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Tese de doutoramento. Engenharia Química e Biológica. Universidade do Porto. Faculdade de Engenharia. 2011

With the rapid development of portable electronics, hybrid-electric and electric cars, there is great interest in utilization of sulfur as cathodes for rechargeable lithium batteries. Lithium/sulfur batteries implement inexpensive, the earth-abundant elements at the cathode while offering up to a five-fold increase in energy density compared with the present Li-ion batteries. However, electrically insulating character of sulfur and solubility of intermediate polysulfides in organic liquid electrolytes, which causes rapid capacity loss upon repeated cycling, restrict the practical application of Li/S batteries. In this thesis, the gel polymer and solid polymer electrolytes were synthesized and applied in Li/S batteries. A gel polymer electrolyte (GPE) was formed by trapping 1 M lithium bistrifluoromethane-sulfonamide (LiTFSI) in tetraethylene glycol dimethyl ether (TEGDME) electrolyte in a poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) /poly(methylmethacrylate) (PMMA) polymer matrix. The electrochemical properties of the resulting GPE were investigated in lithium/sulfur battery. The gel polymer battery exhibited a high specific capacity of 753.8 mAh gˉ¹ at the initial cycle, stable reversible cycling and a capacity retention about 80% over 40 cycles along with a high Coulombic efficiency. Comparative studies conducted with the 1 M LiTFSI liquid electrolyte cell demonstrated that a cell with liquid electrolyte has remarkably low capacity retention and Coulombic efficiency compared with the GPE cell. In the further studies, a solid polymer electrolyte (SPE) based on poly- (ethylene-oxide)/nanoclay composite was prepared and used to assemble an all-solid-state lithium/sulfur battery. The ionic conductivity of the optimized electrolyte has achieved about 3.22×10ˉ¹ mS cmˉ¹ at 60 °C. The Li/S cell with this SPE delivered an initial discharge capacity of 998 mAh gˉ¹ when operated at 60 °C, and retained a reversible capacity of 634 mAh gˉ¹ after 100 cycles. These studies has revealed that the electrochemical performance of lithium/sulfur cells, including charge-discharge cyclability and Coulombic efficiency, can be significantly improved by replacing liquid electrolytes with solid polymer and gel polymer electrolytes, which reduce the polysulfide shuttle effect and could protect the lithium anode from the deposition of the electrochemical reaction, leading to higher sulfur utilization in the cell.

The driving force behind research in alternative clean and renewable energy has been the desire to reduce emissions and dependence on fossil fuels. In the United States, ground vehicles account for 30% of total carbon emission, and significantly contribute to other harmful emissions. This issue causes environmental concerns and threat to human health. On the other hand, the demand on fossil fuel grows with the increasing energy consumption worldwide. Particularly in the United States of America, transportation absorbs 75% of this energy source. There is an urgent need to reduce the transportation dependence on fossil fuel for the purpose of national security. Polymer electrolyte membrane (PEM) fuel cells are strong potential candidates to replace the traditional combustion engines. Even though research effort has transferred the fuel cell technology into real‒world vehicle applications, there are still several challenges hindering the fuel cell technology commercialization, such as hydrogen supply infrastructure, cost of the fuel cell vehicles, on‒board hydrogen storage, public acceptance, and more importantly the performance, durability, and reliability of the PEM fuel cell vehicles themselves. One of the key factors that affect the fuel cell performance and life is the run‒time thermal and water management. The temperature directly affects the humidification of the fuel cell stack and plays a critical role in avoiding liquid water flooding as well as membrane dehydration which affect the performance and long term reliability. There are many models exists in the literature. However, there are still lacks of control‒oriented modeling techniques that describe the coupled heat and mass transfer dynamics, and experimental validation is rarely performed for these models. In order to establish an in‒depth understanding and enable control design to achieve optimal performance in real‒time, this research has explored modeling techniques to describe the coupled heat and mass transfer dynamics inside a PEM fuel cell. This dissertation is to report our findings on modeling the temperature dynamics of the gas and liquid flow in the porous media for the purpose of control development. The developed thermal model captures the temperature dynamics without using much computation power commonly found in CFD models. The model results agree very well with the experimental validation of a 1.5 kW fuel cell stack after calibrations. Relative gain array (RGA) was performed to investigate the coupling between inputs and outputs and to explore the possibility of using a single‒input single‒output (SISO) control scheme for this multi‒input multi‒output (MIMO) system. The RGA analyses showed that SISO control design would be effective for controlling the fuel cell stack alone. Adding auxiliary components to the fuel cell stack, such as compressor to supply the pressurized air, requires a MIMO control framework. The developed model of describing water transport in porous media improves the modeling accuracy by adding catalyst layers and utilizing an empirically derived capillary pressure model. Comparing with other control‒oriented models in the literature, the developed model improves accuracy and provides more insights of the liquid water transport during transient response. / text

With the rapid development of portable electronics, hybrid-electric and electric cars, there is great interest in utilization of sulfur as cathodes for rechargeable lithium batteries. Lithium/sulfur batteries implement inexpensive, the earth-abundant elements at the cathode while offering up to a five-fold increase in energy density compared with the present Li-ion batteries. However, electrically insulating character of sulfur and solubility of intermediate polysulfides in organic liquid electrolytes, which causes rapid capacity loss upon repeated cycling, restrict the practical application of Li/S batteries. In this thesis, the gel polymer and solid polymer electrolytes were synthesized and applied in Li/S batteries. A gel polymer electrolyte (GPE) was formed by trapping 1 M lithium bistrifluoromethane-sulfonamide (LiTFSI) in tetraethylene glycol dimethyl ether (TEGDME) electrolyte in a poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) /poly(methylmethacrylate) (PMMA) polymer matrix. The electrochemical properties of the resulting GPE were investigated in lithium/sulfur battery. The gel polymer battery exhibited a high specific capacity of 753.8 mAh gˉ¹ at the initial cycle, stable reversible cycling and a capacity retention about 80% over 40 cycles along with a high Coulombic efficiency. Comparative studies conducted with the 1 M LiTFSI liquid electrolyte cell demonstrated that a cell with liquid electrolyte has remarkably low capacity retention and Coulombic efficiency compared with the GPE cell. In the further studies, a solid polymer electrolyte (SPE) based on poly- (ethylene-oxide)/nanoclay composite was prepared and used to assemble an all-solid-state lithium/sulfur battery. The ionic conductivity of the optimized electrolyte has achieved about 3.22×10ˉ¹ mS cmˉ¹ at 60 °C. The Li/S cell with this SPE delivered an initial discharge capacity of 998 mAh gˉ¹ when operated at 60 °C, and retained a reversible capacity of 634 mAh gˉ¹ after 100 cycles. These studies has revealed that the electrochemical performance of lithium/sulfur cells, including charge-discharge cyclability and Coulombic efficiency, can be significantly improved by replacing liquid electrolytes with solid polymer and gel polymer electrolytes, which reduce the polysulfide shuttle effect and could protect the lithium anode from the deposition of the electrochemical reaction, leading to higher sulfur utilization in the cell.

>Magister Scientiae - MSc / The objective of the thesis is to develop highly efficient catalysts for solid polymer electrolyte (SPE) electrolyser anodes.The anode is the primary cause of the large overpotential of SPE electrolysers and also adds significantly to the cost of the electrolysers. Currently, unsupported IrO2 is a widely used anode catalyst as it exhibits the best stability during the oxygen evolution reaction. The activity of IrO2 needs to be improved significantly to address the high cost and efficiency issues of the SPE electrolyser. Developments aimed at improving the activity of unsupported IrO2 are however limited due to the limitations of the wellknown supports under the operating conditions of electrolysers, leading to their oxidation.In this study binary metal oxides based on IrO2 were developed and optimized as anode catalysts for the SPE electrolyser and compared to the ‘state-of-art’ commercial IrO2 catalyst. The Adams fusion method was adapted and used to synthesize the catalysts.The activities of the catalysts were determined using half-cell studies. Optimum conditions for the preparation of unsupported IrO2 catalysts were found to be 350 oC and 2 hours. The resulting catalysts had twice the activity of the ‘state-of-art’ commercial IrO2 catalyst. Secondary metals were carefully selected, after carrying out both a literature study and an experimental study. Binary metal oxides were then developed using the optimum synthesis conditions. Four binary metal oxides were studied to identify the best/most efficient catalyst for electrolysis. The catalysts were characterized using XRD, TEM, SEM and EDS analyses, in efforts to understand and correlate the activity of the catalysts to its physical properties and obtain information that could be useful for the further development of efficient catalysts.Although all the binary metal oxides studied showed improved activity compared to IrO2, the catalytic activity of Ir0.7Ru0.3O2 was found to be significantly better than the commercial catalyst: it was over 5 times more active than the ‘state-of-art’ commercial IrO2 catalyst. Ir-Pd mixed oxides also proved to be highly efficient as anode catalysts for SPE electrolysers.

Foram realizadas pesquisas em uma série de condutores iônicos que apresentam aplicações na área dos dispositivos eletroquímicos de estado sólido, utilizando-se basicamente a técnica de Ressonância Magnética Nuclear (RMN). A primeira parte deste trabalho é dedicada aos compostos de intercalação baseados na matriz de dissulfeto de molibdênio (MoS2) onde as espécies intercalantes (íons de lítio e moléculas de aminas) são inseridas num espaço de dimensionalidade reduzida gerado pela matriz. Já a segunda parte envolve os condutores iônicos poliméricos do tipo compósitos, nos quais foram utilizadas nanopartículas de carbono (Carbon Black) e titânio (TiO2) no eletrólito formado pelo poli(óxido de etileno) (POE) e o perclorato de lítio (LiClO4). Todos estes sistemas apresentam, em geral, uma considerável complexidade estrutural, o que significa que os movimentos moleculares e de difusão iônica se produzem num meio semicristalino (caso dos compósitos) ou num meio de dimensionalidade reduzida (caso dos intercalados). A espectroscopia de RMN dos núcleos de 7Li e 1H é uma técnica conveniente para o estudo destes materiais, pois é possível avaliar, através dos resultados obtidos das medidas dos tempos de relaxação e formas de linha, os efeitos provocados pela baixa dimensionalidade dos movimentos em estruturas laminares (caso dos intercalados), bem como identificar e aferir as interações e os mecanismos de relaxação resultantes dos diferentes graus de liberdade dos movimentos (iônicos e moleculares), fornecer parâmetros estruturais (distâncias interatômicas) que auxiliam na proposta de possíveis modelos estruturais e caracterizar completamente a escala temporal dos movimentos iônicos e moleculares. / Nuclear Magnetic Resonance (RMN) techniques were used to study a series of ionic conductor materials, which present applications in the area of the solid-state electrochemical devices. The first part of this work is dedicated to the study of intercalation compounds based on the molybdenum disulfide matrix (MoS2), where the intercalated species (lithium ion and amine molecule) are inserted in the low-dimensionality space generated by the matrix. The second part involves the study of a composite polymer electrolyte, employing fillers like Carbon Black and titanium dioxide (Tio2) nano particles in the electrolyte formed by the poly(ethy1ene oxide) and a lithium salt (LiClO4). In general, these systems present a considerable structural complexity, meaning that the molecular movements and ionic diffusion are produced in a semicrystalline environment (case of the composites) or in an environment of reduced dimensionality (case of intercalates). The 7Li and 1H NMR spectroscopy is a convenient technique for the study of these materials. Relaxation time and line shape measurements may provide a tool to investigate the effects provoked by the low-dimensionality of the movements in laminate structures (case of intercalate), to identify the interactions and relaxation mechanisms of the ionic and molecular motions, to supply structural parameters (interatomic distances) that would help the proposal of possible structural models, and finally, to characterize the time scale of the ionic and molecular movements completely.

Tese de doutoramento. Engenharia Química e Biológica. Faculdade de Engenharia. Universidade do Porto. 2009

Supercapacitors are one of the most important electrochemical energy storage and conversion devices, however low ionic conductivity of solid state polymer electrolytes and the poor accessibility of the ions to the active sites in the porous electrode will cause low performance for all-solid-state supercapacitors and will limit their application. The objective of the dissertation is to improve the performance of all-solid-state supercapactor by improving electrolyte conductivity and solving accessibility problem of the ions to the active sites. The low ionic conductivity (10-8 S/cm) of poly(ethylene oxide) (PEO) limits its application as an electrolyte. Since PEO is a semicrystal polymer and the ion conduction take place mainly in the amorphous regions of the PEO/Lithium salt complex, improvements in the percentage of amorphous phase in PEO or increasing the charge carrier concentration and mobility could increase the ionic conductivity of PEO electrolyte. Hot pressing along with the additions of different lithium salts, inorganic fillers and plasticizers were applied to improve the ionic conductivity of PEO polymer electrolytes. Four electrode methods were used to evaluate the conductivity of PEO based polymer electrolytes. Results show that adding certain lithium salts, inorganic fillers, and plasticizers could improve the ionic conductivity of PEO electrolytes up 10-4 S/cm. Further hot pressing treatment could improve the ionic conductivity of PEO electrolytes up to 10-3 S/cm. The conductivity improvement after hot pressing treatment is elucidated as that the spherulite crystal phase is convert into the fringed micelle crystal phase or the amorphous phase of PEO electrolytes. PEO electrolytes were added into active carbon as a binder and an ion conductor, so as to provide electrodes with not only ion conduction, but also the accessibility of ion to the active sites of electrodes. The NaI/I2 mediator was added to improve the conductivity of PEO electrolyte and provide pseudocapacitance for all-solid-state supercapacitors. Impedance, cyclic voltammetry, and gavalnostatic charge/discharge measurements were conducted to evaluate the electrochemical performance of PEO polymer electrolytes based all-solid-state supercapacitors. Results demonstrate that the conductivity of PEO electrolyte could be improved to 0.1 S/cm with a mediator concentration of 50wt%. A high conductivity in the PEO electrolyte with mediator is an indication of a high electron exchange rate between the mediator and mediator. The high electron exchange rates at mediator carbon interface and between mediator and mediator are essential in order to obtain a high response rate and high power. This automatically solves the accessibility problem. With the addition of NaI/I2 mediator, the specific capacitance increased more than 30 folds, specific power increased almost 20 folds, and specific energy increased around 10 folds. Further addition of filler to the electrodes along with the mediator could double the specific capacitor and specific power of the all-solid-state supercapacitor. The stability of the corresponded supercapacitor is good within 2000 cycles.

The development of improved water management strategies for the polymer electrolyte membrane fuel cell (PEMFC) could stand to benefit from an improved understanding of the surface and internal structure of the gas diffusion layer (GDL). The GDL is a fibrous porous material enabling mass transport between the PEMFC catalyst layer and flow fields. Fluorescence-based visualizations of liquid water droplet evaporation on GDL surfaces were performed to investigate water droplet pinning behaviours. The heterogeneous in-plane and through-plane porosity distributions of untreated GDLs were studied using computed tomography visualizations. The through-plane porosity distributions were utilized to calculate heterogeneous local tortuosity, relative diffusivity, and permeability distributions. Finally, the heterogeneous through-plane porosity distributions of GDLs treated for increased hydrophobicity were investigated. This work provides new insight into GDL material properties to better inform future PEMFC models.