Journal Articles
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Item Recent trends and insights in nickel chalcogenide nanostructures for water-splitting reactions(Taylor and Francis Ltd., 2021) Bhat, K.S.; Nagaraja, H.S.Developing earth-abundant-electrocatalysts for water-splitting reactions is of great importance to curb contemporary energy demands and to address the important issues such as global warming, pollution and etc. State-of-the-art electrocatalysts (Pt, RuO2 and IrO2) remain the first choice for water electrolysis, however, their high-cost, scarcity and poor stability motivate researchers for the search of abundant, inexpensive and stable enough electrocatalysts for long-term operations. Recently nickel chalcogenides have emerged as a promising substitute for state-of-the-art electrocatalysts, owing to their excellent physiochemical aspects such as high electronic conductivity, chemical stability, specific surface area and porosity. This review paper describes the different parameters to evaluate the electrocatalytic activity, followed by different synthesis methods and strategies employed for the synthesis of nickel chalcogenides. Furthermore, this review provides an overview of some of the important investigations and developments on nickel chalcogenides for water-splitting reactions. At last, the challenges and opportunities for the future exploration of these nickel chalcogenides are put forward and discussed. © 2019 Informa UK Limited, trading as Taylor & Francis Group.Item Porous nickel telluride nanostructures as bifunctional electrocatalyst towards hydrogen and oxygen evolution reaction(Elsevier Ltd, 2017) Bhat, K.S.; Barshilia, H.C.; Nagaraja, H.S.Electrochemical water splitting technology has attracted researchers for the development of next generation fuels. Herein, we report the synthesis of nanostructured porous hollow nickel telluride nanosheets and their use as bifunctional electrocatalyst towards hydrogen and oxygen evolution reaction, anticipating an enhanced performance owing to their 2D sheet like morphology, conductivity, porous nature providing larger catalytic surface for water splitting reaction. In this regard, nickel telluride nanostructures were synthesized via an anion-exchange-reaction between pre-synthesized nickel hydroxide hexagonal nanosheets and tellurium ions under hydrothermal conditions. The as-synthesized nanostructures were characterized for structural, morphological and compositional properties using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Nickel telluride modified electrodes were tested as bifunctional electrocatalyst under acidic and alkaline conditions, through linear sweep voltammetry and constant current chronopotentiometry methods. The modified electrodes revealed an onset potential of ?422 mV and 87.4 mV dec?1 Tafel slope towards HER and overpotential of 679 mV and 151 mV dec?1 Tafel slope towards OER. The lower onset potentials are complimented with excellent electrocatalytic stability. © 2017 Hydrogen Energy Publications LLCItem Porous cobalt chalcogenide nanostructures as high performance pseudo-capacitor electrodes(Elsevier Ltd, 2017) Bhat, K.S.; Shenoy, S.; Nagaraja, H.S.; Sridharan, K.Electrochemical supercapacitor is an essential technology that is pivotal for the development of reliable energy storage devices. Herein, we report the fabrication of supercapacitor electrodes using nanostructured porous cobalt chalcogenide (CoTe2 and CoSe2) electrodes, anticipating an enhanced performance owing to their higher contact area with electrolyte and large pore volume enabling shorter diffusion paths for ion exchange. In this regard, we synthesized CoTe2 and CoSe2 nanostructures via an anion-exchange-reaction between pre-synthesized Co(OH)2 hexagonal nanosheets and chalcogen (tellurium and selenium) ions under hydrothermal conditions. Structural, morphological and compositional properties of the as-synthesized materials are examined using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Pseudo-capacitive properties of CoTe2 and CoSe2 nanostructures as working electrodes are studied through cyclic voltammetry and galvanostatic charge-discharge methods using an electrochemical workstation. CoSe2 electrode delivered a specific capacitance of 951 F g?1 at a scan rate of 5 mV s?1, which surprisingly is almost three times higher in comparison to CoTe2 electrode (360 F g?1). Both CoTe2 and CoSe2 electrodes exhibited good capacitance retention capability for 2500 CV cycles. The superior electrochemical performance of the nanoporous CoSe2 electrode indicate their applicability for high-performance energy storage device applications. © 2017 Elsevier LtdItem Effect of oxygen substitution and phase on nickel selenide nanostructures for supercapacitor applications(Institute of Physics Publishing helen.craven@iop.org, 2018) Bhat, K.S.; Nagaraja, H.S.Electrochemical supercapacitors are the eminent technology for the progress of energy storage devices. The current manuscript deals with the formation of oxygen substituted nickel selenide nanostructures and their use as active electrode material for supercapacitor, expecting an enhanced performance owing to their sheet-like geometry, high specific surface area and porous assembly. In this context, Ni(OH)2 (nickel hydroxide) nanostructures were synthesized employing one-pot hydrothermal method and the ion-exchange reaction of Ni(OH)2 nanostructures with selenium resulted in cubic-NiSe2 (nickel selenide) nanostructures. Further, annealing NiSe2 nanostructures at intermediate pressure (10-3 Torr) has realized the partial oxygen substitution in place of selenium, resulting in NiSe/NiO nanostructures along with phase change from cubic-NiSe2 to hexagonal-NiSe. Supercapacitor electrodes fabricated using NiSe/NiO nanostructures delivered the specific capacitance of 83.5 F g-1 at the scan rate of 2 mV s-1, which is surprisingly more than a double as compared with pristine NiSe2 electrodes (37.4 F g-1). Annealing at intermediate pressure and high temperature significantly enhanced the specific capacitances of the nanostructured electrodes and also accompanied with the good capacitance retention of 94% for 5000 CV cycles. © 2018 IOP Publishing Ltd.Item Nickel selenide nanostructures as an electrocatalyst for hydrogen evolution reaction(Elsevier Ltd, 2018) Bhat, K.S.; Nagaraja, H.S.Electrochemical water splitting has gained momentum for the development of alternative energy sources. Herein, we report the synthesis of two different nickel selenide nanostructures of different morphology and composition employing hydrothermal method. NiSe2 nanosheets were obtained by the anion-exchange reaction of Ni(OH)2 with Se ions for 15 h. On the other hand, NiSe nanoflakes were synthesized by the direct selenization of nickel surface with the reaction time of 2 h. Tested as an electrocatalyst for hydrogen evolution reaction, NiSe2 nanosheets and NiSe nanoflakes can afford a geometric current density of 10 mA cm?2 at an overpotential of 198 mV and 217 mV respectively. The measured Tafel slope values of NiSe nanoflakes are 28.6 mV dec?1, which is three times lower as compared with NiSe2 nanosheets (72.1 mV dec?1). These results indicates the HER kinetics of NiSe nanoflakes are at par with the state-of-the-art Pt/C catalyst and also complimented with the short synthesis time of 2 h. Further, both nickel selenides exhibit ultra-long term stability for 30 h as evident from constant current chronopotentiometry and electrochemical impedance spectroscopy results. © 2018 Hydrogen Energy Publications LLCItem Effect of isoelectronic tungsten doping on molybdenum selenide nanostructures and their graphene hybrids for supercapacitors(Elsevier Ltd, 2019) Bhat, K.S.; Nagaraja, H.S.Electrochemical supercapacitors are vital for the advancement of energy storage devices. Herein, we report the synthesis of molybdenum selenide (MoSe 2 ), tungsten-doped molybdenum selenide (W–MoSe 2 ) and their graphene (G) composites (W–MoSe 2 /G) via a facile hydrothermal method. Physiochemical properties of the as-synthesized samples are examined using X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller measurements, scanning electron microscopy, high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy measurements. Used as working electrodes for supercapacitors, MoSe 2 nanostructures could deliver the specific capacitance of 106 F g ?1 at 2 mV s ?1 scan rate. Further, doping with tungsten (W) demonstrates the variation of specific capacitances with 2 M % of tungsten as the optimum doping amount, delivering the maximum specific capacitance of 147 F g ?1 . Furthermore, graphene composites of these nanostructures deliver the enhanced specific capacitances of 248 F g ?1 and complimented with excellent capacitance retention capability of 102% for 20000 cycles. © 2019 Elsevier LtdItem Modulations of physio-chemical and electronic properties of metalorganic KHO single crystals through Co(OH)2 nanoparticles doping(Springer New York LLC barbara.b.bertram@gsk.com, 2019) Mahendra, K.; Bhat, K.S.; Nagaraja, H.S.; Udayashankar, N.K.Semiorganic crystals play vital role in the design and development of optical devices. In this context, we report the synthesis of metalorganic potassium hydrogen oxalate oxalic acid dihydrate (KHO) single crystals using slow solvent evaporation technique. Further, Co(OH)2 nanoparticles pre-synthesized using hydrothermal method are doped to KHO single crystals. Doping of Co(OH)2 nanoparticles in the crystal system were confirmed through XRD, EDAX and PL measurements. The XRD measurements indicate a shift in the peak positions and variation in the overall intensities. On the other hand, PL measurements also indicate enhanced emission peaks confirming the successful doping of Co(OH)2 nanoparticles in the KHO system. As the effect of doping, the declination in the optical bandgap and improvement of electronic conductivity were also observed. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.Item Performance evaluation of molybdenum dichalcogenide (MoX2; X= S, Se, Te) nanostructures for hydrogen evolution reaction(Elsevier Ltd, 2019) Bhat, K.S.; Nagaraja, H.S.Hydrogen evolution reaction (HER) using transition metal dichalcogenides (TMDs) have gained interest owing to their low-cost, abundancy and predominant conductivity. However, forthright comparisons of transition metal chalcogenides for HER are scarcely conducted. In this work, we report the synthesis of series of molybdenum chalcogenide nanostructures MoX2 (X = S, Se, Te) via a facile hydrothermal method. Used as an electrocatalyst for HER, MoS2 nanograins, MoSe2 nanoflowers and MoTe2 nanotubes could afford the benchmark current densities of 10 mA cm?2 at the overpotentials of ?173 mV, ?208 mV and ?283 mV with the measured Tafel slope values of 109.81 mV dec?1, 65.92 mV dec?1 and 102.06 mV dec?1, respectively. Besides other factors influencing HER, the role of electronic conductivity in HER of these molybdenum dichalcogenides are elucidated. In addition, the presented molybdenum dichalcogenides in this work are also complimented with robustness as determined from high-current density stability measurements. © 2019 Hydrogen Energy Publications LLCItem Morphology-dependent electrochemical performances of nickel hydroxide nanostructures(Indian Academy of Sciences, 2019) Bhat, K.S.; Nagaraja, H.S.Electrochemical capacitors form part of the developing technologies in the field of alternative energy sources. In the present work, nickel hydroxide (Ni(OH) 2) nanosheets and microflowers are hydrothermally prepared employing different chemical precursors. Structure, morphology and chemical analysis are conducted using powder X-ray diffraction, field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy measurements. Electrochemical performances as supercapacitor electrodes of the synthesized nanostructures are evaluated through cyclic voltammetry and galvanostatic charge–discharge measurements with three-electrode configurations. The results indicated the specific capacitance of 180 and 417Fg-1 at a scan rate of 5mVs-1 for Ni(OH) 2 nanosheets and microflowers, respectively. The higher specific capacitances for Ni(OH) 2 microflowers could be attributed to the higher specific surface area, morphology, electronic conductivity and porosity. Both Ni(OH) 2 nanostructures exhibited good capacitance retention for 1500 cycles. © 2019, Indian Academy of Sciences.Item Two-Dimensional Cadmium Hydroxide Nanosheets for Electrochemical Capacitors Under High Operating Voltage(Springer, 2020) Bhat, K.S.; Huvinahalli, B.R.; Nagaraja, H.S.Abstract: Electrochemical capacitors are deemed to be the most prospective energy storage devices in the field of alternative energy sources. Here, cadmium hydroxide (Cd(OH)2) nanosheets are hydrothermally synthesized and used as electrodes for supercapacitors. Physiochemical properties of the as-synthesized materials are examined using powder x-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy measurements. Electrochemical investigations reveal an excellent operating potential window of 1.5 V, with the specific capacitance of ? 71 F g?1 at a scan rate of 2 mV s?1. In addition, the Cd(OH)2 electrodes are complemented by good cyclic retention for 2000 cycles. Further, the analysis of the type of charge-storage mechanism reveals prominent contributions from the diffusion-controlled processes. Graphic Abstract: [Figure not available: see fulltext.]. © 2019, The Minerals, Metals & Materials Society.