Mn3O4 Nanocrystals Research Articles

Synthesis of an amorphous Geobacter-manganese oxide biohybrid as an efficient water oxidation catalyst

Self-decorated Mn2O3 nanocrystals on Geobacter sulfurreducens were synthesized as sustainable and efficient water oxidation catalysts.

Electrodeposition of MnO2 on spray-coated nanostructured carbon framework as high performance material for energy storage

Carbon nanotubes/graphene/manganese dioxide nanostructured thin films are developed as supercapacitors electrodes in this contribution. The effects of nanostructuration and pseudocapacitance on electrochemical performances are investigated. The challenge is to assemble these materials in a composite electrode achieving controlled homogeneity, morphology, and composition. Mixing carbon nanotubes (CNTs) and graphene sheets in a hierarchical organization can increase the accessibility of the electrolyte to the entire active surface by creating a controlled porous network inside the electrode. The carbon nanomaterials are deposited on a current collector by dynamic spray gun deposition, an easily scalable, reproducible and industrially suitable method used for nano-structuration of thin films. MnO2 nanocrystals are obtained by anodic electrodeposition directly on this conductive nanostructured carbon framework. This binder free electrode is characterized by SEM, TEM, XPS, XRD, microporosity, Raman spectroscopy, and electrochemistry. Results show that the specific capacitance of the samples can reach 209 F/g thanks to a strong control of the MnO2 morphology, particle size and to adequate Mn/C ratios. The electrode has very good stability with capacitance retention of 96% after 3000 charge/discharge cycles.

Mechanochemical Synthesis of Mn3O4 Nanocrystals and Their Lithium Intercalation Capability.

Syntheses of Mn3O4 involve either high sintering temperatures to produce well crystallized products or the use of water-soluble precursors, surfactants, and organic solvents to generate nanocrystalline products. Mechanochemical approaches are known to be effective in the preparation of fine-grained or nanoscaled materials, while also being environmentally friendly because no solvents and no sintering at high temperatures are required. We report the solvent free mechanochemical synthesis of Mn3O4 nanocrystals at room temperature, from a mixture of MnO and Mn2O3. The single-phase product was characterized by Rietveld refinement and SEM images. The obtained crystallite size was 14.2(2) nm, which is among the smallest ever produced crystallite sizes of Mn3O4. The obtained product reveals an enormous increase in lithium intercalation capability, which was proven via chemical lithium intercalation at room temperature. More than 50% lithiation of nanocrystalline Mn3O4 is observed after a reaction time of 1 h, while coarse-grained material from a solid-state reaction shows no intercalation under the same reaction conditions. Therefore, the produced manganese oxide has a high potential in lithium battery applications.

Ultrathin Cobalt–Manganese Nanosheets: An Efficient Platform for Enhanced Photoelectrochemical Water Oxidation with Electron‐Donating Effect

AbstractAn ultrathin cobalt–manganese (Co‐Mn) nanosheet, consisting of amorphous Co(OH)x layers and ultrasmall Mn3O4 nanocrystals, is designed as an efficient co‐catalyst on an α‐Fe2O3 film for photoelectrochemical (PEC) water oxidation. The uniformly distributed Co‐Mn nanosheets lead to a remarkable 2.6‐fold enhancement on the photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) and an impressive cathodic shift (≈200 mV) of onset potential compared with bare α‐Fe2O3 film. Furthermore, the decorated photoanode exhibits a prominent resistance against photocorrosion with excellent stability for over 10 h. Detailed mechanism investigation manifests that incorporation of Mn sites in the nanosheets could create electron donation to Co sites and facilitate the activation of the OH group, which drastically increases the catalytic activities for water oxidation. These findings provide valuable guidance for designing high‐performance co‐catalysts for PEC applications and open new avenues toward controlled fabrication of mixed metallic composites.

Synergistic Control of Structural Disorder and Surface Bonding Nature to Optimize the Functionality of Manganese Oxide as an Electrocatalyst and a Cathode for Li-O2 Batteries.

An efficient way to improve the electrocatalyst and Li-O2 battery performances of metal oxide is developed by an exquisite synergistic control over structural disorder and surface bonding nature. The effects of amorphous nature and surface chemical environment on the functionalities of metal oxide are systematically investigated with well-crystalline and amorphous MnO2 nanocrystals with/without surface anchoring of highly oxidized iodate clusters. The amorphous MnO2 nanocrystal containing anchored iodate clusters shows much better performance as an oxygen evolution electrocatalyst and cathode catalyst for Li-O2 batteries than both iodate-free amorphous and well-crystalline homologues, underscoring the remarkable advantage of simultaneous enhancement of structural disorder and surface electron density. In situ X-ray absorption spectroscopic analysis demonstrates the promoted formation of double (MnO) bond, a critical step of oxygen evolution reaction, upon amorphization caused by the poor orbital overlap inside highly disordered crystallites. The beneficial effects of iodate anchoring and amorphization on electrocatalyst functionality are attributable to the alteration of surface bonding character, stabilization of Jahn-Teller active Mn3+ species, and enhanced charge transfer of interfaces. The present study underscores that fine-tuning of structural disorder and surface bonding nature provides an effective methodology to explore efficient metal oxide-based electrocatalysts.

Highly Reversible Lithium Storage of Nitrogen‐Doped Carbon@MnO Hierarchical Hollow Spheres as Advanced Anode Materials

AbstractThe practical implementation of MnO‐based anode materials is still obstructed because of their short cyclic life and unsatisfactory rate performance, attributable to low conductivity, large volume variation and self‐aggregation of MnO during intense cycles. To overcome these obstacles, we successfully fabricated nitrogen‐doped carbon@MnO hierarchical hollow spheres through a novel approach. As anode nanomaterials for Li‐ion batteries, the nanocomposite possesses a highly reversible capacity of 1302 mAh g−1 after the 200th cycle at 0.1 Ag−1 and 964 mAh g−1 after the 500th cycle at 0.5 Ag−1, respectively. The prominent electrochemical performances of the nanocomposite may result from synthetic effects of nitrogen‐doped carbon hollow structures, nanosized MnO and the gradual generation of high‐oxide‐state manganese oxide on nitrogen‐doped carbon hollow spheres during intensive cycles. The aforementioned results also demonstrate that MnO nanocrystals can be well utilized by attaching to nitrogen‐doped carbon hollow spheres, which is a valid way to achieve excellent lithium storage properties for transition metal oxides.

Effect of post-treatment on the selective catalytic reduction of NO with NH3 over Mn3O4

The Mn3O4 nanocrystals were successfully prepared by rapid precipitation method at room temperature from MnSO4, H2SO4 and NH3•H2O, and the influences of post‒treatment temperatures at 200‒500 °C on texture, morphology, crystal face, redox and the selective catalytic reduction of NO with NH3 were investigated. The results show that a single tetragonal phase of Mn3O4 nanocrystals is obtained after dryness and it has a BET surface area of 112 m2/g and exposes the (101) plane. When the post‒treatment temperature increases from 200 to 400 °C, both crystal phase and crystallite sizes of Mn3O4 are almost unchanged, but their main exposed crystal faces change from (101) to (200) and (112). When the post‒treatment temperature increases to 500 °C, Mn2O3 is dissociated from Mn3O4. Mn3O4_300 possesses a developed pore structure with a SBET of 201 m2/g and a pore volume of 0.41 cm3/g. Mn3O4_400 has low reduction peaks at 210 and 467 °C and similar reduction properties after multi‒reaction of NH3‒SCR, but the peak temperature shifts to higher temperature compared to the fresh sample. When the post‒treated sample is applied to the NH3‒SCR reaction, Mn3O4_400 exhibits excellent NO removal efficiency at low temperatures, which may be related to the surface oxidation of Mn3O4 to promote the formation of NO2. Mn3O4_400 has good resistance of SO2 and has 92% removal efficiency of NO when only SO2 is induced in the simulated gas, but NO removal efficiency decreases to 63% in the presence of H2O and SO2 due to the generation of MnSO4•H2O.

Pre-lithiated manganous oxide/graphene aerogel composites as anode materials for high energy density lithium ion capacitors

In this study, a porous manganous oxide/graphene aerogel (MnO/GA) is synthesized via a facile sol-gel method used for the anode material of lithium-ion capacitor (LIC). The crystal and morphology characterizations confirm that the MnO nanocrystal grows directly on the graphene sheets of the three-dimensional graphene aerogel (GA) matrix. In a half-cell, the MnO/GA electrode loaded with 2.9 wt% MnO exhibits high specific capacity of 630 mAh g−1 at a current density of 0.1 A g−1 after 100 cycles. The electrochemical results show that the MnO nanocrystals loaded on the GA can enlarge the average pore size and improve the specific capacity. In addition, the porous GA matrix cannot only facilitate capacitive storage of Li+ ions but also accommodate the volume variation of MnO during conversion process. The assembled LICs using the pre-lithiated MnO/GA anode and lithium iron phosphate cathode deliver high energy densities of 194 and 93 Wh·kg−1 at power densities of 300 and 3600 W kg−1, respectively. Furthermore, the LICs exhibit superior cycling stability after 1000 cycles, with high energy density of 101 Wh·kg−1 at a power density of 300 W kg−1.

Three-dimensional reduced graphene oxide/MnO composite architectures as a high-rate anode for lithium storage

Integrating metal oxides into three-dimensional (3D) reduced graphene oxide (rGO) is beneficial to exploit high-performance anodes for lithium storage. Herein, we design two self-assembly processes to prepare 3D rGO/MnO gel (3D rGO/MnO-G) involving initial self-assembly loading of Mn3O4 nanocrystals on GO and following self-assembly of gel along with the reduction of Mn3O4 to MnO. Due to the interconnected porous network structure and intimate contact between MnO and rGO, as well as sufficient buffer space for lithium storage, the 3D rGO/MnO-G shows excellent electrochemical performance with a discharge capacity of 920.8 mA h g−1 at a current density of 0.5 A g−1 after 100 cycles and superior rate capability of 600.1 mA h g−1 at 3.2 A g−1.

Pseudocapacitive Lithium Storage in Three‐Dimensional Cobalt‐Doped MnO/Nitrogen‐Doped Reduced Graphene Oxide Aerogels as High‐Rate Anode Material

AbstractStructural design and modification are effective measures to improve the lithium storage performance of electrode materials. Herein, three‐dimensional (3D) porous Co‐doped MnO/nitrogen‐doped reduced graphene oxide aerogels (Co−MnO/NG‐G) have been prepared by successive self‐assembly processes, including the self‐assembly nucleation of Co−MnO on GO in H2O/N,N‐Dimethylformamide (DMF) mixed solvent, and the 3D reduction‐assembly of hydrogels accompanied with nucleation‐inducing growth of Co−MnO nanocrystals. Due to high‐efficient electron/ion transport channels dating from the novel 3D porous microstructure and improved electron/ion conductivity deriving from doping MnO with Co, the 3D Co−MnO/NG‐G electrode demonstrates high pseudocapacitive lithium storage behavior with 88.3 % at 2 mV s−1. As an anode in lithium‐ion battery, the 3D Co−MnO/NG‐G shows a high capacity of 982.8 mAh g−1 at 0.5 A g−1 after 100 cycles, outstanding rate capability with 424.0 mAh g−1 at 8 A g−1, as well as superior cycle stability with 508.9 mAh g−1 after 800 cycles at 4 A g−1. This work demonstrates that the synergistic strategy between cation doping and 3D porous channels for electron/ion transport is an effective way to design high‐rate anode materials.

Partial Surface Oxidation of Manganese Oxides as an Effective Treatment To Improve Their Activity in Electrochemical Oxygen Reduction Reaction

Enhancing the electrocatalytic activity of low-cost transition-metal oxides for oxygen reduction reaction (ORR) is a crucial challenge for extensive application of fuel cells. A promising approach demonstrated previously is the formation of catalysts with mixed valent metal active sites. Because catalysis happens primarily on the surface of the catalyst, we hypothesize that creating such active sites only on the surface will be an effective strategy for improving the catalytic activities. Here, we present a partial oxidation approach that grows δ-MnO2 nanoflakes on the surface of octahedron Mn3O4 nanocrystals for increasing their ORR activity. The δ-MnO2/Mn3O4 nanocomposite exhibits significantly improved ORR activity with a half-wave potential of 0.75 V versus reversible hydrogen electrode, which is ∼110 and ∼90 mV lower than those of the Mn3O4 nanocrystal and δ-MnO2 nanoflakes in their pure forms, respectively. The electrochemical impedance spectroscopy reveals that the δ-MnO2/Mn3O4 nanocomposite posses...

Synthesis of Pseudocapacitive Porous Metal Oxide Nanoclusters Anchored on Graphene for Aqueous Energy Storage Devices with High Energy Density and Long Cycling Stability along with Ultrafast Charging Capability

AbstractRealization of safe electrochemical energy storages with high energy density and long cycle life along with the high power density enabling fast charging is a major challenge. Here, a strategy to realize high‐performance aqueous energy storages using porous Mn3O4 (p‐MG) positive and porous Fe2O3 (p‐FG) negative electrodes, where granular nanoclusters composing nanoparticles are produced on graphene through lithiation‐induced conversion and the shortened ion diffusion lengths in p‐MG and p‐FG give fast charging rate and excellent cycle stability is reported. Furthermore, it is found from cyclic voltammetry curves and specific capacitances that porous metal oxide structures play mainly as redox reaction sites, while graphene structures provide electrical conductivity to active sites. Indeed, the full‐cell configuration of p‐MG and p‐FG in a hybrid capacitor exhibits a distinguished high energy density exceeding those of aqueous batteries, in addition to excellent capacity retention over 30 000 redox cycles and the energy density 2.5‐fold higher than that of its counterpart with pristine Mn3O4 and Fe2O3 nanocrystals. Additionally, this capacitor shows the high power density allowing ultrafast charging in that the full cells in series can be charged within several seconds by the rapid USB charger, thus outperforming those of typical aqueous batteries by about 100‐fold.

Solid State Supercapacitor Based on Manganese Oxide@Reduced Graphene Oxide and Polypyrrole Electrodes

AbstractA Mn3O4 nanocrystals@reduced graphene oxide nanocomposite (MRNC) is prepared using a hydrothermal method. Structural analysis of the synthesized MRNC shows the formation of tetragonal hausmannite phase of Mn3O4 nanocrystals and reduced graphene oxide (rGO) sheet/scrolls. As electrode material for supercapacitors, MRNC reaches a specific capacitance of 611 F g−1 with excellent stability showing a cycling stability of 95 % after 3000 cycles. In addition, polypyrrole (PPy) films with various thicknesses are deposited on flexible carbon fiber via electrodeposition method. A solid state asymmetric supercapacitor (ASC) based on MRNC as positive and PPy as negative electrode was assembled. The assembled ASC delivers a maximum energy density and power density of 32 Wh kg−1 and 833 W kg−1, respectively, with excellent cycling stability (90 % capacitance retention after 6000 cycles). The result reveals that the MRNC is a promising candidate as superior electrode material for supercapacitors.

Aqueous Synthesis of Mn3O4 Nanoparticle@MnOOH Nanobelt Heterostructures

A combined heterostructures with Mn3O4 nanoparticles attached to MnOOH nanobelts were prepared via an aqueous oxidation method at 90 °C with H2O2 as oxidant. The crystalline structures and morphologies of the as-prepared samples were detected by X-ray powder diffraction (XRD), field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HRTEM) analysis, and the forming mechanism of Mn3O4@MnOOH nanomaterials was discussed. Crystalline composition and morphologies of samples changed with reaction time and crystallinity of Mn(OH)2 precursor. β-MnOOH nanoplates can be obtained in the initial reaction, then it transformed to γ-MnOOH nanobelts and/or Mn3O4 nanoparticles as reaction time increasing. The ratio of Mn3O4 in Mn3O4@MnOOH nanomaterials increased with the better crystallinity of Mn(OH)2 precursor. The as-prepared Mn3O4@MnOOH nanomaterials with varied compositions were used for degradation of Rhodamine B (RhB) in acid condition. The degradation reactions carried out in acid condition without stimulated light sources. The results showed that the manganese heterostructures had good activity based on synergy of Mn3O4 and MnOOH nanocrystals.

Self-assembled Mn3O4/C nanospheres as high-performance anode materials for lithium ion batteries

A self-assembled Manganese (Mn) based metal organic complex (Mn-MOC) with spherical structure was synthesized via a solvothermal reaction. The Mn-MOC precursor materials were converted to hierarchical porous Mn3O4/C nanospheres through thermal annealing treatment. These nanospheres exhibited a high reversible specific capacity (1237 mAh/g at 200 mA/g), excellent ratability (425 mAh/g at 4 A/g), and extremely long cycle life (no significant capacity fading after 3000 cycles at 4A/g) as an anode in lithium ion batteries. The notable capability for lithium storage is attributed to the unique porous hierarchical structure of the Mn3O4/C nanospheres, which consist of homogeneously distributed Mn3O4 nanocrystals with thin carbon shells. Such a desired nanostructure not only provided large reaction surface area and enhanced electrical conductivity, but also promoted the formation of a stable solid electrolyte interphase (SEI) and accommodated the volume change of the conversion reaction type electrode.

Salt-template assisted preparation and electrochemical performance of MnO/C nanosheet composite electrode for lithium-ion battery

MnO/C nanosheet composites have been prepared by heating NaCl@Mn(Oleate)2 particles at 600 °C with a heating rate of 10 °C min−1 under Ar atmosphere for 3 h, and NaCl@Mn(Oleate)2 particles are obtained by coating NaCl powder with the solution of Mn(Oleate)2/n-hexane. The effects of NaCl amount, heating temperatures and heating rates on the grain size and dispersity of MnO crystals are systematically investigated, respectively. The results show that NaCl plays an important role in providing the surface to fabricate the MnO/C nanosheet composites. The thickness of MnO/C nanosheet is influenced by the amount of NaCl, and its optimal amount is 7.5 g as the weight ratio of 12.146 to Mn(Oleate)2. The dimension and morphology of the MnO nanocrystals embedded on the carbon nanosheets are affected by the heating rates and heating temperatures of NaCl@Mn(Oleate)2 particles as well. On the other hand, the conductivity of the obtained composites is well improved during the lithiation/delithiation processes. Meanwhile, the aggregation of MnO nanoparticles is effectively mitigated by the construction of carbon network structure. MnO/C nanosheet composite electrodes exhibit a high reversible capacity of 783 mAh g−1 in the second cycle at a current density of 0.1 A g−1, excellent cycling stability with a capacity retention of 71.7% and good rate performance as the anode in lithium ion battery. The synthetic procedure in the present study could be extended to prepare other metal oxide/carbon nanosheet composites for energy storage materials.

Alumina-coated and manganese monoxide embedded 3D carbon derived from avocado as high-performance anode for lithium-ion batteries

Derived from avocado fruit, a three dimension (3D) carbon is prepared via a hydrothermal/pyrolysis process followed by embedding with MnO nanoparticles by a wet chemical method and coating with Al2O3 through an atomic layer deposition technique. The obtained material presents a hierarchical structure that MnO nanocrystals wrapped in 3D carbon and then encapsulated in a uniform Al2O3 layer with a thickness of about 5 nm. Benefiting from this hierarchical structure in which 3D carbon offers numerous electronic pathways to enhance the conductivity and Al2O3 nanolayer provide a shelter to keep away from dissolution of Mn4+ and volume changes during charge/discharge process. This material (marked as C/MnO@Al2O3) has exhibited high rate performance and excellent cyclability as an anode for lithium ion batteries. A high specific capacity of about 600 mA h g−1 is achieved at a current density of 1000 mA g−1 and the electrode can still deliver a high specific capacity of about 1165 mA h g−1 at 150 mA g−1 after 100 cycles. These results facilitate a green and high potential of anode materials towards promising devices for advance performance of lithium-ion batteries.

High-performance asymmetric supercapacitors based on monodisperse MnO nanocrystals with high energy densities.

Monodisperse spherical MnO nanocrystals (NCs) with a size of 22.5 nm were synthesized by the thermal decomposition of manganese oleate in the presence of oleic acid and 1-octadecene. The as-synthesized MnO NCs show superior electrochemical performances with a specific capacitance of 736.4 F g-1 at a current density of 1 A g-1 and retain 93.3% of initial specific capacitance after 5000 cycles. The MnO NC electrode was successfully assembled in an asymmetric supercapacitor as the cathode with an activated carbon (AC) electrode as the anode. The as-fabricated device can demonstrate remarkable performance with an energy density of 44.2 W h kg-1, a power density of 900 W kg-1, and excellent cycling stability. This work provides a new direction for MnO nanomaterials towards high-performance energy storage devices.

Tubular Monolayer Superlattices of Hollow Mn3O4 Nanocrystals and Their Oxygen Reduction Activity.

Self-assembled nanocrystal (NC) superlattices are emerging as an important class of materials with rationally modulated properties. Engineering the nanoscale structure of constituent building blocks as well as the mesoscale morphology of NC superlattices is a crucial step in widening their range of applications. Here, we report a template-assisted epitaxial assembly strategy, enabling growth of freestanding, carbon-coated tubular monolayer superlattices (TMSLs). Specifically, we design and construct TMSLs of hollow Mn3O4 NCs (h-Mn3O4-TMSLs) by exploiting structural evolution of MnO NCs. The tubular superlattices obtained possess a number of unique and advantageous structural features unavailable in conventional NC superlattices, rendering them particularly attractive for energy conversion applications. We demonstrate this by employing h-Mn3O4-TMSLs as electrocatalysts for oxygen reduction, the catalytic performance of which is comparable to that of state-of-the-art Pt/C catalysts and superior to that of most manganese oxide-based catalysts reported.

Full reoxidation of CuMn2O4 spinel catalyst triggered by epitaxial Mn3O4 surface nanocrystals

AbstractTransmission electron microscopy was performed on a model system of a novel CuMn2O4 spinel catalyst in order to study the effects of reduction by carbon monoxide and subsequent reoxidation by oxygen both processed at 600°C. A phase transition from CuMn2O4 spinel into separated MnO and Cu‐rich phases, which are CuO or Cu depending on the reduction period, was observed in the reduced samples. Furthermore, a surface coverage of the MnO grains by epitaxially grown, nanocrystalline Mn3O4 was found in all reduced samples. These Mn3O4 nanocrystals are assumed to act as seed crystals in the subsequent reoxidation step completely reversing the phase transitions and reconstituting single‐phase CuMn2O4 spinel. In addition, superlattice reflections due to cation ordering occurred as a typical feature in the as‐prepared spinel. In the reoxidized spinel, the superlattice reflections were absent, hence indicating a disordered cation distribution.