Partial Pressure Gradient Research Articles

Transport Properties of Film and Bulk Sr0.98Zr0.95Y0.05O3−δ Membranes

In electrode-supported solid oxide fuel cells (SOFCs) with a thin electrolyte, the electrolyte performance can be affected by its interaction with the electrode, therefore, it is particularly important to study the charge transport properties of thin electrode-supported electrolytes. The transport numbers of charged species in Ni-cermet supported Sr0.98Zr0.95Y0.05O3−δ (SZY) membranes were studied and compared to those of the bulk membrane. SZY films of 2.5 μm thickness were fabricated by the chemical solution deposition technique. It was shown that the surface layer of the films contained 1.5–2 at.% Ni due to Ni diffusion from the substrate. The Ni-cermet supported 2.5 μm-thick membrane operating in the fuel cell mode was found to possess the effective transport number of oxygen ions of 0.97 at 550 °C, close to that for the bulk SZY membrane (0.99). The high ionic transport numbers indicate that diffusional interaction between SZY films and Ni-cermet supporting electrodes does not entail electrolyte degradation. The relationship between SZY conductivity and oxygen partial pressure was derived from the data on effective conductivity and ionic transport numbers for the membrane operating under two different oxygen partial pressure gradients—in air/argon and air/hydrogen concentration cells.

Determination of Oxygen Diffusion and Surface Exchange Coefficients of Mixed Ionic-Electronic Conductors by Oxygen Semi-Permeation Methods

The understanding of oxygen transport mechanisms through mixed ionic-electronic conductors is of great interest for the development of electrochemical devices at high temperature for energy conversion applications. This study is focused on the determination of two coefficients—the oxygen diffusion coefficient and the surface exchange coefficient which are key parameters for the electrochemical performance of a material. These coefficients can be largely impacted by the measurement conditions. The oxygen semi-permeation method is an adequate method to determine these two fundamental coefficients close to the working conditions of the material, i.e., under small or large oxygen partial pressure gradients.

Water Vapor Permeability through Porous Polymeric Membranes with Various Hydrophilicity as Synthetic and Natural Barriers.

The article is devoted to the analysis of sorption kinetics, permeability, and diffusion of water vapor in porous polymeric membranes of different hydrophilicities and through-porosities. The water transport measurement with a constant gradient of partial pressure allows the authors to obtain reliable characteristics for porous membranes, films, artificial leathers, and fabrics of various chemical natures (synthetic and bio-based) and phase structures. All the kinetic permeability curves were determined and effective diffusion coefficients, as well as their apparent activation energies, were calculated at the stationary and non-stationary stages of the mass transfer. The relationship between the sorption–diffusion characteristics of the polymer barriers and their vapor permeability is traced. Within the framework of a Zolotarev–Dubinin dual dispersive model, an analytical equation is obtained that relates permeability to diffusion coefficients of water vapor in the pore volume, polymer skeleton material using such characteristics as porosity and the solubility coefficient. It is proposed to use this equation to predict the sorption properties for barrier and porous materials of complex architecture specifically in food packaging.

Mapping gas exchanges in headwater streams with membrane inlet mass spectrometry

Using continuous injections of helium coupled to in-situ continuous flow membrane inlet mass spectrometry (CF-MIMS), we mapped the gas exchanges along two low-slope headwater streams having discharges of 25 L s−1 and 90 L s−1. Mean reaeration rate coefficients (k2) were estimated at 130 d−1 and 60 d−1, respectively. Our study revealed that gas exchanges along headwater streams are highly heterogeneous. The variable morphology of the streambed causes gas exchanges to be focused into small areas, namely small cascades made up of stones or wood, with reaeration rate coefficients up to 40 times higher than in low-turbulent zones. As such, cascades appear to be hot spots for both oxygenation and greenhouse gases emissions. Additional O2 and CO2 measurements effectively showed fast exchanges between the stream and the atmosphere in the cascades, following the partial pressure gradients. These cascades allow a fast oxygenation of the eutrophic streams depleted in O2, which sustains respiration. Simultaneously, cascades release the oversaturated CO2 originating from groundwater inputs to the atmosphere. By comparing measured reaeration rate coefficients to ten predictive equations from literature, we showed that all equations systematically underestimate reaeration rate coefficients, with significantly higher discrepancies in cascades than in low-turbulent zones. The inadequate characterization of the processes occurring in cascades causes empirical equations to have poor predictive capabilities, leading to a global underestimation of CO2 emission from headwater streams.

Ceramic Membranes: Materials – Components – Potential Applications

AbstractGas separation in dense ceramic membranes is driven by the partial pressure gradient across the membrane. The mixed conducting materials most commonly used are single‐phase perovskites or fluorites. In recent years, the development of dual‐phase systems combining a mixed ion‐conducting and electron‐conducting phase has increased. The advantage is that a larger number of very stable materials systems is available. The membrane designs currently used include planar, tubular, hollow‐fiber, and honeycomb membranes. Each of these designs has specific advantages and disadvantages, depending on the application. Innovative joining concepts are also often needed due to the high temperatures involved. These usually involve the use of glass‐ceramic sealants or reactive metal brazes. Applications focus either on the separation of gases alone, i.e., the supply of oxygen or hydrogen, or on membrane reactors. In membrane reactors, a chemical reaction occurs on one or both sides of the membrane in addition to gas separation. The supply of gases is of potential interest for power plants, for the cement, steel, and glass industries, for the medical sector, and for mobile applications. Membrane reactors can be used to produce base chemicals or synthetic fuels.

Breathing with fins: do the pectoral fins of larval fishes play a respiratory role?

Convective water flow across respiratory epithelia in water-breathing organisms maintains transcutaneous oxygen (O2) partial pressure (Po2) gradients that drive O2 uptake. Following hatch, larval fishes lack a developed gill and the skin is the dominant site of gas transfer, yet few studies have addressed the contribution of convective water flow to cutaneous O2 uptake in larvae. We hypothesized that the pectoral fins, which can generate water flow across the skin in larvae, promote transcutaneous O2 transfer and thus aid in O2 uptake. In zebrafish (Danio rerio), the frequency of pectoral fin movements increased in response to hypoxia at 4 days postfertilization (dpf), but the response was blunted by 15 dpf, when the gills become the dominant site of O2 uptake, and was absent by 21 dpf. In rainbow trout (Oncorhynchus mykiss), Po2 measured at the skin surface of ventilating larvae was lower when the pectoral fins had been surgically removed, directly demonstrating that fins contribute to convective flow that dissipates cutaneous Po2 boundary layers. Lack of pectoral fins compromised whole animal O2 consumption in trout during hypoxia, but this effect was absent in zebrafish. Overall, our findings support a respiratory role of the pectoral fins in rainbow trout, but their involvement in zebrafish remains equivocal.

Oxygen permeation through single-phase perovskite membrane: Modeling study and comparison with the dual-phase membrane

Dual-phase Ce0.85Sm0.15O1.925-Sm0.6Sr0.4Al0.3Fe0.7O3-δ (SDC-SSAF) mixed ionic-electronic conducting (MIEC) membrane, which is highly stable in CO2-rich atmosphere, is a promising candidate for oxyfuel applications. Here, the oxygen permeation behavior of the single-phase perovskite Sm0.6Sr0.4Al0.3Fe0.7O3-δ (SSAF) membrane is analyzed in comparison to that of SDC-SSAF. The oxygen permeation resistances and kinetic constants of a 0.5 mm-thick SSAF disk were regressed using the Zhu model. At 950 °C and under an oxygen partial pressure (PO2) gradient of 0.21 bar/0.02 bar, the characteristic thickness (Lc) was 0.443 mm. Oxygen permeation in SSAF is generally limited by a mixture of surface exchange reactions and bulk diffusion between 800 and 950 °C at 0.21 bar/0.02 bar (feed/permeate). Overall, temperature is more dominant than PO2 gradient to enhance the oxygen permeation flux. A surface exchange-bulk diffusion transition region that exists between 850 and 875 °C in SDC-SSAF case is absent in SSAF case since oxygen permeation through SSAF is governed by mixed bulk diffusion and surface exchange.

Structure and electrical properties of YSZ-rGO composites and YSZ ceramics, obtained from composite powder

Despite zirconia based ceramics is technologically important and applied as a solid electrolyte, the studies for new compositions with enhanced performance are carried out continuously. Zirconia-graphene composites are regarded as promising oxygen transport membranes having a cross-flow of ionic and electronic charge carriers under oxygen partial pressure gradient and as an alternative to perovskite-based SOFC interconnects. In the work 92ZrO2-8Y2O3 (mol.%, YSZ) ceramics and YSZ-rGO composite ceramics (where rGO is reduced graphene oxide) were obtained from composite precursors by the calcination at 1823 K in air and in vacuum, respectively. In order to determine the optimal amount of rGO addition to the ceramic precursor simple mathematical model was suggested for the first time which is scalable for any ceramics-graphene derivative and metal-graphene derivative systems. Using SEM, EDX, XRD, Raman spectroscopy and Impedance spectroscopy data the effect of rGO and the annealing ambiance on the structure, temperature dependencies of grain and grain boundary conductivities of YSZ ceramics rGO-YSZ and composites were revealed for the first time. The rGO additive acts as grain growth inhibitor, which is completely removed from ceramics during annealing in air resulting in ceramics with the refined microstructure and coinciding temperature dependencies of grain and grain boundary conductivities in all temperature range. Vacuum annealing results in YSZ-rGO composites with high ionic conductivity and high crystallinity.

Net Sea–Air CO$_{2}$ Fluxes and Modeled Partial Pressure of CO$_{2}$ in Open Ocean of Bay of Bengal

The key role of oceans in the global climatic system can be accurately quantified by the measurement of CO $_{2}$ exchange across air–sea interface. The direction and magnitude of this CO $_{2}$ exchange are mainly governed by the gradient of partial pressure of carbon dioxide (pCO $_{2}$ ) at the air–sea interface. The sea surface pCO $_{2}$ is highly variable, primarily regulated by seasonal sea surface temperature (SST) and CO $_{2}$ (aqueous) concentration. In this paper, the moored pCO $_{2}$ observations from autonomous pCO $_{2}$ system deployed at 15 $^{\circ }$ N 90 $^{\circ }$ E in the Bay of Bengal (BOB) were used to derive CO $_{2}$ fluxes to examine variability over time scale at BOB. An attempt has also been made to develop a model using multiple linear regression (MLR) and support vector regression (SVR) to estimate the sea surface pCO $_{2}$ from space-based observations of SST and salinity. pCO $_{2}$ variability was primarily attributed to SST and CO $_{2}$ concentration changes. Derived fluxes showed that BOB is a net annual source (0.440 g-C/m $^{2}$ /year for year 2014) of CO $_{2}$ at mooring location with significant seasonality effects. For the estimation of surface pCO $_{2}$ , SVR model showed better results [root mean square error (RMSE) = 7.68 $\mu$ atm] in comparison to MLR model (RMSE = 12.36 $\mu$ atm). The seasonal climatological pCO $_{2}$ maps from September 2011 to February 2018 showed southward increase in pCO $_{2}$ in all seasons possibly due to southward increase in SST and less influence of river water. These pCO $_{2}$ maps can further be utilized in deriving CO $_{2}$ flux maps for BOB.

Effects of inspired oxygen fractions in rabbits anesthetized with isoflurane or sevoflurane, maintained on spontaneous ventilation

ABSTRACT It is important to identify the best inspired fraction of oxygen in a variety of situations, including sevoflurane or isoflurane anesthesia, in spontaneously breathing rabbits. For this, 64 rabbits were assigned to eight groups: GI100 (FiO2= 1,0 + isoflurane), GS100 (FiO2= 1,0 + sevoflurane), GI80 (FiO2= 0,8 + isoflurane), GS80 (FiO2= 0,8 + sevoflurane), GI60 (FiO2= 0,6 + isoflurane), GS60 (FiO2= 0,6 + sevoflurane), GI21 (FiO2= 0,21 + isoflurane), GS21 (FiO2= 0,21 + sevoflurane). The induction was performed with (2.5MAC) of the anesthetic. The vaporizer was setted at 1.5 MAC and FiO2 as attributed for each group. After the induction, the concentration was changed to 1 MAC. Measurements of parameters were performed 30 minutes after induction (T0), and then at 15 minute intervals (from T15 to T60). The arterial partial pressures of oxygen (PaO2), alveolar oxygen partial pressure (PAO2) and alveolar-arterial oxygen gradient [P(A-a)O2] were higher with the use of high FiO2. The GI80 showed higher levels of PaO2 FiO2 ratio and respiratory index (RI). In conclusion, the FiO2 of 0.21 is not indicated, because it causes hypoxemia. The isoflurane determines better ventilation when compared to sevoflurane, but isoflurane associated with 80% of oxygen promotes intrapulmonary shunt increase.

The case for reporting PO2 (partial pressure of oxygen), in addition to DO (dissolved oxygen), in studies of aquatic systems

In this essay, we question the sole use of mg/L (ppm) and the term DO (dissolved oxygen) when referring to oxygen supply in water, and discuss reasons for also reporting the partial pressure of oxygen (PO2). Oxygen moves from water into organisms by diffusion only, and the rate and direction of diffusion is solely dependent upon partial pressure gradients, not concentration gradients. However, the amount of oxygen moved into an organism is both a function of DO and PO2. Therefore, to have physiological and ecological relevance, the presence of oxygen in water, when reported as DO, should also include PO2.

Effect of net gas volume changes on alveolar and arterial gas partial pressures in the presence of ventilation-perfusion mismatch.

The second gas effect (SGE) occurs when nitrous oxide enhances the uptake of volatile anesthetics administered simultaneously. Recent work shows that the SGE is greater in blood than in the gas phase, that this is due to ventilation-perfusion mismatch, that as mismatch increases, the SGE increases in blood but is diminished in the gas phase, and that these effects persist well into the period of nitrous oxide maintenance anesthesia. These modifications of the SGE are most pronounced with the low soluble agents in current use. We investigate further the effect of net gas volume loss during nitrous oxide uptake on low concentrations of other gases present using partial pressure-solubility diagrams. The steady-state equations of gas exchange were solved assuming a log-normal distribution of ventilation-perfusion ratios using Lebesgue-Stieltjes integration. It was shown that under these conditions the classical partial pressure-solubility diagram must be modified, that for currently used volatile anesthetic agents the alveolar-arterial partial pressure difference is less than that predicted in the past, and that the alveolar-arterial partial pressure difference may even be reversed during uptake in the case of highly insoluble gases such as sulfur hexafluoride. Comparing this with the situation described previously for nitrogen in steady-state air breathing, we show that for nitrogen, the direction of the alveolar-arterial gradient is opposite to the direction of net gas volume movement. Although gas uptake with ventilation-perfusion inequality exceeding that when matching is optimal is shown to be possible, it is less likely than alveolar-arterial partial pressure reversal. NEW & NOTEWORTHY Net uptake of gases administered with nitrous oxide may proceed against an alveolar-arterial partial pressure gradient. The alveolar-arterial gradient for nitrogen in the steady-state breathing air depends not only on the existence of a distribution of ventilation-perfusion ratios in the lung but also on the presence of a net change in gas volume and is opposite in direction to the direction of net gas volume uptake.

Numerical analysis of an ion transport membrane system for oxy–fuel combustion

Ion transport membranes (ITM) have been studied as a promising air separation unit (ASU) technology for oxy–fuel combustion owing to their high oxygen permeability. Even though the power consumption of the ITM is lower than that of cryogenic ASU, it still consumes a high proportion of the overall system power.In this study, a numerical analysis of the ITM system has been conducted using Aspen Plus® to determine the optimal system design for minimizing the power consumption to separate oxygen from air. Since the oxygen permeation through the ITM is driven by the oxygen partial pressure gradient between feed and permeation side, three ITM systems that have different pressure gradients across the membrane have been presented and their performances compared. The effects of the contributing parameters, such as thickness, pressure, temperature, and air flow rate on the oxygen permeation rate have been investigated. ITM performances of the counter and parallel flow configurations have been compared. The system that operates under atmospheric pressure at the feed channel and under vacuum pressure at the permeate channel yields the lowest power consumption for obtaining the same oxygen permeation rate among other pressure conditions.

Methane conversion to syngas and hydrogen in a dual phase Ce0.8Sm0.2O2-δ-Sr2Fe1.5Mo0.5O5+δ membrane reactor with improved stability

Coupling of partial oxidation of methane (POM) with water dissociation in an oxygen transport membrane is a promising technology for methane utilization. However, cobalt-based membrane materials show poor stability under the above harsh conditions. In this work, a nominal 60 wt % Ce0.8Sm0.2O2-δ-40 wt % Sr2Fe1.5Mo0.5O5+δ (CSO-SFMO) dual phase membrane is reported, which was synthesized by using a one-pot EDTA-citric acid complexing method. The phase structure and morphology of the CSO-SFMO membrane were characterized by XRD, SEM and EDXS. It was found that a uniform distribution of CSO phase with a fluorite structure and SFMO phase with a perovskite structure was achieved in the dual phase membrane. The CSO-SFMO membrane exhibited an improved stability compared with cobalt based perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ, (BSCF) membrane under CO2 or reductive gas atmospheres. The oxygen permeation flux of the dual phase membrane was investigated under different oxygen partial pressure gradients: air/He, air/CO2, air/POM, and H2O/POM. At 950 °C, the oxygen permeation fluxes of the CSO-SFMO membrane under air/POM and H2O/POM gradients were 2.7 cm3 (STP) min−1 cm−2 and 0.75 cm3 (STP) min−1 cm−2, respectively, which were much higher than the oxygen flux of 0.1 cm3 (STP) min−1 cm−2 under air/He. Moreover, a CO selectivity of 98%, a CH4 conversion of 97% on the POM side and a H2 production of 1.5 cm3 (STP) min−1 cm−2 on the H2O splitting side were achieved in CSO-SFMO membrane reactor under the oxygen partial pressure gradient of H2O/POM, which was steadily run for 100 h before the measurement was intentionally stopped.

Uncertainty in the global oceanic CO<sub>2</sub> uptake induced by wind forcing: quantification and spatial analysis

Abstract. The calculation of the air–water CO2 exchange (FCO2) in the ocean not only depends on the gradient in CO2 partial pressure at the air–water interface but also on the parameterization of the gas exchange transfer velocity (k) and the choice of wind product. Here, we present regional and global-scale quantifications of the uncertainty in FCO2 induced by several widely used k formulations and four wind speed data products (CCMP, ERA, NCEP1 and NCEP2). The analysis is performed at a 1° × 1° resolution using the sea surface pCO2 climatology generated by Landschützer et al. (2015a) for the 1991–2011 period, while the regional assessment relies on the segmentation proposed by the Regional Carbon Cycle Assessment and Processes (RECCAP) project. First, we use k formulations derived from the global 14C inventory relying on a quadratic relationship between k and wind speed (k = c ⋅ U102; Sweeney et al., 2007; Takahashi et al., 2009; Wanninkhof, 2014), where c is a calibration coefficient and U10 is the wind speed measured 10 m above the surface. Our results show that the range of global FCO2, calculated with these k relationships, diverge by 12 % when using CCMP, ERA or NCEP1. Due to differences in the regional wind patterns, regional discrepancies in FCO2 are more pronounced than global. These global and regional differences significantly increase when using NCEP2 or other k formulations which include earlier relationships (i.e., Wanninkhof, 1992; Wanninkhof et al., 2009) as well as numerous local and regional parameterizations derived experimentally. To minimize uncertainties associated with the choice of wind product, it is possible to recalculate the coefficient c globally (hereafter called c∗) for a given wind product and its spatio-temporal resolution, in order to match the last evaluation of the global k value. We thus performed these recalculations for each wind product at the resolution and time period of our study but the resulting global FCO2 estimates still diverge by 10 %. These results also reveal that the Equatorial Pacific, the North Atlantic and the Southern Ocean are the regions in which the choice of wind product will most strongly affect the estimation of the FCO2, even when using c∗.

Clinical efficacy of transjugular intrahepatic portosystemic shunt in the treatment of hepatopulmonary syndrome.

Transjugular intrahepatic portosystemic shunt (TIPS) reduces the portal venous pressure of patients with hepatopulmonary syndrome (HPS).To describe the patients who underwent TIPS for the treatment of HPS.A retrospective study was performed on 81 patients with HPS and gastrointestinal hemorrhage treated with TIPS. Thirty patients underwent TIPS through the main portal vein (group A), 24 through the left branch of the portal vein (group B), and 27 through the right branch of the portal vein (group C). The partial pressure of arterial oxygen (PaO2), alveolar-to-arterial oxygen partial pressure gradient (A–aPO2), oxygen saturation (SO2), and complications were recorded and compared. The survival rate for each group was calculated.The technical success rate was 100% in the 3 groups. Preoperative portal vein pressure showed no significant differences between the 3 groups, which was decreased post-TIPS operation. In group A, PaO2 and SO2 were higher in 15 days and 3 months postoperative than preoperative (P < .05), whereas A–aPO2 was lower (P < .05). No difference occurred between 12 months post- and preoperative group. In group C, PaO2 and SO2 did not alter significantly at each time point after operation (P > .05), whereas A–aPO2 decreased at 3 months (P = .041) than preoperative. In group B, all indicators at each follow-up time point after TIPS were improved significantly as compared with the preoperative group (P < .05), which showed an excellent effect on hypoxemia treatment. Although the 1-year survival rate of 3 groups of patients was 92.85%, 90.90%, and 91.67%, respectively, the rate of hepatic encephalopathy and hepatic myelopathy was 33.33% (10/30), 16.67% (4/24), and 33.33% (9/27) after TIPS.TIPS reduced the pressure of the portal vein effectively and alleviated hypoxemia in most HPS patients successfully. Thus, the left branch of the portal vein is optimal for TIPS owing to fewer complications and efficacy in improving PaO2 as compared with the main portal vein and right branch.

Asymmetric tubular CaTi0.6Fe0.15Mn0.25O3-δ membranes: Membrane architecture and long-term stability

Three architectures of asymmetric tubular oxygen transport membranes (OTM) based on CaTi0.6Fe0.15Mn0.25O3-δ were fabricated with various thicknesses of the tubular porous supports and the dense membrane layers. This was achieved by a two-step firing method combining water based extrusion and dip-coating. The oxygen flux of the tubular membranes was characterized as a function of temperature and oxygen partial pressure on both feed and sweep sides for the different architectures. The flux exhibits different functional dependencies with respect to the oxygen partial pressure gradient and the membrane architecture. Numerical simulations using a Dusty-gas model were conducted to evaluate the effect of the porous support microstructure and thickness on oxygen partial pressure gradient inside the porous media. Results from this work were used to establish dependency of the flux with respect to bulk transport properties of the material, surface kinetics and architecture of the porous support. Furthermore, long-term stability of the produced tubular asymmetric membrane operated in CO2-containing atmospheres was assessed over half a year. The membrane exhibited a stable oxygen flux without showing significant flux degradation.

Structural and chemical stability of high performance Ce0.8Gd0.2O2-δ – FeCo2O4 dual phase oxygen transport membranes

Ceramic oxide membranes are widely being researched for Carbon Capture and Storage/Utilization sector applications. Foreseen applications of these membranes are oxygen generation for oxyfuel combustion in e.g. power plants, glass-, cement- or steel production. Major drawback with Mixed Ionic and Electronic Conducting (MIEC) perovskite structure membranes is their limited long term stability at high temperatures in aggressive atmospheres. Dual phase composite membranes have been reported to excel overcoming this drawback. In addition to performance evaluation, Ce0.8Gd0.2O2-δ – FeCo2O4 (CGO-FCO) membranes were subjected to stability test in flue gas conditions closely mimicking industrial flue gas atmosphere. The dual phase composites are investigated for their phase stability at the operating temperature of 850°C in a gradient of oxygen chemical potential. The composites were also exposed to a series of gas mixtures over a period of time at their operating temperature to test for the chemical stability. CGO-FCO membranes are identified to possess chemical stability in gas mixtures of CO2, SO2 along with oxygen over a period of 200h at 850°C under oxygen partial pressure gradient.

Pore network modelling of molecular diffusion in a single‐block model during lean gas injection, investigating the effect of throat sorting

Based on analogy between isothermal drying of porous media and molecular diffusion in a single‐block model containing a volatile liquid during lean gas injection, a detailed procedure is presented for two‐dimensional quasi‐static pore network modelling of diffusion. A modified invasion percolation algorithm is used to account for evaporation of liquid clusters formed during gas injection process. Diffusion in gas phase, capillary‐induced flow in liquid phase, and evaporation in gas‐liquid interface were modelled to trace the movement of gas‐liquid interface and desaturation profile. Using a set of throat sizes with normal distribution, 15 cases of a regular pore network model were proposed by rearranging throat positions in the model. Results of this study indicate that diffusion time, breakthrough time, vapour partial pressure gradient, liquid saturation gradient, liquid cluster frequency, gas‐liquid interface frequency, and evaporation potential are strong functions of throat sorting. Correlation coefficient of throat size with position was used to capture the disorder of porous medium. It was shown that such a correlation coefficient is not capable of describing diffusion characteristics.

Diffusion-Related SOFC Stack Degradation

Solid Oxide Fuel Cell systems for stationary applications should exhibit low degradation (<< 1%/1,000h voltage loss) to ensure long-term operation (> 40,000h). The rather high operation temperature of SOFCs (≥ 700°C) is on the one hand beneficial for oxygen conduction through the ceramic-based electrolyte, but on the other hand all diffusion-related, energetically-driven mechanisms are also increased. This counts less for gas diffusion processes but is more pronounced for solid-state diffusion (surface, bulk, grain boundary). For gas diffusion processes other parameters like gas stream, individual partial pressures and partial pressure gradients are dominating. Within two different stack tests with four-plane short stacks and their intensive post-test characterization two varying diffusion-related degradation mechanisms have been investigated. One is a short-term (~1250h) test with two different chromium evaporation protection layers on the air side metallic interconnect and frame and the other one is a long-term endurance test (~ 35,000h). For the first stack two planes were coated with a manganese oxide layer applied by wet powder spraying (WPS) while the other two planes were coated by atmospheric plasma spraying (APS) with a manganese-cobalt-iron-spinel layer. Thus the protection layers do not only differ by coating technology but also by material. The voltage loss in the planes with WPS-coated interconnect was markedly higher than in those coated by APS. Finally it turned out that the layers microstructure plays the key role in Cr evaporation minimization. In this stack, gas-phase diffusion is dominating degradation. In the long-term operated stack, the influence of a diffusing element from the cathodic contact layer towards and through the electrolyte has been characterized. Manganese, presumably originating from the contact layer (but it’s additionally also part of the protection layer), diffuses via solid-state diffusion and reacts while reaching reducing conditions in the fuel compartment with the zirconia-based electrolyte. This interaction forms a secondary sponge-like porous phase at the boundary electrolyte-anode. This secondary phase causes stresses and leads, after growing to big islands of some ten micrometers in diameter, to electrolyte-anode delamination and finally to complete cell failure. For the Mn-related degradation solid state diffusion has been supposed, as the electrolyte is coated by a dense thin-film diffusion barrier layer which suppresses gas diffusion. Both results indicate that volatile elements (which could either evaporate via gas phase or diffuse via solid state diffusion) should be avoided as they can lead to severe cell, stack and system failure even after thousands of hours of high-temperature operation.