Primary cell

Primary cell

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Encyclopedia
A primary cell is any kind of 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...

 in which the electrochemical
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...

 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...

 is not reversible, rendering the cell non-rechargeable. A common example of a primary cell is the disposable battery. Unlike a secondary cell, the reaction cannot be reversed by running a current into the cell; the chemical reactants cannot be restored to their initial position and capacity. Primary batteries use up the materials in one or both of their electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

s.

Comparison with rechargeables


Rechargeable batteries are economical to use when their initially higher cost and cost of a charging system can be spread out over many use cycles; for example, in hand-held power tools, it would be very costly to replace a high-capacity primary battery pack every few hours of use.

Primary batteries are useful where long periods of storage are required; a primary battery can be constructed to have a lower self-discharge
Self-discharge
Self-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes...

 rate than a rechargeable battery, so all its capacity is available for useful purposes. Applications that require a small current for a long time, for example a smoke detector
Smoke detector
A smoke detector is a device that detects smoke, typically as an indicator of fire. Commercial, industrial, and mass residential devices issue a signal to a fire alarm system, while household detectors, known as smoke alarms, generally issue a local audible and/or visual alarm from the detector...

, also use primary batteries since the self-discharge current of a rechargeable battery would exceed the load current and limit service time to a few days or weeks. For example, a flashlight
Flashlight
A flashlight is a hand-held electric-powered light source. Usually the light source is a small incandescent lightbulb or light-emitting diode...

 used for emergencies must work when needed, even if it has sat on a shelf for an extended period of time. Primary cells are also more cost-efficient in this case, as rechargeable batteries would use only a small fraction of available recharge cycles. Reserve batteries achieve very long storage time (on the order of 10 years or more) without loss of capacity, by physically separating the components of the battery and only assembling them at the time of use. Such constructions are expensive but are found in applications like munitions, which may be stored for years before use.

Polarization


A primary cell becomes polarized when in use. This means that 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...

 accumulates at the cathode and reduces the effectiveness of the cell. To remove the hydrogen, a depolarizer
Depolarizer
A depolarizer or depolariser, in electrochemistry, according to an IUPAC definition, is a synonym of electroactive substance, i.e., a substance which changes its oxidation state, or partakes in a formation or breaking of chemical bonds, in a charge-transfer step of an electrochemical reaction.In...

 is used and this may be mechanical, chemical or electrochemical.

Attempts have been made to make simple cells self-depolarizing by roughening the surface of the copper plate to facilitate the detachment of hydrogen bubbles. These attempts have had little success. Chemical depolarization utilizes an oxidizing agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...

, such as manganese dioxide (e.g., Leclanché cell
Leclanché cell
Georges Leclanché invented and patented his battery, the Leclanché cell, in 1866. The battery contained a conducting solution of ammonium chloride, a cathode of carbon, a depolarizer of manganese dioxide, and an anode of zinc...

 and zinc–carbon cell) or nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...

 (e.g., 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 :...

 and Grove cell
Grove cell
The Grove cell was an early electric primary cell named after its inventor, British chemist William Robert Grove, and consisted of a zinc anode in dilute sulfuric acid and a platinum cathode in concentrated nitric acid, the two separated by a porous ceramic pot.-Cell details:The Grove cell voltage...

), to oxidize the hydrogen to water. Electrochemical depolarization exchanges the hydrogen for a metal, such as copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 (e.g., Daniell cell
Daniell cell
The Daniell cell was invented in 1836 by John Frederic Daniell, a British chemist and meteorologist, and consisted of a copper pot filled with a copper sulfate solution, in which was immersed an unglazed earthenware container filled with sulfuric acid and a zinc electrode...

), or silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

 (e.g., Silver-oxide cell).

Anode and cathode


The plate that carries the positive terminal (usually carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

) is termed the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

 and the plate that carries the negative terminal (usually zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

) is termed the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

. This is the reverse of the terminology used in an 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...

. The reason is that the terms are related to the passage of electric current through the 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....

, not the external circuit

Inside the cell the anode is the electrode where chemical oxidation occurs, as it accepts electrons from the electrolyte. The cathode is defined as the electrode where chemical reduction occurs, as it donates electrons to the electrolyte.

Outside the cell, different terminology is used. Since the anode accepts electrons from the electrolyte, it becomes negatively charged
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

 and is therefore connected to the terminal marked "−" on the outside of the cell. The cathode, meanwhile, donates electrons to the electrolyte, so it becomes positively charged and is therefore connected to the terminal marked "+" on the outside of the cell.

Old textbooks sometimes contain different terminology that can cause confusion to modern readers. For example, a 1911 textbook by Ayrton and Mather describes the electrodes as the "positive plate" and "negative plate" in a way that contradicts modern usage.

See also


  • History of the battery
    History of the battery
    The history of the development of electrochemical cells is crucial to the scientific study and industrial applications of electricity, for prior to the rise of electrical grids around the end of the 19th century, they were the main source of electricity...

  • Secondary cell (rechargeable)
  • 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...

  • Battery recycling
    Battery recycling
    Battery recycling is a recycling activity that aims to reduce the number of batteries being disposed as municipal solid waste. Batteries contain a number of heavy metals and toxic chemicals, their dumping has raised concern over risks of soil contamination and water pollution.-Battery recycling by...

  • List of battery types
  • Battery nomenclature
    Battery nomenclature
    Standard battery nomenclature describes portable dry cell batteries that are interchangeable in physical dimensions and electrical characteristics between manufacturers. The long history of disposable dry cells means that many different manufacturer-specific and national standards were used to...


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