Anelectrochemical cell is a device that generateselectrical energy fromchemical reactions. Electrical energy can also be applied to these cells to cause chemical reactions to occur.[1] Electrochemical cells that generate an electric current are called voltaic orgalvanic cells and those that generate chemical reactions, viaelectrolysis for example, are calledelectrolytic cells.[2]
Both galvanic and electrolytic cells can be thought of as having twohalf-cells: consisting of separateoxidation and reduction reactions.
When one or more electrochemical cells are connected in parallel or series they make abattery. Primary cells are single use batteries.
A galvanic cell (voltaic cell), named afterLuigi Galvani (Alessandro Volta), is an electrochemical cell that generates electrical energy from spontaneousredox reactions.[3]
A wire connects two differentmetals (e.g.zinc andcopper). Each metal is in a separate solution; often theaqueoussulphate ornitrate forms of the metal, however more generally metal salts and water which conductcurrent.[4] Asalt bridge or porous membrane connects the two solutions, keeping electric neutrality and the avoidance of charge accumulation. The metal's differences in oxidation/reduction potential drive the reaction untilequilibrium.[1]
Key features:
Galvanic cells consists of two half-cells. Each half-cell consists of anelectrode and anelectrolyte (both half-cells may use the same or different electrolytes).[citation needed]
The chemical reactions in the cell involve the electrolyte, electrodes, and/or an external substance (fuel cells may usehydrogen gas as areactant). In a full electrochemical cell, species from one half-cell lose electrons (oxidation) to their electrode while species from the other half-cell gain electrons (reduction) from their electrode.[citation needed]
Asalt bridge (e.g., filter paper soaked in KNO3, NaCl, or some other electrolyte) is used to ionically connect two half-cells with different electrolytes, but it prevents the solutions from mixing and unwanted side reactions. An alternative to a salt bridge is to allow direct contact (and mixing) between the two half-cells, for example in simpleelectrolysis of water.[citation needed]
As electrons flow from one half-cell to the other through an externalcircuit, a difference in charge is established. If no ionic contact were provided, this charge difference would quickly prevent the further flow of electrons. A salt bridge allows the flow of negative or positive ions to maintain a steady-state charge distribution between the oxidation and reduction vessels, while keeping the contents otherwise separate. Other devices for achieving separation of solutions are porous pots and gelled solutions. A porous pot is used in theBunsen cell.[citation needed]
Each half-cell has a characteristic voltage (depending on the metal and its characteristic reduction potential). Each reaction is undergoing anequilibrium reaction between differentoxidation states of the ions: when equilibrium is reached, the cell cannot provide furthervoltage. In the half-cell performing oxidation, the closer the equilibrium lies to the ion/atom with the more positive oxidation state the more potential this reaction will provide.[1] Likewise, in the reduction reaction, the closer the equilibrium lies to the ion/atom with the morenegative oxidation state the higher the potential.[citation needed]
The cell potential can be predicted through the use ofelectrode potentials (the voltages of each half-cell). These half-cell potentials are defined relative to the assignment of 0volts to thestandard hydrogen electrode (SHE). (Seetable of standard electrode potentials). The difference in voltage between electrode potentials gives a prediction for the potential measured. When calculating the difference in voltage, one must first rewrite the half-cell reaction equations to obtain a balanced oxidation-reduction equation.[citation needed]
Cell potentials have a possible range of roughly zero to 6 volts. Cells using water-based electrolytes are usually limited to cell potentials less than about 2.5 volts due to high reactivity of the powerful oxidizing and reducing agents with water which is needed to produce a higher voltage. Higher cell potentials are possible with cells using othersolvents instead of water. For instance,lithium cells with a voltage of 3 volts are commonly available.[citation needed]
The cell potential depends on theconcentration of the reactants, as well as their type. As the cell is discharged, the concentration of the reactants decreases and the cell potential also decreases.[citation needed]
An electrolytic cell is an electrochemical cell in which applied electrical energy drives a non-spontaneousredox reaction.[5]
They are often used to decompose chemical compounds, in a process calledelectrolysis. (The Greek word "lysis" (λύσις) means "loosing" or "setting free".)[citation needed]
Important examples of electrolysis are the decomposition of water intohydrogen andoxygen, and ofbauxite intoaluminium and other chemicals.Electroplating (e.g. of Copper,Silver,Nickel orChromium) is done using an electrolytic cell. Electrolysis is a technique that uses adirect electric current (DC).[citation needed]
The components of an electrolytic cell are:[citation needed]
When driven by an externalvoltage (potential difference) applied to the electrodes, the ions in the electrolyte are attracted to the electrode with the opposite potential, where charge-transferring (also calledfaradaic or redox) reactions can take place. Only with a sufficient external voltage can an electrolytic cell decompose a normally stable, orinert chemical compound in the solution. Thus the electrical energy provided produces a chemical reaction which would not occur spontaneously otherwise.Key features:[citation needed]
A primary cell produces current by irreversible chemical reactions (ex. small disposable batteries) and is not rechargeable.[citation needed]
They are used for their portability, low cost, and short lifetime.[citation needed]
Primary cells are made in a range of standard sizes to power small household appliances such asflashlights and portable radios.[citation needed]
As chemical reactions proceed in a primary cell, the battery uses up the chemicals that generate the power; when they are gone, the battery stops producing electricity.[citation needed]
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Primary batteries make up about 90% of the $50 billion battery market, but secondary batteries have been gaining market share. About 15 billion primary batteries are thrown away worldwide every year,[6] virtually all ending up in landfills. Due to the toxicheavy metals and strong acids or alkalis they contain, batteries arehazardous waste. Most municipalities classify them as such and require separate disposal. The energy needed to manufacture a battery is about 50 times greater than the energy it contains.[7][8][9][10] Due to their high pollutant content compared to their small energy content, the primary battery is considered a wasteful, environmentally unfriendly technology. Mainly due to the increasing sales ofwireless devices andcordless tools, which cannot be economically powered by primary batteries and come with integral rechargeable batteries, the secondary battery industry has high growth and has slowly been replacing the primary battery in high end products.
A secondary cell produces current by reversible chemical reactions (ex.lead-acid battery car battery) and isrechargeable.[citation needed]
Lead-acid batteries are used in an automobile to start an engine and to operate the car's electrical accessories when the engine is not running. The alternator, once the car is running, recharges the battery.[citation needed]
It can perform as a galvanic cell and an electrolytic cell. It is a convenient way to store electricity: when current flows one way, the levels of one or more chemicals build up (charging); while it is discharging, they reduce and the resulting electromotive force can do work.[citation needed]
They are used for their high voltage, low costs, reliability, and long lifetime.[citation needed]
Afuel cell is an electrochemical cell that reacts hydrogen fuel with oxygen or another oxidizing agent, to convert chemical energy toelectricity.[11]
Fuel cells are different frombatteries in requiring a continuous source of fuel and oxygen (usually from air) to sustain the chemical reaction, whereas in a battery the chemical energy comes from chemicals already present in the battery.[11]
Fuel cells can produce electricity continuously for as long as fuel andoxygen are supplied.[11]
They are used for primary and backup power for commercial, industrial and residential buildings and in remote or inaccessible areas. They are also used to powerfuel cell vehicles, includingforklifts, automobiles, buses, boats, motorcycles and submarines.[citation needed]
Fuel cells are classified by the type of electrolyte they use and by the difference in startup time, which ranges from 1 second forproton-exchange membrane fuel cells (PEM fuel cells, or PEMFC) to 10 minutes forsolid oxide fuel cells (SOFC).[citation needed]
There are many types of fuel cells, but they all consist of:
A related technology areflow batteries, in which the fuel can be regenerated by recharging. Individual fuel cells produce relatively small electrical potentials, about 0.7 volts, so cells are "stacked", or placed in series, to create sufficient voltage to meet an application's requirements.[12] In addition to electricity, fuel cells produce water, heat and, depending on the fuel source, very small amounts ofnitrogen dioxide and other emissions. Theenergy efficiency of a fuel cell is generally between 40 and 60%; however, if waste heat is captured in acogeneration scheme, efficiencies up to 85% can be obtained.[citation needed]
In 2022, the global fuel cell market was estimated to be $6.3 billion, and is expected to increase by 19.9% by 2030.[13] Many countries are attempting to enter the market by setting renewable energyGW goals.[14]
Manufacturing a disposable battery takes about 50 times more energy than the battery provides when used.
