The effect of crystal structure on the surface properties of a series of manganese dioxides (original) (raw)
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Structural–chemical disorder of manganese dioxides
Journal of Colloid and Interface Science, 2003
Relationships between lattice parameters of manganese dioxides and their surface properties at the solid-aqueous solution interface were investigated. The studied series ranged from ramsdellite to pyrolusite and encompassed disordered MD samples. The structural model used takes into account structural defects: Pr (rate of pyrolusite intergrowth) and Tw (rate of microtwinning). Water adsorption isotherms showed that the cross sectional area of water molecules adsorbed in the first monolayer is positively correlated to Pr. Titration of the surface charge of the MD series evidenced a positive linear relationship between the PZC and Pr (Pr = 0, Tw = 0, PZC = 1 for ramsdellite; Pr = 1, Tw = 0, PZC = 7.3 for pyrolusite; γ -MD with intermediate values of Pr (0.2 to 0.45) have increasing PZC values). The rate of microtwinning appeared as a secondary factor for the increase of the PZC. The above correlations are explained by the chemical defects at the origin of the structural disorder, respectively Mn 3+ /Mn 4+ substitution for Pr and Mn vacancies for Tw, which result in proton affinity and thus in increased PZC. The experimental results are compared with data collected in the literature for manganese dioxides as well as for dioxides of transition elements with tetragonal structure. : S 0 0 2 1 -9 7 9 7 ( 0 2 ) 0 0 0 1 3 -9
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.
Journal of Alloys and Compounds, 2009
In this paper, during electrodeposition of electrolytic manganese dioxide (EMD) by anodic deposition from MnSO 4 solution, the changing of pH is studied. The pH shows a decreasing trend. Two samples are electrodeposited at fixed pH (2 and 5) and their physico-chemical properties are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), the Barrett-Joyner-Halenda (BJH) method, and rechargeable alkaline manganese dioxide (RAM) battery tests. The results have confirmed that the pH of the solution has remarkable effects on the nanostructure, crystal structure, porosity, and the electrochemical performances of the EMD. Whereas the electrodeposition at pH 2 creates irregular multi-branched morphology with meso/microporosity, a large number of regular nanospheres with microporosity are obtained at pH 5. The battery tests study has revealed that ␣/␥-MnO 2 electrodeposited at pH 2 exhibits good electrochemical performances, and the ␣-MnO 2 displays very stable cycling behavior.
The Electrical Double Layer at Hydrous Manganese Dioxide/Electrolyte Interface
Journal of Colloid and Interface Science, 2001
The interfacial properties of two hydrous oxides of manganese, namely an amorphous variety and a poorly crystalline synthetic birnessite, in different electrolyte media, have been studied by using a batch potentiometric titration procedure with a 72-h equilibration period. The data obtained by this procedure are more relevant to a natural water system than those obtained by the rapid titration method as reported earlier [J. Colloid Interface Sci. 131, 103 (1989).]. Although the points of zero charge do not show any major variations, the surface dissociation and complexation (with Na + in NaCl) constants differ with the method of titration. Both of the samples are characterized by their high surface charge (low pK a ) and surface potential, and as a result, complexation takes place possibly within the inner helmholtz plane. It is concluded that the triple-layer model is not applicable to this colloid system and that a basic Stern model would be more suitable in this case. C 2001 Academic Press Key Words: interfacial properties; hydrous manganese dioxide; electrical double layer.
We studied the effect of anode substrates such as pure lead (Pb), lead antimony (Pb-Sb), and lead-silver (Pb-Ag) on the structural and electrochemical properties of electrolytic manganese dioxide (EMD). X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and chemical analyses were used to determine the structural and chemical characteristics of the EMD samples. The charge–discharge profile was studied in 9 M KOH using a galvanostatic charge-discharge unit. In all the substrates the current efficiencies were more than 99% except with Pb-Sb where it was 90%. Results revealed the nature of the substrate strongly affected the morphology of the deposited material which in turn affected the electrochemical properties of the EMD samples. XRD analyses revealed that the nature of the anode did not affect the crystal structure of the deposited EMD and all the samples were predominantly γ-MnO 2 , which is electrochemically active for energy storage applications. The E...
Electrochemical and surface properties of nanocrystalline β-MnO2 in aqueous electrolyte
Electrochimica Acta, 2006
Surface and electrochemical properties of micrometric and nanometric tetragonal -MnO 2 of, respectively, 4 and 61 m 2 g −1 surface area were studied. The monodispersed nanocrystalline phase prepared by spray pyrolysis and characterized by transmission electron microscopy (TEM), is made of facetted crystals with 70 nm × 40 nm edges. Three types of energy domains were identified by surface acid-base titration. Slow step linear voltammetry reduction (10 mV/2 h scan rate) showed one current peak at −0.6 V (versus Hg-HgO 1 M KOH) for the micrometric phase, but three current peaks at −0.18, −0.36 and −0.55 V for the nanometric phase. They were assigned to the reduction of: (1) surface Mn atoms related to 1 1 1 and 3 1 1 faces; (2) more stable surface Mn atoms related to 1 0 0 and 1 1 0 faces; (3) bulk Mn atoms, respectively. The presence of new reduction peaks at higher potential is ascribed to the increase in Gibbs free energy caused by the surface energy of the nanometric particle.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2000
Pure d-MnO 2 materials were prepared by calcination at 250 -700°C for 3 h of a parent gel obtained by (i) a sol -gel synthesis involving a mild reduction of KMnO 4 solution; (ii) drying at room temperature; (iii) calcination to 700 or 900°C; and (iv) a subsequent acid treatment at room temperature. The bulk crystalline structure of the d-MnO 2 materials was evidenced by thermogravimetry, X-ray powder diffractometry (XRD) and high-resolution transmission electron microscopy (HRTEM). Then, surface properties were determined by X-ray photoelectron spectroscopy (XPS), scanning and transmission electron microscopy, and N 2 sorptometry. The results revealed that the bulk structure is morphologically organized in the form of needle-like nanofibers. Surfaces thereon exposed were found to assume high specific area (74 m 2 g − 1 ) and heat of adsorption (BET-C =130), as well as a mesoporous texture, when initially calcined to 700°C. These surface properties were also found to withstand subsequent calcination at up to 350°C for 3 h.
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.
Journal of Electroanalytical Chemistry, 1997
Voltammetry and electrochemical quartz crystal microbalance (EQCM) studies of MnO 2 films electrodeposited in acidic media at 85°C show the existence of matter exchange during repeated electrochemical reduction and oxidation in 1 M KOH solution. Usually, these reactions are written without mentioning the exchange of matter. The following are observed: a reversible exchange involving water between the crystal lattice and the solution within each successive sweep potential cycle; an irreversible mass loss caused by low dissolution and slow diffusion of Mn(III) and Mn(II) solubilized species; an irreversible large mass gain during the first cycle due to additional hydration of the material.