Sandipan Maiti

Reversible Lithium Storage in Manganese 1,3,5-Benzenetricarboxylate Metal–Organic Framework with High Capacity and Rate Performance

Metal organic frameworks (MOFs) with diverse structural chemistry are being projected as futuristic electrode materials for Li-ion batteries. In this work, we report synthesis of Mn-1,3,5-benzenetricarboxylate MOF by a simple solvothermal method and its application as an anode material for the first time. Scanning electron microscopy of the synthesized MOF shows a bar shaped morphology where these bars, about 1 μm wide and of varied lengths between 2 and 20 μm, are made of porous sheets containing mesoporous walls and macroporous channels. The MOF anode, when examined in the potential window of 0.01-2.0 V versus Li/Li(+), shows high specific capacities of 694 and 400 mAh g(-1) at current densities of 0.1 and 1.0 A g(-1) along with good cyclability, retention of capacity, and sustenance of the MOF network. Ex situ X-ray diffraction, Fourier transform infrared, and X-ray photoelectron spectroscopy studies on the electrode material at different states of charge suggest that the usual conversion reaction for Li storage might not be applicable in this case. Conjugated carboxylates being weakly electron withdrawing ligands with a stronger π-π interaction, a probable alternative Li storage mechanism has been proposed that involves the organic moiety. The present results show promise for applying Mn-1,3,5-benzenetricarboxylate MOF as high performance <2 V anode.

Electrochemical energy storage in Mn2O3 porous nanobars derived from morphology-conserved transformation of benzenetricarboxylate-bridged metal–organic framework

Tailoring the morphology of mesoporous nanostructures toward performance enhancement plays a key role in developing efficient energy storage devices. Herein, we report the formation of well crystallized Mn2O3 mesoporous nanobars through simple exo-templating of a manganese 1,3,5-benzenetricarboxylate metal–organic framework (Mn-BTC MOF) by thermal treatment whereby the general morphology of the parent MOF is conserved, but with more voids and spaces. The parent Mn-BTC MOF was synthesized by solvothermal reaction of trimesic acid with manganese nitrate in alcoholic solution. The MOF-derived Mn2O3 was characterized by XRD, field-emission SEM, high-resolution TEM, and N2 adsorption/desorption isotherm measurements. When examined as an anode material for lithium-ion batteries in the potential windows of 0.01–3.0 V and 0.01–2.0 V, high reversible specific capacities of 849 and 778 mAh g−1 were obtained. It was found that the electrochemical processes are more reversible when cycled in the 2 V window. A steady capacity of ∼410 mAh g−1 was observed after 300 continuous cycles at ∼C/5.5 exhibiting good cycling stability in the 2 V window. When tested as a pseudocapacitor electrode in a three-electrode configuration, a specific capacitance of 250 F g−1 at 0.2 A g−1 could be achieved. Further, to demonstrate practical applicability, two-electrode asymmetric supercapacitor pouch cells were assembled with Mn2O3 as the positive electrode and commercial activated carbon as the negative electrode which showed an ultrahigh energy density of 147.4 W h kg−1 at a power density of 1004 W kg−1. The present work shows the potential of a MOF derived route for obtaining metal oxides with desired nano-architectures for electrochemical applications with high performance.

Influence of imidazolium-based ionic liquid electrolytes on the performance of nano-structured MnO2 hollow spheres as electrochemical supercapacitor

Use of room temperature ionic liquids (ILs) as electrolytes with a wider potential window offers scope for developing high energy density supercapacitors for efficient energy storage. In this work, a comparative study on the performance of nanostructured MnO2 hollow spheres as electrochemical supercapacitor was made by fabricating activated carbon (AC)//MnO2 asymmetric supercapacitor cells using imidazolium-based ILs as electrolytes. Mesoporous MnO2 hollow spheres were synthesized through a simple low temperature (80 °C) solvothermal method by reduction of KMnO4 under acidic condition in presence of Cu3(1,3,5-benzenetricarboxylate)2 metal organic framework (MOF) in the precursor solution, which acts as the source of Cu2+ playing a crucial role for the formation of the hollow spheres. Four different ILs were investigated from combinations of two different cations [1-ethyl-3-methylimidazolium (EMI+) and 1-butyl-3-methylimidazolium (BMI+)] and four different anions [hexaflurophosphate (PF6−), tetrafluoroborate (BF4−), trifluromethanesulfonate (Otf−) and bis(trifluromethylsulfonyl)imide (TFSI−)]. Influences of the physico-chemical properties such as ionic size, nucleophilicity, viscosity of the IL on the electrochemical properties are discussed. A high energy density of 163 W h kg−1, which is comparable to the energy density of a lithium-ion battery, could be achieved with EMIMBF4 as electrolyte. The present findings would help in further research for developing IL-based supercapacitors.

Large-scale synthesis of porous NiCo2O4 and rGO–NiCo2O4 hollow-spheres with superior electrochemical performance as a faradaic electrode

Synthesis of nanocomposites of metal oxide and reduced graphene oxide (rGO) in a hollow spherical form has been proved to be challenging due to the crumbling effect of rGO. In this paper, we report a simple, cost-effective and large-scale synthetic strategy for producing porous NiCo2O4 hollow spheres as well as rGO–NiCo2O4 hollow spheres through spray drying of respective metal ammonium carbonate complex solutions followed by calcination. During the synthesis process, ammonium carbonate played a pivotal role in hollow sphere formation through easy decomposition into CO2 and NH3 and enhancement of internal pressure of atomized droplets helped to overcome the crumbling effect of rGO. The synthesized hollow spheres are porous, made of 7–12 nm particles with an average diameter of 2–3 μm and a surface area of 76 m2 g−1 for pristine NiCo2O4 and 21 m2 g−1 for rGO–NiCo2O4. Observance of distinct redox peaks in cyclic voltammetry (CV) indicates that the electrochemical charge storage mechanism of NiCo2O4 is non-capacitive and somewhat battery type in nature. The synthesized rGO–NiCo2O4 hollow spheres showed a specific capacity as high as 427 C g−1 (971 F g−1) at a current density of 0.5 A g−1 which is much superior to that of pristine NiCo2O4 hollow spheres (183 C g−1). rGO–NiCo2O4 also exhibited an excellent rate performance with capacities of 385.3, 345.4, 256, 169 and 89 C g−1 at current densities of 1, 2, 5, 10 and 20 A g−1, respectively and 76% retention of capacity after 5000 cycles at 10 A g−1. Furthermore, studies on AC//rGO–NiCo2O4 asymmetric cells show that the energy storage performance of rGO–NiCo2O4 hollow spheres obtained by the present scalable and cost-effective process is quite comparable, or even superior to those reported for NiCo2O4 synthesized through sophisticated and costly synthetic protocols.

Metal hydroxides as a conversion electrode for lithium-ion batteries: a case study with a Cu(OH)2 nanoflower array

Conversion electrodes, the materials of choice for the next generation lithium-ion battery (LIB), are mainly limited to metal oxides. In this work, we have investigated the electrochemical performance of chemically synthesized Cu(OH)2 nanoflower arrays. A 50 : 50 composite of Cu(OH)2 and multiwalled carbon nanotubes (MWCNTs) showed a reversible capacity of 522 mA h g−1 at a current density of 0.1 mA cm−2 with 95% retention of capacity after 50 cycles. The results demonstrate that it can be a competitive choice over the corresponding oxides as an anode for LIB.

Electrochemical energy storage in montmorillonite K10 clay based composite as supercapacitor using ionic liquid electrolyte.

Exploring new electrode materials is the key to realize high performance energy storage devices for effective utilization of renewable energy. Natural clays with layered structure and high surface area are prospective materials for electrical double layer capacitors (EDLC). In this work, a novel hybrid composite based on acid-leached montmorillonite (K10), multi-walled carbon nanotube (MWCNT) and manganese dioxide (MnO2) was prepared and its electrochemical properties were investigated by fabricating two-electrode asymmetric supercapacitor cells against activated carbon (AC) using 1.0M tetraethylammonium tetrafluroborate (Et4NBF4) in acetonitrile (AN) as electrolyte. The asymmetric supercapacitors, capable of operating in a wide potential window of 0.0-2.7V, showed a high energy density of 171Whkg(-1) at a power density of ∼1.98kWkg(-1). Such high EDLC performance could possibly be linked to the acid-base interaction of K10 through its surface hydroxyl groups with the tetraethylammonium cation [(C2H5)4N(+) or TEA(+)] of the ionic liquid electrolyte. Even at a very high power density of 96.4kWkg(-1), the cells could still deliver an energy density of 91.1Whkg(-1) exhibiting an outstanding rate capability. The present study demonstrates for the first time, the excellent potential of clay-based composites for high power energy storage device applications.

TiO2-rGO nanocomposite hollow spheres: large scale synthesis and application as an efficient anode material for lithium-ion batteries

We report here a controllable large-scale synthesis protocol for TiO2-rGO nanocomposites with a hollow spherical morphology by a novel aerosol-assisted spray drying method followed by calcination. The developed strategy is easy to scale-up without significant change in morphology. The precursors used for the synthesis are aqueous titanium ammonium peroxo-carbonate complex (TAPCC) solution, being decomposable at low temperatures, and an aqueous graphene oxide (GO) suspension; no additive or structure-directing agent is required. Both the precursors play vital roles in hollow sphere formation. The enhanced internal pressure inside the atomized droplets built-up in situ through decomposition of TAPCC forming gaseous CO2, NH3, O2 and H2O vapour, helps overcome the crumpling effect of GO. The sheet structure of GO provides sufficient mechanical strength to prevent bursting of the expanded droplets. The synthesized TiO2-rGO hollow spheres are porous, composed of 10–20 nm TiO2 particles dispersed on the surfaces of rGO with a surface area of 86 m2 g−1. The synthesized TiO2-rGO composites showed superior electrochemical performance as lithium-ion battery (LIB) anode with capacity values of 265 mA h g−1 (10 wt% rGO) and 274 mA h g−1 (20 wt% rGO) at 18.8 mA g−1 as compared to 236 mA h g−1 for the pristine sample with a 2D flake-like morphology. This is due to the unique thin-walled hollow-spherical structure of TiO2-rGO composites, which provides a large number of electrochemically accessible active sites, favorable diffusion paths for Li+ ions and enhanced charge transport. Thus, the present synthetic method has great potential for large scale production of TiO2-rGO hollow spheres for application as anode in practical high-rate lithium-ion batteries.

CeO2@C derived from benzene carboxylate bridged metal–organic frameworks: ligand induced morphology evolution and influence on the electrochemical properties as a lithium-ion battery anode

We report herein a facile metal–organic framework (MOF) derived route for the synthesis of carbon embedded CeO2 (CeO2@C) with a pre-designed shape-specific morphology by varying the organic linker and by using PVP as the structure directing agent. It is found that the general morphological features of the parent MOF are mimicked by the derived oxide. Four different linkers have been used to prepare CeO2@C particles with three different shapes – spherical, bar-shaped and thin plate-like. A probable formation mechanism is discussed based on metal–ligand coordination. Influence of the morphology on the electrochemical properties as a lithium-ion battery (LIB) anode has been studied in coin cells vs. Li/Li+. The spherically shaped CeO2@C-14 shows a superior performance with a maximum specific capacity of 715 mA h g−1 at 0.05 mA cm−2, good rate performance (413 mA h g−1 at 0.5 mA cm−2) and cycling stability (∼94% capacity retention after 100 cycles). The present results demonstrate that the major limitations of metal oxide anodes – volume expansion during lithiation/delithiation, rate performance and capacity fading upon cycling – could be overcome to a great extent by adopting the two-way approach of morphology design through the MOF route and in situ embedded carbon matrix.

A facile method for the synthesis of a C@MoO2 hollow yolk–shell structure and its electrochemical properties as a faradaic electrode

Transition-metal oxide hollow yolk–shell micro/nanostructures combined with a conducting substance have gained significant attention as efficient electrode materials for electrochemical energy storage applications due to their large surface area, internal void space, and structural stability. Herein, we report a facile aqueous solution-based soft template method using sucrose–CTAB for the synthesis of a hollow yolk–shell structure of carbon-incorporated MoO2 (C@MoO2) with a diameter of 0.9–1.1 μm, wall thickness of 100 nm, inner yolk size of 400–450 nm, and BET surface area of 40 m2 g−1. During the synthesis process, sucrose plays a dual role, both as a template and a carbon source. The electrochemical charge storage mechanism follows a battery-type behaviour when tested as a faradaic electrode in 3.0 M KOH electrolyte. C@MoO2 exhibits a high specific capacity of 188 C g−1 at the current density of 0.5 A g−1, good rate performance (50.6 C g−1 at 10 A g−1), and 78% retention of capacity after 5000 cycles at 5 A g−1. The obtained performance is superior to those obtained for pure MoO2 hollow spheres (137.1 C g−1 at 0.5 A g−1) as well as previously reported MoO2 and MoO3, indicating the potential applicability of the as-synthesised yolk–shell C@MoO2.