Oxygen Mass Transport Research Articles

Modeling the phenomena of dehydration and flooding of a polymer electrolyte membrane fuel cell

A one-dimensional, two-phase, transient PEM fuel cell model including gas diffusion layer, cathode catalyst layer and membrane is developed. The electrode is assumed to consist of a network of dispersed Pt/C forming spherically shaped agglomerated zones that are filled with electrolyte. Water is modeled in all three phases: vapor, liquid and dissolved in the ionomer to capture the effect of dehydration of the ionomer as well as flooding of the porous media. The anode is modeled as a sophisticated spatially reduced interface. Motivated by environmental scanning electron microscope (ESEM) images of contact angles for microscopic water droplets on fibers of the gas diffusion layer, we introduce the feature of immobile saturation. A step change of the saturation between the catalyst layer and the gas diffusion layer is modeled based on the assumption of a continuous capillary pressure at the interface. The model is validated against voltammetry experiments under various humidification conditions which all show hysteresis effects in the mass transport limited region. The transient saturation profiles clearly show that insufficient liquid water removal causes pore flooding, which is responsible for the oxygen mass transport limitation at high current density values. The simulated and measured current responses from chronoamperometry experiments are compared and analyzed.

Design of well and groove microchannel bioreactors for cell culture

Microfluidic bioreactors have been shown valuable for various cellular applications. The use of micro-wells/grooves bioreactors, in which micro-topographical features are used to protect sensitive cells from the detrimental effects of fluidic shear stress, is a promising approach to culture sensitive cells in these perfusion microsystems. However, such devices exhibit substantially different fluid dynamics and mass transport characteristics compared to conventional planar microchannel reactors. In order to properly design and optimize these systems, fluid and mass transport issues playing a key role in microscale bioreactors should be adequately addressed. The present work is a parametric study of micro-groove/micro-well microchannel bioreactors. Operation conditions and design parameters were theoretically examined via a numerical model. The complex flow pattern obtained at grooves of various depths was studied and the shear protection factor compared to planar microchannels was evaluated. 3D flow simulations were preformed in order to examine the shear protection factor in micro-wells, which were found to have similar attributes as the grooves. The oxygen mass transport problem, which is coupled to the fluid mechanics problem, was solved for various groove geometries and for several cell types, assuming a defined shear stress limitation. It is shown that by optimizing the groove depth, the groove bioreactor may be used to effectively maximize the number of cells cultured within it or to minimize the oxygen gradient existing in such devices. Moreover, for sensitive cells having a high oxygen demand (e.g., hepatocytes) or low endurance to shear (e.g., human embryonic stem cells), results show that the use of grooves is an enabling technology, since under the same physical conditions the cells cannot be cultured for long periods of time in a planar microchannel. In addition to the theoretical model findings, the culture of human foreskin fibroblasts in groove (30 microm depth) and well bioreactors (35 microm depth) was experimentally examined at various flow rates of medium perfusion and compared to cell culture in regular flat microchannels. It was shown that the wells and the grooves enable a one order of magnitude increase in the maximum perfusion rate compared to planar microchannels. Altogether, the study demonstrates that the proper design and use of microgroove/well bioreactors may be highly beneficial for cell culture assays.

Effects of Temperature and Relative Humidity on Oxygen Permeation in Nafion® and Sulfonated Poly(Arylene Ether Sulfone)

Oxygen permeation in Nafion®117 and sulfonated poly(arylene ether sulfone) (BPSH-50) were investigated at different temperatures and relative humidities by a potential step method using a Pt microelectrode. The diffusion coefficient increased, while the solubility decreased with an increased in temperature and relative humidity for both samples. Consequently, the permeability increased with an increased in temperature and relative humidity for both samples. The permeability in BPSH-50 was approximately four times lower than that in Nafion®117, due to a smaller diffusion coefficient and a lower solubility in BPSH-50 than those in Nafion®117. Mass transport of oxygen in these membranes was correlated with microstructural changes of the membranes.

Enhanced Gas−Liquid Mass Transfer of an Oscillatory Constricted-Tubular Reactor

The mass transfer performance has been tested for gas−liquid flow in a new tubular reactor system, the oscillating mesotube (OMT), which features the oscillatory movement of fluid across a series of smooth constrictions located periodically along the vertical 4.4 mm internal diameter tube. The effect of the fluid oscillations (frequency, f, and center-to-peak amplitude, x0, in the range of 0−20 s−1 and 0−3 mm, respectively) on the overall volumetric mass transfer coefficient (kLa) has been tested by measuring the oxygen saturation levels with a fiber-optical microprobe (oxygen micro-optrode), and a mathematical model has been produced to describe the oxygen mass transport in the OMT. The oxygen mass transfer rates were about 1 order of magnitude higher (kLa values up to 0.16 s−1) than those values reported for gas−liquid contacting in a 50 mm internal diameter oscillatory flow reactor (OFR), for the same peak fluid oscillatory velocity, i.e., 2πfx0. This represents remarkable oxygen transfer efficiencies, especially when considering the very low mean superficial gas velocity involved in this work (0.37 mm s−1). The narrower constrictions helped to increase the gas fraction (holdup) by reducing the rise velocity of the bubbles. However, the extent of radial mixing and the detachment of vortex rings from the surface of the periodic constrictions are actually the main causes of bubbles retention and effective gas−liquid contacting and are, thus, responsible for the enhancement of kLa in the OMT.

The effective oxidation pressure of indium–oxygen system

A theoretical model on oxygen transport at the surface of liquid metals has been validated by dynamic surface tension measurements performed on liquid Indium as test metal. The oxygen contamination conditions have been obtained at different oxygen partial pressures under both low total pressure (Knudsen regime) and inert atmospheric pressure (Fick regime) conditions, confirming that an oxide removal regime occurs under an oxygen partial pressure much higher than the equilibrium one (the “Effective Oxidation Pressure”). Experimental results are reported which give a further insight on the relative importance of the various processes due to the oxygen mass transport between the liquid metal and the gas phase. The critical aspects involved in surface tension measurements of liquid metals, related to the problem of liquid metal–oxygen interactions, are also underlined.

The effect of the parasitic current on the Direct Ethanol PEM Fuel Cell Operation

In the present work the effect of the parasitic or leakage current, I p, which is the result of the ethanol crossover through the polymer electrolyte membrane (PEM) from the anode to the cathode side of the cell, on both the cathode activation overpotential and the fuel cell operation is investigated. A one-dimensional (1-D), isothermal mathematical model is developed in order to describe the operation of a Direct Ethanol PEM Fuel Cell (DE-PEMFC) in steady state. The equations used describe the mass transport of both ethanol and humidified oxygen at the anode and the cathode compartment of the cell respectively. The mathematical model is validated against experimental data and a relatively good agreement between the model predictions and the experimental results is found. The direct correlation that exists between the ethanol crossover rate and the parasitic current formation is graphically depicted. Moreover, when the parasitic current is enabled and disabled, the calculation of the cathode activation overpotential shows that the mixed overpotential for a DE-PEMFC poses a serious problem hindering the fuel cell operation. According to the model results, the parasitic current is greater at low current density values due to the greater amounts of the crossovered ethanol. Finally, the effect of both the oxygen feed concentration and the parasitic current formation on the fuel cell operation is also presented and discussed.

126 頸動脈分岐部での酸素物質輸送の数値解析(OS1-4:循環器系・血流の計測とメカニクス(4),OS1:循環器系・血流の計測とメカニクス)

126 頸動脈分岐部での酸素物質輸送の数値解析(OS1-4:循環器系・血流の計測とメカニクス(4),OS1:循環器系・血流の計測とメカニクス)

Electrochemical impedance investigation of proton exchange membrane fuel cells experienced subzero temperature

Polarization losses of the fuel cells with different residual water amount frozen at subzero temperature were investigated by electrochemical impedance spectroscopy (EIS) taking into account the ohmic resistance, charge transfer process, and oxygen mass transport. The potential-dependent impedance before and after eight freeze/thaw cycles suggested that the ohmic resistance did not change, while the change of the charge transfer resistance greatly depended on the residual water amount. Among the four cells, the mass transport resistance of the cell with the largest water amount increased significantly even at the small current density region. According to the thin film-flooded agglomerate model, the interfacial charge transfer process and oxygen mass transport within the agglomerate and through the ionomer thin film in the catalyst layer both contributed to the high frequency impedance arc. From the analysis of the Tafel slopes, the mechanism of the oxygen reduction reaction (ORR) was the same after the cells experienced subzero temperature. The agglomerate diffusion changed a little in all cells and the thin film diffusion effect was obvious for the cell with the largest residual water amount. These results indicated that the slower oxygen diffusion within the catalyst layer (CL) was the main contributor for the evident performance loss after eight freeze/thaw cycles.

Electrochemical Quartz Crystal Microbalance in a Channel Flow Cell: A Study of Copper Dissolution

We describe how a quartz crystal microbalance (QCM) may be incorporated in a channel flow system (CFC). It is demonstrated that although deposition is nonuniform the increase in mass loading of the electrode can be related to the change in the resonance frequency of the system. The QCM−CFC is used to investigate the corrosion of copper in acidic sulfate solutions. It is demonstrated that the rate of corrosion in the system is limited by mass transport of oxygen at low volumetric flow rates and by the oxidation of an adsorbed copper(I) intermediate at high volumetric flow rates. Concentration profiles of reactants and products in the QCM−CFC have been calculated, using the backward implicit finite difference method to solve the convective−diffusion equation and rate constants for the oxidation of the Cu(I) species determined

Investigation of the oxygen evolving electrode in pH-neutral electrolytes: Modelling and experiments of the RDE-cell

A model has been developed to illustrate the complex interplay between the acidifying electrode reactions for oxygen evolution, mass transport and homogeneous reactions in pH-neutral electrolytes. Modelled polarisation curves of the oxygen evolution reaction were verified by polarisation curves experimentally measured in 5 M NaClO 4 on a RDE of DSA material. The conditions in the simulations and in the experiments were similar to those in the chlorate process (high ionic strength, 70 °C, chromate-containing electrolyte, DSA electrode), in which the oxygen evolution reaction is one of the possible side reactions. The model predicted the concentration gradients of H +, OH −, CrO 4 2− and HCrO 4 − during oxygen evolution on the RDE. It was found that an important part of the chromate buffering effect at high current densities occurs in a thin (in the order of nanometers) reaction layer at the anode. From comparisons between the model and experiments, a buffering reaction has been proposed. The most likely reaction for the chromate buffering in the investigated system is CrO 4 2− reacting with water to HCrO 4 − and OH −. In the chlorate process, where chromate is a buffer and oxygen evolution is a side reaction, it is likely that chromate promotes oxygen evolution from OH −.

The Effects of Time Varying Curvature on Species Transport in Coronary Arteries

Alterations in mass transport patterns of low-density lipoproteins (LDL) and oxygen are known to cause atherosclerosis in larger arteries. We hypothesise that the species transport processes in coronary arteries may be affected by their physiological motion, a factor which has not been considered widely in mass transfer studies. Hence, we numerically simulated the mass transport of LDL and oxygen in an idealized moving coronary artery model under both steady and pulsatile flow conditions. A physiological inlet velocity and a sinusoidal curvature waveform were specified as velocity and wall motion boundary conditions. The results predicted elevation of LDL flux, impaired oxygen flux and low wall shear stress (WSS) along the inner wall of curvature, a predilection site for atherosclerosis. The wall motion induced changes in the velocity and WSS patterns were only secondary to the pulsatile flow effects. The temporal variations in flow and WSS due to the flow pulsation and wall motion did not affect temporal changes in the species wall flux. However, the wall motion did alter the time-averaged oxygen and LDL flux in the order of 26% and 12% respectively. Taken together, these results suggest that the wall motion may play an important role in coronary arterial transport processes and emphasise the need for further investigation.

Optimal conditions for simulating microgravity employing NASA designed rotating wall vessels

This article discusses the requirements to successfully simulate certain aspects of microgravity employing the NASA designed HARV in the laboratory. Presence of low fluid shear, a clear understanding of the role of the spin of the microcarrier/cell about its own axis, availability of adequate mass transport of nutrients and oxygen, the elimination of collisions between microcarriers and between the microcarrier and walls of the bioreactor, and the use of adequate and appropriate controls are some of the important aspects that enable a successful simulation of certain aspects of microgravity. Results of numerical simulation of the above aspects have been presented and discussed.

Enzymatic Fluorescent Microsphere Glucose Sensors:Evaluation of Response Under Dynamic Conditions

Most previous attempts at the development of a "smart tattoo" for glucose monitoring in diabetes--implantable fluorescent microspheres that can be implanted intradermally and interrogated transdermally using light--have focused on the encapsulation of a competitive binding assay for glucose within hydrogel microspheres or polyelectrolyte microcapsules. We recently reported on the development of a microsphere sensor based on an enzymatic scheme, combined with an oxygen-quenched fluorescent reporter element. A novel feature of this design is the use of polyelectrolyte multilayer nanofilms, which are assembled stepwise on the surface of the microspheres, for modulation of glucose and oxygen mass transport, allowing them to be designed for sensitive and safe operation within the "oxygen deficit" present in the skin. In this work, a flow-through sensor testing apparatus was used to demonstrate the reversible response of the sensors under controlled, dynamic conditions. The sensors were shown to be sensitive over at least 0-140 mg/dL glucose, with a response time of less than 2 min. This report also identifies some current problems with the approach, and gives potential solutions towards the development of a practical implantable sensor for glucose monitoring in diabetes.

Advanced air-breathing direct methanol fuel cells for portable applications

A novel MEA is fabricated to improve the performance of air-breathing direct methanol fuel cells. A diffusion barrier on the anode side is designed to control methanol transport to the anode catalyst layer and thus suppressing the methanol crossover. A catalyst coated membrane with a hydrophobic gas diffusion layer on the cathode side is employed to improve the oxygen mass transport. It is observed that the maximum power density of the advanced DMFC with 2 M methanol solution achieves 65 mW cm −2 at 60 °C. The value is nearly two times more than that of a commercial MEA. At 40 °C, the power densities operating with 1 and 2 M methanol solutions are over 20 mW cm −2 with a cell potential at 0.3 V.

Canary Current and North Equatorial Current from an inverse box model

The large‐scale Canary Basin circulation is estimated from a box inverse model applied to hydrographic data from a quasi‐synoptic survey carried out in September 2003. The cruise consisted of 76 full depth CTD and oxygen stations. Circulation is required to nearly conserve mass and anomalies of salinity and heat within layers bounded by neutral surfaces. It permits advective and diffusive exchange between layers and an adjustment of the Ekman transport and the freshwater flux divergences. The Canary Current at the thermocline layer transports a net mass of 4.7 ± 0.8 Sv southward north of the Canary Islands from the African coast to 19°W. It is divided into a northward circulation at a rate of 1.1 ± 0.5 Sv between the African coast and Lanzarote Island and a southward transport of 5.8 ± 0.6 Sv. It transports North Atlantic Central Water and organic matters advected offshore by the filaments protruding from the upwelling system off northwest Africa. At 24°N, the Canary Current feeds the North Equatorial Current that transports a mixture of North and South Atlantic Central Waters westward. In the intermediate layer a southwestward flow of 1.2 ± 1.1 Sv transports Mediterranean Water to the Subtropical Gyre, though the highest salt flux is transported by a meddy. Oxygen distribution and mass transport suggest a northeastward deep flow of a water mass colder than 2.2°C consisting of diluted Antarctic Bottom Water. The heat and freshwater divergences and the average dianeutral velocity and diffusion between the sections and the African coast are negligible.

Effects of cathode open area and relative humidity on the performance of air-breathing polymer electrolyte membrane fuel cells

For portable applications, the, characteristics of passive air-breathing PEMFCs were investigated by examining effects of cathode open area and relative humidity on the cell performance. Among the single cells with cathode open area from 52 to 94%, the single cell with the open area of 77% exhibited the highest performance. The cell performance was improved with increasing RH of atmosphere from 20 to 100% in the low current region while lowered in high current region. Those results were related with the mass transport of oxygen from the atmosphere to the catalyst layer and the degree of membrane hydration determining the ionic conductivity of the membrane.

Evaluation of a cathode gas channel with a water absorption layer/waste channel in a PEFC by using visualization technique

The polymer electrolyte fuel cell (PEFC) cathode is a performance-limiting component due to the slower oxygen reduction kinetics and mass transport limitations imposed by water generated in an electrochemical reaction. This water assists the performance of the PEFC by preventing drying of the polymer electrolyte. Conversely, the water hinders the transport of the reactant species by blocking the pores in the gas diffusion layer. Moreover, the effective electrode area is decreased, causing the cathode channel to become clogged with supersaturated water from the gas diffusion layer. This problem is overcome by separating the gas channel and the waste channel, and installing a water absorption layer (WAL). The new “WAL type” gas channel has an installed WAL in which the designed waste channel is compared with the gas flow characteristics of a conventional cathode gas channel by using the visualization technique. Gas flowing into the WAL type separator is barely blocked before the WAL absorbs water condensed in the cathode gas channel. Therefore, the WAL type separator effectively improves the PEFC performance.

Dimensionless numbers and correlating equations for the analysis of the membrane-gas diffusion electrode assembly in polymer electrolyte fuel cells

The Quraishi–Fahidy method [Can. J. Chem. Eng. 59 (1981) 563] was employed to derive characteristic dimensionless numbers for the membrane-electrolyte, cathode catalyst layer and gas diffuser, respectively, based on the model presented by Bernardi and Verbrugge for polymer electrolyte fuel cells [AIChE J. 37 (1991) 1151]. Monomial correlations among dimensionless numbers were developed and tested against experimental and mathematical modeling results. Dimensionless numbers comparing the bulk and surface-convective ionic conductivities, the electric and viscous forces and the current density and the fixed surface charges, were employed to describe the membrane ohmic drop and its non-linear dependence on current density due to membrane dehydration. The analysis of the catalyst layer yielded electrode kinetic equivalents of the second Damköhler number and Thiele modulus, influencing the penetration depth of the oxygen reduction front based on the pseudohomogeneous film model. The correlating equations for the catalyst layer could describe in a general analytical form, all the possible electrode polarization scenarios such as electrode kinetic control coupled or not with ionic and/or oxygen mass transport limitation. For the gas diffusion-backing layer correlations are presented in terms of the Nusselt number for mass transfer in electrochemical systems. The dimensionless number-based correlating equations for the membrane electrode assembly (MEA) could provide a practical approach to quantify single-cell polarization results obtained under a variety of experimental conditions and to implement them in models of the fuel cell stack.

Electrochemical sensors for detection of hydrogen in air: model of the non-Nernstian potentiometric response of platinum gas diffusion electrodes

The potentiometric response of three different platinum gas diffusion electrodes deposited on H 3PO 4 doped polybenzimidazole (PBI) was investigated under humidified atmospheres that contained H 2 or mixtures of H 2 and O 2. Continuum modelling was used to analyse the response. It is shown that the non-Nernstian response under H 2 H 2O N 2 mixtures can be explained by a difference of water activity on both sides of the membrane. Under H 2 O 2 N 2 mixtures, the oxygen mass transport parameters have a strong effect on the electrode sensitivity.

Oxygen Distribution in Packed Bed Membrane Reactors For Partial Oxidation Systems and its Effect On Product Selectivity

Packed bed membrane reactors (PBMRs) are currently considered for the distributive addition of oxygen in partial oxidation systems. Among other advantages the decreased oxygen concentrations in the PBMR can result in improved product selectivities for reaction systems in which the oxygen dependency of the target product formation is less pronounced than that of the waste product formation. Oxidative dehydrogenation (ODH) of methanol to formaldehyde (Diakov et al., 2002) and ethylbenzene to styrene (Shakhnovich et al., 1984) are industrially relevant examples of such a kinetic system.To achieve considerable selectivity improvements the oxygen concentration should be kept small compared to hydrocarbon concentrations. However, a decrease of the oxygen concentration is accompanied with a decrease in the effectiveness of the catalyst particles since the intraparticle oxygen concentration gradients (and not the hydrocarbon concentration gradients) predominantly determine the actual activity and product selectivities of the catalyst, rendering the common effectiveness factors inapplicable for the modeling of a PBMR.Furthermore, concentration profiles over the radius of the packed bed can emerge, if the radial mass transport rate of oxygen from the membrane wall to the center of the bed is insufficient compared to the local oxygen consumption rate. If the transmembrane flux is dominated by convective transport as typical with porous membranes, the radial oxygen concentration profiles result in increased product losses, and the use of a one-dimensional reactor model may result in an overestimation of the product selectivity.In this paper the effect of limitations of the oxygen mass transport in a PBMR – i.e. intraparticle and from the membrane to the centerline of the packed bed – have been discussed for the ODH of methanol and ethylbenzene.