An electrode is a conductor that allows electrical current to pass into or out of a non-metallic medium, such as an electrolyte or a vacuum. The cathode is one of the two main types of electrodes, operating as the destination point for positive charge flow in an electrical circuit. Its function is central to modern power storage and electronics, enabling the chemical reactions that generate or consume electrical energy.
Defining the Cathode: Electrons and Reduction
The most precise definition of a cathode is based on the chemical reaction that takes place at its surface, not its electrical charge. The English scientist Michael Faraday, who coined the term in 1834, derived it from the Greek word kathodos, meaning “way down” or “descent,” originally describing the path of positive current. Today, the cathode is definitively the electrode where reduction occurs in an electrochemical cell.
Reduction is a fundamental chemical process involving the gain of electrons by a chemical species, often resulting in a decrease in its oxidation state. For example, a positively charged ion in an electrolyte solution will migrate to the cathode, accept electrons supplied by the circuit, and become a neutral atom or a less-positively charged ion. This constant chemical role is summarized by the mnemonic “Red Cat,” meaning “Reduction at the Cathode”. The flow of electrons from the external circuit into the cathode drives this reduction reaction.
The Polarity Paradox
A common source of confusion is that a cathode can be either positively or negatively charged, depending on the type of electrochemical cell and its operating mode. While the chemical process of reduction is always constant at the cathode, its electrical sign flips based on whether the cell is producing or consuming energy. This difference is determined by whether the cell is a galvanic cell or an electrolytic cell.
In a galvanic, or voltaic, cell—such as a standard battery during discharge—the chemical reaction occurs spontaneously to generate electrical current. During this process, the cathode is the positive terminal because it attracts electrons from the external circuit to complete the spontaneous reduction reaction.
Conversely, in an electrolytic cell, an external power source is used to force a non-spontaneous chemical reaction to occur, such as when charging a battery or performing electroplating. Here, the external power supply forces electrons onto the cathode, making it the negative terminal. Despite the negative sign, the cathode still serves its chemical role by supplying electrons for the reduction reaction.
Cathodes in Modern Technology
The cathode’s function as the site of reduction is harnessed in a wide variety of modern electrical and electronic devices. The most prevalent example is the lithium-ion (Li-ion) battery, which powers everything from smartphones to electric vehicles. During the battery’s discharge phase, the cathode acts as the positive electrode and is the destination for positively charged lithium ions moving through the electrolyte.
The active material in the Li-ion cathode is typically a lithium metal oxide, such as Lithium Cobalt Oxide (\(\text{LiCoO}_2\)), Lithium Iron Phosphate (\(\text{LiFePO}_4\), or LFP), or Lithium Nickel Manganese Cobalt Oxide (NMC). These materials are chosen for their ability to efficiently store and release lithium ions, which is directly related to their capacity to undergo the reduction and oxidation reactions. The specific chemistry of the cathode material directly influences the battery’s energy density, power output, and safety profile.
Cathodes also play a role in semiconductor devices like diodes, which are components that only allow current to flow in one direction. The cathode terminal of a diode is the point where the conventional current exits the device, consistent with the positive electrode role in a discharging cell. The directional flow of charge in a diode, from the anode to the cathode, relies on the principles of electrochemistry, ensuring the device functions as a one-way electrical valve.