Gas Diffusion Resistance Research Articles

Novel functionally graded acicular electrode for solid oxide cells fabricated by the freeze-tape-casting process

The performance of electrode supported solid oxide cells is often limited by gas transport in the thick electrode support. In this study, a novel functionally graded acicular hydrogen electrode microstructure has been fabricated by the freeze-tape-casting method. The effects of freeze-tape-casting processing parameters such as solid loading, freezing bed temperature and tape pulling rate on the morphology of the hydrogen electrode support have been investigated. The electrochemical performance of the cells having the novel functionally graded acicular hydrogen electrode has been significantly improved. In the fuel cell mode, a high power output of 1.28 W cm−2 and a low polarization resistance of 0.166 Ω cm2 have been achieved at 800 °C with H2 as fuel and ambient air as oxidant using nickel–yttria-stabilized zirconia (YSZ) as the hydrogen electrode, YSZ as the electrolyte, and (La0.75Sr0.25)0.95MnO3–YSZ as the oxygen electrode. In the electrolysis mode, a high current density of 2.3 A cm−2 with 30 vol% absolute humidity in the hydrogen electrode at 800 °C has been achieved with an applied cell voltage of 1.6 V. It has been revealed that the novel acicular hydrogen electrode decreases the gas diffusion resistance, thus enhancing the cell performance.

Effect of PMMA Pore Former on Hydrogen Production Performance of Solid Oxide Electrolysis Cells

To reduce the diffusion resistance, improve the energy efficiency and extend the life of solid oxide electrolysis cells (SOEC), the porosity and microstructure optimization of SOEC cathode support layer were studied. Polymethyl methacrylate(PMMA)was added into the cathode as an alternative of starch for pore formation. The experimental results show that the porosity of cathode reaches 45% with conductivity of 6726 S/cm when the PMMA content is 10wt%. The round micro-pores formed by PMMA are well distributed with an average diameter of about 10 μm, which not only significantly reduce the gas diffusion resistance and increase the mechanical strength of cathode materials, but also improve largely the electrolysis efficiency, stability and hydrogen production performance of the SOEC. After the microstructure modification and optimization of cathode layer, the hydrogen production rate of SOEC using PMMA pore former can reach 175mL/(cm·h), which is 1.5 times of the hydrogen production rate of starch. Also their steam diffusion resistance reduce 50% at 850°C and 1.3V electrolysis voltage, which indicates that PMMA is a very promising candidate for the application of SOEC technology.

Research on the Relationship between the Drilling Cutting Gas Desorption Index △h<sub>2</sub> and Parameters of Gas Occurrence

The drilling cutting gas desorption index △h2is one of the most important indexes for the prediction of coal and gas outburst. Based on the mathematical and physical model of the gas diffusion of coal particles, the analytical solution of △h2was deduced. The relationship between the drilling cutting gas desorption index △h2and parameters of gas occurrence such as the gas diffusion coefficient, adsorption constants (a and b), gas pressure, and gaSubscript textSubscript texts content were studied. The results show that △h2increases and gas diffusion resistance decreases with the gas diffusion coefficient. The drilling cutting gas desorption index △h2increases with the adsorption constant a, and the two meet the linear relationship. △h2increases with the adsorption constant b and the raising rate gets lower gradually. And, △h2increases with the gas pressure, and the two meet the power relationship with very high correlation coefficient.△h2and gas content are in linear relationship.

Microstructure of Platinum-Carbon Agglomerates with Hydrocarbon-Based Binder and Its Effect on the Cathode Performance of PEFC

The relationship between the microstructure of platinum-carbon (Pt/C) agglomerates and cathode performance was investigated for membrane-electrode-assemblies (MEAs) with a hydrocarbon-based (HC) binder and a poly (perfluorosulfonic acid) (PFSA) binder. The MEA with an HC binder exhibited a higher gas diffusion resistance than that with the PFSA binder. SEM, TEM, and pore size distribution measurements showed that the HC binder was likely to cover a larger area of the carbon support surface compared with the PFSA binder, and that a large amount of the HC binder easily penetrated the primary pores inside the Pt/C agglomerates, which decreased the volume of the pores. It seems probable that the HC binder in the primary pores blocked the oxygen diffusion to the cathode catalyst. Based on the above consideration, we focused on increasing the primary pore volume. Consequently, the volume was doubled and the gas diffusion resistance at 0.25 A/cm2 was successfully reduced from 1600 to 410 mΩ·cm2.

Impedance Behavior of SOFC at High Fuel Utilization and a Way of Evaluating Diffusion Contribution

A quantitative evaluation method of gas diffusion resistance is proposed in which measurement of V-I characteristics under constant fuel utilization (Uf) and ac impedance were combined to separate diffusion and electromotive force change contributions from the low frequency semi-circle observed in complex impedance diagrams. Diffusion resistance was evaluated using various hydrogen content mixture diluted with nitrogen, Uf , hydrogen flow rate and current as parameters. From the measurement under various Uf, it was found that the diffusion resistance at low Uf originates mainly from the diffusion of H2O. Also, from the measurement under various hydrogen concentration in inlet fuel, the diffusion resistance was found to become significantly large with decreasing hydrogen content.

Micro-Tubular SOFC Systems - Fabrication, Testing and Analysis of Micro-Tubular SOFC

This report summarizes the testing and analysis results on the electrochemical evaluation of a micro-tubular SOFC supported by a micro anode tube, composed of Sc2O3-doped ZrO2 (ScSZ) and NiO mixture. The cell was fabricated through a co-sintering of the ScSZ electrolyte and NiO-ScSZ anode support, and then a La0.6Sr0.4Co0.2Fe0.8O3-x-Ce0.9Gd0.1O1.95 cathode layer was deposited on the electrolyte film. Evaluation was conducted using a potentio/galvanostat and impedance analyzer under a humidified hydrogen flow. Power densities at 0.7 V were 29.4, 154.3 and 382.1 mW/cm2 at 500, 550 and 600 C, respectively. Impedance analyses showed that with increasing operation temperature, ohmic and electrode activation resistances (semicircle at higher frequency region) became decreased, while gas diffusion resistance (semicircle at lower frequency region) depended weakly upon the temperature above 550 C.

Performance limiting factors in anode-supported cells originating from metallic interconnector design

The combination of advanced characterization techniques and FEM-simulations provided detailed information about losses related to the flowfield geometry of a metallic interconnector (MIC) in a planar SOFC repeat unit. The presented 2-D FEM model is able to predict the repeat unit performance decrease due to the in plane ohmic losses in the electrodes and the contact resistance between electrode and MIC. The performed calculation and measurement showed an increase of ohmic losses of up to 84% when the single cell was contacted with a MIC, The contact resistance adds less than 6% on the cathode side. The in-plane current flow from the contact ribs to the triple phase boundaries under the gas channels caused 94% of the additional ohmic losses. Analysis of impedance spectra by the distribution of relaxation times and a subsequent Complex Nonlinear Least Squares fit separated gas diffusion from the total polarization losses. Depending on flowfield design, the gas diffusion resistance on the cathode side increased up to +750%. For high-performance anode supported cells, the choice of cathode flowfield design added up to 41% power loss, whereas the anodic flowfield design was of minor importance (<1% power loss).

Effect of Pore Size Distribution on Cathode Performance of Membrane-Electrode Assembly with a Hydrocarbon-Based Binder

The relationship between pore size distribution and cathode performance was investigated for membrane-electrode-assemblies (MEAs) with a hydrocarbon-based (HC) binder and a poly (perfluorosulfonic acids) (PFSA) binder. The MEA with an HC binder exhibited a higher gas diffusion resistance than that with the PFSA binder. The pore size distribution measurement revealed that the HC binder was likely to cover a larger area of the carbon support surface compared with a PFSA binder, and that a large amount of the HC binder easily penetrated the primary pores inside Pt/C agglomerates, which decreased the volume of the pores. Conceivably, the HC binder in primary pores blocked the oxygen diffusion to the cathode catalyst. Based on the above consideration, we focused on increasing the primary pore volume. Consequently, the volume was doubled, and therefore, the gas diffusion resistance at 0.25 A cm-2 was successfully reduced from 1600 to 410 mΩ cm2.

Ammonia Gas Sensor Using Polypyrrole‐Coated TiO2/ZnO Nanofibers

AbstractHighly porous polypyrrole (PPy)‐coated TiO2/ZnO nanofibrous mat has been successfully synthesized. The core TiO2/ZnO nanofibers have an average diameter of ca. 100 nm and the shell of ultrathin PPy layer has a thickness of ca. 7 nm. The NH3 gas sensor using the as‐prepared material exhibited a fast response over a wide dynamic range and high sensitivity with a detection limit of 60 ppb (S/N=3). Compared to conventional pristine PPy film, the improved performance in NH3 detection can be attributed to the free access of NH3 to PPy and a minimized gas diffusion resistance through the ultrathin PPy layer.

Zeolite Married to Carbon: A New Family of Membrane Materials with Excellent Gas Separation Performance

Novel carbon/zeolite nanocomposite membranes with tailorable gas transport properties were fabricated by pyrolysis of polyamic acid/zeolite nanocomposites. The microstructure and gas permeability of these chemically robust materials were altered by varying zeolite loading and final pyrolysis temperature. Characterization of the as-synthesized composite membranes by SEM, XRD, and TEM revealed that the zeolite particles adhered well to the carbon matrix and that the microstructure of the zeolite was not destroyed during pyrolysis. Gas permeation tests using small molecules (H2, CO2, O2, N2) indicated that the composite membranes exhibited outstanding permeability together with high selectivity and followed the molecular sieve mechanism. It is believed that the zeolite embedded in the carbon matrix decreased the gas diffusion resistance and improved the gas permeation rate. The excellent gas transport properties for all test gases make the carbon/zeolite composite an attractive membrane material in gas separation areas.

Surface effect on oxygen permeation through dense membrane of mixed-conductive LSCF perovskite-type oxide

La 0.1Sr 0.9Co 0.9Fe 0.1O 3− δ (LSCF1991) dense disks with different thicknesses and surface areas were prepared to investigate the contribution of surface reactions and bulk diffusion in oxygen permeation phenomena. The bulk diffusion controlled situation became significant with increasing the membrane thickness, and the surface reaction controlled situation prevailed at smaller surface area. The increase in surface area at the low P(O 2) (anode) side was more effective to increase the oxygen permeation flux than that at the high P(O 2) (cathode) side. The coating of a porous catalyst layer below the optimum thickness was also effective in enhancing the oxygen permeability due to the increase in surface area, but the coating with too thick layer deteriorated the permeability probably due to the increase in the gas diffusion resistance.

Performance diagnostics of PEFC by current-pulse method

PEFC cannot be operated for extended periods because it has some problems. However, if the deterioration factors can identified on time, PEFC can be operated for extended periods by overcoming these problems. Therefore, performance diagnostics needs to be developed to judge the PEFC deterioration factors instantaneously and simply. MCFC performance diagnostic has now been developed by the current-pulse method. Therefore, a thesis was examined relating to the applicability of the diagnostic to a PEFC characteristic diagnosis. The diagnostic evaluates changes in the seven parameters of the fitting equation led from PEFC's transient response between 100 ms obtained by the current-pulse method. The relationship between these parameters and each deterioration factor is evaluated using a PEFC single cell whose electrode area is 25 cm 2. In this case, deterioration factors include cell temperature, the flooding phenomenon and the gas composition/flow rate. As a result, each of seven parameters in the transient response of PEFC corresponds to an ohmic potential drop, anode/cathode gas diffusion resistance, reactive resistance, three-phase interfacial resistance and electrolyte properties, respectively. When the flooding phenomenon is caused in the cathode channel, the transient response of the PEFC obviously differs from the standard value. The change in each parameter is examined by gradually raising the humidity of the cathode supply gas (flooding gradually being caused in the cathode channel). The influence of the flooding phenomenon clearly appeared in three parameters, and in the same way, the temperature dependency of each parameter is confirmed. Therefore, the proposed performance diagnostic will be able to specify the PEFC deterioration factors while driven, easily and on time.

Photosynthetic consequences of phenotypic plasticity in response to submergence: Rumex palustris as a case study

Survival and growth of terrestrial plants is negatively affected by complete submergence. This is mainly the result of hampered gas exchange between plants and their environment, since gas diffusion is severely reduced in water compared with air, resulting in O2 deficits which limit aerobic respiration. The continuation of photosynthesis could probably alleviate submergence-stress in terrestrial plants, but its potential under water will be limited as the availability of CO2 is hampered. Several submerged terrestrial plant species, however, express plastic responses of the shoot which may reduce gas diffusion resistance and enhance benefits from underwater photosynthesis. In particular, the plasticity of the flooding-tolerant terrestrial species Rumex palustris turned out to be remarkable, making it a model species suitable for the study of these responses. During submergence, the morphology and anatomy of newly developed leaves changed: 'aquatic' leaves were thinner and had thinner cuticles. As a consequence, internal O2 concentrations and underwater CO2 assimilation rates were higher at the prevailing low CO2 concentrations in water. Compared with heterophyllous amphibious plant species, underwater photosynthesis rates of terrestrial plants may be very limited, but the effects of underwater photosynthesis on underwater survival are impressive. A combination of recently published data allowed quantification of the magnitude of the acclimation response in this species. Gas diffusion resistance in terrestrial leaves underwater was about 15,000 times higher than in air. Strikingly, acclimation to submergence reduced this factor to 400, indicating that acclimated leaves of R. palustris had an approximately 40 times lower gas diffusion resistance than non-acclimated ones.

Submergence-induced morphological, anatomical, and biochemical responses in a terrestrial species affect gas diffusion resistance and photosynthetic performance.

Gas exchange between the plant and the environment is severely hampered when plants are submerged, leading to oxygen and energy deficits. A straightforward way to reduce these shortages of oxygen and carbohydrates would be continued photosynthesis under water, but this possibility has received only little attention. Here, we combine several techniques to investigate the consequences of anatomical and biochemical responses of the terrestrial species Rumex palustris to submergence for different aspects of photosynthesis under water. The orientation of the chloroplasts in submergence-acclimated leaves was toward the epidermis instead of the intercellular spaces, indicating that underwater CO(2) diffuses through the cuticle and epidermis. Interestingly, both the cuticle thickness and the epidermal cell wall thickness were significantly reduced upon submergence, suggesting a considerable decrease in diffusion resistance. This decrease in diffusion resistance greatly facilitated underwater photosynthesis, as indicated by higher underwater photosynthesis rates in submergence-acclimated leaves at all CO(2) concentrations investigated. The increased availability of internal CO(2) in these "aquatic" leaves reduced photorespiration, and furthermore reduced excitation pressure of the electron transport system and, thus, the risk of photodamage. Acclimation to submergence also altered photosynthesis biochemistry as reduced Rubisco contents were observed in aquatic leaves, indicating a lower carboxylation capacity. Electron transport capacity was also reduced in these leaves but not as strongly as the reduction in Rubisco, indicating a substantial increase of the ratio between electron transport and carboxylation capacity upon submergence. This novel finding suggests that this ratio may be less conservative than previously thought.

An architectural approach to the oxygen permeability of a La 0.6Sr 0.4Fe 0.9Ga 0.1O 3− δ perovskite membrane

Multilayer membranes based on La 0.6Sr 0.4Fe 0.9Ga 0.1O 3− δ (LSFG) and La 0.6Sr 0.4Co 0.8Fe 0.2O 3− δ (LSCF) perovskite materials were fabricated to study the impact of membrane architecture on the oxygen permeability. Thick dense membrane and asymmetric membranes were shaped by tape casting and stacked to reach the desired architecture. Asymmetric membranes composed of a thin dense LSFG layer (120 μm) and a thick porous support layer (820 μm) of the same material were co-sintered to obtain crack-free and flat membranes. The use of large corn-starch particles (14 μm) as pore forming agent to the tape-casting slurries resulted in a connected porosity in the sintered support layer with low gas diffusion resistance. Oxygen permeation measurements in an air/argon gradient between 800 and 925 °C showed that the thickness of self-supported LSFG membranes was not the determining factor in the membrane performance for our testing conditions. A catalytic layer of La 0.6Sr 0.4Co 0.8Fe 0.2O 3− δ (LSCF), deposited on the membrane surfaces to catalyze the oxygen exchange reactions, leads to a significant increase of oxygen permeation rates. As the membrane thickness had no effect even if a catalyst coating was used, surface-exchange reactions were thought to be still limiting for the oxygen permeation fluxes. Thus, the improvement of surface activity of LSFG membrane was found to be a key point to reach higher oxygen permeation fluxes.

MCFC performance diagnosis by using the current-pulse method

Several problems prevent molten carbonate fuel cells (MCFC) operation for an extended period. However, if the degradation factors can be identified and resolved in a timely manner, MCFC could become a valuable technology. Therefore, a performance diagnosis should be developed which enables the simple and instantaneous determination of MCFC degradation factors. A suitable six parameter equation obtained by a current-pulse method, obtainable from MCFC's transient response in 100 ms, is expressible in an equivalent circuit composed of three sub-circuits. The relationship between these parameters and each degradation factor is evaluated by a single MCFC cell, the electrode area of which is 16 cm 2. Degradation factors include cross-leakage, electrolytic loss, cell temperature distribution and gas composition/flow rate. As a result, each of six parameters in the MCFC transient response corresponds to an ohmic potential drop, anode/cathode gas diffusion resistance, reactive resistance, three-phase interfacial resistance and electrolyte properties, respectively. The proposed performance diagnosis specifies the degradation factors by combining the six parameters. Performance diagnosis was applied to a single MCFC cell of an electrode area of 81 cm in extended operations, and the degradation factor diagnosed. As a result, the diagnosis was able to specify the cell degradation factors from the degradation factor ratio, corresponding to cell voltage, cell resistance and the N 2 concentration of MCFC single cell performance. Therefore, the proposed performance diagnosis is able to easily specify the driven MCFC degradation factors in a timely manner.

Effect of electrode microstructure on gas-phase diffusion in solid oxide fuel cells

The relation between electrode microstructure and gas diffusion has been investigated with different morphologies of Pt electrodes by using AC impedance techniques. The measurements were carried out at temperatures of 873–1273 K and oxygen partial pressure (P O 2 ) of 0.01–1 atm. Gas-phase diffusion was observed only for high-performance electrodes at the high-temperature (1073–1273 K) and low-oxygen-partial-pressure regions (<0.1 atm P O 2 ). Considering the physical and electrochemical characteristics of impedance arcs, it was found that the arc at the frequency of below 1 Hz was related to gas conversion resistance, while the arc at the frequency of around 10 Hz represented pore diffusion resistance through the current-collecting part. For a thick electrode with a low porosity, however, gas diffusion resistance through pores of an electrode was observed at a frequency of around 100 Hz. From the results of a comparison of electrode performances with different electrode microstructures, electrochemical reaction sites (ERS) are supposed to be located at the peripheral line of Pt and YSZ as well as the Pt/YSZ interfaces where reaction gas can easily diffuse.

Nitrate reduction and nitrogen fixation in symbiotic association Rhizobium-legumes.

The inhibitory effect of nitrate on nitrogenase activity in root nodules of legume plants has been known for a long time. The major factor inducing changes in nitrogenase activity is the concentration of free oxygen inside nodules. Oxygen availability in the infected zone of nodule is limited, among others, by the gas diffusion resistance in nodule cortex. The presence of nitrate may cause changes in the resistance to O2 diffusion. The aim of this paper is to review literature data concerning the effect of nitrate on the symbiotic association between rhizobia and legume plants, with special emphasis on nitrogenase activity. Recent advances indicate that symbiotic associations of Rhizobium strains characterized by a high nitrate reductase activity are less susceptible to inhibition by nitrate. A thesis may be put forward that dissimilatory nitrate reduction, catalyzed by bacteroid nitrate reductase, significantly facilitates the symbiotic function of bacteroids.

Morpho-functional analysis of lung tissue in mild interstitial edema.

Mild pulmonary interstitial edema was shown to cause fragmentation of interstitial matrix proteoglycans. We therefore studied compartmental fluid accumulation by light and electron microscopy on lungs of anesthetized rabbits fixed in situ by vascular perfusion after 0.5 ml.kg(-1).min(-1) iv saline infusion for 180 min causing approximately 6% increase in lung weight. Morphometry showed that a relevant portion (44%) of extravascular fluid is detected early in the alveolar septa, 85% of this fluid accumulating in the thick portion of the air-blood barrier. The arithmetic mean thickness of the barrier increased in interstitial edema from 1.06 +/- 0.05 (SE) to 1.33 +/- 0.06 microm. The harmonic mean thickness increased from 0.6 +/- 0.03 to 0.86 +/- 0.07 microm, mostly due to thickening of the thin portion causing an increase in gas diffusion resistance. Despite some structural damage, the air-blood barrier displays a relatively high structural resistance providing a safety factor against the development of severe edema. It is suggested that the increase in extra-alveolar perivascular space occurs as a consequence of fluid accumulation in the air-blood barrier.

Absorption of atmospheric phenol by various tree species and their tolerance to phenol

To assess the effect of tree planting on atmospheric phenol, a study was made on the absorption of phenol by various tree species and the tolerance of these species to phenol. The absorption rates ranged from 21.3 (camellia) to 129 ng dm‐2h‐1 ppb‐1 (Japanese elm) at 1000 μmol of photons m‐2 s‐1, and the absorption rate increased in the following order: coniferous tree species ? evergreen broad‐leaved tree species < deciduous broad‐leaved tree species. When the light intensity was varied, a linear relationship between the phenol absorption rate and the transpiration rate was observed for three tree species. In comparison with the absorption rate estimated from a simplified gas diffusive resistance model, we conclude that phenol is absorbed through the stomata and is metabolized fairly rapidly within the leaf tissue, although the absorption rate is less than the estimated potential absorption rate. At phenol concentrations below 200 ppb, the tree can absorb atmospheric phenol for at least 8 h without any visible foliar injury. Trees in general could act as an important sink for atmospheric phenol at phenol concentrations less than 200 ppb, a concentration about twenty times higher than normal ambient levels.