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.     

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.     

Facilely Tuning Porous NiCo2O4 Nanosheets with Metal Valence‐State Alteration and Abundant Oxygen Vacancies as Robust Electrocatalysts Towards Water Splitting

Great efforts in developing clean electrochemical water splitting technology leads to the rational design and synthesis of highly efficient oxygen evolution reaction (OER) catalysts with low overpotential and fast reaction kinetics. Herein, we focus on the role that morphology and composition play in the OER performance to rationally design freestanding 3D porous NiCo₂O₄ nanosheets with metal valence states alteration and abundant oxygen vacancies as robust electrocatalysts towards water splitting. Besides metal valence‐state alteration, surface modification regarding the evolution of oxygen vacancies is facilely realized upon the sodium borohydride treatment, which is beneficial for the enhanced OER performance. Taking advantage of the porous nanostructures and abundant surface activity sites with high reactivity, the resultant nanostructures exhibit excellent OER activity and stability in alkaline electrolytes that outperform that of pristine NiCo₂O₄ and commercial RuO₂, thus holding great potential for the water splitting.     

氧空位和磷掺杂协同增强铁钴基材料电解水催化性能

为了研发高效、稳定的电解水催化剂,我们以氧空位和磷掺杂为基础,通过原位浸泡生长和两步热处理的方法,在泡沫铁上合成具有氧空位和磷掺杂的纳米花结构作为析氢反应(HER)和析氧反应(OER)双功能电催化剂。CoFe₂O₄已被报道为一种很有前途的OER和氧还原反应(ORR)电催化剂,然而CoFe₂O₄在HER中表现出电导率差、电催化反应慢的特性。CoFe₂O₄中氧空位(Ov)的形成可以有效调控催化剂表面的电子结构,有助于产生更多的缺陷和空位,从而提高OER的活性。随后,引入磷原子填充在空位中,制备的P-Ov-CoFe₂O₄/IF在碱性电催化测试中展现出优异的HER和OER性能,在10 mA·cm^-2电流密度下HER和OER过电位仅为54和191 mV,Tafel斜率分别为57和54 mV·dec^-1,并具有良好的循环稳定性。     

Active Electron Density Modulation of Co3O4‐Based Catalysts Enhances their Oxygen Evolution Performance

Despite the fact that many strategies have been developed to improve the efficiency of the oxygen evolution reaction (OER), the precise modulation of the surface electronic properties of catalysts to improve their catalytic activity is still challenging. Herein, we demonstrate that the surface active electron density of Co₃O₄ can be effectively regulated by an argon‐ion irradiation method. X‐ray photoelectron and synchrotron x‐ray absorption spectroscopy, UV photoelectron spectrometry, and DFT calculations show that the surface active electron density band center of Co₃O₄ has been upshifted, leading to a significantly enhanced absorption capability of the oxo group. The optimized Co₃O₄‐based catalysts exhibit an excellent overpotential of 260 mV at 10 mA cm^?2 and Tafel slope of 54 mV dec^?1, superior to the capability of the benchmark RuO₂, representing one of the best Co‐based OER catalysts. This approach could guide the future rational design and discovery of ideal electrocatalysts.     

Effect of Calcination Temperature on Surface Oxygen Vacancies and Catalytic Performance Towards CO Oxidation of Co3O4 Nanoparticles Supported on SiO2

Co₃O₄/SiO₂ catalysts for CO oxidation were prepared by conventional incipient wetness impregnation followed by calcination at various temperatures. Their structures were char-acterized with X-ray diffraction (XRD), laser Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) and X-ray absorption fine structure (XAFS) spectroscopy. Both XRD and Raman spectroscopy only detect the ex-istence of Co₃O₄ crystallites in all catalysts. However, XPS results indicate that excess Co²⁺ ions are present on the surface of Co₃O₄ in Co₃O₄(200)/SiO₂ as compared with bulk Co₃O₄. Meanwhile, TPR results suggest the presence of surface oxygen vacancies on Co₃O₄ in Co₃O₄(200)/SiO₂, and XAFS results demonstrate that Co₃O₄ in Co₃O₄(200)/SiO₂ con-tains excess Co²⁺. Increasing calcination temperature results in oxidation of excess Co²⁺ and the decrease of the concentration of surface oxygen vacancies, consequently the for-mation of stoichiometric Co₃O₄ on supported catalysts. Among all Co₃O₄/SiO₂ catalysts,Co₃O₄(200)/SiO₂ exhibits the best catalytic performance towards CO oxidation, demon-strating that excess Co²⁺ and surface oxygen vacancies can enhance the catalytic activity of Co₃O₄ towards CO oxidation. These results nicely demonstrate the effect of calcination temperature on the structure and catalytic performance towards CO oxidation of silica-supported Co₃O₄ catalysts and highlight the important role of surface oxygen vacancies on Co₃O₄.     

Synthesis of a Hierarchically Porous C/Co3O4 Nanostructure with Boron Doping for Oxygen Evolution Reaction

Fabricating a low‐cost and highly efficient electrocatalyst is of importance for the development of renewable energy devices. In this work, we have synthesized an ultrafine cobalt oxide nanocatalyst (5–10 nm) doped with boron (BC/Co₃O₄) by using a metal–organic framework as a precursor, which exhibits an excellent catalytic activity for oxygen evolution reaction (OER). Owing to the improvement of accessible active sites by boron doping, the synthesized catalyst can reach a current density of 10 mA cm^?2 at 1.54 V with a low overpotential of 310 mV, superior than those of commercial RuO₂ and N‐doped C/Co₃O₄. This work provides a facile way to develop highly efficient catalysts for electrochemical reactions.     

The Roles of Composition and Mesostructure of Cobalt-Based Spinel Catalysts in Oxygen Evolution Reactions

By using the crystalline precursor decomposition approach and direct co-precipitation the composition and mesostructure of cobalt-based spinels can be controlled. A systematic substitution of cobalt with redox-active iron and redox-inactive magnesium and aluminum in a cobalt spinel with anisotropic particle morphology with a preferred 111 surface termination is presented, resulting in a substitution series including Co₃O₄, MgCo₂O₄, Co₂FeO₄, Co₂AlO₄ and CoFe₂O₄. The role of redox pairs in the spinels is investigated in chemical water oxidation by using ceric ammonium nitrate (CAN test), electrochemical oxygen evolution reaction (OER) and H₂O₂ decomposition. Studying the effect of dominant surface termination, isotropic Co₃O₄ and CoFe₂O₄ catalysts with more or less spherical particles are compared to their anisotropic analogues. For CAN-test and OER, Co³⁺ plays the major role for high activity. In H₂O₂ decomposition, Co²⁺ reveals itself to be of major importance. Redox active cations in the structure enhance the catalytic activity in all reactions. A benefit of a predominant 111 surface termination depends on the cobalt oxidation state in the as-prepared catalysts and the investigated reaction.     

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.     

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.     

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.     

Dual Role of Silver Moieties Coupled with Ordered Mesoporous Cobalt Oxide towards Electrocatalytic Oxygen Evolution Reaction

Herein, we show that the performance of mesostructured cobalt oxide electrocatalyst for oxygen evolution reaction (OER) can be significantly enhanced by coupling of silver species. Various analysis techniques including pair distribution function and Rietveld refinement, X‐ray absorption spectroscopy at synchrotron as well as advanced electron microscopy revealed that silver exists as metallic Ag particles and well‐dispersed Ag₂O nanoclusters within the mesostructure. The benefits of this synergy are twofold for OER: highly conductive metallic Ag improves the charge transfer ability of the electrocatalysts while ultra‐small Ag₂O clusters provide the centers that can uptake Fe impurities from KOH electrolyte and boost the catalytic efficiency of Co–Ag oxides. The current density of mesostructured Co₃O₄ at 1.7 V_RHE is increased from 102 to 211 mA cm^?2 with incorporation of silver spices. This work presents the dual role of silver moieties and demonstrates a simple method to increase the OER activity of Co₃O₄.     

Impact of Surface Defects on LaNiO3 Perovskite Electrocatalysts for the Oxygen Evolution Reaction

Perovskite oxides are regarded as promising electrocatalysts for water splitting due to their cost-effectiveness, high efficiency and durability in the oxygen evolution reaction (OER). Despite these advantages, a fundamental understanding of how critical structural parameters of perovskite electrocatalysts influence their activity and stability is lacking. Here, we investigate the impact of structural defects on OER performance for representative LaNiO₃ perovskite electrocatalysts. Hydrogen reduction of 700 °C calcined LaNiO₃ induces a high density of surface oxygen vacancies, and confers significantly enhanced OER activity and stability compared to unreduced LaNiO₃; the former exhibit a low onset overpotential of 380 mV at 10 mA cm⁻² and a small Tafel slope of 70.8 mV dec⁻¹. Oxygen vacancy formation is accompanied by mixed Ni²⁺/Ni³⁺ valence states, which quantum-chemical DFT calculations reveal modify the perovskite electronic structure. Further, it reveals that the formation of oxygen vacancies is thermodynamically more favourable on the surface than in the bulk; it increases the electronic conductivity of reduced LaNiO₃ in accordance with the enhanced OER activity that is observed.     

Surface Reorganization on Electrochemically‐Induced Zn–Ni–Co Spinel Oxides for Enhanced Oxygen Electrocatalysis

Herein, we highlight redox‐inert Zn²⁺ in spinel‐type oxide (Zn_XNi_1?XCo₂O₄) to synergistically optimize physical pore structure and increase the formation of active species on the catalyst surface. The presence of Zn²⁺ segregation has been identified experimentally and theoretically under oxygen‐evolving condition, the newly formed V_Zn?O?Co allows more suitable binding interaction between the active center Co and the oxygenated species, resulting in superior ORR performance. Moreover, a liquid flow Zn–air battery is constituted employing the structurally optimized Zn_0.4Ni_0.6Co₂O₄ nanoparticles supported on N‐doped carbon nanotube (ZNCO/NCNTs) as an efficient air cathode, which presents remarkable power density (109.1 mW cm^?2), high open circuit potential (1.48 V vs. Zn), excellent durability, and high‐rate performance. This finding could elucidate the experimentally observed enhancement in the ORR activity of Zn_XNi_1?XCo₂O₄ oxides after the OER test.     

Laser-Ablation-Produced Cobalt Nickel Phosphate with High-Valence Nickel Ions as an Active Catalyst for the Oxygen Evolution Reaction

Cost-effective, highly efficient and stable non-noble metal-based catalysts for the oxygen evolution reaction (OER) are very crucial for energy storage and conversion. Here, an amorphous cobalt nickel phosphate (CoNiPO₄), containing a considerable amount of high-valence Ni³⁺ species as an efficient electrocatalyst for OER in alkaline solution, is reported. The catalyst was converted from Co-doped Ni₂P through pulsed laser ablation in liquid (PLAL) and exhibits a large specific surface area of 162.5 m² g⁻¹ and a low overpotential of 238 mV at 10 mA cm⁻² with a Tafel slope of 46 mV dec⁻¹, which is much lower than those of commercial RuO₂ and IrO₂. This work demonstrates that PLAL is a powerful technology for generating amorphous CoNiPO₄ with high-valence Ni³⁺, thus paving a new way towards highly effective OER catalysts.     

Ultrathin Spinel‐Structured Nanosheets Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water Oxidation

Electrochemical water splitting is a clean technology for H₂ fuels, but greatly hindered by the slow kinetics of the oxygen evolution reaction (OER). Herein, a series of spinel‐structured nanosheets with oxygen deficiencies and ultrathin thicknesses were designed to increase the reactivity and the number of active sites of the catalysts, which were then taken as an excellent platform for promoting the water oxidation process. Theoretical investigations showed that the oxygen vacancies confined in the ultrathin nanosheet could lower the adsorption energy of H₂O, leading to increased OER efficiency. As expected, the NiCo₂O₄ ultrathin nanosheets rich in oxygen vacancies exhibited a large current density of 285 mA cm^?2 at 0.8 V and a small overpotential of 0.32 V, both of which are superior to the corresponding values of bulk samples or samples with few oxygen deficiencies and even higher than those of most reported non‐precious‐metal catalysts. This work should provide a new pathway for the design of advanced OER catalysts.     

A Bifunctional Hybrid Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions: Nano-Co3O4-Deposited La0.5Sr0.5MnO3 via Infiltration

For rechargeable metal–air batteries, which are a promising energy storage device for renewable and sustainable energy technologies, the development of cost-effective electrocatalysts with effective bifunctional activity for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) has been a challenging task. To realize highly effective ORR and OER electrocatalysts, we present a hybrid catalyst, Co₃O₄-infiltrated La_0.5Sr_0.5MnO_3-δ (LSM@Co₃O₄), synthesized using an electrospray and infiltration technique. This study expands the scope of the infiltration technique by depositing ~18 nm nanoparticles on unprecedented ~70 nm nano-scaffolds. The hybrid LSM@Co₃O₄ catalyst exhibits high catalytic activities for both ORR and OER (~7 times, ~1.5 times, and ~1.6 times higher than LSM, Co₃O₄, and IrO₂, respectively) in terms of onset potential and limiting current density. Moreover, with the LSM@Co₃O₄, the number of electrons transferred reaches four, indicating that the catalyst is effective in the reduction reaction of O₂ via a direct four-electron pathway. The study demonstrates that hybrid catalysts are a promising approach for oxygen electrocatalysts for renewable and sustainable energy devices.     

Self-Assembled Two-Dimensional NiO/CeO2 Heterostructure Rich in Oxygen Vacancies as Efficient Bifunctional Electrocatalyst for Alkaline Hydrogen Evolution and Oxygen Evolution

The development of high-efficiency bifunctional electrocatalysts toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline surroundings is essential and challenging for the large-scale generation of clean hydrogen. Herein, a novel self-assembled two-dimensional (2 D) NiO/CeO₂ heterostructure (HS) consisting of NiO and CeO₂ nanocrystals is prepared through a facile two-step approach, and utilized as an enhanced bifunctional electrocatalyst for the HER and OER under alkaline conditions. It is concluded that this 2 D NiO/CeO₂ HS, rich in oxygen vacancies, demonstrates attractive electrocatalytic properties for both the HER and OER in 1 m KOH, including low onset overpotential (η₁), η₁₀ and Tafel slope, excellent durability, as well as large active surface area. Therefore, the self-assembled 2 D NiO/CeO₂ HS is believed to be an efficient bifunctional electrocatalyst toward the HER and OER.     

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.     

Active Electron Density Modulation of Co3O4-Based Catalysts Enhances their Oxygen Evolution Performance

Despite the fact that many strategies have been developed to improve the efficiency of the oxygen evolution reaction (OER), the precise modulation of the surface electronic properties of catalysts to improve their catalytic activity is still challenging. Herein, we demonstrate that the surface active electron density of Co₃O₄ can be effectively regulated by an argon-ion irradiation method. X-ray photoelectron and synchrotron x-ray absorption spectroscopy, UV photoelectron spectrometry, and DFT calculations show that the surface active electron density band center of Co₃O₄ has been upshifted, leading to a significantly enhanced absorption capability of the oxo group. The optimized Co₃O₄-based catalysts exhibit an excellent overpotential of 260 mV at 10 mA cm⁻² and Tafel slope of 54 mV dec⁻¹, superior to the capability of the benchmark RuO₂, representing one of the best Co-based OER catalysts. This approach could guide the future rational design and discovery of ideal electrocatalysts.