A metal–air electrochemical cell is an

electrochemical cell An electrochemical cell is a device that either generates electrical energy from chemical reactions in a so called galvanic cell, galvanic or voltaic cell, or induces chemical reactions (electrolysis) by applying external electrical energy in an ...

that uses an

anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...

made from pure

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

and an external

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. Conventional curren ...

of ambient air, typically with an aqueous or

aprotic A polar aprotic solvent is a solvent that lacks an acidic proton and is polar. Such solvents lack hydroxyl and amine groups. In contrast to protic solvents, these solvents do not serve as proton donors in hydrogen bonding In chemistry, a hydr ...

electrolyte An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...

. During discharging of a metal–air electrochemical cell, a reduction reaction occurs in the ambient air cathode while the metal anode is

oxidized Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

. The specific capacity and energy density of metal–air electrochemical cells is higher than that of

lithium-ion batteries A lithium-ion or Li-ion battery is a type of rechargeable battery that uses the reversible intercalation of Li+ ions into electronically conducting solids to store energy. Li-ion batteries are characterized by higher specific energy, energy ...

, making them a prime candidate for use in

electric vehicle An electric vehicle (EV) is a motor vehicle whose propulsion is powered fully or mostly by electricity. EVs encompass a wide range of transportation modes, including road vehicle, road and rail vehicles, electric boats and Submersible, submer ...

s. While there are some commercial applications, complications associated with the metal anodes, catalysts, and electrolytes have hindered development and implementation of metal–air batteries.

Types by anode element

Lithium

The remarkably high

energy density In physics, energy density is the quotient between the amount of energy stored in a given system or contained in a given region of space and the volume of the system or region considered. Often only the ''useful'' or extractable energy is measure ...

of lithium metal (up to 3458 Wh/kg) inspired the design of lithium–air batteries. A lithium–air battery consists of a solid lithium electrode, an electrolyte surrounding this electrode, and an ambient air electrode containing oxygen. Current lithium–air batteries can be divided into four subcategories based on the electrolyte used and the subsequent electrochemical cell architecture. These electrolyte categories are aprotic,

aqueous An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in wat ...

, mixed aqueous/aprotic, and solid state, all of which offer their own distinct advantages and disadvantages. Nonetheless, efficiency of lithium–air batteries is still limited by incomplete discharge at the cathode, charging overpotential exceeding discharge

overpotential In electrochemistry, overpotential is the potential difference (voltage) between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly r ...

, and component stability. During discharge of lithium–air batteries, the superoxide ion (O) formed will react with the electrolyte or other cell components and will prevent the battery from being rechargeable.

Sodium

Sodium–air batteries were proposed with the hopes of overcoming the battery instability associated with superoxide in lithium–air batteries.

Sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

, with an energy density of 1605 Wh/kg, does not boast as high an energy density as lithium. However, it can form a stable superoxide (NaO) as opposed to the superoxide undergoing detrimental secondary reactions. Since NaO will decompose reversibly to an extent back to the elemental components, this means sodium–air batteries have some intrinsic capacity to be rechargeable. Sodium–air batteries can only function with aprotic, anhydrous electrolytes. When a

DMSO Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula . This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is ...

electrolyte was stabilized with sodium trifluoromethanesulfonimide, the highest cycling stability of a sodium–air battery was obtained (150 cycles).

Potassium

Potassium–air batteries were also proposed with the hopes of overcoming the battery instability associated with superoxide in lithium–air batteries. While only two to three charge-discharge cycles have ever been achieved with potassium–air batteries, they do offer an exceptionally low overpotential difference of only 50 mV.

Zinc

Zinc–air batteries are used for hearing aids and film cameras.

Magnesium

A variety of metal–air chemistries are currently being studied. The

homogeneous Homogeneity and heterogeneity are concepts relating to the uniformity of a substance, process or image. A homogeneous feature is uniform in composition or character (i.e., color, shape, size, weight, height, distribution, texture, language, i ...

deposition of Mg metal makes Mg–air systems interesting. However, aqueous Mg–air batteries are seriously limited by the Mg electrode's dissolution. The use of a number of ionic aqueous electrolytes in magnesium–air devices has been recommended. Nevertheless,

electrochemical Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference and identifiable chemical change. These reactions involve electrons moving via an electronically conducting phase (typi ...

fragility affects them all. However, the cell's reversibility is limited, and the especially visible during recharging.

Calcium

Calcium–air(O₂) batteries have been reported.

Aluminium

Aluminium–air batteries have the highest energy density of any other battery, with a theoretical maximum energy density of 6–8 kWh/kg, however, , a maximum of only 1.3 kWh/kg has been achieved. Aluminium battery cells are not rechargeable, so new aluminium anodes must be installed to continue getting power from the battery, which makes them expensive to use and limited to mostly military applications. Aluminium–air batteries have been used for prototypes of electric cars, with one claiming 2000 km of range on a single charge, however none have been available to the public. However, aluminium–air batteries maintain a stable voltage and power output until they run out of power, which could make them useful for electric planes, where full power is always required in case of emergency landings. Due to not having to carry a separate metal anode, the natural low density of aluminium, and the high energy density of aluminium–air batteries, the batteries are very lightweight, which is also beneficial for electric aviation. The scale of airports could also allow for on-site recycling of anodes, which would not be feasible for cars where many small stations are necessary. Aluminium–air batteries are better for the environment compared to traditional lithium-ion batteries. Aluminium is the most abundant metal in the Earth's crust, so mines would not have to be as invasive to find a similar amount of aluminium compared to lithium. Another factor is that aluminium recycling plants already exist, while lithium recycling plants are just starting to emerge and become profitable. Aluminium is a lot more economical to recycle with current technology.

Iron

Iron–air rechargeable batteries are an attractive technology with the potential of grid-scale energy storage. The main raw-material of this technology is

iron oxide An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. Iron ...

(

rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH) ...

), a material that is abundant, non-toxic, inexpensive, and environmentally friendly. Most of the batteries currently being developed utilize iron oxide powders to generate and store hydrogen via the Fe/FeO reduction/oxidation (redox) reaction (Fe + H₂O FeO + H₂). In conjunction with a

fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen fuel, hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most bat ...

, this enables the system to behave as a rechargeable battery, creating H₂O/H₂ via the production and consumption of electricity. Furthermore, this technology has minimal environmental impact, as it could be used to store energy from intermittent or variable energy sources, such as solar and wind, developing an energy system with low carbon dioxide emissions. One way the system can start is by using the Fe/FeO redox reaction. Hydrogen created during the oxidation of iron and of oxygen from the air can be consumed by a fuel cell to create electricity. When electricity must be stored, hydrogen generated from water by operating the fuel cell in reverse is consumed during the reduction of the iron oxide to metallic iron. The combination of both of these cycles is what makes the system operate as an iron–air rechargeable battery. Limitations of this technology come from the materials used, and due to lower

energy conversion efficiency Energy conversion efficiency (''η'') is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The input, as well as the useful output may be chemical, electric power, mechanical work, light (radi ...

. Generally, iron oxide powder beds are selected; however, rapid

sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing process used with metals, ceramics, plas ...

and pulverization of the powders limit the ability to achieve a high number of cycles, which results in diminished capacity. Other methods currently under investigation, such as 3D printing and freeze-casting, seek to enable the creation of architecture materials to allow for high surface area and volume changes during the redox reaction.

Comparison

Notes

References

External links

High-temperature liquid Sn–air energy storage cell
– A metal–air battery: Α new type of a high temperature liquid metal-air energy storage cell based on solid oxide electrolyte {{DEFAULTSORT:Metal-air electrochemical cell Battery types