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, 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.