| 1. |
- Fretz, Samuel Joseph, 1987, et al.
(författare)
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Amine- and Amide-Functionalized Mesoporous Carbons: A Strategy for Improving Sulfur/Host Interactions in Li-S Batteries
- 2020
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Ingår i: Batteries and Supercaps. - : Wiley. - 2566-6223. ; 3:8, s. 757-765
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Tidskriftsartikel (refereegranskat)abstract
- Lithium-sulfur (Li-S) batteries are of great interest due to their potentially high energy density, but the low electronic conductivity of both the sulfur (S-8) cathode active material and the final discharge product lithium sulfide (Li2S) require the use of a conductive host. Usually made of relatively hydrophobic carbon, such hosts are typically ill-suited to retain polar discharge products such as the intermediate lithium polysulfides (LiPs) and the final Li2S. Herein, we propose a route to increase the sulfur utilization by functionalizing the surface of ordered mesoporous carbon CMK3 with polar groups. These derivatized CMK3 materials are made using a simple two-step procedure of bromomethylation and subsequent nucleophilic substitution with amine or amide nucleophiles. We demonstrate that, compared to the unfunctionalized control, these modified CMK3 surfaces have considerably larger binding energies with LiPs and Li2S, which are proposed to aid the electrochemical conversion between S-8 and Li2S by keeping the LiPs species in close proximity to the carbon surface during Li-S battery cycling. As a result, the functionalized cathodes exhibit significantly improved specific capacities relative to their unmodified precursor.
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| 2. |
- Fretz, Samuel Joseph, 1987, et al.
(författare)
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Influence of Iron Salt Anions on Formation and Oxygen Reduction Activity of Fe/N-Doped Mesoporous Carbon Fuel Cell Catalysts
- 2019
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Ingår i: ACS Omega. - : American Chemical Society (ACS). - 2470-1343. ; 4:18, s. 17662-17671
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Tidskriftsartikel (refereegranskat)abstract
- Doping carbon materials with transition metal ions can greatly expand their utility, given these metal ions' unique catalytic activity, for example, in oxygen reduction in proton exchange membrane fuel cells. Unlike main group dopants, a counter anion to the metal cation must be selected and this choice has hitherto received little attention for this synthesis method. Herein, we describe the profound effects that the anion has on the resultant iron/nitrogen-doped ordered mesoporous carbons (Fe-OMC). To increase the iron loading and the number of iron-centered catalytically active sites, we selected three iron salts Fe(OAc)2, Fe(OTf)2, and Fe(BF4)2·6H2O, which show greatly enhanced solubility in the liquid carbon precursor (furfurylamine) compared to FeCl3·6H2O. The increased solubility leads to a significantly higher iron loading in the Fe-OMC prepared with Fe(OTf)2, but the increase in performance as cathode catalysts in fuel cells is only marginal. The Fe-OMCs prepared with Fe(OAc)2 and Fe(BF4)2·6H2O exhibited similar or lower iron loadings compared to the Fe-OMC prepared with FeCl3·6H2O despite their much higher solubilities. Most importantly, the different iron salts affect not only the final iron loading, but also which type of iron species forms in the Fe-OMC with different types showing different catalytic activity.
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| 3. |
- Fretz, Samuel Joseph, 1987
(författare)
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Adding utility to carbon materials: introducing dopants using highly soluble metal salts and functionalizing surfaces via bromomethylation
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbon-based materials have received intense research interest over the past few decades due to their outstanding combination of properties including porosity, non-toxicity, chemical inertness, low density, and electrical conductivity, which has allowed them to find a wide array of applications including supercapacitors, batteries, CO2 capture, fuel cells and catalysis. To expand their utility, a variety of techniques have been developed to enhance their reactivity and functionality. One such method is doping, wherein heteroatoms (i.e. non-carbon elements) are purposefully incorporated into the carbon structure with the goal of introducing new reactivity to the material. In this thesis, several systematic studies were carried out on copper and iron salts as dopants for ordered mesoporous carbons (OMC). It was found that the selection of the counter anion to the metal cation has a profound influence on the resultant OMC’s structure, chemical composition, metal loadings, and the type of metal obtained (i.e. chelated ions or nanoparticles). We applied a host of characterization methods to elucidate the effect that the anion has on the transition metal-doped OMC. Many copper salts were used to create copper-doped OMCs (Cu-OMCs). High copper loadings of about 5-8 wt% were obtained from using Cu(BF4)2-nH2O as the dopant salt, compared to previous loadings of < 1 wt% using iron salts. The copper species was determined to be metallic copper (Cu0) nanoparticles with diameters of about 40-50 nm. The high copper loadings, however, were found to be deleterious for use as sulfur hosts in lithium-sulfur (Li-S) batteries, with reversible capacities about 50% lower than undoped OMCs. The Cu-OMCs were also tested for O2 reduction on rotating disc electrodes (RDEs), but their catalytic performance was found to be quite poor. The same approach of using different anions was applied to iron salts in the context of polymer electrolyte membrane fuel cells (PEMFCs). The anion was found to have a strong effect on the OMCs structure, iron loading, and O2 reduction activity. High iron loadings of above 3 wt% were obtained for some of the soluble salts, but their activity in PEMFCs did not increase appreciably compared to the standard chloride salt. Another method for increasing the utility of carbon materials is grafting or surface functionalization, which consists of covalently attaching small, organic molecules to the carbon surface. In the last part of this thesis, we report a novel grafting method – the bromomethylation reaction. Several carbon materials efficiently and reproducibly undergo this reaction and surface-bound bromomethyl groups are stable for months in ambient conditions. Subsequently, many nucleophiles can substitute bromide resulting in monolayer-functionalized surfaces tailored for a specific application. We employ diallylamine and ethylenediamine as nucleophiles to produce amine-functionalized carbons for use as the conductive additive for sulfur in Li-S batteries. Such carbons exhibit improved performance over their unmodified precursors demonstrating the utility of this two-step scheme for functionalizing carbon surfaces. We hope that this two-step method of introducing organic groups to carbon surfaces will find wide-spread use in many applications.
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| 4. |
- Fretz, Samuel Joseph, 1987
(författare)
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Adding Utility to Carbon Materials: Introducing Dopants Using Highly Soluble Metal Salts and Functionalizing Surfaces via Bromomethylation
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbon-based materials have received intense research interest over the past few decades due to their unique combination of properties including porosity, non-toxicity, chemical inertness, low density, and electrical conductivity, which has allowed them to find a wide array of applications including supercapacitors, batteries, CO2 capture, fuel cells, and catalysis. To expand their utility, a variety of techniques have been developed to enhance their reactivity and functionality. One such method is doping, wherein heteroatoms (i.e. non-carbon elements) are purposefully incorporated into the carbon structure with the goal of introducing new reactivity to the material. The first paper in this thesis focuses on using soluble Fe salts as dopants for iron/nitrogen-doped ordered mesoporous carbons (Fe-OMC). The anion was found to have a strong effect on the structure, Fe loading, and oxygen reduction reaction (ORR) activity of the Fe-OMC. High Fe loadings of above 3 wt% were obtained for one of the soluble salts, but their activity in polymer electrolyte membrane fuel cells (PEMFCs) did not increase appreciably compared to the standard chloride salt. Electron paramagnetic resonance (EPR) was used to gain insight into the structure and ORR activity of the various Fe species within each Fe-OMC. Another method for increasing the utility of carbon materials is grafting or surface functionalization, which consists of covalently attaching small, organic molecules to the carbon surface. In three papers of this thesis, we report a novel two-step method for the surface functionalization of high surface area carbon materials. The carbons are first subjected to the bromomethylation reaction then, in the second step, many nucleophiles can substitute bromide resulting in monolayer-functionalized surfaces that can be tailored for a specific application. Example nucleophiles include azide, amines, iodide, sulfite, and amide enolates. Several carbon materials efficiently and reproducibly undergo these reactions and the surface-bound groups are stable for months under ambient conditions. This two-step scheme has numerous advantages over other surface modification techniques for carbon including use of solution-phase reagents, minimal harm to the carbon framework, monolayer functionalization, and no carbon pretreatment steps. A total of 12 surface groups were synthesized, which demonstrates the synthetic flexibility of this two-step technique. Four of the twelve modified carbons were used as cathodes in lithium-sulfur (Li-S) batteries. When used with an electrolyte containing lithium nitrate (LiNO3), the functionalized cathodes show increased capacities by virtue of utilizing more S. When used with electrolytes lacking LiNO3, the surface groups attenuate the lithium polysulfide (LiPS) shuttle as measured by the much higher initial Coulombic efficiencies (ICEs) recorded for the functionalized cathodes relative to the unfunctionalized control. The observations with both electrolytes evidence strong interactions between the electroactive S and the surface groups. The higher binding energies (BEs) computed by density functional theory (DFT) support strong interactions between the surface groups and various sulfur species while cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) lend evidence for a significantly reduced LiPS shuttle on the functionalized carbon surfaces. Based on these results with Li-S batteries, we hope that this two-step method of introducing organic groups to carbon surfaces will find wide-spread use in many applications.
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| 5. |
- Fretz, Samuel Joseph, 1987, et al.
(författare)
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Bromomethylation of high-surface area carbons as a versatile synthon: Adjusting the electrode-electrolyte interface in lithium-sulfur batteries
- 2019
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 7:34, s. 20013-20025
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Tidskriftsartikel (refereegranskat)abstract
- A two-step procedure for the surface functionalization of carbon materials has been developed. In the first step, mesoporous carbon (CMK3), carbon black (XC-72R Vulcan), and activated carbon (AC) are bromomethylated efficiently under mild conditions using commercially available reagents, resulting in reproducible surface bromine loadings, the concentrations of which correlate to the carbon's surface area. The resulting bromomethylated materials display excellent stability over the course of months when stored under ambient conditions. In the second step, substitution reactions with a variety of nucleophiles proceed efficiently. Example nucleophiles include azide, amines, ammonia and iodide, and exhibit high conversion yields. To demonstrate the application of this two-step functionalization method, bromomethylated CMK3, Br-CMK3, was reacted with ethylenediamine (EN) to form EN-CMK3, which was used as the conductive host for the sulfur cathode in lithium-sulfur (Li-S) batteries. Impregnation of EN-CMK3 with a lithium polysulfide-containing electrolyte with either lithium nitrate (LiNO3) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the supporting electrolyte increases the battery performance relative to pristine CMK3. With LiNO3, the surface-bonded EN allows for increased sulfur use and results in higher capacities of ca. 300 mA h g-1; with LiTFSI, the EN groups attenuate the polysulfide shuttle (LiPS shuttle) and the initial charging efficiency (ICE) is increased substantially from 3% to 73%. These results provide a proof-of-principle of the versatility of bromomethylated carbons as a useful starting material for a variety of functional materials. © 2019 The Royal Society of Chemistry.
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| 6. |
- Fretz, Samuel Joseph, 1987, et al.
(författare)
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Lithium Sulfonate Functionalization of Carbon Cathodes as a Substitute for Lithium Nitrate in the Electrolyte of Lithium–Sulfur Batteries
- 2020
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-3028 .- 1616-301X. ; 30:35
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Tidskriftsartikel (refereegranskat)abstract
- A method for grafting lithium sulfonate (LiSO3) groups to carbon surfaces is developed and the resulting carbons are evaluated for their potential to reduce the lithium polysulfide (LiPS) shuttle in lithium–sulfur (Li–S) batteries, replacing the common electrolyte additive lithium nitrate (LiNO3). The LiSO3 groups are attached to the ordered mesoporous carbon (CMK3) surface via a three-step procedure to synthesize LiSO3-CMK3 by bromomethylation, sodium sulfite (Na2SO3) substitution, and cation exchange. As a comparison, ethylenediamine (EN)-substituted CMK3, EN-CMK3, is also synthesized and tested. When used as a cathode in Li–S batteries, the unfunctionalized CMK3 suffers from strong LiPS shuttling as evidenced by its low initial Coulombic efficiencies (ICEs, <10%) compared to its functionalized derivatives EN-CMK3 and LiSO3-CMK3 (ICEs >75%). Postcycling analysis reveals the benefits of cathode surface functionalization on the lithium anode via an attenuated LiPS shuttle. When monitored at open circuit, the functionalized cathodes maintain their cell voltages much better than the CMK3 control and concurrent electrochemical impedance spectroscopy reveals their higher total cell resistance, which provides evidence for a reduced LiPS shuttle in the vicinity of both electrodes. Overall, such surface groups show promise as cathode-immobilized “lithium nitrate mimics.”.
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