Development of electrochemical devices for hydrocarbon sensing purposes in car exhaust gases
- Autores: Fidel Toldra Reig
- Directores de la Tesis: José Manuel Serra Alfaro (dir. tes.), Antonio Eduardo Palomares Gimeno (tut. tes.)
- Lectura: En la Universitat Politècnica de València ( España ) en 2018
- Idioma: español
- Tribunal Calificador de la Tesis: José Manuel López Nieto (presid.), Carlos Bernuy López (secret.), George Van Der Donk (voc.)
- Programa de doctorado: Programa de Doctorado en Química Sostenible por la Universidad de Castilla-La Mancha; la Universidad de Extremadura; la Universidad Jaume I de Castellón; la Universitat de València (Estudi General) y la Universitat Politècnica de València
- Materias:
- Enlaces
- Tesis en acceso abierto en: RiuNet
- Resumen
The present thesis is focused on the development of solid-state electrochemical devices for the selective detection of hydrocarbons in car exhaust gases. For this purpose, several materials were tested as electrodes and electrolytes. Catalytic activation of the working electrode has also been taken into account to boost the electrochemical reaction of the target analyte.
Ethylene is one of the most abundant hydrocarbons in an exhaust gas and was selected as the target analyte to quantify the total amount of hydrocarbons. Not only the device has to be selective to ethylene but it must also have a low cross-sensitivity toward other pollutants abundant in an exhaust gas such as carbon monoxide, water, other hydrocarbons, nitrogen dioxide, etc. Thus, a solid-state potentiometric sensor was selected based on 8% Ytria-stabilized Zirconia (8YSZ) as electrolyte. Two electrodes were screen-printed on top of each face.
First, several metal oxides were tested as working electrode with platinum (Pt) as reference electrode at 550ºC. Most of the materials were discarded because of their lack of selectivity to ethylene, high cross-sensitivity toward carbon monoxide or problems regarding stability. Fe0.7Cr1.3O3 mixed with 8YSZ was finally selected as the most promising material because of its selective response to ethylene with relatively low cross-sensitivity toward carbon monoxide.
This sensor configuration was then exposed to water and phenanthrene and methylnaphthalene. This led to an increase of the cross-sensitivity of the device toward carbon monoxide making the device not suitable for the purposes of the present thesis. The approach to improve the sensor performance was to modify the reference electrode. Platinum, usually employed in literature as reference electrode, was exchanged for a mixed ionic-electronic conductor active to oxygen: La0.8Sr0.2MnO3 mixed with 8YSZ (LSM/8YSZ). Unfortunately, this increases the device activity toward carbon monoxide increasing its cross-sensitivity.
Several nanoparticles were added onto the working electrode to improve the catalytic activity and boost the electrochemical reaction of ethylene. Nickel, titanium and aluminum (the last two elements combined with nickel) provided the best performance: selectivity to ethylene with low cross-sensitivity toward carbon monoxide, water and phenanthrene.
The effect of the electrolyte thickness was also checked in the range from 0.1 to 1.2 mm. Although there was not a huge difference between them, the cross-sensitivity toward carbon monoxide was slightly lower for the thinnest sensor. Other alternatives to 8YSZ electrolyte were tested at lower working temperatures (400 to 550ºC) with the same electrodes materials: gadolinium-doped cerium oxide (CGO) and 10% scandia-stabilized Zirconia (ScSZ). ScsZ-based device showed a good performance in dry conditions but the addition of water decreased its suitability. Once improved the catalytic activity of the working electrode, both devices showed a good performance at lower temperature in dry conditions for ethylene concentration above 100 ppm but the best response was achieved at 550ºC. Both devices were selective to ethylene with low cross-sensitivity toward carbon monoxide, water and phenanthrene.
The effect of mixing the working electrode with an ionic conductor (8YSZ) was also tested by mixing La0.87Sr0.13CrO3 (LSC) with 8YSZ and no change in response was observed when compared to the bare electrode. Finally, the best sensor configuration Fe0.7Cr1.3O3/8YSZ//8YSZ//LSM/8YSZ (after infiltration with nickel) was exposed to nitrogen dioxide to check the cross-sensitivity. The response was still selective to ethylene even with the addition of nitrogen dioxide plus water.
