Development of a Potentiometric carbon dioxide gas sensor

Abstract

Research was undertaken to develop a fast responding, low power carbon dioxide gas sensor. A review of current technologies was undertaken and the Severinghaus design was selected. The technologies used in the Severinghaus design where analysed and researched section by section so as to be improved upon. This involved looking at the pH-sensing electrode, the reference electrode and the electrolyte as well as overall design/shape of the sensor.

Various metal - metal oxides were looked at for the pH sensing including palladium and iridium and thin film polyaniline was also tested. Tests included linearity, lifetime and speed. Different ways of synthesising the metal oxide electrodes were also investigated, including thermal and electrochemical oxidation. The shape and form of the original metal was also modified from wire, foil, and thin film from sputtering and electroplating. Several electrolytes were tested including amorphous polyethylene oxide (APEO) and a silicone-based polymer. The polymers were tested when made up into the carbon dioxide sensor and exposed to differing relative humidities.

Iridium/iridium oxide and polyaniline were found to be the most stable and reliable pH sensing materials for this purpose. When combined with an Ag/AgCl reference electrode and the silicone polymer in a thin film design layout the best performing sensor was obtained with a T₉₀ time of less than 60 seconds. The best sensor lifetime obtained was 7 days before the polymer electrolyte was poisoned. If the electrolyte was replaced then the sensor's sensitivity was restored.

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