Porous transport layer

The presence of water is critical for operation but in current PEMFCs proper water management is a delicate issue and poor control can greatly reduce the efficiency of the device. An excess of water can flood the catalyst and porous transport layers impeding the transport of reactants and eventually drowning the fuel cell. At low water content, the polymer electrolyte membrane can become a poor conductor and the reactivity at the electrodes is affected. Local hot spots arising due to the inefficient operation result in early degradation of the cell. ... 

Polymer-electrolyte fuel cells (PEFC and DMFC) possess a exceptionally diverse range of applications, since they exhibit high thermodynamic efficiency, low emission levels, relative ease of implementation into existing infrastructures and variability in system size and layout. Their key components are a proton-conducting polymer-electrolyte membrane (PEM) and two composite electrodes backed up by electronically conducting porous transport layers and flow fields, as shown schematically in Fig. 1(a). 

The anodic reaction uses water and methanol to produce protons, electrons and carbon dioxide which partially dissolves in water and partially evaporates. Removing the carbon dioxide from the anodic reaction zone means that we have to support counter flow of water and gas in the porous transport layers. Methanol and water may evaporate, thus demanding to model a gas mixture. 

James JP (2012) Micro-computed tomography reconstruction and analysis of the porous transport layer in polymer electrolyte membrane fuel cells. Master, Queen s University, Kingston... 

Luo, G., Ji, Y, Wang, C., 2010, Modeling liquid water transport in gas diffusion layers by topologically equivalent pore network , Electrochim. Acta, 55 (19) pp. 5332. Medici, E. F., and Allen, J. S., 2010, The effects of morphological and wetting properties of porous transport layers on water movement in PEM fuel cells , J. Electrochem. Soc., 157 (10) pp. B1505. 

Pharoah, J. G., Karan, K., and Sun, W. 2006. On effective transport coefficients in PEM fuel cell electrodes Anisotropy of the porous transport layers. Journal of Power Sources 161 214-224. 

The main contributions to irreversible heat loss, listed in the order of decreasing significance, are due to (i) kinetic losses in the ORR at the cathode (Qorr), including losses due to proton transport in the cathode catalyst layer, (ii) resistive losses due to proton transport in the PEM (Qpem)< (iii) losses due to mass transport by diffusion and convection in porous transport layers (Qmt), (iv) kinetic losses in the HOR at the anode (Qhor), and (v) resistive losses due to electron transport in electrode and metal wires (Qm)- Some of these losses are indicated in Figure 1.4. Energy (heat) loss terms are related to overpotentials by r)i = Qi/F, which will be discussed in the section Potentials. ... 

FIG U RE 3.24 Model representation of a UTCL nanopore. The pore is assumed to he straight and cylindrical with charged Pt walls. The pore is bounded by the PEM at one end and the porous transport layer (MPL or GDL) at the other end. In real pores in UTCLs, Pt could be deposited in the form of nanoparticles at a conductive support or as a continuous layer on an insulating support, as indicated on the left. (Reprinted from Chan, K. and Eikerling, M. 2011. J. Electrochem. Soc., 158(1), B18-B28, Figures 1,2,3,4,5,6. Copyright (2011), the Electrochemical Society. With permission.)... 

The operation of generic catalyst layer is illustrated schematically in Figure 4.2. It shows the shapes of local variables and fluxes in a layer, where the axis x is directed from the interface with the electrolyte medium, viz., the PEM, toward the interface with a porous gas diffusion medium, viz., a porous transport layer (PTL) or gas diffusion layer (GDL). The ionic current arrives or leaves the CL at the electrolyte interface, while the feed molecules are supplied from the porous transport medium (Figure 4.2). 

Recall the general components of a PEFC shown in Figure 6.2. The fuel and oxidizer flow through the flow fields, diffuse through the thin ( 200 00-qm) porous diffusion media (DM) [also known as a gas diffusion layer (GDL) or porous transport layer (PTL)] and to the 5-20-qm-thick porous catalyst layers. 

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