Gas Diffusion In Pores Research Articles

A 3D Simulation of DC-Biased Electrochemical Impedance of Solid Oxide Electrolysis Cell: Effects of Delamination

The demand for green energy sources with near zero-carbon footprints is rising owing to global warming, pollution and increasing world population. Hydrogen is one of the capable energy sources with higher gravimetric energy density than conventional gasoline fuel. Clean hydrogen production can be achieved through water splitting electrolysis processes. High temperature solid oxide electrolysis cells (SOEC) have higher efficiency, which requires less overall electric power compared to other electrolysis cells [1]. Hitherto, SOEC has not been commercialized due to their faster degradation under higher current densities, compared against its performance in fuel cell mode [2]. Delamination of oxygen electrode (OE) from the electrolyzer (EL) is the major reason for the cell degradation [3,4].Half-cell configuration is simulated to understand the delamination of OE coupled with experimental results observed during polarization test ran for 1000 hours. The OE used here is a composite of La1-xSrxCo1-yFeyO3-δ (LSCF) and Gadolinium-doped Ceria (GDC). A buffer layer (GDC) is introduced between the electrolyte (EL) made of 8 mol% Y2O3 doped ZrO2 (YSZ) and composite OE. The OE, buffer layer and EL diameter are 7, 10 and 10 mm with thicknesses of 30, 1 and 10 μm, respectively. The oxygen evolution reaction here is considered as multi-step charge transfer reaction at two phase boundaries (2PB) accompanied by triple phase boundary (3PB) intersection of OE, EL and gas phase [5]. Charged species like vacancies, holes, and electrons in OE, especially inside LSCF, are solved using Nernst-Planck transport equations. Gas diffusion in pores is simulated by Fick’s law. The charge transfer in the transport pathways (2PB and 3PB) are given by Butler-Volmer model [6]. Eventually, the perturbation form and the steady state form of the physics-based model will be coupled to simulate DC-biased impedance under various operating current densities. We will use the model to study the evolution of impedance when the OE/EL interface delamination propagates under long-term tests.The preliminary steady state results are shown in Figure 1, operated at 1 A/cm2. The electronic potential distribution in LSCF can be seen in Figure 1a. This potential is close enough to the applied voltage and the variation of potential along the thickness is negligible. The trend of ionic potential along the half-cell thickness is illustrated in Figure 1b. The linearity in potential is changed to exponential correlation when we move from EL to OE region, which is because of 2PB and 3PB current contribution to GDC. The variation in the vacancy is also minimal in OE, owing to its micro scale dimension. The oxygen partial pressure is maximum at OE-Buffer layer interface due to oxygen evolution. All these trends are plotted as a function of cell thickness along its axis. We will validate the model by both the experimentally measured voltage-current curve and electrochemical impedance spectra (EIS) under DC bias. This study will unveil the underlying degradation mechanism of SOEC by relating EIS with delamination of SOEC under long-term tests.

Deposition and Re-Emission of Atmospheric Elemental Mercury over the Tropical Forest Floor.

Significant knowledge gaps exist regarding the emission of elemental mercury (Hg0) from the tropical forest floor, which limit our understanding of the Hg mass budget in forest ecosystems. In this study, biogeochemical processes of Hg0 deposition to and evasion from soil in a Chinese tropical rainforest were investigated using Hg stable isotopic techniques. Our results showed a mean air-soil flux as deposition of -4.5 ± 2.1 ng m-2 h-1 in the dry season and as emission of +7.4 ± 1.2 ng m-2 h-1 in the rainy season. Hg re-emission, i.e., soil legacy Hg evasion, induces negative transitions of Δ199Hg and δ202Hg in the evaded Hg0 vapor, while direct atmospheric Hg0 deposition does not exhibit isotopic fractionation. Using an isotopic mass balance model, direct atmospheric Hg0 deposition to soil was estimated to be 48.6 ± 13.0 μg m-2 year-1. Soil Hg0 re-emission was estimated to be 69.5 ± 10.6 μg m-2 year-1, of which 63.0 ± 9.3 μg m-2 year-1 is from surface soil evasion and 6.5 ± 5.0 μg m-2 year-1 from soil pore gas diffusion. Combined with litterfall Hg deposition (∼34 μg m-2 year-1), we estimated a ∼12.6 μg m-2 year-1 net Hg0 sink in the tropical forest. The fast nutrient cycles in the tropical rainforests lead to a strong Hg0 re-emission and therefore a relatively weaker atmospheric Hg0 sink.

Low-platinum dissymmetric membrane electrode assemblies for fuel cells suitable for a variety of external humidification conditions

Electrospun fuel cell catalyst layers have been shown to significantly reduce Pt consumption; however, obvious performance degradation often occurs under low external humidification conditions. To overcome this problem, we propose a novel dissymmetric membrane electrode assembly (MEA) that contains a graded pore gas diffusion anode and an electrospun cathode catalyst layer (CCL). The microporous layer containing hydrophilic ionomer has a similar structure as the anode catalyst layer, which improves water management and alleviates the effects of both water flooding and drying on the performance of the MEA. With the high Pt utilization of the electrospun CCL, the total Pt consumption of the MEA is 0.124 g kW−1 (H2/air, 100% RH) and successfully reaches the U.S. Department of Energy 2020 target. Even if the external humidification conditions are reduced to harsh 30%anode/0%cathode RH, the total Pt consumption of the MEA is only 0.156 g kW−1, which is significantly better than current conventional MEAs. In addition, the cathode and anode in this novel dissymmetric MEA can be fabricated separately, and no substrates have to be flipped over during the fabrication process, which reduce the production difficulty and rejection rate, and are beneficial to flow-line production.

극초음속 유도탄 열보호 설계를 위한 탄화성 페놀수지 복합재의 열분해 및 화학적 삭마 해석

A chemical ablation analysis of hypersonic missile for thermal protection design was performed using SAMCEF AMARYLLIS V.17, the specific ablation module of commercial software based on a finite element code. The pyrolysis and surface recession models based on the effects of mass loss, pore gas diffusion, and endothermic reaction energy were applied for non-linearity of material and boundary. The numerical studies were carried out to confirm the tendency of the pyrolysis and chemical ablation of theoretical ablative composite for open testing (TACOT) with a chemical composition similar to charring carbon/phenolic composites. The frequency of variation in surface recession, temperature, density and gas mass flux was reviewed to determine the characteristics of multiple decomposition reactions and oxidation via pyrolysis of gas species.

Sintering behaviors of micron-sized ceramic rod features

When ceramic features decrease to much smaller sizes, such as in the micron range, the dominating sintering mechanism(s) can be different from those of the bulk at large scales. In this work, the sintering behaviors of ZnO rod features of ∼4 μm diameter are studied. The rods and the bulk of the ZnO ceramics have different behaviors in shrinkage, grain growth, and densification. Sintering mechanisms are mainly grain boundary diffusion controlled densification and pore gas diffusion controlled grain growth, but with the major difference associated with a factor Ng/Nb (Ng, average pore number per grain; Nb, average pore number per grain boundary) based on the theoretical modeling and Voronoi diagram simulation. The smaller Ng/Nb for the micron-sized features leads to enhanced grain growth, and the higher exposed surface area leads to larger shrinkage; both can be explained using nominal Dgs/Dgb. Nanoparticle agglomeration can potentially affect grain growth in the rods during sintering and should be further studied.

Mechanisms of direct and in-direct sulfation of limestone

The general desulfurization process referred to as “indirect sulfation” takes place via a two-step process. Calcination makes limestone decompose to calcium oxide (CaO), and then SO2 molecules are adsorbed onto the calcined limestone. However, if the CO2 partial pressure in the system is higher than the equilibrium CO2 pressure over the limestone, an adsorption reaction between limestone and SO2 molecules can take place directly in the uncalcined state. This one-step process, called “direct sulfation,” can apply to desulfurization in pressurized fluid-bed combustion (PFBC) or an oxy–fuel combustion system capturing CO2. Limestone samples were reacted in a TGA apparatus, and the degrees of sulfation were measured under 100% air conditions for indirect sulfation and 80% CO2/20% O2 conditions for direct sulfation to compare the mechanisms and kinetics of the two sulfation processes. SEM images of sulfated particles and EDS analyses showed that the gas diffusion of indirect sulfation takes place readily through macro-pores between the grains at a low conversion rate, and SO2 adsorption occurs over the interior of the limestone particles, whereas SO2 adsorption in direct sulfation takes place at the reaction interface, and the product (CaSO4) layer permeates into the interior from the surface of the limestone particles as direct sulfation proceeds. Thus, the grain model and the shrinking unreacted core model are applicable to describing the kinetics of indirect and direct sulfation, respectively. These model predictions agreed with experimental data, and the kinetic parameters obtained from experiments are consistent with the mechanisms. Furthermore, the high activation energy of the diffusion of direct sulfation and indirect sulfation with high conversion indicate that the diffusion mechanisms in the particles in direct sulfation and indirect sulfation at high conversion are both gas diffusion in pores and solid-state diffusion. These results may contribute to understanding why the characteristics of direct sulfation differ from those of indirect sulfation under certain conditions.

Numerical study of pore structure effect on SO2–CaO reactions

The effect of varying pore structures on the kinetics of SO2–CaO reactions is not fully understood in the previous studies. Combining fractal pore model, gas molecular movement model and two-stage reaction model, a new desulfurization model is established in this paper. Fractal pore model is used to simulate CaO particle and gas molecular movement model is used to simulate gas diffusion in pores. Fractal dimension is used to characterize complexity of pore structure instead of tortuosity factor. It is found that the reaction is significantly affected by pore structures. A modulus ϕ is introduced to characterize the relationship between varying pore structures and apparent reaction parameters. And this relationship is verified by thermo-gravimetric analysis (TGA) data. Comparing to the previous models, the effect of varying pore structure on the kinetics of the reaction is described more accurately by the desulfurization model.

Investigation of the Active Thickness of Solid Oxide Fuel Cell Cathode

In solid oxide fuel cell (SOFC) electrode, electrochemical reaction only occurs in a narrow region near the interface of electrode and electrolyte, which is regarded as the “active thickness”. However, as a key parameter for SOFC electrode microstructure design and optimization, the active thickness of SOFC cathode reported by different research groups vary significantly (from 13 μm [1] to 253 μm [2]) due to different measuring conditions or simulation assumptions.In this study, a detailed SOFC model is developed, in which the dusty gas model (DGM), Ohm’s law and the concentration related Butler-Volmer equation are used to describe the gas diffusion in pores, ion and electron conduction in electrolyte and electrodes, and electrochemical reaction kinetics, respectively. The effective ionic conductivity, electronic conductivity and the TPB length are calculated with the percolation theory and coordinate number theory [3]. Current density distribution in cathode is investigated under various operating conditions and 99% total current density is used as the indicator of the active thickness. This work could provide useful information for SOFC cathode microstructure optimization and for performance enhancement. Keywords: Solid oxide fuel cell; Cathode thickness; Triple phase boundary length[1] Lu, Z., Hardy, J. S., Templeton, J. W., & Stevenson, J. W. (2012).Extended reaction zone of La0.6Sr0. 4Co0.2Fe0. 8O3 cathode for solid oxide fuel cell. Journal of Power Sources, 198: 90-94.[2] Jiang, Z. Y., Xia, C. R., & Chen, F. L. (2010). Efficient thickness of solid oxide fuel cell composite electrode. Chinese Journal of Chemical Physics, 23(2), 217.[3] Chen, D., Bi, W., Kong, W., & Lin, Z. (2010). Combined micro-scale and macro-scale modeling of the composite electrode of a solid oxide fuel cell. Journal of Power Sources, 195(19), 6598-6610.

열분해 및 삭마 환경의 복합재 구조물의 열기계적 연계 해석

A coupled thermomechanical analysis of composite structures in pyrolysis and ablation environments is performed. The pyrolysis and ablation models include the effects of mass loss, pore gas diffusion, endothermic reaction energy, surface recession, etc. The thermal and structural analysis interface is based upon a staggered coupling algorithm by using a commercial finite element code. The characteristics of the proposed method are investigated through numerical experiments with carbon/phenolic composites. The numerical studies are carried out to examine the surface recession rate by chemical and mechanical ablation. In addition, the effects of shrinkage or intumescence during the pyrolysis process are shown.

Effect of Na2O on the Reduction of Fe2O3 Compacts with CO/CO2

Compacts of Fe2O3 and Fe2O3 doped with varying amounts of Na2O were isothermally reduced at several temperatures, using CO/CO2 mixed gas in a vertical resistance furnace. To determine the effect of Na2O on the reduction of Fe2O3 compacts, the mass loss due to oxygen removal was continuously recorded, from which the reduction rate and rate constant were obtained. Na2O was found to retard the reduction of Fe2O3 compacts. The apparent activation energy (E a) of reaction and the mathematical relationship for pore gas diffusion suggested that the reduction behavior at the initial stages was controlled by a combination of pore gas diffusion and interfacial chemical reaction. At the intermediate and late stages of reduction, pore gas diffusion was the sole contributing factor. Morphological examination of the reduced compacts showed the formation of a liquid phase during the reduction process, which appeared to lower the rate of reaction.

Oxygen permeability and stability of asymmetric multilayer Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ ceramic membranes

Perovskite-type Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ (BSCF) is considered as one of most promising materials for the oxygen separation from air. In order to assess the impact of asymmetric architecture on oxygen transport properties and stability, dense ceramics and membranes consisting of two porous and one dense layers were fabricated and tested. The stability of Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ in atmospheres with low oxygen partial pressure is similar to that for SrCo 0.8Fe 0.2O 3 − δ . Analysis of the thickness dependence of oxygen permeation fluxes through symmetric dense membranes unambiguously showed that the permeation is limited by surface exchange kinetics at d ≤ 1 mm. The main limitations for the oxygen permeation through asymmetric membranes are associated with gas diffusion in pores and oxygen desorption process, despite a high open porosity (49%) of the porous layers. The asymmetric membranes demonstrated a stable performance if using inert sweep gas. On exposing to CO 2/H 2O containing atmospheres, the permeation decreases dramatically due to a deep decomposition process on the permeate side, resulting in the formation of carbonates and/or hydroxides causing blocking of oxygen transport.

Behavior of direct reduced iron and hot briquetted iron in the upper blast furnace shaft: Part I. Fundamentals of kinetics and mechanism of oxidation

It is considered that the use of prereduced ferrous materials and sources of metallic iron such as direct reduced iron (DRI) or hot briquetted iron (HBI) improves the productivity of the blast furnace (BF). However, oxidation of DRI/HBI can occur in the upper zone of the BF, which may increase the content of the reducing gases but may not decrease the coke rate substantially. The behavior of DRI and HBI was investigated by measuring the rate of oxidation of the materials in CO2 gas in a temperature range of 400 °C to 900 °C. In addition, the microstructure of “as-received” and oxidized materials was examined. The iron oxide phases formed due to oxidation were determined using X-ray diffraction (XRD) and a vibrating sample magnetometer. The results of isothermal experiments indicated that the kinetics of oxidation of metallic iron is slow at 400 °C. In DRI samples, the initial rate is controlled by the limited mixed control of chemical kinetics at the iron/iron oxide interface and pore mass transfer, whereas gas diffusion in pores is the rate governing step during the final stages of oxidation. The oxidation of wustite from iron is found to be faster than the oxidation of the former to magnetite. The structure of DRI after oxidation resembled a “reverse topochemical-oxide on the surface metal in the center” structure at 600 °C to 700 °C. The final iron oxide phase formed in DRI after oxidation was magnetite and not hematite. The oxidation of HBI was limited to the surface of the samples at lower temperatures; at 900 °C, moderate oxidation was observed and a topochemical iron oxide layer was formed.

Gas-Diffusion Process in a Tubular Cathode Substrate of a SOFC: II: Identification of Gas-Diffusion Process Using AC Impedance Method

The effects of cathodic dc bias, bulk and effective -diffusivity on ac impedance spectra of Siemens Westinghouse Power Corporation’s cathode-supported solid oxide fuel cells were systematically studied over a temperature range of 800 to 1000°C. It was found that the activation process dominated the overall electrode kinetics at 800°C, by which the applied dc bias reduced the electrode resistance considerably. With increasing the temperature to above 900°C, the activation process became effectively activated, leading to a visible arc at the lowest frequency on the impedance spectrum, which is relevant to the pore gas-diffusion process. Under this circumstance, lower bulk and lower effective diffusivity were shown in ac impedance spectra to increase pore gas-diffusion polarization as predicted by the gas-diffusion theory shown in part I of this paper. DC bias was also found to greatly affect the gas-diffusion process as a result of increased dc current. © 2004 The Electrochemical Society. All rights reserved.

Identification of Gas-Diffusion Process in a Thick and Porous Cathode Substrate of SWPC Tubular SOFC Using AC Impedance Method

The effects of cathodic DC bias, bulk Po 2 and effective O 2 -diffusivity on AC impedance spectrum of SWPC's cathode-supported SOFCs were studied over a temperature range of 800 to 1000°C. It was found that the activation process, identified as ionic transfer and O - surface diffusion, dominated the overall electrode kinetics at 800°C. The applied DC bias reduced the electrode resistance considerably. With increasing the temperature to above 900°C, the activation process became effectively activated, leading to a visible contribution of the pore gas-diffusion at the lowest frequency on the impedance spectrum. Under this circumstance, lower bulk Po 2 and lower effective O 2 -diffusivity increase pore gas-diffusion polarization as predicted by the gas-diffusion theory. DC bias also increased gas-diffusion as a result of increased DC current. At any circumstance, no pore gas-diffusion was found in pure O 2 atmosphere.

Modeling char combustion with fractal pore effects

The effects of char pores on char combustion has been studied with fractal theory. The fractal properties of different char samples were derived from mercury porosimetry measurements. Gas diffusion in pores affects combustion in char pores and this diffusion is affected by the geometrical complexity of pores. To explore gas diffusion and combustion in fractal pores, a fractal geometrical factor β is defined with pore fractal dimensions and other fractal geometrical factors. It has been found that both apparent activation energy and pre-exponential factor have linear relations with the factors β and the char combustion can be modeled with the fractal geometrical factor.

Meniscus behavior and oxygen reduction in tapered pore gas diffusion electrodes

The electrochemical reduction of oxygen in 0.88 M KOH at 25°C was studied using two silver plated copper electrodes that contained uniform, parallel, tapered pores. Photolithography, etching, and plating techniques were used to fabricate these electrodes. Exchange current densities of 2 × 10 −5 A cm −2 and 2 × 10 −6 A cm −2 were measured using an initial slope method. Diffusion limiting currents increased with differential pressure across the gas-electrolyte interface which suggests smaller contact angles at higher pressures. A wedge meniscus model that treats activation, ohmic, and concentration overpotential was applied to fit the polarization curves.