Study of amperometric gas sensors for nitric oxide/nitrogen dioxide and hydrogen sulfide

There are two main goals in this research. The first goal is to improve NOx amperometric gas sensor (AGS) performance and second goal is to develop H₂S sensors that operate in the presence of high concentrations of hydrogen for process control in fuel cell applications.; A gold working electrode (WE) was fabricated by an e-beam evaporation of 99.999% gold onto a porous Teflon® substrate. The electrode was tested in two different geometries: a lab-built universal (U) electrochemical cell and a commercial housing (C) used in building sensors for sale and distribution. The optimum bias for NO₂ reduction was determined to be −150 mV, vs. Pt/air quasi reference electrode (Pt/air-QRE) and was +400 mV for NO oxidation. Sensor signals were independent of the gas flow rate in both housings at flow rates over 30 cc/min. The “U” sensor had a LOD (Limit of Detection) for NO₂ of 1.3 ppb (S/n = 3σ).; Cyclic voltammetry (CV) techniques were used to measure the concentration of nitric oxide (NO) and nitrogen dioxide (NO₂) in air using the gold electrode amperometric gas sensor. The LOD was about 4.8 ppm for NO in air or 31 ppm for NO₂ in air for the CV methods. A parameter related to the diffusion coefficient and membrane permeability can be determined from CV measurements and was found to be at the high range, 2.4 × 10 −4 cm2s−1, of reported values of diffusion constants. Simple models of combined effects of gas and liquid mass transport and electrokinetic current limiting mechanisms complicate the explanation of observed currents.; An H₂S AGS has been evaluated at concentrations up to 50% (VN) hydrogen. H₂S AGSs were built with three different reference electrodes (RE), specifically a PbSO₄/PbO₂/Au, an IrOx, and a Pt/TFE composite electrode. The Pt/TFE type reference electrode shows unstable standard potential in the presence of high concentration of hydrogen. At a potential 800 mV vs. PbO₂/PbSO₄/Au, H₂S sensitivity was 340 nA/ppm, in a 50% hydrogen background. At an applied potential of +400 mV anodic to the IrOx RE, the H₂S sensitivity was 104 nA/ppm in a 50% hydrogen background.; With various working electrode (WE), sample compositions were evaluated for sensitivity to H₂S in the presence of up to 50% hydrogen. The WEs were prepared by electrochemical deposition of palladium on the gold film at two different loadings (0.3 and 8.1 mg/cm2). At 800mV (vs. a Pb-mix reference electrode) the H₂S sensitivities in N₂ were 65.1, 16.3, 4947 nA/ppm, for pure gold, 0.03, and 8.1 mg/cm2 palladium, respectively. In 1% hydrogen the corresponding H₂S sensitivities were 38.2, 25.0, 120.9 nA/ppm. In 50% hydrogen the H₂S sensitivities were 32.5 and 14.4 nA/ppm for pure gold and 0.03 mg/cm2, respectively, but at 8.1 mg/cm2 palladium loading no H₂S response was observed. A Pd/Gold electrode increased sensitivity in air but in very high concentration of hydrogen, it was lower than the pure gold electrode. Interpretation of these results has led to advances in the analytical ability of amperometric sensors to measure gases at low levels and in process streams containing hydrogen.