Effect of Crystallographic Structure of MnO₂ on Its Electrochemical Capacitance Properties

Abstract

MnO2 is currently under extensive investigations for its capacitance properties. MnO2 crystallizes into several crystallographic structures, namely, α, β, γ, δ, and λ 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 α-MnO2 prepared by a microemulsion route (α-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: α(m) > α ≅ δ > γ > λ > β. A SC value of 297 F g-1 is obtained for α-MnO2(m), whereas it is 9 F g-1 for β-MnO2. A wide (∼4.6 Å) tunnel size and large surface area of α-MnO2(m) are ascribed as favorable factors for its high SC. A large interlayer separation (∼7 Å) also facilitates insertion of cations in δ-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 β-MnO2. As a result, it provides a very small SC.

β-MnO2 nanorods with exposed tunnel structures as high-performance cathode materials for sodium-ion batteries

“…Therefore, the (1 Â 1) tunnel structure can effectively prevent structural collapse and local volumetric variation during the charge/discharge process, which enhances the structural stability and cycling performance. 27,28 Consequently, the as-prepared b-MnO 2 nanorods with exposed (1 Â 1) tunnel structures can achieve a high-rate performance and durable cyclability as cathodes in Na-ion batteries.…”

Charge Storage in Cation Incorporated α-MnO2

“…48 yielding a value of . Additionally, the dissolved salt used in aqueous MnO 2 ECSCs is typically a weak base such as KCl or Na 2 SO 4 at concentrations  1 M. Therefore, a slightly basic pH of 7.4 arising from the 0.1 M Na 2 SO 4 aqueous electrolyte commonly employed for MnO 2 ECSC systems 23,33 was used to specify .…”

Development of novel MnO2/nanoporous carbon composite electrodes in capacitive deionization technology

“…Through the high fraction of meso-surface area, the pores in the electrodes are better connected that more pores can be accessed by the ions. Also MnO 2 can provide high pseudo-capacitance [25,27,28]. Therefore, composites can be expected to have a better performance than AC in the capacitive desalination process as higher capacitance suggests better electrosorption of salt ions.…”