Transport Of Oxygen Species Research Articles

The CO2 electrolysing mechanism in single-phase mixed-conducting cathode of solid oxide cell.

In the field of solid oxide cells (SOC), unveiling the electrochemical reaction and transfer mechanisms in mixed ionic and electronic conducting (MIEC) electrodes is of great importance. Due to the chemical capacitance effects of MIEC materials, SOC often shows large capacitance current during electrochemical tests, which might interfere with the polarization behaviors. This work presents a numerical multiphysical model based on the transport of oxygen species, which accurately and concisely replicates the current-voltage curves of a solid oxide electrolysis cell (SOEC) with MIEC electrodes under various scanning rates. The scanning IV and electrochemical impedance spectra measurement under different SOEC working conditions are combined to enable the separation of Faradic and charging currents. Thus, both the bulk diffusion and surface gaseous diffusion of the oxygen species are encompassed, which explains how the current being generated due to intertwined chemical capacitance effects and chemical reactions in the MIEC electrodes.

Elucidating the Origin of Oxide Ion Blocking Effect at GDC/Srzr(Y)O3/YSZ Heterointerfaces

Solid oxide fuel cells (SOFCs) have attracted significant attention over recent years due to their promising use as source of clean and efficient power generation. Towards the goal of enhancing the efficiency of these devices, it is important to eludicate the mechanisms governing fundamental electrochemical processes, specifically incorporation and transport of oxygen species across heterostructures. Degradation of cell performance has been attributed to the formation of SrZrO3 (SZ), especially along the GDC/YSZ interface, resulting from cation interdiffusion across LSCF/GDC/YSZ heterostructures. In this study, using a combination of isotope oxygen tracer and secondary ion mass spectroscopy (SIMS) depth profiling, we examined oxide ion transport in dense SZ and Y-doped SrZrO3 (SZY) thin films in conjunction with other oxide materials in a multilayered heterostructure, viz., GDC/SZ(Y)/YSZ. The GDC/SZ(Y) multilayers were prepared on (100) YSZ single crystal substrates using pulsed laser deposition; X-ray diffraction results confirmed that the SZ and SZY films were highly oriented along (h00). SIMS depth profile measurements after isotope exchange revealed that ionic transport is drastically inhibited at the SZ(Y)/YSZ interface, indicating the existence of an oxide ion barrier which blocks ionic flow. A similar behavior was also observed at the GDC/SZ interface; however, this was not observed for GDC/SZY. To elucidate the mechanism for the oxide ion blocking effect, we performed detailed microstructural analyses of the interfaces using transmission electron microscopy (TEM) and spatially resolved electron energy loss spectroscopy (EELS) technique. TEM results showed that the GDC/SZ(Y) and SZ(Y)/YSZ interfaces are atomically sharp and devoid of any secondary phases. The SZ film also appears to contain a high density of microstructural defects such as stacking faults which extend across its thickness. Furthermore, evident changes in the O K-edge EELS spectra measured across the heterostructures suggest the possible existence of oxygen disorder in the YSZ region adjacent to the SZ(Y)/YSZ interface. This may indicate a possible link between oxygen disorder, which can effectively break the oxygen diffusion pathways, and blocking of oxide ion transport across the SZ(Y)/YSZ interface.

(Invited) Probing Oxide Ion Transport in Fluorite and Perovskite Oxides for Solid Oxide Fuel Cells

Oxygen surface exchange reactions occurring at the solid cathode surface, and subsequent incorporation and transport of oxygen species across heterostructures strongly affect the performance of solid oxide fuel cells (SOFCs). Using isotopic exchange in conjunction with secondary ion mass spectroscopy (SIMS) depth profile analysis, we examined some technologically relevant fluorite and perovskite thin film oxides commonly utilized as component materials for intermediate-temperature SOFCs. In general, nanograined thin films exhibited improved oxygen surface exchange kinetics as compared to quasi-single-crystalline thin films, indicating enhancement of oxygen incorporation facilitated via highly dense grain boundaries. Furthermore, results indicated strong dependence of oxygen surface exchange properties on crystalline orientation. These results demonstrate the versatility of the technique to further our understanding of oxide ion transport occurring across various heterostructures and heterointerfaces, which hopefully would lead towards envisaging new structures and interfaces for high-performance SOFCs.

(Invited) Probing Oxide Ion Transport in Fluorite and Perovskite Oxides for Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) have attracted significant attention over recent years due to their promising use as source of clean and efficient power generation. Towards the goal of enhancing the efficiency of these devices, it is important to eludicate the mechanisms governing fundamental electrochemical processes, specifically oxygen surface exchange reactions occurring at the solid cathode surface, and incorporation and transport of oxygen species across heterostructures. Among the available techniques for evaluating oxygen diffusion in mixed ionic–electronic conducting materials, isotopic exchange in conjunction with secondary ion mass spectroscopy (SIMS) depth profile analysis is one of the most extensively used and is capable of providing high-quality results. This technique consists of exposing a material to 18O isotopic tracer at a specified temperature and duration, and its concentration profile as a function of depth into the material is then evaluated through SIMS depth profile analysis. Consequently the oxygen surface exchange coefficient (k*) and the ionic diffusion coefficient (D*) can be extracted from a fitting of the 18O concentration which relates these two parameters through an appropriate solution of Fick’s Law. Using this well-established technique, we examined several relevant fluorite and perovskite oxides commonly utilized as component materials for intermediate-temperature SOFCs, including LSC (LaSrCoO3) and LSCF (LaSrCoFeO3) for cathodes, and GDC (gadolinia-doped ceria) for interlayers in conjunction with yttria-stabilized zirconia (YSZ) electrolytes. To examine the oxygen surface exchange kinetics in these materials, a simplified approach usually consists of fabricating these into dense, thin films by means of epitaxial deposition methods such as pulsed laser deposition (PLD). Adopting this approach, we investigated the effect of film microstructure and crystalline orientation on oxygen surface exchange kinetics and transport across various heterostructures of PLD-grown LSC, LSCF, and GDC thin films of various thicknesses prepared on (100) and (111) YSZ single crystal substrates. We found that nanograined thin films consistently exhibited higher 18O concentration as compared to quasi-single-crystalline thin films, indicating enhancement of oxygen reactivity facilitated via highly dense grain boundaries. Moreover, LSC and LSCF thin films having (100)-pseudocubic orientation exhibited more enhanced oxygen surface exchange kinetics compared to (110)-oriented films, suggesting that the type of surface termination highly influences surface reactivity. Furthermore, as degradation of cell performance has been attributed to the formation of SrZrO3 (SZ) resulting from cation interdiffusion across LSCF/GDC/YSZ, we also examined in detail the effect of this phase on oxide ion transport in multilayered heterostructures. Oxide ion transport appears to be drastically inhibited at the SZ/YSZ interface, indicating the existence of an oxide ion barrier which blocks ionic flow. These results demonstrate the versatility of the technique to further our understanding of oxide ion transport occurring across various heterostructures and heterointerfaces, which hopefully would lead towards envisaging new structures and interfaces for high-performance SOFCs.

Electrochemical growth of porous titanium dioxide in a glycerol-based electrolyte at different temperatures

Anodic TiO2 layers on Ti were prepared via a three-step anodization in glycerol containing NH4F (0.38 wt%) and H2O (1.79 wt%) at the constant anodizing potential of 40V and temperatures ranging from 10 to 40°C. It was confirmed that TiO2 growth is thermally activated and transport of oxygen species across the titanium dioxide layer at the pore bottoms is a rate-limiting step of anodization. The morphological characterizations of received anodic layers were performed for all studied temperatures. The structural features of anodic TiO2 such as pore diameter, cell diameter, porosity, pore density and pore circularity were investigated for various anodizing temperatures. It was found that the oxide thickness, pore diameter, cell diameter and porosity of formed TiO2 layers increase gradually with increasing temperature. The pore density and their arrangement analyses showed that the nanoporous anodic titania layer with the lowest density of pores and best pore arrangement was synthesized at 20°C. In addition, photoelectrochemical properties of formed TiO2 layers were investigated as well.

Diffusion-reaction modeling of silicon oxide interlayer growth during thermal annealing of high dielectric constant materials on silicon

We present the quantitative physicochemical modeling of the thermal annealing of high dielectric constant $(k)$ thin films on silicon in oxygen and/or inert ambient. In particular, we study the kinetics of the ${\text{SiO}}_{2}$ interfacial layer growth at the high-$k$ material structure/Si interface. Upon annealing, the transport of oxygen species in the high-$k$ film to the silicon interface is quantitatively evaluated. One-dimensional unsteady-state diffusion-reaction equations are used to model the time evolution of the interfacial ${\text{SiO}}_{2}$ layer thickness. Because of the continuously increasing interfacial ${\text{SiO}}_{2}$ layer, the proposed model incorporates the moving interface that alters the diffusion length of the oxygen species. The numerical solution of the resulting modeling equations is based on the finite volume analysis method and it results in ${\text{SiO}}_{2}$ thickness profiles that comprise of an early fast growth stage followed by pseudosaturation into a self-limited regime. Our model predictions are found to satisfactorily agree with published experimental results. We also study the use of alumina as a potential oxygen diffusion barrier. Alumina is predicted to be an efficient barrier to oxygen diffusion, which is in agreement with published experimental data.