Co-O Bond Research Articles

DFT-based investigation of electronic-structure, magnetic and thermoelectric properties of Dy2CoMnO6 double perovskite

The density functional theory as a computational approach was used to explore the ferrimagnetic semiconducting behavior of Dy2CoMnO6 double perovskite compound in both spin-up and spin-down directions. Thermodynamic stability was confirmed in a wide pressure range set to 30 GPa. Both scalar and full relativistic calculations are used to estimate the spin–orbit effect. The topological distribution of the charge density and the net effective charge of each atom are studied based on the Quantum Theory of Atoms in Molecules (QTAIM) as implemented in Bader code, Besides this, the different QTAIM atomic basins descriptors such as electron density Laplacian of the electron density potential electronic energy density kinetic electronic energy density and density of the total electronic energy at bond critical points (BCPs) are estimated using CRITIC2 software where the ionic type for Dy-O, Co-O, and Mn-O bonds are evaluated. Holes and electrons have different effective masses, their thermoelectric properties appear high figure of merit (ZT) exceeding 0.5 for temperatures greater than 500 K in the negative chemical potential region, suggesting that doping with holes might be more favorable for thermoelectric efficiency than doping with electrons.

Emergent and robust ferromagnetic-insulating state in highly strained ferroelastic LaCoO3 thin films

Transition metal oxides are promising candidates for the next generation of spintronic devices due to their fascinating properties that can be effectively engineered by strain, defects, and microstructure. An excellent example can be found in ferroelastic LaCoO3 with paramagnetism in bulk. In contrast, unexpected ferromagnetism is observed in tensile-strained LaCoO3 films, however, its origin remains controversial. Here we simultaneously reveal the formation of ordered oxygen vacancies and previously unreported long-range suppression of CoO6 octahedral rotations throughout LaCoO3 films. Supported by density functional theory calculations, we find that the strong modification of Co 3d-O 2p hybridization associated with the increase of both Co-O-Co bond angle and Co-O bond length weakens the crystal-field splitting and facilitates an ordered high-spin state of Co ions, inducing an emergent ferromagnetic-insulating state. Our work provides unique insights into underlying mechanisms driving the ferromagnetic-insulating state in tensile-strained ferroelastic LaCoO3 films while suggesting potential applications toward low-power spintronic devices.

Phase transition mechanism of the solid-state reaction of two variable-valence metal oxides: Cobalt and manganese oxides

Due to the state transition of variable-valence ions during the solid-state reaction, the reaction mechavnism is complex. As one of the function materials in supercapacitors and catalysts, there is much research on the property and function mechanism of MnCo2O4. But it’s necessary to study the chemical reaction and a productive method for realizing wide application. In this paper, solid-state reaction method with high efficiency was applied to produce MnCo2O4 and investigated the reaction process of oxides, the formation mechanism of MnCo2O4, respectively. Thermodynamic and roasting experiments of single metal oxides demonstrated the actual stability of MnO2 and Co3O4 in the air. Furthermore, Roasting experiments on different metal oxide systems revealed Mn2O3 and Mn3O4 were the effective reactants of MnCo2O4, which reacted with Co3O4 respectively to produce MnCo2O4. Moreover, the optimum condition of the preparation of MnCo2O4 was determined to be 1150 ℃ with a 90-minute in a 3:2 molar ratio of MnO2 and Co3O4. MnCo2O4 were composed of Mn2+, Mn3+, Co2+, and Co3+, the property of Mn-O and Co-O bonds at 1.50 Å and 1.54 Å all including tetrahedral and octahedral sites with a bond length of 1.9–1.92 Å and 1.93–1.94 Å, respectively. Mn and Co atoms are located at octahedral sites with two coordinated distances: MeOh-MeOh, MeOh-MeTd. The migration behavior of elements in MnO2-Co3O4 diffusion couple was found to manganese move to Co-based and generate a porous MnCo2O4 at the interface. The formation mechanism of spinel-type MnCo2O4 was explained: While Co3O4 was activated, Mn4+ was decomposed to Mn2+ and Mn3+ in the oxidation roasting process. Further, Mn2+ and Mn3+ replaced part of tetrahedral Co2+ and octahedral Co3+ in [Co(2+)][Co(3+)]2O4 to form a spinel-type [Mn(2+)Co(2+)][Mn(3+)Co(3+)]2O4.

Systematic Electronic Tuning on the Property and Reactivity of Cobalt–(Hydro)peroxo Intermediates

A series of cobalt(III)-peroxo complexes, [CoIII(R2-TBDAP)(O2)]+ (1R2; R2 = Cl, H, and OMe), and cobalt(III)-hydroperoxo complexes, [CoIII(R2-TBDAP)(O2H)(CH3CN)]2+ (2R2), bearing electronically tuned tetraazamacrocyclic ligands (R2-TBDAP = N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)-p-R2-pyridinophane) were prepared from their cobalt(II) precursors and characterized by various physicochemical methods. The X-ray diffraction and spectroscopic analyses unambiguously showed that all 1R2 compounds have similar octahedral geometry with a side-on peroxocobalt(III) moiety, but the O-O bond lengths of 1Cl [1.398(3) Å] and 1OMe [1.401(4) Å] were shorter than that of 1H [1.456(3) Å] due to the different spin states. For 2R2, the O-O bond vibration energies of 2Cl and 2OMe were identical at 853 cm-1 (856 cm-1 for 2H), but their Co-O bond vibration frequencies were observed at 572 cm-1 for 2Cl and 550 cm-1 for 2OMe, respectively, by resonance Raman spectroscopy (560 cm-1 for 2H). Interestingly, the redox potentials (E1/2) of 2R2 increased in the order of 2OMe (0.19 V) < 2H (0.24 V) < 2Cl (0.34 V) according to the electron richness of the R2-TBDAP ligands, but the oxygen-atom-transfer reactivities of 2R2 showed a reverse trend (k2: 2Cl < 2H < 2OMe) with a 13-fold rate enhancement at 2OMe over 2Cl in a sulfoxidation reaction with thioanisole. Although the reactivity trend contradicts the general consideration that electron-rich metal-oxygen species with low E1/2 values have sluggish electrophilic reactivity, this could be explained by a weak Co-O bond vibration of 2OMe in the unusual reaction pathway. These results provide considerable insight into the electronic nature-reactivity relationship of metal-oxygen species.

Chromium-doped inverse spinel electrocatalysts with optimal orbital occupancy for facilitating reaction kinetics of lithium-oxygen batteries

Tailored electrocatalysts that can modulate their electronic structure are highly desirable to facilitate the reaction kinetics of oxygen evolution reaction (OER) and oxidation reduction reaction (ORR) in lithium-oxygen batteries (LOB). Although octahedron predominant inverse spinels (e.g., CoFe2O4) have been proposed as promising candidates for catalytic reactions, their performance has remained unsatisfactory. Herein, the chromium (Cr) doped CoFe2O4 nanoflowers (Cr-CoFe2O4) are elaborately constructed on nickel foam as a bifunctional electrocatalyst that drastically improves the performance of LOB. The results show that the partially oxidized Cr6+ stabilizes the cobalt (Co) sites at high-valence and regulates the electronic structure of Co sites, facilitating the oxygen redox kinetics of LOB due to their strong electron-withdrawing capability. Moreover, DFT calculations and ultraviolet photoelectron spectrometer (UPS) results consistently demonstrate that Cr doping optimizes the eg electron filling state of the active octahedral Co sites, significantly improves the covalency of Co-O bonds, and enhances the degree of Co 3d-O 2p hybrids. As a result, Cr-CoFe2O4 catalyzed LOB can achieve low overpotential (0.48 V), high discharge capacity (22030 mA h g−1) and long-term cycling durability (over 500 cycles at 300 mA g−1). This work promotes the oxygen redox reaction and accelerates the electron transfer between Co ions and oxygen-containing intermediates, highlighting the potential of Cr-CoFe2O4 nanoflowers as bifunctional electrocatalysts for LOB.

Influence of Ni2+ substitution by Co2+ on the morphology and magnetic properties of single domain CoαNi0.9-αZn0.1Fe2O4 nanoparticles

This study presents the effect of Ni2+ substitution by Co2+ on the structure, morphology and magnetic properties of CoαNi0.9-αZn0.1Fe2O4 nanoparticles calcined at different temperatures. Fourier-transform infrared spectroscopy revealed the presence of Fe-O, Co-O, Zn-O and Ni-O bonds in samples calcined at 200 °C, while the presence of the characteristic bonds of the SiO2 confirmed the embedding of CoαNi0.9-αZn0.1Fe2O4 at all calcination temperatures. Single phase poorly crystalline ferrite for samples calcined at 400 °C and ferrite accompanied by traces of Fe2SiO4 and SiO2 for samples calcined at high temperatures (800 and 1200 °C) were identified. The dependence of X-ray diffraction parameters on the Co2+ content of nanoparticles was also investigated. The increase in Co2+ content and calcination temperature led to crystallite growth from 4 to 43 nm, while the porosity decreased with the increase in Co2+ content and calcination temperature. The magnetic properties of the rectangular shape nanoparticles evolved proportionally with the particle size. The magnetization, coercivity and magnetocrystalline anisotropy increased with the increase in Co2+ content and calcination temperature. The shape of hysteresis loops indicated a superparamagnetic-like behavior of the nanoparticles calcined at 400 °C and a ferrimagnetic-like behavior at higher calcination temperatures (800 and 1200 °C). These features make the obtained nanoparticles attractive candidates for various technical applications.

Identification of Co–O–Mo Active Centers on Co-Doped MoS2 Electrocatalyst

Strategies for harmonizing the construction of an active site and the building of electron transport for a hybrid MoS2 catalyst are crucial for its application in electrochemical reactions. In this work, an accurate and facile hydrothermal strategy was proposed to fabricate the active center of Co-O-Mo on a supported MoS2 catalyst by forming a CoMoSO phase on the edge of MoS2, yielding (Co-O)x-MoSy (x = 0, 0.3, 0.6, 1, 1.5, or 2.1). The results show that the electrochemical performances (hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and electrochemical degradation) of the yielded MoS2-based catalysts were positively correlated with the Co-O bonds, verifying the significant role of Co-O-Mo as the active center. The fabricated (Co-O)-MoS0.9 presented an extremely low overpotential and Tafel slope in both HER and OER, and it also demonstrated excellent BPA removal in the electrochemical degradation reaction. As compared with the Co-Mo-S configuration, the configuration of Co-O-Mo not only serves as the active center but also provides a conducting channel to facilitate electron conductivity with more accessible charge transfer at the electrode/electrolyte interface, which is favorable for electrocatalytic reaction. This work offers a new perspective for the active mechanism of metallic-heteroatom-dopant electrocatalysts and further boosts research on the development of noble/non-noble hybrid electrocatalysts in the future.

Activation of Co-O bond in (110) facet exposed Co3O4 by Cu doping for the boost of propane catalytic oxidation

Defects engineering in metal oxide is an important avenue for the promotion of VOCs catalytic oxidation. Herein, the influence of crystal facet of Co3O4 is first investigated for the propane oxidation. An intelligent Cu doping is subsequently performed in the most active (110) facet exposed Co3O4 catalyst. The optimized Cu-Co3O4-110–3 catalyst exhibits a prominently enhanced activity with propane conversion rate of 1.9 μmol g−1 s−1 at reaction temperature of 192 °C and the propane mass space velocity of 60,000 mL g−1 h−1, about 2.4 times that of the pristine Co3O4. Systematic experimental characterizations (XAS, EPR, Raman, TPR, XPS, etc.) combined with density functional theory calculations point out that the incorporated Cu could increase the electrophilicity of nearby O atom and implant beneficial defect structures (lattice distortion, coordination unsaturation, abundant oxygen vacancies, etc.), which could significantly activate Co-O bond in Co3O4, leading to the facilitated generation of active oxygen species as well as promoted oxidation ability. This study could set an illuminating paradigm for the boost of the intrinsic oxidation activity by the precise defect construction in Co3O4 catalyst, which will help drive ahead the pursuit of non-precious metal catalyst for VOCs abatement.

Investigation on selective separation of oxygen-containing compounds in lignite and sub-bitumite

To realize the high value-added utilization of low-rank coal and elucidate the selectivity of solvent structure and oxygen-containing compounds in low-rank coal, Zhaotong (ZT) lignite and Naomaohu (NMH) sub-bitumite were subjected to thermal dissolution in three types of oxygen-containing solvents with different properties, namely ethanol (ET), tetrahydrofuran (THF) and ethyl acetate (EA). The soluble portions (SPs) obtained were separated by column chromatography into three types of fractions: hydrocarbon (HY), non-hydrocarbon (NHY) and asphaltene (ASP). The results indicated that the yields of SPs in all three solvents for each coal was ET > THF > EA, and the ratios of column chromatographic fractions of SPs of the two coals were significantly different, with lower asphaltene content in the SPs of NMH. The thermal dissolution process of the three solvents significantly reduced the oxygen content of low-rank coal, and the deoxygenation capacity was in the order of ET > THF > EA. More medium or low polarity compounds could be obtained from NMH than from ZT through thermal dissolution process. ET showed excellent selectivity for the generation of both phenols and esters, where the phenols were mainly alkylphenols. THF could release highly condensed polycyclic compounds from coal thus more naphthalenols containing substituent groups were detected in phenols. And it was more selective to COO bonds in NMH than ZT from the results of XPS. More esters were detected in the SPs of NMH compared with ZT. EA had a strong selectivity for CO in low-rank coals, and the thermal dissolution process released a large number of esters, which could be divided into alkanoates and aromatic acid esters. This study can provide a theoretical basis for extracting high value-added chemicals from low-rank coal under mild conditions.

Semiclassical Dynamics on Machine-Learned Coupled Multireference Potential Energy Surfaces: Application to the Photodissociation of the Simplest Criegee Intermediate.

Determination of high-dimensional potential energy surfaces (PESs) and nonadiabatic couplings have always been quite challenging. To this end, machine learning (ML) models, trained with a finite set of ab initio data, allow accurate prediction of such properties. To express the PESs in terms of atomic contributions is the cornerstone of any ML based technique because it can be easily scaled to large systems. In this work, we have constructed high fidelity PESs and nonadiabatic coupling terms at the CASSCF level of ab initio data using a machine learning technique, namely, kernel-ridge regression. Additional MRCI-level calculations were carried out to assess the quality of the PESs. We use these machine-learned PESs and nonadiabatic couplings to simulate excited-state molecular dynamics based on Tully's fewest-switches surface hopping method (FSSH). FSSH is a semiclassical method in which nuclei move on the PESs due to the electrons according to the laws of classical mechanics. Nonadiabatic effects are taken into account in terms of transitions between PESs. We apply this scheme to study the O-O photodissociation of the simplest Criegee intermediate (CH2OO). The FSSH trajectories were initiated on the lowest optically bright singlet excited state (S2) and propagated along the three most important internal coordinates, namely, O-O and C-O bond distances and the COO bond angle. Some of the trajectories end up on energetically lower PESs as a result of radiationless transfer through conical intersections. All of the trajectories lead to the dissociation of the O-O bond due to the dissociative nature of the excited PESs through one of the two dissociative channels. The simulation reveals that there is about 88.4% probability of dissociation through the lower channel leading to the H2CO (X1A1) and O (1D) products, whereas there is only 11.6% probability of dissociation through the upper channel leading to H2CO (a3A″) and O (3P) products.

Tailoring Metal-Oxygen Bonds Boosts Oxygen Reaction Kinetics for High-Performance Zinc-Air Batteries.

Metal-oxygen bonds significantly affect the oxygen reaction kinetics of metal oxide-based catalysts but still face the bottlenecks of limited cognition and insufficient regulation. Herein, we develop a unique strategy to accurately tailor metal-oxygen bond structure via amorphous/crystalline heterojunction realized by ion-exchange. Compared with pristine amorphous CoSnO3-y, iron ion-exchange induced amorphous/crystalline structure strengthens the Sn-O bond, weakens the Co-O bond strength, and introduces additional Fe-O bond, accompanied by abundant cobalt defects and optimal oxygen defects with larger pore structure and specific surface area. The optimization of metal-oxygen bond structure is dominated by the introduction of crystal structure and further promoted by the introduction of Fe-O bond and rich Co defect. Remarkably, the Fe doped amorphous/crystalline catalyst (Co1-xSnO3-y-Fe0.021-A/C) demonstrates excellent oxygen evolution reaction and oxygen reduction reaction activities with a smaller potential gap (ΔE = 0.687 V), and the Zn-air battery based with Co1-xSnO3-y-Fe0.021-A/C exhibits excellent output power density, cycle performance, and flexibility.

Unravelling the role of oxygen species in toluene oxidation over Co3O4-base catalysts: In situ DRIFTS coupled with quasi in situ XPS

The present work elucidated the role of oxygen species in toluene oxidation over Co3O4-base catalysts. Co3O4 nanoneedles, nanoflowers, nanocubes, and nanoplates (denoted as Co3O4-n, Co3O4-f, Co3O4-c, and Co3O4-p) were employed to anchor Pt nanoparticles by an electrostatic adsorption method. The catalytic activity of the Pt/Co3O4 catalyst for toluene oxidation was significantly enhanced compared to that of Co3O4. Among these catalysts, Pt/Co3O4-p exhibited outstanding catalytic activity with the temperature for 90 % toluene conversion (T90 = 167 °C) at 82 °C lower than Co3O4-p (T90 = 249 °C). The low-temperature reducibility was obviously enhanced by the Pt nanoparticles, which weakened the Co-O bond energy, and improved the capacity of oxygen mobility and the concentration of surface adsorbed oxygen. The results of in situ DRIFTS demonstrated benzoate was identified as the dominant intermediate in toluene oxidation toward Co3O4-based catalysts. Furthermore, combined with the results of quasi in situ XPS showed that oxidizing toluene largely into benzoate primarily by the active adsorbed oxygen species at low reaction temperature (50 °C), mainly following the Langmuir-Hinshelwood mechanism, while oxidizing toluene into CO2 mostly by the lattice oxygen species at high reaction temperature (180–250 °C), largely following the Mars-van Krevelen mechanism.

A theoretical study on the ORR electrocatalytic activity of axial ligand modified cobalt polyphthalocyanine.

In this work, the catalytic activity in the oxygen reduction reaction (ORR) of cobalt polyphthalocyanine whose central Co atom is coordinated at the axial position by ligands (L = -F, -OH, -OCH3, -N3, -Cl, -Br, -I, -SCN, and -CN) (CoPPc-L) was investigated using theoretical calculations in alkaline medium. Among all CoPPc-L, CoPPc-N3 exhibited the lowest ORR overpotential of 0.23 V vs. a standard hydrogen electrode, which is significantly lower than those of CoPPc (0.48 V) and Pt(111) (0.43 V). There is a good linear relationship between ΔG*OOH and the electronegativity of ligating atoms in axial ligands of CoPPc-L. The greater the electronegativity, the stronger the adsorption of the catalyst to the intermediate. Additionally, the adsorption strength of CoPPc to the intermediate is modified by the axial ligands, which adjust the distribution of anti-bonding electronic states of dz2, dxz, and dyz orbitals near the Fermi level, Ef. A larger Mayer bond order of the Co-L bond resulted in a smaller bond order of the Co-O bond. CoPPc-N3 exhibited a moderate Co-O bond order of 0.737, corresponding to moderate adsorption energy to the OOH intermediate. This study demonstrates that the interaction strength between CoPPc and ORR intermediates can be adjusted by selecting appropriate axial ligands, which can modulate the ORR catalytic activity.

Accelerating the reaction kinetics from nitrate to ammonia by anion substitution in NiCo-based catalysts

The electrocatalytic nitrate reduction reaction (NITRR) plays a critical role in maintaining global nitrogen balance, which is strongly depended on the reactive activity of catalysts. Herein, we develop a simple hydrothermal technology to replace the O ions in NiCo2O4 (NCO) by S ions to form NiCo2S4 (NCS), in which the bonding interactions among Ni-O or Co-O bonds are stronger than Ni-S and Co-S bonds. Therefore, more 3d electrons can be localized at Ni or Co sites that can donate more charges to the intermediates. As a result, the ammonia yield can be enhanced to 642 μg h−1 cm−2 and the selectivity can reach 92.42%, which is obviously excellent than NCO. This work provides a new insight into accelerating the electrocatalytic activity by regulating the orbital interaction between anions and cations.

Novel cobalt-based MOF facilitating reclaiming byproduct in wet magnesia desulfurization with ultrahigh dispersity and catalytic activity

In the wet magnesia desulfurization process, magnesium sulfite oxidation is essential to reclaim and resource utilization the byproducts. Therefore, it is of great significance to create reliable and effective catalysts. In this study, a novel and efficient catalyst, Co-MOF (BTC), was synthesized by an ultrasound assisted one-pot method. The oxidation rate of MgSO3 by Co-MOF (BTC) was 0.112 mmol·L−1·s−1, 13–14 times higher than under non-catalytic conditions, with an apparent activation energy of 26.82 kJ·moL−1. The catalytic efficiency is 4–5 times higher than previously reported Co-SBA-15 catalysts with the same loading. The characterization results of XRD, TEM and XPS confirm that the active component cobalt has a high dispersion on TMLA, and Co (II) plays an important role in improving the oxidation performance of catalysts. The results reveal that the rich hydroxyl group on surface of TMLA is conducive to the dispersion of Co for full contact with the reactants, and the Co (II) of Co-MOF (BTC) is favorable to the formation of Co-O bond, and then promotes its oxidative activity. The kinetics of the MgSO3 oxidation catalyzed by the prepared Co-MOF (BTC) was investigated in a bubble oxidation apparatus, indicating that the oxidation rate was dependent on the reaction temperature, pH value, oxygen partial pressure and air velocity.

Thermal properties of the metallic delafossite PdCoO2 : A combined experimental and first-principles study

Metallic delafossite materials (e.g., $\mathrm{PdCo}{\mathrm{O}}_{2}, \mathrm{PtCo}{\mathrm{O}}_{2}$), have attracted much recent attention due to record-high oxide conductivities, the origins of which remain unclear. Relatively little attention has been paid to their related thermal properties, however. We address this here via wide temperature range experimental studies of the crystal structure, thermal expansion, and specific heat of single-crystal $\mathrm{PdCo}{\mathrm{O}}_{2}$, combined with density-functional theory (DFT) calculations of the electronic and phononic densities of states, and thus thermal properties. $\mathrm{PdCo}{\mathrm{O}}_{2}$ is shown to retain the $R\overline{3}m$ space group from 12 to 1000 K, exhibiting a- and c-axis thermal expansion in good quantitative agreement with DFT-based lattice dynamics calculations. The Co-O bond lengths additionally elucidate the stability of the low-spin state of the nominally ${\mathrm{Co}}^{3+}$ ions, which is a notable difference between the edge-shared Co-O octahedra in $\mathrm{PdCo}{\mathrm{O}}_{2}$ and the corner-shared octahedra in Co-based perovskites. Measurements of specific heat from 1.9 to 400 K provide accurate values for the Debye temperature and Sommerfeld coefficient, the phononic part being describable via a combined Debye-Einstein approach (accounting for high-frequency oxygen-related optical phonons), with excess intermediate-temperature specific heat due to a prominent low-energy peak in the phonon density of states. Most significantly, all electronic and phononic contributions to the specific heat are shown to be remarkably closely reproduced by DFT-based calculations, establishing quantitative understanding of key thermal properties of the metallic delafossite $\mathrm{PdCo}{\mathrm{O}}_{2}$.

Deciphering Ligand Controlled Structural Evolution of Prussian Blue Analogues and Their Electrochemical Activation during Alkaline Water Oxidation.

Herein, we have demonstrated the control over the structure of precatalysts to tune the properties of the active catalysts and their water oxidation activity. The reaction of K3 [Fe(CN)6 ] and Na2 [Fe(CN)5 (NO)] with Co(OH)2 @CC produced precatalysts PC-1 and PC-2, respectively, with distinct structural and electronic features. The replacement of the -CN group with strong π-acceptor -NO modulates the electronic and atomic structure of PC-2. As a result, a facile electrochemical transformation of PC-2 into active catalyst Fe-Co(OH)2 -Co(O)OH (AC-2) has been attained only in 15 CV cycles while 600 CV cycles are required for the electrochemical activation of PC-1 into AC-1. The X-ray absorption studies reveal the contraction of the Co-O and Fe-O bond in AC-2 because of the presence of a higher amount of Co3+ and Fe3+ than in AC-1. The high valent Co3+ and Fe3+ modulates the electronic properties of AC-2 and assists in the O-O bond formation, leading to the improved water oxidation activity.

Identifying the geometric catalytic active sites of crystalline cobalt oxyhydroxides for oxygen evolution reaction

Unraveling the precise location and nature of active sites is of paramount significance for the understanding of the catalytic mechanism and the rational design of efficient electrocatalysts. Here, we use well-defined crystalline cobalt oxyhydroxides CoOOH nanorods and nanosheets as model catalysts to investigate the geometric catalytic active sites. The morphology-dependent analysis reveals a ~50 times higher specific activity of CoOOH nanorods than that of CoOOH nanosheets. Furthermore, we disclose a linear correlation of catalytic activities with their lateral surface areas, suggesting that the active sites are exclusively located at lateral facets rather than basal facets. Theoretical calculations show that the coordinatively unsaturated cobalt sites of lateral facets upshift the O 2p-band center closer to the Fermi level, thereby enhancing the covalency of Co-O bonds to yield the reactivity. This work elucidates the geometrical catalytic active sites and enlightens the design strategy of surface engineering for efficient OER catalysts.

Mn-induced strengthening hybridization effect of Co–O bond for stable oxygen evolution in acidic media

The development of transition metal-based acidic OER electrocatalyst is an important step to realize large-scale hydrogen production in low-cost proton exchange membrane (PEM) electrolytic cell. However, the current challenge is how to overcome the tendency of transition metal oxides dissolving in acidic environment to improve the long-term stability. Here, we prepared a trace amount of Mn-doped cobalt spinel oxide by simple room temperature ion exchange using a special Co-MOF precursor (CoBDC). Detailed physical characterizations combined with electrochemical tests showed that the introduction of infinitesimal Mn caused more particle cavities but did not destroy the crystal structure and intrinsic activity of cobalt spinel oxide. Due to the strengthening hybridization effect of Mn-induced Co-O bond, the stability of Mn0.08-Co3O4-400 is 4–5 times higher than that of Co3O4-400 in strong acidic environment. In addition, the confinement effect of specific metal–organic coordination structure of CoBDC also ensured the dispersity and stability of the highly active cobalt sites. In brief, this work provides an effective strategy for the preparation of stable and efficient non-noble metal-base acidic OER electrocatalysts.

Facile electrochemically induced vacancy modulation of NiCo2O4 cathode toward high-performance aqueous Zn-based battery

Transition metal oxides are highly promising cathode materials in rechargeable Zn-based batteries owing to their low cost, high theoretical capacity and good reversibility. Nevertheless, intrinsically low conductivity and sluggish redox reaction kinetics normally result in their inferior specific capacity and poor rate capability, which critically constrains the practical performance of Zn-based batteries. Herein, an ultrafast and controllable electrochemical activation (EA) process is developed to effectively enhance the electrochemical performance of NiCo2O4 cathode. Systematic studies reveal EA process triggers in situ reconstruction of NiCo2O4 lattice by weakening the coordination of Co-O bonds, resulting in the formation of abundant oxygen vacancies (octahedral Co2+). These oxygen vacancies increase the charge carrier density and endow the superficial metallic active sites with higher electrochemical activity, which synergistically accelerates the electrochemical reaction kinetics. The oxygen-deficient NiCo2O4 exhibits a remarkable capacity of 418.9 mAh/g at 1 A/g, which is nearly 5-fold higher than that of pristine NiCo2O4. Furthermore, the oxygen-deficient NiCo2O4//Zn battery presents an extremely high energy density (682.4 Wh kg−1) and excellent power density (50.8 kW kg−1), surpassing most of the reported aqueous rechargeable batteries. This work provides a facile and effective vacancy modulation strategy for the development of advanced materials utilized in energy and catalysis fields.