Cobalt-manganese-based spinels as multifunctional materials that unify catalytic water oxidation and oxygen reduction reactions

Template-free Synthesis of Stable Cobalt Manganese Spinel Hollow Nanostructured Catalysts for Highly Water-Resistant CO Oxidation

Development of spinel oxides as low-cost and high-efficiency catalysts is highly desirable; however, rational synthesis of efficient and stable spinel systems with precisely controlled structure and components remains challenging. We demonstrate the design of complex nanostructured cobalt-based bimetallic spinel catalysts for low-temperature CO oxidation by a simple template-free method. The self-assembled multi-shelled mesoporous spinel nanostructures provide high surface area (203.5 m2/g) and favorable unique surface chemistry for producing abundant active sites and lead to the creation of robust microsphere configured by 16-nm spinel nanosheets, which achieve satisfactory water-resisting property and catalytic activity. Theoretical models show that O vacancies at exposed {110} facets in cubic spinel phase guarantee the strong adsorption of reactive oxygen species on the surface of catalysts and play a key role in the prevention of deactivation under moisture-rich conditions. The design concept with architecture and composition control can be extended to other mixed transition metal oxide compositions.

Verchenko V, Shevelkov AV, Driess M. Boosting Water Oxidation through In Situ Electroconversion of Manganese Gallide: An Intermetallic Precursor Approach

For the first time, the manganese gallide (MnGa4 ) served as an intermetallic precursor, which upon in situ electroconversion in alkaline media produced high-performance and long-term-stable MnOx -based electrocatalysts for water oxidation. Unexpectedly, its electrocorrosion (with the concomitant loss of Ga) leads simultaneously to three crystalline types of MnOx minerals with distinct structures and induced defects: birnessite δ-MnO2 , feitknechtite β-MnOOH, and hausmannite α-Mn3 O4 . The abundance and intrinsic stabilization of MnIII /MnIV active sites in the three MnOx phases explains the superior efficiency and durability of the system for electrocatalytic water oxidation. After electrophoretic deposition of the MnGa4 precursor on conductive nickel foam (NF), a low overpotential of 291 mV, comparable to that of precious-metal-based catalysts, could be achieved at a current density of 10 mA cm-2 with a durability of more than five days.

Highly Active Core-Shell Carbon/NiCo2 O4 Double Microtubes for Efficient Oxygen Evolution Reaction: Ultralow Overpotential and Superior Cycling Stability

Developing highly efficient electrocatalysts with earth abundant elements for oxygen evolution reaction (OER) is a promising way to store light or electrical energy in the form of chemical energy. Here, a new type of electrocatalyst with core-shell carbon/NiCo₂ O₄ double microtubes architecture is successfully synthesized through a hydrothermal method combined with the calcination process with wet tissues as the template and carbon resource. The outer NiCo₂ O₄ nanosheet arrays contain abundant defects, which come from reduction of the carbon in wet tissues. This indicates that carbon is a very excellent defect inducer. The inner carbon microtubes can act as the robust structure skeleton and these core-shell double microtubes provide abundant diffusion channels for oxygen and electrolyte, both of which contribute to improving the stability by avoiding damage to the electrode from produced O₂ bubbles and the collapse of the outer NiCo₂ O₄ microtubes. Electrochemical results show that the electrode, core-shell carbon/NiCo₂ O₄ double microtubes loaded on carbon cloth, exhibits prominent electrocatalytic activity with an overpotential of only 168 mV at 10 mA cm^-2 and a Tafel slope as low as 57.6 mV dec^-1 in 1.0 mol L^-1 KOH. This new type of electrocatalyst possesses great potential in water electrolyzers and rechargeable metal-air batteries.

Simplistic synthesis of ultrafine CoMnO3 nanosheets: An excellent electrocatalyst for highly sensitive detection of toxic 4-nitrophenol in environmental water samples

Design and fabrication of cost effective analytical tools to monitor toxic organic emissions in eco system is of a great necessity. Nitrophenols are a class of widespread toxic organic pollutant lead to serious adverse effects in biosphere on its consumption. This article reports a high sensitive, cost effective, robust electrochemical sensor for 4-nitrophenol (4-NP) in environmental water samples. A novel sheet like CoMnO₃ (CMO Ns) nanocatalyst was synthesized via oxalic acid assisted co-precipitation technique and employed as electrocatalyst for the high sensitive detection of 4-NP. The physiochemical properties of CMO Ns are studied in detail via XRD, FTIR, TEM, TGA, and XPS. TEM results reviled the protocol is an excellent way for synthesis of a uniformly distributed CMO Ns with lathery surface. Evident to the surface and other physiochemical studies the CMO Ns based sensor holds superior electrocatalytic activity towards 4-NP detection with excellent sensitivity (2.458 μA μM^-1 cm^-2) coupled with nanomolar detection (10 nm) limits. Moreover, the constructed sensor holds reliable long-term durability, good reproducibility, and excellent working stability. The practical applicability of the developed sensor was evaluated by determination of 4-NP in samples acquired from water resources with RSD ± 3.3%.

A, Paul A. Tuning water oxidation reactivity by employing surfactant directed synthesis of porous Co3O4 nanomaterials

We have explored Co₃O₄ based nanomaterials for the oxygen evolution reaction prepared via a surfactant directed soft-templating strategy.

Nickel-Based Bicarbonates as Bifunctional Catalysts for Oxygen Evolution and Reduction Reaction in Alkaline Media

Oxygen electrocatalysis, including the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), is one of the most important electrochemical processes for sustainable energy conversion and storage technologies. Herein, nickel-based bicarbonates are, for the first time, developed as catalysts for oxygen electrocatalysis, and demonstrate superior electrocatalytic performance in alkaline media. Iron doping can significantly tune the real valence of nickel ions, and consequently tailor the electrocatalytic ability of bicarbonates. Among the nickel-based bicarbonates, Ni_0.9 Fe_0.1 (HCO₃ )₂ exhibits the highest bifunctional catalytic activity, with a potential difference of 0.86 V between the OER potential at a current density of 10 mA cm^-2 and the ORR potential at a current density of -1 mA cm^-2 , which outperforms most of the reported precious-metal-free catalysts. The present work provides new insights into exploring efficient catalysts for oxygen electrocatalysis, and it suggests that, in addition to the extensively studied transition metal hydroxides and oxides, bicarbonates and carbonates also show great potential as precious metal-free catalysts.

Oxygen Reduction Reaction Electrocatalysis in Alkaline Electrolyte on Glassy-Carbon-Supported Nanostructured Pr6O11 Thin-Films

In this work, hierarchical nanostructured Pr6O11 thin-films of brain-like morphology were successfully prepared by electrostatic spray deposition (ESD) on glassy-carbon substrates. These surfaces were used as working electrodes in the rotating disk electrode (RDE) setup and characterized in alkaline electrolyte (0.1 M NaOH at 25 ± 2 °C) for the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR) for their potential application in alkaline electrolyzers or in alkaline fuel cells. The electrochemical performances of these electrodes were investigated as a function of their crystallized state (amorphous versus crystalline). Although none of the materials display spectacular HER and OER activity, the results show interesting performances of the crystallized sample towards the ORR with regards to this class of non-Pt group metal (non-PGM) electrocatalysts, the activity being, however, still far from a benchmark Pt/C electrocatalyst.

Photochemical Water Oxidation in a Buffered Tris(2,2'-bipyridyl)ruthenium-Persulfate System Using Iron(III)-Modified Potassium Manganese Oxides as Catalysts

Study of manganese oxides for electrocatalytic and photocatalytic oxidation of water is an active area of research. The starting material in this study is a high-surface-area disordered birnessite-like material with K⁺ in the interlayers (KMnOx). Upon ion-exchange with Fe³⁺, the disordered layer structure collapses (Fe(IE)MnOx), and the surface area is slightly increased. Structural analysis of the Fe(IE)MnOx included examination of its morphology, crystal structure, vibrational spectra, and manganese oxidation states. Using the Ru(bpy)₃ ²⁺-persulfate system, the dissolved and headspace oxygen upon visible light photolysis with highly dispersed Fe(IE)MnOx was measured. The photocatalytic activity for O₂ evolution of the Fe(IE)MnOx was three times better than KMnOx, with the highest rate being 9.3 mmol_O2 mol_Mn ^-1 s^-1. The improvement of the photocatalytic activity was proposed to arise from the increased disorder and interaction of Fe³⁺ with the MnO₆ octahedra. As a benchmark, colloidal IrO₂ was a better photocatalyst by a factor of ∼75 over Fe(IE)MnOx.

Metal Organic Framework Derived Materials: Progress and Prospects for the Energy Conversion and Storage

Exploring new materials with high efficiency and durability is the major requirement in the field of sustainable energy conversion and storage systems. Numerous techniques have been developed in last three decades to enhance the efficiency of the catalyst systems, control over the composition, structure, surface area, pore size, and moreover morphology of the particles. In this respect, metal organic framework (MOF) derived catalysts are emerged as the finest materials with tunable properties and activities for the energy conversion and storage. Recently, several nano- or microstructures of metal oxides, chalcogenides, phosphides, nitrides, carbides, alloys, carbon materials, or their hybrids are explored for the electrochemical energy conversion like oxygen evolution, hydrogen evolution, oxygen reduction, or battery materials. Interest on the efficient energy storage system is also growing looking at the practical applications. Though, several reviews are available on the synthesis and application of MOF and MOF derived materials, their applications for the electrochemical energy conversion and storage is totally a new field of research and developed recently. This review focuses on the systematic design of the materials from MOF and control over their inherent properties to enhance the electrochemical performances.

Spinel MnCo2 O4 Nanoparticles Supported on Three-Dimensional Graphene with Enhanced Mass Transfer as an Efficient Electrocatalyst for the Oxygen Reduction Reaction

The rational design of highly efficient and durable oxygen reduction reaction (ORR) catalysts is critical for the commercial application of fuel cells. Herein, three-dimensional graphene (3D-G) is synthesized by the template method, which used coal tar pitch as the carbon source and nano MgO as the template. Then, spinel MnCo₂ O₄ is in situ supported on the 3D-G by a facile hydrothermal method, giving MnCo₂ O₄ /3D-G. The resultant MnCo₂ O₄ /3D-G retains the multilayered mesoporous graphene structure where MnCo₂ O₄ nanoparticles are deposited on the inner walls of pores in the 3D-G. The catalyst MnCo₂ O₄ /3D-G shows high electrocatalytic activity with a half-wave potential of 0.81 V versus reversible hydrogen electrode, which is clearly superior to those of MnCo₂ O₄ /reduced graphene oxide (0.78 V), MnCo₂ O₄ /carbon nanotubes (0.74 V), MnCo₂ O₄ /C (0.72 V), and 20 wt % Pt/C (0.80 V). The electron transfer number of MnCo₂ O₄ /3D-G indicates a four-electron process of ORR. The durability test demonstrates that the MnCo₂ O₄ /3D-G catalyst has a much better durability than 20 wt % Pt/C. Our work makes an inspiring strategy to prepare high-performance electrocatalysts for the development of fuel cells.

Facile Formation of Nanostructured Manganese Oxide Films as High-Performance Catalysts for the Oxygen Evolution Reaction

The development of inexpensive, earth abundant, and bioinspired oxygen evolution electrocatalysts that are easily accessible and scalable is a principal requirement with regard to the feasibility of water splitting for large-scale chemical energy storage. A unique, versatile, and scalable approach has been developed to fabricate manganese oxide films from single layers to multilayers with a controlled thickness and high reproducibility. The produced MnO_x films are composed of small nanostructures that are assembled closely in the form of porous sponge-like layers. The films were investigated for the electrochemical oxygen evolution reaction in alkaline media and demonstrate a remarkable activity as well as a superior stability of over 60 h. To elucidate the catalytically active species, as well as the striking structural characteristics, the films were further examined in depth by using SEM, TEM, and X-ray photoelectron spectroscopy, as well as quasi in situ extended X-ray absorption fine structure and X-ray absorption near edge structure analysis. The MnO_x catalyst films excel because of a favorably high fraction of Mn³⁺ ions that are retained even after operation at oxidizing potentials. Upon exposure to oxidizing potentials in strongly alkaline aqueous electrolyte, the catalyst material maintains its structural integrity at the nanostructural, morphological, and atomic level.

Multifunctional nanostructured electrocatalysts for energy conversion and storage: current status and perspectives

Electrocatalytic oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) have attracted widespread attention because of their important role in the application of various energy storage and conversion devices, such as fuel cells, metal-air batteries and water splitting devices. However, the sluggish kinetics of the HER/OER/ORR and their dependency on expensive noble metal catalysts (e.g., Pt) obstruct their large-scale application. Hence, the development of efficient and robust bifunctional or trifunctional electrocatalysts in nanodimension for both oxygen reduction/evolution and hydrogen evolution reactions is highly desired and challenging for their commercialization in renewable energy technologies. This review describes some recent developments in the discovery of bifunctional or trifunctional nanostructured catalysts with improved performances for application in rechargeable metal-air batteries and fuel cells. The role of the electronic structure and surface redox chemistry of nanocatalysts in the improvement of their performance for the ORR/OER/HER under an alkaline medium is highlighted and the associated reaction mechanisms developed in the recent literature are also summarized.

Monodisperse Ultrasmall Manganese-Doped Multimetallic Oxysulfide Nanoparticles as Highly Efficient Oxygen Reduction Electrocatalyst

The highly efficient and cheap non-Pt-based electrocatalysts such as transition-based catalysts prepared via facile methods for oxygen reduction reaction (ORR) are desirable for large-scale practical industry applications in energy conversion and storage systems. Herein, we report a straightforward top-down synthesis of monodisperse ultrasmall manganese-doped multimetallic (ZnGe) oxysulfide nanoparticles (NPs) as an efficient ORR electrocatalyst by simple ultrasonic treatment of the Mn-doped Zn-Ge-S chalcogenidometalate crystal precursors in H₂O/EtOH for only 1 h at room temperature. Thus obtained ultrasmall monodisperse Mn-doped oxysulfide NPs with ultralow Mn loading level (3.92 wt %) not only exhibit comparable onset and half-wave potential (0.92 and 0.86 V vs reversible hydrogen electrode, respectively) to the commercial 20 wt % Pt/C but also exceptionally high metal mass activity (189 mA/mg at 0.8 V) and good methanol tolerance. A combination of transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and electrochemical analysis demonstrated that the homogenous distribution of a large amount of Mn(III) on the surface of NPs mainly accounts for the high ORR activity. We believe that this simple synthesis of Mn-doped multimetallic (ZnGe) oxysulfide NPs derived from chalcogenidometalates will open a new route to explore the utilization of discrete-cluster-based chalcogenidometalates as novel non-Pt electrocatalysts for energy applications and provide a facile way to realize the effective reduction of the amount of catalyst while keeping desired catalytic performances.

Tunable Bifunctional Activity of Mnx Co3-x O4 Nanocrystals Decorated on Carbon Nanotubes for Oxygen Electrocatalysis

Noble-metal-free electrocatalysts are attractive for cathodic oxygen catalysis in alkaline membrane fuel cells, metal-air batteries, and electrolyzers. However, much of the structure-activity relationship is poorly understood. Herein, the comprehensive development of manganese cobalt oxide/nitrogen-doped multiwalled carbon nanotube hybrids (Mnx Co3-x O4 @NCNTs) is reported for highly reversible oxygen reduction and evolution reactions (ORR and OER, respectively). The hybrid structures are rationally designed by fine control of surface chemistry and synthesis conditions, including tuning of functional groups at surfaces, congruent growth of nanocrystals with controllable phases and particle sizes, and ensuring strong coupling across catalyst-support interfaces. Electrochemical tests reveal distinctly different oxygen catalytic activities among the hybrids, Mnx Co3-x O4 @NCNTs. Nanocrystalline MnCo2 O4 @NCNTs (MCO@NCNTs) hybrids show superior ORR activity, with a favorable potential to reach 3 mA cm-2 and a high current density response, equivalent to that of the commercial Pt/C standard. Moreover, the hybrid structure exhibits tunable and durable catalytic activities for both ORR and OER, with a lowest overall potential of 0.93 V. It is clear that the long-term electrochemical activities can be ensured by rational design of hybrid structures from the nanoscale.

Superexchange Effects on Oxygen Reduction Activity of Edge-Sharing [Cox Mn1-x O6 ] Octahedra in Spinel Oxide

Mn-Co containing spinel oxides are promising, low-cost electrocatalysts for the oxygen reduction reaction (ORR). Most studies are devoted to the design of porous Mn-Co spinels or to strongly coupled hybrids (e.g., MnCo₂ O₄ /N-doped-rmGO) to maximize the mass efficiency. The lack of analyses by metal oxide intrinsic activity (activity normalized to catalysts' surface area) hinders the development of fundamental understanding of the physicochemical principles behind the catalytic activities. A systematic study on the composition dependence of ORR in ZnCo_x Mn_2-x O₄ (x = 0.0-2.0) spinel is presented here with special attention to the role of edge sharing [Co_x Mn_1-x O₆ ] octahedra in the spinel structure. The ORR specific activity of ZnCo_x Mn_2-x O₄ spans across a potential window of 200 mV, indicating an activity difference of ≈3 orders of magnitude. The curve of composition-dependent ORR specific activity as a function of Co substitution exhibits a volcano shape with an optimum Mn/Co ratio of 0.43. It is revealed that the modulated e_g occupancy of active Mn cations (0.3-0.9), as a consequence of the superexchange effect between edge sharing [CoO₆ ] and [MnO₆ ], reflects the ORR activity of edge sharing [Co_x Mn_1-x O₆ ] octahedra in the ZnCo_x Mn_2-x O₄ spinel oxide. These findings offer crucial insights in designing spinel oxide catalysts with fine-tuned e_g occupancy for efficient catalysis.

Fischer-Tropsch synthesis: Variation of Co/ ${{\gamma}}$ γ -Al2O3 catalyst performance due to changing dispersion, reducibility, acidity and strong metal-support interaction by Ru, Zr and Ce promoters

In this study the effect of (0.1 %Ru, 0–10 % Zr) and (0.1 %Ru, 0–10 % Ce) promoters on dispersion, strong metal-support interaction and reduction behavior of cobalt-supported alumina catalyst in the Fischer-Tropsch synthesis was investigated. The synthesized catalysts were characterized by BET, XRD, ICP, TPR, XPS, and TEM techniques. The results show that addition of Ce to the catalyst leads to the shift of the TPR peaks to lower temperatures which expected that shows higher activity due to higher reducibility, but surprisingly due to SMSI effect results in the lower catalyst activity compared with the unpromoted catalyst. Also, there was a synergistic effect between Ce and Ru in the reduction behavior of Ru-Ce promoted catalyst. The other notable finding of this study was the improvement in the catalyst reducibility in the presence of Zr compared with the unpromoted one that equivalently means the higher catalyst activity. Comparison of the Zr-promoted with the Ce-promoted catalysts show that the former have higher catalyst activity than the latter due to higher acidity and SMSI effect in the presence of the Zr promoter. The C5selectivity of the 0.1Ru10Zr/15Co and 3Ce/15Co catalysts at low pressure have shown more than 50 % improvement compared to the unpromoted one. Based on the activity at atmospheric pressure; the unpromoted, 0.1Ru/15Co, 0.1Ru3Ce/15Co and 0.1Ru10Zr/15Co catalysts were selected for high pressure condition tests, in which the 0.1Ru10Zr/15Co catalyst shows the highest catalyst activity and heavier hydrocarbon selectivity.

Morphology and dispersion of nanostructured manganese–cobalt spinel on various carbon supports: the effect on the oxygen reduction reaction in alkaline media

Carbon carriers play a triple role in the ORR, determining nanoparticle dispersion, faceting, and reaction mechanism.

Mesoporous manganese phthalocyanine-based materials for electrochemical water oxidation via tailored templating

Electrochemical water splitting using non-noble metals as catalysts is of increasing importance for the future energy sector.

X-ray emission spectroscopy: an effective route to extract site occupation of cations

Cation site occupation is an important determinant of materials properties, especially in a complex system with multiple cations such as in ternary spinels. In this work, we show that XES provides not only the site occupation information as EXAFS, but also additional information on the oxidation states of the cations at each site. Additionally, we show that XES is a superior and a far more accurate method than EXAFS.

Mass-Production of Mesoporous MnCo2 O4 Spinels with Manganese(IV)- and Cobalt(II)-Rich Surfaces for Superior Bifunctional Oxygen Electrocatalysis

A mesoporous MnCo₂ O₄ electrode material is made for bifunctional oxygen electrocatalysis. The MnCo₂ O₄ exhibits both Co₃ O₄ -like activity for oxygen evolution reaction (OER) and Mn₂ O₃ -like performance for oxygen reduction reaction (ORR). The potential difference between the ORR and OER of MnCo₂ O₄ is as low as 0.83 V. By XANES and XPS investigation, the notable activity results from the preferred Mn^IV - and Co^II -rich surface. The electrode material can be obtained on large-scale with the precise chemical control of the components at relatively low temperature. The surface state engineering may open a new avenue to optimize the electrocatalysis performance of electrode materials. The prominent bifunctional activity shows that MnCo₂ O₄ could be used in metal-air batteries and/or other energy devices.

Rapid One-Pot Solvothermal Batch Synthesis of Porous Nanocrystal Assemblies Composed of Multiple Transition-Metal Elements

The ability of a rapid-heating solvothermal process to synthesize porous nanocrystal assemblies composed of the multiple transition metals was demonstrated. The rapid heating facilitated the quick formation of nascent nanocrystals to generate homogeneous mixed transition-metal oxides. Systematic studies of the synthesis of mixed-metal oxides under various experimental conditions indicated that the present simple method is suitable to develop a wide variety of binary and ternary transition-metal systems such as Co/Mn, Ni/Mn, and Co/Mn/Fe mixed-metal oxides. The products obtained from the rapid heating process were hierarchically assembled porous nanospheres composed of sub-10 nm nanocrystals, which had an extraordinarily high surface area and nano/mesopores. Electrochemical tests revealed the high catalytic ability of the porous nanocrystal assemblies in water oxidation.

Alkaline electrochemical water oxidation with multi-shelled cobalt manganese oxide hollow spheres

Multi-shelled hollow spheres of cobalt manganese oxides (CMOs) deposited on Ni foam exhibited superior alkaline electrochemical water oxidation activity and surpassed those of bulk CMO and commercial noble metal-based catalysts. A higher amount of cobalt in the spinel structure resulted in the transformation of the tetragonal to the cubic phase with a decrease in the overpotential of oxygen evolution.

Iron-Induced Activation of Ordered Mesoporous Nickel Cobalt Oxide Electrocatalyst for the Oxygen Evolution Reaction

Herein, ordered mesoporous nickel cobalt oxides prepared by the nanocasting route are reported as highly active oxygen evolution reaction (OER) catalysts. By using the ordered mesoporous structure as a model system and afterward elevating the optimal catalysts composition, it is shown that, with a simple electrochemical activation step, the performance of nickel cobalt oxide can be significantly enhanced. The electrochemical impedance spectroscopy results indicated that charge transfer resistance increases for Co₃O₄ spinel after an activation process, while this value drops for NiO and especially for CoNi mixed oxide significantly, which confirms the improvement of oxygen evolution kinetics. The catalyst with the optimal composition (Co/Ni 4/1) reaches a current density of 10 mA/cm² with an overpotential of a mere 336 mV and a Tafel slope of 36 mV/dec, outperforming benchmarked and other reported Ni/Co-based OER electrocatalysts. The catalyst also demonstrates outstanding durability for 14 h and maintained the ordered mesoporous structure. The cyclic voltammograms along with the electrochemical measurements in Fe-free KOH electrolyte suggest that the activity boost is attributed to the generation of surface Ni(OH)₂ species that incorporate Fe impurities from the electrolyte. The incorporation of Fe into the structure is also confirmed by inductively coupled plasma optical emission spectrometry.

Polybenzoxazine-Derived N-doped Carbon as Matrix for Powder-Based Electrocatalysts

In addition to catalytic activity, intrinsic stability, tight immobilization on a suitable electrode surface, and sufficient electronic conductivity are fundamental prerequisites for the long-term operation of particle- and especially powder-based electrocatalysts. We present a novel approach to concurrently address these challenges by using the unique properties of polybenzoxazine (pBO) polymers, namely near-zero shrinkage and high residual-char yield even after pyrolysis at high temperatures. Pyrolysis of a nanocubic prussian blue analogue precursor (K_m Mn_x [Co(CN)₆ ]_y ⋅n H₂ O) embedded in a bisphenol A and aniline-based pBO led to the formation of a N-doped carbon matrix modified with Mn_x Co_y O_z nanocubes. The obtained electrocatalyst exhibits high efficiency toward the oxygen evolution reaction (OER) and more importantly a stable performance for at least 65 h.

Virus-directed formation of electrocatalytically active nanoparticle-based Co3O4 tubes

Spinel-type Co₃O₄ finds applications in a wide range of fields, including clean energy conversion, where nanostructured Co₃O₄ may provide a cost-efficient alternative to platinum- and iridium-based catalysts for electrocatalytic water-splitting. We here describe a novel strategy in which basic cobalt carbonate - a precursor to Co₃O₄ - is precipitated as sheet-like structures and microspheres covered with fine surface protrusions, via ammonium carbonate decomposition at room temperature. Importantly, these mild reaction conditions enable us to employ bio-inspired templating approaches to further control the mineral structure. Rod-like tobacco mosaic viruses (TMV) were used as biotemplates for mineral deposition, where we profit from the ability of Co(ii) ions to mediate the ordered assembly of the virus nanorods to create complex tubular superstructures of TMV/ basic cobalt carbonate. Calcination of these tubules is then achieved with retention of the gross morphology, and generates a hierarchically-structured solid comprising interconnected Co₃O₄ nanoparticles. Evaluation of these Co₃O₄ materials as electrocatalysts for the oxygen evolution reaction (OER) demonstrates that the activity of Co₃O₄ prepared by calcination of ammonia diffusion-grown precursors in both, the absence or presence of TMV exceeds that of a commercial nanopowder.

Co3O4-δ Quantum Dots As a Highly Efficient Oxygen Evolution Reaction Catalyst for Water Splitting

Co₃O_4-δ quantum dots (Co₃O_4-δ-QDs) with a crystallite size of approximately 2 nm and oxygen vacancies were fabricated through multicycle lithiation/delithiation of mesoporous Co₃O₄ nanosheets. Used as an oxygen evolution reaction (OER) electrocatalyst for water splitting, the catalytic performance (an overpotential of 270 mV@10 mA cm^-2 and no decay within 30 h) of Co₃O_4-δ-QDs is superior to that of previously reported Co-based catalysts and the state-of-the-art IrO₂. Compared to that of the Co₃O₄ nanosheets, the enhanced OER activity of Co₃O_4-δ-QDs is attributed to two factors: one is the increased quantity of the Faradaic active sites, including the total active sites (q*_Total), the most accessible active sites (q*_Outer), and their ratio (q*_Outer/q*_Total); the other is the enhanced intrinsic electroactivity per active site evaluated by the turnover frequency and the current density normalized by the most accessible active sites (j/q*_Outer) related to the OER. This multicycle lithiation/delithiation method can be applied to other transition metal oxides as well, offering a general approach to develop high-performance electrocatalysts for water splitting.

Efficient and Durable Bifunctional Oxygen Catalysts Based on NiFeO@MnOx Core-Shell Structures for Rechargeable Zn-Air Batteries

Rechargeable Zn-air battery is limited by the sluggish kinetics and poor durability of the oxygen catalysts. In this Research Article, a new bifunctional oxygen catalyst has been developed through embedding the ultrafine NiFeO nanoparticles (NPs) in a porous amorphous MnO_x layer, in which the NiFeO-core contributes to the high activity for the oxygen evolution reaction (OER) and the amorphous MnO_x-shell functions as active phase for the oxygen reduction reaction (ORR), promoted by the synergistic effect between the NiFeO core and MnO_x shell. The synergistic effect is related to the electron drawing of NiFeO core from MnO_x shell, which decreases the affinity and adsorption energy of oxygen on MnO_x shell and significantly increases the kinetics of ORR. The electrocatalytic activity and durability of NiFeO@MnO_x depends strongly on the NiFeO:MnO_x ratio. NiFeO@MnO_x with NiFeO:MnO_x weight ratio of 1:0.8 shows the best performance for reversible ORR and OER, with a potential gap (ΔE) of 0.792 V to achieve a current density of 3 mA cm^-2 for ORR (E_ORR=3) and 5 mA cm^-2 for OER (E_OER=5) in 0.1 M KOH solution. The high activity of the NiFeO@MnO_x(1:0.8) has been demonstrated in a Zn-air battery. Zn-air battery fabricated using the NiFeO@MnO_x(1:0.8) oxygen electrode shows similar initial performance with that of Pt-Ir/C oxygen electrode but a much better durability under charge and discharge cycles as the result of the structure confinement effect of amorphous MnO_x. The results demonstrate NiFeO@MnO_x as an effective bifunctional oxygen catalysts for rechargeable metal-air batteries.

Microwave-assisted synthesis and prototype oxygen reduction electrocatalyst application of N-doped carbon-coated Fe3O4 nanorods

Fe₃O₄ nanorods coated with nitrogen-doped mesoporous carbon (ND-Fe₃O₄@mC) shells of defined thicknesses have been prepared via a new microwave-assisted approach. Microstructural characterization of these ND-Fe₃O₄@mC structures was performed using x-ray diffraction, x-ray photoelectron spectroscopy, transmission electron microscopy, and scanning electron microscopy. Following identification, the electrochemical performance of the catalysts was evaluated using linear sweep voltammetry with a rotating disc electrode system. The present investigation reveals enhanced oxygen reduction reaction catalytic activity and the carbon layer thickness influences oxygen diffusion to the active Fe₃O₄ nanorod core.

Perovskite/Carbon Composites: Applications in Oxygen Electrocatalysis

Oxygen electrocatalysis, i.e., oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), plays an extremely important role in oxygen-based renewable-energy technologies such as rechargeable metal-air batteries, regenerative fuel cells and water splitting. Perovskite oxides have recently attracted increasing interest and hold great promise as efficient ORR and OER catalysts to replace noble-metal-based catalysts, owing to their high intrinsic catalytic activity, abundant variety, low cost, and rich resources. The introduction of perovskite-carbon interfaces by forming perovskite/carbon composites may bring a synergistic effect between the two phases, thus benefiting the oxygen electrocatalysis. This review provides a comprehensive overview of recent advances in perovskite/carbon composites for oxygen electrocatalysis in alkaline media, aiming to provide insights into the key parameters that influence the ORR/OER performance of the composites, including the physical/chemical properties and morphologies of the perovskites, the multiple roles of carbon, the synthetic method and the synergistic effect. A special emphasis is placed on the origin of the synergistic effect associated with the interfacial interaction between the perovskite and the carbon phases for enhanced ORR/OER performance. Finally, the existing challenges and the future directions for the synthesis and development of more efficient oxygen catalysts based on perovskite/carbon composites are proposed.