Fuel cell resistance

C. Yuh, M. Farooque, R. Johnsen, ERC, "Understanding of Carbonate Fuel Cell Resistances in MCFCs," in Proceedings of the Fourth Annual Fuel Cells Contractors Review Meeting, U.S. DOE/METC, Pgs. 53 - 57, July, 1992. 

Current, voltage, load, and fuel cell resistances. 

High-temperature polymer electrolyte membrane fuel cell. Resistance for the flow of H" through the electrolyte matrix. Kilo watts. 

Chacko, C., Ramasamy, R., Kim, S. et al. 2008. Characteristic behavior of polymer electrolyte fuel cell resistance during cold start. /. Electrochem. Soc. 155 B1145-B1154. 

Intermediate-temperature fuel cells with high power density have not been realized because there is not a good electrolyte with high ion conductivity in this temperature region. So, research has been focused on decreasing electrolyte resistance because a major part of fuel cell resistance is derived from the electrolyte resistance. There have been two main research approaches for intermediate-temperature fuel cells (1) the search for a new electrolyte with high ionic conductivity, and (2) search for a thin-film electrolyte. 

Example 4.6 Estimate Total Fuel Cell Resistance Given the experimental polarization data for a SOFC at 700°C from [13], estimate the ionic resistance of the electrolyte. Is this a maximum or a minimum value for the actual ohmic resistance of the electrolyte ... 

For a large number of applications involving ceramic materials, electrical conduction behavior is dorninant. In certain oxides, borides (see Boron compounds), nitrides (qv), and carbides (qv), metallic or fast ionic conduction may occur, making these materials useful in thick-film pastes, in fuel cell apphcations (see Fuel cells), or as electrodes for use over a wide temperature range. Superconductivity is also found in special ceramic oxides, and these materials are undergoing intensive research. Other classes of ceramic materials may behave as semiconductors (qv). These materials are used in many specialized apphcations including resistance heating elements and in devices such as rectifiers, photocells, varistors, and thermistors. 

Phosphoric Acid Fuel Cell This type of fuel cell was developed in response to the industiy s desire to expand the natural-gas market. The electrolyte is 93 to 98 percent phosphoric acid contained in a matrix of silicon carbide. The electrodes consist of finely divided platinum or platinum alloys supported on carbon black and bonded with PTFE latex. The latter provides enough hydrophobicity to the electrodes to prevent flooding of the structure by the electrolyte. The carbon support of the air elec trode is specially formulated for oxidation resistance at 473 K (392°F) in air and positive potentials. 

Solid Oxide Fuel Cell In SOF(7s the electrolyte is a ceramic oxide ion conductor, such as vttriurn-doped zirconium oxide. The conduetKity of this material is 0.1 S/ern at 1273 K (1832°F) it decreases to 0.01 S/ern at 1073 K (1472°F), and by another order of magnitude at 773 K (932°F). Because the resistive losses need to be kept below about 50 rn, the operating temperature of the... 

Aluminum s low density, wide availability, and corrosion resistance make it ideal for construction and for the aerospace industry. Aluminum is a soft metal, and so it is usually alloyed with copper and silicon for greater strength. Its lightness and good electrical conductivity have also led to its use for overhead power lines, and its negative electrode potential has led to its use in fuel cells. Perhaps one day your automobile will not only be made of aluminum but fueled by it, too. 

Microelectronic circuits for communications. Controlled permeability films for drug delivery systems. Protein-specific sensors for the monitoring of biochemical processes. Catalysts for the production of fuels and chemicals. Optical coatings for window glass. Electrodes for batteries and fuel cells. Corrosion-resistant coatings for the protection of metals and ceramics. Surface active agents, or surfactants, for use in tertiary oil recovery and the production of polymers, paper, textiles, agricultural chemicals, and cement. 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

In practice the situation is less favorable due to losses associated with overpotentials in the cell and the resistance of the membrane. Overpotential is an electrochemical term that, basically, can be seen as the usual potential energy barriers for the various steps of the reactions. Therefore, the practical efficiency of a fuel cell is around 40-60 %. For comparison, the Carnot efficiency of a modern turbine used to generate electricity is of order of 50 %. It is important to realize, though, that the efficiency of Carnot engines is in practice limited by thermodynamics, while that of fuel cells is largely set by material properties, which may be improved. 

The principle of the A-probe is shown in Fig. 10.2. It is a simple oxygen sensor made in a similar manner to the solid oxide fuel cell discussed in Chapter 8. An oxide that allows oxygen ions to be transported is resistively heated to ensure sufficiently high mobility and a short response time ( 1 s.). 

Most solid-state sensors are heated to well above 100°C and can operate in the "cold start" condition in a fuel cell. Another important performance parameter for a hydrogen sensor in a fuel cell is its resistance to water entry. Most fuel cells have excess of liquids including water during operation. It is highly possible that water will splash or penetrate into the hydrogen sensor mounted in the ventilation or outlet of a fuel cell. Hydrophobic... 

Catalytic bead Resistance Fuel Cell Safety Inc. 1-5% Yes Can respond to reducibles (VOC, CO) ... 

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