Making a Battery From Fruits and Vegetables

Fig. 1 - Potato battery

Alessandro Volta and Luigi Galvani pioneered the science behind this experiment. Both realised that current electricity is produced when two different metals are placed in contact with a conducting solution, or "electrolyte," and connected with a wire.

In 1771, Galvani discovered that a frog’s legs twitched when placed between two different metals. Volta concluded in 1800 that electricity could be created in a similar fashion by placing brine solution between alternating copper and zinc discs that were arranged in a pile and connected with wire. The famous "Voltaic Pile" was then further adapted in 1836 by John Daniell, who created the "Daniell Cell."

The Chemistry of Wet Cell Batteries

The Daniell Cell, a typical "wet cell" battery, consists of two half cells; in one, a zinc electrode is immersed in a solution of zinc sulphate, while in the other cell a copper electrode is placed in a solution of copper sulphate. The half cells are connected by a salt bridge (see figure 2).

When the zinc and copper are connected by a wire, electrons flow through the wire from the zinc to the copper while positive and negative ions move through the solutions to balance the resulting charge difference. This phenomenon is the result of an oxidation-reduction reaction between the two metals – as zinc is more active than copper it tends to lose electrons through the wire (oxidation) while the copper accepts these electrons (reduction).

How a Lemon Can Act as a Battery

Lemons and other fruit and vegetables contain acidic juices that can act in a similar way to the electrolytes in the Daniell Cell. If two different metals are pushed into the skin of a lemon, for instance, and connected with an insulated wire, an electrical circuit is produced because positive and negative ions present in the juice can move to each metal to balance the charges.

Unlike a Daniell Cell, however, the less active metal is not reduced: instead, hydrogen ions in the juice are reduced to form hydrogen gas. When the two different metals are a galvanised (zinc) nail and a copper coin, the reactions that occur are:

At the galvanised nail: Zn→ Zn2+ + 2 e-

At the copper coin: 2H++ 2e- → H2

The copper coin in effect acts as an electrode to direct the electrons from the nail to the hydrogen ions.

Making a Lemon Battery – Materials and Teaching Method

A copper coin or a piece of copper wire and a galvanised nail are used as the two different electrodes in this experiment, but other combinations could be trialled. To achieve the maximum amount of voltage both should be rubbed with steel wool prior to the activity. In addition, the more lemons connected in series to each other, the more power will be generated. One single lemon cell will produce up to 0.9 volts if copper wire is used instead of a coin.

The following materials and equipment are required per group of around four students:

•                3 or 4 lemons

•                connecting wires and alligator clips

•                steel wool

•                calculator with dry cell removed or LED bulb

•                galvanised nails

•                copper coins or wire

Students should be instructed to copy down the following directions, which could be followed by a teacher-led explanation.

•                Rub the nails and coins/copper wire with steel wool.

•                Push a nail in one end and a coin in the other end of each lemon. If you are using copper wire, use lengths of around 5cm. Coil each length up into a circular shape and push one free end into the lemon skin.

•                Pull the wires in the battery compartment of the calculator out at each side of the calculator. Connect the negative end to a length of insulated wire. Connect the other end of the insulated wire to the galvanised nail in one of the lemons.

•                Now connect another insulated wire between the nail and the copper metal of this first lemon.

Fig.2 - Galvanic Cell

•                Take a further insulated wire and join the copper electrode of the first lemon to the nail in the second lemon.

•                Repeat the two steps above until you have three or four lemons connected in series. Join the last copper electrode to the positive wire emerging from the calculator.

Alternatively, the free ends of the lemon battery can be connected to a LED light bulb. The flat side of the LED should be attached to the galvanised nail (anode) end of the lemon cell. Potatoes, in which phosphoric acid present in the juice acts as an electrolyte, can be used instead of lemons to create comparable voltages.

The following questions could be written on the board after students write up the experiment and their observations.

1.             What is an electrolyte?

2.             Name the electrolyte present in a) lemons; b) potatoes.

3.             Explain how Galvani made the frog’s leg twitch in his famous "animal electricity" experiment.

4.             Name the two different metals and the electrolyte used in Volta’s "Voltaic Pile."

5.             Define: a) oxidation; b) reduction; c) anode; d) cathode.

6.             What acts as the anode in these experiments?

Making a Lemon Battery – Follow-Up Activities

Students could try using apples, kiwi fruit, pears or tomatoes to create electricity instead of lemons and potatoes. A typical calculator and LED light bulb both require around 1.5 volts. Additional current can be produced by connecting pairs of fruit pieces in parallel and then connecting each pair together in series. Note that the more each electrode is pushed into the fruit, the greater is the surface area exposed to the electrolyte.

References

"Battery History," 2010, inventors.about.com

Construct a Vinegar Battery, 2010, hilaroad.com videos

Hila Research Centre, Lemon Battery, hilaroad.com

Knowlera Media, How to Make a lemon Battery, 2010, monkeysee.com

Knowlera Media, How to Make a Potato Battery, 2010, monkeysee.com