In this study, deposition of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) oxide films on Gd-doped ceria (CGO) substrates by an electrostatic assisted ultrasonic spray pyrolysis (EAUSP) method was demonstrated for the first time. The electrostatic field employed for directing the aerosol stream towards the substrate was shown to be indispensable for film deposition. The XRD result indicates that a single phase of cubic perovskite was obtained in the calcined films. SEM examination reveals that the electric field strength had a profound effect on film porosity with weaker field resulting in higher porosity. The results of impedance measurement on LSCF//CGO//LSCF cells indicate that the area specific resistance (ASR) values of current LSCF films and their activation energies are comparable to that obtained by conventional sample preparation routes. In view of the simplicity, efficiency and economy of film deposition and the sound electrochemical characteristics of the obtained films manifested in current work, it is concluded that EAUSP method is a promising method for preparation of SOFC electrode films. Besides the EAUSP method, electrostatic spray deposition (ESD) method was also employed to deposit LSCF films. The growth mechanism of LSCF films deposited on silicon wafer was studied by examining a series of films obtained with increasing deposition durations. The results show that the film formation mechanism in the initial stage depends on the deposition temperature, and films with a unique porous structure were obtained when a deposition temperature lower than the boiling point of the precursor solution was used. Deposition parameters were also varied systematically to deposit LSCF cathode films on CGO substrates to obtain symmetrical cells. The microstructure and morphology of obtained films were investigated by X-ray diffraction and SEM, and the area specific resistances of the symmetrical cells measured by electrochemical impedance spectroscopy (EIS). The minimum interfacial ASR value associated with the LSCF cathodes was 0.25 ohm¡Ecm2 at700 ¢XC. NiO-SDC (Sm0.2Ce0.8O1.9)/SDC/LSCF (La0.6Sr0.4Co0.2Fe0.8O3-£_) cells with either single layer or double layer cathode were also fabricated and tested. The single layer LSCF cathode was made by stencil printing while the double layer one was prepared by depositing a thin porous layer on the SDC electrolyte by ESD before stencil printing LSCF. The maximum power density increased from 1.04 to 1.18 Wcm-2 at 700¢XC when the LSCF inter-layer was introduced. The results showed that the ASRs of the cells reduced to half with the addition of the LSCF inter-layer.

Solid Oxide Fuel Cells could play a key role in energy systems of the future because they can directly convert the chemical energy of fuels into electrical energy in a reliable and energy efficient way. The choice of materials for the components of fuel cells is crucial for the achievement of the high performance and the low price necessary to establish fuel cell technology in the energy market. Current state of the art anodes consisting of nickel and yttria stabilised zirconia (Ni/YSZ) offer good electrochemical performance but suffer from limitations like carbon deposition, redox instability and sulphur poisoning. This thesis explores the properties of composite fully ceramic anodes consisting of a skeleton of yttria stabilised zirconia (YSZ) or cerium gadolinium oxide (CGO) and a perovskite phase based on B-site doped lanthanum strontium titanate. The perovskite phase was fabricated in situ inside the pores of the skeleton material by the infiltration of an aqueous precursor and subsequent firing (impregnation method). Material characterisation of the composite anodes was carried out by X-ray diffraction and the microstructure investigated by electron microscope techniques. The electrochemical performance was tested by IV-curves and impedance spectroscopy. Particularly the investigation of the connection between the microstructure of the impregnated anodes and their electrochemical performance is a main objective of this work. The electrochemical performance of cells with a CGO skeleton and an impregnated lanthanum strontium titanate phase was found to be inferior compared to cells with a YSZ skeleton, even if the ionic conductivity of CGO is known to be higher than the ionic conductivity of YSZ. The difference was assigned to mass transport problems tightly connected to the different microstructure of the composite anodes. A significant improvement of the performance could be achieved by the utilisation of A-site deficient perovskites as impregnated phase in a YSZ skeleton. Cells with composite anodes of YSZ and La₀.₄Sr₀.₄Ti₀.₉₄Mn₀.₀₆O[sub](3-δ) show power densities of 156.2 mW/cm² at a measuring temperature of 750 °C compared to 58.5 mW/cm² measured in a similar cell with A-site stoichiometric LSTM, both cells having an electrolyte thickness of around 60 μm. The superiority of the performance of anodes with A-site deficient perovskites is mainly due to a lower ohmic resistance of only 0.5 Ω*cm², indicating better conductivity of the composite with A-site deficient perovskites. The investigation of the microstructure of composite anodes with A-site deficient perovskites showed the decoration of the surface with nanoparticles after reduction. These nanoparticles originate from exsolution of ions from the B-site of the perovskite and can't be found in A-site stoichiometric perovskites. The influence of fabrication parameters like firing temperature of the skeleton, firing temperature after impregnation or vacuum impregnation on the microstructure and electrochemical performance of the composite anodes was studied. Particularly the increase of the firing temperature of the skeleton from 1400 °C to 1500 °C resulted in an impressive improvement of total cell resistance and maximal power density.

Made available in DSpace on 2015-04-22T22:02:06Z (GMT). No. of bitstreams: 1 DISSERTACAO VERA LUCIA.pdf: 1075794 bytes, checksum: 33b5d3d47e6787d71b4d7ed96ae837be (MD5) Previous issue date: 2010-04-23 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The direct methanol fuel cells are powers alternatives of energy to do support mainly the batteries. The scientific society come in last decades realizing experiments to overcome the technical barriers that block her commercialization. The effect of methanol crossover, from the anode to the cathode through the membrane, is a very serious problem that that severely reduces the cell voltage, current density, fuel utilization and hence the cell performance. The aim this work is better understand the effect of methanol crossover in this type of technology e try to minimize his negative effects improving the efficiency of fuel cell. To this were prepared electrocatalyts of Pt with an second metal so much in anode as us in cathode using three differents methods: alcohol reduction (MRA), ethylene glycol (MRE) and formic acid (MAF) as reducing agents. The electrodes were prepared following technical of the literature, membrane and electrode assemblies were prepared by pressing an anode and a cathode (Pt onto each side of a Nafion 117 membrane at a pressure of 500 kg cm−2 at 125 ◦C for 180 s. The physical characterization of the electrocatalysts was gone by energy dispersive analysis by X-rays and powder X-ray diffractometer pattern. The effect of methanol crossover was investigate through of electrochemical tests realized in unit cell with PtMo/C and PtRu/C, anode, PtCo/C and PtNi/C, cathode, electrocatalysts. The results showed that alcohol reduction method is better to prepared PtMo/C electrocatalyst, however, its catalytic activity is very lower for methanol oxidation that PtRu/C providing high rate of methanol crossover and reduces cell performance. The results found to the PtCo/C e PtNi/C electrocalysts prepared in this laboratory are approximate the commercials and showed efficient to oxygen reduction reaction and tolerant for methanol oxidation. Thus, we can to affirm that the minimize of effect of methanol crossover lead the increase in cell. / As células a combustível de metanol direto são fontes alternativas de energia para dar suporte principalmente às baterias. A sociedade científica vem nas últimas décadas realizando experimentos para superação das barreiras técnicas que impedem sua comercialização. O efeito crossover de metanol, do anodo para o cátodo através da membrana, é um problema muito sério que reduz severamente o potencial da célula, a densidade de corrente, a utilização de combustível e, portanto o desempenho da célula. O objetivo deste trabalho é compreender melhor o efeito crossover de metanol neste tipo de tecnologia e tentar minimizar os efeitos negativos melhorando a eficiência da célula a combustível. Para isso foram preparados eletrocatalisadores de Pt com a impregnação de um segundo metal tanto para o ânodo como para o cátodo usando três diferentes métodos: redução por álcool (MRA), etilenoglicol (MRE) e ácido fórmico (MAF) como agentes redutores. A fabricação dos eletrodos seguiu a técnica de pintura e o eletrólito empregado foi a membrana de Nafion 117. Para a caracterização física dos eletrocatalisadores modificados, amostras foram submetidos às técnicas de energia dispersiva de raio-x e difração de raios-X. O grau de crossover de metanol através da membrana foi investigado durante os testes eletroquímicos dos eletrodos modificados de PtMo/C, ânodo, e PtCo/C e PtNi/C, cátodo, por meio de curvas de polarização realizados na célula unitária. Os resultados mostraram que o método de redução por álcool é o melhor para impregnação da platina, porém a liga formada por este metal possui atividade catalítica ineficiente para oxidação do metanol o que conduz a elevado efeito crossover de metanol e consequentemente desempenho eletroquímico reduzido. Os eletrodos de PtCo/C e PtNi/C modificados tiveram resultados aproximados aos comerciais e mostram-se eficientes para redução do oxigênio e tolerantes a oxidação de metanol. Assim, podemos afirmar que minimizando os efeitos do crossover de metanol sob o desempenho eletroquímico podemos obter células de metanol direto eficientes e capazes de dar suporte às baterias .

Replacing organic liquid electrolytes with nonflammable solid electrolytes can improve safety, offer higher volumetric and gravimetric energy densities, and lower the cost of lithium-ion batteries. However, today’s all-solid-state batteries suffer from low Li-ion conductivity in the electrolyte, slow Li-ion transport across the electrolyte/electrode interface, and slow solid-state Li-ion diffusion within the electrode. Defect chemistry is critical to understanding ionic conductivity and predicting the charge transport through heterogeneous solid interfaces. The goal of this dissertation is to analyze and improve solid state materials for energy storage applications by understanding their defect structure and transport properties. I have investigated defect chemistry of cubic Li7La3Zr2O12 (c-LLZO), one of the most promising candidate solid electrolytes for all-solid-state lithium batteries. By combining conductivity measurements with defect modeling, I constructed a defect diagram of c-LLZO featuring the intrinsic formation of lithium vacancy-hole pairs. The findings provided insights into tailoring single-phase mixed lithium-ion/electron conducting materials for emerging ionic devices, i.e., composite cathodes requiring both fast electronic and ionic paths in solid-state batteries. I suggested that oxygen vacancies could increase the Li-ion conductivity by reducing the amount of electron holes bound with lithium vacancies. Using a simpler but also attractive solid electrolyte Li2ZrO3 (LZO) as an example, I significantly improved Li-ion conductivity by creating extra oxygen vacancies via cation doping. In particular, Fe-doped LZO shows the highest Li-ion conductivity reported for the family of LZO compounds, reaching 3.3 mS/cm at 300 °C. This study brought attentions to the long-neglected oxygen vacancy defects in lithium-ion conductors and revealed their critical role in promoting Li-ion transport. More importantly, it established a novel defect engineering strategy for designing Li-oxide based solid electrolytes for all-solid-state batteries. I surface-modified LiNi0.6Co0.2Mn0.2O2 cathode material with a LZO coating prepared under dry air and oxygen, and systematically investigated the effect of coating atmosphere on their transport properties and electrochemical behaviors. The LZO coating prepared in oxygen is largely amorphous. It not only provided surface protection against the electrolyte corrosion but also enabled faster lithium-ion transport. Additionally, oxygen atmosphere facilitated Zr diffusion from the surface coating to the bulk of LiNi0.6Co0.2Mn0.2O2, which stabilized the crystal structure and enhanced lithium ion diffusion. Consequently, LiNi0.6Co0.2Mn0.2O2 cathodes coated with Li2ZrO3 in oxygen achieved a significant improvement in high-voltage cycling stability and high-rate performance.

Developing novel electrode materials is a substantial issue to improve the performance of lithium ion batteries. In the present study, single phase Ti–Sn alloys with different Sn contents of 1 to 10 at% were used to fabricate Ti–Sn–O nanotubes via a straight-forward anodic oxidation step in an ethylene glycol-based solution containing NH4F. Various characterization tools such as SEM, EDXS, TEM, XPS and Raman spectroscopy were used to characterize the grown nanotube films. Our results reveal the successful formation of mixed TiO2/SnO2 nanotubes in the applied voltage range of 10–40 V. The as-formed nanotubes are amorphous and their dimensions are precisely controlled by tuning the formation voltage which turns Ti–Sn–O nanotubes into highly attractive materials for various applications. As an example, the Ti–Sn–O nanotubes offer promising properties as anode materials in lithium ion batteries. The electrochemical performance of the grown nanotubes was evaluated against a Li/Li+ electrode at a current density of 504 μA cm−2. The results demonstrate that TiO2/SnO2 nanotubes prepared at 40 V on a TiSn1 alloy substrate display an average 1.4 fold increase in areal capacity with excellent cycling stability over more than 400 cycles compared to the pure TiO2 nanotubes fabricated and tested under identical conditions. This electrode was tested at current densities of 50, 100, 252, 504 and 1008 μA cm−2 exhibiting average capacities of 780, 660, 490, and 405 μA cm−2 (i.e. 410, 345, 305 and 212 mA h g−1), respectively. The remarkably improved electrochemical performance is attributed to enhanced lithium ion diffusion which originates from the presence of SnO2 nanotubes and the high surface area of the mixed oxide tubes. The TiO2/SnO2 electrodes retain their original tubular structure after electrochemical cycling with only slight changes in their morphology.

Indiana University-Purdue University Indianapolis (IUPUI) / The studies of rechargeable lithium-sulfur (Li-S) batteries are included in this thesis. In the first part of this thesis, a linear sweep voltammetry method to study polysulfide transport through separators is presented. Shuttle of polysulfide from the sulfur cathode to lithium metal anode in rechargeable Li-S batteries is a critical issue hindering cycling efficiency and life. Several approaches have been developed to minimize it including polysulfide-blocking separators; there is a need for measuring polysulfide transport through separators. We have developed a linear sweep voltammetry method to measure the anodic (oxidization) current of polysulfides crossed separators, which can be used as a quantitative measurement of the polysulfide transport through separators. The electrochemical oxidation of polysulfide is diffusion-controlled. The electrical charge in Coulombs produced by the oxidation of polysulfide is linearly related to the concentration of polysulfide within a certain range (≤ 0.5 M). Separators with a high porosity (large pore size) show high anodic currents, resulting in fast capacity degradation and low Coulombic efficiencies in Li-S cells. These results demonstrate this method can be used to correlate the polysulfide transport through separators with the separator structure and battery performance, therefore provide guidance for developing new separators for Li-S batteries. The second part includes a study on improving cycling performance of Li/polysulfide batteries by applying a functional polymer on carbon current collector. Significant capacity decay over cycling in Li-S batteries is a major impediment for their practical applications. Polysulfides Li2Sx (3 < x ≤ 8) formed in the cycling are soluble in liquid electrolyte, which is the main reason for capacity loss and cycling instability. Functional polymers can tune the structure and property of sulfur electrodes, hold polysulfides, and improve cycle life. We have examined a polyvinylpyrrolidone-modified carbon paper (CP-PVP) current collector in Li/polysulfide cells. PVP is soluble in the electrolyte solvent, but shows strong affinity with lithium polysulfides. The retention of polysulfides in the CP-PVP current collector is improved by ~50%, which is measured by a linear sweep voltammetry method. Without LiNO3 additive in the electrolyte, the CP-PVP current collector with 50 ug of PVP can significantly improve cycling stability with a capacity retention of > 90% over 50 cycles at C/10 rate. With LiNO3 additive in the electrolyte, the cell shows a reversible capacity of > 1000 mAh g ⁻¹ and a capacity retention of > 80% over 100 cycles at C/5 rate. The third part of this thesis is about a study on a binder-free sulfur/carbon composite electrode prepared by a sulfur sublimation method for Li-S batteries. Sulfur nanoparticles fill large pores in a carbon paper substrate and primarily has a monoclinic crystal structure. The composite electrode shows a long cycle life of over 200 cycles with a good rate performance in Li-S batteries.

No description available.

Developing novel electrode materials is a substantial issue to improve the performance of lithium ion batteries. In the present study, single phase Ti–Sn alloys with different Sn contents of 1 to 10 at% were used to fabricate Ti–Sn–O nanotubes via a straight-forward anodic oxidation step in an ethylene glycol-based solution containing NH4F. Various characterization tools such as SEM, EDXS, TEM, XPS and Raman spectroscopy were used to characterize the grown nanotube films. Our results reveal the successful formation of mixed TiO2/SnO2 nanotubes in the applied voltage range of 10–40 V. The as-formed nanotubes are amorphous and their dimensions are precisely controlled by tuning the formation voltage which turns Ti–Sn–O nanotubes into highly attractive materials for various applications. As an example, the Ti–Sn–O nanotubes offer promising properties as anode materials in lithium ion batteries. The electrochemical performance of the grown nanotubes was evaluated against a Li/Li+ electrode at a current density of 504 μA cm−2. The results demonstrate that TiO2/SnO2 nanotubes prepared at 40 V on a TiSn1 alloy substrate display an average 1.4 fold increase in areal capacity with excellent cycling stability over more than 400 cycles compared to the pure TiO2 nanotubes fabricated and tested under identical conditions. This electrode was tested at current densities of 50, 100, 252, 504 and 1008 μA cm−2 exhibiting average capacities of 780, 660, 490, and 405 μA cm−2 (i.e. 410, 345, 305 and 212 mA h g−1), respectively. The remarkably improved electrochemical performance is attributed to enhanced lithium ion diffusion which originates from the presence of SnO2 nanotubes and the high surface area of the mixed oxide tubes. The TiO2/SnO2 electrodes retain their original tubular structure after electrochemical cycling with only slight changes in their morphology.

Among the obstacles for the commercialization of the MoltenCarbonate Fuel Cell (MCFC), the dissolution of thestate-of-the-art lithiated NiO cathode is considered as aprimary lifetime limiting constraint. Development ofalternative cathode materials is considered as a main strategyfor solving the cathode dissolution problem. LiFeO2and LiCoO2had earlier been reported as the most promisingalternative materials; however, they could not satisfactorilysubstitute the lithiated NiO. On the other hand, ternarycompositions of LiFeO2, LiCoO2and NiO are expected to combine some desirableproperties of each component. The aim of this work was todevelop alternative cathode materials for MCFC in the LiFeO2-LiCoO2-NiO ternary system. It was carried out byinvestigating electronic conductivity of the materials, firstin the form of bulk pellets and then in ex-situ sinteredporous-gas-diffusion cathodes, and evaluating theirelectrochemical performance by short-time laboratory-scale celloperations. Materials in the LiFeO2-NiO binary system and five ternary sub-systems,each with a constant molar ratio of LiFeO2:NiO while varying LiCoO2content, were studied. Powders withcharacteristics appropriate for MCFC cathode fabrication couldbe obtained by the Pechini method. The particle size of LiFeO2-LiCoO2-NiO powders considerably depends on thecalcination temperature and the material composition. Theelectrical conductivity study reveals the ability of preparingLiFeO2-LiCoO2-NiO materials with adequate electricalconductivity for MCFC cathode application. A bimodal pore structure, appropriate for the MCFC cathode,could be achieved in sintered cathodes prepared usingporeformers and sub-micron size powder. Further, this studyindicates the nature of the compromise to be made between theelectrical conductivity, phase purity, pore structure andporosity in optimization of cathodes for MCFC application. Cellperformance comparable to that expected for the cathode in acommercial MCFC could be achieved with cathodes prepared from20 mole% LiFeO2- 20 mole% LiCoO2- 60 mole% NiO ternary composition. It shows aniR-corrected polarization of 62 mV and a iR-drop of 46 mV at acurrent density of 160 mAcm-2at 650 °C. Altogether, this study revealsthe possibility of preparing LiFeO2-LiCoO2-NiO cathode materials suitable for MCFCapplication. Keywords: molten carbonate fuel cell (MCFC), MCFC cathode,LiFeO2-LiCoO2-NiO ternary compositions, electrical conductivity,porous gas diffusion electrodes, polarization, electrochemicalperformance, post-cell characterization.

<p>Among the obstacles for the commercialization of the MoltenCarbonate Fuel Cell (MCFC), the dissolution of thestate-of-the-art lithiated NiO cathode is considered as aprimary lifetime limiting constraint. Development ofalternative cathode materials is considered as a main strategyfor solving the cathode dissolution problem. LiFeO₂and LiCoO₂had earlier been reported as the most promisingalternative materials; however, they could not satisfactorilysubstitute the lithiated NiO. On the other hand, ternarycompositions of LiFeO₂, LiCoO₂and NiO are expected to combine some desirableproperties of each component. The aim of this work was todevelop alternative cathode materials for MCFC in the LiFeO₂-LiCoO₂-NiO ternary system. It was carried out byinvestigating electronic conductivity of the materials, firstin the form of bulk pellets and then in ex-situ sinteredporous-gas-diffusion cathodes, and evaluating theirelectrochemical performance by short-time laboratory-scale celloperations.</p><p>Materials in the LiFeO₂-NiO binary system and five ternary sub-systems,each with a constant molar ratio of LiFeO₂:NiO while varying LiCoO₂content, were studied. Powders withcharacteristics appropriate for MCFC cathode fabrication couldbe obtained by the Pechini method. The particle size of LiFeO₂-LiCoO₂-NiO powders considerably depends on thecalcination temperature and the material composition. Theelectrical conductivity study reveals the ability of preparingLiFeO₂-LiCoO₂-NiO materials with adequate electricalconductivity for MCFC cathode application.</p><p>A bimodal pore structure, appropriate for the MCFC cathode,could be achieved in sintered cathodes prepared usingporeformers and sub-micron size powder. Further, this studyindicates the nature of the compromise to be made between theelectrical conductivity, phase purity, pore structure andporosity in optimization of cathodes for MCFC application. Cellperformance comparable to that expected for the cathode in acommercial MCFC could be achieved with cathodes prepared from20 mole% LiFeO₂- 20 mole% LiCoO₂- 60 mole% NiO ternary composition. It shows aniR-corrected polarization of 62 mV and a iR-drop of 46 mV at acurrent density of 160 mAcm^-2at 650 °C. Altogether, this study revealsthe possibility of preparing LiFeO₂-LiCoO₂-NiO cathode materials suitable for MCFCapplication.</p><p>Keywords: molten carbonate fuel cell (MCFC), MCFC cathode,LiFeO₂-LiCoO₂-NiO ternary compositions, electrical conductivity,porous gas diffusion electrodes, polarization, electrochemicalperformance, post-cell characterization.</p>