Cathodes are generally negatively charged electrodes used in batteries, electrolysis systems, plating, electrowinning, electron emission, and other specialized processes. There exists a misconception that cathodes are always negative, but in fact cathodes polarity mainly depends on the device type, and can even vary according to the mode of operation as per the electric current, direction-based universal definition.
In all electrochemical devices the positively charged cations move towards the cathodes. Cathodes are the electrode of an electrochemical cell at which reduction occurs. Electric current flows out of cathodes, which then attract positive charges or positive ions. Cathodes will give up electrons to positive metal ions, resulting in deposition of the metal ion at the cathodes or a part electrically-connected to the cathodes. Whereas the negatively charged ions move away from the cathodes towards the positive counterpart named the anode.
Cathodes are exclusively used in galvanic cells, which consist of an anode, cathodes, and at least one aqueous electrolyte solution. The electrolyte solution can vary across the system or locally. Galvanic cells are important in batteries, electroplating, and corrosion. Cathodes emit electrons in sputtering systems, cathode ray tubes, and other vacuum tube systems. Other types of equipment in which cathodes are widely used are to release electrons from the cathode by strong electric fields, heat, or by electromagnetic radiation as respectively in field emission, thermionic emission, and photoelectric emission. Cold cathodes and hot cathodes are also common; cathodes used for field emission in vacuum tubes are called cold cathodes, whereas cathodes used as heated electrodes or filaments are called hot cathodes.
Cathodes have several special features. In a sputtering system, positive argon ions impinge on the target attached to the cathode. Target ions are dislodged and transported to the substrate for deposition. Sputtering cathode sources typically incorporate magnetrons to produce strong electric and magnetic fields to trap electrons close to the surface of the target and generate energetic plasma. Fuel cells also contain specialized cathodes and cathode materials. Cathodes are the oxidant electrodes in a fuel cell where oxygen gas (O2) is converted to oxygen ions, which can move through the electrolyte.
Cathodes are widely used in batteries, electrolysis systems, plating, electrochemical devices, fuel cells, electrowinning, electron emission, PVD physical vapor depostion, sputtering, and other specialized processes as well.