Calendar Life of Textile Supercapacitors

The integration of flexible supercapacitors into wearable technologies has seen a steady increase over the previous decade. Offering promising power and energy densities, and significant design freedom, these energy storage devices will enable self-powering garments. The performance of these devices depends on many factors, with the electrode material, configuration and choice of electrolyte all contributing to the final device. One primary performance indicator is the cycle stability, where a device is tested under many full electrochemical cycles and the decay of the performance evaluated. A performance indicator that is often overlooked however, is the calendar stability. Given these devices need to perform for the lifetime of the garment without the possibility of replacement, this omission from the literature seems significant. This work begins the investigation of the stability over time by characterising a textile supported supercapacitor, stored in a number of environments. Under the test condition these devices were found to have a calendar life of 35 days and under non-test conditions were found to have calendar lives of $\lt6$ days. An investigation of the ionic conductivity of the electrolyte soaked textile layer suggests the evaporation of the electrolyte is the primary device failure mechanism. This calls into question the validity of using polyvinyl alcohol as the polymer agent in future quasi-solid state electrolytes.