Remote-LAB GymKT

The Leclanche's cell is an electrochemical source of the system: manganese zinc oxide (MnO2Zn) with a slightly acidic electrolyte with a predominance of NH4Cl (salmon). This is the first type of chemical energy source to be mass-produced. In the Leclanche cell, zinc is used as the anode, manganese dioxide as the cathode and ammonium chloride as the main part of the electrolyte, but some zinc chloride is present in the electrolyte. Zinc is oxidised and each zinc atom involved in the reaction releases two electrons. These electrons go to the outer circuit. At the cathode, MnO2 is reduced to form water and ammonia. However, in this chemical reaction, some of the ammonium ions are directly reduced to electrons to form gaseous ammonia and hydrogen.

Negative electrode: $\mathrm{Zn}\to {\mathrm{Zn}}^{\mathrm{2+}}+2{\text{e}}^{\mathrm{–}}$
Positive electrode: $2{\mathrm{NH}}_4^{+}+2{\text{e}}^{\mathrm{–}}\to 2{\mathrm{NH}}_3+{\text{H}}_2$

This gaseous ammonia is further reacted with zinc chloride (ZnCl₂) to form solid diamminium chloride, and hydrogen gas is reacted with manganese dioxide to form solid manganese dioxide and water.

The subsequent absorption of the gases produced can be recorded by the following reactions (these reactions prevent the accumulation of the battery):

The total reaction can be written:

The disadvantage of this electric cell and its arrangement is that the metal zinc anode (which also forms the cell container) dissipates unevenly during discharge, which can lead to premature leakage of the electrolyte, which could destroy the device. To avoid this, highly toxic mercuric chloride was added. This procedure is forbidden today, but unfortunately we still come across some East Asian batteries.

For this reason, most Leclanche manufacturers are now replacing them with manganese-zinc oxide batteries with a slightly acidic electrolyte, mostly zinc chloride, called zinc chloride cells.

The zinc chloride cell is again an electrochemical source of the system: manganese zinc oxide (MnO₂–Zn) with a slightly acidic electrolyte, but this time with predominant ZnCl₂ (zinc chloride). Its construction is identical to that of the Leclanche cell. Zinc is used as the anode, manganese dioxide as the cathode, and zinc chloride as the electrolyt

The electrochemical process that takes place during discharge differs only in its reaction to the positive electrode:

Negative electrode: ${\mathrm{MnO}}_2+{\text{e}}^{\text{–}}+{\text{H}}_2\text{O}\to \mathrm{MnO}\left(\mathrm{OH}\right)+{\mathrm{OH}}^{\text{–}}$
Positive electrode: $\mathrm{Zn}+2{\mathrm{OH}}^{\text{–}}\to 2{\text{e}}^{\text{–}}+\mathrm{ZnO}+{\text{H}}_2\text{O}$

Other reactions are also followed for this galvanic cell:

$4\mathrm{ZnO}+{\mathrm{Zn}}^{\text{+2}}+2{\mathrm{Cl}}^{\text{–}}+4{\text{H}}_{\text{2}}\text{O}+{\text{H}}_{\text{2}}\text{O}\to {\text{ZnCl}}_{\text{2}}·4\mathrm{ZnO}·5{\text{H}}_{\text{2}}\text{O}$

The overall equation of the cell can be expressed by the following equation:

Comparing equations (1) and (2), we can see that water is produced when discharging ammonium chloride batteries, while water is consumed when discharging zinc chloride batteries. Therefore, zinc chloride batteries are much less prone to electrolyte leakage. Another advantage of these products is a significant extension of the permissible storage life of such batteries (up to 3 years). The technical parameters of zinc chloride cells depend mainly on the type of manganese dioxide used. Replacing the natural oxide with electrolytically prepared manganese dioxide significantly increases the useful properties of the cells, but unfortunately at a higher price.

Alkaline cells are manganese dioxide-zinc (MnO₂–Zn) cells with an alkaline electrolyte. Compared to chloride electrolyte batteries, alkaline batteries are able to deliver much higher currents at low voltage drops, making them suitable for applications where high current loads are required (toys, digital cameras…). Of course, they are also used in installations with low current leakage, especially where it is important to protect the device from electrolyte leakage. The steel cell holder is a positive pole, does not serve as an electrochemically active material and therefore does not participate in electrochemical reactions, which virtually eliminates the possibility of electrolyte leakage due to its corrosion.

In an alkaline cell, zinc powder serves as the negative electrode, manganese dioxide as the positive electrode, and potassium hydroxide as the electrolyte:

Negative electrode: $\mathrm{Zn}+2{\mathrm{OH}}^{\text{–}}\to 2{\text{e}}^{\text{–}}+\mathrm{ZnO}+{\text{H}}_{\text{2}}\text{O}$
Positive electrode: $2{\mathrm{MnO}}_2+{\text{H}}_{\text{2}}\text{O}+2{\text{e}}^{\text{–}}\to {\mathrm{Mn}}_2{\text{O}}_3+2{\text{OH}}^{\text{–}}$

Overall reaction:

2 MnO₂ + Zn → Mn₂O₃ + ZnO

(3)

Sometimes the subsequent reaction of ZnO with the KOH electrolyte is taken into account in the overall reaction, but does not affect the resulting electrical voltage (no change in oxidation number):

The alkaline cell is rated at 1.5 V. The new uncharged alkaline cell shows a voltage of 1.50–1.65 V. The average voltage under load can be 1.1–1.3 V. For example, a classic pencil alkaline cell (type AA) is generally designed for a rated current of up to 700 mA.

The comparison of equations (1), (2) and (3) shows that the advantage of alkaline cells is the higher utilization of MnO₂ due to the reduction of Mn⁴⁺ to Mn²⁺, while in a slightly acidic electrolyte battery the process stops at Mn³⁺.

(4)

(5)

(6)