Electrochromic Prussian Blue Thin Films

Modified by Jason Marmon and George Lisensky from J. J. Garcia-Jareño, D. Benito, J. Navarro-Laboulais, and F. Vicente, "Electrochemical Behavior of Electrodeposited Prussian Blue Films on ITO Electrodes," J. Chem. Educ., 75, 881-884 (1998).

In this experiment K₃[Fe(III)(CN)₆] is electrochemically reduced at a glass electrode to produce K₄[Fe(II)(CN)₆]. The K₄[Fe(II)(CN)₆] at the electrode reacts with Fe(III)Cl₃ in solution to give insoluble Prussian Blue, Fe(III)₄[Fe(II)(CN)₆]₃, on the electrode. The approximately 100 nm thick layer exhibits visible electrochromism, i.e., the coating on the glass changes color with applied voltage. (Prussian Blue has been made by chemists for 300 hundred years. For more information on its history, see http://webexhibits.org/pigments/indiv/overview/prussblue.html.)

Clicking a thumbnail on this web page (http://chemistry.beloit.edu/edetc/nanolab/prussianblue/index.html) shows a movie of that step.

Procedure

Wear eye protection

Add 5 mL 0.05 M HCl, then 10 mL 0.05 M K₃[Fe(CN)₆], and then 10 mL 0.05 M FeCl₃^.6H₂O to a 50 mL beaker. The mixed solution should be prepared just before use.

Identify the conducting side of a tin oxide-coated piece of glass by using a multimeter to measure resistance. The conducting side will have a resistance of 20-30 ohms.

Face the conducting side of the glass towards a platinum wire coil or graphite electrode. Connect the negative lead of a voltage source to the glass and the positive lead to a platinum wire coil or graphite electrode. Dip the electrodes (but not the aligator clips) into the solution prepared above and quickly adjust the voltage to produce 40µA/cm² for 60 seconds. See electrical circuit diagram on another page.

Rinse the electrode with pure water. The approximately 50 nm coating (see calculation) after 60 seconds of deposition can be seen by eye on the conducting surface.

Longer electrolysis times give thicker coatings. In this movie, after passing 80 µA current for 30 seconds (40µA/cm² times an electrode area of 2 cm²) the electrode is rinsed to show the color change. The rinsing steps are repeated after every 30 seconds to show the changing color but would not be needed otherwise unless you are recording visible absorbance (690 nm) as a function of layer thickness. The total electrolysis time for this relative thick coating is 300 seconds.

Place the rinsed electrodes in 25 mL 1.0 M KCl, keeping the clip above the solution.

Changing the applied voltage (next step) will change the color of the thin film.

Record the cylic voltammogram of the coated glass at 20 mV/sec from +550 mV to -250 mV to +1200 mV to +550 mV versus a Ag/AgCl reference electrode. When the voltage matches a redox reaction the current increases.

What color is the reduction product? What is the redox potential for the reaction?
Fe(III)₄[Fe(II)(CN)₆]₃ + 4 K⁺ + 4 e^- = K₄Fe(II)₄[Fe(II)(CN)₆]₃

What color is the oxidation product? What is the redox potential for the reaction?
Fe(III)₄[Fe(II)(CN)₆]₃ + 3 Cl^- = Fe(III)₄[Fe(III)(CN)₆]₃Cl₃ + 3 e^-

What is the source of the K⁺ and Cl^- ions in the redox reactions?

Materials

• 0.05 M HCl (Each preparation needs 5 mL. Dilute 5 mL conc HCl to 250 mL with water.)
• 0.05 M K₃[Fe(CN)₆] (Each preparation needs 10 mL. Dissolve 1.65 g in 100 mL water.)
• 0.05 M FeCl₃^.6H₂O (Each preparation needs 10 mL. Dissolve 1.35 g in 100 mL water with a drop of HCl.)
• 1.0 M KCl (Each preparation needs 25 mL. Dissolve 18.6 g in 250 mL water with two drops of HCl to lower the pH.)

Equipment

• 50 mL beaker
• Conductive Glass (1" x 1" x 2.3mm TEC 15 glass) Hartford Glass Co, 735 E Water Street, Hartford City, IN 47348 Phone: 765-348-1282. Glass can be reused by removing prussian blue coatings with concentrated ammonia (solubility increases with pH).
• Ohmeter
• Pt wire or graphite electrode
• 1.5 Volt battery, holder, and 50 K ohm variable resistor for controlling deposition current. See electrical diagram.
• Electrochemical apparatus for obtaining cyclic voltammograms

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