Electrochemistry

Summary

Electrochemists study the chemical changes produced by electricity, but they are also concerned with the generation of electric currents due to the transformations of chemical substances. Whereas traditional electrochemists investigated such phenomena as electrolysis, modern electrochemists have broadened and deepened their interdisciplinary field to include theories of ionic solutions and solvation. This theoretical knowledge has led to such practical applications as efficient batteries and fuel cells, the production and protection of metals, and the electrochemical engineering of nanomaterials and devices that have great importance in electronics, optics, and ceramics.

Definition and Basic Principles

As its name implies, electrochemistry concerns all systems involving electrical energy and chemical processes. More specifically, this field includes the study of chemical reactions caused by electrical forces as well as the study of how chemical processes give rise to electrical energy. Some electrochemists investigate the electrical properties of certain chemical substances, for instance, these substances' ability to serve as insulators or conductors. Because the atomic structure of matter is fundamentally electrical, electrochemistry is intimately involved in all fields of chemistry from physical and inorganic through organic and biochemistry to such new disciplines as nanochemistry. No matter what systems they study, chemists in some way deal with the appearance or disappearance of electrical energy into the surroundings. On the other hand, electrochemists concentrate on those systems consisting of electrical conductors, which can be metallic, electrolytic, or gaseous.

Because of its close connection with various branches of chemistry, electrochemistry has applications that are multifarious. Early applications centered on electrochemical cells that generated a steady current. Additional metals such as potassium, sodium, calcium, and strontium were discovered by electrolysis of their molten salts. Commercial production of such metals as magnesium, aluminum, and zinc was mainly accomplished by the electrolysis of solutions or melts. An understanding of the electrical nature of chemical bonding led chemists to create many new dyes, drugs, plastics, and artificial plastics. Electroplating has served both aesthetic and practical purposes, and it has certainly decreased the corrosion of several widely used metals.

Electrochemistry played a significant part in the research and development of such modern substances as silicones, fluorinated hydrocarbons, synthetic rubbers, and plastics. Even though semiconductors such as germanium and silicon do not conduct electricity as well as copper, an understanding of electrochemical principles has been important in the invention of various solid-state devices that have revolutionized the electronics industries, from radio and television to computers. Electrochemistry, when it has been applied in the life sciences, has resulted in an expanded knowledge of biological molecules. For example, American physical chemist Linus Pauling used electrophoretic techniques to discover the role of a defective hemoglobin molecule in sickle-cell anemia. A grasp of electrochemical phenomena occurring in the human heart and brain has led to diagnostic and palliative technologies that have improved the quality and length of human lives. Much research and development are being devoted to increasingly sophisticated electrochemical devices for implantation in the human body, and some even predict, such as American inventor Ray Kurzweil, that these “nanobots” will help extend human life indefinitely.

Background and History

Most historians of science trace the origins of electrochemistry to the late eighteenth and early nineteenth centuries, when Italian physician Luigi Galvani studied animal electricity and Italian physicist Alessandro Volta invented the first battery. Volta's device consisted of a pile of dissimilar metals, such as zinc and silver, separated by a moist conductor. This “Voltaic pile” produced a continuous current, and applications followed quickly. Researchers showed that a Voltaic pile could decompose water into hydrogen and oxygen by a process later called electrolysis. English chemist Sir Humphry Davy used the electrolysis of melted inorganic compounds to discover several new elements. The Swedish chemist Jöns Jacob Berzelius used these electrochemical studies to formulate a new theory of chemical combination. In his dualistic theory, atoms are held together in compounds by opposite charges, but his theory declined in favor when it was unable to explain organic compounds or even such a simple molecule as diatomic hydrogen.

Though primarily a physicist, Michael Faraday made basic discoveries in electrochemistry, and, with the advice of others, he developed the terminology of this new science. For example, he introduced the terms “anode” and “anion,” “cathode” and “cation,” “electrode,” “electrolyte,” as well as “electrolysis.” In the 1830s, his invention of a device to measure the quantity of electric current resulted in his discovery of a fundamental law of electrochemistry—that the quantity of electric current that leads to the formation of a certain amount of a particular chemical substance also leads to chemically equivalent amounts of other substances. Even though Faraday's discovery of the relationship between the quantity of electricity and electrochemical equivalents was extraordinarily significant, it was not properly appreciated until much later. Particularly helpful was the work of the Swedish chemist Svante August Arrhenius, whose ionic theory, proposed toward the end of the nineteenth century, contained the surprising new idea that anions and cations are present in dilute solutions of electrolytes.

In the twentieth century, Dutch-American physical chemist Peter Debye, together with German chemist Erich Hückel, corrected and extended the Arrhenius theory by taking into account that, in concentrated solutions, cations have a surrounding shell of anions, and vice versa, causing these ions' movements to be retarded in an electric field. Norwegian American chemist Lars Onsager further refined this theory by taking into account Brownian motion, the movement of these ionic atmospheres due to heat. Other scientists used electrochemical ideas to understand the nature of acids and bases, the interface between dissimilar chemicals in electrochemical cells, and the complexities of oxidation-reduction reactions, whether they occur in electrochemical cells or in living things.

How It Works

Primary and Secondary Cells. The basic device of electrochemistry is the cell, generally consisting of a container with electrodes and an electrolyte designed to convert chemical energy into electrical energy. A primary cell, also known as a galvanic or Voltaic cell, is one that generates electrical current via an irreversible chemical reaction. This means that a discharged primary cell cannot be recharged from an external source. By taking measurements at different temperatures, chemists use primary cells to calculate the heat of reactions, which have both theoretical and practical applications. Such cells can also be used to determine the acidity and alkalinity of solutions. Every primary cell has two metallic electrodes at which electrochemical reactions occur. In one of these reactions, the electrode gives up electrons, and at the other electrode, electrons are absorbed.

In a secondary cell, also known as a rechargeable or storage cell, electrical current is created by chemical reactions that are reversible. This means that a discharged secondary cell may be recharged by circulating through the cell in a quantity of electricity equal to what had been withdrawn. This process can be repeated as often as desired. The manufacture of secondary cells has grown into an immense industry, with such commercially successful products as lead-acid cells and alkaline cells with either nickel-iron or nickel-cadmium electrodes.

Electrolyte Processes. Electrolysis, one of the first electrochemical processes to be discovered, has increased in importance in the twentieth and twenty-first centuries. Chemists investigating electrolysis soon discovered that chemical reactions take place at the two electrodes, but the liquid solution between them remains unchanged. An early explanation was that with the passage of electric current, ions in the solution alternated decompositions and recombinations of the electrolyte. This theory had to be later revised in the light of evidence that chemical components had different motilities in solution.

For more than two hundred years, the electrolysis of water has been used to generate hydrogen gas. In an electrolytic cell with a pair of platinum electrodes, to the water of which a small amount of sulfuric acid has been added (to reduce the high voltage needed), electrolysis begins with the application of an external electromotive force, with bubbles of oxygen gas appearing at the anode (due to an oxidation reaction) and hydrogen gas at the cathode (due to a reduction reaction). If sodium chloride is added to the water, the electrochemical reaction is different, with sodium metal and chlorine gas appearing at the appropriate electrodes. In both these electrolytes, the amounts of hydrogen and sodium produced are in accordance with Faraday's law, the mass of the products being proportional to the current applied to the cell.

Redox Reactions. For many electrochemists, the paramount concern of their discipline is the reduction and oxidation (redox) reaction that occurs in electrochemical cells, batteries, and many other devices and applications. Reduction takes place when an element or radical (an ionic group) gains electrons, such as when a double-positive copper ion in solution gains two electrons to form metallic copper. Oxidation takes place when an element or radical loses electrons, such as when a zinc electrode loses two electrons to form a doubly positive zinc ion in solution. In electrochemical research and applications, the sites of oxidation and reduction are spatially separated. The electrons produced by chemical processes can be forced to flow through a wire, and this electric current can be used in various applications.

Electrodes. Electrochemists employ a variety of electrodes, which can consist of inorganic or organic materials. Polarography, a subdiscipline of electrochemistry dealing with the measurement of current and voltage, uses a dropping mercury electrode, a technique enabling analysts to determine such species as trace amounts of metals, dissolved oxygen, and certain drugs. Glass electrodes, whose central feature is a thin glass membrane, have been widely used by chemists, biochemists, and medical researchers. A reversible hydrogen electrode plays a central role in determining the pH of solutions. The quinhydrone electrode, consisting of a platinum electrode immersed in a quinhydrone solution, can also be used to measure pH (it is also known as an indicator electrode because it can indicate the concentration of certain ions in the electrolyte). Also widely used, particularly in industrial pH measurements, is the calomel electrode, consisting of liquid mercury covered by a layer of calomel (mercurous chloride), and immersed in a potassium chloride solution. Electrochemists have also created electrodes with increasing (or decreasing) power as oxidizing or reducing agents. With this quantitative information, they are then able to choose a particular electrode material to suit a specific purpose.

Applications and Products

Batteries and Fuel Cells. Soon after Volta invented the first electric battery, investigators found applications, first as a means of discovering new elements and then as a way to deepen understanding of chemical bonding. By the 1830s, when new batteries were able to serve as reliable sources of electric current, they began to exhibit utility beyond their initial value for experimental and theoretical science. For example, the Daniell cell was widely adopted by the telegraph industry. Also useful in this industry was the newly invented fuel cell that used the reaction of a fuel such as hydrogen and an oxidant such as oxygen to produce direct-current electricity. However, its requirement of expensive platinum electrodes led to its replacement in the late nineteenth and throughout the twentieth century with the rechargeable lead-acid battery, which came to be extensively used in the automobile industry. In the late twentieth and early twenty-first centuries, many electrochemists predicted a bright future for fuel cells based on hydrogen and oxygen, especially with the pollution problems associated with widespread fossil-fuel use.

Battery technology has also evolved to meet size, weight, cost, and other technological demands, and in the first decades of the twenty-first century, rechargeable nickel-metal and lithium-ion batteries overtook alkaline and lead-acid batteries in many applications. Still, other types, such as flow batteries, began to be used for grid-scale energy storage. Hybrid electric vehicles, plug-in hybrid electric vehicles, and all-electric vehicles, as well as most portable consumer electronics, primarily rely on lithium-ion batteries. Nickel-metal hydride batteries are also commonly used in computers and medical equipment.

Electrodeposition, Electroplating, and Electrorefining. When an electric current passes through a solution (for instance, silver nitrate), the precipitation of a material (silver) at an electrode (the cathode) is called "electrodeposition." A well-known category of electrodeposition is electroplating, when a thin layer of one metal is deposited on another. In galvanization, for example, iron or steel objects are coated with rust-resistant zinc. Electrodeposition techniques have the advantage of being able to coat objects thoroughly, even those with intricate shapes. An allied technique, electrorefining, transforms metals contaminated with impurities to very pure states by anodic dissolution and concomitant redeposition of solutions of their salts. Some industries have used so-called electrowinning techniques to produce salable metals from low-grade ores and mine tailings.

Advances in electrochemical knowledge and techniques have led to evermore sophisticated applications of electrodeposition. For example, knowledge of electrode potentials has made the electrodeposition of alloys possible and commercial. Methods have also been discovered to provide plastics with metal coatings. Similar techniques have been discovered to coat such rubber articles as gloves with a metallic layer. Worn or damaged metal objects can be returned to pristine condition by a process called electroforming. Some commercial metal objects, such as tubes, sheets, and machine parts, have been totally manufactured by electrodeposition (sometimes called "electromachining").

Electrometallurgy. A major application of electrochemical principles and techniques occurs in the manufacture of such metals as aluminum and titanium. Plentiful aluminum-containing bauxite ores exist in large deposits in several countries, but it was not until electrochemical techniques were developed in the United States and France at the end of the nineteenth century that the cost of manufacturing this light metal was sufficiently reduced to make it a commercially valuable commodity. This commercial process involved the electrolysis of alumina (aluminum oxide) dissolved in fused cryolite (sodium aluminum fluoride). During the century that followed this process's discovery, many different uses for this lightweight metal ensued, from airplanes to zeppelins.

Corrosion Control and Dielectric Materials. The destruction of a metal or alloy by oxidation is itself an electrochemical process since the metal loses electrons to the surrounding air or water. A familiar example is the appearance of rust (hydrated ferric oxide) on an iron or steel object. Electrochemical knowledge of the mechanism of corrosion led researchers to ways of preventing or delaying it. Keeping oxidants away from the metallic surface is an obvious means of protection. Substances that interfere with the oxidizing of metals are called "inhibitors." Corrosion inhibitors include both inorganic and organic materials, but they are generally categorized by whether the inhibitor obstructs corrosive reactions at the cathode or anode. Cathodic protection is used extensively for such metal objects as underground pipelines or such structures as ship hulls, which have to withstand the corrosive action of seawater. Similarly, dielectric materials with low electrical conductivity, such as insulators, require long-term protection from high and low temperatures as well as from corrosive forces. An understanding of electrochemistry facilitates the construction of such electrical devices as condensers and capacitors that involve dielectric substances.

Electrochemistry, Molecular Biology, and Medicine. Because of the increasing understanding of electrochemistry as it pertains to plant, animal, and human life, and because of concerns raised by the modern environmental movement, several significant applications have been developed, with the promise of many more to come. For example, electrochemical devices have been made for the analysis of proteins and deoxyribonucleic acid (DNA). Researchers have fabricated DNA sensors as well as DNA chips. These DNA sensors can be used to detect DNA damage. Electrochemistry was involved in the creation of implantable pacemakers designed to regulate heartbeats, thus saving lives. Research is underway to create an artificial heart powered by electrochemical processes within the human body. Neurologists electrically stimulate regions of the brain to help mitigate or even cure certain psychological problems. Developments in electrochemistry have led to the creation of devices that detect various environmental pollutants in air and water. Photoelectrochemistry played a role in helping to understand the dramatic depletion of the ozone layer in the stratosphere and the role that chlorofluorocarbons (CFCs) played in exacerbating this problem. Because a large hole in the ozone layer allows dangerous solar radiation to damage plants, animals, and humans, production of CFC-11 has been globally banned since 2010.

Nanomaterials in Electrochemistry. Miniaturization of electronic technologies became evident and important in the computer industry, where advances have been enshrined in Moore's law, which states that transistor density in integrated circuits doubles every eighteen months. Electrodeposition has proved to be a technique well-suited to the preparation of metal nanostructures, with several applications in electronics, semiconductors, optics, and ceramics. In particular, electrochemical methods have contributed to the understanding and applications of quantum dots, nanoparticles that are so small that they follow quantum rather than classical laws. These quantum dots can be as small as a few atoms, and in the form of ultrathin cadmium-sulfide films they have been shown to generate high photocurrents in solar cells. The electrochemical synthesis of such nanostructured products as nanowires, biosensors, and microelectroanalytical devices has led researchers to predict the ultimate commercial success of these highly efficient contrivances.

Careers and Course Work

Electrochemistry is an immense field with many specialties, though specialized education generally occurs at the graduate level. Undergraduates usually major in chemistry, electrochemistry, chemical engineering, or materials science engineering. The coursework involves introductory and advanced physics, calculus, advanced mathematics, and elementary and advanced chemistry courses. Certain laboratory courses, such as qualitative, quantitative, and instrumental analysis, are often required. Because of the growing sophistication of many electrochemical disciplines, those interested in becoming part of these fields must pursue graduate degrees. Depending on their specialty, graduate students must satisfy core courses, such as electrochemical engineering, and several electives, such as semiconductor devices. Some universities, technical institutes, and engineering schools have programs for students interested in theoretical electrochemistry, electrochemical cells, electrodeposition, nanomaterials, and many others. For a doctoral and often for a master's degree, students must write a thesis under the supervision of a faculty director. Universities may also offer students opportunities to conduct electrochemical research in their laboratories or provide internships.

Career opportunities for electrochemists range from laboratory technicians at small businesses to research professors at prestigious universities. The battery business employs many workers with a relevant bachelor of science degrees to help manufacture, service, and improve various products. Senior electrochemical engineers with advanced degrees may be hired to head research programs to develop new products or to supervise the production of the company's major commercial offerings. Electrochemical engineers are often hired to manage the manufacture of electrochemical components or oversee the electrolytic production of such metals as aluminum and magnesium. Some electrochemists are employed by government agencies, for example, to design and develop fuel cells for the National Aeronautics and Space Administration (NASA). In contrast, others may be hired by pharmaceutical companies to develop new drugs and medical devices.

Social Context and Future Prospects

Though batteries, compared with other energy sources, have been heavier, bigger, and less efficient, they continue to be needed in the twenty-first century. Moreover, as governments increasingly require low-carbon energy solutions to reduce carbon emissions from other energy sources, demand for and investment in battery and fuel cell technology will likely increase. As advanced industrialized societies expand, increasing demand for metals like lithium, beryllium, magnesium, aluminum, titanium, and zirconium will necessarily follow, forcing electrochemists to improve the electrolytic processes for deriving these metals from dwindling sources. If Moore's law holds, computer engineers familiar with electrochemical principles will find new ways to populate integrated circuits with more and better microdevices.

Some prognosticators foresee significant progress in the borderline field between electrochemistry and organic chemistry (sometimes called "organic and bioelectrochemistry"). When ordinary chemical methods have proved inadequate to synthesize desired compounds of high purity, electrolytic techniques have been much better than traditional ones. However, these successes have occurred at the laboratory level, and industrial process development will likely occur. Other new fields, such as photoelectrochemistry, may also mature in the twenty-first century, leading to important applications. The electrochemistry of nanomaterials is well underway, both theoretically and practically, and a robust future has been envisioned as electrochemical engineers create new nanophase materials and devices for potential use in various applications, from electronics to optics. Other areas of research include new methods for nanoparticle immobilization, single-nanoparticle nucleation, electrochemistry with nanoelectrodes, probing tandem electrocatalysis, single-cell electrocatalysis, and single-entity opto-electrochemistry.

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