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, alpha, beta, gamma, delta, and lambda 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 alpha-MnO2 prepared by a microemulsion route (alpha-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: alpha(m) > alpha congruent to 6 > gamma > lambda > beta. A SC value of 297 F g(-1) is obtained for alpha-MnO2(m), whereas it is 9 F g(-1) for beta-MnO2. A wide (-similar to 4.6 angstrom) tunnel size and large surface area of alpha-MnO2(m) are ascribed as favorable factors for its high SC. A large interlayer separation (similar to 7 angstrom) also facilitates insertion of cations in delta-MnO2 resulting in a SC close to 236 F g(-1). A narrow tunnel size (1.89 A) does not allow intercalation of cations into beta-MnO2. As a result, it provides a very small SC.