Amperometric gas sensor technologies

Abstract

This project investigated the electrochemistry of carbon monoxide and various volatile organic compounds (VOCs primary alcohols, ketones and esters) with the aim of understanding the details of the operation of amperometric CO sensors and of developing VOC sensors of similar performance. The electrochemical devices studied were based on commercial CO sensors (supplied by Alphasense Ltd) and specially modified Alphasense sensor devices designed to measure VOCs. In amperometric response to carbon monoxide, all of the tested sensors (kindly provided by Alphasense) showed an increase in current flowing within the sensor. This increase in current was directly proportional to the concentration of analyte within the zero-air carrier gas. The large capacitance of the devices prevents the use of standard potentiodynamic techniques to interrogate the mechanism. Instead we employed concentration-step experiments at constant potential using a gas flow system under digital mass flow control. A diffusion-based model for the sensors was derived and solved. Custom modelling software using nonlinear least squares was developed to fit the experimental data and to derive estimates of the apparent membrane/diffusion layer thicknesses and the apparent diffusion coefficient of the analyte. The standard CO-AF sensors whilst exposed to carbon monoxide were calculated as having (at 20°C) layer thicknesses (L) on the order of 10-1 cm, this is commensurate with the physical distance from the top face of the sensor to the electrolyte boundary of approximately 0.3 (±0.1) cm. It also gave CO diffusion coefficients (D) on the order of 10-3 - 10-2 cm2 s -1 (at 20°C). These values suggest currentlimiting diffusion in the gas phase rather than the thin liquid layer (D approximately 10-6 cm2 s -1 ) covering the working electrode. Increasing the thickness of the semi-permeable membrane in the standard CO-AF sensor gave an increase in L, thus validating the diffusion model. In response to VOCs, the CO-A1 sensors (kindly provided by Alphasense) only gave a significant amperometric response to alcohol and aldehyde functional groups. This is in line with the current literature and was hypothesised to be due to those functional groups being relatively easily chemisorbed and subsequently electrooxidised at platinum under aqueous acidic conditions. The unresponsive VOCs were thought to be due to a combination of lack of solubility and high oxidation potential. Again, the sensors showed directly proportional current responses to the concentration of VOC analyte within the carrier gas. At all temperatures the CO-A1 sensors showed layer thicknesses (L) on the order of µm or smaller, and VOC diffusion coefficients (D) were in the 10-8 cm2 s -1 range or smaller. Thus, indicating that the rate limiting step for the detection of alcohols and aldehydes was within the 5M sulfuric acid electrolyte solution or perhaps a kinetic barrier, e.g., for dissolution at the air/electrolyte interface. Finally, some initial studies on alternative electrolytes for amperometric gas sensors were carried out. The motivation for this is that the standard electrolyte (5M H2SO4) is corrosive and hygroscopic. The choice of electrolyte for amperometric devices is not simple because it must have a negligible rate of evaporation in air over long periods (1-2 years). Most aqueous electrolytes are unsuitable and only a few organic electrolytes (propylene carbonate) are both non-volatile and non-toxic. An alternative is a polyionic hydrogel; polyacrylate was chosen and the electrochemistry of some simple redox compounds were investigated. However, the hydrogels ultimately proved unsuitable for the long-term requirements of the sensors due to weight loss over time.