Fuel Cell Operations

Molten Carbonate Fuel Cell. The electrolyte ia the MCFC is usually a combiaation of alkah (Li, Na, K) carbonates retaiaed ia a ceramic matrix of LiA102 particles. The fuel cell operates at 600 to 700°C where the alkah carbonates form a highly conductive molten salt and carbonate ions provide ionic conduction. At the operating temperatures ia MCFCs, Ni-based materials containing chromium (anode) and nickel oxide (cathode) can function as electrode materials, and noble metals are not required. 

As can be seen from Eigure 11b, the output voltage of a fuel cell decreases as the electrical load is increased. The theoretical polarization voltage of 1.23 V/cell (at no load) is not actually realized owing to various losses. Typically, soHd polymer electrolyte fuel cells operate at 0.75 V/cell under peak load conditions or at about a 60% efficiency. The efficiency of a fuel cell is a function of such variables as catalyst material, operating temperature, reactant pressure, and current density. At low current densities efficiencies as high as 75% are achievable. 

The operating temperature also affects the fuel cell operating potential, A high operating temperature accelerates reaction rates but... 

Cooling strongly depends on fuel cell operating temperature and also depends on the fuel cell s external environment. For low temperature fuel cells, cooling imposes a significant energy debit because pumps need to force coolant out to a heat... 

Operating costs, in contrast, are more straightforward to determine because they depend on system efficiency, which, in turn, is related to voltage and current density (the current generated per unit area of electrolyte). Fuel savings are expected since the fuel cell operates more efficiently than a heat engine, and there may be lower maintenance and repair costs because fuel cells have fewer moving parts to wear out. 

System integration involves numerous miscellaneous development activities, such as control software to address system start-up, shut-down and transient operation, and thermal sub-systems to accomplish heat recovei y, heat rejection and water recoveiy within the constraints of weight, size, capital and operating costs, reliability, and so on. Depending on the application, there will be additional key issues automotive applications, for example, demand robustness to vibrations, impact, and cold temperatures, since if the water freezes it will halt fuel cell operation. 

A hydrogen-oxygen fuel cell operates on the reaction ... 

If the cell is designed to produce 1.5 amp of current and if the hydrogen is contained in a 1.0-L tank at 200 atm pressure and 25°C, how long can the fuel cell operate before the hydrogen runs out Assume that oxygen gas is in excess. 

Figure 1.6. Common features of Heterogeneous Catalysis, Fuel Cell operation, Electrolysis and Electrochemical Promotion 1. Solid state catalyst, 2. Adsorption, 3. AG < 0, 4. Yield control via DC current or voltage application (Adapted from N. A. Anastasijevic).

In the present work, we report results on the fabrication and performance of anode-supported, thin SDC el trolyte fuel cells operated in a single chamber configuration where methane and oxygen served as the gas mixture. 

Two new technologies have reduced the cost of alkali fuel cells to the point where a European company markets taxis that use them. One is the use of CO2 scrubbers to purify the air supply, making it possible to use atmospheric O2 rather than purified oxygen. The other is the development of ultrathin films of platinum so that a tiny mass of this expensive metal can provide the catalytic surface area needed for efficient fuel-cell operation. 

Type of fuel cell Operating fuel and temperature Power rating (kW) Fuel efficiency s (%) Power density (mW/cm ) Lifetime s (hr) Capital cost s ( /kW) Applications... 

Power can be generated when supplying suitable reactants to the electrodes (fuel-cell operation). 

The Pt-Rn catalysts have another important property. In contrast to pure platinum, they are almost insensitive to poisoning by carbon monoxide CO. They can be used, therefore, in the hydrogen electrodes of hydrogen-oxygen fuel cells operated with technical hydrogen containing marked amonnts of CO. 

When surveying the central milestones in the development of electrocatalysis for low temperature fuel cells operating in acidic environments, the following, listed in chronological order, seem to be the most outstanding ... 

It would certainly be desirable to evaluate catalyst performance and understand size and stmctural effects directly under the conditions of fuel cell operation. However, determination of kinetic parameters in a single-cell fuel cell is associated with a number of limitations. Let us consider some of them. 

The choice of immobilization strategy obviously depends on the enzyme, electrode surface, and fuel properties, and on whether a mediator is required, and a wide range of strategies have been employed. Some general examples are represented in Fig. 17.4. Key goals are to stabilize the enzyme under fuel cell operating conditions and to optimize both electron transfer and the efficiency of fuel/oxidant mass transport. Here, we highlight a few approaches that have been particularly useful in electrocatalysis directed towards fuel cell applications. 

Figure 17.17 Schematic representation of a single-compartment glucose/02 enzyme fuel cell built from carbon fiber electrodes modified with Os -containing polymers that incorporate glucose oxidase at the anode and bilirubin oxidase at the cathode. The inset shows power density versus cell potential curves for this fuel cell operating in a quiescent solution in air at pH 7.2, 0.14 M NaCl, 20 mM phosphate, and 15 mM glucose. Parts of this figure are reprinted with permission from Mano et al. [2003]. Copyright (2003) American Chemical Society.

Spiegel, R. J. Preston, J. L., Test results for fuel cell operation on anaerobic digester gas. Journal of Power Sources 2000,86, 283-288. 

A typical problem to fuel cells operating at low temperatures comes from the catalyst, which can be damaged (or poisoned ) by the presence of CO or C02 and needs to be replaced AFC and PEMFC are rather intolerant to C02 and CO, while PAFC is moderately tolerant to CO and MCFC and SOFC are fully tolerant to CO. 

The general requirements for an SOFC anode material include [1-3] good chemical and thermal stability during fuel cell fabrication and operation, high electronic conductivity under fuel cell operating conditions, excellent catalytic activity toward the oxidation of fuels, manageable mismatch in coefficient of thermal expansion (CTE) with adjacent cell components, sufficient mechanical strength and flexibility, ease of fabrication into desired microstructures (e.g., sufficient porosity and surface area), and low cost. Further, ionic conductivity would be beneficial to the extension of... 

Since the conductivity of Ni is more than 5 orders of magnitude greater than that of YSZ under the fuel cell operating conditions, the electrical conductivity of a porous Ni-YSZ cermet anode changes several orders of magnitude, usually from -0.1 S/cm... 

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