A Study on Amperometric NO2 Gas Sensors based on Pt/Nafion® Electrodes

• Main Authors: Wen-Tung Hung, 洪文通
• Other Authors: Kuo-Chuan Ho
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/45016289631959012456

博士 === 國立臺灣大學 === 化學工程學研究所 === 91 === The redox electrochemistry of nitrogen dioxide (NO2) at platinum (Pt)/Nafion® electrodes were studied in the thesis. The Pt/Nafion electrodes were prepared by the impregnation-reduction procedure. The electrochemical behaviors during the oxidation and reduction of nitrogen dioxide at the Pt/Nafion® interface, including the limiting current density, current-potential (I-E) characteristics, sensitivity, response and recovery times, reaction at the sensing electrode, detection limit, and long-term stability were discussed. The effects of flow rate, concentration, and interfering gases (NO and CO) were also studies. The kinetic behavior for NO2 gas reduced at the Pt/Nafion® electrode was discussed. The Pt/Nafion electrode as an anode was maintained at a fixed potential, diffusive regions (or mass transfer controlled regions (0.95~1.20 V)) with respect to a reference electrode. An amperometric gas sensor based on the oxidation of NO2 at the Pt/Nafion anode was tested over the NO2 concentration range from 0 to 500 ppm. A 42 s response time (t95) was attained with a 0 to 100 ppm step change in NO2 concentration whereas the recovery time (t95) was 72 s when the NO2 concentration was stepped from 100 to 0 ppm. The current density response was found to be near-linear with respect to the NO2 concentration up to 100 ppm, but it becomes rather non-linear beyond 100 ppm NO2. The average sensitivities were calculated to be 0.16 µA/ppm (51 nA/ppm/cm2) for 0-100 ppm and 0.30 µA/ppm (94 nA/ppm/cm2) for 150-250 ppm at room temperature. The detection limit of the NO2 gas sensor was found to be around 5 ppm. This NO2 sensor, based on a Pt/Nafion electrode, showed long-term stabilities over 30 days for passing 50 ppm and 250 ppm NO2 concentrations. But the sensor was significantly affected by interfering NO gas, although less affected by CO gas. The cross sensitivities for NO and CO were found to be 11.4~11.9 and 0.2~0.4, respectively, for the sensing devices tested under optimum conditions, i.e. an applied potential of 1.05 V and a fixed flow rate of 200 ml/min. Amperometric NO2 gas sensings at the Pt/Nafion cathode also have been studied and tested over the NO2 concentration range from 0 to 25 ppm. A Nafion membrane was used as a supporting material and the Pt/Nafion cathode was maintained at a fixed potential, which corresponds to the diffusive regions (0.72-0.62 V). A 48 s response time (t95) was attained with a 0 to 5 ppm step change in NO2 concentration whereas the recovery time (t95) was 48 s for a reverse step from 5-0 ppm. The current density response was found to be linear with respect to the NO2 concentration up to 20 ppm, but it deviates from the linearity beyond 20 ppm NO2. The sensitivity in the linear range was -10.4 µA/ppm (-3.3 µA/ppm/cm2) at room temperature. The detection limit of the NO2 gas sensor was determined to be 116 ppb. The NO2 sensor based on a Pt/Nafion cathode showed a long-term stability over 39 days for passing 15 ppm NO2 concentration. For amperometric NO2 sensings at a Pt/Nafion cathode, the cross sensitivities for both CO and NO with respect to NO2 were found to be very small (~ -0.03) in the range of 0-20 ppm NO2 at an applied potential of 0.70 V and a fixed flow rate of 200 ml/min. By performing kinetic study at different scan rates, it was found from cyclic voltammograms that the kinetics of NO2 reduction was much slower than that of NO2 oxidation. The response time of reductive NO2 sensing (t95= 48 s) was longer than that of oxidative NO2 sensing (t95< 30 s) for a step change in NO2 concentration from 0-5 ppm. For NO2 reduction at the Pt/Nafion® cathode, transfer coefficient (), standard rate constant (k0), and exchange current density (i0) were calculated to be 0.5~0.6, 4.0 x 10-6 m/s, and 1.4 A/cm2, respectively. In summary, an amperometric gas sensor based on reduction of NO2 at the Pt/Nafion® electrode has several advantages, including high sensitivity, high selectivity, low interference, and low detection limit. The linear range of the concentration was up to 20 ppm.