PEFC model polymer electrolyte membrane

FIGU RE 5.4 Resistance of a Nafion 112 PEM in an operating PEFC (70°C), as a function of the fuel cell current density. Experimental data (dots) are compared to the hydraulic permeation model for various applied gas pressure differences between cathode and anode. (Reprinted from J. Power Sources, Renganathan, S. et al., 2006. Polymer electrolyte membrane resistance model. 160, 386-397, Figure 5, Copyright (2006), with permission from Elsevier.)... 

In this book, we discuss low-temperature fuel cells with a polymer electrolyte membrane (PEM). Two main representatives of the family of low-T cells are hydrogen-fed polymer electrolyte fuel cells (PEFCs) and liquid-fed direct methanol fuel cells (DMFCs). Though the major part of this book is devoted to materials and performance modeling of PEFCs, some features of DMFCs will also be discussed due to a great potential of these cells for small-scale mobile applications. 

The purpose of the present review is to summarize the current status of fundamental models for fuel cell engineering and indicate where this burgeoning field is heading. By choice, this review is limited to hydrogen/air polymer electrolyte fuel cells (PEFCs), direct methanol fuel cells (DMFCs), and solid oxide fuel cells (SOFCs). Also, the review does not include microscopic, first-principle modeling of fuel cell materials, such as proton conducting membranes and catalyst surfaces. For good overviews of the latter fields, the reader can turn to Kreuer, Paddison, and Koper, for example. 

This volume of Modern Aspects of Electrochemistry is intended to provide an overview of advancements in experimental diagnostics and modeling of polymer electrolyte fuel cells. Chapters by Huang and Reifsnider and Gu et al. provide an in-depth review of the durability issues in PEFCs as well as recent developments in understanding and mitigation of degradation in the polymer membrane and electrocatalyst. 

The water distribution within a polymer electrolyte fuel cell (PEFC) has been modeled at various levels of sophistication by several groups. Verbrugge and coworkers [83-85] have carried out extensive modeling of transport properties in immersed perfluorosulfonate ionomers based on dilute-solution theory. Fales et al. [109] reported an isothermal water map based on hydraulic permeability and electro-osmotic drag data. Though the model was relatively simple, some broad conclusions concerning membrane humidification conditions were reached. Fuller and Newman [104] applied concentrated-solution theory and employed limited earlier literature data on transport properties to produce a general description of water transport in fuel cell membranes. The last contribution emphasizes water distribution within the membrane. Boundary values were set rather arbitrarily. 

The membrane is the heart of the fuel-cell sandwich and hence the entire fuel cell. It is this electrolyte that makes polymer-electrolyte fuel cells (PEFCs) unique and, correspondingly, the electrolyte must have very specific properties. Thus, it needs to conduct protons but not electrons as well as inhibit gas transport in the separator but allow it in the catalyst layers. Furthermore, the membrane is one of the most important items in dealing with water management. It is for these reasons as well as for others that modeling and experiments of the membrane have been pursued more than any other layer [1],... 

Usually, the starting point of model derivation is either a physical description along the channel or across the membrane electrode assembly (MEA). For HT-PEFCs, the interaction of product water and electrolyte deserves special attention. Water is produced on the cathode side of the fuel cell and will either be released to the gas phase or become adsorbed in the electrolyte. As can be derived from electrochemical impedance spectroscopy (EIS) measurements [14], water production and removal are not equally fast Water uptake of the membrane is very fast because the water production takes place inside the electrolyte, whereas the transport of water vapor to the gas channels is difiusion limited. It takes several minutes before a stationary state is reached for a single cell. The electrolyte, which consists of phosphoric add, water, and the membrane polymer, changes composition as a function of temperature and water content [15-18]. As a consequence, the proton conductivity changes as a function of current density [14, 19, 20). 

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