High-performance,Flexible,micro-supercapacitors Based On Printable In-plane Interdigital Electrodes

With the reduction in size,price and power consumption of electronic components,as well as the development of portable wireless communication protocls,micro-electrochemical systems,biosensors,micro-robots and wearable/implantable medical equipment,new efficient energy-storage devices with micro-electrochemical system are to be developed urgently.Micro-supercapcitors have drawn special attention as a promising micro-energy storage unit or micro-power source,due to their high power density,excellent cycle life and fast charging/discharging rate.Therefore,significant efforts have been contributed to the miniaturization of supercapacitors and their integration onto flexible substrates.Micro-supercapacitors with planar interdigital electrodes can be integrated with micro-electronics to work as stand-alone power sources,or as efficient energy storage units coupling with energy harvesters to realize self-powered micro-devices.Despite many advances,research and development of micro-supercapacitors,especially asymmetric micro-supercapacitors are still in their infancy.Currently,fabrication strategies for asymmetric interdigital electrodes are very limited.Most of the reported technologies?such as photolithography,plasma etching,laser scribing and inkjet printing?are only suitable for the preparation of interdigital electrodes with specific materials;some methods are complex and hardly compatible with the ones used in semiconductor industry to enable integration with other electronic components.In this work,a facile,economic,fast,scalable and universal technique,i.e.screen printing,was utilized to fabricate the flexible solid-state asymmetric micro-supercapacitors.Moreover,a highly concentrated Na-alginate bio-hydrogel was used as electrolyte for the micro-supercapacitors for the first time.The nontoxic,environmentally friendly and biocompatible Na-alginate hydrogel can provide an electrochemical stable window up to2.8 V,dramatically boosting the energy density of the device.As-fabricated micro-supercapacitors with MoO_3-x nanorods as cathode fingers,glucose-derived activated carbon nanospheres as anode fingers and Na-alginate bio-hydrogel as electrolyte,exhibit outstanding electrochemical performances.It delivers a high areal capacitance of 47.20 mF cm^-2,a superior energy density of 21.20?Wh cm^-2(47.11 mWh cm^-3)at a power density of0.18 mW cm^-2(0.40 W cm^-3),and excellent capacitance retention?95%after 10,000 cycles?.Owing to the small device dimensions and relatively high operating voltage window,our devices exhibit obviously higher energy densities than those of the other symmetric or asymmetric micro-supercapacitors reported in literatures.In addition,the device also demonstrates good mechanical stability,and can be integrated into any printed circuits.Therefore,such kind of devices are very promising for customized power systems in the Internet of Things and wearable/implantable on-chip electronics with high safety requirement.