Electrochromic polymer

This experiment should take 45 minutes. 

Equipment

Apparatus

• Indium-tin oxide (ITO) conducting glass slide
• Battery pack 
• A red and a black cable with banana plugs
• Crocodile clips to fit banana plugs x 2
• Copper electrode 
• Beakers, 100 cm³ x 2
• Tissues
• Gloves

Chemicals

• Deposition solution, 1 dm³ of 2 M HCl (aq) with 200 cm³ of aniline added
• Testing solution, 1 dm³ of a solution containing 0.5 M HCl (aq) and 0.5 M KCl (aq)

Health, safety and technical notes

• Read our standard health and safety guidance here
• This experiment uses a dilute acidic solution of aniline, which is toxic and a possible carcinogen, see CLEAPSS Hazcard HC004a
• Laboratory coats, safety glasses and gloves should be worn at all times during this experiment.
• If you spill any of the solution on your skin, rinse immediately under the tap with cold water and then wash with soap.
• A risk assessment must be done for this activity.
• A spill clean-up kit should also be available.

Procedure

Polymer deposition

1. Attach a crocodile clip to the black cable and clip it to the copper electrode. Place the electrode into the deposition solution and plug the other end of the cable into the black terminal on the power supply unit.
2. Attach a crocodile clip to the red cable and carefully clip the other end to the conducting glass slide. Place the slide carefully into the deposition solution and make sure that the copper electrode and glass slide do not touch each other
3. Make sure that the dial on the power supply is set to one.
4. Connect the red cable to the red terminal on the power supply.
5. You should now see that a green layer of polyaniline is deposited onto the glass slide. When the green layer is clearly visible, carefully remove the glass slide from the solution.

Testing the electrochromic properties of the polymer

1. Transfer both the electrodes into the testing solution. The glass slide should turn dark blue. This is because the new solution is not acidic unlike the deposition solution and the polymer is sensitive to pH as well as potential.
2. Turn the dial on the power supply to maximum. What happens?
3. Next, turn the dial on the power supply to minimum. Watch carefully the series of colour changes that take place.
4. Experiment with the power supply control and see if you can get the slide to remain permanently in four differently coloured states.

Notes

The biggest problem associated with this experiment is that if the Indium-Tin Oxide (ITO) coated slide is placed at negative potential of -1.5 V the oxide layer is rapidly reduced, destroying the conductivity of the glass.

Thus, any ITO surface that makes contact with the testing solution that is not covered with polymer risks being damaged.

The ITO slides are best wrapped in the centre with a piece of insulation tape so that the section that enters both the deposition and testing solutions is always completely submerged and covered with polymer.

Even with the polymer protection, if the slide is held at negative potential, the ITO will still be reduced.

Thus, it is essential to only apply the negative potential just long enough to cause the colour change and then immediately turn the potential to zero.

This will allow the slides to be reused, although almost certainly the students will damage some of them.

The best way to clean the slides is to wipe them with a tissue immediately after removing them from the testing solution.