Fuel Cells

Type of physical science: Chemistry

Field of study: Environmental chemistry

Fuel cells are electrochemical devices that convert the chemical energy of a combustion reaction directly into electrical energy. The production of electrical energy using fuel cells is generally less wasteful than the production of electrical energy from the heat energy released by direct combustion and is less likely to produce harmful by-products.

Overview

Most of the useful processes that occur in nature and in the world of technology involve the conversion of energy from one form to another. Plants receive light energy from the sun and convert it to chemical energy. When plant matter is eaten by an animal, the chemical energy is converted into energy of motion and heat. If the plant matter is burned, chemical energy is converted into heat, which may be used to power an electric generator, producing electrical energy that can be transported and converted into heat, light, or mechanical energy again. Of the many transformations that energy undergoes, those in which heat energy is transformed into another form are limited in a special way in that only a portion of the heat energy present can be converted into another form. This limitation is a consequence of the nature of heat energy, which involves the random motion of atoms and molecules. It is thus desirable to develop alternative ways of converting the chemical energy contained in fuels to other forms without the direct production of heat. Fuel cells provide one means of accomplishing this task.

Fuel cells are electrochemical cells that allow for a continuous input of the reacting compounds and thus for the continuous generation of electrical energy. For the most part, fuel cells are based on chemical reactions that correspond to combustion in air or in an oxygen atmosphere. The operation of a fuel cell is best described with reference to a specific combustion process. The simplest such process is the combustion of hydrogen gas to form water. When hydrogen burns in air, two diatomic molecules of hydrogen combine with the diatomic molecule of oxygen to form two molecules of water. In a fuel cell, the overall process is broken into two parts, called half-reactions. In the first half-reaction, which can occur at the surface of a suitable inert metal such as platinum immersed in a dilute solution of acid, the neutral hydrogen gas molecule is separated into positively charged hydrogen ions (H⁺), which can enter the solution, and electrons (e-), which can move about within the metal. An electrochemical half-reaction of this type, which results in the release of electrons, is called (for historical reasons) an oxidation reaction; the electrode at which it occurs is called an anode. In the second half-reaction, which will occur at a suitable electrode in contact with oxygen gas and acidic solution to provide the necessary hydrogen ions, oxygen molecules combine with the hydrogen ions and electrons from the metal to form water molecules. Electrochemical half-reactions of this type, which consume electrons, are termed "reduction reactions," and the electrodes at which they occur are termed "cathodes." The half-reactions do not actually occur in a mixture of hydrogen and oxygen gas, but can occur when the gases are in contact with suitable electrodes and an electrolyte.

As an example, consider a container of a dilute solution of acid into which two inert metal electrodes have been placed. Hydrogen gas is bubbled over the surface of the first electrode, while oxygen gas is bubbled over the surface of the second electrode. Because of the electrochemical half-reactions described, the electrode in contact with the hydrogen gas acquires excess electrons and hence a negative charge, while the hydrogen ions produced enter the electrolyte solution. When a large enough negative charge has been acquired, the dissociation of hydrogen comes to a stop. At the electrode supplied with oxygen gas, water is produced and hydrogen ions are consumed until the metal acquires a sufficiently large deficiency of electrons, or positive charge, to prevent the further reaction of hydrogen ions. If a wire is used to connect the two electrodes, the electrons produced at the hydrogen electrode (anode) can flow to the oxygen electrode (cathode), and the reactions can continue. Within the electrolyte, hydrogen ions flow from the hydrogen electrode to the oxygen electrode. Most of the energy released by the overall combination of hydrogen with oxygen is carried by the electrons and can be used to power an electrical device. If a direct current motor is connected between the electrodes, this energy is converted directly into mechanical energy. It should be noted that the operation of the hydrogen-oxygen fuel cell is exactly the reverse of an electrolytic cell, in which current is passed between two electrodes with the aid of an external power source, causing the two reactions to proceed in the reverse direction, breaking up the water molecule into hydrogen and oxygen.

Hydrocarbons are among the most important fuel substances in use. The simplest hydrocarbon is methane, which consists of molecules in which a single carbon atom is bound to four hydrogen atoms. When methane burns in air, one molecule of methane combines with two diatomic molecules of oxygen to form one molecule of carbon dioxide and two molecules of water. More complicated hydrocarbons contain more than one carbon atom and many more hydrogens. The butane molecule, for example, contains four carbon atoms and ten hydrogens. Nevertheless, when butane or any other hydrocarbon burns in air, carbon dioxide and water are formed.

In the presence of a dilute acid, the combustion of a hydrocarbon can occur as the result of two processes: an oxidation process, in which the hydrocarbon molecule combines with water molecules to form carbon dioxide, hydrogen ions, and electrons; and a reduction reaction, in which oxygen molecules combine with hydrogen ions and electrons. By replacing the hydrogen flow in the system described by a flow of methane gas, one obtains a primitive hydocarbon-burning fuel cell, in which one of the above reactions occurs at each electrode.

The same overall combustion reactions can be achieved in fuel cells with different electrolytes. In a hydrogen fuel cell with alkali electrolyte, the oxidation reaction involves the combination of hydrogen gas with hydroxide ions (OH^-) to form water and release electrons, while the reduction reaction involves the combination of oxygen molecules with water and electrons to form hydroxide ions. With a hydrocarbon fuel, the oxidation reaction involves the combination of the hydrocarbon with hydroxide ions to form carbon dioxide, water, and release electrons, while the reduction reaction is the same as in a hydrogen cell.

In addition to water solutions of acids and bases, a number of other electrolytes are used in fuel cells. Often, it is desired to have a rigid electrolyte, especially for applications in which the cell may be moved about. Solid zirconium oxide, ZrO₂, has a crystal structure in which each zirconium ion, Zr⁴⁺, is surrounded by eight oxide ions, O^2-, and each oxide ion has two zirconium ions as its nearest neighbors. By introducing a small amount of yttrium oxide, Y₂O₃, into the zirconium oxide, one obtains a crystal structure in which some of the normal Zr⁴⁺ sites are now occupied by Y³⁺ ions. To maintain electrical balance, a corresponding number of the oxide ion sites is left vacant. The vacant oxide ion sites allow for oxide ions to move from one site to another, and thus the crystal is able to conduct electricity through the motion of oxide ions. For a hydrogen-oxygen fuel cell with a solid oxide electrolyte, the oxidation reaction involves the reaction of gaseous hydrogen with oxide ions to form water and release electrons. The reduction reaction then involves the combination of oxygen molecules with electrons to form oxide ions.

Although hydrocarbons release much energy very rapidly on burning in air, it is often desirable to convert them to hydrogen before use in a fuel cell. In the steam hydrocarbon process, the hydrocarbon is allowed to react with steam to form carbon dioxide and hydrogen gas. The hydrogen can be separated from the carbon dioxide by allowing it to diffuse through a thin membrane of a metal such as palladium, which allows the passage of hydrogen, but not other substances. On a nickel catalyst, the reaction of methane and steam produces carbon monoxide (CO) and hydrogen instead. Both hydrogen and carbon monoxide can be used in molten carbonate fuel cells.

Ionic compounds typically become good electrical conductors upon melting. Molten compounds containing the carbonate ion CO^3- have been employed in a number of fuel cell designs. In a fuel cell with a molten carbonate electrolyte supplied with oxygen and a mixture of carbon monoxide and hydrogen as fuel, both the carbon monoxide and hydrogen are oxidized at the cathode. Carbon monoxide combines with carbonate ion to form carbon dioxide and release electrons, while hydrogen combines with carbonate ion to form water and carbon dioxide, also releasing electrons. The reduction reaction in a molten carbonate fuel cell involves the combination of carbon dioxide and oxygen with electrons to form carbonate ions.

In addition to hydrogen, hydrocarbons, and carbon monoxide, many other substances have been employed as fuels in fuel cells, including a variety of alcohols and such nitrogen-containing compounds as ammonia (NH₃) and hydrazine (N₂H₄). Alcohols are typically allowed to react with steam to form a mixture of hydrogen and carbon dioxide prior to the electrochemical reaction. The latter compounds react with oxygen in alkaline fuel cells to form nitrogen gas and water.

Applications

The half-reactions that power a fuel cell can occur only at those places where the gaseous fuel, the electrode metal, and the electrolyte come into contact. Fuel cell electrodes are thus either porous or thin enough to allow gas diffusion. Much of the challenge in designing fuel cells lies in designing electrode-electrolyte contacts, which are stable under conditions of rapid fuel flow. Minimizing the electrical resistance of the electrolyte is important to prevent loss of power to heating of the electrolyte. Fuel cell power plants generally involve connecting many individual cells together and require separate fuel circulation and cooling systems. In developing fuel cells for application, the choice of fuel, electrodes, and electrolytes is critical. For large-scale energy production, inexpensive materials must be used. For cells that are to be moved about, a rigid electrolyte is desirable. For powering space vehicles, expense becomes a secondary consideration to weight and reliability.

Fuel cells are the energy source of choice for powering manned space vehicles. Although photoelectric cells can provide adequate power for unmanned satellites, and storage batteries were sufficient for the first, very brief, manned flights, practical and reliable fuel cell power plants were developed for the Gemini and Apollo space programs of the 1960's and 1970's. The Gemini power system was a hydrogen-oxygen fuel cell with a solid polymer electrolyte--that is, an acidic organic compound that had been polymerized so that it could be mechanically formed into a membrane of uniform thickness. The electrodes were titanium screens covered with platinum. The full Gemini power plant involved three stacks of thirty-two such cells, each supplied with hydrogen and oxygen and connected to a cooling system. The Gemini cell produced about 1 kilowatt of power and about one-half liter of liquid water per hour. The water produced helped meet the life-support needs of the astronauts.

The Apollo fuel cell system employed hydrogen-oxygen cells with a concentrated alkaline (potassium hydroxide) electrolyte and porous nickel electrodes to which hydrogen and oxygen were supplied under pressure. The Apollo system was designed to operate at 200 degrees Celsius and involved three stacks of thirty-one cells each. During the Apollo 8 Moon flight, the system functioned for more than four hundred hours, producing nearly 300-kilowatt hours of electricity and 100 liters of water. The space shuttle power system is an improved version of the Apollo system; the shuttle system is physically smaller but produces six to eight times the power.

A variety of small-scale portable fuel cells have been developed for terrestrial applications. The majority of these employ a metal hydride as a hydrogen source to eliminate the immediate need for a source of hydrogen gas. A number of transition metals absorb large quantities of hydrogen gas (amounts approaching one hydrogen atom per metal atom) and allow relatively rapid diffusion of the absorbed hydrogen to the surface, where it can be oxidized. Such an electrode can be reversibly charged with hydrogen. Other, generally less expensive, metal hydrides will release their hydrogen when exposed to moisture, but cannot be recharged easily. A number of metal hydride air fuel cell systems have been developed for military applications, as have a number of fuel cell power plants using methanol as a fuel. The relatively quiet operation and light weight of fuel cell power plants compared to diesel or gasoline-powered generators makes them attractive for forward-area military use.

The electric power industry has a natural interest in developing a variety of fuel cell power plants for both primary energy production and load-leveling. Since the demand for electrical power is not constant but changes with the time of day and the season of the year, one must have a power plant that can either produce at the maximum power level required or produce energy at a lower rate, but have some means of storing it for use when the demand exceeds the plant capacity. Fuel cell power plants provide for flexibility in energy production because they can be built economically over quite a range of sizes and can be stopped or started very quickly as demand changes. Reversible hydrogen-oxygen cells can be run as water electrolyzers, allowing energy produced at nuclear or hydroelectric plants during off-peak hours to be stored as hydrogen gas, which can then be used as a fuel during peak demand hours. Molten carbonate cells, high-temperature solid oxide fuel cells, and phosphoric acid cells appear to be the principal candidates for eventual power plant use.

Context

Until the beginning of the Industrial Revolution, the energy needs of human society were largely met by the muscle power of humans and animals, augmented at times by wind and water power. The development of machinery for industrial production required, and was stimulated by, the development of the steam engine, which was introduced in primitive form by the English military engineer Thomas Savery in 1698. It was refined over the course of the eighteenth century through the efforts of English inventor Thomas Newcomen and Scottish instrument maker James Watt. The need for a scientific basis for understanding the operation of the steam engine and other heat engines motivated the research of the French physicist Nicholas-Leonard-Sadi Carnot, who announced in 1824 a fundamental theoretical limitation on the efficiency of any heat engine, a result which became one of the cornerstones of the modern science of thermodynamics. The latter part of the nineteenth century saw the development of several forms of internal combustion engine, which gradually replaced the steam engine in the majority of its applications, but also being heat engines, were subject to the same limitations on their efficiency.

The first operating fuel cell was constructed by the English scientist and lawyer Sir William Robert Grove in 1842. Grove's device, which he called a "gaseous voltaic battery," involved fifty hydrogen-oxygen cells, with platinum foil electrodes coated with spongy platinum immersed in dilute sulfuric acid, generated sufficient electrical power to shock humans, to cause sparks, and to decompose a number of substances by electrolysis. Grove's fuel cell remained a laboratory curiosity for more than half a century. In 1894, the German chemist Wilhelm Ostwald took the opportunity provided by the inaugural meeting of an important scientific society to point out the inherent inefficiency of heat engines and to urge their replacement by fuel cells. By 1900, the German physical chemists Walther Hermann Nernst and Fritz Haber had initiated a search for methods of directly oxidizing coal using fuel cells, which was continued by other scientists for some thirty years but without a satisfactory result.

Renewed interest in the development of practical fuel cells was stimulated by the discovery in 1946 of the oxide ion-conducting properties of zirconium oxide-yttrium oxide mixtures by the Soviet chemist O. K. Davtyan. By the early 1950's, the Dutch chemists G. H. Broers and J. A. A. Ketelaar had developed a molten carbonate fuel cell powered by a mixture of hydrogen and carbon monoxide. In 1959, a fuel cell designed by the English scientist F. T. Bacon using an alkaline electrolyte and supplied by hydrogen and oxygen under pressure achieved an energy output of 5 kilowatts. Bacon's fuel cell formed the basis for the fuel cells used in the Apollo space missions.

The need for low weight reliable power systems for spaceflight was probably the single largest stimulant to the development of fuel cells, but interest in other fuel cell applications grew as well. By 1967, a van powered by an array of fuel cells had been built and tested. The rising price of oil in the 1970's, coupled with concern about pollution from the burning of fossil fuels, served to generate new interest in the use of fuel cells on automobiles and in load-leveling applications.

Principal terms:

COMBUSTION: a chemical reaction in which a fuel reacts with the surrounding atmosphere, releasing substantial heat energy

CURRENT: the motion of electrically charged particles, either ions or electrons, through matter or empty space

ELECTRODE: a metal or other substance that conducts electricity through the motion of electrons and is placed in contact with an electrolyte so that its electrons can participate in an electrochemical reaction

ELECTROLYSIS: the separation of a substance into its constituent chemical elements on passage of an electric current

ELECTROLYTE: a substance that contains ions, which are free to move; also, a substance that forms ions when dissolved in another substance

HYDROCARBONS: chemical compounds composed of hydrogen and carbon only; the major chemical components of petroleum and coal

ION: an atom or molecule that has gained or lost electrons from its electrically neutral state to become either a negatively charged anion or a positively charged cation

OXIDATION: loss of electrons by an atom molecule or ion, resulting in a greater positive charge or a lesser negative charge

REDUCTION: gain of electrons by an atom molecule or ion, resulting in a lesser positive charge or a greater negative charge

Bibliography

Bockris, John O'M., and A. K. N. Reddy. MODERN ELECTROCHEMISTRY. 2 vols. New York: Plenum Press, 1973. This ambitious volume covers a wide range of topics, with an emphasis on the electrode/electrolyte interface and the rates of electrochemical reactions. Presents electrochemistry not only as a specialized field of chemistry but also as an interdisciplinary field with strong connections to physics, chemistry, materials science, and engineering. It is one of the few textbooks in the field that is written at a level accessible to individuals who have completed only introductory level work in physics and chemistry.

Bockris, John O'M., D. A. Rand, and B. J. Welch, eds. TRENDS IN ELECTROCHEMISTRY. New York: Plenum Press, 1976. A number of the early chapters in this volume discuss the role that might be played by fuel cells in a world in which the fossil fuels have been replaced by other sources of energy.

Cameron, Don. "Fuel Cell Energy Generators." CHEMTECH 9 (1978): 633-637. An elementary discussion of some of the practical aspects of designing fuel cells for particular applications. Describes some early attempts to introduce fuel cells into commercial power production.

Kordesch, K. V. "Twenty-five Years of Fuel Cell Development (1951-1976)." JOURNAL OF THE ELECTROCHEMICAL SOCIETY 125 (1978): 77C-91C. This review article describes a variety of fuel cell types developed for use in space vehicles and ordinary automobiles over a twenty-five year period. Readers need not be able to follow the technical details in order to appreciate the challenges faced by the designers of the first cells developed for practical application.

Linden, David, ed. HANDBOOK OF BATTERIES AND FUEL CELLS. New York: McGraw-Hill, 1984. This handbook provides much descriptive material, as well as technical detail, on a very wide range of electrochemical power sources. Chapters 41, 42, and 43, respectively, describe general characteristics of fuel cells, low power cells (including those for space vehicles), and fuel cells for large-scale applications.

Roffia, Sergio, V. Concialini, and C. Paradisi. "The Interconversion of Electrical and Chemical Energy." JOURNAL OF CHEMICAL EDUCATION 65 (1988): 272-273. This elementary article compares the operation of a hydrogen-oxygen fuel cell and the corresponding electrolytic cell.

Electrolysis

Charges and Currents

Electrons and Atoms

Laws of Thermodynamics