Nitrogen-doped Graphene Nanoplatelets Research Articles

Electrocatalytic dechlorination of chloroethylenes using nitrogen-doped graphene electrodes

Electro-dehalogenation for the clean-up of carcinogenic chloroethylenes (CEs) in groundwater is hindered by the high operating costs of metal electrodes and low efficiency due to competing water reduction and hydrogen formation. This study prepared metal-free electrodes with high Faradaic efficiency (FE) up to 50 % in aqueous solution by coating nitrogen-doped graphene nanoplatelets (N-GPs) on carbon paper. Dechlorination rates at these N-GPs electrodes were enhanced by ∼15 times compared with the plain graphene-based electrode, with first-order rate constants of 0.28, 0.33, 0.41, and 0.048 h−1 for tetrachloroethylene (PCE, 22 μM), trichloroethylene (TCE, 22 μM), cis-dichloroethylene (cis-DCE, 22 μM), and vinyl chloride (VC, 11 μM) dechlorination to acetylene and ethene (VC), respectively, at −1.0 V vs standard hydrogen electrode (SHE), and initial neutral pH. Surprisingly, extremely low N or no-N containing GP was detected from highly reactive N-GPs. Despite the N-doping process has been wildly used for improving electrocatalytic reactivity of catalysts by inducing N-functional groups, this study suggests that, the polygonal intrinsic defects formed after N burn-off may act as dechlorination active sites. The N-GP electrodes showed appreciable stable performance over 24 h (five cycles). Simultaneous electrolysis of a mixture of CEs in groundwater without the addition of supporting electrolytes achieved >90 % reduction of PCE, TCE, and cis-DCE and ∼60 % reduction of VC within 24 h at −1.23 V vs SHE, demonstrating its superior performance and great potential of these electrodes in practical applications.

CO Tolerance and Stability of Graphene and N-Doped Graphene Supported Pt Anode Electrocatalysts for Polymer Electrolyte Membrane Fuel Cells

Pt electrocatalysts supported on pristine graphene nanosheets (GNS) and nitrogen-doped graphene nanoplatelets (N-GNP) were prepared through the ethylene glycol process, and a comparison of their CO tolerance and stability as anode materials in polymer electrolyte membrane fuel cells (PEMFCs) with those of the conventional carbon (C)-supported Pt was made. Repetitive potential cycling in a half cell showed that Pt/GNS catalysts have the highest stability, in terms of the highest sintering resistance (lowest particle growth) and the lowest electrochemically active surface area loss. By tests in PEMFCs, the Pt/N-GNP catalyst showed the highest CO tolerance, while the poisoning resistance of Pt/GNS was lower than that of Pt/C. The higher CO tolerance of Pt/N-GNP than that of Pt/GNS was ascribed to the presence of a defect in graphene, generated by N-doping, decreasing CO adsorption energy.

Facile hydrothermal method for synthesizing nitrogen-doped graphene nanoplatelets using aqueous ammonia: dispersion, stability in solvents and thermophysical performances

A simple and green approach has been developed to synthesize nitrogen-doped graphene nanoplatelets (N-doped GNPs) for mass production with a very high stability in different solvents e.g. water, ethylene glycol, methanol, ethanol, and 1-hexanol. The strategy is based on mild oxidation of GNPs using hydrogen peroxide and doping with nitrogen using hydrothermal process. The modification of N-doped GNPs was demonstrated by FTIR, TGA, XPS, Raman spectroscopy and high resolution-transmission electron microscope (HRTEM). Further study was carried out by using N-doped GNPs as an additive to prepare different colloidal dispersions. Water-based N-doped GNPs, methanol-based N-doped GNPs, ethanol-based N-doped GNPs, ethylene-glycol based N-doped GNPs and 1-hexanol-based N-doped GNPs dispersions at 0.01 wt.% shown great colloidal stabilities, indicating 17%, 29%, 33%, 18%, and 43% sedimentations after a 15-days period, respectively. The thermophysical properties e.g., viscosity and thermal conductivity of water-based N-doped GNP nanofluids were also evaluated for different weight concentrations of 0.100, 0.075, 0.050, and 0.025 wt.%. Through this, it is found that the obtained dispersions have great potential to be used as working fluids for industrial thermal systems.

Simple solution-based synthesis of pyridinic-rich nitrogen-doped graphene nanoplatelets for supercapacitors

Pyridinic–rich nitrogen-doped graphene nanoplatelets (PRGOs) were synthesized using an acid–catalyzed dehydration reaction between GO and 3,4-diaminopyridine. Specific pyridinic nitrogen configurations can be efficiently introduced to graphitic network through newly formed imine and pyrizaine linkages. The structure of PRGOs was confirmed by various microscopic and spectroscopic analyses. In addition, the nitrogen content in PRGOs can be readily controlled over a wide range (1.9–9.1wt.%) by simple change in the feed ratios of reactants. Owing to the simultaneous structural restoration and nitrogen doping onto graphitic structure, PRGOs show high performance of supercapacitors, including enhanced specific capacitance and long-term stability. Furthermore, the effect of nitrogen contents on specific capacitance has also been investigated to optimize doping levels of nitrogen atoms in PRGOs. The highest specific capacitance of 214Fg−1 at current density of 0.1Ag−1 has been achieved from PRGO-5 with a moderate nitrogen content of 5.3wt.%, due to its balanced electric double layer capacitance and pseudocapacitance.

Preparation and electrochemical performance of nitrogen-doped graphene nanoplatelets

The highly desirable properties of nitrogen-doped graphene nanomaterial, such as high surface area, good hydrophilicity, and enhanced electrocatalytic activity and charge-transfer property, make it an ideal candidate for electrode materials used in the field of energy conversion and storage. Up to now, methods of synthesizing nitrogen-doped graphene nanomaterials mainly include chemical vapor deposition, thermal annealing graphite oxide with NH3, and graphene treated with nitrogen plasma. However, these methods of producing the nitrogen-doped graphene nanomaterials are either costly for practical applications or involving environmently hazardous reagents, and the full potentials of nitrogen-doped graphene materials are hard to achieve without scalable production at low cost. Therefore, a simple and cost-effective method of producing the nitrogen-doped graphene nanomaterial is desirable. In this paper, nitrogen-doped graphene nanoplatelets are prepared by a simple and eco-friendly mechanochemical pin-grinding process under N2 atmosphere through using natural graphite flake as the precursor at room temperature. The as-prepared nitrogen-doped graphene sample is characterized by X-ray photoelectron spectroscopy, Raman spectra, nitrogen adsorption, SEM, and TEM. The images of SEM and BET (Brunauer-Emmett-Teller) surface area measurements demonstrate an effective and spontaneous delamination of the starting graphite into small graphene nanoplatelets even in the solid state by pin-grinding process. The cleavage of graphitic C-C bonds by pin grinding creates numerous active carbon species, which can directly react with nitrogen. X-ray photoelectron spectroscopy measurements indicate that the active carbon species react with nitrogen to form the aromatic C-N in pyrazole and pyridazine rings at the fresh broken edges of the graphitic frameworks. Both pyrrolic nitrogen and pyridinic nitrogen are at the edge of carbon framework, which can provide chemically active sites to improve the electrochemical performance of carbon material. Electrochemical impedance spectroscopy indicvates that nitrogen-doped graphene nanoplatelets possess excellent electrocatalytic activity for the redox reaction between iodide and triiodide ions, used in dye-sensitized solar cells. The charge-transfer resistance of nitrogen-doped graphene nanoplatelet electrode is 1.1 cm2, which is comparable to that of Pt electrode. The capacitance properties of the as-prepared nitrogen-doped graphene nanoplatelets are also investigated. Cyclic voltammetry and galvanostatic charge-discharge curves show that nitrogen-doped graphene nanoplatelets have good capacitive performance. At a current density of 0.3 A/cm2, the specific capacitance of nitrogen-doped graphene nanoplatelets is 202.8 F/g. The good electrochemical performance of nitrogen-doped graphene nanolplatelet can be attributed to its high surface area and doping nitrogen at the edge. The simple and eco-friendly preparation procedure, low cost, and good electrochemical performance allow the as-prepared nitrogen-doped graphene nanoplatelets to be a promising candidate for the electrode materials in dye-sensitized solar cells and supercapacitors.

Wet-chemical nitrogen-doping of graphene nanoplatelets as electrocatalysts for the oxygen reduction reaction

We developed a facile, versatile method to prepare pyridinic N-rich nitrogen-doped graphene nanoplatelets (PN-GNs), which show unique electrocatalytic activities toward the oxygen reduction reaction with enhanced long-term cycle stability and good tolerance to methanol crossover.

Mechanical exfoliation of graphite in 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) providing graphene nanoplatelets that exhibit enhanced electrocatalysis

A novel production method for graphene nanoplatelets (GPs) with enhanced electrocatalytic behaviour is presented. GPs show improvement in their oxygen reduction reaction (ORR) catalysis after prolonging the grinding of graphite in 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6). Nitrogen doping of the GPs has inferred a further increase in ORR. The ORR onset potential, cathodic current magnitude and electron transfer efficiency have all improved as a direct consequence of increasing the graphite grinding duration from 30 min to 4 h. Atomic force microscopy has confirmed a decrease in the GP diameter and height as the grinding increases. Raman spectroscopy indicates a higher level of defects present after prolonging the graphite grinding in BMIM-PF6, most likely a result of the increased edge plane exposure. This increased edge plane appears to promote a higher level of nitrogen incorporation as the graphite grinding duration increases, as confirmed by X-ray photoelectron spectroscopy analysis. The stability of the cathodic current assessed by chronoamperometry analysis is higher for the GP and nitrogen doped graphene nanoplatelet (N-GP) samples than the platinum on carbon black (Pt/C). This study presents a novel process for the production of nitrogen doped graphene nanoplatelets, constituting a strategy for the up-scaled production of electrocatalysts.

Nitrogen-doped graphene with enhanced oxygen reduction activity produced by pyrolysis of graphene functionalized with imidazole derivatives

Nitrogen-doped graphene nanoplatelets (N-GNP) with 1.6–3.3 at.% nitrogen content were synthesized by thermal annealing of GNP functionalized with a series of imidazole-based nitrogen-containing precursors of different nature, charge and nitrogen content. The imidazole derivatives included one ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) and two polymers, a neutral one, poly(vinylimidazole) (PVI) and a poly(ionic liquid), poly(3-butyl-1-vinylimidazolium bromide) (PBVIBr). N-GNP showed enhanced electrocatalytic activity for oxygen reduction reaction (ORR) in alkaline media compared to pristine GNP, with the number of electrons transferred in oxygen-saturated KOH at low overpotentials following the trend: 2.5 (pristine GNP) < 2.9 (N-GNP from PVI–GNP) < 3.3 (N-GNP from BMIBF4–GNP) < 3.5 (N-GNP from PBVIBr-GNP). Interestingly, ORR catalytic activity did not correlate with total nitrogen content but was more affected by Brunauer–Emmett–Teller (BET) surface area. The most active materials were N-GNP with lowest doping levels and highest surface area resulting from the ionic (imidazolium-based) nitrogen precursors.

Nitrogen-Doped Graphene Nanoplatelets from Simple Solution Edge-Functionalization for n-Type Field-Effect Transistors

The development of a versatile method for nitrogen-doping of graphitic structure is an important challenge for many applications, such as energy conversions and storages and electronic devices. Here, we report a simple but efficient method for preparing nitrogen-doped graphene nanoplatelets via wet-chemical reactions. The reaction between monoketone (C═O) in graphene oxide (GO) and monoamine-containing compound produces imine (Shiff base) functionalized GO (iGO). The reaction between α-diketone in GO and 1,2-diamine (ortho-diamine)-containing compound gives stable pyrazine ring functionalized GO (pGO). Subsequent heat-treatments of iGO and pGO result in high-quality, nitrogen-doped graphene nanoplatelets to be designated as hiGO and hpGO, respectively. Of particular interest, hpGO was found to display the n-type field-effect transistor behavior with a charge neutral point (Dirac point) located at around -16 V. Furthermore, hpGO showed hole and electron mobilities as high as 11.5 and 12.4 cm(2)V(-1)s(-1), respectively.

N-Doped Graphene Nanoplatelets as Superior Metal-Free Counter Electrodes for Organic Dye-Sensitized Solar Cells

Highly efficient counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) were developed using thin films of scalable and high-quality, nitrogen-doped graphene nanoplatelets (NGnP), which was synthesized by a simple two-step reaction sequence. The resultant NGnP was deposited on fluorine-doped SnO2 (FTO)/glass substrates by using electrospray (e-spray) coating, and their electrocatalytic activities were systematically evaluated for Co(bpy)3(3+/2+) redox couple in DSSCs with an organic sensitizer. The e-sprayed NGnP thin films exhibited outstanding performances as CEs for DSSCs. The optimized NGnP electrode showed better electrochemical stability under prolonged cycling potential, and its Rct at the interface of the CE/electrolyte decreased down to 1.73 Ω cm(2), a value much lower than that of the Pt electrode (3.15 Ω cm(2)). The DSSC with the optimized NGnP-CE had a higher fill factor (FF, 74.2%) and a cell efficiency (9.05%), whereas those of the DSSC using Pt-CE were only 70.6% and 8.43%, respectively. To the best of our knowledge, the extraordinarily better current-voltage characteristics of the DSSC-NGnP outperforming the DSSC-Pt for the Co(bpy)3(3+/2+) redox couple (in paticular, FF and short circuit current, Jsc) is highlighted for the first time.

Hydrogen Storage Studies of Palladium Decorated Nitrogen Doped Graphene Nanoplatelets

Hydrogen storage in materials is of significant importance in the present scenario of depleting conventional energy sources. Porous solids such as activated carbon or nanostructured carbon materials have promising future as hydrogen storage media. The hydrogen storage capacity in nanostructured carbon materials can be further enhanced by atomic hydrogen spillover from a supported catalyst. In the present work, the hydrogen storage properties of nitrogen doped graphene nanoplatelets (N-GNP) and palladium decorated nitrogen doped graphene nanoplatelets (Pd/N-GNP) have been investigated. The results show that hydrogen uptake capacity of nitrogen doped graphene nanoplatelets and palladium decorated nitrogen doped graphene nanoplatelets at pressure 32 bar and temperature 25 degrees C is 0.42 wt% and 1.25 wt% respectively. The dispersion of palladium nanoparticles increases the hydrogen storage capacity of nitrogen doped graphene nanoplatelets by 0.83 wt%. This may be due to high dispersion of palladium nanoparticles and strong adhesion between metal and graphene nanoplatelets over the surface of N-GNP, which enhances the spillover mechanism. Thus, an increase in the hydrogen spillover effect and the binding energy between metal nanoparticles and supporting material achieved by nitrogen doping has been observed to result in a higher hydrogen storage capacity of pristine GNP.

Nitrogen doped graphene nanoplatelets as catalyst support for oxygen reduction reaction in proton exchange membrane fuel cell

Graphene nanoplatelets have been synthesized by thermal exfoliation of graphitic oxide and nitrogen doped graphene nanoplatelets have been obtained by nitrogen plasma treatment. Graphene nanoplatelets and nitrogen doped graphene nanoplatelets have been used as a catalyst support for platinum nanoparticles for oxygen reduction reactions in proton exchange membrane fuel cells. Platinum nanoparticles were dispersed over these support materials using the conventional chemical reduction technique. The morphology and structure of the graphene based powder samples were studied using X-ray diffraction, Raman spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. A full cell was constructed with platinum loaded nitrogen doped graphene nanoplatelets and the results have been compared with platinum loaded graphene nanoplatelets. A maximum power density of 440 and 390 mW cm−2 has been obtained with platinum loaded nitrogen doped graphene and platinum loaded graphene nanoplatelets as ORR catalysts respectively. Nitrogen plasma treatment created pyrrolic nitrogen defects, which act as good anchoring sites for the deposition of platinum nanoparticles. The improved performance of fuel cells with N-G as catalyst supports can be attributed to the increased electrical conductivity and improved carbon–catalyst binding.