Heteroatom-doped Graphene Materials Research Articles

(Invited) Synthesis, Characterization and Quality Control of Heteroatom-Doped Graphene

Heteroatom doped graphene materials have gained attention as promising candidates for a variety of energy-related applications. Addition of heteroatoms in the graphene lattice is a powerful tool to modify their electronic, electrochemical, and catalytic properties. This presentation focusses on N-doped reduced graphene oxide and summarizes strategies to engineer the structure and properties of the material through control of the synthetic conditions. The produced N-doped graphene materials are characterized to determine their chemical composition, nitrogen content, degree of graphitization, surface area, electronic and electro-chemical properties. The scalability and manufacturability of the N-doped graphene materials are assessed and various examples of their current and future applications in batteries and supercapacitors are discussed.

N, P-Codoped Carbon Film Derived from Phosphazenes and Its Printing Integration with a Polymer Carpet Via "Molecular Welding" for Flexible Electronics.

Although high-performance graphene-based micro/nano flexible electronic devices have shown promising applications in numerous fields, there are still many problems in converting graphene into practical applications. Heteroatom-doped graphene materials are of huge importance because heteroatom doping can significantly change the electronic structure and introduce the active site, which benefits the integration with a promising substrate and achieves nondestructive transfer of carbon materials. Herein, we analyze in detail the pyrolysis gas composition of heteroatom-enriched phosphazenes with different structures and prepare a series of high-quality in situ N, P-codoped carbon-based films from phosphazene solid sources on a low-cost glass substrate by a convenient one-step method. The N, P-codoped carbon film shows reflectivity, good conductivity, and transparency. In addition, with the help of in situ "molecular welding", we achieve nondestructive transfer of a conductive carbon-based film from a glass substrate to promising layer-polyimide (PI) and prepare a flexible free-standing carbon/PI hybrid film with an excellent binding interface. The flexible conductive hybrid film shows excellent durability under an extremely low temperature environment and superior bending stability after 800 bending cycles. The results suggest that a phosphazene precursor is an amazing choice for constructing high-quality heteroatom-doped conductive carbon films. Besides, this work provides a promising way for nondestructive transfer of the conductive carbon-based films and large-scale preparation of large-area patterned conductive thin films.

Atomic Layer Coated Al2O3 on Nitrogen Doped Vertical Graphene Nanosheets for High Performance Sodium Ion Batteries

Heteroatom doped graphene materials are considered as promising anode for high‐performance sodium‐ion batteries (SIBs). Defective and porous structure especially with large specific surface area is generally considered as a feasible strategy to boost reaction kinetics; however, the unwanted side reaction at the anode hinders the practical application of SIBs. In this work, a precisely controlled Al2O3 coated nitrogen doped vertical graphene nanosheets (NVG) anode material has been proposed, which exhibits excellent sodium storage capacity and cycling stability. The ultrathin Al2O3 coating on the NVG is considered to help construct an advantageous interface between electrode and electrolyte, both alleviating the electrolyte decomposition and enhancing sodium adsorption ability. As a result, the optimal Al2O3 coated NVG materials delivers a high reversible capacity (835.0 mAh g−1) and superior cycling stability (retention of 92.3% after 5000 cycles). This work demonstrates a new way to design graphene‐based anode materials for high‐performance sodium‐ion batteries.

Irreversible Increase in Disorder in Doped Graphene Lithium Ion Battery Anodes during First Discharge

Heteroatom-doped graphene materials show high capacity as Li-ion battery anodes. However, the Li+ reduction potential in doped graphene anodes varies drastically during charging/discharging, which adversely affects battery performance. The varying reduction potential is conjectured to result from graphene sheets aggregating during electrode preparation into a disordered graphite structure, which demonstrate a similar reduction potential profile. To test this conjecture we measured the disorder during charging/discharging via in situ x-ray diffraction. We find that the doped graphene interlayer spacing irreversibly increases during the first discharge prior to solid electrolyte interphase formation, and commensurate with this phenomena we find an increase in double-layer capacitance, suggesting an increase in surface area. These results imply an increase in disorder during the first discharge, which supports the conjecture that doped graphene batteries resemble disordered graphite. However, a significant disordering occurs during the first discharge and not necessarily during electrode preparation.

Highly doped graphene with multi-dopants for high-capacity and ultrastable sodium-ion batteries

Heteroatom-doped graphene materials are one of the most promising electrode materials in energy-related realms. In this work, highly doped graphene nanosheets with tunable N and S codopants (NS-GNS) are prepared via a facile one-pot method and investigated in sodium-ion batteries (SIBs). During the pyrolysis, the starting materials can form planar sacrificial-templates that serve for the growth of 2D graphene structure and further in-situ convert into rich N and S dopant species, successfully solving the general challenge of high doping level of multi-heteroatoms into the graphene matrix. Integrating the abundant N, S and O-induced electrochemical redox centers at different potentials, the NS-GNS electrode demonstrates a high reversible capacity of around 400mAhg−1. Particularly, the ultra-long cycle life of 10,000 cycles and good rate capability of the present NS-GNS electrode make it promising for practical application in large-scale energy storage. This work guilds a direction toward redox-enhanced graphene materials with multi-dopants for energy storage and meanwhile provides a low-cost and scalable method for the mass production of well-defined graphene materials with tunable properties.

Modulating the oxygen reduction activity of heteroatom-doped carbon catalysts via the triple effect: charge, spin density and ligand effect.

To enhance the intrinsic activity of and increase the number of active sites in heteroatom-doped graphene (doped-G), it is necessary to recognize the origin of its catalytic activity, and to search for a universal definition and description of the different active sites. Herein, we report an evaluation of a series of heteroatom-doped graphene materials as oxygen reduction reaction (ORR) catalysts with the aid of density functional theory (DFT) calculations. The results of the DFT calculations indicate that the intrinsic catalytic activity and the ORR mechanism depend on the triple effect, that is, the charge, the spin density and the coordinate state (ligand effect) of the carbon sites. The contribution of the above effects towards increasing the binding energies of *OOH or *OH has the following order: negative charge effect < positive charge effect < low spin effect < ligand effect < high spin effect. For nonmetallic single-heteroatom-doped-G, the triple effect separately activates the carbon sites around the doped atom. On each single carbon active site, only the end-on adsorption of *OOH is preferred, leading to the ORR following the associative mechanism, which has an intrinsic limitation in its overpotential of 0.44 V. However, for metal-doped-G and dual-heteroatom-doped-G, the triple effect can activate double carbon sites, and lead to the ORR following the dissociative mechanism, which breaks through the activity limitation of the associative mechanism, and further enhances the catalytic activity for the ORR. With comprehensive theoretical calculation, we conclude that it is possible for the heteroatom-doped-G, if well-modulated via the triple effect, to exceed Pt-based materials in catalysis of the ORR.

S- and N-Doped Graphene Nanomaterials for the Oxygen Reduction Reaction

In the current work, heteroatom-doped graphene materials containing different atomic ratios of nitrogen and sulphur were employed as electrocatalysts for the oxygen reduction reaction (ORR) in acidic and alkaline media. To this end, the hydrothermal route and different chemical reducing agents were employed to synthesize the catalytic materials. The physicochemical characterization of the catalysts was performed by several techniques, such as X-ray diffraction, Raman spectroscopy and elemental analysis; meanwhile, the electrochemical performance of the materials toward the ORR was analyzed by linear sweep voltammetry (LSV), rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques. The main results indicate that the ORR using heteroatom-doped graphene is a direct four-electron pathway, for which the catalytic activity is higher in alkaline than in acidic media. Indeed, a change of the reaction mechanism was observed with the insertion of N into the graphenic network, by the rate determining step changes from the first electrochemical step (formation of adsorbed OOH) on glassy carbon to the removal of adsorbed O (Oad) from the N-graphene surface. Moreover, the addition of sulphur atoms into the N-graphene structure increases the catalytic activity toward the ORR, as the desorption of Oad is accelerated.

Electrocatalytic Activity and Design Principles of Heteroatom-Doped Graphene Catalysts for Oxygen-Reduction Reaction

Heteroatom-doped graphene materials have emerged as highly efficient and inexpensive and variations of graphene doping structures; however, there is still a lack of fundamental understanding of the trend and mechanisms in their ORR activity, which greatly hinders the development of highly active graphene-based catalysts. Here we use density-functional calculations to study the ORR activity and mechanism of nonmetal-element doped graphene catalysts with different doping configurations. Our results demonstrate that binding energies of ORR intermediates (i.e., *OH) on the catalysts can serve as a good descriptor for the ORR activity, attaining the optimal value at the vicinity of ∼2.6 eV. The analysis of electronic structures indicates that the ORR activity of doped graphene catalysts depends on the abundance of electronic states at the Fermi level, which dominates the charge transfer between ORR intermediates and the catalysts. Using binding energy as a descriptor, we predict the realization of highly activ...

Phosphorus doped and defects engineered graphene for improved electrochemical sensing: synergistic effect of dopants and defects

Heteroatom-doped graphene materials emerged as promising metal-free catalysts have recently attracted a growing interest in electrochemical sensing applications. However, their catalytic activity and sensing performances still need to be further improved. Herein, we reported the development of unique phosphorus (P)-doped and plasma-etched graphene (denoted as PG-E) as an efficient metal-free electrocatalyst for dopamine (DA) sensing. It was demonstrated that introducing both P-dopants and plasma-engineered defects in graphene could synergistically improve the activity toward electrocatalytic oxidation of DA by increasing the accessible active sites and promoting the electron transport capability. The resulting PG-E modified electrode showed exceptional DA sensing performances with low detection limit, high selectivity and good stability. These results suggested that the synergistic effect of dopants and defects might be an important factor for developing the advanced graphene-based metal-free catalysts for electrochemical sensing.

Activity origin and catalyst design principles for electrocatalytic hydrogen evolution on heteroatom-doped graphene

The hydrogen evolution reaction (HER) is a fundamental process in electrocatalysis and plays an important role in energy conversion through water splitting to produce hydrogen. Effective candidates for HER are often based on noble metals or transition metal dichalcogenides, while carbon-based metal-free electrocatalysts generally demonstrate poorer activity. Here we report evaluation of a series of heteroatom-doped graphene materials as efficient HER electrocatalysts by combining spectroscopic characterization, electrochemical measurements, and density functional theory calculations. Results of theoretical computations are shown to be in good agreement with experimental observations regarding the intrinsic electrocatalytic activity and the HER reaction mechanism. As a result, we establish a HER activity trend for graphene-based materials, and explore their reactivity origin to guide the design of more efficient electrocatalysts. We predict that by rationally modifying particular experimentally achievable physicochemical characteristics, a practically realizable graphene-based material will have the potential to exceed the performance of the metal-based benchmark for HER. Metal-free doped-graphene materials are emerging as electrocatalysts for energy conversions, but their activity remains low. Here, Jiao et al. explore the origins of catalytic activity for hydrogen evolution, suggesting pathways to metal-free catalysts with activity to rival metal-containing benchmarks.

Recent Advances in Heteroatom-Doped Graphene Materials as Efficient Electrocatalysts towards the Oxygen Reduction Reaction

Recent Advances in Heteroatom-Doped Graphene Materials as Efficient Electrocatalysts towards the Oxygen Reduction Reaction

A nano-engineered graphene/carbon nitride hybrid for photocatalytic hydrogen evolution

A metal-free photocatalytic hydrogen evolution system was successfully fabricated using heteroatom doped graphene materials as electron-transfer co-catalysts and carbon nitride as a semiconductor. The catalytic role of graphene is significantly dependent on the heteroatom dopant of the graphene, such as O, S, B, N doped/undoped graphene co-catalysts, and N-graphene shows the best catalytic hydrogen evolution rate.

Flexible Boron-Doped Laser-Induced Graphene Microsupercapacitors.

Heteroatom-doped graphene materials have been intensely studied as active electrodes in energy storage devices. Here, we demonstrate that boron-doped porous graphene can be prepared in ambient air using a facile laser induction process from boric acid containing polyimide sheets. At the same time, active electrodes can be patterned for flexible microsupercapacitors. As a result of boron doping, the highest areal capacitance of as-prepared devices reaches 16.5 mF/cm(2), 3 times higher than nondoped devices, with concomitant energy density increases of 5-10 times at various power densities. The superb cyclability and mechanical flexibility of the device are well-maintained, showing great potential for future microelectronics made from this boron-doped laser-induced graphene material.

Single source precursor-based solvothermal synthesis of heteroatom-doped graphene and its energy storage and conversion applications.

Solvothermal processes are considered efficient approaches for the gram-scale production of graphene. Further modification of graphene by chemical doping is an important approach to tailor its properties. In this work, we successfully synthesized sulfur-doped graphene by using a solvothermal method with dimethyl sulfoxide as a precursor, which is a common laboratory reagent. Nitrogen-doped graphene was produced to demonstrate the generality of this process. These heteroatom-doped graphene materials exhibited high surface areas and high contents of heteroatoms. Furthermore, the lithium-ion storage properties and oxygen reduction reaction catalytic activity of these materials were also investigated. The success of this approach might facilitate the development of other advanced graphene-based materials with relative simplicity, scalability, and cost effectiveness for use in various potential applications.

Heteroatom-doped graphene materials: syntheses, properties and applications.

Heteroatom doping can endow graphene with various new or improved electromagnetic, physicochemical, optical, and structural properties. This greatly extends the arsenal of graphene materials and their potential for a spectrum of applications. Considering the latest developments, we comprehensively and critically discuss the syntheses, properties and emerging applications of the growing family of heteroatom-doped graphene materials. The advantages, disadvantages, and preferential doping features of current synthesis approaches are compared, aiming to provide clues for developing new and controllable synthetic routes. We emphasize the distinct properties resulting from various dopants, different doping levels and configurations, and synergistic effects from co-dopants, hoping to assist a better understanding of doped graphene materials. The mechanisms underlying their advantageous uses for energy storage, energy conversion, sensing, and gas storage are highlighted, aiming to stimulate more competent applications.