Author: Jessica Zhang
Editors: Max Ye and He-Hanson Xuan,
Artist: Susan Wu
Here is a simple method to turn fruits into batteries: insert a piece of copper and a piece of zinc on opposite sides of a lemon, then attach each plate with a wire and connect them to an ammeter. The movement of the pointer indicates the presence of an electric current. With a few more units of these fruit batteries, you can power a LED! To explore the secret behind the burst of energy from fruits, we need to understand the principles of galvanic cells.
In oxidation-reduction (redox) reactions, a transfer of electrons occurs between two species ‒ an oxidizing agent and a reducing agent. Electrons are transferred from the reducing agent, which loses electrons, to the oxidizing agent, causing it to gain electrons. However, if the reducing agent and the oxidizing agent are physically separated, there must be an external conducting medium for the transfer of electrons and a shared electrolyte for the transfer of ions. As the redox reaction progresses, the movement of electrons generates electricity. In the galvanic cell, there are two kinds of electrodes: the anode and the cathode. Oxidation (i.e., loss of electrons) occurs at the anode, which is made up of a reducing agent, while reduction (i.e., gain of electrons) occurs at the cathode, which is made up of an oxidizing agent. To complete the electrical circuit, both the anode and cathode must be inserted into connected electrolyte solutions (i.e., salt bridge). So in the solutions, cations flow towards the cathode, while anions flow towards the anode. Otherwise, the accumulation of positive charge in the anode and negative charge in the cathode will soon prevent the cell from operating.
The same principles apply to the previous fruit batteries experiment. Copper and zinc metals are electrodes, and the citric acid found in fruits serves as electrolytes. Since zinc is more reactive than copper, zinc acts as the reducing agent, losing electrons:
Zn(s) → Zn2+ (aq) + 2e-
The electrons then travel through the wire to the piece of copper, causing the copper atoms within to gain additional electrons. Since copper atoms cannot act as oxidizing agents and be reduced, gaining electrons, the hydrogen ions in the fruit accept the electrons:
2H+ (aq)+ 2e- → H2 (g)
So during the reaction, the metallic zinc at the surface of the zinc electrode dissolves into the solution, and the hydrogen molecules form at the surface of the copper electrode. The electrons flow externally from the anode (i.e., zinc plate) through the wire to the cathode (i.e., copper plate), generating electricity.
When a galvanic cell or a series of combined galvanic cells can provide a source of electric current at a constant voltage, batteries are formed. Humans have employed several kinds of batteries in widespread use. The most common one, the dry cell battery, consists of a zinc anode and a graphite cathode, with the electrolyte being a paste made of ammonium chloride and zinc chloride in water. The cell reactions are:
Anode: Zn(s) → Zn2+ (aq) + 2e-
Cathode: 2NH4+ (aq) + 2MnO2 (s) + 2e- → Mn2O3(s) + 2NH3(aq) + H2O(l)
Another kind of battery, the lead storage battery, has a lead anode and a lead (IV) oxide (i.e. PbO2) cathode, with sulfuric acid being the electrolyte. The cell reactions are:
Anode: Pb(s) + SO42- (aq) → PbSO4 (s) + 2e-
Cathode: PbO2 (s) + 4H+ (aq) + SO42- (aq) + 2e- → Mn2O3(s) + 2NH3(aq) + H2O(l)
Unlike the dry cell battery, the lead storage battery is rechargeable. The reaction will be reversed by applying an external voltage to the cathode and anode (i.e., electrolysis), meaning the original materials will be replenished.
Galvanic cells are only a part of the magical topic of electrochemistry. Like fruits, many unique energy sources are waiting to be explored.
Citations:
Shittu, Saheed Adebowale, Sunday Adeola Ajagbe, and Racheal Foluke Oloruntola.
"Conversion of fruit to battery." Int. J. Sci. Eng. Res 9 (2018): 1747-1755.
Chang, Raymond. Chemistry. McGraw-Hill, 2010.