Encapsulation design for Mitigating Self-Discharge in Direct Ink Written Micro-Supercapacitors

Sammanfattning

The rapid development of wearable electronics and distributed sensor networks demands miniaturized, flexible, and stable on-chip energy storage devices. Among them, printed micro-supercapacitors based on conducting polymers such as PEDOT:PSS offer high power density and mechanical flexibility, but their practical use is limited by fast self-discharge caused by electrolyte evaporation and environmental exposure. In this work, a stepwise resin encapsulation method has significantly improved the long-term voltage retention of printed PEDOT:PSS/graphene based micro-supercapacitors. The encapsulation involves first forming a UV-cured resin frame around the electrode area to confine the gel electrolyte, followed by a complete hermetic seal with a second resin layer. The encapsulated single devices retain over 80% of their initial voltage after one hour under open-circuit conditions on the fourth day (self-discharge tests were done on each day), compared with only approximately 60% retention for the unencapsulated controls. For a series array of four devices, encapsulation still ensures over 70% voltage retention. Through nonlinear regression analysis of the self-discharge curves using a combined model adapted from the literature that incorporates ohmic leakage, diffusion-controlled charge redistribution, and Faradaic reactions, an attempt is made to quantitatively interpret the self-discharge behavior of the micro-supercapacitors, thereby offering mechanistic perspectives on the role of encapsulation. For single-unit devices, encapsulation effectively suppresses the aging-induced emergence of ohmic and diffusion mechanisms, preserving Faradaic dominance at approximately 74% throughout the fourday measurement period, whereas unencapsulated devices exhibit a decline in Faradaic contribution from 96.0% to approximately 66% with concurrent emergence of ohmic and diffusion pathways. For series-connected four-unit arrays, the protective effect is even more pronounced: unencapsulated arrays show a significant increase in ohmic contribution from 5.9% to 44.0% over four consecutive daily self-discharge tests, transforming ohmic leakage into the primary failure mode, while encapsulated arrays maintain Faradaic dominance at 72–82% with ohmic contributions suppressed to below 15%. Encapsulation also stabilizes the diffusion mechanism, which otherwise exhibits a declining trend in unencapsulated arrays. Importantly, the encapsulation does not degrade the essential capacitive behavior, with 87.6% capacitance retention after four days for encapsulated single devices, whereas unencapsulated controls retain only 45.0%, as confirmed by cyclic voltammetry tests. For series arrays, encapsulation improves capacitance retention from 72.9% to 87.7%. These findings demonstrate that encapsulation primarily suppresses the activation and growth of ohmic leakage pathways—particularly critical in series configurations where such mechanisms are amplified—while preserving Faradaic reactions as the stable, dominant self-discharge pathway, thereby offering insights into the design and reliability improvement of printed micro-supercapacitors for practical applications.