Dhrubajyoti Bhattacharjya

Phosphorus-Doped Ordered Mesoporous Carbons with Different Lengths as Efficient Metal-Free Electrocatalysts for Oxygen Reduction Reaction in Alkaline Media

Phosphorus-doped ordered mesoporous carbons (POMCs) with different lengths were synthesized using a metal-free nanocasting method of SBA-15 mesoporous silica with different sizes as template and triphenylphosphine and phenol as phosphorus and carbon sources, respectively. The resultant POMC with a small amount of P doping is demonstrated as a metal-free electrode with excellent electrocatalytic activity for oxygen reduction reaction (ORR), coupled with much enhanced stability and alcohol tolerance compared to those of platinum via four-electron pathway in alkaline medium. Interestingly, the POMC with short channel length is found to have superior electrochemical performances compared to those with longer sizes.

Nitrogen-doped carbon nanoparticles by flame synthesis as anode material for rechargeable lithium-ion batteries.

Nitrogen-doped turbostratic carbon nanoparticles (NPs) are prepared using fast single-step flame synthesis by directly burning acetonitrile in air atmosphere and investigated as an anode material for lithium-ion batteries. The as-prepared N-doped carbon NPs show excellent Li-ion stoarage properties with initial discharge capacity of 596 mA h g(-1), which is 17% more than that shown by the corresponding undoped carbon NPs synthesized by identical process with acetone as carbon precursor and also much higher than that of commercial graphite anode. Further analysis shows that the charge-discharge process of N-doped carbon is highly stable and reversible not only at high current density but also over 100 cycles, retaining 71% of initial discharge capacity. Electrochemical impedance spectroscopy also shows that N-doped carbon has better conductivity for charge and ions than that of undoped carbon. The high specific capacity and very stable cyclic performance are attributed to large number of turbostratic defects and N and associated increased O content in the flame-synthesized N-doped carbon. To the best of our knowledge, this is the first report which demonstrates single-step, direct flame synthesis of N-doped turbostratic carbon NPs and their application as a potential anode material with high capacity and superior battery performance. The method is extremely simple, low cost, energy efficient, very effective, and can be easily scaled up for large scale production.

Morphology-dependent Li storage performance of ordered mesoporous carbon as anode material.

Rod-shaped ordered mesoporous carbons (OMCs) with different lengths, prepared by replication method using the corresponding size-tunable SBA-15 silicas with the same rodlike morphology as templates, are explored as anode material for Li-ion battery. All of the as-synthesized OMCs exhibit much higher Li storage capacity and better cyclability along with comparable rate capability as compared with commercial graphite. Particularly, the OMC-3 with the shortest length demonstrates the highest reversible discharge capacity of 1012 mAh g(-1) at 100 mA g(-1) and better cyclability with 86.6% retention of initial capacity after 100 cycles. Although the Coulombic efficiencies of all the OMCs are relatively low at the beginning, they improve promptly and after 10 cycles reach the level comparable to commercial graphite. Based on their specific capacity, cycle efficiency, and rate capability, the OMC-3 outperforms considerably its carbon peers, OMC-1 and OMC-2 with longer length. This behavior is mainly attributed to higher specific surface area, which provides more active sites for Li adsorption and storage along with the larger mesopore volume and shorter mesopore channels, which facilitate fast Li ion diffusion and electrolyte transport. The enhancement in reversible Li storage performance with decrease in the channel length is also supported by low solid electrolyte interphase resistance, contact resistance, and Warburg impedance in electrochemical impedance spectroscopy.

1-Dimensional porous α-Fe2O3 nanorods as high performance electrode material for supercapacitors

Porous α-Fe2O3 nanorods are successfully synthesized without any templates by a simple wet chemical synthesis method using ferrous sulphate (FeSO4·7H2O) and sodium acetate (CH3COONa) as starting materials. In this method, initially obtained α-FeOOH is calcinated at 300 °C for 2 h to form 1-dimensional porous α-Fe2O3 nanorods. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HR-TEM) and a gas sorption analyzer are employed to characterize α-Fe2O3 porous nanorods. Based on the characterization results, a formation mechanism for α-Fe2O3 nanorods is proposed. Electrochemical performance of porous α-Fe2O3 nanorods is studied using cyclic (CV) voltammetry, galvanostatic charge/discharge measurements and electrochemical impedance spectroscopy (EIS) in aqueous H3PO4, (NH4)2SO4 and Na2SO4 electrolytes. Interestingly, the porous α-Fe2O3 nanorod-based electrodes exhibit excellent electrochemical performance, which can be attributed to the high surface area induced by the 1-dimensional porous nanorod structures. The rod shape porous structure facilitates the faster faradic reaction toward electrolytes and delivers highest specific capacitance (308 F g−1) and an excellent long cycle life (upto 1000 cycles) in H3PO4 electrolyte, demonstrating that the porous α-Fe2O3 nanorods can serve as an excellent electrode material for supercapacitors.

Nitrogen and phosphorus co-doped cubic ordered mesoporous carbon as a supercapacitor electrode material with extraordinary cyclic stability

Heteroatoms and porosity both have different, but definite effects on the electrochemical capacitance of carbon materials. These effects are studied in detail by using cubic ordered mesoporous carbons (OMCs) co-doped with N and P. 3-Dimensional (3D) mesoporous silica, KIT-6, with bicontinuous cubic Ia3d symmetry is utilized as a hard template to synthesize the cubic OMCs. Interestingly, although the porosity parameters e.g. surface area and pore volume do not change much with N doping, a significant increase of these values is observed upon P doping. Moreover, the P content does not affect the N doping characteristics on co-doping of both N and P. When tested as a supercapacitor electrode, the N-OMC, despite its much lower porosity parameters, exhibits a similar specific capacitance to that of the P-OMC. The high specific capacitance of N-OMC arises mainly from the pseudocapacitive effect of doped N species, whereas high porosity parameters are the main reason for the specific capacitance shown by P-OMC. The synergistic contribution of both effects enables the NP co-doped OMC to show the highest specific capacitance of 210 F g−1 at 1.0 A g−1. Moreover, excellent retention of specific capacitance with more than 90% of initial capacitance is observed for NP-OMC at a high current density of 10 A g−1 and also for 3000 charge–discharge cycles. This is mainly because of high-surface area hierarchical porous structures with uniform and ordered mesopores in the cubic OMC, which facilitate the unrestricted movement of electrolyte ions to access the active surfaces, as well as the excellent synergistic effect of co-doping of N and P. This is also supported by electrochemical impedance spectroscopic analysis, which shows negligible mass transfer resistance and internal cell resistance. Overall, the synthesized cubic OMC materials are found to be highly promising as electrodes for supercapacitors and other energy-related applications.

Functionalized Agarose Self-Healing Ionogels Suitable for Supercapacitors

Agarose has been functionalized (acetylated/carbanilated) in an ionic liquid (IL) medium of 1-butyl-3-methylimidazolium acetate at ambient conditions. The acetylated agarose showed a highly hydrophobic nature, whereas the carbanilated agarose could be dissolved in water as well as in the IL medium. Thermoreversible ionogels were obtained by cooling the IL sols of carbanilated agarose at room temperature. The ionogel prepared from a protic-aprotic mixed-IL system (1-butyl-3-methylimidazolium chloride and N-(2-hydroxyethyl)ammonium formate) demonstrated a superior self-healing property, as confirmed from rheological measurements. The superior self-healing property of such an ionogel has been attributed to the unique inter-intra hydrogen-bonding network of functional groups inserted in the agarose. The ionogel was tested as a flexible solid electrolyte for an activated-carbon-based supercapacitor cell. The measured specific capacitance was found to be comparable with that of a liquid electrolyte system at room temperature and was maintained for up to 1000 charge-discharge cycles. Such novel functionalized-biopolymer self-healing ionogels with flexibility and good conductivity are desirable for energy-storage devices and electronic skins with superior lifespans and robustness.

Synthesis of hollow TiO2@N-doped carbon with enhanced electrochemical capacitance by an in situ hydrothermal process using hexamethylenetetramine

A unique and novel soft template-based hydrothermal approach was developed for the synthesis of hollow TiO2 and hollow TiO2@N-doped carbon. The synthesis strategy involves the slow hydrolysis of hexamethylenetetramine (HMTA) at 100 °C in the presence of a block copolymer (Pluronic F127) as the surfactant, resorcinol as the polymer precursor and titanium salt as the metal oxide precursor to form a hollow composite nanostructure consisting of TiO2 nanoparticles (NPs) covered with a resorcinol–formaldehyde (RF) polymer shell. Hydrolysis of HMTA provides a gradual and controlled supply of hydroxide ions, formaldehyde and ammonia. The resulting ammonia initiates the polymerization reaction of the generated formaldehyde with resorcinol to produce an RF–polymer framework over the TiO2 NPs thereby generating TiO2@RF polymer particles, which in turn self-assemble to form a hollow TiO2@RF polymer composite nanostructure. Subsequent pyrolysis under an N2 atmosphere produces a hollow TiO2 nanostructure covered with a thin layer of N-doped carbon. The resulting novel nanostructure not only possesses a high surface area of 310 m2 g−1, but also provides a protective N-doped carbon layer. As a result, this hollow TiO2@N-doped carbon material demonstrates high potential as an electrode material for use as an electrochemical capacitor with high specific capacitance and high durability. Interestingly, this work proceeds through a very effective, simple one-pot synthesis route to generate novel hollow TiO2 composite structures, and will enable the synthesis of various active hollow metal oxide@N-doped carbon and/or hollow organic–inorganic hydride nanocomposite materials for many possible applications.

Nitrogen‐Doped Ordered Mesoporous Carbon with Different Morphologies for the Oxygen Reduction Reaction: Effect of Iron Species and Synergy of Textural Properties

Nitrogen‐doped ordered mesoporous carbons (N‐OMCs) with different morphologies are prepared as oxygen reduction reaction (ORR) catalysts through pyrolysis of iron phthalocyanine‐infiltrated SBA‐15 silica with different mesochannel lengths. Excellent ORR activity with a nearly four‐electron transfer process is observed in both alkaline and acidic media. In particular, the difference in half‐wave potential for ORR relative to commercial Pt/C catalyst is only 50 mV negative in acidic medium, whereas it is 50 mV more positive in alkaline medium. Interestingly, it is found that although the use of iron is necessary for the preparation of highly active nitrogen‐doped ORR carbon catalysts, its presence is not necessary for N‐OMC to be active in the ORR in either alkaline or acidic media. In addition, the ORR activity increases gradually with decreasing mesopore channel length, with maximum activity in N‐OMC with short channels, demonstrating the high synergistic influence of structural morphology on ORR in heteroatom‐doped carbon.

Graphene Nanoplatelets with Selectively Functionalized Edges as Electrode Material for Electrochemical Energy Storage

In recent years, graphene-based materials have been in the forefront as electrode material for electrochemical energy generation and storage. Despite this prevalent interest, synthesis procedures have not attained three important efficiency requirements, that is, cost, energy, and eco-friendliness. In this regard, in the present work, graphene nanoplatelets with selectively functionalized edges (XGnPs) are prepared through a simple, eco-friendly and efficient method, which involves ball milling of graphite in the presence of hydrogen (H2), bromine (Br2), and iodine (I2). The resultant HGnP, BrGnP, and IGnP reveal significant exfoliation of graphite layers, as evidenced by high BET surface area of 414, 595, and 772 m(2) g(-1), respectively, in addition to incorporation of H, Br, and I along with other oxygen-containing functional groups at the graphitic edges. The BrGnP and IGnP are also found to contain 4.12 and 2.20 at % of Br and I, respectively in the graphene framework. When tested as supercapacitor electrode, all XGnPs show excellent electrochemical performance in terms of specific capacitance and durability at high current density and long-term operation. Among XGnPs, IGnP delivers superior performance of 172 F g(-1) at 1 A g(-1) compared with 150 F g(-1) for BrGnP and 75 F g(-1) for HGnP because the large surface area and high surface functionality in the IGnP give rise to the outstanding capacitive performance. Moreover, all XGnPs show excellent retention of capacitance at high current density of 10 A g(-1) and for long-term operation up to 1000 charge-discharge cycles.

Green fabrication of 3-dimensional flower-shaped zinc glycerolate and ZnO microstructures for p-nitrophenol sensing

The solvent or reaction medium always plays a lead role in synthesis chemistry. Glycerol has been studied as a green solvent for different organic transformations and is also expected to give interesting control in material synthesis. In this study, we use aqueous glycerol to synthesize zinc glycerolate and the corresponding ZnO micro-flower structures with an intention to encourage the utilization of glycerol as a green reaction medium in material synthesis. A zinc ammonium complex is used as a source of zinc, which converts to zinc glycerolate in the presence of glycerol. Glycerol plays a dual role as a reactant to form zinc glycerolate and as a solvent to control the morphology. The unreacted glycerol is recovered after the reaction and reused further. The flower-structured zinc glycerolate and ZnO are then used for the first time to modify a glassy carbon electrode to make a binder-free non-enzymatic amperometric chemical sensor for p-nitrophenol that is a brutal environmental pollutant. The modified electrode is found to be an excellent alternative for the purpose with respect to sensitivity, selectivity and stability.