Electrochemical cell

Electrochemical cell

Overview
An electrochemical cell is a device capable of either deriving electrical
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 energy from chemical reactions
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...

, or facilitating chemical reactions through the introduction of electrical energy. A common example of an electrochemical cell is a standard 1.5-volt "battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

". (Actually a single "Galvanic cell
Galvanic cell
A Galvanic cell, or Voltaic cell, named after Luigi Galvani, or Alessandro Volta respectively, is an electrochemical cell that derives electrical energy from spontaneous redox reaction taking place within the cell...

"; a battery properly consists of multiple cells, connected in either parallel or series pattern.)


An electrochemical cell consists of two half-cells.
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Encyclopedia
An electrochemical cell is a device capable of either deriving electrical
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 energy from chemical reactions
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...

, or facilitating chemical reactions through the introduction of electrical energy. A common example of an electrochemical cell is a standard 1.5-volt "battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

". (Actually a single "Galvanic cell
Galvanic cell
A Galvanic cell, or Voltaic cell, named after Luigi Galvani, or Alessandro Volta respectively, is an electrochemical cell that derives electrical energy from spontaneous redox reaction taking place within the cell...

"; a battery properly consists of multiple cells, connected in either parallel or series pattern.)

Half-cells



An electrochemical cell consists of two half-cells. Each half-cell consists of an electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

, and an electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....

. The two half-cells may use the same electrolyte, or they may use different electrolytes. The chemical reactions in the cell may involve the electrolyte, the electrodes or an external substance (as in fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...

s which may use hydrogen gas as a reactant). In a full electrochemical cell, species from one half-cell lose electrons (oxidation
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

) to their electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

 while species from the other half-cell gain electrons (reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

) from their electrode. A salt bridge
Salt bridge
A salt bridge, in chemistry, is a laboratory device used to connect the oxidation and reduction half-cells of a galvanic cell , a type of electrochemical cell...

(e.g. filter paper soaked in KNO3) is often employed to provide ionic contact between two half-cells with different electrolytes—to prevent the solutions from mixing and causing unwanted side reactions. As electrons flow from one half-cell to the other, a difference in charge is established. If no salt bridge were used, this charge difference would prevent further flow of electrons. A salt bridge allows the flow of ions to maintain a balance in charge between the oxidation and reduction vessels while keeping the contents of each separate. Other devices for achieving separation of solutions are porous pots and gelled solutions. A porous pot is used in the Bunsen cell
Bunsen cell
The Bunsen cell is a zinc-carbon primary cell composed of a zinc anode in dilute sulfuric acid separated by a porous pot from a carbon cathode in nitric or chromic acid.- Cell details :...

 (right).

Equilibrium reaction


Each half-cell has a characteristic voltage. Different choices of substances for each half-cell give different potential differences. Each reaction is undergoing an equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...

 reaction between different oxidation states of the ions—when equilibrium is reached the cell cannot provide further voltage. In the half-cell which is undergoing oxidation, the closer the equilibrium lies to the ion/atom with the more positive oxidation state the more potential this reaction will provide. Similarly, in the reduction reaction, the further the equilibrium lies to the ion/atom with the more negative oxidation state the higher the potential.

Cell potential


The cell potential can be predicted through the use of electrode potential
Electrode potential
Electrode potential, E, in electrochemistry, according to an IUPAC definition, is the electromotive force of a cell built of two electrodes:* on the left-hand side is the standard hydrogen electrode, and...

s (the voltages of each half-cell). (See table of standard electrode potentials). The difference in voltage between electrode potentials gives a prediction for the potential measured.

Cell potentials have a possible range of about zero to 6 volts. Cells using water-based electrolytes are usually limited to cell potentials less than about 2.5 volts, because the very powerful oxidizing and reducing agents which would be required to produce a higher cell potential tend to react with the water.

Main types


Cells are classified into two broad categories,
  • Primary cells irreversibly (within limits of practicality) transform chemical energy to electrical energy. When the initial supply of reactants is exhausted, energy cannot be readily restored to the electrochemical cell by electrical means.
  • Secondary cells can be recharged; that is, they can have their chemical reactions reversed by supplying electrical energy to the cell, restoring their original composition.

Primary electrochemical cells



Primary electrochemical cells can produce current immediately on assembly. Disposable cells are intended to be used once and discarded. Disposable primary cells cannot be reliably recharged, since the chemical reactions are not easily reversible and active materials may not return to their original forms.

Common types of disposable cells include zinc-carbon cells and alkaline cells. Generally, these have higher energy densities
Energy density
Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...

 than rechargeable cells, but disposable cells do not fare well under high-drain applications with loads under 75 ohms
Ohms
OHMS may refer to:* The plural of ohm, a unit of resistance, named after Georg Ohm* Ohm's Law of electric currents, first proposed by Georg Ohm* O.H.M.S., On His/Her Majesty's Service...

 (75 Ω).

Secondary electrochemical cells


Secondary electrochemical cells must be charged before use; they are usually assembled with active materials in the discharged state. Rechargeable electrochemical cells or secondary electrochemical cells can be recharged by applying electric current, which reverses the chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

s that occur during its use. Devices to supply the appropriate current are called chargers or rechargers.

The oldest form of rechargeable cell is the lead-acid cell. This electrochemical cell is notable in that it contains a liquid in an unsealed container, requiring that the cell be kept upright and the area be well ventilated to ensure safe dispersal of the hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 gas produced by these cells during overcharging. The lead-acid cell is also very heavy for the amount of electrical energy it can supply. Despite this, its low manufacturing cost and its high surge current levels make its use common where a large capacity (over approximately 10Ah) is required or where the weight and ease of handling are not concerns.

An improved type of liquid electrolyte cell is the sealed valve regulated lead acid (VRLA
VRLA
A VRLA battery is a type of low-maintenance lead–acid rechargeable battery. Because of their construction, VRLA batteries do not require regular addition of water to the cells....

) cell, popular in the automotive industry as a replacement for the lead-acid wet cell. The VRLA cell uses an immobilized sulphuric acid electrolyte, reducing the chance of leakage and extending shelf life. VRLA cells have the electrolyte immobilized, usually by one of two means:
  • Gel cells contain a semi-solid electrolyte to prevent spillage.
  • Absorbed Glass Mat (AGM) cells absorb the electrolyte in a special fibreglass matting


Other portable rechargeable cells are (in order of increasing power density
Power density
Power density is the amount of power per unit volume....

 and cost): nickel-cadmium cells
Nickel-cadmium battery
The nickel–cadmium battery ' is a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes....

 (NiCd), nickel metal hydride cells
Nickel metal hydride battery
A nickel–metal hydride cell, abbreviated NiMH, is a type of rechargeable battery similar to the nickel–cadmium cell. The NiMH battery uses a hydrogen-absorbing alloy for the negative electrode instead of cadmium. As in NiCd cells, the positive electrode is nickel oxyhydroxide...

 (NiMH), and lithium-ion cells
Lithium ion battery
A lithium-ion battery is a family of rechargeable battery types in which lithium ions move from the negative electrode to the positive electrode during discharge, and back when charging. Chemistry, performance, cost, and safety characteristics vary across LIB types...

(Li-ion). By far, Li-ion has the highest share of the dry cell rechargeable market. Meanwhile, NiMH has replaced NiCd in most applications due to its higher capacity, but NiCd remains in use in power tool
Power tool
A power tool is a tool that is actuated by an additional power source and mechanism other than the solely manual labour used with hand tools. The most common types of power tools use electric motors. Internal combustion engines and compressed air are also commonly used...

s, two-way radio
Two-way radio
A two-way radio is a radio that can both transmit and receive , unlike a broadcast receiver which only receives content. The term refers to a personal radio transceiver that allows the operator to have a two-way conversation with other similar radios operating on the same radio frequency...

s, and medical equipment
Medical equipment
Medical equipment is designed to aid in the diagnosis, monitoring or treatment of medical conditions.-Types:There are several basic types:* Diagnostic equipment includes medical imaging machines, used to aid in diagnosis...

.

Special types

  • Concentration cell
    Concentration cell
    A concentration cell is a limited form of a galvanic cell that has two equivalent half-cells of the same material differing only in concentrations. One can calculate the potential developed by such a cell using the Nernst Equation. A concentration cell produces a voltage as it attempts to reach...

  • Electrolytic cell
    Electrolytic cell
    An electrolytic cell decomposes chemical compounds by means of electrical energy, in a process called electrolysis; the Greek word lysis means to break up. The result is that the chemical energy is increased...

  • Galvanic cell
    Galvanic cell
    A Galvanic cell, or Voltaic cell, named after Luigi Galvani, or Alessandro Volta respectively, is an electrochemical cell that derives electrical energy from spontaneous redox reaction taking place within the cell...

  • Lemon battery
    Lemon battery
    A lemon battery is a device used in experiments proposed in many science textbooks around the world. It is made by inserting two different metallic objects, for example a galvanized nail and a copper coin, into a lemon. The copper coin serves as the positive electrode or cathode and the galvanized...


See also


  • Alkaline battery
    Alkaline battery
    Alkaline batteries are a type of primary batteries dependent upon the reaction between zinc and manganese dioxide . A rechargeable alkaline battery allows reuse of specially designed cells....

  • Cell notation
    Cell notation
    Cell notation in chemistry is a shorthand way of expressing a certain reaction in an electrochemical cell. The cell anode and cathode are separated by two bars or slashes representing a salt bridge, with the anode on the left and cathode on the right. Individual solid, liquid or aqueous phases...

  • Electrochemical potential
  • Nickel Cadmium battery (NiCad)
  • Nickel metal hydride battery
    Nickel metal hydride battery
    A nickel–metal hydride cell, abbreviated NiMH, is a type of rechargeable battery similar to the nickel–cadmium cell. The NiMH battery uses a hydrogen-absorbing alloy for the negative electrode instead of cadmium. As in NiCd cells, the positive electrode is nickel oxyhydroxide...

     (NiMH)
  • Lithium-ion battery
  • Rechargeable battery
    Rechargeable battery
    A rechargeable battery or storage battery is a group of one or more electrochemical cells. They are known as secondary cells because their electrochemical reactions are electrically reversible. Rechargeable batteries come in many different shapes and sizes, ranging anything from a button cell to...

  • List of battery sizes
  • Activity (chemistry)
    Activity (chemistry)
    In chemical thermodynamics, activity is a measure of the “effective concentration” of a species in a mixture, meaning that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution.By convention, activity...