The overall goal of this procedure is to prepare and test fuel cell catalysts with good reproducibility. Reproducible thin film rotating disk electrode measurements of a standard catalyst can then serve as a benchmark for novel catalysts. Testing fuel cell catalyst with the thin film RDE method requires experience.
This tutorial will help you to avoid the typical experimental pitfalls of this method. The main advantage of this protocol is that it includes the preparation of benchmark catalysts. This protocol provides platinum carbon catalyst activities based on platinum surface area and mass.
Remember to always determine your actual platinum content of your catalyst before testing. To begin the procedure, place in a microwave reaction vessel four milliliters of a 0.4 molar solution of sodium hydroxide in ethylene glycol, and four milliliters of a 40 millimolar solution of chloroplatinic acid in ethylene glycol. Equip the vessel with a stir bar.
Heat the mixture in a microwave reactor for three minutes at 160 degrees celsius while gently stirring. Transfer 7.3 milliliters of the resulting colloidal platinum nanoparticle suspension to a centrifuge tube. Precipitate the two nanometer platinum nano particles with 30 milliliters of one molar hydrochloric acid.
Centrifuge the mixture at 2, 900 times g for five minutes and discard the supernatant. Repeat the centrifugation in hydrochloric acid and removal of the supernatant twice more to finish washing the nanoparticles. Then redisburse the platinum nanoparticles in seven milliliters of acetone by manually shaking the container.
Disperse 27.5 milligrams of carbon black in one milliliter of acetone by sonication. Combine the dispersions in a round bottomed flask. Use a rotary evaporator or Schlenk line to completely remove the acetone.
Dry the resulting platinum on carbon catalyst powder at 120 degrees celsius overnight. Next, fill an ultrasonic bath with cold water. Add 20 milliliters of deionized water to the dried platinum on carbon catalyst powder, and sonicate the mixture in the cold ultrasonic bath for three minutes.
Collect the powder on filter paper with four to seven micrometer pores. Wash the powder with 200 milliliters of deionized water. Then, dry the powder overnight in a vacuum oven at 100 to 120 degrees celsius and 10 kilopascals to obtain the 50 percent by weight platinum on carbon catalyst powder.
Prior to the characterization, dry the platinum on carbon catalyst powder overnight in the vacuum oven at 80 degrees celsius and 10 kilopascals. Weigh the dry catalyst powder in a ceramic crucible with a lid. Place the covered crucible in a muffle furnace.
Heat the powder at 900 degrees celsius for 30 minutes in air to burn off the carbon support. Then, shut down the furnace and allow the powder to cool to room temperature in the furnace overnight. Next, add four milliliters of aqua regia to the powder.
Heat the mixture on a hot plate at 80 degrees celsius for two hours to digest the platinum. Then, dilute the solution to 10 milliliters with deionized water. Mix 0.75 milliliters of two molar hydrochloric acid with 0.25 milliliters of one molar tin(II)chloride prepared in four molar hydrochloric acid in a quartz cuvette, and add one milliliter of the aqua regia sample.
Use a mixture of 1.75 milliliters of two molar hydrochloric acid and 0.25 milliliters of two molar tin chloride in four molar hydrochloric acid as the solvent background. Stir the mixture well with a small stir bar. Acquire a UV vis spectrum of the sample mixture from 700 to 350 nanometers and perform background subtraction.
Determine the platinum concentration in the sample by the standard addition method using a 1000 ppm platinum standard solution. To begin preparing the catalyst ink, mix 6.3 milligrams of 50%by weight platinum on carbon catalyst powder with six milliliters of deionized water. Combine this mixture with two milliliters of isopropyl alcohol.
Add about 10 microliters of one molar potassium hydroxide to adjust the ink pH to approximately 10. Sonicate the ink mixture in a cold ultrasonic bath for 15 minutes. Check the ink pH when finished.
Dilute the catalyst ink 50 fold with a one to three by volume mixture of isopropyl alcohol and deionized water. Use acid or base as needed to adjust the pH back to 10. Measure the zeta potential of the catalyst ink sample using electrophoretic light scattering.
Measure the sizes of the aggregates with dynamic light scattering. Next, polish a five millimeter glassy carbon disk electrode to a mirror finish with 0.3 micrometer and 05 micrometer alumina pastes in sequence. Clean the electrode by sonication in deionized water after each polishing step.
Dry the clean, polished electrode with argon gas. Then, run an argon gas line through a bubbler containing a 17 to three by volume mixture of isopropyl alcohol and deionized water. Pipette five microliters of the catalyst ink onto the polished electrode to achieve a platinum loading of 10 micrograms of platinum per square centimeter.
Slowly dry the ink with a stream of argon gas humidified with the mixture of isopropyl alcohol and deionized water. Obtaining homogeneous thin catalyst film on the glassy carbon electrode is extremely important. We encourage everyone to spend some time trying different conditions and procedures to find out what works best for you.
Once the platinum on the carbon catalyst film is dry, use a CCD camera to verify that the film uniformly covers the glassy carbon electrode surface. To begin the electro chemical measurement set up, assemble a clean three-compartment glass or polytetrafluoroethylene electrochemical cell. Connect a catalyst-coated glassy carbon disk electrode to an electrode rotator.
Fill the electrochemical cell with 0.1 molar perchloric acid at the electrolyte. Place the catalyst coated glassy carbon disk electrode in one compartment of the electrochemical cell as the working electrode. Place a platinum mesh and a trapped hydrogen or saturated calomel electrode fitted with a Luggin capillary in the other two compartments as the counter and reference electrodes, respectively.
Connect the electrodes to a potentiostat. Measure the resistance in the electrolyte solution between the working electrode and the Luggin capillary. Configure the potentiostat to compensate for this resistance by using the analog positive feedback scheme.
After calibrating the reference electrode potential against reversible hydrogen electrode potential and cleaning the catalyst, acquire a cyclic voltammogram in argon saturated electrolyte. Then, purge the electrolyte with oxygen gas for 10 minutes. Start rotating the glassy carbon electrode at 1600 rpm.
Record a cyclic voltammogram with the potential sweep between 05 and 1.10 volts versus reversible hydrogen electrode at 05 volts per second. Next, hold the working electrode at 05 volts. Purge the electrolyte with carbon monoxide gas for five minutes followed by argon gas for 10 minutes.
Then, sweep the potential from 05 volts to 1.10 volts at 05 volts per second. After the carbon monoxide stripping voltammogram has finished, wet a piece of laboratory tissue with deionized water. Press the glassy carbon electrode on the wet paper to transfer the catalyst film.
Photograph the catalyst film after the transfer. Transferring the tested catalyst film onto the wet paper is an easy way to see if the film stayed intact during the measurement. Well-dispersed platinum on carbon catalyst powder was prepared using two nanometer platinum metal particles from a stable colloidal suspension.
When less stable suspensions were used, such as those with a lower sodium hydroxide concentration, significant platinum nanoparticle agglomeration occurred on the carbon support. Drying the platinum on carbon catalyst ink with a flow of humidified argon gas was essential to obtaining a homogeneous thin film on the electrode. The coffee ring effect was observed when the ink dried in ambient air, with the catalyst agglomerating around the outside of the electrode.
The thin film uniformity was further tuned by adjusting the humidification conditions. A higher catalyst ink pH improved both the catalyst ink solution stability and the uniformity of the thin film. Cleaning the thin film catalyst by cycling the electrode potential in argon saturated perchloric acid was important for obtaining well defined peaks during the measurements.
The linear sweep voltammogram in oxygen saturated perchloric acid was highly sensitive to the catalyst film quality. The LSV of a homogeneous film showed an oxygen diffusion-limited current density of about 6 milliamperes per square centimeter, with a sharp shoulder in the LSV curve. A non-homogeneous film showed a smaller current density and a broader shoulder.
Once mastered, the synthesis characterization and electrochemical thin film RDE testing of a standard fuel cell catalyst can be done in two days if performed properly. Don't forget that working with concentrated acid and carbon monoxide can be extremely hazardous. Always wear a lab coat, goggles, and gloves.
Gas sensors for carbon monoxide and hydrogen must always be active in your lab while performing these experiments. While attempting this procedure, remember, the cleanliness is crucial to obtain reliable results in electrochemical measurements. We recommend preparing the catalyst and performing the electrochemical testing in two separate laboratories.
In addition to this procedure, catalysts characterization methods like degradation tests, small angle x-ray scattering, or in situ x-ray absorption spectroscopy can be performed to answer additional questions about the functionality of your catalyst. After its development, the thin film RDE technique paves the way for the researchers in the field of electrocatalysis to develop improved electrocatalysts for sustained energy conversion. After watching this video you should have a good understanding of how to make a benchmark catalyst, how to test a fuel cell catalyst with our thin film RDE technique, and which pitfalls to avoid.
