Diffusion layer characteristics

E. Antolini, R. R. Passos, and E. A. Ticianelli. Effects of the cathode gas diffusion layer characteristics on the performance of polymer electrolyte fuel cells. Journal of Applied Electrochemistry 32 (2002) 383-388. 

Brine and soft water of ultrapure quality is essential for smooth and efficient operation of membrane cell process since Ca2+ and Mg2+ ions can harm the performance of ion-exchange membranes in the following ways. The precipitated Ca(OH)2 and Mg(OH)2 offer increased electrical resistance across the membrane thereby increasing the cell voltage. Furthermore, the anolyte diffusion layer characteristics are affected which would alter the optimum current density. More seriously, the membrane performance is affected... 

Passalacqua E, Squadrito G, Lufrano F, Patti A, Giorgi L. Effects of the diffusion layer characteristics on the performance of polymer electroljfte fuel cell electrodes. J Appl Electrochem 2001 31(4) 449-54. 

The region of the gradual potential drop from the Helmholtz layer into the bulk of the solution is called the Gouy or diffuse layer (29,30). The Gouy layer has similar characteristics to the ion atmosphere from electrolyte theory. This layer has an almost exponential decay of potential with increasing distance. The thickness of the diffuse layer may be approximated by the Debye length of the electrolyte. 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

We see that the expression for the current consists of two terms. The first term depends on time and coincides completely with Eq. (11.14) for transient diffusion to a flat electrode. The second term is time invariant. The first term is predominant initially, at short times t, where diffusion follows the same laws as for a flat electrode. During this period the diffusion-layer thickness is still small compared to radius a. At longer times t the first term decreases and the relative importance of the current given by the second term increases. At very long times t, the current tends not to zero as in the case of linear diffusion without stirring (when is large) but to a constant value. For the characteristic time required to attain this steady state (i.e., the time when the second term becomes equal to the first), we can write... 

Ohmic losses AEohmic originate from (i) membrane resistance, (ii) resistance of CLs and diffusion layers, and (iii) contact resistance between the flow field plates. Although it is common practice to split current-voltage characteristics of an MEA into three regions— kinetic (low currents), ohmic (intermediate currents), and mass transport (high currents) [Winter and Brodd, 2004]—implicit separation of vt Afiohmic is not always straightforward, and thus studies of size and... 

At the external boundary of the constrained diffusion layer, application of the linear diffusion characteristic equations leads to... 

In Eqn. 5-65 it appears that most of the change of electrode potential occurs in the space charge layer with almost no change of potential both in the compact layer and in the diffuse layer. This pattern may be regarded as characteristic of semiconductor electrodes. 

The electrode potential at which the electron energy band is flat in semiconductor electrodes is caUed the flat band potential, . The flat band potential is used as a characteristic potential of individual semiconductor electrodes in the same way as the potential of zero charge is used for metal electrodes. At the flat band potential the space charge, Ogc, is zero but the interfacial charge, + oh + o, is not zero. The electrode interface is composed of only the compact layer at the flat band potential if no diffuse layer exists on the solution side. 

The relationship between the over-potential and Ig U will deviate from the Tafel linear area due to the medium affecting the diffusion layer. The effect will gradually disappear and the polarization curves separate each other obviously when the potential is far from zero electric charge potential. This is the reason that COj and Ca(OH) ions have some surfactant action compared with OH ion to form characteristic adsorption more easily and to bring about the change of the capacitance of the double electric charge layer. 

It is also important to take into account that one of the main issues with the carbon fiber paper or cloth used as the DL is the uncontrolled variation in porosity (and other localized properties) of these manufactured conventional diffusion layers that is, the porosity characteristics between carbon papers are not repeatable [57]. These materials are difficult to improve because only average pore sizes and volume densities can be measured and much of the development has been based on empirical parameters. Thus, extensive work has focused on optimizing the MPL in order to reduce the differences within carbon paper fiber and cloth diffusion layers. 

To design the optimal diffusion layer for a specific fuel cell system, it is important to be able to measure and understand all the parameters and characteristics that have a direct influence on the performance of the diffusion layers. This section will discuss in detail some of the most important properties that affect the diffusion layers, such as thickness, hydrophobicity and hydrophilicity, porosity and permeability (for both gas and liquids), electrical and thermal conductivity, mechanical properties, durability, and flow... 

It is important to note that Vie and Kjelstrup [250] designed a method of measuring fhe fhermal conductivities of different components of a fuel cell while fhe cell was rurming (i.e., in situ tests). They added four thermocouples inside an MEA (i.e., an invasive method) one on each side of the membrane and one on each diffusion layer (on the surface facing the FF channels). The temperature values from the thermocouples near the membrane and in the DL were used to calculate the average thermal conductivity of the DL and CL using Fourier s law. Unfortunately, the thermal conductivity values presented in their work were given for both the DL and CL combined. Therefore, these values are useful for mathematical models but not to determine the exact thermal characteristics of different DLs. 

K. Jiao and B. Zhou, hmovative gas diffusion layers and their water removal characteristics in PEM fuel cell cathode. Journal of Power Sources 169 (2007) 296-314. 

Transport properties of hydrated PFSA membranes strongly depend on nanophase-segregated morphology, water content, and state of water. In an operational fuel cell, these characteristics are indirectly determined by the humidity level of the reactant streams and Faradaic current densities generated in electrodes, as well as the transport properhes of catalyst layers, gas diffusion layers, and flow... 

At Coventry University [20] we have obtained similar results to Tu under conventional (silent) conditions. Our initial thoughts were that if the effect of the tail off was either a consequence of mass transfer to the electrode, or a consequence of some problem with the diffusion layer, then ultrasound might be expected to have an effect and thus improve the plating rate. Investigations in the presence of ultrasound and at various pH values did not significantly affect the plating characteristics i. e. the plateau effect still remained. However, the overall efficiency in the presence of ultrasound was affected (Fig. 6.10). 

Divisek et al. presented a similar two-phase, two-dimensional model of DMFC. Two-phase flow and capillary effects in backing layers were considered using a quantitatively different but qualitatively similar function of capillary pressure vs liquid saturation. In practice, this capillary pressure function must be experimentally obtained for realistic DMFC backing materials in a methanol solution. Note that methanol in the anode solution significantly alters the interfacial tension characteristics. In addition, Divisek et al. developed detailed, multistep reaction models for both ORR and methanol oxidation as well as used the Stefan—Maxwell formulation for gas diffusion. Murgia et al. described a one-dimensional, two-phase, multicomponent steady-state model based on phenomenological transport equations for the catalyst layer, diffusion layer, and polymer membrane for a liquid-feed DMFC. 

---