Results for “Pyrolysis” — Page 86

DOE OSTI2023

Revealing the Sodium Storage Mechanisms in Hard Carbon Pores

Abstract Hard carbon (HC) is the most promising anode for the commercialization of sodium‐ion batteries (NIBs); however, a general mechanism for sodium storage in HC remains unclear, obstructing the development of highly efficient anodes for NIBs. To elucidate the mechanism of sodium storage in the pores, operando synchrotron small‐angle X‐ray scattering, wide‐angle X‐ray scattering, X‐ray absorption near edge structure, Raman spectroscopy, and galvanostatic measurements are combined. The multimodal approach provides mechanistic insights into the sodium pore‐filling process for different HC microstructures including the pore sizes that are preferentially filled, the extent to which different pore sizes are filled, and how the defect concentration influences pore filling. It is observed that sodium in the larger pores has an increased pseudo‐metallic sodium character consistent with larger sodium clusters. Furthermore, it is shown that the HCs prepared at higher pyrolysis temperatures have a larger capacity from sodium stored in the pores and that sodium intercalation between graphene layers occurs simultaneously with the pore filling in the plateau region. Opportunities are outlined to improve the performance of HC anodes by fully utilizing the pores for sodium storage, helping to pave the way for the commercialization of sodium ion batteries.

Kitsu Iglesias, Luis↗

DOE OSTI2018

Learning from the past: Are catalyst design principles transferrable between hydrodesulfurization and deoxygenation?

Not provided.

Engineering↗

DOE OSTI2019

Coarse grained computational fluid dynamic simulation of sands and biomass fluidization with a hybrid drag

We report the bubbling fluidized bed reactor is widely used in fast pyrolysis of biomass. Discrete simulation of this reactor is challenging due to many sand particles and lack of accurate drag corrections accounting for the interaction of two different solid particles with different properties. In this research, the computational cost is reduced by using the coarse-grained computational fluid dynamic-discrete element method, where many sand particles are lumped into a larger numerical parcel. The Syamlal–O'Brien drag model is used for sand, while Ganser correction coupled with Gidaspow model is used for the nonspherical biomass particles. This hybrid approach shows superior behavior over other drag models using pressure drops as a benchmark. The predicted bed height and pressure fluctuating frequencies compare well with experiment. Finally, the mixing of biomass is close to perfect if the superficial velocity is larger than four times the minimum fluidization velocity.

09 BIOMASS FUELS↗

DOE OSTI2019

Fragmentation and film growth in supersonic nanoaggregate aerosol deposition

Aerosol deposition with gas phase-synthesized chain-like nanoaggregates can yield dense coatings from the impaction of particles on a substrate; however, dense coating formation is not well understood. Here, we study coating consolidation at the single nanoaggregate level. Flame spray pyrolysis-made tin oxide nanoaggregates are mobility (size) filtered, accelerated through a de Laval nozzle, and impacted on alumina substrates. TEM images obtained from low velocity collection and supersonic deposition are compared via quantitative image analysis, which reveals that upon supersonic impact nanoaggregates fragment into smaller aggregates. Furthermore, this suggests that fragmentation is a key step in producing coatings denser than the depositing nanoaggregates themselves. We supplement experiments with detailed particle trajectory calculations, which show that the impact energies per atom during nanoaggregate deposition are below 0.2 eV/molecule. These results suggest that fragmentation can only occur at locations where nanoaggregates bonded by van der Waals and capillary interactions.

36 MATERIALS SCIENCE↗

DOE OSTI2018

From Metal–Organic Frameworks to Single‐Atom Fe Implanted N‐doped Porous Carbons: Efficient Oxygen Reduction in Both Alkaline and Acidic Media

Abstract It remains highly desired but a great challenge to achieve atomically dispersed metals in high loadings for efficient catalysis. Now porphyrinic metal–organic frameworks (MOFs) have been synthesized based on a novel mixed‐ligand strategy to afford high‐content (1.76 wt %) single‐atom (SA) iron‐implanted N‐doped porous carbon (Fe SA ‐N‐C) via pyrolysis. Thanks to the single‐atom Fe sites, hierarchical pores, oriented mesochannels and high conductivity, the optimized Fe SA ‐N‐C exhibits excellent oxygen reduction activity and stability, surpassing almost all non‐noble‐metal catalysts and state‐of‐the‐art Pt/C, in both alkaline and more challenging acidic media. More far‐reaching, this MOF‐based mixed‐ligand strategy opens a novel avenue to the precise fabrication of efficient single‐atom catalysts.

Jiao, Long↗

DOE OSTI2018

3D‐Printed Carbon Electrodes for Neurotransmitter Detection

Abstract Implantable neural microsensors have significantly advanced neuroscience research, but the geometry of most probes is limited by the fabrication methods. Therefore, new methods are needed for batch‐manufacturing with high reproducibility. Herein, a novel method is developed using two‐photon nanolithography followed by pyrolysis for fabrication of free‐standing microelectrodes with a carbon electroactive surface. 3D‐printed spherical and conical electrodes were characterized with slow scan cyclic voltammetry (CV). With fast‐scan CV, the electrodes showed low dopamine LODs of 11±1 n m (sphere) and 10±2 n m (cone), high sensitivity to multiple neurochemicals, and high reproducibility. Spherical microelectrodes were used to detect dopamine in a brain slice and in vivo, demonstrating they are robust enough for tissue implantation. This work is the first demonstration of 3D‐printing of free‐standing carbon electrodes; and the method is promising for batch fabrication of customized, implantable neural sensors.

Yang, Cheng↗

DOE OSTI2018

Nitrogen‐doped Carbon–CoO x Nanohybrids: A Precious Metal Free Cathode that Exceeds 1.0 W cm −2 Peak Power and 100 h Life in Anion‐Exchange Membrane Fuel Cells

Abstract Efficient and durable nonprecious metal electrocatalysts for the oxygen reduction (ORR) are highly desirable for several electrochemical devices, including anion exchange membrane fuel cells (AEMFCs). Here, a 2D planar electrocatalyst with CoO x embedded in nitrogen‐doped graphitic carbon (N‐C‐CoO x ) was created through the direct pyrolysis of a metal–organic complex with a NaCl template. The N‐C‐CoO x catalyst showed high ORR activity, indicated by excellent half‐wave (0.84 V vs. RHE) and onset (1.01 V vs. RHE) potentials. This high intrinsic activity was also observed in operating AEMFCs where the kinetic current was 100 mA cm −2 at 0.85 V. When paired with a radiation‐grafted ETFE powder ionomer, the N‐C‐CoO x AEMFC cathode was able to achieve extremely high peak power density (1.05 W cm −2 ) and mass transport limited current (3 A cm −2 ) for a precious metal free electrode. The N‐C‐CoO x cathode also showed good stability over 100 hours of operation with a voltage decay of only 15 % at 600 mA cm −2 under H 2 /air (CO 2 ‐free) reacting gas feeds. The N‐C‐CoO x cathode catalyst was also paired with a very low loading PtRu/C anode catalyst, to create AEMFCs with a total PGM loading of only 0.10 mg Pt‐Ru cm −2 capable of achieving 7.4 W mg −1 PGM as well as supporting a current of 0.7 A cm −2 at 0.6 V with H 2 /air (CO 2 free)—creating a cell that was able to meet the 2019 U.S. Department of Energy initial performance target of 0.6 V at 0.6 A cm −2 under H 2 /air with a PGM loading <0.125 mg cm −2 with AEMFCs for the first time.

Peng, Xiong↗

DOE OSTI2020

Understanding the Origin of Highly Selective CO 2 Electroreduction to CO on Ni,N‐doped Carbon Catalysts

Abstract Ni,N‐doped carbon catalysts have shown promising catalytic performance for CO 2 electroreduction (CO 2 R) to CO; this activity has often been attributed to the presence of nitrogen‐coordinated, single Ni atom active sites. However, experimentally confirming Ni−N bonding and correlating CO 2 reduction (CO 2 R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile‐derived Ni,N‐doped carbon electrocatalysts (Ni‐PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO 2 R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X‐ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square‐planar geometry that strongly resembles the active sites of molecular metal–porphyrin catalysts.

Koshy, David M.↗

DOE OSTI2021

Key Features of Polyimide‐Derived Carbon Molecular Sieves

Abstract Carbon molecular sieve (CMS) membranes have impressive separation properties; however, both chemical and morphology structures need to be understood better. Here we characterize CMS with the simplest polyimide (PI) PMDA/pPDA (PMDA=pyromellitic dianhydride, pPDA= p ‐phenylenediamine), using FTIR, solid‐state 15 N‐NMR and 13 C‐NMR, XPS, XRD, and Raman spectra to study chemical structure. We also compare gas separation properties for this CMS to a CMS derived from a more conventional PI precursor. The detailed characterization shows the presence of aromatic pyridinic, pyrrolic rings as well as graphitic, pyridonic components and a few other groups in both CMS types derived from the very different precursors. The CMS morphologies, while related to precursor and pyrolysis temperature details, show similarities consistent with a physical picture comprising distributed molecular sieving plate‐like structures. These results assist in understanding diverse CMS membrane separation performance.

Qiu, Wulin↗

DOE OSTI2023

Appealing sheath‐core spun high‐performance composite carbon molecular sieve membranes

Abstract Carbon molecular sieve (CMS) membranes are attractive candidates to meet requirements for challenging gas separations. The added ability to maintain such intrinsic properties in an asymmetric morphology with a structure that we term a “Pseudo Wheel+Hub & Spoke” asymmetric form offers new opportunities. For CMS membrane, specifically, the structure provides both selective layer support and low flow resistance even for high feed pressures and fluxes in CO 2 removal from natural gas. This capability is unavailable to even rigid glassy polymers due to the much higher modulus of CMS materials. Combining precursor asymmetric hollow fiber formation and optimized pyrolysis creates a defect free CMS proof‐of‐concept membrane for this application. Facile formation of the sheath‐core spun precursor with a 6FDA‐DAM sheath and Matrimid® core also avoids the need to seal defects before or after the carbonization of the precursors. The composite CMS membrane shows CO 2 /CH 4 (50 : 50) mixed gas feed with an attractive CO 2 /CH 4 selectivity of 64.3 and CO 2 permeance of 232 GPU at 35 °C. A key additional benefit of the approach is reduction in use of the more costly high performance 6FDA‐DAM in a composite sheath‐core CMS membrane with the “Pseudo Wheel+Hub & Spoke” structure.

Cao, Yuhe↗

DOE OSTI2018

From Metal-Organic Frameworks to Single-Atom Fe Implanted N-doped Porous Carbons: Efficient Oxygen Reduction in Both Alkaline and Acidic Media [From MOFs to Single Fe Atoms Implanted N-doped Porous Carbons: Efficient Oxygen Reduction in Both Alkaline and Acidic Media]

Here, it remains highly desired but a great challenge to achieve atomically dispersed metals in high loadings for efficient catalysis. Now porphyrinic metal–organic frameworks (MOFs) have been synthesized based on a novel mixed–ligand strategy to afford high–content (1.76 wt %) single–atom (SA) iron–implanted N–doped porous carbon (Fe SA –N–C) via pyrolysis. Thanks to the single–atom Fe sites, hierarchical pores, oriented mesochannels and high conductivity, the optimized Fe SA –N–C exhibits excellent oxygen reduction activity and stability, surpassing almost all non–noble–metal catalysts and state–of–the–art Pt/C, in both alkaline and more challenging acidic media. More far–reaching, this MOF–based mixed–ligand strategy opens a novel avenue to the precise fabrication of efficient single–atom catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2018

3D‐Printed Carbon Electrodes for Neurotransmitter Detection

Abstract Implantable neural microsensors have significantly advanced neuroscience research, but the geometry of most probes is limited by the fabrication methods. Therefore, new methods are needed for batch‐manufacturing with high reproducibility. Herein, a novel method is developed using two‐photon nanolithography followed by pyrolysis for fabrication of free‐standing microelectrodes with a carbon electroactive surface. 3D‐printed spherical and conical electrodes were characterized with slow scan cyclic voltammetry (CV). With fast‐scan CV, the electrodes showed low dopamine LODs of 11±1 n m (sphere) and 10±2 n m (cone), high sensitivity to multiple neurochemicals, and high reproducibility. Spherical microelectrodes were used to detect dopamine in a brain slice and in vivo, demonstrating they are robust enough for tissue implantation. This work is the first demonstration of 3D‐printing of free‐standing carbon electrodes; and the method is promising for batch fabrication of customized, implantable neural sensors.

Yang, Cheng↗

DOE OSTI2018

Nitrogen‐doped Carbon–CoO x Nanohybrids: A Precious Metal Free Cathode that Exceeds 1.0 W cm −2 Peak Power and 100 h Life in Anion‐Exchange Membrane Fuel Cells

Abstract Efficient and durable nonprecious metal electrocatalysts for the oxygen reduction (ORR) are highly desirable for several electrochemical devices, including anion exchange membrane fuel cells (AEMFCs). Here, a 2D planar electrocatalyst with CoO x embedded in nitrogen‐doped graphitic carbon (N‐C‐CoO x ) was created through the direct pyrolysis of a metal–organic complex with a NaCl template. The N‐C‐CoO x catalyst showed high ORR activity, indicated by excellent half‐wave (0.84 V vs. RHE) and onset (1.01 V vs. RHE) potentials. This high intrinsic activity was also observed in operating AEMFCs where the kinetic current was 100 mA cm −2 at 0.85 V. When paired with a radiation‐grafted ETFE powder ionomer, the N‐C‐CoO x AEMFC cathode was able to achieve extremely high peak power density (1.05 W cm −2 ) and mass transport limited current (3 A cm −2 ) for a precious metal free electrode. The N‐C‐CoO x cathode also showed good stability over 100 hours of operation with a voltage decay of only 15 % at 600 mA cm −2 under H 2 /air (CO 2 ‐free) reacting gas feeds. The N‐C‐CoO x cathode catalyst was also paired with a very low loading PtRu/C anode catalyst, to create AEMFCs with a total PGM loading of only 0.10 mg Pt‐Ru cm −2 capable of achieving 7.4 W mg −1 PGM as well as supporting a current of 0.7 A cm −2 at 0.6 V with H 2 /air (CO 2 free)—creating a cell that was able to meet the 2019 U.S. Department of Energy initial performance target of 0.6 V at 0.6 A cm −2 under H 2 /air with a PGM loading <0.125 mg cm −2 with AEMFCs for the first time.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Creation of Well-Defined “Mid-Sized” Micropores in Carbon Molecular Sieve Membranes

Carbon molecular sieve (CMS) membranes are choices for the separation of organic molecules due to their stability, ability to be scaled at practical form factors, and the avoidance of expensive supports or complex multi-step fabrication processes. A critical challenge is the creation of “mid-range” (e.g., 5–9 Å) microstructures that allow for facile permeation of organic solvents and selection between similarly-sized guest molecules. In this work, we create these microstructures via the pyrolysis of a microporous polymer (PIM-1) under low concentrations of hydrogen gas. The introduction of H 2 inhibits aromatization of the decomposing polymer and ultimately results in the creation of a well-defined bimodal pore network that exhibits an ultramicropore size of 5.1 Å. The H 2 assisted CMS dense membranes show a dramatic increase in p-xylene ideal permeability (≈15 times), with little loss in p-xylene/o-xylene selectivity (18.8 vs. 25.0) when compared to PIM-1 membranes pyrolyzed under a pure argon atmosphere. This method is successfully extended to hollow fiber membranes operating in organic solvent reverse osmosis mode, highlighting the potential of this approach to be translated from the laboratory to the field.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Key Features of Polyimide-Derived Carbon Molecular Sieves

Carbon molecular sieve (CMS) membranes have impressive separation properties; however, both chemical and morphology structures need to be understood better. Here we characterize CMS with the simplest polyimide (PI) PMDA/pPDA (PMDA=pyromellitic dianhydride, pPDA=p-phenylenediamine), using FTIR, solid-state 15 N-NMR and 13 C-NMR, XPS, XRD, and Raman spectra to study chemical structure. Here we also compare gas separation properties for this CMS to a CMS derived from a more conventional PI precursor. The detailed characterization shows the presence of aromatic pyridinic, pyrrolic rings as well as graphitic, pyridonic components and a few other groups in both CMS types derived from the very different precursors. The CMS morphologies, while related to precursor and pyrolysis temperature details, show similarities consistent with a physical picture comprising distributed molecular sieving plate-like structures. These results assist in understanding diverse CMS membrane separation performance.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Experimental Observation of Hydrocarbon Growth by Resonance‐Stabilized Radical–Radical Chain Reaction

Abstract Rapid molecular‐weight growth of hydrocarbons occurs in flames, in industrial synthesis, and potentially in cold astrochemical environments. A variety of high‐ and low‐temperature chemical mechanisms have been proposed and confirmed, but more facile pathways may be needed to explain observations. We provide laboratory confirmation in a controlled pyrolysis environment of a recently proposed mechanism, radical–radical chain reactions of resonance‐stabilized species. The recombination reaction of phenyl (c‐C 6 H 5 ) and benzyl (c‐C 6 H 5 CH 2 ) radicals produces both diphenylmethane and diphenylmethyl radicals, the concentration of the latter increasing with rising temperature. A second phenyl addition to the product radical forms both triphenylmethane and triphenylmethyl radicals, confirming the propagation of radical–radical chain reactions under the experimental conditions of high temperature (1100–1600 K) and low pressure (ca. 3 kPa). Similar chain reactions may contribute to particle growth in flames, the interstellar medium, and industrial reactors.

Couch, David E.↗

DOE OSTI2022

Covalent Organic Framework (COF) Derived Ni–N–C Catalysts for Electrochemical CO 2 Reduction: Unraveling Fundamental Kinetic and Structural Parameters of the Active Sites

Electrochemical CO 2 reduction is a potential approach to convert CO 2 into valuable chemicals using electricity as feedstock. Abundant and affordable catalyst materials are needed to upscale this process in a sustainable manner. Nickel-nitrogen-doped carbon (Ni-N-C) is an efficient catalyst for CO 2 reduction to CO, and the single-site Ni-N x motif is believed to be the active site. However, critical metrics for its catalytic activity, such as active site density and intrinsic turnover frequency, so far lack systematic discussion. In this work, we prepared a set of covalent organic framework (COF)-derived Ni-N-C catalysts, for which the Ni-N x content could be adjusted by the pyrolysis temperature. The combination of high-angle annular dark-field scanning transmission electron microscopy and extended X-ray absorption fine structure evidenced the presence of Ni single-sites, and quantitative X-ray photoemission addressed the relation between active site density and turnover frequency.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Appealing sheath-core spun high-performance composite carbon molecular sieve membranes

Carbon molecular sieve (CMS) membranes are attractive candidates to meet requirements for challenging gas separations. The added ability to maintain such intrinsic properties in an asymmetric morphology with a structure that we term a “Pseudo Wheel+Hub & Spoke” asymmetric form offers new opportunities. For CMS membrane, specifically, the structure provides both selective layer support and low flow resistance even for high feed pressures and fluxes in CO 2 removal from natural gas. Further, this capability is unavailable to even rigid glassy polymers due to the much higher modulus of CMS materials. Combining precursor asymmetric hollow fiber formation and optimized pyrolysis creates a defect free CMS proof-of-concept membrane for this application. Facile formation of the sheath-core spun precursor with a 6FDA-DAM sheath and Matrimid® core also avoids the need to seal defects before or after the carbonization of the precursors. The composite CMS membrane shows CO 2 /CH 4 (50 : 50) mixed gas feed with an attractive CO 2 /CH 4 selectivity of 64.3 and CO 2 permeance of 232 GPU at 35 °C. A key additional benefit of the approach is reduction in use of the more costly high performance 6FDA-DAM in a composite sheath-core CMS membrane with the “Pseudo Wheel+Hub & Spoke” structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗