Printing of Silver Electrode on Para-Aramid Paper for Electrochemical Sensors
Authors: Sato Ryota , Sun He , Khosla Ajit , Sekhar Praveen K. , Yoshida Tsukasa 
Published: February 2020

Introduction: "Paper electronics" is becoming an emerging field of research and development. Paper, as the substrate to furnish functionalities onto it, bears advantages not only for its low cost, light weight and flexibility, but also its potential to improve the performance owing to its porous structure. Especially interesting is its use as a substrate to support electrode in electrochemical applications for battery, electrocatalysis and sensor, since facile mass transport through paper could enhance their performance.
In this work, we have employed para-aramid paper and printed conductive silver paste to check their compatibilities to fabricate paper electrodes. Aramid paper is known for its high mechanical, thermal and chemical stability to withstand its use under harsh environments. The conductivity as well as electrochemical activity for oxygen sensing of the paper electrode are studied.

Experimental: Para-aramid paper (TEIJIN, Twaron®, 58 g/m², 180 μm thick, density = 0.32 g/cm³) was cut into roughly 2 × 5 cm², onto which a 1 cm wide Ag band was printed by spreading commercial Ag paste (TOYOCHEM, REXALPHA®) with a glass rod and by applying two parallel strips of Scotch® tape as spacers. After drying under air for ca. 1 h, they were annealed at temperatures between 373 and 773 K under air or N2 in a tubular furnace for 5 min. The sheet resistance (Ω/sq.) of the printed Ag layers was measured by 4 point probe method. The electrochemical measurements were performed by regulating the effective area to 1 cm2 with a masking tape in a neutral 0.1 M KNO3 aqueous electrolyte under N2 and O2, in comparison with a Ag plate electrode.

Results and Discussion: Ag layer well adherent to the aramid paper could be fabricated to withstand multiple bending. The surface was smooth without cracks and pinholes (Fig. 1(A)). The thickness was approximately 100 μm as determined from cross-section image. The sheet resistance could be reduced by increasing temperature of annealing under air from its initial 0.8 Ω/sq. to hit a minimum of 0.3 Ω/sq. at 523 K, and again went up at higher temperatures (Fig. 1(B)). Curling of aramid paper was observed above 623 K and it was carbonized at 773 K. It is a clear advantage of the aramid paper that it allows a high temperature annealing. Annealing at 523 K under N2 could avoid oxidation of Ag to further reduce the resistance to 0.1 Ω/sq. and this sample was used for the electrochemical measurements.
Oxygen reduction reaction (ORR) was measured at the paper electrode and Ag plate for comparison (Fig. 2). The plateau current density for the ORR at the paper electrode was about twice as large as that at the Ag plate, when projected area in contact with the electrolyte was counted. Also, the steeper rise of cathodic current at more positive potential indicates higher ORR activity of the printed Ag than the bulky Ag. The improved O2 sensing ability can be accounted by the porosity of the aramid paper. Since O2 can travel through the paper substrate, the back side of the printed Ag layer can also act as the effective electrode surface.
Fabrication of microelectrode pattern onto paper can be achieved by use of techniques such as screen printing, to further enhance the sensitivity by employing 3D transport of substances, instead of 2D in the present example. Also, fabrication of electrodes other than Ag, such as Au, Pt and C is the next challenge.

Figure 1