Investigating Mechanisms Underlying Elevated-Temperature-Induced Capacity Fading of Aqueous MnO2 Polymorph Supercapacitors: Cryptomelane and Birnessite

The effects of elevated operating temperature on the charge-storage capacity fading of two MnO2 polymorph pseudocapacitive electrodes, namely cryptomelane and birnessite, in an aqueous K2SO4 electrolyte were investigated. These two polymorphs have been shown to exhibit markedly different volume expansion and contraction behaviors during cycling. An increase in the cycling temperature from 25 to 50 degrees C dramatically increased the fading rate for both the electrodes, but the underlying mechanisms were different. For the cryptomelane electrode, in which the MnO2 structure exhibits negligible volume variations during cycling, an increase in the operating temperature increased the irreversible structural distortion that originated from the Jahn-Teller distortion of MnO6 octahedra. The structural distortion hindered the redox reactions involving K+ intercalation into the MnO2 lattice and led to a significant increase in charge-transfer resistance at the solid-electrolyte interface, thus accelerating capacity fading. For the birnessite electrode, cycling produced a marked increase in the electrical resistivity of active layer. This resistivity increase is attributable to the large cyclic volume variations in the birnessite structure, which cause debonding of binder with the constituent particles of the electrode. Accelerated capacity fading resulting from an increased operating temperature is attributed to deteriorating mechanical strength of the binder with increasing temperature. (C) The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.