(Invited) Fabrication of Aramid Paper Electrodes for Electrochemical Applications
Authors: Hirai Yuji , Nishiumi Nobuyuki , Sato Ryota , Masuhara Akito , Yoshida Tsukasa 
Published: December 2020

In recent years, flexible-electronics has become an emerging field of R&D. While plastics are typical substrates to print the circuitries, paper is an interesting alternative. Recently, paper-based touch sensor with printed silver nanoparticle has been demonstrated to work even after repeated bending and wrapping tests [1].
High porosity is also an interesting feature of paper for its use in wet electrochemistry. If printed conductive matters can work as electrodes, the porosity as well as the flexibility can find advantages especially in their sensor for increased sensitivity and versatility. In this study, we employed a para-aramid paper, Teijin Twaron®, as the substrate to fabricate flexible electrodes by printing conductive pastes. Aramid papers can be stable at high temperatures and under harsh electrochemical environments. The printability, sheet resistance, stability against bending test, as well as electrochemical properties have been evaluated.
Low temperature curing type conductive silver paste (TOYOCHEM, REXALPHA®) was coated to para-aramid paper and photocopy paper by doctor blade method, dried under air for an hour, and then annealed at temperatures between 100 and 500°C under air or N2 for 5 min. in a tube furnace. The coatings were observed by laser microscope and SEM. Sheet resistance was measured by four-point probe method. Adhesion and mechanical stability of the coating were checked by rolling test, by repeatedly rolling the printed paper around a 4 mm diameter glass rod, stretched, and measuring the sheet resistance in between. Linear sweep voltammetry (LSV) was performed at 5 mV/s on the paper electrode in a 0.1 M KNO₃ solution (pH 7.2) under O2 or N2.
Observation of the silver coatings by laser microscope revealed smooth surface free of cracks. The SEM image showed that the layer was made of particles of about 500 nm size and had a porous structure. Although it was difficult to measure the exact film thickness due to penetration of paste in between the fibers, the thickness was roughly 100 μm as judged from the cross-section image. The sheet resistance of the as-coated film was about 1.2 Ω/sq, which was reduced to 0.3 Ω/sq by annealing under air at 250°C, whereas photocopy paper could not withstand any higher temperatures above 200°C. Annealing the coated aramid paper under N2 at the same temperature could further reduce the resistance to 0.1 Ω/sq. since oxidation of silver was prevented. While annealing at higher temperatures resulted in slight increase of the sheet resistance and the aramid film was burnt above 450°C. The rolling test up to seven times caused gradual increase of the sheet resistance, but then remained around 0.2 Ω/sq. by further repetitions, indicating its high mechanical stability. No peeling off of the Ag layer was observed.
LSVs of the paper electrodes under O2 are compared to that of Ag plate in Fig. 1. The one indicated as “both sides” is in fact the free-standing silver coated aramid paper. Plateau current seen more negative than about -0.4 V vs. Ag/AgCl should correspond to the diffusion limited current for oxygen reduction reaction (ORR), which, however, is approximately doubled for the paper electrode compared to that of Ag plate. Transport limited current cannot differ from electrode to electrode. Possible account for the current enlargement of the paper electrode is the contribution of current at the back side of the paper. We then attached a masking tape to only expose either the “front side” or “back side”, as indicated in the figure. Indeed, the back side measured the plateau current as large as that of the Ag plate. It seems that the aramid paper is electrochemically transparent allowing penetration of the electrolyte and transport of oxygen. The front side in fact measures slightly enhanced current, which could possibly be caused by that we cannot deactivate the back-side contribution by means of a masking tape, Therefore, the addition of the front and back side current became somewhat larger than that of the freestanding paper electrode. It is however evident that the Ag layer acts like a floating foil electrode for which both front and back sides are electrochemically active. This immediately means increased sensitivity in sensor applications, for example. Micro electrode array can easily be fabricated by screen printing to make it even a 3D active electrode, rather than double sided 2D electrode.
The first trial to employ combination of Ag paste and aramid paper has turned out to be successful. Testing conductive pastes else than Ag for improved chemical stability is under way.
[1] R. Li et al ACS Appl. Mater. Interface, 6 (23), 21721-21729 (2014).

Figure 1