Gas diffusion layers cathode

C. Xu, T. S. Zhao, and Y. L. He. Effect of cathode gas diffusion layer on water transport and cell performance in direct methanol fuel cells. Journal of Power Sources 171 (2007) 268-274. 

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. 

Figure 3.17. Ohmic overpotential distribution in the anode and cathode gas diffusion layers for three different nominal current densities 0.3 AJcrn (upper) 0.7 AJcrn (middle) 1.2 AJcrn (lower).

Figure 19.1. Schematic diagram of a dual-layer cathode gas diffusion layer...

Antolini E, Passos RR, TicianeUi EA. Effects of the carbon powder characteristics in the cathode gas diffusion layer on the performance of poljmier electrolj te fuel cells. J Power Sources 2002 109(2) 477-82. 

The oxygen pressure in the CCL, which is different from that in the channel due to the pressure drop across the cathode gas diffusion layer, is roughly estimated by Pick s law as... 

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. 

FIGURE 3.24 Sketch with slide positions of a cathodic gas diffusion layer. Section A is located in the flow field structure, whereas sections B-D represent sUces in the gas diffusion layer substrate (left). Water agglomerations were visuaUzed in each section [89]. 

A tri-layer PEMFC consisting of a PEM sandwiched between anode and cathode gas diffusion layer electrodes is shown in Figure 9.6. 

Oxygen domain includes cathode channel, cathode gas diffusion layer, and cathode catalyst layer, with no boundary conditions required at the interfaces between these layers. 

Antolini, E., Passos, R.R. and Ticianelli, E.A. (2002) Effect of Carbon Powder Characteristics in the Cathode Gas Diffusion Layer on the Performance of Polymer Electrolyte Fuel Cells, J. Power Sources, 109, 477-482. 

SOFC electrodes are commonly produced in two layers an anode or cathode functional layer (AFL or CFL), and a current collector layer that can also serve as a mechanical or structural support layer or gas diffusion layer. The support layer is often an anode composite plate for planar SOFCs and a cathode composite tube for tubular SOFCs. Typically the functional layers are produced with a higher surface area and finer microstructure to maximize the electrochemical activity of the layer nearest the electrolyte where the reaction takes place. A coarser structure is generally used near the electrode surface in contact with the current collector or interconnect to allow more rapid diffusion of reactant gases to, and product gases from, the reaction sites. A typical microstructure of an SOFC cross-section showing both an anode support layer and an AFL is shown in Figure 6.4 [24],... 

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. 

Figure 2.1 shows a schematic structure of the fuel cell membrane electrode assembly (MEA), including both anode and cathode sides. Each side includes a catalyst layer and a gas diffusion layer. Between the two sides is a proton exchange membrane (PEM) conducting protons from the anode to the cathode. 

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. 

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.)...

Diffusion medium properties for the PEFC system were most recently reviewed by Mathias et al. The primary purpose of a diffusion medium or gas diffusion layer (GDL) is to provide lateral current collection from the catalyst layer to the current collecting lands as well as uniform gas distribution to the catalyst layer through diffusion. It must also facilitate the transport of water out of the catalyst layer. The latter function is usually fulfilled by adding a coating of hydrophobic polymer such as poly(tet-rafluoroethylene) (PTFE) to the GDL. The hydrophobic polymer allows the excess water in the cathode catalyst layer to be expelled from the cell by gas flow in the channels, thereby alleviating flooding. It is known that the electric conductivity of GDL is... 

Fig. 16. Schematic presentation of the morphological features of gas diffusion electrodes for fuel cells of (A) PTFE-bonded and Pt-activatcd Hi anodes and O2 cathodes used for Oi reduction in acidic and alkaline fuel cells (a) support, (b) hydrophobic gas diffusion layer, (c) hydrophilic electrode layer, (d) electrolyte, (e) magnified schematic of PTFE-bonded soot electrode, (f) adjacent hydrophobic layer, (g) microporous soot particles, (h) gas channels (mesopores), (k) PTFE particles, (I) flooded micro- and mesopores, (B) Schematic presentation of the morphology of PTFE-bonded Raney-nickel anodes used in alkaline fuel cells ol the Siemens technology.

Water content affects many processes within a fuel cell and must be properly managed. Proton conductivity within the polymer electrolyte typically decreases dramatically with decreasing water content (especially for perfhiorinated membranes such as Nation ), while excessive liquid water in the catalyst layers (CLs) and gas diffusion layers (GDLs) results in flooding, which inhibits reactant access to the catalyst sites. Water management is complicated by several types of water transport, such as production of water from the cathode reaction, evaporation, and condensation at each electrode, osmotic drag of water molecules from anode to cathode by... 

A typical PEFC, shown schematically in Fig. 1, consists of the anode and cathode compartments, separated by a proton conducting polymeric membrane. The anode and cathode sides each comprises of gas channel, gas diffusion layer (GDL) and catalyst layer (CL). Despite tremendous recent progress in enhancing the overall cell performance, a pivotal performance/durability limitation in PEFCs centers on liquid water transport and resulting flooding in the constituent components.1,2 Liquid water blocks the porous pathways in the CL and GDL thus causing hindered oxygen transport to the... 

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.)...

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. 

Figure 6.6. Polarization curves of fuel cells with electrodes containing 40 wt% PTFE in the gas diffusion layer. The temperature of the humidifier on the cathode side was maintained at ( ) 65°C and ( ) 80°C. For comparison, the polarization curve of the fuel cell with the electrode containing 30 wt% PTFE in the gas diffusion layer is shown at the cathode humidification temperature of 65°C (A) [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.)...

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