Class time: 40 to 65 minutes

Purpose: Students create, test and optimize batteries using various electrodes and electrolyte compositions.

Notes to the teacher: If you have more time, you can allow the students more design options and let them figure out more things themselves. If you have less time, you can allow the students fewer design options by giving more guidance. You can order electrodes such as these from Home Science Tools or buy materials made from or coated with suitable metals from Home Depot or similar stores. Ideally students should be able to find the configurations that produce the most voltage, and then use those to light a lightbulb. If your time is limited, you could have the students focus on either the voltage measurements or lighting the bulb. This activity would work well for pairs of students working together.    

Use the Best Battery activity guide to lead your students through building their batteries.

Materials:

• Beakers or clear plastic cups (about 250 ml)
• Copper electrodes (or copper tubing or stripped copper wire)
• Zinc electrodes (or zinc or zinc-plated hardware)
• Iron or steel electrodes (or iron or steel hardware)
• Aluminum electrodes (or aluminum foil or strips cut from disposable aluminum pans)
• Vinegar (5% acetic acid, sold by the gallon at many grocery stores)
• Table salt (NaCl, not iodized)
• Optional: other ionic solids to use as electrolytes such as KCl or NaNO₃
• Scales or balances and weigh boats or weigh paper
• Graduated cylinders
• Water (distilled if you have it, otherwise tap water)
• Stirring rods
• Electric multimeter (inexpensive models are sold at Walmart or similar stores)
• 1.5 volt incandescent lightbulbs and sockets (see this example from Home Science Tools)
• Wires with alligator clips (see for example these from American Science & Surplus)
• Table of standard reduction potentials
• Goggles
• Gloves and other protective equipment such as aprons
• Paper towels
• Sandpaper or steel wool

Directions:

1. Have the students wear goggles and gloves (and other protective equipment such as aprons, if available).
2. Show the students how to use the table of standard reduction potentials to predict the voltage difference between two different electrode materials. A Khan Academy video gives a brief summary of how to calculate the overall redox reaction potentials under standard conditions from a standard reduction potential table.
3. Show the students how to use the multimeter to measure voltage. Remind students that the multimeter leads must be in direct contact with the battery electrodes in order for the circuit to be complete. Also, when the voltage is positive, the metal electrode connected to the red lead is considered the positive terminal of the battery (the cathode) and the metal electrode connected to the black lead is the negative terminal (the anode). If the voltage is negative, the student should reverse the leads so that the voltage is positive.
4. Ideally students should determine battery configurations that produce the most voltage, and then use those to light a lightbulb. But if time is limited, determine which variables they should focus on. Students can start with determining the largest possible voltage from different combinations of electrodes (theoretically and experimentally). Once the maximum voltage is determined, students can use this electrode combination to test other variables such as the type and/or concentration of the electrolyte (how much vinegar, salt, water is in solution), the approximate distance between electrodes and the number of cells (depending on available supplies). Students can ultimately test their battery with a lightbulb. If students are more advanced, have them calculate the molarity of their electrolyte solutions.

Note the following:

Tap water may have enough impurities to make a fairly good electrolyte by itself; distilled water is less conductive and would show the students the importance of adding ions to the electrolyte. Salt (especially about 1 to 2 g per 200 ml of water) makes the water function as a much better electrolyte. Vinegar has enough ions to serve as a good electrolyte. Vinegar plus salt can be even better.

The specific electrode combination chosen will have a large effect on the voltage, generally in the ballpark of the standard reduction potentials but with some variation due to the electrolyte. (Standard conditions for the reduction potential table are defined as 25 degrees Celsius, 1 atm for any gas participating in the reaction, and 1 M concentration for each ion participating in the reaction.) The electrolyte chosen will have a smaller but still significant effect on cell potential. The spacing between the electrodes may have a small but measurable effect, depending on the electrodes, electrolyte and volume.

Show the students that they can use the alligator clip leads to connect multiple batteries in series if necessary to light a lightbulb.