LAUSR

Abstract Nowadays, the detection of CO2 gas is relevant in industry because it can provoke respiratory diseases if its concentration is high in the work-places. Also, it contributes to the greenhouse effect. The current CO2 sensors are based on rigid ceramic materials, which require high operating temperatures (200–600 °C), and high energy consumption (30–200 W). Due to these disadvantages, the hybrid and flexible sensors have emerged as an alternative to substitute the old technologies of ceramic-based sensors. These flexible sensors are more compact and light. Also, their energy consumption is very low (mW) and can operate at lower temperatures of 25–80 °C. Due to this new trend for CO2 sensors, we present in this review the general configuration and materials to construct hybrid and flexible CO2 sensors. Polyethylenimine (PEI), poly(3,4-ethylenedioxythiophene) (PEDOT), Polyethylene glycol (PEG) and polyaniline (PANI) polymers have been employed to fabricate hybrid sensors with a detection range from 1 to 10,000 ppm of CO2. In the case of sensors based on carbon materials such as graphene and carbon nanotubes, they have a detection range of 1–4000 ppm for CO2. Moreover, we discussed how the variations of parameters such as temperature, humidity, CO2 concentration and curvature radius (in the case of flexible sensors) affects the response times, recovery times and response percentage of the hybrid/flexible sensors. Furthermore, this review presents the current unsolved challenges for flexible CO2 sensors, which are limiting their use in commercial applications. Finally, our review emphasizes the opportunities of research for hybrid/flexible CO2 sensors.