Energy storage on demand: ultra-high-rate and high-energy-density inkjet-printed NiO micro-supercapacitors†
Micro-supercapacitors are an important class of energy storage devices for portable, self-powered and miniaturized electronics such as sensors, biomedical implants and RFID tags. To address the issue of limited energy density of micro-supercapacitors, pseudocapacitive transition-metal oxides have been used as electrodes at the cost of lower power capability due to their low electronic conductivity. In this work, high-energy-density and high-power-density nickel(II) oxide (NiO) micro-supercapacitors, fabricated through inkjet printing, are demonstrated. The nanoparticle-based thin film NiO electrodes showed up to 14 orders of magnitude higher electrical conductivity than single crystal NiO. The enhanced conductivity of the electrodes was reflected in the low relaxation time constant of just 30 ms, which is among the lowest achieved for any supercapacitor. A magnesium perchlorate-based aqueous electrolyte with extended operating voltage window was developed to enable the operation of the devices up to 1.5 V. The devices showed remarkable areal and volumetric specific capacitances of up to 155 mF cm−2 and 705 F cm−3 respectively, surpassing the state-of-the-art inkjet-printed supercapacitors but also a few of the best micro-supercapacitors known to date. This work provides a compelling platform to simplify the fabrication process of micro-supercapacitors, with focus on digital design, scalable manufacturing, and direct integration with printed electronics.