An in-depth mechanistic insight into the redox reaction and degradation of aqueous Zn-MnO2 batteries

Rechargeable aqueous Zn/MnO₂ batteries raise massive research activities in recent years. However, both the working principle and the degradation mechanism of this battery chemistry are still under debate. Herein, we provide an in-depth electrochemical and structural investigation on this controversial issue based on α-MnO₂ crystalline nanowires. Mechanistic analysis substantiates a two-electron reaction pathway of Mn²⁺/Mn⁴⁺ redox couple from part of MnO₂ accompanying with a reversible precipitation/dissolution of flaky zinc sulfate hydroxide (ZSH) during the discharge/charge processes. The formation of the ZSH layer is double-edged, which passivates the deep dissolution of MnO₂ upon discharging, but promotes the electrochemical deposition kinetics of active MnO₂ upon charging. The cell degradation originates primarily from the corrosion failure of metallic zinc anode and the accumulation of irreversible ZnMn₂O₄ phases on the cathode. The addition of MnSO₄ to the electrolyte could afford supplementary capacity contribution via electro-oxidation of Mn²⁺. However, a high MnSO₄ concentration will expedite the cell failure by corroding the metallic zinc anodes. The present study will shed a fundamental insight on developing new strategies toward practically viable Zn/MnO₂ batteries.     

Effect of synthetical conditions, morphology, and crystallographic structure of MnO2 on its electrochemical behavior

Manganese dioxide nanostructures have been synthesized by hydrothermal synthetical method. The crystallographic structure, morphology, and electrochemical properties of obtained MnO₂ are examined by XRD, TEM, cyclic voltammetry, and galvanostatic charge–discharge tests. The results showed that the electrochemical properties of MnO₂ were strongly affected by the crystallographic structure and morphology. The controlling crystallographic structure of MnO₂ can be obtained by altering the molar ratio of KMnO₄/MnSO₄. The morphology was affected by the hydrothermal dwell time and temperature. The optimal synthetic conditions are as follows: the initial molar ratio of KMnO₄/MnSO₄ is 3:1, the reaction lasts 2 h at 120 °C, and the filling factor is 90%. In these prepared conditions, the MnO₂ with the maximum specific capacitance of 259 F g⁻¹ can be obtained. Prepared δ-MnO₂ has a good layer structure and exhibits nanoflower morphology. The XRD studies show that the crystalline degree of this sample is lower, and the average grain size is about 8.3 nm. These results indicate that the product may have potential applications in areas such as electrode materials of supercapacitor and other new storing energy system.     

Morphologies Controlled Synthesis of MnO2 Nanostructures

Spherical MnO₂ nanosheet aggregates have been synthesized by the reaction between MnSO₄ and KMnO₄ at room temperature. After the resulting products are treated under hydrothermal conditions, MnO₂ nanorods and tower‐like crystals have been prepared in the absence/presence of cetyltrimethylammonium bromide, respectively. The products are characterized by field‐emission scanning electron microscopy and x‐ray diffractometer, respectively.     

Co-electrodeposition of MnO2/graphene oxide coating on carbon paper from phosphate buffer and the capacitive properties

MnO₂/graphene oxide sheet composite (MnO₂/GOS) has been co-electrodeposited on the thermally treated carbon paper (TTCP) in phosphate buffer solution containing GOS and KMnO₄. The resulted samples have been characterized by scanning and transmission electron microscopy, Raman, X-ray diffraction, and X-ray photoelectron energy spectroscopy. The results show that the synthesized MnO₂ may be δ-MnO₂ and the morphology of MnO₂/GOS is very different from that of MnO₂, indicating that the introduction of GOS in electrolyte can influence the morphology during the deposition. The capacitive properties of the samples are investigated by using cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. The specific capacitance of MnO₂ for MnO₂/GOS can reach about 829 F g^?1 at discharged current density of 1.0 A g^?1 in 1 M Na₂SO₄ aqueous solution, which is larger than that of MnO₂ deposited on TTCP. The composite of MnO₂/GOS also exhibits excellent cyclic stability with a decrease of 18.5 % specific capacitance after 1,500 cycles.     

Cyclic Voltammetrically‐Prepared MnO2 Coated on an ITO Glass Substrate

For the first time, nanostructured manganese dioxide was successfully electrodeposited onto an ITO (indium tin oxide) glass substrate by cyclic voltammetry (CV) method from an aqueous solution of 0.1 M Na₂SO₄ containing 5 × 10⁻³ M MnSO₄. The obtained manganese dioxide‐modified ITO glass substrates were characterized by energy dispersive spectrometry (EDS), Fourier transform infrared spectrometry (FTIR) and scanning electron microscopy (SEM), respectively. All results not only proved the existence of MnO₂ on an ITO glass substrate but also demonstrated that the morphology of the obtained MnO₂ was greatly affected by the electrodeposition conditions. Also, this MnO₂‐modified ITO electrode was systematically investigated by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS) in an aqueous electrolyte of 0.1 M Na₂SO₄. The results obtained from electrochemical measurement indicated that this developed MnO₂‐modified ITO electrode has a satisfied specific capacitance value of 264 F·g⁻¹ and exhibits excellent electrochemical stability and reversibility.     

Redox‐Mediated Synthesis of a Fe3O4–MnO2 Nanocomposite for Dye Adsorption and Pseudocapacitance

A logically chosen redox reaction of submerged Fe⁰ in an aqueous KMnO₄ solution has been reported. The template‐free reaction conditions produced gram amounts of a hierarchical flowerlike Fe₃O₄–MnO₂ nanocomposite. More precisely, freshly prepared Fe⁰ nanoparticles were prepared from air‐free hot water under submerged conditions using a door magnet. The black Fe⁰ particles were oxidized in water quantitatively by KMnO₄ in the solution phase and the nanocomposite was prepared. The material has been used as a dye adsorbent and the representative cationic dye uptake, recovery, and recycling of the dye becomes easy owing to the ferromagnetic properties and surface negative charge of the material. The nanocomposite also showed a higher specific capacitance (327 F g^?1 at 10 mV s^?1) than the reported values of pure MnO₂ and Fe₃O₄. The material exhibited a high energy density as well as a high power density, and remained stable even after a large number of charge–discharge cycles.     

Chemiluminescence of Colloidal MnO2 with Luminol and Determination of Polyhydroxyl Compounds

The reaction between luminol and colloidal MnO₂ (prepared by chemical reduction of KMnO₄ with Na₂S₂O₃ under neutral aqueous condition) produced an intense chemiluminescence (CL) emission in alkaline medium. The CL reaction conditions were carefully optimized and the CL reaction mechanism was thoroughly discussed. Manganese(III) was suggested to be involved in the reaction and 3‐aminophthalate anion was the luminophor. Moreover, the effects of 23 compounds on the colloidal MnO₂‐luminol CL system were investigated to explore its possible analytical applications. Polyhydroxyl compounds were observed to inhibit the signal significantly, whereas sulfhydryl compounds enhance it slightly. The analytical figures for five polyhydroxyl compounds, namely ascorbic acid, rutin, pyrogallol, quercetin, and L‐adrenaline, were presented. As a preliminary application, the method was applied to the determination of rutin in pharmaceutical formulations.     

Metal carbonates: alternative to metal oxides for supercapacitor applications? A case study of MnCO<Subscript>3</Subscript> vs MnO<Subscript>2</Subscript>

We report here the potential competency of MnCO₃ versus MnO₂ for supercapacitor applications. MnCO₃ was synthesized by a hydrothermal method using KMnO₄ as a manganese source and either sugar or pyrrole as carbon source. MnCO₃ synthesized using sugar and pyrrole as carbon source is referred hereafter as MnCO₃(s) and MnCO₃(p), respectively. The synthesized products were characterized by powder X-ray diffraction, scanning electron microscopic and transmission electron microscopic studies. Microscopic studies revealed that MnO₂ possesses micro-flower-like morphology constructed by self-assembled nano-petals. While the morphology of MnCO₃(s) is sub-micron size particles of different shape, the morphology of MnCO₃(p) is crystalline particles of 10–20 nm dia. The capacitive characteristics of MnO₂, MnCO₃(s) and MnCO₃(p) were evaluated in aqueous 0.1 M Mg(ClO₄)₂ electrolyte between 0 and 1 V using cyclic voltammetry and galvanostatic charge/discharge cycling. Specific capacitance (SC) values of 216 and 296 F g^?1 obtained for MnCO₃(s) and MnCO₃(p) are 35 and 85 % higher than SC value of 160 F g^?1 obtained for MnO₂, respectively. Besides better capacitive storage characteristics, MnCO₃(s) and MnCO₃(p) have also exhibited better rate capability and cycle life than MnO₂.     

Sorption of plutonium from low level liquid waste using nano MnO2

Nano-crystalline MnO₂ has been synthesized by the method of alcoholic hydrolysis of KMnO₄ and its potential as a sorbent for plutonium present in the low level liquid waste (LLW) solutions was investigated. The kinetic studies on the sorption of Pu by MnO₂ reveal the attainment of equilibrium sorption in 15 h, however 90 % of sorption could be achieved within an hour. In the studies on optimization of the solution conditions for sorption, it was observed that the sorption increases with the pH of the aqueous solution, attains the maximum value of 100 % at pH = 3 and remains constant thereafter. The sorption was found to be nearly independent of the ionic strength (0.01–1.0 M) of the aqueous solutions maintained using NaClO₄, indicating the inner sphere complexation between the Pu⁴⁺ ions and the surface sites on MnO₂. Interference studies with different fission products, viz., Cs⁺, Sr²⁺ and Nd³⁺, revealed decrease in the percentage sorption with increasing pH of the suspension indicating the competition between the metal ions. However, at the metal ion concentrations prevalent in the low level liquid waste solutions, the decrease in the Pu sorption was only marginally decreased to 90 % at pH = 3, the decrease being more in the case of Nd³⁺ than that in the case of Cs⁺. This study, therefore, shows nano-crystalline MnO₂ can be used as a sorbent for separation of Pu from LLW solutions.     

Abtrennung von Submikrogramm-Mengen Arsen aus Ammomumfluoridlösungen

Arsenic has been separated from NH₄F solutions by coprecipitation with MnO(OH)₂, which is formed within the solution by addition of stoichiometric amounts of MnSO₄ and KMnO₄. The precipitate is dissolved in HC1 and As determined by known methods.     

Effect of in situ oxidization with potassium permanganate on the morphologies of SEBS membranes

The morphologies of the asymmetric membranes of polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) prepared and simultaneously oxidized with different substrate solutions were investigated with atomic force microscopy (AFM), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance infrared spectroscopy (ATR-FTIR). We used the KMnO₄ aqueous solution and KMnO₄/H₂SO₄ mixture solution as solvent-casting substrates, as well as oxidized reagents. The surface composition and functional groups of membranes were also measured. The effect of casting substrates on morphological changes was discussed through possible chemical reactions. It was found that the SEBS membranes were transformed from an ordered microphase-separated structure to disordered nodular or sponge-like structures. The former might be contributed to MnO₂ depositions while the latter was caused by the bond interruption, after KMnO₄ or KMnO₄/H₂SO₄ oxidizing.     

Thorium adsorption on graphene oxide nanoribbons/manganese dioxide composite material

A functional graphene oxide nanoribbons/manganese dioxide composite material (MnO₂-GONRs) was synthesized by hydrothermal method using graphene oxide nanoribbons (GONRs) as raw material which were formed by longitudinal unzipping of multi-walled carbon nanotubes with KMnO₄ and H₂SO₄. The microstructure of MnO₂-GONRs was characterized by SEM and FT-IR. The various factors affecting the adsorption of Th(IV) in aqueous solution such as pH, solid–liquid ratio, contact time, initial concentration and temperature were investigated by batch static adsorption experiments, and the adsorption mechanism is also discussed. The results showed that MnO₂-GONRs had a good adsorption effect on Th(IV) with a maximum adsorption of 166.11 mg/g.

     

ELECTROCHEMICAL PROPERTIES OF ASPHALTENE PARTICLES IN AQUEOUS SOLUTIONS

The electrical properties of colloidal asphaltene/water solution interface were determined by carrying out the potentiometric titration and electrokinetic measurements. Asphaltenes in aqueous solutions exhibit typical organic colloid properties i.e. surface charge and electrophoretic mobility. It was considered that the surface charge at the asphaltene particles is a result of protonation and dissociation reactions of surface functional groups. On the base of the surface charge density data vs. pH the surface reaction constants were calculated by numerical method. The agreement of these values with calculated ones, on the base of ζ potential data, is noticeable.

The characteristic feature of the investigated systems is the maximum, appearing on the curve ζ potential vs. electrolyte concentration. This behaviour is explained by _”hair layer ” structure of the asphaltene surface     

Surfactant stabilized nanopetals morphology of α-MnO2 prepared by microemulsion method

α-Manganese dioxide is synthesized in a microemulsion medium by a redox reaction between KMnO₄ and MnSO₄ in presence of sodium dodecyl sulphate as a surface active agent. The morphology of MnO₂ resembles nanopetals, which are spread parallel to the field. The material is further characterized by powder X-ray diffraction, energy dispersive analysis of X-ray, and Brunauer–Emmett–Teller surface area. Supercapacitance property of α-MnO₂ nanopetals is studied by cyclic voltammetry and galvanostatic charge–discharge cycling. High values of specific capacitance are obtained.     

Electrochemical capacitors with KCl electrolyte

Potassium manganese dioxide K_xMnO_{2 + δ}·nH₂O and amorphous MnO₂ in a mild 2 M KCl aqueous electrolyte prove to be excellent electrodes for faradaic electrochemical capacitors. The K_xMnO_{2 + δ}·nH₂O materials were prepared by direct thermal decomposition of KMnO₄ and contained a large amorphous/crystalline ratio. A sample decomposed at 550 °C gave a specific cyclic capacitance between −0.2 and +1.0 V/SCE of 240 F·g⁻¹, which corresponds to nearly one-third of the Mn(IV) ions participating in the faradaic reaction. Excellent cyclability at 12 mA·cm⁻² was found for 100 cycles. On short-circuit, K_0,31MnO_2,12·0,63 H₂O in 2 M KCl and pH 10.6 aqueous solution gave an initial current density of 0.58 A·cm⁻² and a total released charge of 4.6 C·cm⁻² compared with 0.32 A·cm⁻² and 11.1 C·cm⁻² for RuOOH·nH₂O in 5.3 M H₂SO₄. Similar results obtained with amorphous MnO₂ demonstrate that alkali ions can be used as the working ion in a faradaic supercapacitor, which frees the search for new materials from the constraint of working in a strong-acid aqueous medium.     

Schist‐like Nanostructured Manganese Oxides and Their Electrochemical Capacitance Properties

With an aim to develop environment friendly and inexpensive materials for electrochemical capacitor electrodes, nanocrystalline γ‐MnO₂ with layered architecture of schist‐like structure was successfully synthesized by reducing mixed MnSO₄×H₂O and CTAMnO₄ precursors in stoichiometric amounts through a hydrothermal process. The as‐prepared MnO₂ was characterized by field emission scanning electron microscopy, X‐ray diffraction, and thermal gravimetric analysis. Also, cyclic voltammetry was employed to investigate the electrochemical properties of the products in neutral KCl aqueous solution, showing a higher enhanced capacitive performance than that of the sample without a coating of schist‐like γ‐MnO₂, indicating that our MnO₂‐based electrode provides a promising opportunity for high‐performance energy storage devices.     

Ti3C2Tx/MnO2正极在水系锌电池中的电化学性能

二氧化锰(MnO₂)材料具有比容量大、电极电位高、储量丰富以及价格低廉等优势,成为水系锌电池正极最受关注的一类材料,然而其仍然存在着结构稳定性差和电化学储存机理复杂的问题。因此,我们通过两步合成法制备了一种花苞状结构的MnO₂负载在Ti₃C₂T_x表面形成Ti₃C₂T_x/MnO₂复合材料,通过X射线粉末衍射(XRD)、X射线光电子能谱(XPS)、透射电子显微镜(TEM)和高分辨透射电子显微镜(HRTEM)对复合样品的结构、成分和形貌进行表征。通过将Ti₃C₂T_x/MnO₂复合材料作为正极,与锌负极匹配组装成水系锌电池,研究了其分别在2 mol·L^-1 ZnSO₄、2 mol·L^-1 ZnSO₄+0.1 mol·L^-1 MnSO₄、30 mol·L^-1三氟甲基磺酸四乙基铵(TEAOTf)+1 mol·L^-1三氟甲烷磺酸锌(ZnOTf)和3 mol·L^-1 ZnOTf四种电解液中的电化学性能。结果表明,Ti₃C₂T_x/MnO₂在2 mol·L^-1 ZnSO₄中的比容量较高,但循环稳定性很差。将TEAOTf盐和ZnOTf盐共溶于水中,设计了一种新型的含惰性阳离子的超高浓度盐包水电解液(30 mol·L^-1 TEAOTf+1 mol·L^-1 ZnOTf),不仅提高了Ti₃C₂T_x/MnO₂材料的可逆性,而且有效抑制了电极材料在循环过程中的溶解。     

A simple homemade spectrophotometer

A simple portable spectrophotometer was constructed and used to perform analytical experiments. The developed model uses available tools and materials such as light emitting diode lamps, compact discs, plastic and cardboard boxes and a cell phone camera to build a design that illustrates the major components of spectrophotometers. The spectra of serial concentrations of KMnO₄ (50?600 μM) were recorded and the values of absorbance were extracted at λ_max to build the calibration curve. A linear relationship between the concentration and absorbance was obtained with the coefficient of determination 0.994. The model was also utilized to study the spectra of KMnO₄, phenolphthalein and bromothymol blue in comparison with a Shimadzu UV-1800 spectrophotometer as a reference instrument. In spite of the differences in the observed spectra, the recorded λ_max were almost identical to those measured by the developed model. The model was successfully used to determine the concentration of sodium alendronate through ligand-exchange complexation with ferric salicylate.     

Experimental Study on Viscosity of Colloidal Silica in Aqueous Electrolytic Solutions

The relative viscosity of colloidal silica dispersion in aqueous electrolytic solutions as the function of volume fraction of dry particles in the solutions has been experimentally determined in this work, in order to study the effects of pH and electrolytes (Na₂SO₄ and AlCl₃) on the hydration of the silica surfaces in the solutions. The results have shown that the maximum relative viscosity of the silica dispersion and the strongest hydration of the silica in aqueous solutions appeared at neutral pH, while the stronger the acidity and the alkalinity of the aqueous solution, the weaker the hydration. In the presence of the electrolytes (Na₂SO₄ and AlCl₃), the relative viscosity of the silica dispersion reduced and the hydration of the silica in aqueous solutions became weak. The higher the concentration of the electrolytes, the weaker the hydration, indicating that the destabilization of the colloidal silica dispersion in aqueous solutions might be realized through adding the high-valence electrolytes to weaken the hydration of the particle surfaces (hydration forces between the particles). Also, it has been shown that the negative zeta potentials of the colloidal silica in aqueous solutions greatly reduced in the presence of the electrolytes. Therefore, the high-valence electrolytes (Na₂SO₄ and AlCl₃) as the coagulant of colloidal silica in aqueous solutions might be originated from that the presence of the electrolytes simultaneously reduces the electrical double layer repulsive force and the hydration repulsive forces between the particles in aqueous solutions.     

Oxidations with alkaline permanganate using monovalent thallium for the back-titration: Estimation of trivalent arsenic

In absence of Ba⁺² ions arsenite reduces KMnO₄ in alkaline medium to MnO₂ without the appearance of an inflection at the manganate state. Reduction could be checked at the manganate in presence of 1N NaOH and Ba⁺² equal to 3 times that equivalent to MnO₄^-2 and arsenate, and when dilute arsenite solutions are applied viz.0.02N In absence of Ba⁺² ions the end-points are attained later than the MnO₂ stage except in 2–3N NaOH. In presence of telluric acid good results are obtained at all alkalinities whence reduction is checked at Mn⁺⁴.As⁺³ could be estimated also by mixing with KMnO₄ either in the presence of Ba⁺² ions + 1N NaOH or in absence of Ba⁺² ions + I.5–3N NaOH and back-titrating the excess oxidant with monovalent thallium.