Effect of crystallographic structure of MnO2 on its electrochemical capacitance properties
Effect of crystallographic structure of MnO2 on its electrochemical capacitance properties
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DOI：
10.1021/jp7108785
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发表时间：
2008-03-20
影响因子：
3.7
通讯作者：
Munichandraiah, N.
中科院分区：
文献类型：
--
作者：
Devaraj, S.;Munichandraiah, N.
二氧化锰的电容特性目前正受到广泛的研究。MnO2结晶成多种晶体结构，即α、β、γ、δ和λ结构。由于这些结构在MnO6八面体的相互连接方式上有所不同，它们具有具有不同大小间隙的隧道或夹层。由于电容特性是由于质子或阳离子在二氧化锰中的插入/脱嵌，因此只有一些具有足够间隙以容纳这些离子的晶体结构有望用于电容研究。为了研究电容与晶体结构的关系，本研究包括在纳米尺度上制备MnO2的不同晶相，并评估其电容特性。用微乳液法制备的α - mno2 （α - mno2 (m)）的结果也进行了比较。根据晶体结构和制备方法的不同，可以形成约50 nm的球形颗粒，直径30-50 nm的纳米棒，或直径10-20 nm的互锁纤维。MnO2的比电容（SC）与晶体结构密切相关，其减小顺序如下：α (m) > α等于6 > γ > λ > β。α - mno2 (m)的SC值为297 gf(-1)，而β - mno2的SC值为9 gf（-1）。α - mno2的宽（-类似4.6埃）隧道尺寸和大表面积(m)被认为是其高SC的有利因素。大的层间间距（类似7埃）也有利于在δ - mno2中插入阳离子，导致SC接近236℉g（-1）。狭窄的隧道尺寸（1.89 A）不允许阳离子插入到β - mno2中。因此，它提供了非常小的SC。
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.