Capacitor-type energy storage is becoming an attractive option as it can be fabricated with simple methods and allows integration into a flexible substrate for wearable applications. Currently, there is a need for reliable and simple fabrication methods that take into account the characteristics of nanomaterials for their controlled deposition. Inkjet printing presents advantageous features because it allows solution-processed materials to be printed with high accuracy and resolution. Therefore, a number of patterning steps can be performed sequentially, thus minimizing the need for lithography or other fabrication methods. We have fabricated a fully inkjet-printed capacitor as energy storage for wearable sensors.

We have explored several configurations of capacitors. A parallel plate capacitor is composed of carbon nanotubes that are inkjet printed on PET film to form a compact conductive mat structure. The assembly is composed of two layers of carbon nanotubes separated by a dielectric layer of polymer, as illustrated in the figure (left). The advantage of this design is that it does not require an electrolyte to function and can be printed on flexible substrate such as PDMS. On the other hand, a supercapacitor configuration achieves higher energy storage, power density, and is also of simple construction [1]. The supercapacitor type is constructed by printing carbon nanotubes and subsequently by electrodepositing a metal oxide. It is finally assembled with a separator membrane for ion conduction and filled with electrolyte, as shown in the figure (right). The charge storage based on the redox reactions of the metal oxide is mainly responsible for its capacitance [2]. The energy storage and power density of the supercapacitor are greatly improved with the use of carbon nanotube electrodes, because of the high surface area of carbon nanotubes.

We demonstrate the capacitor as a charge storage component that is integrated in a wearable system. Moreover, our inkjet printing system precisely prints a network of carbon nanotubes. We show that the inkjet-printed capacitors provide a rapid and reliable fabrication for wearable systems.

References

[1] M. Zhi, C. Xiang, J. Li, M. Li, and N. Wu, Nanoscale, 5, 72–88 (2013).

[2] X. Zhang et al., ACS Nano, 5, 2013–2019 (2011).