What Is a Redox Battery?

Redox electrode is a kind of chemical reaction. It is an electrode that completes the reaction through the interface between metal and solution phase.

Name: Redox electrode, redox eleCtrode

The completion of the reaction is achieved by the electron transfer across the metal-solution phase interface

Inserting the Zn flakes into the CuSO4 solution will immediately proceed spontaneously as follows:

Reaction formula

At this time, it was observed that the Zn flakes were dissolved, Cu was continuously precipitated on the Zn flakes, and the blue CuSO ₄ solution became shallow. Zn loses electrons and is oxidized, Cu ²⁺ gets electrons and is reduced. Since the Zn flakes are in direct contact with the CuSO ₄ solution, electrons are directly transferred from the surface of the Zn flakes to Cu ²⁺ in the solution. The flow of the electrons is disordered, and a directed electron flow cannot be generated. The chemical energy contained in the substance cannot be turned into electrical energy, but can only be released in the form of thermal energy, which increases the temperature of the reaction solution.

According to the principle of electricity, if a device can be used to convert the electron transfer in the redox reaction into an orderly electron flow, that is, instead of allowing electrons to transfer directly, the electrons lost by the reducing agent are transferred to the oxidant through the wire , You can get electricity. Daniel batteries are the device that makes this possible. In a Daniel cell, the left side is a ZnSO ₄ solution with a Zn plate inserted, and the right side is a CuSO ₄ solution with a Cu plate inserted. The two solutions are separated by a porous membrane that allows ions to pass freely. When the Zn sheet and the Cu sheet are connected by a series of wires with a galvanometer, the pointer of the galvanometer is deflected, which proves that a current flows through the wires.

This type of device that uses a redox reaction to convert chemical energy into electrical energy is called a primary battery, or battery for short.

It is stipulated in the primary battery that the electron flowing out is extremely negative and the electron flowing in is extremely positive. In a Cu-Zn primary cell, electrons flow from a Zn sheet to a Cu sheet, one end of the Zn sheet is a negative electrode, and one end of the Cu sheet is a positive electrode. Cu ²⁺ in CuSO ₄ solution obtains electrons from Cu flakes, reduces to Cu and deposits on Cu flakes, Zn loses electrons and becomes Zn ²⁺ into ZnSO ₄ solution. On both electrodes

The responses are:

Anode: Zn-2eZn ²⁺ (oxidation reaction)

Positive electrode: Cu ²⁺ + 2eCu (reduction reaction)

The two electrode reaction formulas are added to obtain a battery reaction formula: Zn + Cu ²⁺ Zn ²⁺ + Cu.

Because zinc (or copper) electrodes make up only half of the primary cell, each electrode is also called a half cell. The oxidation (or reduction) reaction occurring at each electrode is called a half-cell reaction or half-reaction. Each half reaction is composed of two substances with different oxidation numbers of the same element. Among them, those with high oxidation numbers are called oxidation types, and those with low oxidation numbers are called reduction types. They form a redox couple, referred to as an electric couple. It is represented by a symbol as an oxidation type / reduction type. Such as electricity to Zn ²⁺ / Zn, Cu ²⁺ / Cu.

Theoretically, any spontaneous redox reaction can be designed as a primary cell. In the redox reaction, the oxidant gains electrons to undergo a reduction reaction, and the reducing agent loses electrons to undergo an oxidation reaction. Therefore, when a primary cell is formed, the pair of oxidants and their reduction products is the positive electrode, and the pair of reductants and their oxidation products is negative electrode.

In the Cu-Zn primary battery, with the progress of the battery reaction, Zn ^{2+ in the} ZnSO ₄ solution continuously increases, which makes the solution positively charged, and the heteropolar attraction makes it difficult for Zn to lose electrons and prevents further oxidation of Zn. At the same time, the CuSO ₄ solution is deposited on Cu chips due to Cu ²⁺ becoming Cu, which causes the SO ₄ ^{2+ in} the solution to be negatively charged excessively. The same-phase repulsion prevents electrons from flowing to the Cu chips, preventing further reduction of Cu ²⁺ . The solution on both sides is charged, which will hinder the battery reaction from proceeding. However, the porous membrane between the two solutions allows ions to pass, and the excess Zn ^{2+ in} the left solution diffuses to the right solution; the excess SO ₄ ²⁺ in the right solution diffuses to the left, maintaining the electrical neutrality of the solutions on both sides, so that The battery reaction continues. Because Zn ²⁺ and SO ₄ ²⁺ diffuse at different speeds through the contact interface between the two solutions, a potential difference will occur at the contact interface between the two solutions, called the liquid junction potential. Its presence will affect the accurate determination of the battery emf . To eliminate the liquid junction potential, salt bridges are often used instead of porous membranes. ^[3]

Salt Bridge Primary Battery