Diffusion layer characterized

Determine diffusion coefficient and hydrophobic characteristics of the gas diffusion layer. Characterize two-phase flow mechanism in the fuel cell flow field. 

What benefits and drawbacks to these problems can one expect from the use of cyclic voltammetry instead of RDEV They are related. In a general case, the application of cyclic voltammetry will be more complicated, because playing with the scan rate, one can make the diffusion layer penetrate the film or remain outside, as is the case with RDEV. We have already seen a fruitful application of the first of these possibilities in the use of cyclic voltammetry to the characterization of electron hopping transport within the redox films (Section 4.3.4). In the second situation, cyclic voltammetry may replace RDEV in a manner similar to what has been seen in Section 4.3.2 Each time a term (1 — ///a) is encountered in the analysis, it suffices to replace it by... 

Diffusion (along a concentration gradient) is observed if the solution near the electrode is depleted from a substrate or a product is accumulated. Diffusion is characterized by a diffusion coefficient D (typical value 10 cm /s) and extends over a diffusion layer (thickness 5) that develops from the electrode into the electrolyte. At the outward boundary the concentrations approach their bulk values. 

Since analyte cannot move across this layer - from the bulk solution to the electrode - by convection alone, it must diffuse, with the speed of such diffusion being characterized by the diffusion coefficient D. 

One of the most common ways to characterize the hydrophobicity (or hydrophilicity) of a material is through measurement of the contact angle, which is the angle between the liquid-gas interface and the solid surface measured at the triple point at which all three phases interconnect. The two most popular techniques to measure contact angles for diffusion layers are the sessile drop method and the capillary rise method (or Wihelmy method) [9,192]. 

Issues with mass transport resistance, especially at higher current densities, represent an important hurdle that fuel cells need to overcome to achieve the required efficiencies and power densifies that different applications require. Diffusion layers represenf one of fhe major fuel cell components that have a direct impact on these mass transport issues thus, optimization of the DLs is required through the use of differenf experimental and characterization techniques. 

One of the main parameters that would improve the overall performance of a fuel cell is better mass transport of reactants through the diffusion layer toward the active catalyst zones. In order to quantify and characterize how well the gas mass transport is in a specific DL material and design, it is important to measure the in-plane and through-plane permeabilities. Most of the published permeability results report the viscous permeability... 

Accumulation of water inside the DLs and CLs may cause serious failure modes that can significantly deteriorate the performance and lifetime of a fuel cell. To ensure appropriate water removal, it is vital to understand the water transport mechanism inside a fuel cell, especially in the DLs. Because CFP and CC contain complex structures and porosities, many researchers have developed methods that could facilitate the characterization and design of optimal diffusion layers with proper water removal capabilities. A lot of work has also been performed on mathematical models that attempt to analyze the water flooding and transport inside DLs. A comprehensive review describing these models can be found in Sinha, Mukherjee, and Wang [222]. This section will discuss only examples of the experimental techniques. 

Once the durability testing of the fuel cells is finalized, the internal components are then characterized. For diffusion layers, some of these characterization techniques include SEM to visualize surface changes, porosimetry measurements to analyze any changes in porosity within the DL and MPL, IGC (inverse gas chromatography) to identify relative humidity effects on the hydrophobic properties of the DLs, contact angle measurements to observe any changes in the hydrophobic/hydrophilic coatings of the DL, etc. [254,255]. 

M. V. Williams, R. Begg, L. Bonville, H. R. Kunz, and J. M. Fenton. Characterization of gas diffusion layers for PEMFC. Journal of the Electrochemical Society 151 (2004) A1173-A1180. 

V. Gurau, M. J. Bluemle, E. S. De Castro, et al. Characterization of transport properties in gas diffusion layers for proton exchange membrane fuel cells. 2. Absolute permeability. Journal of Power Sources 165 (2007) 793-802. 

J. P. Feser, A. K. Prasad, and S. G. Advani. Experimental characterization of inplane permeability of gas diffusion layers. Journal of Power Sources 162 (2006) 1226-1231. 

S. Escribano, J. P. Blachot, J. Etheve, A. Morin, and R. Mosdale. Characterization of PEMFCs gas diffusion layers properties. Journal of Power Sources 156 (2006) 8-13. 

The dissolved form of O decays to the final electroinactive product via a volume chemical reaction occnrring in the diffusion layer with the volume rate constant (kv), whereas the adsorbed form participates in the surface chemical reaction confined to the electrode surface, characterized by a surface rate constant (kg). These two chemical reactions proceed with different rates due to significant differences between the chenucal nature of dissolved and adsorbed forms of O. Obviously, the mechanisms (2.172)-(2.174) and (2.177) are only limiting cases of the general mechanism (2.178). 

The importance of materials characterization in fuel cell modeling cannot be overemphasized, as model predictions can be only as accurate as their material property input. In general, the material and transport properties for a fuel cell model can be organized in five groups (1) transport properties of electrolytes, (2) electrokinetic data for catalyst layers or electrodes, (3) properties of diffusion layers or substrates, (4) properties of bipolar plates, and (5) thermodynamic and transport properties of chemical reactants and products. 

Equation (4) states that the linear deposition rate vj is a diffusion controlled boundary layer effect. The quantity Ac is the difference in foulant concentration between the film and that in the bulk flow and c is an appropriate average concentration across the diffusion layer. The last term approximately characterizes the "concentration polarization" effect for a developing concentration boundary layer in either a laminar or turbulent pipe or channel flow. Here, Vq is the permeate flux through the unfouled membrane, 6 the foulant concentration boundary layer thickness and D the diffusion coefficient. 

Co304 pellets used in practice are of 4-5 mm in size. Thus, they are much larger than the diameter of wires in platinum gauzes. For this reason, in contrast to the reaction on gauzes, the reaction on Co304 pellets under atmospheric pressure is characterized by the Reynolds number much larger than 1, the Reynolds number being defined by Re = ul/v where u is the linear velocity of the stream, / is the characteristic dimension, v is the kinetic viscosity coefficient. The thickness of the diffusion layer for such pellets is... 

More recently, a spaghetti model for a swollen matrix was developed to provide mechanistic understanding of the complex release process (Fig. 4.4). This model treats polymer erosion as diffusion of polymer across a diffusion layer adjacent to the gel layer.19,20 Thus two competitive diffusional processes contribute to overall drug release diffusion of polymer across the diffusion layer and diffusion of drug across the gel layer. Two parameters have been identified to characterize their relative contributions. Polymer disentanglement concentration Cp>dis gauges the... 

Nitrides. Nitride formation on the surface of molybdenum has been revealed by high-temperature etching with ammonia, and the nitrides were characterized by X-ray diffraction spectrometry.378 The diffusion layer formed on molybdenum consists of Mo2N at >950°C and MoN at <950°C. The interaction of ammonia with a tungsten surface can generate dense adlayers such as W2N3H.379... 

Optically transparent electrode — (OTE), the electrode that is transparent to UV-visible light. Such an electrode is very useful to couple electrochemical and spectroscopic characterization of systems (- spectroelectro-chemistry). Usually the electrodes feature thin films of metals (Au, Pt) or semiconductors (In203, SnCb) deposited on transparent substrate (glass, quartz, plastic). Alternatively, they are in a form of fine wire mesh minigrids. OTE are usually used to obtain dependencies of spectra (or absorbance at given wavelengths) on applied potentials. When the -> diffusion layer is limited to a thin layer (i.e., by placing another, properly spaced, transparent substrate parallel to the OTE), bulk electrolysis can be completed in a few seconds and, for -> reversible or - quasireversible systems, equilibrium is reached for the whole solution with the electrode potential. Such OTEs are called optically transparent thin-layer electrodes or OTTLE s. 

Unlike the RDE technique, which is quite popular for characterizing catalyst activities, the gas diffusion electrode (GDE) technique is not commonly used by fuel cell researchers in an electrochemical half-cell configuration. The fabrication of a house-made GDE is similar to the preparation of a membrane electrode assembly (MEA). In this fabrication, Nation membrane disks are first hot-washed successively in nitric acid, sulphuric acid, hydrogen peroxide, and ultra-pure water. The membranes are then coated with a very thin active layer and hot-pressed onto the gas diffusion layer (GDL) to obtain a Nation membrane assembly. The GDL (e.g., Toray paper) is very thin and porous, and thus the associated diffusion limitation is small enough to be ignored, which makes it possible to study the specific kinetic behaviour of the active layer [6],... 

Our modeling approach was first used to describe the EDL properties of well-characterized, crystalline oxides ( 1). It was shown that the model accounts for many of the experimentally observed phenomena reported in the literature, e.g. the effect of supporting electrolyte on the development of surface charge, estimates of differential capacity for oxide surfaces, and measurements of diffuse layer potential. It is important to note that a Nernstian dependence of surface potential (iIJq) as a function of pH was not assumed. The interfacial potentials (4>q9 4> 9 in Figure 1) are... 

For the cases of oxides and latices. as mentioned above, there is little or no mobility of the surface charges. In such systems the mutual distance between the charges ( ) becomes a characteristic parameter but It is not the absolute value of t that counts, but its relation to the thickness of the diffuse part of the double layer, characterized by x". For kI 1 the diffuse layer is so much thicker than that the assumption of a smeared-out surface charge remains tenable, whereas for Kt 1 the charges are relatively so far apart that around each of them a hemispherical diffuse layer is formed. As x is the yardstick, which decreases with, it follows that the surface charge has a more discrete nature at higher indifferent electrolyte concentration. 

Figure 3.41. Structure of carbon paper (left) and carbon cloth (right) used for gas diffusion layers in PEM fuel cells. A coating of 20% (by weight) fluorinated ethylene propylene has been applied. (From C. Lim and C-Y. Wang (2004). Effects of hydro-phobic polymer content in GDL on power performance of a PEM fuel cell. Electro chimica Acta 49, 4149-4156 G. Lu and C-Y. Wang (2004). Electrochemical and flow characterization of a direct methanol fuel cell.. Power Sources 134, 33-40. Used with permission from Elsevier.)...

Naturally, the microelectrodes can be placed on the macroelectrodes inside their diffusion layers. Let us consider the model of surface irregularities shown in Fig. 1. The electrode surface irregularities are buried deep in the diffusion layer, which is characterized by a steady linear diffusion to the flat portion of the surface.7,20... 

On the other hand, due to the overlapping of the nucleation exclusion zones,7,35,36 deposition on the partially covered graphite electrode is an excellent illustration of the above discussion. Namely, the diffusion layer on the inert electrode partially covered with grains of active metal can be formed and diffusion control established in the same way as on an electrode of massive active metal if the deposition process is characterized by a large jo/jh-1 If dendrites are formed on the grains, their tips enter the bulk solution and overall control of the deposition process becomes activation or mixed controlled. 

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