Lihu Liu

Facile hydrothermal synthesis and electrochemical properties of orthorhombic LiMnO2 cathode materials for rechargeable lithium batteries

Pure-phase, LiMn2O4-mixed and aluminum-doped orthorhombic LiMnO2 (o-LiMnO2) cathode materials with high discharge capacity and excellent cyclic stability were prepared by one-step hydrothermal reaction of MnCl2, EDTA, LiOH, AlCl3 and NaClO solutions. Chemical composition and aluminum content were affected by temperature and the concentration of LiOH, NaClO and AlCl3. A mixed phase of Mn3O4 and o-LiMnO2, pure-phase o-LiMnO2, and a mixed phase of o-LiMnO2 and LiMn2O4 were formed with increasing the concentration of NaClO from 0.08 to 0.25 mol L−1 at 180 °C for 24 h. Adding EDTA and NaClO facilitated the formation of o-LiMnO2. Al/Mn molar percent ratios in doped o-LiMnO2 were 0.34, 0.58, 0.91, and 1.22 when Al/Mn molar ratios in hydrothermal system were controlled at 0.05, 0.10, 0.15, and 0.20, respectively. Mixing LiMn2O4 and doping Al improved the discharge capacity and cyclic stability of o-LiMnO2. o-LiMnO2, the mixture with an o-LiMnO2/LiMn2O4 mass ratio of 2.45, and doped o-LiMnO2 with an Al/Mn molar percent ratio of 0.58 exhibited initial discharge capacities of 76, 139, and 82 mA h g−1, and cycling capacities of 124, 144, and 156 mA h g−1 after 100 cycles, respectively. This work facilitates the preparation and electrochemical performance improvement of o-LiMnO2.

Facile synthesis of birnessite-type manganese oxide nanoparticles as supercapacitor electrode materials.

Manganese oxides are environmentally benign supercapacitor electrode materials and, in particular, birnessite-type structure shows very promising electrochemical performance. In this work, nanostructured birnessite was facilely prepared by adding dropwise NH2OH·HCl to KMnO4 solution under ambient temperature and pressure. In order to fully exploit the potential of birnessite-type manganese oxide electrode materials, the effects of specific surface area, pore size, content of K(+), and manganese average oxidation state (Mn AOS) on their electrochemical performance were studied. The results showed that with the increase of NH2OH·HCl, the Mn AOS decreased and the corresponding pore sizes and specific surface area of birnessite increased. The synthesized nanostructured birnessite showed the highest specific capacitance of 245Fg(-1) at a current density of 0.1Ag(-1) within a potential range of 0-0.9V, and excellent cycle stability with a capacitance retention rate of 92% after 3000 cycles at a current density of 1.0Ag(-1). The present work implies that specific capacitance is mainly affected by specific surface area and pore volume, and provides a new method for the facile preparation of birnessite-type manganese oxide with excellent capacitive performance.

Cadmium Removal from Aqueous Solution by a Deionization Supercapacitor with a Birnessite Electrode

Birnessite is widely used as an excellent adsorbent for heavy metal ions and as active electrode materials for supercapacitors. The occurrence of redox reactions of manganese oxides is usually accompanied by the intercalation-deintercalation of cations during the charge-discharge processes of supercapacitors. In this study, based on the charge-discharge principle of the supercapacitor and excellent adsorption properties of birnessite, a birnessite-based electrode was used to remove Cd2+ from aqueous solutions. The Cd2+ removal mechanism and the influences of birnessite loading and pH on the removal performance were investigated. The results showed that Cd2+ was adsorbed on the surfaces and interlayers of birnessite, and the maximum electrosorption capacity of birnessite for Cd2+ was about 900.7 mg g-1 (8.01 mmol g-1), which was significantly higher than the adsorption isotherm capacity of birnessite (125.8 mg g-1). The electrosorption specific capacity of birnessite for Cd2+ increased with an increase in initial Cd2+ concentration and decreased with an increase in the loading of active birnessite. In the pH range of 3.0-6.0, the electrosorption capacity increased at first with an increase in pH and then reached equilibrium above pH 4.0. This work provides a new method for the highly efficient adsorption of Cd2+ from polluted wastewater.

Zinc removal from aqueous solution using a deionization pseudocapacitor with a high-performance nanostructured birnessite electrode

Manganese oxides are widely studied as heavy metal ion adsorbents and pseudocapacitor electrode materials. Synthesis methods affect the crystal structure, particle size, micromorphology, and the corresponding physicochemical properties of manganese oxides. In this work, nanostructured birnessite was readily obtained through hydrothermal reaction of KMnO4 and β-cyclodextrin under microwave irradiation. Based on the working principle of pseudocapacitors, the nanostructured birnessite was used as an electrode material for Zn2+ removal from aqueous solution by multi-cycle galvanostatic charge–discharge. The effects of electrolyte pH and birnessite mass on Zn2+ removal capacity were further investigated. The results indicate that the Zn2+ removal capacity increases and decreases with the increase of pH and birnessite mass, respectively. The highest Zn2+ removal capacity reaches 530.0 mg g−1, which is remarkably higher than the adsorption isotherm capacity (56.1 mg g−1). The significant improvement of electrochemical removal capacity can be attributed to the nanostructure and the not fully reversible redox reaction of the birnessite. The result of X-ray absorption fine structure (XAFS) indicates that Zn2+ is adsorbed above/below the vacancies and is inserted into the interlayer of birnessite, leading to the transformation of birnessite to Zn-buserite and hetaerolite during the charge–discharge process. The present study proposes a facile method for the rapid synthesis of nanostructured birnessite and highly efficient removal of Zn2+ from aqueous solution.

In situ detection of intermediates from the interaction of dissolved sulfide and manganese oxides with a platinum electrode in aqueous systems

Environmental context Dissolved sulfide results in soil acidification and subsequent contaminant leaching via oxidation processes, usually involving manganese oxides. In this work, redox processes were monitored in situ by cyclic voltammetry and HS– concentrations were semi-quantitatively determined. The method provides qualitative and semi-quantitative assessment for dissolved sulfide and its oxidation intermediates in aqueous systems. Abstract Dissolved sulfide can be oxidised by manganese oxides in supergene environments, while the intermediates including S0, S2O32– and SO32– are easily oxidised by oxygen in air, resulting in some experimental errors in conventional analyses. In this work, the electrochemical behaviours of HS–, S2O32– and SO32– on a platinum electrode were studied by cyclic voltammetry and constant potential electrolysis, and in situ detection of the intermediates was conducted in aqueous systems of HS– and manganese oxides. The results showed that HS– was first oxidised to S0, and then transformed to SO42–. The peak current for the oxidation of HS– to S0 had a positive linear correlation with the used starting HS– concentration. S2O32– and SO32– were directly electrochemically oxidised to SO42–. The oxidation current peak potentials at 0, 0.45 and 0.7V were respectively observed for HS–, S2O32– and SO32– at pH 12.0. Cyclic voltammetry was conducted to monitor the redox processes of HS– and manganese oxides. The oxidation peak current of HS– to S0 decreased, and that of S2O32– to SO42– was observed to increase as the reaction proceeded. The rate of the decrease of the oxidation peak current of HS– indicated that the oxidation activity followed the order of birnessite>todorokite>manganite.

High-performance Cu 2+ adsorption of birnessite using electrochemically controlled redox reactions

Manganese oxides are proposed as superior adsorbents for heavy metal ions, and their adsorption capacities can be greatly improved by electrochemical methods. In this work, birnessite was used as electrode material for Cu2+ adsorption by multi-cycle electrochemical redox reaction. The effects of solution pH and potential window on Cu2+ electrosorption capacity were further investigated. The results showed that the electrosorption capacity for Cu2+ reached as high as 372.3 mg g-1 by electrochemical redox, which was remarkably larger than the adsorption isotherm capacity (44.3 mg g-1). In addition, birnessite could be reused for many times after electrochemical activation. In the process of electrosorption, the amount of copper electrodeposited on the counter electrode accounted for less than 3.2% of the total removal capacity. The enhancement of Cu2+ adsorption capacity could be attributed to the changes in the chemical composition and the dissolution-recrystallization processes of birnessite during the electrochemical redox reactions. The electrosorption capacity increased with increasing pH from 3.0 to 5.0 and potential window width. The present work shows that controllable redox reaction of birnessite is a promising method for the removal of Cu2+ from wastewater.

Abiotic photomineralization and transformation of iron oxide nanominerals in aqueous systems

The formation and transformation of iron oxide nanominerals in water environments control the migration and conversion of essential and toxic elements and organic pollutants. This study demonstrates the formation of iron oxide nanominerals through the oxidation of Fe2+aq by hydroxyl radicals (OH˙) and superoxide radicals (O2˙−) generated from the photolysis of nitrate. The mineral compositions were affected by the anion species and pH. In the photochemical system, schwertmannite was formed in 5.0 mmol L−1 SO42− solution with the initial pH of 6.0, and a mixture of goethite and lepidocrocite was formed when the SO42− concentration decreased to 0.1 mmol L−1. The particle size of schwertmannite increased with decreasing initial pH from 6.0 to 3.0. When Cl− was used instead of SO42−, single-phase lepidocrocite was formed with the initial pH of 6.0. When the initial pH decreased to 4.5 and 3.0, a mixture of goethite and lepidocrocite was formed, and the relative content of lepidocrocite decreased with decreasing initial pH. Under anoxic conditions, Fe2+aq promoted the transformation of the photochemically synthesized schwertmannite to goethite and lepidocrocite by dissolution–recrystallization. The present work expands our understanding of the generation and transformation of iron oxide nanominerals in nitrate-rich supergene environments.

Photochemical Formation and Transformation of Birnessite: Effects of Cations on Micromorphology and Crystal Structure

As important components with excellent oxidation and adsorption activity in soils and sediments, manganese oxides affect the transportation and fate of nutrients and pollutants in natural environments. In this work, birnessite was formed by photocatalytic oxidation of Mn2+aq in the presence of nitrate under solar irradiation. The effects of concentrations and species of interlayer cations (Na+, Mg2+, and K+) on birnessite crystal structure and micromorphology were investigated. The roles of adsorbed Mn2+ and pH in the transformation of the photosynthetic birnessite were further studied. The results indicated that Mn2+aq was oxidized to birnessite by superoxide radicals (O2•-) generated from the photolysis of NO3- under UV irradiation. The particle size and thickness of birnessite decreased with increasing cation concentration. The birnessite showed a plate-like morphology in the presence of K+, while exhibited a rumpled sheet-like morphology when Na+ or Mg2+ was used. The different micromorphologies of birnessites could be ascribed to the position of cations in the interlayer. The adsorbed Mn2+ and high pH facilitated the reduction of birnessite to low-valence manganese oxides including hausmannite, feitknechtite, and manganite. This study suggests that interlayer cations and Mn2+ play essential roles in the photochemical formation and transformation of birnessite in aqueous environments.

Cd2+ adsorption performance of tunnel-structured manganese oxides driven by electrochemically controlled redox

The heavy metal ion adsorption performance of birnessite (a layer-structured manganese oxide) can be enhanced by decreasing the Mn average oxidation state (Mn AOS) and dissolution-recrystallization during electrochemical redox reactions. However, the electrochemical adsorption processes of heavy metal ions by tunnel-structured manganese oxides are still enigmatic. Here, tunnel-structured manganese oxides including pyrolusite (2.3 Å × 2.3 Å tunnel), cryptomelane (4.6 Å × 4.6 Å tunnel) and todorokite (6.9 Å × 6.9 Å tunnel) were synthesized, and their electrochemical adsorptions for Cd2+ were performed through galvanostatic charge-discharge. The influence of both supporting ion species in the tunnel and tunnel size on the electrochemical adsorption performance was also studied. The adsorption capacity of tunnel-structured manganese oxides for Cd2+ was remarkably enhanced by electrochemical redox reactions. Relative to K+ in the tunnel of cryptomelane, the supporting ion H+ was more favorable to the electrochemical adsorption of Cd2+. With increasing initial pH and specific surface area, the electrochemical adsorption capacity of cryptomelane increased. The cryptomelane electrode could be regenerated by galvanostatic charge-discharge in Na2SO4 solution. Due to the differences in their tunnel size and supporting ion species, the tunnel-structured manganese oxides follow the order of cryptomelane (192.0 mg g-1) > todorokite (44.8 mg g-1) > pyrolusite (13.5 mg g-1) in their electrochemical adsorption capacities for Cd2+.