Morphology-dependent electrochemical supercapacitive characteristics of nanostructured manganese dioxide

The dependence on morphology of the supercapacitive characteristics of manganese dioxide nanospheres (NSs) and nanorods (NRs) was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and a series of electrochemical techniques. Because the nanosized pores in MnO₂ NSs resulted in high surface area, MnO₂ electrodes made of NSs had higher specific capacitance (SC) than those made of NRs at current densities less than 2.0 A g⁻¹. However, at current densities over 2.0 A g⁻¹, the power density of MnO₂ electrodes composed of NRs was better than that of NSs. The high surface area and nanosized pores in MnO₂ NSs increase the number of redox active sites, which leads to high specific capacitance. On the other hand, the small pore size in MnO₂ NSs restricts the rates of charge and discharge, thus limiting their power density.     

An RDE and RRDE study into the electrodeposition of manganese dioxide

Electrodeposition of manganese dioxide has been examined using a combination of rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) voltammetry, with the goal of developing an understanding of the electrodeposition mechanism. Experimental variables that have been examined include electrolyte composition (combined changes within the ranges 0.1-1.0 M MnSO₄ and 0.1-5.0 M H₂SO₄), rotation rate (1000-4000 rpm) and temperature (22-98 °C). Voltammetric data (current peak instead of sigmoidal response, and non-proportional current-concentration data) indicates that already deposited manganese dioxide is a poorer catalytic surface compared to Pt. The overall electrodeposition process revolves on the formation of a Mn(III) intermediate, and whether it is soluble for extended periods of time, as in concentrated H₂SO₄ (>1.0 M), or whether it hydrolyzes rapidly to precipitate as a solid Mn(III) species (e.g. MnOOH) as in more dilute H₂SO₄ solutions (<1.0 M). In the more concentrated acid electrolytes most of the Mn(III) was lost to the bulk electrolyte through convection, with what little manganese dioxide that was formed resulting from chemical disproportionation. However, in dilute acid electrolytes, evidence suggests that the solid hydrolysis product underwent solid state oxidation to manganese dioxide. Activation energies extracted from temperature studies supports the different mechanism under different acid concentrations. Experiments examining the effect of rotation rate also indicate that the overall electrodeposition process is not mass transport limited.     

A novel method to prepare nanostructured manganese dioxide and its electrochemical properties as a supercapacitor electrode

Nanostructured MnO₂ was synthesized by co-precipitation in the presence of Pluronic P123 surfactant and characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscope (SEM) and transmission electron microscope (TEM). The sample without surfactant was spherical with particle size on the submicron scale, whereas P123-assisted samples were all loose clew shapes, consisting of MnO₂ nanowires, 8-20 nm in diameter and 200-400 nm in length. The electrochemical performances of the as-prepared MnO₂ as the electrode materials for supercapacitors were evaluated by cyclic voltammetry and galvanostatic charge-discharge measurements in a solution of 1 M Na₂SO₄. The sample without surfactant exhibited a relatively low specific capacitance of 77 F g⁻¹, whereas the nanostructured MnO₂ prepared with 0.02% (wt%) P123 exhibited excellent pseudocapacitive behavior, with a maximum specific capacitance of 176 F g⁻¹.     

Fabrication of superhydrophobic self-cleaning manganese dioxide coatings on Mg alloys inspired by lotus flower

Superhydrophobicity is evolutionarily adaptive to surrounding environment. Lotus flower is naturally occurring superhydrophobicity. Artificial self-cleaning materials function effectively on superhydrophobic surfaces so that water droplets bead up and roll off the surface taking the contamination particles away. Inspired by lotus flower, we have fabricated superhydrophobic Mg alloy surfaces using manganese dioxide (MnO₂) microspheres encased in stearic acid shells. The prepared superhydrophobic Mg alloy surface provides exceptional self-cleaning ability in air and oil, and remains non-wetting even after dynamic water shear, or exposure to strong acid, strong base, and saline solutions as well as organic solvents showing excellent mechanochemical durability, with broad application prospect.     

化学二氧化锰制备中粗二氧化锰的氧化研究

在化学二氧化锰的制备过程中,对粗二氧化锰的精制氧化是提高产品的振实密度和锰收率的关键。以氯酸钠为氧化剂,对粗二氧化锰进行精制氧化,考察了各工艺因素对产品振实密度及锰收率的影响,得到了最佳工艺条件:硫酸质量浓度为150 g/L、氯酸钠加料量为理论量的120%、氧化反应时间为3 h、反应体系液固体积比为3∶1。在此优化工艺条件下,溶液中Mn2+几乎全部被氧化成二氧化锰,锰收率达99.8%以上,产品振实密度大于2.0 g/cm3。     

Theoretical investigation of the phase behaviour of mixtures of a novel family of perfluoroalkyl-polyoxyethylene ether diblock surfactants in aqueous solutions of carbon dioxide

The accurate knowledge of the thermodynamic properties, with special emphasis on phase equilibria, of aqueous solutions of carbon dioxide is essential from practical and theoretical points of view. These aqueous mixtures are of great interest in different industrial processes, including its use as supercritical solvents, CO₂ sequestration technologies and oil enhanced recovery, among many different applications. From a theoretical point of view, this mixture exhibits a variety of interactions, including dispersive forces, hydrogen bonding between water molecules, and also dipolar and quadrupolar electrical interactions, which determine its phase behaviour. In particular, the mixture exhibits type III phase behaviour, which leads to large regions of liquid-liquid immiscibility. In a number of practical situations, it is important to make miscible this mixture. This can effectively be done by adding some amount of a selected surfactant that stabilizes the mixture. An ideal candidate would be a surfactant formed by hydrophilic and CO₂-philic parts that interact favourably with water and carbon dioxide molecules, respectively. In this work we study a novel family of diblock amphiphiles, the perfluoroalkyl-polyoxyethylene ether non-ionic diblock surfactants, which have the general formula F(CF₂)_i(OCH₂CH₂)_jOH or simply F_iE_j. We employ the sophisticated and versatile Statistical Associating Fluid Theory for potentials of variable range, the so-called SAFT-VR approach, to study the phase behaviour of aqueous solutions of carbon dioxide and the novel surfactants. All molecules are modeled under the united-atom approach, in which different chemical groups are treated as hard-sphere attractive segments. The F_iE_j surfactant is modelled as a diblock compound, which consists of a perfluorinated alkane chain and a polyoxyethylene ether chain. The optimized molecular parameters for H₂O, CO₂, and the perfluoroalkyl, polyoxyethylene ether and the rest of chemical groups forming the surfactant are taken from previous works from the literature. Unlike interactions between different chemical groups are carefully chosen to reproduce the phase behaviour of similar associating surfactants previously studied in the literature. In this work we study the effect of temperature, pressure and composition of a selection of mixtures to understand the microscopic conditions that determine the stabilization of aqueous solutions of carbon dioxide when this kind of surfactants are added. To our knowledge, this is the first time the phase behaviour of mixtures containing this kind of surfactants is studied using a microscopic modelling approach.     

Pulsed electrodeposition of Zn in the presence of surfactants

The preparation of Zn deposits has been performed by galvanostatic pulsed electrolysis, from acidic zinc sulphate solutions, on a stainless steel substrate. The influence of the surfactants (cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulphate (SDS) and octylphenolpoly(ethyleneglycolether)_n, n = 10, Triton X-100) on the voltammetric behaviour, structural and morphological characteristics of the deposits have been investigated. The characterization of the samples was made by X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDS). The experimental data shows that the presence of surfactants affects the zinc deposition process. The electrodeposits are mainly composed by Zn with different texture, crystal shape and size (grain size ranging from 40 to 20 nm). The obtained results led us to conclude that the Zn deposits prepared in the absence of surfactants and in the presence of SDS are more crystalline and with a higher grain size than the ones obtained in the presence of CTAB and Triton X-100. These facts may be justified by the increase on the overpotential deposition as the electrochemical studies confirm.The XRD results show that the deposits prepared, in the absence of surfactant and in the presence of SDS, contain ZnSO₄ and Zn₄SO₄(OH)₆ as oxidation products. ZnO is also detected on the deposits obtained in the presence of CTAB and Triton X-100.     

Facile synthesis and strongly microstructure-dependent electrochemical properties of graphene/manganese dioxide composites for supercapacitors

Graphene has attracted much attention since it was firstly stripped from graphite by two physicists in 2004, and the supercapacitor based on graphene has obtained wide attention and much investment as well. For practical applications of graphene-based supercapacitors, however, there are still many challenges to solve, for instance, to simplify the technological process, to lower the fabrication cost, and to improve the electrochemical performance. In this work, graphene/MnO₂ composites are prepared by a microwave sintering method, and we report here a relatively simple method for the supercapacitor packaging, i.e., dipping Ni-foam into a graphene/MnO₂ composite solution directly for a period of time to coat the active material on a current collector. It is found that the microwave reaction time has a significant effect on the microstructure of graphene/MnO₂ composites, and consequently, the electrochemical properties of the supercapacitors based on graphene/MnO₂ composites are strongly microstructure dependent. An appropriately longer microwave reaction time, namely, 15 min, facilitates a very dense and homogeneous microstructure of the graphene/MnO₂ composites, and thus, excellent electrochemical performance is achieved in the supercapacitor device, including a high specific capacitance of 296 F/g and a high capacitance retention of 93% after 3,000 times of charging/discharging cycles.

PACS

81.05.ue; 78.67.Sc; 88.80.fh     

Self-standing manganese dioxide/graphene carbon nanotubes film electrode for symmetric supercapacitor with high energy density and superior long cycling stability

The low capacitance utilization and capacitance fading of manganese dioxide (MnO₂) is mainly due to poor electro-conductivity and irreversible phase transform. This work proposes a new method of designing hierarchical and binder-free electrode based on MnO₂ material for stable supercapacitor with high specific capacitance. Herein, we fabricated the self-standing electrode of MnO₂ on nitrogen-doped graphene and single wall carbon nanotubes (SWCNTs) self-standing film (NGCF) by electrochemical deposition. As a result, as-prepared MnO₂/NGCF cathode showed excellent electrochemical performance of 489.7 F g^-1 at 1 A g^-1. Assembled symmetric aqueous supercapacitor (SC) manifests high voltage of 2.4 V and presents excellent high energy density of 106.7 Wh kg^-1 at 1200 W kg^-1 and outstanding long-life stability without no decay after 10 000 charge-discharge circuits. This work proposes a new view of designing hierarchical and binder-free electrode with high energy density and long cycling stability based on MnO₂ material for stable symmetric supercapacitor.     

Synthesis of manganese titanate and its precursors from xerogel

An overview of research on the synthesis of manganese titanates is presented. The xerogel of Mn–Ti–O–C–H composition was synthesized from manganese acetate and titanium tetrabutylate via liquid-phase method using organic solvents. The calcination of xerogel in air at 450 °C and 700 °C yielded manganese titanate precursors in the form of a nanostructured mixture of Mn₂O₃ and TiO₂. Annealing at 1000 °C, manganese metatitanate MnTiO₃ was obtained. Reference experiments with initial reagents included, separately, thermal decomposition of Mn(CH₃COO)₂×4H₂O and the product of Ti(OC₄H₉)₄ hydrolysis. The composition, structure, and properties of the products were studied using X-ray diffraction, scanning electron microscopy, elemental analysis, diffuse reflectance IR Fourier spectroscopy, thermogravimetry, and by measuring specific surface area. The data presented by these different techniques are basically consistent with each other (with an increase in the annealing temperature, an increase in globule size and decrease in specific surface area are observed; structuring occurs within the long- and short-range order; the size of the crystallites does not exceed that of the globules; elemental composition correlates with phase composition; the endothermic character of the reaction of MnTiO₃ formation at 900 °C is confirmed by a thermodynamic calculation). Nevertheless, some unexpected effects were revealed (based on the FTIR diffuse reflection spectra, mixed oxide Mn–Ti–O is formed in the surface layer of particles already at 450 °C and 700 °C; etc.). Application of the proposed technique for modifying Al₂O₃ powders, with the aim of implementing low-temperature sintering of corundum ceramics, is discussed.     

Phase diagram and supercritical deoxidation kinetics of manganese oxides in organic solvent N,N-dimethylformamide

Deoxidation of manganese oxides in supercritical N,N-dimethylformamide (SC-DMF) is revealed, contrasting to previous amounts of reports on supercritical water oxidation. Mn₃O₄ and MnO can be produced by the reaction starting from layered manganese oxides (δ-MnO₂) in a uniform supercritical process. Time effect can give rise to the transition to Mn₃O₄ under low temperature, but the prolonged time, under a temperature lower than 200 ̊C, cannot cause this transition. Phase diagram is obtained, including three regions of MnO₂, Mn₃O₄ and MnO as the main material phase. The completion of transition accompanies with an oxygen loss course. Comparison between supercritical oxidation of water and supercritical deoxidation of organic DMF demonstrates the unique mechanisms of supercritical processing. In terms of the surface contacting fluid, deoxidation kinetics has a reciprocal law of growing size for manganese oxides. The new deoxidation procedure using organic solvent is proposed for the processing of metal oxides.     

二氧化硫浸锰制备高纯碳酸锰

采用湘潭低品位的贫软锰矿为原料，对二氧化硫气体湿法浸锰、浸出液除杂、碳化结晶制备高纯碳酸锰的工艺进行了研究。实验得到二氧化硫浸锰的最佳工艺条件为：浸锰温度150～155℃，液固质量比3：1，二氧化硫体积分数6％～7％、流速1．5～2．0mL／min，浸锰时间3．0～3．5h。按该条件浸锰，锰的浸出率可达95％以上。该工艺为贫软锰矿的开发利用及有色金属冶炼厂二氧化硫废气的综合治理开辟了一条新途径。     

Degradation of alkanolamine blends by carbon dioxide

Aqueous solutions of MDEA, MDEA + DEA and MDEA + MEA containing 4.2 kmol/m³ total amine, were contacted with CO₂ at a partial pressure of 2.58 MPa and temperatures ranging from 120 to 180°C, in a stainless steel batch reactor. The reaction products include the known degradation compounds of the amines as well as products formed from secondary interactions in the amine blends. The rate of degradation was first order in the amines and, in magnitude, followed the sequence MDEA < MEA < DEA. Furthermore, the rate constant for MDEA was independent of amine substitution level and blend constituents. From a practical standpoint, MDEA + DEA blends would require frequent DEA make-up to maintain treating efficiency.     

Mass transfer and kinetics study on the sulfite forced oxidation with manganese ion catalyst

Wet limestone scrubbing is the most common flue gas desulfurization process (FGD) for control of sulfur dioxide emissions from the combustion of fossil fuels. Forced oxidation, which controls the overall reaction of the sulfur dioxide absorption, is the key path of the process. Manganese which comes from the coal is one of the catalysts during the forced oxidation process. In the present work, the two-film theory was used to analyze the sulfite forced oxidation reaction with an image boundary recognition technique, and the oxidation rate was experimentally studied by contacting pure oxygen with a sodium sulfite solution. There was a critical sulfite concentration 0.328 mol/L without catalyst or at a constant catalyst concentration value. The kinetics study focused on the active energy of the reaction and the reaction constant k; furthermore, we obtained the order with respect to the sulfite and Mn²⁺ concentrations. When the Mn²⁺ catalyst concentration was kept unchanged, the sulfite oxidation reaction rate was controlled by dual film and the reaction kinetics was first order with respect to sulfite while SO₃²⁻ concentration was below 0.328 mol/L; the sulfite oxidation reaction rate was controlled by gas film only and the reaction kinetics was zero order with respect to sulfite while SO₃²⁻ concentration over 0.328 mol/L. When SO₃²⁻ concentration was kept unchanged, the sulfite oxidation reaction rate depended on gas-liquid mass transfer and the reaction kinetics was different in various stages with respect to Mn²⁺ concentrations. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

层状二氧化锰材料的制备及其电化学性能研究

以高锰酸钾和5-氨基四唑为原料,采用常压加热法,制备了层状二氧化锰材料。应用X-射线衍射和扫描电镜技术对所得材料的结构和形貌进行表征。结果表明,所得材料具有层状结构,为颗粒状;恒流充放电测试显示,制备材料具有较好的电化学性能,在50 mA/g的电流密度下,首次充电比容量为761 mAh/g,循环50圈后,容量保持率为50%。此外,制备材料具有良好的倍率性能。     

苯胺氧化制对苯二酚中二氧化锰的回收利用

苯胺氧化法生产对苯二酚的二氧化锰废水经熟石灰除杂,用氨水调pH至碱性,用双氧水氧化,硫酸精制得到二氧化锰固体粉末。最佳回收条件为:双氧水用量为64%(占废液体积),反应液pH为10.0,反应温度为25℃,反应时间为1 h,二氧化锰的回收率可达到80%,采用回收的二氧化锰氧化苯胺可使对苯醌的收率从72.48%提高到83.90%。     

Adsorption equilibrium and kinetics studies of divalent manganese from phosphoric acid solution by using cationic exchange resin

There are numerous impurities in wet-process phosphoric acid, among which manganese is one of detrimental metallic impurities, and it causes striking negative effects on the industrial phosphoric acid production and downstream commodity. This article investigated the adsorption behavior of manganese from phosphoric acid employing Sinco-430 cationic ion-exchange resin. Resorting FT-IR and XPS characterizations, the adsorption mechanism was proved to be that manganese was combined with sulfonic acid group. Several crucial parameters such as temperature, phosphoric acid content and resin dose were studied to optimize adsorption efficiency. Through optimization, removal percentage and sorption capacity of manganese reached 53.12 wt%, 28.34 mg·g⁻¹, respectively. Pseudo-2nd-order kinetic model simulated kinetics data best and the activation energy was evaluated as 6.34 kJ·mol⁻¹ for the sorption reaction of manganese. In addition, the global adsorption rate was first controlled by film diffusion process and second determined by pore diffusion process. It was found that the resin could adsorb up to 50.24 mg·g⁻¹ for manganese. Equilibrium studies showed that Toth adsorption isotherm model fitted best, followed by Temkin and Langmuir adsorption isotherm models. Thermodynamic analysis showed that manganese adsorption was an endothermic process with enhanced randomness and spontaneity.     

新型二氧化锰还原炉的设计与应用

根据锰系列产品生产的需要和对现有窑炉的研究,设计出新型二氧化锰还原焙烧炉。炉体结构：柱状、立式、多通道、燃烧室与还原室隔离、隔绝空气冷却。配套设施：采用燃烧混合煤气的先进、环保的供热方式;采用数字化仪表显示炉内温度;进出料采用自动化设备。另外,确定了二氧化锰还原焙烧的分析测试方法。特点：生产成本低,工人劳动强度小、环境污染程度小。采用新型焙烧炉,对多个产地的二氧化锰进行还原焙烧,还原率均大于94%。     

溶胶-乳状液-凝胶法制备纳米MnO_2及其催化性质研究

采用溶胶-乳状液-凝胶法制备纳米MnO2,探讨了不同还原剂对MnO2溶胶的稳定性、乳化剂和凝胶剂的用量等对产品形貌、粒径等的影响。结果表明,富马酸是制备MnO2溶胶比较合适的还原剂,Span80和Tween80复配乳化剂的效果最好,在油∶溶胶=4∶1,乳状液∶三乙胺=8∶3(体积比)的条件下制得的纳米MnO2微粒,其形貌为规整的球体,粒径均匀,几乎为单分散性。溶胶-乳状液-凝胶法制备的样品,其催化性能明显高于分析纯的MnO2。     

Electrochemical kinetic behaviour of the aqueous manganese dioxide electrode

The processes involved in the reduction of an electrolytic manganese dioxide (EMD) were investigated and their exchange current densities were determined. The dQ/dE vs E curve was fitted by least squares minimisation with peaks based on the Nernst equation. The derivation of the peak model is described. In the potential range of 0.3 to - 0.4 V (vs Hg/HgO reference), nine processes were found to contribute to the overall reduction. Exchange current densities for three of the main processes were determined from the movement of the deconvoluted peaks during a series of constant current discharge experiments.