Cathode: The Negative Electrode
The Core Concepts: Electrodes and Electron Transfer
To understand the cathode, we must first understand the system it belongs to: the electrochemical cell. This is a device that either generates an electric current from a spontaneous chemical reaction or uses an electric current to drive a non-spontaneous chemical reaction. Every cell has two electrodes: the anode and the cathode.
Electrodes are typically made of a conductive material, like a metal or graphite, and are the points where the electrical circuit connects to the chemical reaction. The chemical reactions happening at these electrodes are called half-reactions.
- Oxidation is the loss of electrons. It always occurs at the anode.
- Reduction is the gain of electrons. It always occurs at the cathode.
A simple mnemonic to remember this is OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
A Tale of Two Cells: Galvanic vs. Electrolytic
The polarity (positive or negative sign) of the cathode is not fixed; it changes based on whether the cell is creating electricity or consuming it. This is the crucial distinction between galvanic (voltaic) cells and electrolytic cells.
| Feature | Galvanic (Voltaic) Cell | Electrolytic Cell |
|---|---|---|
| Function | Converts chemical energy into electrical energy. It is a battery. | Uses electrical energy to drive a chemical reaction. |
| Anode Sign | Negative (-) | Positive (+) |
| Cathode Sign | Positive (+) | Negative (-) |
| Anode Reaction | Oxidation | Oxidation |
| Cathode Reaction | Reduction | Reduction |
| Spontaneity | Spontaneous | Non-spontaneous |
| Example | AA battery, Daniell cell | Charging a battery, electroplating silver |
Notice that in both cases, the cathode is where reduction occurs. The sign of the cathode changes, but its chemical function does not. In a galvanic cell (a battery), the cathode is positive because it attracts the electrons that are being released at the negative anode. In an electrolytic cell (like a battery being charged), the cathode is negative because the external power source is pushing electrons onto it to force a reduction reaction.
The Chemistry of Reduction at the Cathode
Let's look at some specific chemical reactions that take place at the cathode. These reduction reactions always involve a species (an atom or ion) gaining one or more electrons.
Example 1: The Daniell Cell (Galvanic)
This classic cell uses a zinc electrode in zinc sulfate solution and a copper electrode in copper sulfate solution. The copper electrode is the cathode. Here, copper ions from the solution gain electrons and become solid copper metal, which plates onto the electrode. The half-reaction is:
Copper ions gain two electrons to become neutral copper metal atoms.
Example 2: Electrolysis of Water (Electrolytic)
When an electric current is passed through water with a little added electrolyte, it splits into hydrogen and oxygen gas. The cathode is the negative electrode. Here, water molecules gain electrons to form hydrogen gas and hydroxide ions. The half-reaction in a basic solution is:
Water molecules gain electrons, producing hydrogen gas bubbles at the cathode.
Example 3: Recharging a Lithium-Ion Battery (Electrolytic)
When you plug in your phone, the battery becomes an electrolytic cell. The cathode (which was the positive terminal during discharge) is now forced to be negative. Lithium ions ($Li^+$) in the electrolyte migrate to the cathode and are reduced by gaining electrons, embedding themselves back into the cathode material (e.g., lithium cobalt oxide).
Lithium ions and electrons re-enter the cathode material, storing energy for later use.
Cathodes in Action: From Batteries to Electroplating
The principle of reduction at the cathode is not just a textbook idea; it is the driving force behind many technologies we use every day.
1. Batteries Powering Devices: In any disposable or rechargeable battery (AA, AAA, car battery), the cathode is the source of the oxidizing agent that accepts electrons from the external circuit. The specific material used for the cathode defines the battery's voltage, capacity, and properties. Common cathode materials include manganese dioxide ($MnO_2$) in alkaline batteries, lead dioxide ($PbO_2$) in car batteries, and lithium cobalt oxide ($LiCoO_2$) in many li-ion batteries.
2. Electroplating for Beauty and Protection: Electroplating is an electrolytic process used to coat a metal object with a thin layer of another metal. The object to be plated is made the cathode. For example, in silver plating, a spoon is connected as the cathode and placed in a solution containing silver ions ($Ag^+$). When current flows, the reduction reaction $Ag^+ + e^- \rightarrow Ag_{(s)}$ occurs, depositing a shiny, thin layer of metallic silver onto the surface of the spoon.
3. Refining Pure Metals: The same principle is used to purify copper. A large block of impure copper is made the anode, and a thin sheet of pure copper is made the cathode. As current flows, copper from the impure anode oxidizes into solution, and pure copper ions are reduced and deposited onto the pure cathode. Impurities fall to the bottom of the cell.
Important Questions
Q: I always get confused. Is the cathode positive or negative?
A: This is the most common question! The answer is: it depends. The cathode is defined by the reaction (reduction), not its charge. In a battery that is discharging (a galvanic cell), the cathode is the positive terminal. In a battery that is being charged (an electrolytic cell), the cathode is the negative terminal. So, always ask: "Is this cell creating electricity or consuming it?" to determine the sign.
Q: How do electrons and ions move in relation to the cathode?
A: Electrons always flow from the anode to the cathode through the external wire. Inside the cell, in the electrolyte, ions move to balance this charge. Positively charged ions (cations) move toward the cathode to gain electrons and be reduced. Negatively charged ions (anions) move toward the anode to lose electrons. A helpful trick: "CAThodes attract CATions."
Q: Can you have a cell without a cathode?
A: No. An electrochemical cell requires two electrodes to complete the circuit and allow for the separate oxidation and reduction reactions to occur. A cathode, defined as the electrode where reduction happens, is an absolute necessity. Without it, there would be no site for electrons to be consumed, and the flow of electricity would stop instantly.
Conclusion
Footnote
This article uses several scientific terms which are defined here for clarity:
1. Electrode[1]: A conductor through which electricity enters or leaves an object, substance, or region.
2. Electrolyte[2]: A substance that produces an electrically conducting solution when dissolved in a polar solvent, like water. It contains free-moving ions.
3. Galvanic Cell (Voltaic Cell)[3]: An electrochemical cell that derives electrical energy from spontaneous redox reactions taking place within the cell. It is also called a battery.
4. Electrolytic Cell[4]: An electrochemical cell that uses electrical energy to drive a non-spontaneous chemical reaction.
5. Cation[5]: A positively charged ion that migrates to the cathode (negative electrode) in electrolysis.
6. Anion[6]: A negatively charged ion that migrates to the anode (positive electrode) in electrolysis.