Since I know that this is a delicate topic, and many get confused, I will explain very briefly the nomenclature that I used. The IUPAC defines the anode as the electrode at which the oxidation occurs and the cathode as the electrode at which the reduction occurs. Therefore, in a galvanic cell, the negative electrode is oxidized, and the positive one is reduced, while in an electrolytic cell the positive one is oxidized and the negative one is reduced. If you are confused by the electrolytic cell nomenclature, it means you use the chemical nomenclature. I will make a separate poster on the difference between the chemical and engineer nomenclature.
Spontaneity A galvanic cell converts chemical energy to electrical energy without the need for an external source. The RedOx reaction at both electrodes happens spontaneously because the negative electrode has a lower potential (higher energy level of electrons) while the positive electrode has a higher potential (lower energy level of the electrons).
Electron Flow The electrons flow spontaneously from the negative electrode to the positive electrode.
Anode In a galvanic cell, the anode is the negative electrode, which gets oxidized. In the case of a Daniel Cell, the zinc anode is oxidized; it releases electrons into the external circuit and zinc ions into the solution. The metallic zinc is thus consumed throughout the reaction.
Galvanic
Salt Bridge To prevent the copper ions from depositing on the zinc, two separate cells need to be used. To keep the reaction from stopping, a salt bridge is necessary for a Daniell Cell. The salt used is usually potassium chloride (KCl) because the cation and anion have a similar ion mobility, thus maintaining the electroneutrality of the solutions. For intercalation-type materials, a salt bridge is not necessary.
Cathode In a galvanic cell, the cathode is the positive electrode, at which the reduction occurs. In this specific case, a Daniel Cell, the positive electrode material is metallic copper. In solution, a copper salt is necessary to keep the reaction going. The metallic copper spontaneously gains electrons from the external circuit. The copper ions in solution are then reduced on the metal’s surface. The copper metal thus grows with the advancement of the reaction
Current Convention: Historically, the current was thought to be made of positive charges. For this reason, the conventional current is opposite to the electron flow.
Spontaneity An electrolytic cell converts electrical energy to chemical energy with the employment of an external source. The RedOx reaction at both electrodes has to be forced backwards due to the nature of the materials’ electrochemical potentials.
Electron Flow The electrons flow non-spontaneously from the positive electrode to the negative electrode. This is achieved by the use of an external energy source, like a battery or a potentiostat / galvanostat.
Cathode In an electrolytic cell, the cathode is the negative electrode, which gets reduced. In the case of a Daniell Cell, the zinc is forced to gain electrons, and the zinc ions in the solution are reduced at the metal’s surface. Thus, the zinc cathode, in the specific case of a Daniell Cell, will grow throughout the reaction.
Electrolytic
Salt Bridge In the Daniell Cell case, a salt bridge is also required during recharge to prevent the chemistries from mixing. A salt bridge is unnecessary whenever no soluble electroactive species can cross between the electrodes and react parasitically, e.g., when the redox centres stay immobilised in the solid phase, escape as gas, or are common to both half-reactions.
Anode In an electrolytic cell, the anode is the positive electrode, at which the oxidation occurs. In the specific case of a Daniel Cell, the electrons are forced out of the metallic copper, which releases copper ions into the solution. Unlike the galvanic cell, the copper metal in an electrolytic cell is consumed.
A summary where you can quickly see the characteristics of both cells:
| Galvanic | Electrolytic | |
| Energy transformation | Chemical to Electric | Electric to Chemical |
| Spontaneity / ∆G | Spontaneous / ∆G < 0 | Non-Spontaneous / ∆G > 0 |
| Cell potential | Positive | Negative |
| Electron Flow | Negative to positive | Positive to negative |
| Anode polarity | Negative | Positive |
| Cathode polarity | Positive | Negative |
| Number of compartments | Two | One or Two |
| Salt bridge | Depends on materials | Depends on materials |
| Examples | Daniell cell, any discharging battery, fuel cells | Electroplating, water electrolysis, charging battery |
If you want to go deeper, Sara has a full video on YouTube where she explain step by step both cells:
You can download the poster as a pdf file here: Galvanic Cell vs. Electrolytic Cell
Or read a breakdown of the subject on the Saiens Academy blog: Galvanic vs. Electrolytic Cells: How To Stop Confusing Anode and Cathode
| www.saiensacademy.com | Email: sara@saiensacademy.com |
| @saiensacademy – all rights reserved | LinkedIn: www.linkedin.com/in/sara-sfiligoi |
See also: Standard Electrode Potentials
Regarding your poster, in the section about electrolysis: electrons are removed from the copper metal (anode). Since the current flows in the opposite direction of the electrons, isn’t the battery polarity diagram drawn reversed? I have the same diagram in an electrochemistry book, with the battery oriented the other way for electrolysis.
Hello Nicolas,
You are absolutely right. I drew it reversed. I will update the image ASAP. Thank you very much for noticing it!