- Alkaline Battery Components
- Basic electrolytes
- Functioning
- Rechargeable batteries
- Applications
- References
The alkaline battery is a battery in which the pH of its electrolyte composition is basic. This is the main difference between this battery and many others where its electrolytes are acidic; as is the case with zinc-carbon batteries that use NH 4 Cl salts, or even concentrated sulfuric acid in car batteries.
It is also a dry cell, since the basic electrolytes are in the form of a paste with a low percentage of humidity; but enough to allow a migration of the ions participating in the chemical reactions towards the electrodes, and thus, complete the electron circuit.
Source: Mike Mozart via Flickr.
Pictured above is a Duracell 9V battery, one of the best known examples of alkaline batteries. The larger the battery, the longer its life and work capacity (especially if they are used for energy-intensive appliances). For small appliances, you have AA and AAA batteries.
Another difference apart from the pH of their electrolyte composition, is that, rechargeable or not, they generally last longer than acid batteries.
Alkaline Battery Components
In the zinc-carbon battery, there are two electrodes: one of zinc, and the other of graphitic carbon. In its "basic version" one of the electrodes instead of being graphite, consists of manganese (IV) oxide, MnO 2 mixed with graphite.
The surface of both electrodes are being consumed and covered by the solids resulting from the reactions.
Source: Lead holder, from Wikimedia Commons
Also, instead of a tin with a homogeneous zinc surface as the cell container, there is a series of compact discs (top image).
In the center of all the disks lies a rod of MnO 2, at the top of which an insulating washer protrudes and marks the positive terminal (cathode) of the battery.
Note that the discs are covered with a porous and a metallic layer; the latter could also be a thin plastic film.
The base of the cell is the negative terminal, where the zinc oxidizes and releases the electrons; but these need an external circuit to reach the top of the battery, its positive terminal.
The surface of the zinc is not smooth, as is the case with Leclanché cells, but rough; that is, they have many pores and a large surface area that increase the activity of the battery.
Basic electrolytes
The shape and structure of the batteries change according to the type and design. However, all alkaline batteries have in common a basic pH of their electrolyte composition, which is due to the addition of NaOH or KOH to the pasty mixture.
Actually, it is the OH ions - those that participate in the reactions responsible for the electrical energy provided by these objects.
Functioning
When the alkaline battery has been connected to the appliance and turned on, the zinc immediately reacts with the OH - from the paste:
Zn (s) + 2OH - (aq) => Zn (OH) 2 (s) + 2e -
The 2 electrons released by the oxidation of zinc travel to the external circuit, where they are responsible for starting the electronic mechanism of the device.
Then, they return to the battery through the positive terminal (+), the cathode; that is, they travel through the MnO 2 -graphite electrode. As the pasta has a certain humidity, the following reaction takes place:
2MnO 2 (s) + 2H 2 O (l) + 2e - => 2MnO (OH) (s) + 2OH - (aq)
Now the MnO 2 is reduced or gains the electrons of the Zn. It is for this reason that this terminal corresponds to the cathode, which is where the reduction occurs.
Note that the OH - is regenerated at the end of the cycle to restart the oxidation of Zn; in other words, they diffuse into the middle of the paste until they come into contact again with the powdered zinc.
Likewise, gaseous products are not formed, as occurs with the zinc-carbon cell where NH 3 and H 2 are generated.
There will come a point where the entire surface of the electrode will be covered by the solids of Zn (OH) 2 and MnO (OH), ending the useful life of the battery.
Rechargeable batteries
The alkaline battery described is not rechargeable, so once it is “dead” there is no way to use it again. This is not the case with rechargeable ones, which are characterized by having reversible reactions.
To revert the products to reactants, an electrical current must be applied in the opposite direction (not from anode to cathode, but from cathode to anode).
An example of a rechargeable alkaline battery is NiMH. It consists of a NiOOH anode, which loses electrons to the nickel hydride cathode. When the battery is used it discharges, and this is where the well-known phrase "charge the battery" comes from.
Thus, it can be recharged hundreds of times, as needed; however, time cannot be completely reversed and the original conditions reached (which would be unnatural).
Also, it cannot be recharged in an arbitrary way: the manufacturer's recommended guidelines must be followed.
That is why sooner or later these batteries also perish and lose their effectiveness. However, it has the advantage of not being quickly disposable, contributing less to contamination.
Other rechargeable batteries are nickel-cadmium and lithium batteries.
Applications
Source: Pxhere.
Some variants of alkaline batteries are so small that they can be used in watches, remote controls, clocks, radios, toys, computers, consoles, flashlights, etc. Others are bigger than a figurine of a Star Wars clone.
In fact, these are the ones on the market that predominate over other types of batteries (at least for home use). They last longer and generate more electricity than conventional Leclanché batteries.
Although the zinc-manganese battery does not contain toxic substances, other batteries, for example those of mercury, open a debate about their possible impact on the environment.
On the other hand, alkaline batteries work very well in a wide range of temperatures; They can even work below 0 ° C, so they are a good source of electrical energy for those appliances that are surrounded by ice.
References
- Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
- Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
- Bobby. (May 10, 2014). Learn More About Most Dependable Alkaline Batteries. Recovered from: upsbatterycenter.com
- Duracell. (2018). Frequently asked questions: science. Recovered from: duracell.mx
- Boyer, Timothy. (April 19, 2018). What Is the Difference Between Alkaline & Non-Alkaline Batteries? Sciencing. Recovered from: sciencing.com
- Michael W. Davidson and The Florida State University. (2018). The Alkaline-Manganese Battery. Recovered from: micro.magnet.fsu.edu