A metal–air electrochemical cell is an

electrochemical cell An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic o ...

that uses an

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

made from pure

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

and an external

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...

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 hydro ...

electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...

. During discharging of a metal–air electrochemical cell, a

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

occurs in the ambient air cathode while the metal anode is

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

. 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 which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...

, making them a prime candidate for use in

electric vehicle An electric vehicle (EV) is a vehicle that uses one or more electric motors for propulsion. It can be powered by a collector system, with electricity from extravehicular sources, or it can be powered autonomously by a battery (sometimes cha ...

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 amount of energy stored in a given system or region of space per unit volume. It is sometimes confused with energy per unit mass which is properly called specific energy or . Often only the ''useful'' or extract ...

lithium metal Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid ele ...

(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, or sodium chloride (NaCl), in water would be rep ...

, 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 relat ...

, 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 with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...

, 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 ( CH3)2. This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds an ...

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 often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...

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, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...

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.

Aluminum

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 (rust) which 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) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...

, 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 solar and wind power sources, 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. Generally, iron oxide powder beds are selected; however, rapid sintering 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 Freeze-casting, also frequently referred to as ''ice-templating'', or ''freeze alignment'', is a technique that exploits the highly anisotropic solidification behavior of a solvent (generally water) in a well-dispersed slurry to controllably temp ...

, 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 metal–air battery
{{DEFAULTSORT:Metal-air electrochemical cell Battery types