Semi-fuel cells

Metal/air cells are a semi-fuel cell, a hybrid between a battery and a fuel cell. The system combine in situ a metal anode, which dissolve in an alkaline or neutral electrolyte, and a porous diffusion cathode, in which air is continuously admitted. 

In some batteries two different types of electrodes are used on one side a conventional battery electrode with solid reactant and reaction products, and on the other side an electrode with gaseous and/or liquid reactant and products of the type used in fuel cells. Such batteries are called compound batteries or sometimes semi fuel cells (this term is rather poor). 

Pointon K D, Lakerman B, Irvine J T S, Bradley J and Jain S (2006), The development of a carbon-air semi fuel cell ,/ Power Sources, 162,765-772. 

Zn/air, Al/air Metal/air semi-fuel cell R.T.-100 OH Low cost, low operating temperature, mechanically rechargeable and low power applications 10 kW. 

Biological fuel cells, or semi-fuel cells (see Section 9.3), are cells in which at least one of the following two conditions is met ... 

The best known example of a semi-fuel cell is the disposable galvanic zinc-air cell widely used in hearing aids and other small electronic devices. Metallic zinc is the negative electrode in these cells (usually in the form of a highly disperse powder). When current is drawn, the zinc dissolves anodically in a concentrated alkaline solution according to the equation... 

A review paper by Neburchilov et al. (2001) discusses the composition, design, and fabrication methods of air cathodes for alkaline zinc-air cells, one of the few successfully commercialized semi-fuel cells. The more promising compositions for air cathodes are based on individual oxides, or mixtures of such, with a spinel, perovskite, or pyrochlore structure MnOa, C03O4, La203, LaNi03, NiCo204, LaMnOs, LaNi03, and so on. 

In the literature on electrochemical power sources, semi-fuel cells are generally regarded as a variety of ordinary batteries (galvanic cells or storage batteries), rather than as a variety of fuel cells (which have the distinguishing feature of a continuous supply of all reactants). In this section, brief information was given on these half-fuel cells to show the connections between their development and the development of real fuel cells. 

Metal-air batteries are different from conventional batteries because metal-air batteries are connected to the atmosphere, and need this access to operate. Metal-air batteries are also different from fuel cells because metal-air batteries have a self-contained anode within the battery case itself. Metal-air batteries are part conventional" battery and part fuel cell. Sometimes metal-air batteries are called semi-fuel cells. "Conventional" batteries have the active components of both the anode and cathode within the battery case (see Figure 1.1). Fuel cells have both of the "active" components (or fuels) of the anode and cathode supplied from outside the fuel cell case. A metal-air battery, or semi-fuel cell, has the solid anode within the case (like a battery), while the cathode fuel is brought into the cell (like a fuel cell). Oxygen gas... 

FIGURE 1.1 This drawing depicts three energy storage devices a battery, semi-fuel cell, and a fuel cell. The battery has an anode and cathode which are both soUd. The fuel cell (on the right) uses gaseous active components at the electrodes. The semi-fuel cell uses both a soUd anode and oxygen gas as the active cathode reactant. 

The poor efficiencies of coal-fired power plants in 1896 (2.6 percent on average compared with over forty percent one hundred years later) prompted W. W. Jacques to invent the high temperature (500°C to 600°C [900°F to 1100°F]) fuel cell, and then build a lOO-cell battery to produce electricity from coal combustion. The battery operated intermittently for six months, but with diminishing performance, the carbon dioxide generated and present in the air reacted with and consumed its molten potassium hydroxide electrolyte. In 1910, E. Bauer substituted molten salts (e.g., carbonates, silicates, and borates) and used molten silver as the oxygen electrode. Numerous molten salt batteiy systems have since evolved to handle peak loads in electric power plants, and for electric vehicle propulsion. Of particular note is the sodium and nickel chloride couple in a molten chloroalumi-nate salt electrolyte for electric vehicle propulsion. One special feature is the use of a semi-permeable aluminum oxide ceramic separator to prevent lithium ions from diffusing to the sodium electrode, but still allow the opposing flow of sodium ions. 

T.P. Magee, H R. Kunz, M. Krasij, H.A. Cole, "The Effects of Halides on the Performance of Coal Gas-Fueled Molten Carbonate Fuel Cell," Semi-Annual Report, October 1986 -March 1987, prepared by International Fuel Cells for the U.S. Department of Energy, Morgantown, WV, under Contract No. DE-AC21-86MC23136, May 1987. 

Mallouk, T. E., Reddington, E., Pu, C., Ley, K. L., Smotkin, E.S., Discovery of methanol electro-oxidation catalysts by combinatorial analysis. In Fuel Cell Semi-... 

The authors wish to thank the French Research Agency (ANR) and its Hydrogen and Fuel Cells programme (Pan-H) for co-financing the work on the nickelate materials in the framework of the national project SEMI-EHT (contract reference ANR-05-PANH-019). 

In a fuel cell, as the cell current becomes high, which indicates the electrochemical reaction rate on the electrode surface is fast, the mass transfer rate of the reactants is not fast enough to provide enough reactants to the electrode surface. Depletion of reactants at the electrode surface leads to a drop in cell voltage. The calculation of the cell voltage drop in this part is difficult, and a semi-empirical equation is usually used to estimate the mass transfer drop. The most popular expression for the mass transfer drop is... 

Isothermal chemistry in fuel cells. Barclay (2002) wrote a paper which is seminal to this book, and may be downloaded from the author s listed web site. The text and calculations of this paper are reiterated, and paraphrased, extensively in this introduction. Its equations are used in Appendix A. The paper, via an equilibrium diagram, draws attention to isothermal oxidation. The single equilibrium diagram brings out the fact that a fuel cell and an electrolyser which are the thermodynamic inverse of each other need, relative to existing devices, additional components (concentration cells and semi-permeable membranes), so as to operate at reversible equilibrium, and avoid irreversible diffusion as a gas transport mechanism. The equilibrium fuel cell then turns out to be much more efficient than a normal fuel cell. It has a greatly increased Nernst potential difference. In addition the basis of calculation of efficiency obviously cannot be the calorific value of the... 

In Appendix A, calculations show a status, for fuel cell isothermal Faradaic oxidation, of a high vacuum of reactants relative to a high concentration of product. That calculated status cannot even be approached in the laboratory, for lack of adequate semi-permeable membranes and circulators (concentration cells). The equilibrium fuel cell of Figure A.l is dead-ended, whereas the air-breathing open-ended design must have both of its electrodes swept by a parallel flow, with an inlet and an... 

The plant in Figure A.4 can be dealt with in exactly the same way. The reformer and the two fuel cells would be elevated to IT/SOFC conditions, as in Figure A.6. All surplus fuel, heavy hydrocarbons and unoxidised fuel from the three plant sections, together with three hot exhausts, would be swallowed by a gas turbine combustion chamber as above. That would yield a controllable plant, subject to availability of semi-permeable membranes and of isothermal concentration cells, appropriate to IT/SOFC temperatures and gas turbine pressure. 

Electrolytic Cell. A combination of a liquid or semi-liquid electrolyte (soln of a salt, acid or base) and two solids serving as electrodes. The cell generates an electric current when the electrodes are connected by an external wire. Flashlight batteries (dry cells), storage batteries and fuel cells (qv). When electricity is generated in such,a cell chemical changes occur at the electrodes so that either they or both the electrodes and the electrolytes are gradually consumed Ref CondChemDict (1961), 434-R... 

Table 3.2 reports the classification of the different types of fuel cells with some technical characteristics [6]. In this table the different electrolytes are specified together with the type of ions exchanged through them, while the catalysts indicated are those used on both anode and cathode to accelerate the semi-reactions (not necessary for SOFC thanks to their high operative temperature). 

The function of the polymeric membrane electrolyte is to permit the transfer of protons produced in anodic semi-reaction (3.11) from anode to cathode, where they react with reduced oxygen to give water. This process is of course essential for fuel cell operation, as it allows the electric circuit to be closed inside the cell. On the other hand, the membrane must also hinder the mixing between fuel and oxidant, and exhibit chemical and mechanical properties compatible with operative conditions of the fuel cell (temperature, pressures, and humidity). 

The electrodes in a PEM fuel cell have the fundamental function to provide a support where the electrochemical reactions take place. As both the electrochemical semi-reactions have to be catalyzed, to occur at temperatures under 90°C, the... 

Wishart J, Dong Z, Secanell M (2006) Optimization of a PEM fuel cell system based on empirical data and a generalized electrochemical semi-empirical model. J Power Sources 161 1041-1055... 

Advance the technology elements required to develop a semi-conformal, Compressed Hydrogen Gas Integrated Storage System (CH2-ISS) for light-duty fuel cell vehicles (FCVs)... 

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