Understanding the Coffee ring Effect on Self-discharge Behavior of Printed micro-Supercapacitors

Printed micro-supercapacitor exhibits its flexibility in geometry design and integration, showing unprecedented potential in powering the internet of things and portable devices. However, the printing process brings undesired processing defects (e.g., coffee ring effect), resulting in severe self-discharge of the printed micro-supercapacitors. The impact of such problems on device performance is poorly understood, limiting further development of micro-supercapacitors. Herein, by analyzing the self-discharge behavior of fully printed micro-supercapacitors, the severe self-discharge problem is accelerated by the ohmic leakage caused by the coffee ring effect on an ultrathin polymer electrolyte. Based on this understanding, the coffee ring effect was successfully eradicated by introducing graphene oxide in the polymer electrolyte, achieving a decline of 99% in the self-discharge rate. Moreover, the micro-supercapacitors with uniformly printed polymer electrolyte present 7.64 F cm^-3 volumetric capacitance (14.37 mF cm^-2 areal capacitance), exhibiting about 50% increase compared to the one without graphene oxide addition. This work provides a new insight to understand the relationship between processing defects and device performance, which will help improve the performance and promote the application of printed micro-supercapacitors.