Carbon-coated Fe3O4 core-shell super-paramagnetic nanoparticle-based ferrofluid for heat transfer applications.

Herein, we report the investigation of the electrical and thermal conductivity of Fe3O4 and Fe3O4@carbon (Fe3O4@C) core-shell nanoparticle (NP)-based ferrofluids. Different sized Fe3O4 NPs were synthesized via a chemical co-precipitation method followed by carbon coating as a shell over the Fe3O4 NPs via the hydrothermal technique. The average particle size of Fe3O4 NPs and Fe3O4@C core-shell NPs was found to be in the range of ∼5-25 nm and ∼7-28 nm, respectively. The thickness of the carbon shell over the Fe3O4 NPs was found to be in the range of ∼1-3 nm. The magnetic characterization revealed that the as-synthesized small average-sized Fe3O4 NPs (ca. 5 nm) and Fe3O4@C core-shell NPs (ca. 7 nm) were superparamagnetic in nature. The electrical and thermal conductivities of Fe3O4 NPs and Fe3O4@C core-shell NP-based ferrofluids were measured using different concentrations of NPs and with different sized NPs. Exceptional results were obtained, where the electrical conductivity was enhanced up to ∼3222% and ∼2015% for Fe3O4 (ca. 5 nm) and Fe3O4@C core-shell (ca. 7 nm) NP-based ferrofluids compared to the base fluid, respectively. Similarly, an enhancement in the thermal conductivity of ∼153% and ∼116% was recorded for Fe3O4 (ca. 5 nm) and Fe3O4@C core-shell (ca. 7 nm) NPs, respectively. The exceptional enhancement in the thermal conductivity of the bare Fe3O4 NP-based ferrofluid compared to that of the Fe3O4@C core-shell NP-based ferrofluid was due to the more pronounced effect of the chain-like network formation/clustering of bare Fe3O4 NPs in the base fluid. Finally, the experimental thermal conductivity results were compared and validated against the Maxwell effective model. These results were found to be better than results reported till date using either the same or different material systems.

Recent Progress and Perspectives on Electrochemical Regeneration of Reduced Nicotinamide Adenine Dinucleotide (NADH).

NAD is a cofactor that maintains cellular redox homeostasis and has immense industrial and biological significance. It acts as an enzymatic mediator in several biocatalytic electrochemical reactions and undergoes oxidation/reduction to form NAD+ or NADH, respectively. The NAD redox couple (NAD+ /NADH) mostly exists in enzyme-assisted metabolic reactions as a coenzyme during which electrons and protons are transferred. NADH shuttles these charges between the enzyme and the substrate. In order to understand such complex metabolic reactions, it is vital to study the bio-electrochemistry of NADH. In addition, the regeneration of NADH in industries has attracted significant attention due to its vast usage and high cost. To make biocatalysis economically viable, primary methods of NADH regeneration including enzymatic, chemical, photochemical and electrochemical methods are widely used. This review is mainly focused on the electrochemical reduction of NAD+ to NADH with specific details on the mechanism and kinetics of the reaction. It provides emphasis on the different routes (direct and mediated) to electrochemically regenerate NADH from NAD+ highlighting the NAD dimer formation. Also, it describes the electrocatalysts developed until now and the scope for development in this area of research.

Fe-based hybrid electrocatalysts for nonaqueous lithium-oxygen batteries.

Lithium-oxygen batteries promise high energy densities, but are confronted with challenges, such as high overpotentials and sudden death during discharge-charge cycling, because the oxygen electrode is covered with the insulating discharge product, Li2O2. Here, we synthesized low-cost Fe-based nanocomposites via an electrical wire pulse process, as a hybrid electrocatalyst for the oxygen electrode of Li-O2 batteries. Fe3O4-Fe nanohybrids-containing electrodes exhibited a high discharge capacity (13,890 mA h gc-1 at a current density of 500 mA gc-1), long cycle stability (100 cycles at a current rate of 500 mA gc-1 and fixed capacity regime of 1,000 mA h gc-1), and low overpotential (1.39 V at 40 cycles). This superior performance resulted from the good electrical conductivity of the Fe metal nanoparticles during discharge-charge cycling, which could enhance the oxygen reduction reaction and oxygen evolution reaction activities. We have demonstrated the increased electrical conductivity of the Fe3O4-Fe nanohybrids using electrochemical impedance spectroscopy.