Abstract:

Objective Inspired by the dew on the tips of grass, condensation on the surface of materials is a very common phenomenon in nature. However, the accumulation of droplets is mainly attributed to the hydrophobic and waterproof layer on the surface of the materials. Due to the heat loss during the condensation, condensation on the porous electrodes will still affect the electrical response stability of the electrode, even if the porous electrode has a hydrophobic coating. Therefore, the surface of porous electrode should not only have hydrophobic properties, but also be able to maintain durable hydrophobic properties in a humid environment (such as fog, dew, etc.).

Method The conductive layer of carbon black (CB) was loaded on porous polyurethane (PU) sponge by electrostatic layer self-assembly method. Firstly, the PU sponge was coated with polyethyleneimine (PEI), a positively charged polyelectrolyte. Because the CB had opposite charges on their surfaces, a chemical bond could form between CB and PEI through strong electrostatic attraction, allowing the CB to be stably coated onto the PU sponge. The above steps were repeated to complete the assembly process of multi-layer conductive CB to prepare CB/PU electrode. Then the sponge impregnated with OTMS solution was placed in an oven and heated at 70 ℃ for 3 h. Additionally, hydroxyl groups in the CB enabled their combination with OTMS via a silane coupling reaction.

Results The flexible porous electrode was coated with OTMS, showing its surface morphology, chemical groups, superhydrophobicity, pressure response properties, and the synergistic effect of superhydrophobicity, porous and temperature to resist infiltration of tiny droplets. Before CB conductive coating, the polyurethane sponge exhibited a smooth surface morphology. However, after coating with CB and OTMS, the sponge surface became rough. The CB and OTMS can be uniformly loaded on the polyurethane sponge, The chemical structure of the CB/OTMS was determined using FT-IR spectroscopy. After hydrophobization, the water contact angle increased to 152.5°, and the slip angle reached 6.2°, showing good superhydrophobic performance. OTMS coating has almost no effect on the conductivity of the CB/PU electrode, and the droplet accumulation on the OTMS/CB/PU electrode does not affect the electrical properties. As a pressure sensor, the flexible superhydrophobic porous electrode shows high sensitivity (103.18 kPa^-1), short response time (60 ms), and good response stability. Although the droplets aggregation on the flexible porous superhydrophobic electrode does not affect its electrochemical stability, the continuous spraying of droplets will interfere the electrical signal response. Based on the electrical signal response during the intermittent spraying process, it is inferred that the temperature of the droplet may be the main cause of the electrical signal response. In order to reduce the influence of the temperature of the droplets on the electrode, an external 20 V constant-voltage DC source was applied to heat the flexible porous superhydrophobic electrode at about 50 ℃. When the electrode is maintained at 50 ℃, it can compensate for the heat loss caused by the droplet spraying, and finally achieves the accurate pressure response behavior of the electrode material within 600 s. The surface of porous material treated by OTMS can achieve effective droplet aggregation, and temperature compensation can balance the temperature loss caused by droplet spray. Furthermore, the superhydrophobic stability of the porous electrode surface is maintained.

Conclusion The prepared flexible porous superhydrophobic electrode has excellent pressure response performance and anti-fogging performance. The long chain alkyl of OTMS has low surface energy, which can give the porous electrode superhydrophobic properties. In addition, electric heating is beneficial to the long alkyl chain extension of OTMS, and pores on the surface of electrode can provide a large number of sites for the condensation, thus increasing the aggregation efficiency. Morever, electric heating can further prevent the liquid film from penetrating into the porous electrode. The results demonstrate that the OTMS porous electrode has excellent superhydrophobicity and anti-wetting for fogs, and the flexible superhydrophobic porous electrode can be utilized to improve the environmental stability of pressure-responsive components.

Key words: porous conductive material, superhydrophobic, flexible porous electrode, octadecyltrimethoxysilane, anti-fogging performance