ALKALINE FUEL CELLS AFCs

The low-temperature fuel cells described in Chapters 18 and 19 have (acidic) proton-conducting membranes as an electrolyte. On account of corrosion problems, only metals of the platinum group can be used as catalysts for the electrode reactions in such fuel cells. 

In AFCs, like in other electrochemical devices, solutions of potassium hydroxide KOH prepared in different concentrations are usually used as the electrolytes. 

Alkaline electrolytes offer further advantages over acidic electrolytes. Oxygen reduction is considerably faster in alkaline electrolytes, which implies that the working potential of the oxygen electrode is more positive, and a larger cell voltage can be realized. Also, apart from the advantages in catalyst selection, less severe corrosion conditions allow nickel and alloys of iron to be used as structural materials in AFCs. 

alkaline electrolytes also have some severe disadvantages relative to acidic electrolytes. The most important disadvantage is their reactivity with carbon dioxide, which they readily absorb forming carbonates. As is always present in air, an 

Electrochemical Power Sources Batteries, Fuel Cells, and Supercapacitors, First Edition. Vladimir S. Bagotsky, Alexander M. Skundin, and Yurij M. Volfkovich 2015 John Wiley Sons, Inc. Published 2015 by John Wiley Sons, Inc. 

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Alkaline fuel cells (AFCs). The electrolyte is 40 to 70% KOH, the working temperatures are 60 to 240°C. Such systems were used in the spacecraft of the Apollo program and in the U.S. space shuttle. 

There are six different types of fuel cells (Table 1.6) (1) alkaline fuel cell (AFC), (2) direct methanol fuel cell (DMFC), (3) molten carbonate fuel cell (MCFC), (4) phosphoric acid fuel cell (PAFC), (5) proton exchange membrane fuel cell (PEMFC), and (6) the solid oxide fuel cell (SOFC). They all differ in applications, operating temperatures, cost, and efficiency. 

Alkaline Fuel Cell (AFC) The electrolyte in this fuel cell is concentrated (85 wt%) KOH in fuel cells operated at high temperature ( 250°C), or less concentrated (35-50 wt%) KOH for lower temperature (<120°C) operation. The electrolyte is retained in a matrix (usually asbestos), and a wide range of electrocatalysts can be used (e.g., Ni, Ag, metal oxides, spinels, and noble metals). The fuel supply is limited to non-reactive constituents except for hydrogen. CO is a poison, and CO2 will react with the KOH to form K2CO3, thus altering the electrolyte. Even the small amount of CO2 in air must be considered with the alkaline cell. 

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. 

Although several fuel cell technologies are reaching technical maturity, the economics of a fuel cell are not clear. The commercial potential of fuel cells will depend on the ability to reduce catalyst and other expensive materials costs and to manufacture the units at a competitive cost. Many uses of fuel cells place a premium on specific performance characteristics. The relatively simple alkaline fuel cells (AFC)... 

Alkaline fuel cell (AFC) working at 80 °C with concentrated potassium hydroxide as electrolyte, conducting by the OH anion. This kind of fuel cell, developed by IFC (USA), is now used in space shuttles. 

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the US space program to produce electrical energy and water onboard spacecraft. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100°C and 250°C. However, more-recent AFC designs operate at lower temperatures of roughly 23°C to 70°C. 

Fuel cells can be broadly classified into two types high temperature fuel cells such as molten carbonate fuel cells (MCFCs) and solid oxide polymer fuel cells (SOFCs), which operate at temperatures above 923 K and low temperature fuel cells such as proton exchange membrane fuel cells (PEMs), alkaline fuel cells (AFCs) and phosphoric acid fuel cells (PAFCs), which operate at temperatures lower than 523 K. Because of their higher operating temperatures, MCFCs and SOFCs have a high tolerance for commonly encountered impurities such as CO and CO2 (CO c)- However, the high temperatures also impose problems in their maintenance and operation and thus, increase the difficulty in their effective utilization in vehicular and small-scale applications. Hence, a major part of the research has been directed towards low temperature fuel cells. The low temperature fuel cells unfortunately, have a very low tolerance for impurities such as CO , PAFCs can tolerate up to 2% CO, PEMs only a few ppm, whereas the AFCs have a stringent (ppm level) CO2 tolerance. 

In alkaline fuel cells (AFCs), the electrolyte is 34-46% KOH, immobilized on a porous support, and the operating temperature is 60-120 °C. Because the environment is alkaline, Raney nickel (a finely divided form of nickel) can be used in place of expensive platinum. However, the alkali will be neutralized by any CO2 in the hydrogen fuel, so AFCs are not suitable for operation with reformed hydrocarbons but can be fueled with alcohols or hydrazine. AFCs were used successfully on the Apollo space missions. 

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. 

Alkaline Fuel Cell (AFC). This cell follows directly from the one that Bacon and Watson produced at Cambridge in the 1950s and is the basis of cells developed for NASA (by International Fuel Cells and predecessor companies (United Technologies Power Systems Divisions, Pratt and Whitney Aircraft) since the Apollo moon program, where pure H2 fuel is available. 

Figure 8.4 shows a schematic representation of an alkaline fuel cell (AFC). In this case, the oxidation reaction in the cathode is given by... 

Alkaline fuel cells (AFC), with concentrated KOH (in asbestos matrices) electrolyte, conduct OH -anions, generated at an 02/H20 exposed cathode to electro-oxidize H2 (fuel) at the anode at moderate temperatures, and... 

The application of gold as an electrocatalytic component within the fuel cell itself has to date been limited primarily to the historical use of a gold-platinum electrocatalyst for oxygen reduction in the Space Shuttle/Orbiter alkaline fuel cells (AFC)88 and the recent use of gold for borohydride oxidation in the direct borohydride alkaline fuel cell (DBAFC).89,90 Electrocatalysts with lower cost, improved carbon monoxide tolerance and higher... 

Alkaline fuel cells (AFC) — The first practical -+fuel cell (FC) was introduced by -> Bacon [i]. This was an alkaline fuel cell using a nickel anode, a nickel oxide cathode, and an alkaline aqueous electrolyte solution. The alkaline fuel cell (AFC) is classified among the low-temperature FCs. As such, it is advantageous over the protonic fuel cells, namely the -> polymer-electrolyte-membrane fuel cells (PEM) and the - phosphoric acid fuel cells, which require a large amount of platinum, making them too expensive. The fast oxygen reduction kinetics and the non-platinum cathode catalyst make the alkaline cell attractive. 

Without considering batteries and other chemical storage devices, there are effectively six types of primary or direct fuel cell technologies currently being developed alkaline fuel cells (AFC), polymer electrolyte fuel... 

Other fuels were also tried in the early stages of fuel cell development. Coal, the major fuel at that time, was considered as a candidate. Attempts to replace hydrogen with coal resulted in the invention of alkaline fuel cells (AFCs) and molten carbonate fuel cells (MCFCs). Mond used reformate gas from coal, which contained abundant hydrogen, as the fuel, with the intention of scaling up Grove s fuel cell to produce electric power. However, impurities poisoned the catalyst and made Mond s design impractical. 

Alkaline fuel cells (AFCs) were the first type of fuel cell to be widely used in space exploration applications-for example, in NASA s Apollo and space shuttle flights. Figure 1.8 shows a schematic of an AFC stmcture. AFCs use H2 and 02 as fuel and oxidant, respectively. The electrolyte is a concentrated KOH solution absorbed into an asbestos matrix. The temperature for AFCs ranges from 100-250°C and the efficiency can be > 60%. OH ions are transported through the electrolyte from cathode to anode. The reactions are as follows ... 

Whereas the hot systems can consume CO, the cool systems suffer CO-poisoned platinum catalysts, and must have a shift reactor to consume the CO. Platinum poisoning is an irreversibility. The alkaline fuel cell (AFC), although without platinum, is especially incompatible with CO because of its KOH electrolyte. It needs a pure hydrogen fuel, and air with CO removed. The latter two purifications carry their own irreversibilities. 

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

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