Polymer-derived microporous ceramic coated sensors for selective H2/CO sensing

Abstract

Currently, most of the hydrogen is produced through steam reforming of methane. Methane reforming and water gas shift reactions are often not completed, leaving very small concentration of CO (∼50 ppm) in the fuel stream. Trace amount of CO (few ppm) causes a substantial degradation in the fuel cell performance as CO acts as poison for Pt-based catalysts, therefore, residual CO concentration in the hydrogen rich stream should be controlled. Rapid and accurate hydrogen detection in the presence of other interfering gases (e.g. detection of H2 in the presence of CO) is necessary during the production, storage and use of hydrogen. It is also essential for monitoring/controlling the hydrogen concentration in nuclear reactors, coal mines, semiconductor manufacturing, etc. The main goal of the present work is to investigate the suitability of polymer-derived microporous ceramic filters for their applications in enhancing H2/CO gas sensing selectivity of chemiresistor gas sensors by integrating filters with chemiresistors. Polymer-derived ceramics possess thermochemical stability and tunable porosity, hence, can be applied for applications in harsh reducing conditions. In the present work, various types of ceramics are synthesized by polymer-pyrolysis route and their performance in the enhancement of gas sensing selectivity have been evaluated. Commercially available vinyl-functionalized polysiloxane, polysilazane and ally hydrido polycarbosilane have been selected as pre-ceramic polymers and are pyrolyzed at 700 °C, 800 °C and 900 °C in argon atmosphere. Polymer-to-ceramic transformation, structural characterizations and porosity characteristics of the synthesized ceramics are investigated. All synthesized ceramics were x-ray amorphous. Porosity characterization of the synthesized ceramics shows that SiOC ceramics are microporous in nature where as SiCN and SiC derived from polymers are found to be non-porous. Moreover, SiOC ceramics obtained at 700 °C are microporous with a mean pore size of about 4.6 Å as measured using nitrogen physisorption method. Microporous SiOC ceramics layers are coated on SnO2- and GaN-based planar chemiresistors with a thickness of about 5-6 µm by dip-coating of polysiloxane solution on planar chemiresistors followed by pyrolysis at 700 °C under argon atmosphere. The diameter of micropores in SiOC (~4.6 Å) is larger than the kinetic diameter of H2 (2.89 Å) and CO (3.76 Å) molecules, allowing in this way their diffusion towards the bottom sensing layer. Transient response characteristics and sensor signals of uncoated- and two-fold SiOC-coated sensors exposed to CO (50, 70 and 100 ppm) and H2 (50, 500 and 1000 ppm) in nitrogen at 400 °C have been performed. The results indicate that uncoated sensors show high response towards both CO and H2 whereas for microporous SiOC coated gas sensors the sensitivity towards the interfering gas CO is significantly reduced.