Considerations of thermodynamics and kinetics for the effects of relative humidity on the electrolyte in electrochemical toxic gas sensors

In this paper, the physical chemistry of the absorption and desorption of water vapor for electrochemical gas sensors with commonly used sulfuric acid as the electrolyte is investigated. Electrochemical gas sensors are being increasingly used for monitoring toxic gases in the environment, and they are, in principle, simple devices, but in practice, their operation is complex. In particular, changes in atmospheric humidity and temperature can have significant effects on the sensor output. A model has been developed for the calculation of sensor weight changes as humidity varies, which are in good agreement with the analysis of experimental results. This then allows for the calculation of the rather more important electrolyte volume variations. Changes in acid molarity and physical characteristics of the electrolyte have also been determined. The effects on working electrode (WE) electrocatalytic activity are discussed, and potential problems with sensors for environmental monitoring are highlighted. In particular, changes in the electroactive area of the WE and, consequently, of the sensor output, and flooding of the WE catalyst aggregates which can lead to problems with electrolyte leakage from sensors are considered.