Electrode alkaline fuel cells

Aqueous, alkaline fuel cells, as used by NASA for supplemental power in spacecraft, are intolerant to C02 in the oxidant. The strongly alkaline electrolyte acts as an efficient scrubber for any C02, even down to the ppm level, but the resultant carbonate alters the performance unacceptably. This behavior was recognized as early as the mid 1960 s as a way to control space cabin C02 levels and recover and recycle the chemically bound oxygen. While these devices had been built and operated at bench scale before 1970, the first comprehensive analysis of their electrochemistry was put forth in a series of papers in 1974 [27]. The system comprises a bipolar array of fuel cells through whose cathode chamber COz-containing air is passed. The electrolyte, aqueous Cs2C03, is immobilized in a thin (0.25 0.75 mm) membrane. The electrodes are nickel-based fuel cell electrodes, designed to be hydrophobic with PTFE. 

Swette L., Kackley N., McCatty S.A. Oxygen electrodes for rechargeable alkaline fuel cells, J. Power Sources. 1991 36 323 - 39. 

The Alkaline Fuel Cell (AFC) was one of the first modern fuel cells to be developed, beginning in 1960. The application at that time was to provide on-board electric power for the Apollo space vehicle. Desirable attributes of the AFC include its excellent performance on hydrogen (H2) and oxygen (O2) compared to other candidate fuel cells due to its active O2 electrode kinetics and its flexibility to use a wide range of electrocatalysts, an attribute which provides development flexibility. 

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. 

Fuel cells operate in a manner reverse to that of electrolysis, discussed in Chapter 2, combining fuel to make electricity. The basic design consists of two electrodes separated by an electrolyte. The oldest type of fuel cell is the alkaline fuel cell where an alkaline electrolyte like potassium hydroxide is used. The hydrogen enters through the anode compartment and oxygen through the cathode compartment. The hydrogen is ionized by the catalytic activity of the anode material and electrons are released into the external circuit. The protons react with the hydroxyl ions in the electrolyte to form water. The reaction can be written as ... 

Fig. 16. Schematic presentation of the morphological features of gas diffusion electrodes for fuel cells of (A) PTFE-bonded and Pt-activatcd Hi anodes and O2 cathodes used for Oi reduction in acidic and alkaline fuel cells (a) support, (b) hydrophobic gas diffusion layer, (c) hydrophilic electrode layer, (d) electrolyte, (e) magnified schematic of PTFE-bonded soot electrode, (f) adjacent hydrophobic layer, (g) microporous soot particles, (h) gas channels (mesopores), (k) PTFE particles, (I) flooded micro- and mesopores, (B) Schematic presentation of the morphology of PTFE-bonded Raney-nickel anodes used in alkaline fuel cells ol the Siemens technology.

Justi et al. (75) had already introduced Raney-nickel anodes for anodic hydrogen oxidation in alkaline fuel cells in the early sixties. At that time they used sintered electrodes composed of Raney-nickel particles, which are, however, too heavy and too expensive to be of use for commercial cells as too high loadings of the relatively expensive nickel are needed. Today... 

One of the first fuel cell designs was low-temperature alkaline fuel cells (AFCs) used in the U.S. space program. They served to produce both water and electricity on the spacecraft. Some of their disadvantages are that they are subject to carbon monoxide poisoning, are expensive, and have short operating lives. The AFC electrodes are made of porous carbon plates laced with a catalyst. The electrolyte is potassium hydroxide. At the cathode, oxygen forms hydroxide ions, which are recycled back to the anode. At the anode, hydrogen gas combines with the hydroxide ions to produce water vapor and electrons that are forced out of the anode to produce electric current. 

The cathode of the zinc-air cell uses a zinc metal anode and an air electrode similar to that for an alkaline fuel cell. The cell reaction is given in Equation 10.4. The cell is constructed with a pull-off... 

Fig. 13.27. Potential vs. current density plots for state-of-the-art fuel cells, o, proton exchange membrane fuel cell , solid oxide fuel cell , pressurized phosphonic acid fuel cell (PAFC) a, direct methanol fuel cell, direct methanol PAFC , alkaline fuel cell. (Reprinted from M. A. Parthasarathy, S. Srinivasan, and A. J. Appleby, Electrode Kinetics of Oxygen Reduction at Carbon-Supported and Un-supported Platinum Microcrystal-lite/Nafion Interfaces, J. Electroanalytical Chem. 339 101-121, copyright 1992, p. 103, Fig. 1, with permission from Elsevier Science.)...

Further research was performed on alkaline fuel-cell concepts (e.g. characterisation of gas diffusion electrodes) as well as on catalytic burners (reaction kinetics of H2/air mixtures). Experimental investigation of dymanic combustion phenomena was performed. Practical tests were carried out on internal combustion engines, including compression ignition engines (Altman el al., 1997 Schucan, 2000)... 

Electrode preparation methods significantly affect fuel cell performance. A variety of methods have been developed. For example, Bevers et al. [17] produced electrodes using a simple and less costly procedure, a modified rolling technique, formerly used in the production of electrodes for alkaline fuel cells and batteries. With these new electrodes, the same power output was obtained as that using commercial ones. 

Alkaline Fuel Cell The electrolyte for NASA s space shuttle orbiter fuel cell is 35 percent potassium hydroxide. The cell operates between 353 and 363 K (176 and 194°F) at 0.4 MPa (59 psia) on hydrogen and oxygen. The electrodes contain platinum-palladium and platinum-gold alloy powder catalysts bonded with polytetrafluoro-ethylene (PTFE) latex and supported on gold-plated nickel screens for current collection and gas distribution. A variety of materials, including asbestos and potassium titanate, are used to form a micro-porous separator that retains the electrolyte between the electrodes. The cell structural materials, bipolar plates, and external housing are usually nickel, plated to resist corrosion. The complete orbiter fuel cell power plant is shown in Fig. 27-62. 

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),... 

Alkaline fuel cells (AFCs) use an aqueous potassium hydroxide (KOH) solution (around 30%) as electrolyte and have electrode reactions of the form... 

Alkaline fuel cells have been used extensively on early spacecraft imtil they were superseded by more reliable solar cells. The high cost of the space cells and the use of corrosive compoxmds requiring special care in handling have been held against AFCs. Current AFC development employs multi-component electrodes using Ni for structural stability and as catalyst, carbon black as electron conductor and polytetrafluoroethylene (PTFE) pore-forming... 

Alkaline fuel cell (AFC) was used for Apollo and Space Shuttle program. Alkaline fuel cell employs liquid alkaline, e.g., KOH, as an electrolyte so that fuel, as well as air or oxygen, should be free of CO2 because the strong alkaline electrolyte reacts with CO2 to form carbonates, which reduces the ionic conductivity. Electrodes, e.g., Ni, Ag, and metal oxides, are relatively inexpensive compared to that of other fuel cells. 

The monolithic structure, good mechanical strength, high surface area, and electrical conductivity of these carbon materials make them attractive as electrodes for various electrochemical applications. As hydrogen oxidation (or oxygen reduction) catalysts may be incorporated to such porous materials, one specific application to consider is the use of this type of material as alkaline fuel cell electrode. 

The air-hydrogen alkaline fuel cell of Kordesch s Austin produced a tepid 6 kilowatts in conjunction with a bank of ordinary 12-volt lead-acid batteries (for acceleration and hill climbing). The fuel cell s electrodes, donated by Union Carbide, had been used earlier in another ground-breaking fuel cell vehicle, General Motors Electrovan, a six-passenger van that had been converted with Kordesch s help in 1967. 

Normally, the C02 present in the atmosphere poisons the electrodes of alkaline fuel cells—one reason why transportation fuel cell developers have been concentrating on PEM-type fuel cells in the 1990s, since they are largely immune to the problem. Kordesch neatly solved that particular difficulty by installing a soda-lime air scrubber, which removed at least half of the air s C02 content of 0.03 percent. Additionally, his cell s alkaline (KOH) electrolyte absorbed some C02, reducing contamination even more. (Kordesch says the electrolyte could be easily changed when it had taken up too much C02.)... 

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