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Alpha-Nickel Hydroxide Coating of Metallic Nickel for Enhanced Alkaline Hydrogen Evolution

Yunchang Liang

In this work, alkaline hydrogen evolution reaction (HER) processes of three typical nickel-based electrocatalysts [i.e., Ni, alpha-Ni(OH)(2), and beta-Ni(OH)(2)] were investigated to probe critical factors that determine the activity and durability. The HER activity trend was observed as Ni >>alpha-Ni(OH)(2) > beta-Ni(OH)(2), likely attributed to a synergy between metallic Ni and Ni(OH)(2) components on the Ni surface and fast water dissociation kinetics on the alpha-Ni(OH)(2) surface. With the HER proceeding, the metallic Ni surface, however, gradually became alpha-Ni(OH)(2), and alpha-Ni(OH)(2) surface ultimately transformed into beta-phase, leading to a dramatic activity decrease of Ni electrodes. Therefore, Ni electrodes were coated with alpha-Ni(OH)(2) nanosheets to slow down the nickel hydroxylation and optimize the surface ratio of Ni(OH)(2) to metallic Ni. This simple coating procedure enhanced both activity and durability of Ni electrocatalysts.

WILEY-V C H VERLAG GMBH2022

From hidden order to antiferromagnetism: Electronic structure changes in Fe-doped URu2Si2

Ji Dai, Emmanouil Frantzeskakis, Kevin Huang

In matter, any spontaneous symmetry breaking induces a phase transition characterized by an order parameter, such as the magnetization vector in ferromagnets, or a macroscopic many electron wave function in superconductors. Phase transitions with unknown order parameter are rare but extremely appealing, as they may lead to novel physics. An emblematic and still unsolved example is the transition of the heavy fermion compound URu2Si2 (URS) into the so-called hidden-order (HO) phase when the temperature drops below T-0 = 17.5 K. Here, we show that the interaction between the heavy fermion and the conduction band states near the Fermi level has a key role in the emergence of the HO phase. Using angle-resolved photoemission spectroscopy, we find that while the Fermi surfaces of the HO and of a neighboring antiferromagnetic (AFM) phase of well-defined order parameter have the same topography, they differ in the size of some, but not all, of their electron pockets. Such a nonrigid change of the electronic structure indicates that a change in the interaction strength between states near the Fermi level is a crucial ingredient for the HO to AFM phase transition.

NATL ACAD SCIENCES2021

Proximity-Induced Novel Ferromagnetism Accompanied with Resolute Metallicity in NdNiO3 Heterostructure

Marco Caputo, Joël Mesot, Zoran Ristic, Zheng Wang, Anna Zakharova

Employing X-ray magnetic circular dichroism (XMCD), angle-resolved photoemission spectroscopy (ARPES), and momentum-resolved density fluctuation (MRDF) theory, the magnetic and electronic properties of ultrathin NdNiO3 (NNO) film in proximity to ferromagnetic (FM) La0.67Sr0.33MnO3 (LSMO) layer are investigated. The experimental data shows the direct magnetic coupling between the nickelate film and the manganite layer which causes an unusual ferromagnetic (FM) phase in NNO. Moreover, it is shown the metal-insulator transition in the NNO layer, identified by an abrupt suppression of ARPES spectral weight near the Fermi level (E-F), is absent. This observation suggests that the insulating AFM ground state is quenched in proximity to the FM layer. Combining the experimental data (XMCD and AREPS) with the momentum-resolved density fluctuation calculation (MRDF) reveals a direct link between the MIT and the magnetic orders in NNO systems. This work demonstrates that the proximity layer order can be broadly used to modify physical properties and enrich the phase diagram of RENiO3 (RE = rare-earth element).

WILEY2021