Redox Kinetics of Nonstoichiometric Oxides

Cerium oxide (CeO2-δ) and its derivatives are the most attractive materials under consideration for solar-driven thermochemical production of chemical fuels. Understanding the rate-limiting factors in fuel production is essential for maximizing the efficacy of the thermochemical process. The rate of response of the porous ceria structured with architectural features typical of those employed in solar reactors was measured via electrical conductance relaxation methods. A transition from behavior controlled by material surface reaction kinetics to that controlled by sweep-gas supply rates is observed on increasing temperature, increasing volume specific surface area, and decreasing normalized gas flow rate. The transition behavior is relevant not only for optimal reactor operation and architectural design of material, but also for accurate measurement of material properties. The redox kinetics of undoped ceria, CeO2-δ at extreme high temperature (1400 °C) was investigated using the electrical conductivity relaxation method, and those of 10% Pr doped ceria at low temperature (700 °C) were done using the mass relaxation method. It was demonstrated under sufficiently high gas flow rates relative to the mass of the oxide, which is required in order to overcome gas phase limitations and access the material kinetic properties. Furthermore, the surface reaction rate constant of undoped ceria, ,kChem, was investigated at high temperature (1400 °C) in humidified gas atmosphere, in consideration of the operating conditions in thermochemical fuel production system. It was demonstrated that H2O potentially plays a role of oxidants as increasing temperature and/or decreasing oxygen partial pressure; thus in such thermodynamic conditions, pH2O, besides temperature and pO2, needs to be carefully considered in surface reaction study. In addition to relaxation experiments under small driving force for redox reaction, the kinetics of surface related oxidation reaction under large chemical driving force (large ΔpO2 change) was investigated by mass relaxation method. Based on the normalized reaction rates of several possible rate determining steps, the relaxation behavior in oxygen concentration for all possible rate determining steps was computed. On the comparison with the experimental results, the most probable rate determining step was suggested (reduction of diatomic oxygen from neutral oxygen molecule to superoxide), and the oxidation kinetics under large driving force was explained.</p

Gas-phase vs. material-kinetic limits on the redox response of nonstoichiometric oxides

Cerium dioxide, CeO_(2−δ), remains one of the most attractive materials under consideration for solar-driven thermochemical production of chemical fuels. Understanding the rate-limiting factors in fuel production is essential for maximizing the efficacy of the thermochemical process. The rate of response is measured here via electrical conductance relaxation methods using porous ceria structures with architectural features typical of those employed in solar reactors. A transition from behavior controlled by material surface reaction kinetics to that controlled by sweep-gas supply rates is observed on increasing temperature, increasing volume specific surface area, and decreasing normalized gas flow rate. The transition behavior is relevant not only for optimal reactor operation and architectural design of the material, but also for accurate measurement of material properties

Extreme high temperature redox kinetics in ceria: exploration of the transition from gas-phase to material-kinetic limitations

The redox kinetics of undoped ceria (CeO_(2−δ)) are investigated by the electrical conductivity relaxation method in the oxygen partial pressure range of −4.3 ≤ log(pO_2/atm) ≤ −2.0 at 1400 °C. It is demonstrated that extremely large gas flow rates, relative to the mass of the oxide, are required in order to overcome gas phase limitations and access the material kinetic properties. Using these high flow rate conditions, the surface reaction rate constant k_(chem) is found to obey the correlation log(k_(chem)/cm s^(−1)) = (0.84 ± 0.02) × log(pO_2/atm) − (0.99 ± 0.05) and increases with oxygen partial pressure. This increase contrasts the known behavior of the dominant defect species, oxygen vacancies and free electrons, which decrease in concentration with increasing oxygen partial pressure. For the sample geometries employed, diffusion was too fast to be detected. At low gas flow rates, the relaxation process becomes limited by the capacity of the sweep gas to supply/remove oxygen to/from the oxide. An analytical expression is derived for the relaxation in the gas-phase limited regime, and the result reveals an exponential decay profile, identical in form to that known for a surface reaction limited process. Thus, measurements under varied gas flow rates are required to differentiate between surface reaction limited and gas flow limited behavior

Chemical surface exchange of oxygen on CeO_(2−δ) in an O_2/H_2O atmosphere

The chemical surface reaction rate constant controlling the change of oxidation state of undoped ceria, k_(Chem), was measured at 1400 °C in the range of (∼0 ≤ (pH_2O/atm) ≤ 0.163(9)) and (10^(−3.85) ≤ (pO_2/atm) ≤ 10^(−2.86)) via the electrical conductivity relaxation method. In humidified atmospheres, k_(Chem) is fully described as the sum of k_(Chem,O2) and k_(Chem,H2)O, which are, respectively, the rate constants for oxidation by O_2 and by H_2O alone. Using measurements under appropriately controlled gas conditions, the total rate constant is found to follow the correlation k_(Chem)/cm s^(−1) = 10^(−(1.35±0.07)) × (pO_2/atm)^(0.72±0.02) + 10^(−(3.85±0.03)) × (pH_2O/atm)^(0.36±0.03) where the pO_2 and pH_2O values of relevance are explicitly those of the final gas condition. The results suggest that at such high temperatures, the concentrations of surface adsorbed species are too low to influence the independent reaction pathways

Chemical surface exchange of oxygen on CeO_(2−δ) in an O_2/H_2O atmosphere

The chemical surface reaction rate constant controlling the change of oxidation state of undoped ceria, k_(Chem), was measured at 1400 °C in the range of (∼0 ≤ (pH_2O/atm) ≤ 0.163(9)) and (10^(−3.85) ≤ (pO_2/atm) ≤ 10^(−2.86)) via the electrical conductivity relaxation method. In humidified atmospheres, k_(Chem) is fully described as the sum of k_(Chem,O2) and k_(Chem,H2)O, which are, respectively, the rate constants for oxidation by O_2 and by H_2O alone. Using measurements under appropriately controlled gas conditions, the total rate constant is found to follow the correlation k_(Chem)/cm s^(−1) = 10^(−(1.35±0.07)) × (pO_2/atm)^(0.72±0.02) + 10^(−(3.85±0.03)) × (pH_2O/atm)^(0.36±0.03) where the pO_2 and pH_2O values of relevance are explicitly those of the final gas condition. The results suggest that at such high temperatures, the concentrations of surface adsorbed species are too low to influence the independent reaction pathways

Comprehensive understanding of cathodic and anodic polarization effects on stability of nanoscale oxygen electrode for reversible solid oxide cells

Whereas solid oxide cells (SOCs), which perform dual functions of power generation (fuel-cell mode) and energy storage (electrolysis mode) with high efficiency at high temperatures, are considered a potent candidate for future energy management systems, it is yet far from their practical use due to the fact that the stable long-term operations have not been achieved. Particularly, degradations of oxygen-electrode in the both electrolysis and fuel-cell operations are considered as the most imminent issues that should be overcome. Unfortunately, even the origins and mechanisms of degradation in the oxygen-electrode have not been clearly established due to the difficulties in precise assessments of microstructural/compositional changes of porous electrode, which is a typical form in actual solid oxide cells, and due to the diversities in operating conditions, electrode structure and material, fabrication history, and so on. We simultaneously investigated the degradation phenomena in electrolysis and fuel-cell operations for 540h using identical two half cells composed of a geometrically well-defined, nanoscale La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) dense film with a thickness of ~ 70 nm on Ce0.9Gd0.1O2-δ electrolyte. Owing to the benefit of well-defined geometry of LSCF thin film, the microstructural/compositional changes in LSCF films were successfully analyzed in nanoscale, and the correlation between the components of electrochemical impedance and the major origins resulting in degradations was clarified. Furthermore, we suggest the most probable degradation mechanisms, and importantly, it is newly suggested that kinetic demixing/decomposition of LSCF, which is not readily observable in the typical porous-structured electrode, are highly probable to affect the both fuel-cell and electrolysis long-term degradations

Fabrication of Modified MMT/Glass/Vinylester Multiscale Composites and Their Mechanical Properties

Montmorillonite (MMT) may become a preferred filler material for fiber-reinforced polymer (FRP) composites due to its high aspect ratio, large surface area, and low charge density. In the present paper, MMT/glass/vinylester multiscale composites are prepared with untreated and surface-treated MMT clay particles with an MMT content of 1.0 wt%. Effects of surface treatment on mechanical properties of MMT/glass/vinylester multiscale composites are investigated through tensile and bending tests, which revealed enhanced mechanical properties in the case of surface-treated MMT. Thermal properties are studied through thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). X-Ray diffraction is performed to investigate the interaction between MMT and the matrix. Fourier Transform Infrared (FTIR) is also performed for both untreated and surface-treated MMT. Furthermore, Field Emission-Scanning Electron Microscope (FE-SEM) is conducted to investigate the path of fracture propagation within the composite surface, showing that the surface-treated MMT based multiscale composite has better interactions with the host matrix than the untreated MMT multiscale composites. These composites with enhanced mechanical strength can be used for various mechanical applications

Automatic consultation system for patients with cardiac implantable electronic devices undergoing magnetic resonance imaging

Background Safety evaluation for patients with cardiac implantable electronic devices (CIEDs) undergoing magnetic resonance imaging (MRI) scanning is often overlooked. We developed an automatic consultation system (ACS) to improve the screening rate in these patients. Methods ACS was developed by the Hospital Information System Development Department of Seoul National University Bundang Hospital. It was designed to automatically request pre-MRI cardiac evaluation in patients with CIED when MRI orders are issued. The proportion of the patients without pre-MRI cardiologic evaluation was evaluated before and after the ACS application. Results From January 2016 to June 2018, a total of 157 patients with CIEDs [pacemaker 136 (86.6%), ICD or CRT-D 21 (13.4%), MR-conditional 117 (74.5%)] visited the MRI facility. Before the ACS application, 23 out of 84 patients (27.4%) did not have adequate pre-MRI cardiologic evaluation. Despite urgent request for pre-MRI cardiac evaluation, MRI examination was postponed or cancelled in 14 (60.8%) cases. After the ACS application, all 73 patients underwent proper cardiologic evaluation before their MRI examinations (P < 0.001). The proportion of immediate request for pre-MRI evaluation at the moment of MRI order also improved with the ACS application (before ACS 57.1%, after ACS 100%, P < 0.001). Conclusions The newly developed ACS helped the patients with CIED receive MRI scan safely on the schedule, improving the quality of care in this population