Screen-printable and flexible in-plane micro-supercapacitors with fractal electrode design

New microscale energy devices need to be developed to accommodate the evolution of integrated flexible electronic systems. Supercapacitors have the advantages of high power density and long cycle life, and can be used independently or combined with batteries to provide electric energy for future electronics. Screen printing is a simple, cost-effective method for fast and scalable fabrication of micro-supercapacitors (MSCs) on various types of substrates that does not involve complicated process or expensive equipment. Currently, most researches of MSCs have been focused on materials and ink development with conventional interdigital architecture, few studies have been aimed at electrode pattern design. Herein, we demonstrate an in-plane MSCs via screen printing with designable electrode patterns on flexible substrate. The thickness of the device is less than 1 mm, ensuring good flexibility. By replacing the carbon ink with the silver ink as current collectors, the performance is improved due to lower device resistance. Owing to the geometric controllability of the screen printing method, we are able to study the effects of electrode design on the output performance by simply changing the screen mesh patterns. In particular, we demonstrate that by adapting the mathematical fractal concept such as Hilbert curve as electrode patterns, the areal capacitance of MSCs can reach up to 28.7 mF cm−2, which is 46.8% higher than that of the traditional interdigital structure. The results suggest that the fractal-based design represents a universal protocol for producing MSCs. Without changing electrode materials or fabrication facilities, the design strategy is highly compatible with industrial scale screen printing for roll-to-toll processing. This high-capacitance, cost-effective and flexible MSCs open a promising pathway for providing electric energy for the Internet of Things and future wearable systems.