cathode
An electrode, of a cell or other electrically polarized device, through which a positive current of electricity flows outwards (and thus, electrons flow inwards). It can have either a negative or a positive voltage with respect to anode of the same polarized device (depending on whether the device is a load or a source, respectively).
cathode: where electrons enter an electrical device
The cathode is an electrode designed to receive electrons during electrical operation. In any polarized electrical device, current flows out of the cathode into an external circuit, while electrons simultaneously flow into the cathode from that same circuit. This directional flow defines the cathode's role, regardless of whether the device is generating power or consuming it.
The cathode's polarity relative to the anode depends entirely on whether the device acts as a source or a load. In a galvanic cell or battery (power source), the cathode is positive; electrons are released at the anode and travel through the external circuit to enter the cathode. In an electrolytic cell (power consumer), the cathode is negative; an external power supply drives electrons into the cathode, where reduction reactions occur at its surface. This reversal often confuses newcomers, but the definition remains constant: electrons flow in, current flows out.
Common cathode materials and applications
In batteries and electrochemical cells, cathode materials vary by chemistry. Alkaline cells use manganese dioxide; lithium-ion cells use lithium metal oxides such as lithium cobalt oxide or lithium iron phosphate. Electroplating operations use inert cathodes, typically copper or steel, onto which metal deposits build up. Vacuum tubes and cathode ray tubes employ heated filaments as cathodes to emit electrons through thermionic emission; these are sometimes called hot cathodes. Cold cathodes, used in some gas discharge tubes, rely on field emission or secondary emission without applied heat.
Industrial electrochemistry depends on cathode behavior. In metal refining, the cathode collects purified metal deposited from solution. In electrowinning of copper, the cathode is typically a thin copper sheet that accumulates 99.99% pure copper through electrolytic reduction. Surface area and current density affect deposition uniformity; cathodes too small relative to current cause uneven plating, dendritic growth, or powdery deposits. Gas evolution can also occur at the cathode in aqueous systems when water reduction becomes competitive with the desired reduction reaction.
The term cathode derives from the Greek kathodos, meaning descent or way down, because early experimenters observed that the negative pole appeared to be the entry point for electric fluid (electrons, though they did not know it then). The naming is older than electron theory, so the directional meaning predates full understanding of electron flow. In modern practice, always confirm whether you are working with a source or load before identifying which terminal is the cathode; the voltage sign alone can mislead.