Floating-based degradation characterisation of an asymmetric MnOx supercapacitor prototype

High-performance MnOx-based supercapacitors are at the forefront of energy storage innovation, offering rapid charge-discharge capability and long cycle life. However, these devices suffer from degradation and aging of the electrode material, electrolyte and other cell components. Particularly, during parasitic reactions, mechanical and chemical instabilities negatively affect the MnOx electrode in aqueous solutions, resulting in capacitance fading. Therefore, this work focuses on understanding the degradation mechanisms in MnOx-based supercapacitors through accelerated degradation tests. Hence, an asymmetric supercapacitor was developed using birnessite-type MnOx and activated carbon as electrodes, and subjected to different floating potentials (1.5, 1.75, 2.25 and 3V) and temperatures (15, 35, 45 and 60 °C). The results revealed a faster degradation of the electrode with the increase in floating potential, assigned to the dissolution of the MnOx into soluble species (Mn 2+ ) detected through cathodic stripping voltammetry. Moreover, increasing the temperature during floating revealed, surprisingly, slower degradation of the electrode associated with enhanced ion diffusion into the pores. Further XRD analysis showed a crystallographic phase change into todorokite that exhibits lower ionic conductivity in relation to birnessite. This investigation provides new insights into the accelerated degradation of MnOx under aggressive conditions, contributing to a more comprehensive understanding of the mechanism of capacitance fade during floating. • Applied floating tests to study the ageing of a MnOx//AC asymmetric cell. • Higher floating potentials led to faster capacitance fading. • Mn 2+ ions detected in solution proved active material loss. • Partial crystal phase transition to todorokite happens at extreme polarisation. • Higher temperatures tend to lead to slower degradation.