Papers by Keyword: Defect Chemistry

Abstract: Cathode materials based on lithium-metal-oxide compounds are an essential technical component for lithium-ion batteries, which are still being researched and continuously improved. For a fundamental understanding of kinetic processes at and in electrodes the Li diffusion is of high relevance. Most cathode materials are based on the layered LiCoO₂ (LCO) and LiNi_0.33Mn_0.33Co_0.33O₂ (NMC₃₃₃). In the present study Li tracer self-diffusion is investigated in polycrystalline sintered bulk samples of sub-stoichiometric Li_0.9CoO₂ at 145 °C ≤ T ≤ 350 °C and compared to Li_0.9Ni_0.33Mn_0.33Co_0.33O₂ in the temperature range between 110 and 350 °C. For analysis, stable ⁶Li tracers are used in combination with secondary ion mass spectrometry (SIMS). The Li tracer diffusivities D^* of both compounds with a sub-stoichiometric Li concentration are identical within error limits and can be described by the Arrhenius law with an activation enthalpy of (0.76 ± 0.13) eV for LCO and (0.85 ± 0.03) eV for NMC₃₃₃, which is interpreted as the migration energy of a single Li vacancy. This means that a modification of the transition metal (M) layer composition within the LiMO₂ structure does not significantly influence lithium diffusion in the temperature range investigated.

Abstract: Titanium dioxide (TiO₂) has been widely used as photoanodes in photoelectrochemical (PEC) water splitting. However, the typically high density of bandgap trap states results in fast charge carrier recombination and poor electrical conductivity, and thereby weak PEC performance. Rational creation of oxygen vacancy (V_o) in TiO₂ has been demonstrated as an effective method to modify the electronic and optical properties, as well as improved PEC performance. Different strategies have been developed to fabricate oxygen deficient TiO₂ photoanodes, such as hydrogen treatment, thermal annealing, electrochemical reduction, flame reduction, and chemical reduction. In conjunction with oxygen vacancy creation, doping of TiO₂ with elements further enhances the PEC activity by introducing other bandgap states. Various techniques, including ultrafast laser spectroscopy, have been employed to probe the chemical nature and associated charge carrier dynamics of the bandgap states.

Abstract: Depletion of fossil fuel at an alarming rate is a major concern of humankind. Consequently, researchers all over the world are putting a concerted effort for finding alternative and renewable energy. Solid oxide fuel cell (SOFC) is one such system. SOFCs are electrochemical devices that have several advantages over conventional power generation systems like high efficiency of power generation, low emission of green house gases and the fuel flexibility. The major research focus of recent times is to reduce the operating temperature of SOFC in the range of 500 to 700 °C so as to render it commercially viable. This reduction in temperature is largely dependent on finding an electrolyte material with adequate oxygen ion conductivity at the intended operating temperature. One much material is Gadolinia doped Ceria (CGO) that shows very good oxygen ion conductivity at the intended operation temperature. The aim of this overview is to highlight the contribution that materials chemistry has made to the development of CGO as an electrolyte.

Abstract: Formation mechanism of pores in undoped TiO₂ ceramics was investigated through defects chemistry and materials testing methods. The undoped TiO₂ ceramics samples were prepared from anatase TiO₂ powders by a traditional solid-state sintering method. Microstructure, chemistry composition and ionic valence of undoped TiO₂ ceramics were by SEM, EDS and XPS. Formation mechanism of pores was discussed by combination defect chemistry with materials structure measurement. The results show that there exist trivalence Titanium ion (Ti ³⁺) and grain boundaries absorbed oxygen in undoped TiO₂ ceramics samples. Both content of absorbed oxygen in grain boundaries and Ti ³⁺ concentration increase with sintering temperature increasing. There are much gas pores in grains and grain boundarties of undoped TiO₂ ceramics samples. The gas pores are mainly originated from lattice oxygen volatilization and oxygen vacancies segregation during high-temperature sintering.

Abstract: Solid oxide fuel cells (SOFCs) are electrochemical devices that offer advantages over conventional power generation systems in terms of their high efficiency of power generation, low emission of green house gases and the flexibility of fuel usage. The major research focus of recent times is to lower the operating temperature of SOFC in the range of 600 to 800°C so as to make it commercially viable. This reduction in temperature is largely dependent on finding an electrolyte material with adequate oxygen ion conductivity at the intended operating temperature. One much material is pervoskite LaGaO3 doped with Sr- and Mg- La1-xSrxGa1-yMgyO3-δ (LSGM) that shows very good oxygen ion conductivity at intermediate temperature (600-800°C) over a wide range of oxygen partial pressure. The aim of this overview is to highlight the contribution that materials chemistry has made to the development of LSGM based SOFCs.

Abstract: In this paper we review a number of studies of stress-induced diffusional matter transport in perovskites, with an emphasis on creep studies used as a means of studying defect chemistry on the cation sublattices. Studies of diffusional creep in air or fixed atmospheres are reviewed first, and the common characteristics among these perovskites are identified. Creep studies of several perovskiterelated or perovskite-like structures are reviewed next, and the similarities/dissimilarities to perovskites are outlined. The diffusional creep studies in controlled atmosphere are reviewed next, with the emphasis on defect chemistry modeling from creep data. The paper presents a detailed review of two creep studies in oxygen controlled atmosphere that show particularly interesting and remarkedly different behavior from that predicted by standard defect chemistry models. Defect chemistry modeling from creep data is presented for these two cases. The potential and limitations of using creep experiments for studying diffusional matter transport and cation defect chemistry are discussed.