Crystal structure of brucite-type cobalt hydroxide β-Co{O(H,D)}2 — neutron diffraction, IR and Raman spectroscopy

Synthesis of ethane-disulfonate pillared layered cobalt hydroxide towards the electrocatalytic oxygen evolution reaction

We report the synthesis of a hybrid layered cobalt hydroxide sample and its redox behaviors in the electrochemical oxygen evolution reaction (OER). Compound Co₇(OH)₁₂(C₂H₄S₂O₆)·1.6H₂O was synthesized via a homogeneous alkalization reaction using Co(SO₃C₂H₄SO₃) and hexamethylenetetramine. This compound comprises cationic host layers of {[Co₇(OH)₁₂]²⁺}_∞, which comprise octahedrally (Co_Oh) and tetrahedrally (Co_Td) coordinated Co cations at a Co_Oh : Co_Td ratio of 5 : 2. The ethane-disulfonate ions are combined with the cationic host layers by electrostatic attractions and hydrogen bonding as a hybrid pillared layered framework. This hybrid sample can promote the OER in 1 M KOH with an overpotential as low as ∼410 mV (at a current density of 10 mA cm^-2). In situ Raman spectroscopy showed that the sample first evolved into Co(III)-based phases comprising a mixture of layered CoOOH and spinel Co₃O₄, and the Co(III)-based compounds were converted into Co(IV)-O intermediates containing [CoO₆] units at the onsite of the OER. The structural evolution behaviors suggest that the catalyst prefers a topotactic phase transition and the Co_Oh and Co_Td units exhibit different activities in the electrochemical reaction. The electron transfer events involved in the electrochemical reaction were identified by Fourier-transformed alternating current voltammetry.

Ultralow Ru Incorporated Amorphous Cobalt-Based Oxides for High-Current-Density Overall Water Splitting in Alkaline and Seawater Media

Realizing efficiency and stable hydrogen production by water electrolysis under high current densities is essential to the forthcoming hydrogen economy. However, its industrial breakthrough is seriously limited by bifunctional catalysts with slow hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalytic processes. Herein, an ultralow Ru incorporated amorphous cobalt-based oxide (Ru-CoO_x /NF), effectively driving the electrolysis of water at high current densities in alkaline water and seawater, is designed and constructed. In 1 m KOH, to reach the current density of 1000 mA cm^-2 for HER and OER, it only needs 252 and 370 mV overpotentials, respectively, beyond commercial Pt/C and RuO₂ catalysts. At the high current density, it also presents outstanding electrochemical stability. Then the electrolyzer apparatus assembled with Ru-CoO_x /NF, just requires the ultra-low voltage of 2.2 and 2.62 V to support the current density of 1000 mA cm^-2 in alkaline water and seawater electrolysis, respectively, for hydrogen production, better than that of the commercial Pt/C and RuO₂ catalysts. This work demonstrates that Ru-CoO_x /NF is one of the most promising catalysts for industrial applications and provides a possibility for exploration of high-current-density water electrocatalysis by changing the crystallinity of the catalyst.

Electronic transitions in layered hydroxides: Consequences of trigonal distortion of octahedral symmetry

This work describes the assignment of the electronic spectra of metal ions in D_3d coordination symmetry. Layered hydroxides are a class of materials that host transition metal ions such as Ni²⁺, Co²⁺, and Cr³⁺ in D_3d coordination symmetry. The electronic spectra of these ions in the layered hydroxides exhibit significant fine structure which is assigned to transitions arising from D_3d coordination symmetry. Towards this end, the correlation diagrams- complete or partial, and the resultant Tanabe-Sugano like diagrams for D_3d symmetry are obtained from first principles and supported by DFT based computations. The approach engendered here helps in better understanding of the electronic transitions arising due to lower symmetry.

Cobalt Colloid-derived Efficient and Durable Nanoscale Electrocatalytic Films for High-Activity Water Oxidation

Oxygen evolution reaction is of immense importance and is vitally necessary for devices such as electrolyzers, fuel cells, and other solar and chemical energy conversion devices. The major challenges that remain in this quest are due to the lack of effective catalytic assemblages operating with optimum efficiency and obtainable following much simpler setups and easily accessible methods. Here, we demonstrate that the robust electrocatalytic activity toward water oxidation can be achieved employing straightforwardly obtainable nanoscale electrocatalysts derived from easily made colloidal-cobalt nanoparticles (Co-CNPs) prepared in clean carbonate systems. Thin-film non-noble metal nanoscale electrocatalysts such as simple Co-CNPs/FTO and annealed Co-CNPs/FTO250 and Co-CNPs/FTO500 obtained by depositing Co-CNPs on the FTO substrate are shown to initiate water oxidation at much lower overpotentials such as just 240 mV for Co-CNPs/FTO250 under mildly alkaline conditions while demonstrating an impressive Tafel slope of just 40 mV dec-1. Furthermore, the robust catalyst demonstrated a high electrochemical surface area of 91 cm2 and high turnover frequency and mass activity of 0.26 s-1 and 18.84 mA mg-1, respectively, just at 0.35 V, and superior durability during long-term electrolysis. These outstanding catalytic outcomes using easily prepared Co-CNPs/FTO250-type catalytic systems are comparable and even better than other noble and non-noble metal-based nanoscale catalytic assemblages obtained by much difficult methods. Most advantageously, the colloidal route also offers the easiest approach of incorporating carbon contents in the catalytic layer, which can ultimately increase mechanical stability and mass transfer capability of the system.

Crowding out: ligand modifications and their structure directing effects on brucite-like {Mx(μ3-OH)y} (M = Co(ii), Ni(ii)) core growth within polymetallic cages

Previous employment of the ligands 2-methoxy-6-[(methylimino)methyl]phenol (L₁H) and 2-methoxy-6-[(phenylimino)methyl]phenol (L₂H) has resulted in the self-assembly of pseudo metallocalix[6]arene complexes of general formulae: [M₇(μ₃-OH)₆(L_x)₆](NO₃)_y (M = Ni(ii), x = 1, y = 2 (1) and Co(ii/iii), x = 2, y = 3 (2)). Extrapolating upon this work, we report the coordination chemistry of ligands 2-methoxy-6-{[(2-methoxyphenyl)imino]methyl}phenol (L₃H), 2-[(benzylimino)methyl]-6-methoxyphenol (L₄H), 2-[(benzylamino)methyl]-6-methoxyphenol (L₅H) and 2-[(benzylamino)methyl]-4-bromo-6-methoxyphenol (L₆H), whose structures are modifications of ligands L_1-2H. These ligands are employed in the synthesis and characterisation of the dimetallic complex [Ni(ii)₂(L₃)₃(H₂O)](NO₃)·2H₂O·3MeOH (3); the monometallic complexes [Ni(ii)(L₄)₂] (4) and [Co(iii)(L₄)₃]·H₂O·MeOH (5a); and the tetranuclear pseudo metallocalix[4]arene complexes: [(NO₃)⊂Co(ii)₄(μ₃-OH)₂(L₅)₄(H₂O)₂](NO₃)·H₂O (6), [(NO₃)⊂Ni(ii)₄(μ₃-OH)₂(L₅)₄(H₂O)₂](NO₃)·H₂O (7) and [Ni(ii)₄(μ₃-OH)₂(L₆)₄(NO₃)₂]·MeCN (8). The tetrametallic 'butterfly' core topologies in 6-8 are discussed with respect to their structural and topological relationship with their heptanuclear [M₇] (M = Co(ii), Ni(ii)) pseudo metallocalix[6]arene ancestors (1 and 2).

Exceptional Lithium Storage in a Co(OH)2 Anode: Hydride Formation

Current lithium ion battery technology is tied in with conventional reaction mechanisms such as insertion, conversion, and alloying reactions even though most future applications like EVs demand much higher energy densities than current ones. Exploring the exceptional reaction mechanism and related electrode materials can be critical for pushing current battery technology to a next level. Here, we introduce an exceptional reaction with a Co(OH)₂ material which exhibits an initial charge capacity of 1112 mAh g^-1, about twice its theoretical value based on known conventional conversion reaction, and retains its first cycle capacity after 30 cycles. The combined results of synchrotron X-ray diffraction and X-ray absorption spectroscopy indicate that nanosized Co metal particles and LiOH are generated by conversion reaction at high voltages, and Co _xH _y, Li₂O, and LiH are subsequently formed by hydride reaction between Co metal, LiOH, and other lithium species at low voltages, resulting in a anomalously high capacity beyond the theoretical capacity of Co(OH)₂. This is further corroborated by AIMD simulations, localized STEM, and XPS. These findings will provide not only further understanding of exceptional lithium storage of recent nanostructured materials but also valuable guidance to develop advanced electrode materials with high energy density for next-generation batteries.

The magnetic structure of β-cobalt hydroxide and the effect of spin-orientation

Neutron diffraction experiments and DFT+Usimulations assess the magnetic structure of layered β-Co(OH)₂, revealing an out-of-plane spin orientation.

Integrating three-dimensional graphene/Fe3O4@C composite and mesoporous Co(OH)2 nanosheets arrays/graphene foam into a superior asymmetric electrochemical capacitor

A novel high-energy asymmetric electrochemical capacitor is fabricated with 3D porous graphene/Fe₃O₄@C anode and mesoporous Co(OH)₂ nanosheets/graphene foam cathode.

Fabrication of porous β-Co(OH)2 architecture at room temperature: a high performance supercapacitor

A facile, cost-effective, surfactant-free chemical route has been demonstrated for the fabrication of porous β-Co(OH)2 hierarchical nanostructure in gram level simply by adopting cobalt acetate as a precursor salt and ethanolamine as a hydrolyzing agent at room temperature. A couple of different morphologies of β-Co(OH)2 have been distinctly identified by varying the mole ratio of the precursor and hydrolyzing agent. The cyclic voltammetry measurements on β-Co(OH)2 displayed significantly high capacitance. The specific capacitance obtained from charge-discharge measurements made at a discharge current of 1 A/g is 416 F/g for the Co(OH)2 sample obtained at room temperature. The charge-discharge stability measurements indicate retention of specific capacitance about 93% after 500 continuous charge-discharge cycles at a current density of 1 A g(-1). The capacitive behavior of the other synthesized morphology was also accounted. The nanoflower-shaped porous β-Co(OH)2 with a characteristic three-dimensional architecture accompanied highest pore volume which made it promising electrode material for supercapacitor application. The porous nanostructures accompanied by high surface area facilitates the contact and transport of electrolyte, providing longer electron pathways and therefore giving rise to highest capacitance in nanoflower morphology. From a broad view, this study reveals a low-temperature synthetic route of β-Co(OH)2 of various morphologies, qualifying it as supercapacitor electrode material.

Morphology control of cobalt oxide nanocrystals for promoting their catalytic performance

The design and fabrication of nanomaterials is a crucial issue in heterogeneous catalysis to achieve excellent performance. Traditionally, the main theme is to reduce the size of particles as small as possible mainly to increase the activity, so-called size-dependent catalytic chemistry. In recent years, the rapid developments in novel morphological and structural nanomaterials have enabled the fabrication of catalytic materials with exposing more reactive crystal planes, favoring a deep understanding of the active sites. Here, we highlight the recent progress in catalytic materials with unique performance caused by the morphology, by taking Co(3)O(4) nanomaterials as an example. Firstly, we briefly summarize the important synthetic strategies and characteristics of morphology-controlled Co(3)O(4) nanomaterials and their precursors like cobalt hydroxides, including zero- to two-dimensional and hierarchical nanostructures. Then, morphology/plane-dependent catalysis of these cobalt oxides is demonstrated, focusing on CH(4) combustion and CO oxidation in order to elaborate the intrinsic nature of morphology and surface plane. Finally, we outline our personal understanding and perspectives on the morphology-dependent nanocatalysis with metal and metal oxides. These morphology-controlled nanomaterials with more reactive crystal planes exposed are expected to be highly efficient for practical applications based on the deep understanding of the catalytically active sites.

Tetrahedral Co(II) Coordination in α-Type Cobalt Hydroxide: Rietveld Refinement and X-ray Absorption Spectroscopy

We report a Rietveld refinement analysis and X-ray absorption study on a green-color Cl(-)-intercalated alpha-type cobalt hydroxide phase. The refinement clearly demonstrated that one-fifth to one-sixth of the Co(II) at octahedral sites was replaced by pairs of tetrahedrally coordinated Co(II) on each side of the hydroxide plane, represented by a structural formula of [Co(octa)(0.828)Co(tetra)(0.348)(OH)2](0.348+)Cl(0.348).0.456H2O. X-ray absorption spectroscopy also indicated that the divalent cobalt were in local neighboring environments of both octahedral and tetrahedral coordination. Furthermore, UV-vis spectroscopic measurements elucidate the typical green/blue color of an alpha-type cobalt hydroxide.

Selective and Controlled Synthesis of α- and β-Cobalt Hydroxides in Highly Developed Hexagonal Platelets

We report on the controlled synthesis of single-crystal platelets of alpha- and beta-Co(OH)2 via homogeneous precipitation using hexamethylenetetramine as a hydrolysis agent. The alpha- and beta-Co(OH)2 hexagonal platelets of several micrometers in width and about 15 nm in thickness were reproducibly yielded in rather dilute CoCl2 solutions in the presence and absence of NaCl at 90 degrees C, respectively. The phase and size control of the products were achieved by varying both CoCl2 and NaCl concentrations. Polarized optical microscope observations revealed clear liquid crystallinity of colloidal suspensions of these high aspect-ratio platelets. The as-prepared alpha-Co(OH)2 containing interlayer chloride ions was intercalated with various inorganic or organic anions, keeping its high crystallinity and hexagonal platelike morphology.