Solution-Processed CoFe2O4 Nanoparticles on 3D Carbon Fiber Papers for Durable Oxygen Evolution Reaction

A Green Synthesis of CoFe2O4 Decorated ZIF-8 Composite for Electrochemical Oxygen Evolution

Low-cost, sustainable hydrogen production requires noble metal-free electrocatalysts for water splitting. In this study, we prepared zeolitic imidazolate frameworks (ZIF) decorated with CoFe₂O₄ spinel nanoparticles as active catalysts for oxygen evolution reaction (OER). The CoFe₂O₄ nanoparticles were synthesized by converting agricultural bio-waste (potato peel extract) into economically valuable electrode materials. The biogenic CoFe₂O₄ composite showed an overpotential of 370 mV at a current density of 10 mA cm^-2 and a low Tafel slope of 283 mV dec^-1, whereas the ZIF@CoFe₂O₄ composite prepared using an in situ hydrothermal method showed an overpotential of 105 mV at 10 mA cm^-2 and a low Tafel slope of 43 mV dec^-1 in a 1 M KOH medium. The results demonstrated an exciting prospect of high-performance noble metal-free electrocatalysts for low-cost, high-efficiency, and sustainable hydrogen production.

Recent Progress on Bimetallic-Based Spinels as Electrocatalysts for the Oxygen Evolution Reaction

Electrocatalytic water splitting is a promising and viable technology to produce clean, sustainable, and storable hydrogen as an energy carrier. However, to meet the ever-increasing global energy demand, it is imperative to develop high-performance non-precious metal-based electrocatalysts for the oxygen evolution reaction (OER), as the OER is considered the bottleneck for electrocatalytic water splitting. Spinels, in particular, are considered promising OER electrocatalysts due to their unique properties, precise structures, and compositions. Herein, the recent progress on the application of bimetallic-based spinels (AFe₂ O₄ , ACo₂ O₄ , and AMn₂ O₄ ; where A = Ni, Co, Cu, Mn, and Zn) as electrocatalysts for the OER is presented. The fundamental concepts of the OER are highlighted after which the family of spinels, their general formula, and classifications are introduced. This is followed by an overview of the various classifications of bimetallic-based spinels and their recent developments and applications as OER electrocatalysts, with special emphasis on enhancing strategies that have been formulated to improve the OER performance of these spinels. In conclusion, this review summarizes all studies mentioned therein and provides the challenges and future perspectives for bimetallic-based spinel OER electrocatalysts.

Synthesis and Characterisation of Cobalt Ferrite Coatings for Oxygen Evolution Reaction

In this paper, two novel procedures based on powder sedimentation, thermal treatment, and galvanostatic deposition were proposed for the preparation of porous cobalt ferrite (CoFe2O4) coatings with a metallic and organic binder for use as catalysts in the oxygen evolution reaction (OER). The electrochemical properties of the obtained electrode materials were determined as well, using both dc and ac methods. It was found that cobalt ferrite coatings show excellent electrocatalytic properties towards the oxygen evolution reaction (OER) with overpotential measured at a current density of 10 mAcm−2 from 287 to 295 mV and a Tafel slope of 35–45 mVdec−1. It was shown that the increase in the apparent activity of the CoFe2O4 coatings with an organic binder results mainly from a large electrochemically active area. Incorporation of the nickel binder between the CoFe2O4 particles causes an increase in both the conductivity and the electrochemically active area. The Tafel slopes indicate that the same rate-determining step controls the OER for all obtained coatings. Furthermore, it was shown that the CoFe2O4 electrodes exhibit no significant activity decrease after 28 h of oxygen evolution. The proposed coating preparation procedures open a new path to develop high-performance OER electrocatalysts.

Microwave-Assisted Auto-Combustion Synthesis of Binary/Ternary Co x Ni1-x Ferrite for Electrochemical Hydrogen and Oxygen Evolution

Enormous efforts have been dedicated to engineering low-cost and efficient electrocatalysts for both hydrogen evolution and oxygen evolution reactions (HER and OER, respectively). For this, the current contribution reports the successful synthesis of binary/ternary metal ferrites (Co x Ni1-x Ferrite; x = 0.0, 0.1, 0.3, 0.5, 0.7, and 1.0) by a simple one-step microwave technique and subsequently discusses its chemical and electrochemical properties. The X-ray diffraction analysis substantiated the phase purity of the as-obtained catalysts with various compositions. Additionally, the morphology of the nanoparticles was identified via transmission electron microscopy. Further, the vibrating sample magnetometer justified the ferromagnetic character of the as-prepared products. The electrochemical measurements revealed that the as-prepared materials required the overpotentials of 422-600 and 419-467 mV for HER and OER, respectively, to afford current densities of 10 mA cm-2. In the general sense, Ni cation substitution with Co influenced favorably toward both HER and OER. Among all synthesized electrocatalysts, Co0.9Ni0.1Ferrite displayed the highest performance in terms of OER in 1 M KOH solution, which is related to the synergistic effect of multiple parameters including the optimal substitution amount of Co, the highest Brunauer-Emmett-Teller surface area, the smallest particle size among all samples (26.71 nm), and the lowest charge transfer resistance. The successful synthesis of ternary ferrites carried out for the first time via a microwave-assisted auto-combustion route opens up a new path for their applications in renewable energy technologies.

CoFe2O4-Quantum Dots for Synergistic Photothermal/Photodynamic Therapy of Non-small-Cell Lung Cancer Via Triggering Apoptosis by Regulating PI3K/AKT Pathway

Non-small-cell lung cancer (NSCLC) has become the second most diagnosed malignant tumors worldwide. As our long-term interests in seeking nanomaterials to develop strategies of cancer therapies, we herein constructed novel CoFe2O4-quantum dots (QDs) with outstanding synergistic photothermal/photodynamic property which suppressed NSCLC efficiently without apparent toxicity. We showed that the combination of CoFe2O4-QDs + NIR treatment induces apoptosis of NSCLC cells. In addition, the CoFe2O4-QDs + NIR treatment also promotes reactive oxygen species generation to trigger cell death through regulating PI3K/AKT pathway. Moreover, the CoFe2O4-QDs + NIR treatment successfully eliminates tumor xenografts in vivo without apparent toxic effects. Taken together, we reported that the novel nanomaterials CoFe2O4-QDs could exhibit enhanced synergistic photothermal therapy and photodynamic therapy effect on killing NSCLC without toxicity, which could be a promising photosensitizer for NSCLC therapy.

Morphological and Elemental Investigations on Co–Fe–B–O Thin Films Deposited by Pulsed Laser Deposition for Alkaline Water Oxidation: Charge Exchange Efficiency as the Prevailing Factor in Comparison with the Adsorption Process

Abstract

Mixed transition-metals oxide electrocatalysts have shown huge potential for electrochemical water oxidation due to their earth abundance, low cost and excellent electrocatalytic activity. Here we present Co–Fe–B–O coatings as oxygen evolution catalyst synthesized by Pulsed Laser Deposition (PLD) which provided flexibility to investigate the effect of morphology and structural transformation on the catalytic activity. As an unusual behaviour, nanomorphology of 3D-urchin-like particles assembled with crystallized CoFe2O4 nanowires, acquiring high surface area, displayed inferior performance as compared to core–shell particles with partially crystalline shell containing boron. The best electrochemical activity towards water oxidation in alkaline medium with an overpotential of 315 mV at 10 mA/cm2 along with a Tafel slope of 31.5 mV/dec was recorded with core–shell particle morphology. Systematic comparison with control samples highlighted the role of all the elements, with Co being the active element, boron prevents the complete oxidation of Co to form Co3+ active species (CoOOH), while Fe assists in reducing Co3+ to Co2+ so that these species are regenerated in the successive cycles. Thorough observation of results also indicates that the activity of the active sites play a dominating role in determining the performance of the electrocatalyst over the number of adsorption sites. The synthesized Co–Fe–B–O coatings displayed good stability and recyclability thereby showcasing potential for industrial applications.

Graphic Abstract

Earth abundant transition metal ferrite nanoparticles anchored ZnO nanorods as efficient and stable photoanodes for solar water splitting

Poor light absorption, severe surface charge recombination and fast degradation are the key challenges with ZnO nanostructures based electrodes for photoelectrochemical (PEC) water splitting. Here, this study attempts to design an efficient and durable nano-heterojunction photoelectrode by integrating earth abundant chemically stable transition metal spinel ferrites MFe₂O₄ (M = Co and Ni) nano-particles on ZnO Nanorod arrays. The low band gap magnetic ferrites improve the solar energy harvesting ability of the nano-heterojunction electrodes in ultraviolet-visible light region resulting in a maximum increase of 105% and 190% in photocurrent density and applied bias photon-to-current efficiency, respectively, compared to pristine ZnO nanorods. The favourable type-II band alignment at the ferrites/ZnO nano-heterojunction provides significantly enhanced photo-generated carrier separation and transfer, endowing the excellent solar H₂ evolution ability (743 and 891 μmol cm^-2 h^-1for ZnO/CoFe₂O₄ and ZnO/NiFe₂O₄, respectively) of the photoanodes by using sacrificial agent. The hybrid nanostructures deliver long term stability of the electrode against photocorrosion. This work demonstrates an easy but effective strategy to develop low-cost earth abundant ferrites-based heterojunction electrodes, which offers excellent PEC activity and stability.

Oxygen-Defect-Rich Cobalt Ferrite Nanoparticles for Practical Water Electrolysis with High Activity and Durability

The scope of any metal oxide as a catalyst for driving electrocatalytic reactions depends on its electronic structure, which is correlated to its oxygen-defect density. Likewise, to transform a spinel oxide, such as cobalt ferrite (CoFe2 O4 ), into a worthy universal-pH, bifunctional electrocatalyst for the hydrogen and oxygen evolution reactions (HER and OER, respectively), oxygen defects need to be regulated. Prepared by coprecipitation and inert calcination at 650 °C, CoFe2 O4 nanoparticles (NPs) require 253 and 300 mV OER overpotentials to reach current densities of 10 and 100 mA cm-2 , respectively, if nickel foam is used as a substrate. With cost-effective carbon fiber paper, the OER overpotential increases to 372 mV at 10 mA cm-2 at pH 14. The NPs prepared at 550 °C require HER overpotentials of 218, 245, and 314 mV at -10 mA cm-2 in alkaline, acidic, and neutral pH, respectively. The intrinsic activity is reflected from turnover frequencies of >3 O2  s-1 and >5 H2  s-1 at overpotentials of 398 and 259 mV, respectively. If coupled for overall water splitting, the extremely durable two-electrode electrolyzer requires a cell potential of only 1.63 V to reach 10 mA cm-2 at pH 14. The homologous couple also splits seawater at impressively low cell voltages of 1.72 and 1.47 V at room temperature and 80 °C, respectively.

Holey Cobalt-Iron Nitride Nanosheet Arrays as High-Performance Bifunctional Electrocatalysts for Overall Water Splitting

Designing efficient metal nitride electrocatalysts with advantageous nanostructures toward overall water splitting is of great significance for energy conversion. In this work, holey cobalt-iron nitride nanosheet arrays grown on Ni foam substrate (CoFeN_x HNAs/NF) are prepared via a facile hydrothermal and subsequent thermal nitridation method. This unique HNA architecture can not only expose abundant active sites but also facilitate the charge/mass transfer. Resulting from these merits, the CoFeN_x HNAs/NF exhibits high catalytic performance with overpotentials of 200 and 260 mV at 10 mA cm^-2 for the hydrogen evolution reaction (HER) and 50 mA cm^-2 for the oxygen evolution reaction (OER), respectively. Furthermore, when using CoFeN_x-500 HNAs/NF as both anode and cathode, the alkaline electrolyzer could afford a current density of 10 mA cm^-2 at 1.592 V, higher than many other metal nitride-based electrocatalysts. This work signifies a simple approach to prepare holey metal nitride nanosheet arrays, which can be applied in various fields of energy conversion and storage.

Recent Developments for Aluminum–Air Batteries

Abstract

Environmental concerns such as climate change due to rapid population growth are becoming increasingly serious and require amelioration. One solution is to create large capacity batteries that can be applied in electricity-based applications to lessen dependence on petroleum. Here, aluminum–air batteries are considered to be promising for next-generation energy storage applications due to a high theoretical energy density of 8.1 kWh kg−1 that is significantly larger than that of the current lithium-ion batteries. Based on this, this review will present the fundamentals and challenges involved in the fabrication of aluminum–air batteries in terms of individual components, including aluminum anodes, electrolytes and air cathodes. In addition, this review will discuss the possibility of creating rechargeable aluminum–air batteries.

Graphic Abstract

Designing transition-metal-boride-based electrocatalysts for applications in electrochemical water splitting

Investigating renewable and clean energy materials as alternatives to fossil fuels can be foreseen as a potential solution to the global problems of energy shortages and environmental pollution. Recently, transition metal boride (TMB)-based materials have emerged as the rising star as efficient electrocatalysts for hydrogen evolution reaction (HER) and/or oxygen evolution reaction (OER). In this review, an overview of the most recent developments in the use of TMB-based materials as electrocatalysts for HER/OER or overall water splitting has been presented. Initially, we provide a comprehensive introduction of the fundamentals of electrochemical water splitting. Then, the synthesis approaches of TMB materials are summarized and compared. Emphasis is put on the various strategies for further improving the electrocatalytic performance of TMBs. Finally, challenges and future perspectives for TMBs in water-splitting applications are proposed.

Ultrasonically Induced Sulfur-Doped Carbon Nitride/Cobalt Ferrite Nanocomposite for Efficient Sonocatalytic Removal of Organic Dyes

The sulfur-doped carbon nitride/cobalt ferrite nanocomposite (SCN/CoFe2O4) was prepared via ultrasonication and studied for the sonocatalytic degradation of wastewater organic dye pollutants including methylene blue, rhodamine B, and Congo red. The X-ray photoelectron spectroscopy confirmed the presence and atomic ratios of S, C, N, Co, Fe, and O elements and their corresponding bonds with Co2+ and Fe3+ cations. The nanocomposite was found to have aggregated nanoparticles on a sheet-like structure. The bandgap energy was estimated to be 1.85 eV. For the sonocatalytic degradation of 25-ppm methylene blue at 20 kHz, 1 W and 50% amplitude, the best operating condition was determined to be 1 g/L of catalyst dosage and 4 vol % of hydrogen peroxide loading. Under this condition, the sonocatalytic removal efficiency was the highest at 96% within a reaction period of 20 min. SCN/CoFe2O4 outperformed SCN and CoFe2O4 by 2.2 and 6.8 times, respectively. The SCN/CoFe2O4 nanocomposite was also found to have good reusability with a drop of only 7% after the fifth cycle. However, the degradation efficiencies were low when tested with rhodamine B and Congo red due to difference in dye sizes, structural compositions, and electric charges.

Tuning the Electrocatalytic Activity of Co3O4 through Discrete Elemental Doping

To gain constructive insight into the possible effect of doping on the electrocatalytic activity of materials, a catalytic framework with a discrete distribution of dopants is an appropriate model system. Such a system assures well-defined active centers, maximum atom utilization efficiency, and hence enhanced selectivity, catalytic activity, and stability. Herein, a comprehensive investigation of the electrocatalytic activity of iron-doped cobalt oxide (Fe-Co₃O₄) nanosheets is presented. In order to understand the contribution of dopants, a series of materials with controlled doping levels are investigated. By controlled iron inclusion into the structure of Co₃O₄, an apparent improvement in the oxygen evolution reaction activity which is reflected in the decrease of 160 mV in the overpotential to reach the current density of 10 mA/cm² is manifested. Additionally, it is shown that there exists an optimum doping content above which the catalytic activity fades. Further investigation of the system with density functional calculations reveals that, along with the optimization of adsorption energy toward the reaction intermediates, substantial downshift of the Fermi level and delocalization of electron density occurs on introducing iron ions into the structure.

Attuning the Electronic Properties of Two-Dimensional Co-Fe-O for Accelerating Water Electrolysis and Photolysis

Two-dimensional (2D) materials such as layered double hydroxides (LDH) are promising electrocatalysts, especially for water oxidation, owing to their unique physical and electronic properties besides having adequate surface area and availability of unsaturated active metal centers. Herein, we illustrate the high-temperature transformation of bimetallic LDH to semicrystalline 2D metal oxide nanoplates that can maneuver their electronic properties and thereby accelerate the water dissociation reactions. The nanoplates prepared at 300 °C require only 280 ± 13 and 177 ± 7 mV overpotentials for oxygen/hydrogen evolution reactions (OER and HER) to achieve a current density of ±10 mA cm^-2 in 1 M KOH, respectively. In a two-electrode water splitting cell, while this bifunctional catalyst needs 1.69 V to deliver a current density of 10 mA cm^-2, the LDH precursor demands a cell voltage of 1.93 V. However, both the catalysts demonstrate excellent durability for more than 200 h. When the bifunctional metal oxide electrolyzer is connected to perovskite solar cells for unassisted solar-driven water splitting, impressively, such an integrated photovoltaic-electrolyzer can achieve a solar-to-hydrogen (STH) efficiency of 9.3%. The predominantly superior catalytic activity of the nanoplates is due to the abundance of unsaturated oxygen which decreases the free energy of adsorption of the intermediates.

Zirconium-Regulation-Induced Bifunctionality in 3D Cobalt-Iron Oxide Nanosheets for Overall Water Splitting

The design of high-efficiency non-noble bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is paramount for water splitting technologies and associated renewable energy systems. Spinel-structured oxides with rich redox properties can serve as alternative low-cost OER electrocatalysts but with poor HER performance. Here, zirconium regulation in 3D CoFe₂ O₄ (CoFeZr oxides) nanosheets on nickel foam, as a novel strategy inducing bifunctionality toward OER and HER for overall water splitting, is reported. It is found that the incorporation of Zr into CoFe₂ O₄ can tune the nanosheet morphology and electronic structure around the Co and Fe sites for optimizing adsorption energies, thus effectively enhancing the intrinsic activity of active sites. The as-synthesized 3D CoFeZr oxide nanosheet exhibits high OER activity with small overpotential, low Tafel slope, and good stability. Moreover, it shows unprecedented HER activity with a small overpotential of 104 mV at 10 mA cm^-2 in alkaline media, which is better than ever reported counterparts. When employing the CoFeZr oxides nanosheets as both anode and cathode catalysts for overall water splitting, a current density of 10 mA cm^-2 is achieved at the cell voltage of 1.63 V in 1.0 m KOH.

Engineering Ternary Copper-Cobalt Sulfide Nanosheets as High-performance Electrocatalysts toward Oxygen Evolution Reaction

The rational design and development of the low-cost and effective electrocatalysts toward oxygen evolution reaction (OER) are essential in the storage and conversion of clean and renewable energy sources. Herein, a ternary copper-cobalt sulfide nanosheets electrocatalysts (denoted as CuCoS/CC) for electrochemical water oxidation has been synthesized on carbon cloth (CC) via the sulfuration of CuCo-based precursors. The obtained CuCoS/CC reveals excellent electrocatalytic performance toward OER in 1.0 M KOH. It exhibits a particularly low overpotential of 276 mV at current density of 10 mA cm−2, and a small Tafel slope (58 mV decade−1), which is superior to the current commercialized noble-metal electrocatalysts, such as IrO2. Benefiting from the synergistic effect of Cu and Co atoms and sulfidation, electrons transport and ions diffusion are significantly enhanced with the increase of active sites, thus the kinetic process of OER reaction is boosted. Our studies will serve as guidelines in the innovative design of non-noble metal electrocatalysts and their application in electrochemical water splitting

Boosting the Oxygen Evolution Reaction Activity of NiFe2O4 Nanosheets by Phosphate Ion Functionalization

Here, we demonstrate an effective strategy to constitutionally increase the conductivity and electrocatalytic property of NiFe₂O₄ by phosphate ion functionalization. The phosphate-ion-modified NiFe₂O₄ (P-NiFe₂O₄) nanosheets are readily grown on a carbon cloth by a simple hydrothermal method and followed by a phosphating process. The introduction of phosphate ions on the NiFe₂O₄ surface is highly beneficial for increasing the charge transport rate and electrocatalytic active sites. As a result, the as-prepared P-NiFe₂O₄ nanosheets show outstanding electrocatalytic activity toward oxygen evolution reaction (OER), with a low overpotential (231 mV at 10 mA/cm²) and Tafel slope (49 mV/dec). Furthermore, the P-NiFe₂O₄ electrode has a remarkable stability with no activity fading after 50 h. In addition, the as-fabricated water electrocatalysts exhibit excellent flexibility at the foldable state. These features make the phosphate-ion-functionalized NiFe₂O₄ electrodes open a new way to develop OER electrocatalysts with high electrochemical property.

Electrodeposited Nanostructured CoFe2O4 for Overall Water Splitting and Supercapacitor Applications

To contribute to solving global energy problems, a multifunctional CoFe2O4 spinel was synthesized and used as a catalyst for overall water splitting and as an electrode material for supercapacitors. The ultra-fast one-step electrodeposition of CoFe2O4 over conducting substrates provides an economic pathway to high-performance energy devices. Electrodeposited CoFe2O4 on Ni-foam showed a low overpotential of 270 mV and a Tafel slope of 31 mV/dec. The results indicated a higher conductivity for electrodeposited compared with dip-coated CoFe2O4 with enhanced device performance. Moreover, bending and chronoamperometry studies suggest excellent durability of the catalytic electrode for long-term use. The energy storage behavior of CoFe2O4 showed high specific capacitance of 768 F/g at a current density of 0.5 A/g and maintained about 80% retention after 10,000 cycles. These results demonstrate the competitiveness and multifunctional applicability of the CoFe2O4 spinel to be used for energy generation and storage devices.

MnMoO4 nanosheet array: an efficient electrocatalyst for hydrogen evolution reaction with enhanced activity over a wide pH range

We report the preparation of MnMoO₄ nanosheet array on nickel foam (MnMoO₄ NSA/NF) as an excellent 3D hydrogen evolution reaction (HER) electrocatalyst with good catalytic performance applied under basic, acidic and neutral conditions. In 0.5 M H₂SO₄, this MnMoO₄ NSA/NF electrode needs an overpotential of 89 mV to drive current densities of 10 mA cm^-2, to achieve the same current density, it demands overpotentials of 105 mV in 1.0 M KOH, 161 mV in 1.0 M PBS (pH = 7), respectively. After continuous CV scanning for 1000 cycles under different pH conditions, it also demonstrates an excellent stability with ignorable activity decrease. Such preeminent HER performance may be derived from the synergistic effect between manganese (Mn) and molybdenum (Mo) atoms, exposure of more active sites on the nanosheets and effective electron transport along the nanosheets. This MnMoO₄ NSA/NF electrocatalyst provides us a highly efficient material for water splitting devices for industrial hydrogen production.

Enhanced Visible-Light-Driven Photocatalytic Bacterial Inactivation by Ultrathin Carbon-Coated Magnetic Cobalt Ferrite Nanoparticles

Ultrathin hydrothermal carbonation carbon (HTCC)-coated cobalt ferrite (CoFe₂O₄) composites with HTCC coating thicknesses between 0.62 and 4.38 nm were fabricated as novel, efficient, and magnetically recyclable photocatalysts via a facile, green approach. The CoFe₂O₄/HTCC composites showed high magnetization and low coercivity, which favored magnetic separation for reuse. The results show that the close coating of HTCC on CoFe₂O₄ nanoparticles enhanced electron transfer and charge separation, leading to a significant improvement in photocatalytic efficiency. The composites exhibited superior photocatalytic inactivation toward Escherichia coli K-12 under visible-light irradiation, with the complete inactivation of 7 log₁₀ cfu·mL^-1 of bacterial cells within 60 min. The destruction of bacterial cell membranes was monitored by field-effect scanning electron microscopy analysis and fluorescence microscopic images. The bacterial inactivation mechanism was investigated in a scavenger study, and ^•O₂, H₂O₂, and h⁺ were identified as the major reactive species for bacterial inactivation. Multiple cycle runs revealed that these composites had excellent stability and reusability. In addition, the composites showed good photocatalytic bacterial inactivation performance in authentic water matrices such as surface water samples and secondarily treated sewage effluents. The results of this work indicate that CoFe₂O₄/HTCC composites have great potential in large-scale photocatalytic disinfection operations.

Highly efficient oxygen evolution electrocatalysts prepared by using reduction-engraved ferrites on graphene oxide

A simple and efficient method was explored for synthesizing efficient ferrite-based OER electrocatalysts by using reduction-engraved ultrafine ferrite nanoparticles on a conductive GO support.

Crab-shell induced synthesis of ordered macroporous carbon nanofiber arrays coupled with MnCo2O4 nanoparticles as bifunctional oxygen catalysts for rechargeable Zn-air batteries

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are traditionally carried out using noble metals (such as Pt) and metal oxides (such as RuO₂ and IrO₂) as catalysts, respectively. Nevertheless, several key issues such as high cost, poor stability, and detrimental environmental effects limit the catalytic activity of these noble metal- and metal oxide-based catalysts. Herein, we have designed and synthesized macroporous carbon nanofiber arrays by using a natural crab shell template. Subsequently, spinel MnCo₂O₄ nanoparticles were embedded into the nitrogen-doped macroporous carbon nanofiber arrays (NMCNAs) by a hydrothermal method. Accompanied by the good conductivity, large surface area and doping of nitrogen, the as-prepared MnCo₂O₄/NMCNA exhibited remarkable catalytic performance and outstanding stability for both ORR and OER in alkaline media. The macroporous superstructures play vital role in reducing the ion transport resistance and facilitating the diffusion of gaseous products (O₂). Finally, rechargeable Zn-air batteries using the MnCo₂O₄/NMCNA catalyst displayed appreciably lower overpotentials, higher power density and better stability than commercial Pt/C, thus raising the prospect of functional low-cost, non-precious-metal bifunctional catalysts in metal-air batteries.

Position Assignment and Oxidation State Recognition of Fe and Co Centers in Heterometallic Mixed-Valent Molecular Precursors for the Low-Temperature Preparation of Target Spinel Oxide Materials

A series of mixed-valent, heterometallic (mixed-transition metal) diketonates that can be utilized as prospective volatile single-source precursors for the low-temperature preparation of M_xM'_3-xO₄ spinel oxide materials is reported. Three iron-cobalt complexes with Fe/Co ratios of 1:1, 1:2, and 2:1 were synthesized by several methods using both solid-state and solution reactions. On the basis of nearly quantitative reaction yields, elemental analyses, and comparison of metal-oxygen bonds with those in homometallic analogues, heterometallic compounds were formulated as [Fe^III(acac)₃][Co^II(hfac)₂] (1), [Co^II(hfac)₂][Fe^III(acac)₃][Co^II(hfac)₂] (2), and [Fe^II(hfac)₂][Fe^III(acac)₃][Co^II(hfac)₂] (3). In the above heteroleptic complexes, the Lewis acidic, coordinatively unsaturated Co^II/Fe^II centers chelated by two hexafluoroacetylacetonate (hfac) ligands maintain bridging interactions with oxygen atoms of acetylacetonate (acac) groups that chelate the neighboring Fe^III metal ion. Preliminary assignment of Fe and Co positions/oxidation states in 1-3 drawn from X-ray structural investigation was corroborated by a number of complementary techniques. Single-crystal resonant synchrotron diffraction and neutron diffraction experiments unambiguously confirmed the location of Fe and Co sites in the molecules of dinuclear (1) and trinuclear (2) complexes, respectively. Direct analysis in real time mass spectrometry revealed the presence of Fe^III- and Co^II-based fragments in the gas phase upon evaporation of precursors 1 and 2 as well as of Fe^III, Fe^II, and Co^II species for complex 3. Theoretical investigation of two possible "valent isomers", [Fe^III(acac)₃][Co^II(hfac)₂] (1) and [Co^III(acac)₃][Fe^II(hfac)₂] (1'), provided an additional support for the metal site/oxidation state assignment giving a preference of 6.48 kcal/mol for the experimentally observed molecule 1. Magnetic susceptibility measurements data are in agreement with the presence of high-spin Fe^III and Co^II magnetic centers with weak anti-ferromagnetic coupling between those in molecules of 1 and 2. Highly volatile heterometallic complexes 1-3 were found to act as effective single-source precursors for the low-temperature preparation of iron-cobalt spinel oxides Fe_xCo_3-xO₄ known as important materials for diverse energy-related applications.

Unprecedented Activity of Bifunctional Electrocatalyst for High Power Density Aqueous Zinc-Air Batteries

The development of nonprecious metal catalysts with desirable bifunctional activities to supersede noble metal catalysts is of vital importance for high performance aqueous zinc-air batteries. Here, an unprecedented activity of bifunctional electrocatalyst is reported by in situ growth of nitrogen-enriched carbon nanotubes with transition metal composite. The resultant catalyst delivers surprisingly high OER (potential@10 mA cm^-2 of 1.58 V) and ORR (onset potential of 0.97 V, half-wave potential of 0.86 V) performance. The overall oxygen electrode activity (overvoltage between ORR and OER) of the catalyst is as low as 0.72 V. In aqueous Zn-air battery tests, primary batteries demonstrate high maximum power density and two-electrode rechargeable batteries also exhibit good cycle performance. The unprecedented electrocatalyst opens up new avenues for developing highly active nitrogen-doped carbon nanotube-supported electrocatalysts and offers prospects for the next generation of fuel cells, metal-air batteries, and photocatalysis applications.

Core-shell CoFe2O4@Co-Fe-Bi nanoarray: a surface-amorphization water oxidation catalyst operating at near-neutral pH

The exploration of high-performance and earth-abundant water oxidation catalysts operating under mild conditions is highly attractive and challenging. In this communication, core-shell CoFe₂O₄@Co-Fe-Bi nanoarray on carbon cloth (CoFe₂O₄@Co-Fe-Bi/CC) was successfully fabricated by in situ surface amorphization of CoFe₂O₄ nanoarray on CC (CoFe₂O₄/CC). As a 3D water oxidation electrode, CoFe₂O₄@Co-Fe-Bi/CC shows outstanding activity with an overpotential of 460 mV to drive a geometrical catalytic current density of 10 mA cm^-2 in 0.1 M potassium borate (pH 9.2). Notably, it also demonstrates superior long-term durability for at least 20 h with 96% Faradic efficiency. Density functional theory calculations indicate that the conversion from OOH* to O₂ is the rate-limiting step and the high water oxidation activity of CoFe₂O₄@Co-Fe-Bi/CC is associated with the lower free energy of 1.84 eV on a Co-Fe-Bi shell.

Surface Electrochemical Modification of a Nickel Substrate to Prepare a NiFe-based Electrode for Water Oxidation

The slow kinetics of water oxidation greatly jeopardizes the efficiency of water electrolysis for H₂ production. Developing highly active water oxidation electrodes with affordable fabrication costs is thus of great importance. Herein, a Ni^II Fe^III surface species on Ni metal substrate was generated by electrochemical modification of Ni in a ferrous solution by a fast, simple, and cost-effective procedure. In the prepared Ni^II Fe^III catalyst film, Fe^III was incorporated uniformly through controlled oxidation of Fe^II cations on the electrode surface. The catalytically active Ni^II originated from the Ni foam substrate, which ensured the close contact between the catalyst and the support toward improved charge-transfer efficiency. The as-prepared electrode exhibited high activity and long-term stability for electrocatalytic water oxidation. The overpotentials required to reach water oxidation current densities of 50, 100, and 500 mA cm^-2 are 276, 290, and 329 mV, respectively.