Rutile (β-)MnO2 Surfaces and Vacancy Formation for High Electrochemical and Catalytic Performance (original) (raw)
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Effect of crystallographic structure of MnO2 on its electrochemical capacitance properties
MnO2 is currently under extensive investigations for its capacitance properties. MnO2 crystallizes into several crystallographic structures, namely, R, â, ç, ä, and ì structures. Because these structures differ in the way MnO6 octahedra are interlinked, they possess tunnels or interlayers with gaps of different magnitudes. Because capacitance properties are due to intercalation/deintercalation of protons or cations in MnO2, only some crystallographic structures, which possess sufficient gaps to accommodate these ions, are expected to be useful for capacitance studies. In order to examine the dependence of capacitance on crystal structure, the present study involves preparation of these various crystal phases of MnO2 in nanodimensions and to evaluate their capacitance properties. Results of R-MnO2 prepared by a microemulsion route (R-MnO2(m)) are also used for comparison. Spherical particles of about 50 nm, nanorods of 30-50 nm in diameter, or interlocked fibers of 10-20 nm in diameters are formed, which depend on the crystal structure and the method of preparation. The specific capacitance (SC) measured for MnO2 is found to depend strongly on the crystallographic structure, and it decreases in the following order: R(m) > R = ä > ç > ì > â. A SC value of 297 F g-1 is obtained for R-MnO2(m), whereas it is 9 F g-1 for â-MnO2. A wide (4.6 Å) tunnel size and large surface area of R-MnO2(m) are ascribed as favorable factors for its high SC. A large interlayer separation (7 Å) also facilitates insertion of cations in ä-MnO2 resulting in a SC close to 236 F g-1. A narrow tunnel size (1.89 Å) does not allow intercalation of cations into â-MnO2. As a result, it provides a very small SC.
Nanostructuring of β-MnO 2 : The Important Role of Surface to Bulk Ion Migration
Chemistry of Materials, 2013
Manganese oxide materials are attracting considerable interest for clean energy storage applications such as rechargeable Li ion and Li−air batteries and electrochemical capacitors. The electrochemical behavior of nanostructured mesoporous β-MnO 2 is in sharp constrast to the bulk crystalline system, which can intercalate little or no lithium; this is not fully understood on the atomic scale. Here, the electrochemical properties of β-MnO 2 are investigated using density functional theory with Hubbard U corrections (DFT+U). We find good agreement between the measured experimental voltage, 3.0 V, and our calculated value of 3.2 V. We consider the pathways for lithium migration and find a small barrier of 0.17 eV for bulk β-MnO 2 , which is likely to contribute to its good performance as a lithium intercalation cathode in the mesoporous form. However, by explicit calculation of surface to bulk ion migration, we find a higher barrier of >0.6 eV for lithium insertion at the (101) surface that dominates the equilibrium morphology. This is likely to limit the practical use of bulk samples, and demonstrates the quantitative importance of surface to bulk ion migration in Li ion cathodes and supercapacitors. On the basis of the calculation of the electrostatic potential near the surface, we propose an efficient method to screen systems for the importance of surface migration effects. Such insight is valuable for the future optimization of manganese oxide nanomaterials for energy storage devices.
Advanced Functional Materials, 2018
Holey 2D nanosheets of low-valent Mn 2 O 3 can be synthesized by thermally induced phase transition of exfoliated layered MnO 2 nanosheets. The heat treatment of layered MnO 2 nanosheets at elevated temperatures leads not only to transitions to low-valent manganese oxides but also to the creation of surface hole in the 2D nanosheet crystallites. Despite distinct phase transitions, highly anisotropic 2D morphology of the precursor MnO 2 material remains intact upon the heat treatment whereas the diameter of surface hole becomes larger with increasing heating temperature. The obtained holey 2D Mn 2 O 3 nanosheets show promising electrocatalyst performances for oxygen evolution reaction, which are much superior to that of nonporous Mn 2 O 3 crystal. Among the present materials, the holey Mn 2 O 3 nanosheet calcined at 500 °C displays the best electrocatalyst functionality with markedly decreased overpotential, indicating the importance of heating condition in optimizing the electrocatalytic activity. Of prime importance is that this material shows much better catalytic activity for Li-O 2 batteries than does nonporous Mn 2 O 3 , underscoring the critical role of porous 2D morphology in this functionality. This study clearly demonstrates the unique advantage of holey 2D nanosheet morphology in exploring economically feasible transition metal oxide-based electrocatalysts and electrodes for Li-O 2 batteries.
Structural and electronic properties of β-MnO2 employing DFTB technique
Suid-Afrikaanse Tydskrif vir Natuurwetenskap en Tegnologie, 2022
MnO2 is presently under massive review for its capacitance properties. MnO2 recrystallizes into several crystallographic structures such as α, β, γ, δ, and λ structure. These structures vary in the way MnO6 octahedra are connected, they possess tunnels or interlayers with gaps of different magnitudes. However, upon lithium intercalation in β-MnO2, LiMnO2 suffers from capacity loss due to undesirable structural phase transformation into spinel like LixMn2O4. One of the major demands is to modify and strengthen the structural stability of MnO2 to prevent phase transformation during lithium intercalation and rapid capacity fading during cycling. DMol3 is a density functional theory-based program used to calculate the lattice parameter of ferromagnetic MnO2. After successfully parameterized MnO2, the lattice parameters were compared with the results from experiments. Density functional tight-binding (DFTB) was employed to investigate the electronic properties of MnO2 such as density of ...
In this study, a first-principles study using the spin-polarized density functional theory approach within corrected functional was carried out to investigate the electronic features of manganese oxide surfaces under three situations of (a) cation vacancy, (b) intercalation of multi- and univalent ions, and (c) adsorption of a water molecule upon the surface as catalytic performance. The possibility of obtaining the significant absolute magnetic momentum phases from native defects in orthorhombic structures of Mn2O3 and Mn3O4 (001) surface is explored, whereas Mn vacancy provides a transition from the insulating phase into a metal-like nature and modifies the electronic transport. Moreover, bandgap engineering via impurity intercalation has been explored. Ca+2 and Al+3 intercalations have manifested substantial attributes and explain the experimental results as efficient conducting system and catalytic activity. Furthermore, the adsorption of one water molecule and the most stable configuration, adsorption energies and electronic properties were thoroughly discussed. Accordingly, it was explored that H2O: Mn2O3 and Mn3O4 exhibit suitable parameters as efficient catalytic synthesis
Understanding the phase dependent energy storage performance of MnO2 nanostructures
Journal of Applied Physics
We demonstrate charge storage mechanisms of four kinds of MnO 2 polymorphs (α, β, γ, and δ) both in micro-and nanodimensions successfully synthesized by hydrothermal and microwave irradiation techniques. We observed that layered δ-MnO 2 , comprised of self-assembled nanoflakes oriented in different directions, shows a significantly improved capacitive behavior. The maximum achieved specific capacitance is 518 F/g at a current density of 3 A/g in a 3M KOH electrolyte solution, exhibiting a large capacity retention of 83.95% over 2000 consecutive charge/discharge cycles at a current density of 10 A/g. State of the art Density Functional Theory (DFT) simulations have also been performed to support experimental data. The quantum capacitance presented from DFT simulations predicts that the δ phase exhibits highest quantum capacitance, whereas it is lowest for the β phase supporting the experimental trend. Also, the structural features of wide tunnel size (∼7 Å) for the δ phase facilitates favorable insertion of cations, whereas narrow tunnel size (∼1.89 Å) for the β phase restricts the diffusion of charge particles yielding poor capacitance performance.
First-principles study of the structure of stoichiometric and Mn-deficient MnO2
Journal of Solid State Chemistry, 2003
We present an extensive Density Functional Theory study on the phases and magnetic states of MnO 2 , with over 300 calculations of various Mn-vacancy configurations and magnetic spin states. It is shown that the paramagnetic extrapolations of spin-polarized results are essential to correctly reproduce pyrolusite as the ground state of MnO 2. Paramagnetic energies are obtained by fitting a Heisenberg Hamiltonian to the energy of 10-20 magnetic configurations for each of 16 possible MnO 2 polymorphs. Near groundstate degeneracy is shown to occur due to the frustration of otherwise large interactions. While many other structures are found to be near degenerate in energy with pyrolusite, no thermal disorder exists in the system up to several thousand degrees. The thermal disorder is suppressed because the strong correlation of the Mn-vacancy order along the lines of face-sharing octahedra removes any low-energy excitations from the system. Mn vacancies compensated by protons (Ruetschi defects), ubiquitously present in commercial MnO 2 , are shown to have a dramatic effect on phase stability. The stabilizing effects of Ruetschi defects may explain the presence in MnO 2 of ramsdellite and twinning, both of which are unstable in the pure material. We believe Ruetschi defects to be an important source of the structural complexity of synthetic MnO 2 produced either electrochemically or chemically.
Density functional theory study of MnO2, TiO2 and VO2
Suid-Afrikaanse Tydskrif vir Natuurwetenskap en Tegnologie, 2022
We investigate the structural stability of metal oxides β-MnO2, TiO2 and VO2 (MO2) which are used as catalyst in metal air batteries, using the density functional theory (DFT) within the generalized gradient approximation (GGA). Their mechanical property was determined to show the stability trend of the metal oxides catalyst. Cell parameters of the bulk structures of the MO2 are in reasonable agreement with the experimental values (deviations of approximately 0.8% and -3.1% for a and c, respectively, and of 1.6 % in the cell volume). Phonon dispersion curves show that rutile (R) TiO2 is the most stable structure since it does not have vibrations in the negative frequencies.