Electrochemical CO[sub 2] Sensor Based on CO[sub 2] Reduction and Subsequent CO Oxidation on Pt/WO[sub 3] Electrode

Abstract

There is an increasing demand for portable electrochemical CO 2 sensors for agricultural, environmental, medical, and biological applications. Present electrochemical sensors are relatively cumbersome and require frequent changes of electrolyte. A novel method based on the reduction of CO 2 on Pt/WO 3 electrode, followed by the anodic oxidation of adsorbed CO in 0.5 M H 2 SO 4 gave a very good linear correlation between peak CO oxidation current and CO 2 concentration (0-4% CO 2 , R 2 = 0.9928; 4-20% CO 2 , R 2 = 0.9082).Portable sensors for CO 2 monitoring are increasingly required by agriculture, environmental, medical, and biological applications. Two main types of electrochemical sensors have been developed for the above applications. The first type is a coulometric sensor 1 based on the measurement of the reduction in pH value of the 1 M KCl/1 M BaCl 2 electrolyte, when the CO 2 in the gas sample reacts with OH -. After measurement, pulse coulometry is used to restore the pH value. For longer lifetimes, the electrolyte must be replaced. The detection range is limited by the maximum change in pH value (0.02-2% of CO 2 ). The second type is based on amperometric measurement. 2,3 The amperometric method is more complicated since it is necessary to remove the oxygen in the gas sample. Dimethylsulfoxide (DMSO) is used as the electrolyte to ensure that no hydrogen evolution takes place during measurement of CO 2 reduction current. However, there is a need to replace the DMSO electrolyte frequently since it adsorbs moisture. Such a sensor can measure CO 2 concentration over a wide rage (0-100%).Theoretical and Practical Considerations The above sensors are relatively complicated and there is a need to change the electrolyte frequently which is inconvenient. Therefore, a sensor which can measure CO 2 in air over a wide range of CO 2 concentration using invariant electrolyte will be a great improvement. If a stream of air containing CO 2 is first reduced on a suitable electrocatalyst to produce adsorbed CO and then subsequently oxidized at anodic potentials, it may be possible to measure the CO 2 concentration on the same electrode. When CO 2 is reduced on Pt surface, adsorbed CO is the main product. 4 Subsequent anodic oxidation of the adsorbed CO may give a correlation between CO 2 concentration and the peak CO oxidation current. However, since it is well known that Pt is strongly poisoned by adsorbed CO, 5,7 very high anodic potential is required to oxidize the adsorbed CO, leading to Pt oxide formation. Subsequent reduction of the Pt oxide at lower potential leads to changes in Pt area, affecting reproducibility of results. Therefore, it is necessary to use catalysts which can oxidize CO at lower overpotentials. Work by Tseung et al. [5][6][7][8][9][10][11] showed that coelectrodeposited Pt/WO 3 and Pt-Ru/WO 3 are bifunctional anode catalysts, enabling CO and other reaction intermediates to be readily oxidized at much lower overpotentials and these catalysts are relatively immune to poisoning. Experimental All el...

Study of crystallographic, optical and sensing properties of Na2WO4 films deposited by thermal evaporation with several thickness

“…The long-term stability of the metal oxide at higher operating temperatures results from diffusion of oxygen vacancies in the metal oxides [9]. WO 3 has been proposed for detection of different types of gases, such as ethanol [10], H 2 [11], CH 3 SH [12], CO 2 [13] and O 3 [14]. Also, because ammonia gas is dangerous, all industries should install an alarm system that monitors it.…”