α-MnO2 Nanowires Research Articles

Enhancement of the electrochemical performance of free-standing graphene electrodes with manganese dioxide and ruthenium nanocatalysts for lithium-oxygen batteries

Hybrid ternary Graphene/Ruthenium/α-MnO2 (rGO/Ru/α-MnO2) flexible nanocomposite cathodes were fabricated via controlling both reduction and vacuum filtration processes without using a binder and conductive carbon additives for flexible Li-air battery system. To compare the electrochemical performance of the Graphene/Ruthenium/α-MnO2 cathodes, bare rGO and rGO/Ru free-standing cathodes were also manufactured. rGO cathodes with well-dispersed α-MnO2 nanowires and ruthenium nanoparticles were successfully synthesized and shown to dramatically increase (decrease) oxygen reduction (evolution) reactions. The enhancement on the electrochemical performance of the synthesized cathodes was attributed not only to catalysis effect of ruthenium and α-MnO2 but also well-stacked morphology of the nanocomposite architecture which enables increased oxygen flow between the layers and, hence boosted reaction kinetics.Physical characterization of the cathodes was carried out using FESEM, EDS, TEM, XRD, XPS and Raman spectroscopy. The discharge product of the cathodes was also evaluated using TEM and XPS. Electrochemical performances of the cathodes were evaluated by means of CV, EIS, galvanostatic charge-discharge and electrochemical cycling tests. Thanks to the synergetic effect of Ruthenium and α-MnO2 catalysts, our ternary rGO/Ru/α-MnO2 cathodes were shown to serve full discharge capacity of 2225 mAh/g while rGO/Ru can deliver only 1670 mAh/g. Besides, the cycling stability of the ternary rGO/Ru/α-MnO2 cathodes was shown for 50 cycles at 650 mAh/g capacity limited tests in assembled Li–O2 batteries.

High-precision synthesis of α-MnO2 nanowires with controllable crystal facets for propane oxidation

α-MnO2 nanowires with different crystal facets were synthesized. The (110) crystal facet of α-MnO2 is conducive to propane catalysis, due to the lowest electron energy, effectively Mn valence adjustment and abundant oxygen vacancies.

Self-assembly of α-MnO2/Mn3O4 hierarchical structure on carbon cloth for aymmetric supercapacitors

In this work, carbon cloth (CC) was employed as a substrate onto which α-MnO2 nanowires were grown hydrothermally as a secondary support structure. In NaBH4 aqueous solution, the surface of the MnO2 was reduced at room temperature to form α-MnO2/Mn3O4 hierarchical structure. At a current density of 2 A g−1, the specific capacitance of the α-MnO2/Mn3O4@CC was found to be 181 F g−1. The energy density of the asymmetric supercapacitor (ASC) assembled by α-MnO2/Mn3O4@CC and activated carbon reaches 118.3 Wh kg−1. After 4000 cycles at a current density of 1 A g−1, the 88% of the capacitance was retained demonstrating good cycle stability. The designed multi-level structure can effectively prevent the stacking and agglomeration of the nanowires, and provides high energy density and good cycle stability in ASC. These results provide a good experimental foundation for the development of advanced, wearable, and energy storage devices.

Graphene-based nanocomposite cathodes architecture with palladium and α-MnO2 for high cycle life lithium-oxygen batteries

High-efficiency electrocatalysts of palladium and α-MnO2 nanowires supported on reduced graphene oxide (rGO) sheets are developed through an effective process to enhance the electrochemical performance of current lithium-oxygen batteries. Palladium is known as an oxygen evolution reaction (OER) electrocatalyst in Li–O2 cathode to reduce the charge overpotential and exhibit stable cycling performance. On the other hand, MnO2 is an attractive, functional transition metal oxide catalyst in Li–O2 batteries due to its low cost, high catalytic activity, and good oxygen reduction behavior. This study integrates the synergic effects of α-MnO2 nanowires and palladium nanoparticles by decorating on graphene sheets to improve cyclability and capacity to obtain highly efficient performance of Li–O2 cells. As-prepared rGO/Pd/α-MnO2 hybrid nanocomposite cathode indicates an initial discharge capacity of 7500 mA h g−1 and stable cycle life for 50 cycles at a limited capacity of 800 mA h g−1. As a result, although the polarization of the cell dramatically decreases and stable capacity behavior is observed with the contribution of α-MnO2 and Pd catalysts, the limited stable cycle life of 50 is obtained due to the consumption of lithium metal which causes total capacity failure after 60 cycles.

In situ hydrothermal synthesis of α-MnO2 nanowire/activated carbon hollow fibers from cotton stalk composite: dual-effect cyclic visible light photocatalysis performance

MnO2 nanowire/carbon hollow fibers (ACHF) derived from cotton stalks composite (MnO2@ACHF) were synthesized by in situ hydrothermal methods. In this process, the morphology of MnO2 could be controlled from particles with an initial width of approximately 50 nm to α-MnO2 nanowires and flower-sphere-like α-MnO2 nanowires by adjusting the mass ratio of potassium permanganate and ACHF. Acting as photocatalytic materials, MnO2@ACHF exhibited excellent dual-effect cyclic absorption and degradation performance with a degradation rate of 99.8% after 240 min in the degradation of organic pollutants (methylene blue as the simulation solution). Furthermore, the photocatalytic characteristics of the prepared composites could enhance the efficiency and circulation of the photocatalysts, which resulted from the synergetic effect of the hierarchical porous ACHF carrier and the layered structure of α-MnO2. Meanwhile, a new method for removing the influence of adsorption on photocatalytic degradation was proposed.

Fluorinated TiO2 coupling with α-MnO2 nanowires supported on different substrates for photocatalytic VOCs abatement under vacuum ultraviolet irradiation

The particular role of fluorine (F) doping in the photocatalytic activity of nanocrystalline titanium dioxide (TiO2) towards toluene degradation was systematically studied. TiO2 nanosheet catalysts doped with different amounts of F− (denoted by F-TiO2) were synthesized by hydrothermal method. For comparison, calcination was performed on the catalyst to remove F-. The photocatalytic oxidation (PCO) of toluene over F-TiO2 was found to be much higher than those of TiO2 and commercial P25 under the irradiation of vacuum ultraviolet (VUV) light, with a high toluene degradation efficiency of about 90 %. Moreover, the toluene degradation efficiency increased to 96 % with the addition of only 1 wt.% α-MnO2 nanowires, and the residual ozone (O3) in the system can be completely eliminated. In addition, four different substrates were explored for catalyst coating to facilitate the operation and minimize catalyst loading, which can provide a promising strategy for practical volatile organic compounds (VOCs) abatement in industrial applications.

Assembled Ag-doped α-MnO2@δ-MnO2 nanocomposites with minimum lattice mismatch for broadband microwave absorption

Building core@shell nanostructures and doping ions are two important strategies to optimize the performance for the microwave impedance matching and attenuation. In this research, the Ag-doped α-MnO2@δ-MnO2 was successfully synthesized by combining hydrothermal reaction and chemical deposition reaction. Driven by the lowest surface energy principle, all the δ-MnO2 nanosheets nucleate on the surface of α-MnO2 nanowires. During this process, the edge of nanosheets attach on the α-MnO2 nanowire with low lattice mismatch of δ (201)//α (211) (nearly 0) and vertically grow on the nanowires surfaces, then the assembled core-shell composites can be prepared. The core-shell composites exhibit synergistic effect and improved the impedance matching, so that the absorbing performance has been obtained. The composite shows the minimum reflection loss (RL) of −53.67 dB at 13.28 GHz for 1.8 mm thickness, and the broadest effective absorbing bandwidth (RL < −10 dB) is 5.44 GHz as the thickness was 1.9 mm. The excellent performance is come from the preferable impedance matching, the large heterogeneous existence of the interfaces and the numerous transmission paths for microwave.

Harvesting the vibration energy of α-MnO2 nanostructures for complete catalytic oxidation of carcinogenic airborne formaldehyde at ambient temperature

Vibration is one of the most prevalent energy sources in natural environment, which can also be harvested and utilized to drive chemical reaction. Herein, mechanical vibration is used for enhancing the catalytic decomposition of formaldehyde at ambient temperature with the assistance of four well-defined morphologies α-MnO2 (nanowire, nanotube, nanorod and nanoflower). In particular, α-MnO2 nanowire exhibits the best catalytic activity, which can completely mineralize formaldehyde into carbon dioxide at ambient temperature by harvesting the vibration energy. To the best of our knowledge, this may be the first report that α-MnO2, as a non-noble metal catalyst, can completely decompose formaldehyde to carbon dioxide at ambient temperature. The characterization results show that α-MnO2 nanowire has a much higher oxygen vacancy concentration than other three catalysts. In addition, thermal effect generated from friction between nanoparticles induced by ultrasonic vibration may enhance its catalytic activity. More importantly, it is the vibration that effectively promotes the activation of O2 adsorbed on the surface oxygen vacancy to produce more ▪, thus increasing the catalytic decomposition performance. The strategy presented herein demonstrates a new approach for efficient use of mechanical vibration to improve catalytic activity of traditional catalysts.

Tailoring morphology and structure of manganese oxide nanomaterials to enhance oxygen reduction in microbial fuel cells

In this study, 1D structures based on α-MnO2 nanowires and β-MnO2 nanorods were fabricated by a single-step hydrothermal synthesis and evaluated as cathode components for microbial fuel cells (MFCs). The role of crystallinity and morphology on electrocatalytic activity was investigated by combining X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy with an electrochemical analysis of oxygen reduction reaction at the surface of MnO2-based structures. Among the synthesized composites, β-MnO2 nanorods showed higher ORR activity thanks to the synergy between the rod-like morphology and the nitrogen presence in/on the carbon support which provides a more accessible surface for oxygen adsorption and consequent reduction. MFCs assembled with β-MnO2 nanorod cathode allowed achieving a maximum power density of 524 ± 3 mW m−2 and superior cycling stability as compared to α-MnO2 nanowires and control Pt/C.

Li Ion Exchanged α-MnO2 Nanowires as Efficient Catalysts for Li-O2 Batteries

Due to the limited energy densities, which could be achieved by lithium-ion cells, Li-O2 batteries, which could provide a promising super energy storage medium, attract much attention nowadays. For its high activity, high storage and low cost, Mn-based oxides have shown versatile application in various batteries. To enhance the cyclability of Li-O2 batteries, here, we synthesized a kind of α-MnO2 nanowires as a bifunctional catalyst for Li-O2 batteries. The particular structure of α-MnO2 reduces the mass transfer resistance of the battery, and the MnO2 nanowires were ion exchanged by saturated lithium sulfate solution so as to further improve the performance of the catalyst. The exchanged α-MnO2 catalyst showed a high discharge specific capacity(6243 mA·h/g at a current density of 200 mA/g) and significantly improved the cyclability up to the 55th cycle(200 mA/g with capacity of 1000 mA·h/g). The results show that the Li ion exchange method is a promising strategy for improving the performance of MnO2 catalyst for Li-O2 batteries.

Enhancing catalytic toluene oxidation over MnO2@Co3O4 by constructing a coupled interface

Herein, a bottom-down design is presented to successfully fabricate ZIF-derived Co3O4, grown in situ on a one-dimensional (1D) α-MnO2 material, denoted as α-MnO2@Co3O4. The synergistic effect derived from the coupled interface constructed between α-MnO2 and Co3O4 is responsible for the enhanced catalytic activity. The resultant α-MnO2@Co3O4 catalyst exhibits excellent catalytic activity at a T90% (temperature required to achieve a toluene conversion of 90%) of approximately 229 oC, which is 47 and 28 °C lower than those of the pure α-MnO2 nanowire and Co3O4-b obtained via pyrolysis of ZIF-67, respectively. This activity is attributed to the increase in the number of surface-adsorbed oxygen species, which accelerate the oxygen mobility and enhance the redox pairs of Mn4+/Mn3+ and Co2+/Co3+. Moreover, the result of in situ diffuse reflectance infrared Fourier transform spectroscopy suggests that the gaseous oxygen could be more easily activated to adsorbed oxygen species on the surface of α-MnO2@Co3O4 than on that of α-MnO2. The catalytic reaction route of toluene oxidation over the α-MnO2@Co3O4 catalyst is as follows: toluene → benzoate species → alkanes containing oxygen functional group → CO2 and H2O. In addition, the α-MnO2@Co3O4 catalyst shows excellent stability and good water resistance for toluene oxidation. Furthermore, the preparation method can be extended to other 1D MnO2 materials. A new strategy for the development of high-performance catalysts of practical significance is provided.

Silicon–air batteries: progress, applications and challenges

Silicon–air battery is an emerging energy storage device which possesses high theoretical energy density (8470 Wh kg−1). Silicon is the second most abundant material on earth. Besides, the discharge products of silicon–air battery are non-toxic and environment-friendly. Pure silicon, nano-engineered silicon and doped silicon have been found potential candidate for anode. Meso-porous meso-cellular carbon having optimal pore size and imbibed with α-MnO2 nanowires catalyst is found the most promising cathode. Several technical, design and corrosion problems associated with Si–air battery system have to be resolved for its mass scale deployment. This review presents comprehensive information on Si–air battery technology. Graphic abstract

Elimination of atrazine through radical/non-radical combined processes by manganese nano-catalysts/PMS and implications to the structure-performance relationship

In this study, a series of Mn based catalysts with different oxidative states, crystal structures and morphologies was applied to activate peroxymonosulfate (PMS) for the degradation of atrazine (ATZ). Among them, α-MnO2 nanowires (Mn-1) demonstrated the highest efficiency. The structure-performance relationship study revealed that the generation of Mn(III) along with the transformation of lattice oxygen to surface hydroxyl groups played the dominant role in controlling the efficiency, and Mn-1 are prone to undergo electron transfer to achieve the transition from Mn(IV) to Mn(III) or even Mn(II). Both radical (sulfate radical (SO4•−) and hydroxyl radical (•OH)) and non-radical species (1O2) contributed to ATZ degradation and accordingly inhibit the formation of toxic BrO3− from Br−. Sixteen intermediates were identified in proposed degradation pathways. The toxicity tests, the consistent performance with coexisting anions and humic acid in water matrix, and the satisfying reusability implied the sustainable application of Mn-1/PMS in future.

Binder-Free α-MnO2 Nanowires on Carbon Cloth as Cathode Material for Zinc-ion Batteries.

Recently, rechargeable zinc-ion batteries (ZIBs) have gained a considerable amount of attention due to their high safety, low toxicity, abundance, and low cost. Traditionally, a composite manganese oxide (MnO2) and a conductive carbon having a polymeric binder are used as a positive electrode. In general, a binder is employed to bond all materials together and to prevent detachment and dissolution of the active materials. Herein, the synthesis of α-MnO2 nanowires on carbon cloth via a simple one-step hydrothermal process and its electrochemical performance, as a binder-free cathode in aqueous and nonaqueous-based ZIBs, is duly reported. Morphological and elemental analyses reveal a single crystal α-MnO2 having homogeneous nanowire morphology with preferential growth along {001}. It is significant that analysis of the electrochemical performance of the α-MnO2 nanowires demonstrates more stable capacity and superior cyclability in a dimethyl sulfoxide (DMSO) electrolyte ZIB than in an aqueous electrolyte system. This is because DMSO can prevent irreversible proton insertion as well as unfavorable dendritic zinc deposition. The application of the binder-free α-MnO2 nanowires cathode in DMSO can promote follow-up research on the high cyclability of ZIBs.

TiC-modified α-MnO2 nanowires as highly efficient oxygen electrocatalyst for rechargeable Zn-air batteries

One dimensional α-MnO2 nanowires (NWs) have attracted extensive research interests owing to their low cost and excellent activity towards the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, to date, their application is still limited by insufficient electronic conductivity. The widely applied carbon conductive additives are thermodynamically instable during OER process. Herein, we report for the first time an efficient bifunctional oxygen electrocatalyst for Zn-air batteries (ZABs) based on α-MnO2 NWs modified by stable and highly conductive TiC nanoparticles (NPs). TiC NPs are uniformly distributed on α-MnO2 NWs, which effectively compensate for the relatively low electron conductivity of α-MnO2 NWs and thereby facilitate the charge transfer process. TiC-modified α-MnO2 NWs (TiC/α-MnO2NW) delivers a remarkable bifunctional activity with a half-wave potential of 0.78 V for ORR and a low potential of 1.64 V at 10 mA cm−2 for OER, far exceeding the pristine α-MnO2 NWs. Impressively, its ORR activity is comparable to Pt/C and OER activity is superior to IrO2. The ZAB with TiC/α-MnO2NW achieves a higher peak power density of 161 mW cm−2 and a better cycle performance than those for ZAB using Pt/C–IrO2 benchmark. The TiC/α-MnO2NW material suggests a promising strategy for designing active and durable bifunctional oxygen electrocatalysts.

The effects of morphology, microstructure and mixed-valent states of MnO2 on the oxygen evolution reaction activity in alkaline anion exchange membrane water electrolysis

In this work, we focused on the evaluation of oxygen evolution reaction (OER) activity of three different shapes of α-MnO2 nanowires (NWs), nanorods (NRs) and nanotubes (NTs) in alkaline anion exchange water electrolyser. We have attempted to separate the effect of shape, surface area, Mn3+ content and crystal facets on OER activity and stability. X-ray Photoelectron Spectroscopy (XPS) measurements showed that NTs had the highest surface concentration of Mn3+ on the as prepared samples with average Mn oxidation state of 3.33. However, after activation an increase in the average oxidation state of all three shapes to 3.9 was confirmed by XPS. X-Ray Diffraction (XRD) showed surface restructuring after testing. MnO2 NWs showed the highest OER mass activity of 60.6 A g−1 (10 mA cm−2 at 1.67 V (RHE)) due to the higher surface area of 72.2 m2 g−1. While NTs showed the highest specific activity due to highest content of 211 facet, high Mn3+ surface concentration/surface defects. Similar trend was observed in electrolyser testing with 2 mg cm−2 loading. Poor electronic conductivity of MnO2 resulted in decrease in performance with increased loading to 4 mg cm−2. All the studied shapes showed good stability over 36 h of electrolyser testing.

Carbon alternative pseudocapacitive V2O5 nanobricks and δ-MnO2 nanoflakes @ α‐MnO2 nanowires hetero-phase for high-energy pseudocapacitor

The limited energy density of the carbon-based asymmetric supercapacitor forces us to find carbon alternative electrode materials to simultaneous boost the energy and power density for supercapacitor cell. The pseudocapacitive materials hold great promise to provide a high energy at high charge-discharge rate. In present work, we have engineered the pseudocapacitive δ‐MnO2 Nanoflakes @ α-MnO2 Nanowires (δ‐MnO2 NFs@ α-MnO2 NWs) hetero-phase core-shell and V2O5 Nanobricks (NBs) on flexible carbon cloth for all-pseudocapacitive asymmetric supercapacitor. The resulting pseudocapacitive δ‐MnO2 NFs@ α-MnO2 NWs and V2O5 NBs reveal excellent electrochemical features in positive and negative potential with the capacitance of 310 F g−1 (0.06 F cm−2) and 167 F g−1 (0.029 F cm−2), respectively. With advantages of positive and negative potentials of δ‐MnO2 NFs@ α-MnO2 NWs and V2O5 NBs electrodes, all-pseudocapacitive asymmetric supercapacitor are assembled and stably operated in 1.5 V exhibits a specific capacitance of 204 F g−1 (12.2 F cm−3). The fabricated supercapacitor cell exhibits high specific energy of 63 Wh kg−1 at 2600 W kg−1 (3.82 Wh cm−3 at 15.8 W cm−3) with smaller relaxation time constant of 0.78 s, and showed remarkably excellent cyclic stability with ~100% and ~95% columbic efficiency of the original performance over 5000 cycles. In addition, these types of materials provide a major incentive for large-scale and high-performance energy storage devices.

Controllable preparation of faceted Co3O4 nanocrystals@MnO2 nanowires shish-kebab structures with enhanced triethylamine sensing performance

Novel one-dimensional shish-kebab structures composed of active (110)-faceted α-MnO2 nanowires as backbones and (111)-faceted Co3O4 nanocrystals as shells were prepared by a hydrothermal method followed by an in-situ epitaxial attachment growth strategy. A series of characterization revealed the formation of Co3O4@MnO2 heterostructures with closely integrated hetero-interfaces. The growth mechanism study suggested that the multiple functions of hexamethylenetetramine were crucial for the the successful growth of the Co3O4@MnO2 heterostructures. The gas sensing performance of the Co3O4@MnO2 heterostructures was studied for the first time. It was shown that the Co3O4@MnO2 heterostructures exhibited obvious enhanced triethylamine sensing performance compared with bare MnO2 nanowires and Co3O4 nanocrystals, suggesting a strong synergistic effect. The unique one-dimensional shish-kebab structures, increased surface active oxygen species and closely-attached hetero-interface were the key factors to improve the triethylamine sensing performance. The work not only provide an in-situ strategy for the synthesis of one-dimensional heterostructures, but also provide an effective way to improve the inherently low gas response of α-MnO2 through comprehensive design of morphology, crystal facet and hetero-interface. Moreover, thanks to the unique morphology, surface and interface characteristics of the Co3O4@MnO2 heterostructures, they are expected to be used in more fields, such as batteries, supercapacitor and catalysis.

A new strategy for synthesis of hierarchical MnO2–Mn3O4 nanocomposite via reduction-induced exfoliation of MnO2 nanowires and its application in high-performance asymmetric supercapacitor

The intrinsically low electrical conductivity and the poor cycling stability are two major obstacles of manganese oxides to hamper their further application in high-performance supercapacitors. One effective way to enhance their electrochemical properties is construction of appropriate nanocomposite with different structures. Herein, we demonstrated the fabrication of a hierarchical MnO2 nanowires-Mn3O4 nanoparticles nanocomposite by an effective and innovative two-step synthesis strategy. Further, the Mn3O4 nanoparticles were exfoliated from the surface of α-MnO2 nanowires. And the optimum composition was calculated to be ~35% Mn3O4 and ~65% α-MnO2. The capacitance of the optimized MnO2–Mn3O4 nanocomposite electrode reaches 278.2 F g−1 at a current density of 20 A g−1, and the capacitance retained at 99.3% even after 5000 cycles. The asymmetric supercapacitor was also fabricated with optimized MnO2–Mn3O4 as the positive electrode and activated carbon as the negative electrode. It could deliver an excellent energy density of ~41.3 Wh kg−1 at a power density of ~1153W kg−1. And the superior cycling stability achieves as well. The excellent capacitance properties of the MnO2–Mn3O4 nanocomposite can be attributed to the hybrid structure, in which one-dimension α-MnO2 nanowires provide a super highway for electron and ion transfer and Mn3O4 nanoparticles attached offer the abundant pseudocapacitive sites for charge storage.

Creation of controllable cationic and anionic defects in tunnel manganese oxide nanowires for enhanced oxygen evolution reaction

The development of efficient oxygen evolution reaction (OER) catalysts is crucial to the environmental and economic feasibility of electrolysis for the production of hydrogen gas. In this work, two facile chemical treatments, acid leaching and transition metal doping, were shown to modify the chemical and structural properties of low cost and environmentally friendly α-MnO2 nanowire catalysts resulting in increased OER activity. Through a combination of XPS and XRD analyses, it was shown that a molten salt treatment of α-MnO2 nanowires with Co(NO3)2 introduced Co2+ ions into the structural tunnels. The introduction of minimal amounts of cobalt (<6 at. %) more than doubled ultimate current densities achieved in linear sweep voltammetry tests. This result was attributed to a reduction of the average oxidation state of manganese in Co-doped samples and a decrease in Tafel slope below 80 mV·dec−1. Acid leaching, on the other hand, is believed to modify nanowire topography through the creation of oxygen and manganese vacancies, exposing more active sites to participate in catalysis. A characterization approach combining atomic absorption spectroscopy and iodometric titration revealed a 5.1% increase in oxygen vacancies after 72h of interaction with nitric acid. When these controllable defect formation approaches were applied in tandem, the high activity of the cobalt-doped samples was combined with the increased number of exposed active sites achieved through acid leaching producing a highly efficient electrocatalyst with more than a 3-fold increase in OER activity over pristine α-MnO2 nanowires. Our results establish that scalable and easy-to-implement approaches, such as acid leaching and transition metal doping, can lead to more than a three-fold increase in OER activity of low-cost non-toxic manganese oxides. This methodology can be beneficial for other material systems used as OER electrocatalysts.