Effect of solvents on the morphology of NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript>/graphene nanostructures for electrochemical pseudocapacitor application

The large internal surface areas and outstanding electrical and mechanical properties of graphene have prompted to blend graphene with NiCo₂O₄ to fabricate nanostructured NiCo₂O₄/graphene composites for supercapacitor applications. The use of graphene as blending with NiCo₂O₄ enhances the specific capacitance and rate capability and improves the cyclic performance when compared to the pristine NiCo₂O₄ material. Here, we synthesized two different nanostructured morphologies of NiCo₂O₄ on graphene sheets by solvothermal method. It has been suggested that the morphologies of oxides are greatly influenced by dielectric constant, thermal conductivity, and viscosity of solvents employed during the synthesis. In order to test this concept, we have synthesized nanostructured NiCo₂O₄ on graphene sheets by facile solvothermal method using N-methyl pyrrolidone and N,N-dimethylformamide solvents with water. We find that mixture of N-methyl pyrrolidone and water solvent favored the formation of nanonet-like NiCo₂O₄/graphene (NiCoO-net) whereas mixture of N,N-dimethylformamide and water solvent produced microsphere-like NiCo₂O₄/graphene (NiCoO-sphere). Electrochemical pseudocapacitance behavior of the two NiCo₂O₄/graphene electrode materials was studied by cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy techniques. The supercapacitance measurements on NiCoO-net and NiCoO-sphere electrodes showed specific capacitance values of 1060 and 855 F g^?1, respectively, at the current density of 1.5 A g^?1. The capacitance retention of NiCoO-net electrode is 93 % while that of NiCoO-sphere electrode is 77 % after long-term 5000 charge-discharge cycles at high current density of 10 A g^?1.     

NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript>/rGO hybrid nanostructures for efficient electrocatalytic oxygen evolution

We report the synthesis of NiCo₂O₄/reduced graphene oxide (NiCo₂O₄/rGO) hybrid hierarchical structures with unique nanonet and microsphere morphologies by organic polar solvent-assisted solvothermal method. The electrocatalytic oxygen evolution reaction (OER) activity of these materials is studied by cyclic voltammetry, linear sweep voltammetry and chronoamperometry methods in O₂-saturated 0.1 M KOH solution. The NiCo₂O₄/rGO hybrid nanocomposite materials are found to be highly active electrocatalysts for OER at lower overpotentials. The nanonet and microsphere-like NiCo₂O₄/rGO catalysts require overpotentials of 0.450 and 0.530 V at a current density of 10 mA cm^?2, and their corresponding Tafel slopes are 53 and 62 mV dec^?1, which are much lower than values reported for non-precious electrocatalysts. Further, both NiCo₂O₄/rGO catalysts show good catalytic stability with current retention more than 92 % over long period of 15,000 s determined by chronoampirometry and at the end of 1000th cycle determined by linear sweep voltammetry. The enhanced OER activity of nanostructured NiCo₂O₄/rGO hybrid catalysts is attributed to synergistic interaction between rGO and NiCo₂O₄, which seems to be essential for maintaining the large contact area at the electrode-electrolyte interface, better mass, and charge transport and to minimize the aggregation of NiCo₂O₄ nanoparticles.     

A Vanadium(V) Oxide Nanorod Promoted Platinum/Reduced Graphene Oxide Electrocatalyst for Alcohol Oxidation under Acidic Conditions

A Pt‐V₂O₅/rGO ternary hybrid electrocatalyst was designed by using active vanadium(V) oxide (V₂O₅) nanorods and reduced graphene oxide (rGO) components. The V₂O₅ nanorods were synthesized by a simple polyol‐assisted solvothermal method and were incorporated uniformly onto rGO sheets by intermittent microwave heating. Subsequently, Pt nanoparticles (2–3 nm in size) were deposited over the V₂O₅/rGO composite by the conventional polyol reflux method. The electrocatalytic performance of the Pt‐V₂O₅/rGO ternary hybrid and bare Pt/rGO catalysts towards the oxidation of simple alcohols was evaluated in acidic media. The ternary hybrid catalyst exhibited higher electrocatalytic activity than bare Pt/rGO and also showed good stability. The higher electrocatalytic activity of the Pt‐V₂O₅/rGO ternary hybrid was attributed to a synergistic effect among the Pt, V₂O₅, and rGO components. In addition, oxygen‐containing species, such as OH groups, were generated on V₂O₅ at lower potentials. These groups were able to scavenge intermediate species such as CO_ads on the Pt surfaces and helped to regenerate the active sites on the Pt surface more effectively for the routine alcohol oxidation reaction.     

Electrochemical lithium and sodium insertion studies in 3D metal oxy-phosphate framework MoWO3(PO4)2 for battery applications

A new type of three-dimensional (3D) oxy-phosphate materials are explored for the application of Li and Na batteries. The molybdenum tungsten oxy phosphate, MoWO₃(PO₄)₂, was synthesized by solid-state method and evaluated for Li/Na insertion/de-insertion electrode material for the first time. The cell at charged state (vs. Li⁺/Li) showed a discharge capacity of 786 mAh g⁻¹ within the voltage window of 0.3 V with amorphization of crystalline MoWO₃(PO₄)₂ as observed from ex-situ powder XRD analysis. The structural integrity was revealed in this material, even with nearly more than 5 Li⁺ ions into the lattice, leading to the discharge capacity of 250 mAh g⁻¹. The reversible charge/discharge behavior with insertion/de-insertion of 2.4 Li⁺ ions in the voltage range of 1.65 − 3.5 V resulted in 110 and 95 mAh g⁻¹ at C/10 and C/5 rates, respectively. On the other hand, poor cycling performance was noticed for Na ion insertion and desertion, with a discharge capacity of 250 mAh/g within the voltage range of 0.3 − 3.5 V (vs. Na⁺/Na).

     

Surfactant-assisted hydrothermal synthesis of CoMn2O4 nanostructures for efficient supercapacitors

Journal of Solid State Electrochemistry - Mixed transition metal oxides/spinels are excellent energy storage electrode materials that can deliver sizeable specific capacitance, excellent cyclic...