A cathode [ kaˈtoːdə ] (also cathode , from ancient Greek κάθοδος káthodos "return", literally "way down") of a component is an electrode at which electrons are fed to a component. A vacuum (see electron tube ), a gas phase (see gas discharge tube ), a plasma or an electrolyte can be present in the component . According to the definition, reduction reactions can take place at this electrode , e.g. B. positive ions ( cations ) are discharged.
The cathode is the counter electrode to the anode . Ions or free electrons migrate between these electrodes . The cations migrate to the cathode and the anions to the anode when the current passes through.
Depending on the electrical polarity between the electrodes, either a positive pole (+) or a negative pole (-) is assigned to a cathode . In the case of voluntary redox reactions , such as when batteries are discharged , the cathode is the positive electrode. In the case of a redox reaction, such as electrolysis , forced by an applied voltage , the cathode is the negatively polarized electrode.
In the case of rechargeable batteries ( accumulators ), the same electrode works either as an anode or a cathode, depending on whether the accumulator is being charged or discharged.
In chemistry , a cathode is the electrode on which a reduction reaction takes place. Electrons are supplied via an electrical conductor and released to the chemical reaction partners via this electrode. The electrochemical reaction always takes place at the phase boundary between the electrode and the electrolyte . The electrolyte can be a solution, an ion-conducting solid or an electrolytic melt.
Examples of cathode reactions :
- Cu 2+ + 2 e - → Cu 0 (s)
- 2 H 3 O + + 2 e - → H 2 (g) + 2 H 2 O
The cathode is the electrode at which electrons are transferred into the surrounding medium ( electrolyte , vacuum , silicon ). In the component under consideration, electrons move from the cathode to the anode in order to then flow from the anode to the cathode through the external electrical circuit. Since the reference direction for the current flow relates to positive charge carriers and is thus opposite to the direction of movement of electrons, the current flows in the external circuit from the cathode to the anode. Within the component under consideration, the current flows from the anode to the cathode; the circuit is closed.
This statement has nothing to do with whether the potential of the anode is higher or lower than the potential of the cathode (in other words: whether the voltage from anode to cathode is positive or negative). For this there are the terms positive pole and negative pole. The potential of the positive pole is always higher than the potential of the negative pole. Therefore the voltage from the positive pole to the negative pole is always greater than zero. The positive pole and negative pole thus indicate the direction of voltage, while the anode and cathode indicate the direction of the current.
In the case of components where the anode has positive voltage compared to the cathode, electrical energy is converted into another form of energy (heat, chemical energy, ...), e.g. B. a diode, a cathode ray tube or an accumulator that is being charged.
In the case of components in which the anode has negative voltage compared to the cathode, electrical energy is given off to the external electrical circuit at the expense of another form of energy (e.g. chemical energy), e.g. B. in a fuel cell or an accumulator that is being discharged.
- If an applied voltage exceeds the material- specific work function of the negative electrode, electrons emerge from the material ( field emission ); the material becomes the cathode at this point.
- The material specific work function is more easily applied when supplying additional energy - for example, by increasing temperature. An important application of this effect is the hot cathode ( glow emission ) in an amplifier tube .
- If additional energy is supplied by irradiation with light, one speaks of a photocathode (see photo effect ).
Very elaborate cathodes can be found in systems for high-voltage direct current transmission , for example in the form of huge copper rings laid in the sea; also in electroplating plants or also in accumulators (e.g. car batteries) and in cathodic corrosion protection .
- International Electrotechnical vocabulary. IEV. Issued by the International Electrotechnical Commission (IEC).
- Wilhelm Gemoll : Greek-German school and hand dictionary. 9th edition. Freytag, Munich et al. 1965.