Articles discussing new battery electrodes often employ the terms anode and cathode without indicating whether the battery is discharging or charging. However, it is important to note that the terms anode, cathode, positive, and negative electrode are not interchangeable and may at times be misunderstood. This can result in errors which can be avoided by proper understanding and use of the correct terminology.
This article aims to provide clarity and a precise definition of these distinct terms.
What Is an Electrode?
Before delving into the specifics of cathodes and anodes, it's crucial to grasp the fundamental concept of an electrode. At its core, an electrode facilitates the flow of electricity, allowing electric current to pass through a non-metallic substance, such as an electrolytic cell.
In essence, an electrode acts as a a conductor to make electrical contact with a non-metallic component of the circuit (e.g. a semiconductor, an electrolyte, a vacuum or air).
Positive and negative electrodes
The electrodes of a battery or accumulator possess varying potentials. The electrode that has a higher potential is denoted as positive, while the one with a lower potential is labeled as negative. The battery's electromotive force (emf), which is expressed in volts (V), corresponds to the disparity between the potentials of the positive and negative electrodes when the battery is inactive.
Anode vs. Cathode
When discussing anodes or cathodes within a cell or circuit, it is necessary to take into account the two primary reactions: oxidation and reduction. An oxidation reaction is an electrochemical process that generates electrons (loss of electrons for the electroactive species), while a reduction reaction is an electrochemical process that requires consumption of electrons (gain of electrons for the electroactive species).
An anode is an electrode where an oxidation reaction takes place.
A cathode is an electrode where a reduction reaction takes place.
Is the anode always the positive electrode, or is the cathode always the negative electrode?
The answer is not always straightforward. It ultimately depends on whether the battery is being charged or discharged.
During the battery discharge, the present electric current travels from the battery's positive electrode through an external circuit until it reaches the negative electrode, while electrons move in the opposite direction. The underlying reason is that the negative electrode undergoes an oxidation reaction, which is also referred to as the anode. This reaction produces electrons that flow through the external circuit and reach the battery's positive electrode, which is known as the cathode.
During a battery charge, electrons travel from the battery's positive electrode, pass through the charger, and reach the negative electrode of the battery. In order to understand its underlying mechanisms, the positive electrode undergoes an oxidation process, commonly called the anode, that produces electrons which travel across the charger towards the negative electrode, also known as the cathode.
Conclusion
In the typical usage of a rechargeable battery, the positive electrode consistently maintains a higher potential than the negative electrode throughout both the discharge and recharge processes. However, their respective roles are reversed during the discharge/charge cycle.
