Chemical current sources
Any galvanic cell can be a source of direct current. But only a few of them meet the technical requirements that make their use possible.
At the heart of some modern dry batteries that supply hearing aids, pocket lights, portable communication equipment, lies the scheme of the Leclanche element, proposed in 1876:
The convenience of such a portable power source is that all its constituent parts are solid or pasty substances, the packaging of which prevents them from reaching the surrounding objects. The anode of a dry element is its zinc membrane, and the cathode is a graphite rod pressed into a layer of manganese (IV) oxide and carbon. As the electrolyte is used the nasta from zinc chloride, ammonium chloride and water. The following half reactions occur on the electrodes of the dry element.
At the anode:
At the cathode:
The total reaction is described by the equation
The voltage of such an element is 1.5 V. Dry elements are primary chemical sources of current (or a single-action galvanic cell). After discharge, the dry cell can not be used and must be discarded.
The secondary source of current are the batteries. The capacity of a discharged battery can be restored by charging it, e.g. passing through it in the opposite direction the current from an external source (electrolysis). When charging, the battery works as an electrolyzer, and when it is discharged it acts like a galvanic cell. The processes of charge of batteries are carried out repeatedly. In Table. 13.1 shows the characteristics of the most common galvanic cells.
The most common is a lead (acid) battery. The lead accumulator is a system of lead perforated plates filled with spongy lead and being a cathode, and the lead electrode is lead oxide Pb0 2 , pressed into the lead lattice. A 30% solution of sulfuric acid is used as the electrolyte. Battery plan:
When the plate is immersed in sulfuric acid, a hardly soluble lead sulfate PbS () 4 is formed on their surface. In this state, the electrodes have the same chemical composition and the redox interaction is impossible, the battery is discharged. Therefore, pre-charge the battery, passing through it a constant electric current from an external source. The processes occurring during charging are similar to processes during electrolysis.
At the cathode (-), the process of recovery occurs.
Some galvanic cells used in industry and transport
This discharge response corresponds to an electrochemical reaction
The Pb ions are oxidized at the anode (+).
The resulting salt undergoes hydrolysis
followed by the decomposition l l ^ ts. the end result is the formation of lead oxide
Thus, after charging, one battery electrode is sponge metal lead, and the other is lead oxide (IV).
The general chemical equation of the charging process:
When the battery is running (discharging), the processes on the electrodes proceed in the opposite direction.
Oxidation at the anode:
The total response:
During the charging process, the acid concentration increases, and in the process of discharge, the concentration decreases. The relative density of sulfuric acid indicates how much the battery is discharged. The emf of the lead accumulator reaches 2.1 V.
The voltage at a charge is higher than the emf and increases during the charge. At the end, the voltage reaches a value sufficient for the electrolysis of water, then the evolution of hydrogen and oxygen begins, so the release of gas bubbles (boiling) is a sign of the end of the battery charge.
When the battery is discharged, its EMF and voltage drop. If the voltage falls below 1.7 V, the formation of a PbS0 4 film of a special crystalline structure (the so-called sulfation), which isolates the electrodes from the electrolyte, begins on the electrodes. As a consequence, a voltage drop of up to 1.7 V is unacceptable.
Lead battery has high efficiency (= 80%), high EMF, simplicity and low price. Disadvantages - a small specific energy = 20-30 W h/kg and a short service life (from 2 to 5 years).
A kind of a galvanic cell is a fuel cell in which the chemical energy of the oxidation-reduction reaction of combustion of gaseous and liquid fuels is converted directly into electrical energy. The peculiarity of fuel cells is that fuel and oxidizer are added as they are consumed. This ensures the continuity of the work of the current source theoretically for an arbitrarily long time. At the same time, oxidation products are also continuously released.
Oxygen in fuel cells is almost always used either pure oxygen or oxygen of air. Hydrogen, hydrazine, methanol, water and generator gases are used as fuel. The greatest success has been achieved in the development of a hydrogen-oxygen fuel cell.
Consider the operation of such a fuel cell, which consists of two electrodes of special design, immersed in a solution of alkali (KOH). To the surface of one of them, hydrogen (fuel) is continuously supplied, and to the other an oxidizer (oxygen).
When the external value is closed at the anode, the hydrogen oxidation reaction proceeds:
Oxygen is restored at the cathode:
On the external circuit, electrons move from the anode to the cathode, and in the solution the circuit is closed by the motion of OH ions from the cathode to the anode. The total equation of the reaction is reduced to obtaining water.
For effective operation of the fuel cell, catalysts are used that are applied to the electrodes.
The most important problem of the fuel cell is the kinetics of the electrode processes - even at very low current the voltage at the source terminals drops rapidly, since the element strongly polarizes. Only a high rate of oxidation and reduction reaction makes it possible to obtain a rather high fuel utilization factor. To reduce the polarization of the fuel cell, porous electrodes with a highly developed surface, made of powders of metals or coal, having a catalytic effect, are used. As catalysts for electrodes, platinum group metals, silver, specially processed nickel and cobalt (therefore fuel cells are still expensive) are used. The speed of the electrode processes can also be increased by increasing the temperature and pressure.
The fuel cells produce a low-voltage direct current (1 -1, 1 V), provide a rather high efficiency of 70% (compared to thermal machines), operate silently and do not emit harmful products. Fuel cells were used in spacecraft "Gemini" and Apollo & quot ;.
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