Diffuse layer compression

As a consequence, two processes overlap and are directed opposite to each other diffusion layer compression due to the enlargement of the drop, and difhision layer dilation due to the growth of the drop area and dilation of the adsorption layer coverage. Both counteracting processes have been taken into consideration in current theories, diffusion layer compression... 

Ge J, Higier A, Liu H (2006) Effect of gas diffusion layer compression on PEM fuel cell performance. J Power Sources 159 922-927... 

CNF is an industrially produced derivative of carbon formed by the decomposition and graphitization of rich organic carbon polymers (Fig. 14.3). The most common precursor is polyacrylonitrile (PAN), as it yields high tensile and compressive strength fibers that have high resistance to corrosion, creep and fatigue. For these reasons, the fibers are widely used in the automotive and aerospace industries [1], Carbon fiber is an important ingredient of carbon composite materials, which are used in fuel cell construction, particularly in gas-diffusion layers where the fibers are woven to form a type of carbon cloth. 

Ofher diffusion layer approaches can also be found in the literature. Chen-Yang et al. [81] made DLs for PEMFCs out of carbon black and unsintered PTFE comprising PTFE powder resin in a colloidal dispersion. The mixture of fhese materials was then heated and compressed at temperature between 75 and 85°C under a low pressure (70-80 kg/cm ). After this, the DLs were obtained by heating the mixture once more at 130°C for around 2-3 hours. Evenfually, fhe amount of resin had a direct influence on determining the properties of fhe DL. The fuel cell performance of this novel DL was shown to be around a half of that for a CFP standard DL. Flowever, because the manufacturing process of these carbon black/PTFE DLs is inexpensive, they can still be considered as potential candidates. 

One crifical paramefer fhaf affecfs fhe fhickness of fhe diffusion layer is fhe compression force used in fhe fuel cell in order fo avoid any gas leaks and to assure good contact between all the components. However, this compressive force can deform the diffusion layer and hence affect the performance of the cell. More information regarding how the compression forces affect the diffusion layer is discussed in Section 4.4.5. Ideally, the material used as the DL should be able to resist this compression force or pressure without affecting most of its parameters. Figure 4.21 shows a schematic of the cell voltage (performance) at a given current density, resistance, and DL porosity as a function of the cell s compression. 

In order to determine the viscous and inert through-plane gas permeabilities of diffusion layers at varied compression pressures, Gostick et al. [212] designed a simple method in which a circular specimen was sandwiched between two plates that have orifices in the middle, aligned with the location of the material. Pressurized air entered the upper plate, flowed through the DL, and exited the lower plate. The pressure drop between the inlet and the outlet was recorded for at least ten different flow rates for each sample. The inert and viscous permeabilities were then determined by fitting the Forchheimer equation to the pressure drop versus flow rate data as explained earlier. 

After the tests, Djilali s group used mathematical assumptions and equations to correlate the intensity of the dye in the image with the depth in the gas diffusion layer. With this method they were able to study the effect of compression on diffusion layers and how fhaf affects water transport. Water removal in a flow charmel has also been probed with this technique and it was observed that, with a dry DL slug, formation and flooding in the FF channels followed the appearance and detachment of water droplets from the DL. Even though this is an ex situ technique, it provides important insight into water transport mechanisms with different DLs and locations. 

As mentioned by Mathias et al. [9], reliable methods to measure the thermal conductivity of diffusion layers as a function of compression pressures are very scarce in the open literature. Khandelwal and Mench [112] designed an ex situ method to measure accurately the thermal conductivities of different components used in a fuel cell. In their apparatus, the sample materials were placed between two cylindrical rods made out of aluminum bronze (see Figure 4.28). Three thermocouples were located equidistantly in each of the upper and lower cylinders to monitor the temperatures along these components. Two plates located at each end compressed both cylinders together. The temperatures of each plate were maintained by flowing coolant fluids at a high flow rate through channels located inside each of the plates. A load cell was located between two plates at one end so that the compression pressure could be measured. 

The membrane and catalyst layers in a fuel cell are thin and delicate components that require mechanical support in order to prevent rupture or substantial bending when a compression pressure is applied to the whole cell. Therefore, the diffusion layers must provide the necessary mechanical support to those components without affecting the other parameters discussed previously. 

After treating different fuel cells to 100 freeze-thaw cycles (from -40 to 70°C), Kim, Ahn, and Mench [261] concluded that stiffer materials used as diffusion layers improved the uniform compression with the CL, resulting in fewer issues after the freeze and thaw cycles. On the other hand, more flexible DLs failed to improve the compression the CL left open spaces for ice films to be formed, resulting in serious issues after the freeze-thaw cycles. However, even with the stiffer materials tested, such ice films were still evident and caused delamination of the DL and CL, surface damage in the CL, and breakage of the carbon fibers. This resulted in increased electrical and mass transport resistances. 

Graftech Inc. was the first company to look at developing perforated diffusion layers from graphite sheet materials made out of compressed expanded graphite... 

I. Nitta, T. Hottinen, O. Himanen, and M. Mikkola. Inhomogeneous compression of PEMFC gas diffusion layer. Part I. Experiment. Journal of Power Sources 171 (2007) 26-36. 

A. Bazylak, D. Sinton, Z. S. Liu, and N. Djilali. Effect of compression on liquid water transport and microstructure of PEMFC gas diffusion layers. Journal of... 

Equation 7.20 with respect to Vmax is involved, and will be not discussed here. It is, however, readily appreciated that, when the electrolyte concentration is increased, the magnitude of k in the exponent of Vel also increases (compression of diffuse layers), so that the maximum caused by it becomes lower. At a certain value of C, the curve V(h) will become similar to curve b in Figure 7.8 with Vmax = 0. In accordance with all that has been said before, coagulation will become fast starting from this concentration. This is therefore the critical concentration, Ccc. In other words, the critical concentration (Ccc) can be estimated from simultaneous solution of following ... 

The rate at which a compound dissolves is dependent upon its surface area, solubility, solution concentration, rate of reaction and transport rate. These quantities are defined as follows surface area - the surface area of the individual particles if the compound is not compressed or the surface area of a disk if the compound is compressed solubility - the solubility of the polymorphic form in the solid phase solution concentration - the concentration of the compound in the bulk of the solution rate of reaction - the rate at which the solid surface reacts with the solvent or dissolution medium transport rate - the rate at which the compound travels through the diffusion layer. The rate of dissolution, or flux, of a compound can be given as ... 

Volume resistance of a graphite plate is about 17 mfl cm [ 1 ], therefore from an underside is formed metal contact (a position 5). However the most important is minimization of electric contact of a bipolar plate with gas diffusion layer. It is achieved with the help of clamping contact, which is created, as a rule, with the help of connection by bolts. Thus contact resistance 30 mQ cm2 is reached at pressure of compression not less than 120 H cm2 [ 1 ]. 

The potential in the diffuse layer decreases exponentially with the distance to zero (from the Stem plane). The potential changes are affected by the characteristics of the diffuse layer and particularly by the type and number of ions in the bulk solution. In many systems, the electrical double layer originates from the adsorption of potential-determining ions such as surface-active ions. The addition of an inert electrolyte decreases the thickness of the electrical double layer (i.e., compressing the double layer) and thus the potential decays to zero in a short distance. As the surface potential remains constant upon addition of an inert electrolyte, the zeta potential decreases. When two similarly charged particles approach each other, the two particles are repelled due to their electrostatic interactions. The increase in the electrolyte concentration in a bulk solution helps to lower this repulsive interaction. This principle is widely used to destabilize many colloidal systems. 

L /polymer extensibility smectic-layer compressive modulus E E, Finger strain tensor B , Cauchy strain tensor yriso/r, capillary number characteristic ratio, defined by R )q — Ccotib translational diffusivity-------------------------... 

The outer surface of the Stern layer is the shear surface of the micelle. The core and the Stern layer together constitute what is termed the kinetic micelle. Surrounding the Stern layer is a diffuse layer called the Gouy-Chapman electrical double layer, which contains the aN counterions required to neutralise the charge on the kinetic micelle. The thickness of the double layer is dependent on the ionic strength of the solution and is greatly compressed in the presence of electrolyte. 

At low concentrations of dissolved organic matter (DOM) (<0.01 mg of C/L) and at low ionic strength (10-3 M), the hematite particles are positively charged at this pH and are stabilized electrostatically by interacting diffuse layers with characteristic (Debye) lengths of 10 nm. As the ionic strength is increased to 10 1 M at these low DOM concentrations, the diffuse layers are compressed to 1 nm, and attractive van der Waals forces promote attachment in classical Derjaguin-Landau-Verwey-Overbeek (DLVO) destabilization by what has been termed double-layer compression. 

It is well known that the process of diffusion is controlled by thickness of so-called diffusive layer which locates in the layer of a liquid around a bubble. With bubble compression, this layer grows and the concentration gradient decreases with bubble expansion, thickness of the layer decreases and the gradient increases, hence, a rate of gas flow into the bubble increases. 

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