This work was supported by the Rowland Fellows Program at the Rowland Institute of Harvard University. This work was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation under award no. ECS-0335765. The CNS is part of Harvard University.

1. Preparation of Ni Single Atom Catalyst (NiN-GS)
<ol>
	<li>Preparation of electrospinning precursor solution
	<ol>
		<li>Take a 20 mL scintillation vial, dissolve 0.5 g of polyacrylonitrile (Mw=150,000), 0.5 g of polypyrrolidone (Mw=1,300,000), 0.5 g of Ni(NO3)2&#183;6H2O, and 0.1 g of dicyandiamide (DCDA) in 10 mL of dimethylformamide (DMF).</li>
		<li>Heat the DMF mixture to 80 &#176;C and keep the mixture at 80 &#176;C with constant stirring ...

<ol>
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In the above electrospinning process, two important steps should be noted in material synthesis procedures: 1) heating the DMF mixture (step 1.1.2), and 2) the pump rate adjusting (step 1.2.2) to match the spinning rate. The SEM image in Figure 1A shows the obtained carbon nanofibers interconnected with each other (~200 nm in diameter). They were broken into small pieces by ball milling for characterizations as shown in Figure 1B. Ni nanoparticles were uniformly...

Converting CO2 into chemicals or fuels using clean electricity is becoming increasingly important as a potential route to prevent further CO2 emissions1,2,3,4,5,6. However, this practical application is currently challenged by the low activity and selectivity of CO2 reduction reaction (CO2RR) due to the high kinetic barriers and the competition with hydrogen evolution reaction (HER) in aqueous media. Most of the traditional transition metal catalyst, such as Fe, Co, and Ni, exhibit low CO2RR selectivity due to their superb HER activities7,8. Effectively tuning their material properties to change the reaction pathways on these transition metal catalysts becomes critical to improve their CO2RR selectivity. Among different methods to modify the electronic properties of catalysts, dispersing metal atoms into a single-atom morphology attracts intensive attentions recently due to their dramatically changed catalytic behaviors compared to their bulk counterpart9,10,11. However, due to the high mobility of unbounded atoms, it is quite challenging to obtain single metal atoms without the presence of supportive materials. Therefore, a host matrix material with defects created to confine and coordinate with transition metal atoms is necessary. This could open up new opportunities to: 1) tune the electronic properties of transition metals as CO2RR active sites, and 2) at the same time maintain relatively simple atomic coordination for fundamental mechanism studies. In addition, those transition metal atoms trapped in a confined environment cannot be easily moved around during catalysis, which prevents the nucleation or reconstructions of surface atoms observed in many cases12,13,14.
Two-dimensional layered graphene is of particular interest as host for metal single atoms due to their high electron conductivity, chemical stability, and inertness to both CO2 reduction and HER catalytic reactions. More importantly, Fe, Co, and Ni metals were known to be able to catalyze the carbon graphitization process on their surface15. In short, those transition metals would alloy with carbon during the high temperature thermal annealing process. When the temperature drops, carbon starts to precipitate out of the alloying phase and is catalyzed to form graphene layers on the surface of transition metal. During this process, with graphene defects generated, metal single atoms would be trapped in those graphene defects as the active sites for CO2RR16,17,18,19. Here, we report this detailed protocol intending to help new practitioners in the field of single atom catalysis, as well as to provide an explicit demonstration of on-line CO2 reduction product analysis. More information can be found in our recently published article19 and a series of related works20,21,22,23.

<table border="0" cellpadding="0" cellspacing="0" width="829">
	<tbody>
		<tr height="23">
			<td height="23" style="height:23px;width:216px;">syringe pump&#160;</td>
			<td style="width:164px;">KD Scientific</td>
			<td style="width:151px;">KDS-100</td>
			<td style="width:299px;"></td>
		</tr>
		<tr height="23">
			<td height="23" style="height:23px;width:216px;">tube furnance</td>
			<td style="width:164px;">Lindberg/Blue M</td>
			<td style="width:151px;">TF55035A-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">ball miller</td>
			<td style="width:164px;">SPEX SamplePrep</td>
			<td style="width:151px;">5100</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">electrochemical work station</td>
			<td style="width:164px;">BioLogic</td>
			<td style="width:151px;">VMP3</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">pH meter</td>
			<td style="width:164px;">Orion</td>
			<td style="width:151px;">320 PerpHecT&#160;</td>
			<td>2 points calibration before use</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">gas chromatograph</td>
			<td style="width:164px;">Shimadzu</td>
			<td style="width:151px;">GC-2014</td>
			<td>a combined seperation system consisting of molecular sieve 5A, Hayesep Q, Hayesep T, and Hayesep N</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">mass flow controller</td>
			<td>Alicat Scientific&#160;</td>
			<td style="width:151px;">MC-50SCCM-D/5M</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">ultrapure water system</td>
			<td>Millipore</td>
			<td>Synergy</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">vacuum desiccator&#160;</td>
			<td>PolyLab</td>
			<td>55205</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">polyacrylonitrile</td>
			<td style="width:164px;">Sigma-Aldrich</td>
			<td style="width:151px;">181315</td>
			<td>Mw=150,000</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">polypyrrolidone</td>
			<td style="width:164px;">Sigma-Aldrich</td>
			<td style="width:151px;">437190</td>
			<td>Mw=1,300,000</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Ni(NO3)26H2O</td>
			<td style="width:164px;">Sigma-Aldrich</td>
			<td style="width:151px;">244074</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">dicyandiamide</td>
			<td style="width:164px;">Sigma-Aldrich</td>
			<td style="width:151px;">D76609</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">dimethylformamide</td>
			<td style="width:164px;">Sigma-Aldrich</td>
			<td style="width:151px;">227056</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">carbon fiber paper&#160;</td>
			<td style="width:164px;">AvCarb</td>
			<td style="width:151px;">MGL370</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Nafion 117 membrane</td>
			<td style="width:164px;">Fuel Cell Store</td>
			<td style="width:151px;">117</td>
			<td>used as proton exchange membrane in H-cell</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">KHCO3</td>
			<td style="width:164px;">Sigma-Aldrich</td>
			<td style="width:151px;">431583</td>
			<td>further purified by electrolysis</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">platinum foil&#160;</td>
			<td style="width:164px;">Beantown Chemical</td>
			<td style="width:151px;">126580</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">saturated calomel electrode</td>
			<td style="width:164px;">CH Instruments</td>
			<td style="width:151px;">CHI150</td>
			<td></td>
		</tr>
		<tr height="27">
			<td height="27" style="height:27px;width:216px;">glassy carbon electrode</td>
			<td style="width:164px;">HTW GmbH</td>
			<td style="width:151px;">SIGRADUR</td>
			<td>1 cm &#215; 2 cm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">wax</td>
			<td>Apiezon</td>
			<td style="width:151px;">W-W100</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Nafion 117 solution</td>
			<td>Sigma-Aldrich</td>
			<td>70160</td>
			<td>used as ionomer in catalyst ink preparation&#160;</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">forming gas</td>
			<td>Airgas</td>
			<td>UHP</td>
			<td>5% H2 balanced with Ar</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">carbon dioxide</td>
			<td>Airgas</td>
			<td>LaserPlus</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">sandard gas</td>
			<td>Airgas</td>
			<td>customized</td>
			<td>500 ppm CO, 500 ppm CH4, 1000 ppm H2 balanced with Ar</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">sandard gas</td>
			<td>Air Liquide</td>
			<td>customized</td>
			<td>100 ppm H2, 100 ppm CO and other alkanes balanced with Ar</td>
		</tr>
	</tbody>
</table>

synthesis and performance characterizations of transition metal single atom catalyst for electrochemical co2 reduction

This protocol presents both the synthesis method of the Ni single atom catalyst, and the electrochemical testing of its catalytic activity and selectivity in aqueous CO2 reduction. Different from traditional metal nanocrystals, the synthesis of metal single atoms involves a matrix material that can confine those single atoms and prevent them from aggregation. We report an electrospinning and thermal annealing method to prepare Ni single atoms dispersed and coordinated in a graphene shell, as active centers for CO2 reduction to CO. During the synthesis, N dopants play a critical role in generating graphene vacancies to trap Ni atoms. Aberration-corrected scanning transmission electron microscopy and three-dimensional atom probe tomography were employed to identify the single Ni atomic sites in graphene vacancies. Detailed setup of electrochemical CO2 reduction apparatus coupled with an on-line gas chromatography is also demonstrated. Compared to metallic Ni, Ni single atom catalyst exhibit dramatically improved CO2 reduction and suppressed H2 evolution side reaction.

Scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping images are shown in Figure 1 for the morphology characterization of NiN-GS. Three-dimensional atom probe tomography (3D-APT) results are shown in Figure 2 for the direct identification of single Ni sites distribution as well as their neighboring chemical environment. On-line electrochemical G...

Watch this Scientific Journal Video about Synthesis and Performance Characterizations of Transition Metal Single Atom Catalyst for Electrochemical CO2 Reduction at JoVE.com

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

Here, we present a protocol for the synthesis and electrochemical testing of transition metal single atoms coordinated in graphene vacancies as active centers for selective carbon dioxide reduction to carbon monoxide in aqueous solutions.

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

This protocol presents both the synthesis method of the Ni single atom catalyst, and the electrochemical testing of its catalytic activity and selectivity ...

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.

synthesis-performance-characterizations-transition-metal-single-atom

Research

JoVE Journal

Chemistry

17.8K Görüntüleme.  Harvard University. 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 tra...