Cathode and anode in the primary and secondary battery.

A battery that is being used for power supply is basically a galvanic or voltaic cell. In a cell chemical energy is converted into electrical energy by a process known as an Oxidation -Reduction reaction or redox reaction.

For the working of a battery an anode, cathode, and electrolyte are required. Cathode and anode are often referred to as electrodes. Cathode and anode, are important components of a battery.

Cathode and anode:

A battery that is being used for power supply is basically a galvanic or voltaic cell. In a cell chemical energy is converted into electrical energy by a process known as an Oxidation-Reduction reaction or redox reaction. A battery contains important components including a cathode, an anode, and an electrolyte.

Oxidation is an electrochemical reaction where the loss of electrons by an atom, ion, or molecule takes place whereas reduction is an electrochemical reaction where a gain of electrons by an atom, ion, or molecules happens.

An anode is an electrode where an oxidation reaction happens where as a cathode is an electrode where a reduction reaction happens. A substance that oxidizes other is called an oxidizing agent and a substance that reduces others is called a reducing agent or reductant. We may say an oxidant is an acceptor of electrons and a reductant is a donor of electrons.

During discharge positive electrode is a cathode and the negative electrode is an anode whereas during charge the above trend is just reversed. Here positive electrode is the anode and the negative electrode is the cathode. Thus in a battery, both reactions can happen on the same electrodes.

Primary battery: cathode, and anode:

A primary battery is one cell, where a redox reaction occurs once, and after use over a period of time, it becomes dead without any scope of further reuse. Normally such dry cell is used in clocks, watches, transistors, etc. Then what is the role of the cathode and anode in such a cell? Let us discuss this.

Zinc carbon cell:

In such a battery a zinc container act as an anode and a carbon/graphite rod act as a cathode. The space between the electrodes is filled with the electrolyte paste of ammonium chloride (NH₄Cl) and zinc chloride(ZnCl₂).

• During the process, the zinc container acts as an anode and undergoes an oxidation reaction.

Anode: Zn(s)→Zn²⁺+2e-

• In the process, the carbon rod act as a cathode, and the electrolyte paste of ammonium chloride (NH₄Cl) and zinc chloride(ZnCl₂) facilitates the reduction reaction.

Cathode: Mn+4O₂+NH₄⁺⁺e^–→Mn+NH₃+3O(OH)

Zn+2+NH3→[Zn(NH₃₎+1(aq)

During the process of reaction at the cathode, the manganese is reduced from a +4 oxidation state to a +3 oxidation state. The ammonia produced in the process forms a complex structure with Zn²⁺ions to give [Zn(NH₃₎]2⁺. Such a cell has an electric potential of 1.5V.

Mercury cell:

The mercury cell or mercury battery also referred to as mercuric-oxygen battery is a primary electrochemical cell. A cell made up of the reaction of mercury oxide and zinc electrodes with an alkaline electrolyte. On this occasion, zinc-mercury amalgam act as an anode and HgO & carbon paste as a cathode. The paste of KOH(potassium hydroxide) and ZnO served as electrolytes. The chemical reactions at the cathode and anode are as follows:

Anode-Zn(Hg)+2OH^–→ZnO(s)+H₂O+2^e-

Cathode-HgO+H₂O+2e^–→Hg(1)+2OH^–

The overall reaction is presented as follows;

Zn(Hg)+HgO(s)→ZnO(s)+Hg(1)

The mercury cells are suitable for low-current devices like hearing aids, watches, etc., and produce approximately 1.35V that remains constant throughout the lifetime.

Secondary battery; Cathode and anode:

The secondary battery or secondary cell is reusable and rechargeable. A good quality secondary cell can go through charging and discharging cycles numerous times. The important secondary cells include lead storage/lead-acid accumulator, nickel-cadmium (NiCD), lithium-ion (Li-ion), Nickel Zink, etc.

Lead storage/lead-acid accumulator,

The lead storage battery is regarded as the oldest type of rechargeable energy storage cell and still finds its applications in inverters and automobile sectors. Such batteries are reliable but heavy to transport. The lead cell has specific life and toxic material it contains and needs expertise for removal after expiry. In such batteries lead foil acts as an anode and a grid of lead packs combined with lead dioxide(PbO₂₎ as the cathode. Sulphuric acid serves as an electrolyte.

• The electrochemical reactions during discharge are observed at the cathode and anode as follows;

Anode-Pb(s)+HSO₄^–(aq)→PbSO₄(s)+H⁺(aq)+2e^–

Cathode-PbO₂(s)+HSO₄-(aq)+3H⁺(aq)+2e^–→PbSO₄(s)+2H₂O(1)

The overall reaction at the cathode and anode during discharge can be written as;

Pb(s)+PbO₂(s)+2H⁺(aq)+2HSO₄^–(aq)c→2PbSO₄(s)+2H₂O(1)

• During the charging process such a cell witnesses the reverse process of electrochemical reaction.

Anode-PbSO₄(s)+2H₂O(1)→PbO₂(s)+HSO₄^–(aq)+3H⁺(aq)+2e^–

Cathode-PbSO₄(s)+H⁺(aq)+2e^–→Pb(s)+HSO₄^–(aq)

The overall reaction at the cathode and anode can be written as;

2PbSO₄(s)+2H₂O(1)→Pb(s)+PbO₂(s)+2H⁺(aq)+2HSO₄^–(aq).

Nickel-cadmium cell:

Nickel-cadmium battery is another secondary cell having advantages over a lead-acid cell of being small, compact, and potable. Generally, such types of cells are used in toys, calculators, small DC motors, etc. Nickel-cadmium cell has a longer life than lead storage cell but is expensive to manufacture. The operating principle of such a cell is the same as a lead storage cell.

In such a battery, a layer of nickel oxide (NiO₂) is kept around the redox as the first layer. This layer of NiO₂ acts as a cathode layer. Above the first layer, another layer of NaOH/KOH has pasted that acts as a separator.

The separator is needed to be soaked in water or moist for the supply of required OH negative ions for reactions. Another layer of cadmium is kept above the separator, which acts as the anode. The battery is a closed chamber and is never oped for its hazardous effects. The maximum cell voltage during charge may be 1.3V and the average cell voltage may be 1.2V.

• The chemical reaction at the cathode and anode during charge and discharge is represented as follows;

Cathode-2NiOOH+2H₂O+2e^–→2Ni(OH)₂+2OH^–

Anode-Cd+2OH^–→2Cd(OH)₂+2e^–

The overall reaction at the cathode and anode can be written as;

2NiOOH+Cd+2H₂O↔2Ni(OH)₂+Cd(OH)2.

Lithium-ion battery:

The lithium-ion battery is the center of attraction nowadays. Such rechargeable batteries are used in our smartwatches, electric vehicles, laptops, consumer electronics, etc. The battery has both a positive electrode(cathode) and the negative electrode(anode) made of metal and electrolyte.

In such batteries, lithium ions move from the negative electrode to the positive electrode during discharge and when charged the lithium ions are removed from the cathode and travel to the anode through the electrolyte, and energy is stored. In such batteries, electrolytes are non-aqueous solutions.

• The chemical reactions at the cathode and anode are written below;

Positive electrode-LiMO₂↔Li_1-xMO₂+Li+xe^–,M=transit metal.

Negative electrode-C+xLi⁺+xe^–↔Li_xC.

The overall reaction can be written as;

LiMO₂+C ↔ Li_xC+Li_1-xMO₂.

Nickel Zink battery:

Nickel zink cells are rechargeable, non-flammable, and eco-friendly. In a battery, zink acts as a negative electrode(anode) and nickel dioxide is a positive electrode(cathode). Potassium hydroxide(KOH) serves the purpose of an alkaline catalyst or electrolyte. Both nickel and zink are easy to recycle and the cell is fast charging. The working principle is the same as other nickel-metal batteries.

• The electrochemical reaction of such batteries at the cathode and anode is calculated below;

Positive electrode:

2NiO(OH)+2H₂O+2e^–↔2Ni(OH)₂+2OH^–

Negative electrode:

Zn+4OH^–↔ Zn(OH)₄^2-+2e^–

Overall reaction may be written as;

Zn+2NiO(OH)+H₂O ↔ZnO+2Ni(OH)₂.

Conclusion:

The cathode and anode are referred to as electrodes. An anode is an electrode where an oxidation reaction happens and a cathode is an electrode where a reduction reaction happens. In other words, we may say at electrodes redox reactions happen. It is a fact that during discharge positive electrode is the cathode and the anode is the negative electrode however during charge such a trend reversed.

Therefore it is better to refer to as positive electrode and negative electrode instead of cathode and anode or anode and cathode. In the case of primary and secondary batteries, such perceptions are true regarding chemical reactions for better understanding.