Chemical current sources (abbreviated as HIT) are devices in which the energy of a redox reaction is converted into electrical energy. Their other names are electrochemical cell, galvanic cell, electrochemical cell. The principle of their operation is as follows: as a result of the interaction of two reagents, a chemical reaction occurs with the release of energy from a direct electric current. In other current sources, the process of generating electricity occurs according to a multi-stage scheme. First, thermal energy is released, then it is converted into mechanical energy, and only then into electrical energy. The advantage of HIT is the single-stage process, that is, electricity is obtained immediately, bypassing the stages of obtaining thermal and mechanical energy.
History
How did the first current sources appear? Chemical sources are called galvanic cells in honor of the Italian scientist of the eighteenth century - Luigi Galvani. He was a physician, anatomist, physiologist and physicist. One of its directionsresearch was the study of animal reactions to various external influences. The chemical method of generating electricity was discovered by Galvani by chance, during one of the experiments on frogs. He connected two metal plates to the exposed nerve on the frog's leg. This resulted in muscle contraction. Galvani's own explanation of this phenomenon was incorrect. But the results of his experiments and observations helped his compatriot Alessandro Volta in subsequent studies.
Volta outlined in his writings the theory of the emergence of an electric current as a result of a chemical reaction between two metals in contact with the muscle tissue of a frog. The first chemical power source looked like a container of saline, with plates of zinc and copper immersed in it.
HIT began to be produced on an industrial scale in the second half of the nineteenth century, thanks to the Frenchman Leclanche, who invented the primary manganese-zinc cell with s alt electrolyte, named after him. A few years later, this electrochemical cell was improved by another scientist and was the only primary chemical current source until 1940.
Design and principle of operation HIT
The device of chemical current sources includes two electrodes (conductors of the first kind) and an electrolyte located between them (conductor of the second kind, or ionic conductor). An electronic potential arises at the boundary between them. The electrode at which the reducing agent is oxidizedcalled the anode, and the one on which the oxidizing agent is reduced is called the cathode. Together with the electrolyte, they make up the electrochemical system.
A by-product of the redox reaction between electrodes is the generation of electric current. During such a reaction, the reducing agent is oxidized and donates electrons to the oxidizing agent, which accepts them and is thereby reduced. The presence of an electrolyte between the cathode and anode is a necessary condition for the reaction. If you simply mix powders from two different metals together, no electricity will be released, all the energy will be released in the form of heat. An electrolyte is needed to streamline the process of electron transfer. Most often, it is a s alt solution or a melt.
Electrodes look like metal plates or grids. When they are immersed in an electrolyte, an electric potential difference arises between them - an open circuit voltage. The anode tends to donate electrons, while the cathode tends to accept them. Chemical reactions begin on their surface. They stop when the circuit is opened, and also when one of the reagents is used up. Opening of the circuit occurs when one of the electrodes or electrolyte is removed.
Composition of electrochemical systems
Chemical current sources use oxygen-containing acids and s alts, oxygen, halides, higher metal oxides, nitroorganic compounds, etc. as oxidizing agents. Metals and their lower oxides, hydrogen are reducing agents in themand hydrocarbon compounds. How electrolytes are used:
- Aqueous solutions of acids, alkalis, saline, etc.
- Non-aqueous solutions with ionic conductivity, obtained by dissolving s alts in organic or inorganic solvents.
- Molten s alts.
- Solid compounds with an ionic lattice in which one of the ions is mobile.
- Matrix electrolytes. These are liquid solutions or melts located in the pores of a solid non-conductive body - an electron carrier.
- Ion-exchange electrolytes. These are solid compounds with fixed ionogenic groups of the same sign. Ions of the other sign are mobile. This property makes the conductivity of such an electrolyte unipolar.
Galvanic batteries
Chemical current sources consist of galvanic cells - cells. The voltage in one of these cells is small - from 0.5 to 4V. Depending on the need, a galvanic battery is used in HIT, consisting of several series-connected cells. Sometimes a parallel or series-parallel connection of several elements is used. Only identical primary cells or batteries are always included in a series circuit. They must have the same parameters: electrochemical system, design, technological option and standard size. For parallel connection, it is acceptable to use elements of different sizes.
HIT Classification
Chemical current sources differ in:
- size;
- designs;
- reagents;
- the nature of the energy-forming reaction.
These parameters determine the HIT performance properties suitable for a particular application.
Classification of electrochemical elements is based on the difference in the principle of operation of the device. Depending on these characteristics, they distinguish:
- Primary chemical current sources are disposable elements. They have a certain supply of reagents, which is consumed during the reaction. After a full discharge, such a cell loses its functionality. In another way, primary HITs are called galvanic cells. It will be correct to call them simply - element. The simplest examples of a primary power source are "batteries" A-A.
- Rechargeable chemical current sources - batteries (they are also called secondary, reversible HIT) are reusable cells. By passing current from an external circuit in the opposite direction through the battery, after a complete discharge, the spent reagents are regenerated, again accumulating chemical energy (charging). Thanks to the ability to recharge from an external constant current source, this device is used for a long time, with breaks for recharging. The process of generating electrical energy is called battery discharge. These HITs include batteries for many electronic devices (laptops, mobile phones, etc.).
- Thermal chemical current sources - continuous devices. ATin the process of their work, there is a continuous flow of new portions of reagents and the removal of reaction products.
- Combined (semi-fuel) galvanic cells have a stock of one of the reagents. The second is fed into the device from the outside. The life of the device depends on the supply of the first reagent. Combined chemical sources of electric current are used as batteries, if it is possible to restore their charge by passing current from an external source.
- HIT renewable rechargeable mechanically or chemically. For them, it is possible to replace the spent reagents with new portions after a complete discharge. That is, they are not continuous devices, but, like batteries, they are periodically recharged.
HIT Features
The main characteristics of chemical power sources include:
- Open circuit voltage (ORC or discharge voltage). This indicator, first of all, depends on the chosen electrochemical system (combination of reducing agent, oxidizing agent and electrolyte). Also, the NRC is affected by the concentration of the electrolyte, the degree of discharge, temperature, and more. The NRC depends on the value of the current passing through the HIT.
- Power.
- Discharge current - depends on the resistance of the external circuit.
- Capacity - the maximum amount of electricity that the HIT gives off when it is fully discharged.
- Power reserve - the maximum energy received when the device is fully discharged.
- Energy characteristics. For batteries, this is, first of all, a guaranteed number of charge-discharge cycles without reducing the capacity or charge voltage (resource).
- Temperature operating range.
- Shelf life - the maximum allowable time between manufacture and the first discharge of the device.
- Useful life - the maximum allowable total period of storage and operation. For fuel cells, continuous and intermittent service life matters.
- Total energy dissipated over lifetime.
- Mechanical strength against vibration, shock, etc.
- Ability to work in any position.
- Reliability.
- Easy maintenance.
HIT Requirements
The design of electrochemical cells must provide conditions conducive to the most efficient reaction. These conditions include:
- prevent current leakage;
- even work;
- mechanical strength (including tightness);
- separation of reagents;
- good contact between electrodes and electrolyte;
- dissipation of current from the reaction zone to the outer terminal with minimal losses.
Chemical current sources must meet the following general requirements:
- highest values of specific parameters;
- maximum operating temperature range;
- the biggest tension;
- minimum costunits of energy;
- voltage stability;
- charge safety;
- security;
- ease of maintenance, and ideally no need for it;
- long service life.
Exploitation HIT
The main advantage of primary galvanic cells is that they do not require any maintenance. Before you start using them, it is enough to check the appearance, expiration date. When connecting, it is important to observe the polarity and check the integrity of the contacts of the device. More complex chemical current sources - batteries, require more serious care. The purpose of their maintenance is to maximize their service life. Caring for the battery is:
- keep clean;
- open circuit voltage monitoring;
- maintaining the electrolyte level (only distilled water can be used for refilling);
- control of electrolyte concentration (using a hydrometer - a simple device for measuring the density of liquids).
When operating galvanic cells, all requirements relating to the safe use of electrical appliances must be observed.
Classification of HIT by electrochemical systems
Types of chemical current sources, depending on the system:
- lead (acid);
- nickel-cadmium, nickel-iron, nickel-zinc;
- manganese-zinc, copper-zinc, mercury-zinc, zinc chloride;
- silver-zinc, silver-cadmium;
- air-metal;
- nickel-hydrogen and silver-hydrogen;
- manganese-magnesium;
- lithium etc.
Modern application of HIT
Chemical current sources are currently used in:
- vehicles;
- portable appliances;
- military and space technology;
- scientific equipment;
- medicine (pacemakers).
Usual examples of HIT in everyday life:
- batteries (dry batteries);
- batteries for portable household appliances and electronics;
- uninterruptible power supplies;
- car batteries.
Lithium chemical current sources are especially widely used. This is because lithium (Li) has the highest specific energy. The fact is that it has the most negative electrode potential among all other metals. Lithium-ion batteries (LIA) are ahead of all other HEC in terms of specific energy and operating voltage. Now they are gradually mastering a new area - road transport. In the future, the development of scientists related to the improvement of lithium batteries will move towards ultra-thin designs and large heavy-duty batteries.
