Synthesis and Performance Characterizations of Transition Metal Single Atom Catalyst for Electrochemical CO₂ Reduction

The overall goal of this procedure is the preparation in the electrochemical application of Transition Metal Single Atoms coordinated in graphene vacancies towards selective carbon dioxide reduction in aqueous solutions. This method can help answer key questions in the electro-catalytic CO2 reduction field about the preparation of catalysts and the determination of product seal activity. The main advantage of this technique is that single atoms can be trapped in graphene vacancies and the transition metal neno-particles can be tightly surrounded by graphene layers.

To begin the procedure, in a 20 milliliter scintillation vial, combine zero point five grams of polyacrylonitrile, zero point five grams of polyvinylpyrrolidone, zero point five grams of Nickel(II)hexahydrate, and zero point one grams of dicdyandiamide, with 10 milliliters of dimethylformamide. Heat the mixture to 80 degrees Celsius and stir until clear and green. Allow the pre-cursor mixture to cool to room temperature afterwards.

Then, mount an eight centimeter by eight centimeter piece of zero point three seven millimeter thick carbon fiber paper as the collection substrate in a conventional electro spinning apparatus. Draw five milliliters of the pre-cursor mixture into a five milliliter syringe. Equip the syringe with a 19 gauge, two inch needle, as the spinning tip and mount the syringe on a syringe pump.

Configure the apparatus for a 15 centimeter space between the spinning and the collector surface. Connect the positive terminal of a power supply to the needle, and the negative terminal to the collection substrate. Configure the power supply to deliver 15 kilovolts of static voltage tot he spinning tip, and minus four kilovolts against the collection substrate.

Run the syringe pump at one point two milliliters per hour and start electro spinning. Once finished, turn off the power and transfer the polymer fiber coated substrate to a piece of aluminum foil. In a box furnace, heat the coated substrate to 300 degrees Celsius over the course of one point five hours.

Hold the sample at that temperature for 30 minutes to oxidize the polymer fibers which will detach from the substrate as a stand alone film. Remove the substrate and the detached oxidized polymer nano-fiber film from the furnace. Allow the film to cool to room temperature in air.

Then, cut the film into approximately zero point five centimeter by two centimeter pieces and collect the pieces in a quartz boat. Place the boat in a tube furnace in the center of the heating zone. Perch the sample in the furnace atmosphere three times with a gas mixture of five percent hydrogen in argon.

Then, maintain the gas flow at 100SCCM and the furnace pressure at one torque. Ramp the furnace first to 300 degrees Celsius over the course of ten minutes. And then to 750 degrees Celsius over the course of two hours.

Both at constant wrap rates. Hold the sample at 750 degrees Celsius for one hour. Then, turn off the heat and allow the sample to cool to room temperature in the furnace under the 100SCCM flow of five percent hydrogen and argon.

Seal the cooled sample in a six millimeter stainless steel milling ball in a stainless steel milling jar. Ball mill the sample at 3, 000 RPM in 60 hertz for five minutes to obtain the nickel nitrogen graphene shell catalyst as a Nano powder. To begin the measurement procedure, dissolve two point five grams of potassium bicarbonate in 250 milliliters of ultra pure water as the electrolyte.

Electrolize the electrolyte solution between two graphite rods at zero point one milliamps for 24 hours to remove trace metal ions. Then, cover the back of a clean, electrochemically polished, one centimeter by two centimeter glassy carbon electrode with an electrochemically inert hydrophobic wax dissolved in toluene. Next, place in a four milliliter scintillation vial, five milligrams of nickel nitrogen graphene shell catalyst nano powder, one milliliter of ethanol, and 100 microliters of a five percent iodymer solution in isopropyl alcohol.

Sonicate the mixture for 20 minutes to obtain a homogeneous catalyst ink suspension. Apply 80 microliters of the catalyst ink to the surface of the glassy carbon electrode. Dry the catalyst covered electrode in a vacuum desiccator for five to 10 minutes.

Then, equip a gas tight H-type electrochemical cell with a proton exchange membrane. Add a nitrogen gas line to the chamber that will contain the working electrode. Place the catalyst covered working electrode and a saturated calomel reference electrode in one compartment of the cell, and a platinum foil counter electrode in the other.

Add about 25 milliliters of the purified electrolyte solution to each compartment. Connect the electrolytes to a multi-channel potentiostat. Bubble nitrogen gas through electrolyte for 30 minutes at 50SCCM to saturate the electrolyte.

Then, select cyclic voltammetry in the Potentiostat software. Configure the experiment for five continuous CV scans from minus zero point five volts to minus one point eight volts versus SCE at 50 millivolts per second with a working electrode potential range of minus 10 volts to 10 volts, and an automatic current range. Acquire a CV in the nitrogen saturated electrolyte.

Then change the gas line to carbon dioxide and bubble carbon dioxide through the electrolyte at 50SCCM for 30 minutes. Then, while still bubbling carbon dioxide at 50SCCM. Acquire another CV using the same parameters as previously used.

Next, select potentiostatic electrochemical impedance spechtrology in the Potentiostat software. Set the frequency range to zero point one hertz to 200 kilohertz. Use the open circuit potential as the initial potential.

Perform an impeded scan and record the solution resistant value. Correct the measured potentials for the voltage drop using this information. Assemble an H-type electrochemical cell with a nickel nitrogen graphene shell coated working electrode in purified zero point one molar potassium bicarbonate as the electrolyte, as previously described.

Set up a gas chromatograph to use a separation system of five angstrom molecular sieves in a series of columns for separating low molecular weight compounds. Equip the GC with a thermal conductivity detector and a flame ionization detector with a methodizer. Direct the exhaust from the electrochemical cell chamber containing the working electrode to the sample loop of the GC using vinyl tubing with an inner diameter of one 16th of an inch.

Run another length of vinyl tubing from the chamber with the counter electrode to a flask of water as a gas flow monitor. Connect the electrodes to a multi-channel potentiostat and saturate the electrolyte with carbon dioxide gas as previously described. Maintain the carbon dioxide flow rate at precisely 50.0SCCM.

Use the Potentiostat software to report a chrono and parametric curve. While stepping the of working electrode from minus zero point three volts to minus one point zero volts versus RAG. Holding for 15 minutes at each step.

For each potential step after 10 minutes of continuous electrolysis, sample the gas products with the GC using a run time of 16 minutes. Determine the hydrogen monoxide contents in the exhaust from the TCD and FID signals, respectively. Calculate the partial current density and faradaic efficiency for each product.

Nickel nano particles were found to be uniformly distributed in the carbon nano fibers of the catalyst. The nano particles were encapsulated in approximately 10 nanometers thick shells of graphene, preventing direct contact between the nickel nano particles and the aqueous slater, and thus suppressing a hydrogen evolution reaction. Energy dispersive x-ray spectroscopy mapping in atom probe tomography showed that nickel atoms were incorporated into the graphene layers around the nano particles.

Nickel atoms were also dispersed in carbon in areas further from nickel nano particles. Nickel single atoms were found to be coordinated in graphene vacancies with a small percentage also coordinated with nitrogen atoms. Statistical analysis indicated that most of the nickel in this nickel poor area was in single atom form.

The electro-catalytic carbon dioxide reduction reaction performance of the catalyst at various applied potentials was evaluated in real time by a GC connected to the electrochemical cell exhaust. The faradaic efficiencies of carbon monoxide and hydrogen were determined to be approximately 93 percent and 12 percent respectively at minus zero point eight two volts versus RAG. After it's development, this tannic paved the way for researchers in the field of electrolysis to explore these transition metals single atom catalysts in artificial photosynthesis as well as other energy conversion applications.