Papers by syam g krishnan Krishnan
For practical deployment of supercapacitors characterized by high energy density, power density a... more For practical deployment of supercapacitors characterized by high energy density, power density and long cycle life, they must be realized using low cost and environmentally benign materials. Titanium dioxide (TiO 2) is largely abundant in the earth's crust; however, they show inferior supercapacitive electrochemical properties in most electrolytes for practical deployment. In this paper, we show that nickel doped TiO 2 (Ni:TiO 2) nanowires developed by electrospinning showed five times larger capaci-tance (~200 F g À1) than the undoped analogue (~40 F g À1). Electrochemical measurements show that the Ni:TiO 2 nanowires have 100% coulombic efficiency. The electrodes showed no appreciable capacitance degradation for over 5000 cycles. The superior charge storage capability of the Ni:TiO 2 could be due to its high electrical conductivity that resulted in five orders of magnitude higher ion diffusion as determined by cyclic voltammetry and electrochemical impedance spectroscopy measurements.
A B S T R A C T Asymmetric supercapacitors (ASCs) are routinely fabricated using battery-type ele... more A B S T R A C T Asymmetric supercapacitors (ASCs) are routinely fabricated using battery-type electrode materials as a positive electrode and electrochemical double layer materials as a negative electrode; the mass-loading in the electrodes is determined by assuming both to be capacitive charge storage materials. This protocol is erroneous as the cyclic voltammograms and galvanostatic charge-discharge curves of the resulting devices showed dissimilarity in the stored charges of the two electrodes and battery-type behaviors, respectively. Herein, we show by employing two choices of battery-type electrodes as positive electrodes and commercial activated carbon as negative electrode in 3 M LiOH electrolyte that equal mass loading in both electrodes leads to supercapacitive charge storage. The positive electrode to negative electrode mass ratio is varied from 0.75 to 1.5 in a mass interval of 0.25 which includes a mass ratio of the conventional method. The electrochemical studies of the fabricated ASCs show that the charge storage capabilities depend on the electrode mass. Electrochemical impedance spectroscopy studies show that the equal mass ratio has low series and charge transfer resistances and wider frequency dispersion of capacitance.
Nanowires of a composite, Mn 2 O 3-SnO 2 , are synthesized at 100 g scale. The composite nanowire... more Nanowires of a composite, Mn 2 O 3-SnO 2 , are synthesized at 100 g scale. The composite nanowires showed beneficial properties of the constituents. They showed significantly high charge storability and cyclability than the constituents. The superior charge storage are attributed to lower characteristic resistances. a b s t r a c t Large scale production of electrochemical materials in non-conventional morphologies such as nanowires has been a challenging issue. Besides, functional materials for a given application do not often offer all properties required for ideal performance; therefore, a composite is the most sought remedy. In this paper, we report large scale production of a composite nanowire, viz. Mn 2 O 3-SnO 2 , and their constituent binary nanowires by a large scale electrospinning pilot plant consisting of 100 needles. Electrochemical characterization of thus produced composite nanowires showed nearly threefold increase in the discharge capacity compared to their single component counterparts: Mn 2 O 3-SnO 2 $53 mA h g À1 (specific capacitance, C S $384 F g À1); Mn 2 O 3 $18 mA h g À1 (C S $164 F g À1); and SnO 2 $14 mA h g À1 (C S $128 F g À1) at 1 A g À1 in 6 M KOH. The EIS studies showed that the characteristic resistances and time of the composite electrode are appreciably lower than their constituents. Owing to the scalability of the synthesis processes and promising capacitive properties achieved would lead the composite material as a competitive low-cost and high-performance supercapacitor electrode.
A B S T R A C T Three morphologies of magnesium cobaltite (MgCo 2 O 4), viz. cuboidal microcrysta... more A B S T R A C T Three morphologies of magnesium cobaltite (MgCo 2 O 4), viz. cuboidal microcrystals, nanoflowers, and nano-spheres, were synthesized using hydrothermal and molten salt methods and evaluated their electrochemical energy storage properties. Among them cuboidal microcrystal and nanoflowers were obtained by a facile hydrothermal route – the former with ethylene glycol and the latter with hexadecyltrimethylammonium bromide as surfactants. The cuboidal microcrystals showed layered flake microstructure with an appreciable space between the layers (~ 100 nm), which would facilitate ion movement between the flakes. The electrochemical studies of the materials revealed the superiority of MgCo 2 O 4 cuboidal microcrystals as a charge storage medium over the nanoflowers and nanospheres, the reasons for this is deeply investigated and reported herewith. The specific charge stored in the MgCo 2 O 4 cuboidal microcrystal electrode was ~ 345 C g −1 at a specific current of 1 A g −1 which was superior to nanoflowers (~ 178 C g −1) and nanospheres (~ 139 C g −1) at the similar current density in 3 M LiOH electrolyte. The MgCo 2 O 4 cuboidal microcrystals also demonstrated superior charge retention (~ 110%) after 3000 cycles over the other electrodes demonstrating its practical utility as a charge storage material.
A B S T R A C T This paper reports that addition of reduced graphene oxide (rGO) in MgCo 2 O 4 im... more A B S T R A C T This paper reports that addition of reduced graphene oxide (rGO) in MgCo 2 O 4 improves the binding of Mg with Co thereby minimizing magnesium dissolution in aqueous alkaline electrolytes and the resulting MgCo 2 O 4 /rGO electrodes offered impressive improvements in charge storage properties. An isostructural high performing material, MnCo 2 O 4 , is used as a benchmark material in this work. The Mg analogues stored >30% more charges than the Mn-analogues in the 3 M LiOH electrolyte despite the former's lower BET surface area; rGO modification further increased charge storage by >60% than the unmodified analogues. Electrochemical measurements show that a larger surface fraction of the Mg analogue is electrochemically active, irrespective of whether or not rGO is present, which arise from, typically for MgCo 2 O 4 /rGO, lower internal resistance, lower Warburg impedance, and lower charge transfer resistance than the other electrodes.
Manganese cobaltite (MnCo 2 O 4) is currently under screening as a high performance supercapacito... more Manganese cobaltite (MnCo 2 O 4) is currently under screening as a high performance supercapacitor electrode owing to its high theoretical capacitance, improved electrical conductivity and long term cyclic stability. Herein, we report synthesis of MnCo 2 O 4 cuboidal microcrystals using hydrothermal method and compare its performance with its flakes prepared by solid combustion process. Crystal structure, surface properties, and electrochemical properties of the flakes are studied using X-ray diffraction, gas adsorption, field emission scanning electron microscopy, cyclic voltammetry, galvanostatic charge edischarge cycling, and electrochemical impedance spectroscopy. The electrochemical properties of MnCo 2 O 4 flakes synthesized using hydrothermal synthesis are superior to that synthesized using the solid combustion process. Electrochemical properties of the cuboidal microcrystals (~specific capaci-tance, C S ~600 F g À1 @ 0.5 A g À1) are superior to those synthesized by the combustion process (C S ~128 F g À1) due to improved faradic utilization of active surface area, layered cuboidal morphology, faster OH À ion penetration owing to higher diffusion coefficient, and larger voltage range available for elec-trochemical reaction.
For practical deployment of supercapacitors characterized by high energy density, power density a... more For practical deployment of supercapacitors characterized by high energy density, power density and long cycle life, they must be realized using low cost and environmentally benign materials. Titanium dioxide (TiO 2) is largely abundant in the earth's crust; however, they show inferior supercapacitive electrochemical properties in most electrolytes for practical deployment. In this paper, we show that nickel doped TiO 2 (Ni:TiO 2) nanowires developed by electrospinning showed five times larger capaci-tance (~200 F g À1) than the undoped analogue (~40 F g À1). Electrochemical measurements show that the Ni:TiO 2 nanowires have 100% coulombic efficiency. The electrodes showed no appreciable capacitance degradation for over 5000 cycles. The superior charge storage capability of the Ni:TiO 2 could be due to its high electrical conductivity that resulted in five orders of magnitude higher ion diffusion as determined by cyclic voltammetry and electrochemical impedance spectroscopy measurements.
Metal cobaltites have promising electrochemical properties for their application as an energy sto... more Metal cobaltites have promising electrochemical properties for their application as an energy storage
medium.In this paper, usefulness of MgCo2O4
as a supercapacitor electrode is demonstrated and
compared its performance with two other cobaltites, MnCo2O4
and CuCo2O4. The materials are synthesized
using molten salt method and characterized by X-ray diffraction, scanning electron microscopy,
BET surface area, cyclic voltammetry, galvanostatic charge–discharge cycling, and electrochemical
impedance spectroscopy techniques. The MgCo2O4 electrodes show superior charge storage
properties in 3 M LiOH among a diverse choice of electrolytes. The MgCo2O4 show higher theoretical (3122 F/g) and practically achieved capacitance (320 F/g), larger coulombic efficiency, and cycling stability than the other two; therefore, it could be developed as a low-cost energy storage
medium.
Flower shaped birnessite type manganese oxide (d-MnO 2 ) nanostructures are synthesized using a s... more Flower shaped birnessite type manganese oxide (d-MnO 2 ) nanostructures are synthesized using a simple hydrothermal process with an aim to fabricate high performance supercapacitors for energy storage electrode. The studies reveal that layered d-MnO 2 had a basal plane spacing of $0.73 nm and are composed of thin nanosheets of thickness $23 nm. A detailed investigation is undertaken to draw a relationship between the solvated ion size of alkaline electrolytes (LiOH, NaOH and KOH) and pore size in the electrode material favoring high specific capacitance and faster electrode kinetics. The present work not only develops a high performance supercapacitive material but also identifies that by suitably tuning the sizes of solvated ion and the pores, supercapacitive behavior of a single material system can be tailored.
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Papers by syam g krishnan Krishnan
medium.In this paper, usefulness of MgCo2O4
as a supercapacitor electrode is demonstrated and
compared its performance with two other cobaltites, MnCo2O4
and CuCo2O4. The materials are synthesized
using molten salt method and characterized by X-ray diffraction, scanning electron microscopy,
BET surface area, cyclic voltammetry, galvanostatic charge–discharge cycling, and electrochemical
impedance spectroscopy techniques. The MgCo2O4 electrodes show superior charge storage
properties in 3 M LiOH among a diverse choice of electrolytes. The MgCo2O4 show higher theoretical (3122 F/g) and practically achieved capacitance (320 F/g), larger coulombic efficiency, and cycling stability than the other two; therefore, it could be developed as a low-cost energy storage
medium.
medium.In this paper, usefulness of MgCo2O4
as a supercapacitor electrode is demonstrated and
compared its performance with two other cobaltites, MnCo2O4
and CuCo2O4. The materials are synthesized
using molten salt method and characterized by X-ray diffraction, scanning electron microscopy,
BET surface area, cyclic voltammetry, galvanostatic charge–discharge cycling, and electrochemical
impedance spectroscopy techniques. The MgCo2O4 electrodes show superior charge storage
properties in 3 M LiOH among a diverse choice of electrolytes. The MgCo2O4 show higher theoretical (3122 F/g) and practically achieved capacitance (320 F/g), larger coulombic efficiency, and cycling stability than the other two; therefore, it could be developed as a low-cost energy storage
medium.