| 1. |
- Grimler, Henrik
(författare)
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Limiting processes in anion-exchange membrane fuel cells
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fuel cells allow for converting chemical energy stored in hydrogen into electrical energy, with only heat and water as by-products. In a sustainable energy society, hydrogen may play an important role due to its ability to act both as an energy carrier and as a valuable chemical in the process industry. The main remaining obstacles for widely available commercial fuel cells are durability and cost. One way to potentially decrease the cost is to change the fuel cell environment to an alternative chemistry by replacing the proton-exchange membrane (PEM) with an anion-exchange membrane (AEM). This thesis studies the anode reaction, the cathode reaction and water transport in an anion-exchange membrane fuel cell (AEMFC), to investigate where its performance limitations lies in the system. Electrochemical characterisation techniques together with physics-based modelling have been utilised.The results from the study of the anode, shows that the hydrogen reaction proceeds through the Tafel-Volmer pathway, with the Tafel step starting to limit the reaction as the anode overpotential increases. Combining the anode model with a Butler-Volmer expression for the cathode reaction made it possible to model a H2:O2 fuel cell. Comparing the losses from the different processes in the fuel cell shows that the cathode is still the main contributor, but that the anode contribution cannot be neglected when predicting the fuel cell performance. Low ionic conductivity in the electrode was also identified as responsible for part of the overall resistances, leading to uneven current distribution in the catalyst layers and bad utilisation of the catalytic material.Investigating the water transport properties of AEMs showed that not only electroosmotic drag and diffusion, but also an absorption/desorption step between gas phase and membrane phase, are necessary to get a model that can explain the experimental observations. The choice of gas diffusion layers (GDLs) used on the anode and cathode was found to be of similar importance on the water transport as doubling the membrane thickness, showing that not only the membrane is important for water transport. Under most realistic conditions, the risk of local dry-out in a cell was found to be low, as water readily diffuses from the high humidity side of the membrane to the low humidity side.
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| 2. |
- Novalin, Timon
(författare)
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Electrochemical characterization of materials for next generation polymer electrolyte fuel cells
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymer electrolyte fuel cells occupy a key position in implementing the hydrogen economy on a global scale. However, assessments of cost, availability and sustainability of materials currently used to manufacture state-of-the-art fuel cell stacks have given cause for concern. Platinum and platinum-group metals are prohibitively expensive and of low abundance. The benchmark ion-conductive polymer NafionTM and related perfluorosulfonic acid-based polymers are difficult to synthesize and environmentally persistent to extreme degrees. Graphitic and carbon composite bipolar plates are unsuitable for mass production and have low recycling potential. In the compiled works, alternative materials were evaluated both for the acidic and alkaline variations of polymer electrolyte fuel cells. Electrochemical characterization was carried out in single cell tests with a focus on finding the limitations in terms of ohmic, charge transfer and transport resistances in the cell, through polarization and impedance measurements.Carbon coated stainless steel bipolar plates were tested operando in a proton exchange membrane fuel cell (PEMFC) under realistic conditions based on the New European Drive Cycle. Observed trace metal contamination of the MEA was linked to metal dissolution from coating defects caused by manufacturing (Paper I). A theoretical understanding of observed metal dissolution was confirmed experimentally and a concept for preventing metal dissolution was developed for PEMFC bipolar plates (Paper II). The developed concept was extended to uncoated stainless steel bipolar plates and tested successfully for three stainless steel types in operando PEMFC (Paper III)Anion exchange polymers based on poly(arylene piperidinium) (PAP) were tested as both membranes and ionomers in a comparative study with a commercial reference material, showing higher performance and the significance of ionomer-carbon support interactions (Paper IV). PAP-based ionomers with varying ion exchange capacities were studied to optimize electrodes in anion exchange membrane fuel cells (AEMFC). A combination of high ion exchange capacity ionomer on both cathode and anode was best performing, linked to small water transport resistance in cathode and increased kinetic contribution of the anode HOR (Paper V). The effects of modifying the catalyst layer through the introduction of crosslinked PAP particles were studied in operando AEMFCs. A positive impact on charge transfer and diffusion resistances in electrodes containing particles could be observed. (Paper VI).Silver nanoparticles were used as catalyst material in the cathode of previously optimized membrane electrode assemblies in AEMFC. The results showed promising performance compared to platinum electrodes based on monetary and sustainability considerations, but also challenges regarding catalyst stability and detrimental silver-ionomer interactions. (Paper VII).
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| 3. |
- Yang, Guomin
(författare)
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Interfacial properties of calcium montmorillonite in aqueous solutions : Density functional theory and classical molecular dynamics studies on the electric double layer
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The swelling properties of Bentonite are highly affected by clay content and the clay-water interactions that arise from the ion distribution in the diffuse double layer formed near the charged montmorillonite (or smectite) surfaces. Existing continuum models describing the electric double layers, such as classical Poisson-Boltzmann and DLVO theory, ignore the ion-ion correlations, which are especially important for multivalent ions at high surface charge and ionic strength. To better understand the clay-water interactions, atomistic models were developed using both density functional theory of fluids (DFT) as well as classical molecular dynamics (MD) methods. In order to increase our understanding of water-saturated, swelling smectite clays, a DFT, technique was initially developed that allowed more accurate predictions of important thermodynamic properties of the diffuse double layers. This DFT approach was then extended to handle systems with mixtures of different sizes and charges. The extended DFT model was verified against experiments and Monte-Carlo simulations. One practical application was to predict the ion exchange equilibria in Bentonite clays, which have wide practical usage in different areas. Nevertheless, in the DFT work it was realized that DFT demands that the particles, ions in this case, which are described as hard spheres, realistically cannot be described as such at low water loadings, when ion specific hydration forces govern the electric double layer properties. To study how the deformation of the hydration shells of Ca2+ influences the properties of compacted smectite clays, MD simulations using the CLAYFF forcefield were employed in order to account for the deformation of the hydration shells. Comparisons of DFT and MD modeling then allowed to demonstrate under which conditions DFT modeling becomes increasingly inaccurate and when it still can give accurate results.
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| 4. |
- Yucel, Yasemin Duygu
(författare)
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LiFePO4-coated carbon fiber electrodes for structural batteries
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Lithium-ion batteries (LIBs) have a central role in products, from portable devices to large-scale energy storage for the electric grid and continue to undergo rapid development. The surge in electric vehicles has intensified the focus on technological advancements and new-generation technologies. Structural batteries have received considerable attention for their multifunctionality and lightweight properties. These batteries utilize carbon fibers to combine their mechanical strength with battery functionalities in a single structure, consequently reducing overall weight and increasing energy density. Similar to traditional LIBs, structural batteries comprise negative and positive electrodes, reinforced within a structural battery electrolyte (SBE). While extensive research has been conducted on carbon fibers as negative electrodes, there has been a relative scarcity in the development of positive electrodes that align with the structural battery concept. This thesis explores coating methodologies on polyacrylonitrile (PAN)-based carbon fibers (CF) with positive electrode active material, specifically focusing on the utilization of lithium iron phosphate (LFP). Electron microscopy and electrochemical tests were conducted to evaluate the relation between structure with long-term and rate performances of these electrodes in half-cells. Spray coating and siphon impregnation (later referred to as ‘powder impregnation’ in this thesis) techniques were employed to coat the carbon fibers, which serve as current collectors instead of conventional aluminum foil. The spray coating method utilized a standard electrode slurry based on an organic solvent, with efforts made to optimize parameters such as the height of the spray gun and plate temperature. The sprayed coating was quite thin, resulting in excellent rate capability. In the powder impregnation method, a water-based slurry was utilized with polyethylene glycol (PEG) as a binder. Efforts were made to obtain good fiber distribution within a homogeneous matrix of coating in the electrode. The parameters, including slurry viscosity, binder effect, electrode design, cell design, electrode preparation, and drying temperatures, were regulated for the best electrochemical performance and cell life. It was found that a binder is necessary for ensuring robust electrodes. Elevated drying temperatures are essential to eliminate moisture from the water-based process and components. Additionally, conductive carbon additives such as carbon black and graphene were incorporated, and their impact was assessed. A small amount of carbon additive (< 1 wt.%) improved performance at higher cycling rates. The electrodes produced via powder impregnation were finally integrated into double-sided full cells versus uncoated PAN-derived CFs serving as negative electrodes in commercial liquid electrolyte or SBE, respectively. The LFP-coated CF electrodes exhibited good performance in full cells, indicating promising performance for the structural battery. The main limitation was observed in the power losses in the CF negative electrodes and in the ionic conductivity of the SBE. Overall, the thesis shows that the encapsulation of individual PAN-derived carbon fiber filaments using the applied coating methodologies was successful and that the use of carbon fibers as current collectors proved to be effective.
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| 5. |
- Acevedo Gomez, Yasna
(författare)
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On Gas Contaminants, and Bipolar Plates in Proton Exchange Membrane Fuel Cells
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The proton exchange membrane (PEM) fuel cell is an electrochemical device that converts chemical energy into electrical energy through two electrocatalytic reactions. The most common catalyst used is platinum on carbon (Pt/C), which has shown the best performance in the fuel cell until now. However, the drawback of this catalyst is that it does not tolerate impurities, and both hydrogen and oxygen may carry small amounts of impurities depending on the production sources. The purpose of this thesis is to understand the effect of two impurities that are less investigated, i.e., ammonia, which may accompany the hydrogen rich reformates from renewable sources, and nitrogen dioxide, which may come from air pollution. The mechanism of contamination and an adequate recovery method for the respective contaminant are studied. Additionally, electroplated bipolar plates with Ni-Mo and Ni-Mo-P coatings were tested as alternatives to stainless steel and carbon materials.The results show that ammonia not only provokes changes in the polymer membrane but also in the oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR) and catalyst ionomer in both electrodes. The extent of performance recovery after the contamination depends on the concentration used and the exposure time. In contrast, nitrogen dioxide affects the catalyst in the electrode directly; the contamination is related to side reactions that are produced on the catalyst’s surface. However, NO2 is not attached strongly to the catalyst and it is possible to restore the performance by using clean air. The time the recovery process takes depends on the potential applied and the air flow.Finally, the evaluation of electroplated Ni-Mo and Ni-Mo-P on stainless steel by ex situ and in situ studies shows that these coatings reduce the internal contact resistance (ICR) and the corrosion rate of the stainless steel considerably. However, the in situ experiments show that phosphorus addition to the coating does not improve the fuel cell performance; thus, the Ni-Mo alloy is found to be a promising choice for electroplating stainless steel bipolar plates.
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| 6. |
- Eriksson, Björn
(författare)
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Electrochemical evaluation of new materials in polymer electrolyte fuel cells
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymer electrolyte fuel cells (PEFC) convert the chemical energy in hydrogen to electrical energy and heat, with the only exhaust being water. Fuel cells are considered key in achieving a sustainable energy sector. The main obstacles to wide scale commercialization are cost and durability. The aim of this thesis is to evaluate new materials for PEFC to potentially lower cost and increase durability. To lower the amount of expensive platinum catalyst in the fuel cell, the activities of Pt-rare earth metal (REM) alloy catalysts have been tested. To improve the lifetime of the carbon support, the carbon corrosion properties of multi walled carbon nanotubes have been evaluated. To reduce the overall cost of fuel cell stacks, carbon coated and metal coated bipolar plates have been tested. To increase the performance and lifetime of anion exchange membranes, the water transport has been studied.The results show that the Pt-REM catalysts had at least two times higher specific activity than pure platinum, and even higher activities should be obtainable if the surface structures are further refined.Multi-walled carbon nanotubes had lower carbon corrosion than conventional carbon Vulcan XC-72. However, once severely corroded their porous structure collapsed, causing major performance losses.The carbon coated metallic bipolar plates showed no significant increase of internal contact resistance (ICR) by cycling, suggesting that these coatings are stable in fuel cells. The NiMo- and NiMoP coated bipolar plates showed low ICR, however, presence of the coated bipolar plates caused secondary harmful effects on the polymer membrane and ionomer.Considering the water transport through anion exchange membranes it was found that most membranes showed very similar water transport properties, with more water detected at both the anode and cathode when a current was applied. The most significant factor governing the water transport properties was the membrane thickness, with thicker membranes reducing the backflow of water from anode to cathode.The results indicate that all of the new tested materials have the capability to improve the lifetime and reduce cost and thereby improve the overall performance of PEFC.
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| 7. |
- Jacques, Eric, 1985-
(författare)
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Lithium-intercalated Carbon Fibres : Towards the Realisation of Multifunctional Composite Energy Storage Materials
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Lightweight design is a major improvement path for sustainable transport asit contributes to lower vehicles energy consumption and gas emissions. Anovel solution to weight savings is to store energy directly in the mechanicalstructure of the vehicle with a multifunctional material, called structural battery,which could simultaneously bear mechanical loads and store electricalenergy. This is especially possible because the carbon fibre is a high performancemechanical reinforcement for polymer composites and can also be usedas a lithium-intercalating electrode in lithium-ion batteries. In this thesis, theperformance of carbon fibres for use as a lithium-intercalating structural electrodeis investigated.Electrochemical characterisation has shown that intermediate modulus polyacrylonitrile- based carbon fibres which have the highest strength also offerthe most promising electrochemical capacities when compared to other fibregrades with different microstructures. The measured capacity of fibre bundleswas highly dependent on the current rate and at low rate the capacitiesclose to that of graphite electrodes were measured. In a mechanical characterisationthe carbon fibre was not affected by the number of electrochemicalcycles, up to 1000 cycles, but rather by the amount of intercalated lithium.The tensile stiffness appeared to remain unchanged, but during lithation thetensile strength dropped and partly recovered during delithiation due to afirst-cycle irreversible drop. A longitudinal expansion of the carbon fibre wasalso measured during lithiation. An irreversible expansion in the delithiatedfibres highlighted that the first cycle-capacity loss is partly due to intercalatedlithium which is trapped in the carbon fibre. From these results, the carbonfibre is without doubts suitable for structural battery applications.A mechanical-electrochemical coupling in lithium-intercalated carbon fibreswas also measured, highlighting a piezo-electrochemical transducer effect resultingin new functionalities for lithium-intercalated carbon fibres. The longitudinalexpansion strain can be used for mechanical actuation. A responseof the cell open-circuit potential to an applied mechanical strain can be usedfor strain sensing.
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| 8. |
- Leijonmarck, Simon
(författare)
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Electrically Induced Debonding of Adhesives
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electrically induced adhesive debonding is a process where an adhesive can be debonded at command with help of an applied voltage. To make this process function, the adhesive is bonded between two metal substrates. In this study an epoxy adhesive is adhered between two aluminium foils forming a laminate structure. The adhesive is made ionically conductive by an addition of an ionic liquid before the curing. This arrangement forms an electrochemical cell, where the metal substrates act as the electrodes while the ionically conductive adhesive acts as the electrolyte. When a voltage is applied over the laminate, a current passes due to electrochemical reactions at the electrode interfaces and ionic transport in the adhesive. This type of material can potentially be used in a wide range of applications. This includes making adhesive joints in automotives to both reduce the total weight but also to simplify the disassembly after end-of-life, enabling an inexpensive recycling process. Another potenital use for debondable adhesives is within consumer packaging. Here it could be possible to pack and transport goods using less packaging material as well as making the handling easier. The aim of this study was to increase the understanding about the processes leading to debonding. This knowledge is important in the development of new types of debonding adhesives. In this study, the commercial laminate Sinuate® was used as a model system. The experiments were focused on the electrochemical behavior and were performed mainly using galvanostatic polarization and electrochemical impedance spectroscopy. Information about the chemistry of debonding was collected with techniques such as scanning electron microscopy (SEM), mass spectrometry (MS) and Raman spectroscopy. The debonding did always take place at the anodic interface, separating the adhesive and the anode aluminium foil. It was found that the total cell resistance increased drastically during polarization, and that essentially all of this increase originated within the anodic half of the laminate. Examining the resistance behavior with EIS, it was found that the increase in total resistance was reversible. The anodic electrochemical reaction during polarization was determined to consist mainly of an oxidation of aluminium, while the major reaction at the cathodic interface was reduction of water into hydrogen. The debonding process, which took place at the anodic interface, could be related to reaction products formed in the polarization process. These products grew out from the anodic aluminium surface into the adhesive. A debonding mechanism is proposed where these products induce an increase in the adhesive volume, causing stresses at the interface which ultimately result in debonding.
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| 9. |
- Oyarce, Alejandro, 1977-
(författare)
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Electrode degradation in proton exchange membrane fuel cells
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The topic of this thesis is the degradation of fuel cell electrodes in proton exchange membrane fuel cells (PEMFCs). In particular, the degradation associated with localized fuel starvation, which is often encountered during start-ups and shut-downs (SUs/SDs) of PEMFCs. At SU/SD, O2 and H2 usually coexist in the anode compartment. This situation forces the opposite electrode, i.e. the cathode, to very high potentials, resulting in the corrosion of the carbon supporting the catalyst, referred to as carbon corrosion. The aim of this thesis has been to develop methods, materials and strategies to address the issues associated to carbon corrosion in PEMFC.The extent of catalyst degradation is commonly evaluated determining the electrochemically active surface area (ECSA) of fuel cell electrode. Therefore, it was considered important to study the effect of RH, temperature and type of accelerated degradation test (ADT) on the ECSA. Low RH decreases the ECSA of the electrode, attributed to re-structuring the ionomer and loss of contact with the catalyst.In the search for more durable supports, we evaluated different accelerated degradation tests (ADTs) for carbon corrosion. Potentiostatic holds at 1.2 V vs. RHE were found to be too mild. Potentiostatic holds at 1.4 V vs. RHE were found to induce a large degree of reversibility, also attributed to ionomer re-structuring. Triangle-wave potential cycling was found to irreversibly degrade the electrode within a reasonable amount of time, closely simulating SU/SD conditions.Corrosion of carbon-based supports not only degrades the catalyst by lowering the ECSA, but also has a profound effect on the electrode morphology. Decreased electrode porosity, increased agglomerate size and ionomer enrichment all contribute to the degradation of the mass-transport properties of the cathode. Graphitized carbon fibers were found to be 5 times more corrosion resistant than conventional carbons, primarily attributed to their lower surface area. Furthermore, fibers were found to better maintain the integrity of the electrode morphology, generally showing less degradation of the mass-transport losses. Different system strategies for shut-down were evaluated. Not doing anything to the fuel cell during shut-downs is detrimental for the fuel cell. O2 consumption with a load and H2 purge of the cathode were found to give around 100 times lower degradation rates compared to not doing anything and almost 10 times lower degradation rate than a simple air purge of the anode. Finally, in-situ measurements of contact resistance showed that the contact resistance between GDL and BPP is highly dynamic and changes with operating conditions.
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| 10. |
- Peuvot, Kevin, 1992-
(författare)
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Lignin- and PAN-based carbon fibres as negative electrodes for alkali-ion batteries
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The development of sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) have accelerated since they can now reach similar gravimetric energy densities as lithium-ion batteries (LIBs) but with a lower environmental impact. Hard carbon is the most common negative electrode for SIBs and KIBs and can be made from renewable resources such as lignin. Lignin can be then manufactured into fibres which can then be used as free-standing electrodes to push even further the sustainability by reducing the amount of current collector and additives needed in the battery. The concept of structural batteries is defined as a system that can simultaneously carry mechanical load as well as store the electrical energy in form of a battery to decrease the total weight. Polyacrylonitrile-based (PAN-based) carbon fibres are some of the most adapted materials thanks to their outstanding mechanical properties as well as their ability to be used as negative electrode for LIBs. However, a structural model and insertion model for alkali-ion insertion in the PAN-based carbon fibres is still lacking and is necessary to be able to understand the dynamics and fundamentals. This thesis focuses on the use of lignin-based carbon fibres (LCFs) and PAN-based carbon fibres as negative electrodes. The potential of using LCFs as negative electrode for SIBs and KIBs is evaluated by using a combination of electrochemical techniques and material characterization methods. The LCFs have high specific capacity and high initial coulombic efficiency when used as negative electrode for SIBs. The diffusion of potassium-ions into the LCFs is investigated by implementing a numerical model. The investigation on the open circuit voltage curves and the entropy change for potassium-ion insertion suggests that the LCFs structure contains two domains which can explain why the numerical model cannot fully fit the experimental data. The PAN-based carbon fibres are investigated as negative electrode for LIBs and SIBs. For SIBs, the axial expansion is investigated during charge/discharge and shows a staged expansion between the slope region and the plateau region of the charge/discharge profile. For LIBs, a combination of ex-situ Li-NMR and ex-situ wide-angle X-ray scattering isused to determine the insertion mechanism and structure of the PAN-based carbon fibres. A structural model and insertion model for lithium-ions is suggested from our experimental results consisting of three different types of sites: disordered domain in the carbon structure, ordereddomain in the carbon structure, and pore filling.
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