In this work, we present a versatile and a robust electrochemical duplex sensor for detection of CO₂ and humidity using a novel sensing material – Room temperature ionic liquid (RTIL). Rising environmental concerns have led to the development of sensors to monitor environmental conditions such as CO₂, VOC's, particulate matter, temperature, and relative humidity. Requirement of sensor performance metrics such as low power, high stability, increased sensitivity has led to the investigation of RTIL as a suitable candidate for the CO₂ and humidity sensing for usability in a broad range of applications from air quality monitoring in HVAC systems, weather prediction, and monitoring the environment in green houses. RTIL's are essentially solvent free electrolytes consisting of an organic cations and inorganic anions with tunable chemical and physical properties for sensing. The innovation of this study is to interface RTIL's with a metal electrode sensing platform which results in the formation of multi stack electrochemical double layer (EDL). CO₂ or water molecules settle within the interstitial spaces formed between the RTIL moieties on the application of a bias potential. AC perturbation of the system causes charge redistribution in the EDL as a result of CO₂ or water adsorption which is studied through an AC based technique- Electrochemical impedance spectroscopy (EIS). The frequency response of the EDL provides an insight of the impedance changes of the EDL as the environment changes around them.

We investigated the effects of ambient and elevated CO₂ concentrations across a temperature range of 25- 65°C at 45% humidity on two fluorinated RTIL’s- EMIM[TF₂N] and EMIM[FAP]. Performance comparison was based on the impedance response and dynamic behavior of the system based on the changes in the anionic symmetry. Both, EMIM[TF₂N] and EMIM[FAP] behave similarly but EMIM[TF₂N] shows a higher sensitivity to CO₂ across temperatures. We also report the effect of varying humidity from 25% to 65% across temperature range of 25- 65°C on the performance of a non-fluorinated RTIL- MMIM[MeSO₄]. MMIM[MeSO₄] shows a larger change in impedance response with increasing humidity percentages at elevated temperatures. RTIL’s have the potential to be integrated with semiconductor technology for the sensitive detection of CO₂ concentrations and humidity ranges across varying temperatures. Our work demonstrates the potential of a new strategy for achieving low power environmental sensors.