Chemistry
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Synthesis of Platinum-nickel Nanowires and Optimization for Oxygen Reduction Performance
Summary April 27th, 2018
The protocol describes the synthesis and electrochemical testing of platinum-nickel nanowires. Nanowires were synthesized by the galvanic displacement of a nickel nanowire template. Post-synthesis processing, including hydrogen annealing, acid leaching, and oxygen annealing were used to optimize nanowire performance and durability in the oxygen reduction reaction.
Transcript
The overall goal of this procedure is to produce platinum based, extended surface electro-catalysts, that demonstrate a high level of performance and durability in the oxygen reduction reaction. This method could help address key issues in hydrogen fuel cells by reducing the amount of platinum required, and cutting the cost of the catalyst layer. The main advantage of this technique is that produces materials with high electrochemical surface areas, attritional limitation of surface catalysts.
Begin the nanowire synthesis steps with the displacement process. In a 50 milliliter centrifuge tube, add 20 milliliters deionized water, and 40 milliliters nickel nanowires. Place the mixture in a sonicator and sonicate for five minutes.
Meanwhile, prepare a mineral oil bath and an electric stirrer. Retrieve the suspended nanowires from the sonicator, and transfer them to a glass, 250 milliliter round-bottom flask. Add to the flask, 60 milliliters of deionized water.
Then place it in the mineral bath to be heated to 90 degrees Celsius, and simultaneously stirred at 500 RPM. Equip the stirrer with a PTFE paddle on a glass shaft. Now form the platinum nickel nanowires.
Have ready an automated syringe pump, and prepare a syringe. Use a 20 milliliter syringe with about 8 centimeters of tubing on its tip. Next, obtain 15 milliliters of deionized water, and add 8.1 milligrams of potassium tetrachloroplatinate.
Transfer the solution to the syringe. Then place the syringe on the syringe pump. Arrange for the pump to add the solution to the flask containing nanowires.
Set the rate to one milliliter per minute, and start the pump. Once the transfer is complete, continue stirring and heating the flask at 90 degrees Celsius for two hours. After two hours, put the solution in to a centrifuge tube, and centrifuge at 2500 G, for 15 minutes.
Centrifugation gives rise to supernatant in the tube. Pour this off in to a waste container. With the supernatant gone, add fresh deionized water to the solids.
Sonicate the mixture for about 10 seconds. Repeat the centrifuge, supernatant removal, and re-suspension steps, 3 times with the water, and a fourth with 2-Proponal. In the end, put the nanowires in a 20 milliliter, glass scintillation vial.
Take the nanowires to a vacuum oven to dry at 40 degrees Celsius, overnight. First prepare, in a 20 milliliter scintillation vial, what is called the Ink. This is deionized water, a catalyst with 73 micrograms of platinum, and 2-propanol.
Ice the vial for five minutes. After five minutes, add 10 microliters of ionomer to the ink. Return the vial to the container of ice, and move both to the sonicator.
Sonicate the vial for 30 seconds by horn. Follow this with bath sonication for 20 minutes. End with another 30 seconds of horned sonication.
During sonication, prepare a container of graphitized carbon nanofibers. Get the sonicated ink and use a pipette to draw out 7.5 milliliters. Add the drawn out ink to the nanofibers.
Now, sonicate the ink in ice with a horn for 30 seconds. Then by bath for 20 minutes. And by horn again, for 30 seconds.
Have ready, an inverted rotator with a working electrode mounted on it. Here, a glassy carbon electrode is ready for coating. Start the electrode spinning at 100 RPM, and pipette 10 microliters of the sonicated ink on to it.
When done, increase the rotation to 700 RPM. As the electrode drys, repeat the three sonication steps with the ink. With the mixture speed at 100 RPM, pipette and additional 10 micrograms of ink on to the working electrode, before increasing the rotation speed to 700 RPM.
Fill the half cell with one-tenth molar perchloric acid. Now assemble the rotating disk electrode test station. The test station has a working reference, and counter electrode connected to the main testing cell.
Next, connect the working electrode to a modulated speed controller. Then, submerge the working electrode tip in the acid. Set up a potentiostat to take electrochemical measurements.
Prior to electrochemical conditioning, purge the electrolyte with nitrogen for seven minutes. Now, prepare to measure oxygen reduction polarization curves. Start the working electrode, rotating at 2500 RPM.
Purge the electrolyte with oxygen. After seven minutes, set the oxygen purge to blanket the electrolyte. Slow the electrode rotation to 1600 RPM.
Set the parameters for an automated linear sweep photometry measurement and run the experiment. When the measurement is completed, stop the rotation and discard the electrolyte. After refilling the cell with perchloric acid, and immersing the electrode, purge it with oxygen for seven minutes.
Perform the linear sweep photometry measurement again, using the same parameters. These data show the platinum nickel nanowire composition as a function of the amount of the platinum precursor. Potassium tetrachloroplatinate, added to 40 milligrams of nickel nanowires during galvanic displacement.
An increase of platinum precursor during galvanic displacement, increases the platinum content and the resulting catalyst. This plot is the electrochemical surface area versus the platinum displacement. Increasing platinum content, generally results in catalyst with lower platinum based electrochemical surface areas.
Optimal performance occurred at a composition of 7.3%platinum. Here's the mass activity for the oxygen reduction reaction as a function of a kneeling temperature. A temperature of 250 degrees Celsius, produced catalyst with optimal mass activity.
Once mastered, this technique can be completed at approximately one week, if performed properly. While attempting this procedure, it is important to remember that contaminants on a part-per-trillion level are electrochemically relevant. Following this procedure, other methods, including microscopy and membrane electrode assembly testing can be completed to answer additional questions, including kettle structure and device performance.
From these protocols, we have developed extended service catalysts for proton and nano exchanged membrane fuel cells, electrolyzers and alcohol based fuel cells. After watching this video, you should have a good idea how to synthesize and electrochemically test platinum based extended surface electro-catalyst that demonstrate a high level of performance and durability in the ontra production reaction. Don't forget that working with concentrated acid and pressurized gas, particularly, hydrogen and carbon monoxide can be extremely hazardous and precautions such as, using personal protective equipment and ventilation control should always be taken while performing this procedure.
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