Electrode

An electrode is an electrical conductor used to establish contact with a nonmetallic part of a circuit. These nonmetallic parts can include semiconductors, electrolytes, vacuums, or even gases.

Electrodes are essential components in electrochemical cells, which are systems that convert chemical energy into electricity. Because they facilitate the movement of charge, they are fundamental to almost all modern battery technology. The specific material used for an electrode depends on the type of cell being used.

In an electrochemical cell, electrodes are categorized as either an anode or a cathode. This naming is based on the direction of the conventional electric current, not the potential difference. The anode is the electrode where conventional current enters the nonmetallic part of the cell.

At the anode, an oxidation reaction occurs, causing electrons to flow away from it. This makes the anode have a negative electric potential. The cathode is the opposite electrode, where the conventional current exits the cell.

At the cathode, a reduction reaction takes place as electrons are absorbed by an oxidizing agent. This makes the cathode the positive electrode in the system.

There are two primary types of electrochemical cells: primary and secondary cells. A primary cell is designed for single use and is then discarded. This is because the chemical reactions occurring at the electrodes are not reversible. A common example is the alkaline battery used in flashlights, which features a zinc anode and a manganese oxide cathode.

These cells can sometimes be recharged, but manufacturers often advise against it because the zinc oxide formed during use can clump, making the discharge less efficient.

Secondary cells are different because they can be recharged through reversible reactions. The first secondary cell was the lead–acid battery, invented by French physicist Gaston Planté in 1859. This battery uses a lead anode and a lead dioxide cathode.

It remains a widely used technology in automobiles today. Other modern rechargeable options include nickel–cadmium, nickel–metal hydride, and lithium-ion batteries.

The history of the electrode involves many important scientific discoveries. In 1762, Johan Wilcke invented the electrophore to study static electricity. In 1833, Michael Faraday coined the term "electrode," which draws from the Greek words for "amber" and "path." Alessandro Volta later devised the first electrochemical battery, known as the Voltaic cell, using stacks of copper and zinc electrodes separated by brine-soaked paper. However, the Voltaic cell had fluctuating voltage. This led John Frederic Daniell to invent the Daniell cell in 1839, which was the first practical battery and also used a zinc–copper combination.

To understand how electrons move, scientists use Marcus theory. Developed by Nobel laureate Rudolph A. Marcus, this theory explains the rate at which an electron "jumps" between a chemical species and an electrode. This process must follow the law of conservation of energy and the Franck-Condon principle. While classical theory suggests electron transfer stops at near-zero temperatures, the displaced harmonic oscillator model explains why transfer still happens via quantum tunneling.

This shows that electron transfer is a complex interaction between the reactants and the surrounding medium.

Manufacturing an electrode, such as those in lithium-ion batteries, requires precision. It begins by mixing active electrode particles, a conductive agent, and a binder into a solvent to create an "electrode slurry."

This slurry is then coated onto a current collector, typically using aluminum for the anode and copper for the cathode. The coating is dried and pressed to a specific thickness. For the electrode to be efficient, the components must be spread evenly to avoid clumping. High efficiency depends on minimizing contact resistance and ensuring the active material adheres well to the collector so it does not dissolve into the electrolyte.