Ion-Exchange Polymer Metal Composites (IPMC) Membranes
Osaka National Research Institute, AIST, Japan
· Base polymer: Nafion 117 (duPont)
· Aqueous solution of platinum ammine complex ([Pt(NH3)4]Cl2 or [Pt(NH3)6]Cl4) [Can be purchased from: Aldrich Chemical Co., Milwackee, WI phone 800-558-9160, Catalog #275905]
· Sodium borohydride (NaBH4, reducing agent for primary reduction)
· Hydrazine hydrate (NH2NH2- ~1.5H2O, reducing agent for secondary reduction)
· Hydroxylamine hydrochloride (NH2OH-HCl, reducing agent for secondary reduction)
· Dilute ammonium hydroxide solution (NH4OH 5% solution)
· Dilute hydrochloric acid (HCl aq, 2 N solution and 0.1 N solution)
· Deionized water
1. Surface Roughening of the Membrane
a. Mild Sandblast: Sandblasting the surface of the membrane in order to increase the surface area. Use fine glass beads (GP 105A, Toshiba Co. Ltd.) that are blown onto the dry membrane by compressed air. The speed of sandblasting is ~1 sec/cm2 membrane area. It is also possible to use emery paper to sand the material.
b. Ultrasonic Washing: Remove the glass beads and residues by washing the membrane with water preferably using ultrasonic cleaner.
c. Treatment with HCl: Boil the membrane in dilute hydrochloric acid (HCl aq, 2 N solution) for 30 minutes to remove impurities and ions in the membrane. Rinse it with deionized water.
d. Treatment with Water: Boil the membrane in deionized water for 30 minutes to remove acid and to swell the membrane. The roughened membrane can store in deionized water.
2. Ion-exchange (Adsorption)
Prepare a platinum complex ([Pt(NH3)4]Cl2 or [Pt(NH3)6]Cl4) solution of 2 mg Pt/ml. Although the adsorbing amount depends on charge of the complex, either complex gives good electrodes. Immerse the membrane in the solution containing more than 3 mg of Pt per cm2 membrane area. For instance, more than 45 ml of the Pt solution is required for a membrane of 30 cm2. Excess amount of the Pt solution is preferable. After immersing the membrane, add 1 ml of ammonium hydroxide solution (5 %) to neutralize. Keep the membrane in the solution at room temperature for more than 3 hours (one night usually).
3. Primary Plating (Reduction)
Prepare a 5 wt% aqueous solution of sodium borohydride. After rinsing the membrane with water, place the membrane of 30 cm2 in stirring water of 180 ml in a water bath at 40oC. Then, add 2 ml of the sodium borohydride solution (5 wt% NaBH4 aq) every 30 min for 7 times. The amount of the reagent should be proportional to the area of the membrane. In the sequence of addition, raise the temperature up to 60oC gradually. Then, add 20 ml of the reducing agent and stir for 1.5 hr at 60oC. Black layer of fine Pt particles deposits only on the surface of the membrane. Rinse the membrane with water and immerse it in dilute hydrochloric acid (0.1 N) for an hour.
4. Secondary Plating (Developing)
The amount of platinum deposited by the 1st plating (reduction process) is only less than 0.9 mg/cm2, which depends on the ion exchange capacity, thickness of the membrane and the structure of the Pt complex. Additional amount of platinum is plated by developing process on the deposited Pt layer. When you add 2 mg/cm2 of Pt on the area of 60 cm2 (both sides of 30 cm2 of membrane), you need Pt complex solution containing 120 mg of Pt. Prepare a 240 ml aqueous solution of the complex ([Pt(NH3)4]Cl2 or [Pt(NH3)6]Cl4) containing 120 mg of Pt and add 5 ml of the 5% ammonium hydroxide solution. Plating amount is determined by the content of Pt in the solution. Prepare a 5% aqueous solution of hydroxylamine hydrochloride (NH2OH-HCl) and a 20% solution of hydrazine (NH2NH2). Place the membrane in the stirring Pt solution at 40oC. Add 6 ml of the hydroxylamine hydrochloride solution and 3 ml of the hydrazine solution every 30 minutes. In the sequence of addition, raise the temperature up to 60oC gradually for 4 hours, and gray metallic layers will form. At the end of this process, sample a small amount of the solution and boil it with the strong reducing agent (NaBH4) to check the end point. It is dangerous to add NaBH4 powder in a hot solution, because of the gas explosion. So add NaBH4 solution to a cold solution, then, warm the solution on a water bath. If any Pt ion remains in the plating solution, the color of the solution turns to black. In such cases, continue to develop Pt with addition of the NH2OH-HCl and NH2NH2 solutions. If you check that there is none of Pt ion in the chemical plating solution, rinse the membrane with water, and boil in dilute hydrochloric acid (0.1 N) to remove the ammonium cation in the membrane. After washing with water, H+ in the composite can be exchanged for any cation by immersing in a solution of the chloride salt of the cation.
The Editor of the WW-EAP Webhub would like to express his sincere appreciation to Dr. Keisuke Oguro for contributing his fabrication recipe. Dr. Oguro is one of the pioneers of the IPMC and leading EAP scientist. His contribution is a very important milestone for the field of EAP particularly since there is no recommended commercial source of such materials. The Editor would like to express his hope to seeing more scientists who are developing new EAP materials to share their recipe by allowing it to be published on the WW-EAP Webhub to promote progress in this emerging field of EAP.