Fuel cell membranes types

With the increased computational power of today s computers, more detailed simulations are possible. Thus, complex equations such as the Navier—Stokes equation can be solved in multiple dimensions, yielding accurate descriptions of such phenomena as heat and mass transfer and fluid and two-phase flow throughout the fuel cell. The type of models that do this analysis are based on a finite-element framework and are termed CFD models. CFD models are widely available through commercial packages, some of which include an electrochemistry module. As mentioned above, almost all of the CFD models are based on the Bernardi and Verbrugge model. That is to say that the incorporated electrochemical effects stem from their equations, such as their kinetic source terms in the catalyst layers and the use of Schlogl s equation for water transport in the membrane. 

The high-cost of materials and efficiency limitations that chemical fuel cells currently have is a topic of primaiy concern. For a fuel cell to be effective, strong acidic or alkaline solutions, high temperatures and pressures are needed. Most fuel cells use platinum as catalyst, which is expensive, limited in availability, and easily poisoned by carbon monoxide (CO), a by-product of many hydrogen production reactions in the fuel cell anode chamber. In proton exchange membrane (PEM) fuel cells, the type of fuel used dictates the appropriate type of catalyst needed. Within this context, tolerance to CO is an important issue. It has been shown that the PEM fuel cell performance drops significantly with a CO con-... 

Since PEM, like living organisms, need water to function and the amount and state of water are critical for an efficient operation, secondary requirements on this type of fuel cell membranes emerge. These include the necessity of sufficient humidification and the ability to retain water under operation conditions. Associated problems under fuel cell operation include the electroosmotic transport of water to the cathode side accompanied by dehydration at the anode side [45]. In the cathode the accumulation of water at high current densities, typically greater than 1 A cm-2, causes performances losses due to blocking of catalytically active sites and restriction of oxygen transport. 

An avalanche-like rush of new research activities has evolved around new routes in membrane synthesis and strategies in theoretical and computational modeling that could facilitate a dehberate design of highly functionalized fuel cell membranes. Comprehensive reviews on membrane synthesis highlight principles and fabrication of new membranes particularly those apphcable for DMFC [214], those that are feasible for operation at elevated temperatures (up to 200° C under ambient pressures) [215], and those that are based on various modifications of Nafion-type membranes [216]. 

Modification Conductivity Type Measured Conductivity (S/cm) Methanol Permeability (cm2/s) Fuel Cell Membrane Efficiency... 

It is also well documented [2-7] that fuel-cell membranes and ionomers in general typically show a phenomenon known as Schroder s paradox [8]. Schroder s paradox is the difference in water uptake (and therefore other properties) due to the type of reservoir in contact with the membrane. As seen in Figure 5.1, the water content of the membrane, X or moles of water per mole of sulfonic acid sites, for a saturated-vapor reservoir is different from that for a liquid-water reservoir even though the chemical potential is identical. This is seen in practice and the size of the difference depends on the membrane state and history. This effect is an important issue since fuel cells are often operated with humidified gases resulting in situations where there is liquid water on the cathodic side of the membrane but only water vapor on the anodic side. 

The fuel cell membranes are exposed to different types of wear that reduce their lifetime, which can be separated into two areas, chemical degradation, and physical degradation pathway. 

Direct chemical to electrical energy conversion can be accomplished in fuel cells. Many types of fuel cells have been implemented at small scale, including low-temperature (<100 °C) proton exchange membrane fuel cells (PEMFC), direct methanol fuel cells (DMFC), and formic acid fuel... 

All-soKd-state supercapacitors are desirable to prevent or reduce corrosion, self-discharge, low energy density, and bulky design [153]. The two types of electrolytes used in all-solid-state supercapacitors are gel electrolytes and solid polymer electrolytes (typically used in battery and fuel cell membranes). 

Many different types of fuel-cell membranes are currently in use in, e.g., solid-oxide fuel cells (SOFCs), molten-carbonate fuel cells (MCFCs), alkaline fuel eells (AFCs), phosphoric-acid fuel cells (PAFCs), and polymer-electrolyte membrane fuel cells (PEMFCs). One of the most widely used polymers in PEMFCs is Nalion, which is basically a fluorinated teflon-like hydrophobic polymer backbone with sulfonated hydrophilic side chains." Nafion and related sulfonic-add based polymers have the disadvantage that the polymer-conductivity is based on the presence of water and, thus, the operating temperature is limited to a temperature range of 80-100 °C. This constraint makes the water (and temperature) management of the fuel cell critical for its performance. Many computational studies and reviews have recently been pubhshed," and new types of polymers are proposed at any time, e.g. sulfonated aromatic polyarylenes," to meet these drawbacks. 

Aromatic gronps in fuel cell membranes are often hnked via ether bridges. Therefore, it is significant that for pH > 10 phenoxy-type radicals are observed, which do not show any splitting attribntable to the -OCH3 gronp. Obviously, the ether group is not stable under these conditions and becomes snbstitnted by HO. ... 

Althongh perfluorinated or partially fluorinated ionomers are mostly favored as fuel cell membrane materials due to their unique resistance to chemical attack from their surroundings, fundamental solution to overcome inherent shortcomings of Nafion and other fluorinated membranes would be the development of chemically different types of ionomers. Alternative membranes for PFS A can reduce methanol crossover as low as two orders of magnitude. Such membranes are discussed in the following sections. 

Besides the covalent crosslinking, Kerres et al. investigated the properties of fuel cell membranes of ionicaUy crosslinked polysulfonic adds. This type of crosslinking was achieved by blending sulfonated poly(arylene ether sul-fone)s and poly(arylene ether ketone)s with basic polymers, such as polybenzimidazole, poly(ethylene imine), poly(vinyl pyridine), or amino functional-... 

Abstract In this chapter, we discuss the proton conductivity and use of heteropoly acids (HP As) in proton exchange fuel cells. We first review the fundamental aspects of proton conduction in the HPAs and then review liquid HPA-based fuel cells. Four types of composite proton exchange membranes containing HPAs have been identified HPAs imbibed perfluorosulfonic acid membranes, HPAs imbibed hydrocarbon membranes, sol-gel-based membranes, and polymer hybrid polyoxometa-late (polypom)-based membranes. 

As previously described, the membrane system is assumed to have three main components membrane, protons, and water. In addition, the types of fuel cell membrane models stated in literature include microscopic and physical models, diffusive models, hydraulic models, hydraulic-diffusive models, and combination models [10]. 

This chapter, in addition to surveying membrane types and production, overviews applications of gas and liquid membrane separation and polymer films as banier layers. Water purification for reuse and in desalination using reverse osmosis and nano-, ultra-, and microfiltration are discussed. Electrodialysis, dialysis, and hemodialysis are also covered. Membranes in emerging technologies are described including fuel cell membranes, membranes in lithium batteries, conducting polymer membranes, and thin film membranes used in LED and photovoltaic applications. 

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