No abstract

In this work, five types of MnO 2 nanostructres (nanowires, nanotubes, nanoparticles, nanorods, and nanoflowers) were synthesized with a fine control over their α-crystallographic form by hydrothermal method. The electrocatalytic activities of these materials were examined toward oxygen reduction reaction (ORR) in alkaline medium. Numerous characterizations were correlated with the observed activity by analyzing their crystal structure (TGA, XRD, TEM), material morphology (FE-SEM), porosity (BET), inherent structural nature (IR, Raman, ESR), surfaces (XPS), and electrochemical properties (Tafel, Koutecky−Levich plots and % of H 2 O 2 produced). Moreover, X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) analysis were employed to study the structural information on the MnO 2 coordination number as well as interatomic distance. These combined results show that the electrocatalytic activities are significantly dependent on the nanoshapes and follow an order nanowire > nanorod > nanotube > nanoparticle > nanoflower. α-MnO 2 nanowires possess enhanced electrocatalytic activity compared to other shapes, even though the nanotubes possess a much higher BET surface area. In the ORR studies, α-MnO 2 nanowires displayed Tafel slope of 65 mV/decade, n-value of 3.5 and 3.6% of hydrogen peroxide production. The superior ORR activity was attributed to the fact that it possesses active sites composed with two shortened Mn− O bonds along with a Mn−Mn distance of 2.824 Å, which provides an optimum requirement for the adsorbed oxygen in a bridge mode favoring the direct 4 electron reduction. In accordance with the first principles based density functional theory (DFT), the enhancement in ORR activity is due to the less activation energy needed for the reaction by the (211) surface than all other surfaces.

Homogeneous solid solutions of sol−gel-prepared R 2 NiMnO 6 (R = La, Pr, Nd, Sm, Gd, Tb, Dy, Y, and Ho) double perovskites crystallize in a B-site-ordered monoclinic structure (P2 1 /n space group). Monoclinic distortion enhances with decreasing R 3+ ionic radii (r R 3+ ). The magnetic ordering temperature (T C ) decreases from 270 K for La 2 NiMnO 6 to 80 K for Ho 2 NiMnO 6 as r R 3+ decreases from 1.16 Å (La 3+ ) to 1.02 Å (Ho 3+ ). An additional magnetic anomaly is observed in Nd 2 NiMnO 6 , Sm 2 NiMnO 6 , Tb 2 NiMnO 6 , and Dy 2 NiMnO 6 at lower temperatures, which originates from the 3d−4f coupling between Mn−Ni and Nd 3+ /Sm 3+ /Tb 3+ /Dy 3+ magnetic moments. Further, high saturation magnetization is achieved for all samples, indicating that they are atomically ordered and have less antisite disorders. Upon a decrease in the size of R 3+ , the local structure shows an expansion of NiO 6 octahedra and almost unchanged of MnO 6 octahedra. X-ray-absorption near-edge spectroscopy reveals a majority of Ni 2+ and Mn 4+ ions in all samples. Softening of phonon modes results in the elongation of the Ni/Mn−O bond length. Finally, a correlation among lattice parameters, structural distortion, octahedral tilting, superexchange angle, electronic band gap, Curie temperature, and the rare-earth ionic radius is established.

Energy storage and conversion driven by electro-or photocatalyst is a highly exciting field of research, and generations of effective and durable oxide catalysts have received much attention in this field. Here, we report A-site lanthanum-doped oxygen-rich quinary oxide CaLaScRuO 6+δ synthesized by adopting the solid-state reaction method and characterized by various techniques such as powder X-ray diffraction, neutron diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma-atomic emission spectrometry, Raman spectroscopy, and temperature-programmed reduction in the presence of a hydrogen atmosphere (H 2 -TPR). X-ray absorption study confirms the existence of mixed valent Ru ions in the structure, which enhances the oxygen stoichiometry for the partial balance of an extra cationic charge. Neutron powder diffraction and reduction of the material in a hydrogen atmosphere (H 2 -TPR) can confirm the oxygen overstoichiometry of the catalyst. The present material works as an efficient and robust oxygen bifunctional electrocatalyst for ORR/ OER (oxygen evolution reaction/oxygen reduction reaction) followed by four-electron transfer pathway in a strong (1 M KOH) alkaline medium. The catalytic nature of the designed structural and chemical flexible perovskite is a novel example of an electrocatalyst for the oxygen bifunctional activity.

The origin of room temperature (RT) ferromagnetism (