Fuel cells generate electricity from a simple electrochemical reaction in which an oxidizer, typically oxygen from air, and a fuel, typically hydrogen, combine to form a product, which for the typical fuel cell is water. The basic principle of fuel cell operation is that it separates the oxidation and reduction into separate compartments, which are the anode and the cathode (separated by a membrane), thereby forcing the electrons exchanged between the two half reactions to travel through the load. Oxygen (air) continuously passes over the cathode and hydrogen passes over the anode to generate electricity, while the by-products are heat and water. The fuel cell itself has no moving parts, so it is a quiet and reliable source of power. A schematic representation of a fuel cell with the reactant-product gases and the ion conduction flow directions through the cell is shown in Figure 7.9. The basic physical structure or building block of a fuel cell consists of an electrolyte layer in contact with a porous anode and cathode on either side.
Figure 7.9 is a simplified diagram that demonstrates how the fuel cell works. In a typical fuel cell, gaseous fuels are fed continuously to the anode (negative electrode) compartment and an oxidant (i.e., oxygen from air) is fed continuously to the cathode (positive electrode) compartment. At electrodes, the electrochemical reactions take place and produce an electric current. The fuel cell is an energy conversion device that theoretically has the capability of producing electrical energy for as long as the fuel and oxidant are supplied to the electrodes. In reality, degradation, primarily corrosion, or malfunction of components limits the practical operating life of fuel cells (U.S. Department of Energy, 2000).
The electrolyte that separates the anode and cathode is an ion conducting material. At the anode, hydrogen and its electrons are separated so that the hydrogen ions (protons) pass through the electrolyte while the electrons pass through an external electrical circuit as a direct current (DC) that can power
2e- Load rWNn
and product gases OUT -
Electrolyte (ion conductor)
(H+) Positive ions
Negative ions (OH-)
FIGURE 7.9 Schematic diagram of a fuel cell.
Depleted oxidant and product gases OUT
useful devices, usually through an inverter, which converts the DC current into an AC one. The hydrogen ions combine with the oxygen at the cathode and are recombined with the electrons to form water. The reactions taking place in a fuel cell are as follows.
Anode half reaction (oxidation),
Cathode half reaction (reduction),
Overall cell reaction,
To obtain the required power, individual fuel cells are combined into fuel cell stacks. The number of fuel cells in the stack determines the total voltage, and the surface area of each cell determines the total current (FCTec, 2008). Multiplying the voltage by the current yields the total electrical power generated as
Power (watts) = Voltage (volts) X Current (amps) (7.5)
Porous electrodes, mentioned previously, are crucial for good electrode performance. Porous electrodes, used in fuel cells, achieve very high current densities, which are possible because the electrode has a high surface area relative to the geometric plate area, which significantly increases the number of reaction sites; and the optimized electrode structure has favorable mass transport properties. In an idealized porous gas fuel cell electrode, high current densities at reasonable polarization are obtained when the liquid (electrolyte) layer on the electrode surface is sufficiently thin, so that it does not significantly impede the transport of reactants to the electroactive sites and a stable three-phase (gas-electrolyte-electrode surface) interface is established (U.S. Department of Energy, 2000).
Fuel cells are classified by their electrolyte material. Today, several types of fuel cells have been developed for applications as small as a mobile phone (with under 1 W power) to as large as a small power plant for an industrial facility or a small town (in the megawatt range). The fuel cells that exist today are the following.
Alkaline fuel cell (AFc)
Alkaline fuel cells (AFCs) are one of the most developed technologies and have been used since the mid-1960s by NASA in the Apollo and space shuttle programs. The fuel cells on board these spacecraft provide electrical power for onboard systems, as well as drinking water. AFCs are among the most efficient (nearly 70%) in generating electricity. The electrolyte in this fuel cell is an aqueous (water-based) solution of potassium hydroxide (KOH), which can be in concentrated (85 wt%) form for cells operated at high temperature (~250°C) or less concentrated (35-50 wt%) for lower-temperature (<120°C) operation. The electrolyte is retained in a matrix, usually made from asbestos. A wide range of electrocatalysts, such as Ni, Ag, metal oxides, and noble metals, can be used. One characteristic of AFCs is that they are very sensitive to CO2 because this will react with the KOH to form K2CO3, thus altering the electrolyte. Therefore, the CO2 reacts with the electrolyte, contaminating it rapidly and severely degrading the fuel cell performance. Even the small amount of CO2 in air must be considered with the alkaline cell. Therefore, AFCs must run on pure hydrogen and oxygen.
AFCs are the cheapest fuel cells to manufacture. This is because the catalyst required on the electrodes can be selected from a number of materials that are relatively inexpensive compared to the catalysts required for other types of fuel cells (U.S. Department of Energy, 2000).
The charge carrier for an AFC is the hydroxyl ion (OH-) transferred from the cathode to the anode, where it reacts with hydrogen to produce water and electrons. Water formed at the anode is transferred back to the cathode to regenerate hydroxyl ions. When operated, the AFC produces electricity and the by-product is heat.
Phosphoric acid fuel cell (PAFc)
Phosphoric acid fuel cells (PAFCs) were the first fuel cells to be commercialized. They were developed in the mid-1960s and have been field tested since the 1970s, and they have improved significantly in stability, performance, and cost. The efficiency of a PAFC in generating electricity is greater than 40%. Simple construction, low electrolyte volatility, and long-term stability are additional advantages. Phosphoric acid (H3PO4) concentrated to 100% is used for the electrolyte in this fuel cell, which operates at 150-220°C, since the ionic conductivity of phosphoric acid is low at low temperatures. The relative stability of concentrated phosphoric acid is high compared to other common acids. In addition, the use of concentrated acid minimizes the water vapor pressure, so water management in the cell is not difficult. The matrix universally used to retain the acid is silicon carbide and the electrocatalyst in both the anode and cathode is platinum (Pt).
The charge carrier in this type of fuel cell is the hydrogen ion (H+, proton). The hydrogen introduced at the anode is split into its protons and electrons. The protons are transferred through the electrolyte and combine with the oxygen, usually from air, at the cathode to form water. In addition, CO2 does not affect the electrolyte or cell performance and can therefore be easily operated with reformed fossil fuels.
Approximately 75 MW of PAFC generating capacity has been installed and is operating. Typical installations include hotels, hospitals, and electric utilities in Japan, Europe, and the United States (FCTec, 2008).
Polymer electrolyte fuel cell (PEFc)
The electrolyte in polymer electrolyte fuel cells is an ion exchange membrane (fluorinated sulfonic acid polymer or other similar polymer) that is an excellent proton conductor. The only liquid used in this fuel cell is water; thus, corrosion problems are minimal. Water management in the membrane is critical for efficient performance because the fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced, because the membrane must be kept hydrated. Because of the limitation on the operating temperature imposed by the polymer, which is usually less than 120°C, and problems with water balance, an H2-rich gas with minimal or no CO, which is a contaminant at low temperature, is used. Higher catalyst loading (Pt in most cases) is required for both the anode and cathode (U.S. Department of Energy, 2000).
Molten carbonate fuel cell (McFc)
Molten carbonate fuel cells (MCFCs) belong to the class of high-temperature fuel cells. The higher operating temperature allows them to use natural gas directly without the need for a fuel processor. MCFCs work quite differently from other fuel cells. The electrolyte in this fuel cell is composed of a molten mixture of carbonate salts. The fuel cell operates at 600-700°C, at which the alkali carbonates form a highly conductive molten salt, with carbonate ions providing ionic conduction. Two mixtures are currently used: the lithium carbonate and potassium carbonate or the lithium carbonate and sodium carbonate. These ions flow from the cathode to the anode, where they combine with hydrogen to yield water, carbon dioxide, and electrons. These electrons are routed through an external circuit back to the cathode, generating electricity and the by-product, heat. At the high operating temperatures in MCFCs, nickel (anode) and nickel oxide (cathode) are adequate to promote reaction, i.e., noble metals are not required.
Compared to the lower-temperature PAFCs and PEFCs, the higher operating temperature of MCFCs has both advantages and disadvantages (FCTec, 2008). The advantages include:
1. At the higher operating temperature, fuel reforming of natural gas can occur internally, eliminating the need for an external fuel processor.
2. The ability to use standard materials for construction, such as stainless steel sheet, and allow use of nickel-based catalysts on the electrodes.
3. The by-product heat from an MCFC can be used to generate high-pressure steam that can be used in many industrial and commercial applications.
The disadvantages are mainly due to the high temperatures and include:
1. High temperature requires significant time to reach operating conditions and responds slowly to changing power demands. These characteristics make MCFCs more suitable for constant power applications.
2. The carbonate electrolyte can cause electrode corrosion problems.
3. As CO2 is consumed at the anode and transferred to the cathode, its introduction into the air stream and its control are problematic for achieving optimum performance.
Solid oxide fuel cell (SoFC)
Solid oxide fuel cells can be operated over a wide temperature range, from 600-1000°C. The SOFC has been in development since the late 1950s and is currently the highest-temperature fuel cell developed to allow a number of fuels to be used. To operate at such high temperatures, the electrolyte is a thin, solid ceramic material (solid oxide) conductive to oxygen ions (O2~), which is the charge carrier. At the cathode, the oxygen molecules from the air are split into oxygen ions with the addition of four electrons. The oxygen ions are conducted through the electrolyte and combine with hydrogen at the anode, releasing four electrons. The electrons travel an external circuit providing electric power and producing heat as a by-product. The operating efficiency in generating electricity is among the highest of the fuel cells, at about 60% (FCTec, 2008).
The solid electrolyte is impermeable to gas crossover from one electrode to another, in contrast to liquid electrolytes, where the electrolyte is contained in some porous supporting structure.
SOFCs operate at extremely high temperatures, so a significant amount of time is required to reach operating temperature. They also respond slowly to changes in electricity demand; thus, they are suitable for high-power applications, including industrial and large-scale central electricity generating stations.
The advantages of the high operating temperature of SOFCs are that it enables them to tolerate relatively impure fuels, such as those obtained from the gasification of coal or gases from industrial processes, and allows cogeneration applications, such as to create high-pressure steam that can be used in many applications. Furthermore, combining a high-temperature fuel cell with a turbine into a hybrid fuel cell further increases the overall efficiency of generating electricity with a potential of an efficiency of more than 70% (FCTec, 2008). The disadvantage of SOFCs is that the high temperatures require more expensive construction materials.
Proton exchange membrane fuel cell (PEMFc)
Proton exchange membrane fuel cells (PEMFCs), also known as polymer electrolyte membrane fuel cells, are believed to be the best type of fuel cell for automobile applications that could eventually replace the gasoline and diesel internal combustion engines. First used in the 1960s for the NASA Gemini Program, PEMFCs are currently being developed and demonstrated for systems ranging from 1 W to 2 kW (FCTec, 2008).
PEMFCs use a solid polymer membrane in the form of a thin plastic film as the electrolyte. This polymer is permeable to protons when it is saturated with water, but it does not conduct electrons. The fuel for the PEMFC is hydrogen and the charge carrier is the hydrogen ion (proton). At the anode, the hydrogen molecule is split into hydrogen ions (protons) and electrons. The hydrogen ions move across the electrolyte to the cathode while the electrons flow through an external circuit and produce electric power. Oxygen, usually in the form of air, is supplied to the cathode and combines with the electrons and the hydrogen ions to produce water (FCTec, 2008).
The advantages of PEMFCs are that they generate more power, compared to other types of fuel cells, for a given volume or weight of fuel cell. This high power density characteristic makes them compact and lightweight. Because the operating temperature is less than 100°C, rapid start up is achieved.
Since the electrolyte is a solid material, the sealing of the anode and cathode gases is simpler with a solid electrolyte, compared to a liquid; therefore, a lower cost is required to manufacture the cell. The solid electrolyte is also less sensitive to orientation and the corrosion problems are lower, compared to many of the other electrolytes, which leads to a longer cell and stack life.
One major disadvantage of the PEMFC is that the electrolyte must be saturated with water to operate optimally; therefore, careful control of the moisture of the anode and cathode streams is required. The high cost of platinum is another disadvantage.
Other types of fuels cells, not described in this book, include the direct methanol fuel cell (DMFC), regenerative fuel cell (RFC), zinc-air fuel cell (ZARF), intermediate temperature solid oxide fuel cell (ITSOFC), and tubular solid oxide fuel cell (TSOFC). Interested readers can find information on these cells in other publications dedicated to the subject.
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