Oxygen Diffusion Studies in Mixed Ionic Electronic Conducting Compounds

Cobalt-based oxides with perovskite-related structures are important candidate cathode materials for the next generation solid oxide fuel cells (SOFCs) because of their good catalytic properties and high values of electronic and ionic conductivity. The aim of the work described in the thesis is to investigate oxygen transport in Sr3YCo4O10.5 (SYC, the so-called 314 phase) and double perovskites LnBaCoFeO5+δ (Ln = La, Pr, Sm and Gd) with combination of theoretical and experimental methods. The oxygen transport properties of Sr3YCo4O10.5 were studied by three different techniques. Molecular dynamics simulations (MD) predicted an isotropic character of oxygen diffusion with an activation energy of 1.56 eV in the temperature range of 1000 – 1400 K. Values of tracer oxygen diffusion, D*, and the surface exchange coefficient, k*, were measured as a function of temperature (500 – 900 °C) with isotope exchange depth profiling combined with secondary ion mass spectrometry (IEDP/SIMS). Chemical oxygen diffusion coefficients, Dchem, and chemical surface exchange rates, kchem, were obtained from electrical conductivity relaxation experiments (ECR). Good agreement of the results obtained by different techniques was confirmed after application of thermodynamic factors to the ECR data. Electrochemical performance of symmetrical cells with Ce0.9Gd0.1O1.95 (GDC) as electrolyte (SYC/GDC/SYC) in the temperature range 620 – 770 °C was evaluated. Double perovskites LnBaCoFeO5+δ (Ln = La, Pr, Sm and Gd) were studied by combination of ECR, thermogravimetric analysis (TGA) and dilatometry. It was found that size of rare earth element (REE) has strong influence on electrochemical and mechanical properties of the samples. In case of La and Pr-containing compounds, fast degradation of the surface kinetics during ECR experiments at high oxygen partial pressure was associated with surface segregation. ECR measurements at low pO2 switches were performed for La, Pr and Gd-containing samples to avoid large deviation from chemical equilibrium during experiments. For SmBaCoFeO5+δ it was found that a linear model for the exchange kinetics works in a larger pO2(1)/pO2(2) interval. The effect of A-cation ordering in LaBaCoFeO5+δ on oxygen transport properties was investigated. Oxygen diffusion was found to be slower in ordered structure, due to the influence of ordering in A-cation sublattice on anion ordering.Chemistry, Department o

“Hydrotriphylites” Li1-xFe1+x(PO4)1-y(OH)4y as Cathode Materials for Li-ion Batteries

Lithium iron phosphate LiFePO4 triphylite is now one of the core positive electrode (cathode) materials enabling the Li-ion battery technology for stationary energy storage applications, which are important for broad implementation of the renewable energy sources. Despite the apparent simplicity of its crystal structure and chemical composition, LiFePO4 is prone to off-stoichiometry and demonstrates rich defect chemistry owing to variations in the cation content and iron oxidation state, and to the redistribution of the cations and vacancies over two crystallographically distinct octahedral sites. The importance of the defects stems from their impact on the electrochemical performance, particularly on limiting the capacity and rate capability through blocking the Li ion diffusion along the channels of the olivine-type LiFePO4 structure. Up to now the polyanionic (i.e. phosphate) sublattice has been considered idle on this playground. Here, we demonstrate that under hydrothermal conditions up to 16% of the phosphate groups can be replaced with hydroxyl groups yielding the Li1-xFe1+x(PO4)1-y(OH)4y solid solutions, which we term “hydrotriphylites”. This substitution has tremendous effect on the chemical composition and crystal structure of the lithium iron phosphate causing abundant population of the Li-ion diffusion channels with the iron cations and off-center Li displacements due to their tighter bonding to oxygens. These perturbations trigger the formation of an acentric structure and increase the activation barriers for the Li-ion diffusion. The “hydrotriphylite”-type substitution also affects the magnetic properties by progressively lowering the Néel temperature. The off-stoichiometry caused by this substitution critically depends on the overall concentration of the precursors and reducing agent in the hydrothermal solutions, placing it among the most important parameters to control the chemical composition and defect concentration of the LiFePO4-based cathodes

Tailoring electrochemical efficiency of hydrogen evolution by fine tuning of TiO x /RuO x composite cathode architecture

Here we report an approach to design composite cathode based on TiO x nanotubes decorated with RuO x nanowhiskers for efficient hydrogen evolution. We tailor catalytic activity of the cathodes by adjustment of morphology of TiO x nanotubular support layer along with variation of RuO x loaded mass and assess its performance using electrochemical methods and wavelet analysis. The highest energy efficiency of hydrogen evolution is observed in 1 M H 2 SO 4 electrolyte to be ca. 64% at −10 mA/cm 2 for cathodes of the most developed area, i.e. smaller diameter of tubes, with enhanced RuO x loading. The efficiency is favored by detachment of small hydrogen bubbles what is revealed by wavelet analysis and is expressed in pure noise at wavelet spectrum. At increased current density, −50 or −100 mA/cm 2 , better efficiency of composite cathodes is supported by titania nanotubes of larger diameter due to an easier release of large hydrogen bubbles manifested in less periodic events appeared in the frequency region of 5–12 s at the spectra. We have shown that efficiency of the catalysts is determined by a pre-dominant type of hydrogen bubble release at different operation regimes depending on specific surface and a loaded mass of ruthenia.Peer reviewe

Chemical origins of a fast-charge performance in disordered carbon anodes

Fast charging of lithium-ion cells often causes capacity loss and limited cycle life, hindering their use in high-power applications. Our study employs electrochemical analysis and a multiphysics model to identify and quantify chemical and physical constraints during fast charging, comparing state-of-the-art graphite and nanocluster carbon (nC, a disordered carbon) anodes. The combination of modeling material phase separation phenomena with ion-electron transfer theory reveals significant insight. The active material strongly influences charge transfer kinetics and solid-state lithium diffusion. Unlike graphite, nC supports lithium insertion without phase separation, enabling faster lithium diffusion, better volume utilization, and lower charge transfer resistance. We demonstrate practical implications of these material phenomena through multilayer pouch cells made with nC anodes, which withstand over 5000 fast-charge cycles at 2C without significant degradation (<10% at reference 0.2C)