Electrochemistry is the study of electron transfer caused by redox reactions that occur at the interface of two types of conductors (electronic conductors, such as metals or semiconductors; ionic conductors, such as electrolyte solutions). Technologies based on electrochemistry include batteries, semiconductors, etching, electrolysis, and electroplating. Electrochemical reactions refer to the conversion of chemical energy into electrical energy in different systems (such as primary batteries, secondary batteries, and fuel cells).
Electrochemical cells and batteries
The electrochemical unit is the smallest device unit that converts chemical energy into electrical energy. A battery usually has multiple electrochemical cells, but it can also refer to a single electrochemical cell. An electrochemical cell includes two different electrodes and electrolyte. Immersion of two electrodes with different potentials in the electrolyte will produce a potential difference, which is what we usually call electromotive force.
The electromotive force is represented by V, which is the potential energy of a unit charge in the electric field, and the electromotive force is the driving force of the current in the circuit. Under the action of electromotive force, each electrode undergoes an oxidation-reduction reaction, and the generated electrons flow through the external circuit. In order to maintain the electrical neutrality of the electrolyte, the redox reaction of the electrode continues until the cell reaches electrochemical equilibrium.
Battery components and electrodes
As mentioned above, a battery (or electrochemical unit) is a device that realizes the mutual conversion of chemical energy and electrical energy through redox reactions.
As shown in the figure are the components of the battery, including the positive electrode, the negative electrode, the electrolyte and the separator to prevent short circuits between the electrodes.
When an electrochemical oxidation-reduction reaction occurs on the electrode, the ions shuttle back and forth between the negative electrode and the positive electrode through the electrolyte. At the same time, electron transfer occurs between the two electrodes. Electrons migrate in the external circuit connecting the two electrodes, thus forming a closed loop.
When the battery is discharged, the electrochemical oxidation of the electrode (hydrogenation reaction, A→A++e-) occurs at the negative electrode. Discharge is a process of converting the battery’s own chemical energy into electrical energy. The electrons from the negative terminal participate in the reduction reaction on the positive electrode through the external circuit (reduction reaction, B++e-→B). The electrolyte acts as an ionic conductor between two electrodes, which is different from an electronic conductor.
For primary batteries, the redox reaction that occurs on the electrode is irreversible, while in secondary batteries, the reaction is reversible and repeatable. The so-called “reversible” here means that the redox reaction can occur repeatedly on the same electrode. One advantage of secondary batteries over primary batteries is that they can be recharged. For secondary batteries, oxidation reaction and reduction reaction can occur on the same electrode, which means that the cathode in the discharge process can be used as the anode in the charging process. But from a traditional point of view, the terminology of the charging and discharging process is the same, that is, the oxidation electrode is the anode during discharge (chemical reaction of self-generation), and the reducing electrode is the cathode.
Full battery and half battery
Electrochemical cells in the form of full cells or half cells are usually used to analyze the electrochemical performance of the battery. The full battery adopts the complete form of the battery, and the electrochemical reaction is carried out on the positive and negative electrodes, which can directly detect the characteristics and performance of the battery. If an additional reference electrode is used, the full battery can obtain the potential difference between the two electrodes by separately measuring the positive and negative electrodes. On the other hand, the counter electrode of a half-cell is often used as a reference electrode to simplify the measurement and analysis of the reaction process under the working voltage, which helps to understand the basic properties of each electrode material. The experimenter can choose a full battery or half battery according to the purpose of the experiment.