Non-graphitic Carbon Research Articles

Enhanced K-storage boosted by compatible ester-based electrolyte for COFs-derived non-graphitic carbon

Effectiveness of non-graphitic carbon materials with disordered structures in potassium storage has promote it as high-performance K-storage anodes of potassium-ion batteries (PIBs). However, there are defects that have not been adequately addressed, in which unstable K-storage performance along with sluggish reaction kinetics usually result from the severe interfacial side reactions and large K+ radius. Herein, rational electrolyte regulation has been treated as a promising strategy to achieve superior K-storage performance for the developed non-graphitic carbon from covalent-organic-frameworks (COFs). Specifically, it give rise to excellent cycle stability of 250 mAh/g after 500 cycles at 50 mA/g and high rate performance of 170 mAh/g after 400 cycles at 1000 mA/g, respectively. The boosted K-storage performance is ascribed to the fast interfacial reaction kinetics dependent on inorganic-rich and stable solid electrolyte interphase (SEI) that derived from the aggregates-dominated solvation structures in the optimized concentrated electrolyte. These findings suggest opens up an effective strategy from electrolyte chemistry that significantly affected SEI for advancing high-performance PIBs.

Direct evidence of the ultramicroporous structure of carbon molecular sieves

We utilize gas sorption experiments, neutron pair distribution function, and small-angle X-ray scattering studies on carbon molecular sieves (CMSs) to gain new insight into their ultramicroporous structure. CMS materials derived from the pyrolysis of PIM-1 (PIM = polymer of intrinsic microporosity) under an inert atmosphere (PIM-1-CMS) and H2 atmosphere (4% H2-PIM-1-CMS) were studied. Neutron total scattering studies of these materials reveal these CMS materials to be mainly graphene-like ribbons with short-range atomic ordering. Small-angle X-ray scattering and low angle diffraction peaks corroborate the presence of well-defined ultramicroporosity observed in carbon dioxide and cryogenic neon adsorption studies suggesting, expectedly, that these materials are non-graphitizing carbon structures. Our findings provide microscopic evidence of the non-graphitizing and ribbon-like structure of these CMS materials and suggest a possible hypothetical microporous structure.

Improving graphenic quality by oxidative liberation of crosslinks in non-graphitizable carbons

This work assesses the potential for improving the crystallinity of non-graphitizable carbon materials by oxidative liberation of crosslinks. A model non-graphitizable carbon – sucrose char and two commercial carbon blacks of varied nanostructure have been used to study crystallinity improvements at two oxidative burnout levels. The virgin materials have been compared to their partially oxidized counterparts at a series of heat treatment temperatures using X-ray diffraction for bulk crystallinity analysis and high-resolution transmission electron microscopy for nanostructural characterization. Remarkable improvements were observed in the case of sucrose char where non-graphitizable sponge-like particles transformed into highly ordered graphenic flakes by the mechanism of oxidative liberation of crosslinks. The in-plane crystallite size and stacking height tripled after the liberation of the restrictive crosslinks. Molecular level changes after partial oxidation have been assessed using Raman spectroscopy while X-ray photoelectron spectroscopy is used to assess changes in elemental composition. Notable improvements were also observed in carbon blacks with the development of longer and lower tortuosity lamellae accompanied by expanded particle sizes. Fringe analysis algorithms have been applied to obtain lamellae statistics and highlight nuances in the nanostructure changes after partial oxidation and heat treatment.

Pyrolysis temperature dependence of sodium storage mechanism in non-graphitizing carbons

Hard carbons are the most investigated materials as negative electrode for Na-ion batteries, although the exact mechanism of sodium storage remains under debate. This work is focused on the study of the sodiation mechanism of non-graphitizing carbons (NGC) prepared in a wide range of pyrolysis temperatures (1000 °C–2500 °C), thus covering the whole range from hard carbons (HC) to glassy carbons (GC). Structural and textural characterizations show that increasing the pyrolysis temperature leads to NGCs with a more ordered structure, containing fewer heteroatoms and structural defects, and particularly leads to a developed closed microporosity. In this work, in situ Raman spectroscopy at different excitation wavelengths is used to clarify the mechanism of electrochemical sodiation: it is revealed that the intercalation of Na+ between the graphene layers occurs mainly during the sloping part of the galvanostatic profile, whereas the plateau at low voltage (below 0.1 V vs. Na+/Na) can be associated with the filling of micropores with sodium.

Iron Nanoparticles to Catalyze Graphitization of Cellulose for Energy Storage Applications

The production of highly graphitic carbon from bioresources is an environmentally friendly approach to synthesize graphene for energy storage applications. Iron catalytic graphitization of cellulose, the most abundant biopolymer on earth, is an alternative approach as until now, cellulose has been classified as poorly graphitizable material. In this study, the impact of processing temperature and iron impregnation on the extent of graphitization of the cellulose-derived graphitic carbon nanostructure is uncovered by combining Raman spectroscopy, X-ray diffraction, transmission electron microscopy, and X-ray pair distribution function analysis. Raman spectroscopy is used in an innovative way to describe the evolution of the average graphitic phase size where the ash content misguides the X-ray diffraction analysis. A correlation was established between (i) the in-plane crystallite size La and the ID″/IG first-order ratio, (ii) the out-of-plane Lc crystallite size and the IG/Itot′ second-order ratio, and (iii) the second-order Raman IG′/Itot′ ratio and the average number of carbon layers per carbon crystallite. For iron-impregnated cellulose, phase quantification and analysis of the spatial distribution reveal highly crystalline rhombohedral graphite surrounded by a nanocrystalline carbon matrix. We explicitly show that traditionally non-graphitizable carbons can be used to form a graphite-like structure with multilayers of graphene sheets by careful addition of widely available nontoxic metal as catalysts. The study also shows that the impact of the catalyst is much more effective than the temperature in the nanostructure transformation. The proposed approach and the results obtained provide interesting insights that should stimulate further works aimed at extending the knowledge in the field.

Carbon-Nitride Popcorn-A Novel Catalyst Prepared by Self-Propagating Combustion of Nitrogen-Rich Triazenes.

The interest in development of non-graphitic polymeric carbon nitrides (PCNs), with various C-to-N ratios, having tunable electronic, optical, and chemical properties is rapidly increasing. Here the first self-propagating combustion synthesis methodology for the facile preparation of novel porous PCN materials (PCN3-PCN7) using new nitrogen-rich triazene-based precursors is reported. This methodology is found to be highly precursor dependent, where variations in the terminal functional groups in the newly designed precursors (compounds 3-7) lead to different combustion behaviors, and morphologies of the resulted PCNs. The foam-type highly porous PCN5, generated from self-propagating combustion of 5 is comprehensively characterized and shows a C-to-N ratio of 0.67 (C3 N4.45 ). Thermal analyses of PCN5 formulations with ammonium perchlorate (AP) reveal that PCN5 has an excellent catalytic activity in the thermal decomposition of AP. This catalytic activity of PCN5 is further evaluated in a closer-to-application scenario, showing an increase of 18% in the burn rate of AP-Al-HTPB (with 2wt% of PCN5) solid composite propellant. The newly developed template- and additive-free self-propagating combustion synthetic methodology using specially designed nitrogen-rich precursors should provide a novel platform for the preparation of non-graphitic PCNs with a variety of building block chemistries, morphologies, and properties suitable for a broad range of technologies.

Heteroatom dopant strategy triggered high-potential plateau to non-graphitized carbon with highly disordered microstructure for high-performance sodium ion storage

Non-graphitized carbon (NGC) has been extensively utilized as carbonaceous anode in sodium-ion batteries (SIBs). However, more optimization to achieve competitive capacity and stability is still challenging for SIBs. In the study, the dopant strategy is utilized to construct nitrogen/sulfur-doped non-graphitized carbon (N-NGC or S-NGC) shell decorated on three-dimensional graphene foam (GF) as a self-support electrode. The highly disordered microstructures of heteroatom doped carbons are produced by applying a low-temperature pyrolysis treatment to precursors containing nitrogen and sulfur. The DFT calculations of Na-ion adsorption energies at diverse heteroatom sites show marginal-S, pyrrolic N and pyridinic N with more intensive Na-ion adsorption ability than middle-S, CO and pristine carbon. The N-NGC with dominant small graphitic regions delivers adsorption ability to Na-ion, while the S-NGC with significant single carbon lattice stripes demonstrates redox reaction with Na-ion. Evidently, in comparison with only adsorption-driven slope regions at high potential for N-NGC, the redox reaction-generated potential-plateau enables non-graphitized S-NGC superior discharge/charge capacity and cycle-stability in the slope region. This work could provide deep insight into the rational design of non-graphitized carbon with rich microstructure and composition.

The influence of sample mass (scaling effect) on the synthesis and structure of non-graphitizing carbon (biochar) during the analytical pyrolysis of biomass.

The porous non-graphitizing carbon (NGC) known as biochar is derived from the pyrolytic conversion of organic precursors and is widely investigated due to its multifunctional applications. At present, biochar is predominantly synthesized in custom lab-scale reactors (LSRs) to determine the properties of carbon, while a thermogravimetric reactor (TG) is utilized for pyrolysis characterization. This results in inconsistencies in the correlation between the structure of biochar carbon and the pyrolysis process. If a TG reactor can also be used as an LSR for biochar synthesis, then the process characteristics and the properties of the synthesized NGC can be simultaneously investigated. It also eliminates the need for expensive LSRs in the laboratory, improves the reproducibility, and correlatability of pyrolysis characteristics with the properties of the resulting biochar carbon. Furthermore, despite numerous TG studies on the kinetics and characterization of biomass pyrolysis, none have questioned how the properties of biochar carbon vary due to the influence of the starting sample mass (scaling) in the reactor. Herein, with a lignin-rich model substrate (walnut shells), TG is utilized as an LSR, for the first time, to investigate the scaling effect starting from the pure kinetic regime (KR). The changes in the pyrolysis characteristics and the structural properties of the resultant NGC with scaling are concurrently traced and comprehensively studied. It is conclusively proven that scaling influences the pyrolysis process and the NGC structure. There is a gradual shift in pyrolysis characteristics and NGC properties from the KR until an inflection mass of ∼200 mg is reached. After this, the carbon properties (aryl-C%, pore characteristics, defects in nanostructure, and biochar yield) are similar. At small scales (≲100 mg), and especially near the KR (≤10 mg) carbonization is higher despite the reduced char formation reaction. The pyrolysis is more endothermic near KR with increased emissions of CO2 and H2O. For a lignin-rich precursor, at masses above inflection point, TG can be employed for concurrent pyrolysis characterization and biochar synthesis for application-specific NGC investigations.

Biomass-Derived Hard Carbon with Interlayer Spacing Optimization toward Ultrastable Na-Ion Storage.

Hard carbons as a kind of nongraphitized amorphous carbon have been recognized as potential anode materials for sodium-ion batteries (SIBs) due to its large interlayer spacing. However, the issues in terms of onerous synthetic procedure and elusive working mechanism remains critical bottlenecks for practical implement. Herein, we report a facile production of tubular hard carbon through direct carbonization of platanus flosses (FHC) for the first time. Through optimizing the pyrolysis temperatures, the FHC obtained at 1300 °C possesses a key balance between the interlayer spacing and surface area, which can maintain the substantial active sites as well as reduce the irreversible sodium storage. Accordingly, it can deliver a reversible capacity of 324.6 mAh g-1 with a high initial Coulombic efficiency of 80%, superb rate property of 107.2 mAh g-1 at 2 A g-1, and long operating stability over 1000 cycles. Furthermore, the in situ Raman spectroscopic studies certify that sodium ions are stored in FHC following the "adsorption-insertion" mechanism. Our study could provide a promising route for large-scale development of the biomass-derived carbonaceous anodes for high-performance SIBs.

Optimizing anion storage performances of graphite/ non-graphitic carbon composites as cathodes for dual-ion batteries

Anion intercalation capacity of graphite cathode is a limiting factor towards the development of dual-ion energy storage devices. A large portion of electrochemically active sites in graphite lattice remains inaccessible to anions due to the instability of electrolytes beyond 5 V. Strategy to composite graphitic intercalation along with surface storage from non-graphitic carbons to enhance capacity is explored in this work. Optimizations are performed to determine the best ratio of graphitic and non-graphitic carbons, and to find out the blend of physical properties of non-graphitic carbons that aid the surface contribution to the greatest extent. Besides, it is also optimized to obtain the maximum achievable lifetime and efficiency, suitable active material loading for balancing energy-power output, and the safest upper cut-off voltage for trading off capacity against cycle life. Surface area, pore size, functional groups, and doped elements govern the electrochemical properties of non-graphitic carbons. A composite of graphite with high surface area carbon (2477 m2g − 1) in a 75:25 ratio doubles the capacity, whereas the composite of graphite and reduced graphene oxide at the same ratio yields prolonged cycle life at 100 mA g − 1 within 2.0–5.0 V. The capacity improvement is invariably reproducible in dual carbon cell using composite materials as both electrodes.

Caramelization as a Key Stage for the Preparation of Monolithic Hard Carbon with Advanced Performance in Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have found broad applications due to the lower cost of stored energy per kilowatt-hour and the availability of raw materials for the production of these batteries. Nongraphitizable carbons, or hard carbons (HCs), appear to be the material of choice for negative electrodes (anodes) in SIBs. Saccharides, particularly glucose, are among the most used precursors to produce HCs. This work comprehensively explores a pretreatment step known as caramelization which precedes high-temperature pyrolysis. We synthesized monolithic HC with an ultralow specific surface area, demonstrating the initial Coulombic efficiency of 89% with a capacity of 300 mA h g–1 in a sodium half-cell. A strong correlation is revealed between the chemical composition of the intermediates and the microstructure of the HC anode materials as the final product. The pretreatment step conditions are shown to influence the properties of the HC (defectiveness, surface functionalities, and type of microporosity) and its electrochemical performance. The obtained data are important for upscaling the synthesis of HC anode materials with reproducible electrochemical characteristics.

L-valine functionalized nitrogen doped non-graphitic carbon dots as a photoluminescent probe for detection of tetracycline

Here we present l-valine functionalized non-graphitic carbon dots (denoted as N-CDvaline) as a photoluminescent (PL) probe for highly sensitive and selective detection of tetracycline antibiotic. The probe, synthesized by heating of ethylene glycol with l-valine at 170 °C for 10 min, exhibited bright bluish photoluminescence at λemission = 445 nm by exciting at λexcitation = 370 nm with 11.4% photoluminescence quantum yield. The higher resolution transmission electron micrograph, selected area electron diffraction, Fourier transformed infrared spectroscopy and X-ray photoelectron spectroscopy of N-CDvaline suggested highly ordered arrangement of linear transverse chains of oligomers of ethylene glycol and l-valine, attributable to self-assembly. Under optimized conditions, the probe exhibited linear PL quenching by tetracycline in the concentration range between 0.25 and 5.00 μM and satisfied the Stern-Volmer equation, with a detection limit of 0.054 μM. This method is highly selective towards tetracycline detection against host of interfering agents e.g., some common cations, anions and other classes of antibiotics. The PL quenching mechanism is attributed to ground state complex formation between the probe and tetracycline, corroborated by time resolved fluorescence spectroscopy studies and due to inner filter effect (IFE) owing to strong spectral overlap between UV–vis absorption spectrum of tetracycline with excitation and emission spectra of N-CDvaline. The scope for using this method for detection of tetracycline in real samples e.g., river water and milk is successfully demonstrated by spike analysis.

Electrolyte Regulation for Non-Graphitic Carbon to Achieve Stable Long-Cycling K-Storage.

Potassium-ion batteries have been considered as a promising next-generation energy storage system due to low cost but comparable energy density to lithium-ion batteries. However, carbon-based anode materials usually delivered unsatisfactory K-storage capacity as well as long-cycling performance due to poor matching with common electrolytes, thus forming an unstable solid electrolyte interphase (SEI). Herein, a robust KF-rich SEI can be achieved on the as-prepared non-graphitic carbon surface by regulating the electrolyte solvation structures, which can significantly suppress redox reaction of solvents and ensure highly reversible K+ intercalation/deintercalation. As a result, the as-synthesized non-graphitic carbon anode predictably exhibits super long-cycling performance with about 200 mA h/g at 100 mA/g for 1000 cycles and a stable capacity of 135 mA h/g at 500 mA/g for 2000 cycles with negligible capacity decay in the optimized 3 M KFSI/DME electrolyte. This work provides deep insights into further development and improvement of advanced electrolyte systems for next generation energy storage devices.

Nonthermal Plasma‐Modified Carbon‐Carrying Sn‐Based Ternary Nanocatalyst for High‐Performance Direct Dimethyl Ether Fuel Cells

A Vulcan XC72 carbon‐supported Sn‐based ternary metal catalyst (Pt3Pd3Sn2/C) is reported to have yielded the highest specific power density (90 mW mg−1 PGM) as compared with other catalysts tested for direct dimethyl ether (DME) fuel cells. However, the micropores present in Vulcan XC72 limit fuel utilization by causing Pt agglomeration. Vulcan XC72 composed of nongraphitized carbon species is also prone to corrosion. Therefore, herein, carbon supports such as multiwalled carbon nanotubes (MWCNT), black pearl 2000 (BP2000), and their cold N2 plasma‐treated counterparts are tested to further enhance the activity of the catalyst and systematically describe the comparative advantages over the Vulcan XC‐72 carbon. Electroanalytical tests show that Pt3Pd3Sn2/BP2000 exhibit excellent performance in terms of electrochemical active surface area, peak current density, and DME oxidation charge. A beneficial effect of plasma activation on the activity is observed only in the case of MWCNT while having no or negative effect on the other carbons. Laboratory fuel cell test indicates that Pt3Pd3Sn2 nanoparticles supported on optimized binary carbon support containing 75% plasma‐activated MWCNT and 25% BP2000 (Pt3Pd3Sn2/75M25B) provides the highest reported power density of 117 mW mg−1 PGM at 70 °C fuel cell temperature and ambient pressure.

Correlating Structural Properties with Electrochemical Behavior of Non-graphitizable Carbons in Na-Ion Batteries.

We report on a detailed structural versus electrochemical property investigation of the corncob-derived non-graphitizable carbons prepared at different carbonization temperatures using a combination of structural characterization methodology unique to this field. Non-graphitizable carbons are currently the most viable option for the negative electrode in sodium-ion batteries. However, many challenges arise from the strong dependence of the precursor’s choice and carbonization parameters on the evolution of the carbon matrix and its resulting electrochemistry. We followed structure development upon the increase in carbonization temperature with thorough structural characterization and electrochemical testing. With the increase of carbonization temperature from 900 to 1600 °C, our prepared materials exhibited a trend toward increasing structural order, an increase in the specific surface area of micropores, the development of ultramicroporosity, and an increase in conductivity. This was clearly demonstrated by a synergy of small- and wide-angle X-ray scattering, scanning transmission electron microscopy, and electron-energy loss spectroscopy techniques. Three-electrode full cell measurements confirmed incomplete desodiation of Na+ ions from the non-graphitizable carbons in the first cycle due to the formation of a solid–electrolyte interface and Na trapping in the pores, followed by a stable second cycle. The study of cycling stability over 100 cycles in a half-cell configuration confirmed the observed high irreversible capacity in the first cycle, which stabilized to a slow decrease afterward, with the Coulombic efficiency reaching 99% after 30 cycles and then stabilizing between 99.3 and 99.5%. Subsequently, a strong correlation between the determined structural properties and the electrochemical behavior was established.

Brewers’ Spent Grains‐Derived Carbon as Anode for Alkali Metal‐Ion Batteries

Herein, the synthesis of a nongraphitizable carbon through cost‐effective pyrolysis of a brewery waste product is reported. It is demonstrated that it is possible to realize hard carbon (HC) with structural and morphological properties from an abundant biomass source with a suitable application in energy storage devices. Resulting HC displays reversible capacities of 334, 112 , and 140 mAh g−1 after 100 cycles at 1C with high charge/discharge efficiency (>99.6%) in Li‐ion, Na‐ion, and K‐ion batteries, respectively. Thus, HC derived from brewers’ spent grains can be considered as promising materials for the realization of advanced metal‐ion batteries.

Sodium Storage Mechanism of Nongraphitic Carbons: A General Model and the Function of Accessible Closed Pores

Nowadays, the sodium storage mechanism of nongraphitic carbon materials still remains controversial despite their promising application as anode materials in sodium-ion batteries. Herein, a series of hard carbons and soft carbons with different crystallinities and pore structures was prepared from coal tar pitch, and a general mechanism is proposed through the analysis and contrast of their microstructures and electrochemical performances. In particular, it was found that sodium adsorption and insertion take place in the form of sloping capacity for both soft carbons and hard carbons, while sodium closed pore filling only exists below 0.1 V in hard carbons corresponding to the plateau capacity. Moreover, we further propose the concept of accessible closed pores (ACPs) and quantify the surface area of ACPs, which exhibits a high linear correlation with the plateau capacity. This general mechanism model will provide theory instruction for the preparation of anode materials with superior performance.

Aberration-corrected transmission electron microscopy of a non-graphitizing carbon

Non-graphitizing carbons (NGCs) are an important class of solid carbons which cannot be converted into graphite by high-temperature heat treatment. They include commercially valuable materials such as activated carbon and glassy carbon. These carbons have been intensively studied for decades, but there is still no agreement about their detailed atomic structure, or the reasons for their resistance to graphitization. The first models for graphitizing and NGCs were proposed by Rosalind Franklin in the early 1950s, and while these are broadly correct, they are incomplete. Many alternative models of NGCs have been put forward since Franklin's time, but none has received universal acceptance. Diffraction and spectroscopic techniques can provide important insights into the nature of these carbons, but only direct microscopic imaging can reveal their true atomic structure. Here, we apply aberration-corrected transmission electron microscopy to an activated carbon prepared from waste biomass and present evidence for the presence of pentagonal and heptagonal carbon rings. This provides support for a model of the structure of NGC made up of curved fragments in which non-hexagonal rings are dispersed randomly throughout hexagonal networks.

Mesoporous Weaved Turbostratic Nanodomains Enable Stable Na+ Ion Storage and Micropore Filling is Revealed to be More Unsafe than Adsorption and Deintercalation.

Advanced wave-shape non-graphitizable carbon sheets are derived, comprising mesoporous weaved turbostratic micropore enabled stable Na+ ion storage. The non-graphitizable amorphous characteristics are determined from the obtained two broad diffraction peaks at 22.7° and 43.8°. The observed D-band at 1325 cm-1 and G-band at 1586 cm-1 confirm the disordered graphitic structure, attributed to the measured specific surface area of 54 m2 g-1. Mesoporous weaved wave-shape carbon sheet architecture is confirmed by surface morphological studies, showing lattice fringes of disordered graphitic structures and dispersed ring patterns for the non-crystalline characteristics. The predominant stable redox peak at 0.014 V/0.185 V and the broader rectangular shape between 0.9 and 0.15 V depict the adsorption-micropore filling mechanism. The mesoporous hard carbon sheet delivers discharge-charge capacities of 450/311 mAh g-1 (1st cycle) and 263/267 mAh g-1 (250th cycle) at 25 mA g-1, exhibiting a superior anode for sodium-ion batteries. Besides, in situ multimode calorimetry results disclose that the micropore filling Na+ ion storage shows a higher released total heat energy of 721 J g-1 than the adsorption (471 J g-1). Ultimately, differential scanning calorimetry analysis of micropore filling Na+ ion storage (discharged state at 0.01 V) has revealed a predominant exothermic peak at 156 °C with the highest released total heat energy of 2183 J g-1 compared to adsorption (553 J g-1) and deintercalation (85 J g-1), indicating that micropore filling status is more unsafe than the adsorption and deintercalation for SIBs.

Bamboo-like carbon nanotube-carbon nanotube for high-performance sodium-ion batteries

A large non-graphitic carbon nanotube (CNT) is grown on another small crystal CNT through in-situ growth of zeolitic imidazolate framework-8 on CNT, etching of tannic acid and final carbonization. The composite has unique bamboo structure and large specific surface area (476.4 m2 g−1). As anode material of sodium ion batteries, bamboo-like CNT-CNT carbonized at 600 °C has the higher specific capacity (177.9 mAh g−1 at 1 A g−1) and the more stable cycling performance during 2000 cycles than those carbonized at 800 °C, 1000 °C and pure CNTs. This is due to the plentiful defects, very large specific surface area and hollow nanotube-in-nanotube structure that significantly promote transport and storage of Na+.