Synthesis and electrode performance of layered nickel dioxide containing alkaline ions

Several polymorphs of layered nickel dioxide were prepared by using the chemical insertion of alkaline ions into Li_0.10NiO₂. We used aqueous AOH (A = Li, Na, K) solutions as reducing agents. Sodium and potassium insertion resulted in hydrated layered compounds that can be classified as γ-NiOOH with high crystallinity, while lithium insertion occurred without hydration. We discuss the coordination environment around the A⁺ ions for these inserted compounds. The thermal behavior, analyzed using high temperature (HT) X-ray diffraction (XRD) and thermogravimetric (TG) measurements, indicated that heating the hydrate at 150 °C yielded its dehydrate. The electrode performance of the nickelate was studied in lithium cells. We discuss the effect of interlayer water on cell rechargeability and the similarity between these nickelate and hydrated manganese dioxide (birnessite).     

Layered Birnessite Cathode with a Displacement/Intercalation Mechanism for High-Performance Aqueous Zinc-Ion Batteries

Mn-based rechargeable aqueous zinc-ion batteries(ZIBs)are highly promising because of their high operating voltages,attractive energy densities,and eco-friendliness.However,the electrochemical performances of Mn-based cathodes usually suffer from their serious structure transformation upon charge/discharge cycling.Herein,we report a layered sodium-ion/crystal water co-intercalated Birnessite cathode with the formula of Na0.55Mn2O4·0.57H2O(NMOH)for high-performance aqueous ZIBs.A displacement/intercalation electrochemical mechanism was confirmed in the Mn-based cathode for the first time.Na+and crystal water enlarge the interlayer distance to enhance the insertion of Zn^2+,and some sodium ions are replaced with Zn^2+ in the first cycle to further stabilize the layered structure for subsequent reversible Zn^2+/H^+ insertion/extraction,resulting in exceptional specific capacities and satisfactory structural stabilities.Additionally,a pseudo-capacitance derived from the surface-adsorbed Na^+ also contributes to the electrochemical performances.The NMOH cathode not only delivers high reversible capacities of 389.8 and 87.1 mA h g^−1 at current densities of 200 and 1500 mA g^−1,respectively,but also maintains a good long-cycling performance of 201.6 mA h g^−1 at a high current density of 500 mA g^−1 after 400 cycles,which makes the NMOH cathode competitive for practical applications.     

Oxidative-coupling reaction of TNT reduction products by manganese oxide

Abiotic transformation of TNT reduction products via oxidative-coupling reaction was investigated using Mn oxide. In batch experiments, all the reduction products tested were completely transformed by birnessite, one of natural Mn oxides present in soil. Oxidative-coupling was the major transformation pathway, as confirmed by mass spectrometric analysis. Using observed pseudo-first-order rate constants with respect to birnessite loadings, surface area-normalized specific rate constants, ksurf, were determined. As expected, ksurf of diaminonitrotoluenes (DATs) (1.49-1.91L/m2 d) are greater about 2 orders than that of dinitroaminotoluenes (DNTs) (1.15 x 10(-2)-2.09 x 10(-2)L/m2d) due to the increased number of amine group. In addition, by comparing the value of ksurf between DNTs or DATs, amine group on ortho position is likely to be more preferred for the oxidation by birnessite. Although cross-coupling of TNT in the presence of various mediator compounds was found not to be feasible, transformation of TNT by reduction using Fe0 followed by oxidative-coupling using Mn oxide was efficient, as evaluated by UV-visible spectrometry.     

Thorium Uptake on Synthetic Inorganic Ion Exchangers

Birnessite, antimonysilicate, and their cation-exchange derivatives were tested to take up thorium. Sorption experiments were performed in different concentrations of acid, sodium, potassium, and calcium nitrate solutions in order to evaluate the influence of cations that are likely to be present in waste effluents. Variation in the magnitude and mechanism of thorium sorption on the exchangers was ascribed to structural differences and the exchange properties of the materials, as well as the aqueous chemistry of thorium. The work included investigation of thorium solution' pH in controlling the sorption process. In acidic solutions, H-antimonysilicate proved to be the best sorbent. The structure of M-H-birnessite allows facile mobility of the interlayer cations with little structural rearrangement, making it of great importance for ion-exchange use in salt conditions. Potassium had the most, and sodium the least effect on thorium selectivity by birnessites, when they are present as macro components. Conversely, calcium ions did greatly inhibit the sorption behavior of thorium on Ca-antimonysilicate. Studying the effect of thorium solution' pH reflected the fact that the microcrystal modifications of birnessites occurred during experiments. In summary, H-birnessite showed superior uptake for thorium in comparison to other materials investigated in the literature, which reflects the characteristics of the material selectivity.     

Highly flexible supercapacitors with manganese oxide nanosheet/carbon cloth electrode

We report a simple and cost-effective synthesis of hierarchically porous structure composed of Birnessite-type manganese dioxide (MnO₂) nanosheets on flexible carbon cloth (CC) via anodic electrodeposition technique. Petal-shaped MnO₂, having sheet thickness of a few nm and typical width of 100 nm, with a strong adhesion on CC is observed. This hierarchically porous MnO₂–CC hybrid structure dose exhibit not only excellent capacitance properties, such as up to 425 F g⁻¹ in specific capacitance, but also high crack resistance owing to its efficient release of bending stress, as observed by cyclic voltammetry and galvanostatic charge/discharge measurements under different curvature of bending configurations. Furthermore, flexible supercapacitors based on this kind of MnO₂ nanosheet/CC electrode showed significantly improved stability in capacitive performance over 3000 cycles under the bending test, which is highly promising for future applications in flexible energy storage device.     

Structure, morphology and electrochemical behaviour of manganese oxides prepared by controlled decomposition of permanganate

Hydrothermal decomposition of permanganate, conducted in a range of pH-controlled solutions (from strongly acidic to strongly basic), is used to prepare manganese dioxides that are well-suited for use as supercapacitor electrode materials. While permanganate is thermodynamically unstable, the kinetics of its decomposition in an aqueous environment are very slow, until the temperature is raised to 200 °C. Although the resultant materials are relatively crystalline and have low total pore volume, their prominent meso-porosity leads to good electrochemical performance. Best behaviour is obtained for material from permanganate decomposition in 0.01 M H₂SO₄ solution, for which composite electrodes (150 μm thick) yield 150 F g⁻¹ at 5 mV s⁻¹ in a 9 M KOH electrolyte.     

Experimental and theoretical realization of an advanced bifunctional 2D δ-MnO2 electrode for supercapacitor and oxygen evolution reaction via defect engineering

Modulating the intrinsic physicochemical properties of crystalline 2D materials by dint of defect engineering largely enables multi-functionality. Uniform thin layered nanosheets further self-assembled at micro scale forming embossing structures of δ-MnO₂ were fabricated by microwave irradiation technique. The irradiation of UV/O₃ impacts incorporation of oxygen vacancy into the pristine system. Furthermore, detailed structural, morphological, surface analytical and electrochemical investigations evidenced outstanding energy storage and conversion activities. The asymmetric device δ-MnO₂-UVT//F-MWCNT with an extended potential window 1.5 V, exhibited maximum energy density of 39 Wh/kg at a power density of 468.75 W/kg. The defect structural design exhibited excellent electrocatalytic OER activity with lowest overpotential (η₂₀, 300 mV) and Tafel slope (71 mV/dec). The efficiency and stability of the illuminated material showed outstanding performances. To support our experimental findings, we have presented the electronic structures and quantum capacitance for pristine δ-MnO₂ and δ-MnO₂ with O vacancy employing Density Functional Theory (DFT) simulations. Presence of O vacancy makes a semi-conducting to metallic transition. The oxygen vacancies delocalize the neighboring electrons around the low coordinated Mn atoms and these delocalized electrons can be easily moved into the conduction band resulting improved conductivity in the material. In addition, the computed quantum capacitance tendency is as follows, δ-MnO₂-UVT > δ-MnO₂ which associates with the experimental supercapacitance behaviour of these systems.     

Oxidative transformation of aqueous phenolic mixtures by birnessite-mediated catalysis

The catalytic efficiency of birnessite in the removal of catechol, hydroxytyrosol, methylcatechol and m-tyrosol, four phenols commonly present in polluted wastewaters, was studied in mono-substrate solutions or in mixtures of two, three, and four substrates. In single phenolic solutions the transformation order of phenols was catechol>hydroxytyrosol>methylcatechol>m-tyrosol. With phenolic mixtures different responses were observed and the amount of each phenol transformed and the crossing effects among the various phenols depended on the type and number of phenols present in the mixture. In particular, general inhibitory effects were observed for hydroxytyrosol and m-tyrosol that were transformed less when present in combination with the other phenols. By contrast the effects by the presence of more than one phenol were basically annulled for catechol and methylcatechol at 24 h incubation in all the mixtures. A simultaneous, but often no stoichiometric, release of soluble Mn2+ in the reaction mixtures occurred. The multi-substrate systems were designed to mimic birnessite-mediated oxidative processes that could occur under field conditions. Therefore they could be of great interest to environmental and soil science. The use of birnessite as a potential tool for an effective detoxification and recovery of phenol-polluted systems could be also envisaged.     

Effect of phenolic mediators and humic acid on cyprodinil transformation in presence of birnessite

Naturally occurring phenols were evaluated as mediators for the transformation of the fungicide cyprodinil by birnessite. With birnessite alone, cyprodinil transformation was negligible (0.6%). In the presence of mediators, however, it increased considerably (1.5-60.9%). With some exceptions (2,6-dimethoxyphenol, syringic acid), methoxylated phenols showed a substantial capacity for enhancing the transformation. Mass spectrometry indicated that cyprodinil cross-coupled with free radicals of phenols formed at birnessite surfaces. The extent of cyprodinil transformation in the presence of syringaldehyde, m-methoxyphenol, or vanillin increased with the amount of birnessite. In reactions with o- and p-methoxyphenol and vanillic acid, cyprodinil transformation was unaffected by the amount of birnessite, but it increased with increasing phenol concentration. The addition of various humic acids at low concentrations (5-10 mg/L) had little effect on cyprodinil transformation in the presence of o-methoxyphenol or syringaldehyde. At higher concentrations, however, humic acids inhibited the transformation (by 5-20%) when o-methoxyphenol was a mediator, but showed no effect in the presence of syringaldehyde.     

TEM observations of a 30 million year old mountain leather nanofiber mineral composite

An unusual, massive mountain leather from a Llanitos mining district mine in Chihuahua, Mexico has been characterized using XRD, SEM, TEM, and SAED analytical techniques and found to be a complex mineral composite composed of intermixed nanofibers of palygorskite ((Mg, Al)_a(SiO_x)_b(OH)_c·nH₂O) and birnessite (Ca_a(MnO₂)_b·nH₂O) and intercalated with cleavage flakes of hematite (Fe₂O₃). This complex nanofiber mass produced an extremely tough material which could not be crushed in the rock crusher. The material is of interest especially in light of the fact that this natural nanocomposite formed from a hydrothermal regime that dates to roughly 30 million years ago.