Superparamagnetic behaviour of metallic Co nanoparticles according to variable temperature magnetic resonance.

Investigating the size distributions of Co nanoparticle ensembles is an important problem, which has no straightforward solution. In this work, we use the combination of 59Co internal field nuclear magnetic resonance (59Co IF NMR) and ferromagnetic resonance (FMR) spectroscopies on a metallic Co nanoparticle sample with a narrow Co nanoparticle size distribution due to encapsulation within the inner channels of carbon nanotubes. High-resolution transmission electron microscopy (TEM) images showed that the nanoparticles can be represented as prolate spheroids, with the majority of particles having an aspect ratio between 1 and 2. This observation has increased the accuracy of superparamagnetic blocking size calculations from Néel relaxation model by introducing the actual volume of the ellipsoids taken from the image processing. 59Co IF NMR and FMR experiments conducted under different temperatures allowed us to observe the thermal blocking of superparamagnetic particles in full accordance with the TEM particle volume distribution. This proved that these magnetic resonance techniques can be used jointly for characterization of Co nanoparticles in the bulk of the sample.

Dielectric Properties of Hybrid Polyethylene Composites Containing Cobalt Nanoparticles and Carbon Nanotubes.

Polymer composites with electrically conductive inclusions are intensively developed for microwave shielding applications, where lightweight and elastic coatings are necessary. In this paper, dielectric properties of hybrid polyethylene composites containing cobalt nanoparticles and multi-wall carbon nanotubes (MWCNT) were investigated in the wide frequency range of 20-40 GHz for electromagnetic shielding applications. The percolation threshold in the hybrid system is close to 6.95 wt% MWCNT and 0.56 Co wt%. Cobalt nanoparticles (up to highest investigated concentration 4.8 wt%) had no impact on the percolation threshold, and for the fixed total concentration of fillers, the complex dielectric permittivity is higher for composites with bigger MWCNT concentrations. Moreover, the microwave complex dielectric permittivity of composites with high concentration of fillers is quite high (for composites with 13.4 wt% MWCNT and 1.1 wt% Co ε' ≈ ε″ ≈ 20 at 30 GHz, it corresponds to microwave absorption 50% of 1 mm thickness plate); therefore, these composites are suitable for electromagnetic shielding applications.

Nafion-Induced Reduction of Manganese and its Impact on the Electrocatalytic Properties of a Highly Active MnFeNi Oxide for Bifunctional Oxygen Conversion.

Electrocatalysts for bifunctional oxygen reduction (ORR) and oxygen evolution reactions (OER) are commonly studied under hydrodynamic conditions, rendering the use of binders necessary to ensure the mechanical stability of the electrode films. The presence of a binder, however, may influence the properties of the materials under examination to an unknown extent. Herein, we investigate the impact of Nafion on a highly active ORR/OER catalyst consisting of MnFeNi oxide nanoparticles supported on multi-walled carbon nanotubes. Electrochemical studies revealed that, in addition to enhancing the mechanical stability and particle connectivity, Nafion poses a major impact on the ORR selectivity, which correlates with a decrease in the valence state of Mn according to X-ray absorption spectroscopy. These findings call for awareness regarding the use of electrode additives, since in some cases the extent of their impact on the properties of electrode films cannot be regarded as negligible.

Bifunctional Oxygen Reduction/Oxygen Evolution Activity of Mixed Fe/Co Oxide Nanoparticles with Variable Fe/Co Ratios Supported on Multiwalled Carbon Nanotubes.

A facile strategy is reported for the synthesis of Fe/Co mixed metal oxide nanoparticles supported on, and embedded inside, high purity oxidized multiwalled carbon nanotubes (MWCNTs) of narrow diameter distribution as effective bifunctional catalysts able to reversibly drive the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline solutions. Variation of the Fe/Co ratio resulted in a pronounced trend in the bifunctional ORR/OER activity. Controlled synthesis and in-depth characterization enabled the identification of an optimal Fe/Co composition, which afforded a low OER/OER reversible overvoltage of only 0.831 V, taking the OER at 10 mA cm-2 and the ORR at -1 mA cm-2 . Importantly, the optimal catalyst with a Fe/Co ratio of 2:3 exhibited very promising long-term stability with no evident change in the potential for both the ORR and the OER after 400 charge/discharge (OER/ORR) cycles at 15 mA cm-2 in 6 m KOH. Moreover, detailed investigation of the structure, size, and phase composition of the mixed Fe/Co oxide nanoparticles, as well as their localization (inside of or on the surface of the MWCNTs) revealed insight of the possible contribution of the individual catalyst components and their synergistic interaction in the catalysis.