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Wallgrén, Kirsi (2005) Novel amperometric gas sensors. PhD thesis, University of Nottingham.

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Abstract

The electrochemistry of oxygen and methanol at solid polymer electrolyte (SPE)-based amperometric sensors, fabricated according to an all-planar design concept, has been investigated.

The solid protonic conductors used were Nafion®-117 membranes and Polybenzimidazole (PBI) films. The working and counter electrodes were non-porous gold and/or platinum layers (300-1500 nm thick), sputtered on the same face of the solid electrolyte, separated by a gap of the ionic conductor (10-1 mm wide) and in contact with the gas sample. Such all-planar solid-state devices could offer potential advantages over sandwich-type gas sensors namely, reduction in precious metal electrode area and simplified fabrication.

Sensors based on both materials exhibited near-linear response to oxygen concentration changes (in the 0.1-21% v/v range) and response times comparable to those of commercially available sensors, irrespective to sample relative humidity, but the magnitude of the signal did depend on the latter even after ohmic correction or at low currents.

A systematic study of the effect of humidity on oxygen reduction and gold surface electrochemistry reveals, that the fall in the oxygen signal with decreasing humidity cannot be explained simply in terms of decreasing membrane conductivity and increased ohmic losses, but is related to the effect of water on the number of electro active sites, their catalytic activity and oxygen reduction mechanism in general. The latter is further supported by the unusually high Tafel slopes obtained both on gold and platinum electrodes with decreasing levels of test gas humidification.

The shape of the oxygen reduction current-potential curves observed at open all-planar gold-based devices and the magnitude of current at both gold-and platinum-based ones, when compared to those of sandwich-and capillary-type arrangements, point to high mass transport rates and a thin or porous mass transport barrier. Current distribution considerations supported by surface electrochemistry estimates suggest that parts of the deposit closer to the reference and counter electrodes contribute more to the observed currents. Further experimentation by varying the deposit thickness and progressive masking of working electrode areas, revealed that the test gas reacted both at the line formed by the gas/solid electrolyte/metal layer interface (diffusion from the gas phase) and underneath the deposit (diffusion from the back of the sensor and through the Nafion® membrane), but not through the metal layer.

For monitoring of dissolved methanol (0.5-3 M) in acidic solutions using bare platinum micro disc electrodes and of methanol vapours (in eqUilibrium with 2-10% w/w or ca. 0.6-3 M aqueous solutions of methanol) using Nafion®-based all-planar platinum sensors, a simple amperometric method was developed. For both types of sensors a clear voltarnmetric picture was obtained with a good separation of methanol oxidation and oxygen reduction curves. The amperometric response could be correlated to the variations in methanol concentration, demonstrating the suitability of the method for crude monitoring of dissolved methanol levels in a range applicable to the feed of direct methanol fuel cells.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Azzopardi, B.J. Sotiropoulos, S.
Keywords:	Chemical detectors
Subjects:	T Technology > TP Chemical technology
Faculties/Schools:	UK Campuses > Faculty of Engineering
Item ID:	49484
Depositing User:	Airey, Ms Valerie
Date Deposited:	01 Feb 2018 15:45
Last Modified:	06 May 2020 14:16
URI:	https://eprints.nottingham.ac.uk/id/eprint/49484

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