Engineering Sciences and Technology - Publications

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Browsing Engineering Sciences and Technology - Publications by Author "Adams, Stefan"
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    Co < inf > 2 < /inf > Mo < inf > 3 < /inf > O < inf > 8 < /inf > /reduced graphene oxide composite: Synthesis, characterization, and its role as a prospective anode material in lithium ion batteries
    ( 2016-01-01) Marka, Sandeep K. ; Petnikota, Shaikshavali ; Srikanth, Vadali V.S.S. ; Reddy, M. V. ; Adams, Stefan ; Chowdari, B. V.R.
    A Co2Mo3O8/reduced graphene oxide (Co2Mo3O8/rGO) composite was synthesized by following a single step solid state reduction procedure. The prepared Co2Mo3O8/rGO composite was characterized using a multitude of characterization techniques, which confirmed the formation of the composite. Electron micrographs clearly showed that the composite consisted of submicron sized (lateral) and 50 nm thick hierarchical hexagonal nanoplatelets of Co2Mo3O8 attached to thin graphene layers of rGO. Raman scattering analysis not only confirmed the presence of Co2Mo3O8 and rGO in the composite but also revealed that the defects present in rGO are more than that in GO. Through thermogravimetric analysis, the amount of rGO present in the composite was found to be ∼22% by weight. Co 2p, Mo 3d, C 1s and O 1s X-ray photoelectron energy peaks were clearly identified. The analysis of these peaks confirmed the oxidation states of the respective elements in stoichiometric Co2Mo3O8. The as-synthesized Co2Mo3O8/rGO composite was tested as an anode material in half-cell configured lithium ion batteries. When cycled at 60 mA g-1 current density and in the 0.005-3.0 V range, the Co2Mo3O8/rGO composite delivered an excellent reversible specific capacity of ∼954 mA h g-1 that corresponds to 82% capacity retention at the end of the 60th cycle, which is higher than the theoretical capacity of both Co2Mo3O8 and graphene. Moreover, the Co2Mo3O8/rGO composite exhibited excellent rate capability. A reversible specific capacity of 471 mA h g-1 (at a current density of 1000 mA g-1) was delivered at the end of the 31st cycle. The value increased to 1006 mA h g-1 when the current density was switched to 100 mA g-1 at the end of the 36th cycle. Redox peaks in the cyclic voltammetry (CV) curves revealed that electrochemical conversion and electrochemical adsorption and desorption type reaction mechanism are the primary reasons for lithium ion storage. A constant area under the CV curves throughout the tests was noticed, which is an indication of the stable capacity while the CV results are in line with the galvanostatic cycling (GC) results. From the CV and GC results, it is concluded that the higher specific capacity, longer cycle life, and better rate capability are due to the excellent synergy between Co2Mo3O8 and rGO in the composite.
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    Electrochemistry-related aspects of safety of graphene-based non-aqueous electrochemical supercapacitors: A case study with MgO-decorated few-layer graphene as an electrode material
    ( 2019-01-01) Petnikota, Shaikshavali ; Srikanth, Vadali V.S.S. ; Toh, Jun Jie ; Srinivasan, Madhavi ; Bobba, Chowdari V.R. ; Adams, Stefan ; Reddy, Mogalahalli V.
    Herein, electrochemistry-related aspects of safety of graphene-based non-aqueous electrochemical supercapacitors are elucidated by using MgO-decorated few-layer graphene (FLG) as an electrode material. MgO-decorated FLG (MgO/FLG) is chosen in this study because on the one hand it has good physical characteristics making it lucrative as an electrode material in graphene based aqueous and non-aqueous electrochemical supercapacitors and on the other hand in MgO/FLG, MgO is chemically bonded to FLG (through either oxygen-mediation (Mg-O-C) or direct bonding with carbon (Mg-C)) and thereby reduces the possibility of exothermic reactions upon charge-discharge cycling leading to an unsafe failure of the supercapacitor. As anticipated, coin cell supercapacitors fabricated with MgO/FLG symmetric electrodes and a non-aqueous BMIM BF4 electrolyte popped up exothermically (the temperature rise was in the range of 110-165 °C) during the initial charge cycles. Post-cycling X-ray photoelectron spectroscopic analysis of the electrode material revealed that the chemical bonding between MgO and FLG triggered highly exothermic reactions leading to the formation of products such as MgF2 and Mg(OH)2. This study elucidates that the heat production and the gaseous species release during charging-discharging cycles are serious safety concerns if the choice of the electrode material and non-aqueous electrolyte (BMIM BF4 and plausibly PF6 based electrolytes) combination is not properly made. On the other hand when MgO/FLG was used in combination with a 6 M KOH aqueous electrolyte in an aqueous supercapacitor, it delivered a discharge capacitance as high as 168 F g-1 at a current density of 0.5 A g-1. Even at the end of the 5000th cycle with a current rate of 3 A g-1, a discharge capacitance of 101 F g-1 corresponding to 95% retention was recorded.
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    Fe < inf > 2 < /inf > Mo < inf > 3 < /inf > O < inf > 8 < /inf > /exfoliated graphene oxide: Solid-state synthesis, characterization and anodic application in Li-ion batteries
    ( 2018-01-01) Maseed, Hussen ; Petnikota, Shaikshavali ; Srikanth, Vadali V.S.S. ; Srinivasan, Madhavi ; Chowdari, B. V.R. ; Reddy, M. V. ; Adams, Stefan
    An Fe2Mo3O8/exfoliated graphene oxide (EG) composite with unique morphology is synthesized by a novel solid-state reduction method. Graphene oxide (GO), FeC2O4·2H2O and MoO3 are heated together at 750 °C for 8 h under an Ar atmosphere to obtain Fe2Mo3O8/EG as the resultant material. The morphology of the as-synthesized Fe2Mo3O8/EG powder as observed in electron micrographs confirmed the presence of layer-like EG and densely populated Fe2Mo3O8 hexagonal platelets. Thermogravimetric analysis showed that Fe2Mo3O8 and EG are in the composite at 98 and 2 wt%, respectively. The structural analysis of the as-synthesized Fe2Mo3O8/EG confirmed that Fe2Mo3O8 platelets are crystallized in the hcp crystal system. Raman scattering analysis further confirmed the presence of Fe2Mo3O8 and EG in the as-synthesized Fe2Mo3O8/EG composite. X-ray photoelectron spectroscopy confirmed that Fe and Mo elements are in the II and IV oxidation states in the as-synthesized Fe2Mo3O8/EG composite, which when tested as an anode material of a half-cell Li-ion battery, exhibited a high reversible capacity of 945 mA h g-1 at 50 mA g-1 current rate. This work paves the way to synthesize other graphene-metal oxide composites (with unique metal oxide morphologies) for their use as anode materials in Li-ion batteries.
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    Unique reduced graphene oxide as efficient anode material in Li ion battery
    ( 2018-04-01) Puttapati, Sampath Kumar ; Gedela, Venkataramana ; Srikanth, Vadali V.S.S. ; Reddy, M. V. ; Adams, Stefan ; Chowdari, B. V.R.
    Unique reduced graphene oxide named solar reduced graphene oxide (SRGO) was found to be an excellent anode material in Li ion battery. SRGO exhibited first cycle discharge- and charge-capacities as high as 1480 and 880 mAh g-1, respectively. Moreover, the columbic efficiency was found to be > 95% and the specific capacity retention even after 60 cycles was > 500 mAh g-1.