100 mV/s in 1 M H₂SO₄ was achieved. The high rate performance of activated carbon-based electrodes ( ~2 µm thickness) has been enhanced by reducing the contact resistance of electrode/current collector interface and building a well-interconnected and hierachical meso/macro-porous structure. A specific capacitance of over 120 F/g at a scan rate of 600 mV/s or 20 A/g current density in 1 M H₂SO₄ was achieved. The performance of carbon black-based electrodes (~4 µm thickness) in different electrolytes has been studied in both two- and three-electrode cells. High specific capacitances of 260 F/g at 1 A/g was achieved in 6 M KOH, together with energy and power densities of 21 kW/kg and 18 Wh/kg in 1 M Na₂SO₄. Finally, graphite-based electrodes for rechargeable lithium-ion batteries have also been fabricated with controlled film thickness from ~ 900 nm to ~ 40 µm and 98% capacity retention of 371 mA/g after 20 cycles. Spray deposition has been demonstrated to have the potential for scalability in the manufacture of carbon-based thin film electrodes with competitive properties.

A scalable spray processing technique was used to fabricate carbon nanotube (CNT)-based film electrodes and solid-state supercapacitors. The sprayed CNT-based electrodes comprised a randomly interconnected meso-porous network with a high electrical conductivity. Layer-by-layer (LbL) deposition of functionalised and oppositely charged single-wall carbon nanotubes (SWNTs) increased the electrode density and improved charging and discharging kinetics when compared with carboxylic functionalised only SWNT electrodes. The capacitance was further increased to 151 F g-1 at 2 mV s-1 and 120 F g-1 at 100 mV s-1 after vacuum and H2 heat treatments that removed the functional groups, and resulted in a hybrid microstructure of SWNTs and multi-layer graphene sheets from unzipped SWNTs. Flexible solid-state supercapacitors were fabricated by directly spraying multi-wall carbon nanotube (MWNT)-based aqueous suspensions onto both sides of a Nafion membrane and dried. A single cell with MWNT-only electrodes had a capacitance of 57 F g-1 per electrode at 2 mV s-1 and 44 F g-1 at 150 mV s-1. Cells with MWNT/ionomer electrodes showed a higher H+ mobility and a lower charge transfer resistance, and the capacitance increased to 145 F g-1 at 2 mV s-1 and 91 F g-1 at 150 mV s-1. Finally, MWNT/TiO2 nanoparticle/ionomer hybrid electrodes were used in the same solid-state supercapacitor configuration and provided a capacitance of 484 F g-1 per electrode at 5 mV s-1 and 322 F g-1 at 100 mV s-1. A qualitative model of the charge storage mechanism was developed, where TiO2 promoted H+ ions via redox reactions that fed protons into the proton-conducting ionomer coating over the MWNTs (in which the TiO2 was embedded), while electrons were readily conducted through the MWNT scaffold. This solid-state supercapacitor provided both attractive energy (31.8 Wh kg-1) and power (14.9 kW kg-1) densities, where such high energy density is difficult to achieve for MWNTs alone and such high power density is difficult for metal oxides alone, especially in the solid state.

Includes abstract.
Includes bibliographical references.
Following the 2008 severe electricity shortage in South Africa, domestic and industrial users faced incessant periods of blackouts. It is generally believed to be associated with lack of generation capacity. Since then research efforts have been directed towards boosting the generation capacity of the South African network by investing in a mix of power generation projects which include coal, nuclear and renewable energy schemes such as solar and wind. The renewable energy resources are considered a more viable option because of their many advantages such as lower greenhouse gas emissions, inexhaustible, reliable and even cheaper energy cost on the long term. Africa has huge potentials of solar power because of the abundance of direct sunshine in most days of the year. The rising cost of the fossil electricity has made the solar power an attractive option bearing in mind that the cost of the solar power has plummeted steadily in the past few years. Two main technologies are prevalent in the solar power research. These are photovoltaic (PV) systems and the concentrated solar power (CSP). The PV systems are made of solar panels and power electronic circuits. They are mostly economical in small residential units. The CSPs on the other hand which are made of solar field, thermal storage and steam turbine/generator units are economical only in large scale. In this thesis, a 2.5 kW Residential PV system and a 100 MW Molten Salt Power Tower Concentrated Solar Power were developed. The technical model of the photovoltaic panel and the power electronic circuits that connect it to the grid were also developed with Matlab/Simulink while the economic simulation of the PV, as well as the Concentrated Solar Power were carried out with Systems Advisor Model (SAM) using the climate data of Cape Town. The simulation results of this work compared the cost of PV electricity first with Renewable Energy Feed-in Tariff (REFIT) of National Energy Regulator of South Africa (NERSA), and then with the residential tariff charged by the City of Cape Town. Also the cost of electricity using CSP is compared NERSA`s REFIT. Finally the cost of PV electricity is compared with that of CSP. We therefore conclude that, with government incentives, CSP and PV are viable technologies however electricity produced by CSP is cheaper than that of the PV.

A new form of organic energy storage devices (organic capacitors) is presented in the first part of this dissertation. The storage devices are made out of an organic semiconductor material and charge storage elements from synthesized nanoparticles. The semiconducting polymer is obtained by blending poly (vinyl alcohol) and poly (acrylic acid) in crystal state polymers with a known plasticizer; glycerol or sorbitol. Synthesized nanoparticles namely, zinc-oxide (ZnO), erbium (Er), cadmium sulfide (CdS), palladium (Pd) and silver-platinum (AgPt) were used as charge storage elements in fabrication of metal-insulator-semiconductor (MIS) structure. The organic semiconductor and synthesized nanoparticles are tested to evaluate and characterize their electrical performance and properties. Fabrication of the organic capacitors consisted of layer-by-layer deposition and thermal evaporation of the electrode terminals. Capacitance versus voltage (C-V) measurement tests were carried out to observe hysteresis loops with a window gate that would indicate the charging, discharging and storage characteristics. Experimental investigation of various integrated energy harvesting techniques combined with these organic based novel energy storage devices are performed in the second part of this dissertation. The source of the energy is the wind and is harvested by means of miniature wind turbines and vibrations, using piezoelectric transduction. In both cases, the generated electric charge is stored in these capacitors. The performance of the organic capacitors are evaluated through their comparison with commercial capacitors. The results show that the voltage produced from the two energy harvesters was high enough to store the harvested energy in the organic capacitors. The charge and energy levels of the organic capacitors are also reported. The third part of this dissertation focuses on harvesting energy from a self-induced flutter of a thin composite beam. The composite beam consisted of an MFC patch bonded near the clamped end and placed vertically in the center of a wind tunnel test section. The self sustaining energy harvesting from the unimorph composite beam is exploited. The effects of different operational parameters including the optimum angle of attack, wind speed and load resistance are determined.
Ph. D.

Cellulose is the most abundant biopolymer in the world and the main component of paper. Modern society requires electronic devices to be more flexible and environmental friendly, which makes cellulose as a good candidate for the next generation of green electronics. However, lots of researches employed “paper-like” petroleum-based polymers to fabricate electronics rather than using real cellulose paper. Cellulose, as a representative of environmental friendly materials, caught into people's attention because of its sustainable nature, ease of functionality, flexibility and tunable surface properties, etc. There are some general challenges about using cellulose for electronics, such as its non-conductivity, porosity and roughness, but these features can be taken advantages of on certain occasions. This thesis focuses on the study of cellulose-based electronic devices by chemical or physical modification of microfibrillated cellulose (MFC). Particularly, three electronic devices were fabricated, including ionic diodes, electric double layer supercapacitors, pseudocapacitors. In addition, a rational design of dye-sensitized solar cell was investigated, although it was not directly cellulose-based, it led the way to the next generation of cellulose-based solar cells. The extraordinary physical and chemical properties of MFC were successfully leveled in those devices, in addition, inspiring and effective fabrication methods were proposed and carried out to solve the major problems faced by paper-based electronics, such as conductivity, flexibility, packaging and designs.

Within the framework of this thesis, three innovative electrochemical devices have been studied. A part of the work is devoted to an already existing device, laminates which are debonded by the application of a voltage. This type of material can potentially be used in a wide range of applications, including adhesive joints in vehicles to both reduce the total weight and to simplify the disassembly after end-of-life, enabling an inexpensive recycling process. Although already a functioning device, the development and tailoring of this process was slowed by a lack of knowledge concerning the actual electrochemical processes responsible for the debonding. The laminate studied consisted of an epoxy adhesive, mixed with an ionic liquid, bonding two aluminium foils. The results showed that the electrochemical reaction taking place at the releasing anode interface caused a very large increase in potential during galvanostatic polarization. Scanning electron microscopy images showed reaction products growing out from the electrode surface into the adhesive. These reaction products were believed to cause the debonding through swelling of the anodic interface so rupturing the adhesive bond. The other part of the work in this thesis was aimed at innovative lithium ion (Li‑ion) battery concepts. Commercial Li-ion batteries are two-dimensional thin film constructions utilized in most often mechanically rigid products. Two routes were followed in this thesis. In the first, the aim was flexible batteries that could be used in applications such as bendable reading devices. For this purpose, nano-fibrillated cellulose was used as binder material to make flexible battery components. This was achieved through a water-based filtration process, creating flexible and strong papers. These paper-based battery components showed good mechanical properties as well as good rate capabilities during cycling. The drawback using this method was relatively low coulombic efficiencies believed to originate from side-reactions caused by water remnants in the cellulose structure. The second Li-ion battery route comprised an electrochemical process to coat carbon fibers, shown to perform well as negative electrode in Li-ion batteries, from a monomer solution. The resulting polymer coatings were ~500 nm thick and contained lithium ions. This process could be controlled by mainly salt content in the monomer solution and polarization time, yielding thin and apparently pin-hole free coatings. By utilizing the carbon fiber/polymer composite as integrated electrode and electrolyte, a variety of battery designs could possibly be created, such as three-dimensional batteries and structural batteries.

QC 20130403

A long lasting energy storage device with energy density rivalling batteries would be very useful in many applications, especially ones where device replacement is difficult or expensive. Devices based on lithium titanate electrodes are considered promising in this regard, as lithium titanate electrodes have very long cycle lives. In this thesis, the feasibility of using the conducting polymer polyaniline in conjunction with a lithium titanate electrode to build a battery-supercapacitor combination energy storage device is considered, since polyaniline is also expected to have a high cycle life, due to its supercapacitor-like charge storage mechanism. Various methods for fabricating a polyaniline electrode are considered, and the deposition of polyaniline onto a stainless steel substrate from an aqueous solution was used. The polyaniline electrode, upon being tested in a non-aqueous solution containing lithium ions, was found to have a specific capacitance and a specific capacity of roughly 220 F/g and 85 F/g respectively. Nuclear magnetic resonance tests were used to find that the lithium ions do not dope the polyaniline and drive its oxidation state changes; therefore, the electrolyte in the proposed device must accommodate all the lithium ions emitted from the lithium titanate electrode. A simulation is presented, based on experimental data from each electrode tested separately, which estimates the energy density of the complete device to be 22.8 Wh/kg and the cost to be $560/kWh. This energy density is more than two-thirds that of a lead-acid battery and the cost is competitive with lithium-ion batteries, so the device is considered viable in applications where long-lasting devices are of utmost importance.

A reversible solid oxide fuel cell (RSOFC) system could buffer intermittent electrical generation, e.g. wind, wave power by storing electrical energy as hydrogen and heat. RSOFC were fabricated by thermoplastic extrusion of (La₀.₈Sr₀.₂)₀.₉₅MnO[subscript(3−δ)] (LSM) ceramic support tubes, which were microstructurally stable with 55% porosity at 1350°C. A composite oxygen electrode of LSM-YSZ was applied, providing a homogeneous substrate for a 20 μm - 30 μm thick YSZ electrolyte. A dip-coated 8YSZ slurry, and a painted commercial 3YSZ ink gave sintered densities of 90% and nearly 100% at 1350°C, respectively. A porous NiO/YSZ fuel electrode was also painted on. A Ag/Cu reactive air braze was unsuccessful at forming a void-free joint between the RSOFC and a 316 stainless steel gas delivery tube, as the braze did not penetrate the oxidation layer on the steel. Two alumina-based ceramic cements failed to fully seal the cell to an alumina gas delivery tube, due to thermal expansion coefficient mismatches and porosity after curing. Therefore, the maximum open circuit voltage (OCV) obtained during RSOFC testing was 0.8 V at 440°C. LSM-YSZ symmetrical cell performance measurements with oxygen pressure showed a diffusion polarisation, which was assigned to dissociative adsorption and surface diffusion of oxygen species. A collaborative RSOFC system software model showed ohmic and activation losses dominated the RSOFC, and diffusion losses were insignificant. Pressurisation from 1 to 70 bar increased the RSOFC Nernst voltage by 11% at 900°C, and reduced the entropy of the gases, reducing heat production and increasing electrical efficiency. A 500 kg Sn/Cu phase change heat store prevented the system overheating. Over a 16 h discharge-charge RSOFC cycle in the range 5 mol.% - 95 mol.% hydrogen in steam, at 20.4 A per cell or 3250 A m⁻², the electrical energy storage efficiency was 64.4%.

This thesis reports on the development of novel screen-printed electrodes for use as energy-storage (supercapacitor) devices. The thesis covers four primary topics; the first considers the basic fundamentals of electrochemistry, which are essential for understanding and furthering the development of energy-storage devices. Section two reports on the electrode materials, highlighting the important contribution of each material towards the electrochemical mechanisms involved at their surfaces. Furthermore, a novel, yet simplistic methodology for characterising various carbon nanomaterials (in terms of their edge and basal content) without recourse to expensive laboratory equipment is presented. Section three describes the processes involved in screen printing but moreover, illustrates how the modification of the electrode with nanomaterials can transform a seemingly redundant electrode into a highly desirable and sometimes ideal option targeted towards the application of energy-storage. The development of true screenprinted supercapacitors utilising transition metal nanomaterials, shows proof of concept for the future advancement of screen-printed electrodes as thin, highly flexible energy-storage devices. The final section highlights some of the limitations found in electrical impedance spectroscopy that are often overlooked. Lastly, a simple methodology is described that has been found to offer improved accuracy in the galvanostatic (charge/discharge) measurement of capacitance.

In the future grid environment, more sustainable resources will be increasing steadily. Their inherent unpredictable and intermittent characteristics will inevitably cause adverse impacts on the system static, dynamic and economic performance simultaneously. In this context, energy storage (ES) devices have been receiving growing attention because of their significant falling prices. Therefore, how to utilize these ES to help alleviate the problem of renewable energy (RE) sources integration has become more and more attractive. In my thesis, I will try to resolve some of the related problems from several perspectives. First of all, a comprehensive Future Australian transmission network simulation platform is constructed in the software DIgSILENT. Then in-depth research has been done on the aspect of frequency controller design. Based on mathematical reasoning, an advanced robust H∞ Load Frequency Controller (LFC) is developed, which can be used to assist the power system to maintain a stable frequency when accommodating more renewables. Afterwards, I develop a power system sensitivity analysis based-Enhanced Optimal Distributed Consensus Algorithm (EODCA). In the following study, a Modified Consensus Alternating Direction Method of Multipliers (MC-ADMM) is proposed, with this approach it can be verified that the convergence speed is notably accelerated even for complex large dimensional systems. Overall, in the Master thesis, I successfully provide several novel and practical solutions, algorithms and methodologies in regards to tackling both the frequency, voltage and the power flow issues in a future grid with the assistance of energy storage devices. The scientific control and optimal dispatch of these facilities could provide us with a promising approach to mitigate the potential threats that the intermittent renewables posed on the power system in the following decades.

Intermittent electricity generation from renewable energy sources, such as wind energy, ocean energy, and solar energy, has significantly intensified the demand for high-energy-density, high-power, and low-cost energy storage devices. In this regard, tremendous efforts have been devoted to the development of electrode materials, electrolytes, and separators of energy-storage devices to address the fundamental needs of emerging technologies such as electric vehicles, artificial intelligence, and virtual reality. Polymer materials are ubiquitous in fabricating these energy storage devices and are widely used as binders, electrolytes, separators, and other components. However, binders, as an important component in energy-storage devices, are yet to receive sufficient attention. Polyvinylidene fluoride (PVDF) has been the dominant binder in the battery industry for decades despite several well-recognized drawbacks, i.e., limited binding strength due to the lack of chemical bonds with electroactive materials, insufficient mechanical properties, and low electronic and lithium-ion conductivities. The limited binding function cannot meet the inherent demands of emerging electrode materials with high capacities such as silicon anodes and sulfur cathodes. Polymers are also used as electrolyte matrices because they offer the advantages of low cost, lightweight, easy processability, excellent mechanical deformation, and better interfacial contact and compatibility with electrodes. However, the practical implementation of solid polymer electrolytes has been hindered by several challenging issues including low ionic conductivity, low ion transfer number, high-voltage instability, and lithium dendrite growth. Because of the increasingly growing demand for higher performance of energy storage devices, it is necessary to develop novel polymeric binders and solid electrolytes with advanced functionalities to help improve the operation of the currently existing energy storage systems. In the first study, we synthesized a novel self-healing poly(ether-thioureas) (SHPET) polymer with balanced rigidity and softness for the silicon anode. The as-prepared silicon anode with the self-healing binder exhibits excellent structural stability and superior electrochemical performance, delivering a high discharge capacity of 3744 mAh g−1 at a current density of 420 mA g−1, and achieving a stable cycle life with a high capacity retention of 85.6% after 250 cycles at a high current rate of 4200 mA g−1. The success of this work suggests that the proposed SHPET binder facilitates fast self-healing, buffers the drastic volume changes and overcomes the mechanical strain in the course of the charge/discharge process, and could subsequently accelerate the commercialization of the silicon anode. Binders could play crucial or even decisive roles in the fabrication of low-cost, stable, and high-capacity electrodes. This is especially the case for the silicon (Si) anodes and sulfur (S) cathodes that undergo large volume change and active material loss in lithium-ion batteries during prolonged cycles. In the second study, a hydrophilic polymer poly(methyl vinyl ether-alt-maleic acid) (PMVEMA) was explored as a dual-functional aqueous binder for the preparation of high-performance silicon anodes and sulfur cathodes. Benefiting from the dual functions of PMVEMA, i.e., the excellent dispersion ability and strong binding forces, the as-prepared electrodes exhibit improved capacity, rate capability, and long-term cycling performance. In particular, the as-prepared Si electrode delivers a high initial discharge capacity of 1346.5 mAh g-1 at a high rate of 8.4 A g-1 and maintains 834.5 mAh g-1 after 300 cycles at 4.2 A g-1, while the as-prepared S cathode exhibits enhanced cycling performance with high remaining discharge capacities of 711.44 mAh g-1 after 60 cycles at 0.2 C and 487.07 mAh g-1 after 300 cycles at 1 C, respectively. These encouraging results suggest that PMVEMA could be a universal binder to facilitate the green manufacture of both anodes and cathodes for high-capacity energy storage systems. Stable and seamless interfaces among solid components in all‐solid‐state batteries (ASSBs) are crucial for high ionic conductivity and high rate performance. This can be achieved by the combination of functional inorganic material and flexible polymer solid electrolytes. In the third study, a flexible all‐solid‐state composite electrolyte is synthesized based on oxygen‐vacancy‐rich Ca‐doped CeO2 (Ca-CeO2) nanotube, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and poly(ethylene oxide) (PEO), namely Ca-CeO2/LiTFSI/PEO. Ca-CeO2 nanotubes play a key role in enhancing ionic conductivity and mechanical strength while the PEO offers flexibility and assures the stable seamless contact between the solid electrolyte and the electrodes in ASSBs. The as‐prepared electrolyte exhibits high ionic conductivity of 1.3 × 10−4 S cm−1 at 60 °C, a high lithium ion transference number of 0.453, and high‐voltage stability. More importantly, various electrochemical characterizations and density functional theory (DFT) calculations reveal that Ca-CeO2 helps dissociate LiTFSI, produces free Li-ions, and therefore enhances ionic conductivity. The ASSBs based on the as‐prepared Ca-CeO2/LiTFSI/PEO composite electrolyte deliver high‐rate capability and high‐voltage stability. Offering high energy density and high safety, all-solid-state lithium-sulfur batteries (ASSLSBs) have emerged as one of the most promising next-generation energy storage systems. However, there are a series of barriers to their practical applications, including insufficient sulfur utilization, low ionic conductivity and unstable interfaces. In the fourth study, we adopt acetamide to construct a deep eutectic system to suppress electrode passivation, and therefore address the issues of sulfur utilization, and improve the ionic conductivity of the solid polymer electrolytes. Furthermore, we establish a lithium bis(trifluoromethanesulfonyl)imide - lithium oxalyldifluoroborate (LiTFSI-LiDFOB) dual-salt system to facilitate the establishment of a stable and uniform passivation layer, a favorable interface on lithium anode, to prevent lithium dendrite formation and the polysulfide shuttling. Consequently, the as-prepared ASSLSBs deliver a high initial discharge specific capacity of 1012 mAh g-1 at 0.05 C and a stable capacity of 234.84 mAh g-1 after 1000 cycles at 0.1 C. This work suggests that the simultaneous adoption of the deep eutectic system and dual-salt electrolyte could accelerate the practical applications of ASSLSBs. In summary, the high performance of the as-prepared silicon anodes demonstrates potential for addressing the challenges for next-generation anodes by designing self-healing polymers and aqueous hydrophilic polymers. Moreover, the success of the aqueous hydrophilic polymer in lithium-sulfur batteries suggests that such a binder system can be extended to other high-capacity energy storage materials that suffer from severe volume changes. As for the polymer electrolytes, the design of functional inorganic/polymeric composite electrolyte presents a promising strategy to resolve the stubborn barriers (i.e., insufficient contact at the interfaces and ionic conductivity) of ASSBs. Additionally, combining the merits of the deep eutectic system and the dual-salt system, long-term cycling stability and high capacity retention of ASSLSBs can be achieved. These polymeric binders and electrolytes can be further optimized to realize high performance for various energy storage systems.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
Full Text

Since the fabrication of graphene, designing advanced atomically thin nanomaterials (ATMs) with few-atoms thick layers, special electronic structures, and excellent electrochemical properties for next-generation energy storage and conversion devices has attracted worldwide attention. Compared with traditional bulk materials, the ATMs exhibit several advantages: i) Their large specific surface area offers abundant active sites for ion insertion/deinsertion, and increases the contact with the electrolyte; ii) The atomic thickness of ATMs conspicuously shortens the ion diffusion pathway; iii) The distorted crystal lattice of ATMs could lead to increased electrical conductivity, and also facilitate vacancy generation, elemental doping and heterostructure construction; iv) The ATMs are regarded as an ideal 2D platform to explore the connection between electrochemical performance and electronic structures. However, they also face challenges like weak conductivity, large bandgap, and poor chemical activity. To solve these problems, various methods, including doping/phase engineering, vacancies/hole creation, and heterostructure construction have been utilized to optimize their properties. The goal of this thesis is to present a deep understanding of the impact of structural design and engineered defects on the electrochemical performance of ATMs. In the first study, holey graphene (HG) was created through an etching method, which acted as a template for the in-situ growth of atomically thin mesoporous NiCo2O4 nanosheets, leading to a NiCo2O4-HG heterostructure. The atomic-thin thickness and porous structures of NiCo2O4-HG was beneficial for electrolyte diffusion and ions/electrons transfer, and the subsequent numerous accessible surface atoms result in improved redox pseudocapacitance. In addition, the synergistic effect between NiCo2O4 and HG produced a broad interfacial area and increased electrical conductivity, dramatically accelerating the intercalation pseudocapacitance. Both redox and intercalation pseudocapacitive energy storage are beneficial for achieving high energy and power density in lithium-ion batteries (LIBs). Consequently, the NiCo2O4@HG delivered a high specific capacity of 1103.4 mAh g-1 at 0.2 C, ~88.9% contribution from pseudocapacitance at 1 mV s-1 and ultra-long life up to 450 cycles with 931.2 mAh g-1 retention, significantly outperforming previously reported electrodes. Vacancies engineering is an effective way to optimize the properties of ATMs. However, cation vacancies have rarely been reported for batteries because of the challenging creation process. Thus, we applied an alkaline etching strategy to produce Co vacancies (VCo) at the interface of ultra-thin Co3-xO4/graphene@CNT for highenergy/ power LIBs. The existence of VCo were confirmed by HRSTEM, XPS, and ELLS. The Co3-xO4/graphene@CNT showed a high capacity of 1688.2 mAh g-1 at 0.2 C, outstanding rate capability of 83.7% capacity retention at 1 C, excellent cycling performance (1500 cycles with a reversible capacity of 1066.3 mAh g-1), and a large pseudocapacitive contribution (86.5%) induced by VCo at the interface of Co3-xO4/graphene@CNT. Density functional theory (DFT) indicates that the VCo could significantly improve Li adsorption and provide more pathways with a lower energy barrier for Li diffusion, leading to obvious intercalation pseudocapacitive behavior and high-capacity/rate energy storage. Inspired by the effect of VCo on the battery performance of Co3-xO4/graphene@CNT, we also created VCo on the interface of Co1-xSe2/graphene (Co1-xSe2/GE) which was utilized as anode for SIBs. The DFT result indicated that due to the VCo the Co1-xSe2/GE exhibited higher sodium adsorption energy (4.57 eV) and lower sodium diffusion barrier (~1.7 eV) which is beneficial for the intercalation and diffusion of Na+. Experimental results confirmed that the tuned electronic state of Co in Co1-xSe2/GE could result in high specific capacity (626.2 mAh g-1 at 0.2 C), outstanding rate capacity, large pseudocapacitive contribution ratio and an exceptional cycling performance superior to most CoSe2-based anodes. The outstanding energy storage performance of Co1-xSe2/GE may be due to the synergistic effect between ultrathin CoSe2 nanobelts and GE nanosheets, which could provide multiple diffusion pathways, added active sites, and lower Na+ diffusion barriers leading to excellent pseudocapacitance behaviour. This work implied that VCo could stimulate the potential of CoSe2 to facilitate the development of low-cost energy storage devices. In addition to application in batteries, we also explored the potential of ATMs for the oxygen evolution reaction (OER). In the final work, we focused on the role of S on the OER activity of ultrathin FeCoOOH and used DFT to confirm the catalytically active centres. The results suggest the electronic states of Co could be optimized by the synergistic effect between two coordinating S and one adjacent Fe, leading to decreased binding energy of OH* (ΔEOH) while rarely changing ΔEO, thereby dramatically lowering the overpotential of the catalytic activity. Further experimental studies verified the synergistic effect between S and Fe on tuning the electronic structure of Co(OH)2, which greatly improved its catalytic activity with a small overpotential of 225.3 mV to drive to a current density of 20 mA cm-2. This work unveils the origin of the high catalytic activity of transition metal sulfides in the atomic level and provides insights into the prospect of ATMs as efficient OER electrocatalysts. In summary, the research exhibited in this thesis indicated that defect engineering of ATMs could increase the surface active sites, provide an additional pathway for ions/electrons transfer, and increase the pseudocapacitance contribution resulting in outstanding battery performance. ATMs also showed impressive potential for electrocatalysis application. These bottom-up synthesis methods and defect engineering were also validated for other transition metal oxides/ dichalcogenides for different energy storage and conversion devices.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
Full Text

Nowadays, the increasing requirements of portable, implantable, and wearable electronics have greatly stimulated the development of miniaturized energy storage devices (MESDs). Electrochemically active materials and microfabrication techniques are two indispensable parts in MESDs. Particularly, the architecture design of microelectrode arrays is beneficial to the accessibility of two-dimensional (2D) active materials. Therefore, this study reviews the recent advancements in microbatteries and microsupercapacitors based on electrochemically active 2D materials. Emerging microfabrication strategies enable the precise control over the thickness, homogeneity, structure, and dimension in miniaturized devices, which offer tremendous opportunities for achieving both high energy and power densities. Furthermore, smart functions and integrated systems are discussed in detail in light of the emergence of intelligent and interactive modes. Finally, future developments, opportunities, and urgent challenges related to 2D materials, device fabrications, smart responsive designs, and microdevice integrations are provided.

In this thesis different materials have been developed to use them in electrochemical cells. The electrochemical cells studied can be divided into two material big groups: solids oxides and acid salts materials. In the first group, materials to use them in electrodes for fuel cells an electrolyzer based on oxygen ion conductor electrolytes were optimized. Pertaining to this group, the influence of doping the Ba0.5Sr0.5Co0.8Fe0.2O3-d perovskite with 3% of Y, Zr and Sc in B position (ABO3-d) was checked. That optimization could reduce the polarization resistance of electrodes and improve the stability with time. Additionally, the limiting mechanisms in the oxygen reduction reaction were determined, and the influence of CO2 containing atmospheres was checked. La2NiO4+d;, pertaining to the Ruddlesden-Popper serie, is a mixed conductor of electron and oxygen ions. This compound was doped in La position (with Nd and Pr) and in Ni position (with Co). The dopants introduced were able to produce structural change and improve the cell performance, reducing in more than one order of magnitude the La1.5Pr0.5Ni0.8Co0.2O4+d; polarization resistance respect to the reference material (La2NiO4+d). In addition, the properties of an electrode based on the pure electronic conductor, La0.8Sr0.2MnO3-d; (LSM), were optimized. The triple phase boundary was enlarged by the addition of a second phase with ionic conductivity. That strategy made possible to reduce the electrode polarization resistance. In order to improve the oxygen reduction reaction, the addition of different catalysts by infiltration was studied. The different infiltrated oxides changed the electrochemistry properties, being the praseodymium oxide the catalyst which made possible a reduction in two orders of magnitude the electrode polarization resistance respects to the composite without infiltration. Furthermore, the efficiency of the cell working in fuel cell and electrolyzer mode was improved. Concerning the materials selected to use as electrodes on proton conductor electrolytes, the efficiency of electrodes based on LSM was optimized by using a second phase with protonic conductivity (La5.5WO12-d) and varying the sintering temperature of the electrode. Finally, the catalytic activity of the cell was boosted by infiltrating samaria doped ceria nanoparticles, achieving higher power densities for the fuel cell. The materials pertaining to the Ruddlesden-Popper series and studied for ionic conductor electrolytes were also used for cathodes in proton conductor fuel cells. After checking the compatibility with the electrolyte material, the influence of different electrode sintering temperatures and air containing atmospheres (dry, H2O y D2O) on the cathode performance was studied. Finally, the electrochemical cells based on acid salts (CsH2PO4) were designed and optimized. In that way, different cell configurations were studied, enabling to obtain thin and dense electrolytes and active electrodes for the hydrogen reduction/oxidation reactions. The thickness of the electrolyte was reduced by using steel and nickel porous supports. Furthermore, an epoxy resin type was added to the electrolyte material to enhance the mechanical properties. The electrodes configuration was modified from pure electronic conductors to composite electrodes. Moreover, copper was selected as an alternative of the expensive platinum working at high operation pressures. The cells developed were able to work with high pressures and with high content of water steam in fuel cell and electrolyzer modes.
En la presente tesis doctoral se han desarrollado materiales para su uso en celdas electroquímicas. Las celdas electroquímicas estudiadas, se podrían separar en dos grandes grupos: materiales de óxido sólido y sales ácidas. En el primer grupo, se optimizaron materiales para su uso como electrodos en pilas de combustible y electrolizadores, basados en electrolitos con conducción puramente iónica. Dentro de este grupo, se comprobó la influencia de dopar la perovskita Ba0.5Sr0.5Co0.8Fe0.2O3-d, con un 3% de Y, Zr y Sc en la posición B (ABO3-d). Esta optimización llevó a la reducción de la resistencia de polarización así como a una mejora de la estabilidad con el tiempo. Así mismo, se determinaron los mecanismos limitantes en la reacción de reducción de oxígeno, y se comprobó la influencia de la presencia de CO2 en condiciones de operación. El La2NiO4+d perteneciente a la serie de Ruddlesden-Popper, es un conductor mixto de iones oxígeno y electrones. Éste, fue dopado tanto en la posición del La (con Nd y Pr) como en la posición del Ni (con Co). Los dopantes introducidos además de producir cambios estructurales, provocaron mejoras en el rendimiento de la celda, reduciendo para alguno de ellos, como el La1.5Pr0.5Ni0.8Co0.2O4+d, en casi un orden de magnitud la resistencia de polarización del electrodo de referencia (La2NiO4+d). De la misma manera, se optimizaron las propiedades del electrodo basado en el conductor electrónico puro La0.8Sr0.2MnO3-d (LSM). La adición de una segunda fase, con conductividad iónica, permitió aumentar los puntos triples (TPB) en los que la reacción de reducción de oxígeno tiene lugar y reducir la resistencia de polarización. Con el fin de mejorar la reacción de reducción de oxígeno, se estudió la adición de nanocatalizadores mediante la técnica de infiltración. Los diferentes óxidos infiltrados produjeron el cambio de las propiedades electroquímicas del electrodo, siendo el óxido de praseodimio el catalizador que consiguió disminuir en dos órdenes de magnitud la resistencia de polarización del composite no infiltrado. De la misma manera, la mejora de la eficiencia del electrodo infiltrado con Pr, mejoró los resultados de la celda electroquímica trabajando como pila (mayores densidades de potencia) y como electrolizador (menores voltajes). En lo que respecta a los materiales seleccionados para su uso como electrodos en electrolitos con conductividad protónica, se optimizó la eficiencia del cátodo basado en LSM, mediante el uso de una segunda fase conductora protónica (La5.5WO12-d) y variando la temperatura de sinterización del electrodo. Finalmente, se mejoró la actividad catalítica mediante la infiltración de nanopartículas de ceria dopada con samario, produciendo mayores densidades de corriente de la pila de combustible. Los materiales pertenecientes a la serie de Ruddlesden-Popper y usados para cátodos en pilas iónicas, fueron empleados también para cátodos en pilas protónicas. Después de comprobar que el material electrolítico (LWO) era compatible con los compuestos de la serie de Ruddlesden-Popper, se estudió la influencia de la temperatura de sinterización de los electrodos en el rendimiento, así como de la composición de la atmosfera de aire (seca, H2O y D2O). Finalmente, se diseñó y optimizó las celdas electroquímicas basadas en sales ácidas (CsH2PO4). En este sentido, se estudiaron diferentes configuraciones de celda, que permitieran obtener un electrolito denso con el menor espesor posible y unos electrodos activos a la reacción de reducción/oxidación de hidrógeno. Se consiguió reducir el espesor del electrolito soportando la celda en discos de acero y níquel porosos. Se añadió una resina tipo epoxi al material electrolítico para aumentar sus propiedades mecánicas. De la misma manera, se cambió la configuración de los electrodos pasando por conductores electrónicos puros a electrodos compuestos por conductores
En la present tesis doctoral es van desenvolupar materials per al seu ús en cel·les electroquímiques. Les cel·les electroquímiques estudiades poden ser dividides en dos grans grups: materials d'òxid sòlid i sals àcides. En el primer grup, es van optimitzar materials per al seu ús com a elèctrodes en piles de combustible i electrolitzadors, basats en electròlits amb conducció purament iònica. Dins d'este grup, es va comprovar la influència de dopar la perovskita Ba0.5Sr0.5Co0.8Fe0.2O3-d amb un 3% de Y, Zr i Sc en la posició B (ABO3-d;). Esta optimització va portar a la reducció de la resistència de polarització així com a una millora de l'estabilitat amb el temps. Així mateix, es van determinar els mecanismes limitants en la reacció de reducció d'oxigen, i es va comprovar la influència de la presència de CO2 en condicions d'operació. El La2NiO4+d pertanyent a la sèrie de Ruddlesden-Popper, és un conductor mixt d'ions oxigen i electrons. Este, va ser dopat tant en la posició del La (amb Nd i Pr) com en la posició del Ni (amb Co). Els dopants introduïts a més de produir canvis estructurals, van provocar millores en el rendiment de la cel·la, reduint per a algun d'ells, com el La1.5Pr0.5Ni0.8Co0.2O4+d, en quasi un ordre de magnitud la resistència de polarització de l'elèctrode de referència (La2NiO4+d). De la mateixa manera, es van optimitzar les propietats de l'elèctrode basat en el conductor electrònic pur La0.8Sr0.2MnO3-d (LSM). L'addició d'una segona fase, amb conductivitat iònica, va permetre augmentar els punts triples (TPB), en els que la reacció de reducció d'oxigen té lloc, i reduir la resistència de polarització. A fi de millorar la reacció de reducció d'oxigen, es va estudiar l'adició de nanocatalitzadors per mitjà de la tècnica d'infiltració. Els diferents òxids infiltrats van produir el canvi de les propietats electroquímiques de l'elèctrode, sent l'òxid de praseodimi el catalitzador que va aconseguir disminuir en dos ordres de magnitud la resistència de polarització del composite no infiltrat. De la mateixa manera, la millora de l'eficiència de l'elèctrode infiltrat amb Pr, va millorar els resultats de la cel·la electroquímica treballant com a pila (majors densitats de potència) i com a electrolitzador (menors voltatges). Pel que fa als materials seleccionats per al seu ús com a elèctrodes en electròlits amb conductivitat protònica, es va optimitzar l'eficiència del càtode basat en LSM, per mitjà de l'ús d'una segona fase conductora protònica (La5.5WO12-d;) i variant la temperatura de sinterització de l'elèctrode. Finalment, es va millorar l'activitat catalítica mitjançant la infiltració de nanopartícules de ceria dopada amb samari, produint majors densitats de corrent de la pila de combustible. Els materials pertanyents a la sèrie de Ruddlesden-Popper i usats per a càtodes en piles iòniques, van ser empleats també per a càtodes en piles protòniques. Després de comprovar que el material electrolític (LWO) era compatible amb els compostos de la sèrie de Ruddlesden-Popper, es va estudiar la influència de la temperatura de sinterització dels elèctrodes en el rendiment, així com de la composició de l'atmosfera d'aire (seca, H2O i D2O). Finalment, es van dissenyar i optimitzar les cel·les electroquímiques basades en sals àcides (CsH2PO4). En este sentit, es van estudiar diferents configuracions de cel·la, que permeteren obtindre un electròlit dens amb el menor espessor possible i uns elèctrodes actius a la reacció de reducció/oxidació d'hidrogen. Es va aconseguir reduir l'espessor de l'electròlit suportant la cel·la en discos d'acer i níquel porosos. Es va afegir una resina tipus epoxi al material electrolític per a augmentar les seues propietats mecàniques. De la mateixa manera, es va canviar la configuració dels elèctrodes passant per conductors electrònics purs a elèctrodes compostos per conductors protònics
Navarrete Algaba, L. (2017). New electrochemical cells for energy conversion and storage [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/78458
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