Research Progress on Integrated Triboelectric Self-Powered Gas and Humidity Sensors
Gas and humidity sensors are widely applied in various fields such as environmental detection,industrial and agricultural production,and medical health.However,the mainstream reported resistive,capacitive,and optoelectric gas/humidity sensors require external energy supply,not only causing the environment pollution from the battery leakage af-ter frequently changed and maintained,but also restricting the sensor's operation under the energy shortage circumstance.As one of the novel energy harvesting devices,triboelectric nanogenerators(TENG)have been widely applied in mechani-cal energy harvesting and self-powered sensors owing to the merits of low-cost,designable structure,and high energy con-version efficiency.Furthermore,researchers have endowed TENG with the ability of acquiring information from the out-side,which is expected to integrate the energy collection and sensing unit into one device.The above-mentioned integrated triboelectric self-powered sensor is one of the hottest topics in the sensing technique.This article provides a review of the current research status and latest progress of integrated triboelectric self-powered gas/humidity sensors,which can be sum-marized from the following three aspects.(1)The working principle and gas sensing mechanism of integrated triboelectric gas/humidity sensors.Meanwhile,parameters(triboelectric charges density and dielectric constant of dielectric layer,con-ductivity of electrode layer)that affect the sensing performances are discussed based on TENG's equivalent circuit model.Triboelectric charges density can be effectively influenced based on the screening effect of the condensed thin film on the friction surface or the electrons transfer between the friction surface and the gas/water molecules,resulting in the increased/decreased equivalent triboelectric charges;Output changes based on dielectric constant of the dielectric layer(also the tribo-electric layer)usually occur in the situation where there is a significant difference in the dielectric constant between the test sample and the dielectric layer.However,the limited gas/water adsorption causes little changes in dielectric constant,result-ing in the poor sensing response;Variations of the output changes of TENG based on the resistances change of the sensing electrode can be attributed to the Kirchhoff voltage divider law,where the voltage received by each load is directly propor-tional to its impedance in a series circuit.However,owing to the high impedance of the air layer and dielectric layers,signif-icant resistance/impedance changes of sensing electrodes during sensing period are demanded,restricting the material selec-tion.(2)The research progresses of integrated triboelectric gas sensors(TGS)are mainly divided into ammonia(NH3),etha-nol,and other volatile organic compound(VOC)gas sensors according to the detection subjects.Furthermore,the applica-tions of TGS in detecting exhaled gas,food spoilage,and exhaust emissions are introduced.(3)Based on the different ef-fects of humidity on the amplitude of output electrical signals,the research progresses of integrated triboelectric humidity sensors(THS)are mainly divided into the sensors with the performances of humidity induced signal drops and rises.Fur-thermore,the applications of THS in non-contact switches,skin humidity and diaper detection are introduced.Finally,the research status and challenges of TGS and THS are summarized.Meanwhile,the prospects of the future development are il-lustrated,providing references for the future research of integrated triboelectric self-powered gas/humidity sensors.
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