Quaternary Nitrogen Research Articles

Mineralogy and geochemistry of ammonian illite in intra-seam partings in Permo-Carboniferous coal of the Qinshui Coalfield, North China

Ammonian illite is observed to be presented in the intra-seam coal partings of the Permo-Carboniferous coal seams from the Qinshui Coalfield, North China. This paper provides new insights into its geochemical and mineralogical characteristics, as well as the factors influencing its formation and its nitrogen isotope ratios. The interlayer cations of ammonian illite in the samples presented in this study were dominated by NH4+ with a certain amount of K+ and distributed homogeneously in each illite layer. Ammonian illite has an NH4+/(NH4++K+) ratio of 0.87 on average, with a basal spacing (d001, 10.267Å on average) greater than those of potassian illite and muscovite, but lower than that of tobelite. The average stoichiometric formula of the ammonian illite was inferred as (NH40.67,K0.11)(Al1.90,Fe0.06,Mg0.04)(Al0.68,Si3.32)O10(OH)2, and the average Si/AlIV ratio (4.88) was found to be higher than that of tobelite. This indicates that the conversion of ammonian illite to tobelite includes the expulsion of Si from the tetrahedral sheets and the incorporation of Al into the octahedral and tetrahedral sheets. The d005 is influenced not only by the NH4+/(NH4++K+) ratio but also, to some extent, by the entrance of AlIV into tetrahedral sites. It is inferred that the ammonian illite was formed by the incorporation of Si into pre-existing kaolinite during diagenesis, and that its formation was influenced not only by the diagenetic temperature but also by the depositional environment. Brackish water favors the formation of ammonian illite during deposition. The NH4+ of the ammonian illite was mainly derived from pyrrolic (N-5) and pyridinic (N-6) nitrogen groups at the margins of the coal's carbon matrix during diagenesis. On average, it has a δ15N value of +8.0‰, much higher than that of total coal nitrogen (+3.9‰ on average). This is related mainly to the higher δ15N values of N-5 and N-6 than the quaternary nitrogen group (N-Q) within the carbon matrix.

Discovery of a marine-derived bis-indole alkaloid fascaplysin, as a new class of potent P-glycoprotein inducer and establishment of its structure–activity relationship

The screening of IIIM natural products repository for P-gp modulatory activity in P-gp over-expressing human adenocarcinoma LS-180 cells led to the identification of 7 natural products viz. withaferin, podophyllotoxin, 3-demethylcolchicine, agnuside, reserpine, seseberecine and fascaplysin as P-gp inducers. Fascaplysin (6a), a marine-derived bis-indole alkaloid, was the most potent among all of them, showing induction of P-gp with EC50 value of 25 nM. P-gp induction is one of the recently targeted strategy to increase amyloid-β clearance from Alzheimer brains. Thus, we pursued a medicinal chemistry of fascaplysin to establish its structure–activity relationship for P-gp induction activity. Four series of analogs viz. substituted quaternary fascaplysin analogs, D-ring opened quaternary analogs, D-ring opened non-quaternary analogs, and β-carbolinium analogs were synthesized and screened for P-gp induction activity. Among the total of 48 analogs screened, only quaternary nitrogen containing analogs 6a–g and 10a, 10h–l displayed promising P-gp induction activity; whereas non-planar non-quaternary analogs 9a–m, 13a–n, 15a–h were devoid of this activity. The P-gp induction activity of best compounds was then confirmed by western-blot analysis, which indicated that fascaplysin (6a) along with 4,5-difluoro analog of fascaplysin 6f and D-ring opened analog 10j displayed 4–8 fold increase in P-gp expression in LS-180 cells at 1 μM. Additionally, compounds 6a and 6f also showed inhibition of acetylcholinestease (AChE), an enzyme responsible for neuronal loss in Alzheimer's disease. Thus, fascaplysin and its analogs showing promising P-gp induction along with AChE inhibition at 1 μM, with good safety window (LS-180: IC50 > 10 μM, hGF: 4 μM), clearly indicates their promise for development as an anti-Alzheimer agent.

Role of micropores and nitrogen-containing groups in CO2 adsorption on indole-3-butyric acid potassium derived carbons

Nitrogen-rich carbons for CO2 capture have received much attention in recent years, but the role of N-containing groups in CO2 adsorption has not been clearly elucidated. In this study, indole-3-butyric acid potassium (IBAP) as a new precursor was used to prepare the nitrogen-rich carbons via one-step carbonization. The IBAP-derived carbon prepared at 700°C exhibited high CO2 adsorption of 1.48mmol/g at 0.15bar and 4.53mmol/g at 1bar and 25°C, among the highly efficient activated carbons for CO2 adsorption. The high CO2 adsorption was attributed to not only the high volume of effective micropores, but also the effective N-containing groups on the IBAP-derived carbons. When carbonization temperatures increased from 500°C to 900°C, the nitrogen contents in the IBAP-derived carbons decreased from 6.35% to 0.70%. X-ray photoelectron spectroscopy (XPS) analysis confirmed that the pyrrolic-N in IBAP was converted into pyridinic-N, quaternary nitrogen and pyridinic-N-oxide in the carbonization process, and their contents first increased and then decreased with increasing carbonization temperatures. Only the pyrrolic-N was proved to be effective for CO2 adsorption on the IBAP-derived carbons, and its contribution to CO2 adsorption was pronounced for the carbons prepared at low temperatures. For the well-developed porous carbons, the effective micropores were mainly responsible for CO2 adsorption via micropore filling.

Synthesis and mass spectrometry analysis of quaternary cryptando-peptidic conjugates

The bicyclic amines in the form of cryptands, the crown ether analogs, were used in the synthesis of cryptando-peptidic conjugates with simultaneous formation of quaternary ammonium nitrogen moiety. A series of model cryptando-peptidic conjugates at the peptide N-terminus was efficiently prepared by the standard Fmoc solid phase synthesis. Tandem mass spectrometric analysis of the obtained conjugates has shown the specific fragmentation pattern during MS/MS experiment. The obtained cryptandic quaternary ammonium group undergoes the Hofmann elimination during collision-induced dissociation fragmentation followed by the ethoxyl group elimination. The presented quaternization of cryptands by iodoacetylated peptides is relatively easy and compatible with standard solid-phase peptide synthesis. Additionally, the applicability of such peptide derivatives and their isotopologues selectively deuterated at the α-carbon in the quantitative LC-MS analysis was analyzed.

Transformation of nitrogen and sulphur impurities during hydrothermal upgrading of low quality coals

The influence of hydrothermal upgrading on the configuration of nitrogen and sulphur species in coal was investigated in this paper. Hydrothermal modification of three typical Chinese lignite was performed at 200°C, 250°C and 300°C. It was found that hydrothermal modification reduced the moisture content of coal and therefore increased the heating value. X-ray photoelectron spectroscopy (XPS) was used to analyze the speciation of nitrogen and sulphur in coal samples before and after the hydrothermal modification. XPS peak fitting analysis was performed by using the XPSPEAK points-peak software. Results showed that the hydrothermal treatment destroyed micro-structures and altered the functional groups of nitrogen and sulphur in coals. Nitrogen, which originally detected as a pyrollic peak (N-5), was converted to pyridic (N-6) or quaternary nitrogen (N-Q) species after the hydrothermal upgrading. Quaternary nitrogen associated with polycyclic aromatic hydrocarbon was converted to N-6 while N-Q molecules located at the coal edge transformed to nitrogen oxides (N-X). Sulphoxides, sulphones and sulphides were converted to thiophenes; alternatively, sulphides transformed to pyrite. These changes could result in a decrease in SO2 emission. Furthermore, there may be the transformation form sulphoxide and sulphone to sulphonates. Organic sulphur, to some extent, transformed to inorganic sulphur during the hydrothermal process.

Carbon Nanohorn-Derived Graphene Nanotubes as a Platinum-Free Fuel Cell Cathode.

Current low-temperature fuel cell research mainly focuses on the development of efficient nonprecious electrocatalysts for the reduction of dioxygen molecule due to the reasons like exorbitant cost and scarcity of the current state-of-the-art Pt-based catalysts. As a potential alternative to such costly electrocatalysts, we report here the preparation of an efficient graphene nanotube based oxygen reduction electrocatalyst which has been derived from single walled nanohorns, comprising a thin layer of graphene nanotubes and encapsulated iron oxide nanoparticles (FeGNT). FeGNT shows a surface area of 750 m(2)/g, which is the highest ever reported among the metal encapsulated nanotubes. Moreover, the graphene protected iron oxide nanoparticles assist the system to attain efficient distribution of Fe-Nx and quaternary nitrogen based active reaction centers, which provides better activity and stability toward the oxygen reduction reaction (ORR) in acidic as well as alkaline conditions. Single cell performance of a proton exchange membrane fuel cell by using FeGNT as the cathode catalyst delivered a maximum power density of 200 mW cm(-2) with Nafion as the proton exchange membrane at 60 °C. The facile synthesis strategy with iron oxide encapsulated graphitic carbon morphology opens up a new horizon of hope toward developing Pt-free fuel cells and metal-air batteries along with its applicability in other energy conversion and storage devices.

Molecular Modeling of Cetylpyridinium Bromide, a Cationic Surfactant, in Solutions and Micelle.

Cationic surfactants are widely used in biological and industrial processes. Notably, surfactants with pyridinium salts, such as cetylpyridinium bromide (CPB), have diverse applications. The cetylpyridium cation has a quaternary nitrogen in the aromatic heterocyclic ring of the headgroup and 16 carbons in the hydrocarbon tail. At present and in the past, it has been widely used in germicides. Recently, several interesting applications of CPB have been explored, including its use in protein folding, polymerization, enzyme studies, and gene delivery as well as in pharmaceuticals as a drug delivery tool. A molecular-level understanding of CPB and its micelle in solution can enhance its development in such applications. Herein, we have proposed the first united-atom force field for CPB that yields stable micellar aggregates in molecular dynamics (MD) simulations. The force field is validated through classical MD simulations of the CPB monomer in pure water and 1-octanol as well as in an aqueous CPB micelle. We have performed principal component analysis (PCA) and calculated the translational and rotational diffusion coefficients, spatial distribution of solvent, counterion distribution, and rotational correlation time of CPB molecule in solutions and in micelle, comparing these data to previous experimental and theoretical results for a strong validation of the force field. PCA confirms that the pyridinium ring remains planar, whereas the movement of the hydrophobic tail region leads to conformational changes during the simulations. The collective modes of the pyridinum ring were identical for CPB molecule in solution and micelle, but conformational dynamics of the CPB tail were restricted in the micelle relative to motions in water and 1-octanol. Using this force field, a spherical CPB micelle was shown to be stable throughout the course of simulation, and its solvation and structural properties are characterized.

N-phosphonomethyl iminodiacetic acid N-oxide synthesis in the presence of bifunctional catalysts based on tungsten complexes

N-phosphonomethyl iminodiacetic acid (PMIDAA) was oxidized in a two phase liquid system by 32% hydrogen peroxide at 70°С. Reaction was performed in the presence of tungsten oxo- and peroxo- complexes, containing N-hexadecylpyridinium or quaternary ammonium cation. Substrate conversion and N-oxide PMIDAA selectivity were found to depend on anions structure and quaternary nitrogen containing cation properties. The best results were obtained when PMIDAA was oxidized in the presence of N-hexadecylpyridinium tetra(oxodiperoxotungsto)phosphate, substrate conversion being 99%, and target product selectivity being 97%.

Molecular Dynamics Study of N-Dodecyl-N,N-Dimethyl-3-Ammonio-1-Propanesulfonate Mono-Layer Adsorbed at the Air/Water Interface

Molecular dynamics (MD) simulation has been carried out to investigate the properties of a mono-layer of N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB12-3) adsorbed at the air/water interface. The simulation results show that SB12-3 can form a closely packed mono-layer at the air/water interface, the anionic sulfonic groups of the hydrophilic head group have stronger interactions with the water molecules by comparison with the cationic quaternary nitrogen of the hydrophilic head group, and water molecules can gather more easily and aggregate around the sulfonic groups. With the addition of the Ca2+ ions, the number of hydrogen bonds between the SB12-3 surfactant molecules and water molecules remains almost unchanged, which may explain the salt resistance behavior for SB12-3 especially for di-valent ions. The hydrophobic tail in SB12-3 is more ordered in the beginning of the chain.

Charge properties and bacterial contact-killing of hyperbranched polyurea-polyethyleneimine coatings with various degrees of alkylation

Coatings of immobilized-quaternary-ammonium-ions (QUAT) uniquely kill adhering bacteria upon contact. QUAT-coatings require a minimal cationic-charge surface density for effective contact-killing of adhering bacteria of around 1014cm−2. Quaternization of nitrogen is generally achieved through alkylation. Here, we investigate the contribution of additional alkylation with methyl-iodide to the cationic-charge density of hexyl-bromide alkylated, hyperbranched polyurea-polyethyleneimine coatings measuring charge density with fluorescein staining. X-ray-photoelectron-spectroscopy was used to determine the at.% alkylated-nitrogen. Also streaming potentials, water contact-angles and bacterial contact-killing were measured. Cationic-charge density increased with methyl-iodide alkylation times up to 18h, accompanied by an increase in the at.% alkylated-nitrogen. Zeta-potentials became more negative upon alkylation as a result of shielding of cationiccharges by hydrophobic alkyl-chains. Contact-killing of Gram-positive Staphylococci only occurred when the cationic-charge density exceeded 1016cm−2 and was carried by alkylated-nitrogen (electron-binding energy 401.3eV). Gram-negative Escherichia coli was not killed upon contact with the coatings. There with this study reveals that cationic-charge density is neither appropriate nor sufficient to determine the ability of QUAT-coatings to kill adhering bacteria. Alternatively, the at.% of alkylated-nitrogen at 401.3eV is proposed, as it reflects both cationic-charge and its carrier. The at.% N401.3 eV should be above 0.45at.% for Gram-positive bacterial contact-killing.

Metal-organic framework derived hierarchically porous nitrogen-doped carbon nanostructures as novel electrocatalyst for oxygen reduction reaction

The hierarchically porous nitrogen-doped carbon materials, derived from nitrogen-containing isoreticular metal-organic framework-3 (IRMOF-3) through direct carbonization, exhibited excellent electrocatalytic activity in alkaline solution for oxygen reduction reaction (ORR). This high activity is attributed to the presence of high percentage of quaternary and pyridinic nitrogen, the high surface area as well as good conductivity. When IRMOF-3 was carbonized at 950°C (CIRMOF-3-950), it showed four-electron reduction pathway for ORR and exhibited better stability (about 78.5% current density was maintained) than platinum/carbon (Pt/C) in the current durability test. In addition, CIRMOF-3-950 presented high selectivity to cathode reactions compared to commercial Pt/C.

Hydrogen Bonding, (1)H NMR, and Molecular Electron Density Topographical Characteristics of Ionic Liquids Based on Amino Acid Cations and Their Ester Derivatives.

Amino acid ionic liquids (AAILs) have attracted significant attention in the recent literature owing to their ubiquitous applications in diversifying areas of modern chemistry, materials science, and biosciences. The present work focuses on unraveling the molecular interactions underlying AAILs. Electronic structures of ion pairs consisting of amino acid cations ([AA(+)], AA = Gly, Ala, Val, Leu, Ile, Pro, Ser, Thr) and their ester substituted derivatives [AAE(+)] interacting with nitrate anion [NO3(-)] have been obtained from the dispersion corrected M06-2x density functional theory. The formation of ion pair is accompanied by the transfer of proton from quaternary nitrogen to anion facilitated via hydrogen bonding. The [Ile], [Pro], [Ser], and [Thr] and their esters reveal relatively strong inter- as well as intramolecular hydrogen-bonding interactions. Consequently, the hierarchy in binding energies of [AA][NO3] ion pairs and their ester analogues turns out to be [Gly] > [Ala] > [Ser] ∼ [Val] ∼ [Ile] > [Leu] ∼ [Thr] > [Pro]. The work underlines how the interplay of intra- as well as intermolecular hydrogen-bonding interactions in [AA]- and [AAE]-based ILs manifest in their infrared and (1)H NMR spectra. Substitution of -OCH3 functional group in [AA][NO3] ILs lowers the melting point attributed to weaker hydrogen-bonding interactions, making them suitable for room temperature applications. As opposed to gas phase structures, the presence of solvent (DMSO) does not bring about any proton transfer in the ion pairs or their ester analogues. Calculated (1)H NMR chemical shifts of the solvated structures agree well with those from experiment. Correlations of decomposition temperatures in [AA]- and [AAE]-based ILs with binding energies and electron densities at the bond critical point(s) in molecular electron density topography, have been established.

Engineering of Nitrogen-Doped Carbon Nanofibers for the ORR By Chemical Vapor Deposition

Recently, nitrogen doped carbon nanofibers (N-CNFs) prepared in the presence of Fe have shown promising activity for the oxygen reduction reaction (ORR). However, the turn over frequency (TOF) at a given potential is low compared to traditional Pt/C catalysts1. This could be due to lower active site density and poor accessibility of the active sites to oxygen molecules. Therefore, it is necessary to understand the nature of the active sites present in the N-CNFs in order to increase their active site density. It has been proposed that the presence of pyridinic nitrogen may be playing an important role for the ORR activity compared to other nitrogen groups such as quaternary, pyrrolic and pyridinic oxide2. In this work, a chemical vapor deposition method has been employed to selectively tailor the N-type groups present in the surface of the N-CNFs. Characterization of the elemental composition and N-groups present in the surface of the N-CNFs was done by XPS analysis and correlated with the ORR activity. The synthesis was carried out in a tubular quartz reactor by decomposing CO and NH3 over Fe catalyst supported on exfoliated graphite. The synthesis temperature was varied from 550⁰C to 750⁰C while keeping all other synthesis parameters constant. The activities for the ORR were examined by performing linear sweep voltammetry in 0.5M H2SO4. The H2O2 formation during the ORR was analyzed using a RRDE. Table1: ORR activity of the N-CNF catalysts Catalyst Temperature N-content EORR at 0.001mA iORR at 0.7V H2O2 at 0.5V N-CNF_600 600⁰C 3.4 at% 0.87 V 1.3 mA/cm2 36.1% N-CNF_650 650⁰C 3.3 at% 0.90 V 2.9 mA/cm2 39.6% N-CNF_750 750⁰C 1.4 at% 0.78 V 0.1 mA/cm2 49.5% The synthesis performed at the lowest temperature showed no growth of N-CNFs, but by increasing the synthesis temperature the N-CNF yields increased. However, the N-content in the surface of the N-CNFs decreased with increasing the synthesis temperature, where only 1.4at% was obtained for N-CNFs obtained at 750⁰C compared to 3.4at% at 600⁰C. It could therefore seem that higher temperature was not favorable for the incorporation of nitrogen in the carbon matrix. Furthermore, the N-CNFs obtained at 750⁰C contained more quaternary nitrogen, while synthesis at lower temperatures favored the presence of pyridinic nitrogen groups as shown in Figure 1. This could be related to the microstructure of the N-CNFs since CVD synthesis is expected to yield more platelet CNFs at lower temperatures, thus exposing more edges and increasing the amount of pyridinic nitrogen. The most active ORR catalyst in acidic medium was the one obtained at 650⁰C with high current density of 2.9 mA/cm2 at 0.7 V vs. RHE. Further investigations using HR-TEM and by applying post treatment methods on prepared N-CNFs will be employed to explore the active sites and textural properties of N-CNFs further.

The effect of carbonization temperature on the electrocatalytic performance of nitrogen-doped porous carbon as counter electrode of dye-sensitized solar cells

Nitrogen-doped porous carbons (NPC) prepared by direct carbonization of nitrogen-containing polymer were investigated as the counter electrode of dye-sensitized solar cells. The effect of carbonization temperature on the electrocatalytic performance of NPC electrode was discussed. The nitrogen content reasonably decreased with the increase of carbonization temperature for NPC samples. However, the electrocatalytic activity of NPC electrode did not follow the same trend. As carbonization temperature increased from 700 to 800 °C, the charge-transfer resistance of NPC electrode decreased from 8.07 to 1.77 Ω cm2. With further increasing carbonization temperature from 800 to 900 °C, the charge-transfer resistance of NPC electrode increased from 1.77 to 5.08 Ω cm2. X-ray photoelectron spectroscopy analysis identified three kinds of nitrogen in as-prepared NPC, including pyridinic, pyrrolic, and quaternary nitrogen. With the increase of carbonization temperature, the content of pyridinic and pyrrolic nitrogen decreased, while the content of quaternary nitrogen increased. Pyridinic and pyrrolic nitrogen could create the electrocatalytic active sites in NPC, while quaternary nitrogen could improve the electron transfer through NPC. Therefore, the proper ratio among different nitrogen states has a significant effect on the electrolcatalytic activity of NPC. The NPC prepared at moderate carbonization temperature of 800 °C exhibited a superior electrolcatalytic activity because it possessed both relatively higher content of pyridinic nitrogen and higher content of quaternary nitrogen.

Synthesis and Characterization of Piperazinium Polyelectrolytes Carrying Hydrophobic Functionalities: Effect of Counter Ion and Charge Density on Flocculation

Following our recent synthesis and characterization of three new cationic polyelectrolytes with subtle hydrophobic variability, this paper reports their physical and chemical properties in aqueous media in relation to their chemical structure. Aryl substituted cationic polyelectrolytes varying with their charge density are reported for the first time. Viscosity studies show that these polymers display typical polyelectrolytic behavior. The flocculation efficiency of the polyelectrolytes was investigated with different counter ions. The zeta potential of the polyelectrolytes indicates the charge of the mono and diquaternary ammonium salts which is supported by chloride analysis. The morphology of polymer before and after flocculation was investigated. The introduction of methylene group and quaternary nitrogen play an important role in the flocculation process. It was shown that increasing the hydrophobicity and charge density of the aryl substituted polymer affects the flocculation in the industrial tannery effluent and bentonite suspension.

Nitrogen-doped porous carbon prepared by a facile soft-templating process as low-cost counter electrode for High-performance dye-sensitized solar cells

Nitrogen-doping porous carbon (NPC) with high surface area is prepared by a simple soft-templating method and explored as a low-cost counter electrode for dye-sensitized solar cells. Nitrogen adsorption analysis shows that the as-prepared NPC has a high surface area and mesoporous structure. Electrochemical impedance spectroscopy test shows a low charge-transfer resistance of 0.77Ωcm2 for NPC electrode in iodide/triiodide redox electrolyte. Such excellent electrocatalytic activity can be attributed not only to good surface properties including high surface area and mesoporous structure but also to nitrogen doping in NPC framework. Among various nitrogen species in the NPC framework, pyridinic and quaternary nitrogen are considered to contribute significantly to the electrocatalytic activity. A conversion efficiency of 7.09% is achieved for dye-sensitized solar cells with NPC counter electrode at one sun illumination, which is comparable to that of the cell with Pt counter electrode. These results reveal that the NPC electrode is a promising counter electrode candidate for dye-sensitized solar cells.

Adsorption of Cd2+ and Cu2+ ions from aqueous solutions by a cross-linked polysulfonate–carboxylate resin

A new cross-linked polyzwitterion (CPZ) was synthesized through cyclocopolymerization of 3-[diallyl(carboxymethyl)ammonio]propane-1-sulfonate (92.5mol%), and a cross-linker 1,1,4,4-tetraallylpiperazine-1,4-diium chloride (7.5mol%) in the presence of tert-butylhydroperoxide. The cross-linked polyzwitterion/anion (CPZA) was obtained by the basification of CPZ with NaOH. The adsorption data fit both Temkin and Freundlich isotherm models. The adsorption trend on CPZA is as Cu2+>Cd2+ and both followed pseudo-second-order kinetic model. The negative ΔG and positive ΔH values ascertained the spontaneous and endothermic nature of the adsorption process. The effectiveness of a zwitterionic–anionic motif consisting of quaternary nitrogen, sulfonate and carboxylate groups has been tested for the first time for capturing Cd2+ and Cu2+ ions at low concentrations.

Easy method to prepare N-doped carbon nanotubes by ball milling

N-doped carbon materials are promising metal-free catalysts for a number of applications. In this work, a cost effective and easy method to prepare N-doped carbon nanotubes by ball milling was developed, which avoids the use of solvents and production of wastes. Melamine and urea were used as nitrogen precursors. The textural and chemical properties of the N-doped materials were characterized by N2 adsorption at −196°C, temperature programmed desorption, elemental analysis, X-ray photoelectron spectroscopy and thermogravimetric analysis. The highest efficiency of N-doping was obtained with melamine. This method leads to the incorporation of large amounts of N-groups, namely quaternary nitrogen, pyridinic, and pyrrolic groups.

Investigation on the effect of catalyst on the electrochemical performance of carbon felt and graphite felt for vanadium flow batteries

The role of catalysts in vanadium flow batteries (VFBs) has been studied by introducing bismuth (Bi) nanoparticles on carbon felt (CF) and graphite felt (GF). The electrocatalytic activity and VFBs performance of CF and GF before and after modification with Bi nanoparticles are investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and VFB single cell charge–discharge test. The results show that CF exhibits the much higher electrocatalytic activity than GF, due to its higher amount of C–OH and quaternary nitrogen groups and more defect sites. Bi nanoparticles can effectively improve the electrocatalytic activity of CF and GF, especially GF, towards V2+/V3+ redox couple in VFBs. As a result, energy efficiency of a VFB with GF electrodes can be improved significantly by modification with Bi due to the dramatically reduced electrochemical polarization. However, the energy efficiency of a VFB with CF electrodes rarely changes after introduction of Bi nanoparticles, due to the fact that dominant limitation in a VFB with CF electrodes is ohmic polarization, and the reduced charge transfer resistance is not enough to improve the performance of this VFB remarkably. Therefore, CF is a more suitable electrode material for commercialized VFBs due to its higher electrocatalytic activity and lower cost.

Nitrogen-Doped Carbon Membrane Derived from Polyimide as Free-Standing Electrodes for Flexible Supercapacitors.

Nitrogen-doped carbon materials have attracted great interest in the energy storage due to the better electrochemical performances than the pristine carbon materials. In this work, a heterocyclic polyimide containing benzopyrrole and benzimidazole rings is carbonized to fabricate the free-standing and flexible carbon membrane (CarbonPI ) with a high packing density (0.89 cm(-3)), in which the location of nitrogen atoms in the doped configurations is easily controlled. XPS analysis indicates that quaternary nitrogen is the predominant nitrogen-doped configurations. The high content of nitrogen effectively improves the wettability of the electrode materials. The CarbonPI membrane exhibits excellent volumetric capacitance (159.3 F cm(-3) at 1 A g(-1)), high rate capability (127.5 F cm(-3) at 7 A g(-1)), and long cycle life. TEM images reveal the very slight change of the microstructure of graphitic nanosheet of CarbonPI during the long charge/discharge cycles.