Cycle Stability of Manganese Dioxide Polymorph Supercapacitors at Elevated Temperature

Electrochemical supercapacitors are of great interest as complementary electrochemical storage devices in the applications that require high power pulses. Many compounds have been investigated as possible supercapacitor materials, and manganese dioxides are certainly among the most attractive candidates. This is mainly due to their low cost, environmental compatibility and appropriate electrochemical performances. The chemistry of MnO₂ is abundant with a variety of structural types. In the field of supercapacitors, the structural complexities exhibited by the 1D, 2D or 3D arrangement of MnO2 materials result in various charge-storage performances [1]. We have previously revealed the crystallographic evolution of the MnO2 polymorph supercapacitors taking place during the course of electrochemical charge/discharge [2], pointing out different cell expansion/contraction behaviors pertaining to their crystal structures. In many potential applications, such as electric vehicle and stand-alone renewable energy storage, supercapacitor cells are likely to constantly operate under elevate temperatures. Unfortunately, the high-temperature cycling stabilities of many pseudocapacitve  supercapacitors, including MnO₂, have essentially not been looked into in the literature.

  In this study, the high-temperature (50 ^oC) electrochemical performances, in particular cycling stability, of supercapacitos made of three different MnO₂ polymorphs, including cryptomelane, birnessite and spinel, with aqueous K₂SO₄ electrolyte were investigated. The samples were subjected to different analyses, including electrochemistry cyclic voltammetry, synchrotron X-ray diffraction, and synchrotron X-ray absorption spectroscopy, in order to reveal the predominant fading mechanisms both at 25 ^oC and 50 ^oC. It is shown that for all three polymorphs, the capacitance fading rates at 50 ^oC are far greater than those at 25 ^oC. It is further evidenced that their capacitance fading mechanisms at high temperatures are strongly affected by their crystallographic structure and the volumetric variation behavior previously disclosed.

[1] O. Ghodbane, et al. ACS Appl. Mater. 1 (2009) 1130.

[2] O. Ghodbane, et al. J. Power Sources 206 (2012) 454