Layered δ-MnO2 Research Articles

Joint Charge Storage for High-Rate Aqueous Zinc-Manganese Dioxide Batteries.

Aqueous rechargeable zinc-manganese dioxide batteries show great promise for large-scale energy storage due to their use of environmentally friendly, abundant, and rechargeable Zn metal anodes and MnO2 cathodes. In the literature various intercalation and conversion reaction mechanisms in MnO2 have been reported, but it is not clear how these mechanisms can be simultaneously manipulated to improve the charge storage and transport properties. A systematical study to understand the charge storage mechanisms in a layered δ-MnO2 cathode is reported. An electrolyte-dependent reaction mechanism in δ-MnO2 is identified. Nondiffusion controlled Zn2+ intercalation in bulky δ-MnO2 and control of H+ conversion reaction pathways over a wide C-rate charge-discharge range facilitate high rate performance of the δ-MnO2 cathode without sacrificing the energy density in optimal electrolytes. The Zn-δ-MnO2 system delivers a discharge capacity of 136.9 mAh g-1 at 20 C and capacity retention of 93% over 4000 cycles with this joint charge storage mechanism. This study opens a new gateway for the design of high-rate electrode materials by manipulating the effective redox reactions in electrode materials for rechargeable batteries.

Ultrathin δ-MnO2 nanosheets as cathode for aqueous rechargeable zinc ion battery

Ultrathin δ-MnO2 nanosheets were synthesized via in-situ reduction of KMnO4 on graphene oxide using graphene oxide as both reductant and self-sacrificing template. The layered δ-MnO2 nanosheets with a thickness of about 2–4 nm inherit the two-dimensional lamellar morphology of graphene oxide. When used as cathode for aqueous zinc ion batteries, the prepared δ-MnO2 nanosheets exhibit a reversible capacity of 133 mAh/g at a current density of 100 mA/g over 100 cycles and a capacity of 86 mAh/g at 500 mA/g due to the layered structure and ultrathin morphology, which is much better than δ-MnO2 microspheres. The electrochemical and structural investigation indicates a possible mechanism of two-step co-insertion of H+ and Zn2+ into the interlayer of δ-MnO2 during discharge process. It's believed that the unique ultrathin morphology of δ-MnO2 can well facilitate ion diffusion and structure stability during cycling, making ultrathin δ-MnO2 nanosheets a promising cathode in aqueous zinc ion batteries.

Fast Microwave-Assisted Hydrothermal Synthesis of Pure Layered δ-MnO₂ for Multivalent Ion Intercalation.

This work reports on the synthesis of layered manganese oxides (δ-MnO2) and their possible application as cathode intercalation materials in Al-ion and Zn-ion batteries. By using a one-pot microwave-assisted synthesis route in 1.6 M KOH (MnVII:MnII = 0.33), a pure layered δ-MnO2 birnessite phase without any hausmannite traces was obtained after only a 14 h reaction time period at 110 °C. Attempts to enhance crystallinity level of as-prepared birnessite through increasing of reaction time up to 96 h in 1.6 M KOH failed and led to decreases in crystallinity and the emergence of an additional hausmannite phase. The influence of MnII:OH− ratio (1:2 to 1:10) on phase crystallinity and hausmannite phase formation for 96 h reaction time was investigated as well. By increasing alkalinity of the reaction mixture up to 2.5 M KOH, a slight increase in crystallinity of birnessite phase was achieved, but hausmannite formation couldn’t be inhibited as hoped. The as-prepared layered δ-MnO2 powder material was spray-coated on a carbon paper and tested in laboratory cells with Al or Zn as active materials. The Al-ion tests were carried out in EMIMCl/AlCl3 while the Zn-Ion experiments were performed in water containing choline acetate (ChAcO) or a ZnSO4 solution. Best performance in terms of capacity was yielded in the Zn-ion cell (200 mWh g−1 for 20 cycles) compared to about 3 mAh g−1 for the Al-ion cell. The poor activity of the latter system was attributed to low dissociation rate of tetrachloroaluminate ions (AlCl4−) in the EMIMCl/AlCl3 mixture into positive Al complexes which are needed for charge compensation of the oxide-based cathode during the discharge step.

Large-Scale Synthesis and Comprehensive Structure Study of δ-MnO2.

Layered δ-MnO2 (birnessites) are ubiquitous in nature and have also been reported to work as promising water oxidation catalysts or rechargeable alkali-ion battery cathodes when fabricated under appropriate conditions. Although tremendous effort has been spent on resolving the structure of natural/synthetic layered δ-MnO2 in the last few decades, no conclusive result has been reached. In this Article, we report an environmentally friendly route to synthesizing homogeneous Cu-rich layered δ-MnO2 nanoflowers in large scale. The local and average structure of synthetic Cu-rich layered δ-MnO2 has been successfully resolved from combined Mn/Cu K-edge extended X-ray fine structure spectroscopy and X-ray and neutron total scattering analysis. It is found that appreciable amounts (∼8%) of Mn vacancies are present in the MnO2 layer and Cu2+ occupies the interlayer sites above/below the vacant Mn sites. Effective hydrogen bonding among the interlayer water molecules and adjacent layer O ions has also been observed for the first time. These hydrogen bonds are found to play the key role in maintaining the intermediate and long-range stacking coherence of MnO2 layers. Quantitative analysis of the turbostratic stacking disorder in this compound was achieved using a supercell approach coupled with anisotropic particle-size-effect modeling. The present method is expected to be generally applicable to the structural study of other technologically important nanomaterials.

Layered-MnO₂ Nanosheet Grown on Nitrogen-Doped Graphene Template as a Composite Cathode for Flexible Solid-State Asymmetric Supercapacitor.

Flexible solid-state supercapacitors provide a promising energy-storage alternative for the rapidly growing flexible and wearable electronic industry. Further improving device energy density and developing a cheap flexible current collector are two major challenges in pushing the technology forward. In this work, we synthesize a nitrogen-doped graphene/MnO2 nanosheet (NGMn) composite by a simple hydrothermal method. Nitrogen-doped graphene acts as a template to induce the growth of layered δ-MnO2 and improves the electronic conductivity of the composite. The NGMn composite exhibits a large specific capacitance of about 305 F g(-1) at a scan rate of 5 mV s(-1). We also create a cheap and highly conductive flexible current collector using Scotch tape. Flexible solid-state asymmetric supercapacitors are fabricated with NGMn cathode, activated carbon anode, and PVA-LiCl gel electrolyte. The device can achieve a high operation voltage of 1.8 V and exhibits a maximum energy density of 3.5 mWh cm(-3) at a power density of 0.019 W cm(-3). Moreover, it retains >90% of its initial capacitance after 1500 cycles. Because of its flexibility, high energy density, and good cycle life, NGMn-based flexible solid state asymmetric supercapacitors have great potential for application in next-generation portable and wearable electronics.

Unveiling the Charge Storage Mechanism of Layered and Tunnel Structures of Manganese Oxides as Electrodes for Supercapacitors

The development of high performance active materials for supercapacitors that can outperform the state of art RuO2 is highly desirable to meet the next generation high power storage devices. In this regard, manganese oxide (MnO2) based materials have been intensively investigated in pseudocapacitors due to their high theoretical specific capacitance, natural abundance, good chemical and thermal stability, environmental benignity and low cost. Unfortunately, mechanism of electrochemical charge storage of MnO2 is not well understood. Here, we present a detailed charge storage mechanism of layered (δ-MnO2) and tunnel (α-MnO2) structures and the capacitance difference between layered and tunnel structures by preparing MnO2 nanostructures using permanganate and amino acids such as glycine, arginine and glutamic acid. Physico-chemical characterizations were carried out using powder X-ray diffraction, thermogravimetric analysis and infrared spectroscopy. Field emission scanning electron microscopy, transmission electron microscopy and high resolution transmission electron microscopy were conducted to analyze the morphology and structural characteristics of the materials synthesized. The electrochemical performance of MnO2 nanostructures is assessed using cyclic voltammogram (CV) and galvanostatic charge-discharge analysis. Complete physico-chemical and electrochemical investigations strongly support that surface adsorption-desorption of cations is predominant in layered δ-MnO2, while tunnel α-MnO2 is more favorable for cation intercalation-deintercalation process.

Morphology and Electrochemical Properties of α-, β-, γ-, and δ-MnO2 Synthesized by Redox Method

The crystal structures and electrochemical properties of α-, β-, γ-, and δ-MnO2, synthesized by a redox method under various conditions, were studied for the application of MnO2 as a positive electrode in a fuel cell/battery (FCB) system. The effects of potassium ion concentration (0–10 M) and temperature (60–160°C) on the morphology of synthesized MnO2 were investigated by X-ray diffraction, scanning electron microscopy, and the Brunauer-Emmett-Teller method. In addition, the charge and discharge characteristics, and life cycle performance of MnO2 as a positive electrode in an FCB system, were investigated by sweep voltammetry and potentiometry. The results indicate that four different crystal structures were obtained by different synthesis conditions: three tunnel structures (α-, β-, and γ-MnO2) and one layered structure (δ-MnO2). The effects of precipitation conditions were mapped and summarized in a phase diagram. Electrochemical testing showed that MnO2 with small tunnel structures (i.e., β- and γ-MnO2) exhibit better life cycle performance than either large tunnel structure α-MnO2 or layered δ-MnO2. Based on XRD analysis carried out after cycling, a schematic diagram is proposed to explain the degradation of the different MnO2 compounds.

High performance MnO2 nanoflower electrode and the relationship between solvated ion size and specific capacitance in highly conductive electrolytes

Flower shaped birnessite type manganese oxide (δ-MnO2) nanostructures are synthesized using a simple hydrothermal process with an aim to fabricate high performance supercapacitors for energy storage electrode. The studies reveal that layered δ-MnO2 had a basal plane spacing of ∼0.73nm and are composed of thin nanosheets of thickness ∼23nm. A detailed investigation is undertaken to draw a relationship between the solvated ion size of alkaline electrolytes (LiOH, NaOH and KOH) and pore size in the electrode material favoring high specific capacitance and faster electrode kinetics. The present work not only develops a high performance supercapacitive material but also identifies that by suitably tuning the sizes of solvated ion and the pores, supercapacitive behavior of a single material system can be tailored.

Facile chemical synthesis of nanoporous layered δ-MnO2 thin film for high-performance flexible electrochemical capacitors

Layered δ-MnO2 thin films with a three-dimensional nanostructure are successfully fabricated on stainless steel foil substrates for flexible electrochemical capacitors by a facile and effective chemical bath deposition technology from ethanol and potassium permanganate solution at 15°C. The as-prepared thin films display nanoporous morphology and a water contact angle of 20°. Energy-dispersive X-ray spectroscopy, X-ray diffraction, and Fourier transform infrared spectroscopy analyses reveal that the thin films are composed of δ-MnO2. Electrochemical data demonstrate that the δ-MnO2 thin film electrodes can deliver a high special capacitance of 447F/g at 2mV/s, and provide a good capacitance retention ratio of 87% after 1000 continuous cycles at 10mV/s in 0.5M Na2SO4. Compressive and tensile bending tests show that the as-prepared electrodes can steadily work over a wide range of applied curvatures between −2.5cm−1 (tension) and 2.5cm−1 (compression). Only a small decrease in special capacitance (0.9% at a curvature of 2.5cm−1 under compressive strain, or 1.2% at a curvature of −2.5cm−1 under tensile strain) is observed even after bending for 200 cycles, indicating the excellent mechanical flexibility and electrochemical stability of the δ-MnO2 thin film electrodes.

Room Temperature Synthesis Routes to the 2D Nanoplates and 1D Nanowires/Nanorods of Manganese Oxides with Highly Stable Pseudocapacitance Behaviors

The 2D nanoplates of δ-MnO2 and the 1D nanowires/nanorods of α-MnO2 can be synthesized at room temperature via one-pot oxidation reaction of commercially available divalent manganese compounds. Treating the MnO or MnCO3 precursor with persulfate ions for 1–2 days yields layered δ-MnO2 2D nanoplates, whereas the same oxidation reaction for the MnSO4 precursor produces γ-MnO2-structured 3D urchins. As the reaction time is extended for ∼14–21 days, not only δ-MnO2 nanoplates but also γ-MnO2 urchins are changed to well-separated 1D nanostructured α-MnO2 materials with controllable diameters. According to N2 adsorption–desorption isotherm measurements and Mn K-edge X-ray absorption spectroscopy, all the obtained manganate nanostructures show expanded surface areas of ∼50–120 m2 g–1 and the mixed oxidation state of Mn3+/Mn4+, respectively. All the present nanostructured manganese oxides exhibit pseudocapacitance behaviors with large specific capacitance and excellent capacitance retention, highlighting their promising functionality as a supercapacitor electrode. Among the materials under investigation, the δ-MnO2 2D nanoplates show the largest specific capacitance (∼180–210 F g–1). The present finding clearly demonstrates that the room-temperature oxidation reaction of the MnO or MnCO3 precursor can provide a facile soft-chemical route to 2D δ-MnO2 nanoplates and 1D α-MnO2 nanowires/nanorods with highly stable pseudocapacitance behaviors.

Single-Step Synthesis, Characterization, and Application of Nanostructured KxMn1-yCoyO2-δ with Controllable Chemical Compositions and Crystal Structures

Cobalt-substituted manganese oxide one-dimensional (1D) nanowires have been synthesized through one-pot hydrothermal treatment of the ion pair solution of Co2+ and MnO4-, along with their three-dimensional (3D) hierarchically assembled microspheres. The stabilization of cobalt ion in the manganese sites of α- and δ-MnO2 structures was clearly evidenced by Mn K-edge and Co K-edge X-ray absorption spectroscopy as well as by powder X-ray diffraction and elemental analyses. The chemical composition, crystal structure, and morphology of the resultant nanostructures can be easily tailored through the control of reaction condition and precursor composition. That is, 1D nanowires with the tunnel-type α-MnO2 structure can be prepared by the reaction at 140 °C, whereas the lowering of reaction temperature or the increase of Co content gives rise to the synthesis of 3D hierarchical nanostructured microspheres with the layered δ-MnO2 structure. It has been obviously demonstrated that the partial Co substitution for the nanostructured manganese oxides causes the improvement of their catalytic activity with respect to olefin oxidation as well as affects significantly their electrode performances. The present study provides an effective way of controlling the chemical properties and functionalities of nanostructured manganese oxides through cation substitution.