Electrochemical Impedance Spectroscopy (EIS) in the past few years has been used to design gas sensors. EIS based gas sensing has been considered as an ultra-sensitive technique to identify and measure different toxic gases. Their sensing performance has been superior when compared to other conventional techniques. Impedance makes it possible to investigate the finer changes occurring on the surface of sensing layers leading to the identification of the target gases such as H2S. The measurements are carried out by scanning the electrode surface with a mixture of frequencies from 1 Hz to 10 MHz and recording the response as Nyquist and Bode plots. From these plots, the results can be used to elucidate the electrochemical phenomena. The acquired impedance data can be fitted into equivalent circuits and this is further modeled to obtain information about the interfacial phenomena like grain bulk resistance, grain boundary resistance, interface between electrode and sensing film resistance. These phenomena support the sensing of toxic gases from ppm - ppb level. This article focuses on impedance spectroscopy and its relevance to sensors, challenges in detecting toxic gases on the electrode surfaces prepared using semiconducting metal oxides, doped metal oxides and graphene based composites along with the research gaps, current innovations and future directions. EIS based gas sensors have high selectivity, sensitivity, reproducibility, long term stability and they also assist prediction of the electrochemical mechanisms for the analyte gases.

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Hydrogen Sulfide (H2S) is a common toxic gas released into the environment mainly during the energy production process from coal and crude oil. H2S causes several neurological damages to the human body which could also be fatal and thus needs to be monitored. Here, we report the incorporation of reduced graphene oxide (rGO) on β-Ga2O3 (β-Ga2O3/rGO) sensing layers followed by its deposition on alumina substrate by drop casting method for H2S gas sensing application. The structure and phase purity of the synthesized β-Ga2O3, GO and β-Ga2O3/rGO samples were characterized using X-ray diffraction (XRD). The gas sensing properties of the coated sensing layers of β-Ga2O3 and β-Ga2O3/rGO were analyzed by Electrochemical Impedance Spectroscopy (EIS) at 100 °C. The gas sensing results revealed superior sensitivity and selectivity of β-Ga2O3/rGO towards H2S detection when compared with different interfering gases (NH3, SO2, CO2 and CO). Nyquist plots and equivalent circuit fitting values clearly indicate that the grain boundary resistance was highly affected in H2S gas environment (3 ppm) compared to other interfering gases. Based on the findings the gas sensing mechanism is proposed.