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Grammar-school of J. Vrchlicky, Klatovy

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Load characteristic of sources

Theory

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Electrochemical (galvanic) cells

There are a number of physical principles that make it possible to transform other forms of energy into electrical energy. The broadest use among these " surrogate sources " they have electrochemical sources that, either irreversibly (so-called primary cells) or reversible (secondary cells, also accumulators), make it possible to transform the chemical energy of some chemical reactions into electrical energy.

The advantage of electricity is the possibility of simple transmissions even over long distances and a relatively light conversion to other types of energy. However, there are disadvantages. The main thing is the very poor efficiency of electricity storage. And true electrochemical cells have the task of storing electrical energy with the greatest possible efficiency.

Zinc primary cells:

Primary electric cells with a Zn anode are still the most widespread electrochemical sources of current. Their nominal voltage is 1.5 V. This group includes the following commonly produced electrochemical systems:

  1. MnO2/Zn with slightly acidic electrolyte with predominance NH4Cl (Leclanche's cell)
  2. MnO2/Zn with slightly acidic electrolyte with predominance ZnCl2 (Zinc-chloride cell)
  3. MnO2/Zn with alkaline electrolyte (Alkaline cell)

Battery systems of manganese dioxide zinc with a slightly acidic electrolyte are often called zinc carbon batteries, although carbon does not serve as an electrochemically active material, but only as a current collector. Sometimes these batteries are also referred to as zinc-burel cells, which might be more correct. And to make little naming, these cells are also known as dry cells because the electrolyte is immobilized (not in the liquid state).

  1. Leclanche's cell

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 mass-produced type of chemical power source. In the Leclanche cell, zinc is used as an anode, manganese dioxide is used as a cathode and ammonium chloride is used as the main part of the electrolyte, but some zinc chloride is present in the electrolyte. Zinc is oxidized and each zinc atom involved in this reaction releases two electrons. These electrons leave for the outer circuit. On the cathode, MnO2 is reduced by the formation of water and ammonia. But in this chemical reaction, some of the ammonium ions are directly reduced to electrons to form gaseous ammonia and hydrogen.


Negative electrode: Zn → Zn2+ + 2 e
Positive electrode: 2 NH4+ + 2 e → 2 NH3 + H2


zinko-uhlíkový článek
  Fig. 1:  1 – metal cap (+)
2 – insulating plug
3 – zinc casing
4 – electrolyte
5 – carbon electrode
6 – depolarizer
7 – metal bottom (–)

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

Subsequent absorption of the gases produced can be recorded by the following reactions (These reactions prevent the build-up of the battery):

2 NH3 + Zn2+ → Zn(NH3)22+
H2 + 2 MnO2 → Mn2O3 + H2O

The overall reaction can be written:

Zn + 2 MnO2 + 2 NH4Cl → Mn2O3 + [Zn(NH3)2] Cl2 + H2O (1)

The disadvantage of this electrical cell and its arrangement is that the metal zinc anode (simultaneously forming the cell container) dissipates unevenly during discharging and thus may lead to premature leakage of the electrolyte which could destroy the appliance. To avoid this phenomenon, highly toxic mercuric chloride was added to the article. This procedure is forbidden today, unfortunately we can still meet with some East Asian batteries.

For the above reason, most of Leclanche's manufacturers are now replaced by manganese-zinc oxide batteries with a slightly acidic electrolyte, predominantly zinc chloride, referred to as zinc-chloride cells.

  1. Zinc-chloride cell

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 ZnCl 2 (zinc chloride). Its design is identical to the Leclanche's cell. Zinc is used as anode, manganese dioxide is used as a cathode and zinc chloride is used as an electrolyte.

The electrochemical process taking place at discharge is de facto different only by reacting to the positive electrode:


Negative electrode: MnO2 + e + H2O → MnO(OH) + OH–1
Positive electrode: Zn + 2 OH–1 → 2 e + ZnO + H2O


Other reactions also is followed for this galvanic cell:

4 ZnO + Zn+2 +2 Cl–1 + 4 H2O + H2O → ZnCl2 . 4 ZnO . 5 H2O

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

4 Zn + 8 MnO2 + ZnCl2 + 9 H2O → 8 MnO(OH) + ZnCl2 . 4 ZnO . 5 H2O (2)

From the comparison of equations (1) and (2), we can see that when discharging the batteries with ammonium chloride, water is produced, while discharging zinc-chloride batteries, on the contrary, water is consumed. Therefore, zinc-chloride batteries are much less prone to leakage of the electrolyte. Another advantage of these articles is to achieve a significant extension of the allowed storage life of such batteries (up to 3 years). The technical parameters of zinc-chloride cells depend, in particular, on the type of manganese dioxide used. By replacing the natural oxide electrolytically prepared, the useful properties of the cells can be greatly increased, but unfortunately at the price of higher prices.

  1. Alkaline cell

Alkaline cells are cells of the manganese dioxide-zinc (MnO2–Zn) system with alkaline electrolyte. Compared to chlorine electrolyte batteries, alkaline batteries are able to provide much higher currents at low voltage drops, so they are suitable for applications where heavy current loads (toys, digital cameras…) are required. Of course, they are also used in installations with low current outflows, especially where an emphasis is placed on protecting the device from leakage of the electrolyte. The steel cell receptacle is a positive pole, does not serve as an electrochemically active material and therefore does not participate in electrochemical reactions, which virtually prevents the possibility of leakage of the electrolyte due to its corrosion.

n the alkaline cell, powdered zinc serves as a negative electrode, manganese dioxide serves as a positive electrode and potassium hydroxide serves as an electrolyte:


Negative electrode: Zn + 2 OH → 2 e + ZnO + H2O
Positive electrode: 2 MnO2 + H2O + 2 e → Mn2O3 + 2 OH


alkalický článek
  Fig. 2:  1 – metal case
2 – separator
3 – plastic closure
4 – metal cap (+)
5 – MnO2 and powdered carbon
6 – zinek a elektrolyt (gel)
7 – collecting electrode (–)

Overall reaction:

2 MnO2 + Zn → Mn2O3 + ZnO (3)

Sometimes, in the overall reaction, the subsequent reaction of ZnO with the KOH electrolyte is taken into account but no longer affects the resulting electrical voltage (no oxidation number changes):

ZnO + 2 KOH → K2ZnO2 + H2O


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 stress value under the load condition can be 1.1-1.3 V. For example, a classic pencil alkaline cell (type AA) is generally rated for rated current up to 700 mA.

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





Electromotive voltage and source resistivity

Based on the behavior of a real source, we present it as a system composed of an ideal resource associated with a certain resistance inside the resource. Each source is characterized by so-called electromotive voltage Ue (voltage of ideal source) and internal resistivity of the source Ri (the resistance connected in series to the ideal source). Internal resistance affects the behavior of sources in circuits. This causes the voltage at the terminals of the power supply (terminal voltage) Us is not equal to the electromotive voltage after the power supply is connected to the circuit. According to Ohm's law, the terminal voltage against the electromotive voltage is lower by the size of the Ri·I, where I is the electrical current in the circuit.

realny zdroj - schema

Therefore, if we connect the source to a circuit with a resistor of R, passes a current of I whose magnitude is given by Ohm's law for closed circuit

I  =  Ue (4)

 R + Ri 

The relation for the terminal voltage Us:

Us = Ue – Ri·I (5)

From the relationship (5) it follows that if the supply to the circuit does not supply any current (I = 0), the terminal voltage is equal to the electromotive voltage (Us = Ue). On the other hand, for Us = 0 (ie for R = 0 – source short-circuit), the maximum short circuit current flows through the circuit, which is limited by internal resistance only (it follows from the relation (4)):

Ik =   Ue  (6)

 Ri 
zatěžovací charakteristika

The above figure shows the dependence of the Us = Us(I ) on the current passing through the circuit for two different types of sources (alkaline and zinc-chloride). According to the relationship (5) these are lines with different directives (slope), which is given by the internal resistance Ri (given by the construction of the source). In both cases, these are so-called linear sources.

It is possible to directly deduce the basic parameters of the source from the dependencies:

  • Ue – the value of the electromotive voltage as the terminal voltage value at I  = 0;
  • Ik – short-circuit current as the value of the electrical current at the short-circuited source (I.e. when Us = 0 V, see equation (6));
  • Ri – internal resistance of the source as the value of the dependency directive (Ri = ΔU /ΔI ).

The series of measured values of given dependencies can be very well transposed by linear regression. The single regression coefficients then correspond to the parameters of the sources Ue and Ri – see the expression (5). You can use the spreadsheet (MS Excel, Oo Calc, Google Tabs, etc.) to quickly obtain the regression line equation. The short-circuit current can not only be obtained by means of a relation (6), but also by the so-called extrapolation method. This method is based on the fact that by extending the regression line we find the intersection of Us = Us(I ) with the horizontal axis – the value I for which Us = 0 (which is the short-circuit current Ik).

 

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