1. The working principle of primary battery
A primary battery is a device that converts chemical energy into electrical energy. The main conditions for its formation are:
- Redox reactions that can occur spontaneously
- Two electrodes with different activity
- Ionic compounds in electrolyte solution or molten state
The anode of primary battery is usually highly mobile metals. Electrons are lost when oxidation occurs. The cathode is composed of less active metals or non-metals that can conduct electricity, and a reduction reaction occurs.
The electron flow direction of the circuit outside the primary battery is: the anode provides electrons, and electrons flow from the anode along the wire to the cathode (the direction of current is the opposite direction of electron flow).
The electron flow direction of the inner circuit is that the anode is continuously dissolved in the electrolyte solution, the anode ion moves to the node, and the cathode ion moves to the cathode.
2. The cathode and anode of primary battery
For a complete primary battery, there is an external circuit, but no external power supply. It should be noted that cathode and anode are distinguished here according to the direction of electron inflow and outflow, or cathode and voltage anode.
A primary battery anode voltage is generated on the cathode, which attracts electrons to flow in. The accumulation of too many electrons on the electrode attracts cation migration, causing substances in the solution near the electrode to obtain electrons, resulting in a reduction reaction. Anode and vice versa.
In the primary battery, the anode and cathode poles need to maintain a large chemical potential difference when there is no loop. When the circuit is formed, a faster redox reaction can occur immediately on both poles.
One pole produces electrons, and the other pole absorbs electrons, forming a voltage difference between the cathode and anode to generate electrical energy.
● Lifetime of electrode material
The more active metal is the anode, and the less active or non-metal is the cathode. The direction in which electrons or current flow The electron flow direction is anode to cathode, and the current direction is reversed.
● Electrode changes
The oxidation of the electrode is an anode, and conversely, the reduction reaction is a cathode.
If the electrode is dissolved and the mass is reduced, it is an anode, and the weight of the electrode increases or bubbles are formed. The direction in which ions move within the electrolyte cathode ions in the electrolyte move to anode, and anode ions move to cathode.
However, it should be noted that the liveliness of metals is affected by the environment in which they are located. For example, after Mg is connected to Al, it is put into hydrochloric acid, Mg is anode, and Al is cathode.
Put into NaOH solution, Al is anode, and Mg is cathode. Fe and Cu are connected, immersed in dilute HNO3, Fe is anode, Cu is cathode. Immersed in concentrated HNO3, Cu is anode (Fe passivation) and Fe is cathode. At this time, the cathode and anode will change.
3. Corrosion and protection of metals
● Corrosion of metal
The corrosion of the metal can cause battery corrosion. Understanding the nature and different situations of metal corrosion in primary battery can help understand the application of electrochemical principles in real-life production.The essence of metal corrosion is the process by which metal atoms such as iron lose electrons and are oxidized into metal cations, which can be divided into chemical corrosion and electrochemical corrosion.
- Chemical corrosion
Corrosion caused by a direct chemical reaction caused by contact between metal and an oxidant (generally non-electrolyte) is called chemical corrosion. For example, chemical corrosion occurs when iron comes into contact with chlorine.
- Galvanic corrosion
More commonly, the most serious hazard is galvanic corrosion. That is, corrosion is caused by the reaction of primary battery between impure metals or alloys and electrolyte solutions. For example, hydrogen evolution corrosion of primary battery occurs in steel under the condition of strong acidity in the water film to produce hydrogen.
Under the condition that the water film is very acidic or neutral, oxygen absorption corrosion occurs. This is oxygen absorption corrosion and hydrogen evolution corrosion, which are the main types of corrosion and are extensive but occur in some local areas. This tends to corrode the more active metals.
● Protection of metals
Chemical corrosion and electrochemical corrosion often occur at the same time, but electrochemical corrosion is more common than chemical corrosion, and the rate of electrochemical corrosion is faster and the harm is more serious. However, protection may also be performed according to the characteristics of the reaction.
- Prevent chemical corrosion
Cover the metal surface with paint or form a dense oxide film and other protective layers to isolate oxidants.
- Prevent galvanic corrosion
Using the principle of primary batterys, the protected metal acts as a cathode. The active metal connected to it then acts as an anode. For example, to protect a steel bridge, it can be connected to a zinc block so that zinc acts as an anode for galvanized cells. You can also choose to change the internal structure of the metal to enhance corrosion resistance. For example: stainless steel
4. Application of primary battery principle
Depending on the phenomenon of reaction and electron migration during primary battery reaction, primary battery have many applications, including:
- Produce a variety of chemical power sources.
Such as dry batteries, storage batteries, high-energy batteries, fuel cell. Although it is made from the original battery, but each battery is different, such as agm vs lead acid.
- Speed up chemical reactions.
For example, the reaction rate of pure zinc with hydrochloric acid to produce H2 is slower. If a few drops of CuCl2 solution are dropped, the replaced copper is tightly attached to the zinc surface, forming many tiny primary batterys, which can greatly accelerate the chemical reaction.
- Electrochemical protection of metals, cathodic protection method of sacrificing anodes.
- Determination of metal mobility.
● Application of the electrolysis principle
The preparation of substances, such as electrolytic saturated saline solutions, can produce hydrogen, chlorine, and caustic soda. The process of applying the principle of electrolysis to plating a thin layer of other metals or alloys on the surface of some metals or nonmetals.
When electroplating, the plated part is used as the cathode, the coated metal is used as the anode, and the salt solution containing the coated metal cation is selected as the electrolyte solution.
Electrolysis can also refine copper: fine copper as cathode, crude copper as anode, copper sulfate as electrolyte solution, crude copper dissolved anode, copper precipitated cathode, Cu2+ concentration in solution decreased.
5. Comparison of primary battery and electrolytic tank
● Chemical battery
Chemical battery is a device that converts chemical energy directly into electrical energy. Chemical batteries can be divided into primary batteries, secondary batteries, and fuel cells.
● Primary battery
Common primary battery: alkaline zinc-manganese batteries, zinc-silver batteries, lifepo4 battery, etc. The secondary battery can be recharged after discharge to regenerate the active material, can be reused many times; also known as rechargeable battery or battery.
Fuel cell is a kind of primary battery that reacts fuel with an oxidizer to directly generate an electric current. Generally, the end product of the electrochemical reaction that occurs in a fuel cell is the same as the combustion product. The advantages of fuel cells are a high energy conversion rate, less waste, and low operating noise.
Then, for a complete electrolytic cell, there are four elements: cathode, anode, external power supply, and electrolyte, which form a loop. The cathode of the electrolytic cell is connected to the power supply anode, and electrons are obtained, and a reduction reaction occurs.
On the contrary, the anode is connected to the power supply cathode, which loses electrons and undergoes oxidation. The object of electron gain and loss on the cathode and anode refers to the electrode itself. However, with the rapid development of electrochemistry, the situation that the electrode itself does not undergo a redox reaction gradually appears.
At this time, the object of redox refers to the substance in the solution near the electrode, which can be ions, molecules, and is deposited or attached to the electrode after reduction. Chemical energy refers to the energy change that occurs when a substance undergoes chemical changes.
Therefore, in the electrolytic cell, an external power supply is required to apply an electric charge to the cathode and anode, and the electrical energy generated by these charges causes the energy change of the substance on the cathode and anode by promoting chemical changes in the matter, which is the electrolytic cell.
It can be seen that the differences between electrolytic cells and primary battery mainly include:
● External power supply
The electrolytic cell has an external power supply, while the primary battery has no external power supply and has its own power supply. As long as the system is processed by relying on the electrical energy generated by the external power supply, the basic principle of the electrolytic cell can be used to deal with various problems.
● Cathode and anode
The relationship between electrodes in primary battery can be easily remembered based on similarities and differences. There is an external power source to generate electrical energy to promote the redox reaction on the electrode, and the electrode connected to the cathode produces electrons.
The electrode connected to the anode absorbs electrons, so it is an electron reaction at the cathode. The primary battery has no power supply and rely on spontaneous redox reactions on electrodes to provide electrical energy to external circuits. The cathode is the inflow of electrons, which causes the electrode to undergo an electron reduction reaction.
The anode is the outflow of electrons, which promotes the electron-loss oxidation reaction of the electrode. Practical application In fact, all kinds of electrocatalysis are the most typical electrolytic cells.
Through the electrochemical workstation in primary battry, an external power supply is applied to the electrochemical system to promote the transformation of electrical energy into chemical energy in the system. The transformation of chemical energy is accompanied by the redox reaction, and the product obtained by the catalytic reaction, that is, the phase transition, is a way of chemical energy generation and change.
Then, after knowing that electrocatalysis is an electrolytic cell, the analysis of the cyclic voltammetry curve displayed by this system becomes clearer, and the electroreduction looks at the reduction peak, and the electrooxidation looks at the oxidation peak. If a redox peak occurs at the same time, it must be a reversible redox reaction on the electrode.