The demand for energy storage devices and sensors is increasing due to the rapid growth of portable devices and the Internet of Things. Many new technologies require novel materials with remarkable properties such as flexibility, small size, low power and being environment-friendly.

Due to its attractive electrical, mechanical and optical properties, graphene shows great potential in both emerging and established technologies such as flexible electronics, wearable devices, sensors, energy storage, solar cells as well as nano- and micro-electromechanical systems.¹ Particularly, graphene is an ideal gas sensing material since it is capable of detecting single gas molecules² and its high surface area makes it promising for energy storage applications.³

Among the several patterning methods of graphene, of particular interest is the inkjet printing technique which allows direct deposition of graphene onto the substrate in the form of graphene flakes.⁴ This technique has the advantage of being cost-effective and versatile, with resolutions of about 50 µm. In this work, we investigate potential applications of graphene-based inkjet printing for use in sensing and energy storage.

First, an inkjet printed humidity sensor is demonstrated. The working principle is based on the change in resistance given by the direct adsorption of water molecules onto the graphene layer.⁵ The morphology of the graphene layer, constituted by protruding graphene flakes (Figure 1a), helps in maximizing the device surface area thus providing more adsorption sites for the water molecules and contributing to a high sensitivity. Accordingly, the sensors exhibit a linear response over a wide humidity range (Figure 1b). Since humidity was controlled by introducing or evacuating air (which carries the water molecules) into the chamber, the response of the sensor was also tested under dry argon atmosphere, showing no significant cross-sensitivity with pressure (Figure 1c).

Also, graphene-based transparent micro-supercapacitors were printed. The printed graphene layers are transparent conductive films and their transparency can be tailored by tuning the film thickness. Supercapacitors that take full advantage of both the transparency and the high surface area of the films are promising in a number of optoelectronic applications like OLEDs, solar cells and wearable electronics. For these purposes, we developed a technique based on both inkjet printing and O₂ plasma etching for maximizing the uniformity, thus the transparency, while retaining good conductivity of the films. Specifically, we demonstrate interdigitated transparent supercapacitors with among the highest areal capacitances yet demonstrated (13 µF/cm², see Figure 1d) at transmittances of ~86% (at 550 nm of wavelength). These devices have also long cycle-life with a capacitance degradation of only 10% after 10 000 cycles (Figure 1e). Additionally, the supercapacitors don´t require a current collector and the energy storage mechanism is purely electrostatic.

In conclusion, we optimized and adapted the inkjet printing technique of graphene for sensing and energy storage. The cost-effectiveness and versatility of the technique demonstrates the potential of graphene for real-world applications.

References

1. A. C. Ferrari et al., Nanoscale, 7, 4598–4810 (2014).

2. F. Schedin et al., Nat. Mater., 6, 652–655 (2007).

3. R. Raccichini, A. Varzi, S. Passerini, and B. Scrosati, Nat. Mater., 14, 271–279 (2015).

4. J. Li et al., Adv. Mater., 25, 3985–3992 (2013).

5. A. D. Smith et al., Nanoscale, 7, 19099–19109 (2015).

Figure 1. a) SEM image of protruding graphene flakes. b) Resistance change with humidity of inkjet printed graphene sensor. c) Variation of resistance in air (which contains water molecules) and argon atmosphere. d) Cyclic voltammetry curves of transparent inkjet printed graphene supercapacitor. e) Capacitance retention of supercapacitor during 10 000 cycles.