Enhancement of Sensitivity Through Electrochemical Reaction Between Water and Micro/Nano Galvanic Arrays Towards Dew Condensation Prediction

Introduction

Prediction of dew condensation at an early stage is very important to control its side-effects like, metal corrosion, molding, crop disease, glass flogging etc. Conventional techniques such as commercial hygrometer are not appropriate for this purpose in terms of accuracy and response time. In this context, we have developed a small and portable sensing device termed as Moisture Sensor that can detect invisibly small water droplet and distinguish its size with high accuracy and high speed. The sensor has two interdigit structures made of two different metals as electrodes, intercalating each other with gaps of 0.5 to 10 μm. The working principle of this sensor is based on the generation of galvanic current when aggregation of water molecules connects adjacent electrodes. If structure of electrodes and the environment are modified, it is likely that the output current from the sensor can be improved. The output must be enhanced for employing it in wide areas of applications. The purpose of this study was to clarify the effects of cathode metal and oxygen content surrounding the sensor on its output response.

Experimental

Silica layer covered Si - wafer was used as the substrate. Arrays of Al and Au/Pt were alternately arranged on the substrate to form an opposing interdigit structure and used as the electrodes. The thickness of Pt on Au was varied; 10 - 90 nm. The gaps between the adjacent arrays were 0.5 and 10 μm and the number of each electrode was 50. The wafer was cut into a 5mm square chip. Electric current between two electrodes of Al and Au were measured by the hand-made device with precise amperemeter. Surface of the sensor was observed by scanning electron microscopy.

The sensor was placed in a humidity controlled chamber, maintained by a dew generator (50±1%). The sensor surface was cooled with a Peltier device. This controlled the relative humidity on sensor's surface and generated water molecules and droplets on the sensor surface. The response current from Moisture Sensor was measured simultaneously, by changing the thickness of Pt coating on Au electrode and under different O₂ concentrations by using nitrogen, air and oxygen as carrier gas for dew generator.

Result and Discussion

As the relative humidity on the sensor surface exceeded 60%, the output current of the sensor started increasing gradually by water molecules adsorption and stacking on the sensor surface, between the electrodes. When it reaches 100 %, dew condensation should occur on the sensor surface while vaporization should make water molecules leave the sensor surface thermodynamically. Therefore, steady response current was obtained theoretically and actually in this study. Fig. 1 shows response current of the sensor with the electrode gap of 0.5 μm at 100% RH when the thickness of Pt was varied for the cathode. The response current was increased from 10 to 60 nm thickness and further increase showed almost same or decreased response current. SEM images of the sensor surface showed that particles of Pt became smaller with the increase of Pt thickness, (deposition time). From these results, it can be said that response current was enhanced by acceleration of electrochemical reaction on the cathode; O₂ + 2H₂O + 4e⁻ → 4OH⁻ on Pt due to the better catalytic action than on Au. It was further enhanced by the increase in surface area of Pt with decreasing its particle size. As the Pt thickness was increased, however, the thickness of Au was decreased, leading to increase in actual electrical resistance of the cathode. Therefore, the whole response current got saturated or decreased. In addition, an increase in the content of oxygen in the chamber increased the output response.

Conclusion

This study showed that the output current between water and micro/nano galvanic arrays of the moisture sensor was enhanced by coating Au electrode with a thin layer of Pt and increasing the oxygen pressure around the electrode. These modification actuall accelerated electrochemical reaction rate on the cathode. This improved the sensitivity of this sensor.

The outcome of this study will contribute to predict dew condensation more accurately and to widen the area of its application where highly sensitive and accurate moisture detection is desired.

References:

1. J. Kawakita, et al., ECS Trans. 2017, 75, 51.

2. R.G. Shrestha, et al., Sensors, 2019, 19, 4500.

3. Y. Kubota et al., Sensors and Actuators A: Physical, 2020, 303, 111838.

Figure 1