The question of whether a cathode is negative or positive does not have a single, straightforward answer. The electrical polarity of a cathode, an electrode within an electrochemical system, depends on the specific type of system and its function. Electrochemical systems are setups where chemical reactions either generate electricity or are driven by electricity. These systems invariably involve electrodes, which are conductive components where these chemical transformations take place.
The Fundamental Role of the Cathode
Across all electrochemical systems, a cathode is consistently defined by the chemical process that occurs at its surface: reduction. Reduction is a chemical reaction involving the gain of electrons by an atom, ion, or molecule.
In contrast, the other electrode in an electrochemical cell is the anode, where the opposite process, oxidation, takes place. Oxidation involves the loss of electrons. Therefore, while the anode is always the site of oxidation, the cathode is where reduction occurs.
The movement of positively charged ions, known as cations, is always towards the cathode, regardless of its electrical polarity. These cations accept electrons from the cathode to undergo reduction.
Cathode Polarity in Energy-Generating Systems
In electrochemical systems designed to generate electrical energy spontaneously, such as batteries or galvanic (voltaic) cells, the cathode functions as the positive terminal. These systems convert chemical energy directly into electrical energy through spontaneous oxidation-reduction reactions. A common example is a standard AA battery.
Electrons produced at the negative anode flow through an external circuit towards the cathode. This flow occurs because the cathode possesses a higher electrical potential, which attracts the negatively charged electrons. The influx of these electrons facilitates the reduction reaction at the cathode’s surface.
For instance, in a discharging lithium-ion battery, lithium ions move from the anode to the cathode, where they gain electrons and undergo reduction. This electron flow from the anode to the positive cathode drives the electrical current used to power devices.
Cathode Polarity in Energy-Consuming Systems
Conversely, in electrochemical systems that consume electrical energy to drive non-spontaneous chemical reactions, known as electrolytic cells, the cathode assumes a negative polarity. Electrolytic cells require an external power source to force reactions to occur. This includes processes like electrolysis, which uses electricity to split chemical compounds.
During electrolysis, the external power source actively pumps electrons onto the cathode, making it negatively charged. This forced accumulation of electrons at the cathode drives the non-spontaneous reduction reaction. For example, in the electrolysis of water, electrical energy decomposes water into hydrogen and oxygen.
In this process, hydrogen ions (H+) in the water gain electrons at the negative cathode to form hydrogen gas (H2). The external power supply ensures a continuous supply of electrons to this negative terminal, enabling the reduction to proceed.
Understanding Polarity Versus Process
The apparent contradiction in the cathode’s polarity—being positive in energy-generating systems and negative in energy-consuming systems—stems from confusing its electrical sign with its fundamental chemical role. A cathode is consistently defined as the electrode where reduction occurs (electrons are gained).
The electrical polarity, whether positive or negative, merely reflects the direction of electron flow and the energy dynamics of the system. In galvanic cells, the cathode is positive because it attracts electrons from the spontaneous reaction. In electrolytic cells, the cathode is negative because an external source forces electrons onto it to drive a non-spontaneous reaction. Understanding that reduction is always the defining process at the cathode, regardless of its charge, resolves this confusion.