2.3.3 Ink jet printing
Ink-jet printing is a digital printing technology. The ink can be deposited on the substrate according to the designed pattern. Compared with screen printing, ink-jet printing ink is sprayed through quantitative digital control. Therefore, the distribution and thickness of the active material of the paper-based electrode prepared by the technology are more controllable. Huang et al.[112]directly sprayed a large area of MXene conductive ink on the paper surface. Then the interdigital electrode with a certain width was prepared by ultraviolet laser (Figure 12A). The geometry of interdigital electrode was optimized by UV laser. Finally, the paper based device showed 23.4mF cm-2 area specific capacitance and good magnification performance. In order to expand the voltage window and power supply time of the device, the author directly prepared series and parallel paper-based devices through inkjet (Figure 12B). The results show that the voltage window of the device after series connection can reach 1.2V without affecting the discharge time. At the same time, the discharge time of the two devices in parallel also increases by 100%. Compared with symmetrical supercapacitors, asymmetric electrode supercapacitors have larger voltage window and energy density. Based on this, in order to improve the voltage window of the device,Sundriyal et al.[113]prepared an asymmetric miniature paper-based supercapacitor through continuous ink-jet printing. As shown in Figure 12C, the device preparation process is shown. Firstly, rGO ink interdigital electrode is printed on paper. Then active carbon-Bi2O3 ink and rGO-MnO2 ink are printed on the corresponding separate interdigital electrodes as cathode and anode respectively. Finally, PVA/KOH gel electrolyte is also printed on the electrode surface in the form of ink. Thanks to the asymmetric electrodes, the device shows a high voltage window of 1.8V. Similarly,different series and parallel devices are designed in this work, and the LED lamp with the working voltage of 3.2V is successfully lit. Similarly, after ink-jet printing, its energy storage characteristics can be improved through post-processing. For example, Jo et al. [114] prepared a water-based additive free oxidized single-walled carbon nanotube slurry for ink-jet printing. It is worth mentioning that in this work, the ink of ink-jet printing is treated with strong pulsed light to improve the electrochemical energy storage performance of the paper-based electrode. As shown in Figure 12D, under the treatment of intense pulsed light, the released gas generated by the rapid elimination of oxygen containing groups on the surface of carbon nanotubes can produce microporous structures in the ink. This is very beneficial to the diffusion of electrolyte ions and the storage of electrons. In addition, the ink is also suitable for screen printing process. In a word, this work provides a feasible method for the preparation of energy storage porous carbon based ink.
In addition to the construction of paper based supercapacitor by spraying active ink on cellulose paper, the preparation of cellulose based conductive ink has also been studied in recent years. For example, Engquist et al. [115,116] from Linköping University in Sweden reported two research efforts on inkjet printing of cellulose-based inks. As shown in Figure 12E, the team prepared a printable cellulose based ink by mixing CNF with conductive polymer PEDOT:PSS. Firstly, ink is sprayed on PET/Al/Carbon collector by ink-jet printing to form a paper-based electrode. Then the gel electrolyte is deposited on the electrode by the rod coating method. Finally, the two cured paper electrodes are combined into a supercapacitor. It is worth mentioning that in order to prevent the brittleness of the ink after forming, the author perfectly solved this problem by adding an appropriate amount of glycerin. On the basis of this work, the team improved the molding of cellulose/PEDOT:PSS ink again. As shown in Figure 12F, this work prepared a paper based supercapacitor with controllable thickness and large area through air atomization spraying of ink and screen printing of gel electrolyte. In addition, the device is combined with the flexible solar cell module to successfully prepare a self charging flexible paper based supercapacitor based on solar energy (Figure12G). As shown in Figure 3H, the integrated device irradiated by sunlight can charge the paper based supercapacitor to 0.6V in 2000s.