The Gas-Diffusion Layer

The porous structure and the balance of wetting properties of the pores influence transport processes in gas-diffusion layers (GDLs) and, as a consequence. 

STRUCTURAL AND WETTING PROPERTIES OF FUEL CELL COMPONENTS 

TABLE 14.2 Porosity of Toray paper with different amounts of PTFE 

Using the standard contact porosimetry method, Gostick et al. (2009) showed that water-air capillary pressure curves of fibrous gas diffudion media depend on sample thickness, clearly demonstrating that finite size effects are important. Compression of the GDM was found to increase the capillary pressures for water injection and decrease the capillary pressures required for water withdrawal. 

Park and Popov (2009) studied the effect of hydrophobic and structural properties of a single or dual cathode gas diffusion layer on mass transport in PEMFCs using an analytical expression. The simulations indicated that liquid water transport at the cathode is controlled by the fi action of hydrophilic surface and the average pore diameter in the cathode gas-diffusion layer. 

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The main components of a PEM fuel cell are the flow channels, gas diffusion layers, catalyst layers, and the electrolyte membrane. The respective electrodes are attached on opposing sides of the electrolyte membrane. Both electrodes are covered with diffusion layers, and the flow channels/current collectors. The flow channels collect current from the electrodes while providing the fuel or oxidant with access to the electrodes. The gas diffusion layer allows gases to diffuse to the electro-catalysts and provides electrical contact throughout the catalyst layers. Within the anode catalyst layer, the fuel (typically H2) is oxidized to produce electrons and protons. The electrons travel through an external circuit to produce electricity, while the protons pass through the proton conducting electrolyte membrane. Within the cathode catalyst layer, the electrons and protons recombine with the oxidant (usually 02) to produce water. 

An analysis of the individual PEM components offers evidence of almost unbroken R D see Fig. 13.10 (Jochem et al., 2007). The overall importance of the membrane is striking. Furthermore, the numbers of annual applications for bipolar plates (BPP) and the gas-diffusion layer (GDL) decrease after 2002, while the increase in membrane applications flattens out. This correlates with the equally lower number of fuel cell patents in the field of mobile applications. 

The catalyst layer is located between the PEM and the gas diffusion layer (GDL). Protons transfer between the CL and the PEM, and electrons transfer between the catalyst layer and the GDL. Both require good interfacial contact. 

Antoine et al. [28] inveshgated the gradient across the CL and found that the Pt utilization was dependent on the CL porosity. In a nonporous CL, catalyst utilization was increased through the preferential locahon of Pt close to the gas diffusion layer in a porous CL, catalyst utilization efficiency was increased through the preferential location of Pt close to the polymer electrolyte membrane. In PEM fuel cells, fhe CL has a porous structure, and better performance is expected if higher Pf loading is used af preferential locahons close to the membrane/catalyst layer interface. 

Similar to screen printing, the spray coating method [95] is widely used for catalyst fabrication, especially in labs. The major difference between the two is that the viscosity of the ink for spray coating is much lower than that for screen printing. The application apparatus can be a manual spray gun or an auto-spraying system with programmed X-Y axes, movable robotic arm, an ink reservoir and supply loop, ink atomization, and a spray nozzle with adjustable flux and pressure. The catalyst ink can be coated on the gas diffusion layer or cast directly on the membrane. To prevent distortion and swelling of the membrane, either it is converted into Na+ form or a vacuum table is used to fix the membrane. The catalyst layer is dried in situ or put into an oven to remove the solvent. 

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. 

K. Fushinobu, D. Takahashi, and K. Okazaki. Micromachined metallic thin films for the gas diffusion layer of PEFCs. Journal of Power Sources 158 (2006) 1240-1245. 

N. Holmstrom, J. Ihonen, A. Lundblad, and G. Lindbergh. The Influence of the gas diffusion layer on water management in polymer electrolyte fuel cells. Fuel Cells 7 (2007) 306-313. 

G. Y. Lin and T. V. Nguyen. Effect of thickness and hydrophobic polymer content of the gas diffusion layer on electrode flooding level in a PEMFC. Journal of the Electrochemical Society 152 (2005) A1942-A1948. 

H. Nakajima, T. Konomi, and T. Kitahara. Direct water balance analysis on a polymer electrolyte fuel cell (PEFC) Effects of hydrophobic treatment and microporous layer addition to the gas diffusion layer of a PEFC on its performance during a simulated start-up operation. Journal of Power Sources 171 (2007) 457-463. 

J. H. Jang, W. M. Yan, and G. G. Shih. Effects of the gas diffusion-layer parameters on cell performance of PEM fuel cells. Journal of Power Sources 161 (2006) 323-332. 

T. Koido, T. Furusawa, and K. Moriyama. An approach to modeling two-phase transport in the gas diffusion layer of a proton exchange membrane fuel cell. Journal of Power Sources 175 (2008) 127-136. 

O. Ghapuis, M. Prat, M. Quintard, et al. Two-phase flow and evaporation in model fibrous media Application to the gas diffusion layer of PEM fuel cells. Journal of Power Sources 178 (2008) 258-268. 

J. Benziger, J. Nehlsen, D. Blackwell, T. Brennan, and J. Itescu. Water flow in the gas diffusion layer of PEM fuel cells. Journal of Membrane Science 261 (2005) 98-106. 

Figure 8. Effective permeability as a function of capillary pressure for the different two-phase models for the gas-diffusion layer. The lines correspond to the models of (a) Berning and Djilali, (b) You and Liu and Mazumder and Cole, (c) Wang et al., (d) Weber and Newman, (e) Natarajan and Nguyen,and (f) Nam and Kaviany. ...

As can be seen in the different boundary conditions, the main effects of having ribs are electronic conductivity and transport of oxygen and water, especially in the liquid phase. In terms of electronic conductivity, the diffusion media are mainly carbon, a material that is fairly conductive. However, for very hydro-phobic or porous gas-diffusion layers that have a small volume fraction of carbon, electronic conductivity can become important. Because the electrons leave the fuel cell through the ribs, hot spots can develop with large gradients in electron flux density next to the channel. " Furthermore, if the conductivity of the gas-diffusion layer becomes too small, a... 

Overall, the rib effects are important when examining the water and local current distributions in a fuel cell. They also clearly show that diffusion media are necessary from a transport perspective. The effect of flooding of the gas-diffusion layer and water transport is more dominant than the oxygen and electron transport. These effects all result in non-uniform reaction-rate distributions with higher current densities across from the channels. Such analysis can lead to optimized flow fields as well as... 

Figure 19. Liquid saturation and current density of the cathode as a function of position for the case of dry air fed at 60 °C. (a) Liquid saturation in the gas-diffusion layer where the channel goes from x = 0 to 0.05 cm and the rib is the rest the total cathode overpotential is —0.5 V. (b) Current-density distributions for different channel/rib arrangements. (Reproduced with permission from ref 56. Copyright 2001 The Electrochemical Society, Inc.)...

Proper water management in proton exchange membrane fuel cells (PEMFCs) is critical to PEMFC performance and durability. PEMFC performance is impaired if the membrane has insufficient water for proton conduction or if the open pore space of the gas diffusion layer (GDL) and catalyst layer (CL) or the gas flow channels becomes saturated with liquid water, there is a reduction in reactant flow to the active catalyst sites. PEMFC durability is reduced if water is left in the CL during freeze/thaw cycling which can result in CL or GDL separation from the membrane,1 and excess water in contact with the membrane can result in accelerated membrane thinning.2... 

In a second prototype, the reaction temperature was reduced to 250 °C, which reduced the carbon monoxide concentration from 1.2 to < 1%. Later, the first fuel processor prototype was linked to a meso-scale high-temperature fuel cell developed at Case Western University by Holladay et al. [117], which was tolerant to carbon monoxide concentrations up to 10%. Hence no CO clean-up was necessary to run the fuel processor. A 23 mW power output was demonstrated according to Holladay et al. [118], This value was lower than expected, which was attributed to several factors. First, the hydrogen supply was lower in the reformate. Second, the presence of carbon monoxide (2%) lowered the cell voltage. Third, the presence of carbon dioxide (25%) generated a magnified dilution effect at the gas diffusion layer material of the fuel cell, which was considerably less porous than conventional materials. 

A state-of-the-art PEMFC and steady-state current-potential measurements are illustrated in Figure 3.18, which shows a schematic view of the PEMFC geometry, the basic electric circuit of the membrane electrode assembly and the gas diffusion layers at both anode and cathode. 

Figure 3.18. Schematic of the PEMFC geometry and basic electric circuit showing the membrane electrode assembly (MEA) and the gas diffusion layers (GDLs) at both anode and cathode [33], (Reprinted from Electrochimica Acta, 51(13), Tsampas MN, Pikos A, Brosda S, Katsaounis A, Vayenas CG, The effect of membrane thickness on the conductivity of Nafion, 2743-55. 2006, with permission from Elsevier.)...

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],... 

Diffusion of oxygen in the gas diffusion layer of the electrode, particularly when air is used instead of oxygen... 

Fuel cell performance is affected by MEA composition, including catalyst loading, PTFE content in the gas diffusion layer, and Nafion content in the catalyst layer and membrane, each of which affects the performance in different ways, yielding distinct characteristics in the electrochemical impedance spectra. Even different fabrication methods may influence a cell s performance and electrochemical impedance spectra. With the help of the model described above, impedance spectra can provide us with a useful tool to probe structure-performance relationships and thereby optimize MEA structure and fabrication methods. 

Figure 6.4 shows the polarization curves of fuel cells with different PTFE content in the gas diffusion layer of the gas diffusion electrodes. The optimal PTFE content in the gas diffusion layer is 30 wt%. The performance of the fuel cell containing a gas diffusion layer with PTFE content of 40 wt% is similar to one with 10 wt% PTFE content. 

Figure 6.4. The polarization curves of fuel cells with electrodes that contain various PTFE content in the gas diffusion layer ( ) 10 ( ) 20 (A) 30 (+) 40 wt% [5]. (Reprinted from Journal of Power Sources, 94(1), Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method, 78-84, 2001, with permission from Elsevier and the authors.)...

Figure 6.5 shows the AC impedance spectra of the same fuel cells measured at different cathodic potentials. It is evident that as the overpotential increases, the diameter of the kinetic arc decreases due to the increasing kinetic rate. At low overpotential, the kinetics dominates and only the kinetic arc appears. At high overpotentials, the low-frequency region shows additional arcs, which are associated with mass-transport limitations across the gas diffusion layer and within the catalyst layer. 

In Figure 6.5a it can be seen that the kinetic arc for the electrode with 30 wt% PTFE content in the gas diffusion layer has the smallest diameter. Indeed, the spectra for this electrode all have the minimum kinetic loop measured at all three cathode potentials, as seen in Figure 6.5b and c. This result is in agreement with that from the polarization curve measurements however, AC impedance spectra provide more information than polarization curves. This figure shows that the impedance arc due to mass transport in the low-frequency region grows with increasing electrode overpotential and is very sensitive to PTFE content in the gas diffusion layer. 

Song et al. [5] explained that for the electrode with 40 wt% PTFE content in the gas diffusion layer, the increase in the size of the kinetic arc was attributable to the substantial decrease in the active Pt area caused by low water content at the interface of the catalyst layer and the gas diffusion layer. This explanation has been verified by cyclic voltammetric results. A possible solution to improve the performance of this particular electrode is simply to raise the humidification temperature in order to increase the water content at the interface. The results at higher humidification temperatures are shown in Figures 6.6 and 6.7. 

In Song et al. s same work [5], the effect that Nafion content in the catalyst layer had upon electrode performance was also investigated, following their work on the optimization of PTFE content in the gas diffusion layer. The optimization of Nafion content was done by comparing the performance of electrodes with different Nafion content in the catalyst layer while keeping other parameters of the electrode at their optimal values. Figures 6.8 and 6.9 show the polarization curves and impedance spectra of fuel cells with electrodes made of catalyst layers containing various amounts ofNafion . 

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