Alkaline-electrolyte fuel cells (see Fig. 9.7) are one of the most developed fuel cell technologies. They have been in use since the mid-1960s for Apollo and space shuttle programs [3, 6, 18, 19]. The AFCs onboard these spacecraft provide electrical power as well as drinking water. AFCs are among the most efficient electricity-generating fuel cells with an efficiency of nearly 70%. The electrolyte used in the AFC is an alka­line solution in which an OH ion can move freely across the electrolyte.

Electrochemistry of AFCs. The electrolyte used in the AFC is an aqueous (water-based) solution of potassium hydroxide (KOH) retained in a porous stabilized matrix. The concentration of KOH can be varied with the fuel cell operating temperature, which ranges from 65 to 220°C.

The charge carrier for an AFC is the hydroxyl ion (OH-) that migrates from the cathode to the anode, where they react with hydrogen to pro­duce water and electrons. Water formed at the anode migrates back to the cathode to regenerate hydroxyl ions.

Anode reaction: 2H2 + 4OH — ^ 4H2O + 4e-

Cathode reaction: O2 + 2H2O + 4e — ^ 4OH-

Hydroxyl ions are the conducting species in the electrolyte.

Overall cell reaction: 2H2 + O2 ^ 2H2O + heat + electricity

In many cell designs, the electrolyte is circulated (mobile electrolyte) so that heat can be removed and water eliminated by evaporation. Since KOH has the highest conductance among the alkaline hydroxides, it is the preferred electrolyte.

Electrolyte. Concentrated KOH (85 wt.%) is used in cells designed for operation at a high temperature (~260°C). For lower temperature (<120oC) operation, less concentrated KOH (35-50 wt.%) is used. The electrolyte is retained in a matrix (usually asbestos), and a wide range of electrocatalysts can be used (e. g., Ni, Ag, metal oxides, and noble metals). A major advantage of the AFC is the lower activation polar­ization at the cathode, resulting in a higher operating voltage (0.875 V). Another advantage of the AFC is the use of inexpensive electrolyte materials. The electrolyte is replenished through a reservoir on the anode side. The typical performance of this AFC cell is 0.85 V at a cur­rent density of 150 mA/cm2. The AFCs used in the space shuttle orbiter have a rectangular cross-section and weigh 91 kg. They operate at an average power of 7 kW with a peak power rating of 12 kW at 27.5 V. A disadvantage of the AFC is that it is very sensitive to CO2 present in the fuel or air. The alkaline electrolyte reacts with CO2 and severely degrades the fuel cell performance, limiting their application to closed environments, such as space and undersea vehicles, as these cells work well only with pure hydrogen and oxygen as fuel.

Electrodes. A significant cost advantage of alkaline fuel cells is that both anode and cathode reactions can be effectively catalyzed with non­precious, relatively inexpensive metals. The most important character­istics of the catalyst structure are high electronic conductivity and stability (mechanical, chemical, and electrochemical). Both metallic (typ­ically hydrophobic) and carbon-based (typically hydrophilic) electrode structures with multilayers and optimized porosity characteristics for the flow of liquid electrolytes and gases (H2 and O2) have been developed. The kinetics of oxygen reduction in alkaline electrolytes is much faster than in acid media; hence AFCs can use low-level Pt catalysts (about 20% Pt, compared with PEMFCs) on a large surface carbon support [20].

Performance. The AFC development has gone through many changes since 1960. To meet the requirements for space applications, the early AFCs were operated at relatively high temperatures and pressures. Now the focus of the technology is to develop low-cost components for AFCs operating at near-ambient temperature and pressure, with air as the oxi­dant for terrestrial applications. This has resulted in lower perform­ance. The reversible cell potential for an H2 and O2 fuel cell decreases by 0.49 mV/°C under standard conditions. An increase in operating tem­perature reduces activation polarization, mass transfer polarization, and ohmic losses, thereby improving cell performance. Alkaline cells operated at low temperatures (~70°C) show reasonable performance.

Pure hydrogen and oxygen are required in order to operate an AFC. Reformed H2 or air containing even trace amounts of CO2 dramatically affects its performance and lifetime. There is a drastic loss in performance when using hydrogen-rich fuels containing even a small amount of CO2 from reformed hydrocarbon fuels and also from the presence of CO2 in the air (~350 ppm CO2 in ambient air). The CO2 reacts with OH (CO2 + 2OH ^ CO32~ + H2O), thereby decreasing their concentration and thus reducing the reaction kinetics. Other ill effects of the presence of CO2 are:

■ Increase in electrolyte viscosity, resulting in lower diffusion rate and

lower limiting currents.

■ Deposition of carbonate salts in the pores of the porous electrode.

■ Reduction in oxygen solubility.

■ Reduction in electrolyte conductivity.

A higher concentration of KOH decreases the life of O2 electrodes when operating with air containing CO2. However, operation at higher tem­peratures is beneficial because it increases the solubility of CO2 in the elec­trolyte. The operational life of air electrodes polytetrafluoroethylene [PTFE] bonded carbon electrodes on porous nickel substrates) at a current density of 65 mA/cm2 in 9-N KOH at 65°C ranges from 4000 to 5500 h with CO2-free air, and their life decreases to 1600-3400 h when air (350-ppm CO2) is used. For large-scale utility applications, operating times >40,000 h are required, which is a very significant hurdle to com­mercialization of AFC devices for stationary electric power generation.

Another problem with the AFC is that the electrodes and catalysts degrade more on no-load or light-load operation than on a loaded condition, because the high open-circuit voltage causes faster carbon oxidation processes and catalyst changes. The AFC with immobilized KOH electrolyte suffers much more from this as the electrolyte has to stay in the cells caus­ing residual carbonate accumulation, separator deterioration, and gas cross leakage during storage or unloaded periods if careful maintenance is not carried out. In circulating an electrolyte-type AFC, the electrolyte is emp­tied from the cell during nonoperating periods. Shutting off the H2 elec­trodes from air establishes an inert atmosphere. This shutdown also eliminates all parasitic currents and increases life expectancy. The exchangeability of the KOH in a circulating electrolyte-type AFC offers the possibility to operate on air without complete removal of the CO2 [20, 21].