Gas Phase Diffusion Resistance Research Articles

Theoretical evaluation of different factors affecting the HO2 uptake coefficient driven by aqueous-phase first-order loss reaction

The heterogeneous loss on aerosols is an important sink of HO2, affecting the radical chemistry and cycling, and thus it plays a key role in the atmospheric photochemistry. Gaining a reasonable HO2 uptake coefficient (γHO2) would be of great importance in evaluating the heterogeneous loss rate of HO2 on aerosols. This work was motivated by the large variance of reported HO2 mass accommodation coefficients (αHO2) in laboratory studies (0.1–1), which can cause consequent bias in the parameterized HO2 uptake coefficient (γHO2). We conducted a theoretical analysis of the roles of several key factors or parameters in determining γHO2 on a sphere droplet with adjustable Cu2+ ion concentration including αHO2, aqueous-phase acidity, the first-order loss-rate constant KI value, and the aqueous phase production of HO2. The results intuitively demonstrate that utilizing a single γHO2 value for aerosols of different sizes, compositions or hygroscopic states is unsafe in atmospheric models. The theoretical analysis indicated that for a single aerosol experiencing hygroscopic growth, γHO2 decreased with increasing aerosol size, because of the increased gas phase diffusion resistance and dilution of aqueous-phase HO2 consuming ions. Aerosol pH and metal abundance influence γHO2 by determining the aqueous-phase loss-rate constants, and these two factors were found to be only predominant for large particles/droplets (Rp > 1 μm). For small and middle size aerosols, the mass accommodation process plays the determining role in controlling HO2 uptake. Considering ambient aerosols rarely grow to cloud droplet size on sunny days when photochemical budget of HO2 radicals is of more concern, it is crucial to adopt appropriate αHO2 in models, as arbitrarily choosing the αHO2 value can lead to large bias when simulating HO2 heterogeneous process on ambient aerosols.

Temperature effect on the electrode reactions in a molten carbonate fuel cell

A temperature effect on the electrode reactions was investigated in terms of activation energy of overpotential with 100cm2 class molten carbonate fuel cells. The overpotential is attributed to the resistance in the electrochemical reactions on electrodes, thus the activation energy reflects reaction characteristics of the electrodes. The activation energy was measured with electrochemical impedance spectroscopy (EIS) and inert gas step addition (ISA) methods. These methods revealed that the anodic overpotential had positive activation energies showing larger overpotential at higher temperature, whereas the cathodic overpotential had negative activation energies. In particular, the positive activation energy of anodic overpotential is a unique behavior which has not been treated before. The gas phase diffusion resistance in the porous electrode, calling pore diffusion resistance (PDR), would be responsible for the positive activation energy. On the other hand, the temperature dependence of reactant gas dissolution to the carbonate melts and transport through the carbonate film leads to negative activation energy because the cathode is covered by the relatively thick electrolyte film. Insignificant PDR effect is observed at the cathode through the activation energy of gas phase mass transfer resistance at the cathode.

Comparison of the electrochemical properties of impregnated and functionally gradient LaNi0.6Fe0.4O3–Gd0.2Ce0.8O2 composite cathodes for Solid Oxide Fuel Cell

This study investigates the electrochemical properties of three different configurations of LaNi0.6Fe0.4O3 (LNF)-based cathodes for Solid Oxide Fuel Cell (SOFC). The results show functionally gradient LNF cathode and Gd0.2Ce0.8O2 (GDC)-impregnated LNF cathode both reveal the better electrochemical properties compared to that of 70 wt.% LNF–30 wt.% GDC cathode. Functionally gradient LNF cathode with better interface of LNF/GDC/ScSZ can be achieved by the gradual change in composition from electrolyte to cathode, resulting in the decline in charge transfer resistance and gas phase diffusion resistance, therefore the decline in specific cathode polarization resistance (Rp). Whereas, the dramatic decrease in Rp for GDC-impregnated LNF cathode is mainly attributed to the extended triple phase boundary (TPB) and enhanced oxide ion conductivity. The nano-sized GDC particle on LNF backbone with high porosity would allow gas phase molecules to easily diffuse to the LNF/GDC/ScSZ boundaries, which significantly enhances cathode activities for oxygen reduction reaction (ORR). The GDC-impregnated LNF cathode reveals the lowest Rp, the lowest activation energies of Rp and the lowest activation energy of ORR among three cathode configurations. Noticeably, the cathode performance can be significantly improved by impregnating nano-sized GDC particles into LNF porous backbone.

Comparison of the Electrochemical Properties of Infiltrated and Functionally Gradient Sm0.5Sr0.5CoO3−δ-Ce0.8Sm0.2O1.9 Composite Cathodes for Solid Oxide Fuel Cells

This study investigates the electrochemical properties of three different configurations of Sm0.5Sr0.5CoO3−δ (SSC)-based cathodes and attempts to discover a potential approach applicable to solid oxide fuel cells (SOFCs). The functionally gradient SSC cathode and Ce0.8Sm0.2O1.9 (SDC)-infiltrated SSC cathode both reveal the better electrochemical properties compared to that of the 70 SSC- 30 SDC composite cathode. The functionally gradient Sm0.5Sr0.5CoO3−δ cathode with better interface of SSC/SDC may be achieved by the gradual change in composition from electrolyte to cathode, resulting in the decline in charge transfer resistance and gas phase diffusion resistance. Whereas, the dramatic decrease in cathode polarization resistance for SDC-infiltrated SSC cathode is mainly attributed to the creation of SDC/SSC phase boundaries. The newly formed SDC on the SSC surface with highly porous would allow gas-phase molecules to easily diffuse to the SDC/SSC boundaries, which considerably increase the electrochemical sites for oxygen reduction reaction (ORR). The SDC-infiltrated SSC cathode reveals the lowest cathode polarization resistance (RP), the lowest activation energies of cathode polarization resistance and the lowest over activation energy for the ORR among all cathode configurations. Noticeably, the cathode performance in SOFCs can be significantly improved by infiltrating nanoparticles of SDC into an SSC porous backbone.

Electrochemical characterization of gradient Sm 0.5Sr 0.5CoO 3− δ cathodes on Ce 0.8Sm 0.2O 1.9 electrolytes for solid oxide fuel cells

This study investigates Sm 0.5Sr 0.5CoO 3− δ (SSC)–Ce 0.8Sm 0.2O 1.9 (SDC) composite cathodes with a gradual change in composition from electrolyte to the cathode in an attempt to discover a potential approach applicable to solid oxide fuel cells (SOFCs). The gradual change in composition from electrolyte to cathode shows the decline in charge transfer resistance ( R 2) and gas phase diffusion resistance ( R 3). Because the value of R 3 is always larger than R 2 and R 3 significantly dominates the total cathode polarization resistance ( R P) at temperatures within the range of 750–850 °C, i.e., in this temperature range, the rate-determining step is dominated by the diffusion or dissociative adsorption of oxygen. The functionally gradient cathode with a graded interface between cathode and electrolyte reveals both a higher exchange current density ( i 0) and a lower activation energy for oxygen reduction reaction (ORR), which suggests that the ORR kinetics can be improved by using the configuration of a functionally gradient cathode.

Conductivity and electrochemical performance of (Ba 0.5Sr 0.5) 0.8La 0.2CoO 3− δ cathode for intermediate-temperature solid oxide fuel cell

This study reports the successful preparation of a single-phase cubic (Ba 0.5Sr 0.5) 0.8La 0.2CoO 3− δ perovskite by the citrate–EDTA complexing method. Its crystal structure, thermogravimetry, coefficient of thermal expansion, electric conductivity, and electrochemical performance were investigated to determine its suitability as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Its coefficient of thermal expansion shows abnormal expansion at 300 °C, which is associated with the loss of lattice oxygen. The maximum conductivity of a (Ba 0.5Sr 0.5) 0.8La 0.2CoO 3− δ electrode is 689 S/cm at 300 °C. Above 300 °C, the electronic conductivity of (Ba 0.5Sr 0.5) 0.8La 0.2CoO 3− δ decreases due to the formation of oxygen vacancies. The charge-transfer resistance and gas phase diffusion resistance of a (Ba 0.5Sr 0.5) 0.8La 0.2CoO 3− δ –Ce 0.8Sm 0.2O 1.9 composite cathode are 0.045 Ω cm 2 and 0.28 Ω cm 2, respectively, at 750 °C.

Electrochemical properties of La0.8Sr0.2FeO3 − δ –La0.45Ce0.55O2 − δ composite cathodes for intermediate temperature SOFC

The electrochemical properties of La0.8Sr0.2FeO3 − δ (LSF)–La0.45Ce0.55O2 − δ (LDC) composite cathodes coated on LSGM electrolyte were studied by electrochemical impedance spectroscopy and cathodic polarization technique. Results showed that the composite cathodes possessed superior electrochemical performance compared to that of pure LSF cathode. The cathodic overpotential of Cathode C was only 100 mV at 0.3 A cm−2, and the charge transfer resistance and the gas phase diffusion resistance were decreased to 0.105 Ω cm2 and 0.257 Ω cm2, respectively at 800 °C. The improvement of the electrochemical performance is contributed to the increase of the triple-phase boundary, enlargement of the effective area for electrode reaction, and increase of the porosity of the cathode by adding LDC to the cathode material.

Improved electrochemical performance of SrCo 0.8Fe 0.2O 3− δ–La 0.45Ce 0.55O 2− δ composite cathodes for IT-SOFC

The performance of the SrCo 0.8Fe 0.2O 3− δ (SCF)–La 0.45Ce 0.55O 2− δ (LDC) composite cathodes was studied in this paper. The composite cathodes were prepared by screen-printing, and then sintered at 1200 °C for 2 h. Electrochemical impedance spectroscopy (EIS) and cathodic polarization test were carried out to investigate the electrochemical properties of the composite cathodes. The results showed that the composite cathodes had superior electrochemical performance compared to that of the pure SCF cathodes. Through optimizing the structures of composite cathodes, the cathodic overpotential of triple-layer SCF–LDC composite cathodes was only 23 mV at 0.3 A cm −2. The specific ohmic resistance, charge transfer resistance and gas phase diffusion resistance of the triple-layer SCF–LDC cathodes were the lowest for the SCF–LDC composite cathodes, and they were 0.1 Ω cm 2, 0.01 Ω cm 2 and 0.1 Ω cm 2 respectively at 800 °C. The changes were attributable to the enlargement of triple phases boundary (tpb) and enhancement of the adhesion between electrode and electrolyte by adding LDC to the cathode material.

Electrode Kinetics of Porous Mixed‐Conducting Oxygen Electrodes

In this paper we use continuum modeling to analyze the mechanism of the oxygen reduction reaction at a porous mixed‐conducting oxygen electrode. We show that for at 700°C, solid‐state oxygen diffusion and surface exchange dominate the electrochemical behavior, producing effective “chemical” resistances and capacitances. This behavior can be explained both qualitatively and quantitatively in terms of the known bulk and surface properties of the materials. This mechanism appears to be generally valid for mixed conductors with high rates of internal mass transfer, but breaks down for mixed conductors that have poor ionic transport. Our analysis also suggests that, for the best electrode materials, extension of the reaction zone beyond the three‐phase boundary is limited to a few micrometers. We also show that gas phase diffusion resistance can contribute significantly to cell impedance at .

Cardiopulmonary adaptations to pneumonectomy in dogs. II. VA/Q relationships and microvascular recruitment

Ventilation-perfusion relationships, diffusing capacity for O2, and hemodynamic response were measured at rest and during exercise while five adult dogs breathed air and 15% O2 2 mo (Pnx-A, n = 2) or 12 mo (Pnx-B, n = 3) after right pneumonectomy (removal of 58% of lung tissue). Results were compared with those in five sham-operated controls. The multiple inert gas elimination technique was employed. Maximal O2 uptake was reduced by 50% in Pnx-A and by 15% in Pnx-B. Ventilation-perfusion matching was impaired in Pnx-A but not in Pnx-B. The increase in O2 diffusing capacity during exercise was significantly restricted in Pnx-A but was partially restored in Pnx-B. Mean pulmonary arterial pressure at a given blood flow through the remaining lung was normal in Pnx-A but lower than normal in Pnx-B compared with control values for a single lung. Stroke volume and cardiac output were lower in both Pnx-A and Pnx-B than in controls at a given exercise level. All functional abnormalities were more severe at 2 mo than at 12 mo postpneumonectomy. Gas phase diffusion resistance was present in both Pnx-A and Pnx-B but not in controls. We conclude that physiological compensation postpneumonectomy is progressive. Inability to recruit pulmonary vascular bed with incremental perfusion pressure or flow appears to be the major pathophysiological process that limits early functional capacity after right pneumonectomy.

Conducting airway gas exchange: diffusion-related differences in inert gas elimination.

We studied CO2 and inert gas elimination in the isolated in situ trachea as a model of conducting airway gas exchange. Six inert gases with various solubilities and molecular weights (MW) were infused into the left atria of six pentobarbital-anesthetized dogs (group 1). The unidirectionally ventilated trachea behaved as a high ventilation-perfusion unit (ratio = 60) with no appreciable dead space. Excretion of higher-MW gases appeared to be depressed, suggesting a MW dependence to inert gas exchange. This was further explored in another six dogs (group 2) with three gases of nearly equal solubility but widely divergent MWs (acetylene, 26; Freon-22, 86.5; isoflurane, 184.5). Isoflurane and Freon-22 excretions were depressed 47 and 30%, respectively, relative to acetylene. In a theoretical model of airway gas exchange, neither a tissue nor a gas phase diffusion resistance predicted our results better than the standard equation for steady-state alveolar inert gas elimination. However, addition of a simple ln (MW) term reduced the remaining residual sum of squares by 40% in group 1 and by 83% in group 2. Despite this significant MW influence on tracheal gas exchange, we calculate that the quantitative gas exchange capacity of the conducting airways in total can account for less than or equal to 16% of any MW-dependent differences observed in pulmonary inert gas elimination.

Ozone Concentration in Leaf Intercellular Air Spaces Is Close to Zero

Transpiration and ozone uptake rates were measured simultaneously in sunflower leaves at different stomatal openings and various ozone concentrations. Ozone uptake rates were proportional to the ozone concentration up to 1500 nanoliters per liter. The leaf gas phase diffusion resistance (stomatal plus boundary layer) to water vapor was calculated and converted to the resistance to ozone multiplying it by the theoretical ratio of diffusion coefficients for water vapor and ozone in air (1.67). The ozone concentration in intercellular air spaces calculated from the ozone uptake rate and diffusion resistance to ozone scattered around zero. The ozone concentration in intercellular air spaces was measured directly by supplying ozone to the leaf from one side and measuring the equilibrium concentration above the other side, and it was found to be zero. The total leaf resistance to ozone was proportional to the gas phase resistance to water vapor with a coefficient of 1.68. It is concluded that ozone enters the leaf by diffusion through the stomata, and is rapidly decomposed in cell walls and plasmalemma.

Calculation of CO2 gas phase diffusion in leaves and its relation to stomatal resistance

A new theory and experimental method was developed to measure the diffusion resistance to CO2 in the gas phase of mesophyll leaf tissue. Excised leaves were placed in a chamber and their net evaporation and CO2 assimilation rates measured at two different ambient pressures. These data were used to calculate CO2 gas phase diffusion resistances. A variety of field grown leaves were tested and the effects of various experimental errors considered. Increasing the gas phase diffusion resistance decreased transpiration more than it decreased CO2 assimilation. It was concluded that gas phase diffusion resistance associated with CO2 assimilation may sometimes be 100 or 200 s·m(-1) greater than the resistance implied by transpiration rates. This may be due to longer path lengths for the CO2 diffusion, constricted in places by the shape and arrangement of mesophyll cells.