Coupling Magnetic Single‐Crystal Co2Mo3O8 with Ultrathin Nitrogen‐Rich Carbon Layer for Oxygen Evolution Reaction

Transition‐metal oxides as electrocatalysts for the oxygen evolution reaction (OER) provide a promising route to face the energy and environmental crisis issues. Although palmeirite oxide A₂Mo₃O₈ as OER catalyst has been explored, the correlation between its active sites (tetrahedral or octahedral) and OER performance has been elusive. Now, magnetic Co₂Mo₃O₈@NC‐800 composed of highly crystallized Co₂Mo₃O₈ nanosheets and ultrathin N‐rich carbon layer is shown to be an efficient OER catalyst. The catalyst exhibits favorable performance with an overpotential of 331 mV@10 mA cm^?2 and 422 mV@40 mA cm^?2, and a full water‐splitting electrolyzer with it as anode catalyst shows a cell voltage of 1.67 V@10 mA cm^?2 in alkaline. Combined HAADFSTEM, magnetic, and computational results show that factors influencing the OER performance can be attributed to the tetrahedral Co sites (high spin, t₂³e⁴), which improve the OER kinetics of rate‐determining step to form *OOH.     

Tailoring Oxygen Vacancy on Co3O4 Nanosheets with High Surface Area for Oxygen Evolution Reaction?

Electrochemical water splitting requires efficient water oxidation catalysts to accelerate the sluggish kinetics of water oxidation reaction. Here, we designed an efficient Co₃O₄ electrocatalyst using a pyrolysis strategy for oxygen evolution reaction (OER). Morphological characterization confirmed the ultra-thin structure of nanosheet. Further, the existence of oxygen vacancies was obviously evidenced by the X-ray photoelectron spectroscopy and electron spin resonance spectroscopy. The increased surface area of Co₃O₄ ensures more exposed sites, whereas generated oxygen vacancies on Co₃O₄ surface create more active defects. The two scenarios were beneficial for accelerating the OER across the interface between the anode and electrolyte. As expected, the optimized Co₃O₄ nanosheets can catalyze the OER efficiently with a low overpotential of 310 mV at current density of 10 mA/cm² and remarkable long-term stability in 1.0 mol/L KOH.     

Coupling Magnetic Single-Crystal Co2Mo3O8 with Ultrathin Nitrogen-Rich Carbon Layer for Oxygen Evolution Reaction

Transition-metal oxides as electrocatalysts for the oxygen evolution reaction (OER) provide a promising route to face the energy and environmental crisis issues. Although palmeirite oxide A₂Mo₃O₈ as OER catalyst has been explored, the correlation between its active sites (tetrahedral or octahedral) and OER performance has been elusive. Now, magnetic Co₂Mo₃O₈@NC-800 composed of highly crystallized Co₂Mo₃O₈ nanosheets and ultrathin N-rich carbon layer is shown to be an efficient OER catalyst. The catalyst exhibits favorable performance with an overpotential of 331 mV@10 mA cm⁻² and 422 mV@40 mA cm⁻², and a full water-splitting electrolyzer with it as anode catalyst shows a cell voltage of 1.67 V@10 mA cm⁻² in alkaline. Combined HAADFSTEM, magnetic, and computational results show that factors influencing the OER performance can be attributed to the tetrahedral Co sites (high spin, t₂³e⁴), which improve the OER kinetics of rate-determining step to form *OOH.     

Boosting Electrocatalytic Oxygen Evolution over Ce−Co9S8 Core–Shell Nanoneedle Arrays by Electronic and Architectural Dual Engineering

An dual electronic and architectural engineering strategy is a good way to rationally design earth-abundant and highly efficient electrocatalysts of the oxygen evolution reaction (OER) for sustainable hydrogen-based energy devices. Here, a Ce-doped Co₉S₈ core–shell nanoneedle array (Ce−Co₉S₈@CC) supported on a carbon cloth has been designed and developed to accelerate the sluggish kinetics of the OER. Profiting from valance alternative Ce doping, a fine core–shell structure and vertically aligned nanoneedle arrayed architecture, Ce−Co₉S₈@CC integrates modulated electronic structure, highly exposed active sites, and multidimensional mass diffusion channels; together, these afford a favorable catalyzed OER. Ce−Co₉S₈@CC exhibits remarkable performance in the OER in an alkaline medium, where the overpotential requires only 242 mV to deliver a current density of 10 mA cm⁻² for the OER; this is 70 mV superior to that of Ce-free Co₉S₈ catalyst and other counterparts. Good stability and impressive selectivity (nearly 100 % Faradic efficiency) are also demonstrated. When integrated into a two-electrode OER//HER electrolyzer, the as-prepared Ce−Co₉S₈@CC displays a low operation potential of 1.54 V at 10 mA cm⁻² and long-term stability, thus demonstrating great potential for economical water electrolysis.     

Metallic Co4N Porous Nanowire Arrays Activated by Surface Oxidation as Electrocatalysts for the Oxygen Evolution Reaction

Designing highly efficient electrocatalysts for oxygen evolution reaction (OER) plays a key role in the development of various renewable energy storage and conversion devices. In this work, we developed metallic Co₄N porous nanowire arrays directly grown on flexible substrates as highly active OER electrocatalysts for the first time. Benefiting from the collaborative advantages of metallic character, 1D porous nanowire arrays, and unique 3D electrode configuration, surface oxidation activated Co₄N porous nanowire arrays/carbon cloth achieved an extremely small overpotential of 257 mV at a current density of 10 mA cm⁻², and a low Tafel slope of 44 mV dec⁻¹ in an alkaline medium, which is the best OER performance among reported Co‐based electrocatalysts to date. Moreover, in‐depth mechanistic investigations demonstrate the active phases are the metallic Co₄N core inside with a thin cobalt oxides/hydroxides shell during the OER process. Our finding introduces a new concept to explore the design of high‐efficiency OER electrocatalysts.     

Enhanced Electrocatalytic Water Oxidation by Interfacial Phase Transition and Photothermal Effect in Multiply Heterostructured Co9S8/Co3S4/Cu2S Nanohybrids

The rational design of advanced nanohybrids (NHs) with optimized interface electronic environment and rapid reaction kinetics is pivotal to electrocatalytic schedule. Herein, we developed a multiple heterogeneous Co₉S₈/Co₃S₄/Cu₂S nanoparticle in which Co₃S₄ germinates between Co₉S₈ and Cu₂S. Using high-angle annular-dark-field imaging and theoretical calculation, it was found that the integration of Co₉S₈ and Cu₂S tends to trigger the interface phase transition of Co₉S₈, leading to Co₃S₄ interlayer due to the low formation energy of Co₃S₄/Cu₂S (−7.61 eV) than Co₉S₈/Cu₂S (−5.86 eV). Such phase transition not only lowers the energy barrier of oxygen evolution reaction (OER, from 0.335 eV to 0.297 eV), but also increases charge carrier density (from 7.76×10¹⁴ to 2.09×10¹⁵ cm⁻³), and creates more active sites. Compared to Co₉S₈ and Cu₂S, the Co₉S₈/Co₃S₄/Cu₂S NHs also demonstrate notable photothermal effect that can heat the catalyst locally, offset the endothermic enthalpy change of OER, and promote carrier migrate, reaction intermediates adsorption/deprotonation to improve reaction kinetics. Profiting from these favorable factors, the Co₉S₈/Co₃S₄/Cu₂S catalyst only requires an OER overpotential of 181 mV and overall water splitting cell voltage of 1.43 V to driven 10 mA cm⁻² under the irradiation of near-infrared light, outperforming those without light irradiation and many reported Co-based catalysts.     

Controllable 1D and 2D Cobalt Oxide and Cobalt Selenide Nanostructures as Highly Efficient Electrocatalysts for the Oxygen Evolution Reaction

The relationship between controllable morphology and electrocatalytic activity of Co₃O₄ and CoSe₂ for the oxygen evolution reaction (OER) was explored in alkaline medium. Based on the time‐dependent growth process of cobalt precursors, 1D Co₃O₄ nanorods and 2D Co₃O₄ nanosheets were successfully synthesized through a facile hydrothermal process at 180 °C under different reaction times, followed by calcination at 300 °C for 2 h. Subsequently, 1D and 2D CoSe₂ nanostructures were derived by selenization of Co₃O₄, which achieved the controllable synthesis of CoSe₂ without templating agents. By comparing the electrocatalytic behavior of these cobalt‐based catalysts in 1 m KOH electrolyte toward the OER, both 2D Co₃O₄ and 2D CoSe₂ nanocrystals have lower overpotentials and better electrocatalytic stability than that of 1D nanostructures. The 2D CoSe₂ nanosheets require overpotentials of 372 mV to reach a current density of 50 mA cm⁻² with a small Tafel slope of 74 mV dec⁻¹. A systematic contrast of the electrocatalytic performances for the OER increase in the order: 1D Co₃O₄<2D Co₃O₄<1D CoSe₂<2D CoSe₂. This work provides fundamental insights into the morphology–performance relationships of both Co₃O₄ and CoSe₂, which were synthesized through the same approach, providing a solid guide for designing OER catalysts.     

Plasma‐Engraved Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction

Co₃O₄, which is of mixed valences Co²⁺ and Co³⁺, has been extensively investigated as an efficient electrocatalyst for the oxygen evolution reaction (OER). The proper control of Co²⁺/Co³⁺ ratio in Co₃O₄ could lead to modifications on its electronic and thus catalytic properties. Herein, we designed an efficient Co₃O₄‐based OER electrocatalyst by a plasma‐engraving strategy, which not only produced higher surface area, but also generated oxygen vacancies on Co₃O₄ surface with more Co²⁺ formed. The increased surface area ensures the Co₃O₄ has more sites for OER, and generated oxygen vacancies on Co₃O₄ surface improve the electronic conductivity and create more active defects for OER. Compared to pristine Co₃O₄, the engraved Co₃O₄ exhibits a much higher current density and a lower onset potential. The specific activity of the plasma‐engraved Co₃O₄ nanosheets (0.055 mA cm^?2_BET at 1.6 V) is 10 times higher than that of pristine Co₃O₄, which is contributed by the surface oxygen vacancies.     

Regulating the Spin State of FeIII by Atomically Anchoring on Ultrathin Titanium Dioxide for Efficient Oxygen Evolution Electrocatalysis

Ferric oxides and (oxy)hydroxides, although plentiful and low‐cost, are rarely considered for oxygen evolution reaction (OER) owing to the too high spin state (e_g filling ca. 2.0) suppressing the bonding strength with reaction intermediates. Now, a facile adsorption–oxidation strategy is used to anchor Fe^III atomically on an ultrathin TiO₂ nanobelt to synergistically lower the spin state (e_g filling ca. 1.08) to enhance the adsorption with oxygen‐containing intermediates and improve the electro‐conductibility for lower ohmic loss. The electronic structure of the catalyst is predicted by DFT calculation and perfectly confirmed by experimental results. The catalyst exhibits superior performance for OER with overpotential 270 mV @10 mA cm^?2 and 376 mV @100 mA cm^?2 in alkaline solution, which is much better than IrO₂/C and RuO₂/C and is the best iron‐based OER catalyst free of active metals such as Ni, Co, or precious metals.     

Photoinduced Cobalt(III)−Trifluoromethyl Bond Activation Enables Arene C−H Trifluoromethylation

Visible‐light capture activates a thermodynamically inert Co^III−CF₃ bond for direct C−H trifluoromethylation of arenes and heteroarenes. New trifluoromethylcobalt(III) complexes supported by a redox‐active [OCO] pincer ligand were prepared. Coordinating solvents, such as MeCN, afford green, quasi‐octahedral [(^SOCO)Co^III(CF₃)(MeCN)₂] ( 2 ), but in non‐coordinating solvents the complex is red, square pyramidal [(^SOCO)Co^III(CF₃)(MeCN)] ( 3 ). Both are thermally stable, and 2 is stable in light. But exposure of 3 to low‐energy light results in facile homolysis of the Co^III−CF₃ bond, releasing ^.CF₃ radical, which is efficiently trapped by TEMPO^. or (hetero)arenes. The homolytic aromatic substitution reactions do not require a sacrificial or substrate‐derived oxidant because the Co^II by‐product of Co^III−CF₃ homolysis produces H₂. The photophysical properties of 2 and 3 provide a rationale for the disparate light stability.     

Autologous Cobalt Phosphates with Modulated Coordination Sites for Electrocatalytic Water Oxidation

The correlation between metal coordination and electrocatalytic water oxidation performance is elusive in many cobalt‐based materials. Herein, we designed an ideal Co phosphate‐based platform to explore the effect of coordination environment on oxygen evolution reaction (OER) activity. The cobalt geometry was modulated from octahedral to tetrahedral by simple removal of water ligands in Co₃(PO₄)₂?8 H₂O. Other features except the coordination structure in the two autologous materials remain similar. The two analogues display the same OER kinetics, but the anhydrous Co₃(PO₄)₂ exhibits a greatly enhanced OER activity. On the basis of Raman and operando XAS results, the higher intrinsic activity of the Co tetrahedral sites is because they facilitate the formation of active high valent cobalt (hydr)oxide intermediates during OER. This work not only brings insights of OER on Co‐based electrocatalysts but also provides a reference system to study the effect of metal geometry on electrocatalysis.     

MOF‐Derived Hollow CoS Decorated with CeOx Nanoparticles for Boosting Oxygen Evolution Reaction Electrocatalysis

Transition‐metal sulfides (TMSs) have emerged as important candidates for oxygen evolution reaction (OER) electrocatalysts. Now a hybrid nanostructure has been decorated with CeO_x nanoparticles on the surface of ZIF‐67‐derived hollow CoS through in situ generation. Proper control of the amount of CeO_x on the surface of CoS can achieve precise tuning of Co²⁺/Co³⁺ ratio, especially for the induced defects, further boosting the OER activity. Meanwhile, the formation of protective CeO_x thin layer effectively inhibits the corrosion by losing cobalt ion species from the active surface into the solution. It is thus a rare example of a hybrid hetero‐structural electrocatalyst with CeO_x NPs to improve the performance of the hollow TMS nanocage.     

SrNb0.1Co0.7Fe0.2O3−δ Perovskite as a Next‐Generation Electrocatalyst for Oxygen Evolution in Alkaline Solution

The perovskite SrNb_0.1Co_0.7Fe_0.2O_3?δ (SNCF) is a promising OER electrocatalyst for the oxygen evolution reaction (OER), with remarkable activity and stability in alkaline solutions. This catalyst exhibits a higher intrinsic OER activity, a smaller Tafel slope and better stability than the state‐of‐the‐art precious‐metal IrO₂ catalyst and the well‐known BSCF perovskite. The mass activity and stability are further improved by ball milling. Several factors including the optimized e_g orbital filling, good ionic and charge transfer abilities, as well as high OH^? adsorption and O₂ desorption capabilities possibly contribute to the excellent OER activity.     

Optimal Geometrical Configuration of Cobalt Cations in Spinel Oxides to Promote Oxygen Evolution Reaction

MgCo₂O₄, CoCr₂O₄, and Co₂TiO₄ were selected, where only Co³⁺ in the center of octahedron (Oh), Co²⁺ in the center of tetrahedron (Td), and Co²⁺ in the center of Oh, can be active sites for the oxygen evolution reaction (OER). Co³⁺(Oh) sites are the best geometrical configuration for OER. Co²⁺(Oh) sites exhibit better activity than Co²⁺(Td). Calculations demonstrate the conversion of O* into OOH* is the rate‐determining step for Co³⁺(Oh) and Co²⁺(Td). For Co²⁺(Oh), it is thermodynamically favorable for the formation of OOH* but difficult for the desorption of O₂. Co³⁺(Oh) needs to increase the lowest Gibbs free energy over Co²⁺(Oh) and Co²⁺(Td), which contributes to the best activity. The coexistence of Co³⁺(Oh) and Co²⁺(Td) in Co₃O₄ can promote the formation of OOH* and decrease the free‐energy barrier. This work screens out the optimal geometrical configuration of cobalt cations for OER and gives a valuable principle to design efficient electrocatalysts.     

Cobalt(II) and Cobalt(III) Tri‐tert‐butoxysilanethiolates. Synthesis,Properties, Crystal and Molecular Structures of [Co{μ‐SSi(OBut)3}{SSi(OBut)3}(NH3)]2 and [Co{SSi(OBut)3}2(NH3)4][SSi(OBut)3] Complexes

The heteroleptic neutral tri‐tert‐butoxysilanethiolate of cobalt(II) incorporating ammonia as additional ligand ( 1 ) has been prepared by the reaction of a cobalt(II) ammine complex with tri‐tert‐butoxysilanethiol in water. Complex 1 , dissolved in hexane, undergoes oxidation in an ammonia saturated atmosphere to the ionic cobalt(III) compound 2 . Molecular and crystal structures of 1 and 2 have been determined by single crystal X‐ray structural analysis. 1 forms a dimeric molecule [Co{μ‐SSi(OBu^t)₃}{SSi(OBu^t)₃}(NH₃)]₂ with a folded central Co₂S₂ ring and distorted tetrahedral ligand arrangement at both Co^II atoms (CoNS₃ core). The product 2 is composed of the octahedral Co^III complex cation [Co{SSi(OBu^t)₃}₂(NH₃)₄]⁺ and the tri‐tert‐butoxysilanethiolate anion. Within the crystal two pairs of ions interact by hydrogen bonds forming well separated entities. 1 and 2 are the first structurally characterized cobalt thiolates where metal is also bonded to ammonia and 2 is the first cobalt(III) silanethiolate.     

Coordination Complexes of a Neutral 1,2,4‐Benzotriazinyl Radical Ligand: Synthesis,Molecular and Electronic Structures,and Magnetic Properties

A series of d‐block metal complexes of the recently reported coordinating neutral radical ligand 1‐phenyl‐3‐(pyrid‐2‐yl)‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl ( 1 ) was synthesized. The investigated systems contain the benzotriazinyl radical 1 coordinated to a divalent metal cation, Mn^II, Fe^II, Co^II, or Ni^II, with 1,1,1,5,5,5‐hexafluoroacetylacetonato (hfac) as the auxiliary ligand of choice. The synthesized complexes were fully characterized by single‐crystal X‐ray diffraction, magnetic susceptibility measurements, and electronic structure calculations. The complexes [Mn( 1 )(hfac)₂] and [Fe( 1 )(hfac)₂] displayed antiferromagnetic coupling between the unpaired electrons of the ligand and the metal cation, whereas the interaction was found to be ferromagnetic in the analogous Ni^II complex [Ni( 1 )(hfac)₂]. The magnetic properties of the complex [Co( 1 )(hfac)₂] were difficult to interpret owing to significant spin–orbit coupling inherent to octahedral high‐spin Co^II metal ion. As a whole, the reported data clearly demonstrated the favorable coordinating properties of the radical 1 , which, together with its stability and structural tunability, make it an excellent new building block for establishing more complex metal–radical architectures with interesting magnetic properties.     

Six‐coordinate Co2+ with imidazole,NH3, and H2O ligands: Approaching spin crossover

Octahedral, six‐coordinate Co²⁺ can exist in two spin states: S = 3/2 and S = 1/2. The difference in energy between high spin (S = 3/2) and low spin (S = 1/2) is dependent on both the ligand mix and coordination stereochemistry. B3LYP calculations on combinations of neutral imidazole, NH₃, and H₂O ligands show that low‐spin isomers are stabilized by axial H₂O ligands and in structures that also include trans pairs of equatorial NH₃ and protonated imidazole ligands, spin crossover structures are predicted from spin state energy differences. Occupied Co d orbitals from the DFT calculations provide a means of estimating effective ligand strength for homoleptic and mixed ligand combinations. These calculations suggest that in a labile biological system, a spin crossover environment can be created. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Non‐3d Metal Modulation of a Cobalt Imidazolate Framework for Excellent Electrocatalytic Oxygen Evolution in Neutral Media

Cobalt imidazolate frameworks are classical electrocatalysts for the oxygen evolution reaction (OER) but suffer from the relatively low activity. Here, a non‐3d metal modulation strategy is presented for enhancing the OER activity of cobalt imidazolate frameworks. Two isomorphous frameworks [Co₄(MO₄)(eim)₆] (M=Mo or W, Heim=2‐ethylimidazole) having Co(eim)₃(MO₄) units and high water stabilities were designed and synthesized. In different neutral media, the Mo‐modulated framework coated on a glassy carbon electrode shows the best OER performances (1 mA cm^?2 at an overpotential of 210 mV in CO₂‐saturated 0.5 m KHCO₃ electrolyte and 2/10/22 mA cm^?2 at overpotential of 388/490/570 mV in phosphate buffer solution) among non‐precious metal catalysts and even outperforms RuO₂. Spectroscopic measurements and computational simulations revealed that the non‐3d metals modulate the electronic structure of Co for optimum reactant/product adsorption and tailor the energy of rate‐determining step to a more moderate value.     

Fe‐Doped Ni3C Nanodots in N‐Doped Carbon Nanosheets for Efficient Hydrogen‐Evolution and Oxygen‐Evolution Electrocatalysis

Uniform Ni₃C nanodots dispersed in ultrathin N‐doped carbon nanosheets were successfully prepared by carburization of the two dimensional (2D) nickel cyanide coordination polymer precursors. The Ni₃C based nanosheets have lateral length of about 200 nm and thickness of 10 nm. When doped with Fe, the Ni₃C based nanosheets exhibited outstanding electrocatalytic properties for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). For example, 2 at % Fe (atomic percent) doped Ni₃C nanosheets depict a low overpotential (292 mV) and a small Tafel slope (41.3 mV dec⁻¹) for HER in KOH solution. An outstanding OER catalytic property is also achieved with a low overpotential of 275 mV and a small Tafel slope of 62 mV dec⁻¹ in KOH solution. Such nanodot‐incorporated 2D hybrid structures can serve as an efficient bifunctional electrocatalyst for overall water splitting.     

Synthesis and characterization of a novel series of metallothiocarbohydrazone polymers and their adducts

A novel linear polymeric pentadentate (O₂N₂S‐sites) ligand (H₃L) bearing both soft and hard donors was prepared by the reaction of a bifunctional carbonyl compound, 4,6‐diacetylresorcinol, with a bifunctional hydrazide compound, thiocarbohydrazide. Mono‐ and binuclear Cu^II and Ni^II complexes/each monomeric unit of the polymeric ligand were obtained depending on the pH of the reaction medium and the metal ion. Adducts with 1,10‐phenanthroline (Phen) and 2,2′‐bipyridyl (Bpy) were obtained. Anomalous dimeric Co^II/Co^III complexes of the polymeric ligand were obtained in which two molecules of the linear polymeric ligand trapped two cobalt ions (Co^II and Co^III) in each monomeric unit. These structures are very interesting in that they contain Co^II/Co^III, side by side, as high‐spin square planar coordinated Co^II ions and low‐spin (diamagnetic) octahedral coordinated Co^III ions. The suggested structures of the complexes have been elucidated on the basis of elemental and thermal analyses, conductance, and magnetic susceptibility measurements as well as spectral studies (electronic, IR, and ESR spectra). © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:100–107, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20239