Simultaneous modulation of surface composition, oxygen vacancies and assembly in hierarchical Co3O4 mesoporous nanostructures for lithium storage and electrocatalytic oxygen evolution

Spontaneous redox reaction-mediated interfacial charge transfer in titanium dioxide/graphene oxide nanoanodes for rapid and durable lithium storage

Titanium dioxide (TiO2) anodes show significant advantages in ion storage owing to their low cost, abundant sources, and small volume change during cycling. However, their intrinsic low electronic conductivity and sluggish ion diffusion coefficient restrict the application of TiO2 anodes, especially at high current densities. The construction of a covalently-bonded interface in TiO2-based composite anodes is an effective approach to solve these issues. Covalent bonds are usually formed in situ during materials synthesis processes, such as high-energy ball milling, solvothermal reactions, plasma-assisted thermal treatment, and addition of a linking agent for covalent coupling. In this study, we demonstrate that a spontaneous redox reaction between defective TiO2 powder and an oxidative graphene oxide (GO) substate can be used to form interfacial covalent bonds in composites. Different structural characterization techniques confirmed the formation of interfacial covalent bonds. Electrochemical measurements on an optimized sample showed that a specific capacity of 281.3 mA h g-1 after 200 cycles can be achieved at a current density of 1 C (1 C = 168 mA g-1). Even at a high rate of 50 C, the electrode maintained a reversible capacity of 97.0 mA h g-1. The good lithium storage performance of the electrode is a result of the uniquely designed composite electrodes with strong interfacial chemical bonds.

Electrocatalytic activity of electrodeposited CoOx thin film on low-carbon unalloyed steel substrate toward electrochemical oxygen evolution reaction (OER)

In this study, we report elaboration of a thin film of CoOx on a low carbon unalloyed steel substrate by electrochemical route and the study of its electrocatalytic performances with respect to the evolution reaction of oxygen (OER) in NaOH medium. The elaborated deposits were well-characterized using X-ray diffraction. Kinetic and thermodynamic parameters such as exchange current density, Tafel slope, reaction order with respect to OH- ions and apparent activation energy were studied. The CoOx displays satisfactory OER performance in an alkaline medium, with a low overvoltage of 362 mV at 10 mA/cm2 and a Tafel slope of 81 mV/dec at 293 K. The apparent kinetic activation energy (= 29.79 kJ/mol) was similar to those obtained for the reported catalytic electrode materials. The O2 gas obtained on the cobalt oxide electrode was 2.865 mmol/s.cm2, which is 28 times higher than that obtained for the platinum electrode (0.102 mmol/s.cm2). Chronoamperometry demonstrates a better electrochemical stability under a polarization potential of 2 V in 1 M NaOH for nearly 25 h. The low cost, the high OER performance, as well as the good stability of the CoOx electrode make it a promising candidate for the industrial-scale water electrolysis.

Defective Metal Oxides: Lessons from CO2 RR and Applications in NOx RR

Sluggish reaction kinetics and the undesired side reactions (hydrogen evolution reaction and self-reduction) are the main bottlenecks of electrochemical conversion reactions, such as the carbon dioxide and nitrate reduction reactions (CO₂ RR and NO₃ RR). To date, conventional strategies to overcome these challenges involve electronic structure modification and modulation of the charge-transfer behavior. Nonetheless, key aspects of surface modification, focused on boosting the intrinsic activity of active sites on the catalyst surface, are yet to be fully understood. Engingeering of oxygen vacancies (OVs) can tune surface/bulk electronic structure and improve surface active sites of electrocatalysts. The continuous breakthroughs and significant progress in the last decade position engineering of OVs as a potential technique for advancing electrocatalysis. Motivated by this, the state-of-the-art findings of the roles of OVs in both the CO₂ RR and the NO₃ RR are presented. The review starts with a description of approaches to constructing and techniques for characterizing OVs. This is followed by an overview of the mechanistic understanding of the CO₂ RR and a detailed discussion on the roles of OVs in the CO₂ RR. Then, insights into the NO₃ RR mechanism and the potential of OVs on NO₃ RR based on early findings are highlighted. Finally, the challenges in designing CO₂ RR/NO₃ RR electrocatalysts and perspectives in studying OV engineering are provided.

In situ grown hierarchical NiO nanosheet@nanowire arrays for high-performance electrochromic energy storage applications

Electrodes with hierarchical nanoarchitectures could promote electrochemical properties due to their largely exposed active sites and quick charge transfer. Herein, in situ grown hierarchical NiO nanosheet@nanowire films are reported by a one-step hydrothermal process followed by heat treatment. The unique NiO hierarchical nanostructures, which are composed of NiO nanowires grown on the surface of a nanosheet array, show improved electrochromic properties such as large optical modulation in different light regions (95% at 550 nm and 50.6% at 1000 nm), fast color change (9.8/5.4 s) and better coloring efficiency (91.2 cm² C^-1) with long-term cycling properties (82.2% after 700 cycles). Simultaneously, the hierarchical nanostructures possess optimal areal capacitance (117.2 mF cm^-2), rate performance and cycling properties. The enhanced electrochemical properties are due to the pretreated seed layer and the synergistic effect between the unique in situ grown ultrathin nanowire and the underlying vertical nanosheet layer which can strengthen the mechanical adhesion of the nanoarray film to the substrate and make both nanosheets and nanowires more exposed to the electrolyte, enhancing charge transfer and mass diffusion. This work provides a promising pathway towards developing high quality electrochromic energy storage devices.

Precisely Tunable Synthesis of Binder-Free Cobalt Oxide-Based Li-Ion Battery Anode Using Scalable Electrothermal Waves

Binder-free transition metal oxide-based anodes for lithium-ion batteries (LIBs), having high capacity and abundance, have received considerable attention. However, their low conductivity and unstable charge-discharge cycles must be addressed, and scalable fabrication routes for binder-free design with optimal phase tuning are necessary. Herein, we report a precisely tunable synthesis of binder-free cobalt oxide-based LIB anodes using scalable electrothermal waves. Needle-like nanoarrays of cobalt hydroxide on nickel foams are prepared as precursors, and Joule-heating-driven electrothermal waves passing through the metal foams cause transition to cobalt oxides with preserved structures and adjustable phase tuning of grains and oxygen vacancies. The rapid heating-cooling environment using electrothermal waves causes extreme input thermal energy over the activation energy of phase transitions and metastable phase trapping. This programmable route completes the selective grain characteristics and vacancy concentrations. The electrothermally tuned binder-free LIB anodes employing the CoO/Co₃O₄@Ni foam-based electrodes exhibit a high-rate capacity (3.7 mAh cm^-2) at 2.4 mA cm^-2 for 70 charge-discharge cycles. Accumulated electrothermal waves from multiple cycles broaden the tunable ranges of the morphological and chemical transitions causing rapid screening of the optimal phases for LIB anodes. This phase-tuning strategy will inspire precise yet efficient synthesis routes for diverse binder-free electrodes and catalysts.

Hydrazine Hydrate Induced Three-Dimensional Interconnected Porous Flower-like 3D-NiCo-SDBS-LDH Microspheres for High-Performance Supercapacitor

Porous structure and surface defects are important to improve the performance of supercapacitors. In this study, a facile pathway was developed for high-performance supercapacitors, which can produce transition metal hydroxides (LDHs) with abundant porous structure and surface defects. The NiCo-SDBS-LDH was prepared by one-step hydrothermal reaction using sodium dodecylbenzene sulfonate (SDBS) as anionic surfactant. And then, three dimensional (3D) interconnected porous flower-like 3D-NiCo-SDBS-LDH microspheres were designed and synthesized using the gas-phase hydrazine hydrate reduction method. Results showed that the hydrazine hydrate reduction not only introduces a large number of pores into 3D-NiCo-SDBS-LDH microspheres and causes the formation of oxygen vacancies, but it also roughens the surface of the microspheres. All these changes contribute to the enhancement of electrochemical activity of 3D-NiCo-SDBS-LDH; the NiCo-SDBS-LDH electrode after hydrazine hydrate treatment (3D-NiCo-SDBS-LDH) exhibits a higher specific capacitance of 1148 F·g^-1 at 1 A·g^-1 (about 1.46 times larger than that of NiCo-SDBS-LDH) and excellent long cycle life with 94% retention after 4000 cycles. Moreover, the assembled 3D-NiCo-SDBS-LDH//AC (active carbon) asymmetric supercapacitor (ASC) achieves remarkable energy density of 73.14 W h·kg^-1 at 800 W·kg^-1 and long-term cycling stability of 95.5% retention for up to 10,000 cycles. The outstanding electrochemical performance can be attributed to the synergy between the rich porous structure and the roughened surface that has been created by the hydrazine hydrate treatment.

Gold Catalyst Anchored to Pre-Reduced Co3O4 Nanorods for the Hydrodeoxygenation of Vanillin Using Alcohols as Hydrogen Donors

The preparation of highly dispersed metal catalysts with strong electronic metal-support interactions (EMSIs) is of great significance. In this study, oxygen vacancies (OVs) were generated on the surfaces of Co₃O₄ nanorods (NRs) through NaBH₄ treatment, and then the generated surface OVs were used to anchor gold clusters. The resulting catalyst was used for the hydrodeoxygenation (HDO) of vanillin based on transfer hydrogenation with alcohol donors. The conversion of vanillin and the selectivity to 2-methoxy-4-methylphenol (MMP) both reached 99% under the optimized reaction conditions, and these values were significantly higher than those obtained for the gold catalyst supported on the untreated Co₃O₄ NRs. The obtained results were verified by theoretical calculations and experimental data and confirmed the existence of strong EMSIs between the OV-enriched Co₃O₄ NRs (Co₃O₄ NRs-OVs) and the gold clusters, which allows electron transfer from the Co₃O₄ NRs to gold. Increasing the number of electrons on the gold surface can promote the catalytic hydrogen transfer of alcohol, in addition to selectively adsorbing the C═O group in vanillin to improve the selectivity toward MMP. This strategy based on the OV-anchoring of metals onto the surface of a support can be extended to other metals, thereby providing a promising method for the design of advanced and highly efficient metal catalysts.

Recent Development of Flexible and Stretchable Supercapacitors Using Transition Metal Compounds as Electrode Materials

Flexible and stretchable supercapacitors (FS-SCs) are promising energy storage devices for wearable electronics due to their versatile flexibility/stretchability, long cycle life, high power density, and safety. Transition metal compounds (TMCs) can deliver a high capacitance and energy density when applied as pseudocapacitive or battery-like electrode materials owing to their large theoretical capacitance and faradaic charge-storage mechanism. The recent development of TMCs (metal oxides/hydroxides, phosphides, sulfides, nitrides, and selenides) as electrode materials for FS-SCs are discussed here. First, fundamental energy-storage mechanisms of distinct TMCs, various flexible and stretchable substrates, and electrolytes for FS-SCs are presented. Then, the electrochemical performance and features of TMC-based electrodes for FS-SCs are categorically analyzed. The gravimetric, areal, and volumetric energy density of SC using TMC electrodes are summarized in Ragone plots. More importantly, several recent design strategies for achieving high-performance TMC-based electrodes are highlighted, including material composition, current collector design, nanostructure design, doping/intercalation, defect engineering, phase control, valence tuning, and surface coating. Integrated systems that combine wearable electronics with FS-SCs are introduced. Finally, a summary and outlook on TMCs as electrodes for FS-SCs are provided.

Advances in hydrogen production from electrocatalytic seawater splitting

As one of the most abundant resources on the Earth, seawater is not only a promising electrolyte for industrial hydrogen production through electrolysis, but also of great significance for the refining of edible salt. Despite the great potential for large-scale hydrogen production, the implementation of water electrolysis requires efficient and stable electrocatalysts that can maintain high activity for water splitting without chloride corrosion. Recent years have witnessed great achievements in the development of highly efficient electrocatalysts toward seawater splitting. Starting from the historical background to the most recent achievements, this review will provide insights into the current state, challenges, and future perspectives of hydrogen production through seawater electrolysis. In particular, the mechanisms of overall water splitting, key features of seawater electrolysis, noble-metal-free electrocatalysts for seawater electrolysis and the underlying mechanisms are also highlighted to provide guidance for fabricating more efficient electrocatalysts toward seawater splitting.

N-Doped Hierarchically Porous CNT@C Membranes for Accelerating Polysulfide Redox Conversion for High-Energy Lithium-Sulfur Batteries

To improve the structural design of electrodes and interlayers for practical applications of Li-S batteries, we report two scalable porous CNT@C membranes for high-energy Li-S batteries. The asymmetric CNT@C (1:2) membrane with both dense and macroporous layers can act as an Al-free cathode for current collection and high sulfur loading, while the symmetric CNT@C (1:1) membrane with hierarchically porous networks can be used as an interlayer to trap lithium polysulfides (LiPSs), thus weakening the shuttle effect by strong adsorption of the N atoms toward LiPSs. The doped N sites in carbon membranes are identified as bifunctional active centers that electrocatalytically accelerate the oxidation of Li₂S and polysulfide conversion. First-principles calculations reveal that the pyridinic and pyrrolic N sites exhibit favorable reactivity for strong adsorption/dissociation of polysulfide species. They lead to greatly reduced energy and kinetic barrier for polysulfide conversion without weakening the polysulfide adsorption on the membrane. Using the synergistic circulation groove with the two membranes, the practical S loading can be tailored from 1.2 to 6.1 mg cm^-2. The Li-S battery can deliver an areal capacity of 4.6 mA h cm^-2 (684 mA h g^-1) at 0.2 C even at an ultrahigh S loading of 6.1 mg cm^-2 and a lean electrolyte to sulfur ratio of 5.3 μL mg^-1. Our work for scalable membrane fabrication and structural design provides a promising strategy for practical applications of high-energy Li-S batteries.

Function and Application of Defect Chemistry in High-Capacity Electrode Materials for Li-Based Batteries

Current commercial Li-based batteries are approaching their energy density limitation, yet still cannot satisfy the energy density demand of the high-end devices. Hence, it is critical to developing advanced electrode materials with high specific capacity. However, these electrode materials are facing challenges of severe structural degradation and fast capacity fading. Among various strategies, constructing defects in electrode materials holds great promise in addressing these issues. Herein, we summarize a series of significant defect engineering in the high-capacity electrode materials for Li-based batteries. The detailed retrospective on defects specification, function mechanism, and corresponding application achievements on these electrodes are discussed from the view of point, line, planar, volume defects. Defect engineering can not only stabilize the structure and enhance electric/ionic conductivity, but also act as active sites to improve the ionic storage and bonding ability of electrode materials to Li metal. We hope this review can spark more perspectives on evaluating high-energy-density Li-based batteries.

Scalable High-Areal-Capacity Li-S Batteries Enabled by Sandwich-Structured Hierarchically Porous Membranes with Intrinsic Polysulfide Adsorption

The key to realizing practical applications of Li-S batteries lies in scalable fabrication of cathode materials with high sulfur-loading and strong binding of lithium polysulfides (LiPSs). We report a scalable CeO₂-CNT@C porous membrane with a large porosity of 90%. Introducing CNTs is critical to increase the porosity and construct porous networks with CNTs as the skeleton and CeO₂-doped carbon as the shell. The macropores can improve the transport of Li⁺ and electrolyte, while the porous networks possess high polysulfide-adsorbing and electron-transferring ability. The CeO₂-CNT@C membrane can serve as an Al foil-free cathode and an interlayer for Li-S batteries. Moreover, CeO₂ can immobilize LiPSs and can alleviate its shuttle effect. The Li-S batteries with a sulfur loading of 6.2 mg cm^-2 deliver a capacity of 847 mA h g^-1 after 100 cycles, showing a high areal capacity of 5.25 mA h cm^-2 at a low electrolyte/sulfur ratio of 5.2 μL mg^-1.

In-situ construction of Bi/defective Bi4NbO8Cl for non-noble metal based Mott-Schottky photocatalysts towards organic pollutants removal

The strategy to improve the photocatalytic performance is still a challenge for the novel Sillen-Aurivillius perovskite type Bi₄NbO₈Cl. Herein, heterostructured Bi/Bi₄NbO₈Cl was fabricated via in-situ solvothermal method, without the additional introduction of Bi-sources. Simultaneously, the amount of oxygen vacancies (OVs) were increased, as the [Bi₂O₂] blocks released in the solvothermal process to serve as precursors for Bi particles. Due to the large work function of Bi, a Schottky barrier formed at the Bi/Bi₄NbO₈Cl interface, promoting photo-induced charge separation generated in the Bi₄NbO₈Cl semiconductor, supplying more holes for the organic compounds decomposition, which could be widely applied in water decontamination. Furthermore, the OVs facilitate the consumption of photo-induced electrons by assisting oxygen activation to produce superoxide radicals (·O₂^-), leaving more holes in the valence band of Bi₄NbO₈Cl, and thus result in the enhancement of Rhodamine B (RhB) degradation by 1.82 times over Bi/Bi₄NbO₈Cl photocatalysts. Through the synergistic effect of Bi and OVs, the Bi/Bi₄NbO₈Cl also exhibits enhanced photocatalytic performance towards various organic water-contaminants, such as methyl orange, acid orange 7, p-nitrophenol and tetracycline hydrochloride.

Ultrafast and Stable Lithium Storage Enabled by the Electric Field Effect in Layer-Structured Tablet-Like NH4TiOF3 Mesocrystals

Design and synthesis of advanced electrode materials with fast and stable ion storage are of importance for energy storage applications. Herein, we propose that introducing the heterogeneous interface in layer-structured mesocrystals is an efficient way to greatly improve the rate capability and cycle stability of lithium-ion battery (LIB) devices. NH₄TiOF₃ mesocrystals were employed as a typical model system to demonstrate the idea. The NH₄TiOF₃ mesocrystals were obtained via the hydrothermal reaction, and the NH₄TiOF₃/TiO₂ interfaces were generated through calcining at different temperatures under an argon atmosphere. Phase composition, microstructure, and chemical analyses show that the as-prepared NH₄TiOF₃ mesocrystals possess "tablet-like" morphology, and the formation of the NH₄TiOF₃/TiO₂ interface can be controlled by the calcination temperature. When evaluated as the anode for LIBs, the optimized sample (NH₄TiOF₃ calcined at 250 °C, NTF-250) shows excellent, fast, and stable lithium storage properties. Specifically, the NTF-250 electrode holds a reversible capacity of 159.5 mA h g^-1 after 200 cycles at 0.2 A g^-1. At a high current density of 20 A g^-1, the electrode still maintains a reversible capacity of 89.6 mA h g^-1 and reaches a reversible capacity of 128.6 mA h g^-1 at a current density of 1 A g^-1 after 2000 cycles. Theoretical and experimental studies show that the synergistic effects of the heterogeneous NH₄TiOF₃/anatase TiO₂ interface in the layer-structured NH₄TiOF₃ mesocrystals lead to the upgraded electrochemical properties. Especially, the local build-in electric field induced by the nonuniform distribution of charge across the NH₄TiOF₃/anatase TiO₂ interface facilitates the charge transport during the charging and discharging cycling. The current electrode design strategy paves a new way in boosting stable ion storage and thus is of great interest in energy storage and conversion.

Mechanistic Investigation into Efficient Water Oxidation by Co-Ni-Based Hybrid Oxide-Hydroxide Flowers

Oxides are envisioned as promising catalysts to facilitate water oxidation, and the benign presence of hydroxide moieties can further enhance the catalyst performance. However, the nature of synergy between oxides and hydroxides remains elusive. In this study, we have designed a one-pot solution growth technique for the synthesis of flower-shaped N-doped-C-enveloped NiCo₂O₄/Ni_xCo_(1-x)(OH)_y catalysts with varying oxide and hydroxide contents and investigated their water oxidation behavior. The correlation between performance-determining parameters involved in water oxidation, such as the onset potential and overpotential with oxide and/or hydroxide content, oxidation states (oxides), and elemental composition (Co/Ni content), and the possible ways to achieve their optimal values are discussed in detail. Our observations conclude that the onset potential and overpotential are minimal for the hybrid oxide-hydroxide bimetallic system compared with pristine hydroxide or oxide. The optimal hybrid catalyst shows excellent current density, low Tafel slope (82 mV/dec), and low onset potential (281 mV at 2 mA/cm²) and overpotential (348 mV at 10 mA/cm²), besides enduring operational stability in alkaline medium. The low Tafel slope suggests the preferable kinetics for water oxidation, and the poisoning study reveals the direct involvement of metal as active sites. The overall study unveils the synergy in the Co-Ni-based binary transition-metal oxide-hydroxide hybrid, which makes it a potential candidate for water oxidation catalysts, and hence, it is expected that the hybrid will find applications in energy conversion devices, such as electrolyzers.

Improved electrochemical performance of 2D accordion-like MnV2O6 nanosheets as anode materials for Li-ion batteries

MnV₂O₆ is a promising anode material for lithium ion batteries with high theoretical specific capacity, abundant reserves and inexpensive constituent elements.

MXene-supported Co3O4quantum dots for superior lithium storage and oxygen evolution activities

A novel hybrid, composed of Co₃O₄quantum dots supported on Ti₃C₂T_x(MXene) nanosheets, exhibits a strong synergetic effect, and shows superior lithium storage (capacity = 766.5 mA h g⁻¹at 2 A g⁻¹after 400 cycles) and oxygen evolution (overpotential = 340 mV at 10 mA cm⁻²) activities.

Room temperature Mg reduction of TiO2: formation mechanism and application in photocatalysis

A Mg/HCl infiltrated metal oxide structure was designed as a facile approach for implanting oxygen vacancies and H atoms into metal oxides.

Oxygen vacancies enhance lithium storage performance in ultralong vanadium pentoxide nanobelt cathodes

Ultralong V₂O₅ nanobelts have been successfully synthesized by a facile hydrothermal oxidation route. Oxygen vacancies are generated in the V₂O₅ nanobelts by annealing under N₂ atmosphere at an elevated temperature. The microstructure and chemical composition of the pristine and annealed V₂O₅ nanobelts are studied by different methods. Compared to the pristine V₂O₅ nanobelts, the annealed V₂O₅ nanobelts sample possesses a higher reversible capacity of 177.8 mAhg^-1 after 50 cycles at a current density of 0.3 Ag^-1, corresponding to ∼0.27% capacity loss per cycle. At a higher current density of 1.2 Ag^-1, the reversible capacity of annealed V₂O₅ electrode can reach 128.5 mAhg^-1, which is two times larger than that of pristine V₂O₅ electrode. Ultralong flexible morphology together with oxygen vacancies in the annealed V₂O₅ electrode is considered to be responsible for the enhanced lithium storage properties.

Simultaneous edge and electronic control of MoS2 nanosheets through Fe doping for an efficient oxygen evolution reaction

Electronic regulation and structural engineering at the atomic level of electrocatalysts is of vital importance to a highly efficient oxygen evolution reaction (OER). Here we report Fe-doped MoS2 (Fe-MoS2) nanosheets in which the Fe dopant influences the synthesis process to simultaneously tune the edge sites and electronic properties of MoS2. Impressively, the Fe-MoS2 nanosheets exhibit greatly improved catalytic activity towards the OER, achieving a current density of 50 mA cm-2 at 290 mV and satisfactory durability in 1.0 M KOH. Transmission electron microscopy analysis confirms that the Fe-MoS2 nanosheets undergo an atomic-scale structural evolution from horizontally aligned planes to vertically aligned planes, maximally exposing their edge sites. Density functional theory calculations reveal that the substitution of Fe at the Mo site in MoS2 has the lowest formation energy, offering a low overpotential of 328 mV for the OER.

Self-supported nickel-cobalt nanowires as highly efficient and stable electrocatalysts for overall water splitting

The development and design of highly active and stable electrocatalysts based on cheap and Earth-abundant materials is critically important to enable water splitting as a desirable renewable energy source. Herein, we fulfill the significant electrochemical water splitting enhancement in both electrocatalytic activity and durability by constructing self-supported nickel-cobalt nanowire catalysts with abundant oxygen vacancies. Specifically, the rich oxygen vacancies can largely promote the oxygen evolution reaction (OER) activity of optimal Ni₁Co₁O₂ NWs with a relatively low overpotential of 248 mV to drive a current density of 10 mA cm^-2. More significantly, after the phosphorization of Ni₁Co₁O₂ NWs, the resultant Ni₁Co₁P NWs can also display excellent electrocatalytic hydrogen evolution reaction (HER) performances with an overpotential of only 101 mV to achieve a current density of 10 mA cm^-2. Furthermore, benefiting from the unique 1D nanowire structure, the synergistic effect, and the optimal Gibbs free energy for hydrogen evolution evolved from the phosphorization, the Ni₁Co₁O₂ NWs//Ni₁Co₁P NWs couple is thus highly active and stable for overall water electrolysis with a low voltage of 1.58 V at 10 mA cm^-2, showing extraordinary promise for practical overall water splitting electrolysis.

Engineering Mesoporous NiO with Enriched Electrophilic Ni3+ and O− toward Efficient Oxygen Evolution

Tremendous efforts have been devoted to develop low-cost and highly active electrocatalysts for oxygen evolution reaction (OER). Here, we report the synthesis of mesoporous nickel oxide by the template method and its application in the title reaction. The as-prepared mesoporous NiO possesses a large surface area, uniform mesopores, and rich surface electrophilic Ni3+ and O− species. The overpotential of meso-NiO in alkaline medium is 132 mV at 10 mA cm−1 and 410 mV at 50 mA cm−1, which is much smaller than that of the other types of NiO samples. The improvement in the OER activity can be ascribed to the synergy of the large surface area and uniform mesopores for better mass transfer and high density of Ni3+ and O− species favoring the nucleophilic attack by OH− to form a NiOOH intermediate. The reaction process and the role of electrophilic Ni3+ and O− were discussed in detail. This results are more conducive to the electrochemical decomposition of water to produce hydrogen fuel as a clean and renewable energy.

Porous Co3O4/SnO2 quantum dot (QD) heterostructures with abundant oxygen vacancies and Co2+ ions for highly efficient gas sensing and oxygen evolution reaction

Porous Co3O4/SnO2 quantum dot (QD) heterojunctions with a strong synergistic effect are successfully synthesized in this paper. Owing to the strong synergistic effect between Co3O4 and SnO2QDs, Co3O4/SnO2QD heterostructures possess more Co2+ ions for a faster Co2+/Co0 redox reaction in the process of sensing of reducing gases and electrochemical reactions, and more oxygen vacancies for more active sites and reduced charge transfer resistance on the surface. These advantages are demonstrated to significantly enhance the gas sensitivity to xylene and greatly improve the catalysis for the oxygen evolution reaction (OER). As a catalyst for the OER, Co3O4/SnO2QD (1 : 1) heterostructures exhibit the highest current density, lowest onset potential, largest active surface area and remarkable durability in alkaline electrolytes. The sensitivity of Co3O4/SnO2QD (1 : 1) heterostructures to 100 ppm xylene is almost 10 times higher than that of pure Co3O4 nanosheets and 3 times higher than that of SnO2QDs. In addition, Co3O4/SnO2QD (1 : 1) heterostructure sensors exhibit excellent gas selectivity, long-term stability and markedly high response to low concentrations of xylene at low operating temperatures.

Three-dimensional iron sulfide-carbon interlocked graphene composites for high-performance sodium-ion storage

Three-dimensional (3D) carbon-wrapped iron sulfide interlocked graphene (Fe7S8@C-G) composites for high-performance sodium-ion storage are designed and produced through electrostatic interactions and subsequent sulfurization. The iron-based metal-organic frameworks (MOFs, MIL-88-Fe) interact with graphene oxide sheets to form 3D networks, and carbon-wrapped iron sulfide (Fe7S8@C) nanoparticles with high individual-particle conductivity are prepared following a sulfurization process, surrounded by interlocked graphene sheets to enhance the interparticle conductivity. The prepared Fe7S8@C-G composites exhibit not only improved individual-particle and interparticle conductivity to shorten electron/ion diffusion pathways, but also enhanced structural stability to prevent the aggregation of active materials and buffer large volume changes during sodiation/desodiation. As a sodium-ion storage material, the Fe7S8@C-G composites exhibit a reversible capacity of 449 mA h g-1 at 500 mA g-1 after 150 cycles and a retention capacity of 306 mA h g-1 under a current density of 2000 mA g-1. The crucial factors related to the structural changes and stability during cycles have been further investigated. These results demonstrate that the high-performance sodium-ion storage properties are mainly attributed to the uniquely designed three-dimensional configuration.