Heptanuclear Co, Ni and mixed Co-Ni clusters as high-performance water oxidation electrocatalysts

Concomitant polymorphs: an alternative to modulate the oxygen evolution performance of mononuclear nickel complexes

Studying two stable reversible transformations of concomitant polymorphs of NiL2 as heterogeneous catalysts, coplanar NiL2-R exhibits better OER activities than zigzag NiL2-G, dependent on different disparities in crystal packing.

Heptanuclear brucite disk with cyanide bridges in a cocrystal and tracking its pyrolysis to an efficient oxygen evolution electrode

The development of efficient oxygen evolution reaction (OER) catalysts is still lacking in exploration of the mechanism of controlled pyrolysis of precursors among new material platforms. Here, a novel Co-based coordination molecular cluster has been first introduced as precursor to obtain metallic cobalt core shelled by N-doped carbon (Co@NC) structure which operates as an oxygen evolution electrode. Specifically, a new cocrystal compound, [Co^II₇(μ₃-CN)₆(mmimp)₆] [Co^IICl₃N(CN)₂]·3CH₃OH (Co₇₊₁, mmimp = 2-methoxy-6-((methylimino)-methyl)phenol), was isolated consisting of Brucite disks of cobalt where the usual bridging μ₃-OH is replaced by μ₃-CN produced by the in-situ decomposition of dicyanamide (N≡C-N-C≡N^-). The cobalt atoms are bonded through the nitrogen atom of the cyanide. Remarkably, time dependent thermogravimetric-mass spectrometry (TG-MS) analysis was utilized to track its pyrolysis process. It allowed us to propose a possible formation process of the Co@NC structure from Co₇₊₁. Interestingly, an extremely superior OER electrode is optimized for Co@NC-600 having the lowest overpotential of 257 mV at 10 mA/cm² in 1 mol/L KOH solution. The present study pins down the importance of clusters of transition metals on realizing distinct nanostructures operating as highly efficient OER electrocatalyst.

Uniaxial magnetic anisotropy energy of bimetallic Co-Ni clusters from a first-principles perspective

Along with the growing precision in the control of matter at increasingly smaller size scales, a field of research, based onto magnetic materials of technical interest, such as bimetallic clusters, has been developed in very recent years. Thereby, here, we report on a complete study of bimetallic clusters composed of cobalt and nickel with up to 7 atoms using ab initio methods in the GGA approach. We applied an unrestricted search method based on the tensor of inertia eigenvalues to find the most stable configurations of the clusters, obtaining a diverse set of structures with different geometric properties. We explored the effect of composition on the structural properties, the chemical stability, the magnetization and the magnetic anisotropy energy (MAE) of the so-obtained systems. Our results indicate that the behavior of the clusters is mainly governed by the Co-Co interaction and to a lesser extent by the Co-Ni and Ni-Ni interactions. Furthermore, for a given cluster size the magnetic moment increases by 2 μB/Co-substitution plus 1 μB/Ni-substitution coming from the cobalt and nickel core d-states, while in some cases unpaired hybrid s-electrons can also give rise to itinerant magnetism. These features have been analyzed with the help of a Jellium model and have important consequences for the magnetism and the magnetic anisotropy of the clusters. The magnetic behavior and MAE present complex and intriguing landscapes, which suggests the possibility of finely controlling the magnetic states, by tuning the cluster composition, aiming at technical implementation in fields such as molecular magnetism or quantum computation. In particular, cases such as Co6Ni, Co4Ni3, CoNi6 Co2Ni5 and Co3Ni3 present high relative stability and enhanced magnetic moments (around 10 μB), what makes them promising candidates for applications such as subnanometer magnetic information storage.