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 until all polymers and salt are dissolved and a clear solution is observed.</li>
	</ol>
	</li>
	<li>Electrospinning to produce polymer fibers
	<ol>
		<li>Set conventional electrospinning parameters as: 15 kV of static electric voltage, 15 cm of air gap distance, a collection substrate of 8 &#215; 8 cm carbon fiber paper (CFP) with - 4 kV electric voltage.</li>
		<li>Extract 5 mL of precursor solution into a 5 mL syringe, start syringe pump at a flow rate of 1.2 mL h&#8722;1, then start the electrospinning. 
		NOTE: A high voltage is used for electrospinning. Even though very low current goes through the system during spinning process, an insulated wood cabinet is suggested.</li>
		<li>Take out the CFP substrate once the electrospinning process is finished. A polymer fiber film covers the CFP surface.</li>
	</ol>
	</li>
	<li>Annealing polymer fibers into carbon nanotubes
	<ol>
		<li>Heat the as-prepared polymer fiber film/CFP to 300 &#176;C in 1.5 h in a box furnace, and keep the temperature for 0.5 h to oxidize the polymers. 
		NOTE: After the oxidization process, the nanofibers (NFs) are self-detached from the carbon paper resulting in the freestanding film.</li>
		<li>Use a pair of scissors to cut those nanofibers into smaller pieces (~ 0.5 cm &#215; 2cm) and place them into a quartz boat.</li>
		<li>Place the quartz boat into a tube furnace, and deaerate it within forming gas (5% H2 in Ar) atmosphere. Keep the gas flow rate as 100 sccm and the pressure as 1 Tor.</li>
		<li>Heat up within 10 min ramping to 300 &#176;C, and 2 h ramping to 750 &#176;C, where it is maintained for another 1 h and followed by the natural cooling down.</li>
	</ol>
	</li>
	<li>Ball mill as-synthesized NiN-GS catalyst for 5 min to nanopowders for catalysis and characterizations.</li>
</ol>
2. Electrochemical CO2 Reduction Measurements
<ol>
	<li>Cell and Electrodes
	<ol>
		<li>To prepare 0.1 M KHCO3 electrolyte for the following measurements, first dissolve 2.5 g of KHCO3 in 250 mL of ultrapure water, then purify the electrolyte by electrolysis between two graphite rods at 0.1 mA for 24 h to remove any trace amount of metal ions.</li>
		<li>Take a fresh (electrochemically) polished glassy carbon (1 cm &#215; 2 cm), and cover its backside with an electrochemically inert, hydrophobic wax, as the working electrode substrate.</li>
		<li>Take a 4 mL scintillation vial, mix 5 mg of as-prepared NiN-GS catalyst with 1 mL of ethanol and 100 &#181;L of ionomer solution (5% in 2-proponal) within the vial, and sonicate for 20 min to get a homogeneous catalyst ink suspension.</li>
		<li>Pipet 80 &#181;L of the catalyst ink onto 2 cm2 glassy carbon surface (0.2 mg cm-2 mass loading), and vacuum dry the catalyst covered electrode in a desiccator prior to usage.</li>
		<li>Use a platinum foil and a saturated calomel electrode (SCE) as the counter and reference electrode, respectively.</li>
		<li>Use a customized gastight H-type glass cell, separated by proton exchange membrane, for the above 3 electrodes assembling as well as the electrochemical tests running.</li>
		<li>Place the working electrode and SCE reference electrode in one compartment of H-cell, and Pt foil electrode in the other chamber. Inject ~ 25 mL of electrolyte in each compartment of H-cell.</li>
		<li>Connect the 3 electrodes in H-cell to the electrochemical work station for potential control.</li>
		<li>Bubble the electrolyte with N2 at 50.0 sccm (monitored by mass flow controller) for 30 min toward N2-saturated 0.1 M KHCO3.</li>
		<li>Select Cyclic Voltammetry (CV) technique in EC-Lab software, set &#34;E Range (potential)&#34; as &#34;-10 V to 10 V&#34;, &#34;I Range (current)&#34; as &#34;Auto&#34;, perform 5 continuous CV scans from -0.5 V to -1.8 V (vs. SCE) at a scan rate of 50 mV/s in N2-saturated 0.1 M KHCO3.</li>
		<li>Change to 50 sccm CO2 gas flow, wait for 30 min toward CO2-saturated 0.1 M KHCO3 electrolyte and maintain the same CO2 flow throughout the following electrolysis.</li>
		<li>Select CV technique in EC-Lab software, set &#34;E Range (potential)&#34; as &#34;-10 V to 10 V&#34;, &#34;I Range (current)&#34; as &#34;Auto&#34;, perform 5 continuous CV scans from -0.5 V to -1.8 V (vs. SCE) at a scan rate of 50 mV/s in CO2-saturated 0.1 M KHCO3.</li>
		<li>Use a pH Meter to determine the pH values of electrolytes, i.e., 0.1 M KHCO3 saturated with either N2 or CO2.</li>
		<li>Convert all potentials measured against SCE to the reversible hydrogen electrode (RHE) scale in this work using E (vs RHE) = E (vs SCE) + 0.244 V + 0.0591 &#215; pH.</li>
		<li>Determine solution resistance (Ru) in EC-Lab software by selecting Potentiostatic Electrochemical Impedance Spectroscopy (PEIS) technique, then set frequency range from 0.1 Hz to 200 kHz, record the resistance value.</li>
		<li>Manually compensate iR-drop as E (iR corrected vs RHE) = E (vs RHE) - Ru &#215; I (amps of average current).</li>
	</ol>
	</li>
	<li>CO2 reduction products analysis by on-line gas chromatograph (GC)
	<ol>
		<li>Employ a GC, equipped with a combination of molecular sieve 5A and micropacked columns, for gas products analysis during CO2RR. 
		NOTE: The detailed GC column types can be found in attached Table of Materials.</li>
		<li>Use a thermal conductivity detector (TCD) to quantify H2 concentration, and a flame ionization detector (FID) with a methanizer to quantitative analysis CO content and/or any other alkane species.</li>
		<li>Use two different standard gases for the calibration curves of H2 and CO concentration (H2: 100 and 1042 ppm; CO: 100 and 496.7 ppm; balanced with Argon).</li>
		<li>During electrolysis, maintain CO2 gas flow rate at 50.0 sccm, deliver CO2 into the cathodic compartment containing CO2-saturated 0.1 M KHCO3 electrolyte, and vent the exhaust into GC.</li>
		<li>Stepwise tune the voltage on working electrode, ranging from -0.3 to -1.0 V vs. RHE, keep ~ 15 min for each potential and record the corresponding chronoamperimetric curve.</li>
		<li>Determine the H2 and CO contents in the exhaust from TCD and FID signals, respectively. 
		NOTE: The gas products are sampled after a continuous electrolysis of ~ 10 min under each potential. The 50 sccm CO2 gas, mixed with continuously produced H2 and CO, flowed through the sampling loop (1 mL) of GC during the electrolysis.</li>
		<li>Calculate the partial current density for a given gas product as below: 
		<img alt="Equation" src="/files/ftp_upload/57380/57380eq2.jpg" /> 
		where xi is the volume fraction of certain product determined by online GC referenced to calibration curves from two standard gas samples (Scott and Airgas), v is the flow rate of 50 sccm, ni is the number of electrons involved, p0 = 101.3 kPa, and F is the Faradaic constant and R is the gas constant.</li>
		<li>Calculate the corresponding Faradaic efficiency (FE) at each potential as <img alt="Equation" src="/files/ftp_upload/57380/57380eq7.jpg" />. 
		NOTE: The overall Faradaic efficiency could be within a range of 90 to 110% due to the errors from gas flow rate, current density, and gas concentration analysis on GC.</li>
	</ol>
	</li>
</ol>

<ol><li>Lewis, N. S., Nocera, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Powering+the+planet%3A+Chemical+challenges+in+solar+energy+utilization%5BTitle%5D)+AND+%22P.+Natl.+Acad.+Sci.+USA%22%5BJournal%5D)">Powering the planet: Chemical challenges in solar energy utilization.</a> P. Natl. Acad. Sci. USA. 103, 15729-15735 (2006).</li>
<li>Appel, A. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Frontiers%2C+Opportunities%2C+and+Challenges+in+Biochemical+and+Chemical+Catalysis+of+CO2+Fixation%5BTitle%5D)+AND+%22Chem.+Rev%22%5BJournal%5D)">Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation.</a> Chem. Rev. 113, 6621-6658 (2013).</li>
<li>Jhong, H. R., Ma, S. C., Kenis, P. J. A. <a target="_blank" href="https://www.sciencedirect.com/science/article/pii/S2211339813000294">Electrochemical conversion of CO2 to useful chemicals: current status, remaining challenges, and future opportunities.</a> Curr. Opin. Chem. Eng. 2, 191-199 (2013).</li>
<li>Ashford, D. L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Molecular+Chromophore-Catalyst+Assemblies+for+Solar+Fuel+Applications%5BTitle%5D)+AND+%22Chem.+Rev%22%5BJournal%5D)">Molecular Chromophore-Catalyst Assemblies for Solar Fuel Applications.</a> Chem. Rev. 115, 13006-13049 (2015).</li>
<li>Nocera, D. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Solar+Fuels+and+Solar+Chemicals+Industry%5BTitle%5D)+AND+%22Accounts.+Chem.+Res%22%5BJournal%5D)">Solar Fuels and Solar Chemicals Industry.</a> Accounts. Chem. Res. 50, 616-619 (2017).</li>
<li>Larrazabal, G. O., Martin, A. J., Perez-Ramirez, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Building+Blocks+for+High+Performance+in+Electrocatalytic+CO2+Reduction%3A+Materials%2C+Optimization+Strategies%2C+and+Device+Engineering%5BTitle%5D)+AND+%22J.+Phys.+Chem.+Lett%22%5BJournal%5D)">Building Blocks for High Performance in Electrocatalytic CO2 Reduction: Materials, Optimization Strategies, and Device Engineering.</a> J. Phys. Chem. Lett. 8, 3933-3944 (2017).</li>
<li>Hori, Y., Wakebe, H., Tsukamoto, T., Koga, O. <a target="_blank" href="https://www.sciencedirect.com/science/article/pii/0013468694851727">Electrocatalytic Process of Co Selectivity in Electrochemical Reduction of Co2 at Metal-Electrodes in Aqueous-Media.</a> Electrochim. Acta. 39, 1833-1839 (1994).</li>
<li>Hori, Y. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aHori%2C+Y+%22Modern+aspects+of+electrochemistry%22">Modern aspects of electrochemistry</a>. , Springer. 89-189 (2008).</li>
<li>Lin, S., et al. <a target="_blank" href="http://science.sciencemag.org/content/349/6253/1208">Covalent organic frameworks comprising cobalt porphyrins for catalytic CO2 reduction in water.</a> Science. 349, 1208-1213 (2015).</li>
<li>Zhang, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Highly+selective+and+active+CO2+reduction+electrocatalysts+based+on+cobalt+phthalocyanine%2Fcarbon+nanotube+hybrid+structures%5BTitle%5D)+AND+%22Nat.+Commun%22%5BJournal%5D)">Highly selective and active CO2 reduction electrocatalysts based on cobalt phthalocyanine/carbon nanotube hybrid structures.</a> Nat. Commun. 8, 14675(2017).</li>
<li>Zhao, C. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Ionic+Exchange+of+Metal+Organic+Frameworks+to+Access+Single+Nickel+Sites+for+Efficient+Electroreduction+of+CO2%5BTitle%5D)+AND+%22J.+Am.+Chem.+Soc%22%5BJournal%5D)">Ionic Exchange of Metal Organic Frameworks to Access Single Nickel Sites for Efficient Electroreduction of CO2.</a> J. Am. Chem. Soc. 139, 8078-8081 (2017).</li>
<li>Manthiram, K., Beberwyck, B. J., Aivisatos, A. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Enhanced+Electrochemical+Methanation+of+Carbon+Dioxide+with+a+Dispersible+Nanoscale+Copper+Catalyst%5BTitle%5D)+AND+%22J.+Am.+Chem.+Soc%22%5BJournal%5D)">Enhanced Electrochemical Methanation of Carbon Dioxide with a Dispersible Nanoscale Copper Catalyst.</a> J. Am. Chem. Soc. 136, 13319-13325 (2014).</li>
<li>Yang, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Catalytically+active+Au-O%28OH%29%28x%29-species+stabilized+by+alkali+ions+on+zeolites+and+mesoporous+oxides%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">Catalytically active Au-O(OH)(x)-species stabilized by alkali ions on zeolites and mesoporous oxides.</a> Science. 346, 1498-1501 (2014).</li>
<li>Manthiram, K., Surendranath, Y., Alivisatos, A. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Dendritic+Assembly+of+Gold+Nanoparticles+during+Fuel-Forming+Electrocatalysis%5BTitle%5D)+AND+%22J.+Am.+Chem.+Soc%22%5BJournal%5D)">Dendritic Assembly of Gold Nanoparticles during Fuel-Forming Electrocatalysis.</a> J. Am. Chem. Soc. 136, 7237-7240 (2014).</li>
<li>Amini, S., Garay, J., Liu, G., Balandin, A. A., Abbaschian, R. <a target="_blank" href="http://aip.scitation.org/doi/abs/10.1063/1.3498815">Growth of large-area graphene films from metal-carbon melts.</a> J. Appl. Phys. 108, 094321(2010).</li>
<li>Krasheninnikov, A. V., Lehtinen, P. O., Foster, A. S., Pyykkö, P., Nieminen, R. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Embedding+Transition-Metal+Atoms+in+Graphene%3A+Structure%2C+Bonding%2C+and+Magnetism%5BTitle%5D)+AND+%22Phys.+Rev.+Lett%22%5BJournal%5D)">Embedding Transition-Metal Atoms in Graphene: Structure, Bonding, and Magnetism.</a> Phys. Rev. Lett. 102, 126807(2009).</li>
<li>Jiang, K., Siahrostami, S., Zheng, T., Hu, Y., Hwang, S., Stavitski, E., Peng, Y., Dynes, J., Gangishetty, M., Su, D., Attenkofer, K., Wang, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Isolated+Ni+Single+Atoms+in+Graphene+Nanosheets+for+High-performance+CO2+Reduction%5BTitle%5D)+AND+%22Energy+Environ.+Sci%22%5BJournal%5D)">Isolated Ni Single Atoms in Graphene Nanosheets for High-performance CO2 Reduction.</a> Energy Environ. Sci. , Advance Article (2018).</li>
<li>Rodríguez-Manzo, J. A., Cretu, O., Banhart, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Trapping+of+Metal+Atoms+in+Vacancies+of+Carbon+Nanotubes+and+Graphene%5BTitle%5D)+AND+%22ACS+Nano%22%5BJournal%5D)">Trapping of Metal Atoms in Vacancies of Carbon Nanotubes and Graphene.</a> ACS Nano. 4, 3422-3428 (2010).</li>
<li>Jiang, K., et al. <a target="_blank" href="https://www.sciencedirect.com/science/article/pii/S2451929417304047">Transition metal atoms in a graphene shell as active centers for highly efficient artificial photosynthesis.</a> Chem. 3, 950-960 (2017).</li>
<li>Jiang, K., Wang, H., Cai, W. B., Wang, H. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Li+Electrochemical+Tuning+of+Metal+Oxide+for+Highly+Selective+CO2+Reduction%5BTitle%5D)+AND+%22ACS+Nano%22%5BJournal%5D)">Li Electrochemical Tuning of Metal Oxide for Highly Selective CO2 Reduction.</a> ACS Nano. 11, 6451-6458 (2017).</li>
<li>Jiang, K., et al. <a target="_blank" href="http://pubs.acs.org/doi/abs/10.1021/acssuschemeng.7b02380">Silver Nanoparticles with Surface-Bonded Oxygen for Highly Selective CO2 Reduction.</a> ACS Sustain Chem. Eng. 5, 8529-8534 (2017).</li>
<li>Siahrostami, S., et al. <a target="_blank" href="http://pubs.acs.org/doi/abs/10.1021/acssuschemeng.7b03031">Theoretical Investigations into Defected Graphene for Electrochemical Reduction of CO2.</a> ACS Sustain Chem. Eng. 5, 11080-11085 (2017).</li>
<li>Jiang, K., et al. <a target="_blank" href="https://www.nature.com/articles/s41929-017-0009-x">Metal Ion Cycling of Cu Foil for Selective C-C Coupling in Electrochemical CO2 Reduction.</a> Nat. Catal. , (2017).</li>
</ol>

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><tbody><tr><td>syringe pump&amp;nbsp;</td><td>KD Scientific</td><td>KDS-100</td><td></td></tr><tr><td>tube furnance</td><td>Lindberg/Blue M</td><td>TF55035A-1</td><td></td></tr><tr><td>ball miller</td><td>SPEX SamplePrep</td><td>5100</td><td></td></tr><tr><td>electrochemical work station</td><td>BioLogic</td><td>VMP3</td><td></td></tr><tr><td>pH meter</td><td>Orion</td><td>320 PerpHecT&amp;nbsp;</td><td>2 points calibration before use</td></tr><tr><td>gas chromatograph</td><td>Shimadzu</td><td>GC-2014</td><td>a combined seperation system consisting of molecular sieve 5A, Hayesep Q, Hayesep T, and Hayesep N</td></tr><tr><td>mass flow controller</td><td>Alicat Scientific&amp;nbsp;</td><td>MC-50SCCM-D/5M</td><td></td></tr><tr><td>ultrapure water system</td><td>Millipore</td><td>Synergy</td><td></td></tr><tr><td>vacuum desiccator&amp;nbsp;</td><td>PolyLab</td><td>55205</td><td></td></tr><tr><td>polyacrylonitrile</td><td>Sigma-Aldrich</td><td>181315</td><td>M&lt;sub&gt;w&lt;/sub&gt;=150,000</td></tr><tr><td>polypyrrolidone</td><td>Sigma-Aldrich</td><td>437190</td><td>M&lt;sub&gt;w&lt;/sub&gt;=1,300,000</td></tr><tr><td>Ni(NO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;6H&lt;sub&gt;2&lt;/sub&gt;O</td><td>Sigma-Aldrich</td><td>244074</td><td></td></tr><tr><td>dicyandiamide</td><td>Sigma-Aldrich</td><td>D76609</td><td></td></tr><tr><td>dimethylformamide</td><td>Sigma-Aldrich</td><td>227056</td><td></td></tr><tr><td>carbon fiber paper&amp;nbsp;</td><td>AvCarb</td><td>MGL370</td><td></td></tr><tr><td>Nafion 117 membrane</td><td>Fuel Cell Store</td><td>117</td><td>used as proton exchange membrane in H-cell</td></tr><tr><td>KHCO&lt;sub&gt;3&lt;/sub&gt;</td><td>Sigma-Aldrich</td><td>431583</td><td>further purified by electrolysis</td></tr><tr><td>platinum foil&amp;nbsp;</td><td>Beantown Chemical</td><td>126580</td><td></td></tr><tr><td>saturated calomel electrode</td><td>CH Instruments</td><td>CHI150</td><td></td></tr><tr><td>glassy carbon electrode</td><td>HTW GmbH</td><td>SIGRADUR</td><td>1 cm &amp;times; 2 cm</td></tr><tr><td>wax</td><td>Apiezon</td><td>W-W100</td><td></td></tr><tr><td>Nafion 117 solution</td><td>Sigma-Aldrich</td><td>70160</td><td>used as ionomer in catalyst ink preparation&amp;nbsp;</td></tr><tr><td>forming gas</td><td>Airgas</td><td>UHP</td><td>5% H&lt;sub&gt;2&lt;/sub&gt; balanced with Ar</td></tr><tr><td>carbon dioxide</td><td>Airgas</td><td>LaserPlus</td><td></td></tr><tr><td>sandard gas</td><td>Airgas</td><td>customized</td><td>500 ppm CO, 500 ppm CH&lt;sub&gt;4&lt;/sub&gt;, 1000 ppm H&lt;sub&gt;2&lt;/sub&gt; balanced with Ar</td></tr><tr><td>sandard gas</td><td>Air Liquide</td><td>customized</td><td>100 ppm H&lt;sub&gt;2&lt;/sub&gt;, 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 ...

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

Research

JoVE Journal

Chemistry

18.1K Views.  Harvard University. Hier presenteren we een protocol voor de synthese en elektrochemische testen van overgangsmetalen enkele atomen gecoördineerd grafeen vacatures als actieve centra voor selectieve kooldioxide te beperken tot controle van de koolmonoxide in waterige oplossingen.

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

Reverse Microemulsion-mediated Synthesis of Monometallic and Bimetallic Early Transition Metal Carbide and Nitride Nanoparticles

A reverse microemulsion is used to encapsulate monometallic or bimetallic early transition metal oxide nanoparticles in microporous silica shells. The silica-encapsulated metal oxide nanoparticles are then carburized in a methane/hydrogen atmosphere at temperatures over 800 &#176;C to form silica-encapsulated early transition metal carbide nanoparticles. During the carburization process, the silica shells prevent the sintering of adjacent carbide nanoparticles while also preventing the deposition of excess surface carbon. Alternatively, the silica-encapsulated metal oxide nanoparticles can be nitridized in an ammonia atmosphere at temperatures over 800 &#176;C to form silica-encapsulated early transition metal nitride nanoparticles. By adjusting the reverse microemulsion parameters, the thickness of the silica shells, and the carburization/nitridation conditions, the transition metal carbide or nitride nanoparticles can be tuned to various sizes, compositions, and crystal phases. After carburization or nitridation, the silica shells are then removed using either a room-temperature aqueous ammonium bifluoride solution or a 0.1 to 0.5 M NaOH solution at 40-60 &#176;C. While the silica shells are dissolving, a high surface area support, such as carbon black, can be added to these solutions to obtain supported early transition metal carbide or nitride nanoparticles. If no high surface area support is added, then the nanoparticles can be stored as a nanodispersion or centrifuged to obtain a nanopowder.

A &#8220;removable ceramic coating method&#8221; is presented in visual format for the synthesis of non-sintered and metal-terminated monometallic and bimetallic early transition metal carbide and nitride nanoparticles with tunable sizes and crystal structures.

A reverse microemulsion is used to encapsulate monometallic or bimetallic early transition metal oxide nanoparticles in microporous silica shells. The ...

On the Preparation and Testing of Fuel Cell Catalysts Using the Thin Film Rotating Disk Electrode Method

We present a step-by-step tutorial to prepare proton exchange membrane fuel cell (PEMFC) catalysts, consisting of Pt nanoparticles (NPs) supported on a high surface area carbon, and to test their performance in thin film rotating disk electrode (TF-RDE) measurements. The TF-RDE methodology is widely used for catalyst screening; nevertheless, the measured performance sometimes considerably differs among research groups. These uncertainties impede the advancement of new catalyst materials and, consequently, several authors discussed possible best practice methods and the importance of benchmarking.
The visual tutorial highlights possible pitfalls in the TF-RDE testing of Pt/C catalysts. A synthesis and testing protocol to assess standard Pt/C catalysts is introduced that can be used together with polycrystalline Pt disks as benchmark catalysts. In particular, this study highlights how the properties of the catalyst film on the glassy carbon (GC) electrode influence the measured performance in TF-RDE testing. To obtain thin, homogeneous catalyst films, not only the catalyst preparation, but also the ink deposition and drying procedures are essential. It is demonstrated that an adjustment of the ink&#39;s pH might be necessary, and how simple control measurements can be used to check film quality. Once reproducible TF-RDE measurements are obtained, determining the Pt loading on the catalyst support (expressed as Pt wt%) and the electrochemical surface area is necessary to normalize the determined reaction rates to either surface area or Pt mass. For the surface area determination, so-called CO stripping, or the determination of the hydrogen underpotential deposition (Hupd) charge, are standard. For the determination of the Pt loading, a straightforward and cheap procedure using digestion in aqua regia with subsequent conversion of Pt(IV) to Pt(II) and UV-vis measurements is introduced.

Preparing and testing Pt/C fuel cell catalysts is subject to continuous discussion in the scientific community with respect to reproducibility and best practice. With the presented work, we intend to present a step-by-step tutorial to make and test Pt/C catalysts, which can serve as benchmark for novel catalyst systems.

We present a step-by-step tutorial to prepare proton exchange membrane fuel cell (PEMFC) catalysts, consisting of Pt nanoparticles (NPs) supported on a ...

Environment

Simple Methods for the Preparation of Non-noble Metal Bulk-electrodes for Electrocatalytic Applications

The rock material pentlandite with the composition Fe4.5Ni4.5S8 was synthesized via high temperature synthesis from the elements. The structure and composition of the material was characterized via powder X-ray diffraction (PXRD), M&#246;ssbauer spectroscopy (MB), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and energy dispersive X-ray spectroscopy (EDX). Two preparation methods of pentlandite bulk electrodes are presented. In the first approach a piece of synthetic pentlandite rock is directly contacted via a wire ferrule. The second approach utilizes pentlandite pellets, pressed from finely ground powder, which is immobilized in a Teflon casing. Both electrodes, whilst being prepared by an additive-free method, reveal high durability during electrocatalytic conversions in comparison to common drop-coating methods. We herein showcase the striking performance of such electrodes to accomplish the hydrogen evolution reaction (HER) and present a standardized method to evaluate the electrocatalytic performance by electrochemical and gas chromatographic methods. Furthermore, we report stability tests via potentiostatic methods at an overpotential of 0.6 V to explore the material limitations of the electrodes during electrolysis under industrial relevant conditions.

A facile preparation method of electrodes using the bulk material Fe4.5Ni4.5S8 is presented. This method provides an alternative technique to conventional electrode fabrication and describes prerequisites for unconventional electrode materials including a straightforward electrocatalytic testing method.

The rock material pentlandite with the composition Fe4.5Ni4.5S8 was synthesized via high temperature synthesis from the elements. The structure and ...

Synthesis of Platinum-nickel Nanowires and Optimization for Oxygen Reduction Performance

Platinum-nickel (Pt-Ni) nanowires were developed as fuel cell electrocatalysts, and were optimized for the performance and durability in the oxygen reduction reaction. Spontaneous galvanic displacement was used to deposit Pt layers onto Ni nanowire substrates. The synthesis approach produced catalysts with high specific activities and high Pt surface areas. Hydrogen annealing improved Pt and Ni mixing and specific activity. Acid leaching was used to preferentially remove Ni near the nanowire surface, and oxygen annealing was used to stabilize near-surface Ni, improving durability and minimizing Ni dissolution. These protocols detail the optimization of each post-synthesis processing step, including hydrogen annealing to 250 &#176;C, exposure to 0.1 M nitric acid, and oxygen annealing to 175 &#176;C. Through these steps, Pt-Ni nanowires produced increased activities more than an order of magnitude than Pt nanoparticles, while offering significant durability improvements. The presented protocols are based on Pt-Ni systems in the development of fuel cell catalysts. These techniques have also been used for a variety of metal combinations, and can be applied to develop catalysts for a number of electrochemical processes.

&#160; 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.

Platinum-nickel (Pt-Ni) nanowires were developed as fuel cell electrocatalysts, and were optimized for the performance and durability in the oxygen ...

Chemical Precipitation Method for the Synthesis of Nb2O5 Modified Bulk Nickel Catalysts with High Specific Surface Area

We demonstrate a method for the synthesis of NixNb1-xO catalysts with sponge-like and fold-like nanostructures. By varying the Nb:Ni ratio, a series of NixNb1-xO nanoparticles with different atomic compositions (x = 0.03, 0.08, 0.15, and 0.20) have been prepared by chemical precipitation. These NixNb1-xO catalysts are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The study revealed the sponge-like and fold-like appearance of Ni0.97Nb0.03O and Ni0.92Nb0.08O on the NiO surface, and the larger surface area of these NixNb1-xO catalysts, compared with the bulk NiO. Maximum surface area of 173 m2/g can be obtained for Ni0.92Nb0.08O catalysts. In addition, the catalytic hydroconversion of lignin-derived compounds using the synthesized Ni0.92Nb0.08O catalysts have been investigated.

Chemical Precipitation Method for the Synthesis of Nb2O5 Modified Bulk Nickel Catalysts with High Specific Surface Area

A protocol for the synthesis of sponge-like and fold-like Ni1-xNbxO nanoparticles by chemical precipitation is presented.

We demonstrate a method for the synthesis of NixNb1-xO catalysts with sponge-like and fold-like nanostructures. By varying the Nb:Ni ratio, a series of ...

Preparation of Polyoxometalate-based Photo-responsive Membranes for the Photo-activation of Manganese Oxide Catalysts

This paper presents a method to prepare charge-transfer chromophores using polyoxotungstate (PW12O403-), transition metal ions (Ce3+ or Co2+), and organic polymers, with the aim of photo-activating oxygen-evolving manganese oxide catalysts, which are important components in artificial photosynthesis. The cross-linking technique was applied to obtain a self-standing membrane with a high PW12O403- content. Incorporation and structure retention of PW12O403- within the polymer matrix were confirmed by FT-IR and micro-Raman spectroscopy, and optical characteristics were investigated by UV-Vis spectroscopy, which revealed successful construction of the metal-to-metal charge transfer (MMCT) unit. After deposition of MnOx oxygen evolving catalysts, photocurrent measurements under visible light irradiation verified the sequential charge transfer, Mn &#8594; MMCT unit &#8594; electrode, and&#160;the photocurrent intensity was consistent with the redox potential of the donor metal (Ce or Co). This method provides a new strategy for preparing integrated systems involving catalysts and photon-absorption parts for use with photo-functional materials.

Here, we present a protocol to prepare charge transfer chromophores based on a polyoxometalate/polymer composite membrane.

This paper presents a method to prepare charge-transfer chromophores using polyoxotungstate (PW12O403-), transition metal ions (Ce3+ or Co2+), and organic ...

CO2 Photoreduction to CH4 Performance Under Concentrating Solar Light

We demonstrate a method for the enhancement of CO2 photoreduction. As the driving force of a photocatalytic reaction is from solar light, the basic idea is to use concentration technology to raise the incident solar light intensity. Concentrating a large-area light onto a small area cannot only increase light intensity, but also reduce the catalyst amount, as well as the reactor volume, and increase the surface temperature. The concentration of light can be realized by different devices. In this manuscript, it is realized by a Fresnel lens. The light penetrates the lens and is concentrated on a disc-shaped catalyst. The results show that both the reaction rate and the total yield are efficiently increased. The method can be applied to most CO2 photoreduction catalysts, as well as to similar reactions with a low reaction rate at natural light.

CO2 Photoreduction to CH4 Performance Under Concentrating Solar Light

We present a protocol for improving the performance of CO2 photoreduction to CH4 by heightening the incident light intensity via concentrating solar energy technology.

We demonstrate a method for the enhancement of CO2 photoreduction. As the driving force of a photocatalytic reaction is from solar light, the basic idea ...

Engineering

Multiscale Sampling of a Heterogeneous Water/Metal Catalyst Interface using Density Functional Theory and Force-Field Molecular Dynamics

A significant number of heterogeneously-catalyzed chemical processes occur under liquid conditions, but simulating catalyst function under such conditions is challenging when it is necessary to include the solvent molecules. The bond breaking and forming processes modeled in these systems necessitate the use of quantum chemical methods. Since molecules in the liquid phase are under constant thermal motion, simulations must also include configurational sampling. This means that multiple configurations of liquid molecules must be simulated for each catalytic species of interest. The goal of the protocol presented here is to generate and sample trajectories of configurations of liquid water molecules around catalytic species on flat transition metal surfaces in a way that balances chemical accuracy with computational expense. Specifically, force field molecular dynamics (FFMD) simulations are used to generate configurations of liquid molecules that can subsequently be used in quantum mechanics-based methods such as density functional theory or ab initio molecular dynamics. To illustrate this, in this manuscript, the protocol is used for catalytic intermediates that could be involved in the pathway for the decomposition of glycerol (C3H8O3). The structures that are generated using FFMD are modeled in DFT in order to estimate the enthalpies of solvation of the catalytic species and to identify how H2O molecules participate in catalytic decompositions.

The goal of the protocol presented here is to generate and sample trajectories of configurations of liquid water molecules around catalytic species on a flat transition metal surface. The sampled configurations can be used as starting structures in quantum mechanics-based methods.

A significant number of heterogeneously-catalyzed chemical processes occur under liquid conditions, but simulating catalyst function under such conditions ...

Tuning the Acidity of Pt/ CNTs Catalysts for Hydrodeoxygenation of Diphenyl Ether

We herein present a method for the synthesis of HNbWO6, HNbMoO6, HTaWO6 solid acid nanosheet modified Pt/CNTs. By varying the weight of various solid acid nanosheets, a series of Pt/xHMNO6/CNTs with different solid acid compositions (x = 5, 20 wt%; M = Nb, Ta; N = Mo, W) have been prepared by carbon nanotube pretreatment, protonic exchange, solid acid exfoliation, aggregation and finally Pt particles impregnation. The Pt/xHMNO6/CNTs are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and NH3-temperature programmed desorption. The study revealed that HNbWO6 nanosheets were attached on CNTs, with some edges of the nanosheets being bent in shape. The acid strength of the supported Pt catalysts increases in the following order: Pt/CNTs &#60; Pt/5HNbWO6/CNTs &#60; Pt/20HNbMoO6/CNTs &#60; Pt/20HNbWO6/CNTs &#60; Pt/20HTaWO6/CNTs. In addition, the catalytic hydroconversion of lignin-derived model compound: diphenyl ether using the synthesized Pt/20HNbWO6 catalyst has been investigated.

A protocol for the synthesis of HNbWO6, HNbMoO6, HTaWO6 solid acid nanosheet modified Pt/CNTs is presented.

We herein present a method for the synthesis of HNbWO6, HNbMoO6, HTaWO6 solid acid nanosheet modified Pt/CNTs. By varying the weight of various solid acid ...

Synthesis of Metal Nanoparticles Supported on Carbon Nanotube with Doped Co and N Atoms and its Catalytic Applications in Hydrogen Production

A method for facile synthesis of nanostructured catalysts supported on carbon nanotubes with atomically dispersed cobalt and nitrogen dopant is presented herein. The novel strategy is based on a facile one-pot pyrolysis treatment of cobalt (II) acetylacetonate and nitrogen-rich organic precursors under Ar atmosphere at 800 &#176;C, resulting in the formation of Co- and N- co-doped carbon nanotube with earthworm-like morphology. The obtained catalyst was found to have a high density of defect sites, as confirmed by Raman spectroscopy. Here, cobalt (II) nanoparticles were stabilized on the atomically dispersed cobalt- and nitrogen-doped carbon nanotubes. The catalyst was confirmed to be effective in the catalytic hydrolysis of ammonia borane, in which the turnover frequency was 5.87 mol H2&#183;molCo-1&#183;min-1, and the specific hydrogen generation rate was determined to be 2447 mL H2&#183;gCo-1&#183;min-1. A synergistic function between the Co nanoparticle and the doped carbon nanotubes was proposed for the first time in the catalytic hydrolysis of ammonia borane reaction under a mild condition. The resulting hydrogen production with its high energy density and minimal refueling time could be suitable for future development as energy sources for mobile and stationary applications such as road trucks and forklifts in transport and logistics.

Here, we present a protocol to synthesize Co nanoparticles supported on carbon nanotubes with Co- and N- dopants for hydrogen productions.

A method for facile synthesis of nanostructured catalysts supported on carbon nanotubes with atomically dispersed cobalt and nitrogen dopant is presented ...

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

Samenvatting

Meer video's verkennen