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.