Classification of fuel cells

Fuel cells are classified primarily according to the types of electrolyte employed.

This determines the types of chemical reactions that take place in the cell, the types of catalysts required, the temperature range in which cell operates, the fuel required, and other factors. These characteristics, in turn, affect the appropriate applications of these cells. There are several types of fuel cells currently under development, each with its own advantages, limitations, and potential applications. According to electrode types, several most promising types include proton exchange membrane fuel cell (PEMFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC). Details of these fuel cells are described as

followed.

1.2.1.1 Polymer electrolyte membrane fuel cell

Polymer electrolyte membrane (PEM) fuel cells, also called proton exchange membrane fuel cells, deliver high power density and have the advantages of low weight and volume, compared to other fuel cells. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing platinum catalysts. They need only hydrogen, oxygen from the air, and water to operate and do not require corrosive fluids as some other fuel cells. They are typically fueled with pure hydrogen supplied from storage tanks or onboard reformers.

Polymer electrolyte membrane fuel cells operate at relatively low temperatures, around 80°C. Low temperature operation initiates quickly and results in less wear on system components and better durability. However, the necessity of noble-metal catalysts to separate electrons and protons of hydrogen costs high for systems. The platinum catalyst is also extremely sensitive to CO poisoning, and it is necessary to employ an additional reactor to reduce CO in the fuel gas if hydrogen are derived from alcohol or hydrocarbon fuels. This also increases the cost. Developers are currently exploring Pt-Ru catalysts that are more resistant to CO. PEM fuel cells are used primarily for transportation applications and some stationary applications. Due to the fast startup, low sensitivity to orientation and favorable power-to-weight ratio, PEM fuel cells are particularly suitable for passenger vehicles, such as cars and buses. A significant barrier of using these fuel cells in vehicles is hydrogen storage.

Most fuel cell vehicles powered by pure hydrogen must store hydrogen onboard as compressed gases in pressurized tanks. Due to the low energy density of hydrogen, it is difficult to store enough hydrogen onboard to allow vehicles to travel the same distance as gasoline-powered vehicles before refueling, typically 300-400 miles.

Higher-density liquid fuels such as methanol, ethanol, natural gas, liquefied petroleum

gas and gasoline seem to be available, but vehicles must have an onboard fuel processor to reform methanol to hydrogen. This increases costs and maintenances.

The reformer also releases carbon dioxide, though less than that emitted from current gasoline-powered engines.

1.2.1.2 Phosphoric acid fuel cell

Phosphoric acid fuel cells use liquid phosphoric acid as an electrolyte with the acid contained in a Teflon-bonded silicon carbide matrix and porous carbon electrodes containing platinum catalysts. Chemical reactions that take place in the cell are shown in the diagram to the right. Phosphoric acid fuel cell (PAFC) is considered the "first generation" of modern fuel cells. It is one of the most mature cell types and the first to be used commercially with over 200 units in current use. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses. PAFCs are more tolerant of impurities in the reformate than PEM cells, which are easily poisoned by CO2—CO2 attaching to the platinum catalyst at the anode as a result of decreasing fuel cell's efficiency. They are efficient by 85% for co-generations of electricity and heat, but less efficient for electricity generation only. This is slightly more efficient than combustion-based power plants with typical operation efficiency of 33 to 35%. PAFCs are also less powerful than other fuel cells, given the same weight and volume. As a result, these fuel cells are normally large and heavy. PAFCs are also expensive. Like PEM fuel cells, PAFCs require an expensive platinum catalyst, which raises the cost of the fuel cell.

1.2.1.3 Alkaline fuel cell

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and were the first type widely used in the U.S. space program to generate electrical energy and water onboard spacecraft. These fuel cells use the aqueous solution of

potassium hydroxide as the electrolyte and use a variety of non-precious metals as catalysts at the anode and cathode. High-temperature AFCs operate at temperatures between 100ºC and 250ºC. However, more-recent AFC designs operate at lower temperatures from 23ºC to 70ºC.

AFCs are high-performance fuel cells due to the rate at which chemical reactions take place in the cell. They are also very efficient, reaching efficiencies of 60% in space applications. The disadvantage of this fuel cell is that it is easily poisoned by carbon CO2. In fact, even a little CO2 in the air can affect the cell's operation, and it is necessary to purify both hydrogen and oxygen in the cell. This purification process is expensive. Susceptibility to poisoning also affects the cell's lifetime and raises further the cost. Cost is less of a factor for remote locations such as space or under the sea.

However, for effective competition in most mainstream commercial markets, these fuel cells have to become more effective in cost. AFC stacks have been shown to maintain sufficiently stable operation for more than 8,000 operating hours. To be economically viable in large-scale utility applications, these fuel cells need to reach operating time exceeding 40,000 hours. This is possibly the most significant obstacle in commercializing this fuel cell technology.

1.2.1.4 Molten carbonate fuel cell

Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial and military applications.

MCFCs are high-temperature fuel cells that use electrolytes composed of molten carbonate salt mixture suspended in a porous and chemically inert ceramic lithium aluminum oxide (LiAlO2) matrix.

Since they operate at extremely high temperatures of 650ºC and above, non-precious metals can be used as catalysts at the anode and cathode to reduce costs.

Improved efficiency is another reason why MCFCs offer significant cost reductions

over phosphoric acid fuel cells (PAFCs). Molten carbonate fuel cells can reach efficiencies of 60%, considerably higher than the 37-42% efficiencies of a phosphoric acid fuel cell plant. When waste heats are utilized, overall fuel efficiencies can be as high as 85%. Unlike alkaline, phosphoric acid and polymer electrolyte membrane fuel cells, MCFCs don't require an external reformer to convert more energy-dense fuels to hydrogen. Due to the high operation temperatures, these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming, which also reduces the cost. Molten carbonate fuel cells are not prone to CO or CO2 poisoning, making them more attractive for fueling with gases made from coals. Although they are more resistant to impurities than other fuel cell types, scientists are looking for ways to make MCFCs resistant enough to impurities from coals, such as sulfurs and particulates. The primary disadvantage of current MCFC technology is durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Scientists are currently exploring corrosion-resistant materials for components and fuel cell designs to increase cell life without decreasing performances.

1.2.1.5 Solid oxide fuel cell

Solid oxide fuel cells (SOFCs) use hard non-porous ceramic compounds as electrolytes. Since the electrolyte is a solid, the cell does not have to be constructed in the typical plate-like configuration of other fuel cell types. SOFCs are expected to be around 50-60% efficient at converting fuel to electricity. In applications designed to recover and utilize the system waste heat, overall fuel efficiencies could top 80-85%.

Solid oxide fuel cells operate at very high temperatures. Cost reduction is valid because it is unnecessary to use precious metal catalysts at high temperatures. SOFCs also reform fuels internally and can use a variety of fuels to reduce the cost associated with adding a reformer to the system. SOFCs are the most sulfur-resistant fuel cell

type and tolerate more sulfurs than other cell types by several orders in magnitude. In addition, they are not poisoned by CO, which can even be used as fuel. This allows SOFCs to use gases made from coals. High-temperature operation has disadvantages.

It results in a slow startup and requires considerable thermal shielding to retain heat and to protect personnel. This may be acceptable for utility applications but not for transportation and small portable applications. High operating temperatures also place stringent durability requirements on materials. The development of low-cost materials with high durability at cell operating temperatures is the key technical challenge. Scientists are currently developing lower-temperature SOFCs operating at or below 800ºC that have fewer durability problems and cost less. Lower-temperature SOFCs produce less electrical power, however, and stack materials that will function in this lower temperature range have not been identified. The differences and features of these fuel cells are summarized in Table 1-3.