Effective Diffusion Layer Research Articles

Soft Microelectrode Linear Array for Scanning Electrochemical Microscopy

A linear array of eight individual addressable microelectrodes has been developed in order to perform high-throughput scanning electrochemical microscopy (SECM) imaging of large sample areas in contact regime. Similar to previous reports, the soft microelectrode array was fabricated by ablating microchannels on a polyethylene terephthalate (PET) film and filling them with carbon ink. Improvements have been achieved by using a 5 μm thick Parylene coating that allows for smaller working distances, as the probe was mounted with the Parylene coating facing the sample surface. Additionally, the application of a SECM holder allows scanning in contact regime with a tilted probe, reducing the topographic effects and assuring the probe bending direction. The main advantage of the soft microelectrode array is the considerable decrease in the experimental time needed for imaging large sample areas. Additionally, soft microelectrode arrays are very stable and can be used several times, since the electrode surface can be regenerated by blade cutting. Cyclic voltammograms and approach curves were recorded in order to assess the electrochemical properties of the device. An SECM image of a gold on glass chip was obtained with high resolution and sensitivity, proving the feasibility of soft microelectrode arrays to detect localized surface activity. Finite element method (FEM) simulations were performed in order to establish the effect of diffusion layer overlapping between neighboring electrodes on the respective approach curves.

Influence of anisotropic bending stiffness of gas diffusion layers on the electrochemical performances of polymer electrolyte membrane fuel cells

The effects of gas diffusion layer’s (GDL’s) anisotropic bending stiffness on the electrochemical performances of polymer electrolyte membrane fuel cells have been investigated for carbon fiber-felt and -paper GDLs. The bending stiffness values of all GDLs in the machine direction are higher than those in the cross-machine direction. We have prepared GDL sheet samples such that the machine direction of GDL roll is aligned with the major flow field direction of a metallic bipolar plate at angles of 0° (parallel: ‘0° GDL’) and 90° (perpendicular: ‘90° GDL’). The I–V performances of all the 5-cell stacks with 90° GDLs are higher than those with 0° GDLs. All the 5-cell stacks with 90° GDLs show lower values of high-frequency resistance (HFR) than those with 0° GDLs. However, the gas pressure differences at both anode and cathode of 5-cell stacks with 90° GDLs appear to be similar to or slightly lower than those with 0° GDLs, making the lower HFR as a dominant factor for the improved I–V performances. This may result from the reduced intrusion of 90° GDLs into gas channels than 0° GDLs as observed by less thickness reduction under compression of 90° GDLs. A 45° GDL (skew alignment) also shows better performances than the 0° GDL.

Three dimensional numerical investigations for the effects of gas diffusion layer on PEM fuel cell performance

Gas diffusion layer (GDL) is an important component of a proton exchange membrane fuel cell (PEMFC) to take part in the interplay of the transport of different species. It has been found that the performance of a PEMFC depends upon the morphology of the GDL. The performance of PEM fuel cell varies with different porosity and thickness of the GDL. Hence, a three dimensional model is simulated to find out the effects of porosity and thickness of GDL on PEMFC performance using a commercial code CFD-ACE+. It was observed that high porosity gave high current density by allowing more reactants to reach the reaction site. Similarly greater thickness of the GDL gives reactant species to increase the consumption rate at the GDL/catalyst layer interface. The simulation results showed that the connection of bipolar plate with the GDL played an important role for reducing the amount of reactants to reach the catalyst layer especially under the land area of the bipolar plate. However, this effect seems to decrease with an increase of overall porosity and the thickness of the GDL.

Effect of diffuse layer and pore shapes in mesoporous carbon supercapacitors

In the spirit of the theoretical evolution from the Helmholtz model to the Gouy–Chapman–Stern model for electric double-layer capacitors, we explored the effect of a diffuse layer on the capacitance of mesoporous carbon supercapacitors by solving the Poisson–Boltzmann (PB) equation in mesopores of diameters from 2 to 20 nm. To evaluate the effect of pore shape, both slit and cylindrical pores were considered. We found that the diffuse layer does not affect the capacitance significantly. For slit pores, the area-normalized capacitance is nearly independent of pore size, which is not experimentally observed for template carbons. In comparison, for cylindrical pores, PB simulations produce a trend of slightly increasing area-normalized capacitance with pore size, similar to that depicted by the electric double-cylinder capacitor model proposed earlier. These results indicate that it is appropriate to approximate the pore shape of mesoporous carbons as being cylindrical and the electric double-cylinder capacitor model should be used for mesoporous carbons as a replacement of the traditional Helmholtz model.

Investigation of freeze/thaw durability in polymer electrolyte fuel cells

This study aims to investigate the effect of different gas diffusion layers (GDLs) on freeze/thaw condition durability in polymer electrolyte fuel cells (PEFCs). Three kinds of GDLs–cloth, felt and paper type—with similar basic properties except thickness and bending stiffness were used. The changes in the properties and cell performance were investigated from the −30 to 70 °C range of freeze/thaw cycles. The I–V performance degradation was observed to be negligible for the felt GDL whereas the cloth and paper GDLs showed a marked I–V performance loss. No distinctive correlation between the changes in electrochemical properties, such as active metal surface area, hydrogen crossover rates and decreased I–V performance, was observed except an increase in ohmic resistance revealed by ac-impedance spectroscopy. The physical destruction of electrodes was also shown by scanning electron microscope (SEM) analysis. The present study found that sufficient mechanical supporting force between the interfaces of materials enhances PEFC durability in sub-zero temperature conditions.

J0802-1-1 EXPERIMENTAL AND NUMERICAL INVESTIGATION OF GAS FLOW IN POLYMER ELECTROLYTE FUEL CELL

In this study, the deformation effect of gas diffusion layer (GDL) on the pressure drop of polymer electrolyte fuel cell (PEFC) separator channel has been investigated both numerically and experimentally. Gas channel and gas diffusion layers are regarded as two important parts of PEFC because they transport reactant gases to the catalyst layer and also byproduct from the catalyst layer. The deformation of GDL caused by the compression pressure plays an important role in the performance of PEFC since it affects the physical properties of the GDL, such as porosity, permeability and the cross sectional area of the gas channel. The flow behavior in the separator channel and GDL of PEFC has been investigated by using a transient, isothermal and three-dimensional numerical model. To develop the numerical simulation of PEFC, verification experiments and data acquisition of physical parameters were conducted by mechanical measurements.

The influence of stack clamping pressure on the performance of PEM fuel cell stack

The effect of gas diffusion layer (GDL) compression on fuel cell performance was experimentally studied in a 5-cell proton exchange membrane fuel cell (PEMFC) stack. Two stacks with different GDL compression, 15% and 30%, were prepared using SGL 10BC carbon-fiber felt GDL and Gore 57 series MEA. The PEMFC stack with high GDL compression (30%) showed better performance than low GDL compression (15%) at all current ranges. This implies that the decrease of contact resistance by high GDL compression affects more dominantly on the stack performance than the increase of mass transport resistance in the present stack configurations. The degree of GDL compression also affected the stack stability, particularly, under the reformed gas conditions as anode fuel. The stack with low GDL compression could not be operated at low reformed gas flow rates showing sudden decrease of stack voltage. Hence, in case of reformed gas condition, GDL compression should be higher than the case of pure hydrogen for the stable stack operation and to enhance the gas utilization at anode.

Diffuse layer effects on the current in galvanic cells containing supporting electrolyte

We study the effect of an inert supporting electrolyte on the steady-state ionic current through galvanic cells by solving the full Poisson–Nernst–Planck transport equation coupled to the generalized Frumkin–Butler–Volmer boundary equation for the electrochemical charge transfer at the electrodes. Consequently, the model presented here allows for non-zero space charge densities locally at the electrodes, thus extending the frequently used models based on the local electroneutrality condition by including diffuse layer (DL) effects. This extension is necessary since the DLs determine the ion concentration and electrical field at the reaction planes, which uniquely determine the charge transfer at the electrodes. In this work we present numerical results for systems which contain added inert supporting electrolyte using finite element discretization and compare those with semi-analytical results obtained using singular perturbation theory (limit of negligibly thin DLs). In case of negligibly thin DLs the presence of supporting electrolyte will introduce a limiting current below the classical diffusion-limiting current. Just as for systems without supporting electrolyte, the supporting electrolyte induced limiting current formally does not occur for systems having non-negligibly thin double DLs. For thin, however still finite, double layers this limit can still be seen as a steepening of the polarization curve for current vs. voltage.

A three-dimensional analysis of the effect of anisotropic gas diffusion layer(GDL) thermal conductivity on the heat transfer and two-phase behavior in a proton exchange membrane fuel cell(PEMFC)

A three-dimensional and two-phase model was employed to investigate the effect of the anisotropic GDL thermal conductivity on the heat transfer and liquid water removal in the PEMFCs with serpentine flow field and semi-counter flow operation. The GDL with different anisotropic thermal conductivity in the three directions (x, y, z) was simulated for four cases. As a result, the water saturation, temperature, species, current, potential distribution and proton conductivity were obtained. According to the comparison between the results of each case, some new conclusions are obtained and listed as below: (1) The anisotropic GDL produces the high temperature difference than that of isotropic case, and the in-plane thermal conductivity perpendicular to the gas channels is more important than that of along channels, which may produce the larger temperature difference. (2) Water saturation decreases due to the large temperature difference in the anisotropic case, but some water vapor may condense in the area neighbor to the channel ribs due to the cool function of the current collector and the great temperature difference. (3) The anisotropic thermal conductivity in the through-plane direction and the in-plane direction perpendicular to the gas channels can lead to the decrease of the membrane conductivity. (4) The isotropic GDL is better than that of anisotropic one for the uniform current density. Also, in-plane thermal conductivity perpendicular to the channels has more negative effect on the current density distribution in the membrane than that of the along channels one.

A Droplet Size Dependent Multiphase Mixture Model and Three-dimensional Simulation of PEM Fuel Cells

A droplet size dependent multiphase mixture model is developed for the two-phase transport in PEM fuel cells. And the droplet size in the gas channel can be considered as a parameter in this mixture model, which includes the effect of gas diffusion layer(GDL) properties and the gas drag function. The three-dimensional non-isothermal two phase simulation of the PEMFCs with a straight flow field is performed. The results are shown for oxygen fraction, water saturation, water vapor fraction and temperature. The comparison between simulation results and experimental results is performed for the I-V curve, which shows the present model is better than the PEMFC module in Fluent® software. Then it provides a new way for the simulation of PEMFCs. The results for different flow pattern show there are large temperature difference and water saturation in the PEMFCs operated under coflow pattern compared under counter flow pattern.

A droplet size dependent multiphase mixture model for two phase flow in PEMFCs

A droplet size dependent multiphase mixture model is developed in this paper, and the droplet size in the gas channel can be considered as a parameter in this multiphase mixture model, which includes the effect of gas diffusion layer (GDL) properties and the gas drag function and cannot be considered in the commonly used multiphase mixture model in the references. The three-dimensional two phase and non-isothermal simulation of the PEMFCs with a straight flow field is performed. The effect of droplet size on the liquid remove, the effect of liquid water on the heat transfer and the effect of gas flow pattern on the heat and mass transfer are mainly investigated. The simulation results show that the large droplet is hard to be dragged by the gas, so it produces large water saturation. The results of the heat transfer show that the liquid water hinders the heat transfer in the GDL and catalyst layer, so it produces the large relative high temperature area, and there are large temperature difference and water saturation in the PEMFCs operated with coflow pattern compared with counter flow pattern.

Two-Phase Flow Maldistribution and Mitigation in Polymer Electrolyte Fuel Cells

Flow maldistribution among polymer electrolyte fuel-cell (PEFC) channels is of concern because this leads to nonuniform distributions of fuel and oxidizer, which in turn result in nonuniform reaction rates in the catalyst layers and thus detrimentally affect PEFC performance and durability. Channels with low flow rates risk flooding by liquid water. This can cause catalyst support corrosion and hence the undesirably accelerated aging of PEFCs. Multiphase flow computations are performed to examine the effects of gas diffusion layer (GDL) intrusion and manifold design on reducing flow maldistribution. Velocity field, hydrodynamic pressure, and liquid saturations are computed in the parallel gas channels using the multiphase-mixture formulation in order to quantify the flow nonuniformity or maldistribution among PEFC channels. It is shown that, when channel flow is in single phase, employing two splitter plates in the header manifold can bring down the flow maldistribution to less than half of that for the case with 20% area maldistribution due to the GDL intrusion. When channel flow occurs in the two-phase regime, the liquid-water front can be pushed downstream and the effect of GDL intrusion on the maximum liquid saturation can be decreased by more than one-third by using flow splitters.

Effect of hydrophobic gas diffusion layers on the performance of the polymer exchange membrane fuel cell

This study uses fuel cell gas diffusion layers (GDLs) fabricated in the laboratory from carbon fiber cloth with different concentrations of hydrophobic agents in proton exchange membrane fuel cells (PEMFCs), and investigates the relationship between the hydrophobic agent content of the carbon fiber cloth and fuel cell performance. The paper examines the effect of hydrophobic agent content on GDL thickness, contact angle, air permeability, and surface and through-plane resistivity. Carbon fiber cloth is impregnated with hydrophobic agent concentrations of 0, 3, 5, 10, 30, and 50 wt%, and the resulting GDLs are subjected to performance tests. When the test piece area is 25 cm 2, the test temperature 80 °C, the gasket thickness 0.36 mm, and the hydrophobic agent content 5 wt%, a fuel cell using the GDL has a current density of 1430 mA cm −2 at 0.3 V.

1-D DSMC simulation of Io's atmospheric collapse and reformation during and after eclipse

A one-dimensional Direct Simulation Monte Carlo (DSMC) model is used to examine the effects of a non-condensable species on Io's sulfur dioxide sublimation atmosphere during eclipse and just after egress. Since the vapor pressure of SO 2 is extremely sensitive to temperature, the frost-supported dayside sublimation atmosphere had generally been expected to collapse during eclipse as the surface temperature dropped. For a pure SO 2 atmosphere, however, it was found that during the first 10 min of eclipse, essentially no change in the atmospheric properties occurs at altitudes above ∼ 100 km due to the finite ballistic/acoustic time. Hence immediately after ingress the auroral emission morphology above 100 km should resemble that of the immediate pre-eclipse state. Furthermore, the collapse dynamics are found to be greatly altered by the presence of even a small amount of a non-condensable species which forms a diffusion layer near the surface that prevents rapid collapse. It is found that after 10 min essentially no collapse has occurred at altitudes above ∼ 20 km when a nominal mole fraction of non-condensable gas is present. Collapse near the surface occurs relatively quickly until a static diffusion layer many mean free paths thick of the non-condensable gas builds up which then retards further collapse of the SO 2 atmosphere. For example, for an initial surface temperature of 110 K and 35% non-condensable mole-fraction, the ratio of the SO 2 column density to the initial column density was found to be 0.73 after 10 min, 0.50 after 30 min, and 0.18 at the end of eclipse. However, real gas species (SO, O 2) may not be perfectly non-condensable at Io's surface temperatures. If the gas species was even weakly condensable (non-zero sticking/reaction coefficient) then the effect of the diffusion layer on the dynamics was dramatically reduced. In fact, if the sticking coefficient of the non-condensable exceeds ∼0.25, the collapse dynamics are effectively the same as if there were no non-condensable present. This sensitivity results because the loss of non-condensable to the surface reduces the effective diffusion layer size, and the formation of an effective diffusion layer requires that the layer be stationary; this does not occur if the surface is a sink. Upon egress, vertical stratification of the condensable and non-condensable species occurs, with the non-condensable species being lifted (or pushed) to higher altitudes by the sublimating SO 2 after the sublimating atmosphere becomes collisional. Stratification should affect the morphology and intensity of auroral glows shortly after egress.

Kinetics of Heterogeneous Electron Transfer Reactions at the Externally Polarized Water/o-Nitrophenyl Octyl Ether Interface

The heterogeneous electron transfer (ET) reactions between dimethylferrocenium (DiMFc+) or ferrocenium (Fc+) produced in situ in o-nitrophenyl octyl ether (NPOE) and hexacyanoferrite [Fe(CN)64−] in aqueous phase have been investigated by scanning electrochemical microscopy (SECM). The potential difference across the water/NPOE (W/NPOE) interface was controlled externally to maintain a certain driving force for the ET reactions. The standard kinetic rate constants (k120) for the above bimolecular ET reactions were measured to be 0.075 and 0.050 cm s−1 M−1 when DiMFc and Fc were employed as the respective organic reactants. The transfer coefficients (α) were calculated to be 1.09 and 1.62, respectively. Those abnormal values of α were analyzed and ascribed to the Frumkin effect (or diffuse layer effect) at both sides of the interfaces. Some of the previous reports are summarized and also analyzed based on the Frumkin effect. The possible effect of ion transfer on the ET process is discussed in detail.

A numerical study of flow crossover between adjacent flow channels in a proton exchange membrane fuel cell with serpentine flow field

The focus of this paper is to study the flow crossover between two adjacent flow channels in a proton exchange membrane (PEM) fuel cell with serpentine flow field design in bipolar plates. The effect of gas diffusion layer (GDL) deformation on the flow crossover due to the compression in a fuel cell assembly process is particularly investigated. A three-dimensional structural mechanics model is created to study the GDL deformation under the assembly compression. A three-dimensional PEM fuel cell numerical model is developed in the aforementioned deformed domain to study the flow crossover between the adjacent channels in the presence of the GDL intrusion. The models are solved in COMSOL Multiphysics—a finite element-based commercial software package. The pressure, velocity, oxygen mass fraction and local current density distribution are presented. A parametric study is conducted to quantitatively investigate the effect of the GDL’s transport related parameters such as porosity and permeability on the flow crossover between the adjacent flow channels. The polarization curves are also examined with and without the assembly compression considered. It is found that the compression effect is evident in the high current density region. Without considering the assembly compression, the fuel cell model tends to over-predict the fuel cell’s performance. The proposed method to simulate the crossover with the deformed computational domain is more accurate in predicting the overall performance.

Theoretical comparison of hydrodynamic diffusion layer models used for dissolution simulation in drug discovery and development

The effective hydrodynamic diffusion layer thickness ( h eff) of a drug particle dissolving into an agitated fluid is of great importance for oral absorption simulation. The purpose of the present study was to: (1) introduce a h eff estimation method based on the fluid dynamic theory (FD model), and (2) compare the FD model with the non-FD-based approximation models previously reported by Hintz and Johnson (HJ model) and Wang and Flanagan (WF model). In the FD model, the relative velocity of a particle suspended in an agitated fluid was estimated from the terminal slip velocity and the microeddy effect. For small particles (particle radius ( r p) < ca. 15 μm), the HJ, WF and FD models resulted in the similar h eff values, whereas they resulted in different h eff values for large particles ( r p > ca. 15 μm). One of the merits of the FD model is that it provides the a priori theoretical estimation of h eff from particle radius, drug density, agitation strength, fluid viscosity, and diffusion coefficient. The hydrodynamic conditions in the gastrointestinal (GI) tract differ among human and animals, the GI sites, and fasted/fed conditions, etc. Therefore, the FD model could provide a more comprehensive and sophisticated simulation of oral absorption.

Effect of diffusion layers and defect layer on acoustic phonons transport through the structure consisting of different films

By using the continuum elastic approximation model and the transfer matrix method, we investigate the effect of diffusion layers and defect layer on acoustic phonons transport through the structure consisting of different films. Our work show that most acoustic phonons can easily pass the structure, but some only have much less transmission probabilities and form corresponding dips in the transmission spectrum. With the change of the structure parameters such as the width of diffusion layers and defect layer, the number of unit cell and the density of containing Al in diffusion layers and defect layer, the magnitude of the frequencies of acoustic phonons corresponding to the dips almost remain unchanged, but the transmission coefficients corresponding to the dips change at different degree, and the transmission probabilities of some frequencies are very sensitive to the variation of the above-mentioned structure parameters. These results can provide some references in controlling the transmission coefficients of acoustic phonons, devising parts of acoustic apparatus and theoretical investigation related.

Effect of cathode gas diffusion layer on water transport and cell performance in direct methanol fuel cells

The optimal design of the cathode gas diffusion layer (GDL) for direct methanol fuel cells (DMFCs) is not only to attain better cell performance, but also to achieve better water management for the DMFC system. In this work, the effects of both the PTFE loading in the cathode backing layer (BL) as well as in the micro-porous layer (MPL) and the carbon loading in the MPL on both water transport and cell performance were investigated experimentally. The experimental data showed that with the presence of a hydrophobic MPL in the GDL, the water-crossover flux through the membrane decreased slightly with increasing the PTFE loading in the BL. However, a higher PTFE loading in the BL not only lowered cell performance, but also resulted in an unstable discharging process. It was also found that the PTFE loading in the MPL had little effect on the water-crossover flux, but its effect on cell performance was substantial: the 40-wt% PTFE loading in the MPL was found to be the optimal value to achieve the best performance. The experimental results further showed that increasing the carbon loading in the MPL significantly lowered the water-crossover flux, but a too high carbon loading would decrease the cell performance as the result of the increased oxygen transport resistance; the 2.0-mg C cm −2 carbon loading was found to exhibit the best performance.

Apparent Michaelis constant of the enzyme modified porous electrode

Plate-gap model of enzyme doped porous electrode was utilized in order to calculate apparent Michaelis constants ( $$K_{\rm M}^{\rm app}$$ ) and apparent maximal currents ( $$I_{\rm max}^{\rm app}$$ ) of modeled amperometric biosensor for the wide range of given reaction/diffusion parameters. It was found that $$K_{\rm M}^{\rm app}$$ of plate-gap biosensor linearly depends on $$I_{\rm max}^{\rm app}$$ when rates of enzymatic reaction are lower than critical. Theoretically predicted linear correlation between apparent parameters was observed experimentally for the case of carbon paste electrodes, which were modified by PQQ-dependent alcohol dehydrogenases. At overcritical rates (or apparent maximal currents), $$K_{\rm M}^{\rm app }$$ is practically independent on Michaelis constant of soluble enzyme. Therefore, apparent Michaelis constant can be regarded as biosensor’s topology representing parameter which, in fact, is not related to the specificity of enzyme kinetics. High and rate-independent values of $$K_{\rm M}^{\rm app}$$ indicate that reaction proceeds at substrate-exposed top layer of the gap. In this case, reaction–diffusion system formally is stratified into separate reaction (top) and diffusion (bottom) zones. Topology of such reaction–diffusion system reminds “inverted” planar electrode, which contains diffusion layer below reaction layer. The net effect of plate-gap topology of working electrode on apparent Michaelis constant is similar to the effect of diffusion layer covering enzymatic planar electrode.