Membrane fuel cell, components electrode

Calculating the structural properties of the component from its chemical composition (e.g., for the case of polymer electrolyte membrane fuel cell (PEMFC) electrodes, by using coarse-grain molecular dynamics (CGMD)) (Fig. 10)... 

Composites as Fuel Cell Components, Electrodes and Membrane... 

Sol-gel techniques have been widely used to prepare ceramic or glass materials with controlled microstructures. Applications of the sol-gel method in fabrication of high-temperature fuel cells are steadily reported. Modification of electrodes, electrolytes or electrolyte/electrode interface of the fuel cell has been also performed to produce components with improved microstructures. Recently, the sol-gel method has expanded into inorganic-organic hybrid membranes for low-temperature fuel cells. This paper presents an overview concerning current applications of sol-gel techniques in fabrication of fuel cell components. 

Sol-gel techniques have been successfidly applied to form fuel cell components with enhanced microstructures for high-temperature fuel cells. The apphcations were recently extended to synthesis of hybrid electrolyte for PEMFC. Although die results look promising, the sol-gel processing needs further development to deposit micro-structured materials in a selective area such as the triple-phase boundary of a fuel cell. That is, in the case of PEMFC, the sol-gel techniques need to be expanded to form membrane-electrode-assembly with improved microstructures in addition to the synthesis of hybrid membranes to get higher fuel cell performance. 

The first key component of a membrane fuel cell is the membrane electrolyte. Its central role lies in the separation of the two electrodes and the transport of ionic species (e.g. hydroxyl ion, OH , in an AEM), between them. In general, quaternary ammonium groups are used as anion-exchange groups in these materials. However, due to their low stability in highly alkaline media [43,44], only a few membranes have been evaluated for use as solid polymer electrolytes in alkaline fuel cells. 

An electrolyte is an essential component within fuel cells, used to facilitate the selective migration of ions between the electrodes. Fuel cells are typically classified according to the electrolytes used alkaline fuel cell (AFC), polymer electrolyte (or proton exchange membrane) fuel cell (PEMFC), phosphoric acid fuel cell (PAFC),... 

It is critical to minimize the component manufacturing variations in order to build a reliable PEM fuel cell system. We demonstrated that even within the same commercial MEA sample, the thicknesses of the electrode layers and membrane could vary greatly from one region to another. With the current scale of PEM fuel cell production, commercial-grade fuel cell components often display substantial deviations from their product specifications. Such component manufacturing issues hamper the overall effort toward improving commercial PEM fuel cell system reliability. Without adequate MEA quality control, it is difficnlt to interpret antopsy resnlts and to link apparent membrane/electrode problems of nsed MEAs to a particnlar failnre mechanism. Part of the problem is the lack of a nondestrnctive in-line MEA qnality control method to ensnre batch-to-batch consistency. 

A detailed cost analysis for a polymer electrolyte membrane fuel cell power plant of 5 kW was provided in 2006 by Kamarudin et al. According to their data, the total cost of such a plant will be about 1200 of which 500 is for the actual fuel-cell stack and 700 for the auxiliary equipment (pumps, heat exchangers, etc.). The cost of the fuel-cell stack is derived from the components as 55 /kW for the membranes, 52 /kW for the platinum, 128 /kW for the electrodes, and 148 /kW for the bipolar plates. 

Little information is currently available on the porosity or even pore size distribution of fuel-cell components such as the GDL. Only a few methods are capable of obtaining this information since a large range of pore size radii have to be measured. The range starts from pores a few nanometers in diameter, the so-called nanopores, which are essential especially close to the membrane, up to macropores with diameters of several micrometers, which are needed for a good supply of gas from the flow-field channels to the electrode layer. Among these methods are BET (Brunauer-Emmett-Teller) measurements, mercury intrusion, and some specialized methods. 

This chapter reviews a new type of solid electrolyte low-temperature fuel cell, the alkaline membrane fuel cell. The principles and main components of this fuel cell technology are described, with a major focus on the electrocatalysts for both electrodes. Finally, the latest published results on operation of the first developed alkaline membrane fuel cells are reviewed. 

PEMFCs are very clean systems and act as filters for impurities introduced from ambient air, fuel used, and even degradation products from the cell materials. Both the membrane and the catalyst are susceptible to cmitamination and poisoning. Electrode degradation of PEMFCs can occur as a result of various impurities found in the fuel feed, air stream, as well as corrosimi by-products from fuel cell components such as the bipolar plate, catalysts, or membrane. 

A fuel cell decays with time, and the rate of decay determines its durability. The decay is related to the aging of the fuel cell components, especially the membrane electrolyte, the catalysts, and the catalyst support. The decay of the membrane will cause its thinning and mechanical property deterioration. The loss of its mechanical properties often causes a fuel cell to fail prematurely and catastrophically. The decay of the catalyst is normally due to the particle size increase and the particle dissolution and redistribution. Catalyst decay rarely causes a sudden failure of a cell. The decay of the catalyst-support is often related to its corrosion. Corrosion makes the electrode more prone to flooding and accelerates the growth and redistribution of the catalyst particles. 

As shown in Fig. 5 7(b) the solid polymer electrolyte cell comprises a membrane, fuel cell type, porous electrodes and three further components z carbon collector, a platinized titanium anode support and a cathode support made from carbon-fibre paper The collector is moulded in graphite with a fluorocarbon polymer binder A 25 pm thick platinized titanium foil is moulded to the anode side to prevent oxidation. The purpose of the collector is to bnsure even fluid distribution over the active electrode area, to act as the main structural component of the cell, to provide sealing of fluid ports and the reactor and to carry current from one cell to the next E>emineralized water is carried across the cell via a number of channels moulded into the collector These channels terminate in recessed manifold areas each of which is fed from six drilled ports. The anode support is a porous conducting sheet of platinized titanium having a thickness of approximately 250 pm. The purpose of the support is to distribute current and fluid uniformly over the active electrode area. It also prevents masking of those parts of the electrode area which would be covered by the... 

The single characteristic that encompasses phenomena at all scales and in all fuel cell components is the fuel cell polarization curve. The polarization curve of a membrane-electrode assembly (MEA), a single cell, or a fuel cell stack furnishes the link between microscopic structure and physicochemical properties of distinct cell components on the one hand and macroscopic cell engineering on the other. It thus condenses an exuberant number of parameters, which lies in the 50s to 100s, into a single response function. Analysis of parametric dependencies in the polariz-tion curve could be extremely powerful at the same time, it could as well be highly misleading if applied blindly. ... 

High-temperature proton exchange membrane fuel cells (HT-PEM fuel cells), which use modified perfluorosulfonic acid (PFSA) polymers [1—3] or acid-base polymers as membranes [4—8], usually operate at temperatures from 90 to 200 °C with low or no humidity. The development of HT-PEM fuel cells has been pursued worldwide to solve some of the problems associated with current low-temperature PEM fuel cells (LT-PEM fuel cells, usually operated at <90 °C) these include sluggish electrode kinetics, low tolerance for contaminants (e.g. carbon monoxide (CO)), and complicated water and heat management [4,5]. However, operating a PEM fuel cell at >90 °C also accelerates degradation of the fuel cell components, especially the membranes and electrocatalysts [8]. 

At the heart of a PEM fuel cell is a polymer membrane that has some unique capabilities. It is impermeable to gases but it conducts protons (hence the name, proton exchange membrane). The membrane that acts as the electrolyte is squeezed between the two porous, electrically conductive electrodes. These electrodes are typically made out of carbon cloth or carbon fiber paper. At the interface between the porous electrode and the polymer membrane there is a layer with catalyst particles, typically platinum supported on carbon. A schematic diagram of cell configuration and basic operating principles is shown in Figure 1-10. Chapter 4 deals in greater detail with those major fuel cell components, their materials, and their properties. 

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