Types of fuel cell

Fuel cells can be classified into diverse types, where the most frequent categorization is by the type of electrolyte used in the cells, namely [6-9,11]  

The Physical Chemistry of Materials Energy and Environmental Applications 

These fuel cells are listed in the order of their approximate operating temperature, ranging from 80°C for PEFCs, 100°C for AFCs, 200°C for PAFCs, 650°C for MCFCs, and 800°C-1000°C for SOFCs. 

Fuel cells are classified by the electrolyte used by the module. The following four distinct types of fuel cells are gaining great interest for applications as a power source  

Research and development activities pursued by various fuel cell companies indicate that PEM fuel cells offer simple design, improved reliability, reduced procurement, low operating cost, and a small footprint. The Dow Chemical Company and Ballard Power Systems, Inc. are dedicated to commercializing the PEM fuel cells for distributed power-generating markets. Regardless of who manufactures the fuel cells, the fuel cells exhibit the following unique characteristics  

Because of their ultra-high reliability, as early as I960, fuel cells were used to provide onboard electrical power for manned spacecraft, and the exhaust produced safe drinking water for the astronauts. Within the past few years, the U.S. military has provided significant research and development in fuel cell support for possible applications in battlefield applications. Chapter 3 provides specific design concepts and material requirements for various types of fuel cells. 

The chapter provides brief descriptions of primary and secondary (rechargeable) batteries. Performance capabilities and limitations of rechargeable batteries are discussed with a particular emphasis on reliability and longevity. Battery requirements for EVs and HEVs are defined in terms of cost per kilometer of travel. Estimates of specific energy density and safe power levels are provided for specific applications. 

Courtney E. Howard, Electronics miniaturization, Military and Aerospace Electronics (June 2009), p. 32. 

Different attributes can be used to distinguish fuel cells  

There are several different types of fuel cells, which are classified primarily by the electrolyte used. This in turn drives requirements around operating temperature range, oxidant and fuel used, types of catalyst, materials requirements, and tolerance to contaminants. Each type of fuel cell may be best suited for certain applications, and will have particular technology challenges to overcome. Table 1.1 from the U.S. DOE H2 program provides a comparison of the main fuel cell types [11]. 

The fuel cell can be classified on the basis of efficiency, operating temperature, nature of electrolyte used and applications. Some of the commonly used fuel cells are given in T able 1.1. The research is focused on the improvisation of fuel cells and their components in order to obtain long-term performance with high efficiency. The following sections deal with the detailed explanation of some common fuel cells along with their working principle and construction. 

Based on the electrolyte used, there are five types of fuel cells. Their key features are listed in Table 1.2. 

A proton exchange membrane fuel cell (PEMFC) uses a solid membrane that transports protons. It can operate from about 0°C to 80°C with the output power ranging from a few watts to several hundred kilowatts. H2 is the best fuel for a PEMFC, and the anode and cathode reactions are shown in Reactions 1.1 and 1.2, respectively. 

When methanol is oxidized directly at the anode as the fuel, the fuel cell is called a direct methanol fuel cell (DMFC). The anode, the cathode, and the overall reactions are shown in Reactions 1.4, 1.5, and 1.6, respectively. It is important to note that methanol oxidation needs the presence of water. In other words, if there is no water at the anode, methanol will not be oxidized. So, supplying enough water to the anode is a necessity for a DMFC. 

When a DMFC and a DFAFC use a PEM as the electrolyte, as presented in Reactions 1.4 to 1.9, they can be regarded as special cases of PEMFCs. 

Electrons flow in the external circuit during these reactions. The oxygen ions recombine with protons to form water  

The product of this reaction is water that is formed at the cathode in fuel cells with proton-conducting membranes. It can be formed at the anode, if an oxygen ion (or carbonate)-conducting electrolyte is used instead, as is the case for high-temperature fuel cells. 

Fuel cells are usually classified by the electrolyte employed in the cell. An exception to this classification is DMFC (direct methanol fuel cell) that is a fuel cell in which methanol is directly fed to the anode. The electrolyte of this cell does not determine the class. The operating temperature for each of the fuel cells can also determine the class. There are, thus, low- and high-temperature fuel cells. Low-temperature fuel cells are alkaline fuel cells (AFCs), polymer electrolyte membrane fuel cells (PEMFCs), DMFC, and phosphoric acid fuel cells (PAFCs). The high-temperature fuel cells operate at temperatures —600-1000 °C and two different types have been developed, molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFCs). AU types of fuel cells are presented in the following sections in order of increasing operating temperature. An overview of the fuel cell types is given in Table 1.1 [1,5-7]. 

Operating trmpendure rc 100 120 60 120 160 220 600 800 800 lOOOi lower leniprrainte (000-600) poaable 

Realized pcmei Siiull plants Snull plams Small plants Small-medium sued plants. Small power ptiats. Small power pkmls. 

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Another even more significant use of methyl alcohol can be as a fuel in its own right in fuel cells. In recent years, in cooperation with Caltech s Jet Propulsion Laboratory (JPL), we have developed an efficient new type of fuel cell that uses methyl alcohol directly to produce electricity without the need to first catalytically convert it to produce hydrogen. 

Another important potential appHcation for fuel cells is in transportation (qv). Buses and cars powered by fuel cells or fuel cell—battery hybrids are being developed in North America and in Europe to meet 2ero-emission legislation introduced in California. The most promising type of fuel cell for this appHcation is the SPEC, which uses platinum-on-carbon electrodes attached to a soHd polymeric electrolyte. 

Graphs of operating potential versus current density are called polarization curves, which reflect the degree of perfection that any particular fuel cell technology has attained. High cell operating potentials are the result of many years of materials optimization. Actual polarization curves will be shown below for several types of fuel cell. 

Phosphoric Acid Fuel Cell This type of fuel cell was developed in response to the industiy s desire to expand the natural-gas market. The electrolyte is 93 to 98 percent phosphoric acid contained in a matrix of silicon carbide. The electrodes consist of finely divided platinum or platinum alloys supported on carbon black and bonded with PTFE latex. The latter provides enough hydrophobicity to the electrodes to prevent flooding of the structure by the electrolyte. The carbon support of the air elec trode is specially formulated for oxidation resistance at 473 K (392°F) in air and positive potentials. 

As with batteries, differences in electrolytes create several types of fuel cells. The automobile s demanding requirements for compactness and fast start-up have led to the Proton Exchange Membrane (PEM) fuel cell being the preferred type. This fuel cell has an electrolyte made of a solid polymer. 

There are a whole variety of types of fuel cell, named after the electrolyte used, each operating at a preferred temperature range with its own feedstock purity criteria (Table 6.3). 

Natural Gas—Its Important Role as a Primary Fuel for All Types of Fuel Cells... 

Type of fuel cell Operating fuel and temperature Power rating (kW) Fuel efficiency s (%) Power density (mW/cm ) Lifetime s (hr) Capital cost s ( /kW) Applications... 

The enthusiasm for developing DMFCs (the fuel cell researcher s dream) evolved in the 1960s, which was really the boom period for R D activities on all types of fuel cell technologies, mainly because of NASA s vital need for fuel cell power plants for space vehicles. As early as the 1960s it was recognized that the major challenges in developing DMFCs... 

The DMFC is the most attractive type of fuel cell as a powerplant for electric vehicles and as a portable power source, because methanol is a liquid fuel with values for the specific energy and energy density being about equal to half those for liquid hydrocarbon fuels (gasoline and diesel fuel). 

The situation changed drastically in the mid-1990s in view of the considerable advances made in the development of membrane hydrogen-oxygen (air) fuel cells, which could be put to good use for other types of fuel cells. At present, most work in methanol fuel cells utilizes the design and technical principles known from the membrane fuel cells. Both fuel-cell types use Pt-Ru catalyst at the anode and pure platinum catalyst at the cathode. The membranes are of the same type. 

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. 

Different fuel cell types exist. They operate at different temperatures and are generally distinguished by their electrolytes. The status of development differs widely for each type. Table 6.4 provides a comparison of the major types of fuel cells currently under development. 

In general, technical developments will lead to a decrease in overall costs of this technology per unit of installed generating capacity (kWe). Some types of fuel cells need to achieve higher power densities per kg weight or m3 most need to increase lifetimes of stacks or other plant components. For smaller applications, the technology must reach a reliable level sufficient to allow the plants to operate unattended. 

SOFC. Further research on this type of fuel cells is especially needed on ... 

Although platinum is the metal of choice for PEM fuel cell cathodes, Paul Matter, Elizabeth Biddinger, and Umit Ozkan (Ohio State University) show that nonprecious metals will have to be developed for this type of fuel cell to become practical and widely used. Although few materials have the electrochemical properties needed to replace platinum, this review discusses candidates such as macrocycle compounds, non-marcrocyclic pyrolyzed carbons, conducting polymers, chalcogen-ides, and heteropolyacids. 

Proton Exchange Membrane Fuel Cells (PEMFCs) are being considered as a potential alternative energy conversion device for mobile power applications. Since the electrolyte of a PEM fuel cell can function at low temperatures (typically at 80 °C), PEMFCs are unique from the other commercially viable types of fuel cells. Moreover, the electrolyte membrane and other cell components can be manufactured very thin, allowing for high power production to be achieved within a small volume of space. Thus, the combination of small size and fast start-up makes PEMFCs an excellent candidate for use in mobile power applications, such as laptop computers, cell phones, and automobiles. 

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