SELF-POWERED HUMIDITY SENSORS
Humidity measurement is very important for every industry. Relative Humidity (RH) of the environment significantly affects human comfort. If RH of the air is high, then sweat does not evaporate and we feel discomfort. Hygrometers are used to measure moisture content of the air. These devices contain a sensing element (or material), whose electrical resistance or capacitance changes with variation in the RH. Thus, this property can be utilized to use various materials for Humidity-sensing applications. Although, many Humidity sensors are commercially available, but they do not generate any electric power while measuring humidity.
Concept of Self-powered device comes from the fact that most electronic sensors run on battery power. Batteries require frequent charging (thus consuming electricity) and replacement, which causes serious impact on the environment. Today, we are facing issues like global warming, extreme weather conditions, along with energy crisis. All of these problems are being handled by diverting our attention towards Renewable sources of energy. One such source is Water, having a tremendous amount of energy in different forms. Development of a device, capable of generating electricity from moisture and at the same time, measuring the humidity, is of great importance for the modern world. Huge loss of energy from water takes place due to evaporation. If we manage to extract this latent energy with high efficiency, then we would meet the global energy requirements easily. Nano-structured materials are able to generate volt-level electrical voltages upon direct interaction with water, this effect is termed as the Hydro-voltaic effect. For this purpose, we have used Nano-structured silicon di-oxide as the sensing material for our device. Two main reasons for using Silica (SiO2) are hydrophillic nature of silica and abundance in nature.
Various properties and the response of Nanostructured Silica (SiO2) particles in the presence of water has been extensively studied in this project to understand the behavior of SiO2 particles in the presence of humidity (present in the air). The electrical response has been recorded and presented in this report to show that Nano-scale Silica particles are a promising candidate for electricity generation from moisture and that they can be used to create self-powered devices. Furthermore, Device parameter optimization has been carried out with the help of interface engineering and device engineering for the development of cost effective and highly efficient self-powered humidity sensors.
Quincke (1859) found out that flow of electrolytes through a Nano-channel generates voltage in the flow, due to pressure gradient. The obtained voltage is called as Streaming potential and the process is based on Electro-kinetic theory. Nano-channels improve the electro-kinetic conversion efficiency due to atomically smooth walls, this has been extensively studied by Van der Heyden et al. (2006). Hydro-voltaic effect is analogous with Photo-voltaic effect, but is more diverse in terms of energy generation capability. Among other Nano-structured materials, Carbon Nano-materials synthesized as graphene and CNTs (Carbon Nano-tubes) have shown excellent performance with water. They are capable of producing electricity from flowing, dropping, waving and evaporating water, as reported by F. Yang et al. (2014). This property has been explained by the electronic coupling behavior shown by carbon atoms.
Hamid Farahani et al. (2014) reported that the generation of current through interaction effect in Nano-materials is mainly due to Proton transport. Self-ionization of water releases protons, which transport within the hydrogen-bonded network to carry current. This is similar to the mechanism of hole-carrier transport in semiconductors. The maximum achievable electro-kinetic conversion efficiency has been reported as 12% in theory. However, the mechanism of adsorption of water molecules and capillary action of the Nano-channel mass, limit the maximum achievable energy conversion efficiency to 3% only.
The experimental setup involves a substrate (FTO coated glass or Microscopic slide), Nano-porous silica paste (to be coated over substrate) and Source-meter to measure OCV (Open-circuit voltage) and SCC (Short-circuit current) across the two electrodes of the prepared sample. After cleaning the glass slides in an Ultra-sonic cleaner, they were heated to 4500C to remove any impurity on the surface. Doctor-blading method was used to coat a film of silica over the substrate and was then sintered at 4500C to vaporize organic impurities from the film, to develop a nice porous structure. After cooling the samples, V and I readings were taken (by dropping water droplets over the surface of our Device) with the help of source-meter and data was plotted in Origin pro software.
The first image shows our device structure, 2nd and 3rd images depict the result obtained by dropping 100 μL water over the surface. 4th and 5th images depict the result obtained by continuous dropping of water over the surface.
Device produces more Streaming Voltage (1V) and less Current (60 nA) as compared to Evaporation Voltage (0.6V) and Current (110 nA) respectively.
The lower current from the device is due to the pore-size of the material (200 nm), hence we need to reduce the pore-size and modify the pore-structure to gain a higher value of current.
Output is maintained for long durations and profile was regular with constant polarity of electrodes, due to better capillary action and increased effective area for evaporation, hence they can be used to fabricate self-powered devices.