A two dimensional Co(OH)2 catalytic gravity-driven membrane for water purification: a green and facile fabrication strategy and excellent water decontamination performance

Cobalt-based materials are reported to be the most efficient catalysts in sulfate radical advanced oxidation processes (SR-AOPs). A green and facile method was developed in this work to prepare uniform Co(OH)2 hexagonal nanosheets, which was void of any organic solvents via mere ambient temperature stirring. The obtained nanosheets were assembled into a catalytic gravity-driven membrane, through which the removal efficiency of a typical pharmaceutical contaminant, ranitidine (RNTD), could reach ∼100% within 20 min. Meanwhile, the catalytic membrane also demonstrated effective removal performance towards various pollutants. In order to augment the long-term stability of catalytic membranes, Co(OH)2/rGO composites were fabricated using the same strategy, and a Co(OH)2/rGO catalytic membrane was prepared correspondingly. The Co(OH)2/rGO membrane could maintain a ∼100% removal of RNTD over a constant reaction period lasting for up to 165 hours, which was approximately 11 times that of the sole Co(OH)2 membrane (15 h). Analysis of element chemical states, metal ion concentration in filtrates, and quenching experiments suggested that the combination with rGO could promote the electron transfer to accelerate the Co(II) regeneration, restrain the cobalt dissolution to alleviate the active site loss, and contribute to the production of 1O2via synergistic effects of oxygen-containing groups in rGO. Toxicity assessment was performed on RNTD and its degradation intermediates to confirm the reduction in ecotoxicity of the treated feed. Overall, this work not only offered guidance for the application of nanosheets in AOP membranes, but also had implications for the environmentally-friendly preparation protocol to obtain functional metal hydroxides.

A novel cobalt(ii) acetate complex bearing lutidine ligand: a promising electrocatalyst for oxygen evolution reaction

Developing cost-effective electrocatalysts using earth-abundant metal as an alternative to expensive precious metal catalyst remains a key challenge for researchers. Several strategies are being researched/tested for making low-cost transition metal complexes with controlled electron-density and coordination flexibility around the metal center to enhance their catalytic activity. Herein, we report a novel lutidine coordinated cobalt(ii) acetate complex [(3,5-lutidine)2Co(OAc)2(H2O)2] (1) as a promising electrocatalyst for oxygen evolution reaction (OER). Complex 1 was characterized by FT-IR, elemental analysis, and single crystal X-ray diffraction data. The structure optimization of complex 1 was also done using DFT calculation and the obtained geometrical parameters were found to be in good agreement with the parameters obtained from the solid state structure obtained through single crystal X-ray diffraction data. Further, the molecular electrostatic potential (MEP) maps analysis of complex 1 observed electron rich centers that were found to be in agreement with the solid-state structure. It was understood that the coordination of lutidine as a Lewis base and acetate moiety as a flexible ligand will provide more coordination flexibility around the metal center to facilitate the catalytic reaction. Further, the electron rich centers around metal center will also support the enhancement of their catalytic activity. Complex 1 shows impressive OER activity, even better than the state-of-the-art IrO2 catalyst, in terms of turnover frequency (TOF: 0.05) and onset potential (1.50 V vs. RHE). The TOF for complex 1 is two and half times higher, while the onset potential is ca. 20 mV lower, than the benchmark IrO2 catalyst studied under identical conditions.

Hierarchical Hollow CoWO4-Co(OH)2 Heterostructured Nanoboxes Enabling Efficient Water Oxidation Electrocatalysis

Rational design and construction of well-defined hollow heterostructured nanomaterials assembled by ultrathin nanosheets overtakes crucial role in developing high-efficiency oxygen evolution reaction (OER) electrocatalysts. Herein, a reliable metal-organic framework-mediated and cation-exchange strategy to tune the geometric structure and multicomponent heterostructures has been proposed for the fabrication of hollow CoWO₄-Co(OH)₂ hierarchical nanoboxes assembled by rich ultrathin nanosheets. Benefiting from the hierarchical hollow nanostructure, the CoWO₄-Co(OH)₂ nanoboxes offer plenty of metal active centers available for reaction intermediates. Moreover, the well-defined nanointerfaces between CoWO₄ and Co(OH)₂ can function as the bridge for boosting the efficient electron transfer from CoWO₄ to Co(OH)₂. As a consequence, the optimized CoWO₄-Co(OH)₂ nanoboxes can exhibit outstanding electrocatalytic performance toward OER by delivering 10 mA cm^-2 with a low overpotential of 280 mV and a small Tafel slope of 70.6 mV dec^-1 as well as outstanding electrochemical stability. More importantly, this CoWO₄-Co(OH)₂ heterostructured nanocatalyst can couple with Pt/C to drive overall water splitting to achieve 10 mA cm^-2 with a voltage of 1.57 V.

Facile synthesis of hexagonal α-Co(OH)2 nanosheets and their superior activity in the selective reduction of nitro compounds

Novel hexagonal α-cobalt hydroxide nanosheets are synthesized through a 2-methylimidazole-induced hydrolysis strategy with cetyltrimethylammonium bromide (CTAB) as a surfactant. The weak alkaline environment provides favorable conditions for the formation of metastable α-Co(OH)₂, while the same raw material will produce β-Co(OH)₂ when a strong alkali solution is used. CTAB plays a vital role not only in hexagonal oriented growth, but also in the formation of the hydrotalcite-like structure of α-Co(OH)₂ with high crystallinity. The crystallinity of both α- and β-Co(OH)₂ is very poor without CTAB as a surfactant. The Co in this Co(OH)_2-x layer presents most of the Co^II and a small part of the Co^III, and the interlayer nitrate anion balances the positive charge of the host layer. The redox function produced by the Co^II and Co^III of α-Co(OH)₂ together with the large layer spacing jointly promotes the electron and mass transfer. The use of hydrazine hydrate for transfer hydrogenation involves the transport of protons and electrons produced by decomposition, and the rapid transport is bound to be conducive to the reduction process. Nitro compounds with varieties of functional groups can be smoothly reduced to the corresponding amines with high selectivity, when α-Co(OH)₂ was used as a catalyst under mild conditions.

Impact of Single-Pulse, Low-Intensity Laser Post-Processing on Structure and Activity of Mesostructured Cobalt Oxide for the Oxygen Evolution Reaction

Herein, we report nanosecond, single-pulse laser post-processing (PLPP) in a liquid flat jet with precise control of the applied laser intensity to tune structure, defect sites, and the oxygen evolution reaction (OER) activity of mesostructured Co3O4. High-resolution X-ray diffraction (XRD), Raman, and X-ray photoelectron spectroscopy (XPS) are consistent with the formation of cobalt vacancies at tetrahedral sites and an increase in the lattice parameter of Co3O4 after the laser treatment. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) further reveal increased disorder in the structure and a slight decrease in the average oxidation state of the cobalt oxide. Molecular dynamics simulation confirms the surface restructuring upon laser post-treatment on Co3O4. Importantly, the defect-induced PLPP was shown to lower the charge transfer resistance and boost the oxygen evolution activity of Co3O4. For the optimized sample, a 2-fold increment of current density at 1.7 V vs RHE is obtained and the overpotential at 10 mA/cm2 decreases remarkably from 405 to 357 mV compared to pristine Co3O4. Post-mortem characterization reveals that the material retains its activity, morphology, and phase structure after a prolonged stability test.

Co-Zn-MOFs Derived N-Doped Carbon Nanotubes with Crystalline Co Nanoparticles Embedded as Effective Oxygen Electrocatalysts

The oxygen reduction reaction (ORR) is a crucial step in fuel cells and metal-air batteries. It is necessary to expand the range of efficient non-precious ORR electrocatalysts on account of the low abundance and high cost of Pt/C catalysts. Herein, we synthesized crystalline cobalt-embedded N-doped carbon nanotubes (Co@CNTs-T) via facile carbonization of Co/Zn metal-organic frameworks (MOFs) with dicyandiamide at different temperatures (t = 600, 700, 800, 900 °C). Co@CNTs- 800 possessed excellent ORR activities in alkaline electrolytes with a half wave potential of 0.846 V vs. RHE (Reversible Hydrogen Electrode), which was comparable to Pt/C. This three-dimensional network, formed by Co@CNTs-T, facilitated electron migration and ion diffusion during the ORR process. The carbon shell surrounding the Co nanoparticles resulted in Co@CNTs-800 being stable as an electrocatalyst. This work provides a new strategy to design efficient and low-cost oxygen catalysts.

Interfacial modification of Co(OH)2/Co3O4 nanosheet heterostructure arrays for the efficient oxygen evolution reaction

The development of efficient, stable and low-cost oxygen evolution reaction (OER) catalysts in anodes is essential for the production of hydrogen resources by electrolyzing water.

Improved photocatalytic performance of metal-organic frameworks for CO2 conversion by ligand modification

Here we demonstrate that the utilization of 2,4,6-tris(4-pyridyl)pyridine (tpy) for metal-organic framework modification can greatly improve the photocatalytic performance for CO₂ reduction. The electron-donating nature of tpy enables the charge transfer effect, which induces strong CO₂ binding affinity, facilitates *COOH formation and promotes CO₂-to-CO conversion.

Synergistic tuning of oxygen vacancies and d-band centers of ultrathin cobaltous dihydroxycarbonate nanowires for enhanced electrocatalytic oxygen evolution

Simple and controllable synthesis of efficient and robust non-noble metal electrocatalysts towards the oxygen evolution reaction (OER) is highly desired and challenging in the development of sustainable energy conversion technologies. Herein, we report a facile one-step solvothermal synthesis of cobaltous dihydroxycarbonate nanowires (Co-OCH NWs) with a tunable diameter ranging from 8.7 to 16.7 nm, which were able to exhibit an interesting diameter-dependent catalytic activity towards the OER. It should be highlighted that the thinnest nanowires (8.7 nm) demonstrated the best OER catalytic activity among the as-prepared nanowires, showing an overpotential of only 307 mV at 10 mA cm-2 and a Tafel slope of 75 mV dec-1 in 1.0 M KOH solution. Based on comprehensive analysis, the excellent electrocatalytic activity of Co-OCH NWs was ascribed to the simultaneous achievement of an enlarged specific surface area, increased oxygen vacancy concentration and favorable position of the 3d-band center for the Co-OCH NWs with the continuous decrease of their diameters. More importantly, this work has emphasized that synergistic tuning of the oxygen vacancy concentration and d-band center position of nanomaterials via facile size control enables boosting their electrocatalytic performance substantially, thereby opening a simple route to design and prepare Earth-abundant electrocatalysts with higher efficiency and lower cost.

The NHx Group Induced Formation of 3D α-Co(OH)2 Curly Nanosheet Aggregates as Efficient Oxygen Evolution Electrocatalysts

Recently, the curly structure attracts researchers' attention due to the strain effect, electronic effect, and improved surface area, which exhibits enhanced electrocatalytic activity. However, the synthesis of metastable curved structures is very difficult. Herein, a simple room temperature coprecipitation method is proposed to synthesize 3D cobalt (Co) hydroxide (α-Co(OH)₂ ) electrocatalysts that consist of curly 2D nanosheets. The formation process of curly nanosheets is elaborated systematically and the results demonstrate that the NH_x group has great effect on the formation of curly structure. Combining the advantage of 2D curly nanosheet and 3D aggregate structure, the as-prepared α-Co(OH)₂ curly nanosheet aggregates show the best water oxidation activity with an overpotential of 269 mV at j = 10 mA cm^-2 in 1.0 m KOH. The electrocatalytic process studies demonstrate that the formation of Co^IV O species is the rate-determining step. Theoretical calculations further confirm the beneficial effect of the bent structure on the conductivity, the adsorption of OH^- and the formation of OOH* species.

Spontaneous Synthesis of Silver-Nanoparticle-Decorated Transition-Metal Hydroxides for Enhanced Oxygen Evolution Reaction

The fabrication of metal-supported hybrid structures with enhanced properties typically requires external energy input, such as pyrolysis, photolysis, and electrodeposition. In this study, silver-nanoparticle-decorated transition-metal hydroxide (TMH) composites were synthesized by an approach based on a spontaneous redox reaction (SRR) at room temperature. The SRR between silver ions and TMH provides a simple and facile route to establish effective and stable heterostructures that can enhance the oxygen evolution reaction (OER) activity. Ag@Co(OH)_x grown on carbon cloth exhibits outstanding OER activity and durability, even superior to IrO₂ and many previously reported OER electrocatalysts. Experimental and theoretical analysis demonstrates that the strong electronic interaction between Ag and Co(OH)₂ activates the silver clusters as catalytically OER active sites, effectively optimizing the binding energies with reacted intermediates and facilitating the OER kinetics.

General Water-Induced Self-Exfoliation Strategy for the Ultrafast and Large-Scale Synthesis of Metal Hydroxide Nanosheets

Metal hydroxides nanosheets of atomic thickness have attracted much interest due to their great potentials in catalysis, energy storage devices, and so on. However, the lack of efficient synthesis of 2D nanosheets has critically impeded their practical applications. Herein, we develop a general water-induced self-exfoliation (WISE) strategy to achieve the fast synthesis of metal hydroxide ultrathin nanosheets with almost single-layer atom thickness in a large scale. In a typical process of layered cobalt hydroxide (LCH) nanosheets, the presynthesized cobalt acetate hydroxide precursor is directly exfoliated to form nanosheets under the attack of H₂O in a few seconds. The water-induced self-exfoliation mechanism has also been proposed based on the analysis of the designed alcohol-mediated slow-down process. In addition, the used solutions and effluents can be recycled making WISE a green, efficient, and surfactant-free method. Furthermore, this general strategy can also be applied to synthesize other layered metal hydroxide nanosheets.

Facile Synthesis of 3d Transition-Metal-Doped α-Co(OH)2 Nanomaterials in Water-Methanol Mediated with Ammonia for Oxygen Evolution Reaction

Layered cobalt hydroxides are cost-efficient electrocatalysts for oxygen evolution reaction (OER) in the field of energy conversion. Herein, we developed a facile synthesis method of 3d transition-metal-doped α-Co(OH)₂ nanomaterials mediated with ammonia in water-methanol at room temperature. The doping of Cu²⁺ and Ni²⁺ leads to flower-like nanostructures similar to pure α-Co(OH)₂, whereas the doping of Fe²⁺ produces nanoparticles with more than 2 times larger surface area in comparison with the Cu²⁺- and Ni²⁺-doped nanoflowers. The obtained dispersion with the addition of Nafion can be used directly as an electrocatalyst for OER with excellent catalytic activity, especially that the overpotential of Fe²⁺ doped is as low as 290 mV at 10 mA cm^-2 and the turnover frequency is improved by 3 times as compared with that of α-Co(OH)₂. Furthermore, the catalyst can be loaded onto foam nickel, which presents excellent durability with the current density unchanged under continuous chronoamperometry reaction for as long as 12 h and almost quantitative faradaic efficiency. The superior electrocatalytic properties combined with the simple synthesis without the tedious purification procedure is very promising for OER.

Interface Electronic Coupling in Hierarchical FeLDH(FeCo)/Co(OH)2 Arrays for Efficient Electrocatalytic Oxygen Evolution

The oxygen evolution reaction (OER) with sluggish kinetics is the key half-cell reaction for several sustainable energy systems, such as electrochemical water splitting, fuel cells, and rechargeable metal-air batteries. Two-dimensional transition-metal hydroxides have good prospects for the OER. Herein, 2D hierarchical FeLDH(FeCo)/Co(OH)₂ (LDH=layered double hydroxide) arrays were fabricated by growing 2D-ZIF-67 (ZIF=zeolitic imidazolate framework) on carbon cloth, transformation of 2D-ZIF-67 into Co(OH)₂ , and electrodeposition of FeLDH(FeCo) on Co(OH)₂ at ambient temperature. The optimized hierarchical catalyst exhibits high OER activity that requires a small overpotential of only 242 mV to drive 10 mA cm^-2 (279 mV for 100 mA cm^-2 ) and prolonged durability for 100 h at 20 mA cm^-2 in 1 m KOH. The FeLDH(FeCo)/Co(OH)₂ interfaces are observed to be the electrocatalytically active centers for the OER. The interfaces contribute to accelerating the OER kinetics owing to fast transfer of intermediate oxygen species. Furthermore, the FeCo alloy promotes electron transfer among the newly formed interfaces related to CoOOH in the OER process, which leads to improved durability. This work gives insight into the design and synthesis of hierarchical bimetallic hydroxide arrays with high OER activity and durability, as well as understanding of the origin of the OER promotion by metals and metal hydroxides.

Shape-controlled synthesis of Co3O4 for enhanced electrocatalysis of the oxygen evolution reaction

One-dimensional Co₃O₄ nanorods, two-dimensional nanosheets and three-dimensional nanocubes were synthesized; the effect of the morphology on their electrocatalytic activities was studied.

Selective synthesis of single layer translucent cobalt hydroxide for the efficient oxygen evolution reaction

Remarkable water oxidation reactivity exhibited by a translucent single layer Co(OH)₂ nanosheet synthesized from α-Co(OH)2 by liquid phase exfoliation.

Ni/Ni3C core–shell nanoparticles encapsulated in N-doped bamboo-like carbon nanotubes towards efficient overall water splitting

A novel electrocatalyst of Ni/Ni₃C core–shell nanoparticles embedded in bamboo-like N-doped carbon nanotubes has been successfully synthesized, which exhibits superior overall water splitting performance in alkaline solution.

Chlorine-doped α-Co(OH)2 hollow nano-dodecahedrons prepared by a ZIF-67 self-sacrificing template route and enhanced OER catalytic activity

A ZIF-67-self-sacrificing template strategy was designed for the synthesis of undoped/Cl-doped α-Co(OH)₂ hollow nano-dodecahedrons with enhanced OER performance.