Alkaline fuel cells (AFCs) was one of the first extensively researched fuel cell types and was used by NASA for the Gemini, Apollo, and Space Shuttle missions. The first alkali electrolyte fuel cell was built by Francis Thomas Bacon (1904–1992) in 1939. He used potassium hydroxide for the electrolyte and porous “gas-diffusion” electrodes instead of the acid electrolytes and solid electrodes used in previous fuel cell prototypes. Like today’s fuel cells, Bacon also used pressurized gases to keep the electrolyte from “flooding” the electrodes. During World War II, he thought the alkali electrolyte fuel cell would provide a good source of power for the Royal Navy submarines instead of the dangerous storage batteries used at the time. For the next 20 years, he created large-scale demonstrations with his alkali cells using potassium hydroxide as the electrolyte. One of the first demonstrations was a 1959 Allis–Chalmers farm tractor that pulled a weight of 3000 pounds powered by a stack of 1008 alkaline cells. Allis–Chalmers continued fuel cell research for many years, and also demonstrated a fuel cell–powered golf cart, submersible vehicle, and forklift. In the late 1950s and 1960s, Union Carbide also experimented with alkali cells. Karl Kordesch and his colleagues designed alkali cells with carbon gas–diffusion electrodes based upon the work of G. W. Heise and E. A. Schumacher in the 1930s. They demonstrated a fuel cell–powered mobile radar set for the U.S. Army, as well as a fuel cell–powered motorbike. Pratt & Whitney licensed the Bacon patents in the early 1960s and won the National Aeronautics and Space Administration (NASA) contract to power the Apollo spacecraft with alkali cells.
The operating temperature of alkaline fuel cells range between room temperature to approximately 250 ˚C and can achieve power-generating efficiencies of up to 70 percent. A diagram of the alkaline fuel cell is shown in Figure 1. The chemical reactions that occur in an AFC are as follows:
Anode: 2H2 (g) + 4(OH)- (aq) → 4H2O (l) + 4e-
Cathode: O2 (g) + 2H2O (l)+ 4e- → 4(OH)-(aq)
Overall: 2H2 (g) + O2 (g) → 2H2O (l)
Figure 1. Chemical Reactions in an Alkaline Fuel Cell.
In AFCs, the oxygen reacts at the cathode to produce either hydroxide (OH-) or a carbonate ion (CO32-), depending upon the electrolyte composition. The ion travels through the electrolyte to react with hydrogen at the cathode. AFCs use lower cost materials compared with other fuel cells.
The catalyst layer can use either platinum or nonprecious metal catalysts such as nickel and the electrolyte has historically consisted of liquid KOH or a KOH filled matrix. A disadvantage of AFCs is that pure hydrogen and oxygen have to be fed into the fuel cell because it cannot tolerate the small amount of carbon dioxide from the atmosphere.
The electrolyte layer is essential for a fuel cell to work properly. In low temperature fuel cells, when the fuel in the fuel cell travels to the catalyst layer, the fuel molecule gets broken into protons (H+) and electrons. The electrons travel to the external circuit to power the load, and the hydrogen proton (ions) travel through the electrolyte until it reaches the cathode to combine with oxygen to form water. In AFMs, the reactions are similar, but they occur in the opposite direction, with the O2 producing the hydroxide and then reacting with hydrogen at the anode.
Traditionally, the electrolyte for alkaline fuel cells is an aqueous solution of alkaline potassium hydroxide soaked in a matrix with normalities ranging from 6 to 9. The solution must be as pure as possible to prevent the impurities from contaminating the catalyst. The electrolyte can be in a mobile or immobile form. The mobile electrolyte is pumped through the cells and removes water and waste heat from the fuel cell. These cells typically have large 2 to 3 mm flow channels to allow rapid flow. The large thickness increases the ohmic polarization which is a major design consideration for AFCs.
In AFCs that use immobile electrolyte, the KOH/H2O solution is held in an asbestos matrix. The electrolyte layer can be as thin as 0.05 mm; therefore, ohmic polarization is not as much of an issue in this type of AFC. The electrolyte typically used is 30-percent potassium hydroxide, which yields the optimal ionic conductivity when AFCs are operated at 60 to 80 C. Increasing the KOH concentration helps AFC performance, but it is not practical and feasible to use high concentrations of KOH in water due to the nonuniformity of KOH concentrations in operating cells. The oxygen reduction reaction may reduce the water concentration near the cathode and then the electrolyte solution may solidify, thus preventing reactant transport.
As with other fuel cell types, the electrolyte must meet the following requirements:
• High ionic conductivity
• Present an adequate barrier to the reactants
• Be chemically and mechanically stable
• Low electronic conductivity
• Ease of manufacturability/availability
• Preferably low-cost
Finding an electrolyte material that meets all of these requirements is tough. The toughest requirements are high ionic conductivity, and a material that is stable in both an oxidizing and reducing environment. There are many research groups around the world that are investigating solutions to overcome these limitations. One solution is to use solid electrolyte layers such as anion exchange membranes (AEMs) instead of liquid or matrix electrolytes.
The AFC electrodes can be hydrophobic or hydrophilic. The hydrophobic electrodes are carbon-based with PTFE, while the hydrophilic electrodes are usually made of metallic materials such as nickel and nickel-based alloys. The electrodes usually have several layers with different porosities for the liquid electrolyte, fuel, and oxidant. AFCs can use both precious and nonprecious metal catalysts. The precious metal catalysts used are platinum or platinum alloys that are deposited on carbon supports or manufactured on nickel-based metallic electrodes. The catalyst loading is typically 0.25 mg Pt/cm2 and up. The most commonly used nonprecious metal catalysts are Raney nickel for the anodes at a loading of 120 mg Ni/cm2, and silver-based powders for the cathodes with a loading between 1.5 to 2 mg Ag/cm2.
Sintered nickel powder. When the alkaline fuel cell was originally designed, it was made to use precious metal catalysts and employ low-cost materials. The electrodes were made of porous powdered nickel that was sintered to make it a rigid structure. To ensure good three-phase contact between the reactant gas, the liquid electrolyte, and the solid electrode, the nickel was made of two layers of different sizes of nickel powders. This structure gave good results, but it was hard to optimize the fuel cell catalyst layer at the time due to limited coating technology and instrumentation to investigate the properties of materials. This structure is still sometimes used today with and without additional catalysts.
Raney metals. Raney metals can be used to achieve very active and porous forms of a metal. These are prepared by mixing the active metal (nickel) with an inactive metal such as aluminum. The mixing is accomplished so that the two metals are not mixed to form an alloy, but the regions and properties of both metals are maintained. A strong alkali is then applied to the mixture to dissolve the aluminum. This leaves a porous material with a very high surface area. This process allows the pore size to be changed by altering the degree of mixing between the two metals.
AFCs offer potential benefits over PEMFCs due to non-noble metal catalysts and the ability to use cheaper and more versatile hydrocarbon-based membranes. PEMFCs face an issue of H2 and O2 crossover because of electro-osmosis and diffusion; however, for AEMFCs, the transport of hydroxide occurs from cathode to anode while water moves from anode to cathode, therefore, the crossover problems are solved in AEMFCs. The alkaline environment of AEMFCs allow the use of non-precious metal catalysts such as iron, cobalt, silver, and graphene, which significantly reduces the cost of the fuel cell system. Water management issues can also potentially be solved by tuning the properties of the polymer to allow for water diffusion from the anode to the cathode.