The Etymology and Meaning of Anode and Cathode

The terms “anode” and “cathode” were first published by Michael Faraday, F.R.S. in 1834. A delightful (and highly recommended) historical account of how these words were conceived by Faraday and his associates can be found in Faraday Consults the Scholars: The Origins of the Terms of Electrochemistry by Sydney Ross [1]. As scientists have learned about how electrochemistry works, the definitions have evolved somewhat. The following is a brief summary of their etymology and their meaning as it stands today in electrochemical circuits. In the course of my own study of electrochemistry, I thought other students may find this information helpful in keeping everything straight.

A galvanic cell powering a robot.

The word anode comes from the Greek ἄνοδος (anodos), meaning ‘ascent’ or ‘the way upward’ [1], which today refers to the movement of electrons up into the circuit after being released from the electrode material by oxidation. Meanwhile, in a galvanic cell, the positive ions resulting from the same oxidation enter into the electrolyte solution. Thus, the anode is the electrode where oxidation occurs [2]. The anode is also called the negative terminal in a galvanic cell, or the positive terminal in an electrolytic cell.

The word cathode comes from the Greek κάθοδος (kathodos), meaning ‘descent’ or ‘the way downward’ [1], which today refers to the movement of electrons down from the circuit into the electrode where they facilitate electrochemical reduction of the positive ions in the electrolyte solution at the surface of the cathode. Thus, the cathode is the electrode where reduction occurs [2]. The cathode is also called the positive terminal in a galvanic cell, or the negative terminal in an electrolytic cell.

It can be confusing that the anode and cathode can be either the negative or positive terminal, depending on whether the cell is galvanic or electrolytic. They key point to remember is that a galvanic cell produces electrical energy, while an electrolytic cell consumes electrical energy. The flow of electrons is always from anode to cathode, and current moves opposite to the flow of electrons. If the current is flowing out of the cell to power a circuit, it flows out from the cathode so the cathode is positive. If the current is flowing into the cell from a power source, then it flows into the anode, and the anode is positive.

Notes

1. Ross, S. Notes and Records of the Royal Society of London 1961, 16, 187–220.
2. McNaught, A.D.; Wilkinson, A.; Jenkins, A.D.; International Union of Pure and Applied Chemistry, IUPAC Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”); Blackwell Scientific Publications: Oxford, 1997.